Clinical Science ( 1 982) 62,26 1-27 1
26 1
EDITORIAL REVIEW
Cholesterol transport through the plasma
N . B. M Y A N T
MRC Lipid Metabolism Unit, Hammersmith Hospiral, London
Introduction
In this review I shall deal with the routes by
which cholesterol enters and leaves the plasma
and the changes it undergoes as a consequence of
the metabolism of liproproteins within the
plasma. In the final section the quantitative
relationship between the flux of cholesterol into
and out of the plasma and that through the whole
body will be considered. As far as possible the
emphasis will be on man and on the more recent
and controversial aspects of the subject, particularly with regard to the receptor-mediated
uptake and catabolism of the various cholesterolcarrying lipoproteins. For general reviews of
lipoprotein structure and metabolism and of the
LDL receptor, see references [ 11-16].
Plasma lipoproteins
Cholesterol is transported into and through the
plasma in association with lipoprotein particles.
These are spherical aggregates of lipid and
protein, consisting essentially of a non-polar core
of esterified cholesterol and triglyceride surrounded by a polar shell of free (unesterified)
cholesterol, phospholipid and protein. The lipoproteins of plasma are most conveniently classified according to their hydrated density into:
(1) Chylomicrons, secreted into the plasma by
the small intestine during absorption of fat.
(2) VLDL, secreted into the plasma by the
Abbreviations: ACAT, acyl-CoA:cholesterol
0-acyltransferase; apoA (B etc.), apolipoprotein
A (B etc.); HDL, high-density lipoprotein (density 1.063-1 -21 g/ml); IDL, intermediatedensity
lipoprotein (density 1406-1.019 g/ml); LCAT,
lecithin: cholesterol acyltransferase; LDL, lowdensity lipoprotein (density 1.0 19-1 -063 g/ml);
VLDL, very-low-density lipoprotein (density
0.95-1.006 g/ml).
0143-5221/82/030261-11$01.50/1
liver and intestine; since triglyceride is the major
component of chylomicrons and VLDL, these two
lipoproteins are together known as triglyceriderich lipoproteins.
(3) IDL, or ‘remnants’, formed within the
blood circulation by the partial hydrolysis of
triglyceride-rich lipoproteins by the action of
lipoprotein lipase. Some authors restrict the term
‘IDL‘to VLDL remnants.
(4) LDL, formed from IDL by a mechanism
that is not yet understood.
(5) HDL, comprising several subfractions, of
which the most important are HDL, and HDL,.
HDL, contains more than 50% protein, whereas
HDL, has more than 50% lipid.
At least 12 apoproteins have been identified in
the lipoproteins of human plasma. From the point
of view of cholesterol transport the most important are: apoA-I, the major component of HDL
protein and a cofactor for LCAT; apoB, existing
in two forms, one of which is the sole apoprotein
of human LDL; apoC-I, apoC-I1 and apoC-111,
major constituents of the protein of chylomicrons and VLDL; apoE (of which there are at
least three isoforms), a constituent of IDL protein
and of the protein of triglyceride-rich
lipoproteins.
Routes of entry of cholesterol into plasma
In newly secreted lipoproteins
Cholesterol enters the plasma by bulk transfer
mainly in chylomicrons and VLDL secreted by
the intestine, in VLDL secreted by the liver and in
HDL secreted by the liver and intestine. Since
lipoproteins undergo very rapid changes in lipid
and protein composition as soon as they enter the
circulation, in considering them as vehicles in
which cholesterol is carried into the plasma,
nascent particles secreted by the liver or intestine
@ 1982 The Biochemical Society and the Medical Research Society
262
N . B. Myant
must by distinguished from the mature particles
that make up the bulk of the lipoprotein present
in plasma. The nascent triglyceride-rich lipoproteins secreted by the intestine contain free
cholesterol in their polar shell and cholesterol
esterified with long-chain fatty acids in the
non-polar core, this esterified cholesterol being
formed by the action of ACAT in the intestinal
mucosa. In nascent VLDL particles secreted by
the human liver the bulk of the cholesterol is
probably unesterified, though nascent VLDL
isolated from the Golgi apparatus of rat liver
contains substantial amounts of esterified
cholesterol, presumably formed by the action of
hepatic ACAT. The HDL particles formed in
liver and intestine are secreted as discoidal
bilayers of free cholesterol and phospholipid,
each surrounded by an annulus of protein; the
major apoprotein is apoE in nascent hepatic HDL
and apoA-I in nascent HDL secreted by the gut.
By reverse cholesterol transport
In addition to these channels of entry,
cholesterol is thought to enter the plasma by a
prctcess, not involving secretion of newly formed
lipoproteins, known as reverse cholesterol transport. Many extrahepatic tissues synthesize
cholesterol in situ or, as discussed below, they
may take up cholesterol from the extracellular
fluids by ingesting lipoproteins. Yet the liver is the
only organ in the body capable of metabolizing or
excreting cholesterol in significant quantities.
Hence, if extrahepatic tissues are to remain in
balance with respect to cholesterol there must be
a mechanism for transferring it from the tissues
to the liver via the plasma. Removal of
cholesterol from the arterial walls during regression of atheromatous lesions must also require
the transport of cholesterol into the plasma other
than by secretion of lipoproteins.
Little is known about the mechanisms responsible for reverse cholesterol transport in vivo.
However, studies of cholesterol efflux from
tissues in uitro have established that cells other
than those that secrete lipoproteins excrete
cholesterol only in unesterified form and only if
there is a suitable acceptor for free cholesterol in
the external medium. The nature of the physiological acceptor for tissue free cholesterol is
clearly a question of considerable interest. If it is
a lipoprotein, it must be one that is present in
interstitial fluid adjacent to cell surfaces and it
must be capable of increasing its load of
cholesterol per particle without loss of stability.
Human HDL, especially HDL,. is an efficient
acceptor of free cholesterol from cells in vitro and
has been shown to be present in human
peripheral lymph, a fluid known to reflect closely
the composition of interstitial fluid. Furthermore,
radioactive free cholesterol is transferred preferentially from the tissues of the human foot to
an HDL-like lipoprotein in peripheral lymph 171
and Nestel & Miller [81 have demonstrated the
transfer of radioactive cholesterol from adipose
tissue to plasma HDL in obese human subjects
undergoing weight reduction.
As we shall see, HDL, in plasma may act as
acceptor for free cholesterol and phospholipid
released from the surfaces of triglyceride-rich
particles during their hydrolysis by lipoprotein
lipase. The addition of lipid to HDL, converts it
into HDL,, each HDL, particle containing about
100 more molecules of cholesterol than an HDL,
particle. This may be analogous to what happens
in the interstitial fluid. Possibly, HDL, crosses the
capillary wall, picks up free cholesterol from cell
surfaces and returns to the plasma as HDL, via
the lymphatic system. This idea is difficult to test
by an experiment involving the intravenous
injection of labelled HDL,, owing to exchange of
apoproteins between HDL, and HDL,. An
alternative approach is to compare the size
distribution of HDL particles in interstitial fluid
(or peripheral lymph) with that in plasma. If
HDL, is converted into larger, more lipid-rich
particles after it has crossed the walls of the blood
capillaries into the interstitial fluid, one would
expect to find a higher ratio of large to small
HDL particles in peripheral lymph than in
plasma; the well-established sieving effect of the
capillary walls on macromolecules would, of
course, produce the opposite effect. Using
density-gradient electrophoresis as a means of
seperating HDL particles on the basis of size,
Reichl et al. [91 have shown that the ratio of large
to small apoA-I-containing particles is higher in
lymph than in plasma. This is far from proving a
precursor-product relationship between HDL,
and HDL, within interstitial fluid, but it is
consistent with the hypothesis suggested above.
