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Abstract

Objective

The cyclooxygenase (COX) metabolite, 15-deoxy-Δ12,14-prostaglandin J2 (15d-PGJ2), has been reported to inhibit the expression of a number of genes involved in the pathogenesis of arthritis. However, its effects on COX-2 remain controversial. We undertook this study to investigate the effects of 15d-PGJ2 on interleukin-1β (IL-1β)–induced COX-2 expression in human synovial fibroblasts (HSFs).

Methods

HSFs were cultured with IL-1β in the absence or presence of 15d-PGJ2, and the levels of COX-2 protein and messenger RNA (mRNA) expression were evaluated using Western blotting and real-time reverse transcriptase–polymerase chain reaction, respectively. COX-2 promoter activity was analyzed in transient transfection experiments. Chromatin immunoprecipitation assays were performed to evaluate the level of histone acetylation and the recruitment of histone deacetylases (HDACs) 1, 2, and 3 and histone acetylase (HAT) p300 to the COX-2 promoter.

Results

IL-1β–induced COX-2 protein and mRNA expression, as well as COX-2 promoter activation, were inhibited by 15d-PGJ2. Troglitazone, a selective peroxisome proliferator–activated receptor γ (PPARγ) ligand, enhanced COX-2 expression, while GW9662, a specific PPARγ antagonist, relieved the suppressive effect of 15d-PGJ2. IL-1β–induced histone H3 acetylation was selectively blocked by 15d-PGJ2. The reduction of histone H3 acetylation did not correlate with the recruitment of HDACs to the COX-2 promoter. Also, treatment with the specific HDAC inhibitor, trichostatin A, did not relieve the suppressive effect of 15d-PGJ2, indicating that HDACs are not involved in the inhibitory effect of 15d-PGJ2. Furthermore, 15d-PGJ2 blocked IL-1β–induced recruitment of p300 to the COX-2 promoter, which may be the mechanism for decreased histone H3 acetylation and COX-2 expression. In accordance with this, overexpression of p300, but not of a mutant p300 lacking HAT activity, relieved the inhibitory effect of 15d-PGJ2 on COX-2 promoter activation.

Conclusion

These data suggest that 15d-PGJ2 can inhibit IL-1β–induced COX-2 expression by an HDAC-independent mechanism, probably by interfering with HAT p300.