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PubMed1995

Insulin-dependent regulation of Glut4 gene expression in ventricular cardiomyocytes: evidence for a direct effect on Glut4 transcription.

The present study examined the effect of insulin on Glut4 transcription in isolated ventricular cardiomyocytes. Upon exposure to the hormone (2 x 10(-7) M) the cellular Glut4 mRNA content increased to 224 +/- 46% of control within 3.5 hours. Nuclear run-on analysis indicated a parallel increase of transcription of the Glut4 gene to 208 +/- 49% of control. Direct incubation of cardiac nuclei with insulin resulted in a comparably significant increase of Glut4 transcription. Cross-linkage experiments with 125I-labelled insulin demonstrated the absence of soluble nuclear insulin receptors, which were readily detected in plasma and microsomal membranes. These findings suggest that expression of the cardiac Glut4 gene is subject to regulation by insulin at the transcriptional level, a process possibly involving nuclear association of the hormone.

Animals↗
PubMed1996

Cleavage of vesicle-associated membrane protein (VAMP)-2 and cellubrevin on GLUT4-containing vesicles inhibits the translocation of GLUT4 in 3T3-L1 adipocytes.

We have identified VAMP isoforms, VAMP-2 and cellubrevin, on GLUT4-containing vesicle membranes isolated from 3T3-Ll adipocytes. These proteins translocate from a low density microsomal fraction to the plasma membrane upon insulin stimulation in a fashion similar to GLUT4. VAMP-1 was not detected in this low density microsomal fraction nor on purified GLUT4-containing vesicles. In streptolysin-O permeabilized 3T3-L1 adipocytes, both VAMP-2 and cellubrevin were cleaved with botulinum neurotoxin isoform B, BoNTx/B. In addition, BoNTx/B partially inhibited insulin-stimulated GLUT4 translocation and glucose transport activity. We conclude that the synaptobrevin isoforms are important components of the insulin-dependent translocation of GLUT4 to the cell surface in adipocytes.

3T3 Cells↗
PubMed1997

A myosin-derived peptide C109 binds to GLUT4-vesicles and inhibits the insulin-induced glucose transport stimulation and GLUT4 recruitment in rat adipocytes.

The yeast-based two-hybrid screening of a human cardiac myocyte cDNA library revealed a peptide, C109 that interacted with the C-terminal cytoplasmic domain of GLUT4 (GLUT4C). cDNA-deduced amino acid sequence of C109 was identical to the human cardiac muscle myosin heavy chain beta isoform sequence 1469-1909. GST-fusion protein of C109 (GST-C109) bound synthetic GLUT4C-peptide in vitro, but not GLUT1C-peptide. GST-C109 avidly bound to the GLUT4-vesicles isolated from basal rat adipocytes but not those isolated from insulin treated adipocytes. Furthermore, the incorporation of C109 into rat adipocytes greatly reduced the plasma membrane GLUT4 level and the 3-O-methyl D glucose flux in host cells without affecting total cellular GLUT4 content. These findings suggest that myosin or a myosin-like protein plays a key role in insulin-regulated movement of GLUT4 to the plasma membrane in rat adipocytes.

Adipocytes↗
PubMed1999

Fluoromicroscopic detection of myc-tagged GLUT4 on the cell surface. Co-localization of the translocated GLUT4 with rearranged actin by insulin treatment in CHO cells and L6 myotubes.

We earlier developed a novel method to detect translocation of glucose transporter type 4 (GLUT 4) directly, quantitatively and simply using c-MYC epitope-tagged GLUT4 (GLUT4myc) Kanai F, Nishioka Y, Hayashi H, Kamohara S, Todaka M, Ebina Y: J Biol Chem 268: 14523-14526, 1993). We further developed the method to visualize GLUT4myc on the cell surface++ by fluorescence microscope using a highly sensitive immunochemical detection system in tissue culture cells stably expressing GLUT4myc. The translocation of GLUT4myc was observed on stimulation with insulin in 3T3-L1 adipocytes, CHO cells and L6 myotubes stably expressing GLUT4myc. Platelet-derived growth factor (PDGF), norepinephrine and bradykinin also triggered GLUT4 translocation in CHO-GLUT4myc cells stably expressing each receptor. To observe the distribution of GLUT4 and actin after insulin treatment, double staining for GLUT4myc and actin was performed. Translocated GLUT4myc on the cell surface was co-localized with rearranged actin in CHO cells and L6 myotubes. This result suggests that a correlation exists between GLUT4 translocation and actin rearrangement.

