Emerging evidence suggests that
circular RNAs (
circRNAs) have crucial roles in various processes, including
cancer development and progression. However, the functional roles of
circRNAs in
breast cancer remain to be elucidated. In this study, we identified a novel
circRNA (named circBMPR2) whose expression was lower in
breast cancer tissues with
metastasis. Moreover, circBMPR2 expression was negatively associated with the motility of
breast cancer cells and significantly downregulated in human
breast cancer tissues. Functionally, we found that circBMPR2 knockdown effectively enhanced cell proliferation, migration, and invasion. Moreover, circBMPR2 knockdown promoted
tamoxifen resistance of
breast cancer cells through inhibiting
tamoxifen-induced apoptosis, whereas circBMPR2 overexpression led to decreased
tamoxifen resistance. Mechanistically, we demonstrated that circBMPR2 could abundantly sponge miR-553 and that miR-553 overexpression could attenuate the inhibitory effects caused by circBMPR2 overexpression. We also found that
ubiquitin-specific protease 4 (USP4) was a direct target of miR-553, which functions as a
tumor suppressor in
breast cancer. Our findings demonstrated that circBMPR2 might function as a miR-553 sponge and then relieve the suppression of USP4 to inhibit the progression and
tamoxifen resistance of
breast cancer. Targeting this newly identified
circRNA may help us to develop potential novel
therapies for
breast cancer patients.