Role of proteasome on posttranscriptional downregulation of BCRP by imatinib. (A) Effects of the proteasomal inhibitor MG132 on cell surface BCRP expression. K562/BCRP-MX10 cells were treated with 1 μM MG132 for 8 hours and then BCRP expression was measured by binding of the 5D3 antibody, as determined by flow cytometry. Each bar represents the mean value with SEM of at least 3 individual experiments. **P < .01, Student t test. (B) Effectiveness of MG132 in causing proteasomal inhibition; effects of imatinib. K562/BCRP-MX10 cells in culture were exposed to the indicated concentrations of MG132 or imatinib for 8 hours. Then, the ubiquitination of proteins in total cell lysates was determined by Western blot, using rabbit polyclonal antibody to ubiquitin. The 8-kDa band represents free, unconjugated ubiquitin ligand. (C) Proteasomal inhibition does not block the down-regulation of BCRP by imatinib. K562/BCRP-MX10 cells in culture were exposed to the indicated concentrations of MG132 or imatinib for 8 hours. Then, BCRP expression was measured by flow cytometry as in panel A. Each bar represents the mean value with SEM of at least 3 individual experiments. *P < .05, ***P < .001 by Student t test.