From the Department of Biochemistry, Albert Einstein
College of Medicine, Bronx, New York 10461 and ¶ Department of
Biochemistry and Molecular Biology, University of Oklahoma Health
Sciences Center, Oklahoma City, Oklahoma 73190
A decade has passed since the cloning of the
insulin-responsive glucose transporter, GLUT4. Numerous
studies have demonstrated the complex hormonal and metabolic regulation
of GLUT4 gene expression in adipose tissue and muscle.
Careful dissection of the regulatory elements in the GLUT4
promoter has provided insight into the intricate control of this
central gene of glucose homeostasis. Genetic manipulation of mice has
provided further insight into the role of GLUT4 in carbohydrate and
lipid metabolism at the whole body and tissue-specific levels. Analysis
of GLUT4+/ Proof that the facilitative glucose transporter, GLUT4, is the
primary effector molecule for insulin-mediated glucose disposal comes
from the use of transgenic animals. Mice that are genetically engineered to generally overexpress an exogenous GLUT4 gene,
or specifically in skeletal muscle or adipose tissue, display enhanced insulin responsiveness and peripheral glucose utilization (for review
see Ref. 1). The high levels of transporters are able to enhance
insulin responsiveness in genetic and experimental models of diabetes.
Thus, expression of the GLUT4 gene is a clinically relevant
molecule to target for treatment of insulin-resistant disease states.
Expression of GLUT4 mRNA is subject to tissue-specific,
hormonal, and metabolic regulation (for review see Ref. 2).
GLUT4 mRNA expression is largely restricted to both
brown and white adipose tissue, skeletal and cardiac muscle, although
GLUT4 mRNA have been detected in specialized cell types
of other tissues. Changes in GLUT4 gene expression are
observed in physiologic states of altered glucose homeostasis and vary
in a tissue-specific manner, occurring much more rapidly in adipose
tissue than skeletal muscle (3). In general, GLUT4 mRNA
expression is down-regulated in states of relative insulin deficiency
such as streptozotocin
(STZ)1-induced diabetes and
chronic fasting (for review see Ref. 2). Chronic fasting markedly
reduces GLUT4 mRNA levels in adipose tissue, while
having either no effect or slightly increasing GLUT4 mRNA in skeletal muscle (4). Changes in steady state levels of
GLUT4 mRNA result from changes in the rate of synthesis
of GLUT4 mRNA (gene transcription) and changes in
degradation of the mRNA (5, 6). Transcription rates using nuclear
run-on transcription assays have demonstrated that the GLUT4
mRNA transcription rate is decreased in both adipose tissue and
skeletal muscle in STZ-induced diabetic animals (5, 6), whereas it is
increased in skeletal muscle of fasted animals (6). Thus, changes in GLUT4 mRNA steady state levels reflect changes in the
rate of mRNA synthesis.
The molecular basis for regulation of GLUT4 gene expression
in states of relative insulin deficiency in vivo has been
very difficult to solve. Insulin deficiency in vivo is
complicated by the fact that compensatory counter-regulatory hormones
are elevated. In addition, insulin deficiency is tightly coupled to plasma glucose levels and intracellular glucose utilization. For instance, STZ-diabetic animals are hyperglycemic and insulinopenic whereas fasted animals are hypoglycemic and insulinopenic, suggesting that insulin rather than circulating glucose levels are responsible for
regulation of adipose tissue GLUT4 expression. This
hypothesis was supported in studies using phlorizin to increase urinary
output of glucose in diabetic rats (2). In contrast to insulin,
phlorizin-induced normalization of glycemia in these insulin-deficient
animals was unable to restore GLUT4 mRNA expression in
adipose tissue. On the other hand, skeletal muscle GLUT4
mRNA is not down-regulated by insulinopenia associated with
fasting, implying that insulin levels do not directly regulate
GLUT4 gene expression (4).
Both chronically fasted and STZ-diabetic animals represent states of
insulin deficiency where peripheral glucose metabolism is inhibited.
