Proteasome-mediated degradation of the coactivator p300 impairs cardiac transcription

p300 provides a corepressor function by cooperating with YY1 and HDAC3 to repress c-Myc

We showed earlier that p300/CBP plays an important role in G1 progression by negatively regulating c-Myc and thereby preventing premature G1 exit. Here, we have studied the mechanism by which p300 represses c-Myc and show that in quiescent cells p300 cooperates with histone deacetylase 3 (HDAC3) to repress transcription. p300 and HDAC3 are recruited to the upstream YY1-binding site of the c-Myc promoter resulting in chromatin deacetylation and repression of c-Myc transcription. Consistent with this, ablation of p300, YY1 or HDAC3 expression results in chromatin acetylation and induction of c-Myc. These three proteins exist as a complex in vivo and form a multiprotein complex with the YY1-binding site in vitro. The C-terminal region of p300 is both necessary and sufficient for the repression of c-Myc. These and other results suggest that in quiescent cells the C-terminal region of p300 provides corepressor function and facilitates the recruitment of p300 and HDAC3 to the YY1-binding site and represses the c-Myc promoter. This corepressor function of p300 prevents the inappropriate induction of c-Myc and S phase.

Atrogin-1 ubiquitin ligase is upregulated by doxorubicin via p38-MAP kinase in cardiac myocytes

AIMS

Doxorubicin (DOX) is one of the most effective anti-neoplastic agents; however, its clinical use is limited by drug-induced cardiomyopathy. The molecular mechanisms responsible for this toxicity remain to be fully addressed. In the present study, we investigated the involvement of atrogin-1, one of the muscle-specific ubiquitin ligases, in DOX-induced cardiotoxicity.

METHODS AND RESULTS

This method involved intraperitoneal administration of DOX-induced atrogin-1 in the hearts and skeletal muscles of C57BL/6 mice. Consistently, atrogin-1 mRNA was upregulated with DOX treatment in cultured rat neonatal cardiomyocytes. Adenoviral transfer of atrogin-1 induced a reduction in cell size that was ameliorated by the ubiquitin proteasome inhibitor, MG-132. The transduction of constitutively active Akt (caAkt), a serine/threonine protein kinase, inhibited the DOX-mediated induction of atrogin-1. The phosphorylation status of Akt and its downstream target, FOXO, was not affected by DOX. DOX treatment did not activate the atrogin-1 promoter that contains FOXO-binding sites, suggesting that DOX induced atrogin-1 without modulating the Akt/FOXO pathway; importantly, DOX activated p38-mitogen-activated protein kinase (MAPK) and c-Jun N-terminal kinase (JNK). Furthermore, pharmacological inhibition of p38-MAPK, but not JNK, abrogated DOX-mediated induction of atrogin-1. Finally, adenoviral transfer of caAkt inhibited the DOX-induced p38-MAPK activation.

CONCLUSIONS

DOX induces atrogin-1 through a p38-MAPK-dependent pathway in cardiac myocytes. Constitutive activation of Akt negatively regulates DOX-mediated atrogin-1 induction by inhibiting p38-MAPK activity as a novel mechanism.

Carboxyl Terminus of NKX2.5 Impairs its Interaction with p300

The transcription factor Nkx2.5 plays critical roles in controlling cardiac-specific gene expression. Previous reports demonstrated that Nkx2.5 is only a modest transactivator due to the auto-inhibitory effect of its C-terminal domain. Deletion of the C-terminal domain, mimicking conformational change, evokes vigorous transactivation activity. Here, we show that a C-terminal defective mutant of Nkx2.5 improves the occupation of p300 at the ANF promoter compared with full-length Nkx2.5, leading to hyperacetylation of histone H4. We reveal that p300 is a cofactor of Nkx2.5, markedly potentiating Nkx2.5-dependent transactivation, whereas E1A antigen impairs Nkx2.5 activity. Furthermore, p300 can acetylate Nkx2.5 and display an acetyltransferase-independent mechanism to coactivate Nkx2.5. Physical interaction between the N-terminal activation domain of Nkx2.5 and the C/H3 domain of p300 are identified by GST pull-down assay. Point mutants of the N-terminal modify the transcriptional activity of Nkx2.5 and interaction with p300. Deletion of the C-terminal domain greatly facilitates p300 binding and improves the susceptibility of Nkx2.5 to histone deacetylase inhibitor. These results establish that p300 acts as an Nkx2.5 cofactor and facilitates increased Nkx2.5 activity by relieving the conformational impediment of its inhibitory C-terminal domain.

