Use of transgenic mice reveals cell-specific transformation by a simian virus 40 T-antigen amino-terminal mutant

Thymic and Postthymic Regulation of Naïve CD4(+) T-Cell Lineage Fates in Humans and Mice Models

Our understanding of how thymocytes differentiate into many subtypes has been increased progressively in its complexity. At early life, the thymus provides a suitable microenvironment with specific combination of stromal cells, growth factors, cytokines, and chemokines to induce the bone marrow lymphoid progenitor T-cell precursors into single-positive CD4⁺ and CD8⁺ T effectors and CD4⁺CD25⁺ T-regulatory cells (Tregs). At postthymic compartments, the CD4⁺ T-cells acquire distinct phenotypes which include the classical T-helper 1 (Th1), T-helper 2 (Th2), T-helper 9 (Th9), T-helper 17 (Th17), follicular helper T-cell (Tfh), and induced T-regulatory cells (iTregs), such as the regulatory type 1 cells (Tr1) and transforming growth factor-β- (TGF-β-) producing CD4⁺ T-cells (Th3). Tregs represent only a small fraction, 5–10% in mice and 1-2% in humans, of the overall CD4⁺ T-cells in lymphoid tissues but are essential for immunoregulatory circuits mediating the inhibition and expansion of all lineages of T-cells. In this paper, we first provide an overview of the major cell-intrinsic developmental programs that regulate T-cell lineage fates in thymus and periphery. Next, we introduce the SV40 immortomouse as a relevant mice model for implementation of new approaches to investigate thymus organogenesis, CD4 and CD8 development, and thymus cells tumorogenesis.

Association Between SV40 and Non-Hodgkin's Lymphoma

Millions of people worldwide were inadvertently exposed to live simian virus 40 (SV40) between 1955 and 1963 through immunization with SV40-contaminated polio vaccines. Although the prevalence of SV40 infections in humans is not known, numerous studies suggest that SV40 is a pathogen resident in the human population today. SV40 is a potent DNA tumor virus that is known to induce primary brain cancers, bone cancers, mesotheliomas, and lymphomas in laboratory animals. SV40 oncogenesis is mediated by the viral large tumor antigen (T-ag), which inactivates the tumor suppressor proteins p53 and pRb. During the last decade, independent studies using different molecular biology techniques have shown the presence of SV40 DNA, T-ag, or other viral markers in primary human brain and bone cancers and malignant mesotheliomas. Evidence suggests that there may be geographic differences in the frequency of these virus-positive tumors. Recent large independent controlled studies have shown that SV40 T-ag DNA is significantly associated with human non-Hodgkin's lymphoma (NHL). In our study, we analyzed systemic NHL from 76 HIV-1-positive and 78 HIV-1-negative patients, and nonmalignant lymphoid samples from 79 HIV-1-positive and 107 HIV-1-negative patients without tumors; 54 colon and breast carcinoma samples served as cancer controls. We used polymerase chain reaction (PCR) followed by Southern blot hybridization and DNA sequence analysis to detect DNAs of polyomaviruses and herpesviruses. SV40-specific DNA sequences were detected in 64 (42%) of 154 NHL, none of 186 nonmalignant lymphoid samples, and none of 54 control cancers. For NHL from HIV-1-positive patients, 33% contained SV40 DNA and 39% Epstein Barr virus (EBV) DNA, whereas NHLs from HIV-1-negative patients were 50% positive for SV40 and 15% positive for EBV. Few tumors were positive for both SV40 and EBV. Human herpesvirus type 8 was not detected. SV40 sequences were found most frequently in diffuse large B cell and follicular-type lymphomas. We conclude that SV40 is significantly associated with some types of NHL and that lymphomas should be added to the types of human cancers associated with SV40.

Update on human polyomaviruses and cancer

Over 50 years of polyomavirus research has produced a wealth of insights into not only general biologic processes in mammalian cells, but also, how conditions can be altered and signaling systems tweaked to produce transformation phenotypes. In the past few years three new members (KIV, WUV, and MCV) have joined two previously known (JCV and BKV) human polyomaviruses. In this review, we present updated information on general virologic features of these polyomaviruses in their natural host, concentrating on the association of MCV with human Merkel cell carcinoma. We further present a discussion on advances made in SV40 as the prototypic model, which has and will continue to inform our understanding about viruses and cancer.

