Abstract: Objective To investigate the effect of microRNA-124（miR124）mediated by circ_MTHFD2 on the resistance of lung cancer against pemetrexed.
Methods The pemetrexedresistant human lung cancer cell line was established by exposing pemetrexed repeatedly to highlevel concentration. The xenograft model of pemetrexed resistant nude mice by intratumoral injection was established. MiR-124 mimics， miR-124 inhibitors and negative controls were transfected into A549 and A549／PEM cells by Lipofectamine liposome method，and divided into miR-124 upregulation group， miR-124 inhibitory group and negative control group； then the sensitivity to pemetrexed was estimated by CCK-8 assay and flow cytometry. Real time fluorescent quantitative PCR（QPCR） and gene chip were used to detect the expression of miR-124 and circular RNA. Sequencing and bioinformatic analysis were used to detect the expression of circular RNA in different cells and tissue and to predict the binding sites between miR124 and circular RNA.Results The IC50 of parental strains A549 was （30.78±1.97）μmol／L， and the resistant strains A549／PEM was （913.53±14.1）μmol／L. The resistance index （RI） of pemetrexed-resistant human lung cancer cell line A549 named A549／PEM was 29.69±1.49. The expression of miR-124 in drug-resistant cell line A549／PEM was significantly decreased， which was about 145.952 times lower than that in A549 cells. Compared with the negative control group， the sensitivity to pemetrexed was significantly reduced in miR-124 inhibitor group， and the IC50 was（9.94±1.90）μmol／L and （80.45±3.50）μmol／L respectively， with significant difference between two groups（P＜0.05）. Flow cytometry showed that up-regulation of miR-124 expression could induce the apoptosis of A549 and 549／PEM cells compared with the control group. The sequencing indicated that the expression of many circular RNAs were upregulated in resistant cells and tissues， and up-regulated circ_MTHFD2 was proposed to bind miR-124. Conclusion Circ_MTHFD2 may be involved in the development of pemetrexed resistance in lung cancer by regulating the expression of miR-124.