Inhibition of mitochondrial respiration elevates oxygen concentration but leaves regulation of hypoxia-inducible factor (HIF) intact

The transcription factor hypoxia-inducible factor-1 (HIF-1) is critical for erythropoietin and other factors involved in the adaptation of the organism to hypoxic stress. Conflicting results have been published regarding the role of the mitochondrial electron transport chain (ETC) in the regulation of HIF-1α. We assessed cellular hypoxia by pimonidazole staining and blotting of the O₂-labile HIF-1 α-subunit in human osteosarcoma cell cultures (U2OS and 143B). In conventional, gas-impermeable cell culture dishes, ETC inhibitors had no effect on pimonidazole staining or HIF-1α abundance in a 20% O₂ atmosphere; both parameters were undetectable. Pimonidazole staining and HIF activity were substantial in 0.1% O₂ irrespective of ETC inhibition. At an intermediate oxygen concentration (3% O₂) pimonidazole staining and HIF-α expression were detectable but strongly reduced after ETC inhibition in conventional cell cultures. All effects of ETC inhibition on HIF-1α regulation were eliminated in gas-permeable dishes. As shown in a 143B subclone deficient in mitochondrial DNA (206ρ0), genetic inactivation of the ETC led to similar responses with respect to HIF-1α regulation as ETC inhibitors. Our data demonstrate that reduction of oxygen consumption reduces the O₂ gradient in conventional cell cultures, causing elevation of the cellular O₂ concentration, which leads to degradation of HIF-α.