- Prolyl hydroxylase domain proteins (PHDs), also known as EGLNs (Egl-9 homologs), are a family of oxygen-, iron-, and 2-oxoglutarate–dependent dioxygenases that play a central role in the regulation of cellular oxygen sensing.
- They are best known for hydroxylating specific proline residues on the α-subunits of hypoxia-inducible factors (HIF-α), thereby marking them for recognition and degradation by the von Hippel–Lindau (pVHL) E3 ubiquitin ligase complex under normal oxygen conditions. In this way, PHDs serve as the molecular oxygen sensors that couple environmental oxygen levels to transcriptional programs governing angiogenesis, metabolism, erythropoiesis, and cell survival.
- There are three main PHD isoforms in mammals—PHD1 (EGLN2), PHD2 (EGLN1), and PHD3 (EGLN3)—each encoded by distinct genes but sharing a conserved catalytic domain. PHD2 is considered the primary regulator of HIF stability under normoxia, while PHD1 and PHD3 have more specialized and tissue-specific functions. All PHDs require molecular oxygen, Fe²⁺, 2-oxoglutarate (α-ketoglutarate), and ascorbate as cofactors to catalyze the hydroxylation reaction. This dependence on oxygen concentration allows PHD activity to fluctuate directly with oxygen availability, making them finely tuned sensors of hypoxic stress.
- Under normoxic conditions, PHDs hydroxylate conserved proline residues in the oxygen-dependent degradation domain (ODD) of HIF-α. This modification enables pVHL to bind HIF-α, recruit the ubiquitin machinery, and target it for proteasomal degradation. When oxygen levels fall, PHD activity is inhibited, preventing HIF hydroxylation. Stabilized HIF-α then accumulates, translocates to the nucleus, and activates hypoxia-responsive genes. This mechanism allows PHDs to function as the “oxygen rheostats” of the cell, ensuring precise control of hypoxia signaling.
- Each PHD isoform has distinct regulatory roles beyond HIF degradation. PHD1 is highly expressed in the testes and plays roles in metabolic regulation, particularly influencing oxidative phosphorylation and glycolysis. PHD2 is ubiquitously expressed and critical for embryonic development, with complete loss being embryonically lethal due to uncontrolled HIF activation. PHD3 is strongly induced by hypoxia and has been implicated in apoptosis, neuronal differentiation, and adaptation to prolonged low-oxygen environments. This functional diversity highlights the broader importance of PHDs in physiology and disease.
- Clinically, dysregulation of PHD activity is implicated in a wide range of pathologies. Overactive PHD function may impair adaptation to ischemia, while insufficient activity contributes to chronic HIF stabilization and tumor progression. Therapeutically, pharmacological inhibition of PHDs has become a major focus, particularly in the treatment of anemia associated with chronic kidney disease. Small-molecule PHD inhibitors (such as roxadustat, daprodustat, and vadadustat) stabilize HIF, increase endogenous erythropoietin production, and enhance iron metabolism, representing a novel class of hypoxia-inducible factor prolyl hydroxylase inhibitors (HIF-PHIs). Conversely, strategies to enhance PHD activity are being investigated in oncology to suppress aberrant HIF-driven tumorigenesis.
