miRNA let-7a inhibits invasion, migration, anchorage-independent growth by suppressing EZH2 and promotes mesenchymal to epithelial transition in MDAMB-231

Enhancer of zeste homolog 2 (EZH2) is a transcriptional repressor belonging to the polycomb group protein family (PRC) and is frequently linked with aggressive breast cancer. Micro RNAs (miRNAs) are single-stranded 19-25 nucleotide long RNAs that control gene expression at the post-transcriptional level. Previous reports indicated that epigenetic modulation through EZH2-mediated histone methylation can silence the miR-200 cluster. Therefore, we attempted to identify the miRNAs that regulate the expression of EZH2 in metastatic breast cancer. Our results with microarray studies showed that upon knock-down of EZH2 lead to enhanced expression of tumor-suppressive miRNAs including miR-203, miR-200a, miR-200b, and let-7a in MDAMB-231, a metastatic breast cancer cell line. Further studies to confirm the expression of EZH2 upon overexpression of the mentioned miRNAs showed that only let-7a could post-transcriptionally regulate EZH2 expression significantly. Luciferase-based 3 ' UTR (untranslated region) reporter assay of EZH2 confirmed a significant reduction in the reporter activity upon co-transfection of let-7a and reporter construct 3 ' UTR of EZH2. Interestingly, over-expression of let-7a leads to decreased expression of another PRC2 group of protein SUZ12 and produced a lowered expression of H3K27me3 methylation. Additionally, let-7a miRNA promoted mesenchymal to epithelial transition i.e., by downregulating the key metastatic markers including Vimentin, Zeb, N-cadherin, and Snail, thus leading to reduced migration and invasion ability in MDA-MB-231 cells. This data poses an impact on understanding and developing therapeutics for breast cancer.