Erythropoietin and bone health: Single high-dose administration triggers bone loss in mice.

Erythropoietin (EPO) is a key regulator of erythropoiesis, and it is mainly used to treat anemia. However, it is also administered prophylactically to non-anemic patients in certain clinical settings and is known to be used illicitly by athletes. The effect of EPO is controversial but emerging evidence indicates that EPO treatment induces bone loss in healthy mice. Here, we investigated the immediate and short-term skeletal effects of a single high-dose EPO injection in young mature (9 weeks) female mice. Cellular and molecular markers of bone turnover were evaluated at multiple time points post-injection. EPO administration led to a rapid increase in macrophage colony-stimulating factor (M-CSF) levels within the bone marrow (BM) microenvironment and in the serum, accompanied by an increase in BM CD115-positive cells and osteoclast precursors, as assessed by flow cytometry. This early cellular response to EPO was followed by an increase in tartrate-resistant acid phosphatase 5b (TRAP5b) and a decrease in procollagen type 1 N-terminal propeptide (P1NP), as determined by serum ELISA analyses, suggesting increased osteoclast numbers and decreased bone formation, respectively. Micro-computed tomography (μCT) revealed a significant reduction in trabecular bone volume. These findings demonstrate that even a single high-dose EPO injection disrupts bone homeostasis and induces significant bone loss through early modulation of the BM niche and osteoclastogenic pathways. Our results have important clinical implications for the prophylactic use of EPO and highlight potential skeletal risks.