Acetylation of cAMP-responsive element-binding protein (CREB) by CREB-binding protein enhances CREB-dependent transcription

Transcriptional repression and DNA looping associated with a novel regulatory element in the final exon of the lymphotoxin-β gene

Transcriptional regulation has a critical role in the coordinate and context-specific expression of a cluster of genes encoding members of the tumour necrosis factor (TNF) superfamily found at chromosome 6p21, comprising TNF, LTA (encoding lymphotoxin-α) and LTB (encoding lymphotoxin-β). This is important, as dysregulated expression of these genes is implicated in susceptibility to many autoimmune, inflammatory and infectious diseases. We describe here a novel regulatory element in the fourth exon of LTB, which is highly conserved, localises to the only CpG island in the locus, and is associated with a DNase I hypersensitive site and specific histone modifications. We find evidence of binding by Yin Yang 1 (YY1), cyclic AMP response element (CRE)-binding protein (CREB) and CCCTC-binding factor (CTCF) to this region in Jurkat T cells, which is associated with transcriptional repression on reporter gene analysis. Chromatin conformation capture experiments show evidence of DNA looping, involving interaction of this element with the LTB promoter, LTA promoter and TNF 3′ untranslated region (UTR). Small interfering RNA (siRNA) experiments demonstrate a functional role for YY1 and CREB in LTB expression. Our findings provide evidence of additional complexity in the transcriptional regulation of LTB with implications for coordinate expression of genes in this important genomic locus.

Wnt/β-catenin signaling is stimulated by α-melanocyte-stimulating hormone in melanoma and melanocyte cells: implication in cell differentiation

Wnt/β-catenin signaling plays important roles in many developmental processes including neural crest-derived melanocyte development and migration. However, the effective contribution of Wnt/β-catenin pathway in melanogenesis in adult human melanocytes has not been fully elucidated. Here, we report that in melanoma cells and in normal human melanocytes, melanogenesis stimulation by α-melanocyte-stimulating hormone (α-MSH) induces phosphorylation of β-catenin-Ser675 and stabilization of β-catenin protein. Activation of protein kinase A by α-MSH attenuates glycogen synthase kinase-3β, which regulates ubiquitin-dependent degradation of β-catenin, suggesting a coordinated mechanism of β-catenin activity stimulation. Consistent with increased nuclear β-catenin, cyclic adenosine monophosphate (cAMP) elevation facilitates β-catenin-dependent transactivation of many Wnt target genes. Moreover, chromatin immunoprecipitation assays demonstrated an increased association of β-catenin with the proximal promoter of microphthalmia-associated transcription factor, the master regulator of pigmentation. These results demonstrate the existence of cross talk between the cAMP and Wnt pathways in melanocytes, suggesting that β-catenin could play a key role in the physiological regulation of epidermal melanogenesis.

CREB: a multifaceted regulator of neuronal plasticity and protection

Since its initial characterization over 20 years ago, there has been intense and unwavering interest in understanding the role of the transcription factor cAMP-responsive element binding protein (CREB) in nervous system physiology. Through an array of experimental approaches and model systems, researchers have begun to unravel the complex and multifaceted role of this transcription factor in such diverse processes as neurodevelopment, synaptic plasticity, and neuroprotection. Here we discuss current insights into the molecular mechanisms by which CREB couples synaptic activity to long-term changes in neuronal plasticity, which is thought to underlie learning and memory. We also discuss work showing that CREB is a critical component of the neuroprotective transcriptional network, and data indicating that CREB dysregulation contributes to an array of neuropathological conditions.

Histone deacetylases and mood disorders: epigenetic programming in gene-environment interactions

Epigenetics involves molecular mechanisms related to gene expression independent of DNA sequence, mostly mediated by modification of chromatin histones. It has recently been suggested that these transcriptional changes may be implicated in the pathophysiology of mood disorders. In addition, histone deacetylase (HDAC) inhibitors have been shown to control epigenetic programming associated with the regulation of cognition and behavior, and may reverse dysfunctional epigenetic regulation associated with early life events in preclinical models. In this context, the active and continuous adaptation of chromatin, and the access of gene promoters to transcription factor mechanisms may represent a potential therapeutic target in the treatment of mood disorders such as bipolar disorder (BD) and major depressive disorder (MDD). Notably, the standard mood stabilizer valproate (VPA) has been shown to modulate the epigenome by inhibiting HDACs. However, several potential limitations are associated with this class of agents, including lack of selectivity for specific HDAC isoforms as well as risk of potentially serious side effects. Further studies regarding the potential role of chromatin remodeling in the mechanism of action of antidepressants and mood stabilizers are necessary to clarify the potential role of this class of agents as therapeutics for mood disorders.

