Activities of SV40 T antigen necessary for the induction of tetraploid DNA content in permissive CV-1 cells

To determine the role of SV40 T antigen in stimulating multiple rounds of DNA synthesis in permissive cells, CV-1 cells were transfected with plasmids expressing mutant or wt T antigen in the presence or absence of the SV40 origin of replication. Induction of cells with > G2 DNA content (tetraploid DNA content) and levels of T antigen protein were detected and analyzed by flow cytometry. The mutant T antigen proteins demonstrated the expected phenotypes as determined by immunoprecipitation. Elevated levels of T antigen protein were detected in each transfection, but full-length T antigen alone was responsible for the tetraploid DNA content. The studies show that full-length T antigen with point mutations to reduce binding to the cellular proteins p53 and/or Rb were capable of inducing > G2 DNA content though the induction by these mutants was greatly enhanced by the presence of the SV40 origin of replication. Truncated T antigen (aa 1-259) could induce cells with tetraploid DNA content only in the presence of the SV40 origin of replication and the absence of Rb binding. These studies suggest that multiple functions of T antigen are involved in the stimulation of the second round of cellular DNA synthesis.

Selective proliferation of lymphoid cells from lineage-c-Kit+ Sca-1+ cells by a clonal bone marrow stromal cell line

To understand the regulatory mechanisms involved in the development of hematopoietic stem cells, we cultured lineage-negative, c-Kit+ Sca-1+ stem cells sorted from bone marrow cells by a fluorescence-activated cell sorter (FACS) on layers of bone marrow stromal cell lines established from SV40 T-antigen gene transgenic mice. We previously reported that the TBR59 stromal cell line induced two sequential cobblestone formations: the first formation committed to the myeloid and the second to the lymphoid lineage. After examination of many other bone marrow stromal cell lines, we found that TBR31-1 stromal cells supported only lymphoid development of the sorted stem cells. The sorted stem cells proliferated by forming cobblestones and the cells were released from the cobblestones. Most released cell populations were B220-positive lymphoid cells; cell production continued for 2 months. Addition of G-CSF or M-CSF produced only a slight effect on myeloid development. FACS analysis of the released cells showed that the B-lymphoid-committed progenitors developed into mature B-cells by expressing surface immunoglobulin M. These results indicate that TBR31-1 bone marrow stromal cells selectively support B-lymphoid development, whereas TBR59 cells support both myeloid and lymphoid development of stem cells.

Simian virus 40 as a vector: recombinant viruses expressing individual polyoma T antigens

We constructed simian virus 40 (SV40)/polyomavirus recombinants by replacing in SV40 the T antigen coding region with polyoma early region sequences, either cDNAs encoding small, middle or large T antigen or the wild-type sequence coding all three proteins. The recombinants maintained the SV40 late region and origin of replication and were propagated in COS cells yielding recombinant virus preparations with titers of 10(6)-10(7) infectious particles per milliliter. These viruses were characterized in productive infections of COS cells by analyzing early and late mRNA levels and by following synthesis of polyoma early proteins. In the absence of viral DNA replication, i.e. in infected monkey or mouse cells, expression of the polyoma T antigens was weak. Further experiments indicated that this was mostly due to high genomic instability during amplification, to lower levels of cDNA transcripts as compared to spliced mRNA, and possibly also to lower infectivity of the recombinant virions. It remains to be determined, whether these handicaps are unique to SV40/polyoma recombinants or whether SV40 is in general inadequate as a viral vector.

