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

ELK3 destabilization by speckle-type POZ protein suppresses prostate cancer progression and docetaxel resistance

Accumulating evidence demonstrates that the activity regulation of ELK3, a member of the E26 transformation-specific oncogene family, is critical to regulating cell proliferation, migration, and survival in human cancers. However, the molecular mechanisms of how ELK3 induces chemoresistance in prostate cancer (PCa) have not been elucidated. In this study, we found that SPOP and ELK3 are an interacting partner. The interaction between SPOP and ELK3 resulted in increased ELK3 ubiquitination and destruction, assisted by checkpoint kinase-mediated ELK3 phosphorylation. Notably, the modulation of SPOP-mediated ELK3 protein stability affected the c-Fos-induced cell proliferation and invasion of PCa cells. The clinical involvement of the SPOP-ELK3 axis in PCa development was confirmed by an immunohistochemical assay on 123 PCa tissues, with an inverse correlation between increased ELK3 and decreased SPOP being present in ~80% of the specimens. This observation was supported by immunohistochemistry analysis using a SPOP-mutant PCa specimen. Finally, docetaxel treatment induced cell death by activating checkpoint kinase- and SPOP-mediated ELK3 degradation, while SPOP-depleted or SPOP-mutated PCa cells showed cell death resistance. Notably, this observation was correlated with the protein levels of ELK3. Taken together, our study reveals the precise mechanism of SPOP-mediated degradation of ELK3 and provides evidence that SPOP mutations contribute to docetaxel resistance in PCa.

Molecular and clinical features of a potential immunotherapy target ELK3 in glioma

Glioma represents the most prevalent malignant primary brain cancer, and its treatment remains a tremendous challenge. Novel and efficient molecular targets are therefore required for improving diagnosis, survival prediction, and treatment outcomes. Additionally, some studies have shown that immunity is highly associated with glioma progression. Our study aimed to investigate the clinicopathological features, prognostic significance, and immunotherapeutic targetability of ELK3, a member of the erythroblast transformation-specific transcription factor family, in glioma using bioinformatics analyses. ELK3 transcript levels in glioma tissues were evaluated using the Gene Expression Omnibus and The Cancer Genome Atlas databases. Clinical and transcriptomic data of The Cancer Genome Atlas glioma patients were analyzed to identify the molecular and clinical characterizations of ELK3. The prognostic significance of ELK3 was assessed using Cox regression and Kaplan-Meier analysis. The biological pathways related to ELK3 expression were identified by gene set enrichment analysis. The relationships between ELK3 and inflammatory responses, immune cell infiltration, and immune checkpoints were explored using canonical correlation analysis and gene set variation analysis. ELK3 was upregulated in gliomas, and its high expression was correlated with advanced clinicopathologic features and unfavorable prognosis. Gene set enrichment analysis revealed that several immune-related pathways were tightly linked to high ELK3 expression. gene set variation analysis and correlograms demonstrated that ELK3 was robustly associated with inflammatory and immune responses. Correlation analyses indicated that ELK3 was positively associated with infiltrating immune cells and synergistic with several immune checkpoints. ELK3 may serve as a novel marker of poor prognosis and a potential immunotherapeutic target in glioma.

ELK3 Mediated by ZEB1 Facilitates the Growth and Metastasis of Pancreatic Carcinoma by Activating the Wnt/β-Catenin Pathway

Rapid progression and metastasis are the major causes of death in patients with pancreatic ductal adenocarcinoma (PDAC). ELK3, a member of the ternary complex factor (TCF), has been associated with the initiation and progression of various cancers. However, the role of ELK3 in PDAC is not yet fully understood. Online databases and immunohistochemistry were used to analyze the ELK3 levels in PDAC tissues. The function of ELK3 was confirmed by a series of in vivo and in vitro studies. Western blotting and immunofluorescence were used to detect the molecular mechanisms of PDAC. ChIP-qPCR was used to study the mechanism responsible for the elevation of ELK3 expression in PDAC. The ELK3 levels were higher in PDAC tissues than in adjacent normal tissues. Functionally, we demonstrated that ELK3 acted as an oncogene to promote PDAC tumorigenesis and metastasis. Further study suggested that ELK3 promoted PDAC cell migration and invasion by activating the Wnt/β-catenin pathway, and proved that ZEB1 could directly bind to the promoter of ELK3 to increase its transcription. Finally, both were associated with the patients’ clinicopathological features and worse overall survival. Conclusively, our findings enrich the role of ELK3 in PDAC, and provide potential avenues for exploring more effective biomarkers and therapeutic strategies for the treatment of PDAC.

