Stimulation of glucose transport by guanine nucleotides in permeabilized rat adipocytes

Insulin receptor signals regulating GLUT4 translocation and actin dynamics

In skeletal muscle and adipose tissue, insulin-stimulated glucose uptake is dependent upon translocation of the insulin-responsive glucose transporter GLUT4 from intracellular storage compartments to the plasma membrane. This insulin-induced redistribution of GLUT4 protein is achieved through a series of highly organized membrane trafficking events, orchestrated by insulin receptor signals. Recently, several key molecules linking insulin receptor signals and membrane trafficking have been identified, and emerging evidence supports the importance of subcellular compartmentalization of signaling components at the right time and in the right place. In addition, the translocation of GLUT4 in adipocytes requires insulin stimulation of dynamic actin remodeling at the inner surface of the plasma membrane (cortical actin) and in the perinuclear region. This results from at least two independent insulin receptor signals, one leading to the activation of phosphatidylinositol (PI) 3-kinase and the other to the activation of the Rho family small GTP-binding protein TC10. Thus, both spatial and temporal regulations of actin dynamics, both beneath the plasma membrane and around endomembranes, by insulin receptor signals are also involved in the process of GLUT4 translocation.

Insulin stimulates guanine nucleotide exchange on Rab4 via a wortmannin-sensitive signaling pathway in rat adipocytes

Rab4, a member of the Rab family of Ras-related small GTP-binding proteins, has been shown to be associated with GLUT4-containing vesicles and implicated in the insulin action on glucose transport in rat adipocytes. In the present study, we investigated the insulin effects on the guanine nucleotide exchange on Rab4. In electrically permeabilized rat adipocytes, the amount of [35S]guanosine 5'-O-(3-thiotrisphosphate) (GTPgammaS) bound to Rab4 increased in a time-dependent manner during 45 min of the incubation period. Addition of insulin resulted in about a 2-fold stimulation of the binding of [35S]GTPgammaS to Rab4, indicating that insulin stimulated the guanine nucleotide exchange on the GTPase. Pretreatment of the cells with wortmannin, a specific inhibitor of phosphatidylinositol 3-kinase, completely abolished the stimulatory effect of insulin on [35S]GTPgammaS binding to Rab4. Wortmannin also attenuated the nucleotide binding to Rab4 in the basal cells, suggesting that phosphatidylinositol 3-kinase activity may be essential for regulation of guanine nucleotide exchange on the GTPase and insulin may up-regulate the exchange activity by stimulating the lipid kinase. Insulin-induced subcellular redistribution of Rab4 from the microsomal fraction to the soluble fraction was also inhibited by wortmannin. These results suggest that insulin stimulates the guanine nucleotide exchange on Rab4 via a phosphatidylinositol 3-kinase-dependent signaling pathway and that Rab4 is one of possible targets of insulin action on intracellular vesicle traffic in rat adipocytes.

Overexpression of Rad inhibits glucose uptake in cultured muscle and fat cells

Rad is a Ras-like GTPase that was isolated by subtraction cloning of human muscle and shown to have increased expression in some individuals with Type II diabetes. To ascertain the potential role of Rad in insulin-mediated signaling, we have overexpressed Rad in myocyte and adipocyte cell lines. Expression of Rad resulted in a 50-90% reduction in insulin-stimulated 2-deoxyglucose glucose uptake in C2C12 murine myotubes, L6 rat myotubes, and 3T3-L1 adipocytes and a 25% reduction in 3-O-methylglucose uptake in 3T3-L1 adipocytes. This occurred despite unaltered levels of glucose transporter expression, with no detectable change in Glut4 translocation and with no alteration in insulin receptor or substrate phosphorylation or phosphatidylinositol 3-kinase activity. These data indicate that Rad is a negative regulator of glucose uptake and that this effect may be due to a decrease in the intrinsic activity of the transporter molecules, rather than an effect on the translocation of Glut4.

A synthetic peptide corresponding to the Rab4 hypervariable carboxyl-terminal domain inhibits insulin action on glucose transport in rat adipocytes

The present study was conducted to examine the involvement of Rab4, a low molecular weight GTP-binding protein, in the action of insulin on glucose transport. A synthetic peptide corresponding to the Rab4 hypervariable carboxyl-terminal domain, Rab4-(191-210), was successfully transferred into rat adipocytes by electroporation and inhibited insulin-stimulated glucose transport by about 50% without affecting the basal transport activity. In contrast, synthetic peptides corresponding to the Rab3C and Rab3D carboxyl-terminal hypervariable domain had little effect on insulin action on glucose transport. The Rab4-(191-210) peptide also reduced insulin-induced GLUT4 translocation from the intracellular pool to the plasma membrane. Furthermore, the Rab4-(191-210) peptide reduced both insulin-induced glucose transport and GLUT4 translocation in the presence of a major histocompatibility complex class I antigen-derived peptide, D(k)-(62-85), which is a potent inhibitor of GLUT4 internalization, suggesting that the peptide inhibited exocytotic recruitment of GLUT4-containing vesicles. The Rab4-(191-210) peptide also inhibited GTP gamma S-stimulated glucose transport. In addition, insulin-stimulated glucose transport was inhibited by the addition of anti-Rab4 antibody. These results suggest that Rab4 protein plays a crucial role in insulin action on GLUT4 translocation, especially in exocytotic recruitment by the hormone of the glucose transporter to the plasma membrane from the intracellular retention pool.

