The tissue-specific toxicity of methimazole in the mouse olfactory mucosa is partly mediated through target-tissue metabolic activation by CYP2A5

Neurons for infant social behaviors in the mouse zona incerta

Understanding the neural basis of infant social behaviors is crucial for elucidating the mechanisms of early social and emotional development. In this work, we report a specific population of somatostatin-expressing neurons in the zona incerta (ZISST) of preweaning mice that responds dynamically to social interactions, particularly those with their mother. Bidirectional neural activity manipulations in pups revealed that widespread connectivity of preweaning ZISST neurons to sensory, emotional, and cognitive brain centers mediates two key adaptive functions associated with maternal presence: the reduction of behavior distress and the facilitation of learning. These findings reveal a population of neurons in the infant mouse brain that coordinate the positive effects of the relationship with the mother on an infant's behavior and physiology.

Social recognition in laboratory mice requires integration of behaviorally-induced somatosensory, auditory and olfactory cues

In humans, discrimination between individuals, also termed social recognition, can rely on a single sensory modality, such as vision. By analogy, social recognition in rodents is thought to be based upon olfaction. Here, we hypothesized that social recognition in rodents relies upon integration of olfactory, auditory and somatosensory cues, hence requiring active behavior of social stimuli. Using distinct social recognition tests, we demonstrated that adult male mice do not exhibit recognition of familiar stimuli or learn the identity of novel stimuli that are inactive due to anesthesia. We further revealed that impairing the olfactory, somatosensory or auditory systems prevents behavioral recognition of familiar stimuli. Finally, we found that familiar and novel stimuli generate distinct movement patterns during social discrimination and that subjects react differentially to the movement of these stimuli. Thus, unlike what occurs in humans, social recognition in mice relies on integration of information from several sensory modalities.

Streptococcus agalactiae Infects Glial Cells and Invades the Central Nervous System via the Olfactory and Trigeminal Nerves

Streptococcus agalactiae causes neonatal meningitis and can also infect the adult central nervous system (CNS). S. agalactiae can cross the blood-brain barrier but may also reach the CNS via other paths. Several species of bacteria can directly invade the CNS via the olfactory and trigeminal nerves, which extend between the nasal cavity and brain and injury to the nasal epithelium can increase the risk/severity of infection. Preterm birth is associated with increased risk of S. agalactiae infection and with nasogastric tube feeding. The tubes, also used in adults, can cause nasal injuries and may be contaminated with bacteria, including S. agalactiae. We here investigated whether S. agalactiae could invade the CNS after intranasal inoculation in mice. S. agalactiae rapidly infected the olfactory nerve and brain. Methimazole-mediated model of nasal epithelial injury led to increased bacterial load in these tissues, as well as trigeminal nerve infection. S. agalactiae infected and survived intracellularly in cultured olfactory/trigeminal nerve- and brain-derived glia, resulting in cytokine production, with some differences between glial types. Furthermore, a non-capsulated S. agalactiae was used to understand the role of capsule on glial cells interaction. Interestingly, we found that the S. agalactiae capsule significantly altered cytokine and chemokine responses and affected intracellular survival in trigeminal glia. In summary, this study shows that S. agalactiae can infect the CNS via the nose-to-brain path with increased load after epithelial injury, and that the bacteria can survive in glia.

Chlamydia pneumoniae can infect the central nervous system via the olfactory and trigeminal nerves and contributes to Alzheimer's disease risk

