The human papillomavirus type 16 E7 oncogene is required for the productive stage of the viral life cycle

Intraepithelial DNA immunisation with a plasmid encoding a codon optimised COPV E1 gene sequence, but not the wild-type gene sequence completely protects against mucosal challenge with infectious COPV in beagles

DNA plasmids encoding the open reading frames of canine oral papillomavirus (COPV) nonstructural early genes E1, E2, or E7 protein were delivered into both oral mucosal and cutaneous epithelial sites in beagle dogs using particle-mediated immunotherapeutic delivery (PMID) technology. Control dogs were vaccinated with plasmid encoding either hepatitis B virus surface antigen (HBVs) or COPV L1. Using a prophylactic immunisation protocol, a priming dose of plasmid DNA was followed by a booster dose 6 weeks later. Four weeks after boost, all dogs were challenged with infectious COPV particles. Following viral challenge, as shown previously (M. A. Stanley et al., 2001, Vaccine 19, 2783-2792), mucosal papillomas developed in the negative-control HBVs vaccinated dogs, but all animals in the COPV L1 group were fully protected from disease development. In the early gene-vaccinated groups five of six in the E1-vaccinated dogs, two of six in E2-vaccinated dogs, and three of six in the E7-vaccinated beagles developed oral papillomas. Compared to the HBVs negative-control group the oral papillomas that did develop in the early-gene vaccinated beagles were significantly smaller, shorter in duration, and fewer in number. Taken together the disease burden was markedly reduced and this was statistically significant. In a second experiment one group of animals was vaccinated with plasmid encoding the wild-type COPV E1 gene, and a separate group was vaccinated with plasmid encoding a synthetic codon-optimised COPV E1 gene sequence. None of the codon-optimised E1-vaccinated animals developed papillomas at any challenge site. However, all animals vaccinated with wild-type E1 had papillomas. These data suggest that immunisation by PMID with papillomavirus early genes can significantly impact upon subsequent disease development and that full protection can be achieved using improved vectors encoding codon-optimised gene sequences perhaps emphasizing the importance of antigen load in the generation of protective responses to papillomavirus proteins.

The role of human papillomavirus oncoproteins E6 and E7 in apoptosis

The oncogenic potential of 'high risk' human papillomaviruses can be mainly attributed to two small proteins called E6 and E7. Even these oncoproteins have a low molecular size, they are highly promiscuous and are capable to interact with a whole variety of host cellular regulator proteins to elicit cellular immortalization and ultimately complete malignant transformation. To avoid reiterations in summarizing the biochemical and molecular biological properties of E6/E7 in terms of their influence on cell cycle control, the present review is mainly an attempt to describe some regulatory principles by which human papillomavirus (HPV) oncoproteins can interfere with apoptosis in order to escape immunological surveillance during progression to cervical cancer. The models derived from these basic cellular and molecular studies are relevant to our understanding of HPV-induced carcinogenesis. Conversely, experimental procedures aimed at relieving apoptosis resistance, can facilitate the eradication of immunologically suspicious cells and may prevent the accumulation of cervical intraepithelial cell abnormalities in future prophylactic or therapeutic approaches.

Human papillomavirus immortalization and transformation functions

The high risk HPVs (such as HPV-16 and HPV-18) that are associated with specific anogenital cancers encode two oncoproteins E6 and E7, which are expressed in the HPV positive cancers. The E7 protein functions in cellular transformation, at least in part, through interactions with pRB and the other pRB related 'pocket proteins'. The major target of the E6 oncoprotein encoded by the genital tract, cancer associated human papillomaviruses is p53. Several lines of evidence suggest that E6 and E7 have additional targets important to the oncogenic potential of the virus. Work from a number of laboratories has focused on determining other activities of HPV relevant to carcinogenesis and identifying additional cellular targets of E6 and E7. This paper will review the state of the field at the time of the 19th International Papillomavirus Workshop in September 2001 with respect to the HPV encoded oncoproteins.

Genetic analysis of high-risk e6 in episomal maintenance of human papillomavirus genomes in primary human keratinocytes

Papillomaviruses possess small DNA genomes that encode five early (E) proteins. Transient DNA replication requires activities of the E1 and E2 proteins and a DNA segment containing their binding sites. The E6 and E7 proteins of cancer-associated human papillomavirus (HPV) transform cells in culture. Recent reports have shown that E6 and E7 are necessary for episomal maintenance of HPV in primary keratinocytes. The functions of E6 necessary for viral replication have not been determined, and to address this question we used a recently developed transfection system based on HPV31. To utilize a series of HPV16 E6 mutations, HPV31 E6 was replaced by its HPV16 counterpart. This chimeric genome was competent for both transient and stable replication in keratinocytes. Four HPV16 E6 mutations that do not stimulate p53 degradation were unable to support stable viral replication, suggesting this activity may be necessary for episomal maintenance. E7 has also been shown to be essential for episomal maintenance of the HPV31 genome. A point mutation in the Rb binding motif of HPV E7 has been reported to render HPV31 unable to stably replicate. Interestingly, HPV31 genomes harboring two of the three p53 degradation-defective E6 mutations combined with this E7 mutation were maintained as replicating episomes. These findings imply that the balance between E6 and E7 functions in infected cells is critical for episomal maintenance of high-risk HPV genomes. This model will be useful to dissect the activities of E6 and E7 necessary for viral DNA replication.

