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

Multiple ETS family proteins regulate PF4 gene expression by binding to the same ETS binding site

In previous studies on the mechanism underlying megakaryocyte-specific gene expression, several ETS motifs were found in each megakaryocyte-specific gene promoter. Although these studies suggested that several ETS family proteins regulate megakaryocyte-specific gene expression, only a few ETS family proteins have been identified. Platelet factor 4 (PF4) is a megakaryocyte-specific gene and its promoter includes multiple ETS motifs. We had previously shown that ETS-1 binds to an ETS motif in the PF4 promoter. However, the functions of the other ETS motifs are still unclear. The goal of this study was to investigate a novel functional ETS motif in the PF4 promoter and identify proteins binding to the motif. In electrophoretic mobility shift assays and a chromatin immunoprecipitation assay, FLI-1, ELF-1, and GABP bound to the −51 ETS site. Expression of FLI-1, ELF-1, and GABP activated the PF4 promoter in HepG2 cells. Mutation of a −51 ETS site attenuated FLI-1-, ELF-1-, and GABP-mediated transactivation of the promoter. siRNA analysis demonstrated that FLI-1, ELF-1, and GABP regulate PF4 gene expression in HEL cells. Among these three proteins, only FLI-1 synergistically activated the promoter with GATA-1. In addition, only FLI-1 expression was increased during megakaryocytic differentiation. Finally, the importance of the −51 ETS site for the activation of the PF4 promoter during physiological megakaryocytic differentiation was confirmed by a novel reporter gene assay using in vitro ES cell differentiation system. Together, these data suggest that FLI-1, ELF-1, and GABP regulate PF4 gene expression through the −51 ETS site in megakaryocytes and implicate the differentiation stage-specific regulation of PF4 gene expression by multiple ETS factors.

B-Myb regulates the A(2B) adenosine receptor in vascular smooth muscle cells

The A(2B) adenosine receptor (A(2B)AR) has been described to control various vascular functions, including inhibition of smooth muscle cell proliferation. Here, we sought to understand the regulation of A(2B)AR gene expression in aortic vascular smooth muscle cells (VSMCs), with a focus on the proliferation phase. Assays with A(2B)AR-beta-gal aortic VSMCs, in which the endogenous A(2B)AR gene promoter drives the expression of prokaryotic beta-galactosidase (beta-gal) instead of the endogenous A(2B)AR gene, show that beta-gal expression is upregulated when the cells are induced to exit from cell cycle arrest. Similarly, the level of A(2B)AR mRNA is upregulated in proliferating primary aortic VSMCs. In search of related mechanisms, it was noted that the A(2B)AR gene promoter contains several putative binding sites for the proliferation-inducing transcription factor, B-Myb. Using a clone of the 5' region upstream of the mouse A(2B)AR gene linked to a reporter gene, B-Myb site deletion mutants were generated. It was determined that B-Myb upregulates the A(2B)AR gene promoter, and specific promoter binding sites were identified as functional. In accordance, B-Myb also elevates endogenous A(2B)AR mRNA and receptor activity, and this activity decreases cell proliferation. Our data are novel in that they show that this proliferation-inhibiting A(2B)AR is itself an inducible receptor regulated by B-Myb.

The A2b adenosine receptor protects against vascular injury

The A2b adenosine receptor (A2bAR) is highly abundant in bone marrow macrophages and vascular smooth muscle cells (VSMC). To examine the functional significance of this receptor expression, we applied a femoral artery injury model to A2bAR knockout (KO) mice and showed that the A2bAR prevents vascular lesion formation in an injury model that resembles human restenosis after angioplasty. While considering related mechanisms, we noted higher levels of TNF-alpha, an up-regulator of CXCR4, and of VSMC proliferation in the injured KO mice. In accordance, CXCR4, which is known to attract progenitor cells during tissue regeneration, is up-regulated in lesions of the KO mice. In addition, aortic smooth muscle cells derived from A2bAR KO mice display greater proliferation in comparison with controls. Bone marrow transplantation experiments indicated that the majority of the signal for lesion formation in the null mice originates from bone marrow cells. Thus, this study highlights the significance of the A2bAR in regulating CXCR4 expression in vivo and in protecting against vascular lesion formation.

