Involvement of the Ca(2+) signaling pathway in osteoprotegerin inhibition of osteoclast differentiation and maturation

The purpose of this study was to determine whether the Ca(2+) signaling pathway is involved in the ability of osteoprotegerin (OPG) to inhibit osteoclast differentiation and maturation. RAW264.7 cells were incubated with macrophage colony-stimulating factor (M-CSF) + receptor activator of nuclear factor-κB ligand (RANKL) to stimulate osteoclastogenesis and then treated with different concentrations of OPG, an inhibitor of osteoclast differentiation. The intracellular Ca(2+) concentration [Ca(2+)](i) and phosphorylation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in the different treatment groups were measured by flow cytometry and Western blotting, respectively. The results confirmed that M-CSF + RANKL significantly increased [Ca(2+)](i) and CaMKII phosphorylation in osteoclasts (p < 0.01), and that these effects were subsequently decreased by OPG treatment. Exposure to specific inhibitors of the Ca(2+) signaling pathway revealed that these changes varied between the different OPG treatment groups. Findings from the present study indicated that the Ca(2+) signaling pathway is involved in both the regulation of osteoclastogenesis as well as inhibition of osteoclast differentiation and activation by OPG.