Erythropoietic response is controlled not only by erythropoietin but also by iron. In addition to its role in iron delivery, transferrin also functions as a signaling molecule, with effects on both iron homeostasis and erythropoiesis. We investigated hematologic parameters, iron status and expression of key proteins, including the hepatic iron regulatory protein hepcidin and the suppressive erythroid factor , in mice subject to dietary iron deficiency with and without anemia. The acute effect of iron on these parameters was investigated by administration of exogenous iron-loaded transferrin (holoTf) in each of the mouse models. Serum iron in mice with iron deficiency (ID) is modestly lower with hematologic parameters maintained by utilization of iron stores in mice with ID. As expected, erythropoietin expression and concentration, along with marrow are unaffected in ID mice. Administration of holoTf restores serum iron and Tf saturation levels to those observed in control mice and results in an increase in hepcidin compared to ID mice not treated with holoTf. The expression of the Bmp signaling molecule is not significantly increased following Tf treatment in ID mice. Thus, the expression level of the gene encoding hepcidin, , is increased relative to expression in ID mice following treatment with holoTf, leading us to speculate that Tf saturation may influence Bmp sensitivity. In mice with iron deficiency anemia (IDA), decreased hematologic parameters were accompanied by pronounced decreases in serum and tissue iron concentrations, and an increase in serum erythropoietin. In the absence of exogenous holoTf, the greater serum erythropoietin was not reflected by an increase in marrow expression. HoloTf administration did not acutely change serum Epo in IDA mice. Marrow expression was, however, markedly increased in IDA mice following holoTf, plausibly accounting for the lack of an increase in following holoTf treatment in the IDA mice. The increase in despite no change in erythropoietin suggests that Tf acts to increase erythropoietin sensitivity. These observations underscore the importance of Tf in modulating the erythropoietic response in recovery from iron deficiency anemia, with implications for other stress erythropoiesis conditions.

Based on the relationship between the intracellular concentration of sickle hemoglobin S (HbS) and the delay that occurs prior to the onset of sickling following deoxygenation, targeting the intracellular HbS concentration is a recognized therapeutic approach for sickle cell disease (SCD). We and others have shown that restricting iron by dietary or pharmacologic means improves hematologic parameters, inflammation, and organ damage in mouse models of SCD. Clinical evidence corroborating these findings is confined to case reports and small case series studies, none of which account for treatment or -thalassemia. We hypothesize that increased transferrin saturation is associated with increased mean cellular hemoglobin concentration (MCHC) which in turn is associated with decreased red cell counts and worsening anemia. To investigate this hypothesis, we examined the relationships between transferrin saturation and MCHC with each of the parameters that define MCHC in sickle patients (HbSS without -thalassemia) and healthy volunteers (HVs). Results indicate that transferrin saturation and MCHC are positively correlated with each other in sickle patients and HV. In patients with SCD, MCHC and transferrin saturation are negatively correlated with RBC count and are not correlated with hemoglobin, whereas each is positively associated with HV. Transferrin saturation and MCHC are each positively correlated with the hemolysis marker, lactate dehydrogenase. These observations support a model where increased transferrin saturation contributes to higher intracellular HbS concentrations with subsequent increases in sickling and hemolysis in sickle patients, suggesting that pharmacologic approaches to decrease serum iron may provide a therapeutic approach for patients with SCD. This study was registered with ClinicalTrials.gov identifiers: NCT00011648, NCT00081523, and NCT04817670.