CYP2A13 expressed in human bladder metabolically activates 4-aminobiphenyl

Genetic and Enzymatic Characteristics of CYP2A13 in Relation to Lung Damage

Cytochrome P450 2A13 is an omitted brother of CYP2A6 that has an important role in the drug metabolism of liver. Due to extrahepatic expression, it has gained less attention than CYP2A6, despite the fact that it plays a significant role in toxicant-induced pulmonary lesions and, therefore, lung cancer. The purpose of this mini-review is to summarize the basic knowledge about this enzyme in relation to the substrates, inhibitors, genetic polymorphisms, and transcriptional regulation that are known so far (September 2021).

Mutagenicity of N-hydroxy-4-aminobiphenyl in human TP53 knock-in (Hupki) mouse embryo fibroblasts

TP53 harbors somatic mutations in more than half of human tumors with some showing characteristic mutation spectra that have been linked to environmental exposures. In bladder cancer, a unique distribution of mutations amongst several codons of TP53 has been hypothesized to be caused by environmental carcinogens including 4-aminobiphenyl (4-ABP). 4-ABP undergoes metabolic activation to N-hydroxy-4-aminobiphenyl (N-OH-4-ABP) and forms pre-mutagenic adducts in DNA, of which N-(deoxyguanosin-8-yl)-4-ABP (dG-C8-4-ABP) is the major one. Human TP53 knock-in mouse embryo fibroblasts (HUFs) are a useful model to study the influence of environmental carcinogens on TP53-mutagenesis. By performing the HUF immortalization assay (HIMA) TP53-mutant HUFs are generated and mutations can be identified by sequencing. Here we studied the induction of mutations in human TP53 after treatment of primary HUFs with N-OH-4-ABP. In addition, mutagenicity in the bacterial lacZ reporter gene and the formation of dG-C8-4-ABP, measured by ³² P-postlabelling analysis, were determined in N-OH-4-ABP-treated primary HUFs. A total of 6% TP53-mutants were identified after treatment with 40 μM N-OH-4-ABP for 24 hr (n = 150) with G>C/C>G transversion being the main mutation type. The mutation spectrum found in the TP53 gene of immortalized N-OH-4-ABP-treated HUFs was unlike the one found in human bladder cancer. DNA adduct formation (~40 adducts/10⁸ nucleotides) was detected after 24 hr treatment with 40 μM N-OH-4-ABP, but lacZ mutagenicity was not observed. Adduct levels decreased substantially (sixfold) after a 24 hr recovery period indicating that primary HUFs can efficiently repair the dG-C8-4-ABP adduct possibly before mutations are fixed. In conclusion, the observed difference in the N-OH-4-ABP-induced TP53 mutation spectrum to that observed in human bladder tumors do not support a role of 4-ABP in human bladder cancer development.

Bioactivation of the tobacco carcinogens 4-aminobiphenyl (4-ABP) and 2-amino-9H-pyrido[2,3-b]indole (AαC) in human bladder RT4 cells

Occupational and tobacco exposure to aromatic amines (AAs) including 4-aminobiphenyl (4-ABP) and 2-naphthylamine (2-NA) are associated with bladder cancer (BC) risk. Several epidemiological studies have also reported a possible role for structurally related heterocyclic aromatic amines (HAAs) formed in tobacco smoke or cooked meats with BC risk. We had screened for DNA adducts of 4-ABP, 2-NA, and several prominent HAAs formed in tobacco smoke or grilled meats including 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 2-amino-3,8-dimethylmidazo[4,5-f]quinoxaline (MeIQx), and 2-amino-9H-pyrido[2,3-b]indole (AαC) in the bladder DNA of BC patients, using liquid chromatography/mass spectrometry. We detected DNA adducts of 4-ABP, but not adducts of the other carcinogens. In this study, we have examined the capacity of RT4 cells, an epithelial human bladder cell line, to bioactivate AAs and HAAs to DNA damaging agents, which may contribute to BC. 4-ABP and AαC formed DNA adducts, but DNA adducts of 2-NA, PhIP, and MeIQx were not detected. 4-ABP DNA adducts were formed at tenfold higher levels than AαC adducts. Pretreatment of RT4 cells with α-naphthoflavone (1-10 µM), a specific cytochrome P450 1 (CYP1) inhibitor, decreased AαC adduct formation by 50% but did not affect the level of 4-ABP adducts. However, cell pretreatment with 8-methoxypsoralen (0.1-1 µM), a potent inhibitor of CYP2A, resulted in a 90% decrease of 4-ABP DNA adducts levels. These data signify that CYP2A and CYP1A isoforms expressed in the target urothelium bioactivate 4-ABP and AαC, respectively, and may be a critical feature of aromatic amine-induced urinary bladder carcinogenesis. The bioactivation of other tobacco and environmental AAs by bladder CYPs and their ensuing bladder DNA damage warrants further study.

Protein interaction disruption in cancer

Background

Most methods that integrate network and mutation data to study cancer focus on the effects of genes/proteins, quantifying the effect of mutations or differential expression of a gene and its neighbors, or identifying groups of genes that are significantly up- or down-regulated. However, several mutations are known to disrupt specific protein-protein interactions, and network dynamics are often ignored by such methods. Here we introduce a method that allows for predicting the disruption of specific interactions in cancer patients using somatic mutation data and protein interaction networks.

Methods

We extend standard network smoothing techniques to assign scores to the edges in a protein interaction network in addition to nodes. We use somatic mutations as input to our modified network smoothing method, producing scores that quantify the proximity of each edge to somatic mutations in individual samples.

Results

Using breast cancer mutation data, we show that predicted edges are significantly associated with patient survival and known ligand binding site mutations. In-silico analysis of protein binding further supports the ability of the method to infer novel disrupted interactions and provides a mechanistic explanation for the impact of mutations on key pathways.

Conclusions

Our results show the utility of our method both in identifying disruptions of protein interactions from known ligand binding site mutations, and in selecting novel clinically significant interactions.Supporting website with software and data: https://www.cs.cmu.edu/~mruffalo/mut-edge-disrupt/.

Electronic supplementary material

The online version of this article (10.1186/s12885-019-5532-5) contains supplementary material, which is available to authorized users.

