ERP, a new member of the ets transcription factor/oncoprotein family: cloning, characterization, and differential expression during B-lymphocyte development

B Lymphocyte Development in the Bursa of Fabricius of Young Broilers is Influenced by the Gut Microbiota

Chickens have been used as a valuable and traditional model for studies on basic immunology. B lymphocytes were first identified in the bursa of Fabricius (BF) of broilers. The microbiota is important for immune system development and function. However, the effect of the microbiota on mediating B cell development and its regulatory mechanism is poorly elucidated. Here, we show that the gut microbiota is associated with the development of bursal B cells in young chickens. Changing patterns of both the alpha diversity and the expression of the B cell marker Bu-1α in the gut microbiota were related to the ages of chickens at different growth phases. Further correlation analysis revealed the marked correlation between the relative abundances of Intestinimonas, Bilophila, Parasutterella, Bacteroides, Helicobacter, Campylobacter, and Mucispirillum and the expression of Bu-1α. In antibiotic-treated chickens, BF and B cell development had aberrations as the relative abundance of the microbiota in early life decreased. These findings were consistent with Spearman's correlation results. Single-cell transcriptome analysis indicated that the heterogeneity in the cellular composition and developmental trajectory of bursal B cells from antibiotic-treated chickens was large. We found a novel subpopulation of unnamed B cells and identified Taf1 as a new pivotal regulator of B cell lineage differentiation. Therefore, we provide novel insights into the regulatory role of the gut microbiota in B cell development in early life and the maturation of host humoral immunity. IMPORTANCE In this study, we used young broilers to investigate the relationship between their gut microbiota and bursal B cell development. We characterized the important variables, microbes, B cells, and immunoglobulins during the posthatch development of birds. We also identified several candidate taxa in the cecal contents associated with B cells. Our study provides a rich resource and cell-cell cross talk model supporting B cell differentiation from the bursa in vitro at single-cell resolution. Furthermore, we determined a new pivotal regulator (Taf1) of B cell differentiation. We believe that our study makes a significant contribution to the literature because our findings may elucidate the role of the gut microbiota in B cell differentiation. This study also serves as a basis for developing new strategies that modulate B cell differentiation to prevent diseases.

A genome-wide association study identifies risk loci for childhood acute lymphoblastic leukemia at 10q26.13 and 12q23.1

Genome-wide association studies (GWASs) have shown that common genetic variation contributes to the heritable risk of childhood acute lymphoblastic leukemia (ALL). To identify new susceptibility loci for the largest subtype of ALL, B-cell precursor ALL (BCP-ALL), we conducted a meta-analysis of two GWASs with imputation using 1000 Genomes and UK10K Project data as reference (totaling 1658 cases and 7224 controls). After genotyping an additional 2525 cases and 3575 controls, we identify new susceptibility loci for BCP-ALL mapping to 10q26.13 (rs35837782, LHPP, P=1.38 × 10-11) and 12q23.1 (rs4762284, ELK3, P=8.41 × 10-9). We also provide confirmatory evidence for the existence of independent risk loci at 9p21.3, but show that the association marked by rs77728904 can be accounted for by linkage disequilibrium with the rare high-impact CDKN2A p.Ala148Thr variant rs3731249. Our data provide further insights into genetic susceptibility to ALL and its biology.

Elk-3 is a KLF4-regulated gene that modulates the phagocytosis of bacteria by macrophages

ETS family proteins play a role in immune responses. A unique member of this family, Elk-3, is a transcriptional repressor that regulates the expression of HO-1. Elk-3 is very sensitive to the effects of inflammatory mediators and is down-regulated by bacterial endotoxin (LPS). In the present study, exposure of mouse macrophages to Escherichia coli LPS resulted in decreased, full-length, and splice-variant isoforms of Elk-3. We isolated the Elk-3 promoter and demonstrated that LPS also decreased promoter activity. The Elk-3 promoter contains GC-rich regions that are putative binding sites for zinc-finger transcription factors, such as Sp1 and KLFs. Mutation of the GC-rich region from bp -613 to -603 blunted LPS-induced down-regulation of the Elk-3 promoter. Similar to the LPS response, coexpression of KLF4 led to repression of Elk-3 promoter activity, whereas coexpression of Sp1 increased activity. ChIP assays revealed that KLF4 binding to the Elk-3 promoter was increased by LPS exposure, and Sp1 binding was decreased. Thus, down-regulation of Elk-3 by bacterial LPS is regulated, in part, by the transcriptional repressor KLF4. Overexpression of Elk-3, in the presence of E. coli bacteria, resulted in decreased macrophage phagocytosis. To determine whether limited expression of HO-1 may contribute to this response, we exposed HO-1-deficient bone marrow-derived macrophages to E. coli and found a comparable reduction in bacterial phagocytosis. These data suggest that down-regulation of Elk-3 and the subsequent induction of HO-1 are important for macrophage function during the inflammatory response to infection.

Elk-1 a transcription factor with multiple facets in the brain

The ternary complex factor (TCF) Elk-1 is a transcription factor that regulates immediate early gene (IEG) expression via the serum response element (SRE) DNA consensus site. Elk-1 is associated with a dimer of serum response factor (SRF) at the SRE site, and its phosphorylation occurs at specific residues in response to mitogen-activated protein kinases (MAPKs), including c-Jun-N terminal kinase (JNK), p38/MAPK, and extracellular-signal regulated kinase (ERK). This phosphorylation event is critical for triggering SRE-dependent transcription. Although MAPKs are fundamental actors for the instatement and maintenance of memory, and much investigation of their downstream signaling partners have been conducted, no data yet clearly implicate Elk-1 in these processes. This is partly due to the complexity of Elk-1 sub-cellular localization, and hence functions, within neurons. Elk-1 is present in its resting state in the cytoplasm, where it colocalizes with mitochondrial proteins or microtubules. In this particular sub-cellular compartment, overexpression of Elk-1 is toxic for neuronal cells. When phosphorylated by the MAPK/ERK, Elk-1 translocates to the nucleus where it is implicated in regulating chromatin remodeling, SRE-dependent transcription, and neuronal differentiation. Another post-translational modification is the conjugation to SUMO (Small Ubiquitin-like MOdifier), which relocalizes Elk-1 in the cytoplasm. Thus, Elk-1 plays a dual role in neuronal functions: pro-apoptotic within the cytoplasm, and pro-differentiation within the nucleus. To address the role of Elk-1 in the brain, one must be aware of its multiple facets, and design molecular tools that will shut down Elk-1 expression, trafficking, or activation, in specific neuronal compartments. We summarize in this review the known molecular functions of Elk-1, its regulation in neuronal cells, and present evidence of its possible implication in model systems of synaptic plasticity, learning, but also in neurodegenerative diseases.

The expression of ELK transcription factors in adult DRG: Novel isoforms, antisense transcripts and upregulation by nerve damage

ELK transcription factors are known to be expressed in a number of regions in the nervous system. We show by RT-PCR that the previously described Elk1, Elk3/Elk3b/Elk3c and Elk4 mRNAs are expressed in adult dorsal root ganglia (DRG), together with the novel alternatively spliced isoforms Elk1b, Elk3d and Elk4c/Elk4d/Elk4e. These isoforms are also expressed in brain, heart, kidney and testis. In contrast to Elk3 protein, the novel Elk3d isoform is cytoplasmic, fails to bind ETS binding sites and yet can activate transcription by an indirect mechanism. The Elk3 and Elk4 genes are overlapped by co-expressed Pctk2 (Cdk17) and Mfsd4 genes, respectively, with the potential formation of Elk3/Pctaire2 and Elk4/Mfsd4 sense-antisense mRNA heteroduplexes. After peripheral nerve injury the Elk3 mRNA isoforms are each upregulated approximately 2.3-fold in DRG (P<0.005), whereas the natural antisense Pctaire2 isoforms show only a small increase (21%, P<0.01) and Elk1 and Elk4 mRNAs are unchanged.

