Species differences in the metabolism of N-nitroso(2-hydroxypropyl)(2-oxopropyl)amine

Carcinogenic Susceptibility to N-bis(2-hydroxypropyl)nitrosamine (DHPN) in rasH2 Mice

To evaluate the susceptibility of rasH2 mice to N-bis(2-hydroxypropyl)nitrosamine (DHPN), a potent carcinogen targeting the lung, liver, thyroid, and kidney, male, 6-week old, rasH2 mice and wild-type Iittermates (non-Tg mice) were given DHPN in drinking water at 0, 20 or 200 ppm, and 0 or 200 ppm, respectively, for 26 weeks. The experiment using rasH2 mice given 200 ppm DHPN and non-Tg mice given 200 and 0 ppm DHPN was completed at 20 weeks, since mortality in these groups was remarkably increased due to hemangiosarcomas of the liver. Histologically, tumors developed in the lung and liver in both rasH2 and non-Tg mice treated with DHPN. In addition, proliferative lesions were observed in the forestomach, urethra, and excretory duct of salivary glands in rasH2 mice given 200 ppm DHPN. RT-PCR analysis showed no marked difference in expression of mRNAs for the transgene and the endogenous mouse ras gene between the whole lung tissue containing a neoplasm and normal lung tissue. Our results suggest that rasH2 mice are highly susceptible to DHPN, the target organs including the forestomach, salivary gland and urethra, which have not been found to develop tumors in previous long-term carcinogenicity studies of DHPN in rats and mice.

Hydrolysis of 3,4-methylenedioxymethamphetamine (MDMA) metabolite conjugates in human, squirrel monkey, and rat plasma

Characterizing the formation of metabolites of 3,4-methylenedioxymethamphetamine (MDMA, "Ecstasy") in different species (rat, squirrel monkey, and human) may provide insight into mechanisms of MDMA neurotoxicity. Two prominent MDMA metabolites, 3,4-dihydroxymethamphetamine (HHMA) and 4-hydroxy-3-methoxymethamphetamine (HMMA), are conjugated with glucuronic or sulfuric acid, but reference standards are not available; therefore, quantification is only possible after conjugate cleavage. Different concentrations of HHMA and HMMA were obtained in human, squirrel monkey, and rat plasma specimens when acid or enzymatic cleavage was performed. Our data document that these differences are due to species-specific influences on conjugate cleavage. Acidic hydrolysis should be used for analyzing free HHMA and HMMA in human or squirrel monkey plasma, while enzymatic hydrolysis with glucuronidase or sulfatase maximizes recovery of free HHMA and HMMA in rat plasma. Optimization of cleavage conditions showed that sulfate conjugates were more readily cleaved by acid hydrolysis and glucuronides by glucuronidase.

Modification by curcumin of mutagenic activation of carcinogenic N-nitrosamines by extrahepatic cytochromes P-450 2B1 and 2E1 in rats

To elucidate the mechanism underlying suppression by curcumin of esophageal carcinogenesis induced by NMBA, we evaluated the CYP level and mutagenic activation of environmental carcinogens, by immunoblot analyses and Ames preincubation test, respectively, and bilirubin, 4-nitrophenol and testosterone UDPGT activities in F344 rats treated with curcumin and/or NMBA. No significant alterations in the hepatic levels of constitutive CYP proteins, mutagenic activation by liver S9 or hepatic UDPGT activities were produced by subcutaneous treatment with 0.5 mg/kg NMBA for 5 weeks and/or feeding of 0.05% and 0.2% curcumin for 6 weeks. In contrast, gavage of 0.2% curcumin decreased esophageal CYP2B1 and 2E1 by up to 60%, compared with vehicle control. Similarly, intragastric treatment with 270 mg/kg curcumin decreased esophageal and gastric CYP2B1 and CYP2E1, but not in lung, kidney or intestine. Conversely, large intestinal CYP2B1 was 2.8-fold higher in the treated rats than in control rats. Mutagenic activities of NOC, including NMBA, in the presence of esophagus and stomach S9 were markedly decreased in the treated rats, whereas those in the presence of large intestine S9 were 2.2-3.0-fold above control. These results show that modifying effects of curcumin on esophageal carcinogenesis can be attributed to a decrease in metabolic activation of NMBA by esophageal CYP2B1 during the initiation phase, without the contribution of metabolic activation and inactivation by liver. Further, the present findings suggest the potential of curcumin for modification of gastric and intestinal carcinogenesis initiated with NOC.

