OVEREXPRESSION OF HA-RAS SELECTIVELY IN ADIPOSE-TISSUE OF TRANSGENIC MICE - EVIDENCE FOR ENHANCED SENSITIVITY TO INSULIN

To determine the role of Ras-dependent signaling pathways in adipocyte function, we created transgenic mice that overexpress Ha-ras in adipo cytes using the aP2 fatty acid-binding protein promoter/enhancer ligat ed to the human genomic ras sequence. ras mRNA was increased 8-17-fold and Ras protein 4-5-fold in white and brown fat, with no overexpressi on in other tissues. The subcellular distribution of overexpressed Ras paralleled that of endogenous Ras. [U-C-14]Glucose uptake into isolat ed adipocytes was increased similar to 2-fold in the absence of insuli n, and the ED(50) for insulin was reduced 70%, with minimal effect on maximally stimulated glucose transport. Expression of Glut4 protein wa s unaltered in transgenic adipocytes, but photoaffinity labeling of tr ansporters in intact cells with )benzoyl]-1,3-bis-(D-mannos-4-yloxy)-2 -propylamine revealed 1.7-2.6-fold more cell-surface Glut4 in the abse nce of insulin and at half-maximal insulin concentration (0.3 nM) comp ared with nontransgenic adipocytes. With maximal insulin concentration (80 nM), cell-surface Glut4 in nontransgenic and transgenic adipocyte s was similar. Glut1 expression and basal cell-surface Glut1 were incr eased 2-2.9-fold in adipocytes of transgenic mice. However, Glut1 was much less abundant than Glut4, making its contribution to transport ne gligible. These in vitro changes were accompanied by in vivo alteratio ns including increased glucose tolerance, decreased plasma insulin lev els, and decreased adipose mass. We conclude that ras overexpression i n adipocytes leads to a partial translocation of Glut4 in the absence of insulin and enhanced Glut4 translocation at physiological insulin c oncentration, but no effect with maximally stimulating insulin concent rations.