Protective effect of heme oxygenase 1 (HO-1) on human chondrocytes. (a) Cell death was induced by treating chondrocytes with 1 mM sodium nitroprusside (SNP) for 24 hours. For HO-1 induction, chondrocytes were treated with 50 μM cobalt protoporphyrin (CoPP) 14 hours prior to treating them with 1 mM SNP. For HO-1 inhibition, chondrocytes were treated with 1 μM zinc protoporphyrin (ZnPP) along with 0.1 mM SNP. Cell death was quantitated using the 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazdium bromide assay. Cell survival in control culture was set at 100%. Data shown are the means and standard deviations of triplicate experiments from at least four different donors. * P < 0.05 versus control. (b) Induction of HO-1 by 0.1 mM SNP treatment in human chondrocytes was analyzed by western blotting. Protein was extracted from chondrocytes after the indicated incubation periods and 20 μg each protein sample was separated by 12% SDS-PAGE and blotted with anti-HO-1 antibody. Data are representative of two samples from different donors. (c) Upregulation of HO-1 by pretreating chondrocytes with 0.1 mM SNP, 50 μM CoPP, or 1 mM dibutylyl guanosine-3',5'-cyclic monophosphate (DBcGMP). Chondrocytes were treated or not treated with the indicated chemicals for 14 hours and were then treated with 1 mM SNP for 2 hours. Protein was extracted from chondrocytes and 20 μg each protein sample was separated by 12% SDS-PAGE and blotted with anti-HO-1 antibody. The data shown are representative of five samples from different donors.