T antigen transgenic mouse models

The study of polyomavirus has benefited immensely from two scientific methodologies, cell culture and in vitro studies on one side and the use of transgenic mice as experimental models on the other. Both approaches allowed us to identify cellular products targeted by the viruses, the consequences of these interactions at the phenotypic and molecular level, and thus the potential roles of the targets within their normal cellular context. In particular, cell culture and in vitro reports suggest a model explaining partially how SV40 large T antigen contributes to oncogenic transformation. In most cases, T antigen induces cell cycle entry by inactivation of the Rb proteins (pRb, p130, and p107), thus activating E2F-dependent transcription and subsequent S-phase entry. Simultaneously, T antigen blocks p53 activity and therefore prevents the ensuing cell-cycle arrest and apoptosis. For the most part, studies of T antigen expression in transgenic mice support this model, but the use of T antigen mutants and their expression in different tissue and cell type settings have expanded our knowledge of the model system and raised important questions regarding tumorigenic mechanisms functioning in vivo.

Expression of a gonadotropin-releasing hormone receptor-simian virus 40 T-antigen transgene has sex-specific effects on the reproductive axis

The GnRH receptor (GnRHR) responds to pulsatile GnRH signals to coordinate pituitary gonadotropin synthesis and secretion. Previously, a 1.2-kb fragment of the 5'-flanking region isolated from the mouse GnRHR gene was shown to target expression to pituitary gonadotropes in vivo. The 1.2-kb gene promoter fused to the simian virus 40 large T antigen (TAg) was used to generate transgenic mice that form gonadotrope-derived pituitary tumors at 4-5 months of age. Transgenic female mice have hypogonadotropic hypogonadism, infantile gonads, and are infertile throughout their life span, whereas males remain reproductively intact until their tumors become large. We hypothesized that the targeted TAg expression causes a sex-specific disruption of the reproductive axis at the level of the pituitary gland. To test this hypothesis, we characterized the pituitary gonadotropin beta-subunit and TAg expression patterns, and measured plasma gonadotropin and gonadal steroid levels in female and male mice before and after pituitary tumor development. TAg expression was observed in transgenic females and males 15 d of age, before tumor development. Interestingly, and in contrast to the transgenic males, pituitary LH beta and FSH beta subunit protein levels, and plasma LH and FSH levels, were reduced in transgenic females. Reproductive organs in transgenic female mice remained underdeveloped but were normal in transgenic males. We conclude that the expression of the TAg transgene driven by the GnRHR gene promoter results in female-specific infertility due to disruption of gonadotropin production and secretion even before tumor development.

SV40: Cell transformation and tumorigenesis

The story of SV40-induced tumorigenesis and cellular transformation is intimately entwined with the development of modern molecular biology. Because SV40 and other viruses have small genomes and are relatively easy to manipulate in the laboratory, they offered tractable systems for molecular analysis. Thus, many of the early efforts to understand how eukaryotes replicate their DNA, regulate expression of their genes, and translate mRNA were focused on viral systems. The discovery that SV40 induces tumors in certain laboratory animals and transforms many types of cultured cells offered the first opportunity to explore the molecular basis for cancer. The goal of this article is to highlight some of the experiments that have led to our current view of SV40-induced transformation and to provide some context as to how they contributed to basic research in molecular biology and to our understanding of cancer.

