Biomedical subjects

Kevin D Bunting

Publications and source records attributed to Kevin D Bunting.

18 recordsLinked to original sources

PubMed2006

Stat5 expression is required for IgE-mediated mast cell function.

The mast cell (MC) inflammatory response is now linked not only to atopy, but also to arthritis, multiple sclerosis, heart disease, and resistance to bacterial infection. In the current study, we demonstrate that the signal transducer and activator of transcription 5 (Stat5) is rapidly activated by IgE cross-linkage, and that its expression is critical to the MC response. Stat5-deficient (Stat5KO) MC demonstrated a significant decrease in IgE-mediated degranulation, leukotriene B4 production, cytokine secretion, and survival signals. The defect in cytokine production may be caused by decreased cytokine mRNA stability. Stat5KO MC-induced cytokine mRNAs normally following IgE cross-linkage, but these mRNAs were not sustained over time and were degraded at twice the rate observed in WT cells. Interestingly, the RNA destabilizing protein tristetraprolin was induced following IgE cross-linkage in Stat5KO but not wild-type cells. Moreover, reducing tristetraprolin expression via short hairpin RNA transfection significantly increased IL-13 production in Stat5KO MC. Our work demonstrates that Stat5 is a critical factor in IgE-induced MC activation, acting in part via posttranscriptional control of cytokine mRNA stability. These data have a direct impact on MC-associated inflammatory and autoimmune diseases.

Animals↗

PubMed2005

Effects of a leukemia-associated gain-of-function mutation of SHP-2 phosphatase on interleukin-3 signaling.

Mutations in SHP-2 phosphatase that cause hyperactivation of its catalytic activity have been identified in human leukemias, particularly juvenile myelomonocytic leukemia, which is characterized by hypersensitivity of myeloid progenitor cells to granulocyte macrophage colony-stimulating factor and interleukin (IL)-3. However, the molecular mechanisms by which gain-of-function (GOF) mutations of SHP-2 induce hematopoietic malignancies are not fully understood. Our previous studies have shown that SHP-2 plays an essential role in IL-3 signal transduction in both catalytic-dependent and -independent manners and that overexpression (5-6-fold) of wild type (WT) SHP-2 attenuates IL-3-mediated hematopoietic cell function through accelerated dephosphorylation of STAT5. These results raised the possibility that SHP-2-associated leukemias are not solely attributed to the increased catalytic activity of GOF mutant SHP-2. GOF mutant SHP-2 must have gained additional capacities. To test this possibility, we investigated effects of a GOF mutation of SHP-2 (SHP-2 E76K) on hematopoietic cell function and IL-3 signal transduction by comparing with those of overexpressed WT SHP-2. Our results showed that SHP-2 E76K mutation caused myeloproliferative disease in mice, while overexpression of WT SHP-2 decreased hematopoietic potential of the transduced cells in recipient animals. The E76K mutation in the N-terminal Src homology 2 domain increased interactions of mutant SHP-2 with Grb2, Gab2, and p85, leading to hyperactivation of IL-3-induced Erk and phosphatidylinositol 3-kinase (PI3K) pathways. In addition, despite the substantial increase in the catalytic activity, dephosphorylation of STAT5 by SHP-2 E76K was dampened. Furthermore, catalytically inactive SHP-2 E76K with an additional C459S mutation retained the capability to increase the interaction with Gab2 and to enhance the activation of the PI3K pathway. Taken together, these studies suggest that in addition to the elevated catalytic activity, fundamental changes in physical and functional interactions between GOF mutant SHP-2 and signaling partners also play an important role in SHP-2-related leukemigenesis.

Adaptor Proteins, Signal Transducing↗

PubMed2005

Bone marrow transplantation combined with gene therapy to induce antigen-specific tolerance and ameliorate EAE.

Hematopoietic stem cell (HSC) transplantation is a potential therapy that can offer multiple sclerosis patients a radical, potentially curative treatment. Using experimental autoimmune encephalomyelitis (EAE) as a model, we previously reported that retrovirally transduced B cells expressing myelin basic protein (MBP), MBP Ac1-11, or myelin oligodendrocyte glycoprotein p35-55 induced tolerance and reduced symptoms. Here, we extend our tolerance approach using bone marrow (BM) cells expressing full-length phospholipid protein (PLP) in a model for relapsing, remitting EAE. Using GFP expression as a marker, we found that up to 50% of cells were positive for transgene expression in peripheral blood after 900 rad irradiation and transduced BM transplantation, and expression was stable in hematopoietic lineages for over 10 weeks. Upon challenge, T cell proliferation in response to PLP p139-151 was reduced and EAE was completely abolished in a pretreatment protocol. In addition, protection from EAE could be achieved with PLP-transduced BM cells given on day 12 after immunization, a potential therapeutic protocol. Finally, the protective effect of PLP-expressing BM could also be observed using a nonmyeloablative protocol, albeit with lower efficacy. Our results suggest that HSC may be useful to achieve long-lasting tolerance to protect mice from EAE and possibly to promote CNS repair in ongoing EAE.

