Immunocytochemical evidence for a nuclear and a cytoplasmic O6-methylguanine repair mechanism in cultured rat hepatocytes

The localization of DNA and RNA adducts was studied at the ultrastructural level using antibodies directed against O6-metG and the protein A-gold technique. Primary rat hepatocyte cultures were exposed for 2-24 h to 5 mM N-nitrosodimethylamine (NDMA) or 0.1 mM N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). In both cases, the O6-metG immunoreactive sites were concentrated in the nucleus and in the rough endoplasmic reticulum (RER) rich cytoplasmic regions. The highest gold labeling density measured was observed at 2 h of NDMA or MNNG treatment. However, after a 24-h exposure, very little labeling was observed in both the nuclear and the cytoplasmic compartments. The rate of disappearance of immunoreactive sites was faster in the cytoplasm than in the nucleus, Untreated control preparations showed no specific immunogold labeling. Furthermore, when cells were exposed first to NDMA and MNNG for a few hours and then to culture medium containing no genotoxin, and subsequently were reexposed to NDMA or MNNG for a few hours, very little labeling of both the nuclear and cytoplasmic compartments was observed. Control preparations without a second genotoxin exposure showed a normal labeling pattern. Control preparations without genotoxin showed no gold labeling. Our results provide evidence for the existence of a cytoplasmic O6-metG repair mechanism that behaves like its nuclear counterpart.

Induction of endometrial adenocarcinomas in persistent estrous Donryu rats by a single intra-uterine administration of N-ethyl-N'-nitro-N-nitrosoguanidine

Induction of endometrial adenocarcinomas in persistent estrous Donryu rats by a single intra-uterine administration of N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG) was examined. Female Donryu rats were divided into four groups at 7 weeks of age. Rats in Croup 1 were controls under a 12-h light/12-h dark regimen while Groups 2-4 were persistent estrous animals maintained under 24-h light conditions. At 10 weeks of age, rats in Group 1 and 2 were given a single intra-uterine administration of polyethylene glycol(PEG), and those in Groups 3 and 4 received ENNG(20 mg/kg body weight), dissolved in PEG, in the same manner. Group 4 rats also received an s.c. 17 beta-estradiol(E2) implant one week thereafter. At the termination of the experiment (week 42 after the ENNG-treatment), all surviving rats were killed and the development of uterine proliferative lesions assessed. All groups demonstrated hyperplasias, the incidence and severity being higher in ENNG-treated animals. The incidence of uterine adenocarcinomas in Group 2(2/20, 10%) was slightly higher than that in Group 1(0/16, 0%), although this difference was not significant. In Groups 3(7/19, 37%) and 4(6/20, 30%) statistically significant induction of adenocarcinomas was noted, two lesions in the latter case being very large with prominent invasion of tumor cells into the perimetrium. At 10 and 25 weeks after ENNG-treatment, the E2-progesterone(E2:P) ratios in Groups 2, 3 and 4 were significantly higher than in Group 1.(ABSTRACT TRUNCATED AT 250 WORDS)

Antimutagenicity of fermented milk

Reconstituted nonfat dry milk was fermented by Lactobacillus helveticus CH65, Lactobacillus acidophilus BG2FO4, Streptococcus salivarius ssp. thermophilus CH3, Lactobacillus delbrueckii ssp. bulgaricus 191R, and by a mixture of the latter two organisms. The fermented milks were then freeze-dried, extracted in acetone, dissolved in dimethylsulfoxide, and assayed for antimutagenicity in the Ames test (Salmonella typhimurium TA 100) against N-methyl, N'-nitro, N-nitroso-guanidine, and 3,2'-dimethyl-4-amino-biphenyl. Dose-dependent activity was significant against both mutagens in all extracts. Maximal inhibitory activity against 3,2'-dimethyl-4-aminobiphenyl and N-methyl, N'-nitro, N-nitroso-guanidine was 2- and 2.7-fold greater, respectively, than that exhibited by extracts of unfermented milk. Extracts of milk fermented by L. delbrueckii ssp. bulgaricus 191R were examined further. Compounds that were responsible for activity against both mutagens were less soluble in aqueous solutions than in dimethylsulfoxide. Adjustment of milk fermented by L. delbrueckii ssp. bulgaricus 191R to pH 3, 7.6, or 13 prior to freeze-drying and acetone extraction did not significantly alter the activity specific for 3,2'-dimethyl-4-aminobiphenyl. In contrast, compounds with activity specific for N-methyl, N'-nitro, N-nitrosoguanidine were less extractable at pH 7.6. The weak antimutagenicity of unfermented milk was not increased by addition of 2% L-lactic acid.

