The SV40 T-antigen induces premature apoptotic mammary gland involution during late pregnancy in transgenic mice

Genetically engineered mice as experimental tools to dissect the critical events in breast cancer

Elucidating the mechanism of pathogenesis of breast cancer has greatly benefited from breakthrough advances in both genetically engineered mouse (GEM) models and xenograft transplantation technologies. The vast array of breast cancer mouse models currently available is testimony to the complexity of mammary tumorigenesis and attempts by investigators to accurately portray the heterogeneity and intricacies of this disease. Distinct molecular changes that drive various aspects of tumorigenesis, such as alterations in tumor cell proliferation and apoptosis, invasion and metastasis, angiogenesis, and drug resistance have been evaluated using the currently available GEM breast cancer models. GEM breast cancer models are also being exploited to evaluate and validate the efficacy of novel therapeutics, vaccines, and imaging modalities for potential use in the clinic. This review provides a synopsis of the various GEM models that are expanding our knowledge of the nuances of breast cancer development and progression and can be instrumental in the development of novel prevention and therapeutic approaches for this disease.

Comparison of gene expression data from human and mouse breast cancers: identification of a conserved breast tumor gene set

The aim of our work was to establish a database for breast cancer gene expression data in order to compare human and mouse breast cancer. We identified human and mouse homologues genes and compared the expression profile of 24 human breast tumors with 6 WAP-SVT/t breast tumors (WAP-SVT/t animals, line 8). Our studies confirmed the heterogeneity in gene expression of human as well as mouse breast cancer cells. However, 63 genes were found to be differentially expressed (upregulated: 40; downregulated: 23 genes) in at least 75% of the breast tumors of both species. To differentiate between early and late events in tumor formation, we compared the 63 differentially expressed genes with a mouse data set obtained from hyperplastic mammary glands. This revealed that the majority of the early deregulated genes are cell proliferation specific. These early changes seem to be necessary although not sufficient for breast cancer formation. Late alterations concern mainly genes belonging to the category of cell communication and metabolism. Interestingly, most of the 63 conserved genes are commonly associated with tumorigenesis.

Targeting the death machinery in mammary epithelial cells: Implications for breast cancer from transgenic and tissue culture experiments

Apoptosis plays important roles in the development of the mammary gland, and its impairment has been speculated to promote breast cancer. In mammary epithelial cells apoptosis is triggered via the intrinsic pathway which is controlled by interactions between pro- and anti-apoptotic members of the Bcl-2 protein family. The impact of impairing this pathway on the development of breast cancer has been addressed experimentally using transgenic mouse models. Neither overexpression of anti-apoptotic Bcl-2 nor a deficiency of pro-apoptotic Bax were tumorigenic on their own in mammary glands of transgenic mice. Both ways of impairing apoptosis, however, promoted mammary tumorigenesis elicited by c-myc or SV40 T antigen. Likewise, inhibition of the intrinsic pathway in a three-dimensional mammary tissue culture model was insufficient to generate solid aggregates resembling early breast cancer stages but required the concomitant activity of proliferation-stimulating oncogenes. These two experimental approaches have thus substantiated the concept of apoptosis acting as a tumor suppressor mechanism, however point towards a complex picture in which alternative routes to cell death may be involved.

The induction of growth arrest in fibroblasts by SV40 T antigen

DNA tumor viruses such as SV40, Ras and papillomaviruses are the most commonly used agents in immortalization of non-hematopoietic cells, but the results are quite different. Some of them even lead instead to a senescence-like state. To verify the potential of SV40 T antigen-mediated immortalization or properties and functions of it to regulate cell growth, human dermal fibroblasts were cultured and then transfected with eukaryotic expressing plasmid psv3-neo which containing SV40 T DNA. We found that expression of oncogenic SV40 T in human dermal fibroblasts resulted in growth, arrest, earlier than the occurrence of control cell senescence, although telomerase was positive and cells grew faster than control ones in early stage following transfection. These observations suggest that SV40 T antigen can activate growth arrest in human dermal fibroblasts under normal growth condition instead of always prolonging the lifespan of fibroblasts. Moreover, high rate of cell division in early stage after transfection may be associated with the expression of telomerase activity.

