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PubMed1994

Cloning of a pig glucose transporter 4 cDNA fragment: use in developing a sensitive ribonuclease protection assay for quantifying low-abundance glucose transporter 4 mRNA in porcine adipose tissue.

A 246-bp fragment of porcine glucose transporter 4 (GLUT4) cDNA was cloned by polymerase chain reaction (PCR) from porcine adipose tissue RNA. Nucleotide sequences 1-138 and 139-246 of the GLUT4 cDNA share 78% sequence identity with exon 4a and 91% sequence identity with exon 4b of the human GLUT4 gene, respectively. The GLUT4 cDNA fragment was subcloned into pGEM-4Z vector to synthesize a highly specific riboprobe that hybridized only to human GLUT4 cDNA but not to human glucose transporter 1 (GLUT1) cDNA. Northern blot analysis of total RNA revealed the presence of a single transcript of 2.8 kb in porcine adipose tissue. Cloning a fragment of the GLUT4 cDNA enabled us to develop a ribonuclease protection assay for detecting porcine GLUT4 mRNA. The ribonuclease (RNase) protection assay is highly reproducible and retains a sensitivity level to as little as 2 pg of GLUT4 mRNA. The standard curve was linear between 2 and 128 pg of sense-strand GLUT4 RNA (r = .994). The ability to detect small quantities of GLUT4 mRNA is important when the abundance of GLUT4 mRNA is low and the quantity of tissue is limiting (e.g., when RNA is extracted from cultured adipose tissue). When porcine adipose tissue explants were cultured in the presence of insulin (10 ng/mL), GLUT4 mRNA abundance was increased. Development of a sensitive assay to quantify GLUT4 mRNA in porcine adipose tissue will enable us to conduct studies to increase our understanding of the molecular mechanisms by which porcine somatotropin (pST) regulates GLUT4 gene expression.

Adipose Tissue↗
PubMed2002

PAX3/forkhead homolog in rhabdomyosarcoma oncoprotein activates glucose transporter 4 gene expression in vivo and in vitro.

Increased levels of glucose uptake and increased expression of the glucose transporter (GLUT) genes are characteristic features of tumors. In the muscle-derived tumor alveolar rhabdomyosarcoma (ARMS), a chromosomal translocation t(2:13) generates the PAX3/forkhead homolog in rhabdomyosarcoma (FKHR) oncoprotein. In muscle tissues, glucose transport is primarily mediated by GLUT4. However, the mechanisms that regulate GLUT4 gene expression in tumor tissues are largely unknown. Therefore, we evaluated the role of PAX3/FKHR in the regulation of GLUT4 gene expression in muscle tumorigenesis. GLUT4 mRNA and protein were detected in ARMS-derived human biopsies and in ARMS-derived RH30 myoblasts, which both express the PAX3/FKHR chimeric protein, but not in either C2C12 or embryonal rhabdomyosarcoma-derived myoblasts. GLUT4 was functionally active in RH30 cells, because insulin induced a 1.4-fold stimulation of basal 2-deoxyglucose uptake rates. Coexpression of PAX3/FKHR increased basal transcriptional activity from a GLUT4 promoter reporter (GLUT4-P) in C2C12, SaOS-2, and Chinese hamster ovary-K1 cells in a dose-dependent and tissue-specific manner. PAX3/FKHR mutants with deletions in either the homeodomain (DeltaHD) or the FKHR-derived activation domain (DeltaFKHR), or in which the PAX3-derived paired domain (PD) was point-mutated (PD-R56L), were unable to activate GLUT4-P. Progressive 5'-deletion analysis of GLUT4-P further identified a specific region of the promoter, -66/+163 bp, which retained about 65% of the full transactivation effect. EMSA studies established that the PAX3/FKHR protein directly and specifically binds to this region and to a shorter fragment, -4/+36 bp, that contains potential binding sites for HD and PD, but not to a -4/+36-bp fragment whose HD and PD sites have been mutated. Thus, the functional interaction of PAX3/FKHR with GLUT4-P appears to require all of the functional domains of PAX3/FKHR, as well as a -4/+36-bp region within the GLUT4 promoter. Taken together, the data suggest that the GLUT4 gene is a downstream target of PAX3/FKHR and that GLUT4 is aberrantly transactivated by this oncoprotein both in vivo and in vitro.

