Adenovirus E1A inhibits cardiac myocyte-specific gene expression through its amino terminus

Anacardic acid attenuates pressure-overload cardiac hypertrophy through inhibiting histone acetylases

Cardiac hypertrophy has become a major cardiovascular problem wordwide and is considered the early stage of heart failure. Treatment and prevention strategies are needed due to the suboptimal efficacy of current treatment methods. Recently, many studies have demonstrated the important role of histone acetylation in myocardium remodelling along with cardiac hypertrophy. A Chinese herbal extract containing anacardic acid (AA) is known to possess strong histone acetylation inhibitory effects. In previous studies, we demonstrated that AA could reverse alcohol‐induced cardiac hypertrophy in an animal model at the foetal stage. Here, we investigated whether AA could attenuate cardiac hypertrophy through the modulation of histone acetylation and explored its potential mechanisms in the hearts of transverse aortic constriction (TAC) mice. This study showed that AA attenuated hyperacetylation of acetylated lysine 9 on histone H3 (H3K9ac) by inhibiting the expression of p300 and p300/CBP‐associated factor (PCAF) in TAC mice. Moreover, AA normalized the transcriptional activity of the heart nuclear transcription factor MEF2A. The high expression of cardiac hypertrophy‐linked genes (ANP, β‐MHC) was reversed through AA treatment in the hearts of TAC mice. Additionally, we found that AA improved cardiac function and survival rate in TAC mice. The current results further highlight the mechanism by which histone acetylation is controlled by AA treatment, which may help prevent and treat hypertrophic cardiomyopathy.

Repression of miR-142 by p300 and MAPK is required for survival signalling via gp130 during adaptive hypertrophy

An increase in cardiac workload, ultimately resulting in hypertrophy, generates oxidative stress and therefore requires the activation of both survival and growth signal pathways. Here, we wanted to characterize the regulators, targets and mechanistic roles of miR-142, a microRNA (miRNA) negatively regulated during hypertrophy. We show that both miRNA-142-3p and -5p are repressed by serum-derived growth factors in cultured cardiac myocytes, in models of cardiac hypertrophy in vivo and in human cardiomyopathic hearts. Levels of miR-142 are inversely related to levels of acetyltransferase p300 and MAPK activity. When present, miR-142 inhibits both survival and growth pathways by directly targeting nodal regulators p300 and gp130. MiR-142 also potently represses multiple components of the NF-κB pathway, preventing cytokine-mediated NO production and blocks translation of α-actinin. Forced expression of miR-142 during hypertrophic growth induced extensive apoptosis and cardiac dysfunction; conversely, loss of miR-142 fully rescued cardiac function in a murine heart failure model. Downregulation of miR-142 is required to enable cytokine-mediated survival signalling during cardiac growth in response to haemodynamic stress and is a critical element of adaptive hypertrophy.

O-GlcNAc signaling attenuates ER stress-induced cardiomyocyte death

We previously demonstrated that the O-linked beta-N-acetylglucosamine (O-GlcNAc) posttranslational modification confers cardioprotection at least partially through mitochondrial-dependent mechanisms, but it remained unclear if O-GlcNAc signaling interfered with other mechanisms of cell death. Because ischemia/hypoxia causes endoplasmic reticulum (ER) stress, we ascertained whether O-GlcNAc signaling could attenuate ER stress-induced cell death per se. Before induction of ER stress (with tunicamycin or brefeldin A), we adenovirally overexpressed O-GlcNAc transferase (AdOGT) or pharmacologically inhibited O-GlcNAcase [via O-(2-acetamido-2-deoxy-d-glucopyranosylidene) amino-N-phenylcarbamate] to augment O-GlcNAc levels or adenovirally overexpressed O-GlcNAcase to reduce O-GlcNAc levels. AdOGT significantly (P < 0.05) attenuated the activation of the maladaptive arm of the unfolded protein response [according to C/EBP homologous protein (CHOP) activation] and cardiomyocyte death (reflected by percent propidium iodide positivity). Moreover, pharmacological inhibition of O-GlcNAcase significantly (P < 0.05) mitigated ER stress-induced CHOP activation and cardiac myocyte death. Interestingly, overexpression of GCA did not alter ER stress markers but exacerbated brefeldin A-induced cardiomyocyte death. We conclude that enhanced O-GlcNAc signaling represents a partially proadaptive response to reduce ER stress-induced cell death. These results provide new insights into a possible interaction between O-GlcNAc signaling and ER stress and may partially explain a mechanism of O-GlcNAc-mediated cardioprotection.