3T3 Cells↗
PubMed1993

Direct demonstration of insulin-induced GLUT4 translocation to the surface of intact cells by insertion of a c-myc epitope into an exofacial GLUT4 domain.

Stimulation of glucose transport is the main physiological effect of insulin in target tissues. This effect is linked to translocation of the GLUT4 glucose transporter from an intracellular pool to the cell surface. To elucidate the molecular mechanisms involved in this effect, we developed a simple direct sensitive method to detect GLUT4 immunologically on the cell surface. cDNA containing GLUT4 inserted by a c-myc epitope in the first ectodomain (GLUT4myc) was constructed without disrupting the functions of GLUT4 and was expressed in 3T3-L1 and Chinese hamster ovary fibroblast cells. In response to insulin, the GLUT4myc expressed in 3T3-L1 adipocytes was translocated to the cell surface from the intracellular pool, as shown by assays of exofacial antibody binding against the myc epitope and of the uptake of 2-deoxyglucose. Insulin, guanosine 5'-O-(3-thiotriphosphate), guanylyl imidodiphosphate, NaF, and phorbol 12-myristate 13-acetate also induced the translocation of GLUT4myc in Chinese hamster ovary cells coexpressing the human insulin receptor.

3T3 Cells↗
PubMed2003

Snares for GLUT4--mechanisms directing vesicular trafficking of GLUT4.

Fusion of GLUT4 vesicles with the plasma membrane is a key terminal step in insulin-regulated glucose transport. This fusion event is mediated by SNARE proteins, syntaxin 4, SNAP23 and VAMP2, through a process regulated by accessory proteins whose roles are still unclear. Munc18c is a key regulatory protein of syntaxin, and possibly of whole-SNARE complex cycling. Rab GTPases, through interactions with tethering molecules and SNAREs, provide targeting specificity in the GLUT4 secretory pathway. We review here insulin-mediated signaling of glucose transport, and particularly the role played by SNAREs and their accessory proteins.

Animals↗
PubMed2005

GLUT4 facilitative glucose transporter specifically and differentially contributes to agonist-induced vascular reactivity in mouse aorta.

OBJECTIVE: We hypothesized that GLUT4 is a predominant facilitative glucose transporter in vascular smooth muscle cells (VSMCs), and GLUT4 is necessary for agonist-induced VSMC contraction. METHODS AND RESULTS: Glucose deprivation and indinavir, a GLUT4 antagonist, were used to assess the role of GLUT4 and non-GLUT4 transporters in vascular reactivity. In isolated endothelium-denuded mouse aorta, approximately 50% of basal glucose uptake was GLUT4-dependent. Norepinephrine-mediated contractions were dependent on both GLUT4 and non-GLUT4 transporters, serotonin (5-HT)-mediated contractions were mainly GLUT4-dependent, and prostaglandin (PG) F(2alpha)-mediated contractions were dependent on non-GLUT4 transporters, whereas indinavir had no effect in GLUT4 knockout vessels. We also observed a 46% decrease in GLUT4 expression in aortas from angiotensin II hypertensive mice. Indinavir caused a less profound attenuation of maximal 5-HT-mediated contraction in these vessels, corresponding to the lower GLUT4 levels in the hypertensive aortas. Finally, and somewhat surprisingly, chronic GLUT4 knockout was associated with increased vascular reactivity compared with that in wild-type animals, suggesting that chronic absence or reduction of GLUT4 expression in VSMCs leads to opposite effects observed with acute inhibition of GLUT4. CONCLUSIONS: Thus, we conclude that GLUT4 is constitutively expressed in large arteries and likely participates in basal glucose uptake. In addition, GLUT4, as well as other non-GLUT4 facilitative glucose transporters, are necessary for agonist-induced contraction, but each transporter participates in VSMC contraction selectively, depending on the agonist, and changes in GLUT4 expression may account for some of the functional changes associated with vascular diseases like hypertension.

Angiotensin II↗
PubMed2004

The transcription factor CCAAT/enhancer-binding protein alpha is required for the intracellular retention of GLUT4.

