Abstract: Objective To investigate the expression of circular-RNA DDX5(circ-DDX5) in breast cancer tissues and its relationship with the clinical stage of breast cancer patients, and to analyze the regulatory mechanism of circ-DDX5 on the proliferation and invasion of human breast cancer cell line. Methods The expression level of circ-DDX5 in breast cancer tissues and its correlation with the clinical stage of breast cancer patients were analyzed by TCGA database. Bioinformatics analysis and dual-luciferase reporter gene experiments verified the targeting relationship between circ-DDX5 and miR-3940. The correlation between circ-DDX5 and miR-3940 expression in breast cancer tissues was analyzed by TCGA database. The expression level of circ-DDX5 in breast cancer SK-BR-3, MDA-MB-231, BT-549, MCF-7, and HCC-1937 cells was detected by RT-qPCR. The circ-DDX5 over-expression plasmid and negative control plasmid were transfected into MDA-MB-231 cells, which were named circ-DDX5 group and NC group, respectively. The proliferation and invasion of MDA-MB-231 cells in the circ-DDX5 group and the NC group were detected by colony formation assay and Transwell assay. The expressions of proliferation phenotype protein and invasion phenotype protein of MDA-MB-231 cells were detected by Western blot. The expression level of miR-3940 in MDA-MB-231 cells of circ-DDX5 group and NC group was detected by RT-qPCR. Results The expression of circ-DDX5 in breast cancer tissues was lower than that in adjacent tissues (P＜0.01) and the expression level of circ-DDX5 was negatively correlated with the clinical stage of breast cancer patients (P＜0.01). There was a targeting relationship between circ-DDX5 and miR-3940 (P＜0.01). The expression of circ-DDX5 and miR-3940 in breast cancer tissue was negatively correlated (P＜0.01). The expression of circ-DDX5 in human breast cancer cell lines was lower than that in immortalized breast epithelial cells MCF-10A (P＜0.05 or P＜0.01). Compared with the NC group, the over-expression of circ-DDX5 could significantly inhibit the proliferation and invasion of MDA-MB-231 cells(P＜0.01), as well as the proliferation phenotype proteins (cyclin C, CDK3) and invasion phenotype proteins (Snail, vimentin) expression (P＜0.01) and miR-3940 expression (P＜0.01). Conclusions The expression of circ-DDX5 in breast cancer tissues and cells is low. circ-DDX5 inhibits the proliferation and invasion of breast cancer MDA-MB-231 cells by targeting the expression of miR-3940.

Key words: breast cancer, circular-RNA DDX5, miR-3940, proliferation, invasion