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PubMed2003

Changes in dietary sodium consumption modulate GLUT4 gene expression and early steps of insulin signaling.

Previous studies have shown that chronic salt overload increases insulin sensitivity, while chronic salt restriction decreases it. In the present study we investigated the influence of dietary sodium on 1) GLUT4 gene expression, by No the n and Western blotting analysis; 2) in vivo GLUT4 protein translocation, by measuring the GLUT4 protein in plasma membrane and microsome, before and after insulin injection; and 3) insulin signaling, by analyzing basal and insulin-stimulated tyrosine phosphorylation of insulin receptor (IR)-beta, insulin receptor substrate (IRS)-1, and IRS-2. Wistar rats we e fed no mal-sodium (NS-0.5%), low-sodium (LS-0.06%), o high-sodium diets (HS-3.12%) fo 9 wk and were killed under pentobarbital anesthesia. Compared with NS ats, HS ats inc eased (P < 0.05) the GLUT4 protein in adipose tissue and skeletal muscle, whereas GLUT4 mRNA was increased only in adipose tissue. GLUT4 expression was unchanged in LS ats compared with NS ats. The GLUT4 translocation in HS ats was higher (P < 0.05) both in basal and insulin-stimulated conditions. On the other hand, LS ats did not increase the GLUT4 translocation after insulin stimulus. Compared with NS ats, LS ats showed reduced (P < 0.01) basal and insulin-stimulated tyrosine phosphorylation of IRS-1 in skeletal muscle and IRS-2 in live, whereas HS ats showed enhanced basal tyrosine phosphorylation of IRS-1 in skeletal muscle (P < 0.05) and of IRS-2 in live. In summary, increased insulin sensitivity in HS ats is elated to increased GLUT4 gene expression, enhanced insulin signaling, and GLUT4 translocation, whereas decreased insulin sensitivity of LS ats does not involve changes in GLUT4 gene expression but is elated to impaired insulin signaling.

Adipose Tissue↗
PubMed2005

Adipocytes from Munc18c-null mice show increased sensitivity to insulin-stimulated GLUT4 externalization.

Insulin-stimulated glucose uptake in adipocytes is mediated by translocation of vesicles containing the glucose transporter GLUT4 from intracellular storage sites to the cell periphery and the subsequent fusion of these vesicles with the plasma membrane, resulting in the externalization of GLUT4. Fusion of the GLUT4-containing vesicles with the plasma membrane is mediated by a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex consisting of vesicle-associated membrane protein 2 (VAMP2), 23-kDa synaptosomal-associated protein (SNAP23), and syntaxin4. We have now generated mouse embryos deficient in the syntaxin4 binding protein Munc18c and show that the insulin-induced appearance of GLUT4 at the cell surface is enhanced in adipocytes derived from these Munc18c-/- mice compared with that in Munc18c+/+ cells. Wortmannin, an inhibitor of PI3K, inhibited insulin-stimulated GLUT4 externalization, without affecting GLUT4 translocation to the cell periphery, in Munc18c+/+ adipocytes, but it did not affect GLUT4 externalization in Munc18c-/- cells. Phosphatidylinositol 3-phosphate, which induced GLUT4 translocation to the cell periphery without externalization in Munc18c+/+ cells, elicited GLUT4 externalization in Munc18c-/- cells. These findings demonstrate that Munc18c inhibits insulin-stimulated externalization of GLUT4 in a wortmannin-sensitive manner, and they suggest that disruption of the interaction between syntaxin4 and Munc18c in adipocytes might result in enhancement of insulin-stimulated GLUT4 externalization.

Adipocytes↗
PubMed2006

Insulin and leptin induce Glut4 plasma membrane translocation and glucose uptake in a human neuronal cell line by a phosphatidylinositol 3-kinase- dependent mechanism.