Phenylbutyrate, a histone deacetylase inhibitor, protects against Adriamycin-induced cardiac injury

Cardiac injury is a major complication for oxidative-stress-generating anticancer agents exemplified by Adriamycin (ADR). Recently, several histone deacetylase inhibitors (HDACIs) including phenylbutyrate (PBA) have shown promise in the treatment of cancer with little known toxicity to normal tissues. PBA has been shown to protect against oxidative stress in normal tissues. Here, we examined whether PBA might protect heart against ADR toxicity in a mouse model. The mice were i.p. injected with ADR (20 mg/kg). PBA (400 mg/kg/day) was i.p. injected 1 day before and daily after the ADR injection for 2 days. We found that PBA significantly decreased the ADR-associated elevation of serum lactate dehydrogenase and creatine kinase activities and diminished ADR-induced ultrastructural damages of cardiac tissue by more than 70%. Importantly, PBA completely rescued ADR-caused reduction of cardiac functions exemplified by ejection fraction and fraction shortening, and increased cardiac manganese superoxide dismutase (MnSOD) protein and activity. Our results reveal a previously unrecognized role of HDACIs in protecting against ADR-induced cardiac injury and suggest that PBA may exert its cardioprotective effect, in part, by the increase of MnSOD. Thus, combining HDACIs with ADR could add a new mechanism to fight cancer while simultaneously decrease ADR-induced cardiotoxicity.

Molecular and cellular mechanisms of anthracycline cardiotoxicity

The molecular and cellular mechanisms that cause cumulative dose-dependent anthracycline-cardiotoxicity remain controversial and incompletely understood. Studies examining the effects of anthracyclines in cardiac myocytes inA vitro have demonstrated several forms of cellular injury. Cell death in response to anthracyclines can be observed by one of several mechanisms including apoptosis and necrosis. Cell death by apoptosis can be inhibited by dexrazoxane, the iron chelator that is known to prevent clinical development of heart failure at high cumulative anthracycline exposure. Together with clinical evidence for myocyte death after anthracycline exposure, in the form of elevations in serum troponin, make myocyte cell death a probable mechanism for anthracycline-induced cardiac injury. Other mechanisms of myocyte injury include the development of cellular \'sarcopenia\' characterized by disruption of normal sarcomere structure. Anthracyclines suppress expression of several cardiac transcription factors, and this may play a role in the development of myocyte death as well as sarcopenia. Degradation of the giant myofilament protein titin may represent an important proximal step that leads to accelerated myofilament degradation. Titin is an entropic spring element in the sarcomere that regulates length-dependent calcium sensitivity. Thus titin degradation may lead to impaired diastolic as well as systolic dysfunction, as well as potentiate the effect of suppression of transcription of sarcomere proteins. An interesting interaction has been noted clinically between anthracyclines and newer cancer therapies that target the erbB2 receptor tyrosine kinase. Studies of erbB2 function in viro suggest that signaling through erbB2 by the growth factor neuregulin may regulate cardiac myocyte sarcomere turnover, as well as myocyte-myocyte/myocyte-matrix force coupling. A combination of further in vitro studies, with more careful monitoring of cardiac function after exposure to these cancer therapies, may help to understand to what extent these mechanisms are at work during clinical exposure of the heart to these important pharmaceuticals.

