Osteoclast differentiation induced by synthetic octacalcium phosphate through receptor activator of NF-κB ligand expression in osteoblasts

Synthetic octacalcium phosphate (OCP) has a potential to enhance new bone formation and exhibits biodegradable characteristics when implanted in experimentally created bone defects. The precise mechanisms of OCP biodegradation remain unclear, though histological observations have revealed that bone-resorbing osteoclasts appear and resorb implanted OCP. To investigate how osteoclasts develop around implanted OCP, we examined osteoclast differentiation using OCP crystals in vitro. Coculturing of mouse bone marrow cells and osteoblasts in OCP-coated cell culture plates induced osteoclast differentiation, whereas that did not occur without coating. Further, addition of bone morphogenetic protein-2 significantly increased the number of osteoclasts in the OCP-coated wells. In the presence of OCP, osteoblasts expressed receptor activator of NF-κB ligand (RANKL), an osteoclast differentiation factor. In addition, when half of each culture well was coated with OCP, osteoclasts were formed in both coated and noncoated areas, suggesting that soluble factors mediate osteoclast differentiation induced by OCP. Also, calcium levels in culture medium were significantly decreased in the presence of OCP, while experimental reduction of calcium from 8.0 to 5.0 mg/dL significantly induced RANKL mRNA expression. These results suggest that OCP itself decreases calcium levels around implanted OCP, which induces osteoclast differentiation through RANKL expression by osteoblasts.