Sequence-specific DNA binding of the proto-oncoprotein ets-1 defines a transcriptional activator sequence within the long terminal repeat of the Moloney murine sarcoma virus

PKA/CREB signaling triggers initiation of endothelial and hematopoietic cell differentiation via Etv2 induction

Ets family protein Etv2 (also called ER71 or Etsrp) is a key factor for initiation of vascular and blood development from mesodermal cells. However, regulatory mechanisms and inducing signals for Etv2 expression have been largely unknown. Previously, we revealed that cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) signaling enhanced differentiation of vascular progenitors into endothelial cells (ECs) and hematopoietic cells (HPCs) using an embryonic stem cell (ESC) differentiation system. Here, we show that PKA activation in an earlier differentiation stage can trigger EC/HPC differentiation through Etv2 induction. We found Etv2 was markedly upregulated by PKA activation preceding EC and HPC differentiation. We identified two cAMP response element (CRE) sequences in the Etv2 promoter and 5'-untranslated region and confirmed that CRE-binding protein (CREB) directly binds to the CRE sites and activates Etv2 transcription. Expression of a dominant negative form of CREB completely inhibited PKA-elicited Etv2 expression and induction of EC/HPCs from ESCs. Furthermore, blockade of PKA significantly inhibited Etv2 expression in ex vivo whole-embryo culture using Etv2-Venus knockin mice. These data indicated that PKA/CREB pathway is a critical regulator for the initiation of EC/HPC differentiation via Etv2 transcription. This early-stage molecular linkage between a triggering signal and transcriptional cascades for differentiation would provide novel insights in vascular and blood development and cell fate determination.

The ETS family member TEL binds to nuclear receptors RAR and RXR and represses gene activation

Retinoic acid receptor (RAR) signaling is important for regulating transcriptional activity of genes involved in growth, differentiation, metabolism and reproduction. Defects in RAR signaling have been implicated in cancer. TEL, a member of the ETS family of transcription factors, is a DNA-binding transcriptional repressor. Here, we identify TEL as a transcriptional repressor of RAR signaling by its direct binding to both RAR and its dimerisation partner, the retinoid x receptor (RXR) in a ligand-independent fashion. TEL is found in two isoforms, created by the use of an alternative startcodon at amino acid 43. Although both isoforms bind to RAR and RXR in vitro and in vivo, the shorter form of TEL represses RAR signaling much more efficiently. Binding studies revealed that TEL binds closely to the DNA binding domain of RAR and that both Helix Loop Helix (HLH) and DNA binding domains of TEL are mandatory for interaction. We have shown that repression by TEL does not involve recruitment of histone deacetylases and suggest that polycomb group proteins participate in the process.

Activation of Ets-2 by oxidative stress induces Bcl-xL expression and accounts for glial survival in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease characterized by selective degeneration of motor neurons and glial activation. Cell-specific transcriptional regulation induced by oxidative stress may contribute to the survival and activation of astrocytes in the face of motor neuron death. In the present study, we demonstrate an age-dependent increase in Bcl-xL and Ets-2 immunoreactivity that correlates with an increase of glial fibrillary acidic protein (GFAP)-positive cells in the ventral horn of the spinal cord in both ALS transgenic mice [mutant SOD1 (G93A)] and affected humans. Chromatin immunoprecipitation (ChIP) analysis verified that Ets-2 preferentially occupies the Ets-2 binding element in the promoter of Bcl-xL in primary astrocytes under oxidative stress conditions as well as in G93A spinal cords. Ets-2 small-interfering RNA down-regulated the transcriptional activity of Bcl-xL. In primary glial cultures, Bcl-xL overexpression and mutant SOD1 (G93A) both conferred resistance to oxidative stress-induced cell death. Our findings suggest that Ets-2 transcription factor activation of Bcl-xL gene may protect glia from constitutive oxidative stress that is thought to be a key mechanism contributing to the pathogenesis of ALS. This survival pathway may contribute to the glial survival and activation seen in the spinal cord of ALS patients.

Monoallele deletion of CBP leads to pericentromeric heterochromatin condensation through ESET expression and histone H3 (K9) methylation

Chromatin remodeling is tightly controlled under physiological conditions. Alterations in chromatin structure are involved in the pathogenesis of neuronal systems. We found that the monoallelic deletion of CREB binding protein (CBP) results in the induction of ERG-associated protein with SET domain (ESET) and increases trimethylation of histone H3 (K9) and condensation of pericentromeric heterochromatin structure in neurons. Nested deletion and mutational analysis of the ESET promoter further demonstrated that the Ets-2 transcription factor regulates transcriptional activity of the ESET gene. In CBP+/- mice, Ets-2 occupancy in the ESET promoter DNA was markedly elevated. Our results suggest that CBP is a transcriptional repressor of ESET gene expression by limiting Ets-2 transcriptional activity, while CBP siRNA enhances basal and Ets-2-dependent ESET transcriptional activity. Altered expression of the ESET gene and hypertrimethylation of H3 (K9) correlate with striatal neuron atrophy and dysfunction in CBP+/- mice. These results establish an alternative pathway that loss of CBP leads to the pericentric heterochromatin condensation through ESET expression and trimethylation of H3 (K9).

The MN1-TEL myeloid leukemia-associated fusion protein has a dominant-negative effect on RAR-RXR-mediated transcription

The translocation t(12;22)(p13;q11) creates an MN1-TEL fusion gene leading to acute myeloid leukemia. MN1 is a transcription coactivator of the retinoic acid and vitamin D receptors, and TEL (ETV6) is a member of the E26-transformation-specific family of transcription factors. In MN1-TEL, the transactivating domains of MN1 are combined with the DNA-binding domain of TEL. We show that MN1-TEL inhibits retinoic acid receptor (RAR)-mediated transcription, counteracts coactivators such as p160 and p300, and acts as a dominant-negative mutant of MN1. Compared to MN1, the same transactivation domains in MN1-TEL are poorly stimulated by p160, p300 or histone deacetylase inhibitors, indicating that the block of RAR-mediated transcription by MN1-TEL is caused by dysfunctional transactivation domains rather than by recruitment of corepressors. The mechanism leading to myeloid leukemia in t(12;22) thus differs from the translocations that involve RAR itself.

Characterization and functional analysis of the p42Ets-1 variant of the mouse Ets-1 transcription factor

We have identified the mouse exon VII splice variant of the Ets-1 transcription factor. The variant is expressed in all cell lines which express ets-1, at lower levels, it is also expressed in the mouse embryo in vivo. The corresponding protein, p42Ets-1, is a transcription factor as it is able to bind to specific DNA sequences and to transactivate a bona fide ETS reporter vector. A comparison of optimal DNA-binding sites shows that p42Ets-1 binds to more various DNA sequences than p51Ets-1; p42Ets-1 recognizes the same optimal consensus sequence as p51Ets-1, but also many variations of it, mainly at base -1, which is located just prior to the GGAA/T core sequence. The binding differences were quantified by surface plasmon resonance analyses and the protein region responsible for the differences in DNA sequence recognition located in the Val280-Glu302 fragment, which is encoded by exon VII. The specific DNA-binding properties of each isoform translates into clear differences in activity, p42Ets-1 transactivates the natural VE-cadherin gene promoter through both ETS-binding site (EBS)2 and EBS4 whereas p51Ets-1 is mainly active on EBS4. Altogether, our data suggest that p42Ets-1 acts as a distinct transcription factor from p51Ets-1.