Escherichia coli induces apoptosis and proliferation of mammary cells

Mammary cell apoptosis and proliferation were assessed after injection of Escherichia coli into the left mammary quarters of six cows. Bacteriological analysis of foremilk samples revealed coliform infection in the injected quarters of four cows. Milk somatic cell counts increased in these quarters and peaked at 24 h after bacterial injection. Body temperature also increased, peaking at 12 h postinjection. The number of apoptotic cells was significantly higher in the mastitic tissue than in the uninfected control. Expression of Bax and interleukin-1beta converting enzyme increased in the mastitic tissue at 24 h and 72 h postinfection, whereas Bcl-2 expression decreased at 24 h but did not differ significantly from the control at 72 h postinfection. Induction of matrix metalloproteinase-9, stromelysin-1 and urokinase-type plasminogen activator was also observed in the mastitic tissue. Moreover, cell proliferation increased in the infected tissue. These results demonstrate that Escherichia coli-induced mastitis promotes apoptosis and cell proliferation.

The SV40 small t-antigen prevents mammary gland differentiation and induces breast cancer formation in transgenic mice; truncated large T-antigen molecules harboring the intact p53 and pRb binding region do not have this effect

We report here for the first time, that the SV40 small t-antigen inhibits mammary gland differentiation during mid-pregnancy and that about 10% of multiparous WAP-SVt transgenic animals develop breast tumors with latencies ranging from 10-17 months. Cyclin D1 is deregulated and over expressed in the small t-antigen positive mammary gland epithelial cells (ME-cells) and in the breast tumor cells. SV40 small t-antigen immortalized ME-cells (t-ME-cells) exhibit a strong intranuclear cyclin D1 staining, also in the absence of external growth factors and the cells continue to divide for several days without serum. In addition, the expression rate of cyclin E and p21(Waf1) but not of p53 is increased. Coimmunoprecipitation experiments revealed that p21(Waf1) is mainly associated with the cyclin D/CDK4 but not with the cyclin E/CDK2 complex. WAP-SVT transgenic animals exhibit an almost regular mammary gland development until late pregnancy but the majority of the ME-cells are eliminated by apoptosis during the early lactation period. Tumor formation is delayed and less efficient than in T/t-antigen positive animals. Sequestration of p53 and pRb by the N-terminal truncated T-antigen molecules (T1-antigen and T2-antigen) does not affect mammary gland differentiation and the transgenic animals (WAP-SVBst-Bam) do not develop breast tumors.

SV40 T/t-antigens sensitize mammary gland epithelial cells to oxidative stress and apoptosis

As shown recently, the SV40 T/t-antigens (T/t-ag) exert a strong apoptotic activity in mouse mammary gland epithelial cells (ME-cells) leading to premature gland involution at late pregnancy. This high spontaneous cell death rate (20%) is also maintained in T/t-ag positive ME-tissue culture cell lines (e.g., 8/61-A), but not in those ME-cells that have switched off the SV40 T/t-transgene expression. In this study, we demonstrate for the first time that the T/t-ag sensitize ME-cells to oxidative stress leading to apoptosis. Treatment of the 8/61-A ME-cells with catalase, a scavenger of H2O2, completely blocked spontaneous cell death, which was linked to downregulation of caspase-3 activity. Furthermore, exposure of the cells to low concentrations of H2O2 highly increased the apoptosis rate. These findings suggest that the T/t-ag positive ME-cells contain either elevated levels of reactive oxygen species or reduced antioxidant activities. During spontaneous and H2O2-induced apoptosis, the activity of caspase-3 is significantly increased. In addition, the 8/61-A cells accumulated p21 and Bax proteins while the level of the anti-apoptotic protein Bcl-2 decreased implying a posttranscriptional regulation of apoptosis.

WAP-TAg transgenic mice and the study of dysregulated cell survival, proliferation, and mutation during breast carcinogenesis