Targeted and Untargeted Detection of DNA Adducts of Aromatic Amine Carcinogens in Human Bladder by Ultra-Performance Liquid Chromatography-High-Resolution Mass Spectrometry

Epidemiological studies have linked aromatic amines (AAs) from tobacco smoke and some occupational exposures with bladder cancer risk. Several epidemiological studies have also reported a plausible role for structurally related heterocyclic aromatic amines present in tobacco smoke or formed in cooked meats with bladder cancer risk. DNA adduct formation is an initial biochemical event in bladder carcinogenesis. We examined paired fresh-frozen (FR) and formalin-fixed paraffin-embedded (FFPE) nontumor bladder tissues from 41 bladder cancer patients for DNA adducts of 4-aminobiphenyl (4-ABP), a bladder carcinogen present in tobacco smoke, and 2-amino-9 H-pyrido[2,3- b]indole, 2-amino-1-methyl-6-phenylimidazo[4,5- b]pyridine and 2-amino-3,8-dimethylimidazo[4,5- f]quinoxaline, possible human carcinogens, which occur in tobacco smoke and cooked meats. These chemicals are present in urine of tobacco smokers or omnivores. Targeted DNA adduct measurements were done by ultra-performance liquid chromatography-electrospray ionization multistage hybrid Orbitrap MS. N-(2'-Deoxyguanosin-8-yl)-4-ABP ( N-(dG-C8)-4-ABP) was the sole adduct detected in FR and FFPE bladder tissues. Twelve subjects (29%) had N-(dG-C8)-4-ABP levels above the limit of quantification, ranging from 1.4 to 33.8 adducts per 109 nucleotides (nt). DNA adducts of other human AA bladder carcinogens, including 2-naphthylamine (2-NA), 2-methylaniline (2-MA), 2,6-dimethylaniline (2,6-DMA), and lipid peroxidation (LPO) adducts, were screened for in bladder tissue, by our untargeted data-independent adductomics method, termed wide-selected ion monitoring (wide-SIM)/MS2. Wide-SIM/MS2 successfully detected N-(dG-C8)-4-ABP, N-(2'-deoxyadenosin-8-yl)-4-ABP and the presumed hydrazo linked adduct, N-(2'-deoxyguanosin- N2-yl)-4-ABP, and several LPO adducts in bladder DNA. Wide-SIM/MS2 detected multiple DNA adducts of 2-NA, 2-MA, and, 2,6-DMA, when calf thymus DNA was modified with reactive intermediates of these carcinogens. However, these AA-adducts were below the limit of detection in unspiked human bladder DNA (<1 adduct per 108 nt). Wide-SIM/MS2 can screen for many types of DNA adducts formed with exogenous and endogenous electrophiles and will be employed to identify DNA adducts of other chemicals that may contribute to the etiology of bladder cancer.

Method for Biomonitoring DNA Adducts in Exfoliated Urinary Cells by Mass Spectrometry

Tobacco smoking contributes to about 50% of the bladder-cancer (BC) cases in the United States. Some aromatic amines in tobacco smoke are bladder carcinogens; however, other causal agents of BC are uncertain. Exfoliated urinary cells (EUCs) are a promising noninvasive biospecimen to screen for DNA adducts of chemicals that damage the bladder genome, although the analysis of DNA adducts in EUCs is technically challenging because of the low number of EUCs and limiting quantity of cellular DNA. Moreover, EUCs and their DNA adducts must remain viable during the time of collection and storage of urine to develop robust screening methods. We employed RT4 cells, a well-differentiated transitional epithelial bladder cell line, as a cell-model system in urine to investigate cell viability and the chemical stability of DNA adducts of two prototypical bladder carcinogens: 4-aminobiphenyl (4-ABP), an aromatic amine found in tobacco smoke, and aristolochic acid I (AA-I), a nitrophenanthrene found in Aristolochia herbaceous plants used for medicinal purposes worldwide. The cell viability of RT4 cells pretreated with 4-ABP or AA-I in urine exceeded 80%, and the major DNA adducts of 4-ABP and AA-I, quantified by liquid chromatography-mass spectrometry, were stable for 24 h. Thereafter, we successfully screened EUCs of mice treated with AA-I to measure DNA adducts of AA-I, which were still detected 25 days following treatment with the carcinogen. EUCs are promising biospecimens that can be employed for the screening of DNA adducts of environmental and dietary genotoxicants that may contribute to the development of BC.

Functional characterization of 9 CYP2A13 allelic variants by assessment of nicotine C-oxidation and coumarin 7-hydroxylation

Cytochrome P450 2A13 (CYP2A13) is responsible for the metabolism of chemical compounds such as nicotine, coumarin, and tobacco-specific nitrosamine. Several of these compounds have been recognized as procarcinogens activated by CYP2A13. We recently showed that CYP2A13*2 contributes to inter-individual variations observed in bladder cancer susceptibility because CYP2A13*2 might cause a decrease in enzymatic activity. Other CYP2A13 allelic variants may also affect cancer susceptibility. In this study, we performed an in vitro analysis of the wild-type enzyme (CYP2A13.1) and 8 CYP2A13 allelic variants, using nicotine and coumarin as representative CYP2A13 substrates. These CYP2A13 variant proteins were heterologously expressed in 293FT cells, and the kinetic parameters of nicotine C-oxidation and coumarin 7-hydroxylation were estimated. The quantities of CYP2A13 holoenzymes in microsomal fractions extracted from 293FT cells were determined by measuring reduced carbon monoxide-difference spectra. The kinetic parameters for CYP2A13.3, CYP2A13.4, and CYP2A13.10 could not be determined because of low metabolite concentrations. Five other CYP2A13 variants (CYP2A13.2, CYP2A13.5, CYP2A13.6, CYP2A13.8, and CYP2A13.9) showed markedly reduced enzymatic activity toward both substrates. These findings provide insights into the mechanism underlying inter-individual differences observed in genotoxicity and cancer susceptibility.

CYP2A13 Genetic Polymorphisms in Relation to the Risk of Bladder Cancer in Japanese Smokers

Tobacco-specific nitrosamines including 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N-nitrosonornicotine (NNN), which can be activated by the metabolic enzyme CYP2A13, are potent procarcinogens. Smoking plays a role in carcinogenesis in the human bladder, which expresses CYP2A13 at a relatively high level. Numerous genetic polymorphisms of CYP2A13 causing amino acid substitution might reduce CYP2A13 metabolic activity toward NNK and NNN, resulting in decreased susceptibility to bladder cancer. The aim of this study was to reveal any association between bladder cancer development and CYP2A13 genetic polymorphisms in Japanese smokers. The CYP2A13 genotype of each subject (163 bladder cancer patients and 161 controls) was determined by next-generation sequencing (NGS) of the full CYP2A13 gene. All samples were genotyped for five CYP2A13 variant alleles (CYP2A13*2, *3, *4, *6, *7). Based on biological logistic regression, the odds ratio (95% confidence interval) for the CYP2A13*1/*2 genotype was 0.34 (0.17-0.69). Thus, CYP2A13 genetic polymorphisms might play important roles in the development of bladder cancer in Japanese smokers.

Tumour suppressor protein p53 regulates the stress activated bilirubin oxidase cytochrome P450 2A6

Human cytochrome P450 (CYP) 2A6 enzyme has been proposed to play a role in cellular defence against chemical-induced oxidative stress. The encoding gene is regulated by various stress activated transcription factors. This paper demonstrates that p53 is a novel transcriptional regulator of the gene. Sequence analysis of the CYP2A6 promoter revealed six putative p53 binding sites in a 3kb proximate promoter region. The site closest to transcription start site (TSS) is highly homologous with the p53 consensus sequence. Transfection with various stepwise deletions of CYP2A6-5'-Luc constructs--down to -160bp from the TSS--showed p53 responsiveness in p53 overexpressed C3A cells. However, a further deletion from -160 to -74bp, including the putative p53 binding site, totally abolished the p53 responsiveness. Electrophoretic mobility shift assay with a probe containing the putative binding site showed specific binding of p53. A point mutation at the binding site abolished both the binding and responsiveness of the recombinant gene to p53. Up-regulation of the endogenous p53 with benzo[α]pyrene--a well-known p53 activator--increased the expression of the p53 responsive positive control and the CYP2A6-5'-Luc construct containing the intact p53 binding site but not the mutated CYP2A6-5'-Luc construct. Finally, inducibility of the native CYP2A6 gene by benzo[α]pyrene was demonstrated by dose-dependent increases in CYP2A6 mRNA and protein levels along with increased p53 levels in the nucleus. Collectively, the results indicate that p53 protein is a regulator of the CYP2A6 gene in C3A cells and further support the putative cytoprotective role of CYP2A6.