Protein acetylation in synaptic plasticity and memory

Posttranslational modifications of proteins regulate various processes in the cells. The seminal role of phosphorylation in synaptic plasticity and memory has been established using several different model systems. Recently, an important role for another posttranslational modification, acetylation, particularly of histones, has emerged in these processes. This review focuses on the role of activity-dependent protein acetylation in synaptic plasticity and memory.

Putting the 'HAT' back on survival signalling: the promises and challenges of HDAC inhibition in the treatment of neurological conditions

Decreased histone acetyltransferase activity and transcriptional dysfunction have been implicated in almost all neurodegenerative conditions. Increasing net histone acetyltransferase activity through inhibition of the histone deacetylases (HDACs) has been shown to be an effective strategy to delay or halt progression of neurological disease in cellular and rodent models. These findings have provided firm rationale for Phase I and Phase II clinical trials of HDAC inhibitors in Huntington's disease, spinal muscular atrophy, and Freidreich's ataxia. In this review, we discuss the current findings and promise of HDAC inhibition as a strategy for treating neurological disorders. Despite the fact that HDAC inhibitors are in an advanced stage of development, we suggest other approaches to modulating HDAC function that may be less toxic and more efficacious than the canonical agents developed so far.

Peroxisome proliferator-activated receptor alpha activates cyclooxygenase-2 gene transcription through bile acid transport in human colorectal cancer cell lines

BACKGROUND

Evidence is accumulating that bile acids are involved in colon cancer development, but their molecular mechanisms remain unexplored. Bile acid has been reported to be associated with induction of the cyclooxygenase-2 (COX-2) gene. Because the human liver-specific organic anion transporter-2 (LST-2/OATP8/OATP1B3) is expressed in gastrointestinal cancers and might transport bile acids to the intracellular space, we studied the molecular mechanisms by which bile acids induce the transcription of COX-2, and the role of LST-2 in colonic cell lines.

METHODS

Transcriptional activity of COX-2 was measured using a human COX-2 promoter-luciferase assay under various concentrations of bile acids. Electrophoresis mobility shift assays (EMSAs) for peroxisome proliferators-activated receptor (PPAR) alpha and cyclic AMP responsive element (CRE) were performed.

RESULTS

The COX-2 promoter was induced by lithocholic acid (LCA), deoxycholic acid (DCA), and chenodeoxycholic acid (CDCA). Deletion and site-directed mutation analyses showed that CRE is the responsive element for LCA. An adenovirus expression system revealed that LST-2 is responsible for induction of COX-2. By EMSA using oligonucleotides of CRE, we observed formation of a specific protein-DNA complex, which was inhibited by a specific antibody against PPARalpha and CRE. A PPARalpha-specific agonist induced transcription of COX-2.

CONCLUSION

These results indicate that COX-2 is transcriptionally activated by the addition of LCA, CDCA, and DCA and that LST-2 plays an important role by transporting bile acid to the intracellular space. Moreover, LCA-dependent COX-2 gene activation consists of a transcriptional complex including PPARalpha and CRE-binding protein. Thus, this induction of COX-2 may participate in carcinogenesis and progression of colorectal cancer cells.

Acetylation and deacetylation regulate CCAAT/enhancer binding protein beta at K39 in mediating gene transcription

The transcription factor CCAAT/enhancer binding protein beta (C/EBPbeta) contains multiple acetylation sites, including lysine (K) 39. Mutation of C/EBPbeta at K39, an acetylation site in the transcriptional activation domain, impairs transcription of C/EBPbeta target genes in a dominant-negative fashion. Further, K39 of C/EBPbeta can be deacetylated by HDAC1, and HDAC1 may decrease C/EBPbeta-mediated transcription, suggesting that acetylation of C/EBPbeta at K39 is dynamically regulated in mediating gene transcription. Acetylation of endogenous C/EBPbeta at K39 is detected in adipose tissue, and also occurs in 3T3-L1 cells undergoing adipocyte conversion. In addition, mutation of K39 in C/EBPbeta impairs activation of its target genes encoding C/EBPalpha and PPARgamma, essential mediators of adipogenesis, as well as adipocyte genes for leptin and Glut4. These findings suggest that acetylation of C/EBPbeta at K39 is an important and dynamic regulatory event that contributes to its ability to transactivate target genes, including those associated with adipogenesis and adipocyte function.

