Thrombopoietin-induced expression of the glycoprotein IIb gene involves the transcription factor PU.1/Spi-1 in UT7-Mpl cells

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.

Gene profiling of the erythro- and megakaryoblastic leukaemias induced by the Graffi murine retrovirus

Background

Acute erythro- and megakaryoblastic leukaemias are associated with very poor prognoses and the mechanism of blastic transformation is insufficiently elucidated. The murine Graffi leukaemia retrovirus induces erythro- and megakaryoblastic leukaemias when inoculated into NFS mice and represents a good model to study these leukaemias.

Methods

To expand our understanding of genes specific to these leukaemias, we compared gene expression profiles, measured by microarray and RT-PCR, of all leukaemia types induced by this virus.

Results

The transcriptome level changes, present between the different leukaemias, led to the identification of specific cancerous signatures. We reported numerous genes that may be potential oncogenes, may have a function related to erythropoiesis or megakaryopoiesis or have a poorly elucidated physiological role. The expression pattern of these genes has been further tested by RT-PCR in different samples, in a Friend erythroleukaemic model and in human leukaemic cell lines.

We also screened the megakaryoblastic leukaemias for viral integrations and identified genes targeted by these integrations and potentially implicated in the onset of the disease.

Conclusions

Taken as a whole, the data obtained from this global gene profiling experiment have provided a detailed characterization of Graffi virus induced erythro- and megakaryoblastic leukaemias with many genes reported specific to the transcriptome of these leukaemias for the first time.

Historical review: megakaryopoiesis and thrombopoiesis

The study of thrombopoiesis has evolved greatly since an era when platelets were termed "the dust of the blood," only about 100 years ago. During this time megakaryocytes were identified as the origin of blood platelets; marrow-derived megakaryocytic progenitor cells were functionally defined and then purified; and the primary regulator of the process, thrombopoietin, was cloned and characterized and therapeutic thrombopoietic agents developed. During this journey we continue to learn that the physiologic mechanisms that drive proplatelet formation can be recapitulated in cell-free systems and their biochemistry evaluated; the molecular underpinnings of endomitosis are being increasingly understood; the intracellular signals sent by engagement of a large number of megakaryocyte surface receptors have been defined; and many of the transcription factors that drive megakaryocytic fate determination have been identified and experimentally manipulated. While some of these biologic processes mimic those seen in other cell types, megakaryocytes and platelets possess enough unique developmental features that we are virtually assured that continued study of thrombopoiesis will yield innumerable clinical and scientific insights for many decades to come.

Tescalcin is an essential factor in megakaryocytic differentiation associated with Ets family gene expression

We show here that the process of megakaryocytic differentiation requires the presence of the recently discovered protein tescalcin. Tescalcin is dramatically upregulated during the differentiation and maturation of primary megakaryocytes or upon PMA-induced differentiation of K562 cells. This upregulation requires sustained signaling through the ERK pathway. Overexpression of tescalcin in K562 cells initiates events of spontaneous megakaryocytic differentiation, such as expression of specific cell surface antigens, inhibition of cell proliferation, and polyploidization. Conversely, knockdown of this protein in primary CD34+ hematopoietic progenitors and cell lines by RNA interference suppresses megakaryocytic differentiation. In cells lacking tescalcin, the expression of Fli-1, Ets-1, and Ets-2 transcription factors, but not GATA-1 or MafB, is blocked. Thus, tescalcin is essential for the coupling of ERK cascade activation with the expression of Ets family genes in megakaryocytic differentiation.

Differentiation of murine committed megakaryocytic progenitors isolated by a novel strategy reveals the complexity of GATA and Ets factor involvement in megakaryocytopoiesis and an unexpected potential role for GATA-6

OBJECTIVE

The differentiation of megakaryocytes is characterized by polyploidization and cytoplasmic maturation leading to platelet production. Studying these processes is hindered by the paucity of bone marrow megakaryocytes and their precursors. We describe a method for the expansion and purification of committed megakaryocyte progenitors and demonstrate their usefulness by studying changes in the expression of Ets and GATA family transcription factors throughout megakaryocytopoiesis.

METHODS

A two-step serum-free method was developed. Cells isolated using this method were analyzed for surface marker expression by flow cytometry, and for their ability to differentiate using single-cell culture. Purified progenitors were induced to differentiate and analyzed with respect to their ploidy by flow cytometry and expression of specific genes by RT-PCR.

RESULTS

A population of Lin- c-kit+ CD45+ CD41+ CD31+ CD34low CD9low FcgammaRII/IIIlow Sca-1med/low committed megakaryocyte progenitors was purified. These cells could be differentiated efficiently, achieving ploidy of up to 128N. Analysis of RNA demonstrated the expected increases in expression of key megakaryocyte-associated genes. RT-PCR analysis also revealed that a range of Ets and GATA factors are expressed, their individual levels and patterns of expression varying widely. Surprisingly, we find that GATA-6 is specifically expressed in late differentiated megakaryocytes and has the potential to regulate megakaryocyte-expressed genes in cooperation with Ets factors.

CONCLUSION

Purified primary megakaryocytic progenitors are able to differentiate as a cohort into fully mature megakaryocytes. The number of cells obtainable, and the synchrony of the differentiation process, facilitates analysis of the dynamics of molecular processes involved in megakaryocytopoiesis. The expression pattern of Ets and GATA family transcription factors reveals the complexity of the involvement of these key megakaryocytic regulators. The finding of GATA-6 expression and demonstration of its functional activity suggests a novel mechanism for the regulation of certain genes late in megakaryocytopoiesis.

Hepatocyte growth factor and inducible nitric oxide synthase are involved in multidrug resistance-induced angiogenesis in hepatocellular carcinoma cell lines

Based on literature, it is possible to hypothesize that multidrug resistance (MDR) and angiogenic phenotypes are linked to each other in human liver cancer cells. Our goal is to assess whether MDR cells trigger angiogenesis and to study the possible molecular mechanisms involved. Conditioned medium from parental drug-sensitive P5 cells (P5-CM) and MDR-positive P1(0.5) cells [P1(0.5)-CM] stimulated human umbilical vein endothelial cells (HUVEC) survival, proliferation, migration, and microtubular structure formation, but P1(0.5)-CM had a significantly greater effect than P5-CM. Cell implants were done in the rabbit avascular cornea to measure angiogenesis in vivo: P1(0.5) cells induced an important neovascular response in rabbit cornea after 1 week, whereas P5 cells had no effect. P1(0.5) and P5 cells produced vascular endothelial growth factor, but only P1(0.5) secreted hepatocyte growth factor (HGF) into the medium, and small interfering RNA specific for MDR1 clearly reduced HGF production in P1(0.5) cells. The transcription factor Ets-1 and the HGF receptor c-Met were up-regulated in P1(0.5) cells and in HUVEC cultured in P1(0.5)-CM. Inducible nitric oxide synthase (iNOS) seemed to play a major role in the proangiogenic effect of P1(0.5), and its inhibition by 1400W blunted the capacity of P1(0.5) cells to stimulate HUVEC proliferation, migration, and Ets-1 expression. In conclusion, these data show that development of MDR and angiogenic phenotypes are linked to each other in MDR cells. HGF production, Ets-1 and c-Met up-regulation, and iNOS expression can be part of the molecular mechanisms that enhance the angiogenic activity of the MDR-positive hepatocellular carcinoma cell line.

