Microsomal prostaglandin E2 synthase 1 expression in basic calcium phosphate crystal-stimulated fibroblasts: role of prostaglandin E2 and the EP4 receptor.

OBJECTIVE: Basic calcium phosphate (BCP) crystals have been implicated in the pathogenesis of osteoarthritis (OA), in part because of their ability to upregulate cyclooxygenase and prostaglandin E(2) (PGE(2)) production. The aim of this work was to investigate the expression of terminal PGE(2) synthases and PGE(2) receptors (EP) in BCP crystal-stimulated fibroblasts. METHODS: Cultured fibroblasts were stimulated with BCP crystals in vitro. mRNA expression was measured by real-time polymerase chain reaction, and protein production by western blotting. RESULTS: Basal expression of microsomal prostaglandin E(2) synthase 1 (mPGES1) in osteoarthritic synovial fibroblasts (OASF) was found to be 30-fold higher than in human foreskin fibroblasts (HFF). BCP crystals increased mPGES1 expression fourfold in HFF, but not in OASF. EP4 expression was downregulated twofold by BCP crystals in OASF, but not in HFF. Exogenous PGE(2) also downregulated EP4 expression; this effect was blocked by co-administration of L-161,982, a selective EP4 antagonist. While administration of exogenous PGE(2) significantly upregulated mPGES1 expression in OASF, mPGES1 expression was threefold higher in the OASF treated with BCP crystals and PGE(2) as compared with OASF treated with PGE(2) alone. CONCLUSIONS: The differing effects of BCP crystals on mPGES1 expression in HFF and OASF may be explained by BCP crystal-induced EP4 downregulation in OASF, likely mediated via PGE(2). These data underline the complexity of the pathways regulating PGE(2) synthesis and suggest the existence of a compensatory mechanism whereby mPGES1 expression can be diminished, potentially reducing the stimulus for further PGE(2) production.