Differential effect of sepsis on ability of leucine and IGF-I to stimulate muscle translation initiation

Signals for Muscular Protein Turnover and Insulin Resistance in Critically Ill Patients: A Narrative Review

Sarcopenia in critically ill patients is a highly prevalent comorbidity. It is associated with a higher mortality rate, length of mechanical ventilation, and probability of being sent to a nursing home after the Intensive Care Unit (ICU). Despite the number of calories and proteins delivered, there is a complex network of signals of hormones and cytokines that affect muscle metabolism and its protein synthesis and breakdown in critically ill and chronic patients. To date, it is known that a higher number of proteins decreases mortality, but the exact amount needs to be clarified. This complex network of signals affects protein synthesis and breakdown. Some hormones regulate metabolism, such as insulin, insulin growth factor glucocorticoids, and growth hormone, whose secretion is affected by feeding states and inflammation. In addition, cytokines are involved, such as TNF-alpha and HIF-1. These hormones and cytokines have common pathways that activate muscle breakdown effectors, such as the ubiquitin-proteasome system, calpain, and caspase-3. These effectors are responsible for protein breakdown in muscles. Many trials have been conducted with hormones with different results but not with nutritional outcomes. This review examines the effect of hormones and cytokines on muscles. Knowing all the signals and pathways that affect protein synthesis and breakdown can be considered for future therapeutics.

Glutamine and leucine administration attenuates muscle atrophy in sepsis

AIMS

This study investigated whether l-glutamine (Gln) and/or l-leucine (Leu) administration could attenuate muscle atrophy in a mouse model of cecal ligation and puncture (CLP)-induced sepsis.

MATERIALS AND METHODS

Septic mice were given a daily intraperitoneal injection of Gln, Leu, or Gln plus Leu, and mice were sacrificed on either day 1 or 4 after CLP. Blood and muscles were collected for analysis of amino acid contents and markers related to protein degradation, muscle regeneration, and protein synthesis.

KEY FINDINGS

Leu treatment alone increased both muscle mass and total muscle protein content on day 4 after CLP. Gln administration reduced muscular Gln contents on day 1 and enhanced plasma Gln levels on day 4. Higher plasma branched-chain amino acid (BCAA) abundances and lower muscular BCAA levels were observed in Leu-treated mice on day 4. Gln and Leu individually suppressed muscle expressions of the E3 ubiquitin ligase genes, Trim63 and Fbxo32, on day 4 after CLP. As to muscle expressions of myogenic genes, both Gln and Leu upregulated Myog expression on day 1, but Leu alone enhanced Myf5 gene expression, whereas Gln plus Leu increased MyoD and Myog expression levels on day 4. Akt/mammalian target of rapamycin (mTOR) signaling was only activated by Gln and Leu when individually administered.

SIGNIFICANCE

Gln and/or Leu administration reduces sepsis-induced muscle degradation and promotes myogenic gene expressions. Leu treatment alone had more-pronounced effects on maintaining muscle mass during sepsis. A combination of Gln and Leu failed to show synergistic effects on alleviating sepsis-induced muscle atrophy.

Protective Effects of Glutamine and Leucine Supplementation on Sepsis-Induced Skeletal Muscle Injuries

This study investigated the effects of l-glutamine (Gln) and/or l-leucine (Leu) administration on sepsis-induced skeletal muscle injuries. C57BL/6J mice were subjected to cecal ligation and puncture to induce polymicrobial sepsis and then given an intraperitoneal injection of Gln, Leu, or Gln plus Leu beginning at 1 h after the operation with re-injections every 24 h. All mice were sacrificed on either day 1 or day 4 after the operation. Blood and muscles were collected for analysis of inflammation and oxidative damage-related biomolecules. Results indicated that both Gln and Leu supplementation alleviated sepsis-induced skeletal muscle damage by reducing monocyte infiltration, calpain activity, and mRNA expression levels of inflammatory cytokines and hypoxia-inducible factor-1α. Furthermore, septic mice treated with Gln had higher percentages of blood anti-inflammatory monocytes and muscle M2 macrophages, whereas Leu treatment enhanced the muscle expressions of mitochondrion-related genes. However, there were no synergistic effects when Gln and Leu were simultaneously administered. These findings suggest that both Gln and Leu had prominent abilities to attenuate inflammation and degradation of skeletal muscles in the early and/or late phases of sepsis. Moreover, Gln promoted the switch of leukocytes toward an anti-inflammatory phenotype, while Leu treatment maintained muscle bioenergetic function.

