| Nearly all deaths from breast cancer are a result of metastasis, highlighting the need for improved therapies to combat distant-stage disease. Our lab has previously demonstrated that Six1 mediates breast cancer metastasis. We have further shown that Six1 can upregulate the tumor promotional effects of TGF-β signaling, while simultaneously inhibiting the tumor suppressive functions of this cytokine. In this work, I demonstrate that Six1 regulates the miR-106b-25 cluster of miRNAs, which have previously been identified in the impairment of TGF-β growth suppression through inhibition of p21 and Bim, thus providing a mechanism for the Six1-mediated inhibition of TGF-β tumor suppression. I also show, for the first time, that these miRNAs are able to target the inhibitory Smad7 protein, leading to increased levels of the TGF-β Type I receptor (TβRI), and ultimately activation of this pathway, thereby implicating one miRNA cluster in the TGF-β oncogenic switch. I demonstrate that the miR-106b-25 miRNAs are sufficient to induce both EMT and TIC phenotypes in breast cancer cells, and that this miRNA cluster is required for TIC phenotypes downstream of Six1. Importantly, I expand these findings into human breast cancers, demonstrating the relevance of a Six1/miR-106b/TGF-β axis in human breast cancer, and revealing a correlation between high miR-106b and miR-93 expression and shortened time to relapse in breast cancer patients. Interestingly, I also demonstrate that miR-106b/93 expression is higher in metastatic lesions as compared to matched primary breast tumors, suggesting a possible role for these miRNA in breast cancer metastasis. I show that overexpression of the miR-106b-25 cluster is not sufficient to induce late stage metastasis, but additional work is needed to determine whether these miRNA play a role in early stage metastasis or in Six1-mediated metastasis. Finally, I also demonstrate that the miR-106b-25 cluster can activate Notch signaling, which may contribute to the oncogenic and cancer stem cell phenotypes observed with expression of these miRNA. Overall, these findings expand the oncogenic spectrum of the miR-106b-25 miRNAs, and uncover new miRNA-based therapeutic avenues for the treatment of breast cancer. |
