Discovery and Characterization of Active CBP/EP300 Degraders Targeting the HAT Domain

Proteolysis Targeting Chimeras (PROTACs) are bifunctional molecules that simultaneously bind an E3 ligase and a protein of interest, inducing degradation of the latter via the ubiquitin-proteasome system. Here we present the development of degraders targeting CREB-binding protein (CBP) and E1A-associated protein (EP300)-two homologous multidomain enzymes crucial for enhancer-mediated transcription. Our PROTAC campaign focused on CPI-1612, a reported inhibitor of the histone acetyltransferase (HAT) domain of these two proteins. A novel asymmetric synthesis of this ligand was devised, while PROTAC-SAR was explored by measuring degradation, target engagement, and ternary complex formation in cellulo. Our study demonstrates that engagement of Cereblon (CRBN) and a sufficiently long linker between the E3 and CBP/EP300 binders (≥21 atoms) are required for PROTAC-mediated degradation using CPI-1612 resulting in a new active PROTAC dCE-1. Lessons learned from this campaign, particularly the importance of cell-based assays to understand the reasons underlying PROTAC performance, are likely applicable to other targets to assist the development of degraders.

EZH2 targeting reduces medulloblastoma growth through epigenetic reactivation of the BAI1/p53 tumor suppressor pathway

Medulloblastoma (MB) is a malignant pediatric brain tumor for which new therapies are urgently needed. We demonstrate that treatment with EPZ-6438 (Tazemetostat), an enhancer of zeste homolog 2 (EZH2) inhibitor approved for clinical trials, blocks MB cell growth in vitro and in vivo, and prolongs survival in orthotopic xenograft models. We show that the therapeutic effect is dependent on epigenetic reactivation of adhesion G-protein-coupled receptor B1 (BAI1/ADGRB1), a tumor suppressor that controls p53 stability by blocking Mdm2. Histone 3 trimethylated on lysine 27 (H3K27me3), a marker of silent chromatin conformation is present at the ADGRB1 promoter, and inhibition of EZH2, the catalytic component of the Polycomb Repressive complex 2 (PRC2) that methylates H3K27, switches the gene into an active chromatin status and reactivates BAI1 expression. Mechanistically, targeting EZH2 promotes transition from H3K27me3 to H3K27ac at the promoter, recruits the C/EBPβ (CREB-binding protein) and CBP transcription factors and activates ADGRB1 gene transcription. Taken together, our results identify key molecular players that regulate ADGRB1 gene expression in MB, demonstrate that reactivation of BAI1 expression underlies EPZ-6438 antitumorigenic action, and provide preclinical proof-of-principle evidence for targeting EZH2 in patients with MB.

Crebbp Loss Drives Small Cell Lung Cancer and Increases Sensitivity to HDAC Inhibition

CREBBP, encoding an acetyltransferase, is among the most frequently mutated genes in small cell lung cancer (SCLC), a deadly neuroendocrine tumor type. We report acceleration of SCLC upon Crebbp inactivation in an autochthonous mouse model. Extending these observations beyond the lung, broad Crebbp deletion in mouse neuroendocrine cells cooperated with Rb1/Trp53 loss to promote neuroendocrine thyroid and pituitary carcinomas. Gene expression analyses showed that Crebbp loss results in reduced expression of tight junction and cell adhesion genes, including Cdh1, across neuroendocrine tumor types, whereas suppression of Cdh1 promoted transformation in SCLC. CDH1 and other adhesion genes exhibited reduced histone acetylation with Crebbp inactivation. Treatment with the histone deacetylase (HDAC) inhibitor Pracinostat increased histone acetylation and restored CDH1 expression. In addition, a subset of Rb1/Trp53/Crebbp-deficient SCLC exhibited exceptional responses to Pracinostat in vivo Thus, CREBBP acts as a potent tumor suppressor in SCLC, and inactivation of CREBBP enhances responses to a targeted therapy.Significance: Our findings demonstrate that CREBBP loss in SCLC reduces histone acetylation and transcription of cellular adhesion genes, while driving tumorigenesis. These effects can be partially restored by HDAC inhibition, which exhibited enhanced effectiveness in Crebbp-deleted tumors. These data provide a rationale for selectively treating CREBBP-mutant SCLC with HDAC inhibitors. Cancer Discov; 8(11); 1422-37. ©2018 AACR. This article is highlighted in the In This Issue feature, p. 1333.