Selective modification of eukaryotic initiation factor 4F (eIF4F) at the onset of cell differentiation: recruitment of eIF4GII and long-lasting phosphorylation of eIF4E

mRNA translation is mainly regulated at the level of initiation, a process that involves the synergistic action of the 5' cap structure and the 3' poly(A) tail at the ends of eukaryotic mRNA. The eukaryote initiation factor 4G(eIF4G) is a pivotal scaffold protein that forms a critical link between mRNA cap structure, poly(A) tail, and the small ribosomal subunit. There are two functional homologs of eIF4G in mammals, the original eIF4G, renamed eIF4GI, and eIF4GII that functionally complements eIF4GI. To date, biochemical and functional analysis have not identified differential activities for eIF4GI and eIF4GII. In this report, we demonstrate that eIF4GII, but not eIF4GI, is selectively recruited to capped mRNA at the onset of cell differentiation. This recruitment is coincident with a strong and long-lasting phosphorylation of eIF4E and the release of 4E-BP1, a suppressor of eIF4E function, from the cap structure, without a concomitant change in 4E-BP1's phosphorylation. Our data further indicate that cytokines such as thrombopoietin can differentially regulate eIF4GI/II activities. These results provide the first evidence that eIF4GI/II does fulfill selective roles in mammalian cells.

Extracellular signal-regulated kinase induces the megakaryocyte GPIIb/CD41 gene through MafB/Kreisler

Extracellular signal-regulated kinase (ERK) facilitates cell cycle progression in most mammalian cells, but in certain cell types prolonged signaling through this pathway promotes differentiation and lineage-specific gene expression through mechanisms that are poorly understood. Here, we characterize the transcriptional regulation of platelet GPIIb integrin (CD41) by ERK during megakaryocyte differentiation. ERK-dependent transactivation involves the proximal promoter of GPIIb within 114 bp upstream of the transcriptional start site. GATA, Ets, and Sp1 consensus sequences within this region are each necessary and function combinatorially in ERK-activated transcription. MafB/Kreisler is induced in response to ERK and synergizes with GATA and Ets to enhance transcription from the proximal promoter. The requirement for MafB in promoter regulation is demonstrated by inhibition of transactivation following dominant-negative or antisense suppression of MafB function. Thus, ERK promotes megakaryocyte differentiation by coordinate regulation of nuclear factors that synergize in GPIIb promoter regulation. These results establish a novel role for MafB as a regulator of ERK-induced gene expression.

STAT5 and Oct-1 form a stable complex that modulates cyclin D1 expression

Signal transducer and activator of transcription 5 (STAT5) is activated by numerous cytokines that control blood cell development. STAT5 was also shown to actively participate in leukemogenesis. Among the target genes involved in cell growth, STAT5 had been shown to activate cyclin D1 gene expression. We now show that thrombopoietin-dependent activation of the cyclin D1 promoter depends on the integrity of a new bipartite proximal element that specifically binds STAT5A and -B transcription factors. We demonstrate that the stable recruitment of STAT5 to this element in vitro requires the integrity of an adjacent octamer element that constitutively binds the ubiquitous POU homeodomain protein Oct-1. We observe that cytokine-activated STAT5 and Oct-1 form a unique complex with the cyclin D1 promoter sequence. We find that STAT5 interacts with Oct-1 in vivo, following activation by different cytokines in various cellular contexts. This interaction involves a small motif in the carboxy-terminal region of STAT5 which, remarkably, is similar to an Oct-1 POU-interacting motif present in two well-known partners of Oct-1, namely, OBF-1/Bob and SNAP190. Our data offer new insights into the transcriptional regulation of the key cell cycle regulator cyclin D1 and emphasize the active roles of both STAT5 and Oct-1 in this process.

Homeodomain proteins MEIS1 and PBXs regulate the lineage-specific transcription of the platelet factor 4 gene

Platelet factor 4 (PF4) is expressed during megakaryocytic differentiation. We previously reported that GATA-1 and ETS-1 regulate the rat PF4 promoter and transactivate the PF4 gene. For the present study, we investigated the regulatory elements and their transcription factors responsible for the lineage-specific expression of the PF4 gene. The promoter activities of deletion constructs were evaluated, and a novel regulatory element termed TME (tandem repeat of MEIS1 binding element) (-219 to -182) was defined. Binding proteins to TME were strongly detected in HEL nuclear extracts by electrophoresis mobility shift assay (EMSA), and they were purified by DNA affinity chromatography. By performing Western blottings and supershift assays, the binding proteins were identified as homeodomain proteins, MEIS1, PBX1B, and PBX2. These factors are expressed in megakaryocytes differentiated from CD34+ cells in human cord blood. MEIS1 and PBXs bind to the TME as MEIS1/PBX complexes and activate the PF4 promoter. In nonmegakaryocytic HepG2 cells, GATA-1 and ETS-1 activate the PF4 promoter approximately 10-fold. Surprisingly, we found that additional expression of both MEIS1 and PBX2 multiplied this major activation another 2-fold. This activation was not observed when MEIS1 binding sites in the TME were disrupted. Furthermore, inhibition of the binding of endogenous MEIS1/PBX complexes to the TME decreased the promoter activity by almost one half, in megakaryocytic HEL cells. Thus, these studies demonstrate that the homeodomain proteins, MEIS1, PBX1B, and PBX2, play an important role in megakaryocytic gene expression.

Cloning and analysis of the thrombopoietin-induced megakaryocyte-specific glycoprotein VI promoter and its regulation by GATA-1, Fli-1, and Sp1

The exposure of collagen fibers at sites of vascular injury results in the adherence of platelets and their subsequent activation. The platelet collagen receptor glycoprotein (GP)(1) VI plays a crucial role in platelet activation and thrombus formation and decreased levels or defective GPVI may lead to excessive bleeding. In addition, elevated levels of collagen receptors may predispose individuals to coronary heart disease or strokes. GPVI expression is restricted to platelets and their precursor cell, the megakaryocyte. In this study we investigate the regulation of GPVI expression and show that thrombopoietin induces its expression in the megakaryocytic cell line UT-7/TPO. A 5'-region flanking the transcription start point of the GPVI gene was cloned (-694 to +29) and we report that this putative GPVI promoter bestows megakaryocye-specific expression. Deletion analyses and site-directed mutagenesis identified Sp1(227), GATA(177), and Ets(48) sites as essential for GPVI expression. We show that transcription factors GATA-1, Fli-1, and Sp1 can bind to and activate this promoter. Finally, GPVI mRNA was detected only in megakaryocytic cell lines expressing both Fli-1 and GATA-1, and we show that overexpression of Fli-1 in a stable cell line (which expresses endogenous GATA-1 and Sp1) results in expression of the endogenous GPVI gene.

Elevated expression of integrin alpha(IIb) beta(IIIa) in drug-naïve, first-episode schizophrenic patients

BACKGROUND

Patients with schizophrenia have an increased risk over the general public of developing cardiovascular illness. It is unknown if there are functional changes in platelet surface receptors in schizophrenia. We therefore analyzed the surface expression of glycoprotein (GP)Ib, the integrin receptor alpha(IIb)beta(IIIa), CD62 (P-selectin), and CD63, and investigated platelet function in schizophrenic patients compared with healthy volunteers.

METHODS

Nineteen drug-naive, first-episode patients with a DSM IV diagnosis of paranoid schizophrenia were compared with matched healthy controls. Flow cytometry was used to assess platelet surface expression levels of GPIb, alpha(IIb)beta(IIIa), CD62, and CD63. Adenosine diphosphate-induced platelet aggregation was assayed.