Insulin modulates glucose uptake into adipocytes by regulating the trafficking of the GLUT4 glucose transporter. GLUT4 is mostly excluded from the surface of unstimulated cells because it is much more slowly exocytosed than it is endocytosed. GLUT4 traffics through an adipocyte-specific, specialized endosomal recycling pathway that only partially overlaps with compartments of the general endosomal recycling pathway. Insulin stimulates GLUT4 exocytosis and partially inhibits its endocytosis, resulting in GLUT4 redistribution to the cell surface. Insulin does not stimulate glucose uptake into adipocytes lacking the CCAAT/enhancer-binding protein alpha (C/EBPalpha) transcription factor. Here we show that these adipocytes do not properly traffic GLUT4. In these adipocytes, GLUT4 was rapidly exocytosed in basal conditions, resulting in an accumulation of GLUT4 on the plasma membrane. Although the kinetics of GLUT4 trafficking were altered, GLUT4 was still targeted to specialized intracellular compartments in adipocytes lacking C/EBPalpha, demonstrating an uncoupling of the targeting of GLUT4 to a specialized, adipocyte-specific insulin-regulated pathway from the regulation of the movement of GLUT4 through this pathway. Re-expression of C/EBPalpha in adipocytes lacking C/EBPalpha restored normal GLUT4 trafficking. We propose that C/EBPalpha controls the expression of the proteins that determine the basal, slow exocytosis of GLUT4, but not the proteins required to make the adipocyte-specific compartments through which GLUT4 traffics. Furthermore, these data support a model in which insulin stimulates GLUT4 exocytosis by releasing an inhibitor of GLUT4 movement to the cell surface, and it is this clamp on basal exocytosis that is missing in adipocytes lacking C/EBPalpha.

3T3-L1 Cells↗
PubMed1995

Insulin-sensitive targeting of the GLUT4 glucose transporter in L6 myoblasts is conferred by its COOH-terminal cytoplasmic tail.

The GLUT4 glucose transporter appears to be targeted to a unique insulin-sensitive intracellular membrane compartment in fat and muscle cells. Insulin stimulates glucose transport in these cell types by mediating the partial redistribution of GLUT4 from this intracellular compartment to the plasma membrane. The structural basis for the unique targeting behavior of GLUT4 was investigated in the insulin-sensitive L6 myoblast cell line. Analysis of immunogold-labeled cells of independent clonal lines by electron microscopy indicated that 51-53% of GLUT1 was present in the plasma membrane in the basal state. Insulin did not significantly affect this distribution. In contrast, only 4.2-6.1% of GLUT4 was present in the plasma membrane of basal L6 cells and insulin increased this percentage by 3.7-6.1-fold. Under basal conditions and after insulin treatment, GLUT4 was detected in tubulovesicular structures, often clustered near Golgi stacks, and in endosome-like vesicles. Analysis of 25 chimeric transporters consisting of reciprocal domains of GLUT1 and GLUT4 by confocal immunofluorescence microscopy indicated that only the final 25 amino acids of the COOH-terminal cytoplasmic tail of GLUT4 were both necessary and sufficient for the targeting pattern observed for GLUT4. A dileucine motif present in the COOH-terminal tail of GLUT4 was found to be necessary, but not sufficient, for intracellular targeting. Contrary to previous studies, the NH2 terminus of GLUT4 did not affect the subcellular distribution of chimeras. Analysis of a chimera containing the COOH-terminal tail of GLUT4 by immunogold electron microscopy indicated that its subcellular distribution in basal cells was very similar to that of wild-type GLUT4 and that its content in the plasma membrane increased 6.8-10.5-fold in the presence of insulin. Furthermore, only the chimera containing the COOH terminus of GLUT4 enhanced insulin responsive 2-deoxyglucose uptake. GLUT1 and two other chimeras lacking the COOH terminus of GLUT4 were studied by immunogold electron microscopy and did not demonstrate insulin-mediated changes in subcellular distribution. The NH2-terminal cytoplasmic tail of GLUT4 did not confer intracellular sequestration and did not cause altered subcellular distribution in the presence of insulin. Intracellular targeting of one chimera to non-insulin-sensitive compartments was also observed. We conclude that the COOH terminus of GLUT4 is both necessary and sufficient to confer insulin-sensitive subcellular targeting of chimeric glucose transporters in L6 myoblasts.

Amino Acid Sequence↗
PubMed1997

Muscle-specific transgenic complementation of GLUT4-deficient mice. Effects on glucose but not lipid metabolism.