Role of PAR1-Egr1 in the Initiation of Thoracic Aortic Aneurysm in Fbln4-Deficient Mice

Objective:

Remodeling of the extracellular matrix plays a vital role in cardiovascular diseases. Using a mouse model of postnatal ascending aortic aneurysms (termed Fbln4 SMKO ), we have reported that abnormal mechanosensing led to aneurysm formation in Fbln4 SMKO with an upregulation of the mechanosensitive transcription factor, Egr1 (Early growth response 1). However, the role of Egr1 and its upstream regulator(s) in the initiation of aneurysm development and their relationship to an aneurysmal microenvironment are unknown.

Approach and Results:

To investigate the contribution of Egr1 in the aneurysm development, we deleted Egr1 in Fbln4 SMKO mice and generated double knockout mice ( DKO , Fbln4 SMKO ; Egr1 −/− ). Aneurysms were prevented in DKO mice (42.8%) and Fbln4 SMKO ; Egr1 +/− mice (26%). Ingenuity Pathway Analysis identified PAR1 (protease-activated receptor 1) as a potential Egr1 upstream gene. Protein and transcript levels of PAR1 were highly increased in Fbln4 SMKO aortas at postnatal day 1 before aneurysm formed, together with active thrombin and MMP (matrix metalloproteinase)-9, both of which serve as a PAR1 activator. Concordantly, protein levels of PAR1, Egr1, and thrombin were significantly increased in human thoracic aortic aneurysms. In vitro cyclic stretch assays (1.0 Hz, 20% strain, 8 hours) using mouse primary vascular smooth muscle cells induced marked expression of PAR1 and secretion of prothrombin in response to mechanical stretch. Thrombin was sufficient to induce Egr1 expression in a PAR1-dependent manner.

Conclusions:

We propose that thrombin, MMP-9, and mechanical stimuli in the Fbln4 SMKO aorta activate PAR1, leading to the upregulation of Egr1 and initiation of ascending aortic aneurysms.

Genome-wide scans identify known and novel regions associated with prolificacy and reproduction traits in a sub-Saharan African indigenous sheep (Ovis aries)

Maximizing the number of offspring born per female is a key functionality trait in commercial- and/or subsistence-oriented livestock enterprises. Although the number of offspring born is closely associated with female fertility and reproductive success, the genetic control of these traits remains poorly understood in sub-Saharan Africa livestock. Using selection signature analysis performed on Ovine HD BeadChip data from the prolific Bonga sheep in Ethiopia, 41 candidate regions under selection were identified. The analysis revealed one strong selection signature on a candidate region on chromosome X spanning BMP15, suggesting this to be the primary candidate prolificacy gene in the breed. The analysis also identified several candidate regions spanning genes not reported before in prolific sheep but underlying fertility and reproduction in other species. The genes associated with female reproduction traits included SPOCK1 (age at first oestrus), GPR173 (mediator of ovarian cyclicity), HB-EGF (signalling early pregnancy success) and SMARCAL1 and HMGN3a (regulate gene expression during embryogenesis). The genes involved in male reproduction were FOXJ1 (sperm function and successful fertilization) and NME5 (spermatogenesis). We also observed genes such as PKD2L2, MAGED1 and KDM3B, which have been associated with diverse fertility traits in both sexes of other species. The results confirm the complexity of the genetic mechanisms underlying reproduction while suggesting that prolificacy in the Bonga sheep, and possibly African indigenous sheep is partly under the control of BMP15 while other genes that enhance male and female fertility are essential for reproductive fitness.

