A functional Ets DNA-binding domain is required to maintain multipotency of hematopoietic progenitors transformed by Myb-Ets

Single-cell reconstruction of differentiation trajectory reveals a critical role of ETS1 in human cardiac lineage commitment

BACKGROUND

Cardiac differentiation from human pluripotent stem cells provides a unique opportunity to study human heart development in vitro and offers a potential cell source for cardiac regeneration. Compared to the large body of studies investigating cardiac maturation and cardiomyocyte subtype-specific induction, molecular events underlying cardiac lineage commitment from pluripotent stem cells at early stage remain poorly characterized.

RESULTS

In order to uncover key molecular events and regulators controlling cardiac lineage commitment from a pluripotent state during differentiation, we performed single-cell RNA-Seq sequencing and obtained high-quality data for 6879 cells collected from 6 stages during cardiac differentiation from human embryonic stem cells and identified multiple cell subpopulations with distinct molecular features. Through constructing developmental trajectory of cardiac differentiation and putative ligand-receptor interactions, we revealed crosstalk between cardiac progenitor cells and endoderm cells, which could potentially provide a cellular microenvironment supporting cardiac lineage commitment at day 5. In addition, computational analyses of single-cell RNA-Seq data unveiled ETS1 (ETS Proto-Oncogene 1) activation as an important downstream event induced by crosstalk between cardiac progenitor cells and endoderm cells. Consistent with the findings from single-cell analysis, chromatin immunoprecipitation followed by high-throughput sequencing (ChIP-Seq) against ETS1 revealed genomic occupancy of ETS1 at cardiac structural genes at day 9 and day 14, whereas ETS1 depletion dramatically compromised cardiac differentiation.

CONCLUSION

Together, our study not only characterized the molecular features of different cell types and identified ETS1 as a crucial factor induced by cell-cell crosstalk contributing to cardiac lineage commitment from a pluripotent state, but may also have important implications for understanding human heart development at early embryonic stage, as well as directed manipulation of cardiac differentiation in regenerative medicine.

Notch1-promoted TRPA1 expression in erythroleukemic cells suppresses erythroid but enhances megakaryocyte differentiation

The Notch1 pathway plays important roles in modulating erythroid and megakaryocyte differentiation. To screen the Notch1-related genes that regulate differentiation fate of K562 and HEL cells, the expression of transient receptor potential ankyrin 1 (TRPA1) was induced by Notch1 receptor intracellular domain (N1IC), the activated form of Notch1 receptor. N1IC and v-ets erythroblastosis virus E26 oncogene homolog 1 (Ets-1) bound to TRPA1 promoter region to regulate transcription in K562 cells. Transactivation of TRPA1 promoter by N1IC depended on the methylation status of TRPA1 promoter. N1IC and Ets-1 suppressed the DNA methyltransferase 3B (DNMT3B) level in K562 cells. Inhibition of TRPA1 expression after Notch1 knockdown could be attenuated by nanaomycin A, an inhibitor of DNMT3B, in K562 and HEL cells. Functionally, hemin-induced erythroid differentiation could be suppressed by TRPA1, and the reduction of erythroid differentiation of both cells by N1IC and Ets-1 occurred via TRPA1. However, PMA-induced megakaryocyte differentiation could be enhanced by TRPA1, and the surface markers of megakaryocytes could be elevated by nanaomycin A. Megakaryocyte differentiation could be reduced by Notch1 or Ets-1 knockdown and relieved by TRPA1 overexpression. The results suggest that Notch1 and TRPA1 might be critical modulators that control the fate of erythroid and megakaryocyte differentiation.

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.

