The E2A and tal-1 helix-loop-helix proteins associate in vivo and are modulated by Id proteins during interleukin 6-induced myeloid differentiation

E47 upregulates ΔNp63α to promote growth of squamous cell carcinoma

Targeted therapy has greatly improved both survival and prognosis of cancer patients. However, while therapeutic treatment of adenocarcinoma has been advanced greatly, progress in treatment of squamous cell carcinoma (SCC) has been slow and ineffective. Therefore, it is of great importance to decipher mechanisms and identify new drug targets involved in squamous cell carcinoma development. In this study, we demonstrate that E47 plays the distinctive and opposite roles on cell proliferation in adenocarcinoma and squamous cell carcinoma. While E47 suppresses cell proliferation in adenocarcinoma cells, it functions as a oncoprotein to promote cell proliferation and tumor growth of squamous cell carcinoma. Mechanistically, we show that E47 can directly bind to the promoter and transactivate ΔNp63 gene expression in squamous cell carcinoma cells, resulting in upregulation of cyclins D1/E1 and downregulation of p21, and thereby promoting cell proliferation and tumor growth. We further show that expression of E2A (E12/E47) is positively correlated with p63 and that high expression of E2A is associated with poor outcomes in clinical samples of squamous cell carcinoma. These results highlight that the E47-ΔNp63α axis may be potential therapeutic targets for treatment of squamous cell carcinoma.

Induction of basic helix-loop-helix protein-containing complexes during erythroid differentiation

The involvement of basic helix-loop-helix (bHLH) transcription factors in erythroid differentiation and development has been established by forced expression of the proteins TAL1 and Id1 in cultured cell lines and by targeted disruption of the mouse TAL1 gene. To better understand the mechanism by which bHLH proteins regulate erythropoiesis, we have investigated HLH protein-DNA interactions in mouse erythroleukemia (MEL) cells before and during chemically induced differentiation. Three bHLH (E-box) binding activities were found to be induced in nuclei from differentiating MEL cells. Using specific antisera, we have demonstrated that these complexes are dimers of TAL1 and ubiquitous E proteins. Similar complexes were detected in nuclear extracts from a human erythroid cell line, K562, and from mouse fetal liver. All three bHLH complexes were disrupted in vitro by Id1, a dominant-negative HLH protein that we and others have previously shown to antagonize MEL cell differentiation. During differentiation of an Id1-overexpressing MEL cell line, induction of a complex containing TAL1 and E2A was not only blocked but reduced below the levels seen in undifferentiating cells. These observations are consistent with the idea that TAL1 and Id1 have opposing effects on erythroid differentiation and that the level of TAL1/E2A heterodimer and/or another E protein-containing complex may influence the decision of a cell to terminally differentiate.

Human Cytomegalovirus Immediate-Early 1 Protein Rewires Upstream STAT3 to Downstream STAT1 Signaling Switching an IL6-Type to an IFNγ-Like Response

The human cytomegalovirus (hCMV) major immediate-early 1 protein (IE1) is best known for activating transcription to facilitate viral replication. Here we present transcriptome data indicating that IE1 is as significant a repressor as it is an activator of host gene expression. Human cells induced to express IE1 exhibit global repression of IL6- and oncostatin M-responsive STAT3 target genes. This repression is followed by STAT1 phosphorylation and activation of STAT1 target genes normally induced by IFNγ. The observed repression and subsequent activation are both mediated through the same region (amino acids 410 to 445) in the C-terminal domain of IE1, and this region serves as a binding site for STAT3. Depletion of STAT3 phenocopies the STAT1-dependent IFNγ-like response to IE1. In contrast, depletion of the IL6 receptor (IL6ST) or the STAT kinase JAK1 prevents this response. Accordingly, treatment with IL6 leads to prolonged STAT1 instead of STAT3 activation in wild-type IE1 expressing cells, but not in cells expressing a mutant protein (IE1dl410-420) deficient for STAT3 binding. A very similar STAT1-directed response to IL6 is also present in cells infected with a wild-type or revertant hCMV, but not an IE1dl410-420 mutant virus, and this response results in restricted viral replication. We conclude that IE1 is sufficient and necessary to rewire upstream IL6-type to downstream IFNγ-like signaling, two pathways linked to opposing actions, resulting in repressed STAT3- and activated STAT1-responsive genes. These findings relate transcriptional repressor and activator functions of IE1 and suggest unexpected outcomes relevant to viral pathogenesis in response to cytokines or growth factors that signal through the IL6ST-JAK1-STAT3 axis in hCMV-infected cells. Our results also reveal that IE1, a protein considered to be a key activator of the hCMV productive cycle, has an unanticipated role in tempering viral replication.

Our previous work has shown that the human cytomegalovirus (hCMV) major immediate-early 1 protein (IE1) modulates host cell signaling pathways involving proteins of the signal transducer and activator of transcription (STAT) family. IE1 has also long been known to facilitate viral replication by activating transcription. In this report we demonstrate that IE1 is as significant a repressor as it is an activator of host gene expression. Many genes repressed by IE1 are normally induced via STAT3 signaling triggered by interleukin 6 (IL6) or related cytokines, whereas many genes activated by IE1 are normally induced via STAT1 signaling triggered by interferon gamma (IFNγ). Our results suggest that the repression of STAT3- and the activation of STAT1-responsive genes by IE1 are coupled. By targeting STAT3, IE1 rewires upstream STAT3 to downstream STAT1 signaling. Consequently, genes normally induced by IL6 are repressed while genes normally induced by IFNγ become responsive to IL6 in the presence of IE1. We also demonstrate that, by switching an IL6 to an IFNγ-like response, IE1 tempers viral replication. These results suggest an unanticipated dual role for IE1 in either promoting or limiting hCMV propagation and demonstrate how a key viral regulatory protein merges two central cellular signaling pathways to divert cytokine responses relevant to hCMV pathogenesis.

Ccndbp1 is a new positive regulator of skeletal myogenesis

Skeletal myogenesis is a multistep process in which basic helix-loop-helix (bHLH) transcription factors, such as MyoD (also known as MyoD1), bind to E-boxes and activate downstream genes. Ccndbp1 is a HLH protein that lacks a DNA-binding region, and its function in skeletal myogenesis is currently unknown. We generated Ccndbp1-null mice by using CRISPR-Cas9. Notably, in Ccndbp1-null mice, the cross sectional area of the skeletal tibialis anterior muscle was smaller, and muscle regeneration ability and grip strength were impaired, compared with those of wild type. This phenotype resembled that of myofiber hypotrophy in some human myopathies or amyoplasia. Ccndbp1 expression was upregulated during C2C12 myogenesis. Ccndbp1 overexpression promoted myogenesis, whereas knockdown of Ccndbp1 inhibited myogenic differentiation. Co-transfection of Ccndbp1 with MyoD and/or E47 (encoded by TCF3) significantly enhanced E-box-dependent transcription. Furthermore, Ccndbp1 physically associated with MyoD but not E47. These data suggest that Ccndbp1 regulates muscle differentiation by interacting with MyoD and enhancing its binding to target genes. Our study newly identifies Ccndbp1 as a positive modulator of skeletal myogenic differentiation in vivo and in vitro, providing new insights in order to decipher the complex network involved in skeletal myogenic development and related diseases.

