Synergistic transcriptional-activation by the papillomavirus E2 protein occurs after DNA binding and correlates with a change in chromatin structure

Epigenetic regulation of human papillomavirus transcription in the productive virus life cycle

Human papillomaviruses (HPV) are a large family of viruses which contain a circular, double-stranded DNA genome of approximately 8000 base pairs. The viral DNA is chromatinized by the recruitment of cellular histones which are subject to host cell-mediated post-translational epigenetic modification recognized as an important mechanism of virus transcription regulation. The HPV life cycle is dependent on the terminal differentiation of the target cell within epithelia-the keratinocyte. The virus life cycle begins in the undifferentiated basal compartment of epithelia where the viral chromatin is maintained in an epigenetically repressed state, stabilized by distal chromatin interactions between the viral enhancer and early gene region. Migration of the infected keratinocyte towards the surface of the epithelium induces cellular differentiation which disrupts chromatin looping and stimulates epigenetic remodelling of the viral chromatin. These epigenetic changes result in enhanced virus transcription and activation of the virus late promoter facilitating transcription of the viral capsid proteins. In this review article, we discuss the complexity of virus- and host-cell-mediated epigenetic regulation of virus transcription with a specific focus on differentiation-dependent remodelling of viral chromatin during the HPV life cycle.

Persistence of an Oncogenic Papillomavirus Genome Requires cis Elements from the Viral Transcriptional Enhancer

Human papillomavirus (HPV) genomes are replicated and maintained as extrachromosomal plasmids during persistent infection. The viral E2 proteins are thought to promote stable maintenance replication by tethering the viral DNA to host chromatin. However, this has been very difficult to prove genetically, as the E2 protein is involved in transcriptional regulation and initiation of replication, as well as its assumed role in genome maintenance. This makes mutational analysis of viral trans factors and cis elements in the background of the viral genome problematic and difficult to interpret. To circumvent this problem, we have developed a complementation assay in which the complete wild-type HPV18 genome is transfected into primary human keratinocytes along with subgenomic or mutated replicons that contain the minimal replication origin. The wild-type genome provides the E1 and E2 proteins in trans, allowing us to determine additional cis elements that are required for long-term replication and partitioning of the replicon. We found that, in addition to the core replication origin (and the three E2 binding sites located therein), additional sequences from the transcriptional enhancer portion of the URR (upstream regulatory region) are required in cis for long-term genome replication.

Human papillomaviruses infect cutaneous and mucosal epithelial cells of the host, and this results in very-long-lived, persistent infection. The viral genomes are small, circular, double-stranded DNA molecules that replicate extrachromosomally in concert with cellular DNA. This replication strategy requires that the virus has a robust mechanism to partition and retain the viral genomes in dividing cells. This has been difficult to study, because viral transcription, replication, and partitioning are regulated by the same viral proteins and involve overlapping elements in the viral genome. We developed a complementation assay that allows us to separate these functions and define the elements required for long-term replication and stable maintenance replication of the HPV genome. This has important implications, as disruption of viral maintenance replication can eliminate viral genomes from infected cells, thus curing persistent HPV infection.

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.

Bovine papillomavirus type 1 E2 protein heterodimer is functional in papillomavirus DNA replication in vivo

Papillomaviruses are small DNA viruses that induce epithelial lesions in their host. The viral life cycle is regulated by the family of proteins encoded by the E2 open reading frame. In addition to the full-length E2 protein, the BPV-1 genome encodes two truncated E2 proteins, E2C and E8/E2, which maintain the DNA-binding-dimerization domains, but lack the activation domain. Heterodimers formed between the full-length E2 and truncated E2 proteins serve as activators of E2-dependent transcription and papillomavirus DNA replication. We show that the single activation domain of E2 is sufficient for interaction with viral helicase E1 and for initiation of DNA replication from different papillomavirus origins. Single-chain E2 heterodimer is able to activate papillomavirus DNA replication in the context of entire BPV genome in the absence of other E2 proteins. These data suggest that E2 heterodimers with single activation domain are functional in initiation of papillomavirus replication in vivo.