Maturation stage-specific regulation of megakaryopoiesis by pointed-domain Ets proteins

Numerous megakaryocyte-specific genes contain signature Ets-binding sites in their regulatory regions. Fli-1 (friend leukemia integration 1), an Ets transcription factor, is required for the normal maturation of megakaryocytes and controls the expression of multiple megakaryocyte-specific genes. However, in Fli-1-/- mice, early megakaryopoiesis persists, and the expression of the early megakaryocyte-specific genes, alphaIIb and cMpl, is maintained, consistent with functional compensation by a related Ets factor(s). Here we identify the Ets protein GABPalpha (GA-binding protein alpha) as a regulator of early megakaryocyte-specific genes. Notably, GABPalpha preferentially occupies Ets elements of early megakaryocyte-specific genes in vitro and in vivo, whereas Fli-1 binds both early and late megakaryocyte-specific genes. Moreover, the ratio of GABPalpha/Fli-1 expression declines throughout megakaryocyte maturation. Consistent with this expression pattern, primary fetal liver-derived megakaryocytes from Fli-1-deficient murine embryos exhibit reduced expression of genes associated with late stages of maturation (glycoprotein [GP] Ibalpha, GPIX, and platelet factor 4 [PF4]), whereas GABPalpha-deficient megakaryocytes were mostly impaired in the expression of early megakaryocyte-specific genes (alphaIIb and cMpl). Finally, mechanistic experiments revealed that GABPalpha, like Fli-1, can impart transcriptional synergy between the hematopoietic transcription factor GATA-1 and its cofactor FOG-1 (friend of GATA-1). In concert, these data reveal disparate, but overlapping, functions of Ets transcription factors at distinct stages of megakaryocyte maturation.

The A2B adenosine receptor protects against inflammation and excessive vascular adhesion

Adenosine has been described as playing a role in the control of inflammation, but it has not been certain which of its receptors mediate this effect. Here, we generated an A2B adenosine receptor-knockout/reporter gene-knock-in (A2BAR-knockout/reporter gene-knock-in) mouse model and showed receptor gene expression in the vasculature and macrophages, the ablation of which causes low-grade inflammation compared with age-, sex-, and strain-matched control mice. Augmentation of proinflammatory cytokines, such as TNF-alpha, and a consequent downregulation of IkappaB-alpha are the underlying mechanisms for an observed upregulation of adhesion molecules in the vasculature of these A2BAR-null mice. Intriguingly, leukocyte adhesion to the vasculature is significantly increased in the A2BAR-knockout mice. Exposure to an endotoxin results in augmented proinflammatory cytokine levels in A2BAR-null mice compared with control mice. Bone marrow transplantations indicated that bone marrow (and to a lesser extent vascular) A2BARs regulate these processes. Hence, we identify the A2BAR as a new critical regulator of inflammation and vascular adhesion primarily via signals from hematopoietic cells to the vasculature, focusing attention on the receptor as a therapeutic target.

Properties of ets-1 binding to chromatin and its effect on platelet factor 4 gene expression

Ets-1 is important for transcriptional regulation in several hematopoietic lineages, including megakaryocytes. Some transcription factors bind to naked DNA and chromatin with different affinities, while others do not. In the present study we used the megakaryocyte-specific promoters platelet factor 4 (PF4), and glycoprotein IIb (GPIIb) as model systems to explore the properties of Ets-1 binding to chromatin. Chromatin immunoprecipitation assays indicated that Ets-1 binds to proximal regions in the PF4 and GPIIb promoters in vivo. In vitro and in vivo experiments showed that Ets-1 binding to chromatin on lineage-specific promoters does not require lineage-specific factors. Moreover, this binding shows the same order of affinity as the binding to naked DNA and does not require ATP-dependent or Sarkosyl-sensitive factors. The effect of Ets-1 binding on promoter activity was examined using the PF4 promoter as a model. We identified a novel Ets-1 site (at -50), and a novel Sarkosyl-sensitive DNase I-hypersensitive site generated by Ets-1 binding to chromatin, which significantly affect PF4 promoter activity. Taken together, our results suggest a model by which Ets-1 binds to chromatin without the need for lineage-specific accessory factors, and Ets-1 binding induces changes in chromatin and affects transactivation, which are essential for PF4 promoter activation.

