Potentiation of tumor necrosis factor α-induced secreted phospholipase A2 (sPLA2)-IIA expression in mesangial cells by an autocrine loop involving sPLA2 and peroxisome proliferator-activated receptor α activation

Abstract

In rat mesangial cells, exogenously added secreted phospholipases A2 (sPLA2s) potentiate the expression of pro-inflammatory sPLA2-IIA first induced by cytokines like tumor necrosis factor-α (TNFα) and interleukin-1β. The transcriptional pathway mediating this effect is, however, unknown. Because products of PLA2 activity are endogenous activators of peroxisome proliferator-activated receptor α (PPARα), we postulated that sPLA2s mediate their effects on sPLA2-IIA expression via sPIA2 activity and subsequent PPARα activation. This study shows that various sPLA2s, including venom enzymes, human sPLA2-UA, and wild-type and catalytically inactive H48Q mutant of porcine pancreatic sPIA2-IB, enhance the TNFα-induced sPIA2-IIA expression at the mRNA and protein levels. In cells transfected with luciferase sPIA2-IIA promoter constructs, sPIA2s are active only when the promoter contains a functional PPRE-1 site. The effect of exogenous sPIA2s is also blocked by the PPARα inhibitor MK886. Interestingly, the expression of sPIA2-IIA induced by TNFα alone is also attenuated by MK886, by the sPIA2-IIA inhibitor LY311727, by heparinase, which prevents the binding of sPLA2-IIA to heparan sulfate proteoglycans, and by the specific cPLA2-α inhibitor pyrrolidine-1. Together, these data indicate that sPIA2-IIA released from mesangial cells by TNFα stimulates its own expression via an autocrine loop involving cPLA2 and PPARα. This signaling pathway is also used by exogenously added sPIA2s including pancreatic sPLA2-IB and is distinct from that used by TNFα.