Effects of tumor necrosis factor-binding protein on hepatic protein synthesis during chronic sepsis

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

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 alpha-subunit of eIF2 on Ser(51) following acute intoxication. The increase in eIF2alpha 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. eIF2alpha phosphorylation alone appears sufficient to maximally inhibit hepatic protein synthesis. Indeed, pretreatment with Salubrinal, an inhibitor of eIF2alpha(P) phosphatase, before ethanol treatment does not further inhibit protein synthesis or increase eIF2alpha phosphorylation, suggesting that acute ethanol intoxication causes maximal eIF2alpha 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 eIF2alpha phosphorylation is a hallmark of excessive alcohol intake leading to inhibition of protein synthesis. Enhanced phosphorylation of eIF2alpha represents a unique response of liver to alcohol intoxication, because the ethanol-induced elevation of eIF2alpha(P) is not observed in skeletal muscle or heart.

Effect of central administration of interleukin-1 receptor antagonist on protein synthesis in skeletal muscle, kidney, and liver during sepsis

Inflammatory cytokines may mediate the host response to infection via central nervous system (CNS), endocrine, and/or paracrine pathways. The purpose of the present study was to determine whether intracerebroventricular (ICV) infusion of interleukin-1 receptor antagonist (IL-1ra) influences the effects of sepsis on protein metabolism in peripheral organs (skeletal muscle, kidney, and liver). A constant ICV infusion of IL-1ra (100 microg/h) or saline was begun immediately before the induction of sepsis or sterile inflammation and continued for 5 days. ICV infusion of IL-1ra did not alter protein metabolism in animals with a sterile abscess. Sepsis reduced muscle weight, protein content, and rates of protein synthesis in gastrocnemius. ICV infusion of IL-1ra attenuated the sepsis-induced loss of muscle mass and protein and the inhibition of protein synthesis in gastrocnemius by augmenting the translational efficiency. Similar results were observed in kidney, with respect to kidney weight, total protein, rates of protein synthesis, and translational efficiency. However, central infusion of IL-1ra did result in a small (12%) increase in the renal RNA content in either sterile or septic abscess rats. In liver, ICV infusion of IL-1ra prevented the sepsis-induced inhibition of protein synthesis and reduction in translational efficiency. These results suggest that central administration IL-1ra can modulate protein metabolism in peripheral organs during sepsis by preventing the sepsis-induced defects in the translational efficiency.

Endotoxin induces differential regulation of mTOR-dependent signaling in skeletal muscle and liver of neonatal pigs

In the present study, differential responses of regulatory proteins involved in translation initiation in skeletal muscle and liver during sepsis were studied in neonatal pigs treated with lipopolysaccharide (LPS). LPS did not alter eukaryotic initiation factor (eIF) 2B activity in either tissue. In contrast, binding of eIF4G to eIF4E to form the active mRNA-binding complex was repressed in muscle and enhanced in liver. Phosphorylation of eIF4E-binding protein, 4E-BP1, and ribosomal protein S6 kinase, S6K1, was reduced in muscle during sepsis but increased in liver. Finally, changes in 4E-BP1 and S6K1 phosphorylation were associated with altered phosphorylation of the protein kinase mammalian target of rapamycin (mTOR). Overall, the results suggest that translation initiation in both skeletal muscle and liver is altered during neonatal sepsis by modulation of the mRNA-binding step through changes in mTOR activation. Moreover, the LPS-induced changes in factors that regulate translation initiation are more profound than previously reported changes in global rates of protein synthesis in the neonate. This finding suggests that the initiator methionyl-tRNA-rather than the mRNA-binding step in translation initiation may play a more critical role in maintaining protein synthesis rates in the neonate during sepsis.