Streptococcus Mutans: A Potential Risk Factor in Recurrent Hemorrhagic Stroke

Stroke is the fifth leading cause of death and is responsible for approximately nine percent of all deaths worldwide. Cases of Streptococcus mutans (S. mutans)-induced intracerebral hemorrhage as a result of bloodstream infections have seldom been reported. New reports show that bacteria with specific collagen binding proteins (CBPs), such as the Cnm type produced by S. mutans, may inhibit platelet aggregation and cause bleeding. In this article, we report on a 62-year-old man with a recent history of left frontal intracerebral hemorrhage (ICH) who presented to the emergency department after a fall due to suspected seizure while in rehabilitation. Computed tomography (CT) scan of the brain showed a right cerebellar hemorrhage with surrounding edema and mass effect on the fourth ventricle. A suboccipital craniotomy to evacuate the cerebellar ICH was completed without complication. Radiologic and angiographic assessments regarding the etiology of this patient's stroke did not reveal any evidence of vascular pathology or mycotic aneurysms to explain his recurrent intracranial hemorrhages. Through persistent patient and family interviews, it came to light that a few weeks prior to the patient's first ICH, he was diagnosed with a bloodstream infection by S. mutans. Bacteremia is known to be associated with embolic stroke, but only recently has it been shown that bacteremia can also be implicated in hemorrhagic stroke. S. mutans of the k serotype have specific CBPs that are attracted to exposed collagen in previously damaged small vessel walls. These bacterial proteins can interrupt the blood clotting cascade through the prevention of platelet aggregation, increasing the risk of intracerebral hemorrhage.

Transcriptional coactivator CBP upregulates hTERT expression and tumor growth and predicts poor prognosis in human lung cancers

Upregulated expression and activation of human telomerase reverse transcriptase (hTERT) is a hallmarker of lung tumorigenesis. However, the mechanism underlying the aberrant hTERT activity in lung cancer cells remains poorly understood. In this study, we found the transcriptional co-activator CBP as a new hTERT promoter-binding protein that regulated hTERT expression and tumor growth in lung adenocarcinoma cells using a biotin-streptavidin-bead pulldown technique. Chromatin immunoprecipitation assay verified the immortalized cell and tumor cell-specific binding of CBP on hTERT promoter. Overexpression of exogenous CBP upregulated the expression of the hTERT promoter-driven luciferase and endogenous hTERT protein in lung cancer cells. Conversely, inhibition of CBP by CBP-specific siRNA or its chemical inhibitor repressed the expression of hTERT promoter-driven luciferase and endogenous hTERT protein as well as telomerase activity. Moreover, inhibition of CBP expression or activity also significantly reduced the proliferation of lung cancer cells in vitro and tumor growth in an xenograft mouse model in vivo. Immunohistochemical analysis of tissue microarrays of lung cancers revealed a positive correlation between CBP and hTERT. Importantly, the patients with high CBP and hTERT expression had a significantly shorter overall survival. Furthermore, CBP was found to interact with and acetylate transactivator Sp1 in lung cancer cells. Inhibition of CBP by CBP-specific siRNA or its chemical inhibitor significantly inhibited Sp1 acetylation and its binding to the hTERT promoter. Collectively, our results indicate that CBP contributes to the upregulation of hTERT expression and tumor growth, and overexpression of CBP predicts poor prognosis in human lung cancers.

Small-molecule inhibition of CBP/catenin interactions eliminates drug-resistant clones in acute lymphoblastic leukemia

