Abstract 5550: Autophagic potentiation of radiosensitization by vitamin D3 and the analog, EB1089

Introduction: While chemotherapy and radiation treatment may be initially effective at suppressing breast cancer growth, disease recurrence and tumor metastasis are major problems in the clinic. A number of strategies have been developed in an attempt to enhance the response to radiation and prevent tumor cell recovery by interfering with cytoprotective signaling. Our current studies are designed to investigate the nature of cell death when 1-25 Vitamin D3 (1,25D3) is utilized to enhance sensitivity to fractionated radiation (FIR) in the MCF-7, BT474, and Hs578t breast cancer cell lines. Results: Previous studies in our laboratory indicate that a 72 hour exposure of MCF-7 cells to 100nM of the Vitamin D3 analog, EB1089, prior to 5×2Gy FIR resulted in an 80% reduction in viable cell number over 96 hours post-irradiation. EB1089 also sensitized MCF-7 cells to FIR in terms of clonogenic survival. Although apoptosis, mitotic catastrophe and senescence are all evident under these experimental conditions, our studies strongly suggest that autophagy plays a central role in the radiosensitization effects of EB1089 as well as attenuation of the proliferative recovery that is observed with irradiation alone. To further define the role of autophagy in radiation sensitization by EB 1089, cells were exposed to the autophagy inhibitor, chloroquine, prior to EB1089 and irradiation. Chloroquine had no effect on sensitivity to radiation alone and enhanced sensitivity to tamoxifen, consistent with the cytoprotective effects of autophagy in the case of this estrogen antagonist. Inhibition by chloroquine of autophagy induced by EB1089 + FIR did not interfere with the apparent radiosensitization by EB1089, as the cells appeared to shift to an apoptotic mode of cell death. Studies are in progress utilizing genetic knockdown of the expression of autophagy related genes such as ATG5 and Beclin-1 to confirm that interference with autophagy drives the cells towards alternative modes of cell death. Additional studies in progress are designed to evaluate the effects of 1,25D3 treatment on radiation sensitivity in breast tumor cells that are resistant to irradiation such as Hs578t breast cancer cells, in addition focusing on breast tumor cells that over express Her-2/neu (MCF7/HER2, isogenic with the parental MCF-7 cell line, as well as p53 mutant BT474 cells with endogenous over expression of Her-2). Conclusions: Vitamin D irreversibly commits MCF7 breast tumor cells towards cell death upon subsequent exposure to irradiation. Inhibition of autophagy drives the cell towards alternative modes of cell death, primarily apoptosis (but possibly mitotic catastrophe). EB1089 and 1,25D3 may prove to also promote sensitivity to radiation (and chemotherapy) in breast tumor cells that are intrinsically resistant to treatment through the overexpression of cytoprotective signaling pathways. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5550.