Human papillomavirus (HPV) E7 induces prolonged G2 following S phase reentry in differentiated human keratinocytes

The human papillomavirus late life cycle and links to keratinocyte differentiation

Regulation of human papillomavirus (HPV) gene expression is tightly linked to differentiation of the keratinocytes the virus infects. HPV late gene expression is confined to the cells in the upper layers of the epithelium where the virus capsid proteins are synthesized. As these proteins are highly immunogenic, and the upper epithelium is an immune-privileged site, this spatial restriction aids immune evasion. Many decades of work have contributed to the current understanding of how this restriction occurs at a molecular level. This review will examine what is known about late gene expression in HPV-infected lesions and will dissect the intricacies of late gene regulation. Future directions for novel antiviral approaches will be highlighted.

IDR-targeting compounds suppress HPV genome replication via disruption of phospho-BRD4 association with DNA damage response factors

Compounds binding to the bromodomains of bromodomain and extra-terminal (BET) family proteins, particularly BRD4, are promising anticancer agents. Nevertheless, side effects and drug resistance pose significant obstacles in BET-based therapeutics development. Using high-throughput screening of a 200,000-compound library, we identified small molecules targeting a phosphorylated intrinsically disordered region (IDR) of BRD4 that inhibit phospho-BRD4 (pBRD4)-dependent human papillomavirus (HPV) genome replication in HPV-containing keratinocytes. Proteomic profiling identified two DNA damage response factors-53BP1 and BARD1-crucial for differentiation-associated HPV genome amplification. pBRD4-mediated recruitment of 53BP1 and BARD1 to the HPV origin of replication occurs in a spatiotemporal and BRD4 long (BRD4-L) and short (BRD4-S) isoform-specific manner. This recruitment is disrupted by phospho-IDR-targeting compounds with little perturbation of the global transcriptome and BRD4 chromatin landscape. The discovery of these protein-protein interaction inhibitors (PPIi) not only demonstrates the feasibility of developing PPIi against phospho-IDRs but also uncovers antiviral agents targeting an epigenetic regulator essential for virus-host interaction and cancer development.

Interplay between the DNA damage response and the life cycle of DNA tumor viruses

Approximately 20 % of human cancers are associated with virus infection. DNA tumor viruses can induce tumor formation in host cells by disrupting the cell's DNA replication and repair mechanisms. Specifically, these viruses interfere with the host cell's DNA damage response (DDR), which is a complex network of signaling pathways that is essential for maintaining the integrity of the genome. DNA tumor viruses can disrupt these pathways by expressing oncoproteins that mimic or inhibit various DDR components, thereby promoting genomic instability and tumorigenesis. Recent studies have highlighted the molecular mechanisms by which DNA tumor viruses interact with DDR components, as well as the ways in which these interactions contribute to viral replication and tumorigenesis. Understanding the interplay between DNA tumor viruses and the DDR pathway is critical for developing effective strategies to prevent and treat virally associated cancers. In this review, we discuss the current state of knowledge regarding the mechanisms by which human papillomavirus (HPV), merkel cell polyomavirus (MCPyV), Kaposi's sarcoma-associated herpesvirus (KSHV), and Epstein-Barr virus (EBV) interfere with DDR pathways to facilitate their respective life cycles, and the consequences of such interference on genomic stability and cancer development.

The potential of PCNA inhibition as a therapeutic strategy in cervical cancer

Cervical cancers are the fourth most common and most deadly cancer in women worldwide. Despite being a tremendous public health burden, few novel approaches to improve care for these malignancies have been introduced. We discuss the potential for proliferating cell nuclear antigen (PCNA) inhibition to address this need as well as the advantages and disadvantages for compounds that can therapeutically inhibit PCNA with a specific focus on cervical cancer.

For Better or Worse: Modulation of the Host DNA Damage Response by Human Papillomavirus

High-risk human papillomaviruses (HPVs) are associated with several human cancers. HPVs are small, DNA viruses that rely on host cell machinery for viral replication. The HPV life cycle takes place in the stratified epithelium, which is composed of different cell states, including terminally differentiating cells that are no longer active in the cell cycle. HPVs have evolved mechanisms to persist and replicate in the stratified epithelium by hijacking and modulating cellular pathways, including the DNA damage response (DDR). HPVs activate and exploit DDR pathways to promote viral replication, which in turn increases the susceptibility of the host cell to genomic instability and carcinogenesis. Here, we review recent advances in our understanding of the regulation of the host cell DDR by high-risk HPVs during the viral life cycle and discuss the potential cellular consequences of modulating DDR pathways.

Mus musculus papillomavirus 1 E8^E2 represses expression of late protein E4 in basal-like keratinocytes via NCoR/SMRT-HDAC3 co-repressor complexes to enable wart formation in vivo

High-risk human papillomaviruses (PV) account for approximately 600,000 new cancers per year. The early protein E8^E2 is a conserved repressor of PV replication, whereas E4 is a late protein that arrests cells in G2 and collapses keratin filaments to facilitate virion release. While inactivation of the Mus musculus PV1 (MmuPV1) E8 start codon (E8-) increases viral gene expression, surprisingly, it prevents wart formation in FoxN1nu/nu mice. To understand this surprising phenotype, the impact of additional E8^E2 mutations was characterized in tissue culture and mice. MmuPV1 and HPV E8^E2 similarly interact with cellular NCoR/SMRT-HDAC3 co-repressor complexes. Disruption of the splice donor sequence used to generate the E8^E2 transcript or E8^E2 mutants (mt) with impaired binding to NCoR/SMRT-HDAC3 activates MmuPV1 transcription in murine keratinocytes. These MmuPV1 E8^E2 mt genomes also fail to induce warts in mice. The phenotype of E8^E2 mt genomes in undifferentiated cells resembles productive PV replication in differentiated keratinocytes. Consistent with this, E8^E2 mt genomes induced aberrant E4 expression in undifferentiated keratinocytes. In line with observations for HPV, MmuPV1 E4-positive cells displayed a shift to the G2 phase of the cell cycle. In summary, we propose that in order to enable both expansion of infected cells and wart formation in vivo, MmuPV1 E8^E2 inhibits E4 protein expression in the basal keratinocytes that would otherwise undergo E4-mediated cell cycle arrest. IMPORTANCE Human papillomaviruses (PVs) initiate productive replication, which is characterized by genome amplification and expression of E4 protein strictly within suprabasal, differentiated keratinocytes. Mus musculus PV1 mutants that disrupt splicing of the E8^E2 transcript or abolish the interaction of E8^E2 with cellular NCoR/SMRT-HDAC3 co-repressor complexes display increased gene expression in tissue culture but are unable to form warts in vivo. This confirms that the repressor activity of E8^E2 is required for tumor formation and genetically defines a conserved E8 interaction domain. E8^E2 prevents expression of E4 protein in basal-like, undifferentiated keratinocytes and thereby their arrest in G2 phase. Since binding of E8^E2 to NCoR/SMRT-HDAC3 co-repressor is required to enable expansion of infected cells in the basal layer and wart formation in vivo, this interaction represents a novel, conserved, and potentially druggable target.

