Translational step inhibited in vivo by aflatoxin B1 in rat-liver polysomes

Effects of dietary milk thistle on blood parameters, liver pathology, and hepatobiliary scintigraphy in white carneaux pigeons (Columba livia) challenged with B1 aflatoxin

Milk thistle (Silybum marianum) has been used in humans for the treatment of liver disease because of its antioxidant properties and its ability to stabilize cell membranes and regulate cell permeability. To investigate possible hepatoprotective effects in birds, standardized extracts (80%) of silymarin from milk thistle were tested in white Carneaux pigeons (Columba livia). Pigeons were separated into 3 groups and fed diets formulated to provide milk thistle at a level of 0, 10, or 100 mg/kg body weight per day. After acclimation, the birds were challenged with B1 aflatoxin (3 mg/kg body weight for 2 consecutive days) by oral gavage. Liver function then was assessed by hematologic testing and plasma biochemical analysis, liver histopathology, and hepatobiliary scintigraphy. Results of histopathology and hepatobiliary scintigraphy showed no protective effects from milk-thistle administration. Aflatoxin challenge resulted in hepatic inflammation and necrosis, biliary-duct hyperplasia, and lymphocyte infiltration. All hepatobiliary scintigraphy elements increased significantly after aflatoxin challenge. Bile acid levels and plasma enzyme concentrations of aspartate aminotransferase, lactate dehydrogenase, alanine aminotransferase, and creatine phosphokinase all increased after aflatoxin exposure and were mostly unchanged with consumption of milk thistle. Only birds fed 10 mg/kg body weight milk thistle showed significant reductions in lactate dehydrogenase, alanine aminotransferase, and creatine phosphokinase concentrations after aflatoxin exposure. Our results show that consumption of milk thistle is not associated with hepatoprotective effects against acute B1 aflatoxin exposure in pigeons.

Inhibition of the biosynthesis of SRP polypeptides and secretory proteins by aflatoxin B1 can disrupt protein targeting

Cell culture and western blotting studies revealed that aflatoxin B(1) (AFB(1)) inhibits the biosynthesis of two of the constituent polypeptides of signal recognition particle (SRP) (SRP54 and 72). SRP escorts polyribosomes carrying signal peptides from free form in the cytosol to the bound form on endoplasmic reticulum (ER) membrane during protein targeting. These effects of AFB(1) on SRP biosynthesis may inhibit the formation of functional SRP. Our experiments have further shown that AFB(1) also inhibits the biosynthesis/translocation of a secretory protein, preprolactin, which fails to appear in the lumen of ER consequent to the treatment with this hepatocarcinogen. The results of the experiments presented in this article therefore enable us to infer for the first time that aflatoxin B(1) may inhibit the functioning of SRP as an escort and deplete the ER of polyribosomes for secretory protein synthesis. As these secretory proteins are important components of the plasma membrane, gap junctions and intercellular matrix, their absence from these locations could disturb cell to cell communication leading to tumorigenesis.

The in vivo disposition of aflatoxin B1 in rat liver

The disposition of a non-toxic i.p. dose of [3H]-aflatoxin B1 (0.70 micrograms/kg) in the blood, plasma, and liver was studied in male Wistar rats. Uptake into the blood, plasma, and liver was biphasic; there was an initial rapid rise (0-2 hr) followed by a second phase (2-12 hr) of a gradual increase. Most of the radioactivity in the blood was bound noncovalently to albumin. Distribution of radioactivity in the subcellular fractions of liver showed that the microsomes exhibited the highest labeling which increased over the time course; labeling of the cytosol reached a maximum at 2 hr then decreased to a new steady state, whereas the mitochondria and nuclei reached a plateau. When the content of aflatoxin B1 in the nuclear subfractions was examined, greater than 92% of the total radioactivity was found in the deoxyribonucleoprotein fraction, and 84% of this was bound noncovalently. These results suggest that aflatoxin B1 is transported from the site of injection through the blood to the liver and its subcellular and subnuclear fractions primarily in a noncovalent form.

