Co-mutagenicity of coumarin (1,2-benzopyrone) with aflatoxin B1 and human liver S9 in mammalian cells

Cancer Hazard Identification Integrating Human Variability: The Case of Coumarin

Coumarin is a naturally occurring sweet-smelling benzopyrone that may be extracted from plants or synthesized for commercial uses. Its uses include as a flavoring agent, fragrance enhancer, and odor-masking additive. We reviewed and evaluated the scientific evidence on the carcinogenicity of coumarin, integrating information from carcinogenicity studies in animals with mechanistic and other relevant data, including data from toxicogenomic, genotoxicity, and metabolism studies, and studies of human variability of a key enzyme, CYP2A6. Increases in tumors were observed in multiple studies in rats and mice in multiple tissues. Our functional pathway analysis identified several common cancer-related biological processes/pathways affected by coumarin in rat liver following in vivo exposure and in human primary hepatocytes exposed in vitro. When coumarin 7-hydroxylation by CYP2A6 is compromised, this can lead to a shift in metabolism to the 3,4-epoxidation pathway and increased generation of electrophilic metabolites. Mechanistic data align with 3 key characteristics of carcinogens, namely formation of electrophilic metabolites, genotoxicity, and induction of oxidative stress. Considerations of metabolism, human variability in CYP2A6 activity, and coumarin hepatotoxicity in susceptible individuals provide additional support for carcinogenicity concern. Our analysis illustrates the importance of integrating information on human variability in the cancer hazard identification process.

Strategies for Avoiding Benzopyrone Hepatotoxicity in Lymphedema Management-The Role of Pharmacogenetics, Metabolic Enzyme Gene Identification, and Patient Selection

INTRODUCTION

Benzopyrones are plant-derived chemicals which have an evidenced degree of clinical efficacy in lymphedema management indicated in past trials. Unfortunately, in some of these cases idiosyncratic hepatotoxicity have been documented in a minority of patients. This review aims to tackle the problem of benzopyrone (particularly coumarin) toxicity by considering their metabolic pathways and identifying relevant alleles needed to take a targeted pharmacogenetic approach in its future use.

METHODS AND RESULTS

The nontoxic 7-hydroxylation and the toxic heterocyclic "ring-splitting" epoxidation pathways are the two main detoxification pathways in the hepatometabolism of coumarin, the former catalyzed by CYP2A6 and the latter by possibly CYP1A and CYP2E. Acetaldehyde dehydrogenase (ALDH) clears toxic aldehyde intermediates. CYP2A6 polymorphism screening methods, including genotyping, by real-time polymerase chain reaction and chromatography-mass spectroscopy functional metabolite assays; efficiency of these techniques are continually improving. ALDH polymorphisms have also been implicated, with clinically viable screening tests, rapid genotyping, and sensitive questionnaires already available for ALDH2*1/ALDH2*2. Dysfunctional polymorphisms of the above genes and others are significantly more prevalent in Eastern Asian populations, uncommon in Caucasian populations. The role of other enzymes/genes in the pathway is yet to be clarified.

CONCLUSION

Although screening techniques are becoming increasingly clinically feasible, uncertainty remains on the link between the genotype, metabolic phenotype, and the exact gene products involved. These must be elucidated further before a targeted pharmacogenomic approach is fully viable. In the meantime, treatment should be avoided in those with vulnerable familial and ethnic descents if used.

Product identification and safety evaluation of aflatoxin B1 decontaminated by electrolyzed oxidizing water

In this study with aflatoxin-contaminated peanuts, the effectiveness of electrolyzed oxidizing water (EOW) in the decontamination of aflatoxin B(1) was investigated. The aflatoxin B(1) content was markedly reduced upon treatment with EOW, particularly with neutral electrolyzed oxidizing water (NEW). The conversion product of EOW treatment was isolated and identified as 8-chloro-9-hydroxy aflatoxin B(1) (compound 1), which is an amphiphilic molecule, in contrast to fat-soluble aflatoxin B(1). A mutagenic response study revealed that the number of revertants per plate after treatment of bacterial strains TA-97, TA-98, TA-100, and TA-102 with NEW was within the standard value range. The HepG2 cell viability assay yielded an IC(50) value of compound 1 approximately 150 mM. This study indicates that EOW had the ability to decontaminate aflatoxin B(1), and the conversion product, compound 1, did not exhibit mutagenic activity or cytotoxic effects.