Degradation of NFAT5, a transcriptional regulator of osmotic stress-related genes, is a critical event for doxorubicin-induced cytotoxicity in cardiac myocytes

Nuclear factor-activated T cell 5 (NFAT5), a novel member of the NFAT family of proteins, was originally identified as a transcriptional factor responsible for adaptation to hyperosmotic stress. Though NFAT5 is ubiquitously expressed, the biological functions of NFAT5 remain to be clarified, especially in the tissues that are not exposed to hypertonicity, including hearts. In the present study, we focused on the cardioprotective roles of NFAT5 against the cardiotoxic anti-tumor agent doxorubicin (Dox). In cultured cardiomyocytes, transcripts of the hypertonicity-inducible genes, such as taurine transporter (TauT) and sodium/myo-inositol transporter, were down-regulated by Dox. Interestingly, NFAT5 protein, but not mRNA, was decreased in cardiomyocytes exposed to Dox. Treatment of proteasome inhibitors, MG-132 or proteasome-specific inhibitor 1, prevented the Dox-mediated decrease of NFAT5 protein. Further, ubiquitin-conjugated NFAT5 was not detected in cultured cardiomyocytes treated with MG-132 and/or Dox, as assessed by immunoprecipitation assay, suggesting Dox-induced degradation through ubiquitin-independent proteasome pathway. Importantly, inhibition of NFAT5 with overexpression of dominant-negative NFAT5 decreased cell viability and increased creatine kinase leakage into culture medium. Consistently, small interfering RNA targeting NFAT5 gene enhanced myocyte death. These findings suggest that Dox promoted the degradation of NFAT5 protein, reducing cell viability in cardiomyocytes. This is the first demonstration that NFAT5 is a positive regulator of cardiomyocyte survival.

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

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

Hypoxia-activated metabolic pathway stimulates phosphorylation of p300 and CBP in oxygen-sensitive cells

Transcription co-activators and histone acetyltransferases, p300 and cyclic AMP responsive element-binding protein-binding protein (CBP), participate in hypoxic activation of hypoxia-inducible genes. Here, we show that exposure of PC12 and cells to 1-10% oxygen results in hyperphosphorylation of p300/CBP. This response is fast, long lasting and specific for hypoxia, but not for hypoxia-mimicking agents such as desferioxamine or Co2+ ions. It is also cell-type specific and occurs in pheochromocytoma PC12 cells and the carotid body of rats but not in hepatoblastoma cells. The p300 hyperphosphorylation specifically depends on the release of intracellular calcium from inositol 1,4,5-triphosphate (IP3)-sensitive stores. However, it is not inhibited by pharmacological inhibitors of any of the kinases traditionally known to be directly or indirectly calcium regulated. On the other hand, p300 hyperphosphorylation is inhibited by several different inhibitors of the glucose metabolic pathway from generation of NADH by glyceraldehyde 3-phosphate dehydrogenase, through the transfer of NADH through the glycerol phosphate shuttle to ubiquinone and complex III of the mitochondrial respiratory chain. Inhibition of IP3-sensitive calcium stores decreases generation of ATP, and this inhibition is significantly stronger in hypoxia than in normoxia. We propose that the NADH glycerol phosphate shuttle participates in generating a pool of ATP that serves either as a co-factor or a modulator of the kinases involved in the phosphorylation of p300/CBP during hypoxia.

Phosphorylation-dependent degradation of p300 by doxorubicin-activated p38 mitogen-activated protein kinase in cardiac cells

p300 and CBP are general transcriptional coactivators implicated in different cellular processes, including regulation of the cell cycle, differentiation, tumorigenesis, and apoptosis. Posttranslational modifications such as phosphorylation are predicted to select a specific function of p300/CBP in these processes; however, the identification of the kinases that regulate p300/CBP activity in response to individual stimuli and the physiological significance of p300 phosphorylation have not been elucidated. Here we demonstrate that the cardiotoxic anticancer agent doxorubicin (adriamycin) induces the phosphorylation of p300 in primary neonatal cardiomyocytes. Hyperphosphorylation precedes the degradation of p300 and parallels apoptosis in response to doxorubicin. Doxorubicin-activated p38 kinases alpha and beta associate with p300 and are implicated in the phosphorylation-mediated degradation of p300, as pharmacological blockade of p38 prevents p300 degradation. p38 phosphorylates p300 in vitro at both the N and C termini of the protein, and enforced activation of p38 by the constitutively active form of its upstream kinase (MKK6EE) triggers p300 degradation. These data support the conclusion that p38 mitogen-activated protein kinase regulates p300 protein stability and function in cardiomyocytes undergoing apoptosis in response to doxorubicin.

