Estradiol downregulates miR-21 expression and increases miR-21 target gene expression in MCF-7 breast cancer cells

Select changes in microRNA (miRNA) expression correlate with estrogen receptor (ER) expression in breast tumors. miR-21 is higher in ER positive than negative tumors, but no one has examined how estradiol (E-2) regulates miR-21 in breast cancer cells. Here we report that E-2 inhibits miR-21 expression in MCF-7 human breast cancer cells. The E-2-induced reduction in miR-21 was inhibited by 4-hydroxytamoxifen (4-OHT), ICI 182 780 (Faslodex), and siRNA ER indicating that the suppression is ER-mediated. ER and ER agonists PPT and DPN inhibited and 4-OHT increased miR-21 expression. E-2 increased luciferase activity from reporters containing the miR-21 recognition elements from the 3-UTRs of miR-21 target genes, corroborating that E-2 represses miR-21 expression resulting in a loss of target gene suppression. The E-2-mediated decrease in miR-21 correlated with increased protein expression of endogenous miR-21-targets Pdcd4, PTEN and Bcl-2. siRNA knockdown of ER blocked the E-2-induced increase in Pdcd4, PTEN and Bcl-2. Transfection of MCF-7 cells with antisense (AS) to miR-21 mimicked the E-2-induced increase in Pdcd4, PTEN and Bcl-2. These results are the first to demonstrate that E-2 represses the expression of an oncogenic miRNA, miR-21, by activating estrogen receptor in MCF-7 cells.