Direct interaction between Sp1 and the BPV enhancer E2 protein mediates synergistic activation of transcription

AP-1 and SP1 trans-activate the expression of hepatic CYP1A1 and CYP2A6 in the bioactivation of AFB1 in chicken

Dietary exposure to aflatoxin B1 (AFB1) is harmful to the health and performance of domestic animals. The hepatic cytochrome P450s (CYPs), CYP1A1 and CYP2A6, are the primary enzymes responsible for the bioactivation of AFB1 to the highly toxic exo-AFB1-8,9-epoxide (AFBO) in chicks. However, the transcriptional regulation mechanism of these CYP genes in the liver of chicks in AFB1 metabolism remains unknown. Dual-luciferase reporter assay, bioinformatics and site-directed mutation results indicated that specificity protein 1 (SP1) and activator protein-1 (AP-1) motifs were located in the core region -1,063/-948, -606/-541 of the CYP1A1 promoter as well as -636/-595, -503/-462, -147/-1 of the CYP2A6 promoter. Furthermore, overexpression and decoy oligodeoxynucleotide technologies demonstrated that SP1 and AP-1 were pivotal transcriptional activators regulating the promoter activity of CYP1A1 and CYP2A6. Moreover, bioactivation of AFB1 to AFBO could be increased by upregulation of CYP1A1 and CYP2A6 expression, which was trans-activated owing to the upregulalion of AP-1, rather than SP1, stimulated by AFB1-induced reactive oxygen species. Additionally, nano-selenium could reduce ROS, downregulate AP-1 expression and then decrease the expression of CYP1A1 and CYP2A6, thus alleviating the toxicity of AFB1. In conclusion, AP-1 and SP1 played important roles in the transactivation of CYP1A1 and CYP2A6 expression and further bioactivated AFB1 to AFBO in chicken liver, which could provide novel targets for the remediation of aflatoxicosis in chicks.

The structure of the extended E2 DNA-binding domain of the bovine papillomavirus-1

Bovine papillomavirus proteins were extensively studied as a prototype for the human papillomavirus. Here, the crystal structure of the extended E2 DNA-binding domain of the dominant transcription regulator from the bovine papillomavirus strain 1 is described in the space group P3₁ 21. We found two protein functional dimers packed in the asymmetric unit. This new protein arrangement inside the crystal led to the reduction of the mobility of a previously unobserved loop directly involved in the protein-DNA interaction, which was then modeled for the first time.

The interaction of human papillomaviruses and adeno-associated viruses in suppressive co-infections

Human papillomavirus (HPV) is one of the most common oncogenic viruses which cause malignancy in different epithelial surfaces of the human body and its infection is the main cause of cervical cancer. However, research suggests that this virus might not be the sole cause of infection in target cells. It is believed that, other infectious agents could co-infect the same cell with HPV including; bacteria, viruses, and parasites, which may have different effects on the carcinogenesis of HPV infections. One of the most important viruses is adeno-associated virus (AAV), which comes from the parvoviridae family. The function of this virus is associated with several stages of HPV carcinogenicity, which leads to the suppression of HPV oncogenesis. The inhibition effects of AAV are exerted not only in viral parts but also in cellular parts. This suppression illuminates a new therapeutic approach in the way of HPV-associated cervical cancer. In the present review we consider the exact roles of AAV infection in this suppression.

miR-182 aids in receptive endometrium development in dairy goats by down-regulating PTN expression

Increasing evidence has shown that miRNAs play important roles in endometrium development during the menstrual cycle in humans and many other animals. Our previous data indicated that miR-182 levels increase 15.55-fold and pleiotrophin (PTN) levels decrease 20.97-fold in the receptive endometrium (RE, D15) compared with the pre-receptive endometrium (PE, D5) in dairy goats. The present study shows that miR-182 is widely expressed in different tissues of dairy goats and that its expression levels are regulated by E2 and P4 in endometrial epithelium cells (EECs). We confirmed that PTN is a target of miR-182 and that miR-182 regulates the protein levels of AKT, Bcl-2, FAS, MAPK, Caspase-3 and SP1 in EECs. Furthermore, miR-182 up-regulates or maintains the expression levels of osteopontin (OPN), cyclooxygenase-2 (COX-2) and prolactin receptor (PRLR) in EECs, suggesting that miR-182 is an important regulatory factor in the construction of endometrial receptivity in dairy goats. In conclusion, miR-182 participates in the development of endometrial receptivity by down-regulating PTN and affecting the expression of select apoptosis-related genes and increasing or maintaining the expression levels of OPN, COX-2 and PRLR in the EECs of dairy goats.

HPV16-E2 protein modifies self-renewal and differentiation rate in progenitor cells of human immortalized keratinocytes

BACKGROUND

Cervical cancer is the fourth cause of death worldwide by cancer in women and is a disease associated to persistent infection with human papillomavirus (HPV), particularly from two high-risk types HPV16 and 18. The virus initiates its replicative cycle infecting cells located in the basal layer of the epithelium, where a small population of epithelial stem cells is located performing important functions of renewal and maintenance of the tissue. Viral E2 gene is one of the first expressed after infection and plays relevant roles in the replicative cycle of the virus, modifying fundamental processes in the infected cells. Thus, the aim of the present study was to demonstrate the presence of hierarchic subpopulations in HaCaT cell line and evaluate the effect of HPV16-E2 expression, on their biological processes.

METHODS

HaCaT-HPV16-E2 cells were generated by transduction of HaCaT cell line with a lentiviral vector. The α6-integrin-CD71 expression profile was established by immunostaining and flow cytometric analysis. After sorting, cell subpopulations were analyzed in biological assays for self-renewal, clonogenicity and expression of stemness factors (RT-qPCR).

RESULTS

We identified in HaCaT cell line three different subpopulations that correspond to early differentiated cells (α6-integrin^dim), transitory amplifying cells (α6-integrin^bri/CD71^bri) and progenitor cells (α6-integrin^bri/CD71^dim). The last subpopulation showed stem cell characteristics, such as self-renewal ability, clonogenicity and expression of the well-known stem cell factors SOX2, OCT4 and NANOG, suggesting they are stem-like cells. Interestingly, the expression of HPV16-E2 in HaCaT cells changed its α6-integrin-CD71 immunophenotype modifying the relative abundance of the cell subpopulations, reducing significantly the percentage of α6-integrin^bri/CD71^dim cells. Moreover, the expression of the stem cell markers was also modified, increasing the expression of SOX2 and NANOG, but decreasing notably the expression of OCT4.

CONCLUSIONS

Our data demonstrated the presence of a small subpopulation with epithelial "progenitor cells" characteristics in the HaCaT cell line, and that HPV16-E2 expression on these cells induces early differentiation.

