Insulin receptor substrate 1 mediates the stimulatory effect of insulin on GLUT4 translocation in transfected rat adipose cells

Insulin signaling is initiated at least in part by activation of the insulin receptor tyrosine kinase and subsequent phosphorylation of cellular substrates such as insulin receptor substrate 1 (IRS-1). Previous studies have focused on the role of IRS-1 in the mitogenic actions of insulin. We have now investigated the possible role of IRS-1 in mediating the effect of insulin to stimulate glucose transport in a physiologically relevant insulin target tissue. In this study, we transfected rat adipose cells in primary culture with an antisense ribozyme directed against rat IRS-1. Expression of the ribozyme in these cells caused a 4.4-fold increase in the concentration of insulin required to achieve half-maximal stimulation of the translocation of cotransfected epitope-tagged GLUT4 without changing the maximal insulin response. Overexpression of human IRS-1 increased the basal cell surface GLUT4 to nearly the maximal level in the absence of insulin. When the ribozyme (specific to rat IRS-1) was cotransfected along with human IRS-1, the insulin dose-response curve was shifted to the left when compared with cells transfected with the ribozyme alone. These data provide strong support for the hypothesis that IRS-1 plays a role in insulin-stimulated glucose transport in insulin-responsive cells.

Subcellular localization and trafficking of the GLUT4 glucose transporter isoform in insulin-responsive cells

The rate-limiting step in the uptake and metabolism of D-glucose by insulin target cells is thought to be glucose transport mediated by glucose transporters (primarily the GLUT4 isoform) localized to the plasma membrane. However, subcellular fractionation, photolabelling and immunocytochemical studies have shown that the pool of GLUT4 present in the plasma membrane is only one of many subcellular pools of this protein. GLUT4 has been found in occluded vesicles at the plasma membrane, clathrin-coated pits and vesicles, early endosomes, and tubulo-vesicular structures; the latter are analogous to known specialized secretory compartments. Tracking the movement of GLUT4 through these compartments, and defining the mechanism and site of action of insulin in stimulating this subcellular trafficking, are major topics of current investigation. Recent evidence focuses attention on the exocytosis of GLUT4 as the major site of insulin action. Increased exocytosis may be due to decreased retention of glucose transporters in an intracellular pool, or possibly to increased assembly of a vesicle docking and fusion complex. Although details are unknown, the presence in GLUT4 vesicles of a synaptobrevin homologue leads us to propose that a process analogous to that occurring in synaptic vesicle trafficking is involved in the assembly of GLUT4 vesicles into a form suitable for fusion with the plasma membrane. Evidence that the pathways of signalling from the insulin receptor and of GLUT4 vesicle exocytosis may converge at the level of the key signalling enzyme, phosphatidylinositol 3-kinase, is discussed.

Low density lipoprotein receptors in rat adipose cells: subcellular localization and regulation by insulin

The distribution of LDL receptors within subcellular compartments of isolated rat adipose cells and the effects of insulin on their expression have been assessed. By immunoblotting with specific anti-rat LDL receptor antibodies, LDL receptors were 2.3- and 4.5-fold enriched in endoplasmic reticulum-rich high-density microsomes (HDM) and Golgi complex-rich low-density microsomes (LDM), respectively, compared to plasma membranes (PM). This distribution was similar in cultured cells in which total receptors were increased 2.5-fold compared to freshly isolated cells. After correction for enzyme recoveries, LDL receptors were distributed approximately 4% in HDM, approximately 73% in LDM, and approximately 23% in PM. Insulin decreased total LDL receptors in adipose cells approximately 44%, with a 48% and 49% decrease in HDM and LDM, respectively, without any changes in PM. In contrast, insulin caused an increase of glucose transporters in PM while also decreasing glucose transporters in LDM. When adipose cells were depleted of potassium to inhibit receptor-mediated endocytosis, insulin again caused a decrease of LDL receptors in LDM but now increased LDL receptors in PM. Insulin increased the rate of LDL receptor synthesis approximately 24%, but decreased their half life approximately 40%. Thus, in isolated adipose cells the majority of LDL receptors appear to be located in an intracellular compartment that co-sediments with the Golgi complex rather than located in the PM. The LDL receptors localized in intracellular compartments seem to be functionally regulated as insulin acutely diminishes the number of receptors by apparently accelerating their rate of degradation through, as yet, incompletely determined mechanisms.

