The human papillomavirus type 16 E6 oncoprotein can down-regulate p53 activity by targeting the transcriptional coactivator CBP/p300

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.

Warts, cancer and ubiquitylation: lessons from the papillomaviruses

Certain human papillomaviruses (such as HPV-16 and HPV-18) are associated with specific anogenital cancers, most notably cervical cancer. These viruses encode two oncoproteins, E6 and E7, which are expressed in the HPV positive cancers. E7 functions in cellular transformation, at least in part, through inactivation of pRB, and the other pRB related "pocket proteins" p107 and p130. The major target of the E6 oncoprotein encoded by the genital tract, cancer-associated human papillomaviruses is the p53 tumor suppressor protein. E6 binding to p53 is mediated by a cellular protein, the E6-associated protein (E6AP). In the presence of E6, E6AP catalyses the ubiquitylation and proteolysis of p53. E6AP is an E3 ubiquitin protein ligase and is not normally involved in the regulation of p53 stability in the absence of E6. E6AP is the prototype for the HECT domain family of E3 ubiquitin protein ligases.

Pitx2a binds to human papillomavirus type 18 E6 protein and inhibits E6-mediated P53 degradation in HeLa cells

Binding of high risk human papillomavirus (HPV) E6 protein to E6-associated protein (E6AP), a cellular ubiquitin-protein ligase, enables E6AP to ubiquitinate p53, leading to p53 degradation in cervical cancer cells such as HeLa cells. Here we report that Pitx2a, a bicoid-type homeodomain transcription factor, can bind to HPV E6 protein and inhibit E6/E6AP-mediated p53 degradation. Deletion of the Pitx2a homeodomain abrogates its ability to bind to HPV E6 protein and to induce p53 accumulation in HeLa cells, suggesting that the homeodomain of Pitx2a is essential for inhibition of E6/E6AP-mediated p53 degradation. Recombinant Pitx2a can also block E6/E6AP-mediated p53 degradation in vitro, indicating that this function of Pitx2a is independent of its transcription activity. Pitx2a does not regulate Hdm2-mediated p53 degradation, because Pitx2a does not affect p53 protein levels in HPV-negative cells, such as HCT116, U2OS, and C33A cells. In addition, Pitx2a-induced p53 is transcriptionally active and maintains its specific DNA binding activity in HeLa cells. Taken together, these findings suggest that, by binding to E6, Pitx2a interferes with E6/E6AP-mediated p53 degradation, leading to the accumulation of functional p53 protein in HeLa cells.

Epigenetics of cervical cancer. An overview and therapeutic perspectives

Cervical cancer remains one of the greatest killers of women worldwide. It is difficult to foresee a dramatic increase in cure rate even with the most optimal combination of cytotoxic drugs, surgery, and radiation; therefore, testing of molecular targeted therapies against this malignancy is highly desirable. A number of epigenetic alterations occur during all stages of cervical carcinogenesis in both human papillomavirus and host cellular genomes, which include global DNA hypomethylation, hypermetylation of key tumor suppressor genes, and histone modifications. The reversible nature of epigenetic changes constitutes a target for transcriptional therapies, namely DNA methylation and histone deacetylase inhibitors. To date, studies in patients with cervical cancer have demonstrated the feasibility of reactivating the expression of hypermethylated and silenced tumor suppressor genes as well as the hyperacetylating and inhibitory effect upon histone deacetylase activity in tumor tissues after treatment with demethylating and histone deacetylase inhibitors. In addition, detection of epigenetic changes in cytological smears, serum DNA, and peripheral blood are of potential interest for development of novel biomolecular markers for early detection, prediction of response, and prognosis.

BRCA1 Interaction with Human Papillomavirus Oncoproteins

Previously, we reported that BRCA1 strongly represses the transcriptional activity of estrogen receptor-alpha (ER-alpha) in human breast and prostate cancer cells but only weakly inhibits ER-alpha in cervical cancer cells. We now report that introduction of the human papillomavirus E7 or E6 oncogenes into human papillomavirus-negative cells rescues the BRCA1 repression of ER-alpha activity and that the E7 and E6 oncoproteins interact directly with BRCA1 in vitro and associate with BRCA1 in vivo in cultured cells. This interaction involves at least two contact points on BRCA1, one within an N-terminal site shown previously to interact with ER-alpha and the other in a C-terminal region of BRCA1 containing the first BRCA1 C-terminal domain. Point mutations within the zinc finger domains of E7 and E6 inactivated the binding to the N terminus of BRCA1 and reduced their ability to rescue BRCA1 inhibition of ER-alpha. E6 and E7 also antagonized the ability of BRCA1 to inhibit c-Myc E-box-mediated transactivation and human telomerase reverse transcriptase promoter activity, in a manner dependent upon the zinc finger domains. Finally, the ability of E6 and E7 to antagonize BRCA1 did not involve proteolytic degradation of BRCA1. These findings suggest functional interactions of BRCA1 with E7 and E6. The potential significance of these findings is discussed.

Advances in the development of therapeutic nucleic acids against cervical cancer

Cervical cancer is the second most common neoplastic disease affecting women worldwide. Basic, clinical and epidemiological analyses indicate that expression of high-risk human papillomaviruses (HPVs) E6/E7 genes is the primary cause of cervical cancer and represent ideal targets for the application of therapeutic nucleic acids (TNAs). Antisense oligodeoxyribonucleotides (AS-ODNs) and ribozymes (RZs) are the most effective TNAs able to inhibit in vivo tumour growth by eliminating HPV-16 and HPV-18 E6/E7 transcripts. Expression of multiple RZs directed against alternative target sites by triplex expression systems may result in the abrogation of highly variable HPVs. More recently, RNA interference (RNAi) gene knockdown phenomenon, induced by small interfering RNA (siRNA), has demonstrated its potential value as an effective TNA for cervical cancer. siRNA and aptamers as TNAs will have a place in the armament for cervical cancer. TNAs against cervical cancer is in a dynamic state, and clinical trials will define the TNAs in preventive and therapeutic roles to control tumour growth, debulk tumour mass, prevent metastasis and facilitate immune interaction.

The human papillomavirus 16 E6 protein can either protect or further sensitize cells to TNF: effect of dose

High-risk strains of human papillomavirus, including HPV 16, cause human cervical carcinomas, due in part to the activity of their E6 oncogene. E6 interacts with a number of cellular proteins involved in host-initiated apoptotic responses. Paradoxically, literature reports show that E6 can both protect cells from and sensitize cells to tumor necrosis factor (TNF). To examine this apparent contradiction, E6 was transfected into U2OS cells and stable clones were treated with TNF. Intriguingly, clones with a high level of E6 expression displayed an increased sensitivity to TNF by undergoing apoptosis, while those with low expression were resistant. Furthermore, TNF treatment of cells in which the expression of E6 was regulated by the addition of doxycycline demonstrated clearly that while low levels of E6 protect cells from TNF, high levels sensitize cells. Together, these results demonstrate that virus-host interactions can be complex and that both quantitative and qualitative aspects are important in determining outcome.