Animals↗
PubMed1997

Glucose transporter levels in a male spontaneous non-insulin-dependent diabetes mellitus rat of the Otsuka Long-Evans Tokushima Fatty strain.

Otsuka Long-Evans Tokushima Fatty (OLETF) rats are a new strain of spontaneous non-insulin-dependent diabetes mellitus (NIDDM) models. To evaluate the role of glucose transporters (GLUT) in the development of diabetes in this model, we examined the action of insulin on the translocation of GLUT4 and GLUT1 in isolated adipocytes, and the GLUT4 protein levels in muscles. Long-Evans Tokushima Otsuka (LETO) rats were used as a control strain. In adipocytes, the GLUT4 protein levels in OLETF rats at 30 weeks of age (diabetic stage) were considerably lower than those in LETO rats at the same age. At a pre-diabetic stage (7 weeks), there were no significant differences in GLUT4 protein levels in adipocytes between LETO and OLETF rats. However, the degree of GLUT4 translocation in OLETF rats was lower than that in LETO rats at 7 weeks of age. There were no differences in GLUT1 levels in adipocytes between the two strains. In muscles, the decrease in GLUT4 protein was observed in OLETF rats at 30 weeks of age. Whether such a difference is under the influence of hyperglycemia was also examined using rats rendered diabetic by 70% pancreatectomy. OLETF rats aged 7 weeks were subjected to partial pancreatectomy (Px) and sham pancreatectomy (sham). At 4 weeks after surgery, GLUT4 protein levels in adipose tissues and skeletal muscles were determined. GLUT4 decrease was observed for both tissues of hyperglycemic Px rats compared with euglycemic sham. Moreover, we examined the direct effect of glucose on GLUT4 protein using primary cultured adipocytes of OLETF rats at 5 weeks of age. After 7-day culture with normal (5.6 mmol/l) or high (25 mmol/l) concentrations of glucose, the GLUT4 protein levels in adipocytes decreased at 25 mmol/l glucose compared with 5.6 mmol/l glucose. These findings suggest an early defect in the insulin resistance of OLETF rats probably reflects impaired GLUT4 translocation. The GLUT4 decrease, which occurs later in the process appears to be a consequence, rather than a cause of diabetes in OLETF rats.

Adipocytes↗
PubMed2007

The glucose transporter 4-regulating protein TUG is essential for highly insulin-responsive glucose uptake in 3T3-L1 adipocytes.

Insulin stimulates glucose uptake in fat and muscle by redistributing GLUT4 glucose transporters from intracellular membranes to the cell surface. We previously proposed that, in 3T3-L1 adipocytes, TUG retains GLUT4 within unstimulated cells and insulin mobilizes this retained GLUT4 by stimulating its dissociation from TUG. Yet the relative importance of this action in the overall control of glucose uptake remains uncertain. Here we report that transient, small interfering RNA-mediated depletion of TUG causes GLUT4 translocation and enhances glucose uptake in unstimulated 3T3-L1 adipocytes, similar to insulin. Stable TUG depletion or expression of a dominant negative fragment likewise stimulates GLUT4 redistribution and glucose uptake, and insulin causes a 2-fold further increase. Microscopy shows that TUG governs the accumulation of GLUT4 in perinuclear membranes distinct from endosomes and indicates that it is this pool of GLUT4 that is mobilized by TUG disruption. Interestingly, in addition to translocating GLUT4 and enhancing glucose uptake, TUG disruption appears to accelerate the degradation of GLUT4 in lysosomes. Finally, we find that TUG binds directly and specifically to a large intracellular loop in GLUT4. Together, these findings demonstrate that TUG is required to retain GLUT4 intracellularly in 3T3-L1 adipocytes in the absence of insulin and further implicate the insulin-stimulated dissociation of TUG and GLUT4 as an important action by which insulin stimulates glucose uptake.

3T3-L1 Cells↗
PubMed1992

Effects of hyperglycemia on glucose transporters of the muscle: use of the renal glucose reabsorption inhibitor phlorizin to control glycemia.

