Protein Kinase Cα Activates c-Src and Induces Podosome Formation via AFAP-110

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Abstract
We report that the actin filament-associated protein AFAP-110 is required to mediate protein kinase Cα (PKCα) activation of the nonreceptor tyrosine kinase c-Src and the subsequent formation of podosomes. Immunofluorescence analysis demonstrated that activation of PKCα by phorbol 12-myristate 13-acetate (PMA), or ectopic expression of constitutively activated PKCα, directs AFAP-110 to colocalize with and bind to the c-Src SH3 domain, resulting in activation of the tyrosine kinase. Activation of c-Src then directs the formation of podosomes, which contain cortactin, AFAP-110, actin, and c-Src. In a cell line (CaOV3) that has very little or no detectable AFAP-110, PMA treatment was unable to activate c-Src or effect podosome formation. Ectopic expression of AFAP-110 in CaOV3 cells rescued PKCα-mediated activation of c-Src and elevated tyrosine phosphorylation levels and subsequent formation of podosomes. Neither expression of activated PKCα nor treatment with PMA was able to induce these changes in CAOV3 cells expressing mutant forms of AFAP-110 that are unable to bind to, or colocalize with, c-Src. We hypothesize that one major function of AFAP-110 is to relay signals from PKCα that direct the activation of c-Src and the formation of podosomes.