Functional consequences of directed mutations in human papillomavirus E6 proteins: abrogation of p53-mediated cell cycle arrest correlates with p53 binding and degradation in vitro

p53 degradation activity, expression, and subcellular localization of E6 proteins from 29 human papillomavirus genotypes

Human papillomaviruses (HPVs) are the etiological agents of cervical cancer and other human malignancies. HPVs are classified into high- and low-risk genotypes according to their association with cancer. Host cell transformation by high-risk HPVs relies in part on the ability of the viral E6 protein to induce the degradation of p53. We report the development of a cellular assay that accurately quantifies the p53 degradation activity of E6 in vivo, based on the fusion of p53 to Renilla luciferase (RLuc-p53). This assay was used to measure the p53 degradation activities of E6 proteins from 29 prevalent HPV types and variants of HPV type 16 (HPV16) and HPV33 by determining the amount of E6 expression vector required to reduce by half the levels of RLuc-p53 (50% effective concentration [EC₅₀]). These studies revealed an unexpected variability in the p53 degradation activities of different E6 proteins, even among active types whose EC₅₀s span more than 2 log units. Differences in activity were greater between types than between variants and did not correlate with differences in the intracellular localization of E6, with most being predominantly nuclear. Protein and mRNA expression of the 29 E6 proteins was also examined. For 16 high-risk types, spliced transcripts that encode shorter E6*I proteins of variable sizes and abundances were detected. Mutation of the splice donor site in five different E6 proteins increased their p53 degradation activity, suggesting that mRNA splicing can limit the activity of some high-risk E6 types. The quantification of p53 degradation in vivo represents a novel tool to systematically compare the oncogenic potentials of E6 proteins from different HPV types and variants.

A plant lignan, 3′-O-Methyl-Nordihydroguaiaretic acid, suppresses papillomavirus E6 protein function, stabilizes p53 protein, and induces apoptosis in cervical tumor cells

Persistent infection with oncogenic human papillomaviruses (HPVs) is the most important factor in the induction of uterine cervical cancer, a leading cause of cancer mortality in women worldwide. Upon cell transformation, continual expression of the viral oncogenes is required to maintain the transformed phenotype. The viral E6 protein forms a ternary complex with the cellular E6-AP protein and p53 protein which promotes the rapid degradation of p53. Recent studies have revealed that lignans from the creosote bush (3'-O-methyl-nordihydroguaiaretic acid) can repress the viral promoter responsible for E6 gene expression. Work reported here shows that the lignan can subvert viral oncogene function resulting in stabilized p53 protein within treated HPV-containing tumor cells. The stabilized p53 is transcriptionally active as demonstrated by a luciferase reporter vector and induction of genes for Bax and PUMA proteins. Apoptosis is detected by annexin V binding to treated cells as analyzed by flow cytometry. Programmed cell death is confirmed by the induction of active caspases and TUNEL assay. Initiator caspase-9 is activated first, followed later by the effector caspase-3 enzyme. The stabilization and induced apoptosis are not observed within treated HPV-negative cervical tumor cells. Quantitative real time RT-PCR analysis of endogenous E6 gene transcription from the integrated HPV 16 promoter shows at least a fivefold repression of expression as compared to untreated cells. These results indicate that the loss of E6 protein in treated cells could be, in part, responsible for the stabilization of p53 within the lignan treated cells.

Human papillomaviruses and cervical cancer

Carcinoma of the uterine cervix, a leading cause of cancer death in women worldwide, is initiated by infection with high-risk types of human papillomaviruses (HPVs). This review summarizes laboratory studies over the past 20 years that have elucidated the major features of the HPV life cycle, identified the functions of the viral proteins, and clarified the consequences of HPV infection for their host cells. This information has allowed the development of various strategies to prevent or treat infections, including prophylactic vaccination with virus-like particles, therapeutic vaccination against viral proteins expressed in cancer cells, and antiviral approaches to inhibit virus replication, spread, or pathogenesis. These strategies have the potential to cause a dramatic reduction in the incidence of cervical carcinoma and serve as the prototype for comprehensive efforts to combat virus-induced tumors.