Mammalian DNA topoisomerase I activity and poisoning by camptothecin are inhibited by simian virus 40 large T antigen

DNA topoisomerase I (top1) is a ubiquitous enzyme that forms reversible DNA single-strand breaks (cleavage complexes) and plays a role in transcription, DNA replication, and repair. Top1 is the target of camptothecins which selectively trap top1 cleavage complexes and represent a novel class of anticancer drugs active against human solid tumors. The present study demonstrates that recombinant large T antigen (T-Ag), a virus encoded helicase with strong affinity for tumor suppressors and cell cycle- and replication-related proteins, suppresses top1 cleavage complexes and top1 catalytic activity. This top1 suppressive activity is probably not due to T-Ag binding to DNA, as a T-Ag truncation mutant containing only the first 246 amino acids and deficient in DNA binding also inhibited top1, and the inhibition was independent of ATP. T-Ag also antagonized and reversed the trapping of top1 cleavage complexes by camptothecin. These results demonstrate a functional interaction between T-Ag and top1: they also suggest the importance of top1-protein interactions for the regulation of DNA replication and modulation of camptothecin activity.

Hibernoma development in transgenic mice identifies brown adipose tissue as a novel target of aldosterone action

Aldosterone is a major regulator of salt balance and blood pressure, exerting its effects via the mineralocorticoid receptor (MR). To analyze the regulatory mechanisms controlling tissue-specific expression of the human MR (hMR) in vivo, we have developed transgenic mouse models expressing the SV40 large T antigen (TAg) under the control of each of the two promoters of the hMR gene (P1 or P2). Unexpectedly, all five P1-TAg founder animals died prematurely from voluminous malignant liposarcomas originating from brown adipose tissue, as evidenced by the expression of the mitochondrial uncoupling protein ucp1, indicating that the proximal P1 promoter was transcriptionally active in brown adipocytes. No such hibernoma occurred in P2-TAg transgenic mice. Appropriate tissue-specific usage of P1 promoter sequences was confirmed by demonstrating the presence of endogenous MR in both neoplastic and normal brown adipose tissue. Several cell lines were derived from hibernomas; among them, the T37i cells can undergo terminal differentiation into brown adipocytes, which remain capable of expressing ucp1 upon adrenergic or retinoic acid stimulation. These cells possess endogenous functional MR, thus providing a new model to explore molecular mechanisms of mineralocorticoid action. Our data broaden the known functions of aldosterone and suggest a potential role for MR in adipocyte differentiation and regulation of thermogenesis.

Induction of retinoblastoma gene expression during terminal growth arrest of a conditionally immortalized fetal rat lung epithelial cell line and during fetal lung maturation

The process by which fetal lung epithelial cells differentiate into type 1 and type 2 cell is largely unknown. In order to study lung epithelial cell proliferation and differentiation we have infected 20-day fetal lung epithelial cells with a retrovirus carrying a temperature-sensitive SV40 T antigen (T Ag) and isolated several immortalized fetal epithelial cell lines. Cell line 20-3 has characteristics of lung epithelial cells including the presence of distinct lamellar bodies, tight junctions, keratin 8 and 18 mRNA, HFH8, and T1 alpha mRNA and low levels of surfactant protein A mRNA. At 33 degrees C 20-3 grows with a doubling time of 21 h. At 40 degrees C the majority of cells cease to proliferate. Growth arrest is accompanied by significant morphological changes including an increase in cell size, transition to a squamous phenotype that resembles type 1 cells, and an increase in the number of multinucleated cells within the population. Greater than 95% of the cells incorporate [3H]thymidine into DNA at 33 degrees C whereas at 40 degrees C label incorporation drops to less than 20%. When shifted down to 33 degrees C 40% of the cells remain terminally growth arrested. In addition, cells plated at 40 degrees C have a reduced ability to form colonies when replated at 33 degrees C. Treatment with TGF-beta increases the percentage of cells that terminally growth arrest to greater than 80%. Growth arrest is accompanied by an increase in the levels of c-jun, jun D, cyclin D1, C/EBP-beta, transglutaminase type II, and retinoblastoma (Rb) mRNA and an induction of p105, the hypophosphorylated, growth regulatory form of Rb. Evaluation of Rb mRNA in fetal lung indicates that it is induced 2.5-fold between 17 and 21 days of gestation. These studies indicate that 20-3 terminally growth arrests in culture at the nonpermissive temperature and that it may be useful in studying changes in gene expression that accompany terminal growth arrest during lung development.

Does preventive vaccination with engineered tumor cells work in cancer-prone transgenic mice?