Drug resistance in acute lymphoblastic leukemia (ALL) remains a major problem warranting new treatment strategies. Wnt/catenin signaling is critical for the self-renewal of normal hematopoietic progenitor cells. Deregulated Wnt signaling is evident in chronic and acute myeloid leukemia; however, little is known about ALL. Differential interaction of catenin with either the Kat3 coactivator CREBBP (CREB-binding protein (CBP)) or the highly homologous EP300 (p300) is critical to determine divergent cellular responses and provides a rationale for the regulation of both proliferation and differentiation by the Wnt signaling pathway. Usage of the coactivator CBP by catenin leads to transcriptional activation of cassettes of genes that are involved in maintenance of progenitor cell self-renewal. However, the use of the coactivator p300 leads to activation of genes involved in the initiation of differentiation. ICG-001 is a novel small-molecule modulator of Wnt/catenin signaling, which specifically binds to the N-terminus of CBP and not p300, within amino acids 1-110, thereby disrupting the interaction between CBP and catenin. Here, we report that selective disruption of the CBP/β- and γ-catenin interactions using ICG-001 leads to differentiation of pre-B ALL cells and loss of self-renewal capacity. Survivin, an inhibitor-of-apoptosis protein, was also downregulated in primary ALL after treatment with ICG-001. Using chromatin immunoprecipitation assay, we demonstrate occupancy of the survivin promoter by CBP that is decreased by ICG-001 in primary ALL. CBP mutations have been recently identified in a significant percentage of ALL patients, however, almost all of the identified mutations reported occur C-terminal to the binding site for ICG-001. Importantly, ICG-001, regardless of CBP mutational status and chromosomal aberration, leads to eradication of drug-resistant primary leukemia in combination with conventional therapy in vitro and significantly prolongs the survival of NOD/SCID mice engrafted with primary ALL. Therefore, specifically inhibiting CBP/catenin transcription represents a novel approach to overcome relapse in ALL.

The role of histone acetylation in cocaine-induced neural plasticity and behavior

How do drugs of abuse, such as cocaine, cause stable changes in neural plasticity that in turn drive long-term changes in behavior? What kind of mechanism can underlie such stable changes in neural plasticity? One prime candidate mechanism is epigenetic mechanisms of chromatin regulation. Chromatin regulation has been shown to generate short-term and long-term molecular memory within an individual cell. They have also been shown to underlie cell fate decisions (or cellular memory). Now, there is accumulating evidence that in the CNS, these same mechanisms may be pivotal for drug-induced changes in gene expression and ultimately long-term behavioral changes. As these mechanisms are also being found to be fundamental for learning and memory, an exciting new possibility is the extinction of drug-seeking behavior by manipulation of epigenetic mechanisms. In this review, we critically discuss the evidence demonstrating a key role for chromatin regulation via histone acetylation in cocaine action.

The organochlorine o,p'-DDT plays a role in coactivator-mediated MAPK crosstalk in MCF-7 breast cancer cells

BACKGROUND

The organochlorine dichlorodiphenyltrichloroethane (DDT), a known estrogen mimic and endocrine disruptor, has been linked to animal and human disorders. However, the detailed mechanism(s) by which DDT affects cellular physiology remains incompletely defined.

OBJECTIVES

We and others have shown that DDT activates cell-signaling cascades, culminating in the activation of estrogen receptor-dependent and -independent gene expression. Here, we identify a mechanism by which DDT alters cellular signaling and gene expression, independent of the estrogen receptor.

METHODS

We performed quantitative polymerase chain reaction array analysis of gene expression in MCF-7 breast cancer cells using either estradiol (E₂) or o,p´-DDT to identify distinct cellular gene expression responses. To elucidate the mechanisms by which DDT regulates cell signaling, we used molecular and pharmacological techniques.

RESULTS

E₂ and DDT treatment both altered the expression of many of the genes assayed, but up-regulation of vascular endothelial growth factor A (VEGFA) was observed only after DDT treatment, and this increase was not affected by the pure estrogen receptor α antagonist ICI 182780. Furthermore, DDT increased activation of the HIF-1 response element (HRE), a known enhancer of the VEGFA gene. This DDT-mediated increase in HRE activity was augmented by the coactivator CBP (CREB-binding protein) and was dependent on the p38 pathway.

CONCLUSIONS

DDT up-regulated the expression of several genes in MCF-7 breast cancer cells that were not altered by treatment with E₂, including VEGFA. We propose that this DDT-initiated, ER-independent stimulation of gene expression is due to DDT's ability to initiate crosstalk between MAPK (mitogen-activated protein kinase) signaling pathways and transcriptional coactivators.

