Altered DNA binding and replication activities of JC virus T-antigen mutants

Ten mutations were introduced into the JC virus (JCV) T antigen within a region corresponding to the SV40 T-antigen DNA binding domain (SV40 amino acids 131 to 220); nine of these increased homology between the two proteins in sequences critical for SV40 T antigen DNA binding. All mutant JCV T antigens bound to JCV and SV40 origins of DNA replication. Binding efficiency relative to the of wild-type JCV T antigen ranged from 83 to 301% for the JCV binding sites and from 44 to 240% for the SV40 binding sites. Nine mutant proteins promoted viral DNA replication in primary human fetal glial (PHFG) and CV-1 cells. In PHFG cells, promotion of DNA replication ranged from 26 to 220% relative to that of wild-type T antigen; in CV-1 cells it ranged from 14 to 522%. Coding sequences for five mutant proteins were transferred into the hybrid virus M1 (SV40) [M1(SV40) contains coding sequences from JCV and regulatory sequences from SV40]. Wild-type T antigen promoted replication weakly from the SV40 origin in these hybrid viruses in CV-1 cells (2% that from the JCV origin); replication driven by the mutant proteins ranged from 110 to 412% of that induced by the wild-type protein. Efficient specific DNA binding by a mutant T antigen was not a reliable indicator of that mutant protein's ability to promote DNA replication.

The zinc finger region of simian virus 40 large T antigen is needed for hexamer assembly and origin melting

Simian virus 40 large T antigen contains a single sequence element with an arrangement of cysteines and histidines that is characteristic of a zinc finger motif. The finger region maps from amino acids 302 through 320 and has the sequence C-302 L K C-305 I K K E Q P S H Y K Y H-317 E K H-320. Previous genetic analysis has shown that the cysteine and histidine sequences and the contiguous S H Y K Y region in the finger are important for DNA replication in vivo. We show here that representative mutations in either of these elements of the finger prevent the assembly of large T antigen into stable hexamers in vitro. These same mutations have a characteristic effect on the interaction of T antigen with the simian virus 40 core origin of replication. The mutant T antigens bind to the central pentanucleotide domain of the core origin but fail to melt the adjacent inverted repeat domain and to untwist the adenine-thymine domain. These defects would prevent the formation of a replication bubble and the initiation of DNA replication. Finger mutations have lesser effects on the helicase function of T antigen and no observable effect on binding of T antigen to the mouse p53 protein. We propose that the zinc finger region contributes to protein-protein interactions essential for the assembly of stable T-antigen hexamers at the origin of replication and that hexamers are needed for subsequent alterations in the structure of origin DNA. We cannot exclude the possibility that the zinc finger region also makes specific contacts with components of origin DNA.

Inhibition of DNA replication of adenovirus type 5 and simian virus 40 by tunicamycin

Addition of tunicamycin, a glycosylation inhibitor, to SV40-infected CV-1 cells or Ad5-infected HeLa cells at the beginning of infection was found to inhibit the accumulation of viral DNA at late times after infection. However, tunicamycin did not block viral DNA replication when added to the infected cells at late times after infection. The inhibitory effect of tunicamycin was partially reversible in the presence of acetylglucosamine, suggesting that the effect was due to glycosylation. In spite of diminished amounts of viral DNA accumulated at the late phase of infection in the presence of tunicamycin, the transcription rate of Ad5 late RNA and the amount of adenovirus late proteins and mRNA were not significantly affected by tunicamycin treatment. The inhibitory effect of tunicamycin on Ad5 DNA replication was much reduced in 293 cells which provide E1a gene products in trans. Similar observation was obtained for the replication of SV40 DNA in Cos-7 cells which provide SV40 early gene products in trans. These results suggest that tunicamycin inhibits a glycosylation event induced by the early gene products of Ad5 and SV40 viruses during the early phase of infection.

