Pathophysiology of cancer-associated anemia. (A) EPO transcription is dependent upon HIF-2⍺, which under normoxic (physiologic oxygenation) conditions is controlled by proteasomal degradation of HIF-2⍺ through a ubiquitination (Ub) process mediated by the von Hippel-Lindau (vHL) protein. With hypoxia, prolyl hydroxylases (PHD) are unable to hydroxylate proline residues on HIF-2α required for vHL binding. This permits HIF-2⍺ to accumulate, dimerize with HIF-1β and bind the EPO promoter region of DNA in renal EPO-producing cells. Secreted EPO migrates to the bone marrow, and upon binding to the EPO receptor located on early erythroid cells (colony forming units-erythroid and proerythroblasts), induces dimerization and phosphorylation of its Janus kinase receptor, leading to erythroblast proliferation.⁶  Erythrocyte production is closely regulated by the concentration of transferrin-bound iron and may also be diminished by certain anti-neoplastic agents, radiotherapy, or cancer itself.^14,15  Upon division, basophilic and polychromatophilic erythroblasts are capable of secreting erythroferrone (ERFE).¹⁶  (B) Regulation of iron metabolism involves several rate-limiting steps affecting RBC development. The key regulator of iron metabolism is hepcidin, a small peptide that inhibits iron flux, in both the gastrointestinal tract and splenic macrophages. EPO-dependent induction of ERFE produced by erythroid precursors during stress erythropoiesis functions to suppress hepcidin transcription via bone morphogenetic proteins BMP-5, -6, and -7 to allow for rapid recovery from blood loss.^16-18  Hepcidin production is also regulated by iron, with iron excess increasing hepcidin expression (resulting in hypoferremia), and iron deficiency decreasing hepcidin expression.^19,20  Hepcidin production is further regulated by inflammation, with cytokines such as interleukin-6 increasing hepcidin synthesis via STAT3 upregulation.^21,22  Iron-dependent induction of hepcidin requires hemojuvelin (HJV), a BMP co-receptor, also found on the hepatocyte.¹⁹  When serum iron concentrations rise and with the help of HJV, BMP-6 and to some extent BMP-2, bind to their cognate receptor to activate HAMP gene (encodes hepcidin) transcription via the SMAD pathway.¹⁹  HFE (high iron) and TFR2 also play a role in BMP signaling.^23-25  Hepcidin transcription is reduced when the transmembrane serine protease TMPRSS6 (also known as matriptase-2) cleaves HJV to deactivate the BMP receptor (BMPR). (C) Dietary iron, once reduced (Fe²⁺), is absorbed via the divalent metal transporter 1 (DMT1) on the apical surface of the intestinal lumen.²²  However, once inside this cell, hepcidin, located on the opposite basolateral surface, regulates iron egress into the bloodstream. Hephaestin (not shown) oxidizes Fe²⁺ to Fe³⁺ which is required by transferrin for iron docking.²²  Inhibition of iron flux occurs when hepcidin binds to ferroportin of gastrointestinal cells and macrophages, leading to ferroportin internalization and degradation.²⁶  (D) Increased levels of hepcidin block mobilization of iron from storage sites, including macrophages in the red pulp of the spleen. (E) Exogenous rhEPO or drug-induced (eg, roxadustat, a prolyl hydroxylase inhibitor [PHDI]) endogenously-produced EPO may bind to the ephrin-B4 receptor on tumor tissue to promote tumor growth. Professional illustration by Patrick Lane, ScEYEnce Studios.