Unravelling the Impact of miR-21 Overexpression on the MicroRNA Network and Cancer Pathways

Abstract MicroRNAs (miRNA, miRs) are small noncoding RNAs that are ubiquitously expressed in all mammalian cells. Their primary function is the regulation of nascent RNA transcripts by direct binding to regions on the target. There is now exciting data to suggest that these miRNAs can bind to other miRNAs, and this may have a broader impact on gene regulation in disease states. The oncomiR miR-21 is one of the highest-expressing miRNAs in cancer cells, and in this study, we characterise which miRNAs could be potential targets of miR-21. In cancer cells delivered with a miR-21 mimic, there was an observable shift of the miRNA milieu. We demonstrate that the miR-17-92a cluster, which harbours six miRNA members, may be a target of miR-21 regulation. Additionally, the primary transcript of miR-17-92a was reduced in the presence of miR-21. In the broader context of gene regulation, overexpression of miR-21 shifted the expression of more than 150 miRNAs, including those known to regulate genes encoding proteins in cancer pathways such as the MAPK signalling and FoxO pathways. This study expands upon our limited understanding of miR:miR regulatory network and reinforces the concept that miRNAs can regulate each other, thereby influencing broader gene regulatory networks.