Regulation of Glucose Transport and Insulin Signaling by Troglitazone or Metformin in Adipose Tissue of Type 2 Diabetic Subjects

Abstract

Type 2 diabetic subjects failing glyburide therapy were randomized to receive additional therapy with either metformin (2,550 mg/day) or troglitazone (600 mg/day) for 3–4 months. Biopsies of subcutaneous abdominal adipose tissue were obtained before and after therapy. Glycemic control was similar with both treatments. Metformin treatment increased insulin-stimulated whole-body glucose disposal rates by 20% (P < 0.05); the response to troglitazone was greater (44% increase, P < 0.01 vs. baseline, P < 0.05 vs. metformin). Troglitazone-treated subjects displayed a tendency toward weight gain (5 ± 2 kg, P < 0.05), increased adipocyte size, and increased serum leptin levels. Metformin-treated subjects were weight-stable, with unchanged leptin levels and reduced adipocyte size (to 84 ± 4% of control, P < 0.005). Glucose transport in isolated adipocytes from metformin-treated subjects was unaltered from pretreatment. Glucose transport in both the absence (321 ± 134% of pre-Rx, P < 0.05) and presence of insulin (418 ± 161%, P < 0.05) was elevated after troglitazone treatment. Metformin treatment had no effect on adipocyte content of GLUT1 or GLUT4 proteins. After troglitazone treatment, GLUT4 protein expression was increased twofold (202 ± 42%, P < 0.05). Insulin-stimulated serine phosphorylation of Akt was augmented after troglitazone (170 ± 34% of pre-Rx response, P < 0.05) treatment and unchanged by metformin. We conclude that the ability of troglitazone to upregulate adipocyte glucose transport, GLUT4 expression, and insulin signaling can contribute to its greater effect on whole-body glucose disposal.