We have taken the approach of introducing the muscle-specific myosin light chain (MLC)-GLUT4 transgene into the GLUT4-null background to assess the relative role of muscle and adipose tissue GLUT4 in the etiology of the GLUT4-null phenotype. The resulting MLC-GLUT4-null mice express GLUT4 predominantly in the fast-twitch extensor digitorum longus (EDL) muscle. GLUT4 is nearly absent in female white adipose tissue (WAT) and slow-twitch soleus muscle of both sexes of MLC-GLUT4-null mice. GLUT4 content in male MLC-GLUT4-null WAT is 20% of that in control mice. In transgenically complemented EDL muscle, 2-deoxyglucose (2-DOG) uptake was restored to normal (male) or above normal (female) levels. In contrast, 2-DOG uptake in slow-twitch soleus muscle of MLC-GLUT4-null mice was not normalized. With the normalization of glucose uptake in fast-twitch skeletal muscle, whole body insulin action was restored in MLC-GLUT4-null mice, as shown by the results of the insulin tolerance test. These results demonstrate that skeletal muscle GLUT4 is a major regulator of skeletal muscle and whole body glucose metabolism. Despite normal skeletal muscle glucose uptake and insulin action, the MLC-GLUT4-null mice exhibited decreased adipose tissue deposits, adipocyte size, and fed plasma FFA levels that are characteristic of GLUT4-null mice. Together these results indicate that the defects in skeletal muscle and whole body glucose metabolism and adipose tissue in GLUT4-null mice arise independently.

Animals↗
PubMed2001

GLUT4 overexpression in db/db mice dose-dependently ameliorates diabetes but is not a lifelong cure.

We previously reported that overexpression of GLUT4 in lean, nondiabetic C57BL/KsJ-lepr(db/+) (db/+) mice resulted in improved glucose tolerance associated with increased basal and insulin-stimulated glucose transport in isolated skeletal muscle. We used the diabetic (db/db) litter mates of these mice to examine the effects of GLUT4 overexpression on in vivo glucose utilization and on in vitro glucose transport and GLUT4 translocation in diabetic mice. We examined in vivo glucose disposal by oral glucose challenge and hyperinsulinemic-hyperglycemic clamps. We also evaluated the in vitro relationship between glucose transport activity and cell surface GLUT4 levels as assessed by photolabeling with the membrane-impermeant reagent 2-N-(4-(1-azi-2,2,2-trifluoroethyl)benzoyl)-1,3-bis(D-mannose-4-yloxy)-2-propylamine in extensor digitorum longus (EDL) muscles. All parameters were examined as functions of animal age and the level of GLUT4 overexpression. In young mice (age 10-12 weeks), both lower (two- to threefold) and higher (four- to fivefold) levels of GLUT4 overexpression were associated with improved glucose tolerance compared to age-matched nontransgenic (NTG) mice. However, glucose tolerance deteriorated with age in db/db mice, although less rapidly in transgenic mice expressing the higher level of GLUT4. Glucose infusion rates during hyperinsulinemic-hyperglycemic clamps were increased with GLUT4 overexpression, compared with NTG mice in both lower and higher levels of GLUT4 overexpression, even in the older mice. Surprisingly, isolated EDL muscles from diabetic db/db mice did not exhibit alterations in either basal or insulin-stimulated glucose transport activity or cell surface GLUT4 compared to nondiabetic db/+ mice. Furthermore, both GLUT4 overexpression levels and animal age are associated with increased basal and insulin-stimulated glucose transport activities and cell surface GLUT4. However, the observed increased glucose transport activity in older db/db mice was not accompanied by an equivalent increase in cell surface GLUT4 compared to younger animals. Thus, although in vivo glucose tolerance is improved with GLUT4 overexpression in young animals, it deteriorates with age; in contrast, insulin responsiveness as assessed by the clamp technique remains improved with GLUT4 overexpression, as does in vitro insulin action. In summary, despite an impairment in whole-body glucose tolerance, skeletal muscle of the old transgenic GLUT4 db/db mice is still insulin responsive in vitro and in vivo.

Animals↗
PubMed2005

GLUT4 distribution between the plasma membrane and the intracellular compartments is maintained by an insulin-modulated bipartite dynamic mechanism.

