Quantitative correlation between simian virus 40 T-antigen synthesis and late viral gene expression in permissive and nonpermissive cells.

The time-course of intranuclear Simian virus 40 (SV40) tumor (T) antigen synthesis and accumulation in permissive CV1 monkey cells and nonpermissive 3T3 mouse cells has been studied by immunofluorescence and cytofluorometry. CV1 cells accumulate T antigen continuously over a period of 48 h after infection, whereas in 3T3 cells the T-antigen content remains about constant and at a comparatively low level. Only those CV1 cells which have attained a threshold concentration of intranuclear T antigen synthesize viral capsid proteins (V antigen). In nonpermissive 3T3 cells, the T-antigen threshold value for the onset of V-antigen synthesis is higher than in CV1 cells and is never reached by infected cells. However, 3T3 cells microinjected with sufficient amounts of SV40 DNA easily surpass this value and behave permissively.

Chemotherapy resistance of mouse WAP-SVT/t breast cancer cells is mediated by osteopontin, inhibiting apoptosis downstream of caspase-3.

Impairment of the complex regulatory network of cell death and survival is frequently the reason for therapy resistance of breast cancer cells and a major cause of tumor progression. We established two independent cell lines from a fast growing mouse breast tumor (WAP-SVT/t transgenic animal). Cells from one line (ME-A cells) are sensitive to apoptotic stimuli such as growth factor depletion or treatment with antitumor agents (e.g. doxorubicin). Cells from the second line (ME-C cells), which carry a missense mutation at the p53 codon 242, are very insensitive to apoptotic stimuli. Co-cultivation experiments revealed that the ME-C cells mediate cell death resistance to the ME-A cells. Microarray and Western blot analysis showed that osteopontin (OPN) is selectively overexpressed by the ME-C cells. This glycoprotein is the most abundant protein secreted by the ME-C cells and we obtained strong indications that OPN is the main antiapoptotic factor. However, the OPN containing ME-C cell medium does not alter the expression level of pro- or antiapoptotic genes or known inhibitors of apoptosis (IAPs). Its signaling involves mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK)1/2 as the kinase inhibitor PD98059 restores apoptosis but not the Akt inhibitor. In the ME-A cells, mitochondrial cytochrome c release occurs with and without external apoptotic stimuli. OPN containing ME-C cell medium does not prevent the mitochondrial cytochrome c release and caspase-9 processing. In serum starved ME-A cells, the OPN containing ME-C cell medium prevents caspase-3 activation. However, in doxorubicin-treated cells, although apoptosis is blocked, it does not inhibit caspase-3. This indicates that the ME-A cells distinguish between the initial apoptotic stimuli and that the cells possess a further uncharacterized control element acting downstream from caspase-3.

Methylation of single sites within the herpes simplex virus tk coding region and the simian virus 40 T-antigen intron causes gene inactivation.

In order to determine whether partial methylation of the herpes simplex virus (HSV) tk gene prevents tk gene expression, the HSV tk gene was cloned as single-stranded DNA. By in vitro second-strand DNA synthesis, specific HSV tk gene segments were methylated, and the hemimethylated DNA molecules were microinjected into thymidine kinase-negative rat2 cells. Conversion of the hemimethylated DNA into symmetrical methylated DNA and integration into the host genome occurred early after gene transfer, before the cells entered into the S phase. HSV tk gene expression was inhibited either by promoter methylation or by methylation of the coding region. Using the HindIII-SphI HSV tk DNA fragment as a primer for in vitro DNA synthesis, all cytosine residues within the coding region, from +499 to +1309, were selectively methylated. This specific methylation pattern caused inactivation of the HSV tk gene, while methylation of the cytosine residues within the nucleotide sequence from +811 to +1309 had no effect on HSV tk gene activity. We also methylated single HpaII sites within the HSV tk gene using a specific methylated primer for in vitro DNA synthesis. We found that of the 16 HSV tk HpaII sites, methylation of 6 single sites caused HSV tk inactivation. All six of these "methylation-sensitive" sites are within the coding region, including the HpaII-6 site, which is 571 bp downstream from the transcription start site. The sites HpaII-7 to HpaII-16 were all methylation insensitive. We further inserted separately the methylation-sensitive HSV tk HpaII-6 site and the methylation-insensitive HpaII-13 site as DNA segments (32-mer) into the intron region of the simian virus 40 T antigen (TaqI site). Methylation of these HpaII sites caused inhibition of simian virus 40 T-antigen synthesis.

Altered phosphorylation at specific sites confers a mutant phenotype to SV40 wild-type large T antigen in a flat revertant of SV40-transformed cells.

