Function for p300 and not CBP in the apoptotic response to DNA damage

High Throughput miRNA Screening Identifies miR-574-3p Hyperproductive Effect in CHO Cells

CHO is the cell line of choice for the manufacturing of many complex biotherapeutics. The constant upgrading of cell productivity is needed to meet the growing demand for these life-saving drugs. Manipulation of small non-coding RNAs—miRNAs—is a good alternative to a single gene knockdown approach due to their post-transcriptional regulation of entire cellular pathways without posing translational burden to the production cell. In this study, we performed a high-throughput screening of 2042-human miRNAs and identified several candidates able to increase cell-specific and overall production of Erythropoietin and Etanercept in CHO cells. Some of these human miRNAs have not been found in Chinese hamster cells and yet were still effective in them. We identified miR-574-3p as being able, when overexpressed in CHO cells, to improve overall productivity of Erythropoietin and Etanercept titers from 1.3 to up to 2-fold. In addition, we validated several targets of miR-574-3p and identified p300 as a main target of miR-574-3p in CHO cells. Furthermore, we demonstrated that stable CHO cell overexpressing miRNAs from endogenous CHO pri-miRNA sequences outperform the cells with human pri-miRNA sequences. Our findings highlight the importance of flanking genomic sequences, and their secondary structure features, on pri-miRNA processing offering a novel, cost-effective and fast strategy as a valuable tool for efficient miRNAs engineering in CHO cells.

Role of Cbp, p300 and Akt in valproic acid induced neural tube defects in CD-1 mouse embryos

Valproic acid (VPA), an antiepileptic and mood-stabilizing drug, is prescribed to women of reproductive age. VPA is associated with a 1-2% increase in neural tube defects in offspring following gestational exposure and results in epigenetic modifications induced by perturbations in transcription cofactors. Cbp and p300, two transcription cofactors, play key roles in embryonic neural development. p300 is a downstream target of Akt, a protein kinase B associated with cell survival and anti-apoptotic mechanisms, as part of the Akt-p300 axis. We examined the effects of in utero VPA exposure on Cbp, p300, and Akt in gestational day (GD)9, GD10 and GD13 CD-1 mouse embryos following a teratogenic maternal dose of 400 mg/kg. Embryos were collected at 0, 1, 3 and 6 h post-dosing on GD9, 24 h post-dosing on GD10 and on GD13. GD10 embryos were grouped according to the status of neural tube closure in control, closed and open groups. GD13 heads were grouped as control, exposed but non-exencephalic and exencephalic. Our data indicate that Cbp, p300 and Akt mRNA levels were downregulated at 1 and 3 h post-exposure in GD9 embryos while Cbp and p300 protein levels remained stable. Akt protein levels were significantly increased 1 h post-exposure. No significant changes were observed in either mRNA or protein expression in embryos with closed or open neural tubes compared to the control group at GD10. Downregulated expression of Cbp, p300, and Akt may play a key role in VPA-induced neural tube defects considering their vitally important role in embryonic development.

Challenges in the Structural-Functional Characterization of Multidomain, Partially Disordered Proteins CBP and p300: Preparing Native Proteins and Developing Nanobody Tools

The structural and functional characterization of large multidomain signaling proteins containing long disordered linker regions represents special methodological and conceptual challenges. These proteins show extreme structural heterogeneity and have complex posttranslational modification patterns, due to which traditional structural biology techniques provide results that are often difficult to interpret. As demonstrated through the example of two such multidomain proteins, CREB-binding protein (CBP) and its paralogue, p300, even the expression and purification of such proteins are compromised by their extreme proteolytic sensitivity and structural heterogeneity. In this chapter, we describe the effective expression of CBP and p300 in a eukaryotic host, Sf9 insect cells, followed by their tandem affinity purification based on two terminal tags to ensure their structural integrity. The major focus of this chapter is on the development of novel accessory tools, single-domain camelid antibodies (nanobodies), for structural-functional characterization. Specific nanobodies against full-length CBP and p300 can specifically target their different regions and can be used for their marking, labeling, and structural stabilization in a broad range of in vitro and in vivo studies. Here, we describe four high-affinity nanobodies binding to the KIX and the HAT domains, either mimicking known interacting partners or revealing new functionally relevant conformations. As immunization of llamas results in nanobody libraries with a great sequence variation, deep sequencing and interaction analysis with different regions of the proteins provide a novel approach toward developing a panel of specific nanobodies.

Tumour suppressor EP300, a modulator of paclitaxel resistance and stemness, is downregulated in metaplastic breast cancer

PURPOSE

We have previously described a novel pathway controlling drug resistance, epithelial-to-mesenchymal transition (EMT) and stemness in breast cancer cells. Upstream in the pathway, three miRs (miR-106b, miR-93 and miR-25) target EP300, a transcriptional activator of E-cadherin. Upregulation of these miRs leads to the downregulation of EP300 and E-cadherin with initiation of an EMT. However, miRs regulate the expression of many genes, and the contribution to EMT by miR targets other than EP300 cannot be ruled out.

METHODS

We used lentiviruses expressing EP300-targeting shRNA to downregulate its expression in MCF-7 cells as well as an EP300-knocked-out colon carcinoma cell line. An EP300-expression plasmid was used to upregulate its expression in basal-like CAL51 and MDA-MB-231 breast cancer cells. Drug resistance was determined by short-term proliferation and long-term colony formation assays. Stemness was determined by tumour sphere formation in both soft agar and liquid cultures as well as by the expression of CD44/CD24/ALDH markers. Gene expression microarray analysis was performed in MCF-7 cells lacking EP300. EP300 expression was analysed by immunohistochemistry in 17 samples of metaplastic breast cancer.

RESULTS

Cells lacking EP300 became more resistant to paclitaxel whereas EP300 overexpression increased their sensitivity to the drug. Expression of cancer stem cell markers, as well as tumour sphere formation, was also increased in EP300-depleted cells, and was diminished in EP300-overexpressing cells. The EP300-regulated gene signature highlighted genes associated with adhesion (CEACAM5), cytoskeletal remodelling (CAPN9), stemness (ABCG2), apoptosis (BCL2) and metastasis (TGFB2). Some genes in this signature were also validated in a previously generated EP300-depleted model of breast cancer using minimally transformed mammary epithelial cells. Importantly, two key genes in apoptosis and stemness, BCL2 and ABCG2, were also upregulated in EP300-knockout colon carcinoma cells and their paclitaxel-resistant derivatives. Immunohistochemical analysis demonstrated that EP300 expression was low in metaplastic breast cancer, a rare, but aggressive form of the disease with poor prognosis that is characterized by morphological and physiological features of EMT.

