Cytochrome P450 2A of nasal epithelium: regulation and role in carcinogen metabolism

Differential distribution of ivacaftor and its metabolites in plasma and human airway epithelia

Ivacaftor is the first clinically approved monotherapy potentiator to treat CFTR channel dysfunction in people with cystic fibrosis. Ivacaftor (Iva) is a critical component for all current modulator therapies, including highly effective modulator therapies. Clinical studies show that CF patients on ivacaftor-containing therapies present various clinical responses, off-target effects, and adverse reactions, which could be related to metabolites of the compound. In this study, we reported the concentrations of Iva and two of its major metabolites (M1-Iva and M6-Iva) in capillary plasma and estimated M1-Iva and M6-Iva metabolic activity via the metabolite parent ratio in capillary plasma over 12 h. We also used the ratio of capillary plasma versus human nasal epithelial cell concentrations to evaluate entry into epithelial cells in vivo. M6-Iva was rarely detected by LC-MS/MS in epithelial cells from participants taking ivacaftor, although it was detected in plasma. To further explore this discrepancy, we performed in vitro studies, which showed that M1-Iva, but not M6-Iva, readily crossed 16HBE cell membranes. Our studies also suggest that metabolism of these compounds is unlikely to occur in airway epithelia despite evidence of expression of metabolism enzymes. Overall, our data provide evidence that there are differences between capillary and cellular concentrations of these compounds that may inform future studies of clinical response and off-target effects.

Anticancer Effect of Rosiglitazone, a PPAR-γ Agonist against Diethylnitrosamine-Induced Lung Carcinogenesis

Multiple effects on cancer cells are exerted by the peroxisome proliferator-activated receptor γ (PPAR-γ). Recent studies have shown that rosiglitazone, a synthetic PPAR-γ ligand, inhibits the growth of cells. This research was designed to assess the impact of rosiglitazone on diethylnitrosamine (DENA)-induced lung carcinogenesis in Wistar rats and to study the underlying molecular mechanism. A total of 40 adult male Wistar rats were separated into four groups as follows: group 1 is known as a control. Group 2 is known as the DENA group (150 mg/kg, i.p.). Group 3 and group 4 denote DENA-induced rats treated with 5 and 10 mg/kg rosiglitazone, respectively. Lipid peroxidation, various antioxidant enzymes, histological perceptions, and caspase-3, Bcl2, and Bax gene expression were measured in lung tissues. Rosiglitazone treatment reverted the DENA-induced changes in the expression of these genes, inflammatory cytokines, and oxidative stress. However, blotting analysis discovered reduced caspase-3 and BAX expressions and elevated Bcl-2 expression in DENA-induced rats. The expression of such proteins causing DENA lung cancer was restored by rosiglitazone therapy.

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.

Toxicological effects of aflatoxins in horses

Aflatoxins are a group of mycotoxins principally produced by Aspergillus flavus and A. parasiticus, which are both natural contaminants of food and feedstuff. Aflatoxin B(1) is the most prevalent member of this group that is normally detected and is the most powerful hepatocarcinogen known. Few naturally occurring episodes of aflatoxicosis in horses have been reported in the literature. Indeed, the published information about aflatoxin exposure, metabolism and the effects on horses is limited and controversial, possibly indicating a lack of awareness rather than the rarity of the occurrence. The target organ in horses, as in other animal species, is the liver and horses suffering from aflatoxicosis show signs of inappetence, depression, fever, tremor, ataxia and cough. Necropsy findings include a yellow-brown liver with centrilobular necrosis, icterus, haemorrhage, tracheal exudates and brown urine. A possible link between aflatoxin exposure and chronic obstructive pulmonary disease has been hypothesised.

Different modes of inhibition of mouse Cyp2a5 and rat CYP2A3 by the food-derived 8-methoxypsoralen

CYP2A enzymes are responsible for nicotine metabolism and for activating tobacco-related carcinogens. Inhibition of CYP2A is a promising approach in chemoprevention, which could lead to a decrease in cigarette consumption and to a reduction in tobacco-related cancer risk. 8-Methoxypsoralen (8-MOP) is a mechanism-based inhibitor of human CYP2A6 and CYP2A13. 8-MOP is also an inhibitor of Cyp2a5, but the mode of this inhibition is unknown. There is no published data on the inhibition of CYP2A3 by 8-MOP. The objective of this work was to investigate the characteristics of 8-MOP inhibition on mouse hepatic Cyp2a5 and rat nasal CYP2A3, in order to determine the best experimental model for chemoprevention studies using 8-MOP. The results show that 8-MOP inhibits CYP2a5 through three different mechanisms: competitive, non-competitive (K(iu)=1.7 microM), and mechanism-based (K(inactivation) of 0.17 min(-1)). By contrast, 8-MOP was able to inhibit CYP2A3-mediated coumarin 7-hydroxylase only in a non-competitive way (K(iu)=0.22 microM). In conclusion, we showed that 8-MOP inhibits Cyp2a5 and CYP2A3 through different mechanisms.

