MicroRNA-125b upregulation confers aromatase inhibitor resistance and is a novel marker of poor prognosis in breast cancer (Breast Cancer Research)

IntroductionIncreasing evidence indicates micro RNAs (miRNAs) being important players in oncogenesis. Considering the wide-spread use of aromatase inhibitors (AIs) in endocrine therapy as a first-line treatment for post-menopausal endocrine receptor ?-positive breast cancer patients, identifying deregulated expression levels of miRNAs in association with AI resistance is of utmost importance. Methods: To gain further insight into the molecular mechanisms underlying the AI resistance, we performed miRNA microarray experiments using a new model of acquired resistance to letrozole (Res-Let cells), obtained by long-term exposure of aromatase-overexpressing MCF-7 cells (MCF-7aro cells) to letrozole, and a model of acquired anastrozole resistance (Res-Ana cells). Three miRNAs (miR-125b, miR-205 and miR-424) similarly deregulated in both AI-resistant cell lines were then investigated in terms of their functional role in AI resistance development, breast cancer cell aggressiveness and their clinical relevance using a cohort of 65 primary breast tumor samples. Results: We identified the deregulated expression of 33 miRNAs in Res-Let cells and of 18 miRNAs in Res-Ana cells compared to the sensitive MCF-7aro cell line. The top-ranked Kyoto Encyclopedia of Genes and Genomes pathways delineated by both miRNA signatures converged on the AKT pathway, which was found constitutively activated in both AI-resistant cell lines. We report for the first time that ectopic overexpression of either miR-125b or miR-205, or the silencing of miR-424 expression in the sensitive MCF-7aro cell line were sufficient to confer resistance to letrozole and to anastrozole, to target and to activate the AKT/mTOR pathway, and to increase formation capacity of cancer-initiating-like cells possessing self-renewing properties. Increasing miR-125b expression levels was also sufficient to confer estrogen-independent growth properties to the sensitive MCF-7aro cell line. Finally, elevated miR-125b expression levels were a novel marker for poor prognosis in breast cancer, and targeting miR-125b in Res-Let cells overcame letrozole resistance. Conclusion: This study highlights that acquisition of specific deregulated miRNAs is a newly discovered alternative mechanism developed by AI-resistant breast cancer cells to achieve constitutive activation of the AKT/mTOR pathway and develop AI resistance. It also highlights that miR-125b is a new biomarker of poor prognosis and a candidate therapeutic target of AI-resistant breast cancers.