Structure and transcription of the mouse erythropoietin receptor gene

The effect of erythropoietin on normal and neoplastic cells

Erythropoietin (Epo) is an essential hormone that binds and activates the Epo receptor (EpoR) resident on the surface of erythroid progenitor cells, thereby promoting erythropoiesis. Recombinant human erythropoietin has been used successfully for over 20 years to treat anemia in millions of patients. In addition to erythropoiesis, Epo has also been reported to have other effects, such as tissue protection and promotion of tumor cell growth or survival. This became of significant concern in 2003, when some clinical trials in cancer patients reported increased tumor progression and worse survival outcomes in patients treated with erythropoiesis-stimulating agents (ESAs). One of the potential mechanisms proffered to explain the observed safety issues was that functional EpoR was expressed in tumors and/or endothelial cells, and that ESAs directly stimulated tumor growth and/or antagonized tumor ablative therapies. Since then, numerous groups have performed further research evaluating this potential mechanism with conflicting data and conclusions. Here, we review the biology of endogenous Epo and EpoR expression and function in erythropoiesis, and evaluate the evidence pertaining to the expression of EpoR on normal nonhematopoietic and tumor cells.

Erythroid Kruppel-like factor (EKLF) is recruited to the gamma-globin gene promoter as a co-activator and is required for gamma-globin gene induction by short-chain fatty acid derivatives

OBJECTIVES

The erythroid Kruppel-like factor (EKLF) is an essential transcription factor for beta-type globin gene switching, and specifically activates transcription of the adult beta-globin gene promoter. We sought to determine if EKLF is also required for activation of the gamma-globin gene by short-chain fatty acid (SCFA) derivatives, which are now entering clinical trials.

METHODS

The functional and physical interaction of EKLF and co-regulatory molecules with the endogenous human globin gene promoters was studied in primary human erythroid progenitors and cell lines, using chromatin immunoprecipitation (ChIP) assays and genetic manipulation of the levels of EKLF and co-regulators.

RESULTS AND CONCLUSIONS

Knockdown of EKLF prevents SCFA-induced expression of the gamma-globin promoter in a stably expressed microLCRbeta(pr)R(luc) (A)gamma(pr)F(luc) cassette, and prevents induction of the endogenous gamma-globin gene in primary human erythroid progenitors. EKLF is actively recruited to endogenous gamma-globin gene promoters after exposure of primary human erythroid progenitors, and murine hematopoietic cell lines, to SCFA derivatives. The core ATPase BRG1 subunit of the human SWI/WNF complex, a ubiquitous multimeric complex that regulates gene expression by remodeling nucleosomal structure, is also required for gamma-globin gene induction by SCFA derivatives. BRG1 is actively recruited to the endogenous gamma-globin promoter of primary human erythroid progenitors by exposure to SCFA derivatives, and this recruitment is dependent upon the presence of EKLF. These findings demonstrate that EKLF, and the co-activator BRG1, previously demonstrated to be required for definitive or adult erythropoietic patterns of globin gene expression, are co-opted by SCFA derivatives to activate the fetal globin genes.

A global role for EKLF in definitive and primitive erythropoiesis

Erythroid Kruppel-like factor (EKLF, KLF1) plays an important role in definitive erythropoiesis and beta-globin gene regulation but failure to rectify lethal fetal anemia upon correction of globin chain imbalance suggested additional critical EKLF target genes. We employed expression profiling of EKLF-null fetal liver and EKLF-null erythroid cell lines containing an inducible EKLF-estrogen receptor (EKLF-ER) fusion construct to search for such targets. An overlapping list of EKLF-regulated genes from the 2 systems included alpha-hemoglobin stabilizing protein (AHSP), cytoskeletal proteins, hemesynthesis enzymes, transcription factors, and blood group antigens. One EKLF target gene, dematin, which encodes an erythrocyte cytoskeletal protein (band 4.9), contains several phylogenetically conserved consensus CACC motifs predicted to bind EKLF. Chromatin immunoprecipitation demonstrated in vivo EKLF occupancy at these sites and promoter reporter assays showed that EKLF activates gene transcription through these DNA elements. Furthermore, investigation of EKLF target genes in the yolk sac led to the discovery of unexpected additional defects in the embryonic red cell membrane and cytoskeleton. In short, EKLF regulates global erythroid gene expression that is critical for the development of primitive and definitive red cells.

Oncogene activation in myeloid leukemias by Graffi murine leukemia virus proviral integration

The Graffi murine leukemia virus (MuLV) is a nondefective retrovirus that induces granulocytic leukemia in BALB/c and NFS mice. To identify genes involved in Graffi MuLV-induced granulocytic leukemia, tumor cell DNAs were examined for genetic alterations at loci described as common proviral integration sites in MuLV-induced myeloid, lymphoid, and erythroid leukemias. Southern blot analysis revealed rearrangements in c-myc, Fli-1, Pim-1, and Spi-1/PU.1 genes in 20, 10, 3.3, and 3.3% of the tumors tested, respectively. These results demonstrate for the first time the involvement of those genes in granulocytic leukemia.

Transcription of the HS2 enhancer toward a cis-linked gene is independent of the orientation, position, and distance of the enhancer relative to the gene

The locus control region (LCR) regulates transcription of the downstream beta-like globin genes 10 to 50 kb away. Among hypersensitive sites HS4, -3, -2, and -1, which define the LCR in erythroid cells, HS2 possesses prominent enhancer function. The mechanism by which the HS2 enhancer and other functional components of the LCR act over the distance is not clear. We have used reverse transcription-PCR and RNase protection assays to analyze the transcriptional statuses of both the endogenous and the transfected HS2 enhancer in erythroid K562 cells. A novel pattern of HS2 enhancer transcription was observed. The endogenous HS2 enhancer was transcribed predominantly in the direction toward the downstream globin genes. The HS2 enhancer in transfected recombinant chloramphenicol acetyltransferase (CAT) plasmids was also transcribed predominantly toward the CAT gene, regardless of whether the enhancer was placed (i) in the genomic or reverse genomic orientation, (ii) in a position 5' or 3' to the gene, or (iii) at various distances up to 6 kb from the gene. The orientation, position, and distance independence in gene-tropic transcription of the HS2 enhancer correlates with the observed orientation, position, and distance independence of HS2 enhancer function and suggests that enhancer transcription may play a role in enhancer function.

