Bap31 is a novel target of the human papillomavirus E5 protein

The human papillomavirus type 16 E5 oncoprotein translocates calpactin I to the perinuclear region

The human papillomavirus type 16 (HPV-16) E5 oncoprotein is embedded in membranes of the endoplasmic reticulum and nuclear envelope with its C terminus exposed to the cytoplasm. Among other activities, E5 cooperates with the HPV E6 oncoprotein to induce koilocytosis in human cervical cells and keratinocytes in vitro. The effect of E5 on infected cells may rely on its interactions with various cellular proteins. In this study we identify calpactin I, a heterotetrameric, Ca(2+)- and phospholipid-binding protein complex that regulates membrane fusion, as a new cellular target for E5. Both the annexin A2 and p11 subunits of calpactin I coimmunoprecipitate with E5 in COS cells and in human epithelial cell lines, and an intact E5 C terminus is required for binding. Moreover, E5-expressing cells exhibit a perinuclear redistribution of annexin A2 and p11 and show increased fusion of perinuclear membrane vesicles. The C terminus of E5 is required for both the perinuclear redistribution of calpactin I and increased formation of perinuclear vacuoles. These results support the hypothesis that the E5-induced relocalization of calpactin I to the perinuclear region promotes perinuclear membrane fusion, which may underlie the development of koilocytotic vacuoles.

Suppression of STAT-1 expression by human papillomaviruses is necessary for differentiation-dependent genome amplification and plasmid maintenance

High-risk human papillomaviruses (HPVs) infect stratified epithelia to establish persistent infections that maintain low-copy-number episomes in infected basal cells. Amplification of viral genomes occurs upon keratinocyte differentiation, followed by virion synthesis. During persistent HPV infections, viral proteins act to evade surveillance by both innate and adaptive immune responses. One of the primary pathways regulating the innate immune response is the JAK/STAT pathway. Our studies indicate that the expression of STAT-1, but not other members of interferon (IFN)-stimulated gene factor 3 (ISGF-3) complex such as STAT-2 and IFN regulatory factor 9 (IRF9), is selectively suppressed by HPV proteins at the level of transcription. Both E6 and E7 oncoproteins independently suppress the expression of STAT-1, and mutational analyses indicate that the E6 targeting E6-associated protein (E6AP) is responsible for suppression. The levels of STAT-1 proteins increase upon differentiation of both normal and HPV-positive cells but are still significantly reduced in the latter cells. Transient restoration of STAT-1 levels in HPV-positive cells using recombinant retroviruses significantly impaired viral amplification upon differentiation while long-term increases abrogated maintenance of episomes. Similarly, increased levels of STAT-1 induced by gamma interferon treatment inhibited HPV genome amplification upon differentiation. Overall, our findings demonstrate that suppression of STAT-1 expression by HPV proteins is necessary for genome amplification and maintenance of episomes, suggesting an important role for this activity in viral pathogenesis.

HPV16 E5 affects the KGFR/FGFR2b-mediated epithelial growth through alteration of the receptor expression, signaling and endocytic traffic

The E5 oncoprotein of the human papillomavirus type 16 (HPV16 E5) cooperates in cervical carcinogenesis and in epithelial transformation deregulating cell growth, survival and differentiation through the modulation of growth factor receptors. Among the epithelial receptor tyrosine kinases, the keratinocyte growth factor receptor/fibroblast growth factor receptor 2b (KGFR/FGFR2b) is a major paracrine mediator of epithelial homeostasis and appears to have an unique and unusual role in epithelial tissues, exerting a tumor-suppressive function in vitro and in vivo. With the aim to better elucidate the molecular events involved in the pathological activity of 16E5, we investigated if the viral protein would be able to affect the KGFR expression, signaling and turnover by interference with its degradative and recycling endocytic pathways. Quantitative reverse transcriptase-PCR and biochemical approaches on human keratinocytes transfected with 16E5-HA showed that E5 protein is able to induce KGFR down-modulation at both transcript and protein levels. Immunofluorescence microscopy in double-transfected cells expressing both E5 and KGFR revealed that the viral protein alters the receptor endocytic trafficking and triggers its endosomal sorting to the indirect juxtanuclear recycling pathway. The shift from lysosomal degradation to recycling at the plasma membrane correlates with a reduced phosphorylation of the fibroblast growth factor receptor substrate-2α tyrosine 196, the major docking site for Grb2-Cbl complexes responsible for receptor ubiquitination and degradation. 5'-Bromo-deoxyuridine incorporation assay demonstrated that expression of 16E5 induces a decrease in the growth response to the receptor ligands as a consequence of KGFR down-modulation, suggesting that 16E5 might have a role on HPV infection in perturbing the KGFR-mediated physiological behavior of confluent keratinocytes committed to differentiation.

