FIG 4.

Cell viability assays with TDF and methotrexate in the presence and absence of the specific inhibitor MK-571. (A) Specific inhibitor MK-571 at various concentrations did not reduce MRP8-overexpressing and parental cell viability. (B) Cytotoxic effects of TDF on MRP8-overexpressing and parental cells. (C) MK-571 further reduced viability of the MRP8-overexpressing cells, but not parental cells, treated with TDF. (D) MK-571 also enhanced cytotoxicity of methotrexate only in MRP8-overexpressing cells. (E and F) Cytotoxicity assays showing methotrexate and TDF concentrations that reduced cell viability by 50% (CC₅₀) in MRP8-overexpressing LLC-PK1 or parental cells with or without the specific inhibitor MK-571. Statistical significance was analyzed by a two-way ANOVA multiple comparison assuming equal variance (*, P < 0.01; **, P < 0.001; ***, P < 0.0001). All values are the means ± standard deviations from five independent experiments.