Unique hexosaminidase reduces metabolic survival signal and sensitizes cardiac myocytes to hypoxia/reoxygenation injury

Metabolic signaling through the posttranslational linkage of N-acetylglucosamine (O-GlcNAc) to cellular proteins represents a unique signaling paradigm operative during lethal cellular stress and a pathway that we and others have recently shown to exert cytoprotective effects in vitro and in vivo. Accordingly, the present work addresses the contribution of the hexosaminidase responsible for removing O-GlcNAc (ie, O-GlcNAcase) from proteins. We used pharmacological inhibition, viral overexpression, and RNA interference of O-GlcNAcase in isolated cardiac myocytes to establish its role during acute hypoxia/reoxygenation. Elevated O-GlcNAcase expression significantly reduced O-GlcNAc levels and augmented posthypoxic cell death. Conversely, short interfering RNA directed against, or pharmacological inhibition of, O-GlcNAcase significantly augmented O-GlcNAc levels and reduced posthypoxic cell death. On the mechanistic front, we evaluated posthypoxic mitochondrial membrane potential and found that repression of O-GlcNAcase activity improves, whereas augmentation impairs, mitochondrial membrane potential recovery. Similar beneficial effects on posthypoxic calcium overload were also evident. Such changes were evident without significant alteration in expression of the major putative components of the mitochondrial permeability transition pore (ie, voltage-dependent anion channel, adenine nucleotide translocase, cyclophilin D). The present results provide definitive evidence that O-GlcNAcase antagonizes posthypoxic cardiac myocyte survival. Moreover, such results support a renewed approach to the contribution of metabolism and metabolic signaling to the determination of cell fate.

Quantitative control of adaptive cardiac hypertrophy by acetyltransferase p300

BACKGROUND

Acetyltransferase p300 is essential for cardiac development and is thought to be involved in cardiac myocyte growth through MEF2- and GATA4-dependent transcription. However, the importance of p300 in the modulation of cardiac growth in vivo is unknown.

METHODS AND RESULTS

Pressure overload induced by transverse aortic coarctation, postnatal physiological growth, and human heart failure were associated with large increases in p300. Minimal transgenic overexpression of p300 (1.5- to 3.5-fold) induced striking myocyte and cardiac hypertrophy. Both mortality and cardiac mass were directly related to p300 protein dosage. Heterozygous loss of a single p300 allele reduced pressure overload-induced hypertrophy by approximately 50% and rescued the hypertrophic phenotype of p300 overexpressers. Increased p300 expression had no effect on total histone deacetylase activity but was associated with proportional increases in p300 acetyltransferase activity and acetylation of the p300 substrates histone 3 and GATA-4. Remarkably, a doubling of p300 levels was associated with the de novo acetylation of MEF2. Consistent with this, genes specifically upregulated in p300 transgenic hearts were highly enriched for MEF2 binding sites.

CONCLUSIONS

Small increments in p300 are necessary and sufficient to drive myocardial hypertrophy, possibly through acetylation of MEF2 and upstream of signals promoting phosphorylation or nuclear export of histone deacetylases. We propose that induction of myocardial p300 content is a primary rate-limiting event in the response to hemodynamic loading in vivo and that p300 availability drives and constrains adaptive myocardial growth. Specific reduction of p300 content or activity may diminish stress-induced hypertrophy and forestall the development of heart failure.

Non-canonical glycosyltransferase modulates post-hypoxic cardiac myocyte death and mitochondrial permeability transition

O-linked beta-N-acetylglucosamine (O-GlcNAc) is a dynamic, inducible, and reversible post-translational modification of nuclear and cytoplasmic proteins on Ser/Thr amino acid residues. In addition to its putative role as a nutrient sensor, we have recently shown pharmacologic elevation of O-GlcNAc levels positively affected myocyte survival during oxidant stress. However, no rigorous assessment of the contribution of O-GlcNAc transferase has been performed, particularly in the post-hypoxic setting. Therefore, we hypothesized that pharmacological or genetic manipulation of O-GlcNAc transferase (OGT), the enzyme that adds O-GlcNAc to proteins, would affect cardiac myocyte survival following hypoxia/reoxygenation (H/R). Adenoviral overexpression of OGT (AdOGT) in cardiac myocytes augmented O-GlcNAc levels and reduced post-hypoxic damage. Conversely, pharmacologic inhibition of OGT significantly attenuated O-GlcNAc levels, exacerbated post-hypoxic cardiac myocyte death, and sensitized myocytes to mitochondrial membrane potential collapse. Both genetic deletion of OGT using a cre-lox approach and translational silencing via RNAi also resulted in significant reductions in OGT protein and O-GlcNAc levels, and, exacerbated post-hypoxic cardiac myocyte death. Inhibition of OGT reduced O-GlcNAc levels on voltage dependent anion channel (VDAC) in isolated mitochondria and sensitized to calcium-induced mitochondrial permeability transition pore (mPTP) formation, indicating that mPTP may be an important target of O-GlcNAc signaling and confirming the aforementioned mitochondrial membrane potential results. These data demonstrate that OGT exerts pro-survival actions during hypoxia-reoxygenation in cardiac myocytes, particularly at the level of mitochondria.

Coordinate Inhibition of Cytokine-mediated Induction of Ferritin H, Manganese Superoxide Dismutase, and Interleukin-6 by the Adenovirus E1A Oncogene

Adenovirus E1A sensitizes cells to the cytotoxic action of tumor necrosis factor alpha (TNF-alpha). This effect has been attributed to direct blockade of NF-kappaB activation, as well as to increased activation of components of the apoptotic pathway and decreases in inhibitors of apoptosis. In this report we evaluated the mechanism by which E1A modulates the expression of the cytokine-inducible cytoprotective genes manganese superoxide dismutase (MnSOD), interleukin-6 (IL-6), and ferritin heavy chain (FH). We observed that E1A blocks induction of MnSOD, IL-6, and FH by TNF-alpha or IL-1alpha. Because NF-kappaB plays a role in cytokine-dependent induction of MnSOD, IL-6, and FH, we assessed the effect of E1A on NF-kappaB in cells treated with TNF. IkappaB, the inhibitor of NF-kappaB, was degraded similarly in the presence and absence of E1A. TNF induced a quantitatively and temporally equivalent activation of NF-kappaB in control and E1A-transfected cells. However, TNF-dependent acetylation of NF-kappaB was diminished in cells expressing E1A. E1A mutants unable to bind p400 or the Rb family proteins were still capable of repressing TNF-dependent induction of FH. However, mutants of E1A that abrogated binding of p300/CBP blocked the ability of E1A to repress TNF-dependent induction of FH. These results suggest that p300/CBP is a critical control point in NF-kappaB-dependent transcriptional regulation of cytoprotective genes by cytokines.