CONCLUSIONS

EP300 plays a major role in the reprogramming events, leading to a more malignant phenotype with the acquisition of drug resistance and cell plasticity, a characteristic of metaplastic breast cancer.

Unique epigenetic influence of H2AX phosphorylation and H3K56 acetylation on normal stem cell radioresponses

Normal stem cells from tissues often exhibiting radiation injury are highly radiosensitive and exhibit a muted DNA damage response, in contrast to differentiated progeny. These radioresponses can be attributed to unique epigenetic regulation in stem cells, identifying potential therapeutic targets for radioprotection.

Normal tissue injury resulting from cancer radiotherapy is often associated with diminished regenerative capacity. We examined the relative radiosensitivity of normal stem cell populations compared with non–stem cells within several radiosensitive tissue niches and culture models. We found that these stem cells are highly radiosensitive, in contrast to their isogenic differentiated progeny. Of interest, they also exhibited a uniquely attenuated DNA damage response (DDR) and muted DNA repair. Whereas stem cells exhibit reduced ATM activation and ionizing radiation–induced foci, they display apoptotic pannuclear H2AX-S139 phosphorylation (γH2AX), indicating unique radioresponses. We also observed persistent phosphorylation of H2AX-Y142 along the DNA breaks in stem cells, which promotes apoptosis while inhibiting DDR signaling. In addition, down-regulation of constitutively elevated histone-3 lysine-56 acetylation (H3K56ac) in stem cells significantly decreased their radiosensitivity, restored DDR function, and increased survival, signifying its role as a key contributor to stem cell radiosensitivity. These results establish that unique epigenetic landscapes affect cellular heterogeneity in radiosensitivity and demonstrate the nonubiquitous nature of radiation responses. We thus elucidate novel epigenetic rheostats that promote ionizing radiation hypersensitivity in various normal stem cell populations, identifying potential molecular targets for pharmacological radioprotection of stem cells and hopefully improving the efficacy of future cancer treatment.

WDR5 Expression Is Prognostic of Breast Cancer Outcome

WDR5 is a core component of the human mixed lineage leukemia-2 complex, which plays central roles in ER positive tumour cells and is a major driver of androgen-dependent prostate cancer cell proliferation. Given the similarities between breast and prostate cancers, we explore the potential prognostic value of WDR5 gene expression on breast cancer survival. Our findings reveal that WDR5 over-expression is associated with poor breast cancer clinical outcome in three gene expression data sets and BreastMark. The eQTL analysis reveals 130 trans-eQTL SNPs whose genes mapped with statistical significance are significantly associated with patient survival. These genes together with WDR5 are enriched with “cellular development, gene expression, cell cycle” signallings. Knocking down WDR5 in MCF7 dramatically decreases cell viability, but does not alter tumour cell response to doxorubicin. Our study reveals the prognostic value of WDR5 expression in breast cancer which is under long-range regulation of genes involved in cell cycle, and anthracycline could be coupled with treatments targeting WDR5 once such a regimen is available.

Epigenetic mechanisms of Rubinstein-Taybi syndrome

Rubinstein-Taybi syndrome (RTS) is an incurable genetic disorder with combination of mental retardation and physical features including broad thumbs and toes, craniofacial abnormalities, and growth deficiency. While the autosomal dominant mode of transmission is limitedly known, the majority of cases are attributable to de novo mutations in RTS. The first identified gene associated with RTS is CREB-binding protein (CREBBP/CBP). Alterations of the epigenetic 'histone code' due to dysfunction of the CBP histone acetyltransferase activity deregulate gene transcriptions that are prominently linked to RTS pathogenesis. In this review, we discuss how CBP mutation contributes to modifications of histone and how histone deacetylase inhibitors are therapeutically applicable to epigenetic conditioning in RTS. Since most genetic mutations are irreversible and therapeutic approaches are limited, therapeutic targeting of reversible epigenetic components altered in RTS may be an ideal strategy. Expeditious further study on the role of the epigenetic mechanisms in RTS is encouraged to identify novel epigenetic markers and therapeutic targets to treat RTS.

Long-range transcriptional regulation of breast cancer genes

Breast cancer is a major health problem and understanding the genetic basis of this disease is crucial for predicting risk and developing effective targeted therapeutics. Several breast cancer predisposing genes have been identified, but mutations in the coding regions of these genes only accounts for a small proportion of risk. Research now suggests that combinations of multiple non-coding changes in breast cancer susceptibility genes, which cause moderate alterations in gene expression, will be responsible for the remaining inherited risk. These non-coding changes will include variants in proximal and distal transcriptional and post-transcriptional regulatory elements and may affect the levels and function of trans-acting factors, including proteins and RNAs, which act on these elements. Somatic changes in such elements and factors have also been associated with breast cancer progression. With the recent advent of techniques allowing the detection of long-range DNA interactions spanning the human genome, it has become increasingly clear that long-range regulatory elements constitute an important mechanism for gene regulation. Recent studies have identified several such elements that are important for regulating genes involved in breast cancer, raising the possibility that defects in these sequences may contribute to breast cancer predisposition and progression. In this review, we discuss the emerging functions of cis-regulatory elements and a subset of trans-acting factors in breast tumorigenesis. We also discuss some recent progress in our understanding of how dysregulation in these transcriptional components may contribute to breast cancer, and the potential implications for molecular diagnosis, prognosis prediction, and the treatment of this disease.

Mechanisms and management of doxorubicin cardiotoxicity

Doxorubicin is an effective anti-tumor agent with a cumulative dose-dependent cardiotoxicity. In addition to its principal toxic mechanisms involving iron and redox reactions, recent studies have described new mechanisms of doxorubicin-induced cell death, including abnormal protein processing, hyper-activated innate immune responses, inhibition of neuregulin-1 (NRG1)/ErbB(HER) signalling, impaired progenitor cell renewal/cardiac repair, and decreased vasculogenesis. Although multiple mechanisms involved in doxorubicin cardiotoxicity have been studied, there is presently no clinically proven treatment established for doxorubicin cardiomyopathy. Iron chelator dexrazoxane, angiotensin converting enzyme (ACE) inhibitors, and β-blockade have been proposed as potential preventive strategies for doxorubicin cardiotoxicity. Novel approaches such as anti-miR-146 or recombinant NRG1 to increase cardiomyocyte resistance to toxicity may be of interest in the future.

