Elk-1 can recruit SRF to form a ternary complex upon the serum response element

CREB-mediated transcriptional activation of NRMT1 drives muscle differentiation

The N-terminal methyltransferase NRMT1 is an important regulator of protein/DNA interactions and plays a role in many cellular processes, including mitosis, cell cycle progression, chromatin organization, DNA damage repair, and transcriptional regulation. Accordingly, loss of NRMT1 results in both developmental pathologies and oncogenic phenotypes. Though NRMT1 plays such important and diverse roles in the cell, little is known about its own regulation. To better understand the mechanisms governing NRMT1 expression, we first identified its predominant transcriptional start site and minimal promoter region with predicted transcription factor motifs. We then used a combination of luciferase and binding assays to confirm CREB1 as the major regulator of NRMT1 transcription. We tested which conditions known to activate CREB1 also activated NRMT1 transcription, and found CREB1-mediated NRMT1 expression was increased during recovery from serum starvation and muscle cell differentiation. To determine how NRMT1 expression affects myoblast differentiation, we used CRISPR/Cas9 technology to knock out NRMT1 expression in immortalized C2C12 mouse myoblasts. C2C12 cells depleted of NRMT1 lacked Pax7 expression and were unable to proceed down the muscle differentiation pathway. Instead, they took on characteristics of C2C12 cells that have transdifferentiated into osteoblasts, including increased alkaline phosphatase and type I collagen expression and decreased proliferation. These data implicate NRMT1 as an important downstream target of CREB1 during muscle cell differentiation.

ELK1 has a dual activating and repressive role in human embryonic stem cells

Background: The ERK MAPK pathway plays a pivotal role in regulating numerous cellular processes during normal development and in the adult but is often deregulated in disease scenarios. One of its key nuclear targets is the transcription factor ELK1, which has been shown to play an important role in controlling gene expression in human embryonic stem cells (hESCs). ELK1 is known to act as a transcriptional activator in response to ERK pathway activation but repressive roles have also been uncovered, including a putative interaction with the PRC2 complex.

Methods: Here we probe the activity of ELK1 in hESCs by using a combination of gene expression analysis in hESCs and during differentiation following ELK1 depletion and also analysis of chromatin occupancy of transcriptional regulators and histone mark deposition that accompany changes in gene expression.

Results: We find that ELK1 can exert its canonical activating activity downstream from the ERK pathway but also possesses additional repressive activities. Despite its co-binding to PRC2 occupied regions, we could not detect any ELK1-mediated repression at these regions. Instead, we find that ELK1 has a repressive role at a subset of co-occupied SRF binding regions.

Conclusions: ELK1 should therefore be viewed as a dichotomous transcriptional regulator that can act through SRF to generate both activating and repressing properties at different genomic loci.

ELK1 has a dual activating and repressive role in human embryonic stem cells

Background: The ERK MAPK pathway plays a pivotal role in regulating numerous cellular processes during normal development and in the adult but is often deregulated in disease scenarios. One of its key nuclear targets is the transcription factor ELK1, which has been shown to play an important role in controlling gene expression in human embryonic stem cells (hESCs). ELK1 is known to act as a transcriptional activator in response to ERK pathway activation but repressive roles have also been uncovered, including a putative interaction with the PRC2 complex. Methods: Here we probe the activity of ELK1 in hESCs by using a combination of gene expression analysis in hESCs and during differentiation following ELK1 depletion and also analysis of chromatin occupancy of transcriptional regulators and histone mark deposition that accompany changes in gene expression. Results: We find that ELK1 can exert its canonical activating activity downstream from the ERK pathway but also possesses additional repressive activities. Despite its co-binding to PRC2 occupied regions, we could not detect any ELK1-mediated repression at these regions. Instead, we find that ELK1 has a repressive role at a subset of co-occupied SRF binding regions. This latter repressive role appears not to be exerted through competition with MRTF family co-activators. Conclusions: ELK1 should therefore be viewed as a dichotomous transcriptional regulator that can act through SRF to generate both activating and repressing properties at different genomic loci.

A Positive Feedback Loop of SLP2 Activates MAPK Signaling Pathway to Promote Gastric Cancer Progression

Rationale: This study is to validate the clinicopathologic significance and potential prognostic value of SLP2 in gastric cancer (GC), to investigate the biological function and regulation mechanism of SLP2, and to explore potential therapeutic strategies for GC. Methods: The expression of SLP2 in GC tissues from two cohorts was examined by IHC. The biological function and regulation mechanism of SLP2 and PHB was validated via loss-of-function or gain-of-function experiments. In vitro proliferation detection was used to evaluate the therapeutic effects of Sorafenib. Results: We validated that SLP2 was significantly elevated in GC tissues and its elevation was associated with poor prognosis of patients. Loss of SLP2 drastically suppressed the proliferation of GC cells and inhibited the tumor growth, while SLP2 overexpression promoted the progression of GC. Mechanistically, SLP2 competed against E3 ubiquitin ligase SKP2 to bind with PHB and stabilized its expression. Loss of SLP2 significantly suppressed phosphorylation of Raf1, MEK1/2, ERK1/2 and ELK1. Furthermore, phosphorylated ELK1 could in turn activate transcription of SLP2. Finally, we demonstrated that a Raf1 inhibitor, Sorafenib, was sufficient to inhibit the proliferation of GC cells. Conclusion: Our findings demonstrated a positive feedback loop of SLP2 which leads to acceleration of tumor progression and poor survival of GC patients. This finding also provided evidence for the reason of SLP2 elevation. Moreover, we found that sorafenib might be a potential therapeutic drug for GC and disrupting the interaction between SLP2 and PHB might also serve as a potential therapeutic target in GC.

