Predicted alpha-helix/beta-sheet secondary structures for the zinc-binding motifs of human papillomavirus E7 and E6 proteins by consensus prediction averaging and spectroscopic studies of E7

Structural dynamic studies on identification of EGCG analogues for the inhibition of Human Papillomavirus E7

High risk human papillomaviruses are highly associated with the cervical carcinoma and the other genital tumors. Development of cervical cancer passes through the multistep process initiated from benign cyst to increasingly severe premalignant dysplastic lesions in an epithelium. Replication of this virus occurs in the fatal differentiating epithelium and involves in the activation of cellular DNA replication proteins. The oncoprotein E7 of human papillomavirus expressed in the lower epithelial layers constrains the cells into S-phase constructing an environment favorable for genome replication and cell proliferation. To date, no suitable drug molecules exist to treat HPV infection whereas anticipation of novel anti-HPV chemotherapies with distinctive mode of actions and identification of potential drugs are crucial to a greater extent. Hence, our present study focused on identification of compounds analogue to EGCG, a green tea molecule which is considered to be safe to use for mammalian systems towards treatment of cancer. A three dimensional similarity search on the small molecule library from natural product database using EGCG identified 11 potential small molecules based on their structural similarity. The docking strategies were implemented with acquired small molecules and identification of the key interactions between protein and compounds were carried out through binding free energy calculations. The conformational changes between the apoprotein and complexes were analyzed through simulation performed thrice demonstrating the dynamical and structural effects of the protein induced by the compounds signifying the domination. The analysis of the conformational stability provoked us to describe the features of the best identified small molecules through electronic structure calculations. Overall, our study provides the basis for structural insights of the identified potential identified small molecules and EGCG. Hence, the identified analogue of EGCG can be potent inhibitors against the HPV 16 E7 oncoprotein.

A Preliminary Study of the Virome of the South American Free-Tailed Bats (Tadarida brasiliensis) and Identification of Two Novel Mammalian Viruses

Bats provide important ecosystem services as pollinators, seed dispersers, and/or insect controllers, but they have also been found harboring different viruses with zoonotic potential. Virome studies in bats distributed in Asia, Africa, Europe, and North America have increased dramatically over the past decade, whereas information on viruses infecting South American species is scarce. We explored the virome of Tadarida brasiliensis, an insectivorous New World bat species inhabiting a maternity colony in Rosario (Argentina), by a metagenomic approach. The analysis of five pooled oral/anal swab samples indicated the presence of 43 different taxonomic viral families infecting a wide range of hosts. By conventional nucleic acid detection techniques and/or bioinformatics approaches, the genomes of two novel viruses were completely covered clustering into the Papillomaviridae (Tadarida brasiliensis papillomavirus type 1, TbraPV1) and Genomoviridae (Tadarida brasiliensis gemykibivirus 1, TbGkyV1) families. TbraPV1 is the first papillomavirus type identified in this host and the prototype of a novel genus. TbGkyV1 is the first genomovirus reported in New World bats and constitutes a new species within the genus Gemykibivirus. Our findings extend the knowledge about oral/anal viromes of a South American bat species and contribute to understand the evolution and genetic diversity of the novel characterized viruses.

High prevalence of Gammapapillomaviruses (Gamma-PVs) in pre-malignant cutaneous lesions of immunocompetent individuals using a new broad-spectrum primer system, and identification of HPV210, a novel Gamma-PV type

Genus Gammapapillomavirus (Gamma-PV) is the most diverse and largest clade within the Papillomaviridae family. A novel set of degenerate primers targeting the E1 gene was designed and further used in combination with the well-known CUT PCR assay to assess HPV prevalence and genus distribution in a variety of cutaneous samples from 448 immunocompetent individuals. General HPV, Gamma-PV and mixed infections prevalence were significantly higher in actinic keratosis with respect to benign and malignant neoplasms, respectively (p = 0.0047, p = 0.0172, p = 0.00001). Gamma-PVs were significantly more common in actinic keratosis biopsies than Beta- and Alpha-PVs (p = 0.002). The full-length genome sequence of a novel putative Gamma-PV type was amplified by 'hanging droplet' long-range PCR and cloned. The novel virus, designated HPV210, clustered within species Gamma-12. This study provides an additional tool enabling detection of HPV infections in skin and adds new insights about possible early roles of Gamma-PVs in the development of cutaneous malignant lesions.

