ASSOCIATION BETWEEN GRB2 SOS AND INSULIN-RECEPTOR SUBSTRATE-1 IS NOT SUFFICIENT FOR ACTIVATION OF EXTRACELLULAR SIGNAL-REGULATED KINASES BYINTERLEUKIN-4 - IMPLICATIONS FOR RAS ACTIVATION BY INSULIN/

Insulin receptor substrate 1 (IRS-1) mediates the activation of a vari ety of signaling pathways by the insulin and insulin-like growth facto r 1 receptors by serving as a docking protein for signaling molecules with SH2 domains, We and others have shown that in response to insulin stimulation IRS-1 binds GRB2/Sos and have proposed that this interact ion is important in mediating Pas activation by the insulin receptor, Recently, it has been shown that the interleukin (IL)-4 receptor also phosphorylates IRS-1 and an IRS-1-related molecule, 4PS. Unlike insuli n, however, IL-4 fails to activate Ras, extracellular signal-regulated kinases (ERKs), or mitogen-activated protein kinases, We have reconst ituted the IL-4 receptor into an insulin-responsive L6 myoblast cell l ine and have shown that IRS-1 is tyrosine phosphorylated to similar de grees in response to insulin and IL-4 stimulation in this cell line, I n agreement with previous findings, IL-4 failed to activate the ERKs i n this cell line or to stimulate DNA synthesis, whereas the same respo nses were activated by insulin, Surprisingly, IL-4's failure to activa te ERKs was not due to a failure to stimulate the association of tyros ine phosphorylated IRS-1 with GRB2/Sos; the amounts of GRB2/Sos associ ated with IRS-1 were similar in insulin- and IL-4-stimulated cells, Mo reover, the amounts of phosphatidylinositol 3-kinase activity associat ed with IRS-1 were similar in insulin- and IL-4-stimulated cells, In c ontrast to insulin, however, IL-4 failed to induce tyrosine phosphoryl ation of She or association of She with GRB2, Thus, ERK activation cor relates with She tyrosine phosphorylation and formation of an Shc/GRB2 complex, Previous studies have indicated that activation of ERKs in t his cell line is dependent upon Ras since a dominant-negative Ras (Asn -17) blocks ERK activation by insulin, Our findings, taken in the cont ext of previous work suggest that binding of GRB2/Sos to She may be th e predominant mechanism whereby insulin as well as cytokine receptors activate Ras.