Adeno-associated virus type 2 rep protein inhibits human papillomavirus type 16 E2 recruitment of the transcriptional coactivator p300

The interaction of human papillomaviruses and adeno-associated viruses in suppressive co-infections

Human papillomavirus (HPV) is one of the most common oncogenic viruses which cause malignancy in different epithelial surfaces of the human body and its infection is the main cause of cervical cancer. However, research suggests that this virus might not be the sole cause of infection in target cells. It is believed that, other infectious agents could co-infect the same cell with HPV including; bacteria, viruses, and parasites, which may have different effects on the carcinogenesis of HPV infections. One of the most important viruses is adeno-associated virus (AAV), which comes from the parvoviridae family. The function of this virus is associated with several stages of HPV carcinogenicity, which leads to the suppression of HPV oncogenesis. The inhibition effects of AAV are exerted not only in viral parts but also in cellular parts. This suppression illuminates a new therapeutic approach in the way of HPV-associated cervical cancer. In the present review we consider the exact roles of AAV infection in this suppression.

High-risk human papillomavirus targets crossroads in immune signaling

Persistent infections with a high-risk type human papillomavirus (hrHPV) can progress to cancer. High-risk HPVs infect keratinocytes (KCs) and successfully suppress host immunity for up to two years despite the fact that KCs are well equipped to detect and initiate immune responses to invading pathogens. Viral persistence is achieved by active interference with KCs innate and adaptive immune mechanisms. To this end hrHPV utilizes proteins encoded by its viral genome, as well as exploits cellular proteins to interfere with signaling of innate and adaptive immune pathways. This results in impairment of interferon and pro-inflammatory cytokine production and subsequent immune cell attraction, as well as resistance to incoming signals from the immune system. Furthermore, hrHPV avoids the killing of infected cells by interfering with antigen presentation to antigen-specific cytotoxic T lymphocytes. Thus, hrHPV has evolved multiple mechanisms to avoid detection and clearance by both the innate and adaptive immune system, the molecular mechanisms of which will be dealt with in detail in this review.

Adeno-associated virus type 2 infection activates caspase dependent and independent apoptosis in multiple breast cancer lines but not in normal mammary epithelial cells

Background

In normal cells proliferation and apoptosis are tightly regulated, whereas in tumor cells the balance is shifted in favor of increased proliferation and reduced apoptosis. Anticancer agents mediate tumor cell death via targeting multiple pathways of programmed cell death. We have reported that the non-pathogenic, tumor suppressive Adeno-Associated Virus Type 2 (AAV2) induces apoptosis in Human Papillomavirus (HPV) positive cervical cancer cells, but not in normal keratinocytes. In the current study, we examined the potential of AAV2 to inhibit proliferation of MCF-7 and MDA-MB-468 (both weakly invasive), as well as MDA-MB-231 (highly invasive) human breast cancer derived cell lines. As controls, we used normal human mammary epithelial cells (nHMECs) isolated from tissue biopsies of patients undergoing breast reduction surgery.

Results

AAV2 infected MCF-7 line underwent caspase-independent, and MDA-MB-468 and MDA-MB-231 cell lines underwent caspase-dependent apoptosis. Death of MDA-MB-468 cells was marked by caspase-9 activation, whereas death of MDA-MB-231 cells was marked by activation of both caspase-8 and caspase-9, and resembled a mixture of apoptotic and necrotic cell death. Cellular demise was correlated with the ability of AAV2 to productively infect and differentially express AAV2 non-structural proteins: Rep78, Rep68 and Rep40, dependent on the cell line. Cell death in the MCF-7 and MDA-MB-231 lines coincided with increased S phase entry, whereas the MDA-MB-468 cells increasingly entered into G2. AAV2 infection led to decreased cell viability which correlated with increased expression of proliferation markers c-Myc and Ki-67. In contrast, nHMECs that were infected with AAV2 failed to establish productive infection or undergo apoptosis.