The GLUT4 glucose transporter is predominantly retained inside basal fat and muscle cells, and it is rapidly recruited to the plasma membrane with insulin stimulation. There is controversy regarding the mechanism of basal GLUT4 retention. One model is that GLUT4 retention is dynamic, based on slow exocytosis and rapid internalization of the entire pool of GLUT4 (Karylowski, O., Zeigerer, A., Cohen, A., and McGraw, T. E. (2004) Mol. Biol. Cell 15, 870-882). In this model, insulin increases GLUT4 in the plasma membrane by modulating GLUT4 exocytosis and endocytosis. The second model is that GLUT4 retention is static, with approximately 90% of GLUT4 stored in compartments that are not in equilibrium with the cell surface in basal conditions (Govers, R., Coster, A. C., and James, D. E. (2004) Mol. Cell Biol. 24, 6456-6466). In this model, insulin increases GLUT4 in the plasma membrane by releasing it from the static storage compartment. Here we show that under all experimental conditions examined, basal GLUT4 retention is by a bipartite dynamic mechanism involving slow efflux and rapid internalization. To establish that the dynamic model developed in studies of the extreme conditions of >100 nm insulin and no insulin also describes GLUT4 behavior at more physiological insulin concentrations, we characterized GLUT4 trafficking in 0.5 nm insulin. This submaximal insulin concentration promotes an intermediate effect on both GLUT4 exocytosis and endocytosis, resulting in an intermediate degree of redistribution to the plasma membrane. These data establish that changes in the steady-state surface/total distributions of GLUT4 are the result of gradated, insulin-induced changes in GLUT4 exocytosis and endocytosis rates.

3T3-L1 Cells↗
PubMed1991

Intracellular targeting of the insulin-regulatable glucose transporter (GLUT4) is isoform specific and independent of cell type.

Insulin stimulates glucose transport in adipocytes via the rapid redistribution of the GLUT1 and GLUT4 glucose transporters from intracellular membrane compartments to the cell surface. Insulin sensitivity is dependent on the proper intracellular trafficking of the glucose transporters in the basal state. The bulk of insulin-sensitive transport in adipocytes appears to be due to the translocation of GLUT4, which is more efficiently sequestered inside the cell and is present in much greater abundance than GLUT1. The cell type and isoform specificity of GLUT4 intracellular targeting were investigated by examining the subcellular distribution of GLUT1 and GLUT4 in cell types that are refractory to the effect of insulin on glucose transport. Rat GLUT4 was expressed in 3T3-L1 fibroblasts and HepG2 hepatoma cells by DNA-mediated transfection. Transfected 3T3-L1 fibroblasts over-expressing human GLUT1 exhibited increased glucose transport, and laser confocal immunofluorescent imaging of GLUT1 in these cells indicated that the protein was concentrated in the plasma membrane. In contrast, 3T3-L1 fibroblasts expressing GLUT4 exhibited no increase in transport activity, and confocal imaging demonstrated that this protein was targeted almost exclusively to cytoplasmic compartments. 3T3-L1 fibroblasts expressing GLUT4 were unresponsive to insulin with respect to transport activity, and no change was observed in the subcellular distribution of the protein after insulin administration. Immunogold labeling of frozen ultrathin sections revealed that GLUT4 was concentrated in tubulo-vesicular elements of the trans-Golgi reticulum in these cells. Sucrose density gradient analysis of 3T3-L1 homogenates was consistent with the presence of GLUT1 and GLUT4 in discrete cytoplasmic compartments. Immunogold labeling of frozen thin sections of HepG2 cells indicated that endogenous GLUT1 was heavily concentrated in the plasma membrane. Sucrose density gradient analysis of homogenates of HepG2 cells expressing rat GLUT4 suggested that GLUT4 is targeted to an intracellular location in these cells. The density of the putative GLUT4-containing cytoplasmic membrane vesicles was very similar in HepG2 cells, 3T3-L1 fibroblasts, 3T3-L1 adipocytes, and rat adipocytes. These data indicate that the intracellular trafficking of GLUT4 is isoform specific. Additionally, these observations support the notion that GLUT4 is targeted to its proper intracellular locale even in cell types that do not exhibit insulin-responsive glucose transport, and suggest that the machinery that regulates the intracellular targeting of GLUT4 is distinct from the factors that regulate insulin-dependent recruitment to the cell surface.

Adipose Tissue↗
PubMed1998

GLUT4 and transferrin receptor are differentially sorted along the endocytic pathway in CHO cells.

