Telomerase activation in cervical keratinocytes containing stably replicating human papillomavirus type 16 episomes

Characterization of 3D organotypic epithelial tissues reveals tonsil-specific differences in tonic interferon signaling

Three-dimensional (3D) culturing techniques can recapitulate the stratified nature of multicellular epithelial tissues. Organotypic 3D epithelial tissue culture methods have several applications, including the study of tissue development and function, drug discovery and toxicity testing, host-pathogen interactions, and the development of tissue-engineered constructs for use in regenerative medicine. We grew 3D organotypic epithelial tissues from foreskin, cervix, and tonsil-derived primary cells and characterized the transcriptome of these in vitro tissue equivalents. Using the same 3D culturing method, all three tissues yielded stratified squamous epithelium, validated histologically using basal and superficial epithelial cell markers. The goal of this study was to use RNA-seq to compare gene expression patterns in these three types of epithelial tissues to gain a better understanding of the molecular mechanisms underlying their function and identify potential therapeutic targets for various diseases. Functional profiling by over-representation and gene set enrichment analysis revealed tissue-specific differences: i.e., cutaneous homeostasis and lipid metabolism in foreskin, extracellular matrix remodeling in cervix, and baseline innate immune differences in tonsil. Specifically, tonsillar epithelia may play an active role in shaping the immune microenvironment of the tonsil balancing inflammation and immune responses in the face of constant exposure to microbial insults. Overall, these data serve as a resource, with gene sets made available for the research community to explore, and as a foundation for understanding the epithelial heterogeneity and how it may impact their in vitro use. An online resource is available to investigate these data (https://viz.datascience.arizona.edu/3DEpiEx/).

Single cell transcriptomic analysis of HPV16-infected epithelium identifies a keratinocyte subpopulation implicated in cancer

Persistent HPV16 infection is a major cause of the global cancer burden. The viral life cycle is dependent on the differentiation program of stratified squamous epithelium, but the landscape of keratinocyte subpopulations which support distinct phases of the viral life cycle has yet to be elucidated. Here, single cell RNA sequencing of HPV16 infected compared to uninfected organoids identifies twelve distinct keratinocyte populations, with a subset mapped to reconstruct their respective 3D geography in stratified squamous epithelium. Instead of conventional terminally differentiated cells, an HPV-reprogrammed keratinocyte subpopulation (HIDDEN cells) forms the surface compartment and requires overexpression of the ELF3/ESE-1 transcription factor. HIDDEN cells are detected throughout stages of human carcinogenesis including primary human cervical intraepithelial neoplasias and HPV positive head and neck cancers, and a possible role in promoting viral carcinogenesis is supported by TCGA analyses. Single cell transcriptome information on HPV-infected versus uninfected epithelium will enable broader studies of the role of individual keratinocyte subpopulations in tumor virus infection and cancer evolution.

Characterization of 3D organotypic epithelial tissues reveals tonsil-specific differences in tonic interferon signaling

Three-dimensional (3D) culturing techniques can recapitulate the stratified nature of multicellular epithelial tissues. Organotypic 3D epithelial tissue culture methods have several applications, including the study of tissue development and function, drug discovery and toxicity testing, host-pathogen interactions, and the development of tissue-engineered constructs for use in regenerative medicine. We grew 3D organotypic epithelial tissues from foreskin, cervix, and tonsil-derived primary cells and characterized the transcriptome of these in vitro tissue equivalents. Using the same 3D culturing method, all three tissues yielded stratified squamous epithelium, validated histologically using basal and superficial epithelial cell markers. The goal of this study was to use RNA-seq to compare gene expression patterns in these three types of epithelial tissues to gain a better understanding of the molecular mechanisms underlying their function and identify potential therapeutic targets for various diseases. Functional profiling by over-representation and gene set enrichment analysis revealed tissue-specific differences: i.e. , cutaneous homeostasis and lipid metabolism in foreskin, extracellular matrix remodeling in cervix, and baseline innate immune differences in tonsil. Specifically, tonsillar epithelia may play an active role in shaping the immune microenvironment of the tonsil balancing inflammation and immune responses in the face of constant exposure to microbial insults. Overall, these data serve as a resource, with gene sets made available for the research community to explore, and as a foundation for understanding the epithelial heterogeneity and how it may impact their in vitro use. An online resource is available to investigate these data ( https://viz.datascience.arizona.edu/3DEpiEx/ ).

Chlamydia trachomatis Serovars Drive Differential Production of Proinflammatory Cytokines and Chemokines Depending on the Type of Cell Infected

Chlamydia trachomatis serovars A-C infect conjunctival epithelial cells and untreated infection can lead to blindness. D-K infect genital tract epithelial cells resulting in pelvic inflammatory disease, ectopic pregnancy, and sterility while L1-L3 infect epithelial cells and macrophages, causing an invasive infection. Despite some strains of Chlamydia sharing high nucleotide sequence similarity, the bacterial and host factors that govern tissue and cellular tropism remain largely unknown. Following introduction of C. trachomatis via intercourse, epithelial cells of the vagina, foreskin, and ectocervix are exposed to large numbers of the pathogen, yet their response to infection and the dynamics of chlamydial growth in these cells has not been well-characterized compared to growth in more permissive cell types that harbor C. trachomatis. We compared intracellular replication and inclusion development of representative C. trachomatis serovars in immortalized human conjunctival epithelial, urogenital epithelial, PMA stimulated THP-1 (macrophages), and HeLa cells. We demonstrate that urogenital epithelial cells of the vagina, ectocervix, and foreskin restrict replication of serovar A while promoting robust replication and inclusion development of serovar D and L2. Macrophages restrict serovars D and A while L2 proliferates in these cells. Furthermore, we show that GM-CSF, RANTES, GROα, IL-1α, IL-1β, IP-10, IL-8, and IL-18 are produced in a cell-type and serovar-specific manner. Collectively we have established a series of human cell lines that represent some of the first cell types to encounter C. trachomatis following exposure and show that differential production of key cytokines early during infection could regulate serovar-host cell specificity.