Chlamydia pneumoniae is a respiratory tract pathogen but can also infect the central nervous system (CNS). Recently, the link between C. pneumoniae CNS infection and late-onset dementia has become increasingly evident. In mice, CNS infection has been shown to occur weeks to months after intranasal inoculation. By isolating live C. pneumoniae from tissues and using immunohistochemistry, we show that C. pneumoniae can infect the olfactory and trigeminal nerves, olfactory bulb and brain within 72 h in mice. C. pneumoniae infection also resulted in dysregulation of key pathways involved in Alzheimer’s disease pathogenesis at 7 and 28 days after inoculation. Interestingly, amyloid beta accumulations were also detected adjacent to the C. pneumoniae inclusions in the olfactory system. Furthermore, injury to the nasal epithelium resulted in increased peripheral nerve and olfactory bulb infection, but did not alter general CNS infection. In vitro, C. pneumoniae was able to infect peripheral nerve and CNS glia. In summary, the nerves extending between the nasal cavity and the brain constitute invasion paths by which C. pneumoniae can rapidly invade the CNS likely by surviving in glia and leading to Aβ deposition.

Role of CYP2A6 in Methimazole Bioactivation and Hepatotoxicity

Methimazole (MMI) is a widely used antithyroid drug, but it can cause hepatotoxicity by unknown mechanisms. Previous studies showed that the hepatic metabolism of MMI produces N-methylthiourea, leading to liver damage. However, the specific enzyme responsible for the production of the toxic metabolite N-methylthiourea is still unclear. In this study, we screened cytochromes P450 (CYPs) in N-methylthiourea production from MMI. CYP2A6 was identified as the key enzyme in catalyzing MMI metabolism to produce N-methylthiourea. When mice were pretreated with a CYP2A6 inhibitor, formation of N-methylthiourea from MMI was remarkably reduced. Consistently, the CYP2A6 inhibitor prevented MMI-induced hepatotoxicity. These results demonstrated that CYP2A6 is essential in MMI bioactivation and hepatotoxicity.

Single or Repeated Ablation of Mouse Olfactory Epithelium by Methimazole

Odor-detecting olfactory sensory neurons residing in the nasal olfactory epithelium (OE) are the only neurons in direct contact with the external environment. As a result, these neurons are subjected to chemical, physical, and infectious insults, which may be the underlying reason why neurogenesis occurs in the OE of adult mammals. This feature makes the OE a useful model for studying neurogenesis and neuronal differentiation, with the possibility for systemic as well as local administration of various compounds and infectious agents that may interfere with these cellular processes. Several different chemical compounds have been shown to cause toxic injury to the OE, which can be used for OE ablation. We, and others, have found that the systemic administration of the hyperthyroid drug, methimazole, reliably causes olfactotoxicity as a side effect. Here, we outline an OE lesioning protocol for single or repeated ablation by methimazole. A single methimazole administration can be used to study neuroepithelial regeneration and stem cell activation, while repeated ablation and regeneration of OE enable the study of tissue stem cell exhaustion and generation of tissue metaplasia.

MATERNAL GRAVES DISEASE AND ABNORMAL CYP2D6 GENOTYPE WITH FETAL HYPERTHYROIDISM

Objective

Fetal hyperthyroidism is a rare yet potentially fatal complication of past or present maternal Graves disease (GD). Our objective was to present a unique case of fetal hyperthyroidism in a mother with a prior history of GD and a cytochrome P450 2D6 (CYP2D6) polymorphism.

Methods

The clinical course in addition to serial laboratory and imaging results are presented. These include thyroid-stimulating hormone, free thyroxine, and thyrotropin receptor antibody levels, as well as fetal ultrasound, doppler fetal heart rate, and cordocentesis testing.

Results

A 27-year-old with a history of GD previously treated with radioiodine and a known cytochrome P450 polymorphism was referred to an endocrinology clinic at 17 weeks gestation for evaluation and management of fetal thyrotoxicosis. Despite close follow-up with a multidisciplinary care team and an aggressive "block and replace" treatment approach, progressive disease resulted in intrauterine fetal demise at 28 weeks gestation.

Conclusion

To our knowledge, this is the first published case report of fetal hyperthyroidism accompanied by a maternal CYP2D6 polymorphism. We hypothesize that the maternal CYP2D6 poor metabolizer phenotype prevents formation of antithyroid drug (ATD) metabolites and thus decreases the efficacy of ATD treatment. We suggest this as an area of future research.

