Amino acids augment muscle protein synthesis in neonatal pigs during acute endotoxemia by stimulating mTOR-dependent translation initiation

Abstract

In skeletal muscle of adults, sepsis reduces protein synthesis by depressing translation initiation and induces resistance to branched-chain amino acid stimulation. Normal neonates maintain a high basal muscle protein synthesis rate that is sensitive to amino acid stimulation. In the present study, we determined the effect of amino acids on protein synthesis in skeletal muscle and other tissues in septic neonates. Overnight-fasted neonatal pigs were infused with endotoxin (LPS, 0 and 10 g ⅐ kg Ϫ1 ⅐ h Ϫ1 ), whereas glucose and insulin were maintained at fasting levels; amino acids were clamped at fasting or fed levels. In the presence of fasting insulin and amino acids, LPS reduced protein synthesis in longissimus dorsi (LD) and gastrocnemius muscles and increased protein synthesis in the diaphragm, but had no effect in masseter and heart muscles. Increasing amino acids to fed levels accelerated muscle protein synthesis in LD, gastrocnemius, masseter, and diaphragm. LPS stimulated protein synthesis in liver, lung, spleen, pancreas, and kidney in fasted animals. Raising amino acids to fed levels increased protein synthesis in liver of controls, but not LPS-treated animals. The increase in muscle protein synthesis in response to amino acids was associated with increased mTOR, 4E-BP1, and S6K1 phosphorylation and eIF4G-eIF4E association in control and LPS-infused animals. These findings suggest that amino acids stimulate skeletal muscle protein synthesis during acute endotoxemia via mTOR-dependent ribosomal assembly despite reduced basal protein synthesis rates in neonatal pigs. However, provision of amino acids does not further enhance the LPS-induced increase in liver protein synthesis. growth; sepsis; mammalian target of rapamycin; eukaryotic initiation factor 4G; ribosomal protein S6 kinase THE SYSTEMIC INFLAMMATORY RESPONSE and sepsis syndromes elicited in the host in response to a bacterial pathogen or its products (35) are associated with hypermetabolism, increased oxygen consumption and energy expenditure, activation of peripheral protein catabolism, and alteration in the flux of metabolic substrates between skeletal muscle and liver (4, 39). Derangements in skeletal muscle glucose and amino acid metabolism evoked by sepsis result from the body's attempt to maintain the balance between peripheral substrate stores and the increased metabolic demands required to activate the immune system and the stress response (4, 37). These adaptative responses are different from those in the normal fasting person in that the immune response is the drive for the metabolic enhancement (11,59). In an attempt to establish homeostasis, amino acids are released from peripheral tissues and shifted to the liver for promotion of hepatic protein synthesis, gluconeogenesis, and urea synthesis (24, 25). Plasma amino acid concentrations in patients with sepsis or disease are lower than in persons without sepsis (17,30,49), and attempts have been made to use the extent of this difference as a marker of the severity and prognosis of the ...

Preserved protein synthesis in the heart in response to acute fasting and chronic food restriction despite reductions in liver and skeletal muscle

“…We observed larger reductions in the phosphorylation of 4E-BP1 in liver and skeletal muscle compared with heart (i.e., an ϳ4-fold decreases vs. an ϳ2-fold decrease, in calorierestricted vs. ad libitum-fed rats, respectively). The observed changes in the phosphorylation status of 4E-BP1 by food restriction in the three tissues are in agreement with dietary studies in whole animals (7,9,24,31,37). However, the magnitude and direction of changes in the phosphorylation of eIF2␣ in liver and skeletal muscle vs. heart (i.e., an ϳ2-fold increases vs. an ϳ1.5-fold decrease in calorie-restricted vs. ad libitum-fed rats, respectively) were surprisingly different, suggesting that the heart sustains protein synthesis during food restriction via molecular mechanisms that do not increase eIF2␣ phosphorylation (48).…”

“…In addition, hyperphosphorylation of 4E-BP1, releases eIF4E, thus increasing cap-dependent translation initiation. Stress conditions (e.g., nutrient limitation) have been associated with the phosphorylation of eIF2 (4,48) and the dephosphorylation of 4E-BP1, which would lead to a reduction in protein synthesis (19,24,31). It isunknown, however, whether/how the molecular control of protein synthesis would respond in different tissues when exposed to the same milieu.…”

Development aggravates the severity of skeletal muscle catabolism induced by endotoxemia in neonatal pigs

“…In healthy neonatal pigs, insulin augments whole body AA disposal and induces a fall in total AA concentrations to allow protein deposition in muscle, but it does not affect liver protein synthesis (24,27). In contrast, LPS increases insulin and decreases BCAA plasma concentrations and neonatal pigs (28,40), likely due to LPS-induced pancreatic insulin secretion (14) and an increase in whole body AA utilization due to increased liver protein synthesis (26). Therefore, the appearance of higher total plasma AA concentrations in LPS-infused 26-dayold pigs may be driven by the systemic inflammatory response.…”

“…Muscle homogenates were separated on PAGE and were analyzed at the same time in triple-wide gels (C.B.S. Scientific, Del Mar, CA) for protein electrophoresis and antibody immunoblotting, as previously described (26). Phosphorylated forms of the signaling proteins were compared with their abundance, i.e., the total content of the respective protein, for normalization.…”

GROWTH AND DEVELOPMENT SYMPOSIUM: Endotoxin, inflammation, and intestinal function in livestock1,2

“…Either way, if the amounts are too great, they can ultimately antagonize anabolic growth (Kimball et al, 2003;Orellana et al, 2007) or lead to septic shock and death (Moore and Morris, 1992). Endotoxin is released during bacterial death and during growth and division, tnaking it a ubiquitous contaminant (Petsch and Anspach, 2000;Yaron et al, 2000).…”