High levels of p105 (NFKB1) and p100 (NFKB2) proteins in HPV16-transformed keratinocytes: role of E6 and E7 oncoproteins

We have previously shown that functional components of the NF-kappaB signaling pathway are up-regulated and sequestered in the cytoplasm of human papillomavirus 16 (HPV16)-transformed cell lines leading to a reduced activity of NF-kappaB. In this study, we examined the expression of the NF-kappaB precursors p100 and p105 in keratinocytes transformed or not by HPV16. Western immunoblotting experiments demonstrated high levels of p100 and p105 proteins not only in HPV16+ cervical carcinoma-derived keratinocytes but also in keratinocytes stably transfected by HPV16 E6 or E7 oncogenes. Moreover, p100 and p105 proteins were predominantly cytoplasmic and nuclear in keratinocytes expressing E7 and E6, respectively. A predominantly cytoplasmic localization of E7 protein was also detected in all keratinocytes expressing E7. Our results suggest that HPV16 E6 and E7 proteins modulate the expression and the subcellular localization of p100 and p105 NF-kappaB precursors.

HTLV-1 HBZ suppresses AP-1 activity by impairing both the DNA-binding ability and the stability of c-Jun protein

Disruption of transcriptional control of cellular genes by human T-cell leukemia virus type-1 (HTLV-1) is thought to be associated, at least in part, with the development of adult T-cell leukemia. It has been reported that activating protein-1 (AP-1) is dysregulated by HTLV-1 infection. HTLV-1-encoded Tax elevates AP-1 activity through the induction of AP-1 family member gene expression, including c-Jun, JunD, c-Fos, and Fra-1. However, the precise mechanism by which HTLV-1 regulates AP-1 activity remains to be addressed. Recently, a novel viral protein named HTLV-1 basic leucine-zipper factor, HBZ, has been shown to interact with c-Jun and repress c-Jun-mediated transcription by abrogating its DNA-binding activity. In the course of investigating HBZ function, we found that HBZ reduced the steady-state levels of c-Jun, and the levels were restored by treatment with a proteasome inhibitor. Together, this indicates that HBZ promotes c-Jun degradation through a proteasome-dependent pathway. Furthermore, HBZ deletion mutants revealed that both the N-terminal and leucine-zipper region of HBZ were required for the elimination of c-Jun. These results suggest dual effects of HBZ on the suppression of AP-1 activity by inhibiting c-Jun function, which may contribute to the dysregulation of cell proliferation.

Downregulation of Bax mRNA expression and protein stability by the E6 protein of human papillomavirus 16

Previous studies have shown that human papillomavirus (HPV) 16 E6 inhibits apoptosis induced during terminal differentiation of primary human keratinocytes (PHKs) triggered by serum and calcium. E6 inhibition of apoptosis was accompanied with prolonged expression of Bcl-2 and reduced elevation of Bax levels. In the present study, the effect of E6 on Bax mRNA expression and protein stability was investigated. These studies indicate that stable E6 expression in differentiating keratinocytes reduced the steady-state levels of Bax mRNA and shortened the half-life of Bax protein. These results were confirmed in transiently transfected 293T cells where E6 degraded Bax in a dose-dependent manner. Bax degradation was also exhibited in Saos-2 cells that lack p53, indicating its p53 independence. E6 did not form complexes with Bax and did not induce Bax degradation in vitro under experimental conditions where p53 was degraded. Finally, E6 aa 120–132 were shown to be necessary for Bax destabilization and, more importantly, for abrogating the ability of Bax to induce cellular apoptosis, highlighting the functional consequences of the E6-induced alterations in Bax expression.

The E6AP ubiquitin ligase is required for transactivation of the hTERT promoter by the human papillomavirus E6 oncoprotein

Most human cancer cells display increased telomerase activity that appears to be critical for continued cell proliferation and tumor formation. The E6 protein of malignancy-associated human papillomaviruses increases cellular telomerase in primary human keratinocytes at least partly via transcriptional activation of the telomerase catalytic subunit, hTERT. In the present study, we investigated whether E6AP, a ubiquitin ligase well known for binding and mediating some of the activities of the E6 oncoprotein, participated in the transactivation of the hTERT promoter. Our results demonstrate that E6 mutants that fail to bind E6AP are also defective for increasing telomerase activity and transactivating the hTERT promoter. More importantly, E6AP knock-out mouse cells and small interfering RNA techniques demonstrated that E6AP was required for hTERT promoter transactivation in both mouse and human cells. Neither E6 nor E6AP bound to the hTERT promoter or activated the promoter in the absence of the partner protein. With all transactivation-competent E6 proteins, induction of the hTERT promoter was dependent upon E box elements in the core promoter. It appears, therefore, that E6-mediated activation of the hTERT promoter requires a complex of E6-E6AP to engage the hTERT promoter and that activation is dependent upon Myc binding sites in the promoter. The recruitment of a cellular ubiquitin ligase to the hTERT promoter during E6-mediated transcriptional activation suggests a role for the local ubiquitination (and potential degradation) of promoter-associated regulatory proteins, including the Myc protein.

Macrophages kill human papillomavirus type 16 E6-expressing tumor cells by tumor necrosis factor alpha- and nitric oxide-dependent mechanisms

The expression of adenovirus serotype 2 or 5 (Ad2/5) E1A sensitizes cells to killing by NK cells and activated macrophages, a property that correlates with the ability of E1A to bind the transcriptional coadaptor proteins p300-CBP. The E6 oncoproteins derived from the high-risk human papillomaviruses (HPV) interact with p300 and can complement mutant forms of E1A that cannot interact with p300 to induce cellular immortalization. Therefore, we determined if HPV type 16 (HPV16) E6 could sensitize cells to killing by macrophages and NK cells. HPV16 E6 expression sensitized human (H4 and C33A) and murine (MCA-102) cell lines to lysis by macrophages but not by NK cells. The lysis of cells that expressed E6 by macrophages was p53 independent but dependent on the production of tumor necrosis factor alpha (TNF-alpha) or nitric oxide (NO) by macrophages. Unlike cytolysis assays with macrophages, E6 expression did not significantly sensitize cells to lysis by the direct addition of NO or TNF-alpha. Like E1A, E6 has been reported to sensitize cells to lysis by TNF-alpha by inhibiting the TNF-alpha-induced activation of NF-kappaB. We found that E1A, but not E6, blocked the TNF-alpha-induced activation of NF-kappaB, an activity that correlated with E1A-p300 binding. In summary, Ad5 E1A and HPV16 E6 sensitized cells to lysis by macrophages. Unlike E1A, E6 did not block the ability of TNF-alpha to activate NF-kappaB or sensitize cells to lysis by NK cells, TNF-alpha, or NO. Thus, there appears to be a spectrum of common and unique biological activities that result as a consequence of the interaction of E6 or E1A with p300-CBP.

