Deoxypodophyllotoxin induces cytoprotective autophagy against apoptosis via inhibition of PI3K/AKT/mTOR pathway in osteosarcoma U2OS cells.

BACKGROUND: A natural compound deoxypodophyllotoxin (DPT) possesses potent anti-proliferative and anti-tumor properties on several cancer types. It triggers cell cycle arrest followed by apoptosis through various cellular processes. However, it is limited to the action mechanism of DPT-mediated cell death modes via apoptosis and autophagy.

METHODS: Cell viability assay, morphological changes, annexin-V/propidium iodide (PI) assay, reactive oxygen species (ROS), acridine orange staining, and Western blot analyses were evaluated.

RESULTS: We demonstrated that DPT induced both apoptosis and autophagy via production of mitochondrial reactive oxygen species (ROS). DPT suppressed the PI3K/AKT/mTOR signaling cascades to lead autophagy process, resulting from conversion of light chain 3-I (LC3-I) into LC3-II and acidic vesicular organelles (AVOs) formation. Even if DPT-induced ROS were occurred in both apoptosis and autophagy, inhibition of ROS generation enhanced cell viability. Otherwise, 3-methyladeine (3-MA) impeding on autophagy accelerated an apoptotic response caused by DPT. Therefore, these findings suggest that DPT triggers cytoprotective autophagy against cytotoxic apoptosis.

CONCLUSION: Autophagy is required for cell survival by inhibition of apoptosis through down-regulation of PI3K/AKT/mTOR pathway against DPT-induced apoptosis in U2OS cells.