A role for ceramide, but not diacylglycerol, in the antagonism of insulin signal transduction by saturated fatty acids

Dietary trans-fatty acids alter adipocyte plasma membrane fatty acid composition and insulin sensitivity in rats

The present study was designed to investigate the effects of dietary trans-fatty acids (TFA) present in Indian vanaspati (partially hydrogenated vegetable oils) in comparison with saturated fatty acids (SFA) on adipocyte plasma membrane fatty acid composition, fluidity, and insulin action. The effects of 3% energy (% en) TFA was studied at 2% and 4% en of linoleic acid (18:2 n-6). WNIN male weanling rats were divided into 4 groups and fed casein-based diet containing 10% groundnut oil control (CON), palmolein (SFA), blend of vanaspati and safflower oil (3% en TFA and 2% en 18:2 n-6, TFA-1), or blend of vanaspati and safflower oil (3% en TFA and 4% en 18:2 n-6, TFA-2) for 12 weeks. Compared with CON, rats fed TFA and SFA diets had high levels of fasting plasma insulin and triglycerides. Both TFA- and SFA-fed groups had low levels of arachidonic acid (20:4 n-6) in adipocyte plasma membrane phospholipids. However, adipocyte plasma membrane fluidity decreased only in TFA-fed rats. Norepinephrine-stimulated lipolysis was high, whereas the antilipolytic effect of insulin and insulin-stimulated glucose transport were low in the adipocytes of SFA- and TFA-fed rats. However, the extent of decrease in the antilipolytic effect of insulin and insulin-stimulated glucose transport was greater in TFA-fed rats. These findings suggest that diet providing approximately 10% en SFA (PUFA/SFA [P/S] ratio 0.2) decreased adipocyte insulin sensitivity in rats. In these diets, replacement of approximately 2% en SFA (16:0) and approximately 1% en monounsaturated fatty acid (18:1 cis) with TFA decreased adipocyte insulin sensitivity to a greater extent. However, increasing dietary 18:2 n-6 did not prevent or reduce the TFA-induced adipocyte insulin resistance.

Cellular mechanisms of insulin resistance, lipodystrophy and atherosclerosis induced by HIV protease inhibitors

Accumulating clinical evidence now links HIV protease inhibitors (HPIs) to the pathogenesis of insulin resistance, dyslipidaemia, lipodystrophy and atherosclerosis associated with highly active anti-retroviral therapy. Here we briefly describe the evidence for a distinct causative role for HPIs, and explore the cellular mechanisms proposed to underlie these side-effects. Acute inhibition of GLUT4-mediated glucose transport, and defective insulin signalling induced by chronic exposure to nelfinavir, are described as cellular mechanisms of insulin resistance. Interference with adipogenesis and adipocyte apoptosis and nelfinavir-induced activation of lipolysis are discussed as potential mechanisms of HPI-induced lipodystrophy. HPI-induced free radical production, apoptosis and increased glucose utilization in vascular smooth muscle cells are presented as possible novel mechanisms for atherosclerosis. Common pathways and cause-effect relationships between the various cellular mechanisms presented are then discussed, with emphasis on the role of insulin resistance, free radical production and enhanced lipolysis. Understanding the cellular mechanisms of HPI-induced side-effects will enhance the search for improved anti-retroviral therapy, and may also shed light on the pathogenesis of common forms of insulin resistance, dyslipidaemia and atherosclerosis.

Neurotensin negatively modulates Akt activity in neurotensin receptor-1-transfected AV12 cells

Neurotensin (NT) regulates a variety of biological processes primarily through interaction with neurotensin receptor-1 (NTR1), a heterotrimeric G-protein-coupled receptor (GPCR). Stimulation of NTR1 has been linked to activation of multiple signaling transduction pathways via specific coupling to G(q), G(i/o), or G(s), in various cell systems. However, the function of NT/NTR1 in the regulation of the Akt pathway remains unknown. Here, we report that activation of NTR1 by NT inhibits Akt activity as determined by the dephosphorylation of Akt at both Ser473 and Thr308 in AV12 cells constitutively expressing human NTR1 (NTR1/AV12). The inactivation of Akt by NT was rapid and dose-dependent. This effect of NT was completely blocked by the specific NTR1 antagonist, (S)-(+)-[1-(7-chloro-4-quinolinyl)-5-(2,6-dimethoxyphenyl)pyrazol-3-yl)-carbonylamino] cyclohexylacetic acid (SR 48527), but unaffected by the less active enantiomer ((R)-(-)-[1-(7-chloro-4-quinolinyl)-5-(2,6-dimethoxyphenyl)pyrazol-3-yl)-carbonylamino] cyclohexylacetic acid (SR 49711)), indicating the stereospecificity of NTR1 in the negative regulation of Akt. In addition, NT prevented insulin- and epidermal growth factor (EGF)-mediated Akt activation. Our results provide insight into the role of NT in the modulation of Akt signaling and the potential physiological significance of Akt regulation by NT.

