The E6 oncoproteins of high-risk papillomaviruses bind to a novel putative GAP protein, E6TP1, and target it for degradation

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.

SPAL, a Rap-specific GTPase activating protein, is present in the NMDA receptor-PSD-95 complex in the hippocampus

BACKGROUND

The PSD-95 family of proteins possesses multiple protein binding domains, including three PDZ domains, an SH3 domain, a HOOK domain and a guanylate kinase-like (GK) domain. The PSD-95 proteins function as scaffolding proteins that link ion channels such as the N-methyl-d-aspartate-receptors (NMDA-Rs) with cytoskeletal networks and signalling molecules, thereby controlling synaptic plasticity and learning.

RESULTS

We found that the PSD-95 family proteins interact via their GK domains with SPA-1-like protein (SPAL), a GTPase-activating protein (GAP) that is specific for Rap1. SPAL was contained within the NMDA-R-PSD-95 complex, and co-localized with PSD-95 and NMDA-R at the synapses in cultured hippocampal neurones. Furthermore, NMDA stimulation induced the dephosphorylation of SPAL in cultured hippocampal neurones.

CONCLUSION

Our findings suggest that SPAL may be involved in the NMDA-mediated organization of cytoskeletal networks and signal transduction.

Rap-specific GTPase activating protein follows an alternative mechanism

Rap1 is a small GTPase that is involved in signal transduction cascades. It is highly homologous to Ras but it is down-regulated by its own set of GTPase activating proteins (GAPs). To investigate the mechanism of the GTP-hydrolysis reaction catalyzed by Rap1GAP, a catalytically active fragment was expressed in Escherichia coli and characterized by kinetic and mutagenesis studies. The GTPase reaction of Rap1 is stimulated 10(5)-fold by Rap1GAP and has a k(cat) of 6 s(-1) at 25 degrees C. The catalytic effect of GAPs from Ras, Rho, and Rabs depends on a crucial arginine which is inserted into the active site. However, all seven highly conserved arginines of Rap1GAP can be mutated without dramatically reducing V(max) of the GTP-hydrolysis reaction. We found instead two lysines whose mutations reduce catalysis 25- and 100-fold, most likely by an affinity effect. Rap1GAP does also not supply the crucial glutamine that is missing in Rap proteins at position 61. The Rap1(G12V) mutant which in Ras reduces catalysis 10(6)-fold is shown to be efficiently down-regulated by Rap1GAP. As an alternative, Rap1(F64A) is shown by kinetic and cell biological studies to be a Rap1GAP-resistant mutant. This study supports the notion of a completely different mechanism of the Rap1GAP-catalyzed GTP-hydrolysis reaction on Rap1.

Deregulated cell cycle control in lens epithelial cells by expression of inhibitors of tumor suppressor function

Previous studies have shown that cell cycle proteins such as retinoblastoma protein (pRB) are essential for cell cycle withdrawal in differentiating lens cells. However, little is known about which factors are critical for cell cycle control in the lens epithelial cells. Here we use the K14 promoter to direct expression of E6 and E7, oncogenes from human papillomavirus type 16, which are known to bind and inactivate p53 and pRB, as molecular tools to study cell cycle regulation in the lens epithelium of transgenic mice. Expression of either gene resulted in increased proliferation and apoptosis, and in the case of E6, a unique epithelial phenotype characterized by multilayering and intercellular vacuoles was observed. Lenses from mice expressing E7 mutants, which are defective in inactivating pRB proteins, were normal and the lens phenotype in the E6 mice was p53-independent. Thus, cell proliferation in the lens epithelium is controlled by multiple factors including, but not necessarily limited to, the pRB family.

Inhibition of serum- and calcium-induced terminal differentiation of human keratinocytes by HPV 16 E6: study of the association with p53 degradation, inhibition of p53 transactivation, and binding to E6BP

Transfection of the E6 gene of human papillovirus (HPV) 16 into primary human keratinocytes (PHKs) generates proliferating cell colonies which are resistant to serum- and calcium-induced terminal differentiation. The extreme C-terminus of E6 was shown to be dispensable for this activity. To map further the amino acid sequences required for inducing resistance to serum and calcium, and to address the functional significance of E6 interactions with p53 and E6BP (ERC-55) in this function, we evaluated the activities of a series of E6 mutants. Small deletions within the central portion of the second putative zinc-finger abolished, or markedly reduced, E6 biological activity, while mutations affecting the cysteine residues in the base of the finger were less effective in this respect. When these mutants were assayed for their ability to degrade p53 in vitro and in vivo and to inhibit p53 transcriptional activation (TA), we found that there was a dissociation of these activities in some mutants. We mapped one mutant which was highly efficient in p53 degradation and inhibition of p53 TA, yet displayed severely reduced activity in the biological assay, and conversely, a subset of mutants that showed moderate activities in the colony assay while being severely impaired in p53 degradation and inhibition of p53 TA. These data argue that p53 inactivation or even elimination are not sufficient, and may not be essential, for altering the response of PHKs to serum and calcium. When these E6 mutants were evaluated for E6BP binding in vitro, there was a similar dissociation between the biological and biochemical activities of several mutants. We mapped mutants with moderate activity in the biological assay that lacked the ability to bind to E6BP and a mutant that showed high biological activity with only marginal binding to E6BP. Thus, there is no absolute correlation between the ability of E6 mutant proteins to induce alterations in keratinocyte differentiation responses to calcium and serum and to induce p53 degradation, inhibit p53 mediated transactivation, or bind E6BP. Evidently there are additional cellular targets for E6 which mediate this alteration in cellular differentiation.

Thymic hyperplasia and lung carcinomas in a line of mice transgenic for keratin 5-driven HPV16 E6/E7 oncogenes

Human Papillomavirus type 16 (HPV-16) is the cause of both benign lesions and ano-genital cancers. In HPV-associated cancers the transforming properties of the expressed viral E6 and E7 proteins have been revealed by a number of different assays. We have generated transgenic mice expressing HPV-16 E6/E7 genes under the control of the murine keratin 5 gene promoter, which should confer cell-type specific expression in the basal cells of squamous stratified epithelia. Transgenic mice developed thymic hyperplasia and lung neoplasia with 100% frequency, the thymus showing a size increase at 2 months and reaching the maximum dimension at 6 months, when lung carcinomas appeared. After this time the size of hyperplastic thymi decreased, while malignant formations invaded the mediastinal area. Hepatic metastasis could be also observed in some of the animals at the autopsy and death invariably occurred around 10-11 months of age.