Insulin-stimulated GLUT4 glucose transporter recycling. A problem in membrane protein subcellular trafficking through multiple pools

The subcellular trafficking of GLUT4 in isolated rat adipose cells and 3T3-L1 adipocytes exhibits many of the properties observed in regulated secretory processes and neurosecretion. GLUT4 is sorted and sequestered from endosomes into a specialized secretory compartment in the basal state and the initial stimulation of its exocytosis by insulin is more rapid than its recycling through the endosomes and secretory compartment during the steady-state response to insulin. We present a mathematical analysis which shows that this behavior is inconsistent with a simple 2-pool model with one plasma membrane and one intracellular compartment, but that a 3-pool model, with two intracellular compartments, can simulate these properties. We extend this model to include the presence of occluded pools in the plasma membrane. Our analysis compares the behavior expected when these occluded pools are precursors in stimulation and/or clathrin-associated-like intermediates in endocytosis. The presence of a precursor occluded pool can account for a lag between the appearance of GLUT4 in the membrane and before the full stimulation of glucose transport activity. The analysis also shows that since the pool size of the occluded GLUT4 is relatively small, the formation of endocytic occluded intermediates such as GLUT4 in clathrin-coated pits is likely to be slow compared with the rate of endocytosis of the coated vesicles.

Regulation of the GLUT1 glucose transporter in cultured myocytes: total number and subcellular distribution as determined by photoaffinity labelling

We have used the impermeant photoaffinity label 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-[2-3H] 1,3-bis-(D-mannos-4-yloxy)-2-propylamine (ATB-[2-3H]BMPA) to identify and quantify the glucose transporters on the surface of BC3H-1 cells, a continuously cultured skeletal-muscle cell line lacking the MyoD transcription factor required for cell fusion. ATB-[2-3H]BMPA was used in combination with immunoprecipitation of the GLUT1 glucose transporter, the only isoform expressed in these cells. The total cellular GLUT1 content was also determined by photolabelling and immunoprecipitation after cell permeabilization with digitonin (0.025%). In glucose-starved cells, 85% of the glucose transporters were present at the cell surface in the basal state, with little change in response to insulin (200 nM), correlating with lack of additional 2-deoxyglucose uptake in response to insulin. Feeding the cells with glucose (25 mM) for 24 h resulted in an 80% decrease in the total GLUT1 content relative to starved cells, of which only 25% were present on the cell surface. This was associated with an 85% decrease in 2-deoxyglucose uptake. In addition, acute stimulation of the fed cells with insulin or phorbol 12-myristate 13-acetate (PMA) led to an increase in GLUT1 at the cell surface, and, in correspondence, an increase in 2-deoxyglucose uptake by approx. 2- and 4-fold respectively. We conclude that exofacial photoaffinity labelling of glucose transporters with ATB-[2-3H]BMPA in the presence and absence of digitonin, followed by specific immunoprecipitation, provides an accurate measure of total and cell-surface glucose transporters in differentiated BC3H-1 muscle cells. This technique demonstrates that glucose pre-feeding (1) decreases the total number of GLUT1 and (2) redistributes the majority of the remaining transporters to an intracellular site, where they can now be translocated to the cell surface in response to insulin and PMA.

Tyrosine kinase-deficient mutant human insulin receptors (Met1153-->Ile) overexpressed in transfected rat adipose cells fail to mediate translocation of epitope-tagged GLUT4

Insulin regulates essential pathways for growth, differentiation, and metabolism in vivo. We report a physiologically relevant system for dissecting the molecular mechanisms of insulin signal transduction related to glucose transport. This is an extension of our recently reported method for transfection of DNA into rat adipose cells in primary culture. In the present work, cDNA coding for GLUT4 with an epitope tag (HA1) in the first exofacial loop is used as a reporter gene so that GLUT4 translocation can be studied exclusively in transfected cells. Insulin stimulates a 4.3-fold recruitment of transfected epitope-tagged GLUT4 to the cell surface. Cells cotransfected with the reporter gene and the human insulin receptor gene show an increase in cell surface GLUT4 in the basal state (no insulin) to levels comparable to those seen with maximal insulin stimulation of cells transfected with the reporter gene alone. In contrast, cells overexpressing a naturally occurring tyrosine kinase-deficient mutant insulin receptor (Met1153-->Ile) show no increase in the basal cell surface GLUT4 and no shift in the insulin dose-response curve relative to cells transfected with the reporter gene alone. These results demonstrate that insulin receptor tyrosine kinase activity is essential in insulin-stimulated glucose transport in adipose cells.