Ceramide down-regulates System A amino acid transport and protein synthesis in rat skeletal muscle cells

Skeletal muscle is a major insulin target tissue and has a prominent role in the control of body amino acid economy, being the principal store of free and protein-bound amino acids and a dominant locus for amino acid metabolism. Interplay between diverse stimuli (e.g., hormonal/nutritional/mechanical) modulates muscle insulin action to serve physiological need through the action of factors such as intramuscular signaling molecules. Ceramide, a product of sphingolipid metabolism and cytokine signaling, has a potent contra-insulin action with respect to the transport and deposition of glucose in skeletal muscle, although ceramide effects on muscle amino acid turnover have not previously been documented. Here, membrane permeant C2-ceramide is shown to attenuate the basal and insulin-stimulated activity of the Na+-dependent System A amino acid transporter in rat muscle cells (L6 myotubes) by depletion of the plasma membrane abundance of SNAT2 (a System A isoform). Concomitant with transporter down-regulation, ceramide diminished both intramyocellular amino acid abundance and the phosphorylation of translation regulators lying downstream of mTOR. The physiological outcome of ceramide signaling in this instance is a marked reduction in cellular protein synthesis, a result that is likely to represent an important component of the processes leading to muscle wasting in catabolic conditions.

Rosiglitazone enhances glucose tolerance by mechanisms other than reduction of fatty acid accumulation within skeletal muscle

We hypothesized that improved glucose tolerance with rosiglitazone treatment would coincide with decreased levels of i.m. triacylglycerol (IMTG), diacylglycerol, and ceramide. Obese Zucker rats were randomly divided into two experimental groups: control (n = 9) and rosiglitazone (n = 9), with lean Zucker rats (n = 9) acting as a control group for obese controls. Rats received either vehicle or 3 mg/kg rosiglitazone for 6 wk. Glucose tolerance was impaired (P < 0.01) in obese compared with lean rats, but was normalized after rosiglitazone treatment. IMTG content was higher in obese compared with lean rats (70.5 +/- 5.1 vs. 27.5 +/- 2.0 micromol/g dry mass; P < 0.05) and increased an additional 30% (P < 0.05) with rosiglitazone treatment. Intramuscular fatty acid composition shifted toward a higher proportion of monounsaturates (P < 0.05) in obese rosiglitazone-treated rats due to an increase in palmitoleate (16:1; P < 0.05). Rosiglitazone treatment increased (P < 0.05) skeletal muscle diacylglycerol and ceramide levels by 65% and 100%, respectively, compared with obese rats, but elevated muscle diacylglycerol was not associated with changes in the total or membrane contents of the diacylglycerol-sensitive protein kinase C isoforms theta;, delta, alpha, and beta. In summary, we observed a disassociation among skeletal muscle IMTG, diacylglycerol and ceramide content, and glucose tolerance with rosiglitazone treatment in obese Zucker rats. Our data suggest, therefore, that rosiglitazone enhances glucose tolerance by mechanisms other than reduction of fatty acid accumulation within skeletal muscle.