Design of peptides as inhibitors of human papillomavirus 16 transcriptional regulator E1-E2

Human papillomavirus 16 (HPV 16) is a DNA virus that is capable of infecting humans and causing cervical cancer. HPV16 E2 plays an important role in viral gene regulation. This work aims to predict the binding conformations and interactions between the dodecapeptides and HPV16 E2 as well as to design novel peptide inhibitors that are capable of binding to HPV16 E2 and disrupt the transcriptional regulator E1-E2 complex formation, using computational protein design techniques. Based on previously reported peptide4 (TWFWPYPYPHLP), novel peptide inhibitors were designed and five peptides that showed lower binding energy to HPV16 E2 than that of peptide4, were selected for in vitro experiments. Enzyme-linked immunosorbent (ELISA) assay showed that Y6R, W4H_Y6R, and W4H peptides bound to HPV16 E2 with higher affinity than peptide4 did. Moreover, Y6R, W4H_Y6R, and W4H peptides more effectively inhibited E1-E2 complex formation than peptide4. This work revealed important interactions between the peptides and E1-E2 complex, suggesting a strategy for development of more potent peptide inhibitors.

The Role of Crowding Forces in Juxtaposing β-Globin Gene Domain Remote Regulatory Elements in Mouse Erythroid Cells

The extremely high concentration of macromolecules in a eukaryotic cell nucleus indicates that the nucleoplasm is a crowded macromolecular solution in which large objects tend to gather together due to crowding forces. It has been shown experimentally that crowding forces support the integrity of various nuclear compartments. However, little is known about their role in control of chromatin dynamics in vivo. Here, we experimentally addressed the possible role of crowding forces in spatial organization of the eukaryotic genome. Using the mouse β-globin domain as a model, we demonstrated that spatial juxtaposition of the remote regulatory elements of this domain in globin-expressing cells may be lost and restored by manipulation of the level of macromolecular crowding. In addition to proving the role of crowding forces in shaping interphase chromatin, our results suggest that the folding of the chromatin fiber is a major determinant in juxtaposing remote genomic elements.

CCCTC-binding factor recruitment to the early region of the human papillomavirus 18 genome regulates viral oncogene expression

Host cell differentiation-dependent regulation of human papillomavirus (HPV) gene expression is required for productive infection. The host cell CCCTC-binding factor (CTCF) functions in genome-wide chromatin organization and gene regulation. We have identified a conserved CTCF binding site in the E2 open reading frame of high-risk HPV types. Using organotypic raft cultures of primary human keratinocytes containing high-risk HPV18 genomes, we show that CTCF recruitment to this conserved site regulates viral gene expression in differentiating epithelia. Mutation of the CTCF binding site increases the expression of the viral oncoproteins E6 and E7 and promotes host cell proliferation. Loss of CTCF binding results in a reduction of a specific alternatively spliced transcript expressed from the early gene region concomitant with an increase in the abundance of unspliced early transcripts. We conclude that high-risk HPV types have evolved to recruit CTCF to the early gene region to control the balance and complexity of splicing events that regulate viral oncoprotein expression.

IMPORTANCE

The establishment and maintenance of HPV infection in undifferentiated basal cells of the squamous epithelia requires the activation of a subset of viral genes, termed early genes. The differentiation of infected cells initiates the expression of the late viral transcripts, allowing completion of the virus life cycle. This tightly controlled balance of differentiation-dependent viral gene expression allows the virus to stimulate cellular proliferation to support viral genome replication with minimal activation of the host immune response, promoting virus productivity. Alternative splicing of viral mRNAs further increases the complexity of viral gene expression. In this study, we show that the essential host cell protein CTCF, which functions in genome-wide chromatin organization and gene regulation, is recruited to the HPV genome and plays an essential role in the regulation of early viral gene expression and transcript processing. These data highlight a novel virus-host interaction important for HPV pathogenicity.

The HPV E2-Host Protein-Protein Interactions: A Complex Hijacking of the Cellular Network

Over 100 genotypes of human papillomaviruses (HPVs) have been identified as being responsible for unapparent infections or for lesions ranging from benign skin or genital warts to cancer. The pathogenesis of HPV results from complex relationships between viral and host factors, driven in particular by the interplay between the host proteome and the early viral proteins. The E2 protein regulates the transcription, the replication as well as the mitotic segregation of the viral genome through the recruitment of host cell factors to the HPV regulatory region. It is thereby a pivotal factor for the productive viral life cycle and for viral persistence, a major risk factor for cancer development. In addition, the E2 proteins have been shown to engage numerous interactions through which they play important roles in modulating the host cell. Such E2 activities are probably contributing to create cell conditions appropriate for the successive stages of the viral life cycle, and some of these activities have been demonstrated only for the oncogenic high-risk HPV. The recent mapping of E2-host protein-protein interactions with 12 genotypes representative of HPV diversity has shed some light on the large complexity of the host cell hijacking and on its diversity according to viral genotypes. This article reviews the functions of E2 as they emerge from the E2/host proteome interplay, taking into account the large-scale comparative interactomic study.

Mitochondria-localized glutamic acid-rich protein (MGARP) gene transcription is regulated by Sp1

Background

Mitochondria-localized glutamic acid-rich protein (MGARP) is a novel mitochondrial transmembrane protein expressed mainly in steroidogenic tissues and in the visual system. Previous studies showed that MGARP functions in hormone biosynthesis and its expression is modulated by the HPG axis.

Methodology/Principal Findings

By bioinformatics, we identified two characteristic GC-rich motifs that are located proximal to the transcription start site (TSS) of MGARP, and each contains two Specificity protein 1 (Sp1) binding elements. We then determined that the −3 kb proximal MGARP promoter is activated in a Sp1-dependent manner using reporter assays and knockdown of Sp1 led to decreased expression of endogenous MGARP messages. We also demonstrated that one of the two GC-rich motifs, GC-Box1, harbors prominent promoter activity mediated by Sp1, and that it requires both GC boxes for full transcriptional activation. These findings suggest a dominant role for these GC boxes and Sp1 in activating the MGARP promoter through a synergistic mechanism. Consistently, the results of an Electrophoretic Mobility Gel Shift Assay (EMSA) and Chromatin Immunoprecipitation (ChIP) confirmed that Sp1 specifically interacts with the GC-rich region. We further found that estrogen receptor α (ERα), a known Sp1 co-activator, could potentiate GC-boxes containing MGARP promoter activity and this effect is mediated by Sp1. Knockdown of Sp1 significantly diminished the MGARP promoter transactivation and the expression of endogenous MGARP mediated by both Sp1 and ERα.

Conclusions/Significance

The present study identified a proximal core sequence in the MGARP promoter that is composed of two enriched Sp1 binding motifs and established Sp1 as one major MGARP transactivator whose functions are synergistic with ERα, providing a novel understanding of the mechanisms of MGARP gene transcriptional regulation.

