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Human Erythrocytes Express GLUT5 and Transport Fructose
By Ilona I. Concha, Fernando V. Velásquez, Juan M. MartıB nez, Constanza Angulo, Andrea Droppelmann,
Alejandro M. Reyes, Juan Carlos Slebe, Juan Carlos Vera, and David W. Golde
Although erythrocytes readily metabolize fructose, it has not
been known how this sugar gains entry to the red blood
cell. We present evidence indicating that human erythrocytes express the fructose transporter GLUT5, which is the
major means for transporting fructose into the cell. Immunoblotting and immunolocalization experiments identified
the presence of GLUT1 and GLUT5 as the main facilitative
hexose transporters expressed in human erythrocytes, with
GLUT2 present in lower amounts. Functional studies allowed the identification of two transporters with different
kinetic properties involved in the transport of fructose in
human erythrocytes. The predominant transporter (GLUT5)
showed an apparent Km for fructose of approximately 10
mmol/L. Transport of low concentrations of fructose was
not affected by 2-deoxy–D-glucose, a glucose analog that is
transported by GLUT1 and GLUT2. Similarly, cytochalasin
B, a potent inhibitor of the functional activity of GLUT1 and
GLUT2, did not affect the transport of fructose in human
erythrocytes. The functional properties of the fructose transporter present in human erythrocytes are consistent with a
central role for GLUT5 as the physiological transporter of
fructose in these cells.
q 1997 by The American Society of Hematology.
C
there are about 300,000 per cell.6 As a consequence of this
high level of expression, the initial studies that led to the
functional characterization of the facilitative glucose transporters were performed using human erythrocytes and later
using highly purified preparations of GLUT1 reconstituted
in lipid vesicles. Although current evidence suggests that
GLUT1 is universally expressed in mammalian cells, this
isoform is still known as the ‘‘erythrocyte’’ glucose transporter.
The high glucose transport capacity of the human erythrocyte is consistent with its dependence on glucose for generating energy. The erythrocyte is also capable of metabolizing
fructose by at least two routes.7 Fructose is phosphorylated
to fructose-6-phosphate by hexokinase,8,9 an enzyme that has
an important role in the metabolism of glucose. Fructose3–phosphate, a phosphorylated sugar that is not a known
intermediary of glucose metabolism, has also been identified
in human erythrocytes.10 Although the physiological source
of the intracellular fructose was not identified in these studies, generation of fructose-3–phosphate in vitro in erythrocytes incubated in the presence of extracellular fructose is
compatible with the existence of fructose transporters operating in the erythrocyte plasma membrane. Two facilitative
hexose transporter isoforms have the capacity to transport
fructose, GLUT2, and GLUT5. GLUT2 is a low affinity
transporter of glucose that also transports fructose with lower
affinity. The hexose transporter GLUT5 is unable to transport
glucose and is instead a high affinity transporter of fructose
with a Km approximately 10 times lower than GLUT2.11
In the course of studies of the expression of facilitative
hexose transporters in human breast tumors, we12 and others13 have noted that erythrocytes in blood vessels show
strong immunohistochemical reactivity for GLUT5. We have
studied in detail the expression of facilitative hexose transporters on human erythrocytes and kinetically and functionally characterized the transport and uptake of fructose. The
immunolocalization studies indicated the presence of
GLUT2 and GLUT5 in human erythrocytes. The kinetic
characteristics of the transport of fructose by human erythrocytes show that they express a high affinity transporter of
fructose, and because the transport of fructose is minimally
affected by compounds known to block the function of
GLUT1 and GLUT2, our results indicate that GLUT5 is the
primary physiological transporter of fructose in these cells.
ELLULAR GLUCOSE uptake is mediated by specific
transporters that transfer it through the lipid barrier.
Two glucose transporter families have been identified in
mammalian cells that differ both in structure and function;
the sodium-glucose cotransporters and the facilitative glucose transporters.1-4 The sodium-glucose cotransporters correspond to a transport system that is restricted in expression
to the small intestine and kidney and consist of a family
of proteins with the capacity to transport glucose against a
concentration gradient.4 The transport of glucose is coupled
to the simultaneous transport of sodium ions down a concentration gradient. The facilitative glucose transporters
(GLUTs) allow the transport of glucose down a concentration gradient (facilitated transport) and are present in all cells
and tissues.1-3 Six facilitative glucose transporter isoforms
have been characterized in mammalian cells, of which five,
GLUT1 through GLUT5, are mainly expressed on the cell
membrane, and one, GLUT7, is restricted in expression to
the internal membranes of the endoplasmic reticulum.5 Each
glucose transporter has a characteristic tissue distribution and
mammalian cells generally express more than one transporter
isoform concomitantly.
