Abstract 2489: Targeting pro-survival BCL-2 family members increases radiation sensitivity in breast cancer cells

Abstract

Abstract In previous studies we have shown that BCL-2 expression was associated with an increased risk of local recurrence in patients with early stage breast cancer (Yang Q, Breast Cancer Res Treat, 2009). Additionally, BCL-2-overexpressing lymphoma and prostate cancer cell lines were resistant to radiation-induced apoptosis. The BCL-2 family encompasses two groups: the pro-survival group (BCL-2, BCL-XL, BCL-w, MCL-1, A1) and pro-apoptotic group (BAK, BAX, BAD, BIK, etc.) ABT-737 is a BH3 mimetic compound that targets the pro-survival members excluding MCL-1. Targeting pro-survival BCL-2 family members presents an attractive novel adjuvant therapeutic option. The purpose of this study was to explore an approach to overcoming radiation resistance by targeting pro-survival BCL-2 family proteins in breast cancer cells. First, siRNAs were employed to target BCL-2 in MCF-7 and BT474 breast cancer cells. Apoptosis was determined by detecting cleaved PARP after siRNA and radiation treatment. Cleaved PARP in BCL-2 siRNA treated MCF-7 cells increased compared to non-targeting siRNA treated cells 72 hours after 8Gy of radiation, indicating the targeting BCL-2 alone increased apoptotic cell death. Despite evidence of increased radiation related apoptosis, cell viability showed there was no significant difference between BCL-2 siRNA and non-targeting siRNA treated MCF-7 cells at 2, 4, 6 and 8Gy, p>0.05, indicating that targeting the BCL-2 alone did not enhance radiation sensitivity. However, the combination of radiation (8Gy) and ABT-737 (variable doses) enhanced BAK and cleaved PARP at 24hr, 48hr and 72hr. Further, the cell viability of the combination of radiation (2Gy) and ABT-737 (5µM) significantly decreased 16.5%, p<0.01 and 26.5%, p<0.01, compared with radiation alone or ABT-737 alone treated MCF-7 cells, respectively. Similar results were observed in T47D and MDA-MB231 cells. Finally, after knocking down MCL-1 by siRNA, cell viability went down in 14.5% (p<0.05) in MCL-1 siRNA treated MCF-7 cells compared with non-targeting siRNA treated cells at 5µm of ABT-737. In addition, MCL-1 protein level decreased after treatment of radation alone and the combination of radiation and ABT-737 in MCF-7, MDA-MB 231 and ZR75-1 cells. Together, our results showed that synergistic effect of radiation and ABT-737 on breast cancer cell lines through down-regulating MCL-1 and activating bak-apoptotic pathway. These results support the combination of radiation and pro-survival BCL-2 family inhibitor as a potential novel therapeutic strategy in breast cancer treatment. Supported by the Breast Cancer Research Foundation Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2489. doi:10.1158/1538-7445.AM2011-2489