With the production of transgenic mice overexpressing the
GLUT4 gene, a model of insulinopenia in which peripheral
glucose utilization is enhanced has been made available. Overexpression of human GLUT4 protein markedly enhanced glucose uptake and utilization in the fed state resulting in hypoglycemia and hypoinsulinemia (7).
Despite the relative hypoinsulinemia, expression of the endogenous
mouse GLUT4 mRNA was unaffected by the presence of the
human GLUT4 protein (8).2
This suggests the predominate metabolic control of GLUT4
gene expression is linked to intracellular glucose metabolism. The divergent effect of hypoinsulinemia of fasting compared with
STZ-diabetes in skeletal muscle may be linked to differences in energy
metabolism that occur in muscle in these different states. In
vitro models for studying GLUT4 expression are limited
by the small number of cultured cell models that express the
GLUT4 gene. Differentiated murine 3T3-L1 and F442A
adipocytes express relatively high levels of
GLUT4 mRNA and protein similar to that found
in primary isolated adipocytes or adipose tissue (9-11). The
appearance of GLUT4 mRNA in these cells first occurs
about 4 days after the onset of differentiation (9). Unlike primary
adipocytes or adipose tissue, 3T3-L1 adipocytes also express high
levels of the GLUT1 glucose transporter isoform (10). These cell lines
have been used as in vitro models to study several aspects
of glucose transporter regulation, including regulation of gene
expression. Interestingly, chronic exposure of 3T3-L1 in
vitro cells or adipose tissue in vivo to insulin has
differential effects on GLUT4 gene expression. Animals
chronically treated with insulin show increased GLUT4
mRNA in adipose tissue (12, 13) whereas chronic insulin treatment
of 3T3-L1 adipocytes has resulted in either no change or in a marked
reduction in GLUT4 mRNA levels (14, 15). The different
responses of GLUT4 mRNA to chronic insulin treatment
in vivo and in vitro suggest GLUT4 mRNA does not respond directly to insulin action on adipose tissue. On the other hand, incubation of 3T3-L1 adipocytes in glucose-free medium down-regulates GLUT4 mRNA about 10-fold and is
accompanied by the up-regulation of GLUT1 mRNA (16).
Re-addition of glucose to the starved adipocytes restored GLUT4
mRNA levels. Supplementation of glucose-free medium with
either fructose or pyruvate as an alternative energy source maintained
the steady state level of GLUT4 mRNA. These data are
consistent with a metabolic rather than hormonal regulation of
GLUT4 gene expression.
To understand how the factors described above exert their
influence on GLUT4 gene expression, it is necessary to
identify the molecular elements (cis-DNA sequences and trans-acting
factors) responsible for the coordinate transcriptional control of gene expression. In the case of GLUT4, a complex pattern of gene
expression is observed in various physiologic states that are difficult
to mimic in vitro using traditional methods of studying the
function of gene promotors in model cell culture systems. Without a
clear understanding of the molecular basis for GLUT4 gene
expression, it is difficult to develop a suitable in vitro
model to understand how the GLUT4 gene is regulated in
vivo. To circumvent these difficulties, promoter analysis using
human GLUT4 reporter genes was performed in transgenic mice.
This system allows analysis of transcriptional activity of the
GLUT4 promoter in a natural physiologic context, which is
essential for a gene that is subjected to a complex mixture of
tissue-specific and nutritional/metabolic factors.
To date, 12 different transgenic constructs have been analyzed for
appropriate tissue-specific and hormone-dependent
GLUT4 gene regulation (Fig.
1). The first established transgenic line was engineered to express a human GLUT4 minigene consisting
of the entire coding region of the gene and 5.3 kb of 5'-flanking DNA
(8). Expression of this construct demonstrated that the human gene, in
a mouse background, was subject to the same regulation as the mouse
GLUT4 gene. Furthermore, the complex pattern of human GLUT4 transcription initiation site selection was also
observed in these transgenic mice. A second line of transgenic mice
carrying a construct in which expression of the chloramphenicol
acetyltransferase (CAT) reporter gene was driven by 2.4 kb of
5'-flanking DNA demonstrated that this region of DNA was sufficient to
confer not only tissue-specific expression but also regulated
expression of the GLUT4 gene in chronic fasted and
STZ-induced diabetic mice (8, 17).