The insulin-sensitive glucose transporter Glut4 is expressed in brain areas that regulate energy homeostasis and body adiposity. In contrast with peripheral tissues, however, the impact of insulin on Glut4 plasma membrane (PM) translocation in neurons is not known. In this study, we examined the role of two anorexic hormones (leptin and insulin) on Glut4 translocation in a human neuronal cell line that express endogenous insulin and leptin receptors. We show that insulin and leptin both induce Glut4 translocation to the PM of neuronal cells and activate glucose uptake. Wortmannin, a specific inhibitor of phosphatidylinositol 3-kinase, totally abolished insulin- and leptin-dependent Glut4 translocation and stimulation of glucose uptake. Thus, Glut4 translocation is a phosphatidylinositol 3-kinase-dependent mechanism in neuronal cells. Next, we investigated the impact of chronic insulin and leptin treatments on Glut4 expression and translocation. Chronic exposure of neuronal cells to insulin or leptin down-regulates Glut4 proteins and mRNA levels and abolishes the acute stimulation of glucose uptake in response to acute insulin or leptin. In addition, chronic treatment with either insulin or leptin impaired Glut4 translocation. A cross-desensitization between insulin and leptin was apparent, where exposure to insulin affects leptin-dependent Glut4 translocation and vice versa. This cross-desensitization could be attributed to the increase in suppressor of cytokine signaling-3 expression, which was demonstrated in response to each hormone. These results provide evidence to suggest that Glut4 translocation to neuronal PM is regulated by both insulin and leptin signaling pathways. These pathways might contribute to an in vivo glucoregulatory reflex involving a neuronal network and to the anorectic effect of insulin and leptin.

Androstadienes↗
PubMed1998

Role of SNARE's in the GLUT4 translocation response to insulin in adipose cells and muscle.

Insulin stimulates glucose transport in skeletal muscle, heart, and adipose tissue by promoting the appearance of GLUT4, the major glucose transporter isoform present in these tissues, on the cell surface. This is achieved by differentially modulating GLUT4 exocytosis and endocytosis, between a specialized intracellular compartment and the plasma membrane. Ligands which activate the heterotrimeric GTP-binding proteins Gs and Gi appear to modulate insulin-stimulated glucose transport through effects on the fusion of docked GLUT4-containing vesicles with the plasma membrane. In insulin resistance states, reduced cellular GLUT4 levels in adipose cells fully account for the decreased glucose transport response to insulin in these cells. In contrast, although insulin-stimulated GLUT4 translocation is also impaired in muscle, total cellular levels of GLUT4 are not altered. The defect in muscle has been attributed to a GLUT4 trafficking problem and thus studies of this mechanism could provide clues as to the nature of the impairment. The movement of GLUT4-containing vesicles from an intracellular storage site to the plasma membrane and the fusion of docked GLUT4-containing vesicles with the plasma membrane are conceptually similar to some secretory processes. A general hypothesis called the SNARE hypothesis (soluble NSF attachment protein receptors where NSF stands for N-ethylmaleimide-sensitive fusion protein) postulates that the specificity of secretory vesicle targeting is generated by complexes that form between membrane proteins on the transport vesicle (v-SNARE's) and membrane proteins located on the target membrane (t-SNARE's). Several v- and t-SNARE's have been identified in adipose cells and muscle. VAMP2 and VAMP3/cellubrevin (v-SNARE's) have been shown to interact with the t-SNARE's syntaxin 4 and SNAP-23. The cytosolic protein NSF has the characteristic of binding to the v-/t-SNARE complex through its interaction with alpha-SNAP, another soluble factor. Furthermore, recent studies have demonstrated that VAMP2/3, syntaxin 4, SNAP-23, and NSF are functionally involved in insulin-stimulated GLUT4 translocation in adipose cells and thus are likely to be involved in the Gs- and Gi-mediated modulation of the glucose transport response to insulin as well. This review summarizes recent advances on the normal mechanism of GLUT4 translocation and discusses how this process could be affected in insulin resistant states such as type II diabetes.

