Human p300 protein is a coactivator for the transcription factor MyoD

The epigenetic network regulating muscle development and regeneration

This review focuses on our current knowledge of the epigenetic changes regulating gene expression at the chromatin and DNA level, independently on the primary DNA sequence, to reprogram the nuclei of muscle precursors during developmental myogenesis and muscle regeneration. These epigenetic marks provide the blueprint by which the extra-cellular cues are interpreted at the nuclear level by the transcription machinery to select the repertoire of tissue-specific genes to be expressed. The reversibility of some of these changes necessarily reflects the dynamic nature of skeletal myogenesis, which entails the progression through two antagonistic processes--proliferation and differentiation. Other epigenetic modifications are instead associated to events conventionally considered as irreversible--e.g. maintenance of lineage commitment and terminal differentiation. However, recent results support the possibility that these events can be reversed, at least upon certain experimental conditions, thereby revealing a dynamic nature of many of the epigenetic modifications underlying skeletal myogenesis. The elucidation of the epigenetic network that regulates transcription during developmental myogenesis and muscle regeneration might provide the information instrumental to devise pharmacological interventions toward selective manipulation of gene expression to promote regeneration of skeletal muscles and possibly other tissue.

Nuclear receptor mediated gene regulation through chromatin remodeling and histone modifications

Nuclear steroid/thyroid vitamin A/D receptor genes form a gene superfamily and encode DNA-binding transcription factors that control the transcription of target genes in a ligand-dependent manner. It has become clear that chromatin remodeling and the modification of histones, the main components of chromatin, play crucial roles in gene transcription, and many distinct classes of NR-interacting co-regulators have been identified that perform significant roles in gene transcription. Since NR dysfunction can lead to the onset or progression of endocrine disease, elucidation of the mechanisms of gene regulation mediated by NRs, as well as the identification and characterization of co-regulator complexes (especially chromatin remodeling and histone-modifying complexes), is essential not only for better understanding of NR ligand function, but also for pathophysiological studies and the development of therapeutic interventions in humans.

Nuclear Rho kinase, ROCK2, targets p300 acetyltransferase

Rho-associated coiled-coil protein kinase (ROCK) is an effector for the small GTPase Rho and plays a pivotal role in diverse cellular activities, including cell adhesion, cytokinesis, and gene expression, primarily through an alteration of actin cytoskeleton dynamics. Here, we show that ROCK2 is localized in the nucleus and associates with p300 acetyltransferase both in vitro and in cells. Nuclear ROCK2 is present in a large protein complex and partially cofractionates with p300 by gel filtration analysis. By immunofluorescence, ROCK2 partially colocalizes with p300 in distinct insoluble nuclear structures. ROCK2 phosphorylates p300 in vitro, and nuclear-restricted expression of constitutively active ROCK2 induces p300 phosphorylation in cells. p300 acetyltransferase activity is dependent on its phosphorylation status in cells, and p300 phosphorylation by ROCK2 results in an increase in its acetyltransferase activity in vitro. These observations suggest that nucleus-localized ROCK2 targets p300 for phosphorylation to regulate its acetyltransferase activity.

Histone deacetylase inhibitors repress the transactivation potential of hypoxia-inducible factors independently of direct acetylation of HIF-alpha

Hypoxia-inducible factors (HIFs) are heterodimeric transcription factors regulating the oxygen supply, glucose metabolism, and angiogenesis. HIF function requires the recruitment of p300/CREB-binding protein, two coactivators with histone acetyltransferase activity, by the C-terminal transactivation domain of HIF-alpha (HIF-alphaCAD). Histone deacetylase inhibitors (HDAIs) induce differentiation or apoptosis and repress tumor growth and angiogenesis, hence being explored intensively as anti-cancer agents. Using combined pharmacological, biochemical, and genetic approaches, here we show that HDAIs repress the transactivation potential of HIF-alphaCAD. This repression is independent of the function of tumor suppressors von Hippel-Lindau or p53 or the degradation of HIF-alpha. We also demonstrate the sufficiency of low concentrations of HDAIs in repression of HIF target genes in tumor cells. We further show that HDAIs induce hyperacetylation of p300 and repress the HIF-1alpha.p300 complex in vivo. In vitro acetylation analysis reveals that the p300CH1 region, but not HIF-alphaCAD, is susceptible to acetylation. Taken together, our data demonstrate that a deacetylase activity is indispensable for the transactivation potential of HIF-alphaCAD and support a model that acetylation regulates HIF function by targeting HIF-alpha.p300 complex, not by direct acetylating HIF-alpha. The demonstration that HDAIs repress both HIF-1alpha and HIF-2alpha transactivation potential independently of von Hippel-Lindau tumor suppressor and p53 function indicates that HDAIs may have biological effects in a broad range of tissues in addition to tumors.

A Constitutively Active Hypoxia-Inducible Factor-1α/VP16 Hybrid Factor Activates Expression of the Human B-Type Natriuretic Peptide Gene

Hypoxia-inducible factor-1 (HIF-1) is a primary regulator of the physiological response to hypoxia. A recombinant adenovirus expressing a constitutively active hybrid form of the HIF-1alpha subunit (Ad2/HIF-1alpha/VP16) is being evaluated as a gene therapy for the treatment of peripheral vascular disease. Ad2/HIF-1alpha/VP16 up-regulates known HIF-1-responsive genes, including those involved in angiogenesis. Expression profile analysis revealed that the brain natriuretic peptide (BNP) gene was significantly up-regulated in response to HIF-1alpha/VP16 in human fetal cardiac cells. Real-time reverse transcription-polymerase chain reaction analyses confirmed transcriptional activation of the BNP gene by HIF-1alpha/VP16 in human but not rat cardiac cells. Because hypoxia itself did not increase human BNP gene expression in these analyses, the mechanism of the HIF-1alpha/VP16 effect was determined. Analyses of promoter deletion mutants suggested that the cis-acting sequence in the human BNP promoter mediating activation by HIF-1alpha/VP16 was a putative HIF-1 responsive element (HRE) located at -466. An SV40 basal promoter-luciferase plasmid containing a minimal BNP HRE was up-regulated by HIF-1alpha/VP16, whereas a similar construct carrying a mutation within the HIF-1 binding site was not. Mutation of an E-box motif within the BNP HRE reduced HIF-1alpha/VP16-mediated transcriptional activation by 50%. Gel-shift analyses showed that both the native HIF-1alpha and HIF-1alpha/VP16 are able to bind to a probe containing the HIF-1 binding site. These experiments demonstrate the existence of a functional HRE in the BNP promoter and further define the scope and mechanism of action of Ad2/HIF-1alpha/VP16.

