Objective To investigate the mechanism by which hypoxia-inducible factor(HIF)-1α and circ-UBE2G1 mutually regulate and promote thyroid cancer(THCA)metastasis under hypoxic microenvironment.Methods The GEPIA database was used to analyze the expression characteristics and correlation between circ-UBE2G1 and HIF-1α in THCA.The relationship of circ-UBE2G1 and miR-330-3p with survival rate was analyzed using Kaplan-Meier survival curve.After THCA cells were exposed to hypoxia,HIF-1α was silenced by transfection to analyze its regulation for circ-UBE2G1 transcription.The targeting relationship between miR-330-3p and either circ-UBE2G1 or HIF-1α was verified by sequence prediction and dual luciferase reporter assay.The effects of HIF-1α overexpression and circ-UBE2G1 silencing on THCA cell migration and invasion were analyzed after corresponding transfections.A tumor-bearing nude mouse model was established by subcutaneous injection of THCA cells with HIF-1α overexpression and circ-UBE2G1 silencing,respectively.THCA tissues and adjacent normal samples were clinically collected to analyze the expression levels and correlations of circ-UBE2G1,miR-330-3p,and HIF-1α.Results Bioinformatics analysis showed that circ-UBE2G1 was positively correlated with HIF-1α(P<0.01),and its high expression was associated with a low survival rate in THCA patients(P=0.024).Inhibition of HIF-1α blocked the promotive effect of hypoxia on circ-UBE2G1(P<0.05).Silencing circ-UBE2G1 inhibited the migration and invasion of THCA cells(P<0.05),and reversed the promotive effect of HIF-1α on these processes(P<0.05).Dual luciferase reporter gene assay revealed that miR-330-3p targeted both circ-UBE2G1 and HIF-1α(P<0.05).Silencing circ-UBE2G1 led to increased levels of miR-330-3p(P<0.05),whereas increasing miR-330-3p inhibited the expression of HIF-1α(P<0.05).In clinical THCA samples,both circ-UBE2G1 and HIF-1α were increased and positively correlated(P<0.05),while miR-330-3p was lowly expressed and negatively correlated with both circ-UBE2G1 and HIF-1α(P<0.05).Conclusion Hypoxia promotes the transcription of circ-UBE2G1 by inducing HIF-1α expression,and circ-UBE2G1 promotes HIF-1α expression by targeting miR-330-3p,thereby promoting THCA metastasis.