Differences of immune disorders between Alzheimer's disease and breast cancer based on transcriptional regulation

Although chronic inflammation and immune disorders are of great importance to the pathogenesis of both dementia and cancer, the pathophysiological mechanisms are not clearly understood. In recent years, growing epidemiological evidence and meta-analysis data suggest an inverse association between Alzheimer’s disease (AD), which is the most common form of dementia, and cancer. It has been revealed that some common genes and biological processes play opposite roles in AD and cancer; however, the biological immune mechanism for the inverse association is not clearly defined. An unsupervised matrix decomposition two-stage bioinformatics procedure was adopted to investigate the opposite behaviors of the immune response in AD and breast cancer (BC) and to discover the underlying transcriptional regulatory mechanisms. Fast independent component analysis (FastICA) was applied to extract significant genes from AD and BC microarray gene expression data. Based on the extracted data, the shared transcription factors (TFs) from AD and BC were captured. Second, the network component analysis (NCA) algorithm in this study was presented to quantitatively deduce the TF activities and regulatory influences because quantitative dynamic regulatory information for TFs is not available via microarray techniques. Based on the NCA results and reconstructed transcriptional regulatory networks, inverse regulatory processes and some known innate immune responses were described in detail. Many of the shared TFs and their regulatory processes were found to be closely related to the adaptive immune response from dramatically different directions and to play crucial roles in both AD and BC pathogenesis. From the above findings, the opposing cellular behaviors demonstrate an invaluable opportunity to gain insights into the pathogenesis of these two types of diseases and to aid in developing new treatments.

Natural history of human papillomavirus infection of sun-exposed healthy skin of immunocompetent individuals over three climatic seasons and identification of HPV209, a novel betapapillomavirus

We present the first longitudinal study reporting the natural history of human papillomavirus (HPV) infection in sun-exposed skin of healthy individuals living in a geographical area in which solar UV radiation is influenced by the ozone content of the atmosphere. During three climatic seasons, skin swab samples were obtained from 78 healthy individuals and the prevalence of cutaneous HPVs was assessed with broad-spectrum FAP and CUT primers and determined at 54, 45 and 47 % in spring, summer and winter, respectively. Frequencies of mixed HPV infections were significantly higher in spring with respect to summer and winter (P=0.02). Seventy-one different HPV types/putative types were identified. While 62 volunteers were HPV-infected in at least one season, 23 had persistent infections. β-PVs (β-1) were the most prevalent and persistent. Age was associated with both the infection status (P=0.01) and the type of HPV infection (no infection, indeterminate/transient, persistent P=0.02). The molecular/phylogenetic analysis of the newly identified β-PV, officially designated as HPV209, showed that the virus has a typical genomic organization of cutaneous HPVs with five early (E6, E7, E1, E2 and E4) and two late genes (L2 and L1), which clusters to the species β-2. This provides useful data on cutaneous HPV infections in high UV-exposed regions.

Zinc induces apoptosis on cervical carcinoma cells by p53-dependent and -independent pathway

OBJECTIVE

There is evidence that the mineral zinc is involved in the apoptotic cell death of various carcinoma cells. In this study, we aim to determine whether zinc in the form of CIZAR induces apoptosis in cervical carcinoma cells by increasing intracellular zinc concentration.

STUDY DESIGN

CaSki and HeLa cervical carcinoma cells and HPV-16 DNA-transformed keratinocyte (CRL2404) were treated with different concentrations of CIZAR. The cell viability test was carried out, the intracellular level of zinc was determined, and apoptosis was confirmed by flow cytometry after propidium iodide (PI) staining and fluorescence microscopy under DAPI staining. The expression of cell-cycle regulators was analyzed by Western blot, including the knock down of p53 and expression of HPV E6 and E7 genes by RT-PCR.

RESULTS

Intracellular zinc accumulation induced the down-regulation of E6/E7 proteins through targeting of the specific transcriptional factors in the upstream regulatory region. p53 was induced after CIZAR treatment and p53-dependent apoptosis did not occur after knock down by p53 siRNA. In cervical carcinoma cells, regardless of HPV-infection, CIZAR induces apoptosis by the activation of the p53-independent pathways through the up-regulation of p21waf1, the down-regulation of c-Myc, and by decreasing the Bcl-2/Bax ratio.

CONCLUSIONS

CIZAR induces apoptosis not only through the restoration of p53/Rb-dependent pathways in HPV-positive cells, but also through the activation of p53/Rb-independent pathways and the mitochondrial death-signal pathway in cervical carcinoma cells regardless of HPV-infection.

Characterization of novel human papillomavirus types 157, 158 and 205 from healthy skin and recombination analysis in genus γ-Papillomavirus

Gammapapillomavirus (γ-PV) is a diverse and rapidly expanding genus, currently consisting of 79 fully characterized human PV (HPV) types. In this study, three novel types, HPV157, HPV158 and HPV205, obtained from healthy sun-exposed skin of two immunocompetent individuals, were amplified by the "Hanging droplet" long PCR technique, cloned, sequenced and characterized. HPV157, HPV158 and HPV205 genomes comprise 7154-bp, 7192-bp and 7298-bp, respectively, and contain four early (E1, E2, E6 and E7) and two late genes (L1 and L2). Phylogenetic analysis of the L1 ORF placed all novel types within the γ-PV genus: HPV157 was classified as a new member of species γ-12 while HPV158 and HPV205 belong to species γ-1. We then explored potential recombination events in genus γ-PV with the RDP4 program in a dataset of 74 viruses (71 HPV types with available full-length genomes and the 3 novel types). Two events, both located in the E1 ORF, met the inclusion criterion (p-values <0.05 with at least four methods) and persisted in different ORF combinations: an inter-species recombination in species γ-8 (major and minor parents: species γ-24 and γ-11, respectively), and an intra-species recombination in species γ-7 (recombinant strain: HPV170; major and minor parents: HPV-109 and HPV-149, respectively). These findings were confirmed by phylogenetic tree incongruence analysis. An additional incongruence was found in members of species γ-9 but it was not detected by the RDP4. This report expands our knowledge of the family Papillomaviridae and provides for the first time in silico evidence of recombination in genus γ-PV.