SV40 large T antigen up-regulates the retrotransposition frequency of viral-like 30 elements

The regulation of non-autonomous retrotransposition is not known. A recombinant bearing a hygromycin gene and a viral-like 30 (VL30) retrotransposon tagged with an enhanced green fluorescent protein (EGFP) gene-based retrotransposition cassette was constructed and used for detection of retrotransposition events. Transfection of this recombinant produced retrotransposition events, detected both by EGFP fluorescence and PCR analysis, in hygromycin-selected clones of two established simian virus 40 (SV40)-transformed mouse NIH3T3 cell lines but not in normal NIH3T3 cells. The retrotransposition potential of this recombinant, as a provirus, was studied in stably transfected NIH3T3 clones. Transfection of these clones with either a wild-type or a mutant LE1135T SV40 large T antigen gene, not expressing small t protein, induced retrotransposition events at high frequencies as measured by fluorescence-activated cell scanning (FACS). In addition, measuring retrotransposition frequencies over a period of nine days following infection with isolated SV40 particles, revealed that the frequency of retrotransposition was time-dependent and induced as early as 24 h, increasing exponentially to high levels (>10(-2) events per cell per generation) up to nine days post-infection. Furthermore, ectopic expression of a cloned MoMLV-reverse transcriptase gene also produced retrotransposition events and suggested that the large T antigen most likely acted through induction of expression of endogenous reverse transcriptase genes. Our results show a direct correlation between SV40-cell transformation and VL30 retrotransposition and provide for the first time strong evidence that SV40 large T antigen up-regulates the retrotransposition of VL30 elements.

pRb inactivation in mammary cells reveals common mechanisms for tumor initiation and progression in divergent epithelia

Retinoblastoma 1 (pRb) and the related pocket proteins, retinoblastoma-like 1 (p107) and retinoblastoma-like 2 (p130) (pRb_f, collectively), play a pivotal role in regulating eukaryotic cell cycle progression, apoptosis, and terminal differentiation. While aberrations in the pRb-signaling pathway are common in human cancers, the consequence of pRb_f loss in the mammary gland has not been directly assayed in vivo. We reported previously that inactivating these critical cell cycle regulators in divergent cell types, either brain epithelium or astrocytes, abrogates the cell cycle restriction point, leading to increased cell proliferation and apoptosis, and predisposing to cancer. Here we report that mouse mammary epithelium is similar in its requirements for pRb_f function; Rb_f inactivation by T₁₂₁, a fragment of SV40 T antigen that binds to and inactivates pRb_f proteins, increases proliferation and apoptosis. Mammary adenocarcinomas form within 16 mo. Most apoptosis is regulated by p53, which has no impact on proliferation, and heterozygosity for a p53 null allele significantly shortens tumor latency. Most tumors in p53 heterozygous mice undergo loss of the wild-type p53 allele. We show that the mechanism of p53 loss of heterozygosity is not simply the consequence of Chromosome 11 aneuploidy and further that chromosomal instability subsequent to p53 loss is minimal. The mechanisms for pRb and p53 tumor suppression in the epithelia of two distinct tissues, mammary gland and brain, are indistinguishable. Further, this study has produced a highly penetrant breast cancer model based on aberrations commonly observed in the human disease.

Inactivation of the three retinoblastoma genes in the mouse mammary gland provides a new animal model for human breast cancer

Functional abrogation of p53 is required for T-Ag induced proliferation in cardiomyocytes

Targeted expression of the SV40 large T-antigen oncoprotein (T-Ag) induces cardiomyocyte proliferation in the atria and ventricles of transgenic mice. Previous studies have identified the p53 tumor suppressor, p107 (a homologue of the retinoblastoma tumor suppressor), and p193 (a novel BH3 only proapoptosis protein) as prominent TAg binding proteins in cardiomyocyte cell lines derived from these transgenic mice. To further explore the significance of these protein-protein interactions in the regulation of cardiomyocyte proliferation, a transgene comprising the human atrial natriuretic factor (ANF) promoter and sequences encoding a mutant T-Ag lacking the p53 binding domain was generated. Repeated micro-injection of this DNA gave rise to genetically mosaic animals with minimal transgene content, suggesting that widespread cardiac expression of mutant T-Ag was deleterious. This notion was supported by the observation that the transgene was selectively lost from the cardiac myocytes (but not the cardiac fibroblasts) in the mosaic animals. Crosses between the mosaic mice and animals expressing a cardiac restricted dominant negative p53 resulted in transgene transmission with ensuing overt cardiac tumorigenesis. Transfection of the mutant T-Ag in embryonic stem (ES) cell-derived cardiomyocytes resulted in wide-spread cell death with characteristics typical of apoptosis. Co-transfection with a dominant negative p53 transgene rescued mutant TAg-induced cell death in the ES-derived cardiomyocyte cultures, resulting in a marked proliferative response similar to that seen in vivo with the rescued transgenic mouse study. These results indicate that T-Ag expression in the absence of p53 functional abrogation results in cardiomyocyte death.