SCL/TAL1 in Hematopoiesis and Cellular Reprogramming

SCL, a transcription factor of the basic helix-loop-helix family, is a master regulator of hematopoiesis. Scl specifies lateral plate mesoderm to a hematopoietic fate and establishes boundaries by inhibiting the cardiac lineage. A combinatorial interaction between Scl and Vegfa/Flk1 sets in motion the first wave of primitive hematopoiesis. Subsequently, definitive hematopoietic stem cells (HSCs) emerge from the embryo proper via an endothelial-to-hematopoietic transition controlled by Runx1, acting with Scl and Gata2. Past this stage, Scl in steady state HSCs is redundant with Lyl1, a highly homologous factor. However, Scl is haploinsufficient in stress response, when a rare subpopulation of HSCs with very long term repopulating capacity is called into action. SCL activates transcription by recruiting a core complex on DNA that necessarily includes E2A/HEB, GATA1-3, LIM-only proteins LMO1/2, LDB1, and an extended complex comprising ETO2, RUNX1, ERG, or FLI1. These interactions confer multifunctionality to a complex that can control cell proliferation in erythroid progenitors or commitment to terminal differentiation through variations in single component. Ectopic SCL and LMO1/2 expression in immature thymocytes activates of a stem cell gene network and reprogram cells with a finite lifespan into self-renewing preleukemic stem cells (pre-LSCs), an initiating event in T-cell acute lymphoblastic leukemias. Interestingly, fate conversion of fibroblasts to hematoendothelial cells requires not only Scl and Lmo2 but also Gata2, Runx1, and Erg, indicating a necessary collaboration between these transcription factors for hematopoietic reprogramming. Nonetheless, full reprogramming into self-renewing multipotent HSCs may require additional factors and most likely, a permissive microenvironment.

Development, Homeostasis, and Heterogeneity of NK Cells and ILC1

Natural killer (NK) cells are a population of cytotoxic innate lymphocytes that evolved prior to their adaptive counterparts and constitute one of the first lines of defense against infected or mutated cells. NK cells are rapidly activated, expressing an array of germ-line encoded receptors that allow them to scan for protein irregularities on cells and kill those deemed "altered-self." NK cells rapidly produce a broad range of cytokines and chemokines following activation by virus, bacterial, or parasitic infection and are thus key in orchestrating inflammation. NK cells have previously been viewed to represent a relatively homogeneous group of IFN-γ-producing cells that express the surface markers NK1.1 and natural killer cell p46-related protein (NKp46 or NCR1 encoded by Ncr1) and depend on the transcription factor T-bet for their development. Recently, a second subset of T-bet-dependent innate cells, the group 1 innate lymphoid cells (ILC1), has been discovered which share many attributes of conventional NK (cNK) cells. Despite the similarities between ILC1 and cNK cells , they differ in several important aspects including their localization, transcriptional regulation, and phenotype suggesting each subset has distinct origins and functions in immune responses. Previously, the ability to detect and spontaneously kill cells that exhibit "altered-self" which is central to tumor and viral immunity has been thought to be an attribute restricted solely to cNK cells. The identification of ILC1 challenges this notion and suggests that key contributions from ILC1 may have gone unrecognized. Thus, understanding the different rules that govern the behavior of ILC1 and cNK cells in immune responses may potentially open unexpected doorways to uncover novel strategies to manipulate these cells in treating disease. Here, we review recent advances in our understanding of peripheral cNK cell and ILC1 heterogeneity in terms of their development, phenotype, homeostasis, and effector functions.

Gfi-1 regulates the erythroid transcription factor network through Id2 repression in murine hematopoietic progenitor cells

Growth factor independence 1 (Gfi-1) is a part of the transcriptional network that regulates the development of adult hematopoietic stem and progenitor cells. Gfi-1-null (Gfi-1(-/-)) mice have reduced numbers of hematopoietic stem cells (HSCs), impaired radioprotective function of hematopoietic progenitor cells (HPCs), and myeloid and erythroid hyperplasia. We found that the development of HPCs and erythropoiesis, but not HSC function, was rescued by reducing the expression of inhibitor of DNA-binding protein 2 (Id2) in Gfi-1(-/-) mice. Analysis of Gfi-1(-/-);Id2(+/-) mice revealed that short-term HSCs, common myeloid progenitors (CMPs), erythroid burst-forming units, colony-forming units in spleen, and more differentiated red cells were partially restored by reducing Id2 levels in Gfi-1(-/-) mice. Moreover, short-term reconstituting cells, and, to a greater extent, CMP and megakaryocyte-erythroid progenitor development, and red blood cell production (anemia) were rescued in mice transplanted with Gfi-1(-/-);Id2(+/-) bone marrow cells (BMCs) in comparison with Gfi-1(-/-) BMCs. Reduction of Id2 expression in Gfi-1(-/-) mice increased the expression of Gata1, Eklf, and EpoR, which are required for proper erythropoiesis. Reducing the levels of other Id family members (Id1 and Id3) in Gfi-1(-/-) mice did not rescue impaired HPC function or erythropoiesis. These data provide new evidence that Gfi-1 is linked to the erythroid gene regulatory network by repressing Id2 expression.

Diversity, function, and transcriptional regulation of gut innate lymphocytes

The innate immune system plays a critical early role in host defense against viruses, bacteria, and tumor cells. Until recently, natural killer (NK) cells and lymphoid tissue inducer (LTi) cells were the primary members of the innate lymphocyte family: NK cells form the front-line interface between the external environment and the adaptive immune system, while LTi cells are essential for secondary lymphoid tissue formation. More recently, it has become apparent that the composition of this family is much more diverse than previously appreciated and newly recognized populations play distinct and essential functions in tissue protection. Despite the importance of these cells, the developmental relationships between different innate lymphocyte populations remain unclear. Here we review recent advances in our understanding of the development of different innate immune cell subsets, the transcriptional programs that might be involved in driving fate decisions during development, and their relationship to NK cells.

NKX3.1 is a direct TAL1 target gene that mediates proliferation of TAL1-expressing human T cell acute lymphoblastic leukemia

TAL1 (also known as SCL) is expressed in >40% of human T cell acute lymphoblastic leukemias (T-ALLs). TAL1 encodes a basic helix-loop-helix transcription factor that can interfere with the transcriptional activity of E2A and HEB during T cell leukemogenesis; however, the oncogenic pathways directly activated by TAL1 are not characterized. In this study, we show that, in human TAL1–expressing T-ALL cell lines, TAL1 directly activates NKX3.1, a tumor suppressor gene required for prostate stem cell maintenance. In human T-ALL cell lines, NKX3.1 gene activation is mediated by a TAL1–LMO–Ldb1 complex that is recruited by GATA-3 bound to an NKX3.1 gene promoter regulatory sequence. TAL1-induced NKX3.1 activation is associated with suppression of HP1-α (heterochromatin protein 1 α) binding and opening of chromatin on the NKX3.1 gene promoter. NKX3.1 is necessary for T-ALL proliferation, can partially restore proliferation in TAL1 knockdown cells, and directly regulates miR-17-92. In primary human TAL1-expressing leukemic cells, the NKX3.1 gene is expressed independently of the Notch pathway, and its inactivation impairs proliferation. Finally, TAL1 or NKX3.1 knockdown abrogates the ability of human T-ALL cells to efficiently induce leukemia development in mice. These results suggest that tumor suppressor or oncogenic activity of NKX3.1 depends on tissue expression.

Repression of Id2 expression by Gfi-1 is required for B-cell and myeloid development

The development of mature blood cells from hematopoietic stem cells requires coordinated activities of transcriptional networks. Transcriptional repressor growth factor independence 1 (Gfi-1) is required for the development of B cells, T cells, neutrophils, and for the maintenance of hematopoietic stem cell function. However, the mechanisms by which Gfi-1 regulates hematopoiesis and how Gfi-1 integrates into transcriptional networks remain unclear. Here, we provide evidence that Id2 is a transcriptional target of Gfi-1, and repression of Id2 by Gfi-1 is required for B-cell and myeloid development. Gfi-1 binds to 3 conserved regions in the Id2 promoter and represses Id2 promoter activity in transient reporter assays. Increased Id2 expression was observed in multipotent progenitors, myeloid progenitors, T-cell progenitors, and B-cell progenitors in Gfi-1(-/-) mice. Knockdown of Id2 expression or heterozygosity at the Id2 locus partially rescues the B-cell and myeloid development but not the T-cell development in Gfi-1(-/-) mice. These studies demonstrate a role of Id2 in mediating Gfi-1 functions in B-cell and myeloid development and provide a direct link between Gfi-1 and the B-cell transcriptional network by its ability to repress Id2 expression.