Experience-facilitated improvements in pup retrieval; evidence for an epigenetic effect

The quality and quantity of maternal care received during infancy are highly predictive of successful infant development. It has been well established, primarily in rats, that the combination of hormonal and infant stimuli at birth modifies neural circuits that regulate maternal responsiveness. During subsequent interactions, infant stimuli are more likely to elicit rapid maternal responsiveness. Some species, such as humans, can display maternal care in the absence of the endocrine events of pregnancy and birth. Similarly, virgin C57BL/6J female mice, display maternal care toward infants, and experience with infants elicits long-lasting increases in maternal care. We hypothesized that these experience-induced changes in behavior may be mediated by chromatin modifications, which in turn change expression of genes that promote maternal care. One site of action is the medial preoptic area (MPOA). To test our hypothesis we treated virgin female mice with sodium butyrate, a histone deacetylase inhibitor. This treatment potentiated maternal responsiveness as well as the expression of several genes: estrogen receptor β (Esr2), oxytocin (Oxt), and cyclicAMP response element binding protein (CREB) binding protein (Crebbp; a histone acetyltransferase) in the MPOA. These data suggest that experience induces high levels of maternal care via epigenetic modifications.

Histone deacetylase 6 (HDAC6) deacetylates survivin for its nuclear export in breast cancer

Survivin is an oncogenic protein that is highly expressed in breast cancer and has a dual function that is dependent on its subcellular localization. In the cytosol, survivin blocks programmed cell death by inactivating caspase proteins; however, in the nucleus it facilitates cell division by regulating chromosomal movement and cytokinesis. In prior work, we showed that survivin is acetylated by CREB-binding protein (CBP), which restricts its localization to the nuclear compartment and thereby inhibits its anti-apoptotic function. Here, we identify histone deacetylase 6 (HDAC6) as responsible for abrogating CBP-mediated survivin acetylation in the estrogen receptor (ER)-positive breast cancer cell line, MCF-7. HDAC6 directly binds survivin, an interaction that is enhanced by CBP. In quiescent breast cancer cells in culture and in malignant tissue sections from ER+ breast tumors, HDAC6 localizes to a perinuclear region of the cell, undergoing transport to the nucleus following CBP activation where it then deacetylates survivin. Genetically modified mouse embryonic fibroblasts that lack mhdac6 localize survivin predominantly to the nuclear compartment, whereas wild-type mouse embryonic fibroblasts localize survivin to distinct cytoplasmic structures. Together, these data imply that HDAC6 deacetylates survivin to regulate its nuclear export, a feature that may provide a novel target for patients with ER+ breast cancer.

Hippocampal focal knockout of CBP affects specific histone modifications, long-term potentiation, and long-term memory

To identify the role of the histone acetyltransferase (HAT) CREB-binding protein (CBP) in neurons of the CA1 region of the hippocampus during memory formation, we examine the effects of a focal homozygous knockout of CBP on histone modifications, gene expression, synaptic plasticity, and long-term memory. We show that CBP is critical for the in vivo acetylation of lysines on histones H2B, H3, and H4. CBP's homolog p300 was unable to compensate for the loss of CBP. Neurons lacking CBP maintained phosphorylation of the transcription factor CREB, yet failed to activate CREB:CBP-mediated gene expression. Loss of CBP in dorsal CA1 of the hippocampus resulted in selective impairments to long-term potentiation and long-term memory for contextual fear and object recognition. Together, these results suggest a necessary role for specific chromatin modifications, selectively mediated by CBP in the consolidation of memories.

Epigenetic regulation in substance use disorders

Substance use disorder is a chronic condition of compulsive drug seeking and use that is mediated by stable changes in central reward pathways. Repeated use of abused drugs causes persistent alterations in gene expression responsible for the long-term behavioral and structural changes. Recently, it has been suggested that epigenetic mechanisms are responsible in part for these drug-induced changes in gene expression. One of the alluring aspects of epigenetic regulation of gene expression is that epigenetic mechanisms may provide transient and potentially stable conditions that in turn may ultimately participate in the molecular mechanisms required for neuronal changes subserving long-lasting changes in behavior. This review describes epigenetic mechanisms of gene regulation and then discusses the emerging role of epigenetics in drug-induced plasticity and behavior. Understanding these mechanisms that establish and maintain drug-dependent plasticity changes may lead to deeper understanding of substance use disorders as well as novel approaches to treatment.

