Doxycycline Enhances Anticancer Activity of Zoledronic Acid via Inducing ROS and Autophagy in Osteosarcoma Cell Lines

Zoledronic acid (ZOL) is an inhibitor of osteoclast-mediated bone resorption. It is used to treat osteoporosis and skeletal complications in patients with tumor-induced osteolysis. ZOL is also demonstrated to possess anti-cancer activity in several tumors via apoptosis induction. Doxycycline is well-known antibiotic used in treatment of infections caused by bacteria and certain parasites. In this study, we evaluated the possibility if doxycycline could be used as an effective adjuvant to ZOL against osteosarcoma cells. The data showed that co-treatment with doxycycline at non-toxic dose could significantly increase the anti-viability effect of ZOL in osteosarcoma HOS and MG-63 cells in MTT assay and colony formation assay, and largely increased the levels of apoptotic markers, cleaved caspase 3 and PARP, in ZOL-treated cells. Furthermore, as co-treatment with doxycycline, the levels of ROS and autophagy were enhanced in ZOL-treated cells. Administration of <i>N</i>-acetyl-L-cysteine, a reactive oxygen species (ROS) inhibitor, or autophagy inhibitor chloroquine both reduced anti-growth effect of this combined treatment, indicating that the increased ROS and autophagy should be involved in anti-viability effect of combined treatment with ZOL and doxycycline. Taken together, our findings suggested that combined treatment with ZOL and doxycycline may serve as a potential strategy for treating osteosarcoma.