Hypoxia and heat inhibit inducible nitric oxide synthase gene expression by different mechanisms in rat hepatocytes

Ischemia/reperfusion contributes to the hepatic injury in resection and transplantation of the liver. However, the precise mechanisms involved in hypoxia stress remain to be clarified. Pro-inflammatory cytokines including interleukin 1beta (IL-1beta) induce a gene expression of inducible nitric oxide synthase (iNOS) and produce nitric oxide, which exerts either a cytoprotective or toxic effect. In this report, we found that hypoxia and heat markedly inhibited the induction of nitric oxide production stimulated by IL-1beta in rat cultured hepatocytes. Both treatments also abolished the induction of iNOS protein and mRNA. However, hypoxia could not prevent either degradation of an inhibitory protein (IkappaBalpha) of nuclear factor-kappaB (NF-kappaB) or translocation of NF-kappaB to the nucleus, whereas heat inhibited both of the IkappaBalpha degradation and NF-kappaB translocation. Transfection experiments with iNOS promoter construct revealed that hypoxia as well as heat significantly inhibited the transactivation of iNOS gene. Further, a hypoxia-response element located in the promoter was not involved in the inhibition of iNOS induction by hypoxia. These results indicate that hypoxia and heat suppress iNOS gene induction at the transcriptional level through different mechanisms. Reduction of nitric oxide production under hypoxic conditions may be implicated in the cellular damage or protection during hepatic ischemia/reperfusion.