Lactate induces insulin resistance in skeletal muscle by suppressing glycolysis and impairing insulin signaling

Elevation of plasma lactate levels induces peripheral insulin resistance, but the underlying mechanisms are unclear. We examined whether lactate infusion in rats suppresses glycolysis preceding insulin resistance and whether lactate-induced insulin resistance is accompanied by altered insulin signaling and/or insulin-stimulated glucose transport in skeletal muscle. Hyperinsulinemic euglycemic clamps were conducted for 6 h in conscious, overnight-fasted rats with or without lactate infusion (120 micromol x kg(-1) x min(-1)) during the final 3.5 h. Lactate infusion increased plasma lactate levels about fourfold. The elevation of plasma lactate had rapid effects to suppress insulin-stimulated glycolysis, which clearly preceded its effect to decrease insulin-stimulated glucose uptake. Both submaximal and maximal insulin-stimulated glucose transport decreased 25-30% (P < 0.05) in soleus but not in epitrochlearis muscles of lactate-infused rats. Lactate infusion did not alter insulin's ability to phosphorylate the insulin receptor, the insulin receptor substrate (IRS)-1, or IRS-2 but decreased insulin's ability to stimulate IRS-1- and IRS-2-associated phosphatidylinositol 3-kinase activities and Akt/protein kinase B activity by 47, 75, and 55%, respectively (P < 0.05 for all). In conclusion, elevation of plasma lactate suppressed glycolysis before its effect on insulin-stimulated glucose uptake, consistent with the hypothesis that suppression of glucose metabolism could precede and cause insulin resistance. In addition, lactate-induced insulin resistance was associated with impaired insulin signaling and decreased insulin-stimulated glucose transport in skeletal muscle.     

Inhibition of IRS-1 phosphorylation and the alterations of GLUT4 in isolated adipocytes from cachectic tumor-bearing rats

Cellular and molecular mechanisms of insulin resistance in isolated adipocytes from methylcholanthrene-induced sarcoma-bearing rats were investigated by measuring 3-O-[14C]methyl glucose transport activity, glucose transporter-4 (GLUT4) protein in both plasma membrane and low-density microsomes, and insulin-stimulated tyrosine phosphorylation of the insulin receptor (IR) and insulin receptor substrate-1 (IRS-1). Compared to both pair-fed and freely fed controls, tumor-bearing rats (TBR) had a decreased insulin-stimulated glucose transport activity with a lower Vmax and a higher EC50. GLUT4 protein in low-density microsomes from adipocytes maintained at the basal state was less in TBR than in controls. In insulin-stimulated adipocytes, GLUT4 protein in plasma membranes was also less in tumor-bearing rats than in controls. Insulin-induced tyrosine phosphorylation of IRS-1 was less in TBR than controls, but that of the IR was similar among the three groups. These data suggest that the insulin resistance seen in adipose cells of these tumor-bearing rats was caused in part by a decreased amount of GLUT4 protein in both basal and insulin-stimulated states resulting from the selective inhibition of insulin-stimulated phosphorylation of IRS-1.     

Reduced glucose uptake precedes insulin signaling defects in adipocytes from heterozygous GLUT4 knockout mice.