Interaction between aflatoxin B1 and oxytetracycline in isolated rat hepatocytes

Isolated rat hepatocytes were used as an in vitro model to investigate a possible interaction between oxytetracycline (OXT) and aflatoxin B1 (AFB1). LDH leakage, RNA and protein synthesis and glycogen accumulation were measured in the presence of both drugs, either separately or in combination. The evolution of LDH leakage during the incubation was identical in untreated and treated cells. AFB1 inhibited RNA and protein synthesis at a concentration of 10(-7) M and 10(-6) M, respectively, and higher, whereas OXT did not influence RNA synthesis but inhibited protein synthesis at the highest tested concentration, 10(-3) M. As far as glycogen is concerned, rats were injected with glucagon before sacrifice in order to obtain a constant synthesis rate in isolated hepatocytes. AFB1 inhibited the accumulation of glycogen from 10(-6) M upward. This effect was never observed before 90 min of incubation. OXT had no effect on glycogen synthesis. In the presence of both drugs, no interaction was demonstrated as far as RNA and protein synthesis were concerned, but OXT opposed the inhibition induced by AFB1 on glycogen accumulation. If the "in vivo" protection, provided by OXT against AFB1-induced toxicity, is due to a direct interference in the toxic mechanisms of the mycotoxin, these results show that OXT does not influence the AFB1-inhibition of RNA and protein synthesis. The latter are early and sensitive parameters inhibited by AFB1. On the contrary, taking into consideration the results on glycogen accumulation, it seems more interesting to investigate further this metabolism.

Effects of aflatoxin on the immune system of the chick

A decreased antibody response to injected sheep red blood cells and weight losses to the extent of 25-38% were observed in the bursa Fabricii and thymus in chicks fed an aflatoxin diet (10% w/w) for 3 weeks. The immune organs did not show any significant histopathological changes. The antibody titre decreased from day 6 of treatment onward until the 15th day, when the antibody titre had disappeared completely in the test chicks. The body weight gain was also less in test chicks (9.7-37.3%) given test feed or test feed along with Salmonella pullorum in water (10(8) cfu ml 1) in comparison to chicks given toxin-free diet and water after 3 weeks of feeding trials. The disappearing component in the test chicks' liver homogenate was found to be antigenic by immunoelectrophoretic studies.

Modification of protein synthetic components by aflatoxin B1

Molecular sites of perturbation by the hepatocarcinogen aflatoxin B1 (AFB1) in the protein synthesis initiation complex were assessed using isolated hepatocytes and a cell-free activating system containing microsomes and cytoplasmic ribonucleoprotein complexes (cRPC). Ribosomal proteins showed no detectable modification by the toxin in either system. With hepatocytes, initiation factors demonstrated only slight modification by AFB1. RNAs from both hepatocytes and the cell-free system with microsomes and cRPC were modified, with poly(A)-containing RNA exhibiting at least a 5-fold higher modification than poly(A)-lacking RNA. The poly(A)-lacking RNAs were modified in the order 28S rRNA greater than 18S rRNA greater than 5-6S rRNA greater than 4S tRNA. Guanine was the target base of AFB1, but only 10% of the AFB1-GMP adducts were on guanines located in a poly(G) region. These results suggest that guanine modification in RNAs may be responsible for the observed inhibition of translational initiation by AFB1 to a greater extent than modification of either ribosomal intrinsic or associated proteins.

3H-amino acid incorporation into proteins during chronic aflatoxin induced coagulation defects in rabbits

Eighteen New Zealand White rabbits were divided equally into three groups and administered either 0.2 ml dimethyl sulfoxide (DMSO)/day, 0.06 mg/kg/day Aflatoxin B1 (AFB1) in DMSO, or 0.09 mg/kg/day AFB1 in DMSO. Incorporation of 3H-amino acids into total serum protein, fibrinogen and platelet proteins was determined during the intoxication and developing coagulation defect. Ten of 18 rabbits administered AFB1 in DMSO died or developed prolonged coagulation times. There was no significant difference in specific incorporation of 3H-amino acids into total serum protein, fibrinogen or platelet proteins between groups, nor was there a difference in incorporation between rabbits with normal coagulation times and those with prolonged coagulation times. Correlation between plasma fibrinogen concentration and specific incorporation of 3H-amino acid into fibrinogen was not significant. In vivo inhibition of 3H-amino acid incorporation into plasma proteins or platelets could not be demonstrated.

