Role of CYP2A13 in the bioactivation and lung tumorigenicity of the tobacco-specific lung procarcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone: in vivo studies using a CYP2A13-humanized mouse model

One-Week Kava Dietary Supplementation Increases Both Urinary N- and O-Glucuronides of NNAL, a Lung Carcinogen Major Metabolite, among Smokers

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (commonly known as NNK) is one of the most prevalent and potent pulmonary carcinogens in tobacco products that increases the human lung cancer risk. Kava has the potential to reduce NNK and tobacco smoke-induced lung cancer risk by enhancing urinary excretion of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL, the major metabolite of NNK) and thus reducing NNK-induced DNA damage. In this study, we quantified N-glucuronidated NNAL (NNAL-N-gluc), O-glucuronidated NNAL (NNAL-O-gluc), and free NNAL in the urine samples collected before and after 1-week kava dietary supplementation. The results showed that kava increased both NNAL-N-glucuronidation and O-glucuronidation. Since NNAL-N-glucuronidation is dominantly catalyzed by UGT2B10, its representative single-nucleotide polymorphisms (SNPs) were analyzed among the clinical trial participants. Individuals with any of the four analyzed SNPs appear to have a reduced basal capacity in NNAL-N-glucuronidation. Among these individuals, kava also resulted in a smaller extent of increases in NNAL-N-glucuronidation, suggesting that participants with those UGT2B10 SNPs may not benefit as much from kava with respect to enhancing NNAL-N-glucuronidation. In summary, our results provide further evidence that kava enhances NNAL urinary detoxification via an increase in both N-glucuronidation and O-glucuronidation. UGT2B10 genetic status has not only the potential to predict the basal capacity of the participants in NNAL-N-glucuronidation but also potentially the extent of kava benefits.

Acceleration of infectious disease drug discovery and development using a humanized model of drug metabolism

A key step in drug discovery, common to many disease areas, is preclinical demonstration of efficacy in a mouse model of disease. However, this demonstration and its translation to the clinic can be impeded by mouse-specific pathways of drug metabolism. Here, we show that a mouse line extensively humanized for the cytochrome P450 gene superfamily ("8HUM") can circumvent these problems. The pharmacokinetics, metabolite profiles, and magnitude of drug-drug interactions of a test set of approved medicines were in much closer alignment with clinical observations than in wild-type mice. Infection with Mycobacterium tuberculosis, Leishmania donovani, and Trypanosoma cruzi was well tolerated in 8HUM, permitting efficacy assessment. During such assessments, mouse-specific metabolic liabilities were bypassed while the impact of clinically relevant active metabolites and DDI on efficacy were well captured. Removal of species differences in metabolism by replacement of wild-type mice with 8HUM therefore reduces compound attrition while improving clinical translation, accelerating drug discovery.

Luteolin Alleviates Oxidative Stress in Chronic Obstructive Pulmonary Disease Induced by Cigarette Smoke via Modulation of the TRPV1 and CYP2A13/NRF2 Signaling Pathways

The current study aims to investigate the therapeutic potential of luteolin (Lut), a naturally occurring flavonoid found in various medicinal plants, for treating chronic obstructive pulmonary disease (COPD) through both in vitro and in vivo studies. The results demonstrated that Lut increased body weight, reduced lung tissue swelling and lung damage indices, mitigated systemic oxidative stress levels, and decreased alveolar fusion in cigarette smoke (CS)- and lipopolysaccharide (LPS)-induced COPD mice. Additionally, Lut was observed to downregulate the expression of the TRPV1 and CYP2A13 proteins while upregulating SIRT6 and NRF2 protein expression in CS + LPS-induced COPD mice and cigarette smoke extract (CSE)-treated A549 cells. The concentrations of total reactive oxygen species (ROS) and mitochondrial ROS in A549 cells induced by CSE significantly increased. Moreover, CSE caused a notable elevation of intracellular Ca2+ levels in A549 cells. Importantly, Lut exhibited inhibitory effects on the inward flow of Ca2+ and attenuated the overproduction of mitochondrial and intracellular ROS in A549 cells treated with CSE. In conclusion, Lut demonstrated a protective role in alleviating oxidative stress and inflammation in CS + LPS-induced COPD mice and CSE-treated A549 cells by regulating TRPV1/SIRT6 and CYP2A13/NRF2 signaling pathways.

Detection of Transgene Location in the CYP2A13/2B6/2F1-transgenic Mouse Model using Optical Genome Mapping Technology

Most transgenic mouse models are generated through random integration of the transgene. The location of the transgene provides valuable information for assessing potential effects of the transgenesis on the host and for designing genotyping protocols that can amplify across the integration site, but it is challenging to identify. Here, we report the successful utility of optical genome mapping technology to identify the transgene insertion site in a CYP2A13/2B6/2F1-transgenic mouse model, which produces three human cytochrome P450 (P450) enzymes (CYP2A13, CYP2B6, and CYP2F1) that are encoded by neighboring genes on human chromosome 19. These enzymes metabolize many drugs, respiratory toxicants, and chemical carcinogens. Initial efforts to identify candidate insertion sites by whole genome sequencing was unsuccessful, apparently because the transgene is located in a region of the mouse genome that contains highly repetitive sequences. Subsequent utility of the optical genome mapping approach, which compares genome-wide marker distribution between the transgenic mouse genome and a reference mouse (GRCm38) or human (GRCh38) genome, localized the insertion site to mouse chromosome 14, between two marker positions at 4451324 base pair and 4485032 base pair. A transgene-mouse genome junction sequence was further identified through long-polymerase chain reaction amplification and DNA sequencing at GRCm38 Chr.14:4484726. The transgene insertion (∼2.4 megabase pair) contained 5-7 copies of the human transgenes, which replaced a 26.9-33.4 kilobase pair mouse genomic region, including exons 1-4 of Gm3182, a predicted and highly redundant gene. Finally, the sequencing results enabled the design of a new genotyping protocol that can distinguish between hemizygous and homozygous CYP2A13/2B6/2F1-transgenic mice. SIGNIFICANCE STATEMENT: This study characterizes the genomic structure of, and provides a new genotyping method for, a transgenic mouse model that expresses three human P450 enzymes, CYP2A13, CYP2B6, and CYP2F1, that are important in xenobiotic metabolism and toxicity. The demonstrated success in applying the optical genome mapping technology for identification of transgene insertion sites should encourage others to do the same for other transgenic models generated through random integration, including most of the currently available human P450 transgenic mouse models.

