Identification and characterization of a novel tissue-specific transcriptional activating element in the 5'-flanking region of the CYP2A3 gene predominantly expressed in rat olfactory mucosa

Cytochrome P450 expression and regulation in the brain

The regulation of brain cytochrome P450 enzymes (CYPs) is different compared with respective hepatic enzymes. This may result from anatomical bases and physiological functions of the two organs. The brain is composed of a variety of functional structures built of different interconnected cell types endowed with specific receptors that receive various neuronal signals from other brain regions. Those signals activate transcription factors or alter functioning of enzyme proteins. Moreover, the blood-brain barrier (BBB) does not allow free penetration of all substances from the periphery into the brain. Differences in neurotransmitter signaling, availability to endogenous and exogenous active substances, and levels of transcription factors between neuronal and hepatic cells lead to differentiated expression and susceptibility to the regulation of CYP genes in the brain and liver. Herein, we briefly describe the CYP enzymes of CYP1-3 families, their distribution in the brain, and discuss brain-specific regulation of CYP genes. In parallel, a comparison to liver CYP regulation is presented. CYP enzymes play an essential role in maintaining the levels of bioactive molecules within normal ranges. These enzymes modulate the metabolism of endogenous neurochemicals, such as neurosteroids, dopamine, serotonin, melatonin, anandamide, and exogenous substances, including psychotropics, drugs of abuse, neurotoxins, and carcinogens. The role of these enzymes is not restricted to xenobiotic-induced neurotoxicity, but they are also involved in brain physiology. Therefore, it is crucial to recognize the function and regulation of CYP enzymes in the brain to build a foundation for future medicine and neuroprotection and for personalized treatment of brain diseases.

Neil2-null Mice Accumulate Oxidized DNA Bases in the Transcriptionally Active Sequences of the Genome and Are Susceptible to Innate Inflammation

Why mammalian cells possess multiple DNA glycosylases (DGs) with overlapping substrate ranges for repairing oxidatively damaged bases via the base excision repair (BER) pathway is a long-standing question. To determine the biological role of these DGs, null animal models have been generated. Here, we report the generation and characterization of mice lacking Neil2 (Nei-like 2). As in mice deficient in each of the other four oxidized base-specific DGs (OGG1, NTH1, NEIL1, and NEIL3), Neil2-null mice show no overt phenotype. However, middle-aged to old Neil2-null mice show the accumulation of oxidative genomic damage, mostly in the transcribed regions. Immuno-pulldown analysis from wild-type (WT) mouse tissue showed the association of NEIL2 with RNA polymerase II, along with Cockayne syndrome group B protein, TFIIH, and other BER proteins. Chromatin immunoprecipitation analysis from mouse tissue showed co-occupancy of NEIL2 and RNA polymerase II only on the transcribed genes, consistent with our earlier in vitro findings on NEIL2's role in transcription-coupled BER. This study provides the first in vivo evidence of genomic region-specific repair in mammals. Furthermore, telomere loss and genomic instability were observed at a higher frequency in embryonic fibroblasts from Neil2-null mice than from the WT. Moreover, Neil2-null mice are much more responsive to inflammatory agents than WT mice. Taken together, our results underscore the importance of NEIL2 in protecting mammals from the development of various pathologies that are linked to genomic instability and/or inflammation. NEIL2 is thus likely to play an important role in long term genomic maintenance, particularly in long-lived mammals such as humans.

Single nucleotide polymorphisms in monocyte chemoattractant protein-1 and its receptor act synergistically to increase the risk of carotid atherosclerosis

BACKGROUND

Monocyte chemoattractant protein 1 (MCP-1), acting in concert with its receptor chemokine receptor 2 (CCR2), promotes recruitment of macrophages into atherosclerotic plaque. We examined whether single nucleotide polymorphism (SNP) variants in the MCP-1 or CCR2 genes independently or in combination are associated with carotid artery atherosclerosis in an African American population at increased risk of vascular disease.

