Bovine papillomavirus E2 repressor mutant displays a high-copy-number phenotype and enhanced transforming activity

Experimental Support for Human Papillomavirus Genome Amplification Early after Infectious Delivery

Even though replication and transcription of human papillomavirus type 16 (HPV16) has been intensively studied, little is known about immediate-early events of the viral life cycle due to the lack of an efficient infection model allowing genetic dissection of viral factors. We employed the recently developed infection model (Bienkowska-Haba M, Luszczek W, Myers JE, Keiffer TR, et al. 2018. PLoS Pathog 14:e1006846) to investigate genome amplification and transcription immediately after infectious delivery of viral genome to nuclei of primary keratinocytes. Using 5-ethynyl-2'-deoxyuridine (EdU) pulse-labeling and highly sensitive fluorescence in situ hybridization, we observed that the HPV16 genome is replicated and amplified in an E1- and E2-dependent manner. Knockout of E1 resulted in failure of the viral genome to replicate and amplify. In contrast, knockout of the E8^E2 repressor led to increased viral genome copy number, confirming previous reports. Genome copy control by E8^E2 was confirmed for differentiation-induced genome amplification. Lack of functional E1 had no effect on transcription from the early promoter, suggesting that viral genome replication is not required for p97 promoter activity. However, infection with an HPV16 mutant virus defective for E2 transcriptional function revealed a requirement of E2 for efficient transcription from the early promoter. In the absence of the E8^E2 protein, early transcript levels are unaltered and even decreased when normalized to genome copy number. Surprisingly, a lack of functional E8^E2 repressor did not affect E8^E2 transcript levels when normalized to genome copy number. These data suggest that the main function of E8^E2 in the viral life cycle is to control genome copy number. IMPORTANCE It is being assumed that human papillomavirus (HPV) utilizes three different modes of replication during its life cycle: initial amplification during the establishment phase, genome maintenance, and differentiation-induced amplification. However, HPV16 initial amplification was never formally proven due to a lack of an infection model. Using our recently established infection model (Bienkowska-Haba M, Luszczek W, Myers JE, Keiffer TR, et al. 2018. PLoS Pathog 14:e1006846), we demonstrate herein that viral genome is indeed amplified in an E1- and E2-dependent manner. Furthermore, we find that the main function of the viral repressor E8^E2 is to control viral genome copy number. We did not find evidence that it regulates its own promoter in a negative feedback loop. Our data also suggest that the E2 transactivator function is required for stimulation of early promoter activity, which has been debated in the literature. Overall, this report confirms the usefulness of the infection model for studying early events of the HPV life cycle using mutational approaches.

Functions of Papillomavirus E8^E2 Proteins in Tissue Culture and In Vivo

Papillomaviruses (PV) replicate in undifferentiated keratinocytes at low levels and to high levels in differentiated cells. The restricted replication in undifferentiated cells is mainly due to the expression of the conserved viral E8^E2 repressor protein, a fusion protein consisting of E8 and the hinge, DNA-binding, and dimerization domain of E2. E8^E2 binds to viral genomes and represses viral transcription and genome replication by recruiting cellular NCoR/SMRT-HDAC3 corepressor complexes. Tissue culture experiments have revealed that E8^E2 modulates long-term maintenance of extrachromosomal genomes, productive replication, and immortalization properties in a virus type-dependent manner. Furthermore, in vivo experiments have indicated that Mus musculus PV1 E8^E2 is required for tumor formation in immune-deficient mice. In summary, E8^E2 is a crucial inhibitor whose levels might determine the outcome of PV infections.

Control of viral replication and transcription by the papillomavirus E8^E2 protein

Human papillomaviruses have adjusted their replication levels to the differentiation state of the infected keratinocyte. PV genomes replicate in undifferentiated cells at low levels and to high levels in differentiated cells. Genome replication requires the viral E1 helicase and the viral E2 transcription/replication activator. The limited replication in undifferentiated cells is predominantly due to the expression of the highly conserved E8^E2 viral repressor protein, which is a fusion between E8 and the C-terminal half of the E2 protein. E8^E2 is a sequence-specific DNA binding protein that inhibits viral gene expression and viral genome replication. The E8 domain is required for repression activities, which are mainly due to the interaction with cellular NCoR/SMRT corepressor complexes. In the case of HPV16, the most carcinogenic HPV type, E8^E2 not only limits genome replication in undifferentiated cells but also productive replication in differentiated epithelium. E8^E2 is expressed from a separate promoter that is controlled by unknown cellular factors and the viral transcription and replication regulators E1, E2 and E8^E2. In summary, E8^E2 is an important negative regulator whose levels may be critical for the outcome of HPV infections.

