Triggering of death receptor apoptotic signaling by human papillomavirus 16 E2 protein in cervical cancer cell lines is mediated by interaction with c-FLIP

Non-apoptotic cell death programs in cervical cancer with an emphasis on ferroptosis

BACKGROUND

Cervical cancer, a pernicious gynecological malignancy, causes the mortality of hundreds of thousands of females worldwide. Despite a considerable decline in mortality, the surging incidence rate among younger women has raised serious concerns. Immortality is the most important characteristic of tumor cells, hence the carcinogenesis of cervical cancer cells pivotally requires compromising with cell death mechanisms.

METHODS

The current study comprehensively reviewed the mechanisms of non-apoptotic cell death programs to provide possible disease management strategies.

RESULTS

Comprehensive evidence has stated that focusing on necroptosis, pyroptosis, and autophagy for disease management is associated with significant limitations such as insufficient understanding, contradictory functions, dependence on disease stage, and complexity of intracellular pathways. However, ferroptosis represents a predictable role in cervix carcinogenesis, and ferroptosis-related genes demonstrate a remarkable correlation with patient survival and clinical outcomes.

CONCLUSION

Ferroptosis may be an appropriate option for disease management strategies from predicting prognosis to treatment.

Apoptosis and Phagocytosis as Antiviral Mechanisms

Viruses are infectious entities that make use of the replication machinery of their hosts to produce more progenies, causing disease and sometimes death. To counter viral infection, metazoan hosts are equipped with various defense mechanisms, from the rapid-evoking innate immune responses to the most advanced adaptive immune responses. Previous research demonstrated that cells in fruit flies and mice infected with Drosophila C virus and influenza, respectively, undergo apoptosis, which triggers the engulfment of apoptotic virus-infected cells by phagocytes. This process involves the recognition of eat-me signals on the surface of virus-infected cells by receptors of specialized phagocytes, such as macrophages and neutrophils in mice and hemocytes in fruit flies, to facilitate the phagocytic elimination of virus-infected cells. Inhibition of phagocytosis led to severe pathologies and death in both species, indicating that apoptosis-dependent phagocytosis of virus-infected cells is a conserved antiviral mechanism in multicellular organisms. Indeed, our understanding of the mechanisms underlying apoptosis-dependent phagocytosis of virus-infected cells has shed a new perspective on how hosts defend themselves against viral infection. This chapter explores the mechanisms of this process and its potential for developing new treatments for viral diseases.

Viral-mediated activation and inhibition of programmed cell death

Viruses are ubiquitous intracellular genetic parasites that heavily rely on the infected cell to complete their replication life cycle. This dependency on the host machinery forces viruses to modulate a variety of cellular processes including cell survival and cell death. Viruses are known to activate and block almost all types of programmed cell death (PCD) known so far. Modulating PCD in infected hosts has a variety of direct and indirect effects on viral pathogenesis and antiviral immunity. The mechanisms leading to apoptosis following virus infection is widely studied, but several modalities of PCD, including necroptosis, pyroptosis, ferroptosis, and paraptosis, are relatively understudied. In this review, we cover the mechanisms by which viruses activate and inhibit PCDs and suggest perspectives on how these affect viral pathogenesis and immunity.

Regulation of innate immune responses by cell death-associated caspases during virus infection

Viruses are obligate intracellular pathogens that rely on cellular machinery for successful replication and dissemination. The host cells encode a number of different strategies to sense and restrict the invading viral pathogens. Caspase-mediated programmed cell death pathways that are triggered by virus infection, such as apoptosis and pyroptosis, provide a means for the infected cells to limit viral proliferation, leading to suicidal cell death (apoptosis) or lytic cell death and alerting uninfected cells to mount anti-viral responses (pyroptosis). However, some viruses can employ activated caspases to dampen the anti-viral responses and facilitate viral replication through cleavage of critical molecules of the innate immune pathways. The regulation of innate immune responses by caspase activation during virus infection has recently become an important topic. In this review, we briefly introduce the characteristics of different classes of caspases and the cell death pathways regulated by these caspases. We then describe how viruses trigger or dampen caspase activation during infection and how these activated caspases regulate three major innate immune response pathways of viral infections: the retinoic acid-inducible gene I-like receptor, toll-like receptor and cyclic GMP-AMP synthase-stimulator of interferon genes pathways.

