Disruption of Ca2+-dependent cell-matrix adhesion enhances c-Src kinase activity, but causes dissociation of the c-Src/FAK complex and dephosphorylation of tyrosine-577 of FAK in carcinoma cells

The nonreceptor tyrosine kinase c-Src is activated in most invasive cancers. Activated c-Src binds to FAK in the focal adhesion complex, resulting in the activation of the c-Src/FAK signaling cascade, which regulates cytoskeletal functions. However, the mechanisms by which c-Src/FAK signaling is regulated during conditions of anchorage-independent growth, a hallmark of tumor progression, are not clearly known. Here, an in vivo approach to measure c-Src activity was studied using phospho-specific antibodies against phosphorylated Y418 of c-Src (SrcpY418]), an autophosphorylation site of c-Src, and phosphorylated Y577 of FAK (FAKpY577]), a known substrate of c-Src. Using genetic and pharmacological approaches to modulate c-Src activity, we showed that the levels of SrcpY418] and FAKpY577], and the formation of a c-Src/FAKpY577] complex correlated with the activation state of c-Src in adherent cells. Interestingly, both the in vivo level of SrcpY418] and in vitro c-Src kinase activity were increased in carcinoma cells following disruption of Ca(2+)-dependent cell-matrix adhesion. In contrast, the level of FAKpY577] and its association with c-Src were reduced in suspended cells. The amount of FAKpY577] in suspended cells was recovered following attachment of rounded cells to fibronectin-coated polystyrene beads, indicating that cell spreading was not required for phosphorylation of FAK. Moreover, cells expressing activated c-Src showed sustained SrcY418] phosphorylation, but required Ca(2+)-dependent cell adhesion for phosphorylation of FAKY577] and association of c-Src with FAKpY577]. These findings indicate an important role of integrin-based cell-matrix adhesion in regulating c-Src/FAK signaling under decreased anchorage conditions.