Abstract 2918: Interaction between MUC1 and STAT1 drives IFITM1 overexpression in aromatase inhibitor-resistant breast cancer cells

Aromatase inhibitors (AIs) successfully treat many estrogen receptor expressing breast cancers by depleting circulating estrogen levels. Unfortunately, many patients eventually develop resistance to this treatment. In order to elucidate the mechanism(s) of AI-resistance, our lab has developed an AI-resistant breast cancer cell line MCF-7:5C which was clonally derived from parental MCF-7 cells following long-term estrogen deprivation. Unlike MCF-7 cells, the AI-resistant MCF-7:5C cell line grows robustly in the absence of estrogen and is highly aggressive. We have previously reported that interferon induced transmembrane protein 1 (IFITM1) is overexpressed in MCF-7:5C cells and in AI-resistant breast tumors. Additionally, we have shown that loss of IFITM1 leads to cell death. What remains to be addressed is how IFITM1 expression is regulated in AI-resistant breast cancer cells. IFITM1 is a type 1 interferon (IFN) stimulated gene which is not expressed in normal tissue and is only induced by the type1 IFNs (IFNα/β). In previous studies we have found that MCF-7:5C cells produce elevated levels of IFNα, which binds to the type 1 IFN receptor, IFNAR, and induces JAK/STAT signaling, resulting in the overexpression of IFITM1. In this study, we further characterized the mechanisms of IFITM1 overexpression in AI-resistant MCF-7:5C cells. We have found that mucin 1 (MUC1) associates with STAT1 and stabilizes its phosphorylated form, thereby enhancing IFITM1 expression. This interaction is disrupted by treatment with the JAK inhibitor, Ruxolitinib. MUC1 is a transmembrane O-glycosylated protein that is overexpressed in 90% of breast cancers and known to promote the survival of the epithelium through formation of mucous, interaction with transcription factors and inhibition of apoptosis. A luciferase reporter determined that MUC1 contributes directly to IFITIM1 transcription. MUC1 expression is hormonally controlled and is normally enhanced by estrogen stimulation. In this study we found that MUC1 expression is dysregulated in AI-resistant MCF-7:5C cells and is instead reduced by estrogen stimulation. In contrast with the AI-sensitive MCF-7 cells, western blot and immunofluorescent staining showed that MUC1 expression was enhanced by treatment with the pure anti-estrogen, fulvestrant. Additionally, Annexin V/PI staining and PARP cleavage indicated that loss of MUC1 expression induced cell death in MCF-7:5C cells. We found that treatment with fulvestrant protects MCF-7:5C cells from this phenomenon, indicating that loss of MUC1 induces apoptosis in an ERα dependent manner. MCF-7:5C cells are sensitive to apoptosis following exposure to estrogen, which can be enhanced by loss of IFITM1 expression. We report that loss of MUC1 expression similarly enhances estrogen-induced apoptosis, suggesting that the communication between MUC1 and STAT1 also influences estrogen signaling in AI-resistant breast cancer. Citation Format: Asona Lui, Joan Lewis-Wambi. Interaction between MUC1 and STAT1 drives IFITM1 overexpression in aromatase inhibitor-resistant breast cancer cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2918.