Human papillomavirus E7 proteins stimulate proliferation independently of their ability to associate with retinoblastoma protein

HPV16 E7 Nucleotide Variants Found in Cancer-Free Subjects Affect E7 Protein Expression and Transformation

The human papillomavirus (HPV) type 16 E7 oncogene is critical to carcinogenesis and highly conserved. Previous studies identified a preponderance of non-synonymous E7 variants amongst HPV16-positive cancer-free controls compared to those with cervical cancer. To investigate the function of E7 variants, we constructed full-length HPV16 E7 genes and tested variants at positions H9R, D21N, N29S, E33K, T56I, D62N, S63F, S63P, T64M, E80K, D81N, P92L, and P92S (found only in controls); D14E, N29H cervical intraepithelial neoplasia (CIN2), and P6L, H51N, R77S (CIN3). We determined the steady-state level of cytoplasmic and nuclear HPV16 E7 protein. All variants from controls showed a reduced level of E7 protein, with 7/13 variants having lower protein levels. In contrast, 2/3 variants from the CIN3 precancer group had near-wild type E7 levels. We assayed the activity of representative variants in stably transfected NIH3T3 cells. The H9R, E33K, P92L, and P92S variants found in control subjects had lower transforming activity than D14E and N29H variants (CIN2), and the R77S (CIN3) had activity only slightly reduced from wild-type E7. In addition, R77S and WT E7 caused increased migration of NIH3T3 cells in a wound-healing assay compared with H9R, E33K, P92L, and P92S (controls) and D14E (CIN2). These data provide evidence that the E7 variants found in HPV16-positive cancer-free women are partially defective for transformation and cell migration, further demonstrating the importance of fully active E7 in cancer development.

Human papillomavirus E7 induces p63 expression to modulate DNA damage response

Cervical cancer is the third most common malignancy diagnosed in women worldwide. The major aetiological factor underlying the malignant transformation of cervical cells is the persistent infection with high-risk human papillomaviruses (HR-HPV), with more than 99% of cases expressing viral sequences. Here, we report a previously unknown mechanism driven by high-risk human papillomavirus E7 protein to modulate response to DNA damage in cervical cancer cells. Our data shows that HR-HPV E7 oncoprotein induces the transcription of the p53-family member p63, which modulates DNA damage response pathways, to facilitate repair of DNA damage. Based on our findings, we proposed a model, where HR-HPV could interfere with the sensitivity of transformed cells to radiation therapy by modulating DNA damage repair efficiency. Importantly, we have shown for the first time a critical role for p63 in response to DNA damage in cervical cancer cells.

The biology of beta human papillomaviruses

The beta genus comprises more than 50 beta human papillomavirus (HPV) types that are suspected to be involved, together with ultraviolet (UV) irradiation, in the development of non-melanoma skin cancer (NMSC), the most common form of human cancer. Two members of the genus beta, HPV5 and HPV8, were first identified in patients with a genetic disorder, epidermodysplasia verruciformis (EV), that confers high susceptibility to beta HPV infection and NMSC development. The fact that organ transplant recipients (OTRs) with an impaired immune system have an elevated risk of NMSC raised the hypothesis that beta HPV types may also be involved in skin carcinogenesis in non-EV patients. Epidemiological studies have shown that serological and viral DNA markers are weakly, but significantly, associated with history of NMSC in OTRs and the general population. Functional studies on mucosal high-risk (HR) HPV types have clearly demonstrated that the products of two early genes, E6 and E7, are the main viral oncoproteins, which are able to deregulate events closely linked to transformation, such as cell cycle progression and apoptosis. Studies on a small number of beta HPV types have shown that their E6 and E7 oncoproteins also have the ability to interfere with the regulation of key pathways/events associated with cellular transformation. However, the initial functional data indicate that the molecular mechanisms leading to cellular transformation are different from those of mucosal HR HPV types. Beta HPV types may act only at early stages of carcinogenesis, by potentiating the deleterious effects of other carcinogens, such as UV radiation.

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.

HPV 16E7 and 48E7 proteins use different mechanisms to target p130 to overcome cell cycle block

Background

Retinoblastoma like protein 2 (RBL2) or p130 is a member of the pocket protein family, which is infrequently mutated in human tumours. Its expression is posttranscriptionally regulated and largely G0 restricted. We have previously shown that E6/E7 oncoproteins encoded by human papillomavirus (HPV) type 16, which is a high-risk type for cervical cancer development, must target p130 to promote the host cell to exit from quiescence (G0) state and enter S phase of the cell cycle. P130 is associated with the DREAM (DP, RB-like, E2F and MuvB) complex in G0/G1, which prevents S phase progression by repressing transcription of E2F-regulated genes. E7 proteins could potentially disrupt the p130-DREAM complex through two known mechanisms: direct interaction with p130 or induction of cyclin dependent kinase 2 (CDK2) phosphorylation by interacting with its inhibitor, p21^CIP1.