Ethyl tertiary-butyl ether: a toxicological review

A number of oxygenated compounds (oxygenates) are available for use in gasoline to reduce vehicle exhaust emissions, reduce the aromatic compound content, and avoid the use of organo-lead compounds, while maintaining high octane numbers. Ethyl tertiary-butyl ether (ETBE) is one such compound. The current use of ETBE in gasoline or petrol is modest but increasing, with consequently similar trends in the potential for human exposure. Inhalation is the most likely mode of exposure, with about 30% of inhaled ETBE being retained by the lungs and distributed around the body. Following cessation of exposure, the blood concentration of ETBE falls rapidly, largely as a result of its metabolism to tertiary-butyl alcohol (TBA) and acetaldehyde. TBA may be further metabolized, first to 2-methyl-1,2-propanediol and then to 2-hydroxyisobutyrate, the two dominant metabolites found in urine of volunteers and rats. The rapid oxidation of acetaldehyde suggests that its blood concentration is unlikely to rise above normal as a result of human exposure to sources of ETBE. Single-dose toxicity tests show that ETBE has low toxicity and is essentially nonirritant to eyes and skin; it did not cause sensitization in a maximization test in guinea pigs. Neurological effects have been observed only at very high exposure concentrations. There is evidence for an effect of ETBE on the kidney of rats. Increases in kidney weight were seen in both sexes, but protein droplet accumulation (with alpha(2u)-globulin involvement) and sustained increases in cell proliferation occurred only in males. In liver, centrilobular necrosis was induced in mice, but not rats, after exposure by inhalation, although this lesion was reported in some rats exposed to very high oral doses of ETBE. The proportion of liver cells engaged in S-phase DNA synthesis was increased in mice of both sexes exposed by inhalation. ETBE has no specific effects on reproduction, development, or genetic material. Carcinogenicity studies have been conducted with ETBE, TBA, and ethanol (included in this review as an endogenous precursor of acetaldehyde in the absence of TBA). A single experiment with ETBE in rats and several experiments with ethanol in rats and mice were not considered adequate for an evaluation of ETBE carcinogenicity. In male rats only, TBA induced alpha(2u)-globulin nephropathy-related renal tubule adenomas. These are generally considered to have no human relevance. In addition, increases in thyroid follicular cell adenoma incidence were associated with TBA treatment in female mice. This result lacks independent confirmation and is not supported by experiments in which similar or higher internal doses of TBA were delivered.

Nasal cytotoxic and carcinogenic activities of systemically distributed organic chemicals

Toxicity and carcinogenicity in the mucosa of the nasal passages in rodents has been produced by a variety of organic chemicals which are systemically distributed. In this review, 14 such chemicals or classes were identified that produced rodent nasal cytotoxicity, but not carcinogenicity, and 11 were identified that produced nasal carcinogenicity. Most chemicals that affect the nasal mucosa were either concentrated in that tissue or readily activated there, or both. All chemicals with effects in the nasal mucosa that were DNA-reactive, were also carcinogenic, if adequately tested. None of the rodent nasal cytotoxins has been identified as a human systemic nasal toxin. This may reflect the lesser biotransformation activity of human nasal mucosa compared to rodent and the much lower levels of human exposures. None of the rodent carcinogens lacking DNA reactivity has been identified as a nasal carcinogen or other cancer hazard to humans. Some DNA-reactive rodent carcinogens that affect the nasal mucosa, as well as other tissues, have been associated with cancer at various sites in humans, but not the nasal cavity. Thus, findings in only the rodent nasal mucosa do not necessarily predict either a toxic or carcinogenic hazard to that tissue in humans.

Metabolism of dopamine by the nasal mucosa

The nasal route of administration offers several advantages over oral and intravenous administration, including the ability to avoid hepatic first pass metabolism. Dopamine deficiency has been associated with several neurological disorders; it has been shown to have good systemic bioavailability and significant uptake into the CNS following intranasal administration. The purpose of these studies was to investigate the limiting role of mucosal metabolism of dopamine during nasal absorption. In vitro transport and initial rate studies were carried out using nasal mucosal explants to study dopamine permeability and metabolism. Dihydroxyphenylacetic acid (DOPAC) was the only metabolite detected. Monoamine oxidase (MAO), the enzyme responsible for DOPAC formation, was localized to the submucosal region of the nasal explants. The amount of DOPAC formed during the transport studies was less than 0.5% of the initial amount of dopamine placed into the system. Iproniazid, an MAO inhibitor, blocked DOPAC formation but had no effect on dopamine transport. The limited extent of dopamine metabolism compared to its mucosal transport demonstrates that nasal dopamine transport is not significantly reduced by mucosal metabolism and suggests that the nasal route may be promising for the efficient delivery of dopamine to the CNS.

