miR-205/RunX2 axis negatively regulates CD44⁺/CD24^- breast cancer stem cell activity

Breast Cancer stem cells (BCSCs) have been extensively studied and have been used directly as a therapeutic target, but how the BCSCs themselves are regulated remain unclear. Here we reported identification of miR-205 that may act as a tumor suppressor and negatively-regulate BCSCs stemness and tumor malignance. By qRT-PCR analysis, we have shown that miR-205 was decreased in CD44⁺/CD24^-/low BCSCs compared with non-BCSCs. We have also shown that miR-205 expression level was very low in MB-231 cells with high BCSC percentage, while relatively high in MCF-7 cells with low BCSC percentage. We then overexpressed miR-205 in MB-231 and SUM-149 cells and knocked it down in MCF-7 and BT-474 cells respectively. Our results showed that overexpression of miR-205 could reduce CD44⁺/CD24^-/low population percentage in MB-231 cells. The mechanism might associate with mesenchymal-epithelial transition (MET). Finally, we found an important transcriptional factor and oncogene, RunX2, was a target gene of miR-205. miR-205 overexpression could inhibit breast cancer malignancy by regulating RunX2 both in vitro and in vivo. A rescue experiment by cotransfection of RunX2 and miR-205 into the MCF-7 cell line attenuate cell proliferation, invasion, migration, CD44⁺/CD24^-/low population, mammosphere formation abilities and xengraft tumor formation. Together, our results support that miR-205 is a tumor suppressor during breast cancer development.