Hypoxia-inducible factor-1α (HIF-1α) escapes O2-driven proteasomal degradation irrespective of its subcellular localization:: nucleus or cytoplasm

Eukaryotic cells sense oxygen and adapt to hypoxia by regulating a number of genes. Hypoxia-inducible factor 1 (HIF-1) is the 'master' in this pleiotypic response. HIF-1 comprises two members of the basic helix-loop-helix transcription factor family, HlF-1 alpha and HlF-1 beta. The HIF-1 alpha protein is subject to drastic O-2-dependent proteasomal control. However, the signalling components regulating the 'switch' for 'escaping' proteasomal degradation under hypoxia are still largely unknown. The rapid nuclear translocation of HIF-1 alpha could represent an efficient way to escape from this degradation. We therefore asked, where in the cell is HIF-1 alpha degraded. To address this question, we trapped HIF-1 alpha either in the cytoplasm, by fusing HIF-1 alpha to the cytoplasmic domain of the Na+-H+ exchanger (NHE-1), or in the nucleus, by treatment with leptomycin B. Surprisingly, we found that HlF-1 alpha is stabilized by hypoxia and undergoes O-2-dependent proteasomal degradation with an identical half-life (5-8 min) in both cellular compartments. Therefore, HIF-1 alpha entry into the nucleus is not, as proposed, a key event that controls its stability. This result markedly contrasts with the mechanism that controls p53 degradation via MDM2.