Insulin signaling and glucose transport in insulin resistant human skeletal muscle

Insulin increases glucose uptake and metabolism in skeletal muscle by signal transduction via protein phosphorylation cascades. Insulin action on signal transduction is impaired in skeletal muscle from Type 2 diabetic subjects, underscoring the contribution of molecular defects to the insulin resistant phenotype. This review summarizes recent work to identify downstream intermediates in the insulin signaling pathways governing glucose homeostasis, in an attempt to characterize the molecular mechanism accounting for skeletal muscle insulin resistance in Type 2 diabetes. Furthermore, the effects of pharmaceutical treatment of Type 2 diabetic patients on insulin signaling and glucose uptake are discussed. The identification and characterization of pathways governing insulin action on glucose metabolism will facilitate the development of strategies to improve insulin sensitivity in an effort to prevent and treat Type 2 diabetes mellitus.     

Molecular mechanisms by which aerobic exercise induces insulin sensitivity

Insulin resistance is a key feature of Type 2 diabetes and an important therapeutic target to address glycemic control to prevent diabetic complications. Lifestyle advice is the first step in the ADA/EASD consensus guidelines followed by metformin therapy. Aerobic exercise (AE) can increase insulin sensitivity by several molecular pathways including upregulation of insulin transporters in the cellular membrane of insulin-dependent cells. In addition, AE improves insulin sensitivity by amelioration of the pathophysiologic pathways involved in insulin resistance such as the reduction of adipokines, inflammatory and oxidative stress responses, and improvement of insulin signal transduction via different molecular pathways. This review details the molecular pathways by which AE induces beneficial effects on insulin resistance     

Glucose and insulin are needed for optimal defensin expression in human cell lines

Many infections are associated with diabetes, as the ability of the body to fight pathogens is impaired. Recently, low levels of defensins have been found in diabetic rodents. However, whether hyperglycemia and/or insulin deficiency/insensitivity is the reason for the reduced defensin levels is still unknown. To study the functionality of the innate immune system during hyperglycemia, the expression levels of human beta-defensin-1 (hBD-1) was measured in human embryonic kidney (HEK-293) and colon adenocarcinoma (HCT-116) cells treated with different concentrations of glucose and insulin. Increasing concentrations of glucose enhanced hBD-1 expression and these levels were further elevated after insulin treatment. Insulin treatment also led to the up-regulation of human sodium/glucose transporter 1 (hSGLT1), which further increases intracellular glucose levels. Thus, our findings suggest for the first time that insulin signaling is important for hBD-1 optimal expression by elevating intracellular glucose levels and by mediating gene expression.     

Role of Insulin Signaling in Maintaining Energy Homeostasis

ObjectiveTo examine the role that insulin signaling plays in modulating metabolic functions involving both peripheral and hypothalamic systems.MethodsWe review the literature regarding insulin signaling as it relates to energy homeostasis.ResultsInsulin signaling in the periphery is known to affect hepatic glucose production and glucose uptake in muscle and adipose tissue. In the brain, insulin is involved in a variety of signaling pathways that control positive and negative aspects of food intake and energy metabolism. Disruption of insulin signaling can affect key cellular pathways that serve to maintain energy balance and glucose homeostasis, which can then lead to insulin resistance and progression toward various metabolic disorders, including cardiovascular disease, obesity, and type 2 diabetes. The use of exogenous insulin as therapy for patients with type 2 diabetes is traditionally associated with increases in weight.ConclusionAn enhanced understanding of how these insulin signaling pathways function may provide answers about how to control weight gain associated with exogenous insulin use. Pharmacologic agents, such as the long-acting insulin analogues and particularly insulin detemir, that may reduce these weight effects hold considerable advantage. (Endocr Pract. 2008;14:373-380)     

Hyperglycemia does not increase basal hypothalamo-pituitary-adrenal activity in diabetes but it does impair the HPA response to insulin-induced hypoglycemia