Cooperative assembly of simian virus 40 T-antigen hexamers on functional halves of the replication origin

The cofactor ATP stimulates the formation of T-antigen double hexamers on the simian virus 40 core origin of replication (I. A. Mastrangelo, P. V. C. Hough, J. S. Wall, M. Dodson, F. B. Dean, and J. Horwitz, Nature [London] 338:658-662, 1989). We report here the pathway for the assembly of hexamers and double hexamers on the core origin. ATP triggers the cooperative assembly of hexamers on the early and late halves of the origin even when they are completely isolated. Hexamer assembly nucleates at T-antigen recognition pentanucleotides in the early half of the origin. In intact origins, assembly of the first hexamer on the early half of the origin cooperatively stimulates the assembly of a second hexamer on the adjacent late half of the origin. Thus, monomer-monomer and hexamer-hexamer interactions of T antigen, allosterically activated by ATP, constitute two distinct types of cooperative interaction with the origin. Finally, we show that the assembly of T-antigen hexamers on isolated half origins leads to the same array of structural changes that T antigen induces in intact origins. We conclude that the origin is divided into complementary halves that each promote the assembly of functional T-antigen hexamers.

Retinoblastoma protein and simian virus 40-dependent immortalization of human fibroblasts

Transformation and immortalization of human diploid fibroblasts by simian virus 40 (SV40) is at least a two-stage process, since transformants have a limited lifespan in culture. We have isolated immortalized derivatives (AR5 and HAL) from transformants generated with an origin-defective SV40 genome encoding a heat-labile large T protein (T antigen) and reported that both preimmortal and immortal transformants are continuously dependent on T antigen function for growth as determined by temperature shift experiments. In this study, we demonstrate complex formation between T antigen and the retinoblastoma susceptibility gene product (Rb) at 35 degrees C and observed a reduction in complexes under conditions of loss of T antigen function and growth inhibition at 39 degrees C. Viral oncogenes (polyomavirus large T protein and adenovirus E1A 12S protein) known to bind Rb were introduced into AR5 and HAL cells, both stably by gene transfer and transiently by virus vectors. Such double transformants are still unable to proliferate at 39 degrees C, although complex formation with the newly introduced oncogenes was demonstrated. We suggest that T antigen interacts with other cellular processes in addition to Rb to transform and immortalize human cells in culture. Our finding that p53-T antigen complexes are also temperature dependent in AR5 and HAL cells could provide such an additional mechanism.

Derivation of lifespan extended cell line from simian virus 40 infected human uterine endocervical epithelial cells cultured on collagen gel

Primary culture of human uterine endocervical epithelial cells was performed. When cells were cultured on a plastic dish, these cells developed into a flat form, then lost their proliferative ability in a short time during continuous subculture. On the other hand, when cells were cultured on collagen gel and subcultured by explanation of this cell containing gel, the cells took on a roughly ovoid rather than a flat shape and active proliferation was attained which maintained confluency for as long as 8 weeks. Furthermore by adopting the Simian Virus 40 in the culture system for these cells, it was possible to extend the lifespan to approximately twice that of the control cell group.

In vitro rescue of an integrated hybrid adeno-associated virus/simian virus 40 genome

In an in vitro simian virus 40 (SV40) DNA replication assay, we have observed excision of a hybrid adeno-associated virus (AAV)/SV40 insert from a plasmid construct. The excision was dependent on the presence of the palindromic AAV terminal repeat and greatly enhanced by the addition of the SV40 T antigen to the reaction. Analysis of the excision product supports a model in which the palindromic terminal sequences of AAV form a cruciform structure (equivalent to a Holliday recombination intermediate), which is cleaved and resealed so that the excision products are linear duplex pBR322 and linear duplex AAV/SV40 insert. Both the excised linear duplex pBR322 and the excised linear duplex AAV/SV40 insert have each terminus covalently crosslinked by one copy of the palindromic region of the AAV terminal repeat region folded on itself. The excision process may be a model system for cellular homologous recombination. The process as observed was either concomitant with or subsequent to DNA replication.