AFB(1) -induced mutagenesis of the gpt gene in AS52 cells

Aflatoxin B(1) (AFB(1) ) is a potent mutagen and an important risk factor for hepatocellular carcinoma (HCC) in humans. Transgenic mouse strains and cells in culture have been used to detect different types of mutations caused by AFB(1) and investigate the molecular determinants of their location and frequency. The AFB(1) mutational spectrum in the gpt gene was markedly different in AS52 cells compared with the liver in gpt delta B6C3F1 transgenic mice. The results demonstrate the importance of metabolism, chromosomal location, transcription and selection conditions on mutational spectra.

The mutagenic, DNA-damaging and antioxidative properties of bark and leaf extracts from Coutarea hexandra (Jacq.) K. Schum

Coutarea hexandra is a species commonly known in Brazil as quina, and its bark is used in folk medicine. In this study, we assess the mutagenic and DNA-damaging effects of ethanol extracts from C. hexandra stem bark (SCH) and leaves (LCH) by employing the Ames test on the TA98 and TA100 strains of Salmonella typhimurium in addition to a plasmid treatment test. Furthermore, we performed a phytochemical analysis by TLC and HPLC, a quantification of the phenolic constituents and an assessment of the antioxidative activity. SCH and LCH showed mutagenic action in the Ames test for TA98 strains after metabolic activation. LCH also showed mutagenicity for the TA100 strain after metabolic activation. The findings from the plasmid treatment test did not indicate any DNA-damaging activity for either of the extracts with the tested dosages. SCH showed greater flavonoid content and greater antioxidative potential in relation to LCH. This study suggests that caution is advisable in the use of this plant. However, in vivo studies should be conducted to confirm these data.

Mikania glomerata Sprengel (Asteraceae) influences the mutagenicity induced by doxorubicin without altering liver lipid peroxidation or antioxidant levels

As shown in numerous studies, natural compounds may exert adverse effects, mainly when associated with some drugs. The hydroalcoholic extract of Mikania glomerata is the pharmaceutical form present in commercially available syrup used for the treatment of respiratory diseases in popular Brazilian medicine. The objective of the present investigation was (1) to evaluate the preventive effects of standardized hydroalcoholic extract of M. glomerata (MEx) against antitumoral drug doxorubicin (DXR)-induced micronucleated polychromatic erythrocytes (MNPCE) in a subchronic assay in mice, and (2) to determine the liver content of malondialdehyde (MDA) and the antioxidants glutathione (GSH) and vitamin E (VE). Male Swiss mice were treated for 30 d with MEx added to drinking water, combined or not with DXR (90 mg/kg body weight) injected intraperitoneally (ip) 24 h before analysis. The results demonstrated that MEx produced no genotoxic damage, but significantly increased the frequency of MNPCE induced by DXR, indicating a drug-drug interaction. This rise was not accompanied by lipid peroxidation or antioxidants level reduction, as measured by MDA, GSH, and VE. Despite the presence of coumarin (a known antioxidant), MEx may exert adverse effects probably in association with mutagenic compounds, although this effect on DNA damage did not involve oxidative stress.

Allelochemical induction of cytochrome P450 monooxygenases and amelioration of xenobiotic toxicity in Helicoverpa zea

Polyphagous herbivores encounter allelochemicals as complex mixtures in their host plants, and the toxicity of an individual compound may be influenced by the chemical matrix in which it is encountered. Certain plant constituents may reduce toxicity of cooccurring compounds by inducing detoxification systems, including cytochrome P450s, which can metabolize a broad range of substances. The polyphagous corn earworm Helicoverpa zea encounters a diversity of plant allelochemicals in its many host plants and, as well, can encounter aflatoxins, mycotoxins produced by Aspergillus flavus and Aspergillus parasiticus that infect damaged grains. Dietary supplementation of each of three plant allelochemicals that are frequently (coumarin, COU), occasionally (indole-3-carbinol, 13C), or rarely (xanthotoxin, XAN) encountered by H. zea larvae substantially reduced the toxicity of aflatoxin B1 (AFB1) to H. zea. Compared to fourth instars on diets containing 1 microg/g AFB1 that failed to develop and pupate, fourth instars on diets containing I3C and XAN increased in mass by 216.1 and 700% after 6 days, and pupated at rates of 40 and 88%, respectively. Diets containing COU or XAN also significantly reduced the mortality rates of caterpillars exposed to the insecticides, diazinon and carbaryl. Diets containing COU and XAN increased CYP6B8 transcripts 2.6-fold; CYP321A1 transcripts increased 20.7, 8.3, and 10.6-fold in response to COU, I3C, and XAN, respectively. These results indicate that consumption of plant allelochemicals can ameliorate toxicity of natural and synthetic toxins encountered by insects, and they suggest that P450s induced by these allelochemicals contribute to detoxification of these chemicals in H. zea.