Adipocytes↗
PubMed2004

Abnormal subcellular distribution of GLUT4 protein in obese and insulin-treated diabetic female dogs.

The GLUT4 transporter plays a key role in insulin-induced glucose uptake, which is impaired in insulin resistance. The objective of the present study was to investigate the tissue content and the subcellular distribution of GLUT4 protein in 4- to 12-year-old control, obese and insulin-treated diabetic mongrel female dogs (4 animals per group). The parametrial white adipose tissue was sampled and processed to obtain both plasma membrane and microsome subcellular fractions for GLUT4 analysis by Western blotting. There was no significant difference in glycemia and insulinemia between control and obese animals. Diabetic dogs showed hyperglycemia (369.9 +/- 89.9 mg/dl). Compared to control, the plasma membrane GLUT4, reported per g tissue, was reduced by 55% (P < 0.01) in obese dogs, and increased by 30% (P < 0.05) in diabetic dogs, and the microsomal GLUT4 was increased by approximately 45% (P < 0.001) in both obese and diabetic animals. Considering the sum of GLUT4 measured in plasma membrane and microsome as total cellular GLUT4, percent GLUT4 present in plasma membrane was reduced by approximately 65% (P < 0.001) in obese compared to control and diabetic animals. Since insulin stimulates GLUT4 translocation to the plasma membrane, percent GLUT4 in plasma membrane was divided by the insulinemia at the time of tissue removal and was found to be reduced by 75% (P < 0.01) in obese compared to control dogs. We conclude that the insulin-stimulated translocation of GLUT4 to the cell surface is reduced in obese female dogs. This probably contributes to insulin resistance, which plays an important role in glucose homeostasis in dogs.

Adipocytes↗
PubMed1996

Enhanced insulin action due to targeted GLUT4 overexpression exclusively in muscle.

Dysregulation of GLUT4, the insulin-responsive glucose transporter, is associated with insulin resistance in skeletal muscle. Although skeletal muscle is the major target of insulin action, muscle GLUT4 has not been linked causally to whole-body insulin sensitivity and regulation of glucose homeostasis. To address this, we generated a line of transgenic mice that overexpresses GLUT4 in skeletal muscle. We demonstrate that restricted overexpression of GLUT4 in fast-twitch skeletal muscles of myosin light chain (MLC)-GLUT4 transgenic mice induces a 2.5-fold increase in insulin-stimulated 2-deoxyglucose uptake in transgene-overexpressing cells. Consequently, glycogen content is increased in the fast-twitch skeletal muscles under insulin action (5.75 +/- 1.02 vs. 3.24 +/- 0.26 mg/g). This indicates that insulin-stimulated glucose transport is partly rate-limiting for glycogen synthesis. At the whole-body level, insulin-stimulated glucose turnover is increased 2.5-fold in unconscious MLC-GLUT4 mice. Plasma glucose and insulin levels in MLC-GLUT4 mice are altered as a result of increased insulin action. In 2- to 3-month-old MLC-GLUT4 mice, fasting insulin levels are decreased (0.43 +/- 0.05 vs. 0.74 +/- 0.10 microgram/l), whereas normal fasting glycemia is maintained. Conversely, 7- to 9-month-old MLC-GLUT4 mice exhibit decreased fasting glycemia (5.75 +/- 0.73 vs. 8.11 +/- 0.57 mmol/l) with normal insulin levels. Fasting plasma lactate levels are elevated in both age groups (50-100%). Additionally lipid metabolism is affected by skeletal muscle GLUT4 overexpression. This is indicated by changes in plasma free fatty acid and beta-hydroxybutyrate levels. These studies underscore the importance of GLUT4 in the regulation of glucose homeostasis and its interaction with lipid metabolism.

Animals↗
PubMed1999

Acute exercise induces GLUT4 translocation in skeletal muscle of normal human subjects and subjects with type 2 diabetes.