The use of genetically modified tumor cells as vaccines has been successful in numerous animal models of grafted syngenic tumors and has provided the groundwork for many clinical trials of gene therapy in cancer patients. To investigate the real efficacy of ex vivo gene therapy-based vaccines, we used transgenic mice that express the SV40 large T and small t antigens under the control of hepatic antithrombin III (ASV-B)-regulatory sequences. These mice systematically develop hepatocarcinoma. Hepatoma cells, derived from ASV-B transgenic mice, were gene-transduced to express either interleukin-2, interleukin-4, the granulocyte-macrophage colony-stimulating factor, or the T-cell costimulatory molecule B7.1. First, we demonstrated the vaccine potential of engineered hepatoma cells by immunizing nontransgenic mice with these cells, which prevented the growth of subsequent grafted nontransduced hepatoma cells. However, vaccination of pretumoral transgenic animals with various combinations of engineered hepatoma cells failed to inhibit hepatoma onset and progression. Rather, tumor development in ASV-B mice appears to be dependent on the immune system, since neonatal induction of immunotolerance to tumor in ASV-B mice cells was associated with a moderate, but significant, acceleration of tumor development. These results seriously call into question the efficacy of this strategy of active vaccinotherapy against natural tumors.

Novel mechanisms of E2F induction by BK virus large-T antigen: requirement of both the pRb-binding and the J domains

E2F activity is regulated in part by the retinoblastoma family of tumor suppressor proteins. Viral oncoproteins, such as simian virus 40 (SV40) large-T antigen (TAg), adenovirus E1A, and human papillomavirus E7, can disrupt the regulation of cellular proliferation by binding to pRb family members and dissociating E2F-pRb family protein complexes. BK virus (BKV), which infects a large percentage of the human population and has been associated with a variety of human tumors, encodes a TAg homologous to SV40 TAg. It has been shown that BKV TAg, when expressed at low levels, does not detectably bind to pRb family members, yet it induces a serum-independent phenotype and causes a decrease in the overall levels of pRb family proteins. The experiments presented in this report show that, despite the lack of TAg-pRb interactions, BKV TAg can induce transcriptionally active E2F and that this induction does in fact require an intact pRb-binding domain as well as an intact J domain. In addition, E2F-pRb family member complexes can be detected in both BKV and SV40 TAg-expressing cells. These results suggest the presence of alternate cellular mechanisms for the release of E2F in addition to the well-established model for TAg-pRb interactions. These results also emphasize a role for BKV TAg in the deregulation of cellular proliferation, which may ultimately contribute to neoplasia.

Simian virus 40 large T antigen interacts with human TFIIB-related factor and small nuclear RNA-activating protein complex for transcriptional activation of TATA-containing polymerase III promoters

The TATA-binding protein (TBP) is common to the basal transcription factors of all three RNA polymerases, being associated with polymerase-specific TBP-associated factors (TAFs). Simian virus 40 large T antigen has previously been shown to interact with the TBP-TAFII complexes, TFIID (B. Damania and J. C. Alwine, Genes Dev. 10:1369-1381, 1996), and the TBP-TAFI complex, SL1 (W. Zhai, J. Tuan, and L. Comai, Genes Dev. 11: 1605-1617, 1997), and in both cases these interactions are critical for transcriptional activation. We show a similar mechanism for activation of the class 3 polymerase III (pol III) promoter for the U6 RNA gene. Large T antigen can activate this promoter, which contains a TATA box and an upstream proximal sequence element but cannot activate the TATA-less, intragenic VAI promoter (a class 2, pol III promoter). Mutants of large T antigen that cannot activate pol II promoters also fail to activate the U6 promoter. We provide evidence that large T antigen can interact with the TBP-containing pol III transcription factor human TFIIB-related factor (hBRF), as well as with at least two of the three TAFs in the pol III-specific small nuclear RNA-activating protein complex (SNAPc). In addition, we demonstrate that large T antigen can cofractionate and coimmunoprecipitate with the hBRF-containing complex TFIIIB derived from HeLa cells infected with a recombinant adenovirus which expresses large T antigen. Hence, similar to its function with pol I and pol II promoters, large T antigen interacts with TBP-containing, basal pol III transcription factors and appears to perform a TAF-like function.