Thrombopoietin rescues in vitro erythroid colony formation from mouse embryos lacking the erythropoietin receptor

The interaction of the hormone erythropoietin and its receptor (EpoR) is though to be required for normal hematopoiesis. To define the role of EpoR in this process, the murine EpoR was disrupted by homologous recombination. Mice lacking the EpoR died in utero at embryonic day 11-12.5 with severe anemia. Embryonic erythropoiesis was markedly diminished, while fetal liver hematopoiesis was blocked at the proerythroblast stage. Other cell types known to express EpoR, including megakaryocytes, mast, and neural cells were morphologically normal. Reverse transcription-coupled PCR analysis of RNA from embryonic yolk sac, peripheral blood, and fetal liver demonstrated near normal transcripts levels for EKLF, thrombopoietin (Tpo), c-MPL, GATA-1, GATA-2, and alpha- and embryonic beta H1-globin but non for adult beta maj-globin. While colony-forming unit-erythroid (CFU-E) and burst-forming unit-erythroid (BFU-E) colonies were not present in cultures derived from EpoR-/- liver or yolk sac cells, hemoglobin-containing BFU-E colonies were detected in cultures treated with recombinant Tpo and Kit ligand or with Tpo and interleukin 3 and 11. Rescued BFU-E colonies expressed adult beta-globin and c-MPL and appeared morphologically normal. Thus, erythroid progenitors are formed in vivo in mice lacking the EpoR, and our studies demonstrate that a signal transmitted through the Tpo receptor c-MPL stimulates proliferation and terminal differentiation of these progenitors in vitro.

Spi-1/PU.1 transgenic mice develop multistep erythroleukemias

Insertional mutagenesis of the spi-1 gene is associated with the emergence of malignant proerythroblasts during Friend virus-induced acute erythroleukemia. To determine the role of spi-1/PU.1 in the genesis of leukemia, we generated spi-1 transgenic mice. In one founder line the transgene was overexpressed as an unexpected-size transcript in various mouse tissues. Homozygous transgenic animals gave rise to live-born offspring, but 50% of the animals developed a multistep erythroleukemia within 1.5 to 6 months of birth whereas the remainder survived without evidence of disease. At the onset of the disease, mice became severely anemic. Their hematopoietic tissues were massively invaded with nontumorigenic proerythroblasts that express a high level of Spi-1 protein. These transgenic proerythroblasts are partially blocked in differentiation and strictly dependent on erythropoietin for their proliferation both in vivo and in vitro. A complete but transient regression of the disease was observed after erythrocyte transfusion, suggesting that the constitutive expression of spi-1 is related to the block of the differentiation of erythroid precursors. At relapse, erythropoietin-independent malignant proerythroblasts arose. Growth factor autonomy could be partially explained by the autocrine secretion of erythropoietin; however, other genetic events appear to be necessary to confer the full malignant phenotype. These results reveal that overexpression of spi-1 is essential for malignant erythropoiesis and does not alter other hematopoietic lineages.

Regulation of transcription of the human erythropoietin receptor gene by proteins binding to GATA-1 and Sp1 motifs

Erythropoietin (Epo), the primary regulator of the production of erythroid cells, acts by binding to a cell surface receptor (EpoR) on erythroid progenitors. We used deletion analysis and transfection assays with reporter gene constructs to examine the transcription control elements in the 5' flanking region of the human EpoR gene. In erythroid cells most of the transcription activity was contained in a 150 bp promoter fragment with binding sites for transcription factors AP2, Sp1 and the erythroid-specific GATA-1. The 150 bp hEpoR promoter exhibited high and low activity in erythroid OCIM1 and K562 cells, respectively, reflecting the high and low levels of constitutive hEpoR expression. The GATA-1 and Sp1 binding sites in this promoter lacking a TATA sequence were necessary for a high level of transcription activation. Protein-DNA binding studies suggested that Sp1 and two other CCGCCC binding proteins from erythroid and non-erythroid cells could bind to the Sp1 binding motif. By increasing GATA-1 levels via co-transfection, we were able to transactivate the hEpoR promoter in K562 cells and non-erythroid cells, but not in the highly active OCIM1 cells, although GATA-1 mRNA levels were comparable in OCIM1 and K562. Interestingly, when we mutated the Sp1 site, resulting in a marked decrease in hEpoR promoter activity, we could restore transactivation by increasing GATA-1 levels in OCIM1 cells. These data suggest that while GATA-1 can transactivate the EpoR promoter, the level of hEpoR gene expression does not depend on GATA-1 alone. Rather, hEpoR transcription activity depends on coordination between Sp1 and GATA-1 with other cell-specific factors, including possibly other Sp1-like binding proteins, to provide high level, tissue-specific expression.

Functional synergy and physical interactions of the erythroid transcription factor GATA-1 with the Krüppel family proteins Sp1 and EKLF

An unresolved aspect of current understanding of erythroid cell-specific gene expression relates to how a limited number of transcriptional factors cooperate to direct high-level expression mediated by cis-regulatory elements separated over large distances within globin loci. In this report, we provide evidence that GATA-1, the major erythroid transcription factor, activates transcription in a synergistic fashion with two Krüppel family factors, the ubiquitous protein Sp1 and the erythroid-restricted factor EKLF (erythroid Krüppel-like factor), which recognize GC and/or GT/CACC motifs. Binding sites for both GATA-1 and these Krüppel proteins (especially Sp1) are found in close association in the promoters and enhancers of numerous erythroid cell-expressed genes and appear to cooperate in directing their expression. We have shown that GATA-1 interacts physically with Sp1 and EKLF and that interactions are mediated through their respective DNA-binding domains. Moreover, we show that GATA-1 and Sp1 synergize from a distance in constructs designed to mimic the architecture of globin locus control regions and downstream globin promoters. Finally, the formation of GATA-1-SP1 complexes was demonstrated in vivo by the ability of Sp1 to recruit GATA-1 to a promoter in the absence of GATA-binding sites. These experiments provide the first evidence for functionally important protein-protein interactions involved in erythroid cell-specific expression and suggest a mechanism by which DNA loops between locus control regions and globin promoters (or enhancers) might be formed or stabilized.

Self-association of the erythroid transcription factor GATA-1 mediated by its zinc finger domains

GATA-1, the founding member of a distinctive family of transcription factors, is expressed predominantly in erythroid cells and participates in the expression of numerous erythroid cell-expressed genes. GATA-binding sites are found in the promoters and enhancers of globin and nonglobin erythroid genes as well as in the alpha- and beta-globin locus control regions. To elucidate how GATA-1 may function in a variety of regulatory contexts, we have examined its protein-protein interactions. Here we show that GATA-1 self-associates in solution and in whole-cell extracts and that the zinc finger region of the molecule is sufficient to mediate this interaction. This physical interaction can influence transcription, as GATA-1 self-association is able to recruit a transcriptionally active but DNA-binding-defective derivative of GATA-1 to promoter-bound GATA-1 and result in superactivation. Through in vitro studies with bacterially expressed glutathione S-transferase fusion proteins, we have localized the minimal domain required for GATA-1 self-association to 40 amino acid residues within the C-terminal zinc finger region. Finally, we have detected physical interaction of GATA-1 with other GATA family members (GATA-2 and GATA-3) also mediated through the zinc finger domain. These findings have broad implications for the involvement of GATA factors in transcriptional control. In particular, the interaction of GATA-1 with itself and with other transcription factors may facilitate its function at diverse promoters in erythroid cells and also serve to bring together, or stabilize, loops between distant regulatory elements, such as the globin locus control regions and downstream globin promoters. We suggest that the zinc finger region of GATA-1, and related proteins, is multifunctional and mediates not only DNA binding but also important protein-protein interactions.

