Regulation of gene transcription by a constitutively active mutant of activating transcription factor 2 (ATF2)

ATF4 Regulates MYB to Increase γ-Globin in Response to Loss of β-Globin

β-Hemoglobinopathies can trigger rapid production of red blood cells in a process known as stress erythropoiesis. Cellular stress prompts differentiating erythroid precursors to express high levels of fetal γ-globin. However, the mechanisms underlying γ-globin production during cellular stress are still poorly defined. Here, we use CRISPR-Cas genome editing to model the stress caused by reduced levels of adult β-globin. We find that decreased β-globin is sufficient to induce robust re-expression of γ-globin, and RNA sequencing (RNA-seq) of differentiating isogenic erythroid precursors implicates ATF4 as a causal regulator of this response. ATF4 binds within the HBS1L-MYB intergenic enhancer and regulates expression of MYB, a known γ-globin regulator. Overall, the reduction of ATF4 upon β-globin knockout decreases the levels of MYB and BCL11A. Identification of ATF4 as a key regulator of globin compensation adds mechanistic insight to the poorly understood phenomenon of stress-induced globin compensation and could inform strategies to treat hemoglobinopathies.

p38 (Mapk14/11) occupies a regulatory node governing entry into primitive endoderm differentiation during preimplantation mouse embryo development

During mouse preimplantation embryo development, the classically described second cell-fate decision involves the specification and segregation, in blastocyst inner cell mass (ICM), of primitive endoderm (PrE) from pluripotent epiblast (EPI). The active role of fibroblast growth factor (Fgf) signalling during PrE differentiation, particularly in the context of Erk1/2 pathway activation, is well described. However, we report that p38 family mitogen-activated protein kinases (namely p38α/Mapk14 and p38β/Mapk11; referred to as p38-Mapk14/11) also participate in PrE formation. Specifically, functional p38-Mapk14/11 are required, during early-blastocyst maturation, to assist uncommitted ICM cells, expressing both EPI and earlier PrE markers, to fully commit to PrE differentiation. Moreover, functional activation of p38-Mapk14/11 is, as reported for Erk1/2, under the control of Fgf-receptor signalling, plus active Tak1 kinase (involved in non-canonical bone morphogenetic protein (Bmp)-receptor-mediated PrE differentiation). However, we demonstrate that the critical window of p38-Mapk14/11 activation precedes the E3.75 timepoint (defined by the initiation of the classical ‘salt and pepper’ expression pattern of mutually exclusive EPI and PrE markers), whereas appropriate lineage maturation is still achievable when Erk1/2 activity (via Mek1/2 inhibition) is limited to a period after E3.75. We propose that active p38-Mapk14/11 act as enablers, and Erk1/2 as drivers, of PrE differentiation during ICM lineage specification and segregation.

Resveratrol stimulates cyclic AMP response element mediated gene transcription

SCOPE

Many intracellular effects have been attributed to resveratrol, a polyphenolic phytoalexin found in grapes and in other plants, including the direct regulation of transcription. Here, we have analyzed the impact of resveratrol on gene transcription regulated by the cyclic AMP response element (CRE).

METHODS AND RESULTS

Transcription of a chromatin-embedded reporter gene with CREs in its regulatory region was significantly elevated in resveratrol-treated 293 human embryonic kidney cells, hepatoma cells and neural stem cells. The CRE thus functions as resveratrol-responsive element. The polyphenols quercetin and naringenin also stimulated CRE-mediated gene transcription, but not in the range of resveratrol. The polyphenol curcumin, in contrast, had no effect upon CRE-regulated transcription. In addition, resveratrol stimulation upregulated the transcriptional activation potentials of the CRE-binding proteins (CREB) and activating transcription factor 2 (ATF2).

CONCLUSION

CREB exhibits cytoprotective activity by stimulating CRE-regulated genes, while ATF2 has been identified as a tumor suppressor. The fact that resveratrol upregulates CRE-mediated gene transcription and enhances the transcriptional activation potentials of CREB and ATF2 suggests that cytoprotective and tumor suppressive activities of resveratrol may rely-at least in part-on the stimulation of CREB- and ATF2-controlled target genes.

JNK suppresses tumor formation via a gene-expression program mediated by ATF2

JNK and p38 phosphorylate a diverse set of substrates and, consequently, can act in a context-dependent manner to either promote or inhibit tumor growth. Elucidating the functions of specific substrates of JNK and p38 is therefore critical for our understanding of these kinases in cancer. ATF2 is a phosphorylation-dependent transcription factor and substrate of both JNK and p38. Here, we show ATF2 suppresses tumor formation in an orthotopic model of liver cancer and cellular transformation in vitro. Furthermore, we find that suppression of tumorigenesis by JNK requires ATF2. We identify a transcriptional program activated by JNK via ATF2 and provide examples of JNK- and ATF2-dependent genes that block cellular transformation. Significantly, we also show that ATF2-dependent gene expression is frequently downregulated in human cancers, indicating that amelioration of JNK-ATF2-mediated suppression may be a common event during tumor development.

Resveratrol stimulates AP-1-regulated gene transcription

SCOPE

Many intracellular functions have been attributed to resveratrol, a polyphenolic phytoalexin found in grapes and in other plants, including the regulation of transcription. Here, we have analyzed the impact of resveratrol on the activity of the transcription factor activator protein-1 (AP-1).

