1-C-glucuronidation of N-nitrosodiethylamine and N-nitrosomethyl-n-pentylamine in vivo and in primary hepatocytes from rats pretreated with inducers

UDP-glucuronosyltransferase polymorphisms affect diethylnitrosamine-induced carcinogenesis in humanized transgenic mice

UDP‐glucuronosyltransferase (UGT) 1A enzymes detoxify a broad array of exogenous compounds including environmental toxins and carcinogens. Case‐control studies identified genetic variations in UGT1A genes leading to reduced glucuronidation activity, which were associated with hepatocellular carcinoma (HCC) formation and progression. The aim of the study was therefore to examine the direct effect of common UGT1A polymorphisms (SNPs) on HCC development and outcome in a diethylnitrosamine (DEN)‐induced mouse model. Therefore, a single intraperitoneal DEN injection (20 mg/kg) was administered to 15‐day‐old htgUGT1A‐WT and htgUGT1A‐SNP mice (containing a human haplotype of 10 common UGT1A SNPs) either receiving water or coffee cotreatment for the following 39 weeks. After this time, tumor incidence, size (>1 mm), histology, liver‐body ratio, serum aminotransferase activities, and UGT1A regulation and activity levels were determined. In DEN‐treated htgUGT1A‐SNP mice, a markedly higher number of tumors with a bigger cumulative diameter were detected. The relative liver weight and aminotransferase activity levels were also significantly higher in mice carrying UGT1A SNPs. After coffee + DEN cotreatment, susceptibility for tumor development and growth considerably decreased in both mouse lines, but was still higher in htgUGT1A‐SNP mice. In conclusion, our study provides experimental evidence for the protective role of UGT1A enzymes in neoplastic transformation. These data confirm case‐control studies implicating impaired UGT1A‐mediated carcinogen detoxification as a risk factor for individual cancer disposition. Coffee treatment, which is able to activate UGT1A expression and activity, reduced HCC development and provides an explanation for the protective properties of coffee on liver diseases including liver cancer.

Expression of Genes Encoding for Xenobiotic Metabolism After Exposure to Dialkylnitrosamines in the Chicken Egg Genotoxicity Alternative Model

The Chicken Egg Genotoxicity Assay (CEGA) demonstrated responsiveness to various DNA-reactive chemicals requiring metabolic activation, which implies broad bioactivation capability. To assess potential metabolic competence, expression profiles of metabolic genes in the embryo-chicken fetal liver were determined using microarray technology. Fertilized chicken eggs were injected under the CEGA protocol with vehicle (deionized water [DW]), the activation-dependent carcinogens, diethylnitrosamine (DEN), and N-nitrosodiethanolamine (NDELA) at doses producing no effect on survival. Previously in CEGA, DEN produced DNA damage, whereas NDELA did not. Expressions of 463 genes known to encode for phase I and II of endo- and xenobiotic metabolism were detected on the array. DW did not affect the expression of the selected genes, deregulating less than 1% of them. In contrast, DEN at 2 mg/egg and NDELA at 4 mg/egg produced significant transcriptomic alterations, up-regulating up to 41% and down-regulating over 31% of studied genes. Both nitrosamines modulated the majority of the genes in a similar manner, sharing 64 up-regulated and 93 down-regulated genes with respect to control group, indicating similarity in the regulation of their metabolism by avian liver. Differences in gene expression between DEN and NDELA were documented for several phase I CYP 450 genes that are responsible for nitrosamine biotransformation, as well as for phase II genes that regulate detoxication reactions. These findings could underlie the difference in genotoxicity of DEN and NDELA in CEGA. In conclusion, the analysis of gene expression profiles in embryo-chicken fetal liver dosed with dialkylnitrosamines demonstrated that avian species possess a complex array of inducible genes coding for biotransformation.

Evaluation of Nitrosamide Formation in the Cytochrome P450-Mediated Metabolism of Tobacco-Specific Nitrosamines

