Tetrandrine enhances radiosensitivity through the CDC25C/CDK1/cyclin B1 pathway in nasopharyngeal carcinoma cells

The increasing resistance of nasopharyngeal carcinoma to irradiation makes the exploration of effective radiosensitizers necessary. Tetrandrine is known to be an antitumor drug, but little is known regarding its radiosensitization effect on nasopharyngeal carcinoma. We investigated the effect of combined treatment of irradiation and maximum non-cytotoxic doses of tetrandrine on the nasopharyngeal carcinoma cell lines CNE1 and CNE2. The maximum non-cytotoxic doses of tetrandrine in CNE1 and CNE2 cells were assessed using the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay. The radiosensitization of cells receiving the maximum non-cytotoxic doses of tetrandrine was assessed by evaluating cell proliferation and DNA damage repair using MTT, clonogenic, comet assays and detection of caspase-3 and phosphorylated histone H2AX (γ-H2AX). The cell cycle was assessed by flow cytometry, and protein expression was detected by western blot analysis. The maximum non-cytotoxic doses of tetrandrine in CNE1 and CNE2 cells were 1.5 μmol/L and 1.8 μmol/L, respectively. When cells were exposed to irradiation and the maximum non-cytotoxic doses of tetrandrine, the survival fraction was decreased. DNA damage and γ-H2AX levels markedly increased. Moreover, tetrandrine abrogated the G2/M phase arrest caused by irradiation. Combined treatment with the maximum non-cytotoxic dose of tetrandrine and irradiation caused suppression of the phosphorylation of CDK1 and CDC25C and increase in the expression of cyclin B1. The study in vivo also showed that the maximum non-cytotoxic dose of tetrandrine could reduce tumor growth in xenograft tumor model. Our results suggest that the maximum non-cytotoxic dose of tetrandrine can enhance the radiosensitivity of CNE1 and CNE2 cells and that the underlying mechanism could be associated with abrogation of radiation-induced G2/M arrest via activation of the CDC25C/CDK1/Cyclin B1 pathway.