The Replicative Consequences of Papillomavirus E2 Protein Binding to the Origin Replication Factor ORC2

The origin recognition complex (ORC) coordinates a series of events that lead to initiation of DNA strand duplication. As a nuclear double stranded DNA plasmid, the papillomavirus (PV) genome resembles a mini-chromosome in infected cells. To initiate its replication, the viral E2 protein binds to and recruits the E1 DNA helicase at the viral origin. PV genome replication program exhibits three stages: initial amplification from a single genome upon infection to a few copies per cell, a cell cycle linked maintenance phase, and a differentiation dependent late stage where the genome is amplified to thousands of copies. Involvement of ORC or other pre-replication complex (pre-RC) factors has not been described. We report that human PV (HPV) and bovine PV (BPV-1) E2 proteins bind to ORC2, however, ORC2 was not detected at the viral origin. Depletion of ORC2 enhanced PV replication in a transient replication model and in keratinocytes stably maintaining viral episomes, while there was no effect on copy number in a cell line with integrated HPV genomes. Consistent with this, occupancy of E1 and E2 at the viral origin increased following ORC2 silencing. These data imply that ORC2 is not necessary for activation of the PV origin by E1 and E2 but instead suppresses E2 replicative function. Furthermore, we observed that over-expression of HPV E2 decreased ORC2 occupation at two known mammalian origins of replication, suggesting that E2 restricts pre-ORC assembly that could otherwise compete for host replication complexes necessary for viral genome amplification. We infer that the ORC2 complex with E2 restricts viral replication in the maintenance phase of the viral replication program and that elevated levels of E2 that occur during the differentiation dependent amplification stage subvert ORC loading and hence DNA synthesis at cellular origins.

Papillomavirus genome replication occurs during three distinct stages that are linked to the differentiation state of the infected epithelium. The viral proteins E1 and E2 recognize the viral origin and initiate a process that attracts host DNA replication factors. The origin recognition complex (ORC) coordinates initiation of chromosome duplication. While ORC2 binds to the E2 protein, its depletion does not impair PV genome replication. Instead, depletion of ORC2 stimulates viral replication, while over-expression of E2 protein decreases ORC2 occupancy at mammalian origins. We propose that the relative abundance of E2 and ORC2 in complex regulates viral and cellular origin licensing.

Levels of the E2 interacting protein TopBP1 modulate papillomavirus maintenance stage replication

The evolutionarily conserved DNA topoisomerase II beta-binding protein 1 (TopBP1) functions in DNA replication, DNA damage response, and cell survival. We analyzed the role of TopBP1 in human and bovine papillomavirus genome replication. Consistent with prior reports, TopBP1 co-localized in discrete nuclear foci and was in complex with papillomavirus E2 protein. Similar to E2, TopBP1 is recruited to the region of the viral origin of replication during G1/S and early S phase. TopBP1 knockdown increased, while over-expression decreased transient virus replication, without affecting cell cycle. Similarly, using cell lines harboring HPV-16 or HPV-31 genome, TopBP1 knockdown increased while over-expression reduced viral copy number relative to genomic DNA. We propose a model in which TopBP1 serves dual roles in viral replication: it is essential for initiation of replication yet it restricts viral copy number.

Interferon treatment of human keratinocytes harboring extrachromosomal, persistent HPV-16 plasmid genomes induces de novo viral integration

Interferons (IFNs) have been used to treat epithelial lesions caused by human papillomavirus (HPV) persistence. Here, we exposed primary human keratinocytes (HFKs) immortalized by persistently replicating HPV-16 plasmid genomes to increasing levels of IFN-γ. While untreated HFKs retained replicating HPV-16 plasmids for up to 60-120 population doublings, IFN led to rapid HPV-16 plasmid loss. However, treated cultures eventually gave rise to outgrowth of clones harboring integrated HPV-16 genomes expressing viral E6 and E7 oncogenes from chimeric virus-cell mRNAs similar to those in cervical and head and neck cancers. Surprisingly, every HPV-16 integrant that arose after IFN exposure stemmed from an independent integration event into a different cellular gene locus, even within parallel cultures started from small cell inocula and cultured separately for ≥ 25 doublings to permit the rise and expansion of spontaneous integrants. While IFN treatment conferred a growth advantage upon preexisting integrants added to mixed control cultures, our results indicate that IFN exposure directly or indirectly induces HPV-16 integration, rather than only selects preexisting, spontaneous integrants that appear to be much less frequent. We estimate that IFN exposure increased integration rates by ≥ 100-fold. IFN-induced HPV-16 integration involved a wide range of chromosomal loci with less apparent selection for recurrent insertions near genes involved in cancer-related pathways. We conclude that IFNs and other potential treatments targeting high-risk HPV persistence that disrupt viral genome replication may promote increased high-risk HPV integration as a step in cancer progression. Therapies against high-risk HPV persistence thus need to be evaluated for their integration-inducing potential.

