Inter-organ protein and carbohydrate metabolic relationships during sepsis: necessary evils or uncanny coincidences?

Parenteral amino acid intakes in critically ill children: a matter of convenience

BACKGROUND

Parenteral and enteral amino acid requirements for nutrition balance and function have not been defined in critically ill children or adults. In addition to playing a role in protein synthesis, amino acids trigger signaling cascades that regulate various aspects of fuel and energy metabolism and serve as precursors for important substrates. Amino acids can also be toxic. In this study, parenteral intakes of essential and nonessential amino acids (EAAs and NEAAs) supplied to critically ill children were assessed as an initial step for further studies aimed at establishing parenteral amino acid requirements.

METHODS

A retrospective review was conducted to assess intakes of parenteral amino acid for 116 critically ill children, and these intakes were compared with EAA intakes recommended by the Institute of Medicine. Because there are no recommended intakes for NEAA, NEAA intakes were compared with mixed muscle protein content in the older children and breast milk amino acid content in the infants.

RESULTS

Parenteral EAAs were provided in amounts that exceeded recommended intakes for healthy children, except for phenylalanine and methionine, which although excessive, were given in less generous amounts. NEAAs were supplied in lower or higher amounts than the content of mixed muscle proteins or breast milk. Parenteral amino acid formulas are limited in taurine, glutamine, and asparagine despite the fact that inflammatory/immune proteins are rich in these amino acids.

CONCLUSIONS

Amino acid composition of parenteral formulas is variable and lacks scientific support. Parenteral amino acid intakes should be based on measured requirements to maintain nutrition and functional balance and on knowledge of toxicity.

IGF-I stimulates protein synthesis in skeletal muscle through multiple signaling pathways during sepsis

Chronic septic abscess formation causes an inhibition of protein synthesis in gastrocnemius not observed in rats with a sterile abscess. Inhibition is associated with an impaired mRNA translation initiation that can be ameliorated by elevating IGF-I but not insulin. The present study investigated the ability of IGF-I signaling to stimulate protein synthesis in gastrocnemius by accelerating mRNA translation initiation. Experiments were performed in perfused hindlimb preparations from rats 5 days after induction of a septic abscess. Protein synthesis in gastrocnemius from septic rats was accelerated twofold by the addition of IGF-I (10 nM) to perfusate. IGF-I increased the phosphorylation of translation repressor 4E-binding protein-1 (4E-BP1). Hyperphosphorylation of 4E-BP1 in response to IGF-I resulted in its dissociation from the inactive eukaryotic initiation factor (eIF) 4E.4E-BP1 complex. Assembly of the active eIF4F complex (as assessed by the association eIF4G with eIF4E) was increased twofold by IGF-I in the perfusate. In addition, phosphorylation of eIF4G and ribosomal protein S6 kinase-1 (S6K1) was also enhanced by IGF-I. Activation of mammalian target of rapamycin, an upstream kinase implicated in phosphorylating both 4E-BP1 and S6K1, was also observed. Thus the ability of IGF-I to accelerate protein synthesis during sepsis may be related to a stimulation of signaling to multiple steps in translation initiation with an ensuing increased phosphorylation of eIF4G, eIF4E availability, and S6K1 phosphorylation.

Diminished ERK 1/2 and p38 MAPK phosphorylation in skeletal muscle during sepsis

Sepsis induces weight loss and the loss of skeletal muscle proteins, in part through an inhibition of protein synthesis secondary to an inhibition of the key steps controlling mRNA translation in skeletal muscle. We have previously shown that sepsis decreases the phosphorylation of eIF4E. The present study examines the phosphorylation of Erk 1/2 MAPK and p38 MAPK in skeletal muscle of rats with a chronic (5-day) intra-abdominal septic abscess. Mnk1 catalyzes the phosphorylation of eIF4E, and Mnk1 is activated by phosphorylation via Erk1/2 MAPK and p38 MAPK. Sepsis resulted in a significant decrease in the steady-state phosphorylation of Erk 1/2 and p38 MAPKs compared with sterile inflammation. To examine the mediators responsible for decreased phosphorylation of Erk 1/2 and p38 MAPKs, rats were treated with TNF binding protein (TNFbp) or infused for 24 h with TNF. Treatment of septic rats with TNFbp resulted in an increase in the phosphorylation of both Erk 1/2 and p38 MAPKs in skeletal muscle. This was associated with enhanced phosphorylation of eIF4E. In contrast, constant intravenous infusion of TNF-alpha for 24 h resulted in a complete inhibition of p38 MAPK phosphorylation while Erk 1/2 MAPK phosphorylation was increased. The net effect was a modest increase in eIF4E phosphorylation. The results suggest altered regulation of Erk 1/2 and p38 MAPK signal translation pathways by endogenously produced TNF, or some compound dependent on TNF may modulate, in part, the phosphorylation state of eIF4E in skeletal muscle during sepsis.

