Cell-type-specific separate regulation of the E6 and E7 promoters of human papillomavirus type 6a by the viral transcription factor E2

Relevance of BET Family Proteins in SARS-CoV-2 Infection

The recent pandemic we are experiencing caused by the coronavirus disease 2019 (COVID-19) has put the world's population on the rack, with more than 191 million cases and more than 4.1 million deaths confirmed to date. This disease is caused by a new type of coronavirus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). A massive proteomic analysis has revealed that one of the structural proteins of the virus, the E protein, interacts with BRD2 and BRD4 proteins of the Bromodomain and Extra Terminal domain (BET) family of proteins. BETs are essential to cell cycle progression, inflammation and immune response and have also been strongly associated with infection by different types of viruses. The fundamental role BET proteins play in transcription makes them appropriate targets for the propagation strategies of some viruses. Recognition of histone acetylation by BET bromodomains is essential for transcription control. The development of drugs mimicking acetyl groups, and thereby able to displace BET proteins from chromatin, has boosted interest on BETs as attractive targets for therapeutic intervention. The success of these drugs against a variety of diseases in cellular and animal models has been recently enlarged with promising results from SARS-CoV-2 infection studies.

Use of Cap Analysis Gene Expression to detect human papillomavirus promoter activity patterns at different disease stages

Transcription of human papillomavirus (HPV) genes proceeds unidirectionally from multiple promoters. Direct profiling of transcription start sites (TSSs) by Cap Analysis Gene Expression (CAGE) is a powerful strategy for examining individual HPV promoter activity. The objective of this study was to evaluate alterations of viral promoter activity during infection using CAGE technology. We used CAGE-based sequencing of 46 primary cervical samples, and quantitatively evaluated TSS patterns in the HPV transcriptome at a single-nucleotide resolution. TSS patterns were classified into two types: early promoter-dominant type (Type A) and late promoter-dominant type (Type B). The Type B pattern was more frequently found in CIN1 and CIN2 lesions than in CIN3 and cancer samples. We detected transcriptomes from multiple HPV types in five samples. Interestingly, in each sample, the TSS patterns of both HPV types were the same. The viral gene expression pattern was determined by the differentiation status of the epithelial cells, regardless of HPV type. We performed unbiased analyses of TSSs across the HPV genome in clinical samples. Visualising TSS pattern dynamics, including TSS shifts, provides new insights into how HPV infection status relates to disease state.

Regulation of HPV transcription

Human papillomavirus infection is associated with the development of malignant and benign neoplasms. Approximately 40 viral types can infect the anogenital mucosa and are categorized into high- and low-risk oncogenic human papillomavirus, depending on their association with the development of cervical carcinoma. High-risk human papillomavirus 16 and 18 are detected in 55% and 15% of all invasive cervical squamous cell carcinomas worldwide, respectively. Low-risk human papillomavirus 6 and 11 are responsible for 90% of genital warts and are also associated with the development of recurrent respiratory papillomatosis. Human papillomavirus preferentially infects mitotic active cells of the basal layer from both mucosal and cutaneous epithelium through microabrasions. The viral life cycle synchronizes with the epithelial differentiation program, which may be due, in part, to the binding of differentially expressed cellular transcription factors to the long control region throughout the various epithelial layers. This review aimed to summarize the current knowledge regarding the mechanisms by which viral gene expression is regulated and the influence of human papillomavirus heterogeneity upon this phenomenon. A better understanding of the regulatory mechanisms may elucidate the particularities of human papillomavirus-associated pathogenesis and may provide new tools for antiviral therapy.

Regulation of hepatitis B virus expression in progenitor and differentiated cell types: evidence for negative transcriptional control in nonpermissive cells

Mechanisms regulating cell type-specific gene expression are not completely understood. We utilized hepatitis B virus (HBV) enhancer I and preS1 promoter sequences, which exhibit cell type specificity, to analyze transcriptional control in pluripotential murine embryonic stem (ES) cells, bipotential HBC-3 progenitor liver cells, mature hepatocytes, and fibroblasts. In transient transfection assays, HBV sequences were most active in primary hepatocytes, followed by HBC-3 and ES cells, and became inactive in fibroblasts. Cotransfections with HNF-3 or C/EBP plasmids increased expression of HBV sequences in hepatocytes and HBC-3 cells. However, increased HBV expression was not observed in ES cells and HBV remained inactive in fibroblasts, suggesting different transcriptional controls, which was compatible with alterations in the abundance of endogenous transcription factors. Analysis in somatic hybrid cells created from NIH 3T3 fibroblasts and Hepa1-6 mouse hepatocytes with expression of albumin and selected hepatic transcription factors showed that HBV sequences were expressed weakly but without increased expression following transfection of HNF-1, HNF-3, and C/EBP plasmids. These findings indicated that repression of HBV in nonpermissive cells involved inactivation of transcription factor activity. Expression of HBV in stem cells is relevant for mechanisms concerning viral persistence and oncogenesis, as well as analysis of hepatocytic differentiation in progenitor cells.

