The role of translational inhibitor in ethionine-induced inhibition of protein synthesis

Translational initiation regulators are hypophosphorylated in rat liver during ethionine-mediated ATP depletion

Administration of ethionine to female rats is known to inhibit hepatic protein synthesis by reducing the level of hepatic ATP. Administration of methionine and/or adenine rapidly restores the ATP levels and protein synthesis. The ethionine administration causes a progressive disaggregation of hepatic polysomes, suggesting that the initiation step of protein synthesis is inhibited. Recent studies indicate that changes in initiation are associated with alterations in the phosphorylation states of translational initiation regulators such as eukaryotic initiation factor (eIF) 4E, eIF4E-binding protein 1 (4E-BP1), and the 70-kDa ribosomal protein S6 kinase (S6K1). We found that these initiation regulators are hypophosphorylated in rat liver during ethionine-mediated ATP depletion (60% of the control value). Furthermore, the restoration of the ATP levels by the administration of methionine and adenine brought about a complete recovery of the phosphorylation states of all these regulators. The present data suggest that hypophosphorylation of various initiation regulators represents the primary event in the ethionine-induced breakdown of polysomes and inhibition of protein synthesis in the liver. Possible involvement of mammalian target of rapamycin (mTOR), as a sensor of intracellular ATP level, was also discussed.

Ribonuclease activity of rat liver perchloric acid-soluble protein, a potent inhibitor of protein synthesis

Rat liver perchloric acid-soluble protein (L-PSP) is a potent inhibitor of cell-free protein synthesis; however, its mechanism of action is not known. Here we show that the protein is a unique ribonuclease and that this activity is responsible for the inhibition of translation. The addition of perchloric acid-soluble protein to a rabbit reticulocyte cell-free system at a concentration of 6.2 microM led to an almost complete inhibition of protein synthesis. The kinetics are unlike those of hemin-controlled inhibitor, a protein that acts at the initiation step. The inhibition appears to be due to an endoribonucleolytic activity of perchloric acid-soluble protein because L-PSP directly affects mRNA template activity and induces disaggregation of the reticulocyte polysomes into 80 S ribosomes, even in the presence of cycloheximide. These effects were observed with authentic as well as recombinant L-PSP. Analysis by thin-layer chromatography of [alpha-32P]UTP-labeled mRNA incubated with the protein showed production of the ribonucleoside 3'-monophosphates Ap, Gp, Up, and Cp, providing direct evidence that the protein is an endoribonuclease. When either 5'- or 3'-32P-labeled 5 S rRNA was the substrate, L-PSP cleaved phosphodiester bonds only in the single-stranded regions of the molecule.

Inhibition of the initiation of hepatic protein synthesis during ethionine mediated ATP depletion in vivo: modification to ribosomal subunits, evidence of impaired ternary complex formation and a subcellular redistribution of eIF-2 alpha

Acute ethionine intoxication is known to induce a reversible hepatic injury in female rats by reducing the level of hepatic ATP. The injury indirectly impairs the initiation of hepatic protein synthesis, with resultant polysome disaggregation. Administration of adenine rapidly restores the ATP levels and protein synthesis. Analysis of liver polysome and ribosomal subunits reveals that polysome disaggregation occurs following 3 h of the intoxication, and reaggregation occurs following the administration of adenine. Inactive hepatic ribosomes accumulate as monomers and disomes when analysed by sucrose gradient sedimentation in low-salt buffers. High-salt buffers dissociate the inactive ribosomes into the component 40 S and 60 S subunits. The level of higher density, 1.48 g/cc, 40 S subunit increases during the inhibition of protein synthesis, while the lower density, 1.41 g/cc, 40 S subunit species does not change significantly. Hepatic microsomal and cytosolic extracts examined for their ability to support the formation of the ternary complex of eIF-2-GTP and [35S]Met-tRNAi demonstrate that during acute ethionine intoxication, ternary complex formation in the two extracts decrease 65% and 85%, respectively. These changes are coincident with polysome disaggregation. Administration of adenine to reverse the intoxication restores the ternary complex forming ability of the cytosolic extract, but does not affect the activity of the microsomal salt wash extracts. Mixing experiments indicate the accumulation of an inhibitor of ternary complex formation in the microsomal salt wash fraction. The application of quantitative western blotting demonstrates that the level of antigenic eIF-2 alpha in the microsomal salt wash extract increases 31% during the inhibition. These observations are consistent with the idea that the inhibition of the initiation of hepatic protein synthesis induced by ethionine is mediated by eIF-2 alpha phosphorylation. The latter results in an inhibition of ternary complex formation, redistribution of eIF-2 to the microsome fraction, polysomal disaggregation, and accumulation of inactive ribosomal subunits.

The mechanism of action of the cytotoxic lectin from Phoradendron californicum: the RNA N-glycosidase activity of the protein

A toxic lectin from Phoradendron californicum (PCL) was found to inactivate catalytically 60 S ribosomal subunits of rabbit reticulocytes, resulting in the inhibition of protein synthesis. To study the mechanism of action of PCL, rat liver ribosomes were treated with the toxin and the extracted rRNA was treated with aniline. A fragment containing about 450 nucleotides was released from the 28 S rRNA. Analysis of the nucleotide sequence of the fragment revealed that the aniline-sensitive phosphodiester bond was between A4324 and G4325 of the 28 S rRNA. These results indicate that PCL inactivates the ribosomes by cleaving an N-glycosidic bond at A4324 of 28 S rRNA in the ribosomes as does ricin A-chain.

Enhancement of biotinidase activity in mouse serum by inhibitors of methylation

DL-ethionine increases the activity of liver biotinidase, an enzyme which hydrolyzes biotinylesters and biotinylpeptides. Chronic DL-ethionine feeding increases transiently the activity of biotinidase in mouse and rat liver, after which it remains elevated in the serum. In the present work we show that both isomers of DL-ethionine are equally good enhancers of the liver biotinidase, while, 3-ethylthiopropionate, the toxic metabolite of DL-ethionine, has no effect on the biotinidase activity of either liver or serum. We have also employed two different combinations of inhibitors of the hydrolytic pathway of SAH, a transmethylation product and potent inhibitor of methylation. It was found that these inhibitors (EHNA and Ara-A, 2-deoxycoformycin and adenosine) increase the activity of serum biotinidase as was the case with ethionine. Because SAH does not ethylate biomolecules, these changes in biotinidase activity, which can not be prevented by adenine, biotin or lecithin are most probably related to the inhibition of methylation.

Effect of ethionine on the in vitro synthesis and degradation of mitochondrial translation products in yeast

The effect of ethionine, an amino acid analog of methionine, has been studied in Saccharomyces cerevisiae in relation to cell growth, oxygen consumption, in vitro protein synthesis of mitochondrial translation products (MTPs) and the degradation of those mitoribosomally made proteins by an ATP-dependent process present within the organelle. Ethionine was found to increase the generation time of those cells already committed to cell division and to abolish the initiation of new cell cycles. Oxygen consumption of cultures grown in the presence of the analog was drastically reduced. Ethionine was also found to impair the incorporation of methionine and leucine into mitochondrial translation products, however the synthesis of proteins was not totally blocked and, apparently, mitochondria utilized ethionine as a precursor amino acid. MTPs synthesized by isolated mitochondria in the presence of ethionine were rapidly degraded inside the organelle at a faster rate compared with the normal proteins synthesized under identical conditions in the mitochondria. It is also shown that these in vitro synthesized proteins are degraded by an ATP-stimulated proteolytic system, as has been previously established.