Lysophosphatidic acid stimulates tyrosine phosphorylation of focal adhesion kinase, paxillin, and p130. Signaling pathways and cross-talk with platelet-derived growth factor.

Abstract

Addition of 1-oleoyl-lysophosphatidic acid (LPA) induces tyrosine phosphorylation of multiple substrates in Swiss 3T3 cells including bands of M(r) 110,000-130,000 and M(r) 70,000-80,000. An increase in tyrosine phosphorylation of the M(r) 110,000-130,000 cluster of bands was detected as soon as 30 s after LPA stimulation reaching a maximum within 1 min. LPA stimulated tyrosine phosphorylation of all bands in a concentration-dependent fashion; a half-maximal effect occurred at 30 nM. Immunoprecipitation of lysates of LPA-treated cells with monoclonal antibodies that specifically recognize focal adhesion kinase (p125FAK), paxillin, and p130 revealed that these proteins are prominent substrates for LPA-stimulated tyrosine phosphorylation. Down-regulation of protein kinase C (PKC) by prolonged pretreatment with phorbol 12,13-dibutyrate, selective inhibition of PKC by GF109203X, or depletion of the intracellular Ca2+ pool by thapsigargin had no effect on LPA-stimulated tyrosine phosphorylation. Thus, protein tyrosine phosphorylation by LPA is largely independent of either the PKC or Ca2+ pathways. In contrast, pretreatment of the cells with cytochalasin D, which selectively disrupts the network of the actin filaments, completely inhibited LPA-induced tyrosine phosphorylation. Furthermore, tyrosine phosphorylation of p125FAK induced by LPA was completely prevented when cells were stimulated in the presence of platelet-derived growth factor at a concentration (30 ng/ml) that causes disruption of actin stress fibers. This suggests that the integrity of the actin cytoskeleton is essential for LPA-induced tyrosine phosphorylation and reveals a novel cross-talk between LPA and platelet-derived growth factor on p125FAK tyrosine phosphorylation.