Characterization of mouse flavin-containing monooxygenase transcript levels in lung and liver, and activity of expressed isoforms

Genetic structure and selective sweeps in Kirghiz sheep using SNP50K bead chip

The objective of this study is to analyze environmental genetic selection signals in large-scale sheep populations with conflicting environmental adaptations, aiming to identify and isolate genes associated with environmental adaptations in sheep populations. Kirghiz sheep, which inhabit high-altitude environments year-round, demonstrate the ability to adapt to extreme conditions. In this study, 42 Kirghiz sheep, 24 Tien-Shan in Kyrgyzstan sheep, 189 Qira black sheep, and 160 Chinese Merino sheep were genotyped using Illumina Ovine SNP50K chip. Regions exhibiting a selection signal threshold of 5%, as well as PI analysis and haplotype statistical scanning gene data were annotated, and intersecting genes were identified as candidate genes. Through Fst and haplotype statistical analysis revealed the key gene PDGFD and its vicinity's impact on fat deposition in sheep tails. Additionally, Fst and PI analysis uncovered genes related to high-altitude adaptation as well as those linked to animal growth and reproduction.Further GO and KEGG enrichment pathway analyses unveiled pathways associated with high-altitude adaptation such as negative regulation of peptidyl-tyrosine phosphorylation and xenobiotic metabolism processes.This investigation into the adaptability of Kirghiz sheep provides theoretical support and practical guidance for the conservation and genetic enhancement of Kirghiz sheep germplasm resources.

Flavin containing monooxygenase 2 regulates renal tubular cell fibrosis and paracrine secretion via SMURF2 in AKI‑CKD transformation

In the follow‑up of hospitalized patients with acute kidney injury (AKI), it has been observed that 15‑30% of these patients progress to develop chronic kidney disease (CKD). Impaired adaptive repair of the kidneys following AKI is a fundamental pathophysiological mechanism underlying renal fibrosis and the progression to CKD. Deficient repair of proximal tubular epithelial cells is a key factor in the progression from AKI to CKD. However, the molecular mechanisms involved in the regulation of fibrotic factor paracrine secretion by injured tubular cells remain incompletely understood. Transcriptome analysis and an ischemia‑reperfusion injury (IRI) model were used to identify the contribution of flavin‑containing monooxygenase 2 (FMO2) in AKI‑CKD. Lentivirus‑mediated overexpression of FMO2 was performed in mice. Functional experiments were conducted using TGF‑β‑induced tubular cell fibrogenesis and paracrine pro‑fibrotic factor secretion. Expression of FMO2 attenuated kidney injury induced by renal IRI, renal fibrosis, and immune cell infiltration into the kidneys. Overexpression of FMO2 not only effectively blocked TGF secretion in tubular cell fibrogenesis but also inhibited aberrant paracrine activation of pro‑fibrotic factors present in fibroblasts. FMO2 negatively regulated TGF‑β‑mediated SMAD2/3 activation by promoting the expression of SMAD ubiquitination regulatory factor 2 (SMURF2) and its nuclear translocation. During the transition from AKI to CKD, FMO2 modulated tubular cell fibrogenesis and paracrine secretion through SMURF2, thereby affecting the outcome of the disease.

Proteomic analysis of ITPR2 as a new therapeutic target for curcumin protection against AFB1-induced pyroptosis

