let-7 MicroRNAs Induce Tamoxifen Sensitivity by Downregulation of Estrogen Receptor α Signaling in Breast Cancer
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.
This work is fully supported by grants awarded to GG Xiao from the Bone Biology Program of the Cancer and Smoking Related Disease Research Program and the Nebraska Tobacco Settlement Biomedical Research Program (LB692, LB595 and LB506). This work was partially supported by OncomiR Biotechnologies Inc. (Los Angeles, CA, USA) to GG Xiao, and by National Institute of Health Grant DK84328 to Z Wang. We would like to thank Xintian Zhang and Lianguo Kang for technical help and fruitful discussion; Xianming Chen, Aiyu Gong and Guoku Hu for help with plasmid construct and luciferase assay.
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