Nicotine Inhibits the Cytotoxicity and Genotoxicity of NNK Mediated by CYP2A13 in BEAS-2B Cells

Both tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and nicotine can be metabolized by cytochrome P450 2A13 (CYP2A13). Previous studies have shown that nicotine has a potential inhibitory effect on the toxicity of NNK. However, due to the lack of CYP2A13 activity in conventional lung cell lines, there had been no systematic in vitro investigation for the key target organ, the lung. Here, BEAS-2B cells stably expressing CYP2A13 (B-2A13 cells) were constructed to investigate the effects of nicotine on the cytotoxicity and genotoxicity of NNK. The results showed more sensitivity for NNK-induced cytotoxicity in B-2A13 cells than in BEAS-2B and B-vector cells. NNK significantly induced DNA damage, cell cycle arrest, and chromosomal damage in B-2A13 cells, but had no significant effect on BEAS-2B cells and the vector control cells. The combination of different concentration gradient of nicotine without cytotoxic effects and a single concentration of NNK reduced or even counteracted the cytotoxicity and multi-dimensional genotoxicity in a dose-dependent manner. In conclusion, CYP2A13 caused the cytotoxicity and genotoxicity of NNK in BEAS-2B cells, and the addition of nicotine could inhibit the toxicity of NNK.

Cytochrome P450 27C1 Level Dictates Lung Cancer Tumorigenicity and Sensitivity towards Multiple Anticancer Agents and Its Potential Interplay with the IGF-1R/Akt/p53 Signaling Pathway

Cytochrome P450 enzymes (CYP450s) exert mighty catalytic actions in cellular metabolism and detoxication, which play pivotal roles in cell fate determination. Preliminary data shows differential expression levels of CYP27C1, one of the "orphan P450s" in human lung cancer cell lines. Here, we study the functions of CYP27C1 in lung cancer progression and drug endurance, and explore its potential to be a diagnostic and therapeutic target for lung cancer management. Quantitative real-time PCR and immunoblot assays were conducted to estimate the transcription and protein expression level of CYP27C1 in human lung cancer cell lines, which was relatively higher in A549 and H1975 cells, but was lower in H460 cells. Stable CYP27C1-knockdown A549 and H1975 cell lines were established, in which these cells showed enhancement in cell proliferation, colony formation, and migration. In addition, aberrant IGF-1R/Akt/p53 signal transduction was also detected in stable CYP27C1-knockdown human lung cancer cells, which exhibited greater tolerance towards the treatments of anticancer agents (including vinorelbine, picropodophyllin, pacritinib, and SKLB610). This work, for the first time, reveals that CYP27C1 impacts lung cancer cell development by participating in the regulation of the IGF-1R/Akt/p53 signaling pathway, and the level of CYP27C1 plays indispensable roles in dictating the cellular sensitivity towards multiple anticancer agents.

Assessing cytochrome P450 function using genetically engineered mouse models

The ability to knock out and/or humanize different genes in experimental animals, globally or in cell- and tissue-specific patterns, has revolutionized scientific research in many areas. Genetically engineered mouse models, including knockout models, transgenic models, and humanized models, have played important roles in revealing the in vivo functions of various cytochrome P450 (CYP) enzymes. These functions are very diverse, ranging from the biotransformation of drugs and other xenobiotics, events that often dictate their pharmacokinetic or toxicokinetic properties and the associated therapeutic or adverse actions, to the metabolism of endogenous compounds, such as steroid hormones and other bioactive substances, that may determine susceptibility to many diseases, such as cancer and metabolic diseases. In this review, we provide a comprehensive list of Cyp-knockout, human CYP-transgenic, and CYP-humanized mouse models that target genes in the CYP1-4 gene families, and highlight their utility in assessing the in vivo metabolism, bioactivation, and toxicity of various xenobiotic compounds, including therapeutic agents and chemical carcinogens. We aim to showcase the advantages of utilizing these mouse models for in vivo drug metabolism and toxicology studies, and to encourage and facilitate greater utility of engineered mouse models to further improve our knowledge of the in vivo functions of various P450 enzymes, which is integral to our ability to develop safer and more effective therapeutics and to identify individuals predisposed to adverse drug reactions or environmental diseases.

Cytochromes P450: Role in Carcinogenesis and Relevance to Cancers

Abstracts:

Cancer is a leading factor of mortality globally. Cytochrome P450 (CYP) enzymes play a pivotal role in the biotransformation of both endogenous and exogenous compounds. Evidence from numerous epidemiological, animal, and clinical studies points to instrumental role of CYPs in cancer initiation, metastasis, and prevention. Substantial research has found that CYPs are involved in activating different carcinogenic chemicals in the environment, such as polycyclic aromatic hydrocarbons and tobacco-related nitrosamines. Electrophilic intermediates produced from these chemicals can covalently bind to DNA, inducing mutation and cellular transformation that collectively result in cancer development. While bioactivation of procarcinogens and promutagens by CYPs has long been established, the role of CYP-derived endobiotics in carcinogenesis has emerged in recent years. Eicosanoids derived from arachidonic acid via CYP oxidative pathways have been implicated in tumorigenesis, cancer progression and metastasis. The purpose of this review is to update on the current state of knowledge about the cancer molecular mechanism involving CYPs with focus on the biochemical and biotransformation mechanisms in the various CYP-mediated carcinogenesis, and the role of CYP-derived reactive metabolites, from both external and endogenous sources, on cancer growth and tumour formation.