Total GLUT4 content in skeletal muscle from individuals with type 2 diabetes is normal; however, recent studies have demonstrated that translocation of GLUT4 to the plasma membrane is decreased in response to insulin stimulation. It is not known whether physical exercise stimulates GLUT4 translocation in skeletal muscle of individuals with type 2 diabetes. Five subjects (two men, three women) with type 2 diabetes and five normal control subjects (5 men), as determined by a standard 75-g oral glucose tolerance test, were recruited to determine whether an acute bout of cycle exercise activates the translocation of GLUT4 to the plasma membrane in skeletal muscle. Each subject had two open biopsies of vastus lateralis muscle; one at rest and one 3-6 weeks later from the opposite leg after 45-60 min of cycle exercise at 60-70% of VO2max. Skeletal muscle plasma membranes were prepared by subcellular fractionation, and GLUT4 content was determined by Western blotting. Plasma membrane GLUT4 increased in each subject in response to exercise. The mean increase in plasma membrane GLUT4 for the subjects with type 2 diabetes was 74 +/-20% above resting values, and for the normal subjects the increase was 71+/-18% above resting values. Although plasma membrane GLUT4 content was approximately 32% lower at rest and after exercise in the muscle of the subjects with type 2 diabetes, the differences were not statistically significant. We conclude that in contrast to the previously reported defect in insulin-stimulated GLUT4 translocation in skeletal muscle of individuals with type 2 diabetes, a single bout of exercise results in the translocation of GLUT4 to the plasma membrane in skeletal muscle of individuals with type 2 diabetes. These data provide the first direct evidence that GLUT4 translocation is an important cellular mechanism through which exercise enhances skeletal muscle glucose uptake in individuals with type 2 diabetes.

5'-Nucleotidase↗
PubMed2000

The transferrin receptor defines two distinct contraction-responsive GLUT4 vesicle populations in skeletal muscle.

Insulin and contraction increase glucose transport in an additive fashion in skeletal muscle. However, it is still unclear whether they do so by inducing the recruitment of GLUT4 transporters from the same or distinct intracellular compartments to the plasma membrane and the T-tubules. Using the transferrin receptor as a recognized marker of recycling endosomes, we have examined whether insulin and/or contraction recruit GLUT4 from this pool to either the plasma membranes or T-tubules, isolated by subcellular fractionation of perfused hindlimb muscles. Either stimulus independently increased GLUT4 translocation from an intracellular fraction to both the plasma membrane and T-tubules. The combination of insulin and contraction induced a marked (approximately threefold) and almost fully additive increase in GLUT4 content, but only in the plasma membrane. Insulin did not stimulate transferrin receptor recruitment from the GLUT4-containing intracellular fraction to either the plasma membrane or the T-tubules. In contrast, contraction stimulated the recruitment of the transferrin receptor from the same GLUT4-containing intracellular fraction to the plasma membrane but not to the T-tubules. Contraction-induced recruitment of the transferrin receptor was also observed from immunopurified GLUT4 vesicles. It is concluded that muscle contraction stimulates translocation of GLUT4 from two distinct intracellular compartments: 1) a population of recycling endosomes that is selectively recruited to the plasma membrane and 2) from GLUT4 storage vesicles that are also insulin-responsive and recruited to both the plasma membrane and the T-tubules. The lack of additive translocation of GLUT4 to the T-tubules may be linked to the failure of GLUT4-containing recycling endosomes to be recruited to these structures.

Animals↗
PubMed1997

Molecular cloning and mRNA expression of the bovine insulin-responsive glucose transporter (GLUT4).