Distinguishable live erythroid and myeloid cells in beta-globin ECFP x lysozyme EGFP mice

We previously described a mouse line that contains green myelomonocytic cells due to the knock-in of enhanced green fluorescence protein (EGFP) into the lysozyme M gene.(1) We have now created a transgenic line with fluorescent erythroid cells using a beta-globin locus control region driving the enhanced cyan fluorescence protein (ECFP) gene. These mice exhibit cyan fluorescent cells specifically in the erythroid compartment and in megakaryocyte-erythroid progenitors. Crossing the animals with lysozyme EGFP mice yielded a line in which live erythroid and myeloid cells can readily be distinguished by fluorescence microscopy and by fluorescence-activated cell-sorter scanner. This cross allowed unambiguous identification of unstained mixed erythroid-myeloid colonies for the first time. The new mouse lines should become useful tools to dissect the branching between erythroid and myelomonocytic cells during in vitro differentiation of definitive multipotent progenitors.

E26 leukemia virus converts primitive erythroid cells into cycling multilineage progenitors

Acute chicken leukemia retroviruses, because of their capacity to readily transform hematopoietic cells in vitro, are ideal models to study the mechanisms governing the cell-type specificity of oncoproteins. Here we analyzed the transformation specificity of 2 acute chicken leukemia retroviruses, the Myb-Ets- encoding E26 virus and the ErbA/ErbB-encoding avian erythroblastosis virus (AEV). While cells transformed by E26 are multipotent (designated "MEP" cells), those transformed by AEV resemble erythroblasts. Using antibodies to separate subpopulations of precirculation yolk sac cells, both viruses were found to induce the proliferation of primitive erythroid progenitors within 2 days of infection. However, while AEV induced a block in differentiation of the cells, E26 induced a gradual shift in their phenotype and the acquisition of the potential for multilineage differentiation. These results suggest that the Myb-Ets oncoprotein of the E26 leukemia virus converts primitive erythroid cells into proliferating definitive-type multipotent hematopoietic progenitors.

Transient expression of PU.1 commits multipotent progenitors to a myeloid fate whereas continued expression favors macrophage over granulocyte differentiation

OBJECTIVES

The Ets-family transcription factor PU.1 is expressed specifically in the hematopoietic system, in which it is absolutely required for the generation of B lymphocytes and macrophages. In contrast, overexpression of PU.1 blocks terminal differentiation of the erythroid lineage, in which it can act as an oncogene. In this study we used a multipotential progenitor cell line to examine the effects of PU.1 overexpression on myeloerythroid commitment within a single model system.

MATERIALS AND METHODS

PU.1 cDNA was introduced transiently and stably into the multipotent, nonleukemic hemopoietic cell line FDCPmix. Transiently transfected cells were isolated by fluorescence-activated cell sorting within 18 hours of transfection. Stable transfectants were selected by antibiotic resistance over a number of weeks. The effects of short- and long-term overexpression of PU.1 on self-renewal, proliferation, and differentiation were investigated.

RESULTS

A transient pulse of expression in multipotent progenitor cells eliminated the options of self-renewal and erythroid differentiation, resulting in commitment to the myeloid lineage. However, this transient pulse of expression did not affect the subsequent lineage choice of bipotent granulocyte/macrophage progenitors. In contrast, continuous expression of PU.1 resulted in a strong bias toward macrophage rather than granulocyte differentiation.

CONCLUSIONS

These results demonstrate promyeloid effects of PU.1 at two distinct stages of hematopoiesis.

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.

The leucine zipper region of Myb oncoprotein regulates the commitment of hematopoietic progenitors

The development of blood cells proceeds from pluripotent stem cells through multipotent progenitors into mature elements belonging to at least 8 different lineages. The lineage choice process during which stem cells and progenitors commit to a particular lineage is regulated by a coordinated action of extracellular signals and transcription factors. Molecular mechanisms controlling commitment are largely unknown. Here, the transcription factor v-Myb and its leucine zipper region (LZR) are identified as regulators of the commitment of a common myeloid progenitor and progenitors restricted to the myeloid lineage. It is demonstrated that wild-type v-Myb with the intact LZR directs development of progenitors into the macrophage lineage. Mutations in this region compromise commitment toward myeloid cells and cause v-Myb to also support the development of erythroid cells, thrombocytes, and granulocytes, similar to the c-Myb protein. In agreement with that, the wild-type v-Myb induces high expression of myeloid factors C/EBP beta, PU.1, and Egr-1 in its target cells, whereas SCL, GATA-1, and c-Myb are more abundant in cells expressing the v-Myb LZR mutant. It is proposed that Myb LZR can function as a molecular switch, affecting expression of lineage-specifying transcription factors and directing the development of hematopoietic progenitors into either myeloid or erythroid lineages.

