Do highly oncogenic group A human adenoviruses cause human cancer? Analysis of human tumors for adenovirus 12 transforming DNA sequences

Molecular mimicry of host short linear motif-mediated interactions utilised by viruses for entry

Viruses are obligate intracellular parasites that depend on host cellular machinery for performing even basic biological functions. One of the many ways they achieve this is through molecular mimicry, wherein the virus mimics a host sequence or structure, thereby being able to hijack the host's physiological interactions for its pathogenesis. Such adaptations are specific recognitions that often confer tissue and species-specific tropisms to the virus, and enable the virus to utilise previously existing host signalling networks, which ultimately aid in further steps of viral infection, such as entry, immune evasion and spread. A common form of sequence mimicry utilises short linear motifs (SLiMs). SLiMs are short-peptide sequences that mediate transient interactions and are major elements in host protein interaction networks. This work is aimed at providing a comprehensive review of current literature of some well-characterised SLiMs that play a role in the attachment and entry of viruses into host cells, which mimic physiological receptor-ligand interactions already present in the host. Considering recent trends in emerging diseases, further research on such motifs involved in viral entry can help in the discovery of previously unknown cellular receptors utilised by viruses, as well as help in the designing of targeted therapeutics such as vaccines or inhibitors directed towards these interactions.

Interaction of viral oncogenic proteins with the Wnt signaling pathway

It is estimated that up to 20% of all types of human cancers worldwide are attributed to viruses. The genome of oncogenic viruses carries genes that have protein products that act as oncoproteins in cell proliferation and transformation. The modulation of cell cycle control mechanisms, cellular regulatory and signaling pathways by oncogenic viruses, plays an important role in viral carcinogenesis. Different signaling pathways play a part in the carcinogenesis that occurs in a cell. Among these pathways, the Wnt signaling pathway plays a predominant role in carcinogenesis and is known as a central cellular pathway in the development of tumors. There are three Wnt signaling pathways that are well identified, including the canonical or Wnt/β-catenin dependent pathway, the noncanonical or β-catenin-independent planar cell polarity (PCP) pathway, and the noncanonical Wnt/Ca2+ pathway. Most of the oncogenic viruses modulate the canonical Wnt signaling pathway. This review discusses the interaction between proteins of several human oncogenic viruses with the Wnt signaling pathway.

Hacking the Cell: Network Intrusion and Exploitation by Adenovirus E1A

As obligate intracellular parasites, viruses are dependent on their infected hosts for survival. Consequently, viruses are under enormous selective pressure to utilize available cellular components and processes to their own advantage. As most, if not all, cellular activities are regulated at some level via protein interactions, host protein interaction networks are particularly vulnerable to viral exploitation. Indeed, viral proteins frequently target highly connected “hub” proteins to “hack” the cellular network, defining the molecular basis for viral control over the host. This widespread and successful strategy of network intrusion and exploitation has evolved convergently among numerous genetically distinct viruses as a result of the endless evolutionary arms race between pathogens and hosts. Here we examine the means by which a particularly well-connected viral hub protein, human adenovirus E1A, compromises and exploits the vulnerabilities of eukaryotic protein interaction networks. Importantly, these interactions identify critical regulatory hubs in the human proteome and help define the molecular basis of their function.

Insights into adenovirus host cell interactions from structural studies

Human adenoviruses cause a significant number of acute respiratory, enteric and ocular infections, however they have also served as useful model systems for uncovering fundamental aspects of cell and molecular biology. In addition, replication-defective forms of adenovirus are being used in gene transfer and vaccine clinical trials. Over the past decade, steady advances in structural biology techniques have helped reveal important insights into the earliest events in the adenovirus life cycle as well as virus interactions with components of the host immune system. This review highlights the continuing use of structure-based approaches to uncover the molecular features of adenovirus-host interactions.

Viruses associated with human cancer

It is estimated that viral infections contribute to 15-20% of all human cancers. As obligatory intracellular parasites, viruses encode proteins that reprogram host cellular signaling pathways that control proliferation, differentiation, cell death, genomic integrity, and recognition by the immune system. These cellular processes are governed by complex and redundant regulatory networks and are surveyed by sentinel mechanisms that ensure that aberrant cells are removed from the proliferative pool. Given that the genome size of a virus is highly restricted to ensure packaging within an infectious structure, viruses must target cellular regulatory nodes with limited redundancy and need to inactivate surveillance mechanisms that would normally recognize and extinguish such abnormal cells. In many cases, key proteins in these same regulatory networks are subject to mutation in non-virally associated diseases and cancers. Oncogenic viruses have thus served as important experimental models to identify and molecularly investigate such cellular networks. These include the discovery of oncogenes and tumor suppressors, identification of regulatory networks that are critical for maintenance of genomic integrity, and processes that govern immune surveillance.