Antiinflammatory effects of the ETS factor ERG in endothelial cells are mediated through transcriptional repression of the interleukin-8 gene

ERG (Ets-related gene) is an ETS transcription factor that has recently been shown to regulate a number of endothelial cell (EC)-restricted genes including VE-cadherin, von Willebrand factor, endoglin, and intercellular adhesion molecule-2. Our preliminary data demonstrate that unlike other ETS factors, ERG exhibits a highly EC-restricted pattern of expression in cultured primary cells and several adult mouse tissues including the heart, lung, and brain. In response to inflammatory stimuli, such as tumor necrosis factor-alpha, we observed a marked reduction of ERG expression in ECs. To further define the role of ERG in the regulation of normal EC function, we used RNA interference to knock down ERG. Microarray analysis of RNA derived from ERG small interfering RNA- or tumor necrosis factor-alpha-treated human umbilical vein (HUV)ECs revealed significant overlap (P<0.01) in the genes that are up- or downregulated. Of particular interest to us was a significant change in expression of interleukin (IL)-8 at both protein and RNA levels. Exposure of ECs to tumor necrosis factor-alpha is known to be associated with increased neutrophil attachment. We observed that knockdown of ERG in HUVECs is similarly associated with increased neutrophil attachment compared to control small interfering RNA-treated cells. This enhanced adhesion could be blocked with IL-8 neutralizing or IL-8 receptor blocking antibodies. ERG can inhibit the activity of the IL-8 promoter in a dose dependent manner. Direct binding of ERG to the IL-8 promoter in ECs was confirmed by chromatin immunoprecipitation. In summary, our findings support a role for ERG in promoting antiinflammatory effects in ECs through repression of inflammatory genes such as IL-8.

Genome-wide array comparative genomic hybridization analysis reveals distinct amplifications in osteosarcoma

Background

Osteosarcoma is a highly malignant bone neoplasm of children and young adults. It is characterized by extremely complex karyotypes and high frequency of chromosomal amplifications. Currently, only the histological response (degree of necrosis) to therapy represent gold standard for predicting the outcome in a patient with non-metastatic osteosarcoma at the time of definitive surgery. Patients with lower degree of necrosis have a higher risk of relapse and poor outcome even after chemotherapy and complete resection of the primary tumor. Therefore, a better understanding of the underlying molecular genetic events leading to tumor initiation and progression could result in the identification of potential diagnostic and therapeutic targets.

Methods

We used a genome-wide screening method – array based comparative genomic hybridization (array-CGH) to identify DNA copy number changes in 48 patients with osteosarcoma. We applied fluorescence in situ hybridization (FISH) to validate some of amplified clones in this study.

Results

Clones showing gains (79%) were more frequent than losses (66%). High-level amplifications and homozygous deletions constitute 28.6% and 3.8% of tumor genome respectively. High-level amplifications were present in 238 clones, of which about 37% of them showed recurrent amplification. Most frequently amplified clones were mapped to 1p36.32 (PRDM16), 6p21.1 (CDC5L, HSPCB, NFKBIE), 8q24, 12q14.3 (IFNG), 16p13 (MGRN1), and 17p11.2 (PMP22 MYCD, SOX1,ELAC27). We validated some of the amplified clones by FISH from 6p12-p21, 8q23-q24, and 17p11.2 amplicons. Homozygous deletions were noted for 32 clones and only 7 clones showed in more than one case. These 7 clones were mapped to 1q25.1 (4 cases), 3p14.1 (4 cases), 13q12.2 (2 cases), 4p15.1 (2 cases), 6q12 (2 cases), 6q12 (2 cases) and 6q16.3 (2 cases).

Conclusions

This study clearly demonstrates the utility of array CGH in defining high-resolution DNA copy number changes and refining amplifications. The resolution of array CGH technology combined with human genome database suggested the possible target genes present in the gained or lost clones.

Glutamate signaling to Ras-MAPK in striatal neurons: mechanisms for inducible gene expression and plasticity

Extracellular signals can regulate mitogen-activated protein kinase (MAPK) cascades through a receptor-mediated mechanism in postmitotic neurons of adult mammalian brain. Both ionotropic and metabotropic glutamate receptors (mGluRs) are found to possess such an ability in striatal neurons. NMDA and AMPA receptor signals seem to share a largely common route to MAPK phosphorylation which involves first activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) via Ca2+ influx, followed by subsequent induction of phosphoinositide 3-kinase (PI3-kinase). Through its lipid and protein kinase activity, active PI3-kinase may transduce signals to Ras-MAPK cascades via at least two distinct pathways. A novel, Ca(2+)-independent pathway is believed to mediate mGluR signals to Ras-MAPK activation. As an information superhighway between the surface membrane and the nucleus, Ras-MAPK cascades, through activating their specific nuclear transcription factor targets, are actively involved in the regulation of gene expression. Emerging evidence shows that MAPK-mediated genomic responses in striatal neurons to drug exposure contribute to the development of neuroplasticity related to addictive properties of drugs of abuse.

Mice deficient for the ets transcription factor elk-1 show normal immune responses and mildly impaired neuronal gene activation

The transcription factor Elk-1 belongs to the ternary complex factor (TCF) subfamily of Ets proteins. TCFs interact with serum response factor to bind jointly to serum response elements in the promoters of immediate-early genes (IEGs). TCFs mediate the rapid transcriptional response of IEGs to various extracellular stimuli which activate mitogen-activated protein kinase signaling. To investigate physiological functions of Elk-1 in vivo, we generated Elk-1-deficient mice by homologous recombination in embryonic stem cells. These animals were found to be phenotypically indistinguishable from their wild-type littermates. Histological analysis of various tissues failed to reveal any differences between Elk-1 mutant and wild-type mice. Elk-1 deficiency caused no changes in the proteomic displays of brain or spleen extracts. Also, no immunological defects could be detected in mice lacking Elk-1, even upon infection with coxsackievirus B3. In mouse embryonic fibroblasts, Elk-1 was dispensable for c-fos and Egr-1 transcriptional activation upon stimulation with serum, lysophosphatidic acid, or tetradecanoyl phorbol acetate. However, in brains of Elk-1-deficient mice, cortical and hippocampal CA1 expression of c-fos, but not Egr-1 or c-Jun, was markedly reduced 4 h following kainate-induced seizures. This was not accompanied by altered patterns of neuronal apoptosis. Collectively, our data indicate that Elk-1 is essential neither for mouse development nor for adult life, suggesting compensatory activities by other TCFs.

Net-targeted mutant mice develop a vascular phenotype and up-regulate egr-1

The ternary complex factors (TCFs) Net, Elk-1 and Sap-1 regulate immediate early genes through serum response elements (SREs) in vitro, but, surprisingly, their in vivo roles are unknown. Net is a repressor that is expressed in sites of vasculogenesis during mouse development. We have made gene-targeted mice that express a hypomorphic mutant of Net, Net delta, which lacks the Ets DNA-binding domain. Strikingly, homozygous mutant mice develop a vascular defect and up-regulate an immediate early gene implicated in vascular disease, egr-1. They die after birth due to respiratory failure, resulting from the accumulation of chyle in the thoracic cage (chylothorax). The mice have dilated lymphatic vessels (lymphangiectasis) as early as E16.5. Interestingly, they express more egr-1 in heart and pulmonary arteries at E18.5. Net negatively regulates the egr-1 promoter and binds specifically to SRE-5. Egr-1 has been associated with pathologies involving vascular stenosis (e.g. atherosclerosis), and here egr-1 dysfunction could possibly be associated with obstructions that ultimately affect the lymphatics. These results show that Net is involved in vascular biology and egr-1 regulation in vivo.

