Human papillomavirus type 16 E6-induced degradation of E6TP1 correlates with its ability to immortalize human mammary epithelial cells

Development of a Spontaneous HPV16 E6/E7-Expressing Head and Neck Squamous Cell Carcinoma in HLA-A2 Transgenic Mice

Human papillomavirus (HPV)-associated head and neck squamous cell carcinoma (HNSCC) is a growing global health problem. HPV16 has been attributed to a majority of HPV-associated HNSCCs. In order to test candidate immunotherapies, we developed a spontaneous HPV16-driven HNSCC model in HLA-A2 (AAD) transgenic mice. We sought to eliminate the confounding effects of dominant HPV antigen presentation through murine major histocompatibility complex class I (MHC-I) via epitope mutagenesis (without compromising tumorigenicity). We generated HPV16 E6(R55K)(delK75) and E7(N53S) expression constructs with mutations in known dominant H-2D^b epitopes and characterized their presentation through murine and human MHC-I molecules using in vitro and in vivo activation of HPV16 E6/E7 antigen-specific CD8⁺ T cells. In addition, we tested the ability of E6(R55K)(delK75) and E7(N53S) for oncogenicity. The mutated E7(N53S) abolished the presentation of murine H-2D^b-restricted HPV16 E7 peptide (i.e., amino acids [aa] 49 to 57) cytotoxic T lymphocyte (CTL) epitope and resulted in HLA-A2-restricted presentation of the HPV16 E7 (aa 11 to 20)-specific CTL epitope. The mutated E6(R55K)(delK75) abolished the activation of murine MHC-I-restricted E6-specific CD8⁺ T cell-mediated immune responses in C57BL/6 mice. In addition, the vaccination led to the activation of human HLA-A2-restricted E6-specific CD8⁺ T cell-mediated immune responses in HLA-A2 (AAD) transgenic mice. Injection of DNA plasmids encoding LucE7(N53S)E6(R55K)(delK75), AKT, c-Myc, and SB100 followed by electroporation results in development of squamous cell carcinoma in the oral/pharyngeal cavity of all of the HLA-A2 (AAD) transgenic mice (5/5), with 2/5 tumor-bearing mice developing metastatic carcinoma in the neck lymph nodes. IMPORTANCE Our data indicate that mutated HPV16 E6(R55K)(delK75) and mutated HPV16 E7(N53S) DNA abolishes the presentation of HPV16 E6 and E7 through murine MHC-I and results in their presentation through human HLA-A2 molecules. Additionally, the mutated HPV16 E6 and E7 remain oncogenic. Our approach is potentially applicable to different human MHC-I transgenic mice for the identification of human MHC-I restricted HPV16 E6/E7-specific CTL epitopes as well as the generation of spontaneous HPV E6/E7-expressing oral/pharyngeal carcinoma.

Catching viral breast cancer

Abstract

We have considered viruses and their contribution to breast cancer.

Mouse mammary tumour virus

The prevalence of mouse mammary tumour virus (MMTV) is 15-fold higher in human breast cancer than in normal and benign human breast tissue controls. Saliva is the most plausible means of transmission. MMTV has been identified in dogs, cats, monkeys, mice and rats. The causal mechanisms include insertional oncogenesis and mutations in the protective enzyme ABOBEC3B.

Human papilloma virus

The prevalence of high risk human papilloma viruses (HPV) is frequently six fold higher in breast cancer than in normal and benign breast tissue controls. Women who develop HPV associated cervical cancer are at higher than normal risk of developing HPV associated breast cancer. Koilocytes have been identified in breast cancers which is an indication of HPV oncogenicity. The causal mechanisms of HPVs in breast cancer appear to differ from cervical cancer. Sexual activity is the most common form of HPV transmission. HPVs are probably transmitted from the cervix to the breast by circulating extra cellular vesicles.

Epstein Barr virus

The prevalence of Epstein Barr virus (EBV) is five fold higher in breast cancer than in normal and benign breast tissue controls. EBV is mostly transmitted from person to person via saliva. EBV infection predisposes breast epithelial cells to malignant transformation through activation of HER2/HER3 signalling cascades. EBV EBNA genes contribute to tumour growth and metastasis and have the ability to affect the mesenchymal transition of cells.

Bovine leukemia virus

Bovine leukemia virus (BLV) infects beef and dairy cattle and leads to various cancers. The prevalence of BLV is double in human breast cancers compared to controls. Breast cancer is more prevalent in red meat eating and cow’s milk consuming populations. BLV may be transmitted to humans from cattle by the consumption of red meat and cow’s milk.

Conclusion

The evidence that MMTV, high risk HPVs and EBVs have causal roles in human breast cancer is compelling. The evidence with respect to BLV is more limited but it is likely to also have a causal role in human breast cancer.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13027-021-00366-3.

Development of DNA Vaccine Targeting E6 and E7 Proteins of Human Papillomavirus 16 (HPV16) and HPV18 for Immunotherapy in Combination with Recombinant Vaccinia Boost and PD-1 Antibody

Immunotherapy for cervical cancer should target high-risk human papillomavirus types 16 and 18, which cause 50% and 20% of cervical cancers, respectively. Here, we describe the construction and characterization of the pBI-11 DNA vaccine via the addition of codon-optimized human papillomavirus 18 (HPV18) E7 and HPV16 and 18 E6 genes to the HPV16 E7-targeted DNA vaccine pNGVL4a-SigE7(detox)HSP70 (DNA vaccine pBI-1). Codon optimization of the HPV16/18 E6/E7 genes in pBI-11 improved fusion protein expression compared to that in DNA vaccine pBI-10.1 that utilized the native viral sequences fused 3' to a signal sequence and 5' to the HSP70 gene of Mycobacterium tuberculosis Intramuscular vaccination of mice with pBI-11 DNA better induced HPV antigen-specific CD8+ T cell immune responses than pBI-10.1 DNA. Furthermore, intramuscular vaccination with pBI-11 DNA generated stronger therapeutic responses for C57BL/6 mice bearing HPV16 E6/E7-expressing TC-1 tumors. The HPV16/18 antigen-specific T cell-mediated immune responses generated by pBI-11 DNA vaccination were further enhanced by boosting with tissue-antigen HPV vaccine (TA-HPV). Combination of the pBI-11 DNA and TA-HPV boost vaccination with PD-1 antibody blockade significantly improved the control of TC-1 tumors and extended the survival of the mice. Finally, repeat vaccination with clinical-grade pBI-11 with or without clinical-grade TA-HPV was well tolerated in vaccinated mice. These preclinical studies suggest that the pBI-11 DNA vaccine may be used with TA-HPV in a heterologous prime-boost strategy to enhance HPV 16/18 E6/E7-specific CD8+ T cell responses, either alone or in combination with immune checkpoint blockade, to control HPV16/18-associated tumors. Our data serve as an important foundation for future clinical translation.IMPORTANCE Persistent expression of high-risk human papillomavirus (HPV) E6 and E7 is an obligate driver for several human malignancies, including cervical cancer, wherein HPV16 and HPV18 are the most common types. PD-1 antibody immunotherapy helps a subset of cervical cancer patients, and its efficacy might be improved by combination with active vaccination against E6 and/or E7. For patients with HPV16+ cervical intraepithelial neoplasia grade 2/3 (CIN2/3), the precursor of cervical cancer, intramuscular vaccination with a DNA vaccine targeting HPV16 E7 and then a recombinant vaccinia virus expressing HPV16/18 E6-E7 fusion proteins (TA-HPV) was safe, and half of the patients cleared their lesions in a small study (NCT00788164). Here, we sought to improve upon this therapeutic approach by developing a new DNA vaccine that targets E6 and E7 of HPV16 and HPV18 for administration prior to a TA-HPV booster vaccination and for application against cervical cancer in combination with a PD-1-blocking antibody.

