LAUSR

BACKGROUND Doxorubicin, a first-line chemotherapeutic drug for breast cancer, kills cancer cells by inducing DNA-crosslinking damage. Dysregulated micro-RNA (miRNA) is associated with the drug resistance of tumors. However, little is known about the effect of miRNA-140-3p on DOX resistance of breast cancer. METHODS The miRNA microarray was used to sequence the transcripts of DOX-chemoresistant breast cancer tissues and DOX-chemosensitive tissues. Then, the breast cancer tissue chip in the GEO database was also analyzed to screen the target gene. Flow cytometry, in situ hybridisation (ISH), immunohistochemistry (IHC), Western blot, cell proliferation assay, real-time PCR analyses (qRT-PCR), and pull-down assay were used to explore the effects of miRNA-140-3p and programmed death ligand-1 (PD-L1) on the chemoresistance of DOX-resistant breast cancer cells treated with DOX. In vivo, the DOX-resistant breast cancer cell lines treated with miRNA-140-3p overexpression were injected subcutaneously into mice to construct breast cancer subcutaneous xenograft tumor models. RESULTS Based on miRNA microarray, GEO database, and bioinformatics analysis, it was found that miRNA-140-3p and PD-L1 are the core molecules in the DOX resistance regulatory network in breast cancer, and lower miRNA-140-3p and higher PD-L1 expression levels were observed in DOX-resistant breast cancer tissues and cells. IHC results showed that compared with breast cancer tissues with high miRNA-140-3p expression, PD-L1 protein expression levels in breast cancer tissues with low miRNA-140-3p were significantly higher (P<0.01). Moreover, compared with DOX-sensitive tissues, the levels of PD-L1 protein expression in DOX-resistant tissues were significantly higher (P<0.01). In in vitro and in vivo experiments, the introduction of miRNA-140-3p decreased PD-L1 expression. Mechanically, we found that the MCF-7/DOX and HS598T/DOX cells pretreated with miRNA-140-3p inhibitor or exosomes containing PD-L1 have higher stemness and lower apoptosis rate, which can be abrogated by co-treating cells with anti-PD-L1 antibody or miRNA-140-3p mimic. CONCLUSIONS MiRNA-140-3p can suppress PD-L1 expression in breast cancer cell-derived exosomes, thereby attenuating the chemoresistance induced by DOX in breast cancer.