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 tra nsplantation of the liver. However, the precise mechanisms involved in hypo xia stress remain to be clarified. Pro-inflammatory cytokines including int erleukin 1 beta (IL-1 beta) induce a gene expression of inducible nitric ox ide synthase (iNOS) and produce nitric oxide, which exerts either a cytopro tective or toxic effect. In this report, we found that hypoxia and heat mar kedly inhibited the induction of nitric oxide production stimulated by IL-1 beta in rat cultured hepatocytes. Both treatments also abolished the induc tion of iNOS protein and mRNA, However, hypoxia could not prevent either de gradation of an inhibitory protein (I kappaB alpha) of nuclear factor-kappa B (NF-kappaB) or translocation of NF-kappaB to the nucleus, whereas heat in hibited both of the I kappaB alpha degradation and NF-kappaB translocation, Transfection experiments with iNOS promoter construct revealed that hypoxi a as well as heat significantly inhibited the transactivation of iNOS gene. Further, a hypoxia-response element located in the promoter was not involv ed in the inhibition of iNOS induction by hypoxia, These results indicate t hat hypoxia and heat suppress iNOS gene induction at the transcriptional le vel through different mechanisms. Reduction of nitric oxide production unde r hypoxic conditions may be implicated in the cellular damage or protection during hepatic ischemia/reperfusion.