Involvement of Brd4 in different steps of the papillomavirus life cycle

Bromodomain-containing protein 4 (Brd4) is a cellular chromatin-binding factor and transcriptional regulator that recruits sequence-specific transcription factors and chromatin modulators to control target gene transcription. Papillomaviruses (PVs) have evolved to hijack Brd4's activity in order to create a facilitating environment for the viral life cycle. Brd4, in association with the major viral regulatory protein E2, is involved in multiple steps of the PV life cycle including replication initiation, viral gene transcription, and viral genome segregation and maintenance. Phosphorylation of Brd4, regulated by casein kinase II (CK2) and protein phosphatase 2A (PP2A), is critical for viral gene transcription as well as E1- and E2-dependent origin replication. Thus, pharmacological agents regulating Brd4 phosphorylation and inhibitors blocking phospho-Brd4 functions are promising candidates for therapeutic intervention in treating human papillomavirus (HPV) infections as well as associated disease.

Methylation status of HPV16 E2-binding sites classifies subtypes of HPV-associated oropharyngeal cancers

BACKGROUND

The human papillomavirus (HPV) E2 protein is a transcriptional repressor of the oncogenes E6/E7 and loss of E2 function is considered a key step in carcinogenesis. Integration of HPV into the host genome may disrupt the E2 gene. Furthermore, methylation of CpG dinucleotides in E2-binding sites (E2BSs) in the HPV upstream regulatory region may interfere with transcriptional repression of E6 and E7 by E2. The authors hypothesized that the CpG methylation status of E2BS identifies subtypes of HPV type 16 (HPV16)-associated oropharyngeal squamous cell cancers (OPSCC) in association with E2 gene integrity and viral integration.

METHODS

Methylation of 10 CpG dinucleotides within the upstream regulatory region, encompassing E2BSs 1, 2, 3, and 4, was quantitatively analyzed by bisulfite pyrosequencing in 57 HPV16-associated OPSCC cases. E2 status was analyzed by gene amplification and quantitative real-time reverse transcriptase-polymerase chain reaction. Viral integration was determined by integration-specific polymerase chain reaction methods.

RESULTS

Three subgroups with differential methylation at E2BS3 and E2BS 4 were identified: 1) complete methylation (>80%) associated with the presence of integrated HPV genomes with an intact E2 gene; 2) intermediate methylation levels (20%-80%) with predominantly episomal HPV genomes with intact E2; and 3) no methylation (<20%) with a disrupted E2 gene. Patients with high methylation levels tended to have a worse 5-year overall survival compared with patients with intermediate methylation (hazard ratio, 3.23; 95% confidence interval, 1.13-9.24 [P = .06]).

CONCLUSIONS

Methylation of E2BS3 and E2BS4 in OPSCC is associated with E2 integrity and viral physical status. It might explain deregulated viral oncogene expression in the presence of E2. The prognostic significance of E2BS methylation for patients with HPV-associated OPSCC needs to be analyzed further.

HPV 16 E2 binding sites 1 and 2 become more methylated than E2 binding site 4 during cervical carcinogenesis

E2 protein binding to the four E2 binding sites (E2BSs) at the long control region of Human Papillomavirus (HPV) 16/18 genome may exert either transcriptional activation/repression on E6 and E7 oncoproteins. Methylation status at the E2BSs may affect the relative binding of E2 protein to them. In this study, methylation percentage at E2BS 1, 2 (promoter-proximal), and 4 (promoter-distal) were assessed by pyrosequencing and compared among HPV 16/18-positive cervical cancer, high-grade, and low-grade Cervical Intraepithelial Neoplasia, Atypical Squamous Cells of Undetermined Significance, and normal cervical epithelium. HPV 16 E2BS1&2 were more methylated than HPV 16 E2BS4 in cervical cancer whereas in cervical premalignant lesions and normal epithelium, HPV 16 E2BS1&2 were less methylated than HPV 16 E2BS4. HPV 18 E2BS1&2 remained more methylated than E2BS4 in all histological groups. HPV 16 E2BS1&2 methylation increased from high-grade lesions to cervical cancer (P < 0.001). HPV 16 E2BS4 methylation increased from low-grade to high-grade premalignant lesions (P = 0.041). Both HPV 18 E2BS1&2 and E2BS4 methylation increased from low-grade to high-grade Cervical Intraepithelial Neoplasia (P = 0.019 and 0.001 respectively) and further increased form high-grade lesions to cervical cancer (P < 0.001 and 0.005 respectively). Conclusively, HPV 16 E2BS1&2 (for transcriptional repression of E6/E7 oncoproteins) became more heavily methylated than E2BS4 (for transcriptional activation of E6/E7) in cervical cancer, favouring the differential binding of E2 protein to E2BS4. Increasing methylation at HPV 16/18 E2BSs are potentially useful adjunctive molecular markers for predicting progression from low-grade to high-grade cervical premalignant lesions and from high-grade lesions to cervical cancer.

