Cell-specific inhibition of inducible nitric oxide synthase activation by leflunomide

The influence of a novel immunomodulating drug, leflunomide, on iNOS-depend ent nitric oxide (NO) production in rodent macrophages and fibroblasts was investigated. Leflunomide's active metabolite A77 1726 caused a dose-depend ent decrease of NO production in IFN-gamma-treated L929 fibroblasts. The ob served effect was cell-specific, as well as stimulus-specific, since A77 17 26 did not affect NO production in IFN-gamma-stimulated murine peritoneal m acrophages or db-cAMP-treated L929 cells. A77 1726 reduced expression of IF N-gamma-induced iNOS and IRF-1 mRNA in L929 cells, while iNOS enzymatic act ivity remained unchanged. Specific inhibitor of MAP kinase kinase (MEK), PD 98059, but not unselective protein kinase inhibitor genistein, completely m imicked cell-type-specific and stimulus-specific NO-inhibitory action of le flunomide. Therefore, the recently described inhibition of MEK/MAP pathway by leflunomide could present a possible mechanism for its suppression of iN OS activation in L929 fibroblasts. Finally, a similar inhibitory effect of A77 1726 on both NO production and iNOS mRNA expression was observed also i n IFN-gamma + LPS-activated murine and rat primary fibroblasts. (C) 2000 Ac ademic Press.