Control of megakaryocyte-specific gene expression by GATA-1 and FOG-1: role of Ets transcription factors

The transcription factor GATA-1 and its cofactor FOG-1 are essential for the normal development of erythroid cells and megakaryocytes. FOG-1 can stimulate or inhibit GATA-1 activity depending on cell and promoter context. How the GATA-1-FOG-1 complex controls the expression of distinct sets of gene in megakaryocytes and erythroid cells is not understood. Here, we examine the molecular basis for the megakaryocyte-restricted activation of the alphaIIb gene. FOG-1 stimulates GATA-1-dependent alphaIIb gene expression in a manner that requires their direct physical interaction. Transcriptional output by the GATA-1-FOG-1 complex is determined by the hematopoietic Ets protein Fli-1 that binds to an adjacent Ets element. Chromatin immunoprecipitation experiments show that GATA-1, FOG-1 and Fli-1 co-occupy the alphaIIb promoter in vivo. Expression of several additional megakaryocyte-specific genes that bear tandem GATA and Ets elements in their promoters also depends on the physical interaction between GATA-1 and FOG-1. Our studies define a molecular context for transcriptional activation by GATA-1 and FOG-1, and may explain the occurrence of tandem GATA and Ets elements in the promoters of numerous megakaryocyte-expressed genes.

A role for cyclin D3 in the endomitotic cell cycle

Platelets, essential for thrombosis and hemostasis, develop from polyploid megakaryocytes which undergo endomitosis. During this cell cycle, cells experience abrogated mitosis and reenter a phase of DNA synthesis, thus leading to endomitosis. In the search for regulators of the endomitotic cell cycle, we have identified cyclin D3 as an important regulatory factor. Of the D-type cyclins, cyclin D3 is present at high levels in megakaryocytes undergoing endomitosis and is markedly upregulated following exposure to the proliferation-, maturation-, and ploidy-promoting factor, Mpl ligand. Transgenic mice in which cyclin D3 is overexpressed in the platelet lineage display a striking increase in endomitosis, similar to changes seen following Mpl ligand administration to normal mice. Electron microscopy analysis revealed that unlike such treated mice, however, D3 transgenic mice show a poor development of demarcation membranes, from which platelets are believed to fragment, and no increase in platelets. Thus, while our model supports a key role for cyclin D3 in the endomitotic cell cycle, it also points to the unique role of Mpl ligand in priming megakaryocytes towards platelet fragmentation. The role of cyclin D3 in promoting endomitosis in other lineages programmed to abrogate mitosis will need further exploration.

p45 NF-E2 regulates expression of thromboxane synthase in megakaryocytes

Transcription factor p45 NF-E2 is highly expressed in the erythroid and megakaryocytic lineages. Although p45 recognizes regulatory regions of several erythroid genes, mice deficient for this protein display only mild dyserythropoiesis but have abnormal megakaryocytes and lack circulating platelets. A number of megakaryocytic marker genes have been extensively studied, but none of them is regulated by NF-E2. To find target genes for p45 NF-E2 in megakaryopoiesis, we used an in vivo immunoselection assay: genomic fragments bound to p45 NF-E2 in the chromatin of a megakaryocytic cell line were immunoprecipitated with an anti-p45 antiserum and cloned. One of these fragments belongs to the second intron of the thromboxane synthase gene (TXS). We demonstrate that the TXS gene, which is mainly expressed in megakaryocytes, is indeed directly regulated by p45 NF-E2. First, its promoter contains a functional NF-E2 binding site; second, the intronic NF-E2 binding site is located within a chromatin-dependent enhancer element; third, p45-null murine megakaryocytes do not express detectable TXS mRNA, although TXS expression can be detected in other cells. These data, and the structure of the TXS promoter and enhancer, suggest that TXS belongs to a distinct subgroup of genes involved in platelet formation and function.

