Cell-Specific Inhibition of Inducible Nitric Oxide Synthase Activation by Leflunomide

The influence of a novel immunomodulating drug, leflunomide, on iNOS-dependent nitric oxide (NO) production in rodent macrophages and fibroblasts was investigated. Leflunomideʹs active metabolite A77 1726 caused a dose-dependent decrease of NO production in IFN-γ-treated L929 fibroblasts. The observed effect was cell-specific, as well as stimulus-specific, since A77 1726 did not affect NO production in IFN-γ-stimulated murine peritoneal macrophages or db-cAMP-treated L929 cells. A77 1726 reduced expression of IFN-γ-induced iNOS and IRF-1 mRNA in L929 cells, while iNOS enzymatic activity remained unchanged. Specific inhibitor of MAP kinase kinase (MEK), PD98059, but not unselective protein kinase inhibitor genistein, completely mimicked cell-type-specific and stimulus-specific NO-inhibitory action of leflunomide. Therefore, the recently described inhibition of MEK/MAP pathway by leflunomide could present a possible mechanism for its suppression of iNOS activation in L929 fibroblasts. Finally, a similar inhibitory effect of A77 1726 on both NO production and iNOS mRNA expression was observed also in IFN-γ + LPS-activated murine and rat primary fibroblasts.