Role of the amino-terminal domain of simian virus 40 early region in inducing tumors in secretin-expressing cells in transgenic mice

BACKGROUND & AIMS

The early region of simian virus 40 (SV40) encodes 2 transforming proteins, large T (Tag) and small t antigen, that produce neuroendocrine tumors in the intestine and the pancreas when expressed in secretin cells of transgenic mice.

METHODS

Two SV40 early-region transgenes containing a deletion that eliminated expression of the small t antigen were expressed in transgenic mice under control of the secretin gene. The 2 lines of mice, one expressing the native large T antigen and the other T antigen with a mutation in its N-terminal J domain, were examined to determine which biological activities of the SV40 early region were required for tumorigenesis.

RESULTS

Most animals expressing wild-type large T antigen developed pancreatic insulinomas and lymphomas and died between 3 and 6 months of age. However, small intestinal neoplasms were extremely rare in the absence of small t antigen expression. Transgenic lines expressing the J domain mutant failed to develop tumors.

CONCLUSIONS

Transformation of secretin-producing enteroendocrine cells by SV40 requires functional cooperation between intact large T and small t oncoproteins. In contrast, large T antigen alone is sufficient to induce tumors in the endocrine pancreas and thymus.

Different functions are required for initiation and maintenance of immortalization of rat embryo fibroblasts by SV40 large T antigen

We have used two different, but complementary assays to characterize functions of SV40 T antigen that are necessary for its ability to immortalize rat embryo fibroblasts. In accordance with previous work, we found that several functions were required. These include activities that map to the p53 binding domain and the amino terminal 176 amino acids which contain the J domain as well as the CR1 and CR2 domain required for binding and sequestering the RB family of pocket proteins. Moreover, we found that even though activities dependent only upon the amino terminus were sufficient for immortalization they were unable to maintain it. This suggests that immortalization by these amino terminal functions requires either additional events or immortalization of a subset of cells within the heterogeneous rat embryo fibroblast population. We further found that an activity dependent upon amino acids 17 - 27 which remove a portion of the CR1 domain and the predicted alpha-1 helix of the J domain was not necessary to maintain growth but was required for direct immortalization suggesting that at least one of the functions required initially was not required to maintain the immortal state. This represents the first demonstration that some of the functions required for maintenance of the immortal state differ from those required for initiation of immortalization.

Cell and molecular biology of simian virus 40: implications for human infections and disease

Simian virus 40 (SV40), a polyomavirus of rhesus macaque origin, was discovered in 1960 as a contaminant of polio vaccines that were distributed to millions of people from 1955 through early 1963. SV40 is a potent DNA tumor virus that induces tumors in rodents and transforms many types of cells in culture, including those of human origin. This virus has been a favored laboratory model for mechanistic studies of molecular processes in eukaryotic cells and of cellular transformation. The viral replication protein, named large T antigen (T-ag), is also the viral oncoprotein. There is a single serotype of SV40, but multiple strains of virus exist that are distinguishable by nucleotide differences in the regulatory region of the viral genome and in the part of the T-ag gene that encodes the protein's carboxyl terminus. Natural infections in monkeys by SV40 are usually benign but may become pathogenic in immunocompromised animals, and multiple tissues can be infected. SV40 can replicate in certain types of simian and human cells. SV40-neutralizing antibodies have been detected in individuals not exposed to contaminated polio vaccines. SV40 DNA has been identified in some normal human tissues, and there are accumulating reports of detection of SV40 DNA and/or T-ag in a variety of human tumors. This review presents aspects of replication and cell transformation by SV40 and considers their implications for human infections and disease pathogenesis by the virus. Critical assessment of virologic and epidemiologic data suggests a probable causative role for SV40 in certain human cancers, but additional studies are necessary to prove etiology.