Suppression of 8-oxo-2'-deoxyguanosine formation and carcinogenesis induced by N-nitrosobis (2-oxopropyl)amine in hamsters by esculetin and esculin

Effects of esculetin (6,7-dihydroxycoumarin) and its glycoside, esculin, on 8-oxo-2'-deoxyguanosine (8-oxodG) formation and carcinogenesis induced by a chemical carcinogen, N-nitrosobis(2-oxopropyl)amine (BOP), were examined in the pancreas of female Syrian golden hamsters. Animals were administered esculetin by gastric intubation into the stomach 30 min before BOP administration or ingestion of a diet containing esculin for 7 days before BOP administration, and killed 1 or 4h after BOP treatment, and the contents of thiobarbituric acid-reacting substrates (TBARS) and 8-oxodG in the pancreas were determined. Both compounds suppressed significantly the BOP-induced increases in 8-oxodG and TBARS contents in hamster pancreas. We further investigated the effect of esculin on pancreatic carcinogenesis by the rapid production model induced by augmentation pressure with a choline-deficient diet, ethionine, methionine and BOP. Esculin was given ad libitum as a 0.05% aqueous solution in either the initiation or promotion phases. The incidence of invasive tumors in animals given esculin during the initiation phase was significantly smaller than in the control group, while esculin given during the promotion phase showed no apparent effects. These results suggest that the intake of esculin has an inhibitory effect on BOP-induced oxidative DNA damage and carcinogenesis in hamster pancreas.

Expression of drug-metabolizing enzymes in the pancreas of hamster, mouse, and rat, responding differently to the pancreatic carcinogenicity of BOP

BACKGROUND/METHODS

N-nitroso-bis(2-oxopropyl)amine (BOP) induces pancreatic ductal adenocarcinoma in Syrian golden hamsters, but not in rats or mice. To examine whether this difference is due to the diversity in the presence and distribution of enzymes involved in the metabolism of BOP, the cellular expression of nine cytochrome P-450 isozymes (CYP1A1, CYP1A2, CYP2B6, CYP2C8,9,19, CYP2D1, CYP2E1, CYP3A1, CYP3A2, and CYP3A4) and of three glutathione S-transferase isozymes (GST-pi, GST-alpha, and GST-mu) was investigated in the pancreas of hamsters, rats, and mice by immunohistochemistry.

RESULTS

We found a wide species variation in the presence and cellular localization of the enzymes and a lack of several enzymes, including GST-alpha in islets, CYP2B6, CYP2C8,9,19, CYP3A1 in acinar cells, and CYP3A4 in ductal cells, in the rat as compared with hamster and mouse.

CONCLUSION

Although the results could not clarify the reasons for the species differences in the pancreatic carcinogenicity of BOP, the presence of most of the cytochrome P-450 isozymes in pancreatic islets of all three species highlights the important role of the islets in drug metabolism.

Dose- and sex-related carcinogenesis by N-bis(2-hydroxypropyl)nitrosamine in Wistar rats

An initiation-promotion medium-term bioassay for detection of chemical carcinogens, developed in the male F344 rat, uses 0.1% N-bis(2-hydroxypropyl)nitrosamine (DHPN) among five genotoxic chemicals for the initiation of carcinogenesis in multiple organs. To establish this bioassay in the Wistar strain, the effects of two dose levels of DHPN were evaluated on the main DHPN rat target organs: lung, thyroid gland, kidneys and liver. Four groups of male and female animals were studied: Control -- untreated group; Multi-organ initiated group (also referred to as DMBDD, based on the initials of the five initiators) -- treated sequentially with N-diethylnitrosamine (DEN, i.p.), N-methyl-N-nitrosourea (MNU, i.p.), N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN, drinking water), N, N'-dimethylhydrazine (DMH, s.c.) and DHPN (drinking water) for 4 weeks; a third group treated with 0.1% DHPN in drinking water for 2 weeks and the last group treated with 0.2% DHPN in drinking water for 4 weeks. The animals were sacrificed after 30 weeks. DHPN at 0. 2% induced preneoplasia in the liver and kidneys of rats of both sexes, the number and area of the putative preneoplastic liver glutathione S-transferase-positive hepatocyte foci being significantly increased in these animals. It also induced benign and malignant tumors in female and in male rats. However, there was no relationship between the increased incidence of preneoplastic lesions and tumor development in the 0.2% DHPN-exposed groups of both sexes. DHPN at 0.1% induced only a few preneoplastic lesions in the liver and kidney and no tumors in both male and female rats. A clear dose and sex-related carcinogenic activity of DHPN was registered, although Wistar rats of both sexes showed a relative resistance to the carcinogenic activity of this compound.

