Effect of selection with mitoxantrone on BCRP and MDR1 expression. (A) The effect of mitoxantrone selection on BCRP expression. K562/BCRP cells were selected with 10 nM of mitoxantrone for 1 hour immediately prior to each passage. BCRP expression was determined by Western blots. Forty micrograms of total protein lysates was loaded onto each lane. Blots were probed with the BXP-21 antibody to BCRP. Wt indicates wild type. (B-C) The effect of mitoxantrone selection on MDR1 mRNA and P-gp expression was examined by quantitative RT-PCR (qRT-PCR) (B) or Western blot, using the F4 antibody to P-gp (C), respectively. For panel B, each bar represents the mean value (n = 2). HL60/vinc cells, which are known to overexpress P-gp, were used as positive controls.