Radiation induced COX-2 expression and mutagenesis at non-targeted lung tissues of gpt delta transgenic mice.

BACKGROUND: Although radiation-induced bystander effects have been confirmed using a variety of endpoints, the mechanism(s) underlying these effects are not well understood, especially for in vivo study. METHODS: A 1-cm(2) area (1 cm × 1 cm) in the lower abdominal region of gpt delta transgenic mice was irradiated with 5 Gy of 300 keV X-rays, and changes in out-of-field lung and liver were observed. RESULTS: Compared with sham-treated controls, the Spi(-) mutation frequency increased 2.4-fold in non-targeted lung tissues at 24 h after partial body irradiation (PBIR). Consistent with dramatic Cyclooxygenase 2 (COX-2) induction in the non-targeted bronchial epithelial cells, increasing levels of prostaglandin, together with 8-hydroxydeoxyguanosine, in the out-of-field lung tissues were observed after PBIR. In addition, DNA double-strand breaks and apoptosis were induced in bystander lung tissues after PBIR. CONCLUSION: The PBIR induces DNA damage and mutagenesis in non-targeted lung tissues, especially in bronchial epithelial cells, and COX-2 has an essential role in bystander mutagenesis.

Comprehensive toxicity study of safrole using a medium-term animal model with gpt delta rats.

In order to investigate a medium-term animal model using reporter gene transgenic rodents in which general toxicity, genotoxicity and carcinogenicity are evaluated, F344 gpt delta rats were given a diet containing 0.1% and 0.5% (a carcinogenic dose) safrole for 13 weeks. Serum biochemistry and histopathological examinations revealed overt hepatotoxicity of safrole, in line with previous reports. In the current study, safrole treatment possibly resulted in renal toxicity in male rats. In the in vivo mutation assays, an increase or a tendency to increase of the gpt mutant frequencies (MFs) was observed in both sexes at the carcinogenic dose. The number and area of foci of glutathione S-transferase placental form (GST-P) positive hepatocytes, ratio of proliferating cell nuclear antigen (PCNA)-positive hepatocytes and 8-hydroxydeoxyguanosine (8-OHdG) levels in liver DNA were significantly increased in both sexes of the 0.5% group. The overall data suggested that the present model might be a promising candidate for investigating comprehensive toxicities of the agents. In addition, data demonstrating the base modification and cell proliferation due to exposure to safrole could contribute to understanding safrole-induced hepatocarcinogenesis, which imply expanding in application of this model.

Report of the IWGT working group on strategy/interpretation for regulatory in vivo tests II. Identification of in vivo-only positive compounds in the bone marrow micronucleus test.

A survey conducted as part of an International Workshop on Genotoxicity Testing (IWGT) has identified a number of compounds that appear to be more readily detected in vivo than in vitro. The reasons for this property varies from compound to compound and includes metabolic differences; the influence of gut flora; higher exposures in vivo compared to in vitro; effects on pharmacology, in particular folate depletion or receptor kinase inhibition. It is possible that at least some of these compounds are detectable in vitro if a specific in vitro test is chosen as part of the test battery, but the 'correct' choice of test may not always be obvious when testing a compound of unknown genotoxicity. It is noted that many of the compounds identified in this study interfere with cell cycle kinetics and this can result in either aneugenicity or chromosome breakage. A decision tree is outlined as a guide for the evaluation of compounds that appear to be genotoxic agents in vivo but not in vitro. The regulatory implications of these findings are discussed.

Report of the IWGT working group on strategies and interpretation of regulatory in vivo tests I. Increases in micronucleated bone marrow cells in rodents that do not indicate genotoxic hazards.

In vivo genotoxicity tests play a pivotal role in genotoxicity testing batteries. They are used both to determine if potential genotoxicity observed in vitro is realised in vivo and to detect any genotoxic carcinogens that are poorly detected in vitro. It is recognised that individual in vivo genotoxicity tests have limited sensitivity but good specificity. Thus, a positive result from the established in vivo assays is taken as strong evidence for genotoxic carcinogenicity of the compound tested. However, there is a growing body of evidence that compound-related disturbances in the physiology of the rodents used in these assays can result in increases in micronucleated cells in the bone marrow that are not related to the intrinsic genotoxicity of the compound under test. For rodent bone marrow or peripheral blood micronucleus tests, these disturbances include changes in core body temperature (hypothermia and hyperthermia) and increases in erythropoiesis following prior toxicity to erythroblasts or by direct stimulation of cell division in these cells. This paper reviews relevant data from the literature and also previously unpublished data obtained from a questionnaire devised by the IWGT working group. Regulatory implications of these findings are discussed and flow diagrams have been provided to aid in interpretation and decision-making when such changes in physiology are suspected.