METHODS

Four SNPs in MCP-1 and 1 in CCR2 were genotyped. Carotid artery duplex ultrasonography was used to identify the presence or absence of carotid plaque >1 mm. The study population included 325 apparently healthy 30- to 59-year-old black siblings of 185 probands with premature coronary artery disease (<60 years old). Associations between each independent SNP and the presence of carotid plaque were examined using multivariate logistic regression models adjusted for age, sex, educational level, diabetes, smoking, hypertension, obesity, low-density lipoprotein cholesterol and non-independence within families. Interactions between SNPs in the MCP-1 gene and the SNP in the CCR2 gene were examined by multivariate analysis.

RESULTS

Siblings were 32% males, with a mean age of 46 +/- 7 years, and 77 (24%) demonstrated carotid plaque. In multivariate analyses, the CC genotype of MCP-1 SNP rs2857656 was independently associated with plaque (p = 0.05). Subjects who had both the MCP-1 CC genotype and were heterozygotic or homozygotic for the CCR2 V64I genotype (rs1799864; n = 12) had an even higher risk of carotid atherosclerosis (odds ratio 6.14, 95% confidence interval 1.82-20.73; p = 0.0037).

CONCLUSION

The MCP-1 rs2857656 CC genotype is independently associated with carotid artery plaque in African American from families with premature coronary artery disease. The combination of the MCP-1 CC homozygous genotype and the homozygotic or heterozygote CCR2 V64I genotype is associated with a particularly high prevalence of carotid artery plaque.

Transcriptional Regulation of Human CYP2A13 Expression in the Respiratory Tract by CCAAT/Enhancer Binding Protein and Epigenetic Modulation

CYP2A13, which is highly active in the metabolic activation of tobacco-specific nitrosamines, is selectively expressed in the respiratory tract, in which it is believed to play an important role in chemical carcinogenesis. The aim of this study was to determine the basis for tissue-specific regulation of CYP2A13 gene expression. We have shown that expression of CYP2A3, the rat homolog of CYP2A13, is regulated by nuclear factor I (NFI) in a tissue-specific manner. In the present study, we found that the transcriptional regulation of human CYP2A13 gene involves CCAAT/enhancer binding protein (C/EBP) transcription factors instead of NFI. DNase I footprinting and gel-shift assays with human lung nuclear extract identified two DNA elements bound by C/EBP. Reporter gene assays using a 216-base pair CYP2A13 promoter fragment confirmed the activation of CYP2A13 by transfected C/EBP factors, and results from chromatin immunoprecipitation assays indicated that C/EBP is associated with CYP2A13 promoter in vivo in the olfactory mucosa of CYP2A13-transgenic mice. In NCI-H441 human lung cancer cells, we discovered that CYP2A13 expression can be induced by a combined treatment with 5-aza-2'-deoxycytosine, a DNA demethylation agent, and trichostatin, a histone deacetylation inhibitor. In 5-aza-2'-deoxycytosine/trichostatin-treated NCI-H441 cells, overexpression of C/EBPdelta, a lung-enriched C/EBP, led to additional increases in CYP2A13 expression, whereas C/EBPdelta knockdown by small interference RNA suppressed CYP2A13 expression, findings that confirm a role for C/EBP in CYP2A13 regulation. Our findings pave the way for further studies of the regulation of the CYP2A13 gene, particularly the gene's potential suppression by airway inflammation, and the role of epigenetic modulation in the gene's tissue-selective expression.

Effect of N-nitrosomethylbenzylamine on nasal mucosa in rats

N-nitrosomethylbenzylamine (NMBA) is one of the most potent organ-specific carcinogens routinely used in rat esophageal tumorigenesis. The aim of the study was to evaluate NMBA effect on nasal mucosa, one of the non-target organs. NMBA was administered subcutaneously to 20 male albino rats of Wistar strain for 5 weeks (0.5mg/kg/dose; three doses/week). The experiment was terminated on week 22. In each case, seven standard frontal sections of the nose were taken after fixation for assessment of all the parts of the nasal mucosa. Microscopic examination revealed one small squamous cell papilloma located on the ventro-lateral surface of the left superior nasal concha, one focus on simple hyperplasia and two foci of squamous epithelium dysplasia within the mucosa covering nasal vestibule near the respiratory part of the nasal cavity. Furthermore, statistically significant increase of proliferation activity in both lesional and non-lesional nasal squamous epithelium in NMBA-exposed animals was also found. These phenomena could be potentially induced by carcinogen exposure.