Selective roles for cAMP response element-binding protein binding protein and p300 protein as coregulators for androgen-regulated gene expression in advanced prostate cancer cells

The protein acetyltransferases p300 and cAMP response element-binding protein binding protein (CBP) are homologous, ubiquitously expressed proteins that interact with hundreds of proteins involved in transcriptional regulation and are involved globally as transcriptional coregulators. Although these two proteins acetylate and interact with overlapping sets of proteins, we found that p300 and CBP contribute to androgen-induced regulation of distinct sets of genes in C4-2B prostate cancer cells, a model of advanced prostate cancer. CBP cannot compensate for the loss of p300 to support androgen-induced expression of many genes, such as TMPRSS2 and PSA. Global gene expression analysis indicated that 47% of androgen-regulated genes are p300-dependent in these cells, whereas, surprisingly, only 0.3% of them are CBP-dependent. Chromatin immunoprecipitation analysis after depletion of cellular p300 indicated that p300 is required for androgen-induced acetylation of histones H3 and H4, methylation of histone H3 at Lys-4, and recruitment of TATA box binding protein (TBP) and RNA polymerase II, but not recruitment of the androgen receptor, on the TMPRSS2 gene in response to androgen. Thus, p300 is the dominant coregulator of the CBP/p300 pair for androgen-regulated gene expression in C4-2B cells. p300 is required at an early stage of chromatin remodeling and transcription complex assembly after binding of androgen receptor to the gene but before many critical histone modifications occur.

Beta-HPV 5 and 8 E6 promote p300 degradation by blocking AKT/p300 association

The E6 oncoprotein from high-risk genus alpha human papillomaviruses (α-HPVs), such as HPV 16, has been well characterized with respect to the host-cell proteins it interacts with and corresponding signaling pathways that are disrupted due to these interactions. Less is known regarding the interacting partners of E6 from the genus beta papillomaviruses (β-HPVs); however, it is generally thought that β-HPV E6 proteins do not interact with many of the proteins known to bind to α-HPV E6. Here we identify p300 as a protein that interacts directly with E6 from both α- and β-HPV types. Importantly, this association appears much stronger with β-HPV types 5 and 8-E6 than with α-HPV type 16-E6 or β-HPV type 38-E6. We demonstrate that the enhanced association between 5/8-E6 and p300 leads to p300 degradation in a proteasomal-dependent but E6AP-independent manner. Rather, 5/8-E6 inhibit the association of AKT with p300, an event necessary to ensure p300 stability within the cell. Finally, we demonstrate that the decreased p300 protein levels concomitantly affect downstream signaling events, such as the expression of differentiation markers K1, K10 and Involucrin. Together, these results demonstrate a unique way in which β-HPV E6 proteins are able to affect host-cell signaling in a manner distinct from that of the α-HPVs.

Inhibition of HIV-1 Tat-mediated transcription by a coumarin derivative, BPRHIV001, through the Akt pathway

The human immunodeficiency virus type 1 (HIV-1)-encoded RNA-binding protein Tat is known to play an essential role in viral gene expression. In the search for novel compounds to inhibit Tat transactivity, one coumarin derivative, BPRHIV001, was identified, with a 50% effective concentration (EC(50)) against HIV-1 at 1.3 nM. BPRHIV001 is likely to exert its effects at the stage after initiation of RNAPII elongation since Tat protein expression and the assembly of the Tat/P-TEFb complex remained unchanged. Next, a reduction of the p300 protein level, known to modulate Tat function through acetylation, was observed upon BPRHIV001 treatment, while the p300 mRNA level was unaffected. A concordant reduction of phosphorylated Akt, which was shown to be closely related to p300 stability, was observed in the presence of BPRHIV001 and was accompanied by a decrease of phosphorylated PDPK1, a well-known Akt activator. Furthermore, the docking analysis revealed that the reduced PDPK1 phosphorylation likely resulted from the allosteric effect of interaction between BPRHIV001 and PDPK1. With strong synergistic effects with current reverse transcriptase inhibitors, BPRHIV001 has the potential to become a promising lead compound for the development of a novel therapeutic agent against HIV-1 infection.

BCL6 repression of EP300 in human diffuse large B cell lymphoma cells provides a basis for rational combinatorial therapy

B cell lymphoma 6 (BCL6), which encodes a transcriptional repressor, is a critical oncogene in diffuse large B cell lymphomas (DLBCLs). Although a retro-inverted BCL6 peptide inhibitor (RI-BPI) was recently shown to potently kill DLBCL cells, the underlying mechanisms remain unclear. Here, we show that RI-BPI induces a particular gene expression signature in human DLBCL cell lines that included genes associated with the actions of histone deacetylase (HDAC) and Hsp90 inhibitors. BCL6 directly repressed the expression of p300 lysine acetyltransferase (EP300) and its cofactor HLA-B-associated transcript 3 (BAT3). RI-BPI induced expression of p300 and BAT3, resulting in acetylation of p300 targets including p53 and Hsp90. Induction of p300 and BAT3 was required for the antilymphoma effects of RI-BPI, since specific blockade of either protein rescued human DLBCL cell lines from the BCL6 inhibitor. Consistent with this, combination of RI-BPI with either an HDAC inhibitor (HDI) or an Hsp90 inhibitor potently suppressed or even eradicated established human DLBCL xenografts in mice. Furthermore, HDAC and Hsp90 inhibitors independently enhanced RI-BPI killing of primary human DLBCL cells in vitro. We also show that p300-inactivating mutations occur naturally in human DLBCL patients and may confer resistance to BCL6 inhibitors. Thus, BCL6 repression of EP300 provides a basis for rational targeted combinatorial therapy for patients with DLBCL.

The role of p300 histone acetyltransferase in UV-induced histone modifications and MMP-1 gene transcription

Matrix metalloproteinase (MMP)-1 promotes ultraviolet (UV)-triggered long-term detrimental effects such as cancer formation and premature skin aging. Although histone modifications may play a crucial role in the transcriptional regulation of MMP-1, the relationship between UV-induced histone modification and MMP-1 expression is not completely understood. Here, we identify regulators of histone acetylation that may link UV-mediated DNA damage and MMP-1 induction by UV in cultured human dermal fibroblasts (HDFs) in vitro. UV irradiation of HDFs induced MMP-1 expression and increased the level of phosphorylation of H2AX (γ-H2AX), p53 and the acetylation of histone H3 (acetyl-H3). Total histone deacetylase (HDAC) enzymatic activity was decreased by UV irradiation, while histone acetyltransferase (HAT) activity was increased. Suppression of p300 histone acetyltransferase (p300HAT) activity by the p300HAT inhibitor anacardic acid (AA) or by down-regulation of p300 by siRNA prevented UV-induced MMP-1 expression and inhibited UV-enhanced γ-H2AX, p53 level, and acetyl-H3. Using chromatin immunoprecipitation assays, we observed that γ-H2AX, p53, acetyl-H3, p300 and c-Jun were consistently recruited by UV to a distinct region (−2067/−1768) adjacent to the p300 binding site (−1858/−1845) in the MMP-1 promoter. In addition, these recruitments of γ-H2AX, p53, acetyl-H3, p300 and c-Jun to the p300-2 site were significantly abrogated by post-treatment with AA. Furthermore, overexpression of p300 increased the basal and UV-induced MMP-1 promoter activity. Our results suggest that p300HAT plays a critical role in the transcriptional regulation of MMP-1 by UV.