Synthetic activity of Sso DNA polymerase Y1, an archaeal DinB-like DNA polymerase, is stimulated by processivity factors proliferating cell nuclear antigen and replication factor C.

DNA replication efficiency is dictated by DNA polymerases (pol) and their associated proteins. The recent discovery of DNA polymerase Y family (DinB/UmuC/RAD30/REV1 superfamily) raises a question of whether the DNA polymerase activities are modified by accessory proteins such as proliferating cell nuclear antigen (PCNA). In fact, the activity of DNA pol IV (DinB) of Escherichia coli is enhanced upon interaction with the beta subunit, the processivity factor of DNA pol III. Here, we report the activity of Sso DNA pol Y1 encoded by the dbh gene of the archaeon Sulfolobus solfataricus is greatly enhanced by the presence of PCNA and replication factor C (RFC). Sso pol Y1 per se was a distributive enzyme but a substantial increase in the processivity was observed on poly(dA)-oligo(dT) in the presence of PCNA (039p or 048p) and RFC. The length of the synthesized DNA product reached at least 200 nucleotides. Sso pol Y1 displayed a higher affinity for DNA compared with pol IV of E. coli, suggesting that the two DNA polymerases have distinct reason(s) to require the processivity factors for efficient DNA synthesis. The abilities of pol Y1 and pol IV to bypass DNA lesions and their sensitive sites to protease are also discussed.

Roles of chromosomal and episomal dinB genes encoding DNA pol IV in targeted and untargeted mutagenesis in Escherichia coli.

DNA polymerase IV (pol IV) in Escherichia coli is a member of a novel family of DNA polymerases (the DinB/UmuC/Rad30/Rev1 super-family or the DNA polymerase Y family). Although expression of the dinB gene encoding DNA pol IV is known to result in an enhancement of untargeted mutagenesis, it remains uncertain whether DNA pol IV is involved in a variety of lesion-induced mutagenesis (targeted mutagenesis), and the relationship between expression levels of dinB and the mutagenesis that DNA pol IV promotes has not been investigated thoroughly. Here, we report that DNA pol IV is involved in -1 frameshift mutagenesis induced by 4-nitroquinoline N-oxide (4-NQO) and that the expression level of the chromosomal pol IV gene is 6-12 times higher than those for other SOS-inducible DNA polymerases in E. coli, i.e., DNA pol II (PolB) or DNA pol V (UmuDC), respectively. Interestingly, the dinB gene is present not only on the chromosome but also on the F' plasmid in the E. coli CC108 strain. In this strain, 750 molecules of DNA pol IV are expressed from the F' dinB gene in the uninduced state and 250 molecules are expressed from the chromosomal gene. These cellular expression levels strongly affect -1 frameshifts induced by 4-NQO in runs of six guanine bases: mutagenicity was highest in the strain CC108, followed by strains YG2242 (chromosome deltadinB/F' dinB+), YG2247 (chromosome dinB+/F' deltadinB) and FC1243 (chromosome deltadinB/F' deltadinB). The incidence of untargeted -1 frameshifts was reduced by two-thirds on deletion of dinB from the F' episome. The chromosomal dinB gene appeared to have little or no effect on the untargeted mutagenesis. These results suggest that DNA pol IV efficiently mediates targeted mutagenesis by 4-NQO, and that the cellular levels of expression substantially affect targeted and untargeted mutagenesis.

The effectiveness of the O(6)-alkylguanine-DNA alkyltransferase encoded by the ogt(ST) gene from S. typhimurium in protection against alkylating drugs, resistance to O(6)-benzylguanine and sensitisation to dibromoalkane genotoxicity.

Here we demonstrate that the Ogt(ST) from Salmonella typhimurium is a highly efficient O(6)-alkylguanine-DNA alkyltransferase (AGT) in affording protection against antitumour chloroethylating drugs (1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU)). In addition, Ogt(ST) is refractory to O(6)-benzylguanine (BG) inactivation and its expression provides only minor sensitisation to genotoxicity by environmental dibromoalkanes (DBE). No other of the assayed bacterial or human AGTs displayed such advantageous properties for chemoprotective gene therapy strategy. Our observations indicate that the Ogt(ST) AGT might be, under some circumstances, of potential use to improve cancer chemotherapy. At least, its properties may provide further insight into the design of human AGT variants that could be expressed in normal or tumour cells to provide either protection or ablation.

