The miR-106b-25 cluster targets Smad7, activates TGF-β signaling, and induces EMT and tumor initiating cell characteristics downstream of Six1 in human breast cancer

Oncogene. 2012 Dec 13;31(50):5162-71. doi: 10.1038/onc.2012.11. Epub 2012 Jan 30.


The role of TGF-β signaling in tumorigenesis is paradoxical: it can be tumor suppressive or tumor promotional, depending on context. The metastatic regulator, Six1, was recently shown to mediate this switch, providing a novel means to explain this elusive 'TGF-β paradox'. Herein, we identify a mechanism by which Six1 activates the tumor promotional arm of TGF-β signaling, via its ability to upregulate the miR-106b-25 microRNA cluster, and further identify a novel function for this cluster of microRNAs. Although expression of the miR-106b-25 cluster is known to overcome TGF-β-mediated growth suppression via targeting p21 and BIM, we demonstrate for the first time that this same cluster can additionally target the inhibitory Smad7 protein, resulting in increased levels of the TGF-β type I receptor and downstream activation of TGF-β signaling. We further show that the miR-106b-25 cluster is sufficient to induce an epithelial-to-mesenchymal transition and a tumor initiating cell phenotype, and that it is required downstream of Six1 to induce these phenotypes. Finally, we demonstrate a significant correlation between miR-106b, Six1, and activated TGF-β signaling in human breast cancers, and further show that high levels of miR-106b and miR-93 in breast tumors significantly predicts shortened time to relapse. These findings expand the spectrum of oncogenic functions of miR-106b-25, and may provide a novel molecular explanation, through the Six1 regulated miR-106b-25 cluster, by which TGF-β signaling shifts from tumor suppressive to tumor promoting.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Breast Neoplasms / genetics*
  • Breast Neoplasms / metabolism
  • Breast Neoplasms / pathology
  • Cell Line, Tumor
  • Cell Transformation, Neoplastic / genetics
  • Cell Transformation, Neoplastic / metabolism
  • Cell Transformation, Neoplastic / pathology
  • Epithelial-Mesenchymal Transition / genetics*
  • Female
  • Gene Expression Regulation, Neoplastic
  • Homeodomain Proteins / genetics
  • Homeodomain Proteins / metabolism*
  • Humans
  • MCF-7 Cells
  • MicroRNAs / genetics*
  • MicroRNAs / metabolism*
  • Protein Serine-Threonine Kinases / genetics
  • Protein Serine-Threonine Kinases / metabolism
  • Receptor, Transforming Growth Factor-beta Type I
  • Receptors, Transforming Growth Factor beta / genetics
  • Receptors, Transforming Growth Factor beta / metabolism
  • Signal Transduction
  • Smad7 Protein / genetics
  • Smad7 Protein / metabolism*
  • Transforming Growth Factor beta / genetics
  • Transforming Growth Factor beta / metabolism*
  • Up-Regulation


  • Homeodomain Proteins
  • MIRN106 microRNA, human
  • MIRN93 microRNA, human
  • MicroRNAs
  • Receptors, Transforming Growth Factor beta
  • SIX1 protein, human
  • SMAD7 protein, human
  • Smad7 Protein
  • Transforming Growth Factor beta
  • Protein Serine-Threonine Kinases
  • Receptor, Transforming Growth Factor-beta Type I