Hierarchical and stage-specific regulation of murine cardiomyocyte maturation by serum response factor

After birth, cardiomyocytes (CM) acquire numerous adaptations in order to efficiently pump blood throughout an animal's lifespan. How this maturation process is regulated and coordinated is poorly understood. Here, we perform a CRISPR/Cas9 screen in mice and identify serum response factor (SRF) as a key regulator of CM maturation. Mosaic SRF depletion in neonatal CMs disrupts many aspects of their maturation, including sarcomere expansion, mitochondrial biogenesis, transverse-tubule formation, and cellular hypertrophy. Maintenance of maturity in adult CMs is less dependent on SRF. This stage-specific activity is associated with developmentally regulated SRF chromatin occupancy and transcriptional regulation. SRF directly activates genes that regulate sarcomere assembly and mitochondrial dynamics. Perturbation of sarcomere assembly but not mitochondrial dynamics recapitulates SRF knockout phenotypes. SRF overexpression also perturbs CM maturation. Together, these data indicate that carefully balanced SRF activity is essential to promote CM maturation through a hierarchy of cellular processes orchestrated by sarcomere assembly.

Role of hippocampal activity-induced transcription in memory consolidation

Experience-dependent changes in the strength of connections between neurons in the hippocampus (HPC) are critical for normal learning and memory consolidation, and disruption of this process drives a variety of neurological and psychiatric diseases. Proper HPC function relies upon discrete changes in gene expression driven by transcription factors (TFs) induced by neuronal activity. Here, we describe the induction and function of many of the most well-studied HPC TFs, including cyclic-AMP response element binding protein, serum-response factor, AP-1, and others, and describe their role in the learning process. We also discuss the known target genes of many of these TFs and the purported mechanisms by which they regulate long-term changes in HPC synaptic strength. Moreover, we propose that future research in this field will depend upon unbiased identification of additional gene targets for these activity-dependent TFs and subsequent meta-analyses that identify common genes or pathways regulated by multiple TFs in the HPC during learning or disease.

Acute Stress Enhances Epigenetic Modifications But Does Not Affect the Constitutive Binding of pCREB to Immediate-Early Gene Promoters in the Rat Hippocampus

The immediate early genes (IEGs) c-Fos and Egr-1 are rapidly and transiently induced in sparse neurons within the hippocampus after exposure to an acute stressor. The induction of these genes is a critical part of the molecular mechanisms underlying successful behavioral adaptation to stress. Our previous work has shown that transcriptional activation of c-Fos and Egr-1 in the hippocampus requires formation of a dual histone mark within their promoter regions, the phosphorylation of serine 10 and acetylation of lysine 9/14 of histone H3. In the present study, using chromatin immuno-precipitation (ChIP), we found that an increase in the formation of H3K9ac-S10p occurs within the c-Fos and Egr-1 promoters after FS stress in vivo and that these histone modifications were located to promoter regions containing cAMP Responsive Elements (CREs), but not in neighboring regions containing only Serum Responsive Elements (SREs). Surprisingly, however, subsequent ChIP analyses showed no changes in the binding of pCREB or CREB-binding protein (CBP) to the CREs after FS. In fact, pCREB binding to the c-Fos and Egr-1 promoters was already highly enriched under baseline conditions and did not increase further after stress. We suggest that constitutive pCREB binding may keep c-Fos and Egr-1 in a poised state for activation. Possibly, the formation of H3K9ac-S10p in the vicinity of CRE sites may participate in unblocking transcriptional elongation through recruitment of additional epigenetic factors.

Discovery and validation of information theory-based transcription factor and cofactor binding site motifs

Data from ChIP-seq experiments can derive the genome-wide binding specificities of transcription factors (TFs) and other regulatory proteins. We analyzed 765 ENCODE ChIP-seq peak datasets of 207 human TFs with a novel motif discovery pipeline based on recursive, thresholded entropy minimization. This approach, while obviating the need to compensate for skewed nucleotide composition, distinguishes true binding motifs from noise, quantifies the strengths of individual binding sites based on computed affinity and detects adjacent cofactor binding sites that coordinate with the targets of primary, immunoprecipitated TFs. We obtained contiguous and bipartite information theory-based position weight matrices (iPWMs) for 93 sequence-specific TFs, discovered 23 cofactor motifs for 127 TFs and revealed six high-confidence novel motifs. The reliability and accuracy of these iPWMs were determined via four independent validation methods, including the detection of experimentally proven binding sites, explanation of effects of characterized SNPs, comparison with previously published motifs and statistical analyses. We also predict previously unreported TF coregulatory interactions (e.g. TF complexes). These iPWMs constitute a powerful tool for predicting the effects of sequence variants in known binding sites, performing mutation analysis on regulatory SNPs and predicting previously unrecognized binding sites and target genes.

A Signaling Network Controlling Androgenic Repression of c-Fos Protein in Prostate Adenocarcinoma Cells

The transcription factor c-Fos controls many important cellular processes, including cell growth and apoptosis. c-Fos expression is rapidly elevated in the prostate upon castration-mediated androgen withdrawal through an undefined mechanism. Here we show that androgens (5α-dihydrotestosterone and R1881) suppress c-Fos protein and mRNA expression induced by 12-O-tetradecanoylphorbol-13-acetate (TPA) or EGF in human prostate cancer (PCa) cell lines. Such suppression transpires through a transcriptional mechanism, predominantly at the proximal serum response element of the c-fos promoter. We show that androgen signaling suppresses TPA-induced c-Fos expression through repressing a PKC/MEK/ERK/ELK-1 signaling pathway. Moreover, our results support the hypothesis that p38(MAPK), PI3K, and PKCδ are involved in the androgenic regulation of c-Fos through controlling MEK/ERK. Stable silencing of c-Fos and PKCδ with shRNAs suggests that R1881 promotes cell death induced by low-dose TPA through a mechanism that is dependent on both PKCδ and loss of c-Fos expression. Reciprocally, loss of either PKCδ or c-Fos activates p38(MAPK) while suppressing the activation of ERK1/2. We also provide the first demonstration that R1881 permits cell death induced by low-dose TPA in the LNCaP androgen-dependent PCa cell line and that TPA-induced cell death is independent of exogenous androgen in the castration-resistant variants of LNCaP, C4-2 and C4-2B. Acquisition of androgen-independent killing by TPA correlates with activation of p38(MAPK), suppression of ERK1/2, and loss of c-Fos. These results provide new insights into androgenic control of c-Fos and use of PKC inhibitors in PCa therapy.