Stable T-p53 complexes are not required for replication of simian virus 40 in culture or for enhanced phosphorylation of T antigen and p53

We generated a number of simian virus 40 (SV40) mutants with single amino acid substitutions in T antigen between residues 388 and 411. All but one mutant (398LV) replicated like wild-type SV40 and gave rise to normal-size plaques. Three different mutations at residue 402 (Asp to Glu, Asn, or His) totally prevented the formation of stable complexes with the cellular protein p53 in monkey cells but had no effect on virus replication. Only one other mutation in this region, involving residue 401 (Met to Thr), slightly inhibited the formation of T-monkey p53 complexes. The three mutant T antigens with substitutions at residue 402 also formed no stable complexes with human p53 but generated low levels of complexes with mouse p53. These results indicate that residue 402 is critical for binding to monkey and human p53 proteins and is important for binding to mouse p53. We suggest that it is one of several points of contact. In cells infected with any one of the three residue 402 mutant viruses. T antigen and p53 became increasingly phosphorylated, as they were in cells infected with wild-type virus. Our data therefore show that stable T-p53 complexes are not required for replication of SV40 in culture or for enhanced phosphorylation of either protein.

Stable growth of simian virus 40 recombinants containing multimerized enhancers

Multiple copies of each of three genetically defined simian virus 40 protoenhancers, A, B, and C, were able to substitute for the wild-type simian virus 40 enhancer. Although the recombinant viruses grew poorly, they could be propagated without the accumulation of enhancer rearrangements that might improve viability. Mutations that inactivate the multimerized B and C protoenhancers abolished virus growth, but, unexpectedly, a mutation that inactivates the octamer-enhanson within the B protoenhancer increased virus viability. This positive effect may reflect loss of repression of the B protoenhancer by the ubiquitous octamer-motif-binding protein Oct-1.

Sequence requirements for activation of replication by the SV40 transcriptional promoter or enhancer elements

Previous studies have demonstrated that the 21- or the 72-bp repeat transcriptional control elements enhance the efficiency of SV40 DNA replication in vivo, provided either of these repeats is located near the end of the core replication origin containing the 17-bp A + T-containing sequence. Using two sets of point mutants we have investigated the contributions of the various sequence motifs present in the 21- or the 72-bp repeats toward activation of replication. Regarding the contribution of the six GC motif components of the 21-bp repeats, we find that GC motif I, located closest to the core origin, is dispensable for activation of replication. A mutation in GC-I in fact causes an increase in replication efficiency. We also find that GC motifs I and II present in the nontandem copy of the 21-bp repeats are not sufficient to activate replication. Our present study indicates that a combination of three GC motifs such as II, III, and IV (including one of the two perfect, tandem copies of the 21-bp repeats) is important for activation of replication. Regarding the 72-bp repeat transcriptional enhancer region, we find mutations in a number of its individual motifs to have a negative consequence on replication, with mutations in the GT-I*/TC-II and Sph-II/octamer motifs exhibiting the most negative effects. Overall, we find that the replication activation effects of the 21- and the 72-bp repeats require the participation of multiple motifs present in them. Cellular factors binding to these motifs are expected to mediate their replication activation effects. For the most part, the motifs required for activation of replication are the same as those reported in earlier studies to be important for efficient early and late viral mRNA transcription.

Dissociation of Rb-binding and anchorage-independent growth from immortalization and tumorigenicity using SV40 mutants producing N-terminally truncated large T antigens

The large T antigen of SV40 is both necessary and sufficient for conversion of primary mouse cells to cells with fully transformed phenotype. In this investigation, the influence of the N-terminal portion of T antigen on individual transformed cell characteristics was probed by using mutants bearing deletions in the 5'T antigen coding sequence. Specifically, DNA constructs expected to produce T antigens missing the first 109, 127, 150, or 176 amino acids or internal amino acid segments between 117 and 250 were tested for the ability to immortalize C57Bl/6 mouse embryo fibroblasts. The transformed cell properties displayed by clonally derived cell lines were then examined. The results indicated that neither the first 127 amino acids nor amino acids 127-250 of T antigen were necessary for efficient immortalization of primary cells or for their tumorigenicity. Functions mapped within these regions, including binding of the retinoblastoma susceptibility gene product (Rb) and transactivation of heterologous promoters, therefore, were not required to confer either of these growth properties. In addition the results showed that anchorage-independent growth was separable genetically from tumorigenicity and that removal of amino acids within the first 250 residues of T antigen compromised other transformed cell growth properties.