Positive regulators of the lineage-specific transcription factor GATA-1 in differentiating erythroid cells

The zinc finger transcription factor GATA-1 is a major regulator of gene expression in erythroid, megakaryocyte, and mast cell lineages. GATA-1 binds to WGATAR consensus motifs in the regulatory regions of virtually all erythroid cell-specific genes. Analyses with cultured cells and cell-free systems have provided strong evidence that GATA-1 is involved in control of globin gene expression during erythroid differentiation. Targeted mutagenesis of the GATA-1 gene in embryonic stem cells has demonstrated its requirement in normal erythroid development. Efficient rescue of the defect requires an intact GATA element in the distal promoter, suggesting autoregulatory control of GATA-1 transcription. To examine whether GATA-1 expression involves additional regulatory factors or is maintained entirely by an autoregulatory loop, we have used a transient heterokaryon system to test the ability of erythroid factors to activate the GATA-1 gene in nonerythroid nuclei. We show here that proerythroblasts and mature erythroid cells contain a diffusible activity (TAG) capable of transcriptional activation of GATA-1 and that this activity decreases during the terminal differentiation of erythroid cells. Nuclei from GATA-1- mutant embryonic stem cells can still be reprogrammed to express their globin genes in erythroid heterokaryons, indicating that de novo induction of GATA-1 is not required for globin gene activation following cell fusion.

Positive regulators of the lineage-specific transcription factor GATA-1 in differentiating erythroid cells

The zinc finger transcription factor GATA-1 is a major regulator of gene expression in erythroid, megakaryocyte, and mast cell lineages. GATA-1 binds to WGATAR consensus motifs in the regulatory regions of virtually all erythroid cell-specific genes. Analyses with cultured cells and cell-free systems have provided strong evidence that GATA-1 is involved in control of globin gene expression during erythroid differentiation. Targeted mutagenesis of the GATA-1 gene in embryonic stem cells has demonstrated its requirement in normal erythroid development. Efficient rescue of the defect requires an intact GATA element in the distal promoter, suggesting autoregulatory control of GATA-1 transcription. To examine whether GATA-1 expression involves additional regulatory factors or is maintained entirely by an autoregulatory loop, we have used a transient heterokaryon system to test the ability of erythroid factors to activate the GATA-1 gene in nonerythroid nuclei. We show here that proerythroblasts and mature erythroid cells contain a diffusible activity (TAG) capable of transcriptional activation of GATA-1 and that this activity decreases during the terminal differentiation of erythroid cells. Nuclei from GATA-1- mutant embryonic stem cells can still be reprogrammed to express their globin genes in erythroid heterokaryons, indicating that de novo induction of GATA-1 is not required for globin gene activation following cell fusion.

Different domains regulate the human erythropoietin receptor gene transcription

To analyse the 5'-flanking sequences required for the tissue specific transcription of the human erythropoietin receptor (hEpo-R) gene, a DNA region spanning nucleotides -1050 to +135 relative to the transcription initiation site (+1) was explored. Our studies indicate that a minimum promoter (-76/+33) containing GATA and SP1 binding sites at positions -45 and -20 is not sufficient to confer erythroid specific expression to a reporter gene. Erythroid specificity of the promoter was observed either with the (-1050/+33 construct) which contains a cluster of Alu repetitive elements or with the addition of the 135 bp down to the transcription initiation site (-76/+135 construct) which exert a negative control on the promoter activity with a major effect in non erythroid tissues. The latter region can be subdivided on two distinct domains: the +1/+78 region that exerts a positive effect and the +79/+135 region that has a negative effect on the Epo-R promoter activity measured by CAT assays. The first region contains three CANNTG motifs, whereas the second contains an SP1/CACCC motif at position +85. These findings reveal a complex regulation of the hEpo-R gene and provide a working model useful to explain how the minimal promoter, containing GATA/SP1, can be positively and negatively regulated during erythroid differentiation.

c-Myc selectively regulates the latent period and erythroid-specific genes in murine erythroleukemia cell differentiation

During the latent period of murine erythroleukemia (MEL) cell differentiation, c-myc levels showed a significant change and the overexpression of the transferred c-myc gene inhibited the commitment and differentiation of MEL cells, suggesting that c-Myc may be a key molecule for the commitment. Since c-Myc may function as a DNA binding transcription factor, we examined whether c-Myc regulates the latent period genes (hsp and hsc70, MER5, Id and Spi-1 genes) and the erythroid-specific genes [beta-globin, glycophorin, delta-aminolevulinic acid synthase (ALAS-E), GATA-1 and erythropoietin receptor (EpoR)] in the MEL cell transformant having transferred c-myc gene. The overexpression of c-myc gene affected the latent period genes in different ways: hsc and hsp 70 genes and Id gene were positively regulated, while expression of MER5 gene was repressed. While c-myc is thought to be involved in DNA replication, its overexpression showed no effect on the expression of proliferating cell specific nuclear antigen or DNA polymerase a. The overexpression of c-myc repressed the expression of glycophorin, ALAS-E and beta-globin genes, of the five erythroid-specific genes, but had no effect on expression of GATA-1 or EpoR gene. These results suggest that c-Myc differentially regulates the expression of the latent period and erythroid-specific genes.

DNA-binding specificity of GATA family transcription factors

GATA-binding proteins constitute a family of transcription factors that recognize a target site conforming to the consensus WGATAR (W = A or T and R = A or G). Here we have used the method of polymerase chain reaction-mediated random site selection to assess in an unbiased manner the DNA-binding specificity of GATA proteins. Contrary to our expectations, we show that GATA proteins bind a variety of motifs that deviate from the previously assigned consensus. Many of the nonconsensus sequences bind protein with high affinity, equivalent to that of conventional GATA motifs. By using the selected sequences as probes in the electrophoretic mobility shift assay, we demonstrate overlapping, but distinct, sequence preferences for GATA family members, specified by their respective DNA-binding domains. Furthermore, we provide additional evidence for interaction of amino and carboxy fingers of GATA-1 in defining its binding site. By performing cotransfection experiments, we also show that transactivation parallels DNA binding. A chimeric protein containing the finger domain of areA and the activation domains of GATA-1 is capable of activating transcription in mammalian cells through GATA motifs. Our findings suggest a mechanism by which GATA proteins might selectively regulate gene expression in cells in which they are coexpressed.

