CCAR1 5′ UTR as a natural miRancer of miR-1254 overrides tamoxifen resistance

MicroRNAs (miRNAs) typically bind to unstructured miRNA-binding sites in target RNAs, leading to a mutual repression of expression. Here, we report that miR-1254 interacts with structured elements in cell cycle and apoptosis regulator 1 (CCAR1) 5′ untranslated region (UTR) and this interaction enhances the stability of both molecules. miR-1254 can also act as a repressor when binding to unstructured sites in its targets. Interestingly, structured miR-1254-targeting sites act as both a functional RNA motif-sensing unit, and an independent RNA functional unit that enhances miR-1254 expression. Artificially designed miRNA enhancers, termed “miRancers”, can stabilize and enhance the activity of miRNAs of interest. We further demonstrate that CCAR1 5′ UTR as a natural miRancer of endogenous miR-1254 re-sensitizes tamoxifen-resistant breast cancer cells to tamoxifen. Thus, our study presents a novel model of miRNA function, wherein highly structured miRancer-like motif-containing RNA fragments or miRancer molecules specifically interact with miRNAs, leading to reciprocal stabilization.