Stress protein (hsp73)-mediated, TAP-independent processing of endogenous, truncated SV40 large T antigen for Db-restricted peptide presentation

Transporter associated with antigen processing (TAP)-competent and TAP-deficient cell lines were transfected with expression plasmids encoding either the wild-type (wt) large tumor antigen (T-Ag) of SV40, or a truncated cytoplasmic variant (cT-Ag) of this viral protein. Stable expression of comparable levels of both forms of the viral protein was observed in different transfectants. The truncated cT-Ag variant, but not the wtT-Ag was stably associated with the constitutively expressed, cytosolic heat shock protein (hsp)73 chaperone. Two Db-binding peptides and one Kb-binding peptide of T-Ag were presented to cytotoxic T lymphocyte lines (CTLL) by TAP-competent transfectants expressing either wtT-Ag or cT-Ag. TAP-deficient transfectants expressing the wtT-Ag did not present any of these epitopes to CTLL. In contrast, TAP-deficient transfectants expressing the truncated hsp73-associated cT-Ag, presented the two Db-binding epitopes, but not the Kb-binding T-Ag epitope to CTLL. Regurgitation of peptides by transfectants was not detectable. The described data indicate that a pool of post-Golgi Db molecules is available for 2-3 h in TAP-deficient transfectants for loading with peptides released during endolysosomal processing of hsp73-associated, endogenous antigen.

Tiny T antigen: an autonomous polyomavirus T antigen amino-terminal domain

Three mRNAs from the murine polyomavirus early region encode the three well-characterized tumor antigens. We report the existence of a fourth alternatively spliced mRNA which encodes a fourth tumor antigen, tiny T antigen, which comprises the amino-terminal domain common to all of the T antigens but is extended by six unique amino acid residues. The amount of tiny T antigen in infected cells is small because of its short half-life. Tiny T antigen stimulates the ATPase activity of Hsc70, most likely because of its DnaJ-like motif. The common amino-terminal domain may interface with chaperone complexes to assist the T antigens in carrying out their diverse functions of replication, transcription, and transformation in the appropriate cellular compartments.

Suppression of gonadotropins inhibits gonadal tumorigenesis in mice transgenic for the mouse inhibin alpha-subunit promoter/simian virus 40 T-antigen fusion gene

We have previously developed a transgenic (TG) mouse model expressing the Simian virus 40 T-antigen (Tag), driven by a 6-kb fragment of the mouse inhibin alpha-subunit promoter (inh-alpha). The mice develop metastasizing gonadal tumors, of granulosa/theca or Leydig cell origin, with 100% penetrance by the age of 5-8 months. In the present study, we examined whether the appearance and growth of the gonadal tumors are dependent on gonadotropins. Gonadotropin suppression was achieved either by treatment of 3-month-old mice for 2-3 months with a GnRH antagonist (Cetrorelix, SB-75), or by cross-breeding the TG mice to the genetic background of the gonadotropin-deficient hypogonadal mutant mouse (hpg). Gonadal tumor growth was clearly inhibited by SB-75 treatment in one of the TG mouse lines (IT6-M), as indicated by the absence of macroscopically visible tumors and by reduced gonadal weights. Despite the suppressed gonadotropin secretion and Tag expression, hyperplasia of testicular Leydig, and ovarian stromal cells persisted in some of the treated mice. In another TG mouse line (IT6-F), with more aggressive tumorigenesis, the SB-75 treatment only partially inhibited gonadal tumor growth. None of the hypogonadotropic TG mice, homozygous for the hpg mutation, developed gonadal tumors. Their gonadal histology was indistinguishable from that of the non-TG hpg mice, suggesting total inhibition of gonadal tumorigenesis in the absence of gonadotropin stimulation. Tag expression and Leydig cell hyperplasia were apparent already in the postnatal TG mice but absent in those TG mice homozygous for the hpg mutation. In conclusion, the present results indicate that the gonadal tumorigenesis in our TG mouse model starts in early age as hyperplasia in specific somatic cells. Both this, and the subsequent malignant tumor growth, are gonadotropin dependent. A sufficient level of Tag expression, a prerequisite for gonadal tumorigenesis, only occurs upon gonadotropin stimulation.