Insulin-response glucose transporter GLUT4 is a member of the glucose transporter family (GLUT) and is present exclusively in muscle and adipose tissue. It is a target of insulin action in humans and rodents. To clarify the molecular structure of bovine GLUT4, its GLUT4 cDNA was cloned by the RT-PCR method. Several cDNA clones corresponding to the different regions of GLUT4 were obtained by amplifying reverse-transcriptase products of RNA extracted from Holstein cattle skeletal muscle. Nucleotide sequence analysis of the cDNA clones revealed that bovine GLUT4 cDNA was composed of 2,656 base pairs with a coding region for a 509 amino acid protein. The deduced amino acid sequence was 64% and 92% identical with bovine GLUT1 (GLUT ubiquitously expressed in all tissues) and rat GLUT4, respectively. Although the amino acid sequence of the GLUT4 COOH-terminal region is highly conserved among the species so far reported, one amino acid (Asp) of the region was replaced by His in bovine GLUT4. The tissue distribution of GLUT4 was also examined by Northern blot analysis using a probe prepared from the bovine cDNA. GLUT4 mRNA was detected in skeletal muscle, heart, and adipose tissue, but not in liver, kidney, lung, brain, or spleen. Such a distribution is essentially the same as in humans and rodents, suggesting that GLUT4 is an insulin-responsive glucose transporter in cattle.

Adipose Tissue↗
PubMed2005

[Regulation of insulin sensitivity by subcellular GLUT4 targeting].

SUMO conjugating enzyme Ubc9 has been shown to upregulate GLUT4 in muscle cells although the mechanism of action is unknown. We investigated the physiological significance of Ubc9 in GLUT4 turnover and subcellular targeting by adenovirus vector-mediated overexpression and by siRNA-mediated gene silencing of Ubc9 in 3T3-L1 adipocytes. Overexpression of Ubc9 resulted in inhibition of GLUT4 degradation and promotion of its targeting to the non-endosomal, non-TGN GLUT4 storage vesicles (GSV), leading to an up-regulation of GLUT4 expression level and insulin-responsive glucose transport. While long-term insulin stimulation caused GLUT4 down-regulation by 40-50%, which was inhibited with lysosomal inhibitors and was associated with a selective reduction in GLUT4 in GSV, overexpression of Ubc9 antagonized these long-term effects of insulin. By contrast, Ubc9 gene silencing with siRNA caused a marked decrease in the GLUT4 level, whereas overexpression of the catalytically inactive mutant of Ubc9 increased GLUT4 and insulin-stimulated glucose transport to the level comparable to that with wild-type Ubc9. These results suggest that Ubc9 up-regulates GLUT4 by inhibition of lysosomal sorting and promotes GLUT4 targeting to GSV by a mechanism unrelated to its catalytic activity. Thus, Ubc9 plays an indispensable role in the expression and maintenance of the insulin-sensitive glucose transport system in adipocytes.

Adipocytes↗
PubMed1993

Transcriptional regulation of the gene for glucose transporter GLUT4 in skeletal muscle. Effects of diabetes and fasting.

GLUT4 glucose transporter protein and mRNA levels in rat skeletal muscle are decreased with streptozotocin (STZ)-induced diabetes and increased by fasting, indicating that GLUT4 expression may be regulated at the pretranslational level. The purpose of the present study was to determine whether GLUT4 is subject to transcriptional regulation in skeletal muscle under the altered metabolic conditions of diabetes and fasting. Nuclei were isolated from red and white portions of the quadriceps and gastrocnemius/plantaris muscles of control, 7-day STZ-diabetic, and 3-day fasted rats. STZ-induced diabetes resulted in a 35% reduction in GLUT4 transcription in red skeletal muscle and thus accounted for a major portion of the corresponding 50% reduction in GLUT4 mRNA observed in red skeletal muscle. STZ-induced diabetes had no significant effect on GLUT4 transcription or mRNA in white skeletal muscle. Fasting, however, significantly increased both GLUT4 transcription (2.2-fold) and mRNA (2.9-fold) in white skeletal muscle with no change detected for either parameter in red skeletal muscle. The nearly 2-fold higher steady-state GLUT4 mRNA in red versus white skeletal muscle of control rats was not associated with any difference in basal transcription. These findings demonstrate that expression of the GLUT4 glucose transporter protein in skeletal muscle is subject to regulation in vivo at the level of transcription of the GLUT4 gene. In addition, GLUT4 transcription is regulated in a fiber type-specific manner in response to the metabolic challenges elicited by STZ-induced diabetes and fasting.