HPV episomal copy number closely correlates with cell size in keratinocyte monolayer cultures

W12E keratinocytes maintaining episomal copies of HPV DNA were separated according to size by centrifugal elutriation. HPV DNA copy number was greatly increased in the largest, most differentiated cells of the population. The large cells with the highest HPV copy number also showed evidence of endoreduplication of host cell DNA. Other cell lines maintaining episomal copies of HPV18 and HPV31 were also tested with all lines showing similar results. The results demonstrate that increase in HPV DNA copy number correlates well with increased cell size, a fundamental marker of keratinocyte differentiation. The results also indicate that simple monolayer cultures may be useful for studying the relationship between differentiation, HPV DNA replication, and cell-cycle events.

Infectious virions produced from a human papillomavirus type 18/16 genomic DNA chimera

The organotypic raft culture system has allowed the study of the differentiation-dependent aspects of the human papillomavirus (HPV) life cycle. However, genetic strategies to more completely understand the HPV life cycle are limited. The generation of chimeric viruses has been a useful tool in other virus systems to analyze infection and replication. To investigate the specificity of the interaction of nonstructural genes of one HPV type with the structural genes of another HPV type, we have replaced the L2 and L1 open reading frames (ORFs) of HPV type 18 (HPV18) with the L2 and L1 ORFs of HPV type 16 (HPV16). The resulting HPV18/16 chimeric construct was introduced into primary keratinocytes, where it was stably maintained episomally at a range of 50 to 100 copies of HPV genomic DNA, similar to that typically found in HPV-infected cells in vivo. The integrity of the HPV18/16 genomic DNA chimera was demonstrated. Upon differentiation in raft cultures, late viral functions, including viral DNA amplification, capsid gene expression, and virion morphogenesis, occurred. Chimeric HPV18/16 virions were purified from the raft cultures and were capable of infecting keratinocytes in vitro. Additionally, infection was specifically neutralized with human HPV16 virus-like particle (VLP)-specific antiserum and not with human HPV18 VLP-specific antiserum. Our data demonstrate that the nonstructural genes of HPV18 functionally interact with the structural genes of HPV16, allowing the complete HPV life cycle to occur. We believe that this is the first report of the propagation of chimeric HPV by normal life cycle pathways.

Human papillomavirus type 16 E7 binds to E2F1 and activates E2F1-driven transcription in a retinoblastoma protein-independent manner

The human papillomavirus (HPV) E7 oncoprotein can immortalize primary human cells and induce tumor formation. These properties of E7 depend on its ability to inhibit the activity of retinoblastoma protein (pRB), which in turn affects E2F function. E2F proteins control the expression of genes involved in differentiation, development, cell proliferation, and apoptosis. By using genetic and biochemical approaches, the present study shows that E7 binds to E2F1 in vivo and in vitro and that both proteins co-localize in the nucleus. Importantly, the binding of the high risk group HPV E7 to E2F1 is tighter than the binding of the low risk group HPV E7 to E2F1. Although E7 of the high risk group HPVs activates E2F1-dependent transcription strongly in C33A or 293T cells, E7 of the low risk group HPVs activates transcription only weakly. By using electrophoretic mobility shift assay, we also showed that E7 binds to E2F1-DNA complexes. Furthermore, we show that these activities of E7 are independent of pRB by using E7 and E2F1 mutants that cannot bind to pRB. Taken together, these data suggest that E7 contributes to the deregulation of pRB-dependent E2F1 repression and to the further activation of E2F1 independently of pRB.