Human papillomavirus type 16 E6 promotes retinoblastoma protein phosphorylation and cell cycle progression

We show that E6 proteins from benign human papillomavirus type 1 (HPV1) and oncogenic HPV16 have the ability to alter the regulation of the G(1)/S transition of the cell cycle in primary human fibroblasts. Overexpression of both viral proteins induces cellular proliferation, retinoblastoma (pRb) phosphorylation, and accumulation of products of genes that are negatively regulated by pRb, such as p16(INK4a), CDC2, E2F-1, and cyclin A. Hyperphosphorylated forms of pRb are present in E6-expressing cells even in the presence of ectopic levels of p16(INK4a). The E6 proteins strongly increased the cyclin A/cyclin-dependent kinase 2 (CDK2) activity, which is involved in pRb phosphorylation. In addition, mRNA and protein levels of the CDK2 inhibitor p21(WAF1/CIP1) were strongly down-regulated in cells expressing E6 proteins. The down-regulation of the p21(WAF1/CIP1) gene appears to be independent of p53 inactivation, since HPV1 E6 and an HPV16 E6 mutant unable to target p53 were fully competent in decreasing p21(WAF1/CIP1) levels. E6 from HPV1 and HPV16 also enabled cells to overcome the G(1) arrest imposed by oncogenic ras. Immunofluorescence staining of cells coexpressing ras and E6 from either HPV16 or HPV1 revealed that antiproliferative (p16(INK4a)) and proliferative (Ki67) markers were coexpressed in the same cells. Together, these data underline a novel activity of E6 that is not mediated by inactivation of p53.

Identification of a novel telomerase repressor that interacts with the human papillomavirus type-16 E6/E6-AP complex

The critical immortalizing activity of the human papillomavirus (HPV) type-16 E6 oncoprotein is to induce expression of hTERT, the catalytic and rate-limiting subunit of telomerase. Additionally, E6 binds to a cellular protein called E6-associated protein (E6-AP) to form an E3 ubiquitin ligase that targets p53 for proteasome-dependent degradation. Although telomerase induction and p53 degradation are separable and distinct functions of E6, binding of E6 to E6-AP strongly correlated with the induction of hTERT. Here, we demonstrate using shRNAs to reduce E6-AP expression that E6-AP is required for E6-mediated telomerase induction. A yeast two-hybrid screen to find new targets of the E6/E6-AP E3 ubiquitin ligase complex identified NFX1. Two isoforms of NFX1 were found: NFX1-123, which coactivated with c-Myc at the hTERT promoter, and NFX1-91, which repressed the hTERT promoter. NFX1-91 was highly ubiquitinated and destabilized in epithelial cells expressing E6. Furthermore, knockdown of NFX1-91 by shRNA resulted in derepression of the endogenous hTERT promoter and elevated levels of telomerase activity. We propose that the induction of telomerase by the HPV-16 E6/E6-AP complex involves targeting of NFX1-91, a newly identified repressor of telomerase, for ubiquitination and degradation.

Human ADA3 binds to estrogen receptor (ER) and functions as a coactivator for ER-mediated transactivation

We have recently identified the hADA3 protein, the human homologue of yeast transcriptional coactivator yADA3, as a novel HPV16 E6 target. Using ectopic expression approaches, we further demonstrated that hADA3 directly binds to the 9-cis retinoic acid receptors alpha and beta, and functions as a coactivator for retinoid receptor-mediated transcriptional activation. Here, we examined the role of endogenous hADA3 as a coactivator for estrogen receptor (ER), an important member of the nuclear hormone receptor superfamily. We show that ADA3 directly interacts with ER alpha and ER beta. Using the chromatin immunoprecipitation assay, we also show that hADA3 is a component of the activator complexes bound to the native ER response element within the promoter of the estrogen-responsive gene pS2. Furthermore, using an ER response element-luciferase reporter, we show that overexpression of ADA3 enhances the ER alpha- and ER beta-mediated sequence-specific transactivation. Reverse transcription-PCR analysis showed an ADA3-mediated increase in estrogen-induced expression of the endogenous pS2 gene. More importantly, using RNA interference against hADA3, we demonstrate that inhibition of endogenous hADA3 inhibited ER-mediated transactivation and the estrogen-induced increase in the expression of pS2, cathepsin D, and progesterone receptor, three widely known ER-responsive genes. The HPV E6 protein, by targeting hADA3 for degradation, inhibited the ER alpha-mediated transactivation and the protein expression of ER target genes. Thus, our results demonstrate that ADA3 directly binds to human estrogen receptor and enhances the transcription of ER-responsive genes, suggesting a broader role of mammalian hADA3 as a coactivator of nuclear hormone receptors and the potential role of these pathways in HPV oncogenesis.

Human Papillomavirus 16 E6 Oncoprotein Interferences with Insulin Signaling Pathway by Binding to Tuberin

Tuberous sclerosis complex (TSC) is a genetic disorder caused by mutations in either TSC1 or TSC2 tumor suppressor gene. TSC1 and TSC2 products, Harmatin and Tuberin, form the functional complex to serve as the negative regulator for insulin-induced phosphorylation of S6 kinase and elF4E-binding protein 1. High-risk human papillomavirus (HPV) infection is the necessary cause for cervical cancer. E6 oncoprotein encoded by HPV plays a pivotal role in carcinogenesis by interference with the host intracellular protein functions. In this study, we show that HPV16 E6 interacts with tumor suppressor gene TSC2 product, Tuberin, and results in the phosphorylation of S6 kinase and S6 even in the absence of insulin. The overexpression of Tuberin overcomes the effect of E6 on S6 kinase phosphorylation. Binding with HPV16 E6 causes the proteasome-mediated degradation of Tuberin. A DILG motif and an ELVG motif located in the carboxyl-terminal of Tuberin are required for E6 binding. In addition, the Tuberin interaction region in E6 has been mapped in the amino-terminal portion of HPV16 E6, which is different from the binding domain with p53. These results provide a possible link between E6-induced oncogenesis and the insulin-stimulated cell proliferation signaling pathway.

Hpv Proteins as Targets for Therapeutic Intervention

Human papillomaviruses (HPV) are the aetiological agents of several types of anogenital tumours, particularly cervical carcinoma. Recent evidence also suggests a role for HPV in the development of squamous cell carcinomas of the skin, especially in immunocompromised individuals. HPV infection also produces a number of non-malignant, but nonetheless cosmetically unpleasant lesions. Therefore, any effective therapeutic treatment for HPV-induced diseases would be extremely beneficial both on humanitarian grounds as well as being economically very attractive. In this review, we will discuss the functions of the viral proteins that appear to be the most appropriate for the development of therapeutics aimed at the treatment of viral infection and virus-induced cancers.

p300/CBP and cancer

p300 and cyclic AMP response element-binding protein (CBP) are adenoviral E1A-binding proteins involved in multiple cellular processes, and function as transcriptional co-factors and histone acetyltransferases. Germline mutation of CBP results in Rubinstein-Taybi syndrome, which is characterized by an increased predisposition to childhood malignancies. Furthermore, somatic mutations of p300 and CBP occur in a number of malignancies. Chromosome translocations target CBP and, less commonly, p300 in acute myeloid leukemia and treatment-related hematological disorders. p300 mutations in solid tumors result in truncated p300 protein products or amino-acid substitutions in critical protein domains, and these are often associated with inactivation of the second allele. A mouse model confirms that p300 and CBP function as suppressors of hematological tumor formation. The involvement of these proteins in critical tumorigenic pathways (including TGF-beta, p53 and Rb) provides a mechanistic route as to how their inactivation could result in cancer.

