Organ-specific genotoxicity of the potent rodent bladder carcinogens o-anisidine and p-cresidine

DNA-damaging activities of twenty-four structurally diverse unsubstituted and substituted cyclic compounds in embryo-fetal chicken livers

DNA-damaging activities of twenty-four structurally diverse unsubstituted and substituted cyclic compounds were assessed in embryo-fetal chicken livers. Formation of DNA adducts and strand breaks were measured using the nucleotide ³²P-postlabelling (NPL) and comet assays, respectively. Unsubstituted monocyclic benzene, polycyclic fused ring compound naphthalene, covalently connected polycyclic ring compound biphenyl, and heterocyclic ring compound fluorene did not produce DNA damage. Amino-substituted monocyclic compounds, aniline and p-phenylenediamine, as well as polycyclic 1-naphthylamine were also negative. In contrast, carcinogenic monocyclic methyl-substituted anilines: o-toluidine, 2,6-xylidine, 3,4-dimethylaniline, 4-chloro-o-toluidine; 2 methoxy-substituted methylaniline: p-cresidine; 2,4 and 2,6 diamino- or dinitro- substituted toluenes all produced DNA damage. Genotoxic polycyclic amino-substituted 2-naphthylamine, 4-aminobiphenyl, benzidine, methyl-substituted 3,2'-dimethyl-4-aminobiphenyl and 4-dimethylaminoazobenzene as well as amino- and nitro- fluorenes substituted at the 1 or 2 positions also were positive in at least one of the assays. Overall, the DNA damaging activity of cyclic compounds in embryo-fetal chicken livers reflected the type and position of the substitution on the aromatic ring. Additionally, substituted polycyclic compounds exhibited higher DNA-damaging potency compared to monocyclic chemicals. These results are congruent with in vivo findings in other species, establishing chicken eggs as a reliable system for structure-activity assessment of members of groups of related chemicals.

Evaluation of the Carcinogenic Potential of Clofibrate in the p53+/− Mouse

This study was conducted as part of International Life Sciences Institute (ILSI) program to evaluate the carcinogenic potential of clofibrate, a nongenotoxic, peroxisome proliferator-activated receptor (PPAR) α agonist, following oral administration to p53+ /− heterozygous mice for a minimum of 26 weeks. p-Cresidine, a urinary bladder carcinogen, was given orally at 400 mg/kg/day as a positive control. Initial clofibrate doses were 50, 250, and 400 mg/kg/day for males and 50, 200, and 500 mg/kg/day for females. Due to unexpected mortality during the first week of dosing, clofibrate doses were lowered to 25, 75, and 100 mg/kg/day for males and 25, 75, and 125 mg/kg/day for females. Clinical signs and mortality were greater in p53+ /− than wild-type (WT) mice. With the exception of liver weights, no marked differences in any other parameters either between the sexes or between WT and p53+ /− mice were noted. Moderate increases in liver weights noted in WT males given 100 mg/kg/day clofibrate were not associated with any microscopic changes. No neoplastic response was observed in p53+ /−mice after 6 months of exposure to clofibrate at doses up to 100 mg/kg/day for males and 125 mg/kg/day for females. Transitional-cell hyperplasia and carcinoma of the urinary bladder were noted in both sexes given p-cresidine, demonstrating that the p53+ /− mouse responded to a known mouse carcinogen as expected. Clofibrate produced non-neoplastic findings in the adrenals, pancreas, and prostate, whereas p-cresidine affected the kidney, liver, pancreas, and spleen.

The urinary bladder carcinogen propoxur does not produce genotoxic effects in the urinary bladder of Wistar male rats