Conclusion

AAV2 regulated enrichment of cell cycle check-point functions in G1/S, S and G2 phases could create a favorable environment for Rep protein expression. Inherent Rep associated endonuclease activity and AAV2 genomic hair-pin ends have the potential to induce a cellular DNA damage response, which could act in tandem with c-Myc regulated/sensitized apoptosis induction. In contrast, failure of AAV2 to productively infect nHMECs could be clinically advantageous. Identifying the molecular mechanisms of AAV2 targeted cell cycle regulation of death inducing signals could be harnessed for developing novel therapeutics for weakly invasive as well as aggressive breast cancer types.

A prominent β-hairpin structure in the winged-helix domain of RECQ1 is required for DNA unwinding and oligomer formation

RecQ helicases have attracted considerable interest in recent years due to their role in the suppression of genome instability and human diseases. These atypical helicases exert their function by resolving a number of highly specific DNA structures. The crystal structure of a truncated catalytic core of the human RECQ1 helicase (RECQ1^49–616) shows a prominent β-hairpin, with an aromatic residue (Y564) at the tip, located in the C-terminal winged-helix domain. Here, we show that the β-hairpin is required for the DNA unwinding and Holliday junction (HJ) resolution activity of full-length RECQ1, confirming that it represents an important determinant for the distinct substrate specificity of the five human RecQ helicases. In addition, we found that the β-hairpin is required for dimer formation in RECQ1^49–616 and tetramer formation in full-length RECQ1. We confirmed the presence of stable RECQ1^49–616 dimers in solution and demonstrated that dimer formation favours DNA unwinding; even though RECQ1 monomers are still active. Tetramers are instead necessary for more specialized activities such as HJ resolution and strand annealing. Interestingly, two independent protein–protein contacts are required for tetramer formation, one involves the β-hairpin and the other the N-terminus of RECQ1, suggesting a non-hierarchical mechanism of tetramer assembly.

Cervical cancer isolate PT3, super-permissive for adeno-associated virus replication, over-expresses DNA polymerase delta, PCNA, RFC and RPA

BACKGROUND

Adeno-associated virus (AAV) type 2 is an important virus due to its use as a safe and effective human gene therapy vector and its negative association with certain malignancies. AAV, a dependo-parvovirus, autonomously replicates in stratified squamous epithelium. Such tissue occurs in the nasopharynx and anogenitals, from which AAV has been clinically isolated. Related autonomous parvoviruses also demonstrate cell tropism and preferentially replicate in oncogenically transformed cells. Combining these two attributes of parvovirus tropism, squamous and malignant, we assayed if AAV might replicate in squamous cervical carcinoma cell isolates.

RESULTS

Three primary isolates (PT1-3) and two established cervical cancer cell lines were compared to normal keratinocytes (NK) for their ability to replicate AAV. One isolate, PT3, allowed for high levels of AAV DNA replication and virion production compared to others. In research by others, four cellular components are known required for in vitro AAV DNA replication: replication protein A (RPA), replication factor C (RFC), proliferating cell nuclear antigen (PCNA), and DNA polymerase delta (POLD1). Thus, we examined PT3 cells for expression of these components by DNA microarray and real-time quantitative PCR. All four components were over-expressed in PT3 over two representative low-permissive cell isolates (NK and PT1). However, this super-permissiveness did not result in PT3 cell death by AAV infection.

CONCLUSION

These data, for the first time, provide evidence that these four cellular components are likely important for AAV in vivo DNA replication as well as in vitro. These data also suggest that PT3 will be a useful reagent for investigating the AAV-permissive transcriptome and AAV anti-cancer effect.

Acetylation by GCN5 regulates CDC6 phosphorylation in the S phase of the cell cycle

In eukaryotic cells, the cell-division cycle (CDC)-6 protein is essential to promote the assembly of pre-replicative complexes in the early G1 phase of the cell cycle, a process requiring tight regulation to ensure that proper origin licensing occurs once per cell cycle. Here we show that, in late G1 and early S phase, CDC6 is found in a complex also containing Cyclin A, cyclin-dependent kinase (CDK)-2 and the acetyltransferase general control nonderepressible 5 (GCN5). GCN5 specifically acetylates CDC6 at three lysine residues flanking its cyclin-docking motif, and this modification is crucial for the subsequent phosphorylation of the protein by Cyclin A-CDKs at a specific residue close to the acetylation site. GCN5-mediated acetylation and site-specific phosphorylation of CDC6 are both necessary for the relocalization of the protein to the cell cytoplasm in the S phase, as well as to regulate its stability. This two-step, intramolecular regulatory program by sequential modification of CDC6 seems to be essential for proper S-phase progression.