The TAL1/SCL transcription factor regulates cell cycle progression and proliferation in differentiating murine bone marrow monocyte precursors

Monocytopoiesis involves the stepwise differentiation in the bone marrow (BM) of common myeloid precursors (CMPs) to monocytes. The basic helix-loop-helix transcription factor TAL1/SCL plays a critical role in other hematopoietic lineages, and while it had been reported to be expressed by BM-derived macrophages, its role in monocytopoiesis had not been elucidated. Using cell explant models of monocyte/macrophage (MM) differentiation, one originating with CMPs and the other from more committed precursors, we characterized the phenotypic and molecular consequences of inactivation of Tal1 expression ex vivo. While Tal1 knockout had minimal effects on cell survival and slightly accelerated terminal differentiation, it profoundly inhibited cell proliferation and decreased entry into and traversal of the G(1) and S phases. In conjunction, steady-state levels of p16(Ink4a) mRNA were increased and those of Gata2 mRNA decreased. Chromatin immunoprecipitation analysis demonstrated the association of Tal1 and E47, one of its E protein DNA-binding partners, with an E box-GATA sequence element in intron 4 of the Gata2 gene and with three E boxes upstream of p16(Ink4a). Finally, wild-type Tal1, but not a DNA binding-defective mutant, rescued the proliferative defect in Tal1-null MM precursors. These results document the importance of this transcription factor in cell cycle progression and proliferation during monocytopoiesis and the requirement for direct DNA binding in these processes.

Inhibitor of differentiation 4 (Id4) is a potential tumor suppressor in prostate cancer

Background

Inhibitor of differentiation 4 (Id4), a member of the Id gene family is also a dominant negative regulator of basic helix loop helix (bHLH) transcription factors. Some of the functions of Id4 appear to be unique as compared to its other family members Id1, Id2 and Id3. Loss of Id4 gene expression in many cancers in association with promoter hypermethylation has led to the proposal that Id4 may act as a tumor suppressor. In this study we provide functional evidence that Id4 indeed acts as a tumor suppressor and is part of a cancer associated epigenetic re-programming.

Methods

Data mining was used to demonstrate Id4 expression in prostate cancer. Methylation specific polymerase chain reaction (MSP) analysis was performed to understand molecular mechanisms associated with Id4 expression in prostate cancer cell lines. The effect of ectopic Id4 expression in DU145 cells was determined by cell cycle analysis (3H thymidine incorporation and FACS), expression of androgen receptor, p53 and cyclin dependent kinase inhibitors p27 and p21 by a combination of RT-PCR, real time-PCR, western blot and immuno-cytochemical analysis.

Results

Id4 expression was down-regulated in prostate cancer. Id4 expression was also down-regulated in prostate cancer line DU145 due to promoter hyper-methylation. Ectopic Id4 expression in DU145 prostate cancer cell line led to increased apoptosis and decreased cell proliferation due in part by an S-phase arrest. In addition to S-phase arrest, ectopic Id4 expression in PC3 cells also resulted in prolonged G2/M phase. At the molecular level these changes were associated with increased androgen receptor (AR), p21, p27 and p53 expression in DU145 cells.

Conclusion

The results suggest that Id4 acts directly as a tumor suppressor by influencing a hierarchy of cellular processes at multiple levels that leads to a decreased cell proliferation and change in morphology that is possibly mediated through induction of previously silenced tumor suppressors.

Efficient retroviral transduction of human B-lymphoid and myeloid progenitors: marked inhibition of their growth by the Pax5 transgene

We applied a coculture system for the genetic manipulation of human B-lymphoid and myeloid progenitor cells using murine bone marrow stromal cell support, and investigated the effects of forced Pax5 expression in both cell types. Cytokine-stimulated cord blood CD34⁺ cells could be transduced at 85% efficiency and 95% cell viability by a single 24-h infection with RD114-pseudotyped retroviral vectors, produced by the packaging cell line Plat-F and bicistronic vector plasmids pMXs-Ig, pMYs-Ig, or pMCs-Ig, encoding EGFP. Infected CD34⁺ cells were seeded onto HESS-5 cells in the presence of stem cell factor and granulocyte colony-stimulating factor, allowing the extensive production of B progenitors and granulocytic cells. We examined the cell number and CD34, CD33, CD19, and CD20 lambda and kappa expressions by flow cytometry. Ectopic expression of Pax5 in CD34⁺ cells resulted in small myeloid progenitors coexpressing CD33 and CD19 and inhibited myeloid differentiation. After 6 weeks, the number of Pax5-transduced CD19⁺ cells was 40-fold lower than that of control cells. However, the expression of CD20 and the κ/λ chain on Pax5-transduced CD19⁺ cells suggests that the Pax5 transgene may not interfere with their differentiation. This report is the first to describe the effects of forced Pax5 expression in human hematopoietic progenitors.

Molecular pathogenesis of T-cell leukaemia and lymphoma

T-cell acute lymphoblastic leukaemia (T-ALL) is induced by the transformation of T-cell progenitors and mainly occurs in children and adolescents. Although treatment outcome in patients with T-ALL has improved in recent years, patients with relapsed disease continue to have a poor prognosis. It is therefore important to understand the molecular pathways that control both the induction of transformation and the treatment of relapsed disease. In this Review, we focus on the molecular mechanisms responsible for disease induction and maintenance. We also compare the physiological progression of T-cell differentiation with T-cell transformation, highlighting the close relationship between these two processes. Finally, we discuss potential new therapies that target oncogenic pathways in T-ALL.

Id2 intrinsically regulates lymphoid and erythroid development via interaction with different target proteins

Inhibitors of DNA binding (Id) family members are key regulators of cellular differentiation and proliferation. These activities are related to the ability of Id proteins to antagonize E proteins and other transcription factors. As negative regulators of E proteins, Id proteins have been implicated in lymphocyte development. Overexpression of Id1, Id2, or Id3 has similar effects on lymphocyte development. However, which Id protein plays a physiologic role during lymphocyte development is not clear. By analyzing Id2 knock-out mice and retroviral transduced hematopoietic progenitors, we demonstrated that Id2 is an intrinsic negative regulator of B-cell development. Hematopoietic progenitor cells overexpressing Id2 did not reconstitute B-cell development in vivo, which resembled the phenotype of E2A null mice. The B-cell population in bone marrow was significantly expanded in Id2 knock-out mice compared with their wild-type littermates. Knock-down of Id2 by shRNA in hematopoietic progenitor cells promoted B-cell differentiation and induced the expression of B-cell lineage-specific genes. These data identified Id2 as a physiologically relevant regulator of E2A during B lymphopoiesis. Furthermore, we identified a novel Id2 function in erythroid development. Overexpression of Id2 enhanced erythroid development, and decreased level of Id2 impaired normal erythroid development. Id2 regulation of erythroid development is mediated via interacting with transcription factor PU.1 and modulating PU.1 and GATA-1 activities. We conclude that Id2 regulates lymphoid and erythroid development via interaction with different target proteins.