RESULTS

The schizophrenic patients had a significantly (p < .0001) increased number of 68,145 +/- 8,260.1 alpha(IIb)beta(IIIa) receptors, platelet compared with 56,235 +/- 8,079.4 receptors, platelet in healthy controls.

CONCLUSIONS

Patients with schizophrenia have increased platelet expression of alpha(IIb)beta(IIIa), which may contribute to their increased risk of cardiovascular illness compared with the general population.

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 selective effect of Mpl ligand on mRNA stabilization during megakaryocyte differentiation

Megakaryocytes, the platelet precursors, are induced to differentiate in response to Mpl ligand. Here we report that stability of the megakaryocyte-specific platelet factor 4 (PF4) mRNA is substantially augmented in the presence of Mpl ligand. This stabilization requires protein synthesis, but the 3'-untranslated region of PF4 mRNA is not sufficient for granting the effect. This cytokine also significantly or mildly stabilizes Mpl receptor or GAPDH mRNAs, respectively, in contrast to a previously reported lack of effect on P2Y(1) receptor mRNA. Our study is the first to suggest that Mpl ligand-induced lineage specification is also determined by message stabilization.

Mpl ligand increases P2Y1 receptor gene expression in megakaryocytes with no concomitant change in platelet response to ADP

The P2Y(1) receptor is responsible for the initiation of platelet aggregation in response to ADP and plays a key role in thrombosis. Although this receptor is expressed early in the platelet lineage, the regulation of its expression during megakaryocyte differentiation is unknown. In the mouse megakaryocytic cell line Y10/L8057, we detected P2Y(1) mRNA of three sizes (2.5, 4.4, and 7.4 kb). These cells have previously been shown to respond to Mpl ligand, the pivotal regulator of megakaryocytopoiesis, by increasing their expression of differentiation markers. Mpl ligand enhanced levels of P2Y(1) mRNAs in Y10/L8057 cells and this effect was selective: the same cytokine did not increase levels of A2a adenosine receptor mRNA. Although Mpl ligand did not affect the short half-lives of the P2Y(1) mRNAs, it enhanced transcription of the P2Y(1) gene. It also increased cell size and the number of cell surface P2Y(1) receptors, but not P2Y(1) receptor density. Injection of Mpl ligand into mice up-regulated P2Y(1) receptor mRNAs in megakaryocytes, as shown by in situ hybridization. However, platelets isolated from these mice did not exhibit a higher P2Y(1) receptor density or increased reactivity to ADP. This correlates with the finding that Mpl ligand increases GPIIb mRNA in megakaryocytes but not the density of the protein per platelet. Thus, the enhancement of P2Y(1) receptor expression induced by Mpl ligand in megakaryocytes may be an integral feature of their differentiation, whereas clinical use of this compound might not be associated with platelet hyper-reactivity to ADP.

Localization of distal regulatory domains in the megakaryocyte-specific platelet basic protein/platelet factor 4 gene locus

The genes for the related human (h) chemokines, PBP (platelet basic protein) and PF4 (platelet factor 4), are within 5.3 kilobases (kb) of each other and form a megakaryocyte-specific gene locus. The hypothesis was considered that the PBP and PF4 genes share a common distal regulatory region(s) that leads to their high-level megakaryocyte-specific expression in vivo. This study examined PBP and PF4 expression in transgenic mice using 4 distinct human PBP/PF4 gene locus constructs. These studies showed that within the region studied there was sufficient information to regulate tissue-specific expression of both hPBP and hPF4. Indeed this region contained sufficient DNA information to lead to expression levels of PBP and PF4 comparable to the homologous mouse genes in a position-independent, copy number-dependent fashion. These studies also indicated that the DNA domains that led to this expression were distinct for the 2 genes; hPBP expression is regulated by a region that is 1.5 to 4.4 kb upstream of that gene. Expression of hPF4 is regulated by a region that is either intergenic between the 2 genes or immediately downstream of the hPF4 gene. Comparison of the available human and mouse sequences shows conserved flanking region domains containing potential megakaryocyte-related transcriptional factor DNA-binding sites. Further analysis of these regulatory regions may identify enhancer domains involved in megakaryopoiesis that may be useful in the selective expression of other genes in megakaryocytes and platelets as a strategy for regulating hemostasis, thrombosis, and inflammation. (Blood. 2001;98:610-617)

Different expression of CD41 on human lymphoid and myeloid progenitors from adults and neonates

The glycoprotein (Gp) IIb/IIIa integrin, also called CD41, is the platelet receptor for fibrinogen and several other extracellular matrix molecules. Recent evidence suggests that its expression is much wider in the hematopoietic system than was previously thought. To investigate the precise expression of the CD41 antigen during megakaryocyte (MK) differentiation, CD34(+) cells from cord blood and mobilized blood cells from adults were grown for 6 days in the presence of stem cell factor and thrombopoietin. Two different pathways of differentiation were observed: one in the adult and one in the neonate cells. In the neonate samples, early MK differentiation proceeded from CD34(+)CD41(-) through a CD34(-)CD41(+)CD42(-) stage of differentiation to more mature cells. In contrast, in the adult samples, CD41 and CD42 were co-expressed on a CD34(+) cell. The rare CD34(+)CD41(+)CD42(-) cell subset in neonates was not committed to MK differentiation but contained cells with all myeloid and lymphoid potentialities along with long-term culture initiating cells (LTC-ICs) and nonobese diabetic/severe combined immune-deficient repopulating cells. In the adult samples, the CD34(+)CD41(+)CD42(-) subset was enriched in MK progenitors, but also contained erythroid progenitors, rare myeloid progenitors, and some LTC-ICs. All together, these results demonstrate that the CD41 antigen is expressed at a low level on primitive hematopoietic cells with a myeloid and lymphoid potential and that its expression is ontogenically regulated, leading to marked differences in the surface antigenic properties of differentiating megakaryocytic cells from neonates and adults. (Blood. 2001;97:2023-2030)

Analysis of hematopoietic stem cell reprogramming with toxigenicity

The molecular mechanisms by which a stem cell is committed to individual lineage are largely unknown. Two different models, though not mutually exclusive, are currently debated. The first describes the temporal and hierarchical coordination of lineage-specific transcriptional programs. The second suggests that multilineage genes are expressed in a self-renewing and undifferentiated cell prior to lineage commitment. To challenge these two models in in vivo-appropriate conditions, the expression of an exogenous toxigene was used to create transgenic animals in which an inducible, reversible cell knock-out at a specific stage of differentiation could be achieved. Both additional transgenesis using the megakaryocyte specific alphaIIb promoter and targeted transgenesis were used to express the herpes virus thymidine kinase (tk) gene in the megakaryocytic lineage. When the tk gene was targeted to the locus of the megakaryocyte-specific alphaIIb gene, a typical Glanzman thrombasthenic syndrome was created. Despite this bleeding disorder, the lack of expression of the alphaIIb gene did not affect the development of the mice. In both transgenic and targeted animals, all progenitor cells were sensitive to the effect of the gancyclovir (GCV), both in vivo and ex vivo. Long-term bone marrow cell cultures on stromal layers indicated that most of the very early progenitor cells expressed the enzyme. All the results obtained with this inducible toxic phenotype indicated that genetic programs that are in control of the expression of lineage-specific genes are operative in a totipotent stem cell prior to lineage commitment and strongly support the concept that stem cells express a multilineage transcriptome.