Fate of cholesterol within the plasma
Although a considerable proportion of the
cholesterol that enters the plasma in nascent
lipoproteins is unesterified, more than two-thirds
of that in mature circulating lipoprotein is
esterified with long-chain fatty acids. The change
in the proportion of esterified to total cholesterol
is due to the presence of LCAT in plasma. This
enzyme, either directly or indirectly, converts the
free cholesterol in nascent HDL and triglyceriderich lipoproteins into esterified cholesterol, the
Plasma cholesterol transport
263
free cholesterol of triglyceride-rich lipoproteins
being made available to LCAT as a consequence
of the action of lipoprotein lipase.
Each remnant particle is thought to retain
essentially all the apoB present in the particle
from which it originated [21.
Lipoprotein lipase, LCAT and the plasma
cholesterol
Fate of remnants
The hydrolytic action of lipoprotein lipase on
the non-polar core of chylomicrons and VLDL
leads to the release of free cholesterol, phospholipid and apoC from their polar surfaces. There is
some doubt as to whether these three components
are released together as particles with the density
of HDL, which then act as substrate for LCAT,
or whether the cholesterol and phospholipid
become incorporated into pre-existing HDL
particles. In either case, the effect of lipoprotein
lipase is to convert the free cholesterol and
lecithin of triglyceride-rich lipoproteins (which
are not substrates for LCAT) into a form that
can be acted upon by LCAT.
As soon as nascent HDL enters the circulation
it is acted upon by LCAT, with the formation of
esterified cholesterol by the transfer of a fatty
acid residue from lecithin to free cholesterol in the
discoidal particle. Some of the cholesteryl ester
enters the potential space between the two layers
of the discoid, converting it into a spherical
HDL, particle. The continued action of LCAT
may then result in the further conversion of
HDL, into HDL,, the additional substrate arising
from the polar surfaces of triglyceride-rich
lipoproteins. As cholesteryl ester molecules are
formed in the cholesterol/phospholipid monolayer of HDL they either move into the centre of
the particle or are transported to VLDL and
LDL by a specific cholesteryl-ester transfer
protein 1101. Thus HDL and LCAT may be
thought of as a system for esterifying the free
cholesterol of triglyceride-rich lipoproteins.
The biological significance of this process is
revealed when the system fails. In familial LCAT
deficiency the free cholesterol released during the
hydrolysis of VLDL and chylomicrons fails to be
converted into a non-polar form that can be
incorporated into the core lipid of lipoprotein
particles. Instead, together with phospholipid,
albumin and other proteins, it forms abnormal
lipoproteins that cause pathological changes in a
variety of tissues.
The remnants resulting from the action of
lipoprotein lipase on triglyceride-rich lipoproteins are depleted of triglyceride and apoC
and are relatively enriched with esterified
cholesterol and apoE, some of the latter probably
being transferred from nascent hepatic HDL to
remnants during some stage of their formation.
In rats, the bulk of the remnants formed from
VLDL and chylomicrons are removed from the
circulation by the liver, as discussed below, only a
small proportion being converted into LDL [ 11
121. In man, however, the fate of remnants
derived from chylomicrons differs markedly from
that of VLDL remnants. The apoB, and presumably the esterified cholesterol, of chylomicron
remnants is rapidly and completely removed from
the circulation without conversion into LDL [ 131.
On the other hand a substantial proportion of
VLDL remnants is converted into LDL, estimates from different laboratories ranging from
50% I141 to more than 90% [ 151 in normal man.
During this process, which for convenience is
considered below (‘Removal as remnants’),
nearly half the esterified cholesterol and all the
apoE of each particle is removed, whereas all the
apoB is retained in the LDL resulting from its
metabolism.
Exit of cholesterol from the plasma
Cholesterol is transported in bulk from the
plasma predominantly as a consequence of the
tissue uptake and catabolism of LDL, remnants
and HDL. In each case the site and extent of
uptake seem to be determined largely by the
tissue distribution of receptors with specificity for
a given type of lipoprotein and by the accessibility of different tissues to the circulating
lipoproteins of a given particle size. Selective
removal of cholesterol from lipoproteins without
tissue uptake of the whole lipoprotein particle
may also occur, but the extent of this process is
difficult to assess (see below: ‘Balance between
inflow and outflow’).
Removal as LDL
The major route for the bulk transfer of
cholesterol out of the circulation is the tissue
uptake and catabolism of LDL, the lipoprotein
that carries 60-709s of the total cholesterol in
human plasma. The rate at which LDL-apoB is
removed from the plasma may be determined
from the plasma LDL concentration and the
fractional rate of turnover of a trace of labelled
autologous LDL injected intravenously. In normal men the absolute rate of turnover is usually
between 10 and 15 mg of apoB day-’ kg-I. This
264
N . B. Myant
is equivalent to the irreversible removal of
0.75-1.0 g of LDL-apoB, or 1 4 - 1 . 5 g of LDL
cholesterol (free plus esterified), per day in the
whole body.
Information as to how and where LDL and its
associated cholesterol is removed from the
circulation has been obtained by studying the
interaction between LDL and isolated cells or
their subcellular membranes and by measuring
the turnover of normal and chemically modified
LDL under various conditions in vivo.
The LDL-receptor pathway. Cultured cells
derived from human and animal tissues, including
skin fibroblasts, smooth muscle cells and blood
mononuclear cells, develop specific high-affinity
receptors for LDL when incubated in a lipoprotein-deficient medium. The properties of these
receptors have been dealt with in several recent
reviews (see IS, 6, 161), so that only a brief
summary of the more significant features need be
given here.
LDL receptors are clustered at specialized
regions of the plasma membrane known as
coated pits, each about 500 nm in diameter. The
coated pits continually invaginate to form endocytotic vesicles enclosing the surface receptors
and any LDL particles they may have bound.
The vesicles then fuse with primary lysosomes to
form secondary lysosomes containing acid-pH
enzymes, including hydrolases capable of hydrolysing the apoB and cholesteryl esters of LDL.
The free cholesterol originally present in the
internalized LDL, together with that formed by
the hydrolysis of LDL cholesteryl ester, after
moving out of the lysosomes suppresses the synthesis of cholesterol from acetyl-CoA and stimulates the microsomal esterification of cholesterol
by ACAT. The uptake of LDL cholesterol and its
release from secondary lysosomes also leads to
suppression of the synthesis of new LDL receptors and, hence, to a decrease in the number per
cell. Thus the receptor pathway for internalizing
LDL may be regarded as a self-regulating
mechanism by which certain cells satisfy their
varying requirements for cholesterol by taking up
LDL from the medium.
The binding of LDL by the LDL receptor
depends upon the recognition of apoB by the
receptor, this requiring the integrity of a limited
number of arginyl and lysyl residues in the
peptide chain. When these are blocked by
cyclohexanedione or reductive methylation respectively, receptor-mediated uptake of LDL is
completely inhibited. LDL receptors also
recognize apoE, their affinity for apoE being
more than ten times that for apoB 1171. Hence
lipoproteins containing apoE but not apoB, such
as the abnormal lipoprotein (HDL,) present in
the plasma of cholesterol-fed dogs, are bound to
the LDL receptor with high affinity. The possible
relevance of this property to the uptake of
chylomicron remnants is considered in the next
section.
