Hepatic uptake and disposition of aflatoxin B1 in isolated perfused rat liver

Comparative metabolism of aflatoxin B1 in mouse, rat and human primary hepatocytes using HPLC-MS/MS

Aflatoxin B1 (AFB1) is a highly hepatotoxic and carcinogenic mycotoxin produced by Aspergillus species. The compound is mainly metabolized in the liver and its metabolism varies between species. The present study quantified relevant AFB1- metabolites formed by mouse, rat, and human primary hepatocytes after treatment with 1 µM and 10 µM AFB1. The use of liquid chromatographic separation coupled with tandem mass spectrometric detection enabled the selective and sensitive determination of phase I and phase II metabolites of AFB1 over incubation times of up to 24 h. The binding of AFB1 to macromolecules was also considered. The fastest metabolism of AFB1 was observed in mouse hepatocytes which formed aflatoxin P1 as a major metabolite and also its glucuronidated form, while AFP1 occurred only in traces in the other species. Aflatoxin M1 was formed in all species and was, together with aflatoxin Q1 and aflatoxicol, the main metabolite in human cells. Effective epoxidation led to high amounts of DNA adducts already 30 min post-treatment, especially in rat hepatocytes. Lower levels of DNA adducts and fast DNA repair were found in mouse hepatocytes. Also, protein adducts arising from reactive intermediates were formed rapidly in all three species. Detoxification via glutathione conjugation and subsequent formation of the N-acetylcysteine derivative appeared to be similar in mice and in rats and strongly differed from human hepatocytes which did not form these metabolites at all. The use of qualitative reference material of a multitude of metabolites and the comparison of hepatocyte metabolism in three species using advanced methods enabled considerations on toxification and detoxification mechanisms of AFB1. In addition to glutathione conjugation, phase I metabolism is strongly involved in the detoxification of AFB1.

Cypermethrin-induced toxic effect on glycogen metabolism in estuarine clam, marcia opima (gmelin, 1791) of ratnagiri coast, maharashtra

Cypermethrin is a synthetic pyrethroid class of insecticide. Toxic effects of cypermethrin were studied by selecting Marcia opima as an animal model. Cypermethrins effect on the total glycogen content of mantle, gill, foot, hepatopancreas, male gonad and a female gonad of an estuarine clam, Marcia opima was examined. The clams were exposed to 1.58 ppm cypermethrin for acute and 1/10th of that concentration for chronic treatment. It was found that there was a decrease in glycogen content in various tissues as compared to control. In LC(0) and LC(50) groups, glycogen was decreased in all tissues except in hepatopancreas compared to control. This decrease is greater in mantle, gill, and foot in LC(50) group than the decrease in those tissues of LC(0) group. In chronic exposure it was found that glycogen was decreased in mantle, foot, male gonad, and female gonad when compared to the control group except in gill and hepatopancreas. Decrease in glycogen content indicates greater utilization of glycogen for metabolic purposes and too combat with cypermethrin stress. The significant increase in glycogen content in gill and hepatopancreas may be a reaction to the increase in energy demand.

Case-control study of an acute aflatoxicosis outbreak, Kenya, 2004

OBJECTIVES

During January-June 2004, an aflatoxicosis outbreak in eastern Kenya resulted in 317 cases and 125 deaths. We conducted a case-control study to identify risk factors for contamination of implicated maize and, for the first time, quantitated biomarkers associated with acute aflatoxicosis.

DESIGN

We administered questionnaires regarding maize storage and consumption and obtained maize and blood samples from participants.

PARTICIPANTS

We recruited 40 case-patients with aflatoxicosis and 80 randomly selected controls to participate in this study.

EVALUATIONS/MEASUREMENTS

We analyzed maize for total aflatoxins and serum for aflatoxin B1-lysine albumin adducts and hepatitis B surface antigen. We used regression and survival analyses to explore the relationship between aflatoxins, maize consumption, hepatitis B surface antigen, and case status.

RESULTS

Homegrown (not commercial) maize kernels from case households had higher concentrations of aflatoxins than did kernels from control households [geometric mean (GM) = 354.53 ppb vs. 44.14 ppb; p = 0.04]. Serum adduct concentrations were associated with time from jaundice to death [adjusted hazard ratio = 1.3; 95% confidence interval (CI), 1.04-1.6]. Case patients had positive hepatitis B titers [odds ratio (OR) = 9.8; 95% CI, 1.5-63.1] more often than controls. Case patients stored wet maize (OR = 3.5; 95% CI, 1.2-10.3) inside their homes (OR = 12.0; 95% CI, 1.5-95.7) rather than in granaries more often than did controls.