Lyl1 interacts with CREB1 and alters expression of CREB1 target genes

The basic helix-loop-helix (bHLH) transcription factor family contains key regulators of cellular proliferation and differentiation as well as the suspected oncoproteins Tal1 and Lyl1. Tal1 and Lyl1 are aberrantly over-expressed in leukemia as a result of chromosomal translocations, or other genetic or epigenetic events. Protein-protein and protein-DNA interactions described so far are mediated by their highly homologous bHLH domains, while little is known about the function of other protein domains. Hetero-dimers of Tal1 and Lyl1 with E2A or HEB, decrease the rate of E2A or HEB homo-dimer formation and are poor activators of transcription. In vitro, these hetero-dimers also recognize different binding sites from homo-dimer complexes, which may also lead to inappropriate activation or repression of promoters in vivo. Both mechanisms are thought to contribute to the oncogenic potential of Tal1 and Lyl1. Despite their bHLH structural similarity, accumulating evidence suggests that Tal1 and Lyl1 target different genes. This raises the possibility that domains flanking the bHLH region, which are distinct in the two proteins, may participate in target recognition. Here we report that CREB1, a widely-expressed transcription factor and a suspected oncogene in acute myelogenous leukemia (AML) was identified as a binding partner for Lyl1 but not for Tal1. The interaction between Lyl1 and CREB1 involves the N terminal domain of Lyl1 and the Q2 and KID domains of CREB1. The histone acetyl-transferases p300 and CBP are recruited to these complexes in the absence of CREB1 Ser 133 phosphorylation. In the Id1 promoter, Lyl1 complexes direct transcriptional activation. We also found that in addition to Id1, over-expressed Lyl1 can activate other CREB1 target promoters such as Id3, cyclin D3, Brca1, Btg2 and Egr1. Moreover, approximately 50% of all gene promoters identified by ChIP-chip experiments were jointly occupied by CREB1 and Lyl1, further strengthening the association of Lyl1 with Cre binding sites. Given the newly recognized importance of CREB1 in AML, the ability of Lyl1 to modulate promoter responses to CREB1 suggests that it plays a role in the malignant phenotype by occupying different promoters than Tal1.

HSPG modulation of BMP signaling in fibrodysplasia ossificans progressiva cells

Cell surface heparan sulfate proteoglycans (HSPGs) play important roles in morphogen gradient formation and cell signaling. Bone morphogenetic protein (BMP) signaling is dysregulated in fibrodysplasia ossificans progressiva (FOP), a disabling disorder of progressive heterotopic bone formation. Here, we investigated the role of HSPG glycosaminoglycan (GAG) side chains on BMP signaling and found increased total and HSPG-specific GAG chain levels and dysregulation in HSPG modulation of BMP signaling in FOP lymphoblastoid cells (LCLs). Specifically, HSPG profiling demonstrated abundant mRNA and protein levels of glypican 1 and syndecan 4 on control and FOP LCLs, with elevated core protein levels on FOP cells. Targeted downregulation of glypican 1 core protein synthesis by siRNA enhanced BMP signaling in control and FOP cells, while reduction of syndecan 4-core protein synthesis decreased BMP signaling in control, but not FOP cells. These results suggest that FOP cells are resistant to the stimulatory effects of cell surface HSPG GAG chains, but are susceptible to the inhibitory effects, as shown by downregulation of glypican 1. These data support that HSPG modulation of BMP signaling is altered in cells from patients with FOP and that altered HSPG-related BMP signaling may play a role in the pathogenesis of the disease.

C/EBPalpha determines hematopoietic cell fate in multipotential progenitor cells by inhibiting erythroid differentiation and inducing myeloid differentiation

C/EBPalpha is an essential transcription factor required for myeloid differentiation. While C/EBPalpha can act as a cell fate switch to promote granulocyte differentiation in bipotential granulocyte-macrophage progenitors (GMPs), its role in regulating cell fate decisions in more primitive progenitors is not known. We found increased numbers of erythroid progenitors and erythroid cells in C/EBPalpha(-/-) fetal liver (FL). Also, enforced expression of C/EBPalpha in hematopoietic stem cells resulted in a loss of erythroid progenitors and an increase in myeloid cells by inhibition of erythroid development and inducing myeloid differentiation. Conditional expression of C/EBPalpha in murine erythroleukemia (MEL) cells induced myeloid-specific genes, while inhibiting erythroid-specific gene expression including erythropoietin receptor (EpoR), which suggests a novel mechanism to determine hematopoietic cell fate. Thus, C/EBPalpha functions in hematopoietic cell fate decisions by the dual actions of inhibiting erythroid and inducing myeloid gene expression in multipotential progenitors.

Dysregulation of the BMP-p38 MAPK signaling pathway in cells from patients with fibrodysplasia ossificans progressiva (FOP)

FOP is a disabling disorder in which skeletal muscle is progressively replaced with bone. Lymphocytes, our model system for examining BMP signaling, cannot signal through the canonical Smad pathway unless exogenous Smad1 is supplied, providing a unique cell type in which the BMP-p38 MAPK pathway can be examined. FOP lymphocytes exhibit defects in the BMP-p38 MAPK pathway, suggesting that altered BMP signaling underlies ectopic bone formation in this disease.

INTRODUCTION

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder characterized by progressive heterotopic ossification of connective tissues. Whereas the primary genetic defect in this condition is unknown, BMP4 mRNA and protein and BMP receptor type IA (BMPRIA) protein are overexpressed in cultured lymphocytes from FOP patients, supporting that altered BMP signaling is involved in this disease. In this study, we examined downstream signaling targets to study the BMP-Smad and BMP-p38 mitogen-activated protein kinase (MAPK) pathways in FOP.

MATERIALS AND METHODS

Protein phosphorylation was assayed by immunoblots, and p38 MAPK activity was measured by kinase assays. To examine BMP target genes, the mRNA expression of ID1, ID3, and MSX2 was determined by quantitative real-time PCR. Statistical analysis was performed using Student's t-test or ANOVA.

RESULTS

FOP lymphocytes exhibited increased levels of p38 phosphorylation and p38 MAPK activity in response to BMP4 stimulation. Furthermore, in response to BMP4, FOP cells overexpressed the downstream signaling targets ID1 by 5-fold and ID3 by 3-fold compared with controls. ID1 and ID3 mRNA induction was specifically blocked with a p38 MAPK inhibitor, but not extracellular signal-related kinase (ERK) or c-Jun N-terminal kinase (JNK) inhibitors. MSX2, a known Smad pathway target gene, is not upregulated in control or FOP cells in response to BMP, suggesting that lymphocytes do not use this limb of the BMP pathway. However, introduction of Smad1 into lymphocytes made the cells competent to regulate MSX2 mRNA after BMP4 treatment.

CONCLUSIONS

Lymphocytes are a cell system that signals primarily through the BMP-p38 MAPK pathway rather than the BMP-Smad pathway in response to BMP4. The p38 MAPK pathway is dysregulated in FOP lymphocytes, which may play a role in the pathogenesis of FOP.

Transcriptional regulatory networks downstream of TAL1/SCL in T-cell acute lymphoblastic leukemia

Aberrant expression of 1 or more transcription factor oncogenes is a critical component of the molecular pathogenesis of human T-cell acute lymphoblastic leukemia (T-ALL); however, oncogenic transcriptional programs downstream of T-ALL oncogenes are mostly unknown. TAL1/SCL is a basic helix-loop-helix (bHLH) transcription factor oncogene aberrantly expressed in 60% of human T-ALLs. We used chromatin immunoprecipitation (ChIP) on chip to identify 71 direct transcriptional targets of TAL1/SCL. Promoters occupied by TAL1 were also frequently bound by the class I bHLH proteins E2A and HEB, suggesting that TAL1/E2A as well as TAL1/HEB heterodimers play a role in transformation of T-cell precursors. Using RNA interference, we demonstrated that TAL1 is required for the maintenance of the leukemic phenotype in Jurkat cells and showed that TAL1 binding can be associated with either repression or activation of genes whose promoters occupied by TAL1, E2A, and HEB. In addition, oligonucleotide microarray analysis of RNA from 47 primary T-ALL samples showed specific expression signatures involving TAL1 targets in TAL1-expressing compared with -nonexpressing human T-ALLs. Our results indicate that TAL1 may act as a bifunctional transcriptional regulator (activator and repressor) at the top of a complex regulatory network that disrupts normal T-cell homeostasis and contributes to leukemogenesis.