Interaction of zyxin, a focal adhesion protein, with the e6 protein from human papillomavirus type 6 results in its nuclear translocation

Zyxin, a focal adhesion molecule, interacts specifically with the E6 protein from human papillomavirus (HPV) type 6 in a yeast two-hybrid screen of a cDNA library prepared from human keratinocytes. Zyxin does not interact significantly with E6 proteins from HPV types 11, 16, or 18. The interaction was confirmed by in vitro and in vivo analyses and it requires the LIM domains (Lin-11, Isl-1, and Mec-3 [G. Freyd, S. K. Kim, and H. R. Horvitz, Nature 344:876-879, 1990]) found at the carboxyl terminus of zyxin. Cotransfection of E6 from HPV ((6)E6) and zyxin results in the accumulation of zyxin in the nucleus where it can function as a transcriptional activator. (6)E6 can also mobilize endogenous zyxin to the nucleus.

The human papillomavirus E6 protein and its contribution to malignant progression

The human papillomavirus (HPV) E6 protein is one of three oncoproteins encoded by the virus. It has long been recognized as a potent oncogene and is intimately associated with the events that result in the malignant conversion of virally infected cells. In order to understand the mechanisms by which E6 contributes to the development of human malignancy many laboratories have focused their attention on identifying the cellular proteins with which E6 interacts. In this review we discuss these interactions in the light of their respective contributions to the malignant progression of HPV transformed cells.

Formation of soluble inclusion bodies by hpv e6 oncoprotein fused to maltose-binding protein

Many polypeptides overexpressed in bacteria are produced misfolded and accumulate as solid structures called inclusion bodies. Inclusion-body-prone proteins have often been reported to escape precipitation when fused to maltose-binding protein (MBP). Here, we have examined the case of HPV 16 oncoprotein E6. The unfused sequence of E6 is overexpressed as inclusion bodies in bacteria. By contrast, fusions of E6 to the C-terminus of MBP are produced soluble. We have analyzed preparations of soluble MBP-E6 fusions by using three independent approaches: dynamic light scattering, lateral turbidimetry, and sandwich ELISA. All three methods showed that MBP-E6 preparations contain highly aggregated material. The behavior of these soluble aggregates under denaturating conditions suggests that they are formed by agglomeration of misfolded E6 moieties. However, precipitation is prevented by the presence of the folded and highly soluble MBP moieties, which maintain the aggregates in solution. Therefore, the fact that a protein or protein domain is produced soluble when fused to the C-terminus of a carrier protein does not guarantee that the protein of interest is properly folded and active. We suggest that aggregation of fusion proteins should be systematically assayed, especially when these fusions are to be used for binding measurements or activity tests.

Regulation of dendritic spine morphology by SPAR, a PSD-95-associated RapGAP

The PSD-95/SAP90 family of scaffold proteins organizes the postsynaptic density (PSD) and regulates NMDA receptor signaling at excitatory synapses. We report that SPAR, a Rap-specific GTPase-activating protein (RapGAP), interacts with the guanylate kinase-like domain of PSD-95 and forms a complex with PSD-95 and NMDA receptors in brain. In heterologous cells, SPAR reorganizes the actin cytoskeleton and recruits PSD-95 to F-actin. In hippocampal neurons, SPAR localizes to dendritic spines and causes enlargement of spine heads, many of which adopt an irregular appearance with putative multiple synapses. Dominant negative SPAR constructs cause narrowing and elongation of spines. The effects of SPAR on spine morphology depend on the RapGAP and actin-interacting domains, implicating Rap signaling in the regulation of postsynaptic structure.

E box-dependent activation of telomerase by human papillomavirus type 16 E6 does not require induction of c-myc

Human papillomavirus type 16 (HPV-16) E6 activates telomerase specifically in epithelial cells. The oncogene c-myc has also been shown to activate telomerase in several cell types. Here we show that while both HPV-16 E6 and c-myc require intact E boxes to transactivate the hTERT promoter, E6 does not induce hTERT transcription simply by inducing expression of c-myc. Moreover, hTERT transactivation by HPV-16 E6 correlates with its ability to bind the cellular E6-associated protein (E6AP), suggesting that E6 and E6AP may target a regulator of hTERT expression.

Both E6 and E7 oncoproteins of human papillomavirus 16 inhibit IL-18-induced IFN-gamma production in human peripheral blood mononuclear and NK cells

Cervical carcinoma is the predominant cancer among malignancies in women throughout the world, and human papillomavirus (HPV) 16 is the most common agent linked to human cervical carcinoma. The present study was performed to investigate the mechanisms of immune escape in HPV-induced cervical cancer cells. The presence of HPV oncoproteins E6 and E7 in the extracellular fluids of HPV-containing cervical cancer cell lines SiHa and CaSki was demonstrated by ELISA. The effect of HPV 16 oncoproteins E6 and E7 on the production of IFN-gamma by IL-18 was assessed. E6 and E7 proteins reduced IL-18-induced IFN-gamma production in both primary PBMCs and the NK0 cell line. FACS analysis revealed that the viral oncoproteins reduced the binding of IL-18 to its cellular surface receptors on NK0 cells, whereas there was no effect of oncoproteins on IL-1 binding to its surface IL-1 receptors on D10S, a subclone of the murine Th cell D10.G4.1. In vitro pull-down assays also revealed that the viral oncoproteins and IL-18 bound to IL-18R alpha-chain competitively. These results suggest that the extracellular HPV 16 E6 and E7 proteins may inhibit IL-18-induced IFN-gamma production locally in HPV lesions through inhibition of IL-18 binding to its alpha-chain receptor. Down-modulation of IL-18-induced immune responses by HPV oncoproteins may contribute to viral pathogenesis or carcinogenesis.

Antibodies in haystacks: how selection strategy influences the outcome of selection from molecular diversity libraries

Antibodies against most antigens can be isolated from high quality phage antibody libraries. However, not all antibodies binding a particular antigen are necessarily found when standard selections are performed. Here we investigate the effect of two different selection strategies on the isolation of antibodies against a number of different antigens, and find that these different strategies tend to select different antibodies, with little overlap between them. This indicates that the full diversity of these libraries is not tapped by a single selection strategy and that each selection strategy imposes different selective criteria in addition to that of antigen binding. To fully exploit such libraries, therefore, many different selection strategies should probably be employed for each antigen. The use of alternative strategies should be considered when selection apparently fails, or when the number of different antibodies recognizing an antigen needs to be maximised. Furthermore, the microtitre selection strategy developed is likely to prove useful in the application of phage antibody libraries to the human genome project, allowing the high throughput selection of antibodies against multiple antigens simultaneously.

