Exacerbation of Aging-Associated and Instability-Induced Murine Osteoarthritis With Deletion of D Prostanoid Receptor 1, a Prostaglandin D2 Receptor.

OBJECTIVE: D prostanoid receptor 1 (DP1), a receptor for prostaglandin D2 , plays important roles in inflammation and cartilage metabolism. However, its role in the pathogenesis of osteoarthritis (OA) remains unknown. This study was undertaken to explore the roles of DP1 in the development of OA in murine models and to evaluate the efficacy of a DP1 selective agonist in the treatment of OA. METHODS: The development of aging-associated OA and destabilization of the medial meniscus (DMM)-induced OA was compared between DP1-deficient (DP1-/- ) and wild-type (WT) mice. The progression of OA was assessed by histology, immunohistochemistry, and micro-computed tomography. Cartilage explants from DP1-/- and WT mice were treated with interleukin-1α (IL-1α) ex vivo, to evaluate proteoglycan degradation. The effect of intraperitoneal administration of the DP1 selective agonist BW245C on OA progression was evaluated in WT mice. RESULTS: Compared to WT mice, DP1-/- mice had exacerbated cartilage degradation in both models of OA, and this was associated with increased expression of matrix metalloproteinase 13 and ADAMTS-5. In addition, DP1-/- mice demonstrated enhanced subchondral bone changes. Cartilage explants from DP1-/- mice showed enhanced proteoglycan degradation following treatment with IL-1α. Intraperitoneal injection of BW245C attenuated the severity of DMM-induced cartilage degradation and bony changes in WT mice. CONCLUSION: These findings indicate a critical role for DP1 signaling in OA pathogenesis. Modulation of the functions of DP1 may constitute a potential therapeutic target for the development of novel OA treatments.

Expression of peroxisome proliferator-activated receptors α, ß, γ, and H- and L-prostaglandin D synthase during osteoarthritis in the spontaneous hartley guinea pig and experimental dog models.

OBJECTIVE: To investigate the expression of peroxisome proliferator-activated receptors (PPAR) α, ß, and γ, and hematopoietic and lipocalin-type prostaglandin D synthase (H- and L-PGDS) over the course of osteoarthritis (OA) in the spontaneous Hartley guinea pig and the anterior cruciate ligament transection dog models. METHODS: Guinea pigs were sacrificed at 2 (control group), 4, 8, and 12 months of age (n = 5 per group). Non-operated (control) and operated dogs were sacrificed at 4, 8, and 12 weeks postsurgery. Cartilage was evaluated histologically using the Osteoarthritis Research Society International (OARSI) guidelines. The expression of PPAR-α, ß, γ, and H- and L-PGDS was evaluated by real-time PCR and immunohistochemistry. The nonparametric Spearman test was used for correlation analysis. RESULTS: PPAR-α, ß, and γ were detected in medial tibial plateau from control animals in both the spontaneous and surgical models. Levels of PPAR-α and ß did not change over the course of OA, whereas PPAR-γ levels decreased during progression of disease. We also observed that the expression of H-PGDS remained unchanged, whereas L-PGDS increased over the course of OA. PPAR-γ levels correlated negatively, whereas L-PGDS levels correlated positively, with the histological score of OA. CONCLUSION: The level of PPAR-γ decreased, whereas level of L-PGDS increased during the progression of OA. These data suggest that reduced expression of PPAR-γ may contribute to the pathogenesis of OA, whereas enhanced expression of L-PGDS may be part of a reparative process.

Egr-1 contributes to IL-1-mediated down-regulation of peroxisome proliferator-activated receptor γ expression in human osteoarthritic chondrocytes.

INTRODUCTION: Peroxisome proliferator-activated receptor (PPAR)γ has been shown to exhibit anti-inflammatory and anti-catabolic properties and to be protective in animal models of osteoarthritis (OA). We have previously shown that interleukin-1ß (IL-1) down-regulates PPARγ expression in human OA chondrocytes. However, the mechanisms underlying this effect have not been well characterized. The PPARγ promoter harbors an overlapping Egr-1/specificity protein 1 (Sp1) binding site. In this study, our objective was to define the roles of Egr-1 and Sp1 in IL-1-mediated down-regulation of PPARγ expression. METHODS: Chondrocytes were stimulated with IL-1 and the expression levels of Egr-1 and Sp1 mRNAs and proteins were evaluated using real-time reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blotting, respectively. The role of de novo protein synthesis was evaluated using the protein synthesis inhibitor cycloheximide (CHX). The recruitment of Sp1 and Egr-1 to the PPARγ promoter was evaluated using chromatin immunoprecipitation (ChIP) assays. The PPARγ promoter activity was analyzed in transient transfection experiments. The roles of Egr-1 and Sp1 were further evaluated using small interfering RNA (siRNA) approaches. The level of Egr-1 in cartilage was determined using immunohistochemistry. RESULTS: Down-regulation of PPARγ expression by IL-1 requires de novo protein synthesis and was concomitant with the induction of the transcription factor Egr-1. Treatment with IL-1 induced Egr-1 recruitment and reduced Sp1 occupancy at the PPARγ promoter. Overexpression of Egr-1 potentiated, whereas overexpression of Sp1 alleviated, the suppressive effect of IL-1 on the PPARγ promoter, suggesting that Egr-1 may mediate the suppressive effect of IL-1. Consistently, Egr-1 silencing prevented IL-1-mediated down-regulation of PPARγ expression. We also showed that the level of Egr-1 expression was elevated in OA cartilage compared to normal cartilage. CONCLUSIONS: Our results indicate that induction and recruitment of Egr-1 contributed to the suppressive effect of IL-1 on PPARγ expression. They also suggest that modulation of Egr-1 levels in the joint may have therapeutic potential in OA.