High extracellular Ca2+ alone stimulates osteoclast formation but inhibits in the presence of other osteoclastogenic factors

Abstract

High ambient Ca(2+) at bone resorption sites have been implicated to play an important role in the regulation of bone remodeling. The present study was performed to clarify the mode of high extracellular Ca(2+) (Ca(2+)(e))-induced modulation of osteoclastogenesis and the expression of receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG), thereby to define its role in osteoclast formation. Mouse bone marrow cells were cocultured with osteoblastic cells in the absence or presence of osteoclastogenic factors such as 1,25-dihydroxyvitaminD(3) (1,25-(OH)(2)vitD(3)) and macrophage colony-stimulating factor/soluble RANKL. Ca(2+) concentration in media (1.8 mM) was adjusted to 3, 5, 7 or 10 mM. Osteoclast formation was confirmed by the appearance of tartrate-resistant acid phosphatase (TRAP)-positive multinuclear cells and the expression of osteoclast phenotypic markers (calcitonin receptor, vitronectin receptor, cathepsin K, matrix metalloproteinase-9, carbonic anhydrase 2). High Ca(2+)(e) alone significantly stimulated osteoclast formation in a dose-dependent manner. However, in the presence of highly osteoclastogenic factors, high Ca(2+)(e) significantly inhibited osteoclastogenesis. High Ca(2+)(e) alone continuously up-regulated RANKL expression while only transiently increased OPG expression. However, in the presence of 1,25-(OH)(2)vitD(3), high Ca(2+)(e) did not change the 1,25-(OH)(2)vitD(3)-induced RANKL expression while increased OPG expression. Taken together, these findings suggest that high Ca(2+)(e) alone increase osteoclastogenesis but inhibit in the presence of other osteoclastogenic factors. In addition, high Ca(2+)(e)-induced osteoclastogenesis may be mediated by osteoblasts via up-regulation of RANKL expression. Meanwhile up-regulated OPG might participate in the inhibitory effect of high Ca(2+)(e) on 1,25-(OH)(2)vitD(3)-induced osteoclastogenesis.