Coxsackievirus A6 Induces Necroptosis for Viral Production

Hand, foot, and mouth disease (HFMD) is a febrile exanthematous disease with typical or atypical symptoms. Typical HFMD is usually caused by enterovirus 71 (EV71) or coxsackievirus A16, while atypical HFMD is usually caused by coxsackievirus A6 (CA6). In recent years, worldwide outbreaks of CA6-associated HFMD have dramatically increased, although the pathogenic mechanism of CA6 is still unclear. EV71 has been established to induce caspase-dependent apoptosis, but in this study, we demonstrate that CA6 infection promotes a distinct pathway of cell death that involves loss of cell membrane integrity. Necrostatin-1, an inhibitor of necroptosis, blocks the cell death induced by CA6 infection, but Z-DEVD-FMK, an inhibitor of caspase-3, has no effect on CA6-induced cell death. Furthermore, CA6 infection up-regulates the expression of the necroptosis signaling molecule RIPK3. Importantly, necrostatin-1 inhibits CA6 viral production, as assessed by its ability to inhibit levels of VP1 protein and genomic RNA and infectious particles. CA6-induced necroptosis is not dependent on the generation of reactive oxygen species; however, viral 3D protein can directly bind RIPK3, which is suggestive of a direct mechanism of necroptosis induction. Therefore, these results indicate that CA6 induces a mechanism of RIPK3-dependent necroptosis for viral production that is distinct from the mechanism of apoptosis induced by typical HFMD viruses.

New biomarkers of human papillomavirus infection in acute cervical intraepithelial neoplasia

Acute human papillomavirus (HPV) infection of the cervix (cervical intraepithelial neoplasia, CIN) is marked by high copy episomal viral DNA and L1/L2 capsid protein expression (productive infection) in the cells towards the surface that facilitate sexual viral transmission. Viral DNA is low copy and not associated with viral capsid protein expression in the less differentiated lower part of the CIN (nonproductive infection). The purpose of this study was to examine the host response in these two areas. Serial section and co-localization analyses demonstrated that in 29/33 (88%) of cases the NF-κB pathway was activated and localized to the suprabasal nonproductively infected cells in the CIN lesions. There was a concomitant increased expression of importin-β, exportin-5, Mcl1, p16, Ki67 and cFLIP in 32/33 (96%) of CIN lesions that likewise localized primarily to the nonproductively infected cells. Only Ki67 and exportin-5 were expressed, though much less so, in the adjacent, normal squamous epithelia. The viral proteins E1^E4 and L1 were localized to productively infected cells whereas E6/E7 protein/RNA was rarely present in early CIN. It is concluded that the host viral response to acute cervical HPV infection includes strong increased expression of proteins besides p16 and Ki67. These include importin-β, exportin-5, Mcl1, and cFLIP in cells with low copy and relatively quiescent viral DNA that, in turn, may serve as new biomarkers of this disease.

C/EBPβ enhances platinum resistance of ovarian cancer cells by reprogramming H3K79 methylation

Chemoresistance is a major unmet clinical obstacle in ovarian cancer treatment. Epigenetics plays a pivotal role in regulating the malignant phenotype, and has the potential in developing therapeutically valuable targets that improve the dismal outcome of this disease. Here we show that a series of transcription factors, including C/EBPβ, GCM1, and GATA1, could act as potential modulators of histone methylation in tumor cells. Of note, C/EBPβ, an independent prognostic factor for patients with ovarian cancer, mediates an important mechanism through which epigenetic enzyme modifies groups of functionally related genes in a context-dependent manner. By recruiting the methyltransferase DOT1L, C/EBPβ can maintain an open chromatin state by H3K79 methylation of multiple drug-resistance genes, thereby augmenting the chemoresistance of tumor cells. Therefore, we propose a new path against cancer epigenetics in which identifying and targeting the key regulators of epigenetics such as C/EBPβ may provide more precise therapeutic options in ovarian cancer.

