Erythropoietin in bone - Controversies and consensus.

Erythropoietin (Epo) is the main hormone that regulates the production of red blood cells (hematopoiesis), by stimulating their progenitors. Beyond this vital function, several emerging roles have been noted for Epo in other tissues, including neurons, heart and retina. The skeletal system is also affected by Epo, however, its actions on bone are, as yet, controversial. Here, we review the seemingly contradicting evidence regarding Epo effects on bone remodeling. We also discuss the evidence pointing to a direct versus indirect effect of Epo on the osteoblastic and osteoclastic cell lineages. The current controversy may derive from a context-dependent mode of action of Epo, namely opposite skeletal actions during bone regeneration and steady-state bone remodeling. Differences in conclusions from the published in-vitro studies may thus relate to the different experimental conditions. Taken together, these studies indicate a complexity of Epo functions in bone cells.