SEPT9-i1 is required for the association between HIF-1α and importin-α to promote efficient nuclear translocation

Septin 9 isoform 1 (SEPT9-i1) protein associates with hypoxia-inducible factor (HIF)-1α to augment HIF-1 transcriptional activity. The first 25 amino acids of SEPT9-i1 (N₂₅) are unique compared with other members of the mammalian septin family. This N₂₅ domain is critical for HIF-1 activation by SEPT9-i1 but not essential for the protein-protein interaction. Here, we show that expression of N₂₅ induces a significant dose-dependent inhibition of HIF-1 transcriptional activity under normoxia and hypoxia without influencing cellular HIF-1α protein levels. In vivo, N₂₅ expression inhibits proliferation, tumor growth and angiogenesis concomitant with decreased expression levels of intratumoral HIF-1 downstream genes. Depletion of endogenous SEPT9-i1 or the exogenous expression of N ₂₅ fragment reduces nuclear HIF-1α levels accompanied by reciprocal accumulation of HIF-1α in the cytoplasm. Mechanistically, SEPT9-i1 binds to importin-α through N₂₅ depending on its bipartite nuclear localization signal, to scaffold the association between HIF-1α and importin-α, which leads to facilitating HIF-1α nuclear translocation. Our data explore a new and a previously unrecognized role of a septin protein in the cytoplasmic-nuclear translocation process. This new level in the regulation of HIF-1α translocation is critical for efficient HIF-1 transcriptional activation that could be targeted for cancer therapeutics.