Sp1 and Sp3 regulate basal transcription of the human CYP2F1 gene

Selective transcription of the human CYP2F1 gene in lung tissues may control the susceptibilities of this organ to diverse pneumotoxicants and lung carcinogens. However, the mechanisms responsible for CYP2F1 organ-selective transcription have not been elucidated. The objectives of the current studies were to identify and characterize basal transcription elements within the TATA-less promoter region of CYP2F1. Four putative Sp1-like sites were identified in the CYP2F1 promoter. Competitive electrophoretic mobility shift assay analysis with mutated oligonucleotide probes and lung A549 cell nuclear extract, along with supershift studies using antibodies to either Sp1 or Sp3 proteins, demonstrated that all four sites formed three specific protein-DNA complexes. Mutations in any of the four core Sp1-like motifs abolished protein-DNA binding. Western blot analysis of both human tissues and cells showed that Sp1 was considerably higher in lung than liver and that Sp3 was much higher in liver than lung. Promoter activation of a luciferase reporter construct was sequentially increased by addition of each of the four Sp1-like motifs in lung A549 cells but not in liver HepG2 cells. Cotransfection of a Sp1 expression vector with the reporter construct dramatically increased luciferase activity in either A549 cells or Sp1-deficient Drosophila Schneider line 2 (SL-2) cells. However, similar cotransfections with an Sp3 expression vector failed to increase activity. Cotransfection of both the Sp1 and Sp3 expression vectors considerably decreased Sp1-mediated activity in A549 cells and abolished activity in SL-2 cells. Thus, these studies demonstrated that four Sp1-dependent proximal promoter elements drive organ-selective CYP2F1 gene transcription, and that Sp1 and Sp3 factors interact to modulate constitutive CYP2F1 transcription in lung cells.

Regulation of Cyp2a5 transcription in mouse primary hepatocytes: roles of hepatocyte nuclear factor 4 and nuclear factor I

The cytochrome P4502a5 (Cyp2a5) gene is expressed principally in liver and olfactory mucosa. In the present study, the transcriptional mechanisms of hepatocyte-specific expression of Cyp2a5 were studied in mouse primary hepatocytes. The Cyp2a5 5'-flanking region -3033 to +10 was cloned in front of a luciferase reporter gene and transfected into hepatocytes. Deletion analysis revealed two major activating promoter regions localized at proximal 271 bp and at a more distal area from -3033 to -2014 bp. The proximal activation region was characterized further by DNase I footprinting, and a single clear footprint was detected in the studied area centred over a sequence similar to the NF-I (nuclear factor I)-binding site. The binding of NF-I was confirmed using an EMSA (electrophoretic mobility-shift assay). A putative HNF-4 (hepatocyte nuclear factor 4)-binding site was localized at the proximal promoter by computer analysis of the sequence, and HNF-4alpha was shown to interact with the site using an EMSA. The functional significance of HNF-4 and NF-I binding to the Cyp2a5 promoter was evaluated by site-directed mutagenesis of the binding motifs in reporter constructs. Both mutations strongly decreased transcriptional activation by the Cyp2a5 promoter in primary hepatocytes, and double mutation almost completely abolished transcriptional activity. Also, the functionality of the distal activation region was found to be dependent on the intact HNF-4 and NF-I sites at the proximal promoter. In conclusion, these results indicate that HNF-4 and NF-I play major roles in the constitutive regulation of hepatic expression of Cyp2a5.

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.

