Promotion by Helicobacter hepaticus-induced hepatitis of hepatic tumors initiated by N-nitrosodimethylamine in male A/JCr mice

A new murine Helicobacter species, Helicobacter hepaticus, infects the livers of mice, causing a progressive chronic active hepatitis culminating in hepatocellular tumors. To examine the role of chronic H. hepaticus infection in carcinogenesis, H. hepaticus-infected male infant mice of A/JCr strain were given a single i.p. dose of N-nitrosodimethylamine (NDMA). Noninfected A/J mice similarly treated with NDMA served as controls. The effect of hepatitis induced by H. hepaticus was studied for 64 wk. At 31-36 wk, the incidence of hepatocellular adenomas in infected mice was significantly higher than in noninfected mice (82 vs 52%; p = 0.05). The multiplicity of hepatocellular tumors was also significantly higher in infected mice compared to noninfected mice (3.2 +/- 0.09 vs 0.09 +/- 0.2; p = 0.03). At 51-64 wk, many (10/18) infected mice developed hepatocellular carcinomas while only 2 of 19 control mice developed such tumors (p = 0.005). Overexpression of cyclin D was observed in hepatocytes as well as adenomas induced by NDMA in H. hepaticus-infected mice, suggesting its role in inflammation, abnormal cell growth, and early neoplasia. High molecular weight keratins were highly expressed in hyperplastic oval cells in hepatitis and in liver tumors in mice with hepatitis, establishing a reliable marker for oval cells in formalin-fixed, paraffin-embedded tissue. Thus, chronic H. hepaticus infection significantly stimulated cyclin D expression, accelerated the development of liver tumors, increased the multiplicity of such lesions, and enhanced the progression of benign to malignant tumors.

DNA adducts and the mechanism of carcinogenesis and cytotoxicity of methylating agents of environmental and clinical significance

DNA adducts are covalent complexes formed between genotoxic carcinogens and DNA bases, and constitute a critical early intermediate on the pathway of chemical carcinogenesis. Their accumulation in different tissues reflects the amount of activated carcinogen reaching DNA, and can therefore serve as an index of the biologically relevant dose reaching the target tissues or cells. Methylating agents are of interest in view of their occurrence in the environment and their use as cytotoxic drugs in cancer chemotherapy. Current evidence indicates that O6-methylguanine plays a particularly important role in the mutagenic, carcinogenic, and cytotoxic activities of methylating agents. O6-Methylguanine is repaired efficiently by the enzyme O6-alkylguanine-DNA alkyltransferase (AGT). Lack of this enzyme results in excessive accumulation of O6-methylguanine and recent evidence suggests that significant quantitative effects on adduct accumulation may be linked to conditions of very low AGT levels. This would be important from the point of view of clinical practice, since modulation of AGT is under investigation as a means of enhancing the therapeutic efficacy of clinical agents acting via the production of O6-methylguanine and related adducts, such as, for example, procarbazine, dacarbazine, and some nitrosoureas. The measurement of O6-methylguanine in human DNA has been employed as a tool to investigate the role of environmental methylating agents in human carcinogenesis. While the nature and origin of the methylating agents responsible for these adducts is currently unknown, recent studies in patas monkeys have shown that N-nitrosodimethylamine, a methylating carcinogen to which human exposure is well documented, is capable of efficiently generating O6-methylguanine in most tissues, including fetal tissues. Furthermore, it has been found that this damage is substantially enhanced by the coadministration of ethyl alcohol which acts by inhibiting the liver first-pass metabolism of the carcinogen, an observation which supports the hypothesis that alcohol consumption may act as a risk factor in human carcinogenesis by augmenting the action of nitrosamines.

Proliferative lesions of oviduct and uterus in CD-1 mice exposed prenatally to tamoxifen

Tamoxifen (TAM) is widely used as adjuvant breast cancer therapy after surgery and as a chemopreventive agent in women of child-bearing age. However, TAM therapy has been shown to result in an increased incidence of endometrial carcinoma in women. The present study was designed to investigate the effects of TAM (5 mg/kg and 7.5 mg/kg body wt) given i.g. to pregnant CD-1 mice (1x/day, days 12 through 18 of gestation) on their female offspring. Progressive proliferative hyperplasia of the oviduct was frequently seen in TAM-exposed offspring, reaching 100% incidence by 52 weeks in both treatment groups. These females also developed progressive proliferative uterine lesions, including moderate/severe cystic endometrial hyperplasia (34-50%) and polypoid adenomas (27-30%) between 53 and 78 weeks. Deciduomas (15%) occurred at young ages (12 and 24 weeks) while leiomyomas (14%), a malignant leiomyosarcoma, and ovarian granulosa cell tumors (14%), were found between 72 and 78 weeks. Our findings thus suggest a strong association between transplacental TAM and reproductive tract abnormalities in female CD-1 mice.