B56 regulatory subunit of protein phosphatase 2A mediates valproic acid-induced p300 degradation

Transcriptional coactivator p300 is required for embryonic development and cell proliferation. Valproic acid, a histone deacetylase inhibitor, is widely used in the therapy of epilepsy and bipolar disorder. However, it has intrinsic teratogenic activity through unidentified mechanisms. We report that valproic acid stimulates proteasome-dependent p300 degradation through augmentation of gene expression of the B56gamma regulatory subunits of protein phosphatase 2A. The B56gamma3 regulatory and catalytic subunits of protein phosphatase 2A interact with p300. Overexpression of the B56gamma3 subunit leads to proteasome-mediated p300 degradation and represses p300-dependent transcriptional activation, which requires the B56gamma3 interaction domain of p300. Conversely, silencing of the B56gamma subunit expression by RNA interference increases the stability and transcriptional activity of the coactivator. Our study establishes the functional interaction between protein phosphatase 2A and p300 activity and provides direct evidence for signal-dependent control of p300 function.

Integrin alphav-mediated inactivation of p53 controls a MEK1-dependent melanoma cell survival pathway in three-dimensional collagen

Integrin αv is required for melanoma cell survival and tumor growth in various models. To elucidate integrin αv-mediated melanoma cell survival mechanisms, we used a three-dimensional (3D) collagen gel model mimicking the pathophysiological microenvironment of malignant melanoma in the dermis. We found that integrin αv inactivated p53 and that suppression of p53 activity by dominant negative p53 or p53-small interfering RNA obviated the need for integrin αv for melanoma cell survival in 3D-collagen and for tumor growth in vivo. This indicates that integrin αv-mediated inactivation of p53 functionally controls melanoma cell survival. Furthermore, we found that melanoma cell integrin αv was required for MAPK kinase (MEK) 1 and extracellular signal-regulated kinase (ERK)1/2 activity in 3D-collagen, whereas inhibition of MEK1 activity induced apoptosis. Surprisingly, MEK1 and ERK1/2 activities were restored in integrin αv-negative melanoma cells by suppression of p53, whereas concomitant block of MEK1 induced apoptosis. This suggests that integrin αv controls melanoma cell survival in 3D-collagen through a pathway involving p53 regulation of MEK1 signaling.

New cellular and molecular approaches for the treatment of cardiac disease

Similar to the kidney in uremia, end-stage cardiac failure is an outcome common to many disparate disease processes including hypertension, various inflammatory pathologies, as well as ischemic loss of tissue. In regard to the heart, cellular and molecular mechanisms responsible for heart failure have been investigated with renewed intensity over the past several years with newer techniques of molecular genetics, genomic analysis, and cell biology. Although this article reviews some recent advances made in our understanding of molecular and cellular events in the heart leading to heart failure and explores possible new targets for therapeutics, the main point is to stress the importance of investigative interactions between organ physiologists and molecular and cellular biologists. These interactions between organ physiologists and molecular geneticists is stressed and supported as a mechanism for rapid advancement for both understanding the underlying pathophysiology of human disease and the development of therapeutic strategies.

