Analysis of a cAMP-responsive activator reveals a two-component mechanism for transcriptional induction via signal-dependent factors

Metformin Treatment Leads to Increased HIV Transcription and Gene Expression through Increased CREB Phosphorylation and Recruitment to the HIV LTR Promoter

Antiretroviral therapy has effectively suppressed HIV infection and replication and prolonged the lifespan of HIV-infected individuals. In the meantime, various complications including type 2 diabetes associated with the long-term antiviral therapy have shown steady increases. Metformin has been the front-line anti-hyperglycemic drug of choice and the most widely prescribed medication for the treatment of type 2 diabetes. However, little is known about the effects of Metformin on HIV infection and replication. In this study, we showed that Metformin treatment enhanced HIV gene expression and transcription in HIV-transfected 293T and HIV-infected Jurkat and human PBMC. Moreover, we demonstrated that Metformin treatment resulted in increased CREB expression and phosphorylation, and TBP expression. Furthermore, we showed that Metformin treatment increased the recruitment of phosphorylated CREB and TBP to the HIV LTR promoter. Lastly, we showed that inhibition of CREB phosphorylation/activation significantly abrogated Metformin-enhanced HIV gene expression. Taken together, these results demonstrated that Metformin treatment increased HIV transcription, gene expression, and production through increased CREB phosphorylation and recruitment to the HIV LTR promoter. These findings may help design the clinical management plan and HIV cure strategy of using Metformin to treat type 2 diabetes, a comorbidity with an increasing prevalence, in people living with HIV.

Salmonella Typhimurium induces genome-wide expression and phosphorylation changes that modulate immune response, intracellular survival and vesicle transport in infected neutrophils

Salmonella Typhimurium is a food-borne pathogen that causes salmonellosis. When in contact with the host, neutrophils are rapidly recruited to act as first line of defense. To better understand the pathogenesis of this infection, we used an in vitro model of neutrophil infection to perform dual RNA-sequencing (both host and pathogen). In addition, and given that many pathogens interfere with kinase-mediated phosphorylation in host signaling, we performed a phosphoproteomic analysis. The immune response was overall diminished in infected neutrophils, mainly JAK/STAT and toll-like receptor signaling pathways. We found decreased expression of proinflammatory cytokine receptor genes and predicted downregulation of the mitogen-activated protein (MAPK) signaling pathway. Also, Salmonella infection inhibited interferons I and II signaling pathways by upregulation of SOCS3 and subsequent downregulation of STAT1 and STAT2. Additionally, phosphorylation of PSMC2 and PSMC4, proteasome regulatory proteins, was decreased in infected neutrophils. Cell viability and survival was increased by p53 signaling, cell cycle arrest and NFkB-proteasome pathways activation. Combined analysis of RNA-seq and phosphoproteomics also revealed inhibited vesicle transport mechanisms mediated by dynein/dynactin and exocyst complexes, involved in ER-to-Golgi transport and centripetal movement of lysosomes and endosomes. Among the overexpressed virulence genes from Salmonella we found potential effectors responsible of these dysregulations, such as spiC, sopD2, sifA or pipB2, all of them involved in intracellular replication. Our results suggest that Salmonella induces (through overexpression of virulence factors) transcriptional and phosphorylation changes that increases neutrophil survival and shuts down immune response to minimize host response, and impairing intracellular vesicle transport likely to keep nutrients for replication and Salmonella-containing vacuole formation and maintenance.

Tyrosine hydroxylase gene promoter activity is upregulated in female catecholaminergic neuroblastoma cells following activation of a Gαq-coupled designer receptor

Tyrosine hydroxylase is the rate-limiting enzyme of catecholamine biosynthesis that catalyzes the conversion of L-tyrosine to L-3,4-dihydroxyphenylalanine. The tyrosine hydroxylase gene is regulated by extracellular signaling molecules such as epidermal growth factor, nerve growth factor and steroids. Here, we investigated whether the activity of the tyrosine hydroxylase gene promoter is upregulated by activation of G protein-coupled receptors, the largest group of plasma membrane receptors. We used catecholaminergic neuroblastoma cells as a cellular model and chromatin-integrated tyrosine hydroxylase promoter-luciferase reporter genes. The results show that stimulation of Rαq, a Gαq-coupled designer receptor, triggered transcription of a reporter gene driven by the tyrosine hydroxylase promoter. Transcription was attenuated by overexpression of regulator of G-protein signaling-2, which activates the GTPase activity of the G protein α-subunit, and by a truncated, dominant-negative mutant of phospholipase Cβ3. Extracellular signal-regulated protein kinase was identified as the signal transducer. At the transcriptional level, tyrosine hydroxylase promoter activity was found to be controlled by the transcription factor CREB. Expression experiments with the adenoviral regulator protein E1A, an inhibitor of CBP/p300 histone acetyltransferases, showed that transcription of the reporter gene controlled by the tyrosine hydroxylase is under epigenetic control. We identified the protein phosphatases MAP kinase phosphatase-1 and calcineurin as part of a shutdown device of the signaling cascade linking Rαq designer receptor activation to tyrosine hydroxylase gene transcription. We conclude that tyrosine hydroxylase promoter activity is controlled by Gαq-coupled receptors.

Taking the road less traveled - the therapeutic potential of CBP/β-catenin antagonists

AREAS COVERED

This perspective discusses the challenges of targeting the Wnt signaling cascade, the safety, efficacy, and therapeutic potential of specific CBP/β-catenin antagonists and a rationale for the pleiotropic effects of CBP/β-catenin antagonists beyond Wnt signaling.

EXPERT OPINION

CBP/β-catenin antagonists can correct lineage infidelity, enhance wound healing, both normal and aberrant (e.g. fibrosis) and force the differentiation and lineage commitment of stem cells and cancer stem cells by regulating enhancer and super-enhancer coactivator occupancy. Small molecule CBP/β-catenin antagonists rebalance the equilibrium between CBP/β-catenin versus p300/β-catenin dependent transcription and may be able to treat or prevent many diseases of aging, via maintenance of our somatic stem cell pool, and regulating mitochondrial function and metabolism involved in differentiation and immune cell function.

CREB Family Transcription Factors Are Major Mediators of BDNF Transcriptional Autoregulation in Cortical Neurons

BDNF signaling via its transmembrane receptor TrkB has an important role in neuronal survival, differentiation, and synaptic plasticity. Remarkably, BDNF is capable of modulating its own expression levels in neurons, forming a transcriptional positive feedback loop. In the current study, we have investigated this phenomenon in primary cultures of rat cortical neurons using overexpression of dominant-negative forms of several transcription factors, including CREB, ATF2, C/EBP, USF, and NFAT. We show that CREB family transcription factors, together with the coactivator CBP/p300, but not the CRTC family, are the main regulators of rat BDNF gene expression after TrkB signaling. CREB family transcription factors are required for the early induction of all the major BDNF transcripts, whereas CREB itself directly binds only to BDNF promoter IV, is phosphorylated in response to BDNF-TrkB signaling, and activates transcription from BDNF promoter IV by recruiting CBP. Our complementary reporter assays with BDNF promoter constructs indicate that the regulation of BDNF by CREB family after BDNF-TrkB signaling is generally conserved between rat and human. However, we demonstrate that a nonconserved functional cAMP-responsive element in BDNF promoter IXa in humans renders the human promoter responsive to BDNF-TrkB-CREB signaling, whereas the rat ortholog is unresponsive. Finally, we show that extensive BDNF transcriptional autoregulation, encompassing all major BDNF transcripts, occurs also in vivo in the adult rat hippocampus during BDNF-induced LTP. Collectively, these results improve the understanding of the intricate mechanism of BDNF transcriptional autoregulation.SIGNIFICANCE STATEMENT Deeper understanding of stimulus-specific regulation of BDNF gene expression is essential to precisely adjust BDNF levels that are dysregulated in various neurological disorders. Here, we have elucidated the molecular mechanisms behind TrkB signaling-dependent BDNF mRNA induction and show that CREB family transcription factors are the main regulators of BDNF gene expression after TrkB signaling. Our results suggest that BDNF-TrkB signaling may induce BDNF gene expression in a distinct manner compared with neuronal activity. Moreover, our data suggest the existence of a stimulus-specific distal enhancer modulating BDNF gene expression.