Transcriptional regulation of human papillomavirus type 18 P105 promoter by the co-activator CBP

Human papillomaviruses (HPVs) are the etiological agents of cervical cancer, with HPV-16 and 18 being the representative types of the higher risk group. The expression of the viral genes with transforming activity (E6 and E7) is controlled by the upstream regulatory region (URR), a segment of the viral genome that contains elements recognized by several transcription factors.

OBJECTIVE

We have analyzed the participation of the cellular co-activator CBP on the transcriptional regulation of the HPV-18 URR.

METHODS

We generated mutants and 5' end deletion constructs derived from the HPV-18 URR and evaluated their transcriptional activity performing transient co-transfection assays on C-33A cells with a plasmid that over-expresses the co-activator CBP. We also performed quantitative chromatin immunoprecipitation assays to analyze the participation of the co-activator CBP on the HPV-18 P105 promoter.

RESULTS

Our results demonstrate that in C-33A cells CBP acts as a strong activator of the HPV-18 P105 promoter by a mechanism that depends on the integrity of the SP1-binding site, directly correlating with the acetylation of the histone H3 that is involved in nucleosomal stability.

CONCLUSION

We propose a mechanism of regulation of the HPV-18 P105 promoter by the cellular co-activator CBP, recruited by the transcription factor SP1.

Characterization of the functional activities of the bovine papillomavirus type 1 E2 protein single-chain heterodimers

Papillomaviruses are small DNA viruses which establish persistent infection in the epithelial tissue of various animal species. Three papillomavirus proteins encoded by the bovine papillomavirus type 1 E2 open reading frame have a common C-terminal DNA binding and dimerization domain and function as dimeric proteins in the regulation of viral gene expression, genome replication, and maintenance. The full-length E2 protein, expressed usually at the lowest level of the three, is an activator, while shorter forms of E2, lacking the transactivation domain, serve as repressors of replication and transcription. In virally infected cells, the full-length E2 protein forms heterodimers with repressor forms of the E2 protein and the biological activities of such heterodimers are poorly known. In order to study the functionality of E2 heterodimers, we joined the full-length E2 protein and E2 repressor by a flexible polypeptide hinge so that they formed a single-chain intramolecular dimer. The single-chain E2 heterodimers folded correctly to form genuine pseudodimers capable of binding to the specific E2 protein binding site with high affinity. Characterization of the activities of this protein in transcription showed that it functions as an effective transcriptional activator, which is comparable to what was found for the full-length E2 protein. The single-chain heterodimer is dependent to some extent on Brd4 protein and is able to support papillomavirus origin replication; however, it does not support the partitioning of the multimeric E2 binding site containing plasmids in dividing cells. Our results suggest that E2 heterodimers serve as activators of transcription and replication during the viral life cycle.

Direct interaction between nucleosome assembly protein 1 and the papillomavirus E2 proteins involved in activation of transcription

Using a yeast two-hybrid screen, we identified human nucleosome assembly protein 1 (hNAP-1) as a protein interacting with the activation domain of the transcriptional activator encoded by papillomaviruses (PVs), the E2 protein. We show that the interaction between E2 and hNAP-1 is direct and not merely mediated by the transcriptional coactivator p300, which is bound by both proteins. Coexpression of hNAP-1 strongly enhances activation by E2, indicating a functional interaction as well. E2 binds to at least two separate domains within hNAP-1, one within the C terminus and an internal domain. The binding of E2 to hNAP-1 is necessary for cooperativity between the factors. Moreover, the N-terminal 91 amino acids are crucial for the transcriptional activity of hNAP-1, since deletion mutants lacking this N-terminal portion fail to cooperate with E2. We provide evidence that hNAP-1, E2, and p300 can form a ternary complex efficient in the activation of transcription. We also show that p53 directly interacts with hNAP-1, indicating that transcriptional activators in addition to PV E2 interact with hNAP-1. These results suggest that the binding of sequence-specific DNA binding proteins to hNAP-1 may be an important step contributing to the activation of transcription.