Analyzing the Human Papillomavirus (HPV) Life Cycle in Primary Keratinocytes with a Quantitative Colony-Forming Assay

Papillomavirus genomes replicate as extrachromosomal plasmids within infected keratinocytes, requiring the regulated expression of early viral gene products to initially amplify the viral genomes and subvert cell growth checkpoints as part of a complex path to immortalization. Building on contemporary keratinocyte transfection and culture systems, the methods described in this unit form a detailed approach to analyzing critical events in the human papillomavirus (HPV) life cycle, utilizing physiologic levels of viral gene products expressed from their native promoter(s) in the natural host cells for HPV infection. A quantitative colony-forming assay permits comparison of the capacities of various transfected HPV types and mutant HPV genomes to initially form colonies and immortalize human keratinocytes. In conjunction with additional methods, these protocols enable examination of genomic stability, viral and cellular gene expression, viral integration, and differentiation patterns influenced by HPV persistence in clonal human keratinocytes that effectively mimic early events in HPV infection.

Cellular transformation by human papillomaviruses: lessons learned by comparing high- and low-risk viruses

The oncogenic potential of papillomaviruses (PVs) has been appreciated since the 1930s yet the mechanisms of virally-mediated cellular transformation are still being revealed. Reasons for this include: a) the oncoproteins are multifunctional, b) there is an ever-growing list of cellular interacting proteins, c) more than one cellular protein may bind to a given region of the oncoprotein, and d) there is only limited information on the proteins encoded by the corresponding non-oncogenic PVs. The perspective of this review will be to contrast the activities of the viral E6 and E7 proteins encoded by the oncogenic human PVs (termed high-risk HPVs) to those encoded by their non-oncogenic counterparts (termed low-risk HPVs) in an attempt to sort out viral life cycle-related functions from oncogenic functions. The review will emphasize lessons learned from the cell culture studies of the HPVs causing mucosal/genital tract cancers.

Human papillomavirus type 16 (HPV-16) genomes integrated in head and neck cancers and in HPV-16-immortalized human keratinocyte clones express chimeric virus-cell mRNAs similar to those found in cervical cancers

Many human papillomavirus (HPV)-positive high-grade lesions and cancers of the uterine cervix harbor integrated HPV genomes expressing the E6 and E7 oncogenes from chimeric virus-cell mRNAs, but less is known about HPV integration in head and neck cancer (HNC). Here we compared viral DNA status and E6-E7 mRNA sequences in HPV-16-positive HNC tumors to those in independent human keratinocyte cell clones derived from primary tonsillar or foreskin epithelia immortalized with HPV-16 genomes. Three of nine HNC tumors and epithelial clones containing unintegrated HPV-16 genomes expressed mRNAs spliced from HPV-16 SD880 to SA3358 and terminating at the viral early gene p(A) signal. In contrast, most integrated HPV genomes in six HNCs and a set of 31 keratinocyte clones expressed HPV-16 major early promoter (MEP)-initiated mRNAs spliced from viral SD880 directly to diverse cellular sequences, with a minority spliced to SA3358 followed by a cellular DNA junction. Sequence analysis of chimeric virus-cell mRNAs from HNC tumors and keratinocyte clones identified viral integration sites in a variety of chromosomes, with some located in or near growth control genes, including the c-myc protooncogene and the gene encoding FAP-1 phosphatase. Taken together, these findings support the hypothesis that HPV integration in cancers is a stochastic process resulting in clonal selection of aggressively expanding cells with altered gene expression of integrated HPV genomes and potential perturbations of cellular genes at or near viral integration sites. Furthermore, our results demonstrate that this selection also takes place and can be studied in primary human keratinocytes in culture.

hTERT promoter activity and CpG methylation in HPV-induced carcinogenesis

Background

Activation of telomerase resulting from deregulated hTERT expression is a key event during high-risk human papillomavirus (hrHPV)-induced cervical carcinogenesis. In the present study we examined hTERT promoter activity and its relation to DNA methylation as one of the potential mechanisms underlying deregulated hTERT transcription in hrHPV-transformed cells.

Methods

Using luciferase reporter assays we analyzed hTERT promoter activity in primary keratinocytes, HPV16- and HPV18-immortalized keratinocyte cell lines and cervical cancer cell lines. In the same cells as well as cervical specimens we determined hTERT methylation by bisulfite sequencing analysis of the region spanning -442 to +566 (relative to the ATG) and quantitative methylation specific PCR (qMSP) analysis of two regions flanking the hTERT core promoter.

Results

We found that in most telomerase positive cells increased hTERT core promoter activity coincided with increased hTERT mRNA expression. On the other hand basal hTERT promoter activity was also detected in telomerase negative cells with no or strongly reduced hTERT mRNA expression levels. In both telomerase positive and negative cells regulatory sequences flanking both ends of the core promoter markedly repressed exogenous promoter activity.

By extensive bisulfite sequencing a strong increase in CpG methylation was detected in hTERT positive cells compared to cells with no or strongly reduced hTERT expression. Subsequent qMSP analysis of a larger set of cervical tissue specimens revealed methylation of both regions analyzed in 100% of cervical carcinomas and 38% of the high-grade precursor lesions, compared to 9% of low grade precursor lesions and 5% of normal controls.

Conclusions

Methylation of transcriptionally repressive sequences in the hTERT promoter and proximal exonic sequences is correlated to deregulated hTERT transcription in HPV-immortalized cells and cervical cancer cells. The detection of DNA methylation at these repressive regions may provide an attractive biomarker for early detection of cervical cancer.

