Osteogenic effects of rapamycin on bone marrow mesenchymal stem cells via inducing autophagy

BACKGROUND: While autophagy is essential for stem cells’ self-renewal and differentiation, its effect on bone marrow mesenchymal stem cells (BMSCs) remains unclear. This study aimed to investigate the interaction between autophagy and osteogenic differentiation using rapamycin (RAPA), a classical autophagy agonist with osteo-regulatory effects. METHODS: Rat BMSC’s autophagy was analyzed after osteoinduction (0, 7, 14, and 21 d) by western blotting, immunofluorescence, and real-time quantitative polymerase chain reaction (RT-qPCR). In addition, we evaluated osteogenic differentiation using alizarin red staining, alkaline phosphatase assays, and RT-qPCR/Western blotting quantification of bone sialoprotein, type 1 collagen, alkaline phosphatase, osteopontin, and Runt-related transcription factor 2 mRNA and protein levels. RESULTS: The BMSC’s basal autophagy level gradually decreased during osteogenic differentiation with a decrease in BECN1 level and the lipidated (LC3-II) to unlipidated (LC3-I) microtubule-associated protein 1 light chain 3 ratio and an increase in the expression of selective autophagic target p62. In contrast, it increased with increasing RAPA concentration. Furthermore, while 2 nM RAPA promoted BMSC osteogenic differentiation on days 7 and 14, 5 nM RAPA inhibited osteogenesis on days 14 and 21. Inhibition of autophagy by the inhibitor 3-methyladenine could impair RAPA’s osteogenesis-enhancing effect on BMSCs. CONCLUSIONS: The BMSC’s basal autophagy level decreased over time during osteogenic differentiation. However, an appropriate RAPA concentration promoted BMSC osteogenic differentiation via autophagy activation.