Expression of GCIP in transgenic mice decreases susceptibility to chemical hepatocarcinogenesis

Transcription factors with helix-loop-helix (HLH) motif play critical roles in controlling the expression of genes involved in lineage commitment, cell fate determination, proliferation, and tumorigenesis. To examine whether the newly identified HLH protein GCIP/CCNDBP1 modulates cell fate determination and plays a role in hepatocyte growth, proliferation, and hepatocarcinogenesis, we generated transgenic mice with human GCIP gene driven by a liver-specific albumin promoter. We demonstrated that in GCIP transgenic mice, the overall liver growth and regeneration occurred normally after liver injury induced by carbon tetrachloride (CCl4). In the diethylnitrosamine (DEN)-induced mouse hepatocarcinogenesis, we demonstrated that overexpression of GCIP in mouse liver suppressed DEN-induced hepatocarcinogenesis at an early stage of tumor development. The number of hepatic adenomas at 24 weeks was significantly lower or not detected in GCIP transgenic male mice compared to the control mice under the same treatment. Although GCIP has little inhibition on the number of hepatic tumors at later stages (40 weeks), hepatocellular tumors in GCIP transgenic mice are smaller and well-differentiated compared to the poorly differentiated tumors in wild-type mice. Furthermore, we demonstrate that GCIP functions as a transcriptional suppressor, regulates the expression of cyclin D1, and inhibits anchorage-independent cell growth and colony formation in HepG2 cells, suggesting a significant role of GCIP in tumor initiation and development.

Mect1-Maml2 fusion oncogene linked to the aberrant activation of cyclic AMP/CREB regulated genes

Malignant salivary gland tumors can arise from a t(11;19) translocation that fuses 42 residues from Mect1/Torc1, a cyclic AMP (cAMP)/cAMP-responsive element binding protein (CREB)-dependent transcriptional coactivator, with 982 residues from Maml2, a NOTCH receptor coactivator. To determine if the Mect1-Maml2 fusion oncogene mediates tumorigenicity by disrupting cAMP/CREB signaling, we have generated in-frame deletions within the CREB-binding domain of Mect1/Torc1 for testing transformation activity and have also developed a doxycycline-regulated Mect1-Maml2 mammalian expression vector for global gene expression profiling. We observed that small deletions within the CREB-binding domain completely abolished transforming activity in RK3E epithelial cells. Further, we have shown that the ectopic induction of Mect1-Maml2 in HeLa cells strongly activated the expression of a group of known cAMP/CREB-regulated genes. In addition, we detected candidate cAMP-responsive element sites within 100 nucleotides of the transcriptional start sites of other genes activated by Mect1-Maml2 expression. In contrast, we did not observe alterations of known Notch-regulated target genes in these expression array profile experiments. We validated the results by reverse transcription-PCR in transfected HeLa, RK3E, and H2009 lung tumor cells and in mucoepidermoid cancer cells that endogenously express the fusion oncopeptide. Whereas overexpression of components of the cAMP pathway has been associated with a subset of human carcinomas, these data provide a direct genetic link between deregulation of cAMP/CREB pathways and epithelial tumorigenesis and suggest future therapeutic strategies for this group of salivary gland tumors.

SCL expression at critical points in human hematopoietic lineage commitment

The stem cell leukemia (SCL or tal-1) gene was initially identified as a translocation partner in a leukemia that possessed both lymphoid and myeloid differentiation potential. Mice that lacked SCL expression showed a complete block in hematopoiesis; thus, SCL was associated with hematopoietic stem cell (HSC) function. More recent studies show a role for SCL in murine erythroid differentiation. However, the expression pattern and the role of SCL during early stages of human hematopoietic differentiation are less clear. In this study we chart the pattern of human SCL expression from HSCs, through developmentally sequential populations of lymphoid and myeloid progenitors to mature cells of the hematopoietic lineages. Using recently defined surface immunophenotypes, we fluorescence-activated cell-sorted (FACS) highly purified populations of primary human hematopoietic progenitors for reverse transcription-polymerase chain reaction (RT-PCR) analysis of SCL expression. Our data show that SCL mRNA is easily detectable in all hematopoietic populations with erythroid potential, including HSCs, multipotential progenitors, common myeloid progenitors, megakaryocyte/erythrocyte progenitors, and nucleated erythroid lineage cells. SCL mRNA expression was present but rapidly downregulated in the common lymphoid progenitor and granulocyte/monocyte progenitor populations that lack erythroid potential. SCL expression was undetectable in immature cells of nonerythroid lineages, including pro-B cells, early thymic progenitors, and myeloid precursors expressing the M-CSF receptor. SCL expression was also absent from all mature cells of the nonerythroid lineages. Although low levels of SCL were detected in lymphoid- and myeloid-restricted progenitors, our studies show that abundant SCL expression is normally tightly linked with erythroid differentiation potential.

OLIG2 (BHLHB1), a bHLH transcription factor, contributes to leukemogenesis in concert with LMO1

OLIG2 (originally designated BHLHB1) encodes a transcription factor that contains the basic helix-loop-helix motif. Although expression of OLIG2 is normally restricted to neural tissues, overexpression of OLIG2 has been shown in patients with precursor T-cell lymphoblastic lymphoma/leukemia (pre-T LBL). In the current study, we found that overexpression of OLIG2 was not only found in oligodendroglioma samples and normal neural tissue but also in a wide spectrum of malignant cell lines including leukemia, non-small cell lung carcinoma, melanoma, and breast cancer cell lines. To investigate whether enforced expression of OLIG2 is oncogenic, we generated transgenic mice that overexpressed OLIG2 in the thymus. Ectopic OLIG2 expression in the thymus was only weakly oncogenic as only 2 of 85 mice developed pre-T LBL. However, almost 60% of transgenic mice that overexpressed both OLIG2 and LMO1 developed pre-T LBL with large thymic tumor masses. Gene expression profiling of thymic tumors that developed in OLIG2/LMO1 mice revealed up-regulation of Notch1 as well as Deltex1 (Dtx1) and pre T-cell antigen receptor alpha (Ptcra), two genes that are considered to be downstream of Notch1. Of note, we found mutations in the Notch1 heterodimerization or proline-, glutamic acid-, serine-, and threonine-rich domain in three of six primary thymic tumors. In addition, growth of leukemic cell lines established from OLIG2/LMO1 transgenic mice was suppressed by a gamma-secretase inhibitor, suggesting that Notch1 up-regulation is important for the proliferation of OLIG2-LMO1 leukemic cells.

ID1 and ID2 are retinoic acid responsive genes and induce a G0/G1 accumulation in acute promyelocytic leukemia cells

Acute promyelocytic leukemia (APL) is uniquely sensitive to treatment with all-trans retinoic acid (ATRA), which results in the expression of genes that induce the terminal granulocytic differentiation of the leukemic blasts. Here we report the identification of two ATRA responsive genes in APL cells, ID1 and ID2. These proteins act as antagonists of basic helix-loop-helix (bHLH) transcription factors. ATRA induced a rapid increase in ID1 and ID2, both in the APL cell line NB4 as well as in primary patient cells. In addition, a strong downregulation of E2A was observed. E2A acts as a general heterodimerization partner for many bHLH proteins that are involved in differentiation control in various tissues. The simultaneous upregulation of ID1 and ID2, and the downregulation of E2A suggest a role for bHLH proteins in the induction of differentiation of APL cells following ATRA treatment. To test the relevance of this upregulation, ID1 and ID2 were overexpressed in NB4 cells. Overexpression inhibited proliferation and induced a G0/G1 accumulation. These results indicate that ID1 and ID2 are important retinoic acid responsive genes in APL, and suggest that the inhibition of specific bHLH transcription factor complexes may play a role in the therapeutic effect of ATRA in APL.

