Involvement of Brd4 in different steps of the papillomavirus life cycle

Targeting BRD4: Potential therapeutic strategy for head and neck squamous cell carcinoma (Review)

As a member of BET (bromodomain and extra-terminal) protein family, BRD4 (bromodomain‑containing protein 4) is a chromatin‑associated protein that interacts with acetylated histones and actively recruits regulatory proteins, leading to the modulation of gene expression and chromatin remodeling. The cellular and epigenetic functions of BRD4 implicate normal development, fibrosis and inflammation. BRD4 has been suggested as a potential therapeutic target as it is often overexpressed and plays a critical role in regulating gene expression programs that drive tumor cell proliferation, survival, migration and drug resistance. To address the roles of BRD4 in cancer, several drugs that specifically target BRD4 have been developed. Inhibition of BRD4 has shown promising results in preclinical models, with several BRD4 inhibitors undergoing clinical trials for the treatment of various cancers. Head and neck squamous cell carcinoma (HNSCC), a heterogeneous group of cancers, remains a health challenge with a high incidence rate and poor prognosis. Conventional therapies for HNSCC often cause adverse effects to the patients. Targeting BRD4, therefore, represents a promising strategy to sensitize HNSCC to chemo‑ and radiotherapy allowing de‑intensification of the current therapeutic regime and subsequent reduced side effects. However, further studies are required to fully understand the underlying mechanisms of action of BRD4 in HNSCC in order to determine the optimal dosing and administration of BRD4‑targeted drugs for the treatment of patients with HNSCC.

Inhibition of CK2 Diminishes Fibrotic Scar Formation and Improves Outcomes After Ischemic Stroke via Reducing BRD4 Phosphorylation

Fibrotic scars play important roles in tissue reconstruction and functional recovery in the late stage of nervous system injury. However, the mechanisms underlying fibrotic scar formation and regulation remain unclear. Casein kinase II (CK2) is a protein kinase that regulates a variety of cellular functions through the phosphorylation of proteins, including bromodomain-containing protein 4 (BRD4). CK2 and BRD4 participate in fibrosis formation in a variety of tissues. However, whether CK2 affects fibrotic scar formation remains unclear, as do the mechanisms of signal regulation after cerebral ischemic injury. In this study, we assessed whether CK2 could modulate fibrotic scar formation after cerebral ischemic injury through BRD4. Primary meningeal fibroblasts were isolated from neonatal rats and treated with transforming growth factor-β1 (TGF-β1), SB431542 (a TGF-β1 receptor kinase inhibitor) or TBB (a highly potent CK2 inhibitor). Adult SD rats were intraperitoneally injected with TBB to inhibit CK2 after MCAO/R. We found that CK2 expression was increased in vitro in the TGF-β1-induced fibrosis model and in vivo in the MCAO/R injury model. The TGF-β1 receptor kinase inhibitor SB431542 decreased CK2 expression in fibroblasts. The CK2 inhibitor TBB reduced the increases in proliferation, migration and activation of fibroblasts caused by TGF-β1 in vitro, and it inhibited fibrotic scar formation, ameliorated histopathological damage, protected Nissl bodies, decreased infarct volume and alleviated neurological deficits after MCAO/R injury in vivo. Furthermore, CK2 inhibition decreased BRD4 phosphorylation both in vitro and in vivo. The findings of the present study suggested that CK2 may control BRD4 phosphorylation to regulate fibrotic scar formation, to affecting outcomes after ischemic stroke.

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.

Protein Kinase CK2α', More than a Backup of CK2α

The serine/threonine protein kinase CK2 is implicated in the regulation of fundamental processes in eukaryotic cells. CK2 consists of two catalytic α or α' isoforms and two regulatory CK2β subunits. These three proteins exist in a free form, bound to other cellular proteins, as tetrameric holoenzymes composed of CK2α2/β2, CK2αα'/β2, or CK2α'2/β2 as well as in higher molecular forms of the tetramers. The catalytic domains of CK2α and CK2α' share a 90% identity. As CK2α contains a unique C-terminal sequence. Both proteins function as protein kinases. These properties raised the question of whether both isoforms are just backups of each other or whether they are regulated differently and may then function in an isoform-specific manner. The present review provides observations that the regulation of both CK2α isoforms is partly different concerning the subcellular localization, post-translational modifications, and aggregation. Up to now, there are only a few isoform-specific cellular binding partners. The expression of both CK2α isoforms seems to vary in different cell lines, in tissues, in the cell cycle, and with differentiation. There are different reports about the expression and the functions of the CK2α isoforms in tumor cells and tissues. In many cases, a cell-type-specific expression and function is known, which raises the question about cell-specific regulators of both isoforms. Another future challenge is the identification or design of CK2α'-specific inhibitors.

ARV-825 Showed Antitumor Activity against BRD4-NUT Fusion Protein by Targeting the BRD4

Objective

The bromodomain-containing 4 (BRD4) is a member of the bromodomain and extra terminal domain (BET) family, which is an important epigenetic reader. It is currently a promising oncology target. In some tumors, BET bromodomain inhibitors have demonstrated promising results. Proteolysis-targeting methods (PROTAC), which rapidly and effectively degrade BRD4, have displayed considerable potential in the treatment of tumors in recent years. The purpose of this study is to examine the potential impact of BRD4 PROTAC compounds ARV-825 on oncogene BRD4-NUT fused protein in NUT carcinoma.

Methods

The effectiveness of ARV-825 was evaluated at the cellular level using the cell counting kit 8 test, wound healing, cell transfection, western blotting analysis, and RNA sequencing. The effectiveness of ARV-825 was also examined in vivo using a xenograft model.

