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NEOPLASIA
From the Center for Cell and Gene Therapy, Baylor
College of Medicine; and Molecular Hematology and Therapy, M. D. Anderson Cancer Center, Houston, TX.
The hematopoietic stem cell underlying acute myeloid leukemia (AML)
is controversial. Flow cytometry and the DNA-binding dye Hoechst 33342 were previously used to identify a distinct subset of murine
hematopoietic stem cells, termed the side population (SP), which
rapidly expels Hoechst dye and can reconstitute the bone marrow of
lethally irradiated mice. Here, the prevalence and pathogenic
role of SP cells in human AML were investigated. Such cells were found
in the bone marrow of more than 80% of 61 patients and had a
predominant CD34low/ The demonstration that acute myeloid leukemia (AML)
is a disease of hematopoietic stem cells (HSCs), rather than of
committed myeloid progenitors, has raised fundamental questions
concerning the true target of leukemic transformation in this
disease.1-3 In most instances, the HSCs capable of
initiating human AML in nonobese diabetic-severe combined
immunodeficient (NOD/SCID) mice or other animal models have been
exclusively CD34+CD38 We have selected murine bone marrow cells on the basis of their rapid
efflux of fluorescent Hoechst dye, using flow cytometry to identify a
small subset of these cells (0.05% of all marrow cells), which we have
termed the side population (SP).10,12 Such cells have many
of the properties of recognized HSCs, including long-term repopulating
activity in mice as well as specific lineage-marker negativity.
Importantly, SP cells were also found in a variety of mammalian
species, including humans, where their frequency ranges from 0.01 to
0.09% (average, 0.03%).12 If leukemic SP cells are
present in AML patients, their characteristics would be expected to
greatly affect the response to therapy. For example, the ability of SP
cells to expel the Hoechst dye might be associated with drug resistance
through rapid efflux of cytotoxic drugs, and their repopulating
activity could lead to disease recurrence from small numbers of
leukemic cells. To test these hypotheses, we studied the bone marrow
and peripheral blood mononuclear cells of children and adults with
primary AML or AML secondary to myelodysplasia (MDS), both in relapse
and in remission. The findings suggest that SP cells constitute a
previously unrecognized population of human leukemic stem cells with
potential clinical significance.
Patients
Cell separation, immunophenotyping, and fluorescence in situ
hybridization analysis
SP cells were immunophenotyped simultaneously by means of fluorescein isothiocyanate (FITC), phycoerythrin (PE), and allophycocyanin (APC) as fluorescent agents excited from the second and third lasers at 488 nm (FITC and PE) and 633 nm (APC). Fluorochrome-conjugated antibodies (all from Becton Dickinson, San Jose, CA) were used to detect the antigens CD13, CD33, CD38, CD45, and CD71, thimine (Thy)-1, and glycophorin A, as well as for the appropriate isotype controls. For the sensitive detection of CD34, we employed a biotinylated first antibody against class II antigen (QBEnd10) (Coulter-Immunotech, Miami, FL) and a streptavidin-APC conjugate (Molecular Probes, Eugene, OR). Multidrug resistance (MDR)-1 was detected with the antibody MRK16 (Kamiya, Seattle, WA) in an indirect immunofluorescence procedure. All antibody incubations were performed at 4°C following the Hoechst 33342 staining at 37°C. Propidium iodide (2 µg/mL) (Sigma) was added to the samples following Hoechst and antibody staining to facilitate dead cell discrimination. All data acquired on the MoFlow were analyzed with the FlowJo analysis program (Tree Star, San Carlos, CA). The cells were sorted directly onto slides to facilitate fluorescence in situ hybridization (FISH) analysis, or into culture medium for all other puposes. Immunostaining of whole bone marrow preparations from NOD/SCID mice to detect human leukemia engraftment was performed with directly conjugated antibodies against human CD45 (FITC) and human CD34 (PE) (Coulter-Immunotech). Analysis was performed on a FACScan flow cytometer (Becton Dickinson) and analyzed with either the CellQuest (Becton Dickinson) or the FlowJo program. Immunocytologic and immunohistochemical studies for human cells with the CD45-specific hybridoma clones 2B11 and PD7/26 (Dako, Carpinteria, CA) and diaminobenzidine were performed on sorted cell populations after cytocentrifugation and fixation in acetone (4°C, 10 minutes) and on formalin-fixed tissue specimens after conventional paraffin embedding and sectioning. FISH analysis followed standard protocols specifying SpectrumOrange-labeled alpha-satellite probes for chromosome 8 (D8Z2) (Vysis, Downers