THE PROTOONCOGENE PRODUCT P120(CBL) LINKS C-SRC AND PHOSPHATIDYLINOSITOL 3'-KINASE TO THE INTEGRIN SIGNALING PATHWAY

Integrin-mediated cell adhesion triggers intracellular signaling casca des, including tyrosine phosphorylation of intracellular proteins, We show in this report that p120(cbl) (Cbl), the 120-kDa c-cbl proto-onco gene product, becomes tyrosine-phosphorylated during integrin-mediated macrophage cell adhesion to extracellular matrix substrata and anti-i ntegrin antibodies. This tyrosine phosphorylation does not occur when cells attach to polylysine, to which cells adhere in a nonspecific fas hion, It also does not take place when adhesion-induced reorganization of the cytoskeleton is inhibited with cytochalasin D. In contrast to the rapid and transient tyrosine phosphorylation of Cbl by CSF-1 stimu lation, tyrosine phosphorylation of Cbl by cell attachment was gradual and persistent, Tyrosine-phosphorylated Cbl was found to form complex es with the SH2 domain-containing signaling proteins Src and phosphati dylinositol 3-kinase; in vitro kinase assays demonstrated that these k inases were active in the Cbl complexes following integrin ligand bind ing. Furthermore, Cbl was found to translocate to the plasma membrane in response to cell adhesion to fibronectin. These observations sugges t that Cbl serves as a docking protein and may transduce signals to do wnstream signaling pathways following integrin-mediated cell adhesion in macrophages.