In addition to cell uptake and catabolism
mediated by LDL receptors, there must be other
pathways for the removal of LDL from the
circulation, since LDL is catabolized at a
considerable rate (though at abnormally high
plasma LDL concentrations) in patients with
familial hypercholesterolaemia in the homozygous form, a condition in which functional
LDL receptors are virtually absent. The
nature of these non-LDL-receptor pathways has
not been elucidated, despite the fact that in
quantitative terms they must be more important
than the LDL-receptor pathway in the disposal of
LDL cholesterol in vivo. One possibility is that,
at high plasma LDL concentrations, significant
amounts of LDL are ingested and catabolized by
non-adsorptive pinocytosis, known to occur in a
wide variety of cells, especially those of the
reticulo-endothelial system. An additional possibility is that LDL that has been modified during
its life in the plasma and interstitial fluid is bound
and catabolized by specific receptors (other than
the LDL receptor) on macrophages. This system
might be analogous to the specific binding and
internalization of acetylated LDL by human
monocyte/macrophage cells I181.
LDL receptors in the liver. The question as to
whether or not there are functioning LDL
receptors in the liver has been much discussed but
is not yet settled. Membranes from the livers of
oestrogen-treated rats have receptors for LDL
similar to those identified on fibroblasts I191 and
these receptors are responsible for the increased
rate of removal of LDL from the plasma
observed in the oestrogen-treated rat [201. LDL
receptors have also been demonstrated in the
liver membranes of young dogs treated with
colestipol and mevinolin 1211 and of normal
rabbits 1221. However, attempts to demonstrate
LDL receptors on intact liver cells in suspension
or culture have not given consistent results. LDL
receptors have been shown to be present in
cultured hepatocytes of newborn pigs 1231.
High-affinity binding of homologous LDL to rat
hepatocytes in suspension has also been reported
(24, 251, but the receptors responsible for this
binding are not necessarily identical with classical
LDL receptors.
Some of the discrepancies between these
findings may be explained by the recent observation of Hui et al. 1261 that classical LDL
Plasma cholesterol transport
receptors are present in membranes from the
livers of normal young dogs but not of adult
dogs. If, however, adult dogs are treated with
cholestyramine, LDL receptors are expressed in
their liver membranes. This raises the possibility
that the livers of young animals of many species
have LDL receptors, but that in some species
they cease to be expressed as the animals mature
unless the liver is stimulated by oestrogen, or
cholestyramine or other drugs. (The presence of a
specific apoE receptor in adult dog liver is
considered below.) The LDL receptors in the
livers of normal adult rabbits are probably
controlled by the gene coding for the fibroblast
receptor, since they are not detectable in the livers
of rabbits with an inherited absence of LDL
receptors in their cultured fibroblasts ('Watanabe
heritable hyperlipidaemic rabbits') I221. If, in
fact, there are species differences in the extent to
which hepatic LDL receptors are expressed in the
unstimulated adult animal, it would clearly be
unwise to draw conclusions about the presence of
hepatic LDL receptors in normal man from
observations on experimental animals.
Contribution of LDL receptors ' t o LDL
catabolism in vivo. LDL catabolism in vivo is
markedly defective in the inherited absence of
LDL receptors, as in homozygous FH I271 and
in the Watanabe rabbit [221. This shows clearly
that the LDL receptor makes a significant
contribution to the degradation of LDL in the
intact organism under physiological conditions.
However, it is not possible to use information
derived from the study of cultured cells or cell
membranes to estimate how much LDL is
catabolized by the receptor pathway in vivo. To
do this it would be necessary to know the
concentration of LDL at the surfaces of extravascular cells and the extent to which receptor
activity is expressed in different cells in vivo;
what little information we have suggests that the
concentration of LDL in human interstitial fluid
[28] is high enough to cause near-maximal
suppression of LDL receptors on fibroblasts.
An alternative approach to the problem is to
compare the rate of catabolism, in vivo, of native
LDL with that of LDL in which the receptorrecognition sites have been blocked, e.g. by treatment with cyclohexanedione. If the chemical
modification is assumed to have no effect on the
catabolism of LDL by the non-LDL-receptor
pathway (an assumption that has not yet been
validated), the rate of catabolism by the nonreceptor pathway should be equal to that of
modified LDL, and that by the receptor pathway
should be equal to the difference between the two
rates. Using this method, Mahley et al. I291 have
265
shown that receptor-mediated catabolism makes
a significant contribution to total LDL catabolism in rats and monkeys, and Shepherd et al. 1301
have shown that the LDL-receptor pathway
accounts for about 30% of the total LDL
catabolized in normal human subjects. In
homozygous FH, on the other hand, the rates of
catabolism of native and cyclohexanedionetreated LDL are virtually identical 1311. This
method has much potential value in the study of
genetic and non-genetic influences on LDL
catabolism and has already been used to explore
the effects of drugs and hormones on LDL
catabolism in man. Thus cholestyramine [321 and
thyroid hormone [311 have been shown to
enhance LDL catabolism in human subjects,
mainly by stimulating the LDL-receptor
pathway.
Sites of L D L catabolism. Although the above
observations provide information about the contribution of the LDL-receptor pathway in the
body as a whole, they tell us little about the sites
of LDL catabolism. In particular, they do not
show how much LDL is catabolized in the liver,
the principal route through which cholesterol is
removed from the plasma.
The contributions of different tissues to LDL
catabolism in vivo may be determined by
assaying the radioactivity present in the tissues of
animals killed after an intravenous injection of
[14Clsucrosecoupled to LDL 1331. Since sucrose
is not hydrolysed by lysosomal enzymes, this
provides an estimate of the cumulative amount of
LDL taken up and catabolized by endocytosis in
each tissue during the interval between injection
and death of the animal. Using this method,
Pittman et al. [33] have shown that LDL
catabolism occurs in many organs and that about
50% of LDL catabolism in pigs occurs in the
liver. From a comparison of the tissue uptake of
native and cyclohexanedione-treatedLDL coupled to ~'4Clsucrose,Carew et al. (34) concluded
that the LDL-receptor pathway accounts for
more than half the LDL catabolized by the liver
in normal rats. The observations of Slater et al.
[ 351 suggest that LDL-receptor-mediated uptake
by the liver makes a similar contribution to LDL
catabolism in the normal adult rabbit and that the
increase in LDL catabolism in the whole animal
induced %y cholestyramine is due largely to
increased LDL-receptor activity in the liver.
Recent observations on monkeys [361 also
suggest that partial ileal bypass (a procedure that,
like cholestyramine, stimulates the catabolism of
cholesterol in the liver) enhances LDL catabolism
by promoting LDL-receptor-mediated catabolism
in the liver.
266
N . B. Myant
Two other independent lines of evidence, both
obtained under experimental conditions in which
LDL receptors are deleted, point to a significant
contribution from LDL receptors in particular
tissues to LDL catabolism in vivo. Uptake of
LDL by the mouse adrenal in vivo can be
blocked by an intravenous injection of antibody
to the LDL receptor 1371. Similarly, uptake of
LDL by the liver and adrenal cortex is markedly
diminished in Watanabe rabbits, compared with
that in normal rabbits 1221; in this case the
deletion of LDL receptors is due to a single-gene
mutation.
