Transcriptional regulation by human papillomaviruses

Epigenetics of oropharyngeal squamous cell carcinoma: opportunities for novel chemotherapeutic targets

Epigenetic modifications are heritable changes in gene expression that do not directly alter DNA sequence. These modifications include DNA methylation, histone post-translational modifications, small and non-coding RNAs. Alterations in epigenetic profiles cause deregulation of fundamental gene expression pathways associated with carcinogenesis. The role of epigenetics in oropharyngeal squamous cell carcinoma (OPSCC) has recently been recognized, with implications for novel biomarkers, molecular diagnostics and chemotherapeutics. In this review, important epigenetic pathways in human papillomavirus (HPV) positive and negative OPSCC are summarized, as well as the potential clinical utility of this knowledge.This material has never been published and is not currently under evaluation in any other peer-reviewed publication.

FOXM1 (Forkhead box M1) in tumorigenesis: overexpression in human cancer, implication in tumorigenesis, oncogenic functions, tumor-suppressive properties, and target of anticancer therapy

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor and is also intimately involved in tumorigenesis. FOXM1 stimulates cell proliferation and cell cycle progression by promoting the entry into S-phase and M-phase. Additionally, FOXM1 is required for proper execution of mitosis. In accordance with its role in stimulation of cell proliferation, FOXM1 exhibits a proliferation-specific expression pattern and its expression is regulated by proliferation and anti-proliferation signals as well as by proto-oncoproteins and tumor suppressors. Since these factors are often mutated, overexpressed, or lost in human cancer, the normal control of the foxm1 expression by them provides the basis for deregulated FOXM1 expression in tumors. Accordingly, FOXM1 is overexpressed in many types of human cancer. FOXM1 is intimately involved in tumorigenesis, because it contributes to oncogenic transformation and participates in tumor initiation, growth, and progression, including positive effects on angiogenesis, migration, invasion, epithelial-mesenchymal transition, metastasis, recruitment of tumor-associated macrophages, tumor-associated lung inflammation, self-renewal capacity of cancer cells, prevention of premature cellular senescence, and chemotherapeutic drug resistance. However, in the context of urethane-induced lung tumorigenesis, FOXM1 has an unexpected tumor suppressor role in endothelial cells because it limits pulmonary inflammation and canonical Wnt signaling in epithelial lung cells, thereby restricting carcinogenesis. Accordingly, FOXM1 plays a role in homologous recombination repair of DNA double-strand breaks and maintenance of genomic stability, that is, prevention of polyploidy and aneuploidy. The implication of FOXM1 in tumorigenesis makes it an attractive target for anticancer therapy, and several antitumor drugs have been reported to decrease FOXM1 expression.

Expression of the HPV11 E2 gene in transgenic mice does not result in alterations of the phenotypic pattern

The E2 early protein of human papillomaviruses (HPV) has been found associated with the mitotic spindle therefore being implicated in the partition of the replicated viral DNA to daughter cells. In addition, E2 proteins bind to the upstream regulatory region of the virus and to cellular promoters modulating thereby cellular transcription and differentiation. In many cervical cancers, the E2 reading frame is interrupted upon incorporation of the viral genome into the host DNA. This results in the loss of the E2 mediated transcriptional repression and uncontrolled expression of the viral oncogenes. All these results have been obtained in transfected cells but no information is available on the E2 effects in the context of the entire organism. Transgenic mice were generated expressing the E2 protein of HPV11 under the control of the Ubiquitin C promoter. E2 mRNA is present in all mice tissues analysed and the E2 protein expressed in the skin (the target tissue of HPV11) was shown by Western blotting, albeit at a very low level. Analysis of the transgenic mice shows no major histological changes in the skin or all other tissues investigated. These data indicate that in transgenic mice the human papillomavirus type 11 E2 does not grossly modulate cellular proliferation or differentiation events.

Rat cytomegalovirus gene expression in cardiac allograft recipients is tissue specific and does not parallel the profiles detected in vitro

Rat cytomegalovirus (RCMV) is a beta-herpesvirus with a 230-kbp genome containing over 167 open reading frames (ORFs). RCMV gene expression is tightly regulated in cultured cells, occurring in three distinct kinetic classes (immediate early, early, and late). However, the extent of viral-gene expression in vivo and its relationship to the in vitro expression are unknown. In this study, we used RCMV-specific DNA microarrays to investigate the viral transcriptional profiles in cultured, RCMV-infected endothelial cells, fibroblasts, and aortic smooth muscle cells and to compare these profiles to those found in tissues from RCMV-infected rat heart transplant recipients. In cultured cells, RCMV expresses approximately 95% of the known viral ORFs with few differences between cell types. By contrast, in vivo viral-gene expression in tissues from rat heart allograft recipients is highly restricted. In the tissues studied, a total of 80 viral genes expressing levels twice above background (5,000 to 10,000 copies per mug total RNA) were detected. In each tissue type, there were a number of genes expressed exclusively in that tissue. Although viral mRNA and genomic DNA levels were lower in the spleen than in submandibular glands, the number of individual viral genes expressed was higher in the spleen (60 versus 41). This finding suggests that the number of viral genes expressed is specific to a given tissue and is not dependent upon the viral load or viral mRNA levels. Our results demonstrate that the profiles, as well as the amplitude, of viral-gene expression are tissue specific and are dramatically different from those in infected cultured cells, indicating that RCMV gene expression in vitro does not reflect viral-gene expression in vivo.

