Recruitment of an RNA polymerase II complex is mediated by the constitutive activation domain in CREB, independently of CREB phosphorylation

Glutamine-rich regions of the disordered CREB transactivation domain mediate dynamic intra- and intermolecular interactions

The cyclic AMP response element (CRE) binding protein (CREB) is a transcription factor that contains a 280-residue N-terminal transactivation domain and a basic leucine zipper that mediates interaction with DNA. The transactivation domain comprises three subdomains, the glutamine-rich domains Q1 and Q2 and the kinase inducible activation domain (KID). NMR chemical shifts show that the isolated subdomains are intrinsically disordered but have a propensity to populate local elements of secondary structure. The Q1 and Q2 domains exhibit a propensity for formation of short β-hairpin motifs that function as binding sites for glutamine-rich sequences. These motifs mediate intramolecular interactions between the CREB Q1 and Q2 domains as well as intermolecular interactions with the glutamine-rich Q1 domain of the TATA-box binding protein associated factor 4 (TAF4) subunit of transcription factor IID (TFIID). Using small-angle X-ray scattering, NMR, and single-molecule Förster resonance energy transfer, we show that the Q1, Q2, and KID regions remain dynamically disordered in a full-length CREB transactivation domain (CREBTAD) construct. The CREBTAD polypeptide chain is largely extended although some compaction is evident in the KID and Q2 domains. Paramagnetic relaxation enhancement reveals transient long-range contacts both within and between the Q1 and Q2 domains while the intervening KID domain is largely devoid of intramolecular interactions. Phosphorylation results in expansion of the KID domain, presumably making it more accessible for binding the CBP/p300 transcriptional coactivators. Our study reveals the complex nature of the interactions within the intrinsically disordered transactivation domain of CREB and provides molecular-level insights into dynamic and transient interactions mediated by the glutamine-rich domains.

Shared and unique phosphoproteomics responses in skeletal muscle from exercise models and in hyperammonemic myotubes

Skeletal muscle generation of ammonia, an endogenous cytotoxin, is increased during exercise. Perturbations in ammonia metabolism consistently occur in chronic diseases, and may blunt beneficial skeletal muscle molecular responses and protein homeostasis with exercise. Phosphorylation of skeletal muscle proteins mediates cellular signaling responses to hyperammonemia and exercise. Comparative bioinformatics and machine learning-based analyses of published and experimentally derived phosphoproteomics data identified differentially expressed phosphoproteins that were unique and shared between hyperammonemic murine myotubes and skeletal muscle from exercise models. Enriched processes identified in both hyperammonemic myotubes and muscle from exercise models with selected experimental validation included protein kinase A (PKA), calcium signaling, mitogen-activated protein kinase (MAPK) signaling, and protein homeostasis. Our approach of feature extraction from comparative untargeted "omics" data allows for selection of preclinical models that recapitulate specific human exercise responses and potentially optimize functional capacity and skeletal muscle protein homeostasis with exercise in chronic diseases.

Transcription Regulation of Tceal7 by the Triple Complex of Mef2c, Creb1 and Myod

Simple Summary

We have previously reported a striated muscle-specific gene during embryogenesis, Tceal7. Our studies have characterized the 0.7 kb promoter of the Tceal7 gene, which harbors important E-box motifs driving the LacZ reporter in the myogenic lineage. However, the underlying mechanism regulating the dynamic expression of Tceal7 during skeletal muscle regeneration is still elusive. In the present work, we have defined a cluster of Mef2#3–CRE#3–E#4 motifs through bioinformatic analysis and transcription assays. Our studies suggested that the triple complex of Mef2c, Creb1 and Myod binds to the Mef2#3–CRE#3–E#4 cluster region, therefore driving the dynamic expression of Tceal7 during skeletal muscle regeneration. The novel mechanism may throw new light on understanding transcription regulation in skeletal muscle myogenesis.

Abstract

Tceal7 has been identified as a direct, downstream target gene of MRF in the skeletal muscle. The overexpression of Tceal7 represses myogenic proliferation and promotes cell differentiation. Previous studies have defined the 0.7 kb upstream fragment of the Tceal7 gene. In the present study, we have further determined two clusters of transcription factor-binding motifs in the 0.7 kb promoter: CRE#2–E#1–CRE#1 in the proximal region and Mef2#3–CRE#3–E#4 in the distal region. Utilizing transcription assays, we have also shown that the reporter containing the Mef2#3–CRE#3–E#4 motifs is synergistically transactivated by Mef2c and Creb1. Further studies have mapped out the protein–protein interaction between Mef2c and Creb1. In summary, our present studies support the notion that the triple complex of Mef2c, Creb1 and Myod interacts with the Mef2#3–CRE#3–E#4 motifs in the distal region of the Tceal7 promoter, thereby driving Tceal7 expression during skeletal muscle development and regeneration.