Transcription factor Elk-1 participates in the interleukin-1β-dependent regulation of expression of immediate early response gene 3 (IER3)

Immediate early response gene 3 (IER3) encodes a protein involved in the regulation of apoptosis and differentiation. Recently the role of IER3 in the regulation of extracellular signal-regulated kinases (ERKs) was discovered. IER3 prolongs ERKs activation by inhibition of phosphatase PP2A. Here we show that interleukin-1β (IL-1β)-induced IER3 expression is mediated by the ERK1/2 target, transcription factor Elk-1. We identified sequences in the IER3 promoter responsible for its ERKs-dependent activation, namely ETS5/6. Elk-1 binds to these sequences and is phosphorylated following IL-1β stimulation. Mutation of ETS5/6 binding site abolishes activation of IER3 promoter by IL-1β as well as by the constitutively active form of Elk-1 (Elk-VP16). Thus IER3 acts not only as a regulator of ERKs activation, but also as a ERKs-Elk-1-dependent downstream effector.

Inositol polyphosphate multikinase is a coactivator for serum response factor-dependent induction of immediate early genes

Inositol polyphosphate multikinase (IPMK) is a notably pleiotropic protein. It displays both inositol phosphate kinase and phosphatidylinositol kinase catalytic activities. Noncatalytically, IPMK stabilizes the mammalian target of rapamycin complex 1 and acts as a transcriptional coactivator for CREB-binding protein/E1A binding protein p300 and tumor suppressor protein p53. Serum response factor (SRF) is a major transcription factor for a wide range of immediate early genes. We report that IPMK, in a noncatalytic role, is a transcriptional coactivator for SRF mediating the transcription of immediate early genes. Stimulation by serum of many immediate early genes is greatly reduced by IPMK deletion. IPMK stimulates expression of these genes, an influence also displayed by catalytically inactive IPMK. IPMK acts by binding directly to SRF and thereby enhancing interactions of SRF with the serum response element of diverse genes.

Molecular mechanisms of ETS transcription factor-mediated tumorigenesis

The E26 transformation-specific (ETS) family of transcription factors is critical for development, differentiation, proliferation and also has a role in apoptosis and tissue remodeling. Changes in expression of ETS proteins therefore have a significant impact on normal physiology of the cell. Transcriptional consequences of ETS protein deregulation by overexpression, gene fusion, and modulation by RAS/MAPK signaling are linked to alterations in normal cell functions, and lead to unlimited increased proliferation, sustained angiogenesis, invasion and metastasis. Existing data show that ETS proteins control pathways in epithelial cells as well as stromal compartments, and the crosstalk between the two is essential for normal development and cancer. In this review, we have focused on ETS factors with a known contribution in cancer development. Instead of focusing on a prototype, we address cancer associated ETS proteins and have highlighted the diverse mechanisms by which they affect carcinogenesis. Finally, we discuss strategies for ETS factor targeting as a potential means for cancer therapeutics.

EGF activates TTP expression by activation of ELK-1 and EGR-1 transcription factors

Background

Tristetraprolin (TTP) is a key mediator of processes such as inflammation resolution, the inhibition of autoimmunity and in cancer. It carries out this role by the binding and degradation of mRNA transcripts, thereby decreasing their half-life. Transcripts modulated by TTP encode proteins such as cytokines, pro-inflammatory agents and immediate-early response proteins. TTP can also modulate neoplastic phenotypes in many cancers. TTP is induced and functionally regulated by a spectrum of both pro- and anti-inflammatory cytokines, mitogens and drugs in a MAPK-dependent manner. So far the contribution of p38 MAPK to the regulation of TTP expression and function has been best described.

Results

Our results demonstrate the induction of the gene coding TTP (ZFP36) by EGF through the ERK1/2-dependent pathway and implicates the transcription factor ELK-1 in this process. We show that ELK-1 regulates ZFP36 expression by two mechanisms: by binding the ZFP36 promoter directly through ETS-binding site (+ 883 to +905 bp) and by inducing expression of EGR-1, which in turn increases ZFP36 expression through sequences located between -111 and -103 bp.

Conclusions

EGF activates TTP expression via ELK-1 and EGR-1 transcription factors.

Fibroblast growth factor 21 induces glucose transporter-1 expression through activation of the serum response factor/Ets-like protein-1 in adipocytes

Fibroblast growth factor 21 (FGF21) is a liver-secreted endocrine factor with multiple beneficial effects on obesity-related disorders. It enhances glucose uptake by inducing the expression of glucose transporter-1 (GLUT1) in adipocytes. Here we investigated the signaling pathways that mediate FGF21-induced GLUT1 expression and glucose uptake in vitro and in animals. Quantitative real-time PCR and a luciferase reporter assay showed that FGF21 induced GLUT1 expression through transcriptional activation. The truncation of the GLUT1 promoter from -3145 to -3105 bp, which contains two highly conserved serum response element (SRE) and E-Twenty Six (ETS) binding motif, dramatically decreased FGF21-induced promoter activity of the GLUT1 gene. A chromatin immunoprecipitation assay demonstrated that the transcription factors serum response factor (SRF) and Ets-like protein-1 (Elk-1) were recruited to the GLUT1 promoter upon FGF21 stimulation. The siRNA-mediated knockdown of either SRF or Elk-1 resulted in a marked attenuation in FGF21-induced GLUT1 expression and glucose uptake in adipocytes. In C57 lean mice, a single intravenous injection of FGF21 induced phosphorylation of Elk-1 at Ser(383) and SRF at Ser(103) and also led to the recruitment of Elk-1 and SRF to the GLUT1 promoter in epididymal fats. By contrast, such effects of in vivo FGF21 administration were blunted in high fat diet-induced obese mice. In conclusion, FGF21 induces GLUT1 expression and glucose uptake through sequential activation of ERK1/2 and SRF/Elk-1, which in turn triggers the transcriptional activation of GLUT1 in adipocytes. The impairment in this signaling pathway may contribute to FGF21 resistance in obese mice.

