Interaction between phosphoinositide-phospholipase C and phosphatidylcholine-phospholipase D in plasma membranes of retinal capillary pericytes.

The existence of phosphatidylcholine-specific phospholipase D (PC-PLD) was demonstrated in the isolated plasma membranes of retinal capillary pericytes. We determined PC-PLD activity, taking advantage of the transphosphatidylation activity of PC-PLD in the presence of ethanol, with phosphatidylethanol (PEt) formation. In the membrane environment, both phosphoinositide-specific phospholipase C (PI-PLC) and PC-PLD were activated by GTP gamma S. Neomycin, an inhibitor of PI-PLC, at a concentration (500 microM) capable of inhibiting 90% of the PI-PLC activity, only slightly inhibited the basal level of PC-PLD, but significantly decreased the GTP gamma S-activated PC-PLD by 66%. These findings indicate that the inhibitory effects of neomycin on PC hydrolysis may exert through the regulatory mechanisms of G proteins. Exogenous phosphatidic acid, a product of PC-PLD, stimulated, whereas PC, the substrate of PC-PLD, inhibited PI-PLC. These data implicate one mechanism by which PC-PLD may modulate PI-PLC activity through changing the phospholipid composition in the membrane, specifically the ratio of PA/PC. These findings are useful for the further studies on the communication between membrane-associated PI-PLC- and PC-PLD-mediated signal transduction pathways.