Enhanced expression of Runx2/PEBP2alphaA/CBFA1/AML3 during fracture healing

The cause of the dramatic increase in expression of the osteopontin gene during fracture healing was studied in a mouse experimental model. Semiquantitative reverse transcription-polymerase chain reaction, Northern blotting, and in situ hybridization analysis showed that the enhanced expression took place prior to callus formation. The change in the expression pattern of collagenous and noncollagenous bone matrix proteins in addition to Ets-1 and Runx2, major transcription factors of osteopontin, were examined and compared to that of osteopontin. Although Ets-1 expression showed no significant change during fracture healing, enhanced expression of Runx2 corresponding to that of osteopontin was observed. Furthermore, in situ hybridization demonstrated that osteopontin-expressing cells also express the Runx2 gene. The results indicated the possibility that Runx2 is a major regulator of osteopontin during fracture healing.

PU.1/Spi-B regulation of c-rel is essential for mature B cell survival

PU.1(+/-)Spi-B(-/-) mice exhibit reduced numbers of immature and mature B lymphocytes, which exhibit severe defects in response to BCR-mediated stimulation and poor survival. We found that expression of c-rel, a member of the Rel/NF-kappa B family, is dramatically reduced in PU.1(+/-)Spi-B(-/-) splenic B cells. Analysis of the murine c-rel promoter identified three PU.1/Spi-B binding sites critical for c-rel promoter activity. Furthermore, reintroduction of Rel protein restored wild-type B cell numbers to mice reconstituted with PU.1(+/-)Spi-B(-/-) bone marrow. These findings are the first to demonstrate that a member of the Rel/NF-kappa B family is directly regulated by Ets proteins and dissect the molecular basis for the function of two Ets factors, PU.1 and Spi-B, in promoting B lymphocyte survival.

The regulation of CD5 expression in murine T cells

BACKGROUND

CD5 is a pan-T cell surface marker that is also present on a subset of B cells, B-1a cells. Functional and developmental subsets of T cells express characteristic CD5 levels that vary over roughly a 30-fold range. Previous investigators have cloned a 1.7 Kb fragment containing the CD5 promoter and showed that it can confer similar lymphocyte-specific expression pattern as observed for endogenous CD5 expression.

RESULTS

We further characterize the CD5 promoter and identify minimal and regulatory regions on the CD5 promoter. Using a luciferase reporter system, we show that a 43 bp region on the CD5 promoter regulates CD5 expression in resting mouse thymoma EL4 T cells and that an Ets binding site within the 43 bp region mediates the CD5 expression. In addition, we show that Ets-1, a member of the Ets family of transcription factors, recognizes the Ets binding site in the electrophoretic mobility shift assay (EMSA). This Ets binding site is directly responsible for the increase in reporter activity when co-transfected with increasing amounts of Ets-1 expression plasmid.We also identify two additional evolutionarily-conserved regions in the CD5 promoter (CD5X and CD5Y) and demonstrate the respective roles of the each region in the regulation of CD5 transcription.

CONCLUSION

Our studies define a minimal and regulatory promoter for CD5 and show that the CD5 expression level in T cells is at least partially dependent on the level of Ets-1 protein. Based on the findings in this report, we propose a model of CD5 transcriptional regulation in T cells.

Expression and purification of recombinant mouse Ets-1 transcription factor

Ets-1 is a transcription factor which belongs to the ETS family. Its mRNA is expressed in the embryo during normal development and also in tumors. In order to sort out functional Ets-1-binding sites among those present in gene promoters, we constructed an expression vector and designed a purification protocol for the production of the 440-amino-acid form of mouse Ets-1, based on heparin-Sepharose affinity and anion-exchange chromatographies. This protocol allows the purification of large amounts of pure recombinant protein as assessed by SDS-PAGE, C18 reverse-phase HPLC, amino-terminal sequencing, and mass spectrometry. The purified protein is recognized by specific anti-Ets-1 antibodies and binds to DNA ETS-binding sites.

The MN1-TEL fusion protein, encoded by the translocation (12;22)(p13;q11) in myeloid leukemia, is a transcription factor with transforming activity

The Tel gene (or ETV6) is the target of the translocation (12;22)(p13;q11) in myeloid leukemia. TEL is a member of the ETS family of transcription factors and contains the pointed protein interaction (PNT) domain and an ETS DNA binding domain (DBD). By contrast to other chimeric proteins that contain TEL's PNT domain, such as TEL-platelet-derived growth factor beta receptor in t(5;12)(q33;p13), MN1-TEL contains the DBD of TEL. The N-terminal MN1 moiety is rich in proline residues and contains two polyglutamine stretches, suggesting that MN1-TEL may act as a deregulated transcription factor. We now show that MN1-TEL type I, unlike TEL and MN1, transforms NIH 3T3 cells. The transforming potential depends on both N-terminal MN1 sequences and a functional TEL DBD. Furthermore, we demonstrate that MN1 has transcription activity and that MN1-TEL acts as a chimeric transcription factor on the Moloney sarcoma virus long terminal repeat and a synthetic promoter containing TEL binding sites. The transactivating capacity of MN1-TEL depended on both the DBD of TEL and sequences in MN1. MN1-TEL contributes to leukemogenesis by a mechanism distinct from that of other chimeric proteins containing TEL.

ETS1 lowers capillary endothelial cell density at confluence and induces the expression of VE-cadherin

Ets1 is a transcription factor expressed in endothelial cells during angiogenesis but its target genes and function in blood vessel formation are still unknown. We have over-expressed Ets1 as a tagged protein in brain capillary endothelial cells and in 3T3 fibroblasts using a retroviral vector. Over-expression of Ets1 reduced by nearly half cell density at confluence of endothelials but not of fibroblasts. As density at confluence is controlled in part by cadherins, this growth arrest could be due to the up-regulation of these cell contact molecules. Indeed, Ets1 increased the expression of the endothelial-specific VE-cadherin without affecting N-cadherin expression levels. In parallel, both a dominant negative mutant of Ets members and an Ets1 anti-sense oligonucleotide inhibited VE-cadherin expression in endothelial cells. Ets1 bound to two Ets-binding sites located in the proximal region of the VE-cadherin promoter. Mutation of these sites abolished Ets1-induced transactivation of the promoter. The present work is the first demonstration of a function of Ets1 in the regulation of a specific endothelial marker based on its endogenous gene and protein expression.