Human Papillomavirus Genome Copy Number Is Maintained by S-Phase Amplification, Genome Loss to the Cytosol during Mitosis, and Degradation in G1 Phase

The current model of human papillomavirus (HPV) replication is comprised of three modes of replication. Following infectious delivery, the viral genome is amplified during the establishment phase to reach up to some hundred copies per cell. The HPV genome copy number remains constant during the maintenance stage. The differentiation of infected cells induces HPV genome amplification. Using highly sensitive in situ hybridization (DNAscope) and freshly HPV16-infected as well as established HPV16-positive cell lines, we observed that the viral genome is amplified in each S phase of undifferentiated keratinocytes cultured as monolayers. The nuclear viral genome copy number is reset to pre-S-phase levels during mitosis. The majority of the viral genome fails to tether to host chromosomes and is lost to the cytosol. Cytosolic viral genomes gradually decrease during cell cycle progression. The loss of cytosolic genomes is blocked in the presence of NH₄Cl or other drugs that interfere with lysosomal acidification, suggesting the involvement of autophagy in viral genome degradation. These observations were also made with HPV31 cell lines obtained from patient samples. Cytosolic viral genomes were not detected in UMSCC47 cells carrying integrated HPV16 DNA. Analyses of organotypic raft cultures derived from keratinocytes harboring episomal HPV16 revealed the presence of cytosolic viral genomes as well. We conclude that HPV maintains viral genome copy numbers by balancing viral genome amplification during S phase with the loss of viral genomes to the cytosol during mitosis. It seems plausible that restrictions to viral genome tethering to mitotic chromosomes reset genome copy numbers in each cell cycle. IMPORTANCE HPV genome maintenance is currently thought to be achieved by regulating the expression and activity of the viral replication factors E1 and E2. In addition, the E8^E2 repressor has been shown to be important for restricting genome copy numbers by competing with E1 and E2 for binding to the viral origin of replication and by recruiting repressor complexes. Here, we demonstrate that the HPV genome is amplified in each S phase. The nuclear genome copy number is reset during mitosis by a failure of the majority of the genomes to tether to mitotic chromosomes. Rather, HPV genomes accumulate in the cytoplasm of freshly divided cells. Cytosolic viral DNA is degraded in G₁ in a lysosome-dependent manner, contributing to the genome copy reset. Our data imply that the mode of replication during establishment and maintenance is the same and further suggest that restrictions to genome tethering significantly contribute to viral genome maintenance.

Regulation of the Innate Immune Response during the Human Papillomavirus Life Cycle

High-risk human papillomaviruses (HR HPVs) are associated with multiple human cancers and comprise 5% of the human cancer burden. Although most infections are transient, persistent infections are a major risk factor for cancer development. The life cycle of HPV is intimately linked to epithelial differentiation. HPVs establish infection at a low copy number in the proliferating basal keratinocytes of the stratified epithelium. In contrast, the productive phase of the viral life cycle is activated upon epithelial differentiation, resulting in viral genome amplification, high levels of late gene expression, and the assembly of virions that are shed from the epithelial surface. Avoiding activation of an innate immune response during the course of infection plays a key role in promoting viral persistence as well as completion of the viral life cycle in differentiating epithelial cells. This review highlights the recent advances in our understanding of how HPVs manipulate the host cell environment, often in a type-specific manner, to suppress activation of an innate immune response to establish conditions supportive of viral replication.

p16 Represses DNA Damage Repair via a Novel Ubiquitin-Dependent Signaling Cascade

Squamous cell carcinoma driven by human papillomavirus (HPV) is more sensitive to DNA-damaging therapies than its HPV-negative counterpart. Here, we show that p16, the clinically used surrogate for HPV positivity, renders cells more sensitive to radiotherapy via a ubiquitin-dependent signaling pathway, linking high levels of this protein to increased activity of the transcription factor SP1, increased HUWE1 transcription, and degradation of ubiquitin-specific protease 7 (USP7) and TRIP12. Activation of this pathway in HPV-positive disease led to decreased homologous recombination and improved response to radiotherapy, a phenomenon that can be recapitulated in HPV-negative disease using USP7 inhibitors in clinical development. This p16-driven axis induced sensitivity to PARP inhibition and potentially leads to "BRCAness" in head and neck squamous cell carcinoma (HNSCC) cells. Thus, these findings support a functional role for p16 in HPV-positive tumors in driving response to DNA damage, which can be exploited to improve outcomes in both patients with HPV-positive and HPV-negative HNSCC.

SIGNIFICANCE

In HPV-positive tumors, a previously undiscovered pathway directly links p16 to DNA damage repair and sensitivity to radiotherapy via a clinically relevant and pharmacologically targetable ubiquitin-mediated degradation pathway.

Jak HPV wysokiego ryzyka indukuje optymalne środowisko dla własnej replikacji w różnicującym się nabłonku

Wirusy brodawczaka ludzkiego (HPV) są często czynnikami wywołującymi niegroźne dla człowieka infekcje, ale przetrwałe zakażenie niektórymi typami HPV jest poważnym zagrożeniem dla zdrowia, ponieważ jest związane z wieloma nowotworami, w tym z rakiem szyjki macicy oraz rosnącą liczbą nowotworów głowy i szyi. Cykl replikacyjny HPV jest ściśle zależny od różnicowania komórek wielowarstwowego nabłonka, co oznacza, że genom wirusa musi być replikowany za pomocą różnych mechanizmów na różnych etapach różnicowania komórek. Ustanowienie infekcji i utrzymywanie genomu wirusa zachodzi w proliferujących komórkach nabłonka, gdzie dostępność czynników replikacji jest optymalna dla wirusa. Jednak produktywna faza cyklu rozwojowego wirusa, w tym produktywna replikacja, późna ekspresja genów i wytwarzanie wirionów, zachodzi w wyniku różnicowania się nabłonka w komórkach, które prawidłowo opuszczają cykl komórkowy. Wirus wykorzystuje wiele szlaków sygnalizacyjnych komórki, w tym odpowiedź na uszkodzenia DNA (DDR, DNA damage response) do realizacji produktywnej replikacji własnego genomu. Zrozumienie mechanizmów związanych z cyklem replikacyjnym HPV jest potrzebne do ustalenia właściwego podejścia terapeutycznego do zwalczania chorób powodowanych przez HPV.

Chlamydia and HPV induce centrosome amplification in the host cell through additive mechanisms

Based on epidemiology studies, Chlamydia trachomatis has been proposed as a co-factor for human papillomavirus (HPV) in the development of cervical cancer. These two intracellular pathogens have been independently reported to induce the production of extra centrosomes, or centrosome amplification, which is a hallmark of cancer cells. We developed a cell culture model to systematically measure the individual and combined effects of Chlamydia and HPV on the centrosome in the same host cell. We found that C. trachomatis caused centrosome amplification in a greater proportion of cells than HPV and that the effects of the two pathogens on the centrosome were additive. Furthermore, centrosome amplification induced by Chlamydia, but not by HPV, strongly correlated with multinucleation and required progression through mitosis. Our results suggest that C. trachomatis and HPV induce centrosome amplification through different mechanisms, with the chlamydial effect being largely due to a failure in cytokinesis that also results in multinucleation. Our findings provide support for C. trachomatis as a co-factor for HPV in carcinogenesis and offer mechanistic insights into how two infectious agents may cooperate to promote cancer. TAKE AWAYS: • Chlamydia and HPV induce centrosome amplification in an additive manner. • Chlamydia-induced centrosome amplification is linked to host cell multinucleation. • Chlamydia-induced centrosome amplification requires cell cycle progression. • Chlamydia and HPV cause centrosome amplification through different mechanisms. • This study supports Chlamydia as a co-factor for HPV in carcinogenesis.