Burkholderia pseudomallei invades the olfactory nerve and bulb after epithelial injury in mice and causes the formation of multinucleated giant glial cells in vitro

The infectious disease melioidosis is caused by the bacterium Burkholderia pseudomallei. Melioidosis is characterised by high mortality and morbidity and can involve the central nervous system (CNS). We have previously discovered that B. pseudomallei can infect the CNS via the olfactory and trigeminal nerves in mice. We have shown that the nerve path is dependent on mouse strain, with outbred mice showing resistance to olfactory nerve infection. Damage to the nasal epithelium by environmental factors is common, and we hypothesised that injury to the olfactory epithelium may increase the vulnerability of the olfactory nerve to microbial insult. We therefore investigated this, using outbred mice that were intranasally inoculated with B. pseudomallei, with or without methimazole-induced injury to the olfactory neuroepithelium. Methimazole-mediated injury resulted in increased B. pseudomallei invasion of the olfactory epithelium, and only in pre-injured animals were bacteria found in the olfactory nerve and bulb. In vitro assays demonstrated that B. pseudomallei readily infected glial cells isolated from the olfactory and trigeminal nerves (olfactory ensheathing cells and trigeminal Schwann cells, respectively). Bacteria were degraded by some cells but persisted in other cells, which led to the formation of multinucleated giant cells (MNGCs), with olfactory ensheathing cells less likely to form MNGCs than Schwann cells. Double Cap mutant bacteria, lacking the protein BimA, did not form MNGCs. These data suggest that injuries to the olfactory epithelium expose the primary olfactory nervous system to bacterial invasion, which can then result in CNS infection with potential pathogenic consequences for the glial cells.

Association between genetic polymorphisms of SLCO1B1 and susceptibility to methimazole-induced liver injury

Methimazole (MMI) has been used in the therapy of Grave's disease (GD) since 1954, and drug-induced liver injury (DILI) is one of the most deleterious side effects. Genetic polymorphisms of drug-metabolizing enzymes and drug transporters have been associated with drug-induced hepatotoxicity in many cases. The aim of this study was to investigate genetic susceptibility of the drug-metabolizing enzymes and drug transporters to the MMI-DILI. A total of 44 GD patients with MMI-DILI and 118 GD patients without MMI-DILI development were included in the study. Thirty-three single nucleotide polymorphisms (SNPs) in twenty candidate genes were genotyped. We found that rs12422149 of SLCO2B1, rs2032582_AT of ABCB1, rs2306283 of SLCO1B1 and rs4148323 of UGT1A1 exhibited a significant association with MMI-DILI; however, no significant difference existed after Bonferroni correction. Haplotype analysis showed that the frequency of SLCO1B1*1a (388A521T) was significantly higher in MMI-DILI cases than that in the control group (OR = 2.21, 95% CI = 1.11-4.39, P = 0.023), while the frequency of SLCO1B1*1b (388G521T) was significantly higher in the control group (OR = 0.52, 95% CI = 0.29-0.93, P = 0.028). These results suggested that genetic polymorphisms of SLCO1B1 were associated with susceptibility to MMI-DILI. The genetic polymorphism of SLCO1B1 may be important predisposing factors for MMI-induced hepatotoxicity.

Caloric restriction reduces basal cell proliferation and results in the deterioration of neuroepithelial regeneration following olfactotoxic mucosal damage in mouse olfactory mucosa

