Phospholipase D mimics platelet-derived growth factor as a competence factor in vascular smooth muscle cells

Recent studies have suggested the importance of phosphatidylcholine (PC) metabolism in growth factor-stimulated cells. In these cells, PC is hydrolyzed not only by PC-specific phospholipase C but also by phospholipase D (PLD). In the present investigation, we show that the simple addition of PC-hydrolyzing PLD from Streptomyces chromofuscus to the culture medium of vascular smooth muscle cells elicits choline release into the medium accompanied by the formation of phosphatidic acid. In the presence of ethanol, this treatment elicits a formation of phosphatidylethanol (PEt) at the expense of phosphatidic acid. Furthermore, we show here that exogenous addition of S. chromofuscus PLD induces a marked DNA synthesis in quiescent vascular smooth muscle cells. This DNA synthesis induced by S. chromofuscus PLD is, like platelet-derived growth factor (PDGF)-elicited DNA synthesis, largely dependent on the presence of insulin. In addition, S. chromofuscus PLD-induced PEt formation and DNA synthesis were not affected by protein kinase C down-regulation, whereas PDGF-induced PEt formation and DNA synthesis were significantly inhibited. These observations strongly suggest that protein kinase-dependent activation of PLD is involved in mitogenic signal in PDGF-stimulated cells and that exogenously added PLD acts as a competence factor in the same way as PDGF.