The B-box dominates SAP-1-SRF interactions in the structure of the ternary complex

The serum response element (SRE) is found in several immediate-early gene promoters. This DNA sequence is necessary and sufficient for rapid transcriptional induction of the human c-fos proto-oncogene in response to stimuli external to the cell. Full activation of the SRE requires the cooperative binding of a ternary complex factor (TCF) and serum response factor (SRF) to their specific DNA sites. The X-ray structure of the human SAP-1-SRF-SRE DNA ternary complex was determined (Protein Data Bank code 1hbx). It shows SAP-1 TCF bound to SRF through interactions between the SAP-1 B-box and SRF MADS domain in addition to contacts between their respective DNA-binding motifs. The SAP-1 B-box is part of a flexible linker of which 21 amino acids become ordered upon ternary complex formation. Comparison with a similar region from the yeast MATalpha2-MCM1-DNA complex suggests a common binding motif through which MADS-box proteins may interact with additional factors such as Fli-1.

Peptides derived from pro-growth hormone-releasing hormone activate p38 mitogen-activated protein kinase in GH3 pituitary cells

Posttranslational processing of the pro-growth hormone-releasing hormone (proGHRH) peptide can result in the formation of at least two peptide products: GHRH and the C-terminal peptide, GHRH-related peptide (GHRH-RP). While cyclic adenosine monophosphate transduces many of the actions of GHRH, other pathways also have been implicated in its actions. The aims of this study were to examine and characterize the activation of mitogen-activated protein kinase (MAPK) pathways by GHRH, and GHRH-RP in pituitary-derived GH3 cells, as well as the activation of the transcription factors that serve as substrates for these kinases. GHRH rapidly increased p44/p42 MAPK activity in GH3 cells in a protein kinase A-dependent and a protein kinase C-independent manner and stimulated the activation of the transcription factor Elk-1. By contrast, GHRH-RP and p75-92NH2 had no effect on p44/p42 MAPK phosphorylation in these cells. Additionally, we determined that all three peptides, GHRH, GHRH-RP, and p75-92NH2, rapidly and specifically increase phosphorylation of p38 MAPK and stimulate the activation of the nuclear factor CHOP. These are the first studies to demonstrate the activation of Elk-1 by GHRH and the activation of p38 MAPK and CHOP by GHRH, GHRH-RP, and p75-92NH2. We conclude that members of the GHRH family of peptides differentially activate multiple intracellular signaling pathways and suggest that the biologic actions of GHRH may be far more diverse than previously thought.

c-Fos oncogene regulator Elk-1 interacts with BRCA1 splice variants BRCA1a/1b and enhances BRCA1a/1b-mediated growth suppression in breast cancer cells

Elk-1, a c-Fos protooncogene regulator, which belongs to the ETS-domain family of transcriptional factors, plays an important role in the induction of immediate early gene expression in response to a variety of extracellular signals. In this study, we demonstrate for the first time the in vitro and in vivo interaction of Elk-1 with BRCA1 splice variants BRCA1a and BRCA1b using GST-pull down assays, co-imunoprecipitations/Western blot analysis of cell extracts from breast cancer cells and mammalian two-hybrid assays. We have localized the BRCA1 interaction domain of Elk-1 protein to the conserved ETS domain, a motif involved in DNA binding and protein-protein interactions. We also observed binding of BRCA1 proteins to other ETS-domain transcription factors SAP1, ETS-1, ERG-2 and Fli-1 but not to Elk-1 splice variant DeltaElk-1 and c-Fos protooncogene. Both BRCA1a and BRCA1b splice variants function as growth suppressors of human breast cancer cells. Interestingly, our studies reveal that although both Elk-1 and SAP-1 are highly homologous members of a subfamily of ETS domain proteins called ternary complex factors, it is only Elk-1 but not SAP-1 that can augment the growth suppressive function of BRCA1a/1b proteins in breast cancer cells. Thus Elk-1 could be a potential downstream target of BRCA1 in its growth control pathway. Furthermore, we have observed inhibition of c-Fos promoter activity in BRCA1a transfected stable breast cancer cells and over expression of BRCA1a/1b attenuates MEK-induced SRE activation in vivo. These results demonstrate for the first time a link between the growth suppressive function of BRCA1a/1b proteins and signal transduction pathway involving Elk-1 protein. All these results taken together suggest that one of the mechanisms by which BRCA1a/1b proteins function as growth/tumor suppressors is through inhibition of the expression of Elk-1 target genes like c-Fos.

Proteins of the ETS family with transcriptional repressor activity

ETS proteins form one of the largest families of signal-dependent transcriptional regulators, mediating cellular proliferation, differentiation and tumorigenesis. Most of the known ETS proteins have been shown to activate transcription. However, four ETS proteins (YAN, ERF, NET and TEL) can act as transcriptional repressors. In three cases (ERF, NET and TEL) distinct repression domains have been identified and there are indications that NET and TEL may mediate transcription via Histone Deacetylase recruitment. All four proteins appear to be regulated by MAPKs, though for YAN and ERF this regulation seems to be restricted to ERKs. YAN, ERF and TEL have been implicated in cellular proliferation although there are indications suggesting a possible involvement of YAN and TEL in differentiation as well. Other ETS-domain proteins have been shown to repress transcription in a context specific manner, and there are suggestions that the ETS DNA-binding domain may act as a transcriptional repressor. Transcriptional repression by ETS domain proteins adds an other level in the orchestrated regulation by this diverse family of transcription factors that often recognize similar if not identical binding sites on DNA and are believed to regulate critical genes in a variety of biological processes. Definitive assessment of the importance of this novel regulatory level will require the identification of ETS proteins target genes and the further analysis of transcriptional control and biological function of these proteins in defined pathways.

Signal transduction and the Ets family of transcription factors

Cellular responses to environmental stimuli are controlled by a series of signaling cascades that transduce extracellular signals from ligand-activated cell surface receptors to the nucleus. Although most pathways were initially thought to be linear, it has become apparent that there is a dynamic interplay between signaling pathways that result in the complex pattern of cell-type specific responses required for proliferation, differentiation and survival. One group of nuclear effectors of these signaling pathways are the Ets family of transcription factors, directing cytoplasmic signals to the control of gene expression. This family is defined by a highly conserved DNA binding domain that binds the core consensus sequence GGAA/T. Signaling pathways such as the MAP kinases, Erk1 and 2, p38 and JNK, the PI3 kinases and Ca2+-specific signals activated by growth factors or cellular stresses, converge on the Ets family of factors, controlling their activity, protein partnerships and specification of downstream target genes. Interestingly, Ets family members can act as both upstream and downstream effectors of signaling pathways. As downstream effectors their activities are directly controlled by specific phosphorylations, resulting in their ability to activate or repress specific target genes. As upstream effectors they are responsible for the spacial and temporal expression or numerous growth factor receptors. This review provides a brief survey of what is known to date about how this family of transcription factors is regulated by cellular signaling with a special focus on Ras responsive elements (RREs), the MAP kinases (Erks, p38 and JNK) and Ca2+-specific pathways and includes a description of the multiple roles of Ets family members in the lymphoid system. Finally, we will discuss other potential mechanisms and pathways involved in the regulation of this important family of transcription factors.

C/EBP beta and Elk-1 synergistically transactivate the c-fos serum response element

BACKGROUND

The serum response element (SRE) in the c-fos promoter is a convergence point for several signaling pathways that regulate induction of the c-fos gene. Many transcription factors regulate the SRE, including serum response factor (SRF), ternary complex factor (TCF), and CCAAT/enhancer binding protein-beta (C/EBPbeta). Independently, the TCFs and C/EBPbeta have been shown to interact with SRF and to respond to Ras-dependent signaling pathways that result in transactivation of the SRE. Due to these common observations, we addressed the possibility that C/EBPbeta and Elk-1 could both be necessary for Ras-stimulated transactivation of the SRE.

RESULTS

In this report, we demonstrate that Elk-1 and C/EBPbeta functionally synergize in transactivation of both a Gal4 reporter plasmid in concert with Gal4-SRF and in transactivation of the SRE. Interestingly, this synergy is only observed upon activation of Ras-dependent signaling pathways. Furthermore, we show that Elk-1 and C/EBPbeta could interact both in an in vitro GST-pulldown assay and in an in vivo co-immunoprecipitation assay. The in vivo interaction between the two proteins is dependent on the presence of activated Ras. We have also shown that the C-terminal domain of C/EBPbeta and the N-terminal domain of Elk-1 are necessary for the proteins to interact.