METHODS AND RESULTS

Using a chromosomally embedded AP-1-responsive reporter gene, we show that the AP-1 activity was significantly elevated in resveratrol-treated 293 human embryonic kidney and HepG2 hepatoma cells. The 12-O-tetradecanoylphorbol-13-acetate-responsive element, a binding site for c-Jun and c-Fos, was identified as resveratrol-responsive element. Expression of c-Jun and c-Fos, two proteins that constitute AP-1, is upregulated in resveratrol-stimulated HEK293 cells. On the transcriptional level, c-Jun and the ternary complex factor Elk-1 are essential for the activation of AP-1 in resveratrol-treated cells. In addition, mitogen-activated protein kinases and protein kinase C are required to connect resveratrol stimulation with enhanced AP-1 controlled transcription. Finally, we show that resveratrol increased the activities of the AP-1 responsive cyclin D1 and tumor necrosis factor α promoters.

CONCLUSION

Resveratrol regulates gene transcription via activation of stimulus-regulated protein kinases and the stimulus-responsive AP-1 transcription factors. The fact that resveratrol regulates AP-1 activity may explain many of the pleiotropic intracellular alterations induced by resveratrol.

Nuclear receptor LRH-1/NR5A2 is required and targetable for liver endoplasmic reticulum stress resolution

Chronic endoplasmic reticulum (ER) stress results in toxicity that contributes to multiple human disorders. We report a stress resolution pathway initiated by the nuclear receptor LRH-1 that is independent of known unfolded protein response (UPR) pathways. Like mice lacking primary UPR components, hepatic Lrh-1-null mice cannot resolve ER stress, despite a functional UPR. In response to ER stress, LRH-1 induces expression of the kinase Plk3, which phosphorylates and activates the transcription factor ATF2. Plk3-null mice also cannot resolve ER stress, and restoring Plk3 expression in Lrh-1-null cells rescues ER stress resolution. Reduced or heightened ATF2 activity also sensitizes or desensitizes cells to ER stress, respectively. LRH-1 agonist treatment increases ER stress resistance and decreases cell death. We conclude that LRH-1 initiates a novel pathway of ER stress resolution that is independent of the UPR, yet equivalently required. Targeting LRH-1 may be beneficial in human disorders associated with chronic ER stress.

DOI:http://dx.doi.org/10.7554/eLife.01694.001

A protein can only work properly if it has been folded into the correct shape. However, it is estimated that about one third of new proteins have the wrong shape. This is a major challenge for cells because misfolded proteins are often toxic, and cause many neurodegenerative and metabolic disorders.

In eukaryotic cells, most protein folding takes place inside a part of the cell called the endoplasmic reticulum (ER). If an incorrectly folded protein is detected, it is prevented from leaving the ER until it is refolded correctly, or destroyed. If too many proteins are misfolded, a process called the unfolded protein response helps the cell to cope with this ‘ER stress’ by expanding the ER and producing more of the molecules that assist protein folding. If this does not relieve the ER stress, the cell self-destructs. Neighboring cells then have to increase protein production to compensate for what would have been produced by the dead cell, thereby increasing the chance that they will also experience ER stress.

Activation of a protein called LRH-1 (short for liver receptor homolog-1) that is produced in the liver, pancreas and intestine can relieve the symptoms of the various metabolic diseases that are associated with chronic ER stress, including type II diabetes and fatty liver disease. However, researchers have been puzzled by the fact that although LRH-1 performs many different roles, its molecular structure provides few clues as to how it can do this.

Mamrosh et al. now confirm the speculated link between LRH-1 and ER stress relief in mice. LRH-1 triggers a previously unknown pathway that can relieve ER stress and is completely independent of the unfolded protein response. Targeting LRH-1 with certain chemical compounds alters its activity, suggesting that drug treatments could be developed to relieve ER stress. As similar targets for drugs have not been found in the unfolded protein response, the discovery of the LRH-1 pathway could lead to new approaches to the treatment of the diseases that result from ER stress.

DOI:http://dx.doi.org/10.7554/eLife.01694.002

Divergent signaling pathways cooperatively regulate TGFβ induction of cysteine-rich protein 2 in vascular smooth muscle cells

Background

Vascular smooth muscle cells (VSMCs) of the arterial wall play a critical role in the development of occlusive vascular diseases. Cysteine-rich protein 2 (CRP2) is a VSMC-expressed LIM-only protein, which functionally limits VSMC migration and protects against pathological vascular remodeling. The multifunctional cytokine TGFβ has been implicated to play a role in the pathogenesis of atherosclerosis through numerous downstream signaling pathways. We showed previously that TGFβ upregulates CRP2 expression; however, the detailed signaling mechanisms remain unclear.