Oxidation of pyrene, 1-hydroxypyrene, 1-nitropyrene and 1-acetylpyrene by human cytochrome P450 2A13

1. The polycyclic hydrocarbons (PAHs), pyrene, 1-hydroxypyrene, 1-nitropyrene and 1-acetylpyrene, were found to induce Type I binding spectra with human cytochrome P450 (P450) 2A13 and were converted to various mono- and di-oxygenated products by this enzyme. 2. Pyrene was first oxidized by P450 2A13 to 1-hydroxypyrene which was further oxidized to di-oxygenated products, i.e. 1,8- and 1,6-dihydroxypyrene. Of five other human P450s examined, P450 1B1 catalyzed pyrene oxidation to 1-hydroxypyrene at a similar rate to P450 2A13 but was less efficient in forming dihydroxypyrenes. P450 2A6, a related human P450 enzyme, which did not show any spectral changes with these four PAHs, showed lower activities in oxidation of these compounds than P450 2A13. 3. 1-Nitropyrene and 1-acetylpyrene were also found to be efficiently oxidized by P450 2A13 to several oxygenated products, based on mass spectrometry analysis. 4. Molecular docking analysis supported preferred orientations of pyrene and its derivatives in the active site of P450 2A13, with lower interaction energies (U values) than observed for P450 2A6 and that several amino acid residues (including Ala-301, Asn-297 and Ala-117) play important roles in directing the orientation of these PAHs in the P450 2A13 active site. In addition, Phe-231 and Gly-329 were found to interact with pyrene to orient this compound in the active site of P450 1B1. 5. These results suggest that P450 2A13 is one of the important enzymes that oxidizes these PAH compounds and may determine how these chemicals are detoxicated and bioactivated in humans.

Relative Contributions of CYP1A2 and CYP2E1 to the Bioactivation and Clearance of 4-Aminobiphenyl in Adult Mice

4-Aminobiphenyl (ABP), a prototypical aromatic amine carcinogen in rodents and humans, requires bioactivation to manifest its toxic effects. A traditional model of ABP bioactivation, based on in vitro enzyme kinetic evidence, had postulated initial N-hydroxylation by the cytochrome P450 isoform CYP1A2. This is followed by phase 2 O-conjugation and hydrolysis to form a reactive nitrenium ion that covalently binds to DNA and produces tumor-initiating mutations. However, Cyp1a2(-/-) mice still possess significant liver ABP N-hydroxylation activity, DNA damage, and incidence of ABP-induced liver tumors, and in vivo induction of CYP1A2 paradoxically reduces levels of ABP-induced DNA damage. Competing ABP detoxification pathways can include N-acetylation by arylamine N-acetyltransferase 1 (NAT1) and/or NAT2; however, wild-type and Nat1/2(-/-) mice have similar in vivo ABP clearance rates. Together, these studies suggest the existence of novel ABP bioactivating and clearance/detoxification enzymes. In the present study, we detected similar reductions in Vmax for ABP N-hydroxylation by liver microsomes from Cyp1a2(-/-) and Cyp2e1(-/-) mice when compared with wild-type mice. In addition, recombinant mouse CYP1A2 and CYP2E1 were both able to N-hydroxylate ABP in mouse hepatoma cells. However, the in vivo clearance of ABP was significantly reduced in Cyp1a2(-/-) but not in Cyp2e1(-/-) mice. Our results support a significant role for CYP2E1 as a novel ABP N-oxidizing enzyme in adult mice, and suggest a more important contribution of CYP1A2 to the in vivo plasma clearance and thus detoxification of ABP.

Generation and characterization of a novel CYP2A13--transgenic mouse model

CYP2A13, CYP2B6, and CYP2F1 are neighboring cytochrome P450 genes on human chromosome 19, and the enzymes that they encode overlap in substrate specificity. A CYP2A13/2B6/2F1-transgenic mouse, in which CYP2A13 and 2F1 are both expressed in the respiratory tract and CYP2B6 is expressed in the liver, was recently generated. We generated a CYP2A13 (only) transgenic mouse so that the specific activity of CYP2A13 can be determined. The CYP2B6 and CYP2F1 genes in the CYP2A13/2B6/2F1 genomic clone were inactivated via genetic manipulations, and CYP2A13 was kept intact. A CYP2A13 (only) transgenic (2A13-TG) mouse was generated using the engineered construct and then characterized to confirm transgene integrity and determine copy numbers. The 2A13-TG mice were normal in gross morphology, development, and fertility. As in the CYP2A13/2B6/2F1-transgenic mouse, CYP2A13 expression in the 2A13-TG mouse was limited to the respiratory tract; in contrast, CYP2B6 and 2F1 proteins were not detected. Additional studies using the CYP2A13-humanized (2A13-TG/Cyp2abfgs-null) mouse produced by intercrossing between 2A13-TG and Cyp2abfgs-null mice confirmed that the transgenic CYP2A13 is active in the bioactivation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a lung procarcinogen. The 2A13-TG mouse should be valuable for assessing specific roles of human CYP2A13 in xenobiotic toxicity in the respiratory tract.

Transcriptional suppression of CYP2A13 expression by lipopolysaccharide in cultured human lung cells and the lungs of a CYP2A13-humanized mouse model

CYP2A13, a human P450 enzyme preferentially expressed in the respiratory tract, is highly efficient in the metabolic activation of tobacco-specific nitrosamines. The aim of this study was to test the hypothesis that inflammation suppresses CYP2A13 expression in the lung, thus explaining the large interindividual differences in CYP2A13 levels previously found in human lung biopsy samples. We first demonstrated that the bacterial endotoxin lipopolysaccharide (LPS) and the proinflammatory cytokine IL-6 can suppress CYP2A13 messenger RNA (mRNA) expression in the NCI-H441 human lung cell line. We then report that an ip injection of LPS (1mg/kg), which induces systemic and lung inflammation, caused substantial reductions in CYP2A13 mRNA (~50%) and protein levels (~80%) in the lungs of a newly generated CYP2A13-humanized mouse model. We further identified two critical CYP2A13 promoter regions, one (major) between -484 and -1008bp and the other (minor) between -134 and -216bp, for the response to LPS, through reporter gene assays in H441 cells. The potential involvement of the nuclear factor NF-κB in LPS-induced CYP2A13 downregulation was suggested by identification of putative NF-κB binding sites within the LPS response regions and effects of an NF-κB inhibitor (pyrrolidine dithiocarbamate) on CYP2A13 expression in H441 cells. Results from gel shift assays further confirmed binding of NF-κB-like nuclear proteins of H441 cells to the major LPS response region of the CYP2A13 promoter. Thus, our findings strongly support the hypothesis that CYP2A13 levels in human lung can be suppressed by inflammation associated with disease status in tissue donors, causing underestimation of CYP2A13 levels in healthy lung.