Xeroderma pigmentosum group E binding factor recognizes a broad spectrum of DNA damage

Xeroderma pigmentosum complementation group E binding factor (XPE-BF) is a damaged DNA binding protein that is deficient in a subset of patients from complementation group E of xeroderma pigmentosum. The protein recognizes various forms of DNA damage including some cyclobutane pyrimidine dimers, 6-4 photoproducts, cis-diamminedichloroplatinum(II) adducts, and single-stranded DNA. We now show that it also recognizes damage induced by nitrogen mustard; N-methyl-N'-nitro-N-nitrosoguanidine, and depurination, but has no detectable affinity for DNA adducts generated by trans-diamminedichloroplatinum(II), 4-nitroquinoline-N-oxide, 8-methoxypsoralen, or enzymatically methylated cytosine and adenine. The failure to recognize 4-nitroquinoline-N-oxide and 8-methoxypsoralen adducts is consistent with previous reports that XPE cells carry out wild-type levels of repair synthesis after DNA damage by those drugs. These results demonstrate that XPE-BF is a versatile damage recognition protein, but suggest that other proteins must contribute to the recognition of DNA lesions for the human excision repair pathway.

Differential effects of milk proteins, BSA and soy protein on 4NQO- or MNNG-induced SCEs in V79 cells

The possible antimutagenic effects of five different proteins against the mutagen 4-nitroquinoline 1-oxide (4NQO) were assessed in a mammalian cell system, using the sister chromatid exchange (SCE) test in Chinese hamster cells (V79). For this purpose the proteins casein, bovine serum albumin (BSA), soy protein, total whey protein and beta-lactoglobulin were studied, as well as pepsin-hydrolysed casein. In addition, the effect of casein on 1-methyl-1-nitroso-3-nitroguanidine (MNNG) was studied. The proteins were tested at a concentration of 1.15% (w/v). Casein was studied over a concentration range of 0 to 1.15% (w/v). A non-toxic concentration was used for the mutagens. Casein hydrolysis by pepsin took place in vitro, simulating human stomach conditions, which resulted in 84% non-casein-N and 16% remaining casein-N. Casein significantly inhibited SCE induction by 4NQO (inhibition 78%, at 1.15% casein, P < 0.05) and by MNNG (83%, at 1.15% casein, P < 0.01). BSA also significantly inhibited 4NQO-induced SCEs (94%, at 1.15% BSA; P < 0.01). However, soy protein, the whey protein fraction of milk and beta-lactoglobulin showed no inhibitory effects. Pepsin-hydrolysed casein inhibited SCE induction by 4NQO and MNNG to a similar extent as non-hydrolysed casein. It is concluded that casein, its pepsin hydrolysis products and BSA may protect mammalian cells against certain genotoxic compounds, whereas other milk proteins, such as whey protein, beta-lactoglobulin and soy protein, do not have this protective action. Although the mechanism of antimutagenicity is unknown, it seems plausible that the protein acts as a blocking agent by chemical or physical interaction with the mutagens. The accessibility of protein molecules and the presence of nucleophilic binding sites may be decisive factors in determining antimutagenic properties of proteins.

Investigations of the genotoxicity and cell proliferative activity of dichlorvos in mouse forestomach

This study investigated the possible mechanism by which dichlorvos may have caused forestomach tumours in mice in a chronic corn oil gavage cancer bioassay [NTP (1989) Toxicology and carcinogenesis studies of dichlorvos in F344/N rats and B6C3F1 mice (gavage studies). National Toxicology Program Technical Report 342, NIH Publ. No 89-2598]. For this purpose, a method has been developed to assess the genotoxicity of irritant substances on mouse forestomach epithelium. Groups of five B6C3F1 mice were given a single oral dose of dichlorvos, the genotoxic forestomach carcinogen 1-methyl-3-nitro-1-nitrosoguanidine (MNNG) or the irritant, non-genotoxic forestomach carcinogen butylated hydroxyanisole (BHA). After periods of 2-48 h, three parameters were assessed: unscheduled DNA synthesis (UDS) by autoradiography of tissue sections, replicative DNA synthesis (RDS) also by autoradiography of incorporated [3H]thymidine, and histopathological changes, including hyperplasia. MNNG induced UDS but not RDS or hyperplasia in forestomach epithelium, consistent with its genotoxic mode of action. BHA and dichlorvos did not induce UDS, consistent with absence of genotoxic activity in the forestomach after in vivo exposure. In contrast, BHA and dichlorvos induced RDS and subsequent hyperplasia, which is likely to result from irritant damage. These data suggest that the chronic effects of dichlorvos on mouse forestomach epithelium in the oral gavage bioassay were mediated via enforced cell proliferation, rather than by a genotoxic mechanism.

