Impaired insulin activation and dephosphorylation of glycogen synthase in skeletal muscle of women with polycystic ovary syndrome is reversed by pioglitazone treatment

Abstract

Context: Insulin resistance is a major risk factor for type 2 diabetes in women with polycystic ovary syndrome (PCOS). The molecular mechanisms underlying reduced insulin-mediated glycogen synthesis in skeletal muscle of patients with PCOS have not been established. Subjects and Methods: We investigated protein content, activity, and phosphorylation of glycogen synthase (GS) and its major upstream inhibitor, GS kinase (GSK)-3 in skeletal muscle biopsies from 24 PCOS patients (before treatment) and 14 matched control subjects and 10 PCOS patients after 16 wk treatment with pioglitazone. All were metabolically characterized by euglycemic-hyperinsulinemic clamps and indirect calorimetry. Results: Reduced insulin-mediated glucose disposal (P < 0.05) was associated with a lower insulin-stimulated GS activity in PCOS patients (P < 0.05), compared with controls. This was, in part, explained by absent insulin-mediated dephosphorylation of GS at the NH2-terminal sites 2 + 2a, whereas dephosphorylation at the COOH-terminal sites 3a + 3b was intact in PCOS subjects (P < 0.05). Consistently, multiple linear regression analysis showed that insulin activation of GS was dependent on dephosphorylation of sites 3a + 3b in women with PCOS. No significant abnormalities in GSK-3α or -3β were found in PCOS subjects. Pioglitazone treatment improved insulin-stimulated glucose metabolism and GS activity in PCOS (all P < 0.05) and restored the ability of insulin to dephosphorylate GS at sites 2 and 2a. Conclusions: Impaired insulin activation of GS including absent dephosphorylation at sites 2 + 2a contributes to insulin resistance in skeletal muscle in PCOS. The ability of pioglitazone to enhance insulin sensitivity, in part, involves improved insulin action on GS activity and dephosphorylation at NH2-terminal sites. Copyright © 2008 by The Endocrine Society.