Background and purpose:Human epidermal growth factor receptor 2(HER2)serves as one of the paramount drivers of breast cancer metastasis,with roughly 20%-30%of breast cancer patients exhibiting high expression of HER2.The expression level of HER2 is regulatable at multiple molecular levels and determines the metastatic potential of breast cancer cells;however,the manner in which HER2 expression is modulated at the mRNA level remains ambiguous.Circ-0007766 is a circRNA originated from the coding gene ERBB2 for HER2,and whether circ-0007766 can regulate HER2 expression via the ceRNA mechanism has not been reported.This study aimed to analyze whether circ-0007766 acts as a miR-1972 sponge to promote breast cancer cell migration and invasion via upregulation of HER2 expression.Methods:In this study,a high-throughput circRNA chip was employed to screen for circRNAs that exhibited highly specific expression in HER2-positive breast cancer cells.RNA fluorescence in situ hybridization(FISH)was utilized to detect the subcellular localization of circ-0007766.The BaseScope experiment was conducted to analyze the expression level of circ-0007766 in breast cancer tissues and its clinical diagnostic significance.Breast cancer cell models with overexpression and knockdown of circ-0007766 were constructed by transfecting cloning plasmids and siRNA in vitro.The effect of circ-0007766 on the migration and invasion of breast cancer cells was assessed using transwell migration and invasion experiments,and the migration and invasion abilities of MDA-MB-231 and SK-BR-3 cells were measured.Additionally,it was evaluated whether circ-0007766 could promote the migration and invasion of breast cancer cells through miR-1972.A dual luciferase reporter gene assay was used to verify whether circ-0007766 could regulate HER2 expression by binding to miR-1972.The direct interaction between circ-0007766 and miR-1972 was further verified through the RAP experiment.RIP detection was performed in MDA-MB-231 cells,and the relative 3'UTR of HER2 mRNA was measured by real-time fluorescence quantitative polymerase chain reaction(RTFQ-PCR).Western blot was used to detect the protein expressions.Results:Circ-0007766 was conspicuously highly expressed in HER2-positive breast cancer cells and distributed in both the cytoplasm and nucleus of cells,with the preponderance being in the cytoplasm.The expression level of circ-0007766 was strikingly higher in breast cancer tissues than in para-cancerous tissues.The expression of circ-0007766 was significantly elevated in HER2-positive breast cancer samples compared with HER2-negative samples.The overexpression(knockdown)of circ-0007766 in HER2-negative breast cancer cells(in HER2-positive breast cancer cells)was capable of promoting(inhibiting)the migration and invasion of breast cancer cells.Circ-0007766 directly bound to miR-1972,which inhibited breast cancer cell migration and invasion,thereby forming an endogenous competitive RNA(ceRNA)regulatory network and impeding the downregulation of HER2 mRNA and protein expression mediated by miR-1972.Circ-0007766 could potentiate the inhibitory effect of miR-1972 on HER2-mediated breast cancer cell migration and invasion that was negatively regulated by miR-1972.CircRNAs sequestered miRNAs to function as ceRNAs,thereby regulating gene expression at both the transcriptional and translational levels.Finally,we discovered that the expressions of circ-0007766 and HER2 were positively correlated in breast cancer cell and tissue samples,while the expression levels of miR-1972 and HER2 were negatively correlated.Circ-0007766 could specifically target miR-1972 to hinder its regulatory effect on HER2 expression.Conclusion:This study discovers that circ-0007766 facilitates the migration and invasion of breast cancer cells via the miR-1972/HER2 signal axis,offering a novel biomarker and potential therapeutic target for patients with metastatic HER2-positive breast cancer.