A novel recombinant papillomavirus genome enabling in vivo RNA interference reveals that YB-1, which interacts with the viral regulatory protein E2, is required for CRPV-induced tumor formation in vivo

YB-1 is considered a negative prognostic marker for different types of cancer. Increased YB-1 protein levels in tumor cells indicate a worse prognosis. In a preceding study comparing the transcripts of CRPV-induced benign papillomas to mRNA levels of malignant epithelial tumors, we identified YB-1 as a gene that is up-regulated in papillomavirus-associated carcinomas and which causes an invasive phenotype in CRPV-positive cells in vitro. Here we demonstrate that YB-1 is a previously unknown factor required for papillomavirus-induced tumor development in the rabbit animal model system. By infecting the animals with a novel recombinant shRNA-expressing CRPV genome, we show that knock-down of YB-1 dramatically reduces papillomavirus-dependent tumor formation in vivo. Consistent with previous reports showing a nuclear distribution of YB-1 proteins as a hallmark of malignancy, we demonstrate a predominantly nuclear localization of YB-1 in CRPV-immortalized cells. Furthermore we give evidence of YB-1 regulating the CRPV URR and thereby viral gene expression and we identified YB-1 as a novel interactor of the CRPV regulatory protein E2. Taken together we hypothesize that YB-1 is essential for papillomavirus-induced tumor formation probably by regulating viral gene expression including expression of the oncogenes E6 and E7.

Differential methylation of E2 binding sites in episomal and integrated HPV 16 genomes in preinvasive and invasive cervical lesions

Enhanced expression of the HPV 16 E6-E7 oncogenes may trigger neoplastic transformation of the squamous epithelial cells at the uterine cervix. The HPV E2 protein is a key transcriptional regulator of the E6-E7 genes. It binds to four E2 binding sites (E2BSs 1-4) in the viral upstream regulatory region (URR). Modification of E2 functions, for example, by methylation of E2BSs is hypothesized to trigger enhanced expression of the viral E6-E7 oncogenes. In the majority of HPV-transformed premalignant lesions and about half of cervical carcinomas HPV genomes persist in an extra-chromosomal, episomal state, whereas they are integrated into host cells chromosomes in the remaining lesions. Here we compared the methylation profile of E2BSs 1-4 of the HPV 16 URR in a series of 18 HPV16-positive premalignant lesions and 33 invasive cervical cancers. CpGs within the E2BSs 1, 3, and 4 were higher methylated in all lesions with only episomal HPV16 genomes compared with lesions displaying single integrated copies. Samples with multiple HPV16 integrated copies displayed high methylation levels for all CpGs suggesting that the majority of multiple copies were silenced by extensive methylation. These data support the hypothesis that differential methylation of the E2BSs 1, 3 and 4 is related to the activation of viral oncogene expression in cervical lesions as long as the viral genome remains in the episomal state. Once the virus becomes integrated into host cell chromosomes these methylation patterns may be substantially altered due to complex epigenetic changes of integrated HPV genomes.

Differential methylation of the HPV 16 upstream regulatory region during epithelial differentiation and neoplastic transformation

High risk human papillomaviruses are squamous epitheliotropic viruses that may cause cervical and other cancers. HPV replication depends on squamous epithelial differentiation. Transformation of HPV-infected cells goes along with substantial alteration of the viral gene expression profile and preferentially occurs at transformation zones usually at the uterine cervix. Methylation of the viral genome may affect regulatory features that control transcription and replication of the viral genome. Therefore, we analyzed the methylation pattern of the HPV16 upstream regulatory region (URR) during squamous epithelial differentiation and neoplastic transformation and analyzed how shifts in the HPV URR methylome may affect viral gene expression and replication. HPV 16 positive biopsy sections encompassing all stages of an HPV infection (latent, permissive and transforming) were micro-dissected and DNA was isolated from cell fractions representing the basal, intermediate, and superficial cell layers, each, as well as from transformed p16^INK4a-positive cells. We observed fundamental changes in the methylation profile of transcription factor binding sites in the HPV16 upstream regulatory region linked to the squamous epithelial differentiation stage. Squamous epithelial transformation indicated by p16^INK4a overexpression was associated with methylation of the distal E2 binding site 1 leading to hyper-activation of the HPV 16 URR. Adjacent normal but HPV 16-infected epithelial areas retained hyper-methylated HPV DNA suggesting that these viral genomes were inactivated. These data suggest that distinct shifts of the HPV 16 methylome are linked to differentiation dependent transcription and replication control and may trigger neoplastic transformation.

Codon usage roles in human papillomavirus

Human papillomavirus (HPV) genomes, similar to other virus genomes, frequently have a G + C content significantly different from their host species. The HPV genomes show a strong codon usage bias to 18 codons, with 14 showing T at the third position amongst degenerately encoded amino acids. The codon usage pattern in HPV genome plays an important role, which regulates low or non-translational expression of the viral capsid genes and results in very weak protein expression of oncogenes in a wide range of mammalian cells. Codon modification has been proved to be a powerful technology to overcome the translational blockage and weak expression of both HPV capsid genes and oncogenes in different expression systems. Furthermore, keratinocytes are the host cells of HPV infection; the codon usage in HPV capsid genes matches available aminoacyl-tRNAs in differentiated keratinocytes to modulate their protein expression. HPV DNA vaccines with codon optimization have been shown to have higher immunogenicity and induce both strong cellular and humoral responses in animal models, which may be a promising form of therapeutic HPV vaccines.