Regulating the human HECT E3 ligases

Ubiquitination, the covalent attachment of ubiquitin to proteins, by E3 ligases of the HECT (homologous to E6AP C terminus) family is critical in controlling diverse physiological pathways. Stringent control of HECT E3 ligase activity and substrate specificity is essential for cellular health, whereas deregulation of HECT E3s plays a prominent role in disease. The cell employs a wide variety of regulatory mechanisms to control HECT E3 activity and substrate specificity. Here, we summarize the current understanding of these regulatory mechanisms that control HECT E3 function. Substrate specificity is generally determined by interactions of adaptor proteins with domains in the N-terminal extensions of HECT E3 ligases. These N-terminal domains have also been found to interact with the HECT domain, resulting in the formation of inhibitory conformations. In addition, catalytic activity of the HECT domain is commonly regulated at the level of E2 recruitment and through HECT E3 oligomerization. The previously mentioned regulatory mechanisms can be controlled through protein-protein interactions, post-translational modifications, the binding of calcium ions, and more. Functional activity is determined not only by substrate recruitment and catalytic activity, but also by the type of ubiquitin polymers catalyzed to the substrate. While this is often determined by the specific HECT member, recent studies demonstrate that HECT E3s can be modulated to alter the type of ubiquitin polymers they catalyze. Insight into these diverse regulatory mechanisms that control HECT E3 activity may open up new avenues for therapeutic strategies aimed at inhibition or enhancement of HECT E3 function in disease-related pathways.

Preclinical Development of Oncolytic Immunovirotherapy for Treatment of HPVPOS Cancers

Immunotherapy for HPV^POS malignancies is attractive because well-defined, viral, non-self tumor antigens exist as targets. Several approaches to vaccinate therapeutically against HPV E6 and E7 antigens have been adopted, including viral platforms such as VSV. A major advantage of VSV expressing these antigens is that VSV also acts as an oncolytic virus, leading to direct tumor cell killing and induction of effective anti-E6 and anti-E7 T cell responses. We have also shown that addition of immune adjuvant genes, such as IFNβ, further enhances safety and/or efficacy of VSV-based oncolytic immunovirotherapies. However, multiple designs of the viral vector are possible—with respect to levels of immunogen expression and method of virus attenuation—and optimal designs have not previously been tested head-to-head. Here, we tested three different VSV engineered to express a non-oncogenic HPV16 E7/6 fusion protein for their immunotherapeutic and oncolytic properties. We assessed their profiles of efficacy and toxicity against HPV^POS and HPV^NEG murine tumor models and determined the optimal route of administration. Our data show that VSV is an excellent platform for the oncolytic immunovirotherapy of tumors expressing HPV target antigens, combining a balance of efficacy and safety suitable for evaluation in a first-in-human clinical trial.

In silico modeling of the cryptic E2∼ubiquitin-binding site of E6-associated protein (E6AP)/UBE3A reveals the mechanism of polyubiquitin chain assembly

To understand the mechanism for assembly of Lys⁴⁸-linked polyubiquitin degradation signals, we previously demonstrated that the E6AP/UBE3A ligase harbors two functionally distinct E2∼ubiquitin-binding sites: a high-affinity Site 1 required for E6AP Cys⁸²⁰∼ubiquitin thioester formation and a canonical Site 2 responsible for subsequent chain elongation. Ordered binding to Sites 1 and 2 is here revealed by observation of UbcH7∼ubiquitin-dependent substrate inhibition of chain formation at micromolar concentrations. To understand substrate inhibition, we exploited the PatchDock algorithm to model in silico UbcH7∼ubiquitin bound to Site 1, validated by chain assembly kinetics of selected point mutants. The predicted structure buries an extensive solvent-excluded surface bringing the UbcH7∼ubiquitin thioester bond within 6 Å of the Cys⁸²⁰ nucleophile. Modeling onto the active E6AP trimer suggests that substrate inhibition arises from steric hindrance between Sites 1 and 2 of adjacent subunits. Confirmation that Sites 1 and 2 function in trans was demonstrated by examining the effect of E6APC820A on wild-type activity and single-turnover pulse-chase kinetics. A cyclic proximal indexation model proposes that Sites 1 and 2 function in tandem to assemble thioester-linked polyubiquitin chains from the proximal end attached to Cys⁸²⁰ before stochastic en bloc transfer to the target protein. Non-reducing SDS-PAGE confirms assembly of the predicted Cys⁸²⁰-linked ¹²⁵I-polyubiquitin thioester intermediate. Other studies suggest that Glu⁵⁵⁰ serves as a general base to generate the Cys⁸²⁰ thiolate within the low dielectric binding interface and Arg⁵⁰⁶ functions to orient Glu⁵⁵⁰ and to stabilize the incipient anionic transition state during thioester exchange.

Spontaneous and Vaccine-Induced Clearance of Mus Musculus Papillomavirus 1 Infection

Mus musculus papillomavirus 1 (MmuPV1/MusPV1) induces persistent papillomas in immunodeficient mice but not in common laboratory strains. To facilitate the study of immune control, we sought an outbred and immunocompetent laboratory mouse strain in which persistent papillomas could be established. We found that challenge of SKH1 mice (Crl:SKH1-Hrhr) with MmuPV1 by scarification on their tail resulted in three clinical outcomes: (i) persistent (>2-month) papillomas (∼20%); (ii) transient papillomas that spontaneously regress, typically within 2 months (∼15%); and (iii) no visible papillomas and viral clearance (∼65%). SKH1 mice with persistent papillomas were treated by using a candidate preventive/therapeutic naked-DNA vaccine that expresses human calreticulin (hCRT) fused in frame to MmuPV1 E6 (mE6) and mE7 early proteins and residues 11 to 200 of the late protein L2 (hCRTmE6/mE7/mL2). Three intramuscular DNA vaccinations were delivered biweekly via in vivo electroporation, and both humoral and CD8 T cell responses were mapped and measured. Previously persistent papillomas disappeared within 2 months after the final vaccination. Coincident virologic clearance was confirmed by in situ hybridization and a failure of disease to recur after CD3 T cell depletion. Vaccination induced strong mE6 and mE7 CD8⁺ T cell responses in all mice, although they were significantly weaker in mice that initially presented with persistent warts than in those that spontaneously cleared their infection. A human papillomavirus 16 (HPV16)-targeted version of the DNA vaccine also induced L2 antibodies and protected mice from vaginal challenge with an HPV16 pseudovirus. Thus, MmuPV1 challenge of SKH1 mice is a promising model of spontaneous and immunotherapy-directed clearances of HPV-related disease.IMPORTANCE High-risk-type human papillomaviruses (hrHPVs) cause 5% of all cancer cases worldwide, notably cervical, anogenital, and oropharyngeal cancers. Since preventative HPV vaccines have not been widely used in many countries and do not impact existing infections, there is considerable interest in the development of therapeutic vaccines to address existing disease and infections. The strict tropism of HPV requires the use of animal papillomavirus models for therapeutic vaccine development. However, MmuPV1 failed to grow in common laboratory strains of mice with an intact immune system. We show that MmuPV1 challenge of the outbred immunocompetent SKH1 strain produces both transient and persistent papillomas and that vaccination of the mice with a DNA expressing an MmuPV1 E6E7L2 fusion with calreticulin can rapidly clear persistent papillomas. Furthermore, an HPV16-targeted version of the DNA can protect against vaginal challenge with HPV16, suggesting the promise of this approach to both prevent and treat papillomavirus-related disease.

A DNA vaccine encoding mutated HPV58 mE6E7-Fc-GPI fusion antigen and GM-CSF and B7.1

BACKGROUND

Persistent infection with high-risk human papillomavirus (HPV) is a predominant cause of cervical cancer, and HPV58 is the third most common virus detected in the patients with cervical cancer in Asia. E6 and E7 are the viral oncogenes which are constitutively expressed in HPV-associated tumor cells and can be used as target antigens for related immunotherapy. In this study, we modified the HPV58 E6 and E7 oncogenes to eliminate their oncogenic potential and constructed a recombinant DNA vaccine that coexpresses the sig-HPV58 mE6E7-Fc-GPI fusion antigen in addition to granulocyte-macrophage colony-stimulating factor (GM-CSF) and B7.1 as molecular adjuvants (PVAX1-HPV58 mE6E7FcGB) for the treatment of HPV58 (+) cancer.

METHODS

PVAX1-HPV58 mE6E7FcGB recombinant DNA vaccine was constructed to express a fusion protein containing a signal peptide, a modified HPV58 mE6E7 gene, and human IgG Fc and glycosylphosphatidylinositol (GPI)-anchoring sequences using the modified DNA vaccine vector PVAX1-IRES-GM/B7.1 that coexpresses GM-CSF, and B7.1. C57BL/6 mice were challenged by HPV58 E6E7-expressing B16-HPV58 E6E7 cells, followed by immunization by PVAX1-HPV58 mE6E7FcGB vaccine on days 7, 14, 21 after tumor challenge. The cellular immune responses in immunized mice were assessed by measuring IFN-γ production in splenocytes upon stimulation by HPV58 E6E7-GST protein and the lysis of B16-HPV58 E6E7 target cells by splenocytes after restimulation with HPV58 E6E7-GST protein. The antitumor efficacy was evaluated by monitoring the growth of the tumor.

