1 In human epithelial-like DLD-1 cells, nitric oxide synthase (NOS) II expression was induced by interferon-g (100 u ml 71 ) alone and, to a larger extent, by a cytokine mixture (CM) consisting of interferon-g, interleukin-1b (50 u ml 71 ) and tumor necrosis factor-a (10 ng ml 71 ). 2 CM-induced NOS II expression was inhibited by tyrphostin B42 (mRNA down to 1%; nitrite production down to 0.5% at 300 mM) and tyrphostin A25 (mRNA down to 24%, nitrite production down to 1% at 200 mM), suggesting the involvement of janus kinase 2 (JAK-2). Tyrphostin B42 also blocked the CM-induced JAK-2 phosphorylation (kinase assay) and reduced the CM-stimulated STAT1a binding activity (gel shift analysis). 3 CM reduced the nuclear binding activity of transcription factor AP-1. A heterogenous group of compounds, that stimulated the expression of c-fos/c-jun, enhanced the nuclear binding activity of AP-1. This group includes the protein phosphatase inhibitors calyculin A, okadaic acid, and phenylarsine oxide, as well as the inhibitor of translation anisomycin. All of these compounds reduced CM-induced NOS II mRNA expression (to 9% at 50 nM calyculin A; to 28% at 500 nM okadaic acid; to 18% at 10 mM phenylarsine oxide; and to 19% at 100 ng ml 71 anisomycin) without changing NOS II mRNA stability. In cotransfection experiments, overexpression of c-Jun and c-Fos reduced promoter activity of a 7 kb DNA fragment of the 5'-¯anking sequence of the human NOS II gene to 63%. 4 Nuclear extracts from resting DLD-1 cells showed signi®cant binding activity for transcription factor NF-kB, which was only slightly enhanced by CM. The NF-kB inhibitors dexamethasone (1 mM), 3,4-dichloroisocoumarin (50 mM), panepoxydone (5 mg ml 71 ) and pyrrolidine dithiocarbamate (100 mM) produced no inhibition of CM-induced NOS II induction. 5 We conclude that in human DLD-1 cells, the interferon-g ± JAK-2-STAT1a pathway is important for NOS II induction. AP-1 (that is downregulated by CM) seems to be a negative regulator of NOS II expression. NF-kB, which is probably important for basal activity of the human NOS II promoter, is unlikely to function as a major e ector of CM in DLD-1 cells.