CONCLUSIONS

These data show that C/EBPbeta and Elk-1 synergize in SRF dependent transcription of both a Gal-4 reporter and the SRE. This suggests that SRF, TCF, and C/EBPbeta are all necessary for maximal induction of the c-fos SRE in response to mitogenic signaling by Ras.

Molecular characterization of a zebrafish TCF ETS-domain transcription factor

The ternary complex factor (TCF) subfamily of ETS-transcription factors represent key nuclear targets of the MAP kinase pathways. Members of this subfamily are classified by the presence of several conserved domains for DNA binding, interaction with SRF, interaction with MAP kinases and transcriptional activation. In this study we have isolated a further member of this subfamily (TCF-1) from zebrafish. The protein product of zebrafish TCF-1 (zTCF-1), shares sequence similarity with the mammalian TCFs in all four conserved domains, with highest overall similarity to SAP-1. Zebrafish TCF-1 is expressed throughout zebrafish embryonic development and exhibits typical TCF DNA binding characteristics, with the B-box being required for interaction with SRF. Of the mammalian TCFs, its DNA binding specificity resembles Elk-1. zTCF-1 is a target for both the growth factor/mitogen-activated and stress-activated MAP kinase cascades in vitro and in vivo. However, differential targeting occurs, with the profile of its activation closely resembling that of mammalian SAP-1. Together, our results demonstrate that the TCFs have been functionally conserved during vertebrate development.

The net repressor is regulated by nuclear export in response to anisomycin, UV, and heat shock

The ternary complex factors (TCFs) are targets for Ras/mitogen-activated protein kinase signalling pathways. They integrate the transcriptional response at the level of serum response elements in early-response genes, such as the c-fos proto-oncogene. An important aim is to understand the individual roles played by the three TCFs, Net, Elk1, and Sap1a. Net, in contrast to Elk1 and Sap1a, is a strong repressor of transcription. We now show that Net is regulated by nuclear-cytoplasmic shuttling in response to specific signalling pathways. Net is mainly nuclear under both normal and basal serum conditions. Net contains two nuclear localization signals (NLSs); one is located in the Ets domain, and the other corresponds to the D box. Net also has a nuclear export signal (NES) in the conserved Ets DNA binding domain. Net is apparently unique among Ets proteins in that a particular leucine in helix 1, a structural element, generates a NES. Anisomycin, UV, and heat shock induce active nuclear exclusion of Net through a pathway that involves c-Jun N-terminal kinase kinase and is inhibited by leptomycin B. Nuclear exclusion relieves transcriptional repression by Net. The specific induction of nuclear exclusion of Net by particular signalling pathways shows that nuclear-cytoplasmic transport of transcription factors can add to the specificity of the response to signalling cascades.

A novel 14-base-pair regulatory element is essential for in vivo expression of murine beta4-galactosyltransferase-I in late pachytene spermatocytes and round spermatids

During murine spermatogenesis, beginning in late pachytene spermatocytes, the beta4-galactosyltransferase-I (beta4GalT-I) gene is transcribed from a male germ cell-specific start site. We had shown previously that a 796-bp genomic fragment that flanks the germ cell start site and contains two putative CRE (cyclic AMP-responsive element)-like motifs directs correct male germ cell expression of the beta-galactosidase reporter gene in late pachytene spermatocytes and round spermatids of transgenic mice (N. L. Shaper, A. Harduin-Lepers, and J. H. Shaper, J. Biol. Chem. 269:25165-25171, 1994). We now report that in vivo expression of beta4GalT-I in developing male germ cells requires an essential and previously undescribed 14-bp regulatory element (5'-GCCGGTTTCCTAGA-3') that is distinct from the two CRE-like sequences. This cis element is located 16 bp upstream of the germ cell-specific start site and binds a male germ cell protein that we have termed TASS-1 (transcriptional activator in late pachytene spermatocytes and round spermatids 1). The presence of the Ets signature binding motif 5'-GGAA-3' on the bottom strand of the TASS-1 sequence (underlined sequence) suggests that TASS-1 is a novel member of the Ets family of transcription factors. Additional transgenic analyses established that an 87-bp genomic fragment containing the TASS-1 regulatory element was sufficient for correct germ cell-specific expression of the beta-galactosidase reporter gene. Furthermore, when the TASS-1 motif was mutated by transversion, within the context of the original 796-bp fragment, transgene expression was reduced 12- to 35-fold in vivo.

Id helix-loop-helix proteins inhibit nucleoprotein complex formation by the TCF ETS-domain transcription factors

The Id subfamily of helix-loop-helix (HLH) proteins plays a fundamental role in the regulation of cellular proliferation and differentiation. Id proteins are thought to inhibit differentiation mainly through interaction with other HLH proteins and by blocking their DNA-binding activity. Members of the ternary complex factor (TCF) subfamily of ETS-domain proteins have key functions in regulating immediate-early gene expression in response to mitogenic stimulation. TCFs form DNA-bound complexes with the serum response factor (SRF) and are direct targets of MAP kinase (MAPK) signal transduction cascades. In this study we demonstrate functional interactions between Id proteins and TCFs. Ids bind to the ETS DNA-binding domain and disrupt the formation of DNA-bound complexes between TCFs and SRF on the c-fos serum response element (SRE). Inhibition occurs by disrupting protein-DNA interactions with the TCF component of this complex. In vivo, the Id proteins cause down-regulation of the transcriptional activity mediated by the TCFs and thereby block MAPK signalling to SREs. Therefore, our results demonstrate a novel facet of Id function in the coordination of mitogenic signalling and cell cycle entry.

Glutamate induces phosphorylation of Elk-1 and CREB, along with c-fos activation, via an extracellular signal-regulated kinase-dependent pathway in brain slices

In cell culture systems, the TCF Elk-1 represents a convergence point for extracellular signal-related kinase (ERK) and c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) subclasses of mitogen-activated protein kinase (MAPK) cascades. Its phosphorylation strongly potentiates its ability to activate transcription of the c-fos promoter through a ternary complex assembled on the c-fos serum response element. In rat brain postmitotic neurons, Elk-1 is strongly expressed (V. Sgambato, P. Vanhoutte, C. Pagès, M. Rogard, R. A. Hipskind, M. J. Besson, and J. Caboche, J. Neurosci. 18:214-226, 1998). However, its physiological role in these postmitotic neurons remains to be established. To investigate biochemically the signaling pathways targeting Elk-1 and c-fos in mature neurons, we used a semi-in vivo system composed of brain slices stimulated with the excitatory neurotransmitter glutamate. Glutamate treatment leads to a robust, progressive activation of the ERK and JNK/SAPK MAPK cascades. This corresponds kinetically to a significant increase in Ser383-phosphorylated Elk-1 and the appearance of c-fos mRNA. Glutamate also causes increased levels of Ser133-phosphorylated cyclic AMP-responsive element-binding protein (CREB) but only transiently relative to Elk-1 and c-fos. ERK and Elk-1 phosphorylation are blocked by the MAPK kinase inhibitor PD98059, indicating the primary role of the ERK cascade in mediating glutamate signaling to Elk-1 in the rat striatum in vivo. Glutamate-mediated CREB phosphorylation is also inhibited by PD98059 treatment. Interestingly, KN62, which interferes with calcium-calmodulin kinase (CaM-K) activity, leads to a reduction of glutamate-induced ERK activation and of CREB phosphorylation. These data indicate that ERK functions as a common component in two signaling pathways (ERK/Elk-1 and ERK/?/CREB) converging on the c-fos promoter in postmitotic neuronal cells and that CaM-Ks act as positive regulators of these pathways.

