Overexpression of bax enhances the radiation sensitivity in human breast cancer cells

Bax-alpha, a splice variant of bax which promotes apoptosis, is expressed in many kinds of untransformed cell lines and breast tissue, whereas only weak or no expression could be detected in breast cancer cell lines and malignant breast tissue. Human breast cancer MCF-7 cells, which have a weak bax gene expression, were stably transfected with pCX2neo bax, encoding human bax and neomycin-resistant genes, and two unique clones (MCF-7/bax-1 and MCF-7/bax-2) were thus generated which expressed different levels of bax-alpha. Sensitivity to ionizing radiation (IR) was examined and each was more sensitive to IR than the parental MCF-7 cells. The degree of enhancement in radiosensitivity was dependent on the expression level of bax, and IR was found to induce intranucleosomal DNA fragmentation in stable transfectant but not in parent cells, thus suggesting that this sensitization is due to apoptosis. We suggest that exogenous bax-alpha expression might therefore be one of the factors determining cellular radiosensitivity in MCF-7 breast cancer cells and may potentially have a therapeutic application by enhancing radiation sensitivity in breast cancer cells.