Troglitazone but not Metformin Restores Insulin-Stimulated Phosphoinositide 3-Kinase Activity and Increases p110β Protein Levels in Skeletal Muscle of Type 2 Diabetic Subjects

Abstract

Insulin stimulation of phosphatidylinositol (PI) 3-kinase activity is defective in skeletal muscle of type 2 diabetic individuals. We studied the impact of antidiabetic therapy on this defect in type 2 diabetic subjects who failed glyburide treatment by the addition of troglitazone (600 mg/day) or metformin (2,550 mg/day) therapy for 3–4 months. Improvement in glycemic control was similar for the two groups, as indicated by changes in fasting glucose and HbA1c levels. Insulin action on whole-body glucose disposal rate (GDR) was determined before and after treatment using the hyperinsulinemic (300 mU · m−2 · min−1) euglycemic (5.0–5.5 mmol/l) clamp technique. Needle biopsies of vastus lateralis muscle were obtained before and after each 3-h insulin infusion. Troglitazone treatment resulted in a 35 ± 9% improvement in GDR (P < 0.01), which was greater than (P < 0.05) the 22 ± 13% increase (P < 0.05) after metformin treatment. Neither treatment had any effect on basal insulin receptor substrate-1 (IRS-1)-associated PI 3-kinase activity in muscle. However, insulin stimulation of PI 3-kinase activity was augmented nearly threefold after troglitazone treatment (from 67 ± 22% stimulation over basal pre-treatment to 211 ± 62% post-treatment, P < 0.05), whereas metformin had no effect. The troglitazone effect on PI 3-kinase activity was associated with a 46 ± 22% increase (P < 0.05) in the amount of the p110β catalytic subunit of PI 3-kinase. Insulin-stimulated Akt activity also increased after troglitazone treatment (from 32 ± 8 to 107 ± 32% stimulation, P < 0.05) but was unchanged after metformin therapy. Protein expression of other key insulin signaling molecules (IRS-1, the p85 subunit of PI 3-kinase, and Akt) was unaltered after either treatment. We conclude that the mechanism for the insulin-sensitizing effect of troglitazone, but not metformin, involves enhanced PI 3-kinase pathway activation in skeletal muscle of obese type 2 diabetic subjects.