The trafficking of GLUT4, a facilitative glucose transporter, is examined in transfected CHO cells. In previous work, we expressed GLUT4 in neuroendocrine cells and fibroblasts and found that it was targeted to a population of small vesicles slightly larger than synaptic vesicles (Herman, G.A, F. Bonzelius, A.M. Cieutat, and R.B. Kelly. 1994. Proc. Natl. Acad. Sci. USA. 91: 12750-12754.). In this study, we demonstrate that at 37 degrees C, GLUT4-containing small vesicles (GSVs) are detected after cell surface radiolabeling of GLUT4 whereas uptake of radioiodinated human transferrin does not show appreciable accumulation within these small vesicles. Immunofluorescence microscopy experiments show that at 37 degrees C, cell surface-labeled GLUT4 as well as transferrin is internalized into peripheral and perinuclear structures. At 15 degrees C, endocytosis of GLUT4 continues to occur at a slowed rate, but whereas fluorescently labeled GLUT4 is seen to accumulate within large peripheral endosomes, no perinuclear structures are labeled, and no radiolabeled GSVs are detectable. Shifting cells to 37 degrees C after accumulating labeled GLUT4 at 15 degrees C results in the reappearance of GLUT4 in perinuclear structures and GSV reformation. Cytosol acidification or treatment with hypertonic media containing sucrose prevents the exit of GLUT4 from peripheral endosomes as well as GSV formation, suggesting that coat proteins may be involved in the endocytic trafficking of GLUT4. In contrast, at 15 degrees C, transferrin continues to traffic to perinuclear structures and overall labels structures similar in distribution to those observed at 37 degrees C. Furthermore, treatment with hypertonic media has no apparent effect on transferrin trafficking from peripheral endosomes. Double-labeling experiments after the internalization of both transferrin and surface-labeled GLUT4 show that GLUT4 accumulates within peripheral compartments that exclude the transferrin receptor (TfR) at both 15 degrees and 37 degrees C. Thus, GLUT4 is sorted differently from the transferrin receptor as evidenced by the targeting of each protein to distinct early endosomal compartments and by the formation of GSVs. These results suggest that the sorting of GLUT4 from TfR may occur primarily at the level of the plasma membrane into distinct endosomes and that the organization of the endocytic system in CHO cells more closely resembles that of neuroendocrine cells than previously appreciated.

Animals↗
PubMed2003

Activity, phosphorylation state and subcellular distribution of GLUT4-targeted Akt2 in rat adipose cells.

In this study, fusion of the kinase domain of Akt2 to the cytosolic C terminus of exofacially-HA-tagged GLUT4 is used to investigate the activity, phosphorylation state and subcellular localization of Akt2 specifically targeted to the GLUT4-trafficking pathway in rat adipose cells. Fusion of wild-type (wt) Akt2, but not a kinase-dead (KD) mutant results in constitutive targeting of the HA-GLUT4 fusion protein to the cell surface to a level similar to that of HA-GLUT4 itself in the insulin-stimulated state. Insulin does not further enhance the cell-surface level of HA-GLUT4-Akt2-wt, but does stimulate the translocation of HA-GLUT4-Akt2-KD. Cell-surface HA-GLUT4-Akt2-wt is found to be phosphorylated on Ser474 in both the absence and presence of insulin, and mutation of Ser474 to Ala reduces the increased basal cell-surface localization of the fusion protein. While Ser474 phosphorylation of HA-GLUT4-Akt2-KD is detected only in the insulin-stimulated state, trapping this fusion protein on the cell surface by coexpression of a dominant negative mutant dynamin does not induce Ser474 phosphorylation. Phosphorylation on Thr309 is not detectable in either HA-GLUT4-Akt2-wt or HA-GLUT4-Akt2-KD, in either the basal or insulin-stimulated state, and mutation of Thr309 to Ala does not influence the insulin-independent increases in cell-surface localization and Ser474 phosphorylation. Expression of HA-GLUT4-Akt2-wt stimulates the translocation of cotransfected myc-GLUT4 to a level similar to that in the insulin-stimulated state; this increase is moderately reduced by mutation of Ser474 to Ala and absent with the kinase-dead mutant. These results demonstrate that targeting Akt2 to the GLUT4-trafficking pathway induces Akt2 activation and GLUT4 translocation. Ser474 phosphorylation is an autocatalytic reaction requiring an active kinase, and kinase activity is associated with a plasma membrane localization. Fusion of Akt2 to the C terminus of GLUT4 appears to substitute for Thr309 phosphorylation in activating the autocatalytic process.