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

Recent analyses have identified a number of binding partners for E6, including E6AP, ERC55, paxillin, hDlg, p300, interferon regulatory factor 3, hMCM7, Bak, and E6TP1. Notably, association with E6 targets p53, E6TP1, myc, hMCM7, and Bak for degradation. However, the relative importance of the various E6 targets in cellular transformation remains unclear. E6 alone can dominantly immortalize normal human mammary epithelial cells (MECs), permitting an assessment of the importance of various E6 targets in cellular transformation. Studies in this system indicate that E6-induced degradation of p53 and E6 binding to ERC55 or hDlg do not correlate with efficient immortalization. Here, we have examined the role of E6TP1, a Rap GTPase-activating protein, in E6-induced immortalization of MECs. We tested a large set of human papillomavirus type 16 E6 mutants for their ability to bind and target E6TP1 for degradation in vitro and in vivo. We observed a strict correlation between the ability of E6 protein to target E6TP1 for degradation and its ability to immortalize MECs. Recent studies have identified telomerase as a target of E6 protein. Previous analyses of E6 mutants have revealed this trait to closely correlate with MEC immortalization. We examined our entire panel of E6 mutants for rapid induction of telomerase activity and found in general a strong correlation with immortalizing ability. The tight correlation between E6TP1 degradation and MEC immortalization strongly supports a critical role of functional inactivation of E6TP1 in E6-induced cellular immortalization.

Rap1 signalling: adhering to new models

Ras-like GTPases are ubiquitously expressed, evolutionarily conserved molecular switches that couple extracellular signals to various cellular responses. Rap1, the closest relative of Ras, has attracted much attention because of the possibility that it regulates Ras-mediated signalling. Rap1 is activated by extracellular signals through several regulatory proteins, and it might function in diverse processes, ranging from modulation of growth and differentiation to secretion, integrin-mediated cell adhesion and morphogenesis.

Regulation of the G2/M transition by p53

p53 protects mammals from neoplasia by inducing apoptosis, DNA repair and cell cycle arrest in response to a variety of stresses. p53-dependent arrest of cells in the G1 phase of the cell cycle is an important component of the cellular response to stress. Here we review recent evidence that implicates p53 in controlling entry into mitosis when cells enter G2 with damaged DNA or when they are arrested in S phase due to depletion of the substrates required for DNA synthesis. Part of the mechanism by which p53 blocks cells at the G2 checkpoint involves inhibition of Cdc2, the cyclin-dependent kinase required to enter mitosis. Cdc2 is inhibited simultaneously by three transcriptional targets of p53, Gadd45, p21, and 14-3-3 sigma. Binding of Cdc2 to Cyclin B1 is required for its activity, and repression of the cyclin B1 gene by p53 also contributes to blocking entry into mitosis. p53 also represses the cdc2 gene, to help ensure that cells do not escape the initial block. Genotoxic stress also activates p53-independent pathways that inhibit Cdc2 activity, activation of the protein kinases Chk1 and Chk2 by the protein kinases Atm and Atr. Chk1 and Chk2 inhibit Cdc2 by inactivating Cdc25, the phosphatase that normally activates Cdc2. Chk1, Chk2, Atm and Atr also contribute to the activation of p53 in response to genotoxic stress and therefore play multiple roles. p53 induces transcription of the reprimo, B99, and mcg10 genes, all of which contribute to the arrest of cells in G2, but the mechanisms of cell cycle arrest by these genes is not known. Repression of the topoisomerase II gene by p53 helps to block entry into mitosis and strengthens the G2 arrest. In summary, multiple overlapping p53-dependent and p53-independent pathways regulate the G2/M transition in response to genotoxic stress.

Low level expression of human papillomavirus type 16 (HPV16) E6 in squamous epithelium does not elicit E6 specific B- or T-helper immunological responses, or influence the outcome of immunisation with E6 protein

Mice transgenic for E6/E7 oncogenes of Human Papillomavirus type 16 display life-long expression of E6 in lens and skin epithelium, and develop inflammatory skin disease late in life, which progresses to papillomata and squamous carcinoma in some mice. We asked whether endogenous expression of E6 induced a specific immunological outcome, i.e. immunity or tolerance, or whether the mice remained immunologically naïve to E6. We show that prior to the onset of skin disease, E6 transgenic mice did not develop a spontaneous E6-directed antibody response, nor did they display T-cell proliferative responses to dominant T-helper epitope peptides within E6. In contrast, old mice in which skin disease had arisen, developed antibodies to E6. We also show that following immunisation with E6, specific antibody responses did not differ significantly among groups of E6-transgenic mice of different ages (and therefore of different durations and amounts of exposure to endogenous E6), and non-transgenic controls. Additionally, E6 immunisation-induced T-cell proliferative responses were similar in E6-transgenic and non-transgenic mice. These data are consistent with the interpretation that unimmunised E6-transgenic mice that have not developed inflammatory skin disease remain immunologically naïve to E6 at the B- and Th levels. There are implications for E6-mediated tumorigenesis in humans, and for the development of putative E6 therapeutic vaccines.

Complete switch from Mdm2 to human papillomavirus E6-mediated degradation of p53 in cervical cancer cells

The E6 oncoprotein of human papillomaviruses (HPVs) that are associated with cervical cancer utilizes the cellular ubiquitin-protein ligase E6-AP to target the tumor suppressor p53 for degradation. In normal cells (i.e., in the absence of E6), p53 is also a target of the ubiquitin-proteasome pathway. Under these conditions, however, p53 degradation is mediated by Mdm2 rather than by E6-AP. Here we show in a mutational analysis that, surprisingly, the structural requirements of p53 to serve as a proteolytic substrate differ between E6 proteins derived from different HPV types and, as expected, between Mdm2 and E6 proteins in vitro and in vivo. Stable expression of such mutants in HPV-negative and HPV-positive cell lines demonstrates that in HPV-positive cancer cells, the E6-dependent pathway of p53 degradation is not only active but, moreover, is required for degradation of p53, whereas the Mdm2-dependent pathway is inactive. Because the p53 pathway was reported to be functional in HPV-positive cancer cells, this finding indicates clearly that the ability of the E6 oncoprotein to target p53 for degradation is required for the growth of HPV-positive cancer cells.

Complete switch from Mdm2 to human papillomavirus E6-mediated degradation of p53 in cervical cancer cells

The E6 oncoprotein of human papillomaviruses (HPVs) that are associated with cervical cancer utilizes the cellular ubiquitin-protein ligase E6-AP to target the tumor suppressor p53 for degradation. In normal cells (i.e., in the absence of E6), p53 is also a target of the ubiquitin-proteasome pathway. Under these conditions, however, p53 degradation is mediated by Mdm2 rather than by E6-AP. Here we show in a mutational analysis that, surprisingly, the structural requirements of p53 to serve as a proteolytic substrate differ between E6 proteins derived from different HPV types and, as expected, between Mdm2 and E6 proteins in vitro and in vivo. Stable expression of such mutants in HPV-negative and HPV-positive cell lines demonstrates that in HPV-positive cancer cells, the E6-dependent pathway of p53 degradation is not only active but, moreover, is required for degradation of p53, whereas the Mdm2-dependent pathway is inactive. Because the p53 pathway was reported to be functional in HPV-positive cancer cells, this finding indicates clearly that the ability of the E6 oncoprotein to target p53 for degradation is required for the growth of HPV-positive cancer cells.