Signal transduction and transcriptional regulation of angiogenesis

When quiescent endothelial cells (ECs) are exposed to angiogenic factor such as VEGF; ECs express proteases to degrade extracellular matrices, migrate, proliferate and form new vessels. However, the molecular mechanism of these events is not fully characterized yet. We are studying the signal transduction and transcriptional regulation of angiogenesis. We investigated the properties of two VEGF receptors, Flt-1 and KDR, by using two newly developed blocking monoclonal antibodies (mAbs), i.e., anti-human Flt-1 mAb and anti-human KDR mAb. VEGF elicited induction of transcription factor Ets-1 in human umbilical vein endothelial cells (HUVECs). This induction was mediated by the KDR/Flt-1 heterodimer and the KDR homodimer. The role of transcription factor Ets-1 in angiogenesis was further clarified. We established both high and low Ets-1 expressing EC lines, and compared angiogenic properties of these cell lines with a parental murine EC line, MSS31. The growth rate was almost identical among three cell lines. It appeared that gene expressions of matrix metalloproteinases (MMP-1, MMP-3, and MMP-9) as well as integrin beta 3 were correlated with the level of Ets-1 expression. As a result, the invasiveness was enhanced in high Ets-1 expressing cells and reduced in low Ets-1 expressing cells compared with parental cells, and high Ets-1 expressing cells made more tube-like structures in type 1 collagen gel. These results indicate that Ets-1 is a principle transcription factor converting ECs to the angiogeneic phenotype.

Ets target genes: past, present and future

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

GATA-1- and FOG-dependent activation of megakaryocytic alpha IIB gene expression

FOG is a multitype zinc finger protein that is essential for megakaryopoiesis, binds to the amino-terminal finger of GATA-1, and modulates the transcription of GATA-1 target genes. Presently investigated are effects of FOG and GATA-1 on the transcription of the megakaryocytic integrin gene, alpha IIb. In GATA-1-deficient FDCER cells (in the presence of endogenous FOG), ectopically expressed GATA-1 activated transcription 3-10-fold both from alpha IIb templates and the endogenous alpha IIb gene. The increased expression of FOG increased reporter construct transcription 30-fold overall. Unexpectedly, alphaIIb gene transcription also was stimulated efficiently upon the ectopic expression in of FOG per se. This occurred in the absence of any detectable expression of GATA-1 and was observed in multiple independent sublines for both the endogenous alpha IIb gene and transfected constructs yet proved to depend largely upon conserved GATA elements 457 and 55 base pairs upstream from the transcriptional start site. In 293 cells, FOG plus GATA-1 but not FOG alone only moderately stimulated alpha IIb transcription, and no direct interactions of FOG with the alpha IIb promoter were detectable. Thus, FOG acts in concert with GATA-1 to stimulate alpha IIb expression but also can act via a GATA-1-independent route, which is proposed to involve additional hematopoietic-restricted cofactors (possibly GATA-2).

Biologic significance of GATA-1 activities in Ras-mediated megakaryocytic differentiation of hematopoietic cell lines

Lineage-specific transcription factors play crucial roles in the development of hematopoietic cells. In a previous study, it was demonstrated that Ras activation was involved in thrombopoietin-induced megakaryocytic differentiation. In this study, constitutive Ras activation by H-rasG12V evoked megakaryocytic maturation of erythroleukemia cell lines F-36P and K562, but not of myeloid cell line 32D cl3 that lacks GATA-1. However, the introduction of GATA-1 led to reprogramming of 32D cl3 toward erythrocytic/megakaryocytic lineage and enabled it to undergo megakaryocytic differentiation in response to H-rasG12V. In contrast, the overexpression of PU.1 and c-Myb changed the phenotype of K562 from erythroid to myeloid/monocytic lineage and rendered K562 to differentiate into granulocytes and macrophages in response to H-rasG12V, respectively. In GATA-1–transfected 32D cl3, the endogenous expression of PU.1 and c-Myb was easily detectable, but their activities were reduced severely. Endogenous GATA-1 activities were markedly suppressed in PU.1-transfected and c-myb–transfected K562. As for the mechanisms of these reciprocal inhibitions, GATA-1 and PU.1 were found to associate through their DNA-binding domains and to inhibit the respective DNA-binding activities of each other. In addition, c-Myb bound to GATA-1 and inhibited its DNA-binding activities. Mutant GATA-1 and PU.1 that retained their own transcriptional activities but could not inhibit the reciprocal partner were less effective in changing the lineage phenotype of 32D cl3 and K562. These results suggested that GATA-1 activities may be crucial for Ras-mediated megakaryocytic differentiation and that its activities may be regulated by the direct interaction with other lineage-specific transcription factors such as PU.1 and c-Myb.

Biologic significance of GATA-1 activities in Ras-mediated megakaryocytic differentiation of hematopoietic cell lines

Lineage-specific transcription factors play crucial roles in the development of hematopoietic cells. In a previous study, it was demonstrated that Ras activation was involved in thrombopoietin-induced megakaryocytic differentiation. In this study, constitutive Ras activation by H-rasG12V evoked megakaryocytic maturation of erythroleukemia cell lines F-36P and K562, but not of myeloid cell line 32D cl3 that lacks GATA-1. However, the introduction of GATA-1 led to reprogramming of 32D cl3 toward erythrocytic/megakaryocytic lineage and enabled it to undergo megakaryocytic differentiation in response to H-rasG12V. In contrast, the overexpression of PU.1 and c-Myb changed the phenotype of K562 from erythroid to myeloid/monocytic lineage and rendered K562 to differentiate into granulocytes and macrophages in response to H-rasG12V, respectively. In GATA-1–transfected 32D cl3, the endogenous expression of PU.1 and c-Myb was easily detectable, but their activities were reduced severely. Endogenous GATA-1 activities were markedly suppressed in PU.1-transfected and c-myb–transfected K562. As for the mechanisms of these reciprocal inhibitions, GATA-1 and PU.1 were found to associate through their DNA-binding domains and to inhibit the respective DNA-binding activities of each other. In addition, c-Myb bound to GATA-1 and inhibited its DNA-binding activities. Mutant GATA-1 and PU.1 that retained their own transcriptional activities but could not inhibit the reciprocal partner were less effective in changing the lineage phenotype of 32D cl3 and K562. These results suggested that GATA-1 activities may be crucial for Ras-mediated megakaryocytic differentiation and that its activities may be regulated by the direct interaction with other lineage-specific transcription factors such as PU.1 and c-Myb.

Identification of a GATA-overlapping sequence within the enhancer of the murine GPIIb promoter that induces transcriptional deregulation in human K562 cells

The human and the murine glycoprotein platelet IIb (GPIIb) promoters are megakaryocyte specific in human and murine cell systems, respectively. Here we show that the murine promoter is, however, highly active when transfected in K562 human cells in which the human promoter is almost inactive. A murine promoter, in which the enhancer element was replaced by the human, retrieves its megakaryocytic specificity in human cell lines. The human and murine GATA-binding sites located in the enhancer region display slight sequence divergence next to the consensus GATA core sequence. Gel shift experiments show that, although the murine and the human GATA sequences both bind GATA-1, the murine sequence alone forms an additional complex (B) not detected with the human sequence. When the murine GATA-containing region is replaced by the human in the context of the murine GPIIb promoter, megakaryocyte specificity is restored in the human cell lines. A G nucleotide 3′ to GATA appears crucial because its substitution abrogates B but not GATA-1 binding and restores megakaryocyte specificity to the murine promoter. Conversely, substitution of the human GATA-1 binding sequence by its murine homologue that binds both GATA-1 and complex B induces an abnormal activity for the human promoter in K562 cells. Altogether, our data suggest that limited changes in the GATA-containing enhancer of the GPIIb promoter can induce the recruitment of accessory proteins that could be involved in alteration of a megakaryocyte-restricted gene activation program.