Propoxur (PPX) is a carbamate insecticide which induced urinary bladder cancer in Wistar rats when fed at 5000ppm in Altromin 1321 diet (1321). In the present investigation, PPX was studied for induction of several key events related to modes of action (MOA) of carcinogenicity in urinary bladders (UBs). Wistar rats were administered the compound for 28 days at 8000ppm in Provini Liba SA 3883 diet, which is similar to the 1321 diet. o-Anisidine HCl (AH) was used as a genotoxic UB carcinogenic comparator, and trisodium nitrilotriacetate (NTA) as an epigenetic UB carcinogen comparator. Along with the non-dosed control and three test substance groups (PPX, AH, NTA), four more groups were additionally fed 2% ammonium chloride (AC) in the diet to acidify the urine, since 1321 was reported to increase urinary pH. AC did acidify the urine, as expected, although the 3883 diet itself did not increase pH values above 8. In the alkaline comet assay, AH produced DNA single strand breaks (SSBs) in the UB urothelium (UBU) irrespective of AC administration, whereas PPX and NTA did not. In the nucleotide (32)P-postlabeling assay (NPL), AH produced DNA adducts irrespective of AC administration, whereas PPX and NTA did not. Routine (H&E) histopathology evaluation of the UBU did not reveal any hyperplasia or evidence of luminal microprecipitates or calculi in any of the groups. Assessment of UBU proliferation as measured by immunohistochemistry of proliferating cell nuclear antigen, revealed that NTA and NTA plus AC increased the replicating fraction (RF). Also AH plus AC, but not AH alone, increased the RF of UBU, whereas PPX groups were not significantly different from controls. Thus, the results reveal no evidence for DNA SSBs, binding, or alteration of DNA synthesis in the UBU by PPX, while demonstrating UBU DNA damage by AH and showing that NTA does not damage DNA, but causes increased UBU proliferation. The findings are in accord with a genotoxic MOA for AH, and an epigenetic MOA for NTA. The MOA of PPX does not involve genotoxicity and may be specific to the 1321 diet.

Formation, persistence, and identification of DNA adducts formed by the carcinogenic environmental pollutant o-anisidine in rats

2-Methoxyaniline (o-anisidine) is an industrial and environmental pollutant causing tumors of urinary bladder in rodents. Here, we investigated the formation and persistence of DNA adducts in the Wistar rat. Using the (32)P-postlabeling method, three o-anisidine-derived DNA adducts were found in several organs of rats treated with a total dose of 0.53 mg o-anisidine/kg body wt (0.15, 0.18, and 0.2 mg/kg body wt ip in the first, second, and third day, respectively), of which the urinary bladder had the highest levels. At four posttreatment times (1 day, 13 days, 10 weeks, and 36 weeks), DNA adducts in bladder, liver, kidney, and spleen of rats were analyzed to study their persistence. In all time points, the highest total adduct levels were found in urinary bladder (39 adducts per 10(7) nucleotides after 1 day and 15 adducts per 10(7) nucleotides after 36 weeks) where 39% adducts remained. In contrast to the urinary bladder, no persistence was detected in other organs. All three DNA adducts were identified as deoxyguanosine adducts. When deoxyguanosine was reacted with the oxidative metabolite of o-anisidine, N-(2-methoxyphenyl)hydroxylamine, three adducts could be separated by high-performance liquid chromatography (HPLC) and were identified by mass spectroscopy and/or nuclear magnetic resonance spectrometry. All adducts are products of the nitrenium/carbenium ions, the reactive species generated from N-(2-methoxyphenyl)hydroxylamine. The major adduct was identified to be N-(deoxyguanosin-8-yl)-2-methoxyaniline. Using cochromatography on HPLC, this adduct was found to be identical to the major adduct generated by activation of o-anisidine in vitro and in vivo.

Cytochrome P450-mediated metabolism of N-(2-methoxyphenyl)-hydroxylamine, a human metabolite of the environmental pollutants and carcinogens o-anisidine and o-nitroanisole