Comparison of body weight and adipose tissue in male C57Bl/6J mice fed diets with and without trans fatty acids

The effect of a diet containing trans-fatty acids (tFA) on the fatty acid composition and fat accumulation in adipose tissue was investigated in mice. Male C57Bl/6J mice were fed Control or Trans Diets that were similar, except that 50% of the 18:1, which was all cis in the Control Diet, was replaced by tFA in the Trans Diet. At selected ages, body weight, epididymal fat pad weight, perirenal fat yield, adipose tissue cellularity and fatty acid composition were examined. Over the time period studied (2-24 mon), the proportion of 18:0 and 16:0 tended to decrease while cis-18:1 levels increased. Compared to the Control Diet, the Trans Diet resulted in adipose tissue lipids with higher percentages of 14:0 and 18:2n-6 and lower percentages of cis-18:1 and 20:4n-6. In polar lipids, tFA replaced saturated fatty acids, whereas tFA replaced cis-18:1 in the nonpolar lipids. Body weights at 16 and 24 mon of age and epididymal fat pad weights at 8-24 mon of age were lower in mice fed the Trans Diet as compared to those fed the Control Diet. At the ages studied, the Trans Diet also resulted in lower values for perirenal fat weights, triacylglycerol to polar lipid ratios, and adipose cell size. The data suggest that chronic consumption of tFA affects lipid metabolism and results in decreased fat accumulation in murine adipose tissue.

Insulin stimulation of glucose transport activity in rat skeletal muscle: increase in cell surface GLUT4 as assessed by photolabelling

We have used a photoaffinity label to quantify cell surface GLUT4 glucose transporters in isolated rat soleus muscles. In this system, insulin stimulated an 8.6-fold increase in 3-O-methylglucose glucose transport, while photolabelled GLUT4 increased 8-fold. These results demonstrate that the insulin-stimulated increase in glucose transport activity in skeletal muscle can be accounted for by an increase in surface-accessible GLUT4 content.

Do adipocytes contain high affinity sulfonylurea receptors

Sulfonylureas interact with specific, high affinity receptors on the pancreatic beta-cell to close ATP-sensitive K+ channels, depolarize the cell, activate Ca2+ influx through voltage-dependent Ca2+ channels, and trigger insulin secretion. We tested the hypothesis that sulfonylureas promote glucose uptake into 3T3-L1 cells or isolated rat adipocytes by similar mechanisms. Using 125I-labeled 5-iodo-2-hydroxyglyburide and either equilibrium binding or photoaffinity labeling, a high affinity sulfonylurea receptor was not found on plasma membranes of either the 3T3-L1 cells or rat adipocytes. Furthermore, glyburide did not inhibit 86Rb+ efflux (a marker for ATP-sensitive K+ channel conductance), increase free cytosolic calcium in adipocytes or 3T3-L1 cells, or increase basal or insulin-stimulated glucose uptake into 3T3-L1 cells or rat adipocytes. Parallel studies using a hamster insulin-secreting tumor cell line (HIT cells) easily demonstrated both the receptor and biological effects of glyburide on free cytosolic calcium and insulin secretion. Thus, rat adipocytes and 3T3-L1 cells do not possess the high affinity sulfonylurea receptor or respond to glyburide alone. We conclude that the antidiabetogenic effects of sulfonylureas are not mediated by a direct action of sulfonylureas to increase glucose uptake into adipose tissue and suggest that the major locus for sulfonylurea action is the beta-cell.

Glucose transporter localization in rat skeletal muscle. Autoradiographic study using ATB-[2-3H]BMPA photolabel

Surface glucose transporters of intact muscles were photolabeled with the membrane impermeant ATB-[2-3H]BMPA reagent and localized by autoradiography. We found sparse labeling of the glucose transporters by ATB-[2-3H]BMPA on the sarcolemmal membrane around the muscle fiber. The majority of label was on the interior of the muscle fiber, at a discrete site which matched the distribution of AI junctions and which was presumed to be on the exterior surface of T-tubules. The amount of photolabel on the T-tubule was increased in response to insulin and was blocked by cytochalasin B. These results support the concept that glucose transport may occur predominantly across the T-tubule membrane under basal and insulin-stimulated conditions.