ZEB1 Collaborates with ELK3 to Repress E-Cadherin Expression in Triple-Negative Breast Cancer Cells

ZEB1 has intrinsic oncogenic functions that control the epithelial-to-mesenchymal transition (EMT) of cancer cells, impacting tumorigenesis from its earliest stages. By integrating microenvironment signals and being implicated in feedback regulatory loops, ZEB1 appears to be a central switch that determines EMT and metastasis of cancer cells. Here, we found that ZEB1 collaborates with ELK3, a ternary complex factor belonging to the ETS family, to repress E-cadherin expression. ZEB1 functions as a transcriptional activator of ELK3. We first identified that ELK3 and ZEB1 have a positively correlated expression in breast cancer cells by using multiple databases for correlation analysis. Molecular analysis revealed that ZEB1 functions as a transcriptional activator of ELK3 expression. GST pull-down assay and coimmunoprecipitation analysis of wild-type or domain deletion mutants of ZEB1 and ELK3 showed that these 2 proteins directly bound each other. Furthermore, we demonstrated that ZEB1 and ELK3 collaborate to repress the expression of E-cadherin, a representative protein that initiates EMT. Our finding suggested that ELK3 is a novel factor of the ZEB1/E-cadherin axis in triple-negative breast cancer cells. IMPLICATIONS: ELK3 is a novel factor in the ZEB1/E-cadherin axis and ZEB1 has a dual role in ELK3 as a transcriptional activator and as a collaborator to repress E-cadherin expression in triple-negative breast cancer cells.

Novel function of E26 transformation-specific domain-containing protein ELK3 in lymphatic endothelial cells

Lymphatic endothelial cells (LEC) are major components of the tumor microenvironment and, due to the relative leakiness of lymphatic vessels compared with blood vessels, are essential for tumor dissemination and metastasis. In the present study, small interfering RNA-mediated suppression of E26 transformation-specific domain-containing protein Elk-3 (ELK3) inhibited the proliferation, migration and tube-forming ability of LEC. Suppression of ELK3 decreased vascular endothelial-cadherin expression levels and increased the phosphorylation of β-catenin. Furthermore, vascular endothelial growth factor receptor-3 (VEGFR-3) mRNA and protein expression levels were suppressed by the transfection of LEC with siELK3. As VEGFR-3 serves a major role in lymphangiogenesis, ELK3 may be a novel therapeutic target to inhibit tumor dissemination through the lymphatic system.

Elk3 is essential for the progression from progenitor to definitive neural crest cell

Elk3/Net/Sap2 (here referred to as Elk3) is an Ets ternary complex transcriptional repressor known for its involvement in angiogenesis during embryonic development. Although Elk3 is expressed in various tissues, additional roles for the protein outside of vasculature development have yet to be reported. Here, we characterize the early spatiotemporal expression pattern of Elk3 in the avian embryo using whole mount in situ hybridization and quantitative RT-PCR and examine the effects of its loss of function on neural crest development. At early stages, Elk3 is expressed in the head folds, head mesenchyme, intersomitic vessels, and migratory cranial neural crest (NC) cells. Loss of the Elk3 protein results in the retention of Pax7+ precursors in the dorsal neural tube that fail to upregulate neural crest specifier genes, FoxD3, Sox10 and Snail2, resulting in embryos with severe migration defects. The results putatively place Elk3 downstream of neural plate border genes, but upstream of neural crest specifier genes in the neural crest gene regulatory network (NC-GRN), suggesting that it is critical for the progression from progenitor to definitive neural crest cell.

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.

Capsaicin-induced inactivation of sensory neurons promotes a more aggressive gene expression phenotype in breast cancer cells