Understanding the process of carcinogenesis is key to developing therapies which might interrupt or reverse tumor onset and progression. Cell growth and death signals are dependent not only upon molecular mechanisms within a cell but also upon external stimuli such as hormones, cell - cell signaling, and extracellular matrix. Mouse models can be used to dissect these complex processes, to identify key signaling pathways operating at different stages of tumorigenesis, and to test the strength of specific interventions. In the WAP-TAg mouse model, carcinogenesis is initiated by expression of the Simian Virus 40 T antigen (TAg). TAg expression is triggered by hormonal stimulation, either during estrus or pregnancy. Breast adenocarcinomas (ranging from well to poorly differentiated) develop in 100% of the female mice by approximately 8 - 9 months of age. Three distinct stages of tumorigenesis are easily identified: an initial proliferation, hyperplasia, and adenocarcinoma. The mean time to first palpable tumor in mice which undergo at least one pregnancy is 6 months. The tumorigenic process is marked by a competition between proliferation and apoptosis and is characterized by cellular acquisition of genetic mutations and increased stromal fibrosis. Protein levels of cell cycle control genes cyclin D1, cdk2, and E2F-1 are increased in these adenocarcinomas. c-Fos protein levels are slightly increased in these cancers, while c-Jun levels do not change. Hormonal exposure alters progression. Estrogen plays a role during the early stages of oncogenesis although the growth of the resulting adenocarcinomas is estrogen-independent. Transient hormonal stimulation by glucocorticoids that temporarily increases the rate of cell proliferation results in tetraploidy, premature appearance of irreversible hyperplasia, and early tumor development. Tumor appearance also can be accelerated through over expression of the cell survival protein, Bcl-2. Bcl-2 over expression not only reduces apoptosis during the initial proliferative process but also decreases the total rate of cell proliferation. This block in cell proliferation is lost selectively as the cells transition to adenocarcinoma. The WAP-TAg model can be utilized to investigate how the basic processes of cell proliferation, apoptosis, DNA mutation, and DNA repair are modified by external and internal signals during mammary oncogenesis.

The mammary pathology of genetically engineered mice: the consensus report and recommendations from the Annapolis meeting

NIH sponsored a meeting of medical and veterinary pathologists with mammary gland expertise in Annapolis in March 1999. Rapid development of mouse mammary models has accentuated the need for definitions of the mammary lesions in genetically engineered mice (GEM) and to assess their usefulness as models of human breast disease. The panel of nine pathologists independently reviewed material representing over 90% of the published systems. The GEM tumors were found to have: (1) phenotypes similar to those of non-GEM; (2) signature phenotypes specific to the transgene; and (3) some morphological similarities to the human disease. The current mouse mammary and human breast tumor classifications describe the majority of GEM lesions but unique morphologic lesions are found in many GEM. Since little information is available on the natural history of GEM lesions, a simple morphologic nomenclature is proposed that allows direct comparisons between models. Future progress requires rigorous application of guidelines covering pathologic examination of the mammary gland and the whole animal. Since the phenotype of the lesions is an essential component of their molecular pathology, funding agencies should adopt policies ensuring careful morphological evaluation of any funded research involving animal models. A pathologist should be part of each research team.

Simian virus 40 T antigen induces p53-independent apoptosis but does not suppress erbB2/neu gene expression in immortalized human epithelial cells

Previously, we have shown that simian virus 40 (SV40) T antigen can directly cause apoptosis in immortalized human epithelial cells under normal growth conditions. In this study, we further characterized the mechanism of T-antigen-mediated apoptosis involving p53 and whether T antigen can suppress erbB2/neu gene expression. Our results show the differential regulation of erbB2/neu gene expression in different cell clones in response to T antigen transgene, indicating that the regression of erbB2/neu gene by SV40 T is cell-type dependent. Our previous study reported T-antigen-induced apoptosis in p53 mutant cells; however, in this study we find increased levels of p53 protein in T-antigen-containing cells. Therefore, we examined the transactivation function of p53 in these cells. Our data show the failure to transactivate p53, suggesting that increased p53 protein in T antigen expressing cells is functionless at least for transcriptional activation.

The induction of apoptosis by SV40 T antigen correlates with c-jun overexpression

Simian virus (SV40) T antigen shares many characteristics with adenovirus E1A which is known to induce apoptosis. To verify the potential of SV40 T antigen-mediated apoptosis, we stably expressed T antigen in immortalized human epithelial cells (Z172 and HaCaT). We found that SV40 T antigen could directly cause apoptosis in 22-27% of these cells under normal growth condition as measured by chromatin condensation and nucleosomal fragmentation. The apoptosis of HaCaT cells which contain mutant p53 suggests the p53-independent nature of T antigen-mediated apoptosis. T antigen-induced apoptosis was associated with increased expression of c-Jun protein. Moreover, the overexpression of c-jun alone in these cells also induced apoptosis, indicating that c-jun might play an important role in T antigen-induced apoptosis.