Human papillomavirus type 16 E5 protein induces expression of beta interferon through interferon regulatory factor 1 in human keratinocytes

Crucial steps in high-risk human papillomavirus (HR-HPV)-related carcinogenesis are the integration of HR-HPV into the host genome and loss of viral episomes. The mechanisms that promote cervical neoplastic progression are, however, not clearly understood. During HR-HPV infection, the HPV E5 protein is expressed in precancerous stages but not after viral integration. Given that it has been reported that loss of HPV16 episomes and cervical tumor progression are associated with increased expression of antiviral genes that are inducible by type I interferon (IFN), we asked whether E5, expressed in early phases of cervical carcinogenesis, affects IFN-β signaling. We show that the HPV type 16 (HPV16) E5 protein expression per se stimulates IFN-β expression. This stimulation is specifically mediated by the induction of interferon regulatory factor 1 (IRF-1) which, in turn, induces transcriptional activation of IRF-1-targeted interferon-stimulated genes (ISGs) as double-stranded RNA-dependent protein kinase R (PKR) and caspase 8. Our data show a new and unexpected role for HR-HPV E5 protein and indicate that HPV16 E5 may contribute to the mechanisms responsible for cervical carcinogenesis in part via stimulation of IFN-β and an IFN signature, with IRF-1 playing a pivotal role. HPV16 E5 and IRF-1 may thus serve as potential therapeutic targets in HPV-associated premalignant lesions.

Membrane-Associated RING-CH proteins associate with Bap31 and target CD81 and CD44 to lysosomes

Membrane-associated RING-CH (MARCH) proteins represent a family of transmembrane ubiquitin ligases modulating intracellular trafficking and turnover of transmembrane protein targets. While homologous proteins encoded by gamma-2 herpesviruses and leporipoxviruses have been studied extensively, limited information is available regarding the physiological targets of cellular MARCH proteins. To identify host cell proteins targeted by the human MARCH-VIII ubiquitin ligase we used stable isotope labeling of amino-acids in cell culture (SILAC) to monitor MARCH-dependent changes in the membrane proteomes of human fibroblasts. Unexpectedly, we observed that MARCH-VIII reduced the surface expression of Bap31, a chaperone that predominantly resides in the endoplasmic reticulum (ER). We demonstrate that Bap31 associates with the transmembrane domains of several MARCH proteins and controls intracellular transport of MARCH proteins. In addition, we observed that MARCH-VIII reduced the surface expression of the hyaluronic acid-receptor CD44 and both MARCH-VIII and MARCH-IV sequestered the tetraspanin CD81 in endo-lysosomal vesicles. Moreover, gene knockdown of MARCH-IV increased surface levels of endogenous CD81 suggesting a constitutive involvement of this family of ubiquitin ligases in the turnover of tetraspanins. Our data thus suggest a role of MARCH-VIII and MARCH-IV in the regulated turnover of CD81 and CD44, two ubiquitously expressed, multifunctional proteins.

The human papillomavirus type 16 E5 oncoprotein inhibits epidermal growth factor trafficking independently of endosome acidification