Respiratory Tract

The chapter describes different aspects of the respiratory tract. In preclinical safety studies, pathologies of the respiratory system can be a result of an intercurrent disease or can be induced by systemically administered drugs. Intranasal or inhalation modes of therapy pose particular challenges in terms of the formulations and technologies required to administer a drug. A complex technology is developed to support the assessment of adverse effects of inhaled substances in rodent and nonrodent species, and the extrapolation of experimental findings to humans. The nasal chambers are the structures that are first to be subjected to the effects of inhaled substances, whether microorganisms or chemical substances. In rodents, the relatively small size of the nose and nasal sinuses facilitates a histological examination. Findings show that infectious agents cause inflammation in the nose and nasal sinuses, and this may be associated with inflammation in the conjunctiva, the middle ear, and the oral cavity. It has been observed that a particular response of the rodent nasal mucosa to some irritant substances, including pharmaceutical agents, is the formation of rounded eosinophilic inclusions in the cytoplasm of sustentacular cells of the olfactory epithelium, and to a lesser extent in respiratory and glandular epithelial cells.

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.

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.

Cell-specific expression of CYP2A5 in the mouse respiratory tract: effects of olfactory toxicants

We performed a detailed analysis of mouse cytochrome P450 2A5 (CYP2A5) expression by in situ hybridization (ISH) and immunohistochemistry (IHC) in the respiratory tissues of mice. The CYP2A5 mRNA and the corresponding protein co-localized at most sites and were predominantly detected in the olfactory region, with an expression in sustentacular cells, Bowman's gland, and duct cells. In the respiratory and transitional epithelium there was no or only weak expression. The nasolacrimal duct and the excretory ducts of nasal and salivary glands displayed expression, whereas no expression occurred in the acini. There was decreasing expression along the epithelial linings of the trachea and lower respiratory tract, whereas no expression occurred in the alveoli. The hepatic CYP2A5 inducers pyrazole and phenobarbital neither changed the CYP2A5 expression pattern nor damaged the olfactory mucosa. In contrast, the olfactory toxicants dichlobenil and methimazole induced characteristic changes. The damaged Bowman's glands displayed no expression, whereas the damaged epithelium expressed the enzyme. The CYP2A5 expression pattern is in accordance with previously reported localization of protein and DNA adducts and the toxicity of some CYP2A5 substrates. This suggests that CYP2A5 is an important determinant for the susceptibility of the nasal and respiratory epithelia to protoxicants and procarcinogens.

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.

Expression of CYP2A3 mRNA and its regulation by 3-methylcholanthrene, pyrazole, and beta-ionone in rat tissues

Cytochrome P450 (CYP) 2A enzymes are involved in the metabolism of numerous drugs and hormones and activate different carcinogens. Human CYP2A6, mouse CYP2A5 and rat CYP2A3 are orthologous enzymes that present high similarity in their amino acid sequence and share substrate specificities. However, different from the human and mouse enzyme, CYP2A3 is not expressed in the rat liver. There are limited data about expression of CYP2A3 in extrahepatic tissues and its regulation by typical CYP inducers. Therefore, the objective of the present study was to analyze CYP2A3 mRNA expression in different rat tissues by RT-PCR, and to study the influence of 3-methylcholanthrene, pyrazole and -ionone treatment on its expression. Male Wistar rats were divided into four groups of 5 rats each, and were treated ip for 4 days with 3-methylcholanthrene (25 mg/kg body weight), pyrazole (150 mg/kg body weight), -ionone (1 g/kg body weight), or vehicle. Total RNA was extracted from tissues and CYP2A3 mRNA levels were analyzed by semiquantitative RT-PCR. CYP2A3 mRNA was constitutively expressed in the esophagus, lung and nasal epithelium, but not along the intestine, liver, or kidney. CYP2A3 mRNA levels were increased in the esophagus by treatment with 3-methylcholanthrene and pyrazole (17- and 7-fold, respectively), in lung by pyrazole and -ionone (3- and 4-fold, respectively, although not statistically significant), in the distal part of the intestine and kidney by 3-methylcholanthrene and pyrazole, and in the proximal part of the intestine by pyrazole. CYP2A3 mRNA was not induced in nasal epithelium, liver or in the middle part of the intestine. These data show that, in the rat, CYP2A3 is constitutively expressed in several extrahepatic tissues and its regulation occurs through a complex mechanism that is essentially tissue specific.

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.

Cell-specific activation of aflatoxin B1 correlates with presence of some cytochrome P450 enzymes in olfactory and respiratory tissues in horse

Horses may be exposed to aflatoxin B(1) (AFB(1)) via inhalation of mouldy dust, leading to high exposure of olfactory and respiratory tissues. In the present study the metabolic activation of AFB(1) was examined in olfactory and respiratory tissues in horse. The results showed covalent binding of AFB(1)-metabolites in sustentacular cells and cells of Bowman's glands in the olfactory mucosa, in some cells of the surface epithelium of nasal respiratory, tracheal, bronchial and bronchiolar mucosa and in some glands in these areas. Immunohistochemistry revealed that cells expressing proteins reacting with CYP 3A4- and CYP 2A6/2B6-antibodies had a similar distribution as those having capacity to activate AFB(1). Our data indicate that the cell-specific activation of AFB(1) correlates with presence of some CYP-enzymes in olfactory and respiratory tissues in horse.