The Not-So-Good, the Bad and the Ugly: HPV E5, E6 and E7 Oncoproteins in the Orchestration of Carcinogenesis

Infection with HPV starts with the access of the viral particles to basal cells in the epidermis, potentially via microtraumas to the skin. The basal cells are able to keep away these pathogens in normal circumstances through a robust immune response from the host, as HPV infections are, in general, cleared within 2 to 3 weeks. However, the rare instances of persistent infection and/or in cases where the host immune system is compromised are major risk factors for the development of lesions potentially leading to malignancy. Evolutionarily, obligatory pathogens such as HPVs would not be expected to risk exposing the host to lethal cancer, as this would entail challenging their own life cycle, but infection with these viruses is highly correlated with cancer and malignancy-as in cancer of the cervix, which is almost always associated with these viruses. Despite this key associative cause and the availability of very effective vaccines against these viruses, therapeutic interventions against HPV-induced cancers are still a challenge, indicating the need for focused translational research. In this review, we will consider the key roles that the viral proteins play in driving the host cells to carcinogenesis, mainly focusing on events orchestrated by early proteins E5, E6 and E7-the not-so-good, the bad and the ugly-and discuss and summarize the major events that lead to these viruses mechanistically corrupting cellular homeostasis, giving rise to cancer and malignancy.

The Catastrophic HPV/HIV Dual Viral Oncogenomics in Concert with Dysregulated Alternative Splicing in Cervical Cancer

Cervical cancer is a public health problem and has devastating effects in low-to-middle-income countries (LTMICs) such as the sub-Saharan African (SSA) countries. Infection by the human papillomavirus (HPV) is the main cause of cervical cancer. HIV positive women have higher HPV prevalence and cervical cancer incidence than their HIV negative counterparts do. Concurrent HPV/HIV infection is catastrophic, particularly to African women due to the high prevalence of HIV infections. Although various studies show a relationship between HPV, HIV and cervical cancer, there is still a gap in the knowledge concerning the precise nature of this tripartite association. Firstly, most studies show the relationship between HPV and cervical cancer at genomic and epigenetic levels, while the transcriptomic landscape of this relationship remains to be elucidated. Even though many studies have shown HPV/HIV dual viral pathogenesis, the dual molecular oncoviral effects on the development of cervical cancer remains largely uncertain. Furthermore, the effect of highly active antiretroviral therapy (HAART) on the cellular splicing machinery is unclear. Emerging evidence indicates the vital role played by host splicing events in both HPV and HIV infection in the development and progression to cervical cancer. Therefore, decoding the transcriptome landscape of this tripartite relationship holds promising therapeutic potential. This review will focus on the link between cellular splicing machinery, HPV, HIV infection and the aberrant alternative splicing events that take place in HIV/HPV-associated cervical cancer. Finally, we will investigate how these aberrant splicing events can be targeted for the development of new therapeutic strategies against HPV/HIV-associated cervical cancer.

Dangerous Liaisons: Long-Term Replication with an Extrachromosomal HPV Genome

Papillomaviruses cause persistent, and usually self-limiting, infections in the mucosal and cutaneous surfaces of the host epithelium. However, in some cases, infection with an oncogenic HPV can lead to cancer. The viral genome is a small, double-stranded circular DNA molecule that is assembled into nucleosomes at all stages of infection. The viral minichromosome replicates at a low copy number in the nucleus of persistently infected cells using the cellular replication machinery. When the infected cells differentiate, the virus hijacks the host DNA damage and repair pathways to replicate viral DNA to a high copy number to generate progeny virions. This strategy is highly effective and requires a close association between viral and host chromatin, as well as cellular processes associated with DNA replication, repair, and transcription. However, this association can lead to accidental integration of the viral genome into host DNA, and under certain circumstances integration can promote oncogenesis. Here we describe the fate of viral DNA at each stage of the viral life cycle and how this might facilitate accidental integration and subsequent carcinogenesis.

Oncogenic HPV promotes the expression of the long noncoding RNA lnc-FANCI-2 through E7 and YY1

Long noncoding RNAs (lncRNAs) play diverse roles in biological processes, but their expression profiles and functions in cervical carcinogenesis remain unknown. By RNA-sequencing (RNA-seq) analyses of 18 clinical specimens and selective validation by RT-qPCR analyses of 72 clinical samples, we provide evidence that, relative to normal cervical tissues, 194 lncRNAs are differentially regulated in high-risk (HR)-HPV infection along with cervical lesion progression. One such lncRNA, lnc-FANCI-2, is extensively characterized because it is expressed from a genomic locus adjacent to the FANCI gene encoding an important DNA repair factor. Both genes are up-regulated in HPV lesions and in in vitro model systems of HR-HPV18 infection. We observe a moderate reciprocal regulation of lnc-FANCI-2 and FANCI in cervical cancer CaSki cells. In these cells, lnc-FANCI-2 is transcribed from two alternative promoters, alternatively spliced, and polyadenylated at one of two alternative poly(A) sites. About 10 copies of lnc-FANCI-2 per cell are detected preferentially in the cytoplasm. Mechanistically, HR-HPVs, but not low-risk (LR)-HPV oncogenes induce lnc-FANCI-2 in primary and immortalized human keratinocytes. The induction is mediated primarily by E7, and to a lesser extent by E6, mostly independent of p53/E6AP and pRb/E2F. We show that YY1 interacts with an E7 CR3 core motif and transactivates the promoter of lnc-FANCI-2 by binding to two critical YY1-binding motifs. Moreover, HPV18 increases YY1 expression by reducing miR-29a, which targets the 3' untranslated region of YY1 mRNA. These data have provided insights into the mechanisms of how HR-HPV infections contribute to cervical carcinogenesis.

The human papillomavirus oncoproteins: a review of the host pathways targeted on the road to transformation

Persistent infection with high-risk human papillomaviruses (HR-HPVs) is the causal factor in over 99 % of cervical cancer cases, and a significant proportion of oropharyngeal and anogenital cancers. The key drivers of HPV-mediated transformation are the oncoproteins E5, E6 and E7. Together, they act to prolong cell-cycle progression, delay differentiation and inhibit apoptosis in the host keratinocyte cell in order to generate an environment permissive for viral replication. The oncoproteins also have key roles in mediating evasion of the host immune response, enabling infection to persist. Moreover, prolonged infection within the cellular environment established by the HR-HPV oncoproteins can lead to the acquisition of host genetic mutations, eventually culminating in transformation to malignancy. In this review, we outline the many ways in which the HR-HPV oncoproteins manipulate the host cellular environment, focusing on how these activities can contribute to carcinogenesis.

Persistent Human Papillomavirus Infection

Simple Summary

The success of HPV as an infectious agent lies not within its ability to cause disease, but rather in the adeptness of the virus to establish long-term persistent infection. The ability of HPV to replicate and maintain its genome in a stratified epithelium is contingent on the manipulation of many host pathways. HPVs must abrogate host anti-viral defense programs, perturb the balance of cellular proliferation and differentiation, and hijack DNA damage signaling and repair pathways to replicate viral DNA in a stratified epithelium. Together, these characteristics contribute to the ability of HPV to achieve long-term and persistent infection and to its evolutionary success as an infectious agent.