AAV-2 Rep78 and HPV-16 E1 interact in vitro, modulating their ATPase activity

Adeno-associated virus (AAV) is a nonpathogenic single-stranded human parvovirus which usually requires the presence of a "helper" virus for strong DNA replication. In addition to adeno- and herpes viruses, human papillomavirus (HPV) can serve as an AAV helper. We recently published that HPV type 16 (HPV-16) E1 protein contributes significantly as an individual helper gene for AAV-2 DNA replication and transcription. As Rep78 and E1 are the corresponding DNA helicase/replication proteins of AAV and HPV, respectively, and Rep78 and E1 have a degree of homology, we assayed whether these two proteins interact physically. The full length proteins were purified from bacteria as GST-E1 and MBP-Rep78 and used in five assays to observe Rep78-E1 interactions. All five assays (pull-down, coimmunoprecipitation, enzyme-linked immunosorbent assay (ELISA), chemical cross-linking, and ATPase activity) provided evidence consistent with Rep78-E1 interaction. Most intriguing, an overall decrease in ATPase activity was observed when both proteins were present together. These data strongly suggest that E1 and Rep78 interact and that this interaction modulates at least some of their individual biochemical functions. This study adds to our understanding of AAV-HPV interaction biology, E1's modulation of Rep78 biochemistry, Rep78's modulation of E1 biochemistry and provides initial clues which may lead to the underlying mechanism of HPV E1 helper function for AAV DNA replication.

PRMT11: a new Arabidopsis MBD7 protein partner with arginine methyltransferase activity

Plant methyl-DNA-binding proteins (MBDs), discovered by sequence homology to their animal counterparts, have not been well characterized at the physiological and functional levels. In order better to characterize the Arabidopsis AtMBD7 protein, unique in bearing three MBD domains, we used a yeast two-hybrid system to identify its partners. One of the interacting proteins we cloned is the Arabidopsis arginine methyltransferase 11 (AtPRMT11). Glutathione S-transferase pull-down and co-immunoprecipitation assays confirmed that the two proteins interact with each other and can be co-isolated. Using GFP fluorescence, we show that both AtMBD7 and AtPRMT11 are present in the nucleus. Further analyses revealed that AtPRMT11 acts as an arginine methyltransferase active on both histones and proteins of cellular extracts. The analysis of a T-DNA mutant line lacking AtPRMT11 mRNA revealed reduced levels of proteins with asymmetrically dimethylated arginines, suggesting that AtPRMT11, which is highly similar to mammalian PRMT1, is indeed a type I arginine methyltransferase. Further, AtMBD7 is a substrate for AtPRMT11, which post-translationally modifies the portion of the protein-containing C-terminal methylated DNA-binding domain. These results suggest the existence of a link between DNA methylation and arginine methylation.

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.

Virus-mediated gene transfer to induce therapeutic angiogenesis: where do we stand?

The potential to induce therapeutic angiogenesis through gene transfer has engendered much excitement as a possible treatment for tissue ischemia. After 10 years of clinical experimentation, however, it now appears clear that several crucial issues are still to be resolved prior to achieving clinical success. These include the understanding of whether functional blood vessels might arise as a result of the delivery of a single angiogenic factor or require more complex cytokine combinations, the identification of the proper timing of therapeutic gene expression and, most notably, the development of more efficacious gene delivery tools. Viral vectors based on the adeno-associated virus (AAV) appear particularly suitable to address the last requirement, since they display a specific tropism for skeletal muscle cells and cardiomyocytes, and drive expression of the therapeutic genes in these cells for indefinite periods of time. In this review, I discuss the current applications of gene therapy for cardiovascular disorders, with particular attention to the possible improvements in the technologies involved in virus-mediated gene transfer.