INTRODUCTION
Top
Introduction
References
, GLUT4 null, and muscle-complemented GLUT4 knockout mice has furthered our understanding of
peripheral insulin sensitivity. Additional studies on GLUT4
gene regulation and GLUT4 knockout models are likely to lead
to novel therapies for type II diabetes and other diseases of insulin resistance.
Regulation of GLUT4 mRNA Expression in Vivo and in
Vitro
Transcriptional Regulation of the GLUT4 Promoter
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Fig. 1.
Summary of functional analysis of the human
GLUT4 promoter in transgenic mice. A schematic
depiction of transgenic constructs is shown on the left. The
colored bars indicate various structural domains
of the GLUT4 gene. The bent arrow
refers to the transcription initiation site. Expression of transgenic
mRNA in adipose tissue (brown and white), hindquarter skeletal
muscle, and heart is indicated to the right. A designation
of ++ indicates full expression, indicates no expression, and +/
indicates a low level of expression relative to the endogenous
GLUT4. STZ indicates whether the construct is regulated in
STZ-induced diabetes.
Because all the apparent regulatory cis-DNA elements were located within 2.4 kb of the transcription initiation site, studies have been concentrated on this area of the human GLUT4 promoter to define the functional elements that drive transcription. The initial approach to studying this promoter was to narrowly define the cis-acting DNA sequences required to support a full program of gene expression by generating CAT fusion genes driven by various fragments of the 5'-GLUT4 regulatory region. Using this approach the structure of the GLUT4 promoter and its regulatory regions has begun to be defined (Fig. 1).
A series of deletions in the 5'-end defined the first 900 base pairs upstream of the major transcription initiation site of the human GLUT4 gene as putative regulatory DNA (18). A comparison of sequences of this regulatory region in the human gene with the analogous regions of the rat and mouse GLUT4 gene revealed the existence of two areas with greater than 90% sequence identity referred to as domains I and II (19). A 5' deletion that removes both of these domains left a basal promoter which was ubiquitously expressed at a very low level in all tissues, including those that do not normally express the GLUT4 gene (20). A construct in which most of domain I was deleted leaving domain II intact was able to support a high level of CAT activity in skeletal muscle, but a low level of CAT expression was observed in heart and adipose tissue and tissues that do not normally express the GLUT4 gene (20). In this construct, domain II was not sufficient to support regulated expression of transgenic mRNA in STZ deficiency in skeletal muscle or any other tissue. Although domain II was not sufficient to support full transgenic expression in heart and adipose tissue, deletion of this region ablated transgene expression in these tissues. Thus domain II is necessary, but not sufficient, for full promoter function of the GLUT4 gene (18). Inspection of domain II revealed the existence of a perfectly conserved binding domain for the MEF2 family of transcription factors. A loss of function of this MEF2 binding domain had the same effect as deletion of the entire conserved region, demonstrating that this binding domain was the functional element within these sequences (18).
A DNA binding site functions by binding a specific protein complex. The MEF2 DNA binding site is known to bind isoforms of the MEF2 family (MEF2A, MEF2B, MEF2C, and MEF2D) of DNA binding proteins belonging to a larger family of MADS-box domain transcription factors (21). Although these transcription factors have been studied largely in the context of myogenesis, the expression of these proteins extends beyond muscle tissues. Using isoform-specific antibodies in electrophoretic mobility shift assays, MEF2A and MEF2D isoforms were shown to bind the GLUT4 MEF2 binding domain in skeletal muscle, heart, and adipose tissue (18).2 These studies were the first to establish a role for MEF2 binding activity in gene expression in adipose tissue. Such a role is not unexpected as adipose tissue and skeletal muscle arise from embryonic mesoderm.
The identification and characterization of domain I in the human
GLUT4 promoter are currently under way. Interestingly,
analysis of the mouse GLUT4 promoter in 3T3-L1 adipocytes
revealed an element responsible for insulin-mediated down-regulation of
GLUT4 gene expression in those cells (22). This
insulin-responsive element in mouse coincides with domain I in the
human gene. Although insulin effects in vivo and in
vitro are dissimilar, it remains possible that the
insulin-responsive element defined in 3T3-L1 cells may be functional in
mediating the effects of insulin deficiency in vivo.