Proinflammatory cytokine impairment of insulin-like growth factor I-induced protein synthesis in skeletal muscle myoblasts requires ceramide

GH and IGF-I control over 80% of postnatal growth. We recently established that TNFalpha impairs the ability of IGF-I to increase protein synthesis and promote expression of myogenin in myoblasts. Here we extend these results by showing that ceramide, a second messenger in both TNFalpha and IL-1beta receptor signaling pathways, is a key downstream sphingosine-based lipid that leads to IGF-I resistance. A cell-permeable ceramide analog, C2-ceramide, inhibits IGF-I-induced protein synthesis by 65% and blocks the ability of IGF-I to increase expression of two key myogenic factors, myogenin and MyoD. Identical results were obtained with both TNFalpha and IL-1beta (1 ng/ml). Consistent with these data, neutral sphingomyelinase (N-SMase), an enzyme that catalyzes formation of ceramide from sphingomyelin, blocks IGF-I-induced protein synthesis and expression of both myogenin and MyoD. The possibility that cytokine-induced ceramide production is required for disruption of IGF-I biologic activity was confirmed by treating C2C12 myoblasts with inhibitors of all three ceramide-generating pathways. A N-SMase inhibitor, glutathione, as well as an acidic sphingomyelinase (A-SMase) inhibitor, D609, reverse the cytokine inhibition of IGF-I-induced protein synthesis by 80% and 45%, respectively. Likewise, an inhibitor of de novo ceramide synthesis, FB1, causes a 50% inhibition. Similarly, all three inhibitors significantly impair the ability of both TNFalpha and IL-1beta to suppress IGF-I-driven expression of myogenin. These experiments establish that ceramide, derived both from sphingomyelin and de novo synthesis, is a key intermediate by which proinflammatory cytokines impair the ability of IGF-I to promote protein synthesis and expression of critical muscle-specific transcription factors.

Discordant effects of a chronic physiological increase in plasma FFA on insulin signaling in healthy subjects with or without a family history of type 2 diabetes

Muscle insulin resistance develops when plasma free fatty acids (FFAs) are acutely increased to supraphysiological levels (approximately 1,500-4,000 micromol/l). However, plasma FFA levels >1,000 micromol/l are rarely observed in humans under usual living conditions, and it is unknown whether insulin action may be impaired during a sustained but physiological FFA increase to levels seen in obesity and type 2 diabetes mellitus (T2DM) (approximately 600-800 micromol/l). It is also unclear whether normal glucose-tolerant subjects with a strong family history of T2DM (FH+) would respond to a low-dose lipid infusion as individuals without any family history of T2DM (CON). To examine these questions, we studied 7 FH+ and 10 CON subjects in whom we infused saline (SAL) or low-dose Liposyn (LIP) for 4 days. On day 4, a euglycemic insulin clamp with [3-3H]glucose and indirect calorimetry was performed to assess glucose turnover, combined with vastus lateralis muscle biopsies to examine insulin signaling. LIP increased plasma FFA approximately 1.5-fold, to levels seen in T2DM. Compared with CON, FH+ were markedly insulin resistant and had severely impaired insulin signaling in response to insulin stimulation. LIP in CON reduced insulin-stimulated glucose disposal (Rd) by 25%, insulin-stimulated insulin receptor tyrosine phosphorylation by 17%, phosphatidylinositol 3-kinase activity associated with insulin receptor substrate-1 by 20%, and insulin-stimulated glycogen synthase fractional velocity over baseline (44 vs. 15%; all P < 0.05). In contrast to CON, a physiological elevation in plasma FFA in FH+ led to no further deterioration in Rd or to any additional impairment of insulin signaling. In conclusion, a 4-day physiological increase in plasma FFA to levels seen in obesity and T2DM impairs insulin action/insulin signaling in CON but does not worsen insulin resistance in FH+. Whether this lack of additional deterioration in insulin signaling in FH+ is due to already well-established lipotoxicity, or to other molecular mechanisms related to insulin resistance that are nearly maximally expressed early in life, remains to be determined.

Differential effect of saturated and polyunsaturated fatty acids on hepatic glucose metabolism in humans

