['To explore the mechanisms responsible for in situ induction of chondrocyte death in experimental dog osteoarthritic (OA) cartilage. The roles of 2 mitogen activated protein kinases (MAPK), MEK 1/2 and p38, nuclear factor-kappaB (NF-kappaB), cyclooxygenase-2 (COX-2), inducible nitric oxide synthase (iNOS), and the caspase cascade were investigated.', 'OA knee cartilage was obtained from dogs that had received sectioning of the anterior cruciate ligament and were sacrificed 12 weeks after surgery. Cartilage explants were cultured in different inhibitors: Z-DEVD-FMK (caspase 3 inhibitor), Z-LEHD-FMK (caspase 9 inhibitor), PD 98059 (MEK 1/2 inhibitor). SB 202190 (p38 inhibitor), SN-50 (NF-kappaB inhibitor), NS-398 (COX-2 inhibitor), N-iminoethyl-l-lysine (L-NIL) (iNOS inhibitor). Cartilage specimens were stained for TUNEL reaction and immunostained using specific antibodies for caspase 3, COX-2, iNOS, and nitrotyrosine. Morphometric analyses were performed.', 'The significant level of chondrocyte death in OA cartilage was markedly decreased by caspase 3 and caspase 9 inhibitors. The two MAPK inhibitors, but not the NF-kappaB inhibitor, decreased chondrocyte death concomitant with the levels of caspase 3 and iNOS. COX-2 level was reduced by all 3 inhibitors. Specific inhibition of either COX-2 or iNOS reduced the level of chondrocyte death and caspase 3. There was evidence of crosstalk between these 2 latter systems; specific inhibition of COX-2 reduced the iNOS level, and selective inhibition of iNOS reduced COX-2 expression. COX-2 and iNOS seem to function in a positive autoregulatory manner that triggers transcription of their own biosynthetic machinery, since the specific inhibition of each system downregulates its expression.'] Is chondrocyte death in experimental osteoarthritis mediated by MEK 1/2 and p38 pathways : role of cyclooxygenase-2 and inducible nitric oxide synthase?