MicroRNA-378-mediated suppression of Runx1 alleviates the aggressive phenotype of triple-negative MDA-MB-231 human breast cancer cells

The Runx1 transcription factor, known for its essential role in normal hematopoiesis, was reported in limited studies to be mutated or associated with human breast tumor tissues. Runx1 increases concomitantly with disease progression in the MMTV-PyMT transgenic mouse model of breast cancer. Compelling questions relate to mechanisms that regulate Runx1 expression in breast cancer. Here, we tested the hypothesis that dysregulation of Runx1-targeting microRNAs (miRNAs) allows for pathologic increase of Runx1 during breast cancer progression. Microarray profiling of the MMTV-PyMT model revealed significant downregulation of numerous miRNAs predicted to target Runx1. One of these, miR-378, was inversely correlated with Runx1 expression during breast cancer progression in mice and in human breast cancer cell lines MCF7 and triple-negative MDA-MB-231 that represent early- and late-stage diseases, respectively. MiR-378 is nearly absent in MDA-MB-231 cells. Luciferase reporter assays revealed that miR-378 binds the Runx1 3' untranslated region (3'UTR) and inhibits Runx1 expression. Functionally, we demonstrated that ectopic expression of miR-378 in MDA-MB-231 cells inhibited Runx1 and suppressed migration and invasion, while inhibition of miR-378 in MCF7 cells increased Runx1 levels and cell migration. Depletion of Runx1 in late-stage breast cancer cells resulted in increased expression of both the miR-378 host gene PPARGC1B and pre-miR-378, suggesting a feedback loop. Taken together, our study identifies a novel and clinically relevant mechanism for regulation of Runx1 in breast cancer that is mediated by a PPARGC1B-miR-378-Runx1 regulatory pathway. Our results highlight the translational potential of miRNA replacement therapy for inhibiting Runx1 in breast cancer.