Characterization of signal transduction and glucose transport in skeletal muscle from type 2 diabetic patients.

Abstract

We characterized metabolic and mitogenic signaling pathways in isolated skeletal muscle from well-matched t y p e 2 diabetic and control subjects. Time course studies of the insulin receptor, insulin receptor substrate ( I R S )-1/2, and phosphatidylinositol (PI) 3-k i n a s e revealed that signal transduction through this pathway was engaged between 4 and 40 min. Insulin-stimulated ( 0 . 6 -6 0 nmol/l) tyrosine phosphorylation of the insulin receptor -subunit, mitogen-activated protein (MAP) kinase phosphorylation, and glycogen synthase activity were not altered in type 2 diabetic subjects. In contrast, insulin-stimulated tyrosine phosphorylation of IRS-1 and anti-phosphotyrosine-associated PI 3-kinase activity were reduced 40-55% in type 2 diabetic subjects at high insulin concentrations (2.4 and 60 nmol/l, respectively). Impaired glucose transport activity was noted at all insulin concentrations (0.6-60 nmol/l). Aberrant protein expression cannot account for these insulin-signaling defects because expression of insulin receptor, IRS-1, IRS-2, MAP kinase, or glycogen synthase was similar between type 2 diabetic and control subjects. In skeletal muscle from type 2 diabetic subjects, IRS-1 phosphorylation, PI 3-kinase activity, and glucose transport activity were impaired, whereas insulin receptor tyrosine phosphorylation, MAP kinase phosphorylation, and glycogen synthase activity were normal. Impaired insulin signal transduction in skeletal muscle from type 2 diabetic patients may partly account for reduced insulin-stimulated glucose transport; however, additional defects are likely to play a role. D i a b e t e s 4 9 :2 8 4-292, 2000 S keletal muscle is a primary site of insulin-stimulated glucose disposal, which accounts for 70-80% of postprandial glucose disposal (1). In vivo studies reveal that insulin resistance in skeletal muscle is one of the first measurable defects associated with type 2 diabetes (2,3). The molecular basis for the development of whole-body insulin resistance remains unclear, although decreased insulin-stimulated glucose transport activity has been observed in isolated skeletal muscle from lean and obese people with type 2 diabetes (4-8). Because glucose transport is an early step in peripheral glucose utilization, a defect in glucose transport most likely plays a major role in the pathogenesis of peripheral insulin resistance (9). Thus, an understanding of the mechanisms that control glucose transport into insulin-sensitive tissues is essential to develop strategies for reestablishing normal glucose homeostasis in people with type 2 diabetes.Insulin-stimulated glucose transport is achieved by translocation of the major insulin-responsive glucose transp o r t e r, GLUT4, from an intracellular vesicle storage site to the plasma membrane and transverse tubules (10-12). Reduced glucose transport activity in skeletal muscle from people with type 2 diabetes may be a consequence of impaired insulin signal transduction (13,14) and/or alterations in the traffic and translocation of GLUT4 to the...

Insulin signalling in skeletal muscle of subjects with or without Type II-diabetes and first degree relatives of patients with the disease

“…Finally, because at the high insulin concentration used by investigators [22], glucose disposal is maximally stimulated at least in control subjects [46,49], consequences of an altered signalling for glucose disposal are not clear. The notion that a reduction in PI3′-kinase activity in diabetic subjects might occur at high, but not at physiological, insulin concentrations is also supported by a study with muscle strips from non-diabetic and diabetic subjects that were incubated in vitro [12], and where no difference in PI3′-kinase activity was detected below 2.4 nmol/l insulin.…”

“…Because potential alterations of this insulin receptor -IRS-1 interaction in diabetes could have been missed using the synthetic polymer, our data not only confirm but also extend our previous findings. Other investigators have previously described normal [12] or impaired [20,46] insulin receptor phosphorylation or activation in diabetic subjects, and, most recently, no impairment of insulin receptor phosphorylation was shown in FDR of dia- a Uridine diphosphate glucose incorporated in glycogen/mg protein/min in the presence of 10 mmol/l G6P betic subjects [4]. Potential reasons for the different findings by different groups include differences in methodologies and study cohorts and have been discussed extensively in our previous paper [18].…”

“…Insulin action on IRSassociated PI3′-kinase activity has been reported to be reduced in Type II diabetes [12,21,22]. In two of these studies the insulin concentrations at which the reductions were observed, were, however very high [12,22]. In another study [23] the reduced insulin effect in obese Type II diabetic patients compared to lean non-diabetic subjects appeared to be mainly due to their obesity because no difference was observed compared to obese non-diabetic subjects.…”

“…These include insulin receptor substrates-1 to 4, Gab 1, and shc [11]. In skeletal muscle, IRS-1 appears to be most important [12]. Binding of tyrosine-phosphorylated insulin receptor substrates to the 85 kDa regulatory subunit of the lipid kinase phosphatidylinositol-3′(PI3′)-kinase then results in an activation of this enzyme, which is necessary for the insulin-mediated stimulation of glucose uptake and glycogen synthase [13].…”

“…Activation of insulin receptor kinase has been found to be either normal [12,18] or decreased [19,20] compared to BMI-matched control groups. Insulin action on IRSassociated PI3′-kinase activity has been reported to be reduced in Type II diabetes [12,21,22]. In two of these studies the insulin concentrations at which the reductions were observed, were, however very high [12,22].…”