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ERβ1 is often downregulated in cancer compared with normal cells, suggesting that it may function as a tumour suppressor and could play an important role in carcinogenesis. The expression of ERβ1 is regulated by multiple mechanisms such as methylation. Five splice variants of ERβ mRNA have been identified, ERβ1 to ERβ5. However, it is unclear whether and how the full-length version, ERβ1, is regulated post-transcriptionally.
MicroRNAs are a class of nonprotein coding small RNAs that regulate expression of genes at post-transcriptional levels. Using rapid amplification of 3' complementary DNA ends (3'RACE), we have confirmed 3' untranslated region (3'UTR) expression and sequences of ERβ1 mRNA in MCF-7 cells. Based on miRNA expression profiling of human breast cancer studies, we found that miR-92 is upregulated in malignant breast. In silico analysis using the miRGen database and RNA hybrid predicted that there are two putative miR-92 target sequences within the 3'UTR of human ERβ1 mRNA. Firstly, we profiled the expression of ERβ1 mRNA and miR-92 in breast cancer tissue and cell lines. miR-92 levels were higher in ERβ1-negative MDA-MB-453 cells than ERβ1-positive MCF-7 cells. We observed miR-92 upregulation in breast tumours while ERβ1 mRNA expression was decreased compared with matched adjacent normal tissues. We found a significant negative correlation between miR-92 and ERβ1 mRNA and protein in breast tumour (r = -0.5, P = 0.001 and r = -0.39, P = 0.037), respectively.
Transfection of MCF-7 cells with anti-miR-92 increased endogenous ERβ1 mRNA and reduced cell proliferation. EGFP report experiment also confirms that the 3'UTR of ERβ1 carries the directly binding sites of miR-92. Finally, we showed that miR-92 expression is modulated by the ER ligands 17β-estradiol and tamoxifen in MCF-7 cells. These findings prove that ERβ1 expression is negatively regulated at a post-transcriptional level by miR-92. This miRNA could be considered a potential therapeutic target in breast cancer.