Animals↗
PubMed1993

GLUT4 phosphorylation and inhibition of glucose transport by dibutyryl cAMP.

To investigate the mechanism responsible for the inhibition of glucose transport by dibutyryl cAMP (Bt2cAMP), two different transporter isoforms (GLUT1 and GLUT4) and several GLUT1/4 chimeric transporters were expressed in Chinese hamster ovary (CHO) cells by using a Sindbis virus expression system. Bt2cAMP inhibited GLUT4-mediated 2-deoxy[3H]glucose (2DOG) uptake by 50% but was without effect on GLUT1-mediated uptake. When the subcellular distribution of GLUT4 was assessed by quantitative immunocytochemistry, neither the overall concentration of GLUT4 nor the regional distribution of GLUT-4 within the plasma membrane was found to be altered by Bt2cAMP. Thus, inhibition of 2DOG uptake by Bt2cAMP appears to be due to a decrease in transporter activity rather than a decrease in the number of transporters exposed at the plasma membrane. By using chimeric transporters, a region of GLUT4 necessary for the inhibitory effect of Bt2cAMP was localized to the last 29 amino acids in the COOH terminus. This intracellular region contains the site (Ser488) phosphorylated in vitro by cAMP-dependent protein kinase (cAdPK). Changing Ser488 to an Ala abolished phosphorylation of GLUT4; however, the inhibitory effect of Bt2cAMP on glucose transport was not diminished by this mutation. Therefore, phosphorylation of GLUT4 was not required for the inhibition. The effects of other nucleotides on GLUT4 transport activity were assessed to investigate the role of cAdPK. Uptake of 2DOG by GLUT4 was inhibited by 8-bromo-AMP, but not by 8-bromo-cAMP, suggesting that the inhibitory effect did not involve activation of cAdPK. Results consistent with this interpretation were obtained with CHO cells (line 10248), which express a cAdPK that is resistant to activation by cAMP. No difference in the concentrations of Bt2cAMP required to inhibit GLUT4-mediated transport was observed in normal CHO cells and 10248 cells. The results presented suggest that the inhibitory effects of Bt2cAMP could be mediated by direct binding of a nucleotide to GLUT4 at a site involving the intracellular COOH terminus of the transporter.

Animals↗
PubMed1993

Insulin action on the internalization of the GLUT4 glucose transporter in isolated rat adipocytes.

A novel method was developed to measure relative amounts of the GLUT4 glucose transporter on the surface of intact fat cells and to monitor the action of insulin on cell surface glucose transporters as they internalize into intracellular membranes. The method takes advantage of two predicted trypsin cleavage sites in the major exofacial loop of this transporter protein. Treatment of cyanide-poisoned rat adipocytes with 1 mg/ml trypsin at 37 degrees C for 30 min produced an immunoreactive GLUT4 protein species in subsequently isolated plasma membranes that migrated with higher mobility (apparent M(r) = 35,000) than native GLUT4 (apparent M(r) = 46,000) on SDS-polyacrylamide gel electrophoresis. This proteolyzed GLUT4 protein was absent in the intracellular low density microsomes. Insulin treatment of adipocytes for 20 min prior to sequential additions of cyanide and trypsin caused a 16-fold increase in the proteolytically cleaved GLUT4 species. Incubation of fresh fat cells with trypsin caused a rapid and progressive appearance of the proteolyzed GLUT4 species in the intracellular low density membranes as well as plasma membranes. After 5 min of trypsinization, 66% of the total cleaved GLUT4 in these cells had moved into the low density membranes. Insulin treatment markedly decreased the internalized cleaved GLUT4 to 20% of the total. These data indicate the following: 1) trypsinization of the GLUT4 transporter protein on intact fat cells is a convenient means to monitor the extent of transporter recruitment to the plasma membrane by insulin, as well as to estimate GLUT4 internalization rates; and 2) the action of insulin on glucose transporter redistribution to the cell surface is associated with a marked inhibition of the fraction of cell surface GLUT4 transporters internalized per unit time.

