Regulation of adolescent rat intestinal epithelial inducible nitric oxide synthase expression in endotoxin tolerance: Modulation of signal transduction

Endotoxin/lipopolysaccharide (LPS) tolerance is a state of induced hyporesponsiveness to endotoxin or LPS characterized by alterations in the release of inflammatory mediators. As the gut is both a source of infection and target of injury, we tested the hypothesis that alterations in intestinal epithelial signal transduction would account for the acquisition of endotoxin tolerance as defined by decreased induction of a key mediator of gut injury, inducible nitric oxide synthase (iNOS). Rats (15 days of age) were injected with saline or LPS (1 mug/g i.p.). Tissue was harvested after 1, 4, or 6 h for assessment of signaling and iNOS expression. Other animals received a second dose of LPS 1 to 7 days after the initial dose. Selected animals received the p38 inhibitor, SB203580 (10 mug/g), which was co-administered with the first dose of LPS. Induction of iNOS mRNA and protein was significantly attenuated after repeated LPS administration. Epithelial cells from LPS-tolerant rats showed a minimal level of iNOS expression by immunohistochemistry. The down-regulation of intestinal iNOS was not gender dependent. p38 inhibition enhanced tolerance rather than blocking it. LPS-mediated activation of NF-kappaB was attenuated in a manner consistent with a primary role in the induction of tolerance. Endotoxin tolerance can be demonstrated in intestinal epithelial cells using an in vivo model. Modulation of NF-kappaB signaling may be key in the down-regulation of LPS effect seen in tolerance.