Results

The BRD4-NUT fusion gene was overexpressed in 3T3 cells, and the pathogenic fusion gene was simulated. The results showed that the overexpression of BRD4-NUT could promote the proliferation and migration of 3T3 cells, but the expression of BRD4 protein was degraded after the addition of the novel cereblon-based PROTAC compound ARV-825 against BRD4, resulting in inhibition of BRD4-NUT 3T3 cell proliferation and migration. Further RNA-seq analysis showed that overexpression of BRD4-NUT was accompanied by increased expression of gene (e.g., Myc, E2F, TRAFs, Wnt, Gadd45g, and Sox6) with significantly enriched pathway (e.g., small cell lung cancer, NF-kappa B signaling pathway, and breast cancer), promoted cell cycle from G 1 phase to S phase, and increased cell proliferation and migration, activated the antiapoptosisi signal, led to abnormal cell growth, and ultimately led to tumorigenesis. The addition of ARV-825 effectively rescued this process and effectively inhibited cell vitality, proliferation, and migration. In vivo studies demonstrated that treatment with ARV-825 greatly suppressed tumor growth without causing harmful side effects and downregulated the BRD4-NUT expression level.

Conclusion

Through the induction of BRD4 protein degradation, ARV-825 can successfully limit BRD4-NUT 3T3 cell proliferation in vitro and in vivo. These findings suggested that the BRD4 inhibitor ARV-825 would be an effective therapeutic strategy for treating NUT carcinoma that with the genetic feature of BRD4-NUT fusion event.

BRD4 as a potential target for human papillomaviruses associated cancer

Around 99% of cervical cancer and 5%-10% of human cancer are associated with human papillomaviruses (HPV). Notably, the life-cycle of HPV begins by low-level infection of the basal cells of the stratified epithelium, where the viral genomes are replicated and passed on to the daughter proliferating basal cells. The production of new viral particles remains restricted to eventually differentiated cells. HPVs support their persistent infectious cycle by hijacking pivotal pathways and cellular processes. Bromodomain-containing protein 4 (BRD4) is one of the essential cellular factors involved in multiple stages of viral transcription and replication. In this review, we demonstrate the role of BRD4 in the multiple stages of HPV infectious cycle. Also, we provide an overview of the intense research about the cellular functions of BRD4, the mechanism of action of bromodomain and extra terminal inhibitors, and how it could lead to the development of antiviral/anticancer therapies.

Targeted inhibition of BET proteins in HPV-16 associated head and neck squamous cell carcinoma reveals heterogeneous transcription response

Integrated human papillomavirus (HPV-16) associated head and neck squamous cell carcinoma (HNSCC) tumors have worse survival outcomes compared to episomal HPV-16 HNSCC tumors. Therefore, there is a need to differentiate treatment for HPV-16 integrated HNSCC from other viral forms. We analyzed TCGA data and found that HPV+ HNSCC expressed higher transcript levels of the bromodomain and extra terminal domain (BET) family of transcriptional coregulators. However, the mechanism of BET protein-mediated transcription of viral-cellular genes in the integrated viral-HNSCC genomes needs to be better understood. We show that BET inhibition downregulates E6 significantly independent of the viral transcription factor, E2, and there was overall heterogeneity in the downregulation of viral transcription in response to the effects of BET inhibition across HPV-associated cell lines. Chemical BET inhibition was phenocopied with the knockdown of BRD4 and mirrored downregulation of viral E6 and E7 expression. Strikingly, there was heterogeneity in the reactivation of p53 levels despite E6 downregulation, while E7 downregulation did not alter Rb levels significantly. We identified that BET inhibition directly downregulated c-Myc and E2F expression and induced CDKN1A expression. Overall, our studies show that BET inhibition provokes a G1-cell cycle arrest with apoptotic activity and suggests that BET inhibition regulates both viral and cellular gene expression in HPV-associated HNSCC.

Protein-DNA Interactions Regulate Human Papillomavirus DNA Replication, Transcription, and Oncogenesis

Human papillomavirus (HPV) is a group of alpha papillomaviruses that cause various illnesses, including cancer. There are more than 160 types of HPV, with many being "high-risk" types that have been clinically linked to cervical and other types of cancer. "Low-risk" types of HPV cause less severe conditions, such as genital warts. Over the past few decades, numerous studies have shed light on how HPV induces carcinogenesis. The HPV genome is a circular double-stranded DNA molecule that is approximately 8 kilobases in size. Replication of this genome is strictly regulated and requires two virus-encoded proteins, E1 and E2. E1 is a DNA helicase that is necessary for replisome assembly and replication of the HPV genome. On the other hand, E2 is responsible for initiating DNA replication and regulating the transcription of HPV-encoded genes, most importantly the E6 and E7 oncogenes. This article explores the genetic characteristics of high-risk HPV types, the roles of HPV-encoded proteins in HPV DNA replication, the regulation of transcription of E6 and E7 oncogenes, and the development of oncogenesis.

Viral Hijacking of BET Proteins

Proteins of the bromodomain and exterminal domain (BET) family mediate critical host functions such as cell proliferation, transcriptional regulation, and the innate immune response, which makes them preferred targets for viruses. These multidomain proteins are best known as transcriptional effectors able to read acetylated histone and non-histone proteins through their tandem bromodomains. They also contain other short motif-binding domains such as the extraterminal domain, which recognizes transcriptional regulatory proteins. Here, we describe how different viruses have evolved to hijack or disrupt host BET protein function through direct interactions with BET family members to support their own propagation. The network of virus-BET interactions emerges as highly intricate, which may complicate the use of small-molecule BET inhibitors-currently in clinical development for the treatment of cancer and cardiovascular diseases-to treat viral infections.

hnRNP G/RBMX enhances HPV16 E2 mRNA splicing through a novel splicing enhancer and inhibits production of spliced E7 oncogene mRNAs

