PLATELET-ACTIVATING-FACTOR STIMULATES MULTIPLE SIGNALING PATHWAYS IN CULTURED RAT MESANGIAL CELLS

We have previously reported that platelet-activating factor (PAF) elevates cytosolic free calcium concentration ([Ca2+]i) in fura-2-loaded glomerular mesangial cells. To confirm that this increase in [Ca2+]i is a result of receptor-mediated activation of phospholipase C, we investigated hydrolysis of phosphatidylinositol-4,5-bisphosphate (Ptdlns-4,5-P2) in PAF-treated mesangial cells. PAF (10(-7) M) stimulated a rapid and transient formation of inositol trisphosphate. In concomitant experiments, PAF stimulated a biphasic accumulation of H-3-arachidonate-labeled 1,2-diacylglycerol (DAG). The secondary elevation in DAG was coincident with a rise in H-3-phosphorylcholine (PC) and H-3-phosphorylethanolamine (PE) suggesting that PAF stimulates delayed phospholipase activities which hydrolyze alternate phospholipids besides the polyphosphoinositides. This PAF-stimulated elevation in H-3-water soluble phosphorylbases was seen at 5 min but not at 15 sec suggesting that the initial rise in DAG as well as the initial elevation in [Ca2+]i are due primarily to PtdIns-4,5-P2 hydrolysis. PAF also stimulated PGE2 as well as H-3-arachidonic acid and H-3-lyso phosphatidylcholine (PtdCho) formation. We suggest that arachidonate released specifically from PtdCho via phospholipase A2 is a source of this PAF-elevated PGE2. it has been postulated that anti-inflammatory prostaglandins may antagonize the contractile and proinflammatory effects of PAF via activation of adenylate cyclase. Surprisingly, exogenous PAF reduced basal and receptor-mediated cAMP concentration indicating that PAF-stimulated transmembrane signaling pathways may oppose receptor-mediated activation of adenylyl cyclase. We have taken advantage of the different sensitivities of phospholipases A2 and C(s) to PMA, EGTA, and pertussis toxin to dissociate phospholipase A2 and C activities. Acute PMA-treatment enhanced PAF-stimulated PGE2 formation, reduced PAF-induced elevations in [Ca2+]i and had no effect upon PAF-stimulated H-3-PE. We have also demonstrated that phospholipase A2, but not Ptdlns-specific phospholipase C, was sensitive to external calcium concentration. The role of a GTP-binding protein to couple PAF-receptors to the Ptdlns-specific phospholipase C was confirmed as GTPgammaS synergistically elevated PAF-stimulated inositol phosphate formation. We also demonstrated that pertussis toxin ADP-ribosylates a single protein of an apparent 42 kD mass and that PAF pretreatment reduced subsequent ADP-ribosylation in a time-dependent manner. However, pertussis toxin had no effect upon phospholipase C-generated water soluble phosphorylbases or inositol phosphates. In contrast, PAF-stimulated phospholipase A2 and PAF-inhibited adenylyl cyclase activities were sensitive to pertussis toxin. These results suggest that a pertussis toxin-sensitive GTP binding protein(s) may couple PAF receptors to both phospholipase A2 and adenylyl cyclase which is distinct from a pertussis toxin-insensitive GTP binding protein that links PAF receptors to phospholipase C(s). Thus, we conclude that PAF activates rat mesangial cells through multiple signaling pathways.