Abstract 265: STAT-1 activation enhances phospholipid scramblase 1-mediated apoptosis in aromatase inhibitor resistant breast cancer cells

Abstract

The signal transducer and activator of transcription 1 (STAT-1) protein is essential for signaling by interferons (IFNs), which, in addition to their role in innate immunity, serve as potent inhibitors of growth and promoters of apoptosis. Likewise, the phospholipid scramblase 1 (PLSCR-1) protein is also induced by interferon and is responsible for the translocation of phospholipids between the lipid bilayer of a cell membrane which occurs during apoptosis. While there is increasing evidence to suggest that STAT-1 and PLSCR-1 play a direct role in apoptosis, the molecular mechanism by which these proteins regulate apoptotic cell death remains unclear. Previously, our laboratory has reported the development of an estrogen receptor alpha (ERα)-positive breast cancer cell line, MCF-7:5C, which is resistant to long term estrogen deprivation (i.e resistant to aromatase inhibitors) but undergoes apoptosis in the presence of physiologic concentrations of 17β-estradiol (E2). Global gene expression profile of MCF-7:5C cells has previously revealed that estradiol-induced apoptosis is associated with significant upregulation of several proinflammatory genes including; IFNs, STAT1, and PLSCR-1. In the present study, we investigated the role of STAT-1 and PLSCR-1 in estradiol-induced apoptosis in MCF-7:5C breast cancer cells. We found that PLSCR-1 and STAT-1 proteins were constitutively overexpressed by ∼20- and 10-fold, respectively, in MCF-7:5C cells compared to hormone-responsive MCF-7 and T47D breast cancer cells and siRNA suppression of PLSCR-1 or STAT-1 expression in MCF-7:5C cells completely blocked their ability to undergo apoptosis in the presence of estradiol and/or interferon-alpha (IFNα) but not other apoptosis-inducing agents such as taxol and etoposide. Immunofluoresence analysis of MCF-7:5C cells indicated that PLSCR-1 and STAT-1 were overexpressed and localized primarily in the cytoplasm of these cells; however, in the presence of IFNα and E2, a significant portion of PLSCR-1 and STAT-1 translocated to the nucleus and was associated with apoptosis. We also found that increased STAT-1 and PLSCR1 activation inhibited the expression of prosurvival (BCL2, BCL-xL) and induced the expression of proapoptotic members (BAK, mitochondrial BAX) of the BCL2 family and was associated with increased mitochondrial membrane permeability and activation of CASP7, CASP8, and CASP9, as well as suppression of pAKT and NF-κB. Overall, our data demonstrate that STAT-1 and PLSCR-1 play a critical role in sensitizing aromatase inhibitor resistant breast cancer cells to estradiol-induced apoptosis and that IFNα combined with E2 might be an effective treatment option for patients with endocrine resistant disease. This work was supported by the NIH Career Development Grant K01CA120051 01A2 and the Hollenbach Foundation Grant (JSLW). Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 265. doi:1538-7445.AM2012-265