Domain structure of human HIF-α and HIF-1β. HIF-α (HIF-1α, HIF-2α, HIF-3α, IPAS) and HIF-1β belong to the bHLH and PAS protein family. HIF-α contains an ODDD that mediates oxygen-regulated stability through the hydroxylation of two proline (P) residues and the acetylation of a lysine (K). The proline residues are conserved in HIF-2α and HIF-3α. HIF-1α and HIF-2α also contain two transaction domains (C-TAD and N-TAD), whereas HIF-1β has only one TAD. The total number of amino acids of each subunit is marked at the end of the domain structure.
Oxygen-dependent regulation of HIF-1 stabilization and transactivation. In normoxia (left), two proline residues of HIF-1α (P402 and P564) and asparagine (N803) are hydroxylated by PHDs and FIH-1, respectively, in an O2, 2-OG, and Fe2+-dependent manner. Hydroxylated HIF-1α proteins bind to the E3 ubiquitin ligase VHL complex, leading to its degradation by the proteasome. Acetylation of lysine (K532) by ARD1 favors the interaction of HIF-1α with VHL. Hydroxylated N803 blocks the recruitment of transcriptional coactivator CBP/p300. In hypoxia (right), the activities of PHDs and FIH-1 are inhibited due to lack of O2, resulted in no proline and asparagine hydroxylation. Therefore, there is no VHL binding and HIF-1α is stabilized. Stabilized HIF-1α proteins translocate to the nucleus and bind to HIF-1β. HIF-1β may bind preferentially to the MAPK-induced phosphorylated form of HIF-1α. Nonhydroxylated N803 of HIF-1α allows CBP/p300 recruitment to the target genes, resulting in gene transcription. In addition, the expression of ARD1 is decreased under hypoxia, causing less acetylated HIF-1α.