Prolonged infusions of lipid and heparin that achieve high physiological free fatty acid (FFA) concentrations inhibit hepatic (and peripheral) insulin sensitivity in humans. These infusions are composed largely of polyunsaturated fatty acids (PUFA; linoleic and linolenic). It is not known whether fatty acid composition per se affects hepatic glucose metabolism in humans. To address this issue, we examined the impact of enteral infusions of either palm oil (48% palmitic, 35% oleic, and 8% linoleic acids) or safflower oil (6% palmitic, 12% oleic, 74% linoleic acids) in 14 obese nondiabetic subjects. (2)H(2)O was administered to determine the contribution of gluconeogenesis to endogenous glucose production (EGP), and a primed continuous infusion of [6,6-(2)H]glucose was administered to assess glucose appearance. As a result of the lipid infusions, plasma FFA concentrations increased significantly in both the palm oil (507.5 +/- 47.4 to 939.3 +/- 61.3 micromol/l, P < 0.01) and safflower oil (588.2.0 +/- 43.0 to 857.8 +/- 68.7 micromol/l, P < 0.01) groups after 4 h. EGP was similar at baseline (12.4 +/- 1.8 vs. 11.2 +/- 1.0 micromol x kg FFM(-1) x min(-1)). During a somatostatin-insulin clamp, the glucose infusion rate was significantly lower (AUC glucose infusion rate 195.8 +/- 50.7 vs. 377.8 +/- 38.0 micromol/kg FFM, P < 0.01), and rates of EGP were significantly higher (10.7 +/- 1.4 vs. 6.5 +/- 1.5 micromol x kg FFM(-1) x min(-1), P < 0.01) after palm oil compared with safflower oil, respectively. Baseline rates of gluconeogenesis and glycogenolysis were also similar. However, after lipid infusion, rates of glycogenolysis were suppressed by safflower oil but not by palm oil. Thus these studies demonstrate, for the first time in humans, a differential effect of saturated fatty acids and PUFA on hepatic glucose metabolism.

Fatty acid-induced insulin resistance in L6 myotubes is prevented by inhibition of activation and nuclear localization of nuclear factor kappa B

Recent studies have implicated inhibitor of kappaB kinase (IKK) in mediating fatty acid (FA)-induced insulin resistance. How IKK causes these effects is unknown. The present study addressed the role of nuclear factor kappaB (NFkappaB), the distal target of IKK activity, in FA-induced insulin resistance in L6 myotubes, an in vitro skeletal muscle model. A 6-h exposure of myotubes to the saturated FA palmitate reduced insulin-stimulated glucose uptake by approximately 30%, phosphatidylinositol-3 kinase and protein kinase B phosphorylation by approximately 40%, and stimulated inhibitor of kappaBalpha degradation and the nuclear translocation of NFkappaB. On the other hand, the Omega-3 polyunsaturated FA linolenate neither induced insulin resistance nor promoted nuclear localization of NFkappaB. Supporting the hypothesis that IKK acts through NFkappaB to cause insulin resistance, the IKK inhibitors acetylsalicylate and parthenolide prevented FA-induced reductions in insulin-stimulated glucose uptake and NFkappaB nuclear translocation. Most importantly, NFkappaB SN50, a cell-permeable peptide that inhibits NFkappaB nuclear translocation downstream of IKK, was sufficient to prevent palmitate-induced reductions in insulin-stimulated glucose uptake. Acetylsalicylate, but not NFkappaB SN50, prevented FA effects on phosphatidylinositol-3 kinase activity and protein kinase B phosphorylation. We conclude that FAs induce insulin resistance and activates NFkappaB in L6 cells. Furthermore, inhibition of NFkappaB activation, indirectly by preventing IKK activation or directly by inhibiting NFkappaB nuclear translocation, prevents the detrimental effects of palmitate on the metabolic actions of insulin in L6 myotubes.

Regulation of insulin action by ceramide: dual mechanisms linking ceramide accumulation to the inhibition of Akt/protein kinase B

The sphingolipid ceramide negatively regulates insulin action by inhibiting Akt/protein kinase B (PKB), a serine/threonine kinase that is a central regulator of glucose uptake and anabolic metabolism. Despite considerable attention, the molecular mechanism accounting for this action of ceramide has remained both elusive and controversial. Herein we utilized deletion constructs encoding two different functional domains of Akt/PKB to identify which region of the enzyme conferred responsiveness to ceramide. Surprisingly the findings obtained with these separate domains reveal that ceramide blocks insulin stimulation of Akt/PKB by two independent mechanisms. First, using the isolated pleckstrin homology domain, we found that ceramide specifically blocks the translocation of Akt/PKB, but not its upstream activator phosphoinositide-dependent kinase-1, to the plasma membrane. Second, using a construct lacking this pleckstrin homology domain, which does not require translocation for activation, we found that ceramide stimulates the dephosphorylation of Akt/PKB by protein phosphatase 2A. Collectively these findings identify at least two independent mechanisms by which excessive ceramide accumulation in peripheral tissues could contribute to the development of insulin resistance. Moreover the results obtained provide a unifying theory to account for the numerous dissenting reports investigating the actions of ceramide toward Akt/PKB.