The human papillomavirus type 16 E5 oncoprotein (16E5) enhances acute, ligand-dependent activation of the epidermal growth factor receptor (EGFR) and concomitantly alkalinizes endosomes, presumably by binding to the 16-kDa "c" subunit of the V-ATPase proton pump (16K) and inhibiting V-ATPase function. However, the relationship between 16K binding, endosome alkalinization, and altered EGFR signaling remains unclear. Using an antibody that we generated against 16K, we found that 16E5 associated with only a small fraction of endogenous 16K in keratinocytes, suggesting that it was unlikely that E5 could significantly affect V-ATPase function by direct inhibition. Nevertheless, E5 inhibited the acidification of endosomes, as determined by a new assay using a biologically active, pH-sensitive fluorescent EGF conjugate. Since we also found that 16E5 did not alter cell surface EGF binding, the number of EGFRs on the cell surface, or the endocytosis of prebound EGF, we postulated that it might be blocking the fusion of early endosomes with acidified vesicles. Our studies with pH-sensitive and -insensitive fluorescent EGF conjugates and fluorescent dextran confirmed that E5 prevented endosome maturation (acidification and enlargement) by inhibiting endosome fusion. The E5-dependent defect in vesicle fusion was not due to detectable disruption of actin, tubulin, vimentin, or cytokeratin filaments, suggesting that membrane fusion was being directly affected rather than vesicle transport. Perhaps most importantly, while bafilomycin A(1) (like E5) binds to 16K and inhibits endosome acidification, it did not mimic the ability of E5 to inhibit endosome enlargement or the trafficking of EGF. Thus, 16E5 alters EGF endocytic trafficking via a pH-independent inhibition of vesicle fusion.

The HPV-16 E5 protein represses expression of stress pathway genes XBP-1 and COX-2 in genital keratinocytes

The HPV-16 E5 protein resides in membranes of the endoplasmic reticulum (ER) and modulates cell growth and viral replication. In order to help define its biological activities, we analyzed E5-induced changes in human keratinocyte gene expression. Our studies identified the downregulation of spliced XBP-1 transcripts, a key player in the ER stress response, as a biochemical marker of E5 expression. IRE1alpha, the endoribonuclease responsible for XBP-1 RNA splicing, was also downregulated. Furthermore, cDNA microarray analysis revealed the repression of COX-2, another member of the ER stress pathway. In contrast, these genes were not altered either by the low-risk HPV-6b E5, or a C-terminal HPV-16 E5 mutant, in which the histidine and alanine residues (conserved in high-risk HPVs) were replaced with tyrosine and isoleucine (conserved in low-risk HPVs). HPV-16 E5 was also able to lower COX-2 mRNA levels in cells co-expressing E6/E7, suggesting that it might exert similar activity during viral replication. Interestingly, the E6/E7 genes were independently able to lower COX-2 transcripts compared to vector cells, indicating that multiple pathways of COX-2 repression exist. COX-2 downregulation by E5 could be overcome by thapsigargin or tunicamycin treatments, which initiate ER stress via calcium fluxes and abnormal protein glycosylation respectively, making it unlikely that E5 specifically tempers these pathways. Overall, our data indicate that E5 represses the cellular ER stress response and suggest a potential role for E5 during productive HPV infection.

Human papillomavirus oncoproteins: pathways to transformation

An association between human papillomavirus (HPV) infection and the development of cervical cancer was initially reported over 30 years ago, and today there is overwhelming evidence that certain subtypes of HPV are the causative agents of these malignancies. The p53 and retinoblastoma proteins are well-characterized targets of the HPV E6 and E7 oncoproteins, but recent studies have shown that the alteration of additional pathways are equally important for transformation. These additional factors are crucial regulators of cell cycle progression, telomere maintenance, apoptosis and chromosomal stability. Understanding how HPV oncoproteins modify these activities provides novel insights into the basic mechanisms of oncogenesis.

Viral strategies for the evasion of immunogenic cell death

Viral strategies for the evasion of immunogenic cell death (Symposium). J Intern Med 2010; 267: 526-542. Driven by co-evolutionary forces, viruses have refined a wide arsenal of strategies to interfere with the host defences. On one hand, viruses can block/retard programmed cell death in infected cells, thereby suppressing one of the most ancient mechanisms against viral dissemination. On the other hand, multiple viral factors can efficiently trigger the death of infected cells and uninfected cells from the immune system, which favours viral spreading and prevents/limits an active antiviral response, respectively. Moreover, several viruses are able to inhibit the molecular machinery that drives the translocation of calreticulin to the surface of dying cells. Thereby, viruses block the exposure of an engulfment signal that is required for the efficient uptake of dying cells by dendritic cells and for the induction of the immune response. In this review, we discuss a variety of mechanisms by which viruses interfere with the cell death machinery and, in particular, by which they subvert immunogenic cell death.