Expression of cytochrome P450 2A13 in human non-small cell lung cancer and its clinical significance

Lung cancer is one of the most important causes of cancer-related mortality worldwide. Human cytochrome P450 2A13 enzyme (CYP2A13) is predominantly expressed in the respiratory tract and could catalyze various carcinogens. In this study, we quantified CYP2A13 expression in non-small cell lung cancer (NSCLC) tissues and examined the relation between CYP2A13 and clinicopathologic factors. Thirty-five paired lung cancer and normal tissues were studied for the expression of the CYP2A13 gene by using real-time PCR and Western blotting assays. We also investigated the relationship between CYP2A13 expression and clinicopathologic factors such as age, gender, histology and lymph node status in tumor tissues. SPSS (17.0) statistical software was applied for data analysis. The real-time PCR results showed that there was no significant difference in the CYP2A13 mRNA transcript levels between tumor and paired normal tissues in the 35 samples and in 12 paired squamous cell carcinomas. In adenocarcinoma, the expression of CYP2A13 mRNA in tumor tissues was 12.5% of that in adjacent tissues (P < 0.05) and it was not associated with age, gender, histology and lymph node status of the patients. The amounts of CYP2A13 proteins detected by Western blotting assays correlated well with those of the corresponding mRNAs. In conclusion, the expression of CYP2A13 was downregulated in lung adenocarcinoma. CYP2A13 may be involved in the development and progression of lung adenocarcinoma.

Understanding the gender disparity in bladder cancer risk: the impact of sex hormones and liver on bladder susceptibility to carcinogens

It has long been known that bladder cancer (BC) incidence is approximately four-fold higher in men than in women in the United States, and a similar disparity also exists in other countries. The reason for this phenomenon is not known, which impedes progress in BC prevention. However, BC incidence is also significantly higher in male animals than in their female counterparts after treatment with aromatic amines, which are principal human bladder carcinogens. These animal studies and related studies in the context of available human data provide significant insight into what may drive the excessive BC risk in men, which is the focus of this article. The carcinogenicity and biotransformation of bladder carcinogens as well as the impact of sex hormones on these processes are discussed, highlighting the novel concept that the gender disparity in BC risk may result primarily from the interplay of androgen, estrogen, and liver, with the liver functioning via its metabolic enzymes as the main decider of bladder exposure to carcinogens in the urine and the male and female hormones exerting opposing effects on carcinogenesis in the bladder and likely also on liver enzymes handling bladder carcinogens. The findings may facilitate further investigation into the mechanism of gender disparity in BC risk and may also have important implications for BC prevention.

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) metabolism-related enzymes gene polymorphisms, NNK metabolites levels and urothelial carcinoma

Gene polymorphisms of the 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) metabolism-related enzymes-cytochrome P450 (CYP) monooxygenase 2A13 (CYP2A13) and UDP-glucuronosyltransferases (UGT)-2B7 could contribute to the levels of NNK-related metabolites in urine, thereby increasing the susceptibility to urothelial carcinoma (UC). Therefore, our study aimed to evaluate the roles of two gene polymorphisms (CYP2A13 and UGT2B7) of NNK metabolism-related enzymes in the carcinogenesis of UC in Taiwan. A hospital-based pilot case-control study was conducted. There were 121 UC cases and 121 age- and sex-matched healthy participants recruited from March 2007 to April 2009. Urine samples were analyzed for NNK-related metabolites using the liquid chromatography-tandem mass spectrometry method. Genotyping was conducted using a polymerase chain reaction-restriction fragment length polymorphism technique. ANCOVA and multivariate logistic regression were applied for data analyses. In healthy controls, former smokers had significantly higher total NNAL and higher NNAL-Gluc than never smokers or current smokers. Subjects carrying the UGT2B7 268 His/Tyr or Tyr/Tyr genotype had significantly lower total NNAL than those carrying His/His genotype. However, no association was seen between gene polymorphisms of CYP2A13 and UGT2B7 and UC risk after adjustment for age and sex. Significant dose -response associations between total NNAL, free NNAL, the ratios of free NNAL/total NNAL and NNAL-Gluc/total NNAL and UC risk were observed. In the future, large-scale studies will be required to verify the association between the single nucleotide polymorphisms of NNK metabolism-related enzymes and UC risk.

Differential distribution of CYP2A6 and CYP2A13 in the human respiratory tract

BACKGROUND

Human CYP2A6 and CYP2A13 play important roles in metabolic activation of many pulmonary carcinogens and thus their expression and distribution may determine the pulmonary susceptibility to metabolically activated carcinogens and the following lung cancer development. Because of the 93.5% of amino acid identity between CYP2A6 and CYP2A13, generation of antibodies specific to CYP2A6 or CYP2A13 has limited immunohistochemical (IHC) analysis of CYP2A6 and CYP2A13 levels in the respiratory tract.

OBJECTIVES

This study aimed to determine the differential distribution of CYP2A6 and CYP2A13 in human respiratory tissue with IHC analysis.

METHODS

With computer-aided protein sequence analyses, candidate epitopes of 15 amino acids in the C-terminal domains of CYP2A6 and CYP2A13 were selected for antibody generation. Specificity of these two antibodies was confirmed with immunoblot and immunofluorescence analyses. With these two selective antibodies, the differential distribution of CYP2A6 and CYP2A13 in human respiratory tissues, including tracheae, bronchi, bronchioles and alveoli, was determined.

RESULTS

IHC results showed that both CYP2A6 and CYP2A13 were markedly expressed in epithelial cells of tracheae and bronchi and that only CYP2A6 was detected in bronchiolar epithelial cells of peripheral lungs. A limitation of the present study is the cross-reactivity of our CYP2A6 antibody to the functional inactive CYP2A7.

CONCLUSIONS

The differential distribution patterns of CYP2A6 and CYP2A13 in the respiratory tract are of importance in considering the pulmonary susceptibility to carcinogens and the following lung cancer development.

Contributions of human enzymes in carcinogen metabolism

Considerable support exists for the roles of metabolism in modulating the carcinogenic properties of chemicals. In particular, many of these compounds are pro-carcinogens that require activation to electrophilic forms to exert genotoxic effects. We systematically analyzed the existing literature on the metabolism of carcinogens by human enzymes, which has been developed largely in the past 25 years. The metabolism and especially bioactivation of carcinogens are dominated by cytochrome P450 enzymes (66% of bioactivations). Within this group, six P450s--1A1, 1A2, 1B1, 2A6, 2E1, and 3A4--accounted for 77% of the P450 activation reactions. The roles of these P450s can be compared with those estimated for drug metabolism and should be considered in issues involving enzyme induction, chemoprevention, molecular epidemiology, interindividual variations, and risk assessment.

