MicroRNA-145 Regulation of Breast Cancer Stem Cell Self-Renewal An important issue challenging the therapeutic management of breast cancer is drug resistance. The cancer stem cell (CSC) hypothesis argues for existence within tumors of a small subpopulation of cancer cells driving tumor growth, capable of self-renewal, and responsible for drug-resistance and recurrence. A subpopulation (CD44high/CD24low) within heterogeneous breast cancers has been identified that is highly tumorigenic, capable of self-renewal, and possess drug-resistant characteristics. Extensive study of this subpopulation has revealed new details concerning tumorigenesis and drug resistance, however, little is known concerning possible microRNA (miR) regulation of these breast cancer stem cells. Many miRs are dysregulated in breast cancer where they contribute to tumorigenesis by regulating expression of genes in important cancer-related pathways. Recently, miR-145 was shown to critically regulate embryonic stem (ES) cell renewal and differentiation. Furthermore, miR-145 has also been found to regulate mesenchymal stem cell differentiation. However, nothing is currently known concerning potential miR-145 regulation of neoplastic stem cells. The hypothesis of the current study is that down-regulation of miR-145 within breast CSCs allows up-regulation of genes important in CSC self-renewal. We will test this hypothesis with experiments designed to address our specific aims: (1) to determine a molecular mechanism for miR-145 down-regulation in cancer stem cells and (2) to examine the impact of miR-145 regulation of breast CSC self-renewal and tumorigenesis in vivo. Completion of the proposed studies will be accompanied with new understanding of the importance of miR regulation in breast CSC function. Additionally, proposed studies will allow an examination, in breast cancer, of regulatory mechanisms that control pluripotency and master regulatory genes in normal and neoplastic stem cells, which in turn will provide a link between CSCs and ES cells. Finally, understanding pathways controlling breast CSCs may reveal new therapeutic strategies for overcoming drug resistance.