CONCLUSION

Aflatoxin concentrations in maize, serum aflatoxin B1-lysine adduct concentrations, and positive hepatitis B surface antigen titers were all associated with case status.

RELEVANCE

The novel methods and risk factors described may help health officials prevent future outbreaks of aflatoxicosis.

Discriminating different classes of toxicants by transcript profiling

Male rats were treated with various model compounds or the appropriate vehicle controls. Most substances were either well-known hepatotoxicants or showed hepatotoxicity during preclinical testing. The aim of the present study was to determine if biological samples from rats treated with various compounds can be classified based on gene expression profiles. In addition to gene expression analysis using microarrays, a complete serum chemistry profile and liver and kidney histopathology were performed. We analyzed hepatic gene expression profiles using a supervised learning method (support vector machines; SVMs) to generate classification rules and combined this with recursive feature elimination to improve classification performance and to identify a compact subset of probe sets with potential use as biomarkers. Two different SVM algorithms were tested, and the models obtained were validated with a compound-based external cross-validation approach. Our predictive models were able to discriminate between hepatotoxic and nonhepatotoxic compounds. Furthermore, they predicted the correct class of hepatotoxicant in most cases. We provide an example showing that a predictive model built on transcript profiles from one rat strain can successfully classify profiles from another rat strain. In addition, we demonstrate that the predictive models identify nonresponders and are able to discriminate between gene changes related to pharmacology and toxicity. This work confirms the hypothesis that compound classification based on gene expression data is feasible.

Dose-related increase in liver heme catabolism during rabbit aflatoxicosis

Aflatoxin B1 (AFB1) has been reported to decrease microsomal hepatic cytochrome P450 (P450) content and increase both total plasma bilirubin concentration and liver heme oxygenase activity. The purposes of this study were to determine whether liver hemoproteins contents and heme catabolizing enzymes were affected by the mycotoxin and whether these alterations were linked to hyperbilirubinemia. Male New Zealand rabbits were divided into three groups of five animals, each receiving for 5 days either arabic gum as vehicle or AFB1 at a daily oral dose of 0.05 or 0.10 mg/kg. These treatments affected neither cytochrome b5 content nor NADPH-cytochrome reductase activity. A linear dose-dependent decrease in cytochrome P450 content and increases in both heme oxygenase and biliverdin reductase activities were observed. Bilirubin UDP-glucuronyltransferase activity was dramatically decreased at both doses, whereas cholestasis occurred only at 0.10 mg/kg. An exponential dose-dependent increase in plasma bilirubin concentration was also observed. Both the simultaneous exponential increase in bilirubinemia associated to a reduced bilirubin UDP-glucuronyltransferase activity and the absence of cholestasis at 0.05 mg/kg, suggested that the hyperbilirubinemia is more probably related to an increased heme catabolism than to an altered bile duct permeability.

Hepatocellular uptake of aflatoxin B1 by non-ionic diffusion. Inhibition of bile acid transport by interference with membrane lipids

Aflatoxin B1 permeates isolated rat hepatocytes by non-ionic diffusion. Its uptake is neither saturable nor influenced by metabolic energy and not inhibited by treatment of cells with proteases. The initial rate of aflatoxin B1 uptake measured at 7 degrees C is between 40 and 50% compared to that at 37 degrees C. However, after an incubation period of 7 minutes identical equilibrium uptake is reached at both temperatures. The apparent activation energies, calculated for aflatoxin B1 uptake by Arrhenius diagrams ranged between 1.69 and 4.5 kcal/mol. A Q10 value of 1.34 was calculated for a temperature interval of 7-17 degrees C but decreased to 1.05 for the interval of 27-37 degrees C. Liposomes or lipoproteins added to the cell suspension inhibited the aflatoxin B1 uptake into hepatocytes. Liposomes mainly composed of unsaturated fatty acids bind twice as much aflatoxin B1 as those composed of saturated ones, indicating that the lipophilicity of the mycotoxin is crucial in the determination of its uptake into liver cells. At concentrations above 5 micrograms/ml, aflatoxin B1 inhibited the carrier-mediated uptake of cholic acid and of phalloidin into hepatocytes. This effect was reversible and abolished by washing the cells after preincubation with aflatoxin. In concentrations below 5 micrograms/ml the uptake of phallotoxin and cholic acid was however stimulated by 15-25%. These results indicate, that a carrier-mediated uptake into hepatocytes via the multispecific bile salt transporter is not responsible for the organoselective clearance of aflatoxins by the liver. On the other hand, the cholestatic effect of aflatoxin B1 results at least partially from the inhibition of the multispecific bile acid transport system. This inhibition may arise from affinity of aflatoxins to lipid domains of the cell membrane.

