Spi-1/PU.1 is a positive regulator of the Fli-1 gene involved in inhibition of erythroid differentiation in friend erythroleukemic cell lines

The complex landscape of haematopoietic lineage commitments is encoded in the coarse-grained endogenous network

Haematopoietic lineage commitments are presented by a canonical roadmap in which haematopoietic stem cells or multipotent progenitors (MPPs) bifurcate into progenitors of more restricted lineages and ultimately mature to terminally differentiated cells. Although transcription factors playing significant roles in cell-fate commitments have been extensively studied, integrating such knowledge into the dynamic models to understand the underlying biological mechanism remains challenging. The hypothesis and modelling approach of the endogenous network has been developed previously and tested in various biological processes and is used in the present study of haematopoietic lineage commitments. The endogenous network is constructed based on the key transcription factors and their interactions that determine haematopoietic cell-fate decisions at each lineage branchpoint. We demonstrate that the process of haematopoietic lineage commitments can be reproduced from the landscape which orchestrates robust states of network dynamics and their transitions. Furthermore, some non-trivial characteristics are unveiled in the dynamical model. Our model also predicted previously under-represented regulatory interactions and heterogeneous MPP states by which distinct differentiation routes are intermediated. Moreover, network perturbations resulting in state transitions indicate the effects of ectopic gene expression on cellular reprogrammes. This study provides a predictive model to integrate experimental data and uncover the possible regulatory mechanism of haematopoietic lineage commitments.

To bi or not to bi: Acute erythroid leukemias and hematopoietic lineage choice

Acute erythroid leukemia (AEL) is an acute leukemia characterized by erythroid lineage transformation. The World Health Organization (WHO) 2008 classification recognized two subtypes of AEL: bilineage erythroleukemia (erythroid/myeloid leukemia) and pure erythroid leukemia. The erythroleukemia subtype was removed in the updated 2016 WHO classification, with about half of cases reclassified as myelodysplastic syndrome (MDS) and half as acute myeloid leukemia (AML). Diagnosis and classification are currently based on morphology using standard blast cutoffs, without integration of underlying genomic and other molecular features. Key outstanding questions are therefore whether AEL can be accurately diagnosed based solely on morphology or whether genetic or other molecular criteria should be included in its classification, and whether considering AEL as an entity distinct from AML and MDS is clinically relevant. We discuss recent work on the molecular basis of AEL, including the identification of mutations causative of AEL and of transcriptional and epigenetic features that can be used to distinguish AEL from MDS and nonerythroid AML, and the prognostic value of these molecular features.

Lack of a site-specific phosphorylation of Presenilin 1 disrupts microglial gene networks and progenitors during development

Microglial cells play a key role in brain homeostasis from development to adulthood. Here we show the involvement of a site-specific phosphorylation of Presenilin 1 (PS1) in microglial development. Profiles of microglia-specific transcripts in different temporal stages of development, combined with multiple systematic transcriptomic analysis and quantitative determination of microglia progenitors, indicate that the phosphorylation of PS1 at serine 367 is involved in the temporal dynamics of microglial development, specifically in the developing brain rudiment during embryonic microgliogenesis. We constructed a developing brain-specific microglial network to identify transcription factors linked to PS1 during development. Our data showed that PS1 functional connections appear through interaction hubs at Pu.1, Irf8 and Rela-p65 transcription factors. Finally, we showed that the total number of microglia progenitors was markedly reduced in the developing brain rudiment of embryos lacking PS1 phosphorylation compared to WT. Our work identifies a novel role for PS1 in microglial development.

C/EBPα and GATA-2 Mutations Induce Bilineage Acute Erythroid Leukemia through Transformation of a Neomorphic Neutrophil-Erythroid Progenitor

Acute erythroid leukemia (AEL) commonly involves both myeloid and erythroid lineage transformation. However, the mutations that cause AEL and the cell(s) that sustain the bilineage leukemia phenotype remain unknown. We here show that combined biallelic Cebpa and Gata2 zinc finger-1 (ZnF1) mutations cooperatively induce bilineage AEL, and that the major leukemia-initiating cell (LIC) population has a neutrophil-monocyte progenitor (NMP) phenotype. In pre-leukemic NMPs Cebpa and Gata2 mutations synergize by increasing erythroid transcription factor (TF) expression and erythroid TF chromatin access, respectively, thereby installing ectopic erythroid potential. This erythroid-permissive chromatin conformation is retained in bilineage LICs. These results demonstrate that synergistic transcriptional and epigenetic reprogramming by leukemia-initiating mutations can generate neomorphic pre-leukemic progenitors, defining the lineage identity of the resulting leukemia.

Transcription factor PU.1 is involved in the progression of glioma

Glioma is a severe disease of the central nervous system. Although previous studies have identified the important role of the immune response in association with tumor intervention, it is still unknown whether PU.1, a transcription factor known for its role in myeloid differentiation and immune responses, is involved in the progression of glioma. In the present study, we found a significant increase in SPI1, the gene that encodes PU.1, in samples from patients with glioma. Through genotype-phenotype association analysis several candidate factors that may mediate the role of PU.1 in glioma were identified. To further validate the association between PU.1 and glioma we found that the expression of BTK, a potential target of PU.1, was also upregulated in patients with glioma. We also demonstrated that various biological pathways could be involved in PU.1-associated glioma by analyzing these potential targets in the Reactome database. These results provide evidence that PU.1 could serve a role in the progress of glioma through its transcriptional targets in multiple signaling pathways. Therefore, in addition to its role in hematopoietic linage development and leukemia, PU.1 appears to be involved in the regulation of glioma and potentially in other malignant cancers.

Erythroleukemia-historical perspectives and recent advances in diagnosis and management

Acute erythroleukemia is a rare form of acute myeloid leukemia recognized by its distinct phenotypic attribute of erythroblastic proliferation. After a century of its descriptive history, many diagnostic, prognostic, and therapeutic implications relating to this unique leukemia subset remain uncertain. The rarity of the disease and the simultaneous involvement of its associated myeloid compartment have complicated in vitro studies of human erythroleukemia cell lines. Although murine and cell line erythroleukemia models have provided valuable insights into pathophysiology, translation of these concepts into treatment are not forthcoming. Integration of knowledge gained through a careful study of these models with more recent data emerging from molecular characterization will help elucidate key mechanistic pathways and provide a much needed framework that accounts for erythroid lineage-specific attributes. In this article, we discuss the evolving diagnostic concept of erythroleukemia, translational aspects of its pathophysiology, and promising therapeutic targets through an appraisal of the current literature.

NLS-tagging: an alternative strategy to tag nuclear proteins

The characterization of transcription factor complexes and their binding sites in the genome by affinity purification has yielded tremendous new insights into how genes are regulated. The affinity purification requires either the use of antibodies raised against the factor of interest itself or by high-affinity binding of a C- or N-terminally added tag sequence to the factor. Unfortunately, fusing extra amino acids to the termini of a factor can interfere with its biological function or the tag may be inaccessible inside the protein. Here, we describe an effective solution to that problem by integrating the ‘tag’ close to the nuclear localization sequence domain of the factor. We demonstrate the effectiveness of this approach with the transcription factors Fli-1 and Irf2bp2, which cannot be tagged at their extremities without loss of function. This resulted in the identification of novel proteins partners and a new hypothesis on the contribution of Fli-1 to hematopoiesis.