Identification of a GATA-overlapping sequence within the enhancer of the murine GPIIb promoter that induces transcriptional deregulation in human K562 cells

The human and the murine glycoprotein platelet IIb (GPIIb) promoters are megakaryocyte specific in human and murine cell systems, respectively. Here we show that the murine promoter is, however, highly active when transfected in K562 human cells in which the human promoter is almost inactive. A murine promoter, in which the enhancer element was replaced by the human, retrieves its megakaryocytic specificity in human cell lines. The human and murine GATA-binding sites located in the enhancer region display slight sequence divergence next to the consensus GATA core sequence. Gel shift experiments show that, although the murine and the human GATA sequences both bind GATA-1, the murine sequence alone forms an additional complex (B) not detected with the human sequence. When the murine GATA-containing region is replaced by the human in the context of the murine GPIIb promoter, megakaryocyte specificity is restored in the human cell lines. A G nucleotide 3′ to GATA appears crucial because its substitution abrogates B but not GATA-1 binding and restores megakaryocyte specificity to the murine promoter. Conversely, substitution of the human GATA-1 binding sequence by its murine homologue that binds both GATA-1 and complex B induces an abnormal activity for the human promoter in K562 cells. Altogether, our data suggest that limited changes in the GATA-containing enhancer of the GPIIb promoter can induce the recruitment of accessory proteins that could be involved in alteration of a megakaryocyte-restricted gene activation program.

Megakaryocyte-targeted synthesis of the integrin β3-subunit results in the phenotypic correction of Glanzmann thrombasthenia

Glanzmann thrombasthenia is an inherited bleeding disorder characterized by qualitative or quantitative defects of the platelet-specific integrin, IIbβ3. As a result, IIbβ3 cannot be activated and cannot bind to fibrinogen, leading to a loss of platelet aggregation. Thrombasthenia is clinically characterized by mucocutaneous hemorrhage with episodes of intracranial and gastrointestinal bleeding. To develop methods for gene therapy of Glanzmann thrombasthenia, a murine leukemia virus (MuLV)-derived vector, −889PlA2β3, was transduced into peripheral blood CD34+ cells from 2 patients with thrombasthenia with defects in the β3 gene. The human IIb promoter was used in this vector to drive megakaryocyte-targeted expression of the wild-type β3 subunit. Proviral DNA and IIbβ3 biosynthesis were detected after in vitro differentiation of transduced thrombasthenic CD34+ cells with megakaryocyte growth and development factor. Flow cytometric analysis of transduced patient samples indicated that 19% of megakaryocyte progeny expressed IIbβ3 on the surface at 34% of normal receptor levels. Treatment of transduced megakaryocytes with a combination of agonists including epinephrine and the thrombin receptor-activating peptide induced the IIbβ3 complex to form an activated conformation capable of binding fibrinogen as measured by PAC-1 antibody binding. Transduced cells retracted a fibrin clot in vitro similar to megakaryocytes derived from a normal nonthrombasthenic individual. These results demonstrate ex vivo phenotypic correction of Glanzmann thrombasthenia and support the potential use of hematopoietic CD34+ cells as targets for IIb promoter-driven MuLV vectors for gene therapy of platelet disorders.

Megakaryocyte-targeted synthesis of the integrin β3-subunit results in the phenotypic correction of Glanzmann thrombasthenia

Glanzmann thrombasthenia is an inherited bleeding disorder characterized by qualitative or quantitative defects of the platelet-specific integrin, IIbβ3. As a result, IIbβ3 cannot be activated and cannot bind to fibrinogen, leading to a loss of platelet aggregation. Thrombasthenia is clinically characterized by mucocutaneous hemorrhage with episodes of intracranial and gastrointestinal bleeding. To develop methods for gene therapy of Glanzmann thrombasthenia, a murine leukemia virus (MuLV)-derived vector, −889PlA2β3, was transduced into peripheral blood CD34+ cells from 2 patients with thrombasthenia with defects in the β3 gene. The human IIb promoter was used in this vector to drive megakaryocyte-targeted expression of the wild-type β3 subunit. Proviral DNA and IIbβ3 biosynthesis were detected after in vitro differentiation of transduced thrombasthenic CD34+ cells with megakaryocyte growth and development factor. Flow cytometric analysis of transduced patient samples indicated that 19% of megakaryocyte progeny expressed IIbβ3 on the surface at 34% of normal receptor levels. Treatment of transduced megakaryocytes with a combination of agonists including epinephrine and the thrombin receptor-activating peptide induced the IIbβ3 complex to form an activated conformation capable of binding fibrinogen as measured by PAC-1 antibody binding. Transduced cells retracted a fibrin clot in vitro similar to megakaryocytes derived from a normal nonthrombasthenic individual. These results demonstrate ex vivo phenotypic correction of Glanzmann thrombasthenia and support the potential use of hematopoietic CD34+ cells as targets for IIb promoter-driven MuLV vectors for gene therapy of platelet disorders.

CDP/Cut DNA binding activity is down-modulated in granulocytes, macrophages and erythrocytes but remains elevated in differentiating megakaryocytes

DNA binding by the CCAAT-displacement protein, the mammalian homologue of the Drosophila melanogaster Cut protein, was previously found to increase sharply in S phase, suggesting a role for CDP/Cut in cell cycle progression. Genetic studies in Drosophila indicated that cut plays an important role in cell-type specification in several tissues. In the present study, we have investigated CDP/Cut expression and activity in a panel of multipotent hematopoietic cell lines that can be induced to differentiate in vitro into distinct cell types. While CDP/Cut DNA binding activity declined in the pathways leading to macrophages, granulocytes and erythrocytes, it remained elevated in megakaryocytes. CDP/Cut was also highly expressed in primary megakaryocytes isolated from mouse, and some DNA binding activity could be detected. Altogether, these results raise the possibility that CDP/Cut may be a determinant of cell type identity downstream of the myelo-erythroid precursor cell. Another possibility, which does not exclude a role in lineage identity, is that CDP/Cut activity in megakaryocytes is linked to endomitosis. Indeed, elevated CDP/Cut activity in differentiating megakaryocytes and during the S phase of the cell cycle suggests that it may be required for DNA replication.

Characterization of hematopoietic lineage-specific gene expression by ES cell in vitro differentiation induction system

The continuous generation of mature blood cells from hematopoietic progenitor cells requires a highly complex series of molecular events. To examine lineage-specific gene expression during the differentiation process, we developed a novel method combiningLacZ reporter gene analysis with in vitro hematopoietic differentiation induction from mouse embryonic stem cells. For a model system using this method, we chose the erythroid and megakaryocytic differentiation pathways. Although erythroid and megakaryocytic cells possess distinct functional and morphologic features, these 2 lineages originate from bipotential erythro-megakaryocytic progenitors and share common lineage-restricted transcription factors. A portion of the 5′ flanking region of the human glycoprotein IIb (IIb) integrin gene extending from base −598 to base +33 was examined in detail. As reported previously, this region is sufficient for megakaryocyte-specific gene expression. However, previous reports that used human erythro-megakaryocytic cell lines suggested that one or more negative regulatory regions were necessary for megakaryocyte-specific gene expression. Our data clearly showed that an approximately 200-base enhancer region extending from −598 to −400 was sufficient for megakaryocyte-specific gene expression. This experimental system has advantages over those using erythro-megakaryocytic cell lines because it recapitulates normal hematopoietic cell development and differentiation. Furthermore, this system is more efficient than transgenic analysis and can easily examine gene expression with null mutations of specific genes.