Regulation of adeno-associated virus DNA replication by the cellular TAF-I/set complex

The Rep proteins of the adeno-associated virus (AAV) are required for viral replication in the presence of adenovirus helper functions and as yet poorly characterized cellular factors. In an attempt to identify such factors, we purified Flag-Rep68-interacting proteins from human cell lysates. Several polypeptides were identified by mass spectrometry, among which was ANP32B, a member of the acidic nuclear protein 32 family which takes part in the formation of the template-activating factor I/Set oncoprotein (TAF-I/Set) complex. The N terminus of Rep was found to specifically bind the acidic domain of ANP32B; through this interaction, Rep was also able to recruit other members of the TAF-I/Set complex, including the ANP32A protein and the histone chaperone TAF-I/Set. Further experiments revealed that silencing of ANP32A and ANP32B inhibited AAV replication, while overexpression of all of the components of the TAF-I/Set complex increased de novo AAV DNA synthesis in permissive cells. Besides being the first indication that the TAF-I/Set complex participates in wild-type AAV replication, these findings have important implications for the generation of recombinant AAV vectors since overexpression of the TAF-I/Set components was found to markedly increase viral vector production.

Interaction between the HPV E7 oncoprotein and the transcriptional coactivator p300

Infection with high-risk human papillomaviruses (HPV) can lead to the development of cervical cancer. This process depends on the interaction of the virus-encoded oncoproteins, E6 and E7, with a variety of host regulatory proteins. As E7 shares both functional and structural similarities with the Adenovirus E1a (Ad E1a) protein, we were interested in investigating the possible interactions between E7 and the transcriptional coactivator p300, since it was originally identified as a target of Ad E1a. Using a variety of assays, we show that E7s from both high- and low-risk HPV types interact with p300. Mutational analysis of E7 maps the site of the interaction to a region spanning the pRb-binding domain and the CKII phosphorylation site. We also map the site of interaction on p300 largely to the CH1 domain. In addition, we demonstrate that the binding between 16E7 and p300 is direct, and can be detected in vivo by coimmunoprecipitation and mammalian two-hybrid assays. Finally, we show that E7 can abolish the p300-mediated E2 transactivation function, suggesting that complex formation between E7 and p300 may contribute to the regulation of E2 transcriptional activity.

Recruitment of human cyclin T1 to nuclear bodies through direct interaction with the PML protein

Human cyclin T1, the cyclin partner of Cdk9 kinase in the positive transcription elongation factor b (P-TEFb), is an essential cellular cofactor that is recruited by the human immunodeficiency virus type 1 (HIV-1) Tat transactivator to promote transcriptional elongation from the HIV-1 long terminal repeat (LTR). Here we exploit fluorescence resonance energy transfer (FRET) to demonstrate that cyclin T1 physically interacts in vivo with the promyelocytic leukaemia (PML) protein within specific subnuclear compartments that are coincident with PML nuclear bodies. Deletion mutants at the C-terminal region of cyclin T1 are negative for FRET with PML and fail to localize to nuclear bodies. Cyclin T1 and PML are also found associated outside of nuclear bodies, and both proteins are present at the chromatinized HIV-1 LTR promoter upon Tat transactivation. Taken together these results suggest that PML proteins regulate Tat- mediated transcriptional activation by modulating the availability of cyclin T1 and other essential cofactors to the transcription machinery.

Temporal acceleration of the human papillomavirus life cycle by adeno-associated virus (AAV) type 2 superinfection in natural host tissue