NFI-Ski interactions mediate transforming growth factor beta modulation of human papillomavirus type 16 early gene expression

Human papillomaviruses (HPVs) are present in virtually all cervical cancers. An important step in the development of malignant disease, including cervical cancer, involves a loss of sensitivity to transforming growth factor beta (TGF-beta). HPV type 16 (HPV16) early gene expression, including that of the E6 and E7 oncoprotein genes, is under the control of the upstream regulatory region (URR), and E6 and E7 expression in HPV16-immortalized human epithelial cells is inhibited at the transcriptional level by TGF-beta. While the URR contains a myriad of transcription factor binding sites, including seven binding sites for nuclear factor I (NFI), the specific sequences within the URR or the transcription factors responsible for TGF-beta modulation of the URR remain unknown. To identify potential transcription factors and binding sites involved in TGF-beta modulation of the URR, we performed DNase I footprint analysis on the HPV16 URR using nuclear extracts from TGF-beta-sensitive HPV16-immortalized human keratinocytes (HKc/HPV16) treated with and without TGF-beta. Differentially protected regions were found to be located around NFI binding sites. Electrophoretic mobility shift assays, using the NFI binding sites as probes, showed decreased binding upon TGF-beta treatment. This decrease in binding was not due to reduced NFI protein or NFI mRNA levels. Mutational analysis of individual and multiple NFI binding sites in the URR defined their role in TGF-beta sensitivity of the promoter. Overexpression of the NFI family members in HKc/HPV16 decreased the ability of TGF-beta to inhibit the URR. Since the oncoprotein Ski has been shown to interact with and increase the transcriptional activity of NFI and since cellular Ski levels are decreased by TGF-beta treatment, we explored the possibility that Ski may provide a link between TGF-beta signaling and NFI activity. Anti-NFI antibodies coimmunoprecipitated endogenous Ski in nuclear extracts from HKc/HPV16, confirming that NFI and Ski interact in these cells. Ski levels dramatically decreased upon TGF-beta treatment of HKc/HPV16, and overexpression of Ski eliminated the ability of TGF-beta to inhibit the URR. Based on these studies, we propose that TGF-beta inhibition of HPV16 early gene expression is mediated by a decrease in Ski levels, which in turn dramatically reduces NFI activity.

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.

Expression of genes encoding for drug metabolising cytochrome P450 enzymes and P-glycoprotein in the rat small intestine; comparison to the liver

The level of expression of genes encoding for nine major xenobiotic metabolising Cytochrome P450s (CYPs) and the P-glycoprotein (Pgp) was determined in three different regions of the small intestine of male and female Sprague Dawley rats and the expression was compared with that in the liver. A semi-quantitative RT-PCR method, using the total RNA from the tissues, was established for the determination of the level of gene expression. Four of the CYP genes: the CYP2B1, CYP2C6, CYP2C11 and CYP2D1 and the Pgp were expressed at as high levels in the small intestine as in the liver. The expression of the other CYP genes was remarkably different in the two organs. The CYP1A2, CYP2A3, CYP2E1 and CYP3A1 showed a strong expression in the liver but only a comparatively weak or no expression in the small intestine. The CYP1A1 on the other hand exhibited a stronger expression in the small intestine than in the liver. With the exception of the CYP2A3, none of the genes showed a clear regional distribution in their small intestinal expression. Furthermore, no obvious sex difference in the expression of the CYP and Pgp genes could be observed. Our results indicate that several of the enzymes, central for drug metabolism are differently expressed in the liver and in the small intestine of the rat which should be taken into account when using rat as a model for the bioavailability and organ specific toxicity studies of orally administered xenobiotics. The apparently strong small intestinal expression of the CYP2C genes suggests that these enzymes could play a key role in the intestinal drug metabolism in rats and therefore affect the bioavailability of those orally used drugs which are substrates of the CYP2Cs. This possibility should be investigated in more detail both in rats and humans.

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.

