Introduction: Previous studies in our laboratory have demonstrated that the active form of vitamin D, 1,25-di hydroxy vitamin D3 (1,25 D3), or its analogs such as EB 1089, can confer enhanced sensitivity to ionizing radiation (IR). Sensitization by EB 1089 appeared to be a consequence of the promotion of autophagic cell death. The current work was designed to extend these findings to ZR-75-1 breast tumor cells, also a p53 wild type ER positive cell line, utilizing 1,25- D3 combined with IR.

Experimental Procedure : In vitro studies assessed the effects of 100nM 1,25 D3 alone, fractionated IR (5 × 2 Gy) delivered over a period of 3 days, and the combination treatment of 100nM 1,25 D3 72hrs prior to IR.

Reults: Radiation alone produced an approximately 30% reduction in total cell number. Pretreatment with 1,25D3 enhanced radiation sensitivity such that an approximately 75% reduction in total cell number was observed. There was no evidence of apoptosis in cells treated with IR alone or pretreated with 1,25 D3 (lack of nuclear fragmentation or chromatin condensation or cleavage of caspase-3). Cell death was succeeded by a prolonged interval of growth arrest with either IR alone or IR preceded by 1,25 D3. Positive β-galactosidase staining, cell flattening and enlargement indicated that residual surviving cells (with IR alone or IR preceded by 1,25 D3) were in a state of senescence. Acridine orange staining and p62 degradation were indicative of autophagy with IR alone; however, autophagy was reduced when IR was preceded by pretreatment with 1,25 D3; there was minimal evidence of autophagy with 1,25 D3 alone. Clearly, autophagy does not appear to be responsible for radiation sensitization in this experimental model. Unexpectedly, the formation of binucleated cells with micronuclei, morphology indicative of mitotic catastrophe, was evident primarily in cells pretreated with 1,25 D3 prior to IR but not in cells treated with IR alone.

Conclusion: Pretreatment with vitamin D3 has the capacity to radiosensitize ZR-75-1 breast cancer cells by increasing the rate and extent of cell killing. Residual surviving cells after both radiation treatment alone and vitamin D pre-treatment appear to be in a state of prolonged growth arrest/senescence. Furthermore, in this system autophagy may prove to be cytoprotective instead of cytotoxic and could be involved in regulating senescence. While the initial mode of cell death has yet to be determined, it appears that mitotic catastrophe may play a role in vitamin D3's radiosensitization of ZR-75-1 breast cancer cells.

Implications. Taken together with our previous work, these studies support the concept that 1,25 D3 and/or its analogs enhance radiation sensitivity in breast tumor cells through the promotion of multiple cell death 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 1405.