The papillomavirus E2 proteins

The papillomavirus E2 proteins are pivotal to the viral life cycle and have well characterized functions in transcriptional regulation, initiation of DNA replication and partitioning the viral genome. The E2 proteins also function in vegetative DNA replication, post-transcriptional processes and possibly packaging. This review describes structural and functional aspects of the E2 proteins and their binding sites on the viral genome. It is intended to be a reference guide to this viral protein.

Cellular factors are required to activate bovine papillomavirus-1 early gene transcription and to establish viral plasmid persistence but are not required for cellular transformation

Transcription from the major upstream early gene promoter, P89, of bovine papillomavirus (BPV)-1 is detectable in transfected cells lacking viral gene products yet also responds to viral E2 proteins. In contrast to human papillomaviruses (HPVs), the BPV upstream regulatory region (URR) functions as a transcriptional enhancer in epithelial cells and fibroblasts of bovine, murine or human origin. Mutations of Sp1 and/or two novel transcriptional enhancer factor (TEF)-1 sites within the 5' URR of the intact BPV-1 genome dramatically reduced P89-initiated mRNA levels, leading to decreased BPV-1 plasmid amplification and inefficient formation of transformed cell foci. However, cell lines transformed with wt or mutant BPV-1 genomes contained similar levels of unintegrated BPV-1 DNA, P89 mRNA and E2-dependent transactivation. We conclude that cellular factors necessary for activating viral early gene transcription, establishment of viral plasmid replication and cell immortalization are not required during the maintenance phase of BPV-1 infection.

Replication and partitioning of papillomavirus genomes

Papillomaviruses establish persistent infection in the dividing, basal epithelial cells of the host. The viral genome is maintained as a circular, double-stranded DNA, extrachromosomal element within these cells. Viral genome amplification occurs only when the epithelial cells differentiate and viral particles are shed in squames that are sloughed from the surface of the epithelium. There are three modes of replication in the papillomavirus life cycle. Upon entry, in the establishment phase, the viral genome is amplified to a low copy number. In the second maintenance phase, the genome replicates in dividing cells at a constant copy number, in synchrony with the cellular DNA. And finally, in the vegetative or productive phase, the viral DNA is amplified to a high copy number in differentiated cells and is destined to be packaged in viral capsids. This review discusses the cis elements and protein factors required for each stage of papillomavirus replication.

The E8--E2 gene product of human papillomavirus type 16 represses early transcription and replication but is dispensable for viral plasmid persistence in keratinocytes

A conserved E8(wedge)E2 spliced mRNA is detected in keratinocytes transfected with human papillomavirus type 16 (HPV-16) plasmid DNA. Expression of HPV-16 E8--E2 (16-E8--E2) is independent of the major early promoter, P97, and is modulated by both specific splicing events and conserved cis elements in the upstream regulatory region in a manner that differs from transcriptional regulation of other early viral genes. Mutations that disrupt the predicted 16-E8--E2 message also increase initial HPV-16 plasmid amplification 8- to 15-fold and major early gene (P97) transcription 4- to 5-fold over those of the wild type (wt). Expressing the 16-E8--E2 gene product from the cytomegalovirus (CMV) promoter represses HPV-16 early gene transcription from P97 in a dose-dependent manner, as detected by RNase protection assays. When expressed from the CMV promoter, 16-E8--E2 also inhibits the amplification of an HPV-16 plasmid and a heterologous simian virus 40 (SV40) ori plasmid that contains E2 binding sites in cis. In contrast, cotransfections with HPV-16 wt genomes that express physiologic levels of 16-E8--E2 are sufficient to repress HPV-16 plasmid amplification but are limiting and insufficient for the repression of SV40 amplification. 16-E8--E2-dependent repression of HPV-16 E1 expression is sufficient to account for this observed inhibition of initial HPV-16 plasmid amplification. Unlike with other papillomaviruses, primary human keratinocytes immortalized by the HPV-16 E8 mutant genome contain more than eightfold-higher levels of unintegrated plasmid than the wt, demonstrating that 16-E8(wedge)E2 limits the viral copy number but is not required for plasmid persistence and maintenance.

The papillomavirus E8-E2C protein represses DNA replication from extrachromosomal origins

Carcinogenic DNA viruses such as high-risk human papillomaviruses (HPV) and Epstein-Barr-Virus (EBV) replicate during persistent infections as low-copy-number plasmids. EBV DNA replication is restricted by host cell replication licensing mechanisms. In contrast, copy number control of HPV genomes is not under cellular control but involves the viral sequence-specific DNA-binding E2 activator and E8-E2C repressor proteins. Analysis of HPV31 mutant genomes revealed that residues outside of the DNA-binding/dimerization domain of E8-E2C limit viral DNA replication, indicating that binding site competition or heterodimerization among E2 and E8-E2C proteins does not contribute to copy number control. Domain swap experiments demonstrated that the amino-terminal 21 amino acids of E8-E2C represent a novel, transferable DNA replication repressor domain, whose activity requires conserved lysine and tryptophan residues. Furthermore, E8-E2C (1-21)-GAL4 fusion proteins inhibited the replication of the plasmid origin of replication of EBV, suggesting that E8-E2C functions as a general replication repressor of extrachromosomal origins. This finding could be important for the development of novel therapies against persistent DNA tumor virus infections.