Results

TGFβ treatment of VSMCs activated both Smad2/3 and ATF2 phosphorylation. Individually knocking down Smad2/3 or ATF2 pathways with siRNA impaired the TGFβ induction of CRP2, indicating that both contribute to CRP2 expression. Inhibiting TβRI kinase activity by SB431542 or TβRI knockdown abolished Smad2/3 phosphorylation but did not alter ATF2 phosphorylation, indicating while Smad2/3 phosphorylation was TβRI-dependent ATF2 phosphorylation was independent of TβRI. Inhibiting Src kinase activity by SU6656 suppressed TGFβ-induced RhoA and ATF2 activation but not Smad2 phosphorylation. Blocking ROCK activity, the major downstream target of RhoA, abolished ATF2 phosphorylation and CRP2 induction but not Smad2 phosphorylation. Furthermore, JNK inhibition with SP600125 reduced TGFβ-induced ATF2 (but not Smad2) phosphorylation and CRP2 protein expression while ROCK inhibition blocked JNK activation. These results indicate that downstream of TβRII, Src family kinase-RhoA-ROCK-JNK signaling pathway mediates TβRI-independent ATF2 activation. Promoter analysis revealed that the TGFβ induction of CRP2 was mediated through the CRE and SBE promoter elements that were located in close proximity.

Conclusions

Our results demonstrate that two signaling pathways downstream of TGFβ converge on the CRE and SBE sites of the Csrp2 promoter to cooperatively control CRP2 induction in VSMCs, which represents a previously unrecognized mechanism of VSMC gene induction by TGFβ.

Transcriptional response to calcium-sensing receptor stimulation

Elevated extracellular Ca(2+) concentrations stimulate the G-protein coupled receptor calcium-sensing receptor. Here we show that this stimulation induces the expression of biologically active early growth response protein 1 (Egr-1), a zinc finger transcription factor. Expression of a dominant-negative mutant of the ternary complex factor Ets-like protein-1 (Elk-1), a key transcriptional regulator of serum response element-driven gene transcription, prevented Egr-1 expression, indicating that Elk-1 or related ternary complex factors connect the intracellular signaling cascade elicited by activation of calcium-sensing receptors with transcription of the Egr-1 gene. These data were corroborated by the fact that stimulation of calcium-sensing receptors increased the transcriptional activation potential of Elk-1. In addition, activator protein-1 (AP-1) transcriptional activity was significantly elevated after the stimulation of calcium-sensing receptors. The expression of a dominant-negative mutant of Elk-1 reduced c-Fos expression and prevented the up-regulation of AP-1 activity as a result of calcium-sensing receptor stimulation, indicating that ternary complex factors control both Egr-1- and AP-1-regulated transcription. In addition, AP-1 activity was reduced after the expression of a dominant-negative mutant of c-Jun in cells expressing an activated calcium-sensing receptor. Stimulus-transcription coupling leading to the up-regulation of Egr-1 and AP-1 controlled transcription in cells expressing calcium-sensing receptors required the protein kinases Raf and ERK, whereas the overexpression of MAPK phosphatase-1 interrupted the signaling cascade connecting calcium-sensing receptor stimulation with transcription of Egr-1 and AP-1 controlled genes. The fact that calcium-sensing receptor stimulation activates the transcription factors Egr-1, Elk-1, and AP-1 indicates that regulation of gene transcription is an integral part of calcium-sensing receptor induced signaling.

Immediate-early transcriptional response to angiotensin II in human adrenocortical cells

Angiotensin II binds to the angiotensin II receptors type 1 (AT1 receptors) in adrenocortical cells and triggers an intracellular signaling cascade leading to changes in the gene expression pattern. Here, we show that stimulation with angiotensin II induces the expression of biologically active early growth response (Egr)-1, a zinc finger transcription factor, in human H295R adrenocortical cells. Expression of a dominant-negative mutant of the ternary complex factor Elk-1, a key transcriptional regulator of serum response element-driven gene transcription, prevented Egr-1 expression in angiotensin II-stimulated H295R cells, indicating that Ets-like protein-1 (Elk-1) or related ternary complex factors connect the intracellular signaling cascade elicited by activation of AT1 receptors with transcription of the Egr-1 gene. These data were corroborated by the fact that angiotensin II stimulation increased the transcription activation potential of Elk-1. In addition, activator protein-1 transcriptional activity was significantly elevated in angiotensin II-treated H295R cells. Expression of c-Jun and c-Fos was increased as well as the transcription activation potential of c-Fos. Expression of a dominant-negative mutant of Elk-1 reduced c-Fos expression in angiotensin II-stimulated adrenocortical cells, suggesting that the serum response element within the c-Fos promoter functions as an angiotensin II-response element. Expression of a dominant-negative mutant of c-Jun reduced activator protein-1 activity in angiotensin II-stimulated adrenocortical cells and reduced the up-regulation of c-Jun after angiotensin II stimulation. Thus, c-Jun regulates its own expression in adrenocortical cells. Together, the data show that angiotensin II stimulation activates the transcription factors Egr-1, Elk-1, c-Jun, and c-Fos in adrenocortical cells, leading to stimulus-dependent changes in the gene expression pattern.