Establishing functional region(s) of this domain will pave the way for
solving the complex regulation of the GLUT4 gene. Understanding the molecular basis for expression of the
GLUT4 gene will be useful for targeting the expression of
this gene in a manner appropriate for treatment of insulin-resistant
glucose transport.
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GLUT4+/![]() |
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Knockout mice with one null allele of GLUT4
(GLUT4+/) have been generated that exhibit reduced GLUT4
expression (23). Though all mice have the same mutation, namely one
disrupted allele of GLUT4, a spectrum of phenotypes is
observed with age. Specifically, when considering serum-fed glucose and
insulin levels, GLUT4+/
mice could be divided into three
distinct groups: normal glycemia with normal insulin levels (N/N),
normal glycemia with high insulin levels (N/H), or hyperglycemic with
high insulin levels (H/H). The diabetes present in GLUT4+/
mice is not coincident with alterations in body weight or fat pad
weight. Curiously, adipocytes from diabetic H/H GLUT4+/
mice are significantly enlarged. Secretion of cytokines such as tumor
necrosis factor
and leptin that influence insulin sensitivity and
energy balance is altered in obese adipocytes (24), and this may
contribute to the diabetic phenotype of the H/H mice. Additionally,
neither pancreatic insufficiency, dyslipidemia, nor hepatic insulin
resistance is associated with the GLUT4+/
phenotype.
Diabetic H/H GLUT4+/
mice display a 40% increase in peak
arterial blood pressure but a normal response to inotropic challenge,
demonstrating that ventricular performance was not compromised.
However, histopathological analysis revealed diabetic cardiomyopathies
including cardiomyocyte hypertrophy, focal cell necrosis, interstitial
fibrosis, and vascular sclerosis. Oddly, despite the lack of
dyslipidemia in H/H GLUT4+/
mice, micro- and
macro-steatosis similar to that seen in diabetics was noted throughout
the liver.
Beginning at 2 months of age all male GLUT4+/ mice have
reduced adipose and muscle GLUT4 expression (75 and 25-46%,
respectively) (23). Decreased adipocyte GLUT4 content in
GLUT4+/
mice is the first measurable cellular defect to
occur, the significance of which is yet to be realized. In
vitro insulin-stimulated glucose uptake into soleus and extensor
digitorum longus (EDL) muscles of prediabetic N/H GLUT4+/
mice is significantly diminished (23). Using
euglycemic/hyperinsulinemic clamps it was demonstrated that the
in vivo peripheral insulin resistance of prediabetic (N/H) GLUT4+/
mice was as severe as in uncontrolled type II
diabetes (25). Additionally, the rate of insulin-stimulated glycogen synthesis was significantly impaired directly because of reduced muscle
GLUT4-mediated glucose uptake and not defective activation of glycogen
synthase. The selective primary reduction in GLUT4 protein is the most
direct proof to date of the rate-limiting role of glucose transport in
whole body and skeletal muscle insulin-stimulated glucose uptake. This
supports the idea that spontaneous variations in GLUT4 gene
expression in human muscle may be a major determinant of peripheral
insulin sensitivity. In vivo tracer studies also demonstrated that prediabetic N/H GLUT4+/
mice exhibit a
modest reduction in postabsorptive hepatic glucose production and
normal intrahepatic distribution of glucose fluxes through
glucose cycling, gluconeogenesis, and glycogenolysis (25). These data
confirm the relatively minor role GLUT4 plays in basal glucose homeostasis.