In vitro progression of human papillomavirus 16 episome-associated cervical neoplasia displays fundamental similarities to integrant-associated carcinogenesis

An important event in the development of cervical squamous cell carcinoma (SCC) is deregulated expression of high-risk human papillomavirus (HR-HPV) oncogenes, most commonly related to viral integration into host DNA. Mechanisms of development of the approximately 15% of SCCs that contain extrachromosomal (episomal) HR-HPV are poorly understood due to limited longitudinal data. We therefore used the W12 model to study mechanisms of cervical carcinogenesis associated with episomal HPV16. In vitro progression of W12 normally occurs through selection of cells containing integrated HPV16. However, in one long-term culture, keratinocytes developed a selective growth advantage and invasive phenotype while retaining HPV16 episomes at increased copy number in the absence of transcriptionally active integrants. Longitudinal investigations revealed similarities between the episome- and integrant-associated routes of neoplastic progression. Most notable were dynamic changes in viral early gene expression in episome-retaining cells, consistent with continually changing selective pressures. An early increase in viral transcription preceded elevated episome copy number and was followed by a reduction to near baseline after the development of invasiveness. Episomal transcriptional deregulation did not require selection of a specific sequence variant of the HPV16 upstream regulatory region, although increased levels of acetylated histone H4 around the late promoter implicated a role for altered chromatin structure. Interestingly, invasive episome-retaining cells showed high levels of HPV16 E2/E6 proteins (despite decreased transcript levels) and reduced expression of IFN-stimulated genes, adaptations that support viral persistence and cell survival. Our findings suggest a unified working model for events important in cervical neoplastic progression regardless of HR-HPV physical state.

The immortalizing and transforming ability of two common human papillomavirus 16 E6 variants with different prevalences in cervical cancer

Persistent infection with high-risk human papillomaviruses (HPVs), especially type 16 has been undeniably linked to cervical cancer. The Asian-American (AA) variant of HPV16 is more common in the Americas than the prototype in cervical cancer. The different prevalence is based on three amino acid changes within the E6 protein denoted Q14H/H78Y/L83V. To investigate the mechanism(s) behind this observation, both E6 proteins, in the presence of E7, were evaluated for their ability to extend the life span of and transform primary human foreskin keratinocytes (PHFKs). Long-term cell culture studies resulted in death at passage 9 of vector-transduced PHFKs (negative control), but survival of both E6 PHFKs to passage 65 (and beyond). Compared with E6/E7 PHFKs, AA/E7 PHFKs were significantly faster dividing, developed larger cells in monolayer cultures, showed double the epithelial thickness and expressed cytokeratin 10 when grown as organotypic raft cultures. Telomerase activation and p53 inactivation, two hallmarks of immortalization, were not significantly different between the two populations. Both were resistant to anoikis at later passages, but only AA/E7 PHFKs acquired the capacity for in vitro transformation. Proteomic analysis revealed markedly different protein patterns between E6/E7 and AA/E7, particularly with respect to key cellular metabolic enzymes. Our results provide new insights into the reasons underlying the greater prevalence of the AA variant in cervical cancer as evidenced by characteristics associated with higher oncogenic potential.

Human papillomavirus (HPV) type 18 induces extended growth in primary human cervical, tonsillar, or foreskin keratinocytes more effectively than other high-risk mucosal HPVs

Mucosal high-risk (HR) human papillomaviruses (HPVs) that cause cervical and other anogenital cancers also are found in approximately 25% of head and neck carcinomas (HNCs), especially those arising in the oropharynx and the tonsils. While many HR HPV types are common in anogenital cancer, over 90% of HPV-positive HNCs harbor HPV type 16 (HPV-16). Using a quantitative colony-forming assay, we compared the ability of full-length mucosal HPV genomes, i.e., the low-risk HPV-11 and HR HPV-16, -18, and -31, to persist in and alter the growth of primary human keratinocytes from the foreskin, cervix, and tonsils. The HR HPV types led to the formation of growing keratinocyte colonies in culture independent of the site of epithelial origin. However, HPV-18 induced colony growth in all keratinocytes >4-fold more effectively than HPV-16 or HPV-31 and >20-fold more efficiently than HPV-11 or controls. HPV-11-transfected or control colonies failed to expand beyond 32 to 36 population doublings postexplantation. In contrast, individual HR HPV-transfected clones exhibited no apparent slowdown of growth or "crisis," and many maintained HPV plasmid persistence beyond 60 population doublings. Keratinocyte clones harboring extrachromosomal HR HPV genomes had shorter population doubling times and formed dysplastic stratified epithelia in organotypic raft cultures, mirroring the pathological features of higher-grade intraepithelial lesions, yet did not exhibit chromosomal instability. We conclude that, in culture, the HR HPV type, rather than the site of epithelial origin of the cells, determines the efficacy of inducing continued growth of individual keratinocytes, with HPV-18 being the most aggressive mucosal HR HPV type tested.

Proliferative defects in dyskeratosis congenita skin keratinocytes are corrected by expression of the telomerase reverse transcriptase, TERT, or by activation of endogenous telomerase through expression of papillomavirus E6/E7 or the telomerase RNA component, TERC

Dyskeratosis congenita (DC) is characterized by the triad of reticulate skin pigmentation, nail dystrophy and leukoplakia. Epidermal atrophy, hair growth defects, bone marrow failure and increased risk of cancer are also common in DC patients. DC is caused by mutations in genes encoding for telomerase complex factors. Although there is an association of epidermal abnormalities with DC, epidermal cells from DC donors have not been previously characterized. We have isolated skin keratinocytes from affected members of a family with an autosomal dominant form of DC that is caused by a mutation in the RNA component of telomerase, TERC. Here, we demonstrate that, similar to DC fibroblasts from these donors, DC keratinocytes have short telomeres and a short lifespan. DC keratinocytes also exhibited impaired colony forming efficiency (CFE) and migration capacity. Exogenous expression of the reverse transcriptase (RT) component of telomerase, TERT, activated telomerase levels to half that of TERT expressing normal cells and maintained telomeres at a short length with concomitant extension of lifespan. Unlike fibroblasts, transduction of human papillomavirus type 16 E6/E7 genes into DC keratinocytes activated telomerase to half that of E6/E7 expressing normal cells, and robust proliferation was observed. While expression of TERC has no measurable effect on telomerase in fibroblasts, expression of TERC in keratinocytes upregulated telomerase activity and, rarely, allowed rescue of proliferative defects. Our results point to important differences between DC fibroblasts and keratinocytes and show, for the first time, that expression of TERC can increase the lifespan of primary human epithelial cells.

