Regulation of prostaglandin E synthases:: Effects of siRNA-mediated inhibition of microsomal prostaglandin E synthase-1

Prostaglandin E-2 (PGE(2)) is a key mediator involved in several inflammatory conditions. In this study, we investigated the expression and regulation of the terminal PGE(2) synthesizing enzyme prostaglandin E synthases (mPGES-1, mPGES-2 and cPGES) in gingival fibroblasts stimulated with pro-inflammatory cytokines. We used siRNA knockdown of mPGES-1 to elucidate the impact of mPGES-1 inhibition on mPGES-2 and cPGES expression, as well as on PGE(2) production. The cytokines TNF alpha and IL-1 beta increased protein expression and activity of mPGES-1, accompanied by increased COX-2 expression and PGE(2) production. The isoenzymes mPGES-2 and OGES, constitutively expressed at mRNA and protein levels, were unaffected by the pro-inflammatory cytokines. We show for the first time that treatment with mPGES-1 siRNA down-regulated the cytokine-induced mPGES-1 protein expression and activity. Interestingly, mPGES-1 siRNA did not affect the cytokine-stimulated PGE(2) production, whereas PGF(2 alpha) levels were enhanced. Neither mPGES-2 nor cPGES expression was affected by siRNA silencing of mPGES-1. Dexamethasone and MK-886 both inhibited the cytokine-induced mPGES-1 expression while mPGES-2 and cPGES expression remained unaffected. In conclusion, mPGES-1 siRNA down-regulates mPGES-1 expression, and neither mPGES-2 nor cPGES substituted for MPGES-1 in a knockdown setting in gingival fibroblasts. Moreover, mPGES-1 siRNA did not affect PGE(2) levels, whereas PGF(2 alpha) increased, suggesting a compensatory pathway of PGE(2) synthesis when mPGES-1 is knocked down. (C) 2007 Elsevier B.V. All rights reserved.