Regulation of expression of the human beta-1,2-N-acetylglucosaminyltransferase II gene (MGAT2) by Ets transcription factors

Oncogenic transformation of fibroblasts by the src oncogene has long been known to cause an increase in the size of cell-surface protein-bound oligosaccharides, owing primarily to increased N-glycan branching mediated by increased beta-1,6-N-acetylglucosaminyltransferase V (GnT V) activity. The src-responsive element of the GnT V promoter was localized to Ets-binding sites and the promoter was transcriptionally stimulated by both ets-1 and ets-2 expression [Buckhaults, Chen, Fregien and Pierce (1997) J. Biol. Chem. 272, 19575-19581; Kang, Saito, Ihara, Miyoshi, Koyama, Sheng and Taniguchi (1996) J. Biol. Chem. 271, 26706-26712]. Because GnT V action requires the prior action of beta-1,2-N-acetylglucosaminyltransferase II (GnT II) and the human GnT II promoter contains four putative Ets-binding sites [Chen, Zhou, Tan and Schachter (1998) Glycoconj. J. 15, 301-308], GnT II might also be under oncogenic control via Ets transcription factors. We now report that co-transfection into HepG2 or COS-1 cells of either ets-1 or ets-2 expression plasmids together with chimaeric GnT II promoter-chloramphenicol acetyltransferase plasmids results in a 2-4-fold stimulation of promoter activity. Mobility-shift assays and South-Western blots localized the functional Ets-binding site to one of the four putative sites on the GnT II promoter. The GnT II promoter, unlike the GnT V promoter, is not activated by either src or neu. Therefore although both promoters are stimulated by a member of the Ets family of transcription factors, the functional role of this Ets transcriptional control seems to be different for the two genes.

The Ets factors PU.1 and Spi-B regulate the transcription in vivo of P2Y10, a lymphoid restricted heptahelical receptor

To investigate the in vivo functions of PU.1 and Spi-B, two highly related Ets transcription factors, we previously generated PU. 1(+/+)Spi-B(-/-) and PU.1(+/-)Spi-B(-/-) mice and demonstrated a significant decrease in B-cell receptor (BCR) signaling in mutants. Major components of BCR signaling appear to be expressed at normal levels in these mice, implying that PU.1 and Spi-B cooperate in the transcription of additional target genes important for antigen receptor signaling. We used subtractive hybridization to identify novel in vivo PU.1/Spi-B target genes and determined that the expression of a heptahelical receptor, P2Y10, is dramatically reduced in PU.1(+/-)Spi-B(-/-) B-cells. Further analysis shows that P2Y10 expression is restricted to lymphoid cells and parallels that of Spi-B in B-lymphocytes. Lastly, the P2Y10 promoter contains a PU. 1/Spi-B binding site functionally required for efficient transcription in B-cells. Thus, P2Y10 is likely to be a direct in vivo transcriptional target for PU.1 and Spi-B and provides a unique model to explore transcriptional regulation by this Ets factor subfamily. Furthermore, P2Y10 suggests an intriguing connection between heterotrimeric G-proteins and BCR signaling.

Regulation of E-cadherin gene expression during tumor progression: the role of a new Ets-binding site and the E-pal element

A new regulatory region (-108 to -86), named CE, containing potential CRE- and Ets-binding sites has been identified in the murine E-cadherin promoter. The Ets-binding site (at -97 position) negatively modulates the activity of the E-cadherin promoter in expressing keratinocyte cell lines and was responsible for the specific retarded complexes obtained with the CE region. Analysis of the methylation status of the endogenous E-cadherin promoter indicated that silencing of E-cadherin expression in malignant keratinocytes cannot be explained by hypermethylation mechanisms. Furthermore, treatment with 5'-aza-2'-deoxycytidine was unable to induce the expression of E-cadherin in deficient keratinocytes. However, in vivo footprinting analysis of the endogenous E-cadherin promoter showed a very distinct pattern in expressing and nonexpressing keratinocytes. Extensive interactions in the previously postulated proximal regulatory elements and in the CE region were detected in expressing cells, while only some nucleotides of the E-pal element and of the CE region were protected in nonexpressing keratinocytes. These results indicate a complex regulation of the mouse E-cadherin promoter and support a model where the combination of positive (CCAAT-box and GC-rich region) and negative (E-pal element and CE region) cis-acting elements contribute to the final level of E-cadherin gene expression. In addition, our results show that downregulation of E-cadherin expression in transformed epidermal keratinocytes is mainly exerted through the interaction of repressor factor(s) with the E-pal element and to the lack of interaction of positive acting factors with the proximal regions.

SPI-B activates transcription via a unique proline, serine, and threonine domain and exhibits DNA binding affinity differences from PU.1

SPI-B is a B lymphocyte-specific Ets transcription factor that shares a high degree of similarity with PU.1/SPI-1. In direct contrast to PU.1(-/-) mice that die in utero and lack monocytes, neutrophils, B cells, and T cells, Spi-B-/- mice are viable and exhibit a severe B cell proliferation defect. Since PU.1 is expressed at wild type levels in Spi-B-/- B cells, the mutant mice provide genetic evidence that SPI-B and PU.1 have at least some non-redundant roles in B lymphocytes. To begin to understand the molecular basis for these defects, we delineated functional domains of SPI-B for comparison to those of PU.1. By using a heterologous co-transfection system, we identified two independent transactivation domains in the N terminus of SPI-B. Interestingly, only one of these domains (amino acids 31-61), a proline/serine/threonine-rich region, unique among Ets proteins, is necessary for transactivation of the immunoglobulin lambda light chain enhancer. This transactivation motif is in marked contrast to PU.1, which contains acidic and glutamine-rich domains. In addition, we describe a functional PU.1 site within the c-FES promoter which SPI-B fails to bind efficiently and transactivate. Finally, we show that SPI-B interacts with the PU.1 cofactors Pip, TBP, c-Jun and with lower affinity to nuclear factor interleukin-6beta and retinoblastoma. Taken together, these data suggest that SPI-B binds DNA with a different affinity for certain sites than PU.1 and harbors different transactivation domains. We conclude that SPI-B may activate unique target genes in B lymphocytes and interact with unique, although currently unidentified, cofactors.

HpEts, an ets-related transcription factor implicated in primary mesenchyme cell differentiation in the sea urchin embryo

The mechanism of micromere specification is one of the central issues in sea urchin development. In this study we have identified a sea urchin homologue of ets 1 + 2. HpEts, which is maternally expressed ubiquitously during the cleavage stage and which expression becomes restricted to the skeletogenic primary mesenchyme cells (PMC) after the hatching blastula stage. The overexpression of HpEts by mRNA injection into fertilized eggs alters the cell fate of non-PMC to migratory PMC. HpEts induces the expression of a PMC-specific spicule matrix protein, SM50, but suppresses of aboral ectoderm-specific arylsulfatase and endoderm-specific HpEndo16. The overexpression of dominant negative delta HpEts which lacks the N terminal domain, in contrast, specifically represses SM50 expression and development of the spicule. In the upstream region of the SM50 gene there exists an ets binding site that functions as a positive cis-regulatory element. The results suggest that HpEts plays a key role in the differentiation of PMCs in sea urchin embryogenesis.