Individuals with non-insulin dependent or insulin-dependent diabetes mellitus present insulin resistance in peripheral tissues. This is reflected in a subnormal whole body insulin-dependent glucose utilization, largely dependent on skeletal muscle. Glucose transport across the cell membrane of this tissue is rate limiting in the utilization of the hexose. Therefore, it is possible that a defect exists in insulin-dependent glucose transport in skeletal muscle in diabetic states. This review focuses on two questions: is there a defect at the level of glucose transporters in skeletal muscle of diabetic animal models, and is this a consequence of abnormal insulin or glucose levels? The latter question arises from the fact that these parameters usually vary inversely to each other. Glucose transport into skeletal muscle occurs by two membrane proteins, the GLUT1 and GLUT4 gene products. By subcellular fractionation and Western blotting with isoform-specific antibodies, it was determined that isolated plasma membranes (PM) contain GLUT4 and GLUT1 proteins at a molar ratio of 3.5:1 and that an intracellular fraction (internal membranes; IM) different from sarcoplasmic reticulum contains only GLUT4 transporters. The IM furnishes transporters to the PM in response to insulin. Both transporter isoforms bind cytochalasin B in a D-glucose-protectable fashion. In streptozocin-induced diabetes of the rat with normal fasting insulin levels and marked hyperglycemia, the number of cytochalasin B-binding sites and of GLUT4 proteins diminishes in the PM whereas the GLUT1 proteins increase to a new ratio of about 1.5:1 GLUT4:GLUT1. In the IM, the levels of GLUT4 protein drop, as does the cellular GLUT4 mRNA. To investigate if these changes are associated with hyperglycemia, glucose levels were corrected back to normal values for a 24-h period with sc injections of phlorizin to block proximal tubule glucose reabsorption. This treatment restored cytochalasin B binding, restored GLUT4 and GLUT1 values back to normal levels in the PM, and partly restored cytochalasin B binding but not GLUT4 levels in the IM, consistent with only a partial recovery of GLUT4 mRNA. It is concluded that GLUT4 protein in the PM correlates inversely whereas GLUT1 protein correlates directly with glycemia. It is proposed that the decrease in GLUT4 levels is a protective mechanism, sparing skeletal muscle from gaining glucose and experiencing diabetic complications, albeit at the expense of becoming insulin resistant.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

Interaction of insulin and exercise on glucose transport in muscle.

Glucose transport is the rate-limiting step for glucose utilization in muscle. In muscle and adipose tissue, glucose transport is acutely regulated by such factors as insulin and exercise. Translocation of glucose transporters (GLUT4) from an intracellular domain to the cell surface is the major mechanism for this regulation. Using immunocytochemistry, the intracellular distribution of GLUT4 under resting conditions is similar in adipocytes and myocytes. GLUT4 is concentrated in tubulovesicular structures either in the trans-Golgi region or in the cytosol, often close to the cell surface but not on the cell surface. After stimulation, cell surface GLUT4 labeling is increased by as much as 40-fold. GLUT4 is chronically regulated by altered gene expression. Neural and/or contractile activity regulates GLUT4 expression in muscle: 1) GLUT4 levels differ among muscles of different fiber type; 2) GLUT4 levels in muscle are increased with exercise training and decreased with denervation; and 3) cultured muscle cells, which lack an intact nerve supply, express very low levels of GLUT4. GLUT4 expression appears to be regulated in parallel with many oxidative enzymes in muscle, suggesting that there may be a unified developmental program that determines the overall metabolic properties of a particular muscle. Preliminary evidence suggests that impaired GLUT4 expression in muscle is not the primary defect associated with insulin resistance. Nevertheless, it is conceivable that the adaptive increase in muscle GLUT4 that is found with exercise training may have beneficial effects in insulin-resistant states such as non-insulin-dependent diabetes.

Animals↗
PubMed1996

Regulation of hexokinase II gene expression by glucose flux in skeletal muscle.

