Rapid changes in glucose metabolism following the administration of ethionine: its role in regulating hepatic protein synthesis

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.

Genetic analysis of L-ethionine-mediated induction of alpha-fetoprotein in mice

Two genetic loci regulate hepatic alpha-fetoprotein (AFP) mRNA levels in adult mice. The raf locus controls basal levels and the Rif locus determines the degree of induction during liver regeneration. We have investigated the function of each locus during L-ethionine-mediated AFP induction using adult female mice with different Rif/raf genotypes. A single intraperitoneal injection of L-ethionine (0.5 mg/g body weight) resulted in significant triglyceride accumulation in hepatic parenchymal cells and increased AFP synthesis 48-96 h following injection. Hepatic AFP mRNA levels in Balb/cJ mice (high basal level/high induction level during regeneration) were 10- to 30-fold higher than Balb/cCRBL or C3H/He mice (low basal level/high induction level) following ethionine injection, indicating that raf-mediated differences persisted throughout the course of acute ethionine poisoning. The magnitude of this induction was similar to that seen during carbon tetrachloride-induced regeneration. In contrast, C57BL/6 mice (low basal level/low induction level during regeneration) contained hepatic AFP mRNA levels similar to Balb/cCRBL and C3H/He mice following ethionine injection. Thus, Rif-dependent differences seen during liver regeneration were not seen during acute ethionine poisoning. This leads us to conclude that (1) hepatic AFP mRNA induction by ethionine may not be mediated by the Rif locus if Rif is a transcriptional inducer, or (2) if Rif is a transcriptional repressor, it is inactivated equally in all strains during acute ethionine poisoning, unlike during liver regeneration. Hepatic albumin mRNA levels were not affected by ethionine treatment in vivo. L-Ethionine elevated AFP mRNA levels in primary mouse hepatocyte cultures; however, ethionine treatment also increased albumin mRNA levels in vitro.