Adipocytes↗
PubMed1993

Multiple defects in the adipocyte glucose transport system cause cellular insulin resistance in gestational diabetes. Heterogeneity in the number and a novel abnormality in subcellular localization of GLUT4 glucose transporters.

Mechanisms causing cellular insulin resistance in gestational diabetes mellitus are not known. We, therefore, studied isolated omental adipocytes obtained during elective cesarean sections in nondiabetic (control) and GDM gravidas. Cellular insulin resistance was attributed to impaired stimulation of glucose transport; compared with control subjects, basal and maximally insulin-stimulated transport rates (per surface area) were reduced 38 and 60% in GDM patients, respectively. To determine underlying mechanisms, we assessed the number, subcellular distribution, and translocation of GLUT4, the predominant insulin-responsive glucose transporter isoform. The cellular content of GLUT4 was decreased by 44% in GDM patients as assessed by immunoblot analysis of total postnuclear membranes. However, GDM patients segregated into two subgroups; half expected profound (76%) cellular depletion of GLUT4 and half had GLUT4 levels in the normal range. Cellular GLUT4 was negatively correlated with adipocyte size in the control subjects and GDM patients with normal GLUT4 (r = 0.60), but fell way below this continuum in GDM patients with low GLUT4, indicating that heterogeneity was not caused by differences in obesity. All GDM. distribution. In basal cells, increased amounts of GLUT4 were detected in membranes fractionating with (such that the plasma membrane GLUT4 level in GDM (such that the plasma membrane GLUT4 level in GDM patients was equal to that observed in insulin-stimulated cells from control subjects). Furthermore, insulin stimulation induced translocation of GLUT4 from low-density microsomes to plasma membranes in control subjects but did not alter subcellular distribution in GDM patients. In other experiments, cellular content of GLUT1 was normal in GDM patients, and GLUT1 did not undergo insulin-mediated recruitment to plasma membranes in either control subjects or GDM patients. A faint signal was detected for GLUT3 only in low-density microsomes and only with one of two different antibodies. In GDM, we conclude that insulin resistance in adipocytes involves impaired stimulation of glucose transport and arises from a heterogeneity of defects intrinsic to the glucose transport effector system. GLUT4 content in adipocytes is profoundly depleted in approximately 50% of GDM patients, whereas all patients are found to exhibit a novel abnormality in GLUT4 subcellular distribution. This latter defect is characterized by accumulation of GLUT4 in membranes cofractionating with plasma membranes and high-density microsomes in basal cells and absence of translocation in response to insulin. The data suggest that abnormalities in cellular traffic or targeting relegate GLUT4 to a membrane compartment from which insulin cannot recruit transporters to the cell surface and have important implications regarding skeletal muscle insulin resistance in GDM and NIDDM.

Adipocytes↗
PubMed1995

Cardiac and adipose tissue abnormalities but not diabetes in mice deficient in GLUT4.

The insulin-sensitive glucose transporter, GLUT4, is the most abundant facilitative glucose transporter in muscle and adipose tissue, the major sites for postprandial glucose disposal. To assess the role of GLUT4 in glucose homeostasis, we have disrupted the murine GLUT4 gene. Because GLUT4 has been shown to be dysregulated in pathological states such as diabetes and obesity, it was expected that genetic ablation of GLUT4 would result in abnormal glucose homeostasis. The mice deficient in GLUT4 (GLUT4-null) are growth-retarded and exhibit decreased longevity associated with cardiac hypertrophy and severely reduced adipose tissue deposits. Blood glucose levels in female GLUT4-null mice are not significantly elevated in either the fasting or fed state; in contrast, male GLUT4-null mice have moderately reduced glycaemias in the fasted state and increased glycaemias in the fed state. However, both female and male GLUT4-null mice exhibit postprandial hyperinsulinaemia, indicating possible insulin resistance. Increased expression of other glucose transporters is observed in the liver (GLUT2) and heart (GLUT1) but not skeletal muscle. Oral glucose tolerance tests show that both female and male GLUT4-null mice clear glucose as efficiently as controls, but insulin tolerance tests indicate that these mice are less sensitive to insulin action. The GLUT4-null mice demonstrate that functional GLUT4 protein is not required for maintaining nearly normal glycaemia but that GLUT4 is absolutely essential for sustained growth, normal cellular glucose and fat metabolism, and expected longevity.

Adipose Tissue↗