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).

Identification of Key Functional Modules and Immunomodulatory Regulators of Hepatocellular Carcinoma

Despite the advances in the treatment of hepatocellular carcinoma (HCC), the prognosis of HCC patients remains unsatisfactory due to postsurgical recurrence and treatment resistance. Therefore, it is important to reveal the mechanisms underlying HCC and identify potential therapeutic targets against HCC, which could facilitate the development of novel therapies. Based on 12 HCC samples and 12 paired paracancerous normal tissues, we identified differentially expressed mRNAs and lncRNAs using the "limma" package in R software. Moreover, we used the weighted gene coexpression network analysis (WGCNA) to analyze the expression data and screened hub genes. Furthermore, we performed pathway enrichment analysis based on the Kyoto Encyclopedia of Genes and Genomes (KEGG) database. In addition, the relative abundance of a given gene set was estimated by single-sample Gene Set Enrichment Analysis. We identified 687 differentially expressed mRNAs and 260 differentially expressed lncRNAs. A total of 6 modules were revealed by WGCNA, and MT1M and MT1E genes from the red module were identified as hub genes. Moreover, pathway analysis revealed the top 10 enriched KEGG pathways of upregulated or downregulated genes. Additionally, we also found that CD58 might act as an immune checkpoint gene in HCC via PD1/CTLA4 pathways and regulate the levels of tumor-infiltrating immune cells in HCC tissues, which might be an immunotherapeutic target in HCC. Our research identified key functional modules and immunomodulatory regulators for HCC, which might offer novel diagnostic biomarkers and/or therapeutic targets for cancer immunotherapy.

Human Family 1-4 cytochrome P450 enzymes involved in the metabolic activation of xenobiotic and physiological chemicals: an update

This is an overview of the metabolic activation of drugs, natural products, physiological compounds, and general chemicals by the catalytic activity of cytochrome P450 enzymes belonging to Families 1-4. The data were collected from > 5152 references. The total number of data entries of reactions catalyzed by P450s Families 1-4 was 7696 of which 1121 (~ 15%) were defined as bioactivation reactions of different degrees. The data were divided into groups of General Chemicals, Drugs, Natural Products, and Physiological Compounds, presented in tabular form. The metabolism and bioactivation of selected examples of each group are discussed. In most of the cases, the metabolites are directly toxic chemicals reacting with cell macromolecules, but in some cases the metabolites formed are not direct toxicants but participate as substrates in succeeding metabolic reactions (e.g., conjugation reactions), the products of which are final toxicants. We identified a high level of activation for three groups of compounds (General Chemicals, Drugs, and Natural Products) yielding activated metabolites and the generally low participation of Physiological Compounds in bioactivation reactions. In the group of General Chemicals, P450 enzymes 1A1, 1A2, and 1B1 dominate in the formation of activated metabolites. Drugs are mostly activated by the enzyme P450 3A4, and Natural Products by P450s 1A2, 2E1, and 3A4. Physiological Compounds showed no clearly dominant enzyme, but the highest numbers of activations are attributed to P450 1A, 1B1, and 3A enzymes. The results thus show, perhaps not surprisingly, that Physiological Compounds are infrequent substrates in bioactivation reactions catalyzed by P450 enzyme Families 1-4, with the exception of estrogens and arachidonic acid. The results thus provide information on the enzymes that activate specific groups of chemicals to toxic metabolites.

Structure-activity relationship and in vitro inhibition of human cytochrome CYP2A6 and CYP2A13 by flavonoids

Cigarette smoking is one of the major risk factors of various diseases including respiratory diseases and lung cancer. While the liver-specific CYP2A6 is associated with the nicotine clearance and smoking addiction, the metabolic activation of the tobacco-specific nitrosamine by lung-specific CYP2A13 can lead to lung tumorigenesis.It has been reported that inhibition of CYP2A6 and CYP2A13 enzymes by flavonoids constituents could be an aids in smoking cessation. This study demonstrates the inhibition activity of kaempferol and myricetin and the structure-function relationship of these two flavonoids and previously isolated flavonoids from Vernonia cinerea and Pluchea indica against both enzymes.Kaempferol could inhibit CYP2A6 with K_ic value of 1.77 ± 0.47 µM while inhibit CYP2A13 with K_ic value of 0.12 ± 0.01 µM. Myricetin could inhibit CYP2A6 with K_ic value of 4.06 ± 0.52 µM while inhibit CYP2A13 with K_ic value of 1.88 ± 0.03 µM.Molecular docking indicated that CYP2A13 enzyme has strong hydrophobic interaction with ring B of flavonoids compared to CYP2A6 enzyme. The presence of the hydroxyl group at C3 position of ring C and the hydroxyl group at C5' of ring B affected inhibitory activity on both enzymes.

Role of mouse cytochrome P450 enzymes of the CYP2ABFGS subfamilies in the induction of lung inflammation by cigarette smoke exposure

BACKGROUND

Many constituents of tobacco smoke (TS) require bioactivation to exert toxic effects; however, few studies have examined the role of bioactivation enzymes in the adverse effects of TS exposure. This knowledge gap is a major source of uncertainty for risk assessment and chemoprevention efforts.

