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Open Access Research article

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

Sarah-Salwa Nebbaki1, Fatima Ezzahra El Mansouri1, Hassan Afif1, Mohit Kapoor1, Mohamed Benderdour2, Nicolas Duval3, Jean-Pierre Pelletier1, Johanne Martel-Pelletier1 and Hassan Fahmi1*

Author Affiliations

1 Osteoarthritis Research Unit, Research Centre of the University of Montreal Hospital Center (CR-CHUM), Notre-Dame Hospital, 1560 Sherbrooke Street East, J.A. DeSève Pavillion, Y-2628, and Department of Medicine, University of Montreal, Montreal, QC H2L 4M1, Canada

2 Research Centre, Sacré-Coeur Hospital, 5400 Gouin Boulevard West, Montreal, QC H4J 1C5, Canada

3 Centre de Convalescence, de Charmilles Pavillion, 1487 des Laurentides Boulevard, Montreal, QC H7M 2Y3, Canada

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Arthritis Research & Therapy 2012, 14:R69  doi:10.1186/ar3788

Published: 28 March 2012

Abstract

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-1β (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.