RESULTS

PVAX1-HPV58 mE6E7FcGB elicited varying levels of IFN-lsgdB58onn T-cell immune responses and lysis of target cell in mice in response to the recombinant antigen HPV58 E6E7-GST. Furthermore, the vaccine also induced antitumor responses in the HPV58 (+) B16-HPV58 E6E7 tumor challenge model as evidenced by delayed tumor development.

CONCLUSION

The recombinant DNA vaccine PVAX1-HPV58 mE6E7FcGB efficiently generates cellular immunity and antitumor efficacy in immunized mice. These data provide a basis for the further study of this recombinant vaccine as a potential candidate vaccine.

Control of HPV-associated tumors by innovative therapeutic HPV DNA vaccine in the absence of CD4+ T cells

Human papillomavirus (HPV) infections are particularly problematic for HIV + and solid organ transplant patients with compromised CD4+ T cell-dependent immunity as they produce more severe and progressive disease compared to healthy individuals. There are no specific treatments for chronic HPV infection, resulting in an urgent unmet need for a modality that is safe and effective for both immunocompromised and otherwise normal patients with recalcitrant disease. DNA vaccination is attractive because it avoids the risks of administration of live vectors to immunocompromised patients, and can induce potent HPV-specific cytotoxic T cell responses. We have developed a DNA vaccine (pNGVL4a-hCRTE6E7L2) encoding calreticulin (CRT) fused to E6, E7 and L2 proteins of HPV-16, the genotype associated with approximately 90% vaginal, vulvar, anal, penile and oropharyngeal HPV-associated cancers and the majority of cervical cancers. Administration of the DNA vaccine by intramuscular (IM) injection followed by electroporation induced significantly greater HPV-specific immune responses compared to IM injection alone or mixed with alum. Furthermore, pNGVL4a-hCRTE6E7L2 DNA vaccination via electroporation of mice carrying an intravaginal HPV-16 E6/E7-expressing syngeneic tumor demonstrated more potent therapeutic effects than IM vaccination alone. Of note, administration of the DNA vaccine by IM injection followed by electroporation elicited potent E6 and E7-specific CD8+ T cell responses and antitumor effects despite CD4+ T cell-depletion, although no antibody response was detected. While CD4+ T cell-depletion did reduce the E6 and E7-specific CD8+ T cell response, it remained sufficient to prevent subcutaneous tumor growth and to eliminate circulating tumor cells in a model of metastatic HPV-16+ cancer. Thus, the antibody response was CD4-dependent, whereas CD4+ T cell help enhanced the E6/E7-specific CD8+ T cell immunity, but was not required. Taken together, our data suggest that pNGVL4a-hCRTE6E7L2 DNA vaccination via electroporation warrants testing in otherwise healthy patients and those with compromised CD4+ T cell immunity to treat HPV-16-associated anogenital disease and cancer.

De-oncogenic HPV E6/E7 vaccine gets enhanced antigenicity and promotes tumoricidal synergy with cisplatin

In order to develop more effective therapeutic vaccines against cancers with high-risk human papillomavirus (HPV) infection, it is crucial to enhance the immunogenicity, eliminate the oncogenicity of oncoproteins, and take a combination of E7- and E6-containing vaccines. It has been shown recently that PE(ΔIII)-E7-KDEL3 (E7), a fusion protein containing the HPV16 oncoprotein E7 and the translocation domain of Pseudomonas aeruginosa exotoxin A, is effective against TC-1 tumor cells inoculated in mice, therefore, we engineered PE(ΔIII)-E6-CRL-KDEL3 (E6), the de-oncogenic versions of the E7 and E6 fusion proteins [i.e. PE(ΔIII)-E7(d)-KDEL3, E7(d), and PE(ΔIII)-E6(d)-CRL-KDEL3, E6(d)] and tested the immunoefficacies of these fusion proteins as mono- and bivalent vaccines. Results indicated that the E7(d) get higher immunogenicity than its wild type and the E6 fusion proteins augmented the immunogenicity and antitumor effects of their E7 counterparts. Furthermore, the bivalent vaccine system E7(d) plus E6(d), in the presence of cisplatin, showed the best tumoristatic and tumoricidal effects against established tumors in vivo. Therefore, it can be concluded that this novel therapeutic vaccine system, upon further optimization, may shed new light on clinical management of HPV-related carcinomas.

E6AP/UBE3A ubiquitin ligase harbors two E2~ubiquitin binding sites

By exploiting (125)I-polyubiquitin chain formation as a functional readout of enzyme activity, we have quantitatively examined the mechanism of human E6AP/UBE3A for the first time. Initial rate studies identify UbcH7 as the cognate E2 carrier protein for E6AP, although related Ubc5 isoforms and the ISG15-specific UbcH8 paralog also support E6AP with reduced efficacy due to impaired binding and catalytic competence. Initial rates of polyubiquitin chain formation displayed hyperbolic kinetics with respect to UbcH7 concentration (K(m) = 57.6 ± 5.7 nM and kcat = 0.032 ± 0.001 s(-1)) and substrate inhibition above 2 μM. Competitive inhibition by an isosteric UbcH7C86S-ubiquitin oxyester substrate analog (K(i) = 64 ± 18 nM) demonstrates that Km reflects intrinsic substrate affinity. In contrast, noncompetitive inhibition by a UbcH7C86A product analog (K(i) = 7 ± 0.7 μM) and substrate inhibition at high concentrations require two functionally distinct E2∼ubiquitin substrate binding sites. The kinetics of polyubiquitin chain formation reflect binding at a cryptic Site 1 not previously recognized that catalyzes E6AP∼ubiquitin thioester formation. Subsequent binding of E2∼ubiquitin at the canonical Site 2 present in the extant crystal structure is responsible for polyubiquitin chain elongation. Other rate studies show that the conserved -4 Phe(849) residue is required for polyubiquitin chain formation rather than target protein conjugation as originally suggested. The present studies unambiguously preclude earlier models for the mechanism of Hect domain-catalyzed conjugation through the canonical binding site suggested by the crystal structure and define a novel two-step mechanism for formation of the polyubiquitin degradation signal.

Gene expression profiles are altered in human papillomavirus-16 E6 D25E-expressing cell lines

Previously, we have reported that the human papillomavirus (HPV) type 16 E6 D25E is the most prevalent variant in Korean women at high risk for cervical cancers. Several studies have identified an association between the increased frequency of this variant and the elevated risk of cervical intraepithelial neoplasia and invasive cervical carcinoma. To investigate whether the HPV-16 E6 D25E variant might influence cervical cancer progression, we used an oligonucleotide microarray approach to identify transcriptionally altered gene expression patterns in recombinant wild-type E6 or E6 D25E variant-expressing HPV-negative cancer cells. We found that 211 genes were significantly up- or down-regulated (at least 1.5-fold, p < 0.05). We identified 14 genes, nine down-regulated and five up-regulated upon E6 D25E expression, compared with wild-type E6 expression. These results further emphasize the unique biological activity of the HPV-16 E6 D25E variant.

Immunogenicity in mice and rhesus monkeys vaccinated with recombinant vaccinia virus expressing bivalent E7E6 fusion proteins from human papillomavirus types 16 and 18

BACKGROUND

Persistent infection with high-risk human papillomavirus (HPV) is a predominant cause of cervical cancer, and HPV16 and HPV18 occur in 50% and 20% of cervical cancer cases, respectively. The viral oncogenes E6 and E7 are constitutively expressed by HPV-associated tumour cells and can therefore be used as target antigens for immunotherapy. In this study, we constructed a recombinant vaccinia virus co-expressing the HPV16/18 E7E6 fusion proteins (rVVJ16/18E7E6) for use as a therapeutic vaccine for the treatment of HPV16⁺ and HPV18⁺ cancers.

METHODS

We constructed a bivalent recombinant vaccinia virus expressing modified E7E6 fusion proteins of HPV type 16 and 18 (rVVJ16/18E7E6) based on the vaccinia virus Tiantan strain. We then defined the cellular immune responses to the virus in mice and rhesus monkeys and assessed antitumour efficacy of these responses in mice using the TC-1 tumour challenge model.