Immortalization of rat embryo fibroblasts by a 3'-untranslated region

We have exploited a cross-species expression screen to search for cellular immortalizing activities. A newt blastemal cDNA expression library was transfected into rat embryo fibroblasts and immortal cell lines were selected. This identified a 1-kb cDNA fragment which has a low representation in the cDNA library and is derived from the 3'-UTR of an alpha-glucosidase-related mRNA. Expression of this sequence in rat embryo fibroblasts has shown that it is active in promoting colony formation and immortalization. It is also able to cooperate with an immortalization-defective deletion mutant of SV40 T antigen, indicating that it can exert its growth-stimulatory activity in the pathway activated by a viral immortalizing oncogene. This is the first example of an immortalizing activity mediated by an RNA sequence, and further analysis of its mechanism should provide new insights into senescence and immortalization.

Ependymal and choroidal cells in culture: characterization and functional differentiation

During the past 10 years, our teams developed long-term primary cultures of ependymal cells derived from ventricular walls of telencephalon and hypothalamus or choroidal cells (modified ependymal cells) derived from plexuses dissected out of fetal or newborn mouse or rat brains. Cultures were established in serum-supplemented or chemically defined media after seeding on serum-, fibronectin-, or collagen-laminin-coated plastic dishes or semipermeable inserts. To identify and characterize cell types growing in our cultures, we used morphological features provided by phase contrast, scanning, and transmission electron microscopy. We used antibodies against intermediate filament proteins (vimentin, glial fibrillary acidic protein, cytokeratin, desmin, neurofilament proteins), actin, myosin, ciliary rootlets, laminin, and fibronectin in single or double immunostaining, and monoclonal antibodies against epitopes of ependymal or endothelial cells, to recognize ventricular wall cell types with immunological criteria. Ciliated or nonciliated ependymal cells in telencephalic cultures, tanycytes and ciliated and nonciliated ependymal cells in hypothalamic cultures always exceeded 75% of the cultured cells under the conditions used. These cells were characterized by their cell shape and epithelial organization, by their apical differentiations observed by scanning and transmission electron microscopy, and by specific markers (e.g., glial fibrillary acidic protein, ciliary rootlet proteins, DARPP 32) detected by immunofluorescence. All these cultured ependymal cell types remarkably resembled in vivo ependymocytes in terms of molecular markers and ultrastructural features. Choroidal cells were also maintained for several weeks in culture, and abundantly expressed markers were detected in both choroidal tissue and culture (Na+-K+-dependent ATPase, DARPP 32, G proteins, ANP receptors). In this review, the culture models we developed (defined in terms of biological material, media, substrates, duration, and subculturing) are also compared with those developed by other investigators during the last 10 years. Focusing on morphological and functional approaches, we have shown that these culture models were suitable to investigate and provide new insights on (1) the gap junctional communication of ependymal, choroidal, and astroglial cells in long-term primary cultures by freeze-fracture or dye transfer of Lucifer Yellow CH after intracellular microinjection; (2) some ionic channels; (3) the hormone receptors to tri-iodothyronine or atrial natriuretic peptides; (4) the regulatory effect of tri-iodothyronine on glutamine synthetase expression; (5) the endocytosis and transcytosis of proteins; and (6) the morphogenetic effects of galactosyl-ceramide. We also discuss new insights provided by recent results reported on in vitro ependymal and choroidal expressions of neuropeptide-processing enzymes and neurosecretory proteins or choroidal expression of transferrin regulated through serotoninergic activation.

