Egr-1 is required for IL-1-mediated suppression of PPARγ expression. Chondrocytes were transfected with 100 nM of control scrambled siRNA, Sp1 siRNA, or Egr-1 siRNA. At 24 hours after transfection, cells were washed, reincubated for another 24 hours, and left untreated or treated with 100 pg/mL IL-1 for 1 or 18 hours. Cell lysates were prepared and analyzed for Egr-1, Sp1 (1-hour treatment), and PPARγ (18-hour treatment) protein expression by Western blotting. In the lower panels, the blots were stripped and reprobed with a specific anti-β-actin antibody. The blots are representative of similar results obtained from four independent experiments. Egr-1, early growth response gene 1; IL, interleukin; PPARγ, peroxisome proliferator-activated receptor gamma; siRNA, small interfering RNA; Sp1, specificity protein 1.