Decreased GLUT4 expression, impaired insulin receptor (IR), IRS-1, and pp60/IRS-3 tyrosine phosphorylation are characteristics of adipocytes from insulin-resistant animal models and obese NIDDM humans. However, the sequence of events leading to the development of insulin signaling defects and the significance of decreased GLUT4 expression in causing adipocyte insulin resistance are unknown. The present study used male heterozygous GLUT4 knockout mice (GLUT4(+/-)) as a novel model of diabetes to study the development of insulin signaling defects in adipocytes with the progression of whole body insulin resistance and diabetes. Male GLUT4(+/-) mice with normal fed glycemia and insulinemia (N/N), normal fed glycemia and hyperinsulinemia (N/H), and fed hyperglycemia with hyperinsulinemia (H/H) exist at all ages. The expression of GLUT4 protein and the maximal insulin-stimulated glucose transport was 50% decreased in adipocytes from all three groups. Insulin signaling was normal in N/N adipose cells. From 35 to 70% reductions in insulin-stimulated tyrosine phosphorylation of IR, IRS-1, and pp60/IRS-3 were noted with no changes in the cellular content of IR, IRS-1, and p85 in N/H adipocytes. Insulin-stimulated protein tyrosine phosphorylation was further decreased to 12-23% in H/H adipose cells accompanied by 42% decreased IR and 80% increased p85 expression. Insulin-stimulated, IRS-1-associated PI3 kinase activity was decreased by 20% in N/H and 68% reduced in H/H GLUT4(+/-) adipocytes. However, total insulin-stimulated PI3 kinase activity was normal in H/H GLUT4(+/-) adipocytes. Taken together, these results strongly suggest that hyperinsulinemia triggers a reduction of IR tyrosine kinase activity that is further exacerbated by the appearance of hyperglycemia. However, the insulin signaling cascade has sufficient plasticity to accommodate significant changes in specific components without further reducing glucose uptake. Furthermore, the data indicate that the cellular content of GLUT4 is the rate-limiting factor in mediating maximal insulin-stimulated glucose uptake in GLUT4(+/-) adipocytes.     

Impact of genetic background and ablation of insulin receptor substrate (IRS)-3 on IRS-2 knock-out mice

Although we and others have generated IRS-2 knock-out (IRS-2(-/-)) mice, significant differences were seen between the two lines of IRS-2(-/-) mice in the severity of diabetes and alterations of beta-cell mass. It has been reported that although IRS-1 and IRS-3 knock-out mice showed normal blood glucose levels, IRS-1/IRS-3 double knock-out mice exhibited marked hyperglycemia. Thus, IRS-1 and IRS-3 compensate each other's functions in maintaining glucose homeostasis. To assess the effect of genetic background and also ablation of IRS-3 on IRS-2(-/-), we generated IRS-2/IRS-3 double knock-out (IRS-2(-/-)IRS-3(-/-)) mice by crossing IRS-3(-/-) mice (129/Sv and C57Bl/6 background) with our IRS-2(-/-) mice (CBA and C57Bl/6 background). Intercrosses of IRS-2(+/-)IRS-3(+/-) mice yielded nine genotypes, and all of them including IRS-2(-/-)IRS-3(-/-) mice were apparently healthy and showed normal growth. However, at 10-20 weeks of age, 20-30% mice carrying a null mutation for the IRS-2 gene, irrespective of the IRS-3 genotype, developed diabetes. When mice with diabetes were excluded from the analysis of glucose and insulin tolerance test, IRS-2(-/-)IRS-3(-/-) showed a degree of glucose intolerance and insulin resistance similar to those of IRS-2(-/-) mice. Both IRS-2(-/-) and IRS-2(-/-)IRS-3(-/-) mice had moderately reduced beta-cell mass despite having insulin resistance. Insulin-positive beta-cells were decreased to nearly zero in IRS-2(-/-) mice with diabetes. Although Pdx1 and glucose transporter 2 expressions were essentially unaltered in islets from IRS-2(-/-) mice without diabetes, they were dramatically decreased in IRS-2(-/-) mice with diabetes. Taken together, these observations indicate that IRS-3 does not play a role compensating for the loss of IRS-2 in maintaining glucose homeostasis and that the severity of diabetes in IRS-2(-/-) mice depends upon genetic background, suggesting the existence of modifier gene(s) for diabetes in mice of the 129/Sv genetic strain.     

Oxidant stress-induced loss of IRS-1 and IRS-2 proteins in rat skeletal muscle: Role of p38 MAPK