Structural and functional analysis of E6 oncoprotein: insights in the molecular pathways of human papillomavirus-mediated pathogenesis

Oncoprotein E6 is essential for oncogenesis induced by human papillomaviruses (HPVs). The solution structure of HPV16-E6 C-terminal domain reveals a zinc binding fold. A model of full-length E6 is proposed and analyzed in the context of HPV evolution. E6 appears as a chameleon protein combining a conserved structural scaffold with highly variable surfaces participating in generic or specialized HPV functions. We investigated surface residues involved in two specialized activities of high-risk genital HPV E6: p53 tumor suppressor degradation and nucleic acid binding. Screening of E6 surface mutants identified an in vivo p53 degradation-defective mutant that fails to recruit p53 to ubiquitin ligase E6AP and restores high p53 levels in cervical carcinoma cells by competing with endogeneous E6. We also mapped the nucleic acid binding surface of E6, the positive potential of which correlates with genital oncogenicity. E6 structure-function analysis provides new clues for understanding and counteracting the complex pathways of HPV-mediated pathogenesis.

Human papillomavirus type 16 E5 protein as a therapeutic target

Cervical cancer is a progressive disease with an onset of one to two decades on average. During the productive replication stage, the Human papillomavirus (HPV) genome is maintained episomally in the infected cervical epithelium and early gene products, including E5, are expressed. Therefore, E5 has a potential to contribute to the HPV-associated carcinogenic process. In invasive malignancies, the HPV genomes are commonly integrated into the host genome, and E6 and E7 genes remain intact. However, the E5 is lost or, if present, under-expressed as compared with the E6 and E7 proteins. This suggests that E5 may play a critical role in the genesis of cervical cancer but less of a role in its persistence or progression. In the initiation of neoplasia and the premalignant stage, there are fewer malignant cells than in the invasive malignancies. Moreover, cells in the invasive malignant stage are found to have a very low level of MHC class I and II, which could hamper the presentation of the antigen and lead to a decreased immune response. Since the E5 protein is likely to play a role during the early tumorigenesis stage, a therapeutic vaccine to target and eliminate the E5-expressing cells may be a good strategy to prevent premalignant lesions from progressing toward invasive cervical cancers. This paper provides an overview of HPV-induced cervical carcinogenesis and strategies for designing prophylactic and therapeutic vaccines to prevent and cure the cervical cancer. In particular, focus will be on the rationale of targeting the E5 protein to develop therapeutic vaccines.

Prophylactic, therapeutic and anti-metastatic effects of an HPV-16mE6Δ/mE7/TBhsp70Δ fusion protein vaccine in an animal model

Human papillomaviruses (HPVs), particularly HPV-16, are not only causally linked to cervical cancers but also play an important role in the development of other cancers. The oncoproteins, E6 and E7, are consistently coexpressed in the majority of HPV-containing carcinomas and their metastatic lesions, and are critical to the induction and maintenance of malignant phenotype, and also can cause tumor metastasis. Therefore, E6 and E7 represent ideal tumor-specific antigens for the development of immunotherapy to prevent and treat HPV-associated cancers and their metastases. The powerful antigenic nature of Mycobacterium tuberculosis heat shock protein 70 (TBhsp70) is emphasized by evidence that mammals are capable of recognizing murine and human multiple B and T cell epitopes in this protein, and therefore allows it to be used as an adjuvant-free carrier to stimulate the immune response to a covalently linked fusion partner. In our present study, we developed a recombinant TBhsp70Delta protein expression vector that permits the production of other protein fused to TBhsp70Delta. A recombinant HPV-16mE6Delta/mE7/TBhsp70Delta fusion protein was expressed and purified, and immunization with the fusion protein in the absence of adjuvant was capable of providing strong protection to C57BL/6 mice against challenge and rechallenge with TC-1 cells, but not HPV negative Lewis lung cancer cells, and induced established TC-1 tumor regression and led to long-term survival. Consistent with the in vivo results, the fusion protein immunization in the absence of adjuvant induced cytolytic T lymphocytes recognized specifically TC-1 tumor cells in vitro. We also demonstrated that immunization with the fusion protein in the absence of adjuvant was effective in both preventing and treating TC-1 metastatic lesions in the lung metastasis model. In particular, immunization with the fusion protein caused regression of established lung metastatic lesions in 50% of immunized animals. This study represents an instance of tumor therapy with a TBhsp70Delta fusion protein and provides the scientific basis for the clinical application of the HPV16mE6Delta/mE7/TBhsp70Delta fusion protein in the treatment of HPV-associated cancers and their metastases.