Persistent immune responses induced by a human immunodeficiency virus DNA vaccine delivered in association with electroporation in the skin of nonhuman primates

Strategies to improve vaccine efficacy are still required, especially in the case of chronic infections, including human immunodeficiency virus (HIV). DNA vaccines have potential advantages over conventional vaccines; however, low immunological efficacy has been demonstrated in many experiments involving large animals and in clinical trials. To improve the immunogenicity of DNA vaccines, we have designed a plasmid vector exploiting the binding capacity of the bovine papillomavirus E2 protein and we have used electroporation (EP) to increase DNA uptake after intradermal inoculation. We demonstrated, in nonhuman primates (NHPs), efficient induction of anti-HIV immunity with an improved DNA vaccine vector encoding an artificial fusion protein, consisting of several proteins and selected epitopes from HIV-1. We show that a DNA vaccine delivery method combining intradermal injection and noninvasive EP dramatically increased expression of the vaccine antigen selectively in the epidermis, and our observations strongly suggest the involvement of Langerhans cells in the strength and quality of the anti-HIV immune response. Although the humoral responses to the vaccine were transient, the cellular responses were exceptionally robust and persisted, at high levels, more than 2 years after the last vaccine boost. The immune responses were characterized by the induction of significant proportions of T cells producing both interferon-gamma and interleukin-2 cytokines, in both subpopulations, CD4(+) and CD8(+). This strategy is an attractive approach for vaccination in humans because of its high efficacy and the possible use of newly developed devices for EP.

Human papillomavirus type 16 E2 protein transcriptionally activates the promoter of a key cellular splicing factor, SF2/ASF

Human papillomavirus (HPV) gene expression is regulated in concert with the epithelial differentiation program. In particular, expression of the virus capsid proteins L1 and L2 is tightly restricted to differentiated epithelial cells. For HPV16, the capsid proteins are encoded by 13 structurally different mRNAs that are produced by extensive alternative splicing. Previously, we demonstrated that upon epithelial differentiation, HPV16 infection upregulates hnRNP A1 and SF2/ASF, both key factors in alternative splicing regulation. Here we cloned a 1-kb region upstream of and including the transcriptional start site of the SF2ASF gene and used it in in vivo transcription assays to demonstrate that the HPV16 E2 transcription factor transactivates the SF2/ASF promoter. The transactivation domain but not the DNA binding domain of the protein is necessary for this. Active E2 association with the promoter was demonstrated using chromatin immunoprecipitation assays. Electrophoretic mobility shift assays indicated that E2 interacted with a region 482 to 684 bp upstream of the transcription initiation site in vitro. This is the first time that HPV16 E2 has been shown to regulate cellular gene expression and the first report of viral regulation of expression of an RNA processing factor. Such E2-mediated control during differentiation of infected epithelial cells may facilitate late capsid protein expression and completion of the virus life cycle.

Modification of papillomavirus E2 proteins by the small ubiquitin-like modifier family members (SUMOs)

Papillomavirus E2 proteins are critical regulatory proteins that function in replication, genome segregation, and viral transcription, including control of expression of the viral oncogenes, E6 and E7. Sumoylation is a post-translational modification that has been shown to target and modulate the function of many transcription factors, and we now demonstrate that E2 proteins are sumoylated. Both bovine and human papillomavirus E2 proteins bind to the SUMO conjugation enzyme, Ubc9, and using in vitro and E. coli sumoylation systems, these E2 proteins were readily modified by SUMO proteins. In vivo experiments further confirmed that E2 can be sumoylated by SUMO1, SUMO2, or SUMO3. Mapping studies identified lysine 292 as the principal residue for covalent conjugation of SUMO to HPV16 E2, and a lysine 292 to arginine mutant showed defects for both transcriptional activation and repression. The expression levels, intracellular localization, and the DNA-binding activity of HPV16 E2 were unchanged by this K292R mutation, suggesting that the transcriptional defect reflects a functional contribution by sumoylation at this residue. This study provides evidence that sumoylation has a role in the regulation of papillomavirus E2, and identifies a new mechanism for the modulation of E2 function at the post-translational level.

Differential regulation of native estrogen receptor-regulatory elements by estradiol, tamoxifen, and raloxifene

Estrogen receptors (ERs) regulate gene transcription by interacting with regulatory elements. Most information regarding how ER activates genes has come from studies using a small set of target genes or simple consensus sequences such as estrogen response element, activator protein 1, and Sp1 elements. However, these elements cannot explain the differences in gene regulation patterns and clinical effects observed with estradiol (E(2)) and selective estrogen receptor modulators. To obtain a greater understanding of how E(2) and selective estrogen receptor modulators differentially regulate genes, it is necessary to investigate their action on a more comprehensive set of native regulatory elements derived from ER target genes. Here we used chromatin immunoprecipitation-cloning and sequencing to isolate 173 regulatory elements associated with ERalpha. Most elements were found in the introns (38%) and regions greater than 10 kb upstream of the transcription initiation site (38%); 24% of the elements were found in the proximal promoter region (<10 kb). Only 11% of the elements contained a classical estrogen response element; 23% of the elements did not have any known response elements, including one derived from the naked cuticle homolog gene, which was associated with the recruitment of p160 coactivators. Transfection studies found that 80% of the 173 elements were regulated by E(2), raloxifene, or tamoxifen with ERalpha or ERbeta. Tamoxifen was more effective than raloxifene at activating the elements with ERalpha, whereas raloxifene was superior with ERbeta. Our findings demonstrate that E(2), tamoxifen, and raloxifene differentially regulate native ER-regulatory elements isolated by chromatin immunoprecipitation with ERalpha and ERbeta.

Molecular Mechanisms of Transactivation and Doxorubicin-mediated Repression of survivin Gene in Cancer Cells

Human maintenance DNA cytosine methyltransferase (DNMT1) regulates gene expression in a methylation-dependent and -independent manner. Anti-apoptotic survivin gene down-regulation is mediated by p53 recruitment of DNMT1 to its promoter. Survivin inhibits programmed cell death, regulates cell division, and is expressed in cancer cells. The survivin gene promoter is CG-rich containing several Sp1 canonical, Sp1-like, cell cycle-dependent element/cell cycle gene homology region, and p53-binding sites. Here we demonstrate that Sp1 transcription factor(s) play a role in transcriptional activation of the survivin promoter in Drosophila and human cells. Sp1 inhibition in vivo by mithramycin A leads to down-regulation of a luciferase reporter driven by the human survivin promoter in transfected cells. Mithramycin A or Sp1-specific short interfering RNA down-regulated the endogenous survivin gene expression, confirming Sp1 as the primary determinant for transcriptional activation. Furthermore, immobilized DNMT1 ligand bound to seven consensus amino acids corresponding to the N-terminal region of the Sp class of transcription factors in a phage display analysis. In the co-immunoprecipitation assay, the endogenous Sp1 or Sp3 pulled down DNMT1 and methyltransferase activity. Similarly, a glutathione S-transferase pulldown assay between DNMT1 and Sp1 demonstrates a direct interaction between the two proteins. Fluorescent fusions of DNMT1 and Sp1 co-localized in the mammalian nucleus, thus supporting binary complex formation between both the proteins. The kinetics of survivin promoter occupancy via chromatin immunoprecipitation following doxorubicin treatment show the presence of Sp1 and gradual accumulation of transcriptional repressors p53, DNMT1, histone methyltransferase G9a, and HDAC1 onto the promoter along with histone H3K9me2. These data suggest that the Sp1 transcription factor acts as a platform for recruitment of transcriptional repressors.