Immunogenic tumor cell death for optimal anticancer therapy: the calreticulin exposure pathway

In response to some chemotherapeutic agents such as anthracyclines and oxaliplatin, cancer cells undergo immunogenic apoptosis, meaning that their corpses are engulfed by dendritic cells and that tumor cell antigens are presented to tumor-specific CD8(+) T cells, which then control residual tumor cells. One of the peculiarities of immunogenic apoptosis is the early cell surface exposure of calreticulin (CRT), a protein that usually resides in the lumen of the endoplasmic reticulum (ER). When elicited by anthracyclines or oxaliplatin, the CRT exposure pathway is activated by pre-apoptotic ER stress and the phosphorylation of the eukaryotic translation initiation factor eIF2alpha by the kinase PERK, followed by caspase-8-mediated proteolysis of the ER-sessile protein BAP31, activation of the pro-apoptotic proteins Bax and Bak, anterograde transport of CRT from the ER to the Golgi apparatus and exocytosis of CRT-containing vesicles, finally resulting in CRT translocation onto the plasma membrane surface. Interruption of this complex pathway abolishes CRT exposure, annihilates the immunogenicity of apoptosis, and reduces the immune response elicited by anticancer chemotherapies. We speculate that human cancers that are incapable of activating the CRT exposure pathway are refractory to the immune-mediated component of anticancer therapies.

All 4 di-leucine motifs in the first hydrophobic domain of the E5 oncoprotein of human papillomavirus type 16 are essential for surface MHC class I downregulation activity and E5 endomembrane localization

The E5 oncoprotein of human papillomavirus type 16 downregulates surface MHC Class I and interacts with the heavy chain of the MHC complex via the first hydrophobic domain, believed to form the first helical transmembrane region (TM1) of E5. TM1 contains 4 equally spaced di-leucine (LL1-LL4) motifs. Di-leucine motifs have been implicated in protein-protein interactions and as localization signals. To see if any of the 4 di-leucine motifs of TM1 are involved in MHC downregulation by E5, we mutated each LL pair into valine pairs (VV1-VV4), as mutation of leucine to valine is not expected to cause major structural alterations in E5. We found that all 4 mutations disrupted the intracellular location of E5 and abrogated its MHC I downregulating activity; however VV2 and VV4 mutants were still able to interact physically with the MHC I heavy chain (HC) in vitro, while VV1 and VV3 mutants had lost this activity. We conclude that LL1 and LL3 are necessary for the interaction with HC, but LL2 and LL4 are not. However all 4 LL motifs are responsible for the proper localization of E5 in the Golgi/ER, and the displacement of E5 from this location contributes to the abrogation of MHC I downregulation. LL1 and LL3 motifs are expected to be on one face of the TM1 helix and LL2 and LL4 on the opposite face. We propose that E5 interacts with HC via LL1 and LL3 and that all 4 di-leucine motifs act as a targeting signal.

Membrane orientation of the human papillomavirus type 16 E5 oncoprotein

The E5 protein of human papillomavirus type 16 is a small, hydrophobic protein that localizes predominantly to membranes of the endoplasmic reticulum (ER). To define the orientation of E5 in these membranes, we employed a differential, detergent permeabilization technique that makes use of the ability of low concentrations of digitonin to selectively permeabilize the plasma membrane and saponin to permeabilize all cellular membranes. We then generated a biologically active E5 protein that was epitope tagged at both its N and C termini and determined the accessibility of these termini to antibodies in the presence and absence of detergents. In both COS cells and human ectocervical cells, the C terminus of E5 was exposed to the cytoplasm, whereas the N terminus was restricted to the lumen of the ER. Finally, the deletion of the E5 third transmembrane domain (and terminal hydrophilic amino acids) resulted in a protein with its C terminus in the ER lumen. Taken together, these topology findings are compatible with a model of E5 being a 3-pass transmembrane protein and with studies demonstrating its C terminus interacting with cytoplasmic proteins.

