HYPOXIA-INDUCIBLE FACTOR (HIF)-3α2 SERVES AS AN ENDOTHELIAL CELL FATE EXECUTOR DURING CHRONIC HYPOXIA

The adaptive response to hypoxia involves the transcriptional induction of three transcription factors called hypoxia inducible factor alpha 1, 2 and 3 (HIF-1 alpha, HIF-2 alpha, and HIF-3 alpha) which dimerize with constitutively expressed beta chains that together form the HIF-1,-2 and-3 transcription factors. During normoxic conditions, the alpha chain is expressed at low levels since its stability is regulated by prolyl-hydroxylation that promotes subsequent ubiquitination and degradation. During hypoxic conditions, however, the prolyl hydroxylases are less active, and the alpha chain accumulates through elevated protein stability and the elevated induction of expression. Two of the three HIFs isoforms present in mammals, HIF-1 and HIF-2, are well characterized and have overlapping functions that promote cell survival, whereas HIF-3's role remains less clear. The HIF-3 response is complicated because the HIF3A gene can utilize different promotors and alternate splicing sites that result in a number of different HIF-3 alpha isoforms. Here, using human umbilical vein endothelial cells (HUVECs), we demonstrate that one of the isoforms of HIF-3 alpha, isoform 2 (HIF-3 alpha 2) accumulates at a late stage of hypoxia and induces the expression of DNA damage inducible transcript 3 (DDIT4), a gene known to promote apoptosis. We also demonstrate that caspase 3/7 activity is elevated, supporting that the role of the HIF-3 alpha 2 isoform is to promote apoptosis. Furthermore, we provide evidence that HIF-3 alpha 2 is also expressed in seven other primary endothelial cell types, suggesting that this may be a common feature of HIF-3 alpha 2 in endothelial cells.