Comprehensive comparison of the interaction of the E2 master regulator with its cognate target DNA sites in 73 human papillomavirus types by sequence statistics

Mucosal human papillomaviruses (HPVs) are etiological agents of oral, anal and genital cancer. Properties of high- and low-risk HPV types cannot be reduced to discrete molecular traits. The E2 protein regulates viral replication and transcription through a finely tuned interaction with four sites at the upstream regulatory region of the genome. A computational study of the E2–DNA interaction in all 73 types within the alpha papillomavirus genus, including all known mucosal types, indicates that E2 proteins have similar DNA discrimination properties. Differences in E2–DNA interaction among HPV types lie mostly in the target DNA sequence, as opposed to the amino acid sequence of the conserved DNA-binding alpha helix of E2. Sequence logos of natural and in vitro selected sites show an asymmetric pattern of conservation arising from indirect readout, and reveal evolutionary pressure for a putative methylation site. Based on DNA sequences only, we could predict differences in binding energies with a standard deviation of 0.64 kcal/mol. These energies cluster into six discrete affinity hierarchies and uncovered a fifth E2-binding site in the genome of six HPV types. Finally, certain distances between sites, affinity hierarchies and their eventual changes upon methylation, are statistically associated with high-risk types.

Thiothymidine plus low-dose UVA kills hyperproliferative human skin cells independently of their human papilloma virus status

The thymidine analogue 4-thiothymidine (S(4)TdR) is a photosensitizer for UVA radiation. The UV absorbance spectrum of S(4)TdR and its incorporation into DNA suggests that it might act synergistically with nonlethal doses of UVA to selectively kill hyperproliferative or cancerous skin cells. We show here that nontoxic concentrations of S(4)TdR combine with nonlethal doses of UVA to kill proliferating cultured skin cells. Established cell lines with a high fraction of proliferating cells were more sensitive than primary keratinocytes or fibroblasts to apoptosis induction by S(4)TdR/UVA. Although S(4)TdR plus UVA treatment induces stabilization of p53, cell death, as measured by apoptosis or clonal survival, occurs to a similar extent in both p53 wild-type and p53-null backgrounds. Furthermore, different types of human papilloma virus E6 proteins, which protect against UVB-induced apoptosis, have little effect on killing by S(4)TdR/UVA. S(4)TdR/UVA offers a possible therapeutic intervention strategy that seems to be applicable to human papilloma virus-associated skin lesions.

Interactions of the cellular CCAAT displacement protein and human papillomavirus E2 protein with the viral origin of replication can regulate DNA replication

Previously, we and others have shown that CCAAT displacement protein (CDP) negatively regulates the papillomavirus promoters. Overexpression of CDP has been shown to inhibit high-risk human papillomavirus virus (HPV) and bovine papillomavirus DNA replication in vivo presumably through reduction in expression of viral replication proteins, E1 and E2. Sequence analysis of the HPV origin indicates several potential CDP-binding sites with one site overlapping the E1-binding site. Therefore, CDP could also negatively regulate papillomavirus replication directly by preventing the loading of the initiation complex. We show here that purified CDP inhibits in vitro HPV DNA replication. Footprint analysis demonstrated that CDP binds the E1-binding site and the TATA box, and that the binding of purified CDP to the E1-binding site is decreased by the addition of purified E2 protein. Consistent with this, E2-independent in vitro HPV replication is inhibited by CDP to a greater extent than E2-dependent replication. These results suggest that binding of E2 at the E2-binding site may play an important role in overcoming the inhibition of E1 initiation complex formation caused by the binding of negative regulators like CDP to the origin of replication.

Genetic analysis of the human papillomavirus type 31 differentiation-dependent late promoter

Human papillomaviruses infect stratifying squamous epithelia, causing benign and malignant lesions. Upon differentiation of the host keratinocyte, the virus undergoes a dramatic increase in both DNA replication and transcription from the late promoter, leading to expression of late genes and virion morphogenesis. In human papillomavirus type 31 (HPV31), the late promoter is designated p742 and includes multiple start sites embedded within the E7 gene. In this report, we mapped viral DNA elements that control transcriptional activity from p742. Enhancer elements in the viral upstream regulatory region positively regulate this promoter. The region containing the transcriptional start sites is dispensable for activity, and at least two separate elements in the E6/E7 region are capable of supporting transcription. Of these, we mapped one to a 150-bp region of the E7 open reading frame and designate it the core p742 promoter. Using GF109203X, an inhibitor of protein kinase C signaling, we show that p742 activation is independent of viral genome amplification. Finally, we mapped elements in the region of p742 that confer responsiveness to differentiation and show that the upstream regulatory region does not contribute to the differentiation response of p742. These studies are an important step toward understanding the functioning and regulation of this multiple-start promoter.

Human papillomavirus type 77 E6 protein selectively inhibits p53-dependent transcription of proapoptotic genes following UV-B irradiation

DNA damage, such as that elicited by UV-B, can induce either a cell cycle arrest or apoptosis that can be signalled by the p53 protein through the activation of a number of downstream cellular target genes. In contrast to oncogenic anogenital human papillomaviruses (HPVs), which mediate proteolytic degradation of p53, the E6 protein of cutaneous HPVs, such as HPV 77, do not promote p53 degradation. We have previously shown, however, that expression of HPV 77 E6 can effectively block UV-induced apoptosis in cells that have UV-activated p53. Here, we report that expression of the E6 protein from the cutaneous HPV 77 attenuates the UV-induced transactivation of p53-regulated proapoptotic genes Fas, PUMAbeta, Apaf-1, PIG3. This inhibition of p53-activation of proapoptotic genes by HPV77 E6 is exerted selectively, as the increased expression of p53 target genes involved in cell cycle arrest or regulatory functions regulation, such as p21 and Hdm2, is unaffected. Our data suggest that HPV 77 E6 may play an important role in specifically deregulating p53-dependent transactivation of proapoptotic genes upon UV-B irradiation.