Methods

In this study we have used p130 mutants deficient in binding the E7 LXCXE domain (p130mE7), unphosphorylatable by CDK2 (p130PM22) or a combination of both (p130PM22/mE7) to investigate these mechanisms used by E7 proteins to disrupt the p130-DREAM complex and promote cell cycle progression.

Results

We found that HPV16 E7 binding to p130 through its LXCXE domain was absolutely required to disrupt p130-DREAM to promote S phase of the cell cycle, as HPV16 E7 was unable to suppress p130mE7 but could suppress p130PM22. In contrast, the E7 protein encoded by a cutaneous HPV type that lacks a functional LXCXE domain, HPV 48 E7, was also able to disrupt p130-DREAM to promote cell cycling, but through the alternative mechanism. Thus, HPV48 E7 could suppress a cell cycle block imposed by p130mE7, but was unable to suppress p130PM22.

Conclusions

Overall, these results indicate that suppression of p130 is required for HPV-induced cell cycling, and that different HPV E7 proteins can use alternative mechanisms to achieve this.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-015-0460-8) contains supplementary material, which is available to authorized users.

A functional interaction of E7 with B-Myb-MuvB complex promotes acute cooperative transcriptional activation of both S- and M-phase genes. (129 c)

High-risk human papillomaviruses are causative agents of cervical cancer. Viral protein E7 is required to establish and maintain the pro-oncogenic phenotype in infected cells, but the molecular mechanisms by which E7 promotes carcinogenesis are only partially understood. Our transcriptome analyses in primary human fibroblasts transduced with the viral protein revealed that E7 activates a group of mitotic genes via the activator B-Myb-MuvB complex. We show that E7 interacts with the B-Myb, FoxM1 and LIN9 components of this activator complex, leading to cooperative transcriptional activation of mitotic genes in primary cells and E7 recruitment to the corresponding promoters. E7 interaction with LIN9 and FoxM1 depended on the LXCXE motif, which is also required for pocket protein interaction and degradation. Using E7 mutants for the degradation of pocket proteins but intact for the LXCXE motif, we demonstrate that E7 functional interaction with the B-Myb-MuvB complex and pocket protein degradation are two discrete functions of the viral protein that cooperate to promote acute transcriptional activation of mitotic genes. Transcriptional level of E7 in patient's cervical lesions at different stages of progression was shown to correlate with those of B-Myb and FoxM1 as well as other mitotic gene transcripts, thereby linking E7 with cellular proliferation and progression in cervical cancer in vivo. E7 thus can directly activate the transcriptional levels of cell cycle genes independently of pocket protein stability.

Human papillomaviruses and the specificity of PDZ domain targeting

The human papillomavirus (HPV) E6 oncoprotein is fundamental to the ability of these viruses to induce human malignancy. A defining characteristic of the HPV E6 oncoproteins found in cancer-causing HPV types is the presence of a PDZ binding motif at their extreme C-terminus. Through this motif, E6 is able to interact with a large number of cellular proteins that contain PDZ domains. Many of these cellular proteins are involved in regulation of processes associated with the control of cell attachment, cell proliferation, cell polarity and cell signaling. How E6 targets multiple proteins containing the same recognition domain is still an open question. In this review, we highlight aspects of E6 function and biology that help to answer this question, and thereby provide insight into the role of these substrates during development of HPV-induced malignancy.

A humanized mouse model of HPV-associated pathology driven by E7 expression

Human papillomavirus (HPV) is the causative agent of human cervical cancer and has been associated with oropharyngeal squamous cell carcinoma development. Although prophylactic vaccines have been developed, there is a need to develop new targeted therapies for individuals affected with malignant infected lesions in these locations, which must be tested in appropriate models. Cutaneous beta HPV types appear to be involved in skin carcinogenesis. Virus oncogenicity is partly achieved by inactivation of retinoblastoma protein family members by the viral E7 gene. Here we show that the E7 protein of cutaneous beta HPV5 binds pRb and promotes its degradation. In addition, we described an in vivo model of HPV-associated disease in which artificial human skin prepared using primary keratinocytes engineered to express the E7 protein is engrafted onto nude mice. Expression of E7 in the transplants was stably maintained for up to 6 months, inducing the appearance of lesions that, in the case of HPV16 E7, histologically resembled human anogenital lesions caused by oncogenic HPVs. Moreover, it was confirmed through biomarker expression analysis via immunodetection and/or quantitative PCR from mRNA and miRNA that the 16E7-modified engrafted skin shares molecular features with human HPV-associated pretumoral and tumoral lesions. Finally, our findings indicate a decrease of the in vitro capacity of HPV5 E7 to reduce pRb levels in vivo, possibly explaining the phenotypical differences when compared with 16E7-grafts. Our model seems to be a valuable platform for basic research into HPV oncogenesis and preclinical testing of HPV-associated antitumor therapies.