Equilibrium analysis of high affinity interactions using BIACORE

BIACORE biosensors are useful for measuring reaction kinetics and calculating affinity constants for macromolecular interactions. However, one drawback with the flow system used in these instruments is that the standard injection procedures limit the amount of time available to collect association-phase data. This is especially problematic during equilibrium analysis of high affinity interactions. Using protein-DNA interactions as a model system, we demonstrate a simple method for overcoming this limitation. By placing the analyte directly into the running buffer we were able to deliver a continuous supply of protein to the sensor surfaces for greater than 12 h at a time. Complete equilibrium binding profiles were generated by changing the concentration of analyte and allowing the surface reactions to reequilibrate. Analyte concentrations were also decreased to demonstrate that the binding reactions were fully reversible. This method of analysis is a simple and convenient way of directly measuring equilibrium dissociation constants for very high affinity interactions.

In vivo footprinting of the enhancer sequences in the upstream long terminal repeat of Moloney murine leukemia virus: differential binding of nuclear factors in different cell types

The enhancer sequences in the Moloney murine leukemia virus (M-MuLV) long terminal repeat (LTR) are of considerable interest since they are crucial for virus replication and the ability of the virus to induce T lymphomas. While extensive studies have identified numerous nuclear factors that can potentially bind to M-MuLV enhancer DNA in vitro, it has not been made clear which of these factors are bound in vivo. To address this problem, we carried out in vivo footprinting of the M-MuLV enhancer in infected cells by in vivo treatment with dimethyl sulfate (DMS) followed by visualization through ligation-mediated PCR (LMPCR) and gel electrophoresis. In vivo DMS-LMPCR footprinting of the upstream LTR revealed evidence for factor binding at several previously characterized motifs. In particular, protection of guanines in the central LVb/Ets and Core sites within the 75-bp repeats was detected in infected NIH 3T3 fibroblasts, Ti-6 lymphoid cells, and thymic tumor cells. In contrast, factor binding at the NF-1 sites was found in infected fibroblasts but not in T-lymphoid cells. These results are consistent with the results of previous experiments indicating the importance of the LVb/Ets and Core sequences for many retroviruses and the biological importance especially of the NF-1 sites in fibroblasts and T-lymphoid cells. No evidence for factor binding to the glucocorticoid responsive element and LVa sites was found. Additional sites of protein binding included a region in the GC-rich sequences downstream of the 75-bp repeats (only in fibroblasts), a hypersensitive guanine on the minus strand in the LVc site (only in T-lymphoid cells), and a region upstream of the 75-bp repeats. These experiments provide concrete evidence for the differential in vivo binding of nuclear factors to the M-MuLV enhancers in different cell types.

Fine-mapping of the mouse T lymphocyte fraction (Tlf) locus on chromosome 9: association with autoimmune diabetes

Tlf (T lymphocyte fraction) defines a locus that governs the unusually high fraction of circulating T lymphocytes in the nonobese diabetic (NOD) mouse. We previously mapped Tlf to proximal Chromosome 9 in BC1 mice. Here, Tlf was tine-mapped on Chromosome 9 using 8 markers covering the 43 cM interval from D9Mit90 at 9 cM to D9Mit35 at 52 cM. Markers for diabetic genes on Chromosomes 3, 4, 5, 6, and 17 were also examined for effects on the Tlf phenotype. By both parametric and nonparametric tests. Tlf associated with two areas on Chromosome 9, one with the segment bounded by D9Mit66 (15 cM) and D9Mit2 (17 cM) and a second region near D9Mit71 (29 cM). This linkage pattern was observed both in BC1 and F2 populations. Thus, the Tlf phenotype is possibly governed by two genes on Chromosome 9. An influence by sex on the penetrance of Tlf was evident in that linkage was strongest for female F2 mice and male BC1 mice. One locus controlling the T lymphocyte fraction may be Idd2 since historically a subline of NOD mice with a low T cell fraction showed a low incidence of diabetes. Candidate genes for Tlf are Ets1 and Fli1, proximally and Igif distally.

Regulatory Elements in the Promoter of a Murine TCRD V Gene Segment

TCRD V segments rearrange in an ordered fashion during human and murine thymic development. Recombination requires the accessibility of substrate gene segments, and transcriptional enhancers and promoters have been shown to regulate the accessible chromatin configuration. We therefore investigated the regulation of TCRD V rearrangements by characterizing the promoter of the first TCRD V segment to be rearranged, DV101S1, under the influence of its own enhancer. Sequences required for full promoter activity were identified by transient transfections of normal and mutated promoters into a human γδ lymphoma, and necessary elements fall between −86 and +66 nt, relative to the major transcription start site. They include a cAMP responsive element (CRE) at −62, an Ets site at −39, a TATA box at −26, the major transcriptional start site sequence (−8 to −5 and −2 to +11), and a downstream sequence (+12 to +33). Gel shift analyses and in vitro DNase I footprinting showed that nuclear proteins bind to the functionally relevant CRE, Ets, +1 to +10 sequence, and the +17 to +21 sequence. Nuclear proteins also bind to an E box at −52, and GATA-3 binds to a GATA motif at −5, as shown by Ab ablation-supershift experiments, but mutations that abrogated protein binding to these sites failed to affect DV101S1 promoter activity. We conclude that not all protein-binding sites within the DV101S1 minimal promoter are important for enhancer driven TCRD gene transcription. Further, the possibility remains that the GATA and E box sites function in enhancer independent DV101S1 germline transcription.

A 5′ Regulatory Sequence Containing Two Ets Motifs Controls the Expression of the Wiskott-Aldrich Syndrome Protein (WASP) Gene in Human Hematopoietic Cells

The recently-identified Wiskott-Aldrich syndrome protein gene (WASP) is responsible for the Wiskott-Aldrich X-linked immunodeficiency as well as for isolated X-linked thrombocytopenia (XLT). To characterize the regulatory sequences of the WASP gene, we have isolated, sequenced and functionally analyzed a 1.6-Kb DNA fragment upstream of the WASP coding sequence. Transfection experiments showed that this fragment is capable of directing efficient expression of the reporter chloramphenicol acetyltransferase (CAT) gene in all human hematopoietic cell lines tested. Progressive 5′ deletions showed that the minimal sequence required for hematopoietic-specific expression consists of 137 bp upstream of the transcription start site. This contains potential binding sites for several hematopoietic transcription factors and, in particular, two Ets-1 consensus that proved able to specifically bind to proteins present in nuclear extracts of Jurkat cells. Overexpression of Ets-1 in HeLa resulted in transactivation of the CAT reporter gene under the control of WASP regulatory sequences. Disruption of the Ets-binding sequences by side-directed mutagenesis abolished CAT expression in Jurkat cells, indicating that transcription factors of the Ets family play a key role in the control of WASP transcription.