Interferon-beta treatment increases human papillomavirus early gene transcription and viral plasmid genome replication by activating interferon regulatory factor (IRF)-1

Interferons (IFNs) have been used to treat mucosal lesions caused by human papillomavirus (HPV) infection, such as intraepithelial precursor lesions to cancer of the uterine cervix, genital warts or recurrent respiratory papillomatosis, to potentially reduce or eliminate replicating HPV plasmid genomes. Mucosal HPVs have evolved mechanisms that impede IFN-β synthesis and downregulate genes induced by IFN. Here we show that these HPV types directly subvert a cellular transcriptional response to IFN-β as a potential boost in infection. Treatment with low levels of human IFN-β induced initial amplification of HPV-16 and HPV-11 plasmid genomes and increased HPV-16 or HPV-31 DNA copy numbers up to 6-fold in HPV-immortalized keratinocytes. IFN treatment also increased early gene transcription from the major early gene promoters in HPV-16, HPV-31 and HPV-11. Furthermore, mutagenesis of the viral genomes and ectopic interferon regulatory factor (IRF) expression in transfection experiments using IRF-1 ^−/− , IRF-2 ^−/− and dual knockout cell lines determined that these responses are due to the activation of IRF-1 interaction with a conserved interferon response element demonstrated in several mucosal HPV early gene promoters. Our results provide a molecular explanation for the varying clinical outcomes of IFN therapy of papillomatoses and define an assay for the modulation of the HPV gene program by IFNs as well as other cytokines and signaling molecules in infection and therapy.

Gene expression profiling to identify markers associated with deregulated hTERT in HPV-transformed keratinocytes and cervical cancer

Although high-risk human papillomavirus (HPV) infection plays a major role in the development of cervical cancer, additive oncogenic events are involved as well. One key event involves increased activity of telomerase resulting from a deregulated expression of its catalytic subunit hTERT. Our previous microcell-mediated chromosome transfer studies revealed that introduction of human chromosome 6 in the HPV16-immortalized keratinocyte cell line FK16A and in the HPV16-containing cervical cancer cell line SiHa induced growth arrest, resulting from a repression of hTERT mRNA expression and telomerase activity. Here, this model was used to analyze expression profiles associated with hTERT deregulation in HPV-transformed cells. Microarray expression analysis of 12 FK16A/chromosome 6 hybrids, 4 of which were negative for endogenous hTERT and 8 of which were positive for endogenous hTERT, resulted in the identification of 164 differentially expressed genes. Differential expression of a selection of 5 genes was verified by real-time RT-PCR. Of these 164 genes, 32 were also differentially expressed in other HPV transformed cells with deregulated hTERT. For 2 of these genes, encoding AQP3 and MGP, altered expression in hTERT positive cervical carcinomas was confirmed by real-time RT-PCR and immunohistochemistry, respectively. Moreover, increased MGP protein expression was significantly more frequent in high-grade cervical premalignant lesions with elevated hTERT mRNA expression compared to those without. In summary, we identified 32 candidate biomarkers for deregulated hTERT mRNA expression, which may enable the identification of cervical premalignant lesions that are at highest risk to progress to invasive cancer.

Human papillomavirus causes an angiogenic switch in keratinocytes which is sufficient to alter endothelial cell behavior

One of the requirements for tumor growth is the ability to recruit a blood supply, a process known as angiogenesis. Angiogenesis begins early in the progression of cervical disease from mild to severe dysplasia and on to invasive cancer. We have previously reported that expression of human papillomavirus type 16 E6 and E7 (HPV16 E6E7) proteins in primary foreskin keratinocytes (HFKs) decreases expression of two inhibitors and increases expression of two angiogenic inducers [Toussaint-Smith, E., Donner, D.B., Roman, A., 2004. Expression of human papillomavirus type 16 E6 and E7 oncoproteins in primary foreskin keratinocytes is sufficient to alter the expression of angiogenic factors. Oncogene 23, 2988-2995]. Here we report that HPV-induced early changes in the keratinocyte phenotype are sufficient to alter endothelial cell behavior both in vitro and in vivo. Conditioned media from HPV16 E6E7 expressing HFKs as well as from human cervical keratinocytes containing the intact HPV16 were able to stimulate proliferation and migration of human microvascular endothelial cells. In addition, introduction of the conditioned media into immunocompetent mice using a Matrigel plug model resulted in a clear angiogenic response. These novel data support the hypothesis that HPV proteins contribute not only to the uncontrolled keratinocyte growth seen following HPV infection but also to the angiogenic response needed for tumor formation.

Telomerase activity as an adjunct to high-risk human papillomavirus types 16 and 18 and cytology screening in cervical cancer

Telomerase is a ribonucleoprotein comprising an RNA template, the telomerase-associated protein and its catalytic subunit, human telomerase reverse transcriptase (hTERT). Telomerase activation is a critical step in cellular immortalisation and development of cancer. Enhanced telomerase activity has been demonstrated in cervical cancer. In the present study telomerase activity and hTERT mRNA expression were evaluated and correlated with the presence of human papillomavirus (HPV) infection and cytological changes in the cervical lesions. Telomerase activity was assayed by telomeric repeat amplification protocol, hTERT mRNA expression by reverse transcriptase polymerase chain reaction and presence of high risk HPV (HR-HPV) infection by polymerase chain reaction. Out of 154 cervical samples of different cytology, 90 (58.44%) were positive for HR-HPV types 16/18, while among 55 normal cervical scrapes, 10 (18.18%) were HPV DNA positive. All 59 invasive cancer samples showed a very high telomerase activity. Among dysplasia, seven (63.6%) mild dysplasia, 18 (100%) of moderate, 20 (100%) of severe dysplasia and 6 (100%) carcinoma in situ (CIS) samples were positive with mild to moderate to high to very high telomerase activity respectively. Seven (12.7%) samples of apparently normal cervical scrapes were weakly positive for telomerase activity. We observed a good correlation (P<0.001) between telomerase activity and HR-HPV 16/18 positivity with a sensitivity of 88.1% for HPV and 100% for telomerase activity. It is suggested that telomerase activity may be used as an adjunct to cytology and HPV DNA testing in triaging women with cervical lesions.