Increased oxidative DNA damage and hepatocyte overexpression of specific cytochrome P450 isoforms in hepatitis of mice infected with Helicobacter hepaticus

A recently discovered bacterium, Helicobacter hepaticus, infects the intrahepatic bile canaliculi of mice, causing a severe chronic hepatitis culminating in liver cancer. Thus, it affords an animal model for study of bacteria-associated tumorigenesis including H. pylori-related gastric cancer. Reactive oxygen species are often postulated to contribute to this process. We now report that hepatitis of male mice infected with H. hepaticus show significant increases in the oxidatively damaged DNA deoxynucleoside 8-hydroxydeoxyguanosine, with the degree of damage increasing with progression of the disease. Perfusion of infected livers with nitro blue tetrazolium revealed that superoxide was produced in the cytoplasm of hepatocytes, especially in association with plasmacytic infiltrates near portal triads. Contrary to expectations, Kupffer cells, macrophages, and neutrophils were rarely involved. However, levels of cytochrome P450 (CYP) isoforms 1A2 and 2A5 in hepatocytes appeared to be greatly increased, as indicated by the number of cells positive in immunohistochemistry and the intensity of staining in many cells, concomitant with severe hepatitis. The CYP2A5 immunohistochemical staining co-localized with formazan deposits resulting from nitro blue tetrazolium reduction and occurred in nuclei as well as cytoplasm. These findings suggest that CYP2A5 contributes to the superoxide production and 8-hydroxydeoxyguanosine formation, although reactive oxygen species from an unknown source in the hepatocytes leading to CYP2A5 induction or coincidental occurrence of these events are also possibilities. Three glutathione S-transferase isoforms, mGSTP1-1 (pi), mGSTA1-1 (YaYa), and mGSTA4-4, also showed striking increases evidencing major oxidative stress in these livers.

Uptake and tissue distribution of chromium(III) in mice after a single intraperitoneal or subcutaneous administration

The tissue levels of chromium were followed after single intraperitoneal or subcutaneous injection of 1 mmol CrCl3/kg body wt. in Swiss male mice. Blood levels were similar after both treatment modes, with half-lives of 31-41 h. Organs not directly exposed by i.p. treatment contained similar amounts in the two groups, with kidneys > lungs > heart > brain. However, after i.p. treatment peritoneal organs (liver, spleen, pancreas and testis) had 40- to 200-fold more chromium compared with s.c. Assay of subsurface liver tissue and of testes removed via the scrotum indicated infiltration of the organs, rather than surface adsorption, of peritoneal chromium. Relative chromium concentrations after i.p. treatment were liver > pancreas = spleen > testis and after s.c. liver > spleen > testis > pancreas. Thus, s.c. treatment with CrCl3 is as effective as i.p. in terms of absorption into the blood. Treatment i.p., leading to direct uptake into peritoneal organs, is an effective way to deliver high chromium doses to these organs, but does not model likely human exposure.

Promotion of dimethylbenz[a]anthracene-initiated mammary carcinogenesis by iron in female Sprague-Dawley rats

Iron body-stores and iron dietary intake have been sporadically reported to increase the risk of cancer in humans. To investigate the effect of iron on the development of mammary tumors, female Sprague-Dawley rats were given dimethylbenz[a]anthracene (DMBA) (5 mg/kg, i.g., 1x) at 55 days of age. Eight days later, rats received iron(II) sulfate s.c. (50 micromol/kg, 2x/week) for 53 weeks. Mammary tumors started to appear 6-8 weeks after DMBA initiation. At 20 weeks after DMBA treatment, iron(II) increased mammary tumor frequency twofold (11/30 versus 5/30 with DMBA alone). Tumor frequency increased with time and was significantly higher in iron-promoted rats after 40 weeks of treatment (24/30 versus 11/30, P = 0.001). Also, mammary tumors in iron-promoted rats were significantly larger than in DMBA-only rats at 20 weeks after initiation (P = 0.04) and this difference remained significant through the observation time point at 40 weeks. Iron could be detected histochemically in the stromal connective tissue, but not in the epithelial cells of mammary carcinomas. Mammary tumors in the DMBA-only group were mostly adenomas and adenocarcinomas, while those promoted by iron sulfate included fibroadenomas, adenomas and adenocarcinomas. Thus, iron(II) administered s.c. subsequent to DMBA initiation, greatly accelerated mammary carcinogenesis, implying its promoting activity for mammary tissue of female rats.