In ovarian cancer, the mechanism of chemoresistance is a key question. Here, the authors demonstrate that C/EBPβ and DOT1L together increase methylation of H3K79, which upregulates expression of oncogenic genes and drives poor platinum response and poor survival in ovarian cancer.

Virus Infection and Death Receptor-Mediated Apoptosis

Virus infection can trigger extrinsic apoptosis. Cell-surface death receptors of the tumor necrosis factor family mediate this process. They either assist persistent viral infection or elicit the elimination of infected cells by the host. Death receptor-mediated apoptosis plays an important role in viral pathogenesis and the host antiviral response. Many viruses have acquired the capability to subvert death receptor-mediated apoptosis and evade the host immune response, mainly by virally encoded gene products that suppress death receptor-mediated apoptosis. In this review, we summarize the current information on virus infection and death receptor-mediated apoptosis, particularly focusing on the viral proteins that modulate death receptor-mediated apoptosis.

Insights into the mechanism of human papillomavirus E2-induced procaspase-8 activation and cell death

High-risk human papillomavirus (HR-HPV) E2 protein, the master regulator of viral life cycle, induces apoptosis of host cell that is independent of its virus-associated regulatory functions. E2 protein of HR-HPV18 has been found to be involved in novel FADD-independent activation of caspase-8, however, the molecular basis of this unique non-death-fold E2-mediated apoptosis is poorly understood. Here, with an interdisciplinary approach that involves in silico, mutational, biochemical and biophysical probes, we dissected and characterized the E2-procasapse-8 binding interface. Our data demonstrate direct non-homotypic interaction of HPV18 E2 transactivation domain (TAD) with α2/α5 helices of procaspase-8 death effector domain-B (DED-B). The observed interaction mimics the homotypic DED-DED complexes, wherein the conserved hydrophobic motif of procaspase-8 DED-B (F122/L123) occupies a groove between α2/α3 helices of E2 TAD. This interaction possibly drives DED oligomerization leading to caspase-8 activation and subsequent cell death. Furthermore, our data establish a model for E2-induced apoptosis in HR-HPV types and provide important clues for designing E2 analogs that might modulate procaspase-8 activation and hence apoptosis.

Cleaved c-FLIP mediates the antiviral effect of TNF-α against hepatitis B virus by dysregulating hepatocyte nuclear factors

BACKGROUND & AIMS

Cytokines are key molecules implicated in the defense against virus infection. Tumor necrosis factor-alpha (TNF-α) is well known to block the replication of hepatitis B virus (HBV). However, the molecular mechanism and the downstream effector molecules remain largely unknown.

METHODS

In this study, we investigated the antiviral effect and mechanism of p22-FLIP (FLICE-inhibitory protein) by ectopic expression in vitro and in vivo. In addition, to provide the biological relevance of our study, we examined that the p22-FLIP is involved in TNF-α-mediated suppression of HBV in primary human hepatocytes.

RESULTS

We found that p22-FLIP, a newly discovered c-FLIP cleavage product, inhibited HBV replication at the transcriptional level in both hepatoma cells and primary human hepatocytes, and that c-FLIP conversion to p22-FLIP was stimulated by the TNF-α/NF-κB pathway. p22-FLIP inhibited HBV replication through the upregulation of HNF3β but downregulation of HNF4α, thus inhibiting both HBV enhancer elements. Finally, p22-FLIP potently inhibited HBV DNA replication in a mouse model of HBV replication.

CONCLUSIONS

Taken together, these findings suggest that the anti-apoptotic p22-FLIP serves a novel function of inhibiting HBV transcription, and mediates the antiviral effect of TNF-α against HBV replication.

