Effect of imatinib on BCRP expression in plasma membrane. (A) MCF-7/AdrVp, Igrov1/T8, or K562/BCRP-MX10 cells were exposed to 5 μM of imatinib for 14 hours at 37°C and then stained with PE-conjugated 5D3 antibody to BCRP. Cellular fluorescence was assessed by flow cytometry. Mean intensities of fluorescence (MIF) obtained from control experiments were normalized to 100%. These experiments were performed twice on different days, with duplicate studies for each experimental condition on a given day. Each bar represents the mean value of 4 individual assays. ***P < .001, Student t test. M1 indicates the channels that exclude 98% of the isotype (IgG) control. (B) K562/BCRP-MX10 cells were cultured for 14 hours with a concentration range of imatinib or mitoxantrone, then BCRP expression on the cell surface was measured as described for panel A. Each bar represents the mean MIF value from at least 2 individual experiments; each dot represents an individual data point. (C) K562/BCRP-MX10 cells were cultured for periods of time up to 120 hours with 0, 0.1, 0.25, or 5 μM imatinib. BCRP expression on the cell surface was then measured as described for panel A. Each point represents the mean value ± SEM of MIFs relative to controls (0 μM imatinib) done at the same time point. Experiments were repeated 2 to 3 times. The lines drawn for the experimental points were obtained by linear regression analysis. The correlation coefficients (R²) were 0.94, 0.91, and 0.89 for cells treated with 0.1, 0.25, and 5 μM imatinib, respectively.