In human erythrocytes, the transport of glucose is mediated by the glucose transporter isoform GLUT1, of which
From the Instituto de BioquıB mica, Facultad de Ciencias, Universidad Austral de Chile, Valdivia, Chile; and the Program in Molecular
Pharmacology and Therapeutics, Memorial Sloan-Kettering Cancer
Center, New York, NY.
Submitted September 25, 1996; accepted January 15, 1997.
Supported in part by Grant No. S-95-24 from the Universidad
Austral de Chile, Grant No. 196-0485 from FONDECYT, Santiago,
Chile, Grants No. R01 CA30388, R01 HL42107, and P30 CA08748
from the National Institutes of Health, and by Memorial SloanKettering Institutional Funds.
Address reprint requests to Ilona I. Concha, PhD, Instituto de
BioquıB mica, Facultad de Ciencias, Universidad Austral de Chile,
Casilla 567, Campus Isla Teja, Valdivia, Chile.
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be hereby marked
‘‘advertisement’’ in accordance with 18 U.S.C. section 1734 solely to
indicate this fact.
q 1997 by The American Society of Hematology.
0006-4971/97/8911-0025$3.00/0
Blood, Vol 89, No 11 (June 1), 1997: pp 4190-4195
4190
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GLUT5 IN HUMAN ERYTHROCYTES
4191
Immunofluorescent detection of the glucose transporters in blood
vessels was studied in normal human tissue sections prepared from
paraffin-archived tissue blocks.
Uptake assays. Erythrocytes were diluted in incubation buffer
(15 mmol/L HEPES [pH 7.4], 135 mmol/L NaCl, 5 mmol/L KCl,
1.8 mmol/L CaCl2 , 0.8 mmol/L MgCl2 ). Unless otherwise indicated,
uptake assays were performed at 47C in 200 mL of incubation buffer
Fig 1. The fructose transporter GLUT5 is expressed in human
erythrocytes. For immunofluorescence, erythrocyte smears were reacted with GLUT antibodies or with antibodies preabsorbed with the
corresponding peptide (P) and visualized using fluorescein-coupled
secondary antibodies.
MATERIALS AND METHODS
Erythrocytes. Outdated human blood was obtained from the
Blood Bank at Hospital Regional, Valdivia. Blood samples were
washed three times in phosphate-buffered saline (PBS) pH 7.4 by
centrifugation at 2,000g for 5 minutes. The final pellet was resuspended in PBS and stored at 47C until use. Erythrocytes were used
within 2 weeks of preparation.
Western blot analysis. The glucose transporter antibodies were
from East Acres Biologicals (Southbridge, MA). The specificity of
the antibodies was tested using human and rat tissues. Erythrocyte
membranes were isolated as previously described.14 Membrane proteins were separated by sodium dodecyl sulfate-polyacrylamide gel
electrophoresis (SDS-PAGE), transferred to nitrocellulose membranes, and incubated with antibodies to the glucose transporters or
preimmune sera diluted 1:500, followed by alkaline phosphataseantirabbit IgG treatment, and color developed with 4-nitroblue tetrazolium chloride and 5-bromo–4-chloro-3–indolyl phosphate.
Immunocytochemistry. Erythrocytes were spread onto gelatincoated slides and fixed with Histochoice (Amresco, Inc, Solon, OH).
The smears were incubated with 5% skim milk, 1% bovine serum
albumin (BSA), 0.3% Triton X-100 in PBS (pH 7.4) for 1 hour, and
then with the glucose transporter antibodies or preimmune rabbit
serum diluted 1:100 in the same solution for 1 hour. The erythrocytes
were washed with PBS (pH 7.4), incubated with goat antirabbit IgG
(H-L)–fluorescein isothiocyanate (FITC) conjugate (GIBCO-BRL,
Gaithersburg, MD) at a dilution of 1:30 for 1 hour, washed with
PBS (pH 7.4), mounted, and observed by fluorescence microscopy.
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Fig 2. Immunolocalization of hexose transporters in blood vessel
erythrocytes. For immunofluorescence, tissue sections were reacted
with GLUT antibodies or with preimmune serum (PI) and visualized
using fluorescein-coupled secondary antibodies.