GSK3 beta-dependent phosphorylation of the alpha NAC coactivator regulates its nuclear translocation and proteasome-mediated degradation

c-Jun is an immediate-early gene whose degradation by the proteasome pathway is required for an efficient transactivation. In this report, we demonstrated that the c-Jun coactivator, nascent polypeptide associated complex and coactivator alpha (alphaNAC) was also a target for degradation by the 26S proteasome. The proteasome inhibitor lactacystin increased the metabolic stability of alphaNAC in vivo, and lactacystin, MG-132, or epoxomicin treatment of cells induced nuclear translocation of alphaNAC. We have shown that the ubiquitous kinase glycogen synthase kinase 3beta (GSK3beta) directly phosphorylated alphaNAC in vitro and in vivo. Inhibition of the endogenous GSKappa3beta activity resulted in the stabilization of this coactivator in vivo. We identified the phosphoacceptor site in the C-terminal end of the coactivator, on position threonine 159. We demonstrated that the inhibition of GSK3beta activity by treatment of cells with the inhibitor 5-iodo-indirubin-3'-monoxime, as well as with a dominant-negative GSK3beta mutant, induced the accumulation of alphaNAC in the nuclei of cells. Mutation of the GSK3beta phosphoacceptor site on alphaNAC induced a significant increase of its coactivation potency. We conclude that GSK3beta-dependent phosphorylation of alphaNAC was the signal that directed the protein to the proteasome. The accumulation of alphaNAC caused by the inhibition of the proteasome pathway or the activity of GSK3beta contributes to its nuclear translocation and impacts on its coactivating function.

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.

Essential role of GATA-4 in cell survival and drug-induced cardiotoxicity

In recent years, significant progress has been made in understanding cardiomyocyte differentiation. However, little is known about the regulation of myocyte survival despite the fact that myocyte apoptosis is a leading cause of heart failure. Here we report that transcription factor GATA-4 is a survival factor for differentiated, postnatal cardiomyocytes and an upstream activator of the antiapoptotic gene Bcl-X. An early event in the cardiotoxic effect of the antitumor drug doxorubicin is GATA-4 depletion, which in turn causes cardiomyocyte apoptosis. Mouse heterozygotes for a null Gata4 allele have enhanced susceptibility to doxorubicin cardiotoxicity. Genetic or pharmacologic enhancement of GATA-4 prevents cardiomyocyte apoptosis and drug-induced cardiotoxicity. The results indicate that GATA-4 is an antiapoptotic factor required for the adaptive stress response of the adult heart. Modulation of survival/apoptosis genes by tissue-specific transcription factors may be a general paradigm that can be exploited effectively for cell-specific regulation of apoptosis in disease states.

Cardiac histone acetylation – therapeutic opportunities abound

Diverse etiologic factors trigger a cardiac remodeling process in which the heart becomes abnormally enlarged with a consequent decline in cardiac function and eventual heart failure. Heart failure is traditionally treated with drugs that antagonize early signaling events at or near the cell membrane. Although such approaches have short-term efficacy, the five-year mortality rate for patients with late-stage heart failure continues to exceed 50%. Because of the redundant nature of the signaling networks that drive cardiac pathogenesis, targeting the common downstream elements of the cascades would be a more effective therapeutic strategy. Recent studies point to the importance of enzymes that control histone acetylation as stress-responsive regulators of gene expression in the heart. Given their role as nuclear integrators that couple divergent upstream signals to the gene program for cardiac remodeling, we propose that these chromatin-modifying factors represent auspicious targets for the pharmacological manipulation of cardiac disease.

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 diverse superfamily of lysine acetyltransferases and their roles in leukemia and other diseases

Acetylation of the epsilon-amino group of lysine residues, or N(epsilon)-lysine acetylation, is an important post-translational modification known to occur in histones, transcription factors and other proteins. Since 1995, dozens of proteins have been discovered to possess intrinsic lysine acetyltransferase activity. Although most of these enzymes were first identified as histone acetyltransferases and then tested for activities towards other proteins, acetyltransferases only modifying non-histone proteins have also been identified. Lysine acetyltransferases form different groups, three of which are Gcn5/PCAF, p300/CBP and MYST proteins. While members of the former two groups mainly function as transcriptional co-activators, emerging evidence suggests that MYST proteins, such as Esa1, Sas2, MOF, TIP60, MOZ and MORF, have diverse roles in various nuclear processes. Aberrant lysine acetylation has been implicated in oncogenesis. The genes for p300, CBP, MOZ and MORF are rearranged in recurrent leukemia-associated chromosomal abnormalities. Consistent with their roles in leukemogenesis, these acetyltransferases interact with Runx1 (or AML1), one of the most frequent targets of chromosomal translocations in leukemia. Therefore, the diverse superfamily of lysine acetyltransferases executes an acetylation program that is important for different cellular processes and perturbation of such a program may cause the development of cancer and other diseases.