Animals↗

PubMed2005

Leukemias following retroviral transfer of multidrug resistance 1 (MDR1) are driven by combinatorial insertional mutagenesis.

Previous studies have demonstrated leukemic complications in mice after high-copy retroviral gene transfer of the multidrug resistance 1 (MDR1) cDNA, encoding a membrane-located efflux pump expressed in hematopoietic stem cells. In contrast, no such complications or MDR1-associated alterations of hematopoiesis were observed in numerous other studies exploring MDR1 gene transfer into cell lines, mice, dogs, nonhuman primates, and human subjects. Here, we show that leukemias associated with retroviral expression of MDR1 depend on high vector dose, and involve the selection of clones with combinatorial insertional mutagenesis of proto-oncogenes or other signaling genes. Compared with insertion patterns in normal long-term repopulating hematopoietic cells, such hits were overrepresented in leukemic clones, pointing to a causal role. A similar constellation of insertion sites was also observed in a leukemia arising after high-copy retroviral gene transfer of a fluorescent protein. Spectral karyotyping demonstrated additional chromosomal translocations in a subset of cases, indicative of secondary genetic instability. We also show that insertional mutants can be amplified in vitro prior to transplantation. On the basis of these findings, we suggest the use of preclinical dose-escalation studies to define a therapeutic index for retroviral transgene delivery.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed2005

Stat5 tetramer formation is associated with leukemogenesis.

Activation of Stat5 is frequently found in leukemias. To study the mechanism and role of Stat5 activation, we introduced a constitutively activated Stat5a mutant, cS5F, into murine bone marrow (BM) cells. BM transplantation with cS5F-transfected cells caused development of multilineage leukemias in lethally irradiated wild-type or nonirradiated Rag2(-/-) mice. The leukemic cells showed strongly enhanced levels of cS5F tetramers but unchanged cS5F dimer levels in a DNA binding assay. Moreover, Stat5a mutants engineered to form only dimers, but not tetramers, failed to induce leukemias. In addition, Stat5 tetramers were found to accumulate in excess compared to dimers in various human leukemias. These data suggest that Stat5 tetramers are associated with leukemogenesis.

Animals↗

PubMed2004

Tissue inhibitor of matrix metalloproteinase-1 overexpression in M1 myeloblasts impairs IL-6-induced differentiation.

The balance between matrix metalloproteinase (MMP) and tissue inhibitor of matrix metalloproteinase (TIMP) is important for extracellular matrix interactions of hematopoietic cells. MMP-independent growth modulating activity for TIMP-1 on B lymphocytes and erythroid progenitors has also been described, but a role for TIMP-1 in myelomonocytic differentiation has not been previously reported. In this study, we demonstrate that TIMP-1 overexpression impairs differentiation of the myeloblastic M1 cell line following interleukin (IL)-6 stimulation. We generated retroviral vectors coexpressing human TIMP-1 and the green fluorescent protein (GFP) and stably transduced murine M1 myeloid cells. TIMP-1 expressing cells showed a large reduction in IL-6-induced macrophage differentiation in vitro that was reversible with a specific monoclonal antibody. The differentiation delay in M1/TIMP-1 cells was also specifically reversible by pharmacologic phosphatidylinositol-3 kinase (PI3-K) inhibition. Additionally, overexpression of a TIMP-1/GFP fusion protein also impaired M1 differentiation and this protein was localized to the cell surface, consistent with an autocrine receptor-mediated mechanism. Surprisingly, TIMP-1 transduced cells had a selective advantage for growth in IL-6, indicating that functional effects on growth and differentiation of M1 cells were primarily through an autocrine mechanism. Intrinsic TIMP-1 expression in myeloid leukemia cells might thus impact upon survival or differentiation.

Bone Marrow Cells↗

PubMed2004

A STAT5 modifier locus on murine chromosome 7 modulates engraftment of hematopoietic stem cells during steady-state hematopoiesis.

