Oxygen Tolerance and Coupling of Mitochondrial Electron Transport

Abstract

Oxygen is critical to aerobic metabolism, but excessive oxygen (hyperoxia) causes cell injury and death. An oxygen-tolerant strain of HeLa cells, which proliferates even under 80% O 2 , termed "HeLa-80," was derived from wild-type HeLa cells ("HeLa-20") by selection for resistance to stepwise increases of oxygen partial pressure. Surprisingly, antioxidant defenses and susceptibility to oxidant-mediated killing do not differ between these two strains of HeLa cells. However, under both 20 and 80% O 2 , intracellular reactive oxygen species (ROS) production is significantly (ϳ2-fold) less in HeLa-80 cells. In both cell lines the source of ROS is evidently mitochondrial. Although HeLa-80 cells consume oxygen at the same rate as HeLa-20 cells, they consume less glucose and produce less lactic acid. Most importantly, the oxygen-tolerant HeLa-80 cells have significantly higher cytochrome c oxidase activity (ϳ2-fold), which may act to deplete upstream electron-rich intermediates responsible for ROS generation. Indeed, preferential inhibition of cytochrome c oxidase by treatment with n-methyl protoporphyrin (which selectively diminishes synthesis of heme a in cytochrome c oxidase) enhances ROS production and abrogates the oxygen tolerance of the HeLa-80 cells. Thus, it appears that the remarkable oxygen tolerance of these cells derives from tighter coupling of the electron transport chain.Oxygen is crucial to aerobic metabolism, but excess oxygen or, more likely, reactive oxygen species (ROS) 1 generated under hyperoxic conditions, will cause cell injury and death. Damage to cells and tissues caused by hyperoxia is clinically important. For example, both lung damage and retrolental fibroplasia occur in premature infants given oxygen as therapy for pulmonary insufficiency. The risk of these complications is further amplified by the immaturity of cellular antioxidant defenses in premature infants (1). Similar pulmonary damage is also observed in adults who, on prolonged exposure to high partial pressures of inhaled O 2 , exhibit cough, shortness of breath, decreased vital capacity, and increased alveolar-capillary permeability (2-4).Despite decades of work, it is still not known precisely how hyperoxia causes damage to cells and tissues. Nonetheless, it is commonly believed that free radicals play a key role in the pathophysiology of oxygen toxicity and cellular damage is probably mediated by increased production of ROS (5). This excessive production of ROS likely derives from the mitochondria that, under conditions of high oxygen, exhibit increased electron leak from the electron transport chain (5-7). We have approached the question of the nature of hyperoxic cell damage using a special line of HeLa cells selected for resistance to stepwise increases in the partial pressure of O 2 . These oxygentolerant HeLa cells are able to survive and grow under 80% O 2 , a partial pressure of oxygen under which normal HeLa cells and most other mammalian cells not only stop growing but die (8).If enhanced ROS production is, in fact, a...

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