Abstract 828: Mechanisms underlying differential response to estrogen-induced apoptosis in long-term estrogen-deprived breast cancer cell lines.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC Models of long-term estrogen-deprived breast cancer cells are utilized in the laboratory to mimic clinical aromatase inhibitor-resistant breast cancer and serve as a tool to discover new therapeutic strategies. The MCF-7:5C and MCF-7:2A subclones are derived from the parental MCF-7 cell line through long-term estrogen deprivation (Sweeney, et al. HMBCI 2012). In addition to the different responses to antiestrogens observed between MCF-7:5C and MCF-7:2A cells, they also have different apoptotic responses to estrogen (E2); the MCF-7:5C cells undergo apoptosis and die during the first week of E2 treatment, whereas the “late-apoptosis” MCF-7:2A cells die after two weeks of E2 treatment. To discover the mechanisms by which MCF-7:2A cells survive two weeks of E2 treatment, systematic experiments were performed in this study. E2 increased the cleavages of poly (ADP-ribose) polymerase (PARP) and Caspase 9 in both cell lines within 72 hours. RT-PCR microarrays of apoptosis-related genes also showed the gene expression pattern in MCF-7:2A cells was similar to that of MCF-7:5C cells after E2 treatment. Importantly, E2 caused apoptosis in both cell lines after 72 hours of E2 treatment, detected by Annexin V binding assay through flow cytometry, but to varying degrees. The MCF-7:5C cells showed approximately double the amount of Annexin V binding (and thereby apoptosis) compared to the MCF-7:2A cells. Further experiments showed that E2 induced similar responses of the unfolded protein response (UPR), demonstrated by the markers eukaryotic initiation factor subunit alpha (eIF2α) and inositol-requiring enzyme 1 alpha (IRE1α), although MCF-7:2A cells have higher basal levels of phosphorylated eIF2α compared with MCF-7:5C. The question arises, then, of why E2 does not kill the 2A cells until the second week of treatment, when their molecular responses closely mirror the 5C cells. Our microarray database demonstrates increased superoxide dismutase 2 (SOD2) in response to E2 in MCF-7:2A but not in MCF-7:5C cells, as well as higher basal levels of glutathione. These could represent protective mechanisms in the MCF-7:2A cells to prevent E2-induced cell death. We are currently investigating whether MCF-7:2A cells are resistant to the oxidative stress induced by E2. All of these preliminary data imply that the antioxidant system may play a critical role to defend MCF-7:2A cells from apoptosis induced by E2. VCJ is supported by the Department of Defense Breast Program under Award number W81XWH-06-1-0590 Center of Excellence; subcontract under the SU2C (AACR) Grant number SU2C-AACR-DT0409; the Susan G Komen For The Cure Foundation under Award number SAC100009; GHUCCTS CTSA (Grant # UL1RR031975) and the Lombardi Comprehensive Cancer Center Support Grant (CCSG) Core Grant NIH P30 CA051008. The views and opinions of the author(s) do not reflect those of the US Army or the Department of Defense. Citation Format: Elizabeth E. Sweeney, Ping Fan, V. Craig Jordan. Mechanisms underlying differential response to estrogen-induced apoptosis in long-term estrogen-deprived breast cancer cell lines. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 828. doi:10.1158/1538-7445.AM2013-828