The relationship of certain blood parameters to aflatoxin resistance in Japanese quail

Blood chemistry profiles were obtained for two lines of Japanese quail selected for resistance to aflatoxin, and for a nonselected control line. Nine of the 18 plasma components measured in samples taken at 4 weeks of age changed as a result of selection. Plasma concentrations of total protein, albumin, cholesterol, and potassium, and the activities of lactic dehydrogenase, glutamic-oxalacetic transaminase, and cholinesterase were significantly elevated in aflatoxin-resistant quail compared with the nonselected line. The activities of beta-glucuronidase and alpha-amylase changed most dramatically; both activities were much lower in the resistant lines than in the control line. In another experiment, serum total protein, albumin, alpha-amylase, and beta-glucuronidase were tested as identifiers of aflatoxin-resistant individuals within a nonselected population of quail. Serum samples obtained from 150 nonselected quail immediately before and 24 hr after administration of an oral dose of aflatoxin were analyzed for each of the four components. The acute toxicosis decreased body weight, serum alpha-amylase activity, total protein, and albumin; whereas, serum beta-glucuronidase activity and the coefficients of variation for each parameter were increased. Correlations between measurements made prior to dosing and parameters reflecting aflatoxin susceptibility were not significant. However, postdose determinations of albumin, protein, and beta-glucuronidase were significantly related to susceptibility parameters. These data indicate that the four blood components tested cannot be utilized to identify resistant birds within a nonselected population of quail without an aflatoxin challenge; but albumin, protein, and beta-glucuronidase are correlated with resistance when measured during an aflatoxin stress.

Effect of inhibitors of microsomal enzymes on aflatoxin B1-induced cytotoxicity and inhibition of RNA synthesis in isolated rat hepatocytes

Previous studies conducted in this laboratory demonstrated that AFB1 activation and deactivation was effectively inhibited by metyrapone and TCPO in isolated hepatocytes. The present study was undertaken to study the toxic effect of AFB1 on hepatocyte and RNA synthesis, and to assess the influence of the inhibitors on AFB1-induced cytotoxicity and AFB1-inhibited RNA synthesis. AFB1 at 50 microM was toxic and inhibited macromolecular synthesis by greater than 70% at 180 min of incubation whereas at lower concentrations of AFB1 (0.05-10 microM) dose-and time-dependent decreases in cell viability, protein and RNA synthesis were observed. Using [3H]-AFB1 (0.1.5 microM), the uptake and covalent binding of the toxin were also dose-and time-dependent. Initial rates of these processes to reach half-maximum was found to be 0.25 microM AFB1. In cells treated with AFB1 (5 microM) and metyrapone (1.0 mM) or SKF-525A (10 microM), the cell viability was similar to the control and [3H]-uridine incorporation was significantly higher than AFB1 treated cells. AFB1 and TCPO (0.5 mM) treated cells exhibited further decreases in cell viability and RNA synthesis. Results suggest that the binding of AFB1 to DNA and impairment of transcriptional activity may lead to cell death.