Tissue-restricted expression of the cardiac alpha-myosin heavy chain gene is controlled by a downstream repressor element containing a palindrome of two ets-binding sites

The expression of the alpha-myosin heavy chain (MHC) gene is restricted primarily to cardiac myocytes. To date, several positive regulatory elements and their binding factors involved in alpha-MHC gene regulation have been identified; however, the mechanism restricting the expression of this gene to cardiac myocytes has yet to be elucidated. In this study, we have identified by using sequential deletion mutants of the rat cardiac alpha-MHC gene a 30-bp purine-rich negative regulatory (PNR) element located in the first intronic region that appeared to be essential for the tissue-specific expression of the alpha-MHC gene. Removal of this element alone elevated (20- to 30-fold) the expression of the alpha-MHC gene in cardiac myocyte cultures and in heart muscle directly injected with plasmid DNA. Surprisingly, this deletion also allowed a significant expression of the alpha-MHC gene in HeLa and other nonmuscle cells, where it is normally inactive. The PNR element required upstream sequences of the alpha-MHC gene for negative gene regulation. By DNase I footprint analysis of the PNR element, a palindrome of two high-affinity Ets-binding sites (CTTCCCTGGAAG) was identified. Furthermore, by analyses of site-specific base-pair mutation, mobility gel shift competition, and UV cross-linking, two different Ets-like proteins from cardiac and HeLa cell nuclear extracts were found to bind to the PNR motif. Moreover, the activity of the PNR-binding factor was found to be increased two- to threefold in adult rat hearts subjected to pressure overload hypertrophy, where the alpha-MHC gene is usually suppressed. These data demonstrate that the PNR element plays a dual role, both downregulating the expression of the alpha-MHC gene in cardiac myocytes and silencing the muscle gene activity in nonmuscle cells. Similar palindromic Ets-binding motifs are found conserved in the alpha-MHC genes from different species and in other cardiac myocyte-restricted genes. These results are the first to reveal a role of the Ets class of proteins in controlling the tissue-specific expression of a cardiac muscle gene.

Serum response factor is essential for mesoderm formation during mouse embryogenesis

The transcription factor serum response factor (SRF), a phylogenetically conserved nuclear protein, mediates the rapid transcriptional response to extracellular stimuli, e.g. growth and differentiation signals. DNA- protein complexes containing SRF or its homologues function as nuclear targets of the Ras/MAPK signalling network, thereby directing gene activities associated with processes as diverse as pheromone signalling, cell-cycle progression (transitions G0-G1 and G2-M), neuronal synaptic transmission and muscle cell differentiation. So far, the activity of mammalian SRF has been studied exclusively in cultured cells. To study SRF function in a multicellular organism we generated an Srf null allele in mice. SRF-deficient embryos (Srf -/-) have a severe gastrulation defect and do not develop to term. They consist of misfolded ectodermal and endodermal cell layers, do not form a primitive streak or any detectable mesodermal cells and fail to express the developmental marker genes Bra (T), Bmp-2/4 and Shh. Activation of the SRF-regulated immediate early genes Egr-1 and c-fos, as well as the alpha-Actin gene, is severely impaired. Our study identifies SRF as a new and essential regulator of mammalian mesoderm formation. We therefore suggest that in mammals Ras/MAPK signalling contributes to mesoderm induction, as is the case in amphibia.

Cloning and characterization of GETS-1, a goldfish Ets family member that functions as a transcriptional repressor in muscle

An Ets transcription factor family member, GETS-1, was cloned from a goldfish retina cDNA library. GETS-1 contains a conserved Ets DNA-binding domain at its N-terminus and is most similar to ternary complex factor (TCF) serum-response-factor protein-1a (SAP-1a). GETS-1 is expressed in many tissues, but is enriched in retina and brain. As with the TCFs SAP-1a and ets-related protein (ERP), overexpression of the GETS-1 promoter suppresses nicotinic acetylcholine receptor epsilon-subunit gene expression in cultured muscle cells. A consensus Ets binding site sequence in the promoter of the epsilon-subunit gene is required for GETS-1-mediated repression. GETS-1 repressor activity is abrogated by overexpression of an activated Ras/mitogen-activated protein kinase (MAP kinase) or by mutation of Ser-405, a MAP kinase phosphorylation site in GETS-1. Fusion proteins created between GETS-1 and the Gal4 DNA-binding domain show that, like other TCFs, GETS-1 contains a C-terminal activation domain that is activated by a Ras/MAP kinase signalling cascade. Interestingly, mutation of Ser-405 located within this activation domain abrogated transcriptional activation of the fusion protein.

Synapse-specific and neuregulin-induced transcription require an ets site that binds GABPalpha/GABPbeta

Localization of acetylcholine receptors (AChRs) to neuromuscular synapses is mediated by multiple pathways. Agrin, which is the signal for one pathway, stimulates a redistribution of previously unlocalized AChRs to synaptic sites. The signal for a second pathway is not known, but this signal stimulates selective transcription of AChR genes in myofiber nuclei located near the synaptic site. Neuregulin (NRG) is a good candidate for the extracellular signal that induces synapse-specific gene expression, since NRG is concentrated at synaptic sites and activates AChR gene expression in cultured muscle cells. Previous studies have demonstrated that 181 bp of 5' flanking DNA from the AChR delta-subunit gene are sufficient to confer synapse-specific transcription in transgenic mice and NRG responsiveness in cultured muscle cells, but the critical sequences within this cis-acting regulatory region have not been identified. We transfected AChR delta-subunit-hGH gene fusions into a muscle cell line, and we show that a potential binding site for Ets proteins is required for NRG-induced gene expression. Furthermore, we produced transgenic mice carrying AChR delta-subunit-hGH gene fusions with a mutation in this NRG-response element (NRE), and we show that this NRE is necessary for synapse-specific transcription in mice. The NRE binds proteins in myotube nuclear extracts, and nucleotides that are important for NRG responsiveness are likewise critical for formation of the protein-DNA complex. This complex contains GABPalpha, an Ets protein, and GABPbeta, a protein that lacks an Ets domain but dimerizes with GABPalpha, because formation of the protein-DNA complex is inhibited by antibodies to either GABPalpha or GABPbeta. These results demonstrate that synapse-specific and NRG-induced gene expression require an Ets-binding site and suggest that GABPalpha/GABPbeta mediates the transcriptional response of the AChR delta-subunit gene to synaptic signals, including NRG.

Implication of a multisubunit Ets-related transcription factor in synaptic expression of the nicotinic acetylcholine receptor

In adult muscle, transcription of the nicotinic acetylcholine receptor (AChR) is restricted to the nuclei located at the neuromuscular junction. The N-box, a new promoter element, was identified recently and shown to contribute to this compartmentalized synaptic expression of the AChR delta- and epsilon-subunits. We demonstrate that the N-box mediates transcriptional activation in cultured myotubes and identify the transcription factor that binds to the N-box as a heterooligomer in myotubes and adult muscle. The GABP (GA-binding protein) alpha-subunit belongs to the Ets family of transcription factors, whereas the beta-subunit shares homology with IkappaB and Drosophila Notch protein. GABP binding specificity to mutated N-box in vitro strictly parallels the sequence requirement for beta-galactosidase targeting to the endplate in vivo. In situ hybridization studies reveal that the mRNAs of both GABP subunits are abundant in mouse diaphragm, with preferential expression of the alpha-subunit at motor endplates. In addition, heregulin increases GABPalpha protein levels and regulates phosphorylation of both subunits in cultured chick myotubes. Finally, dominant-negative mutants of either GABPalpha or GABPbeta block heregulin-elicited transcriptional activation of the AChR delta and epsilon genes. These findings establish the expected connection with a presynaptic trophic factor whose release contributes to the accumulation of AChR subunit mRNAs at the motor endplate.

The Elk-1 ETS-domain transcription factor contains a mitogen-activated protein kinase targeting motif

The phosphorylation of transcription factors by mitogen-activated protein kinases (MAP) is a pivotal event in the cellular response to the activation of MAP kinase signal transduction pathways. Mitogenic and stress stimuli activate different pathways and lead to the activation of distinct groups of target proteins. Elk-1 is targeted by three distinct MAP kinase pathways. In this study, we demonstrate that the MAP kinase ERK2 is targeted to Elk-1 by a domain which is distinct from, and located N-terminally to, its phosphoacceptor motifs. Targeting via this domain is essential for the efficient and rapid phosphorylation of Elk-1 in vitro and full and rapid activation in vivo. Specific residues involved in ERK targeting have been identified. Our data indicate that the targeting of different classes of MAP kinases to their nuclear substrates may be a common mechanism to increase the specificity and efficiency of this signal transduction pathway.