Adipose Tissue↗
PubMed2000

Localization of exercise- and denervation-responsive elements in the mouse GLUT4 gene.

Exercise training increases the expression of GLUT4 in skeletal muscle. Previous studies demonstrated that the exercise-responsive element(s) of the murine GLUT4 gene are located between bases -1001 and -442 relative to the transcription start site. To further characterize the regulatory elements in the GLUT4 gene, the regulation of GLUT4 minigenes containing -701, -551, -442, or -423 bp of the 5'-flanking region was studied in transgenic mice. All minigenes studied showed significant expression in skeletal muscle and heart, including the -423 GLUT4 minigene that lacked the myocyte enhancer factor 2 (MEF2)-binding domain (-CTAAAAATAG-) located between bases -437 and -428. The -701- and -551-bp constructs were expressed in brown adipose tissues while the -442 and -423 constructs were not. In skeletal muscle, either swimming or treadmill running up-regulated GLUT4 minigene mRNA levels in -701 and -551 transgenic mice, but not in the -442 and -423 transgenic mice. Denervation of the gastrocnemius muscle by sectioning of the sciatic nerve down-regulated minigene and endogenous GLUT4 mRNAs in all -701, -551, -442, and -423 transgenic mice. These data indicate that exercise-responsive element(s) and brown adipocyte specific element(s) are located within 109 bp between bases -551 and -442 of the GLUT4 gene, but that the cis-element for denervation-induced down-regulation of the GLUT4 gene is located downstream of base -423. Finally, the MEF2 binding site between bases -437 and -428 is not necessary for expression of GLUT4 in skeletal muscles and heart; the cis-element mediating this effect is also located downstream of base -423.

5' Untranslated Regions↗
PubMed2001

GLUT4 ablation in mice results in redistribution of IRAP to the plasma membrane.

Glucose transporter (GLUT) 4 is the insulin responsive glucose transporter in adipose tissue, skeletal muscle, and heart. Insulin elicits increased glucose uptake by recruiting GLUT4 from a specialized intracellular storage site to the cell surface. Expression of various proteins that colocalize with GLUT4 and/or are involved in insulin-stimulated GLUT4 translocation was examined in adipocytes as well as skeletal and cardiac muscles from GLUT4 null mice. Our data demonstrate that expression of insulin-regulated aminopeptidase (IRAP) is divergently regulated in GLUT4 null tissues, e.g., upregulated 1.6-fold in GLUT4 null adipocytes and downregulated in GLUT4 null skeletal muscle (40%) and heart (60%). IRAP exhibited abnormal subcellular distribution and impaired insulin-stimulated translocation in GLUT4-deficient tissues. We propose the compartment containing IRAP and proteins normally associated with GLUT4 vesicle traffics constitutively to the cell surface in GLUT4 null adipocytes and skeletal muscle.

Adaptor Proteins, Vesicular Transport↗
PubMed1995

Marked depletion of GLUT4 glucose transporters in transverse tubules of skeletal muscle from streptozotocin-induced diabetic rats.

