Liver DNA alkylation after a single carcinogenic dose of dimethylnitrosamine to newborn and adult CFW Swiss mice

Genotoxic sensitivity of the developing hematopoietic system

Genotoxic sensitivity seems to vary during ontogenetic development. Animal studies have shown that the spontaneous mutation rate is higher during pregnancy and infancy than in adulthood. Human and animal studies have found higher levels of DNA damage and mutations induced by mutagens in fetuses/newborns than in adults. This greater susceptibility could be due to reduced DNA repair capacity. In fact, several studies indicated that some DNA repair pathways seem to be deficient during ontogenesis. This has been demonstrated also in murine hematopoietic stem cells. Genotoxicity in the hematopoietic system has been widely studied for several reasons: it is easy to assess, deals with populations cycling also in the adults and may be relevant for leukemogenesis. Reviewing the literature concerning the application of the micronucleus test (a validated assay to assess genotoxicity) in fetus/newborns and adults, we found that the former show almost always higher values than the latter, both in animals treated with genotoxic substances and in those untreated. Therefore, we draw the conclusion that the genotoxic sensitivity of the hematopoietic system is more pronounced during fetal life and decreases during ontogenic development.

Gene-TEQ--a standardized comparative assessment of effects in the comet assay using genotoxicity equivalents

Existing methods for the comparison of genotoxic effects in the comet assay bear considerable disadvantages such as the problem to link information about concentration dependence and severity of effects. Moreover, given the lack of standardized protocols and the use of various standards, it may be extremely difficult to compare different studies. In order to provide a method for standardized comparative assessment of genotoxic effects, the concept of genotoxicity equivalents (Gene-TEQ) was developed. As potential reference compounds for genotoxic effects, three directly acting (N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), methyl-methanesulfonate, and N-methyl-N-nitrosourea) and three indirectly acting (cyclophosphamide, dimethylnitrosamine, and 4-nitroquinoline-oxide) genotoxic substances were compared with respect to their cytotoxic (neutral red) and genotoxic (comet assay) concentration-response profiles in the permanent fish cell line RTL-W1. For further comparison, two sediment extracts from the upper Danube River were investigated as environmental samples. Based on the results of cytotoxicity and genotoxicity testing, MNNG was selected as the reference compound. At several exposure levels and durations, genotoxic effects of both the other pure substances and the environmental samples were calculated as percentages of the maximum MNNG effect and related to the absolute MNNG effect (EC values). Thus, genotoxicity equivalent factors (Gene-TEQs) relative to MNNG could be calculated. Gene-TEQs can easily be applied to pure substances, mixtures and field samples to provide information about their toxicity relative to the reference compound. Furthermore, the Gene-TEQ concept allows a direct comparison of environmental samples from different laboratories.

Differential carcinogenicity of cigarette smoke in mice exposed either transplacentally, early in life or in adulthood

Cigarette smoke (CS) plays a dominant role in the epidemiology of human cancer. However, it is difficult to reproduce its carcinogenicity in laboratory animals. Recently, we showed that CS becomes a potent carcinogen in mice when exposure starts soon after birth. In our study, we comparatively evaluated the carcinogenic response to mainstream CS in mice at different ages. Neonatal mice were exposed daily for 4 months to CS, starting within 12 hr after birth, and sacrificed at 8 months. Adult mice were exposed for the same time period (3-7 months) and sacrificed at 11 months. Other mice were exposed transplacentally or both transplacentally and early in life. A total of 351 neonatal mice and 80 adult Swiss H mice were used. With varying intensity depending on age, CS induced pulmonary emphysema, bronchial and alveolar epithelial hyperplasia, blood vessel proliferation and hemangiomas and microadenomas in lung as well as parenchymal degeneration of liver. Histopathological alterations of kidney were only observed in mice exposed to CS early in life. Lung adenomas and malignant tumors of various histopathological nature were detected in neonatally exposed mice but not in adults. Transplacental CS induced the formation of lung adenomas in the offspring 8 months after birth. Previous exposure during pregnancy attenuated CS-related alveolar epithelial hyperplasia induced after birth. In conclusion, the carcinogenic response to CS varies depending on the developmental stage. The early postnatal life and the prenatal life are particularly at risk for the later development of CS-related tumors.