Relationship between insulin sensitivity and sphingomyelin signaling pathway in human skeletal muscle

In vitro studies revealed that insulin resistance might be associated with the intracellular formation of ceramide, the second messenger in the sphingomyelin signaling pathway. The aim of the present study was to examine the content and composition of fatty acids in ceramide and sphingomyelin in human muscle and to evaluate their relationships with insulin sensitivity. The study was conducted on 27 male subjects with normal glucose tolerance. Euglycemic-hyperinsulinemic clamps and biopsies of vastus lateralis muscle were performed. In 10 subjects, additional biopsies were taken after a 4-h clamp and after a clamp with concurrent Intralipid/heparin infusion. We identified 13 ceramides and sphingomyelins according to fatty acid residues. Insulin sensitivity was related to total ceramide content (r = -0.49, P = 0.01) and to ceramide consisting of palmitic (r = -0.48, P = 0.011), palmitoleic (r = -0.45, P = 0.019), mirystic (r = -0.42, P = 0.028), and nervonic acid (r = -0.39, P = 0.047). Hyperinsulinemia did not affect estimated muscle parameters. Intralipid/heparin infusion resulted in a 24.73% decrease in insulin sensitivity (P = 0.007) and a 47.81% increase in ceramide content (P = 0.005). These changes were significantly related to each other (r = -0.64, P = 0.046). A relationship with the decrease in insulin sensitivity was also observed for ceramides consisting of palmitic (r = -0.68, P = 0.03) and linoleic (r = -0.66, P = 0.038) acid. Our data indicate that the sphingomyelin signaling pathway in muscle might be an important factor determining the development of insulin resistance in humans.

Palmitate, but not unsaturated fatty acids, induces the expression of interleukin-6 in human myotubes through proteasome-dependent activation of nuclear factor-kappaB

Circulating interleukin-6 (IL-6), insulin, and free fatty acid (FFA) concentrations are associated with impaired insulin action in obese and type 2 diabetic individuals. However, a causal relationship between elevated plasma FFAs and IL-6 has not been shown. Because skeletal muscle represents a major target of impaired insulin action, we studied whether FFAs may affect IL-6 expression in human myotubes. We demonstrate that specifically saturated FFAs, e.g. palmitate (0.25 mm), induce IL-6 mRNA expression and protein secretion by a proteasome-dependent mechanism that leads to a rapid and chronic activation of nuclear factor-kappaB. Insulin, high glucose concentrations, or unsaturated FFAs did not activate IL-6 expression. In fact, the unsaturated FFA linoleate inhibited palmitate-induced IL-6 production. Because inhibition of palmitate metabolism by the acyl-CoA synthetase inhibitor triacsin C did not abolish IL-6 expression, it appears that the palmitate molecule per se exerts the observed effects. Furthermore, we show that in human myotubes, IL-6 activates the phosphorylation of signal transducer and activator of transcription 3 in concentrations similar to hepatocytes. However, no inhibitory effect of IL-6 on insulin action, determined as phosphatidylinositol 3-kinase association with insulin receptor substrate-1, Akt phosphorylation, and glycogen synthesis, was detected. We conclude that IL-6 expression may be modulated by the composition of circulating FFA, e.g. by diet, and that skeletal muscle cells could be target cells for IL-6.

Muscle fat oxidative capacity is not impaired by age but by physical inactivity: association with insulin sensitivity

The study aimed at determining whether aging and/or sedentariness impairs muscle fat oxidative capacity (OXFA) and whether this was associated with increased risk to develop insulin resistance. We first examined muscle mitochondrial functions, OXFA and insulin sensitivity (ISI; evaluated during an oral glucose tolerance test) in a cross-sectional study with 32 sedentary (S) and endurance-trained (T), young (Y) and elderly (E) men (24.2+/-2.6 vs. 66.6+/-3.2 yr). As for mitochondrial functions, OXFA was higher in T than in S but similar between age groups (SY 41.8+/-11.3, TY 68.0+/-17.7, SE 40.1+/-14.1, TE 73.1+/-20.1 palmitate x min(-1) x g wet tissue(-1); activity P<0.0001, age P=NS, activity x age P=NS). Similar results were obtained with ISI (SY 6.2+/-2.2, TY 11.4+/-4.4, SE 5.9+/-1.5, TE 11.0+/-3.5, activity P<0.001, age P=NS, activity x age P=NS). Stepwise regression showed that, among body composition, VO2max and muscle biochemical characteristics, OXFA was the main predictor of ISI (r=0.60, P<0.001). We subsequently showed in eight sedentary elderly subjects (63.5+/-3.3 yr) that OXFA and insulin sensitivity (measured using insulin clamp) improved in parallel after 8 weeks of endurance training (r=0.79, P<0.01). We concluded that mitochondrial functions, OXFA and ISI, are not impaired by age but by physical inactivity and are closely correlated.

