Suppression of apoptosis allows differentiation and development of a multipotent hemopoietic cell line in the absence of added growth factors

Transcript-Level Dysregulation of BCL2 Family Genes in Acute Myeloblastic Leukemia

The expression of apoptosis-related BCL2 family genes, fine-tuned in normal cells, is dysregulated in many neoplasms. In acute myeloid leukemia (AML), this problem has not been studied comprehensively. To address this issue, RNA-seq data were used to analyze the expression of 26 BCL2 family members in 27 AML FAB M1 and M2 patients, divided into subgroups differently responding to chemotherapy. A correlation analysis, analysis of variance, and Kaplan-Meier analysis were applied to associate the expression of particular genes with other gene expression, clinical features, and the presence of mutations detected by exome sequencing. The expression of BCL2 family genes was dysregulated in AML, as compared to healthy controls. An upregulation of anti-apoptotic and downregulation of pro-apoptotic genes was observed, though only a decrease in BMF, BNIP1, and HRK was statistically significant. In a group of patients resistant to chemotherapy, overexpression of BCL2L1 was manifested. In agreement with the literature data, our results reveal that BCL2L1 is one of the key players in apoptosis regulation in different types of tumors. An exome sequencing data analysis indicates that BCL2 family genes are not mutated in AML, but their expression is correlated with the mutational status of other genes, including those recurrently mutated in AML and splicing-related. High levels of some BCL2 family members, in particular BIK and BCL2L13, were associated with poor outcome.

Regulating Stem Cell-Related Genes Induces the Plastic Differentiation of Cancer Stem Cells to Treat Breast Cancer

Relapse of cancer is associated with multidirectional differentiation and unrestricted proliferative replication potential of cancer stem cells. Herein, we propose the plastic differentiation strategy for irreversible differentiation of cancer stem cells; further, salinomycin and its newly constructed functional liposomes are used to implement this strategy. Whole gene, cancer stem cell-related RNA, and protein expression analyses reveal that salinomycin induces the cancer stem cells into normal cells, dormant cells, and mature cancer cells. Besides, the results indicate that the gatekeeper is related to the inhibition of the protein kinase C (PKC) α signaling pathway. The differentiated normal or dormant cells are incorporated into normal tissue, whereas the rest are killed by chemotherapy. The findings would offer the evidence for plastic differentiation of cancer stem cells and propose a novel strategy for cancer therapy.

Identification of transcript regulatory patterns in cell differentiation

Motivation

Studying transcript regulatory patterns in cell differentiation is critical in understanding its complex nature of the formation and function of different cell types. This is done usually by measuring gene expression at different stages of the cell differentiation. However, if the gene expression data available are only from the mature cells, we have some challenges in identifying transcript regulatory patterns that govern the cell differentiation.

Results

We propose to exploit the information of the lineage of cell differentiation in terms of correlation structure between cell types. We assume that two different cell types that are close in the lineage will exhibit many common genes that are co-expressed relative to those that are far in the lineage. Current analysis methods tend to ignore this correlation by testing for differential expression assuming some sort of independence between cell types. We employ a Bayesian approach to estimate the posterior distribution of the mean of expression in each cell type, by taking into account the cell formation path in the lineage. This enables us to infer genes that are specific in each cell type, indicating the genes are involved in directing the cell differentiation to that particular cell type. We illustrate the method using gene expression data from a study of haematopoiesis.

Availability and implementation

R codes to perform the analysis are available in http://www1.maths.leeds.ac.uk/∼arief/R/CellDiff/.

Contact

a.gusnanto@leeds.ac.uk.

Supplementary information

Supplementary data are available at Bioinformatics online.

Growth and Differentiation Factors

This review is restricted to neutrophilic granulocytes (granulocytes), monocytes (macrophages), and eosinophils, with only passing reference to cells that are also usually included in the "myeloid" category-megakaryocytes, mast cells, and erythroid cells. Although some dendritic cells are of myeloid origin, they are discussed elsewhere. The validity of the information to be described depends on two assumptions: (a) that in vitro data are applicable to events in vivo and (b) that mouse data reflect events in man. Both assumptions are likely to be broadly correct.

Instruction of hematopoietic lineage choice by cytokine signaling

Hematopoiesis is the cumulative consequence of finely tuned signaling pathways activated through extrinsic factors, such as local niche signals and systemic hematopoietic cytokines. Whether extrinsic factors actively instruct the lineage choice of hematopoietic stem and progenitor cells or are only selectively allowing survival and proliferation of already intrinsically lineage-committed cells has been debated over decades. Recent results demonstrated that cytokines can instruct lineage choice. However, the precise function of individual cytokine-triggered signaling molecules in inducing cellular events like proliferation, lineage choice, and differentiation remains largely elusive. Signal transduction pathways activated by different cytokine receptors are highly overlapping, but support the production of distinct hematopoietic lineages. Cellular context, signaling dynamics, and the crosstalk of different signaling pathways determine the cellular response of a given extrinsic signal. New tools to manipulate and continuously quantify signaling events at the single cell level are therefore required to thoroughly interrogate how dynamic signaling networks yield a specific cellular response.

Real-time monitoring of caspase cascade activation in living cells

We introduce a simple, versatile and robust one-step technique that enables real-time imaging of multiple intracellular caspase activities in living cells without the need for complicated synthetic protocols. Conventional fluorogenic probes or recently reported activatable probes have been designed to target various proteases but are limited to extracellular molecules. Only a few have been applied to image intracellular proteases in living cells because most of these probes have limited cell-permeability. Our platform does not need complicated synthetic processes; instead it involves a straightforward peptide synthesis and a simple mixing step with a commercial transfection agent. The transfection agent efficiently delivered the highly quenched fluorogenic probes, comprised of distinctive pairs of dyes and quenchers, to the initiator caspase-8 and the effector caspase-3 in MDA-MB-435 cells, allowing dual-imaging of the activities of both caspases during the apoptotic process induced by TNF-related apoptosis induced ligand (TRAIL). With the combination of multiple fluorogenic probes, this simple platform can be applied to multiplexed imaging of selected intracellular proteases to study apoptotic processes in pathologies or for cell-based high throughput screening systems for drug discovery.