Metabolic activation and bacterial mutagenicity of aflatoxin B1 in two different test systems

The metabolism of aflatoxin B1 (AFB1) by liver post-mitochondrial supernatant fraction from phenobarbitone treated male albino rats in liquid suspension and soft-agar plate incorporation bacterial mutation assay systems has been studied. The AFB1 concentration used was in the range of those added to bacterial mutagenicity tests with this mycotoxin. Three oxidative metabolites of AFB1 viz the Tris derivative of AFB1 8,9-diol (derived from the 8,9-epoxide), aflatoxin Q1 (AFQ1) and M1 (AFM1) were observed. The metabolite profile and time course of formation were qualitatively similar in both assay systems. The rate and overall formation of metabolites in the soft-agar system was approximately one half that in the liquid suspension system which was reflected in decreased AFB1 induced bacterial mutation in the former system. The AFB1 metabolite profile in these in vitro systems did not mirror completely the reported in vivo profile seen in the rat, as aflatoxin P1 (AFP1) and a glutathione conjugate were not detected. No evidence was found for a prolongation of metabolism in the soft-agar system.

Sequential biochemical and histological changes in rats treated with aflatoxin B1

Thirteen biochemical parameters (viz. glucose, calcium, inorganic phosphorous, urea nitrogen, uric acid, cholesterol, albumin, total protein, total bilirubin, alkaline phosphatase, lactate dehydrogenase, aspartate aminotransferase, and alanine aminotransferase) were determined in serum and partly in liver of rats 1-28 days after i.p. aflatoxin B1 (AFB) (3 mg/kg). Histological examinations of the liver were also made in parallel to the biochemical studies. In the serum, enzyme activities and total bilirubin level increased and peaked on the 2nd day, while other activities of aspartate aminotransferase and alanine aminotransferase in the liver significantly decreased and reached a minimum on the 2nd day after AFB administration. The depression of the liver enzyme activities persisted over 7 days. The liver protein content also reduced transiently during 1-1.5 days. However, all biochemical parameters returned to normal levels 2 weeks after treatment, and remained so throughout the rest of experimental period. Histological changes in the liver were very similar to those reported by other.

Some high-performance liquid-chromatographic studies of the metabolism of aflatoxins by rat liver microsomal preparations

The metabolism of aflatoxin B1 in vitro was examined in rat liver microsomal preparations. 2. H.p.l.c. (high-performance liquid-chromatographic) systems were used. A silica column was used to separate non-polar metabolites. A system utilizing a reversed-phase column which separates both poar and non-polar metabolites was also developed. 3. The principal metabolites of aflatoxin B1 found were aflatoxin M1, aflatoxin Q1 and a compound which co-chromatographed with a degradation product of aflatoxin B1 2,3-dihydrodiol. 4. The time course of metabolism of aflatoxin B1 by microsomal preparations isolated from control and phenobarbitone-pretreated rats was examined. The rate and extent of metabolism was greater with microsomal preparations from the latter. The formation of aflatoxin Q1 was enhanced 4--5-fold by phenobarbitone pretreatment, whereas the production of aflatoxin M1 was only increased 1--2-fold. The formation of the degradation product of aflatoxin B1 2,3-dihydrodiol was increased 4--5-fold by the pretreatment with phenobarbitone. 5. The microsomal metabolism of aflatoxins M1, P1 and Q1 was examined. Aflatoxin M1 apparently underwent very limited microsomal metabolism to more polar compounds. Aflatoxin P1 was not metabolized. The situation with aflatoxin Q1 was complicated in that it was metabolized in the absence of NADPH to an unidentified metabolite. Aflatoxin B1 appeared as a metabolite of aflatoxin Q1 only when NADPH was present, and the formation of more polar metabolites was also then observed.