Gps2, a protein partner for human papillomavirus E6 proteins

We have used the yeast two-hybrid system to screen a cDNA library prepared from normal human epidermal keratinocytes and identified protein partners for human papilloma virus (HPV) E6 proteins. A clone that encoded Gps2 interacted with E6 proteins from HPVs of high and low oncogenic risk. The specificity of these reactions was verified and the regions of E6 that were required for interaction were mapped. Steady-state and pulse-chase analyses of cells cotransfected with DNAs expressing E6 from either HPV6 or HPV18 and Gps2 demonstrated that the E6 proteins induced the degradation of Gps2 in vivo but not in vitro. Gps2 exhibited transcriptional activation activity, and high-risk E6 suppressed this activity.

Multiple genetic changes are required for efficient immortalization of different subtypes of normal human mammary epithelial cells

Multiple Genetic Changes Are Required for Efficient Immortalization of Different Subtypes of Normal Human Mammary Epithelial Cells. Breast cancer is the second leading cause of cancer-related deaths of women in the U.S. About 180,000 new cases of breast cancer are diagnosed each year, a quarter of them fatal. Early detection is the key to the survival of these patients. However, there are no molecular markers to detect breast cancer at very early stages. A hurdle in understanding the early molecular changes in breast cancer has been the difficulty in establishing premalignant lesions and primary breast tumors as in vitro cell cultures. Normal epithelial cells grow for a finite life span and then senesce. Immortalization is defined by continuous growth of otherwise senescing cells and is believed to represent an early stage in tumor progression. To examine these early stages, we and others have developed in vitro models of mammary epithelial cell immortalization. These models have been extremely important in understanding the role of various tumor suppressor pathways that maintain the normal phenotypes of mammary epithelial cells. In this paper, we describe the establishment of these models and their relevance to understanding the molecular changes that occur in early breast cancer. These models have helped to identify molecular changes that occur in early breast cancers and appear to be well suited to identify novel markers for early diagnosis of breast cancer.

Membrane-associated guanylate kinase with inverted orientation (MAGI)-1/brain angiogenesis inhibitor 1-associated protein (BAP1) as a scaffolding molecule for Rap small G protein GDP/GTP exchange protein at tight junctions

BACKGROUND

Membrane-associated guanylate kinase (MAGUK) with inverted orientation (MAGI)-1/brain angiogenesis inhibitor 1-associated protein (BAP1), is a member of the MAGUK family that has multiple PDZ domains and interacts with many transmembrane proteins, including receptors and channels, through these domains. MAGI-1/BAP1 is ubiquitously expressed and localized at tight junctions in epithelial cells. It is an isoform of the neurone-specific synaptic scaffolding molecule (S-SCAM), which is known to interact with NMDA receptors and neuroligins. We have recently found that S-SCAM also interacts with a signalling molecule, a GDP/GTP exchange protein (GEP) that is specific for Rap1 small G protein, Rap GEP, which has also recently been referred to as RA-GEF/PDZ-GEFI/CNras-GEF. In this study, we have examined whether MAGI-1/BAP1 also interacts with and serves as a scaffolding molecule for Rap GEP at tight junctions in epithelial cells.

RESULTS

MAGI-1/BAP1 similarly interacted with Rap GEP in cell-free and intact cell systems. A Northern blot analysis revealed that Rap GEP was expressed in most tissues examined. However, neither postsynaptic density (PSD)-95/synapse-associated protein (SAP) 90 (another member of the MAGUK family) nor SAP97/human discs-large tumour suppressor gene product (another ubiquitously expressed MAGUK localizing to adherens junctions in epithelial cells and the isoform of PSD-95/SAP90) interacted with Rap GEP.

CONCLUSION

These results indicate that MAGI-1/BAP1 serves as a scaffolding molecule for Rap GEP at tight junctions in epithelial cells.

Human scribble (Vartul) is targeted for ubiquitin-mediated degradation by the high-risk papillomavirus E6 proteins and the E6AP ubiquitin-protein ligase

The high-risk human papillomavirus (HPV) E6 proteins stimulate the ubiquitination and degradation of p53, dependent on the E6AP ubiquitin-protein ligase. Other proteins have also been shown to be targeted for degradation by E6, including hDlg, the human homolog of the Drosophila melanogaster Discs large (Dlg) tumor suppressor. We show here that the human homolog of the Drosophila Scribble (Vartul) (hScrib) tumor suppressor protein is also targeted for ubiquitination by the E6-E6AP complex in vitro and that expression of E6 induces degradation of hScrib in vivo. Characterization of the E6AP-E6-hScrib complex indicated that hScrib binds directly to E6 and that the binding is mediated by the PDZ domains of hScrib and a carboxyl-terminal epitope conserved among the high-risk HPV E6 proteins. Green fluorescent protein-hScrib was localized to the periphery of MDCK cells, where it colocalized with ZO-1, a component of tight junctions. E6 expression resulted in loss of integrity of tight junctions, as measured by ZO-1 localization, and this effect was dependent on the PDZ binding epitope of E6. Thus, the high-risk HPV E6 proteins induce the degradation of the human homologs of two Drosophila PDZ domain-containing tumor suppressor proteins, hDlg and hScrib, both of which are associated with cell junction complexes. The fact that Scrib/Vart and Dlg appear to cooperate in a pathway that controls Drosophila epithelial cell growth suggests that the combined targeting of hScrib and hDlg is an important component of the biologic activity of high-risk HPV E6 proteins.

Inactivation of the human papillomavirus-16 E6 oncoprotein by organic disulfides

We are investigating compounds that could be useful in the treatment of neoplastic lesions of the cervix by acting on the oncoprotein E6 of human papillomavirus-16. The E6 protein contains two potential zinc-binding domains that are required for most of its functions. We have published tests that measure (i) the release of zinc ions after chemical alteration of the cysteine groups of these zinc-binding domains (TSQ assay), (ii) the interaction of E6 with the cellular proteins E6AP and E6BP (BIACORE assay), and (iii) the viability of tumor cell lines that require the continuous expression of HPV oncoproteins (WST1 assay). Based on these tests, we identified 4.4'-dithiodimorpholine as a potential lead compound. In this study we examined whether the dithiobisamine moiety of 4,4'-dithiodimorpholine may be an important molecular prerequisite for further drug development in this system. We have evaluated 59 new substances including organic disulfides and those containing the dithiobisamine moiety, as well as structural analogues. The compounds with significant reactivity in all three assays were observed only for dithiobisamine derivatives with saturated cyclic amines and aryl substituted piperazines. The identity of these substances suggests that the N-S-S-N moiety is necessary but not sufficient for reactivity in our assays, and that dithiobisamine based substances are useful as lead compounds that target the cysteine groups of HPV-16 E6 zinc fingers.

