Physiol Rev 86: 205–243, 2006;
doi:10.1152/physrev.00023.2004.
Skeletal Muscle Lipid Metabolism
in Exercise and Insulin Resistance
BENTE KIENS
Copenhagen Muscle Research Centre, Department of Human Physiology, Institute of Exercise
and Sport Sciences, University of Copenhagen, Copenhagen, Denmark
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
I. Perspective
II. Long-Chain Fatty Acid Utilization During Exercise
A. Arterial long-chain fatty acid concentration
B. Utilization of long-chain fatty acids during exercise
III. Regulation of Long-Chain Fatty Acid Utilization in Skeletal Muscle During Exercise
A. Supply of long-chain fatty acids
B. Transport from circulation to cytosol
C. Transport in the cytosol
D. Metabolism in mitochondria
IV. Circulating Very-Low-Density Lipoprotein-Triacylglycerol
A. Skeletal muscle lipoprotein lipase
V. Intramyocellular Triacylglycerol
A. Factors affecting intramyocellular triacylglycerol content
B. Methodological considerations
C. Intramyocellular triacylglycerol utilization during exercise
D. Triacylglycerol stored in adipocytes associated with muscle
E. Is intramyocellular triacylglycerol breakdown during exercise influenced by training?
VI. Regulation of Intramyocellular Triacylglycerol Hydrolysis
A. Hormone-sensitive lipase activity during exercise
B. Molecular mechanisms
VII. Relation Between Lipids and Insulin Resistance
A. Intramyocellular triacylglycerols and insulin resistance
B. Lipid intermediates and insulin resistance
C. Protein kinase C
VIII. Conclusion and Future Directions
206
206
206
206
208
208
210
214
215
218
220
222
222
223
225
227
227
228
228
229
230
230
231
233
234
Kiens, Bente. Skeletal Muscle Lipid Metabolism in Exercise and Insulin Resistance. Physiol Rev 86: 205–243,
2006; doi:10.1152/physrev.00023.2004.—Lipids as fuel for energy provision originate from different sources:
albumin-bound long-chain fatty acids (LCFA) in the blood plasma, circulating very-low-density lipoproteinstriacylglycerols (VLDL-TG), fatty acids from triacylglycerol located in the muscle cell (IMTG), and possibly fatty
acids liberated from adipose tissue adhering to the muscle cells. The regulation of utilization of the different
lipid sources in skeletal muscle during exercise is reviewed, and the influence of diet, training, and gender is
discussed. Major points deliberated are the methods utilized to measure uptake and oxidation of LCFA during
exercise in humans. The role of the various lipid-binding proteins in transmembrane and cytosolic transport of
lipids is considered as well as regulation of lipid entry into the mitochondria, focusing on the putative role of
AMP-activated protein kinase (AMPK), acetyl CoA carboxylase (ACC), and carnitine during exercise. The
possible contribution to fuel provision during exercise of circulating VLDL-TG as well as the role of IMTG is
discussed from a methodological point of view. The contribution of IMTG for energy provision may not be large,
covering ⬃10% of total energy provision during fasting exercise in male subjects, whereas in females, IMTG may
cover a larger proportion of energy delivery. Molecular mechanisms involved in breakdown of IMTG during
exercise are also considered focusing on hormone-sensitive lipase (HSL). Finally, the role of lipids in development of insulin resistance in skeletal muscle, including possible molecular mechanisms involved, is discussed.
www.prv.org
0031-9333/06 $18.00 Copyright © 2006 the American Physiological Society
205
206
BENTE KIENS
I. PERSPECTIVE
II. LONG-CHAIN FATTY ACID UTILIZATION
DURING EXERCISE
At the onset of exercise, there is a large increase in
uptake and oxidation of LCFA in skeletal muscle. It is also
well recognized that prolonged exercise for several hours
at a low intensity induces a gradual decrease in the respiratory quotient and hence an enhanced lipid utilization at
Physiol Rev • VOL
A. Arterial Long-Chain Fatty Acid Concentration
The level of the arterial concentration of LCFA during exercise is dependent on the preexercise diet, on the
time elapsed since the last meal (73, 197), and on whether
carbohydrates are consumed during exercise (194, 200).
In general, the higher the fat content of the preexercise
diet and the longer after the last meal, the higher the
concentration of LCFA in plasma, whereas carbohydrate
feeding inhibits the exercise-induced increase in arterial
LCFA concentration due to an increase in arterial plasma
insulin concentration (194) and consequently a decrease
in adipose tissue lipolysis (129). During exercise an initial
decline in the LCFA concentration is often observed followed by a slow increase. Therefore, if exercise is prolonged, the arterial plasma concentration of LCFA may
increase to values markedly above resting levels, but at
the most to resting levels during more intense exercise (3,
30, 35, 78, 79, 162, 239, 300). The initial drop in arterial
LCFA concentration during exercise is most likely caused
by an imbalance between slow mobilization of fatty acids
from adipose tissue and rapidly increased extraction of
LCFA by skeletal muscle.
B. Utilization of Long-Chain Fatty Acids
During Exercise
Whole body LCFA uptake during exercise can be
measured by tracer techniques, infusing either radioactive
or stable isotopes. Combining tracer data and indirect
calorimetry, findings generally indicate that ⬃55– 65% of
total whole body fat utilization during moderate-intensity
exercise is derived from plasma fatty acids (79, 117, 235,
239, 312). During prolonged submaximal exercise, the
contribution of LCFA to energy provision increases with
time (3, 314). If palmitate is labeled on the carbon molecules (14C or 13C), it is also possible to measure LCFA
oxidation, provided that the bicarbonate pool has been
prelabeled. Experiments have been conducted using infusion of 13C- or 14C-labeled acetate to correct for the
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
By the end of the 19th century it was suggested by
Chauveau and Kaufmann (37) that glucose was the dominating energy substrate during exercise and that lipids
were not directly oxidized but first converted into glucose
and glycogen in the liver. In those years it was, however,
of debate whether glucose was the essential energy substrate for the oxidative metabolism in muscle as Zuntz
(345) argued that a mixture of substrates contributed as
energy sources.
The carefully conducted experiments by Krogh and
Lindhard resolved this debate (165). Krogh’s idea was that
the respiratory exchange ratio (RER) measured during
identical exercise bouts for 2 h following the ingestion of
different diets would indicate the preferential use of substrate for combustion. The main findings of Krogh and
Lindhard were 1) that lipids were used as energy substrate and that subjects performed poorly during severe
exercise when lipids were the preferential energy fuel, 2)
that the preceding diet influenced metabolism during rest
and in the postabsorptive state, and 3) that RER values
increased with increasing exercise intensities, indicating
a greater reliance on carbohydrate as energy fuel.
Subsequently, Christensen and Hansen (40, 41) by
measuring oxygen uptake and respiratory quotient further
described how diet, training, and exercise intensity and
duration affected carbohydrate and lipid utilization.
Today it is well known that lipids, in addition to being
necessary as fuel for the organism, have been found to
have fundamental roles as messengers and regulators of
transcription of genes involved in lipid metabolism. Furthermore, evidence is accumulating that lipids are implicated in the pathogenesis of several common human diseases including the metabolic syndrome, cardiovascular
disease, and type 2 diabetes.
Lipids as fuel for energy provision originate from
three different sources: albumin-bound long-chain fatty
acids (LCFA) in the blood plasma, very-low-density lipoprotein-triacylglycerols (VLDL-TG), fatty acids from triacylglycerol located in the muscle cell (IMTG), and possibly fatty acids liberated from adipose tissue adhering to
the muscle cells. This review focuses on the role of lipids
in fuel provision in skeletal muscle during exercise and in
insulin resistance of skeletal muscle.
the expense of carbohydrates as energy fuel. Moreover,
when the exercise intensity increases, a shift in fuel selection appears towards an increase in carbohydrate and
decrease in fat utilization, and following endurance training a shift towards an enhanced lipid utilization is evident
at least when exercise is performed at the same absolute
work load in the untrained and trained state. It is interesting to note, however, that this training-induced shift in
fuel selection is prevented when a carbohydrate-rich diet
is consumed (116, 119). Even though a change in lipid
utilization under different conditions is well described,
the regulatory mechanisms controlling fatty acid uptake
and oxidation are not yet completely known.
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
Physiol Rev • VOL
Concluding from the available evidence, it is evident
that a major part of plasma LCFA taken up during exercise is oxidized in the body, and the fate of the remaining
LCFA is likely to be esterification in noncontracting muscles or other tissues not directly involved in the exercise.
In agreement with this assumption, it has been reported in
humans that muscle contraction diverts leg uptake of
LCFA to oxidation rather than to reesterification in IMTG
(241) and that during leg exercise nonexercising muscle
may in fact take up and reesterify LCFA to IMTG (262).
Along these lines, it is also possible that esterification
takes place in the nonactive motor units in a muscle
working at a submaximal exercise intensity, whereas esterification of LCFA in the actual contracting fibers probably is less likely.
An important question is, however, the extent to
which data obtained in whole body measurements reflect
what is occurring locally in the exercising muscle. This
question is obviously mostly relevant when large muscle
groups are involved in exercise such as in bicycle ergometer exercise. To address this question, it is necessary to
simultaneously measure whole body and leg uptake and
oxidation of LCFA. Our own studies at 60 – 68% of peak
pulmonary oxygen uptake show that only 32– 45% of the
systemic plasma LCFA total uptake (Rd) occurred in the
two legs (117, 235, 237). Supplementing these data, the
study by Burguera et al.(30) showed that during exercise
at 45% of maximal oxygen consumption the uptake in the
legs was ⬃60% of whole body LCFA turnover during
exercise. That such a relatively small fraction of systemic
plasma LCFA total uptake is extracted in the active muscles implies that about one-half of the systemic plasma
LCFA presumably is taken up in adipose tissue, the heart,
liver, inactive or slightly active upper body muscles, and
possibly other organs during bicycle exercise. However,
looking at LCFA utilization at rest and during exercise, it
appears that at rest there is a relatively high whole body
lipid turnover, and the increase with submaximal bicycle
exercise is about two- to threefold (30, 117, 235, 237). In
contrast, leg LCFA uptake at rest is very low and the
increase with exercise is on the order of 5- to 15-fold (30,
235). Thus another way to compare whole body and leg
LCFA uptake is to compare the increase in utilization with
exercise (Table 1). With an examination of data from the
study by Roepstorff et al. (235), a gender comparison is
also possible. It appears that 60 –76% in endurance-trained
females and males, respectively, of the exercise-induced
increase in whole body LCFA Rd can be accounted for by
uptake in the legs during submaximal bicycle exercise at
60% peak oxygen uptake, and no major gender effect was
demonstrated in these endurance-trained individuals (Table 1). Thus 24 – 40% of LCFA Rd is taken up elsewhere
than the exercising muscles. During one-legged exercise
in which only a relatively small muscle mass (2–3 kg) is
engaged in exercise, ⬃55% of the increase in whole body
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
amount of labeled carbon lost in side reactions in the
tricarboxylic acid (TCA) cycle or fixed in the bicarbonate
pool following oxidation of labeled LCFA. Different whole
body acetate recovery values have appeared in the literature as this value obviously depends on exercise intensity and the location of the labeling on acetate. Accordingly, the whole body acetate recovery values obtained
during exercise varied from 66 to 94% at exercise intensities ranging from 23 to 84% of maximal oxygen consumption when 1-14C-labeled acetate or [1-13C]acetate
was infused (268, 296). When [1,2-13C]acetate was infused
during bicycle exercise, whole body recovery ranged from
69 to 100% at exercise intensities from 40 to 75% of
maximal oxygen consumption (237, 259, 313). The highest
values were measured in the trials with the highest exercise intensity. During knee-extensor exercise during
which only ⬃2–3 kg muscle is engaged in the exercise, the
whole body recovery of infused [1,2-13C]acetate was
⬃85% (309). It is interesting to note that when the label is
on the 2-position of acetate only, recovery is markedly
less (296) and not representative for either 1-C or
[U-13C]palmitate due to the higher tendency of the 2-carbon of acetate to participate in exchange reactions in the
TCA cycle compared with the 1-carbon (339). When measurements are performed using arteriovenous leg balance
methods during exercise, acetate recovery was close to
100% (237, 309).
When applying correction factors obtained by [1,213
C]acetate, the available investigations utilizing infusion
of [U-13C]palmitate demonstrate that 80 –96% of whole
body LCFA rate of disappearance (Rd) were oxidized
during bicycle exercise at intensities ranging from 40 to
75% of maximal oxygen uptake (235, 312, 314). In one
study there was a clear trend towards the highest percentage oxidation values at the highest work loads (312). It
was also shown that when preexercise muscle glycogen
content is low and fat oxidation during exercise consequently is high, the percentage of whole body LCFA Rd,
which is oxidized is higher than when preexercise glycogen content is high, suggesting an influence of glycogen
stores on oxidation of the LCFA taken up (237).
When [1-13C]palmitate was infused, ⬃70% of the systemic plasma LCFA was oxidized during exercise at 68%
of maximal oxygen uptake (117) when a mean acetate
correction factor suitable for [1-13C]palmitate was applied. In studies in trained and untrained men, 58 and 76%
of [1-13C]oleate were oxidized during exercise, representing 40 and 80% of peak oxygen uptake, respectively, also
using a mean acetate correction factor suitable for
[1-13C]oleate (269). The lower percentage of plasma LCFA
oxidized in these studies obtained with 1-C- than with
U-C-labeled tracers is probably due to loss of label inside
reactions when using 1-C tracers, and all together, these
observations tend to indicate that uniformly labeled tracers are preferable to 1-C-labeled tracers.
207
208
BENTE KIENS
1. Exercise-induced increase (⌬) in mean whole
body and leg uptake of LCFA and percentage of
increase in whole body uptake of LCFA
that occurs across the legs
TABLE
⌬ WB FA uptake, ␮mol/min
⌬ Leg FA uptake, ␮mol/min
⌬Leg/⌬WB, %
Females
Males
600
360
60
500
380
76
WB, whole body; FA, fatty acid; LCFA, long-chain fatty acid. [Data
from Roepstorff et al. (235) and C. Roepstorff and B. Kiens, unpublished
data.]
Physiol Rev • VOL
III. REGULATION OF LONG-CHAIN FATTY
ACID UTILIZATION IN SKELETAL MUSCLE
DURING EXERCISE
A. Supply of Long-Chain Fatty Acids
There are several steps involved in the pathway from
LCFA release either from adipose tissue or from the triacylglycerol-rich lipoproteins in the circulation into their
final oxidation in the mitochondria which could play a
significant role in regulation of LCFA oxidation (Fig. 1).
Earlier studies pointed to the uptake and oxidation of
LCFA being largely determined by the rate of lipolysis in
adipose tissue as a linear relationship was usually described between LCFA plasma concentration and the rate
of plasma LCFA uptake and oxidation (3, 100, 102, 107,
209). To evaluate lipid metabolism more specifically in a
well-defined human muscle group (5), a series of human
experiments were performed in which exercise was allocated to the knee-extensors of the thigh. Subjects exercise-trained the knee-extensors of one thigh for 8 wk
while the knee-extensors of the other leg served as control. Then acute prolonged exercise was performed with
the trained knee-extensors 1 day and with the untrained
control knee-extensors another day, in a randomized order (147). Despite a continuous exercise-induced increase
in plasma LCFA concentration, uptake of LCFA in the
untrained thigh only increased initially during exercise
and then leveled off, whereas when subjects exercised
with the trained thigh at the same absolute work load as in
the untrained thigh, a continuous increase in uptake of
LCFA occurred (147). Also, when untrained and trained
subjects performed dynamic knee-extensions with one leg at
the same relative work load, palmitate uptake and oxidation
in the knee-extensors leveled off with time in the untrained
subjects but continued to increase in the trained subjects
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
palmitate oxidation was accounted for by increase in leg
palmitate oxidation (310). Collectively the data on whole
body and leg uptake of LCFA indicate that because of the
relatively high whole body resting uptake (Rd) of LCFA,
the Rd of LCFA during exercise only to a limited extent
reflects uptake of LCFA in active muscle during exercise.
It is worthwhile to note that due to simultaneous uptake
and release of LCFA, true tracer-determined LCFA uptake
is two to three times higher than net uptake (101, 117, 235,
300). In this respect, it might also be worth noting that
while it has been suggested, based on experiments in
three subjects, that net leg LCFA uptake is underestimated when the femoral venous catheter is placed in the
normal antegrade (pointing towards the heart) direction
rather than the retrograde direction (pointing towards the
knee), tracer-derived leg LCFA uptake is not affected by
catheter placement (310).
In experiments where leg uptake and oxidation of
plasma LCFA is determined, it is also possible to measure
how much of the LCFA taken up across the legs was
directly oxidized. Values ranging from 72 to 100% of tracerderived LCFA uptake in the legs during exercise have
been reported, and they vary with gender (235), preexercise muscle glycogen stores (237), and exercise mode
(310). These values are within the relative wide span of
values obtained in older studies, revealing that 60 –100%
of radioactive 14C-labeled palmitate or oleate taken up in
the muscles were directly oxidized (102, 103, 108, 109,
300). The large variation in these findings is possibly
caused by the fact that the intensity and duration of
exercise, the training status of the subjects, and the experimental models vary in the studies. It should be noted
that these older values are not corrected for acetate recovery. However, because the acetate recovery across the
legs is not far from 100% during exercise (237, 308), the
data collectively indicate that in most cases more than
75% of the LCFA taken up in the leg during dynamic
exercise is oxidized. What happens to the remainder of
the LCFA is probably reesterification into IMTG as reported (96, 241). This is probably more likely to happen
during submaximal exercise especially at low intensities,
because a large fraction of the motor units in the leg will be
inactive. Obviously, the lower the relative exercise intensity,
the higher the proportion of inactive fibers and the higher
the possibility of incorporation of plasma LCFA into IMTG.
Summarizing the available data, it seems fair to conclude that during submaximal exercise of 90 min or less
only a relatively minor part (35– 60%) of the whole body
LCFA turnover occurs in the exercising muscle. On the
other hand, the exercise-induced increase in leg uptake of
LCFA reflects the measured exercise-induced increase in
whole body Rd fairly well, although the former underestimates the latter to some extent. The underestimation
may be ascribed to LCFA uptake in accessory exercising
muscle other than the legs. Moreover, it appears that the
higher the exercise intensity, the closer to 100% oxidation
of leg uptake of LCFA is achieved, whereas at low exercise intensities, an incorporation of the LCFA into IMTG
may occur probably in inactive motor units.
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
209
(300) (Fig. 2). Similarly, in the isolated, perfused rat skeletal
muscle, palmitate uptake at rest (298) and during contractions (299) displayed saturation kinetics when plotted
against the unbound perfusate palmitate concentration. The
studies lend weight to the idea that the arterial concentration of LCFA to some extent is of importance for extraction
of LCFA in contracting muscle, but it is likely that other
factors inherent in the muscle are of importance too.
FIG. 2. Arterial plasma concentration (A), fractional uptake (B), uptake
(C), and total oxidation (D) of free fatty
acids (FFA ⫽ long-chain fatty acids)
across the thigh during rest and 3 h of
knee extension exercise in trained and
untrained subjects. Values are means ⫾
SE of 6 subjects in trained and untrained
groups. *P ⬍ 0.05 compared with untrained; †P ⬍ 0.05 compared with previous value. [From Turcotte et al. (300).]
Physiol Rev • VOL
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
FIG. 1. Schematic representation of
the likely routes taken by fatty acids from
the capillary to the mitochondria. VLDL,
very-low-density lipoprotein; ALBFA, albumin bound fatty acid; LPL, lipoprotein
lipase; ALBR, albumin receptor; LBP,
lipid binding protein; ACS, acyl CoA synthetase; FABPc, cytosolic fatty acid binding protein; ACBP, acyl CoA binding protein; TG, triacylglycerol; LCFA, longchain fatty acids; CD36, fatty acid
translocase; FABPpm, membrane fatty
acid binding protein.
210
BENTE KIENS
Further support for this notion is found in the study by
Romijn et al. (240). In their study intralipid plus heparin
was infused during exercise to increase the concentration
of LCFA in plasma. During exercise at 85% of maximal
oxygen uptake, the uptake of LCFA was only 27% higher
in the intralipid trial compared with the control trial, even
though the plasma LCFA concentration averaged 2.13 mM
in the intralipid trial in contrast to 0.29 mM in the control
trial.
B. Transport From Circulation to Cytosol
1. FAT/CD36
Recent evidence supports an important role for FAT/
CD36 in uptake of LCFA in rodent skeletal muscle. For
instance, overexpression of FAT/CD36 in mice reduced
plasma triacylglycerol and LCFA concentrations and increased palmitate oxidation in contracting soleus muscle
compared with wild-type muscles (135). Conversely, FA
uptake was reduced in FAT/CD36 null mice (71). When
plasma membranes were isolated, using the giant vesicle
preparation, a higher FAT/CD36 protein expression was
obtained in muscles from streptozotocin-induced diabetic
rats (181) and in muscles from obese Zucker rats (182),
which display increased fatty acid transport. Conversely,
when rats were treated with leptin for 14 days, a reduction was obtained in sarcolemmal FAT/CD36 protein content in association with a decrease in FA transport (282).
In humans, a fat-rich diet induced an increase in the
expression of FAT/CD36 measured in homogenates from
the vastus lateralis muscle (32, 238).
Both triacylglycerol content and FAT/CD36 protein
and gene expression measured in the vastus lateralis muscle appeared to be higher in women than in men (155,
281) (Fig. 4). Furthermore, an increase in triacylglycerol
storage was found in skeletal muscle (117, 148) in concert
with an increase in FAT/CD36 protein induced by a highfat diet (238). Moreover, in obese and type 2 diabetic men
and women, the amount of FAT/CD36 protein, measured
in giant vesicles prepared from the rectus abdominus
muscle, was higher compared with lean controls, and the
FAT/CD36 protein expression correlated with the IMTG
content (25). On the basis of these findings of simultaneous high expression of FAT/CD36 and IMTG content, it
could be speculated that an enhanced amount of FAT/
CD36 would be of benefit for clearance of circulating
LCFA. Once taken up in the skeletal muscle, the LCFA
could be either oxidized in the mitochondria or directed
towards resynthesis into triacylglycerol. If the need for
energy is low compared with the uptake of LCFA, the
latter will be directed towards resynthesis into IMTG. In
FIG. 3. Representative Western blots of fatty
acid translocase (FAT)/CD36, plasma membranebound fatty acid binding protein (FABPpm), cytosolic fatty acid binding protein (FABPc), and acyl CoA
binding protein (ACBP) in human skeletal muscle
total crude membranes (FAT/CD36, FABPpm, and
ACBP) or cytosolic (FABPc) fractions showing a
single band at ⬃88, 43, 15, and 10 kDa, respectively.
Physiol Rev • VOL
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
From the circulation LCFA must pass the endothelium, the interstitial space, the plasma membrane, the
cytosol, and the mitochondrial membranes for their final
oxidation in the mitochondria (Fig. 1). The elaborate
chain of reactions provides a number of possible points of
regulating the supply of LCFA for oxidation. It has been a
matter of debate whether the transendothelial and/or
transsarcolemma transport of LCFA is a passive process
dependent on the rate of cellular metabolism or occurs
via plasma membrane protein-mediated transport. However, from recent studies it seems that both mechanisms
are operating (22, 219).
Within recent years three putative fatty acid binding
proteins located at the plasma membrane have been identified. These are 1) the plasma membrane-bound fatty
acid binding protein (FABPpm), 2) fatty acid translocase
(FAT/CD36), and 3) the fatty acid transport protein
(FATP). FABPpm is a ⬃43-kDa protein located peripherally on the plasma membrane, which in fact is identical to
the mitochondrial enzyme aspartate aminotransferase
(mAAT) (26, 288). FAT/CD36 is a 88-kDa integral membrane glycoprotein with two predicted transmembrane
domains, and it is found to be 85% homologous to the
glycoprotein IV or CD36 of human blood platelets and
leukocytes (2). FATP is a 63-kDa integral protein with six
predicted transmembrane domains (253), and recently a
family of FATPs was identified (122). Both FABPpm and
FAT/CD36 are present in most metabolic tissues including
human skeletal muscle (24, 31, 32, 149, 155, 234) (Fig. 3),
and the transcript of FATP1 mRNA has also been detected
in human muscle (24, 155). To date, the mechanism of
action of these lipid binding proteins is not well known,
but increasing evidence is emerging that they are involved
in fatty acid uptake.
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
211
other hand, a FAT/CD36 pool located beneath and in close
association with plasma membrane cannot be excluded
from the morphological studies. Recently, by subcellular
fractionation, the presence of FAT/CD36 protein expression was demonstrated in subsarcolemmal and intramyofibrillar located mitochondria in white and red gastrocnemius muscle of female rats (33). Considering the immunohistochemical data, this finding is surprising but may
possibly indicate that FAT/CD36 located in the mitochondria may have the epitope, against which the antibody is
raised, buried in the mitochondrial membrane, and that
the epitope is only exposed when membrane structures
are degraded during lysate production for Western blot.
accordance with this suggestion, overexpression of FAT/
CD36 in C2C12 myotubes increased intracellular triacylglycerol content in the presence of excess (750 ␮M)
palmitate but not when palmitate in the medium was low
(80 ␮M) (11).
The localization of FAT/CD36 in human skeletal muscle was recently investigated morphologically (143, 320)
(Fig. 5). In cryosections from the vastus lateralis muscle,
immunofluorescently labeled with anti-FAT/CD36, it was
shown by confocol microscopy that FAT/CD36 was highly
expressed in endothelial cells and to a lesser degree in the
sarcolemma (143, 320). From these studies it also appeared that FAT/CD36 was more abundant in type 1 than
type 2 muscle fibers (143, 320). These findings are in
agreement with morphological studies in rat (343) and
equine muscle (306).
Interestingly, while it was found that FAT/CD36 was
associated with the plasma and endothelial membrane, no
staining of FAT/CD36 was seen in intracellular compartments (320, 343), whereas others reported that intracellular clusters of FAT/CD36 could be identified in type 1
fibers only (143, 306). Even so, the intracellular labeling
was very weak and not associated with mitochondria
(143). That FAT/CD36 was localized to the sarcolemma
with no apparent staining in the cytosol seems to be in
contrast to findings from fractionation studies in rat skeletal and heart muscle where FAT/CD36 was detected in
both an intracellular depot and in the plasma membrane
and that translocation between these two pools can take
place in the resting state and upon various stimuli such as
insulin, leptin, AICAR, and electric stimulation (23, 183,
185, 188, 282) and hence acutely regulate LCFA uptake.
However, detection of intracellular FAT/CD36 may also
be due to processing of newly synthesized proteins on
their way to the plasma membrane rather than mature
protein sequestered in a storage compartment (4). On the
Physiol Rev • VOL
2. FABPpm
With regard to FABPpm, this protein may also play
a role in the transport of LCFA across the sarcolemma.