RT-PCR method specific for the detection of transgenic CFTR mRNA in the presence of transgene plasmid DNA and endogenous CFTR mRNA

A major problem with experiments involving the correction of cystic fibrosis by gene transfer has been reliably detecting the transgenic message and distinguishing this from an endogenous message and from vector DNA. We have exploited the SV40 small T antigen intron present in an expression vector containing the CFTR cDNA (pCFAS) using a primer directed to the spliced sequence to allow specific and precise detection of the transgeneic message.

Expression and mutation of the tumor suppressor gene p53 in AIDS-associated Kaposi's sarcoma

Alteration of the p53 gene is the most frequent event reported in human cancer, and p53 mutations have been observed in various neoplasms, including certain forms of skin cancer. Therefore, we postulated that p53 may also be involved in Kaposi's sarcoma associated with AIDS (AIDS-KS). Expression of the p53 gene was examined in freshly isolated tumor biopsy specimens from 15 patients with AIDS-KS. p53 mRNA was detected by reverse transcriptase-polymerase chain reaction (RT-PCR) in both the AIDS-KS tumors and in normal skin control samples. p53 protein was detected in 4 of the 15 AIDS-KS specimens by immunohistochemical staining. Single-strand conformation polymorphism analysis PCR-products (PCR-SSCP) was used for detection of mutations of the p53 gene. One of the p53 positive AIDS-KS samples showed mobilized shifts in exon 6 suggestive of a mutation. Sequencing data showed the mutation to be located in codon 210. We examined other mechanisms that could stabilize p53 protein. SV40 large T antigen and adenovirus E1B protein were not found in the AIDS-KS specimens. MDM2, a p53-binding protein, was also detected in five of the AIDS-KS specimens, two of which also contained p53-positive cells. These observations suggest that the tumor suppressor gene p53 may be involved in the pathogenesis of AIDS-KS.

The amino-terminal transforming region of simian virus 40 large T and small t antigens functions as a J domain

Simian virus 40 (SV40) encodes two proteins, large T antigen and small t antigen that contribute to virus-induced tumorigenesis. Both proteins act by targeting key cellular regulatory proteins and altering their function. Known targets of the 708-amino-acid large T antigen include the three members of the retinoblastoma protein family (pRb, p107, and p130), members of the CBP family of transcriptional adapter proteins (cap-binding protein [CBP], p300, and p400), and the tumor suppressor p53. Small t antigen alters the activity of phosphatase pp2A and transactivates the cyclin A promoter. The first 82 amino acids of large T antigen and small t antigen are identical, and genetic experiments suggest that an additional target(s) important for transformation interacts with these sequences. This region contains a motif similar to the J domain, a conserved sequence found in the DnaJ family of molecular chaperones. We show here that mutations within the J domain abrogate the ability of large T antigen to transform mammalian cells. To examine whether a purified 136-amino-acid fragment from the T antigen amino terminus acts as a DnaJ-like chaperone, we investigated whether this fragment stimulates the ATPase activity of two hsc70s and discovered that ATP hydrolysis is stimulated four- to ninefold. In addition, ATPase-defective mutants of full-length T antigen, as well as wild-type small t antigen, stimulated the ATPase activity of hsc70. T antigen derivatives were also able to release an unfolded polypeptide substrate from an hsc70, an activity common to DnaJ chaperones. Because the J domain of T antigen plays essential roles in viral DNA replication, transcriptional control, virion assembly, and tumorigenesis, we conclude that this region may chaperone the rearrangement of multiprotein complexes.