UNLABELLED: The in vivo studies of transcriptional regulation by glucose, in general, have yielded ambiguous interpretations due to the closed loop relationship between insulin and glucose. Insulin cannot be held as a constant since elevated glucose levels will elicit a corresponding rise in insulin and current animal models of insulinopenia are associated with a plethora of counter-regulatory hormone responses. One potential solution to increase intracellular glucose flux without a further increase in insulin was achieved by transgenic overexpression of the insulin-sensitive glucose transporter, GLUT4, in specific skeletal muscles (previously described in Tsao, T.-S., Burcelin, R., Katz, E. B., Huang, L., and Charron, M. J. (1996) Diabetes 45, 28-36). Using these MLC-GLUT4 transgenic mice as a model, we investigated the effects of increased glucose flux on hexokinase II (HK II) gene expression in skeletal muscle. Under conditions where blood glucose levels were normal and insulin levels decreased by 36%, HK II mRNA level was reduced in non-GLUT4-overexpressing tissues (i.e. heart and adipose tissue) of 2-4-month-old male MLC-GLUT4 transgenic mice. This reduction in HK II mRNA was prevented in skeletal muscle, where overexpression of GLUT4 caused a 2.5-fold increase in basal and insulin-stimulated glucose uptake. The levels of HK II mRNA in heart, muscle, and adipose tissue are paralleled by HK II enzymatic activity. IN CONCLUSION: 1) due to relative mild insulinopenia, HK II expression is decreased in non-GLUT4-overexpressing tissues of MLC-GLUT4 mice compared to age/sex-matched controls, and 2) GLUT4-mediated increase in cellular glucose flux can prevent the decrease in HK II expression (in GLUT4-overexpressing tissues) as a result of relative mild insulinopenia. Indeed, during the process of aging, the return of circulating insulin levels of MLC-GLUT4 mice to normal levels is associated with the normalization of HK II expression in all tissues of MLC-GLUT4 and age/sex-matched control mice. We propose that: 1) glucose flux has an amplifying effect on the ability of insulin to stimulate skeletal muscle HK II gene expression and 2) insulin-dependent glucose flux may be a potential mechanism by which HK II gene expression is regulated by sensitivity to insulin.

Adipose Tissue↗
PubMed2004

Intracellular delivery of phosphatidylinositol (3,4,5)-trisphosphate causes incorporation of glucose transporter 4 into the plasma membrane of muscle and fat cells without increasing glucose uptake.

Insulin stimulates glucose uptake into muscle and fat cells by translocating glucose transporter 4 (GLUT4) to the cell surface, with input from phosphatidylinositol (PI) 3-kinase and its downstream effector Akt/protein kinase B. Whether PI 3,4,5-trisphosphate (PI(3,4,5)P(3)) suffices to produce GLUT4 translocation is unknown. We used two strategies to deliver PI(3,4,5)P(3) intracellularly and two insulin-sensitive cell lines to examine Akt activation and GLUT4 translocation. In 3T3-L1 adipocytes, the acetoxymethyl ester of PI(3,4,5)P(3) caused GLUT4 migration to the cell periphery and increased the amount of plasma membrane-associated phospho-Akt and GLUT4. Intracellular delivery of PI(3,4,5)P(3) using polyamine carriers also induced translocation of myc-tagged GLUT4 to the surface of intact L6 myoblasts, demonstrating membrane insertion of the transporter. GLUT4 translocation caused by carrier-delivered PI(3,4,5)P(3) was not reproduced by carrier-PI 4,5-bisphosphate or carrier alone. Like insulin, carrier-mediated delivery of PI(3,4,5)P(3) elicited redistribution of perinuclear GLUT4 and Akt phosphorylation at the cell periphery. In contrast to its effect on GLUT4 mobilization, delivered PI(3,4,5)P(3) did not increase 2-deoxyglucose uptake in either L6GLUT4myc myoblasts or 3T3-L1 adipocytes. The ability of exogenously delivered PI(3,4,5)P(3) to augment plasma membrane GLUT4 content without increasing glucose uptake suggests that input at the level of PI 3-kinase suffices for GLUT4 translocation but is insufficient to stimulate glucose transport.

3T3-L1 Cells↗
PubMed2006

Microtubule network is required for insulin signaling through activation of Akt/protein kinase B: evidence that insulin stimulates vesicle docking/fusion but not intracellular mobility.