Cell transformation by human adenoviruses

The last 40 years of molecular biological investigations into human adenoviruses have contributed enormously to our understanding of the basic principles of normal and malignant cell growth. Much of this knowledge stems from analyses of their productive infection cycle in permissive host cells. Also, initial observations concerning the carcinogenic potential of human adenoviruses subsequently revealed decisive insights into the molecular mechanisms of the origins of cancer, and established adenoviruses as a model system for explaining virus-mediated transformation processes. Today it is well established that cell transformation by human adenoviruses is a multistep process involving several gene products encoded in early transcription units 1A (E1A) and 1B (E1B). Moreover, a large body of evidence now indicates that alternative or additional mechanisms are engaged in adenovirus-mediated oncogenic transformation involving gene products encoded in early region 4 (E4) as well as epigenetic changes resulting from viral DNA integration. In particular, detailed studies on the tumorigenic potential of subgroup D adenovirus type 9 (Ad9) E4 have now revealed a new pathway that points to a novel, general mechanism of virus-mediated oncogenesis. In this chapter, we summarize the current state of knowledge about the oncogenes and oncogene products of human adenoviruses, focusing particularly on recent findings concerning the transforming and oncogenic properties of viral proteins encoded in the E1B and E4 transcription units.

Preferential transformation of human neuronal cells by human adenoviruses and the origin of HEK 293 cells

The 293 cell line was derived by transformation of primary cultures of human embryonic kidney (HEK) cells with sheared adenovirus (Ad)5 DNA. A combination of immunostaining, immunoblot, and microarray analysis showed that 293 cells express the neurofilament (NF) subunits NF-L, NF-M, NF-H, and a-internexin as well as many other proteins typically found in neurons. Three other independently derived HEK lines, two transformed by Ad5 and one by Ad12, also expressed NFs, as did one human embryonic retinal cell line transformed with Ad5. Two rodent kidney lines transformed with Ad12 were also found to express NF proteins, although several rodent kidney cell lines transformed by Ad5 DNA and three HEK cell lines transformed by the SV40 early region did not express NFs. These results suggest that human Ads preferentially transform human neuronal lineage cells. We also demonstrate that the widely used HEK293 cells have an unexpected relationship to neurons, a finding that may require reinterpretation of many previous studies in which it was assumed that HEK293 cells resembled more typical kidney epithelial cells.

Adenoviruses as vectors for the transfer of genetic information and for the construction of new type vaccines

At present many types of corpuscular nondefective, conditional-defective and helper-dependent expressing adenoviral vectors are available which can be used in constructing gene-engineered live or inactivated viral vaccines. In particular, promising results have been obtained with live recombinant human adenoviruses expressing the S antigen of hepatitis B virus, capsid protein of rotaviruses and gB protein of herpes virus. These recombinants are proper candidates for testing as corresponding vaccine strains, a good alternative to well-known recombinant vaccine virus.

Analysis of retinoblastoma for human adenovirus type 12 genome

Adenovirus type 12 has high oncogenic potential in newborn rodents. Moreover, adenovirus 12 induces retinoblastoma-like tumours in baboons and transforms in vitro human embryo retinoblasts. Since adenovirus-transformed cells contain adenovirus transforming gene sequences, the detection of adenovirus 12 transforming gene in tumour cell DNA can provide evidence for or against a possible aetiological role of adenovirus 12 in retinoblastoma. In this experiment, cell DNAs from six retinoblastomas were assayed for adenovirus 12 transforming gene sequences by spot hybridization and Southern blot hybridization, using the labelled EcoRI-C fragment of adenovirus 12 DNA as a probe (the far left 16.5% of the viral genome). No adenovirus 12 transforming gene sequences were detected at the level of 0.1 or 0.5 copy of the probe per diploid cell DNA in all of six retinoblastomas.

Adenovirus-related RNA sequences in human neurogenic tumours

Thirty two human tumours, mainly neurogenic, have been investigated for the presence of adenovirus-related RNA sequences. 3H-labelled tumour virus DNA probes derived from human adenoviruses types 2 and 12, bovine adenovirus type 3, and avian adenovirus CELO were hybridized in-situ on tumour kryostat sections under conditions that detect complementary RNA. Tumour virus-related RNA was detected in 62% of all tumours tested, but was not detectable in normal human brain tissues. Expression of tumour virus-related RNA was found in 2/4 astrocytomas, 2/4 metastatic brain carcinomas, 2/2 glioblastomas, 1/1 melanoma, 5/7 meningiomas, 4/4 neurinomas, 1/2 oligodendrogliomas, and 1/1 rhabdomyosarcoma. The presence of adenovirus-related RNA in the majority of human neurogenic tumours may reflect a viral involvement in the pathogenesis of these tumours.