CYP2E1 genetic polymorphisms and risk of nasopharyngeal carcinoma in Taiwan

BACKGROUND

Nasopharyngeal carcinoma occurs disproportionately among individuals of Chinese descent. The cytochrome P450 2E1 enzyme (CYP2E1) is known to activate nitrosamines and other carcinogens that are possibly involved in the development of this disease. Certain alleles of the CYP2E1 gene are thought to be more highly expressed than others, and their distribution varies between Asian and Caucasian populations. We conducted a case-control study to investigate whether such variations affect the risk of developing nasopharyngeal cancer.

METHODS

Three hundred sixty-four patients with nasopharyngeal carcinoma (96% of 378 eligible patients) and 320 control subjects (86% of 374 eligible subjects) were studied. A risk factor questionnaire was administered to participants to assess factors postulated to be linked to nasopharyngeal carcinoma. Peripheral blood was obtained from all subjects and DNA was purified from nucleated cells. A polymerase chain reaction-based restriction fragment length polymorphism assay that used the restriction enzymes Rsa I and Dra I was used to detect wild-type and variant forms of the CYP2E1 gene.

RESULTS

Individuals homozygous for an allele of the CYP2E1 gene that is detected by Rsa I digestion (c2 allele) were found to have an increased risk of nasopharyngeal carcinoma (relative risk [RR] = 2.6; 95% confidence interval [CI] = 1.2-5.7); this effect was limited to nonsmokers (RR = 9.3; 95% CI = 2.7-32) and was not affected by alcohol consumption.

CONCLUSIONS

Our findings suggest that the CYP2E1 genotype is a determinant of nasopharyngeal carcinoma risk.

Enzymes involved in the bioactivation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone in patas monkey lung and liver microsomes

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a potent tobacco-specific carcinogen in animals. Our previous studies indicated that there are differences between rodents and humans for the enzymes involved in the activation of NNK. To determine if the patas monkey is a better animal model for the activation of NNK in humans, we investigated the metabolism of NNK in patas monkey lung and liver microsomes and characterized the enzymes involved in the activation. In lung microsomes, the formation of 4-oxo-1-(3-pyridyl)-1-butanone (keto aldehyde), 4-(methylnitrosamino)-1-(3-pyridyl-N-oxide)-1-butanone (NNK-N-oxide), 4-hydroxy-1-(3-pyridyl)-1-butanone (keto alcohol), and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) was observed, displaying apparent Km values of 10.3, 5.4, 4.9, and 902 microM, respectively. NNK metabolism in liver microsomes resulted in the formation of keto aldehyde, keto alcohol, and NNAL, displaying apparent Km values of 8.1, 8.2, and 474 microM, respectively. The low Km values for NNK oxidation in the patas monkey lung and liver microsomes are different from those in human lung and liver microsomes showing Km values of 400-653 microM, although loss of low Km forms from human tissue as a result of disease, surgery or anesthesia cannot be ruled out. Carbon monoxide (90%) significantly inhibited NNK metabolism in the patas monkey lung and liver microsomes by 38-66% and 82-91%, respectively. Nordihydroguaiaretic acid (a lipoxygenase inhibitor) and aspirin (a cyclooxygenase inhibitor) decreased the rate of formation of keto aldehyde and keto alcohol by 10-20 % in the monkey lung microsomes. Alpha-Napthoflavone and coumarin markedly decreased the oxidation of NNK in monkey lung and liver microsomes, suggesting the involvement of P450s 1A and 2A6. An antibody against human P450 2A6 decreased the oxidation of NNK by 12-16% and 22-24% in the patas monkey lung and liver microsomes, respectively. These results are comparable to that obtained with human lung and liver microsomes. Coumarin hydroxylation was observed in the patas monkey lung and liver microsomes at a rate of 16 and 4000 pmol/min/mg protein, respectively, which was 5-fold higher than human lung and liver microsomes, respectively. Immunoblot analysis demonstrated that the P450 2A level in the individual patas monkey liver microsomal sample was 6-fold greater than in an individual human liver microsomal sample. Phenethyl isothiocyanate, an inhibitor of NNK activation in rodents and humans, decreased NNK oxidation in the monkey lung and liver microsomes displaying inhibitor concentration resulting in 50% inhibition of the activity (IC50) values of 0.28-0.8 microM and 4.2-6.8 microM, respectively. The results demonstrate the similarities and differences between species in the metabolic activation of NNK. The patas monkey microsomes appear to more closely resemble human microsomes than mouse or rat enzymes and may better reflect the activation of NNK in humans.