Age-related changes of cardiac gene expression following myocardial ischemia/reperfusion

Young and old (4 and 25 months of age, respectively) Fisher 344/Brown Norway hybrid female rats were subjected to four 3 min episodes of ischemia separated by 5 min of reperfusion. Corresponding open-chest sham-operated groups received 32 min of no intervention. All rats were allowed to recover, and 24h later hearts were removed and frozen in liquid nitrogen. Global gene profiling in the ischemic and the non-ischemic areas and in the sham-operated hearts as well was carried out by using Affymetrix Gene Chips. Young ischemic hearts demonstrated down-regulation of gene expression associated with early-remodeling including down-regulation of tissue inhibitor of metalloproteinase 1, decorin, collagen, tropoelastin, and fibulin, as well as decreases in hypertrophy-related transcripts. In contrast, old hearts showed a unique injury-related response, which included up-regulation of mRNAs for proteins associated with hypertrophy or apoptosis (including H36-alpha7 integrin, alpha-actin, tubulin, filamin, connective tissue growth factor, calcineurin, serine protease, and apoptosis inducing factor). These injury-related changes in gene expression could in part explain increased gravity of outcomes of ischemia and myocardial infarction in elderly hearts.

Concomitant increase of histone acetyltransferase activity and degradation of p300 during retinoic acid-induced differentiation of F9 cells

The p300 and closely related CBP histone acetyltransferases (HAT) function as global transcriptional co-activators that play roles in many cell differentiation and signal transduction pathways. Despite their similarities, p300 and CBP have distinct functions during retinoic acid-induced differentiation of mouse F9 embryonal carcinoma cells. F9 cells constitute a well established model system for investigating the first steps of early development and retinoic acid signaling ex vivo. p300, but not CBP, was shown to be essential for F9 differentiation. In this study we have investigated the regulation of p300 during F9 differentiation. We report a dramatic decrease of p300, but not CBP protein levels, after 48 h of retinoic acid treatment. p300 is degraded via the ubiquitin-proteasome pathway. Although the large majority of p300 is degraded, its global HAT activity stays constant during F9 differentiation, which means that its specific HAT activity increases considerably. p300 is strongly phosphorylated in both undifferentiated and differentiated F9 cells; its HAT activity, however, is independent of phosphorylation before differentiation and becomes dependent on phosphorylation during differentiation. Furthermore, we show that protein kinase A affects p300 HAT activity both in vivo and in vitro as well as p300 phosphorylation in differentiated cells. Thus, we show that p300 is differentially phosphorylated in undifferentiated versus differentiated cells and that the changes in phosphorylation affect its HAT activity. Moreover, our study suggests an explanation for the functional switch of p300-mediated repression versus activation during F9 differentiation.

A point mutation in the first splice donor leads to reduced oncogenic properties of the adenovirus serotype 12 E1A gene

Cells transformed by proteins of early regions 1A (E1A) and 1B (E1B) of oncogenic adenovirus serotype 12 (Ad12) grow to tumours in syngeneic, immunocompetent rodents. To gain insight into the mechanisms of oncogenic transformation, we point mutated the first splice donor in the Ad12-E1A gene, leading to the loss of the Ad12-E1A(9.5S) and Ad12-E1A(11S/10S) proteins and to a conservative amino acid (aa) exchange at position aa 30 (valine vs. leucine) in the Ad12-E1A(13S) and Ad12-E1A(12S ) proteins. BMK cells transformed by mutant Ad12-E1A (Ad12-E1Am) plus Ad12-E1B via retrovirus-mediated gene transfer showed features comparable to wild-type Ad12-E1A (Ad12-E1Awt) plus Ad12-E1B-transformed cells: they formed foci in soft agar and produced tumours in immunodeficient nude mice, although after a prolonged latency period. These results suggest that Ad12-E1A(9.5S) and Ad12-E1A(11S/10S) are dispensable for cellular transformation. However, in contrast to Ad12-E1Awt cells, Ad12-E1Am cells failed to grow to tumours in syngeneic, immunocompetent rodents, with the exception of one cell line, which produced tumours in about 50% of the immunocompetent animals. Interestingly, the concentration of the putative tumour suppressor and co-activator p300 was elevated in cell lines expressing high levels of Ad12-E1A and Ad12-E1B due to an increased half-life. These results indicate that p300 is stabilized in Ad12-E1-transformed BMK cells, probably by a mechanism linked to high expression of Ad12-E1A/E1B.