OBJECTIVES

Our aim is to test the hypothesis that cytochrome P450 (P450) enzyme mediated bioactivation is essential to the development of TS exposure-induced lung toxicity, by determining the contributions of P450 enzymes in the mouse Cyp2abfgs gene subfamilies to environmental tobacco smoke (ETS)-induced lung inflammation.

METHODS

Adult female wildtype (WT) and Cyp2abfgs-null mice (both on C57BL/6J background) were exposed to filtered air or ETS, intermittently, for 1 or 2 weeks. Lung inflammation was assessed by quantification of inflammatory cells, cytokines, chemokines, proteins in bronchoalveolar lavage fluid (BALF), and histopathological analysis. Glutathione (GSH) conjugates of two ETS constituents, naphthalene (NA) and 3-methylindole (3MI), were measured in mice exposed to ETS for four hours.

RESULTS

Persistent macrophagic and neutrophilic lung inflammation was observed in ETS-exposed WT mice; the extent of which was significantly reduced in ETS-exposed Cyp2abfgs-null mice. Levels of proinflammatory cytokines and chemokines, along with the total protein concentration, were increased in cell-free BALF from ETS-exposed WT mice, but not Cyp2abfgs-null mice. Additionally, GSH-conjugates of NA and 3MI were detected in the lungs of WT, but not Cyp2abfgs-null, mice following ETS exposure.

CONCLUSIONS

These results provide the first in vivo evidence that the mouse Cyp2abfgs gene cluster plays an important role in ETS-induced lung inflammation.

Effects of cellular differentiation in human primary bronchial epithelial cells: Metabolism of 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanone

Many of the toxicants in tobacco smoke undergo biotransformation in the lungs of smokers, both to reactive and to detoxified derivatives. Human air-liquid-interface (ALI) airway tissue models have emerged as an advanced in vitro model for evaluating the toxicity of inhaled substances; however, the metabolic potential of these cultures has not been evaluated extensively. In this study, we compared the metabolic activities of an ALI tissue model to the undifferentiated normal human primary bronchial epithelial (NHBE) cells from which it was derived. Measurement of the basal levels of gene expression for 84 phase I drug metabolism enzymes indicated that most genes were upregulated in ALI cultures compared to NHBE cells. Furthermore, the enzymatic activities of three cytochrome P450s involved in the bioactivation of tobacco-specific nitrosamines were higher in the ALI cultures, and the bioactivation of 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanone (NNK), as measured by the formation of two of its major metabolites, i.e., keto acid and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), was significantly greater in the ALI cultures. Finally, NNK was a direct-acting genotoxicant in the ALI cultures, while the genotoxicity of NNK was detected in NHBE cells only in the presence of an exogenous liver S9 activation system. Taken together, our findings demonstrate the greater metabolic potential of well-differentiated ALI cultures than primary NHBE cells, supporting the potential use of ALI airway cultures as an alternative in vitro model for evaluating inhaled toxicants that require metabolic transformation.

HapMap-based study: CYP2A13 may be a potential key metabolic enzyme gene in the carcinogenesis of lung cancer in non-smokers

Background

The aim of this study was to evaluate the association between CYP2A13 polymorphisms and lung cancer susceptibility using the HapMap database.

Methods

A case‐control analysis of 532 subjects with lung cancer and 614 controls with no personal history of the disease was performed. The tag SNPs rs1645690 and rs8192789 for CYP2A13 were selected, and the genetic polymorphisms were confirmed experimentally through real‐time PCR, cloning, and sequencing assay.

Results

SNP frequency in this study was consistent with the HapMap Project database of Han‐Chinese and lung cancer risk was associated with CYP2A13 polymorphisms in non‐smokers. CYP2A13 shares a 93.5% identity with CYP2A6 in the amino acid sequence and the homologous sequences may interfere with the study of SNPs of CYP2A13.

Conclusions

CYP2A13 may be a potential key metabolic enzyme gene in the carcinogenesis of lung cancer in non‐smokers. The common polymorphisms of CYP2A13 may be candidate biomarkers for lung cancer susceptibility in Han‐Chinese.

Carcinogens and DNA damage

Humans are variously and continuously exposed to a wide range of different DNA-damaging agents, some of which are classed as carcinogens. DNA damage can arise from exposure to exogenous agents, but damage from endogenous processes is probably far more prevalent. That said, epidemiological studies of migrant populations from regions of low cancer risk to high cancer risk countries point to a role for environmental and/or lifestyle factors playing a pivotal part in cancer aetiology. One might reasonably surmise from this that carcinogens found in our environment or diet are culpable. Exposure to carcinogens is associated with various forms of DNA damage such as single-stand breaks, double-strand breaks, covalently bound chemical DNA adducts, oxidative-induced lesions and DNA-DNA or DNA-protein cross-links. This review predominantly concentrates on DNA damage induced by the following carcinogens: polycyclic aromatic hydrocarbons, heterocyclic aromatic amines, mycotoxins, ultraviolet light, ionising radiation, aristolochic acid, nitrosamines and particulate matter. Additionally, we allude to some of the cancer types where there is molecular epidemiological evidence that these agents are aetiological risk factors. The complex role that carcinogens play in the pathophysiology of cancer development remains obscure, but DNA damage remains pivotal to this process.

