Troglitazone Treatment Increases Protein Kinase B Phosphorylation in Skeletal Muscle of Normoglycemic Subjects at Risk for the Development of Type 2 Diabetes

Abstract

We investigated whether the effect of troglitazone on glucose disposal is associated with altered insulin signaling. Nondiabetic first-degree relatives of type 2 diabetic patients (age 30 ± 2 years, BMI 30 ± 1 kg/m²; n = 20) were randomized in a double-blind manner to 3 months of troglitazone (200 mg/day) or placebo treatment. Before and after treatment, 3-h euglycemic-hyperinsulinemic glucose clamps (40 mU · m⁻² · min⁻¹) were performed, and muscle biopsies were obtained immediately before and after the clamps. In the biopsies, insulin receptor kinase (IRK) activity, insulin receptor substrate (IRS)-1-associated phosphatidylinositol 3-kinase (PI3K) activity, Ser⁴⁷³ and Thr³⁰⁸ phosphorylation of protein kinase B (PKB), and protein expression of IRS-1, IRS-2, phosphoinositol-dependent kinase-1 (PDK-1), PKB, and GLUT-4 were determined. After troglitazone treatment, insulin-stimulated glucose disposal was increased compared with pretreatment and placebo (279 ± 37 vs. 211 ± 26 and 200 ± 25 mg · m⁻² · min⁻¹; both P < 0.05). IRK and PI3K activities were not altered by troglitazone, but PKB Ser⁴⁷³ phosphorylation was enhanced compared with pretreatment and placebo at the clamp insulin level (138 ± 36 vs. 77 ± 16 and 55 ± 13 internal standard units; both P < 0.05) and with pretreatment at the basal level (31 ± 9 vs. 14 ± 4 internal standard units; P < 0.05). PKB Thr³⁰⁸ phosphorylation also tended to be higher, but this was not statistically significant. Troglitazone did not alter insulin receptor number or IRS-1, IRS-2, PKB, PDK-1, or GLUT-4 protein expression. We conclude that increased PKB phosphorylation may contribute to the insulin-sensitizing effects of thiazolidinediones in human skeletal muscle.