Regulation of platelet-derived growth factor isoform-mediated expression of prostaglandin G/H synthase in mesangial cells

Abstract

Incubation of rat renal mesangial cells with platelet-derived growth factor (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 incubation, but was enhanced about twofold after 8 to 12 hours incubation with PDGF-AB or -BB. Enhanced PGHS activity was still observed after 24 hours. Nevertheless, PGE2 release from mesangial cells was not enhanced by PDGF, hinting to the availability of arachidonic acid as rate-limiting step. PDGF receptors are coupled to multiple signaling pathways, among them phospholipase C gamma PDGF-BB rapidly phoshorylated PLC gamma, while phosphorylation by PDGF-AB was barely detectable. The differential effect of PDGF-BB and PDGF-AB was also seen with respect to calcium signaling: PDGF-BB but not PDGF-AB induced release of Ca2+ from internal stores. Activation of PLC and the resulting transient release of Ca2+ were not considered to be essential for PGHS-2 mRNA induction as both PDGF isoforms were equally effective in mRNA induction. Both PDGF isoforms led to a Ca2+ influx resulting in a long lasting elevation of [Ca2+]i. Enhanced [Ca2+]i seemed to be related to PGHS-2 mRNA expression, because PDGF-induced PGHS-2 mRNA was significantly reduced under Ca2+ free conditions. Diacylglycerol, liberated by PLC, is an activator of protein kinase C (PKC). Down-regulation of PKC by overnight incubation with phorbol ester (0.1 microM) attenuated PGHS-2 mRNA induction by PDGF-AB and -BB. Involvement of PKC was substantiated by the PKC inhibitor H7, which interfered with PDGF-mediated PGHS-2 mRNA expression, while HA1004, a considerably specific inhibitor of protein kinases A and G, was without effect. Taken together, signaling pathways other than PLC gamma seem to be involved in activation of PKC and elevation of [Ca2+]i, which were shown to be essential elements of PDGF-mediated induction of PGHS-2 mRNA expression in mesangial cells.