ABCG2 (BCRP) expression in normal and malignant hematopoietic cells

CD34+ Cells from Acute Myeloid Leukemia, Myelodysplastic Syndromes, and Normal Bone Marrow Display Different Apoptosis and Drug Resistance–Associated Phenotypes

Myelodysplastic syndromes and acute myeloid leukemia (AML) are heterogeneous disorders in which conflicting results in apoptosis and multidrug resistance (MDR) have been reported. We have evaluated by multiparameter flow cytometry the expression of apoptosis- (APO2.7, bcl-2, and bax) and MDR-related proteins [P-glycoprotein (P-gp), multidrug resistance protein (MRP), and lung resistance protein (LRP)] specifically on bone marrow (BM) CD34+ cells, and their major CD32-/dim and CD32+ subsets, in de novo AML (n=90), high-risk myelodysplastic syndrome (n=9), and low-risk myelodysplastic syndrome (n=21) patients at diagnosis, and compared with normal BM CD34+ cells (n=6). CD34+ myeloid cells from AML and high-risk myelodysplastic syndrome patients displayed higher expression of bcl-2 (P <0.0001) and lower reactivity for APO2.7 (P=0.002) compared with low-risk myelodysplastic syndrome and normal controls. Similar results applied to the two predefined CD34+ myeloid cell subsets. No significant differences were found in the expression of P-gp, MRP, and LRP between low-risk myelodysplastic syndrome patients and normal BM, but decreased expression of MRP (P <0.03) in AML and high-risk myelodysplastic syndromes and P-gp (P=0.008) in high-risk myelodysplastic syndromes were detected. Hierarchical clustering analysis showed that low-risk myelodysplastic syndrome patients were clustered next to normal BM samples, whereas high-risk myelodysplastic syndromes were clustered together and mixed with the de novo AML patients. In summary, increased resistance to chemotherapy of CD34+ cells from both AML and high-risk myelodysplastic syndromes would be explained more appropriately in terms of an increased antiapoptotic phenotype rather than a MDR phenotype. In low-risk myelodysplastic syndromes abnormally high apoptotic rates would be restricted to the CD34- cell compartments.

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Identification of a human heme exporter that is essential for erythropoiesis

FLVCR, a member of the major facilitator superfamily of transporter proteins, is the cell surface receptor for feline leukemia virus, subgroup C. Retroviral interference with FLVCR display results in a loss of erythroid progenitors (colony-forming units-erythroid, CFU-E) and severe anemia in cats. In this report, we demonstrate that human FLVCR exports cytoplasmic heme and hypothesize that human FLVCR is required on developing erythroid cells to protect them from heme toxicity. Inhibition of FLVCR in K562 cells decreases heme export, impairs their erythroid maturation and leads to apoptosis. FLVCR is upregulated on CFU-E, indicating that heme export is important in primary cells at this stage. Studies of FLVCR expression in cell lines suggest this exporter also impacts heme trafficking in intestine and liver. To our knowledge, this is the first description of a mammalian heme transporter.

Human Breast Cancer Resistance Protein: Interactions with Steroid Drugs, Hormones, the Dietary Carcinogen 2-Amino-1-methyl-6-phenylimidazo(4,5-b)pyridine, and Transport of Cimetidine

The breast cancer resistance protein (BCRP/ABCG2) is an ATP-binding cassette drug efflux transporter that extrudes xenotoxins from cells, mediating drug resistance and affecting the pharmacological behavior of many compounds. To study the interaction of human wild-type BCRP with steroid drugs, hormones, and the dietary carcinogen 2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine (PhIP), we expressed human BCRP in the murine MEF3.8 fibroblast cell line, which lacks Mdr1a/1b P-glycoprotein and Mrp1, and in the polarized epithelial MDCKII cell line. We show that PhIP was efficiently transported by human BCRP in MDCKII-BCRP cells, as was found previously for murine Bcrp1. Furthermore, we show that six out of nine glucocorticoid drugs, corticosterone, and digoxin increased the accumulation of mitoxantrone in the MEF3.8-BCRP cell line, indicating inhibition of BCRP. In contrast, aldosterone and ursodeoxycholic acid had no significant effect on BCRP. The four most efficiently reversing glucocorticoid drugs (beclomethasone, 6alpha-methylprednisolone, dexamethasone, and triamcinolone) and 17beta-estradiol showed a significantly reduced BCRP-mediated transepithelial transport of PhIP by MDCKII-BCRP cells, with the highest reduction of PhIP transport ratio for beclomethasone (from 25.0 +/- 1.1 to 2.7 +/- 0.0). None of the tested endogenous steroids or synthetic glucocorticoids or digoxin, however, were transported substrates of BCRP. We also identified the H(2)-receptor antagonist drug cimetidine as a novel efficiently transported substrate for human BCRP and mouse Bcrp1. The generated BCRP-expressing cell lines thus provide valuable tools to study pharmacological and toxicological interactions mediated by BCRP and to identify new BCRP substrates.

Hematopoietic cells and osteoblasts are derived from a common marrow progenitor after bone marrow transplantation

Bone and bone marrow are closely aligned physiologic compartments, suggesting that these tissues may represent a single functional unit with a common bone marrow progenitor that gives rise to both osteoblasts and hematopoietic cells. Although reports of multilineage engraftment by a single marrow-derived stem cell support this idea, more recent evidence has challenged claims of stem cell transdifferentiation and therefore the existence of a multipotent hematopoietic/osteogenic progenitor cell. Using a repopulation assay in mice, we show here that gene-marked, transplantable marrow cells from the plastic-nonadherent population can generate both functional osteoblasts/osteocytes and hematopoietic cells. Fluorescent in situ hybridization for the X and Y chromosomes and karyotype analysis of cultured osteoblasts confirmed the donor origin of these cells and excluded their generation by a fusion process. Molecular analysis demonstrated a common retroviral integration site in clonogenic hematopoietic cells and osteoprogenitors from each of seven animals studied, establishing a shared clonal origin for these ostensibly independent cell types. Our findings indicate that the bone marrow contains a primitive cell able to generate both the hematopoietic and osteocytic lineages. Its isolation and characterization may suggest novel treatments for genetic bone diseases and bone injuries.