MicroRNA Signatures of Fulvestrant-Treated Luminal Breast Cancer Cells: Identification of Therapeutic Targets Regulated by miR-374b-5p.

Estrogen receptor (ER)-positive breast cancer (BrCa) accounts for two-thirds of all BrCa cases worldwide. Therefore, ER-targeted endocrine therapy is the standard treatment for this disease. There has been a recent trend towards developing combination therapies using molecularly targeted drugs to improve outcomes. This study aimed to identify therapeutic targets demonstrating efficacy when combined with fulvestrant (a selective ER downregulator/degrader). We generated microRNA (miRNA) signatures from fulvestrant-treated MCF-7 cells by RNA sequencing. From the signature, we evaluated miR-374b-5p because its expression was elevated by fulvestrant treatment in MCF-7 cells. Also, in expression analysis by subtype of BrCa patients, miR-374b-5p expression was suppressed only in luminal BrCa. Ectopic expression assays revealed that miR-374b-5p attenuated the malignant phenotypes of MCF-7 cells. We searched for genes regulated by miR-374b-5p and discovered that 11 (NEK2, NUF2, HMMR, DEPDC1B, FOXM1, ELOVL6, KIF20A, NCAPH, CENPK, FAM83D, and KIAA0101) are closely involved in BrCa molecular pathogenesis. Among these target genes, we focused on forkhead box M1 (FOXM1), a transcription factor regulating cell cycle progression and division. Notably, combination therapy with fulvestrant and a FOXM1 inhibitor significantly suppressed MCF-7 cell proliferation. From the miRNA signature established in this study, we identified antitumor miR-374b-5p and its target genes and used these findings to explore candidate drugs with potential efficacy when combined with fulvestrant.