Salt Inducible Kinase Signaling Networks: Implications for Acute Kidney Injury and Therapeutic Potential

A number of signal transduction pathways are activated during Acute Kidney Injury (AKI). Of particular interest is the Salt Inducible Kinase (SIK) signaling network, and its effects on the Renal Proximal Tubule (RPT), one of the primary targets of injury in AKI. The SIK1 network is activated in the RPT following an increase in intracellular Na⁺ (Na⁺_in), resulting in an increase in Na,K-ATPase activity, in addition to the phosphorylation of Class IIa Histone Deacetylases (HDACs). In addition, activated SIKs repress transcriptional regulation mediated by the interaction between cAMP Regulatory Element Binding Protein (CREB) and CREB Regulated Transcriptional Coactivators (CRTCs). Through their transcriptional effects, members of the SIK family regulate a number of metabolic processes, including such cellular processes regulated during AKI as fatty acid metabolism and mitochondrial biogenesis. SIKs are involved in regulating a number of other cellular events which occur during AKI, including apoptosis, the Epithelial to Mesenchymal Transition (EMT), and cell division. Recently, the different SIK kinase isoforms have emerged as promising drug targets, more than 20 new SIK2 inhibitors and activators having been identified by MALDI-TOF screening assays. Their implementation in the future should prove to be important in such renal disease states as AKI.

Muscarinic receptor antagonists activate ERK-CREB signaling to augment neurite outgrowth of adult sensory neurons

A major cellular effector activated by G protein coupled receptors is extracellular signal-regulated kinase (ERK). The ERK signaling cascade regulates a variety of cellular processes including growth and proliferation. Both G protein and β-arrestin-mediated signaling lead to ERK activation by phosphorylation through different kinases. Recently, we have shown muscarinic acetylcholine type 1 receptor (M₁R) antagonists, muscarinic toxin 7 (MT7) and pirenzepine, elevated neurite outgrowth and protected from small and large fiber neuropathy in adult sensory neurons in various animal models. Thus, we tested the novel hypothesis that muscarinic antagonists could drive neurite outgrowth through altered M₁R-ERK signaling. We have used two dimensional isoelectric focusing/SDS-PAGE combined with analysis using multiple phospho-epitope specific antibodies to study ERK1/2 phosphorylation and activation of its downstream nuclear effector cyclic response element binding protein (CREB). Activated CREB is known to exhibit neuroprotective and growth promoting effects. One hour of treatment with MT7 and pirenzepine activated ERK through M₁R and induced a significant increase in levels of pCREB(S133) in cultured sensory neurons. Further, pharmacological blockade or siRNA based knockdown of ERK abolished the MT7 and pirenzepine mediated neuritogenic effect. In addition, we have shown drug-induced alterations of charged protein fractions that may possess additional post-translationally modified forms of ERK and CREB. For the first time we show that long-term treatment, e.g. 1 h, with muscarinic antagonists selective or specific for M₁R can activate a biased β-arrestin dependent ERK-CREB signal cascade. Our study gives novel insight into muscarinic antagonist-mediated modulation of M₁R-ERK-CREB signaling which could be exploited for therapy in neuropathic diseases.

pKID Binds to KIX via an Unstructured Transition State with Nonnative Interactions

Understanding the detailed mechanism of interaction of intrinsically disordered proteins with their partners is crucial to comprehend their functions in signaling and transcription. Through its interaction with KIX, the disordered pKID region of CREB protein is central in the transcription of cAMP responsive genes, including those involved in long-term memory. Numerous simulation studies have investigated these interactions. Combined with experimental results, these can provide valuable and comprehensive understanding of the mechanisms involved. Here, we probe the transition state of this interaction experimentally through analyzing the kinetic effect of mutating both interface and solvent exposed residues in pKID. We show that very few specific interactions between pKID and KIX are required in the initial binding process. Only a small number of weak interactions are formed at the transition state, including nonnative interactions, and most of the folding occurs after the initial binding event. These properties are consistent with computational results and also the majority of experimental studies of intrinsically disordered protein coupled folding and binding in other protein systems, suggesting that these may be common features.

Molecular cloning, characterization and expression analysis of (B-cell lymphoma-2) Bcl-2 in the orange-spotted grouper (Epinephelus coioides)

Bcl-2 is a pro-survival member of Bcl-2 like superfamily, playing an important role in regulating the apoptotic process. In this study, the full-length Bcl-2 (EcBcl-2) was obtained, consisting of a 5'UTR of 290 bp, an ORF of 699 bp and a 3'UTR of 920 bp. EcBcl-2 gene encoded a polypeptide of 232 amino acids with an estimated molecular mass of 26.12 KDa and a predicted isoelectric point (pI) of 6.93. The deduced amino acid sequence analysis showed that EcBcl-2 consisted of the conserved residues and characteristic domains known to the critical functionality for Bcl-2. qRT-PCR analysis revealed that EcBcl-2 transcript was expressed in all the examined tissues, while the strongest expression level was observed in liver, followed by the expression in blood, gill, kidney, spleen, heart, intestine and muscle. The groupers challenged with V. alginolyticus showed a significant increase of EcBcl-2 mRNA in immune tissues. In addition, western blotting analysis confirmed that the up-regulation of EcBcl-2 protein expression was detected in liver. Subcellular localization analysis revealed that EcBcl-2 was localized in both nucleus and cytoplasm. Overexpression of EcBcl-2 can inhibit the LPS-induced apoptosis and activate the transcription activity of NF-κB and AP-1, while the deletion of BH1, BH2, BH3 or BH4 domain from EcBcl-2 can impede the signaling transduction. These results indicate that EcBcl-2 may play a regulatory role in the apoptotic process.

Ferulic acid prevents LPS-induced up-regulation of PDE4B and stimulates the cAMP/CREB signaling pathway in PC12 cells

AIM

Phosphodiesterase 4 (PDE4) isozymes are involved in different functions, depending on their patterns of distribution in the brain. The PDE4 subtypes are distributed in different inflammatory cells, and appear to be important regulators of inflammatory processes. In this study we examined the effects of ferulic acid (FA), a plant component with strong anti-oxidant and anti-inflammatory activities, on lipopolysaccharide (LPS)-induced up-regulation of phosphodiesterase 4B (PDE4B) in PC12 cells, which in turn regulated cellular cAMP levels and the cAMP/cAMP response element binding protein (CREB) pathway in the cells.

METHODS

PC12 cells were treated with LPS (1 μg/mL) for 8 h, and the changes of F-actin were detected using laser scanning confocal microscopy. The levels of pro-inflammatory cytokines were measured suing ELISA kits, and PDE4B-specific enzymatic activity was assessed with a PDE4B assay kit. The mRNA levels of PDE4B were analyzed with Q-PCR, and the protein levels of CREB and phosphorylated CREB (pCREB) were determined using immunoblotting. Furthermore, molecular docking was used to identify the interaction between PDE4B2 and FA.

RESULTS

Treatment of PC12 cells with LPS induced thick bundles of actin filaments appearing in the F-actin cytoskeleton, which were ameliorated by pretreatment with FA (10-40 μmol/L) or with a PDE4B inhibitor rolipram (30 μmol/L). Pretreatment with FA dose-dependently inhibited the LPS-induced production of TNF-α and IL-1β in PC12 cells. Furthermore, pretreatment with FA dose-dependently attenuated the LPS-induced up-regulation of PDE4 activity in PC12 cells. Moreover, pretreatment with FA decreased LPS-induced up-regulation of the PDE4B mRNA, and reversed LPS-induced down-regulation of CREB and pCREB in PC12 cells. The molecular docking results revealed electrostatic and hydrophobic interactions between FA and PDE4B2.

CONCLUSION

The beneficial effects of FA in PC12 cells might be conferred through inhibition of LPS-induced up-regulation of PDE4B and stimulation of cAMP/CREB signaling pathway. Therefore, FA may be a potential therapeutic intervention for the treatment of neuroinflammatory diseases such as AD.