Multi-PDZ domain protein MUPP1 is a cellular target for both adenovirus E4-ORF1 and high-risk papillomavirus type 18 E6 oncoproteins

A general theme that has emerged from studies of DNA tumor viruses is that otherwise unrelated oncoproteins encoded by these viruses often target the same important cellular factors. Major oncogenic determinants for human adenovirus type 9 (Ad9) and high-risk human papillomaviruses (HPV) are the E4-ORF1 and E6 oncoproteins, respectively, and although otherwise unrelated, both of these viral proteins possess a functional PDZ domain-binding motif that is essential for their transforming activity and for binding to the PDZ domain-containing and putative tumor suppressor protein DLG. We report here that the PDZ domain-binding motifs of Ad9 E4-ORF1 and high-risk HPV-18 E6 also mediate binding to the widely expressed cellular factor MUPP1, a large multi-PDZ domain protein predicted to function as an adapter in signal transduction. With regard to the consequences of these interactions in cells, we showed that Ad9 E4-ORF1 aberrantly sequesters MUPP1 within the cytoplasm of cells whereas HPV-18 E6 targets this cellular protein for degradation. These effects were specific because mutant viral proteins unable to bind MUPP1 lack these activities. From these results, we propose that the multi-PDZ domain protein MUPP1 is involved in negatively regulating cellular proliferation and that the transforming activities of two different viral oncoproteins depend, in part, on their ability to inactivate this cellular factor.

Uncoupling between phenotypic senescence and cell cycle arrest in aging p21-deficient fibroblasts

Irreversible G(1) arrest in senescent human fibroblasts is mediated by two inhibitors of cyclin-dependent kinases (Cdks), p21(Cip1/SDI1/WAF1) and p16(Ink4A). To determine the physiological and molecular events that specifically require p21, we studied senescence in human diploid fibroblasts expressing the human papillomavirus type 16 E6 oncogene, which confers low p21 levels via enhanced p53 degradation. We show that in late-passage E6 cells, high Cdk activity drives the cell cycle, but population expansion is slowed down by crisis-like events, probably owing to defective cell cycle checkpoints. At the end of lifespan, terminal-passage E6 cells exhibited several aspects of the senescent phenotype and accumulated unphosphorylated pRb and p16. However, both replication and cyclin-Cdk2 kinase activity were still not blocked, demonstrating that phenotypic and replicative senescence are uncoupled in the absence of normal p21 levels. At this stage, E6 cells also failed to upregulate p27 and inactivate cyclin-Cdk complexes in response to serum deprivation. Eventually, irreversible G(1) arrest occurred coincident with inactivation of cyclin E-Cdk2 owing to association with p21. Similarly, when p21(-/-) mouse embryo fibroblasts reached the end of their lifespan, they had the appearance of senescent cells yet, in contrast to their wild-type counterparts, they were deficient in downregulating bromodeoxyuridine incorporation, cyclin E- and cyclin A-Cdk2 activity, and inhibiting pRb hyperphosphorylation. These data support the model that the critical event ensuring G(1) arrest in senescence is p21-dependent Cdk inactivation, while other aspects of senescent phenotype appear to occur independently of p21.

Requirement of wild-type p53 protein for maintenance of chromosomal integrity

Chromosomal double-strand breaks (DSBs) occurring in mammalian cells can initiate genomic instability, and their misrepairs result in chromosomal deletion, amplification, and translocation, common findings in human tumors. The tumor-suppressor protein p53 is involved in maintaining genomic stability. In this study, we demonstrate that the deficiency of wild-type p53 protein may allow unrepaired DSBs to initiate chromosomal instability. The human lymphoblastoid cell line TK6-E6 was established by transfection with human papilloma virus 16 (HPV16) E6 cDNA into parental TK6 cells via a retroviral vector. Abrogation of p53 function by E6 resulted in an increase in the spontaneous mutation frequencies at the heterozygous thymidine kinase (TK) locus but not at the hemizygous hypoxanthine phosphoribosyl transferase (HPRT) locus. Almost all TK-deficient mutants from TK6-E6 cells exhibited loss of heterozygosity (LOH) with the hemizygous TK allele. LOH analysis with microsatellite loci spanning the long arm of chromosome 17, which harbors the TK locus, showed that LOH extended over half of 17q toward the terminal end. Cytogenetic analysis of LOH mutants by chromosome painting indicated a mosaic of chromosomal aberrations involving chromosome 17, in which partial chromosome deletions, amplifications, and multiple translocations appeared heterogeneously in a single mutant. We speculate that spontaneous DSBs trigger the breakage-fusion bridge cycle leading to such multiple chromosome aberrations. In contrast, no chromosomal alterations were observed in TK-deficient mutants from TK6-20C cells expressing wild-type p53. In wild-type p53 cells, spontaneous DSBs appear to be promptly repaired through recombination between homologous chromosomes. These results support a model in which p53 protein contributes to the maintenance of genomic integrity through recombinational repair.

Human papillomavirus type 16 E6 induces self-ubiquitination of the E6AP ubiquitin-protein ligase

The E6 protein of the high-risk human papillomaviruses (HPVs) and the cellular ubiquitin-protein ligase E6AP form a complex which causes the ubiquitination and degradation of p53. We show here that HPV16 E6 promotes the ubiquitination and degradation of E6AP itself. The half-life of E6AP is shorter in HPV-positive cervical cancer cells than in HPV-negative cervical cancer cells, and E6AP is stabilized in HPV-positive cancer cells when expression of the viral oncoproteins is repressed. Expression of HPV16 E6 in cells results in a threefold decrease in the half-life of transfected E6AP. E6-mediated degradation of E6AP requires (i) the binding of E6 to E6AP, (ii) the catalytic activity of E6AP, and (iii) activity of the 26S proteasome, suggesting that E6-E6AP interaction results in E6AP self-ubiquitination and degradation. In addition, both in vitro and in vivo experiments indicate that E6AP self-ubiquitination results primarily from an intramolecular transfer of ubiquitin from the active-site cysteine to one or more lysine residues; however, intermolecular transfer can also occur in the context of an E6-mediated E6AP multimer. Finally, we demonstrate that an E6 mutant that is able to immortalize human mammary epithelial cells but is unable to degrade p53 retains its ability to bind and degrade E6AP, raising the possibility that E6-mediated degradation of E6AP contributes to its ability to transform mammalian cells.

Induction of apoptosis in human papillomaviruspositive cancer cells by peptide aptamers targeting the viral E6 oncoprotein

Certain types of human papillomaviruses (HPVs) are closely linked to the development of human cancers. Herein, it is shown that intracellular targeting of the HPV16 E6 oncoprotein by E6-binding peptide aptamers resulted in the apoptotic elimination of HPV16-positive cancer cells, whereas HPV-negative cells were not affected. These results provide direct experimental evidence that the HPV E6 oncoprotein has antiapoptotic activity in HPV-positive tumor cells that is required for their survival. The E6-targeting molecules identified herein have implications for the development of therapeutic strategies for the treatment of HPV-associated dysplasias and cancers.