Selective expression of CYP2A13 in human pancreatic α-islet cells

Exposure to cigarette smoke is an etiological factor of human pancreatic cancer and has been associated with an increased risk of pancreatic diseases, including pancreatitis and diabetes. The toxicants in cigarette smoke can reach pancreatic tissue, and most of the toxicants require cytochrome P450 (P450)-mediated metabolic activation to exert their toxicity. Among all the human P450 enzymes, CYP2A13 is the most efficient enzyme in the metabolic activation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a major tobacco-specific toxicant and a suspected human carcinogen. It also metabolically activates 4-aminobiphenyl, another toxicant in cigarette smoke. Immunohistochemical analysis in this study demonstrated that CYP2A13 was selectively expressed in the islets but not in the exocrine portion of adult human pancreas. Further study using dual immunofluorescence labeling technique showed that CYP2A13 protein was mainly expressed in the α-islet but not in β-islet cells. The selective expression of CYP2A13 in human pancreatic α-islet cells suggests that these islet cells could be damaged by the toxicants existing in cigarette smoke through CYP2A13-mediated in situ metabolic activation. Our result provides a mechanistic insight for human pancreatic diseases that have been associated with cigarette smoke exposure.

Generation and characterization of a CYP2A13/2B6/2F1-transgenic mouse model

CYP2A13, CYP2B6, and CYP2F1, which are encoded by neighboring cytochrome P450 genes on human chromosome 19, are active in the metabolic activation of many drugs, respiratory toxicants, and chemical carcinogens. To facilitate studies on the regulation and function of these human genes, we have generated a CYP2A13/2B6/2F1-transgenic (TG) mouse model (all *1 alleles). Homozygous transgenic mice are normal with respect to gross morphological features, development, and fertility. The tissue distribution of transgenic mRNA expression agreed well with the known respiratory tract-selective expression of CYP2A13 and CYP2F1 and hepatic expression of CYP2B6 in humans. CYP2A13 protein was detected through immunoblot analyses in the nasal mucosa (NM) (∼100 pmol/mg of microsomal protein; similar to the level of mouse CYP2A5) and the lung (∼0.2 pmol/mg of microsomal protein) but not in the liver of the TG mice. CYP2F1 protein, which could not be separated from mouse CYP2F2 in immunoblot analyses, was readily detected in the NM and lung but not the liver of TG/Cyp2f2-null mice, at levels 10- and 40-fold, respectively, lower than that of mouse CYP2F2 in the TG mice. CYP2B6 protein was detected in the liver (∼0.2 pmol/mg of microsomal protein) but not the NM or lung (with a detection limit of 0.04 pmol/mg of microsomal protein) of the TG mice. At least one transgenic protein (CYP2A13) seems to be active, because the NM of the TG mice had greater in vitro and in vivo activities in bioactivation of a CYP2A13 substrate, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (a lung carcinogen), than did the NM of wild-type mice.

Mechanism-based inhibition of cytochrome P450 (CYP)2A6 by chalepensin in recombinant systems, in human liver microsomes and in mice in vivo

BACKGROUND AND PURPOSE

Chalepensin is a pharmacologically active furanocoumarin compound found in rue, a medicinal herb. Here we have investigated the inhibitory effects of chalepensin on cytochrome P450 (CYP) 2A6 in vitro and in vivo.

EXPERIMENTAL APPROACH

Mechanism-based inhibition was studied in vitro using human liver microsomes and bacterial membranes expressing genetic variants of human CYP2A6. Effects in vivo were studied in C57BL/6J mice. CYP2A6 activity was assayed as coumarin 7-hydroxylation (CH) using HPLC and fluorescence measurements. Metabolism of chalepensin was assessed with liquid chromatography/mass spectrometry (LC/MS).

KEY RESULTS

CYP2A6.1, without pre-incubation with NADPH, was competitively inhibited by chalepensin. After pre-incubation with NADPH, inhibition by chalepensin was increased (IC(50) value decreased by 98%). This time-dependent inactivation (k(inact) 0.044 min(-1) ; K(I) 2.64 µM) caused the loss of spectrally detectable P450 content and was diminished by known inhibitors of CYP2A6, pilocarpine or tranylcypromine, and by glutathione conjugation. LC/MS analysis of chalepensin metabolites suggested an unstable epoxide intermediate was formed, identified as the corresponding dihydrodiol, which was then conjugated with glutathione. Compared with the wild-type CYP2A6.1, the isoforms CYP2A6.7 and CYP2A6.10 were less inhibited. In mouse liver microsomes, pre-incubation enhanced inhibition of CH activity. Oral administration of chalepensin to mice reduced hepatic CH activity ex vivo.

CONCLUSIONS AND IMPLICATIONS

Chalepensin was a substrate and a mechanism-based inhibitor of human CYP2A6. Formation of an epoxide could be a key step in this inactivation. 'Poor metabolizers' carrying CYP2A6*7 or *10 may be less susceptible to inhibition by chalepensin. Given in vivo, chalepensin decreased CYP2A activity in mice.

Metabolism and biomarkers of heterocyclic aromatic amines in molecular epidemiology studies: lessons learned from aromatic amines

Aromatic amines and heterocyclic aromatic amines (HAAs) are structurally related classes of carcinogens that are formed during the combustion of tobacco or during the high-temperature cooking of meats. Both classes of procarcinogens undergo metabolic activation by N-hydroxylation of the exocyclic amine group to produce a common proposed intermediate, the arylnitrenium ion, which is the critical metabolite implicated in toxicity and DNA damage. However, the biochemistry and chemical properties of these compounds are distinct, and different biomarkers of aromatic amines and HAAs have been developed for human biomonitoring studies. Hemoglobin adducts have been extensively used as biomarkers to monitor occupational and environmental exposures to a number of aromatic amines; however, HAAs do not form hemoglobin adducts at appreciable levels, and other biomarkers have been sought. A number of epidemiologic studies that have investigated dietary consumption of well-done meat in relation to various tumor sites reported a positive association between cancer risk and well-done meat consumption, although some studies have shown no associations between well-done meat and cancer risk. A major limiting factor in most epidemiological studies is the uncertainty in quantitative estimates of chronic exposure to HAAs, and thus, the association of HAAs formed in cooked meat and cancer risk has been difficult to establish. There is a critical need to establish long-term biomarkers of HAAs that can be implemented in molecular epidemioIogy studies. In this review, we highlight and contrast the biochemistry of several prototypical carcinogenic aromatic amines and HAAs to which humans are chronically exposed. The biochemical properties and the impact of polymorphisms of the major xenobiotic-metabolizing enzymes on the biological effects of these chemicals are examined. Lastly, the analytical approaches that have been successfully employed to biomonitor aromatic amines and HAAs, and emerging biomarkers of HAAs that may be implemented in molecular epidemiology studies are discussed.