N'-Nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) are carcinogenic tobacco-specific nitrosamines believed to play a vital role in the initiation of tobacco-related cancers. For their carcinogenicities to be exhibited, both NNN and NNK must be metabolically activated by cytochrome P450s, specifically P450 2A6 and P450 2A13, respectively. Prior research has focused on α-hydroxylation, which leads to the formation of several DNA adducts that have been identified and quantified in vivo. However, some studies indicate that P450s can retain substrates within their active sites and perform processive oxidation. For nitrosamines, this would oxidize the highly unstable α-hydroxynitrosamines to potentially more stable nitrosamides, which could also alkylate DNA. Thus, we hypothesized that both NNN and NNK are processively oxidized in vitro to nitrosamides by P450 2A6 and P450 2A13, respectively. To test this hypothesis, we synthesized the NNN- and NNK-derived nitrosamides, determined their half-lives at pH 7.4 and 37 °C, and monitored for nitrosamide formation in an in vitro P450 system with product analysis by LC/NSI+-HRMS/MS. Half-lives of the nitrosamides were determined by HPLC-UV and ranged from 7-35 min, which is more than 40 times longer than the corresponding α-hydroxynitrosamines. Incubation of NNN in the P450 2A6 system resulted in the formation of the nitrosamide N'-nitrosonorcotinine (NNC) at low levels. Similarly, the nitrosamide 4-(methylnitrosamino)-1-(3-pyridyl)-1,4-butanedione (CH2-oxo-NNK) was detected in low amounts in the incubation of NNK with the P450 2A13 system. The other possible NNK-derived nitrosamide, 4-(nitrosoformamido)-1-(3-pyridyl)-1-butanone (CH3-oxo-NNK), was not observed in the P450 2A13 reactions. CH2-oxo-NNK readily formed O6meGua in reactions with dGuo and calf thymus DNA. These results demonstrate that NNC and CH2-oxo-NNK are novel metabolites of NNN and NNK, respectively. Though low-forming, their increased stability may allow for mutagenic DNA damage in vivo. More broadly, this study provides the first account of a cytochrome P450-mediated conversion of nitrosamines to nitrosamides, which warrants further studies to determine how general this phenomenon is in nitrosamine metabolism.

The enhancing effect of ethanol on the mutagenic activation of N-nitrosomethylbenzylamine by cytochrome P450 2A in the rat oesophagus

Alcohol consumption is frequently associated with various cancers and the enhancement of the metabolic activation of carcinogens has been proposed as a mechanism underlying this relationship. The ethanol-induced enhancement of N-nitrosodiethylamine (DEN)-mediated carcinogenesis can be attributed to an increase in hepatic activity. However, the mechanism of elevation of N-nitrosomethylbenzylamine (NMBA)-induced tumorigenesis remains unclear. To elucidate the mechanism underlying the role of ethanol in the enhancement of NMBA-induced oesophageal carcinogenesis, we evaluated the hepatic and extrahepatic levels of the cytochrome P450 (CYP) and mutagenic activation of environmental carcinogens by immunoblot analyses and Ames preincubation test, respectively, in F344 rats treated with ethanol. Five weeks of treatment with 10% ethanol added to the drinking water or two intragastric treatments with 50% ethanol, both resulted in elevated levels of CYP2E1 (1.5- to 2.3-fold) and mutagenic activities of DEN, N-nitrosodimethylamine and N-nitrosopyrrolidine in the presence of rat liver S9 (1.5- to 2.4-fold). This was not the case with CYP1A1/2, CYP2A1/2, CYP2B1/2 or CYP3A2, nor with the activities of 2-amino-3-methylimidazo[4,5-f]quinoline, 3-amino-1-methyl-5H-pyrido[4,3-b]indole, aflatoxin B(1) or other N-nitroso compounds (NOCs), including NMBA. Ethanol-induced elevations of CYP2A and CYP2E1 were observed in the oesophagus (up to 1.7- and 2.3-fold) and kidney (up to 1.5- and 1.8-fold), but not in the lung or colon. In oesophagus and kidney, the mutagenic activities of NMBA and four NOCs were markedly increased (1.3- to 2.4-fold) in treated rats. The application of several CYP inhibitors revealed that CYP2A were likely to contribute to the enhancing effect of ethanol on NMBA activation in the rat oesophagus and kidney, but that CYP2E1 failed to do so. These results showed that the enhancing effect of ethanol on NMBA-induced oesophageal carcinogenesis could be attributed to an increase in the metabolic activation of NMBA by oesophageal CYP2A during the initiation phase, and that this occurred independently of CYP2E1.

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.

Differential expression of genes related to levels of mucosal cell proliferation among multiple rat strains by using oligonucleotide microarrays