WAF1 retards S-phase progression primarily by inhibition of cyclin-dependent kinases

The p21(WAF1/CIP1/sdi1) gene product (WAF1) inhibits DNA replication in vitro (J. Chen, P. Jackson, M. Kirschner, and A. Dutta, Nature 374:386-388, 1995; S. Waga, G. Hannon, D. Beach, and B. Stillman, Nature 369:574-578, 1994), but in vivo studies on the antiproliferative activity of WAF1 have not resolved G1-phase arrest from potential inhibition of S-phase progression. Here, we demonstrate that elevated WAF1 expression can retard replicative DNA synthesis in vivo. The WAF1-mediated inhibitory effect could be antagonized by cyclin A, cyclin E, or the simian virus 40 small-t antigen with no decrease in the levels of WAF1 protein in transfected cells. Proliferating-cell nuclear antigen (PCNA) overexpression was neither necessary nor sufficient to antagonize WAF1 action. Expression of the N-terminal domain of WAF1, responsible for cyclin-dependent kinase (CDK) interaction, had the same effect as full-length WAF1, while the PCNA binding C terminus exhibited modest activity. We conclude that S-phase progression in mammalian cells is dependent on continuing cyclin and CDK activity and that WAF1 affects S phase primarily through cyclin- and CDK-dependent pathways.

Accumulation of p16INK4a in mouse fibroblasts as a function of replicative senescence and not of retinoblastoma gene status

Viral transformation of mouse and human fibroblasts has very different effects on the composition of cyclin-dependent kinase (Cdk) complexes. In human cells transformed by the large T-antigen of simian virus 40 (SV40 T-Ag) and human tumour cell lines that lack a functional retinoblastoma gene product (pRb) no cyclin D1-Cdk4 complexes can be detected because all the available Cdk4 is associated with the Cdk-inhibitor p16INK4a. In contrast, SV40-transformed mouse cells and fibroblasts from Rh1-nullizygous mouse embryos contain normal levels of cyclin D1-Cdk4 complexes. To investigate this species difference, we have compared the biochemical properties and expression of mouse p16INK4a with that of its human counterpart. There is a marked increase in p16 RNA and protein levels as primary embryo fibroblasts approach their finite lifespan in culture, but mouse p16 expression does not appear to be influenced by the status of pRb. Transformed or spontaneously immortalized mouse cells therefore do not achieve the very high levels of p16 characteristic of pRb-negative human cell lines. We suggest that these differences may be related to the different frequencies with which mouse and human cells can be immortalized in culture.

Nuclear export of the E1B 55-kDa and E4 34-kDa adenoviral oncoproteins mediated by a rev-like signal sequence

The E1B 55-kDa and E4 34-kDa oncoproteins of adenovirus type 5 (abbreviated here as E1B-55kD and E4-34kD) promote the export of viral mRNA and inhibit the export of most cellular mRNA species. We show that the intracellular complex containing E1B-55kD and E4-34kD continuously shuttles between the nucleus and the cytoplasm, and may thus serve as a nucleocytoplasmic transporter for viral mRNA. We present evidence that within this complex, it is the E4-34kD protein that directs both nuclear import and nuclear export. E4-34kD contains a functional nuclear export signal similar to corresponding sequences found in the retroviral proteins rev and rex. This sequence element is required for nuclear export of the complex, and it can function autonomously when fused to a carrier protein and microinjected in HeLa cell nuclei. When E4-34kD is expressed alone, a portion of the protein that contains a predicted arginine-rich amphipathic alpha-helical structure mediates nuclear retention of the protein. This retention, however, can be abolished by the association with E1B-55kD or by a specific point mutation within the arginine-rich motif. The export of E4-34kD can be blocked by an HTLV-rex derived competitive inhibitor and overexpressed E4-34kD inhibits rev-mediated transport, suggesting that the export pathways accessed by the adenoviral and retroviral proteins share components. The interplay between two polypeptides as well as the involvement of a dominant nuclear retention domain are novel features that might contribute to the efficiency and regulation of the adenovirus export system.