In vivo expression and regulation of Elk-1, a target of the extracellular-regulated kinase signaling pathway, in the adult rat brain

The transcription factor Elk-1, a nuclear target of extracellular-regulated kinases (ERKs), plays a pivotal role in immediate early gene induction by external stimuli. Notably, the degree of phosphorylation of Elk-1 is tightly correlated with the level of activation of transcription of c-fos by proliferative signals. No data yet indicate the role of Elk-1 in the adult brain in vivo. To address this question, we have analyzed in the present work (1) Elk-1 mRNA and protein expression in the adult rat brain, and (2) the regulation of Elk-1 (i.e., its phosphorylation state) in an in vivo model of immediate early gene (IEG) induction: an electrical stimulation of the cerebral cortex leading to c-fos and zif268 mRNA induction in the striatum. Using in situ hybridization, we show that Elk-1 mRNA is expressed in various brain structures of adult rat, and that this expression is exclusively neuronal. We demonstrate by immunocytochemistry using various specific Elk-1 antisera that the protein is not only nuclear (as shown previously in transiently transfected cell lines) but is also present in soma, dendrites, and axon terminals. On electrical stimulation of the glutamatergic corticostriatal pathway, we show a strict spatiotemporal correspondence among ERK activation, Elk-1 phosphorylation, and IEG mRNA induction. Furthermore, both activated proteins, analyzed by immunocytochemistry, are found in cytosolic and nuclear comparments of neuronal cells in the activated area. Our data suggest that the ERK signaling pathway plays an important role in regulating genes controlled by serum response element sites via phosphorylation of Elk-1 in vivo.

Cross-cascade activation of ERKs and ternary complex factors by Rho family proteins

Mitogens promote cell growth through integrated signal transduction networks that alter cellular metabolism, gene expression and cytoskeletal organization. Many such signals are propagated through activation of MAP kinase cascades partly regulated by upstream small GTP-binding proteins. Interactions among cascades are suspected but not defined. Here we show that Rho family small G proteins such as Rac1 and Cdc42hs, which activate the JNK/SAPK pathway, cooperate with Raf-1 to activate the ERK pathway. This causes activation of ternary complex factors (TCFs), which regulate c-fos gene expression through the serum response element. Examination of ERK pathway kinases shows that neither MEK1 nor Ras will synergize with Rho-type proteins, and that only MEK1 is fully activated, indicating that MEKs are a focal point for cross-cascade regulation. Rho family proteins utilize PAKs for this effect, as expression of an active PAK1 mutant can substitute for Rho family small G proteins, and expression of an interfering PAK1 mutant blocks Rho-type protein stimulation of ERKs. PAK1 phosphorylates MEK1 on Ser298, a site important for binding of Raf-1 to MEK1 in vivo. Expression of interfering PAK1 also reduces stimulation of TCF function by serum growth factors, while expression of active PAK1 enhances EGF-stimulated MEK1 activity. This demonstrates interaction among MAP kinase pathway elements not previously recognized and suggests an explanation for the cooperative effect of Raf-1 and Rho family proteins on cellular transformation.

Net-b, a Ras-insensitive factor that forms ternary complexes with serum response factor on the serum response element of the fos promoter

The Ras signalling pathway targets transcription factors such as the ternary complex factors that are recruited by the serum response factor to form complexes on the serum response element (SRE) of the fos promoter. We have identified a new ternary complex factor, Net-b. We report the features of the net gene and show that it produces several splice variants, net-b and net-c. net-b RNA and protein are expressed in a variety of tissues and cell lines. net-c RNA is expressed at low levels, and the protein was not detected, raising the possibility that it is a cryptic splice variant. We have studied the composition of ternary complexes that form on the SRE of the fos promoter with extracts from fibroblasts (NIH 3T3) cultured under various conditions and pre-B cells (70Z/3) before and after differentiation with lipopolysaccharide (LPS). The fibroblast complexes contain mainly Net-b followed by Sap1 and Elk1. Net-b complexes, as well as Sap1 and Elk1, are induced by epidermal growth factor (EGF) stimulation of cells cultured in low serum. Pre-B-cell complexes contain mainly Sap1, with less of Net-b and little of Elk1. There is little change upon LPS-induced differentiation compared to the increase with EGF in fibroblasts. We have also found that Net-b is a nuclear protein that constitutively represses transcription. Net-b is not activated by Ras signalling, in contrast to Net, Sap1a, and Elk1. We have previously reported that down-regulation of Net proteins with antisense RNA increases SRE activity. The increase in SRE activity is observed at low serum levels and is even greater after serum stimulation, showing that the SRE is under negative regulation by Net proteins and the level of repression increases during induction. Net-b, the predominant factor in ternary complexes in fibroblasts, may both keep the activity of the SRE low in the absence of strong inducing conditions and rapidly shut the activity off after stimulation.

Isolation and characterization of a novel epithelium-specific transcription factor, ESE-1, a member of the ets family

We report here the isolation of a novel, highly tissue-restricted member of the ets transcription factor/oncogene family, ESE-1 (for epithelium-specific Ets), which has features distinct from those of any other ets-related factor. ESE-1 contains two putative DNA binding domains: an ETS domain, which is unique in that the 5' half shows relatively weak homology to known ets factors, and an A/T hook domain, found in HMG proteins and various other nuclear factors. In contrast to any known ets factors, ESE-1 is expressed exclusively in epithelial cells. ESE-1 expression is induced during terminal differentiation of the epidermis and in a primary human keratinocyte differentiation system. The keratinocyte terminal differentiation marker gene, SPRR2A, is a putative target for ESE-1, since SPRR2A expression during keratinocyte differentiation correlates with induction of ESE-1 expression, and ESE-1 binds with high affinity to and transactivates the ets binding site in the SPRR2A promoter. ESE-1 also binds to and transactivates the enhancer of the Endo A gene, a potential target for ESE-1 in simple epithelia. Due to the important role that other ets factors play in cellular differentiation, ESE-1 is expected to be a critical regulator of epithelial cell differentiation.

Combinatorial determinants of tissue-specific transcription in B cells and macrophages

A tripartite domain of the immunoglobulin mu heavy-chain gene enhancer that activates transcription in B cells contains binding sites for PU.1, Ets-1, and a leucine zipper-containing basic helix-loop-helix factor. Because PU.1 is expressed only in B cells and macrophages, we tested the activity of a minimal mu enhancer fragment in macrophages by transient transfections. The minimal mu enhancer activated transcription in macrophages, and the activity was dependent on all three sites. Analysis of mutated enhancers, in which spacing and orientation of the ETS protein binding sites had been changed, suggested that the mechanisms of enhancer activation were different in B cells and macrophages. Thus, ETS protein binding sites may be combined in different ways to generate tissue-specific transcription activators. Despite the activity of the minimal enhancer in macrophages, a larger mu enhancer fragment was inactive in these cells. We propose that formation of the nucleoprotein complex that is formed on the minimal enhancer in macrophages cannot be helped by the neighboring muE elements that are essential for activity of the monomeric enhancer.

Molecular characterization of the B-box protein-protein interaction motif of the ETS-domain transcription factor Elk-1

The ternary complex factor (TCF) subfamily of ETS-domain transcription factors form ternary complexes with the serum response factor (SRF) and the c-fos SRE. Extracellular signals are relayed via MAP kinase signal transduction pathways through the TCF component of the ternary complex. Protein-protein interactions between TCFs and SRF play an essential role in formation of this ternary complex. A 30 amino acid sequence encompassing the TCF B-box is sufficient to mediate interactions with SRF. In this study we have identified amino acids which are critical for this interaction and derived a molecular model of the SRF binding interface. Alanine scanning of the Elk-1 B-box reveals five predominantly hydrophobic residues which are essential for binding to SRF and for ternary complex formation in vitro and in vivo. These amino acids are predicted to lie on one face of an alpha-helix. Peptides encompassing the B-box retain biological activity and have helix-forming propensity. alpha-Helix and ternary complex formation is disrupted by the introduction of helix-breaking proline residues. Our results are consistent with a model in which the Elk-1 B-box forms an inducible alpha-helix which presents a hydrophobic face for interaction with SRF. We discuss the wider applicability of our results to similar short protein-protein interaction motifs found in other transcription factors.

