In order to understand how microRNAs (miRNAs) regulate breast cancer tumorigenesis, a miRNA expression microarray screening was performed using RNA from formalin-fixed paraffin-embedded (FFPE) breast tissues, which included benign (n = 13), ductal carcinoma in situ (DCIS) (n = 16), and invasive ductal carcinoma (IDC) (n = 15). Twenty-five differentially expressed miRNAs (P < 0.01) were identified, of which let-7 family miRNAs were down-regulated in human breast cancer tissues at stages of DCIS and IDC compared to benign stage. We further found that there was an inverse correlation between ER-α expression and several members of let-7 family in the FFPE tissues. Next, we performed bioinformatics analysis and found that let-7 miRNA sequences match sequence in the 3-UTR of estrogen receptor alpha (ER-α), suggesting ER-α may be a target of let-7, which was further confirmed by a number of experimental assays, including luciferase assay, protein expression, and mRNA expression. Overexpression of let-7 miRNAs in ER-positive breast cancer MCF7 cell line negatively affected ER-α activity. As expected, dampening of the ER-α signaling by let-7 miRNAs inhibited cell proliferation, and subsequently triggered the cell apoptotic process in MCF7 cells. In conclusion, our findings indicate a new regulatory mechanism of let-7 miRNAs in ER-α mediated cellular malignant growth of breast cancer.
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Acknowledgments We thank Dr. Zhaoyi Wang, Dr. Xintian Zhang, and Dr. Lianguo Kang for their technical help and fruitful discussion and Dr. Xianming Chen, Dr. Aiyu Gong, and Dr. Guoku Hu for their help with plasmid construct and luciferase assay. This article was supported by Bone Biology Program of the Cancer, Smoking Related Disease Research Program, and the Nebraska Tobacco Settlement Biomedical Research Program (LB692, LB595, and LB506 to G. G. Xiao).
- Breast cancer
- Estrogen receptor-α signaling
- Let-7 family microRNAs