Identification of 5-hydroxymethylfurfural in cigarette smoke extract as a new substrate metabolically activated by human cytochrome P450 2A13

Cytochrome P450 2A13 (CYP2A13) is an extrahepatic enzyme mainly expressed in the human respiratory system and is reported to mediate tobacco-specific N-nitrosamines (TSNA) metabolism in cigarette smoke. This study aimed to identify other new substrates of CYP2A13 in cigarette smoke and their corresponding respiratory toxicity. Following separation by HPLC, GC-MS/MS, NMR and cytotoxicity assays in BEAS-2B cells stably expressing CYP2A13 (B-2A13), 5-Hydroxymethylfurfural (5-HMF) was screened and identified in the 4-5 min section of cigarette smoke extract (CSE). In vitro metabolism results showed that CYP2A13 mediated the fast clearance of 5-HMF and formed the metabolite 5-HMF acid (5-HMFA). CSE 5-HMF (CSE-5-HMF) showed cytotoxicity similar to that of standard 5-HMF in B-2A13 and B-2A5 cells, which was inhibited by 8-methoxypsoralen (8-MOP), a CYP enzyme inhibitor. Mouse CYP2A5, a homologous CYP enzyme to CYP2A13, shares many substrates with CYP2A13 in cigarette smoke. Thus, CYP2A5^-/- mice were generated to explore the role of CYP2A5 in 5-HMF bioactivation. Compared with CYP2A5^-/- mice, WT mice showed serious histological lung and nasal olfactory mucosa damage, as well as increased inflammatory cells and elevated TNF-α and IL-6 levels in bronchoalveolar lavage fluid. Besides, nasal microsomes undertook fast 5-HMFA formation in WT mice than that in CYP2A5^-/- mice, which could be inhibited by 8-MOP. This study is the first to identify 5-HMF as a new toxic substrate of human CYP2A13 in cigarette smoke, it may play a potential role in cigarette smoke-induced respiratory injuries.

P450-Humanized and Human Liver Chimeric Mouse Models for Studying Xenobiotic Metabolism and Toxicity

Preclinical evaluation of drug candidates in experimental animal models is an essential step in drug development. Humanized mouse models have emerged as a promising alternative to traditional animal models. The purpose of this mini-review is to provide a brief survey of currently available mouse models for studying human xenobiotic metabolism. Here, we describe both genetic humanization and human liver chimeric mouse models, focusing on the advantages and limitations while outlining their key features and applications. Although this field of biomedical science is relatively young, these humanized mouse models have the potential to transform preclinical drug testing and eventually lead to a more cost-effective and rapid development of new therapies.

Chemopreventive action of Imatinib, a tyrosine kinase inhibitor in the regulation of angiogenesis and apoptosis in rat model of lung cancer

The present study explored the events of angiogenesis and apoptosis in 7,12-dimethyl benz(a)anthracene (DMBA)-induced lung cancer in rat and its chemoprevention with Imatinib, a receptor tyrosine kinase inhibitor. Further, it includes  lipopolysaccharide (LPS) mediating inflammation along with DMBA for the promotion of lung carcinogenesis. The animals received a single intratracheal instillation of DMBA (20 mg/kg body weight) in olive oil and LPS (0.6 mg/kg body weight) to induce tumors in 16 weeks. Besides morphology and histology of the lung tissues, RT-PCR, western blots, and immunofluorescence were performed for the expression of apoptotic and angiogenic proteins. Apoptosis was studied by mitochondrial Bcl-2/Bax ratio and staining with the dyes Acridine orange/ethidium bromide of the isolated Broncho epithelial cells. Also, mitochondrial membrane potential (ΔΨ_M) was studied by JC-1. The study revealed that the expression of VEGF, MMP-2, MMP-9, and the chemokine MCP-1 to be very high in DMBA and DMBA + LPS groups, while Bcl-2 also shows an elevated expression. These results were restored with Imatinib treatment. The pro-apoptotic proteins, Bax, Bad, Apaf-1, and Caspase-3 were highly diminished in DMBA and DMBA + LPS groups which were recovered with Imatinib treatment.

A CYP2B6-humanized mouse model and its potential applications

CYP2B6 is a human microsomal cytochrome P450 enzyme with broad substrate selectivity. CYP2B6 is the only functional member of the human CYP2B gene subfamily, which differs from the situation in rodents, such as mouse, where multiple functional Cyp2b genes are expressed. Recent studies with Cyp2b knockout or knockdown mouse models have yielded insights into the in vivo roles of mouse CYP2B enzymes in drug disposition and xenobiotic toxicity. A CYP2B6-humanized mouse model (CYP2A13/2B6/2F1-transgenic/Cyp2abfgs-null), which expresses human CYP2B6 in the liver, and human CYP2A13 and CYP2F1 in the respiratory tract, but not any of the mouse Cyp2b genes, has also been established. In the CYP2B6-humanized mouse, the CYP2B6 transgene is expressed primarily in the liver, where it was found to be active toward prototype CYP2B6 substrate drugs. The regulatory elements of the CYP2B6 transgene appear to be compatible with mouse nuclear receptors that mediate CYP2B induction. Therefore, the CYP2B6-humanized mouse is a valuable animal model for studying the impact of CYP2B6 expression or induction on drug metabolism, drug efficacy, drug-drug interaction, and drug/xenobiotic toxicity. In this mini-review, we provide a brief background on CYP2B6 and the Cyp2b-knockout and CYP2B6-humanized mice, and discuss the potential applications and limitations of the current models.

Cytochrome P450s and Alcoholic Liver Disease

Alcohol consumption causes liver diseases, designated as Alcoholic Liver Disease (ALD). Because alcohol is detoxified by alcohol dehydrogenase (ADH), a major ethanol metabolism system, the development of ALD was initially believed to be due to malnutrition caused by alcohol metabolism in liver. The discovery of the microsomal ethanol oxidizing system (MEOS) changed this dogma. Cytochrome P450 enzymes (CYP) constitute the major components of MEOS. Cytochrome P450 2E1 (CYP2E1) in MEOS is one of the major ROS generators in liver and is considered to be contributive to ALD. Our labs have been studying the relationship between CYP2E1 and ALD for many years. Recently, we found that human CYP2A6 and its mouse analog CYP2A5 are also induced by alcohol. In mice, the alcohol induction of CYP2A5 is CYP2E1-dependent. Unlike CYP2E1, CYP2A5 protects against the development of ALD. The relationship of CYP2E1, CYP2A5, and ALD is a major focus of this review.

