miR-129-5p inhibits anchorage-independent growth through silencing of ACTN1 and the ELK4/c-FOS axis in HPV-transformed keratinocytes

A persistent infection with human papillomavirus (HPV) can induce precancerous lesions of the cervix that may ultimately develop into cancer. Cervical cancer development has been linked to altered microRNA (miRNA) expression, with miRNAs regulating anchorage-independent growth being particularly important for the progression of precancerous lesions to cancer. In this study, we set out to identify and validate targets of miR-129-5p, a previously identified tumor suppressive miRNA involved in anchorage-independent growth and HPV-induced carcinogenesis. We predicted 26 potential miR-129-5p targets using online databases, followed by KEGG pathway enrichment analysis. RT-qPCR and luciferase assays confirmed that 3'UTR regions of six genes (ACTN1, BMPR2, CAMK4, ELK4, EP300, and GNAQ) were targeted by miR-129-5p. Expressions of ACTN1, CAMK4, and ELK4 were inversely correlated to miR-129-5p expression in HPV-transformed keratinocytes, and their silencing reduced anchorage-independent growth. Concordantly, miR-129-5p overexpression decreased protein levels of ACTN1, BMPR2, CAMK4 and ELK4 in anchorage-independent conditions. Additionally, c-FOS, a downstream target of ELK4, was downregulated upon miR-129-5p overexpression, suggesting regulation through the ELK4/c-FOS axis. ACTN1 and ELK4 expression was also upregulated in high-grade precancerous lesions and cervical cancers, supporting their clinical relevance. In conclusion, we identified six targets of miR-129-5p involved in the regulation of anchorage-independent growth, with ACTN1, BMPR2, ELK4, EP300, and GNAQ representing novel targets for miR-129-5p. For both ACTN1 and ELK4 functional and clinical relevance was confirmed, indicating that miR-129-5p-regulated ACTN1 and ELK4 expression contributes to HPV-induced carcinogenesis.

Hsa_circ_0119412 Contributes to Development of Retinoblastoma by Targeting miR-186-5p/ELK4 Axis

Increasing evidences indicate the crucial role of circRNAs in tumorigenesis, but little is understood about their molecular mechanism in retinoblastoma (RB). This paper was designed for exploring the circ_0119412 function in cases with RB and the potential mechanism. RT-qPCR was utilized to ascertain circ_0119412 and miR-186-5p levels in RB tissues and cells, and western blotting was used to quantify ELK4 in RB cells. In addition, CCK-8 and scratch assays were carried out for evaluation of cell proliferation and migration, respectively. Apoptosis-related proteins levels (Bax and Bcl-2) were measure by western blotting. Tumor growth in vivo was detected utilizing xenograft tumor experiment. The targeting relationship between circ_0119412, miR-186-5p, and ELK4 was validated using a dual-luciferase reporter assay and an RNA immunoprecipitation (RIP) assay. In RB tissues and cells, Circ_0119412 and ELK4 expression were upregulated, while miR-186-5p expression was downregulated. In vitro assay revealed that downregulating circ_0119412 accelerated the cell apoptosis of RB cells and slowed down their migration and proliferation, and the in vivo assay indicated that circ_0119412 downregulation reduced the weight and volume of tumor in nude mice. In addition, miR-186-5p interference promoted the malignant behavior of RB cells, while ELK4 silencing showed an opposite trend. Mechanically, circ_0119412 can promote RB malignant phenotypes via miR-186-5p/ELK4 axis. Circ_0119412 was found to be upregulated in RB, and could accelerate the progression of RB via the miR-186-5p/ELK4 axis, indicating circ_0119412 may serve a promising clinical therapeutic target of RB.

Modulating the DNA Affinity of Elk-1 with Computationally Selected Mutations

In order to regulate gene expression, transcription factors must first bind their target DNA sequences. The affinity of this binding is determined by both the network of interactions at the interface and the entropy change associated with the complex formation. To study the role of structural fluctuation in fine-tuning DNA affinity, we performed molecular dynamics simulations of two highly homologous proteins, Elk-1 and SAP-1, that exhibit different sequence specificity. Simulation studies show that several residues in Elk have significantly higher main-chain root-mean-square deviations than their counterparts in SAP. In particular, a single residue, D69, may contribute to Elk's lower DNA affinity for P(c-fos) by structurally destabilizing the carboxy terminus of the recognition helix. While D69 does not contact DNA directly, the increased mobility in the region may contribute to its weaker binding. We measured the ability of single point mutants of Elk to bind P(c-fos) in a reporter assay, in which D69 of wild-type Elk has been mutated to other residues with higher helix propensity in order to stabilize the local conformation. The gains in transcriptional activity and the free energy of binding suggested from these measurements correlate well with stability gains computed from helix propensity and charge-macrodipole interactions. The study suggests that residues that are distal to the binding interface may indirectly modulate the binding affinity by stabilizing the protein scaffold required for efficient DNA interaction.

Loss of Net as Repressor Leads to Constitutive Increased c-fos Transcription in Cervical Cancer Cells

We have investigated the expression of c-fos in cervical carcinoma cells and in somatic cell hybrids derived therefrom. In malignant cells, c-fos was constitutively expressed even after serum starvation. Dissection of the c-fos promoter showed that expression was mainly controlled by the SRE motif, which was active in malignant cells, but repressed in their non-malignant counterparts. Constitutive SRE activity was not mediated by sustained mitogen-activated protein kinase activity but because of inefficient expression of the ternary complex factor Net, which was either very low or even barely discernible. Chromatin immunoprecipitation assays revealed that Net directly binds to the SRE nucleoprotein complex in non-tumorigenic cells, but not in malignant segregants. Small interfering RNA targeted against Net resulted in enhanced c-fos transcription, clearly illustrating its repressor function. Conversely, stable ectopic expression of Net in malignant cells negatively regulated endogenous c-fos, resulting in a disappearance of the c-Fos protein from the AP-1 transcription complex. These data indicate that loss of Net and constitutive c-fos expression appear to be a key event in the transformation of cervical cancer cells.

Ternary complex factor SAP-1 is required for Erk-mediated thymocyte positive selection

Thymocyte selection and differentiation requires extracellular signal-regulated kinase (Erk) signaling, but transcription factor substrates of Erk in thymocytes are unknown. We have characterized the function of SAP-1 (Elk4), an Erk-regulated transcription factor, in thymocyte development. Early thymocyte development was normal, but single-positive thymocyte and peripheral T cell numbers were reduced, reflecting a T cell-autonomous defect. T cell receptor-induced activation of SAP-1 target genes such as Egr1 was substantially impaired in double-positive thymocytes, although Erk activation was normal. Analysis of T cell receptor transgenes showed that positive selection was reduced by 80-90% in SAP-1-deficient mice; heterozygous mice showed a moderate defect. Negative selection was unimpaired. SAP-1 thus directly links Erk signaling to the transcriptional events required for thymocyte positive selection.

