Tamoxifen is the most commonly used endocrine therapy for patients with hormone receptor (HR)-positive breast cancer. Despite its initial therapeutic efficacy, many patients eventually develop drug resistance, which remains a serious clinical challenge. To investigate roles of circular RNAs (circRNAs) in tamoxifen resistance, a tamoxifen-resistant MCF-7 cell line was established and screened for its circRNA expression profile by RNA sequencing. hsa_circ_0025202, a circRNA that was significantly downregulated, was selected for further investigation. Using a large cohort of clinical specimens, we found that hsa_circ_0025202 exhibited low expression in cancer tissues and was negatively correlated with lymphatic metastasis and histological grade. Gain- and loss-of-function assays indicated that hsa_circ_0025202 could inhibit cell proliferation, colony formation, and migration and increase cell apoptosis and sensitivity to tamoxifen. Bioinformatics and luciferase reporter assays verified that hsa_circ_0025202 could act as a miRNA sponge for miR-182-5p and further regulate the expression and activity of FOXO3a. Functional studies revealed that tumor inhibition and tamoxifen sensitization effects of hsa_circ_0025202 were achieved via the miR-182-5p/FOXO3a axis. Moreover, in vivo experiments confirmed that hsa_circ_0025202 could suppress tumor growth and enhance tamoxifen efficacy. Taken together, hsa_circ_0025202 served an anti-oncogenic role in HR-positive breast cancer, and it could be exploited as a novel marker for tamoxifen-resistant breast cancer.
