MiR-99a antitumor activity in human breast cancer cells through targeting of mTOR expression

PLoS One. 2014 Mar 17;9(3):e92099. doi: 10.1371/journal.pone.0092099. eCollection 2014.


MicroRNAs (miRNAs) play an important role in human tumorigenesis as oncogenes or tumor suppressors. miR-99a has been reported as a tumor suppressor gene in various cancers in humans. However, only limited information about the function of miR-99a in human breast cancers is available. Here we investigated the expression of miR-99a in breast cancer tissue specimens and its antitumor activity in breast cancer cells. We initially identified that the expression of miR-99a was significantly reduced in four breast cancer cell lines. More importantly, we found downregulation of miR-99a in breast cancer specimens from ten different patients. We then analyzed the mechanism of miR-99a in inhibiting tumorigenesis. Cell-based assays that showed overexpression of miR-99a not only reduced breast cancer cell viability by inducing accumulation of cells at sub-G1 phase and cell apoptosis, but also inhibited tumorigenicity in vivo. As a critical miR-99a target, we have shown that the function of mammalian target of rapamycin (mTOR) was greatly inhibited by miR-99a-based Luciferase report assay; overexpression of miR-99a reduced the expression of mTOR and its downstream phosphorylated proteins (p-4E-BP1 and p-S6K1). Similar to restoring miR-99a expression, mTOR downregulation suppressed cell viability and increased cell apoptosis, whereas restoration of mTOR expression significantly reversed the inhibitory effects of miR-99a on the mTOR/p-4E-BP1/p-S6K1 signal pathway and the miR-99a antitumor activity. In clinical specimens and cell lines, mTOR was commonly overexpressed and its protein levels were statistically inversely correlated with miR-99a expression. Taken together, these results have demonstrated that miR-99a antitumor activity is achieved by targeting the mTOR/p-4E-BP1/p-S6K1 pathway in human breast cancer cells. This study suggests a potential therapeutic strategy to effectively control breast cancer development.

MeSH terms

  • Adaptor Proteins, Signal Transducing / metabolism
  • Animals
  • Antineoplastic Agents / metabolism*
  • Apoptosis / genetics
  • Base Sequence
  • Breast Neoplasms / genetics*
  • Breast Neoplasms / pathology*
  • Cell Cycle Proteins
  • Cell Line, Tumor
  • Cell Survival / genetics
  • Down-Regulation
  • Female
  • Gene Expression Profiling
  • Gene Expression Regulation, Neoplastic
  • Humans
  • Mice
  • MicroRNAs / genetics
  • MicroRNAs / metabolism*
  • Molecular Sequence Data
  • Phosphoproteins / metabolism
  • Phosphorylation
  • Ribosomal Protein S6 Kinases, 70-kDa / metabolism
  • Signal Transduction / genetics
  • TOR Serine-Threonine Kinases / antagonists & inhibitors
  • TOR Serine-Threonine Kinases / genetics*
  • TOR Serine-Threonine Kinases / metabolism
  • Up-Regulation / genetics
  • Xenograft Model Antitumor Assays


  • Adaptor Proteins, Signal Transducing
  • Antineoplastic Agents
  • Cell Cycle Proteins
  • EIF4EBP1 protein, human
  • MIRN99 microRNA, human
  • MicroRNAs
  • Phosphoproteins
  • MTOR protein, human
  • Ribosomal Protein S6 Kinases, 70-kDa
  • TOR Serine-Threonine Kinases
  • ribosomal protein S6 kinase, 70kD, polypeptide 1

Grants and funding

The authors have no support or funding to report.