Parkin is transcriptionally regulated by ATF4: evidence for an interconnection between mitochondrial stress and ER stress

Loss of parkin function is responsible for the majority of autosomal recessive parkinsonism. Here, we show that parkin is not only a stress-protective, but also a stress-inducible protein. Both mitochondrial and endoplasmic reticulum (ER) stress induce an increase in parkin-specific mRNA and protein levels. The stress-induced upregulation of parkin is mediated by ATF4, a transcription factor of the unfolded protein response (UPR) that binds to a specific CREB/ATF site within the parkin promoter. Interestingly, c-Jun can bind to the same site, but acts as a transcriptional repressor of parkin gene expression. We also present evidence that mitochondrial damage can induce ER stress, leading to the activation of the UPR, and thereby to an upregulation of parkin expression. Vice versa, ER stress results in mitochondrial damage, which can be prevented by parkin. Notably, the activity of parkin to protect cells from stress-induced cell death is independent of the proteasome, indicating that proteasomal degradation of parkin substrates cannot explain the cytoprotective activity of parkin. Our study supports the notion that parkin has a role in the interorganellar crosstalk between the ER and mitochondria to promote cell survival under stress, suggesting that both ER and mitochondrial stress can contribute to the pathogenesis of Parkinson's disease.

Epidermal-growth-factor-induced proliferation of astrocytes requires Egr transcription factors

Stimulation of astrocytes with epidermal growth factor (EGF) induced proliferation and triggered the biosynthesis of the transcription factor Egr-1, involving the activation of the extracellular signal-regulated protein kinase (ERK) signaling pathway. No differences in the proliferation rate of astrocytes prepared from wild-type or Egr-1-deficient mice were detected. However, expression of a dominant-negative mutant of Egr-1 that interfered with DNA-binding of all Egr proteins prevented EGF-induced proliferation of astrocytes. Site-directed mutagenesis of two crucial cysteine residues within the zinc finger DNA-binding domain revealed that DNA-binding of the Egr-1 mutant was essential to inhibit proliferation of EGF-stimulated astrocytes. Expression of NAB2 (a negative co-regulator of Egr-1, Egr-2 and Egr-3) or a dominant-negative mutant of Elk-1 (a key regulator of Egr-1 biosynthesis) abolished EGF-induced proliferation of astrocytes. Chromatin immunoprecipitation experiments showed that Egr-1, Egr-2 and Egr-3 bound to the gene expressing basic fibroblast growth factor (bFGF) in EGF-stimulated astrocytes. Egr-2 and Egr-3 also interacted with the bFGF gene in EGF-stimulated astrocytes prepared from Egr-1-deficient mice, indicating that loss of Egr-1 is compensated by other Egr proteins. Together, these data show that Egr transcription factors are essential for conversion of the mitogenic signal of EGF into a proliferative response.

Opposing roles for ATF2 and c-Fos in c-Jun-mediated neuronal apoptosis

The activator protein 1 (AP-1) transcription factor c-Jun is crucial for neuronal apoptosis. However, c-Jun dimerization partners and the regulation of these proteins in neuronal apoptosis remain unknown. Here we report that c-Jun-mediated neuronal apoptosis requires the concomitant activation of activating transcription factor-2 (ATF2) and downregulation of c-Fos. Furthermore, we have observed that c-Jun predominantly heterodimerizes with ATF2 and that the c-Jun/ATF2 complex promotes apoptosis by triggering ATF activity. Inhibition of c-Jun/ATF2 heterodimerization using dominant negative mutants, small hairpin RNAs, or decoy oligonucleotides was able to rescue neurons from apoptosis, whereas constitutively active ATF2 and c-Jun mutants were found to synergistically stimulate apoptosis. Bimolecular fluorescence complementation analysis confirmed that, in living neurons, c-Fos downregulation facilitates c-Jun/ATF2 heterodimerization. A chromatin immunoprecipitation assay also revealed that c-Fos expression prevents the binding of c-Jun/ATF2 heterodimers to conserved ATF sites. Moreover, the presence of c-Fos is able to suppress the expression of c-Jun/ATF2-mediated target genes and, therefore, apoptosis. Taken together, our findings provide evidence that potassium deprivation-induced neuronal apoptosis is mediated by concurrent upregulation of c-Jun/ATF2 heterodimerization and downregulation of c-Fos expression. This paradigm demonstrates opposing roles for ATF2 and c-Fos in c-Jun-mediated neuronal apoptosis.

Expression of the transcriptional repressor ATF3 in gonadotrophs is regulated by Egr-1, CREB, and ATF2 after gonadotropin-releasing hormone receptor stimulation

Stimulation of GnRH receptors enhances expression of activating transcription factor (ATF) 3 in a pituitary gonadotroph cell line. The signaling pathway requires elevated cytosolic Ca2+ levels and activation of ERK and c-Jun N-terminal protein kinase. The signaling cascade was blocked by overexpression of either MAPK phosphatase (MKP)-1 or MAPK phosphatase-5 that dephosphorylate nuclear ERK and c-Jun N-terminal protein kinase. In addition, ATF3 biosynthesis was impaired after lentiviral-mediated expression of a constitutively active mutant of calcineurin A. Thus, MKP-1, MKP-5, and calcineurin may function as shut-off devices for GnRH receptor signaling. Expression of dominant-negative mutants of early growth response protein (Egr)-1, cAMP response element binding protein (CREB), and ATF2 blocked the biosynthesis of ATF3, indicating that these transcription factors connect the intracellular signaling cascade elicited by activation of GnRH receptors with transcription of the ATF3 gene. This view was corroborated by chromatin immunoprecipitation experiments revealing that Egr-1 and the phosphorylated forms of CREB and ATF2 bound to the 5'-upstream region of the ATF3 gene in buserelin-stimulated gonadotrophs. Together the data indicate that the ATF3 gene is a bona fide target gene of Egr-1, CREB, and ATF2 in gonadotrophs. Moreover, we show that in gonadotrophs ATF3 bound to its own promoter under physiological conditions. The analysis of a lentiviral-transmitted ATF3 promoter/luciferase reporter gene, embedded into the chromatin of the cells, revealed that ATF3 blocked the activity of its own promoter. We additionally identified the chromogranin B gene as bona fide target gene of ATF3 in gonadotrophs.

