Effects of high calcium concentration on the functions and interactions of osteoblastic cells and monocytes and on the formation of osteoclast-like cells

The present study was performed to clarify the role of high calcium concentration and the appearance of mononuclear cells at the resorptive site in bone remodeling. Our recent study revealed that the high concentration of extracellular calcium ([Ca2+]e) stimulated DNA synthesis in osteoblastic MC3T3-E1 cells not only directly but also indirectly via monocytes. Human monocyte-conditioned medium (CM) significantly stimulated DNA synthesis and inhibited alkaline phosphatase (ALP) activity. In contrast, when monocytes were cultured at high [Ca2+]e concentrations (more than 3 mM), CM from these monocytes significantly stimulated ALP activity in MC3T3-E1 cells. Such stimulatory effect of CM was not observed at a high magnesium concentration (Mg2+, 5 mM). Treatment of monocytes with the calcium ionophore A23187 did not affect the CM-induced effect on DNA synthesis and ALP activity in these cells. To determine the migration potency of MC3T3-E1 cells and monocytes toward the high [Ca2+]e, chemotaxis assay was performed. The increasing [Ca2+]e (more than 3 mM) induced a chemotactic response of MC3T3-E1 cells as well as monocytes, but the high concentration of Mg2+ (5 mM) did not induce it. On the other hand, treatment with high [Ca2+]e (more than 3 mM) or CM significantly inhibited the 1,25-(OH)2D3-induced formation of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells (MNC) from their precursors derived from mouse spleen cells. The present study indicated that an increase in [Ca2+]e stimulated DNA synthesis and ALP activity of osteoblasts via monocytes, induced chemotaxis of osteoblasts as well as monocytes, and inhibited the formation of TRAP-positive MNC, suggesting the importance of the high Ca2+ concentration and mononuclear cells at the resorptive site in bone remodeling.