Mammary tumors induced by human adenovirus type 9: a role for the viral early region 4 gene

Large Animal Models of Breast Cancer

In this mini review the status, advantages, and disadvantages of large animal modeling of breast cancer (BC) will be discussed. While most older studies of large animal BC models utilized canine and feline subjects, more recently there has been interest in development of porcine BC models, with some early promising results for modeling human disease. Widely used rodent models of BC were briefly reviewed to give context to the work on the large animal BC models. Availability of large animal BC models could provide additional tools for BC research, including availability of human-sized subjects and BC models with greater biologic relevance.

Infectivity period of mice inoculated with human adenoviruses

Due to non-productive infections, mice are not a good model to study some human adenoviruses. However, mice provide an excellent model to study the metabolic effects of human adenovirus, Ad36. Research interest in Ad36 is increasing rapidly, and consequently an increase in the use of mice as a model is anticipated. However, little is known about the transmission potential of Ad36 from infected mice to other laboratory animals or personnel. While underestimating the infectivity could promote inadvertent spread of Ad36, overstating it could drain valuable laboratory resources and animals. Therefore, we determined the duration of infectivity in female C57BL/6J mice that were experimentally infected with human adenoviruses Ad36 or Ad2. Other uninfected mice were co-housed for one week with the experimentally-infected animals, four or eight weeks postinfection. Additionally, uninfected mice were housed in the cages of mice that were infected with Ad36, 12 weeks earlier. The presence of viral DNA in tissues was used to indicate infection of mice. Although experimentally-infected mice harboured viral DNA at least up to 12 weeks, the horizontal transmission of infection was observed in co-housed mice only up to four weeks postinfection. Thus, Ad36-infected mice should be considered potentially infective for eight weeks and appropriate handling and barrier containment should be used. After eight week postinfection, horizontal transmission appears unlikely. This information may provide guidelines for animal handling, and experimental design using Ad36, which may increase safety for laboratory personnel and reduce the number of mice required for experiments.

Cell polarity proteins: common targets for tumorigenic human viruses

Loss of polarity and disruption of cell junctions are common features of epithelial-derived cancer cells, and mounting evidence indicates that such defects have a direct function in the pathology of cancer. Supporting this idea, results with several different human tumor viruses indicate that their oncogenic potential depends in part on a common ability to inactivate key cell polarity proteins. For example, adenovirus (Ad) type 9 is unique among human Ads by causing exclusively estrogen-dependent mammary tumors in experimental animals and in having E4 region-encoded open reading frame 1 (E4-ORF1) as its primary oncogenic determinant. The 125-residue E4-ORF1 protein consists of two separate protein-interaction elements, one of which defines a PDZ domain-binding motif (PBM) required for E4-ORF1 to induce both cellular transformation in vitro and tumorigenesis in vivo. Most notably, the E4-ORF1 PBM mediates interactions with a selected group of cellular PDZ proteins, three of which include the cell polarity proteins Dlg1, PATJ and ZO-2. Data further indicate that these interactions promote disruption of cell junctions and a loss of cell polarity. In addition, one or more of the E4-ORF1-interacting cell polarity proteins, as well as the cell polarity protein Scribble, are common targets for the high-risk human papillomavirus (HPV) E6 or human T-cell leukemia virus type 1 (HTLV-1) Tax oncoproteins. Underscoring the significance of these observations, in humans, high-risk HPV and HTLV-1 are causative agents for cervical cancer and adult T-cell leukemia, respectively. Consequently, human tumor viruses should serve as powerful tools for deciphering mechanisms whereby disruption of cell junctions and loss of cell polarity contribute to the development of many human cancers. This review article discusses evidence supporting this hypothesis, with an emphasis on the human Ad E4-ORF1 oncoprotein.