Characterization of the bovine angiotensin converting enzyme promoter: essential roles of Egr-1, ATF-2 and Ets-1 in the regulation by phorbol ester

The protease angiotensin converting enzyme (ACE) is a key regulator of blood pressure homeostasis, and is responsible for proteolytic activation of angiotensin I to angiotensin II (Ang II), a potent vasoconstrictor, and proteolytic inactivation of bradykinin, a vasodilator. Recent studies have also implicated ACE and Ang II dysregulation in the progression of fibrotic tissue diseases. Although many studies have utilized bovine tissues and cells for investigating the regulation of ACE gene expression, the bovine ACE promoter has not been previously characterized. Here we present the analysis of the bovine ACE promoter. We investigated cis elements regulated by phorbol 12-myristate 13-acetate (PMA). Sequence analysis shows that the bovine ACE promoter contains several putative binding sites for the transcription factors ATF-2, Ets-1, Egr-1 and SP1/SP3. Chromatin immunoprecipitation (ChIP) indicated that the activation of the bovine ACE promoter by PMA involves histone H4 acetylation, and that PMA induced Egr-1 and ATF-2 binding to the ACE promoter, whereas Ets-1 binding was suppressed by PMA. The regulatory roles of these transcription factors in the bovine ACE gene regulation were confirmed by co-expression of either wild type or dominant negative transcription factors with the luciferase reporter constructs. The bovine and human ACE promoters share similarities in binding sites for transcription factors and PMA regulation within the core regions but contain significant differences in the distal promoter regions.

The role of ATF-2 in oncogenesis

Activating Transcription Factor-2 is a sequence-specific DNA-binding protein that belongs to the bZIP family of proteins and plays diverse roles in the mammalian cells. In response to stress stimuli, it activates a variety of gene targets including cyclin A, cyclin D and c-jun, which are involved in oncogenesis in various tissue types. ATF-2 expression has been correlated with maintenance of a cancer cell phenotype. However, other studies demonstrate an antiproliferative or apoptotic role for ATF-2. In this review, we summarize the signaling pathways that activate ATF-2, as well as its downstream targets. We examine the role of ATF-2 in carcinogenesis with respect to other bZIP proteins, using data from studies in human cancer cell lines, human tumours and mouse models, and we propose a potential model for its function in carcinogenesis, as well as a theoretical basis for its utility in anticancer drug design.

Functional interaction of regulatory factors with the Pgc-1alpha promoter in response to exercise by in vivo imaging

Real-time optical bioluminescence imaging is a powerful tool for studies of gene regulation in living animals. To elucidate exercise-induced signaling/transcriptional control of the peroxisome proliferator-activated receptor-gamma coactivator-1alpha (Pgc-1alpha) gene in skeletal muscle, we combined this technology with electric pulse-mediated gene transfer to cotransfect the Pgc-1alpha reporter gene with plasmid DNA encoding mutant/deletion forms of putative regulatory factors and, thereby, assess the responsiveness of the promoter to skeletal muscle contraction. We show that each of the myocyte enhancer factor 2 sites on the Pgc-1alpha promoter is required for contractile activity-induced Pgc-1alpha transcription. The responsiveness of the Pgc-1alpha promoter to contractile activity could be completely blocked by overexpression of the dominant-negative form of activating transcription factor 2 (ATF2), the signaling-resistant form of histone deacetylase (HDAC) 5 (HDAC5), or protein kinase D (PKD), but not by HDAC4. These findings provide in vivo evidence for functional interactions between PKD/HDAC5 and ATF2 regulatory factors and the Pgc-1alpha gene in adult skeletal muscle.

Role of the repressor JDP2 in the amino acid-regulated transcription of CHOP

The transcriptional activation of CHOP (C/EBP-homologous protein) by amino acid deprivation involves ATF2 and ATF4 binding at the amino acid response element within the promoter. In this report, we investigate the role of JDP2 (Jun Dimerization Protein 2) in the amino acid control of CHOP transcription following amino acid starvation. Our results show that JDP2 binds to the CHOP AARE in unstimulated cells and that its binding decreases following amino acid starvation. We demonstrate that JDP2 acts as a repressor and suggest that it could be functionally associated with HDAC3 to inhibit CHOP transcription.

ATF2 on the double - activating transcription factor and DNA damage response protein

Signal transduction pathways play a key role in the regulation of key cellular processes, including survival and death. Growing evidence points to changes in signaling pathway that occur during skin tumor development and progression. Such changes impact the activity of downstream substrates, including transcription factors. The activating transcription factor 2 (ATF2) has been implicated in malignant and non-malignant skin tumor developments. ATF2 mediates both transcription and DNA damage control, through its phosphorylation by JNK/p38 or ATM/ATR respectively. Here, we summarize our present understanding of ATF2 regulation, function and contribution to malignant and non-malignant skin tumor development.