Prenatal N-acetylcysteine prevents cigarette smoke-induced lung cancer in neonatal mice

Certain adult diseases may have their origin early in life, and perinatal exposures may contribute to cancers both during childhood and later in life. We recently demonstrated that mainstream cigarette smoke (MCS) induces a potent carcinogenic response in mice when exposure starts soon after birth. We also showed that the antioxidant N-acetylcysteine (NAC) prevents the extensive nucleotide and gene expression alterations that occur 'physiologically' at birth in mouse lung. The present study was designed to evaluate whether administration of NAC during pregnancy may affect the yield of tumors in mice exposed to MCS, starting after birth and continuing for 120 days. The results obtained showed that 210 days after birth, one adenoma only was detectable in sham-exposed mice. In contrast, as much as the 61.1% (33/54) of MCS-exposed mice born from untreated dams had lung tumors, including both benign tumors and bronchoalveolar carcinomas. Treatment with NAC during pregnancy strikingly inhibited the formation of benign lung tumors and totally prevented occurrence of carcinomas. In addition, prenatal NAC inhibited the MCS-induced hyperplasia of the urinary bladder epithelium. These findings demonstrate for the first time that treatment during pregnancy with an antioxidant chemopreventive agent can affect the induction of tumors consequent to exposure to a carcinogen after birth.

Use of the alkaline comet assay for the detection of transplacental genotoxins in newborn mice

Several lines of evidence show that in utero exposure to different toxicants has greater consequences than their exposure during adult life. This may be due to involvement of critical developmental stages, physiological immaturity and the long later-life span over which disease may initiate, develop and progress. The in vivo alkaline comet (single-cell gel electrophoresis) assay has been favoured by the scientific community for the evaluation of genotoxins. The objective of this study was to demonstrate the suitability of alkaline comet assay in detecting transplacental genotoxins using newborn mice. Here, we report the successful use of the comet assay in detecting multi-organ genotoxicity of known transplacental genotoxins in newborn mice. Three well known transplacental genotoxic agents, cyclophosphamide (CP), mitomycin-C (MMC) and zidovudine (AZT) were tested in pregnant Swiss mice. These compounds were administered in the late gestational period (16-20th days of pregnancy) and the comet assay was performed with lymphocytes, bone marrow, liver and kidney cells of newborn mice. Significant DNA damage was observed in all the tissues with tested transplacental genotoxins. The results of the comet assay were confirmed by the micronucleus (MN) assay of the peripheral blood of newborn mice. The results of this study provide sufficient evidence that the comet assay can be applied successfully for the detection of transplacental genotoxins in newborn mice.

Environmental genotoxicants/carcinogens and childhood cancer: Bridgeable gaps in scientific knowledge

Cancer in children is a major concern in many countries. An important question is whether these childhood cancers are caused by something, or are just tragic random events. Causation of at least some children's cancers is suggested by direct and indirect evidence, including epidemiological data, and animal studies that predict early life sensitivity of humans to carcinogenic effects. Candidate risk factors include genotoxic agents (chemicals and radiation), but also diet/nutrition, and infectious agents/immune responses. With regard to likelihood of risks posed by genotoxicants, there are pros and cons. The biological properties of fetuses and infants are consistent with sensitivity to preneoplastic genotoxic damage. Recent studies of genetic polymorphisms in carcinogen-metabolizing enzymes confirm a role for chemicals. On the other hand, in numerous epidemiological studies, associations between childhood cancers and exposure to genotoxicants, including tobacco smoke, have been weak and hard to reproduce. Possibly, sensitive genetic or ontogenetic subpopulations, and/or co-exposure situations need to be discovered to allow identification of susceptible individuals and their risk factors. Among the critical knowledge gaps needing to be bridged to aid in this effort include detailed tissue and cellular ontogeny of carcinogen metabolism and DNA repair enzymes, and associations of polymorphisms in DNA repair enzymes with childhood cancers. Perinatal bioassays in animals of specific environmental candidates, for example, benzene, could help guide epidemiology. Genetically engineered animal models could be useful for identification of chemical effects on specific genes. Investigations of interactions between factors may be key to understanding risk. Finally, fathers and newborn infants should receive more attention as especially sensitive targets.