Correlation between dietary factors and the risk of pancreatic cancer

AIM: To explore the association between dietary factors and the risk of pancreatic cancer to provide a scientific basis for prevention of pancreatic cancer through diet and lifestyle changes.

METHODS: A case-control study involving 97 patients with pancreatic cancer and 194 controls was conducted. Controls were matched to cases for age and sex. All of them were interviewed with uniform questionnaires. Conditional logistic regression was used for univariate and multivariate analysis.

RESULTS: The development of pancreatic cancer was positively associated with intake of desserts (OR = 4.706), but negatively with intake of onion (OR = 0.068), yam (OR = 0.301), sweet potato (OR = 0.178), and fruit (OR = 0.299).

CONCLUSION: Dietary factors may play an important role in the development of pancreatic cancer.

The essential function for serum response factor in T-cell development reflects its specific coupling to extracellular signal-regulated kinase signaling

Serum response factor (SRF) recruits members of two families of signal-regulated coactivators, the extracellular signal-regulated kinase (ERK)-regulated ternary complex factors (TCFs) and the actin-regulated myocardin-related transcription factors (MRTFs), to its target genes through its DNA-binding domain. Whether coactivator association is required for SRF function in vivo and whether particular SRF functions reflect specific coupling to one or the other signal pathway have remained largely unexplored. We show that SRF is essential for thymocyte positive selection and thymic T(reg) and NK T-cell development but dispensable for early thymocyte development and negative selection. Expression of wild-type SRF, or mutants lacking the N-terminal phosphorylation sites or C-terminal transcriptional activation domain, restores positive selection in SRF null thymocytes. In contrast, SRF.V194E, which cannot recruit TCF or MRTF family members, is inactive, although it is recruited to target genes. Fusion of a TCF C-terminal activation domain to SRF.V194E effectively restores ERK-dependent single-positive (SP) thymocyte development. The resulting SP thymocytes exhibit normal surface marker expression and proliferation following T-cell receptor cross-linking. Thus, ERK signaling through the TCF pathway to SRF is necessary and sufficient for SRF function in thymocyte positive selection.

Characterization of nuclear factors modulating the apolipoprotein D promoter during growth arrest: implication of PARP-1, APEX-1 and ERK1/2 catalytic activities

Human Apolipoprotein D (apoD) is upregulated under several stress conditions and pathological situations such as neurodegenerative diseases and cancers. We previously showed that apoD mRNA expression is induced in growth-arrested cells and demonstrated the specific binding of nuclear proteins to the region −514 to −475 of the promoter. Such region contains a pair of Serum Responsive Elements (SRE), an Ets-Binding Site (EBS) and a Glucocorticoid Responsive Element (GRE). In this study, we show that Parp-1, HnRNP-U, CBF-A, BUB-3, Kif4, APEX-1 and Ifi204 bind these regulatory elements of the apoD promoter. Specific binding of HnRNP-U and Parp-1 was confirmed by Electrophoretic Mobility Shift Assay (EMSA). In a biotin pull-down assay, Kif4 and BUB-3 bind preferentially the SRE1 and the EBS-GRE sites, respectively, while APEX-1 seems recruited indirectly to these elements. We found that the mRNA expression of some of these binding factors is upregulated in growth-arrested cells and that these proteins also transactivate the apoD promoter. In agreement with these results, mutants of APEX-1 and of Parp-1 defective for their DNA-binding and catalytic activities could not transactivate the promoter. The knockdown of Parp-1 and HnRNP-U and the use of specific inhibitors of MEK1/2 and of Parp-1 also inhibited the induction of apoD gene expression. Moreover, ERK1/2 was found activated in a biphasic manner post serum-starvation and the inhibition of Parp-1 causes a sustained activation of ERK2 but not ERK1 for up to 2 h. Altogether, these findings demonstrate the importance of Parp-1, APEX-1 and ERK1/2 catalytic activities in the growth arrest-induced apoD gene expression.

Transcription factors Elk-1 and SRF are engaged in IL1-dependent regulation of ZC3H12A expression

Background

MCPIP is a novel CCCH zinc finger protein described as an RNase engaged in the regulation of immune responses. The regulation of expression of the gene coding for MCPIP - ZC3H12A is poorly explored.

Results

Here we report that the proinflammatory cytokine IL-1β rapidly induces the synthesis of MCPIP in primary monocyte-derived macrophages and HepG2 cells. This up-regulation takes place through the MAP kinase pathway and following activation of the transcription factor Elk-1. Using a ZC3H12A reporter construct we have shown that a ZC3H12A promoter region, stretching from -76 to +60, mediates activation by IL-1β. This region contains binding sites for Elk-1 and its partner SRF. Chromatin immunoprecipitation analysis confirms in vivo binding of both transcription factors to this region of the ZC3H12A promoter.

Conclusions

We conclude that the transcription factor Elk-1 plays an important role in the activation of ZC3H12A expression in response to IL-1β stimulation.