Aflatoxin B1 (AFB1) is extremely carcinogenic and can cause liver cancer in humans and animals with continued ingestion. As a natural compound, curcumin (Cur) exhibits excellent anti-inflammatory, and anti-cancer properties with few side effects. In this study, a total of 60 male mice (6-week-olds, 15 per group). After one week of acclimatization feeding, the mice were divided into control group (Con), AFB1 group, curcumin group (Cur), and AF+Cur group. The mice were gavaged with curcumin (Cur, 100 mg/kg) and/or AFB1 (0.75 mg/kg). To identify a new therapeutic target for AFB1-induced pyroptosis, we performed proteomic profiling for curcumin alleviating liver injury caused by AFB1 to further validate the targets through volcano plot analysis, Venn analysis, heatmap analysis, correlation, cluster analysis, GO and KEGG enrichment. AFB1 exposure resulted in the loss of hepatocyte membrane, swelling of the endoplasmic reticulum, and a significant increase in transaminase (ALT and AST) contents, while curcumin greatly improved these changes. We found that differentially expressed proteins are enriched in the endoplasmic reticulum membrane and identified ITPR2 as a target of curcumin that alleviates AFB1-induced liver injury by proteomics. Furthermore, ITPR2 expression was detected by immunofluorescence, and qRT-PCR for mRNA expression of genes downstream of ITPR2 (calpain1, calpain2, caspase-12, caspase-3). ITPR2-activated endoplasmic reticulum stress-related proteins (calpain1, calpaini2, bcl-2, BAX, cl-caspase-12, cl-caspase-3), apoptosis (PARP) and pyroptosis (DFNA5) related proteins were examined by western blotting. The analysis showed that it effectively prevents AFB1-induced pyroptosis by lowering endoplasmic reticulum stress via interfering with ITPR2 and its downstream proteins (calpain1, calpain2, bcl-2, Bax) and inhibiting caspase-12/caspase-3 pathway. Conclusively, this study applied proteomic profiling to elucidate ITPR2 as a new target, which might give a new perspective on the mechanism of curcumin alleviating AFB1-induced pyroptosis.

Brown adipose tissue dysfunction promotes heart failure via a trimethylamine N-oxide-dependent mechanism

Low body temperature predicts a poor outcome in patients with heart failure, but the underlying pathological mechanisms and implications are largely unknown. Brown adipose tissue (BAT) was initially characterised as a thermogenic organ, and recent studies have suggested it plays a crucial role in maintaining systemic metabolic health. While these reports suggest a potential link between BAT and heart failure, the potential role of BAT dysfunction in heart failure has not been investigated. Here, we demonstrate that alteration of BAT function contributes to development of heart failure through disorientation in choline metabolism. Thoracic aortic constriction (TAC) or myocardial infarction (MI) reduced the thermogenic capacity of BAT in mice, leading to significant reduction of body temperature with cold exposure. BAT became hypoxic with TAC or MI, and hypoxic stress induced apoptosis of brown adipocytes. Enhancement of BAT function improved thermogenesis and cardiac function in TAC mice. Conversely, systolic function was impaired in a mouse model of genetic BAT dysfunction, in association with a low survival rate after TAC. Metabolomic analysis showed that reduced BAT thermogenesis was associated with elevation of plasma trimethylamine N-oxide (TMAO) levels. Administration of TMAO to mice led to significant reduction of phosphocreatine and ATP levels in cardiac tissue via suppression of mitochondrial complex IV activity. Genetic or pharmacological inhibition of flavin-containing monooxygenase reduced the plasma TMAO level in mice, and improved cardiac dysfunction in animals with left ventricular pressure overload. In patients with dilated cardiomyopathy, body temperature was low along with elevation of plasma choline and TMAO levels. These results suggest that maintenance of BAT homeostasis and reducing TMAO production could be potential next-generation therapies for heart failure.

Flavin Containing Monooxygenase 2 Prevents Cardiac Fibrosis via CYP2J3-SMURF2 Axis

BACKGROUND

Cardiac fibrosis is a common pathological feature associated with adverse clinical outcome in postinjury remodeling and has no effective therapy. Using an unbiased transcriptome analysis, we identified FMO2 (flavin-containing monooxygenase 2) as a top-ranked gene dynamically expressed following myocardial infarction (MI) in hearts across different species including rodents, nonhuman primates, and human. However, the functional role of FMO2 in cardiac remodeling is largely unknown.

METHODS

Single-nuclei transcriptome analysis was performed to identify FMO2 after MI; FMO2 ablation rats were generated both in genetic level using the CRISPR-cas9 (clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeat-associated 9) technology and lentivirus-mediated manner. Gain-of-function experiments were conducted using postn-promoter FMO2, miR1a/miR133a-FMO2 lentivirus, and enzymatic activity mutant FMO2 lentivirus after MI.