Effect of cigarette smoke on the mutagenic activation of various carcinogens in hamster

Male Syrian golden hamsters were exposed for 1 or 2 weeks to smoke produced by commercial non-filter cigarettes for 5 consecutive days in a Hamburg type II smoking machine. Postmitochondrial fractions (S9) prepared from the liver, lungs, and pancreas were used in the Ames liquid incubation assay, in order to assess the effect of cigarette smoke (CS) on the metabolic activation of four groups of procarcinogens. The mutagenic activities of five heterocyclic amines on strain TA98 in the presence of liver S9 mix were induced up to 3.7 times above controls including sham smoke control, while no significant alteration of mutagenicity was observed with 3'-hydroxymethyl-N,N-dimethyl-4-aminoazobenzene and benzo[a]pyrene on TA98 or with N-nirosobis(2-oxopropyl)amine (BOP) on TA100. A similar stimulation of metabolic activation was also observed for 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1) with S9 from the lungs but not from the pancreas. The mutagenic potential of 11 carcinogens including aflatoxin B1 (AFB1) and two other heterocyclic amines was also examined using liver S9 from male hamsters pretreated with phenobarbital (PB) or 3-methylcholanthrene (MC). The numbers of revertant colonies were much higher (2-20-fold) in the presence of MC-treated liver S9 than in the presence of PB-treated liver S9, except in the case of AFB1 which showed a higher mutagenicity with PB-induced S9. 7,8-Benzoflavone considerably inhibited the activities of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and Trp-P-1 in the presence of either untreated, MC- or CS-treated liver S9, whereas metyrapone was totally lacking this effect, indicating that cytochrome P450(CYP)1A1/1A2 isoforms of hamster liver are predominantly involved in the metabolic activation of these carcinogens. CS exposure of hamsters might selectively induce hepatic CYP1A2 which cannot activate BOP. Consequently, the present findings could explain, in part, the anticarcinogenic effect of CS on BOP-induced pancreatic tumors in hamsters. The findings further support the idea that CS markedly stimulates the metabolic activation of food-derived carcinogens, which may contribute to the overall carcinogenic effects of cigarette smoking.

Carcinogen activation by sulfate conjugate formation

The foregoing pages presented a substantial body of data that established that sulfotransferase conjugation can transform many xenobiotics into agents that can modify cellular macromolecules. However, activation by sulfation is rarely the only metabolic pathway that is open to these compounds; other pathways can become more important in response to a variety of factors. This metabolic switching can be produced by substrate concentration, cofactor availability, kinetic factors that dictate the velocity of the various possible conjugation reactions, and, in some cases, competition between Phase-I and Phase-II metabolism. Also, it is important to realize that demonstration of activation by sulfate ester formation in vitro does not necessarily mean that a similar activation process will occur in vivo. Experience also teaches that argument by analogy can be very misleading in the case of sulfate activation. Small structural differences can upset the delicate balance between sulfate activation and the various other competing pathways. Nevertheless, sulfation is an important mechanism by which a number of chemicals are transformed to their activated forms.

Involvement of lauric acid hydroxylase in the activation of beta-substituted nitrosamines

The mutagenicity of N-nitrosobis (2-hydroxypropyl) amine (BHP), N-nitrosobis(2-oxopropyl)amine (BOP) and N-nitroso-(2-hydroxy-propyl) (2-oxopropyl) amine (HPOP) was measured in V79 cells. Hepatocytes, used to metabolize (activate) the nitrosamines, were isolated from untreated Syrian hamsters (control) and hamsters treated with clofibrate (CLO) or dehydroepiandrosterone (DHEA) in vivo. BHP and HPOP mutagenicity increased 3- and 2-fold when hepatocytes from CLO- and DHEA-treated hamsters were used. BOP mutagenicity did not increase. 10-Undecynoic acid, a lauric acid hydroxylase inhibitor, inhibited the increase in BHP and HPOP mutagenicity by 80-90% but did not affect that of BOP. Antimycin A1, a fatty acyl coenzyme A beta-oxidase inhibitor did not affect the mutagenicity of these nitrosamines. Lauric acid hydroxylase, probably omega-1 hydroxylase (cytochrome P-450 IVA2), appears to be involved in the activation of BHP and HPOP.