Human papillomavirus type 16 (HPV16) E2 is an essential HPV16 protein. We have investigated how HPV16 E2 expression is regulated and have identifed a splicing enhancer that is required for production of HPV16 E2 mRNAs. This uridine-less splicing enhancer sequence (ACGAGGACGAGGACAAGGA) contains 84% adenosine and guanosine and 16% cytosine and consists of three ‘AC(A/G)AGG’-repeats. Mutational inactivation of the splicing enhancer reduced splicing to E2-mRNA specific splice site SA2709 and resulted in increased levels of unspliced E1-encoding mRNAs. The splicing enhancer sequence interacted with cellular RNA binding protein hnRNP G that promoted splicing to SA2709 and enhanced E2 mRNA production. The splicing-enhancing function of hnRNP G mapped to amino acids 236–286 of hnRNP G that were also shown to interact with splicing factor U2AF65. The interactions between hnRNP G and HPV16 E2 mRNAs and U2AF65 increased in response to keratinocyte differentiation as well as by the induction of the DNA damage response (DDR). The DDR reduced sumoylation of hnRNP G and pharmacological inhibition of sumoylation enhanced HPV16 E2 mRNA splicing and interactions between hnRNP G and E2 mRNAs and U2AF65. Intriguingly, hnRNP G also promoted intron retention of the HPV16 E6 coding region thereby inhibiting production of spliced E7 oncogene mRNAs.

Identification of high protein kinase CK2α in HPV(+) oropharyngeal squamous cell carcinoma and correlation with clinical outcomes

Background

Oropharyngeal squamous cell carcinoma (OPSCC) incidence is rising worldwide, especially human papillomavirus (HPV)-associated disease. Historically, high levels of protein kinase CK2 were linked with poor outcomes in head and neck squamous cell carcinoma (HNSCC), without consideration of HPV status. This retrospective study examined tumor CK2α protein expression levels and related clinical outcomes in a cohort of Veteran OPSCC patient tumors which were determined to be predominantly HPV(+).

Methods

Patients at the Minneapolis VA Health Care System with newly diagnosed primary OPSCC from January 2005 to December 2015 were identified. A total of 119 OPSCC patient tumors were stained for CK2α, p16 and Ki-67 proteins and E6/E7 RNA. CK2α protein levels in tumors and correlations with HPV status and Ki-67 index were assessed. Overall survival (OS) analysis was performed stratified by CK2α protein score and separately by HPV status, followed by Cox regression controlling for smoking status. To strengthen the limited HPV(−) data, survival analysis for HPV(−) HNSCC patients in the publicly available The Cancer Genome Atlas (TCGA) PanCancer RNA-seq dataset was determined for CSNK2A1.

Results

The patients in the study population were all male and had a predominant history of tobacco and alcohol use. This cohort comprised 84 HPV(+) and 35 HPV(−) tumors. CK2α levels were higher in HPV(+) tumors compared to HPV(−) tumors. Higher CK2α scores positively correlated with higher Ki-67 index. OS improved with increasing CK2α score and separately OS was significantly better for those with HPV(+) as opposed to HPV(−) OPSCC. Both remained significant after controlling for smoking status. High CSNK2A1 mRNA levels from TCGA data associated with worse patient survival in HPV(−) HNSCC.

Conclusions

High CK2α protein levels are detected in HPV(+) OPSCC tumors and demonstrate an unexpected association with improved survival in a strongly HPV(+) OPSCC cohort. Worse survival outcomes for high CSNK2A1 mRNA levels in HPV(−) HNSCC are consistent with historical data. Given these surprising findings and the rising incidence of HPV(+) OPSCC, further study is needed to understand the biological roles of CK2 in HPV(+) and HPV(−) HNSCC and the potential utility for therapeutic targeting of CK2 in these two disease states.

The binding mechanism of NHWD-870 to bromodomain-containing protein 4 based on molecular dynamics simulations and free energy calculation

Bromodomain and extra-terminal (BET) proteins (BRD2, BRD3, BRD4, and BRDT) are epigenetic readers with tandem bromodomains.

Spatial and Functional Organization of Human Papillomavirus Replication Foci in the Productive Stage of Infection

The life cycle of human papillomavirus (HPV) depends on keratinocyte differentiation as the virus modulates and takes advantage of cellular pathways to replicate its genome and assemble viral particles in differentiated cells. Viral genomes are amplified in nuclear replication foci in differentiated keratinocytes, and DNA repair factors from the DNA damage response signaling pathway are recruited to replicate viral DNA. The HPV genome is associated with cellular histones at all stages of the infectious cycle, and here, we show that the histone variant macroH2A1 is bound to the HPV genome and enriched in viral replication foci in differentiated cells. macroH2A1 isoforms play important roles in cellular transcriptional repression, double-strand break repair, and replication stress. The viral E8^E2 protein also binds to the HPV genome and inhibits viral replication and gene expression by recruiting NCoR/SMRT complexes. We show here that E8^E2 and SMRT also localize within replication foci, though independently from macroH2A1. Conversely, transcription complexes containing RNA polymerase II and Brd4 are located on the surface of the foci. Foci generated with an HPV16 E8^E2 mutant genome are not enriched for SMRT or macroH2A1 but contain transcriptional complexes throughout the foci. We propose that both the cellular macroH2A1 protein and viral E8^E2 protein help to spatially separate replication and transcription activities within viral replication foci. IMPORTANCE Human papillomaviruses are small DNA viruses that cause chronic infection of cutaneous and mucosal epithelium. In some cases, persistent infection with HPV can result in cancer, and 5% of human cancers are the result of HPV infection. In differentiated cells, HPV amplifies viral DNA in nuclear replication factories and transcribes late mRNAs to produce capsid proteins. However, very little is known about the spatial organization of these activities in the nucleus. Here, we show that repressive viral and cellular factors localize within the foci to suppress viral transcription, while active transcription takes place on the surface. The cellular histone variant macroH2A1 is important for this spatial organization.