RESULTS

A significant downregulation of FMO2 was consistently observed in hearts after MI in rodents, nonhuman primates, and patients. Single-nuclei transcriptome analysis showed cardiac expression of FMO2 was enriched in fibroblasts rather than myocytes. Elevated spontaneous tissue fibrosis was observed in the FMO2-null animals without external stress. In contrast, fibroblast-specific expression of FMO2 markedly reduced cardiac fibrosis following MI in rodents and nonhuman primates associated with diminished SMAD2/3 phosphorylation. Unexpectedly, the FMO2-mediated regulation in fibrosis and SMAD2/3 signaling was independent of its enzymatic activity. Rather, FMO2 was detected to interact with CYP2J3 (cytochrome p450 superfamily 2J3). Binding of FMO2 to CYP2J3 disrupted CYP2J3 interaction with SMURF2 (SMAD-specific E3 ubiquitin ligase 2) in cytosol, leading to increased cytoplasm to nuclear translocation of SMURF2 and consequent inhibition of SMAD2/3 signaling.

CONCLUSIONS

Loss of FMO2 is a conserved molecular signature in postinjury hearts. FMO2 possesses a previously uncharacterized enzyme-independent antifibrosis activity via the CYP2J3-SMURF2 axis. Restoring FMO2 expression exerts potent ameliorative effect against fibrotic remodeling in postinjury hearts from rodents to nonhuman primates. Therefore, FMO2 is a potential therapeutic target for treating cardiac fibrosis following injury.

Non-cytochrome P450 enzymes involved in the oxidative metabolism of xenobiotics: Focus on the regulation of gene expression and enzyme activity

Oxidative metabolism is one of the major biotransformation reactions that regulates the exposure of xenobiotics and their metabolites in the circulatory system and local tissues and organs, and influences their efficacy and toxicity. Although cytochrome (CY)P450s play critical roles in the oxidative reaction, extensive CYP450-independent oxidative metabolism also occurs in some xenobiotics, such as aldehyde oxidase, xanthine oxidoreductase, flavin-containing monooxygenase, monoamine oxidase, alcohol dehydrogenase, or aldehyde dehydrogenase-dependent oxidative metabolism. Drugs form a large portion of xenobiotics and are the primary target of this review. The common reaction mechanisms and roles of non-CYP450 enzymes in metabolism, factors affecting the expression and activity of non-CYP450 enzymes in terms of inhibition, induction, regulation, and species differences in pharmaceutical research and development have been summarized. These non-CYP450 enzymes are detoxifying enzymes, although sometimes they mediate severe toxicity. Synthetic or natural chemicals serve as inhibitors for these non-CYP450 enzymes. However, pharmacokinetic-based drug interactions through these inhibitors have rarely been reported in vivo. Although multiple mechanisms participate in the basal expression and regulation of non-CYP450 enzymes, only a limited number of inducers upregulate their expression. Therefore, these enzymes are considered non-inducible or less inducible. Overall, this review focuses on the potential xenobiotic factors that contribute to variations in gene expression levels and the activities of non-CYP450 enzymes.

Flavin-Containing Monooxygenases Are Conserved Regulators of Stress Resistance and Metabolism

Flavin-Containing Monooxygenases are conserved xenobiotic-detoxifying enzymes. Recent studies have revealed endogenous functions of FMOs in regulating longevity in Caenorhabditis elegans and in regulating aspects of metabolism in mice. To explore the cellular mechanisms of FMO's endogenous function, here we demonstrate that all five functional mammalian FMOs may play similar endogenous roles to improve resistance to a wide range of toxic stresses in both kidney and liver cells. We further find that stress-activated c-Jun N-terminal kinase activity is enhanced in FMO-overexpressing cells, which may lead to increased survival under stress. Furthermore, FMO expression modulates cellular metabolic activity as measured by mitochondrial respiration, glycolysis, and metabolomics analyses. FMO expression augments mitochondrial respiration and significantly changes central carbon metabolism, including amino acid and energy metabolism pathways. Together, our findings demonstrate an important endogenous role for the FMO family in regulation of cellular stress resistance and major cellular metabolic activities including central carbon metabolism.

