Repeat Treatment of Obese Mice With BRL 49653, a New Potent Insulin Sensitizer, Enhances Insulin Action in White Adipocytes: Association With Increased Insulin Binding and Cell-Surface GLUT4 as Measured by Photoaffinity Labeling

Abstract

(±)-5-([4-[2-Methyl-2(pyridylamino)ethoxy]phenyl]methyl) 2,4-thiazolidinedione (BRL 49653) is a new potent antidiabetic agent that improves insulin sensitivity in animal models of NIDDM. In C57BL/6 obese (ob/ob) mice, BRL 49653, included in the diet for 8 days, improved glucose tolerance. The half-maximal effective dose was 3 μmol/kg diet, which is equivalent to ∼0.1 mg/kg body wt. Improvements in glucose tolerance were accompanied by significant reductions in circulating triacylglycerol, nonesterified fatty acids, and insulin. The insulin receptor number of epididymal white adipocytes prepared from obese mice treated with BRL 49653 (30 μmol/kg diet) for 14 days was increasedtwofold. The affinity of the receptor for insulin was unchanged. In the absence of added insulin, the rates of glucose transport in adipocytes from untreated and BRL 49653-treated obese mice were similar. Insulin (73 nmol/l) produced only a 1.5-fold increase in glucose transport in adipocytes from control obese mice, whereas after BRL 49653 treatment, insulin stimulated glucose transport 2.8-fold. BRL 49653 did not alter the sensitivity of glucose transport to insulin. The increase in insulin responsiveness was accompanied by a 2.5-fold increase in the total tissue content of the glucose transporter GLUT4. Glucose transport in adipocytes from lean littermates was not altered by BRL 49653. To establish the contribution of changes in glucose transporter trafficking to the BRL 49653-mediated increase in insulin action, the cell-impermeant bis-mannose photolabel 2-N-[4-(1-azi-2,2,2-trifluoroethyl)benzoyl]-1,3-bis-(D-mannos-4-yloxy)-2-[2-3H]-propylamine was used to measure adipocyte cell-surface–associated glucose transporters. In these experiments, the increase in maximal insulin-stimulated glucose transport (4.2-fold) produced after BRL 49653 treatment was correlated with a 2.6-fold increase in cell-surface–associated GLUT4. Photolabeled cell-sur-face GLUT1 was not detectable in any adipocyte preparation. These results suggest that the improvement in glycemic control produced by repeated administration of BRL 49653 to obese mice is mediated by increased insulin responsiveness of target tissues. BRL 49653 potentiates insulin-stimulated glucose transport in adipocytes from insulin-resistant obese mice, both by increasing insulin receptor number and by facilitating translocation of GLUT4, from an expanded intracellular pool, to the cell surface. In addition, the increased intrinsic activity of cell-surface glucose transporters may also contribute to an increased insulin responsiveness of adipose tissue.