Cell Cycle-mediated Regulation of Smooth Muscle α-Actin Gene Transcription in Fibroblasts and Vascular Smooth Muscle Cells Involves Multiple Adenovirus E1A-interacting Cofactors

Expression of smooth muscle alpha-actin in growth factor-induced myofibroblasts and in differentiated vascular smooth muscle cells is transcriptionally controlled by multiple positive or negative trans-acting factors interacting with distinct cis-elements in the 5'-flanking region of the gene. Because none of the transcriptional regulators reported to date is smooth muscle cell- or myofibroblast-specific per se, the dynamic interplay among many factors interacting at specific sites along the promoter appears to be a signature feature of smooth muscle alpha-actin gene regulation in these cell types. Herein, the ability of the adenovirus E1A 12 S protein to bind and functionally inactivate specific cell regulatory factors has been exploited to identify several previously unknown coactivators of the mouse smooth muscle alpha-actin promoter in rodent fibroblasts and vascular smooth muscle cells. In transient cotransfection assays, ectopic expression of wild type E1A suppressed promoter activity in a dose- and cis-element-dependent manner. In asynchronous cells, N-terminal E1A mutants defective in CREB-binding protein (CBP) and p300 binding capacity exhibited markedly reduced inhibitory activity toward a smooth muscle alpha-actin promoter driven by a composite TEF-1-, SRF-, and Sp1/3-regulated enhancer. In synchronized cells, however, a more complex mutant E1A inhibitory pattern indicated that collaboration between CBP/p300 and the retinoblastoma family of pocket proteins was required to produce a fully functional enhancer. Cotransfection experiments conducted with Rb(-/-) fibroblasts demonstrated the necessity of pRB in augmenting smooth muscle alpha-actin enhancer/promoter activity. Physical interaction studies with the use of purified wild type and mutant E1A proteins confirmed that CBP, p300, and pRB were targets of E1A binding in nuclear extracts of vascular smooth muscle cells and/or fibroblasts. Collectively, these results suggest that a repertoire of E1A-interacting proteins, namely CBP/p300 and pRB, serve to integrate the activities of multiple trans-acting factors to control smooth muscle alpha-actin gene transcription in a cell type- and cell cycle-dependent manner.

FOG-2 competes with GATA-4 for transcriptional coactivator p300 and represses hypertrophic responses in cardiac myocytes

A multizinc finger protein, FOG-2, associates with a cardiac transcription factor, GATA-4, and represses GATA-4-dependent transcription. GATA-4 is required not only for normal heart development but is also involved in hypertrophic responses in cardiac myocytes; however, the effects of FOG-2 on these responses are unknown. The interaction of GATA-4 with a transcriptional coactivator p300 is required for its full transcriptional activity and the activation of the embryonic program during myocardial cell hypertrophy. We show here that exogenous FOG-2 represses phenylephrine-induced hypertrophic responses such as myofibrillar organization, increases in cell size, and hypertrophy-associated gene transcription. Using immunoprecipitation Western blotting, we demonstrate that FOG-2 physically interacted with p300 and reduced the binding of GATA-4 to p300. In addition, in COS7 cells, in which the function of endogenous p300 is disrupted, FOG-2 is unable to repress the GATA-4-dependent transcriptional activities; however, FOG-2 markedly repressed the p300-mediated increase in the DNA-binding and transcriptional activities of GATA-4 in these cells. Similarly, FOG-2 inhibited a phenylephrine-induced increase in the p300/GATA-4 interaction, the GATA-4/DNA-binding, and transcriptional activities of GATA-4-dependent promoters in cardiac myocytes as well. These findings demonstrate that FOG-2 represses hypertrophic responses in cardiac myocytes and that p300 is involved in these repressive effects.

Transforming growth factor beta1 receptor II is downregulated by E1A in adenovirus-infected cells