CREB at the Crossroads of Activity-Dependent Regulation of Nervous System Development and Function

The central nervous system is a highly plastic network of cells that constantly adjusts its functions to environmental stimuli throughout life. Transcription-dependent mechanisms modify neuronal properties to respond to external stimuli regulating numerous developmental functions, such as cell survival and differentiation, and physiological functions such as learning, memory, and circadian rhythmicity. The discovery and cloning of the cyclic adenosine monophosphate (cAMP) responsive element binding protein (CREB) constituted a big step toward deciphering the molecular mechanisms underlying neuronal plasticity. CREB was first discovered in learning and memory studies as a crucial mediator of activity-dependent changes in target gene expression that in turn impose long-lasting modifications of the structure and function of neurons. In this chapter, we review the molecular and signaling mechanisms of neural activity-dependent recruitment of CREB and its cofactors. We discuss the crosstalk between signaling pathways that imprints diverse spatiotemporal patterns of CREB activation allowing for the integration of a wide variety of stimuli.

Mechanisms of specificity in neuronal activity-regulated gene transcription

The brain is a highly adaptable organ that is capable of converting sensory information into changes in neuronal function. This plasticity allows behavior to be accommodated to the environment, providing an important evolutionary advantage. Neurons convert environmental stimuli into long-lasting changes in their physiology in part through the synaptic activity-regulated transcription of new gene products. Since the neurotransmitter-dependent regulation of Fos transcription was first discovered nearly 25 years ago, a wealth of studies have enriched our understanding of the molecular pathways that mediate activity-regulated changes in gene transcription. These findings show that a broad range of signaling pathways and transcriptional regulators can be engaged by neuronal activity to sculpt complex programs of stimulus-regulated gene transcription. However, the shear scope of the transcriptional pathways engaged by neuronal activity raises the question of how specificity in the nature of the transcriptional response is achieved in order to encode physiologically relevant responses to divergent stimuli. Here we summarize the general paradigms by which neuronal activity regulates transcription while focusing on the molecular mechanisms that confer differential stimulus-, cell-type-, and developmental-specificity upon activity-regulated programs of neuronal gene transcription. In addition, we preview some of the new technologies that will advance our future understanding of the mechanisms and consequences of activity-regulated gene transcription in the brain.

Nucleotide composition-linked divergence of vertebrate core promoter architecture

Transcription initiation involves the recruitment of basal transcription factors to the core promoter. A variety of core promoter elements exists; however for most of these motifs, the distribution across species is unknown. Here we report on the comparison of human and amphibian promoter sequences. We have used oligo-capping in combination with deep sequencing to determine transcription start sites in Xenopus tropicalis. To systematically predict regulatory elements, we have developed a de novo motif finding pipeline using an ensemble of computational tools. A comprehensive comparison of human and amphibian promoter sequences revealed both similarities and differences in core promoter architecture. Some of the differences stem from a highly divergent nucleotide composition of Xenopus and human promoters. Whereas the distribution of some core promoter motifs is conserved independently of species-specific nucleotide bias, the frequency of another class of motifs correlates with the single nucleotide frequencies. This class includes the well-known TATA box and SP1 motifs, which are more abundant in Xenopus and human promoters, respectively. While these motifs are enriched above the local nucleotide background in both organisms, their frequency varies in step with this background. These differences are likely adaptive as these motifs can recruit TFIID to either CpG island or sharply initiating promoters. Our results highlight both the conserved and diverged aspects of vertebrate transcription, most notably showing co-opted motif usage to recruit the transcriptional machinery to promoters with diverging nucleotide composition. This shows how sweeping changes in nucleotide composition are compatible with highly conserved mechanisms of transcription initiation.