Inhibition of internalization of glucose transporters and IGF-II receptors. Mechanism of action of MHC class I-derived peptides which augment the insulin response in rat adipose cells

Peptides from the alpha 1 domain of the major histocompatibility complex class I antigen (MHC class I), e.g. Dk-(61-85) and Dk-(62-85), have been shown previously to augment glucose uptake in insulin-stimulated cells and to inhibit insulin receptor internalization (Stagsted, J., Reaven, G. M., Hansen, T., Goldstein, A., and Olsson, L. (1990) Cell 62, 297-307). We now report that these peptides inhibit by 80-100% the internalization of glucose transporters (GLUT4) and insulin-like growth factor II (IGF-II) receptors in insulin-stimulated cells and correspondingly double insulin-stimulated glucose transport activity and the number of GLUT4 and IGF-II receptors on the cell surface. In addition, the peptides enhance the apparent affinity about 3-fold of IGF-II binding to its receptor. It is concluded that the effects of the peptides on glucose transport and IGF-II binding are a consequence of the peptide-mediated inhibition of internalization of GLUT4 and IGF-II receptor. The active peptides are derived from the alpha 1 domain of a MHC class I molecule, suggesting that the latter is involved in regulation of internalization of cell surface integral membrane proteins such as the GLUT4 and IGF-II and insulin receptors.

Use of bismannose photolabel to elucidate insulin-regulated GLUT4 subcellular trafficking kinetics in rat adipose cells. Evidence that exocytosis is a critical site of hormone action

The subcellular trafficking of tracer-tagged GLUT4 between the plasma membranes and low-density microsomes of rat adipose cells has been studied. Cell-surface GLUT4 have been initially tracer-tagged in the insulin-stimulated state with the [3H]bismanose photolabel 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis-(D-mannos- 4-yloxy)-2- propylamine. The half-time for internalization of tracer-tagged GLUT4 when insulin is removed by collagenase treatment is similar to that observed for the decrease in immunodetectable GLUT4 in the plasma membranes and the decrease in glucose transport activity in the intact cells. In contrast, internalization of tracer-tagged GLUT4 also occurs when cells are maintained in the continuous presence of insulin even though the plasma membrane level of immunodetectable GLUT4 and glucose transport activity in the intact cells are unaltered. These data show, for the first time, that insulin has little, if any, effect on the rate constant for GLUT4 endocytosis, but instead, primarily increases the rate constant for exocytosis. Tracer-tagged GLUT4 that is returned to the low-density microsomes can be restimulated with fresh insulin to recycle to the plasma membranes and to a steady-state distribution level that is the same as that observed in cells that are maintained in the continuous presence of insulin. These data suggest that the cells' entire complement of GLUT4 is involved in the recycling process. Following insulin stimulation of adipose cells initially in the basal state, the increase in immunodetectable GLUT4 in the plasma membranes precedes the increase in accessibility of GLUT4 to exofacial 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis(D-mannos-4 -yloxy)-2- propylamine photolabeling, and this in turn precedes the increase in cellular glucose transport activity. Such time course data suggest that there may be plasma membrane intermediate states in the GLUT4 trafficking pathway. The kinetic properties of GLUT4 translocation and its recycling have been interpreted in terms of a subcellular trafficking model that identifies exocytosis, possibly involving-hypothetical "docking" and "fusion" steps, as the critical site of hormone action.

Transfection of DNA into isolated rat adipose cells by electroporation: evaluation of promoter activity in transfected adipose cells which are highly responsive to insulin after one day in culture

Isolated adipose cells are among the most insulin responsive cells with respect to glucose transport and metabolism. However, molecular biological techniques such as transfection of DNA have heretofore not been applied successfully in these cells in primary culture. We report a method for transfection of DNA into rat adipose cells by electroporation. Six shocks at 800 V and 25 microF in a 0.4 cm gap cuvette results in efficient transfection. We compared the ability of five promoters to drive expression of a luciferase reporter gene in transfected adipose cells. After one day in culture, promoter activity ranged from no expression to a very high level of expression. These transfected, cultured cells also displayed a 10-fold increase in 3-O-methylglucose transport with maximal insulin stimulation. The ability to transfect DNA into adipose cells which remain insulin responsive after one day in primary culture may be helpful for understanding adipose cell-specific gene regulation and elucidating the molecular mechanisms of insulin action.

Insulinomimetic effect on glucose transport by epidermal growth factor when combined with a major histocompatibility complex class I-derived peptide