Nicotine Component of Cigarette Smoke Extract (CSE) Decreases the Cytotoxicity of CSE in BEAS-2B Cells Stably Expressing Human Cytochrome P450 2A13

Cytochrome P450 2A13 (CYP2A13), an extrahepatic enzyme mainly expressed in the human respiratory system, has been reported to mediate the metabolism and toxicity of cigarette smoke. We previously found that nicotine inhibited 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) metabolism by CYP2A13, but its influence on other components of cigarette smoke remains unclear. The nicotine component of cigarette smoke extract (CSE) was separated, purified, and identified using high-performance liquid chromatography (HPLC) and ultra-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS), splitting CSE into a nicotine section (CSE-N) and nicotine-free section (CSE-O). Cell viability and apoptosis by Cell Counting Kit-8 (CCK-8) and flow cytometry assays were conducted on immortalized human bronchial epithelial (BEAS-2B) cells stably expressing CYP2A13 (B-2A13) or vector (B-V), respectively. Interestingly, CSE and CSE-O were toxic to BEAS-2B cells whereas CSE-N showed less cytotoxicity. CSE-O was more toxic to B-2A13 cells than to B-V cells (IC₅₀ of 2.49% vs. 7.06%), which was flatted by 8-methoxypsoralen (8-MOP), a CYP inhibitor. CSE-O rather than CSE or CSE-N increased apoptosis of B-2A13 cells rather than B-V cells. Accordingly, compared to CSE-N and CSE, CSE-O significantly changed the expression of three pairs of pro- and anti-apoptotic proteins, Bcl-2 Associated X Protein/B cell lymphoma-2 (Bax/Bcl-2), Cleaved Poly (Adenosine Diphosphate-Ribose) Polymerase/Poly (Adenosine Diphosphate-Ribose) Polymerase (C-PARP/PARP), and C-caspase-3/caspase-3, in B-2A13 cells. In addition, recombination of CSE-N and CSE-O (CSE-O/N) showed similar cytotoxicity and apoptosis to the original CSE. These results demonstrate that the nicotine component decreases the metabolic activation of CYP2A13 to CSE and aids in understanding the critical role of CYP2A13 in human respiratory diseases caused by cigarette smoking.

Inhibition of human cytochromes P450 2A6 and 2A13 by flavonoids, acetylenic thiophenes and sesquiterpene lactones from Pluchea indica and Vernonia cinerea

The human liver cytochrome P450 (CYP) 2A6 and the respiratory CYP2A13 enzymes play role in nicotine metabolism and activation of tobacco-specific nitrosamine carcinogens. Inhibition of both enzymes could offer a strategy for smoking abstinence and decreased risks of respiratory diseases and lung cancer. In this study, activity-guided isolation identified four flavonoids 1–4 (apigenin, luteolin, chrysoeriol, quercetin) from Vernonia cinerea and Pluchea indica, four hirsutinolide-type sesquiterpene lactones 5–8 from V. cinerea, and acetylenic thiophenes 9–11 from P. indica that inhibited CYP2A6- and CYP2A13-mediated coumarin 7-hydroxylation. Flavonoids were most effective in inhibition against CYP2A6 and CYP2A13, followed by thiophenes, and hirsutinolides. Hirsutinolides and thiophenes exhibited mechanism-based inhibition and in irreversible mode against both enzymes. The inactivation kinetic K_I values of hirsutinolides against CYP2A6 and CYP2A13 were 5.32–15.4 and 0.92–8.67 µM, respectively, while those of thiophenes were 0.11–1.01 and 0.67–0.97 µM, respectively.

Inhibition of Carcinogen-Activating Cytochrome P450 Enzymes by Xenobiotic Chemicals in Relation to Antimutagenicity and Anticarcinogenicity

A variety of xenobiotic chemicals, such as polycyclic aromatic hydrocarbons (PAHs), aryl- and heterocyclic amines and tobacco related nitrosamines, are ubiquitous environmental carcinogens and are required to be activated to chemically reactive metabolites by xenobiotic-metabolizing enzymes, including cytochrome P450 (P450 or CYP), in order to initiate cell transformation. Of various human P450 enzymes determined to date, CYP1A1, 1A2, 1B1, 2A13, 2A6, 2E1, and 3A4 are reported to play critical roles in the bioactivation of these carcinogenic chemicals. In vivo studies have shown that disruption of Cyp1b1 and Cyp2a5 genes in mice resulted in suppression of tumor formation caused by 7,12-dimethylbenz[a]anthracene and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, respectively. In addition, specific inhibitors for CYP1 and 2A enzymes are able to suppress tumor formation caused by several carcinogens in experimental animals in vivo, when these inhibitors are applied before or just after the administration of carcinogens. In this review, we describe recent progress, including our own studies done during past decade, on the nature of inhibitors of human CYP1 and CYP2A enzymes that have been shown to activate carcinogenic PAHs and tobacco-related nitrosamines, respectively, in humans. The inhibitors considered here include a variety of carcinogenic and/or non-carcinogenic PAHs and acethylenic PAHs, many flavonoid derivatives, derivatives of naphthalene, phenanthrene, biphenyl, and pyrene and chemopreventive organoselenium compounds, such as benzyl selenocyanate and benzyl selenocyanate; o-XSC, 1,2-, 1,3-, and 1,4-phenylenebis( methylene)selenocyanate.

