Nitric oxide inhibits aggrecan degradation in explant cultures of equine articular cartilage

Arthroses are debilitating diseases of articular joints which result in ero sion of the cartilage extracellular matrix. Nitric oxide (NO) is a major co mponent of the inflammatory response, and has been implicated as a mediator of some of the effects of the proinflammatory cytokine, interleukin-1 (IL- 1). In this study, we investigated the role of NO in the regulation of prot eoglycan degradation in equine articular cartilage. NO filly mediated the s uppressive effect of IL-1 on proteoglycan synthesis. However, NO was also a ntagonistic to proteoglycan degradation, irrespective of whether degradatio n was initiated by 10 ng/ml IL-1 or 1 mu mol/l all-trans retinoic acid (RA) which (unlike IL-1) does not elevate NO production. This was confirmed usi ng the NO donor 2,2'-(hydroxynitrosohydrazono) bis-ethanamine (DETA-NONOate ) and the iNOS inhibitor L-N-5-iminoethyl ornithine (dihydrochloride) (L-NI O). The G1 fragments of aggrecan were detected in the media and extracts of cartilage explant cultures treated with all-trans RA, DETA-NONOate and L-N IO. The presence of exogenous NO in culture resulted in a decrease in the a ppearance of the 'aggrecanase' cleavage epitope. Therefore, changes in the appearance of the G1 fragment expressing the 'aggrecanase' cleavage epitope in the media emulated the glycosaminoglycan loss from the tissue. These re sults lend further support to the hypothesis that NO has an anticatabolic r ole in equine cartilage proteoglycan degradation, and suggest that this may be mediated by the regulation of 'aggrecanase' activity. Therefore, any ph armacological intervention using NO as a target must take into account both its catabolic and anticatabolic roles in joint tissue turnover.