Peptides derived from the alpha 1-region of the murine H-2Dk molecule enhance glucose uptake in rat adipose cells above the maximum obtained with insulin stimulation alone (Stagsted, J., Reaven, G. M., Hansen, T., Goldstein, A., and Olsson, L. (1990) Cell 62, 297-307). We now describe that epidermal growth factor (EGF) in combination with the same peptides, Dk-(61-85) and Dk-(62-85), stimulates cellular glucose uptake 5-7 times over the basal level, i.e. to 30-50% of the maximal insulin effect. EGF alone increased glucose uptake by only approximately 50% above basal and the peptide alone by 100% above basal. Maximal effect of EGF and peptide was reached in 10-20 min with 30 microM peptide (EC50 10-15 microM) and 50 nM EGF (EC50 1-2 nM). The effect of EGF and peptide on glucose uptake was additive to that of insulin and peptide until the maximal level attained with insulin and peptide was reached. The combined effect of EGF plus peptide on glucose transport was associated with a recruitment of GLUT4 molecules to the plasma membrane. However, the phosphatidylinositol (PI) kinase which is activated by insulin was not activated by EGF plus peptide. Thus, the effect of EGF plus peptide on glucose uptake seems independent of the activity status of the insulin receptor. 125I-Labeled EGF bound specifically to rat adipose cells with an apparent affinity of approximately 2 nM and Bmax approximately 5 x 10(3). However, the major histocompatibility complex (MHC) peptides did not affect EGF-stimulated internalization of EGF receptor, in contrast to their effect on the insulin receptors. Transforming growth factor alpha had an effect similar to EGF on glucose uptake. Three other peptides derived from other parts of murine MHC class I had no effect on glucose uptake in combination with EGF. Thus, EGF in combination with certain MHC class I-derived peptides is insulinomimetic concerning glucose transport and this effect is independent of the insulin receptor activity.

Cell surface accessibility of GLUT4 glucose transporters in insulin-stimulated rat adipose cells. Modulation by isoprenaline and adenosine

Insulin-stimulated glucose transport activity in rat adipocytes is inhibited by isoprenaline and enhanced by adenosine. Both of these effects occur without corresponding changes in the subcellular distribution of the GLUT4 glucose transporter isoform. In this paper, we have utilized the impermeant, exofacial bis-mannose glucose transporter-specific photolabel, 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis-(D-mannos- 4-yloxy)-2-propylamine (ATB-BMPA) [Clark & Holman (1990) Biochem. J. 269, 615-622], to examine the cell surface accessibility of GLUT4 glucose transporters under these conditions. Compared with cells treated with insulin alone, adenosine in the presence of insulin increased the accessibility of GLUT4 to the extracellular photolabel by approximately 25%, consistent with its enhancement of insulin-stimulated glucose transport activity; the plasma membrane concentration of GLUT4 as assessed by Western blotting was unchanged. Conversely, isoprenaline, in the absence of adenosine, promoted a time-dependent (t1/2 approximately 2 min) decrease in the accessibility of insulin-stimulated cell surface GLUT4 of > 50%, which directly correlated with the observed inhibition of transport activity; the plasma membrane concentration of GLUT4 decreased by 0-15%. Photolabelling the corresponding plasma membranes revealed that these alterations in the ability of the photolabel to bind to GLUT4 are transient, as the levels of both photolabel incorporation and plasma membrane glucose transport activity were consistent with the observed GLUT4 concentration. These data suggest that insulin-stimulated GLUT4 glucose transporters can exist in two distinct states within the adipocyte plasma membrane, one which is functional and accessible to extracellular substrate, and one which is non-functional and unable to bind extracellular substrate. These effects are only observed in the intact adipocyte and are not retained in plasma membranes isolated from these cells when analysed for their ability to transport glucose or bind photolabel.

Development of an intracellular pool of glucose transporters in 3T3-L1 cells

The membrane-impermeant bis-mannose photolabel 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis-(D-mannos- 4-yloxy)-2- propylamine (ATB-BMPA) has been used to study the development of an intracellular pool of glucose transporters in 3T3-L1 cells. The subcellular distributions of the transporter isoforms GLUT1 and GLUT4 were determined by comparing the labeling obtained in cells in which the impermeant reagent only had access to the cell surface and the labeling obtained in digitonin-permeabilized cells. ATB-BMPA labeling showed that only GLUT1 was present in preconfluent fibroblasts and that most of the transporters were distributed to the cell surface. In preconfluent fibroblasts, the 2-deoxy-D-glucose transport activity was approximately 5 times higher than in confluent fibroblasts. ATB-BMPA labeling showed that the decrease in transport as cells reached confluence was associated with a decrease in the proportion of GLUT1 distributed to the cell surface. The sequestration of these transporters was associated with the development of an insulin-responsive transport activity which increased by approximately 2.5-fold compared with unstimulated confluent cells. ATB-BMPA labeling showed that insulin stimulation resulted in an approximately 2-fold increase in surface GLUT1 so that about one-half of the available transporters became recruited to the cell surface. Measurements of the changes in the distribution of both GLUT1 and GLUT4 throughout the differentiation of confluent fibroblasts into adipocytes showed that both transporters were sequestered in parallel. Basal levels of transport and photolabeling remained low throughout the differentiation period when the total pool of transporters (GLUT1 plus GLUT4) was increased by approximately 5-fold. These results suggest that the sequestration process was present before new transporters were synthesized. Thus, the sequestration mechanism develops in confluent growth-arrested fibroblasts although the capacity to sequester additional transporters may increase as differentiation proceeds.