Oxidative stress is characterized as an imbalance between the cellular production of oxidants and the cellular antioxidant defenses and contributes to the development of numerous cardiovascular and metabolic disorders, including hypertension and insulin resistance. The effects of prolonged oxidant stress in vitro on the insulin-dependent glucose transport system in mammalian skeletal muscle are not well understood. This study examined the in vitro effects of low-level oxidant stress (60–90 μM, H₂O₂) for 4 h on insulin-stimulated (5 mU/ml) glucose transport activity (2-deoxyglucose uptake) and on protein expression of critical insulin signaling factors (insulin receptor (IR), IR substrates IRS-1 and IRS-2, phosphatidylinositol 3-kinase, Akt, and glycogen synthase kinase-3 (GSK-3)) in isolated soleus muscle of lean Zucker rats. This oxidant stress exposure caused significant (50%, p < 0.05) decreases in insulin-stimulated glucose transport activity that were associated with selective loss of IRS-1 (59%) and IRS-2 (33%) proteins, increased (64%) relative IRS-1 Ser³⁰⁷ phosphorylation, and decreased phosphorylation of Akt Ser⁴⁷³ (50%) and GSK-3β Ser⁹ (43%). Moreover, enhanced (37%) phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK) was observed. Selective inhibition of p38 MAPK (10 μM A304000) prevented a significant portion (29%) of the oxidant stress-induced loss of IRS-1 (but not IRS-2) protein and allowed partial recovery of the impaired insulin-stimulated glucose transport activity. These results indicate that in vitro oxidative stress in mammalian skeletal muscle leads to substantial insulin resistance of distal insulin signaling and glucose transport activity, associated with a selective loss of IRS-1 protein, in part due to a p38 MAPK-dependent mechanism.     

Exercise-induced muscle damage impairs insulin signaling pathway associated with IRS-1 oxidative modification

Strenuous exercise induces delayed-onset muscle damage including oxidative damage of cellular components. Oxidative stress to muscle cells impairs glucose uptake via disturbance of insulin signaling pathway. We investigated glucose uptake and insulin signaling in relation to oxidative protein modification in muscle after acute strenuous exercise. ICR mice were divided into sedentary and exercise groups. Mice in the exercise group performed downhill running exercise at 30 m/min for 30 min. At 24 hr after exercise, metabolic performance and insulin-signaling proteins in muscle tissues were examined. In whole body indirect calorimetry, carbohydrate utilization was decreased in the exercised mice along with reduction of the respiratory exchange ratio compared to the rested control mice. Insulin-stimulated uptake of 2-deoxy-[(3)H]glucose in damaged muscle was decreased after acute exercise. Tyrosine phosphorylation of insulin receptor substrate (IRS)-1 and phosphatidyl-3-kinase/Akt signaling were impaired by exercise, leading to inhibition of the membrane translocation of glucose transporter 4. We also found that acute exercise caused 4-hydroxy-nonenal modification of IRS-1 along with elevation of oxidative stress in muscle tissue. Impairment of insulin-induced glucose uptake into damaged muscle after strenuous exercise would be related to disturbance of insulin signal transduction by oxidative modification of IRS-1.     

Action of insulin receptor substrate-3 (IRS-3) and IRS-4 to stimulate translocation of GLUT4 in rat adipose cells

The insulin receptor initiates insulin action by phosphorylating multiple intracellular substrates. Previously, we have demonstrated that insulin receptor substrates (IRS)-1 and -2 can mediate insulin's action to promote translocation of GLUT4 glucose transporters to the cell surface in rat adipose cells. Although IRS-1, -2, and -4 are similar in overall structure, IRS-3 is approximately 50% shorter and differs with respect to sites of tyrosine phosphorylation. Nevertheless, as demonstrated in this study, both IRS-3 and IRS-4 can also stimulate translocation of GLUT4. Rat adipose cells were cotransfected with expression vectors for hemagglutinin (HA) epitope-tagged GLUT4 (GLUT4-HA) and human IRS-1, murine IRS-3, or human IRS-4. Overexpression of IRS-1 led to a 2-fold increase in cell surface GLUT4-HA in cells incubated in the absence of insulin; overexpression of either IRS-3 or IRS-4 elicited a larger increase in cell surface GLUT4-HA. Indeed, the effect of IRS-3 in the absence of insulin was approximately 40% greater than the effect of a maximally stimulating concentration of insulin in cells not overexpressing IRS proteins. Because phosphatidylinositol (PI) 3-kinase is essential for insulin-stimulated translocation of GLUT4, we also studied a mutant IRS-3 molecule (IRS-3-F4) in which Phe was substituted for Tyr in all four YXXM motifs (the phosphorylation sites predicted to bind to and activate PI 3-kinase). Interestingly, overexpression of IRS-3-F4 did not promote translocation of GLUT4-HA, but actually inhibited the ability of insulin to stimulate translocation of GLUT4-HA to the cell surface. Our data suggest that IRS-3 and IRS-4 are capable of mediating PI 3-kinase-dependent metabolic actions of insulin in adipose cells, and that IRS proteins play a physiological role in mediating translocation of GLUT4.     

