SEPT9_v1 up-regulates hypoxia-inducible factor 1 by preventing its RACK1-mediated degradation

A critical mediator of the cellular response to hypoxia is hypoxia-inducible factor 1 (HIF-1). Increased levels of HIF-1α are often associated with increased tumor metastasis, therapeutic resistance, and poorer prognosis. We recently identified a novel interaction between HIF-1α and the mammalian septin family member, SEPT9_v1. Septins are a highly conserved family of GTP-binding cytoskeletal proteins that are implicated in multiple cellular functions, including cell division and oncogenesis. SEPT9_v1 binds and stabilizes HIF-1α protein and stimulates HIF-1 transcriptional activity. SEPT9_v1-HIF-1 activation promotes tumor growth and angiogenesis. The structural and functional relationships between SEPT9_v1 and HIF-1α were analyzed. We found that SEPT9_v1 binds specifically with HIF-1α but not with HIF-2α. The GTPase domain of SEPT9_v1 was identified as essential for HIF-1α binding. A GTPase domain-derived polypeptide, corresponding to amino acids 252-379, was able to disrupt HIF-1α-SEPT9_v1 interaction and to inhibit HIF-1 transcriptional activity. SEPT9_v1 also protected HIF-1α from degradation induced by HSP90 inhibition by preventing the interaction of HIF-1α with the RACK1 protein, which promotes its oxygen-independent proteasomal degradation. In conclusion, a new mechanism of oxygen-independent activation of HIF-1 has been identified that is mediated by SEPT9_v1 blockade of RACK1 activity on HIF-1α degradation.