Pharmacokinetic study of nicotine and its metabolite cotinine to clarify possible association between smoking and voiding dysfunction in rats using UPLC/ESI-MS

The present study was undertaken to clarify the possible association between nicotine intake/cigarette smoking and detrusor instability. For pharmacokinetic characterization of nicotine and cotinine (a major and pharmacologically less active metabolite of nicotine), a rapid ultra-performance liquid chromatography/electrospray ionization-mass spectrometry (UPLC/ESI-MS) method was developed that requires only a small amount of sample and simple pretreatment. The UPLC/ESI-MS method was validated with a focus on specificity, sensitivity (limit of detection, 2.5 ng/mL; limit of quantification, 5 ng/mL), linearity (r > 0.998), accuracy (97.2-102.8%), precision (relative standard deviation <8%) and robustness in accordance with ICH guidelines (Q2B Validation of Analytical Procedures: Methodology). The developed method was successfully applied to determine nicotine and cotinine levels in rat biological samples such as plasma, urine and several tissues. After subcutaneous administration of nicotine ditartrate (2 mg/kg of body weight) in rats, the absorbed nicotine was rapidly and extensively metabolized into cotinine. However, nicotine was found to be predominant in cortex and bladder, where nicotinic acetylcholine receptors were expressed for neuronal control of voiding function. Repeated administration of nicotine led to a ca. 3-fold higher accumulation of nicotine than that of cotinine in rat urine. The results of the pharmacokinetic study using the UPLC/ESI-MS method further support the possible involvement of nicotine in increased risk of urinary dysfunction in smokers.

Metabolic effects of CYP2A6 and CYP2A13 on 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)-induced gene mutation--a mammalian cell-based mutagenesis approach

Both cytochrome P450 2A6 (CYP2A6) and cytochrome P450 2A13 (CYP2A13) are involved in metabolic activation of tobacco-specific nitrosamines and may play important roles in cigarette smoking-induced lung cancer. Unlike CYP2A6, effects of CYP2A13 on the tobacco-specific nitrosamine-induced mutagenesis in lung cells remain unclear. This study uses a supF mutagenesis assay to examine the relative effects of CYP2A6 and CYP2A13 on metabolic activation of a tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), and its resulting mutagenesis in human lung cells. A recombinant adenovirus-mediated CYP2A6/CYP2A13 expression system was established to specifically address the relative effects of these two CYPs. Mutagenesis results revealed that both CYP2A6 and CYP2A13 significantly enhanced the NNK-induced supF mutation and that the mutagenic effect of CYP2A13 was markedly higher than that of CYP2A6. Analysis of NNK metabolism indicated that ≥70% of NNK was detoxified to 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), either with or without CYP2A6/CYP2A13 expression. Both CYP2A6 and CYP2A13 significantly enhanced the α-hydroxylation of NNK; and the α-hydroxylation activity of CYP2A13 was significantly higher than that of CYP2A6. Analysis of the NNK-related DNA adduct formation indicated that, in the presence of CYP2A13, NNK treatments caused marked increases in O(6)-methylguanine (O(6)-MeG). The present results provide the first direct in vitro evidence demonstrating the predominant roles of CYP2A13 in NNK-induced mutagenesis, possibly via metabolic activation of NNK α-hydroxylation.

Chemical inhibitors of cytochrome P450 isoforms in human liver microsomes: a re-evaluation of P450 isoform selectivity

The majority of marketed small-molecule drugs undergo metabolism by hepatic Cytochrome P450 (CYP) enzymes (Rendic 2002). Since these enzymes metabolize a structurally diverse number of drugs, metabolism-based drug-drug interactions (DDIs) can potentially occur when multiple drugs are coadministered to patients. Thus, a careful in vitro assessment of the contribution of various CYP isoforms to the total metabolism is important for predicting whether such DDIs might take place. One method of CYP phenotyping involves the use of potent and selective chemical inhibitors in human liver microsomal incubations in the presence of a test compound. The selectivity of such inhibitors plays a critical role in deciphering the involvement of specific CYP isoforms. Here, we review published data on the potency and selectivity of chemical inhibitors of the major human hepatic CYP isoforms. The most selective inhibitors available are furafylline (in co-incubation and pre-incubation conditions) for CYP1A2, 2-phenyl-2-(1-piperidinyl)propane (PPP) for CYP2B6, montelukast for CYP2C8, sulfaphenazole for CYP2C9, (-)-N-3-benzyl-phenobarbital for CYP2C19 and quinidine for CYP2D6. As for CYP2A6, tranylcypromine is the most widely used inhibitor, but on the basis of initial studies, either 3-(pyridin-3-yl)-1H-pyrazol-5-yl)methanamine (PPM) and 3-(2-methyl-1H-imidazol-1-yl)pyridine (MIP) can replace tranylcypromine as the most selective CYP2A6 inhibitor. For CYP3A4, ketoconazole is widely used in phenotyping studies, although azamulin is a far more selective CYP3A inhibitor. Most of the phenotyping studies do not include CYP2E1, mostly because of the limited number of new drug candidates that are metabolized by this enzyme. Among the inhibitors for this enzyme, 4-methylpyrazole appears to be selective.

Human CYP2A6 is regulated by nuclear factor-erythroid 2 related factor 2

Human CYP2A6 is responsible for the metabolism of nicotine and coumarin as well as the metabolic activation of tobacco-related nitrosamines. Earlier studies revealed that CYP2A6 activity was increased by dietary cadmium or cruciferous vegetables, but the underlying mechanisms remain to be clarified. In the present study, we investigated the possibility that Nrf2 might be involved in the regulation of CYP2A6. Real-time RT-PCR analysis revealed that the CYP2A6 mRNA level in human hepatocytes was significantly (P<0.01, 1.4-fold) induced by 10μM sulforaphane (SFN), a typical activator of Nrf2. A computer-based search identified three putative antioxidant response elements (AREs) in the 5'-flanking region of the CYP2A6 gene at positions -1212, -2444, and -3441, termed ARE1, ARE2, and ARE3, respectively. Electrophoretic mobility shift assays demonstrated that Nrf2 bound only to ARE1. Luciferase assays using HepG2 cells revealed that the overexpression of Nrf2 significantly increased the reporter activities of the constructs containing a 30-bp fragment that included ARE1. However, the activity of the construct containing the intact 5'-flanking region (-1 to -1395) including ARE1 was not increased by the overexpression of Nrf2. In contrast, when the reporter construct was injected into mice via the tail vein, the reporter activity in the liver was significantly (P<0.05, 1.9-fold) increased by SFN (1mg/head) administration. In conclusion, we found that human CYP2A6 is regulated via Nrf2, suggesting that CYP2A6 is induced under oxidative stress.

