Selective inhibition of inducible nitric oxide synthase in experimental osteoarthritis is associated with reduction in tissue levels of catabolic factors

Objective. We used the experimental dog model of osteoarthritis (OA) to examine the in vivo effects of N-iminoethyl-L-lysine (L-NIL), a selective inhibitor of the inducible nitric oxide synthase (iNOS), on the tissue level and distribution of interleukin-1β (IL-1β), collagenase-1, stromelysin-1, cyclooxygenase-2 (COX-2), iNOS and nitrotyrosine, through immunohistochemical and morphometric analysis. Methods. Cartilage and synovial membrane specimens were obtained from 3 experimental groups of dogs: Group I - unoperated dogs that received no treatment; Group II - dogs subjected to a sectioning of the anterior cruciate ligament of the right knee and given no treatment; and Group III - operated dogs that received oral treatment with L-NIL (10 mg/kg twice daily/po) for 10 weeks starting immediately after surgery. The operated dogs were killed 10 weeks post-surgery. The tissue distributions of IL-1β, metalloproteases (MMP), COX-2, iNOS and nitrotyrosine were documented by immunohistochemistry using specific antibodies, and quantified by morphometric analysis. Results. In cartilage, the cell scores (percentage of chondrocytes staining positive for the antigen) for iNOS and 3-nitrotyrosine were dramatically enhanced in OA specimens compared to normal (p < 0.0001). However, the cartilage of dogs treated with L-NIL showed significantly lower cell scores for iNOS (p < 0.0001, condyle; p < 0.001, plateau), nitrotyrosine (p < 0.0004; p < 0.0001) and COX-2 (p < 0.0001; p < 0.001) compared to that of untreated OA dogs. Similar findings were observed for collagenase-1 and stromelysin-1, where the increased cell scores of these 2 MMP in OA cartilage were reduced after treatment with L-NIL (collagenase: p < 0.002, condyle; p < 0.0003, plateau; stromelysin: p < 0.006; p < 0.0001). The increased cell scores for the IL-1β, COX-2, iNOS and nitrotyrosine found in the synovial lining and mononuclear cell infiltrate of operated animals were also found to be markedly reduced in dogs treated with L-NIL. Conclusion. Our study demonstrates for the first time in vivo in an experimental model of OA, that a selective inhibition of iNOS by L-NIL and the subsequent decreased production of NO also results in a marked decrease in production of major catabolic factors such as MMP, IL-1β and peroxynitrite, as well as a reduction in COX-2 expression.