Role of p38 and JNK mitogen-activated protein kinases in the activation of ternary complex factors

The transcription factors Elk-1 and SAP-1 bind together with serum response factor to the serum response element present in the c-fos promoter and mediate increased gene expression. The ERK, JNK, and p38 groups of mitogen-activated protein (MAP) kinases phosphorylate and activate Elk-1 in response to a variety of extracellular stimuli. In contrast, SAP-1 is activated by ERK and p38 MAP kinases but not by JNK. The proinflammatory cytokine interleukin-1 (IL-1) activates JNK and p38 MAP kinases and induces the transcriptional activity of Elk-1 and SAP-1. These effects of IL-1 appear to be mediated by Rho family GTPases. To examine the relative roles of the JNK and p38 MAP kinase pathways, we examined the effects of IL-1 on CHO and NIH 3T3 cells. Studies of NIH 3T3 cells demonstrated that both the JNK and p38 MAP kinases are required for IL-1-stimulated Elk-1 transcriptional activity, while only p38 MAP kinase contributes to IL-1-induced activation of SAP-1. In contrast, studies of CHO cells demonstrated that JNK (but not the p38 MAP kinase) is required for IL-1-stimulated Elk-1-dependent gene expression and that neither JNK nor p38 MAP kinase is required for IL-1 signaling to SAP-1. We conclude that (i) distinct MAP kinase signal transduction pathways mediate IL-1 signaling to ternary complex transcription factors (TCFs) in different cell types and (ii) individual TCFs show different responses to the JNK and p38 signaling pathways. The differential utilization of TCF proteins and MAP kinase signaling pathways represents a potential mechanism for the determination of cell-type-specific responses to extracellular stimuli.

Regulation of the cfos serum response element by C/EBPbeta

Serum response element binding protein (SRE BP) is a novel binding factor present in nuclear extracts of avian and NIH 3T3 fibroblasts which specifically binds to the cfos SRE within a region overlapping and immediately 3' to the CArG box. Site-directed mutagenesis combined with transfection experiments in NIH 3T3 cells showed that binding of both serum response factor (SRF) and SRE BP is necessary for maximal serum induction of the SRE. In this study, we have combined size fractionation of the SRE BP DNA binding activity with C/EBPbeta antibodies to demonstrate that homodimers and heterodimers of p35C/EBPbeta (a transactivator) and p20C/EBPbeta (a repressor) contribute to the SRE BP complex in NIH 3T3 cells. Transactivation of the SRE by p35C/EBPbeta is dependent on SRF binding but not ternary complex factor (TCF) formation. Both p35C/EBPbeta and p20C/EBPbeta bind to SRF in vitro via a carboxy-terminal domain that probably does not include the leucine zipper. Moreover, SRE mutants which retain responsiveness to the TCF-independent signaling pathway bind SRE BP in vitro with affinities that are nearly identical to that of the wild-type SRE, whereas mutant SRE.M, which is not responsive to the TCF-independent pathway, has a nearly 10-fold lower affinity for SRE BP. We propose that C/EBPbeta may play a role in conjunction with SRF in the TCF-independent signaling pathway for SRE activation.

Selective response of ternary complex factor Sap1a to different mitogen-activated protein kinase subgroups

Mitogenic and stres signals results in the activation of extracellular signal-regulated kinases (ERKs) and stress-activated protein kinase/c-Jun N-terminal kinases (SAPK/JNKs), respectively, which are two subgroups of the mitogen-activated protein kinases. A nuclear target of mitogen-activated protein (MAP) kinases is the ternary complex factor Elk-1, which underlies its involvement in the regulation of c-fos gene expression by mitogenic and stress signals. A second ternary complex factor, Sap1a, is coexpressed with Elk-1 in several cell types and shares attributes of Elk-1, the significance of which is not clear. Here we show that Sap1a is phosphorylated efficiently by ERKs but not by SAPK/JNKs. Serum response factor-dependent ternary complex formation by Sap1a is stimulated by ERK phosphorylation but not by SAPK/JNKs. Moreover, Sap1a-mediated transcription is activated by mitogenic signals but not by cell stress. These results suggest that Sap1a and Elk-1 have distinct physiological functions.

The amino-terminal domain of the transcription termination factor TTF-I causes protein oligomerization and inhibition of DNA binding

The transcription termination factor TTF-I binds specifically to an 18 bp DNA element in the murine ribosomal gene spacer and mediates termination of RNA polymerase I transcription. In this study, we have compared DNA binding and termination activity of recombinant full-length TTF-I (TTF-Ip130) with two deletion mutants lacking 184 and 322 N-terminal amino acids, respectively. All three proteins exhibit similar termination activity, but the DNA binding of TTF-Ip130 is at least one order of magnitude lower than that of the deletion mutants, indicating that the N-terminus represses the interaction of TTF-I with DNA. The inhibitory effect of the N-terminus can be transferred to a heterologous DNA binding domain and is separable from other activities of TTF-I. We show by several methods that TTF-Ip130, the N-terminal domain alone, and fusions of the N-terminus with the DNA binding domain of Oct2.2 form stable oligomers in solution. Thus, in contrast to previous studies suggesting that activation of TTF-I occurs through proteolysis, we demonstrate that full-length TTF-I mediates termination of rDNA transcription in vivo and in vitro and that the oligomerization state of TTF-I may influence its DNA binding activity.

Characterization of NERF, a novel transcription factor related to the Ets factor ELF-1

We have cloned the gene for a novel Ets-related transcription factor, new Ets-related factor (NERF), from human spleen, fetal liver, and brain. Comparison of the deduced amino acid sequence of NERF with those of other members of the Ets family reveals that the level of homology to ELF-1, which is involved in the regulation of several T- and B-cell-specific genes, is highest. Homologies are clustered in the putative DNA binding domain in the middle of the protein, a basic domain just upstream of this domain, and several shorter stretches of homology towards the amino terminus. The presence of two predominant NERF transcripts in various fetal and adult human tissues is due to at least three alternative splice products, NERF-1a, NERF-1b, and NERF-2, which differ in their amino termini and their expression in different tissues. Only NERF-2 and ELF-1, and not NERF-1a and NERF-1b, function as transcriptional activators of the lyn and blk gene promoters, although all isoforms of NERF bind with affinities similar to those of ELF-1 to a variety of Ets binding sites in, among others, the blk, lck, lyn, mb-1, and immunoglobulin H genes and are expressed at similar levels. Since NERF and ELF-1 are coexpressed in B and T cells, both might be involved in the regulation of the same genes.

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

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

SRE elements are binding sites for the fusion protein EWS-FLI-1

EWS-FLI-1 is a chimeric protein produced in most Ewing's sarcomas. It results from the fusion of the N-terminal-encoding region of the EWS gene to the C-terminal DNA-binding domain (the ETS domain) encoded by the FLI-1 ets family gene. Both EWS-FLI-1 and FLI-1 proteins function as transcription factors that bind specifically to ets sequences (the ets boxes) present in promoter elements. EWS- FLI-1 is a powerful transforming protein, whereas FLI-1 is not. In a search for potential DNA binding sites for these two proteins, we have tested their ability to recognize the serum responsive element (SRE) in the c-fos promoter. This cis element contains an ets box which can be occupied by members of the ETS protein family which do not bind DNA autonomously but form a ternary complex with a second protein, p67SRF (serum responsive factor). We demonstrate here that EWS-FLI-1, but not FLI-1, is able to form a ternary complex on the c-fos SRE. Using a GST pull-down assay, we show that both FLI-1 and EWS-FLI-1 interact in vitro with SRF in the absence of DNA. In electromobility shift assays, EWS-FLI-1 binding to the SRE is detectable in the absence of SRF whereas the binding of FLI-1 is not, suggesting that the interaction with DNA is the step which limits ternary complex formation by FLI-1. Deletion of the N-terminal portion of FLI-1 resulted in a protein which behaved as EWS-FLI-1, suggesting the existence of an N- terminal inhibitory domain in the normal protein. Taken together, our data indicate that there are intrinsic differences in the binding of EWS-FLI-1 and FLI-1 proteins to distinct ets sequences.