DNA-binding specificity of GATA family transcription factors

GATA-binding proteins constitute a family of transcription factors that recognize a target site conforming to the consensus WGATAR (W = A or T and R = A or G). Here we have used the method of polymerase chain reaction-mediated random site selection to assess in an unbiased manner the DNA-binding specificity of GATA proteins. Contrary to our expectations, we show that GATA proteins bind a variety of motifs that deviate from the previously assigned consensus. Many of the nonconsensus sequences bind protein with high affinity, equivalent to that of conventional GATA motifs. By using the selected sequences as probes in the electrophoretic mobility shift assay, we demonstrate overlapping, but distinct, sequence preferences for GATA family members, specified by their respective DNA-binding domains. Furthermore, we provide additional evidence for interaction of amino and carboxy fingers of GATA-1 in defining its binding site. By performing cotransfection experiments, we also show that transactivation parallels DNA binding. A chimeric protein containing the finger domain of areA and the activation domains of GATA-1 is capable of activating transcription in mammalian cells through GATA motifs. Our findings suggest a mechanism by which GATA proteins might selectively regulate gene expression in cells in which they are coexpressed.

Expression of a constitutively active erythropoietin receptor in primary hematopoietic progenitors abrogates erythropoietin dependence and enhances erythroid colony-forming unit, erythroid burst-forming unit, and granulocyte/macrophage progenitor growth

We tested the ability of a constitutively activated erythropoietin receptor [EpoR(R129C)] to alter the growth requirements of primary hematopoietic precursors that terminally differentiate in culture. Two recombinant retroviruses expressing EpoR(R129C), spleen focus-forming virus (SFFVc-EpoR) and myeloproliferative sarcoma virus (MPSVcEpoR), were used to infect fetal liver cells that served as a source of hematopoietic progenitors. Methylcellulose cultures were incubated in the absence of any added growth factors or in combination with selected growth factors. EpoR(R129C) completely abrogated the Epo requirement of erythroid colony-forming units to form erythrocytes after 2-5 days in culture and did not interfere with the differentiation program of these cells. In the absence of added growth factors EpoR(R129C) did not enhance erythroid burst-forming unit development. In contrast to experiments in heterologous cell lines, EpoR(R129C) did not render progenitor cells independent of interleukin 3 or granulocyte/macrophage colony-stimulating factor (GM-CSF). However, when progenitors were cultured with added steel factor, but not with interleukin 3 or GM-CSF, EpoR(R129C) augmented the growth and differentiation of erythroid bursts, mixed erythroid/myeloid, and granulocyte/macrophage (GM) colonies. Furthermore, both viruses were capable of expressing EpoR(R129C) in erythroid, mixed erythroid/myeloid, and GM colonies. Thus an aberrantly expressed and constitutively activated EpoR can stimulate proliferation of some GM progenitors. The ability of EpoR(R129C) to abrogate the Epo requirement of primary hematopoietic cells, but not the requirement for other cytokines, is consistent with the induction of erythroblastosis in vivo.

Transcriptional inhibition of the murine erythropoietin receptor gene by an upstream repetitive element

Transcription of the murine erythropoietin receptor (EpoR) gene is inhibited by a novel repetitive element that is located upstream of the EpoR promoter. Reporter gene studies reveal that the inhibitory effect is both distance and orientation dependent. This element is a member of a family of repetitive elements specific to rodents and is present at approximately 10(5) copies per mouse genome. It encodes approximately 500- to 900-bp-long transcripts in both erythroid and nonerythroid cells. RNase protection analysis with a probe from the 5' flanking murine EpoR gene reveals that the direction of transcription is in the sense orientation, relative to the downstream EpoR gene. We suggest that transcriptional inhibition of the EpoR promoter is mediated by read-through transcripts originating in the upstream repetitive element and that this effect may contribute to the basal level of transcription of the murine EpoR gene in erythroid cells.

Transcriptional inhibition of the murine erythropoietin receptor gene by an upstream repetitive element

Transcription of the murine erythropoietin receptor (EpoR) gene is inhibited by a novel repetitive element that is located upstream of the EpoR promoter. Reporter gene studies reveal that the inhibitory effect is both distance and orientation dependent. This element is a member of a family of repetitive elements specific to rodents and is present at approximately 10(5) copies per mouse genome. It encodes approximately 500- to 900-bp-long transcripts in both erythroid and nonerythroid cells. RNase protection analysis with a probe from the 5' flanking murine EpoR gene reveals that the direction of transcription is in the sense orientation, relative to the downstream EpoR gene. We suggest that transcriptional inhibition of the EpoR promoter is mediated by read-through transcripts originating in the upstream repetitive element and that this effect may contribute to the basal level of transcription of the murine EpoR gene in erythroid cells.

In vitro and in vivo protein--DNA interactions on the rat erythroid-specific L' pyruvate kinase gene promoter

The rat L-type pyruvate kinase gene possesses two alternative tissue-specific promoters, located 472 bp apart; the upstream L' promoter is erythroid-specific and the downstream L promoter is hepatocyte-specific. The erythroid-specific L' promoter is strongly active in fetal liver at day 17 of gestation, while its activity rapidly decreases thereafter. A L' promoter fragment spanning from nucleotide -320 to +10 with respect to the cap site is able to direct a weak but erythroid-specific transcription in a cell-free system. We have used DNAse I footprinting and gel mobility shift assays to characterize and identify the binding of nuclear factors from both 17-day-old fetal liver and adult liver nuclear extracts to a 320 bp fragment of the 5' flanking region of the gene in vitro. Two clusters of erythroid-specific interactions were found. The proximal cluster consists of two GATA-1 binding sites at -50 bp and -65 bp from the transcription initiation site, immediately downstream of a CACC motif and two G/C-rich elements. The distal cluster of cis-elements, located 130 bp upstream, corresponds to two GATA-1 sequences. These two sequences overlap NF1 motifs interacting with ubiquitous NF1 transcriptional factors in presence of adult hepatic extracts. Furthermore, we have examined in vivo protein-DNA interactions by DMS footprinting in livers of 17-day-old rat fetuses and adult rats. We found that the sites characterized in vitro are occupied in vivo. Therefore, in adult liver the L' promoter, although inactive, nevertheless interacts with ubiquitous factors.