Complete sequence and organization of the human adenovirus serotype 46 genome

Out of 51 human adenoviral serotypes recognized to date, 32 of them belong to species D. Members of species D adenoviruses are commonly isolated from immune suppressed patients (organ transplant) and patients suffering from AIDS. The role of species D adenoviruses in pathogenesis is currently unclear. To derive new insights into the genetic content and evolution of species D adenoviruses and as a first step towards development of human adenovirus serotype 46 (Ad46) as vector, the complete nucleotide sequence of the virus was determined. The size of the genome is 35,178 bp in length with a G+C content of 56.9%. All the early and late region genes are present in the expected locations of the genome. The deduced amino acid sequences of all late region genes, with the exception of fiber, exhibited high degree of homology with the corresponding proteins of other adenoviruses. The deduced amino acid sequences of early regions E1, E3 and E4 showed a high degree of homology with the corresponding proteins of adenoviruses belonging to species D and less homology with the corresponding proteins of adenoviruses of other species. The homologues of Ad5 E3 region genes encoding 12.5K, gp19K, 10.4K, 14.5K and 14.7K are conserved in the genome of Ad46. However, the E3 region of Ad46 lacks genes encoding 6.7K and adenovirus death protein (ADP) but contains two additional open reading frames with a coding capacity of 433 and 281 amino acids. The fiber protein of Ad46 is 200 amino acids smaller than the fiber protein of Ad5 and contains only 10 pseudo-repeats in the shaft region. To facilitate the manipulation of the genome, the complete genome of Ad46 was cloned into a single bacterial plasmid. Following transfection into E1 complementing cell lines, the virus was recovered demonstrating the feasibility of viral genome manipulation for generation of recombinant viruses.

Immunobiological Characterization of N-Nitrosomethylurea-Induced Rat Breast Carcinomas: Tumoral IL-10 Expression as a Possible Immune Escape Mechanism

Improvement of immunotherapy-based protocols in cancer requires a better understanding of tumor microenvironment and tumor-host interaction. Stromal and immune cells and molecules such as cytokines, chemokines, growth factors and metalloproteases mediate tumor-host interaction determining, at least in part, tumor development. In the present study, we used an immunohistochemical approach to explore leukocyte sub-populations, cytokine profiles and costimulatory molecule expression in rat N -Nitrosomethylurea (NMU)-induced breast tumors. Our results show a strong leukocyte infiltration mainly composed of macrophages and TCR alphabeta positive T cells. We observed a weak expression of costimulatory molecules (CD80, CD86) and an absence of inflammatory cytokines (IFNgamma, TNFalpha, IP-10) and lymphocyte activation markers (CD25). Interestingly, this immunosuppressed status could be a consequence of IL-10 expression by malignant cells, as demonstrated by immunohistology and western blot analysis, which seems to be an early event during mammary carcinogenesis. Analysis of a cell line derived from an NMU-induced rat breast tumor showed that this cell line also expresses IL-10. This study shows that the NMU model of rat breast cancer could be used to evaluate different immune based therapies as well as to study the role of IL-10 in breast cancer. Furthermore, this rat breast cancer model shows an immunohistological profile similar to that found in human cancer and the fact that it develops like spontaneously arising malignancies make it interesting as a cancer model in immunobiology.

Adenovirus early E4 genes in viral oncogenesis

Previous investigations into potential transforming activities of adenovirus (Ad) early genes were largely overshadowed by the more obvious roles of E1A and E1B products. One exception was an Ad9 E4 protein (ORF1) shown to enhance transformation of cultured cells and promote mammary tumors in female rats. Recently, significant advances in understanding Ad E4 gene products at the molecular level have revealed that these proteins possess an unexpectedly diverse collection of functions, which not only orchestrate many viral processes, but overlap with oncogenic transformation of primary mammalian cells. Operating through a complex network of protein interactions with key viral and cellular regulatory components, Ad E4 products are apparently involved in transcription, apoptosis, cell cycle control, DNA repair, cell signaling, posttranslational modifications and the integrity of nuclear multiprotein complexes known as PML oncogenic domains (PODs). Some of these functions directly relate to known transforming and oncogenic processes, or implicate mechanisms such as modulating the function and subcellular localization of cellular PDZ domain-containing proteins, POD reorganization, targeted proteolytic degradation, inhibition of DNA double-strand break repair and 'hit-and-run' mutagenesis. Here, we summarize the recent data and discuss how E4 gene product interactions may contribute to viral oncogenesis.