Lymphoid and myeloid lineage commitment in multipotent hematopoietic progenitors

Hematopoietic stem cells (HSCs) continuously replenish all classes of blood cells through a series of lineage restriction steps that results in the progressive loss of differentiation potential to other cell lineages. This review focuses on the recent advances in understanding one of the earliest differentiation steps in HSC maturation, which involves the diversification of the lymphoid and myeloid cell lineages, the two major branches of hematopoietic cells. We discuss progress in the identification and characterization of progenitor populations downstream of HSCs, which has been a key to understanding the sequential biological events that take place along the course of differentiation into a certain hematopoietic cell type. We also discuss the importance of bone marrow microenvironment in lymphoid and myeloid lineage choice.

The potential landscape of genetic circuits imposes the arrow of time in stem cell differentiation

Differentiation from a multipotent stem or progenitor state to a mature cell is an essentially irreversible process. The associated changes in gene expression patterns exhibit time-directionality. This "arrow of time" in the collective change of gene expression across multiple stable gene expression patterns (attractors) is not explained by the regulated activation, the suppression of individual genes which are bidirectional molecular processes, or by the standard dynamical models of the underlying gene circuit which only account for local stability of attractors. To capture the global dynamics of this nonequilibrium system and gain insight in the time-asymmetry of state transitions, we computed the quasipotential landscape of the stochastic dynamics of a canonical gene circuit that governs branching cell fate commitment. The potential landscape reveals the global dynamics and permits the calculation of potential barriers between cell phenotypes imposed by the circuit architecture. The generic asymmetry of barrier heights indicates that the transition from the uncommitted multipotent state to differentiated states is inherently unidirectional. The model agrees with observations and predicts the extreme conditions for reprogramming cells back to the undifferentiated state.

The colony-stimulating factors and cancer

The four colony-stimulating factors (CSFs) are glycoproteins that regulate the generation and some functions of infection-protective granulocytes and macrophages. Recombinant granulocyte-CSF (G-CSF) and granulocyte-macrophage-CSF (GM-CSF) have now been used to increase dangerously low white blood cell levels in many millions of cancer patients following chemotherapy. These CSFs also release haematopoietic stem cells to the peripheral blood, and these cells have now largely replaced bone marrow as more effective populations for transplantation to cancer patients who have treatment-induced bone marrow damage.

Reprogramming cell fates: reconciling rarity with robustness

The stunning possibility of "reprogramming" differentiated somatic cells to express a pluripotent stem cell phenotype (iPS, induced pluripotent stem cell) and the "ground state" character of pluripotency reveal fundamental features of cell fate regulation that lie beyond existing paradigms. The rarity of reprogramming events appears to contradict the robustness with which the unfathomably complex phenotype of stem cells can reliably be generated. This apparent paradox, however, is naturally explained by the rugged "epigenetic landscape" with valleys representing "preprogrammed" attractor states that emerge from the dynamical constraints of the gene regulatory network. This article provides a pedagogical primer to the fundamental principles of gene regulatory networks as integrated dynamic systems and reviews recent insights in gene expression noise and fate determination, thereby offering a formal framework that may help us to understand why cell fate reprogramming events are inherently rare and yet so robust.

Metabolic basis of benzene toxicity

Potential metabolic mechanisms underlying the haemopoietic toxicity of benzene include bioactivation of phenolic metabolites of benzene by peroxidases in bone marrow and ring opening reactions to generate muconate derivatives. Peroxidase-mediated activation of phenolic metabolites of benzene generates reactive quinones which can be detoxified by NAD(P)H:quinone acceptor oxidoreductase (NQO1). The major peroxidase enzyme in bone marrow is myeloperoxidase (MPO) and potential target cells for phenolic metabolites of benzene were characterized in bone marrow stroma on the basis of high MPO:NQO1 ratios. MPO was found to be expressed at the level of myeloid progenitor cells in both murine (lineage negative cells) and human (CD34+ cells) systems. This suggests that progenitor cells may be relevant targets of phenolic metabolites of benzene resulting in aberrant haemopoiesis. A polymorphism in NQO1 is also described which leads to a complete lack of NQO1 activity. The toxicological significance of this polymorphism with respect to benzene toxicity is under investigation.

Development of macrophages of cyprinid fish

The innate immune responses of early vertebrates, such as bony fishes, play a central role in host defence against infectious diseases and one of the most important effector cells of innate immunity are macrophages. In order for macrophages to be effective in host defence they must be present at all times in the tissues of their host and importantly, the host must be capable of rapidly increasing macrophage numbers during times of need. Hematopoiesis is a process of formation and development of mature blood cells, including macrophages. Hematopoiesis is controlled by soluble factors known as cytokines, that influence changes in transcription factors within the target cells, resulting in cell fate changes and the final development of specific effector cells. The processes involved in macrophage development have been largely derived from mammalian model organisms. However, recent advancements have been made in the understanding of macrophage development in bony fish, a group of organisms that rely heavily on their innate immune defences. Our understanding of the growth factors involved in teleost macrophage development, as well as the receptors and regulatory mechanisms in place to control them has increased substantially. Furthermore, model organisms such as the zebrafish have emerged as important instruments in furthering our understanding of the transcriptional control of cell development in fish as well as in mammals. This review highlights the recent advancements in our understanding of teleost macrophage development. We focused on the growth factors identified to be important in the regulation of macrophage development from a progenitor cell into a functional macrophage and discuss the important transcription factors that have been identified to function in teleost hematopoiesis. We also describe the findings of in vivo studies that have reinforced observations made in vitro and have greatly improved the relevance and importance of using teleost fish as model organisms for studying developmental processes.