Arsenic-Induced Activation of the Homeodomain-Interacting Protein Kinase 2 (HIPK2) to cAMP-Response Element Binding Protein (CREB) Axis

Cyclic AMP-response element-binding protein (CREB) plays key transcriptional roles in cell metabolism, proliferation, and survival. Ser133 phosphorylation by protein kinase A (PKA) is a well-characterized CREB activation mechanism. Homeodomain-interacting protein kinase (HIPK) 2, a nuclear serine/threonine kinase, activates CREB through Ser271 phosphorylation; however, the regulatory mechanism remains uncharacterized. Transfection of CREB in HEK293 cells together with the kinase demonstrated that HIPK2 phosphorylated CREB at Ser271 but not Ser133; likewise, PKA phosphorylated CREB at Ser133 but not Ser271, suggesting two distinct CREB regulatory mechanisms by HIPK2 and PKA. In vitro kinase assay revealed that HIPK2, and HIPK1 and HIPK3, directly phosphorylated CREB. Cells exposed to 10μM sodium arsenite increased the stability of HIPK1 and HIPK2 proteins, leading to CREB activation via Ser271 phosphorylation. Phospho-Ser271 CREB showed facilitated interaction with the TFIID subunit coactivator TAF4 assessed by immunoprecipitation. Furthermore, a focused gene array between cells transfected with CREB alone and CREB plus HIPK2 over empty vector-transfected control displayed 14- and 32-fold upregulation of cyclin A1, respectively, while no upregulation was displayed by HIPK2 alone. These results suggest that the HIPK2-phospho-Ser271 CREB axis is a new arsenic-responsive CREB activation mechanism in parallel with the PKA-phospho-Ser133 CREB axis.

The cyclic AMP signaling pathway: Exploring targets for successful drug discovery (Review)

During development of disease, complex intracellular signaling pathways regulate an intricate series of events, including resistance to external toxins, the secretion of cytokines and the production of pathological phenomena. Adenosine 3′,5′-cyclic monophosphate (cAMP) is a nucleotide that acts as a key second messenger in numerous signal transduction pathways. cAMP regulates various cellular functions, including cell growth and differentiation, gene transcription and protein expression. This review aimed to provide an understanding of the effects of the cAMP signaling pathway and the associated factors on disease occurrence and development by examining the information from a new perspective. These novel insights aimed to promote the development of novel therapeutic approaches and aid in the development of new drugs.

Genome-wide and single-cell analyses reveal a context dependent relationship between CBP recruitment and gene expression

Genome-wide distribution of histone H3K18 and H3K27 acetyltransferases, CBP (CREBBP) and p300 (EP300), is used to map enhancers and promoters, but whether these elements functionally require CBP/p300 remains largely uncertain. Here we compared global CBP recruitment with gene expression in wild-type and CBP/p300 double-knockout (dKO) fibroblasts. ChIP-seq using CBP-null cells as a control revealed nearby CBP recruitment for 20% of constitutively-expressed genes, but surprisingly, three-quarters of these genes were unaffected or slightly activated in dKO cells. Computationally defined enhancer-promoter-units (EPUs) having a CBP peak near the enhancer-like element were more predictive, with CBP/p300 deletion attenuating expression of 40% of such constitutively-expressed genes. Examining signal-responsive (Hypoxia Inducible Factor) genes showed that 97% were within 50 kilobases of an inducible CBP peak, and 70% of these required CBP/p300 for full induction. Unexpectedly, most inducible CBP peaks occurred near signal-nonresponsive genes. Finally, single-cell expression analysis revealed additional context dependence where some signal-responsive genes were not uniformly dependent on CBP/p300 in individual cells. While CBP/p300 was needed for full induction of some genes in single-cells, for other genes CBP/p300 increased the probability of maximal expression. Thus, target gene context influences the transcriptional requirement for CBP/p300, possibly by multiple mechanisms.

Liver Med23 ablation improves glucose and lipid metabolism through modulating FOXO1 activity

Mediator complex is a molecular hub integrating signaling, transcription factors, and RNA polymerase II (RNAPII) machinery. Mediator MED23 is involved in adipogenesis and smooth muscle cell differentiation, suggesting its role in energy homeostasis. Here, through the generation and analysis of a liver-specific Med23-knockout mouse, we found that liver Med23 deletion improved glucose and lipid metabolism, as well as insulin responsiveness, and prevented diet-induced obesity. Remarkably, acute hepatic Med23 knockdown in db/db mice significantly improved the lipid profile and glucose tolerance. Mechanistically, MED23 participates in gluconeogenesis and cholesterol synthesis through modulating the transcriptional activity of FOXO1, a key metabolic transcription factor. Indeed, hepatic Med23 deletion impaired the Mediator and RNAPII recruitment and attenuated the expression of FOXO1 target genes. Moreover, this functional interaction between FOXO1 and MED23 is evolutionarily conserved, as the in vivo activities of dFOXO in larval fat body and in adult wing can be partially blocked by Med23 knockdown in Drosophila. Collectively, our data revealed Mediator MED23 as a novel regulator for energy homeostasis, suggesting potential therapeutic strategies against metabolic diseases.

Driver mutations of cancer epigenomes

Epigenetic alterations are associated with all aspects of cancer, from tumor initiation to cancer progression and metastasis. It is now well understood that both losses and gains of DNA methylation as well as altered chromatin organization contribute significantly to cancer-associated phenotypes. More recently, new sequencing technologies have allowed the identification of driver mutations in epigenetic regulators, providing a mechanistic link between the cancer epigenome and genetic alterations. Oncogenic activating mutations are now known to occur in a number of epigenetic modifiers (i.e. IDH1/2, EZH2, DNMT3A), pinpointing epigenetic pathways that are involved in tumorigenesis. Similarly, investigations into the role of inactivating mutations in chromatin modifiers (i.e. KDM6A, CREBBP/EP300, SMARCB1) implicate many of these genes as tumor suppressors. Intriguingly, a number of neoplasms are defined by a plethora of mutations in epigenetic regulators, including renal, bladder, and adenoid cystic carcinomas. Particularly striking is the discovery of frequent histone H3.3 mutations in pediatric glioma, a particularly aggressive neoplasm that has long remained poorly understood. Cancer epigenetics is a relatively new, promising frontier with much potential for improving cancer outcomes. Already, therapies such as 5-azacytidine and decitabine have proven that targeting epigenetic alterations in cancer can lead to tangible benefits. Understanding how genetic alterations give rise to the cancer epigenome will offer new possibilities for developing better prognostic and therapeutic strategies.

CREB1 and CREB-binding protein in striatal medium spiny neurons regulate behavioural responses to psychostimulants

RATIONALE

The transcription factor cAMP responsive element-binding protein 1 (CREB1) has a complex influence on behavioural responses to drugs of abuse which varies depending on the brain region in which it is expressed. In response to drug exposure, CREB1 is phosphorylated in the striatum, a structure that is critically involved in reward-related learning.

OBJECTIVE

The present study assessed the role of striatal CREB1 and its coactivator CREB-binding protein (CBP) in behavioural responses to psychostimulants.

METHODS

Using the 'cre/lox' recombination system, we generated mice with a postnatal deletion of CREB1 or CBP directed to medium spiny neurons of the striatum. qRT-PCR and immunohistochemistry were used to confirm the deletion, and mice were assessed with respect to their locomotor response to acute cocaine (20 mg/kg), cocaine sensitization (10 mg/kg), amphetamine-induced stereotypies (10 mg/kg) and ethanol-induced hypnosis (3.5 g/kg).

RESULTS

Here we show that CREB1 mutant mice have increased sensitivity to psychostimulants, an effect that does not generalise to ethanol-induced hypnosis. Furthermore, in the absence of CREB1, there is rapid postnatal upregulation of the related transcription factor CREM, indicating possible redundancy amongst this family of transcription factors. Finally striatal deletion of CBP, a coactivator for the CREB1/CREM signalling pathway, results in an even more increased sensitivity to psychostimulants.

CONCLUSIONS

These data suggest that striatal CREB1 regulates sensitivity to psychostimulants and that CREM acting via CBP is able to partially compensate in the absence of CREB1 signalling.

Histone modifiers in cancer: friends or foes?

Covalent modifications of histones can regulate all DNA-dependent processes. In the last few years, it has become more and more evident that histone modifications are key players in the regulation of chromatin states and dynamics as well as in gene expression. Therefore, histone modifications and the enzymatic machineries that set them are crucial regulators that can control cellular proliferation, differentiation, plasticity, and malignancy processes. This review discusses the biology and biochemistry of covalent histone posttranslational modifications (PTMs) and evaluates the dual role of their modifiers in cancer: as oncogenes that can initiate and amplify tumorigenesis or as tumor suppressors.

