Characterization of monosomes produced by aflatoxin B1

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.

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.