Inhibition of serum- and calcium-induced terminal differentiation of human keratinocytes by HPV 16 E6: study of the association with p53 degradation, inhibition of p53 transactivation, and binding to E6BP

Transfection of the E6 gene of human papillovirus (HPV) 16 into primary human keratinocytes (PHKs) generates proliferating cell colonies which are resistant to serum- and calcium-induced terminal differentiation. The extreme C-terminus of E6 was shown to be dispensable for this activity. To map further the amino acid sequences required for inducing resistance to serum and calcium, and to address the functional significance of E6 interactions with p53 and E6BP (ERC-55) in this function, we evaluated the activities of a series of E6 mutants. Small deletions within the central portion of the second putative zinc-finger abolished, or markedly reduced, E6 biological activity, while mutations affecting the cysteine residues in the base of the finger were less effective in this respect. When these mutants were assayed for their ability to degrade p53 in vitro and in vivo and to inhibit p53 transcriptional activation (TA), we found that there was a dissociation of these activities in some mutants. We mapped one mutant which was highly efficient in p53 degradation and inhibition of p53 TA, yet displayed severely reduced activity in the biological assay, and conversely, a subset of mutants that showed moderate activities in the colony assay while being severely impaired in p53 degradation and inhibition of p53 TA. These data argue that p53 inactivation or even elimination are not sufficient, and may not be essential, for altering the response of PHKs to serum and calcium. When these E6 mutants were evaluated for E6BP binding in vitro, there was a similar dissociation between the biological and biochemical activities of several mutants. We mapped mutants with moderate activity in the biological assay that lacked the ability to bind to E6BP and a mutant that showed high biological activity with only marginal binding to E6BP. Thus, there is no absolute correlation between the ability of E6 mutant proteins to induce alterations in keratinocyte differentiation responses to calcium and serum and to induce p53 degradation, inhibit p53 mediated transactivation, or bind E6BP. Evidently there are additional cellular targets for E6 which mediate this alteration in cellular differentiation.

Biological activities and molecular targets of the human papillomavirus E7 oncoprotein

The human papillomavirus (HPV) E7 protein is one of only two viral proteins that remain expressed in HPV-associated human cancers. HPV E7 proteins share structural and functional similarities with oncoproteins encoded by other small DNA tumor viruses such as adenovirus E1A and SV40 large tumor antigen. The HPV E7 protein plays an important role in the viral life cycle by subverting the tight link between cellular differentiation and proliferation in normal epithelium, thus allowing the virus to replicate in differentiating epithelial cells that would have normally withdrawn from the cell division cycle. The transforming activities of E7 largely reflect this important function.

Cellular changes induced by low-risk human papillomavirus type 11 in keratinocytes that stably maintain viral episomes

Infections by low-risk papillomavirus types, such as human papillomavirus (HPV) type 6 (HPV-6) and HPV-11, induce benign genital warts that rarely progress to malignancy. In contrast, lesions induced by high-risk HPV types have the potential to progress to cancer. Considerable information is available concerning the pathogenesis of high-risk HPV types, but little is known about the life cycle of low-risk HPV types. Although functionally distinct, both high- and low-risk virus types infect keratinocytes and induce virion production upon differentiation. This information suggests that they may share common mechanisms for regulating their productive life cycles. Using tissue culture methods developed to study high-risk HPV types, we examined the ability of HPV-11 to be stably maintained as episomes following transfection of normal human keratinocytes with cloned viral DNA. HPV-11 genomes were found to be maintained in keratinocytes for extended passages in cultures in 14 independent experiments involving transfection of cloned HPV-11 DNA. Interestingly, the HPV-11-positive cells exhibited an extended life span that averaged approximately twofold longer than that of control neomycin-transfected cells. In organotypic cultures, HPV-11-positive cells exhibited altered differentiation patterns, but the extent of disruption was less severe than that seen with high-risk HPV types. In addition, the amplification of HPV-11 DNA, as well as the induction of several viral messages, was observed following differentiation of transfected cells in semisolid media. To determine whether global changes in cellular gene expression induced by HPV-11 were similar to those observed with high-risk HPV-31 (Y. E. Chang and L. A. Laimins, J. Virol. 74:4174-4182, 2000), microarray analysis of 7,075 expressed sequences was performed. A spectrum of cellular genes different from that previously reported for HPV-31 was found to be activated or repressed by HPV-11. The expression of only a small set of genes was similarly altered by both high- and low-risk HPV types. This result suggests that different classes of HPVs have distinct effects on global cellular transcription patterns during infection. The methods described allow for a genetic analysis of HPV-11 in the context of its differentiation-dependent life cycle.

Centrosome abnormalities and genomic instability by episomal expression of human papillomavirus type 16 in raft cultures of human keratinocytes

Primary human keratinocytes with ectopic expression of high-risk human papillomavirus (HPV) E6 and E7 oncoproteins display abnormal centrosome numbers, multipolar mitoses, and aneusomy. However, it has not been explored whether these abnormalities can occur in cells containing HPV episomes where E6 and E7 expression is under viral transcriptional control. Here, we demonstrate that centrosome abnormalities and genomic instability occur in organotypic raft cultures of human keratinocytes with episomal HPV-16 even at low copy numbers. We conclude that HPV-16 DNA, when maintained as an episome, can disturb centrosome homeostasis and subvert genomic integrity of the host cell during early stages of the viral infection.