Erythroblasts highly express the ABC transporter Bcrp1/ABCG2 but do not show the side population (SP) phenotype

Stem cells in various somatic tissues including hematopoietic stem cells can be identified by the "side population (SP)" phenotype based on the efflux of Hoechst33342. Knockout and enforced expression experiments show that the expression of the Bcrp1/ABCG2 gene is an important determinant of the SP phenotype. In this study, we showed that erythroblasts also express a large amount of Bcrp1/ABCG2. The level of expression was increased with maturation, but did not relate to the cell-cycle status. Despite the high expression level of Bcrp1/ABCG2, erythroblasts did not show the "side population" phenotype. Furthermore, a Bcrp1/ABCG2 inhibitor, verapamil, had little effect on the Hoechst33342 staining pattern of erythroblasts. However protoporphyrin IX fluorescence was significantly higher in the presence of verapamil, suggesting that the ABCG2 functions as a transmembrane transporter in erythroblasts. These results indicate that dissociation between Bcrp1/ABCG2 expression and dye efflux function exists in erythroblasts and in stem cells, and that the function of ABCG2 in erythroblasts differs from that in stem cells.