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4192
CONCHA ET AL
containing 0.5 mmol/L fructose and 2 mCi of D-[U– 14C]fructose (6
Ci/mmol; DuPont/NEN, Boston, MA), or 0.5 mmol/L 2-deoxy–Dglucose and 1 mCi of 3H-deoxyglucose (5 mCi/mmol, DuPont NEN).
Uptake was stopped with ice-cold PBS containing HgCl2 and KI.
The cells were dissolved in 0.5 mL lysis buffer (10 mmol/L TrisHCl pH 8.0, 0.2% SDS), and the incorporated radioactivity was
assayed by liquid scintillation spectroscopy. Where appropiate, competitors and inhibitors were added to the uptake assays or preincubated with the cells. Data represent means { standard deviation (SD)
of four samples.
RESULTS
We used a panel of antibodies for each of the five members
of the family of glucose transporters (GLUT1 through
GLUT5) to identify the isoforms expressed in human erythrocytes. Immunolocalization using immunofluorescence
showed the presence of the transporters GLUT1, GLUT2,
and GLUT5 in human erythrocytes (Fig 1). For both GLUT1
and GLUT5, the immune reaction was homogeneously distributed across the cell, with a clear rim of fluorescence
observed at the cell external border. For GLUT2, the level
of fluorescence was consistently lower than that observed
with GLUT1 and GLUT5 antibodies, and the reaction was
distributed in patches across the cell. All cells within an
observation field reacted with the three antibodies. No positive reaction was observed in the case of GLUT3 and GLUT4
antibodies. Control experiments showed that GLUT3 and
GLUT4 antibodies gave positive reactions in tissues that
express the respective transporters. In the cases of GLUT1,
GLUT2, and GLUT5, the immunoreactivity was abolished
by preincubating the antibodies with the respective peptides
used to generate them (Fig 1). No immunoreactivity was
observed using preimmune serum. Thus, the immunocytologic data are compatible with the expression of high levels
of the transporters GLUT1 and GLUT5 in human erythrocytes and low levels of the transporter GLUT2.
To confirm the results obtained with blood smears, we
analyzed the reactivity of the erythrocytes present in blood
vessels of samples of human tissue fixed for routine pathological analysis. Similar to erythrocytes present in the blood
smears, erythrocytes in blood vessels were strongly positive
for GLUT1 and GLUT5 and reacted only weakly with
GLUT2 antibodies (Fig 2). No immunoreactivity was observed in control experiments in which the GLUT1, GLUT2,
and GLUT5 antibodies were preabsorbed with the corresponding peptide used to generate the antibodies. No immunoreactivity was observed when the samples were treated
with GLUT3 and GLUT4 antibodies. Similar results were
observed when using archival tissue (paraffin-embedded) or
fresh samples (frozen tissue). We observed anti-GLUT5 immunoreactivity in erythrocytes of blood vessels from different tissues (testis, brain, breast, prostate, liver, and ovary),
using different detection methods (immunofluorescence, immunoperoxidase, alkaline phosphatase) and different fixation
protocols (formalin, Bouin).
The presence of GLUT1, GLUT2, and GLUT5 in the
human erythrocytes was confirmed by immunoblotting. One
broad immunoreactive band, with an estimated Mr of 45,000,
was observed in total membrane proteins of erythrocytes
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probed with anti-GLUT1 (Fig 3). A weakly reactive band
with an estimated Mr of 50,000 was observed in membranes
reacted with GLUT2 antibodies. As expected from the immunolocalization experiments, no immunoreactive band was
observed in samples probed with GLUT3 and GLUT4 antibodies (Fig 3). On the other hand, a prominent band with
an estimated Mr of 50,000, was observed in membranes
tested with GLUT5 antibodies (Fig 3). Overall, the immunolocalization and the immunoblotting data are consistent with
the expression of three different facilitative hexose transporters in human erythrocytes; the glucose transporter GLUT1,
the low-affinity glucose-fructose transporter GLUT2, and the
high-affinity fructose transporter GLUT5.
Functional studies showed that human erythrocytes had
the capacity to take up fructose. Time-course experiments
showed that the transport of fructose by the erythrocytes
occurred slowly compared with the uptake of deoxyglucose
(Fig 4A and B). Deoxyglucose uptake proceeded rapidly,
approaching steady state in about 10 minutes. Considering
an intracellular exchange volume for the erythrocytes of approximately 0.5 mL per 10 million cells, the deoxyglucose
uptake data indicated that half the expected equilibrium concentration of deoxyglucose was reached in about 2 minutes.