Immunohistochemical analysis of CYP2A13 in various types of human lung cancers

Human CYP2A13, which is expressed in the respiratory tract, is the most efficient enzyme for the metabolic activation of tobacco-specific nitrosamines such as 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). The relevance of CYP2A13 in carcinogenicity and toxicity in the respiratory tract has been suggested, but the expression of CYP2A13 protein in lung cancer tissues remains to be determined. We first prepared a mouse monoclonal antibody against human CYP2A13. The antibody showed no cross reactivity with the other CYP isoforms including CYP2A6. Using the specific antibody, we performed immunohistochemical analysis for human lung carcinomas. In adenocarcinomas (n = 15), all specimens were positive for the staining and five samples showed strong staining. In squamous cell carcinomas (n = 15) and large cell carcinomas (n = 15), each 14 samples were positive for the staining and two and three samples showed strong staining, respectively. In small cell carcinoma samples (n = 15), eight samples were negative for the staining and five samples showed weak or moderate staining. In conclusion, we first found that the expression of CYP2A13 was markedly increased in non-small cell lung carcinomas. The high expression might be associated with the tumor development and progression in non-small cell lung carcinomas.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

The pneumotoxin 3-methylindole is a substrate and a mechanism-based inactivator of CYP2A13, a human cytochrome P450 enzyme preferentially expressed in the respiratory tract

3-Methylindole (3MI), a respiratory tract toxicant, can be metabolized by a number of cytochromes P450 (P450), primarily through either dehydrogenation or epoxidation of the indole. In the present study, we assessed the bioactivation of 3MI by recombinant CYP2A13, a human P450 predominantly expressed in the respiratory tract. Four metabolites were detected, and the two principal ones were identified as indole-3-carbinol (I-3-C) and 3-methyloxindole (MOI). Bioactivation of 3MI by CYP2A13 was verified by the observation of three glutathione (GSH) adducts designated as GS-A1 (glutathione adduct 1), GS-A2 (glutathione adduct 2), and GS-A3 (glutathione adduct 3) in a NADPH- and GSH-fortified reaction system. GS-A1 and GS-A2 gave the same molecular ion at m/z 437, an increase of 305 Da over 3MI. Their structures are assigned to be 3-glutathionyl-S-methylindole and 3-methyl-2-glutathionyl-S-indole, respectively, on the basis of the mass fragmentation data obtained by high-resolution mass spectrometry. Kinetic parameters were determined for the formation of I-3-C (V(max) = 1.5 nmol/min/nmol of P450; K(m) = 14 muM), MOI (V(max) = 1.9 nmol/min/nmol of P450; K(m) = 15 muM) and 3-glutathionyl-S-methylindole (V(max) = 0.7 nmol/min/nmol of P450; K(m) = 13 muM). The structure of GS-A3, a minor adduct with a protonated molecular ion at m/z 453, is proposed to be 3-glutathionyl-S-3-methyloxindole. We also discovered that 3MI is a mechanism-based inactivator of CYP2A13, given that it produced a time-, cofactor-, and 3MI concentration-dependent loss of activity toward 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, with a relatively low K(I) value of approximately 10 muM and a k(inact) of 0.046 min(-1). Thus, CYP2A13 metabolizes 3MI through multiple bioactivation pathways, and the process can lead to a suicide inactivation of CYP2A13.

Structure and characterization of human carboxylesterase 1A1, 1A2, and 1A3 genes

OBJECTIVE

Human carboxylesterase (CES) 1A1 gene (14 exons) and CES1A3 pseudogene (six exons) are inverted and duplicated genes in a reference sequence (NT_010498). In contrast, earlier studies reported the CES1A2 gene (14 exons) instead of the CES1A3 pseudogene. The sequences of the CES1A2 gene downstream and upstream of intron 1 are identical with those of the CES1A1 and CES1A3 genes, respectively. A CES1A1 variant of which exon 1 is converted with that of the CES1A3 gene (the transcript is CES1A2) has recently been identified. We sought to clarify the confusing gene structure of human CES1A.

METHODS

A panel of 55 human liver as well as 318 blood samples (104 Caucasians, 107 African-Americans, and 107 Japanese) was used to clarify the gene structures of CES1A1, CES1A2, and CES1A3. Real-time reverse transcription-PCR and western blot analysis were carried out. Imidapril hydrolase activity in human liver microsomes and cytosol was determined by liquid chromatography-mass spectrometry (LC-MS)/MS.

RESULTS

By PCR analyses, we found that the CES1A2 gene is a variant of the CES1A3 gene. Four haplotypes, A (CES1A1 wild type and CES1A3), B (CES1A1 wild type and CES1A2), C (CES1A1 variant and CES1A3), and D (CES1A1 variant and CES1A2), were demonstrated. Ethnic differences were observed in allele frequencies of CES1A1 variant (17.3% in Caucasians and African-Americans and 25.2% in Japanese) and CES1A2 gene (14.4% in Caucasians, 5.1% in African-Americans, and 31.3% in Japanese). In human livers whose diplotype was A/A and C/C or C/D, no CES1A2 and CES1A1 mRNA was detected, respectively. In the other participants, the CES1A1 mRNA levels were higher than the CES1A2 mRNA levels. The CES1A proteins translated from CES1A1 mRNA and CES1A2 mRNA were detected in both human liver microsomes and cytosol fractions suggesting that the differences in exon 1 encoding a signal peptide did not affect the subcellular localization. Imidapril hydrolase activities reflected the CES1A protein levels.

CONCLUSION

We found that the CES1A2 gene is a variant of the CES1A3 pseudogene. The findings presented here significantly increase our understanding about the gene structure and expression properties of human CES1A.

Key residues controlling phenacetin metabolism by human cytochrome P450 2A enzymes

Cytochrome P450s (P450s) metabolize a large number of diverse substrates with specific regio- and stereospecificity. A number of compounds, including nicotine, cotinine, and aflatoxin B(1), are metabolites of the 94% identical CYP2A13 and CYP2A6 enzymes but at different rates. Phenacetin and 4-aminobiphenyl were identified as substrates of human cytochromes P450 1A2 and 2A13 but not of CYP2A6. The purpose of this study was to identify active site amino acids that are responsible for CYP2A substrate specificity using phenacetin as a structural probe. Ten amino acid residues that differ in the CYP2A13 and CYP2A6 active sites were exchanged between the two enzymes. Phenacetin binding revealed that the six substitution, CYP2A13 S208I, A213S, F300I, A301G, M365V, and G369S decreased phenacetin affinity. Although incorporation of individual CYP2A13 residues into CYP2A6 had little effect on this enzyme's very low levels of phenacetin metabolism, the combination of double, triple, and quadruple substitutions at positions 208, 300, 301, and 369 increasingly endowed CYP2A6 with the ability to metabolize phenacetin. Enzyme kinetics revealed that the CYP2A6 I208S/I300F/G301A/S369G mutant protein O-deethylated phenacetin with a K(m) of 10.3 muM and a k(cat) of 2.9 min(-1), which compare very favorably with those of CYP2A13 (K(m) of 10.7 muM and k(cat) of 3.8 min(-1)). A 2.15 A crystal structure of the mutant CYP2A6 I208S/I300F/G301A/S369G protein with phenacetin in the active site provided a structural rationale for the differences in phenacetin metabolism between CYP2A6 and CYP2A13.