Regulating gene transcription in response to cyclic AMP elevation

Many of the effects of prototypical second messenger cyclic adenosine 3',5'-monophosphate (cAMP) on complex processes such as the regulation of fuel metabolism, spermatogenesis and steroidogenesis are mediated via changes in target gene transcription. A large body of research has defined members of the cAMP-response element binding (CREB) protein family as the principal mediators of positive changes in gene expression in response to cAMP following phosphorylation by cAMP-dependent protein kinase (PKA). However, persistent observations of cAMP-mediated induction of specific genes occurring via PKA-independent mechanisms have challenged the generality of the PKA-CREB pathway. In this review, we will discuss in detail both PKA-dependent and -independent mechanisms that have been proposed to explain how cAMP influences the activation status of multiple transcription factors, and how these influence critical biological processes whose defective regulation may lead to disease.

Functional characterization of the promoter of human kinetochore protein HEC1: novel link between regulation of the cell cycle protein and CREB family transcription factors

HEC1 (highly expressed in cancer), which localizes to kinetochore in cell mitosis, plays an essential role in chromosome segregation for M phase progression. To clarify the mechanism of its transcriptional regulation, we searched out and isolated its 5'-flanking region. Mapping of this region identified that it is a TATA-less promoter and contains several putative binding sites for different transcription factors. The results from HeLa cells transfected with pGL3 luciferase reporter vectors containing progressive deletion of the HEC1 5'-flanking region demonstrated that two elements containing binding sites for cAMP responsive element binding (CREB) protein and activating transcription factor 4 (ATF4 or CREB2) are critical for transcriptional activity. Mutation of the two elements, not downstream E2F box, resulted in a significant reduction of the promoter activity. Gel shift and supershift assays also demonstrated specific binding of transcription factors to their putative binding sites. Furthermore, overexpression of either CREB or ATF4 enhanced the activation of the HEC1 promoter and overexpression of both of them had an additive effect on the activation of the HEC1 transcription. Conversely, overexpression of dominant negative mutants of either CREB or ATF4 resulted in downregulation of HEC1 mRNA significantly. Our study provided a new insight into a potential mechanism of how transcription factors of CREB family are involved in the regulation of kinetochore protein HEC1 in cancer-related cells.

Regulation of the human SOX9 promoter by Sp1 and CREB

The transcription factor SOX9 is essential for multiple steps during skeletal development, including mesenchymal cell chondrogenesis and endochondral bone formation. We recently reported that the human SOX9 proximal promoter region is regulated by the CCAAT-binding factor through two CCAAT boxes located within 100 bp of the transcriptional start site. Here we report that the human SOX9 proximal promoter is also regulated by the cyclic-AMP response element binding protein (CREB) and Sp1. We show by DNaseI protection and EMSA analysis that CREB and Sp1 interact with specific sites within the SOX9 proximal promoter region. By transient transfection analysis we also demonstrate that mutations of the CREB and Sp1 binding sites result in a profound reduction of SOX9 promoter activity. Chromatin immunoprecipitation (ChIP) assay demonstrated that both Sp1 and CREB interact with the SOX9 promoter in vivo. Finally, we demonstrate that IL-1beta treatment of chondrocytes isolated from human normal and osteoarthritic (OA) cartilage down-regulates SOX9 promoter activity, an effect accompanied by a reduction of Sp1 binding to the SOX9 proximal promoter.

Constitutive expression of murine decay-accelerating factor 1 is controlled by the transcription factor Sp1

The complement regulatory protein decay-accelerating factor (DAF or CD55) protects host tissue from complement-mediated injury by inhibiting the classical and alternative complement pathways. Besides its role in complement regulation, DAF has also been shown to be a key player in T cell immunity. Modulation of DAF expression could therefore represent a critical regulatory mechanism in both innate and adaptive immune responses. To identify and characterize key transcriptional regulatory elements controlling mouse Daf1 expression, a 2.5-kb fragment corresponding to the 5' flanking region of the mouse Daf1 gene was cloned. Sequence analysis showed that the mouse Daf1 promoter lacks conventional TATA and CCAAT boxes and displays a high guanine and cytosine content. RACE was used to identify one major and two minor transcription start sites 47, 20, and 17 bp upstream of the translational codon. Positive and negative regulatory regions were identified by transiently transfecting sequential 5'deletion constructs of the 5'flanking region into NIH/3T3, M12.4, and RAW264.7 cells. Mutational analyses of the promoter region combined with Sp1-specific ELISA showed that the transcription factor Sp1 is required for basal transcription and LPS-induced expression of the Daf1 gene. These findings provide new information on the regulation of the mouse Daf1 promoter and will facilitate further studies on the expression of Daf1 during immune responses.