NOS3 is involved in the increased protein and arginine metabolic response in muscle during early endotoxemia in mice

Sepsis is a severe catabolic condition. The loss of skeletal muscle protein mass is characterized by enhanced release of the amino acids glutamine and arginine, which (in)directly affects interorgan arginine and the related nitric oxide (NO) synthesis. To establish whether changes in muscle amino acid and protein kinetics are regulated by NO synthesized by nitric oxide synthase-2 or -3 (NOS2 or NOS3), we studied C57BL6/J wild-type (WT), NOS2-deficient (NOS2-/-), and NOS3-deficient (NOS3-/-) mice under control (unstimulated) and lipopolysaccharide (LPS)-treated conditions. Muscle amino acid metabolism was studied across the hindquarter by infusing the stable isotopes L-[ring-2H5]phenylalanine, L-[ring-2H2]tyrosine, L-[guanidino-15N2]arginine, and L-[ureido-13C,2H2]citrulline. Muscle blood flow was measured using radioactive p-aminohippuric acid dilution. Under baseline conditions, muscle blood flow was halved in NOS2-/- mice (P < 0.1), with simultaneous reductions in muscle glutamine, glycine, alanine, arginine release and glutamic acid, citrulline, valine, and leucine uptake (P < 0.1). After LPS treatment, (net) muscle protein synthesis increased in WT and NOS2-/- mice [LPS vs. control: 13 +/- 3 vs. 8 +/- 1 (SE) nmol.10 g(-1).min(-1) (WT), 18 +/- 5 vs. 7 +/- 2 nmol.10 g(-1).min(-1) (NOS2-/-); P < 0.05 for LPS vs. control]. This response was absent in NOS3-/- mice (LPS vs. control: 11 +/- 4 vs. 10 +/- 2 nmol.10 g(-1).min(-1)). In agreement, the increase in muscle arginine turnover after LPS was also absent in NOS3-/- mice. In conclusion, disruption of the NOS2 gene compromises muscle glutamine release and muscle blood flow in control mice, but had only minor effects after LPS. NOS3 activity is crucial for the increase in muscle arginine and protein turnover during early endotoxemia.

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.

Phosphorylation of eukaryotic initiation factor eIF2Bepsilon in skeletal muscle during sepsis

We reported that the inhibition of protein synthesis in skeletal muscle during sepsis correlated with reduced eukaryotic initiation factor eIF2B activity. The present studies define changes in eIF2Bepsilon phosphorylation in gastrocnemius of septic animals. eIF2B kinase activity was significantly elevated 175% by sepsis compared with sterile inflammation, whereas eIF2B phosphatase activity was unaffected. Phosphorylation of eIF2Bepsilon-Ser(535) was significantly augmented over 2-fold and 2.5-fold after 3 and 5 days and returned to control values after 10 days of sepsis. Phosphorylation of glycogen synthase kinase-3 (GSK-3), a potential upstream kinase responsible for the elevated phosphorylation of eIF2Bepsilon, was significantly reduced over 36 and 41% after 3 and 5 days and returned to control values after 10 days of sepsis. The phosphorylation of PKB, a kinase thought to directly phosphorylate and inactivate GSK-3, was significantly reduced approximately 50% on day 3, but not on days 5 or 10, postinfection compared with controls. Treatment of septic rats with TNF-binding protein prevented the sepsis-induced changes in eIF2Bepsilon and GSK-3 phosphorylation, implicating TNF in mediating the effects of sepsis. Thus increased phosphorylation of eIF2Bepsilon via activation of GSK-3 is an important mechanism to account for the inhibition of skeletal muscle protein synthesis during sepsis. Furthermore, the study presents the first demonstration of changes in eIF2Bepsilon phosphorylation in vivo.

