INSULIN STIMULATES TYROSINE PHOSPHORYLATION OF THE PROTOONCOGENE PRODUCT OF C-CBL IN 3T3-L1 ADIPOCYTES

We report here that the product of the c-Cbl proto-oncogene is promine ntly tyrosine phosphorylated in response to insulin in 3T3-L1 adipocyt es. The tyrosine phosphorylation of c-Cbl reaches a maximum within 1-2 min after stimulation by insulin and gradually declines thereafter. T he tyrosine phosphorylation of c-Cbl was also observed after treatment of 3T3-L1 adipocytes with epidermal growth factor, whereas platelet-d erived growth factor had no effect. After insulin-dependent tyrosine p hosphorylation, c-Cbl specifically associates with fusion proteins con taining the Src homology 2 (SH2) domains of Crk and the Fyn tyrosine k inase, but not with fusion proteins containing the SH2 domains of eith er the p85 subunit of phosphatidylinositol 3'-kinase or the tyrosine p hosphatase SHPTP2/Syp. Furthermore insulin stimulates the association of c-Cbl with endogenous c-Crk and Fyn in intact 3T3-L1 adipocytes. Th e tyrosine phosphorylation of c-Cbl is regulated during adipocyte diff erentiation. Neither insulin-like growth factor I nor insulin stimulat ed the tyrosine phosphorylation of c-Cbl in 3T3-L1 fibroblasts. Moreov er, c-Cbl is not tyrosine phosphorylated in response to insulin in cel ls expressing high levels of the human insulin receptor, or in hepatoc ytes, despite comparable levels of c-Cbl expression. These results sug gest that c-Cbl might have a novel function in the regulation of insul in receptor intracellular signalling in 3T3-L1 adipocytes.