Inhibition of rat liver RNA synthesis by aflatoxin B

Long term effect of aflatoxin B(1) on lipid peroxidation in rat liver and kidney: effect of picroliv and silymarin

Aflatoxin B(1) (AFB(1)) is a potent hepatotoxic and hepatocarcinogenic mycotoxin. The mechanism of cellular damage caused by AFB(1) has not been fully elucidated. Lipid peroxidation is one of the main manifestations of oxidative damage and has been found to play an important role in the toxicity and carcinogenesis of many carcinogens. The present investigation aims at assessing the comparative antioxidant effect of picroliv, a standardized iridoid glycoside fraction of Picrorhiza kurroa and silymarin, a well known standard hepatoprotective, on aflatoxin B(1) induced lipid-peroxidation in rat liver and kidney. Marked increases in lipid peroxide levels and a concomitant decrease in enzymic antioxidant levels were observed in aflatoxin B(1) (2 mg/kg, i.p) -toxicated rats, while drug (picroliv and silymarin both) treatment reversed the condition to near normal levels. The effects of picroliv and silymarin were comparable.

Isolation and characterization of the versicolorin B synthase gene from Aspergillus parasiticus. Expansion of the aflatoxin b1 biosynthetic gene cluster

Versicolorin B synthase catalyzes the side chain cyclization of racemic versiconal hemiacetal to the bisfuran ring system of(-)-versicolorin B, an essential transformation in the aflatoxin biosynthetic pathway of Aspergillus parasiticus. The dihydrobisfuran is key to the mutagenic nature of aflatoxin B1. The protein, which shows 58% similarity and 38% identity with glucose oxidase from Aspergillus niger, possesses an amino-terminal sequence homologous to the ADP-binding region of other flavoenzymes. However, this enzyme does not require flavin or nicotinamide cofactors for its cyclase activity. The 643-amino acid native enzyme contains three potential sites for N-linked glycosylation, Asn-Xaa-Thr or Asn-Xaa-Ser. The cDNA and genomic clones of versicolorin B synthase were isolated by screening the respective libraries with random-primed DNA probes generated from an exact copy of an internal vbs sequence. This probe was created through polymerase chain reaction by using nondegenerate polymerase chain reaction primers derived from the amino acid sequences of peptide fragments of the enzyme. The 1985-base genomic vbs DNA sequence is interrupted by one intron of 53 nucleotides. Southern blotting, nucleotide sequencing, and detailed restriction mapping of the vbs-containing genomic clones revealed the presence of omtA, a methyltransferase active in the biosynthesis, 3.3 kilobases upstream of vbs and oriented in the opposite direction from vbs. The presence of omtA in close proximity to vbs supports the theory that the genes encoding the aflatoxin biosynthetic enzymes in A. parasiticus are clustered.

Interaction of aflatoxin B1 with the rat monooxygenase system and relationships to its acute toxic effect

The effect of aflatoxin B1 (AFB1) on the liver microsomal and nuclear mixed function oxidase system (MFO) of adult male rats was studied at oral doses of 1 and 3 mg/kg. At first both doses increased the activities and concentrations of all P450 components followed. The maximum values were observed between 24 and 48 h and were dose- and enzyme-dependent. After 72 h the values dropped to 15-55% of the initial values. Inhibition of the MFO system lasted even after 120 h, when a trend to return to the normal values was already noticeable. We assume that AFB1 acts as an inductor of the monooxygenase system and the reactive electrophilic intermediate(s) bind irreversibly to or autocatalytically inactivate P450.

Contribution of the glutathione S-transferases to the mechanisms of resistance to aflatoxin B1

The harmful effects of Aflatoxin B1 (AFB1) are a consequence of it being metabolized to AFB1-8,9-epoxide, a compound that serves as an alkylating agent and mutagen. The toxicity of AFB1 towards different cells varies substantially; sensitivity can change significantly during development, can be modulated by treatment with xenobiotics and is decreased markedly in preneoplastic lesions as well as in tumors. Three types of resistance, namely intrinsic, inducible and acquired, can be identified. The potential resistance mechanisms include low capacity to form AFB1-8,9-epoxide, high detoxification activity, increase in AFB1 efflux from cells and high DNA repair capacity. Circumstantial evidence exists that amongst these mechanisms the glutathione S-transferases, through their ability to detoxify AFB1-8,9-epoxide, play a major role in determining the sensitivity of cells to AFB1.

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.

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.

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.