Although lysophosphatidylcholine (LPC)-mediated cellular responses are attributed to the activation of protein kinase C (PKC), relatively little is known about the upstream signaling mechanisms that regulate the activation of PKC and downstream mitogen-activated protein (MAP) kinase. LPC activated p42 MAP kinase and PKC in mesangial cells. LPC-mediated MAP kinase activation was inhibited (but not completely) by PKC inhibition, suggesting additional signaling events. LPC stimulated protein tyrosine kinase (PTK) activity and induced Ras-GTP binding. LPC-induced MAP kinase activity was blocked by the PTK inhibitor genistein. Because LPC increased PTK activity, we examined the involvement of phospholipase C-1 (PLC-1) as a key participant in LPC-induced PKC activation. LPC stimulated the phosphorylation of PLC-1. PTK inhibitors suppressed LPC-induced PKC activity, whereas the same had no effect on phorbol 12-myristate 13-acetate-mediated PKC activity. Other lysophospholipids [e.g., lysophosphatidylinositol and lysophosphatidic acid (LPA)] also induced MAP kinase activity, and only LPA-induced MAP kinase activation was sensitive to pertussis toxin. These results indicate that LPC-mediated PKC activation may be regulated by PTK-dependent activation of PLC-1, and both PKC and PTK-Ras pathways are involved in LPC-mediated downstream MAP kinase activation.