OBJECTIVE: To investigate in situ the relationship between 2 key mediators implicated in osteoarthritic (OA) cartilage: nitric oxide (NO) and interleukin-1-converting enzyme (ICE). Interleukin-18 (IL-18) was also studied and served as reference for the effects of ICE. METHODS: An OA model was created in dogs by sectioning (stab wound) the anterior cruciate ligament of the right stifle joint. Three experimental groups were studied: unoperated untreated dogs, operated untreated dogs (OA), and OA dogs treated with oral N-iminoethyl-L-lysine (L-NIL), a specific inhibitor of inducible nitric oxide synthase (iNOS) (10 mg/kg twice a day starting immediately after surgery). At 12 weeks after surgery, cartilage from the femoral condyles and tibial plateaus were processed for immunohistochemistry for ICE, IL-18, and protease inhibitor 9 (PI-9), a natural inhibitor of ICE, followed by morphometric analysis. Cartilage specimens from the femoral condyles of untreated OA dogs were dissected and incubated with specific inhibitors of different signaling pathways likely to be involved in the OA process: SB 202190 (10 microM; a p38 mitogen-activated protein kinase [MAPK] inhibitor), PD 98059 (100 microM; a MAPK kinase 1/2 [MEK-1/2] inhibitor), NS-398 (10 ng/ml; a specific cyclooxygenase 2 [COX-2] inhibitor), and L-NIL (50 microM). RESULTS: Both ICE and IL-18 were present in situ in the canine cartilage, with a significant increase in the level of these 2 proteins in OA cartilage. In contrast, the level of PI-9 was lower in OA than in normal cartilage (difference not statistically significant). Compared with untreated OA cartilage, oral treatment with L-NIL significantly decreased ICE and IL-18 levels in cartilage from the femoral condyles and tibial plateaus, to values similar to those in normal dogs. L-NIL also increased the PI-9 level in normal dogs compared with OA dogs, reaching statistical significance for femoral condyle cartilage. Interestingly, in vitro experiments demonstrated significant inhibition of ICE levels by p38, MEK-1/2, and COX-2 inhibitors, but not by the iNOS inhibitor. CONCLUSION: This study demonstrated that in situ in OA cartilage, the stimulation of chondrocytes by NO is at least partly responsible for the up-regulation of ICE and IL-18 synthesis while decreasing the level of the ICE inhibitor PI-9. The ICE level is controlled by the activation of at least 2 MAPK pathways, p38 and MEK-1/2. Interestingly, it appears that ICE synthesis is not regulated by the endogenous production of NO. These data highlight the role played by iNOS in regulating the synthesis of major catabolic factors involved in OA cartilage degradation.