Use of dietary phytochemicals for inhibition of trimethylamine N-oxide formation

Trimethylamine-N-oxide (TMAO) has been reported as a risk factor for atherosclerosis development, as well as for other cardiovascular disease (CVD) pathologies. The objective of this review is to provide a useful summary on the use of phytochemicals as TMAO-reducing agents. This review discusses the main mechanisms by which TMAO promotes CVD, including the modulation of lipid and bile acid metabolism, and the promotion of endothelial dysfunction and oxidative stress. Current knowledge on the available strategies to reduce TMAO formation are discussed, highlighting the effect and potential of phytochemicals. Overall, phytochemicals (i.e., phenolic compounds or glucosinolates) reduce TMAO formation by modulating gut microbiota composition and/or function, inhibiting host's capacity to metabolize TMA to TMAO, or a combination of both. Perspectives for design of future studies involving phytochemicals as TMAO-reducing agents are discussed. Overall, the information provided by this review outlines the current state of the art of the role of phytochemicals as TMAO reducing agents, providing valuable insight to further advance in this field of study.

[Elucidation of New Function in Endothelial Cells for Efficient Delivery Strategy of Drug to Tissues]

The author has described two new functions of endothelial cells for efficient delivery of drugs to tissues. First, it was indicated that tight junction (TJ)-associated protein, claudin-1, exerts potent paracellular barrier function in cultured mouse lung microvascular endothelial cells (LMECs). This barrier was instantly and reversibly opened by reduction of TJ proteins expression via histamine H₁ and H₂ receptors. Histamine was biosynthesized by l-histidine decarboxylase from uptaken l-histidine, and biotransformed by type B of monoamine oxidase, suggesting that histamine concentration is controlled in rat brain MECs (BMECs) and LMECs. Moreover, uptake of l-histidine into BMECs and LMECs markedly increased with addition of ZnSO₄. Second, it was suggested that drug-metabolizing enzymes such as CYP and flavin-containing monooxygenase exist in vascular endothelial cells exposed to blood and to aerobic conditions. These cells have the same ability to metabolize drugs as hepatocytes, demonstrating that vascular endothelial cells are a metabolic barrier against tissue transfer of drugs. From these results, it was suggested that reversible opening of TJ and selective inhibition of drug metabolism in vascular endothelial cells may be efficient delivery strategies of drugs to tissues. Finally, I hope that this research will lead to development of new drugs and possible re-evaluation of discontinued drugs.

Enzymatically Produced Trimethylamine N-Oxide: Conserving It or Eliminating It

Trimethylamine N-Oxide (TMAO) is the product of the monooxygenation reaction catalyzed by a drug-metabolizing enzyme, human flavin-containing monooxygenase 3 (hFMO3), and its animal orthologues. For several years, researchers have looked at TMAO and hFMO3 as two distinct molecules playing specific but separate roles, the former to defend saltwater animals from osmotic or hydrostatic stress and the latter to process xenobiotics in men. The presence of high levels of plasmatic TMAO in elasmobranchs and other animals was demonstrated a long time ago, whereas the actual physiological role of hFMO3 is still unknown because the enzyme has been mainly characterized for its ability to oxidize drugs. Recently TMAO was found to be related to several human health conditions such as atherosclerosis, cardiovascular, and renal diseases. This correlation poses a striking question of how other vertebrates (and invertebrates) can survive in the presence of very high TMAO concentrations (micromolar in humans, millimolar in marine mammals and several hundred millimolar in elasmobranchs). Therefore, it is important to address how TMAO, its precursors, and FMO catalytic activity are interconnected.

Endogenous Roles of Mammalian Flavin-Containing Monooxygenases

Flavin-containing monooxygenases (FMOs) catalyze the oxygenation of numerous foreign chemicals. This review considers the roles of FMOs in the metabolism of endogenous substrates and in physiological processes, and focuses on FMOs of human and mouse. Tyramine, phenethylamine, trimethylamine, cysteamine, methionine, lipoic acid and lipoamide have been identified as endogenous or dietary-derived substrates of FMOs in vitro. However, with the exception of trimethylamine, the role of FMOs in the metabolism of these compounds in vivo is unclear. The use, as experimental models, of knockout-mouse lines deficient in various Fmo genes has revealed previously unsuspected roles for FMOs in endogenous metabolic processes. FMO1 has been identified as a novel regulator of energy balance that acts to promote metabolic efficiency, and also as being involved in the biosynthesis of taurine, by catalyzing the S-oxygenation of hypotaurine. FMO5 has been identified as a regulator of metabolic ageing and glucose homeostasis that apparently acts by sensing or responding to gut bacteria. Thus, FMOs do not function only as xenobiotic-metabolizing enzymes and there is a risk that exposure to drugs and environmental chemicals that are substrates or inducers of FMOs would perturb the endogenous functions of these enzymes.