Functional Characterization of the Human Platelet Glycoprotein V Gene Promoter: A Specific Marker of Late Megakaryocytic Differentiation

Glycoprotein V (GPV), a subunit of the platelet GPIb-V-IX receptor for von Willebrand factor and thrombin, is specifically found in platelets and mature megakaryocytes. Studies of the GPV gene can therefore provide insight into the mechanisms governing megakaryocyte differentiation. The human GPV promoter was isolated, and elements important for its tissue specific transcriptional activity were localized using systematic DNase I protection and reporter deletion assays. A −1413/+25 fragment inserted into a luciferase reporter construct displayed promoter activity in Dami and HEL but not in K562, HL60, or HeLa cells. Progressive 5′ to 3′ deletion showed a putative enhancer region in the −1413/−903 segment that contained closely spaced GATA and Ets sites protected from DNase I digestion in Dami extracts. Regions similar to a GPIIb gene repressor were found at −816 and −610, with the first exhibiting repressor activity in Dami and HEL cells and the second protected from DNAse I. Deletions from −362 to −103, an area containing protected sites for Sp1, STAT, and GATA, induced a progressive decrease in activity. The −103/+1 fragment, bearing a proximal Ets footprinted site and a GATA/Ets tandem footprint, displayed 75% activity relative to the full-length promoter and retained cell specificity. In summary, this work defines several regions of the GPV gene promoter important for its activity. It contains megakaryocyte-specific signals, including erythro-megakaryocytic GATA, and Ets cis-acting elements, GPIIb-like repressor domains, and binding sites for ubiquitous factors such as Sp1, ETF, and STAT.

Developmental Expression of Plasminogen Activator Inhibitor-1 Associated With Thrombopoietin-Dependent Megakaryocytic Differentiation

Plasminogen activator inhibitor-1 (PAI-1) is present in the platelet -granule and is released on activation. However, there is some debate as to whether the megakaryocyte and platelet synthesize PAI-1, take it up from plasma, or both. We examined the expression of PAI-1 in differentiating megakaryocytic progenitor cells (UT-7) and in CD34+/CD41− cells from cord blood. UT-7 cells differentiated with thrombopoietin (TPO) resembled megakaryocytes (UT-7/TPO) with respect to morphology, ploidy, and the expression of glycoprotein IIb-IIIa. PAI-1 messenger RNA (mRNA) expression was upregulated and PAI-1 protein synthesized in the UT-7/TPO cells accumulated in the cytoplasm without being released spontaneously. In contrast, erythropoietin (EPO)-stimulated UT-7 cells (UT-7/EPO) did not express PAI-1 mRNA after stimulation with TPO because they do not have endogenous c-Mpl. After cotransfection with human wild-typec-mpl, the cells (UT-7/EPO-MPL) responded to phorbol 12-myristate 13-acetate (PMA), tumor necrosis factor- (TNF-), and interleukin-1β (IL-1β) with enhanced PAI-1 mRNA expression within 24 to 48 hours. However, induction of PAI-1 mRNA in UT-7/EPO-MPL cells by TPO required at least 14-days stimulation. UT-7/EPO cells expressing c-Mpl changed their morphology and the other characteristics similar to the UT-7/TPO cells. TPO also differentiated human cord blood CD34+/CD41− cells to CD34−/CD41+ cells, generated morphologically mature megakaryocytes, and induced the expression of PAI-1 mRNA. These results suggest that both PAI-1 mRNA and de novo PAI-1 protein synthesis is induced after differentiation of immature progenitor cells into megakaryocytes by TPO.

Developmental Expression of Plasminogen Activator Inhibitor-1 Associated With Thrombopoietin-Dependent Megakaryocytic Differentiation

Plasminogen activator inhibitor-1 (PAI-1) is present in the platelet -granule and is released on activation. However, there is some debate as to whether the megakaryocyte and platelet synthesize PAI-1, take it up from plasma, or both. We examined the expression of PAI-1 in differentiating megakaryocytic progenitor cells (UT-7) and in CD34+/CD41− cells from cord blood. UT-7 cells differentiated with thrombopoietin (TPO) resembled megakaryocytes (UT-7/TPO) with respect to morphology, ploidy, and the expression of glycoprotein IIb-IIIa. PAI-1 messenger RNA (mRNA) expression was upregulated and PAI-1 protein synthesized in the UT-7/TPO cells accumulated in the cytoplasm without being released spontaneously. In contrast, erythropoietin (EPO)-stimulated UT-7 cells (UT-7/EPO) did not express PAI-1 mRNA after stimulation with TPO because they do not have endogenous c-Mpl. After cotransfection with human wild-typec-mpl, the cells (UT-7/EPO-MPL) responded to phorbol 12-myristate 13-acetate (PMA), tumor necrosis factor- (TNF-), and interleukin-1β (IL-1β) with enhanced PAI-1 mRNA expression within 24 to 48 hours. However, induction of PAI-1 mRNA in UT-7/EPO-MPL cells by TPO required at least 14-days stimulation. UT-7/EPO cells expressing c-Mpl changed their morphology and the other characteristics similar to the UT-7/TPO cells. TPO also differentiated human cord blood CD34+/CD41− cells to CD34−/CD41+ cells, generated morphologically mature megakaryocytes, and induced the expression of PAI-1 mRNA. These results suggest that both PAI-1 mRNA and de novo PAI-1 protein synthesis is induced after differentiation of immature progenitor cells into megakaryocytes by TPO.

Mpl Ligand Enhances the Transcription of the Cyclin D3 Gene: A Potential Role for Sp1 Transcription Factor

Cyclin D3 plays a major role in the development of polyploidy in megakaryocytes. The expression of cyclin D3 gene and the level of cyclin D3 protein are increased by the Mpl ligand in the Y10/L8057 megakaryocytic cell line, as indicated by Northern and Western blot analyses, and by nuclear run-on assays and transfection experiments with cyclin D3 promoter constructs. DNase I footprinting of the promoter region showed protected segments, at −75 to −60 bp and at −134 to −92 bp, which display binding sites for the Sp family of transcription factors. Gel mobility shift assay and supershifts with specific antibodies indicate that Sp1 binds to these regions in the cyclin D3 promoter and that Sp1 binding activity is significantly increased by Mpl ligand. Mutation of either Sp1 site both decreases the basal promoter activity and eliminates the induction by Mpl ligand. We find that the nonphosphorylated form of SP1 has greater affinity for the cyclin D3 promoter and that the majority of Sp1 in the cells is nonphosphorylated. Mpl ligand treatment results in increased levels of Sp1 protein, which also appears as nonphosphorylated. Okadaic acid, which inhibits protein phosphatase 1 (PP1) and shifts Sp1 to a phosphorylated form, decreases cyclin D3 gene expression and suppresses Mpl ligand induction. Our data point to the potential of Mpl ligand to activate at once several Sp1-dependent genes during megakaryopoiesis.