Capsaicin-induced inactivation of sensory neurons has been reported to enhance metastasis of a murine breast cancer cell line, specifically enhancing myocardial metastases. Here we characterized changes in gene expression patterns in primary tumors which developed in capsaicin-treated vs. control mice. We identified a small cohort of genes (17) which all showed significant decreases in expression levels. All of the identified genes have been linked to cell growth, differentiation, and/or cancer progression. Three representative genes, Caspase-7 (an executor of apoptosis), ADAM-10 (A Disintegrin and Metalloprotease), and Elk-3 (a transcriptional repressor of the ternary factor subfamily of the Ets factors) were further investigated. All three showed dramatic downregulation at the protein level in primary tumors from capsaicin-treated animals compared with control (vehicle-treated) animals, and their expression was also lost in cell culture. Elk-3 and Caspase-7 were not expressed in vitro in cultured cell lines, suggesting that their expression was induced by the tumor microenvironment. Loss of Caspase-7 expression can be expected to result in loss of function of apoptotic pathways. At first glance, loss of ADAM-10 expression would be expected to result in decreased invasive capability, due to loss of matrix metalloprotease activity. However, just the opposite appears to be true. We found that ADAM-10 actually hydrolyzes Substance P. Specifically ADAM-10 produces the same growth-inhibitory products from Substance P (i.e., SP (1-7)) that Neprilysin does, so that loss of ADAM-10 expression actually results in loss of production of growth inhibitory peptides from Substance P. Similarly, ADAM-10 also efficiently hydrolyzes Calcitonin Gene-Related Peptide, which may act in concert with Substance P. Finally, overactivity of Ets transcriptional suppressor functions has been linked to inhibition of tumorigenesis (e.g., Erf and Mef), and in addition loss of Elk-3 expression might also be be linked to tumorigenesis via loss of its putative anti-inflammatory activities. There is anecdotal evidence in the literature to indicate that the rest of the down-regulated genes may also contribute to development of a more aggressive phenotype in this breast cancer model.

Endotoxin-induced down-regulation of Elk-3 facilitates heme oxygenase-1 induction in macrophages

Heme oxygenase-1 (HO-1) is a cytoprotective enzyme that is acutely induced by inflammatory stimuli, and the products of HO-1-mediated heme degradation have anti-inflammatory properties. In many different pathophysiologic states, the up-regulation of HO-1 has been shown to be beneficial in combating the detrimental consequences of increased inflammation. Ets transcription factors are known to be important mediators of inflammatory responses, and the ternary complex factor subfamily of Ets proteins has both transcriptional activation and repression activity. The present study demonstrates that of several ternary complex factor subfamily members, only Elk-3 represses HO-1 promoter activity in macrophages. Endotoxin administration to macrophages led to a dose-dependent decrease in endogenous Elk-3 mRNA levels, and this reduction in Elk-3 preceded the LPS-mediated up-regulation of HO-1 message. Analogous results also occurred in lung tissue of mice exposed to endotoxin. Two putative Ets binding sites (EBS1 and EBS2) are present in the downstream region of the murine HO-1 promoter (bp -125 and -93, respectively), and we recently showed that the EBS2 site is essential for HO-1 induction by endotoxin. In contrast, the present study demonstrates that the repressive effect of Elk-3 on HO-1 promoter activity is dependent on the EBS1 site. Taken together, our data reveal that Elk-3 serves as an important repressor of HO-1 gene transcription and contributes to the tight control of HO-1 gene regulation in the setting of inflammatory stimuli.

Elk3 from hamster--a ternary complex factor with strong transcriptional repressor activity

Elk3 belongs to the Ets family of transcription factors, which are regulated by the Ras/mitogen-activated protein kinase-signaling pathway. In the absence of Ras, this protein is a strong inhibitor of transcription and may be directly involved in regulation of growth by downregulating the transcription of genes that are activated during entry into G1. We have isolated the Cricetulus griseus Elk3 gene from the Chinese hamster ovary (CHO) cell line and investigated the transcriptional potential of this factor. Transient transfections revealed that, in addition to its regulation of the c-fos promoter, Elk3 from CHO cells seems to inhibit other promoters controlling expression of proteins involved in G1/S phase progression; Cyclin D1 and DHFR. As has been described for the Elk3 homologs Net (Mouse) and Sap-2 (Human), the results of the present study further indicate that hamster Elk3 is a target of the Ras-Raf-MAPK pathway, and cotransfections with constitutively active H-ras relieves its negative transcriptional activity. No cells stably expressing exogenous Elk3 could be obtained, possibly due to an unspecified toxic or growth retarding effect. These findings support a possible role for Elk3 in growth regulation and reveal a high degree of homology for this protein across species.