Dynamic transcription factor activity profiles reveal key regulatory interactions during megakaryocytic and erythroid differentiation

The directed differentiation toward erythroid (E) or megakaryocytic (MK) lineages by the MK-E progenitor (MEP) could enhance the ex vivo generation of red blood cells and platelets for therapeutic transfusions. The lineage choice at the MEP bifurcation is controlled in large part by activity within the intracellular signal transduction network, the output of which determines the activity of transcription factors (TFs) and ultimately gene expression. Although many TFs have been implicated, E or MK differentiation is a complex process requiring multiple days, and the dynamics of TF activities during commitment and terminal maturation are relatively unexplored. Herein, we applied a living cell array for the large-scale, dynamic quantification of TF activities during MEP bifurcation. A panel of hematopoietic TFs (GATA-1, GATA-2, SCL/TAL1, FLI-1, NF-E2, PU.1, c-Myb) was characterized during E and MK differentiation of bipotent K562 cells. Dynamic TF activity profiles associated with differentiation towards each lineage were identified, and validated with previous reports. From these activity profiles, we show that GATA-1 is an important hub during early hemin- and PMA-induced differentiation, and reveal several characteristic TF interactions for E and MK differentiation that confirm regulatory mechanisms documented in the literature. Additionally, we highlight several novel TF interactions at various stages of E and MK differentiation. Furthermore, we investigated the mechanism by which nicotinamide (NIC) promoted terminal MK maturation using an MK-committed cell line, CHRF-288-11 (CHRF). Concomitant with its enhancement of ploidy, NIC strongly enhanced the activity of three TFs with known involvement in terminal MK maturation: FLI-1, NF-E2, and p53. Dynamic profiling of TF activity represents a novel tool to complement traditional assays focused on mRNA and protein expression levels to understand progenitor cell differentiation.

Spi-1, Fli-1 and Fli-3 (miR-17-92) oncogenes contribute to a single oncogenic network controlling cell proliferation in friend erythroleukemia

Clonal erythroleukemia developing in susceptible mice infected by Friend virus complex are associated with highly recurrent proviral insertions at one of three loci called Spi-1, Fli-1 or Fli-3, leading to deregulated expression of oncogenic Spi-1 or Fli-1 transcription factors or miR-17-92 miRNA cluster, respectively. Deregulated expression of each of these three oncogenes has been independently shown to contribute to cell proliferation of erythroleukemic clones. Previous studies showed a close relationship between Spi-1 and Fli-1, which belong to the same ETS family, Spi-1 activating fli-1 gene, and both Spi-1 and Fli-1 activating multiple common target genes involved in ribosome biogenesis. In this study, we demonstrated that Spi-1 and Fli-1 are also involved in direct miR-17-92 transcriptional activation through their binding to a conserved ETS binding site in its promoter. Moreover, we demonstrated that physiological re-expression of exogenous miR-17 and miR-20a are able to partially rescue the proliferation loss induced by Fli-1 knock-down and identified HBP1 as a target of these miRNA in erythroleukemic cells. These results establish that three of the most recurrently activated oncogenes in Friend erythroleukemia are actually involved in a same oncogenic network controlling cell proliferation. The putative contribution of a similar ETS-miR-17-92 network module in other normal or pathological proliferative contexts is discussed.

Transcription factors Fli1 and EKLF in the differentiation of megakaryocytic and erythroid progenitor in 5q- syndrome and in Diamond-Blackfan anemia

Friend leukemia virus integration 1 (Fli1) and erythroid Krüppel-like factor (EKLF) participate under experimental conditions in the differentiation of megakaryocytic and erythroid progenitor in cooperation with other transcription factors, cytokines, cytokine receptors, and microRNAs. Defective erythropoiesis with refractory anemia and effective megakaryopoiesis with normal or increased platelet count is typical for 5q- syndrome. We decided to evaluate the roles of EKLF and Fli1 in the pathogenesis of this syndrome and of another ribosomopathy, Diamond-Blackfan anemia (DBA). Fli1 and EKLF mRNA levels were examined in mononuclear blood and bone marrow cells from patients with 5q- syndrome, low-risk MDS patients with normal chromosome 5, DBA patients, and healthy controls. In 5q- syndrome, high Fli1 mRNA levels in the blood and bone marrow mononuclear cells were found. In DBA, Fli1 expression did not differ from the controls. EKLF mRNA level was significantly decreased in the blood and bone marrow of 5q- syndrome and in all DBA patients. We propose that the elevated Fli1 in 5q- syndrome protects megakaryocytic cells from ribosomal stress contrary to erythroid cells and contributes to effective though dysplastic megakaryopoiesis.

Important genes in the pathogenesis of 5q- syndrome and their connection with ribosomal stress and the innate immune system pathway

Myelodysplastic syndrome (MDS) with interstitial deletion of a segment of the long arm of chromosome 5q [del(5q)] is characterized by bone marrow erythroid hyperplasia, atypical megakaryocytes, thrombocythemia, refractory anemia, and low risk of progression to acute myeloid leukemia (AML) compared with other types of MDS. The long arm of chromosome 5 contains two distinct commonly deleted regions (CDRs). The more distal CDR lies in 5q33.1 and contains 40 protein-coding genes and genes coding microRNAs (miR-143, miR-145). In 5q-syndrome one allele is deleted that accounts for haploinsufficiency of these genes. The mechanism of erythroid failure appears to involve the decreased expression of the ribosomal protein S14 (RPS14) gene and the upregulation of the p53 pathway by ribosomal stress. Friend leukemia virus integration 1 (Fli1) is one of the target genes of miR145. Increased Fli1 expression enables effective megakaryopoiesis in 5q-syndrome.

PU.1 and Haematopoietic Cell Fate: Dosage Matters

The ETS family transcription factor PU.1 is a key regulator of haematopoietic differentiation. Its expression is dynamically controlled throughout haematopoiesis in order to direct appropriate lineage specification. Elucidating the biological role of PU.1 has proved challenging. This paper will discuss how a range of experiments in cell lines and mutant and transgenic mouse models have enhanced our knowledge of the mechanisms by which PU.1 drives lineage-specific differentiation during haematopoiesis.

A large gene network in immature erythroid cells is controlled by the myeloid and B cell transcriptional regulator PU.1

PU.1 is a hematopoietic transcription factor that is required for the development of myeloid and B cells. PU.1 is also expressed in erythroid progenitors, where it blocks erythroid differentiation by binding to and inhibiting the main erythroid promoting factor, GATA-1. However, other mechanisms by which PU.1 affects the fate of erythroid progenitors have not been thoroughly explored. Here, we used ChIP-Seq analysis for PU.1 and gene expression profiling in erythroid cells to show that PU.1 regulates an extensive network of genes that constitute major pathways for controlling growth and survival of immature erythroid cells. By analyzing fetal liver erythroid progenitors from mice with low PU.1 expression, we also show that the earliest erythroid committed cells are dramatically reduced in vivo. Furthermore, we find that PU.1 also regulates many of the same genes and pathways in other blood cells, leading us to propose that PU.1 is a multifaceted factor with overlapping, as well as distinct, functions in several hematopoietic lineages.