RESULTS

Our data demonstrated that rVVJ16/18E7E6 was able to elicit varying levels of CD8⁺ T cell immune responses and lysis of target cells in mice in response to peptides HPV16E7₄₉₋₅₇ and HPV18E6₆₇₋₇₅. Furthermore, the virus was also able to induce anti-tumour responses in the HPV16⁺ TC-1 tumour challenge model, including partial protection (30-40%) and delayed tumour appearance. In addition, the virus was able to induce immune responses in rhesus monkeys.

CONCLUSIONS

The recombinant vaccinia virus rVVJ16/18E7E6 can generate clear and significant cellular immunity in both mice and rhesus monkeys. These data provide a basis for the use of this recombinant virus as a potential vaccine candidate for further study.

Histone deacetylase inhibitors upregulate Rap1GAP and inhibit Rap activity in thyroid tumor cells

Increases in Rap activity have been associated with tumor progression. Although activating mutations in Rap have not been described, downregulation of Rap1GAP is frequent in human tumors including thyroid carcinomas. In this study, we explored whether endogenous Rap1GAP expression could be restored to thyroid tumor cells. The effects of deacetylase inhibitors and a demethylating agent, individually and in combination, were examined in four differentiated and six anaplastic thyroid carcinoma (ATC) cell lines. Treatment with the structurally distinct histone deacetylase (HDAC) inhibitors, sodium butyrate and trichostatin A, increased Rap1GAP expression in all the differentiated thyroid carcinoma cell lines and in four of the six ATC cell lines. The demethylating agent, 5-aza-deoxycytidine, restored Rap1GAP expression in one anaplastic cell line and enhanced the effects of HDAC inhibitors in a second anaplastic cell line. Western blotting indicated that Rap2 was highly expressed in human thyroid cancer cells. Importantly, treatment with HDAC inhibitors impaired Rap2 activity in both differentiated and anaplastic tumor cell lines. The mechanism through which Rap activity is repressed appears to entail effects on the expression of multiple Rap regulators, including RapGEFs and RapGAPs. These results suggest that HDAC inhibitors may provide a tractable approach to impair Rap activity in human tumor cells.

HPV E6 proteins target Ubc9, the SUMO conjugating enzyme

The human papillomavirus oncogenic protein, E6, interacts with a number of cellular proteins, and for some targets, E6 directs their degradation through the ubiquitin-proteasome pathway. Post-translational modification with ubiquitin-like modifiers, such as SUMO, also influences protein activities, protein-protein interactions, and protein stability. We report that the high risk HPVE6 proteins reduce the intracellular quantity of the sole SUMO conjugation enzyme, Ubc9, concomitant with decreased host sumoylation. E6 did not significantly influence transcription of Ubc9, indicating that the effects were likely at the protein level. Consistent with typical E6-mediated proteasomal degradation, E6 bound to Ubc9 in vitro, and required E6AP for reduction of Ubc9 levels. Under stable E6 expression conditions in differentiating keratinocytes there was a decrease in Ubc9 and a loss of numerous sumoylated targets indicating a significant perturbation of the normal sumoylation profile. While E6 is known to inhibit PIASy, a SUMO ligase, our results suggest that HPV E6 also targets the Ubc9 protein to modulate host cell sumoylation, suggesting that the sumoylation system may be an important target during viral reproduction and possibly the subsequent development of cervical cancer.

Viral oncogenes, noncoding RNAs, and RNA splicing in human tumor viruses

Viral oncogenes are responsible for oncogenesis resulting from persistent virus infection. Although different human tumor viruses express different viral oncogenes and induce different tumors, their oncoproteins often target similar sets of cellular tumor suppressors or signal pathways to immortalize and/or transform infected cells. Expression of the viral E6 and E7 oncogenes in papillomavirus, E1A and E1B oncogenes in adenovirus, large T and small t antigen in polyomavirus, and Tax oncogene in HTLV-1 are regulated by alternative RNA splicing. However, this regulation is only partially understood. DNA tumor viruses also encode noncoding RNAs, including viral microRNAs, that disturb normal cell functions. Among the determined viral microRNA precursors, EBV encodes 25 from two major clusters (BART and BHRF1), KSHV encodes 12 from a latent region, human polyomavirus MCV produce only one microRNA from the late region antisense to early transcripts, but HPVs appears to produce no viral microRNAs.

Downregulation of Rap1GAP in human tumor cells alters cell/matrix and cell/cell adhesion

Rap1GAP expression is decreased in human tumors. The significance of its downregulation is unknown. We show that Rap1GAP expression is decreased in primary colorectal carcinomas. To elucidate the advantages conferred on tumor cells by loss of Rap1GAP, Rap1GAP expression was silenced in human colon carcinoma cells. Suppressing Rap1GAP induced profound alterations in cell adhesion. Rap1GAP-depleted cells exhibited defects in cell/cell adhesion that included an aberrant distribution of adherens junction proteins. Depletion of Rap1GAP enhanced adhesion and spreading on collagen. Silencing of Rap expression normalized spreading and restored E-cadherin, beta-catenin, and p120-catenin to cell/cell contacts, indicating that unrestrained Rap activity underlies the alterations in cell adhesion. The defects in adherens junction protein distribution required integrin signaling as E-cadherin and p120-catenin were restored at cell/cell contacts when cells were plated on poly-l-lysine. Unexpectedly, Src activity was increased in Rap1GAP-depleted cells. Inhibition of Src impaired spreading and restored E-cadherin at cell/cell contacts. These findings provide the first evidence that Rap1GAP contributes to cell/cell adhesion and highlight a role for Rap1GAP in regulating cell/matrix and cell/cell adhesion. The frequent downregulation of Rap1GAP in epithelial tumors where alterations in cell/cell and cell/matrix adhesion are early steps in tumor dissemination supports a role for Rap1GAP depletion in tumor progression.

Therapeutic HPV DNA vaccines

Human papillomavirus (HPV) has been associated with several human cancers, including cervical cancer, vulvar cancer, vaginal and anal cancer, and a subset of head and neck cancers. The identification of HPV as an etiological factor for HPV-associated malignancies creates the opportunity for the control of these cancers through vaccination. Currently, the preventive HPV vaccine using HPV virus-like particles has been proven to be safe and highly effective. However, this preventive vaccine does not have therapeutic effects, and a significant number of people have established HPV infection and HPV-associated lesions. Therefore, it is necessary to develop therapeutic HPV vaccines to facilitate the control of HPV-associated malignancies and their precursor lesions. Among the various forms of therapeutic HPV vaccines, DNA vaccines have emerged as a potentially promising approach for vaccine development due to their safety profile, ease of preparation and stability. However, since DNA does not have the intrinsic ability to amplify or spread in transfected cells like viral vectors, DNA vaccines can have limited immunogenicity. Therefore, it is important to develop innovative strategies to improve DNA vaccine potency. Since dendritic cells (DCs) are key players in the generation of antigen-specific immune responses, it is important to develop innovative strategies to modify the properties of the DNA-transfected DCs. These strategies include increasing the number of antigen-expressing/antigen-loaded DCs, improving antigen processing and presentation in DCs, and enhancing the interaction between DCs and T cells. Many of the studies on DNA vaccines have been performed on preclinical models. Encouraging results from impressive preclinical studies have led to several clinical trials.

HPV E6, E6AP and cervical cancer

Every year, approximately 470,000 new cases of cervical cancer are diagnosed and approximately 230,000 women worldwide die of the disease, with the majority (approximately 80%) of these cases and deaths occurring in developing countries. Human papillomaviruses (HPVs) are the etiological agents in nearly all cases (99.7%) of cervical cancer, and the HPV E6 protein is one of two viral oncoproteins that is expressed in virtually all HPV-positive cancers. E6 hijacks a cellular ubiquitin ligase, E6AP, resulting in the ubiquitylation and degradation of the p53 tumor suppressor, as well as several other cellular proteins. While the recent introduction of prophylactic vaccines against specific HPV types offers great promise for prevention of cervical cancer, there remains a need for therapeutics. Biochemical characterization of E6 and E6AP has suggested approaches for interfering with the activities of these proteins that could be useful for this purpose. PUBLICATION HISTORY : Republished from Current BioData's Targeted Proteins database (TPdb; http://www.targetedproteinsdb.com).

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.

Deletion of the PDZ motif of HPV16 E6 preventing immortalization and anchorage-independent growth in human tonsil epithelial cells

BACKGROUND

Human papillomavirus 16 (HPV16) has been associated with head and neck squamous cell carcinoma (HNSCC) in up to 60% of sampled specimens.

METHODS

To understand better the viral genes required to transform human tonsil epithelial cells (HTEC), we isolated HTEC's and transduced them with retroviral vectors containing HPV16 E6 and E7.