A narrative review of skeletal muscle atrophy in critically ill children: pathogenesis and chronic sequelae

Muscle wasting is now recognized as a growing, debilitating problem in critically ill adults, resulting in long-term deficits in function and an impaired quality of life. Ultrasonography has demonstrated decreases in skeletal muscle size during pediatric critical illness, although variations exist. However, muscle protein turnover patterns during pediatric critical illness are unclear. Understanding muscle protein turnover during critical illness is important in guiding interventions to reduce muscle wasting. The aim of this review was to explore the possible protein synthesis and breakdown patterns in pediatric critical illness. Muscle protein turnover studies in critically ill children are lacking, with the exception of those with burn injuries. Children with burn injuries demonstrate an elevation in both muscle protein breakdown (MPB) and synthesis during critical illness. Extrapolations from animal models and whole-body protein turnover studies in children suggest that children may be more dependent on anabolic factors (e.g., nutrition and growth factors), and may experience greater muscle degradation in response to insults than adults. Yet, children, particularly the younger ones, are more responsive to anabolic agents, suggesting modifiable muscle wasting during critical illness. There is a lack of evidence for muscle wasting in critically ill children and its correlation with outcomes, possibly due to current available methods to study muscle protein turnover in children-most of which are invasive or tedious. In summary, children may experience muscle wasting during critical illness, which may be more reversible by the appropriate anabolic agents than adults. Age appears an important determinant of skeletal muscle turnover. Less invasive methods to study muscle protein turnover and associations with long-term outcome would strengthen the evidence for muscle wasting in critically ill children.

Perioperative physical and nutritional therapy for tongue cancer-induced malnutrition and muscle atrophy: A case report

Malnutrition is a common complication in patients with tongue cancer who experience dysphagia and can steadily lead to skeletal muscle atrophy. Additionally, skeletal muscle loss commonly occurs in patients after invasive surgery. Therefore, patients with tongue cancer are at high risk of skeletal muscle atrophy during the perioperative phase of treatment. Over time, physical and nutritional therapy are expected to increase skeletal muscle mass and improve nutritional status. However, immediate benefits for patients in the perioperative phase of treatment are largely unknown. This case report aimed to evaluate the combined effects of physical and nutritional therapy for a patient in the perioperative phase of treatment for tongue cancer. We describe a 48-year-old woman diagnosed with tongue cancer. Her increasing difficulty with eating and swallowing led to malnutrition. After hospital admission for oral surgery, physical and nutritional therapy were initiated. Skeletal muscle mass measured by body composition analyzer and ultrasound apparatus showed increases, whereas blood tests to indicate nutritional status showed no improvement. This case suggests that physical and nutritional therapy are effective for increasing skeletal muscle during perioperative phase treatment in malnourished patients with tongue cancer and assessment of skeletal muscle mass is a reliable method for clinical evaluation.

Effects of dietary leucine supplementation and immune system stimulation on plasma AA concentrations and tissue protein synthesis in starter pigs

