Why Ineffective Erythropoiesis Suppresses Hepcidin
Ineffective erythropoiesis suppresses hepcidin primarily through erythroferrone (ERFE), a hormone produced by EPO-stimulated erythroblasts that directly inhibits hepatic BMP/SMAD signaling by trapping BMP ligands (specifically BMP5, BMP6, and BMP7), thereby blocking hepcidin transcription and mobilizing iron for attempted—though ultimately futile—red blood cell production. 1
The Molecular Mechanism
The suppression occurs through a well-defined erythroid-hepatic axis:
EPO stimulation drives ERFE production: When erythropoiesis is stimulated (whether effective or ineffective), erythropoietin triggers erythroblasts to synthesize and release ERFE into circulation 1
ERFE acts as a BMP ligand trap: ERFE specifically binds and sequesters BMP5, BMP6, and BMP7—the key BMPs responsible for activating hepatic hepcidin transcription. This binding prevents these BMPs from engaging their receptors on hepatocytes 2
BMP/SMAD pathway inhibition: By trapping BMP ligands, ERFE prevents phosphorylation of SMAD1/5/8 proteins and blocks formation of the SMAD complex that would normally translocate to the nucleus to activate hepcidin gene (HAMP) transcription 1, 2
The Pathologic Paradox in Ineffective Erythropoiesis
This represents a maladaptive response in conditions like β-thalassemia, where the body's iron-sensing mechanism is hijacked by dysfunctional erythropoiesis 3, 4:
- The expanded but apoptotic erythroid precursors continue signaling for more iron through ERFE-mediated hepcidin suppression
- Despite already having iron overload, hepcidin remains inappropriately low
- This creates a vicious cycle: low hepcidin → increased iron absorption and release from stores → worsening iron overload → organ toxicity 5
Additional Erythroid Suppressors
While ERFE is the primary mediator, GDF15 and TWSG1 also contribute to hepcidin suppression specifically in iron-loading anemias 1. However, these appear to play secondary roles compared to ERFE's dominant effect 6.
Clinical Implications
The matriptase-2 (TMPRSS6) enzyme normally suppresses hepcidin in true iron deficiency by cleaving hemojuvelin (HJV), a BMP co-receptor 1, 7. In ineffective erythropoiesis, both TMPRSS6 and ERFE pathways converge to create profound hepcidin suppression that is disproportionate to actual iron needs.
In chronically transfused β-thalassemia patients, transfusions partially correct this by suppressing the ineffective erythropoiesis drive, allowing hepcidin levels to rise closer to normal 8. This explains why transfusion-dependent patients have higher hepcidin than non-transfused patients with the same disorder—the transfusions reduce the erythropoietic stimulus and ERFE production 8.
Therapeutic Relevance
Understanding this mechanism has enabled development of targeted therapies:
- Iron restriction strategies through hepcidin mimetics or ferroportin inhibitors can counteract the inappropriate hepcidin suppression 3
- Luspatercept reduces ineffective erythropoiesis by inhibiting SMAD2/3 signaling, indirectly normalizing the hepcidin-ERFE axis 3
- Gene therapy approaches (like exagamglogene autotemcel) that correct the underlying defect normalize ERFE and hepcidin levels by eliminating ineffective erythropoiesis 9
The key clinical pitfall is recognizing that ferritin levels in these patients reflect iron overload, but the suppressed hepcidin indicates the body is paradoxically still trying to absorb more iron—making iron chelation essential despite the erythropoietic drive 5, 10.