The effects of caloric restriction (CR) on cell dynamics and gene expression in the mouse olfactory neuroepithelium are evaluated. Eight-week-old male C57BL/6 mice were fed either control pellets (104 kcal/week) or CR pellets (67 kcal/week). The cytoarchitecture of the olfactory neuroepithelium in the uninjured condition and its regeneration after injury by an olfactotoxic chemical, methimazole, were compared between mice fed with the control and CR diets. In the uninjured condition, there were significantly fewer olfactory marker protein (OMP)-positive olfactory receptor neurons and Ki67-positive proliferating basal cells at 3 months in the CR group than in the control group. The number of Ki67-positive basal cells increased after methimazole-induced mucosal injury in both the control and the CR groups, but the increase was less robust in the CR group. The recovery of the neuroepithelium at 2 months after methimazole administration was less complete in the CR group than in the control group. These histological changes were region-specific. The decrease in the OMP-positive neurons was prominent in the anterior region of the olfactory mucosa. Gene expression analysis using a DNA microarray and quantitative real-time polymerase chain reaction demonstrated that the expression levels of two inflammatory cytokines, interleukin-6 and chemokine ligand 1, were elevated in the olfactory mucosa of the CR group compared with the control group. These findings suggest that CR may be disadvantageous to the maintenance of the olfactory neuroepithelium, especially when it is injured.

Pharmacologic modulation of nasal epithelium augments neural stem cell targeting of glioblastoma

Glioblastoma (GBM) remains the most lethal and untreatable central nervous system malignancy. The challenges to devise novel and effective anti-tumor therapies include difficulty in locating the precise tumor border for complete surgical resection, and rapid regrowth of residual tumor tissue after standard treatment. Repeatable and non-invasive intranasal application of neural stem cells (NSCs) was recently shown to enable clinically relevant delivery of therapy to tumors. Treatment with chemotactic NSCs demonstrated significant survival benefits when coupled with radiation and oncolytic virotherapy in preclinical models of GBM. In order to further augment the clinical applicability of this novel therapeutic platform, we postulate that the FDA-approved compound, methimazole (MT), can be safely utilized to delay the nasal clearance and improve the ability of NSCs to penetrate the olfactory epithelium for robust in vivo brain tumor targeting and therapeutic actions. METHODS: To examine the role of reversible reduction of the olfactory epithelial barrier in non-invasive intranasal delivery, we explored the unique pharmacologic effect of MT at a single dosage regimen. In our proof-of-concept studies, quantitative magnetic resonance imaging (MRI), immunocytochemistry, and survival analysis were performed on glioma-bearing mice treated with a single dose of MT prior to intranasal anti-GBM therapy using an oncolytic virus (OV)-loaded NSCs. RESULTS: Based on histology and in vivo imaging, we found that disrupting the olfactory epithelium with MT effectively delays clearance and allows NSCs to persist in the nasal cavity for at least 24 h. MT pretreatment amplified the migration of NSCs to the tumor. The therapeutic advantage of this enhancement was quantitatively validated by tissue analysis and MRI tracking of NSCs loaded with superparamagnetic iron oxide nanoparticles (SPIOs) in live animals. Moreover, we observed significant survival benefits in GBM-bearing mice treated with intranasal delivery of oncolytic virus-loaded NSCs following MT injection. Conclusion: Our work identified a novel pharmacologic strategy to accelerate the clinical application of the non-invasive NSCs-based therapeutic platform to tackle aggressive brain tumors.

Methimazole-induced liver injury overshadowed by methylprednisolone pulse therapy: Case report

RATIONALE

Treatment choices are limited, when deciding how to manage thyrotoxicosis and moderate to severe Graves ophthalmopathy (GO) with suspected optic nerve damage in patients with elevated liver transaminase levels. The situation become even more complicated, if methimazole induced hepatotoxicity is suspected and intravenous methylprednisolone is co-administrated.

PATIENT CONCERNS

A 74-year-old woman presented with spontaneous retro-bulbar pain, eyelid swelling and inconstant diplopia.

DIAGNOSES

Thyrotoxicosis and severe GO with suspected optic nerve damage and drug induced liver injury (DILI).

INTERVENTIONS

Intravenous methylprednisolone pulse therapy was administered to treat GO and methimazole was continued for thyrotoxicosis. Dose of methimazole was reduced after exclusion of concurrent infection and active liver disease.

