Regulation of glucose transport by pioglitazone in cultured muscle cells

Abstract

Recent evidence suggests that pioglitazone, a thiazolidinedione hypoglycemic agent, acts by increasing insulin responsiveness at the peripheral level. We studied the effect of pioglitazone (1 to 50 μg/mL) on the glucose transporter and glucose transport in BC3H-1 cells, a continuously cultured skeletal muscle cell line lacking the myoD transcription factor required for cell fusion. Glucose-fed cells (25 mmol/L) responded to insulin with a more than twofold increase in 2-deoxyglucose (2-DOG) uptake as compared with baseline. Treating these cells with pioglitazone alone for 24 hours resulted in a dose-dependent increase in hexose uptake, reaching twofold at 50 μg/mL. Combining long-term pioglitazone (10 μg/mL for 24 hours) and short-term insulin treatment resulted in an additive effect on 2-DOG uptake over a wide range of insulin concentrations (0.1 to 100 nmol/L) without the desensitization to 2-DOG uptake seen in other systems following long-term insulin administration. To determine the basis of the increased glucose uptake response, the level of specific mRNA and immunoreactive glucose transporter protein was determined. Northern and Western blot studies on glucose-treated cells (25 mmol/L) showed that glucose transporter mRNA and protein increased in parallel following treatment with either pioglitazone or insulin alone. The combination of insulin with pioglitazone resulted in an additive stimulation of glucose transporter mRNA and protein. In summary, pioglitazone stimulates hexose uptake both independently and in combination with insulin in BC3H-1 myocytes. These effects are largely accounted for by increases in glucose transporter mRNA and protein, indicating its potential efficacy in the treatment of non-insulin-dependent diabetes mellitus (NIDDM). These studies suggest that the hypoglycemic effect of pioglitazone is exerted in peripheral tissues at the pretranslational level, and provide a basis for studies of the molecular mechanisms of hypoglycemic agent action on glucose transporter gene transcription and mRNA stability.