Adjacent sequence controls the response polarity of nitric oxide-sensitive Sp factor binding sites

Nitric oxide (NO*) and cAMP-dependent protein kinase (PKA) inhibitors up-regulate tumor necrosis factor alpha (TNFalpha) by decreasing Sp1 binding to a proximal GC box element. Here, elements flanking GC boxes were tested for their role in determining whether Sp sites act as activators or repressors. Promoter studies in receptive human cell lines demonstrated that NO* down-regulated endothelial NO* synthase (eNOS) but up-regulated TNFalpha. Like TNFalpha, Sp1 binding to the eNOS promoter was decreased by NO* and a PKA inhibitor, H89, and increased by a PKA activator, dibutyryl cAMP (Bt2cAMP). For either promoter, mutation of Sp sites abolished NO* responses. In contrast, mutation of an upstream AP1 site in the TNFalpha promoter (not present in eNOS) maintained NO* responsiveness, but reversed the direction of NO* and cAMP effects. Using artificial constructs, NO* increased transcription when Sp and AP1 sites were both present (TNFalpha-like response), but decreased it when the adjacent AP1 site was disrupted (eNOS-like response). NO*, H89, and Bt2cAMP were found to produce reciprocal protein binding changes at contiguous AP1 and Sp sites (p < 0.0001 for an interaction). Chromatin immunoprecipitation assays demonstrated that Sp1 and to a lesser extent Sp3 bound to the GC box regions of eNOS and TNFalpha in intact cells. Thus, this NO*- and cAMP-responsive regulatory module has a Sp site sensor variably coupled to an adjacent element that determines response polarity. These results define a composite element that can utilize secondary inputs to convert off signals to on, thereby conferring complex functionalities to the same DNA binding motif.

Isomer-specific bioactivation and toxicity of dichlorophenyl methylsulphone in rat olfactory mucosa

This study aimed to explain the isomer- and site-specific toxic effects of dichlorophenyl methylsulphone in the olfactory mucosa of rats. A single ip dose of the 2,6-chlorinated isomer (16 or 65 mg/kg) induced necrosis preferentially in the Bowman's glands and neuroepithelium in the dorsomedial part of the olfactory region. Only minor damage occurred at this site in rats dosed with the 2,5-chlorinated isomer (65 mg/kg). A strong concentration- and time-dependent covalent binding of the (14)C-labeled 2,6-isomer to rat olfactory microsomes was demonstrated. In contrast, no significant covalent binding of the (14)C-labeled 2,5-isomer was observed. The cytochrome P450 (CYP) inhibitors metyrapone, tranylcypromine and acetonitrile inhibited covalent binding of the 2,6-isomer to olfactory microsomes. Glutathione (GSH) appeared to play a protective role as a scavenger of a reactive intermediate whereas methyl-GSH did not alter covalent binding to olfactory microsomes. As determined by microautoradiography, binding of the 2,6-chlorinated isomer in the olfactory mucosa was confined to the Bowman's glands. Both isomers showed a low binding to liver microsomes and caused no liver injury. We suggest that a CYP2A-catalyzed activation of the 2,6-chlorinated dichlorophenyl methylsulphone to a reactive intermediate and adduct formation in the Bowman's glands will initiate a site-specific toxicity of this isomer in the olfactory mucosa.

Characterization of the human lung CYP2F1 gene and identification of a novel lung-specific binding motif

The CYP2F1 gene encodes a cytochrome P450 enzyme capable of bioactivating a number of pulmonary-selective toxicants. The expression of CYP2F1 is highly tissue-selective; the highest expression is observed in the lung with little or no hepatic expression. The objective of these studies was to elucidate the mechanisms that govern the unique tissue-specific regulation of CYP2F1. Cosmid and bacterial artificial chromosome clones were screened and sequenced to identify a gene that spanned 14 kbp containing 10 exons, including an untranslated exon 1. Primer extension analysis and 5'-rapid amplification of cDNA ends were used to identify the transcription start site. Several sequences homologous to known cis-elements were identified in the 5'-upstream region of the CYP2F1 promoter. Transient transfection studies with luciferase reporter constructs demonstrated a significant functional lung cell-specific CYP2F1 promoter region (from position -129 to +115). DNase footprinting analysis of 1.6 kbp of the upstream sequence with nuclear extracts from human lung tissues revealed one strong DNA-protein complex at -152 to -182. This nuclear protein (called lung-specific factor, LSF) was present only in lung but not liver or heart tissues. Competitive electrophoretic mobility shift assays characterized a DNA consensus site, within the LSF-binding domain, that was highly similar to two E box motifs, but no known "E box" trans-factors were identified. These studies identified a novel LSF and its consensus sequence that may control tissue-specific expression of CYP2F1.