Abstract

Persistent infection with oncogenic human papillomavirus (HPV) types is responsible for ~5% of human cancers. The HPV infectious cycle can sustain long-term infection in stratified epithelia because viral DNA is maintained as low copy number extrachromosomal plasmids in the dividing basal cells of a lesion, while progeny viral genomes are amplified to large numbers in differentiated superficial cells. The viral E1 and E2 proteins initiate viral DNA replication and maintain and partition viral genomes, in concert with the cellular replication machinery. Additionally, the E5, E6, and E7 proteins are required to evade host immune responses and to produce a cellular environment that supports viral DNA replication. An unfortunate consequence of the manipulation of cellular proliferation and differentiation is that cells become at high risk for carcinogenesis.

Cyclin-dependent Kinases as Emerging Targets for Developing Novel Antiviral Therapeutics

Besides its prominent role in cell proliferation, cyclin-dependent kinases (CDKs) are key players in viral infections as both DNA and RNA viruses modify CDK function to favor viral replication. Recently, a number of specific pharmacological CDK inhibitors have been developed and approved for cancer treatment. The repurposing of these specific CDK inhibitors for the treatment of viral infections may represent a novel effective therapeutic strategy to combat old and emergent viruses. In this review, we describe the role, mechanisms of action, and potential of CDKs as antiviral drug targets. We also discuss the current clinical state of novel specific CDK inhibitors, focusing on their putative use as antivirals, especially against new emerging viruses.

Histone Modifications in Papillomavirus Virion Minichromosomes

An unusual feature of papillomaviruses is that their genomes are packaged into virions along with host histones. Viral minichromosomes were visualized as “beads on a string” by electron microscopy in the 1970s but, to date, little is known about the posttranslational modifications of these histones. To investigate this, we analyzed the histone modifications in HPV16/18 quasivirions, wart-derived bovine papillomavirus (BPV1), and wart-derived human papillomavirus type 1 (HPV1) using quantitative mass spectrometry. The chromatin from all three virion samples had abundant posttranslational modifications (acetylation, methylation, and phosphorylation). These histone modifications were verified by acid urea polyacrylamide electrophoresis and immunoblot analysis. Compared to matched host cell controls, the virion minichromosome was enriched in histone modifications associated with active chromatin and depleted for those commonly found in repressed chromatin. We propose that the viral minichromosome acquires specific histone modifications late in infection that are coupled to the mechanisms of viral replication, late gene expression, and encapsidation. We predict that, in turn, these same modifications benefit early stages of infection by helping to evade detection, promoting localization of the viral chromosome to beneficial regions of the nucleus, and promoting early transcription and replication.

Tampering of Viruses and Bacteria with Host DNA Repair: Implications for Cellular Transformation

A reduced ability to properly repair DNA is linked to a variety of human diseases, which in almost all cases is associated with an increased probability of the development of cellular transformation and cancer. DNA damage, that ultimately can lead to mutations and genomic instability, is due to many factors, such as oxidative stress, metabolic disorders, viral and microbial pathogens, excess cellular proliferation and chemical factors. In this review, we examine the evidence connecting DNA damage and the mechanisms that viruses and bacteria have evolved to hamper the pathways dedicated to maintaining the integrity of genetic information, thus affecting the ability of their hosts to repair the damage(s). Uncovering new links between these important aspects of cancer biology might lead to the development of new targeted therapies in DNA-repair deficient cancers and improving the efficacy of existing therapies. Here we provide a comprehensive summary detailing the major mechanisms that viruses and bacteria associated with cancer employ to interfere with mechanisms of DNA repair. Comparing these mechanisms could ultimately help provide a common framework to better understand how certain microorganisms are involved in cellular transformation.

WDHD1 facilitates G1 checkpoint abrogation in HPV E7 expressing cells by modulating GCN5

Background

Genomic instability is a hallmark of cancer. The G1 checkpoint allows cells to repair damaged DNA that may lead to genomic instability. The high-risk human papillomavirus (HPV) E7 gene can abrogate the G1 checkpoint, yet the mechanism is still not fully understood. Our recent study showed that WDHD1 (WD repeat and high mobility group [HMG]-box DNA-binding protein 1) plays a role in regulating G1 checkpoint of E7 expressing cells. In this study, we explored the mechanism by which WDHD1 regulates G1 checkpoint in HPV E7 expressing cells.

Methods

NIKS and RPE1 derived cell lines were used. Real-time PCR, Rescue experiment, FACS and BrdU labeling experiments were performed to examine role of GCN5 in G1 checkpoint abrogation in HPV-16 E7 expressing cells.

Results

In this study, we observed that WDHD1 facilitates G1 checkpoint abrogation by modulating GCN5 in HPV E7 expressing cells. Notably, depletion of WDHD1 caused G1 arrest while overexpression of GCN5 rescued the inhibitory effects of WDHD1 knockdown on G1/S progression. Furthermore, siWDHD1 significantly decreased cell cycle proliferation and DNA synthesis that was correlated with Akt phosphorylation (p-Akt), which was reversed by GCN5 overexpression in HPV E7 expressing cells.

Conclusions

In summary, our data identified a WDHD1/GCN5/Akt pathway leading to the abrogation of G1 checkpoint in the presence of damaged DNA, which may cause genomic instability and eventually HPV induced tumorigenesis.

Epigenetic Regulation of the Human Papillomavirus Life Cycle

Persistent infection with certain types of human papillomaviruses (HPVs), termed high risk, presents a public health burden due to their association with multiple human cancers, including cervical cancer and an increasing number of head and neck cancers. Despite the development of prophylactic vaccines, the incidence of HPV-associated cancers remains high. In addition, no vaccine has yet been licensed for therapeutic use against pre-existing HPV infections and HPV-associated diseases. Although persistent HPV infection is the major risk factor for cancer development, additional genetic and epigenetic alterations are required for progression to the malignant phenotype. Unlike genetic mutations, the reversibility of epigenetic modifications makes epigenetic regulators ideal therapeutic targets for cancer therapy. This review article will highlight the recent advances in the understanding of epigenetic modifications associated with HPV infections, with a particular focus on the role of these epigenetic changes during different stages of the HPV life cycle that are closely associated with activation of DNA damage response pathways.

