ABCG2 expression increases in response to doxorubicin exposure. ABCG2 functional expression was assayed by flow cytometry in 8226 and 8226MR MM cells after exposure to 1 μM doxorubicin for 20 minutes. Higher ABCG2-expressing 8226MR cells were able to efflux doxorubicin more efficiently than parental 8226 cells. The ABCG2-specific inhibitor tryprostatin A decreased efflux in the 8226MR cell line but not the 8226 parental cell line, indicating that doxorubicin efflux was mediated by ABCG2. Myeloma cells treated with low-dose doxorubicin, 0.1 μM in 8226MR cells and 1.0 μM in 8226 cells, exhibit an increase in protein expression as determined by Western analysis (inset of each graph). Equal amounts of protein (25 μg) were assayed. Both 8226 and 8226MR cells demonstrated a 1.7-fold increase in ABCG2 protein after low-dose doxorubicin treatment.