Epigenetic Regulation during B Cell Differentiation Controls CIITA Promoter Accessibility

B cell to plasma cell maturation is marked by the loss of MHC class II expression. This loss is due to the silencing of the MHC class II transcriptional coactivator CIITA. In this study, experiments to identify the molecular mechanism responsible for CIITA silencing were conducted. CIITA is expressed from four promoters in humans, of which promoter III (pIII) controls the majority of B cell-mediated expression. Chromatin immunoprecipitation assays were used to establish the histone code for pIII and determine the differences between B cells and plasma cells. Specific histone modifications associated with accessible promoters and transcriptionally active genes were observed at pIII in B cells but not in plasma cells. A reciprocal exchange of histone H3 lysine 9 acetylation to methylation was also observed between B cells and plasma cells. The lack of histone acetylation correlated with an absence of transcription factor binding to pIII, particularly that of Sp1, PU.1, CREB, and E47. Intriguingly, changes in chromatin architecture of the 13-kb region encompassing all CIITA promoters showed a remarkable deficit in histone H3 and H4 acetylation in plasma cells, suggesting that the mechanism of silencing is global. When primary B cells were differentiated ex vivo, most of the histone marks associated with pIII activation and expression were lost within 24 h. The results demonstrate that CIITA silencing occurs by controlling chromatin accessibility through a multistep mechanism that includes the loss of histone acetylation and transcription factor binding, and the acquisition of repressive histone methylation marks.

Cloning and expression of a novel CREB mRNA splice variant in human testis

Identification of genes specifically expressed in adult and fetal testis is important in furthering our understanding of testis development and function. In this study, a novel human transcript, designated human testis cAMP-responsive element-binding protein (htCREB), was identified by hybridization of adult and fetal human testis cDNA probes with a human cDNA microarray containing 9216 clones. The htCREB transcript (GenBank Accession no. AY347527) was expressed at 2.35-fold higher levels in adult human testes than in fetal testes. Sequence and ntBLAST analyses against the human genome database indicated that htCREB was a novel splice variant of human CREB. RT-PCR-based tissue distribution experiments demonstrated that the htCREB transcript was highly expressed in adult human testis and in healthy sperm, but not in testes from patients with Sertoli cell-only syndrome. Taken together, these results suggest that the htCREB transcript is chiefly expressed in germ cells and is most likely involved in spermatogenesis.

Synergistic activation of CREB-mediated transcription by forskolin and phorbol ester requires PKC and depends on the glutamine-rich Q2 transactivation domain

Recruitment of a RNA polymerase II complex by the glutamine-rich Q2 domain of cAMP response element-binding protein (CREB) allows basal transcriptional activity, while recruitment of CBP/p300 through signal-induced phosphorylation of the kinase-inducible domain at serine-133 enhances CREB-dependent transcription. Here we demonstrate that co-administration of forskolin and phorbol ester TPA to NIH3T3 cells provoked a dose-dependent increase in phosphoserine-133. CREB- and Q2-dependent transcription, as well as transcription by other glutamine-rich transcription factors, but not by transcription factors lacking glutamine-rich regions, augmented synergistically in the presence of both stimuli. Synergistic activation was abograted by specific inhibition of protein kinase C (PKC), but not of PKA. Co-stimulation increased the basal activity of a minimal, CREB-independent promoter. Therefore, Q2, which directly interacts with the RNA polymerase II initiation complex, may transmit the increased basal promoter activity provoked by these stimuli to CREB, thereby contributing to synergistic activation of CREB-mediated transcription. This synergism may have important implications on glutamine-rich transcription factor-target genes.

Tyrosine hydroxylase transcription depends primarily on cAMP response element activity, regardless of the type of inducing stimulus

In neurons and neuroendocrine cells, tyrosine hydroxylase (TH) gene expression is induced by stimuli that elevate cAMP, by depolarization, and by hypoxia. Using these stimuli, we examined TH promoter mutants, cAMP response element binding protein (CREB) phosphorylation site mutants, and transcriptional interference with dominant negative transcription factors to assess the relative contributions of CREB/AP-1 family members to the regulation of basal and inducible TH transcription in PC12 cells. We found that basal transcription depends on transcription factor activity at the partial dyad (-17 bp), CRE (-45 bp), and AP1 (-205 bp) elements. Induced transcription is regulated primarily by activity at the CRE, with only small contributions from the AP1 or hypoxia response element 1 (HRE1; -225 bp) elements, regardless of inducing stimulus. CREB, ATF-1, and CREMtau all mediate CRE-dependent transcription, with CREB and CREMtau being more effective than ATF-1. Phosphorylation of CREB on Ser133, but not on Ser142 or Ser143, is required for induced transcription, regardless of inducing stimulus.