RESULTS

Immortalization and anchorage-independent growth of HTEC's only occurred with expression of E6 and E7 with resultant degradation of p53. However, cells expressing E6 lacking the PSD-95/disc-large/Zo-1 (PDZ) motif did not immortalize or grow anchorage independent. Telomerase activity and degradation of p53 were similar for wild-type and mutant E6.

CONCLUSION

The mechanism of oncogenic transformation by E6 in HTEC's is dependent on the PDZ binding motif. Identification of pathways affected by the interaction of E6 and PDZ domain containing proteins will further our understanding of how HPV causes HNSCC and will provide potential therapeutic targets.

Differential regulation of elafin in normal and tumor-derived mammary epithelial cells is mediated by CCAAT/enhancer binding protein beta

CCAAT/enhancer binding protein beta (C/EBP beta) is a transcription factor implicated in the control of development, differentiation, and proliferation of mammary epithelial cells. However, it remains unclear how C/EBP beta is involved in tumor suppression through its interaction with specific downstream genes in breast cancer. Tumor cells overexpress serine proteases, which play crucial roles in tumor invasion and metastasis. Elafin is an endogenous serine protease inhibitor and is transcriptionally down-regulated in most tumor cell lines. In this study, we show that C/EBP beta is differentially expressed in normal versus tumor cell lines and normal adjacent versus tumor tissues obtained from breast cancer patients. We identified elafin as a downstream effector of C/EBP beta and show that elafin is also differentially regulated between normal and tumor cells. The mechanism by which C/EBP beta regulates elafin expression is through its direct interaction with the elafin promoter. There are three C/EBP beta binding sites involved in the elafin promoter activity, and the overexpression of C/EBP beta transactivates the elafin gene through these sites in tumor cells. RNA interference studies in normal cells further evidenced the requirement of the C/EBP beta for the elafin expression and negative feedback loop between C/EBP beta and elafin. We suggest that elafin is a novel substrate of C/EBP beta, and alterations in C/EBP beta isoforms result in their differential binding to the elafin promoter, leading to the altered expression of the elafin between normal and tumor cells.

Corneal epithelial cell cultures as a tool for research, drug screening and testing

Understanding of visual system function and the development of new therapies for corneal diseases and damages depend upon comprehension of the biological roles of the tissue. The in vitro cultivation of corneal epithelial cells and cell lines derived from them has become a powerful tool to analyze and understand such issues. Currently, researchers have developed well-defined and precisely described culture protocols and a collection of corneal epithelial cell lines. These cell lines have been obtained through different experimental approaches: (1) the ectopic expression of oncogenes, (2) the inactivation of p16 and p53 pathways and hTERT expression, and (3) the spontaneous establishment after serial cultivation of cells. The advantages or disadvantages for these approaches are discussed. In conclusion, the availability of several culture protocols and immortalized cell lines that express corneal epithelial phenotype will be useful for investigating issues such as gene regulation and tissue development, or for validating alternative methods in toxicology.

Generation and characterization of a preventive and therapeutic HPV DNA vaccine

Cervical cancer is one of the most common cancers in women worldwide. Persistent infection with human papillomavirus (HPV) is considered to be the etiological factor for cervical cancer. Therefore, an effective vaccine against HPV infections may lead to the control of cervical cancer. An ideal HPV vaccine should aim to generate both humoral immune response to prevent new infections as well as cell-mediated immunity to eliminate established infection or HPV-related disease. In the current study, we have generated a potential preventive and therapeutic HPV DNA vaccine using human calreticulin (CRT) linked to HPV16 early proteins, E6 and E7 and the late protein L2 (hCRTE6E7L2). We found that vaccination with hCRTE6E7L2 DNA vaccine induced a potent E6/E7-specific CD8+ T cell immune response, resulting in a significant therapeutic effect against E6/E7 expressing tumor cells. In addition, vaccination with hCRTE6E7L2 DNA generated significant L2-specific neutralizing antibody responses, protecting against pseudovirion infection. Thus, the hCRTE6E7L2 DNA vaccines are capable of generating potent preventive and therapeutic effects in vaccinated mice. Our data has significant clinical implications.

Binding of human papillomavirus type 16 E6 to E6AP is not required for activation of hTERT

The human papillomavirus (HPV) type 16 (HPV16) E6 protein stimulates transcription of the catalytic subunit of telomerase, hTERT, in epithelial cells. It has been reported that binding to the ubiquitin ligase E6AP is required for this E6 activity, with E6 directing E6AP to the hTERT promoter. We previously reported two E6AP binding-defective HPV16 E6 mutations that induced immortalization of human mammary epithelial cells. Because activation of hTERT is proposed to be necessary for epithelial cell immortalization, we sought to further characterize the relationship between E6/E6AP association and telomerase induction. We demonstrate that while these E6 mutants do not bind E6AP, they retain the capability to stimulate the expression of hTERT. Chromatin immunoprecipitation assays confirmed the presence of Myc, wild-type E6, and the E6AP binding-defective E6 mutants, but not E6AP itself, at the endogenous hTERT promoter. Interestingly, an immortalization-defective E6 mutant localized to the hTERT promoter but failed to increase transcription. We conclude that binding to E6AP is not necessary for E6 localization to or activation of the hTERT promoter and that another activity of E6 is involved in hTERT activation.

Rap1 integrates tissue polarity, lumen formation, and tumorigenic potential in human breast epithelial cells

Maintenance of apico-basal polarity in normal breast epithelial acini requires a balance between cell proliferation, cell death, and proper cell-cell and cell-extracellular matrix signaling. Aberrations in any of these processes can disrupt tissue architecture and initiate tumor formation. Here, we show that the small GTPase Rap1 is a crucial element in organizing acinar structure and inducing lumen formation. Rap1 activity in malignant HMT-3522 T4-2 cells is appreciably higher than in S1 cells, their nonmalignant counterparts. Expression of dominant-negative Rap1 resulted in phenotypic reversion of T4-2 cells, led to the formation of acinar structures with correct polarity, and dramatically reduced tumor incidence despite the persistence of genomic abnormalities and baseline growth. The resulting acini contained prominent central lumina not observed when other reverting agents were used. Conversely, expression of dominant-active Rap1 in T4-2 cells inhibited phenotypic reversion and led to increased invasiveness and tumorigenicity. Thus, Rap1 acts as a central regulator of breast architecture, with normal levels of activation instructing polarity during acinar morphogenesis, and increased activation inducing tumor formation and progression to malignancy.

A Wnt-CKIvarepsilon-Rap1 pathway regulates gastrulation by modulating SIPA1L1, a Rap GTPase activating protein

Noncanonical Wnt signals control morphogenetic movements during vertebrate gastrulation. Casein kinase I epsilon (CKIvarepsilon) is a Wnt-regulated kinase that regulates Wnt/beta-catenin signaling and has a beta-catenin-independent role(s) in morphogenesis that is poorly understood. Here we report the identification of a CKIvarepsilon binding partner, SIPA1L1/E6TP1, a GAP (GTPase activating protein) of the Rap small GTPase family. We show that CKIvarepsilon phosphorylates SIPA1L1 to reduce its stability and thereby increase Rap1 activation. Wnt-8, which activates CKIvarepsilon, enhances the CKIvarepsilon-dependent phosphorylation and degradation of SIPA1L1. In early Xenopus or zebrafish development, inactivation of the Rap1 pathway results in abnormal gastrulation and a shortened anterior-posterior axis. Although CKIvarepsilon also transduces Wnt/beta-catenin signaling, inhibition of Rap1 does not alter beta-catenin-regulated gene expression. Our data demonstrate a role for CKIvarepsilon in noncanonical Wnt signaling and indicate that Wnt regulates morphogenesis in part through CKIvarepsilon-mediated control of Rap1 signaling.