Transactivation of erythroid transcription factor GATA-1 by a myb-ets-containing retrovirus

ME26 virus is a recombinant mouse retrovirus construct homologous to the avian E26 virus. Both encode a 135-kDa gag-myb-ets fusion protein which is localized in the nucleus. We have recently shown that ME26 virus can induce erythropoietin (Epo) responsiveness in hematopoietic cells. Mice infected with ME26 virus develop a hyperplasia of Epo-dependent hematopoietic precursor cells from which permanent cell lines can be established. In vitro, ME26 virus specifically induces Epo responsiveness in the interleukin-3-dependent myeloid cell line FDC-P2 by enhancing expression of the Epo receptor (EpoR). In the present study we demonstrate that ME26 virus infection of FDC-P2 cells also results in enhanced expression of beta-globin and the erythroid-specific transcription factor GATA-1, a protein which can transactivate both the EpoR promoter and globin genes. In addition, these cells exhibit a down-regulation of c-myb expression similar to that seen in differentiating erythroid cells. To determine the molecular basis for activation of erythroid genes in ME26 virus-infected cells, we carried out transient expression assays with DNA constructs of either the EpoR promoter of the GATA-1 promoter linked to reporter genes. Our results indicate that while ME26 virus did not directly enhance expression from the EpoR promoter, both it and its avian parent, E26, transactivated the GATA-1 promoter. Furthermore, ME26 virus cooperates with the GATA-1 protein to enhance expression of the EpoR gene. We propose that the mechanism by which ME26 virus induces erythroleukemia involves transactivation of the GATA-1 gene, thus positively regulating the expression of the EpoR and leading to the proliferation of a unique population of Epo-responsive cells. By specifically inducing Epo responsiveness in hematopoietic cells via transactivation of a transcription factor, ME26 virus utilizes a novel mechanism for retrovirus pathogenesis.

The gene for erythropoietin receptor is expressed in multipotential hematopoietic and embryonal stem cells: evidence for differentiation stage-specific regulation

The principal regulator of erythropoiesis is the glycoprotein erythropoietin, which interacts with a specific cell surface receptor (EpoR). A study aimed at analyzing EpoR gene regulation has shown that both pluripotent embryonal stem cells and early multipotent hematopoietic cells express EpoR transcripts. Commitment to nonerythroid lineages (e.g., macrophage or lymphocytic) results in the shutdown of EpoR gene expression, whereas commitment to the erythroid lineage is concurrent with or followed by dramatic increases in EpoR transcription. To determine whether gene activity could be correlated with chromatin alterations, DNase-hypersensitive sites (HSS) were mapped. Two major HSS located in the promoter region and within the first intron of the EpoR gene are present in all embryonal stem and hematopoietic cells tested, the intensities of which correlate well with EpoR expression levels. In addition, a third major HSS also located within the first intron of the EpoR gene is uniquely present in erythroid cells that express high levels of EpoR. Transfection assays show that sequences surrounding this major HSS impart erythroid cell-specific enhancer activity to a heterologous promoter and that this activity is at least in part mediated by GATA-1. These data, together with concordant expression levels of GATA-1 and EpoR in both early multipotent hematopoietic and committed erythroid cells, support a regulatory role of the erythroid cell-specific transcription factor GATA-1 in EpoR transcription in these cells. However, the lack of significant levels of GATA-1 expression in embryonal stem cells implies an alternative regulatory mechanism of EpoR transcription in cells not committed to the hematopoietic lineage.

The gene for erythropoietin receptor is expressed in multipotential hematopoietic and embryonal stem cells: evidence for differentiation stage-specific regulation

The principal regulator of erythropoiesis is the glycoprotein erythropoietin, which interacts with a specific cell surface receptor (EpoR). A study aimed at analyzing EpoR gene regulation has shown that both pluripotent embryonal stem cells and early multipotent hematopoietic cells express EpoR transcripts. Commitment to nonerythroid lineages (e.g., macrophage or lymphocytic) results in the shutdown of EpoR gene expression, whereas commitment to the erythroid lineage is concurrent with or followed by dramatic increases in EpoR transcription. To determine whether gene activity could be correlated with chromatin alterations, DNase-hypersensitive sites (HSS) were mapped. Two major HSS located in the promoter region and within the first intron of the EpoR gene are present in all embryonal stem and hematopoietic cells tested, the intensities of which correlate well with EpoR expression levels. In addition, a third major HSS also located within the first intron of the EpoR gene is uniquely present in erythroid cells that express high levels of EpoR. Transfection assays show that sequences surrounding this major HSS impart erythroid cell-specific enhancer activity to a heterologous promoter and that this activity is at least in part mediated by GATA-1. These data, together with concordant expression levels of GATA-1 and EpoR in both early multipotent hematopoietic and committed erythroid cells, support a regulatory role of the erythroid cell-specific transcription factor GATA-1 in EpoR transcription in these cells. However, the lack of significant levels of GATA-1 expression in embryonal stem cells implies an alternative regulatory mechanism of EpoR transcription in cells not committed to the hematopoietic lineage.

Induction of erythropoietin responsiveness in murine hematopoietic cells by the gag-myb-ets-containing ME26 virus

ME26 virus, which was generated by inserting the coding region of the acute avian leukemia-inducing virus E26 into a murine retrovirus vector, encodes a 135-kDa gag-myb-ets fusion protein. Amphotropic murine leukemia virus pseudotypes of ME26 virus induce a high incidence of erythroleukemia 2 to 4 months after injection into newborn NFS/N mice. Spleen cells from the majority of these mice proliferate to high levels in the presence of the erythroid hormone erythropoietin (Epo) and can easily be established as permanent Epo-dependent cell lines. The cell lines contain multiple copies of ME26 viral DNA and express viral message and protein. An Epo receptor mRNA of normal size can be detected in these cells, and binding studies reveal a single class of lower-affinity Epo receptor with an affinity for Epo that is in the range of that previously reported for erythroid cells. The ME26 virus-induced Epo-dependent cell lines, however, appear more immature than previously described erythroid cell lines and more closely resemble early hematopoietic precursor cells, suggesting that the virus may be activating the Epo receptor in hematopoietic cells that do not normally express it. Consistent with this idea, we are able to infect an interleukin-3-dependent myeloid cell line, FDC-P2, with ME26 virus and convert it to Epo dependence. The ME26 virus-infected FDC-P2 cells, even before growth on Epo, showed a large increase in the amount of Epo receptor mRNA. However, no ME26 viral integrations can be detected adjacent to the Epo receptor gene, indicating that the virus is not activating the Epo receptor gene by promoter/enhancer insertion. Our results are more consistent with the hypothesis that the gag-myb-ets-encoded viral fusion protein, which is known to bind DNA, is directly or indirectly activating the expression of the Epo receptor gene in these cells.