IL-3 Induces Inhibitor of DNA-Binding Protein-1 in Hemopoietic Progenitor Cells and Promotes Myeloid Cell Development

Hemopoiesis depends on the expression and regulation of transcription factors, which control the maturation of specific cell lineages. We found that the helix-loop-helix transcription factor inhibitor of DNA-binding protein 1 (Id1) is not expressed in hemopoietic stem cells (HSC), but is increased in more committed myeloid progenitors. Id1 levels decrease during neutrophil differentiation, but remain high in differentiated macrophages. Id1 is expressed at low levels or is absent in developing lymphoid or erythroid cells. Id1 expression can be induced by IL-3 in HSC during myeloid differentiation, but not by growth factors that promote erythroid and B cell development. HSC were transduced with retroviral vectors that express Id1 and were transplanted in vivo to evaluate their developmental potential. Overexpression of Id1 in HSC promotes myeloid but impairs B and erythroid cell development. Enforced expression of Id1 in committed myeloid progenitor cells inhibits granulocyte but not macrophage differentiation. Therefore, Id1 may be part of the mechanism regulating myeloid vs lymphoid/erythroid cell fates, and macrophage vs neutrophil maturation.

Role of DNA Sequence in the Binding Specificity of Synthetic Basic-Helix-Loop-Helix Domains

The basic helix-loop-helix (bHLH) domain defines a class of transcription factors that are essential for the regulation of many genes involved in cell differentiation and development. To determine the role of the DNA sequence in driving dimerization specificity of bHLH transcription factors, we analyzed the DNA sequence in and around a consensus hexanucleotide binding site (E-box). The bHLH domains of two transcription factors, E12 and TAL1, were chemically synthesized. The minimal DNA binding domain for both the E12 homodimer and the E12-TAL1 heterodimer was determined, thereby extending the E-box by two base pairs. Additional studies indicate that the presence of a thymine in the first flanking position 5' to the E-box prevents DNA binding of both dimer complexes. The presence of a thymine or cytosine in a flanking position two bases 5' to the E-box decreases the affinity for the E12 homodimer twofold but completely inactivates DNA binding for the E12-TAL1 heterodimer. Access to synthetic DNA and protein enabled the analysis of specific interactions between a conserved arginine residue in the basic helix of each bHLH domain and adenine in a flanking position two bases 5' to the E-box. Our results indicate a key role of the DNA sequence in driving dimerization specificity among bHLH transcription factors.

Combination of all-trans retinoic acid and lithium chloride surmounts a retinoid differentiation block induced by expression of Scl and Rbtn2 transcription factors in myeloid leukemia cells

We have previously shown that forced expression of the transcription factor Scl in WEHI-3B D(+) cells prevents ATRA-induced cell differentiation. We now find that the overexpression of Rbtn2 also interferes with induction of differentiation by ATRA. Addition of LiCl to ATRA treatment restored the capacity of both Scl- and Rbtn2-expressing cells to respond to the retinoid in a synergistic manner. Similar results were obtained with Scl-transfected HL60 cells where its expression diminished responsiveness to ATRA. These findings suggest that if Scl and/or Rbtn2 are involved in the non-responsiveness of AML patients to ATRA-induced differentiation, addition of LiCl may reverse insensitivity.

TAL1/SCL induces leukemia by inhibiting the transcriptional activity of E47/HEB

Activation of the basic-helix-loop-helix (bHLH) gene TAL1 (or SCL) is a frequent gain-of-function mutation in T cell acute lymphoblastic leukemia (T-ALL). To provide genetic evidence that tal1/scl induces leukemia by interfering with E47 and HEB, we expressed tal1/scl in an E2A or HEB heterozygous background. These mice exhibit disease acceleration and perturbed thymocyte development due to repression of E47/HEB target genes. In tal1/scl thymocytes, we find the corepressor mSin3A bound to the CD4 enhancer, whereas an E47/HEB/p300 complex is detected in wild-type thymocytes. Furthermore, tal1/scl tumors are sensitive to pharmacologic inhibition of HDAC and undergo apoptosis. These data demonstrate that tal1/scl induces leukemia by repressing E47/HEB and suggest that HDAC inhibitors may prove efficacious in T-ALL patients who express TAL1/SCL.

SCL: from the origin of hematopoiesis to stem cells and leukemia

In the hematopoietic system, lineage commitment and differentiation is controlled by the combinatorial action of transcription factors from diverse families. SCL is a basic helix-loop-helix transcription factor that is an essential regulator at several levels in the hematopoietic hierarchy and whose inappropriate regulation frequently contributes to the development of pediatric T-cell acute lymphoblastic leukemia. This review discusses advances that have shed important light on the functions played by SCL during normal hematopoiesis and leukemogenesis and have revealed an unexpected robustness of hematopoietic stem cell function. Molecular studies have unraveled a mechanism through which gene expression is tightly controlled, as SCL functions within multifactorial complexes that exhibit an all-or-none switch-like behavior in transcription activation, arguing for a quantal process that depends on the concurrent occupation of target loci by all members of the complex. Finally, variations in composition of SCL-containing complexes may ensure flexibility and specificity in the regulation of lineage-specific programs of gene expression, thus providing the molecular basis through which SCL exerts its essential functions at several branch points of the hematopoietic hierarchy.

Regulation of pT alpha gene expression by a dosage of E2A, HEB, and SCL

The expression of the pT alpha gene is required for effective selection, proliferation, and survival of beta T-cell receptor (beta TCR)-expressing immature thymocytes. Here, we have identified two phylogenetically conserved E-boxes within the pT alpha enhancer sequence that are required for optimal enhancer activity and for its stage-specific activity in immature T cells. We have shown that the transcription factors E2A and HEB associate with high affinity to these E-boxes. Moreover, we have identified pT alpha as a direct target of E2A-HEB heterodimers in immature thymocytes because they specifically occupy the enhancer in vivo. In these cells, pT alpha mRNA levels are determined by the presence of one or two functional E2A or HEB alleles. Furthermore, E2A/HEB transcriptional activity is repressed by heterodimerization with SCL, a transcription factor that is turned off in differentiating thymocytes exactly at a stage when pT alpha is up-regulated. Taken together, our observations suggest that the dosage of E2A, HEB, and SCL determines pT alpha gene expression in immature T cells.

Disruption of pre-TCR expression accelerates lymphomagenesis in E2A-deficient mice

The helix-loop-helix proteins E47 and E12, which are encoded by the E2A gene, regulate several stages of T cell development. In addition, mice deficient for E2A are highly susceptible to thymic lymphoma. Here we report that the development of lymphoma in E2A-deficient mice did not require pre- and recombinase-activating gene expression. Rather, we found that, whereas illegitimate DNA rearrangement did not play a major role in the development of these lymphomas, defects that prevented pre-T cell antigen receptor expression tended to accelerate lymphomagenesis in E2A-deficient mice. These data and previous observations also provide insight into the role of Notch in lymphoma development. Specifically, we propose that Notch activation indirectly modulates E2A activity through induction of pre-Talpha expression, ultimately leading to the development of lymphoma.