Transforming growth factor beta1 (TGF-beta1) signaling is compromised in many tumors, thereby allowing the tumor to escape the growth-inhibitory and proapoptotic activities of the cytokine. Human adenoviruses interfere with a number of cellular pathways involved in cell cycle regulation and apoptosis, initially placing the cell in a "tumor-like" state by forcing quiescent cells into the cell cycle and also inhibiting apoptosis. We report that adenovirus-infected cells resemble tumor cells in that TGF-beta1 signaling is inhibited. The levels of TGF-beta1 receptor II (TbetaRII) in adenovirus-infected cells were decreased, and this decrease was mapped, by using virus mutants, to the E1A gene and to amino acids 2 to 36 and the C-terminal binding protein binding site in the E1A protein. The decrease in the TbetaRII protein was accompanied by a decrease in TbetaRII mRNA. The decrease in TbetaRII protein levels in adenovirus-infected cells was greater than the decrease in TbetaRII mRNA, suggesting that downregulation of the TbetaRII protein may occur through more than one mechanism. Surprisingly in this context, the half-lives of the TbetaRII protein in infected and uninfected cells were similar. TGF-beta1 signaling was compromised in cells infected with wild-type adenovirus, as measured with 3TP-lux, a TGF-beta-sensitive reporter plasmid expressing luciferase. Adenovirus mutants deficient in TbetaRII downregulation did not inhibit TGF-beta1 signaling. TGF-beta1 pretreatment reduced the relative abundance of adenovirus structural proteins in infected cells, an effect that was potentiated when cells were infected with mutants incapable of modulating the TGF-beta signaling pathway. These results raise the possibility that inhibition of TGF-beta signaling by E1A is a means by which adenovirus counters the antiviral defenses of the host.

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.

Transgenic expression in mouse lung reveals distinct biological roles for the adenovirus type 5 E1A 243- and 289-amino-acid proteins

Little is known about the biological significance of human adenovirus type 5 (Ad5) E1A in vivo. However, Ad5 E1A is well defined in vitro and can be detected frequently in the lungs of patients with pulmonary disease. Transgenic expression of the Ad5 E1A gene targeted to the mouse lung reveals distinct biological effects caused by two Ad5 E1A products. Either of two Ad5 E1A proteins was preferentially expressed in vivo in the transgenic lungs. The preferential expression of the Ad5 E1A 243-amino-acid (aa) protein at a moderate level was associated with cellular hyperplasia, nodular lesions of proliferating lymphocyte-like cells, and a low level of p53-dependent apoptosis in the lungs of transgenic mice. In contrast, the preferential expression of the Ad5 E1A 289-aa protein at a moderate level resulted in a proapoptotic injury and an acute pulmonary proinflammation in the lungs of transgenic mice, mediated by multiple apoptotic pathways, as well as an enhancement of the host immune cell response. Expression of the Ad5 E1A 243-aa protein resulted in proliferation-stimulated p53 upregulation, while expression of the Ad5 E1A 289-aa protein led to DNA damage-induced p53 activation. These data suggest that the Ad5 E1A 243- and 289-aa proteins lead to distinct biological roles in vivo.

The transcription factors GATA4 and dHAND physically interact to synergistically activate cardiac gene expression through a p300-dependent mechanism

An intricate array of heterogeneous transcription factors participate in programming tissue-specific gene expression through combinatorial interactions that are unique to a given cell-type. The zinc finger-containing transcription factor GATA4, which is widely expressed in mesodermal and endodermal derived tissues, is thought to regulate cardiac myocyte-specific gene expression through combinatorial interactions with other semi-restricted transcription factors such as myocyte enhancer factor 2, nuclear factor of activated T-cells, serum response factor, and Nkx2.5. Here we determined that GATA4 also interacts with the cardiac-expressed basic helix-loop-helix transcription factor dHAND (also known as HAND2). GATA4 and dHAND synergistically activated expression of cardiac-specific promoters from the atrial natriuretic factor gene, the b-type natriuretic peptide gene, and the alpha-myosin heavy chain gene. Using artificial reporter constructs this functional synergy was shown to be GATA site-dependent, but E-box site-independent. A mechanism for the transcriptional synergy was suggested by the observation that the bHLH domain of dHAND physically interacted with the C-terminal zinc finger domain of GATA4 forming a higher order complex. This transcriptional synergy observed between GATA4 and dHAND was associated with p300 recruitment, but not with alterations in DNA binding activity of either factor. Moreover, the bHLH domain of dHAND directly interacted with the CH3 domain of p300 suggesting the existence of a higher order complex between GATA4, dHAND, and p300. Taken together with previous observations, these results suggest the existence of an enhanceosome complex comprised of p300 and multiple semi-restricted transcription factors that together specify tissue-specific gene expression in the heart.

Suppression of tissue factor expression, cofactor activity, and metastatic potential of murine melanoma cells by the N-terminal domain of adenovirus E1A 12S protein

Tissue factor, the cellular initiator of blood coagulation, has been implicated as a determinant of metastatic potential in human melanoma cells. Here, we report that differential expression of tissue factor in murine melanoma cell lines of known metastatic behavior is mediated by AP-1-dependent and 12S E1A oncoprotein-repressible gene transcription. When compared to weakly metastatic C10 cells, highly metastatic M4 cells possessed elevated levels of tissue factor cofactor activity, transfected promoter activity, and heterodimeric AP-1 DNA-binding complexes containing Fra-1. Transient co-expression of the adenovirus E1A 12S oncoprotein strongly repressed transcription of an AP-1-driven tissue factor reporter gene indicating the additional requirement of N-terminal E1A-interacting coactivators. Stable expression of E1A mutants defective in CBP/p300-binding failed to suppress tissue factor expression and experimental metastasis by M4 cells while clones expressing wild type E1A exhibited greatly reduced tissue factor cofactor activity and metastatic potential in vivo. Overexpression of functional tissue factor in cells containing wild type E1A failed to restore the highly metastatic M4 phenotype suggesting that additional E1A-responsive and CBP/p300-dependent genes are required to facilitate metastasis of murine melanoma cells demonstrating high tissue factor expression and cofactor activity.