Oxidative DNA damage in tissues of pregnant female mice and fetuses caused by the tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)

The tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), induces the promutagenic oxidative-damage DNA lesion, 8-oxo-2'-deoxyguanosine (8-oxo-dG), in adult animals. To investigate whether this alteration occurs in DNA after transplacental exposure, pregnant Swiss mice were administered single or multiple doses of NNK. The 8-oxo-dG was quantified in placenta, and maternal and fetal tissues. In maternal lungs, single and multiple doses of NNK significantly increased levels of 8-oxo-dG by 23% and 32%, respectively. In maternal liver, a significant 38% increase was observed after multiple dose treatment. In the fetuses, a significant 45% increase in 8-oxo-dG levels was observed in liver after multiple doses of NNK. This is the first demonstration of oxidative DNA damage after transplacental exposure to NNK, and supports the concept of maternal smoking as a contributor to the development of childhood cancer.

Lack of p53 and ras mutations in Helicobacter hepaticus-induced liver tumors in A/JCr mice

Helicobacter hepaticus is a recently discovered bacterium that invades mouse liver causing chronic active hepatitis followed by development of preneoplastic hepatocellular foci, hepatocellular adenomas and carcinomas. This establishes a unique animal model for study of the mechanisms of cancer development due to a chronic bacterial infection. A possible mechanism of bacteria-associated tumorigenesis is mutation of oncogenes or tumor suppressor genes. Since mutations in ras oncogenes have been widely detected in a variety of chemically induced and spontaneous mouse liver tumors and specific mutations in the p53 tumor suppressor gene have been associated with human bladder cancers attributed to chronic schistosomal infection, we studied exons 1 and 2 of the N-, K- and H-ras genes and exons 5-8 of the p53 gene for the presence of point mutations in 25 liver tumors from 10 naturally infected A/JCr mice, ranging in age from 16 to 24 months. The 20 adenomas and five carcinomas varied in size from 0.1 to 2.3 cm and arose in livers characterized by a wide assortment of pathological profiles, including hepatitis, inflammation, hyperplasia, hypertrophy, leukocyte infiltration, necrosis and focal phenotypic alteration. DNA samples extracted from formalin-fixed paraffin-embedded tissues were screened by PCR/SSCP analysis and showed no mutations in the analyzed genes. Complete absence of mutations in ras genes in 25 mouse liver tumors is unusual. Other genes may be targeted or H. hepaticus infection causes liver cancer through other pathways than direct damage to DNA.

Transplacental cisplatin exposure induces persistent fetal mitochondrial and genomic DNA damage in patas monkeys

A previous attempt to model transplacental cisplatin exposure and genotoxicity employed several pregnant Erythrocebus patas monkeys; most of the animals were exposed near the end of gestation and cisplatin-DNA adduct analyses included only genomic DNA. Here, both genomic and mitochondrial DNA adduct formation have been determined in fetuses from two pregnant monkeys exposed at the end of the second trimester of gestation. Multiple fetal tissues were obtained after doses of 0.315 mg cisplatin/kg body weight (5.3 mg/m2 total) on days 101 and 106 of gestation. Cesarean sections were performed 24 h after exposure and 27 d after exposure. Cisplatin genomic (g)-DNA adducts were observed in fetal adrenal, brain, heart, kidney, liver, skin, spleen, and thymus. When placentas from the two animals were divided into four concentric regions at increasing distances from the umbilical cord, and g-DNA was assayed, cisplatin DNA adduct levels were similar in all four regions. Mitochondrial (mt)-DNA adducts were higher than g-DNA adducts in maternal liver and fetal liver, brain and kidney, suggesting that the mitochondria may constitute a particular target for cisplatin genotoxicity. The study demonstrates significant fetal genotoxicity in g-DNA and mt-DNA of patas monkeys exposed to cisplatin in utero, suggesting that similarly exposed human fetuses may also sustain drug-induced DNA damage.

Elevated mitochondrial cisplatin-DNA adduct levels in rat tissues after transplacental cisplatin exposure

Although there is evidence that the toxic effects of cis-diamminedichloroplatinum(II) (cisplatin) include morphologically abnormal mitochondria, direct demonstrations of mitochondrial DNA damage by this chemotherapeutic agent have rarely been reported. Here we show that, in rats exposed to a single dose of cisplatin during gestation, cisplatin-DNA binding levels in both maternal and fetal liver and brain mitochondrial DNA are higher than those observed in genomic DNA. Pregnant F344/NCr rats were injected i.p. with either 5 or 15 mg cisplatin/kg body wt at 18 days of gestation and killed 24 h later. Cisplatin-DNA adducts were determined by dissociation-enhanced lanthanide fluoroimmunoassay using a cisplatin-DNA standard modified in the same range as the biological samples. Values for genomic cisplatin-DNA adducts in multiple maternal and fetal tissues have been presented elsewhere. Here, genomic DNA adduct levels for liver, brain, kidney and placenta are reported again for comparison with mitochondrial DNA adduct levels in the same tissues. In maternal and fetal brain, mitochondrial DNA adduct levels were approximately 7- to 50-fold higher than genomic DNA adduct levels, and in fetal liver they were approximately 2- to 16-fold higher than genomic DNA adduct levels. These studies demonstrate extensive cisplatin-DNA adduct formation in brain and liver mitochondria of fetal rats exposed transplacentally and suggest that mitochondrial DNA in some organs may be a particular target for cisplatin genotoxicity.