As noted above, species differences in the
expression of LDL receptors in vivo are such that
conclusions drawn from observations on animals
cannot justifiably be transferred to man. However, Shepherd et al. (321 have shown that
cholestyramine stimulates LDL-receptor-mediated uptake of LDL in human subjects. presumably by stimulating LDL-receptor activity
in the liver, and Brown et a f . [381 have demonstrated the presence of high-affinity receptors for
LDL in liver membranes obtained from a human
foetus. It should be noted, however, that neither
of these observations proves the existence of
functional LDL receptors in the unstimulated
adult human liver.
Removal as remnants
Role of hepatic receptors. There is little direct
evidence on the metabolism of remnants in man,
though extensive information is available on the
uptake and catabolism of VLDL and chylomicron remnants in rats. Observations on intact
rats 1391, isolated perfused rat livers 140, 41 I and
rat-liver cells in culture 1421 have shown that
remnants of triglyceride-rich lipoproteins are
taken up and catabolized selectively by the liver
by a saturable high-affinity process involving
recognition by receptors on hepatocytes; extrahepatic tissues have little or no capacity for
metabolizing chylomicron remnants I43 1. The
nature of the remnant receptors in rat liver is the
subject of a very active controversy. As discussed
above (‘Removal as LDL‘), LDL receptors have
been identified in membranes prepared from the
livers of rats, dogs and rabbits under various
experimental conditions.
Since remnants contain apoB and apoE, and
since LDL receptors recognize both apoproteins,
it would be reasonable to suppose that remnant
receptors in the livers of oestrogen-treated rats
and of dogs treated with colestipol and mevinolin
are, in fact, the classical LDL receptors identified
in fibroblasts. On this view. the failure of
extrahepatic LDL receptors to take up remnants
in the intact rat is explicable by the relatively
large size of remnant particles, which would
prevent them from crossing the walls of the blood
capillaries and thus gaining access to extravascular tissues. However, this leaves open the
question as to how far LDL receptors are
responsible for the very rapid uptake of remnants
by the livers of untreated rats compared with the
much slower uptake of LDL 1391. The answer
may lie in the observation of Hui e f al. 1261 that
hepatic membranes from mature dogs contain
receptors (‘apoE receptors’) that recognize apoE
but not apoB. These apoE receptors specifically
bind HDL, (see above: ‘The LDL-receptor
pathway’), but they do not bind human LDL. As
noted above, LDL receptors (‘apoB, E receptors’)
are present in the livers of normal young dogs,
but not of mature dogs. Evidently, the apoE
receptor recognizes a different site of the apoE
molecule from that recognized by the LDL
receptor. Both recognition sites, it may be noted,
can be blocked by treatment with cyclohexanedione I26 1, so that sensitivity to treatment
with cyclohexanedione can no longer be used as
a criterion for LDL-receptor-mediated uptake of
a lipoprotein containing both apoB and apoE.
The findings of Hui et al. I26 I raise the possibility
that the hepatic remnant receptor functioning in
the mature dog in vivo, and perhaps in the
untreated rat, is the apoE receptor.
In the absence of direct evidence it is difficult
to relate these observations to man. In the
familial disorder known as floating beta disease
(familial type Ill hyperlipoproteinaemia) remnants accumulate in the plasma owing to the
genetic absence of the E-I11 isoform of apoE I44 I,
the apoE component that appears to be necessary
for recognition of remnants by the liver 1451. This
might suggest that in man the removal of
remnants from the plasma is mediated by an
apoE receptor in the liver, similar to that isolated
from the livers of mature dogs. However, if the
E-Ill isoform is also essential for recognition of
apoE by the LDL receptor (a point that has not
yet been investigated), the accumulation of
E-Ill-deficient remnants in broad beta disease
could be explained equally well on the supposition that uptake of remnants in normal man
is mediated by hepatic LDL receptors, whose
existence is suggested by the effects of cholestyramine on LDL-receptor-mediated catabolism of
LDL in man 132). A significant contribution by
hepatic LDL-receptors to remnant removal in
man is indicated by the observation 1461 that the
fractional rate of turnover of VLDL remnants is
higher in normal men than in patients with FH,
Plasma cholesterol transport
an inborn error of metabolism due to a mutation
in the gene coding for the LDL receptor.
Nevertheless, it seems unlikely that LDLreceptor-mediated uptake is the sole channel
through which remnants are removed from the
circulation in man, since, if this were the case,
remnants would be expected to accumulate in the
plasma of homozygous patients with familial
hypercholesterolaemia (who have no LDL receptors) to the same extent as in patients with
E-111-deficient remnants. Uptake of remnants by
a receptor that recognizes apoE but not apoB
would also explain why E-111-deficient remnants
accumulate in broad beta disease despite the fact
that they contain a normal amount of apoB.
The two forms of apoB. The mechanism by
which the human liver distinguishes between
VLDL remnants and chylomicron remnants is
not understood, but it may be related to the
existence of two species of apoB. Human 1471
and rat 1481 plasma has two apoB proteins, one
high-molecular-weight form (‘B- loo’, ‘large
apoB’ or apoB,) and one with lower molecular
weight (‘B-48’, ‘small apoB’ or apoB,). B-48, the
only apoB synthesized by the intestine, is the
predominant apoB in chylomicrons, whereas
B-100 predominates in VLDL and LDL. The
study of monogenic disorders of apoB metabolism in man suggests that B- 100 and B-48 share a
common subunit and that B-48 has an additional
subunit, not present in B- 100 and controlled by a
seperate gene [491. The presence of B-48 in
chylomicrons and its absence from LDL supports
the conclusion, based on other evidence [ 131, that
chylomicron remnants are removed from the
circulation without conversion into LDL and
suggests that LDL is derived solely from the
B- 100-containing VLDL secreted by the liver. In
keeping with this interpretation, Sparks 8c Marsh
1501 have shown that labelled B-48 injected
intravenously into rats is taken up and
catabolized by the liver much more rapidly than
labelled B-100. This raises the possibility that
3-48 in some way contributes to the recognition
of chylomicron remnants by the liver. Recognition
of B-48 could be based on the presence of a
separate receptor in the liver. Alternatively, B-48
could act by modifying the conformation of apoE
in the surface of the particle so that its recognition
by the apoE receptor is enhanced. ApoC, which
appears to inhibit recognition of apoE-containing
lipoproteins by the liver [511, might act upon
apoE in a contrary sense.
Conversion of IDL into LDL. The site of
production of LDL from VLDL remnants is not
known. It has been suggested that IDL is
converted into LDL in the liver with the
267
participation of hepatic triglyceride lipase, an
enzyme attached to the surface of the endothelial
cells of liver sinusoids 1521. If this is the
mechanism by which LDL is produced, it should
be possible to inhibit the conversion of remnants
into LDL by blocking hepatic lipase in vivo.
However, when rats are given intravenous injections of an antiserum to hepatic lipase, sufficient
to cause a marked fall in enzyme activity, the
expected fall in plasma LDL concentration does
not occur [53-551.
Against the view that the liver is necessary for
the conversion of remnants into LDL, Dory et al.