Activation of c-Jun N-terminal kinase promotes survival of cardiac myocytes after oxidative stress

Reperfusion injury occurs when ischaemic tissue is reperfused. It involves the generation and release of reactive oxygen that activates numerous signalling pathways and initiates cell death. Exposure of isolated cardiac myocytes to chronic hypoxia followed by reoxygenation results in the early activation of c-Jun N-terminal kinase (JNK) and death by apoptosis of approx. 30% of the myocytes. Although JNK activation has been described in a number of models of ischaemia/reperfusion, the contribution of JNK activation to cell fate has not been established. Here we report that the activation of JNK by reoxygenation correlates with myocyte survival. Transfection of myocytes with JNK pathway interfering plasmid vectors or infection with adenoviral vectors support the hypothesis that JNK is protective. Transfection or infection with JNK inhibitory mutants increased the rates of apoptosis by almost 2-fold compared with control cultures grown aerobically or subjected to hypoxia and reoxygenation. Caspase 9 activity, measured by LEHD cleavage, increased >3-fold during reoxygenation and this activity was enhanced significantly at all times in cultures infected with dominant negative JNK adenovirus. Hypoxia-reoxygenation mediated a biphasic (2.6- and 2.9-fold) activation of p38 mitogen-activated protein kinase, as well as a small increase of tumour necrosis factor alpha (TNFalpha) secretion, but treatments with the p38 MAPK-specific inhibitor SB203580 or saturating levels of a TNFalpha-1 blocking antibody provided only partial protection against apoptosis. The results suggest that JNK activation is protective and that the pathway is largely independent of p38 MAPK or secreted TNFalpha.

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.

p300 Functions as a coactivator of transcription factor GATA-4

Transcription factor GATA-4 plays critical roles in controlling heart development and cardiac hypertrophy. To understand how GATA-4 functions under diverse conditions, we sought to identify its coactivators. We tested p300 as a coactivator in GATA-4-dependent transient transcription assays in NIH3T3 cells and found that p300 synergistically activated GATA-4-dependent transcription on both synthetic and natural promoters. Direct physical interactions between the N- and C-zinc finger domains of GATA-4 and the cysteine/histidine-rich region 3 (C/H3) of p300 were identified in immunoprecipitation and glutathione S-transferase pull-down experiments. Deletion of the C/H3 region of p300 abolished its coactivator activity indicating that the physical interaction was required for functional synergy. Through the use of a series of GATA-4 zinc finger mutants, the amino acids WRR in the C finger were identified as critical to the interaction. The adenoviral E1A protein or a peptide encoding the C/H3 region of p300 could inhibit GATA-4-dependent transcription, presumably by competing for p300 binding. Furthermore, deletion of the region of p300 encoding the histone acetyltransferase activity abolished its effect on GATA-4-dependent transcriptional activity. These results establish that p300 acts as a GATA-4 coactivator and that the p300 histone acetyltransferase activity is necessary for the functional interaction.

Coexpression of Mutant p53 and p193 Renders Embryonic Stem Cell–Derived Cardiomyocytes Responsive to the Growth-Promoting Activities of Adenoviral E1A

Abstract —Expression of adenoviral E1A in cardiomyocytes results in the activation of DNA synthesis followed by apoptosis. In contrast, expression of simian virus 40 large T antigen induces sustained cardiomyocyte proliferation. Previous studies have shown that T antigen binds to 2 proapoptotic proteins in cardiomyocytes, namely the p53 tumor suppressor and p193 (a new member of the BH3-only proapoptosis subfamily). Structure-function analyses identified a p193 C-terminal truncation mutant that encodes prosurvival activity. This mutant was used to test the role of p193 in E1A-induced cardiomyocyte apoptosis. E1A induced apoptosis in cardiomyocytes derived from differentiating embryonic stem cells. Expression of the prosurvival p193 mutant alone or a mutant p53 alone did not block E1A-induced apoptosis. In contrast, combinatorial expression of mutant p193 and mutant p53 blocked E1A-induced apoptosis, resulting in a proliferative response indistinguishable from that seen with T antigen. These results confirm the hypothesis that there are 2 proapoptotic pathways, encoded by p53 and p193, respectively, which restrict cardiomyocyte cell cycle activity in differentiating embryonic stem cell cultures. Furthermore, these results explain in molecular terms the phenotypic differences of E1A versus T-antigen gene transfer in cardiomyocytes.

Molecular regulation of the endothelin-1 gene by hypoxia. Contributions of hypoxia-inducible factor-1, activator protein-1, GATA-2, AND p300/CBP