The inositol phosphatase SHIP-1 is negatively regulated by Fli-1 and its loss accelerates leukemogenesis

The activation of Fli-1, an Ets transcription factor, is the critical genetic event in Friend murine leukemia virus (F-MuLV)-induced erythroleukemia. Fli-1 overexpression leads to erythropoietin-dependent erythroblast proliferation, enhanced survival, and inhibition of terminal differentiation, through activation of the Ras pathway. However, the mechanism by which Fli-1 activates this signal transduction pathway has yet to be identified. Down-regulation of the Src homology 2 (SH2) domain-containing inositol-5-phosphatase-1 (SHIP-1) is associated with erythropoietin-stimulated erythroleukemic cells and correlates with increased proliferation of transformed cells. In this study, we have shown that F-MuLV-infected SHIP-1 knockout mice display accelerated erythroleukemia progression. In addition, RNA interference (RNAi)-mediated suppression of SHIP-1 in erythroleukemia cells activates the phosphatidylinositol 3-kinase (PI 3-K) and extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) pathways, blocks erythroid differentiation, accelerates erythropoietin-induced proliferation, and leads to PI 3-K-dependent Fli-1 up-regulation. Chromatin immunoprecipitation and luciferase assays confirmed that Fli-1 binds directly to an Ets DNA binding site within the SHIP-1 promoter and suppresses SHIP-1 transcription. These data provide evidence to suggest that SHIP-1 is a direct Fli-1 target, SHIP-1 and Fli-1 regulate each other in a negative feedback loop, and the suppression of SHIP-1 by Fli-1 plays an important role in the transformation of erythroid progenitors by F-MuLV.

Enrichment of Sca1+ hematopoietic progenitors in polycythemic mice inhibits leukemogenesis

Polycythemia vera (PV) is a myeloproliferative disorder characterized by a pronounced increase in the number of erythroid cells. However, despite this aberrant proliferation, the incidence of erythroleukemia is paradoxically rare in PV patients. In this study, we show that the progression of Friend virus-induced erythroleukemia is delayed in a mouse model of primary familial congenital polycythemia in which the wild-type Epo-receptor (EpoR) gene is replaced with a truncated human EPOR gene. Herein, we show that these mice exhibit enrichment of Sca1(+)/cKit(-) progenitors and several mature immune cells, such as dendritic cells and macrophages. In cotransplantation experiments, Sca1(+)/cKit(-) progenitors inhibit the tumorigenicity of Sca1(-)/cKit(+) erythroleukemic cells. A cell line established from Sca1(+)/cKit(-) progenitors is also capable of inhibiting leukemic proliferation in culture and in mice. This phenomenon of leukemic inhibition, also detected in the serum of PV patients, is partially attributed to increased nitric oxide secretion. In addition, the administration of erythropoietin into leukemic mice induces a polycythemia-like state associated with the expansion of Sca1(+)/cKit(-) progenitors and derivative immune cells, thereby inhibiting leukemia progression. This study indicates that a combination therapy incorporating the enrichment of Sca1(+)/cKit(-) progenitors may serve as a novel approach for the treatment of leukemia.

Spi-1 and Fli-1 directly activate common target genes involved in ribosome biogenesis in Friend erythroleukemic cells

Spi-1 and Fli-1 are ETS transcription factors recurrently deregulated in mouse erythroleukemia induced by Friend viruses. Since they share the same core DNA binding site, we investigated whether they may contribute to erythroleukemia by common mechanisms. Using inducible knockdown, we demonstrated that Fli-1 contributes to proliferation, survival, and differentiation arrest of erythroleukemic cells harboring an activated fli-1 locus. Similarly, we used inducible Fli-1 knockdown and either hexamethylenebisacetamide (HMBA)- or small interfering RNA-mediated Spi-1 knockdown to investigate their respective contributions in erythroleukemic cells harboring an activated spi-1 locus. In these cells, simple or double knockdown of both Spi-1 and Fli-1 additively contributed to induce proliferation arrest and differentiation. Transcriptome profiling revealed that virtually all transcripts affected by both Fli-1 knockdown and HMBA are affected in an additive manner. Among these additively downregulated transcripts, more than 20% encode proteins involved in ribosome biogenesis, and conserved ETS binding sites are present in their gene promoters. Through chromatin immunoprecipitation, we demonstrated the association of Spi-1 and Fli-1 on these promoters in Friend erythroleukemic cells. These data lead us to propose that the oncogenicity of Spi-1, Fli-1, and possibly other ETS transcription factors may involve their ability to stimulate ribosome biogenesis.

Multi-stage Friend murine erythroleukemia: molecular insights into oncogenic cooperation

The Friend virus SFFV (Spleen Focus Forming Virus) provokes an acute erythroblastosis in susceptible strains of mice that progresses to overt erythroleukemia by a multi-step process. For virologists, the Friend virus-induced disease has provided deep insights into the host mechanisms influencing susceptibility to retroviral infection and viremia. These insights have contributed to the understanding of HIV and other human retroviral infections. For cell biologists and oncologists, this leukemia has been a powerful experimental model to identify critical oncogenes involved in a multi-stage process, to understand the contribution of host genes to cancer development, and to investigate the mechanisms leading to cell growth autonomy. This model also provided an example of oncogenic reversion since Friend tumor cells can reinitiate their erythroid differentiation program when exposed in vitro to some chemical inducers. This review highlights recent findings demonstrating that the leukemic progression depends on the cooperation of at least two oncogenic events, one interfering with differentiation and one conferring a proliferative advantage. The Friend model of leukemia progression recapitulates the two phases of human acute myeloid leukemia (AML). Coupling of insights from studies on the Friend erythroleukemia with knowledge on AML might allow a better understanding of the molecular mechanisms involved in the evolution of leukemia in mice and men.

Downregulation of the Spi-1/PU.1 oncogene induces the expression of TRIM10/HERF1, a key factor required for terminal erythroid cell differentiation and survival

Sustained expression of the Spi-1/PU.1 and Fli-1 oncoproteins blocks globin gene activation in mouse erythroleukemia cells; however, only Spi-1/PU.1 expression inhibits the inclusion of exon 16 in the mature 4.1R mRNA. This splicing event is crucial for a functional 4.1R protein and, therefore, for red blood cell membrane integrity. This report demonstrates that Spi-1/PU.1 downregulation induces the activation of TRIM10/hematopoietic RING finger 1 (HERF1), a member of the tripartite motif (TRIM)/RBCC protein family needed for globin gene transcription. Additionally, we demonstrate that TRIM10/HERF1 is required for the regulated splicing of exon 16 during late erythroid differentiation. Using inducible overexpression and silencing approaches, we found that: (1) TRIM10/HERF1 knockdown inhibits hemoglobin production and exon splicing and triggers cell apoptosis in dimethylsulfoxide (DMSO)-induced cells; (2) TRIM10/HERF1 upregulation is required but is insufficient on its own to activate exon retention; (3) Fli-1 has no effect on TRIM10/HERF1 expression, whereas either DMSO-induced downregulation or shRNA-knockdown of Spi-1/PU.1 expression is sufficient to activate TRIM10/HERF1 expression; and (4) Spi-1/PU.1 knockdown triggers both the transcription and the splicing events independently of the chemical induction. Altogether, these data indicate that primary Spi-1/PU.1 downregulation acts on late erythroid differentiation through at least two pathways, one of which requires TRIM10/HERF1 upregulation and parallels the Spi-1/PU.1-induced Fli-1 shutoff regulatory cascade.