The microtubule network has been shown to be required for insulin-dependent GLUT4 redistribution; however, the precise molecular function has not been elucidated. In this article, we used fluorescence recovery after photobleaching (FRAP) to evaluate the role of microtubules in intracellular GLUT4 vesicle mobility. A comparison of the rate of fluorescence recovery (t((1/2))), and the maximum fluorescence recovered (F(max)) was made between basal and insulin-treated cells with or without nocodazole treatment to disrupt microtubules. We found that intracellular mobility of fluorescently tagged GLUT4 (HA-GLUT4-GFP) was high in basal cells. Mobility was not increased by insulin treatment. Basal mobility was dependent upon an intact microtubule network. Using a constitutively active Akt to signal GLUT4 redistribution, we found that microtubule-based GLUT4 vesicle mobility was not obligatory for GLUT4 plasma membrane insertion. Our findings suggest that microtubules organize the insulin-signaling complex and provide a surface for basal mobility of GLUT4 vesicles. Our data do not support an obligatory requirement for long range microtubule-based movement of GLUT4 vesicles for insulin-mediated GLUT4 redistribution to the cell surface. Taken together, these findings suggest a model in which insulin signaling targets membrane docking and/or fusion rather than GLUT4 trafficking to the cell surface.

3T3-L1 Cells↗
PubMed2004

Evidence for the presence of glucose transporter 4 in the endometrium and its regulation in polycystic ovary syndrome patients.

Glucose transporter 4 (GLUT4) seems to be involved in the mechanism of insulin resistance in polycystic ovary syndrome (PCOS) patients (PCOSs) in both muscular and adipose tissue. The observation that insulin stimulates glucose oxidation in endometrial cells led us to investigate the presence of GLUT4 in this tissue and whether a defect of GLUT4 is present at the endometrial level in PCOSs. We also investigated whether body weight influences GLUT4 expression in this syndrome. GLUT4 mRNA content was examined by real-time quantitative RT-PCR and immunostaining reaction in the endometrial tissue of nine normal subjects, nine lean and eight obese hyperinsulinemic (h-INS), and eight lean and 10 obese normoinsulinemic (n-INS) PCOSs. GLUT4 mRNA and its positive immunostaining reaction were present in epithelial cell level in the endometrium of both normal and PCOS subjects. Significantly higher levels of GLUT4 were observed in normal and lean n-INS PCOSs in comparison with other groups. In both n-INS and h-INS obese PCOSs, GLUT4 was significantly lower than in lean subjects. However, obese n-INS and lean h-INS PCOSs showed a similar low GLUT4 expression, whereas obese h-INS PCOSs showed the lowest expression when compared with other groups. In conclusion, our data demonstrate that GLUT4 is present in the endometrium of normal and PCOS subjects and that hyperinsulinism and obesity seem to have a negative effect on endometrial GLUT4 expression in PCOS.

Adult↗
PubMed2007

Differential localization of aquaporin-2 and glucose transporter 4 in polarized MDCK cells.

Membrane water channel aquaporin-2 (AQP2) and glucose transporter 4 (GLUT4) exhibit a common feature in that they are stored in intracellular storage compartments and undergo translocation to the plasma membrane upon hormonal stimulation. We compared the intracellular localization and trafficking of AQP2 and GLUT4 in polarized Madin-Darby canine kidney cells stably transfected with human AQP2 (MDCK-hAQP2) by immunofluorescence microscopy. When expressed in MDCK-hAQP2 cells, GLUT4 and GLUT4-EGFP were predominantly localized in the perinuclear region close to and within the Golgi apparatus, similar to endogenous GLUT4 in adipocytes and myocytes. In addition, GLUT4 was occasionally seen in EEA1-positive early endosomes. AQP2, on the other hand, was sequestered in subapical Rab11-positive vesicles. In the basal state, the intracellular storage site of GLUT4 was distinct from that of AQP2. Forskolin induced translocation of AQP2 from the subapical storage vesicles to the apical plasma membrane, which did not affect GLUT4 localization. When forskolin was washed out, AQP2 was first retrieved to early endosomes from the apical plasma membrane, where it was partly colocalized with GLUT4. AQP2 was then transferred to Rab11-positive storage vesicles. These results show that AQP2 and GLUT4 share a common compartment after retrieval from the plasma membrane, but their storage compartments are distinct from each other in polarized MDCK-hAQP2 cells.