Comprehensive gene and pathway analysis of cervical cancer progression

Cervical Cancer is one of the leading causes of cancer-associated mortality in women. The present study aimed to identify key genes and pathways involved in cervical cancer (CC) progression, via a comprehensive bioinformatics analysis. The GSE63514 dataset from the Gene Expression Omnibus database was analyzed for hub genes and cancer progression was divided into four phases (phases I-IV). Pathway enrichment, protein-protein interaction (PPI) and pathway crosstalk analyses were performed, to identify key genes and pathways using a criterion nodal degree ≥5. Gene pathway analysis was determined by mapping the key genes into the key pathways. Co-expression between key genes and their effect on overall survival (OS) time was assessed using The Cancer Genome Atlas database. A total of 3,446 differentially expressed genes with 107 hub genes were identified within the four phases. A total of 14 key genes with 11 key pathways were obtained, following extraction of ≥5 degree nodes from the PPI and pathway crosstalk networks. Gene pathway analysis revealed that CDK1 and CCNB1 regulated the cell cycle and were activated in phase I. Notably, the following terms, 'pathways in cancer', 'focal adhesion' and the 'PI3K-Akt signaling pathway' ranked the highest in phases II-IV. Furthermore, FN1, ITGB1 and MMP9 may be associated with metastasis of tumor cells. STAT1 was indicated to predominantly function at the phase IV via cancer-associated signaling pathways, including 'pathways in cancer' and 'Toll-like receptor signaling pathway'. Survival analysis revealed that high ITGB1 and FN1 expression levels resulted in significantly worse OS. CDK1 and CCNB1 were revealed to regulate proliferation and differentiation through the cell cycle and viral tumorigenesis, while FN1 and ITGB1, which may be developed as novel prognostic factors, were co-expressed to induce metastasis via cancer-associated signaling pathways, including PI3K-Art signaling pathway, and focal adhesion in CC; however, the underlying molecular mechanisms require further research.

Purinergic signaling and tumor microenvironment in cervical Cancer

Cervical cancer is the fourth most common type of cancer incidence in the world female population, and it has become a public health problem worldwide. Several factors are involved in this type of cancer, including intrinsic factors related to the inflammatory process, such as extracellular nucleotides and adenosine-components of the purinergic system. The present review focuses on the role of the purinergic system in cervical cancer, especially regarding the interaction of extracellular nucleotides with their respective receptors expressed in the tumor microenvironment of cervical cancer and their role in the host immune response. The high concentrations of extracellular nucleotides in the tumor microenvironment of cervical cancer interfere in the regulation, proliferation, differentiation, and apoptosis of cancer cells of the uterine cervix through different P1 and P2 receptor subtypes. Such diverse cellular processes that are mediated by adenosine triphosphate and adenosine across the tumor microenvironment and that also have effects on host immune defense will be reviewed here in detail.

Targeting DNA Damage Response as a Strategy to Treat HPV Infections

Mucosotropic human papillomaviruses (HPVs) cause prevalent anogenital infections, some of which can progress to cancers. It is imperative to identify efficacious drug candidates, as there are few therapeutic options. We have recapitulated a robust productive program of HPV-18 in organotypic raft cultures of primary human keratinocytes. The HPV E7 protein induces S phase reentry, along with DNA damage response (DDR) in differentiated cells to support viral DNA amplification. A number of small molecule inhibitors of DDR regulators are in clinical use or clinical trials to treat cancers. Here, we used our raft culture system to examine effects of inhibitors of ATR/Chk1 and ATM/Chk2 on HPV infection. The inhibitors impaired S-phase reentry and progression as well as HPV DNA amplification. The Chk1 inhibitor MK-8776 was most effective, reducing viral DNA amplification by 90-99% and caused DNA damage and apoptosis, preferentially in HPV infected cells. We found that this sensitivity was imparted by the E7 protein and report that MK-8776 also caused extensive cell death of cervical cancer cell lines. Furthermore, it sensitized the cells to cisplatin, commonly used to treat advanced cervical cancer. Based on these observations, the Chk1 inhibitors could be potential effective agents to be re-purposed to treat the spectrum of HPV infections in single or combination therapy.

Autocrine STAT3 activation in HPV positive cervical cancer through a virus-driven Rac1-NFκB-IL-6 signalling axis

Persistent human papillomavirus (HPV) infection is the leading cause of cervical cancer. Although the fundamental link between HPV infection and oncogenesis is established, the specific mechanisms of virus-mediated transformation are not fully understood. We previously demonstrated that the HPV encoded E6 protein increases the activity of the proto-oncogenic transcription factor STAT3 in primary human keratinocytes; however, the molecular basis for STAT3 activation in cervical cancer remains unclear. Here, we show that STAT3 phosphorylation in HPV positive cervical cancer cells is mediated primarily via autocrine activation by the pro-inflammatory cytokine Interleukin 6 (IL-6). Antibody-mediated blockade of IL-6 signalling in HPV positive cells inhibits STAT3 phosphorylation, whereas both recombinant IL-6 and conditioned media from HPV positive cells leads to increased STAT3 phosphorylation within HPV negative cervical cancer cells. Interestingly, we demonstrate that activation of the transcription factor NFκB, involving the small GTPase Rac1, is required for IL-6 production and subsequent STAT3 activation. Our data provides new insights into the molecular re-wiring of cancer cells by HPV E6. We reveal that activation of an IL-6 signalling axis drives the autocrine and paracrine phosphorylation of STAT3 within HPV positive cervical cancers cells and that activation of this pathway is essential for cervical cancer cell proliferation and survival. Greater understanding of this pathway provides a potential opportunity for the use of existing clinically approved drugs for the treatment of HPV-mediated cervical cancer.

HPV type 16 E6 and NFX1-123 augment JNK signaling to mediate keratinocyte differentiation and L1 expression

The HPV life cycle is differentiation-dependent, with cellular differentiation driving initiation of the late, productive stage of the viral life cycle. Here, we identify a role for the protein NFX1-123 in regulating keratinocyte differentiation and events of the late HPV life cycle. NFX1-123 itself increased with differentiation of epithelial cells. Greater NFX1-123 augmented differentiation marker expression and JNK phosphorylation in differentiating 16E6-expressing human foreskin keratinocytes (16E6 HFKs). This was associated with altered expression of MKK4 and MKK7, upstream kinase regulators of JNK phosphorylation. Modulating levels of NFX1-123 in HPV16-positive W12E cells recapitulated the effects on differentiation markers, JNK phosphorylation, and MKK4/7 seen in 16E6 HFKs. Crucially, levels of NFX1-123 also correlated with expression of L1, the capsid protein of HPV. Altogether, these studies define a role for NFX1-123 in mediating epithelial differentiation through the JNK signaling pathway, potentially linking expression of cellular genes and HPV genes during differentiation.

Impact of Replication Stress in Human Papillomavirus Pathogenesis

The inactivation of critical cell cycle checkpoints by the human papillomavirus (HPV) oncoprotein E7 results in replication stress (RS) that leads to genomic instability in premalignant lesions. Intriguingly, RS tolerance is achieved through several mechanisms, enabling HPV to exploit the cellular RS response for viral replication and to facilitate viral persistence in the presence of DNA damage. As such, inhibitors of the RS response pathway may provide a novel approach to target HPV-associated lesions and cancers.

Breaking Bad: How Viruses Subvert the Cell Cycle

Interactions between the host and viruses during the course of their co-evolution have not only shaped cellular function and the immune system, but also the counter measures employed by viruses. Relatively small genomes and high replication rates allow viruses to accumulate mutations and continuously present the host with new challenges. It is therefore, no surprise that they either escape detection or modulate host physiology, often by redirecting normal cellular pathways to their own advantage. Viruses utilize a diverse array of strategies and molecular targets to subvert host cellular processes, while evading detection. These include cell-cycle regulation, major histocompatibility complex-restricted antigen presentation, intracellular protein transport, apoptosis, cytokine-mediated signaling, and humoral immune responses. Moreover, viruses routinely manipulate the host cell cycle to create a favorable environment for replication, largely by deregulating cell cycle checkpoints. This review focuses on our current understanding of the molecular aspects of cell cycle regulation that are often targeted by viruses. Further study of their interactions should provide fundamental insights into cell cycle regulation and improve our ability to exploit these viruses.