APP1 transcription is regulated by inositol-phosphorylceramide synthase 1-diacylglycerol pathway and is controlled by ATF2 transcription factor in Cryptococcus neoformans

Inositol-phosphorylceramide synthase 1 (Ipc1) is a fungal-specific enzyme that regulates the level of two bioactive molecules, phytoceramide and diacylglycerol (DAG). In previous studies, we demonstrated that Ipc1 regulates the expression of the antiphagocytic protein 1 (App1), a novel fungal factor involved in pathogenicity of Cryptococcus neoformans. Here, we investigated the molecular mechanism by which Ipc1 regulates App1. To this end, the APP1 promoter was fused to the firefly luciferase gene in the C. neofor-mans GAL7:IPC1 strain, in which the Ipc1 expression can be modulated, and found that the luciferase activity was indeed regulated when Ipc1 was modulated. Next, using the luciferase reporter assay in both C. neoformans wild-type and GAL7:IPC1 strains, we investigated the role of DAG and sphingolipids in the activation of the APP1 promoter and found that treatment with 1,2-dioctanoylglycerol does increase APP1 transcription, whereas treatment with phytosphingosine or ceramides does not. Two putative consensus sequences were found in the APP1 promoter for ATF and AP-2 transcription factors. Mutagenesis analysis of these sequences revealed that they play a key role in the regulation of APP1 transcription: ATF is an activator, whereas AP-2 in a negative regulator. Finally, we identified a putative Atf2 transcription factor, which is required for APP1 transcription and under the control of Ipc1-DAG pathway. These studies provide novel regulatory mechanisms of the sphingolipid pathway involved in the regulation of gene transcription of C. neoformans.

Specificity of transcriptional regulation by the zinc finger transcription factors Sp1, Sp3, and Egr-1

The transcription factors Sp1, Sp3, and Egr-1 bind with their zinc finger DNA-binding domains to GC-rich sequences in the regulatory regions of their target genes. The similarity of the DNA-binding sites of Sp1, Sp3, and Egr-1 has triggered the hypothesis that they compete for the same DNA-binding site. We have investigated the specificity of transcriptional regulation by Sp1, Sp3, and Egr-1 using dominant-negative mutants that block the DNA-binding site of Sp1, Sp3, or Egr-1, respectively. The results show that constitutive transcription of Sp1 regulated reporter genes, containing Sp1 sites derived from the aldolase C and p21WAF1/Cip1 genes, or the long terminal repeat of HIV-1, was impaired by dominant-negative mutants of Sp1 and Sp3, but not by a dominant-negative Egr-1. Transcription mediated by Egr-1 was induced by transfection of expression vectors encoding wild-type or mutated Egr-1 or by stimulation of the extracellular signal-regulated protein kinase pathway via an inducible B-Raf-estrogen receptor fusion protein. In all cases transcription of Egr-1-regulated reporter genes, containing Egr-1 binding sites derived from the Egr-1 or the synapsin I gene was impaired by a dominant-negative Egr-1, but not by dominant-negative Sp1 or Sp3 mutants. These results show that there are genuine Sp1/Sp3 or Egr-1 controlled genes showing no cross-regulation of Sp1/Sp3 and Egr-1 through the same DNA-binding site. This does not exclude the existence of composite Sp1/Sp3/Egr-1 binding sites, where competition for a common DNA-binding site occurs.

Transcriptional activation of the Egr-1 gene mediated by tetradecanoylphorbol acetate and extracellular signal-regulated protein kinase

Activation of extracellular signal-regulated protein kinase (ERK) triggers the biosynthesis of Egr-1, a zinc finger transcription factor. Likewise, the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) strongly upregulates Egr-1 biosynthesis. Here, we have analyzed the genetic elements involved in the regulation of Egr-1 gene transcription by ERK and TPA in human hepatoma cells. Expression experiments using mitogen-activated protein kinase phosphatase-1 or a dominant-negative mutant of the ternary complex factor Elk-1 revealed that the distal cluster of serum response elements is essential in the TPA-induced enhancement of Egr-1 promoter activity, encompassing two independent TPA-responsive elements. The CRE in the proximal Egr-1 promoter plays, if anything, only a marginal role in TPA-induced stimulus-transcription coupling of the Egr-1 gene. The fact that Egr-1 promoter/reporter gene transcription is upregulated by a constitutively active CREB mutant indicates that the CRE couples other signaling cascades via CREB to the Egr-1 gene.

