Erythropoietin (EPO) is produced primarily in the kidney in response to tissue hypoxia. The cellular mechanism linking hypoxia to EPO gene transcription involves:

• A During normoxia, prolyl hydroxylase domain proteins (PHDs) hydroxylate HIF-1α using O2 as substrate, targeting it for VHL-mediated proteasomal degradation; hypoxia inactivates PHDs, allowing HIF-1α to accumulate, dimerize with HIF-1β, and transactivate the EPO gene ✓
• B Decreased ATP production during hypoxia activates AMPK, which phosphorylates HIF-1α stabilizing it for nuclear translocation
• C Hypoxia activates the iron-responsive element (IRE) binding protein, which stabilizes EPO mRNA
• D Reduced hemoglobin saturation detected by renal peritubular cells directly activates a cAMP-PKA cascade to transcribe EPO

Explanation

Hypoxia-inducible factor (HIF) is the master regulator of the hypoxic response. Under normoxia, PHD1/2/3 enzymes use molecular oxygen to hydroxylate specific proline residues on HIF-1α (and HIF-2α), which are then recognized by the VHL (von Hippel-Lindau) E3 ubiquitin ligase complex, leading to ubiquitination and proteasomal degradation. Under hypoxia, PHDs lack sufficient O2 to function; HIF-1α/2α accumulate, translocate to the nucleus, dimerize with HIF-1β, and bind hypoxia response elements (HREs) in target genes including EPO. VHL mutations (as in VHL syndrome or renal cell carcinoma) constitutively stabilize HIF regardless of oxygen levels, causing polycythemia. This pathway is the basis for HIF-PHD inhibitors (e.g., roxadustat) used to treat anemia in CKD.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.