ERα, microRNAs, and the epithelial–mesenchymal transition in breast cancer

The most common form of breast cancer, luminal A, is estrogen receptor α (ERα)-positive and epithelial, but nevertheless can metastasize. The process of epithelial–mesenchymal transition (EMT) is probably the first step in the metastasis of epithelial cancers. We discuss the characteristics of EMT, including factors that induce EMT, and the relationship of EMT to cancer stem cells (CSCs). Estrogen/ERα signaling maintains an epithelial phenotype and suppresses EMT. An overview of microRNAs in breast cancer is presented, including how microRNA biogenesis is altered in cancer and regulated by ERα. We also discuss the role of the miR-200 family in opposing EMT. Finally, we discuss specific microRNAs that target ERα and regulate EMT in breast cancer, and the role of these microRNAs in breast cancer progression.