Vasopressin-induced polyphosphoinositide and phosphatidylcholine degradation in fibroblasts. Temporal relationship for formation of phospholipase C and phospholipase D hydrolysis products.

Cultured fibroblasts (REF52 cells) were employed to investigate phospholipid degradation in response to vasopressin (VP) treatment. There have been few studies in fibroblasts which characterize the pattern and relationship of phosphatidylinositol 4,5-bisphosphate (PIP2) and non-phosphoinositide hydrolysis elicited by VP. Here we demonstrate that VP-induced PIP2 hydrolysis is closely accompanied by phosphatidylcholine (PC) degradation by phospholipase D. Cells prelabeled with [3H]arachidonic acid showed rapid formation and diminution of [3H]diacylglycerol (DG) (5-15s) when treated with VP; this was accompanied by a reduction in polyphosphoinositide radioactivity. Radiolabeled inositol trisphosphate was generated with a similar time frame. In cells prelabeled with [3H]myristic acid, which is predominantly incorporated into cellular PC, VP elicited the generation of [3H]myristoyl phosphatidate (PA) as early as 15 s, in the absence of an increase in labeled DG. In the presence of ethanol the pattern of [3H]myristoyl phosphatidylethanol (PEt) formation coincided with [3H]myristoyl-PA formation in the absence of ethanol. PEt was similarly formed, in response to VP treatment, in cells prelabeled with 1-O-[3H]hexadecyl-2-lyso-sn-glycero-3-phosphocholine. The formation of PC-derived [3H]myristoyl-DG was characterized by a lag period of approximately 1 min, after which DG increased steadily over a 10-min period. Biphasic formation of DG was observed in cells prelabeled with [3H]arachidonic acid, and the formation of [3H]PA occurred in an uninterrupted fashion. Two protein kinase C agonists, phorbol diester and dioctanoylglycerol, elicited the formation of [3H]myristoyl-PEt. The inclusion of staurosporine, a protein kinase C inhibitor, blocked VP-induced [3H]myristoyl-PEt formation by 88%. These data demonstrate that VP elicits the coordinated hydrolysis of PIP2 by phospholipase C and PC hydrolysis by phospholipase D. This event results in the prolonged generation of PA and biphasic formation of DG. From the time courses shown, we hypothesize that the early generation of PA, heretofore ascribed to products of the polyphosphoinositide cycle, are in part derived from PC by phospholipase D.