Hepatocyte growth factor (HGF) stimulated mitogen-activated protein (MAP) kinases and MAP kinase kinase in primary cultured rat hepatocytes. Inhibitors for protein kinase C (PKC), Ro31-8425, H-7, and calphostin C, reduced HGF-induced MAP kinase activity. A PKC activator, phorbol myristate acetate (PMA), induced MAP kinase activation in a concentration-dependent manner. Protein tyrosine kinase (PTK) inhibitors, genistein, and ST638 also inhibited HGF-induced MAP kinase activation. Furthermore, HGF increased formation of Ras guanosine triphosphate (GTP) complex, indicating Ras activation. Genistein inhibited HGF-induced Ras activation, but Ro31-8425 was without effect. On the other hand, Ro31-8425 decreased HGF-induced [3H]arachidonic acid (AA) release and [3H]thymidine incorporation. Genistein also prevented [3H]AA release and [3H]-thymidine incorporation. Moreover, a commonly used phospholipase A2 (PLA2) inhibitor, quinacrine, decreased HGF- induced [3H]AA release and [3H]thymidine incorporation. The inhibitory profile of [3H]AA release was well correlated with that of [3H]thymidine incorporation in Ro31-8425-, genistein-, and quinacrine- treated cells. A cyclooxygenase inhibitor, indomethacin, which suppressed HGF-induced DNA synthesis, had minimal effect on MAP kinase activation. In contrast, prostaglandin (PG) E1, E2, or F2α, which stimulate [3H]thymidine incorporation to the same level as that caused by HGF in hepatocytes, caused very weak activation of MAP kinases. These results suggest that PTK, Ras, and PKC play roles in MAP kinase activation induced by HGF and that MAP kinase activation resulting in AA release is involved in DNA synthesis in rat hepatocytes.