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.

Regulation of human papillomavirus type 1 replication by the viral E2 protein

The E1 and E2 proteins encoded by papillomaviruses are required for viral replication. Earlier studies have shown that the viral E2 protein plays an important role in replication by targeting the E1 helicase to the origin of replication (ori). We have previously shown that the E1 protein of human papillomavirus (HPV) type 1 is sufficient for the in vivo replication of ori plasmids, although the E2 protein stimulates replication. In this study, we have further analyzed the role of the E2 protein in HPV-1 replication. The optimal ori of HPV-1 contains one putative E1 binding site (E1BS) and two putative E2 binding sites, E2BS-3 and E2BS-4. Plasmid pori171, containing the optimal ori, replicates to much higher levels than plasmid pori312, which includes an additional upstream E2 binding site, E2BS-2, located 75 nucleotides upstream of E2BS-3. To study the possible role of E2BS-2 and other upstream sequences in E2-dependent downregulation of replication, transient replication analysis was done in the presence of increasing levels of the E2 protein. Interestingly, inhibition of pori312 replication was more severe at higher levels of E2, suggesting that this protein may also negatively regulate HPV-1 replication. Deletion of sequences from pori312 containing an additional putative E2BS, E2BS-2A, relieved the repression of replication to a significant extent, while replacement of E2BS-2 with a different sequence of the same length had a modest effect. These results suggest that E2BS-2A plays a major and E2BS-2 a minor role in the negative regulation of HPV-1 replication at high E2 levels. Electrophoretic mobility-shift assays showed that the purified E2 protein bound with high affinity to E2BS-3 and weakly to the other putative E2BSs located within the viral long control region. EMSA using various ori fragments showed the formation of multiple E2-DNA complexes which likely represent binding of E2 to multiple E2BSs present within the HPV-1 ori. Our data are consistent with the assembly of ori-protein complexes at high E2 levels that are impaired for replication and further suggest that E2 may regulate HPV-1 replication by a mechanism involving interaction between the E2 protein bound to E2BSs at a distance.

High and low levels of cottontail rabbit papillomavirus E2 protein generate opposite effects on gene expression

The papillomavirus E2 protein plays an important role in viral transcriptional regulation and replication. We chose to study the cottontail rabbit papillomavirus (CRPV) E2 protein as a transcriptional regulator because of the availability of an animal model for papilloma formation, which may be relevant for human papillomavirus (HPV) infection and replication. We studied the effect of expression levels of E2 on the long control region, which contains transcriptional promoter and enhancer elements, and synthetic E2-dependent artificial promoters in which the E2 was the dominant factor in the transcriptional activation. These experiments indicated that high levels of E2 were inhibitory and low levels were stimulatory for transactivation. In addition, we showed that the complex formed between CRPV E2 and the cognate binding site was less stable than the complex formed between HPV E2 and the same cognate binding site. Furthermore, we showed that CRPV E2 binding to its transcriptional regulatory sequence was stabilized by other proteins such as E1, which produced increments in transcriptional activation of E2-dependent genes. The data may be used to define conditions in which the rabbit model can be used for the screening of drugs which are inhibitory to the HPV and CRPV replication and gene expression.

The E8E2C protein, a negative regulator of viral transcription and replication, is required for extrachromosomal maintenance of human papillomavirus type 31 in keratinocytes

The viral E2 protein is a major regulator of papillomavirus DNA replication. An important way to influence viral replication is through modulation of the activity of the E2 protein. This could occur through the action of truncated E2 proteins, called E2 repressors, whose role in the replication cycle of human papillomaviruses (HPVs) has not been determined. In this study, using cell lines that contain episomal copies of the "high-risk" HPV type 31 (HPV31), we have identified viral transcripts with a splice from nucleotide (nt) 1296 to 3295. These transcripts are similar to RNAs from other animal and human papillomaviruses and have the potential to fuse a small open reading frame (E8) to the C terminus of E2, resulting in an E8E2C fusion protein. E8E2C transcripts were present throughout the complete replication cycle of HPV31. A genetic analysis of E8E2C in the context of the HPV31 genome revealed that mutation of the single ATG of the E8 gene, introduction of a stop codon downstream of the ATG, or disruption of the splice donor site at nt 1296 led to a dramatic 30- to 40-fold increase in the transient DNA replication levels in both normal and immortalized human keratinocytes. High-level expression of E8E2C from heterologous vectors was found to inhibit E1-E2-dependent DNA replication of an HPV31 origin of replication construct as well as to interfere with E2's ability to transactivate reporter gene constructs. In addition, HPV31 E8E2C strongly repressed the basal activity of the major viral early promoter P97 independent of E2. E8E2C may therefore exert its negative effect on viral DNA replication through modulating E2's ability to enhance E1-dependent DNA replication as well as by regulating viral gene expression. Surprisingly, HPV31 genomes that were unable to express E8E2C could not be maintained extrachromosomally in human keratinocytes in long-term assays despite high transient DNA replication levels. This suggests that the E8E2C protein may play a role in copy number control as well as in the stable maintenance of HPV episomes.