Variant mutational activation of the K-ras oncogene in renal mesenchymal tumors induced in newborn F344 rats by methyl(methoxymethyl)nitrosamine

Renal mesenchymal tumors were induced at high incidence in F344 rats by a single intraperitoneal injection of methyl(methoxymethyl)nitrosamine (DMN-OMe) within 48 h after birth. DNAs from 18 of 35 mesenchymal tumors contained transforming ras sequences in NIH3T3 transfection assays: K-ras (17/18) or N-ras (1/18). Single-stranded conformational polymorphism analysis or dideoxy sequencing of polymerase chain reaction-amplified K-ras gene fragments revealed that these neoplasms contained a variety of activating mutations in the K-ras oncogene. Alterations in codon 12 predominated and included GGT --> GAT transitions, GGT --> GTT or TGT transversions, and previously reported insertion mutations, although some tumors expressed more than one mutation and the pattern of mutations even varied within tumors. Mutations were also found in exons 2 and 3. In addition, tumor transplantability into syngeneic hosts correlated positively and significantly with K-ras activation. Renal mesenchymal tumors with transforming mutations in exon 1 were often successfully passaged (10/12) while tumors which lacked mutations in exon 1 were infrequently transplantable (2/14). While the observed base substitutions in K-ras are consistent with adduct formation, the presence of insertion mutations and intratumor heterogeneity of alterations suggest that ras activation in DMN-OMe-induced tumors is not necessarily an early event in tumorigenesis.

Cisplatin-DNA adduct formation in maternal and fetal rat tissues after transplacental cisplatin exposure

Cis-diamminedichloroplatinum (II) (cisplatin), given to pregnant rats at 5 mg/kg body weight (bw) is a trans placental carcinogen for fetal liver, kidney, nervous system and lung, resulting in tumor incidences of 22.5, 10.5, 6.1 and 7.5% respectively, in offspring grown to adulthood (B.A. Diwan et al., 1995, Toxicol. Appl. Pharm., 132, 115). In this study, the capacity of cisplatin to pass through the placental barrier and bind covalently to DNA in maternal and fetal tissues was evaluated. Pregnant F344/NCr rats were injected i.p. with single doses of 5, 10 or 15 mg cisplatin/kg bw at 18 days of gestation and sacrificed 24 h later. Cisplatin-DNA adducts were determined by dissociation-enhanced lanthanide fluoroimmunoassay (DELFIA) using both High (90 pmol/micrograms DNA) and Low (0.50 pmol/ microgram DNA) Modified cisplatin-DNA standards and atomic absorbance spectrometry (AAS). The adduct quantities determined by the two DELFIAs varied in concert, but the DELFIA with Low Modified standard gave actual values similar to those observed with AAS. In maternal and fetal tissues, with the exception of placenta in one experiment and maternal kidney in another experiment, the extent of cisplatin-DNA adduct formation increased with dose. In maternal kidney, the low adduct levels observed at the 15 mg/kg dose may reflect kidney toxicity. Fetal kidney, liver and lung contained fewer cisplatin-DNA adducts than the corresponding maternal tissues. In contrast, at 5 and 15 mg/kg, fetal brain DNA contained higher adduct levels than maternal brain DNA. This study demonstrates the presence of DNA damage induced by cisplatin in multiple maternal and fetal rat tissues at tumorigenic doses of drug; the results are therefore consistent with the hypothesis that genotoxic mechanisms play an important role in the drug-induced tumor incidence.

Liver tumorigenesis by Helicobacter hepaticus: considerations of mechanism

A new animal model for the causation of liver tumors via a bacterial infection presented itself fortuitously in the form of a new species, Helicobacter hepaticus. This species of Helicobacter colonizes the hepatic bile canaliculi in susceptible strains of mice, resulting in hepatitis and hepatocellular and hepatocholangiolar adenomas and carcinomas. The mechanism by which this infection leads to cancer is unknown. Tests with Helicobacter hepaticus have revealed thus far that the bacteria do not secrete a mutagen which is capable of detection by the Ames Assay. Measurement of oxidatively damaged bases in the liver DNA of hepaticus infected mice have shown accumulation of 8-oxodeoxyguanosine with disease progression. Other promutagenic DNA lesions, 7-methylguanine and O6-methylguanine, indicative of nitrosation of endogenous amines by nitric oxide, were not detected. Analysis of carcinomas and adenomas taken from H. hepaticus infected A/JCr mice revealed no mutations in ras oncogenes or in exons 5-8 of the p53 gene. These preliminary results indicate that a non-genotoxic tumor promotion mechanism, possibly implemented by reactive oxygen species from the immune response, is more likely than a genotoxic mechanism.

