Stabilization of Hypoxia-inducible Factor-1α Protein in Hypoxia Occurs Independently of Mitochondrial Reactive Oxygen Species Production *

The transcription factor hypoxia-inducible factor-1α (HIF-1α) is a master regulator of the cellular response to low oxygen. HIF-1α protein accumulates in hypoxia due to inhibition of prolyl hydroxylase enzymes, which under normoxic conditions use molecular oxygen to hydroxylate HIF-1α on two conserved proline residues (Pro 402 and Pro 564 ), thus targeting the protein for 26 S proteasome-dependent degradation. A functional mitochondrial electron transport chain is known to be necessary for HIF-1α stabilization in hypoxia. It has been reported that reactive oxygen species (ROS), produced under hypoxia by complex III of the mitochondrial electron transport chain, play a critical role in the stabilization of the HIF-1α protein, possibly by directly inhibiting prolyl hydroxylase enzymes. In contrast, we found that ROS production by complex III is not required for hypoxia-induced HIF-1α stabilization. Thus, reestablishing mitochondrial oxygen consumption in the presence of a complex III inhibitor by using an artificial electron donor to complex IV or by overexpressing Ciona intestinalis alternative oxidase results in HIF-1α protein stabilization in hypoxia. Furthermore, five inhibitors that target different sites of the mitochondrial electron transport chain have similar effects on the HIF-1α protein half-life in hypoxia but vary in their effects on mitochondrial ROS production. Finally, ROS do not regulate prolyl hydroxylase activity directly. We conclude that HIF-1α protein stabilization in hypoxia occurs independently of mitochondrial ROS production. However, mitochondria can modulate the cellular hypoxic response through altered respiratory activity, likely by regulating the cellular oxygen availability.