Acute and chronic ethanol consumption differentially impact pathways limiting hepatic protein synthesis

Abstract

This review identifies the various pathways responsible for modulating hepatic protein synthesis following acute and chronic alcohol intoxication and describes the mechanism(s) responsible for these changes. Alcohol intoxication induces a defect in global protein synthetic rates that is localized to impaired translation of mRNA at the level of peptide-chain initiation. Translation initiation is regulated at two steps: formation of the 43S preinitiation complex [controlled by eukaryotic initiation factors 2 (eIF2) and 2B (eIF2B)] and the binding of mRNA to the 40S ribosome (controlled by the eIF4F complex). To date, alcohol-induced alterations in eIF2 and eIF2B content and activity are best investigated. Ethanol decreases eIF2B activity when ingested either acutely or chronically. The reduced eIF2B activity most likely is a consequence of twofold increased phosphorylation of the α-subunit of eIF2 on Ser51following acute intoxication. The increase in eIF2α phosphorylation after chronic alcohol consumption is the same as that induced by acute ethanol intoxication, and protein synthesis is not further reduced by long-term alcohol ingestion despite additional reduced expression of initiation factors and elongation factors. eIF2α phosphorylation alone appears sufficient to maximally inhibit hepatic protein synthesis. Indeed, pretreatment with Salubrinal, an inhibitor of eIF2α(P) phosphatase, before ethanol treatment does not further inhibit protein synthesis or increase eIF2α phosphorylation, suggesting that acute ethanol intoxication causes maximal eIF2α phosphorylation elevation and hepatic protein synthesis inhibition. Ethanol-induced inhibition of hepatic protein synthesis is not rapidly reversed by cessation of ethanol consumption. In conclusion, sustained eIF2α phosphorylation is a hallmark of excessive alcohol intake leading to inhibition of protein synthesis. Enhanced phosphorylation of eIF2α represents a unique response of liver to alcohol intoxication, because the ethanol-induced elevation of eIF2α(P) is not observed in skeletal muscle or heart.