The hepcidin regulator erythroferrone is a new member of the erythropoiesis-iron-bone circuitry

Abstract

Background: Erythroblast erythroferrone (ERFE) secretion inhibits hepcidin expression by sequestering several bone morphogenetic protein (BMP) family members to increase iron availability for erythropoiesis. Methods: To address whether ERFE functions also in bone and whether the mechanism of ERFE action in bone involves BMPs, we utilize the Erfe-/- mouse model as well as β–thalassemic (Hbbth3/+) mice with systemic loss of ERFE expression. In additional, we employ comprehensive skeletal phenotyping analyses as well as functional assays in vitro to address mechanistically the function of ERFE in bone. Results: We report that ERFE expression in osteoblasts is higher compared with erythroblasts, is independent of erythropoietin, and functional in suppressing hepatocyte hepcidin expression. Erfe-/- mice display low–bone–mass arising from increased bone resorption despite a concomitant increase in bone formation. Consistently, Erfe-/- osteoblasts exhibit enhanced mineralization, Sost and Rankl expression, and BMP–mediated signaling ex vivo. The ERFE effect on osteoclasts is mediated through increased osteoblastic RANKL and sclerostin expression, increasing osteoclastogenesis in Erfe-/- mice. Importantly, Erfe loss in Hbbth3/+ mice, a disease model with increased ERFE expression, triggers profound osteoclastic bone resorption and bone loss. Conclusions: Together, ERFE exerts an osteoprotective effect by modulating BMP signaling in osteoblasts, decreasing RANKL production to limit osteoclastogenesis, and prevents excessive bone loss during expanded erythropoiesis in β–thalassemia. Funding: Y.Z.G. acknowledges the support of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) (R01 DK107670 to Y.Z.G. and DK095112 to R.F., S.R., and Y.Z.G.). M.Z. acknowledges the support of the National Institute on Aging (U19 AG60917) and NIDDK (R01 DK113627). T.Y. acknowledges the support of the National Institute on Aging (R01 AG71870). S.R. acknowledges the support of NIDDK (R01 DK090554) and Commonwealth Universal Research Enhancement (C.U.R.E.) Program Pennsylvania.

Bone circuitry and interorgan skeletal crosstalk

“…Iron metabolism also plays a significant role in bone remodeling through action of erythroferrone (ERFE), a protein secreted by erythroblasts in bone marrow, a negative regulator of hepcidin. ERFE was shown to bind to BMP family-2, -6, and -2/6 heterodimer ( Castro-Mollo et al, 2021 ; Wang et al, 2020 ). By using Erfe -/- mice and β-thalassemic mice with systemic loss of ERFE expression ( Hbb th 3/+ ; Erfe -/- ), our group showed that ERFE was highly expressed in osteoblasts even compared with erythroblasts, and the absence of ERFE caused high bone turnover and significant bone loss.…”

New Insights Into Pathophysiology of β-Thalassemia

“…In recent years, there is growing evidence of the relationship between erythropoiesis, bone mineral metabolism and iron homeostasis. Recently, a mechanism responsible for the activation of osteoclasts in thalassemic patients has been described that could be associated to cytokine dysregulation and, in particular, to the modification of the RANK/RANKL/OPG axis, which is essential for the regulation of osteoclastogenesis ( 16 , 17 ). The OPG/RANK/RANKL system is essential for the regulation of osteoclastogenesis.…”

“…Therefore, ERFE seems to be key in the recently described erythropoiesis-iron-bone circuit, by modifying the availability of BMP. Therefore, ERFE appears to be an important link between abnormal erythropoiesis, iron metabolism alteration, and loss of BMD in β-thalassemia ( 16 , 17 ).…”

“…In addition, other data indicate that the loss of BMP signaling (high ERFE), increases bone mass through direct inhibition of osteoclasts and activation of the Wnt pathway, predicting that the loss of ERFE would lead to a decrease in bone mass ( 16 , 17 ) by increased expression of RANKL and sclerostin.…”

“…In summary, to date the only known function of ERFE was hepcidin regulation expression through BMP sequestration, contributing to iron overload ( 9 ). However, ERFE seems to have bone metabolisms implications and a new role in bone protection has been described ( 16 ). In conditions of elevated ERFE, such as β-thalassemia and others ineffective erythropoiesis situations, BMP2 and BMP6 proteins are sequestered, decreasing signaling through the BMP/Smad and ERK pathways.…”