A repressor in the proximal human inducible nitric oxide synthase promoter modulates transcriptional activation

The human inducible nitric oxide synthase (iNOS or NOSII) gene is regulated through an extended and complex promoter. In this study, the transcriptional regulation of human NOSII is investigated in the human colon cell line HCT-8R. Stimulation with a cytokine mix (interferon-gamma, interleukin 1-beta, and tumor necrosis factor alpha) induces NOSII mRNA accumulation, as well as promoter activity in these cells. Several random deletions were performed within the proximal 7 kb of the promoter, which led to the identification of a region, whose deletion provokes a marked increase in transcriptional activity upon cytokine stimulation. Furthermore, this region is shown to repress a viral-driven luciferase construct, mainly at basal levels. An AP-1-like sequence present in this region that is specifically recognized by nuclear proteins is shown to be involved in the repressive effect. This element is capable of repressing a viral promoter, and its deletion augments cytokine-stimulated transcription. These findings are confirmed in various cell lines and suggest a general mechanism for the control of basal levels of NOSII expression, to avoid unnecessary toxicity under normal conditions.