Phosphorylation-dependent BRD4 dimerization and implications for therapeutic inhibition of BET family proteins

Bromodomain-containing protein 4 (BRD4) is an epigenetic reader and oncology drug target that regulates gene transcription through binding to acetylated chromatin via bromodomains. Phosphorylation by casein kinase II (CK2) regulates BRD4 function, is necessary for active transcription and is involved in resistance to BRD4 drug inhibition in triple-negative breast cancer. Here, we provide the first biophysical analysis of BRD4 phospho-regulation. Using integrative structural biology, we show that phosphorylation by CK2 modulates the dimerization of human BRD4. We identify two conserved regions, a coiled-coil motif and the Basic-residue enriched Interaction Domain (BID), essential for the BRD4 structural rearrangement, which we term the phosphorylation-dependent dimerization domain (PDD). Finally, we demonstrate that bivalent inhibitors induce a conformational change within BRD4 dimers in vitro and in cancer cells. Our results enable the proposal of a model for BRD4 activation critical for the characterization of its protein-protein interaction network and for the development of more specific therapeutics.

BRD4 in physiology and pathology: ''BET'' on its partners

Bromodomain-containing 4 (BRD4), a member of Bromo and Extra-Terminal (BET) family, recognizes acetylated histones and is of importance in transcription, replication, and DNA repair. It also binds non-histone proteins, DNA and RNA, contributing to development, tissue growth, and various physiological processes. Additionally, BRD4 has been implicated in driving diverse diseases, ranging from cancer, viral infection, inflammation to neurological disorders. Inhibiting its functions with BET inhibitors (BETis) suppresses the progression of several types of cancer, creating an impetus for translating these chemicals to the clinic. The diverse roles of BRD4 are largely dependent on its interaction partners in different contexts. In this review we discuss the molecular mechanisms of BRD4 with its interacting partners in physiology and pathology. Current development of BETis is also summarized. Further understanding the functions of BRD4 and its partners will facilitate resolving the liabilities of present BETis and accelerate their clinical translation.

Relevance of BET Family Proteins in SARS-CoV-2 Infection

The recent pandemic we are experiencing caused by the coronavirus disease 2019 (COVID-19) has put the world's population on the rack, with more than 191 million cases and more than 4.1 million deaths confirmed to date. This disease is caused by a new type of coronavirus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). A massive proteomic analysis has revealed that one of the structural proteins of the virus, the E protein, interacts with BRD2 and BRD4 proteins of the Bromodomain and Extra Terminal domain (BET) family of proteins. BETs are essential to cell cycle progression, inflammation and immune response and have also been strongly associated with infection by different types of viruses. The fundamental role BET proteins play in transcription makes them appropriate targets for the propagation strategies of some viruses. Recognition of histone acetylation by BET bromodomains is essential for transcription control. The development of drugs mimicking acetyl groups, and thereby able to displace BET proteins from chromatin, has boosted interest on BETs as attractive targets for therapeutic intervention. The success of these drugs against a variety of diseases in cellular and animal models has been recently enlarged with promising results from SARS-CoV-2 infection studies.

BRD4S interacts with viral E2 protein to limit human papillomavirus late transcription

The E2 protein encoded by human papillomaviruses (HPV) is a sequence-specific DNA-binding protein that recruits viral and cellular proteins. Bromodomain-containing protein 4 (BRD4) is a highly conserved interactor for E2 proteins that has been linked to E2's functions as transcription modulator, activator of viral replication and segregation factor for viral genomes. In addition to BRD4, a short form of BRD4 (BRD4S) is expressed from the BRD4 gene which lacks the C-terminal domain of BRD4. E2 proteins interact with the C-terminal motif (CTM) of BRD4, but a recent study suggested that the phospho-dependent interaction domain (PDID) and the basic interaction domain (BID) in BRD4 also bind to E2. These domains are also present in BRD4S. We now find that HPV31 E2 interacts with the isolated PDID domain in living cells and also with BRD4S which is present in detectable amounts in HPV-positive cell lines and is recruited into HPV31 E1 and E2 induced replication foci. Overexpression and knockdown experiments surprisingly indicate that BRD4S inhibits activities of E2. In line with that, the specific knockdown of BRD4S in the HPV31-positive CIN612-9E cell line induces mainly late viral transcripts. This occurs only in undifferentiated but not differentiated cells in which the productive viral replication cycle is induced. These data suggest that the BRD4S-E2 interaction is important to prevent HPV late gene expression in undifferentiated keratinocytes which may contribute to immune evasion and HPV persistence.ImportanceHuman papillomaviruses (HPV) have coevolved with their host by using cellular factors like bromodomain-containing protein 4 (BRD4) to control viral processes such as genome maintenance, gene expression and replication. We here show that, in addition to the C-terminal motif in BRD4, the phospho-dependent interaction domain in BRD4 interacts with E2 proteins which enable the recruitment of BRD4S, the short isoform of BRD4, to E2. Knock-down and overexpression of BRD4S reveals that BRD4S is a negative regulator of E2 activities. Importantly, the knockdown of BRD4S induces mainly L1 transcripts in undifferentiated CIN612-9E cells, which maintain replicating HPV31 genomes. Our study reveals an inhibitory role of BRD4S on HPV transcription, which may serve as an immune escape mechanism by the suppression of L1 transcripts and thus contribute to the establishment of persistent HPV infections.

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.