A 5′ Regulatory Sequence Containing Two Ets Motifs Controls the Expression of the Wiskott-Aldrich Syndrome Protein (WASP) Gene in Human Hematopoietic Cells

The recently-identified Wiskott-Aldrich syndrome protein gene (WASP) is responsible for the Wiskott-Aldrich X-linked immunodeficiency as well as for isolated X-linked thrombocytopenia (XLT). To characterize the regulatory sequences of the WASP gene, we have isolated, sequenced and functionally analyzed a 1.6-Kb DNA fragment upstream of the WASP coding sequence. Transfection experiments showed that this fragment is capable of directing efficient expression of the reporter chloramphenicol acetyltransferase (CAT) gene in all human hematopoietic cell lines tested. Progressive 5′ deletions showed that the minimal sequence required for hematopoietic-specific expression consists of 137 bp upstream of the transcription start site. This contains potential binding sites for several hematopoietic transcription factors and, in particular, two Ets-1 consensus that proved able to specifically bind to proteins present in nuclear extracts of Jurkat cells. Overexpression of Ets-1 in HeLa resulted in transactivation of the CAT reporter gene under the control of WASP regulatory sequences. Disruption of the Ets-binding sequences by side-directed mutagenesis abolished CAT expression in Jurkat cells, indicating that transcription factors of the Ets family play a key role in the control of WASP transcription.

A molecular basis for species differences in Thy-1 expression patterns

Thy-1 is a membrane glycoprotein that displays species-specific differences in its pattern of expression. Although it is expressed on thymocytes and splenocytes in mice, it is only expressed on thymocytes in rats. Based on previous studies suggesting that the third intron of the mouse Thy-1 gene is required for its expression in thymocytes, in vivo footprinting analysis was performed on the third introns of both the mouse and rat Thy-1 genes, and led to the identification of homologous 36 bp "footprinted" regions. The mouse 36 bp region was found to be capable of specifically binding an Ets-1-like nuclear factor present in both mouse thymocytes and splenocytes. In contrast, the homologous 36 bp region of the rat which differs from the mouse 36 bp region by three nucleotides resulting in the loss of the Ets-1 binding site, is unable to bind a similar Ets-1-like factor present in rat thymocytes. Instead, this region of the rat third intron binds another nuclear factor which is present in rat thymocytes but not splenocytes. These observations suggest that the differential expression of the mouse and rat Thy-1 genes in thymocytes and splenocytes is the result of differential expression of nuclear factors that bind to this 36 bp region.

Conformational changes induced in Hoxb-8/Pbx-1 heterodimers in solution and upon interaction with specific DNA

Two classes of homeodomain proteins, Hox and Pbx gene products, have the ability to bind cooperatively to DNA. In Hox proteins, the homeodomain and a highly conserved hexapeptide are required for cooperative DNA binding. In Pbx, the homeodomain and a region immediately C terminal of the homeodomain are essential for cooperativity. Using fluorescence and circular dichroism spectroscopy, we demonstrated that Hox and Pbx proteins interact in the absence of DNA. The interaction in solution is accompanied by conformational changes. Furthermore, upon interaction with specific DNA, additional conformational changes are induced in the Pbx-1/Hoxb-8 heterodimer. These data indicate that prior to DNA binding, Hox-Pbx interaction in solution is accompanied by structural alterations. We propose that these conformational changes modulate the DNA binding properties of these proteins, ultimately resulting in cooperative DNA binding.

Human alpha-galactosidase A: high plasma activity expressed by the -30G-->A allele

Human alpha-galactosidase A (EC 3.2.1.22; alpha-Gal A) is the lysosomal exoglycosidase responsible for the hydrolysis of terminal alpha-galactosyl residues from glycoconjugates and is the defective enzyme causing Fabry disease (McKusick 301500). An unusally elevated level of plasma alpha-Gal A activity (> 2.5 times the normal mean) was detected in two unrelated normal males and the elevated activities were inherited as X-linked traits in their families. Sequencing of the alpha-Gal A coding region, intron/exon boundaries and 5'-flanking region from the proband identified a single mutation, a G-->A transition 30 nt upstream from the initiation of translation codon in exon 1. The -30G-->A mutation occurred in a putative NF kappa B/Ets consensus binding site that was recently shown to inhibit protein binding to the 5'-untranslated region of the gene, providing a possible explanation for its high activity. To further characterize the mutation, the mRNA and protein expressed by this variant allele were studied. Purified plasma and lymphoblast alpha-Gal A activity from individuals with the -30G-->A mutation had normal physical and kinetic properties. In vitro translation of mRNAs from the cloned normal and high plasma activity alleles resulted in similar levels of alpha-Gal A protein, indicating that this mutation did not enhance translation. These findings suggest that the -30G-->A mutation in the 5'-untranslated region of the alpha-Gal A gene enhances transcription, presumably by interfering with the binding of negatively-acting transcription factors which normally decrease alpha-Gal A expression in various cells. Preliminary studies of the frequency of the -30G-->A mutation in 395 unrelated normal males of mixed ancestry revealed two additional unrelated individuals who had high plasma enzymatic activity and the mutation, confirming the effect of this mutation on enzyme expression and suggesting that about 0.5% of normal individuals have high plasma alpha-Gal A activity due to this variant allele.

ETS target genes: identification of egr1 as a target by RNA differential display and whole genome PCR techniques

ETS transcription factors play important roles in hematopoiesis, angiogenesis, and organogenesis during murine development. The ETS genes also have a role in neoplasia, for example in Ewing's sarcomas and retrovirally induced cancers. The ETS genes encode transcription factors that bind to specific DNA sequences and activate transcription of various cellular and viral genes. To isolate novel ETS target genes, we used two approaches. In the first approach, we isolated genes by the RNA differential display technique. Previously, we have shown that the overexpression of ETS1 and ETS2 genes effects transformation of NIH 3T3 cells and specific transformants produce high levels of the ETS proteins. To isolate ETS1 and ETS2 responsive genes in these transformed cells, we prepared RNA from ETS1, ETS2 transformants, and normal NIH 3T3 cell lines and converted it into cDNA. This cDNA was amplified by PCR and displayed on sequencing gels. The differentially displayed bands were subcloned into plasmid vectors. By Northern blot analysis, several clones showed differential patterns of mRNA expression in the NIH 3T3-, ETS1-, and ETS2-expressing cell lines. Sixteen clones were analyzed by DNA sequence analysis, and 13 of them appeared to be unique because their DNA sequences did not match with any of the known genes present in the gene bank. Three known genes were found to be identical to the CArG box binding factor, phospholipase A2-activating protein, and early growth response 1 (Egr1) genes. In the second approach, to isolate ETS target promoters directly, we performed ETS1 binding with MboI-cleaved genomic DNA in the presence of a specific mAb followed by whole genome PCR. The immune complex-bound ETS binding sites containing DNA fragments were amplified and subcloned into pBluescript and subjected to DNA sequence and computer analysis. We found that, of a large number of clones isolated, 43 represented unique sequences not previously identified. Three clones turned out to contain regulatory sequences derived from human serglycin, preproapolipoprotein C II, and Egr1 genes. The ETS binding sites derived from these three regulatory sequences showed specific binding with recombinant ETS proteins. Of interest, Egr1 was identified by both of these techniques, suggesting strongly that it is indeed an ETS target gene.