OUTCOMES

The GO symptoms (eyelid swelling, sight loss, proptosis, retro-bulbar pain, diplopia) markedly decreased after the treatment course. Liver transaminases spontaneously returned to normal ranges and remained normal during the next 12 months until the Graves' disease until the treatment was completed.

LESSONS

1. The interaction of methimazole and methylprednisolone may result in DILI. 2. In a patient without concomitant liver diseases MP can be continued if the methimazole dose is reduced if no other treatment options are available.

Effects of methimazole on Drosophila glucolipid metabolism in vitro and in vivo

Methimazole (MMI) is an antithyroid agent widely used in the treatment of hyperthyroidism, and metabolized by cytochrome P450 enzymes and flavin-containing monooxygenases in mammals. However, drug overdose and the inadequate detoxification of the metabolite(s) are responsible for hepatocellular damage and organ dysfunction. Depending on the desired properties, Drosophila melanogaster has recently emerged as an ideal model organism for the study of human diseases. Here we investigated the changes in metabolic profiles and mRNA expressions related to glucolipid metabolism in response to treatment with MMI in Drosophila. Remarkable loss of lifespan occurred in fruit flies fed on the diets containing 10 or 30mM MMI compared to unsupplemented controls. To examine whether MMI affects glucolipid metabolism in vitro and in vivo, fruit flies were fed diets containing 30mM MMI for two weeks and Drosophila S2 cells were incubated with 300μM MMI for 48h. Measurements of metabolites showed that triglyceride content dramatically decreased (30.56% in vivo and 18.13% in vitro), and glycogen content significantly increased (10.7% in vivo and 126.8% in vitro). Quantitative analyses indicated that mRNA expression levels of Dmfmo1, s6k, dilp2, acc and dilp5 genes involved in metabolic homeostasis were remarkably down-regulated in vivo and in vitro. Meanwhile, the addition of MMI could significantly reduce the lipid droplet content in S2 cells by approximately 25% compared to control subjects. These data may provide a biological basis for the study of MMI on disease symptoms and complications, and discovery of therapeutic treatments.

L-Carnitine intake and high trimethylamine N-oxide plasma levels correlate with low aortic lesions in ApoE(-/-) transgenic mice expressing CETP

OBJECTIVE

Dietary l-carnitine can be metabolized by intestinal microbiota to trimethylamine, which is absorbed by the gut and further oxidized to trimethylamine N-oxide (TMAO) in the liver. TMAO plasma levels have been associated with atherosclerosis development in ApoE(-/-) mice. To better understand the mechanisms behind this association, we conducted in vitro and in vivo studies looking at the effect of TMAO on different steps of atherosclerotic disease progression.

METHODS

J774 mouse macrophage cells were used to evaluate the effect of TMAO on foam cell formation. Male ApoE(-/-) mice transfected with human cholesteryl ester transfer protein (hCETP) were fed l-carnitine and/or methimazole, a flavin monooxygenase 3 (FMO3) inhibitor that prevents the formation of TMAO. Following 12 week treatment, l-carnitine and TMAO plasma levels, aortic lesion development, and lipid profiles were determined.

RESULTS

TMAO at concentrations up to 10-fold the Cmax reported in humans did not affect in vitro foam cell formation. In ApoE(-/-)mice expressing hCETP, high doses of l-carnitine resulted in a significant increase in plasma TMAO levels. Surprisingly, and independently from treatment group, TMAO levels inversely correlated with aortic lesion size in both aortic root and thoracic aorta. High TMAO levels were found to significantly correlate with smaller aortic lesion area. Plasma lipid and lipoprotein levels did not change with treatment nor with TMAO levels, suggesting that the observed effects on lesion area were independent from lipid changes.

CONCLUSION

These findings suggest that TMAO slows aortic lesion formation in this mouse model and may have a protective effect against atherosclerosis development in humans.