Induction levels of cell proliferation, in response to gastric mucosal damage by N-methyl- N'-nitro- N-nitrosoguanidine (MNNG), are different among rat strains and correlate with susceptibility to MNNG-induced gastric carcinogenesis. Here, we used oligonucleotide microarrays to search for genes that show expression levels accordant with the extents of cell proliferation among six rat strains. Expression levels of 8,800 probe sets were analyzed in the pylorus of ACI, LEW, WKY (strains with strong cell proliferation), F344, (ACI x BUF)F1, and BUF rats (strains with weak cell proliferation) after 2-week MNNG treatment. No genes showed complete accordance, and 22 genes showed accordance with one or two exceptions. After confirmation by quantitative RT-PCR, four genes--cellular retinoic acid-binding protein II ( Crabp2), fatty acid binding protein 1 ( Fabp1), progastricsin (pepsinogen C, Pgc), and UDP-glucuronosyltransferase 2 family member 5 ( Ugt2b5)--were found to show good accordance with only one exception. Crabp2, Fabp1, and Ugt2b5 were differentially expressed between ACI and BUF rats both before and after MNNG treatment. Although Crabp2 had been identified as one of the 16 genes differentially expressed between ACI and BUF rats with cDNA-RDA, Fabp1 and Ugt2b5 were newly identified in this study. All three genes are known to be involved in retinoic acid-mediated signaling and could be involved in the control of differential induction of cell proliferation.

N-nitrososdimethylamine is activated in microsomes from hepatocytes to reactive metabolites which damage DNA of non-parenchymal cells in rat liver

The liver carcinogen N-nitrosodimethylamine (NDMA) has to be metabolically activated by specific cytochromes before it can react with cellular macromolecules (e.g. proteins or DNA). Although hepatocytes are believed to be responsible for this activation, the liver tumours originate mainly from non-parenchymal cells (NPC). To investigate their activation capacity we determined NDMA-demethylase activity in isolated microsomes from both liver cell types. The results demonstrate that only hepatocytes have activation capacity. Additional experiments were performed with hepatocytes and NPC using the single cell microgel electrophoresis assay (MGE). DNA damage appears in both cell types following in vivo exposure. Tested in vitro, however, the carcinogens induce DNA damages only in hepatocytes (the cells which activate these compounds). N-nitroso-hydroxymethyl-methylamine could be the responsible metabolite as it is stable enough to be transported from hepatocytes to NPC in an intact liver.

Transport of the beta -O-glucuronide conjugate of the tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) by the multidrug resistance protein 1 (MRP1). Requirement for glutathione or a non-sulfur-containing analog

Nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and its metabolite 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) play a crucial role in the induction of lung cancer, and NNAL-O-glucuronide formation and elimination are important steps in detoxification of these compounds. In the present study, we investigated the ATP-binding cassette (ABC) protein, MRP1 (ABCC1), as a candidate transporter responsible for NNAL-O-glucuronide export. MRP1 mediates the active transport of numerous GSH-, sulfate-, and glucuronide-conjugated organic anions and can transport certain xenobiotics by a mechanism that may involve co-transport with GSH. Using membrane vesicles prepared from transfected cells, we found that MRP1 transports [3H]NNAL-O-glucuronide but is dependent on the presence of GSH (Km 39 microm, Vmax 48 pmol x mg(-1) x min(-1)). We also found that the sulfur atom in GSH was dispensable because transport was supported by the GSH analog, gamma-glutamyl-alpha-aminobutyryl-glycine. Despite stimulation of NNAL-O-glucuronide transport by GSH, there was no detectable reciprocal stimulation of [3H]GSH transport. Moreover, whereas the MRP1 substrates leukotriene C4 (LTC4) and 17beta-estradiol 17beta-(d-glucuronide) (E(2)17betaG) inhibited GSH-dependent uptake of [3H]NNAL-O-glucuronide, only [3H]LTC4 transport was inhibited by NNAL-O-glucuronide (+GSH) and the kinetics of inhibition were complex. A mutant form of MRP1, which transports LTC4 but not E(2)17betaG, also did not transport NNAL-O-glucuronide suggesting a commonality in the binding elements for these two glucuronidated substrates, despite their lack of reciprocal transport inhibition. Finally, the related MRP2 transported NNAL-O-glucuronide with higher efficiency than MRP1 and unexpectedly, GSH inhibited rather than stimulated uptake. These studies provide further insight into the complex interactions of the MRP-related proteins with GSH and their conjugated organic anion substrates, and extend the range of xenotoxins transported by MRP1 and MRP2 to include metabolites of known carcinogens involved in the etiology of lung and other cancers.