N-(2-methoxyphenyl)hydroxylamine is a human metabolite of the industrial and environmental pollutants and bladder carcinogens 2-methoxyaniline (o-anisidine) and 2-methoxynitrobenzene (o-nitroanisole). Here, we investigated the ability of hepatic microsomes from rat and rabbit to metabolize this reactive compound. We found that N-(2-methoxyphenyl)hydroxylamine is metabolized by microsomes of both species mainly to o-aminophenol and a parent carcinogen, o-anisidine, whereas 2-methoxynitrosobenzene (o-nitrosoanisole) is formed as a minor metabolite. Another N-(2-methoxyphenyl)hydroxylamine metabolite, the exact structure of which has not been identified as yet, was generated by hepatic microsomes of rabbits, but its formation by those of rats was negligible. To evaluate the role of rat hepatic microsomal cytochromes P450 (CYP) in N-(2-methoxyphenyl)hydroxylamine metabolism, we investigated the modulation of its metabolism by specific inducers of these enzymes. The results of this study show that rat hepatic CYPs of a 1A subfamily and, to a lesser extent those of a 2B subfamily, catalyze N-(2-methoxyphenyl)hydroxylamine conversion to form both its reductive metabolite, o-anisidine, and o-aminophenol. CYP2E1 is the most efficient enzyme catalyzing conversion of N-(2-methoxyphenyl)hydroxylamine to o-aminophenol.

Redox cycling in the metabolism of the environmental pollutant and suspected human carcinogen o-anisidine by rat and rabbit hepatic microsomes

We investigated the ability of hepatic microsomes from rat and rabbit to metabolize 2-methoxyaniline (o-anisidine), an industrial and environmental pollutant and a bladder carcinogen for rodents. Using HPLC combined with electrospray tandem mass spectrometry, we determined that o-anisidine is oxidized by microsomes of both species to N-(2-methoxyphenyl)hydroxylamine, o-aminophenol, and one additional metabolite, the exact structure of which has not been identified as yet. N-(2-Methoxyphenyl)hydroxylamine is either further oxidized to 2-methoxynitrosobenzene (o-nitrosoanisole) or reduced to parental o-anisidine, which can be oxidized again to produce o-aminophenol. To define the role of microsomal cytochromes P450 (P450) in o-anisidine metabolism, we investigated the modulation of this metabolism by specific inducers and selective inhibitors of these enzymes. The results of the studies suggest that o-anisidine is a promiscuous substrate of P450s of rat and rabbit liver; because P450s of 1A, 2B, 2E, and 3A subfamilies metabolize o-anisidine in hepatic microsomes of both studied species. Using purified enzymes of rat and rabbit (P450s 1A1, 1A2, 2B2, 2B4, 2E1, 2C3, 3A1, and 3A6), reconstituted with NADPH:P450 reductase, the ability of P450s 1A1, 1A2, 2B2, 2B4, 2E1, and 3A6 to metabolize o-anisidine was confirmed. In the reconstituted P450 system, rabbit P450 2E1 was the most efficient enzyme metabolizing o-anisidine. The data demonstrate the participation of different rat and rabbit P450s in o-anisidine metabolism and indicate that both experimental animal species might serve as suitable models to mimic the fate of o-anisidine in human.

Nasal cytotoxic and carcinogenic activities of systemically distributed organic chemicals

Toxicity and carcinogenicity in the mucosa of the nasal passages in rodents has been produced by a variety of organic chemicals which are systemically distributed. In this review, 14 such chemicals or classes were identified that produced rodent nasal cytotoxicity, but not carcinogenicity, and 11 were identified that produced nasal carcinogenicity. Most chemicals that affect the nasal mucosa were either concentrated in that tissue or readily activated there, or both. All chemicals with effects in the nasal mucosa that were DNA-reactive, were also carcinogenic, if adequately tested. None of the rodent nasal cytotoxins has been identified as a human systemic nasal toxin. This may reflect the lesser biotransformation activity of human nasal mucosa compared to rodent and the much lower levels of human exposures. None of the rodent carcinogens lacking DNA reactivity has been identified as a nasal carcinogen or other cancer hazard to humans. Some DNA-reactive rodent carcinogens that affect the nasal mucosa, as well as other tissues, have been associated with cancer at various sites in humans, but not the nasal cavity. Thus, findings in only the rodent nasal mucosa do not necessarily predict either a toxic or carcinogenic hazard to that tissue in humans.