An overview on the proposed mechanisms of antithyroid drugs-induced liver injury

Drug-induced liver injury (DILI) is a major problem for pharmaceutical industry and drug development. Mechanisms of DILI are many and varied. Elucidating the mechanisms of DILI will allow clinicians to prevent liver failure, need for liver transplantation, and death induced by drugs. Methimazole and propylthiouracil (PTU) are two convenient antithyroid agents which their administration is accompanied by hepatotoxicity as a deleterious side effect. Although several cases of antithyroid drugs-induced liver injury are reported, there is no clear idea about the mechanism(s) of hepatotoxicity induced by these medications. Different mechanisms such as reactive metabolites formation, oxidative stress induction, intracellular targets dysfunction, and immune-mediated toxicity are postulated to be involved in antithyroid agents-induced hepatic damage. Due to the idiosyncratic nature of antithyroid drugs-induced hepatotoxicity, it is impossible to draw a specific conclusion about the mechanisms of liver injury. However, it seems that reactive metabolite formation and immune-mediated toxicity have a great role in antithyroids liver toxicity, especially those caused by methimazole. This review attempted to discuss different mechanisms proposed to be involved in the hepatic injury induced by antithyroid drugs.

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

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

Essential role of the cytochrome P450 enzyme CYP2A5 in olfactory mucosal toxicity of naphthalene

Naphthalene (NA), a ubiquitous environmental pollutant that can cause pulmonary and nasal toxicity in laboratory animals, requires cytochrome P450 (P450)-mediated metabolic activation to cause toxicity. Our recent study using a Cyp2f2-null mouse showed that CYP2F2 plays an essential role in NA-induced lung toxicity, but not in NA-induced nasal toxicity. The aim of this study was to determine whether mouse CYP2A5, abundantly expressed in nasal olfactory mucosa (OM) and the liver, but less in the lung, plays a major role in the bioactivation and toxicity of NA in the OM. We found, by comparing Cyp2a5-null and wild-type (WT) mice, that the loss of CYP2A5 expression led to substantial decreases in rates of NA metabolic activation by OM microsomes. The loss of CYP2A5 did not cause changes in systemic clearance of NA (at 200 mg/kg, i.p.). However, the Cyp2a5-null mice were much more resistant than were WT mice to NA-induced nasal toxicity (although not lung toxicity), when examined at 24 hours after NA dosing (at 200 mg/kg, i.p.), or to NA-induced depletion of total nonprotein sulfhydryl in the OM (although not in the lung), examined at 2 hours after dosing. Thus, mouse CYP2A5 plays an essential role in the bioactivation and toxicity of NA in the OM, but not in the lung. Our findings further illustrate the tissue-specific nature of the role of individual P450 enzymes in xenobiotic toxicity, and provide the basis for a more reliable assessment of the potential risks of NA nasal toxicity in humans.

Effects of Enzyme Induction and/or Glutathione Depletion on Methimazole-Induced Hepatotoxicity in Mice and the Protective Role of N-Acetylcysteine

PURPOSE

Methimazole is the most convenient drug used in the management of hyperthyroid patients. However, associated with its clinical use is hepatotoxicity as a life threatening adverse effect. The exact mechanism of methimazole-induced hepatotoxicity is still far from clear and no protective agent has been developed for this toxicity.

METHODS

This study attempts to evaluate the hepatotoxicity induced by methimazole at different experimental conditions in a mice model. Methimazole-induced hepatotoxicity was investigated in different situations such as enzyme induced and/or glutathione depleted animals.

RESULTS

Methimazole (100 mg/kg, i.p) administration caused hepatotoxicity as revealed by increase in serum alanine aminotransferase (ALT) activity as well as pathological changes of the liver. Furthermore, a significant reduction in hepatic glutathione content and an elevation in lipid peroxidation were observed in methimazole-treated mice. Combined administration of L-buthionine sulfoximine (BSO), as a glutathione depletory agent, caused a dramatic change in methimazole-induced hepatotoxicity characterized by hepatic necrosis and a severe elevation of serum ALT activity. Enzyme induction using phenobarbital and/or β-naphtoflavone beforehand, deteriorated methimazole-induced hepatotoxicity in mice. N-acetyl cysteine (300 mg/kg, i.p) administration effectively alleviated hepatotoxic effects of methimazole in both glutathione-depleted and/or enzyme induced animals.