HPV16-E6 associated hTERT promoter acetylation is E6AP dependent, increased in later passage cells and enhanced by loss of p300

The E6 oncoprotein from high-risk HPV types activates human telomerase reverse transcriptase (hTERT) transcription in human keratinocytes. Studies on how E6 regulates hTERT have implicated E-box or X-box elements in the hTERT promoter (Veldman et al., Proc Natl Acad Sci USA 2003;100:8211-14; Oh et al., J Virol 2001;75:5559-66; Gewin et al., Genes Dev 2004;18:2269-82), but the mechanism of activation by E6 is still controversial and not well defined. Here, we demonstrate that induction of both hTERT expression and telomerase activity by HPV-16 E6 in early passage keratinocytes is associated with acetylation of histone H3 at the hTERT promoter, is dependent on the E6 associated protein (E6AP) and is not exclusively reliant on E-box or X-box elements. Further increases in histone acetylation of the hTERT promoter and hTERT transcriptional activity in E6 expressing cells that had been passaged extensively in culture were found to occur only with the endogenous promoter and not with an exogenously introduced hTERT promoter construct. Telomerase activity at both early and late passages, however, was dependent on E6AP expression, implying a continued reliance on E6 function for telomerase activity. Our results demonstrate that E6 induces hTERT promoter acetylation, but that further increases in telomerase activity and histone acetylation in later passage E6 expressing cells are independent of E6 activation of the core hTERT promoter. We also provide evidence that the transcription factor p300 is a potential repressor of telomerase activation and histone acetylation in the context of E6 expression. These studies give insight into how immortalization by HPV results in upregulation of hTERT and furthers our understanding of how telomerase is activated during the process of malignant transformation.

Implication of human telomerase reverse transcriptase in cervical carcinogenesis and cancer recurrence

The objective of this study was to evaluate the implication of human telomerase reverse transcriptase (hTERT) in cervical carcinogenesis and cancer recurrence. One hundred three cases of uterine cervix, including 20 normal, 13 low-grade squamous intraepithelial lesion (LSIL), 30 high-grade squamous intraepithelial lesion (HSIL), and 40 squamous cell carcinoma (SCC) tissues, were evaluated for hTERT immunoreactivity. The expressions of hTERT in normal, LSIL, HSIL, and SCC tissues were compared by Fisher exact or Chi-square test. The relationships between hTERT and clinicopathologic variables of SCC were also assessed. Furthermore, SCC patients were subdivided into negative and positive hTERT expression subgroups, and Kaplan-Meier curves were used to plot the cumulative recurrence hazard for 5 years. There was a significant difference for hTERT expression between LSIL and HSIL subgroups (P < 0.001) but no significant difference between normal and LSIL as well as HSIL and SCC subgroups. For SCC patients, hTERT expression was positive in lymph nodes, vagina, and parametrium metastastic cases. However, it did not reach a significant difference. The cumulative recurrence hazard for 5 years was about 29% in positive hTERT expression subgroup compared to 0% in negative hTERT subgroup (P = 0.2866). In conclusion, a point stage of HSIL exists in the progression of cervical carcinogenesis when the hTERT expression increases significantly. Moreover, SCC patients with positive hTERT expression may have higher cumulative recurrence hazard.

Cervical keratinocytes containing stably replicating extrachromosomal HPV-16 are refractory to transformation by oncogenic H-Ras

Ras expression in human epithelial cells with integrated HPV genomes has been shown to cause tumorigenic transformation. The effects of Ras in cells representing early stage HPV-associated disease (i.e., when HPV is extrachromosomal and the oncogenes are under control of native promoters) have not been examined. Here, we used human cervical keratinocyte cell lines containing stably replicating extrachromosomal HPV-16 and present the novel finding that these cells resist transformation by oncogenic H-Ras. Ras expression consistently diminished anchorage-independent growth (AI), reduced E6 and E7 expression, and caused p53 induction in these cells. Conversely, AI was enhanced or maintained in Ras-transduced cervical cells that were immortalized with a 16E6/E7 retrovirus, and minimal effects on E6 and E7 expression were observed. Ras expression with either episomal HPV-16 or LXSN-E6/E7 was insufficient for tumorigenic growth suggesting that other events are needed for tumorigenic transformation. In conclusion, our results indicate that Ras-mediated transformation depends on the context of HPV oncogene expression and that this is an important point to address when developing HPV tumor models.

Gene expression profiles reveal an upregulation of E2F and downregulation of interferon targets by HPV18 but no changes between keratinocytes with integrated or episomal viral genomes

Persistent infections with human papillomaviruses type 18 can result in the development of cervical cancer. HPV18 genomes persist extrachromosomally in low-grade and precancerous lesions but are always integrated in cervical cancers, and this might contribute to the progression of HPV18-induced lesions. To address whether integration induces additional changes in host cells, several keratinocyte lines with wild type and replication-deficient E1 mutant HPV18 (E1C-TTL) genomes were analyzed with high density oligonucleotide arrays. In comparison to normal keratinocytes, wild type and integrated E1C-TTL HPV18 genomes deregulate the expression of 280 annotated genes. However, the comparison of wild type with E1C-TTL cell lines did not reveal any significant differences, indicating that neither the loss of E1 nor viral integration induces additional gene expression changes in low passage HPV18-positive keratinocytes. Half of the deregulated genes have been described as targets of the p16/Rb/E2F, p53, interferon or NFkappaB pathways consistent with the functions ascribed to the viral E6 and E7 oncoproteins, but the other half can currently not be ascribed to certain pathways.