In rat muscle, fasting (48 h) increased FABPpm protein
content, but only in oxidative muscles which are highly
dependent on fatty acid metabolism (301). A role for
FABPpm in transport of long-chain saturated and nonsaturated fatty acids has been suggested from use of antibodies against FABPpm, which leads to inhibition of LCFA
uptake in various cell types and of transport of LCFA into
skeletal muscle giant vesicles in rats in a dose-dependent
manner (263, 274, 287, 303, 344). Similarly, overexpression of FABPpm by transfection in rat skeletal muscle
increased fatty acid transport and metabolism in resting
muscle (42). However, elucidation of the role of FABPpm
in LCFA transport was challenged by the findings that
FABPpm is identical to the mitochondrial isoform of the
enzyme aspartate aminotransferase (mAspAT) (26, 288).
In favor of a role of mAspAT/FABPpm in fatty acid binding, molecular modeling studies of the crystal structure of
mAspAT have identified a pocket, within the larger domain of the enzyme, which is of sufficient size to accommodate the typical LCFA (15). Whether in fact this pocket
serves as a fatty acid binding site remains to be elucidated. Recently, a study using a polyclonal antibody
against rat mAspAT in immunogold electron microscopy
of rat tissue sections has reported a strong labeling of
mitochondria in several cell types (36). Labeling was also
observed in other locations such as endothelial cell surfaces, and it was concluded from these observations that
mAspAT/FABPpm is both a mitochondrial enzyme and a
plasma membrane protein (36).
Dietary manipulations affect FABPpm protein expression. A high-fat diet induced an increase in FABPpm protein expression in vastus lateralis muscle in male volunteers, and a carbohydrate-rich diet decreased the FABPpm
content (238). The upregulation of FABPpm protein was
however only obtained after long-term dietary interventions as short-term dietary interventions did not affect
FABPpm protein expression (32, 238). In human obesity, a
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
FIG. 4. Intramyocellular triacylglycerol (IMTG) (mmol/kg dry wt)
and FAT/CD36 protein (arbitrary units) content in vastus lateralis muscle in male (n ⫽ 24) and female (n ⫽ 24) subjects. *P ⬍ 0.05 between
gender. [Data from Kiens et al. (155) and Steffensen et al. (281).]
212
BENTE KIENS
higher FABPpm protein expression in homogenates from
the vastus lateralis muscle was found compared with lean
subjects, irrespective of gender (271). This is in contrast
to findings by Bonen et al. (25), who showed a reduced or
no difference in protein expression of FABPpm measured
in homogenates or plasma membranes (from giant vesicle
preparations), respectively, from the rectus abdominis
muscle in a gender mixture of obese compared with lean
subjects. The discrepancy between these findings might
be ascribed to the fact that different muscles have been
investigated.
Exercise training is another condition where the capacity for lipid metabolism is changed. Accordingly, a
higher FABPpm protein expression in homogenates from
the vastus lateralis muscle was also demonstrated with
exercise training (149, 155). The training-induced upregulation of FABPpm protein is obviously related to gender as
Physiol Rev • VOL
changes in FAPBpm protein were not obtained in women
(155). Interestingly, gender differences in FABPpm protein
content in the vastus lateralis muscle were not obtained in
nontrained subjects (155, 271) in contrast to findings of
FAT/CD36 protein content, which is higher in females
than in males (155).
3. FATP1
FATP1 is expressed in adipose tissue, heart, and
skeletal muscle of mouse and rat (1, 122, 250, 253) and in
skeletal muscle of humans (24, 155). Evidence for the
importance of FATP in LCFA transport comes from experiments in cultured cells (122, 253) and yeast (67). A
number of studies have reported that FATP1 has acyl CoA
synthase activity (45, 105, 322), indicating a role in conversion of fatty acids to fatty acid acyl CoA. In growing
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
FIG. 5. Distribution of FAT/CD36 in
human skeletal muscle. A: transverse
cryosections from human vastus lateralis
muscle were fixed and labeled with antiFAT/CD36 mouse monoclonal antibody.
B: colocalization of FAT/CD36 and
caveolin-3 in human skeletal muscle.
Longitudinal cryosections from the vastus lateralis muscle were fixed and double labeled with anti-FAT/CD36 mouse
monoclonal antibody and anti-caveolin-3
rabbit polyclonal antibody. A–C: surface
of a single muscle fiber. D–F: magnification of the central part of A–C. Arrows
indicate a spot with a clear colocalization
of FAT/CD36 and caveolin-3. Bars ⫽ 10
␮m. [From Vistisen et al. (320).]
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
4. Caveolins
Within the recent years caveolae and lipid rafts have
been suggested to be involved in fatty acid uptake. Caveolae are 50- to 100-nm flask-shaped invaginations of the
plasma membrane. Caveolae have been well described in
adipocytes, in endothelial cells, in type 1 pneumocytes of
the lung, and in skeletal muscle cells. Caveolae are a
morphological subclass of lipid rafts, specialized microdomains, composed of sphingolipids and cholesterol
which contain caveolin, the protein which is essential for
invagination of the plasma membrane (for review, see
Ref. 46). Three caveolins have been identified of which
caveolin-1 and caveolin-2 are expressed in most cell
types, whereas caveolin-3 is restricted to skeletal muscle
(254, 293).
Recent data in HepG2 and endothelial cells suggest
that caveolae may play a significant role in uptake and
intracellular trafficking of LCFA (217, 231). With regard to
caveolin-1, it was shown that detergent-insoluble fractions from alveolar type II cells were enriched in FAT/
CD36 and caveolin-1 (163) and furthermore that caveolin-1 binding of fatty acids was saturable (295). In the
caveolin-1 null mice, it was found that serum triacylglycerol and fatty acid concentrations were markedly increased compared with the wild type, especially in the
postprandial state (228). Taken together, these findings
strengthen the view that caveolae and caveolin are involved in regulation of lipid transport. We recently demonstrated that FAT/CD36 was abundantly expressed in
endothelial cells surrounding the capillaries in human
skeletal muscle (Fig. 5) (320), and a close correlation was
obtained between muscle content of FAT/CD36 and
caveolin-1 (C. Roepstorff, K. Roepstorff, B. Vistisen, B.
Kondrup, B. vanDeurs, and B. Kiens, unpublished data).
Further support for a link between FAT/CD36 and caveolin-1 in human skeletal muscle was our recent findings of
a coincidental increase in FAT/CD36 and caveolin-1 proPhysiol Rev • VOL
tein expression in human skeletal muscle, during recovery after intense, prolonged whole body exercise (237).
These data suggest that caveolin-1 and FAT/CD36 may be
regulated in a coordinated manner in conditions of increased LCFA utilization.
Besides the indications of an association of caveolin-1 and FAT/CD36 in human skeletal muscle endothelial
cells, we also recently showed, in longitudinal muscle
sections, in which part of a fiber was cut tangentially, a
high degree of colocalization of caveolin-3 and FAT/CD36
in the sarcolemma (320) (Fig. 5). One putative mechanism
whereby caveolin could be involved in cellular fatty acid
uptake is that caveolae may regulate the function of fatty
acid transporters such as FAT/CD36.
The endothelial cells lining the capillary are the first
transport barrier for the circulating fatty acids before
transport into muscle cell. Earlier studies suggested that a
specific interaction of the albumin-FA complex with proteins associated with the endothelial membrane facilitated the dissociation of the FA-albumin complex and,
hence, the transfer of FA from the vascular to the interstitial compartment (307). Because FAT/CD36 is present
abundantly in the capillary endothelium as well as in the
sarcolemma, FAT/CD36 may play a role in the transport
both in the endothelial cells and in the plasma membrane.
Because the protein has adhesion functions (72), it may
be speculated that FAT/CD36 in the endothelium could be
involved in adherence not only of albumin-bound fatty
acids but also of fatty acids released from the hydrolysis
of circulating VLDL-TG by lipoprotein lipase activity.
Summarizing the available data, it would seem that
the fatty acid binding proteins expressed in the endothelium and plasma membrane are involved in fatty
acid uptake and that caveolae and caveolins may be
involved in this process as well. Yet, the precise functional role of and mechanisms by which the proteins
are mediating LCFA uptake is not known, and the mechanisms of LCFA uptake into various mammalian cells may
not be the same. It may seem redundant that muscle cells
express several types of fatty acid transporters, but the
possibility exists that they have distinct roles in the transport process, or that they function in a coordinated manner in the transport of LCFA. However, this is not known
at present.
5. Influence of exercise
Then the question arises whether the transport proteins may play any regulatory role in LCFA uptake and
oxidation in human skeletal muscle during exercise, a
situation where LCFA uptake and oxidation can increase
manyfold. In rat skeletal muscle, uptake of palmitate into
sarcolemmal giant vesicles increased 50 –75% after contractions (23) and correlated with expression of FAT/
CD36 (187). The training-induced increase in LCFA utili-
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
293 cells, fatty acids taken up through FATP1 were preferentially channeled into triacylglycerol synthesis, which
has suggested a functional link between FATP1-mediated
fatty acid uptake and lipid storage (106). Further support
for this contention are the findings in FATP1 knock-out
mice of a marked reduction in triacylglycerol and diacylglycerol content in the quadriceps muscle after 3 wk
high-fat diet compared with wild-type mice (158). FATP1
protein content was markedly higher in homogenates
from the soleus than the gastrocnemius muscle in rats
(192), and high-fat diets caused an elevation in FATP1
protein content in the soleus muscle, but a reduction in
the gastrocnemius muscle. In a group of matched females
and males, vastus lateralis muscle FATP1 mRNA expression was not different despite the findings of significantly
higher IMTG in the females than in the males (155, 281).
213
214
BENTE KIENS
Physiol Rev • VOL
C. Transport in the Cytosol
LCFA taken into cells are activated in the cytosol by
reaction with CoA and ATP to yield long-chain fatty acyl
CoA (LCFA-CoA) catalyzed by long-chain acyl CoA synthetase (ACS) (Fig. 1). In addition to being substrates for
␤-oxidation and triacylglycerol synthesis (48), it has also
been proposed that LCFA CoA esters play a role in enzyme activation (68, 180), vesicular trafficking (273), and
cell signaling (28). The active site of ACS has been located
to the cytosolic surface of the peroxisomal endoplasmic
reticulum and outer mitochondrial membranes (48). It
was recently demonstrated in 3T3-L1 adipocytes that
long-chain ACS is an integral membrane protein also located in the plasma membrane (84), and it was suggested
that incoming LCFAs are immediately esterified at the
plasma membrane. An efficient esterification maintains a
low intracellular LCFA concentration and contributes to
uptake of LCFA. ACS may therefore affect both the rate
and directionality of LCFA movement across membranes
and may be coupled to other proteins that participate in
LCFA uptake such as FATP 1, which was reported to have
ACS activity (45, 84, 105, 252, 322). There is now strong
evidence that the intracellular transport of LCFA moieties
is mediated by a cytoplasmic fatty acid binding protein
(FABPc) (17, 86, 88) and a cytoplasmic acyl CoA binding
protein (ACBP) (226). Very little LCFA and LCFA-CoA
actually exist as free or unbound molecules but are rather
bound to cytosolic FABPc and ACBP. LCFA-CoAs are
amphipathic molecules and bind strongly to phospholipid
membranes. Data indicate that the cytosolic binding proteins act in extracting LCFA-CoA from and prevent their
binding to biological membranes and liposomes and donate the LCFA-CoAs for metabolic ␤-oxidation (87, 226).
Even though LCFA-CoAs are relatively water soluble and
might move through the cytosol in an unbound fashion,
their binding to FABPc and ACBP is believed to be a major
factor in controlling the free concentration of cytosolic
LCFA-CoA. The importance of FABPc in LCFA uptake
was revealed in mice, where the gene of the heart isoform
of FABPc was ablated. This resulted in a reduced uptake
of LCFA in heart of ⬃50% (199). In skeletal muscle, uptake of LCFA was decreased by ⬃45% in homozygous
mice but was unaffected in heterozygous mice (184).
Other conditions altering the content and regulation
of the cytosolic fatty acid binding proteins are interventions leading to changes in lipid metabolism. Dietary manipulation is one such intervention. Available information
is mainly on the cytoplasmic FABPc. Moreover, most data
are from rat studies, where animals have been fed a
fat-rich diet, mainly composed of saturated fatty acids
(44). Collectively, these data have shown that FABPc in
heart and skeletal muscle did not respond to an increase
in saturated dietary fatty acids (44, 285). In contrast, a
study in rat heart and skeletal muscle revealed that inges-
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
zation during exercise (147, 300) (Fig. 2) could theoretically be ascribed to an increased number and/or activity
of the lipid-binding proteins in the trained compared with
the nontrained state, which might facilitate LCFA transport into the myocyte. This view was supported by the
findings of an increase in muscle FABPpm protein expression with endurance exercise training in male subjects
(149, 155). In addition, training of rats led to an increase
in muscle FABPpm together with a contraction-induced
uptake and oxidation of palmitate (302). The traininginduced increase in FABPpm is apparently gender specific
because the FABPpm protein content was similar in untrained and well-trained female volunteers (155). Interestingly, when comparing endurance-trained and untrained
female and male subjects, FAT/CD36 protein content in
vastus lateralis muscle was not different (155). In contrast, short-term training for 9 days (297) as well as even
a single exercise bout (237) increased FAT/CD36 protein
content slightly (20 –25%) in muscle, perhaps suggesting
that increased FAT/CD36 expression is an early adaptation to increased muscle activity that may wane with
sustained increased activity.
Even though there is evidence that the fatty acid
binding proteins are involved in transport of LCFA across
the membrane, this does not necessarily mean that the
transport proteins or the transport process is rate limiting
for fatty acid utilization during exercise. On the one hand,
overexpression of FAT/CD36 increases fatty acid oxidation during electrically induced muscle contractions
(135), supporting a regulatory role of FAT/CD36. On the
other hand, an intracellular accumulation of fatty acids
was demonstrated in human vastus lateralis muscle when
exercise intensity was increased from 65 to 90% of VO2peak
and fat oxidation decreased markedly (154). This increase
in LCFA content within the muscle cell during the intense
exercise was even found in parallel with a significant
decrease in blood plasma LCFA concentration (154).
These findings give support to the notion that when exercise intensity is increased and fat oxidation decreases,
fatty acid oxidation is limited by factors inside the muscle
cell, rather than by limitations in transmembrane transport. In addition, by manipulating the preexercise muscle
glycogen levels, Roepstorff et al. (237) achieved one situation with high fat oxidation and another with high
carbohydrate oxidation in human skeletal muscle during
a submaximal bicycle exercise bout. This resulted in a
100% higher leg plasma LCFA oxidation, measured by the
tracer technique, during the high fat-oxidation trial compared with the high carbohydrate-oxidation trial, but the
leg uptake, clearance, and fractional extraction of circulating LCFA were similar in the two trials. These findings
further strengthen the view that there are several exercise
conditions during which transmembrane transport during
exercise may not be the important limiting step in plasma
LCFA oxidation.
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
D. Metabolism in Mitochondria
Long-chain fatty acyl CoA (LCFA-CoA) is transported
across the mitochondrial membrane for final ␤-oxidation
to generate acetyl CoA for the tricarboxylic acid (TCA)
cycle. Both transport and oxidation are potential sites of
regulation (Fig. 6).
It has been proposed that an increase in the mitochondrial enzymatic activity induced by exercise training
is important for the increased ability of trained muscle to
combust fatty acids during exercise (90). ␤-Hydroxy acyl
CoA dehydrogenase (HAD) is a key enzyme in ␤-oxidation. Studies have shown that the activity of this enzyme
is increased in conditions of increased fatty acid flux
other than exercise such as a fat-rich diet (115). In agreement with a role of the enzymatic potential of the muscle
in regulating fatty acid oxidation during exercise, a positive correlation between maximal LCFA uptake (r ⫽ 0.88,
P ⬍ 0.05) or LCFA oxidation (r ⫽ 0.76, P ⬍ 0.05) and the
activity of HAD was found across the working thigh (Fig.
7) (146). These findings suggest that the enzymatic capacity in the catabolic pathways is important in determining
LCFA oxidation during exercise, although the exact molecular mechanism behind this relationship is not clear.
Regulation of LCFA-CoA flow into mitochondria results from the fact that acyl CoA derivatives cannot pass
the mitochondrial inner membrane directly. First they
have to be converted to their acyl carnitine derivatives.
This reaction is catalyzed by the enzyme carnitine palmitoyltransferase 1 (CPT1) present at the outer mitochon-
FIG. 6. Schematic representation of
the likely routes taken for long-chain
fatty acyl CoA (LCFA CoA) to enter the
mitochondrion for subsequent ␤-oxidation. CPT-1, carnitine palmitoyl transferase 1; CPT-2, carnitine palmitoyl transferase 2; ACS, acyl CoA synthetase; TCA,
tricarboxylic acid cycle.
Physiol Rev • VOL
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
tion of a diet rich in omega-3 fatty acids markedly increased FABPc content (43), suggesting that the length
and degree of saturation of the carbon chain of the fatty
acids are important for regulation of FABPc. In agreement
with these findings, in humans the ingestion of a fat-rich
diet in which the ratio of polyunsaturated to saturated
fatty acids was relatively high (0.6) induced a significant increase in FABPc protein content in the vastus
lateralis muscle, in contrast to the ingestion of carbohydrate-rich diet for 4 wk (238). It has been suggested
that FABPc cooperates with the membrane-associated
lipid binding protein FAT/CD36 in uptake of LCFA in
cardiac and skeletal muscle (186). Interestingly, in a dietary intervention study, a similar increase in FABPc and
FAT/CD36 protein content was observed, in the vastus
lateralis muscle following a fat-rich diet in human volunteers in contrast to when a carbohydrate-rich diet was
consumed (238).
Even though exercise training increases the potential
for a higher lipid oxidation, neither FABPc nor ACBP
levels in vastus lateralis muscle in humans were changed
by exercise training, suggesting that the amounts of these
cytosolic proteins were abundant and sufficient in trafficking LCFA-CoA esters and LCFA within the cell during
exercise (155). These data are supported by recent findings in rats showing that endurance training did not affect
the protein level of ACBP in skeletal muscle (74) even
though the lipid oxidation potential in muscle was increased.
215
216
BENTE KIENS
drial membrane (Fig. 6) and the generated acyl carnitine
can then cross the inner mitochondrial membrane via the
acyl carnitine/carnitine translocase system. Malonyl CoA
is a potent inhibitor of CPT1 (196). Accordingly, a decrease in malonyl CoA concentrations should increase
LCFA-CoA transmitochondrial transport, caused by a
deinhibition of CPT1 as suggested by studies in rodents
(225). A close relationship between an increase in malonyl CoA concentration and decrease in the rate of fatty
acid oxidation under resting conditions has been described in human volunteers during a sequential euglycemic-hyperinsulinemic clamp (12) and in rats during 3–24 h
of refeeding following 48 h of starvation (39).
The formation of malonyl CoA from acetyl CoA is
catalyzed by the enzyme acetyl-CoA carboxylase (ACC),
which exists in two isoforms (␣- and ␤-form) of which
ACC␤ is dominating in skeletal muscle and heart (97)(Fig.
8). Two types of regulation of ACC have been described.
The first is allosteric activation by the cytosolic concentration of citrate. In accordance, studies in rodents and
humans revealed that high glucose availability at rest
induced an elevation in cytosolic citrate concentration
and in muscle malonyl CoA concentration (12, 224, 242,
243). The second type of regulation of ACC involves phosphorylation and inactivation by 5⬘-AMP-activated protein
kinase (AMPK) (225, 242, 334).
AMPK activity is increased by exercise in human
skeletal muscle. Thus an increase in the activity of ␣2- but
not the ␣1-isoform of AMPK was demonstrated in human
thigh muscle after bicycle exercise above 60% of maximal
Physiol Rev • VOL
FIG. 8. Regulation of malonyl CoA. Schematic illustration of how it
is thought malonyl CoA content may be regulated in skeletal muscle. As
discussed in text, it is controversial whether MCD is activated by AMPK.
AMPK, 5⬘-AMP activated protein kinase; ACC, acetyl-CoA carboxylase;
MCD, malonyl CoA decarboxylase; CL, citrate lyase.
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
FIG. 7. Relationship between muscle activity of ␤-hydroxyacylCoA
dehydrogenase (HAD) and the highest rate of oxidation of fatty acids
during 3 h of dynamic knee extensor exercise at 65% of knee extensor
peak oxygen uptake. Each point represents values from a single individual. [From Kiens (146).]
oxygen uptake but not after exercise of 50% of maximal
oxygen uptake (82, 234, 284, 336, 338) unless the exercise
was very prolonged (337). Moreover, during exercise, the
AMPK activity is increased when muscle glycogen concentrations are low (234, 329, 336). During extremely
intense sprint exercise, the ␣1-isoform is also activated
(38). Accordingly, it would be expected that during exercise, activation of AMPK phosphorylates and inhibits
ACC␤, thereby decreasing the concentrations of malonyl
CoA. Another way to decrease the muscle content of
malonyl CoA is by an increased activation of malonyl CoA
decarboxylase (MCD), the enzyme responsible for decarboxylating malonyl CoA to acetyl CoA (Fig. 8). In accordance, it has been reported that electrical stimulation or
exposure to the AMPK activator AICAR of rat skeletal
muscle induced a two- to threefold increase in both MCD
and AMPK activity (244). In contrast, another study
showed that MCD is not a phosphorylation substrate of
active AMPK and is not activated during muscle contractions in rat fast-twitch skeletal muscle (98). Thus, whether
MCD is activated in muscle during exercise is at present
not certain.
By lowering the malonyl CoA content in skeletal
muscle, the inhibitory effect on CPT1 is diminished, and
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
1. Regulation of fat oxidation by carnitine
Carnitine is substrate for CPT1 and is required for
transport of the activated LC fatty acyl CoA across the
inner mitochondrial membrane and is therefore essential
for the ␤-oxidation (Fig. 6). Carnitine could thus play a
Physiol Rev • VOL
role in regulating lipid oxidation. Available data from
studies at rest suggest that fat oxidation in skeletal muscle is not limited by carnitine (221, 234, 275). Carnitine
could, however, be a molecule that has important regulatory roles in adjusting lipid oxidation during exercise. The
classical findings that carnitine can be acetylated by
acetyl CoA (80) makes carnitine a sink for acetyl groups
during conditions where the rate of acetyl CoA formation
from pyruvate exceeds the rate of utilization by the TCA
cycle. Accordingly, it has been demonstrated that the
concentration of acetyl carnitine within the muscle is
enhanced with increasing exercise intensities (49, 121,
203, 246, 312) decreasing the availability of free carnitine
(49, 121, 246, 312), resulting in low CPT1 activity due to
low availability of its substrate free carnitine. In turn, this
will lead to a diminished supply of LCFA-CoA to ␤-oxidation, consequently limiting oxidation of fatty acids during
exercise. This provides a potential mechanism whereby
an increased availability of pyruvate and acetyl CoA formation can downregulate lipid oxidation (246). Supporting this view, an increase in muscle acetyl carnitine and
decreased muscle free carnitine concentrations were observed concomitantly with a decrease in LCFA oxidation
during increasing exercise intensity in male volunteers
(312). Furthermore, we recently showed that when preexercise muscle glycogen concentrations were low,
thereby providing few acetyl groups to acetylate carnitine, muscle free carnitine concentrations and rate of fat
oxidation were markedly higher during submaximal exercise than when preexercise muscle glycogen concentrations were high and free carnitine concentrations in skeletal muscle were low. The findings indicate that the availability of free carnitine may limit fat oxidation during
exercise (234). Interestingly, when collecting data from
various published studies where acetyl carnitine concentrations were measured in skeletal muscle and substrate
utilization was estimated from RER values during exercise, a close positive relationship between acetyl carnitine
concentrations and RER values and a negative relationship between acetyl carnitine and fat oxidation is found
(Fig. 9). Because total carnitine concentration in skeletal
muscle is unaffected by exercise, this relationship suggests that high RER values and low fat oxidation may be
related to the availability of free carnitine in human skeletal muscle during exercise. Putting it differently the positive relationship in Figure 9 suggests a relationship between free carnitine concentration in contracting muscle
and muscle fat oxidation rate. Even though CPT1 no
doubt is the central gatekeeper for entry of fatty acyl
moieties into the mitochondria, it was recently shown in
isolated mitochondria from rat muscle that FAT/CD36 is
located in the mitochondrial membrane and immunoprecipitation with FAT/CD36 antibody coprecipitates CPT1
(33).This is interesting but surprising since morphological
studies do not reveal such localization (143, 320). It was
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
the entry of acylated fatty acids into the mitochondria is
facilitated. Relatively strong evidence has accumulated
indicating that changes in ACC activity and malonyl CoA
concentration regulate fatty acid entry into the mitochondria in resting muscle (reviewed in Ref. 195) and also
during exercise in rodent muscle (225, 333, 334), but the
question is whether the same regulatory mechanisms operate in human skeletal muscle during exercise. Earlier
studies in human volunteers have failed to demonstrate
decreases in malonyl CoA concentrations during prolonged submaximal exercise (202) and during short-term
exercise at various submaximal intensities (203) despite
marked increases in fatty acid oxidation compared with
rest. However, recently we reported that during exercise
at 60% of maximal oxygen uptake, malonyl CoA concentrations decreased compared with resting values, but the
decrease was in fact similar when subjects were preconditioned with either low muscle glycogen levels and subsequently high AMPK activity or with high muscle glycogen levels and low AMPK activity (234), even though lipid
oxidation was markedly different in the two conditions.
These findings suggest that the decrease in malonyl CoA
concentration from rest to exercise may contribute to the
increase in absolute lipid oxidation when commencing
exercise but plays a lesser role in fine tuning lipid oxidation during exercise. In another study exercise was performed with the knee-extensors at 60, 85, and 100% of the
knee-extensor’s maximal oxygen uptake (56). Going from
rest to 60% one-legged exercise, an ⬃50% reduction in
ACC activity was associated with an increase in fatty
acid oxidation, but no detectable change was noticed in
the concentration of malonyl CoA. With increasing exercise intensities, at which the rate of fatty acid oxidation is diminished, the concentration of malonyl CoA
was in fact modestly decreased and ACC activity was
further reduced. From these human studies, it seems
likely that other mechanisms than changes in ACC
activity, AMPK activity, and malonyl CoA concentrations must be involved in the regulation of lipid utilization during exercise. In line with this suggestion are
recent findings from studies performed in the perfused
rat hindquarter in which low-intensity muscle contractions induced an increase in FA uptake and oxidation
and a decrease in malonyl CoA muscle content without
changes in total AMPK and ACC activities, suggesting
that AMPK activation is not critical in the regulation of
FA uptake and oxidation during low-intensity muscle
contraction (223).