Adenovirus infection induces amplification of persistent viral DNA sequences (simian virus 40, hepatitis B virus, bovine papillomavirus) in human and rodent cells

Adenoviruses, types 2 and 12 induce amplification of SV40 DNA sequences in cells of the SV40-transformed human newborn kidney cell line, NB-E. Similarly, integrated hepatitis B virus DNA sequences in the human hepatoma cell line, PLC/*PRF/5, and bovine papillomavirus (BPV) DNA sequences in BPV-transformed mouse cells (ID13) are amplified by adenovirus infection. Thus, similar to herpes group or vaccinia viruses or DNA damaging agents, adenoviruses are able to mediate selective DNA amplification in addition to their reported mutagenic and chromosome damaging effects. The role of amplification of integrated viral DNA sequences in development and progression of specific tumors (e.g. hepatocellular carcinoma) remains to be determined.

The growth of simian virus 40 (SV40) host range/adenovirus helper function mutants in an African green monkey cell line that constitutively expresses the SV40 agnoprotein

The simian virus 40 T-antigen carboxy-terminal mutants, dlA2459 and dlA2475, are cell line and temperature dependent for growth and plaque formation in monkey kidney cells. Although these mutants did form plaques on BSC-1 cells at 37 degrees C, they were about fivefold less efficient for plaque formation than wild-type simian virus 40. These mutants did not grow in CV-1 cells and did not synthesize agnoprotein in those cells. CV-1 cells which constitutively express the agnoprotein were permissive for mutant plaque formation. However, late mRNAs, virion proteins, and progeny virion yields did not accumulate to wild-type levels during mutant infection of the agnoprotein-producing cells.

Cisplatin resistance and mechanism in a viral test system: SV40 isolates that resist inhibition by the antitumor drug have lost regulatory DNA

Isolates of SV40 that have enhanced ability to survive inhibition by the antitumor drug cisplatin were selected by serial drug challenge in vivo. These mutant viruses have acquired specific deletions within the repeated regulatory motif (GGGCGG)6 or GC box. This DNA element was shown previously to be a strong target of drug attack by cisplatin and other anticancer drugs in vitro and is an important viral and cellular DNA control sequence. Thus, drug resistance in this viral test system is dependent on the loss of important target DNA sequences. The results also indicate that drug efficacy may be related to the ability of certain anticancer drugs to attack regulatory DNA sequences containing strings of guanosines.

Host range determinant in the late region of SV40 and RF virus affecting growth in human cells

WtSV40 and its variant EL-SV40 (contains two complementing defective genomes) fail to productively infect human embryonic kidney cells or human fibroblasts. However, early SV40 (E-SV40) genomes can propagate in human cells when complemented by a particular late RF virus (L-RFV) genome or the closely related wtBKV genome. The L-RFV genome (L-RFV clone H) contains a deleted early region, a complete set of BKV capsid genes, and a single SV40 regulatory region (acquired by recombination). In contrast, it was not possible to make the reciprocal genome cross in human cells; late SV40 genomes containing a deleted early region do not complement early RFV or early BKV DNAs. The L-RFV clone H genome was also shown to complement wtSV40 in human cells. However, wtSV40 DNA was rapidly lost and replaced by a defective SV40 genome. The SV40 defective (E-SV40 alpha) contained a deletion of the late region, an intact early region, and paired with L-RFV clone H DNA to form a new hybrid virus. In human cells wtSV40 was also complemented by wtBKV DNA, but after two serial passages SV40 DNA disappeared. These findings indicate that SV40 late or capsid gene sequences, but not SV40 early sequences, generate a block to SV40 growth in human cells. When the SV40 late region is replaced by a RFV or a BKV late region, E-SV40 DNA propagates efficiently in human cells and in some cases more rapidly than wtBKV. Northern blot hybridization indicates that SV40 DNA is poorly transcribed in human cells when the SV40 late region is present.