Grove, IL), chromosome 15 (D15Z3) (Vysis), and chromosome 7 (D7Z1) (Vysis) after pretreating the slides with pepsin (50 µg/mL, pH 2.0) (Sigma) for 5 minutes at 37°C, as previously described.13,14 Among the clinical samples, a frequency that exceeded 2 SDs from the mean healthy volunteer control value (D7Z1, 6% ± 0.79%; D8Z2, 2.4% ± 0.65%; D15Z3, 4.3% ± 0.76%) was considered evidence for a numerical aberration. Efflux studies for daunorubicin and mitoxantrone Freshly prepared or frozen patient MNC specimens were incubated with Hoechst 33342 and daunorubicin (Ben Venue Laboratories, Bedford, OH) at a final concentration of 0.5 µg/mL (37°C for 30 minutes followed by 15 minutes with Hoechst alone) or with mitoxantrone (Immunex, Seattle, WA) at 0.1 µg/mL (37°C for 30 minutes, 15 minutes with Hoechst alone). The drugs were added 60 minutes after the start of the Hoechst staining and removed by centrifugation in a centrifuge prewarmed to 37°C. The fluorescence emitted from daunorubicin after excitation at 488 nm was detected at 575/25 nm, while that from mitoxantrone was detected at 670/40 nm after excitation at 633 nm.Transplantation of AML cells into NOD/SCID mice Six- to 8-week-old NOD/SCID mice, maintained under defined flora conditions, were transplanted according to protocols approved by the Animal Protocol Review Committee of Baylor College of Medicine. At 12 to 24 hours before retro-orbital intraveneous injection of AML cell preparations, the animals were irradiated with 270 cGy from a 137Cs source. In 10 AML cases, the samples were transplanted without human cytokine supplementation or accessory cells. In 19 cases, 1 × 106 cryopreserved whole bone marrow cells from healthy volunteers were irradiated with 15 Gy and used as accessory cells. Human growth factors were also added in these cases. Beginning immediately after transplantation, mice received a total of 5 intraperitoneal injections at 2-day intervals that contained a mixture of recombinant methionyl human stem cell factor (rmetHuSCF, 10 µg per dose) (kindly provided by Amgen, Thousand Oaks, CA); interleukin-3 (rHuIL-3, 6 µg per dose) (generously provided by Immunex); granulocyte-macrophage colony stimulating factor (GM-CSF, sargramostim; 6 µg per dose) (Immunex); or HuIL-3/HuGM-CSF fusion protein (7 µg per dose) (provided by Immunex); granulocyte-colony stimulating factor (G-CSF, filgrastim; 6 µg per dose) (Amgen); and erythropoetin (epogen, 100 U per dose) (Amgen). Mice were killed and examined for engraftment of human cells 3 months after transplantation or earlier when a moribund state was observed. Single-cell preparations from hind-limb bone marrow, spleen, and liver were analyzed by fluorescence-activate cell sorter (FACS) for CD45+ and CD34+ cells. Tissues from spleen and liver were processed by formalin fixation and standard paraffin embedding for CD45 immunohistochemical studies, and DNA was extracted from bone marrow, spleen, and liver with a commercial DNA purification kit (Puregene) (Gentra, Minneapolis, MN) for Southern blot and polymerase chain reaction (PCR) analyses. Engraftment was based on FACS detection of human CD45+ cells (see above), Southern blot detection of human chromosome 17-specific alpha-satellite sequences, and PCR of human endogeneous retrovirus H (HER-H) sequences. Spleens and livers of the transplanted animals were analyzed by FACS, Southern blot, and immunohistochemistry against human CD45. For Southern blot analysis, 10 µg high-molecular-weight DNA per sample was digested with EcoR1 and hybridized to the chromosome 17-specific alpha-satellite probe (p17H8, kindly provided by J. Dick) with standard techniques,15 with the application of a nonradioactive chemoluminescent detection system (AlkPhos Direct) (Amersham, Frederick, MD). In nested PCR analysis, sequences of the type H human endogeneous retrovirus16 were amplified from 1 µg genomic DNA per sample with the outer primers us1 (5'-TCCTACAAGATCTAAATAATTCTTG-3') and ds1 (5'-AGTGGCCAGATTTCTGGCAC-3') at an annealing temperature of 55°C, with 20 cycles yielding a product of 869 base pairs (bp), and the inner primers us2 (5'-TTATACATTGTTCCCTCCCTAG-3') and ds2 (5'-CCTGGCAGCTGCAGTT-3') at an annealing temperature of 55°C, with 35 cycles yielding a product of 735 bp visualized in standard 1.2% ethidium bromide gels. This assay did not amplifiy a product from the bone marrow of 6 control mice irradiated and injected with irradiated human accessory cells 3 months before sacrifice. The sensitivities of the xenograft detection methods were compared by seeding bone marrow MNCs from NOD/SCID mice, which had been irradiated 3 months earlier, with decreasing numbers of the myeloid cell from HL60, KG1, and AML193 lines and analyzing the preparations as described for the experimental samples.