Bax suppresses tumorigenesis and stimulates apoptosis in vivo

The protein p53 is a key tumour-suppressor, as evidenced by its frequent inactivation in human cancers. Animal models have indicated that attenuation of p53-dependent cell death (apoptosis) can contribute to both the initiation and progression of cancer, but the molecular mechanisms are unknown. Although p53-mediated transcriptional activation is one possible explanation, none of the known p53-responsive genes has been shown to function in p53-dependent apoptosis. Here we test the role of the death-promoting gene bax in a transgenic mouse brain tumour, a model in which p53-mediated apoptosis attenuates tumour growth. Inactivation of p53 causes a dramatic acceleration of tumour growth owing to a reduction in apoptosis of over ninety per cent. We show that p53-dependent expression of bax is induced in slow-growing apoptotic tumours. Moreover, tumour growth is accelerated and apoptosis drops by fifty per cent in Bax-deficient mice, indicating that it is required for a full p53-mediated response. To our knowledge this is the first demonstration that Bax acts as a tumour suppressor, and our findings indicate that Bax could be a component of the p53-mediated apoptotic response in this system.

Susceptibility to cell death induced by mutant SV40 T-antigen correlates with Purkinje neuron functional development

Purkinje cells are uniquely susceptible to a number of physical, chemical, and genetic insults both during development and in the mature state. We have previously shown that when the postmitotic state of murine Purkinje cells is altered by inactivation of the retinoblastoma tumor susceptibility protein (pRb), immature as well as mature Purkinje cells undergo apoptosis. DNA synthesis and neuronal loss are induced in postmitotic Purkinje cells dependent upon the pRb-binding portion of SV40 large T antigen (T-ag). In the present study, Purkinje cell targeting of a mutant T-ag, PVU, which does not bind pRb, reveals disparate cerebellar phenotypes dependent upon temporal differences in transgene expression. Strong embryonic and postnatal transgene expression in three lines alters Purkinje cell development and function during the second postnatal week, causing ataxia without Purkinje cell loss. In contrast, two other transgenic lines reveal that PVU T-ag expression following normal Purkinje cell maturation causes rapid Purkinje cell degeneration. The second and third postnatal weeks of cerebellar development, which include the major period of synaptogenesis, appear to be the defining stage for the two PVU-induced phenotypes. These data indicate that Purkinje cell death susceptibility varies with developmental stage.

A novel simian virus 40 early-region domain mediates transactivation of the cyclin A promoter by small-t antigen and is required for transformation in small-t antigen-dependent assays

At least three regions of the simian virus 40 small-t antigen (small-t) contribute to the protein's ability to enhance cellular transformation. As we showed previously for rat F111 cells, one region includes sequences from residues 97 to 103 that are involved in the binding and inhibition of protein phosphatase 2A. In the present study, the role of the protein phosphatase 2A binding region was confirmed in two additional small-t-dependent transformation systems. Second, small-t was found to provide a function previously identified as a large-T transformation domain. Mutations in residues 19 to 28 of large-T affected its transforming ability, but these mutations were complemented by a wild-type small-t. A third region of small-t was also required for efficient transformation. This region, the 42-47 region, is shared by large-T and small-t and contains a conserved HPDKGG hexapeptide. The 42-47 region function could be provided by either small-t or large-T in small-t-dependent systems. Mutations in the 42-47 region reduced the ability of small-t to transactivate the cyclin A promoter, of interest because small-t increased endogenous cyclin A mRNA levels in both human and monkey cells, as well as transactivating the promoter in transient assays.

Modification of an N-terminal regulatory domain of T antigen restores p53-T antigen complex formation in the absence of an essential metal ion cofactor

We have discovered that the ability of the tumor suppressor protein p53 to bind to the viral large T antigen (TAg) oncogene product is regulated by divalent cations. Both proteins were purified from an insect cell line infected with the appropriate baculovirus expression vector. In a two-site capture enzyme-linked immunosorbent assay, complex formation between the purified proteins is strictly dependent on the addition of specific concentrations of divalent metal ions, notably zinc, copper, cadmium, cobalt, manganese, and nickel. In the presence of zinc the pattern of proteolytic fragments obtained when TAg was subjected to proteolysis by endoproteinase Glu-C (V8) was strikingly different, supporting the idea that a conformational change in TAg associated with ion binding is required for it to complex with p53. Monoclonal antibody analysis provides supporting evidence for a conformational change. When TAg was captured onto an enzyme-linked immunosorbent assay plate coated with PAb 419 as opposed to many other anti-TAg antibodies, complex formation was completely independent of the presence of additional divalent cations. Our results suggest that the ability of p53 and TAg to form a stable complex in vitro is dependent upon a regulatory domain residing in the N terminus of TAg, zinc ions or the binding of a specific monoclonal antibody (PAb 419) provoking a conformational change in TAg that facilitates and supports complex formation.

