Abstract
Immunoevasion has been a severe obstacle for the clinical treatment of breast cancer (BC). CD47, known as an anti-phagocytic molecule, plays a key role in governing the evasion of tumor cells from immune surveillance by interacting with signal-regulated protein α (SIRPα) on macrophages. Here, we report for the first time that miR-299-3p is a direct regulator of CD47 with tumor suppressive effects both in vitro and in vivo. miRNA expression profiles and overall survival of BC cohorts from the Cancer Genome Atlas, METABRIC, or GSE19783 datasets showed that miR-299-3p is downregulated in BC tissues and that BC patients with low levels of miR-299-3p have poorer prognoses. Using dual-luciferase reporter, qRT-PCR, Western blot, and phagocytosis assays, we proved that restoration of miR-299-3p can suppress CD47 expression by directly targeting the predicted seed sequence "CCCACAU" in its 3'-UTR, leading to phagocytosis of BC cells by macrophages, whereas miR-299-3p inhibition or deletion reversed this effect. Additionally, Gene Ontology (GO) analysis and a variety of confirmatory experiments revealed that miR-299-3p was inversely correlated with cell proliferation, migration, and the cell cycle process. Mechanistically, miR-299-3p can also directly target ABCE1, an essential ribosome recycling factor, alleviating these malignant phenotypes of BC cells. In vivo BC xenografts based on nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice further proved that restoration of miR-299-3p resulted in a significant suppression of tumorigenesis and a promotion of macrophage activation and infiltration. Overall, our study suggested that miR-299-3p is a potent inhibitor of CD47 and ABCE1 to exhibit bifunctional BC-suppressing effects through immune activation conjugated with malignant behavior inhibition in breast carcinogenesis and thus can potentially serve as a novel therapeutic target for BC.