Molecular regulation and biological function of adenovirus early genes: the E4 ORFs

Over the past few years there have been a number of interesting advances in our understanding of the functions encoded by the adenovirus early transcription unit 4 (Ad E4). A large body of recent data demonstrates that E4 proteins encompass an unexpectedly diverse collection of functions required for efficient viral replication. E4 gene products operate through a complex network of protein interactions with key viral and cellular regulatory components involved in transcription, apoptosis, cell cycle control and DNA repair, as well as host cell factors that regulate cell signaling, posttranslational modifications and the integrity of nuclear multiprotein complexes known as nuclear bodies (NBs) or PML oncogenic domains (PODs). As understood at present, some of the lytic functions overlap with roles in oncogenic transformation of primary mammalian cells. These observations, together with findings that E4 proteins substantially affect cell toxicity and the immune response of the host have profound implications for the development of Ad vectors for gene therapy. In this article we will summarize recent findings regarding the diverse functions of E4 gene products in the context of earlier work. We will emphasize the interaction of E4 proteins with cellular and viral interaction partners, the role of these interactions for lytic virus growth and how these interactions may contribute to viral oncogenesis. Finally, we will discuss their role in Ad vector and adeno-associated virus infections.

Several E4 region functions influence mammary tumorigenesis by human adenovirus type 9

Among oncogenic adenoviruses, human adenovirus type 9 (Ad9) is unique in eliciting exclusively estrogen-dependent mammary tumors in rats and in not requiring viral E1 region transforming genes for tumorigenicity. Instead, studies with hybrid viruses generated between Ad9 and the closely related nontumorigenic virus Ad26 have roughly localized an Ad9 oncogenic determinant(s) to a segment of the viral E4 region containing open reading frame 1 (E4-ORF1), E4-ORF2, and part of E4-ORF3. Although subsequent findings have shown that E4-ORF1 codes for an oncoprotein essential for tumorigenesis by Ad9, it is not known whether other E4 region functions may similarly play a role in this process. We report here that new results with Ad9/Ad26 hybrid viruses demonstrated that the minimal essential Ad9 E4-region DNA sequences include portions of both E4-ORF1 and E4-ORF2. Investigations with Ad9 mutant viruses additionally showed that the E4-ORF1 protein and certain E4-ORF2 DNA sequences are necessary for Ad9-induced tumorigenesis, whereas the E4-ORF2 and E4-ORF3 proteins are not. In fact, the E4-ORF3 protein was found to antagonize this process. Also pertinent was that certain crucial nucleotide differences between Ad9 and Ad26 within E4-ORF1 and E4-ORF2 were found to be silent with respect to the amino acid sequences of the corresponding proteins. Furthermore, supporting a prominent role for the E4-ORF1 oncoprotein in Ad9-induced tumorigenesis, an E1 region-deficient Ad5 vector that expresses the Ad9 but not the Ad26 E4-ORF1 protein was tumorigenic in rats and, like Ad9, promoted solely mammary tumors. These findings argue that the E4-ORF1 oncoprotein is the major oncogenic determinant of Ad9 and that an undefined regulatory element(s) within the E4 region represents a previously unidentified second function likewise necessary for tumorigenesis by this virus.

Rat models of premalignant breast disease

While a number of agents have been shown to induce mammary carcinogenesis in the rat, premalignant stages of the disease have been best characterized in chemically-induced models, specifically those initiated by either 7,12 dimethylbenz[alpha]anthracene (DMBA) or 1-methyl-1-nitrosourea (MNU). In general, it appears that epithelial cells in mammary terminal end buds or terminal ductules are the targets of carcinogenic initiation, and that a series of morphologically identifiable steps are involved in the development of mammary carcinoma. The premalignant steps include ductal hyperplasia of the usual type and carcinoma in situ of the cribriform or comedo type; atypical ductal hyperplasia has not been reported. Thus the histogenesis of lesions occurring in chemically induced mammary carcinogenesis in the rat is similar to that observed in the human; although, the spectrum of lesions observed in the rat is limited. Opportunities to investigate the biological and molecular characteristics of premalignant breast disease in the rat are presented.