The human papillomavirus type 16 E7 oncoprotein targets Myc-interacting zinc-finger protein-1

We demonstrate that HPV-16 E7 forms a complex with Miz-1. UV-induced expression of the CDK-inhibitor p21^Cip1 and subsequent cell cycle arrest depends upon endogenous Miz-1 in HPV-negative C33A cervical cancer cells containing mutated p53. Transient expression of E7 in C33A inhibits UV-induced expression of p21^Cip1 and overcomes Miz-1-induced G1-phase arrest. The C-terminal E7Δ79LEDLL83-mutant with reduced Miz-1-binding capacity was impaired in its capability to repress p21^Cip1 expression; whereas the pRB-binding-deficient E7C24G-mutant inhibited p21^Cip1 expression similar to wild-type E7. Using ChIP, we demonstrate that endogenous E7 is bound to the endogenous p21^Cip1 core-promoter in CaSki cells and RNAi-mediated knock down of Miz-1 abrogates E7-binding to the p21^Cip1 promoter. Co-expression of E7 with Miz-1 inhibited Miz-1-induced p21^Cip1 expression from the minimal-promoter via Miz-1 DNA-binding sites. Co-expression of E7Δ79LEDLL83 did not inhibit Miz-1-induced p21^Cip1 expression. E7C24G retained E7-wild-type capability to inhibit Miz-1-dependent transactivation. These findings suggest that HPV-16 E7 can repress Miz-1-induced p21^Cip1 gene expression.

Nonconserved lysine residues attenuate the biological function of the low-risk human papillomavirus E7 protein

The mucosotrophic human papillomaviruses (HPVs) are classified as high-risk (HR) or low-risk (LR) genotypes based on their neoplastic properties. We have demonstrated previously that the E7 protein destabilizes p130, a pRb-related pocket protein, thereby promoting S-phase reentry in postmitotic, differentiated keratinocytes of squamous epithelia, and that HR HPV E7 does so more efficiently than LR HPV E7. The E7 proteins of LR HPV-11 and -6b uniquely possess lysine residues following a casein kinase II phosphorylation motif which is critical for the biological function of E7. We now show that mutations of these lysine residues elevated the efficiency of S-phase reentry, independent of their charge. An 11E7 K39,42R mutation moderately increased the association with and the destabilization of p130. Unexpectedly, polyubiquitination on these lysine residues did not attenuate E7 activity, as their mutation caused elevated proteasomal degradation and decreased protein stability. In this regard, the biologically more potent HR HPV E7 proteins were also less stable than the LR HPV E7 proteins. We infer that these lysine residues impede functional protein-protein interactions. A G22D mutation of 11E7 at the pocket protein binding motif possessed augmented efficiency in promoting S-phase reentry and strongly enhanced association with p130 and pRb. The combined effects of these two classes of 11E7 mutations exhibited an efficiency of S-phase reentry comparable to that of HR HPV E7. Thus, these nonconserved residues are primarily responsible for the differential abilities of LR and HR HPV E7 proteins to promote unscheduled DNA replication in organotypic raft cultures.

Retinoblastoma-independent antiproliferative activity of novel intracellular antibodies against the E7 oncoprotein in HPV 16-positive cells

Background

"High risk" Human Papillomavirus strains are the causative agents of the vast majority of carcinomas of the uterine cervix. In these tumors, the physical integration of the HPV genome is a frequent, though not invariable occurrence, but the constitutive expression of the E6 and E7 viral genes is always observed, suggesting key roles for the E6 and E7 oncoproteins in the process of malignant transformation. The "intracellular antibody" technology using recombinant antibodies in single-chain format offers the possibility of targeting a protein in its intracellular environment even at the level of definite domains thus representing a valuable strategy to "knock out" the function of specific proteins.

Methods

In this study, we investigate the in vitro activity of two single-chain antibody fragments directed against the "high-risk" HPV 16 E7 oncoprotein, scFv 43M2 and scFv 51. These scFvs were expressed by retroviral system in different cell compartments of the HPV16-positive SiHa cells, and cell proliferation was analyzed by Colony Formation Assay and EZ4U assay. The binding of these scFvs to E7, and their possible interference with the interaction between E7 and its main target, the tumor suppressor pRb protein, were then investigated by immunoassays, PepSet™technology and Surface Plasmon Resonance.