In Vivo beta-adrenergic activation of atrial natriuretic factor (ANF) reporter expression

Isoproterenol (ISO) infusion increases ANF-mRNA levels and control of ANF expression lies at the level of transcription. In neonatal cardiomyocytes, previous investigations determined that the -125 to -100 region of the rat ANF 5' flanking region contained cis-elements critical for control of ISO induced ANF transcription. However, it is unclear if these same cis-elements regulate ANF transcription in vivo. To examine this question, reporter plasmids containing the ANF 5' flanking/promoter region were injected directly into the left ventricle. Following a recovery period, osmotic pumps were implanted to infuse vehicle or ISO (0.2 or 2.0 mg/kg/d). ISO significantly (p < .05) increased the LV/BW ratio in a dose dependent, but not a time dependent manner. ISO significantly (p < .05) increased ANF reporter expression in both a dose-dependent and time dependent manner. Injections into the midwall of the LV or into the apex did not lead to significant differences in ISO-induced ANF reporter expression. Using site-specific mutations of ANF reporter constructs, comparisons were made of ISO induced ANF transcription in vitro in neonatal cardiomyocytes and in vivo in the adult heart. Cis-elements critical for ISO activation in cultured cardiomyocytes were not essential for the increased expression of the ANF reporters in vivo. The results indicate that distinct differences in ANF transcriptional regulation exist in vivo in the adult heart as compared with neonatal cardiomyocytes, and suggest the recruitment of other signaling pathways beyond adrenergic-receptor mediated pathways.

Transcriptional and translational regulation of BACE1 expression--implications for Alzheimer's disease

The proteolytical processing of the amyloid precursor protein (APP) gives rise to beta-amyloid peptides, which accumulate in brains of Alzheimer's disease (AD) patients. Different soluble or insoluble higher molecular weight forms of beta-amyloid peptides have been postulated to trigger a complex pathological cascade that may cause synaptic dysfunction, inflammatory processes, neuronal loss, cognitive impairment, and finally the onset of the disease. The generation of beta-amyloid peptides requires the proteolytical cleavage of APP by an aspartyl protease named beta-site APP-cleaving enzyme 1 (BACE1). The expression and enzymatic activity of BACE1 are increased in brains of AD patients. Here we discuss the importance of a number of recently identified transcription factors as well as post-transcriptional modifications and activation of intracellular signaling molecules for the regulation of BACE1 expression in brain. Importantly, some of these factors are known to be involved in the inflammatory and chronic stress responses of the brain, which are compromised during aging. Moreover, recent evidence indicates that beneficial effects of non-steriodal anti-inflammatory drugs on the progression of AD are mediated--at least in part--by effects on the peroxisome proliferator-activated receptor-gamma response element present in the BACE1 promoter. The identification of the cell type-specific expression and activation of NF-kappaB, Sp1 and YY1 transcription factors may provide a basis to specifically interfere with BACE1 expression and, thereby, to lower the concentrations of beta-amyloid peptides, which may prevent neuronal cell loss and cognitive decline in AD patients.

Direct interaction of C/EBPdelta and Sp1 at the GC-enriched promoter region synergizes the IL-10 gene transcription in mouse macrophage

We previously reported that LPS activates the transcription of the IL-10 gene through the Sp1 and C/EBP binding sites and indicated that Sp1, C/EBPbeta and C/EBPdelta can coactivate the IL-10 gene expression in mouse macrophage cells [Liu Y.-W., Tseng H.-P., Chen L.-C., Chen B.-K., Chang W.-C. J. Immunol. 171: 821-828, 2003]. In the present report, we demonstrated the direct physical interaction between C/EBPdelta and Sp1, and also mapped the interaction domains of these two proteins. C/EBPdelta binds to Sp1 via its basic region leucine zipper domain. The C-terminus of Sp1 was also the major region interacting with C/EBPdelta. However, both glutamine- and serine/threonine-rich homologus regions of Sp1 also interacted with C/EBPdelta. The binding of Sp1 and C/EBPdelta as a complex to the Sp1 binding site on the promoter of IL-10 was further confirmed by using the DNA affinity precipitation assay. By using Sp1-deficient SL2 cells, we also found that the overexpressions of C/EBPdelta and Sp1 synergically activate the transcriptional activity of IL-10 gene. Taken together, our present results revealed a novel mechanism of a superactivation of Sp1 by C/EBPdelta via a direct interaction between these two transcription factors leading to the activation of the IL-10 gene in mouse macrophage cells.

Bromodomain protein 4 mediates the papillomavirus E2 transcriptional activation function

The papillomavirus E2 regulatory protein has essential roles in viral transcription and the initiation of viral DNA replication as well as for viral genome maintenance. Brd4 has recently been identified as a major E2-interacting protein and, in the case of the bovine papillomavirus type 1, serves to tether E2 and the viral genomes to mitotic chromosomes in dividing cells, thus ensuring viral genome maintenance. We have explored the possibility that Brd4 is involved in other E2 functions. By analyzing the binding of Brd4 to a series of alanine-scanning substitution mutants of the human papillomavirus type 16 E2 N-terminal transactivation domain, we found that amino acids required for Brd4 binding were also required for transcriptional activation but not for viral DNA replication. Functional studies of cells expressing either the C-terminal domain of Brd4 that can bind E2 and compete its binding to Brd4 or short interfering RNA to knock down Brd4 protein levels revealed a role for Brd4 in the transcriptional activation function of E2 but not for its viral DNA replication function. Therefore, these studies establish a broader role for Brd4 in the papillomavirus life cycle than as the chromosome tether for E2 during mitosis.

Papillomavirus E2 protein interacts with and stimulates human topoisomerase I

The papillomavirus (PV) E2 protein plays a role in recruiting viral and cellular DNA replication factors, such as PV E1 or RPA to PV genomes. Using both purified proteins and through co-precipitation, it was determined that HPV-11 E2 binds human topoisomerase I. E2 can stimulate topoisomerase I DNA relaxation activity 3- to 4-fold. Conversely, topoisomerase I is unable to stimulate E2 DNA binding. These findings suggest that stimulation of topoisomerase I by E2 may help promote efficient relaxation of the torsional stress induced by PV DNA replication.

2-Deoxyglucose: An anticancer and antiviral therapeutic, but not any more a low glucose mimetic

2-Deoxyglucose (2-DG), a non-metabolizable glucose analogue, blocks glycolysis and inhibits protein glycosylation. It has been tested in multiple studies for possible application as an anticancer or antiviral therapeutic. The inhibitory effect of 2-DG on ATP generation made it a good candidate molecule as a calorie restriction mimetic as well. Furthermore, 2-DG has been utilized in numerous studies to simulate a condition of glucose starvation. Because 2-DG disrupts glucose metabolism, protein glycosylation, and ER quality control at the same time, a cellular or pathologic outcome could be easily misinterpreted without clear understanding of 2-DG's effect on each of these aspects. However, the effect of 2-DG on protein glycosylation has rarely been investigated. A recent study suggested that 2-DG causes hyperGlcNAcylation of proteins, while low glucose supply causes hypoGlcNAcylation. In certain aspects of cellular physiology, this difference could be disregarded, but in others, this may possibly cause totally different outcomes.