GATA-2 functions downstream of BMPs and CaM KIV in ectodermal cells during primitive hematopoiesis

In Xenopus, primitive blood originates from the mesoderm, but extrinsic signals from the ectoderm are required during gastrulation to enable these cells to differentiate as erythrocytes. The nature of these signals, and how they are transcriptionally regulated, is not well understood. We have previously shown that bone morphogenetic proteins (BMPs) are required to signal to ectodermal cells to generate secondary non-cell-autonomous signal(s) necessary for primitive erythropoiesis, and that calmodulin-dependent protein kinase IV (CaM KIV) antagonizes BMP signaling. The current studies demonstrate that Gata-2 functions downstream of BMP receptor activation in these same cells, and is a direct target for antagonism by CaM KIV. We show, using loss of function analysis in whole embryos and in explants, that ectodermal Gata-2 is required for primitive erythropoiesis, and that BMP signals cannot rescue blood defects caused by ectoderm removal or loss of ectodermal GATA-2. Furthermore, we provide evidence that acetylation of GATA-2 is required for its function in primitive blood formation in vivo. Our data support a model in which Gata-2 is a transcriptional target downstream of BMPs within ectodermal cells, while activation of the CaM KIV signaling pathway alters GATA-2 function posttranslationally, by inhibiting its acetylation.

MED25 is distinct from TRAP220/MED1 in cooperating with CBP for retinoid receptor activation

We isolated MED25, which associates with retinoic acid (RA)-bound retinoic acid receptor (RAR) through the C-terminal nuclear hormone receptor (NR) box/LxxLL motif, and increases RAR/RXR-mediated transcription. When tethered to a promoter, MED25 showed intrinsic transcriptional activity in its PTOV domain, which is likely accomplished by direct association with CBP. Reporter assays using dominant negatives of MED25 demonstrated the importance of the N-terminal Mediator-binding and C-terminal domains in CBP and RAR/RXR binding, which affect MED25 activity. Downregulation of MED25 specifically reduced RAR but not thyroid hormone receptor (TR) activity. Stimulation of RAR by MED25 was correlated with enhanced RA cytotoxicity in vivo. Chromatin immunoprecipitation (ChIP) assays revealed the RA-dependent recruitment of MED25 to the RARbeta2 promoter. Recruitment of CBP and TRAP220 was diminished by the overexpression of a MED25 NR box deletion mutant, and by treatment with MED25 siRNA. Time-course ChIP assays indicated that CBP, together with RAR and MED25, is recruited early, whereas TRAP220 is recruited later to the promoter. Our data suggest that MED25, in cooperation with CBP and Mediators through its distinct domains, imposes a selective advantage on RAR/RXR activation.

Individual CREB-target genes dictate usage of distinct cAMP-responsive coactivation mechanisms

CREB is a key mediator of cAMP- and calcium-inducible transcription, where phosphorylation of serine 133 in its Kinase-Inducible Domain (KID) is often equated with transactivation. Phospho-Ser133 is required for CREB to bind the KIX domain of the coactivators CBP and p300 (CBP/p300) in vitro, although the importance of this archetype coactivator interaction for endogenous gene expression is unclear. Here, we show that the CREB interaction with KIX is necessary for only a part of cAMP-inducible transcription and CBP/p300 recruitment. Surprisingly, individual cAMP-inducible genes with CREB bound at their promoters differed in their reliance on KIX and none examined showed complete dependence. Alternatively, we found that arginine 314 (Arg314) in the CREB basic-leucine zipper (bZIP) domain contributed to CBP/p300 recruitment and KIX-independent CREB transactivation function. This implicates Transducer Of Regulated CREB (TORC), an unrelated cAMP-responsive coactivator that binds via Arg314, and which can bind CBP/p300, in these functions. Interestingly, KIX was also required for the full cAMP induction of a gene that did not require CREB. Thus, individual CREB-target gene context dictates the relative contribution of at least two different cAMP-responsive coactivation mechanisms.

