Induction of tetrasomy by human papillomavirus type 16 E7 protein is independent of pRb binding and disruption of differentiation

Human papillomavirus oncoproteins and post-translational modifications: generating multifunctional hubs for overriding cellular homeostasis

Human papillomaviruses (HPVs) are major human carcinogens, causing around 5% of all human cancers, with cervical cancer being the most important. These tumors are all driven by the two HPV oncoproteins E6 and E7. Whilst their mechanisms of action are becoming increasingly clear through their abilities to target essential cellular tumor suppressor and growth control pathways, the roles that post-translational modifications (PTMs) of E6 and E7 play in the regulation of these activities remain unclear. Here, we discuss the direct consequences of some of the most common PTMs of E6 and E7, and how this impacts upon the multi-functionality of these viral proteins, and thereby contribute to the viral life cycle and to the induction of malignancy. Furthermore, it is becoming increasingly clear that these modifications, may, in some cases, offer novel routes for therapeutic intervention in HPV-induced disease.

The human papillomavirus E7 oncoprotein as a regulator of transcription

High-risk human papillomaviruses (HPVs) encode oncoproteins which manipulate gene expression patterns in the host keratinocytes to facilitate viral replication, regulate viral transcription, and promote immune evasion and persistence. In some cases, oncoprotein-induced changes in host cell behavior can cause progression to cancer, but a complete picture of the functions of the viral oncoproteins in the productive HPV life cycle remains elusive. E7 is the HPV-encoded factor most responsible for maintaining cell cycle competence in differentiating keratinocytes. Through interactions with dozens of host factors, E7 has an enormous impact on host gene expression patterns. In this review, we will examine the role of E7 specifically as a regulator of transcription. We will discuss mechanisms of regulation of cell cycle-related genes by E7 as well as genes involved in immune regulation, growth factor signaling, DNA damage responses, microRNAs, and others pathways. We will also discuss some unanswered questions about how transcriptional regulation by E7 impacts the biology of HPV in both benign and malignant conditions.

The papillomavirus E7 proteins

E7 is an accessory protein that is not encoded by all papillomaviruses. The E7 amino terminus contains two regions of similarity to conserved regions 1 and 2 of the adenovirus E1A protein, which are also conserved in the simian vacuolating virus 40 large tumor antigen. The E7 carboxyl terminus consists of a zinc-binding motif, which is related to similar motifs in E6 proteins. E7 proteins play a central role in the human papillomavirus life cycle, reprogramming the cellular environment to be conducive to viral replication. E7 proteins encoded by the cancer-associated alpha human papillomaviruses have potent transforming activities, which together with E6, are necessary but not sufficient to render their host squamous epithelial cell tumorigenic. This article strives to provide a comprehensive summary of the published research studies on human papillomavirus E7 proteins.

Genomic instability and cancer: lessons learned from human papillomaviruses

High-risk HPV E6 and E7 oncoproteins cooperate to subvert critical host cell cycle checkpoint control mechanisms in order to promote viral genome replication. This results not only in aberrant proliferation but also in host cellular changes that can promote genomic instability. The HPV-16 E7 oncoprotein was found to induce centrosome abnormalities thereby disrupting mitotic fidelity and increasing the risk for chromosome missegregation and aneuploidy. In addition, expression of the high-risk HPV E7 oncoprotein stimulates DNA replication stress as a potential source of DNA breakage and structural chromosomal instability. Proliferation of genomically unstable cells is sustained by several mechanisms including the accelerated degradation of claspin by HPV-16 E7 and the degradation of p53 by the high-risk HPV E6 oncoprotein. These results highlight the oncogenic potential of aberrant proliferation and opens new avenues for prevention of malignant progression, not only in HPV-associated cervical cancer but also in non-virally associated malignancies with disrupted cell cycle checkpoint control mechanisms.

Fluorescence spectroscopy of an in vitro model of human cervical neoplasia identifies graded spectral shape changes with neoplastic phenotype and a differential effect of acetic acid

BACKGROUND

The clinical utility of spectroscopic methods for the diagnosis of cervical cancer is limited by significant inter-patient variation in the spectroscopic properties of the cervix. Improved understanding of the contributions of the components of cervical tissue to the observed spectra would therefore be helpful in the development of spectroscopic approaches to the study of cervical disease in vivo.