Improvement of neurological deficits in 6-hydroxydopamine-lesioned rats after transplantation with allogeneic simian virus 40 large tumor antigen gene-induced immortalized dopamine cells

The replacement of dopamine (DA) by DA neuron transplants in the treatment of advanced Parkinson disease (PD) is a rational approach. Because of limitations associated with fetal tissue transplants, a clone (1RB3AN27) of simian virus 40 large tumor antigen (LTa) gene-induced immortalized DA neurons were used in this study. These allogeneic immortalized dopamine neurons, when grafted into striata of normal rats, did not divide, did not form tumors, did not produce LTa, did not extend neurites to host neurons, and were not rejected, for as long as 13 months after transplantation. Grafted cells when recultured in vitro resumed cell proliferation and LTa production, suggesting the presence of a LTa gene-inhibiting factor in the brain. The grafting of undifferentiated and differentiated 1RB3AN27 cells or differentiated murine neuroblastoma (NBP2) cells into striata of 6-hydroxydopamine-lesioned rats (an animal model of PD) caused a time-dependent improvement in neurological deficits (reduction in the methamphetamine-induced turning rate). At 3 months after transplantation, 100% of the animals receiving differentiated 1RB3AN27 cells, 63% of the animals receiving undifferentiated 1RB3AN27 cells, 56% of the animals receiving differentiated NBP2 cells, and 0% of the sham-transplanted animals showed improvements in neurological deficits. At 6 months after transplantation, there was a progressive increase in spontaneous recovery in sham-transplanted animals. These results suggest that immortalized DA neurons should be further studied for their potential use in transplant therapy in advanced PD patients.

Expression of a fusion gene consisting of the mouse growth hormone-releasing hormone gene promoter linked to the SV40 T-antigen gene in transgenic mice

Limited information is available concerning the regulation of growth hormone-releasing hormone (GHRH) gene expression in the hypothalamus, largely because of the lack of a suitable cellular model. In an attempt to immortalize hypothalamic GHRH-producing neurons, we have generated a transgenic mouse model which expresses the simian virus 40 (SV40) T-antigen gene (Tag) under the control of the GHRH gene promoter. The transgene contains approximately 5 kb of mouse GHRH gene sequences, including 3.5 kb of the 5'-flanking region, the entire hypothalamic exon 1 and 1.5 kb of intron 1, fused to the SV40 Tag gene. This construct was microinjected into fertilized oocytes. Fourteen of 96 mice born had integrated the transgene. These mice were fertile and showed no signs of central or peripheral tumors. The pattern of expression of the SV40 Tag gene was analyzed in four different transgenic lines by RT-PCR. The tissues tested include: hypothalamus, pituitary, cortex, cerebellum, spinal cord, adrenal, testis, spleen and lung. Transgene expression was consistently detected in the hypothalamus of all lines. In addition, SV40 Tag expression was also detected in the hypothalamus by Northern blot analysis in two of the transgenic lines. SV40 Tag expression was also detected in the testis of all transgenic lines by RT-PCR. This result was not expected since the GHRH gene sequences present in the transgene do not include the testis-specific transcription initiation site previously described. This suggests that GHRH gene expression in the mouse testis can be directed by regulatory sequences located downstream of the testis specific transcription start site. We conclude that the promoter region of the GHRH gene included in this construct contains the regulatory elements necessary to drive hypothalamic and testis expression in vivo. In addition, all mice from one of the transgenic lines developed cataracts in both eyes. SV40 Tag expression was detected not only in eyes with cataracts, but also, to a lesser extent, in eyes from other transgenic lines. Furthermore, the endogenous GHRH gene was found to be expressed in the eyes of normal mice.