The inhibition of cultured myoblast differentiation by the simian virus 40 large T antigen occurs after myogenin expression and Rb up-regulation and is not exerted by transformation-competent cytoplasmic mutants

We have investigated the mechanism by which the simian virus 40 large T antigen (SVLT) interferes with the differentiation of C2 myoblasts. SVLT mutants, defective either in the Rb binding site, near the N-terminal end, in a region that affects binding to p53, or in the nuclear transport signal, were also employed to determine whether the interference was especially dependent on these functional domains. It was found that wild-type (wt) SVLT strongly inhibited the terminal differentiation of mouse C2 myoblasts, but this arrest occurred only after the synthesis of myogenin, an initial step in biochemical differentiation. Neither the synthesis nor some basic activities of MyoD appeared to be affected by wt SVLT. In these transformants, mitogen depletion elicited an increase in the Rb level comparable to that in normal C2 cells; wt SVLT, however, promoted the phosphorylation of a large part of the induced Rb. Mutations affecting nuclear transport were far more critical for the ability to interfere with myogenic differentiation than were those affecting the transforming potential; cytoplasmic SVLT expression was fully compatible with the terminal differentiation of C2 cells, despite enabling them to grow in semisolid medium, thus showing that the myogenesis-inhibiting property can be dissociated from transforming competence. The remaining SVLT mutants presented different degrees of ability to inhibit differentiation (as shown by the expression of tissue-specific markers in transformants). The inhibiting mutants, including the Rb binding site mutant, were able to promote a higher state of Rb phosphorylation than that observed in either normal cells or cytoplasmic-SVLT transformants.

Simian virus 40 T antigen-induced amplification of pre-parietal cells in transgenic mice. Effects on other gastric epithelial cell lineages and evidence for a p53-independent apoptotic mechanism that operates in a committed progenitor

Gastric units in the glandular epithelium of the mouse stomach contain several types of continuously renewing epithelial cells. Acid-producing parietal cells are derived from a multipotent stem cell that also gives rise to mucus-producing pit cells and pepsinogen- and intrinsic factor-producing zymogenic cells. We used nucleotides -1035 to +24 of the mouse H+/K(+)-ATPase beta subunit gene (H+/K(+)-ATPase beta subunit-1035 to +24) to examine the consequences of expressing simian virus 40 T antigen (SV 40 TAg) in the normally rare, nonproliferating, short-lived pre-parietal cell progenitor. Light and electron microscopic morphologic studies plus multilabel immunohistochemical analyses of postnatal day (P) 14-80-day transgenic mice revealed that SV40 TAg produces a 50-70-fold amplification of pre-parietal cells which become the predominant cell type in gastric units. Differentiation to mature parietal cells is blocked, resulting in hypochlorhydria and an associated systemic iron deficiency. SV40 TAg-induced pre-parietal proliferation is accompanied by apoptosis. Examination of adult transgenic mice homozygous for p53 wild type or p53 null alleles established that the apoptosis occurs through a p53-independent pathway. H+/K(+)-ATPase beta subunit -1035 to +24/SV40 Tag is not expressed during differentiation of the zymogenic lineage. Nonetheless, P28-P80 transgenic mice exhibit an apparent block in the conversion of pre-zymogenic to zymogenic cells. This block appears to be quite specific: conversion of preneck to neck cells and neck to pre-zymogenic cells is not affected. Comparison of normal and transgenic mice that are p53+/+ or p53-/- confirmed that the loss of mature zymogenic cells is not dependent upon p53. Although H+/K(+)-ATPase beta subunit -1035 to +24 is not active in pit cell progenitors or their differentiated descendants, there is a 2-3-fold increase in mature pit cells in transgenic animals. Our findings (i) demonstrate an approach for amplifying and characterizing pre-parietal or other progenitor cell populations in gastric units, (ii) reveal an SV40 TAg-inducible, p53-independent apoptotic mechanism that operates in a committed epithelial progenitor cell, and (iii) provide a transgenic mouse model for defining factors that may mediate progression through specific points in the differentiation programs of the parietal and zymogenic cell lineages or that may influence decisions about allocation to the pit cell lineage.