The principal goal of the present study was to determine the subcellular content of GLUT4 in diabetic rat muscle, and to test the hypothesis that a reduced abundance of the transporter protein in transverse tubules is responsible for impaired glucose utilization in that tissue. GLUT4 protein levels were measured in hindlimb muscle homogenates as well as in subcellular membrane fractions enriched with either plasma membranes, transverse tubules, or GLUT4-containing intracellular membranes from control and diabetic (streptozotocin-induced) rats. GLUT4 protein contents in diabetic muscle homogenates was reduced by 30% as compared to control rats. Subcellular fractionation experiments revealed that GLUT4 contents in transverse tubules-enriched fractions was markedly decreased (by 55-60%) in skeletal muscle of diabetic animals whereas no significant reductions in GLUT4 abundance was observed in the plasma membrane fraction. Moreover, GLUT4 was markedly depleted (by 45%) in the GLUT4-enriched intracellular membrane fraction. These results indicate that GLUT4 is markedly depleted in both the intracellular pool and in the cell surface membranes in muscle of STZ-diabetic rats. Most strikingly, this study demonstrates that transverse tubules and not the plasma membrane are the main sites of cell surface GLUT4 depletion in diabetic muscle.

Animals↗
PubMed1992

Time-dependent regulation of rat adipose tissue glucose transporter (GLUT4) mRNA and protein by insulin in streptozocin-diabetic and normal rats.

Streptozocin (STZ) administration (125 mg/kg) to normal rats resulted in a rapid (24-hour) decrease in circulating insulin levels, marked hyperglycemia, and weight loss. Adipose tissue glucose transporter 4 (GLUT4) mRNA levels decreased approximately eightfold, whereas GLUT4 protein levels were unchanged. However, GLUT4 protein levels decreased approximately 30% by 48 hours and fivefold by 72 hours of insulin deficiency. Although GLUT4 mRNA levels were rapidly restored by insulin therapy (twofold above control levels within 12 hours), GLUT4 protein levels increased only gradually, reaching peak values of 1.5-fold control levels following 7 to 10 days of insulin treatment. Insulin treatment in normal rats increased adipose GLUT4 mRNA levels nearly 100% by 24 hours, while GLUT4 protein levels increased in a more gradual fashion. The delay in GLUT4 protein induction relative to its mRNA was shorter in normal rats treated with insulin than in insulin-treated diabetic rats. These data demonstrate that insulin-induced changes in adipose GLUT4 protein are considerably delayed relative to its mRNA, and that the diabetic state enhances this difference. The known in vivo time-dependent effects of insulin treatment on adipocyte glucose transport activity can be at least partly explained by altered specific expression of GLUT4 protein.

Animals↗
PubMed1996

Insulin-sensitive glucose transporter transcript levels in calf muscles assessed with a bovine GLUT4 cDNA fragment.

Previous studies have shown that the expression of the insulin-sensitive glucose transporter (GLUT4) is lower in oxidative muscles than in glycolytic muscles in bovines and goats in contrast to observations in rats. Additional experiments in this work provide very strong arguments that the immunoreactive band detected does represent GLUT4 protein, which further validates our previous results. Therefore, to determine the level of regulation, the main objective of the present study was to measure GLUT4 mRNA amounts in various bovine muscles. A 241-bp fragment of the bovine GLUT4 cDNA was cloned by polymerase chain reaction (PCR). It shares 80-90% sequence identity with related sequences in other species. This PCR-amplified bovine GLUT4 probe was used to determine the distribution of GLUT4 mRNA in bovine tissues in comparison with that of GLUT1 mRNA. Moreover, GLUT4 mRNA amounts were quantified by Northern-blot analysis in heart and seven skeletal muscles with various oxidative and glycolytic activities from seven ruminant calves. GLUT4 mRNA was detected by Northern-blot analysis only in calf insulin-sensitive tissues. In contrast, GLUT1 mRNA was detected in all tissues studied except liver. GLUT4 mRNA amount was the highest in masseter and heart, which are oxidative muscles (1.67 +/- 0.16 and 1.53 +/- 0.19 units/g wet tissue weight, respectively) and the lowest in glycolytic or oxido-glycolytic muscles (0.31 +/- 0.04 to 1.00 +/- 0.09 units/g wet tissue weight; SEM, n = 7). These data and our previous results provide evidence for translational and/or post-translational control mechanisms of bovine GLUT4 protein expression in a muscle type-specific manner.

Animals↗