E7 proteins of four groups of human papillomaviruses, irrespective of their tissue tropism or cancer association, possess the ability to transactivate transcriptional promoters E2F site dependently

The Dkk3 gene encodes a vital intracellular regulator of cell proliferation

Members of the Dickkopf (Dkk) family of Wnt antagonists interrupt Wnt-induced receptor assembly and participate in axial patterning and cell fate determination. One family member, DKK3, does not block Wnt receptor activation. Loss of Dkk3 expression in cancer is associated with hyperproliferation and dysregulated ß-catenin signaling, and ectopic expression of Dkk3 halts cancer growth. The molecular events mediating the DKK3-dependent arrest of ß-catenin-driven cell proliferation in cancer cells are unknown. Here we report the identification of a new intracellular gene product originating from the Dkk3 locus. This Dkk3b transcript originates from a second transcriptional start site located in intron 2 of the Dkk3 gene. It is essential for early mouse development and is a newly recognized regulator of ß-catenin signaling and cell proliferation. Dkk3b interrupts nuclear translocation ß-catenin by capturing cytoplasmic, unphosphorylated ß-catenin in an extra-nuclear complex with ß-TrCP. These data reveal a new regulator of one of the most studied signal transduction pathways in metazoans and provides a novel, completely untapped therapeutic target for silencing the aberrant ß-catenin signaling that drives hyperproliferation in many cancers.

Identification of key amino acid residues that determine the ability of high risk HPV16-E7 to dysregulate major histocompatibility complex class I expression

High risk human Papillomavirus (HPV) types are the major causative agents of cervical cancer. Reduced expression of major histocompatibility complex class I (MHC I) on HPV-infected cells might be responsible for insufficient T cell response and contribute to HPV-associated malignancy. The viral gene product required for subversion of MHC I synthesis is the E7 oncoprotein. Although it has been suggested that high and low risk HPVs diverge in their ability to dysregulate MHC I expression, it is not known what sequence determinants of HPV-E7 are responsible for this important functional difference. To investigate this, we analyzed the capability to affect MHC I of a set of chimeric E7 variants containing sequence elements from either high risk HPV16 or low risk HPV11. HPV16-E7, but not HPV11-E7, causes significant diminution of mRNA synthesis and surface presentation of MHC I, which depend on histone deacetylase activity. Our experiments demonstrate that the C-terminal region within the zinc finger domain of HPV-E7 is responsible for the contrasting effects of HPV11- and HPV16-E7 on MHC I. By using different loss- and gain-of-function mutants of HPV11- and HPV16-E7, we identify for the first time a residue variation at position 88 that is highly critical for HPV16-E7-mediated suppression of MHC I. Furthermore, our studies suggest that residues at position 78, 80, and 88 build a minimal functional unit within HPV16-E7 required for binding and histone deacetylase recruitment to the MHC I promoter. Taken together, our data provide new insights into how high risk HPV16-E7 dysregulates MHC I for immune evasion.

Activation of the enhancer of zeste homologue 2 gene by the human papillomavirus E7 oncoprotein

The malignant phenotype of human papillomavirus (HPV)-positive cancer cells is maintained by the activity of the viral E6 and E7 genes. Here, we identified the polycomb group gene enhancer of zeste homologue 2 (EZH2) as a novel downstream target for the viral oncogenes in HPV-transformed cells. EZH2 expression was activated by HPV16 E7 at the transcriptional level via E7-mediated release of E2F from pocket proteins. RNA interference analyses showed that continuous EZH2 expression is required for the proliferation of HPV-positive tumor cells by stimulating cell cycle progression at the G1-S boundary. In addition to its growth-promoting activity, EZH2 also contributed to the apoptotic resistance of cervical cancer cells. Furthermore, we found that HPV-positive dysplastic and tumorigenic cervical lesions were characterized by high levels of EZH2 protein in vivo. We conclude that the E7 target gene EZH2 is a major determinant for the proliferation of HPV-positive cancer cells and contributes to their apoptotic resistance. Moreover, EZH2 may serve as a novel therapeutic target for the treatment of cervical cancer.

Activation of c-Myc contributes to bovine papillomavirus type 1 E7-induced cell proliferation

Inactivation of the tumor suppressor pRB by the human papillomavirus (HPV) oncoprotein E7 is a mechanism by which HPV promotes cell growth. The bovine papillomavirus type 1 (BPV-1) E7 does not bind pRB efficiently yet is required for full transformation of murine cells by BPV-1. In the present study, we investigated the mechanism of BPV-1 E7-induced cell proliferation. Our studies indicate that expression of BPV-1 E7 induces DNA synthesis and stimulates cells to enter S phase in quiescent cells. The induction of cell proliferation by BPV-1 E7 can occur in the retinoblastoma gene (Rb)-null cells, suggesting an Rb-independent mechanism. Consistent with this observation, BPV-1 E7 does not efficiently activate the transcription of the E2F family of transcription factors (E2F)-responsive promoters. Notably, c-Myc is able to induce cells to enter S phase in quiescent cells through an Rb/E2F-independent pathway. Significantly, c-Myc levels are increased in BPV-1 E7-expressing cells. Moreover, expression of a dominant negative c-Myc mutant inhibited BPV-1 E7-induced DNA synthesis. Consistent with the notion that c-Myc could down-regulate p27 and activate Cdk2, p27 level is decreased while both cyclin A and cyclin E-associated kinase activities are up-regulated in BPV-1 E7-expressing cells. These studies indicate an important role for c-Myc in BPV-1 E7-induced cell proliferation.