Beyond transcription factors: the role of chromatin modifying enzymes in regulating transcription required for memory

One of the alluring aspects of examining chromatin modifications in the role of modulating transcription required for long-term memory processes is that these modifications may provide transient and potentially stable epigenetic marks in the service of activating and/or maintaining transcriptional processes. These, in turn, may ultimately participate in the molecular mechanisms required for neuronal changes subserving long-lasting changes in behavior. As an epigenetic mechanism of transcriptional control, chromatin modification has been shown to participate in maintaining cellular memory (e.g., cell fate) and may underlie the strengthening and maintenance of synaptic connections required for long-term changes in behavior. Epigenetics has become central to several fields of neurobiology, where researchers have found that regulation of chromatin modification has a significant role in epilepsy, drug addiction, depression, neurodegenerative diseases, and memory. In this review, we will discuss the role of chromatin modifying enzymes in memory processes, as well as how recent studies in yeast genetics and cancer biology may impact the way we think about how chromatin modification and chromatin remodeling regulate neuronal function.

Regulation of arginase-1 expression in macrophages by a protein kinase A type I and histone deacetylase dependent pathway

The aim of the current study was to investigate the cAMP-dependent regulation of arginase-1 (ARG1) expression in RAW-macrophages. Basal ARG1 mRNA expression was low and increased upon incubation with the cAMP analogue Br-cAMP. We used selective agonists of protein kinase A type I (PKAI), type II (PKAII) and exchange protein directly activated by cAMP (EPAC) to determine the pathway responsible for ARG1 expression. Activation of PKAI led to a significant up-regulation of ARG1 mRNA expression and arginase enzyme activity. In contrast, neither activation of PKAII nor activation of EPAC affected ARG1 expression. In addition, it has been shown that histone deacetylase (HDAC) activity plays a critical role in cAMP-dependent transcriptional regulation. Incubation with Br-cAMP and the HDAC inhibitor trichostatin A (TSA) led to a concentration-dependent suppression of ARG1 expression. These data indicate that cAMP-dependent activation of ARG1 expression is mediated by PKAI and requires histone deacetylation.

Acetylation of histone H3 and adrenergic-regulated gene transcription in rat pinealocytes

In this study we investigated the effect of histone acetylation on the transcription of adrenergic-induced genes in rat pinealocytes. We found that treatment of pinealocytes with trichostatin A (TSA), a histone deacetylase inhibitor, caused hyperacetylation of histone H3 (H3) Lys14 at nanomolar concentrations. Hyperacetylation of H3 was also observed after treatment with scriptaid, a structurally unrelated histone deacetylase inhibitor. The effects of TSA and scriptaid were inhibitory on the adrenergic induction of arylalkylamine-n-acetyltransferase (aa-nat) mRNA, protein, and enzyme activity, and on melatonin production. TSA at higher concentrations also inhibited the adrenergic induction of mapk phosphatase-1 (mkp-1) and inducible cAMP early repressor mRNAs. In contrast, the effect of TSA on the norepinephrine induction of the c-fos mRNA was stimulatory. Moreover, the effect of TSA on adrenergic-induced gene transcription was dependent on the time of its addition; its effect was only observed during the active phase of transcription. Chromatin immunoprecipitation with antibodies against acetylated Lys14 of H3 showed an increase in DNA recovery of the promoter regions of aa-nat, mkp-1, and c-fos after treatment with TSA. Together, our results demonstrate that histone acetylation differentially influences the transcription of adrenergic-induced genes, an enhancing effect for c-fos but inhibitory for aa-nat, mkp-1, and inducible cAMP early repressor. Moreover, both inhibitory and enhancing effects appear to be mediated through specific modification of promoter-bound histones during gene transcription.

Assays for pharmacodynamic analysis of histone deacetylase inhibitors

Histone deacetylase inhibitors (HDACIs) are a promising new class of targeted anticancer drugs. The pharmacodynamic (PD) assessment of whether a drug has hit its target is critically important to the successful development of any molecular targeted therapy. In the case of HDACIs there are many issues to be considered in PD development and implementation. Although originally it was thought that measurement of core histone hyperacetylation in peripheral blood mononuclear cells might suffice as a PD marker, as the field is evolving it is becoming evident that other measures may be essential, and are likely to be tumour context specific. This paper provides an overview of the assays that have been performed thus far in HDACI clinical trials, with an analysis of relative strengths and weaknesses, and a delineation of the complexity of HDACI PD analysis. Consideration is given to where new approaches are needed and potential approaches for future monotherapy and combination therapy trials are suggested.