Multi-PDZ domain protein MUPP1 is a cellular target for both adenovirus E4-ORF1 and high-risk papillomavirus type 18 E6 oncoproteins

A general theme that has emerged from studies of DNA tumor viruses is that otherwise unrelated oncoproteins encoded by these viruses often target the same important cellular factors. Major oncogenic determinants for human adenovirus type 9 (Ad9) and high-risk human papillomaviruses (HPV) are the E4-ORF1 and E6 oncoproteins, respectively, and although otherwise unrelated, both of these viral proteins possess a functional PDZ domain-binding motif that is essential for their transforming activity and for binding to the PDZ domain-containing and putative tumor suppressor protein DLG. We report here that the PDZ domain-binding motifs of Ad9 E4-ORF1 and high-risk HPV-18 E6 also mediate binding to the widely expressed cellular factor MUPP1, a large multi-PDZ domain protein predicted to function as an adapter in signal transduction. With regard to the consequences of these interactions in cells, we showed that Ad9 E4-ORF1 aberrantly sequesters MUPP1 within the cytoplasm of cells whereas HPV-18 E6 targets this cellular protein for degradation. These effects were specific because mutant viral proteins unable to bind MUPP1 lack these activities. From these results, we propose that the multi-PDZ domain protein MUPP1 is involved in negatively regulating cellular proliferation and that the transforming activities of two different viral oncoproteins depend, in part, on their ability to inactivate this cellular factor.

Induction of transformation and p53-dependent apoptosis by adenovirus type 5 E4orf6/7 cDNA

Adenovirus (Ad) E4orf6/7, one of the early gene products of human Ads, forms a stable complex with the cellular transcription factor E2F to activate transcription from the Ad E2 promoter. E2F cDNAs have growth-promoting and apoptosis-inducing activities when overexpressed in cells. We cloned Ad5 E4orf6/7 cDNA in both simian virus 40- and human cytomegalovirus-based expression vectors to examine its transforming and apoptotic activities. The cloned E4orf6/7 collaborated with a retinoblastoma protein (RB)-nonbinding and therefore E2F-nonreleasing mutant of Ad5 E1A (dl922/947) to morphologically transform primary rat cells, suggesting that E2F is an important cellular protein functioning downstream of E1A for transformation. In a G418 colony formation assay, E4orf6/7 was shown to suppress growth of untransformed rat cells. Moreover, a recombinant Ad expressing Ad5 E4orf6/7 induced apoptosis in rat cells when coinfected with wild-type p53-expressing Ad. Mutational analysis of E4orf6/7 revealed that both of the domains required for growth inhibition and transformation by E4orf6/7 lay in the C-terminal region, which is essential for transactivation from the upstream sequence of an E2a promoter containing E2F-binding sites. However, the smallest mutant of E4orf6/7, encoding the C-terminal 59 amino acids, failed to complement the RB-nonbinding dl922/947 mutant despite showing growth inhibition and E2F transactivation activities. Thus, it is suggested that a subregion of E4orf6/7 which is required for growth inhibition and transformation in collaboration with dl922/947 overlaps the transactivation domain of E4orf6/7.

Early region 1 transforming functions are dispensable for mammary tumorigenesis by human adenovirus type 9

Some human adenoviruses are tumorigenic in rodents. Subgroup A and B human adenoviruses generally induce sarcomas in both male and female animals, and the gene products encoded within viral early region 1 (E1 region) are both necessary and sufficient for this tumorigenicity. In contrast, subgroup D human adenovirus type 9 (Ad9) induces estrogen-dependent mammary tumors in female rats and requires the E4 region-encoded ORF1 oncoprotein for its tumorigenicity. Considering the established importance of the viral E1 region for tumorigenesis by adenoviruses, we investigated whether this viral transcription unit is also necessary for Ad9 to generate mammary tumors. The nucleotide sequence of the Ad9 E1 region indicated that the gene organization and predicted E1A and E1B polypeptides of Ad9 are closely related to those of other human adenovirus E1 regions. In addition, an Ad9 E1 region plasmid demonstrated focus-forming activity in both low-passage-number and established rat embryo fibroblasts, whereas a large deletion within either the E1A or E1B gene of this plasmid diminished transforming activity. Surprisingly, we found that introducing the same transformation-inactivating E1A and E1B deletions into Ad9 results in mutant viruses that retain the ability to elicit mammary tumors in rats. These results are novel in showing that Ad9 represents a unique oncogenic adenovirus in which the E4 region, rather than the E1 region, encodes the major oncogenic determinant in the rat.