CYP2A6 polymorphisms: is there a role for pharmacogenomics in preventing coumarin-induced hepatotoxicity in lymphedema patients?

Lymphedema is a chronic progressive and significantly disabling disease that affects over 150 million people worldwide. Coumarin is an effective pharmacological treatment, but is banned in some countries due to incidences of hepatotoxicity in rats and mice, and the rare finding of similar hepatotoxicity in humans. Cytochrome P450 (CYP)2A6 is the major enzyme involved in metabolizing coumarin to 7-hydroxycoumarin. A reduction in CYP2A6 activity will lead to shunting of coumarin into other metabolic pathways. In particular, coumarin is metabolized by CYP3A4 to form 3-hydroxycoumarin, the major metabolite in mice and rats. It has been shown that an increase in the 3-hydroxycoumarin ratio is associated with an increased production of the significant cytotoxic product o-hydroxyphenylacetylacetaldehyde (o-HPA), suggesting that a shunting of coumarin metabolism away from 7-hydroxylation is the cause of the toxicity. Hence, poor CYP2A6 metabolizers are more likely to metabolize coumarin via the cytotoxic pathway. Identifying these patients, and not treating them with coumarin, may reduce the incidence of toxicity associated with this drug. The technology to do so exists, but more information is required regarding the mechanism of coumarin toxicity.

Effects of black tea theafulvins on aflatoxin B(1) mutagenesis in the Ames test

Black tea theafulvins, a fraction of thearubigins isolated from black tea aqueous infusions, potentiated the mutagenic activity of the mycotoxin aflatoxin B(1) in the Ames test, in the presence of a hepatic S9 activation system derived from Aroclor 1254-treated rats. In contrast, when the S9 activation system was replaced with isolated microsomes, theafulvins suppressed the mutagenicity of the mycotoxin. When microsomal metabolism was terminated after metabolic activation of the mycotoxin, incorporation of the theafulvins into the activation system reduced the mutagenic activity, whereas if it was added before termination of microsomal activity a potentiation of mutagenic response was observed. In in vitro studies, theafulvins inhibited epoxide hydrolase and glutathione S-transferase activities in a concentration-dependent manner. Finally, the mutagenicity of aflatoxin B(1) was much more pronounced in bacteria that were pre-exposed to theafulvins but from which they were subsequently washed off. It may be inferred from the above studies that the genotoxic synergy between aflatoxin B(1) and black tea theafulvins does not occur during the bioactivation of the carcinogen, but may partly be due to decreased deactivation of the reactive intermediate, aflatoxin B(1) 8,9-oxide, by conjugation with glutathione.

Lack of effect of coumarin on the formation of micronuclei in an in vivo mouse micronucleus assay

Coumarin was tested for its potential to cause genotoxic effects in mouse bone marrow cells using an in vivo micronucleus assay. Male and female Swiss mice were administered a single oral dose of coumarin at 50, 100 or 200 mg/kg by gavage in corn oil vehicle. Control animals received only the vehicle. Groups of male mice were also administered mitomycin C at 0.75 mg/kg and served as positive controls. At 24 h after treatment, mice from all dose levels, and at 48 h after treatment, mice from the high dose level only were sacrificed. Bone marrow cells were collected and assayed for the presence of micronuclei. Coumarin did not cause any increase in the incidence of micronucleated polychromatic erythrocytes in male or female mice at any of the dose levels, the positive control mitomycin C produced a significant increase. There was no evidence of coumarin or mitomycin C treatment related cytotoxicity to bone marrow cells. The results of this study demonstrate that coumarin is negative in the mouse in vivo micronucleus assay.