Aflatoxin inhibition of template activity of rat liver chromatin

Aflatoxin B₁⁻Formamidopyrimidine DNA Adducts: Relationships between Structures, Free Energies, and Melting Temperatures

Thermal stabilities of DNA duplexes containing Gua (g), α- (a) or β-anomer of formamidopyrimidine-N7-9-hydroxy-aflatoxin B₁ (b) differ markedly (T_m: a<g<b), but the underlying molecular origin of this experimentally observed phenomenon is yet to be identified and determined. Here, by employing explicit-solvent molecular dynamics simulations coupled with free-energy calculations using a combined linear-interaction-energy/linear-response-approximation approach, we explain the quantitative differences in Tm in terms of three structural features (bulkiness, order, and compactness) and three energetical contributions (non-polar, electrostatic, and preorganized-electrostatic), and thus advance the current understanding of the relationships between structures, free energies, and thermal stabilities of DNA double helices.

Immunosuppression due to chronic ochratoxicosis in broiler chicks

Crystalline ochratoxin A (OA) was added to the feed of broiler chicks at 0.5 ppm and 2.0 ppm, and humoral and cell-mediated immunity (CMI) were studied. CMI was assessed by skin sensitivity testing, graft versus host (GVH) reaction and T lymphocyte count. Humoral immunity was examined by measuring the haemagglutinin (HA) response to sheep RBC (SRBC). In addition, total lymphocyte counts, total serum protein, albumin and globulin were determined and the phagocytic activity of splenic macrophages was measured in the nitroblue tetrazolium test (NBT). The weights of lymphoid organs were also recorded at post-mortem examination of the birds. Highly significant reductions in CMI were indicated by diminished skin sensitivity, GVH reactions and T lymphocyte counts. On the other hand, only the overall HA titres differed significantly between the various treatment groups. Total lymphocyte counts, total serum protein, serum albumin and serum globulin were significantly depressed on the 21st day of intoxication. The number of NBT-positive cells was drastically reduced in both the intoxicated groups compared with controls (P less than 0.05) and the weights of thymus, bursa of Fabricius and spleen of intoxicated birds were significantly reduced. The study illustrated the immunosuppressive effects of ochratoxicosis in broiler chicks.

Mutagenicity and cytotoxicity of the carcinogen-mutagen aflatoxin B1 in Streptococcus pneumoniae (Pneumococcus) and Salmonella typhimurium: dependence on DNA repair functions

Strains R6, R6x and R6uvr-1 of Streptococcus pneumoniae (Pneumococcus) are sensitive to the cytotoxic effects of the mutagen/carcinogen aflatoxin B1 (AFB1). R6uvr-1 is more prone to the cytotoxic effects of AFB1 than the repair-proficient parental strain, R6. The same differential susceptibility of strains R6, R6x and R6uvr-1 was observed when UV light replaced metabolically activated AFB1. All pneumococcal strains were immutable by AFB1. AFB1 mutagenesis in Salmonella typhimurium strains was dependent on a functional RecA gene product. The enhancing effects of delta uvrB and plasmid pKM101 were found to be additive. Data presented are consistent with the following: (i) AFB1 toxic effects are due mainly to DNA binding of AFB1; (ii) AFB1 mutagenesis is dependent on error-prone DNA repair; (iii) Pneumococcus lacks an active error-prone (SOS) DNA-repair system.

Synthesis of transfer RNA in rat liver after acute and chronic aflatoxin B1 administration

Synthesis of rat liver tRNA following acute and chronic administration of aflatoxin B1 was compared to protein synthesis. Twenty hours after acute administration of aflatoxin B1 total RNA synthesis was inhibited by 91% and tRNA synthesis by 77% while protein synthesis as assayed in vitro was inhibited by 75%. In the chronic (long-term) experiment at 7, 18 and 30 weeks after administration of aflatoxin B1 no inhibition of total RNA and tRNA synthesis was observed but protein synthesis in the cell-free system was inhibited by 16%. Thus, the inhibitory effects on protein synthesis observed in the long-term experiments are not correlated with the inhibition of tRNA synthesis.

Some studies of the effects of aflatoxin B1 in vivo and in vitro on nucleic acid synthesis in rat and mouse