The homeodomain protein PRH influences the differentiation of haematopoietic cells

Haematopoiesis involves the differentiation of a self-renewing stem cell into all of the lineages found in circulating blood. Myb-Ets transformed chicken blastoderm cells (MEPs) have many of the characteristics of multipotent haematopoietic cells and represent a useful model system for the study of haematopoiesis. The proline-rich homeodomain protein (PRH) has previously been shown to be expressed in the haematopoietic compartment. In this report we show that PRH mRNA and protein levels are down regulated as MEPs differentiate along the myelomonocytic and erythrocytic lineages. In contrast, PRH mRNA and protein levels remain high as MEPs differentiate toward the thrombocytic lineage. Over-expression of full length PRH in MEPs inhibits their transformation and/or proliferation. However, the over-expression of N-terminally truncated PRH proteins results in normally proliferating cells that are predominantly differentiated along the myelomonocytic and eosinophilic lineages. These results suggest that PRH plays a role in the proliferation and differentiation of haematopoietic cells.

Antagonism between C/EBPbeta and FOG in eosinophil lineage commitment of multipotent hematopoietic progenitors

The commitment of multipotent cells to particular developmental pathways requires specific changes in their transcription factor complement to generate the patterns of gene expression characteristic of specialized cell types. We have studied the role of the GATA cofactor Friend of GATA (FOG) in the differentiation of avian multipotent hematopoietic progenitors. We found that multipotent cells express high levels of FOG mRNA, which were rapidly down-regulated upon their C/EBPbeta-mediated commitment to the eosinophil lineage. Expression of FOG in eosinophils led to a loss of eosinophil markers and the acquisition of a multipotent phenotype, and constitutive expression of FOG in multipotent progenitors blocked activation of eosinophil-specific gene expression by C/EBPbeta. Our results show that FOG is a repressor of the eosinophil lineage, and that C/EBP-mediated down-regulation of FOG is a critical step in eosinophil lineage commitment. Furthermore, our results indicate that maintenance of a multipotent state in hematopoiesis is achieved through cooperation between FOG and GATA-1. We present a model in which C/EBPbeta induces eosinophil differentiation by the coordinate direct activation of eosinophil-specific promoters and the removal of FOG, a promoter of multipotency as well as a repressor of eosinophil gene expression.

v-Myb can transform and regulate the differentiation of melanocyte precursors

The activity of the c-Myb transcription factor is essential for the development of definitive multi- and uni-lineage progenitors of the haemopoietic system. Reflecting this requirement, c-Myb has been oncogenically activated by transduction in the E26 avian retrovirus which elicits an acute leukaemia by transforming haemopoietic progenitors. Here, we report the novel finding that Myb in cooperation with EGF receptor signalling can be used to generate clonally expanded populations of transformed cells which have the phenotype of melanocyte precursors. Through the use of a conditional temperature sensitive mutant of Myb, we show that in the transformed cells Myb regulates commitment to melanocyte differentiation and possibly proliferation. These results add to our understanding of the roles of c-Myb beyond the haemopoietic system and to our knowledge and means of investigating the importance of transcription factors in the melanocyte lineage.