The APC/C and CBP/p300 cooperate to regulate transcription and cell-cycle progression

The anaphase-promoting complex/cyclosome (APC/C) is a multicomponent E3 ubiquitin ligase that, by targeting protein substrates for 26S proteasome-mediated degradation through ubiquitination, coordinates the temporal progression of eukaryotic cells through mitosis and the subsequent G1 phase of the cell cycle. Other functions of the APC/C are, however, less well defined. Here we show that two APC/C components, APC5 and APC7, interact directly with the coactivators CBP and p300 through protein-protein interaction domains that are evolutionarily conserved in adenovirus E1A. This interaction stimulates intrinsic CBP/p300 acetyltransferase activity and potentiates CBP/p300-dependent transcription. We also show that APC5 and APC7 suppress E1A-mediated transformation in a CBP/p300-dependent manner, indicating that these components of the APC/C may be targeted during cellular transformation. Furthermore, we establish that CBP is required in APC/C function; specifically, gene ablation of CBP by RNA-mediated interference markedly reduces the E3 ubiquitin ligase activity of the APC/C and the progression of cells through mitosis. Taken together, our results define discrete roles for the APC/C-CBP/p300 complexes in growth regulation.

Mutant huntingtin represses CBP, but not p300, by binding and protein degradation

Huntington's disease can be used as a model to study neurodegenerative disorders caused by aggregation-prone proteins. It has been proposed that the entrapment of transcription factors in aggregates plays an important role in pathogenesis. We now report that the transcriptional activity of CBP is already repressed in the early time points by soluble mutant huntingtin, whereas the histone acetylase activity of CBP/p300 is gradually diminished over time. Mutant huntingtin bound much stronger to CBP than normal huntingtin, possibly contributing to repression. Especially at the later time points, CBP protein level was gradually reduced via the proteasome pathway. In sharp contrast, p300 was unaffected by mutant huntingtin. This selective degradation of CBP was absent in spinocerebellar ataxia 3. Thus, mutant huntingtin specifically affects CBP and not p300 both at the early and later time points, via multiple mechanisms. In addition to the reduction of CBP, also the altered ratio of these closely related histone acetyltransferases may affect chromatin structure and transcription and thus contribute to neurodegeneration.

Identification of a novel p300-specific-associating protein, PRS1 (phosphoribosylpyrophosphate synthetase subunit 1)

CBP [CREB (cAMP-response-element-binding protein)-binding protein] and p300 play critical roles in transcriptional co-activation, cell differentiation, proliferation and apoptosis. Multiple transcription factors associate with CBP/p300. With the exception of the SYT oncoprotein, no proteins have been identified that specifically associate with p300, but not CBP. In the present study, we isolated a novel p300-associated protein for which no interaction with CBP was observed by GST (glutathione S-transferase) pull-down assay using Jurkat cell lysates metabolically labelled with [35S]methionine. This protein bound the KIX (kinase-inducible) domain of p300. Following resolution by two-dimensional acrylamide gel electrophoresis, we identified the KIX-domain-bound protein by MS analysis as PRS1 (phosphoribosylpyrophosphate synthetase subunit 1), a protein essential for nucleoside biosynthesis. This is the first report to demonstrate the existence of a p300 KIX-domain-specific-interacting protein that does not interact with CBP. Thus p300 may play a role in the regulation of DNA synthesis through interactions with PRS1.

Dexamethasone upregulates the expression of the nuclear cofactor p300 and its interaction with C/EBPbeta in cultured myotubes

Muscle wasting during sepsis and other catabolic conditions is, at least in part, mediated by glucocorticoids and is associated with upregulated transcription of multiple genes in the ubiquitin-proteasome proteolytic pathway. In addition to transcription factors, nuclear cofactors, including p300, regulate gene transcription. We tested the hypothesis that glucocorticoids upregulate the expression of p300 in muscle cells. Treatment of cultured L6 myotubes, a rat skeletal muscle cell line, with dexamethasone resulted in a dose- and time-dependent increase in p300 protein and mRNA levels. Surprisingly, the effect of dexamethasone on p300 levels was not inhibited by the glucocorticoid receptor (GR) antagonist RU38486 and RU38486 exerted an agonist effect on p300, increasing its expression. Co-immunoprecipitation showed that treatment of the myotubes with dexamethasone resulted in protein-protein interaction between p300 and C/EBPbeta, but not C/EBPdelta. The present results suggest that glucocorticoids upregulate the expression of p300 and its interaction with C/EBPbeta in skeletal muscle. Increased expression and activity of p300 may be involved in the regulation of gene transcription in glucocorticoid-dependent muscle wasting.

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.

Histone deacetylases as transcriptional activators? Role reversal in inducible gene regulation

Histone deacetylation enzymes have often been associated with the suppression of eukaryotic gene transcription. In contrast, recent studies of inducible gene regulation indicate that protein deacetylation can also be required as a transcriptional activation signal. The concept of protein deacetylation as a requirement for transcription activation seems to contradict earlier conclusions about the function of deacetylation in gene suppression. However, in the context of a more global interpretation, these opposing effects of deacetylation imply its dynamic role in the overall control of gene expression. The exact requirement for deacetylation differs among promoters, depending on their specific architecture and regulation scenario.

Inhibition of MuSK expression by CREB interacting with a CRE-like element and MyoD

The type I receptor-like protein tyrosine kinase MuSK is essential for the neuromuscular junction formation. MuSK expression is tightly regulated during development, but the underlying mechanisms were unclear. Here we identified a novel mechanism by which MuSK expression may be regulated. A cyclic AMP response element (CRE)-like element in the 5'-flanking region of the MuSK gene binds to CREB1 (CRE-binding protein 1). Mutation of this element increases the MuSK promoter activity, suggesting a role for CREB1 in attenuation of MuSK expression. Interestingly, CREB mutants unable to bind to DNA also inhibit MuSK promoter activity, suggesting a CRE-independent inhibitory mechanism. In agreement, CREB1 could inhibit a mutant MuSK transgene reporter whose CRE site was mutated. We provide evidence that CREB interacts directly with MyoD, a myogenic factor essential for MuSK expression in muscle cells. Suppression of CREB expression by small interfering RNA increases MuSK promoter activity. These results demonstrate an important role for CREB1 in the regulation of MuSK expression.

Signaling to the chromatin during skeletal myogenesis: Novel targets for pharmacological modulation of gene expression

Cellular differentiation entails an extensive reprogramming of the genome toward the expression of discrete subsets of genes, which establish the tissue-specific phenotype. This program is achieved by epigenetic marks of the chromatin at particular loci, and is regulated by environmental cues, such as soluble factors and cell-to-cell interactions. How the intracellular cascades convert the myriad of external stimuli into the nuclear information necessary to reprogram the genome toward specific responses is a question of biological and medical interest. The elucidation of the signaling converting cues from outside the cells into chromatin modifications at individual promoters holds the promise to unveil the targets for selective pharmacological interventions to modulate gene expression for therapeutic purposes. Enhancing muscle regeneration and preventing muscle breakdown are important goals in the therapy of muscular diseases, cancer-associated cachexia and aging-associated sarcopenia. We will summarize the recent progress of our knowledge of the regulation of gene expression by intracellular cascades elicited by external cues during skeletal myogenesis. And will illustrate the potential importance of targeting the chromatin signaling in regenerative medicine--e.g. to boost muscle regeneration.

