Functional interaction between human papillomavirus type 18 E2 and poly(ADP-ribose) polymerase 1

Polymorphisms and functional analysis of the intact human papillomavirus16 e2 gene

High risk human papillomavirus (HR-HPV) E2 proteins play roles in transcriptional regulation and are commonly functionally disrupted when the HPV genome integrates into host chromosomes. Some 15-40% of cancer cases, however, contain an intact E2 gene or episomal HPV. In these cases, polymorphism of the E2 gene might be involved. This study aimed to determine polymorphisms of the E2 gene in episomal HPV16 detected in high grade squamous intraepithelial lesions and squamous cell carcinomas and altered functions compared to the E2 prototype. The E2 gene was amplified and sequenced. Two expression vectors containing E2 gene polymorphisms were constructed and transfected in SiHa and C33A cells, then E6 gene as well as Il- 10 and TNF-α expression was determined by quantitative RT-PCR. Expression vectors and reporter vectors containing the HPV16 long control region (LCR) were co-transfected and transcriptional activity was determined. The results showed that a total of 32 nucleotides and 23 amino acids were changed in all 20 cases of study, found in the transactivation (TA) domain, hinge (H) region and DNA binding (DB) domain with 14, 5 and 13 nucleotide positions. They mostly caused amino acid change. The expressing vectors containing different E2 gene polymorphisms showed E6 mRNA suppression, TNF-α mRNA suppression and IL-10 induction but no statistically significant differences when compared to the E2 prototype. Moreover, promoter activity in HPV16 LCR was not affected by E2 protein with different gene polymorphisms, in contrast to nucleotide variations in LCR that showed an effect on transcription activity. These results demonstrated that E2 gene polymorphisms of episomal HPV16 did not affect transcriptional regulation and suggested that nucleotide variation as well as epigenetic modification of the LCR might play a role in inducing malignant transformation of cells containing episomal HPV16.

Poly(ADP-ribose) polymerase-1 modulates Nrf2-dependent transcription

The basic leucine zipper transcription factor Nrf2 has emerged as a master regulator of intracellular redox homeostasis by controlling the expression of a battery of redox-balancing antioxidants and phase II detoxification enzymes. Under oxidative stress conditions, Nrf2 is induced at the protein level through redox-sensitive modifications on critical cysteine residues in Keap1, a component of an E3 ubiquitin ligase complex that targets Nrf2 for proteasomal degradation. Poly(ADP-ribose) polymerase-1 (PARP-1) is historically known to function in DNA damage detection and repair; however, recently PARP-1 has been shown to play an important role in other biochemical activities, such as DNA methylation and imprinting, insulator activity, chromosome organization, and transcriptional regulation. The exact role of PARP-1 in transcription modulation and the underlying mechanisms remain poorly defined. In this study, we report that PARP-1 forms complexes with the antioxidant response element (ARE) within the promoter region of Nrf2 target genes and upregulates the transcriptional activity of Nrf2. Interestingly, PARP-1 neither physically interacts with Nrf2 nor promotes the expression of Nrf2. In addition, PARP-1 does not target Nrf2 for poly(ADP-ribosyl)ation. Instead, PARP-1 interacts directly with small Maf proteins and the ARE of Nrf2 target genes, which augments ARE-specific DNA-binding of Nrf2 and enhances the transcription of Nrf2 target genes. Collectively, these results suggest that PARP-1 serves as a transcriptional coactivator, upregulating the transcriptional activity of Nrf2 by enhancing the interaction among Nrf2, MafG, and the ARE.

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

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

Expression of BAG-1 and PARP-1 in precursor lesions and invasive cervical cancer associated with human papillomavirus (HPV)