MLLT3 regulates early human erythroid and megakaryocytic cell fate

Regulatory mechanisms of human hematopoiesis remain largely uncharacterized. Through expression profiling of prospectively isolated stem and primitive progenitor cells as well as committed progenitors from cord blood (CB), we identified MLLT3 as a candidate regulator of erythroid/megakaryocytic (E/Meg) lineage decisions. Through the analysis of the hematopoietic potential of primitive cord blood cells in which MLLT3 expression has been knocked down, we identify a requirement for MLLT3 in the elaboration of the erythroid and megakaryocytic lineages. Conversely, forced expression of MLLT3 promotes the output of erythroid and megakaryocytic progenitors, and analysis of MLLT3 mutants suggests that this capacity of MLLT3 depends on its transcriptional regulatory activity. Gene expression and cis-regulatory element analyses reveal crossregulatory interactions between MLLT3 and E/Meg-affiliated transcription factor GATA-1. Taken together, the data identify MLLT3 as a regulator of early erythroid and megakaryocytic cell fate in the human system.

Cryptic splicing sites are differentially utilized in vivo

It has long been considered that cryptic splice sites are ignored by the splicing machinery in the context of intact genuine splice sites. In the present study, it is shown that cryptic splice sites are utilized in all circumstances, when the authentic site is intact, partially functional or completely abolished. Their use would therefore contribute to a background lack of fidelity in the context of the wild-type sequence. We also found that a mutation at the 5' splice site of beta-globin intron 1 accommodates multiple cryptic splicing pathways, including three previously reported pathways. Focusing on the two major cryptic 5' splice sites within beta-globin exon 1, we show that cryptic splice site selection ex vivo varies depending upon: (a) the cell stage of development during terminal erythroid differentiation; (b) the nature of the mutation at the authentic 5' splice site; and (c) the nature of the promoter. Finally, we found that the two major cryptic 5' splice sites are utilized with differential efficiencies in two siblings sharing the same beta-globin chromosome haplotype in the homozygous state. Collectively, these data suggest that intrinsic, sequence specific factors and cell genetic background factors both contribute to promote a subtle differential use of cryptic splice sites in vivo.

Ets factors and a newly identified polymorphism regulate Fli1 promoter activity in lymphocytes

Fli1 is an Ets family member that is essential for embryonic development. Increasing evidence suggests modulating Fli1 gene expression impacts lymphocyte development/function and is an important mediator in the autoimmune disease lupus. Fli1 is over-expressed in splenic lymphocytes in lupus prone mouse strains and in PBMCs of lupus patients. Presently, it is unknown how Fli1 gene expression is controlled in lymphocytes or how it becomes over-expressed in lupus. Therefore, we examined Fli1 regulation in a murine B cell line and T cell line and identified several cis-regulatory elements within a 230 bp region that contribute to Fli1 promoter activity. Ets factors Elf1, Tel and Fli1 bind in vitro to this region and increase endogenous Fli1 expression when over-expressed in a T cell line. In addition, we determined that a microsatellite located adjacent to the region containing these cis-regulatory elements is polymorphic in three lupus prone mouse strains and that the length of the microsatellite is inversely correlated with promoter activity in a T cell line. These results suggest that several Ets factors, including Fli1 itself, are involved in the transcriptional regulation of Fli1 in lymphocytes. Furthermore, the presence of a polymorphic microsatellite in the Fli1 promoter may contribute to increased Fli1 expression in T cells during lupus disease progression.

Novel role for EKLF in megakaryocyte lineage commitment

Megakaryocytes and erythroid cells are thought to derive from a common progenitor during hematopoietic differentiation. Although a number of transcriptional regulators are important for this process, they do not explain the bipotential result. We now show by gain- and loss-of-function studies that erythroid Krüppel-like factor (EKLF), a transcription factor whose role in erythroid gene regulation is well established, plays an unexpected directive role in the megakaryocyte lineage. EKLF inhibits the formation of megakaryocytes while at the same time stimulating erythroid differentiation. Quantitative examination of expression during hematopoiesis shows that, unlike genes whose presence is required for establishment of both lineages, EKLF is uniquely down-regulated in megakaryocytes after formation of the megakaryocyte-erythroid progenitor. Expression profiling and molecular analyses support these observations and suggest that megakaryocytic inhibition is achieved, at least in part, by EKLF repression of Fli-1 message levels.

p210(BCR-ABL) reprograms transformed and normal human megakaryocytic progenitor cells into erythroid cells and suppresses FLI-1 transcription

The BCR-ABL oncoprotein exhibits deregulated protein tyrosine kinase activity and is implicated in the pathogenesis of Philadelphia chromosome (Ph)-positive human leukemias. Here, we report that ectopic expression of p210(BCR-ABL) in the megakaryoblastic Mo7e cell line and in primary human CD34(+) progenitors trigger erythroid differentiation at the expense of megakaryocyte (MK) differentiation. Clonal culture of purified CD41(+)CD42(-) cells, a population highly enriched in MK progenitors, combined with the conditional expression of p210(BCR-ABL) tyrosine kinase activity by imatinib identified a true lineage reprogramming. In both Mo7e or CD41(+)CD42(-) cells transduced with p210(BCR-ABL), lineage switching was associated with a downregulation of the friend leukemia Integration 1 (FLI-1) transcription factor. Re-expression of FLI-1 in p210(BCR-ABL)-transduced Mo7e cells rescued the megakaryoblastic phenotype. Altogether, these results demonstrate that alteration of signal transduction via p210(BCR-ABL) reprograms MK cells into erythroid cells by a downregulation of FLI-1. In addition, our findings underscore the role of kinases in lineage choice and infidelity in pathology and suggest that downregulation of FLI-1 may have important implications in CML pathogenesis.

Spi-1/PU.1 Oncoprotein Affects Splicing Decisions in a Promoter Binding-dependent Manner

The expression of the Spi-1/PU.1 transcription factor is tightly regulated as a function of the hematopoietic lineage. It is required for myeloid and B lymphoid differentiation. When overexpressed in mice, Spi-1 is associated with the emergence of transformed proerythroblasts unable to differentiate. In the course of a project undertaken to characterize the oncogenic function of Spi-1, we found that Spi-1 interacts with proteins of the spliceosome in Spi-1-transformed proerythroblasts and participates in alternative splice site selection. Because Spi-1 is a transcription factor, it could be hypothesized that these two functions are coordinated. Here, we have developed a system allowing the characterization of transcription and splicing from a single target. It is shown that Spi-1 is able to regulate alternative splicing of a pre-mRNA for a gene whose transcription it regulates. Using a combination of Spi-1 mutants and Spi-1-dependent promoters, we demonstrate that Spi-1 must bind and transactivate a given promoter to favor the use of the proximal 5' alternative site. This establishes that Spi-1 affects splicing decisions in a promoter binding-dependent manner. These results provide new insight into how Spi-1 may act in the blockage of differentiation by demonstrating that it can deregulate gene expression and also modify the nature of the products generated from target genes.