Heterogeneity of embryonic and adult stem cells

New studies suggest that stem cells of embryonic, neural, and hematopoietic origin are heterogeneous, with cells moving between two or more metastable states. These cell states show a bias in their differentiation potential and correlate with specific patterns of transcription factor expression and chromatin modifications.

Eosinophil progenitors in allergy and asthma - do they matter?

Allergic inflammation is associated with marked infiltration of eosinophils in affected tissues. The eosinophil is believed to be a key effector cells in allergen induced asthma pathogenesis. However, the role of eosinophils in the clinical manifestation of asthma has recently been questioned, since therapies directed against eosinophil infiltration (i.e. anti-interleukin-5) failed to improve clinical symptoms such as airways hyper-responsiveness (AHR) in patients with asthma. Although eosinophils in peripheral blood and the airways were largely depleted after anti-IL-5 treatment, residual eosinophilia in lung tissue persisted, which permits speculation that the remaining eosinophils may be sufficient to drive the asthma symptomatology. Furthermore, recent findings suggest that primitive eosinophil progenitor cells traffic from the bone marrow to sites of inflammation in response to allergen exposure. These progenitors may then differentiate in situ and thus provide an ongoing supply of mature pro-inflammatory cells and secretory mediators that augment the inflammatory response. In the present article, we will review the evidence for these findings, and discuss the rationale for targeting hematopoiesis and their migration pathways in the treatment of allergic diseases. Furthermore, this review will highlight the hypothesis that both IL-5- and CCR3-mediated signaling pathways may need to be targeted in order to control the inflammation and AHR associated with asthma.

Gene expression analysis of major lineage-defining factors in human bone marrow cells: effect of aging, gender, and age-related disorders

Adult bone marrow cells (BMCs) include two populations:;mesenchymal stem cells (MSCs), which can differentiate into bone, cartilage, and fat; and hematopoietic stem cells (HSCs), which produce all mature blood lineage. To study the effect of aging, gender, and age-related disorders on lineage differentiation, we performed quantitative RT-PCR to examine mRNA expression of the major factors defining BMC lineage, cbfa1 for osteoblasts, ppar-gamma for adipocytes, sox9 for chondrocytes, and rankl for osteoclasts, in bone marrow from 80 healthy subjects and patients (14-79 years old) with two age-related disorders: osteoarthritis (OA) and rheumatoid arthritis (RA). Two apoptosis-related genes, bcl-2 and drak1, were studied. RANKL and PPAR-Gamma levels exhibited a clear positive correlation with age in female patients, but not in males, with a slight age-related decline in CBFa1 transcripts. DRAK1 expression showed an age-associated ascending trend with significantly greater transcripts of RANKL and DRAK1 in females (p < 0.01). Compared with age-matched controls, RA patients exhibited increased RANKL, PPAR-Gamma, and DRAK1 mRNA levels (p < 0.05), and OA showed the higher RANKL and PPAR-Gamma transcripts (p < 0.05). Furthermore, SOX9 and DRAK1 expressions in the RA group were higher than in the OA group (p < 0.05). Our data indicate that aging and age-related disorders affect gene expressions differently, suggesting that in aging, the lineage of bone marrow cells was modified with prominent changes in decreased bone marrow osteoblastogenesis, increased adipogenesis and osteoclastogenesis, while in age-related disorders, marrow adipogenesis and the activity or number of osteoclasts may play an important role in the pathogenesis of arthritic bone loss.

Steel factor coordinately regulates the molecular signature and biologic function of hematopoietic stem cells

Hematopoietic stem cells (HSCs) regenerated in vivo display sustained differences in their self-renewal and differentiation activities. Variations in Steel factor (SF) signaling are known to affect these functions in vitro, but the cellular and molecular mechanisms involved are not understood. To address these issues, we evaluated highly purified HSCs maintained in single-cell serum-free cultures containing 20 ng/mL IL-11 plus 1, 10, or 300 ng/mL SF. Under all conditions, more than 99% of the cells traversed a first cell cycle with similar kinetics. After 8 hours in the 10 or 300 ng/mL SF conditions, the frequency of HSCs remained unchanged. However, in the next 8 hours (ie, 6 hours before any cell divided), HSC integrity was sustained only in the 300 ng/mL SF cultures. The cells in these cultures also contained significantly higher levels of Bmi1, Lnk, and Ezh2 transcripts but not of several other regulators. Assessment of 21 first division progeny pairs further showed that only those generated in 300 ng/mL SF cultures contained HSCs and pairs of progeny with similar differentiation programs were not observed. Thus, SF signaling intensity can directly and coordinately alter the transcription factor profile and long-term repopulating ability of quiescent HSCs before their first division.

Permissive roles of hematopoietin and cytokine tyrosine kinase receptors in early T-cell development

Although several cytokines have been demonstrated to be critical regulators of development of multiple blood cell lineages, it remains disputed to what degree they act through instructive or permissive mechanisms. Signaling through the FMS-like tyrosine kinase 3 (FLT3) receptor and the hematopoietin IL-7 receptor alpha (IL-7Ralpha) has been demonstrated to be of critical importance for sustained thymopoiesis. Signaling triggered by IL-7 and thymic stromal lymphopoietin (TSLP) is dependent on IL-7Ralpha, and both ligands have been implicated in T-cell development. However, we demonstrate that, whereas thymopoiesis is abolished in adult mice doubly deficient in IL-7 and FLT3 ligand (FLT3L), TSLP does not play a key role in IL-7-independent or FLT3L-independent T lymphopoiesis. Furthermore, whereas previous studies implicated that the role of other cytokine tyrosine kinase receptors in T lymphopoiesis might not involve permissive actions, we demonstrate that ectopic expression of BCL2 is sufficient not only to partially correct the T-cell phenotype of Flt3l(-/-) mice but also to rescue the virtually complete loss of all discernable stages of early T lymphopoiesis in Flt3l(-/-)Il7r(-/-) mice. These findings implicate a permissive role of cytokine receptors of the hematopoietin and tyrosine kinase families in early T lymphopoiesis.