Molecular characterization, expression pattern and metabolic activity of flavin-dependent monooxygenases in Spodoptera exigua

Enhanced detoxification is one of the important mechanisms for insecticide resistance. Most research in this field to date has focused on the role of cytochrome P450s. Our previous work revealed that flavin-dependent monooxygenases (FMOs) were involved in metabolic resistance of Spodoptera exigua. In the present study we investigated the molecular characteristics, expression patterns and oxidative activities of SeFMO on insecticides. Three FMO genes, which encode proteins with the typical FMO motifs, were cloned from S. exigua. The oxidative activities of eukaryotically expressed SeFMO enzymes were verified with the model substrate of FMO. Importantly, the SeFMOs had significantly higher oxidative activities on metaflumizone and lambda-cyhalothrin than on model substrates and other insecticides tested. The three SeFMOs were mainly expressed in the midgut, fat body and Malpighian tubules. The tissues responsible for xenobiotic metabolism and their expression characteristics were similar to those of P450s acting as detoxification genes. The study also revealed that the expression of SeFMOs could be induced by insecticide exposure, and that SeFMOs were over-expressed in a metaflumizone-resistant strain of S. exigua. These results suggest that SeFMOs are important insecticide detoxifying enzymes, and that over-expression of FMO genes may be one of the mechanisms for metabolic resistance in S. exigua.

Drug metabolism by flavin-containing monooxygenases of human and mouse

INTRODUCTION

Flavin-containing monooxygenases (FMOs) play an important role in drug metabolism. Areas covered: We focus on the role of FMOs in the metabolism of drugs in human and mouse. We describe FMO genes and proteins of human and mouse; the catalytic mechanism of FMOs and their significance for drug metabolism; differences between FMOs and CYPs; factors contributing to potential underestimation of the contribution of FMOs to drug metabolism; the developmental and tissue-specific expression of FMO genes and differences between human and mouse; and factors that induce or inhibit FMOs. We discuss the contribution of FMOs of human and mouse to the metabolism of drugs and how genetic variation of FMOs affects drug metabolism. Finally, we discuss the utility of animal models for FMO-mediated drug metabolism in humans. Expert opinion: The contribution of FMOs to drug metabolism may be underestimated. As FMOs are not readily induced or inhibited and their reactions are generally detoxifications, the design of drugs that are metabolized predominantly by FMOs offers clinical advantages. Fmo1^(-/-),Fmo2^(-/-),Fmo4^(-/-) mice provide a good animal model for FMO-mediated drug metabolism in humans. Identification of roles for FMO1 and FMO5 in endogenous metabolism has implications for drug therapy and initiates an exciting area of research.

Flavin-containing monooxygenase S-oxygenation of a series of thioureas and thiones

Mammalian flavin-containing monooxygenase (FMO) is active towards many drugs with a heteroatom having the properties of a soft nucleophile. Thiocarbamides and thiones are S-oxygenated to the sulfenic acid which can either react with glutathione and initiate a redox-cycle or be oxygenated a second time to the unstable sulfinic acid. In this study, we utilized LC-MS/MS to demonstrate that the oxygenation by hFMO of the thioureas under test terminated at the sulfenic acid. With thiones, hFMO catalyzed the second reaction and the sulfinic acid rapidly lost sulfite to form the corresponding imidazole. Thioureas are often pulmonary toxicants in mammals and, as previously reported by our laboratory, are excellent substrates for hFMO2. This isoform is expressed at high levels in the lung of most mammals, including non-human primates. Genotyping to date indicates that individuals of African (up to 49%) or Hispanic (2-7%) ancestry have at least one allele for functional hFMO2 in lung, but not Caucasians nor Asians. In this study the major metabolite formed by hFMO2 with thioureas from Allergan, Inc. was the sulfenic acid that reacted with glutathione. The majority of thiones were poor substrates for hFMO3, the major form in adult human liver. However, hFMO1, the major isoform expressed in infant and neonatal liver and adult kidney and intestine, readily S-oxygenated thiones under test, with Kms ranging from 7 to 160 μM and turnover numbers of 30-40 min(-1). The product formed was identified by LC-MS/MS as the imidazole. The activities of the mouse and human FMO1 and FMO3 orthologs were in good agreement with the exception of some thiones for which activity was much greater with hFMO1 than mFMO1.

