Enhanced efficacy of radiation therapy by the overexpression of chromatin remodeling factor BRG1-bromodomain

Abstract

Objectives: While radiation therapy is widely used to treat some types of cancer, cancer cells easily develop radiation resistance resulting in poor prognosis. Therefore, radiation sensitization is useful for the efficient treatment of cancer cells. Brahma-related gene 1 (BRG1), the catalytic subunit of the SWI/SNF chromatin-remodeling complex, is involved in DNA double-strand break (DSB) repairs. Overexpression of the BRG1 bromodomain (BRD) site and consequent competitive inhibition results in an inability to induce DNA repair, leading to the accumulation of DSBs. This study aims to improve the therapeutic effect of radiation through overexpression of the BRG1 BRD, resulting in increased competitive inhibition and inefficient DSB repair. Methods: For visualizing tumor growth, luciferase expressing tumor cell lines were established. To monitor the therapeutic effect of external beam radiation, HT29 human colon cancer cells were used. Retroviral pMX-BRG1-BRD vectors were transfected into HT29 cells to establish a BRD over-expressing cell line, with both low and high (1.48-times higher) copy numbers. The cells were irradiated using a 137Cs irradiator (IBL 437C) at 9 Gy. The radio-sensitizing effects were measured through a clonogenic assay and analysis of phosphorylated H2AX foci. Tumor cells that were subcutaneously implanted into Balb/c nude mice, imaged by an in vivo imaging system, and measured using calipers. Results: The survival rates of irradiated (9 Gy) cells expressing low and high levels of BRD were 51.4% and 2.2%, respectively, compared to BRD non-transduced HT29 cells. The fluorescence intensity of phosphorylated H2AX foci for cells with low (2.9 times) and high (9.9 times) BRD levels were higher than that in HT29-luc cells. Tumor growth was reduced corresponding to higher BRG1-BRD expression levels in vivo

bioluminescence signals of low and high overexpression of BRG1-BRD in tumors at 28 days after ionizing radiation were found to be 40.77 % (p = 0.048) and 7.37% (p = 0.018) higher, respectively, than that in control tumors. Conclusion: The radiation-sensitizing effect of BRG1-BRD overexpression in human colon cancer cells was successfully demonstrated using molecular imaging techniques.