The transcriptional co-activators CREB-binding protein (CBP) and p300 play a critical role in cardiac hypertrophy that is dependent on their histone acetyltransferase activity

The CBP and p300 proteins are transcriptional co-activators that are involved in a variety of transcriptional pathways in development and in response to specific signaling pathways. We have previously demonstrated that the ability of both these factors to stimulate transcription is greatly enhanced by treatment of cardiac cells with the hypertrophic agent phenylephrine (PE). Here, we show that inhibition of either CBP or p300 with antisense or dominant negative mutant constructs inhibits PE-induced hypertrophy as assayed by atrial naturetic protein production, cardiac cell protein:DNA ratio and cell size. Furthermore, we show that overexpression of CBP or p300 can induce hypertrophy and that this effect requires their histone acetyltransferase (HAT) activity. Moreover, we show that PE can directly enhance CBP HAT activity and that artificial enhancement of HAT activity is sufficient to induce hypertrophy. Hence, CBP and p300 play an essential role in hypertrophy induced by PE, and this effect is mediated via PE-induced enhancement of their HAT activity. This is the first time a role for these factors, and their HAT activity, in hypertrophy has been directly demonstrated.

Attenuation of glucocorticoid signaling through targeted degradation of p300 via the 26S proteasome pathway

The effects of acetylation on gene expression are complex, with changes in chromatin accessibility intermingled with direct effects on transcriptional regulators. For the nuclear receptors, both positive and negative effects of acetylation on specific gene transcription have been observed. We report that p300 and steroid receptor coactivator 1 interact transiently with the glucocorticoid receptor and that the acetyltransferase activity of p300 makes an important contribution to glucocorticoid receptor-mediated transcription. Treatment of cells with the deacetylase inhibitor, sodium butyrate, inhibited steroid-induced transcription and altered the transient association of glucocorticoid receptor with p300 and steroid receptor coactivator 1. Additionally, sustained sodium butyrate treatment induced the degradation of p300 through the 26S proteasome pathway. Treatment with the proteasome inhibitor MG132 restored both the level of p300 protein and the transcriptional response to steroid over 20 h of treatment. These results reveal new levels for the regulatory control of gene expression by acetylation and suggest feedback control on p300 activity.

Elevated myocardial Akt signaling ameliorates doxorubicin-induced congestive heart failure and promotes heart growth

Doxorubicin is a chemotherapeutic agent that can induce cardiotoxicity and congestive heart failure (CHF). In this study we tested whether intracoronary Akt1 gene delivery could inhibit doxorubicin-induced CHF. Saline or a replication defective adenoviral vector expressing constitutively-active Akt1 (myrAkt) or beta-galactosidase (betagal) was delivered to the myocardium of 8 week old rats one day prior to initiating doxorubicin administration. In animals receiving saline or betagal, doxorubicin resulted in significant decreases in cardiac function and retarded post-natal heart growth at the 5 weeks time point. In contrast, transduction of myrAkt protected hearts against doxorubicin-induced decreases in fractional shortening and cardiac index, and improved left ventricular function at 5 weeks time point. Delivery of myrAkt also reversed the doxorubicin-induced reduction in post-natal heart growth and diminished lung edema. These data show that myocardial Akt can inhibit doxorubicin-induced reductions in cardiac function and growth, suggesting that manipulation of this signaling pathway may have utility for the treatment of congestive heart failure.