Aberrant low expression level of bovine beta-lactoglobulin is associated with a C to A transversion in the BLG promoter region

Beta-lactoglobulin (beta-LG) is the major whey protein in cow's milk. It is well established that the predominant 2 genetic variants, beta-LG A and B, are differentially expressed. Extensive investigation of the genetic variation in the promoter region of the BLG gene revealed the existence of specific haplotypes associated with the A and B variants, respectively. However, the genetic basis for the differential expression of BLG A and B alleles is still elusive. We have previously reported a quantitative beta-LG B variant, characterized by a very low beta-LG protein expression level. Here, we report that the corresponding BLG allele (BLG B*) shows a correspondingly low mRNA expression level. Comparative DNA sequencing of 7,670 bp of the BLG B* allele and the established BLG B allele revealed a unique difference of a C to A transversion at position 215 bp upstream of the translation initiation site (g.-215C>A). This mutation segregated perfectly with the differential phenotypic expression in a paternal half-sib family and could be confirmed in 2 independent cases. The sequence of the BLG B allele in the region of the mutation is highly conserved among 4 related ruminant species. The site of the mutation corresponds to a putative consensus-binding sequence for the transcription factors c-Rel and Elk-1 as predicted by searching the TRANSFAC database. The beta-LG B* site might be relevant in the natural production of milk of low beta-LG content.

JNK- and Rac1-dependent induction of immediate early gene pip92 suppresses neuronal differentiation

The immediate early gene pip92 is rapidly and transiently induced by serum, basic fibroblast growth factor (bFGF), nerve growth factor (NGF) and phobol ester, as well as various toxic stimuli. Rho GTPases, such as RhoA, Rac1 and Cdc42, have been implicated in both cytoskeletal rearrangement and cell cycle control. Rac1 and Cdc42 induce neurite outgrowth in many types of neuronal cells. A downstream effector of both Rac1 and Cdc42, p21-activated kinase (Pak1), is highly enriched in neurons. In the present study, we examined the signal transduction pathways involved in pip92 induction, focusing on the involvement of Rho family guanosine 5'-triphosphate (GTP)ases. We also examined the functional role of pip92 expression during FGF-induced neuronal differentiation in embryonic hippocampal cells. Significant and robust activation of c-Jun N-terminal Kinase (JNK), Rac1 and extracellular signal-regulated kinase (ERK) appeared to be important for pip92 induction in response to bFGF. Transient transfection of kinase-inactive MEKK7 or chemical inhibitors of JNK significantly decreased the activation of Rac1 by FGF. However, blockade of Rac1 did not affect JNK activity. Moreover, a MEK-ERK blockade did not affect Rac1 activity. Activation of JNK and Rac1 induced Pak1 activity, which could then phosphorylate and activate transcription factor Elk1. Stimulation of Pak1-dependent Elk1 was required for the bFGF-induced activation of pip92. Suppression of endogenous pip92 expression by siRNA significantly enhanced bFGF-induced neurite outgrowth, while the ectopic expression of pip92 suppressed the neurite extension. Taken together, these data suggest that neurogenic growth factor-induced expression of pip92 is critical for the regulation of neuronal differentiation, occurring through the subsequent activation of Rac1, JNK, Pak1 and Elk1.

The ternary complex factor Net regulates cell migration through inhibition of PAI-1 expression

Net, Elk-1, and Sap-1 are members of the ternary complex factor (TCF) subfamily of Ets transcription factors. They form ternary complexes with serum response factor (SRF) on serum response elements of immediate early genes such as c-fos and egr-1 and mediate responses to growth factors and mitogen-activated protein kinase signaling. Although the TCFs have been extensively studied as intermediates in signaling cascades, surprisingly little is known about their different target genes and physiological functions. We report that Net homozygous mutant mouse embryonic fibroblasts have a defect in cell migration. This defect results at least in part from increased expression of plasminogen activator inhibitor type 1 (PAI-1), a serine protease inhibitor (serpin) that controls extracellular proteolysis and cell matrix adhesion. The defect in cell migration can be reverted by the addition of a PAI-1 blocking antibody. Net represses PAI-1 promoter activity and binds to a specific region of the promoter containing Ets binding sites in the absence of SRF. We conclude that Net is a negative regulator of PAI-1 expression and is thereby involved in cell migration.

Smooth muscle-specific genes are differentially sensitive to inhibition by Elk-1

Understanding the mechanism of smooth muscle cell (SMC) differentiation will provide the foundation for elucidating SMC-related diseases, such as atherosclerosis, restenosis, and asthma. In the current study, overexpression of Elk-1 in SMCs down-regulated expression of several endogenous smooth muscle-restricted proteins, including telokin, SM22alpha, and smooth muscle alpha-actin. In contrast, down-regulation of endogenous Elk-1 in smooth muscle cells increased the expression of only telokin and SM22alpha, suggesting that smooth muscle-specific promoters are differentially sensitive to the inhibitory effects of Elk-1. Consistent with this, overexpression of the DNA binding domain of Elk-1, which acts as a dominant-negative protein by displacing endogenous Elk-1, enhanced the expression of telokin and SM22alpha without affecting expression of smooth muscle alpha-actin. Elk-1 suppressed the activity of smooth muscle-restricted promoters, including the telokin promoter that does not contain a consensus Elk-1 binding site, through its ability to block myocardin-induced activation of the promoters. Gel mobility shift and chromatin immunoprecipitation assays revealed that Elk-1 binds to a nonconsensus binding site in the telokin promoter and Elk-1 binding is dependent on serum response factor (SRF) binding to a nearby CArG box. Although overexpression of the SRF-binding B-box domain of Elk-1 is sufficient to repress the myocardin activation of the telokin promoter, this repression is not as complete as that seen with an Elk-1 fragment that includes the DNA binding domain. In addition, reporter gene assays demonstrate that an intact Elk-1 binding site in the telokin promoter is required for Elk-1 to maximally inhibit promoter activity. Together, these data suggest that the differential sensitivity of smooth muscle-specific genes to inhibition by Elk-1 may play a role in the complex changes in smooth muscle-specific protein expression that are observed under pathological conditions.