Transcriptional regulation of cyclooxygenase 2 by bradykinin and interleukin-1beta in human airway smooth muscle cells: involvement of different promoter elements, transcription factors, and histone h4 acetylation

Bradykinin and interleukin-1beta (IL-1beta) induce cyclooxygenase 2 (COX-2) in human airway smooth muscle cells. Here we extended our study to explore the gene transcriptional regulation. By transfection with various COX-2 promoter reporter constructs, we found that the bp -327-to-+59 promoter region was essential for COX-2 gene transcription by bradykinin and IL-1beta and that the cyclic AMP response element (CRE) was critical in bradykinin-induced transcription, whereas nuclear factor IL-6 and CRE and, to a lesser extent, nuclear factor-kappaB (NF-kappaB) were involved in IL-1beta-induced transcription. An electrophoretic mobility shift assay revealed that both bradykinin and IL-1beta elicited CRE-binding protein-1 (CREB-1) binding, and IL-1beta also elicited CCAAT/enhancer-binding protein beta and NF-kappaB binding to their respective elements in the COX-2 promoter. These transcription factors were associated with the COX-2 promoter, which was dynamically linked to different patterns of histone H4 acetylation by bradykinin and IL-1beta, as demonstrated by chromatin immunoprecipitation. We also revealed that endogenous prostaglandin E(2) was critical in bradykinin-induced COX-2 transcription initiation and involved in IL-1beta-induced COX-2 transcription at a latter stage. Our result provide the first evidence that COX-2 transcriptional regulation by different stimuli involves different promoter elements and transcription factors and is associated with chromatin remodeling after selective histone H4 acetylation in a stimulus-specific manner.

Genome-wide analysis of CREB target genes reveals a core promoter requirement for cAMP responsiveness

We have employed a hidden Markov model (HMM) based on known cAMP responsive elements to search for putative CREB target genes. The best scoring sites were positionally conserved between mouse and human orthologs, suggesting that this parameter can be used to enrich for true CREB targets. Target validation experiments revealed a core promoter requirement for transcriptional induction via CREB; TATA-less promoters were unresponsive to cAMP compared to TATA-containing genes, despite comparable binding of CREB to both sets of genes in vivo. Indeed, insertion of a TATA box motif rescued cAMP responsiveness on a TATA-less promoter. These results illustrate a mechanism by which subsets of target genes for a transcription factor are differentially regulated depending on core promoter configuration.

Attenuation of a phosphorylation-dependent activator by an HDAC-PP1 complex

The second messenger cAMP stimulates transcription with burst-attenuation kinetics that mirror the PKA-dependent phosphorylation and subsequent protein phosphatase 1 (PP1)-mediated dephosphorylation of the cAMP responsive element binding protein (CREB) at Ser133. Phosphorylation of Ser133 promotes recruitment of the co-activator histone acetylase (HAT) paralogs CBP and P300, which in turn stimulate acetylation of promoter-bound histones during the burst phase. Remarkably, histone deacetylase (HDAC) inhibitors seem to potentiate CREB activity by prolonging Ser133 phosphorylation in response to cAMP stimulus, suggesting a potential role for HDAC complexes in silencing CREB activity. Here we show that HDAC1 associates with and blocks Ser133 phosphorylation of CREB during pre-stimulus and attenuation phases of the cAMP response. HDAC1 promotes Ser133 dephosphorylation via a stable interaction with PP1, which we detected in co-immunoprecipitation and co-purification studies. These results illustrate a novel mechanism by which signaling and chromatin-modifying activities act coordinately to repress the activity of a phosphorylation-dependent activator.