Identification of nucleotide sequences that regulate transcription of the MCF13 murine leukemia virus long terminal repeat in activated T cells

The region downstream of the enhancer (DEN) of the long terminal repeat of the mink cell focus-forming murine leukemia virus is important for viral pathogenicity. Another important activity of DEN is its control of transcription in activated T cells, and we have determined that an NF-kappaB site is critical for this activity.

Specific binding of the ETS-domain protein to the interferon-stimulated response element

Interferon (IFN) activation of genes bearing an IFN-stimulated response element (ISRE) is regulated through binding of IFN-stimulated gene factors (ISGF) to the ISRE found in many IFN-stimulated genes. Using a multimerized human 2-5A synthetase ISRE as probe, we screened lambda gt11 expression libraries for cDNA encoding ISRE-binding activity and isolated a clone for murine proto-oncogene ets-1. The Ets-1 protein binds to the 2-5A synthetase ISRE at a site that also binds ISGF3, a multicomponent factor whose ISRE binding correlates with IFN-induced activation of transcription from ISRE-containing promoters. IFN-induced ISGF3 complex formation on the ISRE can be inhibited by specific Ets-1 antibody. Coexpression of Ets-1 represses ISRE-dependent reporter activity, suggesting that one or more members of the Ets protein family may negatively regulate transcriptional activity mediated by the 2-5A synthetase ISRE.

In vivo transcriptional regulation of the human immunodeficiency virus in the central nervous system in transgenic mice

Human immunodeficiency virus type 1 (HIV-1) causes infections of the central nervous system (CNS) and has been implicated as the causative agent of AIDS-associated encephalopathy and the AIDS dementia complex. The development of in vivo models of HIV-1-mediated gene expression has shown that the HIV long terminal repeat (LTR) from the viral isolate HIV(JR-CSF) specifically supports gene expression in adult and developing CNS. To determine the molecular basis for HIV-1 developmental CNS gene expression, in vivo footprinting analysis by the ligation-mediated PCR technique was performed on CNS tissue from the brain stem of a transgenic mouse. The association of cellular proteins in the CNS with sequences in the LTR was found over sequences that defined the TATA region, the Sp-1 and NF-kappaB sites, and two upstream regions (-111 to -150 and -260 to -300). A purine-rich sequence at positions -256 to -296 of the HIV(JR-CSF) LTR but not of the HIV(IIIB) LTR specifically bound protein in nuclear extracts of newborn brain tested in electrophoretic mobility shift assays. No specific protein binding was observed to this region in liver or HeLa cell nuclear extracts. This suggests the presence of a newly identified transcription factor involved in regulation of HIV-1 gene expression in the CNS.

Redox regulation of GA-binding protein-alpha DNA binding activity

We have investigated the reduction/oxidation (redox) regulation of the heteromeric transcription factor GA-binding protein (GABP). GABP, also known as nuclear respiratory factor 2, regulates the expression of nuclear encoded mitochondrial proteins involved in oxidative phosphorylation, including cytochrome c oxidase subunits IV and Vb, as well as the expression of mitochondrial transcription factor 1. GABP is composed of two subunits, the Ets-related GABP-alpha, which mediates specific DNA binding, and GABP-beta, which forms heterodimers and heterotetramers on DNA sequences containing the PEA3/Ets motif ((C/A)GGA(A/T)(G/A)). We demonstrate here that GABP DNA binding activity and GABP-dependent gene expression in 3T3 cells are inhibited by pro-oxidant conditions. DNA binding of recombinant GABP-alpha was activated by chemical reduction (dithiothreitol) and by thioredoxin; however, GSSG inhibited GABP DNA binding activity. Treatment of GABP-alpha, but not GABP-beta1, with sulfhydryl-alkylating agents also inhibited GABP DNA binding activity. Our results suggest that GABP DNA binding activity is redox-regulated in vivo, possibly by thioredoxin-mediated reduction and by GSSG-mediated oxidation of the GABP-alpha subunit. The regulation of GABP (nuclear respiratory factor 2) DNA binding activity by cellular redox changes provides an important link between mitochondrial and nuclear gene expression and the redox state of the cell.

RREB-1, a novel zinc finger protein, is involved in the differentiation response to Ras in human medullary thyroid carcinomas

An activated ras oncogene induces a program of differentiation in the human medullary thyroid cancer cell line TT. This differentiation process is accompanied by a marked increase in the transcription of the human calcitonin (CT) gene. We have localized a unique Ras-responsive transcriptional element (RRE) in the CT gene promoter. DNase I protection indicates two domains of protein-DNA interaction, and each domain separately can confer Ras-mediated transcriptional inducibility. This bipartite RRE was also found to be Raf responsive. By affinity screening, we have cloned a cDNA coding for a zinc finger transcription factor (RREB-1) that binds to the distal RRE. The consensus binding site for this factor is CCCCAAACCACCCC. RREB-1 is expressed ubiquitously in human tissues outside the adult brain. Overexpression of RREB-1 protein in TT cells confers the ability to mediate increased transactivation of the CT gene promoter-reporter construct during Ras- or Raf-induced differentiation. These data suggest that RREB-1 may play a role in Ras and Raf signal transduction in medullary thyroid cancer and other cells.

Molecular cloning of Elk-3, a new member of the Ets family expressed during mouse embryogenesis and analysis of its transcriptional repression activity

We isolated a cDNA clone, Elk-3, that encodes a novel Ets transcription factor from 16-day mouse embryos. The deduced amino acid sequence of the protein was homologous to human ELK-1 and SAP-1. This protein, ELK-1, and SAP-1 shared some unique structural properties such as an Ets DNA-binding site in the amino-terminal region, a serum response factor interacting domain and phosphorylation sites of serine or threonine residues in the carboxy-terminal region. Northern blotting weakly revealed that two transcripts of 4 and 2.1 kb are expressed in the adult ovary and lung and a 2.1-kb transcript predominated in 8- to 14-day embryos. We assayed the transcriptional activities of Elk-3 protein on the cytokeratin EndoA enhancer containing Ets binding sites in endodermal cells. Elk-3 protein strongly repressed enhancer activity but did not affect the activity of the basal promoter in the absence of the enhancer. Furthermore, Elk-3 can suppress the activity of Ets-2 as the transcriptional activator on the EndoA enhancer. These data suggested that the Elk-3 gene product plays a role in transcriptional regulation during embryogenesis.