A new peptide vector for efficient delivery of oligonucleotides into mammalian cells

The development of antisense and gene therapy has focused mainly on improving methods for oligonucleotide and gene delivery into cells. In the present work, we describe a potent new strategy for oligonucleotide delivery based on the use of a short peptide vector, termed MPG (27 residues), which contains a hydrophobic domain derived from the fusion sequence of HIV gp41 and a hydrophilic domain derived from the nuclear localization sequence of SV40 T-antigen. The formation of peptide vector/oligonucleotide complexes was investigated by measuring changes in intrinsic tryptophan fluorescence of peptide and of mansyl-labelled oligonucleotides. MPG exhibits relatively high affinity for both single- and double-stranded DNA in a nanomolar range. Based on both intrinsic and extrinsic fluorescence titrations, it appears that the main binding between MPG and oligonucleotides occurs through electrostatic interactions, which involve the basic-residues of the peptide vector. Further peptide/peptide interactions also occur, leading to a higher MPG/oligonucleotide ratio (in the region of 20/1), which suggests that oligonucleotides are most likely coated with several molecules of MPG. Premixed complexes of peptide vector with single or double stranded oligonucleotides are delivered into cultured mammalian cells in less than 1 h with relatively high efficiency (90%). This new strategy of oligonucleotide delivery into cultured cells based on a peptide vector offers several advantages compared to other commonly used approaches of delivery including efficiency, stability and absence of cytotoxicity. The interaction with MPG strongly increases both the stability of the oligonucleotide to nuclease and crossing of the plasma membrane. The mechanism of cell delivery of oligonucleotides by MPG does not follow the endosomal pathway, which explains the rapid and efficient delivery of oligonucleotides in the nucleus. As such, we propose this peptide vector as a powerful tool for potential development in gene and antisense therapy.

Bi-transgenic mice reveal that K-rasVal12 augments a p53-independent apoptosis when small intestinal villus enterocytes reenter the cell cycle

Studies in cell culture systems have indicated that oncogenic forms of Ras can affect apoptosis. Activating mutations of Ras occur in approximately 30% of all human tumors and 50% of colorectal carcinomas. Since these mutations appear at early or intermediate stages in multistep journeys to neoplasia, an effect on apoptosis may help determine whether initiated cells progress towards a more neoplastic state. We have tested the effects of K-rasVal12 on apoptosis in transgenic mice. A lineage-specific promoter was used to direct expression of human K-rasVal12, with or without wild-type (wt) or mutant SV-40 T antigens (TAg), in postmitotic villus enterocytes, the principal cell type of the small intestinal epithelium. Enterocytes can be induced to reenter the cell cycle by TAgWt. Reentry is dependent upon the ability of TAg to bind pRB and is associated with a p53-independent apoptosis. Analyses of K-rasVal12 x TAgWt bi-transgenic animals indicated that K-rasVal12 can enhance this apoptosis threefold but only in cycling cells; increased apoptosis does not occur when K-rasVal12 is expressed alone or with a TAg containing Glu107,108two head right arrow Lys107, 108 substitutions that block its ability to bind pRB. Analysis of bi-transgenic K-rasVal12 x TAgWt mice homozygous for wild-type or null p53 alleles established that the enhancement of apoptosis occurs through a p53-independent mechanism, is not attributable to augmented proliferation or to an increase in abortive cell cycle reentry (compared to TAgWt mice), and is not associated with detectable changes in the crypt-villus patterns of expression of apoptotic regulators (Bcl-2, Bcl-xL, Bak, and Bax) or mediators of epithelial cell-matrix interactions and survival (e.g., alpha5beta1 integrin and its ligand, fibronectin). Coexpression of K-rasVal12 and TAgWt produces dysplasia. The K-rasVal12-augmented apoptosis is unrelated to this dysplasia; enhanced apoptosis is also observed in cycling nondysplastic enterocytes that produce K-rasVal12 and a TAg with a COOH-terminal truncation. The dysplastic epithelium of K-rasVal12 x TAgWt mice does not develop neoplasms. Our results are consistent with this finding: (a) When expressed in initiated enterocytes with a proliferative abnormality, K-rasVal12 facilitates progression to a dysplastic phenotype; (b) by diminishing cell survival on the villus, the oncoprotein may impede further progression; and (c) additional mutations may be needed to suppress this proapoptotic response to K-rasVal12.