1561 have shown that the isolated perfused rat
heart is capable of converting VLDL into LDL,
and Suri et al. 1571 have reported the formation
of an LDL-like lipoprotein from VLDL in
functionally hepatectomized rats. Deckelbaum et
al. 1581 have also shown that the hydrolysis of
human VLDL by milk lipoprotein lipase in vitro
results in the formation of a cholesteryl-ester-rich
lipoprotein resembling LDL. These observations
indicate that LDL, or a closely similar lipoprotein, can be formed without the intervention of
the liver, though they do not exclude a contribution from the liver to LDL production in the
intact animai. That the liver contributes to LDL
formation in man is suggested by the observation
of Turner et al. [591 that when radioactive IDL
(Sf 12-60) is infused intravenously into human
subjects, about half the labelled IDL-apoB taken
up in the splanchnic bed (which includes the liver)
reappears in the hepatic vein as LDL-apoB. If
hepatic receptors are involved in the conversion
of VLDL remnants into LDL, the remnant
particle may have some chemical or physical
property that causes it to be released after
modification, without the internalization and
lysosomal degradation undergone by chylomicron remnants.
Removal as HDL
HDL turnover. In normal men the rate of
catabolism of HDL protein (apoA-I plus apoA11), estimated from the turnover of intravenously
injected HDL, is about 8 mg day-’ kg-I 1601. If
tissue uptake and catabolism of the protein of
HDL are mediated by uptake of the complete
particle, this would be equivalent to the bulk
removal, from the circulation, of 250-300 mg of
HDL cholesterol per day in the whole body. As
discussed below (‘Balance between inflow and
outflow’), it is conceivable that hepatic uptake of
the lipid of an HDL particle occurs independently of uptake of the protein component.
Sites of removal of HDL. The relative contri-
268
N. B . Myant
bution of different tissues to the catabolism of
HDL has been the subject of controversy. One
reason for this may be heterogeneity of the
lipoproteins that have been used in the study of
HDL metabolism. Mahley et al. I6 11 have shown
that HDL, contains an apoE-rich lipoprotein
(HDL-I) that is recognized by the LDL receptor
and that may therefore interact with LDL
receptors, and perhaps with hepatic remnant
receptors, in vivo and in uitro. The amount of
HDL-I in a given sample of HDL is influenced by
the method of preparation and the species from
which the sample is obtained, rat HDL, having a
larger proportion of apoE-rich lipoproteiiis than
human HDL,.
Sigurdsson et al. I621 concluded, from
measurements of the rate of catabolism of HDL
by the isolated perfused rat's liver, that less than
10% of the plasma HDL in rats is catabolized by
the liver in uiuo. On the other hand, observations
on the uptake of '2SI-labelledHDL by the tissues
of intact rats suggest that the liver catabolizes
more of the plasma HDL than does any other
organ I63, 641, although adrenals, kidney and
spleen also catabolize appreciable amounts [ 641.
According to Rachmilewitz et al. [651, HDL
taken up by the rat's liver in uiuo appears predominantly in parenchymal cells, but other
workers have concluded that hepatic uptake of
HDL per mg of cell protein is as great in Kupffer
cells as in parenchymal cells [661.
In agreement with evidence derived from the
study of intact animals, observations on isolated
liver cells in suspension [25, 64, 671 or monolayer culture 1681 suggest that the liver has a
considerable capacity for binding HDL and
catabolizing its protein by adsorptive endocytosis followed by lysosomal digestion. In at
least one instance [691 the isolated hepatocytes
were shown to internalize the cholesteryl esters of
HDL added to the incubation medium. Whether
the binding of HDL by liver cells is due to the
presence of a distinct high-affinity receptor for
HDL, or to the binding of apoE-rich HDL by
hepatic LDL receptors or apoE receptors, will
not be decided until competition between HDL
and lipoproteins containing apoE as the sole
protein has been investigated.
Observations on the uptake of HDL by
cultured fibroblasts [701, aortic smooth-muscle
cells [71, 721 and vascular endothelial cells 1731
in culture have shown that extrahepatic cells are
capable of catabolizing HDL and may therefore
contribute to the removal of HDL cholesterol
from the pool of HDL in viuo. Of particular
interest in this respect are the tissues in which
cholesterol is required for the synthesis of steroid
hormones. Studies of intact rats and of rat
adrenal cortex and ovarian tissue in uitro suggest
that the cells of these tissues take up HDL
cholesterol from the plasma, possibly by highaffinity binding and internalization of HDL
particles 174-761 (see I38 I for general review).
Thus the plasma HDL may act not only as
acceptor for tissue free cholesterol. It may also
act as a source of cholesterol for certain
specialized tissues, though different HDL subfractions may be involved in these two processes.
Balance between inflow and outflow
In view of the probable role of HDL as the
immediate acceptor for tissue cholesterol, it is
worth considering how far the catabolism of
HDL could contribute to the transport of
cholesterol from the plasma and extrahepatic
tissues to the liver, the pathway through which
most of the exchangeable cholesterol in the whole
body must finally leave the system. Rough
calculations show that in a man eating 100 g of
fat/day the total amount of cholesterol entering
the plasma from the intestine, liver and extrahepatic tissues might well be as high as 5 g/day
[6l. As we have seen, the daily amount of HDL
protein catabolized per day is equivalent to not
more than 300 mg of cholesterol, so that even if
the liver were the sole site of catabolism of HDL
(which is clearly not the case) there would still be
a substantial imbalance between the flow of
cholesterol into the plasma and the outflow via
HDL to the liver.
Three factors may help to explain this discrepancy. In the first place, much of the
cholesterol entering the plasma in triglyceriderich lipoproteins is diverted to the liver by uptake
and catabolism of chylomicron remnants. Secondly, as a consequence of the LCAT reaction and
the cholesteryl-ester transfer protein, cholesterol
is continually redistributed to VLDL and LDL
from HDL. Since up to 50% of the circulating
LDL may be catabolized in the liver, this would
act as an additional mechanism for removing
HDL cholesterol from the plasma, other than by
the catabolism of HDL; the independent turnover of HDL protein and cholesterol is reflected
quantitatively in the very different fractional rates
of turnover of HDL apoprotein (about 0.1 pool
of apoA-I plus apoA-I1 per day [601) and HDL
cholesteryl ester (about 0.8 pool per day 177)).
Finally, there is the possibility that HDL can act
as a shuttle for cholesterol, depositing cholesterol
in the liver without uptake of the protein, the
partially delipidated particles then returning to
the plasma and interstitial fluid for a further load
Plasma cholesterol transport
of cholesterol. There is no direct evidence that
this occurs in uiuo, either in man or in experimental animals. However, Drevon et al. I691
have shown that isolated rat-liver cells internalize
the cholesteryl esters of HDL at a rate much
greater than the equivalent rate of catabolism of
HDL protein in uiuo. It has also been proposed
that, as a consequence of the local hydrolytic
activity of hepatic triglyceride lipase, HDL,
particles deposit some of their esterified
cholesterol in the liver, the phospholipid- and
cholesterol-depleted particles returning to the
plasma as HDL, [ 78 I.
In conclusion, it may be noted that there is a
marked discrepancy between the net flux of
cholesterol through the plasma in man (at least
5 glday) and that through the pool of exchangeable cholesterol in the whole body (about 1 glday
in normal human adults 161). This indicates that,
on average, a given cholesterol molecule enters
the plasma, other than by molecule-for-molecule
exchange, several times during its life in the body.
One way in which this could occur is by repeated
re-utilization of hepatic cholesterol, derived from
plasma lipoproteins, for incorporation into nascent lipoproteins secreted by the liver.