The profile of urinary metabolites of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone in rats is determined by its pulmonary metabolism

Metabolism of the tobacco specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in rats was compared to metabolism in primary lung and liver cells. Untreated rats and rats pretreated with phenobarbital, acetone or phenethyl isothiocyanate (PEITC) were used for all experiments. Also the influence of [-]-1-methyl-2-[3-pyridyl]-pyrrolidine (nicotine) administered concomitantly with NNK, or incubated with isolated cells, upon NNK metabolism was investigated and found to be only marginal upon alpha-hydroxylation and pyridine N-oxidation in vivo. In hepatocytes nicotine inhibited NNK pyridine N-oxidation, alpha-hydroxylation and glucuronidation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), whereas in lung cells the influence of nicotine was not as pronounced. In vivo phenobarbital induced alpha-hydroxylation and pyridine N-oxidation. In vitro the effects of the modulators were most pronounced upon hepatocytes, where phenobarbital greatly induced pyridine N-oxidation and PEITC inhibited alpha-hydroxylation. NNAL was conjugated to its beta-glucuronide in lung cells at four times higher rates than in hepatocytes. The ratios of the sum of N-oxides to the sum of alpha-hydroxylation products in vivo were similar to those in lung cells, especially at low NNK concentrations (1 microM), while in hepatocytes alpha-hydroxylation was more pronounced. The same correlation of metabolism in isolated lung cells with whole rats was observed if oxidative NNAL metabolism was related to oxidative NNK metabolism. Here hepatocytes showed a much higher formation of NNAL oxidation products than either lung cells formed, or rats excreted in urine. This was true despite a lower rate of metabolism in the lung than in liver if based on cell number, the rate based on mg protein was four times higher in lung than liver. Only after phenobarbital treatment was the contribution of hepatic metabolism to excreted metabolites important. In conclusion the lung which is also the target of NNK carcinogenesis, and not the liver, is the organ with the most important contribution to NNK and NNAL metabolism at concentrations relevant to human exposure.

Effects of phenobarbital and 3-methylcholanthrene induction on the formation of three glucuronide metabolites of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, NNK

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a tobacco specific carcinogen believed to be a causative agent for human lung cancer. To exert its carcinogenic potential, NNK must be metabolically activated, by alpha-hydroxylation, at either the methyl or methylene carbons adjacent to the N-nitroso group. We recently reported the presence of a glucuronide conjugate of 4-(hydroxymethylnitrosamino)-1-(3-pyridyl)-1-butanone (alpha-hydroxymethylNNK-Gluc) in the urine of Phenobarbital (PB) treated rats, and in the media of PB induced hepatocytes incubated with NNK. PB induces the alpha-hydroxylation of NNK, which generates the aglycon, as well as several UDP-glucuronosyl transferases. In the study presented here, we compared the metabolism of NNK to alpha-hydroxymethylNNK-Gluc by PB induced, 3-methylcholanthrene (3-MC) induced and control rat hepatocytes. Media was analyzed for the products of alpha-hydroxylation, N-oxidation and glucuronidation by radioflow HPLC. PB induced both N-oxidation and alpha-hydroxylation of NNK. 3-MC did not induce N-oxidation but induced alpha-hydroxylation more than 10-fold. alpha-HydroxymethylNNK-Gluc was not detected (< 0.05% total metabolites) when control hepatocytes were incubated with 1 to 100 microM NNK. When 3-MC and PB induced hepatocytes were incubated with 1-100 microM NNK alpha-hydroxymethylNNK-Gluc, expressed as the average percent of metabolites, accounted for 0.725 +/- 0.27 and 1.35 +/- 0.24% (+/-S.D.) of the NNK metabolites, respectively. The percent of NNK metabolized to alpha-hydroxymethylNNK-Gluc is small. But this glucuronide is potentially important in NNK carcinogenesis, since its formation results in the direct conjugation of an active metabolite responsible for DNA adduct formation. When PB induced rats were injected with NNK the level of NNK hemoglobin adducts, which can serve as surrogates for DNA adducts, decreased 50% compared to control rats administered NNK. Hepatic microsomal metabolism increased 2-fold and urinary alpha-hydroxymethylNNK-Gluc increased more than 10-fold in PB treated rats. One explanation for the decrease in NNK hemoglobin adducts may be a PB induced increase in the glucuronidation of alpha-hydroxymethylNNK, the metabolite responsible for adduct formation.

Recent studies on mechanisms of bioactivation and detoxification of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a tobacco-specific lung carcinogen

This article reviews recent advances in the biochemistry and molecular biology of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a tobacco-specific pulmonary carcinogen believed to be involved in the induction of lung cancer in smokers. Several aspects of NNK bioactivation are addressed, including identification of its metabolites in laboratory animals and humans, cytochrome P450 enzyme involvement in its metabolic activation, DNA and protein adduct formation, biological significance of the major DNA adducts formed, and mutations in oncogenes from tumors induced by NNK. Collectively, the presently available data provide a reasonably clear picture of NNK bioactivation in rodents, although there are still important gaps in our mechanistic understanding of NNK-induced tumorigenesis. The studies in rodents and primates have facilitated development of methods to assess NNK bioactivation in humans, which will be applicable to studies of lung cancer susceptibility and prevention.