The C/H3 domain of p300 is required to protect VRK1 and VRK2 from their downregulation induced by p53

Background

The vaccinia-related kinase 1 (VRK1) protein, an activator of p53, can be proteolytically downregulated by an indirect mechanism, which requires p53-dependent transcription.

Principal Findings

In this work we have biochemically characterized the contribution of several p53 transcriptional cofactors with acetyl transferase activity to the induction of VRK1 downregulation that was used as a functional assay. Downregulation of VRK1 induced by p53 is prevented in a dose dependent manner by either p300 or CBP, but not by PCAF, used as transcriptional co-activators, suggesting that p53 has a different specificity depending on the relative level of these transcriptional cofactors. This inhibition does not require p53 acetylation, since a p53 acetylation mutant also induces VRK1 downregulation. PCAF can not revert the VRK1 protection effect of p300, indicating that these two proteins do not compete for a common factor needed to induce VRK1 downregulation. The protective effect is also induced by the C/H3 domain of p300, a region implicated in binding to several transcription factors and SV40 large T antigen; but the protective effect is lost when a mutant C/H3Del33 is used. The protective effect is a consequence of direct binding of the C/H3 domain to the transactivation domain of p53. A similar downregulatory effect can also be detected with VRK2 protein.

Conclusions/Significance

Specific p53-dependent effects are determined by the availability and ratios of its transcriptional cofactors. Specifically, the downregulation of VRK1/VRK2 protein levels, as a consequence of p53 accumulation, is thus dependent on the levels of the p300/CBP protein available for transcriptional complexes, since in this context this cofactor functions as a repressor of the effect. These observations point to the relevance of knowing the cofactor levels in order to determine one effect or another.

Orphan receptor TR3 attenuates the p300-induced acetylation of retinoid X receptor-alpha

Acetylation modification regulates the functions of histone and nonhistone proteins, including transcriptional activity, protein interaction, and subcellular localization. Although many nuclear receptors have been shown to be modified by acetylation, whether retinoid X receptors (RXRs) are acetylated and how the acetylation is regulated remains unknown. Here, we provide the first evidence of RXRalpha acetylation by p300 on lysine 145. Acetylation of RXRalpha by p300 facilitated its DNA binding and subsequently increased its transcriptional activity. Furthermore, we discovered that TR3, an orphan receptor, exerted a negative regulation on p300-induced RXRalpha acetylation. TR3 significantly reduced the p300-induced RXRalpha acetylation and transcriptional activity, and such inhibition required the interaction of TR3 with RXRalpha. Binding of TR3 to RXRalpha resulted in the sequestration of RXRalpha from p300. 9-cis retinoic acid, a ligand for RXRalpha, enhanced the association of RXRalpha with TR3, rather than acetylation of RXRalpha by p300. Biological function analysis revealed that the mitogenic activity of RXRalpha stimulated by p300 was acetylation dependent and could be repressed by TR3. Upon the treatment of 9-cis retinoic acid, RXRalpha was translocated with TR3 from the nucleus to the mitochondria, and apoptosis was induced. Taken together, our data demonstrate the distinct regulatory mechanisms of p300 and TR3 on RXRalpha acetylation and reveal a previously unrecognized role for orphan receptor in the transcriptional control of retinoid receptors.

Bmf contributes to histone deacetylase inhibitor-mediated enhancing effects on apoptosis after ionizing radiation

Histone deacetylase (HDAC) inhibitors augment ionizing radiation (IR)-induced apoptosis in several cancer cells by undefined mechanism(s). We recently found that the HDAC inhibitors induce a BH3-only protein Bmf in human squamous carcinoma SAS cells. We extended this study and found that 2.5 nM FK228 pretreatment could not induce apoptosis but augmented IR-induced death. The FK228 pretreatment increased Bmf expression level, and siRNA-mediated knockdown of Bmf transcripts strongly inhibited its augmentation of IR-induced cell death, disruption of mitochondrial membrane potential and DNA fragmentation. Another HDAC inhibitor CBHA pretreatment similarly augmented IR-induced apoptosis, and this effect was also inhibited by Bmf knockdown. Bmf overexpression augmented IR-induced death, and the augmented effects of FK228 were similarly observed in another squamous carcinoma HSC2 cells. Overexpression of histone acetyltransferase p300 mimicked the effects of the HDAC inhibitors, i.e., it enhanced IR-induced death, which was mostly abolished by Bmf knockdown. Taken together, histone hyperacetylation may enhance IR-induced death via activation of Bmf transcription, thereby implying Bmf as a key molecule for HDAC inhibitors (FK228 and CBHA)-mediated enhancing effect on IR-induced cell death.

p300 is required for orderly G1/S transition in human cancer cells

The role of the transcriptional coactivator p300 in cell cycle control has not been analysed in detail due to the lack of appropriate experimental systems. We have now examined cell cycle progression of p300-deficient cancer cell lines, where p300 was disrupted either by gene targeting (p300(-) cells) or knocked down using RNAi. Despite significant proliferation defects under normal growth conditions, p300-deficient cells progressed rapidly through G1 with premature S-phase entry. Accelerated G1/S transition was associated with early retinoblastoma (RB) hyperphosphorylation and activation of E2F targets. The p300-acetylase activity was dispensable since expression of a HAT-deficient p300 mutant reversed these changes. Co-immunoprecipitation showed p300/RB interaction occurs in vivo during G1, and this interaction has two peaks: in early G1 with unphosphorylated RB and in late G1 with phosphorylated RB. In vitro kinase assays showed that p300 directly inhibits cdk6-mediated RB phosphorylation, suggesting p300 acts in early G1 to prevent RB hyperphosphorylation and delay premature S-phase entry. Paradoxically, continued cycling of p300(-) cells despite prolonged serum depletion was observed, and this occurred in association with persistent RB hyperphosphorylation. Altogether, these results suggest that p300 has an important role in G1/S control, possibly by modulating RB phosphorylation.