The Rev2 cell line is a cellular revertant of the SV40 wild-type transformed rat cell line SV-52 [Bauer, M., Guhl, E., Graessmann, M. & Graessmann, A. (1987). J. Virol., 61, 1821-1827]. To characterize the level of cellular interference with the SV40 large T antigen (large T)-induced transformation pathway in Rev2 cells, we analysed the biological and biochemical properties of large T expressed in Rev2 cells. We found that Rev2 cells encoded an authentic wild-type large T, with regard to its sequence and its transforming functions. No differences were found in the metabolic stability of large T, or in complex formation with the cellular p53 protein, or in p53 metabolic stabilization. In contrast to SV-52 cells, Rev2 cells showed no association of large T with the chromatin fraction of isolated nuclei. This difference correlated with a reduced affinity of the Rev2 large T to SV40 DNA in vitro. The T proteins from both cell lines were phosphorylated at the same multiple sites. However, in Rev2 cells the phosphorylation of large T at specific serine -residues was significantly reduced. Thus the revertant phenotype of Rev2 cells may be due to an altered phosphorylation state of its large T protein, leading to altered nuclear localization and reduced transforming activity. The alterations of Rev2 large T properties and phosphorylation were very similar to the changes observed with mutant large T in FR(tsA58)A cells, an SV40 tsA58 N-type transformant, when the cells had reverted to the normal phenotype at the non-permissive growth temperature. Thus altered phosphorylation might provide a common structural basis for the biological inactivation of the large T proteins in these cells.

Breast cancer formation in transgenic animals induced by the whey acidic protein SV40 T antigen (WAP-SV-T) hybrid gene.

After injection of the whey acidic protein (WAP)-SV-T hybrid gene into fertilized mouse eggs, eight independent transgenic mouse lines were obtained. Females from three lines developed mammary carcinomas with high frequency, coinciding mostly with lactation. In contrast to the endogenous WAP gene, expression of the hybrid gene continued after lactation. The tumor cells had a very invasive growth characteristic. Tumor regression in vivo was not observed. However, after transfer into tissue culture 25% of the cells ceased to express the hybrid gene and acquired the growth characteristic of normal cells. It was possible to retransform these cells by injection of wild-type SV40 DNA, but not after transfer of the hybrid WAP-SV-T gene. Inactivation of the endogenous WAP and of the WAP-SV-T transgene did not correlate with DNA methylation.

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

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

The second large simian virus 40 T-antigen exon contains the information for maximal cell transformation.

Microinjection of early simian virus 40 (SV40) DNA fragments has shown that maximal transformation of rat cells (Ref 52) is a property of the second SV40 T-antigen exon. Expression of this particular T-antigen region was obtained by coinjection of the Taq/Bam DNA fragment with the early promoter/enhancer HpaII/BglI fragment. Microinjection of the DNA fragment mixture induced two categories of transformants; namely, maximally and minimally transformed cells. The maximally transformed cells synthesize two Taq/Bam-specific polypeptides, and the minimally transformed cells only the lower molecular weight form. Both types of transformants contain the cellular p52 protein at high concentrations. Furthermore, maximal transformation of Ref 52 cells requires the carboxy terminus of the T-antigen. Cells transformed by microinjection of the SV40 Pst A-fragment display different parameters of maximally transformed cells but not anchorage-independent growth.

Molecular characterization of the DNA puff C-8 gene of Rhynchosciara americana.

We have mapped the only transcription unit known to be present in the C-8 DNA puff of Rhynchosciara americana and describe the isolation and sequence of a cDNA clone, pRa C-8-22, which contains a nearly complete copy of the mRNA transcribed from this DNA puff and part of the sequence of genomic clone BSC8-0.9, which contains the promotor region and the remainder of the transcription unit. The characteristics of the protein predicted from the ORF present in the cDNA indicate that it is unique and secreted.

Molecular characterization of the C-3 DNA puff gene of Rhynchosciara americana.

We have mapped a region of about 33 kb which includes the transcription unit of the C-3 DNA puff gene of Rhynchosciara americana. The C-3 TU and a region extending approximately 800 bp upstream of the C-3 promoter were characterized. The TU is composed of three exons and produces a 1.1-kb mRNA whose level in salivary glands increases with the expansion of the C-3 puff. The C-3 messenger appears to undergo rapid deadenylation resulting in an RNA of about 0.95 kb which can still be observed in gland cells 15 h after the puff has regressed. The 1.1-kb mRNA codes for a 32.4-kDa, predominantly alpha-helical polypeptide with three conserved parallel coiled-coil stretches. The aa composition and structure of this polypeptide suggests that it is secreted and contributes to the formation of the cocoon in which the larvae pupate. The region upstream of the promoter contains several A-rich sequences with similarity to the ACS of yeast which might have a role in the initiation of replication/amplification.

In vitro synthesized SV40 cRNA is trans-spliced after microinjection into the nuclei of mammalian cells.

We present for the first time experimental evidence that in vitro synthesized RNA (cRNA) is trans-spliced after microinjection into the nuclei of mammalian tissue culture cells. The template used for cRNA synthesis was the early SV40 BstXl/BamHI DNA fragment. This DNA fragment encodes exclusively for the second T-antigen exon and contains the intact small t-antigen intron. To generate the corresponding mRNA (T1-mRNA) by trans-splicing, the cells utilize a 5' cryptic splice site located within the second T-antigen exon of one cRNA molecule which is spliced to the small t-antigen 3' splice site of another cRNA molecule. Formation of the T1-mRNA by trans-splicing was confirmed by RT-PCR analysis and DNA sequencing. Efficient trans-splicing required that competitive small t-antigen cis-splicing be inhibited by deletion of the small t-antigen 5' splice site. The T1-mRNA was not generated when the cryptic 5' splice site was mutated.