Increased T-cell apoptosis and terminal B-cell differentiation induced by inactivation of the Ets-1 proto-oncogene

The Ets-1 proto-oncogene is a member of a transcription factor family characterized by homology to the v-ets oncogene. In adult mice, Ets-1 is expressed predominantly in lymphoid cells where it has been implicated in regulating transcription of lymphocyte-specific genes. Following T-cell activation, the specific DNA binding activity of Ets-1 is inactivated by transient phosphorylation, suggesting a function in the transition from the resting to activated state. Ets-1 has also been suggested to cooperate with the AP-1 transcription factor complex to mediate cellular growth factor responses. Here we show, by using RAG-2-deficient blastocyst complementation, that Ets-1 deficiency has dramatic, but different, effects on development and function of T- and B-lineage cells. Ets-1-deficient T cells were present in reduced numbers and were highly susceptible to cell death in vitro. In contrast, Ets-1-deficient B cells were present in normal numbers but a large proportion were IgM plasma cells. Our data demonstrate that Ets-1 is essential for maintenance of the normal pool of resting T- and B-lineage cells.

ERM, a PEA3 subfamily of Ets transcription factors, can cooperate with c-Jun

A human cDNA clone for ERM, a member of the ets gene family, has been obtained by polymerase chain reaction with degenerate primers corresponding to highly conserved regions within an Ets DNA binding domain. ERM mRNA is expressed ubiquitously. The gene was mapped to chromosome 3q27. In in vivo transient-expression assays, ERM induced transcription more efficiently from a synthetic element containing both an ets-binding site (EBS) and a cyclic AMP response element (CRE) than from one containing an EBS alone. The activation of a synthetic EBS-CRE site by ERM was likely to involve a leucine zipper protein capable of dimerizing with CRE-BP1 leucine zipper. Indeed, ERM and c-Jun synergistically activated the EBS-CRE without making an apparent ternary complex. The synergy between c-Jun and ERM may be attributed to the enhancing effect of c-Jun on the transcription activity of ERM, because c-Jun increased ERM transcription activity by more than 20-fold in an assay system using a variety of fusion proteins between a Gal4 DNA-binding domain and a portion of ERM. This enhancing effect of c-Jun required the amino-terminal portion of ERM.

Modulation of transcription factor Ets-1 DNA binding: DNA-induced unfolding of an alpha helix

Conformational changes, including local protein folding, play important roles in protein-DNA interactions. Here, studies of the transcription factor Ets-1 provided evidence that local protein unfolding also can accompany DNA binding. Circular dichroism and partial proteolysis showed that the secondary structure of the Ets-1 DNA-binding domain is unchanged in the presence of DNA. In contrast, DNA allosterically induced the unfolding of an alpha helix that lies within a flanking region involved in the negative regulation of DNA binding. These findings suggest a structural basis for the intramolecular inhibition of DNA binding and a mechanism for the cooperative partnerships that are common features of many eukaryotic transcription factors.

The E47 transcription factor binds to the enhancer sequences of recombinant murine leukemia viruses and influences enhancer function

The genomes of most recombinant murine leukemia viruses (MuLVs) inherit pathogenic U3 region sequences from the endogenous xenotropic provirus Bxv-1. However, the U3 regions of about one-third of recombinant MuLVs from CWD mice, such as CWM-T15, have nonecotropic substitutions that are probably derived from an endogenous polytropic provirus. The CWM-T15 U3 region sequences contain five nucleotide substitutions compared with the less pathogenic sequences of the endogenous ecotropic virus parent, Emv-1. Three of these substitutions are located immediately 3' of the enhancer core, and two form part of an E-box motif that is also found in the Bxv-1 sequence. A series of electromobility shift assays revealed that nuclear extracts from S194 cells and the basic helix-loop-helix transcription factor E47 could distinguish between oligonucleotides that contained the core region sequences of CWM-T15 or Emv-1. The E47 homodimers appeared to bind to the CWM-T15 E-box motif and when expressed at high levels in cells transactivated the CWM-T15 but not the Emv-1 enhancer. Taken together, these results suggest that E47 or related basic helix-loop-helix proteins that are expressed in lymphoid cells bind to and transactivate the CWM-T15 enhancer in vivo. This transactivation may explain why the CWM-T15 and Bxv-1 U3 regions accelerate the onset of lymphoid neoplasms and why related enhancer core region sequences are preferentially incorporated into the genomes of recombinant MuLVs and are found in other leukemogenic mammalian retroviruses.

ETS1 suppresses tumorigenicity of human colon cancer cells

We have ectopically expressed transcription factor ETS1 in two different highly tumorigenic human colon cancer cell lines, DLD-1 and HCT116, that do not express endogenous ETS1 protein and have obtained several independent clones. The expression of wild-type ETS1 protein in these colon cancer cells reverses the transformed phenotype and tumorigenicity in a dose-dependent manner. By contrast, expression in DLD-1 cells of a variant form of ETS1, lacking transcriptional activity, did not alter the tumorigenic properties of the cells, suggesting that the reduction in tumorigenicity in these clones was specific for the wild-type ETS1 gene products. Since these colon cancer cells have multiple genetic alterations, the system described in this paper could be a good model to study the suppression of tumorigenicity at a transcriptional level, which could lead to the design and development of novel drugs for cancer treatment.

Regulation of cell-type-specific interleukin-2 receptor alpha-chain gene expression: potential role of physical interactions between Elf-1, HMG-I(Y), and NF-kappa B family proteins

The interleukin 2 receptor alpha-chain (IL-2R alpha) gene is rapidly and potently induced in T cells in response to mitogenic stimuli. Previously, an inducible enhancer between nucleotides -299 and -228 that contains NF-kappa B and CArG motifs was identified. We now report the characterization of a second essential positive regulatory element located between nucleotides -137 and -64 that binds Elf-1 and HMG-I(Y). This element had maximal activity in lymphoid cells, paralleling the cell type specificity of Elf-1 expression. Transcription from the IL-2R alpha promoter was inhibited when either the Elf-1 or the HMG-I(Y) binding site was mutated. Coexpression of both proteins activated transcription of the -137 to -64 element in COS-7 cells. Elf-1 physically associated with HMG-I and with NF-kappa B p50 and c-Rel in vitro, suggesting that protein-protein interactions might functionally coordinate the actions of the upstream and downstream positive regulatory elements. This is the first report of a physical interaction between an Ets family member and NF-kappa B family proteins. These findings provide significant new insights into the protein-protein and protein-DNA interactions that regulate cell-type-specific and inducible IL-2R alpha gene expression and also have implications for other genes regulated by Elf-1 and NF-kappa B family proteins.