Exercise stimulates Pgc-1alpha transcription in skeletal muscle through activation of the p38 MAPK pathway

Peroxisome proliferator-activated receptor gamma co-activator 1alpha (PGC-1alpha) promotes mitochondrial biogenesis and slow fiber formation in skeletal muscle. We hypothesized that activation of the p38 mitogen-activated protein kinase (MAPK) pathway in response to increased muscle activity stimulated Pgc-1alpha gene transcription as part of the mechanisms for skeletal muscle adaptation. Here we report that a single bout of voluntary running induced a transient increase of Pgc-1alpha mRNA expression in mouse plantaris muscle, concurrent with an activation of the p38 MAPK pathway. Activation of the p38 MAPK pathway in cultured C2C12 myocytes stimulated Pgc-1alpha promoter activity, which could be blocked by the specific inhibitors of p38, SB203580 and SB202190, or a dominant negative p38. Furthermore, the p38-mediated increase in Pgc-1alpha promoter activity was enhanced by increased expression of the downstream transcription factor ATF2 and completely blocked by ATF2DeltaN, a dominant negative ATF2. Skeletal muscle-specific expression of a constitutively active activator of p38, MKK6E, in transgenic mice resulted in enhanced Pgc-1alpha and cytochrome oxidase IV protein expression in fast-twitch skeletal muscles. These findings suggest that contractile activity-induced activation of the p38 MAPK pathway promotes Pgc-1alpha gene expression and skeletal muscle adaptation.

cAMP response element binding protein (CREB) activates transcription via two distinct genetic elements of the human glucose-6-phosphatase gene

Background

The enzyme glucose-6-phosphatase catalyzes the dephosphorylation of glucose-6-phosphatase to glucose, the final step in the gluconeogenic and glycogenolytic pathways. Expression of the glucose-6-phosphatase gene is induced by glucocorticoids and elevated levels of intracellular cAMP. The effect of cAMP in regulating glucose-6-phosphatase gene transcription was corroborated by the identification of two genetic motifs CRE1 and CRE2 in the human and murine glucose-6-phosphatase gene promoter that resemble cAMP response elements (CRE).

Results

The cAMP response element is a point of convergence for many extracellular and intracellular signals, including cAMP, calcium, and neurotrophins. The major CRE binding protein CREB, a member of the basic region leucine zipper (bZIP) family of transcription factors, requires phosphorylation to become a biologically active transcriptional activator. Since unphosphorylated CREB is transcriptionally silent simple overexpression studies cannot be performed to test the biological role of CRE-like sequences of the glucose-6-phosphatase gene. The use of a constitutively active CREB2/CREB fusion protein allowed us to uncouple the investigation of target genes of CREB from the variety of signaling pathways that lead to an activation of CREB. Here, we show that this constitutively active CREB2/CREB fusion protein strikingly enhanced reporter gene transcription mediated by either CRE1 or CRE2 derived from the glucose-6-phosphatase gene. Likewise, reporter gene transcription was enhanced following expression of the catalytic subunit of cAMP-dependent protein kinase (PKA) in the nucleus of transfected cells. In contrast, activating transcription factor 2 (ATF2), known to compete with CREB for binding to the canonical CRE sequence 5'-TGACGTCA-3', did not transactivate reporter genes containing CRE1, CRE2, or both CREs derived from the glucose-6-phosphatase gene.

Conclusions

Using a constitutively active CREB2/CREB fusion protein and a mutant of the PKA catalytic subunit that is targeted to the nucleus, we have shown that the glucose-6-phosphatase gene has two distinct genetic elements that function as bona fide CRE. This study further shows that the expression vectors encoding C2/CREB and catalytic subunit of PKA are valuable tools for the study of CREB-mediated gene transcription and the biological functions of CREB.

Identification of promoters bound by c-Jun/ATF2 during rapid large-scale gene activation following genotoxic stress

The NH2-terminal Jun kinases (JNKs) function in diverse roles through phosphorylation and activation of AP-1 components including ATF2 and c-Jun. However, the genes that mediate these processes are poorly understood. A model phenotype characterized by rapid activation of Jun kinase and enhanced DNA repair following cisplatin treatment was examined using chromatin immunoprecipitation with antibodies against ATF2 and c-Jun or their phosphorylated forms and hybridization to promoter arrays. Following genotoxic stress, we identified 269 genes whose promoters are bound upon phosphorylation of ATF2 and c-Jun. Binding did not occur following treatment with transplatin or the JNK inhibitor SP600125 or JNK-specific siRNA. Of 89 known DNA repair genes represented on the array, 23 are specifically activated by cisplatin treatment within 3-6 hr. Thus, the genotoxic stress response occurs at least partly via activation of ATF2 and c-Jun, leading to large-scale coordinate gene expression dominated by genes of DNA repair.

Role of basic region leucine zipper transcription factors cyclic AMP response element binding protein (CREB), CREB2, activating transcription factor 2 and CAAT/enhancer binding protein alpha in cyclic AMP response element-mediated transcription

The transcription factor cAMP response element binding protein (CREB), a member of the basic region leucine zipper (bZIP) family of proteins, is the major cAMP response element (CRE) binding. Other bZIP proteins, including CREB2, activating transcription factor 2 (ATF2), or CAAT/enhancer binding protein (C/EBP) have been reported to transactivate CRE-containing genes or to interfere with transactivation by CREB. We have designed a simple transactivation assay using expression of either a constitutively active CREB mutant or a nuclear targeted mutant of the catalytic subunit of cAMP-dependent protein kinase. In both cases, a striking stimulation of transcription of CRE-containing reporter genes was observed in noradrenergic locus coeruleus-like CATH.a cells. In addition, a constitutively active mutant of ATF2 specifically transactivated a secretogranin II promoter/luciferase reporter gene, but had no effect on the tyrosine hydroxylase promoter. In contrast, CREB2 and C/EBPalpha did not transactivate CRE-containing reporter genes, indicating that these bZIP proteins target distinct genetic elements. Experiments involving dominant-negative bZIP mutants revealed that CREB does not heterodimerize with CREB2, ATF2, c-Jun or C/EBP. Rather, CREB and ATF2 compete for binding to the CRE, and are independently able to up-regulate transcription of genes containing CRE motifs in their regulatory regions.