Vorinostat, a pan-HDAC inhibitor, abrogates productive HPV-18 DNA amplification

Human papillomaviruses (HPVs) cause epithelial proliferative diseases. Persistent infection of the mucosal epithelia by the high-risk genotypes can progress to high-grade dysplasia and cancers. Viral transcription and protein activities are intimately linked to regulation by histone acetyltransferases and histone deacetylases (HDACs) that remodel chromatin and regulate gene expression. HDACs are also essential to remodel and repair replicating chromatin to enable the progression of replication forks. As such, Vorinostat (suberoylanilide hydroximic acid), and other pan-HDAC inhibitors, are used to treat lymphomas. Here, we investigated the effects of Vorinostat on productive infection of the high-risk HPV-18 in organotypic cultures of primary human keratinocytes. HPV DNA amplifies in the postmitotic, differentiated cells of squamous epithelia, in which the viral oncoproteins E7 and E6 establish a permissive milieu by destabilizing major tumor suppressors, the pRB family proteins and p53, respectively. We showed that Vorinostat significantly reduced these E6 and E7 activities, abrogated viral DNA amplification, and inhibited host DNA replication. The E7-induced DNA damage response, which is critical for both events, was also compromised. Consequently, Vorinostat exposure led to DNA damage and triggered apoptosis in HPV-infected, differentiated cells, whereas uninfected tissues were spared. Apoptosis was attributed to highly elevated proapoptotic Bim isoforms that are known to be repressed by EZH2 in a repressor complex containing HDACs. Two other HDAC inhibitors, Belinostat and Panobinostat, also inhibited viral DNA amplification and cause apoptosis. We suggest that HDAC inhibitors are promising therapeutic agents to treat benign HPV infections, abrogate progeny virus production, and hence interrupt transmission.

Human papillomavirus and genome instability: from productive infection to cancer

Infection with high oncogenic risk human papillomavirus types is the etiological factor of cervical cancer and a major cause of other epithelial malignancies, including vulvar, vaginal, anal, penile and head and neck carcinomas. These agents affect epithelial homeostasis through the expression of specific proteins that deregulate important cellular signaling pathways to achieve efficient viral replication. Among the major targets of viral proteins are components of the DNA damage detection and repair machinery. The activation of many of these cellular factors is critical to process viral genome replication intermediates and, consequently, to sustain faithful viral progeny production. In addition to the important role of cellular DNA repair machinery in the infective human papillomavirus cycle, alterations in the expression and activity of many of its components are observed in human papillomavirus-related tumors. Several studies from different laboratories have reported the impact of the expression of human papillomavirus oncogenes, mainly E6 and E7, on proteins in almost all the main cellular DNA repair mechanisms. This has direct consequences on cellular transformation since it causes the accumulation of point mutations, insertions and deletions of short nucleotide stretches, as well as numerical and structural chromosomal alterations characteristic of tumor cells. On the other hand, it is clear that human papillomavirus-transformed cells depend on the preservation of a basal cellular DNA repair activity level to maintain tumor cell viability. In this review, we summarize the data concerning the effect of human papillomavirus infection on DNA repair mechanisms. In addition, we discuss the potential of exploiting human papillomavirus-transformed cell dependency on DNA repair pathways as effective antitumoral therapies.

Alterations in the expression and activity of extracellular matrix components in HPV-associated infections and diseases

Infection with human papillomaviruses is associated with a series of benign and malignant hyperproliferative diseases that impose a heavy burden on human populations. A subgroup of mucosal human papillomavirus types are associated with the majority of cervical cancers and a relevant fraction of vulvar, vaginal, anal, penile and head and neck carcinomas. Human papillomaviruses mediate cell transformation by the expression of two pleiotropic oncoproteins that alter major cellular regulatory pathways. However, these viruses are not complete carcinogens, and further alterations within the infected cells and in their microenvironment are necessary for tumor establishment and progression. Alterations in components of the extracellular matrix for instance, matrix metalloproteinases and some of their regulators such as tissue inhibitors of metalloproteinases, have been consistently reported in human papillomaviruses-associated diseases. Matrix metalloproteinases function by remodeling the extracellular matrix and alterations in their expression levels and/or activity are associated with pathological processes and clinical variables including local tumor invasion, metastasis, tumor relapse and overall patient prognosis and survival. In this review we present a summarized discussion on the current data concerning the impact of human papillomavirus infection on the activity and expression of extracellular matrix components. We further comment on the possibility of targeting extracellular matrix molecules in experimental treatment protocols.

The DNA damage response activates HPV16 late gene expression at the level of RNA processing

We show that the alkylating cancer drug melphalan activated the DNA damage response and induced human papillomavirus type 16 (HPV16) late gene expression in an ATM- and Chk1/2-dependent manner. Activation of HPV16 late gene expression included inhibition of the HPV16 early polyadenylation signal that resulted in read-through into the late region of HPV16. This was followed by activation of the exclusively late, HPV16 splice sites SD3632 and SA5639 and production of spliced late L1 mRNAs. Altered HPV16 mRNA processing was paralleled by increased association of phosphorylated BRCA1, BARD1, BCLAF1 and TRAP150 with HPV16 DNA, and increased association of RNA processing factors U2AF65 and hnRNP C with HPV16 mRNAs. These RNA processing factors inhibited HPV16 early polyadenylation and enhanced HPV16 late mRNA splicing, thereby activating HPV16 late gene expression.

STAT3 activation by E6 is essential for the differentiation-dependent HPV18 life cycle

Human papillomaviruses (HPV) activate a number of host factors to control their differentiation-dependent life cycles. The transcription factor signal transducer and activator of transcription (STAT)-3 is important for cell cycle progression and cell survival in response to cytokines and growth factors. STAT3 requires phosphorylation on Ser727, in addition to phosphorylation on Tyr705 to be transcriptionally active. In this study, we show that STAT3 is essential for the HPV life cycle in undifferentiated and differentiated keratinocytes. Primary human keratinocytes containing high-risk HPV18 genomes display enhanced STAT3 phosphorylation compared to normal keratinocytes. Expression of the E6 oncoprotein is sufficient to induce the dual phosphorylation of STAT3 at Ser727 and Tyr705 by a mechanism requiring Janus kinases and members of the MAPK family. E6-mediated activation of STAT3 induces the transcription of STAT3 responsive genes including cyclin D1 and Bcl-xL. Silencing of STAT3 protein expression by siRNA or inhibition of STAT3 activation by small molecule inhibitors, or by expression of dominant negative STAT3 phosphorylation site mutants, results in blockade of cell cycle progression. Loss of active STAT3 impairs HPV gene expression and prevents episome maintenance in undifferentiated keratinocytes and upon differentiation, lack of active STAT3 abolishes virus genome amplification and late gene expression. Organotypic raft cultures of HPV18 containing keratinocytes expressing a phosphorylation site STAT3 mutant display a profound reduction in suprabasal hyperplasia, which correlates with a loss of cyclin B1 expression and increased differentiation. Finally, increased STAT3 expression and phosphorylation is observed in HPV positive cervical disease biopsies compared to control samples, highlighting a role for STAT3 activation in cervical carcinogenesis. In summary, our data provides evidence of a critical role for STAT3 in the HPV18 life cycle.