Transcriptional regulation of the rat platelet factor 4 gene: interaction between an enhancer/silencer domain and the GATA site

We used various segments of the 5' upstream region of the rat platelet factor 4 (PF4) gene coupled to the human growth hormone gene and heterologous promoters to identify domains which are critical for tissue-specific expression. Transient expression experiments with rat bone marrow cells and other cell lines revealed a complex interplay between a core promoter domain from -97 to the transcriptional start site and an enhancer/silencer domain from -448 to -112. The core promoter contains a GATA site at -31 to -28 whose mutation to TATA or AATA decreases tissue specificity and moderately affects expression in megakaryocytes as well as a positively acting subdomain from -97 to -83 whose removal decreases overall transcription without affecting tissue specificity. The enhancer/silencer domain possesses three positively acting subdomains from -380 to -362, -270 to -257, and -137 to -120 as well as a negatively acting subdomain at -184 to -151 which is able to reduce overall transcription but has no effect on tissue specificity. The subdomain from -380 to -362 is most critical in restricting gene expression driven either by the PF4 promoter or by a heterologous promoter to the megakaryocytic lineage. The subdomains from -270 to -257 and -137 to -120 function together with the subdomain from -380 to -362 to somewhat increase tissue specificity. Simultaneous mutation of the GATA site and deletion of either the whole enhancer/silencer domain or the subdomain from -380 to -362 or -137 to -120 reduce transcription in megakaryocytes by 10- to 30-fold. On the basis of the above-described results, we propose that the megakaryocyte-specific enhancer/silencer domain and the GATA site are responsible for high-level expression of the PF4 gene in a lineage-specific manner.

Sequence and expression of GLN3, a positive nitrogen regulatory gene of Saccharomyces cerevisiae encoding a protein with a putative zinc finger DNA-binding domain

The GLN3 gene of Saccharomyces cerevisiae is required for the activation of transcription of a number of genes in response to the replacement of glutamine by glutamate as source of nitrogen. We cloned the GLN3 gene and constructed null alleles by gene disruption. GLN3 is not essential for growth, but increased copies of GLN3 lead to a drastic decrease in growth rate. The complete nucleotide sequence of the GLN3 gene was determined, revealing one open reading frame encoding a polypeptide of 730 amino acids, with a molecular weight of approximately 80,000. The GLN3 protein contains a single putative Cys2/Cys2 zinc finger which has homology to the Neurospora crassa NIT2 protein, the Aspergillus nidulans AREA protein, and the erythroid-specific transcription factor GATA-1. Immunoprecipitation experiments indicated that the GLN3 protein binds the nitrogen upstream activation sequence of GLN1, the gene encoding glutamine synthetase. Neither control of transcription nor control of initiation of translation of GLN3 is important for regulation in response to glutamine availability.

Response to erythropoietin in erythroid subclones of the factor-dependent cell line 32D is determined by translocation of the erythropoietin receptor to the cell surface

Regulation of the expression of the erythropoietin (Epo) receptor (EpoR) gene is under the control of transcriptional regulatory factor GATA-1. GATA-1 is expressed widely among the nonerythroid, factor-dependent subclones of the interleukin 3-dependent mouse cell line 32D. Consequently, to determine whether GATA-1 and EpoR gene expression are linked even in nonerythroid cells, we have studied the correlation of GATA-1 expression with expression and function of EpoR in these cell lines. EpoR mRNA (by RNase protection analysis) and EpoR protein (by specific antibody immunoprecipitation of metabolically labeled EpoR protein) were detectable not only in 32D and 32D Epo (an Epo-dependent subclone) but also in 32D GM, a subclone dependent for growth on granulocyte/macrophage colony-stimulating factor. EpoR mRNA also was detectable by PCR in 32D G, a subclone dependent for growth on granulocyte colony-stimulating factor. However, only 32D Epo cells bound 125I-labeled Epo and expressed EpoR protein on the cell surface, as determined by immunoprecipitation of surface-labeled proteins. These results indicate that, in these factor-dependent cell lines, the major regulatory step determining the erythroid-specific response to Epo is the efficiency of EpoR protein translocation to the cell surface. Mechanisms that could affect lineage-specific translocation are the presence of a chaperone protein, erythroid-specific editing of EpoR mRNA, or altered processing of the EpoR protein to the cell surface. In this model, lineage-restricted responses to growth factors such as Epo are determined not by expression of the genes for growth factor receptors but, rather, by appropriate processing of the receptor protein.

Transcriptional regulation of the rat platelet factor 4 gene: interaction between an enhancer/silencer domain and the GATA site

We used various segments of the 5' upstream region of the rat platelet factor 4 (PF4) gene coupled to the human growth hormone gene and heterologous promoters to identify domains which are critical for tissue-specific expression. Transient expression experiments with rat bone marrow cells and other cell lines revealed a complex interplay between a core promoter domain from -97 to the transcriptional start site and an enhancer/silencer domain from -448 to -112. The core promoter contains a GATA site at -31 to -28 whose mutation to TATA or AATA decreases tissue specificity and moderately affects expression in megakaryocytes as well as a positively acting subdomain from -97 to -83 whose removal decreases overall transcription without affecting tissue specificity. The enhancer/silencer domain possesses three positively acting subdomains from -380 to -362, -270 to -257, and -137 to -120 as well as a negatively acting subdomain at -184 to -151 which is able to reduce overall transcription but has no effect on tissue specificity. The subdomain from -380 to -362 is most critical in restricting gene expression driven either by the PF4 promoter or by a heterologous promoter to the megakaryocytic lineage. The subdomains from -270 to -257 and -137 to -120 function together with the subdomain from -380 to -362 to somewhat increase tissue specificity. Simultaneous mutation of the GATA site and deletion of either the whole enhancer/silencer domain or the subdomain from -380 to -362 or -137 to -120 reduce transcription in megakaryocytes by 10- to 30-fold. On the basis of the above-described results, we propose that the megakaryocyte-specific enhancer/silencer domain and the GATA site are responsible for high-level expression of the PF4 gene in a lineage-specific manner.

Sequence and expression of GLN3, a positive nitrogen regulatory gene of Saccharomyces cerevisiae encoding a protein with a putative zinc finger DNA-binding domain

The GLN3 gene of Saccharomyces cerevisiae is required for the activation of transcription of a number of genes in response to the replacement of glutamine by glutamate as source of nitrogen. We cloned the GLN3 gene and constructed null alleles by gene disruption. GLN3 is not essential for growth, but increased copies of GLN3 lead to a drastic decrease in growth rate. The complete nucleotide sequence of the GLN3 gene was determined, revealing one open reading frame encoding a polypeptide of 730 amino acids, with a molecular weight of approximately 80,000. The GLN3 protein contains a single putative Cys2/Cys2 zinc finger which has homology to the Neurospora crassa NIT2 protein, the Aspergillus nidulans AREA protein, and the erythroid-specific transcription factor GATA-1. Immunoprecipitation experiments indicated that the GLN3 protein binds the nitrogen upstream activation sequence of GLN1, the gene encoding glutamine synthetase. Neither control of transcription nor control of initiation of translation of GLN3 is important for regulation in response to glutamine availability.