Genetic variation of physiological response to aflatoxin in Gallus domesticus

A pedigreed, commercial broiler population of 31 sire families was administered dietary aflatoxin at levels of either 0.0 or 5.0 μg of aflatoxin per g of diet from 7 to 21 days of age and their response assessed by various physiological parameters.Body weight, gain, packed red blood cell volume (PCV). plasma albumin, plasma protein and cholesterol responses were significantly reduced from control values by the 5.0 μg/g aflatoxin diet. Males had greater body weights and gains in both dietary regimes than females. Females had significantly higher PCV, protein, albumin and cholesterol values in the 5.0 μg/g aflatoxin group than their male counterparts. These differences resulted in significant sex × aflatoxin level interactions for these parameters. Coefficients of variation were increased for all parameters measured in the 5.0 μg/g aflatoxin treatment compared to values for the control group. This increase was greatest for plasma protein, albumin, and cholesterol responses. Heritabilities were calculated for all responses within both treatment groups and were found to be increased in all cases by the 5.0 μg/g aflatoxin diet. Highly significant phenotypic correlations were determined between body weight and gain and between plasma albumin and total plasma protein in both treatment groups. High phenotypic correlations among PCV, plasma cholesterol, plasma protein, and plasma albumin were noted in the 5.0 μg/g aflatoxin group. Significant genetic correlations were determined between body weight and gain and between plasma albumin and plasma protein in the control group. Body weight and gain and plasma protein, albumin, cholesterol and PCV were genetically correlated in the 5.0 μg/g aflatoxin group. Genetic correlations calculated across environments for the same traits were high for PCV, body weight and gain and much lower for plasma albumin, plasma protein, and plasma cholesterol.The results of this study demonstrate that genetic variability for resistance to aflatoxin exists in commercial broiler populations. Strong genetic and phenotypic relationships, and high heritabilities associated with plasma albumin and protein suggest their applicability as selection criteria for aflatoxin resistance. Genetic correlation for these traits across dietary environments indicate that responses for aflatoxin resistance should be measured during aflatoxin challenge and suggest that selection for growth and selection for aflatoxin resistance are not antagonistic.

Toxicological significance of dihydrodiol metabolites

Dihydrodiols are often found as the major organic-extractable metabolites of various olefinic or aromatic xenobiotics in many biological samples. Studies on the chemistry of dihydrodiol metabolites have provided insight into the pharmacokinetic behavior and the mode of action of the parent compound. The toxicology of dihydrodiol is more complex than what can be deduced solely on the basis of diminished bioavailability of the epoxide precursor, and the increased hydrophilicity associated with the dihydrodiol moiety. Dihydrodiols can be intrinsically toxic and may even represent metabolically activated species. Some of the dihydrodiol metabolites may still retain sufficient lipophilic character to serve again as substrates for microsomal oxygenases. Because of the tremendous chemical and biological diversity that existed among the various dihydrodiols, more mechanistic studies are needed to examine the toxicological properties of these compounds. It may be premature to conclude dihydrodiol formation as purely a detoxification route for xenobioties.

Inhibition of rat liver aminoacyl-tRNA synthetases in vitro after acute and chronic aflatoxin B1 administration in vivo

In a cell-free system the influence of acute aflatoxin B1 administration on the fractions of the hepatic microsomal translation complex of female rats was examined and compared with the influence of chronic aflatoxin B1 treatment. Polypeptide synthesis determined by [14C]leucine incorporation with the postmitochondrial supernatant was inhibited by 80% 24 h after acute aflatoxin B1 administration whereas inhibition was only 17% in animals 30 weeks after chronic treatment. After acute aflatoxin B1 administration inhibition of protein synthesis was by 67%, mainly on the polysomal level, whereas inhibition with the pH 5 enzyme was only by 22%. After chronic aflatoxin B1 administration inhibition was by 18%, mainly with the pH 5 enzyme and by only 6% on the polysomes. The inhibition by pH 5 enzyme was further investigated with regard to aminoacyl-tRNA synthetases, components of pH 5 enzyme. Leucine-specific aminoacyl-tRNA synthesis was inhibited by 20% 24 h after acute and by 12% 30 weeks after chronic aflatoxin B1 treatment. Aminoacyl-tRNA synthetase activities in a crude synthetase preparation tested with 5 amino acids were found to be noncompetitively inhibited. It is concluded that the inhibition of protein synthesis after acute and chronic aflatoxin B1 administration is in part due to decreased aminoacyl-tRNA levels caused by inhibited aminoacyl-tRNA synthetases.

The effect of aflatoxin B1 on translation in cells in culture

The effect of aflatoxin B1 (AFB1) upon translation has been studied in two cell lines in culture, which differ markedly in their sensitivity to the toxin. It was found that AFB1 predominantly inhibits initiation of translation in both cell lines.