Physical and functional interactions between members of the tumour suppressor p53 and the Sp families of transcription factors: importance for the regulation of genes involved in cell-cycle arrest and apoptosis

In the present study, we have investigated mechanisms of transcriptional co-operation between proteins that belong to the tumour suppressor p53 and Sp (specificity protein) families of transcription factors. Such mechanisms may play an important role in the regulation of genes containing binding sites for both classes of transcription factors in their promoters. Two of these genes were analysed in the present study: the cyclin-dependent kinase inhibitor p21Cip1 gene and the PUMA (p53-up-regulated mediator of apoptosis) gene. We found that Sp1 and Sp3, but not Sp2, co-operate functionally with p53, p73 and p63 for the synergistic transactivation of the p21Cip1 promoter in Drosophila Schneider SL2 cells that lack endogenous Sp factors. We also found that Sp1 strongly transactivated the PUMA promoter synergistically with p53, whereas deletion of the Sp1-binding sites abolished the transactivation by p53. Using p53 mutant forms in GST (glutathione S-transferase) pull-down assays, we found that the C-terminal 101 amino acids of p53, which include the oligomerization and regulatory domains of the protein, are required for the physical interactions with Sp1 and Sp3, and that deletion of this region abolished transactivation of the p21Cip1 promoter. Utilizing truncated forms of Sp1, we established that p53 interacted with the two transactivation domains A and B, as well as the DNA-binding domain. Our findings suggest that Sp factors are essential for the cellular responses to p53 activation by genotoxic stress. Understanding in detail how members of the p53 and Sp families of transcription factors interact and work together in the p53-mediated cellular responses may open new horizons in cancer chemotherapy.

Association of bovine papillomavirus E2 protein with nuclear structures in vivo

Papillomaviruses are small DNA viruses which have the capacity to establish a persistent infection in mammalian epithelial cells. The papillomavirus E2 protein is a central coordinator of viral gene expression, genome replication, and maintenance. We have investigated the distribution of bovine papillomavirus E2 protein in nuclei of proliferating cells and found that E2 is associated with cellular chromatin. This distribution does not change during the entire cell cycle. The N-terminal transactivation domain, but not the C-terminal DNA-binding domain, of the E2 protein is responsible for this association. The majority of the full-length E2 protein can only be detected in chromatin-enriched fractions but not as a free protein in the nucleus. Limited micrococcal nuclease digestion revealed that the E2 protein partitioned to different chromatin regions. A fraction of the E2 protein was located at nuclear sites that are resistant against nuclease attack, whereas the remaining E2 resided on compact chromatin accessible to micrococcal nuclease. These data suggest that there are two pools of E2 in the cell nucleus: one that localizes on transcriptionally inactive compact chromatin and the other, which compartmentalizes to transcriptionally active nuclear structures of the cell. Our data also suggest that E2 associates with chromatin through cellular protein(s), which in turn is released from chromatin at 0.4 M salt.

Interaction of bovine papillomavirus E2 protein with Brd4 stabilizes its association with chromatin

The bovine papillomavirus E2 protein maintains and segregates the viral extrachromosomal genomes by tethering them to cellular mitotic chromosomes. E2 interacts with a cellular bromodomain protein, Brd4, to mediate the segregation of viral genomes into daughter cells. Brd4 binds acetylated histones and has been observed to diffusely coat mitotic chromosomes in several cell types. In this study, we show that in mitotic C127 cells, Brd4 diffusely coated the condensed chromosomes. However, in the presence of the E2 protein, E2 and Brd4 colocalized in punctate dots that were randomly distributed over the chromosomes. A similar pattern of E2 and Brd4 colocalization on mitotic chromosomes was observed in CV-1 cells, whereas only a faint chromosomal coating of Brd4 was detected in the absence of the E2 protein. Therefore, the viral E2 protein relocalizes and/or stabilizes the association of Brd4 with chromosomes in mitotic cells. The colocalization of E2 and Brd4 was also observed in interphase cells, indicating that this protein-protein interaction persists throughout the cell cycle. The interaction of E2 with Brd4 greatly stabilized the association of Brd4 with interphase chromatin. In both mitotic and interphase cells, this stabilization required a transcriptionally competent transactivation domain, but not the DNA binding function of the E2 protein. Thus, the E2 protein modulates the chromatin association of Brd4 during both interphase and mitosis. This study demonstrates that the segregation of papillomavirus genomes is not simply due to the passive hitchhiking of the E2/genome complex with a convenient cellular chromosomal protein.

DNA looping induced by a transcriptional enhancer in vivo

Enhancers are DNA sequences that can activate gene transcription from remote positions. In yeast, regulatory sequences that are functionally equivalent to the metazoan enhancers are called upstream activating sequences (UASs). UASs show a lower degree of flexibility than their metazoan counterparts, but can nevertheless activate transcription from a distance of >1000 bp from the promoter. One of several models for the mechanism of action of transcriptional enhancers proposes that enhancer-bound activating proteins contact promoter-bound transcription factors and thereby get in close proximity to the promoter region with concomitant looping of the intervening DNA. We tested the mode of enhancer activity in yeast. A polymerase II-transcribed gene was paired with a remote, inducible enhancer. An independent reporter system was inserted next to the promoter to monitor the potential modes of enhancer activity. Our results show that the enhancer activated the reporter system only in the presence of a functional promoter. We also demonstrate that the heterologous expression of GAGA, a factor known to facilitate DNA loop formation, allows enhancer action in yeast over a distance of 3000 bp.

Sp1 elements inSULT2B1bpromoter and 5′-untranslated region of mRNA: Sp1/Sp2 induction and augmentation by histone deacetylase inhibition

The steroid/sterol sulfotransferase gene (SULT2B1) encodes for two isozymes of which one (SULT2B1b) sulfonates cholesterol and is selectively expressed in skin. The human SULT2B1 gene contains neither a TATAAA nor a CCAAT motif upstream of the coding region for SULT2B1b; however, this area is GC-rich. Of five Sp1 elements identified two had regulatory activity utilizing immortalized human keratinocytes: one element is located above the ostensible transcription initiation site, whereas the other is located within the 5'-untranslated region of the SULT2B1b mRNA. Sp1 and Sp2 transcription factors identified by supershift analyses induced reporter gene activity, an effect markedly augmented by histone deacetylase inhibition.

