Manganese superoxide dismutase suppresses hypoxic induction of hypoxia-inducible factor-1α and vascular endothelial growth factor

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Abstract
Hypoxia-inducible factor-1 (HIF-1) is a transcription factor that governs cellular responses to reduced O2 availability by mediating crucial homeostatic processes. HIF-1 is composed of an HIF-1α subunit and an HIF-1β subunit. HIF-1α is degraded following enzyme-dependent hydroxylation of prolines of HIF-1α in the presence of molecular oxygen, Fe2+, α-ketoglutarate, and ascorbate. These cofactors contribute to the redox environment of cells. The antioxidant enzyme manganese superoxide dismutase (MnSOD) also modulates the cellular redox environment. Here we show that MnSOD suppressed hypoxic accumulation of HIF-1α protein in human breast carcinoma MCF-7 cells. This suppression was biphasic depending on MnSOD activity. At low levels of MnSOD activity, HIF-1α protein accumulated under hypoxic conditions. At moderate levels of MnSOD activity (two- to six-fold increase compared to parent cells), these accumulations were blocked. However, at higher levels of MnSOD activity (>6-fold increase), accumulation of HIF-1α protein was again observed. This biphasic modulation was observed under both 1 and 4% O2. Coexpression of mitochondrial hydrogen peroxide-removing proteins prevented the accumulation of HIF-1α protein in cells with high levels of MnSOD; this effect demonstrates that the restabilization of HIF-1α observed in high MnSOD overexpressors is probably due to hydrogen peroxide, most likely produced from MnSOD. Hypoxic induction of vascular endothelial growth factor (VEGF) protein was also suppressed by elevated MnSOD activity and its levels reflected HIF-1α protein levels. These observations demonstrated that HIF-1α accumulation and VEGF expression could be modulated by the antioxidant enzyme MnSOD.