Study population Bone marrow and peripheral blood cells were derived from 71 diagnostic samples of AML from 61 children and adults (Table 1) All French-American-British (FAB) subtypes of AML except M0 were represented in the study population. Five cases were unclassified; AML had developed from MDS in 15 cases. The original specimens were collected before treatment (n = 23), during remission (n = 18), or during relapse or resistance to chemotherapy (n = 18). The vast majority of patients (59 of 61) were classified as intermediate or poor risk by standard cytogenetic criteria.17 The male/female ratio was 2 to 1, and the median age was 48 years (range, 10 months-80 years).Prevalence of SP cells in bone marrow and peripheral blood In normal human bone marrow, SP cells account for approximately 0.03% (range, 0.01%-0.09%) of all MNCs.10 To estimate the prevalence of SP cells in AML, we stained bone marrow and peripheral blood samples with Hoechst 33342 and then used dual-wavelength flow cytometry to identify cells with strong dye efflux activity (Figure 1). In 61 bone marrow and 10 peripheral blood specimens studied, an SP fraction was detected in 39 of the 52 patients with active disease (75%), including 19 of 28 (67%) samples from untreated patients and 20 of 24 (83%) samples from patients with drug-resistant or relapsed disease. Samples from 16 of the 19 specimens from patients (84%) in complete hematologic remission also contained SP cells (Figure 2). Only 4 of 10 leukemic blood samples, 6 from untreated patients and 4 from patients with relapsed or resistant disease, contained SP cells. The proportion of SP cells within the MNC fraction varied widely among patients, from 0.00% to 16% (median, 0.10%) in leukemic marrow and 0.00% to 80% (median, 0.00%) in leukemic blood. Although SP cell frequencies showed less variation in remission marrows, they exceeded the normal range in 10 of 21 samples. The highest SP cell fractions (16%, 71%, and 80%) were found in patients who were resistant to remission induction or salvage therapy. The prevalence of SP cells in bone marrow and peripheral blood samples was not correlated with FAB classification (data not shown) or with age at presentation.
Cell surface phenotype Normal human HSCs are classically defined as CD34+.18,19 Although the human SP fraction does contain some CD34+ cells, the surface phenotype is typically CD34low/ , with virtually no expression of the
markers found on mature or differentiating hematopoietic
cells.10 Because leukemic transformation and progression
can alter the expression of surface antigens, we sought to determine
the extent to which the immunophenotype of SP cells from AML patients
differs from that of normal marrow SP cells. CD34 expression by SP
cells was low or undetectable in more than half the cases of active
AML, compared with the vast majority of the cases in complete
hematologic remission (Figure 3). Six of
the leukemic bone marrow samples had unexpectedly high CD34+ frequencies (53% to 97%). CD38 expression ranged
from undetectable to greater than 85% of SP cells, in both leukemic
and remission samples, in agreement with observations on normal
cells.10 There was no correlation of CD34 and CD38
expression on the SP cells, regardless of the disease status (data not
shown). Tests for Thy-1 (HSC marker), CD13/CD33 (myelomonocytic
differentiation), CD71 (progenitor activation/proliferation), and MDR1,
performed exclusively on samples of active disease, yielded positive
results in 4 of 9 (2% to 63% positivity), 17 of 20 (9% to 100%
positivity), 4 of 8 (3% to 17% positivity), and 8 of 14 (5% to 73%
positivity) cases, respectively. Thus, SP cells in active AML are
characterized by considerable case-to-case heterogeneity in expression
of phenotypic markers, including CD34, while in cases in hematologic
remission, they retain the CD34low/CD38low
phenotype characteristic of normal SP cells.
