Down regulation of anti-apoptotic proteins can promote apoptosis and enhance the radiosensitivity of cancer cells [10–13]. The disruption of anti-apoptotic pathways is a novel target for overcoming radioresistance in breast cancer. ABT-737 is a rationally designed small molecule that binds with high affinity to Bcl-2 and Bcl-xL and antagonizes

their anti-apoptotic function, thereby inducing apoptosis in many cancer cell types [14, 15]. Recently, an increasing number of studies have focused on the role of ABT-737 in cancer therapy.ABT-737 have been shown to reverse acquired paclitaxel resistance in breast cancer cell lines [16]. Combined with rapamycin, ABT-737 has Anti-infection Compound Library been shown to enhance the radiosensitivity Selleckchem MLN8237 of non-small cell lung tumors by inducing apoptosis [16, 17]. To our knowledge, there have been no prior studies investigating the effect of ABT-737 in combination with radiotherapy for the treatment of breast cancer. In the present study, we addressed whether ABT-737 could reverse the acquired radioresistance in breast cancer cells with the

aim of develop a new strategy to address the serious clinical problem of acquired radioresistance in breast cancer. Methods Cell culture, materials and reagents The human breast cancer cell line MDA-MB-231 was purchased from the American Type Culture Collection. The cells were grown in Leibovitz’s L-15 medium (11415–064, GIBCO) supplemented with 10% fetal bovine serum (FBS) (10099–158, GIBCO) and maintained in a humidified 5% CO2 atmosphere at 37°C. ABT-737 was purchased from Santa Cruz Biotechnology, Inc (SC-207242). Generation of radioresistant cells MDA-MB-231 cells (1 × 106) were plated

in 75 cm2 culture flasks and irradiated with 4Gy of γ-rays using a Theratron Cobalt-60 treatment unit at a dose rate of 1 Gy per minute when the cells were at approximately 60% confluence in the culture flask. Immediately following before irradiation, the culture medium was renewed, and the cells were returned to the incubator. When the MDA-MB-231 cells reached approximately 90% confluence, they were trypsinized, counted and passaged into new culture flasks. Again, the cells were treated with 4 Gy γ-rays when they reached approximately 60% confluence. The irradiation was performed 13 times for a total dose of 50 Gy (irradiated with 2 Gy of γ-rays at the final irradiation) over 5 months. The parental cells were trypsinized, counted and passaged under the same conditions without irradiation. Clonogenic assay for radiosensitivity The cells were seeded in 6-well cell culture plates and incubated for 2 weeks at 37°C after the receiving various doses of irradiation. The colonies were fixed with pure ethanol and stained with 1% crystal violet, washed and air-dried. Colonies consisting of 50 or more cells were counted as clonogenic survivors.