Calcium phosphate nanoparticles are associated with inorganic phosphate-induced osteogenic differentiation of rat bone marrow stromal cells.

In the present study, we demonstrated that calcium phosphate (CaP) nanoparticles formed in cell culture media were implicated in the process of high inorganic phosphate (Pi) mediated osteogenic differentiation of rat bone marrow stromal cells (BMSCs). Exposure of BMSCs in vitro to high Pi-containing media reduced alkaline phosphatase (ALP) activity and the expressions of osteoblast-specific genes. The sediments of CaP nanoparticles were observed at the cell surface and some of them were concomitantly found inside cells at high Pi concentration. In addition, treatment the cells with pyrophosphate (PPi), an inhibitor of calcium crystal formation, abrogated the ALP activity induced by high Pi, suggesting the contribution of CaP nanoparticles. Moreover, for isolated CaP nanoparticles, there was a trend of conversion from amorphous calcium phosphate to hydroxyapatite with elevated Pi. The particle size of CaP increased and the surface morphology changed from spherical to irregular due to increased concentrations of serum proteins incorporated into CaP nanoparticles. The study demonstrated that those physicochemical properties of CaP nanoparticles played an important role in modulating BMSCs differentiation. Furthermore, the addition of Pi in the osteogenic media resulted in a dose-dependent increase in matrix mineralization, while treatment of the cells with PPi suppressed Pi-induced calcium deposition. The findings indicated that calcium deposition in the matrix partly came from the spontaneous precipitation of CaP nanoparticles.