Ectopic expression of TAL-1 protein in Ly-6E.1-htal-1 transgenic mice induces defects in B- and T-lymphoid differentiation

The tal-1 gene encodes a basic helix-loop-helix (bHLH) transcription factor required for primitive and definitive hematopoiesis. Additionally, ectopic activation of the tal-1 gene during T lymphopoiesis occurs in numerous cases of human T-cell acute lymphoblastic leukemia. With the use of transgenic mice, we show that, in adult hematopoiesis, constitutive expression of TAL-1 protein causes disorders in the hematopoietic lineages that normally switch off tal-1 gene expression during their differentiation process. Myelopoiesis was characterized by a moderate increase of myeloid precursors and by Sca-1 antigen persistence. Although no lymphoid leukemia was observed, T lymphopoiesis and B lymphopoiesis were severely impaired. Transgenic mice showed reduced thymic cellularity together with a decrease in double-positive cells and a concurrent increase in the single-positive population. B cells exhibited a differentiation defect characterized by a reduction of the B-cell compartment most likely because of a differentiation block upstream of the intermediate pro-B progenitor. B cells escaping this defect developed normally, but transgenic splenocytes presented a defect in immunoglobulin class switch recombination. Altogether, these results enlighten the fine-tuning of TAL-1 expression during adult hematopoiesis and indicate why TAL-1 expression has to be switched off in the lymphoid lineages.

The DNA binding activity of TAL-1 is not required to induce leukemia/lymphoma in mice

Activation of the basic helix-loop-helix (bHLH) gene TAL-1 (or SCL) is the most frequent gain-of-function mutation in pediatric T cell acute lymphoblastic leukemia (T-ALL). Similarly, mis-expression of tal-1 in the thymus of transgenic mice results in the development of clonal T cell lymphoblastic leukemia. To determine the mechanism(s) of tal-1-induced leukemogenesis, we created transgenic mice expressing a DNA binding mutant of tal-1. Surprisingly, these mice develop disease, demonstrating that the DNA binding properties of tal-1 are not required to induce leukemia/lymphoma in mice. However, wild type tal-1 and the DNA binding mutant both form stable complexes with E2A proteins. In addition, tal-1 stimulates differentiation of CD8-single positive thymocytes but inhibits the development of CD4-single positive cells: effects also observed in E2A-deficient mice. Our study suggests that the bHLH protein tal-1 contributes to leukemia by interfering with E2A protein function(s).

Development and characterization of T cell leukemia cell lines established from SCL/LMO1 double transgenic mice

We have established a panel of nine immortal cell lines from T cell malignancies which arose in mice transgenic for the SCL and LMO1 genes. Cells from the primary malignancies initially grew very slowly in vitro, loosely attached to a stromal layer, before gaining the ability to proliferate independently. Upon gaining the ability to proliferate in the absence of a stromal layer, these cell lines grew rapidly, doubling every 14-23 h, to a very high density, approaching 10(7) cells/ml. Whereas the tumors which arise in SCL/LMO1 double transgenic mice are typically diploid or pseudodiploid, the cell lines were all grossly aneuploid, suggesting the possibility that additional genetic events were selected for in vitro. Given that SCL and LMO1 gene activation are both commonly seen in human patients with T cell acute lymphoblastic leukemia, these cell lines may be a useful in vitro model for the human disease.

During ontogeny primitive (CD34+CD38−) hematopoietic cells show altered expression of a subset of genes associated with early cytokine and differentiation responses of their adult counterparts

Comparison of gene expression profiles in closely related subpopulations of primitive hematopoietic cells offers a powerful first step to elucidating the molecular basis of their different biologic properties. Here we present the results of a comparative quantitative analysis of transcript levels for various growth factor receptors, ligands, and transcription factor genes in CD34+CD38− and CD34+CD38+ cells purified from first trimester human fetal liver, cord blood, and adult bone marrow (BM). In addition, adult BM CD34+CD38− cells were examined after short-term exposure to various growth factors in vitro. Transcripts for 19 of the 24 genes analyzed were detected in unmanipulated adult BM CD34+CD38− cells. Moreover, the levels of transforming growth factor beta (TGF-β), gp130, c-fos, and c-jun transcripts in these cells were consistently and significantly different (higher) than in all other populations analyzed, including phenotypically similar but biologically different cells from fetal or neonatal sources, as well as adult BM CD34+ cells still in G0 after 2 days of growth factor stimulation. We have thus identified a subset of early response genes whose expression in primitive human hematopoietic cells is differently regulated during ontogeny and in a fashion that is recapitulated in growth factor-stimulated adult BM CD34+CD38− cells, before their cell cycle progression and independent of their subsequent differentiation response. These findings suggest a progressive alteration in the physiology of primitive hematopoietic cells during development such that these cells initially display a partially “activated” state, which is not maximally repressed until after birth.

During ontogeny primitive (CD34+CD38−) hematopoietic cells show altered expression of a subset of genes associated with early cytokine and differentiation responses of their adult counterparts

Comparison of gene expression profiles in closely related subpopulations of primitive hematopoietic cells offers a powerful first step to elucidating the molecular basis of their different biologic properties. Here we present the results of a comparative quantitative analysis of transcript levels for various growth factor receptors, ligands, and transcription factor genes in CD34+CD38− and CD34+CD38+ cells purified from first trimester human fetal liver, cord blood, and adult bone marrow (BM). In addition, adult BM CD34+CD38− cells were examined after short-term exposure to various growth factors in vitro. Transcripts for 19 of the 24 genes analyzed were detected in unmanipulated adult BM CD34+CD38− cells. Moreover, the levels of transforming growth factor beta (TGF-β), gp130, c-fos, and c-jun transcripts in these cells were consistently and significantly different (higher) than in all other populations analyzed, including phenotypically similar but biologically different cells from fetal or neonatal sources, as well as adult BM CD34+ cells still in G0 after 2 days of growth factor stimulation. We have thus identified a subset of early response genes whose expression in primitive human hematopoietic cells is differently regulated during ontogeny and in a fashion that is recapitulated in growth factor-stimulated adult BM CD34+CD38− cells, before their cell cycle progression and independent of their subsequent differentiation response. These findings suggest a progressive alteration in the physiology of primitive hematopoietic cells during development such that these cells initially display a partially “activated” state, which is not maximally repressed until after birth.

Unilineage hematopoietic differentiation in bulk and single cell culture

The rarity of hematopoietic stem and progenitor cells (HSCs, HPCs) has hampered the analysis of cellular and molecular mechanisms underlying early hematopoiesis. Methodology for HPC purification has partially offset this limitation. A further hurdle has been represented by the heterogeneity of the analyzed HPC/precursor populations: recently, development of unilineage HPC differentiation cultures has provided homogeneous populations of hematopoietic cells, particularly in the early differentiation state, i.e., populations pertaining to a single lineage and a restricted stage of differentiation/maturation, but sufficiently large for cellular/molecular analysis. This report focuses on the development and characterization of the unilineage HPC differentiation culture systems. A section is devoted to selected cellular and molecular mechanisms underlying hematopoiesis, which have been investigated by the HPC unilineage culture approach. Finally, recent advances in the development of HPC unilineage cultures at single cell level are discussed.

Regulation of Id gene expression during embryonic stem cell-derived hematopoietic differentiation

To elucidate the role of helix-loop-helix (HLH) Id proteins in hematopoietic differentiation, we used a model of embryonic stem (ES) cell differentiation in vitro which gives access not only to hematopoietic myeloid progenitor cells but also to the more primitive blast colony-forming cell (BL-CFC), the in vitro equivalent of the hemangioblast that gives rise to blast cell colonies in the presence of VEGF. We first demonstrated that ES cell-derived blast cell colonies could be used as a model to study hematopoietic differentiation and maturation. We next established the expression profile of Id genes in this model. Transcripts of the four Id genes were present in ES cells. Id1, Id3 and Id4 gene expression was down-regulated during the development of blast cell colonies while that of Id2 was maintained. Thus, Id1, Id3, and Id4 proteins are candidates for being negative regulators of hematopoiesis in the model of hematopoietic ES cell differentiation in vitro.

