Autophagy inhibition enhances radiosensitivity of Eca-109 cells via the mitochondrial apoptosis pathway

Autophagy inhibition is crucial for the improvement of the efficacy of radiotherapy in cancer. The aim of the present study was to determine the potential therapeutic value of autophagy and its correlation with mitochondria in human esophageal carcinoma cells following treatment with ionizing radiation (IR). Autophagy in Eca-109 cells was induced under poor nutrient conditions. The formation of autophagic vacuoles was monitored using electron microscopy. In addition, cell apoptosis after IR and mitochondrial membrane potential (MMP) were analyzed by flow cytometry. LC3, beclin-1, cytochrome c and apoptosis-related proteins were assayed by western blotting. A nude mouse xenograft model was also employed to verify the biological effects and mechanisms underlying autophagy in vivo. The formed autophagic vesicles and increased LC3 II/LC3 I ratio indicated marked induction of autophagy by Earle's balanced salt solution (EBSS) in Eca-109 cells. 3-Methyladenine or LY294002 significantly antagonized EBSS-induced autophagy and increased apoptosis of irradiated cells, suggesting that autophagy inhibition conferred radiosensitivity in vitro. Notably, IR induced prominent release of cytochrome c and Bax activation, and decreased Bcl-2 and MMP expression in Eca-109 cells under poor nutrient conditions. Of note, these changes were more prominent following pretreatment with autophagy inhibitors. In vivo, IR treatment mildly delayed tumor growth, but the radiotherapeutic effect was improved significantly by abolishing autophagy. Furthermore, mitochondrial signaling was investigated in the Eca-109 xenograft nude mice model, and the results were consistent with the in vitro study. Therefore, the mitochondrial pathway may be associated with improvement of radiosensitivity in Eca-109 cells.