Radiosensitization of EMT6 mammary carcinoma cells by 2-benzoyl-3-phenyl-6,7-dichloroquinoxatine 1,4-dioxide

Background and purpose: Previously, we have reported that 2-benzoyl-3-phenyl-6,7-dichloroquinoxaline 1,4-dioxide (DCQ) is a radiosensitizer. Here, we investigate the mechanism of radiosensitization. Materials and methods: EMT6 cells were treated with DCQ for 4 h prior to ionizing radiation (IR). Flow cytometry, clonogenic assay, TUNEL, and Western blotting were performed to assess the effect of treatment on cells. Results: Propidium iodide staining of EMT6 cells treated with IR DCQ revealed high numbers of cells with decreased DNA, consistent with an apoptotic response. TUNEL assay revealed apoptosis was 4%, 38%, and 49% 24 h after treatment with IR alone, DCQ alone, and DCQ + IR, respectively. Clonogenic assays revealed that the survival of irradiated EMT6 cells was significantly reduced by DCQ treatment. DCQ treatment abrogated the radiation-induced expression of p21 and p53. The increased apoptosis observed in DCQ + IR-treated cells was correlated to suppression of radiation-induced phosphorylation of Akt and the expression of Bcl-X-L. DCQ also caused the phosphorylation of mitogen-activated protein kinases Erk and Jnk. Conclusions: The radiosensitization effect of DCQ occurs through enhancement of radiation-induced apoptosis, which correlates to the inhibition of p-Akt kinase and Bcl-XL and the activation of Erk and Jnk kinases, but appears independent of p53 induction or modulation of Bax/Bcl-2 gene expression. These data suggest DCQ should be tested as a radiosensitizer in vivo and has potential in the treatment of human solid tumors. (c) 2008 Published by Elsevier Ireland Ltd.