Endothelin-1 (ET-1) is a peptide hormone with potent vasoconstrictor properties which is synthesized and secreted predominantly by vascular endothelial cells. Its production is regulated by numerous stimuli including ischemia and hypoxia, and the enhanced levels that occur during myocardial ischemia may contribute to the progression of heart failure. We reported previously a preliminary characterization of a hypoxia-inducible factor-1 (HIF-1) binding site in the human ET-1 promoter which contributed to the activation of ET-1 expression in endothelial cells. We report here that the HIF-1 binding site alone is not sufficient for the response to hypoxia but requires an additional 50 base pairs of flanking sequence that includes binding sites for the factors activator protein-1 (AP-1), GATA-2, and CAAT-binding factor (NF-1). Mutation of any one of these sites or the HIF-1 site eliminated induction by hypoxia. Mutations of the AP-1 and GATA-2 sites, but not the HIF-1 site, were complemented by overexpressing AP-1, GATA-2, HIF-1alpha, or the activator protein p300/CBP, restoring the response to hypoxia. Binding studies in vitro confirmed physical associations among GATA-2, AP-1, and HIF-1 factors. Overexpression or depletion of p300/CBP modulated the level of ET-1 promoter expression as well as the endogenous ET-1 transcript but did not change the fold induction by hypoxia in either case. Regulation of the ET-1 promoter by hypoxia in non-endothelial cells required overexpression of GATA-2 and HIF-1alpha. The results support essential roles for AP-1, GATA-2, and NF-1 in stabilizing the binding of HIF-1 and promoting recruitment of p300/CBP to the ET-1 hypoxia response complex.

Control of cardiac-specific transcription by p300 through myocyte enhancer factor-2D

The transcriptional integrator p300 regulates gene expression by interaction with sequence-specific DNA-binding proteins and local remodeling of chromatin. p300 is required for cardiac-specific gene transcription, but the molecular basis of this requirement is unknown. Here we report that the MADS (MCM-1, agamous, deficiens, serum response factor) box transcription factor myocyte enhancer factor-2D (MEF-2D) acts as the principal conduit for cardiac transcriptional activation by p300. p300 activation of the native 2130-base pair human skeletal alpha-actin promoter required a single hybrid MEF-2/GATA-4 DNA motif centered at -1256 base pairs. Maximal expression of the promoter in cultured myocytes and in vivo correlated with binding of both MEF-2 and p300, but not GATA-4, to this AT-rich motif. p300 and MEF-2 were coprecipitated from cardiac nuclear extracts by an oligomer containing this element. p300 was found exclusively in a complex with MEF-2D at this and related sites in other cardiac-restricted promoters. MEF-2D, but not other MEFs, significantly potentiated cardiac-specific transcription by p300. No physical or functional interaction was observed between p300 and other factors implicated in skeletal actin transcription, including GATA-4, TEF-1, or SRF. These results show that, in the intact cell, p300 interactions with its protein targets are highly selective and that MEF-2D is the preferred channel for p300-mediated transcriptional control in the heart.

Tight control of exogenous SERCA expression is required to obtain acceleration of calcium transients with minimal cytotoxic effects in cardiac myocytes

Collateral effects of exogenous sarcoendoplasmic reticulum Ca(2+) ATPase (SERCA) expression were characterized in neonatal rat and chicken embryo cardiac myocytes, and the conditions required to produce acceleration of Ca(2+) transients with minimal toxicity were established. Cultured myocytes were infected with adenovirus vector carrying the cDNA of wild-type SERCA1, an inactive SERCA1 mutant, or enhanced green fluorescence protein under control of the cytomegalovirus promoter. Controls were exposed to empty virus vector. Each group was tested with and without phenylephrine (PHE) treatment. Under conditions of limited calf-serum exposure, the infected rat myocytes manifested a more rapid increase in size, protein content, and rate of protein synthesis relative to noninfected controls. These changes were not accompanied by reversal to fetal transcriptional pattern (as observed in hypertrophy triggered by PHE) and may be attributable to facilitated exchange with serum factors. SERCA virus titers >5 to 6 plaque-forming units per cell produced overcrowding of ATPase molecules on intracellular membranes, followed by apoptotic death of a significant number of rat but not chicken myocytes. Enhanced green fluorescence protein virus and empty virus also produced cytotoxic effects but at higher titers than SERCA. Expression of exogenous SERCA and enhancement of Ca(2+) transient kinetics could be obtained with minimal cell damage in rat myocytes if the SERCA virus titer were maintained within 1 to 4 plaque-forming units per cell. Expression of endogenous SERCA was unchanged, but expression of exogenous SERCA was higher in myocytes rendered hypertrophic by treatment with PHE than in nontreated controls.

A novel Rb- and p300-binding protein inhibits transactivation by MyoD

The retinoblastoma protein (Rb) regulates both the cell cycle and tissue-specific transcription, by modulating the activity of factors that associate with its A-B and C pockets. In skeletal muscle, Rb has been reported to regulate irreversible cell cycle exit and muscle-specific transcription. To identify factors interacting with Rb in muscle cells, we utilized the yeast two-hybrid system, using the A-B and C pockets of Rb as bait. A novel protein we have designated E1A-like inhibitor of differentiation 1 (EID-1), was the predominant Rb-binding clone isolated. It is preferentially expressed in adult cardiac and skeletal muscle and encodes a 187-amino-acid protein, with a classic Rb-binding motif (LXCXE) in its C terminus. Overexpression of EID-1 in skeletal muscle inhibited tissue-specific transcription. Repression of skeletal muscle-restricted genes was mediated by a block to transactivation by MyoD independent of G(1) exit and, surprisingly, was potentiated by a mutation that prevents EID-1 binding to Rb. Inhibition of MyoD may be explained by EID-1's ability to bind and inhibit p300's histone acetylase activity, an essential MyoD coactivator. Thus, EID-1 binds both Rb and p300 and is a novel repressor of MyoD function.

