Oleanolic acid (OA) is a natural pentacyclic
triterpenoid that has been used in
traditional medicine as an anticancer and
anti-inflammatory agent. The aim of our study was to investigate whether or not OA increases the radiosensivity of
tumor cells, and the relative mechanism was also investigated. Clonogenic assay was used to observe the radiosensitivity of C6 and A549 cells following different treatments. The alteration of intracellular DNA damage was determined using a micronucleus (MN) assay. In order to identify the mechanism of OA-mediated radiosensitization of
tumor cells, the levels of
glutathione (GSH) in irradiated cells following various pretreatments were determined using
glutathione reductase/5,5'-dithiobis-(2-nitrobenzoic
acid) (
DTNB) recycling assay. Under the same condition, the activities of γ-
glutamylcysteine synthetase (γ-GCS) and GSH synthase (GSS), both key
enzymes for GSH synthesis, were detected using appropriate methods. In order to confirm the
radiosensitizing effect of OA on
cancer cells by attenuating GSH,
N-acetylcysteine (NAC) was added to cells in culture for 12 h before irradiation. The results showed that the combined treatment of radiation with OA significantly decreased the clonogenic growth of
tumor cells and enhanced the numbers of intracellular MN compared to irradiation alone. Furthermore, it was found that the synthesis of cellular GSH was inhibited concomitantly with the downregulation of γ-GCS activity. Therefore, the utilization of OA as a
radiosensitizing agent for irradiation-inducing cell death offers a potential therapeutic approach to treat
cancer.