Organization of human papillomavirus productive cycle during neoplastic progression provides a basis for selection of diagnostic markers

The productive cycle of human papillomaviruses (HPVs) can be divided into discrete phases. Cell proliferation and episomal maintenance in the lower epithelial layers are followed by genome amplification and the expression of capsid proteins. These events, which occur in all productive infections, can be distinguished by using antibodies to viral gene products or to surrogate markers of their expression. Here we have compared precancerous lesions caused by HPV type 16 (HPV16) with lesions caused by HPV types that are not generally associated with human cancer. These include HPV2 and HPV11, which are related to HPV16 (supergroup A), as well as HPV1 and HPV65, which are evolutionarily divergent (supergroups E and B). HPV16-induced low-grade squamous intraepithelial lesions (CIN1) are productive infections which resemble those caused by other HPV types. During progression to cancer, however, the activation of late events is delayed, and the thickness of the proliferative compartment is progressively increased. In many HPV16-induced high-grade squamous intraepithelial lesions (CIN3), late events are restricted to small areas close to the epithelial surface. Such heterogeneity in the organization of the productive cycle was seen only in lesions caused by HPV16 and was not apparent when lesions caused by other HPV types were compared. By contrast, the order in which events in the productive cycle were initiated was invariant and did not depend on the infecting HPV type or the severity of disease. The distribution of viral gene products in the infected cervix depends on the extent to which the virus can complete its productive cycle, which in turn reflects the severity of cervical neoplasia. It appears from our work that the presence of such proteins in cells at the epithelial surface allows the severity of the underlying disease to be predicted and that markers of viral gene expression may improve cervical screening.

Mutation and abnormal expression of the p53 gene in the viral skin carcinogenesis of epidermodysplasia verruciformis

Patients suffering from epidermodysplasia verruciformis are prone to nonmelanoma skin cancers, due to an inherited abnormal susceptibility to the oncogenic human papillomavirus type 5. Genotoxic sunlight ultraviolet B radiations are likely to be a cofactor. Lesions of two human-papillomavirus-type-5-infected epidermodysplasia verruciformis patients collected during an 8 y period were retrospectively studied for p53 mutations in exons 5 through 8 by a polymerase chain reaction single-strand conformation polymorphism technique and/or by DNA sequencing of amplified exons. Mutations were detected in 11 of 26 (42.3%) specimens, including five (62.5%) squamous cell carcinomas, three (33.3%) Bowen's carcinomas in situ, two (40%) actinic keratoses, and one (33%) benign lesion. The nine mutations characterized by sequencing were shown to be missense and to affect mutational hotspots in human cancers. Five were C-->T transitions at dicytidine sites considered as ultraviolet signature mutations. Two were transversions (C-->G and C-->A) at dicytidine sites and two were C-->T transitions at nondipyrimidine sites. A marked p53 immunoreactivity was disclosed in 72.7% of 11 invasive carcinomas, 55.6% of nine carcinomas in situ, 37.5% of eight actinic keratoses, and one of three benign lesions. This includes 81.8% of 11 specimens with a p53 mutation but also 50% of 14 specimens with no mutation detected. A dysfunction of the p53 gene is thus likely to play a part in epidermodysplasia verruciformis carcinogenesis, either due to ultraviolet-B-induced p53 mutations, as in nonmelanoma skin cancers in the general population, or involving other mutagens or mechanisms. The part played by human papillomavirus type 5 proteins expressed in epidermodysplasia verruciformis keratinocytes remains to be determined.

Molecular targets for human papillomaviruses: prospects for antiviral therapy

A substantial medical need exists for the development of antiviral medicines for the treatment of diseases associated with infection by human papillomaviruses (HPVs). HPVs are associated with various benign and malignant lesions including benign genital condyloma, common skin warts, laryngeal papillomas and anogenital cancer. Since treatment options are limited and typically not very satisfactory, the development of safe and effective antiviral drugs for HPV could have substantial clinical impact. In the last few years, exciting advances have been made in our understanding of papillomavirus replication and the effects that the virus has on growth of the host cell. Although still somewhat rudimentary, techniques have been developed for limited virion production in vitro offering the promise of more rapid advances in the dissection and understanding of the virus life cycle. Of the 8-10 HPV gene products that are made during infection, only one encodes enzymatic activities, the E1 helicase. Successful antiviral therapies have traditionally targeted viral enzymes such as polymerases, kinases and proteases. In contrast, macromolecular interactions which mediate the functions of E6, E7 and E2 are thought to be more difficult targets for small molecule therapy.