NITRIC-OXIDE SYNTHESIS AND ITS REGULATION BY RABBIT SYNOVIOCYTES

Objective. To determine whether rabbit synovial fibroblasts can synthe size nitric oxide (NO) and, if so, how production is regulated by cyto kines. Methods. Primary cultures of synovial fibroblasts (type B synov iocytes) were established from synovia excised from the knee joints of New Zealand white rabbits. Synthesis of NO was measured as nitrite ac cumulation in conditioned media in the presence or absence of various cytokines and other activators. Results. Resting cultures of synoviocy tes normally produced little or no NO. However, production of this fre e radical was induced by interleukin 1 (IL-1), tumor necrosis factor a lpha (TNF-alpha) or the phagocytosis of latex beads; in some cultures, the synthesis of NO occurred spontaneously. In each case, NO synthesi s began approximately 9 h after the addition of cytokines, suggesting the involvement of an inducible form of NO synthase. Antagonists of th is phenomenon included interferon gamma (IFN-gamma), which weakly inhi bited NO production, and transforming growth factor beta (TGF-beta), a very strong inhibitor. Platelet derived growth,factor (PDGF) inhibite d NO synthesis by cells stimulated with IL-1, but not by cells stimula ted with TNF-alpha. Synovial autocrine factors (CAF) modestly induced NO synthesis, but inhibited synthesis by IL-1; TGF-beta was identified as an inhibitory component of CAF. Phorbol myristate acetate (PMA) ha d only a small inductive effect, and inhibited induction by IL-1. Howe ver, the protein kinase inhibitor staurosporin was a strong inducer. M odulators of cyclic nucleotides, in contrast, had relatively modest ef fects on NO synthesis. Inhibition of NO biosynthesis by N-G-monomethyl -L-arginine (NMA) had no effect upon the increase in the production of prostaglandin E(2) (PGE(2)), matrix metalloproteinases (MMP) or lacta te by synoviocytes responding to IL-1. The rabbit synoviocyte cell lin e, HIG-82, did not synthesize detectable NO under any of the culture c onditions tested. Conclusion. Synoviocytes are a potential source of N O in arthritic joints.