Human Papillomaviruses Preferentially Recruit DNA Repair Factors to Viral Genomes for Rapid Repair and Amplification

High-risk human papillomaviruses (HPVs) activate the ataxia telangiectasia mutated-dependent (ATM) DNA damage response as well as the ataxia telangiectasia mutated-dependent DNA-related (ATR) pathway in the absence of external DNA damaging agents for differentiation-dependent genome amplification. Through the use of comet assays and pulsed-field gel electrophoresis, our studies showed that these pathways are activated in response to DNA breaks induced by the viral proteins E6 and E7 alone and independently of viral replication. The majority of these virally induced DNA breaks are present in cellular DNAs and only minimally in HPV episomes. Treatment of HPV-positive cells with inhibitors of both ATM and ATR leads to the generation of DNA breaks and the fragmentation of viral episomes, indicating that DNA breaks are introduced into HPV genomes. These breaks, however, are rapidly repaired through the preferential recruitment of homologous recombination repair enzymes, such as RAD51 and BRCA1, to viral genomes at the expense of cellular DNAs. When HPV-positive cells are treated with hydroxyurea, this recruitment of RAD51 and BRCA1 to viral genomes is greatly enhanced with little recruitment to damaged cellular DNAs and with retention of the ability of viral genomes to amplify. Overall, our studies demonstrated that human papillomaviruses induce breaks into cellular and viral DNAs and that the preferential repair of these lesions in viral episomes leads to genome amplification.

High-risk human papillomaviruses (HPVs) are the etiologic agents of cervical cancer and are linked to the development of many other anogenital and oropharyngeal cancers. Replication of high-risk HPVs requires the activation of the ataxia telangiectasia-mutated (ATM) and ATM- and Rad3-related (ATR) DNA repair pathways. Our studies have shown that HPVs activate these pathways by inducing double-strand breaks primarily in cellular DNAs and minimally in viral genomes. Breaks are induced in viral genomes but are rapidly repaired through the preferential recruitment of homologous repair factors such as RAD51 and BRCA1 to HPV episomes. The preferential repair of breaks in viral genomes leads to amplification. Our study identified a novel mechanism by which human papillomaviruses manipulate DNA repair pathways to productively replicate viral genomes. The induction of genetic instability in cellular DNAs likely contributes to the generation of mutations that lead to the development of cancers.

How Human Papillomavirus Replication and Immune Evasion Strategies Take Advantage of the Host DNA Damage Repair Machinery

The DNA damage response (DDR) is a complex signalling network activated when DNA is altered by intrinsic or extrinsic agents. DDR plays important roles in genome stability and cell cycle regulation, as well as in tumour transformation. Viruses have evolved successful life cycle strategies in order to ensure a chronic persistence in the host, virtually avoiding systemic sequelae and death. This process promotes the periodic shedding of large amounts of infectious particles to maintain a virus reservoir in individual hosts, while allowing virus spreading within the community. To achieve such a successful lifestyle, the human papilloma virus (HPV) needs to escape the host defence systems. The key to understanding how this is achieved is in the virus replication process that provides by itself an evasion mechanism by inhibiting and delaying the host immune response against the viral infection. Numerous studies have demonstrated that HPV exploits both the ataxia-telangiectasia mutated (ATM) and ataxia-telangiectasia and rad3-related (ATR) DDR pathways to replicate its genome and maintain a persistent infection by downregulating the innate and cell-mediated immunity. This review outlines how HPV interacts with the ATM- and ATR-dependent DDR machinery during the viral life cycle to create an environment favourable to viral replication, and how the interaction with the signal transducers and activators of transcription (STAT) protein family and the deregulation of the Janus kinase (JAK)-STAT pathways may impact the expression of interferon-inducible genes and the innate immune responses.

Sp100 colocalizes with HPV replication foci and restricts the productive stage of the infectious cycle

We have shown previously that Sp100 (a component of the ND10 nuclear body) represses transcription, replication and establishment of incoming human papillomavirus (HPV) DNA in the early stages of infection. In this follow up study, we show that Sp100 does not substantially regulate viral infection in the maintenance phase, however at late stages of infection Sp100 interacts with amplifying viral genomes to repress viral processes. We find that Sp100 localizes to HPV16 replication foci generated in primary keratinocytes, to HPV31 replication foci that form in differentiated cells, and to HPV16 replication foci in CIN 1 cervical biopsies. To analyze this further, Sp100 was down regulated by siRNA treatment of differentiating HPV31 containing cells and levels of viral transcription and replication were assessed. This revealed that Sp100 represses viral transcription and replication in differentiated cells. Analysis of Sp100 binding to viral chromatin showed that Sp100 bound across the viral genome, and that binding increased at late stages of infection. Therefore, Sp100 represses the HPV life cycle at both early and late stages of infection.

DNA damage response is hijacked by human papillomaviruses to complete their life cycle

The DNA damage response (DDR) is activated when DNA is altered by intrinsic or extrinsic agents. This pathway is a complex signaling network and plays important roles in genome stability, tumor transformation, and cell cycle regulation. Human papillomaviruses (HPVs) are the main etiological agents of cervical cancer. Cervical cancer ranks as the fourth most common cancer among women and the second most frequent cause of cancer-related death worldwide. Over 200 types of HPVs have been identified and about one third of these infect the genital tract. The HPV life cycle is associated with epithelial differentiation. Recent studies have shown that HPVs deregulate the DDR to achieve a productive life cycle. In this review, I summarize current findings about how HPVs mediate the ataxia-telangiectasia mutated kinase (ATM) and the ATM-and RAD3-related kinase (ATR) DDRs, and focus on the roles that ATM and ATR signalings play in HPV viral replication. In addition, I demonstrate that the signal transducer and activator of transcription-5 (STAT)-5, an important immune regulator, can promote ATM and ATR activations through different mechanisms. These findings may provide novel opportunities for development of new therapeutic targets for HPV-related cancers.

Kinase Activity of Fibroblast Growth Factor Receptor 3 Regulates Activity of the Papillomavirus E2 Protein

The papillomavirus (PV) E2 protein is a DNA binding, protein interaction platform that recruits viral and host factors necessary for transcription and replication. We recently discovered phosphorylation of a tyrosine (Y102) in bovine PV (BPV) E2. To identify the responsible factor, we tested several candidate tyrosine kinases that are highly expressed in keratinocytes for binding to BPV-1 E2. Fibroblast growth factor receptor 3 (FGFR3) coimmunoprecipitated with the BPV-1 E2 protein, as did human papillomavirus 31 (HPV-31) E2, which also colocalized with FGFR3 within the nucleus. A constitutively active mutant form of FGFR3 decreased BPV-1 and HPV-31 transient replication although this result also occurred in a BPV-1 E2 mutant lacking a previously identified phosphorylation site of interest (Y102). Furthermore, FGFR3 depletion in cell lines that maintain HPV-31 episomes increased viral copy number. These results suggest that FGFR3 kinase activity may regulate the PV reproductive program through phosphorylation of the E2 protein although this is unlikely to occur through the Y102 residue of HPV E2.IMPORTANCE The papillomavirus (PV) is a double-stranded DNA tumor virus infecting cervix, mouth, and throat tissues. The viral protein E2 is responsible for the replication of the virus. Understanding the mechanisms of the replicative life cycle of the virus may bring to light direct targets and treatments against viral infection. We recently found that the fibroblast growth factor receptor 3 (FGFR3) interacts with and mediates PV E2 function through phosphorylation of the E2 protein. Our study suggests that the function of the E2 protein may be regulated through a direct FGFR3 target during the maintenance stage of the PV life cycle.

