Abstract: ObjectiveTo investigate the mechanism of microRNA320（miR320） in regulating radiosensitivity in esophageal squamous cell carcinoma（ESCC） cells. 
MethodsRealtime fluorescence quantitative PCR（QPCR） was used to detect the expressions of miR320 and forkhead box protein M1（FOXM1） in normal esophageal epithelial cells HEEC and ESCC cell lines of KYSE150， TE13 and Eca109. MiR320 mimics and its negative control （NC） were transfected into Eca109 cells by Lipofectamine 2000 and assigned to miR320 transfection group and control group， respectively. QPCR and Western blotting were used to detect the mRNA and protein levels of FOXM1 to evaluate the regulatory effect of miR320. Clone formation assay and immunofluorescence assay were performed to evaluate the cloneforming ability of Eca109 and the formation of γH2AX. The apoptotic rates of Eca109 cells were detected by flow cytometry and the expression levels of XIAP， Bax, Bcl2 and cleaved caspase3 were detected by Western blotting. Dual luciferase reporter assay was performed to verify the targeting relationship between miR320 and FOXM1. 
ResultsCompared with HEEC cells， the expression of miR320 in Eca109 cells was the lowest， while the expression of FOXM1 was the highest（P＜005）， and Eca109 cells were chosen for the following experiments. Fortyeight hours after transfection， transfection group showed an increased miR320 expression compared with control group； and the mRNA and protein levels of FOXM1 in transfection group were both lower than control group（P＜005）. The colonyforming ability of Eca109 cells in transfection group was lower than that in control group（P＜005）.The number of γH2AX foci in transfection group at 1， 6 and 24 h after irradiation were significantly higher than those in control group（P＜005）. The apoptotic rate of Eca109 cells in transfection group was significantly higher than that in control group（P＜005）. Compared with control group， the expression of Bax and cleaved caspase3 increased and the expression of XIAP and Bcl2 were decreased in transfection group（P＜005）. The luciferase activity of Eca109 cells transfected with wildtype FOXM1 3’UTR plasmid and miR320 mimics was significantly lower than that of transfected with NC. No significant difference was observed between cells transfected with mutanttype FOXM1 3’UTR plasmid. 
ConclusionmiR320 may enhance the radiosensitivity of ESCC cells and the underlying mechanism may associated with inhibition of FOXM1.

Key words: Esophageal squamous cell carcinoma, Radiosensitivity, MicroRNA320（miR320）, Forkhead box protein M1（FOXM1）