Cytomegalovirus: detection in human colonic and circulating mononuclear cells in association with gastrointestinal disease

The specificity and strength of the reported association between cytomegalovirus (CMV) and colonic adenocarcinoma were tested by analysis of a consecutive series of surgically resected colons for: CMV-DNA by a DNA-DNA reassociation kinetics (hybridization) procedure; latent virus by co-cultivation of fresh tissue with indicator fibroblasts; and CMV viral antigens by immunofluorescence. Ten of 13 cancer patients whose colonic tissue was able to be examined by all techniques showed evidence of active or prior CMV infection. Four cancer specimens were CMV-DNA (hybridization)- positive; an additional specimen from a cancerous colon was culture-positive. In six instances, CMV DNA was detected in mucosal cells adjacent to colon adenocarcinoma. In tissue from one of three patients with ulcera tive colitis and two of seven patients with other non-neoplastic colonic disease, CMV DNA was also detected. No fresh colonic tissues were demonstrated to have CMV surface or nuclear antigens when examined by immunofluorescence. Culture of peripheral lymphocytes was positive for CMV in three of 14 cancer patients. A CMV specific defect in humoral immunity could not be documented in that most cancer patients, as well as cancer-free patients, exhibited circulating specific antibody to CMV and had a normal capacity for CMV-specific antibody-dependent cellular cytotoxicity. We conclude that CMV, probably in a latent form, is readily detectable in colonic cells of man, including those derived from malignant, pre-malignant and non-malignant tissues. Neither preferential replication in damaged tissue nor carriage of CMV by peripheral lymphocytes homing to gut appear to explain the presence of CMV in colon cells.

Analysis of human tonsil and cancer DNAs and RNAs for DNA sequences of group C (serotypes 1, 2, 5, and 6) human adenoviruses

Group C human adenoviruses (Ads) of serotypes 1, 2, 5, and 6 infect most children and commonly cause latent infections of lymphoid tissues. Ads transform cells into a malignant-like phenotype; the oncogenic genetic information is in the left 8% of the viral genome, in the HindIII-G DNA fragment. We have investigated the molecular basis for group C Ad latent infections in human tonsils as well as whether these viruses are linked to human cancer. Tonsil or cancer DNAs and RNAs were assayed for Ad sequences by liquid-phase saturation-hybridization with in vitro-labeled Ad5 HindIII-G fragment. About 25% of the 52 tonsils analyzed contained DNA or RNA sequences specific to HindIII-G, indicating that Ad transforming sequences are expressed as RNA in tonsils. Southern blotting analysis of four tonsil DNAs revealed multiple copies of the complete Ad genome in a free state and provided evidence for an unusual form of the Ad genome, possibly Ad DNA integrated into cellular DNA. In assays of human cancers, no Ad sequences were detected in DNAs from 26 squamous cell carcinomas (Cas), 3 adenocarcinomas, 4 oat cell Cas, 5 stomach Cas, 5 small intestine Cas, 15 colon Cas, 6 rectum Cas, 5 Hodgkin and 6 non-Hodgkin lymphomas, and 2 breast Cas. Reconstruction experiments indicated that the HindIII-G probe could detect 1 copy per cell of 0.2-0.3% of the viral genome. No HindIII-G-specific sequences were detected in RNAs from 21 squamous cell Cas, 3 oat cell Cas, 2 stomach Cas, or 18 colon Cas. In six other experiments using the complete Ad2 genome as probe, no Ad sequences were found in DNAs from 6 lung Cas, 12 normal lung tissues, 33 gastrointestinal Cas, 19 normal gastrointestinal tissues, 6 Hodgkin lymphomas, 3 breast Cas, or 4 kidney Cas, at a sensitivity of about 1 copy per tumor cell of 5-10% of the Ad2 genome. All Ad-induced cancer cells should contain at least 1 copy of 1-6% of the viral genome, the minimal size of the transforming region, and probably should contain multiple copies of more of the genome. Therefore, our data are definitive evidence against group C Ads being the cause of the cancers tested, which represent about 50% of the cancer incidence in the United States. Of additional interest, we did not detect Ad2 sequences in RNAs from 7 human placentas, 12 normal lungs, or 19 normal gastrointestinal tissues (nor in 44 cancer or 23 tonsil RNAs). Thus, we did not confirm a recent report of the presence of Ad2 RNA in RNAs from human placentas; the possibility that a small population of cells in placenta expresses group C "related" sequences is not ruled out.