PROTEIN-TYROSINE PHOSPHORYLATION-INDUCED BY LYSOPHOSPHATIDIC ACID IN RAT-1 FIBROBLASTS - EVIDENCE THAT PHOSPHORYLATION OF MAP KINASE IS MEDIATED BY THE GI-P21RAS PATHWAY

Lysophosphatidic acid (LPA) is a platelet-derived phospholipid that se rves as a mitogen for fibroblasts. LPA activates its own G protein-cou pled receptor(s) leading to stimulation of phospholipase C and inhibit ion of adenylate cyclase. Furthermore, LPA rapidly activates p21ras th rough a pertussis toxin-sensitive pathway. In this study, we have exam ined LPA-induced protein tyrosine phosphorylation in Rat-1 fibroblasts . LPA action was compared with that of endothelin, which is a stronger activator of phospholipase C than LPA but fails to activate p21ras an d to stimulate DNA synthesis in these cells. LPA and, more effectively , endothelin rapidly stimulate tyrosine phosphorylation of proteins of 110-130, 95, and 65-75 kDa. The effect of LPA is dose- and time-depen dent, being half-maximal at 3-30 nm and peaking after 2-5 min. Among t he 110-130-kDa group of phosphotyrosyl proteins is the 125-kDa ''focal adhesion kinase'' (p125FAK) but not the 120-kDa p21ras GTPase-activat ing protein. Furthermore, LPA, like epidermal growth factor, causes ty rosine phosphorylation and activation of the p42/p44 mitogen-activated protein (MAP) kinases, paralleling p21ras activation. In contrast, en dothelin fails to phosphorylate MAP kinase. Treatment of the cells wit h pertussis toxin blocks LPA-induced MAP kinase phosphorylation withou t affecting the other tyrosine phosphorylations. The kinase inhibitor staurosporine (1 muM) blocks LPA-induced, but not epidermal growth fac tor-induced, activation of p21ras and MAP kinase, consistent with an i ntermediate protein kinase linking the LPA receptor to p21ras activati on. The results support a model in which LPA-induced phosphorylation o f MAP kinase is mediated by p21ras and tyrosine phosphorylation of the other substrates, including p125FAK, is associated with phospholipase C activation.