Functional role of cytochrome p-450 2a3 in N-nitrosomethylbenzylamine metabolism in rat esophagus

Previous in vitro studies demonstrated that the rat esophageal carcinogen N-nitrosomethylbenzylamine (NMBA) is metabolically activated by cytochrome P-450s (CYP) 2A3 and 2E1. However, the in vivo role of these P-450s in the metabolism of NMBA has not been fully evaluated. In this study, the effects of single and multiple doses of NMBA were investigated on CYP2A3 and CYP2E1 mRNA expression in the rat esophagus and lung. Seven- to 8-wk old male Fischer 344 rats were administered a single subcutaneous dose of NMBA at either 0.5 mg/kg or 2 mg/kg body weight, after which the rats were sacrificed at 1, 3, 6, 12, 24, 48, and 72 h. In the multiple-dose experiment, 2 groups of rats were dosed with 0.5 mg/kg body weight NMBA 3 times per week for 1 wk or 3 wk. The animals were sacrificed 24 h following the last treatment. Semiquantitative reverse-transcription polymerase chain reaction (RT-PCR) analysis demonstrated a reduction of CYP2A3 mRNA expression in lung and esophagus from NMBA-treated animals compared to dimethyl sulfoxide (DMSO)-treated vehicle controls. This reduction in CYP2A3 mRNA was significant at 48 h in the esophagus and at 24 and 48 h in the lung following a single dose of 2 mg/kg body weight NMBA. In contrast, CYP2E1 mRNA expression remained unchanged in rat lung following NMBA treatment and no consistent pattern of expression could be observed in the esophagus. In the multiple-dose study, a 32% and 25% reduction in esophageal CYP2A3 mRNA expression was observed at 1 and 3 wk, respectively. Similar reductions in CYP2A3 mRNA expression were also observed in the lung. Further, esophageal explants derived from animals pretreated with NMBA in vivo demonstrated a reduced ability to metabolize the carcinogen in vitro as compared to explants from vehicle control animals. Taken together, these data provide further support for a potential role of CYP2A3 in NMBA metabolism in the rat esophagus. Data suggest that CYP2A3 levels in the rat esophagus can be a determinant of its ability to metabolize this carcinogen in vivo.

Expression of the rat CYP2A3 gene in transgenic mice

Rat CYP2A3 and mouse CYP2A5 are predominantly expressed in the olfactory mucosa. CYP2A3 is also expressed in the lung at a low level, whereas CYP2A5 is expressed in several additional tissues. To better understand the transcriptional regulation of the CYP2A genes, transgenic mice were generated with a full-length CYP2A3 gene fragment containing 3.4 kilobases of the 5'-flanking region. CYP2A3 mRNA was detected in the brain and olfactory bulb in four transgenic mouse lines, in the olfactory mucosa in three lines, and in kidney, liver, lung, and small intestine in two lines. Thus, the expression of the CYP2A3 transgene mimicked the tissue distribution pattern of mouse CYP2A5 rather than that of rat CYP2A3. Furthermore, the levels of CYP2A3 mRNA were very low in all lines examined, suggesting that more distal regulatory regions may be involved in the abundant expression of the CYP2A genes in the olfactory mucosa.

Activation of the NPTA element of the CYP2A3 gene by NFI-A2, a nasal mucosa-selective nuclear factor 1 isoform

The aim of this study was to determine whether the NPTA element of the olfactory mucosa-predominant CYP2A3 gene can be activated by NFI-A2, a recently identified member of the nuclear factor 1 family of transcription factors. Isoform-specific RNA-PCR confirmed that NFI-A2 is mainly expressed in rat olfactory mucosa. A full-length NFI-A2 cDNA was isolated from a cDNA library of rat olfactory mucosa and was used for preparation of a construct encoding a fusion protein of NFI-A2 with the yeast GAL4 activation domain. Expression of the fusion protein in yeast was detected with an antibody to NFI-A. The fusion protein activated the expression of a LacZ reporter gene in yeast one-hybrid assays with a reporter construct containing the NPTA element, but not with other constructs lacking the NPTA element. These findings suggest that NFI-A2 may be involved in the tissue-selective transcriptional activation of the CYP2A3 gene in the olfactory mucosa.

