INVOLVEMENT OF SHC IN INSULIN-INDUCED AND EPIDERMAL GROWTH FACTOR-INDUCED ACTIVATION OF P21(RAS)

Shc proteins are phosphorylated on tyrosine residues and associate wit h growth factor receptor-bound protein 2 (Grb2) upon treatment of cell s with epidermal growth factor (EGF) or insulin. We have studied the r ole of Shc in insulin- and EGF-induced activation of p21(ras) in NIH 3 T3 cells overexpressing human insulin receptors (A14 cells). A14 cells are equally responsive to insulin and EGF with respect to activation of p21(ras). Analysis of Shc immunoprecipitates revealed that (i) both insulin and EGF treatment resulted in Shc tyrosine phosphorylation an d (ii) Shc antibodies coimmunoprecipitated both Grb2 and mSOS after in sulin and EGF treatment. The induction of tyrosine phosphorylation of Shc and the presence of Grb2 and mSOS in Shc immunoprecipitates follow ed similar time courses, with somewhat higher levels after EGF treatme nt. In mSOS immunoprecipitates, Shc could be detected as well. Further more, Shc immune complexes contained guanine nucleotide exchange activ ity toward p21(ras) in vitro. From these results, we conclude that aft er insulin and EGF treatment, Shc associates with both Grb2 and mSOS a nd therefore may mediate, at least in part, insulin- and EGF-induced a ctivation of p21(ras). In addition, we investigated whether the Grb2-m SOS complex associates with the insulin receptor or with insulin recep tor substrate 1 (IRS1). Although we observed association of Grb2 with IRS1, we did not detect complex formation between mSOS and IRS1 in exp eriments in which the association of mSOS with Shc was readily detecta ble. Furthermore, whereas EGF treatment resulted in the association of mSOS with the EGF receptor, insulin treatment did not result in the a ssociation of mSOS with the insulin receptor. These results indicate t hat the association of Grb2-mSOS with Shc may be an important event in insulin-induced, mSOS-mediated activation of p21(ras).