Metabolism and pharmacokinetics of the anti-tuberculosis drug ethionamide in a flavin-containing monooxygenase null mouse

Multiple drug resistance (MDR) in Mycobacterium tuberculosis (mTB), the causative agent for tuberculosis (TB), has led to increased use of second-line drugs, including ethionamide (ETA). ETA is a prodrug bioactivated by mycobacterial and mammalian flavin-containing monooxygenases (FMOs). FMO2 is the major isoform in the lungs of most mammals, including primates. In humans a polymorphism exists in the expression of FMO2. FMO2.2 (truncated, inactive) protein is produced by the common allele, while the ancestral allele, encoding active FMO2.1, has been documented only in individuals of African and Hispanic origin, at an incidence of up to 50% and 7%, respectively. We hypothesized that FMO2 variability in TB-infected individuals would yield differences in concentrations and ratios of ETA prodrug and metabolites. In this study we assessed the impact of the FMO2 genetic polymorphism on the pharmacokinetics of ETA after administration of a single oral dose of ETA (125 mg/kg) to wild type and triple Fmo1/2/4-null mice, measuring levels of prodrug vs. metabolites in plasma collected from 0 to 3.5 h post-gavage. All mice metabolized ETA to ETA S-oxide (ETASO) and 2-ethyl-4-amidopyridine (ETAA). Wild type mice had higher plasma concentrations of metabolites than of parent compound (p = 0.001). In contrast, Fmo1/2/4-null mice had higher plasma concentrations of parent compound than of metabolites (p = 0.0001). Thus, the human FMO2 genotype could impact the therapeutic efficacy and/or toxicity of ETA.

Long-term administration of the histone deacetylase inhibitor vorinostat attenuates renal injury in experimental diabetes through an endothelial nitric oxide synthase-dependent mechanism

Epigenetic changes in gene expression play a role in the development of diabetic complications, including nephropathy. Histone deacetylases (HDACs) are a group of enzymes that exert epigenetic effects by altering the acetylation status of histone and nonhistone proteins. In the current study, we investigated the action of the clinically available HDAC inhibitor vorinostat in a mouse model of diabetic nephropathy, with the following aims: to define its effect on the progression of renal injury and to explore its mechanism of action by focusing on its role in regulating the expression of endothelial nitric oxide synthase (eNOS). Control and streptozotocin-diabetic wild-type and eNOS(-/-) mice were treated with vorinostat by daily oral dosing for 18 weeks. Without affecting either blood glucose concentration or blood pressure, vorinostat decreased albuminuria, mesangial collagen IV deposition, and oxidative-nitrosative stress in streptozotocin-wild-type mice. These attenuating effects were associated with a >50% reduction in eNOS expression in mouse kidneys and in cultured human umbilical vein endothelial cells. Vorinostat treatment had no effect on albuminuria, glomerular collagen IV concentration, or mesangiolysis in diabetic mice genetically deficient in eNOS. These observations illustrate the therapeutic efficacy of long-term HDAC inhibition in diabetic nephropathy and emphasize the importance of the interplay between eNOS activity and oxidative stress in mediating these effects.