BDNF-promoted increases in proximal dendrites occur via CREB-dependent transcriptional regulation of cypin

Alterations in dendrite branching and morphology are present in many neurodegenerative diseases. These variations disrupt postsynaptic transmission and affect neuronal communication. Thus, it is important to understand the molecular mechanisms that regulate dendritogenesis and how they go awry during disease states. Previously, our laboratory showed that cypin, a mammalian guanine deaminase, increases dendrite number when overexpressed and decreases dendrite number when knocked down in cultured hippocampal neurons. Here, we report that exposure to brain-derived neurotrophic factor (BDNF), an important mediator of dendrite arborization, for 72 h but not for 24 h or less increases cypin mRNA and protein levels in rat hippocampal neurons. BDNF signals through cypin to regulate dendrite number, since knocking down cypin blocks the effects of BDNF. Furthermore, BDNF increases cypin levels via mitogen-activated protein kinase and transcription-dependent signaling pathways. Moreover, the cypin promoter region contains putative conserved cAMP response element (CRE) regions, which we found can be recognized and activated by CRE-binding protein (CREB). In addition, exposure of the neurons to BDNF increased CREB binding to the cypin promoter and, in line with these data, expression of a dominant negative form of CREB blocked BDNF-promoted increases in cypin protein levels and proximal dendrite branches. Together, these studies suggest that BDNF increases neuronal cypin expression by the activation of CREB, increasing cypin transcription leading to increased protein expression, thus identifying a novel pathway by which BDNF shapes the dendrite network.

Role of mu-opioids as cofactors in human immunodeficiency virus type 1 disease progression and neuropathogenesis

About one third of acquired immunodeficiency syndrome cases in the USA have been attributed to the use of injected addictive drugs, frequently involving opioids like heroin and morphine, establishing them as significant predisposing risk factors for contracting human immunodeficiency virus type 1 (HIV-1). Accumulating evidence from in vitro and in vivo experimental systems indicates that opioids act in concert with HIV-1 proteins to exacerbate dysregulation of neural and immune cell function and survival through diverse molecular mechanisms. In contrast, the impact of opioid exposure and withdrawal on the viral life cycle and HIV-1 disease progression itself is unclear, with conflicting reports emerging from the simian immunodeficiency virus and simian-human immunodeficiency virus infection models. However, these studies suggest a potential role of opioids in elevated viral production. Because human microglia, astrocytes, CD4+ T lymphocytes, and monocyte-derived macrophages express opioid receptors, it is likely that intracellular signaling events triggered by morphine facilitate enhancement of HIV-1 infection in these target cell populations. This review highlights the biochemical changes that accompany prolonged exposure to and withdrawal from morphine that synergize with HIV-1 proteins to disrupt normal cellular physiological functions especially within the central nervous system. More importantly, it collates evidence from epidemiological studies, animal models, and heterologous cell systems to propose a mechanistic link between such physiological adaptations and direct modulation of HIV-1 production. Understanding the opioid-HIV-1 interface at the molecular level is vitally important in designing better treatment strategies for HIV-1-infected patients who abuse opioids.

Unique aspects of transcription regulation in male germ cells

Spermatogenesis is a complex and ordered differentiation process in which the spermatogonial stem cell population gives rise to primary spermatocytes that undergo two successive meiotic divisions followed by a major biochemical and structural reorganization of the haploid cells to generate mature elongate spermatids. The transcriptional regulatory programs that orchestrate this process have been intensively studied in model organisms such as Drosophila melanogaster and mouse. Genetic and biochemical approaches have identified the factors involved and revealed mechanisms of action that are unique to male germ cells. In a well-studied example, cofactors and pathways distinct from those used in somatic tissues mediate the action of CREM in male germ cells. But perhaps the most striking feature concerns the paralogs of somatically expressed transcription factors and of components of the general transcription machinery that act in distinct regulatory mechanisms in both Drosophila and murine spermatogenesis.

Dynamic acetylation of all lysine-4 trimethylated histone H3 is evolutionarily conserved and mediated by p300/CBP

Histone modifications are reported to show different behaviors, associations, and functions in different genomic niches and organisms. We show here that rapid, continuous turnover of acetylation specifically targeted to all K4-trimethylated H3 tails (H3K4me3), but not to bulk histone H3 or H3 carrying other methylated lysines, is a common uniform characteristic of chromatin biology in higher eukaryotes, being precisely conserved in human, mouse, and Drosophila. Furthermore, dynamic acetylation targeted to H3K4me3 is mediated by the same lysine acetyltransferase, p300/cAMP response element binding (CREB)-binding protein (CBP), in both mouse and fly cells. RNA interference or chemical inhibition of p300/CBP using a newly discovered small molecule inhibitor, C646, blocks dynamic acetylation of H3K4me3 globally in mouse and fly cells, and locally across the promoter and start-site of inducible genes in the mouse, thereby disrupting RNA polymerase II association and the activation of these genes. Thus, rapid dynamic acetylation of all H3K4me3 mediated by p300/CBP is a general, evolutionarily conserved phenomenon playing an essential role in the induction of immediate-early (IE) genes. These studies indicate a more global function of p300/CBP in mediating rapid turnover of acetylation of all H3K4me3 across the nuclei of higher eukaryotes, rather than a tight promoter-restricted function targeted by complex formation with specific transcription factors.

Regulation of γ-globin gene expression involves signaling through the p38 MAPK/CREB1 pathway

In response to sodium butyrate and trichostatin A treatment in erythroid cells, p38 mitogen activated protein kinase (MAPK) mediates fetal hemoglobin (HbF) induction by activating cAMP response element binding protein 1 (CREB1). To expand on this observation, we completed studies to determine the role of p38 MAPK in steady-state γ-globin regulation. We propose that p38 signaling regulates Gγ-globin transcription during erythroid maturation through its downstream effector CREB1 which binds the Gγ-globin cAMP response element (G-CRE). We demonstrated that a loss of p38 or CREB1 function by siRNA knockdown resulted in target gene silencing. Moreover, gain of p38 or CREB1 function augments γ-globin transcription. These regulatory effects were conserved under physiological conditions tested in primary erythroid cells. When the G-CRE was mutated in a stable chromatin environment Gγ-globin promoter activity was nearly abolished. Furthermore, introduction of mutations in the G-CRE abolished Gγ-globin activation via p38 MAPK/CREB1 signaling. Chromatin immunoprecipitation assays (ChIP) demonstrated that CREB1 and its binding partner CREB binding protein (CBP) co-localize at the G-CRE region. These data support the role of p38 MAPK/CREB1 signaling in Gγ-globin gene transcription under steady-state conditions.

Kaposi's sarcoma-associated herpesvirus viral protein kinase interacts with RNA helicase a and regulates host gene expression

RNA helicase A (RHA) containing the DExH motif is a human homolog of maleless protein that regulates expression of genes located in the Drosophila X chromosome during dosage compensation. RHA exerts helicase activity that unwinds double-stranded RNA and DNA to a single-strand form. The protein acts as a bridging factor mediating interactions of CBP/p300 and RNA pol II, and consequently affects gene expression. Kaposi's sarcoma-associated herpesvirus (KSHV) is a member of the gamma-herpesvirus subfamily that causes several disorders. The majority of herpesviruses commonly encode predicted viral protein kinases. KSHV open reading frame 36 (ORF36) codes for protein kinase domains, and functions as a serine/threonine protein kinase. KSHV ORF36 is classified as a late gene, as it is expressed during lytic replication and localized in the nuclei of KSHV-infected cells. Recent studies show that viral protein kinase (vPK) interacts with cellular proteins. In this study, we determined the cellular localization of vPK in KSHV-infected BCBL-1 cells using confocal microscopy. Proteomic analysis indicates that cellular proteins interacted with vPK, and co-immunoprecipitation reactions further reveal interactions between vPK and RHA. Moreover, KSHV vPK appeared to regulate the transcriptional activation of Cre promoter, and plays an important role in cellular transcription of RHA.