Mutagenic Potential ofBos taurus Papillomavirus Type 1 E6 Recombinant Protein: First Description

Bovine papillomavirus (BPV) is considered a useful model to study HPV oncogenic process. BPV interacts with the host chromatin, resulting in DNA damage, which is attributed to E5, E6, and E7 viral oncoproteins activity. However, the oncogenic mechanisms of BPV E6 oncoprotein per se remain unknown. This study aimed to evaluate the mutagenic potential of Bos taurus papillomavirus type 1 (BPV-1) E6 recombinant oncoprotein by the cytokinesis-block micronucleus assay (CBMNA) and comet assay (CA). Peripheral blood samples of five calves were collected. Samples were subjected to molecular diagnosis, which did not reveal presence of BPV sequences. Samples were treated with 1 μg/mL of BPV-1 E6 oncoprotein and 50 μg/mL of cyclophosphamide (positive control). Negative controls were not submitted to any treatment. The samples were submitted to the CBMNA and CA. The results showed that BPV E6 oncoprotein induces clastogenesis per se, which is indicative of genomic instability. These results allowed better understanding the mechanism of cancer promotion associated with the BPV E6 oncoprotein and revealed that this oncoprotein can induce carcinogenesis per se. E6 recombinant oncoprotein has been suggested as a possible vaccine candidate. Results pointed out that BPV E6 recombinant oncoprotein modifications are required to use it as vaccine.

Characterization of Human Papillomavirus Subtype 72b

We report the characterization of human papillomavirus (HPV) subtype 72b of the genus Alphapapillomavirus isolated from an oral rinse sample of a healthy woman. The HPV72b L1 open reading frame (ORF) was 90.2% identical to that of HPV72, indicating a subtype close to the border of a novel HPV type.

Complete genome sequences of three novel human papillomavirus types, 175, 178, and 180

We report the characterization of three novel human papillomavirus (HPV) types of the genus Gammapapillomavirus. HPV175 and HPV180 were isolated from a condyloma. HPV178 was isolated from healthy skin adjacent to an actinic keratosis.

Characterization of human papillomavirus type 154 and tissue tropism of gammapapillomaviruses

The novel human papillomavirus type 154 (HPV154) was characterized from a wart on the crena ani of a three-year-old boy. It was previously designated as the putative HPV type FADI3 by sequencing of a subgenomic FAP amplicon. We obtained the complete genome by combined methods including rolling circle amplification (RCA), genome walking through an adapted method for detection of integrated papillomavirus sequences by ligation-mediated PCR (DIPS-PCR), long-range PCR, and finally by cloning of four overlapping amplicons. Phylogenetically, the HPV154 genome clustered together with members of the proposed species Gammapapillomavirus 11, and demonstrated the highest identity in L1 to HPV136 (68.6%). The HPV154 was detected in 3% (2/62) of forehead skin swabs from healthy children. In addition, the different detection sites of 62 gammapapillomaviruses were summarized in order to analyze their tissue tropism. Several of these HPV types have been detected from multiple sources such as skin, oral, nasal, and genital sites, suggesting that the gammapapillomaviruses are generalists with a broader tissue tropism than previously appreciated. The study expands current knowledge concerning genetic diversity and tropism among HPV types in the rapidly growing gammapapillomavirus genus.

Relevance of infection with human papillomavirus: the role of the p53 tumor suppressor protein and E6/E7 zinc finger proteins (Review)

Human papillomaviruses (HPV) are small circular, double-stranded DNA viruses infecting epithelial tissues. HPV types can be classified both as high-risk or low-risk. Of the more than 120 different identified types of HPV, the majority are involved in infections of the genital tract, cancer of the cervix, vulva, vagina and penis, and of non-anogenital localizations, such as the head and neck areas. From the point of view of the infection, human papillomaviruses have developed several molecular mechanisms to enable infected cells to suppress apoptosis. This review provides a comprehensive and critical summary of the current literature that focuses on cervical carcinoma and cancer of the head and neck caused by HPV. In particular, we discuss HPV virology, the molecular mechanisms of carcinogenesis, the role of the tumor suppressor protein p53 and the E6/E7 zinc finger proteins. Classification of HPV according to diagnosis is also described.