Regulation of immune responses and hematopoiesis by the Rap1 signal

Rap1 (Ras-proximity 1), a member of the Ras family of small guanine triphosphatases (GTPases), is activated by diverse extracellular stimuli. While Rap1 has been discovered originally as a potential Ras antagonist, accumulating evidence indicates that Rap1 per se mediates unique signals and exerts biological functions distinctly different from Ras. Rap1 plays a dominant role in the control of cell-cell and cell-matrix interactions by regulating the function of integrins and other adhesion molecules in various cell types. Rap1 also regulates MAP kinase (MAPK) activity in a manner highly dependent on the context of cell types. Recent studies (including gene-targeting analysis) have uncovered that the Rap1 signal is integrated crucially and unpredictably in the diverse aspects of comprehensive biological systems. This review summarizes the role of the Rap1 signal in developments and functions of the immune and hematopoietic systems as well as in malignancy. Importantly, Rap1 activation is tightly regulated in tissue cells, and dysregulations of the Rap1 signal in specific tissues result in certain disorders, including myeloproliferative disorders and leukemia, platelet dysfunction with defective hemostasis, leukocyte adhesion-deficiency syndrome, lupus-like systemic autoimmune disease, and T cell anergy. Many of these disorders resemble human diseases, and the Rap1 signal with its regulators may provide rational molecular targets for controlling certain human diseases including malignancy.

Disruption of the retinoblastoma pathway by small interfering RNA and ectopic expression of the catalytic subunit of telomerase lead to immortalization of human ovarian surface epithelial cells

The risk of developing ovarian cancer is about 1% over a lifetime, but it is the most deadly gynecologic cancer, in part due to lack of diagnostic markers for early-stage disease and cell model system for studying early neoplastic changes. Most existing immortal human ovarian surface epithelial cells were achieved by using viral protein such as SV40 T/t antigen or E6/E7, which inactivate multiple cellular pathways. In the current study, we used a small interfering RNA (siRNA) against the retinoblastoma gene (pRb) and ectopic expression of human telomerase reverse transcriptase (hTERT) to immortalize the primary ovarian epithelial cell line OSE137 and two additional human ovarian surface epithelial cells. The immortalized OSE137 showed increased telomerase activity, lengthened telomeres, increased G2/M phase, altered cell-cycle regulatory proteins but nontumorigenic. As both Rb and hTERT pathways are commonly altered in human ovarian cancer and these genetic changes are faithfully modeled in these cells without using viral protein, these immortal cells represent an authentic in vitro model system with which to study the initiation and progression of human ovarian cancer.

A combination of DNA vaccines targeting human papillomavirus type 16 E6 and E7 generates potent antitumor effects

Human papillomavirus (HPV) infects large numbers of women worldwide and is present in more than 99% of all cervical cancers. HPV E6 and E7 are two viral oncoproteins that are consistently expressed in HPV infections and HPV-associated malignancies. We have previously developed DNA vaccines encoding calreticulin (CRT) linked either to HPV type 16 (HPV-16) E6 or to HPV-16 E7, both of which generated significant antitumor effects against E6- and E7-expressing tumors. In this study, we demonstrate that simultaneous vaccination of C57BL/6 mice or HLA-A2 transgenic mice with both CRT/E6 and CRT/E7 DNA vaccines generates significant E6- and E7-specific T-cell immune responses in vaccinated mice. Furthermore, combined vaccination with both CRT/E6 and CRT/E7 DNA generates significantly better therapeutic antitumor effects against HPV E6- and E7-expressing tumors than vaccination with either CRT/E6 DNA or CRT/E7 DNA alone. Our data suggest that it may be desirable to combine DNA vaccines targeting E6 with DNA vaccines targeting E7 to develop effective immunotherapeutic strategies for control of HPV infection and HPV-associated lesions in a clinical setting.

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.

Involvement of a cellular ubiquitin-protein ligase E6AP in the ubiquitin-mediated degradation of extensive substrates of high-risk human papillomavirus E6

Human scribble (hScrib), which was identified as substrate of human papillomavirus (HPV) E6 for ubiquitin-mediated degradation dependent on ubiquitin-protein ligase E6AP, is a human homolog of Drosophila neoplastic tumor suppressor scribble, in which mutation causes loss of polarity and overgrowth of epithelia. Drosophila discs large (Dlg) is one of neoplastic tumor suppressors, which genetically links to scribble. E6 also targets human Dlg (hDlg) for ubiquitin-mediated degradation. Ubiquitin-protein ligase involved in this process has not been identified thus far. Here we investigated mechanism underlying degradation of three target proteins of E6, hScrib, hDlg, and p53 by using eighteen HPV 16 E6 mutants with single amino acid substitution. In vitro degradation ability of each E6 mutant was equivalent for these tumor suppressors. We investigated whether E6AP is involved in ubiquitin-mediated degradation of hDlg. In vitro binding assay revealed that hDlg formed ternary complex with E6-E6AP complex. The ability of E6 mutants to degrade these tumor suppressors was correlated with their ability to interact with E6AP. Furthermore, hDlg was targeted for in vitro ubiquitination in the presence of both E6 and E6AP. These data revealed that E6AP is extensively involved in the ubiquitin-mediated degradation of E6-dependent substrates as a cellular E3 ubiquitin-protein ligase.

Characterization of HPV-16 E6 DNA vaccines employing intracellular targeting and intercellular spreading strategies

Human papillomavirus (HPV) E6 and E7 are consistently expressed and are responsible for the malignant transformation of HPV-associated lesions. Thus, E6 and E7 represent ideal targets for therapeutic HPV vaccine development. We have previously used the gene gun approach to test several intracellular targeting and intercellular spreading strategies targeting HPV-16 E7. These strategies include the use of the sorting signal of lysosome-associated membrane protein (LAMP-1), Mycobacterium tuberculosis heat shock protein 70 (HSP70), calreticulin (CRT) and herpes simplex virus type 1 (HSV-1) VP22 proteins. All of these strategies have been shown to be capable of enhancing E7-DNA vaccine potency. In the current study, we have characterized DNA vaccines employing these intracellular targeting or intercellular spreading strategies targeting HPV-16 E6 for their ability to generate E6-specific CD8+ T cell immune responses and antitumor effects against an E6-expressing tumor cell line, TC-1, in C57BL/6 mice. We found that all the intracellular targeting strategies (CRT, LAMP-1, HSP70) as well as the intercellular spreading strategy (VP22) were able to enhance E6 DNA vaccine potency, although the orientation of HSP70 linked to E6 antigen in the E6 DNA vaccine appears to be important for the HSP70 strategy to work. The enhanced E6-specific CD8+ T cell immune response in vaccinated mice also translated into potent antitumor effects against TC-1 tumor cells. Our data indicate that all of the intracellular targeting and intercellular spreading strategies that have been shown to enhance E7 DNA vaccine potency were also able to enhance E6 DNA vaccine potency.

Down-regulation of p21 contributes to apoptosis induced by HPV E6 in human mammary epithelial cells

Infection with human papillomaviruses (HPV) is strongly associated with the development of cervical cancer. The HPV E6 gene is essential for the oncogenic potential of HPV. E6 induces cell proliferation and apoptosis in cervical cancer precursor lesions and in cultured cells. Although induction of telomerase and inactivation of the tumor suppressor p53 play important roles for E6 to promote cell growth, the molecular basis of E6-induced apoptosis is poorly understood. While it is expected that inactivation of p53 by E6 should lead to a reduction in cellular apoptosis, numerous studies demonstrated that E6 could in fact sensitize cells to apoptosis. Understanding the mechanism of p53-independent apoptosis is of clinical significance. In the present study, we investigated the mechanism of apoptosis during E6-mediated immortalization of primary human mammary epithelial cell (HMEC). E6 by itself is sufficient to immortalize HMECs and is believed to do so at least in part by activation of telomerase. During the process of E6-mediated HMEC immortalization, an increased apoptosis was observed. Mutational analysis demonstrated that E6-induced apoptosis was distinct from its ability to promote cell proliferation, activate telomerase, or degrade p53. While the known pro-apoptotic E6 target proteins such as Bak or c-Myc did not appear to play an important role, down-regulation of the cyclin-dependent kinase inhibitor p21(Waf1/Cip1) (p21) by E6 correlated with its ability to induce apoptosis. Ectopic expression of p21 inhibited E6-induced apoptosis. Moreover, a p53 degradation defective E6 mutant was competent for p21 down-regulation and apoptosis induction. The anti-apoptotic function of p21 may not simply be the result of p21-induced growth arrest. These studies demonstrate an E6 activity to down-regulate p21 that is important for induction of apoptosis.