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

The antithyroid drug methimazole (MMZ) can cause severe, tissue-specific toxicity in mouse olfactory mucosa (OM), presumably through a sequential metabolic activation of MMZ by cytochrome P450 (P450) and flavin monooxygenases (FMO). The aims of this study were to determine whether CYP2A5, one of the most abundant P450 enzymes in the mouse OM, is involved in MMZ metabolic activation, by comparing Cyp2a5-null with wild-type (WT) mice, and whether hepatic microsomal P450 enzymes, including CYP2A5, are essential for MMZ-induced OM toxicity, by comparing liver-Cpr-null (LCN) mice, which have little P450 activity in hepatocytes, with WT mice. We showed that the loss of CYP2A5 expression did not alter systemic clearance of MMZ (at 50 mg/kg, i.p.); but it did significantly decrease the rates of MMZ metabolism in the OM, whereas FMO expression in the OM was not reduced. MMZ induced depletion of nonprotein thiols, as well as pathological changes, in the OM of WT mice; the extent of these changes was much reduced in the Cyp2a5-null mice. Thus, CYP2A5 plays an important role in mediating MMZ toxicity in the OM. In contrast, the rate of systemic clearance of MMZ was significantly reduced in the LCN mice, compared to WT mice, whereas the MMZ-induced OM toxicity was not prevented. Therefore, hepatic P450 enzymes are essential for systemic MMZ clearance, but they are not required for MMZ-induced OM toxicity. We conclude that the tissue-specific toxicity of MMZ is mediated by target tissue metabolic activation, and the reaction is partly catalyzed by CYP2A5 in the OM.

Flavin-containing monooxygenase mRNA levels are up-regulated in als brain areas in SOD1-mutant mice

Flavin-containing monooxygenases (FMOs) are a family of microsomal enzymes involved in the oxygenation of a variety of nucleophilic heteroatom-containing xenobiotics. Recent results have pointed to a relation between Amyotrophic Lateral Sclerosis (ALS) and FMO genes. ALS is an adult-onset, progressive, and fatal neurodegenerative disease. We have compared FMO mRNA expression in the control mouse strain C57BL/6J and in a SOD1-mutated (G93A) ALS mouse model. Fmo expression was examined in total brain, and in subregions including cerebellum, cerebral hemisphere, brainstem, and spinal cord of control and SOD1-mutated mice. We have also considered expression in male and female mice because FMO regulation is gender-related. Real-Time TaqMan PCR was used for FMO expression analysis. Normalization was done using hypoxanthine-guanine phosphoribosyl transferase (Hprt) as a control housekeeping gene. Fmo genes, except Fmo3, were detectably expressed in the central nervous system of both control and ALS model mice. FMO expression was generally greater in the ALS mouse model than in control mice, with the highest increase in Fmo1 expression in spinal cord and brainstem. In addition, we showed greater Fmo expression in males than in female mice in the ALS model. The expression of Fmo1 mRNA correlated with Sod1 mRNA expression in pathologic brain areas. We hypothesize that alteration of FMO gene expression is a consequence of the pathological environment linked to oxidative stress related to mutated SOD1.

Characterization of sulfoxygenation and structural implications of human flavin-containing monooxygenase isoform 2 (FMO2.1) variants S195L and N413K

Catalytically active human flavin-containing monooxygenase isoform 2 (FMO2.1) is encoded by an allele detected only in individuals of African or Hispanic origin. Genotyping and haplotyping studies indicate that S195L and N413K occasionally occur secondary to the functional FMO2*1 allele encoding reference protein Gln472. Sulfoxygenation under a range of conditions reveals the role these alterations may play in individuals expressing active FMO2 and provides insight into FMO structure. Expressed S195L lost rather than gained activity as pH was increased or when cholate was present. The activity of S195L was mostly eliminated after heating at 45 degrees C for 5 min in the absence of NADPH, but activity was preserved if NADPH was present. By contrast, Gln472 was less sensitive to heat, a response not affected by NADPH. A major consequence of the S195L mutation was a mean 12-fold increase in K(m) for NADPH compared with Gln472. Modeling an S213L substitution, the equivalent site, in the structural model of FMO from the Methylophaga bacterium leads to disruption of interactions with NADP(+). N413K had the same pattern of activity as Gln472 in response to pH, cholate, and magnesium, but product formation was always elevated by comparison. N413K also lost more activity when heated than Gln472; however, NADPH attenuated this loss. The major effects of N413K were increases in velocity and k(cat) compared with Gln472. Although these allelic variants are expected to occur infrequently as mutations to the FMO2*1 allele, they contribute to our overall understanding of mammalian FMO structure and function.