Characterization of CYP2A13*2, a variant cytochrome P450 allele previously found to be associated with decreased incidences of lung adenocarcinoma in smokers

CYP2A13, a human cytochrome P450 enzyme expressed mainly in the respiratory tract, is believed to play an important role in the initiation of smoking-induced lung cancer. CYP2A13.1 has high efficiency in the metabolic activation of a major tobacco-specific carcinogenic nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). CYP2A13(*)2, a variant allele, was previously found to be associated with decreased incidence of lung adenocarcinoma in smokers. The aim of the present study was to determine whether the CYP2A13.2 protein has decreased enzyme activity and/or expression levels in the lung, compared with CYP2A13.1. CYP2A13.2 has two sequence variations from CYP2A13.1: R25Q and R257C. We compared the activities of heterologously expressed CYP2A13.1 and CYP2A13.2 toward several known CYP2A13.1 substrates: NNK, N-nitrosomethylphenylamine, N,N-dimethylaniline, 2'-methoxyacetophenone, and hexamethylphosphoramide. Our results indicated that CYP2A13.2 was 20 to 40% less active than CYP2A13.1 with the substrates tested. We also determined the levels of the CYP2A13(*)2 mRNA, relative to the level of the CYP2A13(*)1 mRNA, in the lung tissue from (*)1/(*)2 heterozygotes. We found that the CYP2A13(*)2 allele was associated with a level of allelic expression approximately 40% lower than that of the CYP2A13(*)1 allele. Sequence analysis of the promoter region of the CYP2A13(*)2 allele identified a 26-nucleotide deletion. Functional analysis of a 2-kilobase pair CYP2A13-luciferase promoter construct indicated that the 26-nucleotide deletion causes decreases in CYP2A13 promoter activity in the A549 human lung cell line. These findings suggest that the reported association of the CYP2A13(*)2 allele with decreased incidences of lung adenocarcinoma in smokers can be at least partly explained by a decrease in CYP2A13 function.

CYP2A13: variable expression and role in human lung microsomal metabolic activation of the tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

CYP2A13 is the most efficient cytochrome P450 enzyme in the metabolic activation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a tobacco-specific lung carcinogen. The aims of this study were to determine the levels of CYP2A13 protein in human lung microsomes and to ascertain whether CYP2A13 plays any role in lung microsomal NNK metabolic activation. The expression of CYP2A6 and CYP2A13 was examined using a high-resolution immunoblotting method, following immunopurification with an anti-CYP2A5 antibody. We found that, of 116 human lung microsomal samples analyzed, approximately 90% had detectable CYP2A6, whereas only 12% had detectable CYP2A13 with a detection limit of approximately 2 fmol of CYP2A/mg protein. For the majority of microsomal samples analyzed, the level of CYP2A13 was found to be lower than the level of CYP2A6; overall, the highest level of CYP2A13 found ( approximately 20 fmol/mg protein) was approximately 10-fold lower than the highest level of CYP2A6 detected. Quantitative RNA-polymerase chain reaction analysis confirmed that the highly variable expression of the CYP2A proteins was consistent with variations in the levels of the corresponding CYP2A mRNAs in the same tissue samples. It is noteworthy that the level of CYP2A13, but not CYP2A6, was correlated with lung microsomal NNK metabolic activation activity. Furthermore, the addition of 8-methoxypsoralen, a CYP2A inhibitor, led to greater inhibition of NNK metabolic activation in microsomes containing relatively high levels of CYP2A13 than in samples containing no detectable CYP2A13. Taken together, these data indicate that human lung microsomal CYP2A13 is active in NNK metabolic activation. Therefore, individuals having relatively high levels of CYP2A13 expression will likely have an increased risk of developing smoking-related lung cancer.

Human CYP2A6 Is Induced by Estrogen via Estrogen Receptor

Human CYP2A6, which is predominantly expressed in liver, is a key enzyme responsible for the metabolism of nicotine, coumarin, and some pharmaceutical drugs. CYP2A6 is also expressed in sex steroid-responsive tissues such as breast, ovary, uterus, testis, and adrenal grand. In this study, we examined the regulation of CYP2A6 gene by estrogen. Reverse transcription-polymerase chain reaction (RT-PCR) assays revealed that CYP2A6 mRNA was induced by estradiol in estrogen receptor (ER)-positive MCF-7 (2.9-fold) and HepG2 (1.3-fold) cells, but not in ER-negative MDA-MB-435 cells. Real-time RT-PCR assays revealed the CYP2A6 induction by estradiol in human hepatocytes (1.2- to 1.5-fold). Computer-assisted homology search identified a putative estrogen response element (ERE) at -2436 on the CYP2A6 gene. Electrophoretic mobility shift assays demonstrated specific binding of ERalpha to this element. Luciferase assays using MCF-7 cells revealed that the transcriptional activity of the CYP2A6 promoter was significantly activated by estradiol in an ERalpha-dependent manner, in which ERE was responsible for the activation. Chromatin immunoprecipitation assays verified the in vivo association of ERalpha with the ERE on the CYP2A6 gene. Immunohistochemical analyses using human endometrial tissues indicated that the CYP2A6 protein level in glandular cells was significantly higher in the proliferative phase than in the secretory phase, concomitant with local estrogen secretion during the menstrual cycle. These findings clearly demonstrated that CYP2A6 is directly induced by estrogen in an ERalpha-dependent manner, implying a biological role of CYP2A6 in estrogen-responsive tissues. Furthermore, this mechanism can also explain clinical aspects of increased nicotine metabolism under estrogen-rich environments.

Inhibitory Effects of Neurotransmitters and Steroids on Human CYP2A6

Human CYP2A6 catalyzes the metabolism of nicotine, cotinine, and coumarin as well as some pharmaceutical drugs. CYP2A6 is highly expressed in liver and, also, in brain and steroid-related tissues. In this study, we investigated the inhibitory effects of neurotransmitters and steroid hormones on CYP2A6 activity. We found that coumarin 7-hydroxylation and cotinine 3'-hydroxylation by recombinant CYP2A6 expressed in baculovirus-infected insect cells were competitively inhibited by tryptamine (both K(i) = 0.2 microM), serotonin (K(i) = 252 microM and 167 microM), dopamine (K(i) = 49 microM and 22 microM), and histamine (K(i) = 428 microM and 359 microM). Cotinine formation from nicotine was inhibited by tryptamine (K(i) = 0.7 microM, competitive), serotonin (K(i) = 272 microM, noncompetitive), dopamine, noradrenaline, and adrenaline (K(i) = 11 microM, 54 microM, and 81 microM, uncompetitive). Estrogens (K(i) = 0.6-3.8 microM), androgens (K(i) = 60-149 microM), and corticosterone (K(i) = 36 microM) also inhibited cotinine formation, but coumarin 7-hydroxylation and cotinine 3'-hydroxylation did not. Nicotine-Delta(5'(1'))-iminium ion formation from nicotine was not affected by these steroid hormones, indicating that the inhibition of cotinine formation was due to the inhibitory effects on aldehyde oxidase. The nicotine-Delta(5'(1'))-iminium ion formation was competitively inhibited by tryptamine (K(i) = 0.3 microM), serotonin (K(i) = 316 microM), dopamine (K(i) = 66 microM), and histamine (K(i) = 209 microM). Thus, we found that some neurotransmitters inhibit CYP2A6 activity, being related with inter- and intraindividual differences in CYP2A6-dependent metabolism. The inhibitory effects of steroid hormones on aldehyde oxidase may also contribute to interindividual differences in nicotine metabolism.