The global transcriptional effects of the human papillomavirus E6 protein in cervical carcinoma cell lines are mediated by the E6AP ubiquitin ligase

The function of the human papillomavirus (HPV) E6 protein that is most clearly linked to carcinogenesis is the targeted degradation of p53, which is dependent on the E6AP ubiquitin ligase. Additional functions have been attributed to E6, including the stimulation of telomerase activity and the targeted degradation of other cellular proteins, but in most cases it is unclear whether these activities are also E6AP dependent. While E6 clearly influences the transcriptional program of HPV-positive cell lines through the inactivation of p53, it has been shown that at least a subset of its p53-independent functions are also reflected in the transcriptional program. For this study, we have determined the extent to which E6AP is involved in mediating the set of E6 functions that impact on the global transcriptional program of HPV-positive cell lines. The transcriptional profiles of approximately 31,000 genes were characterized for three cell lines (HeLa, Caski, and SiHa cells) after small interfering RNA (siRNA)-mediated silencing of E6 or E6AP. We found that E6 and E6AP siRNAs elicited nearly identical alterations in the transcriptional profile of each cell line. Some of the expression alterations were apparent secondary effects of p53 stabilization, while the basis of most other changes was not reconcilable with previously proposed E6 functions. While expression changes of the TERT gene (telomerase catalytic subunit) were not revealed by the array, telomerase repeat amplification protocol assays showed that both E6 and E6AP knockouts resulted in a suppression of telomerase activity. Together, these results suggest that E6AP mediates a broad spectrum of E6 functions, including virtually all functions that impact on the transcriptional program of HPV-positive cell lines.

GAP control: regulating the regulators of small GTPases

The small GTPases of the Ras superfamily mediate numerous biological processes through their ability to cycle between an inactive GDP-bound and an active GTP-bound form. Among the key regulators of GTPase cycling are the GTPase-activating proteins (GAPs), which stimulate the weak intrinsic GTP-hydrolysis activity of the GTPases, thereby inactivating them. Despite the abundance of GAPs and the fact that mutations in GAP-encoding genes underlie several human diseases, these proteins have received relatively little attention. Recent studies have addressed the regulatory mechanisms that influence GAP activity. So far, findings suggest that GAP activity is regulated by several mechanisms, including protein-protein interactions, phospholipid interactions, phosphorylation, subcellular translocation and proteolytic degradation.

The GTPase-activating protein Rap1GAP uses a catalytic asparagine

Rap1 is a Ras-like guanine-nucleotide-binding protein (GNBP) that is involved in a variety of signal-transduction processes. It regulates integrin-mediated cell adhesion and might activate extracellular signal-regulated kinase. Like other Ras-like GNBPs, Rap1 is regulated by guanine-nucleotide-exchange factors (GEFs) and GTPase-activating proteins (GAPs). These GAPs increase the slow intrinsic GTPase reaction of Ras-like GNBPs by many orders of magnitude and allow tight regulation of signalling. The activation mechanism involves stabilization of the catalytic glutamine of the GNBP and, in most cases, the insertion of a catalytic arginine of GAP into the active site. Rap1 is a close homologue of Ras but does not possess the catalytic glutamine essential for GTP hydrolysis in all other Ras-like and Galpha proteins. Furthermore, RapGAPs are not related to other GAPs and apparently do not use a catalytic arginine residue. Here we present the crystal structure of the catalytic domain of the Rap1-specific Rap1GAP at 2.9 A. By mutational analysis, fluorescence titration and stopped-flow kinetic assay, we demonstrate that Rap1GAP provides a catalytic asparagine to stimulate GTP hydrolysis. Implications for the disease tuberous sclerosis are discussed.

Immortalization of human mammary epithelial cells is associated with inactivation of the p14ARF-p53 pathway

Inactivation of the ARF-p53 tumor suppressor pathway leads to immortalization of murine fibroblasts. The role of this pathway in immortalization of human epithelial cells is not clear. We analyzed the functionality of the p14(ARF)-p53 pathway in human mammary epithelial cells (MEC) immortalized by human papillomavirus 16 (HPV16) E6, the p53 degradation-defective E6 mutant Y54D, or hTERT. E6-MEC or E6Y54D-MEC maintains high-level expression of p14(ARF). Late-passage hTERT-immortalized MEC express p53 but down-regulate p14(ARF). Enforced expression of p14(ARF) induces p53-dependent senescence in hTERT-MEC, while both E6-MEC and E6Y54D-MEC are resistant. We show that E6Y54D inhibits p14(ARF)-induced activation of p53 without inactivation of the p53-dependent DNA damage response. Hence, p53 degradation and inhibition of p14(ARF) signaling to p53 are independent functions of HPV16 E6. Our observations imply that long-term proliferation of MEC requires inactivation of the p14(ARF)-p53 pathway.

Signals that dictate nuclear localization of human papillomavirus type 16 oncoprotein E6 in living cells

Human papillomavirus (HPV) type 16 E6 (16E6) is an oncogenic, multifunctional nuclear protein that induces p53 degradation and perturbs normal cell cycle control, leading to immortalization and transformation of infected keratinocytes and epithelial cells. Although it is unclear how 16E6 disrupts the epigenetic profile of host genes, its presence in the nucleus is a key feature. The present report describes intrinsic properties of 16E6 that influence its nuclear import in living cells. When the coding region of full-length 16E6 was inserted in frame into the C terminus of green fluorescent protein (GFP), it effectively prevented the 16E6 pre-mRNA from being spliced and led to the expression of a GFP-E6 fusion which localized predominantly to the nucleus. Further studies identified three novel nuclear localization signals (NLSs) in 16E6 that drive the protein to accumulate in the nucleus. We found that all three NLS sequences are rich in positively charged basic residues and that point mutations in these key residues could abolish the retention of 16E6 in the nucleus as well as the p53 degradation and cell immortalization activities of the protein. When inserted into corresponding regions of low-risk HPV type 6 E6, the three NLS sequences described for 16E6 functioned actively in converting the normally cytoplasmic HPV type 6 E6 into a nuclear protein. The separate NLS sequences, however, appear to play different roles in nuclear import and retention of HPV E6. The discovery of three unique NLS sequences in 16E6 provides new insights into the nuclear association of 16E6 which may reveal other novel activities of this important oncogenic protein.

Antitumor efficacy of Venezuelan equine encephalitis virus replicon particles encoding mutated HPV16 E6 and E7 genes

An effective vaccine for treating human papillomavirus (HPV)-associated malignancies such as cervical cancer should elicit strong T cell-mediated immunity (CMI) against the E6 and/or E7 proteins necessary for the malignant state. We have developed Venezuelan equine encephalitis (VEE) virus replicon particle (VRP) vaccines encoding the HPV16 E6 and E7 genes and tested their immunogenicity and antitumor efficacy. The E6 and E7 genes were fused to create one open reading frame and mutated at four or at five amino acid positions to inactivate their oncogenic potential. VRP encoding mutant or wild type E6 and E7 proteins elicited comparable cytotoxic T lymphocyte (CTL) responses to an immunodominant E7(49-57) epitope and generated comparable antitumor responses in several HPV16 E6(+)E7(+) tumor challenge models: protection from either C3 or TC-1 tumor challenge was observed in 100% of VRP-vaccinated mice. Eradication of C3 tumors was observed in approximately 90% of mice following therapeutic VRP vaccination. Eradication of HLF16 tumors lacking the E7(49-57) epitope was observed in 90% of human leukocyte antigen (HLA)-A(*)0201 transgenic mice following therapeutic VRP vaccination. Finally, the predicted inactivation of E6 and E7 oncogenic potential was confirmed by demonstrating normal levels of both p53 and retinoblastoma proteins in human mammary epithelial cells (MEC) infected with VRP expressing mutant E6 and E7 genes. These promising results support the continued development of mutant E6 and E7 VRP as safe and effective candidates for clinical evaluation against HPV-associated disease.

Human papillomavirus type 16 E6 activates TERT gene transcription through induction of c-Myc and release of USF-mediated repression

Human papillomavirus type 16 (HPV-16), a DNA tumor virus, has a causal role in cervical cancer, and the viral oncoproteins E6 and E7 contribute to oncogenesis in multiple ways. E6 increases telomerase activity in keratinocytes through increased transcription of the telomerase catalytic subunit gene (TERT), but the factors involved in this have been elusive. We have found that mutation of the proximal E box in the TERT promoter has an activating effect in luciferase assays. This suggested that a repressive complex might be present at this site. HPV-16 E6 activated the TERT promoter predominantly through the proximal E box, and thus, might be acting on this repressive complex. This site is specific for the Myc/Mad/Max transcription factors as well as USF1 and USF2. Addition of exogenous USF1 or USF2 repressed activation of the TERT promoter by E6, dependent on the proximal E box. Using siRNA against USF1 or USF2 allowed for greater activation of the TERT promoter by E6. Conversely, loss of c-Myc function, through a dominant-negative Myc molecule, reduced activation by E6. Chromatin immunoprecipitations showed that in the presence of E6, there was a reduction in binding of USF1 and USF2 at the TERT promoter proximal E box, and a concomitant increase in c-Myc bound to this site. This shows that a repressive complex containing USF1 and USF2 is present in normal cells with little or no telomerase activity. In E6 keratinocytes, this repressive complex is replaced by c-Myc, which corresponds to higher levels of TERT transcription and consequently, telomerase activity.