CCAAT/enhancer-binding protein (C/EBP) beta is acetylated at multiple lysines: acetylation of C/EBPbeta at lysine 39 modulates its ability to activate transcription

Transcription factor function can be modulated by post-translational modifications. Because the transcription factor CCAAT/enhancer-binding protein (C/EBP) beta associates with the nuclear coactivator p300, which contains acetyltransferase activity, acetylation of C/EBPbeta was examined to understand its regulation and function. C/EBPbeta is acetylated by acetyltransferases p300 and p300/CREB-binding protein associated factor. Endogenous C/EBPbeta in 3T3-F442A preadipocytes is also recognized by an acetyl-lysine-specific antibody. Analysis of truncations of C/EBPbeta and peptides based on C/EBPbeta sequences identified multiple lysines within C/EBPbeta that can be acetylated. Among these, a novel acetylation site at lysine 39 of C/EBPbeta was identified. Mutation of Lys-39 to arginine or alanine impairs its acetylation and the ability of C/EBPbeta to activate transcription at the promoters for C/EBPalpha and c-fos. Different C/EBPbeta-responsive promoters require different patterns of acetylated lysines in C/EBPbeta for transcription activation. Furthermore, C/EBPbeta acetylation was increased by growth hormone, and mutation of Lys-39 impaired growth hormone-stimulated c-fos promoter activation. These data suggest that acetylation of Lys-39 of C/EBPbeta, alone or in combination with acetylation at other lysines, may play a role in C/EBPbeta-mediated transcriptional activation.

Acetylsalicylic acid accelerates the antidepressant effect of fluoxetine in the chronic escape deficit model of depression

Evidence has accumulated suggesting that major depression is associated with dysfunction of inflammatory mediators. Moreover, antidepressants show an anti-inflammatory action possibly related to their clinical efficacy. An improvement in psychiatric symptoms has been recently reported in patients treated with anti-inflammatory drugs for other indications. These data imply that inflammation may be involved in the pathogenesis of depression and that anti-inflammatory drugs may be used as an adjunctive therapy. The aim of the present study was to evaluate the behavioural effect of the co-administration of acetylsalicylic acid (ASA, 45 mg/kg or 22.5 mg/kg) and fluoxetine (FLX, 5 mg/kg) in the chronic escape deficit model of depression. The chronic escape deficit model is based on the modified reactivity of rats to external stimuli induced by exposure to unavoidable stress and allows evaluation of the capacity of a treatment to revert the condition of escape deficit. In this model, FLX alone needs to be administered for at least 3 weeks to revert this condition. Our results show that combined treatment of fluoxetine and ASA completely reverted the condition of escape deficit by as early as 7 days, the effect being already partially present after 4 days. The effect was maintained after 14 and 21 days of treatment. ASA alone was ineffective at any time tested and the effect of fluoxetine was significant only at 21 days. These results, together with clinical data from preliminary results, suggest that ASA might accelerate the onset of action of selective serotonin reuptake inhibitor antidepressants.

Regulation of CRE-dependent transcription by presenilins: prospects for therapy of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder and is characterized by memory loss and other cognitive disabilities. Mutations in the presenilin genes are the major cause of familial AD. Analysis of conditional knockout mice has shown that inactivation of presenilins results in progressive memory impairment and age-dependent neurodegeneration, suggesting that reduced presenilin activity might represent an important pathogenic mechanism. Presenilins positively regulate the transcription of cAMP response element (CRE)-containing genes, some of which are known to be important for memory formation and neuronal survival. Phosphodiesterase 4 and histone deacetylase inhibitors, which can enhance CRE-dependent gene expression, have been shown to ameliorate memory deficits and neurodegeneration in animal models. Thus, modulation of CRE-dependent transcription might be beneficial for the treatment of dementia in AD.

Role of cAMP response element binding protein in cardiovascular remodeling: good, bad, or both?

The cAMP response element binding protein (CREB) is a ubiquitously expressed nuclear transcription factor that is activated by various extracellular stimuli. CREB is known to regulate the expression of genes important to cell proliferation, differentiation, adaptation, and survival in many cell types. Loss of CREB function by transgenic overexpression of dominant negative CREB or targeted deletion of the CREB gene revealed that CREB is involved in the differentiation of T lymphocytes, production of growth hormone, and the long-term potentiation of neuronal memory. The role of CREB in cardiovascular system is incompletely characterized and several controversies remain. A growing body of recent evidence, however, has suggested that CREB plays an important role in the cardiovascular remodeling process, including inflammation, cell migration, and apoptosis. Thus, CREB may be a possible target for the treatment of cardiovascular diseases such as atherosclerosis, restenosis, and reperfusion injury.