Ahr locus phenotype in congenic mice influences hepatic and pulmonary DNA adduct levels of 2-amino-3-methylimidazo[4,5-f]quinoline in the absence of cytochrome P450 induction

The potent food mutagen/carcinogen 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) undergoes metabolic N-hydroxylation by cytochromes P450, including cytochrome P450 1A2, followed by generation of an unstable ester catalyzed by acetyltransferases; promutagenic DNA adducts result. Genetic polymorphisms in these enzymes have been implicated in human cancer risk related to arylamine exposure. We investigated the effects of Ahr locus and acetylator polymorphisms on 32P-postlabeled IQ/DNA adducts in lungs and livers of female C57BL/6 mice congenic for slow acetylation and/or Ahr-nonresponsiveness; some groups were pretreated with beta-naphthoflavone (beta NF), a cytochrome P450 1A inducer. Total adducts in lung were doubled by beta NF pretreatment in Ahr-responsive mice only and consisted of < or = 30% adduct 2 and < or = 60% adduct 3. In contrast, in Ahr-nonresponsive mice, adducts 2 and 3 were each < or= 7% of the total. Livers of noninduced Ahr-responsive mice formed 6-18-fold more adducts than those of nonresponsive mice. This striking difference was not due to altered levels of cyp1a-2, as indicated by specific enzyme assays and immunoblotting, and was not accompanied by a comparable increase in the ability of liver preparations to activate IQ to a mutagen in the Ames test. Pretreatment of responsive mice with beta NF to induce cyp1a-1 and cyp1a-2 led to a reduction in liver adduct levels. Acetylation phenotype also had a significant effect in Ahr-responsive mice, with 3-fold more adducts in slow than in rapid acetylators. These results indicate that in uninduced mice, the normal Ah receptor facilitates formation of IQ/DNA adducts in liver and alters the profile of adducts in lung, via an unknown mechanism, whereas the Ah receptor-dependent enzyme induction reduces adducts in liver, probably due to increased detoxification, but increases them in lung.

Induction of hepatic and renal P4502E1 of neonatal rats exposed translactationally to ethanol

The effect of maternal ethanol intake during lactation on neonatal cytochrome P4502E1 was investigated in Sprague-Dawley rats. Dams were exposed to 15% (v/v) ethanol in drinking water from day 1 of lactation to 4, 7 or 14 days postpartum. Significant (P < 0.01) enhancement of both hepatic and renal N-nitrosodimethylamine (NDMA) demethylase, an activity of P4502E1, was observed in lactating mothers given ethanol in drinking water. Demethylase activity also significantly increased (P < 0.01) in the 7- and 14-day livers of both female and male pups and in the 7- and 14-day female and 14-day male kidneys exposed to ethanol through the transmammary route. Cytochrome P4502E1 protein content, assayed by immunoblotting, increased in the maternal liver and kidney of all groups consuming ethanol. Neonatal P4502E1 protein content increased in the 7- and 14-day livers of both sexes and 14-day female kidneys exposed translactationally to ethanol. No effect of ethanol on enzyme activity or protein content of P4502E1 was observed in the liver or kidney of 4-day-old neonates. These results demonstrate the translactational effect of ethanol on neonatal P4502E1 enzyme, which is involved in the metabolism of many low molecular weight xenobiotics, and indicate the possibility of alterations occurring in the kinetics of neonatal drug and xenobiotic metabolism and also in processes connected with perinatal carcinogenesis.

Nitrosamines, alcohol, and gastrointestinal tract cancer: recent epidemiology and experimentation

Recent epidemiological and experimental data continues to implicate nitrosamines in causation of gastrointestinal cancers. The evidence is strong for pharynx, esophagus, and stomach, and more problematic for liver, pancreas, and colorectum. Substantial levels of the promutagenic DNA adduct, Ob-methylguanine, in DNA from these organs in patas monkeys after a low dose of N-nitrosodimethylamine confirms the capacity for activation of environmental nitrosamines in these primate tissues. Alcohol is both an independent and a tobacco-interactive risk factor, influencing cancer incidence for oropharynx and esophagus strongly, and for stomach, colorectum, and liver more moderately. In a tabulation of experimental effects of ethanol potentially related to cancer-enhancing effects, toxicokinetic inhibition of hepatic first-pass clearance of nitrosamines is quantitatively greatest, and may be a major part of the mechanism of alcohol's effect on cancer risk for oropharnx, esophagus, and colon. Other operative mechanisms supported by experimental data are induction of activating enzymes, inhibition of DNA repair, and tumor promotion.