The SV40 nucleosome-free region is detected throughout the virus life cycle

The structures of SV40 intracellular chromatin complexes and of extracellular virus particles were examined by photolabeling with a radioactive psoralen derivative in order to determine the fate of the exposed origin region during the virus life cycle. We have previously shown that the origin region of intracellular SV40 chromatin is preferentially accessible to psoralen derivatives in vivo, whereas psoralen adducts are uniformly distributed when purified virus particles are photoreacted. We demonstrate here that when virion is photoreacted prior to a freeze-thaw cycle, the exposed regulatory region detected in intracellular nucleoprotein complexes is also found in mature virus particles. In contrast, if the virion is frozen and thawed prior to the photoreaction, the origin is not preferentially exposed to photoaddition. Virus particles that have not been subjected to a freeze-thaw cycle were found to exhibit preferential labeling in the origin region whether they were irradiated intracellularly, in culture medium, or following purification. Banding the virus in CsCl had no significant effect on the relative accessibility of the origin region to psorealen. Our findings indicate that the open regulatory region found on intracellular SV40 chromatin persists throughout the virus life cycle.

UV-enhanced reactivation of UV-damaged SV40 is due to the restoration of viral early gene function

Mammalian cells respond to UV-radiation by inducing an increased ability to support the survival of UV-damaged virus. We have tested whether the induction of enhanced viral reactivation (ER) reflects heightened UV-resistance of specific viral functions. For this, we examined the extent of ER for SV40 containing UV-damage in three functionally distinct regions of the SV40 genome: (i) the viral regulatory region, (ii) the early genes region and (iii) the late genes region. ER corresponding to a dose reduction factor of 43% was observed for damage in the early genes region. No ER was observed for damage in the regulatory or late genes regions. We conclude that ER in SV40 reverses the lethal disruption of an essential function peculiar to the viral early genes region. This function is almost certainly transcription.

Frequency of cell transformation by the small DNA tumor viruses: infection of proliferating cells and quiescent cells

Small DNA-containing tumor viruses (simian virus 40, mouse polyoma-virus, and adenoviruses) malignantly transform fibroblasts of the susceptible rodents. Fibroblasts can exist, in vitro and in vivo, in either of the two states: the proliferating state or the quiescent state. In the present study, we examined whether the state of fibroblasts at the time of exposure to these DNA viruses affects the frequency of transformation. Dense-focus formation in monolayer culture of rat 3Y1 fibroblasts was used to quantitate transformation. Results show that the frequency of transformation by simian virus 40 and mouse polyomavirus was reduced when cells were in the proliferating state at the time of virus inoculation as compared to cells in the quiescent state, whereas that by adenovirus type 12 was similar in the two cellular states. The reduction of the frequency of transformation in proliferating cells infected with simian virus 40 was also observed in BALB/c 3T3 mouse cells. Mechanisms underlying the difference between the two cellular states and the difference between the papovavirus and adenovirus in this aspect of transformation remain to be investigated.

Purification of replication protein C, a cellular protein involved in the initial stages of simian virus 40 DNA replication in vitro

The replication of simian virus 40 (SV40) DNA is dependent upon a single viral protein [tumor (T) antigen] and multiple cellular proteins. To define the required cellular proteins, we have made use of a cell-free system that supports the replication of plasmid DNA molecules containing the SV40 origin of replication. We report here the purification from HeLa cell extracts of replication protein C (RP-C), a previously undescribed protein that is required to reconstitute efficient DNA replication in vitro. Highly purified preparations of RP-C contain two closely related polypeptides of 32 and 34 kDa. Preincubation of purified RP-C with T antigen and the DNA template largely eliminates the delay normally observed before the onset of rapid DNA synthesis. In addition, RP-C stimulates the unwinding of duplex DNA molecules containing the SV40 replication origin in a reaction that requires T antigen and a single-stranded DNA binding protein. These observations suggest that RP-C is involved in the initial steps of SV40 DNA replication in vitro.

