REGULATION OF PLATELET-DERIVED GROWTH-FACTOR ISOFORM-MEDIATED EXPRESSION OF PROSTAGLANDIN G H SYNTHASE IN MESANGIAL CELLS/

Incubation of rat renal mesangial cells with platelet-derived growth f actor (PDGF) -AB or -BB led to a transient increase in prostaglandin G /H synthase-2 (PGHS-2) mRNA expression with a maximum after two hours. Expression of PGHS-1 mRNA remained unchanged during short term incuba tion, but was enhanced about twofold after 8 to 12 hours incubation wi th PDGF-AB or -BB. Enhanced PGHS activity was still observed after 24 hours. Nevertheless. PGE(2) release from mesangial cells was not enhan ced by PDGF, hinting to the availability of arachidonic acid as rate-l imiting step. PDGF receptors are coupled to multiple signaling pathway s, among them phospholipase C-gamma. PDGF-BB rapidly phoshorylated PLC (gamma), while phosphorylation by PDGF-AB was barely detectable. The d ifferential effect of PDCF-BB and PDGF-AB was also seen with respect t o calcium signaling: PDGF-BB but not PDGF-AB induced release of Ca2+ f rom internal stores. Activation of PLC and the resulting transient rel ease of Ca2+ were not considered to be essential for PGHS-2 mRNA induc tion as both PDGF isoforms were equally effective in mRNA induction. B oth PDGF isoforms led to a Ca2+ influx resulting in a long lasting ele vation of [Ca2+](i). Enhanced [Ca2+](i) seemed to be related to PGHS-2 mRNA expression, because PDGF-induced PGHS-2 mRNA was significantly r educed under Ca2+ free conditions. Diacylglycerol, liberated by PLC, i s an activator of protein kinase C (PKC). Down-regulation of PKC by ov ernight incubation with phorbol ester (0.1 mu M) attenuated PGHS-2 mRN A induction by PDGF-AB and -BB. Involvement of PKC was substantiated b y the PKC inhibitor H7, which interfered with PDGF-mediated PGHS-2 mRN A expression, while HA1004, a considerably specific inhibitor of prote in kinases A and G, was without effect. Taken together, signaling path ways other than PLC(gamma) seem to be involved in activation of PKC an d elevation of [Ca2+](i), which were shown to be essential elements of PDGF-mediated induction of PGHS-2 mRNA expression in mesangial cells.