Cytogenetic evidence of SP cell leukemic involvement The alterations in prevalence and immunophenotype observed in the SP cells of patients with AML (Figures 1-2) compared with SP cells in healthy individuals suggests a dysregulation of SP associated with the disease, but does not establish their derivation from the original leukemic clone. We therefore used interphase FISH to test both leukemic and remission samples from 16 cases defined by trisomy 8 (n = 8), monosomy 7 (n = 7), or both (n = 1). In the 11 cases of active disease (blast cell count, 5% or greater), one or both of these cytogenetic markers were present in 9% to 90% (median, 41% ± 29%) of the SP cells in all of the cases, compared with 11% and 12.5% in 2 of the 5 cases in remission (Table 2). The frequency of the defining cytogenetic abnormality among CD34low/ SP cells generally
corresponded to that in the CD34+ non-SP fraction. These
findings indicate a leukemic origin for a significant fraction of SP
cells in most cases of active AML. The presence of marker-positive
cells in some remission samples suggests a role of leukemic SP cells in
disease persistence after chemotherapy.
Efficient efflux of daunorubicin and mitoxantrone from SP cells The ability of SP cells to efficiently expel Hoechst 33342 suggested a mechanism by which such cells could escape the lethal effects of antileukemic drugs. We therefore asked if daunorubicin and mitoxantrone, 2 commonly used agents in the treatment of AML, might be cotransported with this lipophilic dye. Mononuclear bone marrow cells from 2 childhood and 1 adult AML patient were simultaneously incubated with Hoechst 33342 and either daunorubicin or mitoxantrone, and the efflux of drug from the gated SP and non-SP cells was measured by its specific emission fluorescence at 575/25 nm (daunorubicin) or 670/40 nm (mitoxantrone). Comparison of the emission profiles of SP versus non-SP cells demonstrated increased efflux of daunorubicin and mitoxantrone together with Hoechst 33342 in each of the clinical samples of SP cells (Figure 4). Additional testing with leukemia/lymphoma cell lines and bone marrow samples from healthy volunteers confirmed the greater efficiency of cytostatic drug efflux from SP cells (data not shown).
Transplantable leukemic activity of SP cells In competitive repopulation experiments, normal SP cells accounted for virtually all of the HSC activity of transplanted murine bone marrow.10,12 Thus, the SP cells we have identified in human AML should be able to regenerate the primary disease in immunodeficient mice. The limited number of SP cells available for transplantation, together with their likely quiescent state,12 led us to develop assays (see "Patients, materials, and methods") that detect very low levels of human cell engraftment (maximum sensitivity, 0.001%, in a mixture of HL60 and AML193 leukemic blasts with NOD/SCID bone marrow). Subsequently, we injected variable numbers of either CD34low/ SP cells
(range, 3.8 × 101 to 1 × 105; n = 29
cases) or the remainder of the Hoechst-positive cell population (range,
6.5 × 102 to 3.9 × 106; n = 29 cases)
into 21 pairs of NOD/SCID mice (SP- vs non-SP cells).
Three SP cell transplants (from cases with initial SP fractions of
80%, 71%, and 16%) gave rise to AML-like disease in each recipient
(Figure 5). Transplantation of non-SP
cells led to overt leukemia in a single mouse injected with
1 × 105 cells from one of cases, in which SP cells had
regenerated overt AML. Importantly, in mice transplanted with SP cells,
low but persistent numbers of human cells were detected in murine bone marrows by immunocytology (8 of 14 mice) and PCR (6 of 18 mice), as
well as in mice transplanted with non-SP cells (6 of 18 and 8 of 14 mice, respectively). In most of these instances, there was either
engraftment or nonengraftment by each of the paired samples;
engraftment by one but not the other cell type was unusual.