Akt negatively regulates translation of the ternary complex factor Elk-1

Cross-talk between signaling pathways plays an important role in regulation of cell growth, differentiation, survival, and death. Here, we show that Akt regulates the Elk-1 transcription factor, independent of its negative regulation of Raf kinases. Using a constitutively active Mek1 to bypass the regulation of Raf by Akt, we find that the Elk-1 and Sap1a proteins are dramatically decreased in the presence of activated Akt. Akt catalytic activity is required. Also, Mek-dependent activation of a TCF (Elk-1/Sap-1a)-dependent c-fos reporter is decreased by activated Akt. Neither the level of Elk-1 mRNA nor the stability of the Elk-1 protein is altered by activated Akt. Instead, the rate of incorporation of labeled methionine into Elk-1 protein is decreased in the presence of Akt. In addition, the level of the Elk-1 protein but not GFP is significantly decreased in the presence of activated Akt, when GFP is expressed from an IRES element in a bicistronic message with Elk-1. We conclude that Akt negatively regulates translation of the Elk-1 mRNA. A coding region determinant that maps within the first 279 nts of the Elk-1 message is necessary and sufficient for Akt-mediated regulation of Elk-1.

Transcriptional regulation of the cytosolic chaperonin theta subunit gene, Cctq, by Ets domain transcription factors Elk-1, Sap-1a, and Net in the absence of serum response factor

The chaperonin-containing t-complex polypeptide 1 (CCT) is a molecular chaperone that facilitates protein folding in eukaryotic cytosol, and the expression of CCT is highly dependent on cell growth. We show here that transcription of the gene encoding the theta subunit of mouse CCT, Cctq, is regulated by the ternary complex factors (TCFs), Elk-1, Sap-1a, and Net (Sap-2). Reporter gene assay using HeLa cells indicated that the Cctq gene promoter contains a cis-acting element of the CCGGAAGT sequence (CQE1) at -36 bp. The major CQE1-binding proteins in HeLa cell nuclear extract was recognized by anti-Elk-1 or anti-Sap-1a antibodies in electrophoretic mobility shift assay, and recombinant Elk-1, Sap-1a, or Net specifically recognized CQE1. The CQE1-dependent transcriptional activity in HeLa cells was virtually abolished by overexpression of the DNA binding domains of TCFs. Overexpression of full-length TCFs with Ras indicated that exogenous TCFs can regulate the CQE1-dependent transcription in a Ras-dependent manner. PD98059, an inhibitor of MAPK, significantly repressed the CQE1-dependent transcription. However, no serum response factor was detected by electrophoretic mobility shift assay using the CQE1 element. These results indicate that transcription of the Cctq gene is regulated by TCFs under the control of the Ras/MAPK pathway, probably independently of serum response factor.

Regulation of TCF ETS-domain transcription factors by helix-loop-helix motifs

DNA binding by the ternary complex factor (TCF) subfamily of ETS-domain transcription factors is tightly regulated by intramolecular and intermolecular interactions. The helix-loop-helix (HLH)-containing Id proteins are trans-acting negative regulators of DNA binding by the TCFs. In the TCF, SAP-2/Net/ERP, intramolecular inhibition of DNA binding is promoted by the cis-acting NID region that also contains an HLH-like motif. The NID also acts as a transcriptional repression domain. Here, we have studied the role of HLH motifs in regulating DNA binding and transcription by the TCF protein SAP-1 and how Cdk-mediated phosphorylation affects the inhibitory activity of the Id proteins towards the TCFs. We demonstrate that the NID region of SAP-1 is an autoinhibitory motif that acts to inhibit DNA binding and also functions as a transcription repression domain. This region can be functionally replaced by fusion of Id proteins to SAP-1, whereby the Id moiety then acts to repress DNA binding in cis. Phosphorylation of the Ids by cyclin-Cdk complexes results in reduction in protein-protein interactions between the Ids and TCFs and relief of their DNA-binding inhibitory activity. In revealing distinct mechanisms through which HLH motifs modulate the activity of TCFs, our results therefore provide further insight into the role of HLH motifs in regulating TCF function and how the inhibitory properties of the trans-acting Id HLH proteins are themselves regulated by phosphorylation.

Induction of the early growth response gene 1 promoter by TCR agonists and partial agonists: ligand potency is related to sustained phosphorylation of extracellular signal-related kinase substrates

Responses to partial agonist TCR signals include positive selection of thymocytes, survival of naive T cells, and homeostatic proliferation. As part of an effort to understand the molecular basis of these processes, we have determined how agonist and partial agonist ligands act differently to induce a change in gene expression. We have found that the early growth response gene 1 (Egr1) promoter is activated by agonist and partial agonist ligands, but the partial agonist induces 10-fold lower promoter activity. Both agonist and partial agonist ligands require all six serum response elements in the Egr1 promoter to reach maximum induction. Although slightly fewer cells respond to the partial agonist, all of the responding cells have reduced activity compared with the cells responding to agonist. The factors binding to the serum response elements of the Egr1 promoter form a ternary complex (TC) consisting of serum response factor and either Elk-1 or serum response factor accessory protein-1a. Formation of a stable TC and inducible promoter activity are both dependent on extracellular signal-related kinase activation. Examination of TC formation over time reveals that this complex is induced well by partial agonist ligands, but it is not sustained, whereas agonist stimulation induces longer lived TCs. Therefore, the data suggest that both agonist and partial agonist ligands can induce formation of multiple TC on the Egr1 promoter, but the ability of the agonist ligand to maintain these complexes for an extended time results in the increased potency of the agonist.

Specificity determinants in MAPK signaling to transcription factors

One critical component in determining the specificity, fidelity, and efficiency of MAPK substrate phosphorylation is the presence of distinct docking domains in the substrate proteins. These docking domains are found in a range of substrates, including the transcription factors myocyte enhancer factor-2A and SAP-1. However, the sequences of these motifs differ, as does their targeting preferences by MAPKs, with SAP-1 being targeted by both the ERK and p38 isoforms, whereas myocyte enhancer factor-2A is targeted only by certain members of the p38 subfamily. Here, we have investigated the specificity determinants within these motifs and generated a model for how specificity is generated. We demonstrate that residues throughout the docking domains play important roles in the recognition process. However, residues located at different positions are important for discriminating between ERK and p38 MAPKs. Furthermore, the docking domains can be further subdivided into submotifs, which are differentially required for phosphorylation by ERK or p38 MAPKs. We have used loss- and gain-of-function mutagenesis to identify residues that discriminate between ERK and p38 MAPKs, residues that act to promote suboptimal interactions, and regions that are differentially required depending on the kinase involved. A model is proposed to explain how specificity is generated within these short docking domains.