Genetic regulatory networks programming hematopoietic stem cells and erythroid lineage specification

Erythroid cell production results from passage through cellular hierarchies dependent on differential gene expression under the control of transcription factors responsive to changing niches. We have constructed Genetic Regulatory Networks (GRNs) describing this process, based predominantly on mouse data. Regulatory network motifs identified in E. coli and yeast GRNs are found in combination in these GRNs. Feed-forward motifs with autoregulation generate forward momentum and also control its rate, which is at its lowest in hematopoietic stem cells (HSCs). The simultaneous requirement for multiple regulators in multi-input motifs (MIMs) provides tight control over expression of target genes. Combinations of MIMs, exemplified by the SCL/LMO2 complexes, which have variable content and binding sites, explain how individual regulators can have different targets in HSCs and erythroid cells and possibly also how HSCs maintain stem cell functions while expressing lineage-affiliated genes at low level, so-called multi-lineage priming. MIMs combined with cross-antagonism describe the relationship between PU.1 and GATA-1 and between two of their target genes, Fli-1 and EKLF, with victory for GATA-1 and EKLF leading to erythroid lineage specification. These GRNs are useful repositories for current regulatory information, are accessible in interactive form via the internet, enable the consequences of perturbation to be predicted, and can act as seed networks to organize the rapidly accumulating microarray data.

Characterization of a megakaryocyte-specific enhancer of the key hemopoietic transcription factor GATA1

Specification and differentiation of the megakaryocyte and erythroid lineages from a common bipotential progenitor provides a well studied model to dissect binary cell fate decisions. To understand how the distinct megakaryocyte- and erythroid-specific gene programs arise, we have examined the transcriptional regulation of the megakaryocyte erythroid transcription factor GATA1. Hemopoietic-specific mouse (m)GATA1 expression requires the mGata1 enhancer mHS-3.5. Within mHS-3.5, the 3' 179 bp of mHS-3.5 are required for megakaryocyte but not red cell expression. Here, we show mHS-3.5 binds key hemopoietic transcription factors in vivo and is required to maintain histone acetylation at the mGata1 locus in primary megakaryocytes. Analysis of GATA1-LacZ reporter gene expression in transgenic mice shows that a 25-bp element within the 3'-179 bp in mHS-3.5 is critical for megakaryocyte expression. In vitro three DNA binding activities A, B, and C bind to the core of the 25-bp element, and these binding sites are conserved through evolution. Activity A is the zinc finger transcription factor ZBP89 that also binds to other cis elements in the mGata1 locus. Activity B is of particular interest as it is present in primary megakaryocytes but not red cells. Furthermore, mutation analysis in transgenic mice reveals activity B is required for megakaryocyte-specific enhancer function. Bioinformatic analysis shows sequence corresponding to the binding site for activity B is a previously unrecognized motif, present in the cis elements of the Fli1 gene, another important megakaryocyte-specific transcription factor. In summary, we have identified a motif and a DNA binding activity likely to be important in directing a megakaryocyte gene expression program that is distinct from that in red cells.

Regulation of Fli1 gene expression and lupus

Ets transcription factors function throughout development in such varied processes as cellular proliferation, apoptosis, differentiation and migration. Many have been implicated to play important roles in hematopoiesis, vasculogenesis/angiogenesis and myogenesis. Fli1 is an Ets family member that is essential for development and increasing evidence suggests modulating Fli1 gene expression impacts lymphocyte function and is important in the autoimmune disease lupus. Presently, it is unknown how Fli1 gene expression is controlled in lymphocytes. Identifying upstream regulators of Fli1 in lymphocytes will be critical for understanding lymphocyte development and the consequences of dysregulation and may be of value in developing future treatments for lupus.

Overexpression of Ets-1 in human hematopoietic progenitor cells blocks erythroid and promotes megakaryocytic differentiation

Ets-1 is a widely expressed transcription factor implicated in development, tumorigenesis and hematopoiesis. We analyzed Ets-1 gene expression during human erythroid and megakaryocytic (MK) differentiation in unilineage cultures of CD34+ progenitor cells. During erythroid maturation, Ets-1 is downmodulated and exported from the nucleus into the cytoplasm through an active mechanism mediated by a leucine-rich nuclear export signal. In contrast, during megakaryocytopoiesis Ets-1 increases and remains localized in the nucleus up to terminal maturation. Overexpression of Ets-1 in erythroid cells blocks maturation at the polychromatophilic stage, increases GATA-2 and decreases both GATA-1 and erythropoietin receptor expression. Conversely, Ets-1 overexpressing megakaryocytes are characterized by enhanced differentiation and maturation, coupled with upmodulation of GATA-2 and megakaryocyte-specific genes. We show that Ets-1 binds to and activates the GATA-2 promoter, in vitro and in vivo, indicating that one of the pathways through which Ets-1 blocks erythroid and promotes MK differentiation is via upmodulation of GATA-2 expression.

Resistance against Friend leukemia virus-induced leukemogenesis in DNA-dependent protein kinase (DNA-PK)-deficient scid mice associated with defective viral integration at the Spi-1 and Fli-1 site

Retroviral DNA integration is mediated by the viral protein integrase. However, elements of the host DNA repair machinery such as the phosphatidylinositol 3-kinase (PI-3K)-related protein kinase family system would play a role in the integration of viral DNA into the host DNA. Here, we show that a host PI-3K-related protein kinase, DNA-dependent protein kinase (DNA-PK), plays a role in the specific integration of retroviral DNA and induction of retroviral diseases in vivo. DNA-PK-deficient scid mice inoculated with Friend leukemia virus (FLV) exhibited a random integration into their genomic DNA and expressed the viral envelope protein gp70. However, the specific integration of FLV at Spi-1 or Fli-1 sites did not occur in association with the significant resistance of scid mice to FLV-induced leukemogenesis. In contrast, the knockout of another member of the PI-3K-related protein kinase family, encoded by the ataxia telangiectasia mutated (ATM) gene, resulted in mice as sensitive to FLV-induced leukemogenesis as the wild type mice. FLV was specifically integrated into the DNA at Spi-1 and Fli-1 sites with significant expression of these transcription factors. These findings indicated that DNA-PK would be essential for controlling the in vivo integration of FLV at specific sites as well as the susceptibility to FLV-induced leukemogenesis.

Des oncogènes à l’interface entre transcription et épissage

Oncogene activity ranges from transduction signals to transcription factors. Altered expression of oncogenes, either by chromosomal translocation, proviral insertion or point mutations, can lead to tumor formation. More specifically, data accumulated through the last two decades have shown that disregulation of oncogenic transcription factors can interfere with regulatory cascades that control the growth, differentiation, and survival of normal cells. There is also evidence that alterations of oncogene activity are associated with pre-mRNA splicing defects. The insights gained from the pivotal role of RNA polymerase II in coupling transcription and splicing have instigated a new line of research regarding the possible role of oncogenic transcription factors in pre-mRNA splicing regulation. This review focuses on recent advances addressing this question. Understanding the impact of alterations in the expression and/or function of oncogenes have important prognostic implications that can guide the design of new therapeutic drugs to promote differentiation and/or apoptosis over cell proliferation.