Cervical cancer remains persistently the second most common malignancies among women worldwide, responsible for 500,000 new cases annually. Only in Brazil, the estimate is for 18,430 new cases in 2011. Several types of molecular markers have been studied in carcinogenesis including proteins associated with apoptosis such as BAG-1 and PARP-1. This study aims to demonstrate the expression of BAG-1 and PARP-1 in patients with low-grade squamous intraepithelial lesions (LSILs), high-grade squamous intraepithelial lesions (HSILs) and invasive squamous cell carcinomas (SCCs) of the uterine cervix and to verify a possible association with HPV infection. Fifty samples of LSILs, 50 samples of HSILs and 50 samples of invasive SCCs of the uterine cervix were analyzed by immunohistochemistry for BAG-1 and PARP-1 expression. PCR was performed to detect and type HPV DNA. BAG-1 expression levels were significantly different between LSILs and HSILs (p = 0,014) and between LSILs and SCCs (p = 0,014). In regards to PARP-1 expression, we found significant differences between the expression levels in HSILs and SCCs (p = 0,022). No association was found between BAG-1 expression and the presence of HPV. However, a significant association was found between PARP-1 expression and HPV positivity in the HSILs group (p = 0,021). In conclusion our research suggests that BAG-1 expression could contribute to the differentiation between LSIL and HSIL/SCC whereas PARP-1 could be useful to the differentiation between HSIL HPV-related and SCC. Further studies are needed to clarify the molecular aspects of the relationship between PARP-1 expression and HPV infection, with potential applications for cervical cancer prediction.

The human papillomavirus type 8 E2 protein induces skin tumors in transgenic mice

Transgenic mice expressing early genes of the cutaneous human papillomavirus 8 (HPV8) spontaneously develop skin papillomas, epidermal dysplasia, and squamous cell carcinoma (6%). As the HPV8 protein E2 revealed transforming capacity in vitro, we generated three epidermal specific HPV8-E2-transgenic FVB/N mouse lines to dissect its role in tumor development. The rate of tumor formation in the three lines correlated with the different E2-mRNA levels. More than 60% of heterozygous line 2 mice, but none of the HPV8-negative littermates, spontaneously developed ulcerous lesions of the skin over an observation period of up to 144 weeks, beginning on average 74+/-22 weeks after birth. Most lesions presented infundibular hyperplasia and acanthosis combined with low-grade dysplasia. Severe dysplasia of the epidermis occurred in 6%. Two carcinomas revealed a sharply demarcated spindle-cell component. Only 3 weeks after a single UV irradiation, 87% of heterozygous line 2 and 36% of line 35 mice developed skin tumors. A rapidly growing invasive tumor composed of spindle cells arose 10 weeks after irradiation of a line-35 animal. The histology of skin cancers in HPV8-E2 mice is reminiscent of a subset of highly aggressive squamous cell carcinoma in immunosuppressed transplant recipients with a massive spindle-cell component.

Poly(ADP-ribose) polymerase-1 regulates vimentin expression in lung cancer cells

Vimentin is one of the mammalian intermediate filament proteins. It is expressed in cells of mesenchymal origin and is characteristic of proliferating cells at the fetal stage. During malignancy, vimentin expression is activated in certain lung epithelial cells. Examination of a group of lung cancer cells showed a marked difference in their vimentin expression. The difference in vimentin expression among lung cancer cells is due to differential regulation at the transcriptional level. Analysis of the vimentin promoter revealed a 102-bp promoter sequence that is important for promoter activity in a lung cancer cell line in which vimentin is strongly expressed. This promoter region interacts with poly(ADP-ribose) polymerase-1 (PARP-1), which is also a transcription regulator. Exogenous expression of PARP-1 increased vimentin promoter activity. A shortened PARP-1 without the COOH-terminal catalytic domain showed the same promoter activation effect. Treatment of cells with H(2)O(2) reduced PARP-1 and vimentin expression at the protein level. H(2)O(2) also dose dependently suppressed vimentin promoter activity in cells overexpressing PARP-1. These results demonstrate that vimentin expression in lung cancer cells is regulated at the transcriptional level and that PARP-1 binds and activates the vimentin promoter independent of its catalytic domain and may play a role in H(2)O(2)-induced inhibition of vimentin expression.