Methylation patterns of papillomavirus DNA, its influence on E2 function, and implications in viral infection

The biological activities of the papillomavirus E2 protein in transcription, replication, and maintenance of the papillomavirus genome rely on the E2 protein's ability to bind that genome specifically. The E2 binding sites (E2BSs), located within the long control region (LCR) of human papillomavirus (HPV) genomes, contain potential sites for 5'methylation at cytosine (CpG) residues. The E2 protein's capacity to bind E2BS in vitro is inhibited by methylation of these cytosines (59). Herein, we describe experiments to assess the influence of methylation on E2 function in cells. E2's ability to activate transcription was inhibited by the global methylation of CpG dinucleotides in E2-responsive transcriptional templates or when only the CpG dinucleotides within the E2BSs of a transcriptional template were methylated. Thus at least one biological activity of E2 that is dependent on its ability to bind DNA in a site-specific manner is influenced by the methylation status of its cognate binding site. The activity of DNA methylases is influenced by the differentiation status of mammalian cells. The life cycle of HPVs is tied to the differentiation of its host cells within stratified squamous epithelia. To investigate whether methylation of the papillomavirus genomes is influenced by the differentiation status of host epithelial cells, we analyzed HPV16 DNA harvested from a cervical epithelial cell line that was isolated from an HPV16-infected patient. We found, using bisulfite treatment to discriminate between methylated and unmethylated cytosines, that the HPV16 LCR was selectively hypomethylated in highly differentiated cell populations. In contrast, the HPV16 LCR from poorly differentiated, basal cell-like cells contained multiple methylated cytosines and were often methylated at E2BSs, particularly E2BS(2). These experiments indicate that the methylation state of the viral genome, and particular that of E2BSs, may vary during the viral life cycle, providing a novel means for modulating E2 function. These studies also uncovered an extensive pattern of methylation at non-CpG dinucleotides indicative of de novo methylation. The potential implications of this de novo methylation pattern are discussed.

Identification of the E9/E2C cDNA and functional characterization of the gene product reveal a new repressor of transcription and replication in cottontail rabbit papillomavirus

Cottontail rabbit papillomavirus (CRPV) genomes mutated in the trans-activation domain of the E2 protein, which stimulates both viral DNA replication and transcription, are severely impaired in their ability to induce tumors in New Zealand White rabbits. A number of papillomaviruses encode, in addition to full-length E2, a shortened E2 protein or an E2 protein fused to a short stretch of amino acids derived from the small E8 open reading frame that counteract the activities of E2. We identified and cloned the novel cDNA E9/E2C of CRPV from papillomas of New Zealand White and cottontail rabbits and characterized the functions of the encoded gene product. E9/E2C was shown to be a bona fide repressor of minimal viral promoters, with the E9 domain being essential for this activity, and to repress E1/E2-dependent replication of a CRPV origin construct. In addition, E9/E2C counteracted the transactivation effect of the full-length E2 on minimal promoters containing several E2 binding sites. To investigate the role of E9/E2C in tumorigenesis, we constructed two CRPV genomes mutated in E9/E2C. One, designated CRPV-E9atgmut-pLAII, contained a mutation in the unique start codon in the E9 open reading frame, and the second E9/E2C mutant was constructed by the introduction of a stop codon close to the splice donor site at nucleotide 3714 that additionally prevented the correct splicing of the transcript. When we infected New Zealand White rabbits with these constructs, we surprisingly noted no differences in tumor induction efficiency, viral genome copy number, and viral transcription in comparison to wild-type CRPV.

A transactivator function of cottontail rabbit papillomavirus e2 is essential for tumor induction in rabbits

Infection of domestic rabbits with cottontail rabbit papillomavirus (CRPV) causes local papillomas which progress to carcinomas in more than 80% of cases. This animal model system therefore allows the identification of molecular mechanisms required for the induction and progression of epithelial tumors. The viral E2 protein stimulates both viral DNA replication and transcription, and these functions can be genetically separated. We introduced the respective mutations into CRPV E2 and found, in line with published data for other papillomavirus E2 proteins, that mutation of the highly conserved amino acid 37 or 73 resulted in replication-competent but transactivation-deficient E2 proteins, whereas E2 proteins with mutations at residue 39 were replication deficient and transactivation competent. The R37A, I73L, and I73A E2 mutants, showing a loss of transactivation function, and the R37K E2 mutant, which is still transactivation competent, were introduced into the whole genome of CRPV, which was then injected into the skin of rabbits. Strikingly, the ability to induce tumors within 6 weeks was abolished by each of the E2 mutations, in contrast to the tumor induction rate (93%) obtained with wild-type CRPV DNA. Two small papillomas induced by mutant E2 I73A CRPV DNA appeared as late as 12 or 24 weeks postinjection, were significantly smaller, and showed no further extension of growth. These data suggest that functionally conserved amino acids in the transactivation domain of E2 are also required for the induction and growth of epithelial tumors in rabbits infected with CRPV.