Using small molecules to overcome drug resistance induced by a viral oncogene

We used small molecule screening to discover compounds and mechanisms for overcoming E6 oncogene-mediated drug resistance. Using high-throughput screening in isogenic cell lines, we identified compounds that potentiate doxorubicin's lethality in E6-expressing colon cancer cells. Such compounds included quaternary ammonium salts, protein synthesis inhibitors, 11-deoxyprostaglandins, and two additional classes of compounds-analogs of 1,3-bis(4-morpholinylmethyl)-2-imidazolidinethione (a thiourea) and acylated secondary amines that we named indoxins. Indoxins upregulated topoisomerase IIalpha, the target of doxorubicin, thereby increasing doxorubicin lethality. We developed a photolabeling strategy to identify targets of indoxin and discovered a nuclear actin-related protein complex as a candidate indoxin target.

Protein mutagenesis with monodispersity-based quality probing: selective inactivation of p53 degradation and DNA-binding properties of HPV E6 oncoprotein

Interpretation of protein mutagenesis experiments requires the ability to distinguish functionally relevant mutations from mutations affecting the structure. When a protein is expressed soluble in bacteria, properly folded mutants are expected to remain soluble whereas misfolded mutants should form insoluble aggregates. However, this rule may fail for proteins fused to highly soluble carrier proteins. In a previous study, we analysed the biophysical status of HPV oncoprotein E6 fused to the C-terminus of maltose-binding protein (MBP) and found that misfolded E6 moieties fused to MBP formed soluble aggregates of high molecular weight. By contrast, preparations of properly folded E6 fused to MBP were monodisperse. Here, we have used this finding to evaluate the quality of 19 MBP-fused E6 site-directed mutants by using a light scattering assay performed in a fluorimeter. This assay guided us to rule out structurally defective mutants and to obtain functionally relevant E6 mutants selectively altered for two molecular activities: degradation of tumour suppressor p53 and DNA recognition.

Human papillomavirus type 16 E2-specific T-helper lymphocyte responses in patients with cervical intraepithelial neoplasia

T-cell-mediated immune responses against mucosal oncogenic types of human papillomaviruses (HPV) are thought to play a role in the control of the virus infection and its associated cervical lesions. The in vitro production of interleukin-2 by T-helper (Th) cells in response to the C-terminal and N-terminal domains of the HPV-16 E2 protein was determined in 74 women with cytological evidence of premalignant cervical epithelial neoplasia who participated in a non-intervention follow-up (FU) study. Cross-sectional analysis at the end of FU showed that Th cell responses against the C-terminal domain were associated with evidence of previous or present HPV-16 infection as compared to patients with no evidence of any HPV infection (18.9% versus 0%, P = 0.039). Th cell responses against the N-terminal domain were not associated with evidence of HPV-16 infection. No association with disease outcome was observed with Th cell responses against either of the E2 protein domains. However, longitudinal analysis revealed that Th cell responses against the C-terminal domain frequently occur at the time of virus clearance. Whether these responses are responsible for the clearance of the virus is not known.