Oct-1 potentiates CREB-driven cyclin D1 promoter activation via a phospho-CREB- and CREB binding protein-independent mechanism

Cyclin D1, the regulatory subunit for mid-G(1) cyclin-dependent kinases, controls the expression of numerous cell cycle genes. A cyclic AMP-responsive element (CRE), located upstream of the cyclin D1 mRNA start site, integrates mitogenic signals that target the CRE-binding factor CREB, which can recruit the transcriptional coactivator CREB-binding protein (CBP). We describe an alternative mechanism for CREB-driven cyclin D1 induction that involves the ubiquitous POU domain protein Oct-1. In the breast cancer cell line MCF-7, overexpression of Oct-1 or its POU domain strongly increases transcriptional activation of cyclin D1 and GAL4 reporter genes that is specifically dependent upon CREB but independent of Oct-1 DNA binding. Gel retardation and chromatin immunoprecipitation assays confirm that POU forms a complex with CREB bound to the cyclin D1 CRE. In solution, CREB interaction with POU requires the CREB Q2 domain and, notably, occurs with CREB that is not phosphorylated on Ser 133. Accordingly, Oct-1 also potently enhances transcriptional activation mediated by a Ser133Ala CREB mutant. Oct-1/CREB synergy is not diminished by the adenovirus E1A 12S protein, a repressor of CBP coactivator function. In contrast, E1A strongly represses CBP-enhanced transactivation by CREB phosphorylated on Ser 133. Our observation that Oct-1 potentiates CREB-dependent cyclin D1 transcriptional activity independently of Ser 133 phosphorylation and E1A-sensitive coactivator function offers a new paradigm for the regulation of cyclin D1 induction by proliferative signals.

Mechanisms of basal and kinase-inducible transcription activation by CREB

The cAMP response element (CRE)-binding protein (CREB) stimulates basal transcription of CRE-containing genes and mediates induction of transcription upon phosphorylation by protein kinases. The basal activity of CREB maps to a carboxy-terminal constitutive activation domain (CAD), whereas phosphorylation and inducibility map to a central, kinase-inducible domain (KID). The CAD interacts with and recruits the promoter recognition factor TFIID through an interaction with a specific TATA-binding-protein-associated factor (TAF), dTAFII110/ hTAFII135. Interaction between the TAF and the CAD is mediated by a central cluster of hydrophobic amino acids, mutation of which disrupts TAF binding, polymerase recruitment, and transcription activation. Assessment of the contributions of the CAD and KID to recruitment of the polymerase complex versus enhancement of subsequent reaction steps (isomerization, promoter clearance, and reinitiation) showed that the CAD and P-KID act in a concerted mechanism to stimulate transcription. The CAD, but not the KID, mediated recruitment of a complex containing components of a transcription initiation complex, including pol II, IIB, and IID. However, the CAD was relatively ineffective in stimulating subsequent steps in the reaction mechanism. In contrast, phosphorylation of the KID in CREB effectively stimulated isomerization of the recruited polymerase complex and multiple-round transcription. A model for the activation of transcription by phosphorylated CREB is proposed, in which the polymerase is recruited by interaction of the CAD with TFIID and the recruited polymerase is activated further by phosphorylation of the KID in CREB.

Function and regulation of CREB family transcription factors in the nervous system

CREB and its close relatives are now widely accepted as prototypical stimulus-inducible transcription factors. In many cell types, these factors function as effector molecules that bring about cellular changes in response to discrete sets of instructions. In neurons, a wide range of extracellular stimuli are capable of activating CREB family members, and CREB-dependent gene expression has been implicated in complex and diverse processes ranging from development to plasticity to disease. In this review, we focus on the current level of understanding of where, when, and how CREB family members function in the nervous system.

Salt-inducible kinase represses cAMP-dependent protein kinase-mediated activation of human cholesterol side chain cleavage cytochrome P450 promoter through the CREB basic leucine zipper domain

Salt-inducible kinase (SIK), one of the serine/threonine protein kinases, was transiently expressed in Y1 cells during the early phase of the ACTH/cAMP-dependent protein kinase (PKA)-mediated signal transduction. The overexpression of SIK(N), the SIK's N-terminal kinase domain, repressed the expression of the side chain cleavage cytochrome P450 (CYP11A) gene. To elucidate the mechanism of the repression by SIK, several CYP11A promoter constructs were tested for the promoter activities in the presence of PKA and/or SIK(N). A cAMP-response element (CRE)-like sequence present in the promoter was shown to be responsible not only for the PKA-mediated promoter activation but also for the SIK(N)-mediated repression. When the Gal4 DNA binding domain-linked full-length CRE-binding protein (CREB) construct was cotransfected with Gal4 reporter gene, SIK(N) repressed the PKA-induced reporter gene expression. However, SIK(N) could not repress the PKA-induced reporter activity conferred by Gal4 DNA binding domain-linked basic leucine zipper (bZIP)-less CREB or bZIP-disrupted CREB. On the other hand, SIK(N) could repress the kinase-inducible domain-disrupted CREB-dependent reporter gene expression in the presence of PKA. The in vitro kinase reaction studies showed that SIK(N) could not phosphorylate CREB, and PKA failed to phosphorylate SIK(N). Taken together, these results suggest that SIK(N), cooperating with PKA, may act on the CREB's bZIP domain and repress the CREB-mediated transcriptional activation of the CYP11A gene.