Pathogenesis of human papillomaviruses in differentiating epithelia

Human papillomaviruses (HPV) are the etiological agents of cervical and other anogenital malignancies. Over 100 different types of HPVs have been identified to date, and all target epithelial tissues for infection. One-third of HPV types specifically infect the genital tract, and a subset of these are the causative agents of anogenital cancers. Other HPV types that infect the genital tract induce benign hyperproliferative lesions or genital warts. The productive life cycle of HPVs is linked to epithelial differentiation. Papillomaviruses are thought to infect cells in the basal layer of stratified epithelia and establish their genomes as multicopy nuclear episomes. In these cells, viral DNA is replicated along with cellular chromosomes. Following cell division, one of the daughter cells migrates away from the basal layer and undergoes differentiation. In highly differentiated suprabasal cells, vegetative viral replication and late-gene expression are activated, resulting in the generation of progeny virions. Since virion production is restricted to differentiated cells, infected basal cells can persist for up to several decades or until the immune system clears the infection. The E6 and E7 genes encode viral oncoproteins that target Rb and p53, respectively. During the viral life cycle, these proteins facilitate stable maintenance of episomes and stimulate differentiated cells to reenter the S phase. The E1 and E2 proteins act as origin recognition factors as well as regulators of early viral transcription. The functions of the E5 and E1--E4 proteins are still largely unknown, but these proteins have been implicated in modulating late viral functions. The L1 and L2 proteins form icosahedral capsids for progeny virion generation. The characterization of the cellular targets of these viral proteins and the mechanisms regulating the differentiation-dependent viral life cycle remain active areas for the study of these important human pathogens.

The human papillomavirus 16 E6 protein binds to Fas-associated death domain and protects cells from Fas-triggered apoptosis

High risk strains of human papillomavirus (HPV), such as HPV 16, cause human cervical carcinoma. The E6 protein of HPV 16 mediates the rapid degradation of the tumor suppressor p53, although this is not the only function of E6 and cannot completely explain its transforming potential. Previous work in our laboratory has demonstrated that E6 can protect cells from tumor necrosis factor-induced apoptosis by binding to the C-terminal end of tumor necrosis factor R1, thus blocking apoptotic signal transduction. In this study, E6 was shown to also protect cells from apoptosis induced via the Fas pathway. Furthermore, use of an inducible E6 expression system demonstrated that this protection is dose-dependent, with higher levels of E6 leading to greater protection. Although E6 suppresses activation of both caspase 3 and caspase 8, it does not affect apoptotic signaling through the mitochondrial pathway. Mammalian two-hybrid and in vitro pull-down assays were then used to demonstrate that E6 binds directly to the death effector domain of Fas-associated death domain (FADD), with deletion and site-directed mutants enabling the localization of the E6-binding site to the N-terminal end of the FADD death effector domain. E6 is produced in two forms as follows: a full-length version of approximately 16 kDa and a smaller version of about half that size corresponding to the N-terminal half of the full-length protein. Pull-down and functional assays demonstrated that the full-length version, but not the small version of E6, was able to bind to FADD and to protect cells from Fas-induced apoptosis. In addition, binding to E6 leads to degradation of FADD, with the loss of cellular FADD proportional to the amount of E6 expressed. These results support a model in which E6-mediated degradation of FADD prevents transmission of apoptotic signals via the Fas pathway.

Roles of the E6 and E7 proteins in the life cycle of low-risk human papillomavirus type 11

Many important functions have been attributed to the high-risk human papillomavirus (HPV) E6 and E7 proteins, including binding and degradation of p53 as well as interacting with Rb proteins. In contrast, the physiological roles of the low-risk E6 and E7 proteins remain unclear. Previous studies demonstrated that the high-risk E6 and E7 proteins also play roles in the productive life cycle by facilitating the maintenance of viral episomes (J. T. Thomas, W. G. Hubert, M. N. Ruesch, and L. A. Laimins, Proc. Natl. Acad. Sci. USA 96:8449-8454, 1999). In order to determine whether low-risk E6 or E7 is similarly necessary for the stable maintenance of episomes, HPV type 11 (HPV-11) genomes that contained translation termination mutations in E6 or E7 were constructed. Upon transfection into normal human keratinocytes, genomes in which E6 function was abolished were unable to be maintained episomally. Transfection of genomes containing substitution mutations in amino acids conserved in high- and low-risk HPV types suggested that multiple protein domains are involved in this process. Examination of cells transfected with HPV-11 genomes in which E7 function was inhibited were found to exhibit a more complex phenotype. At the second passage following transfection, mutant genomes were maintained as episomes but at significantly reduced levels than in cells transfected with the wild-type HPV-11 genome. Upon further passage in culture, however, the episomal forms of these E7 mutant genomes quickly disappeared. These findings identify important new functions for the low-risk E6 and E7 proteins in the episomal maintenance of low-risk HPV-11 genomes and suggest that they may act in a manner similar to that observed for the high-risk proteins.

The human papillomavirus type 11 and 16 E6 proteins modulate the cell-cycle regulator and transcription cofactor TRIP-Br1

The genital human papillomaviruses (HPVs) are a taxonomic group including HPV types that preferentially cause genital and laryngeal warts ("low-risk types"), such as HPV-6 and HPV-11, or cancer of the cervix and its precursor lesions ("high-risk types"), such as HPV-16. The transforming processes induced by these viruses depend on the proteins E5, E6, and E7. Among these oncoproteins, the E6 protein stands out because it supports a particularly large number of functions and interactions with cellular proteins, some of which are specific for the carcinogenic HPVs, while others are shared among low- and high-risk HPVs. Here we report yeast two-hybrid screens with HPV-6 and -11 E6 proteins that identified TRIP-Br1 as a novel cellular target. TRIP-Br1 was recently detected by two research groups, which described two separate functions, namely that of a transcriptional integrator of the E2F1/DP1/RB cell-cycle regulatory pathway (and then named TRIP-Br1), and that of an antagonist of the cyclin-dependent kinase suppression of p16INK4a (and then named p34SEI-1). We observed that TRIP-Br1 interacts with low- and high-risk HPV E6 proteins in yeast, in vitro and in mammalian cell cultures. Transcription activation of a complex consisting of E2F1, DP1, and TRIP-Br1 was efficiently stimulated by both E6 proteins. TRIP-Br1 has an LLG E6 interaction motif, which contributed to the binding of E6 proteins. Apparently, E6 does not promote degradation of TRIP-Br1. Our observations imply that the cell-cycle promoting transcription factor E2F1/DP1 is dually targeted by HPV oncoproteins, namely (i) by interference of the E7 protein with repression by RB, and (ii) by the transcriptional cofactor function of the E6 protein. Our data reveal the natural context of the transcription activator function of E6, which has been predicted without knowledge of the E2F1/DP1/TRIP-Br/E6 complex by studying chimeric constructs, and add a function to the limited number of transforming properties shared by low- and high-risk HPVs.

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.

