AIM:To investigate the molecular mechanism of inhibition of miR-421 expression promoting radiosensitivity in the cervical cancer cells.METHODS:Cervical cancer lines HeLa, SiHa, C33A and CaSki were transfected with miR-421 inhibitor or negative control nucleotide using Lipofectamine 2000 kit, and the levels of miR-421 expression in the cervical cancer lines and endometrial epithelium cell line ESC were detected by real-time PCR.These cells with transfection were exposed to various doses of X-ray (0, 2, 4, 6 and 8 Gy).After 48 h, the cell viability, LDH leakage rate and apoptotic rate were measured respectively by MTT assay, ELISA and flow cytometry with Annexin V-FITC/PI staining.The protein levels of cleaved caspase-9, caspase-9, cleaved PARP, PARP, Bcl-2 and Bax were monitored by Western blot.RESULTS:Low miR-421 levels was found in the ESC cells, while high miR-421 levels were observed in the HeLa, SiHa, C33A and CaSki cells.The level of miR-421 in the cells transfected with miR-421 inhibitor was significantly lower than that in negative control group (P<0.05).The viability and LDH leakage rate of the cervical cancer cells with low miR-421 expression were notablely lower than those in negative control group, and the apoptotic rate at 72 h was remarkablely increased (P<0.05) under the same conditions.The results of Western blot indicated that, after exposure to ionizing radiation, the protein levels of cleaved caspase-9, cleaved PARP and Bcl-2 were significantly increased, while the protein level of Bax was significantly decreased in the cervical cancer cells with low miR-421 expression compared with negative control group (P<0.05).CONCLUSION:miR-421 is lowly expressed in the normal endometrial epithelial cells, but highly expressed in the cervical cancer cells.Down-regulation of miR-421 expression significantly inhibits the growth and enhances the radiosensitivity of cervical cancer cells at least partly via activating caspase-9 apoptosis pathway, thus promoting Bcl-2 and inhibiting Bax expression.