N-nitrosodimethylamine-derived O(6)-methylguanine in DNA of monkey gastrointestinal and urogenital organs and enhancement by ethanol

N-nitrosodimethylamine (NDMA) is a human cancer initiator suspect. Ethanol, a cancer risk factor, may synergize with nitrosamines by suppressing hepatic clearance, to increase internal exposure. A limitation to these hypotheses is lack of activation of NDMA by many rodent tissues. However, systemtic primate studies are lacking. Patas monkeys were utilized to investigate NDMA activation by primate tissues in vivo, generating the promutagenic DNA lesion 0(6)-methylguanine (0(6)-meG). Adult monkeys received 0. 1 mg/kg NDMA by gavage, in some cases preceded by ethanol. Four hours after NDMA only, 0(6)-meG was detected in DNA from all tissues. Levels were highest in gastric mucosa and liver and were only about 50% lower in DNA from white blood cells, esophagus, ovary, pancreas, urinary bladder and uterus. With ethanol co-exposure, amounts of 0(6)-meG increased at least 2-fold in all tissues except liver. The largest effect was in esophagus (17-fold increase), followed by ovary, large intestine, urinary bladder, spleen and cerebellum (9- to 13-fold increases), and uterus, cerebrum and brain stem (7- to 8-fold increases). Alkylguanine alkyltransferase activities varied over a 30-fold range and were highest in liver and stomach. Thus primate tissues, especially those of the gastrointestinal and urogenital organs, are sensitive targets for DNA adduct damage due to NDMA, and ethanol co-exposure leads to striking increases in adducts. Our data support epidemiology implicating nitrosamines in causation of cancers of stomach and other organs, and alcohol as enhancing internal exposure to nitrosamines.

O6-methylguanine DNA adduct formation and modulation by ethanol in placenta and fetal tissues after exposure of pregnant patas monkeys to N-nitrosodimethylamine

Perinatal nitrosamine exposures may contribute to childhood cancer risk. To test primate fetal susceptibility to formation of cancer initiation-related DNA adducts from nitrosamines, pregnant patas monkeys were given 1.0 or 0.1 mg/kg N-nitrosodimethylamine. Appreciable levels of the promutagenic O6-methylguanine adduct occurred in placental and fetal liver DNA after both doses and were lower but detectable in other fetal tissues after the higher dose. Coadministered ethanol (1.6 g/kg) reduced adducts in placenta and fetal liver by one-half and increased levels in other fetal tissues to the same degree. Thus, primate placenta and fetal tissues have a significant, ethanol-modulated capacity to activate N-nitrosodimethylamine, supporting implication of nitrosamines in human perinatal carcinogenesis and of alcohol as a modulating factor.

Fetal mouse susceptibility to transplacental carcinogenesis: differential influence of Ah receptor phenotype on effects of 3-methylcholanthrene, 12-dimethylbenz[a]anthracene, and benzo[a]pyrene

Genetic backcrosses of C57BL/6 and DBA/2 mice were used to examine the influence of maternal and fetal polymorphisms at the Ahr locus on susceptibility to transplacental carcinogenesis by 3-methylcholanthrene, 7,12-dimethylbenz[a]anthracene, and benzo[a]pyrene. (C57BL/6 x DBA/2) F1 mothers were backcrossed to DBA/2 males, and DBA/2 females to F1 males to produce both Ahr-responsive (Ah+) and nonresponsive (Ah-) fetuses carried by mothers that were themselves either Ah+ or Ah-. 3-Methylcholanthrene was given intragastrically on gestation days 13-18 and 7,12-dimethylbenz[a]anthracene or benzo[a]pyrene on day 17 as a single intraperitoneal dose. Ahr phenotype was determined by the zoxazolamine sleeping time test after beta-naphthoflavone pretreatment at 6 weeks of age. The offspring were examined for tumours at 1 year. Both 3-methylcholanthrene and 7,12-dimethylbenz[a]anthracene treatments resulted in a two- to five-fold greater incidence and multiplicity of lung and liver tumours in the Ah+ offspring compared with that in Ah- littermates. By contrast, there was no difference between Ah+ and Ah- offspring with regard to numbers of tumours caused by benzo[a]pyrene. Maternal Ahr phenotype appeared to play a role also, in that the offspring of the Ahr-responsive F1 mothers developed fewer tumours per unit dose than those of the nonresponsive DBA/2 mothers. The effect of maternal phenotype on risk was three- to five-fold. Fetal and maternal phenotype combined yielded a 10- to 20-fold risk differential for transplacental carcinogenesis by the methylated compounds, with greatest risk experienced by responsive fetuses in nonresponsive mothers, and least by nonresponsive progeny of responsive mothers.

