Disruption of the gastrointestinal tract with bacterial contamination is responsible for a host of surgical conditions. The mechanisms responsible for bacterial resistance of the GI tract have not been fully elucidated. In other tissues, the bactericidal activity of activated macrophages is dependent in part upon expression of inducible nitric oxide synthase (iNOS), the enzyme that catalyzes the synthesis of nitric oxide (NO) from L-arginine and molecular oxygen. Several reports have identified iNOS expression in GI-derived epithelial cell lines, however, a systematic in vivo study of iNOS expression in the GI tract has not been performed. It is our hypothesis that iNOS plays a role in the GI mucosal barrier to bacterial and parasitic invasion.
METHODS: To test this hypothesis, we examined the in vivo expression of iNOS in the GI tract of normal balb/c, germ-free, and g-interferon -/- mice using RNAse protection and a sensitive in situ hybridization assay.
RESULTS: iNOS expression was strongly localized to the epithelium of the GI tract. Increasing steady state levels of iNOS RNA were observed in each more distal segment of the GI tract of normal mice from the stomach (1X), duodenum (4.1X), jejunum (6.3X), caecum (9.1X), to the colon (10.5X), paralleling the degree of bacterial colonization. Consistent with our hypothesis, iNOS mRNA and enzyme activity were not detected in the GI tract of germ-free mice, but rapidly appeared after removal from the germ-free environment or injection with E. Coli LPS. Moreover, the pattern of iNOS expression in the GI tract of g-interferon -/- mice was not detectably changed compared to +/+ littermates. LPS-dependent induction of iNOS expression and production of NO in GI epithelial cells was confirmed using the colon carcinoma-derived cell line, CMT-93. In addition to the GI epithelium, LPS-induced accumulation of iNOS mRNA was also observed in the epithelium covering the renal papilla and preferentially in the periportal region of hepatic lobules.
SUMMARY: iNOS is expressed by the GI epithelium of normal mice and the degree of expression throughout the GI tract parallels the degree of bacterial colonization. Furthermore, iNOS expression is absent in germ-free mice but is reinstated when these mice are removed from their germ-free environment or treated with LPS. These results are consistent with the hypothesis that nitric oxide produced by these tissues in response to bacterial colonization contributes to the barrier that prevents bacterial and parasitic invasion.