CONCLUSION

The severe hepatotoxic effects of methimazole in glutathione-depleted animals, reveals the crucial role of glutathione as a cellular defense mechanism against methimazole-induced hepatotoxicity. Furthermore, the more hepatotoxic properties of methimazole in enzyme-induced mice, indicates the role of reactive intermediates in the hepatotoxicity induced by this drug. The protective effects of N-acetylcysteine could be attributed to its radical/reactive metabolite scavenging, and/or antioxidant properties as well as glutathione replenishment activities.

Mechanisms of permanent loss of olfactory receptor neurons induced by the herbicide 2,6-dichlorobenzonitrile: effects on stem cells and noninvolvement of acute induction of the inflammatory cytokine IL-6

We explored the mechanisms underlying the differential effects of two olfactory toxicants, the herbicide 2,6-dichlorobenzonitrile (DCBN) and the anti-thyroid drug methimazole (MMZ), on olfactory receptor neuron (ORN) regeneration in mouse olfactory epithelium (OE). DCBN, but not MMZ, induced inflammation-like pathological changes in OE, and DCBN increased interleukin IL-6 levels in nasal-wash fluid to much greater magnitude and duration than did MMZ. At 24h after DCBN injection, the population of horizontal basal cells (HBCs; reserve, normally quiescent OE stem cells) lining the DMM became severely depleted as some of them detached from the basal lamina, and sloughed into the nasal cavity along with the globose basal cells (GBCs; heterogeneous population of stem and progenitor cells), neurons, and sustentacular cells of the neuroepithelium. In contrast, the layer of HBCs remained intact in MMZ-treated mice, as only the mature elements of the neuroepithelium were shed. Despite the respiratory metaplasia accompanying the greater severity of the DCBN lesion, residual HBCs that survived intoxication were activated by the injury and contributed to the metaplastic respiratory epithelium, as shown by tracing their descendants in a K5CreEr(T2)::fl(stop)TdTomato strain of mice in which recombination causes HBCs to express TdTomato in advance of the lesion. But, contrary to published observations with MMZ, the HBCs failed to form ORNs. A role for IL-6 in suppressing ORN regeneration in DCBN-treated mice was rejected by the failure of the anti-inflammatory drug dexamethasone to prevent the subsequent respiratory metaplasia in the DMM, suggesting that other factors lead to HBC neuro-incompetence.

Generation and characterization of a novel Cyp2a(4/5)bgs-null mouse model

Knockout mouse models targeting various cytochrome P450 (P450 or CYP) genes are valuable for determining P450's biologic functions, including roles in drug metabolism and chemical toxicity. In this study, a novel Cyp2a(4/5)bgs-null mouse model was generated, in which a 1.2-megabase pair genomic fragment containing nine Cyp genes in mouse chromosome 7 (including, sequentially, Cyp2a5, 2g1, 2b19, 2b23, 2a4, 2b9, 2b13, 2b10, and 2s1) are deleted, through Cre-mediated recombination in vivo. The resultant mouse strain was viable and fertile, without any developmental deficits or morphologic abnormalities. Deletion of the constitutive genes in the cluster was confirmed by polymerase chain reaction analysis of the genes and the mRNAs in tissues known to express each gene. The loss of this gene cluster led to significant decreases in microsomal activities toward testosterone hydroxylation in various tissues examined, including olfactory mucosa (OM), lung, liver, and brain. In addition, systemic clearance of pentobarbital was decreased in Cyp2a(4/5)bgs-null mice, as indicated by >60% increases in pentobarbital-induced sleeping time, compared with wild-type (WT) mice. This novel Cyp2a(4/5)bgs-null mouse model will be valuable for in vivo studies of drug metabolism and chemical toxicities in various tissues, including the liver, lung, brain, intestine, kidney, skin, and OM, where one or more of the targeted Cyp genes are known to be expressed in WT mice. The model will also be valuable for preparation of humanized mice that express human CYP2A6, CYP2A13, CYP2B6, or CYP2S1, and as a knockout mouse model for five non-P450 genes (Vmn1r184, Nalp9c, Nalp4a, Nalp9a, and Vmn1r185) that were also deleted.

