Relationship between RFC gene expression and intracellular drug concentration in methotrexate-resistant osteosarcoma cells

Osteosarcoma is a primary malignant tumor in adolescents, associated with high mortality and morbidity. The high-dose methotrexate (MTX) chemotherapy used to treat this disease may induce primary or secondary drug resistance, resulting in a reduced effect of comprehensive treatment. In this study, the relationship between reduced folate carrier (RFC) gene expression and intracellular drug concentration in MTX-resistant osteosarcoma cells (Saos-2) was investigated. MTX-resistant human osteosarcoma cells (Saos-2/MTX2.2, Saos-2/MTX4.4) were prepared. The sensitivities of Saos-2 (primary cells), Saos-2/MTX2.2, and Saos-2/MTX4.4 cells to MTX, diamminedichloroplatinum (DDP), ifosfamide (IFO), epirubicine (EPI), adriamycin (ADM), theprubicin (THP), and paclitaxel (PTX) were detected by MTT. The median inhibitory concentration (IC50) and resistance index were measured. Semi-quantitative RT-PCR was used to evaluate the expression of RFC gene in cells. The intracellular (3)H-MTX concentration was determined. Results showed that IC50 of Saos-2/MTX2.2 and Saos-2/MTX4.4 was 4.87 and 12.73 times that of Saos-2, respectively. Both Saos-2/MTX2.2 and Saos-2/MTX4.4 had resistance to IFO, ADM, EPI, THP, and PTX, but not DDP. Compared to Saos-2/MTX2.2 and Saos-2/MTX4.4, the expression of RFC mRNA in Saos-2 was significantly higher. The intracellular (3)H-MTX concentration reached a peak at 50 min. After 70 min, the concentration was maintained at a plateau. During this phase, the (3)H-MTX concentration in Saos-2 cells was 2.15 times higher than the concentration in Saos-2/MTX4.4 cells. The reduced RFC mRNA expression in PTX-resistant osteosarcoma cells may be related to the decrease in intracellular (3)H-MTX concentration.