Expression of the rat CYP2A3 gene in transgenic mice

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.

Cytochrome P450 2A isoenzymes in freshly prepared blood lymphocytes isolated from rats and validation as a biomarker for clinical studies in humans

1. The present study aimed to identify the expression of carcinogen metabolizing cytochrome P4502A (CYP2A) isoenzymes in freshly prepared rat peripheral blood lymphocytes (PBL) isolated from adult rats and investigate similarities in the regulation of lymphocyte CYP2A-isoenzymes with the tissue enzyme. 2. qRT-PCR studies demonstrated significant constitutive mRNA expression of CYP2A-isoenzymes in PBL isolated from male and female rats which further increases significantly after pretreatment with nicotine or 3-methylcholanthrene (MC) indicating responsiveness of CYP2A-isoenzymes in PBL. This increase in the CYP2A expression was associated with an increase in the protein expression and CYP2A3-dependent coumarin hydroxylase (COH) activity in PBL. 3. Clinical studies further demonstrated significant increase in the expression of CYP2A6 and associated enzyme activity in PBL isolated from lung cancer patients. Our data thus provided evidence for similarities in the regulation of carcinogen metabolizing CYP2A-isoenzymes in PBL with the tissue enzymes. Further, responsiveness of blood CYP2A6 in human blood lymphocytes isolated from lung cancer patients has led us to suggest that associating expression profiles of CYP2A6 and other polycyclic aromatic hydrocarbons (PAH)-responsive CYPs in PBL with the genotyping data could lead to the development of a possible screen to monitor and predict environment-induced diseases and toxicity in humans.

Ethanol induction of CYP2A5: role of CYP2E1-ROS-Nrf2 pathway

Chronic ethanol consumption was previously shown to induce CYP2A5 in mice, and this induction of CYP2A5 by ethanol was CYP2E1 dependent. In this study, the mechanisms of CYP2E1-dependent ethanol induction of CYP2A5 were investigated. CYP2E1 was induced by chronic ethanol consumption to the same degree in wild-type (WT) mice and CYP2A5 knockout (Cyp2a5 (-/-)) mice, suggesting that unlike the CYP2E1-dependent ethanol induction of CYP2A5, ethanol induction of CYP2E1 is not CYP2A5 dependent. Microsomal ethanol oxidation was about 25% lower in Cyp2a5 (-/-) mice compared with that in WT mice, suggesting that CYP2A5 can oxidize ethanol although to a lesser extent than CYP2E1 does. CYP2A5 was induced by short-term ethanol consumption in human CYP2E1 transgenic knockin (Cyp2e1 (-/-) KI) mice but not in CYP2E1 knockout (Cyp2e1 (-/-)) mice. The redox-sensitive transcription factor nuclear factor-erythroid 2-related factor 2 (Nrf2) was also induced by acute ethanol in Cyp2e1 (-/-) KI mice but not in Cyp2e1 (-/-) mice. Ethanol induction of CYP2A5 in Nrf2 knockout (Nrf2 (-/-)) mice was lower compared with that in WT mice, whereas CYP2E1 induction by ethanol was comparable in WT and Nrf2 (-/-) mice. Antioxidants (N-acetyl-cysteine and vitamin C), which blocked oxidative stress induced by chronic ethanol in WT mice and acute ethanol in Cyp2e1 (-/-) KI mice, also blunted the induction of CYP2A5 and Nrf2 by ethanol but not the induction of CYP2E1 by ethanol. These results suggest that oxidative stress induced by ethanol via induction of CYP2E1 upregulates Nrf2 activity, which in turn regulates ethanol induction of CYP2A5. Results obtained from primary hepatocytes, mice gavaged with binge ethanol or fed chronic ethanol, show that Nrf2-regulated ethanol induction of CYP2A5 protects against ethanol-induced steatosis.

Generation and characterization of a transgenic mouse model with hepatic expression of human CYP2A6

The aim of this study was to prepare and characterize a transgenic mouse model in which CYP2A6, a human P450 enzyme, is expressed specifically in the liver. CYP2A6, which is mainly expressed in human liver, is active toward many xenobiotics. Our transgene construct contained the mouse transthyretin promoter/enhancer, a full-length CYP2A6 cDNA, and a downstream neomycin-resistance gene for positive selection in embryonic stem cells. Hepatic expression of the CYP2A6 transgene was demonstrated by immunoblotting, whereas tissue specificity of CYP2A6 expression was confirmed by RNA-PCR. The transgenic mouse was further characterized after being backcrossed to the B6 strain for six generations. Hepatic microsomes from homozygous transgenic mice had activities significantly higher than those of B6 mice toward coumarin. The in vivo activity of transgenic CYP2A6 was also determined. Systemic clearance of coumarin was significantly higher in the transgenic mice than in B6 controls, consistent with the predicted role of CYP2A6 as the major coumarin hydroxylase in human liver. The CYP2A6-transgenic mouse model should be valuable for studying the in vivo function of this polymorphic human enzyme in drug metabolism and chemical toxicity.