217
218
BENTE KIENS
suggested that FAT/CD36 could play a role in LCFA transfer across the mitochondria membrane likely in combination with CPT1 (33), but further work is needed to investigate this possibility.
Integrating the various findings in muscle during exercise leads to a schematic presentation of the possible
roles of AMPK, ACC, and carnitine in the regulation of
lipid oxidation during exercise (Fig. 10).
2. Other potential regulators of fat oxidation
Other factors involved in the regulation of LCFA
utilization during exercise could be considered. It has
been shown in vitro in human skeletal muscle that a
decline in pH from 7.1 to 6.8 resulted in a significant
decrease of 34 – 40% in CPT1 activity both in sarcolemmal
and interfibrillarly located mitochondria (16, 280). A link
between pH and CPT1 makes sense, as during prolonged
submaximal exercise when lipid utilization is high, only a
small increase in muscle lactate and a small decrease in
muscle pH is observed in contrast to exercise performed
at high intensities, where high levels of muscle lactate
have been obtained in parallel with reductions in pH (245)
and lipid oxidation. Thus an exercise-induced decrease in
pH causing a decrease in CPT1 activity could also seem to
be a likely mechanism contributing to a decrease in lipid
oxidation during high-intensity exercise.
Another step to consider is the ␤-ketoacyl-CoA thiolase, the enzyme responsible for catalyzing the final reaction in ␤-oxidation: ␤-ketoacyl CoA to acetyl CoA and acyl
CoA (Fig. 6). ␤-Oxidation is inhibited mainly by feedback
inhibition and acetyl CoA has been shown to be a potent
inhibitor of ␤-ketoacyl CoA thiolase. Therefore, when
acetyl CoA is accumulating the thiolase is inhibited and
accordingly so is ␤-oxidation. In fact, inactivation of the
thiolase causes the complete inhibition of palmitate ␤-oxidation (177). Thus high concentrations of acetyl CoA, as
Physiol Rev • VOL
observed during intense exercise in muscle (49, 62, 203)
or when muscle glycogen levels are high (221, 234), may
tend to slow ␤-oxidation by inhibition of ␤-keto acyl CoA
thiolase.
Finally, changes in the membrane lipid composition
may affect the kinetic properties of different membraneassociated enzymes and transporters, which could influence the LCFA uptake. Recently it was demonstrated that
exercise training and diet induce modifications of the
phospholipid composition of the membranes in human
skeletal muscle (7, 8, 118). The functional role of a change
in membrane phospholipid composition on transsarcolemmal transport of LCFA should be elucidated.
IV. CIRCULATING VERY-LOW-DENSITY
LIPOPROTEIN-TRIACYLGLYCEROL
Endogenous triacylglycerol (TG) is secreted by the
liver, wrapped up into VLDL. VLDLs are the main carriers
of circulating TG in the postabsorptive state. The hydrolysis of core triacylglycerol in VLDLs is mediated by lipoprotein lipase.
Most authors have neglected the potential contribution of fatty acids derived from circulating VLDL-TG to
energy substrates during exercise. This might be due to
earlier findings where the arteriovenous (a-v) differences
of total serum triacylglycerols (S-TG), both unlabeled and
radiolabeled, were measured in subjects during forearm
exercise (204). The authors concluded that a-v differences
of S-TG did not exceed the level of error of the analytical
methods used in the study. On the other hand, they also
estimated that due to the accuracy of the methods, a-v
differences of up to 20 ␮M could have been missed and
such an a-v difference could, if fully oxidized, account for
⬃25% of the oxygen extraction in the forearm due to the
energy density of circulating TG. Thus the potential for a
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
FIG. 9. Relation between acetyl carnitine and respiratory exchange ratio
(RER) or fat oxidation. Relationship between the acetyl carnitine concentration
in the human vastus lateralis muscle and
RER (A) or fat oxidation (B) during exercise under different conditions. It
should be noted that the sum of acetylcarnitine and free carnitine concentration is constant during exercise. Therefore, an increase in acetylcarnitine concentration coincides with a decrease in
free carnitine concentration. [Data are
extracted from several studies (203, 221,
234, 275, 312, 327) and reproduced from
Roepstorff (233).]
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
219
significant contribution of circulating S-TG was not ruled
out. In addition, findings in the forearm may not be completely representative to findings in the leg. For example,
Havel et al. (109) demonstrated a consistent 4 – 6% arterialfemoral venous difference of circulating triacylglycerol
during exercise in two out of four subjects, whereas in the
other two subjects the differences were small at ⬃1%.
From these data it was concluded that the circulating
triacylglycerols could contribute ⬃10% of the necessary
fuels during exercise. However, in evaluating the possible
contribution of circulating triacylglycerol, it is necessary
to consider that because TG is so energy-dense a significant contribution from circulating VLDL-TG to the oxidative metabolism requires only a very small fractional extraction. Furthermore, because VLDL-TG is the main carrier of TG in the overnight fasted state, it may be more
appropriate to measure VLDL-TG than total serum TG.
Thus, when TG in VLDL was measured in the femoral
artery and vein at rest and during exercise, a consistently
lower VLDL-TG concentration was observed in the femoral vein than in the artery under resting conditions, and
this VLDL-TG degradation was significantly larger when
Physiol Rev • VOL
measured in a trained leg compared with a nontrained
control leg (150). Although consistent femoral a-v differences of VLDL-TG were not found at all time points in all
subjects during submaximal exercise with the knee-extensors, the total net degradation of VLDL-TG during 2 h
of exercise, estimated from the area under the curve of
femoral arterial and venous VLDL-TG multiplied by
plasma flow, averaged 8 mmol in the nontrained leg and
was a little higher in the trained leg (9.3 mmol) (147). The
finding of a degradation of VLDL-TG during exercise was
supported in a recent study, where submaximal bicycle
exercise was performed for 1 h. In that study male volunteers had been on a fat-rich diet [65 energy (E%) fat] for 7
wk while participating in an endurance training program.
After 30 min of exercise, the average femoral a-v difference of VLDL-TG amounted to ⬃100 ␮M, and the estimated energy delivered by VLDL-TG during the hour of
exercise covered ⬃25% of total energy fuels and ⬃40% of
lipid fuels, assuming that all liberated fatty acids were
oxidized (117). When comparable subjects ingested a carbohydrate-rich diet (65 E%) during 7 wk of training, the
degradation of serum VLDL-TG across the leg during ex-
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
FIG. 10. Metabolic interactions during exercise. Metabolic interactions discussed in the text that may determine the fat oxidation rate in human
skeletal muscle during exercise. When exercise induces an increase in glycogenolysis and glycolysis, it will through the action of the PDH complex
lead towards enhanced mitochondrial generation of acetyl CoA and secondly acetyl carnitine. In turn, this will result in a decrease in mitochondrial
free carnitine and consequently in cytosolic carnitine concentrations. A reduction in cytosolic carnitine content results in a decrease in substrate
availability for CPT1 and thereby the generation of LCFA CoA for ␤-oxidation. When, on the other hand, the glycogenolytic and glycolytic rate is
low, the mitochondrial acetyl CoA and acetyl carnitine generation appear to be lower, and a decrease in the carnitine level in the cell does not occur,
and substrate for CPT1 is provided. Exercise also reduces the generation of malonyl CoA due to the increased phosphorylation (inactivation) of
ACC. This reduction may be slightly more pronounced during lower carbohydrate oxidation, due to the lower concentration of cytosolic acetyl CoA.
A reduction of malonyl CoA content with exercise may contribute to the increased fat oxidation appearing from rest to exercise. However, because
a reduction in malonyl CoA content is apparently independent of fuel combusted during exercise, carnitine rather than malonyl CoA seems to be
a likely candidate in fine tuning fat oxidation during exercise.
220
BENTE KIENS
A. Skeletal Muscle Lipoprotein Lipase
As discussed above, it is apparently not the arterial
VLDL-TG concentration that is determining the degradation of VLDL-TG in exercising muscle (117, 198) but
rather local factors in the muscle, of which the lipoprotein
lipase (LPL) activity is important. The activity of LPL
might, on the other hand, also be influenced by the oxidative potential of the skeletal muscle as will be discussed
later.
Since VLDL-TG is degraded by LPL, it is appropriate
to consider whether the activity of muscle LPL is increased by exercise, since this might facilitate breakdown
of VLDL-TG during exercise. LPL is located at the luminal
site of the endothelial cells in the capillary bed of primarily skeletal and cardiac muscle and adipose tissue. LPL is
the rate-limiting enzyme in VLDL-TG hydrolysis, and the
action of LPL makes the liberated fatty acids available for
uptake in the surrounding tissue. LPL is thought to primarily hydrolyze circulating lipoproteins, while it is associated with heparan sulfate proteoglycans (HSPG) to the
capillary endothelium. After synthesis in the myocyte, the
enzymes must transfer to the endothelial cells and then
translocate from the abluminal to the luminal surface of
the capillary endothelial cell. Final posttranslational modifications of LPL involve dimerization and asparaginePhysiol Rev • VOL
linked glycosylation in the endoplasmic reticulum and
Golgi complex (89, 220) to become an enzymatically active enzyme.
To evaluate whether exercise and exercise training
induce an increase in skeletal muscle LPL activity, it is
important to control for the nutritional status, as this has
a significant influence on muscle LPL activity (148, 290).
When dietary control was performed, a 70% higher muscle
LPL activity in the vastus lateralis muscle was demonstrated after 8 wk of regular, dynamic knee-extensor exercise training compared with the untrained muscle in the
same individuals (150), indicating that training by itself
increases muscle LPL activity. This finding was further
supported in a recent cross-sectional study showing that
males and females, who had been engaged in regular
physical exercise training for several years, had significantly higher muscle LPL activity than sedentary matched
males and females (155). It is well known that endurance
training increases the capillarization of skeletal muscle (6,
150, 249). This augmented capillarization will increase the
surface of the endothelium, which offers the possibility to
anchor more muscle LPL to the binding sites at the luminal endothelial cell surface. Accordingly, a close correlation between capillarization and muscle LPL activity has
been demonstrated (150). An increased capillarization
also results in an improved perfusion of the tissue, decreased diffusion distances, and decreased mean transit
time (248). The latter may increase the possibility of a
longer contact time between the VLDLs and LPL, which in
turn may increase lipolysis. Moreover, the capacity to
oxidize lipids is enhanced in the trained state because of
the training-induced increases in the activity of oxidative
enzymes (120, 124, 147, 249). Data have suggested that
when the capacity of tissue to metabolize the liberated
fatty acids is exceeded, hydrolysis slows down, primarily
as a result of product inhibition of LPL (212). Thus an
increased capacity for muscle lipid oxidation may increase the assimilation of fatty acids and result in less
product inhibition of LPL.
Different findings exist as to whether acute exercise
induces changes in muscle LPL activity. In some studies
where one-legged knee-extension exercise was performed for 1–2 h (150, 151) or bicycle exercise for 1 h
(178), muscle LPL activity, measured in biopsies obtained
at termination of exercise, remained unchanged compared with rest. This is in contrast to findings demonstrating a significant increase in muscle LPL activity at termination of exhaustive prolonged bicycle exercise in
healthy male volunteers (153) (Fig. 11). If total energy
expenditure during exercise plays a role for the increase
in muscle LPL activity, that might be the reason for the
discrepant findings. Alternatively, exercise-induced increases in muscle LPL activity may take time to occur.
Accordingly, under conditions where muscle LPL activity
was not enhanced at termination of exercise, a 4-h de-
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
ercise was significantly less in spite of higher serum
VLDL-TG concentrations (117) and contributed only ⬃5%
of total energy fuels and ⬃10% of lipid fuels. Supporting a
role for serum VLDL-TG as a fuel during exercise, an
increase compared with rest in VLDL turnover was observed during moderate-intensity exercise in human volunteers even though plasma VLDL-TG concentrations remained constant during the exercise period (198). Taken
together, the studies suggest that circulating VLDL-TG
may contribute to energy delivery to human skeletal muscle during exercise. The magnitude of this contribution
may vary depending on the habitual diet of the subjects,
exercise intensity, and duration and may reach 25% of
total energy expenditure after adaptation to a fat-rich diet.
Because of the energy-dense nature of VLDL-TG, only a
very small fractional extraction in muscle is sufficient to
cover a substantial amount of energy provision during
exercise. Clearly more research is needed to clarify the
potential role of serum VLDL-TG as a fuel in various
modes and durations of exercise. It is worth considering
that neglecting the contribution of serum VLDL-TG to fuel
provision during exercise will cause an overestimation of
IMTG utilization during exercise in studies where IMTG
utilization is estimated as the difference between total
lipid utilization determined by indirect calorimetry and
whole body isotope-derived LCFA utilization (LCFA Rd),
as described in section VB.
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
layed increase in the enzymatic activity was observed
(Fig. 12) (151), and the increase in muscle LPL activity has
in some situations lasted from several hours up to at least
24 h (Fig. 11) (153, 179, 264).
Recent investigations have shed some light on the
regulatory mechanisms underlying the cellular LPL response to exercise in muscle. Hence, muscle LPL mRNA
levels are not changed immediately after 90 min of submaximal exercise in male or female subjects (155), but
another study revealed an exercise-induced delayed increase (4 h postexercise) in muscle LPL mRNA expression (265) followed by an increase in muscle LPL protein
8 h postexercise (265). These data are supported by the
findings of no elevation in transcription rate of LPL in the
vastus lateralis muscle immediately after a 4-h low-intensity cycling bout or after a 60- to 90-min one-legged kneeextensor exercise (216). However, the transcription rate
increased steadily to three- to sixfold above control levels
after 4 h of recovery. LPL mRNA expression tended to
increase in both conditions but was not statistically significant (216). Thus it is tempting to speculate that the
metabolic demands, induced during exercise, lead to an
activation of the LPL gene and LPL activity in skeletal
muscle to ensure sufficient energy for the muscle during
recovery but also that even small, transient increases in
the transcription rate of muscle LPL generate a cumulative increase in mRNA and subsequently in the protein
content after regular exercise training for several weeks
or months. Accordingly, not only muscle LPL activity but
also muscle LPL mRNA expression was higher in trained
than sedentary males and females (148, 155, 290).
A variety of mechanisms may regulate LPL activity in
skeletal muscle in the postexercise period. During an
euglycemic hyperinsulinemic clamp, we demonstrated a
Physiol Rev • VOL
decrease in resting muscle LPL activity at physiological
insulin concentrations (151), which is in contrast to the
stimulatory effect of insulin on LPL in adipose tissue (Fig.
12) (206). In contrast to these findings, an enhanced muscle LPL activity for at least 24 h postexercise was demonstrated in spite of the ingestion of a carbohydrate-rich
diet (65–70 E%), resulting in enhanced insulin concentrations and high muscle glycogen resynthesis rate (153)
(Fig. 11). Others have also demonstrated a prolonged
increase in muscle LPL activity despite intake of food
(179, 264). These observations may then indicate that
prior exercise makes muscle LPL activity resistant to the
inhibitory effects of insulin. In agreement with this interpretation, we previously demonstrated that whereas insulin decreases muscle LPL activity in resting muscle, the
effect of insulin is apparently counterbalanced by other
mechanisms in muscle after exercise (151) (Fig. 12). One
such “exercise” factor could be a low muscle glycogen
content. However, the increase in muscle LPL activity
during recovery is apparently not directly related to
changes in muscle glycogen concentrations, since we
found that muscle glycogen was decreased at termination
of an exercise bout in parallel with unchanged muscle
FIG. 12. Muscle lipoprotein lipase activity in the nonexercised (F)
and exercised (E) thigh in the control (top) and clamp (bottom) groups.
In the clamp group, the steady-state insulin concentrations were 23
and 44 ␮U/l at 6 and 8 h of recovery, respectively. In the control
group, the lipoprotein lipase activity was measured before (pre) and
after (post) one-legged knee extensions as well as during recovery for
4 and 8 h. *P ⬍ 0.05 compared with contralateral thigh. ‡P ⬍ 0.05
compared with previous measurement. [From Kiens et al. (151),
copyright 2000, The Endocrine Society.]
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
FIG. 11. Intramyocellular triacylglycerol (IMTG) content and muscle
lipoprotein lipase (mLPL) activity in the vastus lateralis muscle obtained at
rest, immediately after exhaustive prolonged glycogen-depleting exercise,
and in the postexercise recovery period. *P ⬍ 0.05 compared with
preexercise values of IMTG. †P ⬍ 0.05 compared with preexercise
values of mLPL activity. [Data from Kiens and Richter (153).]
221
222
BENTE KIENS
diet. When healthy, physically active male volunteers increased their fat intake to 54 E% for 4 wk, a significant
increase in IMTG content of 50% was observed (148).
Similarly, 7 wk of endurance training of healthy male
subjects consuming a fat-rich diet (65 E%) increased
IMTG content by 57%. In contrast, when similar subjects
were subjected to a high-carbohydrate diet (65E% carbohydrates) for 7 wk of training, IMTG content remained
unchanged compared with the pretraining level obtained
on the habitual mixed diet containing 30 E% of fat (117).
Along this line, Coyle et al. (54) demonstrated that a
decrease in dietary fat intake to 2 E% for 3 wk resulted in
a decrease in IMTG content. Other reports also indicate
that IMTG content is dependent on the fat content of the
habitual diet and in the postexercise recovery period (119,
279, 342). Collectively, these data clearly demonstrate
that a fat-rich diet increases and a carbohydrate-rich diet
decreases the triacylglycerol content in skeletal muscle.
The triacylglycerol pool within the muscle receives
LCFAs from circulating albumin-bound fatty acids and
from VLDL-TGs that are degraded by LPL. Thus the dietary influence on IMTG concentrations may be related to
the activity of muscle LPL. The consequence of an increased amount of fat in the diet is an increased activity
of LPL in skeletal muscle along with increased IMTG
(148). On the other hand, a diet rich in carbohydrates for
3 days (139) or 4 wk after the consumption of a diet rich
in fat (148) both reduced LPL activity in skeletal muscle.
The reciprocal variation between dietary carbohydrate
amount and LPL activity could possibly be explained by
the generally higher plasma insulin levels when fed a
carbohydrate-rich diet because insulin is found to decrease LPL activity in skeletal muscle (Fig. 12) (151).
Thus these findings support the notion that diet-induced changes in muscle LPL activity play a significant
role in the dietary-induced changes in IMTG. The likelihood of this possible association is further strengthened
by the fact that when training is performed while ingesting a fat-rich diet (65 E%) the increase in IMTG correlates
tightly with the increase in muscle LPL activity (Fig. 13).
V. INTRAMYOCELLULAR TRIACYLGLYCEROL
2. Muscle fiber type
Triacylglycerol stored within striated muscle cells
(intramyocellular triacylglycerol, IMTG) represents a potentially large energy source, and several factors influence
the IMTG content in skeletal muscle as discussed below.
While the above-described studies make a strong
case for the diet as an important determinant for IMTG
content in resting skeletal muscle, other aspects are of
importance too. For example, as alluded to earlier, concentrations of IMTG are muscle fiber type specific. Previous biochemical analysis on single fibers and histochemical and electron microscopy studies have shown up to a
threefold higher lipid content in type 1 fibers than in type
2 fibers in healthy male subjects (66, 130) and in rats
(277). These data are supported by recent data using oil
red O and immunofluorescence microscopy (316, 317).
Data obtained by 1H-magnetic resonance spectroscopy
A. Factors Affecting Intramyocellular
Triacylglycerol Content
1. Diet
One of the factors determining the content of TG
within human skeletal muscle is the composition of the
Physiol Rev • VOL
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
LPL activity (151). Furthermore, during fasting recovery,
muscle glycogen remained unchanged, whereas LPL activity increased after 4 h and then decreased towards
preexercise levels after 8 h of recovery (151) (Fig. 12) still
in the face of unchanged low muscle glycogen levels.
Interestingly, during the postexercise recovery period, the
maximum activity of muscle LPL activity was observed at
the same time as the muscle TG was decreased the most,
which was 18 h after termination of exercise (153) (Fig.
11). As muscle LPL activity is responsible for VLDL-TG
hydrolysis, it was suggested that the fatty acids liberated
from VLDL-TG hydrolysis might be a potential energy
source during postexercise recovery (153). In accordance
with this hypothesis, Malkova et al. (191) reported a significant clearance of total triacylglycerol and VLDL-TG
across the leg during a postprandial period 14 h after leg
exercise. Similarly, in six healthy male volunteers studied
during 6 h of recovery from intense glycogen-depleting
exercise with one leg, the degradation of VLDL-TG across
the previously exercised leg (4.8 ⫾ 1.0 mmol) was markedly higher than over the nonexercised control leg (1.8 ⫾
0.6 mmol) (Kiens, unpublished observations), and furthermore, VLDL-TG fractional catabolic rate was significantly
higher during the early recovery phase than at rest and
during exercise in human volunteers (198).
Collectively the studies indicate that muscle LPL activity is unchanged during moderate exercise and increased when exercise is strenuous and prolonged, hence
allowing hydrolysis of circulating VLDL-TG to occur. On
the other hand, in the postexercise recovery period after
exhaustive and glycogen-depleting exercise, there seems
to be a marked degradation of VLDL-TG across the legs in
fed subjects despite enhanced plasma insulin levels. This
seems to be caused by a marked increase in muscle LPL
activity, transcription, and translation induced by exercise. Thus it is tempting to speculate that the metabolic
demands, induced during and/or after exercise, lead to an
activation of the LPL gene in skeletal muscle to secure
lipids as energy fuel while the resynthesis of glycogen is
of priority.
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
223
regard to different fiber types by using the 1H-MRS
method are not clear due to the limited number of subjects included in the studies (134, 210).
4. Physical training
(1H-MRS) in skeletal muscle also suggest a fiber type
difference in IMTG. Thus, when the lipid content was
quantified in the soleus and tibialis anterior muscle in
humans, a two- to threefold higher intramyocellular TG
content was found in the soleus muscle relative to that in
tibialis anterior (134, 210, 230). That the amount of triacylglycerol in a muscle is dependent on the fiber type
distribution within the muscle was supported by our
finding in the vastus lateralis muscle of a positive correlation between the percentage of type 1 fibers in the
quadriceps muscle expressed relative to fiber area on one
hand and the IMTG content on the other hand (r ⫽ 0.39,
n ⫽ 41, P ⬍ 0.05) (281).
3. Gender
B. Methodological Considerations
Gender may also play a significant role in triacylglycerol content of skeletal muscle. Recently we demonstrated that females have a higher resting IMTG concentration than male subjects independent of training status
(281). The gender differences could not be ascribed to the
influence of the diet as both female and male subjects had
consumed a similar diet during the preceding 8 days.
Whether the phase of the menstrual cycle plays a role for
the content of lipids in skeletal muscle is not clear. However, in the above-mentioned study, all females were studied in the midfollicular phase of the menstrual cycle. As
mentioned above, previous studies have shown up to a
threefold higher lipid content in type 1 fibers than in type
2 fibers in healthy male subjects (66, 130, 179, 316, 317).
Whether this pattern also exists in females or whether
both type 1 and/or type 2 fibers in females contain an
increased amount of lipids compared with males remains
to be elucidated. Gender differences in lipid content in
Essentially two different methods have been utilized
to measure muscle triacylglycerol content. These are the
muscle biopsy technique with subsequent biochemical or
microscopic determination of IMTG content and the magnetic resonance spectroscopy technique (1H-MRS). For
both techniques a methodological problem is to distinguish between intracellular and intercellular TG. The latter is located in adipocytes associated with the muscle
cells, whereas the former is found in the form of lipid
droplets inside the myocytes.
Physiol Rev • VOL
1. Biochemical determination
The classical method to determine muscle triacylglycerol has been to obtain a piece (5–10 mg wet wt) of
muscle by biopsy and then to freeze-dry it. Subsequently,
the piece of muscle is dissected free of blood, connective
tissue, and adhering adipose tissue under a microscope,
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
FIG. 13. Correlation between the increase in muscle lipoprotein
lipase (mLPL) activity and the increase in intramyocellular triacylglycerol content (IMTG) during 7 wk of fat diet and endurance training. Each
point represents values from a single individual. [Data from Helge et al.
(117) and B. Kiens and J. W. Helge, unpublished observations.]
Longitudinal training studies have shown higher concentrations of IMTG in the vastus lateralis muscle in
healthy subjects after endurance training for 2 wk (261), 6
wk (130), or 31 days (214). Also when exercise training
was allocated to a single muscle group (the knee-extensors) in subjects on a mixed diet for 8 wk, a higher IMTG
content was apparent compared with IMTG content in the
contralateral, untrained leg (147). This indicates that
training by itself may have an enhancing effect on IMTG
levels. Recently, IMTG content, measured in vastus lateralis muscle 48 h after an exercise bout by using the oil
red O soluble dye, was found to be significantly higher in
trained versus untrained, lean male subjects (91), and
similar findings have been made with the MRS technique
(58). In contrast, when female and male volunteers at
different physical activity levels were studied with the
muscle biopsy technique, IMTG content in the vastus
lateralis muscle was found to be independent of training
status (281). The lack of training effect on IMTG content
might very well be caused by the fact that in the latter
study all subjects followed a similar, controlled carbohydrate-rich diet during the preceding 8 days. Because the
IMTG content is strongly dependent on the diet (as discussed above), it is suggested that a training-induced
enhancement of IMTG content in males and females is
dependent on the diet consumed such that ample
amounts of fat in the diet are necessary for an increase in
IMTG with training to occur.
224
BENTE KIENS
in carefully dissected skeletal muscle samples, indicating
no contamination of the muscle with adipose tissue (171)
(Fig. 14B).
A putative explanation for the variability in biochemical determination of IMTG in a muscle biopsy could be
that different biopsies contain different percentages of
type 1 and type 2 muscle fibers, and because the IMTG
content is two to three times higher in type 1 fibers than
in type 2 fibers (66, 130, 316), this could introduce some
variability in the results, especially when muscles with
mixed fiber type distribution are studied. It is, however,
also worth considering that a muscle biopsy obtained
during exercise is a mixture of active and nonactive or
less active fibers, especially during low-intensity exercise.
As reesterification of LCFA into IMTG can take place in
nonexercising muscle (262) and probably also in nonactive fibers, results from a biopsy of mixed muscle will
reflect these different responses and different fiber types,
in turn increasing variation in the results.
To minimize variation due to fiber type differences
between biopsies, one can examine a larger sample
(30 – 40 mg wet wt) that is freeze-dried and dissected as
described above into single fibers, which are then mixed
before the biochemical analysis (152). The advantage of
this method is that an initial large muscle sample from
which pooled single fibers are picked will better represent
FIG. 14. A: Western blots of perilipin
in human adipose tissue (AT) and the
vastus lateralis muscle in female (F) and
male (M) volunteers. Note that muscle
was loaded at 40 ␮g protein, whereas
adipose tissue was loaded at 3 ␮g protein. B: mRNA for various lipid binding
proteins and for the adipocyte specific
protein adipisin in carefully dissected human muscle biopsies. Note absence of
adipsin mRNA in muscle samples. [From
Lapsys et al. (171).]