Sequence-specific binding of recombinant Zbed4 to DNA: insights into Zbed4 participation in gene transcription and its association with other proteins

Zbed4, a member of the BED subclass of Zinc-finger proteins, is expressed in cone photoreceptors and glial Müller cells of human retina whereas it is only present in Müller cells of mouse retina. To characterize structural and functional properties of Zbed4, enough amounts of purified protein were needed. Thus, recombinant Zbed4 was expressed in E. coli and its refolding conditions optimized for the production of homogenous and functionally active protein. Zbed4’s secondary structure, determined by circular dichroism spectroscopy, showed that this protein contains 32% α-helices, 18% β-sheets, 20% turns and 30% unordered structures. CASTing was used to identify the target sites of Zbed4 in DNA. The majority of the DNA fragments obtained contained poly-Gs and some of them had, in addition, the core signature of GC boxes; a few clones had only GC-boxes. With electrophoretic mobility shift assays we demonstrated that Zbed4 binds both not only to DNA and but also to RNA oligonucleotides with very high affinity, interacting with poly-G tracts that have a minimum of 5 Gs; its binding to and GC-box consensus sequences. However, the latter binding depends on the GC-box flanking nucleotides. We also found that Zbed4 interacts in Y79 retinoblastoma cells with nuclear and cytoplasmic proteins Scaffold Attachment Factor B1 (SAFB1), estrogen receptor alpha (ERα), and cellular myosin 9 (MYH9), as shown with immunoprecipitation and mass spectrometry studies as well as gel overlay assays. In addition, immunostaining corroborated the co-localization of Zbed4 with these proteins. Most importantly, in vitro experiments using constructs containing promoters of genes directing expression of the luciferase gene, showed that Zbed4 transactivates the transcription of those promoters with poly-G tracts.

Extraction, purification and characterization of the plant-produced HPV16 subunit vaccine candidate E7 GGG

Several studies indicated that biopharmaceuticals based on the recombinant protein E7 of human papillomavirus (HPV) can serve as therapeutic vaccines preventing the development of cancer in women infected with high-risk types of HPV such as HPV16. Here, we report effective extraction and purification of a plant-produced E7GGG-lichenase fusion protein, an HPV16 subunit vaccine candidate, from Nicotiana benthamiana plants, to a high yield. The target contains the modified HPV16 E7 protein internally fused to the surface loop of a truncated, hexa-His- and KDEL-tagged variant of bacterial lichenase, and has been previously shown to possess anti-cancer activity in an animal model. We purified the protein using a combination of immobilized metal-ion affinity chromatography and gel filtration. The achieved purity of the final product was 99% as confirmed by Coomassie or SYPRO Ruby staining after sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by analytical size exclusion chromatography coupled with multi-angle laser light scattering. The overall yield was 50% corresponding to 0.1g of protein per 1 kg plant biomass. Only slight changes in these parameters were observed during the process scale-up from 50 g to 1 kg of processed leaf biomass.

New generic primer system targeting mucosal/genital and cutaneous human papillomaviruses leads to the characterization of HPV 115, a novel Beta-papillomavirus species 3

We explored the cutaneotropic HPV genetic diversity in 71 subjects from Argentina. New generic primers (CUT) targeting 88 mucosal/cutaneous HPV were designed and compared to FAP primers. Overall, 69 different HPV types/putative types were identified, being 17 of them novel putative types. Phylogenetic analysis of partial L1 sequences grouped 2 novel putative types in the Beta-PV, 14 in the Gamma-PV and 1 in the Mu-PV genera. CUT primers showed broader capacity than FAP primers in detecting different genera/species and novel putative types (p<0.01). Using overlapping PCR, the full-length genome of a Beta-PV putative type was amplified and cloned. The new virus, designated HPV 115, encodes five early genes and two late genes. Phylogenetic analysis indicated HPV 115 as the most divergent type within the genus Beta-PV species 3. This report is the first providing data on cutaneous HPVs circulating in South America and expands our knowledge of the Papillomaviridae family.

Identification of a novel human gammapapillomavirus species

By using random PCR amplification, shotgun sequencing and sequence similarity searches, we analysed nucleic acids present in cell cultures inoculated with samples from unexplained cases of encephalitis. We identified a divergent human papillomavirus (HPV) sequence originating from a rectal swab. The full genome was amplified by inverse PCR and sequenced. The prototype of the sixth gammapapillomavirus species, HPV116, was not found in the patient's cerebrospinal fluid or respiratory secretions, nor in culture supernatants from other unexplained cases of encephalitis, indicating that its identification in an encephalitis patient was accidental.

Four novel human betapapillomaviruses of species 2 preferentially found in actinic keratosis

Recent studies have suggested an association between human papillomaviruses (HPVs), particularly species 2 members of the genus Betapapillomavirus, and squamous cell carcinoma (SCC) of the skin. As most of these viruses are uncharacterized, molecular characterization and epidemiology are needed to advance our understanding of their significance in carcinogenesis. This study determined the complete genomes of four betapapillomaviruses of species 2 from skin lesions designated HPV-107, -110 and -111 and FA75[KI88-03], an isolate of an unpublished HPV type, and analysed their prevalence and viral loads in biopsies from SCC, actinic keratosis (AK), basal cell carcinoma, seborrhoeic keratosis and the healthy skin of 263 immunocompetent patients by HPV type-specific real-time PCR assays. Seventeen patients (6.5 %) harboured at least one of the four HPV types in their lesion, whereas seven patients (2.7 %) harboured one or more of the HPV types in healthy skin. Overall, the four viruses were more common in AK than in healthy skin (odds ratio 5.0, 95 % confidence interval 1.4–17.5), but the prevalence and viral loads were low. This characterization of HPV-107, -110 and -111 and FA75[KI88-03] expands the heterogeneity of members of species 2 of the genus Betapapillomavirus. However, as these types were found in only a few samples and in low amounts, a possible role in carcinogenesis remains elusive.