Spi-1/PU.1 but not Fli-1 inhibits erythroid-specific alternative splicing of 4.1R pre-mRNA in murine erythroleukemia cells

The inclusion of exon 16 in mature protein 4.1R mRNA arises from a stage-specific splicing event that occurs during late erythroid development. We have shown that mouse erythroleukemia (MEL) cells reproduce this erythroid-specific splicing event upon induction of differentiation. We here found that this splicing event is regulated specifically in erythroleukemic cells that have the potential to differentiate and produce hemoglobin, regardless of the nature of the differentiation inducer. Knowing that dysregulated expression of spi-1/pu.1 and fli-1 oncogenes is involved in MEL cell differentiation arrest, we looked at their effect on exon 16 erythroid splicing. We found that exon 16 inclusion requires Spi-1/PU.1 shutdown in MEL cells, and that enforced expression of Spi-1/PU.1 inhibits exon selection, regardless of the presence or absence of a chemical inducer. By contrast, endogenous overexpression or enforced expression of Fli-1 has no effect on exon selection. We further showed that Spi-1/PU.1 acts similarly on the endogenous and on a transfected exon 16, suggesting a promoter-independent effect of Spi-1/PU.1 on splicing regulation. This study provides the first evidence that Spi-1/PU.1 displays the unique property, not shared with Fli-1, to inhibit erythroid-specific pre-mRNA splicing in erythroleukemia cell context.

Erythroblast transformation by FLI-1 depends upon its specific DNA binding and transcriptional activation properties

FLI-1 is a transcriptional regulator of the ETS family of proteins. Insertional activation at the FLI-1 locus is an early event in F-murine leukemia virus-induced erythroleukemia. Consistent with its essential role in erythroid transformation, enforced expression of FLI-1 in primary erythroblasts strongly impairs the response of these cells to erythropoietin (Epo), a cytokine essential to erythropoiesis. We show here that point mutations in the ETS domain that abolished FLI-1 binding to specific DNA elements (ETS-binding sites) suppressed the ability of FLI-1 to transform erythroblasts. The exchange of the entire ETS domain (DNA binding domain) of FLI-1 for that of PU.1 changed the DNA binding specificity of FLI-1 for that of PU.1 and impaired FLI-1 transforming properties. In contrast, ETS domain swapping mutants that maintained the DNA binding specificity of FLI-1 did not affect the ability of FLI-1 to transform erythroblasts. Deletion and swapping mutants that failed to inhibit the DNA binding activity of FLI-1 but impaired its transcriptional activation properties were also transformation-defective. Taken together, these results show that both the ability of FLI-1 to inhibit Epo-induced differentiation of erythroblasts and to confer enhanced cell survival in the absence of Epo critically depend upon FLI-1 ETS-binding site-dependent transcriptional activation properties.

Functional cross-antagonism between transcription factors FLI-1 and EKLF

FLI-1 is an ETS family transcription factor which is overexpressed in Friend erythroleukemia and contributes to the blockage of differentiation of erythroleukemic cells. We show here that FLI-1 represses the transcriptional activity of the beta-globin gene promoter in MEL cells and interacts with two of its critical transactivators, GATA-1 and EKLF. Unexpectedly, FLI-1 enhances the stimulating activity of GATA-1 on a GATA-1-responsive promoter but represses that of EKLF on beta-globin and an EKLF-responsive artificial promoters. This repressive effect of FLI-1 requires the ETS DNA binding domain and its association with either the N- or C-terminal domain, which themselves interact with EKLF but not with GATA-1. Furthermore, the FLI-1 ETS domain alone behaves as an autonomous repression domain when linked to the Gal4 DNA binding domain. Taken together, these data indicate that FLI-1 represses EKLF-dependent transcription due to the repression activity of its ETS domain and its indirect recruitment to erythroid promoters by protein-protein interaction with EKLF. Reciprocally, we also show that EKLF itself represses the FLI-1-dependent megakaryocytic GPIX gene promoter, thus further suggesting that functional cross-antagonism between FLI-1 and EKLF might be involved in the control of the erythrocytic versus megakaryocytic differentiation of bipotential progenitors.

Molecular biology of the Ets family of transcription factors

The Ets family of transcription factors characterized by an evolutionarily-conserved DNA-binding domain regulates expression of a variety of viral and cellular genes by binding to a purine-rich GGAA/T core sequence in cooperation with other transcriptional factors and co-factors. Most Ets family proteins are nuclear targets for activation of Ras-MAP kinase signaling pathway and some of them affect proliferation of cells by regulating the immediate early response genes and other growth-related genes. Some of them also regulate apoptosis-related genes. Several Ets family proteins are preferentially expressed in specific cell lineages and are involved in their development and differentiation by increasing the enhancer or promoter activities of the genes encoding growth factor receptors and integrin families specific for the cell lineages. Many Ets family proteins also modulate gene expression through protein-protein interactions with other cellular partners. Deregulated expression or formation of chimeric fusion proteins of Ets family due to proviral insertion or chromosome translocation is associated with leukemias and specific types of solid tumors. Several Ets family proteins also participate in malignancy of tumor cells including invasion and metastasis by activating the transcription of several protease genes and angiogenesis-related genes.

Leukemia-related transcription factor TEL accelerates differentiation of Friend erythroleukemia cells

TEL belongs to a member of the ETS family transcription factors that represses transcription of target genes such as FLI-1. Although TEL is essential for establishing hematopoiesis in neonatal bone marrow, its role in erythroid lineage is not understood. To investigate a role for TEL in erythroid differentiation, we introduced TEL into mouse erythroleukemia (MEL) cells. Overexpressing wild-type-TEL in MEL cells enhanced differentiation induced by hexamethylene bisacetamide or dimethylsulfoxide, as judged by the increased levels of erythroid-specific delta-aminolevulinate synthase and beta-globin mRNAs. TEL bound to a corepressor mSin3A through the helix-loop-helix domain. A TEL mutant lacking this domain still bound to the ETS binding site, but lost its transrepressional effect. This mutant completely blocked erythroid differentiation in MEL cells. Moreover, it showed dominant-negative effects over TEL-mediated transcriptional repression and acceleration of erythroid differentiation. Endogenous TEL mRNA was found to increase during the first 3 days in differentiating MEL cells and drastically decrease thereafter. All these data suggest that TEL might play some role in erythroid cell differentiation.

Role of Ets-1 in transcriptional regulation of transferrin receptor and erythroid differentiation

High expression of transferrin receptor (TfR) on the membrane of erythroid cells accounts for the high level of iron required to sustain heme synthesis. Several studies indicate that during erythroid differentiation TfR expression is highly dependent on transcriptional regulation. In this study we characterized the minimal region able to confer transcriptional regulation during erythroid differentiation in Friend leukemia cells (FLC). This region of 120 bp, upstream the transcription start site, contains an overlapping consensus recognition sequence for AP1/CREB/ATF transcription factors and for proteins of the Ets family and a GC rich region. Here, we report that both the Ets and the Ap1/CRE like sites are essential for promoter activity during erythroid differentiation. We showed that Ets-1 binds to the EBS-TfR and its binding activity decreases in FLC induced to differentiate and during normal erythroid differentiation. Consistent with this, FLC constitutively expressing Ets-1 show a decrease in TfR gene expression, globin mRNA and hemoglobin synthesis. We conclude that Ets-1 binding activity is modulated during erythroid maturation and that a deregulated expression of this transcription factor interferes with terminal erythroid differentiation.