Chromatin-dependent cooperativity between constitutive and inducible activation domains in CREB

The cyclic AMP (cAMP)-responsive factor CREB induces target gene expression via constitutive (Q2) and inducible (KID, for kinase-inducible domain) activation domains that function synergistically in response to cellular signals. KID stimulates transcription via a phospho (Ser133)-dependent interaction with the coactivator paralogs CREB binding protein and p300, whereas Q2 recruits the TFIID complex via a direct association with hTAF(II)130. Here we investigate the mechanism underlying cooperativity between the Q2 domain and KID in CREB by in vitro transcription assay with naked DNA and chromatin templates containing the cAMP-responsive somatostatin promoter. The Q2 domain was highly active on a naked DNA template, and Ser133 phosphorylation had no additional effect on transcriptional initiation in crude extracts. Q2 activity was repressed on a chromatin template, however, and this repression was relieved by the phospho (Ser133) KID-dependent recruitment of p300 histone acetyltransferase activity to the promoter. In chromatin immunoprecipitation assays of NIH 3T3 cells, cAMP-dependent recruitment of p300 to the somatostatin promoter stimulated acetylation of histone H4. Correspondingly, overexpression of hTAFII130 potentiated CREB activity in cells exposed to cAMP, but had no effect on reporter gene expression in unstimulated cells. We propose that cooperativity between the KID and Q2 domains proceeds via a chromatin-dependent mechanism in which recruitment of p300 facilitates subsequent interaction of CREB with TFIID.

The coactivator dTAF(II)110/hTAF(II)135 is sufficient to recruit a polymerase complex and activate basal transcription mediated by CREB

A specific TATA binding protein-associated factor (TAF), dTAF(II)110/hTAF(II)135, interacts with cAMP response element binding protein (CREB) through its constitutive activation domain (CAD), which recruits a polymerase complex and activates transcription. The simplest explanation is that the TAF is a coactivator, but several studies have questioned this role of TAFs. Using a reverse two-hybrid analysis in yeast, we previously mapped the interaction between dTAF(II)110 (amino acid 1-308) and CREB to conserved hydrophobic amino acid residues in the CAD. That mapping was possible only because CREB fails to activate transcription in yeast, where all TAFs are conserved, except for the TAF recognizing CREB. To test whether CREB fails to activate transcription in yeast because it lacks a coactivator, we fused dTAF(II)110 (amino acid 1-308) to the TATA binding protein domain of the yeast scaffolding TAF, yTAF(II)130. Transformation of yeast with this hybrid TAF conferred activation by the CAD, indicating that interaction with yTFIID is sufficient to recruit a polymerase complex and activate transcription. The hybrid TAF did not mediate activation by VP16 or vitamin D receptor, each of which interacts with TFIIB, but not with dTAF(II)110 (amino acid 1-308). Enhancement of transcription activation by dTAF(II)110 in mammalian cells required interaction with both the CAD and TFIID and was inhibited by mutation of core hydrophobic residues in the CAD. These data demonstrate that dTAF(II)110/hTAF(II)135 acts as a coactivator to recruit TFIID and polymerase and that this mechanism of activation is conserved in eukaryotes.

Regulation of cAMP-responsive element-binding protein-mediated transcription by the SNF2/SWI-related protein, SRCAP