A novel way to induce erythroid progenitor self renewal: cooperation of c-Kit with the erythropoietin receptor

Red blood cells are of vital importance for oxygen transport in vertebrates. Thus, their formation during development and homeostasis requires tight control of both progenitor proliferation and terminal red cell differentiation. Self renewal (i.e. long-term proliferation without differentiation) of committed erythroid progenitors has recently been shown to contribute to this regulation. Avian erythroid progenitors expressing the EGF receptor/c-ErbB (SCF/TGFalpha progenitors) can be induced to long-term proliferation by the c-ErbB ligand transforming growth factor alpha and the steroids estradiol and dexamethasone. These progenitors have not yet been described in mammals and their factor requirements are untypical for adult erythroid progenitors. Here we describe a second, distinct type of erythroid progenitor (EpoR progenitors) which can be established from freshly isolated bone marrow and is induced to self renew by ligands relevant for erythropoiesis, i.e. erythropoietin, stem cell factor, the ligand for c-Kit and the glucocorticoid receptor ligand dexamethasone. Limiting dilution cloning indicates that these EpoR progenitors are derived from normal BFU-E/CFU-E. For a detailed study, mEpoR progenitors were generated by retroviral expression of the murine Epo receptor in bone marrow erythroblasts. These progenitors carry out the normal erythroid differentiation program in recombinant differentiation factors only. We show that mEpoR progenitors are more mature than SCF/TGFalpha progenitors and also do no longer respond to transforming growth factor alpha and estradiol. In contrast they are now highly sensitive to low levels of thyroid hormone, facilitating their terminal maturation into erythrocytes.

Requirement of the mouse I-mfa gene for placental development and skeletal patterning

The bHLH-repressor protein I-mfa binds to MyoD family members, inhibits their activity, and blocks their nuclear import and binding to DNA. In situ hybridization analysis demonstrated that mouse I-mfa was highly expressed in extraembryonic lineages, in the sclerotome, and subsequently within mesenchymal precursors of the axial and appendicular skeleton, before chondrogenesis occurs. Targeted deletion of I-mfa in a C57Bl/6 background resulted in embryonic lethality around E10.5, associated with a placental defect and a markedly reduced number of trophoblast giant cells. Overexpression of I-mfa in rat trophoblast (Rcho-1) stem cells induced differentiation into trophoblast giant cells. I-mfa interacted with the bHLH protein Mash2, a negative regulator of trophoblast giant cell formation, and inhibited its transcriptional activity in cell culture. In contrast, I-mfa did not interfere with the activity of the bHLH protein Hand1, a positive regulator of giant cell differentiation. Interestingly, I-mfa-null embryos on a 129/Sv background had no placental defect, generally survived to adulthood, and exhibited delayed caudal neural tube closure and skeletal patterning defects that included fusions of ribs, vertebral bodies and abnormal formation of spinous processes. Our results indicate that I-mfa plays an important role in trophoblast and chondrogenic differentiation by negatively regulating a subset of lineage-restricted bHLH proteins.

Distinct C/EBP functions are required for eosinophil lineage commitment and maturation

Hematopoietic differentiation involves the commitment of multipotent progenitors to a given lineage, followed by the maturation of the committed cells. To study the transcriptional events controlling these processes, we have investigated the role of C/EBP proteins in lineage choice of multipotent hematopoietic progenitors (MEPs) transformed by the E26 virus. We found that forced expression of either the alpha or beta isoforms of C/EBP in MEPs induced eosinophil differentiation and that in addition, C/EBPbeta could induce myeloid differentiation. Conversely, dominant-negative versions of C/EBPbeta inhibited myeloid differentiation. C/EBP-induced eosinophil differentiation could be separated into two distinct events, lineage commitment and maturation. Thus, eosinophils induced by transactivation-deficient C/EBPbeta alleles were found to be blocked in their maturation, whereas those expressing wild-type C/EBP proteins were not. Likewise, a 1-day activation of a conditional C/EBPbeta allele in multipotent progenitors led to the formation of immature eosinophils, whereas sustained activation produced mature eosinophils. These results show that C/EBP can induce both myeloid and eosinophil lineage commitment and that transactivation independent and dependent C/EBP functions are required during eosinophil lineage commitment and maturation, respectively.