The passage of trimethylamine across rat and human skin

Trimethylamine is a volatile low molecular weight tertiary aliphatic amine that has known toxicity and the potential for human exposure from industrial and environmental sources is considerable. It is generally believed that absorption across the skin is an unimportant route of entry but there is little, if any, supporting evidence for this assumption. Passage across rat and human skin has been investigated employing excised skin circles in an in vitro diffusion cell apparatus. Trimethylamine was found to penetrate readily when applied to the epidermal surface of skin at three different dose levels (0.1, 1.0 and 10 mg per skin membrane 0.32 cm2). The apparent dermal flux was calculated as 3.40 +/- 1.60, 58.3 +/- 30.6 and 265.0 +/- 155.0 microg/cm2/h for rat and 0.98 +/- 0.75, 9.21 +/- 3.06 and 92.7 +/- 31.9 microg/cm2/h for human at the three dose levels, respectively. Both rat and human skin was able to act as a reservoir, with the trimethylamine not remaining in the stratum corneum but passing through. When presented to the underneath of rat and human skin circles, both [U-14C]-trimethylamine and [U-14C]-trimethylamine N-oxide were able to pass from the dermis to the epidermis. Small but detectable amounts of trimethylamine were oxidised to its N-oxide during passage through the skin.

Assessing cancer risks from short-term exposures in children

For the vast majority of chemicals that have cancer potency estimates on IRIS, the underlying database is deficient with respect to early-life exposures. This data gap has prevented derivation of cancer potency factors that are relevant to this time period, and so assessments may not fully address children's risks. This article provides a review of juvenile animal bioassay data in comparison to adult animal data for a broad array of carcinogens. This comparison indicates that short-term exposures in early life are likely to yield a greater tumor response than short-term exposures in adults, but similar tumor response when compared to long-term exposures in adults. This evidence is brought into a risk assessment context by proposing an approach that: (1) does not prorate children's exposures over the entire life span or mix them with exposures that occur at other ages; (2) applies the cancer slope factor from adult animal or human epidemiology studies to the children's exposure dose to calculate the cancer risk associated with the early-life period; and (3) adds the cancer risk for young children to that for older children/adults to yield a total lifetime cancer risk. The proposed approach allows for the unique exposure and pharmacokinetic factors associated with young children to be fully weighted in the cancer risk assessment. It is very similar to the approach currently used by U.S. EPA for vinyl chloride. The current analysis finds that the database of early life and adult cancer bioassays supports extension of this approach from vinyl chloride to other carcinogens of diverse mode of action. This approach should be enhanced by early-life data specific to the particular carcinogen under analysis whenever possible.

Hamster and rat fetal cells have low spontaneous mutation frequencies and rates

Somatic cells of whole Syrian hamster fetuses (gestation day 13) were isolated and tested by an in vivo/in vitro mutation assay for spontaneous mutation frequencies using independent 6-thioguanine (6-TG), diphtheria toxin (DT), and ouabain mutation selection systems. Optimum conditions were ascertained. For 6-TG mutants, a total of 21 mutants were found in cells from 24 litters on 1993 plates, for an overall mutant frequency of 1.8 x 10(-7) per viable cell with 12 positive litters. In all, 26 litters were tested using DT; 77 mutants were found in 840 plates, yielding an overall mutant frequency of 2.6 x 10(-7), with 20 positive litters. No correlations or familial effects were found among 23 litters tested for both DT and 6-TG. Of 14 litters which were tested for ouabain mutants, 4 were positive, with a total of 5 mutants found on 988 plates, for an overall mutant frequency of 7.6 x 10(-8). For 14 F344 rat fetuses, the overall 6-TG spontaneous mutation frequency was determined to be 1.6 x 10(-7). From the data, estimates of mutation rates were calculated. For mutation to 6-TG resistance the rate was 8.3 x 10(-8), for mutation to DT resistance the rate was 8.1 x 10(-8) and for ouabain, the spontaneous mutation rate was 5.7 x 10(-8). For F344 rat, the spontaneous mutation rate was 1.1 x 10(-7). Induced mutant frequencies after in utero exposure to 1 mmol/kg N-ethyl-N-nitrosourea (ENU) were 311, 135 and 200 times the spontaneous value for 6-TG, DT and ouabain, respectively, for Syrian hamster fetal cells and 125 times the spontaneous 6-TG value for fetal F344 rat cells. Both spontaneous mutation frequencies and underlying spontaneous mutation rates are low, consistent with the view that fetal cells exercise extremely tight control over DNA fidelity.