Effects of fluorescein isothiocyanate on insulin actions in rat adipocytes

The effects of fluorescein isothiocyanate II (FITC) on the actions of insulin in rat adipocytes were studied. When adipocytes were incubated with FITC at pH 7.4 (2 mM agent, 8 min), the cells were completely deprived of their specific insulin-binding activity and rendered unresponsive to the hormone. The effect of FITC on the insulin-binding activity was milder at pH 9.0, and cAMP phosphodiesterase in cells exposed to FITC at pH 9.0 was maximally stimulated if the insulin concentration was increased to 100 nM. Under identical conditions, however, glucose transport activity was rendered not only less sensitive but also less responsive to the hormone. When FITC was added to cells after insulin at pH 9.0, the glucose transport activity that had been stimulated by the hormone was considerably reduced. This reduction was largely, but not entirely, prevented if the cells were deprived of ATP, suggesting that FITC (a) elicited the ATP-dependent reversal of the hormonal effect and, simultaneously, (b) mildly inhibited the transport activity per se. Western blot assay of GLUT-4 (a major isoform of glucose transporter in adipocytes) indicated that FITC (a) partially blocked insulin-dependent translocation of GLUT-4 from the intracellular site to the plasma membrane while it (b) induced a mild "insulin-like" effect. It is concluded that FITC at pH 9.0 (a) renders both glucose transport and phosphodiesterase activities less insulin sensitive presumably by modifying the cellular hormone receptor and (b) makes glucose transport activity less responsive to insulin presumably by (i) blocking hormone-dependent translocation of glucose transporter and (ii) mildly inhibiting intrinsic glucose transport activity.

Cholera and pertussis toxins modify regulation of glucose transport activity in rat adipose cells: evidence for mediation of a cAMP-independent process by G-proteins

Adenylyl cyclase in rat adipose cells is stimulated by ligands for Rs receptors (e.g. isoproterenol) and inhibited by ligands for Ri receptors (e.g. adenosine). In contrast, Rs receptors mediate inhibition and Ri receptors mediate augmentation of insulin-stimulated glucose transport activity by a process independent of changes in cellular cAMP-dependent protein kinase activity [Kuroda M., Honnor R. C., Cushman S. W., Londos C. and Simpson I. A. (1987) J. biol. Chem. 262, 245-253]. The present study examines the possible role of G-proteins in the regulation of insulin-stimulated glucose transport activity by Rs and Ri receptors. First, conditions were established that permit intoxication of isolated rat adipocytes by cholera and pertussis toxins without compromising cell integrity. Effectiveness of toxin treatment was monitored by examining adenylyl cyclase activity in isolated plasma membranes. Secondly, neither toxin interfered with the ability of a maximal concentration insulin to initiate the glucose transport response. Thirdly, pertussis toxin eliminated the augmenting effects of adenosine on insulin-stimulated glucose transport activity, but enhanced the inhibitory effects of isoproterenol. Findings with ligands for other Ri receptors (nicotinic acid and prostaglandin E2) mirrored those with adenosine. Finally, cholera toxin elicited a modest depression of transport activity, and only in the absence of an Ri ligand (e.g. adenosine). Furthermore, in contrast to the enhanced stimulation of adenylyl cyclase by isoproterenol and GTP, cholera toxin eliminated the inhibitory effect of isoproterenol on transport activity. The augmentative effects of adenosine on transport activity were unchanged. Measurements of (-/+cAMP) cAMP-dependent protein kinase activity ratios reinforce the notion that modulation of glucose transport activity is independent of changes in cAMP. We conclude that regulation of glucose transport activity by Rs and Ri receptors is mediated by the G-proteins, Gs and Gi (or other toxin substrates), respectively. Inasmuch as such regulation occurs at the plasma membrane and appears to be cAMP-independent, it is suggested that glucose transporters may be direct targets for receptor: G-protein interactions.