Distinct transcriptional regulation and function of the human BACE2 and BACE1 genes

Amyloid beta protein (Abeta) is the principal component of neuritic plaques in Alzheimer's disease (AD). Abeta is derived from beta amyloid precursor protein (APP) by beta- and gamma-secretases. Beta-site APP cleaving enzyme 1 (BACE1) has been identified as the major beta-secretase. BACE2 is the homolog of BACE1. The BACE2 gene is on chromosome 21 and has been implicated in the pathogenesis of AD. However, the function of BACE2 in Abeta generation is controversial. Some studies have shown that BACE2 cleaved APP at the beta-site whereas other studies showed it cleaved around the alpha-secretase site. To elucidate the involvement of BACE2 in AD pathogenesis, we compared BACE2 and BACE1 gene regulation and their functions in Abeta generation. We cloned and functionally characterized the human BACE2 promoter. The BACE2 gene is controlled by a TATA-less promoter. Though Sp1 can regulate both BACE1 and BACE2 genes, comparative sequence analysis and transcription factor prediction showed little similarity between the two promoters. BACE1 increased APP cleavage at the beta-site and Abeta production whereas BACE2 did not. Overexpression of BACE2 significantly increased sAPP levels in conditioned media but markedly reduced Abeta production. Knockdown of BACE2 resulted in increased APP C83. Our data indicate that despite being homologous in amino acid sequence, BACE2 and BACE1 have distinct functions and transcriptional regulation. BACE2 is not a beta-secretase, but processes APP within the Abeta domain at a site downstream of the alpha-secretase cleavage site. Our data argue against BACE2 being involved in the formation of neuritic plaques in AD.

Specific in vitro interaction between papillomavirus E2 proteins and TBP-associated factors

The bovine and human papillomavirus (BPV/HPV) E2 proteins bind specifically to palindromic sequences ACCGN4CGGT that are concentrated within the viral long control region, where they regulate viral oncogene transcription. E2 can activate viral promoters over relatively large distances within the viral genome and was shown to cooperate with a number of cellular transcription factors. Transcriptional activator proteins, such as E2, are thought to act, at least in part, by influencing the assembly and/or stability of preinitiation complexes and it has been suggested that the transcription factor IID, composed by the TATA-binding protein (TBP) and numerous TBP-associated factors (TAFs), is a possible target of this important viral protein. In this paper, we demonstrate that E2 proteins associate in vitro with several TAFs, in particular with TAFII250 and TAFII80. In addition, we observed that the association of TAFII250 with BPV1 E2 is stronger than with HPV18 E2 and that the carboxy terminal domain of both viral proteins is involved in this interaction. On the other hand, TAFII80 binds with similar strength to both E2 proteins through their amino terminal region. These observations may help to explain the different behavior of bovine and human E2 proteins, since BPV E2 is a stronger transcriptional activator than HPV18 E2.

SF2/ASF binds the human papillomavirus type 16 late RNA control element and is regulated during differentiation of virus-infected epithelial cells

Pre-mRNA splicing occurs in the spliceosome, which is composed of small ribonucleoprotein particles (snRNPs) and many non-snRNP components. SR proteins, so called because of their C-terminal arginine- and serine-rich domains (RS domains), are essential members of this class. Recruitment of snRNPs to 5' and 3' splice sites is mediated and promoted by SR proteins. SR proteins also bridge splicing factors across exons to help to define these units and have a central role in alternative and enhancer-dependent splicing. Here, we show that the SR protein SF2/ASF is part of a complex that forms upon the 79-nucleotide negative regulatory element (NRE) that is thought to be pivotal in posttranscriptional regulation of late gene expression in human papillomavirus type 16 (HPV-16). However, the NRE does not contain any active splice sites, is located in the viral late 3' untranslated region, and regulates RNA-processing events other than splicing. The level of expression and extent of phosphorylation of SF2/ASF are upregulated with epithelial differentiation, as is subcellular distribution, specifically in HPV-16-infected epithelial cells, and expression levels are controlled, at least in part, by the virus transcription regulator E2.

Attenuation of estrogen receptor alpha-mediated transcription through estrogen-stimulated recruitment of a negative elongation factor

Estrogen receptor alpha (ERalpha) signaling is paramount for normal mammary gland development and function and the repression of breast cancer. ERalpha function in gene regulation is mediated by a number of coactivators and corepressors, most of which are known to modify chromatin structure and/or influence the assembly of the regulatory complexes at the level of transcription initiation. Here we describe a novel mechanism of attenuating the ERalpha activity. We show that cofactor of BRCA1 (COBRA1), an integral subunit of the human negative elongation factor (NELF), directly binds to ERalpha and represses ERalpha-mediated transcription. Reduction of the endogenous NELF proteins in breast cancer cells using small interfering RNA results in elevated ERalpha-mediated transcription and enhanced cell proliferation. Chromatin immunoprecipitation reveals that recruitment of COBRA1 and the other NELF subunits to endogenous ERalpha-responsive promoters is greatly stimulated upon estrogen treatment. Interestingly, COBRA1 does not affect the estrogen-dependent assembly of transcription regulatory complexes at the ERalpha-regulated promoters. Rather, it causes RNA polymerase II (RNAPII) to pause at the promoter-proximal region, which is consistent with its in vitro biochemical activity. Therefore, our in vivo work defines the first corepressor of nuclear receptors that modulates ERalpha-dependent gene expression by stalling RNAPII. We suggest that this new level of regulation may be important to control the duration and magnitude of a rapid and reversible hormonal response.

The X-ray structure of the papillomavirus helicase in complex with its molecular matchmaker E2

DNA replication of the papillomaviruses is specified by cooperative binding of two proteins to the ori site: the enhancer E2 and the viral initiator E1, a distant member of the AAA+ family of proteins. Formation of this prereplication complex is an essential step toward the construction of a functional, multimeric E1 helicase and DNA melting. To understand how E2 interacts with E1 to regulate this process, we have solved the X-ray structure of a complex containing the HPV18 E2 activation domain bound to the helicase domain of E1. Modeling the monomers of E1 to a hexameric helicase shows that E2 blocks hexamerization of E1 by shielding a region of the E1 oligomerization surface and stabilizing a conformation of E1 that is incompatible with ATP binding. Further biochemical experiments and structural analysis show that ATP is an allosteric effector of the dissociation of E2 from E1. Our data provide the first molecular insights into how a protein can regulate the assembly of an oligomeric AAA+ complex and explain at a structural level why E2, after playing a matchmaker role by guiding E1 to the DNA, must dissociate for subsequent steps of initiation to occur. Building on previously proposed ideas, we discuss how our data advance current models for the conversion of E1 in the prereplication complex to a hexameric helicase assembly.

Identification of the protein Zibra, its genomic organization, regulation, and expression in breast cancer cells

The mRNA that encodes zibra (zinc, in-between-ring finger, ubiquitin-associated domain), previously known as hypothetical protein FLJ10111, or RNF31 is expressed in several distinct cancers. Little is known about the genomic organization, expression, or regulation of zibra. Using RNA ligase-mediated rapid amplification of cDNA ends (RLM-RACE), we cloned the full-length zibra cDNA from a transformed breast cell line. We identified a novel exon, the 5' untranslated region including the +1 start site, and three alternatively spliced zibra transcripts. The zibra protein contains three zinc ring-finger motifs, an ubiquitin-associated domain, and an in-between-ring-finger domain, characteristic of ubiquitin ligases. We obtained an antibody to zibra and confirmed the presence of translated zibra protein for the first time. Promoter studies localized a core element responsible for basal activity to a 14-bp region in the 5' untranslated region. Although there are numerous consensus Ets factor binding sites in the zibra promoter, we found no affect on promoter activity from Ets-1, PDEF, or PEA-3/E1A-F. Treatment of cells with the proteasome inhibitor I (PSI) decreased zibra protein to an undetectable level after 8 h. Zibra remained undetectable even after 32 h, while mRNA levels remained essentially unchanged. In conclusion, zibra is a translationally regulated putative ubiquitin ligase that is frequently overexpressed in different forms of cancer.