 Immune system stimulation (ISS) adversely affects protein and AA metabolism and reduces productivity in pigs. Leucine (Leu) has a regulatory role in skeletal muscle protein turnover, which may be affected by ISS. The objective of this study was to evaluate the effects of ISS and dietary Leu supplementation on the protein fractional synthesis rate (FSR) of various tissues in pigs. Yorkshire barrows were surgically fitted with jugular vein catheters and assigned to one of three dietary treatments: (i) CON, 1.36% standardized ileal digestible (SID) Leu; (ii) LEU-M, 2.04% SID Leu; and (iii) LEU-H, 2.72% SID Leu. The diets were formulated to contain all essential AA 10% above estimated requirements for maximum whole-body protein deposition for this BW range. At the start of the 36-h challenge period (initial BW = 14.5 ± 0.8 kg), ISS was induced in pigs with lipopolysaccharide (ISS+; n = 7, 8, and 7 for CON, LEU-M, and LEU-H pigs, respectively); a subset of CON pigs was injected with sterile saline (ISS-; n = 6). During challenge period, pigs were fed every 4 h and feed intake of ISS- pigs was kept equal to ISS+ pigs. At the end of the challenge period, FSR of liver, plasma, gastrocnemius, and LD proteins were determined with a flooding dose of l-[ring-2H5]phenylalanine (40 mol%). All essential AA, most nonessential AA, and plasma urea-N peaked at 12 h and declined to baseline levels at 36 h after ISS was induced in ISS+ pigs (P < 0.05), whereas plasma AA and urea-N concentrations were constant in ISS- pigs. At 36 h, dietary Leu supplementation resulted in a linear decline in plasma isoleucine, valine, glutamine, and urea nitrogen concentrations (P < 0.05), whereas plasma Leu concentration was unaffected. Liver protein FSR was increased in ISS+ pigs (P < 0.05), whereas plasma and skeletal muscle protein FSR was not affected by ISS. Dietary Leu supplementation tended to diminish liver protein FSR (linear reduction; P = 0.052) and increase gastrocnemius protein FSR (linear increase; P = 0.085) in ISS+ pigs. Leucine supplementation above estimated requirements may support repartitioning of AA from visceral to peripheral protein deposition during ISS.

Effects of low-dose leucine supplementation on gastrocnemius muscle mitochondrial content and protein turnover in tumor-bearing mice

Many forms of cancer are associated with loss of lean body mass, commonly attributed to decreased protein synthesis and stimulation of proteolytic pathways within the skeletal muscle. Leucine has been shown to improve protein synthesis, insulin signaling, and mitochondrial biogenesis, which are key signaling pathways influenced by tumor signaling. The purpose of this study was to examine the effects of leucine supplementation on mitochondrial biogenesis and protein turnover in tumor-bearing mice. Twenty male C57BL/6 mice were divided into 4 groups (n = 5): Chow, leucine (Leu), Lewis lung carcinoma (LLC) implant, and LLC+Leu. At 9-10 weeks of age, mice were inoculated and supplemented with 5% leucine (w/w) in the diet. C2C12 myotubes were treated with 2.5 mmol/L leucine and 25% LLC conditioned media to further elucidate the direct influence of the tumor and leucine on the muscle. Measures of protein synthesis, mitochondrial biogenesis, and inflammation in the gastrocnemius were assessed via Western blot analysis. Gastrocnemius mass was decreased in LLC+Leu relative to LLC (p = 0.040). Relative protein synthesis rate was decreased in LLC mice (p = 0.001). No change in protein synthesis was observed in myotubes. Phosphorylation of STAT3 was decreased in the Leu group relative to the control in both mice (p = 0.019) and myotubes (p = 0.02), but did not significantly attenuate the inflammatory effect of LLC implantation (p = 0.619). LLC decreased markers of mitochondrial content; however, PGC-1α was increased in LLC+Leu relative to LLC (p = 0.001). While leucine supplementation was unable to preserve protein synthesis or mitochondrial content associated with LLC implantation, it was able to increase mitochondrial biogenesis signaling. Novelty This study provides novel insights on the effect of leucine supplementation on mitochondrial biogenesis and protein turnover in tumor-bearing mice. Leucine increased signaling for mitochondrial biogenesis in the skeletal muscle. Leucine supplementation decreased inflammatory signaling in skeletal muscle.

Regulation of Muscle Growth in Early Postnatal Life in a Swine Model

Skeletal muscle growth during the early postnatal period is rapid in the pig and dependent on the capacity of muscle to respond to anabolic and catabolic stimuli. Muscle mass is driven by the balance between protein synthesis and degradation. Among these processes, muscle protein synthesis in the piglet is exceptionally sensitive to the feeding-induced postprandial changes in insulin and amino acids, whereas muscle protein degradation is affected only during specific catabolic states. The developmental decline in the response of muscle to feeding is associated with changes in the signaling pathways located upstream and downstream of the mechanistic target of rapamycin protein complex. Additionally, muscle growth is supported by an accretion of nuclei derived from satellite cells. Activated satellite cells undergo proliferation, differentiation, and fusion with adjacent growing muscle fibers. Enhancing early muscle growth through modifying protein synthesis, degradation, and satellite cell activity is key to maximizing performance, productivity, and lifelong pig health.

