Serum exosomal miR‐3662 down‐regulates the expression of RBMS3 to promote malignant progression and gemcitabine resistance of breast cancer cells

Abstract Breast cancer (BC) is a prevalent malignancy among women worldwide. As an anticancer drug of pyrimidine nucleoside analogs, gemcitabine can be used to treat BC, but its clinical application is restricted due to drug resistance. This study investigated the effect of serum exosomal microRNA‐3662 (miR‐3662) on gemcitabine resistance in BC cells by targeting RNA‐Binding Motif Single‐Stranded Interacting Protein 3 (RBMS3) and related molecular mechanisms. We performed the bioinformatics analyses on the differential miRNAs in BC and predicted the downstream regulators. Quantitative real‐time polymerase chain reaction was conducted to determine miR‐3662 and RBMS3 expression, while dual luciferase was conducted to confirme the regulatory relationship between them. Flow cytometry, cell counting kit‐8, and transwell assays were applied to assess apoptosis, cell viability, invasion, and migration. The expression of marker proteins (TSG101, CD63, and CD81) in patients' serum exosomes was evaluated through western blot, and exosomes were observed using transmission electron microscopy. miR‐3662 expression was significantly upregulated in BC, and miR‐3662 knockdown significantly reduced BC cell viability and gemcitabine resistance. As the downstream gene of miR‐3662, RBMS3 was significantly downregulated in BC, and dual luciferase assay verified the binding of RBMS3‐3′UTR to miR‐3662. Rescue experiments revealed that silencing RBMS3 reversed the inhibitory effect of miR‐3662 knockdown on BC cells. Besides, we also found that miR‐3662 expression was significantly low in serum exosome samples from BC patients and could be transmitted to tumor cells. miR‐3662 was upregulated in serum exosomes and promoted BC cell progression and gemcitabine resistance by targeting RBMS3.