The farnesoid X receptor modulates adiposity and peripheral insulin sensitivity in mice

The farnesoid X receptor (FXR) is a bile acid (BA)-activated nuclear receptor that plays a major role in the regulation of BA and lipid metabolism. Recently, several studies have suggested a potential role of FXR in the control of hepatic carbohydrate metabolism, but its contribution to the maintenance of peripheral glucose homeostasis remains to be established. FXR-deficient mice display decreased adipose tissue mass, lower serum leptin concentrations, and elevated plasma free fatty acid levels. Glucose and insulin tolerance tests revealed that FXR deficiency is associated with impaired glucose tolerance and insulin resistance. Moreover, whole-body glucose disposal during a hyperinsulinemic euglycemic clamp is decreased in FXR-deficient mice. In parallel, FXR deficiency alters distal insulin signaling, as reflected by decreased insulin-dependent Akt phosphorylation in both white adipose tissue and skeletal muscle. Whereas FXR is not expressed in skeletal muscle, it was detected at a low level in white adipose tissue in vivo and induced during adipocyte differentiation in vitro. Moreover, mouse embryonic fibroblasts derived from FXR-deficient mice displayed impaired adipocyte differentiation, identifying a direct role for FXR in adipocyte function. Treatment of differentiated 3T3-L1 adipocytes with the FXR-specific synthetic agonist GW4064 enhanced insulin signaling and insulin-stimulated glucose uptake. Finally, treatment with GW4064 improved insulin resistance in genetically obese ob/ob mice in vivo. Although the underlying molecular mechanisms remain to be unraveled, these results clearly identify a novel role of FXR in the regulation of peripheral insulin sensitivity and adipocyte function. This unexpected function of FXR opens new perspectives for the treatment of type 2 diabetes.     

Metabolic implications of reduced heart-type fatty acid binding protein in insulin resistant cardiac muscle

Insulin resistance is characterized by elevated rates of cardiac fatty acid utilization resulting in reduced efficiency and cardiomyopathy. One potential therapeutic approach is to limit the uptake and oxidation of fatty acids. The aims of this study were to determine whether a quantitative reduction in heart-type fatty acid binding protein (FABP3) normalizes cardiac substrate utilization without altering cardiac function. Transgenic (FABP3(+/-)) and wild-type (WT) littermates were studied following low fat (LF) or high fat (HF) diets, with HF resulting in obese, insulin-resistant mice. Cardiovascular function (systolic blood pressure, % fractional shortening) and heart dimension were measured at weaning and every month afterward for 3 mo. During this period cardiovascular function was the same independent of genotype and diet. Catheters were surgically implanted in the carotid artery and jugular vein for sampling and infusions in mice at 4 mo of age. Following 5 d recovery, mice underwent either a saline infusion or a hyperinsulinemic-euglycemic clamp (4 mU kg(-1) min(-1)). Indices of long chain fatty acid and glucose utilization (R(f), R(g); mumol g wet weight(-1) min(-1)) were obtained using 2-deoxy[(3)H]glucose and [(125)I]-15-rho-iodophenyl)-3-R,S-methylpentadecanoic acid. FABP3(+/-) had enhanced cardiac R(g) compared with WT during saline infusion in both LF and HF. FABP3(+/-) abrogated the HF-induced decrement in insulin-stimulated cardiac R(g). On a HF diet, FABP(+/-) but not WT had an increased reliance on fatty acids (R(f)) during insulin stimulation. In conclusion, cardiac insulin resistance and glucose uptake is largely corrected by a reduction in FABP3 in vivo without contemporaneous deleterious effects on cardiac function.     

