Nano-Hydroxyapatite Modulates Osteoblast Differentiation Through Autophagy Induction via mTOR Signaling Pathway

Hydroxyapatite (HAP) plays an important role in skeleton formation and bone remodeling. Synthetic HAP is similar to naturally occurring HAP and has been widely used in orthopedic and dental implantation. While it is known nano-HAP easily aggregates at high concentrations, the intracellular fate and mechanisms-of-action of dispersed nano-HAP in osteoblast differentiation remain to be fully elucidated. In the present study, we treated the osteoblast cell line MC3T3E1 with autoclaved nano-HAP and added sodium hexametaphosphate to control nanoparticle agglomeration. Dispersed nano-HAP promoted osteoblast differentiation in a dose-dependent manner. Our findings further revealed that the internalized nano-HAP particles were located in typical autophagic vacuoles and increased the ratio of LC3II/LC3I, indicating nano-HAP induced cell autophagy. Moreover, the induction of autophagy by nano-HAP was via mTOR signaling pathway which also concentration dependent. Collectively, these results reveal nano-HAP modulates osteoblast differentiation by mediating autophagy in a dose-dependent manner.