Ets ternary complex transcription factors

The three ternary complex factors (TCFs) Elk-1, Net and Sap-1 form a subfamily of the E twenty-six (Ets) domain transcription factors. Their characteristic property is the ability to form a ternary nucleoprotein complex with the serum response factor (SRF) over the serum response element (SRE) of the c-fos promoter. The molecular mechanisms that underlie the function and regulation of these factors have been extensively studied and the TCFs are a paradigm for the study of transcriptional regulation in response to extracellular signalling through the mitogen-activated protein (MAP) kinase pathway. As final effectors of multiple signalling pathways and components of protein complexes on immediate early promoters, they represent key elements in the complex and dynamic regulation of gene expression. This review summarises the molecular, structural and biochemical studies that have led to the understanding of the functional domains of the TCFs, ternary complex formation, transcriptional regulation, protein partners and target genes in cell lines. Finally, the emerging studies of the biological roles of the TCFs in vivo will be discussed.

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.

Elk-3 is a transcriptional repressor of nitric-oxide synthase 2

The inducible isoform of nitric-oxide synthase (NOS2), a key enzyme catalyzing the dramatic increase in nitric oxide by lipopolysaccharide (LPS), plays an important role in the pathophysiology of endotoxemia and sepsis. Recent evidence suggests that Ets transcription factors may contribute to NOS2 induction by inflammatory stimuli. In this study, we investigated the role of Ets transcription factors in the regulation of NOS2 by LPS and transforming growth factor (TGF)-beta 1. Transient transfection assays in macrophages showed that Ets-2 produced an increase in NOS2 promoter activity, whereas the induction by Ets-1 was modest and NERF2 had no effect. Elk-3 (Net/Erp/Sap-2a) markedly repressed NOS2 promoter activity in a dose-dependent fashion, and overexpression of Elk-3 blunted the induction of endogenous NOS2 message. Mutation of the Net inhibitory domain of Elk-3, but not the C-terminal-binding protein interaction domain, partially alleviated this repressive effect. We also found that deletion of the Ets domain of Elk-3 completely abolished its repressive effect on the NOS2 promoter. LPS administration to macrophages led to a dose-dependent decrease in endogenous Elk-3 mRNA levels, and this decrease in Elk-3 preceded the induction of NOS2 mRNA. In a mouse model of endotoxemia, the expression of Elk-3 in kidney, lung, and heart was significantly down-regulated after systemic administration of LPS, and this down-regulation also preceded NOS2 induction. Moreover, TGF-beta 1 significantly increased endogenous Elk-3 mRNA levels that had been down-regulated by LPS in macrophages. This increase in Elk-3 correlated with a TGF-beta 1-induced down-regulation of NOS2. Taken together, our data suggest that Elk-3 is a strong repressor of NOS2 promoter activity and mRNA levels and that endogenous expression of Elk-3 inversely correlates with NOS2. Thus, Elk-3 may serve as an important mediator of NOS2 gene expression.

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.

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.

Opposing roles of Elk-1 and its brain-specific isoform, short Elk-1, in nerve growth factor-induced PC12 differentiation

The ternary complex factor Elk-1, a major nuclear target of extracellular signal-regulated kinases, is a strong transactivator of serum-responsive element (SRE) driven gene expression. We report here that mature brain neurons and nerve growth factor (NGF)-differentiated PC12 cells also express a second, smaller isoform of Elk-1, short Elk-1 (sElk-1). sElk-1 arises from an internal translation start site in the Elk-1 sequence, which generates a protein lacking the first 54 amino acids of the DNA-binding domain. This deletion severely compromises the ability of sElk-1 to form complexes with serum response factor on the SRE in vitro and to activate SRE reporter genes in the presence of activated Ras. Instead, sElk, but not a mutant that cannot be phosphorylated, inhibits transactivation driven by Elk-1. More pertinent to the neuronal-specific expression of sElk-1, we show it plays an opposite role to Elk-1 in potentiating NGF-driven PC12 neuronal differentiation. Overexpression of sElk-1 but not Elk-1 increases neurite extension, an effect critically linked to its phosphorylation. Interestingly, in the presence of sElk-1, Elk-1 loses its strictly nuclear localization to resemble the nuclear/cytoplasm pattern observed in the mature brain. This is blocked by mutating a normally cryptic nuclear export signal in Elk-1. These data provide new insights into molecular events underlying neuronal differentiation of PC12 cells mediated by the NGF-ERK signaling cascade.