Animals↗
PubMed1995

Possible domains responsible for intracellular targeting and insulin-dependent translocation of glucose transporter type 4.

Translocation of the type 4 glucose transporter (GLUT4) to the cell surface from an intracellular pool is the major mechanism of insulin-stimulated glucose uptake in insulin-target cells. We developed a highly sensitive and quantitative method to detect GLUT4 immunologically on the surface of intact cells, using c-myc epitope-tagged GLUT4 (GLUT4myc). We constructed c-myc epitope-tagged glucose transporter type 1 (GLUT1myc) and found that the GLUT1myc was also translocated to the cell surface of Chinese hamster ovary cells, 3T3-L1 fibroblasts and NIH 3T3 cells, in response to insulin, but the degree of translocation was less than that of GLUT4myc. Since GLUT1 and GLUT4 have different intracellular distributions and different degrees of insulin-stimulated translocation, we examined the domains of GLUT4, using c-myc epitope-tagged chimeric glucose transporters between these two isoforms. The results indicated that, (1) all the cytoplasmic N-terminal region, middle intracellular loop and cytoplasmic C-terminal region of GLUT4 have independent intracellular targeting signals, (2) these sequences for intracellular targeting of GLUT4 were not sufficient to determine GLUT4 translocation in response to insulin, and (3) the N-terminal half of GLUT4 devoid both of cytoplasmic N-terminus and of middle intracellular loop seems to be necessary for insulin-stimulated GLUT4 translocation.

3T3 Cells↗
PubMed1995

High level overexpression of glucose transporter-4 driven by an adipose-specific promoter is maintained in transgenic mice on a high fat diet, but does not prevent impaired glucose tolerance.

High fat feeding is associated with impaired insulin action, an obese body composition, and down-regulation of glucose transporter-4 (GLUT4) expression in adipocytes. We recently showed that overexpression of GLUT4 selectively in adipocytes of transgenic mice using the aP2 (fatty acid-binding protein) promoter/enhancer results in enhanced glucose tolerance and adipocyte hyperplasia. Here, we fed these GLUT4-overexpressing transgenic mice a high fat (55%) or a low fat (10%) diet for 13-15 weeks to determine the role of alterations in GLUT4 expression in adipocytes in the development of insulin resistance and obesity, which are characteristic of high fat consumption. In nontransgenic mice, high fat feeding results in 45-50% reduction of GLUT4 levels in white and brown adipose tissue, with a parallel decrease in insulin-stimulated glucose transport. In transgenic mice receiving the low fat diet, GLUT4 is overexpressed 20-fold in white and 4-fold in brown adipose tissue. Glucose transport in epididymal adipocytes is increased 20-fold in the basal state and 6-fold in the insulin-stimulated state. Even after transgenic mice are fed a high fat diet, GLUT4 expression and glucose transport in their adipocytes remains 14- to 30-fold greater than that in nontransgenic mice receiving the same diet. Despite these marked effects at the adipose cell level, glucose tolerance is not improved, probably due to insulin resistance in skeletal muscle and liver, where the transgene is not expressed. During the low fat diet, transgenic mice have 80% more body lipid than nontransgenics. High fat feeding increases body lipid 76% and adipocyte size 65% in nontransgenic mice, but has no effect in transgenic mice. Thus, overexpression of GLUT4 selectively in adipocytes protects against a further increase in adiposity. Furthermore, by using a heterologous promoter, high level overexpression of GLUT4 can be maintained even under metabolic conditions where it is normally down-regulated in adipocytes. This overexpression results in markedly increased glucose transport at the cellular level, but adipose-specific GLUT4 overexpression does not prevent the decrease in glucose tolerance associated with high fat feeding.

Adipose Tissue↗
PubMed1996

Differential effects of insulin and exercise on Rab4 distribution in rat skeletal muscle.

