K-ras-transformed NIH-3T3 fibroblasts Involvement of intracellular Ca2+ oscillations in phosphatidylcholine breakdown

Bradykinin (BK) induced a biphasic increase in 1,2-diacylglycerol (DAG) in both K-ras-transformed fibroblasts (DT) and the parent NIH-3T3 cells. The first phase was coincident with the increase in Ins(1,4,5)P3 resulting from PtdIns(4,5)P2 hydrolysis, and the second, sustained, phase was derived from phosphatidylcholine (PtdCho) hydrolysis. In NIH-3T3 cells, stimulation by BK induced greater production of choline than phosphocholine in [3H]choline-labelled cells and appreciable phosphatidylethanol (PtdEtOH) formation in [3H]myristic acid-labelled cells, suggesting that PtdCho was hydrolysed mainly by a phospholipase D (PLD) activity. Pretreatment with propranolol, an inhibitor of phosphatidate phosphohydrolase, markedly diminished the second DAG accumulation, supporting the above notion. In DT cells, BK induced predominantly phosphocholine generation and little PtdEtOH formation, indicating that the PtdCho hydrolysis was due to a phospholipase C (PLC) activity. The BK-induced oscillations in intracellular Ca2l concentration ([Ca2+]1) observed in single DT cells [Fu, Sugimoto, Oki, Murakami, Okano & Nozawa (1991) FEBS Lett. 281, 263-266] were detected as a sustained [Ca2+], elevation when assayed in a cell suspension. A receptor-operated Ca2+ channel blocker, SK&F 96365, suppressed both the BK-induced phosphocholine generation and the sustained [Ca2+]i elevation in a similar dose-dependent manner. These results thus suggested that oscillations in [Ca2+], are involved in the activation of PtdChospecific PLC in DT cells.