Micronuclei and cell proliferation as early biological markers of ortho-phenylphenol-induced changes in the bladder of male F344 rats

ortho-Phenylphenol (OPP) and its sodium salt, sodium ortho-phenylphenate (SOPP), are widely used fungicides and antibacterial agents known to cause tumors in the bladders of male F344 rats. Previous studies in our laboratory have shown that micronuclei and cell proliferation were induced in the bladders of treated rats by a high dose of OPP. In our present studies, we investigated the relationship in dose response between these two biomarkers and previously reported tumor formation in the bladders of male F344 rats. Significant non-linear increases in micronuclei (MN) and BrdU-labeling were seen in the bladder cells of rats treated with the 8000 and 12,500 ppm doses of OPP and at 20,000 ppm SOPP. CREST anti-kinetochore staining showed that the micronuclei originated from both chromosomal loss and breakage. In addition, increases in MN were detected in the bladder but not in the bone marrow, underscoring the value of assessing genotoxicity in the target organ. In summary, these studies clearly show that at high doses, OPP and SOPP are genotoxic to the rat bladder. These results also indicate that micronucleus formation and cell proliferation can detect early OPP-induced changes in the rat bladder and may be useful as biomarkers for bladder carcinogens.

Identification of a genotoxic mechanism for the carcinogenicity of the environmental pollutant and suspected human carcinogen o-anisidine

2-methoxyaniline (o-anisidine) is an industrial and environmental pollutant and a bladder carcinogen for rodents. The mechanism of its carcinogenicity was investigated with 2 independent methods, 32P-postlabeling and 14C-labeled o-anisidine, to show that o-anisidine binds covalently to DNA in vitro after its activation by human hepatic microsomes. We also investigated the capacity of o-anisidine to form DNA adducts in vivo. Rats were treated i.p. with o-anisidine (0.15 mg/kg daily for 5 days) and DNA from several organs was analyzed by 32P-postlabeling. Two o-anisidine-DNA adducts, identical to those found in DNA incubated with o-anisidine and human microsomes in vitro, were detected in urinary bladder (4.1 adducts per 10(7) nucleotides), the target organ, and, to a lesser extent, in liver, kidney and spleen. These DNA adducts were identified as deoxyguanosine adducts derived from a metabolite of o-anisidine, N-(2-methoxyphenyl)hydroxylamine. This metabolite was identified in incubations with human microsomes. With 9 human hepatic microsomal preparations, we identified the specific CYP catalyzing the formation of the o-anisidine metabolites by correlation studies and by examining the effects of CYP inhibitors. On the basis of these analyses, oxidation of o-anisidine was attributed mainly to CYP2E1. Using recombinant human CYP (in Supersomes) and purified CYPs, the participation of CYP2E1 in o-anisidine oxidation was confirmed. In Supersomes, CYP1A2 was even more efficient in oxidizing o-anisidine than CYP2E1, followed by CYP2B6, 1A1, 2A6, 2D6 and 3A4. The results, the first report on the potential of the human microsomal CYP enzymes to activate o-anisidine, strongly suggest a carcinogenic potential of this rodent carcinogen for humans.

Toxicity and carcinogenicity studies of chlorpromazine hydrochloride and p-cresidine in the p53 heterozygous mouse model

The carcinogenic potential of chlorpromazine hydrochloride, a psychotropic agent, was assessed in the p53 heterozygous mouse assay. In a 4-week dose range finding study in p53 wild-type mice, doses of 20,40, 60, and 80 mg/kg were poorly tolerated because of mortality secondary to the severe sedative and hypotensive effects of chlorpromazine. Based on 40% mortality at a dose of 20 mg/kg in the dose-range finding study, a high dose of 10 mg/kg was chosen for the 26-week carcinogenicity study in p53 heterozygous mice. Doses of 2.5, 5, and 10 mg/kg chlorpromazine hydrochloride were well tolerated in the 26-week study. The administration of chlorpromazine hydrochloride at dose levels up to and including 10 mg/kg to p53 heterozygous and wild-type mice did not result in a dose-related increase in tumor incidence or in the type of tumors seen in comparison to controls. Findings related to the administration of chlorpromazine in the 26-week study were limited to minimal uterine and ovarian atrophy in p53 wild-type mice dosed with 10 mg/kg chlorpromazine hydrochloride. However, p53 heterozygous mice administered 400 mg/kg p-cresidine, a genotoxic carcinogen commonly used as a positive control for this model, developed urinary bladder tumors. Administration of p-cresidine also resulted in a regenerative anemia, splenic and hepatic hemosiderosis, renal findings, and ovarian and uterine atrophy. This study demonstrated that chlorpromazine hydrochloride, at the doses tolerated, was not carcinogenic in the p53 heterozygous mouse assay.