Immortalization of normal adult human middle ear epithelial cells using a retrovirus containing the E6/E7 genes of human papillomavirus type 16

A human middle ear epithelial cell line (HMEEC-1) was established using human papillomavirus E6/E7 genes. HMEEC-1 has remained morphologically and phenotypically stable, even after 50 passages. The cells are anchorage-dependent and nontumorigenic when injected into nude mice. This cell line thus provides a new tool for the study of normal cell biology and the pathological processes associated with the epithelial cells of the middle ear in otitis media. HMEEC-1 will also be useful in the search for new drugs and biological agents for the treatment of otitis media.

Gene Expression of Genital Human Papillomaviruses and Considerations on Potential Antiviral Approaches

Genital human papillomaviruses (HPVs) are carcinogenic to humans and are associated with most cases of cervical cancer, genital and laryngeal warts, and certain cutaneous neoplastic lesions. Five of the more than 50 known genital HPV types, HPV-6, -11, -16, -18 and -31, have become the models to study gene expression. The comparison of the studies of these five viruses and analyses of the genomic sequences of those genital HPV types that have not been transcriptionally studied make it likely that genital HPVs share most strategies for regulating their transcription. These strategies are quite different from those of unrelated human and animal papillomaviruses. Among these common properties are (i) a specific promoter structure allowing for fine-tuned negative feedback, (ii) a transcriptional enhancer that is specific for epithelial cells, (iii) regulation by progesterone and glucocorticoid hormones, (iv) silencers, whose principal function appears to be transcriptional repression in the basal layer of infected epithelia, (v) specifically positioned nucleosomes that mediate the functions of some enhancer and the silencer factors, (vi) nuclear matrix attachment regions that can, under different conditions, repress or stimulate transcription, and (vii) as yet poorly understood late promoters positioned very remote from the late genes. Most of these properties are controlled by cellular proteins that, due to their simultaneous importance for cellular processes, may not be useful as HPV-specific drug targets. It should be possible, however, to target complex cis-responsive elements unique to these HPV genomes by nucleotide sequence-specific molecules, such as antisense RNA, polyamides and artificial transcription factors. The application of small molecule-based drugs may be restricted to target proteins encoded by the HPV DNA, such as the replication factor E1 and the transcription/replication factor E2.

Displacement of YY1 by differentiation-specific transcription factor hSkn-1a activates the P(670) promoter of human papillomavirus type 16

Transcription from human papillomavirus type 16 (HPV16) P(670), a promoter in the E7 open reading frame, is repressed in undifferentiated keratinocytes but becomes activated upon differentiation. We showed that the transient luciferase expression driven by P(670) was markedly enhanced in HeLa cells cotransfected with an expression plasmid for human Skn-1a (hSkn-1a), a transcription factor specific to differentiating keratinocytes. The hSkn-1a POU domain alone, which mediates sequence-specific DNA binding, was sufficient to activate the expression of luciferase. Electrophoretic mobility shift assay revealed the presence of two binding sites, sites 1 and 2, upstream of P(670), which were shared by hSkn-1a and YY1. Site 1 bound more strongly to hSkn-1a than site 2 did. YY1 complexing with a short DNA fragment having site 1 was displaced by hSkn-1a, indicating that hSkn-1a's affinity with site 1 was stronger than YY1's. Disrupting the binding sites by nucleotide substitutions raised the basal expression level of luciferase and decreased the enhancing effect of hSkn-1a. In HeLa cells transfected with circular HPV16 DNA along with the expression plasmid for hSkn-1a, the transcript from P(670) was detectable, which indicates that the results obtained with the reporter plasmids are likely to have mimicked the regulation of P(670) in authentic HPV16 DNA. The data strongly suggest that the transcription from P(670) is repressed primarily by YY1 binding to the two sites, and the displacement of YY1 by hSkn-1a releases P(670) from the repression.

Expression of a transforming gene (E5) of bovine papillomavirus in sarcoids obtained from horses

OBJECTIVE

To determine expression of a transforming gene (E5) of bovine papillomavirus in sarcoids, other tumors, and normal skin samples collected from horses with and without sarcoids.

SAMPLE POPULATION

23 sarcoids and 6 samples of normal skin obtained from 16 horses with sarcoids, 2 samples of normal skin and 2 papillomas obtained from horses without sarcoids, and 1 papilloma obtained from a cow.

PROCEDURE

Protein was extracted from tissue samples collected from horses and incubated with agarose beads covalently coupled to Staphylococcus aureus protein A and an anti-E5 polyclonal antibody. Following incubation, proteins were eluted from the beads and electrophoresed on a 14% polyacrylamide gel and transferred to a polyvinylidene difluoride membrane. The E5 protein was detected by use of western blot analysis, using a chemiluminescence detection system.

RESULTS

All 23 sarcoids had positive results for expression of E5 protein. Quantity of viral protein appeared to vary among sarcoids. All other tissues examined had negative results for E5 protein. Highest expression for E5 protein was observed in biologically aggressive fibroblastic variants of sarcoids, compared with expression in quiescent tumors.

CONCLUSIONS AND CLINICAL RELEVANCE

This study documented that activation and expression of the E5 gene is evident in sarcoids obtained from horses. These data support the conclusion that infection with bovine papillomavirus is important in the initiation or progression of sarcoids in horses. Treatment strategies designed to increase immune recognition of virally infected cells are warranted.