Homologous disruption of expression of signal transducer and activator of transcription 5a (STAT5a) and STAT5b (STAT5ab(-/-)) in mice results in hematopoietic stem cells (HSCs) that can engraft irradiated hosts alone but are noncompetitive against wild-type HSCs. To explore mechanisms for this phenotype, we crossed the STAT5 mutations onto an HW80 background congenic to the original C57BL/6 that differs in a small chromosome 7 genomic locus. We previously demonstrated that C57BL/6 or HW80 background STAT5ab(-/-) bone marrow (BM) cells showed equal repopulating function either competitively or noncompetitively in irradiated hosts. However, one intraperitoneal injection of wild-type green fluorescent protein (GFP) transgenic BM cells into unconditioned newborn STAT5ab(-/-) recipients of either background was sufficient for high-level donor engraftment. Furthermore, haploinsufficiency of STAT5 (STAT5ab(+/-)) allowed improved engraftment over wild-type recipients, indicating a dose-dependent requirement for STAT5 activation. In reciprocal experiments, STAT5ab(-/-) BM was transplanted into nonirradiated W/W(v) hosts. In these mice, C57BL/6 STAT5ab(-/-) BM cells were 10-fold more defective in long-term engraftment than control wild-type BM cells and HW80 STAT5ab(-/-) BM cells were 5- to 10-fold more defective than C57BL/6 STAT5ab(-/-) BM cells. Therefore, we conclude that STAT5 plays a critical role during steady-state HSC engraftment and a chromosome 7 modifier locus regulates this activity.

Animals↗

PubMed2004

Cutting edge: IL-5 primes Th2 cytokine-producing capacity in eosinophils through a STAT5-dependent mechanism.

Both type-2 CD4(+) Th cells (CD4(+)Th2) and type-2 innate effector cells play critical roles in generating type-2 immunity that can either be protective against parasitic infection or cause tissue damage in allergy and asthma. How innate effector cells acquire the capacity to produce Th2 cytokines is not entirely known. We previously showed that IL-4 induced differentiation of Th2 cytokine-producing eosinophils. To determine whether other Th2 cytokines can also induce Th2 cytokine-producing capacity in innate effector cells, we cultured bone marrow progenitor cells in the presence of various Th2 cytokines. IL-5, but not IL-13 or IL-25, primed bone marrow progenitor cells to differentiate into robust IL-4-producing cells. The majority of IL-4-producing cells induced by IL-5 were eosinophils. Importantly, IL-5 completely depended on STAT5 to promote IL-4-producing capacity in eosinophils. Thus, our study demonstrates that IL-5 functions as a potent factor that drives bone marrow progenitor cells into IL-4-producing eosinophils.

Animals↗

PubMed2004

Increased numbers of committed myeloid progenitors but not primitive hematopoietic stem/progenitors in mice lacking STAT6 expression.

Signal transducer and activator of transcription-6 (STAT6) plays important roles in cytokine signaling via interleukin-4 and -13 receptors (IL-4R and IL-13R). Mice in which STAT6 has been disrupted by homologous recombination show defects in T helper cell type 2 (Th2) lymphocyte production, resulting in an accumulation of Th1 cells. In addition to defects in differentiation and proliferation of T lymphocytes, STAT6-deficient mice show increased cell-cycle activation and frequency of myeloid progenitors. Although this has been shown to be mediated through Oncostatin M production by T cells, IL-4Ralpha and STAT6 have also recently been found to be enriched for expression in primitive hematopoietic stem cells (HSCs) in gene expression-profiling studies. Therefore, we have investigated whether defects in hematopoietic function in mice lacking STAT6 expression extended into the primitive hematopoietic compartments of the bone marrow. Here, we report that STAT6 deficiency increased bone marrow-committed myeloid progenitors but did not alter the number of cells enriched for HSC/multipotent progenitors, primitive cobblestone area-forming cells assayed in vitro, or bone marrow short-term or long-term repopulating cells assayed in vivo. Therefore, the requirement for STAT6 activation during hematopoiesis is limited, and primitive hematopoietic cell types are insulated against possible effects of cytokine stimulation by Th1 cells.

Animals↗

PubMed2004

A negative role of SHP-2 tyrosine phosphatase in growth factor-dependent hematopoietic cell survival.