METHODS

In this study, we used organotypic epithelial raft culture as an in vitro model system to analyse the fluorescence properties of the surface squamous epithelium specifically. The spectrum of cervical dysplasia was modelled by producing rafts lined by primary human keratinocytes (PHKs) and the HaCaT, SiHa and CaSki human keratinocyte cell lines and fluorescence emission spectra were recorded at a wide range of excitation wavelengths.

RESULTS

Statistically significant differences in spectral shape were identified between the different rafts at excitation wavelengths between 250nm and 310nm. A graded, differential effect of acetic acid on fluorescence intensity was also observed, consistent with the visible effects of acetic acid on clinical examination at colposcopy.

CONCLUSION

These data suggest that the development of neoplastic changes in the squamous epithelium of the cervix are associated with alterations in its fluorescence properties and that the application of acetic acid has a demonstrable effect on these properties. Identification of these alterations may aid the discrimination of cervical lesions in vivo.

Molecular mechanisms of cervical carcinogenesis by high-risk human papillomaviruses: novel functions of E6 and E7 oncoproteins

Over the last two decades, since the initial discovery of human papillomavirus (HPV) type 16 and 18 DNAs in cervical cancers by Dr. Harald zur Hausen (winner of the Nobel Prize in Physiology or Medicine, 2008), the HPVs have been well characterised as causative agents for cervical cancer. Viral DNA from a specific group of HPVs can be detected in at least 90% of all cervical cancers and two viral genes, E6 and E7, are invariably expressed in HPV-positive cervical cancer cells. Their gene products are known to inactivate the major tumour suppressors, p53 and retinoblastoma protein (pRB), respectively. In addition, one function of E6 is to activate telomerase, and E6 and E7 cooperate to effectively immortalise human primary epithelial cells. Though expression of E6 and E7 is itself not sufficient for cancer development, it seems to be either directly or indirectly involved in every stage of multi-step carcinogenesis. Epidemiological and biological studies suggest the potential efficacy of prophylactic vaccines to prevent genital HPV infection as an anti-cancer strategy. However, given the widespread nature of HPV infection and unresolved issues about the duration and type specificity of the currently available HPV vaccines, it is crucial that molecular details of the natural history of HPV infection as well as the biological activities of the viral oncoproteins be elucidated in order to provide the basis for development of new therapeutic strategies against HPV-associated malignancies. This review highlights novel functions of E6 and E7 as well as the molecular mechanisms of HPV-induced carcinogenesis.

Early detection of cervical neoplasia by Raman spectroscopy

Early detection of malignant tumours, or their precursor lesions, improves patient outcome. High risk human papillomavirus (HPV), particularly HPV16, infection can lead to the development of uterine cervical neoplasia, and therefore, the identification in clinical samples of the effects of HPV infection may have clinical value. In this report, we apply Raman microspectroscopy to live and fixed cultured cells to discriminate between defined cell types. Raman spectra were acquired from primary human keratinocytes (PHK), PHK expressing the E7 gene of HPV 16 (PHK E7) and CaSki cells, an HPV16-containing cervical carcinoma-derived cell line. Averaged Raman spectra showed variations, mostly in peaks originating from DNA and proteins, consistent with HPV gene expression and cellular changes associated with neoplasia, in both live and fixed cells. Principal component analysis produced good discrimination between the cell types, with sensitivities of up to 100% for the comparison of fixed PHK and CaSki. These results demonstrate the ability of Raman spectroscopy to discriminate between cell types representing different stages of cervical neoplasia. More specifically, this technique was able to identify cells expressing the HPV 16 E7 gene accurately and objectively, suggesting that this approach may be of value in diagnosis. Moreover, the ability to detect the effects of the virus in fixed samples also demonstrates the compatibility of Raman spectroscopy with current cervical screening methods. (c) 2007 Wiley-Liss, Inc.

