let-7 microRNAs induce tamoxifen sensitivity by downregulation of estrogen receptor α signaling in breast cancer

Mol Med. 2011;17(11-12):1233-41. doi: 10.2119/molmed.2010.00225. Epub 2011 Jul 27.

Abstract

MicroRNAs (miRNAs) play an important regulatory role in breast tumorigenesis. Previously, we found that let-7 miRNAs were downregulated significantly in formalin-fixed paraffin-embedded (FFPE) breast cancer tissues. In this study, we further found that endogenous levels of let-7b and let-7i miRNAs are inversely correlated with levels of estrogen receptor (ER)-a36, a new variant of ER-α66, in the FFPE tissue set. Bioinformatic analysis suggested that ER-α36 may be another target of let-7 miRNAs. To test this hypothesis, cotransfection of let-7 mimics or inhibitors together with full-length or a fragment of ER-α36 3'UTR luciferase construct was performed, and we found that let-7b and let-7i mimics suppressed the activity of reporter gene significantly, which was enhanced remarkably by let-7b and let-7i inhibitors. Both mRNA and protein expression of ER-α36 were inhibited by let-7 mimics and enhanced by let-7 inhibitors. Furthermore, ER-α36 mediated nongenomic MAPK and Akt pathways were weakened by let-7b and let-7i mimics in triple negative breast cancer cell line MDA-MB-231. The reverse correlation between let-7 miRNAs and ER-α36 also exists in Tamoxifen (Tam)-resistant MCF7 cell line. Transfection of let-7 mimics to Tam-resistant MCF7 cells downregulated ER-α36 expression and enhanced the sensitivity of MCF7 cells to Tam in estrogen-free medium, which could be restored by overexpression of ER-α36 constructs without 3'UTR. Our results suggested a novel regulatory mechanism of let-7 miRNAs on ER-α36 mediated nongenomic estrogen signal pathways and Tam resistance.

MeSH terms

  • Base Sequence
  • Breast Neoplasms / drug therapy
  • Breast Neoplasms / genetics*
  • Cell Line, Tumor
  • Down-Regulation / drug effects*
  • Down-Regulation / genetics
  • Drug Screening Assays, Antitumor
  • Estrogen Receptor alpha / genetics*
  • Estrogen Receptor alpha / metabolism
  • Female
  • Gene Expression Regulation, Neoplastic / drug effects
  • Gene Knockdown Techniques
  • Genome, Human / genetics
  • Humans
  • MicroRNAs / genetics
  • MicroRNAs / metabolism*
  • Models, Biological
  • Molecular Sequence Data
  • Receptor, ErbB-2 / metabolism
  • Signal Transduction / drug effects*
  • Signal Transduction / genetics
  • Tamoxifen / pharmacology*
  • Tamoxifen / therapeutic use
  • Transfection

Substances

  • ESR1 protein, human
  • Estrogen Receptor alpha
  • MicroRNAs
  • mirnlet7 microRNA, human
  • Tamoxifen
  • ERBB2 protein, human
  • Receptor, ErbB-2