HPV-mediated cervical carcinogenesis: concepts and clinical implications

Persistent infection with a high-risk human papillomavirus (hrHPV) is generally accepted as a necessary cause of cervical cancer. However, cervical cancer is a rare complication of an hrHPV infection since most such infections are transient, not even giving rise to cervical lesions. On average, it takes 12-15 years before a persistent hrHPV infection may ultimately, via consecutive premalignant stages (ie CIN lesions), lead to an overt cervical carcinoma. This argues that HPV-induced cervical carcinogenesis is multi-step in nature. In this review, the data from hrHPV-mediated in vitro transformation studies and those obtained from analysis of clinical specimens have been merged into a cervical cancer progression model. According to this model, a crucial decision maker in the early stages following infection involves individual susceptibility for certain HPV types depending on the genetic make-up of immune surveillance determinants. Once a CIN lesion has developed, altered transcriptional regulation of the viral E6/E7 oncogenes, resulting in genomic instability and distinguishing the process of cell transformation from a productive viral infection, probably provides the subsequent important step towards malignancy. The additional (epi)genetic alterations that subsequently accumulate in high-grade CIN lesions may result in overt malignancy via immortality and growth conditions that gradually become less sensitive to growth-modulating influences mediated by cytokines and cell-cell and cell-matrix adhesions. The potential implications of hrHPV testing and some other biomarkers deduced from this model for cervical screening and the clinical management of CIN disease are also discussed.

Cidofovir incorporation into human keratinocytes with episomal HPV 16 results in nonselective cytotoxicity

OBJECTIVES

Recurrent respiratory papillomatosis (RRP) is caused by human papillomavirus (HPV). Surgical excision is the mainstay of treatment; however, medical therapy including cidofovir, a cytosine analog, has been investigated. Human papillomavirus does not encode a viral DNA polymerase, which is the known target of cidofovir in cytomegalovirus infections.

METHODS

In an effort to better understand the usefulness of cidofovir in the treatment of HPV-related disease, we tested cidofovir's ability to inhibit growth, alter gene expression, and inhibit genome replication.

RESULTS

With the use of carbon 14-labeled cidofovir in episomal HPV 16-containing keratinocytes, there was a minimal increase in cidofovir incorporation into episomal DNA versus genomic DNA. Cidofovir decreased the copies of episomal HPV 16 in keratinocytes; however, the copies per cell returned to baseline levels once cidofovir was removed. Expression of a viral oncogene (HPV 16 E6) in transformed keratinocytes with episomal HPV 16 was not decreased by cidofovir. Cytotoxicity in head and neck squamous cell carcinoma lines exposed to cidofovir correlated with cell doubling time, and not with HPV status. Also, tonsil keratinocytes transformed with episomal HPV 16 did not exhibit greater cidofovir-mediated toxicity than did telomerase-transformed keratinocytes.

CONCLUSIONS

These findings suggest that any potential in vivo benefit of cidofovir therapy results from non-viral-specific cell toxicity at the site of application.

Amplification of the chromosome 20q region is associated with expression of HPV-16 E7 in human airway and anogenital epithelial cells

To study the role of human papillomavirus (HPV) infection in the development of genetic instability, we transduced normal human airway and anogenital epithelial cells with various combinations of HPV-16 E6, E7, and the reverse transcriptase component of telomerase (hTERT). Cell lines generated by co-expression of E7 with E6 and/or hTERT (i.e., E6/E7, E7/hTERT, and E6/E7/hTERT) exhibited extra copies of chromosome 20 and specific amplification of the 20q12-ter region, whereas those generated without E7 (i.e., hTERT alone or E6/hTERT) did not. Co-expression of hTERT and a dominant-negative version of cdk4 that has been shown to inactivate the retinoblastoma (pRb) pathway also resulted in 20q amplification. Interestingly, extra copies of chromosome 20 were observed in early passage keratinocytes that expressed E7 alone, and microarray expression analysis revealed that genes in the 20q region and on chromosome 5 were specifically upregulated in these cells. Our results indicate that chromosome 20q amplification is an early event that may be specifically caused by expression of E7 through inactivation of the pRb pathway in human epithelial cells.

Variant upstream regulatory region sequences differentially regulate human papillomavirus type 16 DNA replication throughout the viral life cycle

While the central role of the viral upstream regulatory region (URR) in the human papillomavirus (HPV) life cycle has been well established, its effects on viral replication factor expression and plasmid replication of HPV type 16 (HPV16) remain unclear. Some nonprototypic variants of HPV16 contain altered URR sequences and are considered to increase the oncogenic risk of infections. To determine the relationship between viral replication and variant URRs, hybrid viral genomes were constructed with the replication-competent HPV16 prototype W12 and analyzed in assays which recapitulate the different phases of normal viral replication. The establishment efficiencies of hybrid HPV16 genomes differed about 20-fold among European prototypes and variants from Africa and America. Generally, European and African genomes exhibited the lowest replication efficiencies. The high replication levels observed with American variants were primarily attributable to their efficient expression of the replication factors E1 and E2. The maintenance levels of these viral genomes varied about fivefold, which correlated with their respective establishment phenotypes and published P(97) activities. Vegetative DNA amplification could also be observed with replicating HPV16 genomes. These results indicate that efficient E1/E2 expression and elevated plasmid replication levels during the persistent stage of infection may comprise a risk factor in HPV16-mediated oncogenesis.

