Potential role of the JNK/SAPK signal transduction pathway in the induction of iNOS by TNF-alpha

Nitric oxide production by macrophages is principally regulated by the calc ium-independent enzyme, inducible nitric oxide synthase (iNOS). Both lipopo lysaccharide and TNF-alpha synergize with IFN-gamma in the expression of iN OS with subsequent production of nitric oxide. Previous work has shown that IL-4 downregulates iNOS and nitric oxide expression by macrophages stimula ted with LPS and IFN-gamma, In this study, we found that IL-4 also downregu lated iNOS and nitric oxide expression induced by IFN-gamma and TNF-alpha a nd in mouse macrophages. Because various members of the mitogen-activated p rotein kinases and their upstream kinases have been shown to directly or in directly activate a number of transcription factors including AP-1 and NF k appa B, we examined the effects of IL-4 on TNF-alpha activation of the MAPK s, Our results show that IL-4 modestly inhibited JNK/SAPK and ERK activatio n by TNF-alpha. Previously, we showed that selective pharmacologic inhibiti on of the ERK and/or p38(mapk) pathway did not affect NO2- expression. Trea tment of cells with the chloride channel blocker 5-nitro-2-(3-phenylpropyla mino) benzoic acid (NPPB) showed a dose-response inhibition of NO2- express ion. NPPB was also found to inhibit ERK and JNK/SAPK activation but not p38 (mapk) With TNF-alpha stimulation. The discordance between the marked degre e of inhibition of iNOS transcript by IL-4 and the modest inhibition of JNK /SAPK and ERK suggests that the mechanism by which IL-4 inhibits iNOS trans cription appears more complex than a mere inhibition of these MAPKs. (C) 19 98 Academic Press.