The amino-terminal C/H1 domain of CREB binding protein mediates zta transcriptional activation of latent Epstein-Barr virus

Stimulation of CREB binding protein nucleosomal histone acetyltransferase activity by a class of transcriptional activators

The transcriptional coactivator CREB binding protein (CBP) possesses intrinsic histone acetyltransferase (HAT) activity that is important for gene regulation. CBP binds to and cooperates with numerous nuclear factors to stimulate transcription, but it is unclear if these factors modulate CBP HAT activity. Our previous work showed that CBP interacts with the Epstein-Barr virus-encoded basic region zipper (b-zip) protein, Zta, and augments its transcriptional activity. Here we report that Zta strongly enhances CBP-mediated acetylation of nucleosomal histones. Zta stimulated the HAT activity of CBP that had been partially purified or immunoprecipitated from mammalian cells as well as from affinity-purified, baculovirus expressed CBP. Stimulation of nucleosome acetylation required the CBP HAT domain, the Zta DNA binding and transcription activation domain, and nucleosomal DNA. In addition to Zta, we found that two other b-zip proteins, NF-E2 and C/EBPalpha, strongly stimulated nucleosomal HAT activity. In contrast, several CBP-binding proteins, including phospho-CREB, JUN/FOS, GATA-1, Pit-1, and EKLF, failed to stimulate HAT activity. These results demonstrate that a subset of transcriptional activators enhance the nucleosome-directed HAT activity of CBP and suggest that nuclear factors may regulate transcription by altering substrate recognition and/or the enzymatic activity of chromatin modifying coactivators.

Viral replication and the coactivators p300 and CBP

Productive viral infection requires coordinate regulation of viral and cellular gene expression. Viruses of different classes have evolved different mechanisms to conform to, adapt to and exploit programs of cellular gene expression. Many viral gene products influence and respond to cellular signals that control differentiation and proliferation Transcriptional coactivators are central to the regulation of the expression of genes controlling these events. p300 and CBP are closely related coactivators that regulate the transcription of specific genes, modify chromatin structure and influence cell cycle progression. In this review, the different molecular interactions of proteins encoded by DNA tumor viruses and lentiviruses with these transcriptional coactivators and related cellular proteins are summarized.

Lytic but not latent replication of epstein-barr virus is associated with PML and induces sequential release of nuclear domain 10 proteins

Nuclear domains called ND10 (nuclear domain 10) are discrete nuclear protein aggregations characterized by a set of interferon-upregulated proteins including Sp100 and PML, where papova-, adeno-, and herpesviruses begin their transcription and DNA replication. Both the alpha- and betaherpesvirus subfamilies disrupt ND10 upon infection by dispersing and/or destroying ND10-associated proteins. We studied the effect of the gammaherpesvirus Epstein-Barr virus (EBV) on ND10 and its spatial distribution in the nucleus of cells during latency and lytic reactivation. In latently infected Burkitt's lymphoma, lymphoblastoid, and D98/HR1 cells, ND10 were intact, as judged by immunofluorescence localization of PML, Sp100, NDP55, and Daxx. Fluorescent in situ hybridization revealed no association between viral episomes and ND10 during latency, implying that the maintenance replication of EBV, which depends on host cell proliferation, occurs independent of ND10. As in mitosis, the EBV genomes were attached to interphase chromosomes, suggesting that they are unable to move freely within the interchromosomal space and thus unable to associate with the interchromosomally located ND10 or other nuclear domains. Upon lytic activation, ND10 became dispersed in cells expressing lytic proteins. Redistribution of ND10 proteins occurred sequentially at different stages of the lytic cycle, with Sp100, Daxx, and NDP55 dispersed before and PML dispersed after the onset of lytic replication. ND10 remnants were retained until the early stages of lytic replication, and replicating EBV genomes were frequently found beside this nuclear domain; the number of replication domains was usually lower than the average latent virus frequency. Thus, latency does not require or induce interaction of EBV with ND10 for transcription and replication, whereas lytic replication triggers dispersion of ND10 proteins and occurs in close association with PML aggregates. The required movement of chromosome-attached latent EBV episomes to ND10 after reactivation from latency might include physical release of the chromosome-bound episomes. Only episomes contacting ND10 after such a release might be able to begin lytic replication.