Ca(2+)- and GTP[gamma S]-induced translocation of the glucose transporter, GLUT-4, to the plasma membrane of permeabilized cardiomyocytes determined using a novel immunoprecipitation method

In cardiomyocytes glucose transport is activated not only by insulin but also by contractile activity that causes translocation of the glucose transporter, GLUT-4, from intracellular vesicles to the plasma membrane. The latter effect may possibly be mediated by intracellular Ca2+, as suggested by previous studies. To investigate the role of Ca2+, we permeabilized neonatal rat myocytes with alpha-toxin and incubated them for 1 h either at a pCa (i.e.--log10 [Ca2+]) of 8 (control) or at a pCa of 5 in the presence of adenosine 5'-triphosphate (ATP). Translocation of GLUT-4 was then monitored by a novel immunoprecipitation method using a peptide antibody directed against an exofacial (extracellular) loop of GLUT-4 (residues 58-80). Incorporation of GLUT-4 into the plasmalemma was stimulated 1.8-fold by 10 microM Ca2+ and 1.7-fold by insulin (as in the case of intact cells). The insulin effect was Ca2+ independent, i.e. it was identical in the absence and presence of Ca2+ (10 microM). Guanosine 5'-O-(3-thio-triphosphate) (GTP[gamma S]), which was inactive in intact cells, also caused translocation of GLUT-4 in permeabilized cardiomyocytes. Thus, incorporation of GLUT-4 into the plasma membrane was enhanced 2.5-fold by 200 microM GTP[gamma S] in the virtual absence of Ca2+ (pCa 8) and even 3.5-fold at 10 microM free Ca2+. We conclude that an increase in intracellular Ca2+ concentration increases GLUT-4 translocation of (permeabilized) cardiomyocytes to a similar extent as do insulin and GTP[gamma S] in the absence of Ca2+, but that the effects of Ca2+ and GTP[gamma S] may be additive.

Dissection of GLUT4 recycling pathway into exocytosis and endocytosis in rat adipocytes. Evidence that GTP-binding proteins are involved in both processes

The effects of guanine nucleotides on either exocytosis or endocytosis of GLUT4 were examined in electrically permeabilized rat adipocytes by using Dk-(62-85), a major histocompatibility complex class I-derived peptide. Reversal of glucose transport activity that had been stimulated with insulin was completely blocked by Dk-(62-85). Likewise, endocytosis of the trypsin-cleaved 35-kDa fragment of GLUT4 was almost completely inhibited by the peptide. Insulin-stimulated glucose transport activity was enhanced about 50% by Dk-(62-85), whereas the basal transport activity was stimulated only slightly. Although guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) augmented glucose transport to the same extent as insulin in the absence of the peptide, glucose transport stimulated by GTP gamma S was only 60% of the insulin effect in the presence of the peptide; the effect of insulin was markedly enhanced by Dk-(62-85), whereas GTP gamma S-induced glucose transport was not affected, suggesting that GTP gamma S has an effect similar to that of the peptide. In fact, endocytosis of the 35-kDa fragment of GLUT4 was markedly inhibited by GTP gamma S. Additionally, GLUT4 endocytosis was accelerated by GTP but was inhibited by guanosine 5'-O-(2-thiodiphosphate). These results indicate that GTP gamma S induces translocation of GLUT4 by both stimulating exocytosis and inhibiting endocytosis. With respect to the dependence on GTP hydrolysis, distinct types of GTP-binding proteins are involved in exocytosis and endocytosis of GLUT4.

Persistence of disturbed adipocyte metabolism in streptozocin-induced diabetic rats despite near-euglycemia with phlorizin

It is widely accepted that hyperglycemia per se incites and perpetuates the diabetic state by adverse effects on beta cell insulin secretion and peripheral insulin action. Examination of the latter locus has revealed glucose-related abnormalities in facilitated glucose transport. Beyond the plasma membrane, however, there is scant data examining whether hyperglycemia influences important intracellular metabolic events. We recently described a sizable reduction in post-transport, in situ metabolism in permeabilized fat cells from streptozocin-induced diabetic rats. Of importance, the diabetes-related deficit was entirely ameliorated by insulin therapy. In this study we examined whether hyperglycemia per se contributes to this altered intracellular metabolic effect. By infusing phlorizin, near euglycemia was achieved for at least four days in streptozocin-induced diabetic rats. The phlorizin-treated diabetic rats had improved (intact cell) rates of insulin-stimulated 2-deoxyglucose uptake. Despite this, permeabilized fat cell studies revealed no improvement or deterioration in diabetic intracellular metabolism as measured by both the oxidation of [6-14C]glucose-6-phosphate via the citric acid cycle or its incorporation into triglyceride. We conclude that hypoinsulinemia, and not hyperglycemia, mediates the disturbance in porous diabetic adipocyte cellular metabolism.