High-yield induction of uterine endometrial adenocarcinomas in Donryu rats by a single intra-uterine administration of N-ethyl-N'-nitro-N-nitrosoguanidine via the vagina

A total of 130 female Donryu rats (10-week-old) were divided into two groups; 80 animals in the experimental group were given a single intra-uterine administration of 20 mg/kg N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG) dissolved in polyethylene glycol (PEG) via the vagina without laparotomy, and 50 animals in the control group received PEG alone in the same manner. Small numbers of animals in both groups were killed at 3, 6, 9 and 12 months after ENNG treatment for sequential histological and endocrinological examination, and at 12.5 experimental months (15 months of age) all survivors were killed. At the termination, endometrial adenocarcinomas were present in 49% of the experimental group, compared to 0% in the control group. Severe endometrial hyperplasias were also found only in the experimental group and sequential histological examination showed first appearance of hyperplasia at 6 months and adenocarcinoma at 9 months. No tumors other than uterine carcinomas were induced by ENNG and the carcinogen treatment did not affect the endocrine environment of rats, persistent estrus appearing at 6 months after the start and increasing with age in both groups. The estradiol-17 beta:progesterone (E:P) ratio was also increased after 6 months, with further elevation at 12 months to about 8 times higher than the level at 6 months. These results indicate that an increased E:P ratio might act as a promoter of development of endometrial proliferative lesions initiated by ENNG in this rat strain. The study indicates that the present simple method using Donryu rats provides a good animal model for endometrial adenocarcinoma development in women.

Effect of transfection of human poly(ADP-ribose)polymerase in Chinese hamster cells on mutagen resistance

Poly(ADP-ribose)polymerase (PARP) is a DNA-binding protein that is activated upon induction of DNA breaks and supposed to play a role in DNA repair. To elucidate the effect of overexpression of PARP on the resistance of cells to mutagens, Chinese hamster ovary cells (both the line CHO-9 and the mutagen-hypersensitive derivative 27-1) were transfected with the human PARP cDNA along with pSV2neo. Treatment of the transfected cell population with a high dose of MNNG and selection with G418 gave rise to a significant increase of neo+ clones, as compared to the control transfection with pSV2neo + salmon sperm DNA. The frequency of survivors in these mass culture experiments was lower, however, than after transfection with the bacterial ada gene encoding the DNA repair protein O6-alkylguanine-DNA alkyltransferase. Thus transfection of PARP cDNA in CHO cells is only weakly effective in inducing alkylation resistance. This was confirmed by analyzing the mutagen resistance of individual PARP transfectant clones derived from CHO-9 and 27-1 cells that expressed increased levels of PARP mRNA, protein and PARP activity. These strains were slightly more resistant to the toxic effect of MMS and showed a reduced frequency of MMS-induced chromosomal aberrations. CHO-9-PARP transfectants also gained resistance to UV. From these data we conclude that, in CHO cells, PARP is limiting in handling critical lesions during the repair process and that increase of the amount of PARP protein can elicit some protection against genotoxic effects of mutagens.

Strand- and sequence-specific attenuation of N-methyl-N'-nitro-N-nitrosoguanidine-induced G.C to A.T transitions by expression of human 6-methylguanine-DNA methyltransferase in Chinese hamster ovary cells

The effect of human O6-methylguanine-DNA methyltransferase (MGMT) on the cytotoxicity, the mutagenicity, and the specific kinds of base substitutions induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) were examined in non-MGMT transfected Chinese hamster ovary cells (CHOM cells) and in those cells which had been transfected with human MGMT complementary DNA (AGT cells). AGT cells containing a high level of human MGMT activity were markedly more resistant to the cytotoxic and mutagenic effects of MNNG than CHOM cells which had no detectable MGMT activity. The dosages of MNNG which reduced to 50% of colony forming ability were estimated to be 0.8 microM for CHOM and 10 microM for AGT cells. The induction frequency of 6-thioguanine-resistant cells was significantly declined in AGT cells. At 4 microM MNNG, this frequency was declined from 273 mutants/10(6) viable CHOM cells to 13 mutants/10(6) viable AGT cells. The entire coding region of the hypoxanthine (guanine) phosphoribosyltransferase (hprt) gene in 37 AGT and 22 CHOM mutants was characterized by direct sequencing of the mRNA-polymerase chain reaction-amplified complementary DNA. Base changes at the intron-exon boundaries of the hprt DNA in the splicing mutants were further examined. Those results indicated that G to A transitions were significantly reduced in MNNG-treated AGT cells (chi 2 test, P < 0.001), suggesting that O6-methylguanine was repaired error free by human MGMT. In contrast, no difference arose in the frequencies of T to C transitions induced by MNNG in these two populations. All of the G to A transitions induced in AGT cells were located on the nontranscribed strand, assuming that the causative lesion was O6-methylguanine (P < 0.05). Such a strand specificity was not observed in CHOM mutants. Most of the G to A transitions observed in CHOM mutants were located at the middle guanine of 5'-GGPu sequences. Transitions observed at these sites, particularly 5'-GGG, were significantly reduced in AGT mutants (P < 0.05). Our results have suggested that human MGMT specifically repairs O6-methylguanine with a preference to remove those located on the transcribed strand and middle guanine of 5'-GGG.