Cytochrome P450s and cytochrome P450 reductase in the olfactory organ of the cotton leafworm Spodoptera littoralis

Cytochrome P450 enzymes (P450s) are involved in many physiological functions in insects, such as the metabolism of signal molecules, adaptation to host plants and insecticide resistance. Several P450s have been reported in the olfactory organs of insects, the antennae, and have been proposed to play a role in odorant processing and/or xenobiotic metabolism. Despite recent transcriptomic analyses in several species, the diversity of antennal P450s in insects has not yet been investigated. Here, we report the identification of 37 putative P450s expressed in the antennae of the pest moth Spodoptera littoralis, as well as the characterization of a redox partner, cytochrome P450 reductase (CPR). Phylogenetic analysis revealed that S. littoralis P450s belong to four clades defined by their conservation with vertebrate P450s and their cellular localization. Interestingly, the CYP3 and CYP4 clans, which have been described to be mainly involved in the metabolism of plant compounds and xenobiotics, were largely predominant. More surprisingly, two P450s related to ecdysteroid metabolism were also identified. Expression patterns in adult and larval tissues were studied. Eight P450s appeared to be specific to the chemosensory organs, ie the antennae and proboscis, suggesting a specific role in odorant and tastant processing. Moreover, exposure of males to a plant odorant down-regulated the transcript level of CPR, revealing for the first time the regulation of this gene by odorants within insect antennae. This work suggests that the antennae of insects are a key site for P450-mediated metabolism of a large range of exogenous and endogenous molecules.

Role of CYP2A5 in the bioactivation of the lung carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone in mice

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a potent lung carcinogen. Previously, we have demonstrated that NNK-induced lung tumorigenesis in mice depends on target-tissue bioactivation by pulmonary cytochrome P450 (P450) enzymes. The present study was designed to test the hypothesis that mouse CYP2A5 plays an essential role in NNK bioactivation in mouse lung. The role of CYP2A5 in NNK bioactivation was studied both in vitro and in vivo, by comparing the kinetic parameters of microsomal NNK metabolism and tissue levels of O(6)-methylguanine (O(6)-mG) (the DNA adduct highly correlated with lung tumorigenesis) between wild-type (WT) and Cyp2a5-null mice. In both liver and lung microsomes, the loss of CYP2A5 resulted in significant increases in the apparent K(m) values for the formation of 4-oxo-4-(3-pyridyl)butanone, which represents the reactive intermediate that produces O(6)-mG in vivo. The loss of CYP2A5 did not change circulating levels of NNK or 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol in mice treated intraperitoneally with NNK at either 20 or 100 mg/kg. However, the levels of lung O(6)-mG were significantly lower in Cyp2a5-null than in WT mice; the extent of the reduction was greater at the 20 mg/kg dose (∼40%) than at the 100 mg/kg dose (∼20%). These results indicate that CYP2A5 is the low-K(m) enzyme for NNK bioactivation in mouse lung. It is noteworthy that the remaining NNK bioactivation activities in the Cyp2a5-null mice could be inhibited by 8-methoxypsoralen, a P450 inhibitor used previously to demonstrate the role of CYP2A5 in NNK-induced lung tumorigenesis. Thus, P450 enzymes other than CYP2A5 probably also contribute to NNK-induced lung tumorigenesis in mice.