The potential of knockout mouse lines in defining the role of flavin-containing monooxygenases in drug metabolism

Genetic toxicology and toxicokinetics of arecoline and related areca nut compounds: an updated review

Areca nut (AN) is consumed by more than 600 million of individuals, particularly in some regions of South Asia, East Africa, and tropical Pacific, being classified as carcinogenic to humans. The most popular way of exposure consists of chewing a mixture of AN with betel leaf, slaked lime, and other ingredients that may also contain tobacco named betel quid (BQ). Arecoline is the principal active compound of AN, and, therefore, has been systematically studied over the years in several in vitro and in vivo genotoxicity endpoints. However, much of this information is dispersed, justifying the interest of an updated and comprehensive review article on this topic. In this sense, it is thus pertinent to describe and integrate the genetic toxicology data available as well as to address key toxicokinetics aspects of arecoline. This review also provides information on the effects induced by arecoline metabolites and related compounds, including other major AN alkaloids and nitrosation derivatives. The complexity of the chemicals involved renders this issue a challenge in genetic toxicology. Overall, positive results in several endpoints have been reported, some of them suggesting a key role for arecoline metabolites. Nevertheless, some negative genotoxicity findings for this alkaloid in short-term assays have also been reported in the literature. Finally, this article also collates information on the potential mechanisms of arecoline-induced genotoxicity, and suggests further approaches to tackle this important toxicological issue.

Flavin-Containing Monooxygenase 1 Catalyzes the Production of Taurine from Hypotaurine

Taurine is one of the most abundant amino acids in mammalian tissues. It is obtained from the diet and by de novo synthesis from cysteic acid or hypotaurine. Despite the discovery in 1954 that the oxygenation of hypotaurine produces taurine, the identification of an enzyme catalyzing this reaction has remained elusive. In large part, this is due to the incorrect assignment, in 1962, of the enzyme as an NAD-dependent hypotaurine dehydrogenase. For more than 55 years, the literature has continued to refer to this enzyme as such. Here we show, both in vivo and in vitro, that the enzyme that oxygenates hypotaurine to produce taurine is flavin-containing monooxygenase (FMO) 1. Metabolite analysis of the urine of Fmo1-null mice by ¹H NMR spectroscopy revealed a buildup of hypotaurine and a deficit of taurine in comparison with the concentrations of these compounds in the urine of wild-type mice. In vitro assays confirmed that human FMO1 catalyzes the conversion of hypotaurine to taurine, utilizing either NADPH or NADH as cofactor. FMO1 has a wide substrate range and is best known as a xenobiotic- or drug-metabolizing enzyme. The identification that the endogenous molecule hypotaurine is a substrate for the FMO1-catalyzed production of taurine resolves a long-standing mystery. This finding should help establish the role FMO1 plays in a range of biologic processes in which taurine or its deficiency is implicated, including conjugation of bile acids, neurotransmitter, antioxidant and anti-inflammatory functions, and the pathogenesis of obesity and skeletal muscle disorders. SIGNIFICANCE STATEMENT: The identity of the enzyme that catalyzes the biosynthesis of taurine from hypotaurine has remained elusive. Here we show, both in vivo and in vitro, that flavin-containing monooxygenase 1 catalyzes the oxygenation of hypotaurine to produce taurine.

Ancestral-sequence reconstruction unveils the structural basis of function in mammalian FMOs

Flavin-containing monooxygenases (FMOs) are ubiquitous in all domains of life and metabolize a myriad of xenobiotics, including toxins, pesticides and drugs. However, despite their pharmacological importance, structural information remains bereft. To further our understanding behind their biochemistry and diversity, we used ancestral-sequence reconstruction, kinetic and crystallographic techniques to scrutinize three ancient mammalian FMOs: AncFMO2, AncFMO3-6 and AncFMO5. Remarkably, all AncFMOs could be crystallized and were structurally resolved between 2.7- and 3.2-Å resolution. These crystal structures depict the unprecedented topology of mammalian FMOs. Each employs extensive membrane-binding features and intricate substrate-profiling tunnel networks through a conspicuous membrane-adhering insertion. Furthermore, a glutamate-histidine switch is speculated to induce the distinctive Baeyer-Villiger oxidation activity of FMO5. The AncFMOs exhibited catalysis akin to human FMOs and, with sequence identities between 82% and 92%, represent excellent models. Our study demonstrates the power of ancestral-sequence reconstruction as a strategy for the crystallization of proteins.

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.