Recently, we established that hypothalamo-pituitary-adrenal (HPA) and counterregulatory responses to insulin-induced hypoglycemia were impaired in uncontrolled streptozotocin (STZ)-diabetic (65 mg/kg) rats and insulin treatment restored most of these responses. In the current study, we used phloridzin to determine whether the restoration of blood glucose alone was sufficient to normalize HPA function in diabetes. Normal, diabetic, insulin-treated, and phloridzin-treated diabetic rats were either killed after 8 days or subjected to a hypoglycemic (40 mg/dl) glucose clamp. Basal: Elevated basal ACTH and corticosterone in STZ rats were normalized with insulin but not phloridzin. Increases in hypothalamic corticotrophin-releasing hormone (CRH) and inhibitory hippocampal mineralocorticoid receptor (MR) mRNA with STZ diabetes were not restored with either insulin or phloridzin treatments. Hypoglycemia: In response to hypoglycemia, rises in plasma ACTH and corticosterone were significantly lower in diabetic rats compared with controls. Insulin and phloridzin restored both ACTH and corticosterone responses in diabetic animals. Hypothalamic CRH mRNA and pituitary pro-opiomelanocortin mRNA expression increased following 2 h of hypoglycemia in normal, insulin-treated, and phloridzin-treated diabetic rats but not in untreated diabetic rats. Arginine vasopressin mRNA was unaltered by hypoglycemia in all groups. Interestingly, hypoglycemia decreased hippocampal MR mRNA in control, insulin-, and phloridzin-treated diabetic rats but not uncontrolled diabetic rats, whereas glucocorticoid receptor mRNA was not altered by hypoglycemia. In conclusion, despite elevated basal HPA activity, HPA responses to hypoglycemia were markedly reduced in uncontrolled diabetes. We speculate that defects in the CRH response may be related to a defective MR response. It is intriguing that phloridzin did not restore basal HPA activity but it restored the HPA response to hypoglycemia, suggesting that defects in basal HPA function in diabetes are due to insulin deficiency, but impaired responsiveness to hypoglycemia appears to stem from chronic hyperglycemia.     

Hyperglycemia-induced insulin resistance in diabetic dyslipidemic Yucatan swine

Hyperglycemia, dyslipidemia, and associated insulin resistance are hallmarks of diabetes mellitus. Purposes of the study reported here were to develop practical methods for assessment of in vivo insulin sensitivity and determine contributions of hyperglycemia and dyslipidemia to insulin resistance in the porcine model of alloxan-induced diabetes mellitus and dyslipidemia. Male Yucatan swine groups were treated for 20 weeks: control (C), high fat-fed (2% cholesterol) hyperlipidemic (H), alloxan-induced diabetic normolipidemic (D), diabetic high fat-fed (diabetic dyslipidemic, DD), and diabetic dyslipidemic treated with the lipid-lowering agent atorvastatin (DDA). Plasma cholesterol concentration increased sixfold in animals of groups H, DD, and DDA, whereas triglyceride concentration increased threefold in animals of group DD only. Diabetics had decreases in glucose tolerance and pancreatic immunostaining for insulin. Use of the gold standard hyperinsulinemic euglycemic clamp procedure indicated that maximal insulin-stimulated glucose uptake was similar to that in humans, but this method was not practical for use in pigs. Instead, a more convenient and valid insulin sensitivity test involving suppression of insulin secretion with somatostatin and a single insulin injection was used. Insulin sensitivity was greatly impaired by anesthesia with isoflurane, but was not affected by use of the anxiolytic agent diazepam. Insulin sensitivity decreased by 75% in diabetics (groups D, DD, DDA), compared with animals of groups C and H, and was inversely related to fasting blood glucose concentration (r = -0.72). Insulin treatment to restore blood glucose values of diabetics (> 250 mg/dl) to near control values (< 100 mg/dl) promptly restored insulin sensitivity to control values. We conclude that hyperglycemia is a major cause of insulin resistance in the porcine model of alloxan-induced diabetes mellitus and dyslipidemia.     

Reversal of insulin resistance in type I diabetes after treatment with continuous subcutaneous insulin infusion.

Insulin responsiveness was studied with the euglycaemic glucose clamp technique in seven patients with type I diabetes and in six control subjects matched for age and weight. The glucose disposal rate was significantly reduced in the diabetic subjects when they were receiving conventional insulin treatment compared with the control group, showing insulin resistance in the diabetics. The diabetic patients were again studied after eight days of intensified metabolic control achieved with continuous subcutaneous insulin infusion. During the infusion a more physiological insulin regimen was used compared with their regular treatment, less of the total insulin dose being given as continuous infusion and more as bolus doses before meals. The insulin resistance in the diabetics was largely reversed after this improved metabolic control. Dose response studies showed an increased glucose disposal rate at all plasma insulin concentrations, including the maximum insulin concentration, indicating a predominant effect of the continuous infusion regimen at the postreceptor level. The improved insulin effect seen with continuous subcutaneous insulin infusion could be due to the improved metabolic control achieved as well as the more physiological regimen.     

