Hypoxia and aerobic metabolism adaptations of human endothelial cells

The goal of our study was to assess the influence of chronic exposure to hypoxia on mitochondrial oxidative metabolism in human umbilical vein endothelial cells (EA.hy926 line) cultured for 6 days at 1% O₂ tension. The hypoxia-induced effects were elucidated at the cellular and isolated mitochondria levels. Hypoxia elevated fermentation but did not change mitochondrial biogenesis or the aerobic respiratory capacity of endothelial cells. In endothelial cells, hypoxia caused a general decrease in mitochondrial respiration during carbohydrate, fatty acid, and amino acid oxidation but increased exclusively ketogenic amino acid oxidation. Hypoxia induced an elevation of intracellular and mitochondrial reactive oxygen species (ROS) formation, although cell viability was unchanged and antioxidant systems (superoxide dismutases SOD1 and SOD2, and uncoupling proteins (UCPs)) were not increased. In mitochondria from hypoxic cells, the opposite change was observed at the respiratory chain level, i.e., considerably elevated expression and activity of complex II, and decreased expression and activity of complex I were observed. The elevated activity of complex II resulted in an increase in succinate-sustained mitochondrial ROS formation, mainly through increased reverse electron transport. A hypoxia-induced decrease in UCP2 expression and activity was also observed. It can be concluded that the exposure to chronic hypoxia induces a shift from aerobic toward anaerobic catabolic metabolism. The hypoxia-induced increase in intracellular and mitochondrial ROS formation was not excessive and may be involved in endothelial signaling of hypoxic responses. Our results indicate an important role of succinate, complex II, and reverse electron transport in hypoxia-induced adjustments in endothelial cells.