Objective: To investigate the effects of dihydrofolate reductase (DHFR) gene expression up-regulating on cisplatin resistance in epithelial ovarian cancer cell lines.
Methods: The cDNA length of DHFR gene was amplified by PCR and was connected to lentiviral vector pWPI, the recombinant retroviral vector DHFR-pWPI was infected SKOV3 cells by lipofectamine 2000. The groups included DHFR-pWPI-SKOV3 cell, pWPI-SKOV3 cell and SKOV3 cell group. Western blot was used to detect the expression of DHFR. Flow cytometry was applied to measure the cell apoptosis rate of 3 groups cells in different cisplatin concentrations (2.5, 5.0, 10.0, 20.0 µg/ml) and at different time period (24, 48 and 72 hours), and half maximal inhibitory concentration (IC50) treated with cisplatin concentration (6.0, 4.0, 4.9 µg/ml). High performance liquid chromatography (HPLC) was applied to test intracellular cisplatin concentration in different cisplatin concentration (4.0, 6.0, 8.0 µg/ml) at 24 and 48 hours. Transmission electron microscope was used to observe ultrastructural changes cells under IC50 cisplatin concentration.
Results: The recombinant plasmid DHFR-pWPI was constructed and then infected into SKOV3 cell successfully. (1) The expression of DHFR detected by western blot in transfection group was higher than those in the negative control group and blank control group (10.280±0.009 vs 2.050±0.003 vs 3.480±0.003; P<0.01). (2) Treated with cisplatin concentration (2.5, 5.0, 10.0, 20.0 µg/ml) at 24, 48 hours, the apoptosis rate detected by flow cytometry results shown that they were lower than those in the negative control group and blank control group (P<0.05), while treated at the concentration of 5.0 and 10.0 µg/ml for 72 hours, whose apoptosis rate in transfection group was higher than those in the negative control group and blank control group (P<0.05). When treated cells under IC50 cisplatin concentration (6.0, 4.0, 4.9 µg/ml) at for 24 and 48 hours, the results indicated that there were mainly G0/G1 stage cell cycle rate in 3 groups, it was obviously higher in transfection group than those in two control groups (P<0.05). However, mainly G2/M, S stage cell cycle rate for 72 hours, and S stage cell cycle rate in transfection group obviously higher than those in two control groups, but G2/M stage cell cycle rate were lower (P<0.01). (3) After treated with cisplatin concentration (4.0 µg/ml) for 24, 48 hours and cisplatin concentration (6.0 µg/ml) for 24 hours, the intracellular cisplatin content tested by HPLC method in the transfection group were significantly lower than those in two control groups (P<0.01). While, at 6.0 µg/ml of cisplatin concentration for 48 hours and 8.0 µg/ml of cisplatin concentration for 24 and 48 hours, the intracellular cisplatin content of transfection group were obviously higher than those two control groups (P<0.05). (4) Treated with IC50 (6.0, 4.0, 4.9 µg/ml) cisplatin concentration at different time to obeserve ultrastructural changes by transmission electron microscopy. The results shown that the microwire gathered together at 24 and 48 hours, and the number and structure of mitochondria had obvious change in transfection group, while there was rare microfilament, the number of mitochondria decreased but structure change was not apparent in two control groups. There were appeared expansion of endoplasmic reticulum and had rare normal organelles among three groups. After treated with cisplatin for 72 hours, there were inordinate microfilament, a part of nuclear membrane disappeared, a lot of ribosomes gathered together in two control groups, and there were rare microfilament in transfection group, nuclear membrane completely disappeared, many white cystic matter were seen in cytoplasm, mitochondrial structure disappeared completely, which seems most cells on the verge of death.
Conclusion: The lentiviral expressing vector harboring human DHFR gene were successfully constructed. When the up- expression of DHFR gene, the drug-resistant in ovarian cancer cell may be increase, which suggest that there were certain contact between resistance increases with microfilament gathered and the change of the mitochondria.