Genetic dissection of cardiac growth control pathways

Cardiac muscle cells exhibit two related but distinct modes of growth that are highly regulated during development and disease. Cardiac myocytes rapidly proliferate during fetal life but exit the cell cycle irreversibly soon after birth, following which the predominant form of growth shifts from hyperplastic to hypertrophic. Much research has focused on identifying the candidate mitogens, hypertrophic agonists, and signaling pathways that mediate these processes in isolated cells. What drives the proliferative growth of embryonic myocardium in vivo and the mechanisms by which adult cardiac myocytes hypertrophy in vivo are less clear. Efforts to answer these questions have benefited from rapid progress made in techniques to manipulate the murine genome. Complementary technologies for gain- and loss-of-function now permit a mutational analysis of these growth control pathways in vivo in the intact heart. These studies have confirmed the importance of suspected pathways, have implicated unexpected pathways as well, and have led to new paradigms for the control of cardiac growth.

The retinoblastoma gene family members pRB and p107 coactivate the AP-1-dependent mouse tissue factor promoter in fibroblasts

Serum-stimulation of quiescent mouse fibroblasts results in transcriptional activation of tissue factor (TF), the cellular initiator of blood coagulation. This requires the rapid entry of c-Fos into specific AP-1 DNA-binding complexes and can be strongly inhibited by the adenovirus EIA 12S gene product. In this study, we utilized a panel of E1A mutants deficient in cellular protein binding to analyse the molecular basis for EIA inhibition of a minimal, c-Fos-dependent TF promoter/ reporter construct in mouse AKR-2B fibroblasts. Mutations which impaired binding of the retinoblastoma tumor suppressor protein family members pRB, p107, and p130 relieved E1A-mediated inhibition of transcription in response to serum-stimulation or c-Fos overexpression. Inhibition was restricted to the G0 to G1 transition, consistent with the specificity of E1A for hypophosphorylated forms of RB proteins. Although E1A mutants deficient in CBP/p300 binding retained the ability to inhibit TF transcription, deletion of the amino-terminal portion of the CBP/p300 interaction domain was required to permit rescue of TF promoter activity by coexpression of pRB. Moreover, ectopic p107 could effectively substitute for pRB in relieving E1A-mediated repression. In primary mouse embryo fibroblasts, activity of the minimal AP-1-dependent TF promoter was suppressed in Rb(-/-) cells compared to parallel Rb(+/-) and Rb(+/+) transfectants. Ectopic expression of either pRB or p107 markedly enhanced TF promoter activity in Rb(-/-) fibroblasts. Collectively, these data imply that pRB and p107 can cooperate with c-Fos to activate TF gene transcription in fibroblasts and suggest a requirement for another, as yet unidentified, E1A-binding protein.

Subcellular compartmentalization of E2F family members is required for maintenance of the postmitotic state in terminally differentiated muscle

Maintenance of cells in a quiescent state after terminal differentiation occurs through a number of mechanisms that regulate the activity of the E2F family of transcription factors. We report here that changes in the subcellular compartmentalization of the E2F family proteins are required to prevent nuclei in terminally differentiated skeletal muscle from reentering S phase. In terminally differentiated L6 myotubes, E2F-1, E2F-3, and E2F-5 were primarily cytoplasmic, E2F-2 was nuclear, whereas E2F-4 became partitioned between both compartments. In these same cells, pRB family members, pRB, p107, and p130 were also nuclear. This compartmentalization of the E2F-1 and E2F-4 in differentiated muscle cells grown in vitro reflected their observed subcellular location in situ. We determined further that exogenous E2F-1 or E2F-4 expressed in myotubes at levels fourfold greater than endogenous proteins compartmentalized identically to their endogenous counterparts. Only when overexpressed at higher levels was inappropriate subcellular location for these proteins observed. At these levels, induction of the E2F-regulated genes, cyclins A and E, and suppression of factors associated with myogenesis, myogenin, and p21(Cip1) was observed. Only at these levels of E2F expression did nuclei in these terminally differentiated cells enter S phase. These data demonstrate that regulation of the subcellular compartmentalization of E2F-family members is required to maintain nuclei in a quiescent state in terminally differentiated cells.

Simian virus 40 large T antigen binds a novel Bcl-2 homology domain 3-containing proapoptosis protein in the cytoplasm

A 193-kDa SV40 large T antigen (T-Ag)-binding protein, designated p193, was identified and cloned. Inspection of the deduced amino acid sequence revealed the presence of a short motif similar to the Bcl-2 homology (BH) domain 3, suggesting that p193 may be a member of a family of apoptosis promoting proteins containing only BH3 motifs. In support of this, p193 expression promoted apoptosis in NIH-3T3 cells. Deletion of the BH3 motif abolished p193 apoptosis activity. p193-induced apoptosis was antagonized by co-expression of Bcl-X(L). Immune cytologic analysis indicated that p193 is localized to the cytoplasm of transfected cells. p193-induced apoptosis was also antagonized by co-expression of T-Ag, which resulted in the cytoplasmic localization of both proteins. The p193 binding site was mapped to an N-terminal region of T-Ag previously implicated in transforming activity. These results suggest that T-Ag possesses an antiapoptosis activity, independent of p53 sequestration, which is actuated by T-Ag/p193 binding in the cytoplasm.

