Caspase-8, -9, and -3 activation by 4-hydroxynonenal (HNE). Chondrocytes were treated with 30 μM HNE for the indicated times, and enzymatic activities of caspase-8 (a), caspase-9 (b), or caspase-3 (c) were determined with commercial kits. (d) Chondrocytes were pre-incubated for 1 hour with or without 200 μM N-acetyl-cysteine (NAC) followed by another incubation for 16 hours with increasing concentrations of HNE (0 to 30 μM). Pro-caspase-8, pro-caspase-9, and pro-caspase-3 were analyzed by Western blot. Data are mean ± standard error of the mean (n = 8) and expressed as a percentage of untreated cells. Statistics: Student unpaired t test; *P < 0.05, **P < 0.01, ***P < 0.001.