CREBBP re-arrangements affect protein function and lead to aberrant neuronal differentiation

Biallelic inactivation of the CREB-binding protein (CREBBP) a transcriptional co-activator produces an embryonic lethal phenotype in mice. In humans, re-arrangements in CREBBP are associated with the Rubinstein-Taybi Syndrome (RSTS) that is characterised by craniofacial, skeletal and neuronal symptoms. Neuronal defects in RSTS can be attributed to genetic re-arrangements in CREBBP, which has been implicated in synaptic plasticity and long-term memory. The present study was designed to investigate the role of CREBBP re-arrangements during neuronal differentiation. Towards this, deletion constructs of pCREBBP, viz. pDeltaCB-HAT and pDeltaHAT-CT were generated and transfected into NT2 cells. Expression profiling of the components of Notch, Wnt, SHH and Retinoid signaling along with screening of the neuronal markers was carried out in the NT2 cells and their mutant derivatives. ChIP-PCRs along with co-immunoprecipitations were also performed in these cells to investigate defects due to inappropriate interaction of mutated CREEBP with the corresponding transcription factor and other transcription regulatory proteins both at steady state as well as during differentiation. Mutant NT2 cells lacking the CREB, BROMO and HAT domains (CB-HAT) were highly proliferative and showed limited differentiation; while mutant NT2 cells expressing CREBBP lacking the HAT and CTAD domains (HAT-CT) are proliferation deficient and differentiate rapidly albeit generating an insufficient number of neurons. Altered CREBBP structure resulted in changes in HAT activity, cell cycle profiles and expression of basal levels of components of Notch, SHH, Wnt and retinoid pathways known to be critical in the proliferation and differentiation of neuronal progenitors. At the chromatin level, aberrant signaling correlated with altered binding affinities of the (CREBBP-transcription factor) complexes to promoter regions of components of these pathways. Thus, differentiation defects are manifested early at the genomic level leading to aberrant transcription of the genes involved in differentiation along the neuronal lineage.

Dynamic bookmarking of primary response genes by p300 and RNA polymerase II complexes

Profiling the dynamic interaction of p300 with proximal promoters of human T cells identified a class of genes that rapidly coassemble p300 and RNA polymerase II (pol II) following mitogen stimulation. Several of these p300 targets are immediate early genes, including FOS, implicating a prominent role for p300 in the control of primary genetic responses. The recruitment of p300 and pol II rapidly transitions to the assembly of several elongation factors, including the positive transcriptional elongation factor (P-TEFb), the bromodomain-containing protein (BRD4), and the elongin-like eleven nineteen lysine-rich leukemia protein (ELL). However, transcription at many of these rapidly induced genes is transient, wherein swift departure of P-TEFb, BRD4, and ELL coincides with termination of transcriptional elongation. Unexpectedly, both p300 and pol II remain accumulated or "bookmarked" at the proximal promoter long after transcription has terminated, demarking a clear mechanistic separation between the recruitment and elongation phases of transcription in vivo. The bookmarked pol II is depleted of both serine-2 and serine-5 phosphorylation of its C-terminal domain and remains proximally positioned at the promoter for hours. Surprisingly, these p300/pol II bookmarked genes can be readily reactivated, and elongation factors can be reassembled by subsequent addition of nonmitogenic agents that, alone, have minimal effects on transcription in the absence of prior preconditioning by mitogen stimulation. These findings suggest that p300 is likely to play an important role in biological processes in which transcriptional bookmarking or preconditioning influences cellular growth and development through the dynamic priming of genes for response to rechallenge by secondary stimuli.

Bipartite functions of the CREB co-activators selectively direct alternative splicing or transcriptional activation

The CREB regulated transcription co-activators (CRTCs) regulate many biological processes by integrating and converting environmental inputs into transcriptional responses. Although the mechanisms by which CRTCs sense cellular signals are characterized, little is known regarding how CRTCs contribute to the regulation of cAMP inducible genes. Here we show that these dynamic regulators, unlike other co-activators, independently direct either pre-mRNA splice-site selection or transcriptional activation depending on the cell type or promoter context. Moreover, in other scenarios, the CRTC co-activators coordinately regulate transcription and splicing. Mutational analyses showed that CRTCs possess distinct functional domains responsible for regulating either pre-mRNA splicing or transcriptional activation. Interestingly, the CRTC1-MAML2 oncoprotein lacks the splicing domain and is incapable of altering splice-site selection despite robustly activating transcription. The differential usage of these distinct domains allows CRTCs to selectively mediate multiple facets of gene regulation, indicating that co-activators are not solely restricted to coordinating alternative splicing with increase in transcriptional activity.

Transcriptional activation by MEIS1A in response to protein kinase A signaling requires the transducers of regulated CREB family of CREB co-activators

The transcription factor encoded by the murine ecotropic integration site 1 gene (MEIS1) is a partner of HOX and PBX proteins. It has been implicated in embryonic patterning and leukemia, and causally linked to restless legs syndrome. The MEIS1A C terminus harbors a transcriptional activation domain that is stimulated by protein kinase A (PKA) in a manner dependent on the co-activator of cAMP response element-binding protein (CREB), CREB-binding protein (CBP). We explored the involvement of another mediator of PKA-inducible transcription, namely the CREB co-activators transducers of regulated CREB activity (TORCs). Overexpression of TORC1 or TORC2 bypassed PKA for activation by MEIS1A. Co-immunoprecipitation experiments demonstrated a physical interaction between MEIS1 and TORC2 that is dependent on the MEIS1A C terminus, whereas chromatin immunoprecipitation revealed PKA-inducible recruitment of MEIS1, PBX1, and TORC2 on the MEIS1 target genes Hoxb2 and Meis1. The MEIS1 interaction domain on TORC1 was mapped to the N-terminal coiled-coil region, and TORC1 mutants lacking this domain attenuated the response to PKA on a natural MEIS1A target enhancer. Thus, TORCs physically cooperate with MEIS1 to achieve PKA-inducible transactivation through the MEIS1A C terminus, suggesting a concerted action in developmental and oncogenic processes.

Minireview: role of protein methylation and demethylation in nuclear hormone signaling

Nuclear hormone receptors (NRs) are transcription factors responsible for mediating the biological effects of hormones during development, metabolism, and homeostasis. Induction of NR target genes is accomplished through the assembly of hormone-bound NR complexes at target promoters and coincides with changes in histone modifications that promote transcription. Some coactivators and corepressors of NR can enhance or inhibit NR function by covalently modifying histones. One such modification is methylation, which plays important roles in transcriptional regulation. Histone methylation is catalyzed by histone methyltransferases and reversed by histone demethylases. Recent studies have uncovered the importance of these enzymes in the regulation of NR target genes. In addition to histones, these enzymes have nonhistone substrates and can methylate and demethylate NRs and coregulatory proteins in order to modulate their function. This review discusses recent progress in our understanding of the role of methylation and demethylation of histones, NRs, and their coregulators in NR-mediated transcription.

A composite element that binds basic helix loop helix and basic leucine zipper transcription factors is important for gonadotropin-releasing hormone regulation of the follicle-stimulating hormone beta gene

Although FSH plays an essential role in controlling gametogenesis, the biology of FSHbeta transcription remains poorly understood, but is known to involve the complex interplay of multiple endocrine factors including GnRH. We have identified a GnRH-responsive element within the rat FSHbeta promoter containing an E-box and partial cAMP response element site that are bound by the basic helix loop helix transcription factor family members, upstream stimulating factor (USF)-1/USF-2, and the basic leucine zipper member, cAMP response element-binding protein (CREB), respectively. Expression studies with CREB, USF-1/USF-2, and activating protein-1 demonstrated that the USF transcription factors increased basal transcription, an effect not observed if the cognate binding site was mutated. Conversely, expression of a dominant negative CREB mutant or CREB knockdown attenuated induction by GnRH, whereas dominant negative Fos or USF had no effect on the GnRH response. GnRH stimulation specifically induced an increase in phosphorylated CREB occupation of the FSHbeta promoter, leading to the recruitment of CREB-binding protein to enhance gene transcription. In conclusion, a composite element bound by both USF and CREB serves to integrate signals for basal and GnRH-stimulated transcription of the rat FSHbeta gene.

Inducible cAMP early repressor splice variants ICER I and IIgamma both repress transcription of c-fos and chromogranin A

Inducible cAMP early repressor (ICER) splice variants are generated upon activation of an alternative, intronic promoter within the CREM gene. ICER is proposed to downregulate both its own expression, and the expression of other genes, containing cAMP-responsive promoter elements. To examine the biological function of the two ICER splice variants, I and IIgamma, in comparable cellular systems, we generated HEK 293 cell variants with controllable overexpression of either ICER I or IIgamma. These two splice variants contain two different variants of DNA binding domains. Overexpression of either ICER I or IIgamma strongly represses CRE-driven reportergene transcription but not AP1- or NFkappaB-driven transcription. Thus, high specificity is maintained even at ICER overexpression. We here show that both ICER I and IIgamma repress Pituitary adenylate cyclase-activating polypeptide (PACAP)-mediated c-fos mRNA induction with similar efficiency, indicating that both splice variants play an important role in modulating PACAP-mediated transcriptional activation of the c-fos gene. ICER I and IIgamma also repress cAMP-mediated activation of chromogranin A (CgA), indicating that these splice variants may function as negative feedback regulators in CgA synthesis. The proliferation rate was not altered in cells overexpressing ICER I or IIgamma. Thus, in the epithelial cells HEK 293, ICER I and IIgamma splice variants seem to exert similar biological function.