Phosphorylation-dependent formation of a quaternary complex at the c-fos SRE

The rapid and transient induction of the human proto-oncogene c-fos in response to a variety of stimuli depends on the serum responses element (SRE). In vivo footprinting experiments show that this promoter element is bound by a multicomponent complex including the serum response factor (SRF) and a ternary complex factor such as Elk-1. SRF is thought to recruit a ternary complex factor monomer into an asymmetric complex. In this report, we describe a quaternary complex over the SRE which, in addition to an SRF dimer, contains two Elk-1 molecules. Its formation at the SRE is strictly dependent on phosphorylation of S-383 in the Elk-1 regulatory domain and appears to involve a weak intermolecular association between the two Elk-1 molecules. The influence of mutations in Elk-1 on quaternary complex formation in vitro correlates with their effect on the induction of c-fos reporter expression in response to mitogenic stimuli in vivo.

Structural coupling of the inhibitory regions flanking the ETS domain of murine Ets-1

Several members of the ets gene family of transcription factors show negative regulation of DNA binding by intramolecular interactions. A structural mechanism for this auto-inhibition is investigated using a 161-residue N-terminal deletion mutant of murine Ets-1, Ets-1 delta N280. This protein shows a similar reduced affinity for DNA as native Ets-1 because it contains the ETS domain in context of the flanking amino- and carboxy-terminal regions that together mediate repression of DNA binding. The secondary structure of Ets-1 delta N280 was determined using NMR chemical shift, NOE, J coupling, and amide hydrogen exchange information. In addition to the winged helix-turn-helix ETS domain, Ets-1 delta N280 contains two alpha-helices in the amino-terminal inhibitory region and one alpha-helix in the carboxy-terminal inhibitory region. Chemical shift comparisons were made between this protein and an activated form of Ets-1 lacking the amino-terminal inhibitory region. The spectral differences demonstrate that the amino- and carboxy-terminal inhibitory sequences are structurally coupled to one another, thus explaining the observation that both regions are required for the repression of DNA binding. Furthermore, these data show that the inhibitory sequences also interact directly with the first helix of the intervening ETS domain, thereby providing a pathway for the repression of DNA binding. These results lead to a model of an inhibitory module in Ets-1 composed of both the amino- and carboxy-terminal regions interfaced with the ETS domain. This establishes the structural framework for understanding the intramolecular inhibition of Ets-1 DNA binding.

The ETS-domain transcription factors Elk-1 and SAP-1 exhibit differential DNA binding specificities

The ETS DNA-binding domain is conserved amongst many eukaryotic transcription factors. ETS-domains bind differentially to specific DNA sites containing a central GGA trinucleotide motif. The nucleotides flanking this motif define the binding specificity of individual proteins. In this study we have investigated binding specificity of the ETS-domains from two members of the ternary complex factor (TCF) subfamily, Elk-1 and SAP-1. The ETS DNA-binding domains of Elk-1 (Elk-93) and SAP-1 (SAP-92) select similar sites from random pools of double stranded oligonucleotides based on the consensus sequence ACCGGAAGTR. However, SAP-92 shows a more relaxed binding site selectivity and binds efficiently to a greater spectrum of sites than does Elk-93. This more relaxed DNA binding site selectivity is most pronounced in nucleotides located on the 3' side of the GGA motif. This differential DNA-binding specificity is also exhibited by longer TCF derivatives and, indeed by the full-length proteins. Our results suggest that the range of potential in vivo target sites for SAP-1 is likely to be greater than for Elk-1. We discuss our results in relation to other similar studies carried out with more divergent ETS-domains.

DNA bending in the ternary nucleoprotein complex at the c-fos promoter

Transcriptional induction of the c-fos proto-oncogene in response to serum growth factors is mediated in part by a ternary complex that forms on the serum response element (SRE) within its promoter. This complex consists of Elk-1, serum response factor (SRF) and the SRE. Elk-1 is phosphorylated by MAP kinase, which correlates with the induction of c-fos transcription. In this study we have investigated the protein-induced DNA bending which occurs during the formation and post-translational modification of the ternary complex that forms at the c-fos SRE. Circular permutation analysis demonstrates that the minimal DNA-binding domain of SRF, which contains the MADS box, is sufficient to induce flexibility into the centre of its binding site within the SRE. Phasing analysis indicates that at least part of this flexibility results in the production of a directional bend towards the minor groove. The isolated ETS domains from Elk-1 and SAP-1 induce neither DNA bending nor increased DNA flexibility. Formation of ternary complexes by binding of Elk-1 to the binary SRF:SRE complex results in a change in the flexibility of the SRE. Phosphorylation of Elk-1 by MAP kinase (p42/ERK2) induces further minor changes in this DNA flexibility. However, phasing analysis reveals that the recruitment of Elk-1 to form the ternary complex affects the SRF-induced directional DNA bend in the SRE. The potential roles of DNA bending at the c-fos SRE are discussed.

Pi 1 binding sites are negative regulators of bcl-2 expression in pre-B cells

The bcl-2 gene is differentially regulated during B-cell development, with low-level expression in pre-B cells and higher-level expression in mature B cells. These changes correlate with susceptibility to cell death by apoptosis and suggest that the Bcl-2 protein may play a role in the control of cell death during B-cell development. We have identified two negative regulatory regions in the human bcl-2 5' flanking and 5' untranslated regions in pre-B cells; these regions have no significant function in mature B cells. Further investigation of these regions revealed two pre-B-cell-specific enhancer elements (pi 1 sites) in the 5' negative regulatory region and one in the 3' negative regulatory region. Mutational analysis confirmed that these three sites functioned as negative regulators of the bcl-2 promoter in the pre-B-cell line Nalm-6. Electrophoretic mobility shift assays with each of the three sites demonstrated a complex of identical mobility to that formed with the immunoglobulin heavy-chain enhancer pi 1 site. UV cross-linking experiments revealed that a protein with a molecular mass of 58 kDa bound to the three bcl-2 sites and to the immunoglobulin enhancer site. This protein reacted with an antibody against Ets family proteins. Constructs with the isolated pi 1 sites linked to the simian virus 40 promoter were used in transient transfection experiments in the pre-B-cell line. The bcl-2 sites decreased expression of the simian virus 40 promoter, while the immunoglobulin enhancer site increased its expression. The pi 1 sites in the bcl-2 gene may play a role in the developmental regulation of bcl-2 expression during B-cell differentiation.

Regulatory role of MEF2D in serum induction of the c-jun promoter

Serum induction of c-jun expression in HeLa cells requires a MEF2 site at -59 in the c-jun promoter. MEF2 sites, found in many muscle-specific enhancers, are bound by a family of transcription factors, MEF2A through -D, which are related to serum response factor in their DNA binding domains. We have found that MEF2D is the predominant protein in HeLa cells that binds to the c-jun MEF2 site. Serum induction of a MEF2 reporter gene was not observed in a line of NIH 3T3 cells which contain low MEF2 site binding activity. Transfection of MEF2D into NIH 3T3 cells reconstituted serum induction, demonstrating that MEF2D is required for the serum response. Deletion analysis of MEF2D showed that its DNA binding domain, when fused to a heterologous transcriptional activation domain, was sufficient for serum induction of a MEF2 reporter gene. This is the domain homologous to that in the serum response factor which is required for serum induction of the c-fos serum response element, suggesting that serum regulation of c-fos and c-jun may share a common mechanism.