PU.1 induces myeloid lineage commitment in multipotent hematopoietic progenitors

Little is known about the transcription factors that mediate lineage commitment of multipotent hematopoietic precursors. One candidate is the Ets family transcription factor PU.1, which is expressed in myeloid and B cells and is required for the development of both these lineages. We show here that the factor specifically instructs transformed multipotent hematopoietic progenitors to differentiate along the myeloid lineage. This involves not only the up-regulation of myeloid-specific cell surface antigens and the acquisition of myeloid growth-factor dependence but also the down-regulation of progenitor/thrombocyte-specific cell-surface markers and GATA-1. Both effects require an intact PU.1 transactivation domain. Whereas sustained activation of an inducible form of the factor leads to myeloid lineage commitment, short-term activation leads to the formation of immature eosinophils, indicating the existence of a bilineage intermediate. Our results suggest that PU.1 induces myeloid lineage commitment by the suppression of a master regulator of nonmyeloid genes (such as GATA-1) and the concomitant activation of multiple myeloid genes.

Regulation of eosinophil-specific gene expression by a C/EBP-Ets complex and GATA-1

The EOS47 antigen is an early and specific marker of eosinophil differentiation in the chicken haematopoietic system. To elucidate the transciptional events controlling commitment to the eosinophil lineage, we studied the regulation of the eosinophil-specific EOS47 promoter. This promoter is TATA-less, and binds trancription factors of the Ets, C/EBP, GATA and Myb families. These sites are contained within a 309 bp promoter fragment which is sufficient for specific high level transcription in an eosinophil cell line. Co-transfection experiments in Q2bn fibroblasts showed cooperative activation of the EOS47 proximal promoter by c-Myb, Ets-1/Fli-1, GATA-1 and C/EBPalpha. The Ets-1/Fli-1 and C/EBPalpha proteins were the most potent activators, and acted with high synergy through juxtaposed binding sites located approximately 60 bp upstream of the transcription start site. The Ets-1 and C/EBPalpha proteins were found to associate physically via their DNA-binding domains and to bind their combined binding site cooperatively. GATA-1 showed biphasic regulation of the EOS47 promoter, activating at low and repressing at high protein concentrations. These results demonstrate combinatorial activation of an eosinophil-specific promoter by ubiquitous and lineage-restricted haematopoietic transcription factors. They also indicate that direct interactions between C/EBPs and specific Ets family members, together with GATA-1, are important for eosinophil lineage determination.

v-Myb of E26 leukemia virus up-regulates bcl-2 and suppresses apoptosis in myeloid cells

Many oncogenes have been shown to be deregulated transcription factors, yet direct target genes mediating cell transformation remain elusive. Here we describe such a target for v-Myb by exploiting a temperature-sensitive mutant of the E26 avian leukemia virus encoding Myb-Ets. Myeloblasts transformed by the mutant differentiate into macrophages or die by apoptosis when shifted to the nonpermissive temperature as a result of inactivation of v-Myb. During this process mRNA of the antiapoptotic oncoprotein Bcl-2 is down-regulated with kinetics similar to those of Mim-1, a differentiation-related protein whose expression is directly regulated by Myb. Forced expression of bcl-2 rescues the cells from apoptosis, without preventing either their withdrawal from the cell cycle or their differentiation. v-Myb appears to act directly on the bcl-2 gene, because a bcl-2 promoter-driven reporter is activated by Myb-Ets and v-Myb-VP16 and requires intact Myb binding sites within the promoter. Surprisingly, inactivation of v-Myb in multipotent progenitors transformed by E26 virus does not induce apoptosis, indicating that bcl-2 regulation by the oncoprotein is required for the transformation of some cell types but not others.