Linking Cancer Cachexia-Induced Anabolic Resistance to Skeletal Muscle Oxidative Metabolism

Cancer cachexia, a wasting syndrome characterized by skeletal muscle depletion, contributes to increased patient morbidity and mortality. While the intricate balance between protein synthesis and breakdown regulates skeletal muscle mass, the suppression of basal protein synthesis may not account for the severe wasting induced by cancer. Therefore, recent research has shifted to the regulation of “anabolic resistance,” which is the impaired ability of nutrition and exercise to stimulate protein synthesis. Emerging evidence suggests that oxidative metabolism can regulate both basal and induced muscle protein synthesis. While disrupted protein turnover and oxidative metabolism in cachectic muscle have been examined independently, evidence suggests a linkage between these processes for the regulation of cancer-induced wasting. The primary objective of this review is to highlight the connection between dysfunctional oxidative metabolism and cancer-induced anabolic resistance in skeletal muscle. First, we review oxidative metabolism regulation of muscle protein synthesis. Second, we describe cancer-induced alterations in the response to an anabolic stimulus. Finally, we review a role for exercise to inhibit cancer-induced anabolic suppression and mitochondrial dysfunction.

Marginal dietary zinc deprivation augments sepsis-induced alterations in skeletal muscle TNF-α but not protein synthesis

Severe zinc deficiency is associated with an increased systemic inflammatory response and mortality after sepsis. However, the impact of mild zinc deficiency, which is more common in populations with chronic illnesses and sepsis, is unknown. In this study, we hypothesized that marginal dietary Zn deprivation (ZM) would amplify tissue inflammation and exacerbate the sepsis-induced decrease in muscle protein synthesis. Adult male C57BL/6 mice were fed a zinc-adequate (ZA) or ZM diet (30 or 10 mg Zn/kg, respectively) over 4 weeks, peritonitis was induced by cecal ligation and puncture (CLP), and mice were examined at either 24 h (acute) or 5 days (chronic) post-CLP Acute sepsis decreased the in vivo rate of skeletal muscle protein synthesis and the phosphorylation of the mTOR substrate 4E-BP1. Acutely, sepsis increased TNF-α and IL-6 mRNA in muscle, and the increase in TNF-α was significantly greater in ZM mice. However, muscle protein synthesis and 4E-BP1 phosphorylation returned to baseline 5 days post-CLP in both ZA and ZM mice. Protein degradation via markers of the ubiquitin proteasome pathway was increased in acute sepsis, yet only MuRF1 mRNA was increased in chronic sepsis and ZM amplified this elevation. Our data suggest that mild zinc deficiency increases TNF-α in muscle acutely after sepsis but does not significantly modulate the rate of muscle protein synthesis.

Leucine supplementation stimulates protein synthesis and reduces degradation signal activation in muscle of newborn pigs during acute endotoxemia

