To study the effect of circular enhancer of zeste homolog 2 (CircEZH2) on oxaliplatin (OXA) resistance in colon cancer cells by regulating the miR-340-5p/Pumilio RNA binding family member 1 (PUM1) axis.

Human colon cancer cell line HCT116 was cultured in vitro and its OXA resistant cell line HCT116/OXA was constructed using concentration gradient method. RT-qPCR and Western blot were applied to detect the expression of CircEZH2, miR-340-5p, and PUM1 in human colon cancer cell line HCT116 and its OXA resistant cell line HCT116/OXA. HCT116/OXA cells were cultured in vitro and randomly grouped into control group, OXA group, OXA+CircEZH2 negative control group, OXA+miR-340-5p negative control group, OXA+CircEZH2 knockdown group, and OXA+ CircEZH2 knockdown + miR-340- 5p inhibitor group. RT-qPCR and Western blot were applied to detect the expression of CircEZH2, miR- 340-5p, and PUM1 of cells in each group. CCK-8 assay and flow cytometry were applied to detect cell proliferation and apoptosis in each group. Western blot was applied to detect the expression of apoptotic proteins (Cleaved Caspase-3, Bax) and drug resistant proteins [breast cancer resistance protein (BCRP), P-glycoprotein (P-gp) ] in each group. HCT116/OXA cells were cultured in vitro and randomlydivided into control group, CircEZH2 knockdown group, negative control group, and CircEZH2 knockdown+miR-340-5p inhibitor group, CCK-8 assay was applied to detect the survival rate of cells in each group and calculating their drug resistance index. The double luciferase reporter gene experiment was applied to detect the targeted regulation of CircEZH2 on miR-340-5p and miR-340-5p on PUM1 in HCT116/OXA cells. One-way ANOVA was used for comparison between multiple groups, and SNK-q test was used for comparison between pairwise groups.

Compared with control group, cell viability[ (45.84 ± 7.63) % vs (100.00 ± 0.00) %] and the expression of P-gp protein (0.31 ± 0.05 vs 0.92 ± 0.20) in the OXA+CircEZH2 knockdown group were decreased, while the apoptosis rate [ (57.92 ± 9.84) % vs (1.60 ± 0.53) %] was increased (all P < 0.05). Compared with the OXA group, cell viability [ (45.84 ± 7.63) % vs (93.46 ± 16.75) %] and the expression of P-gp protein (0.31 ± 0.05 vs 0.78 ± 0.18) in the OXA+CircEZH2 knockdown group were decreased, while the apoptosis rate [ (57.92 ± 9.84) % vs (2.74 ± 0.81) %] was increased (all P < 0.05). Compared with the OXA+CircEZH2 knockdown group, cell viability [ (85.79 ± 18.93 ) % vs (45.84 ± 7.63) %] and the expression of P-gp protein (0.73 ± 0.11 vs 0.31 ± 0.05) in the the OXA+CircEZH2 knockdown+miR-340-5p inhibitor group were increased, while the apoptosis rate [ (3.93 ± 1.22) % vs (57.92 ± 9.84) %] was decreaseds (all P < 0.05). Compared with the control group, the drug resistance index (1.08 ± 0.15 vs 3.19 ± 0.30) of the CircEZH2 knockdown group was decreased significantly (P < 0.05). Compared with the CircEZH2 knockdown group, the drug resistance index (3.06 ± 0.19 vs 1.08 ± 0.15) of the CircEZH2 knockdown+miR-340-5p inhibitor group was increased signiticantly (P < 0.05). CircEZH2 was able to target and down-regulation the expression of miR-340-5p in HCT116/OXA cells, and miR-340- 5p could target and down-regulation the expression of PUM1.

Knocking down CircEZH2 can down-regulate PUM1 expression by up-regulating miR-340-5p, reducing OXA resistance in colon cancer cells, enhancing the killing power of OXA on colon cancer cells, and inducing apoptosis of HCT116/OXA cells under OXA treatment.