Transactivation-competent bovine papillomavirus E2 protein is specifically required for efficient repression of human papillomavirus oncogene expression and for acute growth inhibition of cervical carcinoma cell lines

The papillomavirus E2 proteins can function as sequence-specific transactivators or transrepressors of transcription and as cofactors in viral DNA replication. We previously demonstrated that acute expression of the bovine papillomavirus type 1 (BPV1) E2 protein in HeLa and HT-3 cervical carcinoma cell lines greatly reduced cellular proliferation by imposing a specific G1/S phase growth arrest. In this report, we analyzed the effects of a panel of point mutations in the BPV1 E2 protein to identify the functional requirements for acute growth inhibition. Disruption of E2-specific transactivation by mutations within either the transactivation domain or the DNA binding domain severely impaired E2-mediated growth inhibition in HeLa and HT-3 cells, even though these mutants retain various other E2 activities. This result indicates that functional transactivation activity is required for acute E2-mediated growth inhibition. HeLa cells, which contain a wild-type p53 gene, and HT-3 cells, which contain a transactivation-defective p53 gene, exhibited similar responses to the E2 mutants, suggesting that identical functions of the E2 protein were required for growth arrest regardless of p53 status. Replacement of the E2 transactivation domain with that of the herpes simplex virus VP16 generated a chimeric transactivator that efficiently stimulated expression of an E2-responsive reporter plasmid yet was completely defective for growth inhibition, suggesting that an E2-specific transactivation function is required for growth arrest. Surprisingly, the transactivation-defective E2 mutants were also markedly defective in their ability to repress transcription of the native human papillomavirus type 18 (HPV18) E6/E7 oncogenes in HeLa cells and of the HPV18 promoter present in a transfected reporter plasmid. These mutants were also defective in their ability to increase p53 levels. Therefore, efficient repression of the HPV18 promoter in HeLa cells is not merely a consequence of the binding of an E2 protein to appropriately situated binding sites in the promoter.

Competition for DNA binding sites between the short and long forms of E2 dimers underlies repression in bovine papillomavirus type 1 DNA replication control

Papillomaviruses establish a long-term latency in vivo by maintaining their genomes as nuclear plasmids in proliferating cells. Bovine papillomavirus type 1 encodes two proteins required for viral DNA replication: the helicase E1 and the positive regulator E2. The homodimeric E2 is known to cooperatively bind to DNA with E1 to form a preinitiation complex at the origin of DNA replication. The virus also codes for two short forms of E2 that can repress viral functions when overexpressed, and at least one copy of the repressor is required for stable plasmid maintenance in transformed cells. Employing a tetracycline-regulated system to control E1 and E2 production from integrated loci, we show that the short form of E2 negatively regulates DNA replication. We also found that the short form could repress replication in a cell-free replication system and that the repression requires the DNA binding domain of the protein. In contrast, heterodimers of the short and long forms were activators and, by footprint analysis, were shown to be as potent as homodimeric E2 in loading E1 to its cognate site. DNA binding studies show that when E1 levels are low and are dependent upon E2 for occupancy of the origin site, the repressor can block E1-DNA interactions. We conclude that DNA replication modulation results from competition between the different forms of E2 for DNA binding. Given that heterodimers are active and that the repressor form of E2 shows little cooperativity with E1 for DNA binding, this protein is a weak repressor.

A papillomavirus E2 phosphorylation mutant exhibits normal transient replication and transcription but is defective in transformation and plasmid retention

Papillomavirus DNA persists in infected cells as a nuclear plasmid, causing epithelial lesions in many hosts, including humans. The viral protein E2 is required for both replication and transcription to facilitate this persistence. Bovine papillomavirus E2 protein is phosphorylated at two predominant sites. Phosphorylation of one of these sites (serine 301) inhibits replication of the genome. Using mass spectrometry and Edman sequencing, we have mapped additional phosphorylation sites in tryptic peptides to positions which lie primarily in the putatively unstructured hinge region of E2. Mutation of the major sites facilitates transformation in the absence of viral repressors and only has a minor effect on transformation when the repressors are present. Mutation of the major phosphorylation sites combined with one additional change at a newly discovered site (serine 235) blocks transformation. Transformation can be restored by mutating this residue to aspartic acid, mimicking a phosphorylated amino acid, suggesting that phosphorylation is key to the regulation. Transformation by the mutant genome can also be rescued by ectopic expression of the E2 enhancer protein, demonstrating a loss of function by the mutant protein and not a toxic defect. In transient assays, phosphorylation site mutants of E2 protein were normal for all viral functions tested, including replication, transcriptional activation and repression (by the overlapping mutant repressors), protein accumulation, and surprisingly, viral oncogene E5 promoter activation. While the mutant genome transiently replicated to high levels, stable replication was defective, suggesting that a function of E2 required for plasmid retention is regulated by phosphorylation.