PKN binds and phosphorylates human papillomavirus E6 oncoprotein

The high risk human papillomaviruses (HPVs) are associated with carcinomas of cervix and other genital tumors. Previous studies have identified two viral oncoproteins E6 and E7, which are expressed in the majority of HPV-associated carcinomas. The ability of high risk HPV E6 protein to immortalize human mammary epithelial cells has provided a single gene model to study the mechanisms of E6-induced oncogenic transformation. In recent years, it has become clear that in addition to E6-induced degradation of p53 tumor suppressor protein, other targets of E6 are required for mammary epithelial cells immortalization. Using the yeast two-hybrid system, we have identified a novel interaction of HPV16 E6 with protein kinase PKN, a fatty acid- and Rho small G protein-activated serine/threonine kinase with a catalytic domain highly homologous to protein kinase C. We demonstrate direct binding of high risk HPV E6 proteins to PKN in wheat-germ lysate in vitro and in 293T cells in vivo. Importantly, E6 proteins of high risk HPVs but not low risk HPVs were able to bind PKN. Furthermore, all the immortalization-competent and many immortalization-non-competent E6 mutants bind PKN. These data suggest that binding to PKN may be required but not sufficient for immortalizing normal mammary epithelial cells. Finally, we show that PKN phosphorylates E6, demonstrating for the first time that HPV E6 is a phosphoprotein. Our finding suggests a novel link between HPV E6 mediated oncogenesis and regulation of a well known phosphorylation cascade.

Human papillomavirus types 16 E6 and E7 contribute differently to carcinogenesis

High-risk human papillomaviruses (HPVs) are etiologically implicated in human cervical cancer. Two viral genes, E6 and E7, are commonly found expressed in these cancer cells. We have previously shown that mice transgenic for the HPV-16 E6 gene or E7 gene, in which the E6 or E7 was expressed in the basal layer of epithelia, developed skin tumors. The spectrum of tumors derived from E6 and E7 mice differed, however; although most tumors derived from the E7-transgenic mice were benign, the majority of the tumors from the E6-transgenic mice were malignant. These findings led us to hypothesize that E6 and E7 play different roles in carcinogenesis. To assess at what stages in carcinogenesis E6 and E7 act, we treated the skin of K14E6- and K14E7-transgenic mice with chemical carcinogens known to contribute to distinct stages in carcinogenesis. Both E6 and E7 were found to synergize with chemical carcinogens in causing tumor formation. E6 was found to act weakly at the promotion stage of carcinogenesis in the formation of benign tumors but strongly at the progression stage which involves the malignant conversion of benign tumors. In contrast, E7 primarily affected the promotion stage of carcinogenesis. These results provide direct evidence that E6 and E7 contribute differently to carcinogenesis; E7 promotes the formation of benign tumors, and E6 acts primarily to accelerate progression of these benign tumors to the malignant stage. Consistent with this model, we found E6 and E7 to cooperate in inducing tumor formation in mice expressing both oncogenes.

Molecular mechanism of carcinogenesis by human papillomavirus-16

Human papillomaviruses (HPVs) are common DNA viruses in humans. Recently, epithelial cancers associated with HPV infection have been used as models of virus-induced carcinogenesis. HPVs can be divided into two groups, mucosal and cutaneous. HPV-16 is the most frequent mucosal type associated with cervical cancer. Although the molecular mechanisms of carcinogenesis by HPV-16 have not been completely elucidated, it is apparent that HPV infection is the major risk factor in cervical carcinogenesis. Two viral early genes, E6 and E7, and an upstream regulatory region (URR) are preserved in cervical carcinoma cell lines as well as in clinical samples of cervical cancer, indicating that these regions are important in cancer development. E6 and E7 function as transforming genes. E6 protein binds to and promotes degradation of the tumor suppressor protein, p53, while E7 protein complexes and inactivates the Rb protein; together, they disrupt cell cycle regulation. E6 and E7 are transcribed from a promoter, P97. P97 is regulated by complex interactions between multiple, positive and negative, cellular factors and the viral E2 product. E2 disruption caused by the integration into the cellular genome may induce overexpression of E6 and E7. The E6 and E7 proteins are thought to act as critical factors in cervical carcinogenesis by inactivating the two tumor suppressor proteins, p53 and Rb, which are commonly mutated in other human cancers.

Expression of human papilloma virus E7 protein causes apoptosis and inhibits DNA synthesis in primary hepatocytes via increased expression of p21(Cip-1/WAF1/MDA6)

The impact of human papilloma virus (HPV16) E7 proteins and retinoblastoma (RB) antisense oligonucleotides upon mitogen-activated protein kinase (MAPK)-mediated inhibition of DNA synthesis via p21(Cip-1/WAF1/MDA6) (p21) was determined in primary hepatocytes. Prolonged activation of the MAPK pathway in p21(+/+) or p21(-/-) hepatocytes caused a large decrease and increase, respectively, in DNA synthesis. Either transfection with RB antisense oligonucleotides, expression of wild type E7, or RB binding mutant E7 (C24S) proteins increased p21 levels and reduced DNA synthesis in p21(+/+) hepatocytes. RB antisense oligonucleotides and E7 proteins increased apoptosis in p21(+/+), but not p21(-/-), hepatocytes. Expression of wild type E7 increased DNA synthesis above control levels in p21(-/-) cells, which was additive with prolonged MAPK activation. In contrast, expression of mutant E7 did not alter DNA synthesis above control levels in p21(-/-) cells and was supra-additive with prolonged MAPK activation. Antisense ablation of RB in p21(-/-) hepatocytes had a weak stimulatory effect upon DNA synthesis itself but enhanced the capacity of mutant E7 protein to stimulate DNA synthesis to the same level observed using wild type E7. The ability of prolonged MAPK activation to stimulate DNA synthesis in the presence of mutant E7 and antisense RB was additive. Collectively, the present data demonstrate that loss of RB function together with loss of p21 function plays an important role in the E7- and MAPK-dependent modulation of apoptosis and DNA synthesis in primary hepatocytes.

Identification of a second transforming function in bovine papillomavirus type 1 E6 and the role of E6 interactions with paxillin, E6BP, and E6AP

Papillomavirus E6 oncoproteins transform mammalian cells through interaction with cellular proteins. Bovine papillomavirus type 1 E6 (BE6) interacts with three previously described cellular targets: the E6AP E3 ubiquitin ligase, the calcium-binding protein E6BP (also known as ERC-55), and paxillin, which is a focal adhesion adapter protein. BE6 interacts strongly with each of these proteins in vitro, binding to similar peptide sequences found in E6AP, E6BP, and paxillin. To determine which BE6 interactions are necessary for transformation by BE6, we used a novel selection strategy for temperature-sensitive BE6 mutants in yeast that could discriminate in their interaction between E6AP, E6BP, and paxillin. All BE6 mutants that retained transforming ability retained association with paxillin, while some mutants that were transformation positive failed to interact with E6AP or E6BP. This study demonstrates that oncogene mutants that are temperature sensitive for transformation can be selected in yeast and that the induction of anchorage-independent cell proliferation by BE6 does not require strong association of BE6 with either E6AP or E6BP. Of particular interest is the identification of a BE6 mutant that interacts strongly with the acidic charged leucine motifs of E6AP, E6BP, and paxillin but is devoid of transformation activity, thereby genetically identifying a second essential transformation function in BE6 that is independent of interaction with acidic charged leucine motifs.