The proximal element of the human apolipoprotein A-II promoter increases the enhancer activity of the distal region

We have previously shown that human apolipoprotein A-II (apoA-II) transcription is controlled by a complex set of regulatory elements. In this study, we demonstrate that the distal region of the apoA-II promoter (-911/-614) acts as an enhancer and results in a 6-fold increase in activity when the proximal AB element is inserted between this enhancer and a TATA box. The AB element alone does not display any transcriptional activity. The combination of the proximal AB element and the enhancer is sufficient to activate transcription to the same level as that achieved with the full-length promoter. DNA binding and competition assays, and binding interference experiments allowed us to identify two adjacent binding sites within the AB element. These bind activities designated CIIIB1 and AIIAB3/4, respectively. Mutation on each of these sites showed that the binding site of CIIIB1 within the AB element played a major role in the interaction with the enhancer. Whereas transcriptional activation of other apolipoprotein genes involves the binding of the liver-enriched hepatocyte nuclear factor 4 (HNF4) on their proximal promoter, the present study demonstrates that neither HNF4 nor ApoA-I regulatory protein 1, its antagonistic orphan receptor, was able to bind the AB element. Instead, apoA-II transcription was driven by the interaction of apoA-II enhancer with proximal AB element, which involves an unidentified activity, CIIIB1.

Tissue-specific regulation of the rabbit 15-lipoxygenase gene in erythroid cells by a transcriptional silencer

The 15-lipoxygenase (lox) gene is expressed in a tissue-specific manner, predominantly in erythroid cells but also in airway epithelial cells and eosinophils. We demonstrate in this report that the 5' flanking DNA of the 15-lox gene contains sequences which down-regulate its activity in a variety of non-erythroid cell lines but not in two erythroid cell lines. The element has characteristics of a transcriptional 'silencer' since it functions in both orientations. The main activity of the silencer has been mapped to the first 900 bp of 5' flanking DNA, which contains nine binding sites for a nuclear factor present in non-erythroid cells but not in erythroid cells. These binding sites have similar sequences and multiple copies of the binding sites confer tissue-specific down-regulation when attached to a minimal lox promoter fragment. The 5' flanking DNA also contains a cluster of three binding sites for the GATA family of transcription factors.

The tsA58 simian virus 40 large tumor antigen disrupts megakaryocyte differentiation in transgenic mice

Thrombocytopenia is a condition of multiple etiologies affecting the megakaryocyte lineage. To perturb this lineage in transgenic mice, the tsA58 mutation of the simian virus 40 large tumor antigen was targeted to megakaryocytes using the platelet factor 4 promoter. Ten of 17 transgenic lines generated exhibited low platelet levels, each line displaying a distinct, heritable level of thrombocytopenia. Within a line, the degree of the platelet reduction correlated directly with transgene zygosity. The platelet level could be further reduced by the inactivation of one copy of the endogenous retinoblastoma gene. Western blot analysis detected large tumor antigen protein in the most severely affected lines; less affected lines were below the level of detection. Platelets and megakaryocytes from thrombocytopenic mice exhibited morphological abnormalities. Mice with either normal or reduced platelet levels developed megakaryocytic malignancies with a mean age of onset of about 8 months. There was no correlation between severity of thrombocytopenia and onset of malignancy. These mice provide a defined genetic model for thrombocytopenia, and for megakaryocytic neoplasia, and implicate the retinoblastoma protein in the process of megakaryocyte differentiation.

