Tumor Suppressor MIR-489 Regulates Proliferation, Autophagy and Overcomes Drug-Resistance in Breast Cancer

Despite decades of extensive research, breast cancer remains an untamable disease due to complexity and heterogeneity of disease. It is for this very reason, most efforts targeting a single pathway did not yield satisfactory results. Discovery of a novel therapeutic agents targeting multiple pathways, such as miRNAs, holds promise for future cancer therapy. This study was aimed to explore therapeutic potential of one such microRNA, miR-489. In the first study, we identified autophagy as a novel pathway targeted by miR-489 and reported ULK1 and LAPTM4B to be a direct of miR-489 target. We showed that miR-489 mediated autophagy inhibition and autophagosome accumulation is necessary for its cytotoxic effect. Furthermore, we demonstrated autophagy inhibition and LAPTM4B down regulation as a novel mechanism responsible for miR-489 mediated doxorubicin sensitization. Finally, we found that miR-489 and LAPTM4B levels were inversely correlated in human tumor specimens, and more importantly, miR-489 expression levels predict overall survival in patients with 8q22 amplification (Soni et al.). We then established that miR-489 is an estrogen regulated miRNA and is highly expressed in hormone positive breast tumors and cell lines. For the first time we reported that miR-489 negatively regulates estrogen signaling. Depletion of miR-489 using Anti-miR-489 siRNA or CRISPR-Cas9 significantly increased proliferation, colony formation ability and stem like cell population. Mechanistically, we report that miR-489 modulates estrogen induced estrogen receptor localization into nucleus. We further established that downregulation of p38 MAPK is responsible for miR-489’s effect of estrogen receptor localization. We showed that estrogen activates p38 MAPK and this activated p38 MAPK is necessary for nuclear translocation of estrogen receptor. Furthermore, we also report that miR-489 prevents activation of ERα by inhibiting its phosphorylation. We demonstrated that miR-489 inhibits ERK and AKT activation which are responsible kinase for ERα phosphorylation at Ser 118 and Ser 167 site, respectively. We then demonstrated that miR-489 is lost in tamoxifen resistant breast tumors and cell lines and restoration of miR-489 significantly inhibits its proliferation and overcomes both de novo and acquired tamoxifen resistance. miR-489 exerts this effect through its unique ability to target HER2, MAPK and PI3K-AKT signaling pathway at the same time. These data advocates strongly for therapeutic potential of miR-489 in ERα positive and tamoxifen resistant breast cancer. In summary, our data shows that miR-489 expression is inversely correlated with aggressiveness of breast cancer and drug resistance and proposes miR-489 as a promising prognostic biomarker or therapeutic agent in breast cancer