Cells in behaviourally relevant brain regions coexpress nuclear receptor coactivators and ovarian steroid receptors

Oestradiol and progesterone act in the brain to elicit profound effects on behaviour and physiology. One physiological function of oestradiol is the induction of progesterone receptor (PR) expression in a variety of behaviourally relevant brain regions, including the ventromedial nucleus of the hypothalamus (VMN), the medial preoptic nucleus of the preoptic area (MPOA), the arcuate nucleus (ARC) and the medial central grey (MCG). Ligand-dependent transcriptional activity of steroid receptors, including oestrogen receptors (ER) and Pr, is dramatically influenced by nuclear receptor coactivators. In previous studies, we have found that two of these nuclear receptor coactivators, steroid receptor coactivator-1 (SRC-1) and CREB-binding protein (CBP), are important in ER-mediated induction of PR in the VMN and in steroid-dependent behaviours. For nuclear receptor coactivators to function in hormone-dependent transcription in the brain and regulate behaviour, both receptor and coactivator must be expressed in the same cell. In the present study, we used a dual-label immunohistochemical technique to investigate if individual cells in behaviourally relevant brain regions coexpress nuclear receptor coactivators and steroid receptors. Confocal analysis revealed that in oestrogen-primed rats, most of the E-induced PR cells in the VMN (89.6%), MPOA (63%), ARC (82.6%), and many in the MCG (39%), also express SRC-1. In addition, the majority of the cells containing E-induced PR in the VMN (78.3%), MPOA (83.1%), ARC (83.6%), and MCG (60%) also express CBP. These results, taken together with the findings that virtually all oestradiol-induced PR containing cells in the brain express ER, suggest that these neurones represent sites of functional interaction of nuclear receptor coactivators with ovarian steroid receptors in the brain. The present findings provide neuroanatomical evidence that nuclear receptor coactivators are integral in mediating steroid hormone action in behaviourally relevant brain regions.

Host epigenetic modifications by oncogenic viruses

Epigenetic alterations represent an important step in the initiation and progression of most human cancers, but it is difficult to differentiate the early cancer causing alterations from later consequences. Oncogenic viruses can induce transformation via expression of only a small number of viral genes. Therefore, the mechanisms by which oncogenic viruses cause cancer may provide clues as to which epigenetic alterations are critical in early carcinogenesis.

The bHLH domain of Mistl is sufficient to activate gene transcription

Mist1 is a tissue-specific basic helix-loop-helix (bHLH) transcription factor that plays an essential role in maintaining and organizing the exocrine pancreas. Consequently, mice lacking Mist1 exhibit disrupted acinar cellular polarity and defective zymogen granule trafficking. Despite extensive studies demonstrating a requirement for Mist1 in exocrine pancreas development and function, little is known about the molecular targets for Mist1 interaction and the mechanism(s) of how Mist1 regulates gene transcription. To address these deficiencies, a series of molecular studies was performed to identify the preferred Mist1 dimer complex and to establish the preferred DNA binding site for this bHLH factor. In vivo coimmunoprecipitation assays confirmed that the functional Mist1 complex in pancreatic acinar cells was a Mist1 homodimer that bound to a unique DNA target site known as the TA-E-box. Binding of Mist1 to a TA-E-box-regulated promoter led to transcriptional activation of the target gene. Surprisingly, Mist1 truncations containing only the central bHLH domain retained approximately 80% of transcriptional activity. Coimmunoprecipitation studies demonstrated that the bHLH domain interacted with coactivators belonging to the p300/CBP family, suggesting that Mist1 activates exocrine-specific gene transcription through an acetylation mechanism.

Nuclear receptor coactivators function in estrogen receptor- and progestin receptor-dependent aspects of sexual behavior in female rats