Combinatorial control of the bradykinin B2 receptor promoter by p53, CREB, KLF-4, and CBP: implications for terminal nephron differentiation

Despite a wealth of knowledge regarding the early steps of epithelial differentiation, little is known about the mechanisms responsible for terminal nephron differentiation. The bradykinin B2 receptor (B2R) regulates renal function and integrity, and its expression is induced during terminal nephron differentiation. This study investigates the transcriptional regulation of the B2R during kidney development. The rat B2R 5′-flanking region has a highly conserved cis-acting enhancer in the proximal promoter consisting of contiguous binding sites for the transcription factors cAMP response element binding protein (CREB), p53, and Krüppel-like factor (KLF-4). The B2R enhancer drives reporter gene expression in inner medullary collecting duct-3 cells but is considerably weaker in other cell types. Site-directed mutagenesis and expression of dominant negative mutants demonstrated the requirement of CREB DNA binding and Ser-133 phosphorylation for optimal enhancer function. Moreover, helical phasing experiments showed that disruption of the spatial organization of the enhancer inhibits B2R promoter activity. Several lines of evidence indicate that cooperative interactions among the three transcription factors occur in vivo during terminal nephron differentiation: 1) CREB, p53, and KLF-4 are coexpressed in B2R-positive differentiating cells; 2) the maturational expression of B2R correlates with CREB/p53/KLF-4 DNA-binding activity; 3) assembly of CREB, p53, and KLF-4 on chromatin at the endogenous B2R promoter is developmentally regulated and is accompanied by CBP recruitment and histone hyperacetylation; and 4) CREB and p53 occupancy of the B2R enhancer is cooperative. These results demonstrate that combinatorial interactions among the transcription factors, CREB, p53, and KLF-4, and the coactivator CBP, may be critical for the regulation of B2R gene expression during terminal nephron differentiation.

Multisite protein modification and intramolecular signaling

Post-translational modification is a major mechanism by which protein function is regulated in eukaryotes. Instead of single-site action, many proteins such as histones, p53, RNA polymerase II, tubulin, Cdc25C and tyrosine kinases are modified at multiple sites by modifications like phosphorylation, acetylation, methylation, ubiquitination, sumoylation and citrullination. Multisite modification on a protein constitutes a complex regulatory program that resembles a dynamic 'molecular barcode' and transduces molecular information to and from signaling pathways. This program imparts effects through 'loss-of-function' and 'gain-of-function' mechanisms. Among the latter, covalent modifications specifically recruit a diverse array of modules, including the SH2 domain, 14-3-3, WW domain, Polo box, BRCT repeat, bromodomain, chromodomain, Tudor domain and motifs binding to ubiquitin and other protein modifiers. Such recruitments are often modulated by modifications occurred at neighboring and distant sites. Multisite modification thus coordinates intermolecular and intramolecular signaling for the qualitative and quantitative control of protein function in vivo.

Mass spectroscopic characterization of the coronavirus infectious bronchitis virus nucleoprotein and elucidation of the role of phosphorylation in RNA binding by using surface plasmon resonance

Phosphorylation of the coronavirus nucleoprotein (N protein) has been predicted to play a role in RNA binding. To investigate this hypothesis, we examined the kinetics of RNA binding between nonphosphorylated and phosphorylated infectious bronchitis virus N protein with nonviral and viral RNA by surface plasmon resonance (Biacore). Mass spectroscopic analysis of N protein identified phosphorylation sites that were proximal to RNA binding domains. Kinetic analysis, by surface plasmon resonance, indicated that nonphosphorylated N protein bound with the same affinity to viral RNA as phosphorylated N protein. However, phosphorylated N protein bound to viral RNA with a higher binding affinity than nonviral RNA, suggesting that phosphorylation of N protein determined the recognition of virus RNA. The data also indicated that a known N protein binding site (involved in transcriptional regulation) consisting of a conserved core sequence present near the 5' end of the genome (in the leader sequence) functioned by promoting high association rates of N protein binding. Further analysis of the leader sequence indicated that the core element was not the only binding site for N protein and that other regions functioned to promote high-affinity binding.

Hepatitis C virus core protein regulates p300/CBP co-activation function. Possible role in the regulation of NF-AT1 transcriptional activity

Hepatitis C virus (HCV) core is a viral structural protein; it also participates in some cellular processes, including transcriptional regulation. However, the mechanisms of core-mediated transcriptional regulation remain poorly understood. Oncogenic virus proteins often target p300/CBP, a known co-activator of a wide variety of transcription factors, to regulate the expression of cellular and viral genes. Here we demonstrate, for the first time, that HCV core protein interacts with p300/CBP and enhances both its acetyl-transferase and transcriptional activities. In addition, we demonstrate that nuclear core protein activates the NH2-terminal transcription activation domain (TAD) of NF-AT1 in a p300/CBP-dependent manner. We propose a model in which core protein regulates the co-activation function of p300/CBP and activates NF-AT1, and probably other p300/CBP-regulated transcription factors, by a novel mechanism involving the regulation of the acetylation state of histones and/or components of the transcriptional machinery.