On the safety of e-cigarettes: "I can resist anything except temptation"

Strategic policy decisions are being made about e-cigarettes, based on the plausibility of their greater safety, rather than on essential scientific evidence which would permit a proper risk assessment. If e-cigarettes are really ‘safer’, then their use should be recommended, but only after an intelligent analysis of their risk to human health, based on integrated in silico, in vitro and clinical studies for both scientific and logistical reasons

Dietary Dihydromethysticin Increases Glucuronidation of 4-(Methylnitrosamino)-1-(3-Pyridyl)-1-Butanol in A/J Mice, Potentially Enhancing Its Detoxification

Effective chemopreventive agents are needed against lung cancer, the leading cause of cancer death. Results from our previous work showed that dietary dihydromethysticin (DHM) effectively blocked initiation of lung tumorigenesis by 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in A/J mice, and it preferentially reduced 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL)-derived DNA adducts in lung. This study explored the mechanism(s) responsible for DHM's differential effects on NNK/NNAL-derived DNA damage by quantifying their metabolites in A/J mice. The results showed that dietary DHM had no effect on NNK or NNAL abundance in vivo, indicating that DHM does not affect NNAL formation from NNK. DHM had a minimal effect on cytochrome P450 2A5 (CYP2A5, which catalyzes NNK and NNAL bioactivation in A/J mouse lung), suggesting that it does not inhibit NNAL bioactivation. Dietary DHM significantly increased O-glucuronidated NNAL (NNAL-O-gluc) in A/J mice. Lung and liver microsomes from dietary DHM-treated mice showed enhanced activities for NNAL O-glucuronidation. These results overall support the notion that dietary DHM treatment increases NNAL detoxification, potentially accounting for its chemopreventive efficacy against NNK-induced lung tumorigenesis in A/J mice. The ratio of urinary NNAL-O-gluc and free NNAL may serve as a biomarker to facilitate the clinical evaluation of DHM-based lung cancer chemopreventive agents.

Role of Cytochrome P450 Monooxygenase in Carcinogen and Chemotherapeutic Drug Metabolism

The purpose of this chapter is to provide insight into which human cytochromes P450 (CYPs) may be involved in metabolism of chemical carcinogens and anticancer drugs. A historical overview of this field and the development of literature using relevant animal models and expressed human CYPs have provided information about which specific CYPs may be involved in carcinogen metabolism. Definition of the biochemical properties of CYP activity came from several groups who studied the reaction stoichiometry of butter yellow and benzo[α]pyrene, including their role in induction of these enzyme systems. This chapter will list as much as is known about the human CYPs involved in carcinogen and anticancer drug metabolism, as well as summarize studies with rodent CYPs. A review of three major classes of anticancer drugs and their metabolism in humans is covered for cyclophosphamide, procarbazine, and anthracycline antibiotics, cancer chemotherapeutic compounds extensively metabolized by CYPs. The emerging information about human CYP gene polymorphisms as well as other enzymes involved in foreign compound metabolism provides considerable information about how these genetic variants affect carcinogen and anticancer drug metabolism. With information available from individual's genomic sequences, consideration of populations who may be at risk due to environmental exposure to carcinogens or how to optimize their cancer therapy regimens to enhance efficacy of the anticancer drugs appears to be an important field of study to benefit individuals in the future.

Preclinical Murine Models for Lung Cancer: Clinical Trial Applications

Murine models for the study of lung cancer have historically been the backbone of preliminary preclinical data to support early human clinical trials. However, the availability of multiple experimental systems leads to debate concerning which model, if any, is best suited for a particular therapeutic strategy. It is imperative that these models accurately predict clinical benefit of therapy. This review provides an overview of the current murine models used to study lung cancer and the advantages and limitations of each model, as well as a retrospective evaluation of the uses of each model with respect to accuracy in predicting clinical benefit of therapy. A better understanding of murine models and their uses, as well as their limitations may aid future research concerning the development and implementation of new targeted therapies and chemotherapeutic agents for lung cancer.

Suppression of pulmonary CYP2A13 expression by carcinogen-induced lung tumorigenesis in a CYP2A13-humanized mouse model

CYP2A13 is a human cytochrome P450 (P450) enzyme important in the bioactivation of the tobacco-specific lung procarcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). CYP2A13 expression levels vary dramatically among lung biopsy samples from patients, presumably owing in part to a suppression of CYP2A13 expression by disease-associated inflammation. Here, we determined whether CYP2A13 expression in the lungs of CYP2A13-humanized mice is suppressed by the presence of lung tumors. Tissues from an NNK lung tumor bioassay were examined. CYP2A13-humanized mice (95-100%) had multiple lung tumors at 16 weeks after NNK (30 or 50 mg/kg) treatment; whereas only ∼9% of saline-treated CYP2A13-humanized mice had lung tumor (∼1/lung). Mice with lung tumors, from the NNK-treated groups, were used for dissecting adjacent tumor-free lung tissues; whereas mice without visible lung tumors, from the saline-treated group, were used as controls. Compared with the controls, the levels of CYP2A13 protein and mRNA were both reduced significantly (by ≥50%) in the NNK-treated groups. The levels of mouse CYP2B10 and CYP2F2 mRNAs were also significantly lower in the dissected normal lung tissues from tumor-bearing mice than in lungs from the control mice. Pulmonary tissue levels of three proinflammatory cytokines, tumor necrosis factor alpha, interferon gamma, and interleukin-6, were significantly higher in the tumor-bearing mice than in the controls, indicating occurrence of low-grade lung inflammation at the time of necropsy. Taken together, these findings support the hypothesis that CYP2A13 levels in human lungs can be suppressed by disease-associated inflammation in tissue donors, a scenario causing underestimation of CYP2A13 levels in healthy lungs.

