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 fibroblasts expressing an activated mutant (Lys-61) N-Ras protein exhibited a 3-fold increase in 2-deoxyglucose uptake rates compared with nontransfected cells. Insulin stimulated hexose uptake by ≈2-fold in parental fibroblasts but did not stimulate hexose uptake in the N-Ras61K-expressing fibroblasts. Overexpression of N-Ras61Kalso mimicked the large effect of insulin on 2-deoxyglucose transport in 3T3-L1 adipocytes, 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 GLUT-4 protein was actually lower in the N-Ras61K-expressing compared with parental adipocytes. However, expression of N-Ras61Kin 3T3-L1 adipocytes markedly elevated both GLUT-1 and GLUT-4 in plasma membranes relative to intracellular membranes, and insulin had no further effect. These modulations of glucose transporters by N-Ras61Kexpression are not due to upstream regulation of insulin receptors because receptor tyrosine phosphorylation and association of phosphatidylinositol 3-kinase with tyrosine-phosphorylated proteins were unaffected. These results show that activated Ras mimics the actions of insulin on membrane trafficking of glucose transporters, consistent with the concept that Ras proteins function as intermediates in this insulin signaling pathway.