The general transcription machinery and general cofactors

In eukaryotes, the core promoter serves as a platform for the assembly of transcription preinitiation complex (PIC) that includes TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH, and RNA polymerase II (pol II), which function collectively to specify the transcription start site. PIC formation usually begins with TFIID binding to the TATA box, initiator, and/or downstream promoter element (DPE) found in most core promoters, followed by the entry of other general transcription factors (GTFs) and pol II through either a sequential assembly or a preassembled pol II holoenzyme pathway. Formation of this promoter-bound complex is sufficient for a basal level of transcription. However, for activator-dependent (or regulated) transcription, general cofactors are often required to transmit regulatory signals between gene-specific activators and the general transcription machinery. Three classes of general cofactors, including TBP-associated factors (TAFs), Mediator, and upstream stimulatory activity (USA)-derived positive cofactors (PC1/PARP-1, PC2, PC3/DNA topoisomerase I, and PC4) and negative cofactor 1 (NC1/HMGB1), normally function independently or in combination to fine-tune the promoter activity in a gene-specific or cell-type-specific manner. In addition, other cofactors, such as TAF1, BTAF1, and negative cofactor 2 (NC2), can also modulate TBP or TFIID binding to the core promoter. In general, these cofactors are capable of repressing basal transcription when activators are absent and stimulating transcription in the presence of activators. Here we review the roles of these cofactors and GTFs, as well as TBP-related factors (TRFs), TAF-containing complexes (TFTC, SAGA, SLIK/SALSA, STAGA, and PRC1) and TAF variants, in pol II-mediated transcription, with emphasis on the events occurring after the chromatin has been remodeled but prior to the formation of the first phosphodiester bond.

PARP-1 interaction with VP1 capsid protein regulates polyomavirus early gene expression

Poly(ADP-ribose)polymerases are involved in fundamental cellular events as well as they seem to be associated to some viral infection process. In this work, the poly(ADP-ribose)polymerase-1 (PARP-1) role in the polyomavirus life cycle has been investigated. Early viral transcription was reduced by competitive inhibitors of PARPs in Swiss 3T3 cells and almost abolished in PARP-1 knockout fibroblasts and in wild-type fibroblasts when PARP-1 was silenced by RNA interference. In vivo chromatin immunoprecipitation assays showed that poly(ADP-ribosyl)ation (poly(ADP-ribose)) facilitates the release of the capsid protein viral protein 1 (VP1) from the chromatin of infecting virions. In vitro experiments demonstrated that VP1 stimulates the enzymatic activity of PARP-1 and binds non-covalently both protein-free and PARP-1-bound poly(ADP-ribose). Our studies suggest that PARP-1 promotes the complete VP1 displacement from viral DNA favouring the viral early transcription.

Interactions of the cellular CCAAT displacement protein and human papillomavirus E2 protein with the viral origin of replication can regulate DNA replication

Previously, we and others have shown that CCAAT displacement protein (CDP) negatively regulates the papillomavirus promoters. Overexpression of CDP has been shown to inhibit high-risk human papillomavirus virus (HPV) and bovine papillomavirus DNA replication in vivo presumably through reduction in expression of viral replication proteins, E1 and E2. Sequence analysis of the HPV origin indicates several potential CDP-binding sites with one site overlapping the E1-binding site. Therefore, CDP could also negatively regulate papillomavirus replication directly by preventing the loading of the initiation complex. We show here that purified CDP inhibits in vitro HPV DNA replication. Footprint analysis demonstrated that CDP binds the E1-binding site and the TATA box, and that the binding of purified CDP to the E1-binding site is decreased by the addition of purified E2 protein. Consistent with this, E2-independent in vitro HPV replication is inhibited by CDP to a greater extent than E2-dependent replication. These results suggest that binding of E2 at the E2-binding site may play an important role in overcoming the inhibition of E1 initiation complex formation caused by the binding of negative regulators like CDP to the origin of replication.

PARP-1 binds E2F-1 independently of its DNA binding and catalytic domains, and acts as a novel coactivator of E2F-1-mediated transcription during re-entry of quiescent cells into S phase

