Prolyl hydroxylase domain enzyme (PHD) (PHD)

Prolyl hydroxylase domain (PHD) enzymes, including isoforms PHD1, PHD2, and PHD3, are non-heme iron-dependent dioxygenases that function as the primary oxygen sensors in cells (UniProt: P59722). Under normal oxygen levels (normoxia), these enzymes hydroxylate specific proline residues on the Hypoxia-Inducible Factor 1-alpha (HIF-1-alpha) subunit, a process that requires oxygen, 2-oxoglutarate, and iron (Maxwell et al., Nature, 1999). This hydroxylation allows the von Hippel-Lindau (VHL) E3 ubiquitin ligase complex to recognize and target HIF-1-alpha for rapid proteasomal degradation, maintaining low levels of the protein (Ivan et al., Science, 2001). In response to hypoxia or pharmacological inhibition of PHDs, HIF-1-alpha remains unhydroxylated and stable, allowing it to translocate to the nucleus and dimerize with HIF-1-beta (Semenza, Cell, 2012). The resulting transcription factor complex binds to hypoxia-response elements (HREs) to induce the expression of genes such as erythropoietin (EPO), vascular endothelial growth factor (VEGF), and those involved in glucose metabolism (Haase, Kidney International, 2017). Therapeutic targeting of this pathway using PHD inhibitors (HIF-PHIs) has emerged as a significant strategy for treating anemia in chronic kidney disease by stimulating endogenous EPO production (Sanghani and Haase, Adv Chronic Kidney Dis, 2019). Drugs like Roxadustat and Daprodustat mimic the state of hypoxia to increase red blood cell production without the need for high-dose exogenous erythropoiesis-stimulating agents (Gupta and Wish, Am J Kidney Dis, 2022). However, the broad transcriptional activity of HIF-1-alpha raises concerns regarding potential off-target effects, including the promotion of angiogenesis in tumors and cardiovascular complications (Chen et al., J Am Soc Nephrol, 2019).

Other names

HIF prolyl hydroxylaseHIF-PHEGLNHPHProlyl 4-hydroxylaseEGLN1EGLN2EGLN3

Prolyl hydroxylase domain inhibitors (HIF-PHIs) act as 2-oxoglutarate analogues that competitively inhibit the PHD enzymes, preventing the prolyl hydroxylation of HIF-alpha subunits. This stabilization prevents HIF-alpha degradation via the VHL-proteasome pathway, allowing it to translocate to the nucleus and induce the transcription of erythropoietic and iron-regulatory genes (Haase, Kidney International, 2017).