Results

The expression of the two scFvs in the nucleus and the endoplasmic reticulum of SiHa cells resulted in the selective growth inhibition of these cells. Analysis of binding showed that both scFvs bind E7 via distinct but overlapping epitopes not corresponding to the pRb binding site. Nevertheless, the binding of scFv 43M2 to E7 was inhibited by pRb in a non-competitive manner.

Conclusions

Based on the overall results, the observed inhibition of HPV-positive SiHa cells proliferation could be ascribed to an interaction between scFv and E7, involving non-pRb targets. The study paves the way for the employment of specific scFvs in immunotherapeutic approaches against the HPV-associated lesions.

Specific betapapillomaviruses associated with squamous cell carcinoma of the skin inhibit UVB-induced apoptosis of primary human keratinocytes

Epidemiological studies have shown an association between infections by specific betapapillomaviruses, such as human papillomavirus (HPV) types 5 and 8, and cutaneous squamous cell carcinoma (SCC). The role of betapapillomaviruses in the development of cutaneous SCC is, however, still enigmatic. The ability to inhibit UVB-induced apoptosis, as demonstrated for HPV5 in vitro, may be important in this respect, as survival of DNA-damaged and mutated cells increases the risk of transformation. The aim of this study was to assess whether inhibition of UVB-induced apoptosis is a general property of betapapillomaviruses and to identify apoptotic factors that are potentially involved in this process. Primary human keratinocytes transduced with E6 and E7 of selected betapapillomaviruses (HPV5, HPV8, HPV15, HPV20, HPV24 and HPV38) were characterized and subjected to UVB irradiation. HPV8- and HPV20-expressing keratinocytes in particular showed fewer signs of apoptosis, as demonstrated by lower levels of active caspase 3, less enzymic caspase activity and less DNA fragmentation. The observed inhibition of UVB-induced apoptosis was mediated by E6 and coincided with reduced steady-state expression of the pro-apoptotic protein Bax. In conclusion, E6 of HPV8 and HPV20 reduces the apoptotic responses upon UVB irradiation when expressed in primary human keratinocytes. Infections with HPV8 and HPV20 may therefore augment the carcinogenic effect of UV radiation and potentially contribute to oncogenic transformation of the skin.

Human papilloma virus (HPV) and host cellular interactions

Viral-induced carcinogenesis has been attributed to the ability of viral oncoproteins to target and interact with the host cellular proteins. It is generally accepted that Human papilloma virus (HPV) E6 and E7 function as the dominant oncoproteins of 'high-risk' HPVs by altering the function of critical cellular proteins. Initially it was shown that HPV E6 enhances the degradation of p53, while HPV E7 inactivates the function of the retinoblastoma tumor suppressor protein Rb. However, recent studies during the last decade have identified a number of additional host cellular targets of both HPV E6 and E7 that may also play an important role in malignant cellular transformation. In this review we present the interactions of HPV E6 and E7 with the host cellular target proteins. We also present the role of DNA integration in the malignant transformation of the epithelial cell.

Human papillomavirus type 77 E7 protein is a weak deregulator of cell cycle

Human Papillomavirus type 77 is a skin type found in non-melanoma skin cancers of immuno-compromised individuals. Although, the HPV77 E6 oncoprotein has been well studied, nothing is known about E7. Studies on mucosal HPV types (e.g. HPV16) showed that E7 deregulates the cell cycle by binding to and promoting degradation of retinoblastoma protein (pRb). Here, we characterized the impact of HPV77 E7 on the cell cycle. We observed that HPV77 E7 associated with pRb with a lower affinity than HPV16 E7, promoting weakly its degradation. Although, HPV16 E7 led to cellular proliferation and accumulation of the cell cycle inhibitor p16(INK4a), both events were not clearly observed in HPV77 E7 cells. Together, these data indicate that HPV77 E7 does not efficiently deregulate the cell cycle, in contrast to several E7s of mucosal HPV types.