References
I I I SMITH.L.C.. POWNALL.H.J. & Gorro. A. M. (1978) The
plasma lipoproteins: structure and metabolism. Annual
Reviews in Biochemistry. 47. 75 1-777.
121 EISENBERG.S . & LEVY.R.I. (1975) Lipcprotein metabolism.
Adi,ances in Lipid Research, 13, 1-89.
131 SOUTAR.
A.K. & MYANT.N.B. (1979) Plasma lipoproteins. In:
Chemistry of Macromolecules. I I B. Macromolecular Conplexes. Internarional Review af Biochemistrj: vol. 25. Ed.
OlTord. R. E. University Park Press. Baltimore.
141 GOLDSTEIN.J.L. & BROWN. M.S. (1977) The low-density
lipoprotein pathway and its relation to atherosclerosis. Annual
Reiyiews in Biochemistry. 46.897-930.
151 BROWN. M.S.. KOVANEN.P.T. & GOLDSTEIN.J.L. (1981)
Regulation of plasma cholesterol by lipoprotein receptors.
Science. 212,628-635.
161 MYANT.N.B. (1981) The Biology of Cholesterol and Related
Steroids. William Heinemann Medical Books Ltd. London.
171 REICHL.D..MYANT.N.B..RUDRA.
D.N. & PFLUG.J.J. (1980)
Evidence for the presence of tissue-free cholesterol in low
density and high density lipoproteins of human peripheral
lymph. Atherosclerosis, 3 7 , 4 8 9 4 9 5 .
181 NESTEL. P.J. & MILLER. N.E. (1978) In: High Defisity
Lipoproreins and Alherosclerosis. Ed. Gotto. A.M.. Jr. Miller.
N.E.& Oliver. M.F. Elsevier. Amsterdam.
191 REICHL.D.. RUDRA.D.N. & PFLUG.J. (1981)The distribution
of cholesterol and apolipoprotein A-I among high-density
lipoproteins of human peripheral lymph. Biochemical Society
Transactions, 9.2 I ~ P .
I101 CHAJEK. T. & FIELDING.C.J. (1978) Isolation and characteri
zation of a human serum cholesteryl ester transfer protein.
Proceedings of the National Academj, of Sciences U.S.A.. 75,
3445-3449.
0. & HAVEL. R.J. (1975) Metabolism of
I l l FAERGEMANN,
cholesteryl esters of rat very low density lipoproteins. Journal
of Clinical Inwstigarion. 55, 12 10-12 18.
0..SATA.T.. KANE.J.P. & HAVEL R.J. (1975)
I I2 FAERGEMAN.
Metabolism of apoprotein B of plasma very low density
lipoproteins in the rat. Journal of Clinical Inivstigarion. 56.
1396-1403.
269
I131 SCHAEFER.E.J.. JENKINS.L.L. & BREWER.H.B.. J R (1978)
Human chylomicron apolipoprotein metabolism. Biochemical
and Biophysical Research Communications, 80,405-4 12.
1141 JANUS. E.D.. NICOLL A.. WOOTTON. R.. TURNER.P.R..
MAGILL P.J. & LEWIS. B. (1980) Quantitative studies of very
low density lipoprotein: conversion to low density lipoprotein
in normal controls and primary hyperlipidaemic states and the
role of direct secretion of low density lipoprotein in
heterozygous familial hypercholesterolaemia. European Journal of Clinical Inoesrigation. 10, 149-159.
1151 REARDON. M.F.. FIDGE.N.H. & NESTEL P.J. (1978)
Catabolism of very low density lipoprotein B apoprotein in
man. Journal of Clinical Inoestigarion, 61.850-860.
1161 GOLDSTEIN.J.L. & BROWN. M.S. (1982) The L D L receptor
defect in familial hypercholesterolemia. Medical Clinics of
North America (In press).
1171 INNERARITY. T.L. & MAHLEY.R.W. (1978) Enhanced binding
by cultured human fibroblasts of apo-E-containing lipoproteins as compared with low density lipoproteins. Biochemistry. 17, 1440-1447.
I181 GOLDSTEIN.
J.L.. Ho. Y.K.. BASU. S.K. & BROWN. M.S.
(1979) Binding site on macrophages that mediates uptake and
degradation of acetylated low density lipoprotein. producing
massive cholesterol deposition. Proceedings of the Narional
Academv of Sciences U S A . 76,333-337.
1191 KOVANEN.P.T.. BROWN. M.S. & GOLDSTEIN.J.L. (1979)
Increased binding of low density lipoprotein to liver membranes from rats treated with I7aethinyl estradiol. Journal of
Biological Chemistry. 254, 11367-1 1373.
1201 CHAO. Y-S.. WINDLER.E.E.. CHEN. G.C. & HAVEL. R.J.
(1979) Hepatic catabolism of rat and human lipoproteins in
rats treated with I7u-ethinyl estradiol. Journal of Biological
Chemistry. 254, I 1360- I 1366.
1211 KOVANEN, P.T.. BILHEIMER. D.W.. GOLDSTEIN, J.L..
JARAMILLO.J.J. & BROWN. M.S. (1981) Regulatory role for
hepatic low density lipoprotein receptors in C ~ O Oin the dog.
Proceedings of the National Academy of Sciences U.S-4.. 78.
1194-1 198.
1221 KITA. T.. BROWN. M.S.. WATANABE.Y. & GOLDSTEIN.J.L.
(1981) Deficiency of low density lipoprotein receptors in liver
and adrenal gland of the W H H L rabbit. an animal model of
familial hypercholesterolemia. Proceedings of the National
Academy of Sciences U.S.A.. 78,2268-2272.
1231 PANGBURN.S.H.. NEWTON.R.S.. CHANG.C-M.. WEINSTEIN.
D.B. & STEINBERG.D. (198 I) Receptor-mediated catabolism
of homologous low density lipoproteins in cultured pig
hepatocytes. Journal of Biological Chemistry. 256, 33403347.
1241 VAN BERKEL.T.J.C.. KRUUT.J.K.. VANGENT.T.&VANTOL.
A. (1980) Saturable high affinity binding of low density and
high density lipoprotein by parenchymal and non-parenchymal
cells from rat-liver. Biochemical and Biophysical Research
Communications. 92. 1002- 1008.
1251 OSE.T.. BERG.T.. NORUM.K.R. & OSE L. (1980)Catabolism
of (lzSl) low density lipoproteins in isolated rat liver cells.
Biochemical and Biophwical Research Communications. 97,
192- 199.
1261 HUI. D.Y.. INNERARITY. T.L. & MAHLEY. R.W. (1981)
Lipoprotein binding to canine hepatic membranes.
Metabolically distinct apo-E and apo-B. E receptors. Journal
of Biological Chemistry, 256,5646-5655.
1271 SIMONS.L.A.. REICHL. D.. MYANT. N.B. & MANCINI.M.
(1975) The metabolism of the apoprotein of plasma low
density lipoprotein in familial hyperbetalipoproteinaemia in the
homozygous form. Atherosclerosis. 21,283-298.
1281 REICHL.D.. MVANT.N.B. & PFLUG.J.J. (1977) Concentration
of lipoproteins containing apolipoprotein B in human
peripheral lymph. Biochimica et Biophvsica Acta. 489,
98-105.