Viral oncoproteins target the DNA methyltransferases

Small DNA tumour viruses have evolved a number of mechanisms to drive nondividing cells into S phase. Virally encoded oncoproteins such as adenovirus E1A and human papillomavirus (HPV) E7 can bind an array of cellular proteins to override proliferation arrest. The DNA methyltransferase Dnmt1 is the major mammalian enzyme responsible for maintaining CpG methylation patterns in the cell following replication. One of the hallmarks of tumour cells is disrupted DNA methylation patterns, highlighting the importance of the proper regulation of DNA methyltransferases in normal cell proliferation. Here, we show that adenovirus 5 E1A and HPV-16 E7 associate in vitro and in vivo with the DNA methyltransferase Dnmt1. Consistent with this interaction, we find that E1A and E7 can purify DNA methyltransferase activity from nuclear extracts. These associations are direct and mediated by the extreme N-terminus of E1A and the CR3 zinc-finger domain of E7. Furthermore, we find that a point mutant at leucine 20 of E1A, a residue known to be critical for its transformation functions, is unable to bind Dnmt1 and DNA methyltransferase activity. Finally, both E1A and E7 can stimulate the methyltransferase activity of Dnmt1 in vitro. Our results provide the first indication that viral oncoproteins bind and regulate Dnmt1 enzymatic activity. These observations open up the possibility that this association may be used to control cellular proliferation pathways and suggest a new mechanism by which small DNA tumour viruses can steer cells through the cell cycle.

A plant lignan, 3′-O-Methyl-Nordihydroguaiaretic acid, suppresses papillomavirus E6 protein function, stabilizes p53 protein, and induces apoptosis in cervical tumor cells

Persistent infection with oncogenic human papillomaviruses (HPVs) is the most important factor in the induction of uterine cervical cancer, a leading cause of cancer mortality in women worldwide. Upon cell transformation, continual expression of the viral oncogenes is required to maintain the transformed phenotype. The viral E6 protein forms a ternary complex with the cellular E6-AP protein and p53 protein which promotes the rapid degradation of p53. Recent studies have revealed that lignans from the creosote bush (3'-O-methyl-nordihydroguaiaretic acid) can repress the viral promoter responsible for E6 gene expression. Work reported here shows that the lignan can subvert viral oncogene function resulting in stabilized p53 protein within treated HPV-containing tumor cells. The stabilized p53 is transcriptionally active as demonstrated by a luciferase reporter vector and induction of genes for Bax and PUMA proteins. Apoptosis is detected by annexin V binding to treated cells as analyzed by flow cytometry. Programmed cell death is confirmed by the induction of active caspases and TUNEL assay. Initiator caspase-9 is activated first, followed later by the effector caspase-3 enzyme. The stabilization and induced apoptosis are not observed within treated HPV-negative cervical tumor cells. Quantitative real time RT-PCR analysis of endogenous E6 gene transcription from the integrated HPV 16 promoter shows at least a fivefold repression of expression as compared to untreated cells. These results indicate that the loss of E6 protein in treated cells could be, in part, responsible for the stabilization of p53 within the lignan treated cells.

Identification of differentially expressed genes in HPV-positive and HPV-negative oropharyngeal squamous cell carcinomas

Human papillomaviruses (HPVs) have been implicated in the pathogenesis of a subset of squamous cell carcinoma of the head and neck (SCCHN). The goal of this study was to compare the cellular gene expression profiles of HPV-positive and HPV-negative oropharyngeal carcinomas with those of the normal oral epithelium. Using Affymetrix Human U133A GeneChip, our results showed that 397 genes were differentially expressed in HPV-positive SCCHN compared to the normal oral epithelium. The upregulated genes included those involved in cell cycle regulation (CDKN2A), cell differentiation (SFRP4) and DNA repair (RAD51AP1), while the downregulated genes included those involved in proteolysis (PRSS3). We also found 162 differentially expressed genes in HPV-negative SCCHN compared to the normal oral mucosa. The upregulated genes included those involved in cell proliferation (AKR1C3) and transcription regulation (SNAPC1), while downregulated genes included those involved in apoptosis (CLU) and RNA processing (RBM3). Our studies also identified a subgroup of 59 differentially expressed genes in HPV-positive SCCHN as compared to both HPV-negative SCCHN and normal oral tissues. Such upregulated genes included those involved in nuclear structure and meiosis (SYCP2), DNA repair (RFC5), and transcription regulation (ZNF238). Genes involved in proteolysis (KLK8) and signal transduction (CRABP2) were found to be downregulated in HPV-positive SCCHN. The results of GeneChip experiments were validated by quantitative real-time RT-PCR analysis of a few representative genes. Our results reveal specific gene expression patterns in HPV-positive and HPV-negative oropharyngeal squamous carcinomas that may serve as potential biomarkers for the development of SCCHN.

Viruses and sumoylation: recent highlights

Since its discovery in 1997, SUMO (small ubiquitin-like modifier) has been implicated in a range of activities, indicating that this protein is as important in the cell as ubiquitin is. Although it can function throughout the cell, it appears to be involved more in nuclear functions. The growing list of substrates that are covalently modified by SUMO includes many viral proteins; SUMO appears to facilitate viral infection of cells, making it a possible target for antiviral therapies. It therefore is important to understand how viruses manipulate the cellular sumoylation system and how sumoylation affects viral functions.