Phosphorylation state of the GLUT4 isoform of the glucose transporter in subfractions of the rat adipose cell: effects of insulin, adenosine, and isoproterenol

The acute effects of insulin, adenosine, and isoproterenol on the activity, subcellular distribution, and phosphorylation state of the GLUT4 glucose transporter isoform were investigated in rat adipocytes under conditions carefully controlled to monitor changes in cAMP-dependent protein kinase (A-kinase) activity. In contrast to GLUT1, which has not been shown to be phosphorylated even when cells are exposed to any of the above agents, GLUT4 was partially phosphorylated (0.1-0.2 mol/mol) when the activity of the A-kinase was suppressed, and remained unchanged in response to insulin. Isoproterenol elicited a 64% inhibition of insulin-stimulated glucose transport activity in the absence, but not the presence, of adenosine receptor agonists. However, in either the presence or the absence of agonists, A-kinase was activated as assessed by examining the phosphorylation of the major adipocyte A-kinase substrate, perilipin. Similarly, under either condition, phosphorylation of GLUT4 was enhanced 1.4-fold in the intracellular membranes, but no significant change was observed in the plasma membrane. In the absence of adenosine receptor agonists, isoproterenol exerted a small (14%) but significant inhibition of the insulin-induced translocation of GLUT4 but had no effect on the translocation of GLUT1. Thus, changes in the phosphorylation state and/or subcellular distribution of GLUT4 cannot account for the inhibition of insulin-stimulated glucose activity induced by isoproterenol.

Normalization of blood glucose in diabetic rats with phlorizin treatment reverses insulin-resistant glucose transport in adipose cells without restoring glucose transporter gene expression

Evidence is emerging for a direct role of glucose, independent of changes in insulin, in the regulation of cellular glucose transport and glucose utilization in vivo. In this study we investigate potential cellular and molecular mechanisms for this regulatory effect of glucose by determining how normalization of glycemia without insulin therapy in diabetic rats influences 3-O-methylglucose transport and the expression and translocation of two genetically distinct species of glucose transporters (GTs) in adipose cells. These results are compared with alterations in glucose disposal in vivo measured by euglycemic clamp. In rats rendered diabetic by 90% pancreatectomy, insulin-stimulated glucose transport in adipose cells is decreased 50% in parallel with reduced insulin-mediated glucose disposal in vivo. Levels of adipose/muscle GTs measured by immunoblotting are decreased in adipose cell subcellular membrane fractions, as are the corresponding mRNA levels assessed by Northern blotting of total adipose cell RNA. Normalization of blood glucose in diabetic rats with phlorizin, which impairs renal tubular glucose reabsorption and thus enhances glucose excretion, restores insulin-stimulated glucose transport in adipose cells and insulin-mediated glucose disposal in vivo. Importantly, levels of the adipose/muscle GT protein remain 43% reduced in the low-density microsomes in the basal state and 46% reduced in the plasma membranes in the insulin-stimulated state. Adipose/muscle GT mRNA levels remain approximately 50% depressed. Levels of the HepG2/brain GT protein and mRNA are unaltered by diabetes or phlorizin treatment. Thus, changes in ambient glucose independent of changes in ambient insulin can regulate the glucose transport response to insulin in isolated adipose cells and changes in responsiveness parallel alterations in glucose uptake in vivo. Since this effect can occur without alteration in the expression of the two species of glucose transporters present in adipose cells or in their translocation to the plasma membrane in response to insulin, it may result from changes in GT functional activity.

Role of protein kinase C in the regulation of glucose transport in the rat adipose cell. Translocation of glucose transporters without stimulation of glucose transport activity

The possible role of protein kinase C in the regulation of glucose transport in the rat adipose cell has been examined. Both insulin and phorbol 12-myristate 13-acetate (PMA) stimulate 3-O-methylglucose transport in the intact cell ein association with the subcellular redistribution of glucose transporters from the low density microsomes to the plasma membranes, as assessed by cytochalasin B binding. In addition, the actions of insulin and PMA on glucose transport activity and glucose transporter redistribution are additive. Furthermore, PMA accelerates insulin's stimulation of glucose transport activity, reducing the t1/2 from 3.2 +/- 0.4 to 2.1 +/- 0.2 min (mean +/- S.E.). However, the effect of PMA on glucose transport activity is approximately 10% of that for insulin whereas its effect on glucose transporter redistribution is approximately 50% of the insulin response. Immunoblots of the GLUT1 and GLUT4 glucose transporter isoforms in subcellular membrane fractions also demonstrate that the translocations of GLUT1 in response to PMA and insulin are of similar magnitude whereas the translocation of GLUT4 in response to insulin is markedly greater than that in response to PMA. Thus, glucose transport activity in the intact cell with PMA and insulin correlates more closely with the appearance of GLUT4 in the plasma membrane than cytochalasin B-assayable glucose transporters. Although these data do not clarify the potential role of protein kinase C in the mechanism of insulin action, they do suggest that the mechanisms through which insulin and PMA stimulate glucose transport are distinct but interactive.