HIV-1 Vif blocks the antiviral activity of APOBEC3G by impairing both its translation and intracellular stability

The human immunodeficiency virus type 1 (HIV-1) relies on Vif (viral infectivity factor) to overcome the potent antiviral function of APOBEC3G (apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3G, also known as CEM15). Using an APOBEC3G-specific antiserum, we now show that Vif prevents virion incorporation of endogenous APOBEC3G by effectively depleting the intracellular levels of this enzyme in HIV-1-infected T cells. Vif achieves this depletion by both impairing the translation of APOBEC3G mRNA and accelerating the posttranslational degradation of the APOBEC3G protein by the 26S proteasome. Vif physically interacts with APOBEC3G, and expression of Vif alone in the absence of other HIV-1 proteins is sufficient to cause depletion of APOBEC3G. These findings highlight how the bimodal translational and posttranslational inhibitory effects of Vif on APOBEC3G combine to markedly suppress the expression of this potent antiviral enzyme in virally infected cells, thereby effectively curtailing the incorporation of APOBEC3G into newly formed HIV-1 virions.

Interaction between the HPV E7 oncoprotein and the transcriptional coactivator p300

Infection with high-risk human papillomaviruses (HPV) can lead to the development of cervical cancer. This process depends on the interaction of the virus-encoded oncoproteins, E6 and E7, with a variety of host regulatory proteins. As E7 shares both functional and structural similarities with the Adenovirus E1a (Ad E1a) protein, we were interested in investigating the possible interactions between E7 and the transcriptional coactivator p300, since it was originally identified as a target of Ad E1a. Using a variety of assays, we show that E7s from both high- and low-risk HPV types interact with p300. Mutational analysis of E7 maps the site of the interaction to a region spanning the pRb-binding domain and the CKII phosphorylation site. We also map the site of interaction on p300 largely to the CH1 domain. In addition, we demonstrate that the binding between 16E7 and p300 is direct, and can be detected in vivo by coimmunoprecipitation and mammalian two-hybrid assays. Finally, we show that E7 can abolish the p300-mediated E2 transactivation function, suggesting that complex formation between E7 and p300 may contribute to the regulation of E2 transcriptional activity.

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.

Human papillomavirus E6 and Myc proteins associate in vivo and bind to and cooperatively activate the telomerase reverse transcriptase promoter

The papillomavirus E6 protein binds and directs the ubiquitin-dependent degradation of the p53 tumor suppressor protein. Independent of this p53-degradative function, however, E6 induces cellular telomerase activity. This increase in enzyme activity reflects E6-enhanced transcription of the human telomerase reverse transcriptase (hTERT) catalytic subunit, but the molecular basis for this transactivation is unknown. In the present study, we demonstrate that E6/Myc interactions regulate hTERT gene expression. Mad protein, a specific antagonist of Myc, repressed E6-mediated transactivation of the hTERT promoter and this repression was relieved by Myc overexpression. The proximal Myc/ Max-binding element (E-box) in the hTERT promoter was the major determinant of both E6 and Myc responsiveness in keratinocytes. E6 did not alter Myc protein expression or Myc/Max association, and the induction of hTERT by Myc/E6 was independent of Myc phosphorylation at Thr-58/Ser-62 within the transactivation domain. However, immunoprecipitation studies demonstrated that endogenous Myc protein coprecipitated with E6 protein and chromatin immunoprecipitation analyses demonstrated that both E6 and Myc proteins bound to a minimal 295-bp hTERT promoter. Only the "high-risk" E6 proteins bound to the hTERT promoter, consistent with their preferential ability to induce telomerase. The observation that E6 associates with Myc complexes and activates a Myc-responsive gene identifies a mechanism by which this oncogene can modulate cell proliferation and differentiation.

Domain substructure of HPV E6 oncoprotein: biophysical characterization of the E6 C-terminal DNA-binding domain

E6 is a viral oncoprotein implicated in cervical cancers, produced by high-risk human papillomaviruses (HPVs). Structural data concerning this protein are scarce due to the difficulty of producing recombinant E6. Recently, we described the expression and purification of a stable, folded, and biologically active HPV16 E6 mutant called E6 6C/6S. Here, we analyzed the domain substructure of this mutated E6. Nonspecific proteolysis of full-length E6 6C/6S (158 residues) yielded N-terminal and C-terminal fragments encompassing residues 7-83 and 87-158, respectively. The C-terminal fragment of residues 87-158 was cloned, overexpressed, and purified at concentrations as high as 1 mM. The purified domain retains the selective four-way DNA junction recognition activity of the full-length E6 protein. Using UV absorption, UV fluorescence, circular dichroism, and nuclear magnetic resonance, we show that the peptide is primarily monomeric and folded with equal proportions of alpha-helix and beta-sheet secondary structure.

Advances in the Development of Ribozymes and Antisense Oligodeoxynucleotides as Antiviral Agents for Human Papillomaviruses

Urogenital human papillomavirus (HPV) infections are the most common viral sexually transmitted disease in women. On a worldwide basis cervical cancer is the second most prevalent cancer of women. Although HPV infection is not sufficient to induce cancer, the causal relation between high-risk HPV infection and cervical cancer is well established. Over 99% of cervical cancers are positive for high-risk HPV. Therefore, there is a need for newer approaches to treat HPV infection. Two novel approaches for inactivating gene expression involve ribozymes and oligonucleotides. Methods for identification of target genes involved in neoplastic transformation and tumour growth have been established, and these will lead to therapeutic approaches without any damage to normal cellular RNA molecules, which is often associated with conventional therapeutics. Ribozymes and oligonucleotides represent rational antiviral approaches for inhibiting the growth of cervical lesions and carcinomas by interfering with E6/E7 RNA production. The E6 and E7 genes of high-risk HPVs cooperate to immortalize primary epithelial cells and because they are found in cervical cancer are considered the hallmark of cervical cancer. The use and modification of ribozymes and antisense oligodeoxynucleotides can inhibit the growth of HPV-16 and HPV-18 immortalized cells, and tumour cells by eliminating E6/E7 transcript. Hammerhead and hairpin ribozymes have been widely studied because of their potential use for gene therapy and their place as therapeutic tools for cervical cancer is being evaluated. Although antiviral ribozymes and anti-sense molecules have been effective as in vitro or in vivo inhibitors of high-risk HPV-positive cells, none is currently in clinical trial. There are, however, a number of other antisense therapies in Phase I–III clinical trial for several oncogenes.

Dual role of tumor suppressor p53 in regulation of DNA replication and oncogene E6-promoter activity of epidermodysplasia verruciformis-associated human papillomavirus type 8

Human papillomavirus 8 (HPV8) is a representative of Epidermodysplasia verruciformis (EV)-associated viruses. Transient assays in the human skin keratinocyte cell line RTS3b have shown that its replication depends in trans on expression of the viral proteins E1 and E2, similarly to other HPVs. Using deletion mutants and cloned subfragments of the noncoding region (NCR) of HPV8 we identified a 65-bp sequence in the 3' part of the NCR to be necessary and sufficient to support replication in cis. The origin of replication (ori) of HPV8 is composed of the sequence motifs "CCAAC" (nt 57-73) and M29 (nt 84-112), which are highly conserved among the majority of EV HPVs. Analysis of M29 revealed an unconventional binding site of the E2 protein and an overlapping DNA recognition site of the tumor suppressor protein p53. Both these factors competitively bind to M29. In transient replication assays p53 acted as a potent inhibitor of ori activity, most probably in a DNA-binding-dependent fashion. The minimal ori sequences are also functionally critical for the E6 oncogene promoter P(175). In contrast to its effect on replication, p53 stimulated promoter activity depending on its interaction with M29. Our observations suggest that p53 is involved in controlling the balance between DNA replication and gene expression of HPV8.

