Cell culture and determination of drug sensitivity

The MCF-7 human breast cancer cell line and the MDR-MCF-7 multi-drug resistant cell line were purchased from the Cell Bank of the Chinese Academy of Sciences (Shanghai, China). The cells were cultured in RPMI 1640 medium (Gibco-BRL, Gaithersburg, MD, USA) supplemented with 10% FBS, 105 U/L penicillin and 100 mg/L streptomycin (Gibco Laboratories, Grand Island, NY, USA). All cells were cultured at 37°C in the presence of 5% CO₂. MDR-MCF-7 cells were exposed to 0.05 μM doxorubicin for the maintenance of the MDR phenotype, but cultured in drug-free medium for at least one week prior to experimental study. The cell survival rate was detected using MTT (Sigma-Aldrich, Dorset, UK) assay, then concentration inhibition curves were plotted to calculate the IC₅₀s of doxorubicin, docetaxel and 5-fluorouracil in MCF-7 and MDR-MCF-7 cells (RI = IC₅₀ (resistant cells)/IC₅₀ (sensitive cells)). Experiments were repeated three times.