ABCG2 (BCRP) expression in normal and malignant hematopoietic cells

ABCG2 (BCRP) is a member of the ATP-binding cassette (ABC) family of cell surface transport proteins. ABCG2 expression occurs in a variety of normal tissues, and is relatively limited to primitive stem cells. ABCG2 expression is associated with the side population (SP) phenotype of Hoechst 33342 efflux. The substrate profile of ABCG2 includes the antineoplastic drugs primarily targeting topoisomerases, including anthracyclines and camptothecins. More recently, pheophorbide, a chlorophyll-breakdown product, and protoporhyrin IX have been described as ABCG2 substrates, perhaps indicating a physiologic role of cytoprotection of primitive cells. Also, mice lacking ABCG2 expression have no intrinsic stem cell defects, although there is a remarkable increase in toxicity with antineoplastic drugs that are ABCG2 substrates, and also a photosensitivity resembling protoporphyria. Like other members of the ABC family, such as MDR1 and MRP1, ABCG2 is expressed in a variety of malignancies. Despite numerous reports of ABCG2 expression in AML, there is little evidence that ABCG2 expression is correlated with an adverse clinical outcome. This review will focus on the potential usefulness of ABCG2 as a marker primitive stem cells and possible physiologic roles of ABCG2 in protection of primitive stem cell populations, and potential methods of overcoming ABCG2-associated drug resistance in anticancer therapy.