Transcriptional regulation of human papillomavirus type 18 P105 promoter by the co-activator CBP

Human papillomaviruses (HPVs) are the etiological agents of cervical cancer, with HPV-16 and 18 being the representative types of the higher risk group. The expression of the viral genes with transforming activity (E6 and E7) is controlled by the upstream regulatory region (URR), a segment of the viral genome that contains elements recognized by several transcription factors.

OBJECTIVE

We have analyzed the participation of the cellular co-activator CBP on the transcriptional regulation of the HPV-18 URR.

METHODS

We generated mutants and 5' end deletion constructs derived from the HPV-18 URR and evaluated their transcriptional activity performing transient co-transfection assays on C-33A cells with a plasmid that over-expresses the co-activator CBP. We also performed quantitative chromatin immunoprecipitation assays to analyze the participation of the co-activator CBP on the HPV-18 P105 promoter.

RESULTS

Our results demonstrate that in C-33A cells CBP acts as a strong activator of the HPV-18 P105 promoter by a mechanism that depends on the integrity of the SP1-binding site, directly correlating with the acetylation of the histone H3 that is involved in nucleosomal stability.

CONCLUSION

We propose a mechanism of regulation of the HPV-18 P105 promoter by the cellular co-activator CBP, recruited by the transcription factor SP1.

Surface expression of GABAA receptors is transcriptionally controlled by the interplay of cAMP-response element-binding protein and its binding partner inducible cAMP early repressor

The regulated expression of type A gamma-aminobutyric acid (GABA) receptor (GABA(A)R) subunit genes plays a critical role in neuronal maturation and synaptogenesis. It is also associated with a variety of neurological diseases. Changes in GABA(A) receptor alpha1 subunit gene (GABRA1) expression have been reported in animal models of epilepsy, alcohol abuse, withdrawal, and stress. Understanding the genetic mechanism behind such changes in alpha subunit expression will lead to a better understanding of the role that signal transduction plays in control over GABA(A)R function and brings with it the promise of providing new therapeutic tools for the prevention or cure of a variety of neurological disorders. Here we show that activation of protein kinase C increases alpha1 subunit levels via phosphorylation of CREB (pCREB) that is bound to the GABRA1 promoter (GABRA1p). In contrast, activation of protein kinase A decreases levels of alpha1 even in the presence of pCREB. Decrease of alpha1 is dependent upon the inducible cAMP early repressor (ICER) as directly demonstrated by ICER-induced down-regulation of endogenous alpha1-containing GABA(A)Rs at the cell surface of cortical neurons. Taken together with the fact that there are less alpha1gamma2-containing GABA(A)Rs in neurons after protein kinase A stimulation and that activation of endogenous dopamine receptors down-regulates alpha1 subunit mRNA levels subsequent to induction of ICER, our studies identify a transcriptional mechanism for regulating the cell surface expression of alpha1-containing GABA(A)Rs that is dependent upon the formation of CREB heterodimers.

Synergistic activation of human involucrin gene expression by Fra-1 and p300--evidence for the presence of a multiprotein complex

Involucrin is expressed in the differentiated suprabasal epidermal layers, and an AP1 transcription factor-binding site present in the involucrin promoter distal regulatory region is required for this regulation. This site binds Fra-1, but cofactor interaction at this site has not been adequately characterized. We show that Fra-1 and p300 histone acetyltransferase are present at the AP1 site, as detected by chromatin immunoprecipitation. This interaction is functional, as treating p300 expressing keratinocytes with calcium or 12-O-tetradeconylphorbol-13-acetate, results in a synergistic increase in hINV expression, and this enhanced activation can be reproduced by coexpression of Fra-1 and p300. p300 also co-precipitates with Fra-1, but protein fractionation studies suggest that this interaction requires an additional protein. Fra-1 also interacts with other proteins that interact at the AP1-5 site, including JunD, JunB, Sp1, and P/CAF. Contrary to results in some other systems, Fra-1 functions as a positive transcriptional regulator in human keratinocytes. These studies suggest that a large multiprotein complex, which includes Fra-1, p300, P/CAF, junD, junB, and Sp1 acts at the AP1-5 site to produce a synergistic increase in hINV gene expression.

MHC class II transactivator represses human IL-4 gene transcription by interruption of promoter binding with CBP/p300, STAT6 and NFAT1 via histone hypoacetylation

In addition to its property of enhancing major histocompatibility complex (MHC) class II expression, the class II transactivator (CIITA) was recently demonstrated to be involved in T helper type 1/type 2 (Th1/Th2) differentiation by regulating interleukin-4 (IL-4) gene transcription. There was however, controversy regarding whether CIITA promotes or suppresses IL-4 expression in the experiments with transgenic mice. To clarify the discrepancy by using simpler experimental systems, human Jurkat T cells that express IL-4 but not interferon-gamma, even if stimulated with phorbol 12-myristate 13-acetate plus ionomycin, were used for CIITA transfection. Significant suppression of IL-4 gene expression was demonstrated. Simultaneously, histones H3 and H4 in the IL-4 promoter were hypoacetylated. The suppression could be totally reversed by the histone deacetylatase inhibitor trichostatin A. Furthermore, the IL-4 expression was determined in primarily established human Th1/Th2 cells to which CIITA small interference RNA (siRNA) had been introduced. A substantially increased level of IL-4 was recorded in the CIITA siRNA-transfected Th1 cells, which was in parallel with significantly enhanced acetylation in histone H3 of the IL-4 promoter. Chromatin immunoprecipitation analysis indicated that CIITA abrogated the binding of coactivator CBP/p300 and transcription factors STAT6/NFAT1 to IL-4 promoter in the CIITA-transfected cells. In conclusion, CIITA was active in the repression of transcription activation of human IL-4 gene in both the T-cell line and the primary human CD4 T cells by preventing transcription factors from binding to IL-4 promoter through histone hypoacetylation. Our data confirm a potential significant role of CIITA in controlling Th1/Th2 differentiation via modulation of IL-4 gene activation.

DNA Binding and Phosphorylation Induce Conformational Alterations in the Kinase-inducible Domain of CREB

CREB-mediated activation of target gene transcription is stimulated by protein kinase A (PKA) phosphorylation at serine 133. This is followed by recruitment of the coactivators CREB-binding protein (CBP) or p300. Conversely, the decline in expression during the attenuation phase is linked to CREB dephosphorylation by nuclear phosphatases. The CREB bZIP domain, which promotes dimerization and promoter binding, as well as the kinase-inducible domain (KID), which interacts with the KIX domain of CBP/p300, are both largely unstructured in solution and become more structured once bound to their respective ligands. In this study, we biochemically characterize DNA- and phosphorylation-induced conformational alterations in CREB that may play a role in its transcriptionally poised, activated state. We find that sequence-specific DNA binding of pCREB renders the protein resistant to serine 133 dephosphorylation by protein phosphatase 1. Paradoxically, CREB bound to DNA and chromatin is efficiently phosphorylated by PKA, indicating that the KID region exists in a different conformation depending on its phosphorylation state. Consistent with this observation, we find that phosphorylation of DNA-bound CREB promotes an alternate conformation characterized by an apparent increase in the size or asymmetry of the complex and a qualitative change in proteolytic sensitivity. Together, our data indicate that DNA binding promotes a global conformational change in CREB that alters the structure of KID. PKA phosphorylation of KID in the DNA-bound state induces a phosphatase-resistant conformation that may prolong transcriptional activity.

CREB--a real culprit in oncogenesis

The cAMP response element-binding protein (CREB) is a stimulus-induced transcription factor that responds rapidly to phosphorylation and/or coactivator activation. Regulated activation of CREB has a significant impact on cellular growth, proliferation and survival. To overturn the cellular control of these processes, tumor cells have developed various mechanisms to achieve constitutive activation of CREB, including gene amplification, chromosome translocation, interaction with viral oncoproteins, and inactivation of tumor suppressor genes. These mechanisms converge on the phosphorylation of CREB and/or the activation of transducer of regulated CREB activity (TORC) coactivators to effect uncontrolled proliferation of cells. This minireview summarizes the different lines of existing evidence that support a direct role of CREB in oncogenesis.