Pharmacologic profiling of transcriptional targets deciphers promoter logic

The blueprint for cellular diversity and response to environmental change is encoded in the cis-acting regulatory sequences of most genes. Deciphering this 'cis-regulatory code' requires multivariate data sets that examine how these regions coordinate transcription in response to diverse environmental stimuli and therapeutic treatments. We describe a transcriptional approach that profiles the activation of multiple transcriptional targets against combinatorial arrays of therapeutic and signal transducing agents. Application of this approach demonstrates how cis-element composition and promoter context combine to influence transcription downstream of mitogen-induced signaling networks. Computational dissection of these transcriptional profiles in activated T cells uncovers a novel regulatory synergy between IGF-1 and CD28 costimulation that modulates NF-kappaB and AP1 pathways through signaling cascades sensitive to cyclosporin A and wortmannin. This approach provides a broader view of the hierarchical signal integration governing gene expression and will facilitate a practical design of combinatorial therapeutic strategies for exploiting critical control points in transcriptional regulation.

Cyclin D1 represses p300 transactivation through a cyclin-dependent kinase-independent mechanism

Cyclin D1 encodes a regulatory subunit, which with its cyclin-dependent kinase (Cdk)-binding partner forms a holoenzyme that phosphorylates and inactivates the retinoblastoma protein. In addition to its Cdk binding-dependent functions, cyclin D1 regulates cellular differentiation in part by modifying several transcription factors and nuclear receptors. The molecular mechanism through which cyclin D1 regulates the function of transcription factors involved in cellular differentiation remains to be clarified. The histone acetyltransferase protein p300 is a co-integrator required for regulation of multiple transcription factors. Here we show that cyclin D1 physically interacts with p300 and represses p300 transactivation. We demonstrated further that the interaction of the two proteins occurs at the peroxisome proliferator-activated receptor gamma-responsive element of the lipoprotein lipase promoter in the context of the local chromatin structure. We have mapped the domains in p300 and cyclin D1 involved in this interaction. The bromo domain and cysteine- and histidine-rich domains of p300 were required for repression by cyclin D1. Cyclin D1 repression of p300 was independent of the Cdk- and retinoblastoma protein-binding domains of cyclin D1. Cyclin D1 inhibits histone acetyltransferase activity of p300 in vitro. Microarray analysis identified a signature of genes repressed by cyclin D1 and induced by p300 that promotes cellular differentiation and induces cell cycle arrest. Together, our results suggest that cyclin D1 plays an important role in cellular proliferation and differentiation through regulation of p300.

HIF-1alpha, STAT3, CBP/p300 and Ref-1/APE are components of a transcriptional complex that regulates Src-dependent hypoxia-induced expression of VEGF in pancreatic and prostate carcinomas

Hypoxia stimulates a number of pathways critical to cancer cell survival, including the activation of vascular endothelial growth factor (VEGF) transcription. In normal fibroblasts, hypoxia-induced activation of the protein tyrosine kinase, Src, is required for VEGF expression. We show here in both pancreatic and prostate carcinoma cell lines cobalt chloride (used to mimic hypoxia) -induced VEGF expression requires Src activation and leads to increased steady-state levels of HIF-1alpha and increased phosphorylation of signal and transducer of transcription 3 (STAT3). STAT3 and hypoxia-inducible factor (HIF)-1alpha bind simultaneously to the VEGF promoter, where they form a molecular complex with the transcription coactivators CBP/p300 and Ref-1/APE. Expression of activated Src from an inducible promoter is sufficient to increase VEGF expression and form these STAT3/HIF-1alpha-containing promoter complexes. Inhibition of DNA binding by expression of either STAT3 or HIF-1alpha dominant negative mutants significantly reduces VEGF expression. These data suggest that the binding of both STAT3 and HIF-1alpha to the VEGF promoter is required for maximum transcription of VEGF mRNA following hypoxia.

The steroid receptor co-activator-1 (SRC-1) potentiates TGF-beta/Smad signaling: role of p300/CBP

The three related 160-kDa proteins, SRC-1, TIF-2 and RAC-3, were initially identified as factors interacting with nuclear receptors. They have also been reported to potentiate the activity of other transcription factors such as AP-1 or NF-kappaB. The aim of this work was to identify whether SRC-1 interferes with the TGF-beta/Smad signaling pathway, and if so, to identify its underlying mechanisms of action. Using transient cell transfection experiments performed in human dermal fibroblasts with the Smad3/4-specific (SBE)4-lux reporter construct, as well as the human PAI-1 promoter, we determined that SRC-1 enhances TGF-beta-induced, Smad-mediated, transcription. Likewise, SRC-1 overexpression potentiated TGF-beta-induced upregulation of PAI-1 steady-state mRNA levels. Using a mammalian two-hybrid system, we demonstrated that SRC-1 interacts with the transcriptional co-activators p300/CBP, but not with Smad3. Overexpression of the adenovirus E1A oncoprotein, an inhibitor of CBP/p300 activity, prevented the enhancing effect of SRC-1 on Smad3/4-mediated transcription, indicating that p300/CBP may be required for SRC-1 effect. Such hypothesis was validated, as expression of a mutant form of SRC-1 lacking the CBP/p300-binding site failed to upregulate Smad3/4-dependent transcription, while full-length SRC-1 potentiated p300.Smad3 interactions. These results identify SRC-1 as a novel Smad3/4 transcriptional partner, facilitating the functional link between Smad3 and p300/CBP.

Transcription factor Sox-2 inhibits co-activator stimulated transcription

Previous studies have shown that transcription of the fibroblast growth factor-4 (FGF-4) gene by early embryonic cells is dependent upon a powerful distal enhancer located 3 kb downstream of the transcription start site within the untranslated region of the last exon. The transcription factors Sox-2 and Oct-3 cooperatively bind to critical cis-regulatory elements within the enhancer to synergistically activate transcription. Moreover, the co-activator p300 can mediate the synergistic activity of Sox-2 and Oct-3, and p300 associates with the FGF-4 enhancer in vivo. Embryonal carcinoma (EC) cells have been used extensively as a model system to study the regulation of the FGF-4 gene during early development. Recently, it has been suggested that suboptimal levels of Sox-2 expression in F9 EC cells limit the transcription of the FGF-4 gene. The studies presented in this report argue that Sox-2 levels are not limiting in F9 EC cells. Moreover, overexpression of Sox-2 in F9 EC cells decreases FGF-4 promoter activity. In addition, overexpression of Sox-2 in these cells inhibits activation by the co-activators p300, CBP, and OCA-B in a manner that requires the transactivation domain of Sox-2. These findings suggest that Sox-2 levels in F9 EC cells are regulated carefully to avoid interference with the transcription of critical genes.

Modulation of smooth muscle gene expression by association of histone acetyltransferases and deacetylases with myocardin

Differentiation of smooth muscle cells is accompanied by the transcriptional activation of an array of muscle-specific genes controlled by serum response factor (SRF). Myocardin is a cardiac and smooth muscle-specific expressed transcriptional coactivator of SRF and is sufficient and necessary for smooth muscle gene expression. Here, we show that myocardin induces the acetylation of nucleosomal histones surrounding SRF-binding sites in the control regions of smooth muscle genes. The promyogenic activity of myocardin is enhanced by p300, a histone acetyltransferase that associates with the transcription activation domain of myocardin. Conversely, class II histone deacetylases interact with a domain of myocardin distinct from the p300-binding domain and suppress smooth muscle gene activation by myocardin. These findings point to myocardin as a nexus for positive and negative regulation of smooth muscle gene expression by changes in chromatin acetylation.