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Prevention of Insulin Resistance in GLUT4+/![]() |
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Myosin light chain 1-GLUT4 (MLC-GLUT4)
transgenic mice that specifically overexpress GLUT4 in fast twitch
muscle (26) were mated into the genetic background of the
GLUT4+/ mutation to assess the therapeutic merit of muscle
GLUT4 gene therapy in type II diabetes
(23).3 MLC-GLUT4
transgenic mice have increased insulin sensitivity without hypoglycemia
(26). GLUT4 content and insulin-stimulated glucose uptake were
normalized in fast twitch muscles of MLC-GLUT4+/
mice. Fed
plasma glucose and insulin levels were normal throughout the lives of
MLC-GLUT4+/
mice, and cardiac histopathologies were minimal. In vivo tracer studies demonstrated that whole body
glucose utilization, glycolysis, and glycogen synthesis were normal in MLC-GLUT4+/
mice, confirming the central role of muscle
GLUT4 in peripheral insulin sensitivity and the genesis of diabetic complications. Although these studies strongly suggest muscle GLUT4 gene therapy could be effective in treating type II
diabetes or other diseases of peripheral insulin resistance, they do
not prove that point. To specifically test this hypothesis, design of
an inducible, muscle-specific GLUT4 transgene
(iGLUT4) is required (27). The iGLUT4
transgenically expressed in GLUT4+/
mice could be
activated at various points in the disease (e.g. N/H
prediabetic; H/H diabetic) to determine whether muscle GLUT4
gene therapy can halt or reverse any or all diabetic complications.
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GLUT4 Null Mice Maintain Normal Glycemia without GLUT4 |
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In comparison with GLUT4+/ mice it is both exciting
and perplexing that mice which lack the insulin-sensitive glucose
transporter (GLUT4 null) are not diabetic but do exhibit
abnormalities in glucose and lipid metabolism (1, 28-30).
Surprisingly, blood glucose levels in GLUT4 null mice are
normal under fasted and fed conditions. Although GLUT4 null
mice have normal glucose tolerance, they do exhibit hyperinsulinemia in
the fed state and impaired insulin tolerance, suggesting insulin
resistance. Careful analysis of serum metabolites of GLUT4
null mice reveals a significant reduction in fed lactate and free fatty
acid levels. Additionally, 9-fold reductions in fasting ketones are
noted. GLUT4 null mice are 15-20% growth-retarded and have
severely reduced adipose tissue depots and extreme cardiac hypertrophy.
Northern and Western blot analyses verified that GLUT4 null
mice could compensate for the lack of GLUT4 and maintain normal
circulating glucose levels by a mechanism that did not involve
overexpression of a known facilitative or
Na+-dependent glucose transporter isoform in
skeletal muscle (28, 31).4
Curiously, GLUT2 expression in GLUT4 null liver
is significantly increased. The increase in GLUT2 expression
suggests that the liver is capable of increased hepatic glucose uptake.
This excess glucose subsequently could be converted to fatty acids or glycogen.
Though GLUT4 null hearts display characteristics of
hypertrophy caused by hypertension, they have normal blood pressure
(1). The GLUT4 null heart represents a unique model of
hypertrophy that may be used to study the consequences of altered
substrate utilization in both normal and pathophysiological conditions. Consistent with this notion, GLUT1 expression is increased in GLUT4 null hearts, and serum free fatty acids are reduced,
as are peripheral adipose tissue depots. The GLUT4 null
heart may in fact be glycolytically primed, which could present an
advantage under ischemic conditions. Indeed, preliminary studies
demonstrate that GLUT4 null hearts resist loss of function
following
ischemia/reperfusion.5
GLUT4 ablation also results in an extreme depletion of fat
mass. Gonadal fat pad weights are reduced 10-fold, and fat cells are approximately 50% smaller in size. This reduction in cell size may
affect the secretion of the metabolic modifiers mentioned earlier (23).
Interestingly, endurance exercise training results in reduced fat mass,
smaller fat cell size, cardiac hypertrophy without hypertension, and
reduced fed serum free fatty acids (32). The striking similarity
between GLUT4 null mice and endurance exercise-trained
athletes may suggest that similar adaptive responses are elicited by
these two forms of cellular stress.