On the other hand, fructose uptake proceeded in a linear
fashion for the first 3 minutes, followed by a second, slower
uptake component that lasted for the length of the uptake
assay (Fig 4A). Total fructose uptake accounted for only
about 5% of the uptake of deoxyglucose under the same
conditions. Because these experiments were performed at
47C, the difference in uptake cannot be accounted for by
differences in intracellular metabolism.
Fig 3. Expression of hexose transport proteins in erythrocytes.
For immunoblotting, total erythrocyte membrane proteins were
loaded on each lane of a sodium dodecyl sulfate-containing polyacrylamide gel, electrophoresed, transferred to Immobilon, reacted with
the different anti-GLUT antibodies, and visualized using horseradish
peroxidase antirabbit IgG and enhanced chemiluminescence. Sizes
on the left are kD.
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GLUT5 IN HUMAN ERYTHROCYTES
4193
Fig 4. Kinetic analysis of the
uptake of fructose by erythrocytes. (A) Time course of the uptake of 0.5 mmol/L fructose. (B)
Time course of the uptake of 0.5
mmol/L deoxyglucose. (C) Double-reciprocal plot of the substrate dependence for fructose
transport. Uptake was measured
in the presence of concentrations of fructose ranging from 3
to 40 mmol/L. (D) Semilog plot
of the concentration dependence for inhibition of fructose
transport by deoxyglucose. Measurements were performed at
0.5 mmol/L fructose using 1minute uptake assays. (E) Semilog plot of the concentration
dependence for inhibition of
fructose (●) and deoxyglucose
transport (s) by cytochalasin B.
Measurements were performed
at 0.5 mmol/L fructose or deoxyglucose using 1-minute uptake
assays. Cells were preincubated
for 5 minutes with cytochalasin
B. Where appropriate, data represent the mean Ô SD of at least
four samples.
Dose-response experiments examining uptake at 47C using
incubation times of 5 minutes indicated the presence of two
functional components involved in the transport of fructose in
human erythrocytes. The high-affinity component presented an
apparent Km for the transport of fructose of approximately 10
mmol/L (Fig 4C). The second functional component did not
show evidence of saturation at concentrations of fructose of 50
mmol/L or higher. Because these fructose concentrations
greatly exceeded the normal fructose content of blood even
after a meal rich in fructose, no further attempt to characterize
this activity was undertaken. It is known that GLUT2 transports
fructose with low-affinity, with a Km for transport of about 70
mmol/L. On the other hand, the Km for the transport of fructose
by GLUT5 is in the range of 10 mmol/L. Thus, our data point
to GLUT5 as the main route of entry of fructose in human
erythrocytes. Consistent with this notion was the finding in
competition experiments that uptake of fructose was not decreased in the presence of high concentrations of deoxyglucose
that completely blocked the transport of methylglucose by the
human erythrocytes (Fig 4D). Similarly, cytochalasin B, a potent inhibitor of the functional activities of GLUT1 and GLUT2
that does not inhibit GLUT5, did not affect the uptake of fructose by human erythrocytes at concentrations that caused a total
inhibition of the uptake of deoxyglucose by these cells (Fig
4E). The data are compatible with the concept that GLUT5 is
mainly responsible for the cellular uptake of fructose in human
erythrocytes.
DISCUSSION
We show here that human erythrocytes express the fructose transporter GLUT5, and that this transporter is directly
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involved in the uptake of fructose by these cells. This conclusion is supported by the results of immunolocalization, immunoblotting and transport assays.
The immunolocalization data indicate the presence in human erythrocytes of three members of the family of mammalian facilitative hexose transporters, the proteins GLUT1,
GLUT2, and GLUT5. The glucose transporter GLUT1 present in human erythrocytes has been extensively studied using
whole cells as well as reconstituted systems.1 The results of
the immunofluorescent localization and the immunoblotting
experiments indicating a higher degree of reactivity with
GLUT1 antibodies compared with GLUT5 and GLUT2 antibodies are consistent with the concept that GLUT1 is the
main hexose transporter expressed in human erythrocytes,
followed by the abundant expression of GLUT5 and low
level expression of GLUT2.