Familial type C syndrome of insulin resistance and short stature with possible autosomal dominant transmission

We describe a 17-yr-old girl with insulin resistant diabetes, acanthosis nigricans, hirsutism and short stature. At the age of 14 she was found to have glycosuria and diagnosed as diabetes mellitus. No endocrinological abnormality except transient amenorrhea and exaggerated LH response to LHRH was found. Insulin resistance was demonstrated by fasting hyperinsulinemia, insulin tolerance test and euglycemic glucose clamp test, and large doses of insulin with CSII were not effective in controlling blood glucose. Insulin binding to erythrocytes was normal, suggesting a postbinding defect. The same phenotype of insulin resistant diabetes and short stature was found in her mother who was diagnosed as diabetes mellitus at the age of 31 and died of diabetic nephropathy at the age of 41. Her maternal grandfather and uncle were reportedly affected with the same phenotype. Her father had impaired glucose tolerance, but no hyperinsulinemia. Two sisters had essentially normal glucose tolerance. Insulin binding to erythrocytes of her father and mother was also in the normal range. These results suggest that the present case may be a rare syndrome present together with type C syndrome of insulin resistance, and with short stature which was inherited autosomal dominantly.     

Diabetes: the importance of the liver

Insulin resistance, the hallmark of non-insulin dependent diabetes mellitus, is characterized by the failure of tissues to take up and store glucose in response to insulin. Two recent studies shed new light on the importance of insulin signalling in the liver and how this may become defective in diabetes.     

Oxidative stress--mediated alterations in glucose dynamics in a genetic animal model of type II diabetes

Insulin resistance, characterized by an inexorable decline in skeletal muscle glucose utilization and/or an excessive hepatic glucose production, constitutes a major pathogenic importance in a cluster of clinical disorders including diabetes mellitus, hypertension, dyslipidemia, central obesity and coronary artery disease. A novel concept suggests that heightened state of oxidative stress during diabetes contributes, at least in part, to the development of insulin resistance. Several key predictions of this premise were subjected to experimental testing using Goto-Kakizaki (GK) rats as a genetic animal model for non-obese type II diabetes. Euglycemic-hyperinsulinemic clamp studies with an insulin infusion index of 5 mU/kg bw/min were used to measure endogenous glucose production (EGP), glucose infusion rate (GIR), glucose disposal rate (GDR) and skeletal muscle glucose utilization index (GUI). Moreover, the status of oxidative stress as reflected by the urinary levels of isoprostane and protein carbonyl formation were also assessed as a function of diabetes. Post-absorptive basal EGP and circulating levels of insulin, glucose and free fatty acid (FFA) were elevated in GK rats, compared to their corresponding control values. In contrast, steady state GIR and GDR of the hyperglycemic/hyperinsulinemic animals were reduced, concomitantly with impaired insulin's ability to suppress EGP. Insulin stimulated [3H]-2-deoxyglucose (2-DG) uptake (a measure of glucose transport activity) by various types of skeletal muscle fibers both in vivo and in vitro (isolated muscle, cultured myoblasts) was diminished in diabetic GK rats. This diabetes-related suppression of skeletal muscle glucose utilization was associated with a decrease in insulin's ability to promote the phosphorylation of tyrosine residues of insulin receptor substrate-1 (IRS-1). Similarly, the translocation of GLUT-4 from intracellular compartment to plasma membrane in response to insulin was also reduced in these animals. Oxidative stress-based markers (e.g. urinary isoprostane, carbonyl-bound proteins) were elevated as a function of diabetes. Nullification of the heightened state of oxidative stress in the GK rats with alpha-lipoic acid resulted in a partial amelioration of the diabetes-related impairment of the in vivo and in vitro insulin actions. Collectively, the above data suggest that 1) insulin resistance in GK rats occurs at the hepatic and skeletal muscle levels, 2) muscle cell glucose transport exhibited a blunted response to insulin and it is associated with a major defect in key molecules of both GLUT-4 trafficking and insulin signaling pathways, 3) skeletal muscle insulin resistance in GK rats appears to be of genetic origin and not merely related to a paracrine or autocrine effect, since this phenomenon is also observed in cultured myoblasts over several passages and finally heightened state of oxidative stress may mediate the development of insulin resistance during diabetes.     

Impairment of insulin-stimulated Akt/GLUT4 signaling is associated with cardiac contractile dysfunction and aggravates I/R injury in STZ-diabetic Rats