Hepatitis C virus core protein recruits nucleolar phosphoprotein B23 and coactivator p300 to relieve the repression effect of transcriptional factor YY1 on B23 gene expression

Hepatitis C virus (HCV) core has a pleiotropic effect on various promoters. In this study, we found that the expression of nucleolar phosphoprotein B23 was enhanced in HCV core-expressing cells and, moreover, HCV core interacts directly with the C-terminal end of B23. Using sucrose gradient centrifugation analysis and immunoprecipitation assays, HCV core was found in a large complex containing B23 and its interacting partner transcription factor YY1. Both B23 and HCV core associated with YY1 in the central GA/GK-rich and C-terminal zinc finger domain. These physical interactions between core, B23, and YY1 led to ternary complex formation that was bound to the YY1 response element. In a transient cotransfection experiment, relief of the trans-suppression activity of YY1 on the YY1-response element-driven reporter by core and B23 was found. This is also true when examining the effects of these three constructs on the B23 promoter-driven reporter. Additionally, chromatin immunoprecipitation assays indicated that a transcriptional activation complex consisting of core, together with B23, p300, and YY1, was recruited to the YY1 response element of B23 promoter, and this probably occurred through complex formation between core and these three cellular transcription regulators. This is different from the situation in the absence of core, where YY1 and histone deacetylase 1, but not B23 and p300, were associated on the YY1 element as the transcription repression complex. Together, our results indicate that HCV core can recruit B23 and p300 to relieve the repression effect of YY1 on B23 promoter activity, a property that requires the intrinsic histone acetyltransferase activity of p300. Thus, because these three core-associated cellular transcription regulators have a multitude of cellular interacting proteins and are involved in a versatility of cellular processes, the complex formation described here may partially account for the pleiotropic effects of core protein on gene expression and cellular function in HCV-infected cells.

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.

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.

Role of protein-protein interactions in the antiapoptotic function of EWS-Fli-1

In the majority of Ewing's family tumors, chromosomal translocation t(11;22) leads to aberrant fusion of RNA-binding protein EWS with DNA-binding ETS transcriptional factor Fli-1. EWS-Fli-1 has altered the transcriptional activity and modulating its downstream target genes through this transcriptional activity is thought to be responsible for this tumor. We have previously shown that both EWS-Fli-1 and Fli-1 have antiapoptotic activity against several apoptotic inducers. Here, we show that the transcriptional activity of EWS-Fli-1 and Fli-1 is not essential for its antiapoptotic activity. We also demonstrate that EWS-Fli-1 and Fli-1 interact with CBP through its amino-terminal region and inhibit the CBP-dependent transcriptional activity of RXR. This activity appears to be independent of DNA-binding activity of EWS-Fli-1. Introduction of the dominant-negative form of CBP into Ewing's sarcoma cells sensitizes these cells against genotoxic or retinoic-acid induced apoptosis. These results suggest that the ability of EWS-Fli-1/Fli-1 to target transcriptional cofactor(s) and modulate apoptotic pathways may be responsible for its antiapoptotic and tumorigenic activities.

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.

p300/CBP and cancer

p300 and cyclic AMP response element-binding protein (CBP) are adenoviral E1A-binding proteins involved in multiple cellular processes, and function as transcriptional co-factors and histone acetyltransferases. Germline mutation of CBP results in Rubinstein-Taybi syndrome, which is characterized by an increased predisposition to childhood malignancies. Furthermore, somatic mutations of p300 and CBP occur in a number of malignancies. Chromosome translocations target CBP and, less commonly, p300 in acute myeloid leukemia and treatment-related hematological disorders. p300 mutations in solid tumors result in truncated p300 protein products or amino-acid substitutions in critical protein domains, and these are often associated with inactivation of the second allele. A mouse model confirms that p300 and CBP function as suppressors of hematological tumor formation. The involvement of these proteins in critical tumorigenic pathways (including TGF-beta, p53 and Rb) provides a mechanistic route as to how their inactivation could result in cancer.

Expression of an E1A/E7 chimeric protein sensitizes tumor cells to killing by activated macrophages but not NK cells

Adenovirus (Ad) E1A and human papillomavirus (HPV) E7 express homologous conserved regions (CRs) that mediate their shared biological functions. Despite their similarities, the expression of E1A sensitizes tumor cells to killing by NK cells and macrophages but the expression of E7 does not, a factor that may contribute to the dissimilar oncogenicities of Ad and HPV. This study was undertaken to define molecular differences between E1A and E7 that are responsible for the ability of E1A and the inability of E7 to sensitize cells to killing by NK cells and macrophages. Genetic mapping studies using human fibrosarcoma cells (H4) that stably expressed mutant forms of E1A showed that only those forms of E1A that interacted with the transcriptional coadaptor protein p300 sensitized cells to killing by NK cells and macrophages. E7 lacks the N-terminal p300-binding region present in E1A. Therefore, a chimeric E1A/E7 gene was constructed that included the N terminus and the CR1 (p300-binding) domain of E1A fused to CR2 and the C-terminal sequences of E7. The E1A/E7 protein interacted with p300 and pRb and immortalized primary mouse embryo fibroblasts (MEF). The expression of E1A/E7 sensitized H4 and MEF cells to killing by activated macrophages but not to killing by NK cells. Therefore, N-terminal differences between E1A and E7 that map to the E1A-p300 binding region accounted for differences in their abilities to sensitize cells to killing by macrophages. However, regions in addition to the E1A-p300 binding region are required to sensitize cells to killing by NK cells.

Phylogenetic origin of LI-cadherin revealed by protein and gene structure analysis

The intestine specific LI-cadherin differs in its overall structure from classical and desmosomal cadherins by the presence of seven instead of five cadherin repeats and a short cytoplasmic domain. Despite the low sequence similarity, a comparative protein structure analysis revealed that LI-cadherin may have originated from a five-repeat predecessor cadherin by a duplication of the first two aminoterminal repeats. To test this hypothesis, we cloned the murine LI-cadherin gene and compared its structure to that of other cadherins. The intron-exon organization, including the intron positions and phases, is perfectly conserved between repeats 3-7 of LI-cadherin and 1-5 of classical cadherins. Moreover, the genomic structure of the repeats 1-2 and 3-4 is identical for LI-cadherin and highly similar to that of the repeats 1-2 of classical cadherins. These findings strengthen our assumption that LI-cadherin originated from an ancestral cadherin with five domains by a partial gene duplication event.