Expression of the peptide antibiotics human beta defensin-1 and human beta defensin-2 in normal human skin

Normal human skin is remarkably resistant to infection from the large numbers of microorganisms that routinely colonize its surface. In addition to the role of skin as a mechanical barrier, it has long been recognized that skin and other epithelia can produce a range of anti-microbial chemicals that play an important part in eliminating potential cutaneous pathogens. Anti-microbial peptides are an important evolutionarily conserved innate host defense mechanism in many organisms. Human beta defensin-1 and -2 are cysteine-rich, cationic, low molecular weight anti-microbial peptides that have recently been shown to be expressed in epithelial tissues. In this study, we describe the characterization of human beta defensin-1 and -2 mRNA and peptide expression in normal human skin. Using reverse transcription-polymerase chain reaction we demonstrate that human beta defensin-1 is consistently expressed in skin samples from various body sites. Human beta defensin-2 demonstrates expression that is more variable and is more readily detectable in facial skin and foreskin compared with skin from abdomen and breast. In situ hybridization localizes the human beta defensin-1 and -2 transcripts to keratinocytes within interfollicular skin. Using specific antibodies, we have shown that human beta defensin-1 and -2 peptides are localized to the Malpighian layer of the epidermis and/or stratum corneum and that there are interindividual and site-specific differences in intensity of immunostaining and the pattern of peptide localization. The localization of human beta defensins to the outer layer of the skin is consistent with the hypothesis that human beta defensins play an essential part in cutaneous innate immunity.

The E8 domain confers a novel long-distance transcriptional repression activity on the E8E2C protein of high-risk human papillomavirus type 31

Infections with high-risk human papillomaviruses (HPVs) are the major risk factor for the development of anogenital cancers. Viral E2 proteins are involved in viral DNA replication and regulation of transcription. Repression of the viral P97 promoter by E2 proteins has been implicated in the modulation of the immortalization capacity and DNA replication properties of high-risk HPVs. Analysis of the cis and trans requirements for repression of the HPV type 31 (HPV31) P97 promoter, however, revealed striking differences between the full-length E2 and the E8E2C fusion protein which were due to conserved residues W6 and K7 of the E8 domain. In contrast to E2, E8E2C completely inhibited the P97 promoter from a single promoter-distal E2 binding site. This novel long-distance repression activity of the E8 domain also enabled E8E2C to inhibit the HPV6a P2 promoter and minimal-promoter constructs containing E2 binding sites. Thus, E8E2C may represent the master repressor of viral gene expression during a high-risk HPV infection, and changes in the activity of E8E2C might contribute to the progression of high-risk HPV-induced lesions.

Role of Bak in UV-induced apoptosis in skin cancer and abrogation by HPV E6 proteins

Ultraviolet B (UVB) damage is recognized as the most important etiological factor in the development of skin cancer. Human papillomaviruses (HPV) have also been implicated in the disease, although the mechanism of action of these viruses remains unknown. We present evidence here that Bak protein is involved in signaling apoptosis in the skin in response to UVB damage, and that cutaneous HPV E6 proteins target and abrogate Bak function by promoting its proteolytic degradation both in vitro and in regenerated epithelium. Additionally, HPV positive skin cancers had undetectable levels of Bak in contrast to HPV negative cancers, which expressed Bak. This study supports a link between the virus and UVB in the induction of HPV-associated skin cancer and reveals a survival mechanism of virally infected cells.

The structural basis of DNA target discrimination by papillomavirus E2 proteins

The papillomavirus E2 proteins regulate the transcription of all papillomavirus genes and are necessary for viral DNA replication. Disruption of the E2 gene is commonly associated with malignancy in cervical carcinoma, indicating that E2 has a role in regulating tumor progression. Although the E2 proteins from all characterized papillomaviruses bind specifically to the same 12-base pair DNA sequence, the cancer-associated human papillomavirus E2 proteins display a unique ability to detect DNA flexibility and intrinsic curvature. To understand the structural basis for this phenomenon, we have determined the crystal structures of the human papillomavirus-18 E2 DNA-binding domain and its complexes with high and low affinity binding sites. The E2 protein is a dimeric beta-barrel and the E2-DNA interaction is accompanied by a large deformation of the DNA as it conforms to the E2 surface. DNA conformation and E2-DNA contacts are similar in both high and low affinity complexes. The differences in affinity correlate with the flexibility of the DNA sequence. Preferences of E2 proteins from different papillomavirus strains for flexible or prevent DNA targets correlate with the distribution of positive charge on their DNA interaction surfaces, suggesting a role for electrostatic forces in the recognition of DNA deformability.

