ABCG2 expression increases in response to topotecan chemotherapy. (A-B) Multiple myeloma 8226MR, 8226, and H929 cells treated with low-dose topotecan (B) exhibit an increase in membrane ABCG2 over the no-drug control (A), as shown by immunostaining with ABCG2 antibody (Bcrp1 FITC). (C) Protein expression of ABCG2 was quantified as pixel intensity (from immunofluorescence microscopy), and also assessed by Western analysis (inset of each graph). (D) ABCG2 expression was measured by flow cytometry and showed an increase in log-phase 8226MR and 8226 cells after exposure to low-dose topotecan (1 μM) for 24 hours. The ABCG2-nonexpressing cell line (H929) shows no increase in ABCG2 antibody binding. Red indicates control cells in the absence of topotecan, and green represents myeloma cells exposed to topotecan.