The therapeutic effect of the BRD4-degrading PROTAC A1874 in human colon cancer cells

A1874 is a novel BRD4-degrading proteolysis targeting chimera (PROTAC). In primary colon cancer cells and established HCT116 cells, A1874 potently inhibited cell viability, proliferation, cell cycle progression, as well as cell migration and invasion. The BRD4-degrading PROTAC was able to induce caspase and apoptosis activation in colon cancer cells. Furthermore, A1874-induced degradation of BRD4 protein and downregulated BRD-dependent genes (c-Myc, Bcl-2, and cyclin D1) in colon cancer cells. Significantly, A1874-induced anti-colon cancer cell activity was more potent than the known BRD4 inhibitors (JQ1, CPI203, and I-BET151). In BRD4-knockout colon cancer cells A1874 remained cytotoxic, indicating the existence of BRD4-independent mechanisms. In addition to BRD4 degradation, A1874 cytotoxicity in colon cancer cells was also associated with p53 protein stabilization and reactive oxygen species production. Importantly, the antioxidant N-acetyl-cysteine and the p53 inhibitor pifithrin-α attenuated A1874-induced cell death and apoptosis in colon cancer cells. In vivo, A1874 oral administration potently inhibited colon cancer xenograft growth in severe combined immuno-deficient mice. BRD4 degradation and p53 protein elevation, as well as apoptosis induction and oxidative stress were detected in A1874-treated colon cancer tissues. Together, A1874 inhibits colon cancer cell growth through both BRD4-dependent and -independent mechanisms.

Phosphorylation of the Human Papillomavirus E2 Protein at Tyrosine 138 Regulates Episomal Replication

The papillomavirus (PV) E2 protein is a critical regulator of viral transcription and genome replication. We previously reported that tyrosine (Y) 138 of HPV-31 E2 is phosphorylated by the fibroblast growth factor receptor 3 (FGFR3) kinase. In this study, we generated quasiviruses containing G418-selectable HPV-31 genomes with phosphodeficient phenylalanine mutant E2 Y138F and phosphomimetic glutamic acid mutant Y138E. We observed significantly fewer early viral transcripts immediately after infection with these Y138 mutant genomes even though E2 occupancy at the viral origin was equivalent to that of wild-type E2. Keratinocytes infected with Y138F quasiviruses formed stable colonies, and the genomes were maintained as episomes, while those infected with Y138E quasiviruses did not. We previously reported that the HPV-31 E2 Y138 mutation to glutamic acid did not bind to the Brd4 C-terminal motif (CTM). Here, we demonstrate that HPV-16 E2 Y138E bound to full-length Brd4 but not to the Brd4 CTM. We conclude that association of E2 with the Brd4 CTM is necessary for viral genome replication and suggest that this interaction can be regulated by phosphorylation of E2 Y138.IMPORTANCE Papillomavirus (PV) is a double-stranded DNA tumor virus infecting the cutaneous and mucosal epithelium. The PV E2 protein associates with a number of cellular factors to mediate replication of the HPV genome. Fibroblast growth factor receptor 3 (FGFR3) regulates HPV replication through phosphorylation of tyrosine 138 in the HPV E2 protein. Employing a quasivirus infection model and selection for G418 resistant genomes, we demonstrated that Y138 is a critical residue for Brd4 association and that inability to complex with Brd4 does not support episomal replication.

ARV-825-induced BRD4 protein degradation as a therapy for thyroid carcinoma

Bromodomain-containing protein 4 (BRD4) is overexpressed in thyroid carcinoma, represents as an important therapeutic target. ARV-825 is a novel cereblon-based PROTAC (Proteolysis Targeting Chsimera) compound. It can induce fast and sustained BRD4 protein degradation. Its potential effect in human thyroid carcinoma cells was studied here. In TPC-1 cells and primary human thyroid carcinoma cells, ARV-825 potently inhibited cell viability, proliferation and migration. Furthermore, ARV-825 induced robust apoptosis activation in the thyroid carcinoma cells. ARV-825 induced BRD4 protein degradation and downregulation of its targets, including c-Myc, Bcl-xL and cyclin D1 in thyroid carcinoma cells. It was significantly more potent in inhibiting thyroid carcinoma cells than the known small molecule BRD4 inhibitors. In vivo studies demonstrated that ARV-825 oral administration potently suppressed TPC-1 xenograft tumor growth in severe combined immunodeficient mice. BRD4 protein degradation as well as c-Myc, Bcl-xL and cyclin D1 downregulation were detected in ARV-825-treated TPC-1 tumor tissues. Taken together, ARV-825 induces BRD4 protein degradation and inhibits thyroid carcinoma cell growth in vitro and in vivo.

The homeobox transcription factor HOXC13 upregulates human papillomavirus E1 gene expression and contributes to viral genome maintenance

Human papillomavirus (HPV) infects the basal cells of epithelia and maintains its genome stably as episomes. However, the mechanisms of viral genome maintenance are not fully understood. Here, using normal human immortalized keratinocytes (NIKS), we identified the homeobox transcription factor HOXC13 as a critical host factor for retaining the copy number of HPV genomes in the cell. HOXC13 knockdown in NIKS significantly decreased mRNA levels of the E1 gene, which encodes a DNA helicase required for HPV genome replication, accompanied by a reduction of the viral genome copy number. Chromatin immunoprecipitation assays revealed HOXC13 binding to the long control region that regulates E1 expression. These results indicate that HOXC13 plays invaluable roles in maintaining HPV persistent infection through E1 gene upregulation.

Control of Viral Latency by Episome Maintenance Proteins

The human DNA tumor viruses Epstein-Barr virus (EBV), Kaposi's sarcoma-associated herpesvirus (KSHV), and human papillomavirus (HPV) share the common property of persisting as multicopy episomes in the nuclei of rapidly dividing host cells. These episomes form the molecular basis for viral latency and are etiologically linked to virus-associated cancers. Episome maintenance requires epigenetic programming to ensure the proper control of viral gene expression, DNA replication, and genome copy number. For these viruses, episome maintenance requires a dedicated virus-encoded episome maintenance protein (EMP), namely LANA (KSHV), EBNA1 (EBV), and E2 (HPV). Here, we review common features of these viral EMPs and discuss recent advances in understanding how they contribute to the epigenetic control of viral episome maintenance during latency.