Targetting of the N-terminal domain of the human papillomavirus type 16 E6 oncoprotein with monomeric ScFvs blocks the E6-mediated degradation of cellular p53

The E6 protein of cancer-associated human papillomavirus type 16 (HPV16) binds to cellular p53 and promotes its degradation through the ubiquitin pathway. In an attempt to identify the regions of E6 that could be targetted for functional inhibition, we generated monoclonal antibodies to the HPV16 E6 oncoprotein (16E6) and analysed their effect on E6-mediated p53 in vitro degradation. The isolated antibodies recognize the 16E6 oncoprotein expressed in the CaSki carcinoma cell line and strongly inhibit the proteolysis of p53 in vitro by binding specifically to a region of 10 residues located at the N-terminal end of 16E6. The variable regions of these antibodies were cloned and expressed in E. coli as single chain Fvs (scFvs). Purified scFvs were present in monomeric form and totally abolished 16E6-mediated p53 degradation by preventing the formation of E6/p53 protein complexes. Our results demonstrate that monovalent binding of scFvs to the N-terminal end of 16E6 abrogates the biological mechanisms leading to the degradation of p53, and they suggest that this region of 16E6 may be a useful in vivo target for blocking the oncogenic activity of HPV16 E6 protein.

Molecular events in uterine cervical cancer

OBJECTIVE

To review the literature regarding the molecular events which occur in the development of uterine cervical cancer, with particular reference to human papillomavirus (HPV) infection.

METHODOLOGY

Bibliographic searches of Medline and the ISI citation databases using appropriate keywords, including the following: papillomavirus, cervix, pathology, cyclin, chromosome, heterozygosity, telomerase, smoking, hormones, HLA, immune response, HIV, HSV, EBV.

CONCLUSIONS

It has become clear that most cervical neoplasia, whether intraepithelial or invasive, is attributable in part to HPV infection. However, HPV infection alone is not sufficient, and, in a small proportion of cases, may not be necessary for malignant transformation. There is increasing evidence that HPV gene products interfere with cell cycle control leading to secondary accumulation of small and large scale genetic abnormalities. This may explain the association of viral persistence with lesion progression but, in many patients, secondary factors, such as smoking and immune response, are clearly important. However, the mechanisms involved in the interaction between HPV and host factors are poorly understood.

Inhibition of HPV-16 E6/E7 immortalization of normal keratinocytes by hairpin ribozymes

HPV-16 E6 and E7 genes are required to efficiently immortalize a broad spectrum of cell types including cervical keratinocytes. Therefore, the E6/E7 genes can be considered relevant targets for anti-cancer therapy. We produced several engineered hairpin (HP) ribozymes to specifically disrupt HPV-16 E6/E7 mRNA. After extensive biochemical characterization, one anti-E6 HP ribozyme (R434) was selected for in vivo testing because of its superior catalytic capabilities. When expressed in cis, R434 efficiently inhibited E6 in vitro translation. Cis-expression of the HP ribozyme with HPV-16 E6/E7 genes in normal human keratinocytes reduced the growth rate and prevented immortalization. RNA analysis by reverse transcription-PCR showed that E6/E7 transcripts were cleaved in post-transfected cells and virtually were eliminated after long term expression. Of interest, an inactive version of the HP also was able to significantly affect the immortalizing ability of E6/E7, probably through passive hybridization. The combination of passive and cleaving antisense RNA therefore is established as an effective inhibitor of HPV-16 E6/E7 immortalization.

A comparative analysis of the interactions of the E6 proteins from cutaneous and genital papillomaviruses with p53 and E6AP in correlation to their transforming potential