Crystal structure of a ternary SAP-1/SRF/c-fos SRE DNA complex

Combinatorial DNA binding by proteins for promoter-specific gene activation is a common mode of DNA regulation in eukaryotic organisms, and occurs at the promoter of the c-fos proto-oncogene. The c-fos promoter contains a serum response element (SRE) that mediates ternary complex formation with the Ets proteins SAP-1 or Elk-1 and the MADS-box protein, serum response factor (SRF). Here, we report the crystal structure of a ternary SAP-1/SRF/c-fos SRE DNA complex containing the minimal DNA-binding domains of each protein. The structure of the complex reveals that the SAP-1 monomer and SRF dimer are bound on opposite faces of the DNA, and that the DNA recognition helix of SAP-1 makes direct contact with the DNA recognition helix of one of the two SRF subunits. These interactions facilitate an 82 degrees DNA bend around SRF and a modulation of protein-DNA contacts by each protein when compared to each of the binary DNA complexes. A comparison with a recently determined complex containing SRF, an idealized DNA site, and a SAP-1 fragment containing a SRF-interacting B-box region, shows a similar overall architecture but also shows important differences. Specifically, the comparison suggests that the B-box region of the Ets protein does not significantly influence DNA recognition by either of the proteins, and that the sequence of the DNA target effects the way in which the two proteins cooperate for DNA recognition. These studies have implications for how DNA-bound SRF may modulate the DNA-binding properties of other Ets proteins such as Elk-1, and for how other Ets proteins may modulate the DNA-binding properties of other DNA-bound accessory factors to facilitate promoter-specific transcriptional responses.

Development of a homogeneous MAP kinase reporter gene screen for the identification of agonists and antagonists at the CXCR1 chemokine receptor

Agonist activity at G protein-coupled receptors (GPCRs) that regulate heterotrimeric G proteins of the Galpha(i/o) or Galpha(q) families has been shown to result in activation of the mitogen-activated protein (MAP) kinase cascade. To facilitate compound screening for these classes of GPCR, we have developed a reporter gene that detects the activation of the ternary complex transcription factor Sap1a following MAP kinase activation. In contrast to other reporter gene assays for Galpha(i/o)-coupled GPCRs, the MAP kinase reporter generates an increase in signal in the presence of agonist. The reporter gene has been transfected into Chinese hamster ovary cells to generate a "host" reporter gene-containing cell line. The Galpha(i)-coupled human CXCR1 chemokine receptor was subsequently transfected into this cell line in order to develop a 384-well format screen for both agonists and antagonists of this receptor. Agonists activated the reporter gene with the expected rank order of potency and with similar concentration dependence as seen with the regulation of other signal transduction cascades in mammalian cells: interleukin-8 (IL-8) (pEC(50) = 7.0 +/- 0.1) > GCP-2 (pEC(50) = 6.3 +/- 0.1) > NAP-2 (pEC(50) < 6). CXCR1-mediated activation of MAP kinase was inhibited by pertussis toxin and the MEK inhibitor PD98059, demonstrating that receptor activation of MAP kinase is due to pertussis toxin-sensitive Galpha(i/o)-family G proteins to cause the activation of MEK kinase. Using the 384-well format, assay performance was unaffected by solvent concentrations of 0.5% ethanol, 0.15% glycerol, or 1% DMSO. Signal crosstalk between adjacent wells was less than 1%. The assay exhibited a Z factor of 0.53 and a coefficient of variation of response to repeated application of IL-8 (100 nM) of 15.9%.

Selective targeting of MAPKs to the ETS domain transcription factor SAP-1

MAPK pathways play important roles in regulating the key cellular processes of proliferation, differentiation, and apoptosis. There are multiple MAPK pathways, which are subject to different regulatory cues. It is important that these pathways maintain specificity in signaling to elicit the activation of a specific program of gene expression. MAPK-docking domains in several transcription factors have been shown to play important roles in determining the specificity and efficiency of their phosphorylation by MAPKs. Here we investigate the mechanisms by which MAPKs are targeted to the ETS domain transcription factor SAP-1. We demonstrate that SAP-1 contains two different domains that are required for its efficient phosphorylation in vitro and activation in vivo by ERK2 and a subset of p38 MAPKs. The D-domain is closely related to other MAPK-docking domains, but exhibits a novel specificity and serves to promote selective targeting of ERK2, p38alpha, and p38beta(2) to SAP-1. A second important region, the FXF motif, also plays an important role in directing MAPKs to phosphorylate SAP-1. The FXF motif promotes targeting by ERK2 and, to a lesser extent, p38alpha, but not p38beta(2). Our data therefore demonstrate that a modular system of motifs is responsible for directing specific MAPK subtypes to SAP-1, but also point to important distinctions in the mechanism of action of the D-domain and FXF motif.

Structure of the elk-1-DNA complex reveals how DNA-distal residues affect ETS domain recognition of DNA

SAP-1 and Elk-1 are members of a large group of eukaryotic transcription factors that contain a conserved ETS DNA binding domain and that cooperate with the serum response factor (SRF) to activate transcription of the c-fos protooncogene. Despite the high degree of sequence similarity, which includes an identical amino acid sequence for the DNA recognition helix within the ETS domain of these proteins, they exhibit different DNA binding properties. Here we report the 2.1 ¿ crystal structure of the ETS domain of Elk-1 bound to a high affinity E74 DNA (E74DNA) site and compare it to a SAP-1-E74DNA complex. This comparison reveals that the differential DNA binding properties of these proteins are mediated by non-conserved residues distal to the DNA binding surface that function to orient conserved residues in the DNA recognition helix for protein-specific DNA contacts. As a result, nearly one-third of the interactions between the protein recognition helix and the DNA are different between the SAP-1 and Elk-1 DNA complexes. Taken together, these studies reveal a novel mechanism for the modulation of DNA binding specificity within a conserved DNA binding domain, and have implications for how highly homologous ETS proteins exhibit differential DNA-binding properties.