Both TEL and AML-1 contribute repression domains to the t(12;21) fusion protein

t(12;21) is the most frequent translocation found in pediatric B-cell acute lymphoblastic leukemias. This translocation fuses a putative repressor domain from the TEL DNA-binding protein to nearly all of the AML-1B transcription factor. Here, we demonstrate that fusion of the TEL pointed domain to the GAL4 DNA-binding domain resulted in sequence-specific transcriptional repression, indicating that the pointed domain is a portable repression motif. The TEL pointed domain functioned equally well when the GAL4 DNA-binding sites were moved 600 bp from the promoter, suggesting an active mechanism of repression. This lead us to demonstrate that wild-type TEL and the t(12;21) fusion protein bind the mSin3A corepressor. In the fusion protein, both TEL and AML-1B contribute mSin3 interaction domains. Deletion mutagenesis indicated that both the TEL and AML-1B mSin3-binding domains contribute to repression by the fusion protein. While both TEL and AML-1B associate with mSin3A, TEL/AML-1B appears to bind this corepressor much more stably than either wild-type protein, suggesting a mode of action for the t(12;21) fusion protein.

Fibroblast growth factor receptor signaling activates the human interstitial collagenase promoter via the bipartite Ets-AP1 element

Interstitial collagenases participate in the remodeling of skeletal matrix and are regulated by fibroblast growth factor (FGF). A 0.2-kb fragment of the proximal human interstitial collagenase [matrix metalloproteinase (MMP1)] promoter conveys 4- to 8-fold induction of a luciferase reporter in response to FGF2 in MC3T3-E1 osteoblasts. By 5'-deletion, this response maps to nucleotides -100 to -50 relative to the transcription initiation site. The 63- bp MMP1 promoter fragment -123 to -61 confers this FGF2 response on the rous sarcoma virus minimal promoter. Intact Ets and AP1 cognates in this element are both required for responsiveness. The AP1 site supports basal and FGF-inducible promoter activity. The intact Ets cognate represses basal transcriptional activity in both heterologous and native promoter contexts and is also required for FGF activation. FGF2 up-regulates a DNA-binding activity that recognizes the MMP1 AP1 cognate and contains immunoreactive Fra1 and c-Jun. Both constitutive and FGF-inducible DNA-binding activities are present in MC3T3-E1 cells that recognize the MMP1 Ets cognate; prototypic Ets transcriptional activators are not present in these complexes. Inhibitors of protein kinase C, phosphatidyl inositol 3-OH kinase, and calmodulin-dependent protein kinase do not attenuate MMP1 promoter activation. FGF2 activates ERK1/ERK2 signaling in osteoblasts; however, 25 microM MAPK-ERK kinase (MEK) inhibitor PD98059 (inhibits by > 85% the phosphorylation of ERK1/ERK2) has no effect on MMP1 promoter activation by FGF2. Ligand-activated and constitutively active FGF receptors initiate MMP1 induction. Dominant negative Ras abrogates MMP1 induction by constitutively active FGFR2-ROS, but dominant negative Rho and Rac do not inhibit induction. The mitogen-activated protein kinase (MAPK) phosphatase MKP2 [inactivates extracellular regulated kinase (ERK) = Jun N-terminal kinase (JNK) > p38 MAPK] completely abrogates MMP1 activation, whereas PAC1 (inactivates ERK = p38 > JNK) attenuates but does not completely prevent induction. Thus, a Ras- and MKP2-regulated MAPK pathway, independent of ERK1/ERK2 MAPK activity, mediates FGF2 transcriptional activation of MMP1 in MC3T3-E1 osteoblasts, converging upon the bipartite Ets-AP1 element. The DNA-protein interactions and signal cascades mediating FGF induction of the MMP1 promoter are distinct from two other recently described FGF response elements: the MMP1 promoter (-123 to -61) represents a third FGF-activated transcriptional unit.