Hematopoietic cytokines

The production of hematopoietic cells is under the tight control of a group of hematopoietic cytokines. Each cytokine has multiple actions mediated by receptors whose cytoplasmic domains contain specialized regions initiating the various responses-survival, proliferation, differentiation commitment, maturation, and functional activation. Individual cytokines can be lineage specific or can regulate cells in multiple lineages, and for some cell types, such as stem cells or megakaryocyte progenitors, the simultaneous action of multiple cytokines is required for proliferative responses. The same cytokines control basal and emergency hematopoietic cell proliferation. Three cytokines, erythropoietin, granulocyte colony-stimulating factor, and granulocyte-macrophage colony-stimulating factor, have now been in routine clinical use to stimulate cell production and in total have been used in the management of many millions of patients. In this little review, discussion will be restricted to those cytokines well established as influencing the production of hematopoietic cells and will exclude newer candidate regulators and those active on lymphoid cells. As requested, this account will describe the cytokines in a historical manner, using a sequential format of discovery, understanding, validation, and puzzlement, a sequence that reflects the evolving views on these cytokines over the past 50 years.

Regulation of allergen-induced bone marrow eosinophilopoiesis: role of CD4+ and CD8+ T cells

BACKGROUND

The mechanisms of the distant stimulation of the bone marrow (BM) after airway allergen exposure remain largely obscure. T cells have been implicated in allergic airway inflammation but their role in allergen-induced BM eosinophilopoiesis is poorly understood. The aim of this study was to determine the role of CD4(+) and CD8(+) T cells in allergen-induced BM eosinophilopoiesis.

METHODS

Ovalbumin (OVA)-sensitized wild type (WT), CD4 knockout (CD4-/-) and CD8 knockout (CD8-/-) mice were exposed intranasally to OVA or saline. Bromo-deoxyuridine (BrdU) was used to label newly produced cells. Bone marrow, blood and bronchoalveolar lavage (BAL) were sampled 24 h after the final exposure. Immunostaining for newly produced eosinophils (i.e. BrdU(+)/MBP(+)) and BM eosinophil progenitor [CD34(+)/CD45(+)/interleukin-5 (IL-5)Ralpha(+)] cells was performed.

RESULTS

The number of newly produced BM eosinophils (BrdU(+)/MBP(+) cells) was significantly reduced in allergen exposed CD4-/- or CD8-/- mice compared with allergen exposed WT mice, which was followed by a subsequent decrease in newly produced blood and airway eosinophils. Furthermore, BM eosinophil progenitors were significantly reduced in allergen exposed CD4-/- and CD8-/- mice compared with WT mice. Finally, serum IL-5 and Bronchoalveolar lavage fluid eotaxin-2 levels were abolished in allergen exposed CD4-/- mice to levels seen in saline exposed WT mice.

CONCLUSIONS

These data suggests that both CD4(+) and CD8(+) T cells have a regulatory role in allergen-induced BM eosinophilopoiesis, whereas CD4(+) T cells are obligatory for allergen-induced airway eosinophilia. The subsequent traffic of eosinophils to the airways is likely to be at least partly regulated by a CD4(+) T-cell-dependent local airway eotaxin-2 production.

Expression of Fas and Fas-ligand in donor hematopoietic stem and progenitor cells is dissociated from the sensitivity to apoptosis

OBJECTIVE

The interaction between the Fas receptor and its cognate ligand (FasL) has been implicated in the mutual suppression of donor and host hematopoietic cells after transplantation. Following the observation of deficient early engraftment of Fas and FasL-defective donor cells and recipients, we determined the role of the Fas-FasL interaction.

METHODS

Donor cells were recovered after syngeneic (CD45.1-->CD45.2) transplants from various organs and assessed for expression of Fas/FasL in reference to lineage markers, carboxyfluorescein succinimidyl ester dilution, Sca-1 and c-kit expression. Naïve and bone marrow-homed cells were challenged for apoptosis ex vivo.

RESULTS

The Fas receptor and ligand were markedly upregulated to 40% to 60% (p < 0.001 vs 5-10% in naïve cells) within 2 days after syngeneic transplantation, while residual host cells displayed modest and delayed upregulation of these molecules ( approximately 10%). All lin(-)Sca(+)c-kit(+) cells were Fas(+)FasL(+), including 95% of Sca-1(+) and 30% of c-kit(+) cells. Fas and FasL expression varied in donor cells that homed to bone marrow, spleen, liver and lung, and was induced by interaction with the stroma, irradiation, cell cycling, and differentiation. Bone marrow-homed donor cells challenged with supralethal doses of FasL were insensitive to apoptosis (3.2% +/- 1% vs 38% +/- 5% in naïve bone marrow cells), and engraftment was not affected by pretransplantation exposure of donor cells to an apoptotic challenge with FasL.

CONCLUSION

There was no evidence of Fas-mediated suppression of donor and host cell activity after transplantation. Resistance to Fas-mediated apoptosis evolves as a functional characteristic of hematopoietic reconstituting stem and progenitor cells, providing them competitive engraftment advantage over committed progenitors.

Fas transduces dual apoptotic and trophic signals in hematopoietic progenitors

Stem cells and progenitors are often required to realize their differentiation potential in hostile microenvironments. The Fas/Fas ligand (FasL) interaction is a major effector pathway of apoptosis, which negatively regulates the expansion of differentiated hematopoietic cells. The involvement of this molecular interaction in the function of hematopoietic stem and progenitor cells is not well understood. In the murine syngeneic transplant setting, both Fas and FasL are acutely upregulated in bone marrow-homed donor cells; however, the Fas(+) cells are largely insensitive to FasL-induced apoptosis. In heterogeneous populations of lineage-negative (lin(-)) bone marrow cells and progenitors isolated by counterflow centrifugal elutriation, trimerization of the Fas receptor enhanced the clonogenic activity. Inhibition of caspases 3 and 8 did not affect the trophic signals mediated by Fas, yet it efficiently blocked the apoptotic pathways. Fas-mediated tropism appears to be of physiological significance, as pre-exposure of donor cells to FasL improved the radioprotective qualities of hematopoietic progenitors, resulting in superior survival of myeloablated hosts. Under these conditions, the activity of long-term reconstituting cells was not affected, as determined in sequential secondary and tertiary transplants. Dual caspase-independent tropic and caspase-dependent apoptotic signaling place the Fas receptor at an important junction of activation and death. This regulatory mechanism of hematopoietic homeostasis activates progenitors to promote the recovery from aplasia and converts into a negative regulator in distal stages of cell differentiation. Disclosure of potential conflicts of interest is found at the end of this article.