P53 down-regulates matrix metalloproteinase-1 by targeting the communications between AP-1 and the basal transcription complex

We have previously reported that human matrix metalloproteinase-1 (MMP1) is a p53 target gene subject to down-regulation (Sun et al. [1999]: J Biol Chem 274:11535-11540]. In the present study, we demonstrate that the down-regulation of the human -83MMP1 promoter fragment by p53 was abolished when the -72AP-1 site was eliminated and that a GAL4-cJun-mediated but not a GAL4-Elk1-mediated induction of pFR-luci was effectively inhibited by p53 suggesting an AP-1 dependent but AP-1 binding independent mechanism. Results from gel mobility shift assays were consistent with an AP-1 binding independent mechanism. We also demonstrate that both p300 and TATA box binding proteins cooperated with the transcription factor AP-1 to induce the promoter of MMP1; however, p53 only inhibited the p300-mediated induction of the MMP1 promoter and the inhibition was -72AP-1 dependent. Furthermore, the down-regulation of the MMP1 promoter and mRNA by p53 could be reversed by p300 and by a p53 binding p300 fragment that had no coactivator activity. Taken together, these results indicate that p53 down-regulates MMP1 mainly by disrupting the communications between the transactivator AP-1 and the basal transcriptional complex, which are partially mediated by p300. Finally, by using p53 truncated mutant constructs, we demonstrate that both the N-terminal activation domain and the C-terminal oligomerization domains of p53 were required for the down-regulation of MMP1 transcription.

MyoD recruits the cdk9/cyclin T2 complex on Myogenic-genes regulatory regions

During skeletal myogenesis, muscle-regulatory factors bHLH and MEF2 promote the expression of muscle-specific genes by recruiting several chromatin-modifying complexes on specific DNA regulatory sequences. A number of MyoD-interacting proteins have been reported, but whether they are recruited to the chromatin of myogenic loci, and the relationship with other chromatin bound proteins is unknown. We show that MyoD recruits cdk9/cyclin T2, together with the histone acetyltransferases p300 and PCAF, and the chromatin remodeling complex SWI/SNF, on promoters and enhancers of muscle-specific genes, and that this event correlates with the acetylation of histone tails, remodeling of chromatin, and phosphorylation of the C-terminal domain (CTD) of the RNA polymerase II at these elements.

NF-Y behaves as a bifunctional transcription factor that can stimulate or repress the FGF-4 promoter in an enhancer-dependent manner

NF-Y is a bifunctional transcription factor capable of activating or repressing transcription. NF-Y specifically recognizes CCAAT box motifs present in many eukaryotic promoters. The mechanisms involved in regulating its activity are poorly understood. Previous studies have shown that the FGF-4 promoter is regulated positively by its CCAAT box and NF-Y in embryonal carcinoma (EC) cells where the distal enhancer of the FGF-4 gene is active. Here, we demonstrate that the CCAAT box functions as a negative cis-regulatory element when cis-regulatory elements of the FGF-4 enhancer are disrupted, or after EC cells differentiate and the FGF-4 enhancer is inactivated. We also demonstrate that NF-Y mediates the repression of the CCAAT box and that NF-Y associates with the endogenous FGF-4 gene in both EC cells and EC-differentiated cells. Importantly, we also determined that the orientation and the position of the CCAAT box are critical for its role in regulating the FGF-4 promoter. Together, these studies demonstrate that the distal enhancer of the FGF-4 gene determines whether the CCAAT box of the FGF-4 promoter functions as a positive or a negative cis-regulatory element. In addition, these studies are consistent with NF-Y playing an architectural role in its regulation of the FGF-4 promoter.

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.

Kinetic profiles of p300 occupancy in vivo predict common features of promoter structure and coactivator recruitment

Understanding the language encrypted in the gene regulatory regions of the human genome is a challenging goal for the genomic era. Although customary extrapolations from steady-state mRNA levels have been effective, deciphering these regulatory codes will require additional empirical data sets that more closely reflect the dynamic progression of molecular events responsible for inducible transcription. We describe an approach using chromatin immunoprecipitation to profile the kinetic occupancy of the transcriptional coactivator and histone acetyltransferase p300 at numerous mitogen-induced genes in activated T cells. Comparison of these profiles reveals a class of promoters that share common patterns of inducible expression, p300 recruitment, dependence on selective p300 domains, and sensitivity to histone deacetylase inhibitors. Remarkably, this class also shares an evolutionarily conserved promoter composition and structure that accurately predicts additional human genes with similar functional attributes. This "reverse genomic" approach will have broad application for the genome-wide classification of promoter structure and function.

Deacetylase recruitment by the C/H3 domain of the acetyltransferase p300

The balance between acetylation and deacetylation of histone and nonhistone proteins controls gene expression in a variety of cellular processes, with transcription being activated by acetyltransferases and silenced by deacetylases. We report here the formation and enzymatic characterization of a complex between the acetyltransferase p300 and histone deacetylases. The C/H3 region of p300 was found to co-purify deacetylase activity from nuclear cell extracts. A prototype of class I histone deacetylases, HDAC1, interacts with p300 C/H3 domain in vitro and in vivo. The p300-binding protein E1A competes with HDAC1 for C/H3 binding; and, like E1A, HDAC1 overexpression interferes with either activation of Gal4p300 fusion protein or p300-dependent co-activation of two C/H3-binding proteins, MyoD and p53. The exposure to deacetylase inhibitors could reverse the dominant-negative effect of a C/H3 fragment insulated from the rest of the molecule, on MyoD- and p53-dependent transcription, whereas inhibition by E1A was resistant to trichostatin A. These data support the hypothesis that association between acetyltransferases and deacetylases can control the expression of genes implicated in cellular growth and differentiation, and suggest that the dominant-negative effect of the p300 C/H3 fragment relies on deacetylase recruitment.

A novel complex regulates cardiac actin gene expression through interaction of Emb, a class VI POU domain protein, MEF2D, and the histone transacetylase p300

Expression of the mouse cardiac actin gene depends on a distal enhancer (-7 kbp) which has been shown, in transgenic mice, to direct expression to embryonic skeletal muscle. The presence of this distal sequence is also associated with reproducible expression of cardiac actin transgenes. In differentiated skeletal muscle cells, activity of the enhancer is driven by an E box, binding MyoD family members, and by a 3' AT-rich sequence which is in the location of a DNase I-hypersensitive site. This sequence does not bind MEF2 proteins, or other known muscle transcription factors, directly. Oct1 and Emb, a class VI POU domain protein, bind to consensus sites on the DNA, and it is the binding of Emb which is important for activity. Emb binds as a major complex with MEF2D and the histone transacetylase p300. The form of Emb present in this complex and as a major form in muscle cell extracts is longer (80 kDa) than that previously described. These results demonstrate the importance of this novel complex in the transcriptional regulation of the cardiac actin gene and suggest a potential role in chromatin remodeling associated with muscle gene activation.

