Human MDA-MB-231 breast cancer cells overexpressing miR-183-5p and its isomiRs

Overexpression of miR-183-5p|+2, but not of the other two isomiRs |0 and |+1, was observed to reduce cell cycle and cell proliferation in different triple-negative breast cancer cell lines. Therefore, we hypothesized that the |+2 isoform has targets distinct from the other two isoforms. To test this hypothesis, we overexpressed separately the three different isoforms or negative controls (siAllstar or mimic-Cltr) and performed Mass Spectrometry to identify differentially regulated proteins. Interestingly, a gene set enrichment analysis of the changes in protein expression revealed significant downregulation of transcriptional targets of E2F specifically in cells transfected with the |+2 isoform prompting us to validate the predicted isomiR specific target E2F1. Subsequently, we could show that direct targeting of E2F1 by miR-183-5p|+2 is responsible for the impact of the isomiR on cell cycle and proliferation.