Activity of CK2α protein kinase is required for efficient replication of some HPV types

Inhibition of human papillomavirus (HPV) replication is a promising therapeutic approach for intervening with HPV-related pathologies. Primary targets for interference are two viral proteins, E1 and E2, which are required for HPV replication. Both E1 and E2 are phosphoproteins; thus, the protein kinases that phosphorylate them might represent secondary targets to achieve inhibition of HPV replication. In the present study, we show that CX4945, an ATP-competitive small molecule inhibitor of casein kinase 2 (CK2) catalytic activity, suppresses replication of different HPV types, including novel HPV5NLuc, HPV11NLuc and HPV18NLuc marker genomes, but enhances the replication of HPV16 and HPV31. We further corroborate our findings using short interfering RNA (siRNA)-mediated knockdown of CK2 α and α' subunits in U2OS and CIN612 cells; we show that while both subunits are expressed in these cell lines, CK2α is required for HPV replication, but CK2α' is not. Furthermore, we demonstrate that CK2α acts in a kinase activity-dependent manner and regulates the stability and nuclear retention of endogenous E1 proteins of HPV11 and HPV18. This unique feature of CK2α makes it an attractive target for developing antiviral agents.

Phosphorylation of a Conserved Tyrosine in the Papillomavirus E2 Protein Regulates Brd4 Binding and Viral Replication

The papillomavirus (PV) E2 protein coordinates viral transcription and genome replication. Following a strategy to identify amino acids in E2 that are posttranslationally modified, we reported that tyrosine kinase fibroblast growth factor receptor 3 (FGFR3) complexes with and phosphorylates E2, which inhibits viral DNA replication. Here, we present several lines of evidence indicating that tyrosine (Y) 138 of HPV-31 E2 is a substrate of FGFR3. The active form of FGFR3 bound to and phosphorylated the region of amino acids (aa) 107 to 175 in HPV-31 E2. The E2 phenylalanine (F) mutant Y138F displayed reduced FGFR3-induced phosphotyrosine. A constitutive kinase-active FGFR3 inhibited wild-type (WT) E2-induced E1-dependent DNA replication, while the 138F mutant retained activity. The tyrosine to glutamic acid (E) mutant Y138E, which can mimic phosphotyrosine, failed to induce transient DNA replication, although it maintained the ability to bind and localize the viral DNA helicase E1 to the viral origin. The bromodomain-containing protein 4 (Brd4) binds to E2 and is necessary for initiation of viral DNA synthesis. Interestingly, the Y138E protein coimmunoprecipitated with full-length Brd4 but was defective for association with its C-terminal domain (CTD). These results imply that the activity of the FGFR3 kinase in the infected epithelial cell restricts the HPV replication program through phosphorylation of E2 at Y138, which interferes with E2 binding to the Brd4 CTD, and that this interaction is required for initiation of viral DNA synthesis.IMPORTANCE Human papillomaviruses (HPVs) are highly infectious pathogens that commonly infect the oropharynx and uterine cervix. The idea that posttranslational modifications of viral proteins coordinates viral genome replication is less explored. We recently discovered that fibroblast growth factor receptor 3 (FGFR3) phosphorylates the viral E2 protein. The current study demonstrates that FGFR3 phosphorylates E2 at tyrosine 138, which inhibits association with the C-terminal peptide of Brd4. This study illustrates a novel regulatory mechanism of virus-host interaction and provides insight into the role of Brd4 in viral replication.

SF2523 inhibits human chondrosarcoma cell growth in vitro and in vivo

Developing novel therapeutic agents against chondrosarcoma is important. SF2523 is a PI3K-Akt-mTOR and bromodomain-containing protein 4 (BRD4) dual inhibitor. Its activity in human chondrosarcoma cells is tested. Our results show that SF2523 potently inhibited survival, proliferation and migration, and induced apoptosis activation in SW1353 cells and primary human chondrosarcoma cells. The dual inhibitor was yet non-cytotoxic to the primary human osteoblasts and OB-6 osteoblastic cells. SF2523 blocked Akt-mTOR activation and downregulated BRD4-regulated genes (Bcl-2 and c-Myc) in chondrosarcoma cells. It was more efficient in killing chondrosarcoma cells than other established PI3K-Akt-mTOR and BRD4 inhibitors, including JQ1, perifosine and OSI-027. In vivo, intraperitoneal injection of SF2523 (30 mg/kg) potently inhibited subcutaneous SW1353 xenograft tumor growth in severe combined immunodeficient mice. Akt-mTOR inhibition as well as Bcl-2 and c-Myc downregulation were detected in SF2523-treated SW1353 tumor tissues. In conclusion, targeting PI3K-Akt-mTOR and BRD4 by SF2523 potently inhibited chondrosarcoma cell growth in vitro and in vivo.

Brd4 inhibition suppresses HPV16 E6 expression and enhances chemoresponse: A potential new target in cervical cancer therapy

Although a vast amount of research underlines the roles of the HR HPV E6 and E7 oncogenes in HPV-induced carcinogenesis of cervical cancer, it remains unclear whether these oncogenes are also involved in the resistance of the cancer against chemotherapy. We examined the role of the HPV16 E6 oncogene in cisplatin resistance by analyzing its expression in newly established cisplatin-sensitive versus -resistant cervical cancer cell lines (CC7, CC10). Resistant variants were obtained by interval exposure treatment with 1-2 μM cisplatin for 8-9 months. Our results demonstrate that the expression level of HPV16 E6 directly correlates with the extent of cisplatin resistance in novel as well as established (SiHa) drug resistant cervical cancer cell lines. Overexpression of HPV16 E6 in cisplatin-naïve cells rendered these cells more resistant to cisplatin. Reducing E6 expression by JQ1 treatment reversed the drug resistant phenotype and strongly enhanced chemoresponse only in HPV-positive cisplatin-resistant variants and not in HPV-negative C33A cervical cancer cells. The level of E6 directly correlated with the extent of cisplatin sensitivity and was shown to be increased in newly established drug-resistant cell line variants, while reducing E6 expression using Brd4-inhibitors enhanced chemoresponse when co-delivered with cisplatin. Inhibition of Brd4 could represent a new therapeutic option by increasing treatment response in cervical cancer cells and might allow lower cisplatin dosages, thus reducing negative side effects.

