Activation of peroxisome proliferator-activated receptor γ inhibits interleukin-1β-induced membrane-associated prostaglandin E2 synthase-1 expression in human synovial fibroblasts by interfering with Egr-1

Abstract

Membrane-associated prostaglandin (PG) E2 synthase-1 (mPGES-1) catalyzes the conversion of PGH2 to PGE2, which contributes to many biological processes. Peroxisome proliferator-activated receptor γ (PPARγ) is a ligand-activated transcription factor and plays an important role in growth, differentiation, and inflammation in different tissues. Here, we examined the effect of PPARγ ligands on interleukin-1β (IL-1β)-induced mPGES-1 expression in human synovial fibroblasts. PPARγ ligands 15-deoxy-Δ12,14 prostaglandin J2 (15d-PGJ2) and the thiazolidinedione troglitazone (TRO), but not PPARγ ligand Wy14643, dose-dependently suppressed IL-1β-induced PGE2 production, as well as mPGES-1 protein and mRNA expression. 15d-PGJ2 and TRO suppressed IL-1β-induced activation of the mPGES-1 promoter. Overexpression of wild-type PPARγ further enhanced, whereas overexpression of a dominant negative PPARγ alleviated, the suppressive effect of both PPARγ ligands. Furthermore, pretreatment with an antagonist of PPARγ, GW9662, relieves the suppressive effect of PPARγ ligands on mPGES-1 protein expression, suggesting that the inhibition of mPGES-1 expression is mediated by PPARγ. We demonstrated that PPARγ ligands suppressed Egr-1-mediated induction of the activities of the mPGES-1 promoter and of a synthetic reporter construct containing three tandem repeats of an Egr-1 binding site. The suppressive effect of PPARγ ligands was enhanced in the presence of a PPARγ expression plasmid. Electrophoretic mobility shift and supershift assays for Egr-1 binding sites in the mPGES-1 promoter showed that both 15d-PGJ 2 and TRO suppressed IL-1β-induced DNA-binding activity of Egr-1. These data define mPGES-1 and Egr-1 as novel targets of PPARγ and suggest that inhibition of mPGES-1 gene transcription may be one of the mechanisms by which PPARγ regulates inflammatory responses.