1291 MAHLEY. R.W.. WEISGRABER.K.H.. MELCHIOR G.W..
INNERARIN,T.L. & HOLCOMBE,K.S. (1980) Inhibition of
receptor-mediated clearance of lysine and arginine-modified
lipoproteins from the plasma of rats and monkeys Proceedings
of the National Academy of Sciences U.S.A. 77,225-229.
1301 SHEPHERD.
J.. BICKER.S.. LORIMER.A.R. & PACKARD.C.J.
( 1979) Receptor-mediated low density lipoprotein catabolism
in man. JournalofLipid Research. 20.999-1006.
270
N . B . Mvant
131 I THOMPSON.G.R.. SOUTAR.A.K.. SPENGEL. F.A.. JADHAV.A..
GAVIGAN.
S.J.P. & MYANT.N.B. (1981) Defects of receptor
mediated low density lipoprotein catabolism in homozygous
familial hypercholesterolemia and hypothyroidism in uiro.
Proceedings of the Notional Acadetny of Sciences U.S.A.. 78,
2591-2595.
J.. PACKARD.
C.J.. BICKER.S.. LAWRIE.T.D.V. &
1321 SHEPHERD.
MORGAN. H.G. ( 1980) Cholestyramine promotes receptor
mediated low-density lipoprotein catabolism. New England
Journal of Medicine. 302, I 2 19- 1222.
D.
133 PITTMAN.R.C.. ATTIE. A.D.. CAREW.T.E. & STEINBERG.
(1979) Tissue sites of degradation of low density lipoprotein:
application of a method for determining the fate of plasma
proteins. Proceedings of rhe National Academy OJ Sciences
U.S.A..76,5345-5349.
D. (1980) Tissue
I34 CAREW.T.E.. FORAN.W.A. & STEINBERG.
sites of degradation of unmodified and reductively modified
low density lipoprotein in the rat. Circulation. 6 2 (Suppl. 11).
111-17.
1351 SLATER,H.R.. PACKARD.C.J.. BICKER.S. & SHEPHERD.
J.
( 1980) Effects of cholestyramine on receptor~mediated
clearance and tissue uptake of human low density lipoproteins
in the rabbit. Journal of Biological Chemisty. 255. 1021010213.
1361 SPENGEL. F.. MYANT. N.B.. DUFFIELD.R.. WOOD. C. &
THOMPSON.G.R. (1981) Effects of partial ileal bypass on
receptor-mediated low density lipoprotein catabolism. Clinic01
Science. 61, 1 9 ~ .
1371 KITA.T.. BEISIEGEL.
U., GOLDSTEIN,
J.L.. SCHNEIDER.
W. &
BROWN.M.S. (1981) Antibody against low density lipoprotein
receptor blocks uptake of low density lipoprotein (but not high
density lipoprotein) hy adrenal gland of the mouse i n cirw
Journal ofBiologicol Chemistry. 256,470 1-4703.
1381 BROWN,M.S.. KOVANEN.P.T. & GOLDSTEIN.J.L. (1979)
Receptor-mediated uptake of lipoproteimcholesterol and its
utilization for steroid synthesis in the adrenal cortex. In:
Rerenr Progress in Hormone Research. vol. 35. pp. 215-257.
Ed. Creep. R.O. Academic Press. New York.
1391 ANDERSEN.
J.M.. NERVI.F.O. & DIETSCHY
J.M. (1977) Rate
constants for the uptake of cholesterol from various intestinal
and serum lipoprotein fractions by the liver of the rat i n r i r ~ i .
Biochimica er Biophysica Acta. 486,298-307.
1401 COOPER.A.D. & Yu. P.Y.S. (1978) Rates of removal and
degradation of chylomicron remnants by isolated perfused rat
liver. Journal uJ'Lipid Researrh. 19,635-643.
141 I SHFKKILL.
B.C. & DIETSCHY.
J.M. (1978) Characterization of
the sinusoidal transport process responsible for uptake of
chylomicrons by the liver. Jorrriial o f Biolugical Chernistr?.
253. 1859-1867.
1421 F L O R ~ NC.H.
.
& NILSSON.A. (1977) Binding. interiorization
and degradation of cholesteryl ester-labelled chylomicron
remnant particles by rat hepatocyte monolayerr. Biochrnticul
Journal. 168.483-494.
1431 ANDERSEN.J.M. & DIETSCHY.J.M. (1977) Regulation of
sterol synthesis in I6 tissues of rat. I . Effect of diurnal light
cycling. fasting. stress. manipulation of enterohepatic cir
culation and administration of chylomicrons and triton.
Jourrrul oJBiulugica1 Chemistry. 252.3646-365 I.
1441 UTERMANN.
G.. JAESCHKE.
M. & MENZEL.J. (1975) Familial
hyperlipoproteinemia type 111: deficiency of a specific apolipo
protein IapoE 111) in the very low~densitylipoproteins. F E B S
Letters. 56. 352-355.
1451 HAVEL.R.J.. CHAO.Y~S..WINDLER.
E.E.. K O ~ I T EL.. & Guo.
L.S.S. ( 1980) lsoprotein specificity in the hepatic uptake of
apolipoprotein E and the pathogenesis of familial dysbeta
lipoproteinemia. Proceedings OJ the Narional 4 codenix 01
Scrences U.S.A..77.4349-4353.
1461 SOUTAR.A.K.. MYANT.N.B. & THOMPSON.G.R. (1981)The
metabolism of very-low-density and intermediate-density lipo
proteins in patients with familial hypercholesterolaemia.
(Unpublished).
1471 KANE. J.P.. HARDMAN.D.A. & PAULUS. H.E. (1980)
Heterogeneity of apolipoprotein B: isolation of a new species
from human chylomicrons. Proceedrngs q / . the ,N~itiiirrirl
Acadi,riir qfSiciences L'.S.A .. 77. 2465-2469.
1481 K R I S H N A I A HK.V..
.
WALKER. L.F.. BORENSLIAJN.J..
SCHONFELD.G. & GETZ. G.S. (1980) Apolipoprotein B
variant derived from rat intestine. Proceedings oJ'the Norioncrl
Acadein? qfSciences. L'.S.A.. 77,3806-38 10.
1491 MALLOY.M.J.. KANE.J.P.. HARDMAN.
A,. HAMILTON.
R.L. &
DALAL. K.B. ( 198 I ) Normotriglyceridemic abetalipoproteinemia. Absence of the B 100 apolipoprotein. Journal of
Clinical Incesrigarioir. 67, 144 1-1450.
1501 SPARKS,C.E. & MARSH.J.B. (1981) Metabolic heterogeneity
of apolipoprotein B in the rat. Journal of Lipid Research. 22.
519-527.
I S 1 I SHELBURNE.
F.. HANKS.J., MEYERS.W. & QUARFORDT.
S.
(1980) Effect of apoproteins on hepatic uptake of triglyceride
emulsions in the rat. Journal of Clinical 1nt.esrigarion. 65,
652-658.
P.K.J.
1521 Kuusi, T.. NIKKILA.E.A., VIRTANEN,1. & KINNUNEN.
(1979) Localization of the heparin-releasable lipase in situ in
the rat liver. Biochemical Journal. 181,245-246.
1531 Kuusi. T.. KINNUNEN.
P.K.J. & NIKKILA.E.A. (1979) Hepatic
endothelial lipase antiserum influences rat plasma low and high
density lipoproteins in riao. FEES Letters. 104.384-399.