Ligation-mediated amplification of RNA from murine erythroid cells reveals a novel class of beta globin mRNA with an extended 5'-untranslated region

Ligation-mediated RNA amplification was developed as a tool for analysis and determination of the termini of RNA molecules [Volloch et al. (1991) Proc. Natl. Acad. Sci. USA 88: 10671-10675]. In this approach, T4 RNA ligase is used to join cellular RNA with a defined ribo-oligonucleotide. Although several additional enzymatic steps are involved in this type of analysis, the reliability of the entire procedure is determined by the initial ligation step, which marks and preserves the termini of cellular RNA molecules. We applied this approach to the analysis of the 5' terminus of beta globin mRNA in various murine erythroid cells. As expected, we detected RNA molecules with 5' ends terminating at the regular cap site as well as globin RNA molecules truncated at the 5' end. Unexpectedly, we also detected a class of beta globin mRNA which is identical to regular beta globin mRNA in every respect but contains 17, 29, or 31 additional nucleotides 5' to the regular cap site. These extensions correspond precisely to the genomic segments just upstream of the regular cap site and are probably generated by initiation of transcription of the globin gene upstream from the regular cap site. It is likely that the extended globin RNA is transcribed not from the TATA promoter, which regulates the transcription of regular murine globin mRNA, but from the GATA regulatory element located 30 nucleotides upstream of the 31-nucleotide extension, in a position identical to that of the active GATA promoter of the TATA-less chicken beta globin gene. The evolutionary conservation of this relationship suggests the importance of the GATA promoter element of the mouse beta globin gene and its possible involvement in developmental regulation of expression of this gene.

Endothelial-cell-specific regulation of von Willebrand factor gene expression

In both tissue sections and cell culture, the endothelial nature of a cell is most commonly determined by demonstration of its expression of von Willebrand factor (vWf) protein and/or mRNA. Thus, the mechanism of cell-type-specific transcriptional regulation of the vWf gene is central to studying the basis of endothelial-cell-specific gene expression. In this study, deletion analyses were carried out to identify the region of the vWf gene which regulates its endothelial-cell-specific expression. A 734-bp fragment which spans the sequence from -487 to +247 relative to the transcription start site was identified as the cell-type-specific promoter. It consists of a minimal core promoter located between -90 and +22, a strong negative regulatory element located upstream of the core promoter (ca. -500 to -300), and a positive regulatory region located downstream of the core promoter in the first exon. The activity of the core promoter is not cell type specific, and the negative regulatory region is required to inhibit its activity in all cell types. The positive regulatory region relieves this inhibition only in endothelial cells and results in endothelial-cell-specific gene expression. The positive regulatory region contains sequences predicting possible SP1, GATA, and octamer binding sites. Mutations in either the SP1 or octamer sequence have no effect on transcriptional activity, while mutation in the GATA binding element totally abolishes the promoter activity. Evidence that a GATA factor is involved in this interaction is presented. Thus, the positive regulatory region with an intact GATA binding site is required to overcome the inhibitory effect of the negative regulatory element and activate vWf gene expression in an endothelial-cell-specific manner.

Endothelial-cell-specific regulation of von Willebrand factor gene expression

In both tissue sections and cell culture, the endothelial nature of a cell is most commonly determined by demonstration of its expression of von Willebrand factor (vWf) protein and/or mRNA. Thus, the mechanism of cell-type-specific transcriptional regulation of the vWf gene is central to studying the basis of endothelial-cell-specific gene expression. In this study, deletion analyses were carried out to identify the region of the vWf gene which regulates its endothelial-cell-specific expression. A 734-bp fragment which spans the sequence from -487 to +247 relative to the transcription start site was identified as the cell-type-specific promoter. It consists of a minimal core promoter located between -90 and +22, a strong negative regulatory element located upstream of the core promoter (ca. -500 to -300), and a positive regulatory region located downstream of the core promoter in the first exon. The activity of the core promoter is not cell type specific, and the negative regulatory region is required to inhibit its activity in all cell types. The positive regulatory region relieves this inhibition only in endothelial cells and results in endothelial-cell-specific gene expression. The positive regulatory region contains sequences predicting possible SP1, GATA, and octamer binding sites. Mutations in either the SP1 or octamer sequence have no effect on transcriptional activity, while mutation in the GATA binding element totally abolishes the promoter activity. Evidence that a GATA factor is involved in this interaction is presented. Thus, the positive regulatory region with an intact GATA binding site is required to overcome the inhibitory effect of the negative regulatory element and activate vWf gene expression in an endothelial-cell-specific manner.