Further characterization and validation of gpt delta transgenic mice for quantifying somatic mutations in vivo.

The utility of any mutation assay depends on its characteristics, which are best discovered using model mutagens. To this end, we report further on the characteristics of the lambda-based gpt delta transgenic assay first described by Nohmi et al. ([1996]: Environ Mol Mutagen 28:465-470). Our studies show that the gpt transgene responds similarly to other transgenic loci, specifically lacZ and cII, after treatment with acute doses of N-ethyl-N-nitrosourea (ENU). Because genetic neutrality is an important factor in the design of treatment protocols for mutagenicity testing, as well as for valid comparisons between different tissues and treatments, a time-course study was conducted. The results indicate that the gpt transgene, like cII and lacZ, is genetically neutral in vivo. The sensitivities of the loci are also equivalent, as evidenced by spontaneous mutant frequency data and dose- response curves after acute treatment with 50, 150, or 250 mg/kg ENU. The results are interesting in light of transgenic target size and location and of host genetic background differences. Based on these studies, protocols developed for other transgenic assays should be suitable for the gpt delta. Additionally, a comparison of the gpt and an endogenous locus, Dlb-1, within the small intestine of chronically treated animals (94 microg/mL ENU in drinking water daily) shows differential accumulation of mutations at the loci during chronic exposure. The results further support the existence of preferential repair at endogenous, expressed genes relative to transgenes.

Reporter gene transgenic mice as a tool for analyzing the molecular mechanisms underlying experimental carcinogenesis.

Carcinogenic compounds are classified into 2 categories, genotoxic and non-genotoxic, which are basically judged from in vitro genotoxicity data. However, it is well documented that genotoxicants do not necessarily exert in vivo carcinogenicity in rodents, partly because of a discrepancy between in vitro and in vivo mutagenicities. Recently, transgenic animal models with reporter genes such as lacI, lacZ and gpt have been developed as a tool for assessing in vivo mutagenicity as well as carcinogenicity. In this article, data using lacI transgenic mice and gpt delta mice are presented and their application is discussed. In lacI transgenic mice, dimethylnitrosamine (DMN) treatment significantly increased lacI mutant frequency (MF) in the liver, kidenys and lungs, but not in other non-target organs. Repeated dose ip administration of DMN was more effective than single dose treatment in the induction of lacI MF. The spectrum of mutant plaques induced by DMN was characterized by deletions as well as GC to AT base transitions. The remaining mice receiving DMN proved to have liver adenomas at a high frequency after 78 weeks. Meanwhile, dietary 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MelQx) significantly increased lacI and gpt MFs in the liver and colon. The characteristic spectrum of mutant plaques induced by MeIQx was a GC to TA base transversion in both the lacI and gpt mutations. Our results thus strongly suggest that these reporter gene transgenic animal models could offer a useful tool for analyzing molecular mechanisms underlying experimental carcinogenesis and for assessing the carcinogenic risk of environmental chemicals.

Molecular nature of ultraviolet B light-induced deletions in the murine epidermis.

Depletion of the stratospheric ozone layer leads to an increase in ambient UV loads, which are expected to raise skin cancer incidences. Tumor development in the skin could be a multistep process in which various genetic alterations, such as point mutations and deletions, occur successively. Here, we demonstrate that UVB irradiation efficiently induces deletions in the epidermis using a novel transgenic mouse, gpt delta. In this mouse model, deletions in lambda DNA integrated in the chromosome are preferentially selected as Spi(-) (sensitive to P2 interference) phages, which can then be subjected to molecular analysis. The mice were exposed to UVB at single doses of 0.3, 0.5, 1.0, 1.5, and 2.0 kJ/m(2). After 4 weeks, lambda phage was rescued from the genomic DNA of the epidermis by in vitro packaging reactions. The mutant frequencies of Spi(-) with large deletions in the epidermis increased >15-fold at a UVB dose of 0.5 kJ/m(2) over the control. Molecular sizes of most of the large deletions were >1000 bp. More than one-half of the large deletions occurred between short direct-repeat sequences from 1 to 6 bp, and the remainder had flush ends. In the unirradiated mouse, almost all of the Spi(-) mutants were 1-bp frameshifts in runs of identical bases. These results suggest that UVB irradiation induces deletions in the murine epidermis, and most of the deletions are generated through end-joining of double strand breaks in DNA.

Mutation induction by heavy-ion irradiation of gpt delta transgenic mice.