HPV positive bronchopulmonary carcinomas in women with previous high-grade cervical intraepithelial neoplasia (CIN III)

A significant higher incidence of some cancers, especially lung cancer, has been found in women with previous HPV-related (human papillomavirus) urogenital and anal neoplasias than in individuals without this particular clinical history. The aim of our study was to investigate whether HPV is present in both CIN III (cervical intraepithelial neoplasia) lesions and bronchopulmonary second primary cancers in women with a clinical history of both diseases. Paraffin-embedded tumour tissue from 75 patients with bronchopulmonary carcinomas was examined using the polymerase chain reaction (PCR) technique and in situ hybridization for the presence of human HPV. In total, 51 primary tumours without metastases, 11 primary tumours with metastases and 13 lymph node metastases without available tissue from primary tumours were analysed. In our study 37/75 primary bronchopulmonary tumours (49%) were identified as HPV positive by the PCR method: 18 cases were purely HPV 16 positive (49%), 12 were purely HPV 6 positive (32%), 5 cases were HPV 16/6 positive (14%), 1 case was HPV 16/11 positive (2%) and 1 case was HPV 16/18 positive (2%). Fourteen metastases were HPV positive, and HPV 16, 11 and 6 were detected in both regional and distant metastases. Two of the HPV 16-positive metastases were brain metastases from two separate HPV 16-positive primary tumours; 35% of the HPV-positive cases were adenocarcinomas, 30% squamous cell carcinomas, 22% oat cell carcinomas, 5% large cell carcinomas, 3% anaplastic carcinoma, 3% low-differentiated carcinoma, and 3% malignant cylindroma. The CIN III lesions from 34 of the 37 HPV-positive bronchopulmonary carcinomas were analysed by PCR. The overall HPV positivity in the CIN III lesions was 74% (25/34 cases): 48% were purely HPV 16 positive, 24% purely HPV 6 positive, 24% HPV 16/6 positive and 4% were HPV 18 positive. Our results indicate that HPV is also involved in the development of bronchopulmonary cancers in women with a history of CIN III lesions.

Nucleotide sequence and characterization of human papillomavirus type 83, a novel genital papillomavirus

Studies of human papillomaviruses (HPV) are hampered by the lack of a conventional culture system, because HPV completes its life cycle only in fully differentiated human tissue. To overcome this obstacle, the athymic mouse xenograft system has been used to study the pathogenesis of a limited number of HPV types. We recently reported the propagation of a novel HPV type in the mouse xenograft system and the cloning of its genome. Consensus primer PCR had previously identified this virus as MM7, LVX82, or PAP291. Here we report the nucleotide sequence of the 8104-bp genome of this virus, now called HPV 83. HPV 83 is most closely related to HPV 61 and HPV 72, placing it in the papillomavirus genome homology group A3. Based on limited epidemiological data, the histological appearance of infected human foreskin implants, and the structure of the predicted HPV 83 E7 protein, this virus is probably of at least intermediate cancer risk. Like other papillomaviruses, HPV 83 produces an E1 E4, E5 transcript, but the position of the splice acceptor differs from that of other HPVs. The presence of an E5 open reading frame in the HPV 83 genome is uncertain; the most likely candidate to be the HPV 83 E5 protein has some structural similarity to the bovine papillomavirus 1 E5 oncoprotein, and is unlike most other HPV E5 proteins. HPV 83 is a relatively prevalent genital papillomavirus that has the largest genome of any characterized HPV and several other novel structural features that merit further study.

CCAAT displacement protein binds to and negatively regulates human papillomavirus type 6 E6, E7, and E1 promoters

Expression of human papillomavirus genes increases as the target cell, the keratinocyte, differentiates. CCAAT displacement protein (CDP) is a cellular protein which has been shown in other cell types to negatively regulate gene expression in undifferentiated cells but not in differentiated cells. We have previously shown that a 66-bp purine-thymidine-rich sequence (the 66-mer) binds CDP and negatively regulates the human papillomavirus type 6 (HPV-6) E6 promoter (S. Pattison, D. G. Skalnik, and A. Roman, J. Virol. 71:2013-2022, 1997). Cotransfection experiments with a plasmid expressing luciferase from the HPV-6 E6, E7, or E1 regulatory region and a plasmid carrying the CDP gene indicate that CDP represses transcription from all three HPV-6 promoters. Using electrophoretic mobility shift assays (EMSAs), we have shown that CDP binds HPV-6 both upstream and downstream of the E6, E7, and E1 transcription initiation start sites. Furthermore, when keratinocytes were induced to differentiate, all three promoter activities increased. Consistent with this, immunoblotting and EMSAs revealed that endogenous nucleus CDP and, correspondingly, DNA binding activity decreased when keratinocytes were induced to differentiate. The elevated promoter activities were abrogated by exogenously transfected CDP. Our data demonstrate that CDP fulfills the requirement of a differentiation-dependent negative regulator that could tie the HPV life cycle to keratinocyte differentiation.

Evidence for the separate regulation of the human papillomavirus type 11 E7 and E6 promoters by Viral cis sequences near the E6 promoter

The human papillomavirus type 11 (HPV-11) E7 protein can modulate host cell functions and is required for papilloma formation, but little is known concerning the regulation of its expression. This study was designed to determine whether the viral upstream regulatory region controlled expression from the E7 promoter and whether cis sequences differentially regulated E6 and E7 expression in laryngeal mucosal keratinocytes, the natural target cells for this virus. Reporter constructs were designed to study expression of the luciferase gene from the HPV-11 E7 promoter in its natural position downstream of a functional E6 promoter. E7 expression, like E6 expression, required upstream regulatory sequences. However, E7 expression was less sensitive to repression by viral E2 protein and to mutation of the Spl binding site adjacent to the E2 binding site. Moreover, there was differential sensitivity of the two promoters to mutation of the E6 TATA box, with E7 expression more affected than E6 expression. These findings show that, in the normal host cells for this virus, the E6 and E7 promoters can be independently regulated by the cis regulatory region adjacent to the E6 promoter.