The elevation of cellular phosphatidic acid levels caused by polyomavirus transformation can be disassociated from the activation of phospholipase D

Middle T (mT), the oncogene of murine polyomavirus, causes transformation of rat fibroblasts by activating a number of signal transducing pathways usually used by polypeptide growth factors and their receptors. Here, we report data regarding the activation of signal transducing pathways involving phospholipase D (PL-D). The hydrolysis of phospholipids by PL-D produces phosphatidic acid (PA), a compound with multiple biological effects. The PA content of cells expressing wild-type mT, introduced via a number of different methods, is approximately 50% higher than their untransformed counterparts. This increase in cellular PA content is associated with an approximately 65% increase in PL-D activity in cells expressing wild-type mT. We have also examined the effects of a number of site-directed mutants of mT, on both cellular PA levels and on PL-D activity. Mutants that do not produce mT (Py808A) or that produce a truncated, nonmembrane bound mT (Py1387T) have PA levels similar to that of control cells. Cells expressing the 322YF mutant of mT (which abolishes interaction of mT with phospholipase C gamma1) show increases in both PA levels and PL-D activity that are similar to those seen with wild-type mT. Expression of mutants that abolish the interaction of mT with either shc or with phosphatidylinositol 3-kinase (250YS and 315YF, respectively) cause an increase in PL-D activity comparable to that seen with wild-type mT. However, the PA content of cells expressing these mutants is not elevated. These results suggest that mT causes activation of cellular PL-D, but this activation alone is not sufficient to cause an increase in cellular PA content. Therefore, wild-type mT must affect another, as yet unknown, step in PA metabolism.

Simian virus 40 T antigen can regulate p53-mediated transcription independent of binding p53

A simian virus 40 (SV40) T-antigen mutant containing only the N-terminal 136 amino acids, able to bind to Rb and p300 but not p53, partially inhibited p53-mediated transcription without affecting the ability of p53 to bind DNA. These results suggest that SV40 T antigen can regulate p53-mediated transcription either directly through protein-protein association or indirectly through interaction with factors which may function to confer p53-mediated transcription.

Immortalized gastric epithelial cell line GSM06 synthesizes hyaluronan under the influence of simian virus 40 large T-antigen expression

GSM06 is a cell line established from the stomach of transgenic mouse harboring a temperature-sensitive simian virus 40 (SV40) large T-antigen gene. 3H-labeled macromolecules produced by the cells incubated with [3H] glucosamine were characterized to examine whether or not GSM06 cells synthesize mucin (mucus glycoprotein). The GSM06 cells grew until a confluent monolayer formed at 33 degrees C (the permissive temperature for SV40 large T-antigen expression), and the 3H-labeled macromolecules appeared in both cell extract and medium during culture for at least 1 week. Unexpectedly, almost all 3H-labeled macromolecules, which were excluded from a column of Sepharose CL-4B, were identified as hyaluronan by analyses using Sepharose CL-2B chromatography, cesium trifluoroacetate equilibrium centrifugation, treatment with dithiothreitol, and trypsin, hyaluronidase, and chondroitinase ABC digestion. At a nonpermissive temperature (39 degrees C), GSM06 cells grew only slightly, but produced much more hyaluronan than at 33 degrees C. The results indicate that GSM06 cells produce not mucin, but hyaluronan, and that the expression of large T-antigen may influence hyaluronan synthesis in GSM06 cells.

pp60c-src binding to polyomavirus middle T-antigen (MT) requires residues 185 to 210 of the MT sequence