Cloning and characterization of DIP1, a novel protein that is related to the Id family of proteins

Using human cyclin D1 as the "bait" in a yeast two-hybrid system, together with a HL60 cDNA library, we identified a novel human nuclear protein designated DIP1. This protein is expressed in a variety of cell types, and in fibroblasts its level remains constant throughout the cell cycle. However, the level of this protein increases severalfold during the differentiation of HL60 cells. The DIP1 protein can be phosphorylated in vitro by a cellular kinase and this activity reaches its maximum in extracts obtained from cells in the G1 phase of the cell cycle. DIP1 contains a helix-loop-helix motif but lacks an adjacent basic DNA-binding domain, thus resembling the Id family of proteins. The dip1 gene is located on human chromosome 16p11.2-12, a locus that is amplified in several types of human cancer. These results suggest that DIP1 may be involved in the control of gene expression and differentiation, but its precise function remains to be determined.

The t(14;21)(q11.2;q22) chromosomal translocation associated with T-cell acute lymphoblastic leukemia activates the BHLHB1 gene

We have cloned the genomic breakpoints for a balanced t(14;21)(q11. 2;q22) chromosomal translocation associated with T-cell acute lymphoblastic leukemia. Sequence analysis of the genomic breakpoints indicated that the translocation had been mediated by an illegitimate V(D)J recombination event that disrupted the T-cell receptor (TCR) alpha locus and placed the TCR alpha locus enhancer on the derivative 21 chromosome. We identified a previously unreported transcript, designated BHLHB1 (for basic domain, helix-loop-helix protein, class B, 1) that had been activated by the translocation. BHLHB1 mapped to the region of chromosome 21 that has been proposed to be responsible, at least in part, for the learning deficits seen in children with Down's syndrome. Although BHLHB1 expression normally is restricted to neural tissues, T-cell lymphoblasts with the t(14;21)(q11.2;q22) also expressed high levels of BHLHB1 mRNA. Expression of BHLHB1 dramatically inhibited E2A-mediated transcription activation in NIH 3T3 fibroblasts and Jurkat T cells. This observation suggests that BHLHB1, similar to SCL/TAL1, may exert a leukemogenic effect through a functional inactivation of E2A or related proteins.

Fetal Alz-50 clone 1, a novel zinc finger protein, binds a specific DNA sequence and acts as a transcriptional regulator

Fetal Alz-50 clone 1 (FAC1) is a novel, developmentally regulated gene that exhibits changes in protein expression and subcellular localization during neuronal development and neurodegeneration. To understand the functional implications of altered subcellular localization, we have established a normal cellular function of FAC1. The FAC1 amino acid sequence contains regional homology to transcriptional regulators. Using the polymerase chain reaction-assisted binding site selection assay, we have identified a DNA sequence recognized by recombinant FAC1. Mutation of any 2 adjacent base pairs in the identified binding site dramatically reduced the binding preference of FAC1, demonstrating that the binding is specific for the identified site. Nuclear extracts from neural and non-neural cell lines contained a DNA-binding activity with similar specificity and nucleotide requirements as the recombinant FAC1 protein. This DNA-binding activity can be attributed to FAC1 since it is dependent upon the presence of FAC1 and behaves identically on a nondenaturing polyacrylamide gel as transiently transfected FAC1. In NIH3T3 cells, luciferase reporter plasmids containing the identified binding site (CACAACAC) were repressed by cotransfected FAC1 whether the binding site was proximal or distal to the transcription initiation site. This study indicates that FAC1 is a DNA-binding protein that functions as a transcription factor when localized to the nucleus.

Id1 and Id3 are required for neurogenesis, angiogenesis and vascularization of tumour xenografts

Id proteins may control cell differentiation by interfering with DNA binding of transcription factors. Here we show that targeted disruption of the dominant negative helix-loop-helix proteins Id1 and Id3 in mice results in premature withdrawal of neuroblasts from the cell cycle and expression of neural-specific differentiation markers. The Id1-Id3 double knockout mice also display vascular malformations in the forebrain and an absence of branching and sprouting of blood vessels into the neuroectoderm. As angiogenesis both in the brain and in tumours requires invasion of avascular tissue by endothelial cells, we examined the Id knockout mice for their ability to support the growth of tumour xenografts. Three different tumours failed to grow and/or metastasize in Id1+/- Id3-/- mice, and any tumour growth present showed poor vascularization and extensive necrosis. Thus, the Id genes are required to maintain the timing of neuronal differentiation in the embryo and invasiveness of the vasculature. Because the Id genes are expressed at very low levels in adults, they make attractive new targets for anti-angiogenic drug design.

Transcription factors in hematopoiesis

The advent of gene targeting in the mouse has led to rapid advances in the identification of factors controlling gene expression that are essential for normal hematopoietic development. Recent work has also uncovered roles for some of these factors in leukemogenesis and in the global regulation of chromatin structure.

p300 functions as a transcriptional coactivator for the TAL1/SCL oncoprotein

Activation of the TAL1 (or SCL) gene, originally identified through its involvement by a recurrent chromosomal translocation, is the most frequent gain-of-function mutation recognized in T-cell acute lymphoblastic leukemia (T-ALL). The TAL1 proteins contain a basic helix - loop - helix (bHLH) motif characteristic of a large family of transcription factors that control transcription from an E box target element as heterodimers with the E2A- and HEB-encoded gene products. Gene knockout studies in mice indicate that this transcription factor is required for embryonic and adult hematopoiesis, and considerable evidence suggests it has specific functions in terminal erythroid differentiation. We investigated whether the broadly expressed nuclear protein p300, known to function as a coactivator for other bHLH proteins involved in cellular differentiation, also interacts with TAL1. p300 was found to coimmunoprecipitate with Tal1 in extracts from murine erythroleukemia (MEL) cells induced to differentiate with dimethylsulfoxide (DMSO), and p300 and Tal1 were observed in a common E box DNA-binding complex in extracts from differentiating MEL cells. p300 also interacted with Tal1 in protein pulldown assays, suggesting this was a direct interaction. Finally, p300 augmented transcription by Tal1 from an E box-containing promoter and by a GAL4-Tal1 fusion from a promoter containing the GAL4 DNA-binding element. Deletion analysis identified the bHLH domain of Tal1 and amino-terminal sequences of p300 as necessary for p300-stimulated transactivation and Tal1-p300 interaction in vitro. These results indicate that recruitment of the transcriptional coactivator p300 can positively regulate TAL1-directed gene expression. The dependence of their interaction in MEL cells on addition of a differentiation inducer suggests, further, that this TAL1-p300 complex may have an important role in terminal erythroid differentiation.

Disordered T-cell development and T-cell malignancies in SCL LMO1 double-transgenic mice: parallels with E2A-deficient mice

The gene most commonly activated by chromosomal rearrangements in patients with T-cell acute lymphoblastic leukemia (T-ALL) is SCL/tal. In collaboration with LMO1 or LMO2, the thymic expression of SCL/tal leads to T-ALL at a young age with a high degree of penetrance in transgenic mice. We now show that SCL LMO1 double-transgenic mice display thymocyte developmental abnormalities in terms of proliferation, apoptosis, clonality, and immunophenotype prior to the onset of a frank malignancy. At 4 weeks of age, thymocytes from SCL LMO1 mice show 70% fewer total thymocytes, with increased rates of both proliferation and apoptosis, than control thymocytes. At this age, a clonal population of thymocytes begins to populate the thymus, as evidenced by oligoclonal T-cell-receptor gene rearrangements. Also, there is a dramatic increase in immature CD44(+) CD25(-) cells, a decrease in the more mature CD4(+) CD8(+) cells, and development of an abnormal CD44(+) CD8(+) population. An identical pattern of premalignant changes is seen with either a full-length SCL protein or an amino-terminal truncated protein which lacks the SCL transactivation domain, demonstrating that the amino-terminal portion of SCL is not important for leukemogenesis. Lastly, we show that the T-ALL which develop in the SCL LMO1 mice are strikingly similar to those which develop in E2A null mice, supporting the hypothesis that SCL exerts its oncogenic action through a functional inactivation of E proteins.