Helpers for efficient encapsidation of SV40 pseudovirions

Plasmid DNA that carries the simian virus 40 (SV40) ori can be packaged as SV40 pseudovirions. The pseudovirions are very efficient in gene transmission into a variety of cell types, including human hemopoietic cells. They are routinely prepared with wild-type (wt) SV40 as a helper. In the present study, several parameters required for the helper function were investigated. Plasmids that carry pBR322 sequences in addition to the late genes of SV40 were inefficient in providing helper functions, presumably because the prokaryotic sequences interfered with expression of the SV40 late genes. Efficient helpers were plasmid pSVPiC [Villarreal and Soo, Mol. Appl. Genet. 3 (1985) 62-71] and an SV40 defective virus SLT3 (presently constructed). Plasmid pSVPiC carries a duplication of the SV40 ori and enhancer regions, and pi AN7 sequences. Because of its large size it was not packaged into virion particles. However, it underwent extensive recombination generating infective SV40 particles. Almost no prokaryotic sequences are included in SLT3, that carries the SV40 late gene. In spite of its small size (3.5 kb) it was packaged efficiently, creating defective (T-antigen-negative) SV40 virions. The availability of T-antigen positive and negative pseudovirion mixtures enabled us to suggest that T-antigen drives gene amplification in the target human hemopoietic cells.

Azapyrimidine analogues: inhibition of viral DNA synthesis and protein synthesis in SV40 infected BSC-1 cells

The replication of simian virus 40 DNA and protein synthesis in BSC-1 cells was analyzed in vitro after treatment with 5,6-dihydro-5-azacytidine (DH-5-AzaCR) or 5-azacytidine (5-AzaCR). Results demonstrated that after a 3-h treatment period with 100 micrograms/ml, DH-5-AzaCR exhibited a 77% inhibition of viral DNA synthesis, whereas 5-AzaCR resulted in a 50% inhibition. Stimulation of DNA synthesis occurred when infected cultures were treated with low doses (0.1 to 0.5 microgram/ml) of 5-AzaCR for 3 h and after 1 and 2 h of treatment with 100 micrograms/ml of 5-AzaCR; however, stimulation did not occur with DH-5-AzaCR. DNA synthesized in the presence of either drug demonstrated altered conformations when analyzed on agarose gels; however this alteration was negligible after DH-5-AzaCR treatment, but more pronounced in the presence of 5-AzaCR. Restriction enzyme analysis suggests that DH-5-AzaCR may not be a hypomethylating agent as is 5-AzaCR. Inhibition of total protein synthesis (cellular and viral) was essentially complete 6 h after treatment with DH-5-AzaCR, whereas 5-AzaCR completely inhibited protein synthesis after 3 h. These data indicate that 5-AzaCR does not exhibit a direct dose relationship to the inhibition of DNA synthesis whereas DH-5-AzaCR may show some dose relationship, and that DH-5-AzaCR is a more potent inhibitor of DNA synthesis as compared to 5-AzaCR.

Active ras and myc oncogenes can be compatible, but Sv40 large T antigen is specifically suppressed with normal differentiation of mouse embryonic stem cells