Mechanisms by which HPV Induces a Replication Competent Environment in Differentiating Keratinocytes

Human papillomaviruses (HPV) are the causative agents of cervical cancer and are also associated with other genital malignancies, as well as an increasing number of head and neck cancers. HPVs have evolved their life cycle to contend with the different cell states found in the stratified epithelium. Initial infection and viral genome maintenance occurs in the proliferating basal cells of the stratified epithelium, where cellular replication machinery is abundant. However, the productive phase of the viral life cycle, including productive replication, late gene expression and virion production, occurs upon epithelial differentiation, in cells that normally exit the cell cycle. This review outlines how HPV interfaces with specific cellular signaling pathways and factors to provide a replication-competent environment in differentiating cells.

The human papillomavirus replication cycle, and its links to cancer progression: a comprehensive review

HPVs (human papillomaviruses) infect epithelial cells and their replication cycle is intimately linked to epithelial differentiation. There are over 200 different HPV genotypes identified to date and each displays a strict tissue specificity for infection. HPV infection can result in a range of benign lesions, for example verrucas on the feet, common warts on the hands, or genital warts. HPV infects dividing basal epithelial cells where its dsDNA episomal genome enters the nuclei. Upon basal cell division, an infected daughter cell begins the process of keratinocyte differentiation that triggers a tightly orchestrated pattern of viral gene expression to accomplish a productive infection. A subset of mucosal-infective HPVs, the so-called ‘high risk’ (HR) HPVs, cause cervical disease, categorized as low or high grade. Most individuals will experience transient HR-HPV infection during their lifetime but these infections will not progress to clinically significant cervical disease or cancer because the immune system eventually recognizes and clears the virus. Cancer progression is due to persistent infection with an HR-HPV. HR-HPV infection is the cause of >99.7% cervical cancers in women, and a subset of oropharyngeal cancers, predominantly in men. HPV16 (HR-HPV genotype 16) is the most prevalent worldwide and the major cause of HPV-associated cancers. At the molecular level, cancer progression is due to increased expression of the viral oncoproteins E6 and E7, which activate the cell cycle, inhibit apoptosis, and allow accumulation of DNA damage. This review aims to describe the productive life cycle of HPV and discuss the roles of the viral proteins in HPV replication. Routes to viral persistence and cancer progression are also discussed.

Playing with fire: consequences of human papillomavirus DNA replication adjacent to genetically unstable regions of host chromatin

Papillomaviruses are small DNA viruses that replicate persistently in the stratified epithelial surfaces of the host. They have minimal coding capacity and must hijack many cellular processes to complete their life cycle. For example, viral genomes are tethered to host chromatin to ensure that they are effectively partitioned in dividing cells, and the host DNA damage and repair pathways are usurped to replicate viral DNA in differentiated cells. These processes result in the close juxtaposition of viral DNA with host DNA that is undergoing replication stress. This could explain the propensity of oncogenic human papillomaviruses (HPVs) to accidently integrate into common fragile sites in host DNA.

Mechanisms and strategies of papillomavirus replication

Animal and human papillomaviruses (HPVs) replicate persistently in specific types of stratified epithelia of their host. After the initial infection, the viral genome replicates at low levels in the dividing cells of the epithelium, and these cells form a reservoir of infection that can last for decades. When the infected cells differentiate, viral genomes replicate to high levels to form progeny virus that is released from the surface of the epithelium. This complex life cycle requires several different modes of viral DNA replication, but papillomaviruses are masters at hijacking key cellular processes to facilitate their own reproduction.

Identification of several high-risk HPV inhibitors and drug targets with a novel high-throughput screening assay

Human papillomaviruses (HPVs) are oncogenic viruses that cause numerous different cancers as well as benign lesions in the epithelia. To date, there is no effective cure for an ongoing HPV infection. Here, we describe the generation process of a platform for the development of anti-HPV drugs. This system consists of engineered full-length HPV genomes that express reporter genes for evaluation of the viral copy number in all three HPV replication stages. We demonstrate the usefulness of this system by conducting high-throughput screens to identify novel high-risk HPV-specific inhibitors. At least five of the inhibitors block the function of Tdp1 and PARP1, which have been identified as essential cellular proteins for HPV replication and promising candidates for the development of antivirals against HPV and possibly against HPV-related cancers.

Human papillomaviruses are causative agents of many different cancers; they are most commonly associated with cervical cancer which leads to about quarter of a million deaths each year. Regardless of extensive studies for decades there is no specific cure against HPV infection. During this research, we have engineered modified HPV marker genomes that express Renilla luciferase reporter gene which expression level correlates directly with viral genome copy number. We have used such modified HPV genome in high-throughput screening of NCI Diversity Set IV chemical library and have identified a number of novel high-risk HPV-specific chemical compounds and drug targets. Such Renilla-expressing marker genomes could be used in various cell systems suitable for HPV replication studies to conduct high-throughput screens and quantify viral genome copy number quickly and effectively.

Modulation of the DNA damage response during the life cycle of human papillomaviruses

Human papillomavirus (HPV) is the most common sexually transmitted viral infection. Infection with certain types of HPV pose a major public health risk as these types are associated with multiple human cancers, including cervical cancer, other anogenital malignancies and an increasing number of head and neck cancers. The HPV life cycle is closely tied to host cell differentiation with late viral events such as structural gene expression and viral genome amplification taking place in the upper layers of the stratified epithelium. The DNA damage response (DDR) is an elaborate signaling network of proteins that regulate the fidelity of replication by detecting, signaling and repairing DNA lesions. ATM and ATR are two kinases that are major regulators of DNA damage detection and repair. A multitude of studies indicate that activation of the ATM (Ataxia telangiectasia mutated) and ATR (Ataxia telangiectasia and Rad3-related) pathways are critical for HPV to productively replicate. This review outlines how HPV interfaces with the ATM- and ATR-dependent DNA damage responses throughout the viral life cycle to create an environment supportive of viral replication and how activation of these pathways could impact genomic stability.

HPV31 utilizes the ATR-Chk1 pathway to maintain elevated RRM2 levels and a replication-competent environment in differentiating Keratinocytes

Productive replication of human papillomaviruses (HPV) is restricted to the uppermost layers of the differentiating epithelia. How HPV ensures an adequate supply of cellular substrates for viral DNA synthesis in a differentiating environment is unclear. Here, we demonstrate that HPV31 positive cells exhibit increased dNTP pools and levels of RRM2, a component of the ribonucleotide reductase (RNR) complex, which is required for de novo synthesis of dNTPs. RRM2 depletion blocks productive replication, suggesting RRM2 provides dNTPs for viral DNA synthesis in differentiating cells. We demonstrate that HPV31 regulates RRM2 levels through expression of E7 and activation of the ATR-Chk1-E2F1 DNA damage response, which is essential to combat replication stress upon entry into S-phase, as well as for productive replication. Our findings suggest a novel way in which viral DNA synthesis is regulated through activation of ATR and Chk1 and highlight an intriguing new virus/host interaction utilized for viral replication.