Hypoxic but not anoxic stabilization of HIF-1α requires mitochondrial reactive oxygen species

The molecular mechanisms by which cells detect hypoxia (1.5% O₂), resulting in the stabilization of hypoxia-inducible factor 1α (HIF-1α) protein remain unclear. One model proposes that mitochondrial generation of reactive oxygen species is required to stabilize HIF-1α protein. Primary evidence for this model comes from the observation that cells treated with complex I inhibitors, such as rotenone, or cells that lack mitochondrial DNA (ρ⁰-cells) fail to generate reactive oxygen species or stabilize HIF-1α protein in response to hypoxia. In the present study, we investigated the role of mitochondria in regulating HIF-1α protein stabilization under anoxia (0% O₂). Wild-type A549 and HT1080 cells stabilized HIF-1α protein in response to hypoxia and anoxia. The ρ⁰-A549 cells and ρ⁰-HT1080 cells failed to accumulate HIF-1α protein in response to hypoxia. However, both ρ⁰-A549 and ρ⁰-HT1080 were able to stabilize HIF-1α protein levels in response to anoxia. Rotenone inhibited hypoxic, but not anoxic, stabilization of HIF-1α protein. These results indicate that a functional electron transport chain is required for hypoxic but not anoxic stabilization of HIF-1α protein.