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

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 C alpha (PKC alpha) 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 PKC alpha prior to activation of FAK. Inhibiting PKC alpha 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 PKC alpha and expression of mda-9/syntenin were interdependent. First, mda-9/syntenin inhibition suppressed basal or FN-induced phosphorylation of PKC alpha at Thr(638/641), whereas PKC alpha inhibition suppressed basal or FN-induced expression of mda-9/syntenin. Second, inhibiting either mda-9/syntenin or PKC alpha suppressed FN-induced formation of integrin-beta(1)/FAK/c-Src signaling complexes. Third, inhibiting either mda-9/syntenin or PKC alpha suppressed FN-induced phosphorylation of FAK Tyr(397) and c-Src Tyr(416) and the induction of downstream effector signals to p38 and mitogen-activated protein kinase, Cdc42, and NF-kappa B. In summary, our findings offer evidence that mda-9/syntenin acts as a molecular adaptor linking PKC alpha and FAK activation in a pathway of FN adhesion by human breast cancer and melanoma cells. Cancer Res; 70(4); 1645-55. (C) 2010 AACR.