Csx/Nkx2-5 is required for homeostasis and survival of cardiac myocytes in the adult heart

Csx/Nkx2-5, which is essential for cardiac development of the embryo, is abundantly expressed in the adult heart. We here examined the role of Csx/Nkx2-5 in the adult heart using two kinds of transgenic mice. Transgenic mice that overexpress a dominant negative mutant of Csx/Nkx2-5 (DN-TG mice) showed degeneration of cardiac myocytes and impairment of cardiac function. Doxorubicin induced more marked cardiac dysfunction in DN-TG mice and less in transgenic mice that overexpress wild type Csx/Nkx2-5 (WT-TG mice) compared with non-transgenic mice. Doxorubicin induced cardiomyocyte apoptosis, and the number of apoptotic cardiomyocytes was high in the order of DN-TG mice, non-transgenic mice, and WT-TG mice. Overexpression of the dominant negative mutant of Csx/Nkx2-5 induced apoptosis in cultured cardiomyocytes, while expression of wild type Csx/Nkx2-5 protected cardiomyocytes from doxorubicin-induced apoptotic death. These results suggest that Csx/Nkx2-5 plays a critical role in maintaining highly differentiated cardiac phenotype and in protecting the heart from stresses including doxorubicin.

Proteasomal inhibition enhances glucocorticoid receptor transactivation and alters its subnuclear trafficking

The ubiquitin-proteasome pathway regulates the turnover of many transcription factors, including steroid hormone receptors such as the estrogen receptor and progesterone receptor. For these receptors, proteasome inhibition interferes with steroid-mediated transcription. We show here that proteasome inhibition with MG132 results in increased accumulation of the glucocorticoid receptor (GR), confirming that it is likewise a substrate for the ubiquitin-proteasome degradative pathway. Using the mouse mammary tumor virus (MMTV) promoter integrated into tissue culture cells, we found that proteasome inhibition synergistically increases GR-mediated transactivation. This increased activation was observed in a number of cell lines and on various MMTV templates, either as transiently transfected reporters or stably integrated into chromatin. These observations suggest that the increase in GR-mediated transcription due to proteasome inhibition may occur downstream of the initial chromatin remodeling step. In support of this concept, the increase in transcription did not correlate with an increase in chromatin remodeling, as measured by restriction enzyme hypersensitivity, or transcription factor loading, as exemplified by nuclear factor 1. To investigate the relationship between GR turnover, transcription, and subnuclear trafficking, we examined the effect of proteasome inhibition on the mobility of the GR within the nucleus and association of the GR with the nuclear matrix. Blocking GR turnover reduced the mobility of the GR within the nucleus, and this correlated with increased association of the receptor with the nuclear matrix. As a result of proteasome inhibition, GR mobility within the nucleus was reduced while its association with the nuclear matrix was increased. Thus, while altered nuclear mobility of steroid receptors may be a common feature of proteasome inhibition, GR is unique in its enhanced transactivation activity that results when proteasome function is compromised. Proteasomes may therefore impact steroid receptor action at multiple levels and exert distinct effects on individual receptor types.

The transcriptional co-activators CBP and p300 are activated via phenylephrine through the p42/p44 MAPK cascade

The CBP and p300 co-activators play a key role in many aspects of gene regulation being recruited to the DNA via transcription factors that are targets for specific signaling pathways. It has previously been demonstrated that in neuronal cells the ability of CBP and p300 to activate transcription can be directly stimulated by nerve growth factor or calcium-activated signaling pathways. Here we demonstrate that, in cardiac cells, the activity of CBP and p300 is stimulated by phenylephrine (PE) treatment and that they are required for the activation of atrial naturetic factor (ANF) gene expression by PE. Activation of CBP/p300 by PE involves the p42/p44 MAPK pathway and targets primarily the N terminus of p300 and the C terminus of CBP, which are not homologous to one another. To our knowledge, this is the first report of a specific stimulus modulating the activity of CBP and p300 in cardiac cells and it suggests that these factors play an important role in the hypertrophic effect of PE.