Transcriptional regulation of BACE1, the beta-amyloid precursor protein beta-secretase, by Sp1

Proteolytic processing of the beta-amyloid precursor protein (APP) at the beta site is essential to generate Abeta. BACE1, the major beta-secretase involved in cleaving APP, has been identified as a type 1 membrane-associated aspartyl protease. We have cloned a 2.1-kb fragment upstream of the human BACE1 gene and identified key regions necessary for promoter activity. BACE1 gene expression is controlled by a TATA-less promoter. The region of bp -619 to +46 is the minimal promoter to control the transcription of the BACE1 gene. Several putative cis-acting elements, such as a GC box, HSF-1, a PU box, AP1, AP2, and lymphokine response element, are found in the 5' flanking region of the BACE1 gene. Transcriptional activation and gel shift assays demonstrated that the BACE1 promoter contains a functional Sp1 response element, and overexpression of the transcription factor Sp1 potentiates BACE gene expression and APP processing to generate Abeta. Furthermore, Sp1 knockout reduced BACE1 expression. These results suggest that BACE1 gene expression is tightly regulated at the transcriptional level and that the transcription factor Sp1 plays an important role in regulation of BACE1 to process APP generating Abeta in Alzheimer's disease.

Crystal structure of the E2 transactivation domain of human papillomavirus type 11 bound to a protein interaction inhibitor

Interaction between the E2 protein and E1 helicase of human papillomaviruses (HPVs) is essential for the initiation of viral DNA replication. We recently described a series of small molecules that bind to the N-terminal transactivation domain (TAD) of HPV type 11 E2 and inhibits its interaction with E1 in vitro and in cellular assays. Here we report the crystal structures of both the HPV11 TAD and of a complex between this domain and an inhibitor, at 2.5- and 2.4-A resolution, respectively. The HPV11 TAD structure is very similar to that of the analogous domain of HPV16. Inhibitor binding caused no significant alteration of the protein backbone, but movements of several amino acid side chains at the binding site, in particular those of Tyr-19, His-32, Leu-94, and Glu-100, resulted in the formation of a deep hydrophobic pocket that accommodates the indandione moiety of the inhibitor. Mutational analysis provides functional evidence for specific interactions between Tyr-19 and E1 and between His-32 and the inhibitor. A second inhibitor molecule is also present at the binding pocket. Although evidence is presented that this second molecule makes only weak interactions with the protein and is likely an artifact of crystallization, its presence defines additional regions of the binding pocket that could be exploited to design more potent inhibitors.

The type 1 CD10/neutral endopeptidase 24.11 promoter: functional characterization of the 5′-untranslated region

The cell surface zinc metalloproteinase CD10/neutral endopeptidase 24.11 (NEP) is expressed on normal and malignant lymphoid progenitors, granulocytes and a variety of epithelial cells. Because CD10/NEP functions as part of a regulatory loop that controls local concentrations of peptide substrates and associated peptide-mediated signal transduction, its role in each tissue is different depending on the availability of substrate. To characterize further how this widely distributed molecule is regulated differentially in each tissue, we analysed the major type 2 CD10/NEP promoter and found three functionally important transcription factor binding sites, one of which was identical to CCAAT-binding transcription factor/nuclear transcription factor Y. In this report, we analyse the type 1 CD10/NEP promoter and found a functionally important transcription factor binding site in the 5'-untranslated region. The results of the competition and supershift experiments demonstrated that the functionally important transcription factor was identical to Sp1. Our results suggest that ubiquitously expressed Sp1 may play an important role in differentiation stage-specific regulation of CD10/NEP expression in lymphoid lineage.

Interaction between the varicella zoster virus IE62 major transactivator and cellular transcription factor Sp1

The varicella zoster virus (VZV) IE62 protein is involved in the activation of expression of all three kinetic classes of VZV proteins. Analysis of the viral promoter for VZV glycoprotein I has shown that the cellular factor Sp1 is involved in or required for the observed IE62 mediated activation. Co-immunoprecipitation experiments show that the two proteins are present in a complex in VZV-infected cells. Protein affinity pull-down assays using recombinant proteins showed that IE62 and Sp1 interact in the absence of any other viral and cellular proteins. Mapping studies using GST-fusion proteins containing truncations of IE62 and Sp1 have delimited the interacting regions to amino acids 612-778 in Sp1 and amino acids 226-299 in IE62. The region identified in Sp1 is involved in DNA-binding, synergistic Sp1 activation, and Sp1 interaction with cellular transcription factors. The interacting region identified in IE62 overlaps with or borders on sites involved in interactions with the VZV IE4 protein and the cellular factors TBP and TFIIB. Assays using wild-type and mutant promoter elements indicate that Sp1 is involved in recruitment of IE62 to the gI promoter and IE62 enhances Sp1 and TBP binding.

Transcriptional control of the RECK metastasis/angiogenesis suppressor gene

The RECK gene is widely expressed in normal human tissues but is downregulated in tumor cell lines and oncogenically transformed fibroblasts. RECK encodes a membrane-anchored glycoprotein that suppresses tumor invasion and angiogenesis by regulating matrix-metalloproteinases (MMP-2, MMP-9 and MT1-MMP). Understanding of the transcriptional regulation of tumor/metastasis suppressor genes constitutes a potent approach to the molecular basis of malignant transformation. In order to uncover the mechanisms of control of RECK gene expression, the RECK promoter has been cloned and characterized. One of the elements responsible for the Ras-mediated downregulation of mouse RECK gene is the Sp1 site, to which Sp1 and Sp3 factors bind. Other regulatory events, such as DNA methylation of the RECK promoter and histone acetylation/deacetylation have been studied to understand the underlying mechanisms of RECK expression. Understanding of the mechanisms which control RECK gene transcription may lead to the development of new strategies for cancer prevention and treatment.