Methylation of the SV40 HpaII site does not affect late viral gene expression in microinjected tissue culture cells.

Intranuclear coinjection of the late SV40 KpnI/BclI DNA fragment and the early promotor/enhancer HpaII/BglI DNA segment into permissive monkey and non-permissive mouse cells allows late SV40 gene expression without T-antigen synthesis and DNA replication. These conditions were chosen to analyse the effect of DNA methylation on V-antigen synthesis detached from the process of DNA replication. We found that in vitro methylation of a single cytosine nucleotide proximal to the major late mRNA cap site by the HpaII methylase does not block capsid protein synthesis. This result is in contrast to reported data obtained in Xenopus laevis oocyte injection experiments [(1982) Proc. Natl. Acad. Sci. USA 79, 5142-5146].

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

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

After microinjection hemimethylated DNA is converted into symmetrically methylated DNA before DNA replication.

In this investigation we analysed the maintenance methylation activity of the mammalian cell DNA methyltransferase by microinjection of hemimethylated HSV-tk DNA into thymidine kinase-negative rat 2 cells. We found that the hemimethylated DNA was efficiently converted into symmetrical methylated molecules before DNA replication. Furthermore, integration of the trans-DNA into the host genome is an early event after gene transfer.

SV40 T1-mRNA trans-splicing and translation requires that the in vitro synthesized cRNA is capped before microinjection.

The purpose of this investigation was to study the effect on cap structure for trans-splicing in mammalian cells. The early SV40 Bst/Bam pre-mRNA (cRNA) was synthesized in vitro in both capped (cap-Bst/Bam-cRNA) and non-capped (Bst/Bam-cRNA) versions and microinjected into the nuclei of TC7 cells. Trans-splicing was monitored by immunofluorescence staining (T1-antigen) and by RT-PCR analysis. Cap-Bst/Bam-cRNA was trans-spliced with high efficiency, but not the Bst/Bam-cRNA molecules. Northern blot analysis revealed that both the capped and uncapped cRNA molecules had similar stability in the microinjected cells. The coinjected m7G(5')ppp(5')G cap analog did not inhibit the trans-splicing reaction in vivo and did not prevent nuclear export of the mRNA.

SV40 chromatin structure is not essential for viral gene expression.

The biological activity and the fate of SV40 DNA (minichromosomes, DNA I, DNA II, DNA III) were tested in culture cells by immunofluorescence staining and blot analysis. Following microinjection of 2-4 circular SV40 molecules (minichromosomes, DNA I, DNA II) into the cytoplasm or the nuclei of monkey and rat cells, T- and V-antigen synthesis was demonstrable in nearly every recipient cell. Only linear DNA induced T-antigen synthesis with a very low efficiency after cytoplasmic injection. This low activity correlates with a rapid degradation of DNA III in the recipient cells. Further modifications observed immediately after injection are relaxation of superhelical molecules and formation of high-Mr DNA. Assembly of the injected DNA into SV40 chromatin-like structure, however, occurred only late after early viral gene expression.

Heterologous HIV-nef mRNA trans-splicing: a new principle how mammalian cells generate hybrid mRNA and protein molecules.

Heterologous trans-splicing is a messenger RNA (mRNA) processing mechanism, that joins RNA segments from separate transcripts to generate functional mRNA molecules. We present here for the first time experimental evidence that the proximal segment of the HIV-nef RNA segment can be trans-spliced to both viral (e.g. SV40 T-antigen) and cellular transcripts. Following either microinjection of in vitro synthesized HIV-nef and SV40 T-antigen pre-mRNA or transfection of the HIV-nef DNA into T-antigen positive cells (CV1-B3; Cos7), it was found that recipient cells synthesized HIV-nef/T-antigen hybrid mRNA and protein molecules. To generate the hybrid mRNA, the cells utilized the 5' cryptic splice sites of the HIV-nef (5'cry 66 and 5'cry 74) and the SV40 T/t-antigen 3' splice site. To demonstrate that heterologous trans-splicing also occurs between the HIV-nef RNA and cellular transcripts, a cDNA library was established from HIV-nef positive CV1-B3 cells (CV1-B3/13 cells) and screened for hybrid mRNA molecules. Reverse transcription-PCR and Northern blot analysis revealed that a significant portion of the HIV-nef transcript is involved in heterologous trans-splicing. To date, eight independent HIV-nef/cellular hybrid mRNA molecules have been identified. Five of these isolates contain segments from known cellular genes (KIAA1454, PTPkappa, Alu and transposon gene families), while three hybrid segments contain sequences of not yet known cellular genes (genes 1-3).