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.