The Papillomavirus E2 protein binds to and synergizes with C/EBP factors involved in keratinocyte differentiation

The papillomavirus life cycle is closely linked to the differentiation program of the host keratinocyte. Thus, late gene expression and viral maturation are restricted to terminally differentiated keratinocytes. A variety of cellular transcription factors including those of the C/EBP family are involved in the regulation of keratinocyte differentiation. In this study we show that the papillomavirus transcription factor E2 cooperates with C/EBPalpha and -beta in transcriptional activation. This synergism was independent of an E2 binding site. E2 and C/EBP factors synergistically transactivated a synthetic promoter construct containing classical C/EBPbeta sites and the C/EBPalpha-responsive proximal promoter of the involucrin gene, which is naturally expressed in differentiating keratinocytes. C/EBPalpha or -beta coprecipitated with E2 proteins derived from human papillomavirus type 8 (HPV8), HPV16, HPV18, and bovine papillomavirus type 1 in vitro and in vivo, indicating complex formation by the cellular and viral factors. The interaction domains could be mapped to the C terminus of E2 and amino acids 261 to 302 located within the bZIP motif of C/EBPbeta. Our data suggest that E2, via its interaction with C/EBP factors, may contribute to enhancing keratinocyte differentiation, which is suppressed by the viral oncoproteins E6 and E7 in HPV-induced lesions.

Molecular evolution of nucleoside diphosphate kinase genes: conserved core structures and multiple-layered regulatory regions

Genomic data regarding the nucleoside diphosphate (NDP) kinase genes have been accumulated from diverged phyla. Comparison of their regulatory sequences have shed light on the multiple facets of gene regulation systems. Phylogenetic studies, including CpG island and intron-mapping, and homologous sequence comparison, have suggested that the regions of the major mammalian genes, the ortholog (rat alpha or nm23-H2) and its paralog (rat beta or nm23-H1), have been constructed by a stepwise gain and loss of alien genes resulting in "multiple-layered" regulatory systems. They contain representative cis-elements for the constitutive, stage/lineage-specific, and early response expression. These elements' binding capacities to nuclear proteins were confirmed by electrophoretic mobility shift assay. Further, these regulatory systems generate heterogeneous mRNA at the 5' untranslated region, which influences their own translation efficiencies. In terms of this process, the transcription system would control another layer of gene expression: posttranscriptional (translational) regulation.

Transcriptional activity among high and low risk human papillomavirus E2 proteins correlates with E2 DNA binding

The full-length E2 protein, encoded by human papillomaviruses (HPVs), is a sequence-specific transcription factor found in all HPVs, including cancer-causing high risk HPV types 16 and 18 and wart-inducing low risk HPV types 6 and 11. To investigate whether E2 proteins encoded by high risk HPVs may function differentially from E2 proteins encoded by low risk HPVs and animal papillomaviruses, we conducted comparative DNA-binding and transcription studies using electrophoretic mobility shift assays and cell-free transcription systems reconstituted with purified general transcription factors, cofactor, RNA polymerase II, and with E2 proteins encoded by HPV-16, HPV-18, HPV-11, and bovine papillomavirus type 1 (BPV-1). We found that although different types of E2 proteins all exhibited transactivation and repression activities, depending on the sequence context of the E2-binding sites, HPV-16 E2 shows stronger transcription activity and greater DNA-binding affinity than those displayed by the other E2 proteins. Surprisingly, HPV-18 E2 behaves more similarly to BPV-1 E2 than HPV-16 E2 in its functional properties. Our studies thus categorize HPV-18 E2 and BPV-1 E2 in the same protein family, a finding consistent with the available E2 structural data that separate the closely related HPV-16 and HPV-18 E2 proteins but classify together the more divergent BPV-1 and HPV-18 E2 proteins.

Regulation of the activity of Sp1-related transcription factors

The initiation of transcription is accomplished via interactions of many different proteins with common and gene-specific regulatory motifs. Clearly, sequence-specific transcription factors play a crucial role in the specificity of transcription initiation. A group of sequence-specific DNA-binding proteins, related to the transcription factor Sp1, has been implicated in the regulation of many different genes, since binding sites for these transcription factors (GC/GT boxes) are a recurrent motif in regulatory sequences such as promoters, enhancers and CpG islands of these genes. The simultaneous occurrence of several homologous GC/GT box-binding factors precludes a straightforward deduction of their role in transcriptional regulation. In this review, we focus on the connection between functional specificity and biochemical properties including glycosylation, phosphorylation and acetylation of Sp1-related factors.

Functional interaction between human papillomavirus type 18 E2 and poly(ADP-ribose) polymerase 1

Human papillomavirus E2 protein is a transcription factor of viral gene expression and DNA replication. Here we show that PARP is a positive regulator of the E2 protein of human papillomavirus type 18 (HPV-18). PARP interacted with the COOH terminal region of HPV-18 E2 in vitro. The E2 interaction domain within PARP is located in the NH(2)-terminal zinc finger motif and the BRCT motif included in the automodification domain. Overexpression of either wild type or the NH(2)-terminal region of PARP containing zinc finger and BRCT stimulated E2-dependent transcription. Gel retardation assay indicates that PARP augments DNA binding activity of E2 in vitro. We also show that PARP-1 is recruited to E2-dependent promoter in vivo using ChIP assay. These results suggest that PARP serves a transcriptional co-activator in E2-dependent transcription by interacting directly with the HPV E2 protein.

Human papillomavirus E2 down-regulates the human telomerase reverse transcriptase promoter

The transcriptional regulation of the human telomerase reverse transcriptase (hTERT) gene is a critical step in transformation and differentiation. Human papillomavirus E2 protein inhibits cell growth in HPV-infected cells and triggers apoptosis in HeLa cells. Because E2 induces cell growth suppression and senescence, we hypothesize that the protein may modulate cellular gene expression related to these processes. In this report, we demonstrate that E2 inhibits the hTERT promoter. The mapping of the E2-responsive region of hTERT reveals that Sp1 is important for E2-mediated repression of this promoter in 293T cells. Site-directed mutagenesis data on the hTERT promoter show that E2 does not abolish E-Box-mediated transcription and represses promoter activity via the Sp1 binding site. Furthermore, chromatin immunoprecipitation assays indicate that E2 is actively recruited to the hTERT promoter region. Our findings provide novel insights into the biological function of human papillomavirus E2.

RelA-associated inhibitor blocks transcription of human immunodeficiency virus type 1 by inhibiting NF-kappaB and Sp1 actions

RelA-associated inhibitor (RAI) is an inhibitor of nuclear factor kappaB (NF-kappaB) newly identified by yeast two-hybrid screen as an interacting protein of the p65 (RelA) subunit. In this study, we attempted to examine the effect of RAI on transcription and replication of human immunodeficiency virus type 1 (HIV-1). We found that RAI inhibited gene expression from the HIV-1 long terminal repeat (LTR) even at the basal level. Upon in vitro DNA-binding reactions, RAI could directly block the DNA-binding of p65 subunit of NF-kappaB but not that of the p50 subunit or AP1. We found that RAI could also inhibit the DNA-binding of Sp1 and thus inhibit the basal HIV-1 promoter activity. We further examined the effects of RAI on Sp1 and found that RAI colocalizes with Sp1 in the nucleus and interacts with Sp1 in vitro and in vivo. Moreover, we found that RAI efficiently blocked the HIV-1 replication when cotransfected with a full-length HIV-1 clone. These findings indicate that RAI acts as an efficient inhibitor of HIV-1 gene expression in which both NF-kappaB and Sp1 play major roles.