Role of the BLT2, a leukotriene B4 receptor, in Ras transformation

Oncogenic Ras is known to drive both the Rac and Raf-MAP-kinase pathways, which act in concert to cause cell transformation. Unlike the Raf-MAP-kinase cascade, however, the downstream elements of Rac pathway are not fully understood. Previously, we showed that cytosolic phospholipase A2 (cPLA2) and subsequent metabolism of arachidonic acid act downstream of Rac to mediate the transformation signaling induced by Ha-Ras(V12). In the present study, we observed that leukotriene B4 (LTB4) and its synthetic enzymes as well as BLT2, the low-affinity LTB4 receptor, are all elevated in Ha-Ras(V12)-transformed cells. In addition, the malignant phenotypes of Ras-transformed cells were markedly inhibited by BLT2 blockade, as was their tumorigenicity in vivo. Finally, in situ hybridization analysis revealed that expression of BLT2 is significantly upregulated in a variety of human cancers. Taken together, our results suggest that an LTB4-BLT2-linked cascade plays a crucial mediatory role in the cell transformation induced by oncogenic Ha-Ras(V12), possibly acting downstream of Rac-cPLA2.

Ternary complex factor-serum response factor complex-regulated gene activity is required for cellular proliferation and inhibition of apoptotic cell death

Members of the ternary complex factor (TCF) subfamily of the ETS-domain transcription factors are activated through phosphorylation by mitogen-activated protein kinases (MAPKs) in response to a variety of mitogenic and stress stimuli. The TCFs bind and activate serum response elements (SREs) in the promoters of target genes in a ternary complex with a second transcription factor, serum response factor (SRF). The association of TCFs with SREs within immediate-early gene promoters is suggestive of a role for the ternary TCF-SRF complex in promoting cell cycle entry and proliferation in response to mitogenic signaling. Here we have investigated the downstream gene regulatory and phenotypic effects of inhibiting the activity of genes regulated by TCFs by expressing a dominantly acting repressive form of the TCF, Elk-1. Inhibition of ternary complex activity leads to the downregulation of several immediate-early genes. Furthermore, blocking TCF-mediated gene expression leads to growth arrest and triggers apoptosis. By using mutant Elk-1 alleles, we demonstrated that these effects are via an SRF-dependent mechanism. The antiapoptotic gene Mcl-1 is identified as a key target for the TCF-SRF complex in this system. Thus, our data confirm a role for TCF-SRF-regulated gene activity in regulating proliferation and provide further evidence to indicate a role in protecting cells from apoptotic cell death.

The ETS domain transcription factor Elk-1 regulates the expression of its partner protein, SRF

The ternary complex factors (TCF) are a subfamily of ETS domain transcription factors that bind and activate serum response elements (SREs) in the promoters of target genes in a ternary complex with a second transcription factor, serum response factor (SRF). Here, we have identified the SRF gene as a target for the TCFs, thereby providing a positive feedback loop whereby TCF activation leads to the enhancement of the expression of its partner protein SRF. The binding of the TCF Elk-1 to the SRF promoter and subsequent regulation of SRF expression occurs in a ternary complex-dependent manner. Our data therefore reveal that SRF is an important target for the ERK and Rho signaling pathways that converge on a ternary TCF-SRF complex at the SRE on the SRF promoter.

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.

Transcriptional activation of immediate-early gene ETR101 by human T-cell leukaemia virus type I Tax

Human T-cell leukaemia virus type I (HTLV-I) Tax regulates viral and cellular gene expression through interactions with multiple cellular transcription pathways. This study describes the finding of immediate-early gene ETR101 expression in HTLV-I-infected cells and its regulation by Tax. ETR101 was persistently expressed in HTLV-I-infected cells but not in HTLV-I uninfected cells. Expression of ETR101 was dependent upon Tax expression in the inducible Tax-expressing cell line JPX-9 and also in Jurkat cells transiently transfected with Tax-expressing vectors. Tax transactivated the ETR101 gene promoter in a transient transfection assay. A series of deletion and mutation analyses of the ETR101 gene promoter indicated that a 35 bp region immediately upstream of the TATA-box sequence, which contains a consensus cAMP response element (CRE) and a G+C-rich sequence, is the critical responsive element for Tax activation. Site-directed mutagenesis analysis of the 35 bp region suggested that both the consensus CRE motif and its upstream G+C-rich sequence were critical for Tax transactivation. Electrophoretic mobility shift analysis (EMSA) using the 35 bp sequence as probe showed the formation of a specific protein-DNA complex in HTLV-I-infected cell lines. EMSA with specific antibodies confirmed that the CREB transcription factor was responsible for formation of this specific protein-DNA complex. These results suggested that Tax directly transactivated ETR101 gene expression, mainly through a CRE sequence via the CREB transcription pathway.

Transcriptional regulators and myelopoiesis: the role of serum response factor and CREB as targets of cytokine signaling

Hematopoiesis is a complex process in which mature myeloid and lymphoid cells are produced from a small population of pluripotent stem cells within the bone marrow. Blood cell formation occurs, in part, by progenitor cell exposure to humoral growth regulators, known as hematopoietic cytokines, as well as by the regulated expression of genes by transcription factors. In this paper, we review two important nuclear proteins, the serum response factor and the cyclic adenosine monophosphate response element-binding protein, as downstream targets of mitogens, with a specific focus on hematopoietic cytokine signaling and the role these proteins play in gene regulation.