Smooth muscle alpha-actin CArG elements coordinate formation of a smooth muscle cell-selective, serum response factor-containing activation complex

Previous studies have shown that multiple serum response factor (SRF)-binding CArG elements were required for smooth muscle cell (SMC)-specific regulation of smooth muscle (SM) alpha-actin expression. However, a critical question remains as to the mechanisms whereby a ubiquitously expressed transcription factor such as SRF might contribute to SMC-specific expression. The goal of the present study was to investigate the hypothesis that SMC-selective expression of SM alpha-actin is due at least in part to (1) unique CArG flanking sequences that distinguish the SM alpha-actin CArGs from other ubiquitously expressed CArG-dependent genes such as c-fos, (2) cooperative interactions between CArG elements, and (3) SRF-dependent binding of SMC-selective proteins to the CArG-containing regions of the promoter. Results demonstrated that specific sequences flanking CArG B were important for promoter activity in SMCs but not in bovine aortic endothelial cells. We also provided evidence indicating that the structural orientation between CArGs A and B was an important determinant of promoter function. Electrophoretic mobility shift assays and methylation interference footprinting demonstrated that a unique SRF-containing complex formed that was selective for SMCs and, furthermore, that this complex was probably stabilized by protein-protein interactions and not by specific interactions with CArG flanking sequences. Taken together, the results of these studies provide evidence that SM alpha-actin expression in SMCs is complex and may involve the formation of a unique multiprotein initiation complex that is coordinated by SRF complexes bound to multiple CArG elements.

Dimer formation by ternary complex factor ELK-1

Ternary complex factors (TCFs), a subgroup of the ets protein family, bind with a dimer of serum response factor to the c-fos serum response element. Both DNA binding and transcriptional activation by TCFs are regulated by mitogen-activated protein kinases. When activated, mitogen-activated protein kinases form homodimers that translocate to the nucleus, where they interact with TCFs via specific docking sites. Here we show by three different criteria that Elk-1 is capable of forming dimers in eukaryotic cells through two distinct interaction domains. These observations are consistent with a dynamic model of TCF-promoter interactions.

Ternary complex factors Elk-1 and Sap-1a mediate growth hormone-induced transcription of egr-1 (early growth response factor-1) in 3T3-F442A preadipocytes

In our search for transcription factors induced by GH, we have analyzed immediate early gene activation in a model of GH-dependent differentiation. Here we describe the activation of early growth response factor-1 (egr-1) in GH-stimulated 3T3-F442A preadipocytes and the transcription factors responsible for its transactivation. Binding activity of egr-1 in electrophoretic mobility shift assay (EMSA) increased transiently 1 h after GH stimulation, accompanied by a concomitant increase in egr-1 mRNA. egr-1 induction appeared not to be related to proliferation since it was amplified in quiescent preadipocytes at a time when cells were refractive to GH-stimulated DNA synthesis. Truncations of the proximal 1 kb of the egr-1 promoter revealed that a 374-bp region (-624 to -250) contributes about 80% of GH inducibility in 3T3-F442A cells and approximately 90% inducibility in CHO-K1 cells. This region contains three juxtaposed SRE (serum response element)/Ets site pairs known to be important for egr-1 activity in response to exogenous stimuli. Site-specific mutations of individual SRE and Ets sites within this region each reduced GH inducibility of the promoter. Use of these site-specific mutations in EMSA showed that disruption of either Ets or SRE sites abrogated ternary complex formation at the composite sites. DNA binding of ternary complexes, but not binary complexes, in EMSA was rapidly and transiently increased by GH. EMSA supershifts indicated these ternary complexes contained serum response factor (SRF) and the Ets factors Elk-1 and Sap-1a. Coexpression of Sap-1a and Elk-1 resulted in a marked increase in GH induction of egr-1 promoter activity, although transfection with expression vectors for either Ets factor alone did not significantly enhance the GH response. We conclude that GH stimulates transcription of egr-1 primarily through activation of these Ets factors at multiple sites on the promoter and that stabilization of ternary complexes with SRF at these sites maximizes this response.

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.

Characterization of the human elk-1 promoter. Potential role of a downstream intronic sequence for elk-1 gene expression in monocytes

To characterize the human elk-1 promoter, we mapped the transcriptional start site and isolated elk-1-specific genomic phage clones that contained extensive upstream and downstream sequences. A TATA-like motif was identified immediately upstream of the transcriptional start site. Functional analyses of DNA fragments containing the TATA element and the identification of a DNase I-hypersensitive chromatin site (HS 1) in close proximity to the TATA box suggest that the identified TATA motif is important for elk-1 transcription in vivo. Sequences upstream and downstream from the TATA box were found to contribute to elk-1 promoter activity. A second hypersensitive site (HS 2) was identified within the first intron in pre-monocytic cells, which express Elk-1 only when differentiating to monocytes. In a variety of other cell types, which display a constitutive Elk-1 expression, HS 2 did not exist, suggesting that inducibility of elk-1 expression is associated with the presence of HS 2. Egr-1 and the serum response factor were found to interact specifically with the intronic sequence at +265 and +448, respectively. Because Egr-1 mRNA and protein levels were observed to increase significantly before induction of elk-1 expression, we propose that Egr-1 is important for the regulation of elk-1 transcription in differentiating monocytes.

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.

Nociception activates Elk-1 and Sap1a following expression of the ORL1 receptor in Chinese hamster ovary cells

Nociceptin stimulation of the ORL1 receptor expressed in Chinese hamster ovary (CHO) cells results in the activation of the extracellular signal regulated kinases ERK1 and ERK2. ERK1/ERK2 activation is inhibited by pertussis toxin, the MEK inhibitor PD 98059 and by transient expression of alpha-transducin, indicating that ORL1 up-regulation of these kinases occurs as a consequence of the release of the G-protein betagamma complex following the activation of pertussis-toxin sensitive Galphai-family G-proteins. Using specific reporter genes we demonstrate that the transcription factors Elk-1 and Sapla are activated in a pertussis toxin-sensitive manner as a consequence of ORL1 upregulation of ERK1/ERK2 to induce changes in gene expression. The activation of these transcription factors is also inhibited following treatment with PD 98059 and following coexpression of alpha-transducin.

Structures of SAP-1 bound to DNA targets from the E74 and c-fos promoters: insights into DNA sequence discrimination by Ets proteins

SAP-1 is a member of the Ets transcription factors and cooperates with SRF protein to activate transcription of the c-fos protooncogene. The crystal structures of the conserved ETS domain of SAP-1 bound to DNA sequences from the E74 and c-fos promoters reveal that a set of conserved residues contact a GGA core DNA sequence. Discrimination for sequences outside this core is mediated by DNA contacts from conserved and nonconserved protein residues and sequence-dependent DNA structural properties characteristic of A-form DNA structure. Comparison with the related PU.1/DNA and GABPalpha/beta/DNA complexes provides general insights into DNA discrimination between Ets proteins. Modeling studies of a SAP-1/SRF/DNA complex suggest that SRF may modulate SAP-1 binding to DNA by interacting with its ETS domain.

