SOS PHOSPHORYLATION AND DISASSOCIATION OF THE GRB2-SOS COMPLEX BY THEERK AND JNK SIGNALING PATHWAYS

Insulin activation of Ras is mediated by the plasma membrane targeting of the guanylnucleotide exchange factor SOS associated with the small adapter protein Grb2, SOS also lies in an insulin-stimulated feedback pathway in which the serine/threonine phosphorylation of SOS results in disassociation of the Grb2-SOS complex thereby limiting the extent of Ras activation, To examine the relative role of the mitogen-activat ed protein kinases in the feedback phosphorylation of SOS we determine d the signaling specificity of insulin, osmotic shock, and anisomycin to activate the ERK (extracellular-signal regulated kinase) and JNK (c -Jun kinase) pathways. In Chinese hamster ovary cells expressing the h uman insulin receptor and murine 3T3L1 adipocytes, insulin specificall y activated ERK with no significant effect on JNK, whereas anisomycin specifically activated JNK but was unable to activate ERK. In contrast , osmotic shock was equally effective in the activation of both kinase pathways. Insulin and osmotic shock, but not anisomycin, resulted in SOS phosphorylation and disassociation of the Grb2-SOS complex, demons trating that the JNK pathway was not involved in the insulin-stimulate d feedback uncoupling of the Grb2-SOS complex, Both the insulin and os motic shock-induced activation of ERK was prevented by treatment of ce lls with the specific MEK inhibitor (PD98059). However, expression of dominant-interfering Ras (N17Ras) inhibited the insulin- but not osmot ic shock-stimulated phosphorylation of ERK and SOS. These data demonst rate that activation of the ERK pathway, but not JNK, is responsible f or the feedback phosphorylation and disassociation of the Grb2-SOS com plex.