Role of Ets-1 in erythroid differentiation

Members of the Ets gene family are known to be expressed in the hematopoietic tissue and some of them play a pivotal role in normal hematopoietic cell development. Ets-1 gene expression was analyzed in Friend Leukemia Cells (FLC) induced to erythroid differentiation by DMSO. We show that the level of Ets-1 protein and its binding activity decreases in FLC along erythroid differentiation of primary human progenitors. The same behavior was observed during normal erythroid differentiation. Moreover, FLC constitutively expressing Ets-1 show a decrease in TfR gene expression, globin mRNA and hemoglobin synthesis. These data indicate that a decrease in Ets-1 binding activity is required for a normal erythroid maturation and that a deregulated expression of this transcription factor may interfere with terminal erythroid differentiation.

Caspase cleavage of the transcription factor FLI-1 during preB leukemic cell death

Programmed cell death (apoptosis) is a complex phenomenon that is mediated in mammals mainly via the selective cleavage of intracellular proteins by the large family of cysteine aspartate protease caspases. Apoptosis is tightly regulated by the competitive effect of numerous proteins displaying either pro-apoptotic or anti-apoptotic activity. The ETS-family transcription factor FLI-1, frequently associated with malignant transformation, has been shown to display anti-apoptotic activity in several cell types including avian erythroblasts, mouse fibroblasts or lymphoid cells. We show here that apoptosis of murine preB leukemic cells is accompanied with the specific cleavage of FLI-1 by a caspase-like activity. We also demonstrate that the two isoforms of FLI-1 are indeed cleaved at three conserved sites by caspase 3 in vitro. The conservation of these cleavage sites among species suggests that the caspase cleavage of the anti-apoptotic transcription factor FLI-1 may represent a critical step to ensure irreversible cell death.

Friend leukemia virus infection enhances DNA damage-induced apoptosis of hematopoietic cells, causing lethal anemia in C3H hosts

Exposure of hematopoietic progenitors to gamma irradiation induces p53-dependent apoptosis. However, host responses to DNA damage are not uniform and can be modified by various factors. Here, we report that a split low-dose total-body irradiation (TBI) (1.5 Gy twice) to the host causes prominent apoptosis in bone marrow cells of Friend leukemia virus (FLV)-infected C3H mice but not in those of FLV-infected DBA mice. In C3H mice, the apoptosis occurs rapidly and progressively in erythroid cells, leading to lethal host anemia, although treatment with FLV alone or TBI alone induced minimal apoptosis in bone marrow cells. A marked accumulation of P53 protein was demonstrated in bone marrow cells from FLV-infected C3H mice 12 h after treatment with TBI. Although a similar accumulation of P53 was also observed in bone marrow cells from FLV-infected DBA mice treated with TBI, the amount appeared to be parallel to that of mice treated with TBI alone and was much lower than that of FLV- plus TBI-treated C3H mice. To determine the association of p53 with the prominent enhancement of apoptosis in FLV- plus TBI-treated C3H mice, p53 knockout mice of the C3H background (C3H p53(-/-)) were infected with FLV and treated with TBI. As expected, p53 knockout mice exhibited a very low frequency of apoptosis in the bone marrow after treatment with FLV plus TBI. Further, C3H p53(-/-) --> C3H p53(+/+) bone marrow chimeric mice treated with FLV plus TBI survived even longer than the chimeras treated with FLV alone. These findings indicate that infection with FLV strongly enhances radiation-induced apoptotic cell death of hematopoietic cells in host animals and that the apoptosis occurs through a p53-associated signaling pathway, although the response was not uniform in different host strains.

Transcriptional regulation of erythropoiesis. Fine tuning of combinatorial multi-domain elements

Haematopoiesis, the differentiation of haematopoietic stem cells and progenitors into various lineages, involves complex interactions of transcription factors that modulate the expression of downstream genes and mediate proliferation and differentiation signals. Commitment of pluripotent haematopoietic stem cells to the erythroid lineage induces erythropoiesis, the production of red blood cells. This process involves a concerted progression through an erythroid burst forming unit (BFU-E), an erythroid colony forming unit (CFU-E), proerythroblast and an erythroblast. The terminally differentiated erythrocytes, in mammals, lose their nucleus yet function several more months. A well-coordinated cohort of transcription factors regulates the formation, survival, proliferation and differentiation of multipotent progenitor into the erythroid lineage. Here, we discuss broad-spectrum factors essential for self-renewal and/or differentiation of multipotent cells as well as specific factors required for proper erythroid development. These factors may operate solely or as part of transcriptional complexes, and exert activation or repression. Sequence comparisons reveal evolutionarily conserved modular composition for these factors; X-ray crystallography demonstrates that they include multidomain elements (e.g. HLH or zinc finger motifs), consistent with their complex interactions with other proteins. Finally, transfections and genomic studies show that the timing of each factor's expression during the hematopoietic process, the cell lineages affected and the existing combination of other factors determine the erythroid cell fate.

Ets-1 regulates fli-1 expression in endothelial cells. Identification of ETS binding sites in the fli-1 gene promoter

To understand the role of the Ets-1 transcription factor during angiogenesis, we have overexpressed it in endothelial cells and analyzed the levels of expression of several candidate target genes involved in angiogenesis. The transcripts levels of the ETS transcription factor fli-1 are specifically up-regulated in endothelial cells, which overexpress Ets-1, but not in fibroblasts. Analysis of the promoter of the mouse fli-1 gene reveals that the 1-kb region that comprises the transcription starts and part of exon 1 is responsible for the response of the promoter to Ets-1. The -270/-41 fragment contains two known Spi-1-responding Ets binding sites (EBS), which are also necessary for the activation by Ets-1. In contrast to Spi-1, a third EBS is necessary for the full response of this promoter fragment to Ets-1. The rest of the promoter activity has been located in the -986/-505 region, where three active EBSs have been identified. Furthermore, endogenous Fli-1 was found to be bound to its own gene promoter and to be able to promote the transactivation of its gene. These results suggest that Ets-1 activates an auto-regulatory loop of expression of fli-1 in endothelial cells, a mechanism that could have significant implications for the endothelial cell fate.

Purification of human recombinant GATA-1 from bacteria: implication for protein-protein interaction studies

GATA-1 is a key regulator of terminal erythroid differentiation in mammals and birds. The structural and biochemical studies of human GATA-1 (hGATA-1) are limited by the difficulty of its purification in a sufficient amount. Here we describe the procedure for obtaining pure bacterial recombinant hGATA-1 in an active functional state. We demonstrate that this protein may be successfully used for preparing an affinity column, producing GATA-1-specific rabbit polyclonal antibodies, and studying DNA-protein and protein-protein interactions.