Role of flavin-containing monooxygenase in drug development

This review summarizes some recent observations and information related to the role of the flavin-containing monooxygenase (FMO) in preclinical drug development. Flavin-containing monooxygenase is a complimentary enzyme system to the cytochrome P450 (CYP) family of enzymes and oxygenates several soft, highly polarizable nucleophilic heteroatom-containing chemicals and drugs. The products of FMO-mediated metabolism are generally benign and highly polar, readily excreted materials. There may be some advantages in designing drugs that are metabolized in part by FMO and not exclusively by CYP. In this review, I describe the practical aspects for the participation of FMO in drug and chemical metabolism including: i) the study of FMO using in vitro preparations; ii) some observations about metabolism of drugs and chemicals by FMO in vivo; and iii) the consequences of studying FMO-related metabolism in various small animal models. Some of the preclinical research and development areas related to FMO are not fully mature areas and there are certain gaps in our knowledge. However, I include discussion of these areas to stimulate further work and invite further discussion.

Metabolism of the anti-tuberculosis drug ethionamide by mouse and human FMO1, FMO2 and FMO3 and mouse and human lung microsomes

Tuberculosis (TB) results from infection with Mycobacterium tuberculosis and remains endemic throughout the world with one-third of the world's population infected. The prevalence of multi-drug resistant strains necessitates the use of more toxic second-line drugs such as ethionamide (ETA), a pro-drug requiring bioactivation to exert toxicity. M. tuberculosis possesses a flavin monooxygenase (EtaA) that oxygenates ETA first to the sulfoxide and then to 2-ethyl-4-amidopyridine, presumably through a second oxygenation involving sulfinic acid. ETA is also a substrate for mammalian flavin-containing monooxygenases (FMOs). We examined activity of expressed human and mouse FMOs toward ETA, as well as liver and lung microsomes. All FMOs converted ETA to the S-oxide (ETASO), the first step in bioactivation. Compared to M. tuberculosis, the second S-oxygenation to the sulfinic acid is slow. Mouse liver and lung microsomes, as well as human lung microsomes from an individual expressing active FMO, oxygenated ETA in the same manner as expressed FMOs, confirming this reaction functions in the major target organs for therapeutics (lung) and toxicity (liver). Inhibition by thiourea, and lack of inhibition by SKF-525A, confirm ETASO formation is primarily via FMO, particularly in lung. ETASO production was attenuated in a concentration-dependent manner by glutathione. FMO3 in human liver may contribute to the toxicity and/or affect efficacy of ETA administration. Additionally, there may be therapeutic implications of efficacy and toxicity in human lung based on the FMO2 genetic polymorphism, though further studies are needed to confirm that suggestion.

Aryl hydrocarbon receptor-dependent induction of flavin-containing monooxygenase mRNAs in mouse liver

Flavin-containing monooxygenases (FMOs) are important in detoxication but generally are considered not to be inducible by xenobiotics. Our recent microarray studies revealed induction of FMO2 and FMO3 mRNAs by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in liver of mice with wild-type aryl hydrocarbon receptor (AHR) but not in Ahr-null mice. The aim of the present study was to delineate mechanisms of FMO regulation. In adult male mice, basal FMO3 mRNA is low but was induced 6-fold at 4 h and 6000-fold at 24 h. The ED50 was approximately 1 microg/kg for FMO2 and FMO3, similar to that for the classic AHR-regulated gene, Cyp1a1. In adult female mice basal FMO3 mRNA is high and was not induced at 4 h but was elevated 8-fold at 24 h. FMO5 mRNA was significantly down-regulated by TCDD in both male and female adult mice. Juvenile mice show no sex difference in response to TCDD; FMO3 was induced 4 to 6-fold by TCDD in both sexes. Chromatin immunoprecipitation demonstrated recruitment of AHR and aryl hydrocarbon nuclear translocator proteins to Fmo3 regulatory regions, suggesting that induction by TCDD is a primary AHR-mediated event. Although FMO2 and FMO3 mRNAs were highly induced by TCDD in adult males, overall FMO catalytic activity increased only modestly. In contrast to the striking up-regulation of FMO2 and FMO3 in mouse liver, TCDD has little effect on FMO mRNA in rat liver. However, FMO2 and FMO3 mRNAs were highly induced in transgenic mice that express wild-type rat AHR, indicating that lack of induction in rat is not due to an incompetent AHR in this species.