Divergent signaling pathways link focal adhesion kinase to mitogen-activated protein kinase cascades -: Evidence for a role of paxillin in c-Jun NH2-terminal kinase activation

Stimulation of a number of cell surface receptors, including integrins and Cf protein-coupled receptors, results in the activation of a non-receptor tyrosine kinase known as focal adhesion kinase (FAK), In turn, this kinase is believed to play a critical role in signaling to intracellular kinase cascades controlling gene expression such as extracellular signal-regulated kinases (ERKs), by a yet poorly defined mechanism, Furthermore, whether this tyrosine kinase also mediates the activation of other mitogen-activated protein kinase family members, such as c-Jun NH2-terminal kinases (JNKs), is still unclear. We show here that the activation of FAK by anchoring to the cell membrane is itself sufficient to stimulate potently both ERK and JNK. These effects were found to be phosphatidylinositol 5-kinase-independent, as FAK effectively stimulated Akt, and wortmannin suppressed Akt but not ERK or JNK: activation. As previously reported by others, activation of ERK correlated with the ability of FAK to induce tyrosine phosphorylation of Shc. Surprisingly, however, stimulation of JNK was not dependent on the kinase activity of FAK or on the ability to induce tyrosine phosphorylation of FAK substrates. Instead, we provide evidence that FAK may stimulate JNK through a novel pathway involving the recruitment of paxillin to the plasma membrane and the subsequent activation of a biochemical route dependent on small GTP-binding proteins of the Rho family.