Brain contains inhibiting factors specific to the large T-antigen gene

SV40 large T-antigen (LTa) gene-induced immortalized rat dopamine-producing nerve cells (IRB3AN27), which produce LTa protein and divide in vitro, do not divide and do not produce LTa protein when transplanted into striatum of adult rats. This suggests the presence of LTa gene-inhibiting factors in brain. Here we report that rat brain soluble fraction (SF) contains factors which specifically inhibit LTa gene activity in vitro. The brain SF inhibited LTa protein levels and the growth of IRB3AN27 cells and 2RSG cells (LTa gene-induced immortalized rat parotid acinar cells) in vitro, but it stimulated the growth of spontaneously immortalized human parotid acinar cells (2HPC8) and had no effect on the proliferation of murine neuroblastoma cells (NBP2) and rat glioma cells (C-6) in culture. In contrast, the liver SF inhibited the growth of all cell lines tested at varying degrees and thus lacked specificity with respect to LTa gene activity. The presence of specific LTa gene-inhibiting factors in the brain and general tumor growth-inhibiting factors in the liver may provide some of the mechanisms of protection against in vivo carcinogenesis.

Immortalization of human marrow stromal cells by retroviral transduction with a temperature sensitive oncogene: identification of bipotential precursor cells capable of directed differentiation to either an osteoblast or adipocyte phenotype

The etiology of osteoporosis is multifactorial, but there is evidence from both animal and human studies that the volume of marrow adipose tissue increases when bone volume is reduced in osteoporosis. The cell-related mechanism that may account for this inverse relationship between the volume of marrow adipose tissue and bone remains to be clarified, although it is known that both adipocytes and osteoblasts are derived from stromal cells precursors in bone marrow. We report that retroviral transduction with a temperature-sensitive oncogene (SV40 large T antigen) can generate bipotential cell lines from human marrow stroma that are capable of directed differentiation, in vitro, down either an osteogenic or adipocytic lineage pathway. One such clone, designated hOP 7, expresses type alpha 1(I) procollagen and has low alkaline phosphatase (AP) activity under basal culture conditions that is reminiscent of an osteoprogenitor cell. Exposure of hOP 7 cells to dexamethasone upregulates AP activity and enables the cells to mineralize their extracellular matrix. Also, treatment with calcitriol induces osteocalcin expression and both PTH and PGE2 induce/augment cAMP formation. Incubation with normal rabbit serum, however, causes the cells to become adipogenic as demonstrated by histological staining with Oil-red-O, expression of mRNA for the early and late adipocyte markers lipoprotein lipase and glycerol 3-phosphate dehydrogenase, respectively, and loss of type alpha 1(I) procollagen mRNA. The generation of homogeneous populations of these cells, as confirmed by Southern blot analysis, demonstrates the capacity of a human clonal cell line to differentiate in either an osteogenic or adipogenic direction.

Simian virus 40-like DNA sequences and large-T antigen-retinoblastoma family protein pRb2/p130 interaction in human mesothelioma

The oncoprotein of the Simian virus 40, SV40 large T-antigen (Tag), is reported to target and inactivate growth-suppressive proteins such as the retinoblastoma (Rb) family and p53 leading to transformation of human cell lines in vitro, to produce tumours in rodents, and to be detected in several human cancers including mesothelioma. In support of the potential role of SV40 Tag in the pathogenesis of certain human cancers, we have found SV40-like sequences in 8/25 bioptic specimens of mesothelioma from patients with exposure to asbestos fibres. We have also demonstrated that the SV40 Tag detected in human mesothelioma binds the retinoblastoma family protein pRb2/p130 in 5/5 specimens studied. We submit that the tumorigenic potential of SV40 Tag in some human mesotheliomas may arise from its ability to interact with and thereby inactivate several tumour and/or growth suppressive proteins in cooperation with asbestos fibres in inducing pleural mesothelioma.