1541 MURASE.T. & ITAKURA. H. (1981) Accumulation of inter
mediate density lipoprotein in plasma after intravenous
administration of hepatic triglyceride lipase antibody in rats.
Atherosclerosis. 39, 293-300.
1551 GROSSER.J.. SCHRECKER.
0. & GREEN. H. (1981) Function
of hepatic lipase in lipoprotein metabolism. Journol of Lipid
Research. 2 2 , 4 3 7 4 4 2 .
1561 DORY. L.. POCOCK. D. & RUBINSTEIN.D. (1978) The
catabolism of human and rat very low density lipoproteins by
perfused rat hearts. Biochimica et Biophvsica Acro. 528,
I 6 1-175.
1571 SURI. B.S.. TARG. M.E. & ROBINSON,D.S. (1979) The
metabolic conversion of very-low-density lipoprotein into
low-density lipoprotein by the extrahepatic tissues of the rat.
Biochemical Journal. 1 7 8 , 4 5 5 4 6 6 .
1581 DECKELBAUM,R.J.
EISENBERG. S.. FAINARU. M..
T. (1979) In rirro produc
BARENHOLZ.Y. & OLIVECRONA.
tion of human plasma low density lipoprotein~likeparticles. A
model for very low density lipoprotein catabolism. Journal uJ'
Riolugical Chemistry. 254.6079-6087.
1591 T ~ J R N E R
P.R..
.
MILLER.N.E., CORTESE.C.. HAZZARD.W..
COt.TARr. J. & LEWIS.B. 11981) Splanchnic metabolism of
plasma apolipoprotein B. Studies of artery-hepatic vein
differences in man and radiolabel in fasted human subjecti.
Jorrrnal o/'Clinrcal Ini-esrigarion. 67, 1678- 1686.
1601 BI.UM. C.B.. LEVY. R.I.. EISENBERG.S.. HALL. M. 111.
GOEBEL.R.H. & BERMAN.
M. (1977) High density lipoprotein
metabolism in man. Journal I!/'Clinicol liii~esrigorioii. 60.
795-807.
1611 MAHLEY. R.W.. WEISGRABER.K.H.. BERSOT. T.P. &
INNEKARITV.
T.L. ( 1978) In: High Densitr Lipoprureins und
.Itherosclerosrs. Ed. Gotto. A.M.. Jr. Miller. N.E. & Oliver.
M.F. Elsevier. Amsterdam.
1621 SIGURDSSON.
G.. NOEL. S-P. & HAVEL.R.J. (1979) Quantifi
cation of the hepatic contribution to the catabolism of high
density lipoproteins in rats. Journal 01Lipid Research. 20,
3 16-324.
1631 ROHEIM.P.S.. RACHMILEWITZ.
D.. STEIN.0. & STEIN. Y.
( 1971) Metabolism of iodinated high density lipoproteins in the
rat. Half-life in thc circulation and uptake by organs
Biochimico et Biophysica Acta. 248.3 15-329.
I641 OSE. L.. OSE.T.. NORUM.K.R. & BERG. T. (1979) Uptake
and degradation of "'I labelled high density lipoproteins in rat
liver cells in r.il.0 and in rirro. Biochrmica et Biophysica Acta.
574,521-536.
1651 RACHMILEWITZ.
D.. STEIN. 0.. ROHEIM.P.S. & STEIN. Y.
( 1972) Metabolism of iodinated high density lipoproteins in the
rat. I I . Autoradiographic localization in the liver. Biochimica
L'/ Biuphnica Acra. 270.4 14-425.
166 V A N BERKEL.T.J.C. & VANTOL. A. (1978) ln r.ii.u uptake of
human and rat low density and high density lipoprotein by
parenchymal and nonparenchymal cells from rat liver. Bio
rhimico et Biophj,sica Acra. 530,299-304.
1671 NAKAI.T.. Orro. P.S.. KENNEDY.
D.L. & WHAVNF.T I . . J R
( lY76) Rat high density lipoprotein subtraction ( H D L , ) uptake
and catabolism by isolated rat livcr parenchymal cell\. Jinrritol
i~l'Biolugicu1Ch1viirsri:i~.
25 I.49 14-49 2 I.
1681 WI.INSTEIN.
D.B.. C A R E W . ~ . EMILI.ER.N.E..
..
PANGBURN.S..
Plasma cholesterol transport
ATTIE. A.. DREVON.
C.A. & STEINBERG.
D. (1978)Interaction
of high density lipoprotein with cultured peripheral cells and
hepatocytes. In: Proceedings of the 69th Annual Meeting of the
American Oil Chemists Society. pp. 66-67. American Oil
Chemists Society.
1691 DREVON.C.A.. BERG,T. & NORUM.K.R. (1977)Uptake and
degradation of cholesterol ester-labelled rat plasma lipoprotein
in purified rat hepatocytes and nonparenchymal liver cells.
Biochimica el Biophysica Acro, 481, 122-236.
1701 MILLER,N.E., WEINSTEIN.D.B. & STEINBERG.D. (1977)
Binding, internalization, and degradation of high density
lipoprotein by cultured normal human fibroblasts. Journal of
Lipid Research, 18,438-450.
171 I BIERMAN,E.L., STEIN, 0. & STEIN. Y. (1974) Lipoprotein
uptake and metabolism by rat aortic smooth muscle cells in
tissue culture. Circulation Research, 35,136-1 50.
T.. HAYES.S.B. & STEINBERG.
D.
1721 CAREW,T.E., KOSCHINSKY,
(1976) A mechanism by which high-density lipoproteins may
slow the atherogenic process. Lancet. i, 1315-1317.
J.P.D., WEINSTEIN.
D.B. & STEINBERG.
D. (1976)
1731 RECKLESS,
Lipoprotein metabolism by rabbit arterial endothelial cells.
Circulation. 54.11-56.
1741 ANDERSEN,J.M. & DIETSCHY.J.M. (1977) Regulation of
27 1
sterol synthesis in 15 tissues of rat. I I Role of rat and human
high and low density plasma lipoproteins and of rat chylomicron remnants. Journal of Biological Chemistry. 252,
3652-3659.
J.T..MAHAFFEE.D., BREWER.H.B. & NEY. R.L.
1751 GWYNNE.
(1976)Adrenal cholesterol uptake from plasma lipoproteins:
regulation by corticotropin. Proceedings of the Nationol
Academy of Sciences U.SA.. 13,4329-4333.
1761 BALASUBRAMANIAM,S.. GOLDSTEIN.J.L.. FAUST, J.R..
BRUNSCHEDE. G.Y. & BROWN. M.S. (1977) Lipoproteinmediated regulation of 3-hydroxy-3-methylglutarylcoenzyme
A reductase activity and cholesteryl ester metabolism in the
adrenal gland of the rat. Journal of Biological Chemistry. 252,
I77 I- 1779.
S.. MOUTAFIS. C.D..
177) MYANT, N.B.. BALASUBRAMANIAM.
MANCINI.M. & SLACK.J. (1973) Turnover of cholesteryl
esters in plasma low-density and high-density lipoproteins in
familial hyperbetalipoproteinaemia. Clinical Science ond
Molecular Medicine. 45. 55 1-560.
W.C. (1980)On the
1781 JANSEN.H..VAN TOL. A. & HULSMANN.
metabolic function of heparin-releasable liver lipase. Biochemical and Biophysical Research Comtnunicorions. 92,
53-59.