Targeted expression of a conditional oncogene in hematopoietic cells of transgenic mice

We have produced two lines of transgenic mice in which the expression of temperature-sensitive SV-40 large T antigen is targeted to bone marrow megakaryocytes via the platelet factor 4 (PF4) tissue-specific promoter. The progeny of these transgenic mice were observed for about 3 mo, and no malignancies were detected over this period of time. The offspring of these transgenic mice, 6- to 12-wk of age, served as a source of bone marrow cells, which upon in vitro cultivation at the permissive temperature yielded immortalized cell lines (MegT). At the permissive temperature, MegT cells exhibit the characteristics of early 2N and 4N megakaryocytes which include the presence of specific gene products such as PF4, glycoprotein IIb, acetylcholinesterase, and CD45 as well as the absence of molecular markers of other cell lineages such as the macrophage marker Mac-1, the T helper cell marker CD4, the mast cell marker IgE, the T cell marker CD2 or the erythroid cell marker alpha-globin. The inactivation of the oncogene by a shift of temperature from 34 degrees to 39.5 degrees C produces a reduction in the frequency of the 2N cells, in conjunction with the appearance of 8N and 16N cells, consisting of 27 and 3% of total cells, respectively. Thus, we have generated hematopoietic cell lines that are trapped in the early stages of megakaryocyte commitment, but able to undergo part of the normal program of terminal differentiation.

C/EBP, NF-kappa B, and c-Ets family members and transcriptional regulation of the cell-specific and inducible macrophage inflammatory protein 1 alpha immediate-early gene

Macrophage inflammatory protein 1 alpha (MIP-1 alpha) cytokine gene expression is restricted to a limited number of cells of hemopoietic origin and is rapidly and transiently induced by serum and endotoxin in macrophages. A single nuclear DNase I-hypersensitive site, which maps to the proximal promoter of the MIP-1 alpha gene, was identified in macrophage cells but was absent in cells which do not express basal levels of MIP-1 alpha mRNA. The proximal promoter sequences (+36 to -220 bp) are sufficient to confer cell-specific and inducible transcription in transfection assays. In vitro DNA-binding studies revealed five major nuclear protein binding sites in the proximal promoter which bind C/EBP, NF-kappa B, and/or c-Ets family members. Cell-specific differences in DNA binding by members of the NF-kappa B and c-Ets families correlate with the cell-specificity of MIP-1 alpha gene expression and the chromosomal conformation of the promoter. Changes in promoter binding by members of the C/EBP and NF-kappa B families correlate with the transcriptional up-regulation observed in serum- or endotoxin-stimulated macrophages in functional studies.

C/EBP, NF-kappa B, and c-Ets family members and transcriptional regulation of the cell-specific and inducible macrophage inflammatory protein 1 alpha immediate-early gene

Macrophage inflammatory protein 1 alpha (MIP-1 alpha) cytokine gene expression is restricted to a limited number of cells of hemopoietic origin and is rapidly and transiently induced by serum and endotoxin in macrophages. A single nuclear DNase I-hypersensitive site, which maps to the proximal promoter of the MIP-1 alpha gene, was identified in macrophage cells but was absent in cells which do not express basal levels of MIP-1 alpha mRNA. The proximal promoter sequences (+36 to -220 bp) are sufficient to confer cell-specific and inducible transcription in transfection assays. In vitro DNA-binding studies revealed five major nuclear protein binding sites in the proximal promoter which bind C/EBP, NF-kappa B, and/or c-Ets family members. Cell-specific differences in DNA binding by members of the NF-kappa B and c-Ets families correlate with the cell-specificity of MIP-1 alpha gene expression and the chromosomal conformation of the promoter. Changes in promoter binding by members of the C/EBP and NF-kappa B families correlate with the transcriptional up-regulation observed in serum- or endotoxin-stimulated macrophages in functional studies.