Physiol Rev • VOL
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
leaving the individual muscle fibers for analysis. The technique has been criticized for having a large coefficient of
variation as described by Wendling et al. (332). This has
been thought to be due to invisible contamination of the
muscle fibers with adipocytes or remnants of adipocytes.
Because the triacylglycerol content in adipocytes is large
compared with the content inside the fibers, contamination with a few adipocytes would greatly influence the
measurements of TG in muscle tissue. Still, it is apparent
from personal experience and from experience from colleagues in the field (315) that after examining many muscle biopsies as well as histochemical sections from
healthy young human subjects, the presence of adipocytes interspersed between muscle fibers is very infrequent. To further elucidate whether contamination of dissected muscle fibers with adipocytes is an issue, we have
performed Western blotting of dissected muscle and of
adipose tissue and have blotted for perilipin, a protein
which is abundant in adipocytes (Fig. 14). As can be seen
from the blot in Figure 14, while a strong signal as expected is found in adipocytes, no signal is found in muscle
loaded at considerably higher protein content, indicating
no or extremely little contamination with adipocytes (Fig.
14A). In agreement with our findings, it was previously
reported that the mRNA expression of adipsin, which is
expressed at a high level in adipocytes, was undetectable
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
2. Morphological determination
Electron microscopy has also been used to examine
sections from muscle biopsies. Whereas this technique
offers unprecedented detailed information, the large magnification makes it difficult to examine a large number of
muscle cells (128). Staining of muscle sections with oil
red O and subsequent analysis by light microscopy has
also been used to estimate muscle IMTG content (91,
190). Recently this technique has been combined with
fluorescence microscopy, which enables direct visualization of both intramyocellular triacylglycerol (IMCL) deposits and muscle fiber type in the same muscle cross
section (57, 316, 317). Applying this method for analyzing
TG content in muscle supported the earlier findings of
fiber type specific IMTG content obtained by biochemical
analysis of single muscle fibers (66). Because the histochemical technique is based on light microscopy, it allows
a large number of fibers to be analyzed from the same
biopsy. However, it should be noted that visualization of
IMTG droplets either by electron microscopy or by light
microscopy only allows a semi-quantitative measure of
IMTG.
3. 1H-MRS
With the 1H-MRS technique, the signal from IMCL can
be at least partially separated from the signal from extramyocellular triacylglycerol (EMCL) because the spectra are slightly different (21). The advantage of the
method is obviously the noninvasive nature of the technique that allows unlimited measurements and apparently
a separation between IMCL and EMCL. However, the
1
H-MRS technique contains a number of limitations and
disadvantages. These include the difficulties in positioning the voxel to avoid vasculature, minimizing inclusion of
subcutaneous adipose depots, and ensuring consistent
Physiol Rev • VOL
orientation of the muscle fibers along the magnetic field.
In addition, the presence of EMCL can cross-contaminate
the IMCL signal whereby the precision of the IMCL estimation will be reduced. Another disadvantage is that the
scans take at least 15 min (20), which in combination with
the time it takes to carefully position the voxel often
causes IMTG determinations to be performed 1–2 h after
termination of exercise and opens the possibility that
measurements reflect postexercise triacylglycerol levels
rather than levels immediately after exercise. Furthermore, until recently, most 1H-MRS studies have been performed on the soleus or the tibialis anterior muscles,
whereas most biopsy studies are performed on the vastus
lateralis muscle making comparisons difficult. However,
recently 1H-MRS studies of the vastus lateralis muscle
have also been published, and the findings from these
studies basically confirm findings in the earlier studies on
the soleus and tibialis anterior muscles (142, 262). A
validation of the 1H-MRS studies has been done in dogs,
where the tissue levels were measured at rest without
prior interventions such as exercise, and the accuracy of
the method was comparable to the biochemical determination (291). In humans, results obtained before and after
exercise showed a good agreement between 1H-MRS data
and electron microscopy of biopsies obtained from the
tibialis anterior muscle, whereas the biochemical extraction method gave different results (131).
4. Indirect measurements
In addition to these methods for measuring IMTG
content in muscle, a third method has been used extensively to estimate utilization of IMTG during exercise.
This is the indirect method based on the difference between whole body lipid oxidation, calculated by indirect
calorimetry, and the plasma rate of disappearance (Rd
FA) or oxidation of fatty acids calculated from tracer
techniques. As discussed in section VC3, this method has
significant limitations.
C. Intramyocellular Triacylglycerol Utilization
During Exercise
For many years it has been debated whether triacylglycerols located in the muscle are utilized during exercise because conflicting results have appeared. With a
closer look at the experimental designs of the different
studies, it is striking that the exercise tests performed
have varied in intensity and duration from minutes to
several hours, and the training status of the subjects has
varied from untrained to trained. Moreover, different exercise models have been employed, and finally the
method by which IMTG was measured differs among
studies. To interpret the role of IMTG as a potential
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
the muscle. With the use of this approach, a much smaller
coefficient of variation (CV) of the IMTG concentration of
4% has been obtained between five samples from the
mixed freeze-dried pool of fibers as described above. This
procedure is in contrast to the situation when a muscle
sample of 40 mg is divided into five small 5–10 mg samples
of wet muscle that are subsequently dissected and analyzed separately resulting in a CV of 31% (281) similar to
the value obtained by Wendling et al. (332).
In several laboratories the Folch method is used for
extraction of lipids from the tissue. Exchanging the laborious and repeated extractions by the Folch method
(where recovery is not 100%) with incubation with tetraethylammonium hydroxide, which selectively cleaves triacylglycerol in tissues (152), also helps reduce variability
in the assay. Performed carefully, the muscle biopsy technique remains the only definitive method for quantification of myocellular triacylglycerol.
225
226
BENTE KIENS
energy source during exercise, these matters have to be
taken into consideration.
1. Studies where the muscle biopsy technique
was applied
Physiol Rev • VOL
When treadmill running at 65–70% V̇O2peak was performed until exhaustion or for 2 h, a decrease by 33% in
the IMTG signal (IMCL in MRS terminology) content in
soleus muscle was obtained in seven males and two females (167) and by 25% in females (172). Treadmill walking at 50% V̇O2peak for 2 h decreased IMCL signal in the
tibialis anterior muscle of males by 22–26% (58), whereas
running at alternating intensities for 90 min or to exhaustion did not decrease the IMCL signal in soleus, gastrocnemius, or tibialis muscles of males (230). In another
study, utilization of IMCL in males was found to be dependent on the intensity of the exercise because running
at 60 –70% of maximal oxygen uptake decreased IMCL
signal in both tibialis anterior and soleus muscles,
whereas running at 80 –90% of maximal oxygen uptake did
not cause changes in IMCL signal in either muscle (27).
Thus the above 1H-MRS studies suggest breakdown of
IMTG during prolonged low- to moderate-intensity exercise, whereas during intermittent or high-intensity exercise, no detectable breakdown occurs. In agreement with
this interpretation, recent 1H-MRS measurements performed on the vastus lateralis muscle reported a 20%
decrease in IMCL signal in trained male cyclists after 3 h
of exercise at 55% of maximal power output (262). Likewise, when strenuous exercise was performed at 70%
V̇O2 max for 3 h in highly trained male cyclists, after the
consumption of either a high- or a low-carbohydrate diet
(48 h), a reduction in IMCL signal of 57 and 64%, respectively, was obtained (142). However, the measurements
were not performed until 1–2 h after termination of exercise. Breakdown of IMTG in the early recovery phase has
been found in some (57, 153) but not in another study
(160), and therefore, it cannot be excluded that a delay of
1–2 h in obtaining postexercise IMTG concentrations may
influence the data obtained.
3. Indirect estimation of myocellular triacylglycerol
utilization during exercise
With the use of the indirect method of quantification
of IMTG utilization during exercise and with the assumption of 100% oxidation of Rd of plasma LCFA, it has been
estimated that nonplasma sources assumed to be intramuscular TG provide 35–50% of total lipid oxidation in
male volunteers during exercise at various durations and
intensities (55, 140, 193, 215, 239). This indirect measure
is, however, based on several assumptions. For instance,
if part of the circulating fatty acids is stored rather than
oxidized, then the apparent contribution of IMTG to fat
oxidation is underestimated. As discussed previously, the
percentage of LCFA-Rd oxidized varies with exercise intensity. Finally, the greatest limitation in the use of the
indirect estimation of triacylglycerol breakdown is the
finding that only 34 – 60% of whole body LCFA disappear-
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
In studies where the muscle biopsy technique was
used in males by Carlson et al. (34) and Costill et al. (53),
a decline of 25% in IMTG was obtained after 99 min of
bicycle exercise and 30% after 147 min of exercise on a
treadmill, respectively. In both of these studies IMTG was
measured on a wet muscle sample from a biopsy, where
dissection of tissues and material other than muscle (such
as connective tissue, blood, and adipocytes) is difficult.
Using a similar procedure of analysis of IMTG, Fröberg
and Mossfeldt (81) and Lithell et al. (179) found a decrease of 50% of IMTG during exercise. However, in those
studies the subjects exercised for 7–9 h (the classical
Swedish Wasa ski-race), and this large energy demand is
expected to recruit all energy sources heavily. Interestingly, the most fit skiers had the largest stores of IMTG
before the race, and they also depleted the stores the
most (179). Findings of an exercise-induced decrease in
muscle triacylglycerol content in males have also been
reported in later studies, where freeze-dried and dissected
muscle tissue has been used for the analysis of IMTG and
exercise was performed for ⬃2 h (64, 133, 214), whereas
other studies did not find detectable changes in IMTG
during exercise (14, 117, 147, 153, 235, 279, 281). Lack of
breakdown of IMTG was also shown indirectly by microdialysis of the vastus lateralis muscle during exercise.
During dynamic knee-extensions at 65 and 85% of kneeextensor peak oxygen uptake in male volunteers, an interstitial water-arterial plasma water gradient of glycerol
was not observed, indicating a lack of breakdown of
muscle triacylglycerol (278). A muscle biopsy at termination of exercise confirmed this finding. It is, however,
noteworthy that females of widely differing training backgrounds in contrast to matched male volunteers utilize a
significant amount of IMTG in the vastus lateralis during
prolonged bicycle exercise (281). The significant utilization of IMTG in females could be caused by the higher
habitual IMTG levels in females than in males and/or that
type 1 muscle fibers expressed per area were more abundant in females than in males (281).
When combining the muscle biopsy technique with
the oil red O staining, it was found in males that prolonged submaximal exercise for 2 h resulted in significant
reduction in area, of IMTG percent, but only in type 1
fibers (57, 317). Interestingly, IMTG breakdown during
exercise occurred only in the fasted state and was completely abolished when subjects were fed carbohydrate
before and during exercise (57).
2. Studies where the 1H-MRS technique was applied
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
D. Triacylglycerol Stored in Adipocytes Associated
With Muscle
A potential source of lipids during exercise might be
fatty acids liberated from adipocytes dispersed between
or along the muscle fibers as suggested previously by us
and others (65, 117, 147, 235). From muscle biopsies it is
not possible to get a good measure of the intercellular TG
pool. However, because the in vivo 1H-MRS studies apparently can differentiate between the IMCL and the
EMCL signal, a possibility may exist to evaluate whether
breakdown of a triacylglycerol compartment, located in
between the muscle fibers, takes place during exercise. In
a recent study in which IMCL signal decreased during
exercise in female runners (172) and in trained male
cyclists (262), no significant change in EMCL signal was
observed. In another 1H-MRS study in which no change in
IMCL with prolonged intermittent exercise was found, no
changes in EMCL were found either (230). However, the
variation in measurements of EMCL apparently is considerable (172, 291), and in most 1H-MRS studies on exercise,
EMCL values are not reported. In fact, Decombaz et al.
Physiol Rev • VOL
(58) state that “because the size of the EMCL signal in a
specific spectrum is not representative for a specific muscle, the size of the EMCL signal cannot be evaluated in
terms of depletion and recovery.” If so, the 1H-MRS technique will not be able to answer the question of whether
extramyocellular fat is utilized during exercise. Intriguing
are the findings by Szczepaniak et al. (291) infusing norepinephrine into dogs and thereby accelerating adipocyte
lipolysis. They observed a trend toward a decreased
EMCL signal in the gastrocnemius and tibialis anterior
muscle in dogs. In contrast, the IMCL increased significantly in both muscles. The interpretation of these findings could give support to the notion that triacylglycerol
located in adipocytes between the muscle fibers may be
recruited for utilization in the muscle cells.
To summarize the available studies, it does not seem
entirely possible to adequately explain the differing results regarding utilization of IMTG during exercise. TG is
very energy dense, and only small amounts contribute
with substantial energy provision, possibly explaining the
difficulties detecting these changes with biochemical determination in muscle biopsies when exercise is not very
prolonged. Of note, however, are the findings that females
break down more IMTG during exercise than males
matched for training status. Recent investigations using
1
H-MRS and oil red O staining of muscle biopsies, however, indicate the use of IMTG during submaximal exercise, and the oil red O technique has also shown that
IMTG breakdown apparently only occurs in type 1 fibers.
This can influence the biochemical IMTG measurements,
especially when a mixed muscle is studied. Furthermore,
it cannot be excluded that some of the LCFA taken up in
the exercising muscle is esterified to IMTG in nonactive
motor units, especially at lower exercise intensities, and
this could influence the data obtained. Nevertheless, the
contribution to energy provision in males is rather low, in
the range of ⬃10% of total energy expenditure in the
fasting state (and likely lower in the fed state), whereas
the IMTG store in females seems to be more dynamic, and
breakdown of IMTGs in females may cover up to 10 –25%
of energy provision during exercise.
E. Is Intramyocellular Triacylglycerol Breakdown
During Exercise Influenced by Training?
It is controversial whether IMTG hydrolysis and utilization during exercise is increased by training. Applying
the muscle biopsy technique, only a few studies have
demonstrated a higher IMTG utilization during exercise in
the trained compared with the untrained situation when
exercise was performed by males at the same absolute
intensity (133, 214), whereas other studies were unable to
obtain a training-induced increase in IMTG utilization in
males (14, 147, 235) or in females (281) at the same
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
ance (Rd) during bicycle exercise can be accounted for by
uptake in the working legs, as discussed in section II. This
means that a major part of Rd for LCFA during exercise in
fact occurs in other parts of the body than the exercising
legs. Furthermore, it is assumed that LCFA is the only
circulating lipid source, thereby ignoring VLDL-TG as a
potential energy source. As discussed in section IV, this
may not be correct. It therefore seems fair to conclude
that this indirect method is inadequate to estimate utilization of intramuscular triacylglycerol during exercise,
and therefore, studies using this method for quantification
of IMTG are not easy to interpret. However, in a recent
study the tracer technique was used combined with histochemical analysis of muscle. During 120 min of exercise
at 60% of V̇O2 max in trained male cyclists, it was shown
that muscle- and lipoprotein-derived TG measured by the
tracer technique covered ⬃15% of total energy expenditure. At the same time, a net decline in muscle lipid
content was demonstrated in the type 1 fibers only, quantitated by the oil red O analysis (314). Unfortunately,
probably due to the semi-quantitative nature of the oil red
O determination of IMTG utilization, a direct comparison
of the two methods for quantification of IMTG utilization
during exercise was not reported by the authors. The
findings in that study also revealed that the rate of LCFA
oxidation increased substantially during exercise concomitantly with a decline in the rate of muscle- and
lipoprotein-derived TG oxidation (314). These observations support earlier suggestions by Kiens et al. (147) that
enhanced utilization of blood-borne fat substrates could
decrease and/or inhibit intramuscular TG net breakdown.
227
228
BENTE KIENS
VI. REGULATION OF INTRAMYOCELLULAR
TRIACYLGLYCEROL HYDROLYSIS
Even though the usage of triacylglycerol for energy
has been investigated intensively through the years, the
regulation of triacylglycerol synthesis and breakdown in
skeletal muscle is poorly understood. It is thought that
hormone sensitive lipase (HSL) is the major enzyme responsible for the hydrolysis of stored triacylglycerol in
skeletal muscle similar to in adipose tissue.
HSL protein or mRNA has been detected in rodent
(126, 170) and in human (236) skeletal muscle but with a
considerable lower expression than in adipose tissue
(236). The HSL protein expression also varies between
fiber types, being higher in oxidative than glycolytic fibers
(170).
HSL is a multifunctional lipase with a broad substrate
specificity that efficiently catalyzes the hydrolysis of triacylglycerol, diacylglycerol, monoacylglycerol, cholesterol esters, and retinyl esters in adipose tissue (13, 331,
340). From in vitro assay systems it is evident that the
enzyme exhibits an ⬃10- and 5-fold higher specific activity
for diacylglycerol and cholesteryl esters compared with
triacylglycerol or monoacylglycerol, respectively (76, 77).
In assays to measure HSL activity, several different substrates have been used. Thus, in adipose tissue or skeletal
muscle, HSL activity has been measured using diacylglycerol as substrate (9, 267) or triacylglycerol (60, 85, 161,
168, 169, 236, 267, 323, 326, 329, 330) or cholesteryl esters
(94, 141, 267) as substrates, which make the findings
somewhat difficult to compare.
Studies on adipocytes have indicated that HSL is rate
limiting in lipolysis (76). This view has been challenged by
studies in HSL knock-out mice that exhibited normal
Physiol Rev • VOL
body weight and had retained 40% of triacylglycerol lipase
activity in white adipose tissue (207). Furthermore, it was
recently shown (99) that HSL-deficient mice accumulated
diacylglycerol in adipose tissue and skeletal muscle, indicating that when HSL is missing this leads to an incomplete hydrolysis of triacylglycerol with an interruption of
the lipolytic cascade at the stage of diacylglycerol hydrolysis. This observation was supported by the findings of a
marked reduction in the formation of fatty acids in skeletal muscle as in several other tissues (99). Furthermore,
in vitro studies with HSL-deficient fat pads demonstrated
an accumulation of diacylglycerol and a drastic reduction
in fatty acid production after ␤-adrenergic stimulation
compared with the wild type (99). Interestingly, when the
specific triacylglycerol hydrolase activity (the enzymatic
conversion of TG to DAG) was calculated, the specific TG
hydrolase activity was reduced 50% in adipose tissue,
whereas no reduction was found in skeletal muscle when
comparing HSL-deficient mice with wild type (99). These
data suggest that HSL is rate-limiting in the catabolism of
DAG but not of TG hydrolysis in skeletal muscle and
furthermore points to the existence of one or more lipases
with considerable activity, specifically to TG hydrolysis in
skeletal muscle. Further support for the existence of
lipases other than HSL involved in basal triacylglycerol
hydrolysis in skeletal muscle appears from recent studies,
where neutral lipase activity in human skeletal muscle
only decreased by ⬃25% when antiserum against HSL was
added to the assay medium (236, 329). When a similar
approach was taken in basal, resting rat soleus muscle
(169, 170), HSL activity was decreased by ⬃60%, indicating that HSL may be less dominating in human skeletal
muscle than in rat skeletal muscle and that more lipases
than HSL are involved in TG hydrolysis in the resting
state.
A. Hormone-Sensitive Lipase Activity
During Exercise
It has been shown that electrically induced muscle
contractions increase the neutral lipase activity of soleus
muscle of rats (169). Studies in humans have likewise
demonstrated an exercise-induced increase in neutral
lipase activity in skeletal muscle (161, 236, 323, 326, 329).
In human muscle, the measured increase in neutral lipase
activity by exercise was completely abolished when muscles were preincubated with anti-HSL before measuring
neutral lipase activity, clearly demonstrating that the increase in neutral lipase activity during exercise was due
to an increase in HSL activity, whereas the remaining
neutral lipases were not affected by exercise (236, 329).
Similar findings have been obtained previously in rat skeletal muscle (169). An intriguing finding in studies where
exercise-induced muscle contractions or electrical stimu-
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
absolute or relative work load. When indirectly estimated
from total lipid oxidation and plasma LCFA Rd or oxidation, the calculated whole body IMTG utilization was
higher in endurance-trained than in untrained subjects at
the same absolute work load (162, 193). However, as
discussed previously, this method of estimation of IMTG
breakdown during exercise is not without limitations.
Finally, a 1H-MRS study showed that during running at
50% of peak oxygen uptake in males, degradation of IMCL
in the tibialis anterior muscle was larger in absolute but
not relative terms in trained compared with untrained
individuals (58). Thus the available data do not allow one
to conclude whether training increases IMTG utilization
during exercise or not, but this may to some extent be
explained by different dietary regimes in the different
studies.
For further discussion on IMTG in skeletal muscle,
the reader is referred to other recent reviews on this topic
(311, 325).
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
B. Molecular Mechanisms
1. Phosphorylation
The molecular mechanisms behind the activation of
HSL in skeletal muscle are quite incompletely known. It
appears from recent studies that HSL activity, both in
adipose tissue and skeletal muscle, is regulated by phosphorylation, by allosteric mechanisms, and by translocation of the enzyme to the fat droplet, although translocation has yet only been demonstrated in adipocytes (13, 85,
125, 127, 267).
With regard to phosphorylation, five phosphorylation
sites on HSL have so far been identified as regulatory
sites. In vitro studies have demonstrated that Ser563,
Ser659, and Ser660 are cAMP-dependent protein kinase A
(PKA) targets on HSL. In adipocytes, all three sites are
phosphorylated both in vivo with isoprenaline and in vitro
when incubated with PKA (9, 125). Although it is well
known that the PKA sites on HSL are phosphorylated by
catecholamines in adipocytes, it has not until recently
been investigated in skeletal muscle (236). Interestingly,
despite a severalfold increase in arterial epinephrine concentrations during exercise, phosphorylation of Ser563 on
HSL in human vastus lateralis muscle was not enhanced
compared with rest (236). These findings suggest that
Ser563 on human skeletal muscle HSL may not be a target
for PKA in skeletal muscle during exercise. Alternatively,
HSL in human skeletal muscle may be constitutively phosphorylated on Ser563 by resting catecholamine levels
and/or very high plasma catecholamine concentrations
are needed to further increase skeletal muscle Ser563
phosphorylation during exercise. These data do not support epinephrine-induced phosphorylation of HSL on
Physiol Rev • VOL
Ser563 to be of importance for regulating muscle HSL
activity during exercise. So far, of the sites on HSL known
to be phosphorylated by epinephrine in adipocytes, only
Ser563 has been investigated in muscle.
Activation of the extracellular signal-regulated kinase (ERK) pathway was shown to increase lipolysis and
HSL activity in adipocytes by phosphorylating HSL on
Ser600 (94). Furthermore, it was recently demonstrated
that in contracting isolated rat soleus muscle, HSL activation required the involvement of ERK and/or protein
kinase C (60).
An additional phosphorylation site on HSL, Ser565,
can be phosphorylated by AMP-activated protein kinase
(AMPK) and Ca2⫹/calmodulin-dependent kinase II
(CaMKII) (85, 205, 267, 340). When adipocytes were preincubated with AICAR, an AMP mimicking agent, isoprenaline-induced lipolysis was inhibited (52, 289). It was also
shown that phosphorylation of HSL by AMPK prevented
subsequent phosphorylation by PKA, and vice versa (85).
Based on this, it was proposed that phosphorylation of
Ser565 exerts an antilipolytic role (85).
AMPK is activated during exercise (38, 59, 82, 236,
284, 318, 329, 334, 336, 338), and the activation is inversely
related to content of glycogen in skeletal muscle (59, 236,
329, 336). The study by Roepstorff et al. (236) is the first
to investigate the effect of AMPK on HSL Ser565 phosphorylation and HSL activity in human skeletal muscle. During
exercise with either high or low muscle glycogen content,
low and high muscle activity of ␣2AMPK, respectively,
was achieved. An expected finding was that in the trial
with the highest muscle ␣2AMPK activity, the highest HSL
Ser565 phosphorylation in muscle was also achieved. Nevertheless, differences were not obtained in HSL activity or
IMTG hydrolysis between trials. These findings are in
accordance with observations in isolated contracting rat
soleus muscle, where it was shown that muscle HSL
activity varied in the face of constant AMPK activity (61).
In contrast, in another study it was in fact reported that
AMPK activation may inhibit HSL activity during exercise,
although no effect on muscle triacylglycerol breakdown
was reported (329). Taken together, the data suggest that
AMPK can phosphorylate HSL on Ser565 in skeletal muscle, but AMPK seems to be of minor importance as a
regulator of HSL activity and muscle triacylglycerol
breakdown in skeletal muscle during muscle contraction/
exercise.
2. Allosteric regulation
TG hydrolysis also appears to be under allosteric
regulation. In in vitro incubated purified bovine adipocytes, oleoyl CoA and oleic acid inhibited HSL activity,
measured against cholesterol oleate. Of these, the fatty
acyl CoA was found to have the greatest effect, decreasing HSL activity by 50% at ⬃0.1 ␮M, versus 50% at ⬃0.5
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
lation have been applied is that the effect of muscle
contractions was transient, as neutral lipase activity and
in some studies actual HSL activity increased rapidly at
initiation of exercise or electrical stimulation, but then
declined towards resting, basal values during continued
exercise or electrical stimulation (169, 236, 323). The
reason for this transient activation is not known. It is
surprising from the point of view that exercise is accompanied by increases in circulating catecholamine concentrations, and there is evidence for activation of HSL by
epinephrine. Thus, in incubated rat soleus muscle, HSL
was activated by epinephrine (170). Also in human skeletal muscle, evidence was provided that ␤-adrenergic
stimulation by infusion of epinephrine increased neutral
lipase activity at rest and during exercise (161, 330). Nevertheless, the role of physiological concentrations of epinephrine in exercise-induced increased HSL activity is not
clear due to the only temporary increase in muscle HSL
activity with exercise despite increasing epinephrine concentrations.