The kinase suppressor of Ras (KSR) modulates growth factor and Ras signaling by uncoupling Elk-1 phosphorylation from MAP kinase activation

The Ras GTPase plays an essential role in many cellular signal transduction events. Activation of the mitogen activated protein (MAP) kinase is a primary consequence of Ras activation and plays a key role in mediating Ras signal transduction. A novel kinase, KSR, has recently been functionally isolated as a positive regulator of Ras signaling in Caenorhabditis elegans vulval induction and Drosophila photoreceptor differentiation. We have examined the effect of KSR on growth factor and Ras-induced MAP kinase signaling in mammalian cells. Surprisingly, we observed that KSR specifically blocks EGF and Ras-induced phosphorylation and activation of ternary complex factors (TCF), physiological substrates of MAP kinases, without affecting the activation of MAP kinase itself. A kinase-deficient mutant of KSR, KSR-RM, appears to function as a dominant interfering mutant which elevates phosphorylation of Elk-1, a member of the TCF family, and Elk-1-dependent transcription. The effect of KSR on Elk-1 was significantly decreased by inhibition of calcineurin, a putative Elk-1 phosphatase. These observations demonstrate that KSR is capable of uncoupling the MAP kinase activation from its target phosphorylation, and thus provide a novel mechanism for modulating the Ras-MAP kinase signaling pathway. This study provides the first evidence that signal output of MAP kinase cascades is subject to regulation at a level independent of MAP kinase activity.

Transforming growth factor-beta1 and protein kinase C synergistically activate the c-fos serum response element in myocardial cells

We previously reported that transforming growth factor-beta1 (TGF-beta1) potentiated alpha1-adrenergic and stretch-induced c-fos mRNA expression and norepinephrine (NE)-induced amino acid incorporation in rat cultured myocardial cells (MCs). In the present study, we attempted to explore the mode of TGF-beta1 action for c-fos gene expression in MCs. In the transient transfection assay, TGF-beta1 potentiated NE- or 12-O-tetradecanoylphorbol-13-acetate (TPA)-activated c-fos promoter/enhancer, but not forskolin-activated c-fos promoter/enhancer. The c-fos serum response element (SRE) and the TPA response element (TRE) were responsible for TGF-beta1-induced potentiation of the NE or TPA action. Although TGF-beta1 activated not only the wild-type c-fos SRE, but also the mutated c-fos SRE, which contains an intact binding site for the serum response factor (SRF) but lacks the ternary complex factor (TCF) binding site, TPA activated the wild-type c-fos SRE but not the mutated c-fos SRE. TGF-beta1 did not potentiate the effects of TPA on the activation of mitogen-activated protein kinase (MAPK) and the phosphorylation of Elk-1 and SAP-1a, which belong to TCF at the c-fos SRE. These results indicate that TGF-betaf potentiates the c-fos SRE activated by PKC through the SRF binding site. TGF-beta1 is involved in the regulation of c-fos gene expression through the c-fos SRE and is subsequently involved in the regulation of the gene which has the TRE in the promoter/enhancer region.

Activation of the c-fos SRE through SAP-1a

TCFs, which are members of the Ets family of transcription factors, are recruited to the Serum Response Element (SRE) in the c-fos promoter by SRF. These Ets proteins, which are substrates for the MAP kinases, are direct targets of the Ras/MAP kinase signal transduction pathway. In this paper, we demonstrate that one of the TCFs, SAP-1a, displays a significant level of autonomous binding to the SRE Ets box. In contrast to previous observations, deletion of the SRF binding domain did not modulate the autonomous binding of SAP-1a. Also, the autonomous binding was not modulated by the phosphorylation of SAP-1a by MAP kinases. The autonomous binding was also detected in live cells: transfected SAP-1a was able to restore the response of a CArG-less SRE in PC12 cells. The response occurred in the absence of SRF recruitment since a mutant of SAP-1a in which the B-box, a domain required for interaction with SRF, had been deleted was still able to transactivate the CArG-less SRE. The transactivation was repressed by a Ras transdominant negative mutant, indicating the involvement of the Ras/MAP kinase pathway. Taken together, these data demonstrate that SAP-1a is capable of binding to the c-fos SRE in the absence of SRF.

Convergence of MAP kinase pathways on the ternary complex factor Sap-1a

The serum response element (SRE), which is pivotal for transcriptional up-regulation of the c-fos protooncogene, is constitutively occupied by a protein complex comprising the serum response factor and a ternary complex factor (TCF). Phosphorylation of the TCFs Elk-1 and Sap-1a by the ERK and JNK subclasses of MAP kinases triggers c-fos transcription. We demonstrate here that Elk-1 is barely activated by a third subclass of MAP kinases (p38), most likely because the critical residues Ser383 and Ser389 are poorly phosphorylated by p38 MAP kinase. In contrast, the TCF Sap-1a is efficiently phosphorylated by p38 MAP kinase in vitro and in vivo on the homologous residues Ser381 and Ser387. Mutation of these sites to alanine severely reduces c-fos SRE-dependent transcription mediated by Sap-1a and p38 MAP kinase. Thus, Sap-1a may be an important target for mitogens, stress and apoptotic signals to elicit a nuclear response. However, signaling from p38 MAP kinase to Sap-1a or from Sap-1a to the basal transcription machinery does not occur in all cell types nor at promoters other than the c-fos SRE, which may ensure the specificity of signaling.

Activation of the Sap-1a transcription factor by the c-Jun N-terminal kinase (JNK) mitogen-activated protein kinase

Ternary complex factors (TCFs) bind to the serum response element in the c-fos promoter and mediate its activation by many extracellular stimuli. Some of these stimuli activate the ERK subclass of mitogen-activated protein kinases (MAPKs) that target the TCF Sap-1a. We show that Sap-1a is also phosphorylated by the stress-activated JNK subclass of MAPKs leading to stimulation of both c-fos serum response element and E74-site-dependent transcription in RK13 cells. Several JNK-1 phosphorylation sites were mapped within Sap-1a, and mutation of these sites affected the transactivation mediated by Sap-1a and JNK-1. The impact of these phosphorylation sites varied at different promoters and was dependent on whether Sap-1a was stimulated by ERK-1 or JNK-1. Additionally, a comparison of Sap-1a with another TCF, Elk-1, revealed that these proteins behaved differently to stimulation by ERK-1 and JNK-1. Furthermore, activation of Sap-1a by JNK-1 was inhibited by the p38(MAPK) in RK13 cells, possibly by competition for a common upstream activator. Altogether, our data suggest that Sap-1a plays an important role in the nuclear response elicited by cellular stress.

