Osteocytes autophagy mediated by mTORC2 activation controls osteoblasts differentiation and osteoclasts activities under mechanical loading

Autophagy is an important mechanosensitive response for cellular homeostasis and survival in osteocytes. However, the mechanism and its effect on bone metabolism have not yet clarified. The objective of this study was to evaluate how compressive cyclic force (CCF) induced autophagic response in osteocytes and to determine the effect of mechanically induced-autophagy on bone cells including osteocytes, osteoblasts, and osteoclasts. Autophagic puncta observed in MLO-Y4 cells increased after exposure to CCF. The upregulated levels of the LC3II isoform and the degradation of p62 further confirmed the increased autophagic flux. Additionally, ATP synthesis and release, osteocalcin (OCN) expression, and cell survival increased in osteocytes as well. The Murine osteoblasts MC3T3-E1 cells and RAW 264.7 macrophage cells were cultured in conditioned medium collected from MLO-Y4 cells subjected to CCF. The concentration of FGF23 increased and the concentrations of SOST and M-CSF and RANKL/OPG ratio decreased significantly in the conditioned medium. Moreover, the promotion of osteogenic differentiation in MC3T3-E1 cells and inhibition of osteoclastogenesis and function in RAW 264.7 cells were significantly attenuated when osteocytes autophagy was inhibited by siAtg7. Our findings suggested that CCF induced protective autophagy in osteocytes and subsequently enhanced osteocytes survival and osteoblasts differentiation and downregulated osteoclasts activities. Further study revealed that CCF induced autophagic response in osteocytes through mechanistic target of rapamycin complex 2 (mTORC2) activation. In conclusion, CCF-induced osteocytes autophagy upon mTORC2 activation promoted osteocytes survival and osteogenic response and decreased osteoclastic function. Thus, osteocytes autophagy will provide a promising target for better understanding of bone physiology and treatment of bone diseases.