Common necessity for all papillomaviruses is to induce DNA synthesis in quiescent cells. This is commonly achieved by the E7 gene product, which interferes with the function of members of the retinoblastoma family controlling transition from the G1-phase to the S-phase of the cell cycle. Uncontrolled entry into S-phase activates, however, negative growth control signals which have to be bypassed to achieve production of progeny viruses. In addition to inherent activities of the E7 protein, high risk genital types encode an E6 protein that overcomes p53-mediated G1-arrest and apoptosis in concert with the cellular factor E6AP by targeting p53 for the enhanced ubiquitin-dependent degradation. The key question, which of these functions of genital E6 and E7 proteins is responsible for the carcinogenic phenotype, is still not completely answered. In contrast to high risk genital types no immortalizing or transforming activities have been found for the E7 proteins of the high risk cutaneous HPV8 and 47. On the other hand the ability of the E6 protein to transform established rodent fibroblasts seems to be a property shared by high risk genital and cutaneous types. To examine the existence of a common E6-mediated transforming pathway for both virus groups we compared the properties of the cutaneous E6 proteins with already known functions of E6 proteins of genital viruses. For this we analyzed the E6 proteins of low nak and high risk cutaneous and genital papillomaviruses with respect to cell transformation, to their abilities to bind, degradate, and influence the activity of human p53, and to bind E6AP. The results of our study demonstrate a clear lack of interaction between the transforming E6 proteins of HPV1 and HPV8 and both cellular proteins p53 and E6AP. In contrast, we found E6AP-independent binding of HPV16 E6 and HPV6 E6 to p53, although both proteins were different in their transforming potential. Of all four proteins investigated, only HPV16 E6 was able to bind to p53 and E6AP and to induce degradation of the p53 protein in the reticulocyte system. When we investigated in frame deletion mutants of the E6 protein of HPV16 for their abilities to bind to p53 or E6AP, degradate, and inhibit the transactivation function of p53 and to transform rodent fibroblasts, no correlation between the different activities could be found. Mutants still able to bind p53 and E6AP lacked transforming ability and other mutants that were transformation-competent were deficient in p53 and E6AP bindings.

Natural variants of the human papillomavirus type 16 E6 protein differ in their abilities to alter keratinocyte differentiation and to induce p53 degradation

Three naturally occurring variant human papillomavirus type 16 (HPV-16) E6 proteins, which contained amino acid substitutions predominantly near the N terminus, exhibited significant differences in their abilities to abrogate keratinocyte differentiation in response to serum and calcium and to induce the degradation of p53 in vitro. One variant surpassed the reference E6 protein in its ability to abrogate keratinocyte differentiation responses, whereas another showed a reduction in this activity. Interestingly, the biological activities of the HPV-16 E6 proteins and their abilities to induce p53 degradation in vitro were directly correlated. These results demonstrate that naturally occurring variants of HPV-16 differ in biological and biochemical properties which might result in differences in pathogenicity.

The E6 variant proteins E6I-E6IV of human papillomavirus 16: expression in cell free systems and bacteria and study of their interaction with p53

Several species of alternatively spliced mRNAs are transcribed from the E6 gene region of human papillomavirus (HPV) 16. These have the coding capacity for either the full length E6 of 151 amino acids (aa) or four truncated variants, E6I-E6IV, of 43-64 aa. As the first step to identify the putative E6 variants and their functions, we generated cDNAs corresponding to the various E6 open reading frames (ORF) and examined their expression employing in vitro transcription/translation systems and the bacterial pET system. In wheat germ extract, in vitro translation resulted in the production of all five proteins, E6 and E6I-E6IV. These proteins were also expressed as stable fusion proteins from the pET16b and pET17 x b vectors in Escherichia coli. Mobilites of the E6 variant proteins on SDS-acrylamide gels were consistent with their predicted sizes. The authenticity of the synthesized proteins was confirmed by immunoprecipitation with specific antibodies directed against epitopes in the N-terminal portion of E6 as well as antibodies raised against the individual variant proteins produced in E. coli. In rabbit reticulocyte lysate, however, only the full length E6 and the E6IV variant were synthesized. This could be due to inefficient translation as well as lower stability of the short variants. E6I-III, in reticulocyte lysate (RTL). The ability of the E6 variants to associate with p53 and target its proteolytic degradation in vitro, was examined in coimmunoprecipitation assays, using in vitro synthesized proteins and monoclonal antibodies to p53. Results of these assays indicated that only the full length E6 efficiently binds to and promotes the degradation of p53. The E6 variants E6I-E6IV, although able to associate with p53 at a low efficiency, were unable to target its degradation.