Simian virus 40 footprints in normal human tissues, brain and bone tumours of different histotypes

SV40 footprints were investigated by PCR in normal human tissues and tumours of different histotypes, followed by Southern blot hybridization with a specific internal oligoprobe for SV40 DNA. Specific SV40 amplification products were detected at high prevalence in primary human brain tumours: 83% of choroid plexus papillomas, 75% ependymomas, 47% astrocytomas and 37% glioblastomas. SV40 footprints were also revealed in primary bone tumours: 35% osteosarcomas and Ewing's tumours. Positive normal tissue samples ranged from 45% of sperm fluids to 8% of brain tissue. Normal bone tissue specimens were SV40 negative. These results indicate that SV40 is associated with human brain and bone neoplasms, whereas normal bone and brain tissues were either SV40 negative or positive at low grade. SV40 footprints were found in other normal samples such as PBC, B- and T-lymphocytes and sperm fluids, indicating that SV40 is latent in these cells. Therefore, these cells may be vectors of SV40 in other host tissues and may spread SV40 infection by blood transfusion and sexual transmission in the human population.

Detection of SV40-like DNA sequences in pleural mesothelioma, bronchopulmonary carcinoma and other pulmonary diseases

Mesotheliomas are pleural-based tumours mainly associated with asbestos exposure (70% of cases) and the incidence is still raising. Recently, a possible viral connection was reported and 60% of mesotheliomas were demonstrated to contain and express SV40-like DNA sequences. In this study, the presence of SV40-like DNA sequences were investigated in mesotheliomas (15 tissue samples and six cell lines) and in 63 additional bronchopulmonary carcinomas, one parietal osteosarcoma and non-malignant lung samples as well as in organizing pleuritis (8). Finally, 163 samples were analysed by the polymerase chain reaction (PCR) with a set of primers PYV.for and PYV.rev to amplify a 173 bp region of the SV40 Tag. and a 179 bp region JC virus (JCV) as well as a 182 bp region BK virus (BKV). PCR amplification and hybridization with a probe specific for SV40 Tag revealed that 47.6% of mesotheliomas, 28.6% of primary bronchopulmonary carcinomas and 16% of non-neoplasic lung diseases contained SV40-like DNA sequences. No statistically significant difference in the occurrence of these DNA sequences was found between malignant mesothelioma and bronchopulmonary carcinoma. However, a significantly higher number of mesothelioma cases exhibited SV40- like DNA sequences in comparison with non-malignant pleural and pulmonary tissues. The DNA sequences were not related to BK and JC virus sequences. These results indicate that SV40-like DNA sequences are present in mesotheliomas as well as in bronchopulmonary carcinomas and non-malignant pleuropulmonary diseases.

Thymic epithelial tumor progression in an SV40T transgenic mouse model. Cortical thymoma-thymic carcinoma sequence

There have been several reports that thymoma in human is a progressive disease, and that thymoma and thymic carcinoma form a continuum. We established a stable line of SV40T transgenic mice, which consistently produced thymic epithelial tumours progressing to thymic carcinoma within a predictable time span. Using this animal model and a morphological approach, thymic epithelial tumour progression was studied with reference to sequential changes at different time points in animals aged from 3 to 32 weeks. At all ages, SV40T was expressed in the nuclei of thymic epithelial cells; in these transgenic mice we observed the entire spectrum from cortical type thymoma to thymic carcinoma. Thymic size tended to increase with ageing in SV40T TG mice. While younger mice had predominantly cortical (organoid) or cortical thymoma, older mice had well-differentiated thymic carcinoma (WDTC) or poorly differentiated thymic carcinoma. When SV40T TG mice (248 line) reached a certain age, carcinoma of the thymus was present in all of them. Cortical-type thymoma became malignant within a predictable time span, suggesting a cortical thymoma-carcinoma sequence. When the mice were 9 weeks of age, the thymuses formed gross masses compatible with cortical thymoma. At 14 weeks of age, WDTC appeared against the background of cortical thymoma. Poorly differentiated thymic carcinoma was found after 15 weeks and affected all animals over 23 weeks of age. Most thymic carcinomas coexisted in varying proportions with cortical-type thymoma. Medullary thymomas did not develop in the mice, and no transition from medullary-type thymomas to thymic carcinomas was observed. In this SV40T transgenic mouse model, thymic carcinoma is clearly preceded by cortical-type thymoma. These transgenic mice may provide an interesting model for the progression from cortical thymoma to WDTC and/or high-grade carcinoma.