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Characterization of a Novel Glucose Transport Activity in GLUT4 Null Soleus Muscle |
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The ability of two GLUT4 null muscle types to take up glucose in the presence of maximally stimulating concentrations of insulin was measured in vitro (29-31). Perhaps as expected, fast twitch EDL muscles of GLUT4 null mice failed to take up more glucose in response to insulin. Surprisingly, a 2-fold increase in insulin-stimulated glucose uptake was noted in female GLUT4 null slow twitch soleus muscles compared with a 3-fold increase in wild type controls. Soleus muscle of GLUT4 null males displayed a 2-fold increase in basal glucose uptake with no further increase following insulin stimulation. The molecular basis for the sexually dimorphic response to GLUT4 ablation in soleus muscle may be linked to the superior insulin sensitivity of female mice. These results indicate that highly oxidative soleus muscle can adapt to ablation of GLUT4 and take up a large amount of glucose, whereas glycolytic EDL muscle cannot. The specificity of glucose uptake was demonstrated by incubating muscles in the presence of 50 µM cytochalasin B, a fungal metabolite that inhibits facilitated D-glucose transport (29). Basal and insulin-stimulated glucose uptake in soleus and EDL muscles from GLUT4 null and control mice was reduced by cytochalasin B to the same extent. This result, combined with the failure to detect increased expression of any known GLUT, led to the hypothesis that a novel glucose transport system is responsible for glucose uptake into highly oxidative muscle, which contributes to euglycemia in GLUT4 null mice (1, 28-31).
Recently glucose uptake was measured in GLUT4 null muscles under normoxic and hypoxic conditions (33). Hypoxia has been shown to stimulate glucose transport in skeletal muscle via a pathway that is independent from that of insulin by recruitment of GLUT4 to the plasma membrane (34). In both soleus and EDL from GLUT4 null mice, hypoxia treatment failed to stimulate glucose uptake to levels above basal normoxic conditions (33). This result proved GLUT4 is essential for hypoxia-induced increase in glucose uptake, and the compensatory glucose transport activity in GLUT4 null soleus does not respond to stimulation by hypoxia in vitro. As hypoxia is a useful model for exercise, the above data suggest that GLUT4 is essential for exercise-stimulated increases in muscle glucose uptake.
Basal and insulin-stimulated insulin receptor tyrosine kinase activity
was shown to be normal in muscles of male GLUT4 null mice
(31). Furthermore, insulin receptor autophosphorylation was also shown
to be unchanged in null muscle. Thus, GLUT4 ablation does
not alter the activation of insulin receptor tyrosine kinase activity
in skeletal muscle. Glycogen synthase, like GLUT4, is a major
downstream target of insulin receptor action (35). Basal glycogen
synthesis was increased in GLUT4 null soleus muscle; however, insulin stimulation resulted in a parallel decrease in glucose
uptake and glycogen synthesis (31). Although GLUT4 null soleus muscles are able to maintain nearly normal steady state levels
of glycogen, acute insulin-stimulated glucose uptake and incorporation
into glycogen appear to be GLUT4-dependent. The increased
glycogen content of female GLUT4 null soleus muscle was
tightly associated with maintenance of ATP and phosphocreatine levels
similar to controls in response to hypoxic stress (33). Though
GLUT4 null muscles take up significantly less glucose, they
maintain normal high energy phosphate stores possibly because of
increased utilization of fatty acids (36). The reduced fed serum free
fatty acids in GLUT4 null mice are consistent with this hypothesis.
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MLT-GLUT4 Null Mice (Muscle Only GLUT4 Expression) |
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Mice expressing GLUT4 only in fast twitch skeletal muscle were generated to assess the role of muscle GLUT4 in whole body glucose disposal, insulin sensitivity, energy homeostasis, and the complex phenotype of the GLUT4 null model. The muscle-specific MLC-GLUT4 transgene (26) was mated into the genetic background of GLUT4 null mice to generate a population of MLC-GLUT4 null mice (37). MLC-GLUT4 null mice have restored GLUT4 expression in glycolytic muscle such as EDL but not in oxidative muscles like soleus. Fed serum lactate levels of MLC-GLUT4 null mice compared with GLUT4 nulls are restored to normal. Unlike GLUT4 null mice, MLC-GLUT4 null mice do not exhibit fed hyperinsulinemia, suggestive of improved insulin sensitivity. Insulin tolerance tests demonstrate that MLC-GLUT4 nulls clear glucose as well as wild type control mice, unlike GLUT4 null mice, which are significantly insulin-resistant. This result was corroborated by a significant increase in insulin-stimulated glucose transport into EDL muscles. Furthermore, MLC-GLUT4 null soleus muscle retained the compensatory glucose transport activity measured in GLUT4 null soleus. When combined, these findings indicate that expression of the compensatory glucose transporter is not dependent upon hyperinsulinemia.