Subcellular distribution and phosphorylation state of insulin receptors from insulin- and isoproterenol-treated rat adipose cells

Rat adipose cells treated with insulin followed by isoproterenol exhibit a change in glucose transporter intrinsic activity (lowered maximal activity) and a decrease in insulin sensitivity (rightward shift of the concentration-response curve) when assayed for 3-O-methylglucose transport. To investigate the latter phenomenon, the distribution and phosphorylation state of insulin receptors was examined. Isoproterenol augmented the effect of insulin to reduce cell surface receptors by 20-30%. These receptors were recovered in microsomal fractions. Isoproterenol also markedly reduced insulin-stimulated [32P]phosphate incorporation into the plasma membrane receptor beta-subunit. These effects may account for the effect of isoproterenol to decrease the sensitivity of the glucose transport response to insulin.

Cell surface labeling of glucose transporter isoform GLUT4 by bis-mannose photolabel. Correlation with stimulation of glucose transport in rat adipose cells by insulin and phorbol ester

A new impermeant photoaffinity label has been used for identifying cell surface glucose transporters in isolated rat adipose cells. This compound is 2-N-4(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis(D-mannos-4- yloxy)-2- propylamine. We have used this reagent in combination with immunoprecipitation by specific antibodies against the GLUT4 and GLUT1 glucose transporter isoforms to estimate the relative abundance of these two transporters on the surface of the intact adipose cell following stimulation by insulin and phorbol 12-myristate 13-acetate (PMA). In the basal state, GLUT4 and GLUT1 are both present at the cell surface but GLUT4 is more abundant than GLUT1. In response to insulin, GLUT4 increases 15-20-fold and GLUT1 increases approximately 5-fold while 3-O-methyl-D-glucose transport is stimulated 20-30-fold. By contrast, PMA only induces a approximately 4-fold increase in GLUT4 while GLUT1 increases approximately 5-fold to the same level as seen with insulin. In addition, PMA stimulates 3-O-methyl-D-glucose transport approximately 3-fold to only 13% of the insulin-stimulated state. Thus GLUT4 is the major glucose transporter isoform under all conditions, and it is selectively and markedly enriched in response to insulin but not PMA which increases GLUT1 and GLUT4 equally. Furthermore, stimulation of glucose transport activity correlates closely with the appearance of GLUT4 on the cell surface in response to both insulin and PMA but does not correlate with the sum of GLUT1 and GLUT4 appearance. These results suggest that GLUT4 may be inherently more active than GLUT1 due to a higher TK (turnover/Km).

Differential regulation of two glucose transporters in adipose cells from diabetic and insulin-treated diabetic rats

At least two genetically distinct glucose transporters (GTs) coexist in adipose cells, one cloned from human hepatoma cells and rat brain (HepG2/brain) and another from rat skeletal muscle, heart, and adipose cells (adipose cell/muscle). Here we demonstrate differential regulation of these two GTs in adipose cells of diabetic and insulin-treated diabetic rats and compare changes in the expression of each GT with marked alterations in insulin-stimulated glucose transport activity. Adipose cell/muscle GTs detected by immunoblotting with the monoclonal antiserum 1F8 (James, D. E., R. Brown, J. Navarro, and P. F. Pilch. 1988. Nature (Lond.). 333:183-185), which reacts with the protein product of the newly cloned adipose cell/muscle GT cDNA, decrease 87% with diabetes and increase to 8.5-fold diabetic levels with insulin treatment. These changes concur qualitatively with previous detection of GTs by cytochalasin B binding and with insulin-stimulated 3-O-methylglucose transport. Northern blotting reveals that the adipose/muscle GT mRNA decreases 50% with diabetes and increases to 6.8-fold control (13-fold diabetic) levels with insulin treatment. In contrast, GTs detected with antisera to the carboxyl terminus of the HepG2 GT or to the human erythrocyte GT show no significant change with diabetes or insulin treatment. The HepG2/brain GT mRNA is unchanged with diabetes and increases threefold with insulin treatment. These results suggest that (a) altered expression of the adipose cell/muscle GT forms the molecular basis for the dysregulated glucose transport response to insulin characteristic of diabetes, (b) the expression of two types of GTs in rat adipose cells is regulated independently, and (c) alterations in mRNA levels are only part of the mechanism for in vivo regulation of the expression of either GT species.