Effects of gain of function of C/EBP binding protein (CHOP) on release of nitric oxide (NO) and matrix metalloproteinase-3 (MMP-3), and on oxidative stress and apoptosis in chondrocytes. Cultured human knee chondrocytes (passage 1) were transfected with pCMV6-hCHOP and the vector control (Vec ctrl) for 48 hours, then stimulated with IL-1β (10 ng/ml) for 18 hours (A, D) or 6 hours (B, C). (A) Expression of CHOP was examined by western blot. Superoxide generation and apoptosis were determined by percentage of cells stained positively for Mitosox Red (B) and annexin V (C). Release of NO and MMP-3 (D) were assessed by Griess reaction and western blot, respectively, in conditioned media. Data representative of experiments on three different knee chondrocyte donors.