The acetyltransferase activity of Drosophila CBP is dispensable for regulation of the Dpp pathway in the early embryo

The CBP protein is a transcriptional co-activator and histone acetyltransferase. Reduced expression of Drosophila CBP (dCBP) in the early embryo specifically impairs signaling by the TGF-beta molecules Dpp and Screw (Scw). This occurs by a failure to activate transcription of the tolloid (tld) gene, which codes for a protease that generates active Dpp and Scw ligands. We show that dCBP directly regulates this gene by binding to the tld enhancer, and that tld expression can be partially rescued with a dCBP transgene. At a slightly later stage of development, Dpp/Scw signaling recovers in mutant embryos, but is unable to turn on expression of the Dpp/Scw-target gene rhomboid (rho). Interestingly, an acetyltransferase (AT)-defective dCBP transgene rescued tld and rho gene expression to an extent comparable to the wild-type transgene, whereas a transgene containing a 130 amino acid deletion rescued tld but not late rho expression. A tracheal phenotype caused by the reduced dCBP levels was also rescued more efficiently with the wild-type dCBP transgene than with this mutant transgene. Our results indicate that separate parts of the dCBP protein are required on different promoters, and that the AT activity of dCBP is dispensable for certain aspects of Dpp signaling. We discuss the similarity of these results to the role of p300/CBP in TGF-beta signaling in the mouse.

PC12 cells regulate inducible cyclic AMP (cAMP) element repressor expression to differentially control cAMP response element-dependent transcription in response to nerve growth factor and cAMP

Both cyclic AMP (cAMP) and nerve growth factor (NGF) have been shown to cause rapid activation of cAMP response element-binding protein (CREB) by phosphorylation of serine 133, but additional regulatory events contribute to CREB-targeted gene expression. Here, we have used stable transfection with a simple cAMP response element (CRE)-driven reporter to address the kinetics of CRE-dependent transcription during neuronal differentiation of PC12 cells. In naive cells, dibutyryl cAMP (dbcAMP) generated a rapid increase in CRE-driven luciferase activity by 5 h that returned to naive levels by 24 h. Luciferase induction after NGF treatment was delayed until 48 h when CRE-driven luciferase expression became TrkA dependent. Blocking histone deacetylase (HDAC) activity accelerated NGF-dependent CRE-driven luciferase expression by at least 24 h and resulted in a sustained cAMP-dependent expression of CRE-driven luciferase beyond 24 h. Inhibition of protein synthesis before stimulation with NGF or dbcAMP indicated that both stimuli induce expression of a transcriptional repressor that delays NGF-dependent and attenuates cAMP-dependent CRE-driven transcription. NGF caused a rapid but transient HDAC-dependent increase in inducible cAMP element repressor (ICER) expression, but ICER expression was sustained with increased cAMP. Depletion of ICER from PC12 cells indicated that HDAC-dependent ICER induction is responsible for the delay in CRE-dependent transcription after NGF treatment.

The general transcription machinery and general cofactors

In eukaryotes, the core promoter serves as a platform for the assembly of transcription preinitiation complex (PIC) that includes TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH, and RNA polymerase II (pol II), which function collectively to specify the transcription start site. PIC formation usually begins with TFIID binding to the TATA box, initiator, and/or downstream promoter element (DPE) found in most core promoters, followed by the entry of other general transcription factors (GTFs) and pol II through either a sequential assembly or a preassembled pol II holoenzyme pathway. Formation of this promoter-bound complex is sufficient for a basal level of transcription. However, for activator-dependent (or regulated) transcription, general cofactors are often required to transmit regulatory signals between gene-specific activators and the general transcription machinery. Three classes of general cofactors, including TBP-associated factors (TAFs), Mediator, and upstream stimulatory activity (USA)-derived positive cofactors (PC1/PARP-1, PC2, PC3/DNA topoisomerase I, and PC4) and negative cofactor 1 (NC1/HMGB1), normally function independently or in combination to fine-tune the promoter activity in a gene-specific or cell-type-specific manner. In addition, other cofactors, such as TAF1, BTAF1, and negative cofactor 2 (NC2), can also modulate TBP or TFIID binding to the core promoter. In general, these cofactors are capable of repressing basal transcription when activators are absent and stimulating transcription in the presence of activators. Here we review the roles of these cofactors and GTFs, as well as TBP-related factors (TRFs), TAF-containing complexes (TFTC, SAGA, SLIK/SALSA, STAGA, and PRC1) and TAF variants, in pol II-mediated transcription, with emphasis on the events occurring after the chromatin has been remodeled but prior to the formation of the first phosphodiester bond.

Mutational and functional analysis of an essential subdomain of the adenovirus E1A N-terminal transcription repression domain

Adenovirus early gene 1A (E1A) possesses a potent transcriptional repression function within the first 80 amino acids (E1A 1-80). Our previous analysis of subdomain 1 (residues 1 to 30) revealed strong correlations between residues required for repression and for disruption of TBP-TATA complexes. Here, we report a functional analysis of subdomain 2 (48 to 60) by alanine-scanning mutagenesis. 53Ala, 54Pro, 55Glu, and 56Asp are required for repression in vitro and in vivo and for efficient interaction with p300 but not for disruption of TBP-TATA. These combined results suggest a model for E1A transcription repression. E1A through subdomains 1 and 2 uses coactivators like p300 as scaffolds to access E1A repressible promoters. At the promoter, subdomain 1 interacts with TBP to disrupt TBP-TATA and abort transcription initiation. In further support of this model, we show that E1A 1-80 bound to the p300-binding site retains the ability to interact with TBP.

Mechanism of Repression of the Inhibin α-Subunit Gene by Inducible 3′,5′-Cyclic Adenosine Monophosphate Early Repressor

The rodent ovary is regulated throughout the reproductive cycle to maintain normal cyclicity. Ovarian follicular development is controlled by changes in gene expression in response to the gonadotropins FSH and LH. The inhibin alpha-subunit gene belongs to a group of genes that is positively regulated by FSH and negatively regulated by LH. Previous studies established an important role for inducible cAMP early repressor (ICER) in repression of alpha-inhibin. These current studies investigate the mechanisms of repression by ICER. It is not clear whether all four ICER isoforms expressed in the ovary can act as repressors of the inhibin alpha-subunit gene. EMSAs demonstrate binding of all isoforms to the inhibin alpha-subunit CRE (cAMP response element), and transfection studies demonstrate that all isoforms can repress the inhibin alpha-subunit gene. Repression by ICER is dependent on its binding to DNA as demonstrated by mutations to ICER's DNA-binding domain. These mutational studies also demonstrate that repression by ICER is not dependent on heterodimerization with CREB (CRE-binding protein). Competitive EMSAs show that ICER effectively competes with CREB for binding to the inhibin alpha CRE in vitro. Chromatin immunoprecipitation assays demonstrate a replacement of CREB dimers bound to the inhibin alpha CRE by ICER dimers in ovarian granulosa cells in response to LH signaling. Thus, there is a temporal association of transcription factors bound to the inhibin alpha-CRE controlling inhibin alpha-subunit gene expression.

A Novel NF-κB-Regulated Site within the Human Iγ1 Promoter Requires p300 for Optimal Transcriptional Activity

Transcriptional activation of germline (GL) promoters occurs through binding of NF-kappaB to three evolutionarily conserved sites within a CD40 response region in the human and mouse GL Igamma and Iepsilon promoters. Here we identify and characterize a novel NF-kappaB binding site (kappaB6) within the human GL Igamma1 promoter that plays an essential role in basal- and CD40-induced transcription. This site is adjacent to identified CREB/activating transcription factor (ATF) sites, present in the Igamma1 but not the Igamma3 promoter, which are important for the amplification of transcription. Our data suggest a cohesive protein complex regulating Igamma1 promoter activity because disruption of any individual NF-kappaB or CREB/ATF site markedly lowers the overall inducible activity of the promoter. In addition, alteration of helical phasing within the promoter indicates spatial orientation of CREB/ATF and NF-kappaB, proteins contributes directly to promoter activity. We found that CREB and p50 transactivators, as well as coactivator p300, interact in vivo with the Igamma1 promoter in the presence and absence of CD40 signaling in Ramos and primary B cells. However, the level of CREB and p300 binding differs as a consequence of activation in primary B cells. Furthermore, overexpression of p300, and not a mutant lacking acetyltransferase activity, significantly increases Igamma1 construct-specific transcription. Together these data support a model whereby CREB and multiple NF-kappaB complexes bind to the Igamma1 promoter and recruit p300. CD40 signals induce p300-dependent changes that result in optimal Igamma1 promoter activity.

