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 3 (GSK-3) in skeletal muscle biopsies from 24 PCOS patients (pre-treatment) and 14 matched control subjects, and from 10 PCOS patients after 16 weeks 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 dephoshorylation 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-3 or -3 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.