Erythroferrone in iron regulation and beyond

In this issue of Blood, Coffey et al¹ generate a new mouse model overexpressing the erythroid hepcidin regulator erythroferrone (ERFE) to demonstrate that increased ERFE levels have multifactorial effects, not only to suppress hepcidin and load iron, but also to impact the development and homeostasis of several organ systems.

How does the body ensure enough iron is available for erythropoiesis? In 2014, ERFE was discovered as a signal that communicates the iron demand of erythroid cells in the bone marrow to the liver and other organs to regulate the iron supply.² Produced and secreted by erythroblasts in response to erythropoietin, ERFE acts on the liver to suppress production of the iron regulatory hormone hepcidin.² In the absence of its ligand hepcidin, the iron exporter ferroportin is stabilized on the surface of duodenal enterocytes, macrophages, and hepatocytes, thereby increasing iron release into circulation from dietary sources and body stores.³ This pathway is not only induced as a physiologic response to anemia but also as a pathophysiologic response to ineffective erythropoiesis, which contributes to dietary iron hyperabsorption and iron overload in β-thalassemia.⁴ Recently, ERFE was proposed to suppress liver hepcidin production by acting as a ligand trap to sequester bone morphogenetic proteins (BMPs), key signaling molecules responsible for regulating hepcidin expression via SMAD transcription factors.^5,6 However, the physiologic functions of BMPs extend well beyond hepcidin regulation.⁷ This raises the possibility that chronic overproduction of ERFE, as seen in β-thalassemia and other disorders of ineffective erythropoiesis, could contribute to other pathologies beyond iron overload.

In the present study, Coffey et al generate several transgenic mouse lines with graded overexpression of Erfe in erythroid cells to gain a better understanding of ERFE function independent of any effects of anemia or erythropoietin. As expected, Erfe-transgenic mice develop relative hepcidin deficiency and iron overload in a dose-dependent fashion, consistent with ERFE’s known role as a hepcidin suppressor. Erfe-transgenic mice also exhibit some defects in liver BMP-SMAD signaling, particularly when analyzed relative to the degree of iron overload, consistent with ERFE’s reported function as a BMP ligand trap. Notably, serum ERFE levels in the lowest-expressing Erfe-transgenic mice are similar to a mouse model of β-thalassemia intermedia (Th3/+). However, although not compared directly, the Erfe-transgenic mice appear to have less severe hepcidin suppression and iron loading than previously reported in Th/3+ mice.⁴ This suggests that ERFE is not the sole erythroid hepcidin regulator contributing to iron overload in β-thalassemia. This concept is also supported by previous reports that Erfe knockout does not completely reverse hepcidin deficiency and iron overload in Th3/+ mice.⁴ Moreover, hepcidin suppression by acute phlebotomy is not fully reversed in Erfe knockout mice.² It is also notable that the changes in markers of liver BMP-SMAD signaling in the present study are relatively weak compared with the changes in hepcidin, a finding that is particularly striking in the fetal liver where hepcidin suppression is most robust. Although there are other potential explanations, this raises the possibility that ERFE may have additional mechanisms to suppress hepcidin beyond functioning as a BMP ligand trap.

Intriguingly, Coffey et al also report several other features in a subset of the highest-expressing Erfe-transgenic mice, including perinatal lethality, impaired growth, reduced kidney size, decreased gonadal fat depots, and neurobehavioral abnormalities. Although it is unclear how ERFE levels in these mice compare with ERFE levels in patients, these findings suggest potential roles for ERFE beyond hepcidin and iron homeostasis regulation. Based on ERFE’s known function as a BMP ligand trap^5,6 and reports of similar types of abnormalities in other animal models with altered BMP signaling,⁷ the authors hypothesize that these nonhematologic findings in Erfe-transgenic mice are related to impaired BMP-SMAD signaling. However, similar to the liver, changes in markers of BMP-SMAD signaling are absent or weak in other tissues studied, including the kidney and bone marrow. There are many potential explanations for these findings. The phenotypic abnormalities in Erfe-transgenic mice could be related to interference with BMP signaling during earlier stages of development or with non-SMAD effects of BMP ligands, which account for many of their biologic functions.⁷ Neither of these possibilities was examined in the current study. As noted above, ERFE may also have other biologic functions beyond its role as a BMP ligand trap. Importantly, the cellular source of ERFE may also be relevant to its biologic effects. The exclusive overexpression of ERFE in erythroid cells in the current study may not fully mirror its expression in physiologic or pathophysiologic conditions, and this may account for some phenotypic differences in Erfe-transgenic mice compared with other models of ERFE excess. Indeed, a recent report suggested that ERFE may also be produced by osteoblasts, where it has a protective role to limit excessive bone loss during expanded erythropoiesis.⁸ 

Together, the findings by Coffey et al solidify an important functional role for ERFE as a hepcidin suppressor, demonstrate that chronically elevated ERFE contributes to iron overload, and suggest that, at very high levels, ERFE may also impact other organ systems (see figure). However, many questions remain to be addressed by future studies. Is ERFE solely functioning as a BMP ligand trap or does it have additional mechanism(s) of action? Why does ERFE preferentially target the liver to inhibit BMP signaling and hepcidin expression, and why are other tissues relatively spared from ERFE-mediated BMP inhibition, at least at lower concentrations? Does ERFE have biologic functions beyond hepcidin and iron homeostasis regulation in patients with β-thalassemia or other diseases of ineffective erythropoiesis? Are there other endogenous sources of ERFE and is the location of ERFE production important for its action? What are the other erythroid regulators of hepcidin and what are their relative roles compared with ERFE? Will targeting ERFE prove a viable therapeutic strategy to limit iron overload and organ dysfunction in patients with β-thalassemia or other anemias of ineffective erythropoiesis? The answers to these questions will provide new insights into the mechanisms of iron overload and organ dysfunction in patients with iron-loading anemias and may pave the way for new therapies for these disorders.