Regulation of cytochrome P450 gene expression in the olfactory mucosa

The mammalian olfactory mucosa (OM) is unique among extrahepatic tissues in having high levels, and tissue-selective forms, of cytochrome P450 (CYP) enzymes. These enzymes may have important toxicological implications, as well as biological functions, in this chemosensory organ. In addition to a tissue-selective, abundant expression of CYP1A2, CYP2A, and CYP2G1, some of the OM CYPs are also known to have an early developmental expression, a resistance to xenobiotic inducers, and a lack of responsiveness to circadian rhythm. Efforts to fully characterize the regulation of CYP expression in the OM, and to identify the underlying mechanisms, are important for our understanding of the physiological functions and toxicological significance of these biotransformation enzymes, and may also shed unique light on the general mechanisms of CYP regulation. The aim of this mini-review is to provide a summary of current knowledge of the various modes of regulation of CYPs expressed in the OM, an update on our mechanistic studies on tissue-selective CYP expression, and a review of the literature on xenobiotic inducibility of OM CYPs. Our goal is to stimulate further studies in this exciting research area, which is of considerable importance, in view of the constant exposure of the human nasal tissues to inhaled, as well as systemically derived, chemicals, the prevalence of olfactory system damage in individuals with neurodegenerative diseases, and the current uncertainty in risk assessments for potential olfactory toxicants.

Transcriptional regulation of rat CYP2A3 by nuclear factor 1: identification of a novel NFI-A isoform, and evidence for tissue-selective interaction of NFI with the CYP2A3 promoter in vivo

Rat CYP2A3 and its mouse and human orthologs are expressed preferentially in the olfactory mucosa. We found previously that an element in the proximal promoter region of CYP2A3 (the nasal predominant transcriptional activating (NPTA) element), which is similar to a nuclear factor 1 (NFI)-binding site, is critical for transcriptional activation of CYP2A3 in vitro. We proposed that this element might be important for tissue-selective CYP2A3 expression. The goals of the present study were to characterize NPTA-binding proteins and to obtain more definitive evidence for the role of NFI in the transcriptional activation of CYP2A3. The NPTA-binding proteins were isolated by DNA-affinity purification from rat olfactory mucosa. Mass spectral analysis indicated that isoforms corresponding to all four NFI genes were present in the purified NPTA-binding fraction. Further analysis of NPTA-binding proteins led to the identification of a novel NFI-A isoform, NFI-A-short, which was derived from alternative splicing of the NFI-A transcript. Transient transfection assay showed that NFI-A2, an NFI isoform previously identified in the olfactory mucosa, transactivated the CYP2A3 promoter, whereas NFI-A-short, which lacks the transactivation domain, counteracted the activation. Chromatin immunoprecipitation assays indicated that NFI proteins are associated with the CYP2A3 promoter in vivo, in rat olfactory mucosa, but essentially not in the liver where the CYP2A3 promoter is hypermethylated and CYP2A3 is not expressed. These data strongly support a role for NFI transcription factors in the transcriptional activation of CYP2A3.

Immunoblot analysis and immunohistochemical characterization of CYP2A expression in human olfactory mucosa

The aim of the present study was to further characterize the expression of the CYP2A genes in human nasal mucosa. Fetal nasal tissues at 12-26 weeks of gestational age and surgical biopsy tissues from various regions of nasal cavity of adult patients were studied to determine whether CYP2A proteins can be detected by immunoblot in adults, whether higher levels of CYP2A proteins are present in adult than in fetal nasal mucosal microsomes, and whether CYP2A13 mRNA is more abundant than CYP2A6 mRNA in fetal nasal mucosa. In adults, immunoblot analysis detected CYP2A proteins in microsomes of the olfactory region from 8 of 10 individuals, but in none of the nasal microsomes of the respiratory region from 47 patients. Quantitative immunoblot analysis confirmed that CYP2A proteins are selectively expressed in the olfactory region in both adult and fetal tissues. Interestingly, the levels of CYP2A proteins in nasal microsomes were generally higher in fetuses than in adults. In the fetus, the level of CYP2A13 mRNA was much higher than that of CYP2A6 mRNA, as has been previously found in adult nasal mucosa. Immunohistochemical studies confirmed that, in the fetus, the CYP2A proteins are expressed in the supporting cells in the olfactory epithelium and in the Bowman's glands in the lamina propria. The prenatal expression of the CYP2A proteins in the olfactory mucosa suggests potential risks of developmental toxicity from maternally derived xenobiotics, since both CYP2A6 and CYP2A13 are known to be efficient in the metabolic activation of tobacco-specific nitrosamines and other respiratory toxicants.

Human extrahepatic cytochromes P450: function in xenobiotic metabolism and tissue-selective chemical toxicity in the respiratory and gastrointestinal tracts

Cytochrome P450 (CYP) enzymes in extrahepatic tissues often play a dominant role in target tissue metabolic activation of xenobiotic compounds. They may also determine drug efficacy and influence the tissue burden of foreign chemicals or bioavailability of therapeutic agents. This review focuses on xenobiotic-metabolizing CYPs of the human respiratory and gastrointestinal tracts, including the lung, trachea, nasal respiratory and olfactory mucosa, esophagus, stomach, small intestine, and colon. Many CYPs are expressed in one or more of these organs, including CYP1A1, CYP1A2, CYP1B1, CYP2A6, CYP2A13, CYP2B6, CYP2C8, CYP2C9, CYP2C18, CYP2C19, CYP2D6, CYP2E1, CYP2F1, CYP2J2, CYP2S1, CYP3A4, CYP3A5, and CYP4B1. Of particular interest are the preferential expression of certain CYPs in the respiratory tract and the regional differences in CYP expression profile in different parts of the gastrointestinal tract. Current research activities on the characterization of CYP expression, function, and regulation in these tissues, as well as future research needs, are discussed.