miR-888 in MCF-7 side population sphere cells directly targets E-cadherin.

Side population (SP) cells are a small subset of cells isolated from a cultured cancer cell line that exhibit characteristics similar to those of cancer stem cells (CSCs), such as high metastatic and tumorigenic potential. The molecular mechanisms that give rise to the malignant properties of SP cells are not clear. We isolated SP cells from the MCF-7 breast cancer cell line and profiled microRNA (miRNA) expression patterns between SP cell-derived spheroids and non-SP cells. SP spheroids were found to possess 42 up-regulated miRNAs and 27 down-regulated ones (above 5-fold changes). One of the up-regulated miRNAs, miR-888 computationally predicted to participate in the adherens junction (AJ) pathway, was investigated. Over-expression of miR-888 in MCF-7 cells reduced the mRNA levels of all four AJ pathway genes (E-cadherin, ACTG1, PTPRT and CDC42) that were selected for testing, whereas knocking down miR-888 reversed the trends. Western blot and flow cytometric quantitation of the membrane E-cadherin levels showed the same trend of change under these treatments. Luciferase reporter assay showed E-cadherin is a direct target of miR-888. As a potential role in intercellular adhesiveness and maintenance of malignant tissue architecture, the results indicate that miR-888 is a repressor of the AJ pathway in MCF-7 cells and that up-regulation of miR-888 contributes to aggressiveness in MCF-7 SP cells.