T antigens encoded by replication-defective simian virus 40 mutants dl1135 and 5080

We present a preliminary biochemical characterization of two simian virus 40 mutants that affect different T antigen replication functions. SV40 T antigen mutants dl1135 (delta 17-27 amino acids) and 5080 (P-L) have been studied extensively with regard to their ability to transform cells in culture and induce tumors in transgenic mice. Both mutants are defective for viral DNA replication in vivo. In order to assess in more detail the molecular basis for the in vivo replication defects of 5080 and dl1135, we expressed the mutant proteins using the baculovirus system and purified them by immunoaffinity chromatography. With each of the purified proteins, we examined some of the biochemical activities of T antigen required for replication, viz. ATPase, binding to the origin of replication (ori) and assembly on ori, DNA helicase and unwinding, and replication in in vitro assays. Consistent with previous studies, we found that the 5080 protein is defective for multiple biochemical activities including ATPase, helicase, ori-specific unwinding, and ATP-induced hexamerization. However, this mutant retains some sequence-specific DNA binding activity. In contrast, the dl1135 protein exhibited significant levels of activity in all assays, including the ability to drive SV40 DNA replication in vitro. Thus, dl1135 is one of several mutants with an altered amino-terminal domain which can replicate DNA in vitro, but not in vivo. Thus, while the 5080 mutation affects a T antigen enzymatic function directly required for viral DNA synthesis, dl1135 may alter an activity required to prepare the cell for viral replication.

The kinetics of simian virus 40-induced progression of quiescent cells into S phase depend on four independent functions of large T antigen

Microinjection of purified simian virus 40 large-T-antigen protein or DNA encoding T antigen into serum-starved cells stimulates them to re-enter the cell cycle and progress through G1 into the S phase. Genetic analysis of T antigen indicated that neither its Rb/p107-binding activity nor its p53-binding activity is essential to induce DNA synthesis in CV1P cells. However, T antigens bearing missense mutations that inactivate either activity induced slower progression of the cells into the S phase than did wild-type T antigen. Inactivation of both activities resulted in a T antigen essentially unable to induce DNA synthesis. Missense mutations in either the DNA-binding region of the N terminus also impaired the ability of full-length T antigen to stimulate DNA synthesis in CV1P cells. The wild-type kinetics of cell cycle progression were restored by genetic complementation after coinjection of plasmid DNAs encoding different mutant T antigens or coinjection of purified mutant T-antigen proteins, suggesting that the four mitogenic functions of T antigen are independent. The maximal rate of induction of DNA synthesis in secondary primate cells and established rodent cell lines required the same four functions of T antigen. A model to explain how four independent activities could cooperate to stimulate cell cycle progression is presented.

p53-dependent apoptosis suppresses tumor growth and progression in vivo

To determine the contribution of p53 loss to tumor progression, we have induced abnormal proliferation in the brain choroid plexus epithelium of transgenic mice using a SV40 T antigen fragment that perturbs pRB family function but does not affect p53 function. Tumors induced by this mutant develop slowly compared with those induced by wild-type T antigen. Suppressed tumor growth is directly attributable to p53 function, since rapid tumor development occurs when the T antigen fragment is expressed in p53-null mice. In p53-heterozygous mice, stochastic loss of the wild-type p53 allele results in the focal emergence of aggressive tumor nodules characteristic of tumor progression. In each case, aggressive tumor development in the absence of p53 function corresponds to a decrease in the level of apoptosis. These results provide in vivo evidence that p53-dependent apoptosis, occurring in response to oncogenic events, is a critical regulator of tumorigenesis.