229
230
BENTE KIENS
VII. RELATION BETWEEN LIPIDS
AND INSULIN RESISTANCE
A. Intramyocellular Triacylglycerols
and Insulin Resistance
The content of IMTG has been proposed to be related
to insulin resistance. Thus Falholt et al. (69) originally
showed that in type 2 diabetic insulin-resistant patients,
the triacylglycerol content in the rectus abdominus muscle was significantly larger compared with healthy individuals, and in rats it was also demonstrated that IMTG
correlated negatively to insulin sensitivity (286). Later,
Pan et al. (208) showed in a cross-sectional study, involving among others overweight Pima Indians [body mass
index (BMI) ⫽ 37 kg/m2] an inverse relationship between
whole body insulin sensitivity (determined by the euglycemic hyperinsulinemic clamp technique) and the content
of muscle triacylglycerol measured in biopsies from the
vastus lateralis muscle. A significant inverse relationship
between insulin activation of glycogen synthase and muscle IMTG has also been described in middle-aged nondiabetic women (213). When indirect methods to measure
muscle content of triacylglycerol like computed tomographic imaging (CT) and 1H-MRS have been applied (91–
93, 272, 291) or morphometry and oil red O staining have
been used (91, 93, 112, 175, 190), an inverse relationship
between muscle content of triacylglycerol and insulin
sensitivity has also been demonstrated. What characterizes these latter studies is that increased muscle triacylglycerol content and decreased insulin sensitivity was
observed in either overweight, insulin-resistant individuals, with and without type 2 diabetes (91–93, 112, 175), in
normal weight insulin-resistant relatives to patients with
type 2 diabetes (210), or in severely obese individuals
(190). A negative association between IMTG content and
whole body insulin-stimulated glucose uptake was, howPhysiol Rev • VOL
ever, also found in normal-weight, healthy individuals
(BMI ⫽ 24.1–25.7 kg/m2) when IMTG was measured by
the 1H-MRS technique in the soleus muscle (166, 210) or
in the vastus lateralis muscle (319).
However, there are instances where the correlation
between insulin sensitivity and IMTG content is not
present. For instance, in healthy young male volunteers
who for 4 wk consumed a diet consisting of 45 E% carbohydrates of which the carbohydrate food items were
either of high or low glycemic index (HGI and LGI, respectively), the biochemical measured content of IMTG in
the vastus lateralis muscle was significantly higher after
the HGI diet compared with after the LGI diet and so was
insulin sensitivity, measured by the euglycemic, hyperinsulinemic clamp procedure (152). Furthermore, endurance-trained subjects who have an increased sensitivity to
insulin are often found to have high IMTG content (58, 91,
130), although this effect is very dependent on the diet
(117, 148, 281) as discussed previously. Recently, it was
reported that 8 wk of moderate exercise training in type 2
diabetic patients (BMI ⫽ 32 kg/m2) decreased IMTG content to values similar to the healthy controls, yet insulin
sensitivity was still markedly lower in the trained diabetics than even untrained controls (29). Also in recent studies, performed in normal-weight, healthy, insulin-sensitive
individuals, using either the 1H-MRS technique (260, 272)
or the muscle biopsy technique (113), no association between insulin sensitivity and IMTG content was demonstrated. Finally, in females, we have recently described
that IMTG content is higher than in males irrespective of
training status (281). If intramuscular triacylglycerols are
indeed directly involved in insulin resistance, one would
assume that females would be insulin resistant compared
with males. However, this is not the case. It has been
demonstrated that women are more sensitive to insulin
than similarly fit men (201).
Summarizing the available data, it is apparent that
IMTG content may not be directly involved in determining
insulin sensitivity but rather may be a marker of decreased insulin sensitivity in populations where lipids are
stored as a consequence of an imbalance between lipid
supply and lipid oxidation rate in skeletal muscle. Accordingly, lipid infusion studies in healthy male volunteers as
well as fat feeding for 3 days caused insulin resistance and
increases in muscle IMTG, quantified by 1H-MRS, in soleus and tibialis anterior muscles after lipid infusion, but
only in the tibialis anterior muscle after the high-fat diet
(10, 19). Thus a likely scenario is that when the level of
physical activity and hence energy utilization is low compared with the intake of lipids, skeletal muscle may partition LCFA towards re-esterification to IMTG rather than
to oxidation (Fig. 15).
From several recent studies in obese subjects and
type 2 diabetic patients, it is also apparent that the intramuscular deposition of lipids is a consequence of an
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
␮M with the fatty acid (141). In homogenates from resting
rat soleus muscle, a ⬃20% reduction in neutral lipase
activity was found when palmitoyl CoA was added (328).
During exercise, an increase in muscle LCFA CoA content
has been observed (323). Also exercise at high intensities
has revealed an increased intramyocellular concentration
of LCFA in human skeletal muscle (154). The cytosolic
concentrations of long-chain fatty acyl CoA and of fatty
acids during exercise might be sufficient to inhibit in vivo
HSL activity (141), whereby TG lipolysis will be reduced.
Thus it is worth considering the allosteric inhibition of TG
hydrolysis in muscle tissue during exercise, and it could
be speculated that the allosteric inhibition in some cases
might override activating phosphorylation during exercise. In addition, phosphorylation of HSL might change
the sensitivity of HSL towards its allosteric regulators.
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
231
imbalance between rate of LCFA uptake and rate of LCFA
oxidation. Thus, in a study by Kelly et al. (144), severely
obese but glucose-tolerant female and male volunteers
(BMI ⫽ 34 kg/m2) were compared with healthy subjects
(BMI ⫽ 23 kg/m2). Despite similar arterial LCFA concentrations and LCFA uptake in both groups, measured
across the leg by infusion of [U-C14]oleic acid, a lower
oxidation of fatty acids was seen in the obese group.
Similarly, an increase in LCFA uptake, measured in muscle strips from the rectus abdominis muscle, was demonstrated in combination with a decrease in LCFA oxidation
and increase in IMTG synthesis in extremely obese
women (132), and in obese, nondiabetic women (283)
compared with lean subjects. Furthermore, similar findings were obtained in insulin-resistant skeletal muscle of
obese Zucker rats where rates of palmitate esterification
were markedly increased concomitantly with a reduced
oxidation of LCFA (304). It has been reported that LCFA
transport rate and FAT/CD36 content in giant sarcolemmal vesicles prepared from human rectus abdominis muscles was upregulated approximately fourfold and associated with an increased IMTG content in obese individuals
and in type 2 diabetics compared with lean subjects (25),
suggesting that sarcolemmal transport capacity of LCFA
is increased in obesity and type 2 diabetes. Taken together, the available evidence supports that in obesity and
type 2 diabetes decreased muscle oxidation and possibly
increased transmembrane transport capacity of LCFA
compared with lean individuals led to increased IMTG
deposition. This picture seems to be most pronounced in
Physiol Rev • VOL
severe obesity and type 2 diabetic patients rather than in
moderately obese subjects.
B. Lipid Intermediates and Insulin Resistance
Several factors could be involved in the direction of
LCFA or actually long-chain acyl CoA (LCA-CoA) towards
esterification rather than oxidation in obesity and type 2
diabetes. Localization of mitochondria more centrally in
the muscle cell will increase the transport distance for
LCA-CoA to the mitochondria for oxidation, increasing
the likelihood for esterification rather than oxidation. In
fact, it was recently described that insulin-resistant obese
and type 2 diabetic subjects displayed decreased subsarcolemmal mitochondria in the vastus lateralis muscle
compared with lean controls (232). It is, however, probably more important that several findings point toward
decreased mitochondrial oxidative capacity as central in
the reduced oxidation rate of LCFA-CoA. Thus a number
of studies have shown that the activity of the key enzymes
citrate synthase (CS) and ␤-hydroxy acyl dehydrogenase
(HAD) are significantly reduced in skeletal muscle in
obesity (47, 112, 132, 145, 159) and type 2 diabetes (18,
112, 145, 270). Furthermore, the muscle activity of CPT1
was also reduced in association with obesity in a number
of studies (132, 145, 159, 271). Because mitochondrial
oxidative capacity is low in insulin-resistant subjects (47,
91, 145, 232, 270, 271), mitochondrial dysfunction and
thereby decreased lipid oxidation has been thought to be
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
FIG. 15. Interactions discussed in
the text whereby decreased lipid oxidation potential may result in partitioning
of LCFA CoA towards reesterification to
IMTG in skeletal muscle. LCFA CoA,
long-chain fatty acyl CoA; CPT, carnitine
palmitoyl transferase; GPAT, glycerol-3phosphate acyl transferase; LPAAT, lysophosphatidate acyltransferase; PPH-1,
phosphatidate phosphohydrolase; DGAT,
diacylglycerol acyltransferase; ER, endoplasmatic reticulum.
232
BENTE KIENS
Zucker rats, LCFA-CoA content was increased by 90%
compared with lean controls (75). Likewise, in mice, overexpression of LPL led to an increase in intramuscular
LCFA-CoA, DAG, and ceramide concentration in skeletal
muscle in combination with muscle-specific insulin resistance (156). Nevertheless, a recent training study of type
2 diabetics showed improved insulin sensitivity after
training but no change in LCFA-CoA content (29). Thus
the role of LCFA CoA in insulin resistance is not totally
clear, although there are a number of identified mechanisms by which LCFA-CoA may influence glucose metabolism. For instance, studies in homogenates from human
and rat soleus muscle in vitro have shown that LCFA-CoA
can inhibit hexokinase activity, which may contribute to a
reduction in glucose metabolism (294) (Fig. 16).
LCFA is also a substrate for the synthesis of ceramide, and increased availability of palmitate has been
found to elevate intracellular ceramide levels. Thus, when
mouse skeletal muscle C2C12 myotubes were incubated
with saturated fatty acids, the content of cellular ceramide was elevated (257). A recent study demonstrated
that 11 ceramide fatty acids were present in human vastus
lateralis muscle (114). Ceramide has been linked to insulin resistance. Hence, increased levels of ceramide have
been found in insulin-resistant muscle of the Zucker rat
(305). Furthermore, when C2C12 myoblasts were pretreated with palmitate or a cell-permeable ceramide analog, insulin-stimulated glycogen synthesis and Akt/PKB
activation were inhibited, without affecting IRS-1 tyrosine
phosphorylation (258). In addition, incubation of L6 myo-
FIG. 16. Mechanisms whereby intramuscular lipids may interfere with the insulin signaling pathway and glucose metabolism in human skeletal muscle. When protein kinase C (PKC) is activated by lipid
intermediates, the insulin receptor is phosphorylated (p) on a serine (S) residue,
whereby initiation of the insulin signaling
cascade is inhibited. Besides, lipid intermediates may inhibit the insulin signaling cascade further downstream as well as early
in the glucose uptake pathway. IMTG, intramuscular triacylglycerol; DAG, diacylglycerol; LCFA CoA, long-chain fatty acyl
CoA; PKC, protein kinase C; IR, insulin
receptor; IRS, insulin receptor substrate;
(p), phosphorylation; S, serine residue; T,
tyrosine residue; PI3K, phosphoinositide
3-kinase; PDK, 3-phosphoinositide-dependent kinase; GSK-3, glycogen synthase kinase 3; GS, glycogen synthase; HK, hexokinase; G-6-P, glucose-6-phosphate.
Physiol Rev • VOL
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
important in development of insulin resistance. Accordingly, a connection between mitochondrial dysfunction,
increased intramuscular triacylglycerol levels, and insulin
resistance has recently been described in insulin-resistant
offspring of patients with type 2 diabetes (211).
Interestingly, it is not only the oxidative capacity of
skeletal muscle that seems to be of importance as an
increased activity of marker enzymes of the glycolytic
pathway: phosphofructokinase (PFK), glyceraldehyde
phosphate dehydrogenase (GAPDH), and hexokinase
(HK) were observed in obese, non-insulin-dependent diabetic patients (270) compared with lean subjects, and the
HK/CS ratio had the strongest correlation with insulin
sensitivity. The findings suggest that a dysregulation between mitochondrial oxidative capacity and capacity for
glycolysis is an important component in the mechanism of
insulin resistance because such a dysregulation contributes to a decreased lipid oxidation rate and a direction
towards reesterification of LCFA by providing glycerol-3phosphate (Fig. 15).
However, the rate of reesterification of LCA-CoA towards triacylglycerol and phospholipids is apparently not
adequate to handle prolonged lipid oversupply, since increased muscle concentrations of LCA-CoA and diacylglycerol (DAG) are associated with lipid oversupply (50,
189). Interestingly, an inverse relationship between total
content of LCFA-CoA in the vastus lateralis muscle and
whole body insulin action was demonstrated in male individuals with varied degrees of BMI (24 –38 kg/m2) and
glucose tolerance (63), and in obese insulin-resistant
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
C. Protein Kinase C
LCFA-CoA may interfere with the insulin signaling
pathway directly by activating protein kinase C (PKC)
(Fig. 16). The PKC family constitutes a group of multifunctional serine/threonine protein kinases involved in
metabolism, mitogenesis, and gene expression (70, 247).
Depending on mode of activation, the PKC family is divided into three subgroups. Conventional isoforms (␣, ␤1,
␤2, ␥) are dependent on both Ca2⫹ and DAG for stimulation of activity, novel isoforms (␦, ⑀, ␪, ␩) are dependent
on DAG, and the atypical isoforms (␨, ␭/␫, where PKC-␭ is
the mouse homolog of human PKC-␫) that are independent on Ca2⫹ and DAG but are activated by phosphatidic
acid and phosphatidylinositol 3,4,5-trisphosphate (PIP3)
(247). During activation, the kinases can translocate
within the cell, and the extent to which a PKC isozyme is
found in the membrane fractions is commonly used as a
measure of its activation. It is believed that PKC inhibits
insulin signaling by phosphorylation of the serine residues on the insulin receptor (138, 292) and insulin receptor substrate-1 (IRS-1) (227) in turn inhibiting the tyrosine
phosphorylation of IRS-1, leading to decreased insulinstimulated phosphatidylinositol 3-kinase activity (95, 341)
(Fig. 16). In this respect, it appears that some conventional and in particular some of the novel PKC isoforms
are involved in downregulating the insulin signaling cascade (95, 341). Recently, it was shown that PKC-␪ can
phosphorylate IRS-1 at Ser-1101 and block tyrosine phosphorylation of IRS-1 and downstream activation of the
Akt pathway (176).
From studies in rats where the plasma concentration
of LCFA was acutely elevated by lipid infusion for 5 h and
insulin resistance was induced, an increased membrane
localization of PKC-␪, and thereby presumably activation
of the isozyme, was found concomitantly with a decrease
in the cytosol (95, 341), whereas no changes were observed in PKC-⑀ (341). When insulin resistance and increased lipid levels were obtained by prolonged glucose
Physiol Rev • VOL
infusion in rats, PKC-⑀ translocation to the membrane
fraction was observed (173). In insulin-resistant obese
Zucker rats, the membrane localization of both PKC-⑀ and
-␪ was increased (222), and the PKC-⑀ expression was
increased in muscle from a diabetes prone-line sand rat
(Psammomys obesus) (266). Furthermore, PKC-␪ knockout mice are protected from fat-induced insulin resistance
(157), further supporting an important role of novel PKCs
in development of lipid-induced insulin resistance. In addition to the acute effect of elevated plasma LCFA levels,
chronic effects of high fat feeding for 3 wk appear to
activate both PKC-␪ and -⑀, which was associated with
insulin resistance (256).
The mechanism by which LCFA-CoA causes activation of PKC likely involves accumulation of DAG and
activation of the DAG-sensitive isoforms of PKC. DAG can
be generated through esterification of LCFA-CoA towards
muscle triacylglycerol, from hydrolysis of muscle triacylglycerol and by breakdown of phospholipids particularly
phosphatidylinositol 4,5-bisphosphate and phosphatidylcholine by the action of phospholipases C and D, respectively (164) (Fig. 16). Cell culture studies indicate that
DAGs with a saturated fatty acid profile generally are poor
PKC activators, whereas DAGs with an unsaturated fatty
acid profile are better activators of DAG-sensitive PKC
isoforms (123, 251, 321). Studies implying a role for DAG
in lipid-induced insulin resistance include the study by Yu
et al. (341) in which intralipid plus heparin infusion in rats
induced an increase in intracellular levels of LCFA-CoA,
and a transient increase in DAG concomitantly with increased activation of the novel PKC isozymes ␪ and ⑀. An
increase in DAG associated with translocation and activation of PKC-␪ and -⑀ was also found in obese Zucker
rats (51, 222). Thus DAG levels in skeletal muscle of rats
have been found to be elevated in several situations of
lipid-induced insulin resistance as well as in transgenic
mice overexpressing LPL in muscle (156) and in several
cases in association with increased activity of PKC-␪ and
-⑀ (222, 255, 341). Thus studies in rats, using different
models, strongly support a link between activation of
especially PKC-⑀ and -␪, serine phosphorylation of IRS-1,
and in turn decreased phosphatidylinositol 3-kinase activity and decreased insulin sensitivity by increased lipid
availability to skeletal muscle. However, one study does
not fit in this picture: when obese Zucker rats were
treated with rosiglitazone, which improved glucose tolerance, skeletal muscle IMTG, DAG, and ceramide content
were increased rather than decreased, and translocation
of novel PKC isoforms to the membrane fraction of muscle was not observed (174). It is likely that roziglitazone
increases glucose tolerance by other mechanisms that
can override the detrimental effects of lipid accumulation
in muscle.
Patients with type 2 diabetes have increased PKC-␪
protein levels and isozyme activity in the rectus abdomi-
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
cytes with palmitate caused marked ceramide synthesis
and decreased insulin sensitivity involving impaired activation of Akt/PKB (104, 218). Nevertheless, despite the
evidence for effects of ceramide on insulin action, infusion of intralipid caused insulin resistance in humans but
no increased muscle ceramide concentrations (137). Furthermore, a single exercise bout, which is known to increase insulin sensitivity (229), apparently increased
rather than decreased muscle ceramide concentrations
(114). Finally, treatment of obese Zucker rats with rosiglitazone increased glucose tolerance but also increased
muscle ceramide levels (174). Thus further studies are
needed to clarify the potential role of ceramides in insulin
resistance.
233
234
BENTE KIENS
VIII. CONCLUSION AND FUTURE DIRECTIONS
It is evident that the control of lipid utilization in
skeletal muscle is exquisitely intricate and is subject to a
plethora of regulatory mechanisms that have different
degrees of importance at rest and during or after exercise.
Recent evidence has implicated the lipid binding proteins
as regulators of transmembrane transport of LCFA, and
intriguingly, caveolin-3 has been found to colocalize with
FAT/CD36 in muscle. Unraveling the possible role of
caveolae and caveolins in lipid transport will be an important further step in our knowledge of lipid metabolism
Physiol Rev • VOL
also in exercise. Still, the importance of fatty acid transporters in limiting utilization of LCFA during exercise
remains unsettled, and regulatory steps in mitochondrial
access of the lipid may seem to be more important. In this
respect, a major regulatory role for carnitine in controlling lipid oxidation during exercise is proposed. Clearly,
further research is needed to clarify the relative importance of the various regulatory steps in lipid oxidation
during exercise.
The metabolic role of LPL has been studied in various
tissues; however, the role of LPL in muscle lipid and
lipoprotein metabolism at rest, during, and after exercise
is an important aspect of current and future research.
Especially the mechanisms involved in posttranscriptional, translational, and posttranslational processes that
regulate LPL maturation and enzyme translocation to the
capillaries in skeletal muscle should be elucidated. IMTG
utilization during exercise depends on preexercise diet,
gender, training status and duration, intensity, and mode
of exercise. The available methods for measuring IMTG
all have their limitations. Because IMTG utilization is fiber
type specific, the fiber type recruitment pattern during
exercise is of importance when measuring IMTG utilization in muscles with mixed fiber type distribution. However, the contribution of IMTG for energy provision may
not be large, covering ⬃10% of energy provision in fasting
exercise in male subjects, whereas in females IMTG may
cover a larger proportion of energy delivery. A further
step is to elucidate how IMTG turnover during exercise is
regulated at the molecular level and in that respect regulation of HSL activity on the molecular level during exercise is now in progress.
Finally, lipids are heavily implicated in development
of insulin resistance in skeletal muscle. This seems to be
linked to an imbalance between lipid supply and lipid
oxidation, the latter being related to decreased mitochondrial oxidative capacity in states of insulin resistance.
Because exercise has the ability to both increase lipid
oxidation and increase muscle mitochondrial oxidative
capacity, it will be important to further establish the
molecular mechanisms behind lipid-induced insulin resistance as well as the role of exercise in preventing and
treating this condition.
ACKNOWLEDGMENTS
Carsten Roepstorff and Erik A. Richter are acknowledged
for fruitful discussions during preparation of this manuscript.
Irene Bech Nielsen, Betina Bolmgren, Winnie Taagerup, and
Nina Pfluzek performed skilled technical assistance.
Address for reprint requests and other correspondence: B.
Kiens, Copenhagen Muscle Research Centre, Dept. of Human
Physiology, Institute of Exercise and Sports Sciences, Univ. of
Copenhagen, 13 Universitetsparken, DK-2100 Copenhagen, Denmark (e-mail: [email protected]).
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
nus muscle (136) and decreased muscle insulin receptor
tyrosine kinase activity, which could be restored by phosphatase treatment in vitro, possibly suggesting increased
serine phosphorylation of the insulin receptor due to
increased PKC activity (138). In only one human study in
healthy, lean male subjects the association between insulin resistance, caused by acute intralipid infusion for 5 h,
and muscle PKC isozyme content has been examined
(137). In this study, elevated plasma LCFA was associated
with increased DAG mass and an increase in membrane
PKC-␦ protein and membrane PKC-␤II, whereas no significant changes were observed in PKC-⑀, -␪, or -␨ measured
in the vastus lateralis (137). These findings in humans are
in contrast to findings in rats using the same model for
generating insulin resistance, where an activation of
PKC-␪ has been found. However, in the human study by
Itani et al. (137), muscle samples were not obtained until
after 6 h of intralipid infusion, and it cannot be excluded
that PKC-␪ was activated at an earlier or perhaps later
stage. Supporting the latter assumption, obese diabetic
patients showed a significantly higher PKC-␪ content and
activity in the rectus abdominus muscle compared with
muscles from obese, normoglycemic controls (138).
In summary, lipid oversupply in relation to lipid expenditure seems to result in decreased insulin action in
muscle by a variety of mechanisms involving accumulation of IMTG, LCA-CoA, and DAG and activation of PKC
and possibly synthesis of ceramide. The net result is a
decrease in insulin signaling and decreased glucose uptake (Fig. 16). Exercise would be expected to be particularly beneficial in preventing these events by increasing
the oxidation of lipids and thereby partitioning lipids
towards oxidation rather than reesterification and accumulation. Whereas studies have shown increased insulin
sensitivity after training in obese or type 2 diabetic subjects improvements in IMTG and LCFA-CoA content in
muscle were not consistently seen (29, 83, 111). Thus the
relation between IMTG and LCFA-CoA and insulin resistance is not clearly apparent when exercise is used as an
intervention to increase insulin sensitivity.
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
GRANTS
Work by the author was supported by grants from the
Danish National Research Foundation (Grant 504 –12); the
Copenhagen Muscle Research Centre; the Danish Sports Research Council; NovoNordisk Research Foundation; the Danish
Medical, Natural Sciences, and Veterinary Research Councils;
and Team Danmark and by an Integrated Project (Contract
Number LSHM-CT-2004 – 005272) from the European Union.
REFERENCES
Physiol Rev • VOL
17. Binas B, Danneberg H, McWhir J, Mullins L, and Clark AJ.
Requirement for the heart-type fatty acid binding protein in cardiac
fatty acid utilization. FASEB J 13: 805– 812, 1999.
18. Blaak EE, Wagenmakers AJ, Glatz JF, Wolffenbuttel BH,
Kemerink GJ, Langenberg CJ, Heidendal GA, and Saris WH.
Plasma FFA utilization and fatty acid-binding protein content are
diminished in type 2 diabetic muscle. Am J Physiol Endocrinol
Metab 279: E146 –E154, 2000.
19. Boden G, Lebed B, Schatz M, Homko C, and Lemieux S. Effects
of acute changes of plasma free fatty acids on intramyocellular fat
content and insulin resistance in healthy subjects. Diabetes 50:
1612–1617, 2001.
20. Boesch C, Decombaz J, Slotboom J, and Kreis R. Observation
of intramyocellular lipids by means of 1H magnetic resonance
spectroscopy. Proc Nutr Soc 58: 841– 850, 1999.
21. Boesch C and Kreis R. Dipolar coupling and ordering effects
observed in magnetic resonance spectra of skeletal muscle. NMR
Biomed 14: 140 –148, 2001.
22. Bonen A, Benton CR, Campbell SE, Chabowski A, Clarke DC,
Han XX, Glatz JF, and Luiken JJ. Plasmalemmal fatty acid
transport is regulated in heart and skeletal muscle by contraction,
insulin and leptin, and in obesity and diabetes. Acta Physiol Scand
178: 347–356, 2003.
23. Bonen A, Luiken JJ, Arumugam Y, Glatz JF, and Tandon NN.
Acute regulation of fatty acid uptake involves the cellular redistribution of fatty acid translocase. J Biol Chem 275: 14501–14508,
2000.
24. Bonen A, Miskovic D, and Kiens B. Fatty acid transporters
(FABPpm, FAT, FATP) in human muscle. Can J Appl Physiol 24:
515–523, 1999.
25. Bonen A, Parolin ML, Steinberg GR, Calles-Escandon J, Tandon NN, Glatz JF, Luiken JJ, Heigenhauser GJ, and Dyck DJ.
Triacylglycerol accumulation in human obesity and type 2 diabetes
is associated with increased rates of skeletal muscle fatty acid
transport and increased sarcolemmal FAT/CD36. FASEB J 18:
1144 –1146, 2004.
26. Bradbury MW and Berk PD. Mitochondrial aspartate aminotransferase: direction of a single protein with two distinct functions to
two subcellular sites does not require alternative splicing of the
mRNA. Biochem J 345: 423– 427, 2000.
27. Brechtel K, Niess AM, Machann J, Rett K, Schick F, Claussen
CD, Dickhuth HH, Haering HU, and Jacob S. Utilisation of
intramyocellular lipids (IMCLs) during exercise as assessed by
proton magnetic resonance spectroscopy (1H-MRS). Horm Metab
Res 33: 63– 66, 2001.
28. Bronfman M, Morales MN, and Orellana A. Diacylglycerol activation of protein kinase C is modulated by long-chain acyl-CoA.
Biochem Biophys Res Commun 152: 987–992, 1988.
29. Bruce CR, Kriketos AD, Cooney GJ, and Hawley JA. Disassociation of muscle triglyceride content and insulin sensitivity after
exercise training in patients with Type 2 diabetes. Diabetologia 47:
23–30, 2004.
30. Burguera B, Proctor D, Dietz N, Guo Z, Joyner M, and Jensen
MD. Leg free fatty acid kinetics during exercise in men and women.
Am J Physiol Endocrinol Metab 278: E113–E117, 2000.
31. Calles-Escandon J, Sweet L, Ljungqvist O, and Hirshman MF.
The membrane-associated 40 KD fatty acid binding protein (Berk’s
protein), a putative fatty acid transporter is present in human
skeletal muscle. Life Sci 58: 19 –28, 1996.
32. Cameron-Smith D, Burke LM, Angus DJ, Tunstall RJ, Cox GR,
Bonen A, Hawley JA, and Hargreaves M. A short-term, high-fat
diet up-regulates lipid metabolism and gene expression in human
skeletal muscle. Am J Clin Nutr 77: 313–318, 2003.