FLI1 and EWS-FLI1 function as ternary complex factors and ELK1 and SAP1a function as ternary and quaternary complex factors on the Egr1 promoter serum response elements

The ETS gene products are a family of transcriptional regulatory proteins that contain a highly conserved and structurally unique DNA binding domain, termed the ETS domain. Several ETS proteins bind to DNA as monomers, however it has been shown that the DNA binding activity is enhanced or modulated in the presence of other factors. By differential display and whole genome PCR techniques, we have recently shown that the Erg1 gene is a target for ETS proteins. The Egr1 promoter contains multiple ETS binding sites, three of which exist as parts of two serum response elements (SREI and SREII). The SRE is a cis-element that regulates the expression of many growth factor responsive genes. ELK1 and SAP1a have been shown to form ternary complexes with SRF on the SRE located in the c-fos promoter. Similarly, we examined whether the ELK1, SAP1a, FLI1, EWS-FLI1, ETS1, ETS2, PEA3 and PU.1 proteins can form ternary complexes with SRF on the Egr1 SREI and II. Our results demonstrate that indeed ELK1, SAPla, FLI1 and EWS-FLI1 are able to form ternary complexes with SRF on Egr1 SREs. In addition, ELK1 and SAP1a can also form quarternary complexes on the Egr1 SREI. However, the proteins ETS1, ETS2, PEA3 and PU.1 were unable to form ternary complexes with SRF on either the Egr1 or c-fos SREs. Our data demonstrate that FLI1 and EWS-FLI1 constitute new members of a subgroup of ETS proteins that can function as ternary complex factors and further implicate a novel function for these ETS transcription factors in the regulation of the Egr1 gene. By amino acid sequence comparison we found that, in fact, 50% of the amino acids present in the B-box of SAP1a and ELK1, which are required for interaction with SRF, are identical to those present in both FLI1 (amino acids 231- 248) and EWS-FLI1 proteins. This B-box is not present in ETS1, ETS2, PEA3 or PU.1 and these proteins were unable to form ternary complexes with SRF and Egrl-SREs or c-fos SRE. Furthermore, deletion of 194 amino terminal amino acids of FLI1 did not interfere with its ability to interact with SRF, in fact, this truncation increased the stability of the ternary complex. The FLI1 protein has a unique R-domain located next to the DNA binding region. This R-domain may modulate the interaction with SRF, providing a mechanism that would be unique to FLI1 and EWS-FLI1, thus implicating a novel function for these ETS transcription factors in the regulation of the Egr1 gene.

Biosynthesis, processing, and targeting of sphingolipid activator protein (SAP )precursor in cultured human fibroblasts. Mannose 6-phosphate receptor-independent endocytosis of SAP precursor

Sphingolipid activator proteins (SAPs) are essential cofactors for the lysosomal degradation of glycosphingolipids with short oligosaccharide chains by acidic exohydrolases. SAP-A, -B, -C, and -D derive from proteolysis of a 73-kDa glycoprotein, the SAP precursor. In the present publication, we studied the intracellular transport and the endocytosis of SAP precursor in human skin fibroblasts. Our data indicate that SAP precursor bears phosphate residues on noncomplex carbohydrate chains linked to the SAP-C and the SAP-D domain and sulfate residues on complex carbohydrate chains located within the SAP-A, -C, and possibly the SAP-D domain. Treatment of fibroblasts with either bafilomycin A1 or 3-methyladenine indicates that proteolytic cleavage of SAP precursor begins as early as in the late endosomes. To determine whether targeting of SAP precursor depends on mannose 6-phosphate residues, we analyzed the processing of SAP precursor in I-cell disease fibroblasts. In these cells nearly normal amounts of newly synthesized SAP-C were found, although secretion of SAP precursor was enhanced 2-3-fold. Moreover, SAP-C could be localized to lysosomal structures by indirect immunofluorescence in normal and in I-cell disease fibroblasts. Mannose 6-phosphate was not found to interfere significantly with endocytosis of SAP precursor. Normal fibroblasts internalized SAP precursor secreted from I-cells nearly as efficiently as the protein secreted from normal cells. To our surprise, deglycosylated SAP precursor was taken up by mannose 6-phosphate receptor double knock out mouse fibroblasts more efficiently than the glycosylated protein. We propose that intracellular targeting of SAP precursor to lysosomes is only partially dependent on mannose 6-phosphate residues, whereas its endocytosis occurs in a carbohydrate-independent manner.

The p38 and ERK MAP kinase pathways cooperate to activate Ternary Complex Factors and c-fos transcription in response to UV light

We investigated the activation of c-fos transcription following UV irradiation, a 'stress' stimulus. In both HeLa TK- and NIH 3T3 cells the Serum Response Element is required for efficient UV-induced c-fos transcription, and in HeLa TK- cells the Ternary Complex Factor (TCF) binding site contributes substantially to activation. Consistent with this, UV irradiation activates LexA-TCF fusion proteins more strongly in HeLa TK- than in NIH 3T3 cells. The TCF C-termini of the TCFs are substrates for UV-induced MAP kinases: both the Elk-1 and SAP-1a C-termini are efficiently phosphorylated by the p38 MAPK, but only the Elk-1 C-terminus is a good substrate for the SAPK/JNKs. The specificity and activation kinetics of TCF C-terminal kinases, and the susceptibility of transcriptional activation by LexA-TCF fusion proteins to specific inhibitors of different MAPK pathways, show that both the ERK and p38 MAPK pathways contribute to TCF activation in response to UV irradiation. Activity of both these pathways is also required for the response of the c-fos gene itself to UV stimulation.