Mouse cyp2g1 gene: promoter structure and tissue-specific expression of a cyp2g1-lacz fusion gene in transgenic mice

The structure of the mouse Cyp2g1 gene was determined to identify regulatory regions important for its olfactory mucosa-specific expression. Two Cyp2g1 genomic clones were isolated and characterized. A 3.6-kilobase 5'-flanking sequence was used to prepare a Cyp2g1--LacZ fusion gene for transgenic mice production. Transgene expression, as determined by beta-galactosidase activity in tissue extracts, was detected in the olfactory mucosa, but not in any other tissues examined, in five different transgenic lines. Thus, the 3.6-kilobase fragment contained regulatory elements sufficient for olfactory mucosa-specific and proper developmental expression of the reporter gene. However, histological and immunohistochemical studies indicated that the expression of the transgene in the olfactory mucosa was patchy and the cellular expression patterns of the transgene did not exactly match that of the endogenous gene. These results implicate the presence of additional regulatory sequences that are necessary for the correct cell type-selectivity within the olfactory mucosa.

Identification and functional characterization of a conserved, nuclear factor 1-like element in the proximal promoter region of CYP1A2 gene specifically expressed in the liver and olfactory mucosa

CYP1A2 is a major cytochrome P-450 isoform in the liver and the olfactory mucosa but is essentially not expressed in other tissues. A nuclear factor 1 (NF-1) -like element was identified in the proximal promoter region of rat, mouse, rabbit, and human CYP1A2 genes through data base analysis. In vitro DNase I footprinting with a -211 to +81 probe from the rat CYP1A2 gene and nuclear extracts from rat liver and olfactory mucosa revealed a single protected region corresponding to the NF-1-like element at -129 to -111. Protein binding to this NF-1-like element was tissue-selective and was confirmed by in vivo footprinting in native chromatin from rat liver. Multiple DNA-binding complexes were detected in gel-shift assays using the CYP1A2 NF-1-like element and nuclear extracts from liver and olfactory mucosa, all of which were supershifted in the presence of an anti-NF1 antibody. The NF-1-like element was essential for transcriptional activity of the CYP1A2 gene in an in vitro transcription assay using nuclear extracts from the two tissues. Thus, members of the NF-1 family of transcription factors may play an important role in the tissue-selective expression of the CYP1A2 gene in the liver and olfactory mucosa.

A novel positive regulatory element that enhances hamster CYP2A8 gene expression mediated by xenobiotic responsive element

CYP2A8 is a major form of cytochrome P-450 inducible by 3-methylcholanthrene in Syrian hamster liver. To identify DNA elements necessary for the transcriptional activation of the CYP2A8 gene, we analyzed the regulatory region of the CYP2A8 gene and conducted transient transfection experiments of CYP2A8-luciferase fusion plasmids in primary cultures of hamster hepatocytes. We analyzed up to -5 kb of the 5'-flanking region and found the region sufficient for the 3-methylcholanthrene-inducible gene expression. This region contained a consensus sequence for xenobiotic responsive element (XRE) between -2366 and -2349, which was shown to be essential for induction of the gene expression. Furthermore, we found a novel positive regulatory element for XRE-mediated gene expression (PREX) located upstream of the XRE. This element is not identified in any genes inducible by 3-methylcholanthrene so far reported. Without PREX, the XRE-mediated promoter activity was enhanced nearly 10-fold, whereas with PREX, the activity was enhanced 20-fold over the basal level. Gel mobility shift assays revealed specific binding of nuclear proteins to PREX. Mutations and deletions of PREX caused a loss of the binding and promoter-enhancing activities, respectively. Moreover, transient expression experiments showed that the enhancing activity of PREX was not observed in Drosophila Schneider's line 2 cells, which were shown to lack the PREX binding proteins.