miR3609 sensitizes breast cancer cells to adriamycin by blocking the programmed death-ligand 1 immune checkpoint

Chemoresistance is the leading cause of breast cancer therapy failure, and studies of the mechanisms underlying chemoresistance and the treatment of drug-resistant tumors remain challenging. In the present study, we discovered a novel microRNA, miR3609, that influences the malignancy of breast cancer. Our results showed miR3609 expression was lower in resistant breast cancer cells than in sensitive breast cancer cells (MCF-7), while PD-L1 expression was higher in resistant breast cancer cells than in sensitive breast cancer cells. Co-transfection of a miR-3609 plasmid with a luciferase construct containing the PD-L1 3'-untranslated region suppressed luciferase activity. Transfection of a miR-3609 mimic markedly suppressed PD-L1 protein expression in MDA-MB-231 and MDA-MB-468 cells in a dose-dependent manner and increased the sensitivity of MCF7/ADR cells to adriamycin, whereas transfection of a miR-3609 inhibitor enhanced PD-L1 protein expression in HBL-100 and MCF-7 cells. In addition, knockdown of PD-L1 by siRNA restored the sensitivity of MCF7/ADR cells to adriamycin. Mice injected with breast cancer cells stably overexpressing miR3609 survived markedly longer and had fewer tumors than mice injected with control miRNA (miR-sc)-transfected cells. Treatment with a CD8(+) blocking antibody (anti-CD8) eliminated these effects, suggesting that CD8(+) T cells are required for the efficacy of miR3609 in breast cancer. Further, low miR-3609 expression and high PD-L1 expression were correlated with poor prognosis in breast cancer patients. Therefore, restoration of miR-3609 expression may sensitize breast cancer to adriamycin by blocking PD-L1 expression.