The transport data are consistent with the expression of
GLUT5 and GLUT2 in human erythrocytes and support the
concept that GLUT5 is the physiological transporter of fructose in these cells. The apparent Km for the transport of
fructose by human erythrocytes is similar to the Km for
transport of fructose by GLUT5.15 The properties of a second
transport system that did not approach saturation at concentrations of fructose as high as 50 mmol/L are compatible
with the expression of low levels of GLUT2 in human erythrocytes. GLUT2 is capable of transporting fructose, but the
Km for fructose transport is greater than 70 mmol/L.3 The
lack of a measurable effect of deoxyglucose on fructose
transport by human erythrocytes is also consistent with a
major role for GLUT5 in the uptake of fructose by these
cells. Deoxyglucose is transported by GLUT1 and GLUT2,
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4194
CONCHA ET AL
but not by GLUT5.16 Thus, it is expected that deoxyglucose
would compete for the transport of fructose if GLUT2 were
the main transporter involved in the transport of fructose by
human erythrocytes. The alkaloid cytochalasin B strongly
inhibits the functional activity of GLUT1, GLUT2, GLUT3,
and GLUT4, but does not inhibit GLUT5-mediated fructose
transport.2,3,16 The lack of effect of cytochalasin B on the
transport of fructose by human erythrocytes is also consistent
with the concept that GLUT5, and not GLUT2, is the main
transporter involved in the uptake of fructose by these cells.
The time-course experiments showed that fructose entered
human erythrocytes at a velocity that was only a fraction of
the velocity of uptake of deoxyglucose by these cells. Although the transport data are consistent with the immunolocalization data that suggest that GLUT5 is not as abundant
as GLUT1 in human erythrocytes, a side-by-side comparison
of the data indicates that the large difference in transport
velocity cannot be explained solely by differences in expression. A plausible explanation is that the membrane translocation of fructose mediated by GLUT5 occurs very slowly. An
example of slow transport of a molecule across the cell
membrane is the transport of a fluorescent glucose analog
by GLUT1.17
The demonstration that human erythrocytes express the
fructose transporter GLUT5 and have the capacity to transport fructose has implications for our understanding of human red blood cell metabolism. There has been an increased
consumption of fructose in modern times due to the widespread use of fructose as a natural sweetener. Fructose
crosses the brush border membrane of the small intestinal
enterocytes through GLUT5.15 In the intact organism and
under normal conditions, the liver is the major site of fructose metabolism, although there is also evidence indicating
that adipose tissue, and to a lesser degree muscle tissue, may
be important sites for fructose metabolism.18 In liver, the
transport of fructose is mediated by GLUT2. On the other
hand, adipose and muscle tissue do not express measurable
amounts of GLUT2, but instead express low levels of
GLUT5.19 It is conceivable then that GLUT5 is directly involved in the transport of fructose in these tissues.
A better understanding of the metabolic pathways involved in the metabolism of fructose by human erythrocytes
is slowly emerging. Similar to liver, it is generally believed
that the first step in the metabolism of fructose in human
erythrocytes is the phosphorylation to fructose-6–phosphate
by hexokinase, an enzyme that primarily phosphorylates glucose, but also has low affinity for fructose.9 In liver, fructose
can be phosphorylated to fructose-1–phosphate by fructokinase, and fructose-1–phosphate is a potent activator of glucose phosphorylation to glucose-6–phosphate.20 It has been
described recently that human erythrocytes accumulate measurable amounts of fructose-3–phosphate when incubated
in the presence of fructose.21 Fructose-3–phosphate and its
metabolic product, 3-deoxyglucosone, which is a potent glycosylating agent, have also been found in the lens of diabetic
rats.22 Although phosphorylated fructose intermediaries are
generated intracellularly from glucose, our results indicate
that fructose of dietary origin can be transported into human
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erythrocytes with the direct participation of the high-affinity
fructose transporter GLUT5. The data presented here are
compatible with the notion that human erythrocytes may be
a significant, and until now unrecognized, component of
nonhepatic clearance of dietary fructose.
ACKNOWLEDGMENT
We thank Dr Luis Norambuena from the Instituto de HistologıB a
y PatologıB a, Universidad Austral de Chile, for tissue samples.
REFERENCES
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1997 89: 4190-4195
Human Erythrocytes Express GLUT5 and Transport Fructose
Ilona I. Concha, Fernando V. Velásquez, Juan M. Marti?nez, Constanza Angulo, Andrea Droppelmann,
Alejandro M. Reyes, Juan Carlos Slebe, Juan Carlos Vera and David W. Golde
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