Lectin-binding profiles in MNNG-induced shrew esophageal carcinomas

The lectin-binding profiles of methacarn-fixed paraffin-embedded sections of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced shrew esophageal lesions (papillomas, dysplasia, and carcinoma) were studied histochemically with biotinylated lectins and compared with those in adjacent normal esophageal epithelia. The lectins used for demonstrating sugar residues were soybean agglutinin (SBA), peanut agglutinin (PNA), Helix pomatia agglutinin (HPA), wheat germ agglutinin (WGA) and Griffonia simplicifolia-I (GS-I). Lectin-binding in normal squamous epithelium showed suprabasal (SBA, PNA and HPA) or all layer (WGA and GS-I) types. Papillomas generally showed similar lectin-binding profiles to normal epithelium, with the exception of the loss of WGA staining in basal cells. There was a significant down-regulation of SBA, PNA, HPA and WGA binding in dysplasia and carcinoma cells, while GS-I binding was preserved.

Effects of arsenic on DNA damage and repair in human fetal lung fibroblasts

Previous studies suggest that arsenic may be both mutagenic and co-mutagenic. In this report, we examined the effects of sodium arsenite (As) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) on unscheduled DNA synthesis (UDS) in human fetal lung fibroblasts (2BS cells) by 3H/14C double-labeling and liquid-scintillation counting techniques. Arsenic at concentrations of 1, 5 and 10 microM increased UDS value, indicating that arsenic directly damaged DNA and did not inhibit DNA repair. In addition, UDS induced by 34 microM MNNG in combination with arsenic was significantly increased by 3 microM As and not affected by 0.1, 0.5, 1.0 and 5 microM As, also indicating that arsenic did not inhibit the excision and polymerization steps of DNA repair. Based on the results and a previous study that 3 microM As is more efficient than 1 and 5 microM As in the induction of DNA-protein crosslinks, we proposed that arsenic may enhance the mutagenicity of other compounds by inducing DNA-protein crosslinks rather than inhibiting DNA repair.

Excision of hypoxanthine from DNA containing dIMP residues by the Escherichia coli, yeast, rat, and human alkylpurine DNA glycosylases

The deamination of adenine residues in DNA generates hypoxanthine, which is mutagenic since it gives rise to an A.T to G.C transition. Hypoxanthine is removed by hypoxanthine DNA glycosylase activity present in Escherichia coli and mammalian cells. Using polydeoxyribonucleotides or double-stranded synthetic oligonucleotides that contain dIMP residues, we show that this activity in E. coli is associated with the 3-methyladenine DNA glycosylase II coded for by the alkA gene. This conclusion is based on the following facts: (i) the two enzymatic activities have the same chromatographic behavior on various supports and they have the same molecular weight, (ii) both are induced during the adaptive response, (iii) a multicopy plasmid bearing the alkA gene overproduces both activities, (iv) homogeneous preparation of AlkA has both enzymatic activities, (v) the E. coli alkA- mutant does not show any detectable hypoxanthine DNA glycosylase activity. Under the same experimental conditions, but using different substrates, the same amount of AlkA protein liberates 1 pmol of 3-methyladenine from alkylated DNA and 1.2 fmol of hypoxanthine from dIMP-containing DNA. The Km for the latter substrate is 420 x 10(-9) M as compared to 5 x 10(-9) M for alkylated DNA. Hypoxanthine is released as a free base during the reaction. Duplex oligodeoxynucleotides containing hypoxanthine positioned opposite T, G, C, and A were cleaved efficiently. ANPG protein, APDG protein, and MAG protein--the 3-methyladenine DNA glycosylases of human, rat, and yeast origin, respectively--were also able to release hypoxanthine from various DNA substrates containing dIMP residues. The mammalian enzyme is by far the most efficient hypoxanthine DNA glycosylase of all the enzymes tested.