Myeloid lineage commitment from the hematopoietic stem cell

Prospective isolation of hematopoietic stem and progenitor cells has identified the lineal relationships among all blood-cell types and has allowed their developmental mechanisms to be assayed at the single-cell level. These isolated cell populations are used to elucidate the molecular mechanism of lineage fate decision and of its plasticity directly by stage-specific enforcement or repression of lineage-instructive signaling in purified cells. With an emphasis on the myeloid lineage, this review summarizes current concepts and controversies regarding adult murine hematopoietic development and discusses the potential mechanisms, operated by single or by multiple transcription factors, of myeloid lineage fate decision.

Activation of mitogen-activated protein kinase kinase (MEK)/extracellular signal regulated kinase (ERK) signaling pathway is involved in myeloid lineage commitment

Common lymphoid progenitors (CLPs) are lymphoid-lineage-committed progenitor cells. However, they maintain a latent myeloid differentiation potential that can be initiated by stimulation with interleukin-2 (IL-2) via ectopically expressed IL-2 receptors. Although CLPs express IL-7 receptors, which share the common gamma chain with IL-2 receptors, IL-7 cannot initiate lineage conversion in CLPs. In this study, we demonstrate that the critical signals for initiating lineage conversion in CLPs are delivered via IL-2 receptor beta (IL-2R beta) intracellular domains. Fusion of the A region of the IL-2R beta cytoplasmic tail to IL-7R alpha enables IL-7 to initiate myeloid differentiation in CLPs. We found that Shc, which associates with the A region, mediates lineage conversion signals through the mitogen activated protein kinase (MAPK) pathway. Because mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) inhibitors completely blocked IL-2-mediated lineage conversion, MAPK activation, specifically via the MEK/ERK pathway, is critically involved in the initiation of this event. Furthermore, formation of granulocyte/macrophage (GM) colonies by hematopoietic stem cells, but not by common myeloid progenitors (CMPs), was severely reduced in the presence of MEK/ERK inhibitors. These results demonstrate that activation of MEK/ERK plays an important role in GM lineage commitment.

Bifurcation dynamics in lineage-commitment in bipotent progenitor cells

Lineage specification of multipotent progenitor cells is governed by a balance of lineage-affiliated transcription factors, such as GATA1 and PU.1, which regulate the choice between erythroid and myelomonocytic fates. But how ratios of lineage-determining transcription factors stabilize progenitor cells and resolve their indeterminacy to commit them to discrete, mutually exclusive fates remains unexplained. We used a simple model and experimental measurements to analyze the dynamics of a binary fate decision governed by a gene-circuit containing auto-stimulation and cross-inhibition, as embodied by the GATA1-PU.1 paradigm. This circuit generates stable attractors corresponding to erythroid and myelomonocytic fates, as well as an uncommitted metastable state characterized by coexpression of both regulators, explaining the phenomenon of "multilineage priming". GATA1 and PU.1 mRNA and transcriptome dynamics of differentiating progenitor cells confirm that commitment occurs in two stages, as suggested by the model: first, the progenitor state is destabilized in an almost symmetrical bifurcation event, resulting in a poised state at the boundary between the two lineage-specific attractors; second, the cell is driven to the respective, now accessible attractors. This minimal model captures fundamental features of binary cell fate decisions, uniting the concepts of stochastic (selective) and deterministic (instructive) regulation, and hence, may apply to a wider range of binary fate decision points.

Sustained JNK signaling by proteolytically processed HPK1 mediates IL-3 independent survival during monocytic differentiation

We studied monocytic differentiation of primary mouse progenitor cells to understand molecular mechanisms of differentiation. We found a tightly controlled non-apoptotic activation of caspase-3 that correlated with differentiation. Although caspase activity was already detected during monocytic differentiation, a caspase-3 target has not been identified yet. We show that hematopoietic progenitor kinase 1 (HPK1) is processed towards its N- and C-terminal fragments during monocytic differentiation. While HPK1 is an immunoreceptor-proximal kinase in T and B cells, its role in myeloid cells is elusive. Here, we show that the N-terminal cleavage product, HPK1-N, comprising the kinase domain, confers progenitor cell survival independent of the growth factor IL-3. Furthermore, HPK1-N causes differentiation of progenitor cells towards the monocytic lineage. In contrast to full-length kinase, HPK1-N is constitutively active causing sustained JNK activation, Bad phosphorylation and survival. Blocking of caspase activity during differentiation of primary mouse progenitor cells leads to reduced HPK1-N levels, suppressed JNK activity and attenuated monocytic differentiation. Our work explains growth factor-independent survival during monocytic differentiation by caspase-mediated processing of HPK1 towards HPK1-N.

Asymmetrical lymphoid and myeloid lineage commitment in multipotent hematopoietic progenitors

The mechanism of lineage commitment from hematopoietic stem cells (HSCs) is not well understood. Although commitment to either the lymphoid or the myeloid lineage is popularly viewed as the first step of lineage restriction from HSCs, this model of hematopoietic differentiation has recently been challenged. The previous identification of multipotent progenitors (MPPs) that can produce lymphocytes and granulocyte/macrophages (GMs) but lacks erythroid differentiation ability suggests the existence of an alternative HSC differentiation program. Contribution to different hematopoietic lineages by these MPPs under physiological conditions, however, has not been carefully examined. In this study, we performed a refined characterization of MPPs by subfractionating three distinct subsets based on Flt3 and vascular cell adhesion molecule 1 expression. These MPP subsets differ in their ability to give rise to erythroid and GM lineage cells but are equally potent in lymphoid lineage differentiation in vivo. The developmental hierarchy of these MPP subsets demonstrates the sequential loss of erythroid and then GM differentiation potential during early hematopoiesis. Our results suggest that the first step of lineage commitment from HSCs is not simply a selection between the lymphoid and the myeloid lineage.

