Let-7 MicroRNAs Induce Tamoxifen Sensitivity by Downregulation of Estrogen Receptor α Signaling in Breast Cancer

Yingchun Zhao,Gary Guishan Xiao,Danjun Ma,Mingxi Guo,Zhaoyi Wang,Weida Lu,Robert R Recker,Caishu Deng,Zoran Gatalica,Jing Xiao

doi:10.2119/molmed.2010.00225

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.

Highlights

The involvement of estrogen in mammary carcinogenesis has been known for more than 100 years [1]
estrogen receptors (ER)-α36 Is Widely Expressed in Breast Cancer Cell Lines
We first checked the ER-α36 expression in some established breast cancer cell lines (Figure 2A) and found that ER-α36 was expressed in both ER-α66 positive and negative cell lines

Summary

Introduction

The involvement of estrogen in mammary carcinogenesis has been known for more than 100 years [1]. The estrogen signaling was thought to be mediated by two major estrogen receptors (ER), ER-α and ER-β [2], which share a common structural architecture. ER-α is a 66 kD protein that functions as a transcription factor and regulates the transcription of estrogenresponsive genes. The A/B region contains a ligand-independent transactivation domain (AF-1). Regions C and E are responsible for DNA and ligand binding, respectively. A ligand-inducible transcription-activating function (AF-2) is present in the ligand-binding domain D/E/F [4]. ER-α46 functions to inhibit the transcriptional activity mediated by the AF-1 domain of ER-α66 [5] and to signal a membrane-initiated estrogen pathway [6]

Methods

Results

Conclusion