Objective　　　 To determine the effect of miR-18a on the radiosensitivity of A549 cells and investigate the underlying molecular mechanisms.　　　 Methods　　　 After A549 cells were treated with 2 Gy X-ray

miR-18a was determined by qRT-PCR. And then miR-18a mimics were transfected into A549 cells

after exposure to X-ray radiation at different doses

surrival fraction was assessed by clonogenic assay. Survival fraction at 2 Gy (SF

2

) and sensitive enhancement ration (SER) were calculated. The protein levels of

ataxia telangiectasia mutated (ATM) kinase and ATM (phospho S1981) were determined by Western blotting after up-regulating miR-18a.　　　 Results　　　 miR-18a was decreased from 1.00±0.04 to 0.18±0.04-0.31±0.09 in A549 cells after exposure to 2 Gy X-ray (P0.05). miR-18a over-expression increased the radiosensitivity in A549 cells. The SF

2

in miR-18a over-expression cells and normal A549 cells was 0.34 and 0.48

respectively. Moreover

the expression levels of ATM and ATM (phospho S1981) were down-regulated in miR-18a over-expressed A549 cells.　　　 Conclusion　　　 In A549 cells

miR-18a enhances radiosensitivity by inhibiting ATM and ATM (phospho S1981).