Pulmonary expression of CYP2A13 and ABCB1 is regulated by FOXA2, and their genetic interaction is associated with lung cancer

Inhaled xenobiotics such as tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone are mainly metabolized by phase I oxidase cytochrome P450, family 2, subfamily A, polypeptide 13 (CYP2A13), phase II conjugate UDP glucuronosyltransferase 2 family, polypeptide B17 (UGT2B17), and phase III transporter ATP-binding cassette, subfamily B (MDR/TAP), member 1 (ABCB1), with genetic polymorphisms implicated in lung cancer. Their genetic interaction and pulmonary expression regulation are largely unknown. We analyzed joint association for CYP2A13 and ABCB1 polymorphisms in 2 independent lung cancer case populations (669 and 566 patients) and 1 common control population (749 subjects), and characterized the trans-acting function of the lung development-related transcription factor forkhead box A2 (FOXA2). We undertook FOXA2 overexpression and down-regulation in lung epithelial cell lines, analyzed functional impact on the transactivation of CYP2A13, UGT2B17, and ABCB1, and measured correlation for their expressions in lung tissues. We found a substantial reduction in cancer risk (OR 0.39; 95% CI 0.25-0.61; Pinteraction = 0.029) associated with combined genotypes for CYP2A13 R257C and a functionary regulatory variant in the cis element of ABCB1 synergistically targeted by GATA binding protein 6 and FOXA2. Genetic manipulation of FOXA2 consistently influenced its binding to and transactivation of the promoters of CYP2A13, UGT2B17, and ABCB1, whose mRNA and protein expressions were all consistently correlated with those of FOXA2 in both tumorous and normal lung tissues. We therefore establish FOXA2 as a core transcriptional modulator for pulmonary xenobiotic metabolic pathways and uncover an etiologically relevant interaction between CYP2A13 and ABCB1, furthering our understanding of expression and function of the xenobiotic metabolism system.

Identification of cytochrome P450 enzymes critical for lung tumorigenesis by the tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK): insights from a novel Cyp2abfgs-null mouse

Cytochrome P450 (P450) enzymes encoded by the mouse Cyp2abfgs gene cluster are preferentially expressed in the respiratory tract. Previous studies have demonstrated that pulmonary P450-mediated bioactivation is necessary for lung tumorigenesis induced by the tobacco-specific lung procarcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), and that CYP2A5 mediates a noteworthy fraction, but not all, of NNK bioactivation in the lung. The aim of this study was to determine whether other P450s encoded by the Cyp2abfgs gene cluster also play significant roles in NNK lung tumorigenesis. A novel Cyp2abfgs-null mouse was generated, in which all Cyp2a, 2b, 2g, 2f and 2s genes are deleted. The Cyp2abfgs-null mouse was viable, fertile and without discernible physiological abnormalities or compensatory increases in the expression of other P450s. NNK bioactivation in vitro and NNK-induced DNA adduction and lung tumorigenesis in vivo were determined for wild-type (WT) and Cyp2abfgs-null mice; the results were compared with previous findings from Cyp2a5-null mice. The Cyp2abfgs-null mice exhibited significantly lower rates of NNK bioactivation in lung and liver microsomes, compared with either WT or Cyp2a5-null mice. The levels of lung O(6)-methyl guanine DNA adduct were also substantially reduced in Cyp2abfgs-null mice, compared with either WT or Cyp2a5-null mice. Moreover, the Cyp2abfgs-null mice were largely resistant to NNK-induced lung tumorigenesis at both low (50mg/kg) and high (200mg/kg) NNK doses, in contrast to the WT or Cyp2a5-null mice. These results indicate for the first time that, collectively, the CYP2A, 2B, 2F, 2G, and 2S enzymes are indispensable for NNK-induced lung tumorigenesis.

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.

Role of hepatic and intestinal p450 enzymes in the metabolic activation of the colon carcinogen azoxymethane in mice

P450-mediated bioactivation of azoxymethane (AOM), a colon carcinogen, leads to the formation of DNA adducts, of which O⁶-methylguanine (O⁶-mG) is the most mutagenic and contributes to colon tumorigenesis. To determine whether P450 enzymes of the liver and intestine both contribute to AOM bioactivation in vivo, we compared tissue levels of AOM-induced DNA adducts, microsomal AOM metabolic activities, and incidences of colonic aberrant crypt foci (ACF) among wild-type (WT), liver-specific P450 reductase (Cpr)-null (LCN), and intestinal epithelium-specific Cpr-null (IECN) mice. At 6 h following AOM treatment (at 14 mg/kg, s.c.), O⁶-mG and N⁷-mG levels were highest in the liver, followed by the colon, and then small intestine in WT mice. As expected, hepatic adduct levels were significantly lower (by >60%) in LCN mice but unchanged in IECN mice, whereas small-intestinal adduct levels were unchanged or increased in LCN mice but lower (by >50%) in IECN mice compared to that in WT mice. However, colonic adduct levels were unchanged in IECN mice compared to that in WT mice and increased in LCN mice (by 1.5–2.9-fold). The tissue-specific impact of the CPR loss in IECN and LCN mice on microsomal AOM metabolic activity was confirmed by rates of formation of formaldehyde and N⁷-mG in vitro. Furthermore, the incidence of ACF, a lesion preceding colon cancer, was similar in the three mouse strains. Thus, AOM-induced colonic DNA damage and ACF formation is not solely dependent on either hepatic or intestinal microsomal P450 enzymes. P450 enzymes in both the liver and intestine likely contribute to AOM-induced colon carcinogenesis.

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.