Infection of rats with Taenia taeniformis metacestodes increases hepatic CYP450, induces the activity of CYP1A1, CYP2B1 and COH isoforms and increases the genotoxicity of the procarcinogens benzo[a]pyrene, cyclophosphamide and aflatoxin B(1)

Infection of rat liver by Taenia taeniformis metacestodes produced an increase in total CYP450 content and induced activity of the CYP1A1, CYP2B1 and COH isoforms. Variations in activity and p450 total content were found with increasing time of infection. During increased activity of p450 isoforms, rats were challenged with carcinogens metabolized by the mentioned isozymes and an increased amount of genotoxic damage was found when benzo[a] pyrene, cyclophosphamide and aflatoxin B(1) were used. No change was seen in CYP2E1 activity. These results support previous findings regarding an increased susceptibility to genotoxic damage of infected organisms.

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.

Rat oesophageal cytochrome P450 (CYP) monooxygenase system: comparison to the liver and relevance in N-nitrosodiethylamine carcinogenesis

N-nitrosodiethylamine (NDEA) is able to induce tumours in the rat oesophagus. It has been suggested that this could be due to tissue specific expression of NDEA activating cytochrome P450 enzymes. We investigated this by characterizing the oesophageal monooxygenase complex of male Wistar rats and comparing it with that of the liver. Total amount of cytochrome P450, NADPH P450 reductase, cytochrome b5 and cytochrome b5 reductase of the oesophageal mucosa was approximately 7% of what was found in the liver. In addition, major differences were found in the cytochrome P450 isoenzyme composition between these organs: CYP 2B1/2B2 and CYP3A were found only in the liver, whereas CYP1A1 was constitutively expressed only in the oesophagus. Of the two well-known nitrosamine metabolizing enzymes, CYP2A3 was found only in the oesophagus whereas CYP2E1 was exclusively expressed in the liver. Catalytic studies, western blotting and RT-PCR analyses confirmed the expression of CYP2A3 in the oesophagus. CYP2A enzymes are known to be good catalysts of NDEA metabolism. Oesophageal microsomes had a K(m) for NDEA metabolism, which was about one-third of that of hepatic microsomes, but they showed similar activities when compared per nmol of total P450. NDEA activity in the oesophagus was significantly increased by coumarin (CO), which also induced oesophageal CYP2A3. Immunoinhibition of the microsomal NDEA activity showed that up to 70% of this reaction is catalysed by CYP2A3 in the oesophagus, whereas no inhibition of the hepatic NDEA activity could be achieved by the anti-CYP2A5 antibody. NDEA, but not N-nitrosodimethylamine (NDMA) inhibited the oesophageal metabolism of CO. The results of the present investigation show major differences in the enzyme composition of the oesophageal and hepatic monooxygenase complexes, and are in accordance with the hypothesis that the NDEA organotropism could, to a large extent, be due to the tissue specific expression of the activating enzymes.

The toxicity of styrene to the nasal epithelium of mice and rats: studies on the mode of action and relevance to humans

Inhaled styrene is known to be toxic to the nasal olfactory epithelium of both mice and rats, although mice are markedly more sensitive. In this study, the nasal tissues of mice exposed to 40 and 160 ppm styrene 6 h/day for 3 days had a number of degenerative changes including atrophy of the olfactory mucosa and loss of normal cellular organisation. Pretreatment of mice with 5-phenyl-1-pentyne, an inhibitor of both CYP2F2 and CYP2E1 completely prevented the development of a nasal lesion on exposure to styrene establishing that a metabolite of styrene, probably styrene oxide, is responsible for the observed nasal toxicity. Comparisons of the cytochrome P-450 mediated metabolism of styrene to its oxide, and subsequent metabolism of the oxide by epoxide hydrolases and glutathione S-transferases in nasal tissues in vitro, have provided an explanation for the increased sensitivity of the mouse to styrene. Whereas cytochrome P-450 metabolism of styrene is similar in rats and mice, the rat is able to metabolise styrene oxide at higher rates than the mouse thus rapidly detoxifying this electrophilic metabolite. Metabolism of styrene to its oxide could not be detected in human nasal tissues in vitro, but the same tissues did have epoxide hydrolase and glutathione S-transferase activities, and were able to metabolise styrene oxide efficiently, indicating that styrene is unlikely to be toxic to the human nasal epithelium.