Establishment and characterization of a retinal Müller cell line

PURPOSE

Primary cultures of Müller cells have proven useful in cell biologic, developmental, and electrophysiological studies of Müller cells. However, the limited lifetime of the primary cultures and contamination from non-neural cells have restricted the utility of these cultures. The aim of this study was to obtain an immortalized cell line that exhibits characteristics of Müller cells.

METHODS

Primary Müller cell cultures were prepared from retinas of rats exposed to 2 weeks of constant light. Cells were immortalized by transfection with simian virus 40. Single clones were obtained by repeatedly passaging cells using cloning wells. Immunocytochemical and immunoblotting studies were carried out with glial fibrillary acidic protein (GFAP)-specific and cellular retinaldehyde-binding protein (CRALBP)-specific antibodies. Transient transfections with CRALBP-luciferase constructs were performed by electroporation.

RESULTS

Oncogene transformation resulted in the establishment of a permanent cell line that could be readily propagated. Immunocytochemical and immunoblotting studies demonstrated that the Müller cell line, rMC-1, expressed both GFAP, a marker for reactive gliosis in Müller cells, and CRALBP, a marker for Müller cells in the adult retina. Transient transfection assays showed that promoter-proximal sequences of the CRALBP gene were able to stimulate reporter gene expression in rMC-1.

CONCLUSIONS

Viral oncogene transformation has been successfully used to isolate a permanent cell line that expresses Müller cell phenotype. The rMC-1 cells continue to express both induced and basal markers found in primary Müller cell cultures as well as in the retina. The availability of rMC-1 should facilitate gene expression studies in Müller cells and improve our understanding of Müller cell-neuron interactions.

Serine 257 phosphorylation regulates association of polyomavirus middle T antigen with 14-3-3 proteins

Polyomavirus middle T antigen (MT) is phosphorylated on serine residues. Partial proteolytic mapping and Edman degradation identified serine 257 as a major site of phosphorylation. This was confirmed by site-directed mutagenesis. Isoelectric focusing of immunoprecipitated MT from transfected 293T cells showed that phosphorylation on wild-type MT occurred at near molar stoichiometry at S257. MT was previously shown to be associated with 14-3-3 proteins, which have been connected to cell cycle regulation and signaling. The association of 14-3-3 proteins with MT depended on the serine 257 phosphorylation site. This has been demonstrated by comparing wild-type and S257A mutant MTs expressed with transfected 293T cells or with Sf9 cells infected with recombinant baculoviruses. The 257 site is not critical for transformation of fibroblasts in vitro, since S257A and S257C mutant MTs retained the ability to form foci or colonies in agar. The tumor profile of a virus expressing S257C MT showed a striking deficiency in the induction of salivary gland tumors. The basis for this defect is uncertain. However, differences in activity for the wild type and mutant MT lacking the 14-3-3 binding site have been observed in transient reporter assays.

Regulation of the expression of the human vimentin gene: application to cellular immortalization

Vimentin is an intermediate filament protein mainly specific of the mesoderm in vivo. In vitro vimentin synthesis is characteristic of proliferating cells, regardless of their embryonal origin, and is switched off upon differentiation of certain precursor cells. Vimentin gene expression is upregulated in some metastatic tumour cells, appearing as a marker of oncogenic progression. The vimentin network has been suggested to participate in several steps of viral infections. The promoter of the vimentin gene is comprised of multiple elements responsible for its complex transcriptional regulation. Among them, an NF-kappa B- and two AP1-binding sites mediate growth factor responsiveness. Two negative elements are present, one of which is deregulated by the HTLV-1 activator protein Tax. Transcription factor PEA3, encoded by a member of the ets oncogene family, activates the vimentin promoter in mammary tumour cells. In vitro, 878 base pairs of the vimentin 5'-regulatory region are sufficient to give high levels of transcription. These sequences were coupled to the SV40 large T antigen-encoding gene to achieve immortalization of new cell lines, either by transfection of primary cultures, or by derivation of cell explants from transgenic mice expressing the vimentin-SV40 construct. This allowed us, for instance, to immortalize endothelial, myogenic or renal epithelial cells, otherwise difficult to maintain in culture without loss of their differentiated phenotypes.