The mouse compared with the rat, is more resistent to the acute toxic action of aflatoxin B1 and is refractory to its hepatocarcinogenic properties. Aflatoxin B1 inhibits DNA synthesis more strongly than RNA synthesis in the rat, and both nucleic acid syntheses more strongly in rat than in the mouse. Mouse hepatic microsomes, like those of the rat, are capable of metabolizing aflatoxin B1 in vitro in the presence of NADPH, to an active form which binds to DNA both in solution and in intact nuclei and also inhibits nuclear RNA synthesis. Non NADPH-dependent binding of aflatoxin B1 to nuclei is not effective in inhibiting RNA polymerase and is largely removed by washing with lipid solvents. Mouse nuclear RNA polymerases particularly Mn 2+ (NH4)2SO4 primed acitivity are more resistant to inhibition in vitro by activated aflatoxin B1 than are the corresponding enzyme activities in rat liver nuclei. This would appear to be due to the bound aflatoxin B1 being less efficient in the case of the mouse nucleus, in inhibiting RNA synthesis. Mouse liver slices exhibit a much lesser degree of inhibition of RNA synthesis by aflatoxin B1 than do rat liver slices. Accompanying this is a lower level of binding of aflatoxin B1 to subcellular particulate fractions in the mouse liver slice compared to the rat, this disparity being most marked in the case of the nuclear fraction. The suggestion is made that the resistance of RNA synthesis in the mouse liver, to aflatoxin B1, and perhaps also resistance to its toxicity, is dependent, not on a lower capacity to activate the toxin, but (a) on a less efficient inhibition of RNA synthesis by nuclear bound toxin, and (b) a detoxifying mechanism at least partially situated in the cytosol fraction.

Ultrastructural alterations and modifications of nuclear RNA of rat liver by the combined action of thioacetamide and aflatoxin

When aflatoxin is administered to thioacetamide-treated rats, the synthesis of nuclear RNA not only stops but the RNA that had accumulated in the nuclei by thioacetamide action disappears, probably by degradation "in situ" as none appears in the cytoplasm. Morphologically, the lesions provoked by aflatoxin add to those caused by thioacetamide. In the gigantic nucleoli that develop upon exposure to thioacetamide, aflatoxin provokes atypical segregations that result in the formation of larger and larger spaces in the nucleoli.

Induction of mutations in DNA-repair deficient bacteria by a liver microsomal metabolite of aflatoxin B1

Certain strains of Salmonella typhimurium and Escherichia coli, particularly those which are very sensitive to u.v. light, are killed when incubated with rat liver mixed function oxidases and aflatoxin B(1). UvrA or recA strains of E. coli are more susceptible than the wild-type strain, while the double mutant uvrA recA is the most sensitive strain yet tested. The aflatoxin B(1) metabolite is also able to induce reverse mutations in 2 histidine auxotrophic strains of S. typhimurium, one strain of which is reverted specifically by frame shift mutagens and the other by agents inducing base pair substitutions.Pretreatment of rats with either 3-methylcholanthrene or benzo(a)pyrene, both inducers of liver microsomal mixed function oxidases, did not alter the amount of lethal aflatoxin B(1) metabolite formed, whereas an increase was observed after phenobarbitone pretreatment. Addition of the nucleophiles methionine, cysteine, glutathione, sodium thiosulphate or sodium sulphide, or the epoxide hydrase inhibitor, cyclohexene oxide to the toxicity assay medium did not alter bacterial killing by the aflatoxin B(1) metabolite. 2,3-Dimercaptopropanol had some protective action.Toxic metabolites were also formed when 5-methoxysterigmatocystin, O-methylsterigmatocystin, parasiticol or versicolorin A, but not vericolorin B, were incubated with mixed function oxidases. The relationship between the metabolite of aflatoxin B(1) lethal to bacteria and that which initiates liver cancer is discussed.

The effect of aflatoxin B 1 on normal and cortisol-stimulated rat liver ribonucleic acid synthesis

1. Aflatoxin B(1), administered in vivo, inhibits the incorporation of [(14)C]orotic acid in vivo into rat liver nuclei, and also inhibits both Mg(2+)- and Mn(2+)-dependent RNA polymerase activities in nuclei assayed in vitro. 2. Aflatoxin B(1) inhibits the cortisol-induced increase in incorporation of [(14)C]leucine in vivo, but does not affect the control value of this activity. 3. Aflatoxin B(1) administered in vivo inhibits the increase in nuclear Mg(2+)-dependent RNA polymerase activity, assayed in vitro, which results from the treatment with cortisol. 4. Adrenalectomy causes a decrease in Mg(2+)-dependent RNA polymerase activity. The effect on this enzymic activity of adrenalectomy plus treatment with aflatoxin B(1) is no greater than that of treatment with aflatoxin B(1) alone. 5. These results suggest that the inhibition of cortisol-stimulated biochemical pathways by aflatoxin B(1) is due to an inhibition of cortisol-stimulated RNA synthesis. 6. The cytoplasmic action of aflatoxin is thought to be due to a competition for receptor sites on the endoplasmic reticulum between steroid hormones and aflatoxin B(1). No evidence was obtained for a similar competition for nuclear receptor sites between [(3)H]cortisol and aflatoxin B(1). 7. No differences were observed between the activities of RNA polymerase preparations solubilized from control or aflatoxin-inhibited nuclei. 8. No differences in ;melting' profiles were observed between DNA and chromatin preparations isolated from control nuclei or from aflatoxin-inhibited nuclei. 9. It is suggested that aflatoxin B(1) exerts its effect on RNA polymerase by decreasing the template capacity of the chromatin and that the aflatoxin ;target' area of the chromatin includes that region which is stimulated by cortisol. This process, however, does not involve inhibiting the movement of cortisol from the outside of the hepatic cell to the nuclear chromatin.