Osteoblast-Derived Erythroferrone Regulates Stress Erythropoiesis

Erythroferrone (ERFE) secreted by erythroblasts is known as an erythroid regulator of iron metabolism through hepcidin suppression. Our earlier studies indicated that ERFE highly expresses in osteoblasts and Erfe^-/- mice displayed decreased bone mineral density, indicating the osteoprotective effect of ERFE in bone homeostasis. However, the contribution of ERFE in osteoblasts to bone formation, iron metabolism, and erythropoiesis is unknown. To explore the role of osteoblast-derived ERFE as a physiological regulator of bone remodeling and as an erythroid regulator of iron homeostasis, we generated osteoblast-specific Erfe knockout mice (Col2.3-Cre;Erfe^fl/fl) by crossing Erfe^fl/fl (floxed) with osteoblast-selective Cre (Col2.3-Cre). Unlike Erfe^-/- mice, skeletal phenotyping revealed that Col2.3-Cre;Erfe^fl/fl mice had a WT-like phenotype with no differences in spleen weight, hemoglobin, reticulocyte counts, and hepcidin expression. Meanwhile, under in vitro studies, Col2.3-Cre;Erfe^fl/fl osteoblasts exhibit high levels of mineralization and induction of Col1a1 and Alp expression, consistent with Erfe^-/- osteoblasts. A previous study has shown that ERFE is highly induced in erythroblasts in response to phlebotomy and erythropoietin (EPO) treatment. Our study is the first to demonstrate that Erfe expression in osteoblasts is stimulated by hypoxia and EPO treatment. In the normal condition, Epor in Col2.3-Cre;Erfe^fl/fl osteoblasts was found higher than wild type osteoblasts. EPO and its receptor, EPOR are crucially involved in the regulation of erythropoiesis. To confirm that ERFE in osteoblasts might participate in stress erythropoiesis, Col2.3-Cre;Erfe^fl/fl mice were phlebotomized to induce anemia. We found that Col2.3-Cre;Erfe^fl/fl mice do not suppress hepcidin and had a decrease in serum ERFE after 16 hours phlebotomy. Interestingly, after 6 day phlebotomy, Col2.3-Cre;Erfe^fl/fl mice had higher levels of red blood cells than control mice (Erfe^fl/fl) with no change in hemoglobin and mean corpuscular hemoglobin (MCR) concentrations. Taken together, our data demonstrate that ERFE in osteoblasts might regulate red blood cell regeneration in response to stress erythropoiesis.