TGFβ inhibits IL-1 -induced iNOS expression and NO production in immortalized chondrocytes

Objective: The balance between anti-inflammatory (e.g. TGFβ) and proinflammatory cytokines (e.g. IL-1 and TNFα), regulates destructive processes in OA cartilage. IL-1 and TNFα enhance nitric oxide (NO) production in OA cartilage through the inducible nitric oxide synthase (iNOS) pathway and NO mediates many of the destructive effects of these cytokines. The aim of the present study was to investigate the effects of TGFβ on NO production in immortalized H4 chondrocytes exposed to IL-1. Results: IL-1 induced NO production in chondrocytes through nuclear factor kappa B (NF-κB) sensitive and dexamethasone insensitive expression of iNOS. TGFβ inhibited IL-1 -induced iNOS expression and NO production in chondrocytes, but it did not have any effect on iNOS mRNA levels. iNOS protein levels were similar in cells treated with IL-1 or IL-1 + TGFβ when measured after 8 h incubation, whereas when measured after 12 h and 24 h incubations, iNOS protein levels were 50% and 80% lower in cells treated with IL-1 + TGFβ than in cells treated with IL-1 alone. Conclusion: TGFβ suppressed IL-1-induced iNOS expression and NO production in chondrocytes, probably by enhancing iNOS protein degradation. This finding suggests an additional mechanism for TGFβ to counteract the destructive effects of IL-1 in OA. Received 24 March 2005; returned for revision 16 June 2005; accepted by J. Hamilton 7 July 2005