Role of the cytosolic phospholipase A2-linked cascade in signaling by an oncogenic, constitutively active Ha-Ras isoform

Activation of Ras signaling by growth factors has been associated with gene regulation and cell proliferation. Here we characterize the contributory role of cytosolic phospholipase A(2) in the oncogenic Ha-Ras(V12) signaling pathway leading to activation of c-fos serum response element (SRE) and transformation in Rat-2 fibroblasts. Using a c-fos SRE-luciferase reporter gene, we showed that the transactivation of SRE by Ha-Ras(V12) is mainly via a Rac-linked cascade, although the Raf-mitogen-activated protein kinase cascade is required for full activation. In addition, Ha-Ras(V12)-induced DNA synthesis was significantly attenuated by microinjection of recombinant Rac(N17), a dominant negative mutant of Rac1. To identify the mediators downstream of Rac in the Ha-Ras(V12) signaling, we investigated the involvement of cytosolic phospholipase A(2). Oncogenic Ha-Ras(V12)-induced SRE activation was significantly inhibited by either pretreatment with mepacrine, a phospholipase A(2) inhibitor, or cotransfection with the antisense oligonucleotide of cytosolic phospholipase A(2). We also found cytosolic phospholipase A(2) to be situated downstream of Ha-Ras(V12) in a signal pathway leading to transformation. Together, these results are indicative of mediatory roles of Rac and cytosolic phospholipase A(2) in the signaling pathway by which Ha-Ras(V12) transactivates c-fos SRE and transformation. Our findings point to cytosolic phospholipase A(2) as a novel potential target for suppressing oncogenic Ha-Ras(V12) signaling in the cell.

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

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

Expression of immediate early gene pip92 during anisomycin-induced cell death is mediated by the JNK- and p38-dependent activation of Elk1

We report here that immediate early gene pip92 is expressed during anisomycin-induced cell death in fibroblast NIH3T3 cells. To determine the mechanism by which this occurs and to identify downstream signaling pathways, we investigated the induction of the pip92 promoter. The activation of pip92 by anisomycin is mediated by the activation of MAP kinases, such as JNK and p38 kinase, but not ERK. Deletion analysis of the pip92 promoter indicated that pip92 activation occurs primarily within the region containing a serum response element (SRE). Further analysis of the SRE using a heterologous thymidine kinase promoter showed that both an Ets and CArG-like site are required for anisomycin-induced pip92 expression. Elk1, which binds to the Ets site, was phosphorylated by the JNK- and p38-dependent pathways and the phosphorylation of Elk1-GAL4 fusion proteins by these pathways was sufficient for the transactivation. Overall, this study suggested that different MAPK pathways are involved in the expression of immediate early gene pip92 by growth factors and environmental stresses.

Granulocyte colony-stimulating factor induces Egr-1 up-regulation through interaction of serum response element-binding proteins

Granulocyte colony-stimulating factor (G-CSF) stimulates the proliferation and maturation of myeloid progenitor cells both in vitro and in vivo. We showed that G-CSF rapidly and transiently induces expression of egr-1 in the NFS60 myeloid cell line. Transient transfections of NFS60 cells with recombinant constructs containing various deletions of the human egr-1 promoter identified the serum response element (SRE) between nucleotides (nt) -418 and -391 as a critical G-CSF-responsive sequence. The SRE (SRE-1) contains a CArG box, the binding site for the serum response factor (SRF), which is flanked at either side by an ETS protein binding site. We demonstrated that a single copy of the wild-type SRE-1 in the minimal promoter plasmid, pTE2, is sufficient to induce transcriptional activation in response to G-CSF and that both the ETS protein binding site and the CArG box are required for maximal transcriptional activation of the pTE2-SRE-1 construct. In electromobility shift assays using NFS60 nuclear extracts, we identified SRF and the ETS protein Fli-1 as proteins that bind the SRE-1. We also demonstrated through electrophoretic mobility shift assays, using an SRE-1 probe containing a CArG mutation, that Fli-1 binds the SRE-1 independently of SRF. Our data suggest that SRE-binding proteins potentially play a role in G-CSF-induced egr-1 expression in myeloid cells.

A systemic administration of NMDA induces immediate early gene pip92 in the hippocampus

In the mammalian CNS, aspartate and glutamate are major excitatory amino acids, and their receptors are believed to mediate a wide range of physiological and pathological processes, including neurotransmission, plasticity, excitotoxicity, and various forms of neurodegeneration. The immediate early gene pip92 has been identified in serum-stimulated BALB/c 3T3 fibroblasts, activated T lymphocytes treated with cycloheximide, and fibroblast growth factor-stimulated hippocampal cells during neuronal differentiation. In this study we have demonstrated that pip92 is expressed in the mouse brain after a single intraperitoneal injection of NMDA. The distribution of pip92 mRNA levels in the NMDA-treated mouse brain was investigated using in situ RT-PCR. The region-specific activation of pip92 in the CNS was observed 3 h after NMDA injection, and high levels of pip92 mRNA were detected in the hippocampal dentate gyrus and piriform cortex regions. In addition, the activation of pip92 by NMDA was mediated by activation of mitogen-activated protein kinases (MAPKs), such as c-Jun N-terminal kinase (JNK) and p38 kinase, but not extracellular signal-regulated kinase (ERK) in the mouse hippocampus and immortalized rat hippocampal progenitor cells. This study suggests that pip92 is likely to play an important role in neuronal cell death induced by excitotoxic NMDA injury in the CNS.