Using the new transgenic mice produced by mating gpt delta with p53 knockout, mutation induction by heavy-ion irradiation and the effect of p53 background on such induction were studied. After the whole body irradiation with 10 Gy of 135 MeV/u carbon-ion beam, the genomic DNA was isolated from the different organs and the lambda DNA was rescued as a phage. Mutations in the transgene on the lambda DNA were determined by the spi(-) selection (deletion assay). The spi(-) mutation was induced by the above irradiation, but enhancement of the mutant frequency by the knockout of p53 gene was found not in the phages recovered from liver but in those from kidney. We are now making an effort to determine the nature of spi(-) mutation to confirm such p53 effect.

Construction of Salmonella typhimurium YG7108 strains, each coexpressing a form of human cytochrome P450 with NADPH-cytochrome P450 reductase.

A series of Salmonella typhimurium (S. typhimurium) YG7108 strains, each coexpressing a form of human cytochrome P450 (CYP) (CYP1A1, CYP1A2, CYP1B1, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP3A4, or CYP3A5) together with human NADPH-cytochrome P450 reductase (OR), was established. The parental S. typhimurium YG7108, derived from TA1535, lacks two O(6)-methylguanine-DNA methyltransferase genes, ada and ogt, and is highly sensitive to the mutagenicity of alkylating agents. The expression levels of CYP holo-protein in the genetically engineered S. typhimurium YG7108 cells, determined by carbon monoxide (CO) difference spectra, ranged from 62 nmol/L culture for CYP2C19 to 169 nmol/L culture for CYP3A4. The expression level of the OR varied, depending on the form of CYP coexpressed, and ranged from 214 to 1029 units/L culture. Each form of CYP expressed in the S. typhimurium YG7108 cells catalyzed the oxidation of a representative substrate at an efficient rate. The rates appeared comparable to the reported activities of CYP expressed in human liver microsomes or CYP in other heterologous systems, indicating that the OR was sufficiently expressed to support the catalytic activity of CYP. These S. typhimurium strains may be useful not only for predicting the metabolic activation of promutagens catalyzed by human CYP but also for identifying the CYP form involved.

Recent advances in the protocols of transgenic mouse mutation assays.

Transgenic mutation assays were developed to detect gene mutations in multiple organs of mice or rats. The assays permit (1) quantitative measurements of mutation frequencies in all tissues/organs including germ cells and (2) molecular analysis of induced and spontaneous mutations by DNA sequencing analysis. The protocols of recently developed selections in the lambda phage-based transgenic mutation assays, i.e. cII, Spi(-) and 6-thioguanine selections, are described, and a data set of transgenic mutation assays, including those using Big Blue and Muta Mouse, is presented.

Development of a Salmonella tester strain sensitive to promutagenic N-nitrosamines: expression of recombinant CYP2A6 and human NADPH-cytochrome P450 reductase in S. typhimurium YG7108.

We developed a new Salmonella tester strain highly sensitive to promutagenic N-nitrosamines by introducing a plasmid carrying human cytochrome P450 2A6 (CYP2A6) and NADPH-cytochrome P450 reductase (OR) cDNA into the ada- and ogt-deficient strain YG7108. The YG7108 2A6/OR cells expressed high levels of CYP2A6 (77+/-8nmol/l) and OR (470+/-20 micromol cytochrome c reduced/min/l). The expressed CYP2A6 efficiently catalyzed coumarin 7-hydroxylation. N-Nitrosodiethylamine (NDEA), N-nitrosomethylphenylamine (NMPhA), and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) were mutagenic in the new strain in the absence of any exogenous activation system. The concentrations of promutagen that caused a two-fold increase in revertants were 7.1, 0.14, and 1.4 microM for NDEA, NMPhA, and NNK, respectively. YG7108 2A6/OR cells showed about 10- and 100-fold higher sensitivity to NDEA and NNK, respectively, than parental YG7108 cells assayed in the presence of rat liver S9 (final concentration, 21% (v/v)). Parental YG7108 cells did not detect NMPhA mutagenicity even in the presence of rat liver S9. We believe that this is the first demonstration that CYP2A6 is responsible for the metabolic activation of NMPhA. The established tester strain may be useful to predict human activation of N-nitrosamine promutagens.

Characterization of mutations induced by 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine in the colon of gpt delta transgenic mouse: novel G:C deletions beside runs of identical bases.

