Hypoxia‐induced upregulation of eNOS gene expression is redox‐sensitive: A comparison between hypoxia and inhibitors of cell metabolism

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Abstract
Several papers report a hypoxia‐induced upregulation of the endothelial nitric oxide synthase (eNOS) mRNA expression. Since there is no known hypoxia‐sensitive element binding site in the eNOS promoter, we reasoned that the effect of hypoxia could be simulated by a metabolically elicited alteration of the redox state. Therefore, cultured porcine aortic endothelial cells (PAEC) were exposed to hypoxia (1–10% O2) or inhibitors of cellular energy metabolism including rotenone, 2, 4 dinitrophenol (DNP) and 2‐deoxyglucose for 6 to 24 h. Additionally, cells were treated with lactate and nicotinic acid to alter the cellular NAD(P)H/NAD(P) ratio without changes of energy supply. The cellular NAD(P)H/NAD(P) ratio was used as an index of the cellular redox state and determined using the MTT‐assay. Hypoxia increased eNOS mRNA transcription and MTT‐reduction in a manner inversely proportional to pO2. Exposure to rotenone, DNP, and lactate increased the NAD(P)H/NAD(P) ratio, MTT‐reduction, and eNOS mRNA also in parallel. In contrast, 2‐deoxyglucose and nicotinic acid attenuated both MTT‐reduction and eNOS mRNA expression. In order to study a potential role of the redox regulated transcription factor complex AP‐1 in hypoxia‐induced eNOS mRNA transcription, c‐jun expression was determined and decoy experiments were performed. c‐jun expression paralleled changes of eNOS mRNA expression and MTT‐reduction. Furthermore, in the presence of oligodeoxynucleotides corresponding to the AP‐1 binding sites of the eNOS promoter, the hypoxia and chemically induced eNOS mRNA expression was completely abolished. We propose that hypoxia, by altering cellular metabolism, leads to an increase in the cellular NAD(P)H/NAD(P) ratio which favors enhanced eNOS expression by redox‐sensitive AP‐1 mediated transcriptional control.