Rapid phosphorylation of Elk-1 transcription factor and activation of MAP kinase signal transduction pathways in response to visual stimulation

The AP-1 transcription factor, which is composed of various combinations of Fos and Jun proteins, is believed to be a key participant in molecular processes that guide activity-dependent changes in gene expression. In this study, we investigated the activity of different MAP kinases that have been implicated in AP-1 activation. We examined the activities of ERK, JNK/SAPK, and p38 MAPK along with their nuclear targets (Elk-1 and c-Jun) in rat visual cortex after light stimulation. The transcription factor Elk-1 (a possible regulator of c-fos expression) was found to be transiently modified by phosphorylation when visual stimulation was applied after a period of dark rearing. In vitro kinase assay with Elk-1 as substrate showed that light stimulation activated MAPK/ERK in visual cortex but not frontal cortex. Furthermore, ERK activation was temporally matched to onset of Elk-1 phosphorylation. The activity of JNK1 (c-Jun N-terminal kinase 1) was elevated at 2-6 h after visual exposure and was also temporally correlated to increase of endogenous P-c-Jun levels and its appearance within the AP-1 DNA-binding complex. The activities of p38 MAP kinases did not change significantly. These results demonstrate the differential engagement of MAPK signaling pathways following sensory stimulation and their relative effects upon AP-1 expression in the intact brain.

Identification of a cell type-specific enhancer in the distal 5'-region of the platelet-derived growth factor A-chain gene

Transient transfection analysis of DNA subfragments from the distal 5'-flanking region of the human platelet-derived growth factor A-chain gene (-18.3 to -1.8 kilobase pairs (kb)) revealed enhancer and silencer elements that contribute significantly to transcriptional regulation. Two adjacent regions (-8.2 to -7.5 kb and -7.5 to -7.0 kb) enhanced transcription of both A-chain and heterologous thymidine kinase promoters, whereas repression was observed in two other nearby regions (-9.9 to -8.2 kb and -7.0 to -5. 9 kb). The -7.5 to -7.0-kb fragment, or J, was the strongest enhancer, and its activity was localized to a 66-base pair element (A-chain cell type-specific enhancer (ACE 66)). ACE66 activity was highly cell type-specific, with greatest activity seen in choriocarcinoma cell lines (4-10-fold enhancement). Progressive 5'- and 3'-deletions of the ACE66 revealed distribution of activity across the element, with nucleotides 1-33 being critical for function. Electrophoretic mobility shift assays revealed cell type-specific patterns of high affinity protein binding to the element. Ethylation interference footprinting of JEG-3 extract localized guanine contacts on nucleotides 1-18 of both strands of the ACE element, whereas more extensive contacts were made with the phosphate backbone (nucleotides 1-32). The ACE66 element is a potent transcriptional regulator in placental cells and represents a valuable model of long distance regulation in a growth factor gene.

Ets transcription factors: nuclear effectors of the Ras-MAP-kinase signaling pathway

The Ets family of transcription factors includes nuclear phosphoproteins that are involved in cell proliferation, differentiation and oncogenic transformation. The family is defined by a conserved DNA-binding domain (the ETS-DBD), which forms a highly conserved, winged, helix-turn-helix structural motif. As targets of the Ras-MAPK signaling pathway, Ets proteins function as critical nuclear integrators of ubiquitous signaling cascades. To direct signals to specific target genes, Ets proteins interact with (other) transcription factors that promote the binding of Ets proteins to composite Ras-responsive elements.

The ETS-domain transcription factor family

During recent years, several significant discoveries have been made concerning the function of ETS-domain transcription factors. This family of transcription factors was originally defined on the basis of the conserved primary sequence of their DNA-binding domains. The ETS DNA-binding domain is also conserved at the structural level and is a divergent member of the winged helix-turn-helix superfamily of DNA binding proteins. This sequence conservation is reflected by their overlapping DNA-binding specificities based on the central GGAA/T motif. In addition to DNA-protein interactions, protein-protein interactions with partner proteins often play major roles in targeting ETS-domain proteins to specific promoters. Several such partner proteins have been identified. ETS-domain proteins function as either transcriptional activators or repressors and their activities are often regulated by signal transduction pathways, including the MAP kinase pathways. Specific links between such pathways and ETS-domain proteins have been established in several different experimental systems. ETS-domain transcription factors regulate a diverse array of biological functions including mammalian haematopoiesis and Drosophila eye development. In vertebrates, many ETS-domain proteins regulate embryonic and adult haematopoiesis. Deregulation of ETS-domain protein activity often leads to tumorigenesis. Future work will uncover further details of how these transcription factors work at the molecular level to regulate specific biological processes.

Determination of floral organ identity by Arabidopsis MADS domain homeotic proteins AP1, AP3, PI, and AG is independent of their DNA-binding specificity

The MADS domain homeotic proteins APETALA1 (AP1), APETALA3 (AP3), PISTILLATA (PI), and AGAMOUS (AG) combinatorially specify the identity of Arabidopsis floral organs. AP1/AP1, AG/AG, and AP3/PI dimers bind to similar CArG box sequences; thus, differences in DNA-binding specificity among these proteins do not seem to be the origin of their distinct organ identity properties. To assess the overall contribution that specific DNA binding could make to their biological specificity, we have generated chimeric genes in which the amino-terminal half of the MADS domain of AP1, AP3, PI, and AG was substituted by the corresponding sequences of human SRF and MEF2A proteins. In vitro DNA-binding assays reveal that the chimeric proteins acquired the respective, and distinct, DNA-binding specificity of SRF or MEF2A. However, ectopic expression of the chimeric genes reproduces the dominant gain-of-function phenotypes exhibited by plants ectopically expressing the corresponding Arabidopsis wild-type genes. In addition, both the SRF and MEF2 chimeric genes can complement the pertinent ap1-1, ap3-3, pi-1, or ag-3 mutations to a degree similar to that of AP1, AP3, PI, and AG when expressed under the control of the same promoter. These results indicate that determination of floral organ identity by the MADS domain homeotic proteins AP1, AP3, PI, and AG is independent of their DNA-binding specificity. In addition, the DNA-binding experiments show that either one of the two MADS domains of a dimer can be sufficient to confer a particular DNA-binding specificity to the complex and that sequences outside the amino-terminal basic region of the MADS domain can, in some cases, contribute to the DNA-binding specificity of the proteins.