Endothelin-1-dependent nuclear factor of activated T lymphocyte signaling associates with transcriptional coactivator p300 in the activation of the B cell leukemia-2 promoter in cardiac myocytes

Endothelin-1 (ET-1) is a potent survival factor that protects cardiac myocytes from apoptosis. ET-1 induces cardiac gene transcription and protein expression of antiapoptotic B cell leukemia-2 (bcl-2) in a calcineurin-dependent manner. A cellular target of adenovirus early region 1A (E1A) oncoprotein, p300 also activates bcl-2 transcription in cardiac myocytes and is required for their survival. p300 acts as a calcineurin-regulated nuclear factors of activated T lymphocytes (NFATc), downstream targets of calcineurin. In addition, the bcl-2 promoter contains multiple NFAT consensus sequences. These findings prompted us to investigate the role of NFATc in ET-1-dependent and p300-dependent bcl-2 transcription in cardiac myocytes. In primary cardiac myocytes prepared from neonatal rats, mutation of 2 NFAT sites within the bcl-2 promoter completely abolished the ET-1- and p300-induced increases in the activity of this promoter. We show here that p300 markedly potentiates the binding of NFATc1 to the bcl-2 NFAT element by interacting with NFATc1 in an E1A-dependent manner. On the other hand, stimulation of cardiac myocytes with ET-1 causes nuclear translocation of NFATc1, which interacts with p300 and increases DNA binding. Expression of E1A did not change the cardiac nuclear localization of NFATc1 but blocked its interaction with p300, DNA binding, and bcl-2 promoter activation. These findings suggest that ET-1-dependent NFATc signaling associates with p300 in the transactivation of bcl-2 gene in cardiac myocytes.

Expression of p300 protects cardiac myocytes from apoptosis in vivo

Doxorubicin is an anti-tumor agent that represses cardiac-specific gene expression and induces myocardial cell apoptosis. Doxorubicin depletes cardiac p300, a transcriptional coactivator that is required for the maintenance of the differentiated phenotype of cardiac myocytes. However, the role of p300 in protection against doxorubicin-induced apoptosis is unknown. Transgenic mice overexpressing p300 in the heart and wild-type mice were subjected to doxorubicin treatment. Compared with wild-type mice, transgenic mice exhibited higher survival rate as well as more preserved left ventricular function and cardiac expression of alpha-sarcomeric actin. Doxorubicin induced myocardial cell apoptosis in wild-type mice but not in transgenic mice. Expression of p300 increased the cardiac level of bcl-2 and mdm-2, but not that of p53 or other members of the bcl-2 family. These findings demonstrate that overexpression of p300 protects cardiac myocytes from doxorubicin-induced apoptosis and reduces the extent of acute heart failure in adult mice in vivo.

The proline repeat domain of p53 binds directly to the transcriptional coactivator p300 and allosterically controls DNA-dependent acetylation of p53

The transcription coactivator p300 cannot acetylate native p53 tetramers, thus revealing intrinsic conformational constraints on p300-catalyzed acetylation. Consensus site DNA is an allosteric effector that promotes acetylation of p53, suggesting that p300 has an undefined conformationally flexible interface within the p53 tetramer. To identify such conformationally responsive p300-binding sites, p300 was subjected to peptide selection from a phage-peptide display library, a technique that can define novel protein-protein interfaces. The enriched p300-binding peptides contained a proline repeat (PXXP/PXPXP) motif, and five proline repeat motifs actually reside within the p53 transactivation domain, suggesting that this region of p53 may harbor the second p300 contact site. p300 binds in vitro to PXXP-containing peptides derived from the proline repeat domain, and PXXP-containing peptides inhibit sequence-specific DNA-dependent acetylation of p53, indicating that p300 docking to both the LXXLL and contiguous PXXP motif in p53 is required for p53 acetylation. Deletion of the proline repeat motif of p53 prevents DNA-dependent acetylation of p53 by occluding p300 from the p53-DNA complex. Sequence-specific DNA places an absolute requirement for the proline repeat domain to drive p53 acetylation in vivo. Chromatin immunoprecipitation was used to show that the proline repeat deletion mutant p53 is bound to the p21 promoter in vivo, but it is not acetylated, indicating that proline-directed acetylation of p53 is a post-DNA binding event. The PXXP repeat expands the basic interface of a p300-targeted transactivation domain, and proline-directed acetylation of p53 at promoters indicates that p300-mediated acetylation can be highly constrained by substrate conformation in vivo.

Quantitative analysis of CBP- and P300-induced histone acetylations in vivo using native chromatin

In vivo, histone tails are involved in numerous interactions, including those with DNA, adjacent histones, and other, nonhistone proteins. The amino termini are also the substrates for a number of enzymes, including histone acetyltransferases (HATs), histone deacetylases, and histone methyltransferases. Traditional biochemical approaches defining the substrate specificity profiles of HATs have been performed using purified histone tails, recombinant histones, or purified mononucleosomes as substrates. It is clear that the in vivo presentation of the substrate cannot be accurately represented by using these in vitro approaches. Because of the difficulty in translating in vitro results into in vivo situations, we developed a novel single-cell HAT assay that provides quantitative measurements of endogenous HAT activity. The HAT assay is performed under in vivo conditions by using the native chromatin structure as the physiological substrate. The assay combines the spatial resolving power of laser scanning confocal microscopy with simple statistical analyses to characterize CREB binding protein (CBP)- and P300-induced changes in global histone acetylation levels at specific lysine residues. Here we show that CBP and P300 exhibit unique substrate specificity profiles, consistent with the developmental and functional differences between the two HATs.

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.

Human papillomavirus oncoprotein E6 inactivates the transcriptional coactivator human ADA3

High-risk human papillomaviruses (HPVs) are associated with carcinomas of the cervix and other genital tumors. The HPV oncoprotein E6 is essential for oncogenic transformation. We identify here hADA3, human homologue of the yeast transcriptional coactivator yADA3, as a novel E6-interacting protein and a target of E6-induced degradation. hADA3 binds selectively to the high-risk HPV E6 proteins and only to immortalization-competent E6 mutants. hADA3 functions as a coactivator for p53-mediated transactivation by stabilizing p53 protein. Notably, three immortalizing E6 mutants that do not induce direct p53 degradation but do interact with hADA3 induced the abrogation of p53-mediated transactivation and G(1) cell cycle arrest after DNA damage, comparable to wild-type E6. These findings reveal a novel strategy of HPV E6-induced loss of p53 function that is independent of direct p53 degradation. Given the likely role of the evolutionarily conserved hADA3 in multiple coactivator complexes, inactivation of its function may allow E6 to perturb numerous cellular pathways during HPV oncogenesis.