SIX1 promotes tumor lymphangiogenesis by coordinating TGFβ signals that increase expression of VEGF-C

Lymphatic vessels are one of the major routes for the dissemination of cancer cells. Malignant tumors release growth factors such as VEGF-C to induce lymphangiogenesis, thereby promoting lymph node metastasis. Here, we report that sine oculis homeobox homolog 1 (SIX1), expressed in tumor cells, can promote tumor lymphangiogenesis and lymph node metastasis by coordinating with TGFβ to increase the expression of VEGF-C. Lymphangiogenesis and lymph node metastasis in cervical cancer were closely correlated with higher expression of SIX1 in tumor cells. By enhancing VEGF-C expression in tumor cells, SIX1 could augment the promoting effect of tumor cells on the migration and tube formation of lymphatic endothelial cells (LEC) in vitro and lymphangiogenesis in vivo. SIX1 enhanced TGFβ-induced activation of SMAD2/3 and coordinated with the SMAD pathway to modulate VEGF-C expression. Together, SIX1 and TGFβ induced much higher expression of VEGF-C in tumor cells than each of them alone. Despite its effect in promoting VEGF-C expression, TGFβ could inhibit lymphangiogenesis by directly inhibiting tube formation by LECs. However, the increased production of VEGF-C not only directly promoted migration and tube formation of LECs but also thwarted the inhibitory effect of TGFβ on LECs. That is, tumor cells that expressed high levels of SIX1 could promote lymphangiogenesis and counteract the negative effects of TGFβ on lymphangiogenesis by increasing the expression of VEGF-C. These findings provide new insights into tumor lymphangiogenesis and the various roles of TGFβ signaling in tumor regulation. Our results also suggest that SIX1/TGFβ might be a potential therapeutic target for preventing lymph node metastasis of tumor.

Sine oculis homeobox homolog 1 promotes DNA replication and cell proliferation in cervical cancer

Malignant proliferation is the fundamental trait of tumor cells. The initiation of DNA replication represents a key process for cell proliferation, and has a marked impact on tumorigenesis and progression. Here we report that Sine oculis homeobox homolog 1 (SIX1) functions as a master regulator in DNA replication of cervical cancer cells. The expression of SIX1 was induced by the E7 oncoprotein of human papillomaviruses in cervical intraepithelial neoplasia and cervical cancer. The increase of SIX1 expression resulted in the upregulation of multiple genes related to the initiation of DNA replication, including the genes coding for the proteins in minichromosome maintenance complex (MCM2, MCM3, MCM6), DNA polymerase α-primase complex (POLA1, PRIM1, PRIM2), clamp loader (RFC3, RFC4, RFC5), DNA polymerase δ complex (POLD3) and DNA polymerase ε complex (POLE2). In line with this, the increase of SIX1 expression enhanced DNA synthesis, accelerated G1 to S phase progression, and promoted the proliferation of cervical cancer cells and the growth of cervical cancer. Consistently, knockdown of SIX1 could hamper DNA synthesis, slow down G1 to S phase progression, and suppress tumor cell proliferation and tumor growth. Importantly, SIX1 could more efficiently promote anchorage-independent cell growth. These results suggest that the increase of SIX1 expression could promote tumorigenesis, progression and invasive growth of cervical cancer by promoting DNA replication, and that targeting SIX1 may have significant therapeutic value in cervical cancer treatment.

HPV16 E2-mediated potentiation of NF-κB activation induced by TNF-α involves parallel activation of STAT3 with a reduction in E2-induced apoptosis