Comparison of mutation spectra induced by N-ethyl-N-nitrosourea in the hprt gene of Mer+ and Mer- diploid human fibroblasts

N-Ethyl-N-nitrosourea (ENU) forms several major adducts upon reaction with DNA, of which ethylation at the O6 position of guanine and the O4, O2 and N3 positions of thymine have been implicated to be mutagenic lesions. To investigate what specific kinds of ENU-induced mutations were affected by the repair ability of O6-alkylguanine-DNA alkyltransferase (AGT), we examined the mutations in the hypoxanthine (guanine) phosphoribosyltransferase gene (hprt) in 87 independent mutants derived from ENU-treated AGT proficient (Mer+) or deficient (Mer-) diploid human fibroblasts. Of the characterized mutations, 97% were single base substitutions. The major difference in the mutation spectra was that the frequency of G.C to A.T transitions was significantly higher in Mer- mutants (16/38) than in Mer+ mutants (4/33). The results indicate that AGT removes O6-ethylguanine, thus protecting human cells from parts of the cytotoxic and mutagenic effects of ENU. A high frequency of T.A to A.T transversions induced by ENU was observed in both Mer+ (52%) and Mer- (34%) mutants. This type of mutation was less frequently observed (10%) in N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced mutants derived from the same Mer+ cells in our previous report (J. Mol. Biol., 221, 421, 1991). Comparison of alkylating lesions formed by MNNG and ENU indicates that O2-ethylthymine and N3-ethylthymine are potent mutational adducts for T to A transversions. The occurrence of ENU-induced T.A base pair transversions showed a strong strand bias; 35/37 were located on the non-transcribed strand, assuming thymine is the mutagenic lesion. The result suggests a difference in repair capacity of ethylthymine on the two strands. In addition, this type of mutation preferentially occurred at 5'-Pu-T sequences.

Inhibition of the bacterial mutagenicity of N-methyl-N'-nitro-N-nitrosoguanidine by ascorbic acid and ascorbyl palmitate

The mechanism of antimutagenic activity of ascorbic acid (AA) and its derivatives was studied using the Salmonella typhimurium TA100 bacterial test system. All substances studied inhibited N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced mutagenesis. Ascorbyl palmitate (AP) markedly decreased the numbers of his+ revertants, behaving as a membrane-active antimutagen. A comparative study of the antioxidative activity of the investigated substances in the methyl oleate (MO) system has demonstrated that AA and its derivatives have pro-oxidant properties within the limits of the concentrations studied. The results obtained do not agree with the common view of the mode of action of these antimutagens, including both inhibition of free radical processes and MNNG reductive inactivation.

Antimutagenic effect of plant flavonoids in the Salmonella assay system

The antimutagenic effects of 27 kinds of plant flavonoids on the mutagenicity of aflatoxin B1(AFB1) and N-methyl-N'-nitro-N-nitrosoguanidine(MNNG) in Salmonella typhimurium TA100 were investigated. In the mixed applications of AFB1 (1 microgram/plate) with the flavonoids (300 micrograms/plate) in the presence of a mammalian metabolic activation system (S9 mix), chrysin, apigenin, luteolin and its glucoside, kaempferol, fisetin, morin, naringenin, hesperetin, persicogenin, (+)-catechin and (-)-epicatechin showed the antimutagenic effect against AFB1 with more than 70% inhibition rate. A little or no antimutagenicities except flavone against MNNG (0.5 microgram/plate) were observed. For the antimutagenicity of the flavonoids on AFB1, the flavonoid structure that contains the free 5-, 7-hydroxyl group seemed to be essential. However, saturation of the 2,3-double bond or elimination of the 4-keto group did not affect the activity.

Antimutagenicity in Euglena gracilis

The genotoxic effect of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and furadantine (Fu) was significantly decreased by standard antimutagens (ascorbic acid, alpha-tocopherol, chlorophyllin and sodium selenite) in the unicellular flagellate Euglena gracilis. The effects of these compounds were verified also by a bacterial test in which three strains of Salmonella typhimurium, TA97, TA100, and TA102, were used. The above compounds were antimutagenic in strains of bacteria used, except for chlorophyllin which had no effect on strain TA102.