Hematopoiesis and stem cell renewal in long-term bone marrow cultures containing catalase

Culturing mouse bone marrow in the presence of catalase dramatically alters hematopoiesis. Granulocyte output is initially increased 4- to 5-fold. This increase is transient and granulocyte production declines as immature (Sca-1+/LIN-) cells accumulate. One third of these immature cells have a phenotype (Sca-1+/c-Kit+) characteristic of hematopoietic stem cells. At 2 to 3 weeks there are greater than 200-fold more Sca-1+/c-Kit+/LIN- cells in treated cultures than in controls. This population contains functional stem cells with both short-term and long-term bone marrow repopulating activity. In addition to myeloid progenitors, this Sca-1+/LIN- population contains a large number of cells that express CD31 and CD34 and have an active Tie-2 promoter, indicating that they are in the endothelial lineage. After 3 to 4 weeks hematopoiesis in treated cultures wanes but if catalase is removed, hematopoiesis resumes. After 7 to 10 days the cultures are indistinguishable from untreated controls. Thus, protected from H2O2, hematopoietic progenitors multiply and become quiescent. This sequence resembles in vivo development in normal marrow. These results make it clear that peroxide-sensitive regulatory mechanisms play an important role in controlling hematopoiesis ex vivo and presumably in vivo as well. They also indicate that manipulation of the peroxide levels can be used to enhance the growth of hematopoietic stem cells in culture.

Distinctive and indispensable roles of PU.1 in maintenance of hematopoietic stem cells and their differentiation

The PU.1 transcription factor is a key regulator of hematopoietic development, but its role at each hematopoietic stage remains unclear. In particular, the expression of PU.1 in hematopoietic stem cells (HSCs) could simply represent "priming" of genes related to downstream myelolymphoid lineages. By using a conditional PU.1 knock-out model, we here show that HSCs express PU.1, and its constitutive expression is necessary for maintenance of the HSC pool in the bone marrow. Bone marrow HSCs disrupted with PU.1 in situ could not maintain hematopoiesis and were outcompeted by normal HSCs. PU.1-deficient HSCs also failed to generate the earliest myeloid and lymphoid progenitors. PU.1 disruption in granulocyte/monocyte-committed progenitors blocked their maturation but not proliferation, resulting in myeloblast colony formation. PU.1 disruption in common lymphoid progenitors, however, did not prevent their B-cell maturation. In vivo disruption of PU.1 in mature B cells by the CD19-Cre locus did not affect B-cell maturation, and PU.1-deficient mature B cells displayed normal proliferation in response to mitogenic signals including the cross-linking of surface immunoglobulin M (IgM). Thus, PU.1 plays indispensable and distinct roles in hematopoietic development through supporting HSC self-renewal as well as commitment and maturation of myeloid and lymphoid lineages.

Enforced Bcl-2 expression overrides serum and feeder cell requirements for mouse embryonic stem cell self-renewal

Leukemia inhibitory factor (LIF) is required, but not sufficient, for pluripotent mouse embryonic stem (ES) cell expansion in vitro in the absence of serum or a feeder cell layer, suggesting that additional signals are provided by serum or feeders that are necessary to support self-renewal. Here we show that transgenic ES cell lines expressing Bcl-2, an antiapoptotic protein, continue to self-renew in serum- and feeder-free conditions when supplemented with LIF; even in the absence of bone morphogenic proteins. Bcl-2-expressing clones sustain the characteristics of undifferentiated, pluripotent ES cells during long-term culture, and maintain their potential to differentiate into mature cell types. These results suggest that LIF and Bcl-2 overexpression are sufficient to expand these mouse pluripotent stem cells in vitro.

c-Myc controls the balance between hematopoietic stem cell self-renewal and differentiation

The activity of adult stem cells is essential to replenish mature cells constantly lost due to normal tissue turnover. By a poorly understood mechanism, stem cells are maintained through self-renewal while concomitantly producing differentiated progeny. Here, we provide genetic evidence for an unexpected function of the c-Myc protein in the homeostasis of hematopoietic stem cells (HSCs). Conditional elimination of c-Myc activity in the bone marrow (BM) results in severe cytopenia and accumulation of HSCs in situ. Mutant HSCs self-renew and accumulate due to their failure to initiate normal stem cell differentiation. Impaired differentiation of c-Myc-deficient HSCs is linked to their localization in the differentiation preventative BM niche environment, and correlates with up-regulation of N-cadherin and a number of adhesion receptors, suggesting that release of HSCs from the stem cell niche requires c-Myc activity. Accordingly, enforced c-Myc expression in HSCs represses N-cadherin and integrins leading to loss of self-renewal activity at the expense of differentiation. Endogenous c-Myc is differentially expressed and induced upon differentiation of long-term HSCs. Collectively, our data indicate that c-Myc controls the balance between stem cell self-renewal and differentiation, presumably by regulating the interaction between HSCs and their niche.