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.

Intratype HPV16 sequence variation within LCR of isolates from asymptomatic carriers and cervical cancers

BACKGROUND

HPV16 is a predominant type of virus identified in genital lesions and strongly associated with the development of genital cancers. Infection with the virus is considered to be the main risk factor in the development of cervical cancer. Based on HPV16 DNA isolated from invasive cancers, a classification of intratype genetic variants was established and the strains were designated according to geographical regions. The HPV16 variants classification was based on isolates derived from cancers.

OBJECTIVES

Analysis of HPV16 LCR variants isolated from asymptomatic carriers for comparison with cervical cancer isolates to examine whether a correlation can be found between cervical epithelium state and variant of HPV16 it carries.

MATERIALS AND METHODS

The HPV16 LCR fragments were amplified by PCR using DNA isolated from cervical swabs and tissue sections then screened for nucleotide changes by SSCP. Polymorphic sites were analysed for regulatory protein binding properties by EMSA.

RESULTS

Comparison of the two groups revealed that isolates from cervical cancers predominantly carry changes in sequences of YY1 binding sites (especially at nucleotide 7519), while variants from asymptomatic carriers contained nucleotide changes within or close to transcription binding sites for AP-1, Oct-1, NF1, Tef-1, Tef-2, Sp1, YY1 and viral E2. EMSA study showed that sequence changes in the segment alter binding and formation of transcriptional complexes in quantitative and/or qualitative manner and so they may inflict viral activity.

CONCLUSION

The results of our study show that there might be HPV16 variants of decreased oncogenic potential therefore infection with such variants can recede.

The chromatin structure of the long control region of human papillomavirus type 16 represses viral oncoprotein expression

The long control region (LCR) of human papillomavirus type 16 (HPV-16) has a size of 850 bp (about 12% of the viral genome) and regulates transcription and replication of the viral DNA. The 5' segment of the LCR contains transcription termination signals and a nuclear matrix attachment region, the central segment contains an epithelial cell-specific enhancer, and the 3' segment contains the replication origin and the E6 promoter. Here we report observations on the chromatin organization of this part of the HPV-16 genome. Treatment of the nuclei of CaSki cells, a cell line with 500 intrachromosomal copies of HPV-16, with methidiumpropyl-EDTA-Fe(II) reveals nucleosomes in specific positions on the LCR and the E6 and E7 genes. One of these nucleosomes, which we termed Ne, overlaps with the center of the viral enhancer, while a second nucleosome, Np16, overlaps with the replication origin and the E6 promoter. The two nucleosomes become positioned on exactly the same segments after in vitro assembly of chromatin on the cloned HPV-16 LCR. Primer extension mapping of DNase I-cleaved chromatin revealed Np16 to be positioned centrally over E6 promoter elements, extending into the replication origin. Ne covers the center of the enhancer but leaves an AP-1 site, one of the strongest cis-responsive elements of the enhancer, unprotected. Np16, or a combination of Np16 and Ne, represses the activity of the E6 promoter during in vitro transcription of HPV-16 chromatin. Repression is relieved by addition of Sp1 and AP-1 transcription factors. Sp1 alters the structure of Np16 in vitro, while no changes can be observed during the binding of AP-1. HPV-18, which has a similar arrangement of cis-responsive elements despite its evolutionary divergence from HPV-16, shows specific assembly in vitro of a nucleosome, Np18, over the E1 binding site and E6 promoter elements but positioned about 90 bp 5' of the position of Np16 on the homologous HPV-16 sequences. The chromatin organization of the HPV-16 and HPV-18 genomes suggests important regulatory roles of nucleosomes during the viral life cycle.