33. Campbell SE, Tandon NN, Woldegiorgis G, Luiken JJ, Glatz
JF, and Bonen A. A novel function for fatty acid translocase
(FAT)/CD36: involvement in long chain fatty acid transfer into the
mitochondria. J Biol Chem 279: 36235–36241, 2004.
34. Carlson LA, Ekelund LG, and Froberg SO. Concentration of
triglycerides, phospholipids and glycogen in skeletal muscle and of
free fatty acids and beta-hydroxybutyric acid in blood in man in
response to exercise. Eur J Clin Invest 1: 248 –254, 1971.
35. Carlson LA and Pernow B. Studies on blood lipids during exercise. J Lab Clin Med 53: 833– 841, 1959.
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
1. Abumrad N, Coburn C, and Ibrahimi A. Membrane proteins
implicated in long-chain fatty acid uptake by mammalian cells:
CD36, FATP and FABPm. Biochim Biophys Acta 1441: 4 –13, 1999.
2. Abumrad NA, El-Maghrabi MR, Amri EZ, Lopez E, and
Grimaldi PA. Cloning of a rat adipocyte membrane protein implicated in binding or transport of long-chain fatty acids that is
induced during preadipocyte differentiation. J Biol Chem 268:
17665–17668, 1993.
3. Ahlborg G, Felig P, Hagenfeldt L, Hendler R, and Wahren J.
Substrate turnover during prolonged exercise in man. Splanchinc
and leg metabolism of glucose, free fatty acids and amino acids.
J Clin Invest 53: 1080 –1090, 1974.
4. Alessio M, De Monte L, Scirea A, Gruarin P, Tandon NN, and
Sitia R. Synthesis, processing, and intracellular transport of CD36
during monocytic differentiation. J Biol Chem 271: 1770 –1775,
1996.
5. Andersen P, Adams RP, Sjogaard G, Thorboe A, and Saltin B.
Dynamic knee extension as model for study of isolated exercising
muscle in humans. J Appl Physiol 59: 1647–1653, 1985.
6. Andersen P and Henriksson J. Capillary supply of the quadriceps femoris muscle of man: adaptive response to exercise.
J Physiol 270: 677– 690, 1977.
7. Andersson A, Sjodin A, Hedman A, Olsson R, and Vessby B.
Fatty acid profile of skeletal muscle phospholipids in trained and
untrained young men. Am J Physiol Endocrinol Metab 279: E744 –
E751, 2000.
8. Andersson A, Sjodin A, Olsson R, and Vessby B. Effects of
physical exercise on phospholipid fatty acid composition in skeletal muscle. Am J Physiol Endocrinol Metab 274: E432–E438, 1998.
9. Anthonsen MW, Ronnstrand L, Wernstedt C, Degerman E,
and Holm C. Identification of novel phosphorylation sites in hormone-sensitive lipase that are phosphorylated in response to isoproterenol and govern activation properties in vitro. J Biol Chem
273: 215–221, 1998.
10. Bachmann OP, Dahl DB, Brechtel K, Machann J, Haap M,
Maier T, Loviscach M, Stumvoll M, Claussen CD, Schick F,
Haring HU, and Jacob S. Effects of intravenous and dietary lipid
challenge on intramyocellular lipid content and the relation with
insulin sensitivity in humans. Diabetes 50: 2579 –2584, 2001.
11. Bastie CC, Hajri T, Drover VA, Grimaldi PA, and Abumrad
NA. CD36 in myocyte channels fatty acids to a lipase-accessible
triglyceride pool that is related to cell lipid and insulin responsiveness. Diabetes 53: 2209 –2216, 2004.
12. Bavenholm PN, Pigon J, Saha AK, Ruderman NB, and Efendic
S. Fatty acid oxidation and the regulation of malonyl-CoA in human
muscle. Diabetes 49: 1078 –1083, 2000.
13. Belfrage P, Jergil B, Stralfors P, and Tornqvist H. Hormonesensitive lipase of rat adipose tissue: identification and some properties of the enzyme protein. FEBS Lett 75: 259 –264, 1977.
14. Bergman BC, Butterfield GE, Wolfel EE, Casazza GA, Lopaschuk GD, and Brooks GA. Evaluation of exercise and training on
muscle lipid metabolism. Am J Physiol Endocrinol Metab 276:
E106 –E117, 1999.
15. Berk PD and Stump DD. Mechanisms of cellular uptake of long
chain free fatty acids. Mol Cell Biochem 192: 17–31, 1999.
16. Bezaire V, Heigenhauser GJ, and Spriet LL. Regulation of CPT
I activity in intermyofibrillar and subsarcolemmal mitochondria
from human and rat skeletal muscle. Am J Physiol Endocrinol
Metab 286: E85–E91, 2004.
235
236
BENTE KIENS
Physiol Rev • VOL
57. De Bock K, Richter EA, Russell AP, Eijnde BO, Derave W,
Ramaekers M, Koninckx E, Leger B, Verhaeghe J, and Hespel
P. Exercise in the fasted state facilitates fibre type-specific intramyocellular lipid breakdown and stimulates glycogen resynthesis in humans. J Physiol 564: 649 – 660, 2005.
58. Decombaz J, Schmitt B, Ith M, Decarli B, Diem P, Kreis R,
Hoppeler H, and Boesch C. Postexercise fat intake repletes
intramyocellular lipids but no faster in trained than in sedentary
subjects. Am J Physiol Regul Integr Comp Physiol 281: R760 –
R769, 2001.
59. Derave W, Ai H, Ihlemann J, Witters LA, Kristiansen S, Richter EA, and Ploug T. Dissociation of AMP-activated protein kinase activation and glucose transport in contracting slow-twitch
muscle. Diabetes 49: 1281–1287, 2000.
60. Donsmark M, Langfort J, Holm C, Ploug T, and Galbo H.
Contractions activate hormone-sensitive lipase in rat muscle by
protein kinase C and mitogen-activated protein kinase. J Physiol
550: 845– 854, 2003.
61. Donsmark M, Langfort J, Holm C, Ploug T, and Galbo H.
Contractions induce phosphorylation of the AMPK site Ser565 in
hormone-sensitive lipase in muscle. Biochem Biophys Res Commun 316: 867– 871, 2004.
62. Dyck DJ, Putman CT, Heigenhauser GJ, Hultman E, and
Spriet LL. Regulation of fat-carbohydrate interaction in skeletal
muscle during intense aerobic cycling. Am J Physiol Endocrinol
Metab 265: E852–E859, 1993.
63. Ellis BA, Poynten A, Lowy AJ, Furler SM, Chisholm DJ, Kraegen EW, and Cooney GJ. Long-chain acyl-CoA esters as indicators of lipid metabolism and insulin sensitivity in rat and human
muscle. Am J Physiol Endocrinol Metab 279: E554 –E560, 2000.
64. Essen B. Studies on the regulation of metabolism in human skeletal muscle using intermittent exercise as an experimental model.
Acta Physiol Scand Suppl 454: 1–32, 1978.
65. Essen B, Hagenfeldt L, and Kaijser L. Utilization of blood-borne
and intramuscular substrates during continuous and intermittent
exercise in man. J Physiol 265: 489 –506, 1977.
66. Essen B, Jansson E, Henriksson J, Taylor AW, and Saltin B.
Metabolic characteristics of fibre types in human skeletal muscle.
Acta Physiol Scand 95: 153–165, 1975.
67. Faergeman NJ, DiRusso CC, Elberger A, Knudsen J, and
Black PN. Disruption of the Saccharomyces cerevisiae homologue
to the murine fatty acid transport protein impairs uptake and
growth on long-chain fatty acids. J Biol Chem 272: 8531– 8538,
1997.
68. Faergeman NJ and Knudsen J. Role of long-chain fatty acyl-CoA
esters in the regulation of metabolism and in cell signalling. Biochem J 323: 1–12, 1997.
69. Falholt K, Jensen I, Lindkaer JS, Mortensen H, Volund A,
Heding LG, Noerskov PP, and Falholt W. Carbohydrate and
lipid metabolism of skeletal muscle in type 2 diabetic patients.
Diabet Med 5: 27–31, 1988.
70. Farese RV. Function and dysfunction of aPKC isoforms for glucose transport in insulin-sensitive and insulin-resistant states. Am J
Physiol Endocrinol Metab 283: E1–E11, 2002.
71. Febbraio M, Abumrad NA, Hajjar DP, Sharma K, Cheng W,
Pearce SF, and Silverstein RL. A null mutation in murine CD36
reveals an important role in fatty acid and lipoprotein metabolism.
J Biol Chem 274: 19055–19062, 1999.
72. Febbraio M, Hajjar DP, and Silverstein RL. CD36: a class B
scavenger receptor involved in angiogenesis, atherosclerosis, inflammation, and lipid metabolism. J Clin Invest 108: 785–791, 2001.
73. Febbraio MA and Stewart KL. CHO feeding before prolonged
exercise: effect of glycemic index on muscle glycogenolysis and
exercise performance. J Appl Physiol 81: 1115–1120, 1996.
74. Franch J, Andersen JL, Jensen J, Pedersen PK, and Knudsen
J. Acyl-coenzyme A binding protein expression is fibre-type specific in rat skeletal muscle but not affected by moderate endurance
training. Pflügers Arch 443: 387–393, 2002.
75. Franch J, Knudsen J, Ellis BA, Pedersen PK, Cooney GJ, and
Jensen J. Acyl-CoA binding protein expression is fiber type-specific and elevated in muscles from the obese insulin-resistant
Zucker rat. Diabetes 51: 449 – 454, 2002.
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
36. Cechetto JD, Sadacharan SK, Berk PD, and Gupta RS. Immunogold localization of mitochondrial aspartate aminotransferase in
mitochondria and on the cell surface in normal rat tissues. Histol
Histopathol 17: 353–364, 2002.
37. Chauveau MA and Kaufmann M. Experiences pour la determination du coefficient de l’activite nutritive et respiratoire des muscles en repos et en travail. C R Acad Sci 104: 1126 –1132, 1887.
38. Chen ZP, McConell GK, Michell BJ, Snow RJ, Canny BJ, and
Kemp BE. AMPK signaling in contracting human skeletal muscle:
acetyl-CoA carboxylase and NO synthase phosphorylation. Am J
Physiol Endocrinol Metab 279: E1202–E1206, 2000.
39. Chien D, Dean D, Saha AK, Flatt JP, and Ruderman NB.
Malonyl-CoA content and fatty acid oxidation in rat muscle and
liver in vivo. Am J Physiol Endocrinol Metab 279: E259 –E265,
2000.
40. Christensen EH and Hansen O. Arbeitsfähigkeit und Ernährung.
Skand Arch Physiol 81: 160 –171, 1939.
41. Christensen EH and Hansen O. Respiratorisher Quotient und O2
-Aufname. Skand Arch Physiol 81: 180 –189, 1939.
42. Clarke DC, Miskovic D, Han XX, Calles-Escandon J, Glatz JF,
Luiken JJ, Heikkila JJ, and Bonen A. Overexpression of membrane-associated fatty acid binding protein (FABPpm) in vivo increases fatty acid sarcolemmal transport and metabolism. Physiol
Gen 17: 31–37, 2004.
43. Clavel S, Farout L, Briand M, Briand Y, and Jouanel P. Effect
of endurance training and/or fish oil supplemented diet on cytoplasmic fatty acid binding protein in rat skeletal muscles and heart.
Eur J Appl Physiol 87: 193–201, 2002.
44. Coe NR and Bernlohr DA. Physiological properties and functions
of intracellular fatty acid-binding proteins. Biochim Biophys Acta
1391: 287–306, 1998.
45. Coe NR, Smith AJ, Frohnert BI, Watkins PA, and Bernlohr
DA. The fatty acid transport protein (FATP1) is a very long chain
acyl-CoA synthetase. J Biol Chem 274: 36300 –36304, 1999.
46. Cohen AW, Hnasko R, Schubert W, and Lisanti MP. Role of
caveolae and caveolins in health and disease. Physiol Rev 84:
1341–1379, 2004.
47. Colberg SR, Simoneau JA, Thaete FL, and Kelley DE. Skeletal
muscle utilization of free fatty acids in women with visceral obesity. J Clin Invest 95: 1846 –1853, 1995.
48. Coleman RA, Lewin TM, and Muoio DM. Physiological and
nutritional regulation of enzymes of triacylglycerol synthesis. Annu
Rev Nutr 20: 77–103, 2000.
49. Constantin-Teodosiu D, Carlin JI, Cederblad G, Harris RC,
and Hultman E. Acetyl group accumulation and pyruvate dehydrogenase activity in human muscle during incremental exercise.
Acta Physiol Scand 143: 367–372, 1991.
50. Cooney GJ, Thompson AL, Furler SM, YEJ, and Kraegen EW.
Muscle long-chain acyl CoA esters and insulin resistance. Ann NY
Acad Sci 967: 196 –207, 2002.
51. Cooper DR, Watson JE, and Dao ML. Decreased expression of
protein kinase-C alpha, beta, and epsilon in soleus muscle of
Zucker obese (fa/fa) rats. Endocrinology 133: 2241–2247, 1993.
52. Corton JM, Gillespie JG, Hawley SA, and Hardie DG. 5-Aminoimidazole-4-carboxamide ribonucleoside. A specific method for
activating AMP-activated protein kinase in intact cells? Eur J Biochem 229: 558 –565, 1995.
53. Costill D, Fink W, Getchell L, Ivy J, and Witzmann F. Lipid
metabolism in skeletal muscle of endurance-trained males and
females. J Appl Physiol 47: 787–791, 1979.
54. Coyle EF, Jeukendrup AE, Oseto MC, Hodgkinson BJ, and
Zderic TW. Low-fat diet alters intramuscular substrates and reduces lipolysis and fat oxidation during exercise. Am J Physiol
Endocrinol Metab 280: E391–E398, 2001.
55. Coyle EF, Jeukendrup AE, Wagenmakers AJ, and Saris WH.
Fatty acid oxidation is directly regulated by carbohydrate metabolism during exercise. Am J Physiol Endocrinol Metab 273: E268 –
E275, 1997.
56. Dean D, Daugaard JR, Young ME, Saha A, Vavvas D, Asp S,
Kiens B, Kim KH, Witters L, Richter EA, and Ruderman N.
Exercise diminishes the activity of acetyl-CoA carboxylase in human muscle. Diabetes 49: 1295–1300, 2000.
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
Physiol Rev • VOL
96. Guo Z, Burguera B, and Jensen MD. Kinetics of intramuscular
triglyceride fatty acids in exercising humans. J Appl Physiol 89:
2057–2064, 2000.
97. Ha J, Lee JK, Kim KS, Witters LA, and Kim KH. Cloning of
human acetyl-CoA carboxylase-beta and its unique features. Proc
Natl Acad Sci USA 93: 11466 –11470, 1996.
98. Habinowski SA, Hirshman M, Sakamoto K, Kemp BE, Gould
SJ, Goodyear LJ, and Witters LA. Malonyl-CoA decarboxylase is
not a substrate of AMP-activated protein kinase in rat fast-twitch
skeletal muscle or an islet cell line. Arch Biochem Biophys 396:
71–79, 2001.
99. Haemmerle G, Zimmermann R, Hayn M, Theussl C, Waeg G,
Wagner E, Sattler W, Magin TM, Wagner EF, and Zechner R.
Hormone-sensitive lipase deficiency in mice causes diglyceride
accumulation in adipose tissue, muscle, and testis. J Biol Chem
277: 4806 – 4815, 2002.
100. Hagenfeldt L. Metabolism of free fatty acids and ketone bodies
during exercise in normal and diabetic man. Diabetes 28 Suppl 1:
66 –70, 1979.
101. Hagenfeldt L and Wahren J. Simultaneous uptake and release of
individual free fatty acids in human forearm muscle during exercise. Life Sci 5: 357–364, 1966.
102. Hagenfeldt L and Wahren J. Human forearm muscle metabolism
during exercise. II. Uptake, release and oxidation of individual FFA
and glycerol. Scand J Clin Lab Invest 21: 263–276, 1968.
103. Hagenfeldt L, Wahren J, Pernow B, Cronestrand R, and
Ekestrom S. Free fatty acid metabolism of leg muscles during
exercise in patients with obliterative iliac and femoral artery disease before and after reconstructive surgery. J Clin Invest 51:
3061–3071, 1972.
104. Hajduch E, Balendran A, Batty IH, Litherland GJ, Blair AS,
Downes CP, and Hundal HS. Ceramide impairs the insulin-dependent membrane recruitment of protein kinase B leading to a
loss in downstream signalling in L6 skeletal muscle cells. Diabetologia 44: 173–183, 2001.
105. Hall AM, Smith AJ, and Bernlohr DA. Characterization of the
Acyl-CoA synthetase activity of purified murine fatty acid transport
protein 1. J Biol Chem 278: 43008 – 43013, 2003.
106. Hatch GM, Smith AJ, Xu FY, Hall AM, and Bernlohr DA.
FATP1 channels exogenous FA into 1,2,3-triacyl-sn-glycerol and
down-regulates sphingomyelin and cholesterol metabolism in
growing 293 cells. J Lipid Res 43: 1380 –1389, 2002.
107. Havel RJ, Ekelund LG, and Holmgren A. Kinetic analysis of the
oxidation of palmitate-1-14C in man during prolonged heavy muscular exercise. J Lipid Res 8: 366 –373, 1967.
108. Havel RJ, Naimark A, and Borchgrevink CF. Turnover rate and
oxidation of free fatty acids of blood plasma in man during exercise: studies during continous infusion of palmitate-1-C14. J Clin
Invest 42: 1054 –1063, 1963.
109. Havel RJ, Pernow B, and Jones NL. Uptake and release of free
fatty acids and other metabolites in the legs of exercising men.
J Appl Physiol 23: 90 –99, 1967.
111. He J, Goodpaster BH, and Kelley DE. Effects of weight loss and
physical activity on muscle lipid content and droplet size. Obesity
Res 12: 761–769, 2004.
112. He J, Watkins S, and Kelley DE. Skeletal muscle lipid content
and oxidative enzyme activity in relation to muscle fiber type in
type 2 diabetes and obesity. Diabetes 50: 817– 823, 2001.
113. Helge JW and Dela F. Effect of training on muscle triacylglycerol
and structural lipids: a relation to insulin sensitivity? Diabetes 52:
1881–1887, 2003.
114. Helge JW, Dobrzyn A, Saltin B, and Gorski J. Exercise and
training effects on ceramide metabolism in human skeletal muscle.
Exp Physiol 89: 119 –127, 2004.
115. Helge JW and Kiens B. Muscle enzyme activity in man: role of
substrate availability and training. Am J Physiol Regul Integr
Comp Physiol 272: R1620 –R1624, 1997.
116. Helge JW, Richter EA, and Kiens B. Interaction of training and
diet on metabolism and endurance during exercise in man.
J Physiol 492: 293–306, 1996.
117. Helge JW, Watt PW, Richter EA, Rennie MJ, and Kiens B. Fat
utilization during exercise: adaptation to a fat-rich diet increases
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
76. Fredrikson G, Stralfors P, Nilsson NO, and Belfrage P. Hormone-sensitive lipase of rat adipose tissue. Purification and some
properties. J Biol Chem 256: 6311– 6320, 1981.
77. Fredrikson G, Tornqvist H, and Belfrage P. Hormone-sensitive
lipase and monoacylglycerol lipase are both required for complete
degradation of adipocyte triacylglycerol. Biochim Biophys Acta
876: 288 –293, 1986.
78. Friedlander AL, Casazza GA, Horning MA, Buddinger TF, and
Brooks GA. Effects of exercise intensity and training on lipid
metabolism in young women. Am J Physiol Endocrinol Metab 275:
E853–E863, 1998.
79. Friedlander AL, Casazza GA, Horning MA, Usaj A, and Brooks
GA. Endurance training increases fatty acid turnover, but not fat
oxidation, in young men. J Appl Physiol 86: 2097–2105, 1999.
80. Friedman S, and Fraenkel G. Reversible enzymatic acetylation
of carnitine. Arch Biochem Biophys 59: 491–501, 1955.
81. Froberg SO and Mossfeldt F. Effect of prolonged strenuous
exercise on the concentration of triglycerides, phospholipids and
glycogen in muscle of man. Acta Physiol Scand 82: 167–171, 1971.
82. Fujii N, Hayashi T, Hirshman MF, Smith JT, Habinowski SA,
Kaijser L, Mu J, Ljungqvist O, Birnbaum MJ, Witters LA,
Thorell A, and Goodyear LJ. Exercise induces isoform-specific
increase in 5⬘AMP-activated protein kinase activity in human skeletal muscle. Biochem Biophys Res Commun 273: 1150 –1155, 2000.
83. Gan SK, Kriketos AD, Ellis BA, Thompson CH, Kraegen EW,
and Chisholm DJ. Changes in aerobic capacity and visceral fat but
not myocyte lipid levels predict increased insulin action after exercise in overweight and obese men. Diabetes Care 26: 1706 –1713,
2003.
84. Gargiulo CE, Stuhlsatz-Krouper SM, and Schaffer JE. Localization of adipocyte long-chain fatty acyl-CoA synthetase at the
plasma membrane. J Lipid Res 40: 881– 892, 1999.
85. Garton AJ, Campbell DG, Carling D, Hardie DG, Colbran RJ,
and Yeaman SJ. Phosphorylation of bovine hormone-sensitive
lipase by the AMP-activated protein kinase. A possible antilipolytic
mechanism. Eur J Biochem 179: 249 –254, 1989.
86. Glatz JF and Storch J. Unravelling the significance of cellular
fatty acid-binding proteins. Curr Opin Lipidol 12: 267–274, 2001.
87. Glatz JF, Van Breda E, and Van der Vusse GJ. Intracellular
transport of fatty acids in muscle. Role of cytoplasmic fatty acidbinding protein. Adv Exp Med Biol 441: 207–218, 1998.
88. Glatz JF, Vork MM, Cistola DP, and Van der Vusse GJ. Cytoplasmic fatty acid binding protein: significance for intracellular
transport of fatty acids and putative role on signal transduction
pathways. Prostaglandins Leukot Essent Fatty Acids 48: 33– 41,
1993.
89. Goldberg IJ and Merkel M. Lipoprotein lipase: physiology, biochemistry, and molecular biology. Front Biosci 6: D388 –D405,
2001.
90. Gollnick PD and Saltin B. Significance of skeletal muscle oxidative enzyme enhancement with endurance training. Clin Physiol 2:
1–12, 1982.
91. Goodpaster BH, He J, Watkins S, and Kelley DE. Skeletal
muscle lipid content and insulin resistance: evidence for a paradox
in endurance-trained athletes. J Clin Endocrinol Metab 86: 5755–
5761, 2001.
92. Goodpaster BH, Thaete FL, Simoneau JA, and Kelley DE.
Subcutaneous abdominal fat and thigh muscle composition predict
insulin sensitivity independently of visceral fat. Diabetes 46: 1579 –
1585, 1997.
93. Goodpaster BH, Theriault R, Watkins SC, and Kelley DE.
Intramuscular lipid content is increased in obesity and decreased
by weight loss. Metabolism 49: 467– 472, 2000.
94. Greenberg AS, Shen WJ, Muliro K, Patel S, Souza SC, Roth
RA, and Kraemer FB. Stimulation of lipolysis and hormonesensitive lipase via the extracellular signal-regulated kinase pathway. J Biol Chem 276: 45456 – 45461, 2001.
95. Griffin ME, Marcucci MJ, Cline GW, Bell K, Barucci N, Lee D,
Goodyear LJ, Kraegen EW, White MF, and Shulman GI. Free
fatty acid-induced insulin resistance is associated with activation
of protein kinase C theta and alterations in the insulin signaling
cascade. Diabetes 48: 1270 –1274, 1999.
237
238
118.
119.
120.
121.
122.
124.
125.
126.
127.
128.
129.
130.
131.
132.
133.
134.
135.
136.
137.
utilization of plasma fatty acids and very low density lipoproteintriacylglycerol in humans. J Physiol 537: 1009 –1020, 2001.
Helge JW, Wu BJ, Willer M, Daugaard JR, Storlien LH, and
Kiens B. Training affects muscle phospholipid fatty acid composition in humans. J Appl Physiol 90: 670 – 677, 2001.
Helge JW, Wulff B, and Kiens B. Impact of a fat-rich diet on
endurance in man: role of the dietary period. Med Sci Sports
Exercise 30: 456 – 461, 1998.
Henriksson J. Training induced adaptations of skeletal muscle
and metabolism during submaximal exercise. J Physiol 270: 661–
675, 1977.
Hiatt WR, Regensteiner JG, Wolfel EE, Ruff L, and Brass EP.
Carnitine and acylcarnitine metabolism during exercise in humans.
Dependence on skeletal muscle metabolic state. J Clin Invest 84:
1167–1173, 1989.
Hirsch D, Stahl A, and Lodish HF. A family of fatty acid transporters conserved from mycobacterium to man. Proc Natl Acad Sci
USA 95: 8625– 8629, 1998.
Hodgkin MN, Pettitt TR, Martin A, Michell RH, Pemberton
AJ, and Wakelam MJ. Diacylglycerols and phosphatidates: which
molecular species are intracellular messengers? Trends Biochem
Sci 23: 200 –204, 1998.
Holloszy JO and Coyle EF. Adaptations of skeletal muscle to
endurance exercise and their metabolic consequences. J Appl
Physiol 56: 831– 838, 1984.
Holm C. Molecular mechanisms regulating hormone-sensitive
lipase and lipolysis. Biochem Soc Trans 31: 1120 –1124, 2003.
Holm C, Belfrage P, and Fredrikson G. Immunological evidence
for the presence of hormone-sensitive lipase in rat tissues other
than adipose tissue. Biochem Biophys Res Commun 148: 99 –105,
1987.
Holm C, Osterlund T, Laurell H, and Contreras JA. Molecular
mechanisms regulating hormone-sensitive lipase and lipolysis.
Annu Rev Nutr 20: 365–393, 2000.
Hoppeler H, Howald H, Conley K, Lindstedt SL, Claassen H,
Vock P, and Weibel ER. Endurance training in humans: aerobic
capacity and structure of skeletal muscle. J Appl Physiol 59: 320 –
327, 1985.
Horowitz JF, Mora-Rodriguez R, Byerley LO, and Coyle EF.
Lipolytic suppression following carbohydrate ingestion limits fat
oxidation during exercise. Am J Physiol Endocrinol Metab 273:
E768 –E775, 1997.
Howald H, Hoppeler H, Claassen H, Mathieu O, and Straub R.
Influences of endurance training on the ultrastructural composition
of the different muscle fiber types in humans. Pflügers Arch 403:
369 –376, 1985.
Howald H, Boesch C, Kreis R, Matter S, Billeter R, EssenGustavsson B, and Hoppeler H. Content of intramyocellular
lipids derived by electron microscopy, biochemical assays, and
1
H-MR spectroscopy. J Appl Physiol 92: 2264 –2272, 2002.
