THE INSULIN-DEPENDENT BIOSYNTHESIS OF GLUT1 AND GLUT3 GLUCOSE TRANSPORTERS IN L6 MUSCLE-CELLS IS MEDIATED BY DISTINCT PATHWAYS - ROLES OF P21(RAS) AND PP70 S6 KINASE

Insulin binding results in rapid phosphorylation of insulin receptor s ubstrate-1 to activate p21(ras) and mitogen-activated protein kinase. Insulin also activates the ribosomal protein S6 kinase (pp70 S6 kinase ) independently of the Ras pathway. Chronic (18 h) treatment of L6 mus cle cells with insulin increases glucose transport activity severalfol d due to biosynthetic elevation of the GLUT1 and GLUT3 but not the GLU T4 glucose transporters. Here we investigate the roles of p21(ras) and pp70 S6 kinase in the insulin-mediated increases in GLUT1 and GLUTS e xpression. L6 cells were transfected with the dominant negative Ras(S1 7N) under the control of a dexamethasone-inducible promoter. Induction of Ras(S17N) failed to block the insulin-mediated increase in GLUT1 g lucose transporter protein and mRNA; however, it abrogated the insulin -mediated increase in GLUTS glucose transporter protein and mRNA, Inhi bition of pp70 S6 kinase by rapamycin, on the other hand, eliminated t he insulin-mediated increase in GLUT1 but had no effect on that of GLU T3 in both parental and Ras(S17N) transfected L6 cells. These results suggest that the biosynthetic regulation of glucose transporters is di fferentially determined, with pp70 S6 kinase and p21(ras) playing acti ve roles in the insulin stimulated increases in GLUT1 and GLUT3, respe ctively.