Human papilloma virus is associated with cervical and other tumors, and several cellular conditions also play an important role in carcinogenesis. Human papilloma virus (HPV)-infected cells exhibit activation of NF-κB and STAT3 (mediators of inflammation), but little is known about their regulation by HPV. This study attempts to understand the role of HPV16 E2, an important early protein of HPV16, in the regulation of NF-κB and STAT3 by reporter assays, quantitative reverse transcriptase-polymerase chain reaction, and immunoblotting. We demonstrate that E2 enhances NF-κB activation induced by TNF-α, a proinflammatory cytokine, in both non-tumor- and tumor-derived epithelial cell lines besides potentiating STAT3 transcriptional activity induced by TNF-α in HEK293 cells. E2 increases the expression of RelA and its transcriptional activation, and retention of E2 was observed in the nucleus with significant interaction with RelA (immunoprecipitation) upon TNF-α treatment. Transfection with shRNA-RelA or pretreatment with a STAT3 inhibitor had a negative effect on the ability of E2 to enhance TNF-α-induced NF-κB activation. Experiments with co-expression of a mutant of STAT3 with E2 also suggested that the activation of STAT3 is indispensible for TNF-α-induced NF-κB activation. Inhibition of STAT3 activation enhanced E2-induced apoptosis, whereas parallel activation of NF-κB and STAT3 by the combined action of E2 and TNF-α increased the expression of their common targets, cyclin-D1, c-Myc, survivin, and Bcl-2, leading to a decrease in E2-induced apoptosis (viability and cell cycle). Our results reveal novel mechanisms by which E2 may regulate NF-κB and STAT3 activation in the presence of TNF-α with implications on the survival of HPV-infected cells.

Targeting the Anti-Apoptotic Protein c-FLIP for Cancer Therapy

Cellular FLICE (FADD-like IL-1beta-converting enzyme)-inhibitory protein (c-FLIP) is a major resistance factor and critical anti-apoptotic regulator that inhibits tumor necrosis factor-alpha (TNF-alpha), Fas-L, and TNF-related apoptosis-inducing ligand (TRAIL)-induced apoptosis as well as chemotherapy-triggered apoptosis in malignant cells. c-FLIP is expressed as long (c-FLIP(L)), short (c-FLIP(S)), and c-FLIP(R) splice variants in human cells. c-FLIP binds to FADD and/or caspase-8 or -10 in a ligand-dependent and-independent fashion, which in turn prevents death-inducing signaling complex (DISC) formation and subsequent activation of the caspase cascade. Moreover, c-FLIP(L) and c-FLIP(S) are known to have multifunctional roles in various signaling pathways, as well as activating and/or upregulating several cytoprotective signaling molecules. Upregulation of c-FLIP has been found in various tumor types, and its downregulation has been shown to restore apoptosis triggered by cytokines and various chemotherapeutic agents. Hence, c-FLIP is an important target for cancer therapy. For example, small interfering RNAs (siRNAs) that specifically knockdown the expression of c-FLIP(L) in diverse human cancer cell lines augmented TRAIL-induced DISC recruitment and increased the efficacy of chemotherapeutic agents, thereby enhancing effector caspase stimulation and apoptosis. Moreover, small molecules causing degradation of c-FLIP as well as decreasing mRNA and protein levels of c-FLIP(L) and c-FLIP(S) splice variants have been found, and efforts are underway to develop other c-FLIP-targeted cancer therapies. This review focuses on (1) the functional role of c-FLIP splice variants in preventing apoptosis and inducing cytokine and drug resistance; (2) the molecular mechanisms that regulate c-FLIP expression; and (3) strategies to inhibit c-FLIP expression and function.

The papillomavirus E2 proteins

The papillomavirus E2 proteins are pivotal to the viral life cycle and have well characterized functions in transcriptional regulation, initiation of DNA replication and partitioning the viral genome. The E2 proteins also function in vegetative DNA replication, post-transcriptional processes and possibly packaging. This review describes structural and functional aspects of the E2 proteins and their binding sites on the viral genome. It is intended to be a reference guide to this viral protein.

Nip the HPV encoded evil in the cancer bud: HPV reshapes TRAILs and signaling landscapes

HPV encoded proteins can elicit ectopic protein–protein interactions that re-wire signaling pathways, in a mode that promotes malignancy. Moreover, accumulating data related to HPV is now providing compelling substantiation of a central role played by HPV in escaping immunosurveillance and impairment of apoptotic response. What emerges is an intricate network of Wnt, TGF, Notch signaling cascades that forms higher-order ligand–receptor complexes routing downstream signaling in HPV infected cells. These HPV infected cells are regulated both extracellularly by ligand receptor axis and intracellularly by HPV encoded proteins and impair TRAIL mediated apoptosis. We divide this review into different sections addressing how linear signaling pathways integrate to facilitate carcinogenesis and compounds that directly or indirectly reverse these aberrant interactions offer new possibilities for therapy in cancer. Although HPV encoded proteins mediated misrepresentation of pathways is difficult to target, improved drug-discovery platforms and new technologies have facilitated the discovery of agents that can target dysregulated pathways in HPV infected cervical cancer cells, thus setting the stage for preclinical models and clinical trials.