Hulver MW, Berggren JR, Cortright RN, Dudek RW, Thompson RP, Pories WJ, MacDonald KG, Cline GW, Shulman GI,
Dohm GL, and Houmard JA. Skeletal muscle lipid metabolism
with obesity. Am J Physiol Endocrinol Metab 284: E741–E747,
2003.
Hurley BF, Nemeth PM, Martin IIIWH, Hagberg JM, Dalsky
GP, and Holloszy JO. Muscle triglyceride utilization during exercise: effect of training. J Appl Physiol 60: 562–567, 1986.
Hwang JH, Pan JW, Heydari S, Hetherington HP, and Stein
DT. Regional differences in intramyocellular lipids in humans observed by in vivo 1H-MR spectroscopic imaging. J Appl Physiol 90:
1267–1274, 2001.
Ibrahimi A, Bonen A, Blinn WD, Hajri T, Li X, Zhong K,
Cameron R, and Abumrad NA. Muscle-specific overexpression of
FAT/CD36 enhances fatty acid oxidation by contracting muscle,
reduces plasma triglycerides and fatty acids, and increases plasma
glucose and insulin. J Biol Chem 274: 26761–26766, 1999.
Itani SI, Pories WJ, MacDonald KG, and Dohm GL. Increased
protein kinase C theta in skeletal muscle of diabetic patients.
Metabolism 50: 553–557, 2001.
Itani SI, Ruderman NB, Schmieder F, and Boden G. Lipidinduced insulin resistance in human muscle is associated with
Physiol Rev • VOL
138.
139.
140.
141.
142.
143.
144.
145.
146.
147.
148.
149.
150.
151.
152.
153.
154.
155.
156.
157.
158.
changes in diacylglycerol, protein kinase C, and IkappaB-alpha.
Diabetes 51: 2005–2011, 2002.
Itani SI, Zhou Q, Pories WJ, MacDonald KG, and Dohm GL.
Involvement of protein kinase C in human skeletal muscle insulin
resistance and obesity. Diabetes 49: 1353–1358, 2000.
Jacobs I, Lithell H, and Karlsson J. Dietary effects on glycogen
and lipoprotein lipase activity in skeletal muscle in man. Acta
Physiol Scand 115: 85–90, 1982.
Jansson E and Kaijser L. Substrate utilization and enzymes in
skeletal muscle of extremely endurance-trained men. J Appl
Physiol 62: 999 –1005, 1987.
Jepson CA and Yeaman SJ. Inhibition of hormone-sensitive
lipase by intermediary lipid metabolites. FEBS Lett 310: 197–200,
1992.
Johnson NA, Stannard SR, Mehalski K, Trenell MI, Sachinwalla T, Thompson CH, and Thompson MW. Intramyocellular
triacylglycerol in prolonged cycling with high- and low-carbohydrate availability. J Appl Physiol 94: 1365–1372, 2003.
Keizer HA, Schaart G, Tandon NN, Glatz JF, and Luiken JJ.
Subcellular immunolocalisation of fatty acid translocase (FAT)/
CD36 in human type-1 and type-2 skeletal muscle fibres. Histochem
Cell Biol 121: 101–107, 2004.
Kelley DE, Goodpaster B, Wing RR, and Simoneau JA. Skeletal
muscle fatty acid metabolism in association with insulin resistance,
obesity, and weight loss. Am J Physiol Endocrinol Metab 277:
E1130 –E1141, 1999.
Kelley DE, He J, Menshikova EV, and Ritov VB. Dysfunction of
mitochondria in human skeletal muscle in type 2 diabetes. Diabetes
51: 2944 –2950, 2002.
Kiens B. Effect of endurance training on fatty acid metabolism:
local adaptations. Med Sci Sports Exercise 29: 640 – 645, 1997.
Kiens B, Essen-Gustavsson B, Christensen NJ, and Saltin B.
Skeletal muscle substrate utilization during submaximal exercise
in man: effect of endurance training. J Physiol 469: 459 – 478, 1993.
Kiens B, Essen-Gustavsson B, Gad P, and Lithell H. Lipoprotein lipase activity and intramuscular triglyceride stores after longterm high-fat and high-carbohydrate diets in physically trained
men. Clin Physiol 7: 1–9, 1987.
Kiens B, Kristiansen S, Richter EA, and Turcotte LP. Membrane associated FABP in human skeletal muscle is increased by
endurance training. Biochem Biophys Res Commun 231: 463– 465,
1997.
Kiens B and Lithell H. Lipoprotein metabolism influenced by
training-induced changes in human skeletal muscle. J Clin Invest
83: 558 –564, 1989.
Kiens B, Lithell H, Mikines KJ, and Richter EA. Effects of
insulin and exercise on muscle lipoprotein lipase activity in man
and its relation to insulin action. J Clin Invest 84: 1124 –1129, 1989.
Kiens B and Richter EA. Types of carbohydrate in an ordinary
diet affect insulin action and muscle substrates in humans. Am J
Clin Nutr 63: 47–53, 1996.
Kiens B and Richter EA. Utilization of skeletal muscle triacylglycerol during postexercise recovery in humans. Am J Physiol
Endocrinol Metab 275: E332–E337, 1998.
Kiens B, Roemen THM, and Van der Vusse GJ. Muscular longchain fatty acid content during graded exercise in humans. Am J
Physiol Endocrinol Metab 276: E352–E357, 1999.
Kiens B, Roepstorff C, Glatz JF, Bonen A, Schjerling P, Knudsen J, and Nielsen JN. Lipid-binding proteins and lipoprotein
lipase activity in human skeletal muscle: influence of physical
activity and gender. J Appl Physiol 97: 1209 –1218, 2004.
Kim JK, Fillmore JJ, Chen Y, Yu C, Moore IK, Pypaert M, Lutz
EP, Kako Y, Velez-Carrasco W, Goldberg IJ, Breslow JL, and
Shulman GI. Tissue-specific overexpression of lipoprotein lipase
causes tissue-specific insulin resistance. Proc Natl Acad Sci USA
98: 7522–7527, 2001.
Kim JK, Fillmore JJ, Sunshine MJ, Albrecht B, Higashimori
T, Kim DW, Liu ZX, Soos TJ, Cline GW, O’Brien WR, Littman
DR, and Shulman GI. PKC-theta knockout mice are protected
from fat-induced insulin resistance. J Clin Invest 114: 823– 827,
2004.
Kim JK, Gimeno RE, Higashimori T, Kim HJ, Choi H, Punreddy S, Mozell RL, Tan G, Stricker-Krongrad A, Hirsch DJ,
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
123.
BENTE KIENS
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
159.
160.
161.
162.
164.
165.
166.
167.
168.
169.
170.
171.
172.
173.
174.
175.
176.
Physiol Rev • VOL
177.
178.
179.
180.
181.
182.
183.
184.
185.
186.
187.
188.
189.
190.
191.
192.
193.
194.
inhibits insulin signaling by phosphorylating IRS1 at Ser(1101).
J Biol Chem 279: 45304 – 45307, 2004.
Liang X, Le W, Zhang D, and Schulz H. Impact of the intramitochondrial enzyme organization on fatty acid oxidation. Biochem
Soc Trans 29: 279 –282, 2001.
Lithell H, Hellsing K, Lundqvist G, and Malmberg P. Lipoprotein-lipase activity of human skeletal-muscle and adipose tissue
after intensive physical exercise. Acta Physiol Scand 105: 312–315,
1979.
Lithell H, Orlander J, Schele R, Sjodin B, and Karlsson J.
Changes in lipoprotein-lipase activity and lipid stores in human
skeletal muscle with prolonged heavy exercise. Acta Physiol Scand
107: 257–261, 1979.
Liu YQ, Tornheim K, and Leahy JL. Fatty acid-induced beta cell
hypersensitivity to glucose. Increased phosphofructokinase activity and lowered glucose-6-phosphate content. J Clin Invest 101:
1870 –1875, 1998.
Luiken JJ, Arumugam Y, Bell RC, Calles-Escandon J, Tandon
NN, Glatz JF, and Bonen A. Changes in fatty acid transport and
transporters are related to the severity of insulin deficiency. Am J
Physiol Endocrinol Metab 283: E612–E621, 2002.
Luiken JJ, Arumugam Y, Dyck DJ, Bell RC, Pelsers MM,
Turcotte LP, Tandon NN, Glatz JF, and Bonen A. Increased
rates of fatty acid uptake and plasmalemmal fatty acid transporters
in obese Zucker rats. J Biol Chem 276: 40567– 40573, 2001.
Luiken JJ, Coort SL, Koonen DP, van der Horst DJ, Bonen A,
Zorzano A, and Glatz JF. Regulation of cardiac long-chain fatty
acid and glucose uptake by translocation of substrate transporters.
Pflügers Arch 448: 1–15, 2004.
Luiken JJ, Koonen DP, Coumans WA, Pelsers MM, Binas B,
Bonen A, and Glatz JF. Long-chain fatty acid uptake by skeletal
muscle is impaired in homozygous, but not heterozygous, hearttype-FABP null mice. Lipids 38: 491– 496, 2003.
Luiken JJ, Koonen DP, Willems J, Zorzano A, Becker C,
Fischer Y, Tandon NN, Van der Vusse GJ, Bonen A, and Glatz
JF. Insulin stimulates long-chain fatty acid utilization by rat cardiac myocytes through cellular redistribution of FAT/CD36. Diabetes 51: 3113–3119, 2002.
Luiken JJ, Schaap FG, Van Nieuwenhoven FA, Van der Vusse
GJ, Bonen A, and Glatz JF. Cellular fatty acid transport in heart
and skeletal muscle as facilitated by proteins. Lipids 34 Suppl:
S169 –S175, 1999.
Luiken JJ, Turcotte LP, and Bonen A. Protein-mediated palmitate uptake and expression of fatty acid transport proteins in heart
giant vesicles. J Lipid Res 40: 1007–1016, 1999.
Luiken JJ, Willems J, Van der Vusse GJ, and Glatz JF. Electrostimulation enhances FAT/CD36-mediated long-chain fatty acid
uptake by isolated rat cardiac myocytes. Am J Physiol Endocrinol
Metab 281: E704 –E712, 2001.
Machann J, Haring H, Schick F, and Stumvoll M. Intramyocellular lipids and insulin resistance. Diabetes Obes Metab 6: 239 –248,
2004.
Malenfant P, Joanisse DR, Theriault R, Goodpaster BH,
Kelley DE, and Simoneau JA. Fat content in individual muscle
fibers of lean and obese subjects. Int J Obes Relat Metab Disord 25:
1316 –1321, 2001.
Malkova D, Evans RD, Frayn KN, Humphreys SM, Jones PR,
and Hardman AE. Prior exercise and postprandial substrate extraction across the human leg. Am J Physiol Endocrinol Metab 279:
E1020 –E1028, 2000.
Marotta M, Ferrer-Martnez A, Parnau J, Turini M, Mace K,
and Gomez Foix AM. Fiber type- and fatty acid compositiondependent effects of high-fat diets on rat muscle triacylglyceride
and fatty acid transporter protein-1 content. Metabolism 53: 1032–
1036, 2004.
Martin WH III, Dalsky GP, Hurley BF, Matthews DE, Bier DM,
Hagberg JM, Rogers MA, King DS, and Holloszy JO. Effect of
endurance training on plasma free fatty acid turnover and oxidation during exercise. Am J Physiol Endocrinol Metab 265: E708 –
E714, 1993.
McConell G, Fabris S, Proietto J, and Hargreaves M. Effect of
carbohydrate ingestion on glucose kinetics during exercise. J Appl
Physiol 77: 1537–1541, 1994.
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
163.
Fillmore JJ, Liu ZX, Dong J, Cline G, Stahl A, Lodish HF, and
Shulman GI. Inactivation of fatty acid transport protein 1 prevents
fat-induced insulin resistance in skeletal muscle. J Clin Invest 113:
756 –763, 2004.
Kim JY, Hickner RC, Cortright RL, Dohm GL, and Houmard
JA. Lipid oxidation is reduced in obese human skeletal muscle.
Am J Physiol Endocrinol Metab 279: E1039 –E1044, 2000.
Kimber NE, Heigenhauser GJ, Spriet LL, and Dyck DJ. Skeletal muscle fat and carbohydrate metabolism during recovery from
glycogen-depleting exercise in humans. J Physiol 548: 919 –927,
2003.
Kjaer M, Howlett K, Langfort J, Zimmerman-Belsing T,
Lorentsen J, Bulow J, Ihlemann J, Feldt-Rasmussen U, and
Galbo H. Adrenaline and glycogenolysis in skeletal muscle during
exercise: a study in adrenalectomised humans. J Physiol 528: 371–
378, 2000.
Klein S, Coyle EF, and Wolfe RR. Fat metabolism during lowintensity exercise in endurance-trained and untrained men. Am J
Physiol Endocrinol Metab 267: E934 –E940, 1994.
Kolleck I, Guthmann F, Ladhoff AM, Tandon NN, Schlame M,
and Rustow B. Cellular cholesterol stimulates acute uptake of
palmitate by redistribution of fatty acid translocase in type II
pneumocytes. Biochemistry 41: 6369 – 6375, 2002.
Koya D and King GL. Protein kinase C activation and the development of diabetic complications. Diabetes 47: 859 – 866, 1998.
Krogh A and Lindhard L. The relative volume of fat and carbohydrate as sources of muscle energy. With appendices on the
correlation between standard metabolism and the respiratory quotient during rest and work. Biochem J 14: 290 –363, 1920.
Krssak M, Falk PK, Dresner A, DiPietro L, Vogel SM, Rothman DL, Roden M, and Shulman GI. Intramyocellular lipid concentrations are correlated with insulin sensitivity in humans: a 1H
NMR spectroscopy study. Diabetologia 42: 113–116, 1999.
Krssak M, Petersen KF, Bergeron R, Price T, Laurent D,
Rothman DL, Roden M, and Shulman GI. Intramuscular glycogen and intramyocellular lipid utilization during prolonged exercise
and recovery in man: a 13C and 1H nuclear magnetic resonance
spectroscopy study. J Clin Endocrinol Metab 85: 748 –754, 2000.
Langfort J, Donsmark M, Ploug T, Holm C, and Galbo H.
Hormone-sensitive lipase in skeletal muscle: regulatory mechanisms. Acta Physiol Scand 178: 397– 403, 2003.
Langfort J, Ploug T, Ihlemann J, Holm C, and Galbo H. Stimulation of hormone-sensitive lipase activity by contractions in rat
skeletal muscle. Biochem J 351: 207–214, 2000.
Langfort J, Ploug T, Ihlemann J, Saldo M, Holm C, and Galbo
H. Expression of hormone-sensitive lipase and its regulation by
adrenaline in skeletal muscle. Biochem J 340: 459 – 465, 1999.
Lapsys NM, Kriketos AD, Lim-Fraser M, Poynten AM, Lowy A,
Furler SM, Chisholm DJ, and Cooney GJ. Expression of genes
involved in lipid metabolism correlates with peroxisome proliferator-activated receptor gamma expression in human skeletal muscle. J Clin Endocrinol Metab 85: 4293– 4297, 2000.
Larson-Meyer DE, Newcomer BR, and Hunter GR. Influence of
endurance running and recovery diet on intramyocellular lipid
content in women: a 1H NMR study. Am J Physiol Endocrinol
Metab 282: E95–E106, 2002.
Laybutt DR, Schmitz-Peiffer C, Saha AK, Ruderman NB, Biden TJ, and Kraegen EW. Muscle lipid accumulation and protein
kinase C activation in the insulin-resistant chronically glucoseinfused rat. Am J Physiol Endocrinol Metab 277: E1070 –E1076,
1999.
Lessard SJ, Giudice SL, Lau W, Reid JJ, Turner N, Febbraio
MA, Hawley JA, and Watt MJ. Rosiglitazone enhances glucose
tolerance by mechanisms other than reduction of fatty acid accumulation within skeletal muscle. Endocrinology 145: 5665–5670,
2004.
Levin K, Daa SH, Alford FP, and Beck-Nielsen H. Morphometric documentation of abnormal intramyocellular fat storage and
reduced glycogen in obese patients with Type II diabetes. Diabetologia 44: 824 – 833, 2001.
Li Y, Soos TJ, Li X, Wu J, Degennaro M, Sun X, Littman DR,
Birnbaum MJ, and Polakiewicz RD. Protein kinase C Theta
239
240
BENTE KIENS
Physiol Rev • VOL
215. Phillips SM, Green HJ, Tarnopolsky MA, Heigenhauser GJF,
Hill RE, and Grant SM. Effects of training duration on substrate
turnover and oxidation during exercise. J Appl Physiol 81: 2182–
2191, 1996.
216. Pilegaard H, Ordway GA, Saltin B, and Neufer PD. Transcriptional regulation of gene expression in human skeletal muscle
during recovery from exercise. Am J Physiol Endocrinol Metab
279: E806 –E814, 2000.
217. Pohl J, Ring A, and Stremmel W. Uptake of long-chain fatty acids
in HepG2 cells involves caveolae: analysis of a novel pathway. J
Lipid Res 43: 1390 –1399, 2002.
218. Powell DJ, Turban S, Gray A, Hajduch E, and Hundal HS.
Intracellular ceramide synthesis and protein kinase Czeta activation play an essential role in palmitate-induced insulin resistance in
rat L6 skeletal muscle cells. Biochem J 382: 619 – 629, 2004.
219. Pownall HJ and Hamilton JA. Energy translocation across cell
membranes and membrane models. Acta Physiol Scand 178: 357–
365, 2003.
220. Preiss-Landl K, Zimmermann R, Hammerle G, and Zechner R.
Lipoprotein lipase: the regulation of tissue specific expression and
its role in lipid and energy metabolism. Curr Opin Lipidol 13:
471– 481, 2002.
221. Putman CT, Spriet LL, Hultman E, Lindinger MI, Lands LC,
McKelvie RS, Cederblad G, Jones NL, and Heigenhauser GJ.
Pyruvate dehydrogenase activity and acetyl group accumulation
during exercise after different diets. Am J Physiol Endocrinol
Metab 265: E752–E760, 1993.
222. Qu X, Seale JP, and Donnelly R. Tissue and isoform-selective
activation of protein kinase C in insulin-resistant obese Zucker rats:
effects of feeding. J Endocrinol 162: 207–214, 1999.
223. Raney MA, Yee AJ, Todd MK, and Turcotte LP. AMPK activation is not critical in the regulation of muscle FA uptake and
oxidation during low intensity muscle contraction. Am J Physiol
Endocrinol Metab 2004.
224. Rasmussen BB, Holmback UC, Volpi E, Morio-Liondore B,
Paddon-Jones D, and Wolfe RR. Malonyl coenzyme A and the
regulation of functional carnitine palmitoyltransferase-1 activity
and fat oxidation in human skeletal muscle. J Clin Invest 110:
1687–1693, 2002.
225. Rasmussen BB and Winder WW. Effect of exercise intensity on
skeletal muscle malonyl-CoA and acetyl-CoA carboxylase. J Appl
Physiol 83: 1104 –1109, 1997.
226. Rasmussen JT, Faergeman NJ, and Kristiansen K. Acyl-CoAbinding protein (ACBP) can mediate intermembrane acyl-CoA
transport and donate acyl-CoA for beta-oxidation and glycerolipid
synthesis. Biochem J 299: 165–170, 1994.
227. Ravichandran LV, Esposito DL, Chen J, and Quon MJ. Protein
kinase C-zeta phosphorylates insulin receptor substrate-1 and impairs its ability to activate phosphatidylinositol 3-kinase in response to insulin. J Biol Chem 276: 3543–3549, 2001.
228. Razani B, Combs TP, Wang XB, Frank PG, Park DS, Russell
RG, Li M, Tang B, Jelicks LA, Scherer PE, and Lisanti MP.
Caveolin-1-deficient mice are lean, resistant to diet-induced obesity, and show hypertriglyceridemia with adipocyte abnormalities.
J Biol Chem 277: 8635– 8647, 2002.
229. Richter EA, Mikines KJ, Galbo H, and Kiens B. Effect of
exercise on insulin action in human skeletal muscle. J Appl Physiol
66: 876 – 885, 1989.
230. Rico-Sanz J, Hajnal JV, Thomas EL, Mierisova S, Ala-Korpela
M, and Bell JD. Intracellular and extracellular skeletal muscle
triglyceride metabolism during alternating intensity exercise in
humans. J Physiol 510: 615– 622, 1998.
231. Ring A, Pohl J, Volkl A, and Stremmel W. Evidence for vesicles
that mediate long-chain fatty acid uptake by human microvascular
endothelial cells. J Lipid Res 43: 2095–2104, 2002.
232. Ritov VB, Menshikova EV, He J, Ferrell RE, Goodpaster BH,
and Kelley DE. Deficiency of subsarcolemmal mitochondria in
obesity and type 2 diabetes. Diabetes 54: 8 –14, 2005.
233. Roepstorff C. Regulation of Lipid Utilization in Human Skeletal
Muscle During Exercise (Dissertation). Denmark: Univ. of Copenhagen, 2004 (ISBN 87 89361 93).
234. Roepstorff C, Halberg N, Hillig T, Saha AK, Ruderman NB,
Wojtaszewski JF, Richter EA, and Kiens B. Malonyl-CoA and
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
195. McGarry JD. Banting lecture 2001: dysregulation of fatty acid
metabolism in the etiology of type 2 diabetes. Diabetes 51: 7–18,
2002.
196. McGarry JD, Mills SE, Long CS, and Foster DW. Observations
on the affinity for carnitine and malonyl-CoA sensitivity of carnitine
palmitoyltransferase I in animal and human tissues. Biochem J 214:
21–28, 1983.
197. Montain SJ, Hopper MK, Coggan AR, and Coyle EF. Exercise
metabolism at different time intervals after a meal. J Appl Physiol
70: 882– 888, 1991.
198. Morio B, Holmback U, Gore D, and Wolfe RR. Increased VLDLTAG turnover during and after acute moderate-intensity exercise.
Med Sci Sports Exercise 36: 801– 806, 2004.
199. Murphy EJ, Barcelo-Coblijn G, Binas B, and Glatz JF. Heart
fatty acid uptake is decreased in heart fatty acid-binding protein
gene-ablated mice. J Biol Chem 279: 34481–34488, 2004.
200. Murray R, Paul GL, Seifert JG, and Eddy DE. Responses to
varying rates of carbohydrate ingestion during exercise. Med Sci
Sports Exercise 23: 713–718, 1991.
201. Nuutila P, Knuuti MJ, Maki M, Laine H, Ruotsalainen U,
Teras M, Haaparanta M, Solin O, and Yki-Jarvinen H. Gender
and insulin sensitivity in the heart and in skeletal muscles. Studies
using positron emission tomography. Diabetes 44: 31–36, 1995.
202. Odland LM, Heigenhauser GJF, Lopaschuk GD, and Spriet
LL. Human skeletal muscle malonyl-CoA at rest and during prolonged submaximal exercise. Am J Physiol Endocrinol Metab 270:
E541–E544, 1996.
203. Odland LM, Howlett RA, Heigenhauser GJ, Hultman E, and
Spriet LL. Skeletal muscle malonyl-CoA content at the onset of
exercise at varying power outputs in humans. Am J Physiol Endocrinol Metab 274: E1080 –E1085, 1998.
204. Olsson AG, Eklund B, Kaijser L, and Carlson LA. Extraction of
endogenous plasma triglycerides by the working human forearm
muscle in the fasting state. Scand J Clin Lab Invest 35: 231–236,
1975.
205. Olsson H and Belfrage P. Phosphorylation and dephosphorylation of hormone-sensitive lipase. Interactions between the regulatory and basal phosphorylation sites. FEBS Lett 232: 78 – 82, 1988.
206. Ong J and Kern P. The role of glucose and glycosylation in the
regulation of lipoprotein lipase synthesis and secretion in rat adipocytes. J Biol Chem 264: 3177–3182, 1989.
207. Osuga J, Ishibashi S, Oka T, Yagyu H, Tozawa R, Fujimoto A,
Shionoiri F, Yahagi N, Kraemer FB, Tsutsumi O, and Yamada
N. Targeted disruption of hormone-sensitive lipase results in male
sterility and adipocyte hypertrophy, but not in obesity. Proc Natl
Acad Sci USA 97: 787–792, 2000.
208. Pan DA, Lillioja S, Kriketos AD, Milner MR, Baur LA, Bogardus C, Jenkins AB, and Storlien LH. Skeletal muscle triglyceride
levels are inversely related to insulin action. Diabetes 46: 983–988,
1997.
209. Paul P. FFA metabolism of normal dogs during steady-state exercise at different work loads. J Appl Physiol 28: 127–132, 1970.
210. Perseghin G, Scifo P, De Cobelli F, Pagliato E, Battezzati A,
Arcelloni C, Vanzulli A, Testolin G, Pozza G, Del Maschio A,
and Luzi L. Intramyocellular triglyceride content is a determinant
of in vivo insulin resistance in humans: a 1H-13C nuclear magnetic
resonance spectroscopy assessment in offspring of type 2 diabetic
parents. Diabetes 48: 1600 –1606, 1999.
211. Petersen KF, Dufour S, Befroy D, Garcia R, and Shulman GI.
Impaired mitochondrial activity in the insulin-resistant offspring of
patients with type 2 diabetes. N Engl J Med 350: 664 – 671, 2004.
212. Peterson J, Bihain BE, Bengtsson-Olivecrona G, Deckelbaum
RJ, Carpentier YA, and Olivecrona T. Fatty acid control of
lipoprotein lipase: a link between energy metabolism and lipid
transport. Proc Natl Acad Sci USA 87: 909 –913, 1990.
213. Phillips DI, Caddy S, Ilic V, Fielding BA, Frayn KN, Borthwick
AC, and Taylor R. Intramuscular triglyceride and muscle insulin
sensitivity: evidence for a relationship in nondiabetic subjects.
Metabolism 45: 947–950, 1996.
214. Phillips SM, Green HJ, Tarnopolsky MA, Heigenhauser GJF,
and Grant SM. Progressive effect of endurance training on metabolic adaptions in working skeletal muscle. Am J Physiol Endocrinol Metab 270: E265–E272, 1996.
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
235.
236.
237.
238.
240.
241.
242.
243.
244.
245.
246.
247.
248.
249.
250.
251.
252.
253.
254.
Physiol Rev • VOL
255.
256.
257.
258.
259.
260.
261.
262.
263.
264.
265.
266.
267.
268.
269.
270.
271.
272.
and association of GLUT4 with caveolin-rich vesicles. J Cell Biol
127: 1233–1243, 1994.
Schmitz-Peiffer C. Signalling aspects of insulin resistance in skeletal muscle: mechanisms induced by lipid oversupply. Cell Signal
12: 583–594, 2000.