Chelating agents stabilize the monomeric state of the zinc binding human papillomavirus 16 E6 oncoprotein

The E6 protein of human papillomavirus 16 is known to be difficult and, when overexpressed, insoluble and agglomerated. It has two putative zinc ion binding sites crucial for its function. No metallochaperone has yet been found to deliver zinc ions to the E6 protein. Here, we report that a specific chelating agent, which we think functionally mimics a metallochaperone, stabilized the soluble monomeric form of E6 and inhibited multimerization in vitro. This effect seemed to depend on the chelating strength of the agent. While strong chelating agents precipitated the E6 protein and weak chelating agents did not favor the monomeric form of E6, chelating agents of intermediate strength [L-penicillamine and ethylene glycol bis(beta-aminoethyl)-N,N,N',N'-tetraacetic acid (EGTA)] effectively support the formation of a monomer. We did not observe formation of a dimer or defined oligomers. Degradation assays imply that the monomer is the biologically active form of the protein. Since EGTA favors the formation of monomeric over agglomerated E6 protein, we propose that chelating agents of appropriate strength could assist zinc delivery to recombinant metalloproteins in vitro and may even destabilize existing agglomerates.

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.

The role of TP53 in Cervical carcinogenesis

Functional loss of the tumor suppressor p53 by alterations in its TP53 gene is a frequent event in cancers of different anatomical regions. Cervical cancer is strongly linked to infection by high-risk human papillomavirus (HPV) types. The viral oncoprotein E6 has the ability to associate with and neutralize the function of p53. E6 interacts with a 100-kDa cellular protein, termed E6 associated protein (E6AP; also called ubiquitin-protein ligase E3A or UBE3A), which functions as an ubiquitin protein ligase. The dimeric complex then binds p53 and E6AP catalyzes multi-ubiquitination and degradation of p53. The ability to promote p53 degradation is an exclusive property of E6 from the high-risk HPV types. Indeed, the low-risk E6 proteins lack this activity, although they can bind p53. Consistent with the E6 function of the high-risk HPV types, the majority of cervical cancer cells have a wild-type p53 gene, but the protein levels are strongly decreased. Several independent studies have shown that in a small percentage of cervical tumors the p53 gene is mutated. However, this event appears to be unrelated to the presence or absence of HPV infection and the nature of the tumor.

Translational approaches to improving cervical screening

Screening programmes for cervical cancer using the current test--the Pap smear--have markedly reduced the incidence of the disease. However, an individual Pap test is of limited sensitivity and is difficult and expensive to perform. Increased understanding of the molecular pathogenesis of cervical cancer indicates that new approaches to screening might offer increased accuracy, affordability and the potential for automation. Such approaches exemplify how improved understanding of the biology of neoplasia might be translated into clinical benefit.

The role of steroid contraceptive hormones in the pathogenesis of invasive cervical cancer: a review

Invasive cervical cancer remains a leading cause of morbidity and mortality, especially among women in the developing world where screening is either deficient or absent. Of all agents linked to the causation of this disease, high-risk human papillomavirus (HPV) appears to be the strongest factor. However, not all women with HPV develop cervical cancer. Steroid contraception has been postulated to be one mechanism whereby HPV exerts its tumorigenic effect on cervical tissue. Steroids are thought to bind to specific DNA sequences within transcriptional regulatory regions on the HPV DNA to either increase or suppress transcription of various genes. Although some earlier studies were reassuring as no increased incidence of cervical cancer was observed, subsequent research has shown a causative association, especially among long-term users. The role of steroids was further enhanced by the discovery of hormone receptors in cervical tissue. Some earlier studies of oral contraceptive steroids found no increased risk, even after controlling for other risk factors, including smoking and number of partners. However, prospective studies have shown a greater progression of dysplasia to carcinoma-in-situ with more than 6 years of oral steroid contraceptive use. Similar findings were also evident from other work, including the Royal College of General Practitioners Oral Contraception Study. The WHO Collaborative Study of Neoplasia and Steroid Contraceptives showed a relative risk of 1.2 for invasive cancer in users of the long-acting progestational contraceptive, depo-medroxyprogesterone acetate. However, in users of more than 5 years duration, an estimate of 2.4 was reported. The upstream regulatory region (URR) of the HPV type 16 viral genome, mediates transcriptional control of the HPV genome and is thought to contain enhancer elements that are activated by steroid hormones. It has been shown that steroid hormones bind to specific glucorticoid-response elements within HPV-DNA. Experimental evidence has revealed that high-risk type HPV 16 are able to stimulate the development of vaginal and cervical squamous cell carcinomas in transgenic mice exposed to slow-release pellets of 17 beta-estradiol in the presence of human keratin-14 promoter. Squamous cell carcinomas developed in a multi-stage pathway only in transgenic mice and not in nontransgenic mice. The E6 oncoprotein of HPV 16 has been shown to bind to the p53 tumor suppressor gene and stimulate its degradation by a ubiquitin-dependent protease system. Steroid hormones are thought to increase the expression of the E6 and E7 HPV 16 oncogenes, which in turn bind to and degrade the p53 gene product, leading to apoptotic failure and carcinogenesis. However, the molecular basis of this remains to be proven.

Cyclic, proteasome-mediated turnover of unliganded and liganded ERalpha on responsive promoters is an integral feature of estrogen signaling

We present an integrated model of hERalpha-mediated transcription where both unliganded and liganded receptors cycle on estrogen-responsive promoters. Using ChIP, FRAP, and biochemical analysis we evaluate hERalpha at several points in these cycles, establishing the ubiquitination status and subnuclear distribution of hERalpha, its mobility, the kinetics of transcriptional activation, and the cyclic recruitment of E3 ligases and the 19S regulatory component of the proteasome. These experiments, together with an evaluation of the inhibition of transcription and proteasome action, demonstrate that proteasome-mediated degradation and hERalpha-mediated transactivation are inherently linked and act to continuously turn over hERalpha on responsive promoters. Cyclic turnover of hERalpha permits continuous responses to changes in the concentration of estradiol.

The viral control of cellular acetylation signaling

It is becoming clear that the post-translational modification of histone and non-histone proteins by acetylation is part of an important cellular signaling process controlling a wide variety of functions in both the nucleus and the cytoplasm. Recent investigations designate this signaling pathway as one of the primary targets of viral proteins after infection. Indeed, specific viral proteins have acquired the capacity to interact with cellular acetyltransferases (HATs) and deacetylases (HDACs) and consequently to disrupt normal acetylation signaling pathways, thereby affecting viral and cellular gene expression. Here we review the targeting of cellular HATs and HDACs by viral proteins and highlight different strategies adopted by viruses to control cellular acetylation signaling and to accomplish their life cycle.

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.