Bovine papillomavirus type 1 E2 transcriptional regulators directly bind two cellular transcription factors, TFIID and TFIIB

The bovine papillomavirus type 1 (BPV-1) E2 translational open reading frame encodes three proteins that regulate viral transcription and DNA replication: the E2 transcriptional activator (E2TA), the E2 transcriptional repressor (E2TR) and the E8/E2 transcriptional repressor (E8/E2TR). E2TA is a strong activator of papillomaviral promoters and is required for viral DNA replication. E2TR and E8/E2TR inhibit the activities of E2TA but also possess weak transactivational properties of their own. Two components of the cellular transcription apparatus, TFIID and TFIIB, have previously been shown to associate with other viral and cellular transcriptional activators. We present evidence here that E2TA, the full-length E2 open reading frame gene product, directly binds both of these transcription factors in vitro. Glutathione S-transferase E2TA (GST-E2TA) fusion protein bound in vitro-synthesized TATA-box-binding protein (TBP), a component of TFIID, and in vitro-synthesized TFIIB. Likewise, GST-E2TA bound TFIID and TFIIB present in a nuclear extract from the human cervical cancer-derived cell line, HeLa. The binding of GST-E2TA to TBP and TFIIB required no additional mammalian factors, as shown by direct binding of GST-E2TA to bacterially synthesized recombinant TBP and recombinant TFIIB. The domain of E2TA required for its interaction with both TBP and TFIIB was localized to the C terminus of E2TA, a region also present in E2TR and E8/E2TR. This domain lies within the region of E2TA previously shown to confer cooperative DNA binding by E2TA and TBP and overlaps with the region of E2TA required for DNA binding and dimerization. Our findings, taken in context with previous studies, lead us to conclude that (i) cooperative DNA binding by E2 proteins and TBP is likely mediated by the direct binding of E2 proteins to TBP, (ii) the weak transcriptional transactivation by E2TR and E8/E2TR may result as a consequence of direct TBP and TFIIB binding by these proteins, and (iii) TBP and/or TFIIB binding may be required but is not sufficient for E2TA's strong transactivational activity.

Mutational analysis of the interaction between the bovine papillomavirus E5 transforming protein and the endogenous beta receptor for platelet-derived growth factor in mouse C127 cells

The bovine papillomavirus E5 protein is a 44-amino-acid membrane-associated protein that forms a stable complex with the endogenous platelet-derived growth factor (PDGF) beta receptor in rodent and bovine fibroblasts, resulting in sustained receptor activation and cell transformation. We report here that high-level expression of the E5 protein caused a reduction in the level of the mature form of the PDGF beta receptor in acutely and stably transformed mouse C127 cells. To explore in more detail the interaction of the E5 protein and the PDGF beta receptor, we tested the abilities of various E5 point mutants to bind the PDGF receptor, to induce PDGF receptor down-regulation and tyrosine phosphorylation, and to transform cells. A transformation-competent mutant, like the wild-type E5 protein, bound the receptor and induced receptor tyrosine phosphorylation and down-regulation. Transformation-defective E5 proteins either failed to interact with the endogenous PDGF beta receptor in mouse fibroblasts or underwent an aberrant interaction with the receptor. Mutation of glutamine at position 17, aspartic acid at position 33, or both carboxyl-terminal cysteine residues required for E5 homodimerization interfered with stable complex formation with the PDGF receptor, tyrosine phosphorylation and down-regulation of the receptor, and cell transformation. Point mutations at several other carboxyl-terminal positions generated transformation-defective E5 proteins that formed a complex with the PDGF receptor and induced receptor tyrosine phosphorylation but did not induce PDGF receptor down-regulation. Either PDGF receptor activation is not sufficient for transformation of C127 cells or the receptors that are tyrosine phosphorylated in response to these mutant E5 proteins are not fully activated and therefore are not able to deliver a mitogenic signal.

