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 exa mine the in vivo effects of N-iminoethyl-L-lysine (L-NIL), a selective inhi bitor of the inducible nitric oxide synthase (iNOS), on the tissue level an d distribution of interleukin-1 beta (IL-1 beta), collagenase-1, stromelysi n-1, cyclooxygenase-2 (COX-2), iNOS and nitrotyrosine, through immunohistoc hemical and morphometric analysis. Methods. Cartilage and synovial membrane specimens were obtained from 3 exp erimental groups of dogs: Group I - unoperated dogs that received no treatm ent; Group II - dogs subjected to a sectioning of the anterior cruciate lig ament of the right knee and given no treatment; and Group III - operated do gs 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-1 beta, metalloproteases (MMP), COX-2, iNOS and nitrotyrosine were documented by immunohistochemist ry 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 e nhanced in OA specimens compared to normal (p < 0.0001). However, the carti lage 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 redu ced 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 f or the IL-1 beta, COX-2, iNOS and nitrotyrosine found in the synovial linin g 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 experim ental model of OA, that a selective inhibition of iNOS by L-NIL and the sub sequent decreased production of NO also results in a marked decrease in pro duction of major catabolic factors such as MMP, IL-1 beta and peroxynitrite , as well as a reduction in COX-2 expression.