Transcriptional regulation of the telomerase hTERT gene as a target for cellular and viral oncogenic mechanisms

Malignant transformation from mortal, normal cells to immortal, cancer cells is generally associated with activation of telomerase and subsequent telomere maintenance. A major mechanism to regulate telomerase activity in human cells is transcriptional control of the telomerase catalytic subunit gene, human telomerase reverse transcriptase (hTERT). Several transcription factors, including oncogene products (e.g. c-Myc) and tumor suppressor gene products (e.g. WT1 and p53), are able to control hTERT transcription when over-expressed, although it remains to be determined whether a cancer-associated alteration of these factors is primarily responsible for the hTERT activation during carcinogenic processes. Microcell-mediated chromosome transfer experiments have provided evidence for endogenous factors that function to repress the telomerase activity in normal cells and are inactivated in cancer cells. At least one of those endogenous telomerase repressors, which is encoded by a putative tumor suppressor gene on chromosome 3p, acts through transcriptional repression of the hTERT gene. The hTERT gene is also a target site for viruses frequently associated with human cancers, such as human papillomavirus (HPV) and hepatitis B virus (HBV). HPV E6 protein contributes to keratinocyte immortalization and carcinogenesis through trans-activation of the hTERT gene transcription. In at least some hepatocellular carcinomas, the hTERT gene is a non-random integration site of HBV genome, which activates in cis the hTERT transcription. Thus, a variety of cellular and viral oncogenic mechanisms converge on transcriptional control of the hTERT gene. Regulation of chromatin structure through the modification of nucleosomal histones may mediate the action of these cellular and viral mechanisms. Further elucidation of the hTERT transcriptional regulation, including identification and characterization of the endogenous repressor proteins, should lead to better understanding of the complex regulation of human telomerase in normal and cancer cells and may open up new strategies for anticancer therapy.

Myeloproliferative stem cell disorders by deregulated Rap1 activation in SPA-1-deficient mice

SPA-1 (signal-induced proliferation-associated gene-1) is a principal Rap1 GTPase-activating protein in hematopoietic progenitors. SPA-1-deficient mice developed a spectrum of myeloid disorders that resembled human chronic myelogenous leukemia (CML) in chronic phase, CML in blast crisis, and myelodysplastic syndrome as well as anemia. Preleukemic SPA-1-deficient mice revealed selective expansion of marrow pluripotential hematopoietic progenitors, which showed abnormal Rap1GTP accumulation. Overexpression of an active form of Rap1 promoted the proliferation of normal hematopoietic progenitors, while SPA-1 overexpression markedly suppressed it. Furthermore, restoring SPA-1 gene in a SPA-1-deficient leukemic blast cell line resulted in the dissolution of Rap1GTP accumulation and concomitant loss of the leukemogenicity in vivo. These results unveiled a role of Rap1 in myeloproliferative stem cell disorders and a tumor suppressor function of SPA-1.

Molecular interactions of 'high risk' human papillomaviruses E6 and E7 oncoproteins: implications for tumour progression

The aetiology of cervical cancer has been primarily attributed to human papillomaviruses (HPVs). These are characterized by the persistent expression of the two oncogenes, E6 and E7. Experimental studies show that E6 and E7 genes of the high risk HPVs deregulate key cell cycle controls. Recent work has uncovered new cellular partners for these proteins that throw light on many of the pathways and processes in which these viral proteins intervene. This review focuses on the regulation of host proteins by the viral oncoproteins and consequence of such interactions on cell survival, proliferation, differentiation and apoptosis.

GAPs galore! A survey of putative Ras superfamily GTPase activating proteins in man and Drosophila

Typical members of the Ras superfamily of small monomeric GTP-binding proteins function as regulators of diverse processes by cycling between biologically active GTP- and inactive GDP-bound conformations. Proteins that control this cycling include guanine nucleotide exchange factors or GEFs, which activate Ras superfamily members by catalyzing GTP for GDP exchange, and GTPase activating proteins or GAPs, which accelerate the low intrinsic GTP hydrolysis rate of typical Ras superfamily members, thus causing their inactivation. Two among the latter class of proteins have been implicated in common genetic disorders associated with an increased cancer risk, neurofibromatosis-1, and tuberous sclerosis. To facilitate genetic analysis, I surveyed Drosophila and human sequence databases for genes predicting proteins related to GAPs for Ras superfamily members. Remarkably, close to 0.5% of genes in both species (173 human and 64 Drosophila genes) predict proteins related to GAPs for Arf, Rab, Ran, Rap, Ras, Rho, and Sar family GTPases. Information on these genes has been entered into a pair of relational databases, which can be used to identify evolutionary conserved proteins that are likely to serve basic biological functions, and which can be updated when definitive information on the coding potential of both genomes becomes available.

Human papillomavirus-induced carcinogenesis and the ubiquitin-proteasome system

Certain types of human papillomaviruses have been etiologically associated with malignant lesions, most notably with cervical cancer. The major oncoproteins of these cancer-associated viruses are encoded by the viral E6 and E7 genes. Thorough characterization of these oncoproteins and their interaction with cellular proteins has shown that both E6 and E7 exploit the ubiquitin-proteasome system to degrade and, thus, to functionally inactivate negative cell-regulatory proteins including members of the p110(RB) family and p53. This act of piracy is assumed to contribute to both the efficient propagation of HPVs and HPV-induced carcinogenesis.

Requirement of E6AP and the features of human papillomavirus E6 necessary to support degradation of p53

E6 oncoproteins from human papillomavirus type 16 (16E6) and Bovine Papillomavirus type 1 (BE6) bind to leucine rich peptides (called charged leucine, LXXLL, or signature peptides) found on target cellular proteins. BE6 and 16E6 both bind the product of the UBE3A gene called E6AP on a charged leucine peptide, LQELL. E6AP is an E3 ubiquitin ligase that together with 16E6 interacts with p53 to target p53 degradation. Although both BE6 and 16E6 bind the LQELL peptide of E6AP, only 16E6 acts as an adapter to then bring p53 to E6AP. In order to determine how E6 proteins function as adapters, 16E6, p53, and E6AP were expressed in yeast, and were shown to form a tri-molecular complex. 16E6 mutants were selected that retained interactions with E6AP yet were defective for interaction with p53. Such 16E6 mutations were typically within the amino-terminus of 16E6. Through the use of E6AP null cells, transfected E6AP was shown to be necessary and sufficient for the degradation of p53 in the presence of 16E6. However, the interaction of 16E6 with E6AP was complex. While BE6 interacts only with the LQELL motif of E6AP, an intact LQELL motif is not necessary either for interaction of 16E6 with E6AP or for p53 degradation. In addition, 16E6 mutants that fail to bind the LQELL motif of E6AP can support p53 degradation. These results indicate that 16E6 may have multiple modes of interaction with E6AP and that assembly of p53 containing complexes for targeted degradation by E6AP may occur in more than one way. These results have implications for potential targeting of the interaction of 16E6 and E6AP in the therapy of HPV-induced cancer.

The high-risk human papillomavirus type 16 E6 counters the GAP function of E6TP1 toward small Rap G proteins

We have recently identified E6TP1 (E6-targeted protein 1) as a novel high-risk human papillomavirus type 16 (HPV16) E6-binding protein. Importantly, mutational analysis of E6 revealed a strong correlation between the transforming activity and its abilities to bind and target E6TP1 for ubiquitin-mediated degradation. As a region within E6TP1 has high homology with GAP domains of known and putative Rap GTPase-activating proteins (GAPs), these results raised the possibility that HPV E6 may alter the Rap small-G-protein signaling pathway. Using two different approaches, we now demonstrate that human E6TP1 exhibits GAP activity for Rap1 and Rap2, confirming recent findings that a closely related rat homologue exhibits Rap-specific GAP activity. Using mutational analysis, we localize the GAP activity to residues 240 to 945 of E6TP1. Significantly, we demonstrate that coexpression of HPV16 E6, by promoting the degradation of E6TP1, enhances the GTP loading of Rap. These results support a role of Rap small-G-protein pathway in E6-mediated oncogenesis.