Selective induction of apoptosis by leptomycin B in keratinocytes expressing HPV oncogenes

Human papillomavirus (HPV) infection is strongly associated with the development of anogenital neoplasia, particularly cervical cancer. It has been estimated that 99.7% of all cervical carcinomas are attributable to infection with HPV, and types 16 and 18 account for the vast majority of such cases. Both of these 'high risk' HPV types encode the oncoproteins E6 and E7, which exert multiple effects on many proteins involved in cell-cycle regulation, including p53. The nuclear export protein inhibitor leptomycin B (LMB) has been shown to cause the nuclear sequestration of p53 in cervical carcinoma cells. We demonstrate that LMB induces apoptosis selectively at nanomolar concentrations in primary human keratinocytes (PHKs) expressing HPV oncogenes. Both monolayer and organotypic raft cultures of transduced PHKs were highly susceptible to treatment with LMB. By contrast, although LMB stimulated p53 accumulation in normal PHKs, no significant induction of apoptosis was detected on Western blots or immunostained monolayer/raft cells, or following pulsed exposure to the drug. Furthermore, topical application of microM concentrations of LMB to mouse skin was non-toxic. These data suggest that the topical application of LMB to HPV-infected intra-epithelial lesions may represent a specific and effective therapeutic strategy against HPV-associated anogenital neoplasia.

Fluorescence spectroscopy of anin vitro model of human cervical precancer identifies neoplastic phenotype

The clinical diagnosis of cervical neoplasia by spectroscopic methods is potentially a reliable, fast and cost-effective alternative to the conventional smear test. However, it is currently limited by significant inter-patient variation in the spectroscopic properties of the cervix. Characterisation of suitable in vitro models of the spectroscopic changes that take place during neoplastic progression may prove to be a significant step towards the successful development of reliable in vivo systems. In this study, we used organotypic epithelial raft culture as an in vitro model of cervical tissue to analyse changes in the fluorescence properties of surface squamous epithelium that are associated with the development of neoplastic disease. Collagen plugs lined by primary human keratinocytes (PHKs) were used to model the normal cervical epithelium, and plugs lined by cells of the SiHa line were used as a model of neoplastic cervical tissue. Fluorescence emission spectra of these rafts were recorded at excitation wavelengths in the 250-330 nm range, complementing previous work published at longer wavelengths. Normalised, truncated emission spectra were analysed using multivariate principal component analysis. We successfully distinguished between in vitro models of normal and neoplastic cervical tissue and demonstrated a differential effect of acetic acid, which enhances the discrimination of normal from neoplastic tissue. Identification of these differences between in vitro organotypic epithelial rafts may ultimately aid the discrimination of cervical lesions in vivo.

Induction of tetraploidy through loss of p53 and upregulation of Plk1 by human papillomavirus type-16 E6

Cancer cells are insensitive to many signals that inhibit growth of untransformed cells. Here, we show that primary human epithelial cells expressing human papillomavirus (HPV) type-16 E6/E7 bypass arrest caused by the DNA-damaging drug adriamycin and become tetraploid. To determine the contribution of E6 in the context of E7 to the resistance of arrest and induction of tetraploidy, we used an E6 mutant unable to degrade p53 or RNAi targeting p53 for knockdown. The E6 mutant fails to generate tetraploidy; however, the presence of E7 is sufficient to bypass arrest while the p53 RNAi permits both arrest insensitivity and tetraploidy. We published previously that polo-like kinase 1 (Plk1) is upregulated in E6/E7-expressing cells. We observe here that abnormal expression of Plk1 protein correlates with tetraploidy. Using the p53 binding-defective mutant of E6 and p53 RNAi, we show that p53 represses Plk1, suggesting that loss of p53 results in tetraploidy through upregulation of Plk1. Consistent with this hypothesis, overexpression of Plk1 in cells generates tetraploidy but does not confer resistance to arrest. These results support a model for transformation caused by HPV-16 where bypass of arrest and tetraploidy are separable consequences of p53 loss with Plk1 required only for the latter effect.

