Objective: To elucidate the mechanism of basic calcium phosphate (BCP) crystal-induced prostaglandin E(2) (PGE(2)) production in human foreskin fibroblasts (HFFs), to identify the signaling pathway involved in the induction of cyclooxygenase 2 (COX-2) messenger RNA (mRNA) by BCP crystals, to examine the effect of BCP crystals on interleukin-1beta (IL-1beta) mRNA expression, and to investigate the potential of phosphocitrate to abrogate the BCP crystal-induced effects.
Methods: PGE(2) levels were quantified using a commercial enzyme immunoassay kit. COX-2 and COX-1 transcript levels were quantified using real-time reverse transcriptase-polymerase chain reaction (RT-PCR). Induction of IL-1beta and COX-2 mRNA was examined by end-point RT-PCR. COX-2 protein expression was assessed by Western blotting.
Results: PGE(2) production measured 4 and 30 hours after BCP crystal treatment was higher in BCP crystal-treated (mean +/- SEM 1,891 +/- 273 pg/microg and 1,792 +/- 233 pg/microg, respectively) than in untreated (88 +/- 5 pg/microg and 205 +/- 93 pg/microg, respectively) HFFs. The PGE(2) produced after 4 hours was sensitive to inhibition with NS398, a selective COX-2 inhibitor, implying that it was COX-2 mediated, whereas the PGE(2) produced at 30 hours could not be completely inhibited by NS398. Real-time RT-PCR demonstrated a 23-fold increase in COX-2 mRNA that was maximal at 4 hours, whereas analysis of mRNA for COX-1 showed up-regulation of transcript peaking at 24 hours poststimulation (1.75-fold increase). The protein kinase C and phosphatidylinositol 3-kinase signal-transduction inhibitors bisindolylmaleimide I and LY294002, respectively, blocked BCP crystal-induced COX-2 mRNA in HFFs. In addition, BCP crystals were found to up-regulate the proinflammatory cytokine IL-1beta (maximal at 8 hours). The induction of both COX-2 and IL-1beta by BCP crystals was attenuated when the cells were treated with phosphocitrate.
Conclusion: These findings indicate that BCP crystals may be an important amplifier of PGE(2) production through induction of the COX enzymes and the proinflammatory cytokine IL-1beta.