Molecular mechanisms of insulin action in human skeletal muscle and adipose tissue : implications for diabetes and obesity

Abstract

Studies of animal models of type 2 diabetes mellitus implicate that impaired insulin signal transduction to glucose transport accounts for whole body insulin resistance in skeletal muscle and adipose tissue. The overall aim of the present thesis was to determine insulin signal transduction to glucose transport in human skeletal muscle and adipose tissue from type 2 diabetic patients and obese insulin resistant subjects.

In vivo hyperinsulinemia was achieved by an insulin infusion in healthy and type 2 diabetic subjects. Skeletal muscle biopsies were obtained before and after insulin infusion. In vivo hyperinsulinemia activated early insulin signaling events in human skeletal muscle, including the insulin receptor substrate (IRS)-1 and phosphatidylinositol (PI) 3-kinase. In type 2 diabetic patients, reduced insulin-stimulated glucose transport was associated with impaired IRS-1 tyrosine phosphorylation and PI 3-kinase activity.

To further characterize insulin signal transduction in type 2 diabetic skeletal muscle we utilized an in vitro approach that allowed incubation of skeletal muscle strips. Phosphorylation of the insulin receptor and the mitogen-activated protein kinase was normal in skeletal muscle from type 2 diabetic subjects. In contrast, signaling via the IRS-1/PI 3kinase pathway was reduced in response to 60 nmol/l insulin, whereas insulin-stimulated glucose transport was reduced at all insulin concentrations studied in the type 2 diabetic subjects. Impaired insulin signaling was not due to altered protein expression, but rather occurs from defective function.

PI 3-kinase is a key mediator of insulin-stimulated glucose transport in skeletal muscle. Defective PI 3-kinase activation, but normal p85alpha protein expression was observed in type 2 diabetic subjects. Full length p85alpha mRNA expression was increased 3,8-fold in type 2 diabetic subjects. p50alpha, but not p55alpha, could be detected in human skeletal muscle using semiquantitative PCR. mRNA expression of p50alpha and protein expression of all p85alpha isoforms were similar between healthy and type 2 diabetic subjects.

Altered glucose transport is observed in adipose tissue from obese people. Insulin action on IRS-1 tyrosine phosphorylation and IRS-2 associated PI 3-kinase activity was similar between obese and lean subjects. In contrast, IRS-1 associated PI 3-kinase activity and p85alpha protein expression were reduced in adipocytes from obese subjects. Defective IRS-1 signaling and reduced GLUT4 protein expression were associated with reduced insulin sensitivity of glucose transport in obese subjects. Short-term very low calorie diet (VLCD) further decreased GLUT4 protein expression and markedly reduced basal and insulin-stimulated glucose transport in adipocytes from obese subjects. Alterations in early components of the insulin signaling pathway are not likely to contribute to deterioration of insulin resistance after VLCD in obese subjects.

Cell surface GLUT4 content and glucose transport in skeletal muscle were stimulated to a similar extent in response to insulin and hypoxia in healthy subjects. In type 2 diabetic subjects, cell surface GLUT4 content and glucose transport were reduced in response to both insulin and hypoxia. A greater impairment was observed for cell surface GLUT4 content compared to glucose transport in skeletal muscle from type 2 diabetic subjects. Thus, defective GLUT4 translocation and insulin signaling defects appear to explain reduced glucose transport in skeletal muscle from type 2 diabetic subjetcs.

In conclusion, obesity and type 2 diabetes mellitus are associated with altered insulin signal transduction to glucose transport in skeletal muscle and adipose tissue. In addition, defective GLUT4 translocation in skeletal muscle may also contribute to impaired glucose homeostasis in type 2 diabetic subjects. Finally, defects in early components of the insulin signaling pathway do not appear to contribute to the deterioration of insulin resistance in adipose tissue from obese subjects after VLCD.