Human papillomavirus type 45 E7 is a transforming protein inducing retinoblastoma protein degradation and anchorage-independent cell cycle progression

High-risk human papillomaviruses (HPV) cause cervical cancer. The biological properties of HPV-45, the third most prevalent high-risk HPV-genotype, are unknown. We demonstrate here that the HPV-45 E7 protein transforms immortalized NIH3T3 fibroblasts, while mutations in either the conserved LXCXE sequence (C28G) or the carboxyl-terminus (Delta87LQQLF91) significantly abolish this activity. To address the mechanisms underlying cell transformation by HPV-45 E7, we investigated its impact on the cell cycle. We show that HPV-45 E7 associates with the hypophosphorylated form of the retinoblastoma protein (pRb) and induces a significant reduction in the pRb half-life which can be blocked by epoxomicin. Moreover, HPV-45 E7 induces anchorage-independent cell cycle progression of NIH3T3 cells and extends the lifespan of primary human keratinocytes. HPV-45 E7C28G did not bind pRb and could neither induce pRb-proteolysis nor promote cell cycle progression. HPV-45 E7Delta87LQQLF91 had intermediate pRb-binding affinity and retained a residual activity to induce the degradation of pRb but lost the capability to promote cell cycle progression in suspension. Another carboxyl-terminal mutant, HPV-45 E7Delta81AEDL84, showed a trend to reduced transforming activity, had reduced pRb-binding activity and lost the capability to induce pRb-degradation; however, this mutant could induce anchorage-independent cell cycle progression with the same efficiency as HPV-45 E7 wild type. In summary, these data suggest that HPV-45 E7 is a transforming protein and that abrogation of cell cycle control contributes to its oncogenic potential.

Characterization of two novel cutaneous human papillomaviruses, HPV93 and HPV96

Two novel human papillomaviruses (HPVs), HPV93 and HPV96, with genomes of 7450 and 7438 bp, respectively, are described. The L1 open reading frame of HPV93 showed highest identity to HPV24 (79%) and that of HPV96 had highest identity to HPV92 (71%). Real-time PCR for HPV92, 93 and 96 on stripped biopsies from tumours and healthy skin from 269 immunocompetent patients found HPV DNA in 2.6% of tumours and in 0.4% of healthy skin samples. Double infections were observed in two tumours. HPV92 was detected in four, HPV93 in two and HPV96 in three tumours. The range of viral loads spanned from one copy per 45 cells to one copy per 10,000 cells. The E7 proteins of HPV92, 93 and 96 were found to bind the retinoblastoma protein (pRb). These results suggest a possible role for these HPV types in skin carcinogenesis that deserves further study.

Protein sequence-based risk classification for human papillomaviruses

Human papillomaviruses (HPVs) are small DNA tumor viruses which infect epithelial tissues and induce hyperproliferative lesions. Infection by high-risk genital HPVs is associated with the development of anogenital cancers. Classification of risk types is important in understanding the mechanisms in infection and in developing novel instruments for medical examination such as DNA microarrays. The sequence-based classification methods are useful in classifying risk types by considering residues in conserved positions. In this paper, we present a machine learning approach to the classification of HPV risk types by using the protein sequences. Our approach is based on the hidden Markov model and the kernel method. The former searches informative subsequence positions and the latter computes efficiently to classify protein sequences. In the experiments, the classifier predicted four unknown HPV types exactly. An additional result shows that the kernel-based classifiers learned with more informative subsequences outperform the classifiers learned with the whole sequence or random subsequences.

Expression of the high-risk human papillomavirus type 18 and 45 E7 oncoproteins in cervical carcinoma biopsies

E7 proteins are major oncoproteins of high-risk human papillomaviruses (HPVs), which play a key role in cervical carcinogenesis. These proteins have been shown to immortalize primary human cells. Due to the absence of antibodies with suitable sensitivity and specificity, little is known about expression of the E7 oncoproteins in naturally infected tissues. Recently, high-level expression of the E7 protein of HPV-16, the most prevalent oncogenic HPV type, was demonstrated in cervical carcinomas by immunohistochemistry; however, approximately 15 additional high-risk HPV types are known to be associated with cervical carcinoma. It is unknown whether the E7 oncoproteins of HPV-18 and -45, the second and third most prevalent HPV types, are expressed in cervical cancers. Using antibodies against HPV-18 and -45 E7 proteins, it is shown here for the first time that the HPV-18 and -45 E7 proteins can be detected in cervical carcinoma biopsies. Together with anti-HPV-16 E7 antibodies, this could create the possibility of detecting E7 oncoproteins in approximately 80 % of all cervical cancers.