Lysine acetylation and the bromodomain: a new partnership for signaling

Lysine acetylation has been shown to occur in many protein targets, including core histones, about 40 transcription factors and over 30 other proteins. This modification is reversible in vivo, with its specificity and level being largely controlled by signal-dependent association of substrates with acetyltransferases and deacetylases. Like other covalent modifications, lysine acetylation exerts its effects through "loss-of-function" and "gain-of-function" mechanisms. Among the latter, lysine acetylation generates specific docking sites for bromodomain proteins. For example, bromodomains of Gcn5, PCAF, TAF1 and CBP are able to recognize acetyllysine residues in histones, HIV Tat, p53, c-Myb or MyoD. In addition to the acetyllysine moiety, the flanking sequences also contribute to efficient recognition. The relationship between acetyllysine and bromodomains is reminiscent of the specific recognition of phosphorylated residues by phospho-specific binding modules such as SH2 domains and 14-3-3 proteins. Therefore, lysine acetylation forges a novel signaling partnership with bromodomains to govern the temporal and spatial regulation of protein functions in vivo.

What turns CREB on?

The transactivation domain of the cAMP response element-binding protein (CREB) consists of two major domains. The glutamine-rich Q2 domain, which interacts with the general transcription factor TAFII130/135, is sufficient for the recruitment of a functional RNA polymerase II complex and allows basal transcriptional activity. The kinase-inducible domain, however, mediates signal-induced activation of CREB-mediated transcription. It is generally believed that recruitment of the coactivators CREB-binding protein (CBP) and p300 after signal-induced phosphorylation of this domain at serine-133 strongly enhances CREB-dependent transcription. Transcriptional activity of CREB can also be potentiated by phosphoserine-133-independent mechanisms, and not all stimuli that provoke phosphorylation of serine-133 stimulate CREB-dependent transcription. This review presents an overview of the diversity of stimuli that induce CREB phosphorylation at Ser-133, focuses on phosphoserine-133-dependent and -independent mechanisms that affect CREB-mediated transcription, and discusses different models that may explain the discrepancy between CREB Ser-133 phosphorylation and activation of CREB-mediated transcription.

Chromatin acetylation, memory, and LTP are impaired in CBP+/- mice: a model for the cognitive deficit in Rubinstein-Taybi syndrome and its amelioration

We studied a mouse model of the haploinsufficiency form of Rubinstein-Taybi syndrome (RTS), an inheritable disorder caused by mutations in the gene encoding the CREB binding protein (CBP) and characterized by mental retardation and skeletal abnormalities. In these mice, chromatin acetylation, some forms of long-term memory, and the late phase of hippocampal long-term potentiation (L-LTP) were impaired. We ameliorated the L-LTP deficit in two ways: (1) by enhancing the expression of CREB-dependent genes, and (2) by inhibiting histone deacetyltransferase activity (HDAC), the molecular counterpart of the histone acetylation function of CBP. Inhibition of HDAC also reversed the memory defect observed in fear conditioning. These findings suggest that some of the cognitive and physiological deficits observed on RTS are not simply due to the reduction of CBP during development but may also result from the continued requirement throughout life for both the CREB co-activation and the histone acetylation function of CBP.

Deacetylase inhibitors and the viral transactivator TaxBLV synergistically activate bovine leukemia virus gene expression via a cAMP-responsive element- and cAMP-responsive element-binding protein-dependent mechanism

Efficient bovine leukemia virus (BLV) transcription requires the virus-encoded transactivator Tax(BLV), which acts through three Tax(BLV)-responsive elements located in the 5' long terminal repeat. It has been proposed that the binding of the CRE-binding protein (CREB) and the activating transcription factor (ATF) to the three imperfect cAMP-responsive elements (CREs) located in each Tax(BLV)-responsive element mediates Tax(BLV) transactivation. Here we demonstrated that deacetylase inhibitors (HDACis) synergistically enhanced the transcriptional activation of the BLV promoter by Tax(BLV) in a CRE-dependent manner. Tax(BLV) was acetylated in vivo at its N(alpha) terminus but not at internal lysine residues. Rather, HDACi potentiation of Tax(BLV) transactivation was mediated by an HDACi indirect action that requires new protein synthesis. Mechanistically, using a dominant-negative form of CREB, we showed that Tax(BLV) and HDACi synergistically activated BLV gene expression via a CREB-dependent mechanism. Moreover, electrophoretic mobility shift assay and Western blot experiments revealed that HDACi increased the in vitro DNA binding activity of CREB/ATF but did not alter CREB/ATF intranuclear presence. Remarkably, chromatin immunoprecipitation assays demonstrated that HDACi treatment increased the level of CREB bound to the BLV promoter in vivo. Our results together suggest that an increase in CREB/ATF occupancy of the viral CREs in response to HDACi potentiates Tax(BLV) transactivation of the BLV promoter.