The role of metals in site-specific DNA damage with reference to carcinogenesis

We reviewed the mechanism of oxidative DNA damage with reference to metal carcinogenesis and metal-mediated chemical carcinogenesis. On the basis of the finding that chromium (VI) induced oxidative DNA damage in the presence of hydrogen peroxide (H2O2), we proposed the hypothesis that endogenous reactive oxygen species play a role in metal carcinogenesis. Since then, we have reported that various metal compounds, such as cobalt, nickel, and ferric nitrilotriacetate, directly cause site-specific DNA damage in the presence of H2O2. We also found that carcinogenic metals could cause DNA damage through indirect mechanisms. Certain nickel compounds induced oxidative DNA damage in rat lungs through inflammation. Endogenous metals, copper and iron, catalyzed ROS generation from various organic carcinogens, resulting in oxidative DNA damage. Polynuclear compounds, such as 4-aminobiphenyl and heterocyclic amines, appear to induce cancer mainly through DNA adduct formation, although their N-hydroxy and nitroso metabolites can also cause oxidative DNA damage. On the other hand, mononuclear compounds, such as benzene metabolites, caffeic acid, and o-toluidine, should express their carcionogenicity through oxidative DNA damage. Metabolites of certain carcinogens efficiently caused oxidative DNA damage by forming NADH-dependent redox cycles. These findings suggest that metal-mediated oxidative DNA damage plays important roles in chemical carcinogenesis.

Mechanism of peroxidase-mediated oxidation of carcinogenic o-anisidine and its binding to DNA

2-Methoxyaniline (o-anisidine) is a urinary bladder carcinogen in both mice and rats. Since the urinary bladder contains substantial peroxidase activity, we investigated the metabolism of this carcinogen by prostaglandin H synthase (PHS), a prominent enzyme in the urinary bladder, and lactoperoxidase as model mammalian peroxidases. Horseradish peroxidase (HRP)-mediated oxidation of o-anisidine was also determined and compared with the reactions catalyzed by mammalian peroxidases. All three peroxidases oxidized o-anisidine via a radical mechanism. Using HPLC combined with electrospray tandem mass spectrometry, we determined that peroxidases oxidized o-anisidine to a diimine metabolite, which subsequently hydrolyzed to form a quinone imine. Two additional metabolites were identified as a dimer linked by an azo bond and another metabolite consisting of three methoxybenzene rings, which exact structure has not been identified as yet. Using [14C]-labeled o-anisidine, we observed substantial peroxidase-dependent covalent binding of o-anisidine to DNA, tRNA and polydeoxynucleotides [poly(dX)]. The 32P-postlabeling assay (a standard procedure and enrichment of adducts by digestion with nuclease P1 or by extraction into 1-butanol prior to 32P-labeling) was employed as the second method to detect and quantitate binding of o-anisidine to DNA. Using these versions of the 32P-postlabeling technique we did not observe any DNA adducts derived from o-anisidine. The o-anisidine-DNA adducts became detectable only when DNA modified by o-anisidine was digested using three times higher concentrations of micrococcal nuclease and spleen phosphodiesterase (MN/SPD). We found deoxyguanosine to be the target for o-anisidine binding in DNA using poly(dX) and deoxyguanosine 3'-monophosphate (dGp). A diimine metabolite of o-anisidine is the reactive species forming adducts in dGp. The results strongly indicate that peroxidases play an important role in o-anisidine metabolism to reactive species, which might be responsible for its genotoxicity, and its carcinogenicity to the urinary bladder in rodents. The limitation of the 32P-postlabeling technique to analyze DNA adducts derived from o-anisidine as a means to estimate its genotoxicity is discussed.