HPV-associated skin disease

Human papillomaviruses (HPVs) are DNA tumour viruses that induce hyperproliferative lesions in cutaneous and mucosal epithelia. The relationship between HPV and non-melanoma skin cancer (NMSC) is important clinically since NMSC is the most common form of malignancy among fair-skinned populations. It is well established that solar ultraviolet (UV) irradiation is the major risk factor for developing NMSC, but a pathogenic role for HPV in the development of NMSC has also been proposed. Recent molecular studies reveal a likely role for HPV infection in skin carcinogenesis as a co-factor in association with UV. This review summarizes the literature describing these data, highlights some of the important findings derived from these studies, and speculates on future perspectives.

Regulation of human papillomavirus type 16 E7 activity through direct protein interaction with the E2 transcriptional activator

In order to ensure a productive life cycle, human papillomaviruses (HPVs) require fine regulation of their gene products. Uncontrolled activity of the viral oncoproteins E6 and E7 results in the immortalization of the infected epithelial cells and thus prevents the production of mature virions. Ectopically expressed E2 has been shown to suppress transcription of the HPV E6 and E7 region in cell lines where the viral DNA is integrated into the host genome, resulting in growth inhibition. However, it has been demonstrated that growth control of these cell lines can also occur independently of HPV E2 transcriptional activity in high-risk HPV types. In addition, E2 is unable to suppress transcription of the same region in cell lines derived from cervical tumors that harbor only episomal copies of the viral DNA. Here we show that HPV type 16 (HPV-16) E2 is capable of inhibiting HPV-16 E7 cooperation with an activated ras oncogene in the transformation of primary rodent cells. Furthermore, we demonstrate a direct interaction between the E2 and E7 proteins which requires the hinge region of E2 and the zinc-binding domain of E7. These viral proteins interact in vivo, and E2 has a marked effect upon both the stability of E7 and its cellular location, where it is responsible for recruiting E7 onto mitotic chromosomes at the later stages of mitosis. These results demonstrate a direct role for E2 in regulating the function of E7 and suggest an important role for E2 in directing E7 localization during mitosis.

Viral carcinogenesis and genomic instability

Oncogenes encoded by human tumor viruses play integral roles in the viral conquest of the host cell by subverting crucial and relatively non-redundant regulatory circuits that regulate cellular proliferation, differentiation, apoptosis and life span. Human tumor virus oncoproteins can also disrupt pathways that are necessary for the maintenance of the integrity of host cellular genome. Some viral oncoproteins act as powerful mutator genes and their expression dramatically increases the incidence of host cell mutations with every round of cell division. Others subvert cellular safeguard mechanisms intended to eliminate cells that have acquired abnormalities that interfere with normal cell division. Viruses that encode such activities can contribute to initiation as well as progression of human cancers.

Human Papillomavirus Infections of the Genital and Respiratory Tracts in Young Children

Human papillomavirus (HPV) causes papillomas (warts) on the skin and respiratory mucosal surfaces (laryngeal and oral papillomas) in addition to condyloma acuminata (anogenital warts). HPV has become one of the most common sexually transmitted diseases in adults. Vertical transmission from mother to infant during birth is well recognized. Laryngeal papillomas are the most common tumors of the larynx in children worldwide, and recurrent lesions are common occurrences. Anogenital warts in children are problematic in that child sexual abuse is a potential means of acquisition, but many cases are acquired perinatally. Postnatal acquisition by nonsexual means also can occur. The likelihood of sexual abuse as the mode of acquisition increases with increasing age in childhood. The virus infects primarily epithelial cells, where it can exist as a long-term latent infection that can reactivate or persist actively (even subclinically), with resultant accumulation of host chromosomal mutations. The latter accounts for the oncogenic potential of a number of HPV types, and childhood infections may lead to neoplasia later in life. Regression of papillomas over the course of months to years is the usual natural course. Numerous treatments are available, but most do not prevent persistent infection or problematic recurrences. Multivalent HPV vaccines have been developed, and early results of clinical trials appear to be very promising.

