Circ_0000033 up-regulates NUAK2 by sequestering miR-378a-3p to promote breast tumorigenesis

Circular RNAs (circRNAs), including circ_0000033, were shown to be abnormally expressed in breast cancer (BC) and play an important regulatory function in the development of this cancer. This study aimed to investigate the action and mechanism of circ_0000033 in BC carcinogenesis. Specifically, levels of genes and proteins were analyzed using quantitative real-time PCR (qRT-PCR) and western blotting. Circ_0000033 was highly expressed in BC tissues and cells. Properties of cells with modified expression of circ_0000033 were characterized using an in vitro colony formation assay, EdU assay, flow cytometry, caspase-3 activity analysis, transwell assay, and tube formation assay, respectively. Functionally, knockdown of circ_0000033 suppressed BC cell proliferation, migration, invasion, angiogenesis, and induced apoptosis and cell cycle arrest in vitro. An in vivo experiment was conducted using a murine xenograft model and showed circ_0000033 silencing also impeded the growth of BC in nude mice. The binding between miR-378a-3p and circ_0000033 or NUAK2 (NUAK Family Kinase 2) was validated using a dual-luciferase reporter assay. Circ_0000033 sequestered miR-378a-3p and resulted in NUAK2 release, indicating a circ_0000033/miR-378a-3p/NUAK2 regulatory network operates in BC cells. Circ_0000033 down-regulation in BC cells was accompanied by decreased NUAK2 and increased miR-378a-3p expression. Moreover, the anticancer effects mediated by circ_0000033 knockdown were abolished by miR-378a-3p inhibition or NUAK2 overexpression in BC cells. Overall, circ_0000033 up-regulates NUAK2 through sequestration miR-378a-3p, which promoted breast tumorigenesis, suggesting circ_0000033 is a promising therapeutic target for BC treatment.