Abstract: ObjectiveTo investigate the effect of microRNA3917 （miR-3917） on the proliferation of lung cancer cells and the expression of biological clock gene Period2 （PER2）. 
MethodsThe recovered A549 cells were cultured to logarithmic growth phase. The pEGFPmiR3917siRNA plasmid vector targeting miR3917 was transfected into A549 cells by 10， 20 and 50 nmol／L liposome. The expression of green fluorescent protein was observed under fluorescence microscope after 48 h transfection. According to the experimental design， they were divided into control group， empty load group and interference group. The plasmid concentration in empty load group and interference group was 50 nmol／L. The realtime quantitative PCR （QPCR） was used to detect the relative expression of miR3917 at 48 h after transfection. The mRNA and protein levels of PER2 were detected by QPCR and Western blotting after at 48 h after transfection， respectively. The inhibitive rates of proliferation at 24， 48 and 72 h after transfection were detected by CCK8 assay. Dual luciferase target assay was employed to verify that PER2 was a direct target of miR3917. 
ResultsThe relative expression levels of miR3917 in control group， empty load group and interference group were 1.007±0.018， 1.068±0.084 and 0171±0.052. The relative expression level of miR3917 in the interference group was lower than that in the control group and empty load group， and the difference was statistically significant （P＜0.05）. The relative expression levels of PER2 were 1.060±0.129， 1.086±0.229 and 2.920±0.927 in the control group， empty load group and interference group， and the protein levels were 0.204±0.046， 0.183±0.043 and 0.512±0.117. The mRNA and protein levels of PER2 in the interference group were higher than those in the control group and empty load group， and the difference was statistically significant （P＜0.05）. Compared with the control group and empty load group， the proliferative ability decreased in interference group. The inhibitive rates of proliferation were （34.192±5.268）％， （33.527±6.603）％ and （30.591±7.788）％ at 24， 48， 72 h in interference group， higher than other two groups， and the difference was statistically significant （P＜0.05）. miR3917 could significantly inhibit the luciferase activity of wildtype PER23’UTR plasmid transfected cells， but had no significant effect on the luciferase activity of mutant PER2-3’UTR plasmid transfected cells. 
ConclusionDownregulation of miR3917 can inhibit the proliferation of lung cancer cells and increase the expression of PER2. miR-3917 has a targeted regulation effect on PER2， and can be used as a new target for the prevention and treatment of lung cancer.

Key words: Lung cancer, A549 cell line, Cell proliferation, MicroRNA3917, Biological clock gene Period2 （PER2）