Amplification of the chromosome 20q region is associated with expression of HPV-16 E7 in human airway and anogenital epithelial cells

To study the role of human papillomavirus (HPV) infection in the development of genetic instability, we transduced normal human airway and anogenital epithelial cells with various combinations of HPV-16 E6, E7, and the reverse transcriptase component of telomerase (hTERT). Cell lines generated by co-expression of E7 with E6 and/or hTERT (i.e., E6/E7, E7/hTERT, and E6/E7/hTERT) exhibited extra copies of chromosome 20 and specific amplification of the 20q12-ter region, whereas those generated without E7 (i.e., hTERT alone or E6/hTERT) did not. Co-expression of hTERT and a dominant-negative version of cdk4 that has been shown to inactivate the retinoblastoma (pRb) pathway also resulted in 20q amplification. Interestingly, extra copies of chromosome 20 were observed in early passage keratinocytes that expressed E7 alone, and microarray expression analysis revealed that genes in the 20q region and on chromosome 5 were specifically upregulated in these cells. Our results indicate that chromosome 20q amplification is an early event that may be specifically caused by expression of E7 through inactivation of the pRb pathway in human epithelial cells.

Update on pathophysiologic mechanisms of human papillomavirus

PURPOSE OF REVIEW

The role of the human papillomavirus in the pathogenesis has been the subject of many publications in the recent literature. The physical state of the human papillomavirus and the role of chromosomal aneuploidy has been reported. This review discusses the recent pathological mechanisms described in the genesis of human papillomavirus-related disease.

RECENT FINDINGS

The mere presence of the human papillomavirus is not sufficient for the development of neoplasia. Genetic and other co-factors seem to be necessary for the expression of the invasive phenotype. The expression of human papillomavirus 16 E6-E7 oncogenes results in chromosomal aneuploidy, favouring the integration of high-risk human papillomavirus genomes into cellular chromosomes. The integration of human papillomavirus 16 may not always be required for the progression to the invasive phenotype unlike human papillomavirus 18 DNA. Such integration sites are randomly distributed over the whole genome. The genetic susceptibility of codon 98 of the fragile histadine triad has been elucidated.

SUMMARY

Various molecular mechanisms of human papillomavirus-associated neoplasia are discussed. The interaction between HIV and human papillomavirus are complex and favour the persistence and progression of cervical disease. Future research should pave the way for therapeutic vaccine development.

Inhibition of human papillomavirus type 16 E7 phosphorylation by the S100 MRP-8/14 protein complex

The human papillomavirus type 16 (HPV16) E7 is a major viral oncoprotein that is phosphorylated by casein kinase II (CKII). Two S100 family calcium-binding proteins, macrophage inhibitory-related factor protein 8 (MRP-8) and MRP-14, form a protein complex, MRP-8/14, that inactivates CKII. The MRP-8/14 protein complex may inhibit CKII-mediated E7 phosphorylation and therefore may alter its interaction with cellular ligands and reduce E7 oncogenic activity. We examined the inhibitory effect of the MRP-8/14 complex on CKII activity and HPV16 E7 phosphorylation. We have shown that CKII activity and HPV16 E7 phosphorylation were inhibited by uptake of exogenous MRP-8/14 and activation of endogenous MRP-8/14. MRP-8/14-mediated inhibition of E7 phosphorylation occurred at the G1 phase of the cell cycle. Analysis of MRP expression in primary keratinocytes and in HPV16- and 18-transformed cervical and foreskin epithelial cell lines showed that expression of MRP-8, MRP-14, and the MRP-8/14 complex was detected only in primary untransformed keratinocytes and not in the HPV-infected immortalized epithelial cells. CKII activity in HPV-immortalized keratinocytes was approximately fourfold higher than in HPV-negative primary keratinocytes. Treatment of HPV-positive immortalized epithelial cells with exogenous MRP-8/14 resulted in E7 hypophosphorylation and complete inhibition of cell growth within 2 weeks, compared with HPV-negative primary and immortalized HPV-negative cervical epithelial cells, which showed 25 and 40% growth inhibition, respectively. Together these results suggests that the MRP-8/14 protein complex in HPV-infected epithelial cells may play an important role in regulation of CKII-mediated E7 phosphorylation and inhibition of its oncogenic activity.