Abstract 2071: Loss of ER-regulated microRNAs is fundamental to hormone-independent breast cancer cells

Proceedings: AACR 101st Annual Meeting 2010‐‐ Apr 17‐21, 2010; Washington, DC Breast cancer is a disease that affects thousands of women every year where death generally occurs due to metastasis and the development of resistance to chemotherapeutic agents. A large subset of breast cancer is attributed to the dysregulation of estrogen receptor (ER) signaling. Postmenopausal women have decreased systemic estrogen, which presumably leads to a repressed ER signaling program. Treatment of postmenopausal breast cancer patients with aromatase inhibitors, which further decrease the level of estrogen by inhibiting its production from androgens, leads to an improved response when compared to anti-estrogen therapy. Long-term estrogen deprived (LTED) MCF7 breast cancer cells, which were generated by limiting the levels of estrogen in culture models this class of cancer. Previous studies have examined gene expression profiles in LTED cells providing clues to how these cancers are regulated. In our study, we set out to further characterize these cancers by examining microRNA (miRNA) expression in LTED cells. As a means to determine miRNAs that may be essential for the regulation of hormone-independent ER-positive breast cancers, we profiled the expression of 738 miRNAs in MCF7 breast cancer cell lines. We compared the miRNA expression levels of common MCF7 cells to a LTED derivative, MCF7:2A. Of the differentially expressed miRNAs, we discovered two different miRNA clusters that are directly regulated by estrogen in MCF7 cells. We observed induced ER binding near the primary-miRNAs (pri-miRNAs) that contain these clusters and that LTED cells have less overall basal ER interaction at these cis elements. This corresponds to a decreased expression of these pri-miRNAs in the LTED cells versus MCF7. Furthermore, treatment of MCF7s with fulvestrant, an anti-estrogen, reduces pri-miRNA expression. Predicted targets of these miRNAs include IGF1R and HER2, proteins known to be upregulated in LTED cells. These proteins have been shown to be fundamental to the growth and survival of estrogen-independent cell lines. We demonstrate modulation of the expression level of these proteins with the addition of miRNAs to LTED cells. Additionally, we can show endogenous association of the mRNAs encoding HER2 and IGF1R with the Ago1 complex in MCF7 cells and reduced association in the LTED cells. These data provide a mechanism by which growth receptors can be induced to express upon estrogen loss in breast cancer. Preliminary analysis of published datasets show a correlation between pri-miRNA expression and metastatic breast cancer where there is decreased miRNA expression in metastatic tumors. This study identifies both ER-regulated miRNAs as well as a potential postmenopausal breast cancer miRNA signature, which may lead to novel therapy of ER-positive breast cancers in postmenopausal women. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2071.