Mechanism by which fatty acids inhibit insulin activation of insulin receptor substrate-1 (IRS-1)-associated phosphatidylinositol 3-kinase activity in muscle

Recent studies have demonstrated that fatty acids induce insulin resistance in skeletal muscle by blocking insulin activation of insulin receptor substrate-1 (IRS-1)-associated phosphatidylinositol 3-kinase (PI3-kinase). To examine the mechanism by which fatty acids mediate this effect, rats were infused with either a lipid emulsion (consisting mostly of 18:2 fatty acids) or glycerol. Intracellular C18:2 CoA increased in a time-dependent fashion, reaching an approximately 6-fold elevation by 5 h, whereas there was no change in the concentration of any other fatty acyl-CoAs. Diacylglycerol (DAG) also increased transiently after 3-4 h of lipid infusion. In contrast there was no increase in intracellular ceramide or triglyceride concentrations during the lipid infusion. Increases in intracellular C18:2 CoA and DAG concentration were associated with protein kinase C (PKC)-theta activation and a reduction in both insulin-stimulated IRS-1 tyrosine phosphorylation and IRS-1 associated PI3-kinase activity, which were associated with an increase in IRS-1 Ser(307) phosphorylation. These data support the hypothesis that an increase in plasma fatty acid concentration results in an increase in intracellular fatty acyl-CoA and DAG concentrations, which results in activation of PKC-theta leading to increased IRS-1 Ser(307) phosphorylation. This in turn leads to decreased IRS-1 tyrosine phosphorylation and decreased activation of IRS-1-associated PI3-kinase activity resulting in decreased insulin-stimulated glucose transport activity.     

Lack of REDD1 reduces whole body glucose and insulin tolerance,and impairs skeletal muscle insulin signaling

A lack of the REDD1 promotes dysregulated growth signaling, though little has been established with respect to the metabolic role of REDD1. Therefore, the goal of this study was to determine the role of REDD1 on glucose and insulin tolerance, as well as insulin stimulated growth signaling pathway activation in skeletal muscle. First, intraperitoneal (IP) injection of glucose or insulin were administered to REDD1 wildtype (WT) versus knockout (KO) mice to examine changes in blood glucose over time. Next, alterations in skeletal muscle insulin (IRS-1, Akt, ERK 1/2) and growth (4E-BP1, S6K1, REDD1) signaling intermediates were determined before and after IP insulin treatment (10 min). REDD1 KO mice were both glucose and insulin intolerant when compared to WT mice, evident by higher circulating blood glucose concentrations and a greater area under the curve following IP injections of glucose or insulin. While the REDD1 KO exhibited significant though blunted insulin-stimulated increases (p < 0.05) in Akt S473 and T308 phosphorylation versus the WT mice, acute insulin treatment has no effect (p < 0.05) on REDD1 KO skeletal muscle 4E-BP1 T37/46, S6K1 T389, IRS-1 Y1222, and ERK 1/2 T202/Y204 phosphorylation versus the WT mice. Collectively, these novel data suggest that REDD1 has a more distinct role in whole body and skeletal muscle metabolism and insulin action than previously thought.     

Muscle damage impairs insulin stimulation of IRS-1, PI 3-kinase, and Akt-kinase in human skeletal muscle