Myb-Ets fusion oncoprotein inhibits thyroid hormone receptor/c-ErbA and retinoic acid receptor functions: a novel mechanism of action for leukemogenic transformation by E26 avian retrovirus

The E26 and avian erythroblastosis virus (AEV) avian retroviruses induce acute leukemia in chickens. E26 can block both erythroid and myeloid differentiation at an early multipotent stage. Moreover, E26 can block erythroid differentiation at the erythroid burst-forming unit/erythroid CFU (BFU-E/CFU-E) stage, which also corresponds to the differentiation stage blocked by AEV. AEV carries two oncogenes, v-erbA and v-erbB, whereas E26 encodes a single 135-kDa Gag-Myb-Ets fusion oncoprotein. v-ErbA is responsible for the erythroid differentiation arrest through negative interferences with both the retinoic acid receptor (RAR) and the thyroid hormone receptor (T3R/c-ErbA). We investigated whether Myb-Ets could block erythroid differentiation in a manner similar to v-ErbA. We show here that Myb-Ets inhibits both RAR and c-ErbA activities on specific hormone response elements in transient-expression assays. Moreover, Myb-Ets abrogates the inactivation of transcription factor AP-1 by RAR and T3R, another feature shared with v-ErbA. Myb-Ets also antagonizes the biological response of erythrocytic progenitor cells to retinoic acid and T3. Analysis of a series of mutants of Myb-Ets reveals that the domains of the oncoprotein involved in these inhibitory activities are the same as those involved in oncogenic transformation of hematopoietic cells. These data demonstrate that the Myb-Ets oncoprotein shares properties with the v-ErbA oncoprotein and that inhibition of ligand-dependent RAR and c-ErbA functions by Myb-Ets is responsible for blocking the differentiation of hematopoietic progenitors.

Excision of Ets by an inducible site-specific recombinase causes differentiation of Myb-Ets-transformed hematopoietic progenitors

BACKGROUND

The Myb-Ets protein encoded by the E26 acute avian leukemia virus is a paradigm for the function of fused transcriptional activator oncoproteins. Myb-Ets transforms hematopoietic progenitor cells (myb-Ets progenitors, MEPs) that can be induced to differentiate into eosinophilic and myeloid cells by the activation of pathways involving Ras and/or protein kinase C. The Ets portion of the fusion protein seems to be required to maintain the multipotency of MEPs: MEPs transformed with a temperature-sensitive E26 mutant with a lesion in Ets (ts 1.1) and shifted to the non-permissive temperature predominantly form erythroid cells, but also form eosinophilic and myeloid cells. This interpretation is complicated, however, by the observation that ts 1.1-transformed MEPs differ from MEPs transformed with wild-type E26 in that they expressed erythroid and eosinophil markers even at the permissive temperature.

RESULTS

In order to alleviate the problems associated with the use of temperature-sensitive mutants we have designed a vector that allows the inducible deletion of the Ets domain. To this end, we introduced FLP recombinase target sites into the E26 virus on the 5' and 3' sides of Ets and included within the same retroviral vector sequences encoding and estrogen-dependent FLP recombinase. This construct, termed FRV-3, is capable of transforming cells to produce a phenotype indistinguishable from that of MEPs obtained with wild-type virus. Hormone treatment of MEPs transformed with FRV-3 induced erythroid differentiation in a subpopulation of the cells; this subpopulation was found to have completely excised ets. However, in contrast to previous results obtained with ts 1.1-transformed MEPs, no differentiation along the eosinophilic and myeloid lineages was seen in hormone-treated FRV-3-transformed MEPs.

CONCLUSIONS

Our results demonstrate the feasibility of using a site-specific recombinase to excise a fused oncoprotein domain encoded by a retrovirus. More specifically, they show that the Ets portion of the Myb-Ets protein selectively inhibits differentiation of MEPs along the erythroid lineage, and suggests that Ets is also required for their differentiation along the eosinophil and, possibly, myeloid lineages.