Comparative transcriptomic analysis of bovine papillomatosis

BACKGROUND

Bovine papillomavirus (BPV) belongs to the Papillomaviridae family and infects epithelial cells of bovines and closely related animals, causing hyperproliferative lesions known as warts or papillomas, which may regress or progress to form benign or malignant tumors. The virus enters the host cell and interacts with it by altering the regulation of genes that are responsible for controlling the cell cycle, thus triggering lesion formation. It is not yet known which host genes are regulated by viral infection. Therefore, the objective of this study was to make use of next-generation RNA sequencing methods to identify differentially expressed genes associated with BPV infection, which might elucidate possible marker genes that could be used to control the disease.

RESULTS

Transcriptome analysis revealed that 1343 genes were differentially regulated (FDR < 0.05). A comparison of gene expression in infected and noninfected cows indicated that 655 genes were significantly upregulated, and 688 genes were significantly downregulated. Most differentially expressed genes were associated with BPV infection pathways, which supports the hypothesis that viral infection was the mechanism associated with this regulation.

CONCLUSIONS

This is the first study that focused on a large-scale evaluation of gene expression associated with BPV infection, which is important to identify possible metabolic pathways regulated by host genes for lesion development. In addition, novel targets could be identified in order to find ligands that interact with BPV, with the aim of interrupting the infection cycle.

Regulation of HPV transcription

Human papillomavirus infection is associated with the development of malignant and benign neoplasms. Approximately 40 viral types can infect the anogenital mucosa and are categorized into high- and low-risk oncogenic human papillomavirus, depending on their association with the development of cervical carcinoma. High-risk human papillomavirus 16 and 18 are detected in 55% and 15% of all invasive cervical squamous cell carcinomas worldwide, respectively. Low-risk human papillomavirus 6 and 11 are responsible for 90% of genital warts and are also associated with the development of recurrent respiratory papillomatosis. Human papillomavirus preferentially infects mitotic active cells of the basal layer from both mucosal and cutaneous epithelium through microabrasions. The viral life cycle synchronizes with the epithelial differentiation program, which may be due, in part, to the binding of differentially expressed cellular transcription factors to the long control region throughout the various epithelial layers. This review aimed to summarize the current knowledge regarding the mechanisms by which viral gene expression is regulated and the influence of human papillomavirus heterogeneity upon this phenomenon. A better understanding of the regulatory mechanisms may elucidate the particularities of human papillomavirus-associated pathogenesis and may provide new tools for antiviral therapy.

Virus DNA Replication and the Host DNA Damage Response

Viral DNA genomes have limited coding capacity and therefore harness cellular factors to facilitate replication of their genomes and generate progeny virions. Studies of viruses and how they interact with cellular processes have historically provided seminal insights into basic biology and disease mechanisms. The replicative life cycles of many DNA viruses have been shown to engage components of the host DNA damage and repair machinery. Viruses have evolved numerous strategies to navigate the cellular DNA damage response. By hijacking and manipulating cellular replication and repair processes, DNA viruses can selectively harness or abrogate distinct components of the cellular machinery to complete their life cycles. Here, we highlight consequences for viral replication and host genome integrity during the dynamic interactions between virus and host.

Virus DNA Replication and the Host DNA Damage Response

Viral DNA genomes have limited coding capacity and therefore harness cellular factors to facilitate replication of their genomes and generate progeny virions. Studies of viruses and how they interact with cellular processes have historically provided seminal insights into basic biology and disease mechanisms. The replicative life cycles of many DNA viruses have been shown to engage components of the host DNA damage and repair machinery. Viruses have evolved numerous strategies to navigate the cellular DNA damage response. By hijacking and manipulating cellular replication and repair processes, DNA viruses can selectively harness or abrogate distinct components of the cellular machinery to complete their life cycles. Here, we highlight consequences for viral replication and host genome integrity during the dynamic interactions between virus and host.

microRNA-608 inhibits human hepatocellular carcinoma cell proliferation via targeting the BET family protein BRD4

Over-expression of the bromodomain and extraterminal (BET) family protein BRD4 is associated with hepatocellular carcinoma (HCC) progression. In the present study, we indentified a novel putative anti-BRD4 microRNA: microRNA-608 ("miR-608"). In HepG2 cells and primary human HCC cells, over-expression of miR-608, using a lentiviral construct, induced BRD4 downregulation and proliferation inhibition. Conversely, transfection of the miR-608 inhibitor increased BRD4 expression to promote HepG2 cell proliferation. Our results suggest that BRD4 is the primary target gene of miR-608 in HepG2 cells. shRNA-mediated knockdown or CRSIPR/Cas9-mediated knockout of BRD4 mimicked and overtook miR-608's actions in HepG2 cells. Furthermore, introduction of a 3'-untranslated region (3'-UTR) mutant BRD4 (UTR-A1718G) blocked miR-608-induced c-Myc downregulation and proliferation inhibition in HepG2 cells. In vivo, HepG2 xenograft tumor growth was significantly inhibited after expressing miR-608 or BRD4 CRSIPR/Cas9-KO construct. Importantly, BRD4 mRNA was upregulated in human HCC tissues, which was correlated with downregulation of miR-608. Together, we conclude that miR-608 inhibits HCC cell proliferation possibly via targeting BET family protein BRD4.