SRCAP (SNF2-related CPB activator protein) belongs to the SNF2 family of proteins whose members participate in various aspects of transcriptional regulation, including chromatin remodeling. It was identified by its ability to bind to cAMP-responsive-binding protein (CREB)-binding protein (CBP), and it increases the transactivation function of CBP. The phosphoenolpyruvate carboxykinase (PEPCK) promoter was used as a model system to explore the role of SRCAP in the regulation of transcription mediated by factors that utilize CBP as a coactivator. We show that transcription of a PEPCK chloramphenicol acetyltransferase (CAT) reporter gene activated by protein kinase A (PKA) is enhanced 7-fold by SRCAP. In the absence of PKA this SRCAP-mediated enhancement does not occur, suggesting that SRCAP functions as a coactivator for PKA-activated factors such as CREB. Replacing the PEPCK promoter binding site for CREB with a binding site for Gal4 (DeltaCRE (cAMP-responsive element) Gal4 PEPCK-CAT reporter gene) blocks the ability of SRCAP to activate transcription despite the presence of PKA. Expression of a Gal-CREB chimera restores the ability of PKA to regulate transcription of the DeltaCRE Gal4 PEPCK gene and restored the ability of SRCAP to stimulate PKA-activated transcription. In addition, SRCAP in the presence of PKA enhances the ability of the Gal-CREB chimera to activate transcription of a Gal-CAT reporter gene that contains only binding sites for Gal4. SRCAP binds to CBP amino acids 280-460, a region that is important for CBP to function as a coactivator for CREB. Overexpression of a SRCAP peptide corresponding to this CBP binding domain acts as a dominant negative inhibitor of CREB-mediated transcription. Structure-function studies were done to explore the mechanism(s) by which SRCAP regulates transcription. These studies indicate that the N-terminal region of SRCAP, which contains five of the seven regions that comprise the ATPase domain, is not needed for activation of CREB-mediated transcription. SRCAP apparently has several domains that participate in the activation of transcription.

Activation of extracellular signal-regulated kinases and CREB/ATF-1 mediate the expression of CCAAT/enhancer binding proteins beta and -delta in preadipocytes

The essential role of CCAAT/enhancer binding proteins (C/EBPs) beta and delta for adipocyte differentiation has been clearly established. In preadipocytes, their expression is up-regulated by the activation of leukemia inhibitory factor receptor (LIF-R) and prostacyclin receptor (IP-R) via the extracellular signal-regulated kinase (ERK) pathway and cAMP production, respectively. However, the molecular mechanisms by which LIF and prostacyclin-induced signals are propagated to the nucleus and the transcription factors mediating ERK and cAMP-induced C/EBP gene expression were unknown. Here we report that both pathways share cAMP responsive element binding protein/activation transcription factor 1 (CREB/ATF-1) as common downstream effectors. LIF-R and IP-R activation induced binding of CREB and/or ATF-1 to C/EBP promoters and CREB-dependent transcription. Expression of dominant negative forms of CREB dramatically reduced the LIF- and prostacyclin-stimulated C/EBP beta and C/EBP delta expression. Upon stimulation of the IP-R, the ERK pathway was activated in a PKA-dependent manner. ERK activation by the PKA pathway was not required for CREB/ATF-1 phosphorylation but rather was necessary for CREB-dependent up-regulation of C/EBPs expression. Our findings suggest that ERK activation is required for CREB transcriptional activity, possibly by recruitment of a coactivator.

Transcriptional regulation by the phosphorylation-dependent factor CREB

The transcription factor CREB -- for 'cyclic AMP response element-binding protein' -- functions in glucose homeostasis, growth-factor-dependent cell survival, and has been implicated in learning and memory. CREB is phosphorylated in response to various signals, but how is specificity achieved in these signalling pathways?

Protein kinase A phosphorylates hepatocyte nuclear factor-6 and stimulates glucose-6-phosphatase catalytic subunit gene transcription

Glucose-6-phosphatase is a multicomponent system that catalyzes the terminal step in gluconeogenesis. To examine the effect of the cAMP signal transduction pathway on expression of the gene encoding the mouse glucose-6-phosphatase catalytic subunit (G6Pase), the liver-derived HepG2 cell line was transiently co-transfected with a series of G6Pase-chloramphenicol acetyltransferase fusion genes and an expression vector encoding the catalytic subunit of cAMP-dependent protein kinase A (PKA). PKA markedly stimulated G6Pase-chloramphenicol acetyltransferase fusion gene expression, and mutational analysis of the G6Pase promoter revealed that multiple cis-acting elements were required for this response. One of these elements was mapped to the G6Pase promoter region between -114 and -99, and this sequence was shown to bind hepatocyte nuclear factor (HNF)-6. This HNF-6 binding site was able to confer a stimulatory effect of PKA on the expression of a heterologous fusion gene; a mutation that abolished HNF-6 binding also abolished the stimulatory effect of PKA. Further investigation revealed that PKA phosphorylated HNF-6 in vitro. Site-directed mutation of three consensus PKA phosphorylation sites in the HNF-6 carboxyl terminus markedly reduced this phosphorylation. These results suggest that the stimulatory effect of PKA on G6Pase fusion gene transcription in HepG2 cells may be mediated in part by the phosphorylation of HNF-6.