Negative regulation of the bovine papillomavirus E5, E6, and E7 oncogenes by the viral E1 and E2 genes

Papillomaviruses induce benign squamous epithelial lesions that infrequently are associated with uncontrolled growth or malignant conversion. The virus-encoded oncogenes are clearly under negative regulation since papillomaviruses can latently infect cells and since different levels of viral oncogene expression are seen within the layers of differentiating infected epitheliomas. We used bovine papillomavirus type 1 (BPV-1) to investigate the mechanisms involved in the negative regulation of transformation. We found that the following two distinct and interacting mechanisms negatively regulate BPV-1 transformation effected by virally encoded trans-acting factors: (i) E2 repressors suppress transformation by the E6 and E7 oncogenes, and (ii) E1 and the E2 transactivator suppress transformation by the E6, E7, and E5 oncogenes. These systems interact in that the E2 repressors function to relieve the transformation suppression effected by the E1 and E2 transactivator genes. A BPV-1 mutant that lacked E2 repressors and E1 had greatly augmented transformation capacity. Analysis of this mutant revealed that the enhanced transformation was due to expression of the E6 and E7 genes in the absence of E5, revealing a previously unappreciated potency and synergy for the BPV-1 E6 and E7 oncogenes.

Papillomavirus contains cis-acting sequences that can suppress but not regulate origins of DNA replication

Bovine papillomavirus (BPV) DNA has been reported to restrict its own replication and that of the lytic simian virus 40 (SV40) origin to one initiation event per molecule per S phase, which suggests BPV DNA replication as a model for cellular chromosome replication. Suppression of the SV40 origin required two cis-acting BPV sequences (NCOR-1 and -2) and one trans-acting BPV protein. The results presented in this paper confirm the presence of two NCOR sequences in the BPV genome that can suppress polyomavirus (PyV) as well as SV40 origin-dependent DNA replication as much as 40-fold. However, in contrast to results of previous studies on SV40, most of the suppression of the PyV origin was due to NCOR-1, a 512-bp sequence that functioned independently of distance or orientation with respect to the PyV origin and that was not required for BPV DNA replication. Moreover, NCOR-1 alone or together with NCOR-2 did not restrict the ability of the PyV ori to reinitiate replication within a single S phase and did not require any BPV protein to exert suppression. Furthermore, NCOR-1 did not suppress BPV origin-dependent DNA replication except in the presence of PyV large tumor antigen (T-ag). Since NCOR-1 suppression of PyV origin activity also varied with T-ag concentration, suppression of origins by NCOR sequences appeared to require papovavirus T-ag. Therefore, it is unlikely that NCOR sequences are involved in regulating BPV DNA replication. When these results are taken together with those from other laboratories, BPV appears to be a slowly replicating version of papovaviruses rather than a model for origins of DNA replication in eukaryotic cell chromosomes.

Inhibition of cervical carcinoma cell line proliferation by the introduction of a bovine papillomavirus regulatory gene

Human papillomavirus (HPV) E6 and E7 oncogenes are expressed in the great majority of human cervical carcinomas, whereas the viral E2 regulatory gene is usually disrupted in these cancers. To investigate the roles of the papillomavirus E2 genes in the development and maintenance of cervical carcinoma, the bovine papillomavirus (BPV) E2 gene was acutely introduced into cervical carcinoma cell lines by infection with high-titer stocks of simian virus 40-based recombinant viruses. Expression of the BPV E2 protein in HeLa, C-4I, and MS751 cells results in specific inhibition of the expression of the resident HPV type 18 (HPV18) E6 and E7 genes and in inhibition of cell growth. HeLa cells, in which HPV gene expression is nearly completely abolished, undergo a dramatic and rapid inhibition of proliferation, which appears to be largely a consequence of a block in progression from the G1 to the S phase of the cell cycle. Loss of HPV18 gene expression in HeLa cells is also accompanied by a marked increase in the level of the cellular p53 tumor suppressor protein, apparently as a consequence of abrogation of HPV18 E6-mediated destabilization of p53. The proliferation of HT-3 cells, a human cervical carcinoma cell line devoid of detectable HPV DNA, is also inhibited by E2 expression, whereas two other epithelial cell lines that do not contain HPV DNA are not inhibited. Thus, a number of cervical carcinoma cell lines are remarkably sensitive to growth inhibition by the E2 protein. Although BPV E2-mediated inhibition of HPV18 E6 and E7 expression may contribute to growth inhibition in some of the cervical carcinoma cell lines, the BPV E2 protein also appears to exert a growth-inhibitory effect that is independent of its effects on HPV gene expression.

Properties of bovine papillomavirus E1 mutants

Ostensibly comparable mutants of bovine papillomavirus type 1 (BPV-1) affecting the E1 open reading frame that were constructed in several laboratories have been reported to exhibit either reduced or increased transformation efficiencies in established mouse cell lines relative to wild-type BPV-1 DNA. To resolve these discrepancies, we have reexamined many of the mutants in mouse C127 cells by using focus formation assays. Our primary conclusions are that all E1 mutants tested consistently generated reduced numbers of transformants and that the reduced transformation was not due to cell toxicity associated with E1 mutations, as had been proposed. Our results can best be explained by the inability of the E1 mutants to replicate extrachromosomally, therefore leading to a rapid loss of the BPV-1 DNA and consequently, reduced transformation. In support of this hypothesis, we demonstrated that the human papillomavirus type 11 E1 protein was able to suppress BPV-1 transformation, probably because of interference with BPV-1 replication. Therefore, we attribute the phenotypic disparities reported by the various laboratories to still undefined differences in assay conditions.

