[Metabolism and Bioenergetics] Detection of Reactive Oxygen Species via Endogenous Oxidative Pentose Phosphate Cycle Activity in Response to Oxygen Concentration: IMPLICATIONS FOR THE MECHANISM OF HIF-1{alpha} STABILIZATION UNDER MODERATE HYPOXIA

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[Metabolism and Bioenergetics] Detection of Reactive Oxygen Species via Endogenous Oxidative Pentose Phosphate Cycle Activity in Response to Oxygen Concentration: IMPLICATIONS FOR THE MECHANISM OF HIF-1{alpha} STABILIZATION UNDER MODERATE HYPOXIA

The oxidative pentose phosphate cycle (OPPC) is necessary to maintain cellular reducing capacity during periods of increased oxidative stress. Metabolic flux through the OPPC increases stoichiometrically in response to a broad range of chemical oxidants, including those that generate reactive oxygen species (ROS). Here we show that OPPC sensitivity is sufficient to detect low levels of ROS produced metabolically as a function of the percentage of O2. We observe a significant decrease in OPPC activity in cells incubated under severe and moderate hypoxia (ranging from <0.01 to 4% O2), whereas hyperoxia (95% O2) results in a significant increase in OPPC activity. These data indicate that metabolic ROS production is directly dependent on oxygen concentration. Moreover, we have found no evidence to suggest that ROS, produced by mitochondria, are needed to stabilize hypoxia-inducible factor 1 (HIF-1) under moderate hypoxia. Myxothiazol, an inhibitor of mitochondrial electron transfer, did not prevent HIF-1 stabilization under moderate hypoxia. Moreover, the levels of HIF-1 that we observed after exposure to moderate hypoxia were comparable between 0 cells, which lack functional mitochondria, and the wild-type cells. Finally, we find no evidence for stabilization of HIF-1 in response to the non-toxic levels of H2O2 generated by the enzyme glucose oxidase. Therefore, we conclude that the oxygen dependence of the prolyl hydroxylase reaction is sufficient to mediate HIF-1 stability under moderate as well as severe hypoxia.