RNA polymerase II transcription is required for human papillomavirus type 16 E7- and hydroxyurea-induced centriole overduplication

Aberrant centrosome numbers are detected in virtually all human cancers where they can contribute to chromosomal instability by promoting mitotic spindle abnormalities. Despite their widespread occurrence, the molecular mechanisms that underlie centrosome amplification are only beginning to emerge. Here, we present evidence for a novel regulatory circuit involved in centrosome overduplication that centers on RNA polymerase II (pol II). We found that human papillomavirus type 16 E7 (HPV-16 E7)- and hydroxyurea (HU)-induced centriole overduplication are abrogated by alpha-amanitin, a potent and specific RNA pol II inhibitor. In contrast, normal centriole duplication proceeded undisturbed in alpha-amanitin-treated cells. Centriole overduplication was significantly reduced by siRNA-mediated knock down of CREB-binding protein (CBP), a transcriptional co-activator. We identified cyclin A2 as a key transcriptional target of RNA pol II during HU-induced centriole overduplication. Collectively, our results show that ongoing RNA pol II transcription is required for centriole overduplication whereas it may be dispensable for normal centriole duplication. Given that many chemotherapeutic agents function through inhibition of transcription, our results may help to develop strategies to target centrosome-mediated chromosomal instability for cancer therapy and prevention.

E6/E7 expression of human papillomavirus type 20 (HPV-20) and HPV-27 influences proliferation and differentiation of the skin in UV-irradiated SKH-hr1 transgenic mice

The functional role of UV irradiation, in combination with the E6 and E7 proteins of the cutaneous human papillomavirus (HPV) types in the malignant conversion of benign papillomatous lesions, has not been elucidated. Transgenic SKH-hr1 hairless mice expressing HPV-20 and HPV-27 E6 and E7 proteins in the suprabasal compartment were generated and exposed to chronic UV irradiation. Histological and immunohistochemical examination of skin samples revealed enhanced proliferation of the epidermal layers and papilloma formation in both transgenic strains in comparison to what was observed with nontransgenic mice. Squamous cell carcinoma developed in the HPV-20 E6/E7 transgenic line as well as in the HPV-27 E6/E7 transgenic line. Several weeks after cessation of UV-B exposure, enhanced proliferation, as measured by BrdU incorporation, was maintained only in HPV-20 transgenic skin. Keratin 6 expression was increased in the transgenic mice throughout all cell layers. Expression of the differentiation markers involucrin and loricrin was reduced and disturbed. p63alpha expression was differentially regulated with high levels of cytoplasmic expression in clusters of cells in the granular layer of the skin in the transgenic lines 20 weeks after cessation of UV-B exposure, in contrast to uninterrupted staining in the nontransgenic lines. p53 was expressed in clusters of cells in nontransgenic and HPV-27 transgenic mice, in contrast to an even distribution in a higher number of cells in HPV-20 transgenic animals.

Retinoblastoma family proteins as key targets of the small DNA virus oncoproteins

RB, the most investigated tumor suppressor gene, is the founder of the RB family of growth/tumor suppressors, which comprises also p107 (RBL1) and Rb2/p130 (RBL2). The protein products of these genes, pRb, p107 and pRb2/p130, respectively, are also known as 'pocket proteins', because they share a 'pocket' domain responsible for most of the functional interactions characterizing the activity of this family of cellular factors. The interest in these genes and proteins springs essentially from their ability to regulate negatively cell cycle processes and for their ability to slow down or abrogate neoplastic growth. The pocket domain of the RB family proteins is dramatically hampered in its functions by the interference of a number of proteins produced by the small DNA viruses. In the last two decades, the 'viral hypothesis' of cancer has received a considerable renewed impulse from the notion that small DNA viruses, such as Adenovirus, Human papillomavirus (HPV) and Polyomavirus, produce factors that can physically interact with major cellular regulators and alter their function. These viral proteins (oncoproteins) act as multifaceted molecular devices that have evolved to perform very specific tasks. Owing to these features, viral oncoproteins have been widely employed as invaluable experimental tools for the identification of several key families of regulators, particularly of the cell cycle homeostasis. Adenovirus early-region 1A (E1A) is the most widely investigated small DNA tumor virus oncoprotein, but relevant interest in human oncology is raised by the E1A-related E7 protein from transforming HPV strains and by Polyomavirus oncoproteins, particularly large and small T antigens from Simian virus 40, JC virus and BK virus.

Papillomaviruses: different genes have different histories

Papillomaviruses (PVs) infect stratified squamous epithelia in vertebrates. Some PVs are associated with different types of cancer and with certain benign lesions. It has been assumed that PVs coevolved with their hosts. However, recently it has been shown that different regions of the genome have different evolutionary histories. The PV genome has a modular nature and appeared after the addition of pre-existent blocks. This order of appearance in the PV genome is evident today in the different evolutionary rates of the different genes, with new genes--E5, E6 and E7--diverging faster than old genes--E1, E2, L2 and L1. Here, we propose an evolutionary framework aiming to integrate genome evolution, PV biology and epidemiology of PV infections.