Mechanism of inhibition of hepatic protein synthesis in rats by the carcinogen, methylazoxymethanol acetate

Treatment of rats with the carcinogen, methylazoxymethanol acetate, results in a rapid, marked inhibition of hepatic protein synthesis and disaggregation of polysomes. Studies were undertaken to learn the mechanism by which this carcinogen induces these effects in rat liver. The data show that the inhibition of endogenous protein synthesis is not due to an effect on the high speed supernatant 'factors' but rather at the level of the polysome, and that both free and membrane-bound polysomes are affected. Poly(U)-directed polyphenylalanine synthesis by native ribosomal subunits is greater in preparations isolated from rats treated with carcinogen than it is in controls. Moreover, the native ribosomal subunit fraction from treated livers in response to added rabbit globin mRNA is able to synthesize a protein similar in molecular weight to globin. These studies show that methylazoxymethanol acetate does not induce significant alterations of ribosomal subunits or of initiation factors and suggest that the inhibition of protein synthesis and disaggregation of polysomes may be the results of an alteration of cytoplasmic mRNA, or its association with ribosomes.

Expression of a bacterial gene turned on by a potent carcinogen

Contrary to mutagenesis, lysogenic induction produced by chemical carcinogens occurs in the majority of a population of lysogenic cells. Such a mass effect can therefore be measured at the biochemical level using an E. coli tester strain in which the galactose operon has been put under the negative control of the lambda repressor. In this publication we show that galactokinase synthesis is turned on by aflatoxin B1 metabolites within an hour after treatment of the tester bacteria. Such a biochemical assay provides a useful means for identifying potential chemical carcinogens.

Aflatoxin B1, a selective inhibitor of DNA synthesis in mammalian cells

Aflatoxin B1 in concentrations between 0.01 and 0.1 microgram/ml inhibits DNA synthesis in African green monkey cells in culture, but has little effect on RNA synthesis and no effect on protein synthesis. The drug even at concentrations up to 1.0 microgram/ml does not interfere with DNA repair promoted by ultraviolet irradiation nor does it induce DNA repair. The inhibition of DNA synthesis attains maximum values 3 h after addition of aflatoxin B1 and is irreversible upon removal of the drug. Profiles of pulselabeled DNA sedimented in alkaline sucrose gradients indicate that aflatoxin B1 blocks initiation of replication rather than elongation.

Action of monovalent cations on the biological properties of PR toxin, a mycotoxin from Penicillium roqueforti

PR toxin impairs liver cell metabolism by inhibiting RNA and protein synthesis. In vitro, the drug inhibits the transcription carried out by isolated rat liver nuclei and the translation promoted by polysomes. The action of monovalent cations on the biological activities of PR toxin has been studied. The increased ionic strength due to the presence of salt in the incubation medium, lowers the inhibitory action of PR toxin on in vitro transcription and translation activities; this action is reversible. Besides the overall effect of the ionic strength, ammonium salts possess a specific ability to suppress irreversibly the biological properties of PR toxin (in vivo toxicity and capacity of inhibiting RNA and protein synthesis). The mechanism of this action is discussed.

Effect of aflatoxin B1 on hepatic polyribosomes and protein synthesis in the rat

This study was concerned with the effect of aflatoxin B1 on protein synthesis in rat liver. Specifically, the effect of the administration of aflatoxin B1 (6.0 mg/kg body weight) on hepatic polyribosomes (free and membranebound), protein synthesis in vitro, and initiation factors activity of rats within 3-12 h was investigated. The results revealed that aflatoxin B1 rapidly led to disaggregation of free and membrane-bound polyribosomes, to inhibition of in vitro protein synthesis by both populations of polyribosomes, to little or no effect on the activities of initiation factors, and to defective ribosomes, particularly the 60S ribosomal subnits, of both types of polyribosomes. Comparative studies on the effects of aflatoxin B1 and actinomycin D revealed progressive disaggregation by both hepatic free and membrane-bound polyribosomes after aflatoxin B1 but not only of the free polyribosomes after actinomycin D.