The E2 binding sites determine the efficiency of replication for the origin of human papillomavirus type 18

Human papillomaviruses (HPV-s) have been shown to possess transforming and immortalizing activity for many different, mainly keratinocyte cell lines and they have been detected in 90% of anogenital cancer tissues, which suggests a causative role in the induction of anogenital and other tumours. We have exploited a quantitative assay to identify and characterize the origin of replication of the human papillomavirus type 18 (HPV-18), one of the most prevalent types in the high-risk HPV group. Replication of HPV origin fragments was studied transiently by cotransfection with a protein expression vector providing replication proteins E1 and E2. We have localized the HPV-18 origin to nucleotides 7767-119. This region contains three E2 binding sites and an essential A/T rich DNA region (nucleotides 9-35) that is partly homologous to the E1 binding site found in bovine papillomavirus type 1 (BPV-1) genome. At least one of the three E2 binding sites was absolutely required for origin function; addition of other E2 sites had cooperative stimulating effect. This is the first quantitative analysis of the E2 binding sites for papillomavirus replication.

Control of human papillomavirus type 11 origin of replication by the E2 family of transcription regulatory proteins

Replication of human papillomavirus type 11 (HPV-11) DNA requires the full-length viral E1 and E2 proteins (C.-M. Chiang, M. Ustav, A. Stenlund, T. F. Ho, T. R. Broker, and L. T. Chow, Proc. Natl. Acad. Sci. USA 89:5799-5803, 1992). Using transient transfection of subgenomic HPV DNA into hamster CHO and human 293 cells, we have localized an origin of replication (ori) to an 80-bp segment in the upstream regulatory region spanning nucleotide 1. It overlaps the E6 promoter region and contains a short A + T-rich segment and a sequence which is homologous to the binding site of the bovine papillomavirus type 1 (BPV-1) E1 protein in the BPV-1 ori. However, unlike the BPV-1 ori, for which half an E2-responsive sequence (E2-RS) or binding site suffices, an intact binding site is essential for the HPV-11 ori. Replication was more efficient when additional E2-RSs were present. The intact HPV-11 genome also replicated in both cell lines when supplied with E1 and E2 proteins. Expression vectors of transcription repressor proteins that lack the N-terminal domain essential for E2 transcriptional trans activation did not support replication in collaboration with the E1 expression vector. Rather, cotransfection with the repressor expression vectors inhibited ori replication by the E1 and E2 proteins. These results demonstrate the importance of the N-terminal domain of the E2 protein in DNA replication and indicate that the family of E2 proteins positively and negatively regulates both viral DNA replication and E6 promoter transcription.

Bovine papillomavirus with a mutation in the E2 serine 301 phosphorylation site replicates at a high copy number

The E2 open reading frame of bovine papillomavirus type 1 (BPV-1) encodes at least three proteins with transcriptional regulatory properties. The full-length E2 open reading frame encodes a transcriptional transactivator, and the 3' region encodes two smaller polypeptides that repress E2-mediated transactivation. The full-length gene product is also required for viral DNA replication. We have demonstrated that the BPV-1 E2 polypeptides are phosphorylated primarily on two serine residues at a site adjacent to the carboxy-terminal DNA binding domain, which is common to all three E2 proteins (A. A. McBride, J. B. Bolen, and P. M. Howley, J. Virol. 63:5076-5085, 1989). These serine residues, at amino acid positions 298 and 301, were substituted with alanine residues in the context of the entire BPV-1 genome. The mutated BPV-1 genomes were introduced into rodent cell lines and assayed for focus formation, viral gene expression, and extrachromosomal viral DNA replication. Viral DNAs containing the E2 serine-to-alanine substitution mutants transformed C127 cells with efficiencies comparable to that of wild-type BPV-1. However, the viral genome containing the serine-to-alanine substitution at position 301 of the E2 polypeptide replicated to a copy number 20-fold higher than that of wild-type DNA.

