Inducible Nitric Oxide Synthase Is Not Required in the Development of Endotoxin Tolerance in Mice

Abstract

We investigated the role of inducible nitric oxide synthase (iNOS) in endotoxin tolerance, which was induced in mice genetically deficient of iNOS (iNOS−/−) and in wild-type littermates. In non-tolerant wild-type mice, endotoxin induced high mortality, elevation of plasma levels of nitrite and nitrate, tumor necrosis factor α (TNFα), and interleukin 10 (IL-10). These events were preceded by degradation of inhibitors κBα (IκBα) and κBβ (IκBβ), and activation of nuclear factor-κB (NF-κB) in the lung. Pretreatment of wild-type mice with a sublethal dose of endotoxin prior to lethal endotoxin administration ameliorated lethality and blunted TNFα production, whereas IL-10, nitrite, and nitrate production was maintained. These events were associated with reduction of IκBα degradation and NF-κB activation in the lung. The kinetics of degradation of IκBβ were also altered. In parallel experiments, nontolerant iNOS−/− mice experienced similar mortality after endotoxin as nontolerant wild-type mice. Plasma levels of nitrite and nitrate were not elevated after lethal endotoxin administration. IL-10 levels were significantly reduced in comparison to nontolerant wild-type mice, whereas TNFα levels were similarly increased. These events were preceded by lesser degradation of IκBα and reduced NF-κB activation in the lung. Pretreatment of iNOS−/− mice with a sublethal endotoxin ameliorated lethality. TNFα production was significantly reduced, whereas IL-10 production was significantly increased when compared to nontolerant iNOS−/− mice. Degradation of IκBα and activation of NF-κB in the lung were not altered by endotoxin tolerance, whereas kinetics of IκBβ degradation was only delayed. Our data suggests that iNOS is not required for the development of endotoxin tolerance, and that other signal transduction pathways, rather than NF-κB, may regulate induction of endotoxin tolerance in the absence of iNOS.