GATA and Ets cis-acting sequences mediate megakaryocyte-specific expression

The human glycoprotein IIB (GPIIB) gene is expressed only in megakaryocytes, and its promoter displays cell type specificity. We show that this specificity involved two cis-acting sequences. The first one, located at -55, contains a GATA binding site. Point mutations that abolish protein binding on this site decrease the activity of the GPIIB promoter but do not affect its tissue specificity. The second one, located at -40, contains an Ets consensus sequence, and we show that Ets-1 or Ets-2 protein can interact with this -40 GPIIB sequence. Point mutations that impair Ets binding decrease the activity of the GPIIB promoter to the same extent as do mutations that abolish GATA binding. A GPIIB 40-bp DNA fragment containing the GATA and Ets binding sites can confer activity to a heterologous promoter in megakaryocytic cells. This activity is independent of the GPIIB DNA fragment orientation, and mutations on each binding site result in decreased activity. Using cotransfection assays, we show that c-Ets-1 and human GATA1 can transactive the GPIIB promoter in HeLa cells and can act additively. Northern (RNA) blot analysis indicates that the ets-1 mRNA level is increased during megakaryocyte-induced differentiation of erythrocytic/megakaryocytic cell lines. Gel retardation assays show that the same GATA-Ets association is found in the human GPIIB enhancer and the rat platelet factor 4 promoter, the other two characterized regulatory regions of megakaryocyte-specific genes. These results indicate that GATA and Ets cis-acting sequences are an important determinant of megakaryocytic specific gene expression.

GATA and Ets cis-acting sequences mediate megakaryocyte-specific expression

The human glycoprotein IIB (GPIIB) gene is expressed only in megakaryocytes, and its promoter displays cell type specificity. We show that this specificity involved two cis-acting sequences. The first one, located at -55, contains a GATA binding site. Point mutations that abolish protein binding on this site decrease the activity of the GPIIB promoter but do not affect its tissue specificity. The second one, located at -40, contains an Ets consensus sequence, and we show that Ets-1 or Ets-2 protein can interact with this -40 GPIIB sequence. Point mutations that impair Ets binding decrease the activity of the GPIIB promoter to the same extent as do mutations that abolish GATA binding. A GPIIB 40-bp DNA fragment containing the GATA and Ets binding sites can confer activity to a heterologous promoter in megakaryocytic cells. This activity is independent of the GPIIB DNA fragment orientation, and mutations on each binding site result in decreased activity. Using cotransfection assays, we show that c-Ets-1 and human GATA1 can transactive the GPIIB promoter in HeLa cells and can act additively. Northern (RNA) blot analysis indicates that the ets-1 mRNA level is increased during megakaryocyte-induced differentiation of erythrocytic/megakaryocytic cell lines. Gel retardation assays show that the same GATA-Ets association is found in the human GPIIB enhancer and the rat platelet factor 4 promoter, the other two characterized regulatory regions of megakaryocyte-specific genes. These results indicate that GATA and Ets cis-acting sequences are an important determinant of megakaryocytic specific gene expression.

Functional analysis of the human vascular cell adhesion molecule 1 promoter

The vascular cell adhesion molecule 1 (VCAM-1) is a 110-kD member of the immunoglobulin gene superfamily expressed on the surface of interleukin 1 beta- or tumor necrosis factor alpha (TNF)-stimulated endothelial cells. The cell surface protein functions as an inducible adhesion receptor for circulating mononuclear leukocytes and some tumor cells. We have previously characterized the genomic organization of the VCAM1 gene and described its chromosomal localization. In this report, the promoter of the VCAM1 gene is characterized. New transcription of the VCAM1 gene occurred when endothelial cells were treated with TNF. Fusion plasmids containing the 5' flanking sequence of the VCAM1 gene and the chloramphenicol acetyltransferase reporter gene were used to identify cis-acting sequences that direct the cytokine-induced transcription. When transfected into bovine aortic endothelial cells, constructs containing 755 bp of the 5' flanking sequence were induced by TNF. Within the cytokine-responsive region of the core promoter were functional NF-kappa B and GATA elements. Upstream of the core promoter, the VCAM1 5' flanking sequence contained a negative regulatory activity. NF-kappa B-mediated activation of VCAM1 gene expression may lead to endothelial expression of a mononuclear leukocyte adhesion molecule associated with initial events in the development of an atherosclerotic lesion.

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.