In this study, we established systemic in-vivo evidence from molecular to organism level to explain how diabetes can aggravate myocardial ischemia-reperfusion (I/R) injury and revealed the role of insulin signaling (with specific focus on Akt/GLUT4 signaling molecules). The myocardial I/R injury was induced by the left main coronary artery occlusion for 1 hr and then 3 hr reperfusion in control, streptozotocin (STZ)-induced insulinopenic diabetes, and insulin-treated diabetic rats. The diabetic rats showed a significant decrease in heart rate, and a prolonged isovolumic relaxation (tau) which lead to decrease in cardiac output (CO) without changing total peripheral resistance (TPR). The phosphorylated Akt and glucose transporter 4 (GLUT 4) protein levels were dramatically reduced in both I/R and non-I/R diabetic rat hearts. Insulin treatment in diabetes showed improvement of contractile function as well as partially increased Akt phosphorylation and GLUT 4 protein levels. In the animals subjected to I/R, the mortality rates were 25%, 65%, and 33% in the control, diabetic, and insulin-treated diabetic group respectively. The I/R-induced arrhythmias and myocardial infarction did not differ significantly between the control and the diabetic groups. Consistent with its anti-hyperglycemic effects, insulin significantly reduced I/R-induced arrhythmias but had no effect on I/R-induced infarctions. Diabetic rat with I/R exhibited the worse hemodynamic outcome, which included systolic and diastolic dysfunctions. Insulin treatment only partially improved diastolic functions and elevated P-Akt and GLUT 4 protein levels. Our results indicate that cardiac contractile dysfunction caused by a defect in insulin-stimulated Akt/GLUT4 may be a major reason for the high mortality rate in I/R injured diabetic rats.     

Regulation of liver and brain hexose monophosphate dehydrogenases by insulin and dietary intake in the female rat

Summary Liver glucose 6-phosphate dehydrogenase and phosphogluconate dehydrogenase activities were significantly decreased in both diabetic and fasted rats. Treatment of diabetic rats with insulin resulted in liver glucose 6-phosphate dehydrogenase and phosphogluconate dehydrogenase activities that were significantly greater than controls. Insulin promoted an increase in food consumption that was blocked by adrenaline. Insulin, when administered together with adrenaline, restored hepatic glucose 6-phosphate dehydrogenase and phosphogluconate dehydrogenas activities of diabetic animals to control values, without altering food consumption. Brain glucose 6-phosphate dehydrogenase and phosphogluconate dehydrogenase activities were not significantly altered by either dietary restriction, diabetes or insulin treatment. These results demonstrate a dissociation between the action of insulin on hepatic glucose 6-phosphate dehydrogenase activity and its action to increase food intake.Abbreviations NADP⁺ oxidoreductase, EC 1.1.1.49 Glucose 6-P dehydrogenase, GPD, D-glucose-6-phosphate - NADP⁺ 2-oxidoreductase (decarboxylating), EC 1.1.1.44 phosphogluconate dehydrogenase, PGD, 6-phospho-D-gluconate     

Respiratory chain dysfunction in skeletal muscle does not cause insulin resistance

Insulin resistance in skeletal muscle is a characteristic feature of diabetes mellitus type 2 (DM2). Several lines of circumstantial evidence suggest that reduced mitochondrial oxidative phosphorylation capacity in skeletal muscle is a primary defect causing insulin resistance and subsequent development of DM2. We have now experimentally tested this hypothesis by characterizing glucose homeostasis in tissue-specific knockout mice with progressive respiratory chain dysfunction selectively in skeletal muscle. Surprisingly, these knockout mice are not diabetic and have an increased peripheral glucose disposal when subjected to a glucose tolerance test. Studies of isolated skeletal muscle from knockout animals show an increased basal glucose uptake and a normal increase of glucose uptake in response to insulin. In summary, our findings indicate that mitochondrial dysfunction in skeletal muscle is not a primary etiological event in DM2.     

Extract of Ocimum canum lowers blood glucose and facilitates insulin release by isolated pancreatic β-islet cells

Aqueous extract of Ocimum canum Sim, (Lamiaceae) is used by some Ghanaians to manage diabetes mellitus. In vivo modulation of levels of fasting blood glucose by 0. canum extract was evaluated in type-II diabetes mellitus using the C57BL/KsJ db/db genetically diabetic animal model, and its effects on glucose-stimulated insulin release in vitro were monitored using isolated rat pancreatic beta-islet cells. The results showed that fasting blood glucose levels and body weight decreased significantly (p < 0.05) in diabetic and non-diabetic C57BL/KsJ mice, which were administered aqueous extract of 0. canum. In vitro, the 0. canum extract significantly enhanced insulin release from isolated rat pancreatic beta-islet cells. Insulin release was found to be dependent on glucose concentration and increased with increasing O. canum concentration in the incubation medium up to an optimum extract concentration of 0.03 mg/ml. Release of the hormone decreased beyond this concentration of extract in the medium. Addition to the medium of Desmodium adscendens, a plant preparation used to manage inflammatory disorders, did not increase but rather inhibited insulin secretion by the pancreatic beta-islet cells. These results could explain the use of 0. canum in Ghanaian folk medicine to manage diabetes mellitus.