Activation of transgene expression by early region 4 is responsible for a high level of persistent transgene expression from adenovirus vectors in vivo

The persistence of transgene expression has become a hallmark for adenovirus vector evaluation in vivo. Although not all therapeutic benefit in gene therapy is reliant on long-term transgene expression, it is assumed that the treatment of chronic diseases will require significant persistence of expression. To understand the mechanisms involved in transgene persistence, a number of adenovirus vectors were evaluated in vivo in different strains of mice. Interestingly, the rate of vector genome clearance was not altered by the complete deletion of early region 4 (E4) in our vectors. The GV11 (E1- E4-) vector genome cleared with a similar kinetic profile as the GV10 (E1-) vector genome in immunocompetent and immunocompromised mice. These results suggest that the majority of adenovirus vector genomes are eliminated from transduced tissue via a mechanism(s) independent of T-cell, B-cell, and NK cell immune mechanisms. While the levels of persistence of transgene expression in liver or lung transduced with GV10 and GV11 vectors expressing beta-galactosidase, cystic fibrosis transmembrane conductance regulator, or secretory alkaline phosphatase were similar in immunocompetent mice, a marked difference was observed in immunocompromised animals. Levels of transgene expression initially from both GV10 and GV11 vectors were the same. However, GV11 transgene expression correlated with loss of vector genome, while GV10 transgene expression persisted at a high level. Coadministration and readministration of GV10 vectors showed that E4 provided in trans could activate transgene expression from the GV11 vector genome. While transgene expression activity per genome from the GV10 vector is clearly activated, expression from a cytomegalovirus promoter expression cassette in a GV11 vector appeared to be further inactivated as a function of time. Understanding the molecular mechanisms underlying these expression effects will be important for developing persistent adenovirus vectors for chronic applications.

A carboxy-terminal region required by the adenovirus type 9 E4 ORF1 oncoprotein for transformation mediates direct binding to cellular polypeptides

Human adenovirus type 9 (Ad9) is unique among oncogenic adenoviruses in that it elicits exclusively mammary tumors in rats and requires the viral E4 region open reading frame 1 (9ORF1) gene for tumorigenicity. The 9ORF1 oncogenic determinant codes for a 14-kDa transforming protein, and three separate regions of this polypeptide, including one at the extreme C terminus, are necessary for transforming activity. In this study, we investigated whether the 9ORF1 transforming protein interacts with cellular factors. Following incubation with cell extracts, a glutathione S-transferase (GST)-9ORF1 fusion protein associated with several cellular phosphoproteins (p220, p180, p160, p155), whereas GST fusion proteins of transformation-defective 9ORF1 C-terminal mutants did not. Similar interactions requiring the 9ORF1 C terminus were revealed with protein-blotting assays, in which a GST-9ORF1 protein probe reacted specifically with cellular polypeptides having gel mobilities resembling those of the 9ORF1-associated cellular phosphoproteins, as well as with additional cellular polypeptides designated p140/p130. In addition, GST fusion proteins containing 9ORF1 C-terminal fragments associated with some of the 9ORF1-associated cellular polypeptides, as did GST fusion proteins of full-length wild-type Ad5 and Ad12 E4 ORF1 transforming proteins. Significantly, the results of coimmunoprecipitation analyses suggested that the same cellular polypeptides also associate with wild-type but not C-terminal-mutant 9ORF1 proteins in vivo. Together, these findings suggest that the 9ORF1 C terminus, which is essential for transformation, participates in specific and direct binding of the 9ORF1 oncoprotein to multiple cellular polypeptides. We propose that interactions with these cellular factors may be responsible, at least in part, for the transforming activity of the 9ORF1 viral oncoprotein.