Identification of a novel mitochondrial protein ("mitoNEET") cross-linked specifically by a thiazolidinedione photoprobe

Thiazolidinediones address underlying causes of type 2 diabetes, although their mechanism of action is not clearly understood. The compounds are thought to function as direct activators of the nuclear receptor PPARgamma (peroxisome proliferator-activated receptor-gamma), although pioglitazone, the weaker agonist of the two thiazolidinediones now in clinical use, seems to have more useful effects on circulating lipids. We have used tritiated pioglitazone and a photoaffinity cross-linker to identify a novel binding site in mitochondria. A saturable binding site for [3H]pioglitazone was solubilized from the membranes with CHAPS and migrated as a large complex by size exclusion chromatography. The binding correlated with a <17-kDa protein (m17), marked by a photoaffinity cross-linker, in both subcellular location and selectivity of competition by analogs. The protein was isolated and identified by mass spectrometry analysis and NH2-terminal sequencing. Three synthetic peptides with potential antigenic properties were synthesized from the predicted nontransmembrane sequence to generate antibodies in rabbits. Western blots show that this protein, which we have termed "mitoNEET," is located in the mitochondrial fraction of rodent brain, liver, and skeletal muscle, showing the identical subcellular location and migration on SDS-PAGE as the protein cross-linked specifically by the thiazolidinedione photoprobe. The protein exists in low levels in preadipocytes, and expression increases exponentially in differentiated adipocytes. The synthetic protein bound to solid phase associated with a complex of solubilized mitochondrial proteins, including the trifunctional beta-oxidation protein. It is possible that thiazolidinedione modification of the function of the mitochondrial target may contribute to lipid lowering and/or antidiabetic actions.

Ceramide content is increased in skeletal muscle from obese insulin-resistant humans

Increased intramyocellular lipid concentrations are thought to play a role in insulin resistance, but the precise nature of the lipid species that produce insulin resistance in human muscle are unknown. Ceramides, either generated via activation of sphingomyelinase or produced by de novo synthesis, induce insulin resistance in cultured cells by inhibitory effects on insulin signaling. The present study was undertaken to determine whether ceramides or other sphingolipids are increased in muscle from obese insulin-resistant subjects and to assess whether ceramide plays a role in the insulin resistance of Akt in human muscle. Lean insulin-sensitive and obese insulin-resistant subjects (n = 10 each) received euglycemic-hyperinsulinemic clamps with muscle biopsies basally and after 30, 45, or 60 min of insulin infusion. The rate of glucose infusion required to maintain euglycemia (reflecting glucose uptake) was reduced by >50%, as expected, in the obese subjects at each time point (P < 0.01). Under basal conditions, total muscle ceramide content was increased nearly twofold in the obese subjects (46 +/- 9 vs. 25 +/- 2 pmol/2 mg muscle, P < 0.05). All species of ceramides were increased similarly in the obese subjects; in contrast, no other sphingolipid was increased. Stimulation of Akt phosphorylation by insulin in the obese subjects was significantly reduced after 30 min (0.96 +/- 0.11 vs. 1.84 +/- 0.38 arbitrary units) or 45-60 min (0.68 +/- 0.17 vs. 1.52 +/- 0.26) of insulin infusion (P < 0.05 for both). Muscle ceramide content was significantly correlated with the plasma free fatty acid concentration (r = 0.51, P < 0.05). We conclude that obesity is associated with increased intramyocellular ceramide content. This twofold increase in ceramide may be involved in the decrease in Akt phosphorylation observed after insulin infusion and could theoretically play a role in the reduced ability of insulin to stimulate glucose uptake in skeletal muscle from obese subjects.