Novel shift of Jak/Stat signalling characterizes the protective effect of aurintricarboxylic acid (ATA) from tumor necrosis factor-alpha toxicity in human B lymphocytes

Previous results demonstrated that the occurrence of death in human peripheral B lymphocytes by TNF-alpha was paralleled by the activation of the cytoplasmic Jak1 and Tyk2 protein kinases, along with the recruitment of transcription factors Stat3 and Stat5b. In this study we demonstrate that the balance of survival signals in the presence of TNF-alpha was altered by the addition of a salicylate compound, the endonuclease inhibitor aurintricarboxylic acid (ATA). Apoptosis effected by TNF-alpha alone was suppressed by ATA and this event was paralleled by phosphorylation and nuclear translocation of Jak2, Stat2, Stat4 and NF-kB, along with inhibition of caspase activation. These results confirm that among the different cellular responses evoked by TNF-alpha in human B cells, recruitment of Jak/Stat proteins and possible related gene modulation represent contributing factors and address the issue of the development of potential therapeutic strategies aimed at the control of systemic or local effects produced by TNF-alpha.

Haemopoietic stem cells

Considerable effort has been made in recent years in understanding the mechanisms that govern stem cell generation, proliferation, self-renewal, commitment and lately plasticity. In the development of the haemopoietic system during embryonic and fetal life the notion of different pools of stem cells arising from the endothelium is gaining consensus. Gene expression profiling of populations of stem cells is bringing to light categories of genes important for self-renewal or commitment. Besides the role of transcription factors in lineage decision, the role of soluble factors and transmembrane proteins, very active at the time of embryo development, are taking central stage in the maintenance and in vitro expansion of haemopoietic stem cells (HSCs). The hierarchical model of haemopoietic development is being questioned with reports of lineage switching and plasticity of haemopoietic stem cells to non-haemopoietic cells. Yet the understanding of the overall process is still very fragmented and hypothetical. This is mainly due to the absence of appropriate markers to enable selection of homogeneous stem cell populations and the need to rely on retrospective functional assays, able only to determine the overall behaviour of a population of cells. This review is intended to be an overview of the haemopoietic system and a critical re-visitation of issues such as plasticity and self-renewal important for therapeutic applications of haemopoietic stem cells.

Chromosomal aberrations in human lymphocytes exposed to the anticholinesterase pesticide isofenphos with mechanisms of leukemogenesis

Human lymphocytes were exposed to the leukemogenic pesticide isofenphos (IFP) to investigate its effects on chromosomal DNA and cholinergic homeostasis using cholinesterase activity as a marker. Isolated peripheral lymphocytes were administered concentrations of IFP ranging from 0.1 ng/ml to 10 microg/ml. The absence (Group 1) and presence (Group 2) of DNA repair inhibitors 4 mM hydroxyurea (HU), 40 microM cytosine arabinoside (ARA-C) and an NADPH regenerating system (NRS) (Group 3) were analyzed at 1, 6 and 24 h by single cell gel electrophoresis using the comet assay. Significant damage to DNA directly from IFP at 1 h by remarkably low concentrations was observed in Group 1, escalating in Group 2 with DNA repair inhibition, while Group 3 disruptions were highest due to the presence of the NRS P-450 microsomal fraction conducive to producing reactive IFP-oxon and N-desalkyl metabolites. The extent of DNA aberrations increased further in parallel within the groups at 6 and 24 h. Male and female chemical sensitivities were similar on average (P < 0.01). Cholinesterase activity measured in a satellite group was inhibited with 0.1 microg/ml IFP by 69, 62, and 48% at 1, 6, and 24 h, respectively, indicating gradual induction of compensatory synthesis. Restoration of cholinergic homeostasis may be exceptionally impaired at higher IFP concentrations from acetyl-CoA depletion [Leuk. Res. 25 (2001) 883]. In summary, these studies reveal that exposure to the organophosphate pesticide isofenphos induces human DNA mutation beyond endogenous repair capacity and disrupts cholinergic nuclear signaling affectively constructing the mutator phenotype of leukemogenesis.

Wnt signaling in the stem cell niche

PURPOSE OF REVIEW

All the cells present in the blood are derived from the hematopoietic stem cell (HSC). Because mature blood cells have a limited life span, HSCs must perpetuate themselves through self-renewal to maintain a functional hematopoietic compartment for the lifetime of an organism. This review focuses on studies that identify the Wnt signaling pathway as a mediator of HSC self-renewal and maintenance and analyzes its potential influence in context of the HSC niche.

RECENT FINDINGS

The Wnt signaling pathway has emerged as a potential regulator of self-renewal for HSCs. Recent findings have demonstrated that Wnt signaling can directly promote HSC self-renewal and ability to reconstitute the hematopoietic system of lethally irradiated mice. The recent findings that osteoblasts are an important regulatory component of the HSC microenvironment, and that elements of the Wnt signaling pathway can influence osteoblast frequency, raise the possibility that Wnt signaling may influence HSC function indirectly through the niche as well.

SUMMARY

In this review, the authors evaluate the experimental evidence for a direct role of Wnt signaling HSCs as well as an indirect role through its influence on the HSC niche. Defining the mechanism of action of Wnt signaling in HSC maintenance in context of the surrounding microenvironment and determining how this signal may integrate with other niche derived signals represents the next challenge HSC biology.

The MEK-1/ERKs signalling pathway is differentially involved in the self-renewal of early and late avian erythroid progenitor cells

Making decisions between self-renewal and differentiation is a central ability of stem cells. Elucidation of molecular networks governing this decision is therefore of prime importance. A model of choice to explore this question is represented by chicken erythroid progenitors, in which self-renewal versus differentiation as well as progenitor maturation are regulated by external factor combinations. We used this system to study whether similar or different signalling pathways were involved in the self-renewal of early, immature or more mature erythroid progenitors. We show that a transforming growth factor (TGF)-alpha-activated Ras/MEK-1/ERK1/2 pathway is strictly required for immature self-renewing cells but becomes fully dispensable when those cells are induced to differentiate. Consequently, pharmacological inhibition of this pathway led to spontaneous differentiation, only dependent on the presence of survival signals. Conversely, ectopic expression of a constitutive form of MEK-1 stimulates renewal and arrests differentiation process. Finally, we demonstrate that the ERK/MAPK signalling pathway is required in early but not in late primary erythroid progenitors, which can be turned into each other by different growth factor combinations specifically driving their renewal. To the best of our knowledge, this is the first description of a central role of ERK/MAPK signalling in regulating progenitor plasticity in the same cell type under different environmental conditions.