The human papillomavirus type 11 and 16 E6 proteins modulate the cell-cycle regulator and transcription cofactor TRIP-Br1

The genital human papillomaviruses (HPVs) are a taxonomic group including HPV types that preferentially cause genital and laryngeal warts ("low-risk types"), such as HPV-6 and HPV-11, or cancer of the cervix and its precursor lesions ("high-risk types"), such as HPV-16. The transforming processes induced by these viruses depend on the proteins E5, E6, and E7. Among these oncoproteins, the E6 protein stands out because it supports a particularly large number of functions and interactions with cellular proteins, some of which are specific for the carcinogenic HPVs, while others are shared among low- and high-risk HPVs. Here we report yeast two-hybrid screens with HPV-6 and -11 E6 proteins that identified TRIP-Br1 as a novel cellular target. TRIP-Br1 was recently detected by two research groups, which described two separate functions, namely that of a transcriptional integrator of the E2F1/DP1/RB cell-cycle regulatory pathway (and then named TRIP-Br1), and that of an antagonist of the cyclin-dependent kinase suppression of p16INK4a (and then named p34SEI-1). We observed that TRIP-Br1 interacts with low- and high-risk HPV E6 proteins in yeast, in vitro and in mammalian cell cultures. Transcription activation of a complex consisting of E2F1, DP1, and TRIP-Br1 was efficiently stimulated by both E6 proteins. TRIP-Br1 has an LLG E6 interaction motif, which contributed to the binding of E6 proteins. Apparently, E6 does not promote degradation of TRIP-Br1. Our observations imply that the cell-cycle promoting transcription factor E2F1/DP1 is dually targeted by HPV oncoproteins, namely (i) by interference of the E7 protein with repression by RB, and (ii) by the transcriptional cofactor function of the E6 protein. Our data reveal the natural context of the transcription activator function of E6, which has been predicted without knowledge of the E2F1/DP1/TRIP-Br/E6 complex by studying chimeric constructs, and add a function to the limited number of transforming properties shared by low- and high-risk HPVs.

Cisplatin restores p53 function and enhances the radiosensitivity in HPV16 E6 containing SiHa cells

Most HPV-positive cervical cancer cells possess wild type p53 gene, but its normal p53 functions are disrupted by expression of HPVs E6. Treatment with 0-20 microM cisplatin for 24 h in HPV16 E6 containing SiHa cells suppressed E6 mRNA, reduced E6 protein, and restored p53 expression in dose-dependent manners. Dual-parameter flow cytometric analysis indicated that sub-G(1) apoptotic cells, but not necrotic cells were the major species for cisplatin-induced cytotoxicity in SiHa cells. After 0-10 microM cisplatin treatment, slightly more apoptotic cells appeared from SiHa cells than those from dominant negative p53-transfected SiHa cells. There was no different ionizing radiation (IR)-induced apoptosis in these two different cells. On the other hand, cisplatin enhanced more IR-induced sub-G(1) apoptosis in SiHa than mp53-SiHa cells. These accompanied with prolonged p53 restoration in irradiated-SiHa cells after 24 h cisplatin treatment and thereafter. In contrast, it was not found in cells after irradiation alone. Similar results were also shown in Mdm2 expression in SiHa cells after combined treatment. Therefore, cisplatin restored p53 expression and prolonged IR-induced p53 restoration would be possible candidates to response more sub-G(1) apoptosis in irradiated SiHa cells. These results provided another new explanation on cisplatin sensitizing radiotherapy for HPV16 E6 containing cancer cells.

Chelating agents stabilize the monomeric state of the zinc binding human papillomavirus 16 E6 oncoprotein

The E6 protein of human papillomavirus 16 is known to be difficult and, when overexpressed, insoluble and agglomerated. It has two putative zinc ion binding sites crucial for its function. No metallochaperone has yet been found to deliver zinc ions to the E6 protein. Here, we report that a specific chelating agent, which we think functionally mimics a metallochaperone, stabilized the soluble monomeric form of E6 and inhibited multimerization in vitro. This effect seemed to depend on the chelating strength of the agent. While strong chelating agents precipitated the E6 protein and weak chelating agents did not favor the monomeric form of E6, chelating agents of intermediate strength [L-penicillamine and ethylene glycol bis(beta-aminoethyl)-N,N,N',N'-tetraacetic acid (EGTA)] effectively support the formation of a monomer. We did not observe formation of a dimer or defined oligomers. Degradation assays imply that the monomer is the biologically active form of the protein. Since EGTA favors the formation of monomeric over agglomerated E6 protein, we propose that chelating agents of appropriate strength could assist zinc delivery to recombinant metalloproteins in vitro and may even destabilize existing agglomerates.