Amrita Bindu--a salt-spice-herbal health food supplement for the prevention of nitrosamine induced depletion of antioxidants

Ayurveda, practised in India, identified a large number of plant components to be used in the diet for the prevention or the delayed development of degenerative disorders. They include some of the commonly used spices, namely pepper and ginger. The Materia Medica includes both naturally occurring and artificially produced salts, as a partial substitute for common salt. Health promoting herbs and spices which are classified pharmacologically as rejuvenating, nourishing, invigorating, cleansing, wound-healing, etc., are used as food additives. Amrita Bindu is a salt-spice-herbal mixture based on these principles and was tested for its effect in maintaining antioxidant defense systems in blood and liver when exposed to a carcinogenic nitrosamine, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Amrita Bindu supplementation prevented MNNG induced depletion of the antioxidant enzymes and the scavenger antioxidants glutathione and vitamins A, C and E. Amrita Bindu provides protection against free radical and reactive oxygen species induced tissue lipid peroxidation and the resultant tissue degeneration.

Glycoinositol phospholipid anchor-defective K562 mutants with biochemical lesions distinct from those in Thy-1- murine lymphoma mutants

Deficient expression of glycoinositol phospholipid (GPI)-anchored surface proteins has been linked to six different genetic defects in Thy-1- murine lymphoma mutants. In this study, human K562 cell mutants defective in GPI anchoring were derived by anti-decay-accelerating factor (CD55) based negative fluorescent cell sorting of N-methyl-N'-nitro-N-nitrosoguanidine pretreated cells. Homologous cell fusions of six clones that complemented a previously described K562 mutant corresponding to one of the Thy-1- mutant classes (Hirose, S., Mohney, R. P., Mutka, S. C., Ravi, L., Singleton, D. R., Perry, G., Tartakoff, A., and Medof, M. E. (1992) J. Biol. Chem. 267, 5272-5278) showed that they segregated into two complementation groups. In heterologous cell fusions, representative clones of each group complemented Thy-1 expression by all of the previously described GPI anchor pathway-defective Thy-1- murine lymphoma classes (A, B, C, E, F, and H) but not class(es) D (and I) defective in the Thy-1 structural gene. Analyses of putative GPI anchor precursors synthesized by the two lines revealed that one mutant exhibited a complete block in deacetylation of N-acetyl-D-glucosamine-inositol phospholipid to glucosamine (GlcN)-inositol phospholipid, whereas the other mutant assembled GlcN-inositol phospholipid and subsequent mannose (Man)-containing intermediates but showed markedly increased amounts of the terminal ethanolamine (EthN)-phosphate (P)-substituted putative anchor precursors, EthN-P-6ManMan(EthN-P-->)ManGlcN- and EthN-P-6Man(EthN-P-6)Man(EthN- P-->)ManGlcN-acylinositol phospholipid (H7 and H8). We designate these new complementation classes J, harboring a defect in N-acetyl-D-glucosamine-inositol phospholipid deacetylation, and K, deficient in a step preliminary to or associated with protein transfer of assembled anchor precursors. The availability of these new mutant classes should aid in characterization of the GPI anchor pathway enzymes providing for these reactions.

On the quantitative relationship between O6-methylguanine residues in genomic DNA and production of sister-chromatid exchanges, mutations and lethal events in a Mer- human tumor cell line

O6-Methylguanine (m6G) is an altered base produced in DNA by SN1 methylating agents such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). This lesion is repaired by the protein O6-methylguanine-DNA methyltransferase (MGMT) in normal human cell lines, but is not repaired in certain human tumor lines that are termed Mex- or Mer-. Compared with repair-proficient cell lines, such repair-deficient tumor lines are hypersensitive to the production by MNNG of sister-chromatid exchanges (SCE), mutations and lethality. We report here that MNNG treatment produces 1 SCE for every 42 +/- 10 m6G formed in the genome of Mer- tumor cells, 1 6TG-resistant mutant for every 8 (range of 5-14) m6G produced statistically in the coding region of the hypoxanthine phosphoribosyltransferase gene, and 1 lethal event per 6650 +/- 1200 m6G. In addition, in vitro base mismatch incision at m6G: BrU pairs was similar to that at m6G: T pairs, the lesions that likely initiate SCE production. We conclude that m6G residues in genomic DNA are very recombinogenic as well as highly mutagenic in Mer- human tumor cells. The results are interpreted in terms of the relationship between methylation-induced SCE and G: T mismatch recognition.