Regulation of macrophage and neutrophil cell fates by the PU.1:C/EBPalpha ratio and granulocyte colony-stimulating factor

Hematopoietic transcription factors are essential for specifying cell fates; however, the function of cytokines in such developmental decisions is unresolved. We demonstrate here that haploinsufficiency for the gene encoding the transcription factor PU.1 partially suppresses the neutropenia of mice deficient in granulocyte colony-stimulating factor. This suppression was due to an increase in granulocytic progenitors and a diminution of monocytic progenitors. With (PU.1+/-) ES cells as well as (PU.1-/-) hematopoietic progenitors, we show that higher expression of PU.1 is needed for macrophage than for neutrophil development. In a (PU.1-/-) progenitor cell line, in which graded activity of PU.1 regulates neutrophil versus macrophage development, granulocyte colony-stimulating factor signaling supported the neutrophil cell fate by increasing expression of the neutrophil transcription factor C/EBPalpha in relation to expression of PU.1. Collectively, these results indicate that cytokines can promote cell fate decisions by altering the relative concentrations of lineage-determining transcriptional regulators.

BCL-2 and BCL-XL restrict lineage choice during hematopoietic differentiation

Differentiation of hematopoietic cells from multipotential progenitors is regulated by multiple growth factors and cytokines. A prominent feature of these soluble factors is promotion of cell survival, in part mediated by expression of either of the anti-apoptotic proteins, BCL-2 and BCL-XL. The complex expression pattern of these frequently redundant survival factors during hematopoiesis may indicate a role in lineage determination. To investigate the latter possibility, we analyzed factor-dependent cell-Patersen (FDCP)-Mix multipotent progenitor cells in which we stably expressed BCL-2 or BCL-XL. Each factor maintained complete survival of interleukin-3 (IL-3)-deprived FDCP-Mix cells but, unexpectedly, directed FDCP-Mix cells along restricted and divergent differentiation pathways. Thus, IL-3-deprived FDCP-Mix BCL-2 cells differentiated exclusively to granulocytes and monocytes/macrophages, whereas FDCP-Mix BCL-XL cells became erythroid. FDCP-Mix BCL-2 cells grown in IL-3 were distinguished from FDCP-Mix and FDCP-Mix BCL-XL cells by a striking reduction in cellular levels of Raf-1 protein. Replacement of the BCL-2 BH4 domain with the related BCL-XL BH4 sequence resulted in a switch of FDCP-Mix BCL-2 cells to erythroid fate accompanied by persistence of Raf-1 protein expression. Moreover, enforced expression of Raf-1 redirected FDCP-Mix BCL-2 cells to an erythroid fate, and prohibited generation of myeloid cells. These results identify novel roles for BCL-2 and BCL-XL in cell fate decisions beyond cell survival. These effects are associated with differential regulation of Raf-1 expression, perhaps involving the previously identified interaction between BCL-2-BH4 and the catalytic domain of Raf-1.

Role of biomodulators and involvement of protein tyrosine kinase on stem cell migration in normal and leukaemic mice

Tyrosine kinase has an important role with regard to self-renewal and as a comitogen in the movement of stem cells out of the haemopoietic stem cell pool into the progeny pool. The present investigation has an objective to evaluate the protein tyrosine kinase (PTK) activity of bone marrow derived pluripotent cells before and after application of biological response modifiers (BRMs) in normal and leukaemic mice. The PTK activity of the cytosolic fraction of bone marrow cells has been determined by the assay kit based on per-oxidase labeled substrate analog and biotin-streptavidin expression. A consequent cell population kinetic study has also been conducted. Results showed a higher activity in the cells of leukaemic mice, which under the influence of interleukin-2 (IL-2) and the non-specific BRM sheep erythrocytes (SRBC) undergo further activation. Interferon-gamma (IFN-gamma) when administered alone showed a suppressive effect and the combination of the three manifested a resultant suppression. Corresponding migration (cell population kinetics) of the bone marrow cells (BMC) also correlated well with the PTK activity of the cells concerned. The observations indicated that the pluripotent BMCs are under regulated control of the PTK activity, which can be manipulated by selective BRMs. The data also suggested the therapeutic benefit of IFN-gamma along with chemotherapeutics against leukaemia and that of IL-2 and SRBC during bone marrow failure.

Bcl-2 promotes premature senescence induced by oncogenic Ras

The expression of the apoptosis inhibitory protein, Bcl-2, is increased in naturally senescing human fibroblasts and upon induction of their senescence-like growth arrest by oxidative stress, implying its role in maintaining their extended viability. Oncogenic Ras(V12) protein induces signaling cascades that result in the premature senescence of primary fibroblast cells, which are insensitive to oncogene-dependent apoptosis. Here we show that constitutive expression of Bcl-2 accelerates selected features of the Ras-induced senescence program in primary human fibroblasts. Yet, Bcl-2 also inhibits fibroblast apoptosis induced by exogenous H(2)O(2), while both signals induce an increased endogenous Bcl-2 expression in these cells. Together, these data suggest a context-dependent phenotypic function of Bcl-2 in the regulation of overlapping cell fate specification programs, with potential implications for both physiology and multistep tumorigenesis.

Disruption of differentiation in human cancer: AML shows the way

Although much is understood about the ways in which transcription factors regulate various differentiation systems, and one of the hallmarks of many human cancers is a lack of cellular differentiation, relatively few reports have linked these two processes. Recent studies of acute myeloid leukaemia (AML), however, have indicated how disruption of transcription-factor function can disrupt normal cellular differentiation and lead to cancer. This model involves lineage-specific transcription factors, which are involved in normal haematopoietic differentiation. These factors are often targeted in AML--either by direct mutation or by interference from translocation proteins. Uncovering these underlying pathways will improve the diagnosis and treatment of AML, and provide a working model for other types of human cancer, including solid tumours.