Objective:To investigate the radiosensitive effect of all trans retinoic acid(ATRA) on lung cancer cell line A549 cultured in vitro and its possible mechanism. Methods: The A549 cells in the logarithmic growth phase were divided into four groups: control group

ATRA group

radiation group

combination group(ATRA combined with radiation). MTT assay was used to detect the growth inhibition rate and to further filter out the best experimental concentration of ATRA. The radiosensitive effect was detected by clone formation test. The cell cycle and apoptosis was measured by flow cytometry. The expression of VEGF mRNA was detected by RT-PCR. The VEGF values in the cell supernatant were detected by ELISA. Results: The optimal dose of ATRA was 10 μmol/L. ATRA can enhance the radiosensitivity of A549 cells. The proportion of G0/G1 phase in ATRA group was significantly increased compared with that in control group（P<0.05）;statistic differences in apoptosis rate were observed between ATRA group and control group（P<0.05）. The expressions of VEGF mRNA were significantly reduced in combination group compared with those in radiation group（P<0.05）. The VEGF levels in the supernatant of combination group were significantly decreased compared with those in radiation group（P<0.05）. Conclusion: All-trans retinoic acid can increase the radiosensitivity of A549 cells. The mechanism may be related with the direct inhibition of ATRA on A549 cells

redistribution of the inducing cell cycle

inducement of the apoptosis and down regulation of the expression of VEGF in tumor cells.