MiR-340 Inhibits Triple-Negative Breast Cancer Progression by Reversing EZH2 Mediated miRNAs Dysregulated Expressions

The anti-tumor efficacy of miR-340 has been recently characterized in cancers. However, the underlying mechanisms of miR-340 inhibited cell growth and invasion in triple-negative breast cancer (TNBC) have not been well elucidated. In this study, we found that miR-340 expression was negatively correlated with EZH2 (Enhancer of zeste homolog 2) expression in TNBC tissues and cell lines. Subsequent luciferase reporter assay confirmed that EZH2 was a novel molecule target of miR-340. Upregulated miR-340 levels by mimics transfection significantly inhibited the MDA-MB-231 and MDA-MB-468 breast cancer cells proliferation, invasion and migration, and induced more cell apoptosis. Meanwhile, miR-340 inhibited the tumor growth in an orthotopic MDA-MB-231 breast cancer mouse model. Furthermore, we found the reduced EZH2 expression by miR-340 mimics transfection decreased the DNMT1, H3K27me3, β-catenin and P-STAT3 expressions, which ultimately resulted in miR-21 activity blockage and miR-200a/b expression upregulation. The results of rescue experiments further confirmed that miR-340 inhibited triple-negative breast cancer progression through targeting EZH2. Taken together, our results identified miR-340 as a tumor suppressor in TNBC, moreover, an EZH2 medicated regulatory loop was established. Post-transcriptional suppression of EZH2 expression not only blocked STAT3 mediated miR-21 trans-activation, but also reversed the miR-200a/b silencing via reducing DNMT1 and H3K27me3 expressions. MiR-21 inhibition and miR-200a/b expression triggered by miR-340 ultimately cooperated in the TNBC progression.