The androgen receptor acetylation site regulates cAMP and AKT but not ERK-induced activity

The androgen receptor (AR) regulates ligand-dependent gene transcription upon binding specific DNA sequences. The AR conveys both trans-activation and trans-repression functions, which together contribute to prostate cellular growth, differentiation, and apoptosis. Like histone H3, the AR is post-translationally modified by both acetylation and phosphorylation. The histone acetyltransferase p300 transactivates the AR and directly acetylates the AR in vitro at a conserved motif. Point mutations of the AR acetylation motif that abrogate acetylation reduce trans-activation by p300 without affecting the trans-repression function of the AR. The current studies assessed the functional relationship between acetylation and phosphorylation of the AR. Herein trans-activation of the AR acetylation site mutants were enhanced by the p42/p44 MAPK pathway but were defective in regulation by protein kinase A (PKA) signaling. PKA inhibition augmented ARwt activity but not AR acetylation mutant gene reporter activity and association at an androgen response element in chromatin immunoprecipitation assays. Mutations of the lysine residues at the AR acetylation site reduced trichostatin A (TSA) responsiveness and ligand-induced phosphorylation of the AR. The AR acetylation site mutant formed ligand-induced phosphorylation-dependent isoforms with distinguishable characteristics from wild type AR as determined with two-dimensional electrophoresis. Conversely, point mutation of a subset of AR phosphorylation sites reduced trichostatin A responsiveness and trans-activation by histone acetyltransferases. Together these studies suggest that acetylation and phosphorylation of the AR are linked events and that the conserved AR lysine motif contributes to a select subset of pathways governing AR activity.

Trans-activators regulating neuronal glucose transporter isoform-3 gene expression in mammalian neurons

The murine facilitative glucose transporter isoform 3 is developmentally regulated and is predominantly expressed in neurons. By employing the primer extension assay, the transcription start site of the murine Glut 3 gene in the brain was localized to -305 bp 5' to the ATG translation start codon. Transient transfection assays in N2A neuroblasts using murine GLUT3-luciferase reporter constructs mapped enhancer activities to two regions located at -203 to -177 and -104 to -29 bp flanking a previously described repressor element (-137 to -130 bp). Dephosphorylated Sp1 and Sp3 proteins from the 1- and 21-day-old mouse brain nuclear extracts bound the repressor elements, whereas both dephosphorylated and phosphorylated cAMP-response element-binding protein (CREB) in N2A, 1- and 21-day-old mouse brain nuclear extracts bound the 5'-enhancer cis-elements (-187 to -180 bp) of the Glut 3 gene, and the Y box protein MSY-1 bound the sense strand of the -83- to -69-bp region. Sp3, CREB, and MSY-1 binding to the GLUT 3 DNA was confirmed by the chromatin immunoprecipitation assay, whereas CREB and MSY-1 interaction was detected by the co-immunoprecipitation assay. Furthermore, small interference RNA targeted at CREB in N2A cells decreased endogenous CREB concentrations, and CREB mediated GLUT 3 transcription. Thus, in the murine brain similar to the N2A cells, phosphorylated CREB and MSY-1 bound the Glut 3 gene trans-activating the expression in neurons, whereas Sp1/Sp3 bound the repressor elements. We speculate that phosphorylated CREB and Sp3 also interacted to bring about GLUT 3 expression in response to development/cell differentiation and neurotransmission.

Loss of CBP causes T cell lymphomagenesis in synergy with p27Kip1 insufficiency

CBP can function as a tumor suppressor, but the mechanisms that govern oncogenesis in its absence are unknown. Here we show that CBP inactivation in mouse thymocytes leads to lymphoma. Although CBP has been implicated in the transactivation functions of p53, development of these tumors does not seem to involve loss of p53 activity. CBP-null tumors show reduced levels of p27Kip1 and increased levels of cyclin E and Skp2, two oncoproteins that can promote p27Kip1 proteolysis. Reduction of p27Kip1 by introduction of a p27Kip1-null allele into CBP knockout mice accelerates lymphomagenesis and seems to obviate the requirement for Skp2 and cyclin E upregulation. These data suggest that CBP loss mediates lymphomagenesis in cooperation with a mechanism that reduces p27Kip1 abundance.

A ‘molecular switchboard’—covalent modifications to proteins and their impact on transcription

Proteins undergo a remarkable variety of posttranslational modifications, with more than 200 distinct modifications identified to date. Increasing evidence suggests that many proteins bear multiple, distinct modifications, and the ability of one modification to antagonize or synergize the deposition of another can have significant biological consequences. Here, we illustrate the importance of posttranslational modifications within the context of transcriptional regulation, and we offer a perspective on the emerging role of combinatorial networks of modifications. Finally, we discuss the potential for chemical approaches to transform our understanding of the field.

