W e have investigated the effect of cis-diamminedichloroplatinum(II) (CDDP) on signal transduction pathways. CDDP treatment did not cause any change in the binding of [3H]-phorbol dibutyrate to PC-9 (human lung adenocarcinoma cell line) cells, a measure of protein kinase C activation. However, 2-h CDDP treatment (20 micrograms/ml) caused approximately 200% increase in 1,2-sn-diacylglycerol (DAG) production and approximately 50% decrease in inositol 1,4,5-triphosphate production. To explore the different source of DAG, we analyzed phospholipids labeled with [14C]choline by TLC and revealed that [14C]choline-labeled phosphatidylcholine (PC) was decreased to 50% by CDDP treatment. This suggested that PC turnover was increased by CDDP-treatment. PC-specific phospholipase C (PC-PLC) activity was increased to 2.5-fold (2.58 +/- 0.28 nmol/mg protein per min) by 2 h CDDP (20 micrograms/ml) treatment compared with control (1.05 +/- 0.24 nmol/mg protein per min). Treatment of CDDP also stimulated PC-PLC in the crude membrane extract from PC-9 cells. CDDP had no effect on the activities of phospholipase A2 and D. Trans-DDP, which has far less cytotoxicity than its stereoisomer, CDDP, did not cause any change in PC-PLC activity. A significant inhibition of DNA synthesis (less than 80%) occurred 4 h after 2 h CDDP (20 micrograms/ml) treatment. These results demonstrated that CDDP-induced PC-PLC activation was an early event in CDDP-induced cytotoxicity and suggested that the effects of CDDP on signal transduction pathways had an important role in CDDP-induced cytotoxicity.