A global approach to energy metabolism in an experimental model of sepsis

Disturbances in energy metabolism during sepsis are not clearly understood. The aim of the study was to globally assess the energy drive in septic rat myocytes, studying both glycolysis rates and mitochondrial maximal activities together, using recent in vitro techniques. Measurements were assessed before (H0) and 4 h after sepsis induction (H4). Hyperlactatemia was observed in all septic animals ([lactate] = 1.2 +/- 0.3 mmol/L at H0 versus 3.3 +/- 0.6 mmol/L at H4; p < 0.001). An enhanced glycolysis rate was observed in both aerobic ( J(A) = 7.2 +/- 0.9 at H0 versus 18.2 +/- 4.1 nmol glucose/min/g at H4; p < 0.05) and anaerobic ( J(B) = 7.5 +/- 1.2 at H0 versus 15.4 +/- 3.4 micromol glucose/min/g at H4; p < 0.05) fluxes, associated with a selective significant pyruvate-malate-dependent oxygen consumption rate decrease (V O(2)-PM = 0.144 +/- 0.008 at H0 versus 0.113 +/- 0.007 micromol O(2)/h/mg at H4; p < 0.05). This oxygen consumption decrease can be interpreted either as a complex I and/or a complex I-ubiquinone relation alteration. Our results are consistent with the hypothesis that an altered mitochondrial function during sepsis is responsible, at least in part, for hyperlactatemia, which is thus a consequence of an increased glycolysis rate.

Inter-organ substrate exchanges in the critically ill

Metabolic inter-organ exchange is a major field of research for improving the treatment of the critically ill. Adapting regional blood flows is the first regulatory step, although the relationships between hypoperfusion and metabolic disorders are matter of controversy. Metabolic steady state results from a vast inter-organ interplay and several nutrients or metabolites are signalling molecules in the regulation of gene transcription. Inter- or intra-organ substrate recycling shares or delays the mandatory need for aerobic ATP synthesis in some conditions. Nitrogen metabolism is highly compartmentalised in an inter-organ co-operation and liver, muscle, kidney and gut are the most important organs. By remodelling the amino acid mixture delivered to peripheral cells after intestinal absorption, the liver plays a determinant role in whole body protein synthesis. Albumin turnover increases after brain injury. Since the location of synthesis is different to that of breakdown this turnover can be viewed as an inter-organ exchange. The metabolic side of pH homeostasis is also an inter-organ exchange mainly shared by liver, kidney and muscle.

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

BACKGROUND

Cytokines are thought to play a role in the stimulation of protein synthesis in liver during inflammation and sepsis. We previously showed that administration of tumor necrosis factor-binding protein (TNFbp) prevents the sepsis-induced inhibition of protein synthesis in skeletal muscle. The purpose of the present set of experiments was to investigate the effect of TNFbp on hepatic protein synthesis and its ability to modulate the mechanisms responsible for increased hepatic protein synthesis during chronic (5-day) intraabdominal sepsis.

MATERIALS AND METHODS

We examined the effects of TNFbp on hepatic protein synthesis during sepsis in four groups of rats: control, control + TNFbp, septic, and septic + TNFbp. Saline (1.0 ml) or TNFbp (1 mg/kg, 1.0 ml) was injected daily starting 4 h prior to the induction of sepsis. The effect of sepsis and TNFbp administration on hepatic protein synthesis in vivo was examined 5 days later.

RESULTS

Sepsis increased the rate of protein synthesis by 35% relative to controls. Accelerated rates of protein synthesis were accompanied by increased total RNA content, eukaryotic initiation factor (eIF) 2alpha content, and phosphorylation of p70S6 kinase. Injection of TNFbp into septic rats for 5 days did not diminish the sepsis-induced stimulation of hepatic protein synthesis. Compared with controls, septic rats treated with TNFbp also showed elevated total RNA content, elF2alpha content, and phosphorylation of p70S6 kinase. No significant differences in any of the parameters measured were observed between untreated and TNFbp-treated septic rats. Treatment of control animals with TNFbp for 5 days was without effect on any of the parameters examined.

CONCLUSIONS

TNFbp did not prevent the sepsis-induced stimulation of hepatic protein metabolism or modulate the septic-induced changes in factors regulating protein synthesis. Global rates of protein synthesis in livers from septic rats are accelerated by increases in the abundance or activity of components of translational apparatus.