Bacterial induction of inducible nitric oxide synthase in cultured human intestinal epithelial cells

Background and Aims: Enterocytes play a major role in the mucosa as a source of proinflammatory cytokines and cytotoxins. We tested the hypothesis that bacteria induce expression of the inducible nitric oxide synthase (iNOS) in cultured human enterocytes. Methods: DLD-1 and Caco-2BBe cell monolayers exposed to Salmonella dublin were analyzed for iNOS up-regulation and nitric oxide production (NO(x)) in the presence of various proinflammatory cytokines. Results: S. dublin augmented NO(x) in interferon gamma (IFN-γ)- primed cells but had no independent effect on iNOS expression. S. dublin- induced NO(x) was not mediated by endotoxin and was augmented by an enteroinvasive phenotype. In DLD-1 cells, S. dublin-mediated NO(x) was blocked by inhibitors of nuclear factor kappa B (NF-κB) and tyrosine kinase activation and was steroid resistant. Cis-acting elements in the human iNOS promoter responsive to endotoxin and S. dublin stimulation of IFN-γ-treated DLD-1 cells were identified between 10.9 and 8.7 kilobases upstream of the transcription initiation site. Conclusions: S. dublin alters the regulation of iNOS messenger RNA in IFN-γ-treated intestinal epithelial cells via a steroid-resistant pathway involving NF-κB and tyrosine kinase activity. Because bacterial interaction with cytokine-primed epithelial cells induces the synthesis of NO, an endogenous antimicrobial agent, these findings may have implications for the regulation of mucosal immunity.