Physiological stress associated with muscle damage results in systemic insulin resistance. However, the mechanisms responsible for the insulin resistance are not known; therefore, the present study was conducted to elucidate the molecular mechanisms associated with insulin resistance after muscle damage. Muscle biopsies were obtained before (base) and at 1 h during a hyperinsulinemic-euglycemic clamp (40 mU x kg(-1) x min(-1)) in eight young (age 24+/-1 yr) healthy sedentary (maximal O(2) consumption, 49.7+/-2.4 ml x kg(-1) x min(-1)) males before and 24 h after eccentric exercise (ECC)-induced muscle damage. To determine the role of cytokines in ECC-induced insulin resistance, venous blood samples were obtained before (control) and 24 h after ECC to evaluate ex vivo endotoxin-induced mononuclear cell secretion of tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, and IL-1beta. Glucose disposal was 19% lower after ECC (P<0.05). Insulin-stimulated insulin receptor substrate (IRS)-1 tyrosine phosphorylation was 45% lower after ECC (P<0.05). Insulin-stimulated phosphatidylinositol (PI) 3-kinase, Akt (protein kinase B) serine phosphorylation, and Akt activity were reduced 34, 65, and 20%, respectively, after ECC (P < 0.05). TNF-alpha, but not IL-6 or IL-1beta production, increased 2.4-fold 24 h after ECC (P<0.05). TNF-alpha production was positively correlated with reduced insulin action on PI 3-kinase (r = 0.77, P = 0.04). In summary, the physiological stress associated with muscle damage impairs insulin stimulation of IRS-1, PI 3-kinase, and Akt-kinase, presumably leading to decreased insulin-mediated glucose uptake. Although more research is needed on the potential role for TNF-alpha inhibition of insulin action, elevated TNF-alpha production after muscle damage may impair insulin signal transduction.     

A novel use of the hyperinsulinemic-euglycemic clamp technique to estimate insulin sensitivity of systemic lipolysis.

The aim of the present study was to assess whether a standard hyperinsulinemic-euglycemic clamp can provide an estimate for the antilipolytic insulin sensitivity. For this purpose, we infused 9 non-obese, healthy volunteers with [2H5]glycerol and used the glycerol rate of appearance (Ra) in plasma as an index for systemic lipolysis during a standard (1 mU/kg x min, 120 min) and a 3-step (0.1, 0.25, 1.0 mU/kg x min) hyperinsulinemic-euglycemic clamp. The insulin concentration, which half-maximally suppressed lipolysis (EC50) in the three-step clamp, was considered to be the gold standard for the antilipolytic insulin sensitivity. Glycerol Ra decreased from 1.53+/-0.11 micromol/kg x min to 0.60+/-0.09 micromol/kg x min (p <0.001) during the standard clamp. The decrease in Ra at most time points during the standard clamp significantly correlated with the EC50. The highest correlation for the % decrease of glycerol Ra from baseline was found at 60 min (r = 0.96, p < 0.001) making this parameter a useful index for the antilipoytic insulin sensitivity. Neither plasma glycerol nor plasma free fatty acid (FFA) concentrations were significantly correlated with the EC50. In conclusion, the standard hyperinsulinemic-euglycemic clamp in combination with isotopic determination of glycerol Ra provides a reasonable estimate for the antilipolytic insulin sensitivity. In healthy subjects, the parameter best suited to estimate the insulin EC50 (by linear correlation) was the percentage decrease of glycerol Ra at 60 min.     

Differential effects of pharmacological liver X receptor activation on hepatic and peripheral insulin sensitivity in lean and ob/ob mice

Liver X receptor (LXR) agonists have been proposed to act as anti-diabetic drugs. However, pharmacological LXR activation leads to severe hepatic steatosis, a condition usually associated with insulin resistance and type 2 diabetes mellitus. To address this apparent contradiction, lean and ob/ob mice were treated with the LXR agonist GW-3965 for 10 days. Insulin sensitivity was assessed by hyperinsulinemic-euglycemic clamp studies. Hepatic glucose production (HGP) and metabolic clearance rate (MCR) of glucose were determined with stable isotope techniques. Blood glucose and hepatic and whole body insulin sensitivity remained unaffected upon treatment in lean mice, despite increased hepatic triglyceride contents (61.7 +/- 7.2 vs. 12.1 +/- 2.0 nmol/mg liver, P < 0.05). In ob/ob mice, LXR activation resulted in lower blood glucose levels and significantly improved whole body insulin sensitivity. GW-3965 treatment did not affect HGP under normo- and hyperinsulinemic conditions, despite increased hepatic triglyceride contents (221 +/- 13 vs. 176 +/- 19 nmol/mg liver, P < 0.05). Clamped MCR increased upon GW-3965 treatment (18.2 +/- 1.0 vs. 14.3 +/- 1.4 ml x kg(-1) x min(-1), P = 0.05). LXR activation increased white adipose tissue mRNA levels of Glut4, Acc1 and Fasin ob/ob mice only. In conclusion, LXR-induced blood glucose lowering in ob/ob mice was attributable to increased peripheral glucose uptake and metabolism, physiologically reflected in a slightly improved insulin sensitivity. Remarkably, steatosis associated with LXR activation did not affect hepatic insulin sensitivity.     