Evidence for nontargeted mutagenesis in a monkey kidney cell line and analysis of its sequence specificity using a shuttle-vector plasmid

Intact pZ189 DNA was allowed to replicate in monkey kidney vero cells that had been pretreated with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The E. coli MBM7070 was transfected with replicated plasmid, and those with mutations in the supF gene were identified. The frequency of mutants that did not contain recognizable changes in the electrophoretic mobility of the plasmid DNA was scored. The frequency of such mutants was 12.2 x 10(-4) (43/35376) and 6.2 x 10(-4) (22/35712) in mutants derived from cells pretreated with 0.2 mumoles/l and 2 mumoles/l MNNG respectively; these values represent an increase of 5.8- and 2.9-fold over the spontaneous mutation frequency of 2.1 x 10(-4) (10/47741) (p < 0.01). Sequence analysis of the supF genes of these mutants showed that 89% (24/27) of base substitutions occurred at G.C base pairs; 59% of the base substitutions (16/27) were transversions, and 41% (11/27) were transitions. The types of base substitutions were predominantly G.C-->T.A and G.C-->A.T. 48% of base substitutions occurred at 6 sites of the supF gene; 4 of these sites consist of 5'-TTNN where N is G or C. Base substitutions never previously reported were found, namely, T-->C at 61, G-->T at 70, G-->T at 99, and G-->C at 103 were found; these have never been reported up to now. In addition, 2 of the 5 frameshifts occurred in the region 99-105 of the supF gene (GGTGGGG), suggesting that this region is a hot spot for nontargeted frameshifts. These results strongly suggest that nontargeted mutagenesis can occur in mammalian cells and shows that the spectrum of mutations induced differs from that of spontaneous and targeted mutations.

Dose response of promotion by butylated hydroxyanisole in chemically initiated tumours of the rat forestomach

The antioxidant food preservative butylated hydroxyanisole (BHA) was tested in an initiation-promotion protocol in which male F344 rats (6 wk old), 27 per group, were gavaged with a single dose of 200 mg N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)/kg. After 3 wk on control diet, test diets containing 0, 60, 300, 1000, 3000, 6000 or 12,000 ppm BHA were fed until termination of the experiment at approximately 110 wk, at which time most animals had died with stomach tumours. MNNG caused a high incidence of tumours in the glandular stomach and forestomach of all groups. Administration of 12,000 and 6000 ppm BHA, but not 3000 ppm or lower doses, caused statistically significant increases in the time-related incidence of MNNG-induced forestomach tumours as analyzed by life table analysis. BHA had no effect on the incidence of tumours in the glandular stomach or oesophagus. Tumour incidences in other organs were not related to BHA dose. No increase in hyperplasia in the oesophagus was evident in the high-dose BHA-treated animals compared with the MNNG-only group. This study provides corroboration that BHA affects only forestomach tumorigenesis and that the dose for enhancement of tumorigenesis is at least 1500-fold greater than human exposure.

DNA-damaging agents stimulate the formation of directed reciprocal translocations in Saccharomyces cerevisiae

DNA-damaging agents can stimulate the formation of directed reciprocal translocations in strains of Saccharomyces cerevisiae containing his3 recombinational substrates to generate chromosomal rearrangements. Such agents were compared with those that can stimulate sister-chromatid recombination. We show that chemicals and environmental agents that produce a variety of DNA lesions, including bulky adduct, thymidine dimers, interstrand cross-links, double-strand breaks alkylated bases, can stimulate recombination to yield reciprocal translocations. Of the agents tested, only the alkylating agents methyl methanesulfonate (MMS) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), and a bifunctional agent that causes bulky DNA adducts, 4-nitroquinoline-N-oxide (4-NQO), significantly stimulate sister-chromatid recombination in our assay. Factors that contribute to the stimulation of interchromosomal recombination include strain genetic background and ploidy.

Accumulation of aluminum in cancers of the liver, stomach, duodenum and mammary glands of rats

The aim of the present investigation was to evaluate abnormal changes in trace element concentrations during carcinogenesis. First, Al, Zn and Cu in the liver tissues of rats were measured by atomic absorption analysis over a half year of hepatocarcinogenesis. Male Wistar rats were given carcinogenic food containing 600 mg/kg of 3'-methyl-4-dimethylaminoazobenzene (3'-MeDAB) in a basal diet for several months. After 4 to 6 months of feeding, hepatocarcinomas developed in the rats. Zn and Cu concentrations in the hepatocarcinomas of the 3'-MeDAB group significantly decreased as compared with normal liver tissues of the control groups. On the other hand, the aluminum concentration in the hepatocarcinomas was more than three times that in the normal liver tissues. The Al and Se contents of developed gastric and mammary cancers were measured in Experiment II. Male and female rats were given 1-methyl-3-nitrothoguanidine(MNNG) and 2,7-dimehtylbenz(a)anthracene(DMBA), respectively. After several months, carcinomas developed in over half of the rats. The Al and Se concentrations in cancers, livers and the blood were determined by atomic absorption analysis. It was shown that both gastric and mammary carcinomas contained a high level of aluminum and very little selenium in comparison with normal liver tissues. The present study demonstrated that aluminum accumulated in experimentally induced carcinomas in rats, i.e., cancers of the liver, stomach, duodenum and mammary glands.