Mutant adenovirus type 9 E4 ORF1 genes define three protein regions required for transformation of CREF cells

Human adenovirus type 9 (Ad9) elicits exclusively estrogen-dependent mammary tumors in rats, and an essential oncogenic determinant for this virus is Ad9 E4 open reading frame 1 (9ORF1), which encodes a 125-residue cytoplasmic protein with cellular growth-transforming activity in vitro. In this study, we engineered 48 different mutant 9ORF1 genes in an attempt to identify regions of this viral protein essential for transformation of the established rat embryo fibroblast cell line CREF. In initial assays with CREF cells, 17 of the 48 mutant 9ORF1 genes proved to be severely defective for generating transformed foci but only 7 of these defective genes expressed detectable amounts of protein. To further examine the defects of the seven mutant proteins, we selected individual cell pools of stable CREF transformants for the wild-type and mutant 9ORF1 genes. Compared to cell pools expressing the wild-type 9ORF1 protein, most cell pools expressing mutant proteins displayed decreased growth in soft agar, and all generated significantly smaller tumors in syngeneic animals. The altered amino acid residues of the seven mutant 9ORF1 polypeptides clustered within three separate regions referred to as region I (residues 34 to 41), region II (residues 89 to 91), and C-terminal region III (residues 122 to 125). By using indirect immunofluorescence, we also assessed whether the mutant proteins localized properly to the cytoplasm of cells. The region I and region II mutants displayed approximately wild-type subcellular localizations, whereas most region III mutants aberrantly accumulated within the nucleus of cells. In summary, we have identified three 9ORF1 protein regions necessary for cellular transformation and have demonstrated that C-terminal region III sequences significantly influence the proper localization of the 9ORF1 polypeptide in cells.

Human adenovirus early region 4 open reading frame 1 genes encode growth-transforming proteins that may be distantly related to dUTP pyrophosphatase enzymes

An essential oncogenic determinant of subgroup D human adenovirus type 9 (Ad9), which uniquely elicits estrogen-dependent mammary tumors in rats, is encoded by early region 4 open reading frame 1 (E4 ORF1). Whereas Ad9 E4 ORF1 efficiently induces transformed foci on the established rat embryo fibroblast cell line CREF, the related subgroup A Ad12 and subgroup C Ad5 E4 ORF1s do not (R. T. Javier, J. Virol. 68:3917-3924, 1994). In this study, we found that the lack of transforming activity associated with non-subgroup D adenovirus E4 ORF1s in CREF cells correlated with significantly reduced protein levels compared to Ad9 E4 ORF1 in these cells. In the human cell line TE85, however, the non-subgroup D adenovirus E4 ORF1s produced protein levels higher than those seen in CREF cells as well as transforming activities similar to that of Ad9 E4 ORF1, suggesting that all adenovirus E4 ORF1 polypeptides possess comparable cellular growth-transforming activities. In addition, searches for known proteins related to these novel viral transforming proteins revealed that the E4 ORF1 proteins had weak sequence similarity, over the entire length of the E4 ORF1 polypeptides, with a variety of organismal and viral dUTP pyrophosphatase (dUTPase) enzymes. Even though adenovirus E4 ORF1 proteins lacked conserved protein motifs of dUTPase enzymes or detectable enzymatic activity, E4 ORF1 and dUTPase proteins were predicted to possess strikingly similar secondary structure arrangements. It was also established that an avian adenovirus protein, encoded within a genomic location analogous to that of the human adenovirus E4 ORF1s, was a genuine dUTPase enzyme. Although no functional similarity was found for the E4 ORF1 and dUTPase proteins, we propose that human adenovirus E4 ORF1 genes have evolved from an ancestral adenovirus dUTPase and, from this structural framework, developed novel transforming properties.

Animal models of breast cancer: their diversity and role in biomedical research

Animal models of breast cancer have been widely used to study various aspects of breast cancer biology, and are remarkably diverse, encompassing chemically and virally induced tumors, human tumor xenografts, and transgenic mouse models. Several novel models have become available during the past few years, including tumors induced by polyomavirus and adenovirus, and a series of human cell line variants. The several following articles describe, in some detail, the characteristics of these models and their relevance to the human disease. Descriptions of each of the major models, e.g., 7,12-dimethylbenz(a)anthracene-induced, MMTV-associated, and human breast cancer cell line xenografts, also are included. The limitations and advantages of several of these models, and some issues relating to the choice of models, are briefly discussed in this overview.