Purification and biophysical characterization of a minimal functional domain and of an N-terminal Zn2+-binding fragment from the human papillomavirus type 16 E6 protein

The E6 Zn(2+)-binding protein of high-risk human papillomaviruses (HPVs) is one of the major transforming proteins encoded by these tumor viruses. A bacterial system was used to express wild type and truncated forms of HPV-16 E6 linked to GST. The recombinant proteins were released from GST through cleavage of a factor Xa site. Functional analysis of these proteins demonstrated that amino acids 2--142 comprise the minimal domain of E6 required to promote the degradation of p53 in vitro in a rabbit reticulocyte lysate. This purified protein, E6(Delta 143--151), required a high salt concentration for maximum solubility, eluted as a monomer on gel filtration, and was shown to bind two Zn(2+) ions by atomic absorption analysis. An N-terminal subdomain of E6 (amino acids 2--77, E6-N) was similarly purified. Unlike E6(Delta 143--151), E6--N was very soluble in low-salt buffers and hence was highly amenable to biophysical characterization. E6-N was shown to bind one Zn(2+) ion by electrospray mass spectrometry and by atomic absorption analysis. UV--visible spectroscopic analysis of Co(2+)-substituted E6--N revealed that four cysteine residues coordinate the metal ion. Mutational studies of all the cysteine residues in E6--N substantiated a critical role for Cys 30, 33, 63, and 66 in Zn(2+) binding and in proper folding of the subdomain. Equilibrium sedimentation of E6-N demonstrated that it is a monomer, like E6(Delta 143--151), at low concentrations, but dimerization occurs at high concentrations (K(d) = 0.1 mM). Finally, circular dichroism studies revealed significant secondary structure for both E6(Delta 143--151) and E6--N. The results support a model of monomeric E6 possessing two functionally critical Zn(2+)-binding motifs.

Interaction with CBP/p300 enables the bovine papillomavirus type 1 E6 oncoprotein to downregulate CBP/p300-mediated transactivation by p53

The E6 oncoprotein of bovine papillomavirus type 1 (BPV-1) can transform cells independently of p53 degradation. The precise mechanisms underlying this transformation are not yet completely understood. Here it is shown that BPV-1 E6 interacts with CBP/p300 in the same way as described for the E6 proteins of oncogenic human papillomaviruses. This interaction results in an inhibition of the transcriptional coactivator function of CBP/p300 required by p53 and probably by other transcription factors. The comparison of the CBP/p300-binding properties of BPV-1 E6 mutants previously characterized in transcription and transformation studies suggests (i) that the E6-CBP/p300 interaction may be necessary, but not sufficient, for cell transformation, and (ii) that the transcriptional activator function, inherent to the E6 protein, is not derived from forming a complex with CBP/p300.

Inactivation of the human papillomavirus-16 E6 oncoprotein by organic disulfides

We are investigating compounds that could be useful in the treatment of neoplastic lesions of the cervix by acting on the oncoprotein E6 of human papillomavirus-16. The E6 protein contains two potential zinc-binding domains that are required for most of its functions. We have published tests that measure (i) the release of zinc ions after chemical alteration of the cysteine groups of these zinc-binding domains (TSQ assay), (ii) the interaction of E6 with the cellular proteins E6AP and E6BP (BIACORE assay), and (iii) the viability of tumor cell lines that require the continuous expression of HPV oncoproteins (WST1 assay). Based on these tests, we identified 4.4'-dithiodimorpholine as a potential lead compound. In this study we examined whether the dithiobisamine moiety of 4,4'-dithiodimorpholine may be an important molecular prerequisite for further drug development in this system. We have evaluated 59 new substances including organic disulfides and those containing the dithiobisamine moiety, as well as structural analogues. The compounds with significant reactivity in all three assays were observed only for dithiobisamine derivatives with saturated cyclic amines and aryl substituted piperazines. The identity of these substances suggests that the N-S-S-N moiety is necessary but not sufficient for reactivity in our assays, and that dithiobisamine based substances are useful as lead compounds that target the cysteine groups of HPV-16 E6 zinc fingers.

Human papillomavirus type 16 E7 oncoprotein binds and inactivates growth-inhibitory insulin-like growth factor binding protein 3

The E7 protein encoded by human papillomavirus type 16 is one of the few viral genes that can immortalize primary human cells and thereby override cellular senescence. While it is generally assumed that this property of E7 depends on its interaction with regulators of the cell cycle, we show here that E7 targets insulin-like growth factor binding protein 3 (IGFBP-3), the product of a p53-inducible gene that is overexpressed in senescent cells. IGFBP-3 can suppress cell proliferation and induce apoptosis; we show here that IGFBP-3-mediated apoptosis is inhibited by E7, which binds to IGFBP-3 and triggers its proteolytic cleavage. Two transformation-deficient mutants of E7 failed to inactivate IGFBP-3, suggesting that inactivation of IGFBP-3 may contribute to cell transformation.

Modulation of type M2 pyruvate kinase activity by the human papillomavirus type 16 E7 oncoprotein

We report here that the E7 oncoprotein encoded by the oncogenic human papillomavirus (HPV) type 16 binds to the glycolytic enzyme type M2 pyruvate kinase (M2-PK). M2-PK occurs in a tetrameric form with a high affinity to its substrate phosphoenolpyruvate and a dimeric form with a low affinity to phosphoenolpyruvate, and the transition between both conformations regulates the glycolytic flux in tumor cells. The glycolytic intermediate fructose 1, 6-bisphosphate induces the reassociation of the dimeric to the tetrameric form of M2-PK. The expression of E7 in an experimental cell line shifts the equilibrium to the dimeric state despite a significant increase in the fructose 1,6-bisphosphate levels. Investigations of HPV-16 E7 mutants and the nononcogenic HPV-11 subtype suggest that the interaction of HPV-16 E7 with M2-PK may be linked to the transforming potential of the viral oncoprotein.