Determinants of DNA-binding specificity of ETS-domain transcription factors

Several mechanisms are employed by members of transcription factor families to achieve sequence-specific DNA recognition. In this study, we have investigated how members of the ETS-domain transcription factor family achieve such specificity. We have used the ternary complex factor (TCF) subfamily as an example. ERK2 mitogen-activated protein kinase stimulates serum response factor-dependent and autonomous DNA binding by the TCFs Elk-1 and SAP-la. Phosphorylated Elk-1 and SAP-la exhibit specificities of DNA binding similar to those of their isolated ETS domains. The ETS domains of Elk-1 and SAP-la and SAP-2 exhibit related but distinct DNA-binding specificities. A single residue, D-69 (Elk-1) or V-68 (SAP-1), has been identified as the critical determinant for the differential binding specificities of Elk-1 and SAP-1a, and an additional residue, D-38 (Elk-1) or Q-37 (SAP-1), further modulates their DNA binding. Creation of mutations D38Q and D69V is sufficient to confer SAP-la DNA-binding specificity upon Elk-1 and thereby allow it to bind to a greater spectrum of sites. Molecular modelling indicates that these two residues (D-38 and D-69) are located away from the DNA-binding interface of Elk-1. Our data suggest a mechanism in which these residues modulate DNA binding by influencing the interaction of other residues with DNA.

MafB is an interaction partner and repressor of Ets-1 that inhibits erythroid differentiation

Using a yeast one-hybrid screen with a DNA-bound Ets-1 protein, we have identified MafB, an AP-1 like protein, as a direct interaction partner. MafB is specifically expressed in myelomonocytic cells and binds to the DNA-binding domain of Ets-1 via its basic region or leucine-zipper domain. Furthermore, it represses Ets-1 transactivation of synthetic promoters containing Ets binding sites and inhibits Ets-1-mediated transactivation of the transferrin receptor, which is known to be essential for erythroid differentiation. Accordingly, overexpression of MafB in an erythroblast cell line down-regulates the endogenous transferrin receptor gene and inhibits differentiation without affecting cell proliferation. These results highlight the importance of inhibitory interactions between transcription factors in regulating lineage-specific gene expression.

Regulation of hematopoietic cell proliferation and differentiation by the myb oncogene family of transcription factors

The myb family of genes include the virally encoded v-myb oncogene, its normal cellular equivalent c-myb and two related members called A-myb and B-myb. They are all transcription factors that recognize the same DNA sequence (PyAACG/TG) and are all involved in the regulation of proliferation and differentiation in different cell types, including hematopoietic cells. C-myb is most highly expressed in hematopoietic cells and its oncogenic activation leads to transformation of these cells. Several lines of evidence have demonstrated that c-myb regulates both the proliferation and differentiation of hematopoietic cells of different lineages. The mechanisms of action of c-myb and v-myb are becoming clearer, mostly through the study of the different genes that are regulated by these transcription factors and the cofactors with which c-myb and v-myb co-operate. More recently the biological and biochemical functions of the B-myb and A-myb gene products have been investigated. Evidence for the function of the different members of the myb family in relation to hematopoietic proliferation and differentiation is presented, and the different roles of the myb genes are discussed.

Synergistic transcriptional activation of the tissue inhibitor of metalloproteinases-1 promoter via functional interaction of AP-1 and Ets-1 transcription factors

The tissue inhibitor of metalloproteinases-1 (TIMP-1) is an inhibitor of the extracellular matrix-degrading metalloproteinases. We characterized response elements that control TIMP-1 gene expression. One contains a binding site that selectively binds c-Fos and c-Jun in vitro and confers a response to multiple AP-1 family members in vivo. Adjacent to this is a binding site for Ets domain proteins. Although c-Ets-1 alone did not activate transcription from this element, it enhanced transcription synergistically with AP-1 either in the context of the natural promoter or when the sequence was linked upstream of a heterologous promoter. Furthermore, a complex of c-Jun and c-Fos interacted with c-Ets-1 in vitro. These results suggest that AP-1 tethers c-Ets-1 to the TIMP-1 promoter via protein-protein interaction to achieve Ets-dependent transcriptional regulation. Collectively, our results indicate that TIMP-1 expression is controlled by several DNA response elements that respond to variations in the level and activity of AP-1 and Ets transcriptional regulatory proteins.