Oxidative DNA damage induced by a metabolite of carcinogenic o-anisidine: enhancement of DNA damage and alteration in its sequence specificity by superoxide dismutase

The mechanism of DNA damage by a metabolite of the carcinogen o-anisidine in the presence of metals was investigated by the DNA sequencing technique using 32P-labeled human DNA fragments. The o-anisidine metabolite, o-aminophenol, caused DNA damage in the presence of Cu(II). The DNA damage was inhibited by catalase and bathocuproine, suggesting the involvement of H2O2 and Cu(I). The formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine by o-aminophenol increased in the presence of Cu(II). We conclude that Cu(II)-mediated oxidative DNA damage by this o-anisidine metabolite seems to be relevant for the expression of the carcinogenicity of o-anisidine. o-Aminophenol plus Cu(II) caused preferential DNA damage at the 5'-site guanine of GG and GGG sequences. When CuZn-SOD or Mn-SOD was added, the DNA damage was enhanced and its predominant cleavage sites were changed into thymine and cytosine residues. We consider that SOD may increase the frequency of mutations due to DNA damage induced by o-aminophenol and thus increase its carcinogenic potential.

The comet assay with multiple mouse organs: comparison of comet assay results and carcinogenicity with 208 chemicals selected from the IARC monographs and U.S. NTP Carcinogenicity Database

The comet assay is a microgel electrophoresis technique for detecting DNA damage at the level of the single cell. When this technique is applied to detect genotoxicity in experimental animals, the most important advantage is that DNA lesions can be measured in any organ, regardless of the extent of mitotic activity. The purpose of this article is to summarize the in vivo genotoxicity in eight organs of the mouse of 208 chemicals selected from International Agency for Research on Cancer (IARC) Groups 1, 2A, 2B, 3, and 4, and from the U.S. National Toxicology Program (NTP) Carcinogenicity Database, and to discuss the utility of the comet assay in genetic toxicology. Alkylating agents, amides, aromatic amines, azo compounds, cyclic nitro compounds, hydrazines, halides having reactive halogens, and polycyclic aromatic hydrocarbons were chemicals showing high positive effects in this assay. The responses detected reflected the ability of this assay to detect the fragmentation of DNA molecules produced by DNA single strand breaks induced chemically and those derived from alkali-labile sites developed from alkylated bases and bulky base adducts. The mouse or rat organs exhibiting increased levels of DNA damage were not necessarily the target organs for carcinogenicity. It was rare, in contrast, for the target organs not to show DNA damage. Therefore, organ-specific genotoxicity was necessary but not sufficient for the prediction of organ-specific carcinogenicity. It would be expected that DNA crosslinkers would be difficult to detect by this assay, because of the resulting inhibition of DNA unwinding. The proportion of 10 DNA crosslinkers that was positive, however, was high in the gastrointestinal mucosa, stomach, and colon, but less than 50% in the liver and lung. It was interesting that the genotoxicity of DNA crosslinkers could be detected in the gastrointestinal organs even though the agents were administered intraperitoneally. Chemical carcinogens can be classified as genotoxic (Ames test-positive) and putative nongenotoxic (Ames test-negative) carcinogens. The Ames test is generally used as a first screening method to assess chemical genotoxicity and has provided extensive information on DNA reactivity. Out of 208 chemicals studied, 117 are Ames test-positive rodent carcinogens, 43 are Ames test-negative rodent carcinogens, and 30 are rodent noncarcinogens (which include both Ames test-positive and negative noncarcinogens). High positive response ratio (110/117) for rodent genotoxic carcinogens and a high negative response ratio (6/30) for rodent noncarcinogens were shown in the comet assay. For Ames test-negative rodent carcinogens, less than 50% were positive in the comet assay, suggesting that the assay, which detects DNA lesions, is not suitable for identifying nongenotoxic carcinogens. In the safety evaluation of chemicals, it is important to demonstrate that Ames test-positive agents are not genotoxic in vivo. This assay had a high positive response ratio for rodent genotoxic carcinogens and a high negative response ratio for rodent genotoxic noncarcinogens, suggesting that the comet assay can be used to evaluate the in vivo genotoxicity of in vitro genotoxic chemicals. For chemicals whose in vivo genotoxicity has been tested in multiple organs by the comet assay, published data are summarized with unpublished data and compared with relevant genotoxicity and carcinogenicity data. Because it is clear that no single test is capable of detecting all relevant genotoxic agents, the usual approach should be to carry out a battery of in vitro and in vivo tests for genotoxicity. The conventional micronucleus test in the hematopoietic system is a simple method to assess in vivo clastogenicity of chemicals. Its performance is related to whether a chemical reaches the hematopoietic system. Among 208 chemicals studied (including 165 rodent carcinogens), 54 rodents carcinogens do not induce micronuclei in mouse hematopoietic system despite the positive finding with one or two in vitro tests. Forty-nine of 54 rodent carcinogens that do not induce micronuclei were positive in the comet assay, suggesting that the comet assay can be used as a further in vivo test apart from the cytogenetic assays in hematopoietic cells. In this review, we provide one recommendation for the in vivo comet assay protocol based on our own data.