Sepsis disrupts skeletal muscle proteostasis and mitigates the anabolic response to leucine (Leu) in muscle of mature animals. We have shown that Leu stimulates muscle protein synthesis (PS) in healthy neonatal piglets. To determine if supplemental Leu can stimulate PS and reduce protein degradation (PD) signaling in neonatal muscle during endotoxemia, overnight-fasted neonatal pigs were infused for 8 h with LPS or saline while plasma amino acids, glucose, and insulin were maintained at fasting levels during pancreatic-substrate clamps. Leu or saline was infused during the last hour. Markers of PS and PD were determined in skeletal muscle. Compared with controls, Leu increased PS in longissimus dorsi (LD), gastrocnemius, and soleus muscles. LPS decreased PS in these three muscles by 36%, 28%, and 38%, but Leu antagonized that reduction by increasing PS by 84%, 81%, and 83%, respectively, when supplemented to LPS. Leu increased eukaryotic translation initiation factor (eIF)3b-raptor interactions, eIF4E-binding protein-1, and S6 kinase 1 phosphorylation as well as eIF4E·eIF4G complex formation in LD, gastrocnemius, and soleus muscles of control and LPS-treated pigs. In LD muscle, LPS increased the light chain (LC)3-II-to-LC3 ratio and muscle-specific RING finger (MuRF-1) abundance but not atrogin-1 abundance or AMP-activated protein kinase-α phosphorylation. Leu supplementation to LPS-treated pigs reduced the LC3-II-to-LC3 ratio, MuRF-1 abundance, and AMP-activated protein kinase-α phosphorylation compared with LPS alone. In conclusion, parenteral Leu supplementation attenuates the LPS-induced reduction in PS by stimulating mammalian target of rapamycin complex 1-dependent translation and may reduce PD by attenuating autophagy-lysosome and MuRF-1 signaling in neonatal skeletal muscle.

Performance of a phosphoflow assay to determine phosphorylation of S6 ribosomal protein as a pharmacodynamic read out for mTOR inhibition

OBJECTIVES

The S6 ribosomal protein (S6RP) is phosphorylated by the mammalian target of rapamycin (mTOR). The objective of this study was to assess the analytical suitability of a commercial kit-based phosphoflow cytometry protocol using whole blood (WBS) to measure the level of phosphorylated S6RP (p-S6RP) in T-cell subsets to study the pharmacodynamic effects of mTOR inhibitors (mTORi).

DESIGN AND METHODS

A kit was used for fixation and permeabilization of mitogen-stimulated cells, and p-S6RP was assessed separately in CD3+CD4+ and CD3+CD8+ cells by employing an anti-phospho-Ser235/236 antibody. Specificity, linearity, within-run precision and stability were investigated in either WBS spiked with everolimus and non-mTORi immunosuppressants or in WBS from patients on immunosuppressive therapy (n=56). In addition, healthy controls (n=10) and patients without immunosuppression (n=10) were included. A comparison (n=15) with an established western blot method based on anti-phospho p70S6 kinase (Thr389) was made by splitting WBS.

RESULTS

Everolimus decreased p-S6RP in vitro concentration dependently (0.00-27.4μg/L). This effect was also confirmed in vivo after a single dose of everolimus to healthy volunteers (n=3). However, spiking WBS with 500μg/L cyclosporine also decreased p-S6RP. The within-run coefficient of variation was <18% in transplant patients and <27% in healthy controls for both cell subsets. Sample stability for p-S6RP analysis was limited (<24h). p-S6RP was significantly decreased in CD3+CD8+ cells of patients treated with sirolimus (p=0.02) but not with everolimus. No significant correlation between the phosphoflow- and western blot method was noted.

CONCLUSION

The phosphoflow assay of p-S6RP performed well analytically, but sample stability, specificity, and method comparison results question its fitness for clinical purposes.

Glycine restores the anabolic response to leucine in a mouse model of acute inflammation

Amino acids, especially leucine, potently stimulate protein synthesis and reduce protein breakdown in healthy skeletal muscle and as a result have received considerable attention as potential treatments for muscle wasting. However, the normal anabolic response to amino acids is impaired during muscle-wasting conditions. Although the exact mechanisms of this anabolic resistance are unclear, inflammation and ROS are believed to play a central role. The nonessential amino acid glycine has anti-inflammatory and antioxidant properties and preserves muscle mass in calorie-restricted and tumor-bearing mice. We hypothesized that glycine would restore the normal muscle anabolic response to amino acids under inflammatory conditions. Relative rates of basal and leucine-stimulated protein synthesis were measured using SUnSET methodology 4 h after an injection of 1 mg/kg lipopolysaccharide (LPS). Whereas leucine failed to stimulate muscle protein synthesis in LPS-treated mice pretreated with l-alanine (isonitrogenous control), leucine robustly stimulated protein synthesis (+51%) in mice pretreated with 1 g/kg glycine. The improvement in leucine-stimulated protein synthesis was accompanied by a higher phosphorylation status of mTOR, S6, and 4E-BP1 compared with l-alanine-treated controls. Despite its known anti-inflammatory action in inflammatory cells, glycine did not alter the skeletal muscle inflammatory response to LPS in vivo or in vitro but markedly reduced DHE staining intensity, a marker of oxidative stress, in muscle cross-sections and attenuated LPS-induced wasting in C2C12 myotubes. Our observations in male C57BL/6 mice suggest that glycine may represent a promising nutritional intervention for the attenuation of skeletal muscle wasting.