GATA-1 reprograms avian myelomonocytic cell lines into eosinophils, thromboblasts, and erythroblasts

The transcription factor GATA-1 is expressed in early hematopoietic progenitors and specifically down-regulated in myelomonocytic cells during lineage determination. Our earlier observation that the differentiation of Myb-Ets-transformed chicken hematopoietic progenitors into myeloblasts likewise involves a GATA-1 down-regulation, whereas expression is maintained in erythroid, thrombocytic, and eosinophilic derivatives, prompted us to study the effect of forced GATA-1 expression in Myb-Ets-transformed myeloblasts. We found that the factor rapidly suppresses myelomonocytic markers and induces a reprogramming of myeloblasts into cells resembling either transformed eosinophils or thromboblasts. In addition, we observed a correlation between the level of GATA-1 expression and the phenotype of the cell, intermediate levels of the factor being expressed by eosinophils and high levels by thromboblasts, suggesting a dosage effect of the factor. GATA-1 can also induce the formation of erythroblasts when expressed in a myelomonocytic cell line transformed with a Myb-Ets mutant containing a lesion in Ets. These cells mature into erythrocytes following temperature-inactivation of the Ets protein. Finally, the factor can reprogram a v-Myc-transformed macrophage cell line into myeloblasts, eosinophils, and erythroblasts, showing that the effects of GATA-1 are not limited to Myb-Ets-transformed myeloblasts. Our results suggest that GATA-1 is a lineage-determining transcription factor in transformed hematopoietic cells, which not only activates lineage-specific genetic programs but also suppresses myelomonocytic differentiation. They also point to a high degree of plasticity of transformed hematopoietic cells.

A residue of the ETS domain mutated in the v-ets oncogene is essential for the DNA-binding and transactivating properties of the ETS-1 and ETS-2 proteins

The c-ets-1 locus encodes two transcription factors, p54c-ets-1 and p68c-ets-1 that recognize purine-rich motifs. The v-ets oncogene of the avian retrovirus E26 differs from its cellular progenitor p68c-ets-1 by two amino acid substitutions (alanine 285 and isoleucine 445 in c-ets-1 both substituted by valine in v-ets, mutations A and B respectively) and its carboxy-terminal end (mutation C). The B mutation affects a well conserved residue in the carboxy-terminal 85 amino acids, ETS DNA-binding domain. To address the biological relevance of the B mutation found between v-ets and c-ets-1, we have randomly mutagenized isoleucine 445 of p68c-ets-1 by polymerase chain reaction. Using in vitro gel mobility shift assays, we show that this residue is crucial for the binding properties of c-ets-1 since the 12 mutations we have generated at this position, all diminish or even abolish the binding, to the 'optimized' Ets-1 binding site (EBS), of 35 kDa proteins corresponding to the 311 carboxy-terminal residues of c-ets-1. Among them, substitutions of isoleucine to glutamic acid, glycine or proline have the highest inhibitory effects. Similar results were obtained when the same mutations were introduced either in full-length p68c-ets-1 protein or into a carboxy-terminal polypeptide of 109 amino acids encompassing the ETS-domain which has previously been shown to display a very high binding activity as compared with the full-length protein. Consistent with the in vitro results, point mutations in p68c-ets-1 that decrease binding activity to EBS abrogate its ability to transactivate reporter plasmids carrying either the TPA Oncogene Response Unit of the Polyoma virus enhancer (TORU) or a sequence derived from the HTLV-1 LTR. Furthermore, as this isoleucine residue is rather well-conserved within the ETS gene family, we show that mutation of the corresponding isoleucine of c-ets-2 into glycine also abrogates its DNA-binding and hence, transactivating properties. Thus, the v-ets B mutation highlights the isoleucine 445 as an essential amino acid of the c-ets-1 and c-ets-2 DNA-binding domains.