SHP-2 tyrosine phosphatase is highly expressed in hematopoietic cells; however, the function of SHP-2 in hematopoietic cell processes is not fully understood. Recent identification of SHP-2 mutations in childhood leukemia further emphasizes the importance of SHP-2 regulation in hematopoietic cells. We previously reported that SHP-2 played a positive role in IL-3-induced activation of Jak2 kinase in a catalytic-dependent manner. Interestingly, enforced expression of wild-type (WT) SHP-2 in Ba/F3 cells enhanced growth factor deprivation-induced apoptosis. Biochemical analyses revealed that although IL-3 activation of Jak2 kinase was increased, tyrosyl phosphorylation of its downstream substrate STAT5 was disproportionately decreased by the overexpression of SHP-2. Following IL-3 deprivation, the tyrosyl phosphorylation of STAT5 that is required for its antiapoptotic activity was rapidly diminished in SHP-2 overexpressing cells. As a result, reduction of the putative downstream targets of STAT5-Bcl-X(L) and pim-1 was accelerated by overexpression of SHP-2. Further investigation showed that SHP-2 associated with STAT5, and that it was indeed able to dephosphorylate STAT5. Finally, overexpression of SHP-2 in primary bone marrow hematopoietic progenitor cells compromised their differentiative and proliferative potential, and enhanced growth factor withdrawal-induced cell death. And, the effect of SHP-2 overexpression on growth factor-dependent survival was diminished in STAT5-deficient hematopoietic cells. Taken together, these results suggest that SHP-2 tyrosine phosphatase negatively regulates hematopoietic cell survival by dephosphorylation of STAT5.

Animals↗

PubMed2004

Safety concerns related to hematopoietic stem cell gene transfer using retroviral vectors.

Endogenous retroviruses have developed efficient methods during their life cycle for stable integration into the host genome. Because of this ability, retroviral vectors were designed with the goal of gene transfer into hematopoietic stem cells (HSCs). The ability to genetically modify HSCs provides a vehicle for durable expression of potentially therapeutic transgenes in all lineages of mature blood cells for the lifetime of the patient. Combined with bone marrow transplant, retroviral gene transfer has many potential applications for a wide range of blood diseases. Advances in the development of oncoretroviral vectors based on murine leukemia viruses (MLV) and more recent development of human immunodeficiency virus (HIV)-based vectors have greatly increased the gene transfer efficiency. Optimization of methods for gene transfer using MLV-based vectors has substantially improved marking levels in mice, with lower levels in large animals and in human clinical trials. With advances in gene transfer technology has also come renewed concern about insertional mutagenesis and activation of oncogenes. Advanced techniques for integration site analysis combined with sequence comparison using mouse and human genome databases has now made it possible to begin to understand the spectrum of possible integration sites for both MLV- and HIV-based vectors. Furthermore, other studies have shown positive and negative dosage-dependent effects of transgene expression in mouse and human cells. Therefore, vector design and safety testing are at the forefront of the field of gene therapy and this review discusses recent developments.

Animals↗

PubMed2003

Hematopoietic-repopulating defects from STAT5-deficient bone marrow are not fully accounted for by loss of thrombopoietin responsiveness.

Signal transducer and activator of transcription-5 (STAT5) plays an important role in repopulating activity of hematopoietic stem cells (HSCs). However, the relationship of STAT5 activation with early acting cytokine receptors is not well established. We have directly compared bone marrow (BM) from mice mutant for STAT5a and STAT5b (STAT5ab(-/-)) with that from mice lacking c-Mpl (c-Mpl(-/-)), the thrombopoietin receptor. Both STAT5 and c-Mpl deficiency only mildly affected committed myeloid progenitors assayed in vitro, but STAT5ab(-/-) BM showed lower Gr-1+ (4.4-fold), B220+ (23-fold), CD4+ (20-fold), and Ter119+ (17-fold) peripheral blood repopulating activity than c-Mpl(-/-) BM against wild-type competitor in long-term repopulating assays in vivo. Direct head-to-head competitions of STAT5ab(-/-) BM and c-Mpl(-/-) BM showed up to a 25-fold reduction in STAT5ab(-/-) contribution. Differences affecting reconstitution of primitive c-Kit+Lin-Sca-1+ multipotent progenitor (MPP)/HSC (1.8-fold) and c-Kit+Lin-Sca-1- oligopotent progenitor BM fractions (3.3-fold) were more modest. In serial transplantation experiments, STAT5ab(-/-) and c-Mpl(-/-) BM both failed to provide consistent engraftment in tertiary hosts and could not radioprotect lethally irradiated quaternary recipients. These results indicate substantial overlap in c-Mpl-STAT5 signaling defects at the MPP/HSC level but indicate that STAT5 is activated independent of c-Mpl to promote multilineage hematopoietic differentiation.