Deactylase inhibitors disrupt cellular complexes containing protein phosphatases and deacetylases

Affinity isolation of protein serine/threonine phosphatases on the immobilized phosphatase inhibitor microcystin-LR identified histone deacetylase 1(HDAC1), HDAC6, and HDAC10 as novel components of cellular phosphatase complexes. Other HDACs, specifically HDAC2, -3, -4, and -5, were excluded from such complexes. In vitro biochemical studies showed that recombinant HDAC6, but not HDAC4, bound directly to the protein phosphatase (PP)1 catalytic subunit. No association was observed between HDAC6 and PP2A, another major protein phosphatase. PP1 binding was mapped to the second catalytic domain and adjacent C-terminal sequences in HDAC6, and treatment of cells with trichostatin A (TSA) disrupted endogenous HDAC6.PP1 complexes. Consistent with the inhibition of tubulin deactylase activity of HDAC6, TSA enhanced cellular tubulin acetylation, and acetylated tubulin was present in the PP1 complexes from TSA-treated cells. Trapoxin B, a weak HDAC6 inhibitor, and calyculin A, a cell-permeable phosphatase inhibitor, had no effect on the stability of the HDAC6.PP1 complexes or on tubulin acetylation. Mutations that inactivated HDAC6 prevented its incorporation into cellular PP1 complexes and suggested that when bound together both enzymes were active. Interestingly, TSA disrupted all the cellular HDAC.phosphatase complexes analyzed. This study provided new insight into the mechanism by which HDAC inhibitors elicited coordinate changes in cellular protein phosphorylation and acetylation and suggested that changes in these protein modifications at multiple subcellular sites may contribute to the known ability of HDAC inhibitors to suppress cell growth and transformation.

Deacetylase activity is required for cAMP activation of a subset of CREB target genes

Many hormones activate transcription by raising the level of cAMP within cells. In one well studied pathway, cAMP induces protein kinase A to phosphorylate the transcription factor CREB, which binds to a consensus sequence, the cAMP-regulated enhancer, found in many target genes. A generally accepted model suggests that phosphorylated CREB recruits the histone acetyltransferase CBP to activate transcription. In contrast, histone deacetylases have been linked to the cessation of CREB-dependent transcription. Here we tested this model in the regulation of endogenous CREB target genes. We used a constitutively active CREB mutant and microarray analysis to identify target genes in PC12 cells. We then tested the role of histone deacetylase activity in cAMP activation of four of these genes (c-FOS, ICER, NOR-1, and NUR77) by treating cells with the histone deacetylase inhibitor trichostatin A. Consistent with the generally accepted model, trichostatin A enhanced activation of c-FOS and NUR77 by cAMP. Surprisingly, trichostatin A blocked activation of ICER and NOR-1. The block of ICER and NOR-1 activation persisted in the presence of cycloheximide, indicating that the trichostatin A effect did not depend on new protein synthesis. This unexpected role of histone deacetylases in transcriptional activation of certain endogenous CREB target genes was not apparent in transfected reporter genes. Chromatin immunoprecipitation analysis indicated that the differential roles of histone deacetylases in activating or repressing CREB target genes was manifested at the level of preinitiation complex recruitment. These data indicate that histone deacetylases differentially regulate CREB target genes by contributing to either activation or cessation of transcription.

Histone deacetylase inhibition is associated with transcriptional repression of the Hmga2 gene

The high-mobility-group A2 protein (HMGA2) plays important functional roles in transcriptional regulation, DNA replication and chromatin structure. In this study, the effect of histone deacetylase inhibition on the transcriptional activity of the Hmga2 gene was investigated in vivo both at the endogenous gene level and in a variety of cell lines using transiently transfected promoter constructs. Trichostatin A (TSA) repressed both transfected murine and human Hmga2 promoter constructs 3-8-fold in NIH3T3, F9 and HeLa cells. Steady-state Hmga2 mRNA levels in NIH3T3 cells decreased 4-5-fold following TSA treatment, while pre- treatment of NIH3T3 cells with the transcriptional inhibitor, actinomycin D, completely blocked TSA mediated repression of the Hmga2 gene. Cross-linked chromatin immunoprecipitation (X-ChIP) analysis revealed a 5-6-fold decrease in endogenous Hmga2 promoter bound Sp1 and Sp3 proteins following TSA treatment in parallel with observed loss of acetylated histone H3 and H4. In addition, the poly-pyrimidine-tract-binding protein (PTB) was observed to bind to the Hmga2 promoter in both TSA treated and untreated NIH3T3 cells. Together, these results suggest TSA treatment leads to a decrease in Hmga2 gene transcription, and a significant decrease in promoter bound Sp1, Sp3 and acetylated histones H3 and H4.