The transcription factor E2F-1 is implicated in the activation of S-phase genes as well as induction of apoptosis, and is regulated by interactions with Rb and by cell cycle-dependent alterations in E2F-1 abundance. We earlier demonstrated a pivotal role for poly(ADP-ribose) polymerase-1 (PARP-1) in the regulation of E2F-1 expression and promoter activity during S-phase re-entry when quiescent cells re-enter the cell cycle. We now investigate the putative mechanism(s) by which PARP-1 may upregulate E2F-1 promoter activity during S-phase re-entry. DNase-1 footprint assays with purified PARP-1 showed that PARP-1 did not directly bind the E2F-1 promoter in a sequence-specific manner. In contrast to p53, a positive acceptor in poly(ADP-ribosyl)ation reactions, E2F-1 was not poly(ADP-ribosyl)ated by wild-type PARP-1 in vitro, indicating that PARP-1 does not exert a dual effect on E2F-1 transcriptional activation. Protein-binding reactions and coimmunoprecipitation experiments with purified PARP-1 and E2F-1, however, revealed that PARP-1 binds to E2F-1 in vitro. More significantly, physical association of PARP-1 and E2F-1 in vivo also occurred in wild-type fibroblasts 5 h after re-entry into S phase, coincident with the increase in E2F-1 promoter activity and expression of E2F-1-responsive S-phase genes cyclin A and c-Myc. Mapping of the interaction domains revealed that full-length PARP-1 as well as PARP-1 mutants lacking either the catalytic active site or the DNA-binding domain equally bind E2F-1, whereas a PARP-1 mutant lacking the automodification domain does not, suggesting that the protein interaction site is located in this central domain. Finally, gel shift analysis with end-blocked E2F-1 promoter sequence probes verified that the binding of PARP-1 to E2F-1 enhances binding to the E2F-1 promoter, indicating that PARP-1 acts as a positive cofactor of E2F-1-mediated transcription.

In vivo regulation of the chicken cardiac troponin T gene promoter in zebrafish embryos

The chicken cardiac troponin T (cTnT) gene is representative of numerous cardiac and skeletal muscle-specific genes that contain muscle-CAT (MCAT) elements within their promoters. We examined the regulation of the chicken cTnT gene in vivo in zebrafish embryos, and in vitro in cardiomyocyte, myoblast, and fibroblast cultures. Defined regions of the cTnT promoter were linked to the green fluorescent protein (GFP) gene for in vivo analysis, and the luciferase gene for in vitro analysis. Injection of the cTnT promoter constructs into fertilized zebrafish eggs resulted in GFP expression in both heart and skeletal muscle cells reproducing the pattern of expression of the endogenous cTnT gene in the chicken embryo. Promoter deletion analysis revealed that the cis-regulatory regions responsible for cardiac and skeletal muscle-specific expression functioned in an equivalent manner in both in vitro and in vivo environments. In addition, we show that mutation of the poly-ADP ribose polymerase-I (PARP-I) binding site adjacent to the distal MCAT element in the chicken cTnT promoter produced a non-cell-specific promoter in vitro and in the zebrafish. Thus, the PARP-I transcriptional regulatory mechanism that governs muscle specificity of the chicken cTnT promoter is conserved across several chordate classes spanning at least 350 million years of evolution.

UVB irradiation reduces the half-life and transactivation potential of the human papillomavirus 16 E2 protein

Human papillomaviruses (HPV) are causative agents of human cancers including those of the cervix and also of the head and neck; HPV16 is the most commonly found type in these diseases. The viral E2 protein regulates transcription from the viral genome by interacting with DNA-binding sequences in the HPV transcriptional control region; it also regulates replication by interacting with and recruiting the HPV replication factor E1 to the viral origin. Therefore, E2 is essential for the viral life cycle. The E2 protein interacts with several proteins involved in the cellular response to DNA damage including p53, TopBP1, and PARP. We therefore set out to establish whether DNA-damaging agents can regulate E2 activity. Here we show that UVB irradiation downregulates transcriptional activity of both HPV16 and HPV8 E2, while hydroxyurea and etoposide do not. This downregulation of E2 activity is independent of p53 function as it occurs in p53 wild type and null cell types as well as in the presence of functional HPV16 E6 that degrades p53. Using stable cell lines expressing E2 we show that this downregulation of E2 function by UVB is due to a reduction of the E2 protein half-life. The identification of the pathway(s) through which UVB downregulates E2 transcriptional activity and protein levels will present a novel target for the treatment of HPV-related diseases.

PARP goes transcription

PARP-1, an enzyme that catalyzes the attachment of ADP ribose units to target proteins, plays at least two important roles in transcription regulation. First, PARP-1 modifies histones and creates an anionic poly(ADPribose) matrix that binds histones, thereby promoting the decondensation of higher-order chromatin structures. Second, PARP-1 acts as a component of enhancer/promoter regulatory complexes. Recent studies have shown that both of these activities are critical for gene regulation in vivo.