Regulation of early gene expression from the bovine papillomavirus genome in transiently transfected C127 cells

Expression of bovine papillomavirus (BPV) early gene products is required for viral DNA replication and establishment of the transformed phenotype. By the use of a highly efficient electroporation system, we have examined for the first time the transcriptional activity of BPV promoters in their natural genomic context in a replication-permissive cell line. We have determined that a qualitatively distinct stage of transcription is not detectable prior to DNA replication in transiently transfected cells. This suggests that the transcriptional activity of the BPV genome in stably transformed cells represents the early stage of BPV gene expression. Quantitative differences in promoter activity between transiently transfected and stably transformed cells suggest that subtle changes in gene expression may control progression of the viral life cycle. Deletion analysis demonstrated that the E2 transactivator protein stimulates all of the early promoters through sequences located in the upstream regulatory region. This E2-dependent enhancer was found to be highly redundant, and particular E2 binding sites did not display a preference for particular promoters. Despite this dependence on a common cis-acting sequence, the various promoters displayed different sensitivities to the E2 transactivator. The findings that E2 regulates all promoters and, with the exception of the E2 repressors, that no other known viral gene product appears to affect transcription indicate that the E2 system functions as the master regulator of BPV early gene expression.

Targeting the E1 replication protein to the papillomavirus origin of replication by complex formation with the E2 transactivator

The mechanism by which transcription factors stimulate DNA replication in eukaryotes is unknown. Bovine papillomavirus DNA synthesis requires the products of the viral E1 gene and the transcriptional activator protein encoded by the E2 gene. Experimental data showed that the 68-kilodalton (kD) E1 protein formed a complex with the 48-kD E2 transcription factor. This complex bound specifically to the viral origin of replication, which contains multiple binding sites for E2. Repressor proteins encoded by the E2 open reading frame failed to complex with E1 suggesting that the 162-amino acid region of E2 that participates in transactivation contained critical determinants for interaction with E1. The physical association between a replication protein and a transcription factor suggests that transcriptional activator proteins may function in targeting replication initiator proteins to their respective origins of replication.

The E2 transactivator of bovine papillomavirus type 1 is expressed from multiple promoters

The E2 proteins of bovine papillomavirus type 1 (BPV-1) are a family of site-specific DNA-binding proteins which regulate viral transcription by repression and activation. Repressors E2-TR and E8/E2 are expressed from promoters P5 (P3080) and P3 (P890), respectively. Previous reports have provided evidence that the transcript for the 48-kilodalton transactivator is initiated from a promoter proximal to the open reading frame encoding this protein (P2440 or P4). Our studies extend these findings and show that the E2 transactivation gene is expressed from multiple promoters. We have described the isolation of a cDNA (N15-2) which represents an RNA species expressed from the P3 promoter. The major exon of this species was produced by splicing to an acceptor located at nucleotide 2558 and contained the complete E2 open reading frame. The acceptor is probably utilized by yet another more abundant mRNA expressed from the P2 promoter (A. Stenlund, J. Zabielski, H. Ahola, J. Moreno-Lopez, and U. Pettersson, J. Mol. Biol. 182:541-554, 1985). Linked to a surrogate promoter, the N15-2 cDNA can transactivate an E2-responsive reporter gene. BPV-1 plasmids containing mutations either in the 2558 splice acceptor or in the P4 promoter showed significantly reduced transforming ability and reduced ability to transactivate an E2-responsive reporter, while a double mutant was inactive in both assays. The transformation defect was complemented by an E2 expression vector, and the BPV genome absolutely required the E2 protein to transactivate in the second assay. Thus, these genetic experiments show that alternate modes of E2 expression contribute to the E2 mRNA pool. Direct analysis of cytoplasmic RNA from transformed cultured cells proves that transcripts containing the 2558 acceptor exon are approximately as abundant as the P4 type E2 mRNAs. Furthermore, analysis of the E2 proteins present in various cell lines harboring specific BPV-1 mutants, including the 2558 acceptor mutant, proves that alternate modes of E2 expression exist. The ability of the E2 activator and repressors to each be independently expressed from multiple E2-responsive promoters probably adds to the resiliency of the latent virus as a plasmid and may be important for its homeostasis within the cell in different environmental or developmental situations.

Phenotypic analysis of bovine papillomavirus type 1 E2 repressor mutants

The bovine papillomavirus type 1 (BPV-1) E2 open reading frame encodes three proteins: the E2 transcriptional transactivator, the E2 transcriptional repressor (E2-TR), and the E8/E2 fusion peptide. In this study, we describe the phenotypes of BPV-1 mutants which are disrupted in their capacity to encode either the E2 transcriptional repressor or the E8/E2 fusion peptide. We also describe experiments which demonstrate that the E8/E2 gene product functions similarly to E2-TR. In the context of the entire viral genome, disruption of E8/E2 expression had little effect on viral processes, whereas disruption of E2-TR expression resulted in a complex phenotype marked by a 10- to 20-fold increase in viral DNA plasmid copy number as well as increased transformation potential. A double mutant, defective in the expression of both E8/E2 and E2-TR proteins, had high levels of E2 transactivation activity yet had reduced plasmid replication capacity and a delayed capacity to transform rodent cells.