Differences between pancreatropic nitrosamine carcinogens and N-nitrosodimethylamine in methylating DNA in various tissues of hamsters and rats

N-Nitrosobis(2-oxopropyl)amine (BOP) and N-nitroso(2-hydroxypropyl)(2-oxypropyl)amine (HPOP) induce pancreatic tumors in the Syrian hamster. BOP and HPOP target the kidneys, esophagus and upper respiratory system in rats, but the pancreas of this species is resistant to the above carcinogens. On the other hand, N-nitrosodimethylamine (DMN) induces hepatic and kidney tumors in the rat, and tumors of the liver and upper respiratory system in the hamster, but it is not known to affect the pancreas of either species. At equimolar doses, ratios of DMN versus BOP or HPOP mediated methylation in hamster liver DNA are 1.6 and 8.1, respectively. Respective ratios in the rat liver are 1.1 and 6.5. However, in both species equitoxic doses of BOP, HPOP and DMN induce similar levels of N7-methylguanine (N7-MeG) in hepatic DNA. At such doses methylation of kidney DNA is 24 and 14 times more extensive in BOP and HPOP than in DMN-treated hamsters. Similarly, ratios of N7-MeG in the pancreas of BOP and HPOP vs. DMN-treated hamsters are 10 and 5, respectively, while in the lung this ratio is 2.2 for both carcinogens. Levels of O6-methylguanine (O6-MeG) in the DNA of extrahepatic tissues are substantially greater in hamsters treated with BOP or HPOP than in those treated with an equitoxic dose of DMN. In rats, equitoxic doses of BOP and DMN induce similar levels of N7-MeG and O6-MeG in hepatic, kidney and lung DNA. However, levels of these adducts in pancreatic DNA are 2 times greater following BOP than DMN administration. Ratios of N7-MeG in pancreas, lung and kidney in HPOP vs. DMN-treated rats are 2.1, 2.7 and 2.1, respectively. Repair of O6-MeG is more effective in rat than in hamster liver, however in other tissues this is not always the case. Levels of O6-MeG in the pancreas of rats are reduced to half of their initial value between 40 and 50 h following the administration of 10, 50 or 20 mg/kg DMN, HPOP or BOP, respectively. However, half-lives for the repair of O6-MeG in hamster pancreas are 28, 62 and greater than 120 h at the respective doses of the above carcinogens. Since the above doses of DMN, HPOP and BOP induce 7, 19 and 41 nmol O6-MeG/mmol of guanine respectively in the hamster pancreas, it is suggested that the rate of repair could be a function of the initial concentration of this adduct. Differences between DMN and BOP or HPOP in methylating pancreatic DNA are sufficient to distinguish the latter two nitrosamines as pancreatic carcinogens for the hamster.

Carcinogenic potency of N-nitrosomethyl(2-hydroxypropyl)amine and other metabolic relatives of N-nitrosobis(2-hydroxypropyl)amine by single intraperitoneal injection on the lung of rats

The carcinogenic effects of a single intraperitoneal injection of N-nitrosobis(2-hydroxypropyl)amine (BHP) or its metabolic relatives, N-nitrosomethyl(2-hydroxypropyl)amine (MHP), N-nitrosobis(2-oxopropyl)amine (BOP), N-nitroso(2-hydroxypropyl)(2-oxopropyl)amine (HPOP) and N-nitroso-2,6-dimethylmorpholine (NDMM), were studied in male Wistar rats. The main target organ of these nitrosamines proved to be the lung, followed by the thyroid. Lung lesions were induced in a dose-dependent manner with total lung tumor incidences reaching 55% to 100%. BHP, MHP, HPOP and NDMM all caused lung carcinomas to develop (22% to 44% incidence), whereas BOP was only associated with adenomas. On the basis of dose administered and incidence of carcinomas, MHP appeared to be the most potent lung carcinogen of the five nitrosamines investigated. Smaller numbers of neoplasms were also induced in the kidney, urinary bladder, esophagus and intestine at differing rates by these nitrosamines.