Elevated extracellular calcium stimulates secretion of bone morphogenetic protein 2 by a macrophage cell line

It has been suggested that macrophages and multinucleated giant cells are responsible for phagocytosis of resorbable calcium phosphate (CaP) compounds implanted in bone defects. However, function of macrophages around the CaP, if continuously exposed to various concentration of extracellular calcium ions ([Ca²⁺]_o), is still unknown. The present study showed that when resorbable octacalcium phosphate was implanted in mouse calvaria, macrophage-like cells were observed around the implant during bone formation. Then, experiments were designed to investigate whether secretion of bone morphogenetic protein 2 (BMP-2) is enhanced by [Ca²⁺]_o in a macrophage cell line (J774A.1) in vitro. The mRNA expression and the secretion of BMP-2 in J774A.1 cells were significantly increased when incubated in the medium with [Ca²⁺]_o up to 14 mM. The promotion of mRNA expression was maintained even when incubated with a small amount of minute CaP crystals. The present results suggest that [Ca²⁺]_o above physiological concentration may stimulate macrophages to induce osteogenic cytokine, such as BMP-2, for bone formation by osteoblast.