Sepsis attenuates the anabolic response to skeletal muscle contraction

Electrically stimulated muscle contraction is a potential clinical therapy to treat sepsis-induced myopathy; however, whether sepsis alters contraction-induced anabolic signaling is unknown. Polymicrobial peritonitis was produced by cecal ligation and puncture (CLP) in male C57BL/6 mice and time-matched, pair-fed controls (CON). At ∼24 h post-CLP, the right hindlimb was electrically stimulated via the sciatic nerve to evoke maximal muscle contractions, and the gastrocnemius was collected 2 h later. Protein synthesis was increased by muscle contraction in CON mice. Sepsis suppressed the rate of synthesis in both the nonstimulated (31%) and stimulated (57%) muscle versus CON. Contraction of muscle in CON mice increased the phosphorylation of mTORC1 (mammalian target of rapamycin [mTOR] complex 1) substrates S6K1 (70-kd ribosomal protein S6 kinase 1) Thr (8-fold), S6K1 ThrSer (7-fold) and 4E-BP1 Ser (11-fold). Sepsis blunted the contraction-induced phosphorylation of S6K1 Thr (67%), S6K1 ThrSer (46%), and 4E-BP1 Ser (85%). Conversely, sepsis did not appear to modulate protein elongation as eEF2 Thr phosphorylation was decreased similarly by muscle contraction in both groups. Mitogen-activated protein kinase signaling was discordant following contraction in septic muscle; phosphorylation of extracellular signal-regulated kinase ThrTyr and p38 ThrTyr was increased similarly in both CON and CLP mice, while sepsis prevented the contraction-induced phosphorylation of JNK ThrTyr and c-JUN Ser. The expression of interleukin 6 and tumor necrosis factor α (TNF-α) mRNA in muscle was increased by sepsis, and contraction increased TNF-α to a greater extent in muscle from septic than CON mice. Injection of the mTOR inhibitor Torin2 in separate mice confirmed that contraction-induced increases in S6K1 and 4E-BP1 were mTOR mediated. These findings demonstrate that resistance to contraction-induced anabolic signaling occurs during sepsis and is predominantly mTORC1-dependent.

Cerium oxide nanoparticle treatment ameliorates peritonitis-induced diaphragm dysfunction

The severe inflammation observed during sepsis is thought to cause diaphragm dysfunction, which is associated with poor patient prognosis. Cerium oxide (CeO2) nanoparticles have been posited to exhibit anti-inflammatory and antioxidative activities suggesting that these particles may be of potential use for the treatment of inflammatory disorders. To investigate this possibility, Sprague Dawley rats were randomly assigned to the following groups: sham control, CeO2 nanoparticle treatment only (0.5 mg/kg iv), sepsis, and sepsis+CeO2 nanoparticles. Sepsis was induced by the introduction of cecal material (600 mg/kg) directly into the peritoneal cavity. Nanoparticle treatment decreased sepsis-associated impairments in diaphragmatic contractile (P(o)) function (sham: 25.6±1.6 N/cm(2) vs CeO2: 23.4±0.8 N/cm(2) vs Sep: 15.9±1.0 N/cm(2) vs Sep+CeO2: 20.0±1.0 N/cm(2), P<0.05). These improvements in diaphragm contractile function were accompanied by a normalization of protein translation signaling (Akt, FOXO-1, and 4EBP1), diminished proteolysis (caspase 8 and ubiquitin levels), and decreased inflammatory signaling (Stat3 and iNOS). Histological analysis suggested that nanoparticle treatment was associated with diminished sarcolemma damage and diminished inflammatory cell infiltration. These data indicate CeO2 nanoparticles may improve diaphragmatic function in the septic laboratory rat.

