Activation of the integrin effector kinase focal adhesion kinase in cancer cells is regulated by crosstalk between protein kinase Cα and the PDZ adapter protein mda-9/syntenin

Abstract

Aberrant adhesion signaling pathways in cancer cells underlie their deadly invasive capabilities. The adhesion-related PDZ adapter protein mda-9/syntenin is a positive regulator of cancer cell progression in breast cancer, melanoma, and other human cancers. In this study, we report that mda-9/syntenin mediates adhesion-mediated activation of protein kinase Cα (PKCα) and focal adhesion kinase (FAK) by fibronectin (FN) in human breast cancer and melanoma cells. FN rapidly stimulated the expression of mda-9/syntenin and the activation of PKCα prior to activation of FAK. Inhibiting PKCα suppressed basal or FN-induced expression of mda-9/syntenin, as well as cell migration and invasion toward FN stimulated by mda-9/syntenin. Several lines of evidence suggested that activation of PKCα and expression of mda-9/syntenin were interdependent. First, mda-9/syntenin inhibition suppressed basal or FN-induced phosphorylation of PKCα at Thr638/641, whereas PKCα inhibition suppressed basal or FN-induced expression of mda-9/syntenin. Second, inhibiting either mda-9/syntenin or PKCα suppressed FN-induced formation of integrin-β1/FAK/c-Src signaling complexes. Third, inhibiting either mda-9/syntenin or PKCα suppressed FN-induced phosphorylation of FAK Tyr397 and c-Src Tyr416 and the induction of downstream effector signals to p38 and mitogen-activated protein kinase, Cdc42, and NF-κB. In summary, our findings offer evidence that mda-9/syntenin acts as a molecular adaptor linking PKCα and FAK activation in a pathway of FN adhesion by human breast cancer and melanoma cells.