Activation of the erythropoietin receptor promoter by transcription factor GATA-1

Erythropoietin, a glycoprotein produced by the kidneys in response to anemia and hypoxia, is a major growth factor for cells of the erythroid lineage. Erythropoietin interacts with high-affinity cell surface receptors (EpoR) present on developing progenitors and is required for their survival. Previously we characterized the gene for EpoR and demonstrated that its promoter acts in a cell-specific manner. Here we show that the hematopoietic-specific transcription factor GATA-1 is necessary, and indeed is sufficient as the sole cell-restricted regulator, for activation of the EpoR promoter in fibroblast transfection assays. Hence, GATA-1, which participates in transcriptional control of the majority of erythroid-expressed genes, also acts on the promoter of an essential lineage-restricted receptor (EpoR). This central contribution of GATA-1 to EpoR promoter function provides a mechanism whereby a cell-restricted regulator may ensure the viability and subsequent maturation of progenitor cells during hematopoietic differentiation.

Unregulated expression of the erythropoietin receptor gene caused by insertion of spleen focus-forming virus long terminal repeat in a murine erythroleukemia cell line

A murine erythroleukemia (MEL) cell line, F5-5, expressed 10,000 binding sites for erythropoietin (EPO) per cell, 10-fold more than was expressed by other murine erythroleukemia cell lines and normal erythroid progenitors. Northern (RNA) and Southern blot analyses revealed overexpression of mRNA for the EPO receptor (EPOR) and rearrangement of one of the EPOR gene alleles in F5-5 cells, respectively. Molecular cloning of F5-5-derived cDNA encoding EPOR revealed that the 5' noncoding region of the EPOR cDNA corresponds to the 3' long terminal repeat sequence of the polycythemic strain of Friend spleen focus-forming virus (F-SFFVP). The aberrant EPOR transcripts containing the 3' long terminal repeat sequence were mainly expressed in F5-5 cells. The same integration upstream of the EPOR gene was also observed in other subclones and the parent cell line. It is possible that overexpression of EPOR by viral promoter insertion will confer growth advantage to an F-SFFVP-infected erythroid progenitor cell, leading to positive clonal selection through further leukemogenic steps.

Unregulated expression of the erythropoietin receptor gene caused by insertion of spleen focus-forming virus long terminal repeat in a murine erythroleukemia cell line

A murine erythroleukemia (MEL) cell line, F5-5, expressed 10,000 binding sites for erythropoietin (EPO) per cell, 10-fold more than was expressed by other murine erythroleukemia cell lines and normal erythroid progenitors. Northern (RNA) and Southern blot analyses revealed overexpression of mRNA for the EPO receptor (EPOR) and rearrangement of one of the EPOR gene alleles in F5-5 cells, respectively. Molecular cloning of F5-5-derived cDNA encoding EPOR revealed that the 5' noncoding region of the EPOR cDNA corresponds to the 3' long terminal repeat sequence of the polycythemic strain of Friend spleen focus-forming virus (F-SFFVP). The aberrant EPOR transcripts containing the 3' long terminal repeat sequence were mainly expressed in F5-5 cells. The same integration upstream of the EPOR gene was also observed in other subclones and the parent cell line. It is possible that overexpression of EPOR by viral promoter insertion will confer growth advantage to an F-SFFVP-infected erythroid progenitor cell, leading to positive clonal selection through further leukemogenic steps.

GATA-1 transactivates erythropoietin receptor gene, and erythropoietin receptor-mediated signals enhance GATA-1 gene expression

Erythropoietin is a cytokine which specifically regulates the proliferation and differentiation of erythroid progenitor cells. The expression of erythropoietin receptor on the cell membrane of the progenitor cells is a critical event during the erythroid differentiation process. In order to clarify the tissue-specific and differentiation stage-specific expression of the erythropoietin receptor gene, its transcriptional regulation was examined by transient expression assay, gel mobility shift assay and DNase I footprinting. The results clearly showed that GATA-1 transactivates the gene expression through a single GATA motif located around -200 bp upstream from the ATG codon in a dose dependent manner. Furthermore, Northern blot analysis revealed that erythropoietin receptor-mediated signals strongly enhanced GATA-1 gene expression in accordance with the appearance of hemoglobin-positive cells. Taken together with other observations, these results suggested the following scheme of erythroid differentiation: 1)GATA-1 is expressed in the early stage of blood cell development; 2) GATA-1 transactivates the erythropoietin receptor gene; 3) erythropoietin binds its receptor and the receptor-mediated signals enhance GATA-1 gene expression in erythroid progenitor cells; and 4) GATA-1 finally transactivates hemoglobin synthesis-related genes and globin genes in relatively matured erythroid cells.

Organization of the murine and human interleukin-7 receptor genes: two mRNAs generated by differential splicing and presence of a type I-interferon-inducible promoter

To better understand the regulation of interleukin-7 receptor (IL-7R) expression, we have pursued a detailed analysis of the structure of the murine and human IL-7R genes. The genes consist of eight exons, the sizes of which are conserved in mouse and human cells, spread out over 24 kbp (murine) and 19 kbp (human). A differential splicing event results in an mRNA encoding a secreted form of the human IL-7R gene. Primer extension and S1 nuclease analysis show a single transcriptional start site for the murine IL-7R gene. The 5'-flanking region of the murine IL-7R gene contains TATA- and CAAT-like sequences. The promoter region also contains a functional interferon regulatory element, to which the interferon-induced nuclear factors IRF-1 and IRF-2 are capable of binding and which is able to confer interferon-inducible expression on a heterologous gene. There are also potential binding sites for the transcription factors AP-1 and AP-2 as well as multiple glucocorticoid response elements. A fusion gene containing 2.5 kb of murine IL-7R 5' regulatory sequence linked to the bacterial chloramphenicol acetyltransferase gene directed expression of chloramphenicol acetyltransferase activity in murine pre-B-cell line 70Z/3 but not in the mouse fibroblast cell line NIH 3T3. Comparison of the murine and human IL-7R exon/intron boundaries with those of other hematopoietin receptor superfamily members whose exon/intron boundaries are also known reveals a conserved evolutionary structure.