Double-blind, placebo-controlled pilot trial of L-Leucine-enriched amino-acid mixtures on body composition and physical performance in men and women aged 65-75 years

Background/Objectives:

Adequate protein intake is essential to retaining muscle and maintaining physical function, especially in the elderly, and L-Leucine has received attention as an essential amino acid (EAA) that enhances protein retention. The study's aim was to compare the efficacy of EAA mixtures on lean tissue mass (LTM) and functional performance (FP) in a healthy elderly population.

Subjects/Methods:

Thirty-six subjects (65–75 years) volunteered to receive capsules with EAAs (Groups A and B containing 20% and 40% L-Leucine, respectively) or placebo (lactose containing 0% L-Leucine, Group C) for 12 weeks. The daily amount ranged from 11 to 21 g (0.21 g/ kg/day) and was taken in two equal dosages alongside food, morning and evening. Main outcomes measured before and after intervention were LTM and FP (30-s arm-curl test; 30-s chair-stand test (30-CST); 6-min walk test (6-WT); and handgrip strength). Secondary outcomes included dietary intakes and physical activity.

Results:

Twenty-five subjects (11 male and 14 female) completed the study (Group A, n=8; Group B, n=8; Group C, n=9). Gains associated with medium effect sizes were noted in LTM (Group B, 1.1 ±1.1%, P=0.003) and FP (Group A in 30-CST (11.0±11.5%, P=0.02) and 6-WT (8.8±10.0%, P=0.02); Group B in 6-WT (5.8±6.6%, P=0.03) and a trend in 30-CST (13.2±16.0, P=0.06)). Significant differences between groups were not observed in secondary outcomes.

Conclusions:

Twice-daily supplementation of EAAs containing 20% or 40% L-Leucine improved aspects of functional status and at the higher level improved LTM. Further work to establish change in a larger sample and palatable supplemental format is now required.

Acute acidosis attenuates leucine stimulated signal transduction and protein synthesis in rat skeletal muscle

BACKGROUND

Critical illnesses are often complicated by acute metabolic acidosis, which if persistent, adversely affects outcome. Among the harmful effects that it might cause are impaired utilization of nutrients, increased proteolysis and depressed protein synthesis, leading to muscle wasting. As the amino acid leucine stimulates protein synthesis by activating mTOR signaling, we explored whether in acidosis, impaired leucine-stimulated signaling might be a contributor to the depressed protein synthesis.

METHODS

Male pair-fed rats were gavaged with NH4Cl (acidosis) or NaCl (control) for 2 days and then gavaged once with leucine and sacrificed 45 min later. Extensor digitorum longus muscles were isolated, incubated with or without leucine and protein synthesis measured. The anterior tibial muscle signaling was analysed by Western immunobloting.

RESULTS

Despite pair-feeding, acidotic rats lost body and muscle weight vs. controls. Moreover, leucine-induced protein synthesis in isolated muscle from acidotic rats was impaired. In-vivo, 45 min after an oral leucine load, anterior tibial muscle mTOR and 4E-BP1 phosphorylation increased significantly and comparably in control and acidotic rats. In contrast, leucine-stimulated phosphorylation of S6K1, a regulator of translation initiation and protein synthesis, was attenuated to approximately 56% of the control value (p < 0.05).

CONCLUSION

This study reveals that an acute metabolic acidosis impairs leucine-stimulated protein synthesis and activation of signaling downstream of mTOR at the level of S6K1. We propose that this S6K1 abnormality may account in part, for the resistance to leucine-stimulated muscle protein synthesis, and may thereby contribute to the impaired nutrient utilization and ultimately the muscle wasting that develops in acidosis.