Regulation of GTP cyclohydrolase I gene transcription by basic region leucine zipper transcription factors

Tetrahydrobiopterin is an essential cofactor for the phenylalanine, tyrosine and tryptophan hydroxylases, and the family of nitric oxide synthases. The initial and rate-limiting enzyme in the biosynthesis of tetrahydrobiopterin is GTP cyclohydrolase I. The proximal promoter of the human GTP cyclohydrolase I gene contains the sequence motif 5'-TGACGCGA-3', resembling a cAMP response element (CRE). The objective of this study was to analyze the regulation of GTP cyclohydrolase I gene transcription by basic region leucine zipper (bZIP) transcription factors. A constitutively active mutant of the cAMP response element binding (CREB) protein strongly stimulated GTP cyclohydrolase I promoter activity, indicating that the CRE in the context of the GTP cyclohydrolase I gene is functional. Likewise, GTP cyclohydrolase I promoter/luciferase gene transcription was stimulated following nuclear expression of the catalytic subunit of cAMP-dependent protein kinase. Constitutively active mutants of activating transcription factor 2 (ATF2) and c-Jun additionally stimulated GTP cyclohydrolase I promoter activity, but to a lesser extent than the constitutively active CREB mutant. The fact that stress-activated protein kinases target the GTP cyclohydrolase I gene was corroborated by expression experiments involving p38 and MEKK1 protein kinases. We conclude that signaling pathways involving either the cAMP-dependent protein kinase or stress-activated protein kinases converge to the GTP cyclohydrolase I gene. Hence, enzymatic reactions that require tetrahydrobiopterin as cofactor are therefore indirectly controlled by signaling cascades involving the signal-responsive transcription factors CREB, c-Jun, and ATF2.

Transient increase in anti-p-ATF2 immunoreactivity in the late secretion ameloblasts apical to the transition zone of rat incisors

Activating transcription factor 2 (ATF2) was localized in the ameloblasts of rat incisors by immunohistochemistry. A specific antibody against phosphorylated ATF2 (p-ATF2), which is an activated form of ATF2, was detected from the proliferation zone to maturation ameloblasts just after the transition. In the secretion zone, a transient increase in p-ATF2 was observed in the late secretion ameloblast nuclei, where a stronger reactivity of p-ATF2 extended from 1 mm apical to the transition to the transition zone, whereas ameloblast nuclei in most of the maturation zone exhibited either weak or no reactivity. A similar pattern was also observed in the case of c-Jun immunohistochemistry, except for in most of the maturation zone, where strong c-Jun reactivity was detected. Thus, ATF-2 and c-Jun are deeply involved in amelogenesis and, in particular, ATF2 is related to the proliferation, differentiation, secretion and transition zones.

How mammalian transcriptional repressors work

Research on the regulation of transcription in mammals initially focused on the mechanism of transcriptional activation and 'positive control' of gene regulation. In contrast, transcriptional repression and 'negative control' of gene transcription was viewed rather as part of the 'prokaryotic book of biology'. However, results obtained in recent years have shown convincingly that transcriptional repression mediated by repressor proteins is a common regulatory mechanism in mammals and may play a key role in many biological processes. In particular, the fact that human diseases, such as Rett and ICF syndromes as well as some human forms of cancer, are connected with the activities of human repressor proteins indicates that transcriptional repression and gene silencing is essential for maintenance of the cellular integrity of a multicellular organism. The wide range of diseases caused by aberration in transcriptional repression sheds light on the importance of understanding how mammalian transcriptional repressor proteins work.

Induction of CHOP expression by amino acid limitation requires both ATF4 expression and ATF2 phosphorylation

The CHOP gene is transcriptionally induced by amino acid starvation. We have previously identified a genomic cis-acting element (amino acid response element (AARE)) involved in the transcriptional activation of the human CHOP gene by leucine starvation and shown that it binds the activating transcription factor 2 (ATF2). The present study was designed to identify other transcription factors capable of binding to the CHOP AARE and to establish their role with regard to induction of the gene by amino acid deprivation. Electrophoretic mobility shift assay and transient transfection experiments show that several transcription factors that belong to the C/EBP or ATF families bind the AARE sequence and activate transcription. Among all these transcription factors, only ATF4 and ATF2 are involved in the amino acid control of CHOP expression. We show that inhibition of ATF2 or ATF4 expression impairs the transcriptional activation of CHOP by amino acid starvation. The transacting capacity of ATF4 depends on its expression level and that of ATF2 on its phosphorylation state. In response to leucine starvation, ATF4 expression and ATF2 phosphorylation are increased. However, induction of ATF4 expression by the endoplasmic reticulum stress pathway does not fully activate the AARE-dependent transcription. Taken together our results demonstrate that at least two pathways, one leading to ATF4 induction and one leading to ATF2 phosphorylation, are necessary to induce CHOP expression by amino acid starvation. This work was extended to the regulation of other amino acid regulated genes and suggests that ATF4 and ATF2 are key components of the amino acid control of gene expression.