Health risks associated with inhaled nasal toxicants

Health risks of inhaled nasal toxicants were reviewed with emphasis on chemically induced nasal lesions in humans, sensory irritation, olfactory and trigeminal nerve toxicity, nasal immunopathology and carcinogenesis, nasal responses to chemical mixtures, in vitro models, and nasal dosimetry- and metabolism-based extrapolation of nasal data in animals to humans. Conspicuous findings in humans are the effects of outdoor air pollution on the nasal mucosa, and tobacco smoking as a risk factor for sinonasal squamous cell carcinoma. Objective methods in humans to discriminate between sensory irritation and olfactory stimulation and between adaptation and habituation have been introduced successfully, providing more relevant information than sensory irritation studies in animals. Against the background of chemoperception as a dominant window of the brain on the outside world, nasal neurotoxicology is rapidly developing, focusing on olfactory and trigeminal nerve toxicity. Better insight in the processes underlying neurogenic inflammation may increase our knowledge of the causes of the various chemical sensitivity syndromes. Nasal immunotoxicology is extremely complex, which is mainly due to the pivotal role of nasal lymphoid tissue in the defense of the middle ear, eye, and oral cavity against antigenic substances, and the important function of the nasal passages in brain drainage in rats. The crucial role of tissue damage and reactive epithelial hyperproliferation in nasal carcinogenesis has become overwhelmingly clear as demonstrated by the recently developed biologically based model for predicting formaldehyde nasal cancer risk in humans. The evidence of carcinogenicity of inhaled complex mixtures in experimental animals is very limited, while there is ample evidence that occupational exposure to mixtures such as wood, leather, or textile dust or chromium- and nickel-containing materials is associated with increased risk of nasal cancer. It is remarkable that these mixtures are aerosols, suggesting that their "particulate nature" may be a major factor in their potential to induce nasal cancer. Studies in rats have been conducted with defined mixtures of nasal irritants such as aldehydes, using a model for competitive agonism to predict the outcome of such mixed exposures. When exposure levels in a mixture of nasal cytotoxicants were equal to or below the "No-Observed-Adverse-Effect-Levels" (NOAELs) of the individual chemicals, neither additivity nor potentiation was found, indicating that the NOAEL of the "most risky chemical" in the mixture would also be the NOAEL of the mixture. In vitro models are increasingly being used to study mechanisms of nasal toxicity. However, considering the complexity of the nasal cavity and the many factors that contribute to nasal toxicity, it is unlikely that in vitro experiments ever will be substitutes for in vivo inhalation studies. It is widely recognized that a strategic approach should be available for the interpretation of nasal effects in experimental animals with regard to potential human health risk. Mapping of nasal lesions combined with airflow-driven dosimetry and knowledge about local metabolism is a solid basis for extrapolation of animal data to humans. However, more research is needed to better understand factors that determine the susceptibility of human and animal tissues to nasal toxicants, in particular nasal carcinogens.

Kinetics of propylene oxide metabolism in microsomes and cytosol of different organs from mouse, rat, and humans

Kinetics of the metabolic inactivation of 1,2-epoxypropane (propylene oxide; PO) catalyzed by glutathione S-transferase (GST) and by epoxide hydrolase (EH) were investigated at 37 degrees C in cytosol and microsomes of liver and lung of B6C3F1 mice, F344 rats, and humans and of respiratory and olfactory nasal mucosa of F344 rats. In all of these tissues, GST and EH activities were detected. GST activity for PO was found in cytosolic fractions exclusively. EH activity for PO could be determined only in microsomes, with the exception of human livers where some cytosolic activity also occurred, representing 1-3% of the corresponding GST activity. For GST, the ratio of the maximum metabolic rate (V(max)) to the apparent Michaelis constant (K(m)) could be quantified for all tissues. In liver and lung, these ratios ranged from 12 (human liver) to 106 microl/min/mg protein (mouse lung). Corresponding values for EH ranged from 4.4 (mouse liver) to 46 (human lung). The lowest V(max) value for EH was found in mouse lung (7.1 nmol/min/mg protein); the highest was found in human liver (80 nmol/min/mg protein). K(m) values for EH-mediated PO hydrolysis in liver and lung ranged from 0.83 (human lung) to 3.7 mmol/L (mouse liver). With respect to liver and lung, the highest V(max)/K(m) ratios were obtained for GST in mouse and for EH in human tissues. GST activities were higher in lung than in liver of mouse and human and were alike in both rat tissues. Species-specific EH activities in lung were similar to those in liver. In rat nasal mucosa, GST and EH activities were much higher than in rat liver.