The role of the E6-p53 interaction in the molecular pathogenesis of HPV

Human papillomaviruses (HPVs) are associated with a number of clinical conditions, of which the most serious is cervical carcinoma. The E6 protein of the oncogenic, mucosal-specific HPV types has been shown to complex with p53 and, as a result, target it for rapid proteasome-mediated degradation. As a consequence, p53's growth-arrest and apoptosis-inducing activities are abrogated. Since p53 is frequently wild type in cervical cancers, unlike other cancers in which it is often mutated, the notion has arisen that E6's activity with respect to p53 is equivalent to an inactivating mutation of p53. In addition, several studies have shown that the pathways both upstream and downstream of p53 are intact in cervical cancers; this suggests the potential importance of the E6 - p53 interaction for therapeutic intervention. However, like all viral oncoproteins, E6 is a multifunctional protein and a plethora of other cellular targets has been identified. Indeed, E6's interactions with some of these additional targets appear to be equally important in the pathogenesis of HPV, and may also represent valid targets for therapeutic intervention.

The interaction between p53 and papillomaviruses

The p53 tumour suppressor is one of the host's principal defences against viral replication and subsequent cell transformation. The human papillomaviruses have evolved an elaborate strategy whereby the viral E6 proteins directly target p53 for ubiquitin mediated degradation and thus overcome the inhibitory effects of p53. However, a more detailed picture of the HPV*b1p53 interaction is now emerging in which there is a complex interplay between both positive and negative effectors of these interactions. This demonstrates the existence of a finely balanced virus*b1host relationship which, on rare occasion, fails and initiates the processes that ultimately lead to malignancy.

Ras and Rap1: two highly related small GTPases with distinct function

The Ras-like family of small GTPases includes, among others, Ras, Rap1, R-ras, and Ral. The family is characterized by similarities in the effector domain. While the function of Ras is, at least in part, elucidated, little is known about other members of the family. Currently, much attention is focused on the small GTPase Rap1. Initially, this member was identified as a transformation suppressor protein able to revert the morphological phenotype of Ras-transformed fibroblasts. This has led to the hypothesis that Rap1 antagonizes Ras by interfering in Ras effector function. Recent analysis revealed that Rap1 is activated rapidly in response to activation of a variety of receptors. Rap1 activation is mediated by several second messengers, including calcium, diacylglycerol, and cAMP. Guanine nucleotide exchange factors (GEFs) have been identified that mediate these effects. The most interesting GEF is Epac, an exchange protein directly activated by cAMP, thus representing a novel cAMP-induced, protein kinase A-independent pathway. Furthermore, Rap1 is inactivated by specific GTPase-activating proteins (GAPs), one of which is regulated through an interaction with Galphai. While Ras and Rap1 may share some effector pathways, evidence is accumulating that Ras and Rap1 each regulate unique cellular processes in response to various extracellular ligands. For Rap1 these functions may include the control of cell morphology.

Cell transformation by the E7 oncoprotein of human papillomavirus type 16: interactions with nuclear and cytoplasmic target proteins

The E7 oncoprotein of human papillomavirus type 16 (HPV-16) has long been known as a potent immortalizing and transforming agent. However, the molecular mechanisms underlying cell transformation and immortalization by E7 remain largely unknown. It is believed that E7 exerts its oncogenic function at least in part by modulating cellular growth regulatory pathways. Increasing experimental evidence suggests that cell transformation by E7 is mediated by the physical association of E7 with cellular regulatory proteins, whose functions are specifically altered by E7, as exemplified by the well-known interaction of E7 with the retinoblastoma protein. In this review, we summarize the available data on the interaction of E7 with cellular regulatory factors and functional consequences of these interactions. We will focus the review on a set of recently identified new target proteins for the E7 oncoprotein, which sheds new light on E7 functions required for cell transformation and immortalization. Similar to the case of the E6 protein of HPV-16, whose interaction with p53 was long considered its major activity, it now appears that the interaction of E7 with the retinoblastoma protein represents just one of many distinct interactions that are relevant for cell transformation.

nRap GEP: a novel neural GDP/GTP exchange protein for rap1 small G protein that interacts with synaptic scaffolding molecule (S-SCAM)

Synaptic scaffolding molecule (S-SCAM) has six PDZ domains through which it interacts with N-methyl-d-aspartate receptors and neuroligin at synaptic junctions. We isolated here a novel S-SCAM-binding protein. This protein has one PDZ, one Ras association, one Ras GDP/GTP exchange protein (Ras GEP) domain, and one C-terminal consensus motif for binding to PDZ domains. We named it nRap GEP (neural Rap GEP). nRap GEP moreover has an incomplete cyclic AMP (cAMP)-binding (CAB) domain. The domain organization of nRap GEP is similar to that of Epac/cAMP-guanine nucleotide exchange factor (GEF) I, except that Epac/cAMP-GEFI has complete CAB and Ras GEP domains but lacks the other two domains and the C-terminal motif. nRap GEP showed GEP activity for Rap1 but did not bind cAMP. nRap GEP was specifically expressed in rat brain. Immunohistochemical analysis revealed that nRap GEP and S-SCAM were localized at synaptic areas of the cerebellum. These results suggest that nRap GEP is a novel neural Rap1-specific GEP which is associated with S-SCAM.

Mechanisms of G2 arrest in response to overexpression of p53

Overexpression of p53 causes G2 arrest, attributable in part to the loss of CDC2 activity. Transcription of cdc2 and cyclin B1, determined using reporter constructs driven by the two promoters, was suppressed in response to the induction of p53. Suppression requires the regions -287 to -123 of the cyclin B1 promoter and -104 to -74 of the cdc2 promoter. p53 did not affect the inhibitory phosphorylations of CDC2 at threonine 14 or tyrosine 15 or the activity of the cyclin-dependent kinase that activates CDC2 by phosphorylating it at threonine 161. Overexpression of p53 may also interfere with the accumulation of CDC2/cyclin B1 in the nucleus, required for cells to enter mitosis. Constitutive expression of cyclin B1, alone or in combination with the constitutively active CDC2 protein T14A Y15F, did not reverse p53-dependent G2 arrest. However, targeting cyclin B1 to the nucleus in cells also expressing CDC2 T14A Y15F did overcome this arrest. It is likely that several distinct pathways contribute to p53-dependent G2 arrest.

