Tyrosine phosphorylation of C-Cbl facilitates adhesion and spreading whilesuppressing anchorage-independent growth of V-Abl-transformed NIH3T3 fibroblasts

The protooncogenic protein c-Cbl becomes tyrosine phosphorylated in normal cells in response to a variety of external stimuli, as well as in cells tra nsformed by oncogenic protein tyrosine kinases. Tyrosine phosphorylation of c-Cbl upregulates its binding to multiple crucial signaling molecules. How ever, the biological consequences of c-Cbl-mediated signaling are insuffici ently understood. To analyse the biological functions of c-Cbl, we overexpr essed wild-type c-Cbl and its tyrosine phosphorylation-defective mutant for m in v-Abl-transformed NIH3T3 fibroblasts. In this system, wild-type c-Cbl facilitated adhesion and spreading of v-Abl-transformed fibroblasts on the extracellular matrix, while reducing anchorage independence of these cells, as measured by their colony-forming efficiency in soft agar. Therefore, ov erexpression of wild-type c-Cbl exhibits an overall transformation-suppress ing effect. By contrast, overexpression of a tyrosine phosphorylation-defec tive form of c-Cbl increases neither adhesion nor anchorage dependence of v -Abl-transformed fibroblasts. Analysis of the role of individual tyrosine p hosphorylation sites of c-Cbl in these phenomena indicates that both phosph atidylinositol-3' kinase and the CrkL adaptor protein may be involved in th e observed effects of c-Cbl. To summarize, the results presented in this re port indicate that c-Cbl is involved in regulation of cell adhesion and cyt oskeletal rearrangements, and that these effects of c-Cbl are dependent on its tyrosine phosphorylation.