A ternary complex factor-dependent mechanism mediates induction of egr-1 through selective serum response elements following antigen receptor cross-linking in B lymphocytes

Induction of the primary response gene egr-1 occurs rapidly following antigen receptor cross-linking in B lymphocytes. Antisense studies have demonstrated that this induction is necessary for their subsequent activation to this signal. The present study examines the molecular mechanism whereby the receptor-generated signals interact with the egr-1 promoter to elicit transcription. Deletion mapping and point mutations have indicated that two of the five serum response elements (SREs) in the egr-1 promoter can mediate induction. Of the two critical SREs, both are capable of mediating maximal induction even in the absence of the other SRE. Our results also indicate that adjacent Ets motifs are necessary for induction. Like the c-fos SRE, the egr-1 SRE/Ets sites are occupied by a multiprotein (ternary) complex containing a homodimer of serum response factor and an unidentified member of the Ets family of transcription factors. The identification of a ternary complex-dependent mechanism of egr-1 induction, along with selective utilization of SREs in B lymphocytes, suggests that a complicated array of signaling cascades interacts with unique combinations of regulatory elements in the egr-1 promoter in different cell types.

ets-1 in astrocytes: expression and transmitter-evoked phosphorylation

The ets-1 protein has been primarily studied as a sequence-specific transcriptional regulator that is predominately expressed in lymphoid cells. In this report, we show that ets-1 is also expressed in astrocytes and astrocytoma cells and is regulated during both signal transduction and differentiation. Both isoforms of ets-1, p51 and p42, were found in astrocytes and astrocytoma cells, but whereas expression of p51 was strong, p42, the alternate splice product previously shown to lack the phosphorylation domain, was difficult to detect and was present at a level 10- to 40-fold lower than that of p51. This differed by roughly an order of magnitude from the ratio generally observable in T cells and thymocytes. In two astrocytoma lines of human origin, CCF and 1321N1, ets-1 phosphorylation was stimulated by bradykinin and carbachol, respectively. Glutamate, norepinephrine, and bradykinin elicited phosphorylation of p51 in cultures of primary rat type 1 astrocytes. ets-1 phosphorylation was dramatically blocked by KT5926, an inhibitor of myosin light-chain kinase, suggesting that this kinase may be involved in phosphorylation of ets-1 in vivo. Investigations of retinoic acid-induced differentiation in P19 cells provided further support for a strong correlation of ets-1 with the pathway for astrocyte differentiation.

Identification of ETS domain proteins in murine T lymphocytes that interact with the Moloney murine leukemia virus enhancer

The enhancer of Moloney murine leukemia virus (Mo-MuLV) contains an array of transcriptional control elements that direct viral gene expression in diverse cell types. The murine transcription factor Ets-1 was shown to bind to the LVb and LVc elements of the enhancer by DNase I protection and methylation interference assays. Enhancers containing disrupted Ets-1 binding sites were tested in transient expression assays in the murine T-cell line EL4.E1; alterations in the LVb element affected constitutive enhancer activity, while mutation of either the LVb or LVc element disrupted phorbol ester-induced enhancer activity. Members of the ets gene family of proteins display similar DNA-binding properties; therefore, we speculated that ets proteins other than Ets-1 also might bind these elements. Crude nuclear extracts of EL4.E1 cells were assayed to identify the protein(s) that potentially functions at the LVb element. The predominant binding activity was not Ets-1 but rather two independent DNA-protein complexes that comigrated in mobility shift assays. UV cross-linking and denaturing gel electrophoresis sized the two DNA-binding species, which we denoted p55 and p100. Immunoprecipitation combined with UV cross-linking identified p55 as the alpha subunit of GA-binding protein. The DNA-binding properties of p100 and several ets proteins were compared. Similarities suggested that p100 is also an ETS domain protein, possibly Elf-1. This strategy could be used to identify other ETS domain proteins in crude nuclear extracts. These findings suggest multiple ETS domain proteins could regulate gene expression of Mo-MuLV.

Identification of ETS domain proteins in murine T lymphocytes that interact with the Moloney murine leukemia virus enhancer

The enhancer of Moloney murine leukemia virus (Mo-MuLV) contains an array of transcriptional control elements that direct viral gene expression in diverse cell types. The murine transcription factor Ets-1 was shown to bind to the LVb and LVc elements of the enhancer by DNase I protection and methylation interference assays. Enhancers containing disrupted Ets-1 binding sites were tested in transient expression assays in the murine T-cell line EL4.E1; alterations in the LVb element affected constitutive enhancer activity, while mutation of either the LVb or LVc element disrupted phorbol ester-induced enhancer activity. Members of the ets gene family of proteins display similar DNA-binding properties; therefore, we speculated that ets proteins other than Ets-1 also might bind these elements. Crude nuclear extracts of EL4.E1 cells were assayed to identify the protein(s) that potentially functions at the LVb element. The predominant binding activity was not Ets-1 but rather two independent DNA-protein complexes that comigrated in mobility shift assays. UV cross-linking and denaturing gel electrophoresis sized the two DNA-binding species, which we denoted p55 and p100. Immunoprecipitation combined with UV cross-linking identified p55 as the alpha subunit of GA-binding protein. The DNA-binding properties of p100 and several ets proteins were compared. Similarities suggested that p100 is also an ETS domain protein, possibly Elf-1. This strategy could be used to identify other ETS domain proteins in crude nuclear extracts. These findings suggest multiple ETS domain proteins could regulate gene expression of Mo-MuLV.

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

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

Pathogenic determinants in the U3 region of recombinant murine leukemia viruses isolated from CWD and HRS/J mice

Recombinant murine leukemia viruses (MuLVs) from high-leukemia-incidence mouse strains typically acquire pathogenic U3 region sequences from the genome of the endogenous xenotropic virus, Bxv-1. However, a recombinant virus isolated from a leukemic HRS/J mouse and another from a CWD mouse contained U3 regions that lacked genetic markers of Bxv-1. The U3 regions of both recombinants were derived from the endogenous ecotropic virus Env-1 and had retained a single enhancer element. However, compared with that of Emv-1, the U3 region of each of the recombinant viruses contained five nucleotide substitutions, one of which was shared. To determine the biological significance of these substitutions, chimeric ecotropic viruses that contained the U3 region from one of the two recombinant viruses or from Emv-1 were injected into NIH Swiss mice. All three of the chimeric ecotropic viruses were leukemogenic following a long latency. Despite the presence of an enhancer core motif that is known to contribute to the leukemogenicity of the AKR MuLV SL3-3, the HRS/J virus U3 region induced lymphomas only slightly more rapidly than the allelic Emv-1 sequences. The chimeric virus with the U3 region of the CWD recombinant caused lymphomas more frequently and more rapidly than either of the other two viruses. The results support the hypothesis that one or more of the five nucleotide substitutions in the U3 regions of the recombinants contribute to viral pathogenicity. Comparison of DNA sequences suggests that the pathogenicity of the CWD virus U3 region was related to a sequence motif that is shared with Bxv-1 and is recognized by the basic helix-loop-helix class of transcription factors.