Acetylation regulates the differentiation-specific functions of the retinoblastoma protein

The retinoblastoma tumor-suppressor protein (pRb) is known to induce growth arrest and cellular differentiation. The molecular determinants of pRb function include protein-protein interactions and post-translational modifications such as phosphorylation. Recently, the co-activator p300 was found to acetylate pRb. The biological significance of pRb acetylation, however, remains unclear. In the present study, we provide evidence that pRb undergoes acetylation upon cellular differentiation, including skeletal myogenesis. In addition to p300, the p300-Associated Factor (P/CAF) can mediate pRb acetylation as pRb interacts directly with the acetyltransferase domain of P/CAF in vitro and can associate with P/CAF in differentiated cells. Significantly, by using a C terminal acetylation-impaired mutant of pRb, we reveal that acetylation does not affect pRb-dependent growth arrest or the repression of E2F transcriptional activity. Instead, acetylation is required for pRb-mediated terminal cell cycle exit and the induction of late myogenic gene expression. Based on these results, we propose that acetylation regulates the differentiation-specific function(s) of pRb.

Structure of the mitochondrial ATP synthase by electron cryomicroscopy

We have determined the structure of intact ATP synthase from bovine heart mitochondria by electron cryomicroscopy of single particles. Docking of an atomic model of the F1-c10 subcomplex into a major segment of the map has allowed the 32 A resolution density to be interpreted as the F1-ATPase, a central and a peripheral stalk and an FO membrane region that is composed of two domains. One domain of FO corresponds to the ring of c-subunits, and the other probably contains the a-subunit, the transmembrane portion of the b-subunit and the remaining integral membrane proteins of FO. The peripheral stalk wraps around the molecule and connects the apex of F1 to the second domain of FO. The interaction of the peripheral stalk with F1-c10 implies that it binds to a non-catalytic alpha-beta interface in F1 and its inclination where it is not attached to F1 suggests that it has a flexible region that can serve as a stator during both ATP synthesis and ATP hydrolysis.

The phosphorylation state of an autoregulatory domain controls PACS-1-directed protein traffic

PACS-1 is a cytosolic sorting protein that directs the localization of membrane proteins in the trans-Golgi network (TGN)/endosomal system. PACS-1 connects the clathrin adaptor AP-1 to acidic cluster sorting motifs contained in the cytoplasmic domain of cargo proteins such as furin, the cation-independent mannose-6-phosphate receptor and in viral proteins such as human immunodeficiency virus type 1 Nef. Here we show that an acidic cluster on PACS-1, which is highly similar to acidic cluster sorting motifs on cargo molecules, acts as an autoregulatory domain that controls PACS-1-directed sorting. Biochemical studies show that Ser278 adjacent to the acidic cluster is phosphorylated by CK2 and dephosphorylated by PP2A. Phosphorylation of Ser278 by CK2 or a Ser278-->Asp mutation increased the interaction between PACS-1 and cargo, whereas a Ser278-->Ala substitution decreased this interaction. Moreover, the Ser278-->Ala mutation yields a dominant-negative PACS-1 molecule that selectively blocks retrieval of PACS-1-regulated cargo molecules to the TGN. These results suggest that coordinated signaling events regulate transport within the TGN/endosomal system through the phosphorylation state of both cargo and the sorting machinery.

hZimp10 is an androgen receptor co-activator and forms a complex with SUMO-1 at replication foci

The androgen receptor (AR) plays a central role in male sexual development and in normal and malignant prostate cell growth and survival. It has been shown that transcriptional activation of AR is regulated through interaction with various co-factors. Here we identify a novel PIAS-like protein, hZimp10, as an AR-interacting protein. The transactivation domain (TAD) of AR and the central region of hZimp10 were found to be responsible for the interaction. A strong intrinsic transactivation domain was identified in the C-terminal, proline-rich region of hZimp10. Endogenous AR and hZimp10 proteins were co-stained in the nuclei of prostate epithelial cells from human tissue samples. In human prostate cancer cells, hZimp10 augmented the transcriptional activity of AR. Moreover, hZimp10 co-localized with AR and SUMO-1 at replication foci throughout S phase, and it was capable of enhancing sumoylation of AR in vivo. Studies using sumoylation deficient AR mutants suggested that the augmentation of AR activity by hZimp10 is dependent on the sumoylation of the receptor. Taken together, these data demonstrate that hZimp10 is a novel AR co-regulator.

EID-2, a novel member of the EID family of p300-binding proteins inhibits transactivation by MyoD

Skeletal muscle differentiation has been shown to be dependent on the expression of Rb and p300. We recently cloned a novel inhibitor of muscle differentiation called EID-1, which interacted with both of these factors. In a database search for related molecules, we have cloned and characterized a new EID-1 family member, EID-2. This 28-kDa protein encodes a 236-amino-acid protein with significant similarity to EID-1 in its C-terminus. EID-2 displays developmentally regulated expression with high levels in adult heart and brain. Overexpression of EID-2 inhibits muscle-specific gene expression through inhibition of MyoD-dependent transcription. This inhibitory effect on gene expression can be explained by EID-2's ability to associate with and inhibit the acetyltransferase activity of p300. These data suggest that EID-1 and -2 represent a novel family of proteins that negatively regulate differentiation through a p300-dependent mechanism.

Cyclic adenosine 3',5'-monophosphate-elevating agents inhibit transforming growth factor-beta-induced SMAD3/4-dependent transcription via a protein kinase A-dependent mechanism

Transforming growth factor-beta (TGF-beta) plays complex roles in carcinogenesis, as it may exert both tumor suppressor and pro-oncogenic activities depending on the stage of the tumor. SMAD proteins transduce signals from the TGF-beta receptors to regulate the transcription of specific target genes. Crosstalks with other signaling pathways may contribute to the specificity of TGF-beta effects. In this report, we have investigated the effects of cyclic adenosine 3',5'-monophosphate (cAMP), a key second messenger in the cellular response to various hormones, on SMAD-dependent signaling in human HaCaT keratinocytes. Using either an artificial SMAD3/4-dependent reporter construct or the natural TGF-beta target, plasminogen activator inhibitor-1, we show that membrane-permeable dibutyryl cAMP, and other intracellular cAMP-elevating agents such as the phosphodiesterase inhibitor isobutyl-methylxanthine, the adenylate cyclase activator forskolin, or exogenous prostaglandin E2 (PGE2), interfere with TGF-beta-induced SMAD-specific gene transactivation. Inhibition of protein kinase A (PKA), the main downstream effector of cAMP, with H-89, suppressed cAMP-dependent repression of SMAD-driven gene expression. Inversely, coexpression of either an active PKA catalytic subunit or that of the cAMP response element (CRE)-binding protein (CREB) blocked SMAD-driven gene transactivation. cAMP-elevating agents did not inhibit nuclear translocation and DNA binding of SMAD3/4 complexes, but abolished the interactions of SMAD3 with the transcription coactivators CREB-binding protein (CBP) and p300 in a PKA-dependent manner. These results suggest that suppression of TGF-beta/SMAD signaling and resulting gene transactivation by cAMP-inducing agents occurs via PKA-dependent, CREB-mediated, disruption of SMAD-CBP/p300 complexes.