Methyl methacrylate toxicity in rat nasal epithelium: studies of the mechanism of action and comparisons between species

Female F344 rats exposed to 200 ppm methyl methacrylate for 6 h developed a lesion in the nasal olfactory epithelium which was characterised by degeneration and atrophy. The severity of the lesion was markedly reduced by pre-treatment of the rats with an intraperitoneal dose of 100 mg/kg bis-(p-nitrophenyl)phosphate, an inhibitor of carboxylesterase enzymes, thus demonstrating that the lesion is caused by the carboxylesterase mediated metabolism of methyl methacrylate to methacrylic acid, an irritant and corrosive metabolite. The distribution of the carboxylesterases in nasal tissues has been investigated and the metabolism of methyl methacrylate to methacrylic acid has been compared in rat, hamster and human nasal tissue fractions in vitro. Histocytochemistry showed that the carboxylesterases are heavily localised in the sustentacular cells and Bowman's glands of the rat olfactory region, but are more generally distributed in human olfactory epithelium. Consistent with this, the enzyme activity in all three species was higher in fractions prepared from olfactory tissue than from respiratory tissue, 3-fold in rat and human and 12-fold in the hamster. The maximum rates (V(max)) of metabolism in rat and hamster olfactory tissue fractions were comparable, whereas those in human olfactory tissue fractions were at least 13-fold lower. The rate of metabolism in rat olfactory tissue was also comparable to that in rat liver whereas in humans, the rate in olfactory tissue was 500-fold lower than that in the liver. In respiratory tissues, the rate in humans was at least 6-fold lower than that in the rat. These results suggest that humans are significantly less sensitive than rodents to the nasal toxicity of methyl methacrylate.

Acetochlor-induced rat nasal tumors: further studies on the mode of action and relevance to humans

The herbicide acetochlor, and its analogue alachlor, have similar toxicological properties, the most significant being the induction of nasal adenomas in rats in 2-year feeding studies. Previous investigations have proposed a mode of action involving metabolism to a quinone-imine, the formation of protein adducts, cell death, and compensatory hyperplasia leading to the observed adenomas. Comparisons between rats and humans of the metabolic cascade leading to the quinone-imine indicate that these chemicals do not pose a threat to humans. Further investigations with acetochlor, presented here, have revealed an additional activation pathway in which a sulfoxide metabolite of acetochlor plays a key role. The sulfoxide was found to be the major plasma metabolite in rats dosed with acetochlor. Whole-body autoradiography studies established that this metabolite selectively accumulates and persists in the olfactory epithelium of rats. Radiolabeling of the sulfoxide molecule in the phenyl ring and in the sulfoxide side-chain demonstrated that the metabolite accumulating in nasal tissues retains the sulfoxide side-chain. The formation of a quinone-imine from the sulfoxide was facilitated by hydroxylation of the phenyl ring by a cytochrome P450 isoenzyme which was specific to the nasal epithelium in the rat. This metabolic conversion could not be detected in 33 fresh human nasal tissue samples, supporting the earlier view that the acetochlor-induced rat nasal tumors do not represent a hazard for humans.

Effect of starvation and chlormethiazole on cytochrome P450s of rat nasal mucosa

Cytochrome P450 (CYP) enzymes of nasal tissue are relatively resistant to induction by classical inducers. In the present study, the effects of starvation on the expression of CYP1A, 2A, 2B, 2C, 2E, 2G, and 3A subfamilies in the nasal mucosa of rat were studied. Fasting for 72 hr caused an increase in 2E1-dependent p-nitrophenol hydroxylase and 1A-dependent ethoxy- (or methoxy) resorufin dealkylase activities, but did not affect either 2A-linked coumarin hydroxylase or the testosterone hydroxylase activity, the latter reaction being a marker of several CYPs including 2G1. Whereas increases in 2E1- and 1A- associated catalytic activities were accompanied by a concomitant increase in the corresponding apoproteins as determined by immunoblotting, immunoactive protein bands reactive with antibodies raised against rat 1A1, 2B1, 2C11, 3A1 or rabbit nasal 2A10/11 and 2G1 were not altered. Fasting also increased CYP2E1 and CYP1A2 on the mRNA level, but did not alter CYP1A1 mRNA as determined by hybridization with cDNA probes selective for these cytochromes. A reiterative administration of chlormethiazole, a specific inhibitor of 2E1 in liver, strongly inhibited many CYPs, including 2E1, 1A2, 2G1, and 2A in the nasal mucosa, but did not influence expression of 2B or 3A as determined by immunoblotting or catalytic activities. The chlormethiazole-mediated inhibition of 1A1 and 2E1 was demonstrated to be at the mRNA level. These results suggest that fasting induces the gene expression of 2E1 and 1A2 and that the mechanisms involved in the regulation of CYPs in the nasal mucosa are tissue-specific. The inducibility of the above-mentioned isoforms may have a significant role in the clearance of drugs and bioactivation of inhaled compounds.