Net (ERP/SAP2) one of the Ras-inducible TCFs, has a novel inhibitory domain with resemblance to the helix-loop-helix motif

The three ternary complex factors (TCFs), Net (ERP/ SAP-2), ELK-1 and SAP-1, are highly related ets oncogene family members that participate in the response of the cell to Ras and growth signals. Understanding the different roles of these factors will provide insights into how the signals result in coordinate regulation of the cell. We show that Net inhibits transcription under basal conditions, in which SAP-1a is inactive and ELK-1 stimulates. Repression is mediated by the NID, the Net Inhibitory Domain of about 50 amino acids, which autoregulates the Net protein and also inhibits when it is isolated in a heterologous fusion protein. Net is particularly sensitive to Ras activation. Ras activates Net through the C-domain, which is conserved between the three TCFs, and the NID is an efficient inhibitor of Ras activation. The NID, as well as more C-terminal sequences, inhibit DNA binding. Net is more refractory to DNA binding than the other TCFs, possibly due to the presence of multiple inhibitory elements. The NID may adopt a helix-loop-helix (HLH) structure, as evidenced by homology to other HLH motifs, structure predictions, model building and mutagenesis of critical residues. The sequence resemblance with myogenic factors suggested that Net may form complexes with the same partners. Indeed, we found that Net can interact in vivo with the basic HLH factor, E47. We propose that Net is regulated at the level of its latent DNA-binding activity by protein interactions and/or phosphorylation. Net may form complexes with HLH proteins as well as SRF on specific promotor sequences. The identification of the novel inhibitory domain provides a new inroad into exploring the different roles of the ternary complex factors in growth control and transformation.

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.

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.

Analysis of SRF, SAP-1 and ELK-1 transcripts and proteins in human cell lines

We have analysed the expression of the genes encoding transcription factors involved in c-fos transcriptional regulation, i.e. the serum response factor (SRF) and the ETS-related proteins ELK-1 and SAP-1, in a variety of human cell lines. RNA was determined by Northern blot analysis, and proteins were detected on Western blots: the two analyses gave essentially identical results. SRF was expressed at similar levels in all cell lines tested. In contrast, SAP-1 and ELK-1 expression varied from one cell line to another. Interestingly, in any given cell line, high levels of one protein were accompained by low levels of the other. Similar results were obtained by electro-mobility shift assays (EMSA), using antibodies directed against the proteins. Thus, our data raise the possibility of a coordinated regulation of the expression of these two Ets genes, at both RNA and protein levels.

Oxidized low density lipoprotein inhibits lipopolysaccharide-induced binding of nuclear factor-kappaB to DNA and the subsequent expression of tumor necrosis factor-alpha and interleukin-1beta in macrophages

A large body of evidence suggests that oxidized LDL (oxLDL) has a role in atherogenesis. One effect is the impact on macrophage function. We have studied the effects of oxLDL and oxysterols on the binding of the transcription factors nuclear factor (NF)-kappaB and AP-1 to DNA. These transcription factors are involved in the regulation of several genes and expressed during activation of macrophages, for example by endotoxin (LPS). OxLDL did not induce binding of NF-kappaB. However, the LPS-induced response to NF-kappaB was substantially reduced after preincubation with oxLDL. Medium and highly oxidized LDL also decreased the constitutive DNA-binding of AP-1. Similar effects on AP-1-binding were seen with the oxysterols, 7beta-hydroxycholesterol, 24- hydroxy-, 25-hydroxy-, and 27-hydroxy-cholesterol. Our data therefore suggest an effect of oxLDL on the DNA-binding of AP-1, which might be mediated by the oxysterol content of oxLDL. A decreased LPS-induced TNF-alpha and IL-1beta mRNA and protein expression were found in macrophages incubated with oxLDL before LPS-exposure. These observations suggest that macrophages that internalize extensively oxidized LDL are suppressed in their response to inflammatory stimulation.

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.

Integration of growth factor signals at the c-fos serum response element

A transcription factor ternary complex composed of serum response factor (SRF) and a second factor, ternary complex factor (TCF), mediates the response of the c-fos Serum Response Element to growth factors and mitogens. In NIH3T3 fibroblasts, TCF binding is required for transcriptional activation by the SRE in response to activation of the Ras-Raf-ERK pathway. We compared the properties of three members of the TCF family, Elk-1, SAP-1 and SAP-2 (ERP/NET). Although all the proteins contain sequences required for ternary complex formation with SRF, only Elk-1 and SAP-1 appear to interact with the c-fos SRE efficiently in vivo. Each TCF contains a C-terminal activation domain capable of transcriptional activation in response to activation of the Ras-Raf-ERK pathway, and this is dependent on the integrity of S/T-P motifs conserved between all the TCF family members. In contrast, activation of the SRE by whole serum and the mitogenic phospholipid LPA requires SRF binding alone. Constitutively activated members of the Rho subfamily of Ras-like GTPases are also capable of inducing activation of the SRE in the absence of TCF; unlike activated Ras itself, these proteins do not activate the TCFs in NIH3T3 cells. At the SRE, SRF- and TCF-linked signalling pathways act synergistically to potentiate transcription.

Signalling pathways: jack of all cascades

The transcription factors that bind the c-fos promoter element SRE are targeted by multiple, independent signalling cascades; the identities of these signalling pathways and their modes of activation are being elucidated.

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.

Integration of MAP kinase signal transduction pathways at the serum response element

The ternary complex factor (TCF) subfamily of ETS-domain transcription factors bind with serum response factor (SRF) to the serum response element (SRE) and mediate increased gene expression. The TCF protein Elk-1 is phosphorylated by the JNK and ERK groups of mitogen-activated protein (MAP) kinases causing increased DNA binding, ternary complex formation, and transcriptional activation. Activated SRE-dependent gene expression is induced by JNK in cells treated with interleukin-1 and by ERK after treatment with phorbol ester. The Elk-1 transcription factor therefore integrates MAP kinase signaling pathways in vivo to coordinate biological responses to different extracellular stimuli.

Comparative analysis of the ternary complex factors Elk-1, SAP-1a and SAP-2 (ERP/NET)

A transcription factor ternary complex composed of Serum Response Factor (SRF) and Ternary Complex Factor (TCF) mediates the response of the c-fos Serum Response Element (SRE) to growth factors and mitogens. Three Ets domain proteins, Elk-1, SAP-1 and ERP/NET, have been reported to have the properties of TCF. Here we compare Elk-1 and SAP-1a with the human ERP/NET homologue SAP-2. All three TCF RNAs are ubiquitously expressed at similar relative levels. All three proteins contain conserved regions that interact with SRF and the c-fos SRE with comparable efficiency, but in vitro complex formation by SAP-2 is strongly inhibited by its C-terminal sequences. Similarly, only Elk-1 and SAP-1a efficiently bind the c-fos SRE in vivo; ternary complex formation by SAP-2 is weak and is substantially unaffected by serum stimulation or v-ras co-expression. All three TCFs contain C-terminal transcriptional activation domains that are phosphorylated following growth factor stimulation. Activation requires conserved S/T-P motifs found in all the TCF family members. Each TCF activation domain can be phosphorylated in vitro by partially purified ERK2, and ERK activation in vivo is sufficient to potentiate transcriptional activation.