To determine the proportions of leukemic cells in the human hemopoietic
xenografts, we applied FISH for monosomy 7 and trisomy 8 to the bone
marrow MNCs of recipient mice. Table 3
shows the proportions of SP and non-SP cells with marker-positive
nuclei in 8 cases of AML with monosomy 7 or trisomy 8. In one case (No. 18), all of the human cells detected carried the relevant leukemic marker, while in the remaining cases, there was a mixture of diploid and cytogenetically aberrant cells. In the samples from a patient with
trisomy 8 and monosomy 15 (No. 20), the engrafted SP cells were nearly
all disomic for chromosome 8, in contrast to the repeated finding of
trisomy 8 during the course of the disease. Finally, in a case of
monosomy 7 (No. 17), the results of SP- and non-SP-cell engraftment
showed a remarkable discrepancy. Whereas the progeny of transplanted
non-SP cells were primarily marker positive (85%), those from SP cells
were essentially disomic for chromosome 7. The morphologic features of
the human diploid cell population infiltrating the bone marrow, spleen,
and retrobulbar fat pad were unequivocally leukemic (data not shown).
Rapid efflux of fluorescent dyes, mainly rhodamine 123 and
Hoechst 33342, has been widely used to identify HSC subpopulations in
mammals.20-23 Analysis of Hoechst 33342 fluorescence at 2 wavelengths reveals a small subset of HSCs (SP cells) characterized by
a CD34low/ Only low percentages of leukemic SP cells expressed CD71 or Thy-1, whereas approximately half expressed CD13 and/or CD33. Since the last 2 antigens were not expressed on the SP cells of healthy individuals, this observation raises the issue of phenotypic promiscuity in the AML stem cell compartment. In normal hematopoiesis, myeloid differentiation markers are generally not expressed on cells that retain the function of primitive HSCs, but this relationship may not be preserved once leukemic transformation has occurred. Most, if not all, of the previously described myeloid leukemia cell lines, capable of unlimited self-renewal in vitro, express myeloid differentiation antigens to some degree. The repopulating activity of leukemic SP cells was tested by
xenotransplantation into NOD/SCID mice. The minimal numbers of AML stem
cells transplanted successfully into NOD/SCID mice in previous dilution
experiments were 4 × 104 by Bonnet and
Dick3 and 1 × 105 by Ailles et
al,26 leading to an estimated frequency of
leukemia-initiating cells of 0.2 to 100 per 106 and 0.7 to
45 per 107 AML peripheral blood cells, respectively. In the
present study, we transplanted an average of 340 CD34 The results of engraftment from non-SP controls were remarkably
similar to those obtained with CD34low/ In contrast to most other definitions of HSCs, the distinguishing phenotype of SP cells is directly linked to a basic cell function, rapid efflux of lipophilic substances probably mediated by an MDR-related mechanism.10 We exploited this property to test a novel hypothesis: that antileukemic drugs might be expelled from SP cells by the same mechanism responsible for the rapid efflux of Hoechst 33342. Our results, obtained with daunorubicin and mitoxantrone in 3 AML samples and myeloid leukemia cell lines (data not shown), clearly support this prediction (Figure 4). In each comparison, cells within the SP population expelled either drug more efficiently than did their non-SP counterparts. This suggests that the increased drug efflux capacity of even small populations of leukemic SP cells could influence the outcome of therapy in patients with AML. Increasing evidence that the AML leukemic clone is organized as a
hierarchy of HSCs and committed progenitor cells with many similarities
to the normal hematopoietic system has provided fresh insight into the
mechanisms that initiate and maintain the myeloid leukemias. The
original work of Bonnet and Dick,3 using AML cells from
peripheral blood, suggested that CD34+ HSCs are the
target for leukemic transformation and therefore the cells most worthy
of exploiting in new therapeutic strategies. The findings we report
demonstrate the involvement of CD34low/
We thank Brian Newsom and Mike Cubbage for their excellent work with the flow cytometer, and John Gilbert for helpful comments on the manuscript; the M. D. Anderson Leukemia Department physicians for clinical samples; and Amgen and Immunex for generously providing samples of human cytokines. M.A.G. is a Fellow Scholar of the American Society of Hematology.
Submitted December 26, 2000; accepted April 11, 2001.
Supported in part by grants from the National Institutes of Health to M.A.G. (RO1 CA81179-01A1) and M.A. (PO1 CA55164, PO1 CA49639, and CA16672), and from the Deutsche Forschungsgemeinschaft (WU 310/1-1) to G.G.W.
The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked "advertisement" in accordance with 18 U.S.C. section 1734.
Reprints: Margaret A. Goodell, Center for Cell and Gene Therapy, Baylor College of Medicine, N1030, One Baylor Plaza, Houston, TX 77030; e-mail: goodell{at}bcm.tmc.edu.