The ovarian hormones, estradiol (E) and progesterone (P) facilitate the expression of sexual behavior in female rats. E and P mediate many of these behavioral effects by binding to their respective intracellular receptors in specific brain regions. Nuclear receptor coactivators, including Steroid Receptor Coactivator-1 (SRC-1) and CREB Binding Protein (CBP), dramatically enhance ligand-dependent steroid receptor transcriptional activity in vitro. Previously, our lab has shown that SRC-1 and CBP modulate estrogen receptor (ER)-mediated induction of progestin receptor (PR) gene expression in the ventromedial nucleus of the hypothalamus (VMN) and hormone-dependent sexual receptivity in female rats. Female sexual behaviors can be activated by high doses of E alone in ovariectomized rats, and thus are believed to be ER-dependent. However, the full repertoire of female sexual behavior, in particular, proceptive behaviors such as hopping, darting and ear wiggling, are considered to be PR-dependent. In the present experiments, the function of SRC-1 and CBP in distinct ER- (Exp. 1) and PR- (Exp. 2) dependent aspects of female sexual behavior was investigated. In Exp. 1, infusion of antisense oligodeoxynucleotides to SRC-1 and CBP mRNA into the VMN decreased lordosis intensity in rats treated with E alone, suggesting that these coactivators modulate ER-mediated female sexual behavior. In Exp. 2, antisense to SRC-1 and CBP mRNA around the time of P administration reduced PR-dependent ear wiggling and hopping and darting. Taken together, these data suggest that SRC-1 and CBP modulate ER and PR action in brain and influence distinct aspects of hormone-dependent sexual behaviors. These findings support our previous studies and provide further evidence that SRC-1 and CBP function together to regulate ovarian hormone action in behaviorally-relevant brain regions.

Mutation analysis of CBP and PCAF reveals rare inactivating mutations in cancer cell lines but not in primary tumours

In this study we screened the histone acetyltransferases CBP and PCAF for mutations in human epithelial cancer cell lines and primary tumours. We identified two CBP truncations (both in cell lines), seven PCAF missense variants and four CBP intronic microdeletions. These data suggest that neither gene is commonly inactivated in human epithelial cancers.

Transcription coactivator CBP has direct DNA binding activity and stimulates transcription factor DNA binding through small domains

CBP and p300 are transcriptional coactivators that physically interact with diverse sequence-specific DNA-binding factors through conserved domains. To further investigate the functional roles of these protein-interaction domains in CBP/p300 regulation, we have identified multiple domains of CBP that interact with FKLF2 and the CH2 domain as a new p53 interacting domain of CBP. Functional studies demonstrate that several domains of CBP are capable of stimulating FKLF2 and p53 DNA binding. In addition, we found that CBP through distinct domain is able to bind DNA directly with no specificity. We identified a 51-residue domain in CBP that is capable of interacting with both transcription factors and DNA. We named this domain PDBD for protein and DNA binding domain. These results unveiled two novel activities of CBP. First, these highly conserved domains of CBP not only function to recruit CBP to the target promoter through interaction with DNA-bound transcription factors, but they also actively regulate the DNA binding activity of their interacting factors. Second, by directly interacting with DNA, CBP may orchestrate the formation of stable and promoter-committed transcriptional complexes through interactions with both proteins and promoter DNA.

Gene dose-dependent control of hematopoiesis and hematologic tumor suppression by CBP

Mice with monoallelic inactivation of the CBP gene develop highly penetrant, multilineage defects in hematopoietic differentiation and, with advancing age, an increased incidence of hematologic malignancies. The latter are characterized, at least in some cases, by loss of heterozygosity (LOH) at the CBP locus. No such pathology was observed in wild-type or p300 heterozygous null mice of the same age and genetic background. Thus, a full complement of CBP, but not p300, is required for normal hematopoietic differentiation. These results also provide the first experimental evidence for the hypothesis that CBP has tumor-suppressing activity.

Acetyltransferases and tumour suppression

The acetyltransferase p300 was first identified associated with the adenoviral transforming protein E1A, suggesting a potential role for p300 in the regulation of cell proliferation. Direct evidence demonstrating a role for p300 in human tumours was lacking until the recently publication by Gayther et al, which strongly supports a role for p300 as a tumour suppressor. The authors identify truncating mutations associated with the loss or mutation of the second allele in both tumour samples and cell lines, suggesting that loss of p300 may play a role in the development of a subset of human cancers.