Human plasma concentrations of trimethylamine N-oxide extrapolated using pharmacokinetic modeling based on metabolic profiles of deuterium-labeled trimethylamine in humanized-liver mice

Medicinal carnitine-derived and dietary-derived malodorous trimethylamine and its non-malodorous metabolite trimethylamine N-oxide were historically regarded as nontoxic. Clinical and toxicological interest has recently arisen because of their potential association with atherosclerosis. We previously reported a human physiologically based pharmacokinetic (PBPK) model for trimethylamine and its primary metabolite, trimethylamine N-oxide, based on reported rat trimethylamine pharmacokinetics. However, rats are poor metabolizers with respect to trimethylamine N-oxygenation, and this species difference was investigated in vitro using substrate depletion rates in rat and human liver microsomes. The current study investigated the pharmacokinetics of deuterium-labeled trimethylamine orally administered to immunodeficient humanized-liver mice transplanted with commercially available human hepatocytes. Trimethylamine N-oxide was extensively formed in vivo in humanized-liver mice, but not in control mice. The experimental pharmacokinetic data of deuterium-labeled trimethylamine and its N-oxide in humanized-liver mice were scaled up for application to a human PBPK model. The human plasma concentration curves generated by the resulting simple PBPK model were consistent with concentrations in humans reported in the literature. The model can also simulate human plasma levels of trimethylamine and trimethylamine N-oxide during treatment with the prescription medicine L-carnitine and in trimethylamine loading tests. The predicted plasma levels were in the ranges that occur under the consumption of daily dietary foodstuff; such levels are associated with few toxicological impacts. The present PBPK model for trimethylamine and trimethylamine N-oxide could estimate daily doses by both forward and reverse dosimetry and could facilitate risk assessment in humans.

Donors FMO3 polymorphisms affect tacrolimus elimination in Chinese liver transplant patients

AIM

Flavin-containing monooxygenase (FMO) variants were potentially involved in tacrolimus metabolism in kidney transplantion. The influences of FMO3 genotypes on tacrolimus elimination in Chinese liver transplant patients remained unclear.

PATIENTS & METHODS

FMO3 SNPs and CYP3A5 rs776746 were analyzed in 110 Chinese patients.

RESULTS

Donor FMO3 rs1800822 allele T and rs909530 allele T were associated with fast tacrolimus elimination. Combination of polymorphisms of donor FMO3 rs1800822 and rs909530 genotype impacted on tacrolimus elimination (p = 0.0221). The number of donor rs1800822 allele T and rs909530 allele T was confirmed to be an independent predictor of the tacrolimus concentration-to-dose ratios for weeks 2, 3 and 4 in the multivariate analysis.

CONCLUSION

Donor's FMO3 polymorphisms might affect tacrolimus elimination.

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.

The phenotype of a knockout mouse identifies flavin-containing monooxygenase 5 (FMO5) as a regulator of metabolic ageing

We report the production and metabolic phenotype of a mouse line in which the Fmo5 gene is disrupted. In comparison with wild-type (WT) mice, Fmo5(-/-) mice exhibit a lean phenotype, which is age-related, becoming apparent after 20 weeks of age. Despite greater food intake, Fmo5(-/-) mice weigh less, store less fat in white adipose tissue (WAT), have lower plasma glucose and cholesterol concentrations and enhanced whole-body energy expenditure, due mostly to increased resting energy expenditure, with no increase in physical activity. An increase in respiratory exchange ratio during the dark phase, the period in which the mice are active, indicates a switch from fat to carbohydrate oxidation. In comparison with WT mice, the rate of fatty acid oxidation in Fmo5(-/-) mice is higher in WAT, which would contribute to depletion of lipid stores in this tissue, and lower in skeletal muscle. Five proteins were down regulated in the liver of Fmo5(-/-) mice: aldolase B, ketohexokinase and cytosolic glycerol 3-phosphate dehydrogenase (GPD1) are involved in glucose or fructose metabolism and GPD1 also in production of glycerol 3-phosphate, a precursor of triglyceride biosynthesis; HMG-CoA synthase 1 is involved in cholesterol biosynthesis; and malic enzyme 1 catalyzes the oxidative decarboxylation of malate to pyruvate, in the process producing NADPH for use in lipid and cholesterol biosynthesis. Down regulation of these proteins provides a potential explanation for the reduced fat deposits and lower plasma cholesterol characteristic of Fmo5(-/-) mice. Our results indicate that disruption of the Fmo5 gene slows metabolic ageing via pleiotropic effects.

In vitro evidence for bakuchiol's influence towards drug metabolism through inhibition of UDP-glucuronosyltransferase (UGT) 2B7

BACKGROUND

Inhibition of drug-metabolizing enzymes (DMEs) has been regarded as one of the most important reason for clinical drug-drug interaction.