Differential expression of cdk inhibitors p16, p21cip1, p27kip1, and cyclin E in cervical cytological smears prepared by the ThinPrep method

Our objective was to correlate p16, p21cip1, p27kip1, and cyclin E protein expression with the degree of dysplasia on ThinPrep Papanicolaou (Pap) smears using a modified immunoperoxidase staining. Smears read as normal, atypical squamous cells of undetermined significance (ASC-US), low-grade squamous intraepithelial lesion (LSIL), or high-grade SIL (HSIL) were identified and tested for high-risk human papillomavirus (HR-HPV). Additional smears were processed for immunoperoxidase for p16, p21cip1, p27kip1, and cyclin E. Thirty-four smears were satisfactory for study. The p16 was positive in all nine HSIL, in four of nine LSIL, and in one of seven ASC-US. The p27kip1 was positive in all nine HSIL, in eight of nine LSIL, and in one of seven ASC-US. The p21cip1 was positive in all nine HSIL, in one of nine LSIL, and in one of seven ASC-US. Cyclin E was positive in seven of nine HSIL and in one of nine LSIL and in none of the ASC-US smears. Normal smears were negative for all the antigens. There was poor correlation of protein expression and HR-HPV infection. We concluded that p16, p21cip1, p27kip1, and cyclin E can be demonstrated on Pap smears and they are expressed differentially in dysplastic cells, with highest expression in HSIL. The p21cip1 and cyclin E showed the greatest correlation with HSIL.

Human papillomavirus in squamous cell carcinoma of esophagus in a high-risk population

AIM: To investigate the relation of human papillomavirus (HPV) and esophageal squamous cell carcinoma (ESCC) in Iranian patients as compared to normal controls.

METHODS: Using MY09/MY11 consensus primers, we compared the prevalence of a HPV L1 gene in tumor tissues from 38 ESCC cases and biopsied tissues from 38 endoscopically normal Iranian individuals. We also compared the presence of HPV16 and HPV18 in the same samples using type-specific E6/E7 primers.

RESULTS: Fourteen (36.8%) of the 38 ESCC samples but only 5 (13.2%) of the 38 control samples were positive for the HPV L1 gene (P = 0.02). Five (13.2%) of the ESCC samples but none of the control samples were positive for the HPV16 E6/E7 gene (P = 0.05). Three (7.9%) of the ESCC samples and 5 (13.2%) of the control samples were positive for the HPV18 E6/E7 gene (P = 0.71).

CONCLUSION: Our data are consistent with HPV DNA studies conducted in other high-risk areas for ESCC. HPV should be considered as a potential factor contributing to the high incidence of ESCC in Iran and other high-incidence areas of the world.

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.

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

We have demonstrated previously that high-risk human papillomaviruses (HPVs) induce tetrasomy in low-grade squamous intraepithelial lesions of the cervix. In this study we show that the E6 and E7 genes of high-risk HPV-16, but not those of low-risk HPV-6, are independently able to induce tetrasomy when constitutively expressed in proliferating monolayer cultures of primary human keratinocytes. Of seven HPV-16 E7 mutants analysed (H2P, Delta6-10, Delta21-24, C24G, S31G/S32G, A50S and S71I), five were severely impaired in their ability to induce tetrasomy in monolayer and raft culture. Only mutant C24G induced tetrasomy to levels comparable with wild-type E7 in monolayer and raft culture. This mutant shows strongly reduced binding to the retinoblastoma gene product pRb. The casein kinase II phosphorylation defective mutant S31G/S32G induced tetrasomy to levels comparable with wild-type E7 in raft culture, but not in monolayer culture, and induction of tetrasomy did not correlate with raft morphology. These results indicate that pRb protein binding is not required for HPV-16 E7 associated tetrasomy and that tetrasomy is not directly related to the ability of this protein to disrupt keratinocyte differentiation.

Lack of the canonical pRB-binding domain in the E7 ORF of artiodactyl papillomaviruses is associated with the development of fibropapillomas

The L-X-C-X-E pRB-binding motif of papillomavirus (PV) E7 proteins has been implicated in the immortalization and transformation of the host cell. However, sequencing of the complete genomes of bovine papillomavirus type 3 (BPV-3), bovine papillomavirus type 5 (BPV-5), equine papillomavirus (EQPV) and reindeer (Rangifer tarandus) papillomavirus (RPV) supports the notion that the pRB-binding motif is not ubiquitous among E7 proteins in the PV proteome. Key among the animal groups that lack the pRB-binding domain are the artiodactyl PVs, including European elk PV (EEPV), deer PV (DPV), reindeer PV (RPV), ovine PVs types 1 and 2 (OvPV-1 and -2) and bovine PVs 1, 2 and 5 (BPV-1, -2 and -5). Whereas the presence of the pRB-binding domain is normally associated with papillomas, the artiodactyl PVs are marked by the development of fibropapillomas on infection. Previous studies emphasized the role of E5 in the pathogenic mechanism of fibropapilloma development, but correlation between the lack of an E7 pRB-binding domain and the unique pathology of the artiodactyl PVs suggests a more complicated mechanism and an early evolutionary divergence from a pRB-binding ancestor.

