Betulin inhibits mTOR and induces autophagy to promote apoptosis in human osteosarcoma cell lines

Betulin is a lupane type pentacyclic triterpenoid, and commonly found in the bark of birch trees. It displays various pharmacological properties, such as antibacterial, anti‐inflammation, antitumor, and antiviral. In this report, we attempted to investigate the anti‐proliferative and pro‐apoptotic effects of betulin on osteosarcoma cell lines. Our results revealed that betulin significantly decreased cell viability and colony formation in osteosarcoma cell lines. Dose‐dependent induction of Annexin V positive cells, activated caspase 8, activated caspase 9, activated caspase 3, and the cleavage of poly (ADP‐ribose) polymerase were observed after the treatment with betulin, indicating betulin induces apoptosis in osteosarcoma cell lines. mTOR has been identified as a key modulator of autophagy in response to different stresses. In this study, we found that the treatment with betulin suppressed the activation of mTOR, and increased the level of LC 3‐II, the autophagy marker, in osteosarcoma cell lines. Co‐administration of the autophagy inhibitor chloroquine significantly rescued the cell viability and the clonogenic activity in betulin‐treated osteosarcoma cell lines. Our data showed that betulin induced autophagy, and the up‐regulated autophagy positively contributed to the apoptosis. Taken together, our findings suggested that betulin may serve as a promising anti‐proliferative agent for treating osteosarcoma.