Animals↗

PubMed2003

Stat5 expression is critical for mast cell development and survival.

Interleukin-3 (IL-3) and stem cell factor (SCF) are important mast cell growth and differentiation factors. Since both cytokines activate the transcription factor signal transducer and activator of transcription 5 (Stat5), a known regulator of proliferation and survival, we investigated the effects of Stat5 deficiency on mast cell development and survival. Bone marrow-derived mast cell (BMMC) populations cultured from Stat5A/B-deficient mice survived in IL-3 + SCF, but not in either cytokine alone. These cells demonstrated reduced expression of Bcl-2, Bcl-x(L), cyclin A2, and cyclin B1, with increased apoptosis and delayed cell cycle progression during IL-3 or SCF culture. Finally, the absence of Stat5 resulted in loss of in vivo mast cell development, as judged by assessments of Stat5-deficient mice and transplantation of Stat5-deficient bone marrow cells to mast cell-deficient recipient mice. These results indicate that Stat5A and Stat5B are critical regulators of in vitro and in vivo mast cell development and survival.

Animals↗

PubMed2003

Integrative molecular and developmental biology of adult stem cells.

Stem cells are believed to be important for regeneration of several adult tissues. Recently, adult stem cells with very broad differentiation potential have been identified although whether they represent vestigial primitive pluripotent stem cells or products of extremely rare de-differentiation events involving tissue-specific stem cells is not known. Transdifferentiation of tissue-specific stem cells across lineage boundaries has also been demonstrated but the relative inefficiency of the process in vivo, even in the presence of tissue damage, questions whether such a mechanism is of physiologic relevance. Interestingly, among adult stem cells, the capacity for lineage switching appears to be greatest in stem cells that can be cultured ex vivo for extended periods. If the normal cell fate decisions of diverse adult stem cell types could be reliably redirected at high frequency in situ, possible regenerative therapies for a wide variety of diseases could be envisioned. An integrated understanding of the transcriptional regulatory networks that comprise the various adult stem cell entities as well as the signaling pathways governing their differentiation into therapeutically useful cell types will facilitate clinical application of these exciting findings.

ADP-ribosyl Cyclase↗

PubMed2002

Cell intrinsic defects in cytokine responsiveness of STAT5-deficient hematopoietic stem cells.

Secreted growth factors are integral components of the bone marrow (BM) niche and can regulate survival, proliferation, and differentiation of committed hematopoietic stem cells (HSCs). However, downstream genes activated in HSCs by early-acting cytokines are not well characterized. To better define intracellular cytokine signaling in HSC function, we have analyzed mice lacking expression of both signal transducer and activator of transcription 5a (STAT5a) and STAT5b (STAT5ab(-/-)). These studies specifically avoided possible autoimmune and/or splenomegaly disease-mediated indirect effects on HSC function by using 2 independent approaches: (1) by crossing onto the C57Bl/6 RAG2(-/-) background, and (2) by generation of wild-type chimeric mice reconstituted with transplanted STAT5ab(-/-) BM cells. These experiments demonstrated that STAT5-deficient HSCs have cell autonomous defects in competitive long-term repopulating activity. Furthermore, in the chimeric mice, injected wild-type BM cells showed a progressive multilineage competitive repopulating advantage in vivo, demonstrating that steady-state hematopoiesis was also highly STAT5-dependent. Consistent with the in vivo repopulating deficiency, when Sca-1(+)c-kit(+)lin(-) (KLS) cells were isolated and stimulated with growth factors in vitro, up to a 13-fold reduced expansion of total nucleated cells was observed in response to cocktails containing interleukin 3 (IL-3), IL-6, stem cell factor (SCF), Flt3 ligand, and thrombopoietin. Notably, a 10-fold reduction in expansion was observed with IL-3 and SCF. However, STAT5 activation was not required for regeneration of the KLS pool in vivo following transplant or for secondary repopulating ability. These studies support a major role for STAT5 activation as a cellular determinant of cytokine-mediated HSC repopulating potential but not self-renewal capacity.

Animals↗

PubMed2002

The tao of hematopoietic stem cells: toward a unified theory of tissue regeneration.

