Tyrosine phosphorylation of p125(Fak) p130(Cas), and paxillin does not require extracellular signal-regulated kinase activation in Swiss 3T3 cells stimulated by bombesin or platelet-derived growth factor

The experiments presented here were designed to examine the contribution of the extracellular signal-regulated mitogen-activated protein kinases (ERKs ) to the tyrosine phosphorylation of the focal adhesion proteins p125(Fak), p130(Cas): and paxillin induced by G protein-coupled receptors (GPCRs) and tyrosine kinase receptors in Swiss 3T3 cells. Stimulation of these cells w ith bombesin, lysophosphatidic acid (LPA), endothelin, and platelet-derived growth factor (PDGF) led to a marked increase in the tyrosine phosphorylat ion or these focal adhesion proteins and in ERK activation. Exposure of the cells to two structurally unrelated mitogen-activated protein kinase or ER K kinase (MEK) inhibitors, PD98059 and U0126, completely abrogated ERK acti vation but did not prevent tyrosine phosphorylation of p125(Fak) p130(Cas) and paxillin. Furthermore, different dose-response relationships were obtai ned for tyrosine phosphorylation of focal adhesion proteins and for ERK act ivation in response to PDGF. Putative upstream Events in the activation of focal adhesion proteins including actin cytoskeletal reorganization and myo sin light chain (MLC) phosphorylation were also not prevented by inhibition of ERK activation. Thus, our results demonstrate that the activation of th er ERK pathway is not necessary For the increase of the tyrosine phosphoryl ation of p125(Fak), p130(Cas), and paxillin induced by tither GPCRs or tyro sine kinase receptors in Swiss 3T3 cells. (C) 2000 Wiley-Liss, Inc.