Insulin and exercise cause the translocation of GLUT4 from an intracellular location to the plasma membrane in skeletal muscle. The purpose of this study was to determine if Rab4, a small GTP binding protein that has been implicated in the insulin-stimulated translocation of GLUT4 in adipose cells, is involved in the regulation of transporter translocation in skeletal muscle. Male rats were injected with insulin (20 U i.p.) or exercised on a treadmill(1 h, 20 m/min, 10% grade). Rats were killed 30 min after insulin injection or immediately after exercise, and the hind limb muscles dissected. Plasma membrane and intracellular microsomal membrane fractions were prepared, and the distribution of GLUT4 and Rab4 was determined by immunoblotting. Both insulin and exercise caused GLUT4 translocation as demonstrated by a decrease in microsomal membrane GLUT4 and an increase in plasma membrane GLUT4. In contrast, only insulin caused a decrease in Rab4 in the microsomal membrane. Rab4 was associated with GLUT4-containing vesicles isolated by immunoprecipitation. Rab4 was not detected in plasma membrane under any condition. These data demonstrate that insulin modulates the subcellular distribution of both GLUT4 and Rab4 in rats skeletal muscle, suggesting that Rab4 may play a role in the insulin-stimulated movement of GLUT4-containing vesicles. Although both insulin and exercise increase skeletal muscle glucose uptake by the translocation of GLUT4, the regulation of translocation may occur by different mechanisms.

Animals↗
PubMed2005

Genetic and nongenetic determinants of skeletal muscle glucose transporter 4 messenger ribonucleic acid levels and insulin action in twins.

CONTEXT: Insulin-stimulated glucose uptake in skeletal muscle is mediated through translocation of the insulin-sensitive glucose transporter 4 (GLUT4)-containing vesicles to the plasma membrane. Thus, skeletal muscle GLUT4 content plays an important role in whole-body insulin sensitivity. OBJECTIVES: The objectives of this study were 1) to examine the relative impact of genetic vs. environmental factors on skeletal muscle GLUT4 mRNA expression using biometric modeling, and 2) to identify factors influencing the expression of GLUT4 and insulin-stimulated whole-body metabolism. DESIGN: We measured GLUT4 mRNA expression in biopsies from young and elderly monozygotic (MZ) and dizygotic (DZ) twins before and during a 2-h hyperinsulinemic euglycemic clamp including 3-(3)H-tritiated glucose and indirect calorimetry. PARTICIPANTS: A random sample of young (22-31 yr; n = 89) and elderly (57-66 yr; n = 69) same sex MZ and DZ twin pairs identified through the Danish Twin Register were studied. RESULTS: We found a major genetic component in the control of basal and insulin-stimulated GLUT4 mRNA expression in young and elderly twins. GLUT4 gene expression increased upon insulin stimulation in both young and elderly twins. Multiple regression analysis revealed that both basal and insulin-stimulated GLUT4 mRNA expressions were positively related to birth weight and total body aerobic capacity and were higher in MZ vs. DZ twins as well as in males vs. females. Both basal and insulin-stimulated expressions of GLUT4 were independently and significantly related to whole-body in vivo insulin action, nonoxidative glucose metabolism, and glucose oxidation. CONCLUSION: We show that skeletal muscle GLUT4 gene expression in twins is significantly and independently related to glucose metabolism and is determined by both genetic and nongenetic factors, including zygosity and birth weight.

Adult↗
PubMed1996

Lilly lecture 1995. Glucose transport: pivotal step in insulin action.

The effect of insulin to acutely stimulate glucose uptake into muscle and adipose tissue is essential for normal glucose homeostasis. The GLUT4 glucose transporter is a major mediator of this action, and insulin recruits GLUT4 from an intracellular pool to the plasma membrane. An important pathologic feature of obesity, NIDDM, and to a lesser extent IDDM is resistance to insulin-stimulated glucose uptake. Investigations of the mechanisms have revealed tissue-specific regulation of GLUT4 with decreased gene expression in adipose cells but not in skeletal muscle. This has led to the hypothesis that alterations in the trafficking of the GLUT4 vesicle or in the exposure or activation of the GLUT4 transporter may cause insulin resistance in skeletal muscle in obesity and diabetes. Exercise training increases GLUT4 expression in muscle in association with enhanced glucose tolerance in vivo. Transgenic mice have been created to investigate other approaches to improve insulin action on glucose transport. Overexpression of GLUT4 in adipocytes of transgenic mice increases the proportion of GLUT4 on the plasma membrane and enhances insulin sensitivity in vivo. Altering insulin signaling by overexpressing p21ras in adipocytes of transgenic mice results in increased GLUT4 on the plasma membrane in the absence of insulin and increases insulin sensitivity in vitro and in vivo. Thus, glucose transport is a pivotal step in whole-body insulin action. Strategies to increase the number of GLUT4 transporters that are functionally inserted in the plasma membrane in muscle and adipocytes may lead to new therapies to treat or prevent NIDDM.