Like GLUT4 null mice, MLC-GLUT4 null mice weigh
approximately 20% less than controls and exhibit significantly
decreased fed serum free fatty acid levels (37). Further,
MLC-GLUT4 null mice also have significantly reduced inguinal
fat pads and adipose cells compared with controls. These data
demonstrate that complementation of fast twitch muscle with GLUT4 does
not correct defects in adipose tissue mass or lipid metabolism because
of GLUT4 ablation. When combined, these data indicate that
muscle GLUT4 is a major regulator of whole body glucose metabolism and
that defects in glucose metabolism and adipose tissue mass and lipid
metabolism in GLUT4 null mice arise independently.
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Conclusion |
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To date GLUT4+/ mice represent the only gene knockout
model where the phenotype of the heterozygote is more severe than that noted in the homozygote (with respect to the diabetes). Additionally, this is the first mouse model that demonstrates that a heterozygous knockout of a single known diabetogene can yield the plethora of
pathologies that characterize type II diabetes with increasing age. In
contrast to GLUT4+/
mice, which express normal amounts of
GLUT4 in their insulin-responsive tissue until 2 months of age,
GLUT4 null mice never express GLUT4 and do not develop a diabetic phenotype. The mechanisms that develop in GLUT4
null mice to help maintain euglycemia may not be apparent in
GLUT4+/
mice because the function of the compensatory
glucose transport system may be masked by the remaining functionally
dominant GLUT4. Perhaps, endurance exercise training that elicits
characteristics seen in GLUT4 null mice (i.e.
reduced adipose tissue, cardiac hypertrophy, reduced serum free fatty
acids) might lead to expression of the glucose transport system seen in
GLUT4 null soleus muscle. This expression could be
responsible in part for the therapeutic effects of exercise by
improving whole body insulin sensitivity in diabetics (32).
Understanding the genetic and molecular basis of the adaptive responses
to GLUT4 ablation responsible for the maintenance of
euglycemia will undoubtedly lead to novel therapies for diseases of
insulin resistance including type II diabetes.
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ACKNOWLEDGEMENTS |
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We thank Drs. R. Burcelin, J. Li, A. E. Stenbit, and T.-S. Tsao for invaluable contributions to these studies and Drs. J. Olefsky, M. Czech, and J. Pessin for critical comments.
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FOOTNOTES |
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* This minireview will be reprinted in the 1999 Minireview Compendium, which will be available in December, 1999. This is the third article of three in the "Insulin-stimulated Glucose Transport Minireview Series." This work was supported by National Institutes of Health Grants DK47425 and HL58119 (to M. J. C.) and DK47894 (to A. L. O.) and by grants from the American Diabetes Association (to M. J. C. and A. L. O.) and the American Heart Association (to M. J. C.).
§ Recipient of an Irma T. Hirschl career scientist award. To whom correspondence should be addressed: Dept. of Biochemistry, Albert Einstein College of Medicine, 1300 Morris Park Ave., Bronx, NY 10461. Tel.: 718-430-2852; Fax: 718-430-8676; E-mail: charron{at}aecom.yu.edu.
The abbreviations used are: STZ, streptozotocin; kb, kilobases; CAT, chloramphenicol acetyltransferase; EDL, extensor digitorum longus; MLC, myosin light chain.
2 A. L. Olson, unpublished data.
3 Tsao, T. S., Stenbit, A. E., Factor, S. M., Chen, W., Rossetti, L., and Charron, M. J. (1999) Diabetes, in press.
4 A. E. Stenbit and M. J. Charron, unpublished observation.
5 A. E. Stenbit, D. L. Geenen, and M. J. Charron, unpublished observations.
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REFERENCES |
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