E1A can provoke G1 exit that is refractory to p21 and independent of activating cdk2

E1A can evoke G1 exit in cardiac myocytes and other cell types by displacing E2F transcription factors from tumor suppressor "pocket" proteins and by a less well-characterized p300-dependent pathway. Bypassing pocket proteins (through overexpression of E2F-1) reproduces the effect of inactivating pocket proteins (through E1A binding); however, pocket proteins associate with a number of molecular targets apart from E2F. Hence, pocket protein binding by E1A might engage mechanisms for cell cycle reentry beyond those induced by E2F-1. To test this hypothesis, we used adenoviral gene transfer to express various E2F-1 and E1A proteins in neonatal rat cardiac myocytes that are already refractory to mitogenic serum, in the absence or presence of several complementary cell cycle inhibitors-p16, p21, or dominant-negative cyclin-dependent kinase-2 (Cdk2). Rb binding by E2F-1 was neither necessary nor sufficient for G1 exit, whereas DNA binding was required; thus, exogenous E2F-1 did not merely function by competing for the Rb "pocket." E2F-1-induced G1 exit was blocked by the "universal" Cdk inhibitor p21 but not by p16, a specific inhibitor of Cdk4/6; p21 was permissive for E2F-1 induction of cyclins E and A, but prevented their stimulation of Cdk2 kinase activity. In addition, E2F-1-induced G1 exit was blocked by dominant-negative Cdk2. Forced expression of cyclin E induced endogenous Cdk2 activity but not G1 exit. Thus, E2F-1-induced Cdk2 function was necessary, although not sufficient, to trigger DNA synthesis in cardiac muscle cells. In contrast, pocket protein-binding forms of E1A induced G1 exit that was resistant to inhibition by p21, whereas G1 exit via the E1A p300 pathway was sensitive to inhibition by p21. Both E1A pathways-via pocket proteins and via p300-upregulated cyclins E and A and Cdk2 activity, consistent with a role for Cdk2 in G1 exit induced by E1A. However, p21 blocked Cdk2 kinase activity induced by both E1A pathways equally. Thus, E1A can cause G1 exit without an increase in Cdk2 activity, if the pocket protein-binding domain is intact. E1A also overrides p21 in U2OS cells, provided the pocket protein-binding domain is intact; thus, this novel function of E1A is not exclusive to cardiac muscle cells. In summary, E1A binding to pocket proteins has effects beyond those produced by E2F-1 alone and can drive S-phase entry that is resistant to p21 and independent of an increase in Cdk2 function. This suggests the potential involvement of other endogenous Rb-binding proteins or of alternative E1A targets.

Hypoxia-activated apoptosis of cardiac myocytes requires reoxygenation or a pH shift and is independent of p53

Ischemia and reperfusion activate cardiac myocyte apoptosis, which may be an important feature in the progression of ischemic heart disease. The relative contributions of ischemia and reperfusion to apoptotic signal transduction have not been established. We report here that severe chronic hypoxia alone does not cause apoptosis of cardiac myocytes in culture. When rapidly contracting cardiac myocytes were exposed to chronic hypoxia, apoptosis occurred only when there was a decrease in extracellular pH ([pH](o)). Apoptosis did not occur when [pH](o) was neutralized. Addition of acidic medium from hypoxic cultures or exogenous lactic acid stimulated apoptosis in aerobic myocytes. Hypoxia-acidosis-mediated cell death was independent of p53: equivalent apoptosis occurred in cardiac myocytes isolated from wild-type and p53 knockout mice, and hypoxia caused no detectable change in p53 abundance or p53-dependent transcription. Reoxygenation of hypoxic cardiac myocytes induced apoptosis in 25-30% of the cells and was also independent of p53 by the same criteria. Finally, equivalent levels of apoptosis, as demonstrated by DNA fragmentation, were induced by ischemia-reperfusion, but not by ischemia alone, of Langendorff-perfused hearts from wild-type and p53 knockout mice. We conclude that acidosis, reoxygenation, and reperfusion, but not hypoxia (or ischemia) alone, are strong stimuli for programmed cell death that is substantially independent of p53.

Regulation of expression and activity of distinct pRB, E2F, D-type cyclin, and CKI family members during terminal differentiation of P19 cells

The cell cycle regulatory proteins, which include cyclin-dependent kinases (cdks), cdk inhibitors (CKIs), cyclins, and the pRB, and E2F families of proteins, constitute a network of interacting factors which govern exit from or passage through the mammalian cell cycle. While the proteins within these families have similar structural characteristics, each family member exhibits distinct expression patterns during embryogenesis and distinct biological activities. In order to begin to understand the tissue-specific roles of these interacting factors, we determined the expression pattern and activity of the pRB, E2F, cyclin, cdk, and CKI families of cell cycle regulatory proteins during retinoic acid-induced (neuronal pathway) and DMSO-induced (cardiac muscle pathway) differentiation of the pluripotent murine embryonal carcinoma cell line, P19. We demonstrate here that P19 terminal differentiation causes lineage-specific changes in the expression and activity of distinct members of the E2F, pRB, cyclin, and CKI families. Furthermore, dynamic changes in the activities of these cell cycle regulatory proteins occur through several overlapping mechanisms, culminating in repression of DNA-binding activity by all of the E2F family members as cells terminally differentiate.