The pathobiological effects of oncogenes on normal differentiation of mouse embryonic stem cells from 4-day embryos were examined by introducing active ras, myc, and SV40 large T genes, all driven by mouse metallothionein I enhancer and promoter. Stem cell clones R5, M3, and T2 for ras, myc, and SV40 T genes, respectively, were particularly chosen for analyses because of their higher levels of transgene expression and their diploid chromosomal constitutions. These stem cells were then introduced into host 4-day embryos and the embryos were allowed to develop in the uterus of foster mothers. The stem cells colonized the tissues as extensively as the parent cells and gave rise to adult chimera with no apparent loss or abnormality of the embryos. The active ras and myc oncogenes introduced were expressed not only in the stem cells, but also in the developing embryos and in a variety of tissues of adult chimeras. However, although T antigen was originally expressed in the stem cells, it was not expressed in either developing embryos or tissues of adult chimeras. Induced by retinoic acid treatment in vitro or by subcutaneous grafting, this suppression of T-gene expression was also confirmed in differentiated progeny cells from several stem cell clones expressing T antigen. Permanent lines of fibroblast-like cells could be established at higher frequency from primary cultures of tissues of chimera, subcutaneous differentiated cells, and in vitro differentiated cells derived from T2 cells, and all these clones reexpressed T antigen. The results suggest that active myc and ras genes can be compatible with normal differentiation of the stem cells, but the expression of T antigen is specifically suppressed with recognition of its coding domain.

Pausing in simian virus 40 DNA replication by a sequence containing (dG-dA)27.(dT-dC)27

A 200 bp sequence including a stretch of 54 base pairs of alternating guanosine and adenosine nucleotide residues [(dG-dA)27.(dT-dC)27] was cloned in the simian virus 40 (SV40) genome between the KpnI and HpaII sites. This sequence was discovered earlier as part of a region limiting the amplification of sequences adjacent to an integrated polyoma virus in a transformed rat cell line. The newly constructed DNA was transfected into African Green monkey kidney CV1 cells and the variant virus was isolated by plaque-purification. The insertion was stably maintained and the variant virus grew more slowly than the wild type, had lower titers and gave smaller plaques. In mixed infection experiments, the variant was found to be stable, though the wild type replicated more rapidly. Pulse labeling experiments indicated that the unusual inserted sequence acts as a pause site for fork progression during DNA replication, as evidenced by the rate of incorporation of radioactively labeled nucleotides into various regions of the SV40 genome. Statistical fit of the experimental curves with theoretically generated curves suggested the pause of fork progression to be about one minute.

Effects of SO2 or NOx on toxic and genotoxic properties of chemical carcinogens. I. In vitro studies

This paper describes in vitro studies on the effects of environmental pollutants (SO2/NOx) in biological systems. Basic physical, chemical and biochemical parameters were analyzed to establish the rate of SO2/NOx absorption by the culture medium. It was shown that the pH remains constant for 24 h of exposure to gas concentrations up to 50 p.p.m. The concentration of ions resulting from absorption of each pollutant in the liquid phase is dependent on their concentration in the gas phase and on exposure time. Short exposure times and high gas dosages resulted in similar doses in the medium as long exposure periods and low gas dosages. The activities of a human serum standard (alkaline phosphatase, ALP; aspartate amino transferase, AST; alanine amino transferase, ALT; gamma-glutamyltransferase, gamma-GT; lactate dehydrogenase, LDH) were determined after gaseous exposure to SO2 and NOx. The results revealed a distinct decrease in the activity of LDH after 1, 3 and 5 h exposure to 200 p.p.m. SO2. The effects of the pollutants were assayed in vitro using fetal hamster lung cells (FHLC), rat hepatocytes and the cell line CO60. For the determination of toxic effects, it was shown that the plating efficiency was a more sensitive parameter than the assay for trypan blue exclusion. Toxicity indicated as an increase of LDH leakage was not observed from FHLC in culture. Instead, a decrease of LDH was found following SO2 exposition. This decrease was similar to that observed for the human serum standard. The induction of DNA single-strand breaks was determined as a measure of genotoxic effects. SO2 application decreased the rate of DNA single-strand breaks induced by N-nitroso-acetoxymethyl-methylamine in both FHLC and in rat hepatocytes. SO2 or NOx treatment of CO60 cells for 1 h did not result in the induction of DNA amplification. HSO3- added directly to the medium as the sodium salt, however, distinctly induced the amplification of SV40 DNA. The amplification rates induced by benzo[a]pyrene or dimethylbenzanthracene were neither influenced by SO2, NOx nor HSO3-. An additive effect of HSO3- with either benzo[a]pyrene or dimethylbenzanthracene for this biological parameter was therefore not observed.