Coexposure to ethanol with N-nitrosodimethylamine or 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone during lactation of rats: marked increase in O(6)-methylguanine-DNA adducts in maternal mammary gland and in suckling lung and kidney

Use of alcoholic beverages increases risk of cancer at several target sites, including the breast. Of several possible mechanisms for this effect, competitive inhibition by ethanol of hepatic clearance of nitrosamines, resulting in increased dose delivery to posthepatic tissues, gives the quantitatively most pronounced enhancement. We investigated whether this effect would pertain to the mammary gland, and to ethanol and nitrosamines delivered translactationally to sucklings. Ethanol (1.6 g/kg) was administered by gavage to nursing Sprague-Dawley rats 10 min before 5 mg/kg N-nitrosodimethylamine (NDMA) or 50 mg/kg 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK); treatment was on postnatal days 1, 7, or 14. Tissues taken 4 h later for analysis of O(6)-methylguanine in DNA were liver, blood, and mammary glands from the mothers, and liver, lung, kidney, and blood from the sucklings. Ethanol cotreatment resulted in a marked, 10-fold increase in O(6)-methylguanine adducts from NDMA in mammary gland, as well as smaller but significant increases in this tissue from NNK and in maternal blood cells from both chemicals; adducts in maternal liver decreased slightly. In the sucklings, ethanol cotreatment also lowered adducts in liver after NDMA or NNK treatment. After NDMA, adducts were also detected in suckling lung and kidney and were increased five- to 10-fold after ethanol coexposure. Adducts from either chemical, with or without ethanol, decreased markedly in all suckling tissues with development from postnatal day 1 to day 14. Thus ethanol coexposure with nitrosamines increases O(6)-methylguanine DNA adducts in mammary gland and strongly influences adduct formation in suckling tissues after translactational delivery.

Metabolic activation capacity of neonatal mice in relation to the neonatal mouse tumorigenicity bioassay

The neonatal mouse tumorigenicity bioassay is a well-developed animal model that has recently been recommended as an alternative tumorigenicity bioassay by the International Conference on Harmonization (ICH) for Technical Requirements for the Registration of Pharmaceuticals for Human Use. There are sufficient data to conclude that this animal model is highly sensitive to genotoxic chemical carcinogens that exert their tumorigenicity through mechanisms involving the formation of covalently bound exogenous DNA adducts that lead to mutation. On the other hand, it is not sensitive to chemical carcinogens that exert tumorigenicity through a secondary mechanism. The metabolizing enzymes present in the neonatal mouse, particularly the cytochromes P450, are critical factors in determining the tumorigenic potency of a chemical tested in this bioassay. However, compared to the metabolizing enzymes of the adult mouse and rat, the study of the metabolizing enzymes in neonatal mouse tissues has been relatively limited.

Susceptibility of C57BL/6 mice to tumorigenicity induced by dimethylnitrosamine and 2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine in the neonatal bioassay

Male C57BL/6 neonates were treated on days 8 and 15 with 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP, 6.5 or 26.2 mg/kg) or dimethylnitrosamine (DMN, 2.6 or 10.5 mg/kg). No tumors were seen in PhIP-treated animals at 15 months of age. Liver and lung tumor incidences in DMN-treated animals were 67-79 and 0-7%, respectively. In comparison with data from other strains, our results indicate that (1) neonatally-treated C57BL/6 mice are resistant to the induction of liver and lung tumors by PhIP and lung tumors by DMN and (2) the susceptibility of this strain to induced liver tumors correlates with the activity of hepatic DMN N-demethylase and PhIP N-hydroxylase in the (untreated) neonates.

Dimethylamine in human urine

The urinary excretion of dimethylamine has been measured in 203 unrelated healthy volunteers (102 male) who maintained their normal diets. The results for female volunteers are the first reported in the literature. The average daily output was 17.43 +/- 11.80 mg (mean +/- S.D.) (21.21 +/- 14.78 male; 13.74 +/- 5.65 female) with values for the majority of the population lying within the 0.68-35.72 mg range. Four male outliers excreted up to 109.2 mg; these large amounts of dimethylamine were presumed to be of dietary origin. The literature pertaining to urinary levels of dimethylamine has been summarised and integrated with the present observations.

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.

Fate of dimethylamine in man

1. The fate of [14C]-dimethylamine was investigated following oral administration to four male volunteers. 2. The major route of excretion was urine, with 94% of the administered radioactivity being voided over 3 days (87% during the first 24 h). Small amounts (1-3%) of radioactivity were found in the faeces and expired air. 3. Metabolism was limited with only 5% being demethylated to methylamine. The remainder of the dose was excreted unchanged. 4. Pharmacokinetic studies indicated rapid (t1/2ab = 8 min) and extensive absorption (bioavailability = 82%) from the gastrointestinal tract followed by widespread distribution and a fairly prompt excretion (t1/2el = 6-7 h) with a plasma clearance of 190 ml/min.