Osteocytes produce erythroferrone to regulate liver hepcidin during stress erythropoiesis

The bone marrow microenvironment has a variety of cell types. Osteocytes regulate bone remodeling and were described to participate in bone marrow functions. However, their role in regulating red blood cell production is not known. To understand their role in red blood cell regulation we isolated osteocytes from mice after acute hemorrhagic anemia and performed bulk RNAseq in comparison to controls. We identified the top upregulated gene in osteocytes as Fam132b or erythroferrone (Erfe). ERFE is known to be produced by erythroblasts in response to erythropoietin (EPO) and mediates hepcidin suppression in the liver during stress erythropoiesis. In our model ERFE expression was 16-fold higher in osteocytes isolated from mice after phlebotomy compared to control mice. The expression level of ERFE in osteocytes was similar to that described in erythroid precursors. Expression in osteocytes was also induced by hypoxia in vitro. In vivo experiments confirmed these findings. When Erfe-/- mice were transplanted with WT BM (thus only the BM can produce Erfe), the hepcidin suppression in the liver was significantly less then in WT mice, when both the osteocytes and the BMcan produce Erfe. Osteocyte Erfe is produced through their Epo receptors sensing the hypoxia. So, we tested if eliminating the Epo receptors from bone cells only would have an effect on liver hepcidin and found that these mice could not suppress liver hepcidin as much as control animals did. These two experiments confirmed our hypothesis, that osteocytes produce Erfe in response to EPO and that the Erfe they release has a significant contribution to suppressing liver hepcidin production. The lowered hepcidin levels make iron available to increase red cell production. Thus osteocytes actively participate in regaining the balance of the bone marrow by increasing red cell production after bleeding.