Multiple functions of human papillomavirus type 16 E6 contribute to the immortalization of mammary epithelial cells

The E6 proteins from cervical cancer-associated human papillomavirus (HPV) types such as HPV type 16 (HPV-16) induce proteolysis of the p53 tumor suppressor protein through interaction with E6-AP. We have previously shown that human mammary epithelial cells (MECs) immortalized by HPV-16 E6 display low levels of p53. HPV-16 E6 as well as other cancer-related papillomavirus E6 proteins also binds the cellular protein E6BP (ERC-55). To explore the potential functional significance of these interactions, we created and analyzed a series of E6 mutants for their ability to interact with E6-AP, p53, and E6BP in vitro. While there was a similar pattern of binding among these E6 targets, a subset of mutants differentiated E6-AP binding, p53 binding, and p53 degradation activities. These results demonstrated that E6 binding to E6-AP is not sufficient for binding to p53 and that E6 binding to p53 is not sufficient for inducing p53 degradation. The in vivo activity of these HPV-16 E6 mutants was tested in MECs. In agreement with the in vitro results, most of these p53 degradation-defective E6 mutants were unable to reduce the p53 level in early-passage MECs. Interestingly, several mutants that showed severely reduced ability for interacting with E6-AP, p53, and E6BP in vitro efficiently immortalized MECs. These immortalized cells exhibited low p53 levels at late passage. Furthermore, mutants defective for p53 degradation but able to immortalize MECs were also identified, and the immortal cells retained normal levels of p53 protein. These results imply that multiple functions of HPV-16 E6 contribute to MEC immortalization.

Human papillomavirus type 16 E6-enhanced susceptibility of L929 cells to tumor necrosis factor alpha correlates with increased accumulation of reactive oxygen species

Human papillomavirus type 16 (HPV-16) E6 has been shown to prevent or enhance apoptosis depending on the stimulus and cell type. Here we present evidence that HPV-16 E6 sensitized murine fibrosarcoma L929 cells to tumor necrosis factor alpha (TNF)-induced cytolysis. The E6-enhanced cytolysis correlated with a precedent increase in reactive oxygen species (ROS) level and antioxidant treatment could completely block the E6-dependent sensitization. These findings represent the first demonstration of a link between a viral oncogene-sensitized cytolysis and ROS. Previous studies have shown conflicting results regarding whether TNF-induced cytolysis of L929 cells is through necrosis or apoptosis. Here we report that, although L929 cells underwent DNA fragmentation after exposure to TNF, they retained the morphology of intact nuclei while gaining permeability to propidium iodide, features characteristic of necrosis rather than apoptosis. We confirmed that the broad spectrum caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone markedly increased the susceptibility of L929 cells to TNF, and further demonstrated that E6 enhanced this susceptibility, which again correlated with increased ROS accumulation. We showed that the expression of E6 in L929 cells did not alter the stability of p53, and the cells retained a p53 response to actinomycin D. Furthermore, two E6 mutants defective for p53 degradation in other systems exhibited differential effects on TNF sensitization. These results suggest that the enhancement of TNF-induced L929 cytolysis by E6 is independent of p53 degradation. We also found that TNF-induced activation of NF-kappaB did not account for the enhanced TNF susceptibility by E6.

Human papillomavirus type 31 oncoproteins E6 and E7 are required for the maintenance of episomes during the viral life cycle in normal human keratinocytes

The E6 and E7 oncoproteins of the high-risk human papillomavirus (HPV) types are able to immortalize human keratinocytes in vitro and likely contribute to the development of anogenital malignancies in vivo. The role of these oncoproteins in the productive viral life cycle, however, is not known. To begin to examine these possible roles, mutations in E6 were introduced in the context of the complete HPV 31 genome. Although transfected wild-type HPV 31 genomes, as well as genomes containing an E6 translation termination linker, an E6 frameshift mutation, and a point mutation in the p53 interacting domain were able to replicate in transient assays, only the wild-type genome was stably maintained as an episome. Interestingly, mutant genomes in either the E6 splice-donor site or splice-acceptor site were reduced in replication ability in transient assays; however, cotransfection of E1 and E2 expression vectors restored this function. In a similar fashion, genomes containing mutant HPV 31 E7 genes, including a translation termination mutant, two Rb-binding site mutants, a casein kinase II phosphorylation site mutant, and a transformation deficient mutant, were constructed. Although transient replication was similar to wild type in all of the E7 mutants, only the casein kinase II mutant had the ability to maintain high copies of episomal genomes. These findings suggest a role for E6 and E7 in the viral life cycle beyond their ability to extend the life span of infected cells.

The human papillomavirus type 16 E6 gene alone is sufficient to induce carcinomas in transgenic animals

High-risk human papillomaviruses (HPVs) are the causative agents of certain human cancers. HPV type 16 (HPV16) is the papillomavirus most frequently associated with cervical cancer in women. The E6 and E7 genes of HPV are expressed in cells derived from these cancers and can transform cells in tissue culture. Animal experiments have demonstrated that E6 and E7 together cause tumors. We showed previously that E6 and E7 together or E7 alone could induce skin tumors in mice when these genes were expressed in the basal epithelia of the skin. In this study, we investigated the role that the E6 gene plays in carcinogenesis. We generated K14E6 transgenic mice, in which the HPV16 E6 gene was directed in its expression by the human keratin 14 promoter (hK14) to the basal layer of the epidermis. We found that E6 induced cellular hyperproliferation and epidermal hyperplasia and caused skin tumors in adult mice. Interestingly, the tumors derived from E6 were mostly malignant, as opposed to the tumors from E7 mice, which were mostly benign. This result leads us to hypothesize that E6 may contribute differently than E7 to HPV-associated carcinogenesis; whereas E7 primarily contributes to the early stages of carcinogenesis that lead to the formation of benign tumors, E6 primarily contributes to the late stages of carcinogenesis that lead to malignancy.

Rap1 GTPase-activating protein SPA-1 negatively regulates cell adhesion

Rap1 GTPase is activated by a variety of stimulations in many types of cells, but its exact functions remain unknown. In this study we have shown that SPA-1 interferes with Rap1 activation by membrane-targeted C3G, C3G-F, in 293T cells through the GTPase activating protein (GAP) activity. SPA-1 transiently expressed in HeLa cells was mostly localized at the cortical cytoskeleton and induced rounding up of the cells, whereas C3G-F conversely induced extensive cell spreading. Conditional SPA-1 overexpression in HeLa cells by tetracycline-regulative system suppressed Rap1 activation upon plating on dishes coated with fibronectin and resulted in the reduced adhesion. When SPA-1 was conditionally induced after the established cell adhesion, the cells gradually rounded up and detached from the dish. Both effects were counteracted by exogenous fibronectin in a dose-dependent manner. Retroviral overexpression of SPA-1 in promyelocytic 32D cells also inhibited both activation of Rap1 and induction of cell adhesion by granulocyte colony stimulating factor without affecting differentiation. These results have indicated that Rap1 GTP is required for the cell adhesion induced by both extracellular matrix and soluble factors, which is negatively regulated by SPA-1.