The ras signaling pathway mimics insulin action on glucose transporter translocation.

Abstract

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-Ras6lK-expressing fibroblasts. Overexpression of N-Ras6lK also 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-Ras6lK_ expressing 3T3-L1 fibroblasts or adipocytes, whereas total GLUT-4 protein was actually lower in the N-Ras6lK-expressing compared with parental adipocytes. However, expression of N-Ras6lK in 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-Ras6lK expression 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.Insulin is a major physiological regulator of many metabolic pathways, including those directing protein synthesis, lipogenesis, and glucose utilization. These biological actions of insulin are initiated by the activation of a specific, heterotetrameric receptor (1) with intrinsic tyrosine kinase activity (2), leading to rapid autophosphorylation of receptor tyrosine residues (3), as well as the tyrosine phosphorylation of other cellular substrates (4). Early cellular events preceding insulin regulation of target metabolic enzymes include the association of phosphatidylinositol 3-kinase with tyrosinephosphorylated proteins (5-8), an increase in the amounts of the protooncogene product Ras present in the active, GTPbound state (9-11), and the activation of numerous protein serine/threonine kinases (12, 13). Some of the insulinactivated protein kinases appear to operate in cascades (14, 15), and recent evidence indicates that phosphatase I modulation by insulin is a result of protein kinase activations (16). It is thought that insulin-regulated protein kinase and phosphatase activities catalyze downstream phosphorylation/ dephosphorylation reactions that acutely regulate insulinsensitive proteins and enzymes. Thus, conversion of the elevated insulin-receptor tyrosine kinase activity to serine/ threonine phosphorylation activity is a key element of signal transmission by insu...

Constitutively active mitogen-activated protein kinase kinase increases GLUT1 expression and recruits both GLUT1 and GLUT4 at the cell surface in 3T3-L1 adipocytes.

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