A mutant of human papillomavirus type 16 E6 deficient in binding alpha-helix partners displays reduced oncogenic potential in vivo

Human papillomaviruses (HPVs) are small DNA tumor viruses that are the causative agent of warts and are associated with many anogenital cancers. The viral gene encoding the E6 protein has been found to be involved in HPV oncogenesis. E6 is known to inactivate the cellular tumor suppressor, p53. In addition, E6 has been shown to bind to a variety of other cellular proteins. The focus of this study was to determine what role the interactions of E6 with a subset of cellular proteins which contain a common alpha-helical domain in their E6 binding region (alpha-helix partners) play in E6-mediated phenotypes. We generated transgenic mice expressing a mutant of E6, E6(I128T), which is defective for binding at least a subset of the alpha-helix partners, including E6AP, the ubiquitin ligase that mediates E6-dependent degradation of the p53 protein, to determine whether binding of alpha-helix partners plays a role in E6-mediated activities in vivo. Unlike mice expressing the wild-type E6 (strain K14E6(WT)), the mice expressing E6(I128T) lacked the ability to alter the radiation-induced block to DNA synthesis and promote the formation of benign skin tumors in conjunction with chemical carcinogens. Additionally, they displayed reduced levels of skin hyperplasia, spontaneous skin tumors, and tumor progression activity compared to those of the K14E6(WT) mice. From these results, we conclude that a domain in E6 that mediates alpha-helix partner binding is critical for E6-induced phenotypes in transgenic mice.

Human papilloma virus 16 E6 oncoprotein inhibits retinoic X receptor-mediated transactivation by targeting human ADA3 coactivator

The expression of human papillomavirus (HPV) E6 oncoprotein is causally linked to high-risk HPV-associated human cancers. We have recently isolated hADA3, the human homologue of yeast transcriptional co-activator yADA3, as a novel E6 target. Human ADA3 binds to the high-risk (cancer-associated) but not the low-risk HPV E6 proteins and to immortalization-competent but not to immortalization-defective HPV16 E6 mutants, suggesting a role for the perturbation of hADA3 function in E6-mediated oncogenesis. We demonstrate here that hADA3 directly binds to the retinoic X receptor (RXR)alpha in vitro and in vivo. Using chromatin immunoprecipitation, we show that hADA3 is part of activator complexes bound to the native RXR response elements within the promoter of the cyclin-dependent kinase inhibitor gene p21. We show that hADA3 enhances the RXR(alpha)-mediated sequence-specific transactivation of retinoid target genes, cellular retinoic acid-binding protein II and p21. Significantly, we demonstrate that E6 inhibits the RXR(alpha)-mediated transactivation of target genes, implying that perturbation of RXR-mediated transactivation by E6 could contribute to HPV oncogenesis.

Human papillomavirus immortalization and transformation functions

The high risk HPVs (such as HPV-16 and HPV-18) that are associated with specific anogenital cancers encode two oncoproteins E6 and E7, which are expressed in the HPV positive cancers. The E7 protein functions in cellular transformation, at least in part, through interactions with pRB and the other pRB related 'pocket proteins'. The major target of the E6 oncoprotein encoded by the genital tract, cancer associated human papillomaviruses is p53. Several lines of evidence suggest that E6 and E7 have additional targets important to the oncogenic potential of the virus. Work from a number of laboratories has focused on determining other activities of HPV relevant to carcinogenesis and identifying additional cellular targets of E6 and E7. This paper will review the state of the field at the time of the 19th International Papillomavirus Workshop in September 2001 with respect to the HPV encoded oncoproteins.

Cooperative activation of human papillomavirus type 8 gene expression by the E2 protein and the cellular coactivator p300

The E2 proteins of papillomaviruses (PV) bind to the coactivator CBP/p300 as do many other transcription factors, but the precise role of CBP/p300 in E2-specific functions is not yet understood. We show that the E2 protein of human PV type 8 (HPV8) directly binds to p300. Activation of HPV8 gene expression by low amounts of HPV8 E2 was stimulated up to sevenfold by coexpression of p300. The interaction between E2 and p300 may play a role in differentiation-dependent activation of PV gene expression, since we can show that the expression level of p300 increases during keratinocyte differentiation. Surprisingly, sequence-specific binding of E2 to its recognition sites within the regulatory region of HPV8 is not necessary for this cooperation, indicating that E2 can be recruited to the promoter via protein-protein interaction. HPV8 E2 binds via its N-terminal activation domain (AD), its C-terminal DNA binding domain (DBD), and its internal hinge region to p300 in vitro. Transient-transfection assays revealed that the AD is necessary and sufficient for cooperative activation with p300. However, we provide evidence that the interaction of the hinge and the DBD of HPV8 E2 with p300 may contribute. Our data suggest an important role of p300 in regulation of HPV8 gene expression and reveal a new mechanism by which E2 may be recruited to a promoter to activate transcription without sequence specific DNA binding.

Down regulation of the interleukin-8 promoter by human papillomavirus type 16 E6 and E7 through effects on CREB binding protein/p300 and P/CAF

Previously, we reported that human papillomavirus (HPV) type 16 E6 binds to C/H1, C/H3, and the C-terminal domains of coactivators p300 and CBP, causing the modulation of the transcription of certain genes controlled by NF-kappaB (p65 or relA) and p53. To establish the biological significance of these observations, we have focused on the transcriptional regulation of interleukin-8 (IL-8), a potent chemoattractant for T lymphocytes and neutrophils, which is also essential for the initiation of the local immune response. The IL-8 promoter is regulated by NF-kappaB/p65 in response to tumor necrosis factor alpha and requires the cooperation of the coactivators CBP/p300 and steroid receptor coactivator 1 (SRC-1) and the p300/CBP-associated factor (P/CAF) for optimal activation. Here we report that, in the presence of HPV-16 E6, the promoter activity of IL-8 was repressed. Moreover, from the mutational analysis of the IL-8 promoter, we found that E6 down-regulates the IL-8 promoter activity through the NF-kappaB/p65 binding site. This inhibition appears to result from the ability of HPV-16 E6 to compete with NF-kappaB/p65 and SRC-1 for binding to the N terminus and C terminus of CBP, respectively. Reporter data also showed that E7 represses IL-8 promoter activity, though to a lesser extent than E6 but, like E6, the repression by E7 is through the NF-kappaB/p65 binding site. E7 was shown for the first time to bind to P/CAF, and the binding was necessary for the down regulation of the IL-8 promoter. E6 and E7 together inhibited transcription of the IL-8 promoter to a greater extent than either alone. Finally, by RNase protection assay, we showed that the synthesis of endogenous IL-8 mRNA was repressed in keratinocytes stably expressing E6 and E7. Taken together, the results provide evidence that E6 and E7 can cooperatively disrupt IL-8 transcription through disruption of transcriptional active complexes, and this may have important consequences for immune responses in infected hosts.

Interaction of oncogenic papillomavirus E6 proteins with fibulin-1

Human papillomavirus (HPV) infection is the primary risk factor for the development of cervical cancer. The papillomavirus E6 gene is essential for virus-induced cellular transformation and the viral life cycle. Important insight into the mechanism of E6 function came from the discovery that cancer-related HPV E6 proteins promote the degradation of the tumor suppressor p53. However, mounting evidence indicates that interaction with p53 does not mediate all E6 activities. To explore the p53-independent functions of E6, we performed a yeast two-hybrid screen and identified fibulin-1 as an E6 binding protein. Fibulin-1 is a calcium-binding plasma and extracellular matrix protein that has been implicated in cellular transformation and tumor invasion. The interaction between E6 and fibulin-1 was demonstrated by both in vitro and in vivo assays. Fibulin-1 is associated specifically with cancer-related HPV E6s and the transforming bovine papillomavirus type 1 E6. Significantly, overexpression of fibulin-1 specifically inhibited E6-mediated transformation. These results suggest that fibulin-1 plays an important role in the biological activities of E6.