Overexpression and ribozyme-mediated targeting of transcriptional coactivators CREB-binding protein and p300 revealed their indispensable roles in adipocyte differentiation through the regulation of peroxisome proliferator-activated receptor gamma

The cAMP-response element-binding protein-binding protein (CBP) and p300 are common coactivators for several transcriptional factors. It has been reported that both CBP and p300 are significant for the activation of peroxisome proliferator-activated receptor gamma (PPARgamma), which is a crucial nuclear receptor in adipogenesis. However, it remains unclear whether CBP and/or p300 is physiologically essential to the activation of PPARgamma in adipocytes and adipocyte differentiation. In this study, we investigated the physiological significance of CBP/p300 in NIH3T3 cells transiently expressing PPARgamma and CBP and in 3T3-L1 preadipocytes stably expressing CBP- or p300-specific ribozymes. In PPARgamma-transfected NIH3T3 cells, induction of expression of PPARgamma target genes such as adipocyte fatty acid-binding protein (aP2) and lipoprotein lipase (LPL) by adding thiazolidinedione was enhanced, depending on the amount of a CBP expression plasmid transfected. Expression of aP2 and LPL genes, as well as glycerol-3-phosphate dehydrogenase activity and triacylglyceride accumulation after adipogenic induction, was largely suppressed in 3T3-L1 adipocytes expressing either the CBP- or p300-specific active ribozyme, but not in inactive ribozyme-expressing cells. These data suggest that both CBP and p300 are indispensable for the full activation of PPARgamma and adipocyte differentiation and that CBP and p300 do not mutually complement in the process.

Phosphorylation of CBP mediates transcriptional activation by neural activity and CaM kinase IV

Activity-regulated transcription has been implicated in adaptive plasticity in the CNS. In many instances, this plasticity depends upon the transcription factor CREB. Precisely how neuronal activity regulates CREB remains unclear. To address this issue, we examined the phosphorylation state of components of the CREB transcriptional pathway. We show that NMDA activates transcription of CREB-responsive genes in hippocampal neurons, with ERK responsible for persistent CREB phosphorylation and CaM kinase IV (CaMKIV) responsible for phosphorylating the CREB coactivator, CBP. Ser301 of CBP was identified as a major target of CaMKIV phosphorylation in vitro and in vivo. CaM kinase inhibitors attenuated phosphorylation at Ser301 and blocked CBP-dependent transcription. Additionally, mutation of Ser301 impaired NMDA- and CaMKIV-stimulated transcription. These findings demonstrate that activity-induced CaMKIV signaling contributes to CREB/CBP-dependent transcription by phosphorylating CBP at Ser301.

DNA damage-dependent acetylation of p73 dictates the selective activation of apoptotic target genes

The tumor suppressor p53 and its close relative p73 are activated in response to DNA damage resulting in either cell cycle arrest or apoptosis. Here, we show that DNA damage induces the acetylation of p73 by the acetyltransferase p300. Inhibiting the enzymatic activity of p300 hampers apoptosis in a p53(-/-) background. Furthermore, a nonacetylatable p73 is defective in activating transcription of the proapoptotic p53AIP1 gene but retains an intact ability to regulate other targets such as p21. Finally, p300-mediated acetylation of p73 requires the protooncogene c-abl. Our results suggest that DNA damage-induced acetylation potentiates the apoptotic function of p73 by enhancing the ability of p73 to selectively activate the transcription of proapoptotic target genes.

High-throughput screening for identification of small molecule inhibitors of histone acetyltransferases using scintillating microplates (FlashPlate)

The role of histone acetyltransferases (HATs) in the regulation of crucial cellular functions, e.g., gene transcription, differentiation, and proliferation, has recently been documented and there is increasing evidence that aberrant expression of these enzymes may have a role to play in the development of the malignant phenotype. The availability of potent and selective small molecule inhibitors of HATs would provide useful proof of principle probes for further validation of these enzymes as drug discovery targets and may also provide lead molecules for clinical drug development. We have developed a microplate assay for HAT activity suitable for high-throughput screening. In the assay, following incubation of histone H3, [3H]acetylCoA, and enzyme (recombinant p300/CBP-associated factor expressed as a glutathione S-transferase fusion protein), radiolabeled histone was captured onto the walls of a scintillating microplate (FlashPlate) generating a scintillation signal. The assay was reproducible, amenable to automation, and generated a wide signal to noise ratio. Although antiacetylated histone antibodies were initially used to capture the radiolabeled product, it was subsequently shown that a signal was effectively produced by histone passively binding to the walls of the FlashPlate. This resulted in a simple "mix and measure" assay that is currently being used for the identification of HAT inhibitors.

p300/CBP proteins: HATs for transcriptional bridges and scaffolds

p300/CBP transcriptional co-activator proteins play a central role in co-ordinating and integrating multiple signal-dependent events with the transcription apparatus, allowing the appropriate level of gene activity to occur in response to diverse physiological cues that influence, for example, proliferation, differentiation and apoptosis. p300/CBP activity can be under aberrant control in human disease, particularly in cancer, which may inactivate a p300/CBP tumour-suppressor-like activity. The transcription regulating-properties of p300 and CBP appear to be exerted through multiple mechanisms. They act as protein bridges, thereby connecting different sequence-specific transcription factors to the transcription apparatus. Providing a protein scaffold upon which to build a multicomponent transcriptional regulatory complex is likely to be an important feature of p300/CBP control. Another key property is the presence of histone acetyltransferase (HAT) activity, which endows p300/CBP with the capacity to influence chromatin activity by modulating nucleosomal histones. Other proteins, including the p53 tumour suppressor, are targets for acetylation by p300/CBP. With the current intense level of research activity, p300/CBP will continue to be in the limelight and, we can be confident, yield new and important information on fundamental processes involved in transcriptional control.

