Cells sense O2 via PHD (prolyl hydroxylase) enzymes that hydroxylate HIF-α (HIF-1α + HIF-2α) on conserved prolines using O2 + α-ketoglutarate. Normoxia: hydroxylated HIF-α is recognized by VHL E3 ligase → ubiquitinated → proteasome. Hypoxia: PHD has no O2 substrate → HIF-α accumulates → binds HIF-β → nuclear → HRE-driven transcription of EPO, VEGF, glycolytic enzymes (Warburg effect), GLUT1. Tumor angiogenesis + Warburg metabolism = pathological HIF activation. HIF-PHD INHIBITORS (roxadustat, vadadustat, daprodustat) stabilize HIF → endogenous EPO synthesis without rhEPO injection → orally treat CKD anemia (alternative to ESA therapy). Phase 3 + post-marketing safety remains under scrutiny (CV signal in some trials).
Organ Systems
immune-hematologic
cardiovascular
renal
Pathway Steps
hif-alpha-protein → hif-alpha-hydroxylated — via PHD1/2/3 prolyl hydroxylation; requires O2 + α-KG + Fe2+ + ascorbate. HIF-α is produced constitutively but, when oxygen is present, is hydroxylated on proline residues by prolyl hydroxylases (PHDs). The PHDs use molecular O2 as a substrate, making them direct cellular oxygen sensors — the molecular basis of how cells “measure” oxygen, recognized by the 2019 Nobel Prize.
hif-alpha-hydroxylated → vhl-degradation — via VHL E3 ligase → ubiquitin → proteasome (normoxia outcome). Hydroxylated HIF-α is recognized by the von Hippel-Lindau (VHL) E3 ligase and rapidly ubiquitinated for proteasomal degradation. So in normoxia HIF-α is destroyed within minutes; VHL loss-of-function (VHL disease, clear-cell renal cancer) stabilizes HIF constitutively, driving tumor angiogenesis.
hif-alpha-stabilized → epo-vegf-glycolytic-transcription — via hypoxia → no PHD action → HIF-α stabilized → HRE binding. In hypoxia, hydroxylation stops, so HIF-α is stabilized, enters the nucleus, dimerizes with HIF-β, and transcribes adaptive genes — erythropoietin (EPO), VEGF (angiogenesis), and glycolytic enzymes. PHD inhibitors that stabilize HIF are now used to treat the anemia of chronic kidney disease.