Mitochondrial respiration inhibitor enhances the anti-tumor effect of high-dose ascorbic acid in castration-resistant prostate cancer.

ABSTRACT

Previous evidences have demonstrated that anti-tumor effect of high-dose ascorbic acid is associated with the generation of reactive oxygen species (ROS) via autoxidation. Hypoxia induces therapy resistance in castration-resistant prostate cancer. As a mitochondrial respiration inhibitor, metformin has the potential to improve tumor oxygenation. In this study, we evaluate the anti-tumor effect of ascorbic acid combined with metformin in prostate cancer. We demonstrated that ascorbic acid inhibits prostate cancer cells proliferation by generating ROS, and metformin enhances the anti-tumor effects of ascorbic acid. Mechanistically, metformin reduces oxygen consumption rate and NADP+/NADPH value in prostate cancer cells, thereby increases the ROS content induced by ascorbic acid. In addition, our data demonstrated that ascorbic acid inhibits p-AKT signaling in a ROS-dependent pathway, leading to inhibition of p-mTOR expression. And metformin inhibits the p-mTOR expression by activating the AMPK signaling pathway, exerting a synergistic effect on tumor suppression with ascorbic acid. Furthermore, metformin improves tumor oxygenation, and the combined treatment effect of ascorbic acid and metformin were demonstrated in a xenograft model of prostate cancer. Taken together, our data demonstrate that metformin enhances the anti-tumor proliferation effect of ascorbic acid by increasing ROS content in castration-resistant prostate cancer. This provides a new strategy for the clinical application of high-dose ascorbic acid as an anti-tumor drug. KEY MESSAGES Ascorbic acid inhibits tumor growth by inducing ROS generation. As a mitochondrial respiration inhibitor, metformin inhibits cellular oxygen consumption rate to improve oxygenation of prostate cancer. Metformin enhances anti-tumor effect of ascorbic acid by increasing ROS content. Ascorbic acid inhibits the mTOR expression via PI3K-AKT pathway, and metformin inhibits the mTOR expression by inhibiting AMPK signaling in prostate cancer cells.