Direct activation of cyclin-dependent kinase 2 by human papillomavirus E7

Addition of human papillomavirus (HPV) E7 CDK2/cyclin A or CDK2/cyclin E, purified from either insect cells or bacteria, dramatically upregulates histone H1 kinase activity. Activation is substrate specific, with a smaller effect noted for retinoblastoma protein (Rb). The CDK2 stimulatory activity is equivalent in high-risk (HPV type 16 [HPV16] and HPV31) and low-risk (HPV6b) E7. Mutational analyses of HPV16 E7 indicate that the major activity resides in amino acids 9 to 38, spanning CR1 and CR2, and does not require casein kinase II or Rb-binding domain functions. Synthetic peptides spanning HPV16 amino acid residues 9 to 38 also activate CDK2. Peptides containing this sequence that carry biotin on the carboxy terminus, as well as a photoactivated cross-linking group (benzophenone), also activate the complex and covalently associate with the CDK2/cyclin A complex in a specific manner requiring UV. Cross-linking studies that use protein monomers detect association of the E7 peptides with cyclin A but not CDK2. Together, our results indicate a novel mechanism whereby E7 promotes HPV replication by directly altering CDK2 activity and substrate specificity.

Human papillomavirus type 32 does not display in vitro transforming properties

Human papillomavirus type 32 (HPV32) is one of the etiological agents of a benign oral condition, focal epithelial hyperplasia. However, the previously characterized properties of its E7 oncoprotein suggest a possible malignant nature for this virus. In this study we characterized the properties of HPV32 E6 and E7. Our data show that HPV32 E7, despite its high affinity for pRb, does not promote degradation of the cellular protein. In addition, HPV32 E6 does not prevent p53-mediated apoptosis and/or cell cycle arrest. Moreover, coexpression of HPV32 E6 and E7 in primary human fibroblasts or keratinocytes does not alter their proliferative state. Together, these data provide evidence of the benign nature of HPV32.

Papillomaviruses: death-defying acts in skin cancer

Ultraviolet (UV) radiation is an environmental agent that has a major impact on humans, and cumulative exposure poses a serious risk in terms of developing skin cancer. Acute doses of UV induce apoptotic cell death in the skin via signalling pathways that are, in part, dependent on the p53 tumour suppressor protein. However, p53-independent mechanisms have also been described. Recent findings show that a high proportion of non-melanoma skin cancers contain human papillomavirus. The viral E6 protein effectively blocks the epidermal apoptotic response to UV and might play a key role in promoting tumour development in cooperation with the mutagenic effects of UV.

Identification of a novel activity of human papillomavirus type 16 E6 protein in deregulating the G1/S transition

In this study we show that E6 of human papillomavirus has the ability to deregulate the cell cycle G1/S transition. In rodent immortalized fibroblasts (NIH3T3) serum deprivation or over-expression of the cyclin-dependent kinase inhibitors, p16(INK4a) or p27(KIP1), leads to G1 cell cycle arrest. HPV16 E6 overcomes the antiproliferative signals, gaining the ability to drive serum-deprived and p16(INK4a) or p27(KIP1) over-expressing cells into S phase. E6 protein from the benign HPV type 1 displays a similar activity to HPV16 E6 to deregulate the G1/S transition. Thus, this activity appears to be conserved between E6 proteins from non-oncogenic and oncogenic HPV types. Furthermore, we show that HPV16 E6 is not able to circumvent a G1 arrest imposed by pRb mutant in which all CDK phosphorylation sites have been mutated. These data indicate that the viral protein acts upstream of pRb and its mechanism in promoting cell cycle progression is dependent on pRb phosphorylation. In summary, this study describes a novel biological function of HPV E6 and shows that the S phase entry, required for viral DNA replication, is not exclusively controlled by E7, but that E6 also is involved in this event.