Interaction with the src family of tyrosine kinases is crucial to the transforming action of polyomavirus middle T-antigen (MT). Association with MT activates the tyrosine kinase activity of pp60(c-src) and, through subsequent MT phosphorylation, creates binding sites for signalling molecules whose stimulation culminates in cell transformation. Despite this importance, and many studies, little is known of the mechanisms by which pp60(c-src) binds to MT. We report here isolation of the first MT mutants that disrupt pp60(c-src) binding without affecting the interaction between MT and protein phosphatase 2A (PP2A). Through deletion analysis we established that interaction with pp60(c-src) requires the sequences between amino acids 185 and 210 of MT, but these residues have no effect on PP2A binding. Cells expressing these mutants showed few altered properties, indicating that the PP2A-MT interaction alone has little influence on cell phenotype. Subcellular location of these mutant MT molecules was indistinguishable by immunofluorescence analysis from that of wild-type MT but was altered markedly on loss of PP2A binding. This suggests a possible role for PP2A in specifying subcellular distribution.

Tissue culture adaptation of natural isolates of simian virus 40: changes occur in viral regulatory region but not in carboxy-terminal domain of large T-antigen

The regulatory region of natural isolates of simian virus 40 (SV40) is different from that of laboratory-adapted strains of the virus. The latter have a nucleotide sequence duplication within the enhancer region which varies slightly with each strain, whereas the duplication is lacking in fresh isolates of SV40, which contain an 'archetypal' regulatory region. Many isolates also display nucleotide differences in the DNA encoding the carboxy terminus of large tumour antigen (T-ag). To determine whether genetic changes in these two regions of the SV40 genome were detectable during laboratory adaptation and long-term passage, low-passage virus stocks of two laboratory strains which had detailed passage histories spanning more than 25 years (Baylor strain and VA45-54) were analysed using PCR, cloning and sequencing assays. Both laboratory and archetypal regulatory regions were present in low-passage stocks. Following duplication in the regulatory region, no additional changes were detectable. The variable region at the T-ag carboxy terminus did not undergo any change with tissue culture passage and may serve as a useful site for taxonomic classification of different strains of SV40. Cloned genomes containing single or duplicated enhancers derived from both SV40 strains were viable in CV-1 cells. Attempts to induce regulatory region duplications by 14 serial passages of SV40 archetypal strains in monkey cells were not successful. The results are compatible with tissue culture adaptation of SV40, reflecting either selection of a rare variant pre-existing in the original sample or generation of a rare regulatory region duplication in infected cells.

Functional consequences of oncogene-induced horizontal cell degeneration in the retinas of transgenic mice

Visual function was evaluated in transgenic mice expressing the simian virus 40 early region under the control of the promoter for phenylethanolamine-N-methyltransferase. These transgenic mice undergo a degeneration of the retinal horizontal cells and the outer plexiform layer. Electroretinograms (ERGs) were recorded under stimulus conditions chosen to elicit both receptoral and postreceptoral responses. The dark-adapted a-waves obtained from transgenic mice were not different from control recordings, indicating that the degenerative process does not interfere with function of the rod photoreceptors. In comparison, the ERG b-wave was markedly reduced in transgenic mice under both dark- and light-adapted conditions. Reproducible visual evoked potentials (VEPs) were recorded from transgenic mice in response to both low luminance stimuli that isolate rod function, and to higher luminance stimuli, indicating that retinal activity is transmitted centrally to the visual cortex. However, VEPs were delayed at all stimulus luminances compared to controls. Analysis of luminance-response functions suggests that the VEP delays could reflect the combination of a decrease in synaptic efficacy and an overall loss in visual sensitivity. These functional abnormalities correlate well with the anatomical abnormalities that have been previously observed in the transgenic retina (Hammang et al., 1993), namely a reduced number of synapses between photoreceptors and second-order neurons.