Transgenic gpt+ V79 cell lines differ in their mutagenic response to clastogens

Several gpt+ transgenic cell lines were derived from hprt V79 cells to study mutagenesis mechanisms in mammalian cells. The G12 cell line was previously shown to be hypermutable by X-rays and UV at the gpt locus compared to the endogenous hprt gene of the parental V79 cells (Klein and Rossman, 1990), and is now shown to be highly mutable by the clastogenic anti-tumor agent bleomycin sulfate. A second transgenic cell line G10, which has a different gpt insertion site, was studied in comparison with G12. Both G12 and G10 cell lines carry the stable gpt locus at a single integration site in the Chinese hamster genome, and neither spontaneously deletes the integrated gpt sequence at a high frequency. Although spontaneous mutation to 6-thioguanine resistance in G10 cells is 3-4 times higher than in G12 cells, the cell lines differ to a much greater extent when mutated by clastogens. In comparison to G12 cells, the gpt locus in G10 cells is up to 13 times more sensitive to bleomycin mutagenesis and 5 times more responsive to X-ray mutagenesis. In contrast, there is much less difference in UV-induced mutagenesis and no differences in mutagenesis induced by alkylating agents such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The dose-dependent decrease in survival of the transgenic cells is the same for all mutagens tested, and does not differ from that of V79 cells. Neither transgenic cell line is generally hypermutable, since mutagenesis at an endogenous gene, Na+K+/ATPase, is similar to that of the parental V79 cell line. Although both cell lines can be induced to delete the transgene following clastogen exposure, deletions are not the only recovered mutations, and the cell lines can also be used to study mutations within the PCR recoverable gpt gene. The utility of these transgenic cells to investigate genome position effects related to mammalian mutagenesis mechanisms is discussed.

Induction and promotion of forestomach tumors by sodium nitrite in combination with ascorbic acid or sodium ascorbate in rats with or without N-methyl-N'-nitro-N-nitrosoguanidine pre-treatment

In experiment I, short-term effects of combined treatment with anti-oxidants, sodium ascorbate (NaAsA) and sodium nitrite (NaNO2) on forestomach cell proliferation were examined in F344 male rats. Groups of 5 animals aged 6 weeks were treated for 4 weeks with 0.8% catechol, 0.8% hydroquinone, 1% tert-butyl-hydroquinone (TBHQ), 2% gallic acid or 2% pyrogallor alone or in combination with 0.3% NaNO2 in the drinking water and/or 1% NaAsA in the diet. The thicknesses of forestomach mucosa in rats treated with anti-oxidants and NaNO2 in combination were greater than those with antioxidant alone and additional NaAsA treatment further enhanced the thickening of mucosa. It was noteworthy that values for mucosae of animals treated with NaNO2 and NaAsA without anti-oxidant were similar to those for anti-oxidants. In experiment 2, effects of combined treatment with NaAsA or ascorbic acid (AsA) and NaNO2 on carcinogenesis were examined in F344 male rats with or without N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) pre-treatment. Groups of 20 or 15 rats, respectively, aged 6 weeks, were given a single intra-gastric administration of 150 mg/kg body weight of MNNG in DMSO:water = 1:1 or the vehicle alone by stomach tube. Starting 1 week later, they received supplements of 1% NaAsA or 1% AsA in the diet and 0.3% NaNO2 in drinking water in combination, each of the individual chemicals alone, or basal diet until the end of week 52. In MNNG-treated animals, incidences of forestomach papillomas and carcinomas were significantly enhanced in the NaNO2 alone group (84 and 47%, respectively) as compared with the basal diet group (30 and 10%), with further significant increase in carcinomas occurring with additional NaAsA (79%, p < 0.05) or AsA (85%, p < 0.05) treatment. In animals without MNNG, all animals in the NaNO2 group demonstrated mild hyperplasia, additional administration of NaAsA or AsA remarkably enhancing the grade of hyperplasia, and resulting in 53% and 20% incidences, respectively, of papillomas. Thus NaNO2 was demonstrated to exert promoter action for forestomach carcinogenesis, with NaAsA and AsA acting as co-promoters. The results strongly indicate that combined treatment with NaAsA or AsA and NaNO2 may induce forestomach carcinomas in the long term.