Interference of CREB-dependent transcriptional activation by expanded polyglutamine stretches--augmentation of transcriptional activation as a potential therapeutic strategy for polyglutamine diseases

On the basis of the hypothesis that the interaction of mutant proteins with expanded polyglutamine stretches with transcriptional co-activator, TAFII130, leads to transcriptional dysregulation, the transcriptional activation of c-Fos and its suppression by expanded polyglutamine stretches was investigated. The phosphorylation of cAMP-responsive element binding protein (CREB) and induction of c-Fos in response to cAMP were strongly suppressed in Neuro2a cells expressing expanded polyglutamine. The suppression of CREB-dependent transcriptional activation was reversibly rescued by increasing the concentration of cAMP. Expanded polyglutamine-induced cytotoxicity was also substantially suppressed by augmenting CREB-dependent transcriptional activation with a high concentration of cAMP. FR901228, a histone deacetylase inhibitor, was also demonstrated as rescuing the expanded polyglutamine-induced suppression of CREB phosphorylation and c-Fos expression. Furthermore, nuclear fragmentation was significantly suppressed by FR901228. The co-expression of dominant-negative CREB vectors considerably abrogated the suppressive effect of cAMP and FR901228 on the expanded polyglutamine-induced nuclear fragmentation, suggesting that these compounds suppress polyglutamine-induced cytotoxicity, largely, via the enhancement of CREB-dependent transcriptional activation. These findings suggest that the interference of CREB-dependent transcriptional activation by expanded polyglutamine stretches is involved in the pathogenetic mechanisms underlying neurodegeneration, and that the augmentation of CREB-dependent transcriptional activation is a potential strategy in treating polyglutamine diseases.

Transcriptional Co-activators CREB-binding protein/p300 increase chondrocyte Cd-rap gene expression by multiple mechanisms including sequestration of the repressor CCAAT/enhancer-binding protein

Cartilage-derived retinoic acid-sensitive protein (CD-RAP) is a small secreted matrix protein expressed in developing and adult cartilage and by chondrocytes in culture. We have previously shown that the expression of Cd-rap, like many other cartilage matrix proteins, is repressed by interleukin 1beta and that the transcription factor CCAAT/enhancer-binding protein (C/EBP) beta plays an important role in the interleukin 1beta-induced repression (Okazaki, K., Li, J., Yu, H., Fukui, N., and Sandell, L. J. (2002) J. Biol. Chem. 277, 31526-31533). The co-activators CREB-binding protein (CBP) and p300 are transcriptional co-regulators that participate in the activities of many different transcription factors including C/EBP. Here we show that CBP/p300 can reverse the inhibitory effect of C/EBP and moreover can stimulate expression of Cd-rap. The mechanism of this effect is shown to involve a unique synergy whereby CBP/p300 stimulate Cd-rap gene expression by at least two mechanisms. First, binding of CBP/p300 to C/EBPbeta leads to sequestration of C/EBP eliminating DNA binding and subsequent repression; second, binding of CBP/p300 to the transcriptional activator Sox9 increases Sox9 DNA binding to the Cd-rap promoter leading to further stimulation of gene transcription. This is an example of a complementary transcriptional network whereby two very different mechanisms act together to confer a functional increase in transcription. This new paradigm is likely generally applicable to cartilage genes as Col2a1 cartilage collagen gene responds similarly.

Glutamine rich and basic region/leucine zipper (bZIP) domains stabilize cAMP-response element-binding protein (CREB) binding to chromatin

We have examined the dynamics of cAMP-response element-binding protein (CREB) binding to chromatin in live cells using fluorescence recovery after photobleaching (FRAP). CREB was found to bind to target sites with a residence time of 100 s, and exposure to a cAMP agonist had no effect on these kinetics. In addition to the basic region/leucine zipper (bZIP) domain, a glutamine-rich trans-activation domain in CREB called Q2 also appeared to be critical for promoter occupancy. Indeed, mutations in Q2 that reduced residence time by FRAP assay disrupted target gene activation via CREB in cells exposed to a cAMP agonist. Notably, insertion of the glutamine-rich B trans-activation domain of SP1 into a mutant CREB polypeptide lacking Q2 stabilized CREB occupancy and rescued target gene activation. These results suggest a novel mechanism by which the family of glutamine-rich activators promotes cellular gene expression.

VP16-dependent association of chromatin-modifying coactivators and underrepresentation of histones at immediate-early gene promoters during herpes simplex virus infection

During infection by herpes simplex virus type 1 (HSV-1), the virion protein VP16 activates the transcription of viral immediate-early (IE) genes. Genetic and biochemical assays have shown that the potent transcriptional activation domain of VP16 can associate with general transcription factors and with chromatin-modifying coactivator proteins of several types. The latter interactions are particularly intriguing because previous reports indicate that HSV-1 DNA does not become nucleosomal during lytic infection. In the present work, chemical cross-linking and immunoprecipitation assays were used to probe the presence of activators, general transcription factors, and chromatin-modifying coactivators at IE gene promoters during infection of HeLa cells by wild-type HSV-1 and by RP5, a viral strain lacking the VP16 transcriptional activation domain. The presence of VP16 and Oct-1 at IE promoters did not depend on the activation domain. In contrast, association of RNA polymerase II, TATA-binding protein, histone acetyltransferases (p300 and CBP), and ATP-dependent remodeling proteins (BRG1 and hBRM) with IE gene promoters was observed in wild-type infections but was absent or reduced in cells infected by RP5. In contrast to the previous evidence for nonnucleosomal HSV-1 DNA, histone H3 was found associated with viral DNA at early times of infection. Interestingly, histone H3 was underrepresented on IE promoters in a manner dependent on the VP16 activation domain. Thus, the VP16 activation domain is responsible for recruiting general transcription factors and coactivators to IE promoters and also for dramatically reducing the association of histones with those promoters.

CBP and p300: HATs for different occasions

The transcriptional coactivators CREB binding protein (CBP) and p300 are key regulators of RNA polymerase II-mediated transcription. Genetic alterations in the genes encoding these regulatory proteins and their functional inactivation have been linked to human disease. Findings in patients, knockout mice and cell-based studies indicate that the ability of these multidomain proteins to acetylate histones and other proteins is critical for many biological processes. Furthermore, despite their high degree of homology, accumulating evidence indicates that CBP and p300 are not completely redundant but also have unique roles in vivo. Recent studies suggest that these functional differences could be due to differential association with other proteins or differences in substrate specificity between these acetyltransferases. Inactivation of the acetyltransferase function of either CBP or p300 in various experimental systems will no doubt teach us more about the specific biological roles of these proteins. Given the wide range of human diseases in which CBP and/or p300 have been implicated, understanding the mechanisms that regulate their activity in vivo could help to develop novel approaches for the development of therapeutic strategies.

Opening the chromatin for transcription

Eukaryotic genomes are packaged into a dynamic hierarchy chromatin structure. In such a particular context, the transition from a repressed compacted chromatin to a rather extended fiber is necessary for transcription. The chromatin opening includes three events, the initial factor getting access to nucleosome DNA, local chromatin opening mediated by activator/coactivator, and transcription associated with extensive chromatin opening. Chromatin dynamics, which is DNA sequence dependent, and also occurs in condensed fiber, provides the opportunity for activators binding to DNA. Coactivators recruited by the activator open the chromatin locally. However, it appears that genes adopt distinct chromatin opening mechanisms according to whether the gene is induced expression, developmental and tissue-specific expression, or constitutive expression. In contrast to transcription initiation-related local chromatin opening, large scale of chromatin opening is associated with a functional enhancer as well as high transcription rate. How the transcription initiated from an enhancer or enhancer like modules, i.e. intergenic transcription, conducts the extensive chromatin opening is discussed. A model for long-range interaction that non-coding transcripts from enhancers may promote efficient communication with promoters is proposed.