Mpl Ligand Enhances the Transcription of the Cyclin D3 Gene: A Potential Role for Sp1 Transcription Factor

Cyclin D3 plays a major role in the development of polyploidy in megakaryocytes. The expression of cyclin D3 gene and the level of cyclin D3 protein are increased by the Mpl ligand in the Y10/L8057 megakaryocytic cell line, as indicated by Northern and Western blot analyses, and by nuclear run-on assays and transfection experiments with cyclin D3 promoter constructs. DNase I footprinting of the promoter region showed protected segments, at −75 to −60 bp and at −134 to −92 bp, which display binding sites for the Sp family of transcription factors. Gel mobility shift assay and supershifts with specific antibodies indicate that Sp1 binds to these regions in the cyclin D3 promoter and that Sp1 binding activity is significantly increased by Mpl ligand. Mutation of either Sp1 site both decreases the basal promoter activity and eliminates the induction by Mpl ligand. We find that the nonphosphorylated form of SP1 has greater affinity for the cyclin D3 promoter and that the majority of Sp1 in the cells is nonphosphorylated. Mpl ligand treatment results in increased levels of Sp1 protein, which also appears as nonphosphorylated. Okadaic acid, which inhibits protein phosphatase 1 (PP1) and shifts Sp1 to a phosphorylated form, decreases cyclin D3 gene expression and suppresses Mpl ligand induction. Our data point to the potential of Mpl ligand to activate at once several Sp1-dependent genes during megakaryopoiesis.

High-Level Expression of Mpl in Platelets and Megakaryocytes Is Independent of Thrombopoietin

Thrombopoietin (TPO) is a hematopoietic growth factor that regulates megakaryocytopoiesis and platelet production through binding to its receptor, Mpl, encoded by the c-mpl proto-oncogene. Circulating levels of TPO are regulated by receptor-mediated uptake and degradation. To better understand this mode of TPO regulation, we examined whether expression of Mpl was regulated by its ligand. Using RNase protection analysis, we found no differences in the levels ofc-mpl transcripts in megakaryocytes (MKs) produced in vitro either in the presence or absence of TPO and in platelets (PLTs) obtained from mice hyperstimulated in vivo by ectopic secretion of TPO. Similarly, Western blot analysis of MKs produced in the presence or absence of TPO showed no difference in Mpl levels. Levels of Mpl, GpIIb, or P-selectin were virtually identical in platelet lysates obtained from normal, TPO knockout and mildly TPO-stimulated mice. In contrast, the expression of Mpl was significantly reduced in PLTs from severely thrombocythemic mice. These results show that TPO does not have a major effect on the transcription or translation of Mpl. However, they do suggest that an excess of circulating TPO can lead to the disappearance of Mpl from PLTs via catabolism.

High-Level Expression of Mpl in Platelets and Megakaryocytes Is Independent of Thrombopoietin

Thrombopoietin (TPO) is a hematopoietic growth factor that regulates megakaryocytopoiesis and platelet production through binding to its receptor, Mpl, encoded by the c-mpl proto-oncogene. Circulating levels of TPO are regulated by receptor-mediated uptake and degradation. To better understand this mode of TPO regulation, we examined whether expression of Mpl was regulated by its ligand. Using RNase protection analysis, we found no differences in the levels ofc-mpl transcripts in megakaryocytes (MKs) produced in vitro either in the presence or absence of TPO and in platelets (PLTs) obtained from mice hyperstimulated in vivo by ectopic secretion of TPO. Similarly, Western blot analysis of MKs produced in the presence or absence of TPO showed no difference in Mpl levels. Levels of Mpl, GpIIb, or P-selectin were virtually identical in platelet lysates obtained from normal, TPO knockout and mildly TPO-stimulated mice. In contrast, the expression of Mpl was significantly reduced in PLTs from severely thrombocythemic mice. These results show that TPO does not have a major effect on the transcription or translation of Mpl. However, they do suggest that an excess of circulating TPO can lead to the disappearance of Mpl from PLTs via catabolism.

Regulation of the Megakaryocytic Glycoprotein IX Promoter by the Oncogenic Ets Transcription Factor Fli-1

Glycoprotein (GP) IX is a subunit of the von Willebrand receptor, GPIb-V-IX, which mediates adhesion of platelets to the subendothelium of damaged blood vessels. Previous characterization of the GPIX promoter identified a functional Ets site that, when disrupted, reduced promoter activity. However, the Ets protein(s) that regulated GPIX promoter expression was unknown. In this study, transient cotransfection of several GPIX promoter/reporter constructs into 293T kidney fibroblasts with a Fli-1 expression vector shows that the oncogenic protein Fli-1 can transactivate the GPIX promoter when an intact GPIX Ets site is present. In addition, Fli-1 binding of the GPIX Ets site was identified in antibody supershift experiments in nuclear extracts derived from hematopoietic human erythroleukemia cells. Comparative studies showed that Fli-1 was also able to transactivate the GPIb and, to a lesser extent, the GPIIb promoter. Immunoblot analysis identified Fli-1 protein in lysates derived from platelets. In addition, expression of Fli-1 was identified immunohistochemically in megakaryocytes derived from CD34+ cells treated with the megakaryocyte differentiation and proliferation factor, thrombopoietin. These results suggest that Fli-1 is likely to regulate lineage-specific genes during megakaryocytopoiesis.

Regulation of the Megakaryocytic Glycoprotein IX Promoter by the Oncogenic Ets Transcription Factor Fli-1

Glycoprotein (GP) IX is a subunit of the von Willebrand receptor, GPIb-V-IX, which mediates adhesion of platelets to the subendothelium of damaged blood vessels. Previous characterization of the GPIX promoter identified a functional Ets site that, when disrupted, reduced promoter activity. However, the Ets protein(s) that regulated GPIX promoter expression was unknown. In this study, transient cotransfection of several GPIX promoter/reporter constructs into 293T kidney fibroblasts with a Fli-1 expression vector shows that the oncogenic protein Fli-1 can transactivate the GPIX promoter when an intact GPIX Ets site is present. In addition, Fli-1 binding of the GPIX Ets site was identified in antibody supershift experiments in nuclear extracts derived from hematopoietic human erythroleukemia cells. Comparative studies showed that Fli-1 was also able to transactivate the GPIb and, to a lesser extent, the GPIIb promoter. Immunoblot analysis identified Fli-1 protein in lysates derived from platelets. In addition, expression of Fli-1 was identified immunohistochemically in megakaryocytes derived from CD34+ cells treated with the megakaryocyte differentiation and proliferation factor, thrombopoietin. These results suggest that Fli-1 is likely to regulate lineage-specific genes during megakaryocytopoiesis.