Molecular cloning, heterologous expression, and characterization of a novel member of CYP2A in the Syrian hamster

The cDNA clone coding for a novel cytochrome P-450 2A subfamily member (CYP2A16) was isolated from a Syrian hamster liver cDNA library. The deduced amino acid sequence of CYP2A16 showed more than 90% identity with those of rat CYP2A3 and mouse CYP2A4/5. The catalytic activity of CYP2A16 was determined by transient expression of its cDNA in transfected COS7 cells and CYP2A16 was found to have the testosterone 2 beta-, 15 alpha-, and 15 beta-hydroxylases, coumarin 7-hydroxylase, and ethoxycoumarin O-deethylase activities. These enzymatic characteristics of CYP2A16 are different from those of other Syrian hamster CYP2A subfamily members, CYP2A8 and CYP2A9. Northern blot analysis showed that CYP2A16 was expressed in kidney and lung while most of the other CYP2A subfamily members have been reported to be expressed in liver and olfactory. These observations indicated that the Syrian hamster CYP2A16 had unique properties compared with those of other CYP2A subfamily members.

Expression of CYP2A genes in human liver and extrahepatic tissues

Members of the human cytochrome P450 2A (CYP2A) subfamily are known to metabolize several promutagens, procarcinogens, and pharmaceuticals. In this study, the expression of the three genes found in the human CYP2A gene cluster was investigated in the liver and several extrahepatic tissues by gene-specific reverse transcriptase-polymerase chain reaction (RT-PCR). All three transcripts (CYP2A6, CYP2A7, and CYP2A13) were found to be present in liver. Quantitative RT-PCR analysis showed that CYP2A6 and CYP2A7 mRNAs were present at roughly equal levels in the liver, while CYP2A13 was expressed at very low levels. Two putative splicing variants of CYP2A7 were found in the liver. Nasal mucosa contained a low level of CYP2A6 and a relatively high level of CYP2A13 transcripts. Kidney, duodenum, lung, alveolar macrophages, peripheral lymphocytes, placenta, and uterine endometrium were negative for all transcripts. This survey gives a comprehensive picture of the expression pattern of CYP2A genes in liver and extrahepatic tissues and constitutes a basis for a search for functional CYP2A forms and their roles in chemical toxicity in liver and nasal mucosa.

Analysis of the inhibition of N-nitroso-dimethylamine activation in the liver by N-nitro-dimethylamine using a new non-linear statistical method

N-nitro-dimethylamine (NTDMA) is carcinogenic to rats: it induces nasal cavity tumours. It can be demethylated to N-nitromethylamine and formaldehyde and reduced to N-nitroso-dimethylamine (NDMA): a potent liver carcinogen and also of the nasal cavity if activation in the liver is blocked. To explain the mechanism of NTDMA carcinogenicity we compared its demethylation with that of NDMA in liver microsomes from female and male rats, untreated, fasted or treated with ethanol to induce cytochrome P450 2E1 (CYP2E1). Kinetic parameters were analysed by nonlinear statistical methods, which yielded unbiased parameter estimates for the calculated Km and Vmax values. Km for both compounds was very similar in females (24-47 microM) whereas Vmax for NTDMA was consistently higher than for NDMA as substrate: 1.07-4.70 nmol formaldehyde/mg microsomal protein x min and 0.52-2.76 nmol, respectively. In liver microsomes from induced male rats NTDMA was found to be a much more effective inhibitor of NDMA activation (KEI 39.6-73.6 microM) than NDMA of NTDMA demethylation (KEI 224-286 microM). Nasal microsomes can demethylate both NDMA and NTDMA but the kinetics are vastly different. NTDMA is demethylated at a linear rate and approximately 10-fold more effectively than NDMA. The mechanism of carcinogenicity of ingested NTDMA, we propose, is a partial reduction to NDMA in the liver and inhibition of NDMA activation in the liver by residual NTDMA, which enables NDMA to reach the nasal mucosa where it is activated to DNA-alkylating species and the observed tumours are formed.

Identification of CYP2A3 as a major cytochrome P450 enzyme in the female peripubertal rat breast

On isolation of rat breast cytochrome P450, one of the proteins whose amino terminus was sequenced was CYP2A3. CYP2A3 was detected by Western blotting in cytochrome P450 fractions isolated from breast of 3-, 6-, and 9-week-old rats but was low during pregnancy and lactation. Reverse transcription-polymerase chain reaction analysis and sequencing of the PCR product confirmed the presence and identity of CYP2A3 in the rat breast. Breast microsomal coumarin-7-hydroxylase activity paralleled the developmental pattern observed for CYP2A3 on Western blots. In the lung, coumarin-7-hydroxylase activity was 10-fold higher than that in the breast, but the developmental pattern was similar to that in the breast. Lung microsomes from 9-week-old rats activated the heterocyclic amine 2-amino-3-methylimidazo[4,5-f]quinoline to mutagenic metabolites which could be detected with the Ames test. This activation could be inhibited by the CYP2A3 antiserum. With breast microsomes, which contain approximately 10% of the cytochrome P450 in the lung, activation of 2-amino-3-methylimidazo[4, 5-f]quinoline could not be reliably measured. Immunohistochemical localization revealed that CYP2A3 was expressed in a limited number of epithelial cells in the ducts of 6-week-old rat breast. Double staining with smooth muscle actin, a marker for myoepithelial cells, showed no staining of CYP2A3 immunoreactive cells, indicating that these cells were not myoepithelial. The data clearly show that a cytochrome P450 that can activate environmental procarcinogens is developmentally regulated and concentrated in specific cells in the breast. The peripubertal period seems to be a window in time when the breast may be more sensitive to procarcinogens that are substrates for CYP2A3.

