Beclin1- and Atg13-dependent autophagy activation and morroniside have synergistic effect on osteoblastogenesis

Morroniside is known to improve osteoporosis by promoting osteoblastogenesis. The activation of PI3K/Akt/mTOR signaling is a significant mechanism in morroniside-promoted osteoblastogenesis. It is well known that protective autophagy is an important factor in osteoblastogenesis. However, the activation of mTOR signaling can inhibit autophagy. This study aimed to investigate the relationship between mTOR signaling and autophagy in morroniside-regulated osteoblastogenesis. In this study, we investigated the effect of morroniside on the autophagic activity (LC3 conversion rate, LC3-puncta formation, and autophagosome number) of differentiated osteoblast precursors (MC3T3-E1 cells). Then, we identified the roles of mTOR knockdown in morroniside-regulated alterations of autophagy and osteogenic parameters in MC3T3-E1 cells. Next, mTOR knockdown and overexpression were used to observe the roles of mTOR in morroniside-regulated alterations of autophagic molecules (Atg7, Atg13, and Beclin1). Subsequently, the additional value of the above autophagic molecules on morroniside-regulated osteogenic parameters in MC3T3-E1 cells was analyzed based on lentiviral transduction. Finally, combined with morroniside and TAT-Beclin1, the roles of Beclin1 upregulation in the in vivo effects of morroniside was investigated. Our experimental data showed that morroniside promoted both the mTOR activity and autophagy in MC3T3-E1 cells. Morroniside-upregulated autophagic activity and Atg13 or Beclin1 protein level in MC3T3-E1 cells were enhanced by mTOR knockdown. Furthermore, Morroniside-upregulated Atg13 and Beclin1 expression was reversed by mTOR overexpression. Importantly, autophagy upregulation with overexpression of the autophagic gene, Atg13 or BECN1 (gene form of Beclin1), significantly promoted osteoblastogenesis regulated by morroniside. The promotional effect of morroniside on bone microarchitecture, bone mass, and bone parameters (including trabecular bone area and OCN expression in trabecular bone) in ovariectomized (OVX) mice was enhanced by TAT-Beclin1 administration. In conclusion, the autophagy-enhancing drugs related to Beclin1 or Atg13 may be an effective adjuvant therapy in the treatment of osteoporosis with morroniside.