Transcriptional regulation of hemopoiesis

The regulation of blood cell formation, or hemopoiesis, is central to the replenishment of mature effector cells of innate and acquired immune responses. These cells fulfil specific roles in the host defense against invading pathogens, and in the maintenance of homeostasis. The development of hemopoietic cells is under stringent control from extracellular and intracellular stimuli that result in the activation of specific downstream signaling cascades. Ultimately, all signal transduction pathways converge at the level of gene expression where positive and negative modulators of transcription interact to delineate the pattern of gene expression and the overall cellular hemopoietic response. Transcription factors, therefore, represent a nodal point of hemopoietic control through the integration of the various signaling pathways and subsequent modulation of the transcriptional machinery. Transcription factors can act both positively and negatively to regulate the expression of a wide range of hemopoiesis-relevant genes including growth factors and their receptors, other transcription factors, as well as various molecules important for the function of developing cells. The expression of these genes is dependent on the complex interactions between transcription factors, co-regulatory molecules, and specific binding sequences on the DNA. Recent advances in various vertebrate and invertebrate systems emphasize the importance of transcription factors for hemopoiesis control and the evolutionary conservation of several of such mechanisms. In this review we outline some of the key issues frequently identified in studies of the transcriptional regulation of hemopoietic gene expression. In teleosts, we expect that the characterization of several of these transcription factors and their regulatory mechanisms will complement recent advances in a number of fish systems where identification of cytokine and other hemopoiesis-relevant factors are currently under investigation.

Defective megakaryopoiesis and abnormal erythroid development in Fli-1 gene-targeted mice

Mouse embryos homozygous for a targeted disruption in the Fli-1 gene show hemorrhage into the neural tube and brain on embryonic day (E)11.0 and die shortly thereafter. Livers from the mutant embryos contain drastically reduced numbers of pronormoblasts, basophilic normoblasts, and colony-forming cells. To determine the nature of impaired hematopoiesis, we carried out cell culture studies of mutant embryonic stem (ES) cells and cells from the aorta-gonad-mesonephros (AGM) region of E10.0 mutant embryos. There was a striking reduction in the number of megakaryocytes in cultures of mutant AGM cells compared with cultures of AGM cells from wild-type or heterozygous embryos. Furthermore, Fli-1 mutant ES cells failed to produce megakaryocyte colonies and multilineage colonies containing megakaryocytes. Consistent with the observed defect in megakaryopoiesis, we also demonstrated the down-regulation of c-mpl in the AGM of mutant embryos. The percentages of pronormoblasts and basophilic normoblasts were significantly reduced in cultures of mutant AGM embryos, which contained primarily polychromatophilic and orthochromatic normoblasts. These results provide further evidence for the role of Fli-1 in the regulation of hematopoiesis and for c-mpl as a Fli-1 target gene.

Role of lymphoid cells in age-related change of susceptibility to Friend leukemia virus-induced leukemia

Susceptibility for Friend leukemia virus (FLV)-induced leukemogenesis was examined in the C3H/He (C3H)-->C57BL/6 (B6) radiation bone marrow chimeras of various age groups, and the effect of aging of host mice on the susceptibility was determined. The bone marrow chimera system provided the various age of FLV-resistant host mice (B6) possessing the same age of FLV-susceptible target cells from C3H mice. Using this system, we could determine the aging effect on the host resistancy against FLV without an influence of the aging effect on target cells. First, the young C3H-->young B6 chimeras and young C3H-->old B6 chimeras were compared. The young-->old chimeras were more susceptible to FLV-induced acute disease than the young-->young chimeras. The spleen CD4+ as well as CD8+ T cells were reduced in young-->old chimeras compared with young-->young chimeras. Similarly, the old C3H-->old B6 chimeras were more susceptible than old-->young chimeras and revealed the lower CD4+ T cell ratio in the spleen. Discussion was made on the possible implication of these findings on the role of T cells in age-related change of resistance to FLV-induced leukemogenesis.

The Ets-transcription factor family in embryonic development: lessons from the amphibian and bird

This chapter reviews the expression and role of Ets-genes during embryogenesis of amphibians and birds. In addition to overlapping expression domains, some of them exhibit cell type-specific expression. Many of them are expressed in migratory cells: neural crest, endothelial, and pronephric duct cells for instance. They are also transcribed in embryonic areas affected by epithelio-mesenchymal transitions. Both processes involve modifications of cellular adhesion. Ets-family genes appear to coordinate changes in the expression of adhesion molecules and degradation of the extracellular matrix upon regulation of matrix metalloproteinases and their specific inhibitors. These functions are essential for physiological processes like tissue remodelling during embryogenesis or wound healing. Unfortunately they also play a harmful role in metastasis. Recent studies in the nervous system showed that Ets-genes contribute to the establishment of a cellular identity. This identity could rely on definite cell-surface determinants, among which cadherins could play an important role. In addition to cell-type specific expression, other factors contribute to the specificity of function of Ets-genes. These genes have a broad specificity of recognition of target sequences in gene promoters, insufficient for accurate control of gene expression. A fine tuning could arise from combinatorial interactions with other Ets- or accessory proteins.

Ets target genes: past, present and future

Ets is a family of transcription factors present in species ranging from sponges to human. All family members contain an approximately 85 amino acid DNA binding domain, designated the Ets domain. Ets proteins bind to specific purine-rich DNA sequences with a core motif of GGAA/T, and transcriptionally regulate a number of viral and cellular genes. Thus, Ets proteins are an important family of transcription factors that control the expression of genes that are critical for several biological processes, including cellular proliferation, differentiation, development, transformation, and apoptosis. Here, we tabulate genes that are regulated by Ets factors and describe past, present and future strategies for the identification and validation of Ets target genes. Through definition of authentic target genes, we will begin to understand the mechanisms by which Ets factors control normal and abnormal cellular processes.

Ets and retroviruses - transduction and activation of members of the Ets oncogene family in viral oncogenesis

Studies of retroviral-induced oncogenesis in animal systems led to the initial discovery of viral oncogenes and their cellular homologs, and provided critical insights into their role in the neoplastic process. V-ets, the founding member of the ETS oncogene family, was originally identified as part of the fusion oncogene encoded by the avian acute leukemia virus E26 and subsequent analysis of virus induced leukemias led to the initial isolation of two other members of the ETS gene family. PU.1 was identified as a target of insertional activation in the majority of tumors induced by the murine Spleen Focus Forming virus (SFFV), while fli-1 proved to be the target of Friend murine leukemia virus (F-MuLV) in F-MuLV induced erythroleukemia, as well as that of the 10A1 and Graffi viruses. The common features of the erythroid and myeloid diseases induced by these viruses provided the initial demonstration that these and other members of the ETS family play important roles in hematopoietic development as well as disease. This review provides an overview of the role of ETS genes in retrovirally induced neoplasia, their possible mechanisms of action, and how these viral studies relate to current knowledge of the functions of these genes in hematopoiesis.

The role of Fli-1 in normal cell function and malignant transformation

Aberrant expression of the Fli-1 transcription factor following genetic mutation has been recognized as a seminal event in the initiation of certain types of malignant transformation. Indeed, the etiology of a number of virally induced leukemias, including Friend virus-induced erythroleukemia, has been associated with Fli-1 overexpression. The clinical relevance of Fli-1 becomes apparent in human Ewing's sarcoma in which Fli-1 is the target of a characteristic chromosomal translocation. As such, Fli-1 has generated considerable interest over the past several years for its role in malignant transformation and tumor progression. This review will present a synopsis of the current research on Fli-1 with emphasis on its function in malignant transformation. Moreover, the possible role of Fli-1 in cellular proliferation, differentiation and survival, as well as the recent development of transgenic and knock-out mice to investigate the function of Fli-1 will be discussed. Finally, the significance of identifying target genes that are regulated by Fli-1 and their role in cellular function will be reviewed.