Dynamical and integrative cell signaling: challenges for the new biology

Years of careful experimental analysis have revealed that signaling molecules are organized into complex networks of biochemical reactions exquisitely regulated in time and space to provide a cell with high-fidelity information about an extremely noisy and volatile environment. A new view of signaling networks as systems consisting of multiple complex elements interacting in a multifarious fashion is emerging, a view that conflicts with the single-gene or protein-centric approach common in biological research. The postgenomic era has brought about a different, network-centric methodology of analysis, suddenly forcing researchers toward the opposite extreme of complexity, where the networks being explored are, to a certain extent, intractable and uninterpretable. Both the cartoons of simple pathways and the very large "hair-ball" diagrams of large intracellular networks are also representations of static worlds, superficially devoid of dynamics and chemistry. These representations are often viewed as being analogous to stably linked computer and neural networks rather than dynamically changing networks of chemical interactions, where the notions of concentration, compartmentalization, and diffusion may be the primary determinants of connectivity. Arguably, the systems biology approach, relying on computational modeling coupled with various experimental techniques and methodologies, will be an essential component of analysis of the behavior of signal transduction pathways. Combining the dynamical view of rapidly evolving responses and the structural view arising from high-throughput analyses of the interacting species will be the best approach toward efforts toward greater understanding of intracellular signaling processes.

p300 regulates the synergy of steroidogenic factor-1 and early growth response-1 in activating luteinizing hormone-beta subunit gene

Tight regulation of luteinizing hormone-beta subunit (LHbeta) expression is critical for differentiation and maturation of mammalian sexual organs and reproductive function. Two transcription factors, steroidogenic factor-1 (SF-1) and early growth response-1 (Egr-1), play a central role in activating LHbeta promoter, and the synergy between these two factors is essential in mediating gonadotropin-releasing hormone stimulation of LHbeta promoter. Here we demonstrate that the transcriptional co-activator p300 regulates this synergy. Overexpression of p300 results in strong stimulation of LHbeta promoter but only in the presence of both SF-1 and Egr-1, and not in the presence of other Egr proteins. Mutation of the binding sites for either SF-1 or Egr-1 completely abolishes the synergy between these two factors, as well as the influence of p300. Importantly, LHbeta promoter is precipitated using p300 antibodies in a chromatin immunoprecipitation assay with LbetaT2 gonadotropes, and this effect is enhanced by gonadotropin-releasing hormone. The influence of p300 on LHbeta promoter is potentiated by steroid receptor co-activator, as well as by E1A proteins, and attenuated by Smad nuclear interacting protein 1. Taken together, these results suggest that p300 is recruited to LHbeta promoter where it coordinates the functional synergy between SF-1 and Egr-1.

Differential role of p300 and CBP acetyltransferase during myogenesis: p300 acts upstream of MyoD and Myf5

Studies in tissue culture cells have implicated p300 and CBP acetyltransferases in myogenic regulatory factor (MRF) mediated transcription and terminal differentiation of skeletal muscle cells. However, in vivo data placing p300 and CBP on myogenic differentiation pathways are not yet available. In this report we provide genetic evidence that p300 but not CBP acetyltransferase (AT) activity is required for myogenesis in the mouse and in embryonic stem (ES) cells. A fraction of embryos carrying a single p300 AT- deficient allele exhibit impaired MRF expression, delayed terminal differentiation and a reduced muscle mass. In mouse embryos lacking p300 protein, Myf-5 induction is severely attenuated. Similarly, ES cells homozygous for a p300 AT or a p300 null mutation fail to activate Myf5 and MyoD transcription efficiently, while Pax3, acting genetically upstream of these MRFs, is expressed. In contrast, ES cells lacking CBP AT activity express MyoD and Myf5 and undergo myogenic differentiation. These data reveal a specific requirement for p300 and its AT activity in the induction of MRF gene expression and myogenic cell fate determination in vivo.

Induction of disease-associated keratin 16 gene expression by epidermal growth factor is regulated through cooperation of transcription factors Sp1 and c-Jun

Overexpression of keratin 16 has been observed in keratinocytes in those skin diseases characterized by hyperproliferation such as psoriasis. Therefore, keratin 16 is usually referred to as a disease-associated keratin. In the present study, we found that epidermal growth factor (EGF) increased the expression of keratin 16 mRNA and protein synthesis in a time-dependent manner in HaCaT cells. Reporter assays revealed that the EGF response region was in the range of -162 to -114 bp. Disruption of the Sp1 site (-127 to -122 bp) and the AP1 site (-148 to -142 bp) of the keratin 16 promoter by site-directed mutagenesis significantly inhibited keratin 16 promoter activity induced by EGF. Furthermore, keratin 16 gene expression induced by Ras activation was also regulated in the same manner as the EGF response. By using the DNA affinity precipitation assay in HaCaT and SL2 cells, Sp1 directly interacted with the Sp1 site of the promoter, and c-Jun and c-Fos precipitated with the Sp1 oligonucleotide was attributable to the interaction between the Sp1 and AP1 proteins. Moreover, cotransfection assays revealed that Sp1 acted synergistically with c-Jun to activate keratin 16. The coactivators p300/CBP could collaborate with Sp1 and c-Jun in the activation of keratin 16 promoter, and EGF-induced promoter activation was blocked by the viral oncoprotein E1A. Taken together, these results suggest that Sp1 and AP1 sites in the essential promoter region are critical for EGF response, and Sp1 showed a functional cooperation with c-Jun and coactivators p300/CBP in driving the transcriptional regulation of EGF-induced keratin 16 gene expression.

Targeting combinatorial transcriptional complex assembly at specific modules within the interleukin-2 promoter by the immunosuppressant SB203580

The proximal promoter sequence of the interleukin-2 (IL-2) gene contains a series of composite sites or modules that controls much of its responsiveness to environmental stimuli. The integrated targeting of these modules is therefore a major mode of regulation. This report describes how multiple functional hierarchies, required for the recruitment of the p300 co-activator to the CD28RE/AP1 (TRE) module of the IL-2 promoter, are selectively disrupted in human T-cells by the immunosuppressive and anti-inflammatory actions of the p38 mitogen-activated protein kinase inhibitor (MAPK), SB203580. The molecular hierarchies targeted by SB203580 include the combinatorial interaction of NF-kappaB and CREB at the CD28RE/AP1 element coupled with the subsequent dynamic co-assembly and activation of p300. Several aspects of this targeting are linked to the ability of SB203580 to inhibit p38 MAPK-controlled pathways. Together, these results provide the molecular basis through which the combinatorial structure and context of the composite elements of the IL-2 promoter dictates mitogen responsiveness and drug susceptibility that are quantitatively and qualitatively distinct from the isolated action of single consensus sequences and/or transcriptional motifs.

