Brief hypoxia differentially regulates LPS-induced IL-1beta and TNF-alpha gene transcription in RAW 264.7 cells

Episodes of tissue hypoxia and reoxygenation frequently occur during gram-negative bacteremia that progresses to septic shock. However, few studies have evaluated modulation by hypoxia and reoxygenation of the proinflammatory cytokine gene expression that is normally induced by gram-negative bacteremia or endotoxemia. In buffer-perfused organs, hypoxia downregulates Escherichia coli-induced expression of tumor necrosis factor (TNF)-alpha and interleukin (IL)-1beta in the liver but upregulates these cytokines in the lungs. To identify molecular mechanisms underlying these events, we investigated the effects of brief (1.5-h) hypoxia on TNF-alpha and IL-1beta expression in cultured RAW 264.7 cells during their continuous exposure to lipopolysaccharide (LPS) endotoxin derived from E. coli (serotype 055:B5) for up to 24 h. IL-1beta and TNF-alpha concentrations in cell lysates and culture supernatants were measured by ELISA, and steady-state mRNA was measured by Northern analysis. LPS-induced IL-1beta synthesis was downregulated by hypoxia at both the protein and mRNA levels despite no change in cellular redox status as measured by levels of GSH. In contrast, LPS-induced TNF-alpha production was unaffected by hypoxia as assessed by cell lysate mRNA and lysate and supernatant protein levels. Nuclear runoff analysis showed that downregulation of IL-1beta gene expression by hypoxia occurred transcriptionally. Allopurinol or catalase treatment did not alter modulation of LPS-induced IL-1beta expression by hypoxia, suggesting that this suppression was not caused by reactive oxygen species. Cycloheximide pretreatment suggested that hypoxia-induced downregulation of IL-1beta expression did not require de novo protein synthesis.