SAP1a is a nuclear target of signaling cascades involving ERKs

The Ets protein SAP1a has been shown to interact with the c-fos serum response element upon recruitment by the serum response factor. We demonstrate that SAP1a is a nuclear protein stimulating transcription via the c-fos serum response element, and additionally via an Ets binding site independently of the serum response factor. However, transactivation has only been observed under conditions leading to the activation of extracellular signal-regulated protein kinases (ERKs). The transcriptional activation domain of SAP1a resides within the C-terminal region, the function of which may be impeded by the N-terminus. Several potential ERK consensus sites within the C-terminal region of SAP1a can modulate its transactivation efficacy, implicating that SAP1a is a direct target of ERKs. Since ERKs are activated by a broad range of signals, SAP1a may play an important role in the transformation of extracellular stimuli into a nuclear response.

Mapping of the human SAP1 (SRF accessory protein 1) gene and SAP2, a gene encoding a related protein, to chromosomal bands 1q32 and 12q23, respectively

SAP1, SAP2, and ELK1 form a related subgroup of ETS-domain proteins that can form ternary complexes with the transcription factor SRF at the c-fos serum response element (SRE). SAP1 was identified by a genetic screen for proteins interacting with SRF expressed in yeast, and SAP2 by its homology with SAP1; ELK1 was previously identified by its homology to the ETS domain. cDNA probes were used to isolate cosmid and phage clones harboring genes encoding SAP1 and SAP2. These clones were subsequently used to map the genes to 1q32 and 12q23, respectively, by fluorescence in situ hybridization.

The role of regulated phosphorylation in the biological activity of transcription factors SRF and Elk-1/SAP-1

The promoters of many genes whose expression is rapidly and transiently increased upon growth factor or mitogen stimulation of susceptible cells harbor a common regulatory element, the serum response element (SRE). The transcription factors that interact with the SRE of these immediate-early genes are being intensively studied so as to elucidate the signal transduction cascades that link cell surface receptors to the SRE, and to determine the mechanisms by which signal-dependent phosphorylation events modulate their function.

Ras/MAP kinase-dependent and -independent signaling pathways target distinct ternary complex factors

Transcriptional activation of the immediate early genes c-fos and egr-1 by extracellular signals appears to be mediated by ternary complex factors (TCFs). In BAC-1 macrophages, growth factor stimulation leads to the retardation of protein-DNA complexes containing distinct TCFs. One TCF is recognized by Elk-1 antisera, whereas the other is immunologically related to SAP-1. The appearance and decay of hyperphosphorylated TCF/Elk-1-containing complexes after stimulation coincide with the activation of mitogen-activated protein kinase (MAPK) and the induction and repression of c-fos and egr-1, whereas modified TCF/SAP-1-containing complexes decay more slowly. Suppression of MAPK activation in macrophages and fibroblasts correlates with the failure to induce TCF/Elk-1 hyperphosphorylation without blocking TCF/SAP-1 modification. Accordingly the modified Elk-1 complex is generated in vitro by activated MAPK, whereas that of SAP-1 is not. Expression of a dominant-negative Ras mutant (RasAsn17) in BAC-1 cells does not affect CSF-1-induced TCF/SAP-1 modification while suppressing TCF/Elk-1 phosphorylation. Neither PKC down-regulation by TPA nor inhibition of Gi proteins by pertussis toxin pretreatment influences CSF-1-induced signaling to TCFs. These data indicate the existence of two separate signaling pathways for the modification of distinct TCFs: one dependent on Ras and MAPK and converging on TCF/Elk-1, and the other targeting TCF/SAP-1 independently of Ras and MAPK.

Ternary complex factors: growth factor regulated transcriptional activators

Members of a family of Ets domain proteins, the ternary complex factors (TCFs), are recruited to the c-fos serum response element by interaction with the serum response factor. Recent findings indicate that phosphorylation of TCFs occurs in response to activation of the MAP kinase pathway, and that regulation of TCF activity is an important mechanism by which the serum response element responds to growth factor signals.

Transcriptional activation domains of elk-1, delta elk-1 and SAP-1 proteins

elk-1, an ets related gene codes for a sequence specific DNA binding transcriptional activator which in association with serum response factor (SRF) forms a ternary complex at the c-fos serum response element (SRE). Recently the C-terminal region of both elk-1 and delta elk-1 proteins was shown to undergo phosphorylation by MAP kinases and function as an activator of MAP kinases. Here we show that delta elk-1 and two other elk-1 related proteins SAP-1a and SAP-1b, like elk-1, can function as transcriptional activators. In this report we have localized the transcriptional activation domain of the SAP-1 proteins (STA) to a large portion of the carboxy terminal region and have identified two autonomous transcriptional activation domains in the elk-1 protein, one at the amino (ETA-1) and the other at the carboxy terminal region (ETA-2). delta elk-1 protein contains only the ETA-2 domain indicating differential usage of activation domains as a result of alternative splicing. We can speculate that the ETA-1 domain can function in vivo independent of ETA-2, but the ETA-2 domain can function either in the absence of ETA-1 (as seen in delta elk-1) or in the presence of accessory proteins like SRF. The role of SRF in the activation of the ternary complex might be to bind to the ETA-1 domain, somehow conceal it's activation domain and in the process unmask the ETA-2 domain (for phosphorylation by MAP kinases) and activate transcription. The ETA-1 domain may be functioning as a negative regulatory transcriptional activation domain for ETA-2. These observations suggest that the elk-1 family of proteins may not only regulate fos and MAP kinases but also other elk-1 target genes that are essential for cellular growth control.

Spatial flexibility in ternary complexes between SRF and its accessory proteins

We investigated the sequence requirements for ternary complex formation by the transcription factor SRF and its Ets domain accessory factors Elk-1 and SAP-1. Ternary complex formation is specified by an SRF consensus site CC(A/T)6GG and a neighbouring Ets motif (C/A)(C/A)GGA(A/T), which is contacted by Elk-1/SAP-1. Both the spacing of these sequences and their relative orientation can be substantially altered with little effect on the efficiency of ternary complex formation. Efficient ternary complex formation by Elk-1 is mediated by the B box, a conserved 21 amino acid region located 50 residues C-terminal to the Ets domain, which also acts to inhibit autonomous DNA binding. Binding studies with the isolated Ets domains indicate that ternary complex formation compensates for low affinity Ets domain-DNA interactions. Several naturally occurring SREs containing Ets motifs at different locations to that in the human c-fos SRE allow SAP-1 and Elk-1 recruitment in vitro. We discuss the mechanism of ternary complex formation.