Abstract 717: Overexpression of interferon-stimulated genes is critical for the survival of aromatase inhibitor-resistant breast cancer cells

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Abstract Aromatase inhibitors (AIs) successfully treat many estrogen receptor expressing breast cancers by depleting circulating estrogen levels. Unfortunately, the majority of patients treated with AIs 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. The AI-resistant MCF-7:5C cells grow robustly in the absence of estrogen, are highly aggressive, and constitutively overexpress a panel of type 1 interferon (IFN) stimulated genes (ISGs) including IFITM1, IFIT1, IFI27, PLSCR1, STAT1/2, OAS1 and MX1 as compared to the AI-sensitive MCF-7 cells. Human tissue microarray analysis also indicates that IFITM1 and PLSCR1 are overexpressed in AI-resistant/recurrence breast tumors as compared to primary tumors. ISGs are not expressed basally in normal tissue and are only induced by type1 IFNs (IFNα and β) to protect the host from viral infections. IFNα signals through a specific receptor, IFNAR1/2, which uses JAK/STAT signaling to produce the ISGs. The significance of constitutive overexpression of ISGs in AI-resistant breast cancer is not known. In this study, we investigated the functional importance of ISGs in the AI-resistant breast cancer cell line, MCF-7:5C, and the mechanism(s) that drive their overexpression in these cells. Using ELISA, we detected a 3-fold higher level of IFNα in AI-resistant MCF-7:5C cells than in the parental MCF-7 cells and FACS analysis revealed that the IFNα was produced by all cells in culture, not a distinct subset of the population. Blockade of IFNα signaling using a neutralizing antibody to IFNAR, as well as transient knockdown of IFNα and its transcription factor IRF7 dramatically reduced ISG expression. Most notably, loss of IFNα and suppression of type 1 IFN signaling resulted in the death of the AI-resistant MCF-7:5C cells, while having no effect on the parental MCF-7 cells, thus confirming its pro-survival function. Apart from IFNα signaling, we also found evidence of post-transcriptional regulation of ISGs in the AI-resistant cells. Specifically, several of the ISGs had a significantly longer mRNA half-life in the MCF-7:5C cells than in the parental MCF-7 cells. Additionally, knockdown of the RNA binding protein Human Antigen R (HuR) significantly reduced the mRNA stability and expression of the ISGs. Electron microscopy confirmed that AI-resistant MCF-7:5C cells exhibit classic morphological signs of anti-viral response and ISG expression, including dilated ER, and enhanced cell-cell adhesion as compared with the AI-sensitive MCF-7 cell line. Overall, we have found that AI-resistant breast cancer cells maintain elevated levels of IFNα in order to drive constitutive overexpression of ISGs which sustains their survival in estrogen deprived conditions. Citation Format: Asona Lui, Hye-Joung Choi, Joan Lewis-Wambi. Overexpression of interferon-stimulated genes is critical for the survival of aromatase inhibitor-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 717. doi:10.1158/1538-7445.AM2015-717