1,25-dihydroxyvitamin D₃ inhibits directly human osteoclastogenesis by down-regulation of the c-Fms and RANK expression

Objective: 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) is a key molecule to maintain calcium homeostasis and bone metabolism. It was recently reported that 1,25(OH)₂D₃ directly inhibited osteoclast differentiation in mouse bone marrow cells and human bone marrow-derived colony-forming unit granulocyte macrophage (CFU-GM) cells. However, the direct effects of 1,25(OH)₂D₃ and its affecting mechanisms on the osteoclast differentiation of human osteoclast precursors remain largely unknown. In this study, we examined the direct effects of 1,25(OH)₂D₃ on the osteoclastogenesis of human peripheral blood (PB) osteoclast precursors. Methods: In vitro osteoclastogenesis assays were performed using osteoclast precursors from normal PB. The gene expressions were analyzed using real-time PCR. The cell surface proteins, including c-Fms and RANK, were measured by flow cytometry. Results: 1,25(OH)₂D₃ strongly inhibited osteoclast differentiation and it suppressed the expression of RANK in the human PB osteoclast precursors. One mechanism of RANK inhibition by 1,25(OH)₂D₃ is down-regulation of the M-CSF receptor c-Fms, which is required for the expression of RANK. In contrast to the previous reports on mouse osteoclast precursors, 1,25(OH)₂D₃ did not affect the expression of c-Fos. Parallel to the inhibition of osteoclastogenesis, 1,25(OH)₂D₃ increased the expression and phosphorylation of CCAAT enhancer-binding protein β (C/EBPβ), which is a recently discovered inhibitor of osteoclastogenesis. Conclusions: Our results show that 1,25(OH)₂D₃ inhibits human osteoclastogenesis by decreasing the RANK+ osteoclast precursors, and we suggest that 1,25(OH)₂D₃ may be a powerful therapeutic agent for treating inflammation-induced bone disease that shows excessive osteoclast activation.