SV40 large tumor antigen (T antigen): database of mutants

The SV40 T antigen database (http://www.pitt.edu/pipaslab/) lists viruses and plasmids expressing mutant forms of large T antigen. Each entry contains information regarding the mutant designation, mutant type, virus strain, nucleotide change, amino acid change and pertinent references. The database is now available as an internet searchable index.

A mouse Sertoli cell line expressing anti-Müllerian hormone and its type II receptor

Anti-Müllerian hormone (AMH) induces the regression of Müllerian ducts in the male foetus; it is secreted by prepubertal testicular Sertoli cells and repressed at puberty. Using an AMH promoter/Simian virus 40 (SV40) oncogene fusion gene, we generated transgenic mouse lines exhibiting heritable Sertoli cell tumorigenesis. One cell line, derived from an adult male, expressed mRNAs characteristic of mature Sertoli cells, but no AMH. Two other cell lines were obtained from pretumoral testes at 6.5 days. One was cloned to yield SMAT1, whose expression pattern was characteristic of prepubertal Sertoli cells, namely no transferrin and high SF-I and AMH expression. SMAT1 also secretes AMH protein into the culture medium and expresses the AMH receptor. To the best of our knowledge, this is the first Sertoli cell line stably expressing AMH and its receptor. Our results show that, in targeted oncogenesis, the timing of cell line derivation plays a critical role even when using a developmentally regulated promoter.

High-frequency generation of transgenic zebrafish which reliably express GFP in whole muscles or the whole body by using promoters of zebrafish origin

Despite a number of reports on transgenic zebrafish, there have been no reports on transgenic zebrafish in which the gene is under the control of a promoter of zebrafish origin. Neither have there been reports on transgenic zebrafish in which the gene is under the control of a tissue-specific promoter/enhancer. To investigate whether it is possible to generate transgenic zebrafish which reliably express a reporter gene in specific tissues, we have isolated a zebrafish muscle-specific actin (alpha-actin) promoter and generated transgenic zebrafish in which the green fluorescent protein (GFP) reporter gene was driven by this promoter. In total, 41 GFP-expressing transgenic lines were generated with a frequency of as high as 21% (41 of 194), and GFP was specifically expressed throughout muscle cells in virtually all of the lines (40 of 41). Nonexpressing transgenic lines were rare. This demonstrates that a tissue-specific promoter can reliably drive reporter gene expression in transgenic zebrafish in a manner identical to the control of the endogeneous expression of the gene. Levels of GFP expression varied greatly from line to line; i.e., fluorescence was very weak in some lines, while it was extremely high in others. We also isolated a zebrafish cytoskeletal beta-actin promoter and generated transgenic zebrafish using a beta-actin-GFP construct. In all of the four lines generated, GFP was expressed throughout the body like the beta-actin gene, demonstrating that consistent expression could also be achieved in this case. In the present study, we also examined the effects of factors which potentially affect the transgenic frequency or expression levels. The following results were obtained: (i) expression levels of GFP in the injected embryo were not strongly correlated to transgenic frequency; (ii) the effect of the NLS peptide (SV40 T antigen nuclear localization sequence), which has been suggested to facilitate the transfer of a transgene into embryonic nuclei, remained to be elusive; (iii) a plasmid vector sequence placed upstream of the construct might reduce the expression levels of the reporter gene.

Introns enable the polyomavirus middle and small T antigens to stimulate the growth of primary rat embryo fibroblasts

We constructed spliceable vectors that separately encode polyomavirus MT and ST. The addition of an intron enables MT to transform and to immortalize more efficiently and ST to transiently stimulate the growth of primary rat embryo fibroblasts.