Activation of the BRLF1 promoter and lytic cycle of Epstein-Barr virus by histone acetylation

Histone acetylation alters the chromatin structure and activates the genes that are repressed by histone deacetylation. This investigation demonstrates that treating P3HR1 cells with trichostatin A (TSA) activates the Epstein-Barr virus (EBV) lytic cycle, allowing the virus to synthesize three viral lytic proteins-Rta, Zta and EA-D. Experimental results indicate that TSA and 12-O:-tetradecanoylphorbol-13-acetate synergistically activate the transcription of BRLF1, an immediate-early gene of EBV. Chromatin immunoprecipitation assay reveals that histone H4 at the BRLF1 promoter is acetylated after P3HR1 cells are treated with TSA, suggesting that histone acetylation activates BRLF1 transcription. Furthermore, results in this study demonstrate that mutation of a YY1-binding site in the BRLF1 promoter activates BRLF1 transcription 1.6- and 2.3-fold in P3HR1 cells and C33A cells, respectively. Real time PCR analysis reveals that the mutation also increases the histone acetylation level of the nucleosomes at the BRLF1 promoter 1. 64- and 3.08-fold in P3HR1 and C33A cells, respectively. Results presented herein suggest that histone deacetylation plays an important role in maintaining the viral latency and histone acetylation at the BRLF1 promoter allows the virus to express Rta and to activate the viral lytic cycle.

Upregulation of tyrosine kinase TKT by the Epstein-Barr virus transactivator Zta

The Zta protein is a key transactivator involved in initiating the Epstein-Barr virus (EBV) lytic cascade. In addition to transactivating many viral genes, Zta has the capacity to influence host cellular signals by binding to promoter regions or by interacting with several important cellular factors. Based on the observation that tyrosine kinases play central roles in determining the fate of cells, a kinase display assay was used to investigate whether cells expressing Zta have an altered pattern of kinase expression. The assay revealed that TRK-related tyrosine kinase (TKT) is expressed at significant levels in Zta transfectants but not in control cells. Additional evidence was obtained from Northern and Western blotting. Importantly, the upregulation of phosphorylated TKT and TKT downstream effector matrix metalloproteinase 1 in Zta transfectants hinted that TKT might initiate a signaling cascade in Zta-expressing cells. In addition, deletion analysis of the Zta protein revealed that the transactivation and dimerization domains were both essential for the upregulation of TKT transcription. Moreover, correlation of expression levels of Zta and TKT transcripts in nasopharyngeal carcinoma biopsy specimens was clearly demonstrated by quantitative PCR (Q-PCR), which provides the first evidence for an effect of Zta on cellular gene expression in vivo. These findings offer insight into the virus-cell interactions and may help us elucidate the role of EBV in tumorigenesis.

Expression of p300-truncated fragments results in the modulation of apoptosis in rat mesangial cells

BACKGROUND

Mesangial cell proliferation, apoptosis, and matrix deposition have pivotal roles in the pathogenesis of renal diseases such as diabetic nephropathy and glomerulonephritis. The behavior of mesangial cells depends on the integration of intracellular signals elicited by hormones and cytokines. We hypothesized that p300 is primarily involved in the integration of signal transduction pathways in rat mesangial cells (RMCs) and that interference with p300 function will alter apoptotic signals.

METHODS

We established an RMC cell line expressing the Tet-activator (tTA). RMC-tTA cells were transiently transfected with vectors coding for either the N-terminal third or the C-terminal third of p300. Expression was induced by the addition of doxycycline [Dox; 1 microg/mL; 5% fetal bovine serum (FBS)]. The percentage of apoptosis was determined using the TUNEL technique. Specific protein-protein interactions were determined by Western blot analysis of immunoprecipitated complexes. Cells were treated with 5% FBS or with H2O2 (500 micromol/L, 1 h) with and without Dox.

