THE RAS SIGNALING PATHWAY MIMICS INSULIN ACTION ON GLUCOSE TRANSPORTER TRANSLOCATION

Recent observations suggest that insulin increases cellular levels of activated, GTP-bound Ras protein. We tested whether the acute actions of insulin on hexose uptake and glucose-transporter redistribution to the cell surface are mimicked by activated Ras. 3T3-L1 flbroblasts exp ressing an activated mutant (Lys-61) N-Ras protein exhibited a 3-fold increase in 2-deoxyglucose uptake rates compared with non-transfected cells. Insulin stimulated hexose uptake by almost-equal-to 2-fold in p arental flbroblasts but did not stimulate hexose uptake in the N-Ras61 K-expressing fibroblasts. Overexpression of N-Ras61K also mimicked the large effect of insulin on 2-deoxyglucose transport in 3T3-L1 adipocy tes, and again the effects of the two agents were not additive. Total glucose transporter protein (GLUT) 1 was similar between parental and N-Ras61K-expressing 3T3-L1 fibroblasts or adipocytes, whereas total GL UT-4 protein was actually lower in the N-Ras61K-expressing compared wi th parental adipocytes. However, expression of N-RaS61K in 3T3-L1 adip ocytes markedly elevated both GLUT-1 and GLUT-4 in plasma membranes re lative to intracellular membranes, and insulin had no further effect. These modulations of glucose transporters by N-Ras61K expression are n ot due to upstream regulation of insulin receptors because receptor ty rosine phosphorylation and association of phosphatidylinositol 3-kinas e with tyrosine-phosphorylated proteins were unaffected. These results show that activated Ras mimics the actions of insulin on membrane tra fficking of glucose transporters, consistent with the concept that Ras proteins function as intermediates in this insulin signaling pathway.