Calcium sparks in the intact gerbil spiral modiolar artery

Background

We and others have demonstrated previously that ghrelin receptor (GhrR) knock out (KO) mice fed a high fat diet (HFD) have increased insulin sensitivity and metabolic flexibility relative to WT littermates. A striking feature of the HFD-fed GhrR KO mouse is the dramatic decrease in hepatic steatosis. To characterize further the underlying mechanisms of glucose homeostasis in GhrR KO mice, we conducted both hyperglycemic (HG) and hyperinsulinemic-euglycemic (HI-E) clamps. Additionally, we investigated tissue glucose uptake and specifically examined liver insulin sensitivity.

Results

Consistent with glucose tolerance-test data, in HG clamp experiments, GhrR KO mice showed a reduction in glucose-stimulated insulin release relative to WT littermates. Nevertheless, a robust 1^st phase insulin secretion was still achieved, indicating that a healthy β-cell response is maintained. Additionally, GhrR KO mice demonstrated both a significantly increased glucose infusion rate and significantly reduced insulin requirement for maintenance of the HG clamp, consistent with their relative insulin sensitivity. In HI-E clamps, both LFD-fed and HFD-fed GhrR KO mice showed higher peripheral insulin sensitivity relative to WT littermates as indicated by a significant increase in insulin-stimulated glucose disposal (Rd), and decreased hepatic glucose production (HGP). HFD-fed GhrR KO mice showed a marked increase in peripheral tissue glucose uptake in a variety of tissues, including skeletal muscle, brown adipose tissue and white adipose tissue. GhrR KO mice fed a HFD also showed a modest, but significant decrease in conversion of pyruvate to glucose, as would be anticipated if these mice displayed increased liver insulin sensitivity. Additionally, the levels of UCP2 and UCP1 were reduced in the liver and BAT, respectively, in GhrR KO mice relative to WT mice.

Conclusions

These results indicate that improved glucose homeostasis of GhrR KO mice is characterized by robust improvements of glucose disposal in both normal and metabolically challenged states, relative to WT controls. GhrR KO mice have an intact 1^st phase insulin response but require significantly less insulin for glucose disposal. Our experiments reveal that the insulin sensitivity of GhrR KO mice is due to both BW independent and dependent factors. We also provide several lines of evidence that a key feature of the GhrR KO mouse is maintenance of hepatic insulin sensitivity during metabolic challenge.     

Regular exercise enhances insulin activation of IRS-1-associated PI3-kinase in human skeletal muscle.

Insulin action in skeletal muscle is enhanced by regular exercise. Whether insulin signaling in human skeletal muscle is affected by habitual exercise is not well understood. Phosphatidylinositol 3-kinase (PI3-kinase) activation is an important step in the insulin-signaling pathway and appears to regulate glucose metabolism via GLUT-4 translocation in skeletal muscle. To examine the effects of regular exercise on PI3-kinase activation, 2-h hyperinsulinemic (40 mU. m(-2). min(-1))-euglycemic (5.0 mM) clamps were performed on eight healthy exercise-trained [24 +/- 1 yr, 71.8 +/- 2.0 kg, maximal O(2) uptake (VO(2 max)) of 56.1 +/- 2.5 ml. kg(-1). min(-1)] and eight healthy sedentary men and women (24 +/- 1 yr, 64.7 +/- 4.4 kg, VO(2 max) of 44.4 +/- 2.7 ml. kg(-1). min(-1)). A [6, 6-(2)H]glucose tracer was used to measure hepatic glucose output. A muscle biopsy was obtained from the vastus lateralis muscle at basal and at 2 h of hyperinsulinemia to measure insulin receptor substrate-1(IRS-1)-associated PI3-kinase activation. Insulin concentrations during hyperinsulinemia were similar for both groups (293 +/- 22 and 311 +/- 22 pM for trained and sedentary, respectively). Insulin-mediated glucose disposal rates (GDR) were greater (P < 0.05) in the exercise-trained compared with the sedentary control group (9.22 +/- 0.95 vs. 6.36 +/- 0.57 mg. kg fat-free mass(-1). min(-1)). Insulin-stimulated PI3-kinase activation was also greater (P < 0.004) in the trained compared with the sedentary group (3.8 +/- 0.5- vs. 1.8 +/- 0.2-fold increase from basal). Endurance capacity (VO(2 max)) was positively correlated with PI3-kinase activation (r = 0.53, P < 0.04). There was no correlation between PI3-kinase and muscle morphology. However, increases in GDR were positively related to PI3-kinase activation (r = 0.60, P < 0.02). We conclude that regular exercise leads to greater insulin-stimulated IRS-1-associated PI3-kinase activation in human skeletal muscle, thus facilitating enhanced insulin-mediated glucose uptake.     

