The insulin-dependent biosynthesis of GLUT1 and GLUT3 glucose transporters in L6 muscle cells is mediated by distinct pathways: Roles of p21ras and pp70 S6 kinase

Abstract

Insulin binding results in rapid phosphorylation of insulin receptor substrate-1 to activate p21ras 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 muscle cells with insulin increases glucose transport activity severalfold due to biosynthetic elevation of the GLUT1 and GLUT3 but not the GLUT4 glucose transporters. Here we investigate the roles of p21ras and pp70 S6 kinase in the insulin-mediated increases in GLUT1 and GLUT3 expression. L6 cells were transfected with the dominant negative Ras(S17N) under the control of a dexamethasone-inducible promoter. Induction of Ras(S17N) failed to block the insulin-mediated increase in GLUT1 glucose transporter protein and mRNA; however, it abrogated the insulin-mediated increase in GLUT3 glucose transporter protein and mRNA. Inhibition of pp70 S6 kinase by rapamycin, on the other hand, eliminated the insulin-mediated increase in GLUT1 but had no effect on that of GLUT3 in both parental and Ras(S17N) transfected L6 cells. These results suggest that the biosynthetic regulation of glucose transporters is differentially determined, with pp70 S6 kinase and p21ras playing active roles in the insulin-stimulated increases in GLUT1 and GLUT3, respectively.