Telomerase Activity as a Potential Diagnostic Marker for Triage of Abnormal Pap Smears

OBJECTIVES

To determine whether there is an association between high levels of telomerase and premalignant cervical disease and to provide a preliminary analysis of telomerase activity as a potential triage strategy.

MATERIALS AND METHODS

Premenopausal women were invited to participate in the study during routine gynecologic visits as well as visits where colposcopy was performed. Samples were taken from the cervix using a broom device and placed in cold phosphate-buffered saline. A total of 92 samples were evaluated. Cells were counted and lysed, and a semiquantitative measure of telomerase activity was determined using a commercially available telomerase enzyme-linked immunosorbent assay kit. The presence of human papillomavirus (HPV) types 16 and 18 was assessed by polymerase chain reaction analysis. One-way analysis of variance was used to test for the association of telomerase activity with cytology, HPV type 16 or 18 status, and colposcopy and/or biopsy findings.

RESULTS

When telomerase levels were analyzed according to Pap smear results, there were no differences among four groups of cytology findings (normal, atypical squamous cells of undetermined significance, low-grade squamous intraepithelial lesion, and high-grade squamous intraepithelial lesion). When colposcopy and/or biopsy results were considered, significantly higher levels of telomerase were detected in cervical intraepithelial neoplasia (CIN) 2,3 samples than in normal Pap smear samples and CIN 1 samples (p = .035). There was no significant difference in telomerase levels between samples that tested positive for HPV type 16 or 18 and those that did not (p = .111).

CONCLUSIONS

Telomerase levels were significantly higher in cytologic samples from women with biopsy-proven CIN 2,3 than in samples from women with normal cytology results or CIN 1. These results warrant larger studies to determine whether telomerase activity may be a useful triage tool for abnormal cytologic findings.

Degradation of p53, not telomerase activation, by E6 is required for bypass of crisis and immortalization by human papillomavirus type 16 E6/E7

Bypass of two arrest points is essential in the process of cellular immortalization, one of the components of the transformation process. Expression of human papillomavirus type 16 E6 and E7 together can escape both senescence and crisis, processes which normally limit the proliferative capacity of primary human keratinocytes. Crisis is thought to be mediated by telomere shortening. Because E6 stimulates telomerase activity and exogenous expression of the TERT gene with E7 can immortalize keratinocytes, this function is thought to be important for E6 to cooperate with E7 to bypass crisis. However, it has also been reported that E6 dissociates increased telomerase activity from maintenance of telomere length and that a dominant-negative p53 molecule can substitute for E6 in cooperative immortalization of keratinocytes with E7. Thus, to determine which functions of E6 are required to allow bypass of crisis and immortalization of keratinocytes with E7, immortalization assays were performed using specific mutants of E6, in tandem with E7. In these experiments, every clone expressing an E6 mutant capable of degrading p53 was able to bypass crisis and immortalize, regardless of telomerase induction. All clones containing E6 mutants incapable of degrading p53 died at crisis. These results suggest that the ability of E6 to induce degradation of p53 compensates for continued telomere shortening in E6/E7 cells and demonstrate that degradation of p53 is required for immortalization by E6/E7, while increased telomerase activity is dispensable.

Immortalization of human mammary epithelial cells is associated with inactivation of the p14ARF-p53 pathway

Inactivation of the ARF-p53 tumor suppressor pathway leads to immortalization of murine fibroblasts. The role of this pathway in immortalization of human epithelial cells is not clear. We analyzed the functionality of the p14(ARF)-p53 pathway in human mammary epithelial cells (MEC) immortalized by human papillomavirus 16 (HPV16) E6, the p53 degradation-defective E6 mutant Y54D, or hTERT. E6-MEC or E6Y54D-MEC maintains high-level expression of p14(ARF). Late-passage hTERT-immortalized MEC express p53 but down-regulate p14(ARF). Enforced expression of p14(ARF) induces p53-dependent senescence in hTERT-MEC, while both E6-MEC and E6Y54D-MEC are resistant. We show that E6Y54D inhibits p14(ARF)-induced activation of p53 without inactivation of the p53-dependent DNA damage response. Hence, p53 degradation and inhibition of p14(ARF) signaling to p53 are independent functions of HPV16 E6. Our observations imply that long-term proliferation of MEC requires inactivation of the p14(ARF)-p53 pathway.

Propagation of infectious human papillomavirus type 16 by using an adenovirus and Cre/LoxP mechanism

Human papillomavirus type 16 (HPV16) infection is a major risk factor for the development of squamous cell cancers of the cervix and of the head and neck. A major barrier to understanding the progression from initial infection to cancer has been the lack of in vitro models that allow infection, replication, and persistence of the viral genome as an episome in differentiated epithelial cells. To overcome this barrier, we designed an adenoviral delivery vector that contained a full HPV16 genome flanked by LoxP homologous recombination sites and a fluorescent reporter that was expressed only after the HPV genome was excised by Cre recombinase. This system delivered circular HPV16 genomes to cervical epithelial cells and well differentiated human airway epithelia. After delivery, the HPV16 genome replicated and persisted as an episome in cervical keratinocytes. These cells developed an immortalized phenotype and a dysplastic epithelial appearance. Moreover, induction of differentiation led to the expression of late genes and production of infectious HPV16 virions. This work provides a means of introducing biologically active HPV genomes into epithelial cells, which are normally difficult to transfect. These methods allow the study of HPV genome replication and gene expression in the earliest stages of HPV genome establishment, and they may provide a means to study nononcogenic HPV viral types.