Insulin receptor substrate 3 (IRS-3) and IRS-4 impair IRS-1- and IRS-2-mediated signaling

To investigate the roles of insulin receptor substrate 3 (IRS-3) and IRS-4 in the insulin-like growth factor 1 (IGF-1) signaling cascade, we introduced these proteins into 3T3 embryonic fibroblast cell lines prepared from wild-type (WT) and IRS-1 knockout (KO) mice by using a retroviral system. Following transduction of IRS-3 or IRS-4, the cells showed a significant decrease in IRS-2 mRNA and protein levels without any change in the IRS-1 protein level. In these cell lines, IGF-1 caused the rapid tyrosine phosphorylation of all four IRS proteins. However, IRS-3- or IRS-4-expressing cells also showed a marked decrease in IRS-1 and IRS-2 phosphorylation compared to the host cells. This decrease was accounted for in part by a decrease in the level of IRS-2 protein but occurred with no significant change in the IRS-1 protein level. IRS-3- or IRS-4-overexpressing cells showed an increase in basal phosphatidylinositol 3-kinase activity and basal Akt phosphorylation, while the IGF-1-stimulated levels correlated well with total tyrosine phosphorylation level of all IRS proteins in each cell line. IRS-3 expression in WT cells also caused an increase in IGF-1-induced mitogen-activated protein kinase phosphorylation and egr-1 expression ( approximately 1.8- and approximately 2.4-fold with respect to WT). In the IRS-1 KO cells, the impaired mitogenic response to IGF-1 was reconstituted with IRS-1 to supranormal levels and was returned to almost normal by IRS-2 or IRS-3 but was not improved by overexpression of IRS-4. These data suggest that IRS-3 and IRS-4 may act as negative regulators of the IGF-1 signaling pathway by suppressing the function of other IRS proteins at several steps.     

ATP-sensitive potassium channels participate in glucose uptake in skeletal muscle and adipose tissue

ATP-sensitive potassium (K(ATP)) channels are known to be critical in the control of both insulin and glucagon secretion, the major hormones in the maintenance of glucose homeostasis. The involvement of K(ATP) channels in glucose uptake in the target tissues of insulin, however, is not known. We show here that Kir6.2(-/-) mice lacking Kir6.2, the pore-forming subunit of these channels, have no K(ATP) channel activity in their skeletal muscles. A 2-deoxy-[(3)H]glucose uptake experiment in vivo showed that the basal and insulin-stimulated glucose uptake in skeletal muscles and adipose tissues of Kir6.2(-/-) mice is enhanced compared with that in wild-type (WT) mice. In addition, in vitro measurement of glucose uptake indicates that disruption of the channel increases the basal glucose uptake in Kir6.2(-/-) extensor digitorum longus and the insulin-stimulated glucose uptake in Kir6.2(-/-) soleus muscle. In contrast, glucose uptake in adipose tissue, measured in vitro, was similar in Kir6.2(-/-) and WT mice, suggesting that the increase in glucose uptake in Kir6.2(-/-) adipocytes is mediated by altered extracellular hormonal or neuronal signals altered by disruption of the K(ATP) channels.