Retinoid receptors and their coregulators

Retinoids regulate gene transcription by binding to the nuclear receptors, the retinoic acid (RA) receptors (RARs), and the retinoid X receptors (RXRs). RARs and RXRs are ligand-activated transcription factors for the regulation of RA-responsive genes. The actions of RARs and RXRs on gene transcription require a highly coordinated interaction with a large number of coactivators and corepressors. This review focuses on our current understanding of these coregulators known to act in concert with RARs and RXRs. The mechanisms of action of these coregulators are beginning to be uncovered and include the modification of chromatin and the recruitment of basal transcription factors. Challenges remain to understand the specificity of action of RARs and RXRs and the formation of specific transcription complexes consisting of the receptors, coregulators, and other unknown factors.

p300 is involved in formation of the TBP-TFIIA-containing basal transcription complex, TAC

We have recently identified a novel basal transcription complex, TAC, that is present and active in embryonal carcinoma (EC) cells but not in other adult cells such as COS7. In the search for factors involved in TAC formation, we found that expression of the adenoviral 12S E1A oncoprotein abolishes TAC formation in EC cells. This effect of E1A depends on its N-terminal domain that is essential for cell differentiation and that targets the transcriptional coactivators p300 and PCAF. Expression of p300 lacking its major E1A interaction domain, CH3, restores TAC formation in the presence of E1A, in a bromodomain- and HAT domain-dependent manner. Consistently, the unprocessed TFIIAalphabeta precursor that is selectively assembled into TAC is acetylated preferentially compared with the processed subunits present in 'free' TFIIA. Intriguingly, expression of p300 in COS7 cells that do not contain detectable levels of TAC instigates formation of TAC from endogenous components. Our data suggest that p300 plays a role in formation of the TBP-TFIIA-containing basal transcription complex, TAC.

Akt regulates basic helix-loop-helix transcription factor-coactivator complex formation and activity during neuronal differentiation

Neural basic helix-loop-helix (bHLH) transcription factors regulate neurogenesis in vertebrates. Signaling by peptide growth factors also plays critical roles in regulating neuronal differentiation and survival. Many peptide growth factors activate phosphatidylinositol 3-kinase (PI3K) and subsequently the Akt kinases, raising the possibility that Akt may impact bHLH protein function during neurogenesis. Here we demonstrate that reducing expression of endogenous Akt1 and Akt2 by RNA interference (RNAi) reduces neuron generation in P19 cells transfected with a neural bHLH expression vector. The reduction in neuron generation from decreased Akt expression is not solely due to decreased cell survival, since addition of the caspase inhibitor z-VAD-FMK rescues cell death associated with loss of Akt function but does not restore neuron formation. This result indicates that Akt1 and Akt2 have additional functions during neuronal differentiation that are separable from neuronal survival. We show that activated Akt1 enhances complex formation between bHLH proteins and the transcriptional coactivator p300. Activated Akt1 also significantly augments the transcriptional activity of the bHLH protein neurogenin 3 in complex with the coactivators p300 or CBP. In addition, inhibition of endogenous Akt activity by the PI3K/Akt inhibitor LY294002 abolishes transcriptional cooperativity between the bHLH proteins and p300. We propose that Akt regulates the assembly and activity of bHLH-coactivator complexes to promote neuronal differentiation.

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.

Biological role of p300 in cardiac myocytes

A cellular target of adenovirus E1A oncoprotein, p300 is a transcriptional coactivator required for the maintenance of differentiated phenotypes in cardiac myocytes. The full transcriptional activities of hypertrophy-responsive transcription factors such as GATA-4 and MEF2 require interaction with p300. A p300 protein also possesses intrinsic histone acetyl transferase activity, which promotes a transcriptionally active chromatin configuration. Here, we review the biological functions of p300 in cardiac myocytes. Although p300 is biologically active in many cell types, this protein appears to play a crucial role in the differentiation, growth and apoptosis of cardiac myocytes. Understanding precise mechanisms of its biological functions will shed light on molecular pathways for heart failure.

A novel EID-1 family member, EID-2, associates with histone deacetylases and inhibits muscle differentiation

An EID-1 (E1A-like inhibitor of differentiation-1) inhibits differentiation by blocking the histone acetyltransferase activity of p300. Here we report a novel inhibitor of differentiation exhibiting homology to EID-1, termed EID-2 (EID-1-like inhibitor of differentiation-2). EID-2 inhibited MyoD-dependent transcription and muscle differentiation. Unlike EID-1, EID-2 did not block p300 activity. Interestingly, EID-2 associated with class I histone deacetylases (HDACs). The N-terminal portion of EID-2 was required for the binding to HDACs. This region was also involved in the transcriptional repression and nuclear localization, suggesting the importance of the involvement of HDACs in the EID-2 function. These results indicate a new family of differentiation inhibitors, although there are several differences in the biochemical mechanisms between EID-2 and EID-1.

Physical and functional interactions among AP-2 transcription factors, p300/CREB-binding protein, and CITED2

The transcriptional co-activators and histone acetyltransferases p300/CREB-binding protein (CBP) interact with CITED2, a transcription factor AP-2 (TFAP2) co-activator. p300/CBP, CITED2, and TFAP2A are essential for normal neural tube and cardiac development. Here we show that p300 and CBP co-activate TFAP2A in the presence of CITED2. TFAP2A transcriptional activity was modestly impaired in p300(+/-) and CBP(+/-) mouse embryonic fibroblasts; this was rescued by ectopic expression of p300/CBP. p300, TFAP2A, and endogenous CITED2 could be co-immunoprecipitated from transfected U2-OS cells indicating that they can interact physically in vivo. CITED2 interacted with the dimerization domain of TFAP2C, which is highly conserved in TFAP2A/B. In mammalian two-hybrid experiments, full-length p300 and TFAP2A interacted only when CITED2 was co-transfected. N-terminal residues of TFAP2A, containing the transactivation domain, are both necessary and sufficient for interaction with p300, and this interaction was independent of CITED2. Consistent with this, N-terminal residues of TFAP2A were required for p300- and CITED2-dependent co-activation. A histone acetyltransferase-deficient p300 mutant (D1399Y) did not co-activate TFAP2A and did not affect the expression or cellular localization of TFAP2A or CITED2. In mammalian two-hybrid experiments p300D1399Y failed to interact with TFAP2A, explaining, at least in part, its failure to function as a co-activator. Our results suggest a model wherein interactions among TFAP2A, CITED2, and p300/CBP are necessary for TFAP2A-mediated transcriptional activation and for normal neural tube and cardiac development.

Cardiac p300 is involved in myocyte growth with decompensated heart failure

A variety of stresses on the heart initiate a number of subcellular signaling pathways, which finally reach the nuclei of cardiac myocytes and cause myocyte hypertrophy with heart failure. However, common nuclear pathways that lead to this state are unknown. A zinc finger protein, GATA-4, is one of the transcription factors that mediate changes in gene expression during myocardial-cell hypertrophy. p300 not only acts as a transcriptional coactivator of GATA-4, but also possesses an intrinsic histone acetyltransferase activity. In primary cardiac myocytes derived from neonatal rats, we show that stimulation with phenylephrine increased an acetylated form of GATA-4 and its DNA-binding activity, as well as expression of p300. A dominant-negative mutant of p300 suppressed phenylephrine-induced nuclear acetylation, activation of GATA-4-dependent endothelin-1 promoters, and hypertrophic responses, such as increase in cell size and sarcomere organization. In sharp contrast to the activation of cardiac MEK-1, which phosphorylates GATA-4 and causes compensated hypertrophy in vivo, p300-mediated acetylation of mouse cardiac nuclear proteins, including GATA-4, results in marked eccentric dilatation and systolic dysfunction. These findings suggest that p300-mediated nuclear acetylation plays a critical role in the development of myocyte hypertrophy and represents a pathway that leads to decompensated heart failure.