Metabolic capacity of nasal tissue interspecies comparisons of xenobiotic-metabolizing enzymes

High levels of xenobiotic-metabolizing enzymes occur in the nasal mucosa of all species studied. In certain species, including rats and rabbits, unique enzymes are present in the nasal mucosa. The function of these enzymes is not well understood, but it is thought that they play a role in protecting the lungs from toxicity of inhalants. The observation that several nasal xenobiotic-metabolizing enzymes accept odorants as substrates may indicate that these enzymes also play a role in the olfactory process. Xenobiotic-metabolizing enzymes were found in the nasal cavity around 15 years ago. Since that time, much has been learned about the nature of the enzymes and the substrates they accept. In the present review, this information is summarized with special attention to species differences in xenobiotic-metabolizing enzymes of the nasal cavity. Such differences may be important in interpreting the results of toxicity assays in animals because rodents are apparently more susceptible to nasal toxicity after exposure to inhalants than are humans.

Intraperitoneal administration of coumarin causes tissue-selective depletion of cytochromes P450 and cytotoxicity in the olfactory mucosa

Coumarin is a naturally occurring fragrant compound widely used in consumer products and also as a therapeutic agent. The effects of intraperitoneal (ip) and oral administration of coumarin on cytochrome P450 (P450) expression in olfactory mucosa were examined. A single ip injection of coumarin at 50 mg/kg resulted in a significant reduction of levels of CYP2A and CYP2G in the olfactory mucosa of Wistar rats and C57BL/6 mice at 48 hr following injection. Dose-response analysis of coumarin effects indicated that Wistar rats were more sensitive than C57BL/6 mice. A significant suppression of nasal CYP2A levels was observed at 25 mg/kg in rats, but not in mice. Depletion of P450 content was not observed in liver of either rats or mice at 50 mg/kg, indicating tissue-selective effects. Decreased P450 levels were observed at 24 hr, 48 hr, and 7 days following treatment, with minimal levels seen at 48 hr. The decrease in P450 levels was accompanied by necrosis, cell loss, and basal cell metaplasia in the olfactory mucosa. Intraperitoneal injection of 7-hydroxycoumarin or 3,4-dihydrocoumarin at 50 mg/kg did not result in depletion of nasal P450, indicating that the toxicity is not mediated by P450-catalyzed coumarin 7-hydroxylation and supporting the hypothesis that the formation of coumarin 3,4-epoxide may be responsible for the toxicity. Oral treatment with coumarin in drinking water led to a small, yet significant induction of CYP2A protein and coumarin hydroxylase activity in the nasal mucosa of mice, but not rats. Thus, ip administration of coumarin causes tissue-selective depletion of P450 and cytotoxicity in the olfactory mucosa of Wistar rats and C57BL/6 mice. It remains to be determined whether similar toxicity occurs following coumarin administration by other routes.

cDNA cloning, heterologous expression, and characterization of mouse CYP2G1, an olfactory-specific steroid hydroxylase

CYP2G1 is expressed specifically in the olfactory mucosa in rabbits and rats. In the present study, a full-length cDNA for mouse CYP2G1 was obtained using a PCR approach with RNA preparations from the olfactory mucosa of C57BL/6 mice. Sequence comparisons indicated that mouse CYP2G1 is highly homologous in deduced amino acid sequence to rabbit (82.4% identity) and rat CYP2G1 (94.9% identity). RNA blot and immunoblot analyses indicated that mouse CYP2G1 is expressed only in the olfactory mucosa. The coding region of the mouse CYP2G1 cDNA was cloned into a baculoviral expression vector for heterologous production of the enzyme in cultured insect cells. Heterologously expressed mouse CYP2G1 was active in a reconstituted system toward testosterone and progesterone, producing all the major metabolites detected in olfactory microsomal reactions, including 15 alpha-, 15 beta-, and 2 beta-hydroxytestosterone from testosterone and two unidentified metabolites from progesterone. Kinetic analysis indicated that mouse CYP2G1 has relatively high affinities toward the steroid substrates, with K(m) values in the micromolar range for both testosterone and progesterone. At a substrate concentration of 10 microM, microsomes of olfactory mucosa had much higher turnover numbers toward testosterone and progesterone than hepatic microsomes, consistent with the olfactory-specific expression of a high-affinity sex steroid hydroxylase. These findings will facilitate further molecular genetics studies on the biological function of CYP2G1 in a mouse model.