The papillomavirus E6 proteins

Specific types of human papillomaviruses (HPV) are strongly associated with the development of cervical cancer. The E6 gene from cancer-related HPVs has exhibited functions in tumorigenesis, regulation of transcription, telomerase, and apoptosis. Cancer-related HPVs E6 proteins bind the tumor suppressor p53 and promotes its degradation through an ubiquitin-dependent pathway. Several additional cellular E6-binding proteins have recently been identified and implicated in playing roles in p53-independent functions of E6.

Identification of an alpha helical motif sufficient for association with papillomavirus E6

We recently identified a cellular protein named E6BP or ERC-55 that binds cancer-related papillomavirus E6 proteins (Chen, J. J., Reid, C. E., Band, V., and Androphy, E. J. (1995) Science 269, 529-531). By construction of a series of deletion mutants, the region of E6BP that is necessary and sufficient for complex formation with human papillomavirus type 16 E6 has been mapped to a 25-amino acid domain. The corresponding peptide was synthesized and found by nuclear magnetic resonance spectroscopy to bind calcium and fold into a classical helix-loop-helix EF-hand conformation. Additional deletion mutagenesis showed that 13 amino acids that form the second alpha helix mediated E6 association. Alanine replacement mutagenesis indicated that amino acids of this helix were most important for E6 binding. Alignment of this alpha helical E6 binding peptide with the 18-amino acid E6 binding region of E6AP (Huibregtse, J. M., Scheffner, M., and Howley, P. M. (1993) Mol. Cell. Biol. 13, 4918-4927) and the first LD repeat of another E6-binding protein, paxillin (Tong, X., and Howley, P. M. (1997) J. Biol. Chem. 272, 33373-33376), revealed substantial similarities among these E6 binding domains. The extent of homology and the mutational data define the peptide as an E6 binding motif.

Interaction of the bovine papillomavirus E6 protein with the clathrin adaptor complex AP-1

The E6 gene of the bovine papillomavirus type 1 (BPV-1) is expressed in fibropapillomas caused by BPV-1 and in tissue culture cells transformed by BPV-1. It encodes one of the two major oncoproteins of BPV-1. In this study, we demonstrate an interaction between the BPV-1 E6 protein and AP-1, the TGN (trans-Golgi network)-specific clathrin adaptor complex. AP-1 is a four-subunit protein complex required for clathrin-mediated cellular transport from the TGN. The AP-1/E6 interaction was observed in vitro and in cells. The E6 binding site on AP-1 was mapped to the N-terminal trunk domain of the gamma subunit. BPV-1 E6 preferentially associated with membrane-bound AP-1 in cells but not with free cytosolic AP-1. BPV-1 E6 was further shown to be recruited to isolated Golgi membranes and to copurify with clathrin-coated vesicles. The recruitment of BPV-1 E6 to Golgi membranes was AP-1 independent, but the E6 interaction with AP-1 was required for its association with clathrin-coated vesicles. Furthermore, AP-1 proteins could compete with BPV-1 E6 for binding to Golgi membranes, suggesting that the recruitment of BPV-1 E6 and AP-1 to Golgi membranes involves a common factor. Taken together, our results suggest that cytosolic BPV-1 E6 is first recruited to the TGN, where it is then recognized by membrane-bound AP-1 and subsequently recruited into TGN-derived clathrin-coated vesicles. We propose that BPV-1 E6, through its interaction with AP-1, can affect cellular processes involving clathrin-mediated trafficking pathway.

The human papillomavirus E7 oncoprotein functionally interacts with the S4 subunit of the 26 S proteasome

Human papillomaviruses (HPV) have been etiologically linked to human cervical cancer. More than 90% of cervical cancer tissues express two HPV-encoded oncoproteins E6 and E7. Both E6 and E7 proteins possess transformation activity. and together they cooperate to transform primary human keratinocytes, fibroblasts. and epithelial cells. The transforming activity of E7 is associated with its ability to bind the retinoblastoma tumor suppressor protein (Rb). However, the carboxyl-terminal mutants of E7 are also defective for transformation, suggesting that other cellular targets for E7 might exist. We screened a human placenta cDNA library by yeast two-hybrid assay using HPV 16 E7 as a bait and identified the subunit 4 (S4) ATPase of the 26 S proteasome as a novel E7-binding protein. E7 binds to S4 through the carboxyl-terminal zinc binding motif, and the binding is independent of E7 sequences involved in binding to Rb. The interaction between S4 and E7 can be easily detected by in vitro protein binding assays. Moreover, we found that E7 increases the ATPase activity of S4. A recent study has shown that, in epithelial cells, E7 degrades Rb through the 26 S proteasome pathway. We hypothesize that E7 might target Rb for degradation by 26 S proteasome through its interaction with the subunit 4 of the proteasome.