Hematopoietic stem cells (HSCs) are the best studied of the tissue-specific stem cells. By definition, HSCs have long been regarded as restricted to formation of blood cells of both the lymphoid and myeloid lineages. HSCs residing in the bone marrow microenvironment have self-renewal capacity and can repopulate the hematopoietic system of irradiated transplant recipients for the lifetime of the individual. Therefore, HSCs are extremely important targets for gene therapy applications aimed toward the treatment of inherited and acquired blood disorders. However, recent studies have suggested that a subpopulation of HSCs may have the ability to contribute to diverse cell types such as hepatocytes, myocytes, and neuronal cells, especially following induced tissue damage. Preclinical amelioration of liver disease and myocardial infarcts by HSC-enriched bone marrow cell populations raises the possibility that HSC transplants have the potential to provide therapeutic benefit for a wide variety of diseases. These surprising findings contradict the dogma that adult stem cells are developmentally restricted. Extrapolation of these findings to the clinic will be facilitated by prospective identification of the stem cells that possess this developmental plasticity. Furthermore, characterization of the signaling pathways and molecular determinants regulating the remarkable transdifferentiation capacity of these stem cells may provide insight into novel approaches for modulating frequency of differentiative potential.

Animals↗

PubMed2002

Reduced lymphomyeloid repopulating activity from adult bone marrow and fetal liver of mice lacking expression of STAT5.

Signal transducers and activators of transcription (STATs) are intracellular mediators of cytokine receptor signals. Because many early-acting growth factors have been implicated in STAT5 activation, this study sought to investigate whether STAT5 may be a transcriptional regulator of hematopoietic stem cell (HSC) long-term repopulating activity. To test this possibility, bone marrow (BM) and fetal liver (FL) cells from mice containing homozygous deletions of both STAT5a and STAT5b genes (STAT5ab(-/-)) were characterized for hematopoietic repopulating activities. BM and FL grafts were capable of repopulating lymphoid and myeloid lineages of lethally irradiated primary and secondary hosts, with defects observed primarily in T-lymphocyte engraftment. Because only a fraction of normal HSC function is required to reconstitute hematopoiesis, competitive repopulation assays of adult BM or FL cells were used against wild type adult BM or FL cells to quantitate stem cell function. In these analyses, average 25-, 28-, 45-, and 68-fold decreases in normal repopulating activity were evident in granulocyte (Gr-1(+)), macrophage (Mac-1(+)), erythroid progenitor (Ter119(+)), and B-lymphocyte (B220(+)) populations, respectively, with T lymphocytes (CD4(+)) always undetectable from the STAT5ab(-/-) graft. Consistent with previous reports of divergence between stem cell phenotype and function in cases of perturbed hematopoiesis, the absolute number of cells within Sca-1(+)c-kit(+)lin(-) or lin(-) Hoechst 33342 side population fractions was not significantly different between wild type and STAT5ab(-/-) BM or FL cells. These results demonstrate that a significant proportion of the growth factor signals required for multilineage reconstitution potential of HSCs is STAT5 dependent.

Animals↗

PubMed2002

ABC transporters as phenotypic markers and functional regulators of stem cells.

Characterization of molecules with tightly controlled expression patterns during differentiation represents an approach to understanding regulation of hematopoietic stem cell commitment. The multidrug resistance-1 (MDR1) gene product, P-glycoprotein, and the breast cancer resistance protein (BCRP) are expressed differentially during hematopoiesis, with the highest levels in primitive bone marrow stem cell populations that are CD34(low) and CD34(-), respectively. Roles for ATP-binding cassette (ABC) transporter superfamily members in conferring drug resistance have been extensively described. However, recent hematopoietic overexpression studies have begun to reveal previously unknown roles for ABC transporter function in normal and malignant hematopoiesis. Expression of MDR1 and BCRP transporters in the myeloid lineage has been reported in blasts from acute myeloid leukemia, but very low to undetectable in normal myelomonocytic cells. Retroviral-mediated dysregulated expression of the MDR1 transporter resulted in increased hematopoietic repopulating activity and myeloproliferative disease in mice. A distinct functional role for the BCRP transporter as a negative regulator of hematopoietic repopulating activity has recently been demonstrated using the same approach. Additionally, the presence of BCRP expression specifically on hematopoietic side-population stem cells and neural stem/progenitors, makes BCRP an attractive candidate marker for isolation of stem cells with the ability to respond to diverse environmental cues. Regulation of stem cell biology by ABC transporters has emerged as an important new field of investigation. In light of these findings, it will be critical to further characterize this family of proteins in hematopoietic lineage-restricted stem cells and in pluripotent stem cells capable of crossing lineage barriers.

ATP Binding Cassette Transporter, Subfamily B, Mem↗