Schmitz-Peiffer C, Browne CL, Oakes ND, Watkinson A, Chisholm DJ, Kraegen EW, and Biden TJ. Alterations in the expression and cellular localization of protein kinase C isozymes epsilon
and theta are associated with insulin resistance in skeletal muscle
of the high-fat-fed rat. Diabetes 46: 169 –178, 1997.
Schmitz-Peiffer C, Craig DL, and Biden TJ. Ceramide generation is sufficient to account for the inhibition of the insulin-stimulated PKB pathway in C2C12 skeletal muscle cells pretreated with
palmitate. J Biol Chem 274: 24202–24210, 1999.
Schmitz-Peiffer C, Craig DL, and Biden TJ. Ceramide generation is sufficient to account for the inhibition of the insulin-stimulated PKB pathway in C2C12 skeletal muscle cells pretreated with
palmitate. J Biol Chem 274: 24202–24210, 1999.
Schrauwen P, Blaak EE, Aggel-Leijssen DP, Borghouts LB,
and Wagenmakers AJ. Determinants of the acetate recovery factor: implications for estimation of [13C]substrate oxidation. Clin
Sci 98: 587–592, 2000.
Schrauwen-Hinderling VB, Schrauwen P, Hesselink MK, van
Engelshoven JM, Nicolay K, Saris WH, Kessels AG, and Kooi
ME. The increase in intramyocellular lipid content is a very early
response to training. J Clin Endocrinol Metab 88: 1610 –1616, 2003.
Schrauwen-Hinderling VB, Schrauwen P, Hesselink MKC, van
Engelshoven JMA, Nicolay K, Saris WHM, Kessels AGH, and
Kooi ME. The increase in intramyocellular lipid content is a very
early response to training. J Clin Endocrinol Metab 88: 1610 –1616,
2003.
Schrauwen-Hinderling VB, Van Loon LJC, Koopman R, Nicolay K, Saris WHM, and Kooi ME. Intramyocellular lipid content
is increased after exercise in nonexercising human skeletal muscle.
J Appl Physiol 95: 2328 –2332, 2003.
Schwieterman W, Sorrentino D, Potter B, Rand J, and Kiang
C. Uptake of oleate by isolated rat adipocytes is mediated by a
40-kDa plasma membrane fatty acid binding protein closely related
to that in liver and gut. Proc Natl Acad Sci USA 85: 359 –363, 1988.
Seip RL, Angelopoulos TJ, and Semenkovich CF. Exercise
induces human lipoprotein lipase gene expression in skeletal muscle but not adipose tissue. Am J Physiol Endocrinol Metab 268:
E229 –E236, 1995.
Seip RL, Mair K, Cole TG, and Semenkovich CF. Induction of
human skeletal muscle lipoprotein lipase gene expression by shortterm exercise is transient. Am J Physiol Endocrinol Metab 272:
E255–E261, 1997.
Shafrir E and Ziv E. Cellular mechanism of nutritionally induced
insulin resistance: the desert rodent Psammomys obesus and other
animals in which insulin resistance leads to detrimental outcome. J
Basic Clin Physiol Pharmacol 9: 347–385, 1998.
Shen WJ, Patel S, Natu V, and Kraemer FB. Mutational analysis
of structural features of rat hormone-sensitive lipase. Biochemistry 37: 8973– 8979, 1998.
Sidossis LS, Coggan AR, Gastaldelli A, and Wolfe RR. A new
correction factor for use in tracer estimations of plasma fatty acid
oxidation. Am J Physiol Endocrinol Metab 269: E649 –E656, 1995.
Sidossis LS, Gastaldelli A, Klein S, and Wolfe RR. Regulation
of plasma fatty acid oxidation during low- and high-intensity exercise. Am J Physiol Endocrinol Metab 272: E1065–E1070, 1997.
Simoneau JA and Kelley DE. Altered glycolytic and oxidative
capacities of skeletal muscle contribute to insulin resistance in
NIDDM. J Appl Physiol 83: 166 –171, 1997.
Simoneau JA, Veerkamp JH, Turcotte LP, and Kelley DE.
Markers of capacity to utilize fatty acids in human skeletal muscle:
relation to insulin resistance and obesity and effects of weight loss.
FASEB J 13: 2051–2060, 1999.
Sinha R, Dufour S, Petersen KF, LeBon V, Enoksson S, Ma YZ,
Savoye M, Rothman DL, Shulman GI, and Caprio S. Assessment of skeletal muscle triglyceride content by (1)H nuclear magnetic resonance spectroscopy in lean and obese adolescents: relationships to insulin sensitivity, total body fat, and central adiposity.
Diabetes 51: 1022–1027, 2002.
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
239.
carnitine in regulation of fat oxidation in human skeletal muscle
during exercise. Am J Physiol Endocrinol Metab 287: E696 –E705,
2004.
Roepstorff C, Steffensen CH, Madsen M, Stallknecht B, Kanstrup IL, Richter EA, and Kiens B. Gender differences in substrate utilization during submaximal exercise in endurance-trained
subjects. Am J Physiol Endocrinol Metab 282: E435–E447, 2002.
Roepstorff C, Vistisen B, Donsmark M, Nielsen JN, Galbo H,
Green KA, Hardie DG, Wojtaszewski JF, Richter EA, and
Kiens B. Regulation of hormone-sensitive lipase activity and
Ser563 and Ser565 phosphorylation in human skeletal muscle during exercise. J Physiol 560: 551–562, 2004.
Roepstorff C, Vistisen B, Roepstorff K, and Kiens B. Regulation of plasma long-chain fatty acid oxidation in relation to uptake
in human skeletal muscle during exercise. Am J Physiol Endocrinol Metab 287: E696 –E705, 2004.
Roepstorff C, Wulff HJ, Vistisen B, and Kiens B. Studies of
plasma membrane fatty acid-binding protein and other lipid-binding proteins in human skeletal muscle. Proc Nutr Soc 63: 239 –244,
2004.
Romijn JA, Coyle EF, Sidossis LS, Gastaldelli A, Horowitz JF,
Endert E, and Wolfe RR. Regulation of endogenous fat and
carbohydrate metabolism in relation to exercise intensity and duration. Am J Physiol Endocrinol Metab 265: E380 –E391, 1993.
Romijn JA, Coyle EF, Sidossis LS, Zhang XJ, and Wolfe RR.
Relationship between fatty acid delivery and fatty acid oxidation
during strenuous exercise. J Appl Physiol 79: 1939 –1945, 1995.
Sacchetti M, Saltin B, Osada T, and van Hall G. Intramuscular
fatty acid metabolism in contracting and non-contracting human
skeletal muscle. J Physiol 540: 387–395, 2002.
Saha AK, Kurowski TG, and Ruderman NB. A malonyl-CoA
fuel-sensing mechanism in muscle: effects of insulin, glucose, and
denervation. Am J Physiol Endocrinol Metab 269: E283–E289,
1995.
Saha AK, Laybutt DR, Dean D, Vavvas D, Sebokova E, Ellis B,
Klimes I, Kraegen EW, Shafrir E, and Ruderman NB. Cytosolic
citrate and malonyl-CoA regulation in rat muscle in vivo. Am J
Physiol Endocrinol Metab 276: E1030 –E1037, 1999.
Saha AK, Schwarsin AJ, Roduit R, Masse F, Kaushik V, Tornheim K, Prentki M, and Ruderman NB. Activation of malonylCoA decarboxylase in rat skeletal muscle by contraction and the
AMP-activated protein kinase activator 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside. J Biol Chem 275: 24279 –24283,
2000.
Sahlin K. Intracellular pH and energy metabolism in skeletal muscle of man. With special reference to exercise. Acta Physiol Scand
Suppl 455: 1–56, 1978.
Sahlin K. Muscle carnitine metabolism during incremental dynamic exercise in humans. Acta Physiol Scand 138: 259 –262, 1990.
Saito N, Kikkawa U, and Nishizuka Y. The family of protein
kinase C and membrane lipid mediators. J Diabetes Complications
16: 4 – 8, 2002.
Saltin B. Hemodynamic adaptations to exercise. Am J Cardiol 55:
42D– 47D, 1985.
Saltin B and Gollnick P. Skeletal muscle adaptability: significance for metabolism and performance. In: Handbook of Physiology. Skeletal Muscle. Bethesda, MD: Am. Physiol. Soc, 1983, sect.
10, p. 555– 631.
Schaap FG, Hamers L, Van der Vusse GJ, and Glatz JF. Molecular cloning of fatty acid-transport protein cDNA from rat. Biochim Biophys Acta 1354: 29 –34, 1997.
Schachter JB, Lester DS, and Alkon DL. Synergistic activation
of protein kinase C by arachidonic acid and diacylglycerols in vitro:
generation of a stable membrane-bound, cofactor-independent
state of protein kinase C activity. Biochim Biophys Acta 1291:
167–176, 1996.
Schaffer JE. Fatty acid transport: the roads taken. Am J Physiol
Endocrinol Metab 282: E239 –E246, 2002.
Schaffer JE and Lodish HF. Expression cloning and characterization of a novel adipocyte long chain fatty acid transport protein.
Cell 79: 427– 436, 1994.
Scherer PE, Lisanti MP, Baldini G, Sargiacomo M, Mastick
CC, and Lodish HF. Induction of caveolin during adipogenesis
241
242
BENTE KIENS
Physiol Rev • VOL
294. Thompson AL and Cooney GJ. Acyl-CoA inhibition of hexokinase in rat and human skeletal muscle is a potential mechanism of
lipid-induced insulin resistance. Diabetes 49: 1761–1765, 2000.
295. Trigatti BL, Anderson RG, and Gerber GE. Identification of
caveolin-1 as a fatty acid binding protein. Biochem Biophys Res
Commun 255: 34 –39, 1999.
296. Trimmer JK, Casazza GA, Horning MA, and Brooks GA. Recovery of 13CO2 during rest and exercise after [1-13C]acetate,
[2-13C]acetate, and NaH13CO3 infusions. Am J Physiol Endocrinol
Metab 281: E683–E692, 2001.
297. Tunstall RJ, Mehan KA, Wadley GD, Collier GR, Bonen A,
Hargreaves M, and Cameron-Smith D. Exercise training increases lipid metabolism gene expression in human skeletal muscle. Am J Physiol Endocrinol Metab 283: E66 –E72, 2002.
298. Turcotte LP, Kiens B, and Richter EA. Saturation kinetics of
palmitate uptake in perfused skeletal muscle. FEBS Lett 279: 327–
329, 1991.
299. Turcotte LP, Petry C, Kiens B, and Richter EA. Contractioninduced increase in Vmax of palmitate uptake and oxidation in
perfused skeletal muscle. J Appl Physiol 84: 1788 –1794, 1998.
300. Turcotte LP, Richter EA, and Kiens B. Increased plasma FFA
uptake and oxidation during prolonged exercise in trained vs.
untrained humans. Am J Physiol Endocrinol Metab 262: E791–
E799, 1992.
301. Turcotte LP, Srivastava AK, and Chiasson JL. Fasting increases plasma membrane fatty acid-binding protein [FABP(PM)]
in red skeletal muscle. Mol Cell Biochem 166: 153–158, 1997.
302. Turcotte LP, Swenberger JR, Tucker MZ, and Yee AJ. Training-induced elevation in FABP(PM) is associated with increased
palmitate use in contracting muscle. J Appl Physiol 87: 285–293,
1999.
303. Turcotte LP, Swenberger JR, Tucker MZ, Yee AJ, Trump G,
Luiken JJ, and Bonen A. Muscle palmitate uptake and binding
are saturable and inhibited by antibodies to FABP(PM). Mol Cell
Biochem 210: 53– 63, 2000.
304. Turcotte LP, Swenberger JR, Zavitz TM, and Yee AJ. Increased fatty acid uptake and altered fatty acid metabolism in
insulin-resistant muscle of obese Zucker rats. Diabetes 50: 1389 –
1396, 2001.
305. Turinsky J, O’Sullivan DM, and Bayly BP. 1,2-Diacylglycerol
and ceramide levels in insulin-resistant tissues of the rat in vivo.
J Biol Chem 265: 16880 –16885, 1990.
306. Van Dam KG, Van Breda E, Schaart G, van Ginneken MM,
Wijnberg ID, Graaf-Roelfsema E, van der Kolk JH, and Keizer
HA. Investigation of the expression and localization of glucose
transporter 4 and fatty acid translocase/CD36 in equine skeletal
muscle. Am J Vet Res 65: 951–956, 2004.
307. Van der Vusse GJ, Glatz JF, Van Nieuwenhoven FA, Reneman
RS, and Bassingthwaighte JB. Transport of long-chain fatty acids across the muscular endothelium. In: Muscle Metabolism in
Exercise and Diabetes, edited by Richter EA, Kiens B, Galbo H, and
Saltin B. New York: Plenum, 1998.
308. Van Hall G. Correction factors for 13C-labelled substrate oxidation
at whole-body and muscle level. Proc Nutr Soc 58: 979 –986, 1999.
309. Van Hall G, Sacchetti M, and Radegran G. Whole body and leg
acetate kinetics at rest, during exercise and recovery in humans.
J Physiol 542: 263–272, 2002.
310. Van Hall G, Sacchetti M, Radegran G, and Saltin B. Human
skeletal muscle fatty acid and glycerol metabolism during rest,
exercise and recovery. J Physiol 543: 1047–1058, 2002.
311. Van Loon LJ. Use of intramuscular triacylglycerol as a substrate
source during exercise in humans. J Appl Physiol 97: 1170 –1187,
2004.
312. Van Loon LJ, Greenhaff PL, Constantin-Teodosiu D, Saris
WH, and Wagenmakers AJ. The effects of increasing exercise
intensity on muscle fuel utilisation in humans. J Physiol 536:
295–304, 2001.
313. Van Loon LJ, Koopman R, Schrauwen P, Stegen J, and Wagenmakers AJ. The use of the [1,2-13C]acetate recovery factor in
metabolic research. Eur J Appl Physiol 89: 377–383, 2003.
314. Van Loon LJ, Koopman R, Stegen JH, Wagenmakers AJ,
Keizer HA, and Saris WH. Intramyocellular lipids form an impor-
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
273. Sleeman MW, Donegan NP, Heller-Harrison R, Lane WS, and
Czech MP. Association of acyl-CoA synthetase-1 with GLUT4containing vesicles. J Biol Chem 273: 3132–3135, 1998.
274. Sorrentino D, Stump D, Potter B, Robinson R, and White R.
Oleate uptake by cardiac myocytes is carrier mediated and involves
a 40-kD plasma membrane fatty acid binding protein similar to that
in liver, adipose tissue and gut. J Clin Invest 82: 928 –935, 1988.
275. Spencer MK, Yan Z, and Katz A. Effect of low glycogen on
carbohydrate and energy metabolism in human muscle during exercise. Am J Physiol Cell Physiol 262: C975–C979, 1992.
277. Spriet LL, Heigenhauser GJ, and Jones NL. Endogenous triacylglycerol utilization by rat skeletal muscle during tetanic stimulation. J Appl Physiol 60: 410 – 415, 1986.
278. Stallknecht B, Kiens B, Helge JW, Richter EA, and Galbo H.
Interstitial glycerol concentrations in human skeletal muscle and
adipose tissue during graded exercise. Acta Physiol Scand 180:
367–377, 2004.
279. Starling RD, Trappe TA, Parcell AC, Kerr CG, Fink WJ, and
Costill DL. Effects of diet on muscle triglyceride and endurance
performance. J Appl Physiol 82: 1185–1189, 1997.
280. Starritt EC, Howlett RA, Heigenhauser GJ, and Spriet LL.
Sensitivity of CPT I to malonyl-CoA in trained and untrained human
skeletal muscle. Am J Physiol Endocrinol Metab 278: E462–E468,
2000.
281. Steffensen CH, Roepstorff C, Madsen M, and Kiens B. Myocellular triacylglycerol breakdown in females but not in males
during exercise. Am J Physiol Endocrinol Metab 282: E634 –E642,
2002.
282. Steinberg GR, Dyck DJ, Calles-Escandon J, Tandon NN, Luiken JJ, Glatz JF, and Bonen A. Chronic leptin administration
decreases fatty acid uptake and fatty acid transporters in rat skeletal muscle. J Biol Chem 277: 8854 – 8860, 2002.
283. Steinberg GR, Parolin ML, Heigenhauser GJ, and Dyck DJ.
Leptin increases FA oxidation in lean but not obese human skeletal
muscle: evidence of peripheral leptin resistance. Am J Physiol
Endocrinol Metab 283: E187–E192, 2002.
284. Stephens TJ, Chen ZP, Canny BJ, Michell BJ, Kemp BE, and
McConell GK. Progressive increase in human skeletal muscle
AMPKalpha2 activity and ACC phosphorylation during exercise.
Am J Physiol Endocrinol Metab 282: E688 –E694, 2002.
285. Storch J and Thumser AE. The fatty acid transport function of
fatty acid-binding proteins. Biochim Biophys Acta 1486: 28 – 44,
2000.
286. Storlien LH, Jenkins AB, Chisholm DJ, Pascoe WS, Khouri S,
and Kraegen EW. Influence of dietary fat composition on development of insulin resistance in rats: relationship to muscle triglyceride and w-3 fatty acids in muscle phospholipid. Diabetes 40:
280 –289, 1991.
287. Stremmel W. Fatty acid uptake by isolated rat heart myocytes
represents a carrier-mediated transport process. J Clin Invest 81:
844 – 852, 1988.
288. Stump DD, Zhou SL, and Berk PD. Comparison of plasma membrane FABP and mitochondrial isoform of aspartate aminotransferase from rat liver. Am J Physiol Gastrointest Liver Physiol 265:
G894 –G902, 1993.
289. Sullivan JE, Brocklehurst KJ, Marley AE, Carey F, Carling D,
and Beri RK. Inhibition of lipolysis and lipogenesis in isolated rat
adipocytes with AICAR, a cell-permeable activator of AMP-activated protein kinase. FEBS Lett 353: 33–36, 1994.
290. Svedenhag J, Lithell H, Juhlin-Dannfelt A, and Henriksson J.
Increase in skeletal muscle lipoprotein lipase following endurance
training in man. Atherosclerosis 49: 203–207, 1983.
291. Szczepaniak LS, Babcock EE, Schick F, Dobbins RL, Garg A,
Burns DK, McGarry JD, and Stein DT. Measurement of intracellular triglyceride stores by H spectroscopy: validation in vivo.
Am J Physiol Endocrinol Metab 276: E977–E989, 1999.
292. Takayama S, White MF, and Kahn CR. Phorbol ester-induced
serine phosphorylation of the insulin receptor decreases its tyrosine kinase activity. J Biol Chem 263: 3440 –3447, 1988.
293. Tang Z, Scherer PE, Okamoto T, Song K, Chu C, Kohtz DS,
Nishimoto I, Lodish HF, and Lisanti MP. Molecular cloning of
caveolin-3, a novel member of the caveolin gene family expressed
predominantly in muscle. J Biol Chem 271: 2255–2261, 1996.
LIPID METABOLISM: EXERCISE AND INSULIN RESISTANCE
315.
316.
317.
318.
320.
321.
322.
323.
325.
326.
327.
328.
329.
Physiol Rev • VOL
330.
331.
332.
333.
334.
336.
337.
338.
339.
340.
341.
342.
343.
344.
345.
can be overridden by AMPK signaling. FASEB J 18: 1445–1446,
2004.
Watt MJ, Stellingwerff T, Heigenhauser GJ, and Spriet LL.
Effects of plasma adrenaline on hormone-sensitive lipase at rest
and during moderate exercise in human skeletal muscle. J Physiol
550: 325–332, 2003.
Wei S, Lai K, Patel S, Piantedosi R, Shen H, Colantuoni V,
Kraemer FB, and Blaner WS. Retinyl ester hydrolysis and retinol
efflux from BFC-1beta adipocytes. J Biol Chem 272: 14159 –14165,
1997.
Wendling PS, Peters SJ, Heigenhauser GJ, and Spriet LL.
Variability of triacylglycerol content in human skeletal muscle
biopsy samples. J Appl Physiol 81: 1150 –1155, 1996.
Winder WW, Arogyasami J, Barton RJ, Elayan IM, and Verhs
PR. Muscle malonyl-CoA decreases during exercise. J Appl Physiol
67: 2230 –2233, 1989.
Winder WW and Hardie DG. Inactivation of acetyl-CoA carboxylase and activation of AMP-activated protein kinase in muscle
during exercise. Am J Physiol Endocrinol Metab 270: E299 –E304,
1996.
Wojtaszewski JF, MacDonald C, Nielsen JN, Hellsten Y, Hardie DG, Kemp BE, Kiens B, and Richter EA. Regulation of
5⬘AMP-activated protein kinase activity and substrate utilization in
exercising human skeletal muscle. Am J Physiol Endocrinol Metab
284: E813–E822, 2003.
Wojtaszewski JF, Mourtzakis M, Hillig T, Saltin B, and Pilegaard H. Dissociation of AMPK activity and ACCbeta phosphorylation in human muscle during prolonged exercise. Biochem Biophys Res Commun 298: 309 –316, 2002.
Wojtaszewski JF, Nielsen P, Hansen BF, Richter EA, and
Kiens B. Isoform-specific and exercise intensity-dependent activation of 5⬘-AMP-activated protein kinase in human skeletal muscle.
J Physiol 528: 221–226, 2000.
Wolfe RR and Jahoor F. Recovery of labeled CO2 during the
infusion of C-1- vs C-2-labeled acetate: implications for tracer studies of substrate oxidation. Am J Clin Nutr 51: 248 –252, 1990.
Yeaman S. Hormone-sensitive lipase, a multipurpose enzyme in
lipid metabolism. Biochim Biophys Acta 1052: 128 –132, 1990.
Yu C, Chen Y, Cline GW, Zhang D, Zong H, Wang Y, Bergeron
R, Kim JK, Cushman SW, Cooney GJ, Atcheson B, White MF,
Kraegen EW, and Shulman GI. Mechanism by which fatty acids
inhibit insulin activation of insulin receptor substrate-1 (IRS-1)associated phosphatidylinositol 3-kinase activity in muscle. J Biol
Chem 277: 50230 –50236, 2002.
Zderic TW, Davidson CJ, Schenk S, Byerley LO, and Coyle EF.
High-fat diet elevates resting intramuscular triglyceride concentration and whole body lipolysis during exercise. Am J Physiol Endocrinol Metab 286: E217–E225, 2004.
Zhang X, Fitzsimmons RL, Cleland LG, Ey PL, Zannettino AC,
Farmer EA, Sincock P, and Mayrhofer G. CD36/fatty acid translocase in rats: distribution, isolation from hepatocytes, and comparison with the scavenger receptor SR-B1. Lab Invest 83: 317–332,
2003.
Zhou SL, Stump D, Sorrentino D, Potter BJ, and Berk PD.
Adipocyte differentiation of 3T3-L1 cells involves augmented expression of a 43-kDa plasma membrane fatty acid-binding protein.
J Biol Chem 267: 14456 –14461, 1992.
Zuntz N. Uber die Bedeutung der vershciedenen Narhstoffe als
Erzeuger der Muskelkraft. Arch Geo Physiol 83: 557–571, 1901.
86 • JANUARY 2006 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
319.
tant substrate source during moderate intensity exercise in endurance-trained males in a fasted state. J Physiol 553: 611– 625, 2003.
Van Loon LJC. Use of intramuscular triacylglycerol as a substrate
source during exercise in humans. J Appl Physiol 97: 1170 –1187,
2004.
Van Loon LJC, Koopman R, Manders R, van der Weegen W,
van Kranenburg GP, and Keizer HA. Intramyocellular lipid content in type 2 diabetes patients compared with overweight sedentary men and highly trained endurance athletes. Endocrinol Metab
287: E558 –E565, 2004.
Van Loon LJC, Koopman R, Stegen JHCH, Wagenmakers
AJM, Keizer HA, and Saris WHM. Intramyocellular lipids form
an important substrate source during moderate intensity exercise
in endurance-trained males in a fasted state. J Physiol 553: 611–
625, 2003.
Vavvas D, Apazidis A, Saha AK, Gamble J, Patel A, Kemp BE,
Witters LA, and Ruderman NB. Contraction-induced changes in
acetyl-CoA carboxylase and 5⬘-AMP-activated kinase in skeletal
muscle. J Biol Chem 272: 13255–13261, 1997.
Virkamaki A, Korsheninnikova E, Seppala-Lindroos A, Vehkavaara S, Goto T, Halavaara J, Hakkinen AM, and Yki-Jarvinen H. Intramyocellular lipid is associated with resistance to in
vivo insulin actions on glucose uptake, antilipolysis, and early
insulin signaling pathways in human skeletal muscle. Diabetes 50:
2337–2343, 2001.
Vistisen B, Roepstorff K, Roepstorff C, Bonen A, van Deurs B,
and Kiens B. Sarcolemmal FAT/CD36 in human skeletal muscle
colocalizes with caveolin-3 and is more abundant in type 1 than in
type 2 fibers. J Lipid Res 45: 603– 609, 2004.
Wakelam MJ. Diacylglycerol—when is it an intracellular messenger? Biochim Biophys Acta 1436: 117–126, 1998.
Watkins PA, Lu JF, Steinberg SJ, Gould SJ, Smith KD, and
Braiterman LT. Disruption of the Saccharomyces cerevisiae
FAT1 gene decreases very long-chain fatty acyl-CoA synthetase
activity and elevates intracellular very long-chain fatty acid concentrations. J Biol Chem 273: 18210 –18219, 1998.
Watt MJ, Heigenhauser GJ, O’Neill M, and Spriet LL. Hormone-sensitive lipase activity and fatty acyl-CoA content in human
skeletal muscle during prolonged exercise. J Appl Physiol 95:
314 –321, 2003.
Watt MJ, Heigenhauser GJ, and Spriet LL. Intramuscular triacylglycerol utilization in human skeletal muscle during exercise: is
there a controversy? J Appl Physiol 93: 1185–1195, 2002.
Watt MJ, Heigenhauser GJ, and Spriet LL. Effects of dynamic
exercise intensity on the activation of hormone-sensitive lipase in
human skeletal muscle. J Physiol 547: 301–308, 2003.
Watt MJ, Heigenhauser GJ, Stellingwerff T, Hargreaves M,
and Spriet LL. Carbohydrate ingestion reduces skeletal muscle
acetylcarnitine availability but has no effect on substrate phosphorylation at the onset of exercise in man. J Physiol 544: 949 –956,
2002.
Watt MJ, Holmes AG, Steinberg GR, Mesa JL, Kemp BE, and
Febbraio MA. Reduced plasma FFA availability increases net
triacylglycerol degradation, but not GPAT or HSL activity, in human skeletal muscle. Am J Physiol Endocrinol Metab 287: E120 –
E127, 2004.
Watt MJ, Steinberg GR, Chan S, Garnham A, Kemp BE, and
Febbraio MA. Beta-adrenergic stimulation of skeletal muscle HSL
243