P120(CBL) IS A MAJOR SUBSTRATE OF TYROSINE PHOSPHORYLATION UPON B-CELL ANTIGEN RECEPTOR STIMULATION AND INTERACTS IN-VIVO WITH FYN AND SYK TYROSINE KINASES, GRB2 AND SHC ADAPTERS, AND THE P85 SUBUNIT OF PHOSPHATIDYLINOSITOL 3-KINASE

We and others have demonstrated that the c-cbl protooncogene product i s one of the earliest targets of tyrosine phosphorylation upon T cell receptor stimulation. Given the similarities in the B and T lymphocyte antigen receptors, and the induction of pre-B leukemias in mice by th e v-cbl oncogene, we examined the potential involvement of Cbl in B ce ll receptor signaling. We demonstrate prominent and early tyrosine pho sphorylation of Cbl upon stimulation of human B cell lines through sur face IgM. Cbl was associated in vivo with Fyn and, to a lesser extent, other Src family kinases. B cell activation also induced a prominent association of Cbl with Syk tyrosine kinase. A substantial fraction of Cbl was constitutively associated with Grb2 and this interaction was mediated by Grb2 SH3 domains. Tyrosine-phosphorylated Shc, which promi nently associated with Grb2, was detected in association with Cbl in a ctivated B cells. Thus, Grb2 and Shc adaptors, which associate with im munoreceptor tyrosine based activation motifs, may link Cbl to the B c ell receptor. B cell activation also induced a prominent association b etween Cbl and the p85 subunit of phosphatidylinositol (PI) 3-kinase r esulting in the association of a substantial fraction of PI 3-kinase a ctivity with Cbl. Thus, Cbl is likely to play an important role to cou ple the B cell receptor to the PI 3-kinase pathway. Our results strong ly suggest a role for p120(cbl) in signaling downstream of the B cell receptor and support the idea that Cbl participates in a general signa l transduction function downstream of the immune cell surface receptor s.