The human papillomavirus oncoprotein E7 attenuates NF-kappa B activation by targeting the Ikappa B kinase complex

Infection with high-risk human papillomaviruses (HPV) can lead to the development of cervical carcinomas. This process critically depends on the virus-encoded E6 and E7 oncoproteins, which stimulate proliferation by manipulating the function of a variety of host key regulatory proteins. Here we show that both viral proteins dose-dependently interfere with the transcriptional activity of NF-kappaB. A variety of experimental approaches revealed that a fraction of the E7 proteins is found in association with the IkappaB kinase complex and attenuates induced kinase activity of IkappaB kinase alpha (IKKalpha) and IKKbeta, thus resulting in impaired IkappaBalpha phosphorylation and degradation. Indirect immunofluorescence shows that E7 impairs TNFalpha-induced nuclear translocation of NF-kappaB, thus preventing NF-kappaB from binding to its cognate DNA. While E7 obviates IKK activation in the cytoplasm, the E6 protein reduces NF-kappaB p65-dependent transcriptional activity within the nucleus. We suggest that HPV oncogene-mediated suppression of NF-kappaB activity contributes to HPV escape from the immune system.

Transduction of the E6 and E7 genes of epidermodysplasia-verruciformis-associated human papillomaviruses alters human keratinocyte growth and differentiation in organotypic cultures

Epidermodysplasia-verruciformis-associated human papilloma virus DNA has been detected in skin cancers, in premalignant and benign skin lesions, and in plucked hairs from immunocompetent and immunosuppressed patients. The role of epidermodysplasia-verruciformis-associated human papilloma virus in the pathogenesis of nonmelanoma skin cancer is still enigmatic. In organotypic cultures we investigated the effects of retroviral transduction of the E6 and E7 genes of epidermodysplasia-verruciformis-associated human papilloma virus types 5, 12, 15, 17, 20, and 38 on the growth and differentiation of human keratinocytes. Differentiation was disturbed to different degrees as revealed by histology and by the expression patterns of differentiation markers keratin 10 and small proline rich protein 2. Conversely, proliferating cell nuclear antigen was induced in some of the suprabasal, differentiated cells to varying extent. No unscheduled DNA synthesis was detected in these cells, however, as probed by 5'-bromo-2'-deoxyuridine incorporation. Most intriguingly, when the E6 and E7 genes of epidermodysplasia-verruciformis-associated human papilloma virus types 15 and 17 were transduced, a broadening layer of basal cells and an accelerated differentiation were observed. In addition, "papilla-like structures" comprising basal-like keratinocytes arose from the basal layer into the differentiated layers. These cells did not express the differentiation markers keratin 10 and small proline rich protein 2, but did actively replicate DNA. These observations warrant further research by using this system to elucidate the replication strategy of epidermodysplasia-verruciformis-associated human papilloma virus types in keratinocytes and to shed light on the role of these human papilloma virus types in the pathogenesis of skin cancer.

The HPV-16 E7 oncoprotein binds Skip and suppresses its transcriptional activity

E7 is the major transforming protein of human papillomavirus (HPV), which is implicated in the development of cervical cancer. The transforming activity of E7 has been attributed in part to its interaction with the retinoblastoma (Rb) tumour suppressor; however, the Rb interaction alone is not sufficient for transformation by E7. In a screen for cellular targets of HPV E7, we identified the Ski interacting protein, Skip, as a new interacting partner of E7. We show that HPV-16 E7 associates with Skip via sequences in its carboxy terminal region, and the evolutionarily conserved proline rich sequences (PRS) of the SNW domain of Skip. E7 functionally targets Skip in vivo and inhibits its transcriptional activation activity. Two transformation defective mutants of E7 were identified that failed both to bind Skip and to inhibit its transcriptional activity. These results suggest that inhibition of Skip function may contribute to cell transformation by HPV-16 E7.

Determination of the binding affinity of different human papillomavirus E7 proteins for the tumour suppressor pRb by a plate-binding assay

A plate-binding assay was developed to quantify the affinity of the E7 oncoprotein from different human papillomavirus (HPV) types for the tumour suppressor pRb. The method is highly reproducible, sensitive and easy to handle. It could be easily adapted for the quantitative study of other interacting proteins and for screenings of inhibitors of protein/protein interactions. The pRb-binding affinity of six different E7 proteins has been quantified. The K(D) values vary from approximately 4.5x10(-9) M for HPV16 E7 to more than 1x10(-7) M for HPV10 and HPV48 E7. Point mutation C24G in the high affinity pRb-binding domain of HPV16 E7 results in a 3-fold affinity reduction. The data indicate that the high affinity pRb-binding domain of E7, LXCXE, is essential for the association between the viral and cellular proteins. However, other E7 domain(s), which appear(s) not to be present in all E7s, contribute to stabilize the E7-pRb association.