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 C Massard, E Deutsch, and J-C Soria Tumour stem cell-targeted treatment: elimination or differentiation Ann. Onc., November 1, 2006; 17(11): 1620 - 1624. [Abstract] [Full Text] [PDF]
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M. Fischer, M. Schmidt, S. Klingenberg, C. J. Eaves, C. von Kalle, and H. Glimm Short-term repopulating cells with myeloid potential in human mobilized peripheral blood do not have a side population (SP) phenotype Blood, September 15, 2006; 108(6): 2121 - 2123. [Abstract] [Full Text] [PDF]
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X. Fan, W. Matsui, L. Khaki, D. Stearns, J. Chun, Y.-M. Li, and C. G. Eberhart Notch Pathway Inhibition Depletes Stem-like Cells and Blocks Engraftment in Embryonal Brain Tumors. Cancer Res., August 1, 2006; 66(15): 7445 - 7452. [Abstract] [Full Text] [PDF]
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P. P. Szotek, R. Pieretti-Vanmarcke, P. T. Masiakos, D. M. Dinulescu, D. Connolly, R. Foster, D. Dombkowski, F. Preffer, D. T. MacLaughlin, and P. K. Donahoe Ovarian cancer side population defines cells with stem cell-like characteristics and Mullerian Inhibiting Substance responsiveness PNAS, July 25, 2006; 103(30): 11154 - 11159. [Abstract] [Full Text] [PDF]
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 B. T. Porse, D. Bryder, K. Theilgaard-Monch, M. S. Hasemann, K. Anderson, I. Damgaard, S. E. W. Jacobsen, and C. Nerlov Loss of C/EBP{alpha} cell cycle control increases myeloid progenitor proliferation and transforms the neutrophil granulocyte lineage J. Exp. Med., July 5, 2005; 202(1): 85 - 96. [Abstract] [Full Text] [PDF]
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N. Y. Frank, A. Margaryan, Y. Huang, T. Schatton, A. M. Waaga-Gasser, M. Gasser, M. H. Sayegh, W. Sadee, and M. H. Frank ABCB5-Mediated Doxorubicin Transport and Chemoresistance in Human Malignant Melanoma Cancer Res., May 15, 2005; 65(10): 4320 - 4333. [Abstract] [Full Text] [PDF]
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F. Behbod and J. M. Rosen Will cancer stem cells provide new therapeutic targets? Carcinogenesis, April 1, 2005; 26(4): 703 - 711. [Abstract] [Full Text] [PDF]
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C. Hirschmann-Jax, A. E. Foster, G. G. Wulf, J. G. Nuchtern, T. W. Jax, U. Gobel, M. A. Goodell, and M. K. Brenner A distinct "side population" of cells with high drug efflux capacity in human tumor cells PNAS, September 28, 2004; 101(39): 14228 - 14233. [Abstract] [Full Text] [PDF]
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 P. Krishnamurthy, D. D. Ross, T. Nakanishi, K. Bailey-Dell, S. Zhou, K. E. Mercer, B. Sarkadi, B. P. Sorrentino, and J. D. Schuetz The Stem Cell Marker Bcrp/ABCG2 Enhances Hypoxic Cell Survival through Interactions with Heme J. Biol. Chem., June 4, 2004; 279(23): 24218 - 24225. [Abstract] [Full Text] [PDF]
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T. Kondo, T. Setoguchi, and T. Taga Persistence of a small subpopulation of cancer stem-like cells in the C6 glioma cell line PNAS, January 20, 2004; 101(3): 781 - 786. [Abstract] [Full Text] [PDF]
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N. Y. Frank, S. S. Pendse, P. H. Lapchak, A. Margaryan, D. Shlain, C. Doeing, M. H. Sayegh, and M. H. Frank Regulation of Progenitor Cell Fusion by ABCB5 P-glycoprotein, a Novel Human ATP-binding Cassette Transporter J. Biol. Chem., November 21, 2003; 278(47): 47156 - 47165. [Abstract] [Full Text] [PDF]
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B. L. Abbott, A.-M. Colapietro, Y. Barnes, F. Marini, M. Andreeff, and B. P. Sorrentino Low levels of ABCG2 expression in adult AML blast samples Blood, December 15, 2002; 100(13): 4594 - 4601. [Abstract] [Full Text] [PDF]
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Top
Abstract
Introduction
)/HLA-DR.
Blood.
1998;92:4325-4335