Estrogen deprivation using aromatase inhibitors (AIs) is the standard of care for postmenopausal women with hormone receptor-positive breast cancer. However, the majority of patients treated with AIs eventually develop resistance, inevitably resulting in patient relapse and, ultimately, death. We have previously reported the development of an AI-resistant breast cancer cell line, MCF-7:5C, which was clonally derived from parental MCF-7 breast cancer cells following long term estrogen deprivation. A unique phenotype of AI-resistant MCF-7:5C cells is that they grow in the absence of estrogen; however, when they are treated with estradiol they undergo apoptosis/cell death through activation of the mitochondrial death pathway. More recently, we have also discovered that AI-resistant MCF-7:5C cells constitutively overexpress the interferon stimulated gene IFITM1. IFITM1 is a cell surface membrane protein that is most well-known for its ability to block viral replication, however, its role in breast cancer and AI-resistance is not known. We found that IFITM1 was constitutively overexpressed at the protein (> 20-fold) and mRNA (> 15-fold) level in AI-resistant MCF-7:5C breast cancer cells compared to parental MCF-7 cells and that its overexpression strongly correlated with enhanced growth and increased aggressiveness. Clinical data also indicated that IFITM1 was overexpressed in 36 out of 40 AI-resistant breast tumor samples compared to AI-sensitive tumors. In this study, we assessed the functional role of IFITM1 in AI-resistant MCF-7:5C cells. Notably, we found that knockdown of IFITM1 significantly induced cell death in MCF-7:5C cells and it reduced cell proliferation. The induction of cell death as a result of IFITM1 knockdown was associated with increases in p21, Bax, Noxa, and Puma expression; however, p53 was not altered in these cells. To validate the effect observed with IFITM1 knockdown in our resistant cells we used a second shRNA targeting IFITM1. We found that shRNA knockdown of IFITM1 induced PARP cleavage and p21, reduced cell proliferation, and induced cell death in AI-resistant MCF-7:5C cells. Moreover, we found that knockdown of IFITM1 dramatically enhanced estradiol-induced cell death in AI-resistant MCF-7:5C cells which was associated with induction of pro-apoptotic proteins and p21 via a p53-independent mechanism. Further analysis revealed that knockdown of STAT1 and STAT2, which are critical regulators of IFITM1, also significantly enhanced estradiol-induced apoptosis in MCF-7:5C cells. Taken together, these findings indicate that overexpression of IFITM1 plays a critical role in breast cancer and AI-resistance and that targeting this gene can sensitize AI-resistant cells to estrogen-induced death. Furthermore, these results suggest that IFITM1 can regulate the mitochondrial pathway through its interactions with anti-apoptotic proteins.

Citation Format: Hye Joung Choi, Joan Lewis-Wambi. Loss of interferon-induced transmembrane protein 1 enhances estrogen-induced cell death in AI-resistant breast cancer cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 25. doi:10.1158/1538-7445.AM2015-25