Organization of the murine and human interleukin-7 receptor genes: two mRNAs generated by differential splicing and presence of a type I-interferon-inducible promoter

To better understand the regulation of interleukin-7 receptor (IL-7R) expression, we have pursued a detailed analysis of the structure of the murine and human IL-7R genes. The genes consist of eight exons, the sizes of which are conserved in mouse and human cells, spread out over 24 kbp (murine) and 19 kbp (human). A differential splicing event results in an mRNA encoding a secreted form of the human IL-7R gene. Primer extension and S1 nuclease analysis show a single transcriptional start site for the murine IL-7R gene. The 5'-flanking region of the murine IL-7R gene contains TATA- and CAAT-like sequences. The promoter region also contains a functional interferon regulatory element, to which the interferon-induced nuclear factors IRF-1 and IRF-2 are capable of binding and which is able to confer interferon-inducible expression on a heterologous gene. There are also potential binding sites for the transcription factors AP-1 and AP-2 as well as multiple glucocorticoid response elements. A fusion gene containing 2.5 kb of murine IL-7R 5' regulatory sequence linked to the bacterial chloramphenicol acetyltransferase gene directed expression of chloramphenicol acetyltransferase activity in murine pre-B-cell line 70Z/3 but not in the mouse fibroblast cell line NIH 3T3. Comparison of the murine and human IL-7R exon/intron boundaries with those of other hematopoietin receptor superfamily members whose exon/intron boundaries are also known reveals a conserved evolutionary structure.

Regulated expression of globin chains and the erythroid transcription factor GATA-1 during erythropoiesis in the developing mouse

Erythropoiesis in vertebrates is characterized by sequential changes in erythropoietic site, erythroblast morphology, and hemoglobin synthesis. We have examined the expression of globin chains and the major erythroid transcription factor GATA-1 (previously known as GF-1/NF-E1/Eryf 1) from days 7.5 to 17.5 of mouse development. mRNAs for embryonic (epsilon y2, beta H1, and zeta) and adult (alpha and beta) globin chains were quantitated by RNase protection assays. Switching of globins within the alpha-globin cluster (alpha and zeta) was not strictly coordinated with that within the beta-globin cluster (epsilon y2, beta H1, and beta). Regulation of globin switches during development was primarily transcriptional. Of particular note, we found two developmental switches (beta H1 to epsilon y2 and epsilon y2 to beta) in the mouse, more analogous than previously thought to shifts found in human development. The erythroid transcription factor GATA-1, believed to be a principal regulator of genes expressed in erythroid cells, first appeared in the embryo in yolk sac at the time of blood island formation and remained at a low level during embryonic erythropoiesis (8 to 11 days) relative to that found later in fetal liver (12 to 15 days). The rise in GATA-1 mRNA in fetal liver paralleled and preceded the rapid accumulation of adult beta-globin RNA. RNase protection assays and a GATA-1-specific peptide antiserum were used to establish that a single GATA-1 polypeptide is expressed throughout mouse development. Overall, these findings suggest that the levels of this erythroid transcription factor during development may contribute to the differential gene activation characteristic of definitive versus primitive erythropoiesis.

Progressive inactivation of the expression of an erythroid transcriptional factor in GM- and G-CSF-dependent myeloid cell lines

The transcriptional binding protein NFE-1 (also called GF-1 and Ery-f1) is thought to play a necessary, but not sufficient, role in the regulation of differentiation-related gene expression in a subset of hematopoietic lineages (erythroid, megakaryocytic, and basophil-mast cell). In order to clarify the mechanism which underlies the lineage-specificity of the NFE-1 expression, as well as the relationship between the expression of this factor and growth factor responsiveness, we have evaluated the capacity of erythropoietin (Epo)-, granulomonocytic (GM)-colony stimulating factor (CSF)-, and granulocyte (G)-CSF-dependent subclones derived from the interleukin 3 (IL-3)-dependent cell line 32D, to express 1) NFE-1 mRNA, 2) NFE-1-related nuclear proteins, and 3) chloramphenicol acetyl transferase (CAT) activity when transfected with a CAT gene under the control of NFE-1 cognate sequences. NFE-1 mRNA was found to be expressed not only in cells with mast cell (IL-3-dependent 32D) and erythroid (Epo-dependent 32D Epo1) phenotypes, but also in cells with predominantly granulocyte/macrophage properties, such as the GM-CSF- (early myelomonocytic) and G-CSF- (myelocytic) dependent subclones of 32D. However, a gradient of expression, correlating with the lineage, the stage of differentiation, and the growth factor responsiveness of the cell lines, was found among the different subclones: Epo greater than or equal to IL-3 greater than GM-CSF greater than G-CSF. Binding experiments demonstrated NFE-1 activity in all cell lines except the G-CSF-dependent line. Function of the NFE-1 protein was assessed by the expression of the CAT gene linked to the SV40 promoter and a mutant (-175 T----C) HPFH gamma-globin promoter. High level CAT expression was seen only in the Epo1 cells although low level expression was also seen in the parent 32D. These results demonstrate that the specificity of the expression of NFE-1 for the erythroid--megakaryocytic--mast cell lineages is obtained by progressive inactivation of its expression in alternative lineages.

Regulated expression of globin chains and the erythroid transcription factor GATA-1 during erythropoiesis in the developing mouse

Erythropoiesis in vertebrates is characterized by sequential changes in erythropoietic site, erythroblast morphology, and hemoglobin synthesis. We have examined the expression of globin chains and the major erythroid transcription factor GATA-1 (previously known as GF-1/NF-E1/Eryf 1) from days 7.5 to 17.5 of mouse development. mRNAs for embryonic (epsilon y2, beta H1, and zeta) and adult (alpha and beta) globin chains were quantitated by RNase protection assays. Switching of globins within the alpha-globin cluster (alpha and zeta) was not strictly coordinated with that within the beta-globin cluster (epsilon y2, beta H1, and beta). Regulation of globin switches during development was primarily transcriptional. Of particular note, we found two developmental switches (beta H1 to epsilon y2 and epsilon y2 to beta) in the mouse, more analogous than previously thought to shifts found in human development. The erythroid transcription factor GATA-1, believed to be a principal regulator of genes expressed in erythroid cells, first appeared in the embryo in yolk sac at the time of blood island formation and remained at a low level during embryonic erythropoiesis (8 to 11 days) relative to that found later in fetal liver (12 to 15 days). The rise in GATA-1 mRNA in fetal liver paralleled and preceded the rapid accumulation of adult beta-globin RNA. RNase protection assays and a GATA-1-specific peptide antiserum were used to establish that a single GATA-1 polypeptide is expressed throughout mouse development. Overall, these findings suggest that the levels of this erythroid transcription factor during development may contribute to the differential gene activation characteristic of definitive versus primitive erythropoiesis.