Adipose Tissue↗
PubMed2000

[Exercise induces increased CLUT4 gene expression and protein content in diabetic rats].

OBJECTIVE: To investigate the effects of exercise on insulin receptor, glucose transporter 4 (GLUT4) gene expression and protein content in skeletal muscle cells of streptozotocin-induced diabetic rats. METHODS: All rats were randomized into three groups: normal, diabetic and diabetic exercise groups. Diabetic exercise rats were swim-trained for 6 weeks. The GLUT4 mRNA of skeletal muscle was determined with dot blot and GLUT4 protein was detected by Western blotting. RESULTS: After 6 weeks of exercise training, plasma glucose level of diabetic rats decreased from 18.5 mmol/L +/- 1.9 mmol/L to 14.0 mmol/L +/- 3.3 mmol/L (P < 0.01). The dot blot revealed that GLUT4 mRNA decreased by 54.9% in skeletal muscle cells of diabetic rats compared with the normal rats. The Western blotting showed that GLUT4 protein of skeletal muscle cell in diabetic rats decreased by 30.7% compared with the control rats. GLUT4 mRNA and the protein of skeletal muscle in diabetic exercise group increased by 56% and 60.8% respectively as compared with the diabetic rats. However, the maximum binding of insulin receptor of skeletal muscle cells in diabetic exercise group decreased significantly compared with diabetic group. CONCLUSION: The weaker expression of GLUT4 gene and the lower level of GLUT4 protein may be one of the causes of hyperglycemia in diabetic rats. Exercise training may raise the GLUT4 gene expression and the amount of GLUT4 protein in diabetic rats, contributing to the main mechanism of improved post-receptor insulin resistance and decreased hyperglycemia in diabetic rats.

Animals↗
PubMed2006

Cerebral glucose transporters expression and spatial learning in the K-ATP Kir6.2(-/-) knockout mice.

K-ATP channels formed of the Sur and Kir subunits are widely distributed in the brain. Sur1-Kir6.2 is the most common combination of K-ATP channel subunits in the brain and Kir6.2 plays an important role in glucose metabolism through pancreatic insulin secretion or hypothalamic glucose sensing. K-ATP channels have also been reported to play a role in memory processing. Therefore, the aim of the present experiment is to assess the gene and protein expression of GLUT1, GLUT3 and GLUT4 in various brain regions of Kir6.2(-/-) K-ATP knockout mice and to test their working memory performance. GLUT4 was measured using two antibodies, one recognizing an intracellular epitope and the other, an extracellular epitope. Relative to their corresponding wild type, semi-quantitative immunohistochemistry showed that GLUT4 protein expression as measured by a GLUT4 antibody recognizing an extracellular epitope was increased in the Kir6.2(-/-) K-ATP mice. However, there was only a small increase in GLUT4 labeling using the GLUT4 antibody recognizing the intracellular epitope. These results suggest a compensatory higher GLUT4 inclusion at the cellular neuronal membrane in the cerebral cortex, hippocampus and cerebellum of the Kir6.2(-/-) K-ATP knockout mice. However, there was no change in GLUT4 gene expression assessed by TaqMan PCR except for a decrease in the cerebellum of these mice. Working memory performance of the Kir6.2(-/-) K-ATP mice was disrupted at age of 12 weeks but not at 5 weeks. The mild glucose intolerance that is observed in the Kir6.2 knockout mice is unlikely to have created the memory deficits observed. Rather, in light of the effects of K-ATP channel modulators on memory, the memory deficits in the Kir6.2(-/-) K-ATP mice are more likely due to the absence of the Kir6.2 and possible disruption of the GLUT4 activity in the brain.

Age Factors↗