RESULTS

The expression of p300-C resulted in increased susceptibility to low serum-induced (20.0 +/- 4.6 vs. 3.0 +/- 1.7%) and to H2O2-induced apoptosis (75.3 +/- 13.3 vs. 50.8 +/- 6.5%) compared with controls. Immunoprecipitation of p300-C showed an interaction with the transcription factor c-Fos, which was enhanced by H2O2 treatment. Expression of the p300-N resulted in a rescue (34.8 +/- 6. 4 vs. 50.8 +/- 6.5%) from H2O2-induced apoptosis compared with controls. P300-N was shown to form a complex with the transcription factor nuclear factor-kappaB (NF-kappaB).

CONCLUSIONS

The data indicate that endogenous p300 is involved in apoptosis in mesangial cells. We propose that interference or enhancement of endogenous p300 function, by expression of exogenous fragments, can alter interactions with c-Fos or NF-kappaB and modulate signals during cellular stress.

Epstein-Barr virus immediate-early proteins BZLF1 and BRLF1 activate the ATF2 transcription factor by increasing the levels of phosphorylated p38 and c-Jun N-terminal kinases

Expression of either Epstein-Barr virus (EBV) immediate-early protein BZLF1 (Z) or BRLF1 (R) is sufficient to convert EBV infection from the latent to lytic form. Disruption of viral latency requires transcriptional activation of the Z and R promoters. The Z and R proteins are transcriptional activators, and each immediate-early protein activates expression of the other immediate-early protein. Z activates the R promoter through a direct binding mechanism. However, R does not bind directly to the Z promoter. In this study, we demonstrate that the ZII element (a cyclic AMP response element site) in the Z promoter is required for efficient activation by R. The ZII element has been shown to be important for induction of lytic EBV infection by tetradecanoyl phorbol acetate and surface immunoglobulin cross-linking and is activated by Z through an indirect mechanism. We demonstrate that both R and Z activate the cellular stress mitogen-activated protein (MAP) kinases, p38 and JNK, resulting in phosphorylation (and activation) of the cellular transcription factor ATF2. Furthermore, we show that the ability of R to induce lytic EBV infection in latently infected cells is significantly reduced by inhibition of either the p38 kinase or JNK pathways. In contrast, inhibition of stress MAP kinase pathways does not impair the ability of Z expression vectors to disrupt viral latency, presumably because expression of Z under the control of a strong heterologous promoter bypasses the need to activate Z transcription. Thus, both R and Z can activate the Z promoter indirectly by inducing ATF2 phosphorylation, and this activity appears to be important for R-induced disruption of viral latency.

Genetic dissection of cell growth arrest functions mediated by the Epstein-Barr virus lytic gene product, Zta

Expression of the Epstein-Barr virus (EBV) latency-associated genes activates cell cycle progression and drives immortalization of the infected cell. In contrast, progression of the EBV replication program occurs most efficiently in growth-arrested cells. Previous studies showed that the EBV-encoded immediate-early transcription factor, Zta, can induce expression of the cyclin-dependent kinase inhibitors, p21 and p27, the tumor suppressor, p53, and cell growth arrest. Moreover, Zta-mediated induction of growth arrest occurs independently of its transcriptional transactivation function. Here we show that substitution of Zta's basic DNA binding domain with the analogous region of the Zta homologue, c-Fos, abrogates Zta's ability to induce growth arrest and to induce p21, p27, or p53 expression, suggesting that protein-protein interactions between this region of Zta and key cell cycle control proteins are involved in signaling cell cycle arrest. We also show that despite the crucial role for Zta's basic domain in eliciting cell growth arrest, its amino terminus is required for efficient induction of p27 and it modulates the level of p53 induction. Last, we provide evidence that Zta-mediated inductions of p21, p27, and p53 occur, at least in part, through distinct pathways. Therefore, Zta interacts with multiple growth arrest pathways, a property which may have evolved partly as a means to ensure that lytic replication occurs in a growth-arrested setting in multiple different tissues in various states of differentiation.