Model SV40-transformed fibroblast lines for metabolic studies of human prosaposin and acid ceramidase deficiencies

Skin fibroblasts from patients with Farber disease (acid ceramidase deficiency) and from two siblings of the only known family affected with prosaposin deficiency were transformed by transfection with a plasmid carrying the SV40 large T antigen. The prosaposin-deficient transformed cell lines conserved their original metabolic defects, and in particular they were free of detectable immunoreactivity when using anti-saposin B and anti-saposin C antisera. Ultrastructurally, the cells contained heterogeneous lysosomal storage products. As found for their parental cell lines, the SV40-transformed fibroblasts exhibited deficient in vitro activities of lysosomal ceramidase and beta-galactosylceramidase, but a normal activity of acid sphingomyelinase. As observed for SV40-transformed fibroblasts from Farber disease, degradation of radioactive glucosylceramide or low density lipoprotein-associated radiolabelled sphingomyelin by the prosaposin-deficient cells in situ showed a clear impairment in the turnover of lysosomal ceramide. Ceramide storage in prosaposin-deficient cells was also demonstrated by ceramide mass determination. In contrast to acid ceramidase deficient cells, both the accumulation of ceramide and the reduced in vitro activity of acid ceramidase in cells from prosaposin deficiency could be corrected by addition of purified saposin D. The data confirm that prosaposin is required for lysosomal ceramide degradation, but not for sphingomyelin turnover. The SV40-transformed fibroblasts will be useful for pathophysiological studies on human prosaposin deficiency.

Immortalization of neuro-endocrine cells from adrenal tumors arising in SV40 T-transgenic mice

Pheochromocytomas are adrenal medullary tumors which arise from the transformation of neural crest-derived cells. In the course of studies of mice transgenic for an SV40 T-gene ectopically expressed in the adrenal medulla, we observed the occurrence of large, mainly bilateral tumors in a high proportion of transgenic animals. From these tumors we established immortalized cell lines which grow in vitro at 32 degrees C (the permissive temperature for the tsA58 T-protein encoded by the transgene), but not at 38 C. These cells demonstrate characteristics of both neuronal (160 kd neurofilament) and endocrine (chromogranins) cells. The expression of Mash-1 and ret supports their initial characterization as early bipotential neuro-endocrine progenitors.

SV40 and adenovirus may act as cocarcinogens by downregulating glutathione S-transferase expression

We have discovered a novel function of the SV40 T antigen and the adenovirus E1A proteins: the ability to downregulate the endogenous expression of an important detoxification enzyme, glutathione S-transferase alpha (GST alpha). GST alpha mRNA is much less abundant in rat and human cells that express SV40 T antigen than in the parental cell lines. This GST alpha downregulation does not require expression of SV40 small t antigen or complex formation between large T antigen and p53, p300, or the pRb family of proteins. As might be predicted, cells that express SV40 T antigen are more sensitive than normal cells to alkylating drugs, which GST alpha is known to detoxify. Finally, GST alpha expression is also downregulated in cells that express the adenovirus E1A proteins. We propose that by downregulating GST alpha expression and inactivating p53 function, SV40 and adenovirus may contribute to the initiation of, or the progression toward, malignancy. Thus, in their quest to establish persistent infections, these viruses may inadvertently make the cellular environment more permissive for tumorigenesis.

Expression of intracisternal A-particle retrotransposons in primary tumors of oncogene-expressing transgenic mice

Intracisternal A-Particle (IAP) sequences are endogenous retrovirus-like mobile elements, present at 1000 copies in the mouse genome. These elements transpose in a replicative manner via an RNA intermediate and its reverse transcription, and their transposition should therefore be tightly controlled by their transcription level. The in vivo pattern of expression of these potentially mutagenic elements had previously been analysed in normal mice, and we have now investigated their expression in transgenic mice carrying different oncogenes (e.g. c-myc, v-Ha-ras, SV40 T-antigen) under tissue-specific promoters and disclosing tumors within the brain, the mammary or salivary glands, or the lymphoid organs. Northern blot analysis of IAP expression within the resulting tumors demonstrates a lack of significant and/or systematic effect of v-Ha-ras and SV40 T-antigen expression, but a systematic IAP induction in the myc-induced tumors. In this case, however, analysis of double transgenic mice obtained by crossing the tumor-prone mice with previously described transgenic mice carrying IAP reporter genes did not provide any evidence for induction of the IAP transgenes, therefore strongly suggesting that c-myc expression had an effect on only a limited number of IAP sequences--most probably depending on their position and/or methylation state. These results strengthen the importance of in vivo studies for a correct appraisal of complex biological processes, and moderate previous conclusions derived from in vitro analyses on the general activation of IAPs by oncogenes and on the role of these transposable elements in tumorigenesis.