Functional role of p35srj, a novel p300/CBP binding protein, during transactivation by HIF-1

Recruitment of p300/CBP by the hypoxia-inducible factor, HIF-1, is essential for the transcriptional response to hypoxia and requires an interaction between the p300/CBP CH1 region and HIF-1alpha. A new p300-CH1 interacting protein, p35srj, has been identified and cloned. p35srj is an alternatively spliced isoform of MRG1, a human protein of unknown function. Virtually all endogenous p35srj is bound to p300/CBP in vivo, and it inhibits HIF-1 transactivation by blocking the HIF-1alpha/p300 CH1 interaction. p35srj did not affect transactivation by transcription factors that bind p300/CBP outside the CH1 region. Endogenous p35srj is up-regulated markedly by the HIF-1 activators hypoxia or deferoxamine, suggesting that it could operate in a negative-feedback loop. In keeping with this notion, a p300 CH1 mutant domain, defective in HIF-1 but not p35srj binding, enhanced endogenous HIF-1 function. In hypoxic cells, p35srj may regulate HIF-1 transactivation by controlling access of HIF-1alpha to p300/CBP, and may keep a significant portion of p300/CBP available for interaction with other transcription factors by partially sequestering and functionally compartmentalizing cellular p300/CBP.

Chromatin, TAFs, and a novel multiprotein coactivator are required for synergistic activation by Sp1 and SREBP-1a in vitro

The promoter selectivity factor Sp1 often cooperates with other enhancer-binding proteins to activate transcription. To study the molecular underpinnings of these regulatory events, we have reconstituted in vitro the synergy observed in vivo between Sp1 and the sterol-regulated factor SREBP-1a at the low density lipoprotein receptor (LDLR) promoter. Using a highly purified human transcription system, we found that chromatin, TAFs, and a novel SREBP-binding coactivator activity, which includes CBP, are all required to mediate full synergistic activation by Sp1 and SREBP-1a. The SREBP-binding domain of CBP inhibits activation by SREBP-1a and Sp1 in a dominant-negative fashion that is both chromatin- and activator-specific. Whereas recombinant CBP alone is not sufficient to mediate activation, a human cellular fraction containing CBP can support high levels of chromatin-dependent synergistic activation. Purification of this activity to near homogeneity resulted in the identification of a multiprotein coactivator, including CBP, that selectively binds to the SREBP-1a activation domain and is capable of mediating high levels of synergistic activation by SREBP/Sp1 on chromatin templates. The development of a reconstituted chromatin transcription system has allowed us to isolate a novel coactivator that is recruited by the SREBP-1a activation domain and that functions in concert with TFIID to coordinate the action of multiple activators at complex promoters in the context of chromatin.

TGF-beta-stimulated cooperation of smad proteins with the coactivators CBP/p300

TGF-beta and activin induce the phosphorylation and activation of Smad2 and Smad3, but how these proteins stimulate gene transcription is poorly understood. We report that TGF-beta receptor phosphorylation of Smad3 promotes its interaction with the paralogous coactivators CBP and p300, whereas CBP/p300 binding to nonphosphorylated Smad3 or its oligomerization partner Smad4 is negatively regulated by Smad-intramolecular interactions. Furthermore, p300 and TGF-beta receptor-phosphorylated Smad3 synergistically augment transcriptional activation. Thus, CBP/p300 are important components of activin/TGF-beta signaling and may mediate the antioncogenic functions of Smad2 and Smad4.

p300 and estrogen receptor cooperatively activate transcription via differential enhancement of initiation and reinitiation

Estrogen- and antiestrogen-regulated, AF-2-dependent transcriptional activation by purified full-length human estrogen receptor (ER) was carried out with chromatin templates in vitro. With this system, the ability of purified human p300 to function as a transcriptional coactivator was examined. In the absence of ligand-activated ER, p300 was found to have little effect (less than twofold increase) on transcription, whereas, in contrast, p300 was observed to act synergistically with ligand-activated ER to enhance transcription. When transcription was limited to a single round, p300 and ER were found to enhance the efficiency of transcription initiation in a cooperative manner. On the other hand, when transcription reinitiation was allowed to occur, ER, but not p300, was able to increase the number of rounds of transcription. These results suggest a two-stroke mechanism for transcriptional activation by ligand-activated ER and p300. In the first stroke, ER and p300 function cooperatively to increase the efficiency of productive transcription initiation. In the second stroke, ER promotes the reassembly of the transcription preinitiation complex. Therefore, ER exhibits distinct, dual functions in transcription initiation and reinitiation.