AIM

The aim of the present study is to evaluate the inhibition of bakuchiol towards UDP-glucuronosyltransferase (UGT) 2B isoforms.

METHODS

In vitro recombinant UGT2B-catalyzed 4-methylumbelliferone glucuronidation was used as the probe reaction. Dixon plot and Lineweaver-Burk plot were employed to determine the inhibition kinetic type, and nonlinear regression of data was utilized to calculate the inhibition kinetic parameter (Ki). In vitro-in vivo extrapolation (IVIVE) was carried out to predict in vivo inhibition magnitude.

RESULTS

Among the tested UGT2B isoforms, UGT2B7 was inhibited by the strongest intensity. The noncompetitive inhibition was demonstrated by the results obtained from Dixon plot and Lineweaver-Burk plot. The Ki value was calculated to be 10.7 µM. In combination with the reported concentration after an intravenous administration of bakuchiol (15 mg/kg) in rats, the high risk of in vivo inhibition of bakuchiol towards UGT2B7-catalyzed metabolism of drugs was indicated.

CONCLUSION

All these results provide an important information for the risk evaluation of the clinical utilization of bakuchiol.

Tolerance to acetaminophen hepatotoxicity in the mouse model of autoprotection is associated with induction of flavin-containing monooxygenase-3 (FMO3) in hepatocytes

Acetaminophen (APAP) pretreatment with a hepatotoxic dose (400 mg/kg) in mice results in resistance to a second, higher dose (600 mg/kg) of APAP (APAP autoprotection). Recent microarray work by our group showed a drastic induction of liver flavin containing monooxygenase-3 (Fmo3) mRNA expression in our mouse model of APAP autoprotection. The role of liver Fmo3, which detoxifies xenobiotics, in APAP autoprotection is unknown. The purpose of this study was to characterize the gene regulation and protein expression of liver Fmo3 during APAP hepatotoxicity. The functional consequences of Fmo3 induction were also investigated. Plasma and livers were collected from male C57BL/6J mice over a period of 72 h following a single dose of APAP (400 mg/kg) to measure Fmo3 mRNA and protein expression. Although Fmo3 mRNA levels increased significantly following APAP treatment, protein expression changed marginally. In contrast, both Fmo3 mRNA and protein expression were significantly higher in APAP autoprotected livers. Unlike male C57BL/6J mice, female mice have ∼80-times higher constitutive Fmo3 mRNA levels and are highly resistant to APAP hepatotoxicity. Coadministration of APAP with the FMO inhibitor methimazole rendered female mice susceptible to APAP hepatotoxicity, with no changes in susceptibility detected in male mice. Furthermore, a human hepatocyte cell line (HC-04) clone over-expressing human FMO3 showed enhanced resistance to APAP cytotoxicity. Taken together, these findings establish for the first time induction of Fmo3 protein expression and function by xenobiotic treatment. Our results also indicate that Fmo3 expression and function plays a role in protecting the liver from APAP-induced toxicity. Although the mechanism(s) of this protection remains to be elucidated, this work describes a novel protective function for this enzyme.

The phenotype of a flavin-containing monooyxgenase knockout mouse implicates the drug-metabolizing enzyme FMO1 as a novel regulator of energy balance

Flavin-containing monooxygenases (FMOs) of mammals are thought to be involved exclusively in the metabolism of foreign chemicals. Here, we report the unexpected finding that mice lacking Fmos 1, 2 and 4 exhibit a lean phenotype and, despite similar food intake, weigh less and store less triglyceride in white adipose tissue (WAT) than wild-type mice. This is a consequence of enhanced whole-body energy expenditure, due mostly to increased resting energy expenditure (REE). This is fuelled, in part, by increased fatty acid β-oxidation in skeletal muscle, which would contribute to depletion of lipid stores in WAT. The enhanced energy expenditure is attributed, in part, to an increased capacity for exercise. There is no evidence that the enhanced REE is due to increased adaptive thermogenesis; instead, our results are consistent with the operation in WAT of a futile energy cycle. In contrast to FMO2 and FMO4, FMO1 is highly expressed in metabolic tissues, including liver, kidney, WAT and BAT. This and other evidence implicates FMO1 as underlying the phenotype. The identification of a novel, previously unsuspected, role for FMO1 as a regulator of energy homeostasis establishes, for the first time, a role for a mammalian FMO in endogenous metabolism. Thus, FMO1 can no longer be considered to function exclusively as a xenobiotic-metabolizing enzyme. Consequently, chronic administration of drugs that are substrates for FMO1 would be expected to affect energy homeostasis, via competition for endogenous substrates, and, thus, have important implications for the general health of patients and their response to drug therapy.