AZD5153, a novel BRD4 inhibitor, suppresses human thyroid carcinoma cell growth in vitro and in vivo

The development of novel anti-papillary thyroid carcinoma agents is urgent. AZD5153 is a novel and specific Bromodomain-containing protein 4 (BRD4) inhibitor. Here, we show that AZD5153 dose-dependently inhibited survival, proliferation and cell cycle progression in TPC-1 cells and primary human thyroid carcinoma cells. Yet, it was non-cytotoxic to the primary thyroid epithelial cells. AZD5153 induced caspase-3/-9 and apoptosis activation in TPC-1 cells and primary cancer cells. Its cytotoxicity in TPC-1 cells was significantly attenuated with co-treatment of the caspase inhibitors. BRD4 expression was elevated in TPC-1 and primary human thyroid carcinoma cells, but was low in the thyroid epithelial cells. BRD4-regulated proteins, including c-Myc, Bcl-2 and cyclin D1, were significantly downregulated following AZD5153 treatment in TPC-1 and primary cancer cells. In vivo, oral administration of AZD5153 at well-tolerated doses significantly inhibited TPC-1 xenograft growth in severe combined immunodeficient (SCID) mice. BRD4-dependent proteins, Myc, Bcl-2 and cyclin D1, were also downregulated in AZD5153-treated tumor tissues. Collectively, the results suggest that targeting BRD4 by AZD5153 inhibits human thyroid carcinoma cell growth in vitro and in vivo.

Epigenetic Regulation of Viral Biological Processes

It is increasingly clear that DNA viruses exploit cellular epigenetic processes to control their life cycles during infection. This review will address epigenetic regulation in members of the polyomaviruses, adenoviruses, human papillomaviruses, hepatitis B, and herpes viruses. For each type of virus, what is known about the roles of DNA methylation, histone modifications, nucleosome positioning, and regulatory RNA in epigenetic regulation of the virus infection will be discussed. The mechanisms used by certain viruses to dysregulate the host cell through manipulation of epigenetic processes and the role of cellular cofactors such as BRD4 that are known to be involved in epigenetic regulation of host cell pathways will also be covered. Specifically, this review will focus on the role of epigenetic regulation in maintaining viral episomes through the generation of chromatin, temporally controlling transcription from viral genes during the course of an infection, regulating latency and the switch to a lytic infection, and global dysregulation of cellular function.

Dual inhibition of BRD4 and PI3K by SF2523 suppresses human prostate cancer cell growth in vitro and in vivo

Bromodomain-containing protein 4 (BRD4) and phosphatidylinositol 3-kinase (PI3K) are both key oncogenic proteins in human prostate cancer. In the current study, we examined the anti-prostate cancer cell activity by SF2523, a BRD4 and PI3K dual inhibitor. We showed that SF2523 potently inhibited survival and proliferation of the primary human prostate cancer cells. SF2523 induced profound apoptosis activation in prostate cancer cells. The dual inhibitor was yet non-cytotoxic to the prostate epithelial cells. At the molecular level, SF2523 downregulated BRD4-regulated genes (cyclin D1, c-Myc and androgen receptor) and almost blocked AKT-S6K1 activation in prostate cancer cells. In vivo, SF2523 intraperitoneal administration at the well-tolerated dose inhibited human prostate cancer xenograft growth in severe combined immunodeficient (SCID) mice. BRD4-regulated genes (cyclin D1, c-Myc and androgen receptor) and AKT-S6K1 activation were inhibited in SF2523-treated tumors. Together, dual inhibition of BRD4 and PI3K by SF2523 suppresses human prostate cancer cell growth in vitro and in vivo.

Dual inhibition of BRD4 and PI3K-AKT by SF2523 suppresses human renal cell carcinoma cell growth

Bromodomain-containing protein 4 (BRD4) and PI3K-AKT are both important for renal cell carcinoma (RCC) development and progression. SF2523 is a BRD4 and PI3K-AKT dual inhibitor. The present study demonstrated that SF2523 was cytotoxic and anti-proliferative to established RCC cell lines (786-O and A498) and primary human RCC cells. SF2523 induced activation of caspase and apoptosis in RCC cells. Further, SF2523 disrupted RCC cell cycle progression and inhibited cell migration in vitro. At the signaling level, SF2523 in-activated PI3K-AKT-mTOR, and downregulated BRD4-dependent proteins, Bcl-2 and Myc, in RCC cells. Remarkably, SF2523 was more efficient than Wortmannin (the PI3K inhibitor) and JQ1 (the BRD4 specific inhibitor) in killing RCC cells. In vivo, SF2523 administration at well-tolerated doses suppressed 786-O xenograft tumor growth in severe combined immunodeficient (SCID) mice. Together, our results suggest that concurrent blockage of BRD4 and PI3K-AKT signalings by SF2523 efficiently inhibits RCC cell growth in vitro and in vivo.

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.

Regulation of the human papillomavirus type 16 late promoter by transcriptional elongation

Transcripts from the late promoter of human papillomavirus type 16 (HPV16) are upregulated upon host cell differentiation. Differentiation-dependent transcript regulation is thought to sequester viral antigens in the uppermost epithelial layers, facilitating immune evasion. The mechanisms regulating late promoter upregulation during differentiation are poorly characterized. We show that the late promoter is upregulated at the transcriptional level and that the viral enhancer stimulates promoter activity. Using kinase inhibition and chromatin immunoprecipitation analysis, we show evidence for differentiation-dependent enhancement of transcript elongation. Three factors that promote transcript elongation, cyclin dependent kinase 9 (CDK9), CDK8 (a subunit of the Mediator complex), and bromodomain containing protein 4 (Brd4) are recruited to viral genomes upon differentiation, and each plays a role in promoter activity. These results shed light on the transcriptional processes utilized by HPV16 for proper regulation of gene expression during the viral life cycle.