Oleanolic acid enhances the radiosensitivity of tumor cells under mimetic hypoxia through the reduction in intracellular GSH content and HIF-1α expression.

We previously found that oleanolic acid (OA), a naturally pentacyclic triterpenoid, enhances the radiosensitizing effect on tumor cells. However, it is unclear whether or not OA enhances the radiosensitivity of hypoxic cells. Therefore, the aim of the present study was to further observe the influence of OA on hypoxic tumor cells, and the relative mechanism was also investigated. The radiosensitivity of rat glioma C6 cells and human lung cancer A549 cells with different treatments, under mimetic hypoxia, was evaluated by clonogenic assay. A micronucleus (MN) test, meanwhile, was utilized to observe the alteration in intracellular DNA damage. For determining the mechanism involved in the OA influence on the radiosensitivity of hypoxic cells, we determined the levels of intracellular reduced glutathione (GSH) using the glutathione reductase/5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) recycling assay. Simultaneously, the activities of γ-glutamylcysteine synthetase (γ-GCS) and GSH synthase (GSS), both enzymes for GSH synthesis, were tested using appropriate methods. Due to the involvement of hypoxia inducible factor-1α (HIF-1α) in the resistence of hypoxic cells to radiation damage, its levels were also observed by western blot method. The results from this study demonstrated that the clonogenic growth of irradiated cells was increased under mimetic hypoxia while the refractory effect of hypoxic cells to radiation was decreased following OA treatment. Moreover, the (MN) frequencies in the hypoxic cells treated with OA were augmented after irradiation compared with the cells without OA treatment. In the subsequent experiment, OA significantly reduced the biosynthesis of intracellular GSH via the attenuation of γ-GCS activity. Additionally, there was an obvious reduction in HIF-1α expression in irradiated cells treated with OA at different concentrations. In conclusion, OA significantly enhanced the radiosensitivity of tumor cells under mimetic hypoxia, through the reduction in intracellular GSH content and HIF-1α expression.