Promotion of N-nitrosodiethylamine-initiated hepatocellular tumors and hepatoblastomas by 2,3,7,8-tetrachlorodibenzo-p-dioxin or Aroclor 1254 in C57BL/6, DBA/2, and B6D2F1 mice

To investigate the hypothesis that tumor promotion by chlorinated aromatic hydrocarbons involves Ah receptor occupation and subsequent induction of cytochromes P-450 1a-1, effects of Aroclor 1254 or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were examined in N-nitrosodiethylamine-initiated mice with different Ah receptor phenotype. Levels of cytochromes P-450 1a and 2b were measured by enzyme assay and Western immunoblots. Males of the C57BL/6, DBA/2, or (C57BL/6 x DBA/2)F1 (hereafter referred to as "B6D2F1") strain were initiated with a single i.p. dose of N-nitrosodiethylamine (90 mg/kg body weight), followed by either multiple doses of TCDD (0.05 micrograms/kg) weekly or Aroclor 1254 chronically in the diet (100 ppm) for 20 weeks, and then no treatment for 24 weeks. Lung tumor incidence or multiplicity was not altered by either of the promoters. Liver tumor incidence was similar among the three strains after N-nitrosodiethylamine alone (14, 21, and 21%, respectively). In DBA/2 mice, TCDD neither induced Cyp 1a nor promoted liver tumors. Aroclor caused an 8-fold induction of hepatic Cyp 2b, which was its maximum at the 12-week time point but did not promote tumors. Inductions of hepatic Cyp 1a by TCDD and 1a and 2b by Aroclor were similar in C57BL/6 and B6D2F1 mice, but tumor promotion responses were quite different. Dietary Aroclor significantly promoted liver tumors in C57BL/6 mice (59 versus 14%) but not in B6D2F1 mice (24 versus 21%). Repeated TCDD promoted only in B6D2F1 mice (52 versus 21%) and not in C57BL/6 mice (19 versus 14%). Thus, whereas these data confirm that a functional Ah receptor is required for liver tumor promotion, the degree of activation as measured by induction of Cyp 1a is not directly related to the degree of tumor-promoting capability. Other genetic factors must play a role in mediating the final tumor outcome.

Dosimetry of O6-methylguanine in rat DNA after low-dose, chronic exposure to N-nitrosodimethylamine (NDMA). Implications for the mechanism of NDMA hepatocarcinogenesis

Groups of female Wistar Furth/NCr rats, aged 6 weeks or 7 months at the start of the experiment, were administered drinking water containing N-nitrosodimethylamine (NDMA) for up to 28 days at concentrations in the range 0.2-2.64 p.p.m., resulting in daily intakes in the range 28-372 micrograms/kg/day at age 10 weeks. The levels of the premutagenic DNA adduct O6-methylguanine (O6-meG) in liver and blood leukocyte DNA were measured at different times during this exposure as well as on the days immediately following cessation of exposure. The adduct was found to accumulate rapidly in both tissues, reaching within 2-7 days steady states in the range 0.08-0.45 mumol/molG, similar in young and adult animals. Accumulation of O6-meG in blood leukocytes was approximately 30% lower than in the liver. Following cessation of NDMA treatment, adducts were lost rapidly from the DNA of both tissues, with an apparent t1/2 of approximately 19-23 h for the liver and 30-35 h for blood leukocytes. No change in liver O6-alkylguanine-DNA alkyltransferase (AGT) took place throughout this treatment. The steady-state adduct levels were approximately linearly related to NDMA dose-rate, except that a clear break (corresponding to a 2.6-fold lower slope) was observed at dose rates > 0.4 p.p.m. (approximately 56 micrograms/kg/day). This dose-response relationship is in contrast to the sharp increase in the liver tumour induction in rats chronically treated with similar concentrations of NDMA reported by Peto et al. (Cancer Res., 51, 6415-6451) and suggests that accumulation of O6-meG cannot by itself account for the hepatocarcinogenic efficacy of NDMA in the rat. Following i.g. administration of single doses of NDMA in a range approximately corresponding to the daily intake during the above mentioned chronic exposure study (25, 50 or 100 micrograms/kg), repair of O6-meG followed sharply biphasic kinetics in both liver and blood leukocytes. In the liver, an initial rapid phase (t1/2 approximately 1.5-1.7 h) was followed by much slower repair (t1/2 approximately 20.7-24.9 h) despite the absence of any change in AGT. Extrapolation of the two segments of the repair kinetics plots back to 0 time suggests that approximately 52-61% of the adducts originally formed in the liver and 33-35% of those formed in blood leukocytes belonged to the rapidly repaired category.(ABSTRACT TRUNCATED AT 400 WORDS)