Breast cancer (BC) is the most diagnosed cancer in women worldwide. In estrogen receptor (ER)-positive disease, antiestrogens and aromatase-inhibitors (AI) improve patient survival, however many patients develop resistance. Dysregulation of apoptosis is a common resistance mechanism, thus agents that can reinstate activity of apoptotic pathways represent promising therapeutics for advanced drug-resistant BC. Emerging targets in this scenario include microRNAs (miRs).

To identify miRs modulating apoptosis in BC endocrine-resistance, high-throughput screening of a miR inhibitor library in hormone-responsive/-resistant BC cells was performed, followed by high-content microscopy for apoptotic markers. Validation demonstrated that a miR-361-3p inhibitor significantly increases early apoptosis and reduces proliferation of drug-responsive (MCF7), plus AI-/antiestrogen-resistant derivatives (LTED, TamR, FulvR), and ER- cells (MDA-MB-231). Importantly, proliferation-inhibitory effects were observed in vivo in a xenograft model, indicating potential clinical application of miR-361-3p inhibition. RNA-seq on tumour xenografts identified FANCA as a direct miR-361-3p target, and validation suggested that miR-361-3p inhibitor increases FANCA mRNA and protein levels. Moreover, pathway analysis revealed that miR-361-3p inhibition induces DNA damage, observed as increased H2AX foci in MCF7 nuclei, as well as p53-mediated G1 cell cycle arrest through activation of p21. Additionally, the miR-361-3p inhibitor significantly decreases BC invasion.

Analysis of publicly-available datasets showed that miR-361-3p expression is significantly higher in primary breast tumours versus paired normal tissue and is associated with decreased overall survival. In addition, miR-361-3p inhibitor treatment of BC patient explants decreased levels of miR-361-3p and proliferation marker, Ki67. Finally, the miR-361-3p inhibitor showed synergistic effects on BC growth when combined with the PARP inhibitor, Olaparib. Together, these studies identify miR-361-3p inhibitor as a potential new treatment for drug-responsive and -resistant advanced BC.