EPO

**Source and regulation** - ~90% made by peritubular interstitial cells (fibroblast-like) in the renal cortex; ~10% by the liver (the main source in fetal life). - Stimulus: tissue hypoxia (high altitude, chronic lung disease, anemia, right-to-left shunt, CO poisoning, high-affinity Hb variants). It is not stimulated by low RBC count per se, but by low O2 delivery/tension. - Mechanism: under normoxia, HIF-1α/HIF-2α is hydroxylated by prolyl hydroxylase (requires O2, Fe2+, α-ketoglutarate) and then degraded via VHL-mediated ubiquitination. Under hypoxia, HIF is stabilized, binds HIF-1β, and enters the nucleus to increase EPO gene transcription (it also increases VEGF, GLUT1, and others).

**Mechanism of action** - EPO binds the EPO receptor (a cytokine receptor on erythroid progenitors, mainly CFU-E and proerythroblasts). The receptor signals via JAK2-STAT5, which promotes survival (anti-apoptosis), proliferation, and differentiation. - Result: increased reticulocyte count and RBC mass, with a response over days.

**Clinical significance** - *Anemia of chronic kidney disease*: decreased EPO leads to normocytic, normochromic anemia with a low reticulocyte count. Treat with recombinant EPO (epoetin alfa) or darbepoetin. Iron stores must be replete or the response is blunted. - *Secondary polycythemia*: appropriately high EPO (hypoxia from COPD, high altitude, OSA, cyanotic heart disease, smoking). Inappropriately high EPO occurs with EPO-secreting tumors: renal cell carcinoma, hepatocellular carcinoma, hemangioblastoma (VHL disease), uterine leiomyoma. Also seen with renal cysts or hydronephrosis, and with exogenous EPO or androgen use. - *Polycythemia vera*: JAK2 V617F mutation causes EPO-independent RBC production, so serum EPO is low (negative feedback). Classic features: pruritus after a hot shower, erythromelalgia, splenomegaly, thrombosis risk, and Budd-Chiari syndrome. Treat with phlebotomy, hydroxyurea, and ruxolitinib (JAK inhibitor). - *Blood doping*: athletes use recombinant EPO to increase O2-carrying capacity, with a risk of hyperviscosity, stroke, and MI. - *Recombinant EPO adverse effects*: hypertension, thromboembolism (higher risk of stroke, MI, and VTE when targeting a high Hb), pure red cell aplasia (anti-EPO antibodies, rare), and possible tumor progression in cancer patients (ESA boxed warning). - *Other uses*: anemia of chemotherapy, zidovudine-related anemia, and anemia of prematurity. - *HIF prolyl hydroxylase inhibitors* (e.g., daprodustat, roxadustat) are oral agents that stabilize HIF to increase endogenous EPO, used for anemia of CKD.

**High-yield pearls** - Altitude: EPO rises within hours to days, and chronic exposure leads to increased Hb and 2,3-BPG (right shift of the oxyhemoglobin curve). - Anemia of chronic disease: hepcidin is increased, and EPO response is blunted (ferritin is high, TIBC is low). - Fetal Hb and the fetal liver: the liver is the primary EPO source in the fetus, and the kidney takes over after birth. - Expected labs: EPO is high in secondary polycythemia (except with high-affinity Hb or tumors, where it may be normal or high), and low in PV. Low O2 saturation points to a hypoxic cause. - Note: "EPO" can also stand for other terms outside this context (e.g., Exclusive Provider Organization in health insurance, or eosinophil peroxidase), but the hormone is the board-relevant meaning.