Safety-modified episomal vectors for human gene therapy

The effectiveness of ongoing gene therapy trials may be limited by the expression characteristics of viral and plasmid-based vectors. To enhance levels of heterologous gene expression, we have developed a safety-modified episomal expression vector that replicates extrachromosomally in human cells. This vector system employs a simian virus 40 (SV40) large T antigen mutant (107/402-T) that is deficient in binding to human tumor suppressor gene products, including p53, retinoblastoma, and p107, yet retains replication competence. These SV40-based episomes replicate to thousands of copies by 2-4 days after gene transfer in multiple types of human cell lines, with lower activity in hamster cells, and no detectable activity in dog, rat, and murine cell lines. Importantly, 107/402-T has enhanced replication activity compared with wild-type T antigen; this finding may be due, in part, to the inability of p53 and retinoblastoma to inactivate 107/402-T function. We demonstrate that the level and duration of 107/402-T expression regulates the observed episomal copy number per cell. Compared with standard plasmid constructs, episomes encoding 107/402-T yield approximately 10- to 100-fold enhanced levels of gene expression in unselected populations of transient transfectants. To determine if 107/402-T-based episomes replicate extrachromosomally in vivo, tumor explants in nude mice were directly injected with liposome/DNA complexes. Using a PCR-based assay, we demonstrate that SV40-based episomes replicate in human cells after direct in vivo gene transfer. These data suggest that safety-modified SV40-based episomes will be effective for cancer gene therapy because high level expression of therapeutic genes in transient transfectants should yield enhanced tumor elimination.

Transcriptional activation of the human c-myc gene by simian virus 40 large T antigen without binding to p53 and RB proteins in the transient expression system

Transcriptional activation of the human c-myc gene by SV40 large T antigen was examined using HepG2 cells by co-transfecting a T antigen expression plasmid with a myc-CAT construct containing the 2.3-kb upstream region from the P1 promoter and the P2 promoter region fused to the CAT gene. T antigen increased the basal activity of the P2 promoter region containing the E2F binding site, but both the P2 promoter region and the upstream region from the P1 promoter were important for overall activation by T antigen. CAT assay using mutated T antigen lacking p53 or the RB binding site indicated that p53 or RB was not mainly involved in transcriptional activation of the c-myc gene. It appears that activation of the c-myc gene by T antigen is probably dependent upon E2F and a cellular factor through a mechanism which is independent of binding of T antigen to p53 and RB.

Multiple neuroendocrine tumours in transgenic mice induced by c-kit-SV40 T antigen fusion genes

Transgenic mice carrying either a 1.008 or a 4.225 kb of the mouse c-kit 5'-flanking sequences linked to the oncogenic large T antigen (TAg) region of the simian virus 40 (SV40) genome were generated to test if the c-kit promoter could be used to develop useful mouse models. Both constructs promote tumourigenesis in the pituitary and the thyroid with high efficiency. The cell types from which each of these tumours derives were identified. Tumours of the pituitary derive from alpha-MSH-expressing cells located in the intermediate lobe. Transformed cells of the thyroid were calcitonin-positive, implying that the tumours derive from C cells or their precursors. Chromogranin A and neuron-specific enolase, general neuroendocrine cell markers, were expressed in both tumour types. Furthermore a variety of tumours appeared in the transgenic mice. Several of them stained positively for chromogranin A and/or neuron-specific enolase. This suggests a previously unsuspected tissue-specificity of the c-kit 5' flanking sequences for neuroendocrine cells. The Kit-TAg transgenic mouse lines may represent a valuable model for the study of the development and the biology of neuroendocrine tumours.