Erythropoietin Mediated Bone Formation Is Regulated by mTOR Signaling

The role of erythropoietin (Epo) and Epo/Epo receptor (EpoR) signaling pathways for production of red blood cells are well established. However, little is known about Epo/EpoR signaling in non-hematopoietic cells. Recently, we demonstrated that Epo activates JAK/STAT signaling in hematopoietic stem cells (HSCs), leading to the production of bone morphogenetic protein 2 (BMP2) and bone formation and that Epo also directly activates mesenchymal cells to form osteoblasts in vitro. In this study, we investigated the effects of mTOR signaling on Epo-mediated osteoblastogenesis and osteoclastogenesis. We found that mTOR inhibition by rapamycin blocks Epo-dependent and-independent osteoblastic phenotypes in human bone marrow stromal cells (hBMSCs) and ST2 cells, respectively. Furthermore, we found that rapamycin inhibits Epo-dependent and-independent osteoclastogenesis in mouse bone marrow mononuclear cells and Raw264.7 cells. Finally, we demonstrated that Epo increases NFATc1 expression and decreases cathepsin K expression in an mTOR-independent manner, resulting in an increase of osteoclast numbers and a decrease in resorption activity. Taken together, these results strongly indicate that mTOR signaling plays an important role in Epo-mediated bone homeostasis. J. Cell. Biochem. 113: 220–228, 2012. 2011 Wiley Periodicals, Inc. KEY WORDS: OSTEOBLASTS; HSCs; OSTEOCLASTS; ERYTHROPOIETIN; mTOR; RAPAMYCIN E rythropoietin (Epo) is a glycoprotein hormone that regulatesred blood cell production in the bone marrow. Under hypoxic conditions, Epo is produced mainly by the adult kidney and secreted Epo binds to the Epo receptor (EpoR) expressed by erythroi