Insulin signals to prenyltransferases via the Shc branch of intracellular signaling

We assessed the roles of insulin receptor substrate-1 (IRS-I) and She in in sulin action on farnesyltransferase (FTase) and geranylgeranyltransferase I (GGTase I) using Chinese hamster ovary (CHO) cells that overexpress wild-ty pe human insulin receptors (CHO-hIR-WT) or mutant insulin receptors lacking the NPEY domain (CHO-Delta NPEY) or 3T3-L1 fibroblasts transfected with ad enoviruses that express the PTB or SAIN domain of IRS-1 and She, the plecks trin homology (PH) domain of IRS-1, or the Src homology 2 (SH2) domain of S he. Insulin promoted phosphorylation of the alpha -subunit of FTase and GGT ase I in CHO-hIR-WT cells, but was without effect in CHO-Delta NPEY cells. Insulin increased FTase and GGTase I activities and the amounts of prenylat ed Pas and RhoA proteins in CHO-hIR-WT (but not CHO-Delta NPEY) cells. Over expression of the PTB or SAIN domain of IRS-l (which blocked both IRS-1 and She signaling) prevented insulin-stimulated phosphorylation of the FTase a nd GGTase I alpha -subunit activation of FTase and GGTase I and subsequent increases in prenylated Res and RhoA proteins. In contrast, overexpression of the IRS-1 PH domain, which impairs IRS-1 (but not She) signaling, did no t alter insulin action on the prenyltransferases, but completely inhibited the insulin effect on the phosphorylation of IRS-1 and on the activation of phosphatidylinositol 3-kinase and Akt. Finally, overexpression of the Shc SH2 domain completely blocked the insulin effect on FTase and GGTase I acti vities without interfering with insulin signaling to MAPK. These data sugge st that insulin signaling from its receptor to the prenyltransferases FTase and GGTase I is mediated by the She pathway, but not the IRS-1/phosphatidy linositol 3-kinase pathway. She-mediated insulin signaling to MAPK may be n ecessary (but not sufficient) for activation of prenyltransferase activity, An additional pathway involving the She SH2 domain may be necessary to med iate the insulin effect on FTase and GGTase I.