Human papillomavirus oncoprotein E6 inactivates the transcriptional coactivator human ADA3

High-risk human papillomaviruses (HPVs) are associated with carcinomas of the cervix and other genital tumors. The HPV oncoprotein E6 is essential for oncogenic transformation. We identify here hADA3, human homologue of the yeast transcriptional coactivator yADA3, as a novel E6-interacting protein and a target of E6-induced degradation. hADA3 binds selectively to the high-risk HPV E6 proteins and only to immortalization-competent E6 mutants. hADA3 functions as a coactivator for p53-mediated transactivation by stabilizing p53 protein. Notably, three immortalizing E6 mutants that do not induce direct p53 degradation but do interact with hADA3 induced the abrogation of p53-mediated transactivation and G(1) cell cycle arrest after DNA damage, comparable to wild-type E6. These findings reveal a novel strategy of HPV E6-induced loss of p53 function that is independent of direct p53 degradation. Given the likely role of the evolutionarily conserved hADA3 in multiple coactivator complexes, inactivation of its function may allow E6 to perturb numerous cellular pathways during HPV oncogenesis.

Cellular steady-state levels of "high risk" but not "low risk" human papillomavirus (HPV) E6 proteins are increased by inhibition of proteasome-dependent degradation independent of their p53- and E6AP-binding capabilities

The group of mucosal epithelia-infecting human papillomaviruses (HPV) can be subdivided in "low" and "high risk" HPV types. Both types induce benign neoplasia (condyloma), but only the infection with a "high risk" HPV type is causally associated with an increased risk of developing anogenital tumors. The oncogenic potential of high risk HPVs resides at least partially in the viral E6 protein. The E6 protein targets the cellular p53 protein for proteasome-dependent degradation, which is associated with the immortalizing and transforming functions of these viruses. Recently the E6-dependent proteasome-mediated destabilization of additional cellular proteins (E6TP1, c-myc, Bak, hMCM7, human scribble, E6AP, MAGI-1) has been described, but the cellular mechanisms controlling the viral E6 protein stability itself have been so far not analyzed. In this study, we transiently expressed the E6 genes of the high risk HPV type 16, the low risk HPV types 6a and 11, and the cutaneous epithelia-infecting HPV types 5 and 8 from a eucaryotic expression vector and compared the cellular steady-state levels of the expressed E6 proteins. We demonstrated that the high risk HPV 16 E6 protein possesses the lowest steady-state level in comparison to the low risk HPV type E6 proteins and the cutaneous epithelia-infecting HPV type E6 proteins. Inhibition of cellular proteasome-dependent protein degradation led to an increase in steady-state levels of high risk but not of low risk E6 proteins. Analysis of functionally deficient HPV 16 E6 proteins in p53 null- and p53 wild-type-expressing cell lines revealed that the cellular steady-state level of this protein is influenced neither by its p53- nor its E6AP-binding abilities.

The human papillomavirus 16 E6 protein binds to tumor necrosis factor (TNF) R1 and protects cells from TNF-induced apoptosis

High risk strains of human papillomavirus (HPV), such as HPV 16, cause human cervical carcinoma. The E6 protein of HPV 16 mediates the rapid degradation of p53, although this is not the only function of E6 and cannot completely explain its transforming potential. Previous work in our laboratory has demonstrated that transfection of HPV 16 E6 into the tumor necrosis factor (TNF)-sensitive LM cell line protects expressing cells from TNF-induced apoptosis in a p53-independent manner, and the purpose of this study was to determine the molecular mechanism underlying this protection. Caspase 3 and caspase 8 activation were significantly reduced in E6-expressing cells, indicating that E6 acts early in the TNF apoptotic pathway. In fact, E6 binds directly to TNF R1, as shown both by co-immunoprecipitation and mammalian two-hybrid approaches. E6 requires the same C-terminal portion of TNF R1 for binding as does TNF R1-associated death domain, and TNF R1/TNF R1-associated death domain interactions are decreased in the presence of E6. HA-E6 also blocked cell death triggered by transfection of the death domain of TNF R1. Together, these results provide strong support for a model in which HPV E6 binding to TNF R1 interferes with formation of the death-inducing signaling complex and thus with transduction of proapoptotic signals. They also demonstrate that HPV, like several other viruses, has developed a method for evading the TNF-mediated host immune response.

The promoter of the human proliferating cell nuclear antigen gene is not sufficient for cell cycle-dependent regulation in organotypic cultures of keratinocytes

The proliferating cell nuclear antigen (PCNA) is essential for DNA replication of mammalian cells and their small DNA tumor viruses. The mechanism of the cell cycle-dependent regulation of the human PCNA promoter is not clear despite extensive investigations. In this report, we employed organotypic cultures of primary human keratinocytes, which closely resemble native skin comprising both proliferating and postmitotic, differentiated cells, to examine the cell cycle-dependent regulation of the human PCNA gene (hPCNA) in the absence or presence of the human papillomavirus type 18 (HPV-18) E7 protein. HPV-18 E7 promotes S phase re-entry in post-mitotic differentiated keratinocytes by abrogating the transcription repression of E2F transcription factors by the retinoblastoma susceptibility protein, pRb. We demonstrated that E7 reactivated the transcription of the endogenous hPCNA in differentiated keratinocytes. In contrast, with or without E7, the expression of a transduced hPCNA promoter-driven reporter did not correlate with that of the endogenous hPCNA gene in either proliferating or differentiated cells. Moreover, in Chinese hamster ovary and L-cells, HPV E7 and the adenovirus E1A protein repressed the transduced hPCNA promoter, but both activated an extended promoter construct spanning the first intron. Mutations of two E2F sites in the intron reduced the basal activity and abolished the response to E7 or E1A. Promoter repression or activation required the CR2 domain of E7 and, to a lesser extent, CR1 as well. However, in organotypic cultures, this extended promoter construct failed to recapitulate the cell cycle-dependent regulation of the endogenous hPCNA gene. Only when a full-length Myc-tagged hPCNA spanning the 5' promoter and all exons and introns was used was the native pattern of expression largely restored, indicative of the complexity of its regulation.

Complementation of a p300/CBP defective-binding mutant of adenovirus E1a by human papillomavirus E6 proteins

Previous studies have shown that the human papillomavirus type 16 (HPV-16) E6 protein binds to p300/CBP and abrogates its transcriptional co-activator function. However, there is little information on the biological consequences of this interaction and discrepancy as to whether the interaction is high-risk E6 specific or not. We performed a series of studies to compare the interactions of HPV-18 and HPV-11 E6 with p300, and showed that both high- and low- risk E6 proteins bind p300. In addition, using a transformation-deficient mutant of adenovirus E1a, which cannot interact with p300, we demonstrated that HPV-16, HPV-18 and, to a lesser extent, HPV-11 E6, can complement this mutant in cell transformation assays. In contrast, a mutant of HPV-16 E6 which does not bind p300 failed to rescue the E1a mutant. These results suggest that the E6-p300 interaction may be important for the ability of HPV E6 to contribute towards cell transformation.