The alkaline single cell electrophoresis assay with eight mouse organs: results with 22 mono-functional alkylating agents (including 9 dialkyl N-nitrosoamines) and 10 DNA crosslinkers

The genotoxicity of 22 mono-functional alkylating agents (including 9 dialkyl N-nitrosoamines) and 10 DNA crosslinkers selected from IARC (International Agency for Research on Cancer) groups 1, 2A, and 2B was evaluated in eight mouse organs with the alkaline single cell gel electrophoresis (SCGE) (comet) assay. Groups of four mice were treated once intraperitoneally at the dose at which micronucleus tests had been conducted, and the stomach, colon, liver, kidney, bladder, lung, brain, and bone marrow were sampled 3, 8, and/or 24 h later. All chemicals were positive in the SCGE assay in at least one organ. Of the 22 mono-functional alkylating agents, over 50% were positive in all organs except the brain and bone marrow. The two subsets of mono-functional alkylating agents differed in their bone marrow genotoxicity: only 1 of the 9 dialkyl N-nitrosoamines was positive in bone marrow as opposed to 8 of the 13 other alkylating agents, reflecting the fact that dialkyl N-nitrosoamines are poor micronucleus inducers in hematopoietic cells. The two groups of mono-functional alkylating agents also differ in hepatic carcinogenicity in spite of the fact that they are similar in hepatic genotoxicity. While dialkyl N-nitrosoamines produce tumors primarily in mouse liver, only one (styrene-7,8-oxide) out of 10 of the other type of mono-functional alkylating agents is a mouse hepatic carcinogen. Taking into consideration our previous results showing high concordance between hepatic genotoxicity and carcinogenicity for aromatic amines and azo compounds, a possible explanation for the discrepancy might be that chemicals that require metabolic activation show high concordance between genotoxicity and carcinogenicity in the liver. A high percent of the 10 DNA crosslinkers were positive in the SCGE assay in the gastrointestinal mucosa, but less than 50% were positive in the liver and lung. In this study, we allowed 10 min alkali-unwinding to obtain low and stable control values. Considering that DNA crosslinking lesions can be detected as lowering of not only positive but also negative control values, low control values by short alkali-treatment might make it difficult to detect DNA crosslinking lesions. In conclusion, although both mono-functional alkylating agents and DNA crosslinkers are genotoxic in mouse multiple organs, the genotoxicity of DNA crosslinkers can be detected in the gastrointestinal organs even though they were given intraperitoneally followed by the short alkali-treatment.

The alkaline single cell gel electrophoresis assay with mouse multiple organs: results with 30 aromatic amines evaluated by the IARC and U.S. NTP