The modulation of apoptosis by oncogenic viruses

Transforming viruses can change a normal cell into a cancer cell during their normal life cycle. Persistent infections with these viruses have been recognized to cause some types of cancer. These viruses have been implicated in the modulation of various biological processes, such as proliferation, differentiation and apoptosis. The study of infections caused by oncogenic viruses had helped in our understanding of several mechanisms that regulate cell growth, as well as the molecular alterations leading to cancer. Therefore, transforming viruses provide models of study that have enabled the advances in cancer research. Viruses with transforming abilities, include different members of the Human Papillomavirus (HPV) family, Hepatitis C virus (HCV), Human T-cell Leukemia virus (HTLV-1), Epstein Barr virus (EBV) and Kaposi’s Sarcoma Herpesvirus (KSHV).

Apoptosis, or programmed cell death, is a tightly regulated process that plays an important role in development and homeostasis. Additionally, it functions as an antiviral defense mechanism. The deregulation of apoptosis has been implicated in the etiology of diverse diseases, including cancer. Oncogenic viruses employ different mechanisms to inhibit the apoptotic process, allowing the propagation of infected and damaged cells. During this process, some viral proteins are able to evade the immune system, while others can directly interact with the caspases involved in apoptotic signaling. In some instances, viral proteins can also promote apoptosis, which may be necessary for an accurate regulation of the initial stages of infection.

FLIP: molecular switch between apoptosis and necroptosis

Cancerous growth is one of the most difficult diseases to target as there is no one clear cause, and targeting only one pathway does not generally produce quantifiable improvement. For a truly effective cancer therapy, multiple pathways must be targeted at the same time. One way to do this is to find a gene that is associated with several pathways; this approach expands the possibilities for disease targeting and enables multiple points of attack rather than one fixed point, which does not allow treatment to evolve over time as cancer does. Inducing programmed cell death (PCD) is a promising method to prevent or inhibit the progression of tumor cells. Intricate cross talk among various programmed cell death pathways including cell death by apoptosis, necroptosis or autophagy plays a critical role in the regulation of PCD. In addition, the complex and overlapping patterns of signaling and lack of understanding of such networks between these pathways generate hurdles for developing effective therapeutic approaches. This review article focuses on targeting FLIP (Fas-associated death domain-like interleukin-1-converting enzyme-like inhibitory protein) signaling as a bridge between various PCD processes as an effective approach for cancer management.

Roles of c-FLIP in Apoptosis, Necroptosis, and Autophagy

Cellular FLICE (FADD-like IL-1β-converting enzyme)-inhibitory protein (c-FLIP) is a major antiapoptotic protein and an important cytokine and chemotherapy resistance factor that suppresses cytokine- and chemotherapy-induced apoptosis. c-FLIP is expressed as long (c-FLIPL), short (c-FLIPS), and c-FLIPR splice variants in human cells. c-FLIP binds to FADD and/or caspase-8 or -10 and TRAIL receptor 5 (DR5). This interaction in turn prevents Death-Inducing Signaling Complex (DISC) formation and subsequent activation of the caspase cascade. c-FLIPL and c-FLIPS are also known to have multifunctional roles in various signaling pathways, as well as activating and/or upregulating several cytoprotective and pro-survival signaling proteins including Akt, ERK, and NF-κB. In addition to its role in apoptosis, c-FLIP is involved in programmed necroptosis (necrosis) and autophagy. Necroptosis is regulated by the Ripoptosome, which is a signaling intracellular cell death platform complex. The Ripoptosome contains receptor-interacting protein-1/Receptor-Interacting Protein-3 (RIP1), caspase-8, caspase-10, FADD, and c-FLIP isoforms involved in switching apoptotic and necroptotic cell death. c-FLIP regulates the Ripoptosome; in addition to its role in apoptosis, it is therefore also involved in necrosis. c-FLIPL attenuates autophagy by direct acting on the autophagy machinery by competing with Atg3 binding to LC3, thereby decreasing LC3 processing and inhibiting autophagosome formation. Upregulation of c-FLIP has been found in various tumor types, and its silencing has been shown to restore apoptosis triggered by cytokines and various chemotherapeutic agents. Hence, c-FLIP is an important target for cancer therapy. This review focuses on (1) the anti-apoptotic role of c-FLIP splice variants in preventing apoptosis and inducing cytokine and chemotherapy drug resistance, as well as its roles in necrosis and autophagy, and (2) modulation of c-FLIP expression as a means to enhance apoptosis and modulate necrosis and autophagy in cancer cells.