Mutations induced by one of the typical dietary mutagens/carcinogens, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), were characterized using gpt delta transgenic mice. This transgenic mouse model has two selection methods to efficiently detect different types of mutations, i.e. 6-thioguanine selection for point mutations and Spi(-) selection for deletions. The mice were fed with a diet containing 400 p.p.m. PhIP for 13 weeks and gpt and Spi(-) mutations were analyzed from the colon, where the highest mutant frequencies were detected. Concerning the types of gpt mutations from PhIP-treated mice, 81% were single base pair substitutions and G:C-->T:A transversions predominated; single base pair deletions at G:C base pairs were also observed. In untreated mice G:C-->A:T transitions predominated and >80% of these events involved 5'-CpG-3' sites. Concerning Spi(-) mutants from PhIP-treated mice, 76% were G:C base pair deletions and more than half of these events occurred in monotonic G or C run sequences. Interestingly, a novel type of frameshift motif, i.e. G:C base pair deletions beside run sequences, was observed. The most frequently observed mutation in this class was the 5'-TTTTTTG-3'-->5'-TTTTTT-3' event. These results suggest that PhIP induces point mutations, such as base substitutions and single base pair deletions, rather than larger deletions in vivo and that run sequences may play an important role in PhIP-induced G:C base pair deletions.

Escherichia coli DNA polymerase IV mutator activity: genetic requirements and mutational specificity.

The dinB gene of Escherichia coli is known to be involved in the untargeted mutagenesis of lambda phage. Recently, we have demonstrated that this damage-inducible and SOS-controlled gene encodes a novel DNA polymerase, DNA Pol IV, which is able to dramatically increase the untargeted mutagenesis of F' plasmid. At the amino acid level, DNA Pol IV shares sequence homologies with E. coli UmuC (DNA Pol V), Rev1p, and Rad30p (DNA polymerase eta) of Saccharomyces cerevisiae and human Rad30A (XPV) proteins, all of which are involved in translesion DNA synthesis. To better characterize the Pol IV-dependent untargeted mutagenesis, i.e., the DNA Pol IV mutator activity, we analyzed the genetic requirements of this activity and determined the forward mutation spectrum generated by this protein within the cII gene of lambda phage. The results indicated that the DNA Pol IV mutator activity is independent of polA, polB, recA, umuDC, uvrA, and mutS functions. The analysis of more than 300 independent mutations obtained in the wild-type or mutS background revealed that the mutator activity clearly promotes single-nucleotide substitutions as well as one-base deletions in the ratio of about 1:2. The base changes were strikingly biased for substitutions toward G:C base pairs, and about 70% of them occurred in 5'-GX-3' sequences, where X represents the base (T, A, or C) that is mutated to G. These results are discussed with respect to the recently described biochemical characteristics of DNA Pol IV.

Metabolic activation of N-alkylnitrosamines in genetically engineered Salmonella typhimurium expressing CYP2E1 or CYP2A6 together with human NADPH-cytochrome P450 reductase.

A Salmonella typhimurium tester strain YG7108 2E1/OR co-expressing human CYP2E1 together with human NADPH-cytochrome P450 reductase (OR) was established. The mutagen-activating capacity of human CYP2E1 for N-alkylnitrosamines was compared with that of CYP2A6 using the YG7108 2E1/OR and the YG7108 2A6/OR strains of SALMONELLA: Salmonella YG7108 2A6/OR is a derivative of YG7108 co-expressing CYP2A6 together with OR. Eight N-alkylnitrosamines, including N-nitrosodimethylamine (NDMA), N-nitrosodiethylamine (NDEA), N-nitrosodipropylamine (NDPA), N-nitrosodibutylamine (NDBA), N-nitrosomethylphenylamine (NMPhA), N-nitrosopyrrolidine (NPYR), N-nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) were examined. CYP2E1 expressed in the YG7108 2E1/OR cells showed mutagen-activating capacity, as indicated by induced revertants/min/pmol cytochrome P450, for NDMA, NDEA, NDPA, NDBA, NPYR and NNK, but not NMPhA and NNN. CYP2A6 activated NDMA, NDEA, NDPA, NDBA, NMPhA, NPYR, NNN and NNK. The ratio of the mutagen-activating capacity seen with CYP2A6 to that seen with CYP2E1 was calculated for each N-alkylnitrosamine. In the case of NDMA, NPYR and NDEA, the ratio was under 1.0, while the ratio was over 1.0 with NDPA, NDBA, NNK, NMPhA and NNN. We conclude that human CYP2E1 is mainly responsible for the metabolic activation of N-nitrosamines with a relatively short alkyl chain(s), whereas CYP2A6 was predominantly responsible for the metabolic activation of N-alkylnitrosamines possessing a relatively bulky alkyl chain(s).