The genotoxicity of 30 aromatic amines selected from IARC (International Agency for Research on Cancer) groups 1, 2A, 2B and 3 and from the U.S. NTP (National Toxicology Program) carcinogenicity database were evaluated using the alkaline single cell gel electrophoresis (SCG) (Comet) assay in mouse organs. We treated groups of four mice once orally at the maximum tolerated dose (MTD) and sampled stomach, colon, liver, kidney, bladder, lung, brain, and bone marrow 3, 8 and 24 h after treatment. For the 20 aromatic amines that are rodent carcinogens, the assay was positive in at least one organ, suggesting a high predictive ability for the assay. For most of the SCG-positive aromatic amines, the organs exhibiting increased levels of DNA damage were not necessarily the target organs for carcinogenicity. It was rare, in contrast, for the target organs not to show DNA damage. Organ-specific genotoxicity, therefore, is necessary but not sufficient for the prediction of organ-specific carcinogenicity. For the 10 non-carcinogenic aromatic amines (eight were Ames test-positive and two were Ames test-negative), the assay was negative in all organs studied. In the safety evaluation of chemicals, it is important to demonstrate that Ames test-positive agents are not genotoxic in vivo. Chemical carcinogens can be classified as genotoxic (Ames test-positive) and putative non-genotoxic (Ames test-negative) carcinogens. The alkaline SCG assay, which detects DNA lesions, is not suitable for identifying non-genotoxic carcinogens. The present SCG study revealed a high positive response ratio for rodent genotoxic carcinogens and a high negative response ratio for rodent genotoxic non-carcinogens. These results suggest that the alkaline SCG assay can be usefully used to evaluate the in vivo genotoxicity of chemicals in multiple organs, providing for a good assessment of potential carcinogenicity.

Detection of nivalenol genotoxicity in cultured cells and multiple mouse organs by the alkaline single-cell gel electrophoresis assay

We tested the genotoxicity of nivalenol (NIV), a potent toxic trichothecene from Fusarium nivale, in cultured CHO cells and in several mouse organs and tissues (liver, kidney, thymus, bone marrow and mucosa of stomach, jejunum, and colon) using the alkaline single-cell gel electrophoresis (SCG, or Comet) assay. NIV at 50 and 100 micrograms/ml damaged the nuclear DNA of CHO cells in the absence of S9 mix, showing that NIV was a direct mutagen. In an in vivo study, mice were sacrificed 2, 4, and 8 h after either oral (20 mg/kg) or intraperitoneal (3.7 mg/kg) administration of NIV. DNA damage was measured by the SCG assay as modified by us. After oral dosing, DNA damage appeared in the kidney and bone marrow at 2 h (returning to almost control level within the following 2 h), and in the stomach, jejunum, and colon at 2, 4, and 8 h, respectively. Liver and thymus DNA were not damaged. After intraperitoneal injection, no DNA damage appeared in any of the organs or tissues tested except for the colon, where extensive DNA damage was observed, as in the oral study, at 8 h. For histopathological examination, mice were sacrificed 2, 4, and 8 h after oral (20 mg/kg) administration of NIV. No necrotic changes were detected in any of the organs where NIV yielded statistically significant DNA damage. To measure the effect of NIV on transport activity in mice, 10 ml/kg (same volume as NIV treatments) of 1% brilliant blue FCF (BB) was administered orally. Thirty minutes later, the BB reached the colon, and simultaneous oral administration of NIV (20 mg/kg, dissolved in 10 ml BB solution) did not affect the dye transport rate. Thus, the strong yet delayed damage to colon DNA may follow from a systemic absorption rather than a topical effect. As a direct mutagen, NIV showed organ specific genotoxicity in mice in time and intensity.

Colon-specific genotoxicity of heterocyclic amines detected by the modified alkaline single cell gel electrophoresis assay of multiple mouse organs

The in vivo genotoxicity of five heterocyclic amines-Trp-P-2 (13 mg/kg), IQ (13 mg/kg), MeIQ (13 mg/kg), MeIQx (13 mg/kg), and PhIP (40 mg/kg)-in the mucosa of gastrointestinal and urinary tract organs (stomach, duodenum, jejunum, ileum, colon, and bladder) was studied by the alkaline single cell gel electrophoresis (SCG) (Comet) assay. Male CD-1 mice were sacrificed 1, 3, and 8 h after intraperitoneal injection. All the heterocyclic amines studied yielded statistically significant DNA damage in the colon but not the small intestine (duodenum, jejunum, and ileum) or urinary bladder. In this study, five heterocyclic amines were injected intraperitoneally to avoid the consequences of ingestion. Thus, the extensive damage to colon DNA was concluded to be due, at least in part, to a systemic effect.