The canine papillomavirus e5 protein signals from the endoplasmic reticulum

The recently discovered Canis familiaris papillomavirus (PV) type 2 (CfPV2) provides a unique opportunity to study PV gene functions in vitro and in vivo. Unlike the previously characterized canine oral PV, CfPV2 contains an E5 open reading frame and is associated with progression to squamous cell carcinoma. In the current study, we have expressed and characterized the CfPV2-encoded E5 protein, a small, hydrophobic, 41-amino-acid polypeptide. We demonstrate that, similar to the E5 protein from high-risk human PV type 16, the CfPV2 E5 protein is localized in the endoplasmic reticulum (ER) and that its expression decreases keratinocyte proliferation and cell life span. E5 expression also increases the percentage of cells in the G(1) phase of the cell cycle, with a concomitant decrease in the percentage of cells in S phase. To identify a potential mechanism for E5-mediated growth inhibition from the ER, we developed a real-time PCR method to quantify the splicing of XBP1 mRNA as a measure of ER stress. We found that the CfPV2 E5 protein induced ER stress and that this, as well as the observed growth inhibition, is tempered significantly by coexpression of the CfPV2 E6 and E7 genes. It is possible that the spatial/temporal regulation of E6/E7 gene expression during keratinocyte differentiation might therefore modulate E5 activity and ER stress.

Effects of human papillomavirus type 16 E5 deletion mutants on epithelial morphology: functional characterization of each transmembrane domain

Human papillomavirus type 16 (HPV-16) is the cause of cervical cancer. The HPV genome encodes three transforming proteins, E5, E6 and E7. E6 and E7 are the main transforming proteins of HPV, while the role of E5 is still poorly understood. Using three dimensional organotypic raft cultures we show that HaCaT human keratinocytes expressing HPV-16 E5 form a very perturbed epithelium, with simultaneous hyperkeratinization of some cells and defective differentiation of other cells. The basal layer is disturbed and many cells invade the collagen matrix. Many cells among the differentiated layers show characteristics of basal cells: progression through the cell cycle, expression of cytokeratin 14, lack of cytokeratin 1 and production of matrix metalloproteases (MMP). Using deletion mutants which encompass the three hydrophobic domains of E5, we have assigned the ability to promote invasion of the matrix to the first hydrophobic domain, and the capacity to induce MMP9 to the C-terminal four amino acids. We also show that invasion and production of MMP9 can be dissociated, as mutants that are still capable of invasion do not produce MMP9 and vice versa.

Human papillomaviruses and the interferon response

Human papillomaviruses (HPV) are small DNA viruses that target stratified keratinocytes for infection. A subset of HPV types infect epithelia in the genital tract and are the causative agents of cervical as well as other anogenital cancers. Interferon treatment of existing genital HPV lesions has had mixed results. While HPV proteins down-regulate the expression of interferon-inducible genes, interferon treatment ultimately induces their high-level transcription after a delay. Cells containing complete HPV genomes that are able to undergo productive replication upon differentiation are sensitive to interferon-induced growth arrest, while cells from high-grade cancers that only express E6 and E7 are resistant. Recent studies indicate this sensitivity is dependent upon the binding of the interferon-inducible factor, p56, to the E1 replication protein. The response to interferon by HPV proteins is complex and results from the action of multiple viral proteins.

The EVER proteins as a natural barrier against papillomaviruses: a new insight into the pathogenesis of human papillomavirus infections

Infections by human papillomaviruses (HPVs) are the most frequently occurring sexually transmitted diseases. The crucial role of genital oncogenic HPV in cervical carcinoma development is now well established. In contrast, the role of cutaneous HPV in skin cancer development remains a matter of debate. Cutaneous beta-HPV strains show an amazing ubiquity. The fact that a few oncogenic genotypes cause cancers in patients suffering from epidermodysplasia verruciformis is in sharp contrast to the unapparent course of infection in the general population. Our recent investigations revealed that a natural barrier exists in humans, which protects them against infection with these papillomaviruses. A central role in the function of this HPV-specific barrier is played by a complex of the zinc-transporting proteins EVER1, EVER2, and ZnT-1, which maintain cellular zinc homeostasis. Apparently, the deregulation of the cellular zinc balance emerges as an important step in the life cycles not only of cutaneous but also of genital HPVs, although the latter viruses have developed a mechanism by which they can break the barrier and impose a zinc imbalance. Herein, we present a previously unpublished list of the cellular partners of EVER proteins, which points to future directions concerning investigations of the mechanisms of action of the EVER/ZnT-1 complex. We also present a general overview of the pathogenesis of HPV infections, taking into account the latest discoveries regarding the role of cellular zinc homeostasis in the HPV life cycle. We propose a potential model for the mechanism of function of the anti-HPV barrier.