The co-activator p300 associates physically with and can mediate the action of the distal enhancer of the FGF-4 gene

Distal enhancers commonly regulate gene expression. However, the mechanisms of transcriptional mediation by distal enhancers remain largely unknown. To better understand distal enhancer-mediated transcription, we examined the regulation of the FGF-4 gene. The FGF-4 gene is regulated during early development by a powerful distal enhancer located downstream of the promoter in exon 3. Sox-2 and Oct-3 bind to the enhancer and are required for the activation of the FGF-4 gene. Previously, we implicated the co-activator p300 as a mediator of Sox-2/Oct-3 synergistic activation of a heterologous promoter, suggesting that p300 may play a role in mediating enhancer activation of the FGF-4 gene. In this study, we provide both functional and physical evidence that p300 plays an important role in the action of the FGF-4 enhancer. Specifically, we show that E1a, but not a mutant form of E1a that is unable to bind p300, inhibits enhancer activation of the FGF-4 promoter. We also demonstrate that Gal4/p300 fusion proteins can stimulate the FGF-4 promoter when bound to the FGF-4 enhancer. Additionally, we present evidence that p300 mediation of the FGF-4 enhancer requires acetyltransferase activity. Importantly, we also show that Sox-2 and p300 are physically associated with the endogenous FGF-4 enhancer but weakly associated with the endogenous FGF-4 promoter. These results are consistent with a model of transitory interaction between the distal enhancer and the FGF-4 promoter. Our results also suggest that intragenic distal enhancers may use mechanisms that differ from extragenic distal enhancers.

Histone acetyltransferases and deacetylases in the control of cell proliferation and differentiation

Histone acetylation and deacetylation are chromatin-modifying processes that have fundamental importance for transcriptional regulation. Transcriptionally active chromatin regions show a high degree of histone acetylation, whereas deacetylation events are generally linked to transcriptional silencing. Many of the acetylating and deacetylating enzymes were originally identified as transcriptional coactivators or repressors. Their histone-modifying enzymatic activity was discovered more recently, opening up a whole new area of research. Histone acetyltransferases such as CREB-binding protein (CBP) and PCAF are involved in processes as diverse as promoting cell cycle progression and regulating differentiation. A controlled balance between histone acetylation and deacetylation seems to be essential for normal cell growth. Both histone acetyltransferases and deacetylases are involved in the development of diseases, including neurodegenerative disorders and cancer. Treatments that target these enzymes are already under clinical investigation.

MyoD stimulates RB promoter activity via the CREB/p300 nuclear transduction pathway

The induction of RB gene transcription by MyoD is a key event in the process of skeletal muscle differentiation, because elevated levels of the retinoblastoma protein are essential for myoblast cell cycle arrest as well as for the terminal differentiation and survival of postmitotic myocytes. We previously showed that MyoD stimulates transcription from the RB promoter independently of direct binding to promoter sequences. Here we demonstrate that stimulation by MyoD requires a cyclic AMP-responsive element (CRE) in the RB promoter, bound by the transcription factor CREB in differentiating myoblasts. We also show that both the CREB protein level and the level of phosphorylation of the CREB protein at Ser-133 rapidly increase at the onset of muscle differentiation and that both remain high throughout the myogenic process. Biochemical and functional evidence indicates that in differentiating myoblasts, MyoD becomes associated with CREB and is targeted to the RB promoter CRE in a complex also containing the p300 transcriptional coactivator. The resulting multiprotein complex stimulates transcription from the RB promoter. These and other observations strongly suggest that MyoD functions by promoting the efficient recruitment of p300 by promoter-bound, phosphorylated CREB.

ERK1/2-dependent contractile protein expression in vascular smooth muscle cells

In vivo, vascular smooth muscle (VSM) cells change their contractile phenotype toward a more proliferative phenotype during the pathogenesis of vascular diseases. Because these dedifferentiated VSM cells may gradually regain contractile functions, we aimed to identify signaling pathways that result in an increased expression of contractile proteins in VSM cells. In vitro, serum and thrombin induced a reversible upregulation of smooth muscle myosin heavy-chain (SM-MHC) in cultured neonatal rat VSM cells. Cotransfection of a SM-MHC-promoter chloramphenicol acetyltransferase-construct with dominant-negative N17Ras or N17Raf or treatment with the mitogen-activated/ERK-activating kinase (MEK) inhibitor PD 98059 concentration dependently decreased the serum- or thrombin-induced SM-MHC promoter activity. Consistently, the serum- or thrombin-induced phosphorylation of MEK and extracellular signal-regulated kinase 1/2 (ERK1/2) coincided with a MEK-dependent nuclear accumulation of phosphorylated ERK1/2 and subsequent nuclear phosphorylation of the transcription factors c-myc and Elk-1. A 5'-deletion analysis of cis-elements within the SM-MHC promoter demonstrated that a conserved region (nucleotide -1346 to -1102) was required for both cell type-specific expression and serum- or thrombin-induced upregulation of the SM-MHC promoter in VSM cells. Within this region, 2 CArG-boxes, a GC-rich element, and a CTF/NF-1 site are critical positively acting cis-elements for the serum- or thrombin-induced upregulation of SM-MHC. We conclude that the serum- or thrombin-induced differentiation requires an intact Ras/Raf/MEK/ERK signaling cascade, nuclear translocation of activated ERK1/2, phosphorylation of transcription factors, and several cis-elements within the SM-MHC promoter.

Recruitment of p300 by C/EBPbeta triggers phosphorylation of p300 and modulates coactivator activity

Transcriptional coactivators such as p300 act as crucial elements in the eukaryotic gene regulation network. These proteins bind to various transcription factors which recruit them to specific gene regions whose chromatin structure subsequently is remodeled. Previously, we have shown that C/EBPbeta recruits p300 by interacting with the E1A-binding site of the coactivator. We now show that C/EBPbeta not only binds to p300 but also triggers massive phosphorylation of p300. This novel activity of C/EBPbeta is dependent on the E1A-binding region of p300 as well as on several subdomains of C/EBPbeta, all of which are involved in the p300-C/EBPbeta interaction. We have identified several sites of C/EBPbeta-inducible phosphorylation within the C-terminal domain of p300. Mutation of these sites substantially impairs the activity of p300 as a coactivator of C/EBPbeta. Interestingly, phosphorylation of p300 is also triggered by other C/EBP family members as well as by various other transcription factors that interact with the E1A-binding domain of p300, suggesting that this novel phosphorylation mechanism may be of general relevance.