Effect of mitogen-activated protein kinase and NF-κB inhibitors on IL-1-induced downregulation of PPARγ1 expression. (a) Osteoarthritis (OA) chondrocytes were exposed to increasing concentrations of SB203580 (p38 mitogen-activated protein kinase inhibitor), SP600125 (c-Jun N-terminal kinase inhibitor) and PD98059 (extracellular signal-regulated kinase inhibitor) for 30 minutes before treatment with or without IL-1 (100 pg/ml). (b) OA chondrocytes were exposed to increasing concentrations of various inhibitors of NF-κB (pyrrolidine dithiocarbamate, MG-132, and SN-50) for 30 minutes before stimulation with or without IL-1 (100 pg/ml). After 24 hours, cell lysates were prepared and analyzed for peroxisome proliferator-activated receptor γ1 (PPARγ1) 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.