title: Egr-1 contributes to IL-1-mediated down-regulation of peroxisome proliferator-activated receptor γ expression in human osteoarthritic chondrocytes.
abstract: BACKGROUND
  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.
authors.affiliation: 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.
authors.forename: Sarah-Salwa
authors.initials: SS
authors.lastname: Nebbaki
authors.forename: Fatima Ezzahra
authors.initials: FE
authors.lastname: El Mansouri
authors.forename: Hassan
authors.initials: H
authors.lastname: Afif
authors.forename: Mohit
authors.initials: M
authors.lastname: Kapoor
authors.forename: Mohamed
authors.initials: M
authors.lastname: Benderdour
authors.forename: Nicolas
authors.initials: N
authors.lastname: Duval
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Pelletier
authors.forename: Johanne
authors.initials: J
authors.lastname: Martel-Pelletier
authors.forename: Hassan
authors.initials: H
authors.lastname: Fahmi
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D002478:Cells, Cultured; D019902:Chondrocytes / Q000378:metabolism*; D015536:Down-Regulation / Q000502:physiology*; D051766:Early Growth Response Protein 1 / Q000502:physiology*; D005260:Female; D006801:Humans; D007375:Interleukin-1 / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D010003:Osteoarthritis / Q000378:metabolism*; D047495:PPAR gamma / Q000037:antagonists & inhibitors* / Q000096:biosynthesis*
chemical_list: C497325:EGR1 protein, human; D051766:Early Growth Response Protein 1; D007375:Interleukin-1; D047495:PPAR gamma