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*
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 PPARα 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-PGJ2 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.
This work was supported in part by the Canadian Institutes of Health Research Grant IMH-63168, the Fonds de Recherche en Santé du Québec (FRSQ) Subvention d'Établissement de Jeune Chercheur # JC2836, and the Fonds de la Recherche du Centre de Recherche du Centre Hospitalier de l'Université de Montréal. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