Unexpected and coordinated expression of Spi-1, Fli-1, and megakaryocytic genes in four Epo-dependent cell lines established from transgenic mice displaying erythroid-specific expression of a thermosensitive SV40 T antigen

Most erythroleukemic cell lines established in vitro coexpress erythrocytic and megakaryocytic markers that often are associated with expression of Spi-1 and/or Fli-1 transcription factors known as transactivators of megakaryocyte-specific promoters. In the present study, we examined the possibility of establishing new cell lines keeping strictly erythroid-specific properties in vitro through the targeted and conditional immortalization of erythrocytic progenitors. For that purpose, we established several lines of transgenic mice displaying erythroid-specific expression of a thermosensitive SV40 T antigen. As expected, these transgenic mice developed splenomegaly due to the massive amplification of Ter 119 positive erythroid nucleated cells expressing T antigen. Despite this drastic effect in vivo, the in vitro immortalization of erythropoietin-dependent erythroid progenitors unexpectedly occurred at low frequency, and all four cell lines established expressed both erythrocytic (globins) and megakaryocytic markers (glycoprotein IIb, platelet factor 4) as well as Spi-1 and Fli-1 transcripts at permissive temperature. Switching the cells to the nonpermissive temperature led to a marked increase in globin gene expression and concomitant decrease in expression of Spi-1, Fli-1, and megakaryocytic genes in an erythropoietin-dependent manner. Interestingly, enhanced expression of Spi-1 and Fli-1 genes already was detected in the Ter 119 positive cell population of transgenic mice spleen in vivo. However, like normal Ter 119 erythroid cells, these Ter 119 positive cells from transgenic mice still expressed high levels of beta-globin and very low or undetectable glycoprotein IIb and platelet factor 4 megakaryocytic transcripts. Taken together, these data indicate that the unexpected expression of megakaryocytic genes is a specific property of immortalized cells that cannot be explained only by enhanced expression of Spi-1 and/or Fli-1 genes.

ETS-1 converts endothelial cells to the angiogenic phenotype by inducing the expression of matrix metalloproteinases and integrin beta3

The transcription factor ETS-1 is induced in endothelial cells (ECs) by angiogenic growth factors and the specific elimination of ETS-1 synthesis by antisense oligodeoxynucleotide inhibited angiogenesis in vitro (Iwasaka et al., 1996, J Cell Physiol 169:522-531). To understand the precise role of ETS-1 in angiogenesis, we established both high and low ETS-1 expression EC lines and compared angiogenic properties of these cell lines with those of the parental murine EC line, MSS-31. Although growth rate was almost identical for each cell line, the invasiveness was markedly enhanced in high ETS-1 expression cells and reduced in low ETS-1 expression cells compared with that of parental cells. The gene expressions of matrix metalloproteinases (MMP-1, MMP-3, and MMP-9) and gelatinolytic activity of MMP-9 were significantly increased in high ETS-1 expression cells. Low ETS-1 expression cells could not spread on a vitronectin substratum, and the phosphorylation of focal adhesion kinase was markedly impaired because of the reduced expression of integrin beta3. These results indicate that ETS-1 is a principal regulator that converts ECs to the angiogenic phenotype.

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

Spi-1/PU.1 and Fli-1 are two members of the ETS family of transcription factors whose expression is deregulated by proviral insertion in most erythroleukemic cell lines induced by the spleen focus-forming virus (SFFV) and Friend murine leukemia virus (F-MuLV) components of the Friend viral complex, respectively. In this study, we present evidence that transcription of the Fli-1 gene is positively regulated by Spi-1/PU.1 in SFFV-transformed cell lines: (i) all SFFV-transformed cell lines expressing Spi-1/PU.1 are characterized by a specific pattern of Fli-1 gene transcripts initiated in the -200 region instead of position -400 as reported for F-MuLV-transformed cell lines; (ii) these Fli-1 transcripts initiated in the -200 region are downregulated in parallel with that of Spi-1/PU.1 during hexamethylenebisacetamide (HMBA) induced differentiation; and (iii) Fli-1 transcription is upregulated in SFFV cells lines following stable transfection of a Spi-1/PU.1 expression vector. Furthermore, we found by transient transfection assays that the -270/-41 region of the Fli-1 gene displays promoter activity which is transactivated by Spi-1/PU.1. This promoter is strictly dependent on the integrity of two highly conserved ETS DNA binding sites that bind the Spi-1/PU.1 protein in vitro. Finally, we show that transfection of constitutive or inducible Fli-1 expression vectors in SFFV-transformed cells inhibits their erythroid differentiation induced by HMBA. Overall, these data indicate that Fli-1 is a target gene of the Spi-1/PU.1 transcription factor in SFFV-transformed cell lines. We further suggest that deregulated synthesis of Fli-1 may trigger a common mechanism contributing to erythroleukemia induced by either SFFV or F-MuLV.

Human Integrin β3 Gene Expression: Evidence for a Megakaryocytic Cell-Specific cis-Acting Element

The human integrin β3 participates in a wide range of adhesive biologic functions and is expressed in a selected subset of tissues, but little is known about the cis-acting DNA elements or trans-acting factors responsible for this regulation. Using cell lines characterized for β3 expression, a number of upstream regulatory regions in the β3 gene were identified. (1) The three regions from −1159 to −584, −290 to −146, and −126 to −115 demonstrated positive, negative, and negative activity, respectively. (2) The region from −115 to +29 of the β3 gene was sufficient for cell-specific activity. Deletion of the sequence from −115 to −89 produced a 6- to 40-fold reduction in reporter gene activity in β3-expressing megakaryocytic cell lines (K562, Dami, and HEL), but only a 1.7- and 2.7-fold reduction, respectively, in β3-expressing endothelial and melanoma cell lines, and 1.3- and 2.8-fold reduction, respectively, in non–β3-expressing Chinese hamster ovary and 293 cell lines. This sequence also bound nuclear proteins in a cell-specific manner in electrophoretic mobility shift assays. Mutational analysis indicated that the sequence GAGGGG (positions −113 to −108) is a megakaryocytic cell line-specificcis-acting element. (3) The region from −89 to +29 promoted lower activity in all cell lines. We also provide evidence that a CCCACCC sequence at position −70 has transcriptional activity, most likely through the Sp1 transcription factor. These data supply the first detailed map of the transcriptional regulatory elements of the 5′ region of the β3 gene, define positive regulatory sequences with potent megakaryocyte preferential activity, and indicate that the ubiquitous transcription factor, Sp1, may augment β3 gene expression.

© 1998 by The American Society of Hematology.

Human Integrin β3 Gene Expression: Evidence for a Megakaryocytic Cell-Specific cis-Acting Element

The human integrin β3 participates in a wide range of adhesive biologic functions and is expressed in a selected subset of tissues, but little is known about the cis-acting DNA elements or trans-acting factors responsible for this regulation. Using cell lines characterized for β3 expression, a number of upstream regulatory regions in the β3 gene were identified. (1) The three regions from −1159 to −584, −290 to −146, and −126 to −115 demonstrated positive, negative, and negative activity, respectively. (2) The region from −115 to +29 of the β3 gene was sufficient for cell-specific activity. Deletion of the sequence from −115 to −89 produced a 6- to 40-fold reduction in reporter gene activity in β3-expressing megakaryocytic cell lines (K562, Dami, and HEL), but only a 1.7- and 2.7-fold reduction, respectively, in β3-expressing endothelial and melanoma cell lines, and 1.3- and 2.8-fold reduction, respectively, in non–β3-expressing Chinese hamster ovary and 293 cell lines. This sequence also bound nuclear proteins in a cell-specific manner in electrophoretic mobility shift assays. Mutational analysis indicated that the sequence GAGGGG (positions −113 to −108) is a megakaryocytic cell line-specificcis-acting element. (3) The region from −89 to +29 promoted lower activity in all cell lines. We also provide evidence that a CCCACCC sequence at position −70 has transcriptional activity, most likely through the Sp1 transcription factor. These data supply the first detailed map of the transcriptional regulatory elements of the 5′ region of the β3 gene, define positive regulatory sequences with potent megakaryocyte preferential activity, and indicate that the ubiquitous transcription factor, Sp1, may augment β3 gene expression.

© 1998 by The American Society of Hematology.