The HPV E2-Host Protein-Protein Interactions: A Complex Hijacking of the Cellular Network

Over 100 genotypes of human papillomaviruses (HPVs) have been identified as being responsible for unapparent infections or for lesions ranging from benign skin or genital warts to cancer. The pathogenesis of HPV results from complex relationships between viral and host factors, driven in particular by the interplay between the host proteome and the early viral proteins. The E2 protein regulates the transcription, the replication as well as the mitotic segregation of the viral genome through the recruitment of host cell factors to the HPV regulatory region. It is thereby a pivotal factor for the productive viral life cycle and for viral persistence, a major risk factor for cancer development. In addition, the E2 proteins have been shown to engage numerous interactions through which they play important roles in modulating the host cell. Such E2 activities are probably contributing to create cell conditions appropriate for the successive stages of the viral life cycle, and some of these activities have been demonstrated only for the oncogenic high-risk HPV. The recent mapping of E2-host protein-protein interactions with 12 genotypes representative of HPV diversity has shed some light on the large complexity of the host cell hijacking and on its diversity according to viral genotypes. This article reviews the functions of E2 as they emerge from the E2/host proteome interplay, taking into account the large-scale comparative interactomic study.

DNA methylation profiling across the spectrum of HPV-associated anal squamous neoplasia

BACKGROUND

Changes in host tumor genome DNA methylation patterns are among the molecular alterations associated with HPV-related carcinogenesis. However, there is little known about the epigenetic changes associated specifically with the development of anal squamous cell cancer (SCC). We sought to characterize broad methylation profiles across the spectrum of anal squamous neoplasia.

METHODOLOGY/PRINCIPAL FINDINGS

Twenty-nine formalin-fixed paraffin embedded samples from 24 patients were evaluated and included adjacent histologically normal anal mucosa (NM; n = 3), SCC-in situ (SCC-IS; n = 11) and invasive SCC (n = 15). Thirteen women and 11 men with a median age of 44 years (range 26-81) were included in the study. Using the SFP(10) LiPA HPV-typing system, HPV was detected in at least one tissue from all patients with 93% (27/29) being positive for high-risk HPV types and 14 (93%) of 15 invasive SCC tissues testing positive for HPV 16. Bisulfite-modified DNA was interrogated for methylation at 1,505 CpG loci representing 807 genes using the Illumina GoldenGate Methylation Array. When comparing the progression from normal anal mucosa and SCC-IS to invasive SCC, 22 CpG loci representing 20 genes demonstrated significant differential methylation (p<0.01). The majority of differentially methylated gene targets occurred at or close to specific chromosomal locations such as previously described HPV methylation "hotspots" and viral integration sites.

CONCLUSIONS

We have identified a panel of differentially methlylated CpG loci across the spectrum of HPV-associated squamous neoplasia of the anus. To our knowledge, this is the first reported application of large-scale high throughput methylation analysis for the study of anal neoplasia. Our findings support further investigations into the role of host-genome methylation in HPV-associated anal carcinogenesis with implications towards enhanced diagnosis and screening strategies.