CBP, a transcriptional coactivator and acetyltransferase

The CREB binding protein (CBP) was first identified as a protein that specifically binds to the active phosphorylated form of the cyclic-AMP response element binding protein (CREB). CBP was initially defined as a transcriptional coactivator that, as a result of its large size and multiple protein binding domain modules, may function as a molecular scaffold. More recently, an acetyltransferase activity, both of histones and nonhistones, has been found to be essential for transactivation. In this review, we will discuss the current understanding of the acetyltransferase specificity and activity of the CBP protein and how it may function to coactivate transcription. We will also examine the regulation of the CBP histone acetyltransferase activity in the cell cycle, by signal-transduction pathways and throughout development.Key words: CBP, acetyltransferase, chromatin, acetylation, p300.

p300/CBP/p53 interaction and regulation of the p53 response

Substantial evidence points to a critical role for the p300/CREB binding protein (CBP) coactivators in p53 responses to DNA damage. p300/CBP and the associated protein P/CAF bind to and acetylate p53 during the DNA damage response, and are needed for full p53 transactivation as well as downstream p53 effects of growth arrest and/or apoptosis. Beyond this simplistic model, p300/CBP appear to be complex integrators of signals that regulate p53, and biochemically, the multipartite p53/p300/CBP interaction is equally complex. Through physical interaction with p53, p300/CBP can both positively and negatively regulate p53 transactivation, as well as p53 protein turnover depending on cellular context and environmental stimuli, such as DNA damage.

Neurohormonal regulation of myocardial cell apoptosis during the development of heart failure

Adult cardiac myocytes are terminally differentiated cells that are no longer able to divide. Accumulating data support the idea that apoptosis in these cells is involved in the transition from cardiac compensation to decompensated heart failure. Since a number of neurohormonal factors are activated in this state, these factors may be involved in the positive and negative regulation of apoptosis in cardiac myocytes. beta1-Adrenergic receptor and angiotensin type 1 receptor pathways, nitric oxide and natriuretic peptides are involved in the induction of apoptosis in these cells, while alpha1- and beta2-adrenergic receptor and endothelin-1 type A receptor pathways and gp130-related cytokines are antiapoptotic. The myocardial protection of the latter is mediated, at least in part, through mitogen-activated protein kinase-dependent pathways, compatible with the findings in other cell types. In contrast, signaling pathways leading to apoptosis in cardiac myocytes are distinct from those in other cell types. The cAMP/PKA pathway induces apoptosis in cardiac myocytes and blocks apoptosis in other cell types. The p300 protein, a coactivator of p53, mediates apoptosis in fibroblasts but appears to play a protective role in differentiated cardiac myocytes. The inhibition of myocardial cell apoptosis in heart failure may be achieved by directly blocking apoptosis signaling pathways or by modulating neurohormonal factors involved in their regulation. These may provide novel therapeutic strategies in some forms of heart failure.

Distinct p300-responsive mechanisms promote caspase-dependent apoptosis by human T-cell lymphotropic virus type 1 Tax protein

The dysregulation of cellular apoptosis pathways has emerged as a critical early event associated with the development of many types of human cancers. Numerous viral and cellular oncogenes, aside from their inherent transforming properties, are known to induce programmed cell death, consistent with the hypothesis that genetic defects are required to support tumor survival. Here, we report that nuclear expression of the CREB-binding protein (CBP)/p300-binding domain of the human T-cell lymphotropic virus type 1 (HTLV-1) transactivator, Tax, triggers an apoptotic death-inducing signal during short-term clonal analyses, as well as in transient cell death assays. Coexpression of the antiapoptotic factor Bcl-2 increased serum stimulation; incubation with the chemical caspase inhibitor z-Val-Ala-DL-Asp fluoromethylketone antagonized Tax-induced cell death. The CBP/p300-binding defective Tax mutants K88A and V89A exhibited markedly reduced cytotoxic effects compared to the wild-type Tax protein. Importantly, nuclear expression of the minimal CBP/p300-binding peptide of Tax induced apoptosis in the absence of Tax-dependent transcriptional activities, while its K88A counterpart did not cause cell death. Further, Tax-mediated apoptosis was effectively prevented by ectopic expression of the p300 coactivator. We also report that activation of the NF-kappaB transcription pathway by Tax, under growth arrest conditions, results in apoptosis that occurs independent of direct Tax coactivator effects. Our results allude to a novel pivotal role for the transcriptional coactivator p300 in determining cell fate and raise the possibility that dysregulated coactivator usage may pose an early barrier to transformation that must be selectively overcome as a prerequisite for the initiation of neoplasia.

p300 interacts with the nuclear proto-oncoprotein SYT as part of the active control of cell adhesion

Complexes containing p300, but not CBP, and the nuclear proto-oncoprotein SYT were detected in confluent cultures of G1-arrested cells but not in sparse cells or during S or G2. SYT sequences constitute the N-terminal segment of a fusion oncogene product, SYT-SSX, routinely detected in synovial sarcoma, an aggressive human tumor. SYT/p300 complex formation promotes cell adhesion to a fibronectin matrix, as reflected by compromise of this process in cells expressing SYT dl mutants that retain p300 binding activity and in the primary fibroblasts of p300 but not CBP heterozygous null mice. The mechanism linking the action of SYT/p300 complexes to adhesion function is, at least in part, transcription activation-independent and results in proper activation of beta1 integrin, a major adhesion receptor.

Synergistic activation of p53-dependent transcription by two cooperating damage recognition pathways

High level activation of p53-dependent transcription occurs following cellular exposure to genotoxic damaging agents such as UV-C, while ionizing radiation damage does not induce a similarly potent induction of p53-dependent gene expression. Reasoning that one of the major differences between UV-C and ionizing radiation damage is that the latter does not inhibit general transcription, we attempted to reconstitute p53-dependent gene expression in ionizing irradiated cells by co-treatment with selected transcription inhibitors that alone do not activate p53. p53-dependent transcription can be dramatically enhanced by the treatment of ionizing irradiated cells with low doses of DRB, which on its own does not induce p53 activity. The mechanism of ionizing radiation-dependent activation of p53-dependent transcription using DRB is more likely due to inhibition of gene transcription rather than prolonged DNA damage, as the non-genotoxic and general transcription inhibitor Roscovitine also synergistically activates p53 function in ionizing irradiated cells. These results identify two distinct signal transduction pathways that cooperate to fully activate p53-dependent gene expression: one responding to lesions induced by ionizing radiation and the second being a kinase pathway that regulates general RNA Polymerase II activity.

Dial 9-1-1 for p53: mechanisms of p53 activation by cellular stress

The tumor suppressor protein, p53, is part of the cell's emergency team that is called upon following cellular insult. How do cells sense DNA damage and other cellular stresses and what signal transduction pathways are used to alert p53? How is the resulting nuclear accumulation of p53 accomplished and what determines the outcome of p53 induction? Many posttranslational modifications of p53, such as phosphorylation, dephosphorylation, acetylation and ribosylation, have been shown to occur following cellular stress. Some of these modifications may activate the p53 protein, interfere with MDM2 binding and/or dictate cellular localization of p53. This review will focus on recent findings about how the p53 response may be activated following cellular stress.