Epidemiologically, certain human papillomaviruses are positively associated with cervical cancer, while adeno-associated viruses (AAV-2) are negatively associated with this same cancer. Both HPV and AAV productively replicate in differentiating keratinocytes of the skin and interact with each other. However, AAV has a relatively fast life cycle, generating infectious progeny by the third to fourth day of an organotypic epithelial raft culture. In contrast, HPV is slow, generating infectious progeny only after 10-12 days. As earlier studies indicated that these two skin-tropic virus types significantly affect each other's life cycle, we investigated if the temporal kinetics of the slow HPV life cycle was affected by the fast AAV in raft cultures. Here it is shown that the presence of AAV-2 at a variety of multiplicities of infection (m.o.i.) resulted in early onset HPV-31b DNA replication. Using plasmids which each expressed only one of the four rep proteins, an enhancement affect was seen for all four rep proteins of AAV, with Rep40 having the highest activity. Furthermore, AAV (m.o.i. of 5) also resulted in a temporally accelerated production of HPV infectious units, seen as early as Day 4, with high levels of viral progeny being produced by Day 6.5. Like earlier studies at Day 12, histological differences were seen at Day 6.5 between AAV-infected and mock-infected HPV/rafts. These data suggest that under specific conditions the AAV rep trans-factors can positively regulate HPV gene expression in addition to the usual negative regulation that has been consistently observed by the rep proteins. These data also suggest that AAV has a significant effect upon the temporal kinetics of the HPV life cycle in natural host tissue. However, it is unclear if or how this AAV-induced fast HPV life cycle mechanistically correlates with lower rates of HPV-associated cervical disease.

Functional interaction between p/CAF and human papillomavirus E2 protein

p300/CREB-binding protein-associated factor (p/CAF), a transcriptional co-activator, interacts with co-activator p300/CBP and acidic transcription factors. p/CAF mediates transcriptional activation by acetylating nucleosomal histones and cellular factors. Previously we reported that CBP binds to human papillomavirus E2 and activates E2-dependent transcription (Lee, D., Lee, B., Kim, J., Kim, D. W., and Choe, J. (2000) J. Biol. Chem. 275, 7045-7051). Here we show that p/CAF binds to the human papillomavirus E2 protein in vivo and in vitro and activates E2-dependent transcription. CBP along with p/CAF synergistically activates E2-dependent transcription. In addition, the histone acetylase activity of p/CAF is required for efficient activation of E2 transcriptional activity. These results suggest that p/CAF is a transcriptional co-activator of the human papillomavirus E2 protein.

Identification of a cellular protein that interacts and synergizes with the RTA (ORF50) protein of Kaposi's sarcoma-associated herpesvirus in transcriptional activation

Lytic reactivation of Kaposi's sarcoma-associated herpesvirus (KSHV), or human herpesvirus 8, from latency requires transcriptional transactivation by the viral protein RTA encoded by the ORF50 gene. Very little is known about how RTA functions and the cellular factors that may be involved in its transactivation function. Using the yeast two-hybrid system, we have identified a human cellular protein that can interact with KSHV RTA. The cellular protein, referred to as the human hypothetical protein MGC2663 by GenBank, is encoded by human chromosome 19. This protein is 554 amino acids (aa) in size and displays sequence similarity with members of the Krueppel-associated box-zinc finger proteins (KRAB-ZFPs). MGC2663 expression could be detected in all primate cell lines tested, and its expression level was neither stimulated nor inhibited by RTA. MGC2663 specifically synergizes with RTA to activate viral transcription, and overexpression of MGC2663 in the presence of RTA further enhances RTA transactivation of several viral promoters that were identified as targets for RTA. Coimmunoprecipitation and pull-down assays further demonstrated that MGC2663 interacts with RTA both in vivo and in vitro, and the N-terminal 273 aa of KSHV RTA and the potential zinc finger domain of MGC2663 are required for their interaction. Our results indicate that this novel human cellular protein, MGC2663, named K-RBP (KSHV RTA binding protein) due to its RTA binding feature, specifically interacts with the KSHV RTA protein and functions as a cellular RTA cofactor to activate viral gene expression. Though its normal cellular function needs to be further studied, K-RBP may play a significant role in mediating RTA transactivation in vivo.

The human papillomavirus E6 protein and its contribution to malignant progression

The human papillomavirus (HPV) E6 protein is one of three oncoproteins encoded by the virus. It has long been recognized as a potent oncogene and is intimately associated with the events that result in the malignant conversion of virally infected cells. In order to understand the mechanisms by which E6 contributes to the development of human malignancy many laboratories have focused their attention on identifying the cellular proteins with which E6 interacts. In this review we discuss these interactions in the light of their respective contributions to the malignant progression of HPV transformed cells.