Sepsis-induced changes in amino acid transporters and leucine signaling via mTOR in skeletal muscle

The present study tested the hypothesis that sepsis-induced leucine (Leu) resistance in skeletal muscle is associated with a down-regulation of amino acid transporters important in regulating Leu flux or an impairment in the formation of the Leu-sensitive mTOR-Ragulator complex. Sepsis in adult male rats decreased basal protein synthesis in gastrocnemius, associated with a reduction in mTOR activation as indicated by decreased 4E-BP1 and S6K1 phosphorylation. The ability of oral Leu to increase protein synthesis and mTOR kinase after 1 h was largely prevented in sepsis. Sepsis increased CAT1, LAT2 and SNAT2 mRNA content two- to fourfold, but only the protein content for CAT1 (20 % decrease) differed significantly. Conversely, sepsis decreased the proton-assisted amino acid transporter (PAT)-2 mRNA by 60 %, but without a coordinate change in PAT2 protein. There was no sepsis or Leu effect on the protein content for RagA-D, LAMTOR-1 and -2, raptor, Rheb or mTOR in muscle. The binding of mTOR, PRAS40 and RagC to raptor did not differ for control and septic muscle in the basal condition; however, the Leu-induced decrease in PRAS40·raptor and increase in RagC·raptor seen in control muscle was absent in sepsis. The intracellular Leu concentration was increased in septic muscle, compared to basal control conditions, and oral Leu further increased the intracellular Leu concentration similarly in both control and septic rats. Hence, while alterations in select amino acid transporters are not associated with development of sepsis-induced Leu resistance, the Leu-stimulated binding of raptor with RagC and the recruitment of mTOR/raptor to the endosome-lysosomal compartment may partially explain the inability of Leu to fully activate mTOR and muscle protein synthesis.

mTor signaling in skeletal muscle during sepsis and inflammation: where does it all go wrong?

The mammalian target of rapamycin (mTOR) is an evolutionarily conserved protein kinase that exquisitely regulates protein metabolism in skeletal muscle. mTOR integrates input from amino acids, growth factors, and intracellular cues to make or break muscle protein. mTOR accomplishes this task by stimulating the phosphorylation of substrates that control protein translation while simultaneously inhibiting proteasomal and autophagic protein degradation. In a metabolic twist of fate, sepsis induces muscle atrophy in part by the aberrant regulation of mTOR. In this review, we track the steps of normal mTOR signaling in muscle and examine where they go astray in sepsis and inflammation.

Sepsis-induced alterations in protein-protein interactions within mTOR complex 1 and the modulating effect of leucine on muscle protein synthesis

Sepsis-induced muscle atrophy is produced in part by decreased protein synthesis mediated by inhibition of mTOR (mammalian target of rapamycin). The present study tests the hypothesis that alteration of specific protein-protein interactions within the mTORC1 (mTOR complex 1) contributes to the decreased mTOR activity observed after cecal ligation and puncture in rats. Sepsis decreased in vivo translational efficiency in gastrocnemius and reduced the phosphorylation of eukaryotic initiation factor (eIF) 4E-binding protein (BP) 1, S6 kinase (S6K) 1, and mTOR, compared with time-matched pair-fed controls. Sepsis decreased T246-phosphorylated PRAS40 (proline-rich Akt substrate 40) and reciprocally increased S792-phosphorylated raptor (regulatory associated protein of mTOR). Despite these phosphorylation changes, sepsis did not alter PRAS40 binding to raptor. The amount of the mTOR-raptor complex did not differ between groups. In contrast, the binding and retention of both 4E-BP1 and S6K1 to raptor were increased, and, conversely, the binding of raptor with eIF3 was decreased in sepsis. These changes in mTORC1 in the basal state were associated with enhanced 5'-AMP activated kinase activity. Acute in vivo leucine stimulation increased muscle protein synthesis in control, but not septic rats. This muscle leucine resistance was associated with coordinated changes in raptor-eIF3 binding and 4E-BP1 phosphorylation. Overall, our data suggest the sepsis-induced decrease in muscle protein synthesis may be mediated by the inability of 4E-BP1 and S6K1 to be phosphorylated and released from mTORC1 as well as the decreased recruitment of eIF3 necessary for a functional 48S complex. These data provide additional mechanistic insight into the molecular mechanisms by which sepsis impairs both basal protein synthesis and the anabolic response to the nutrient signal leucine in skeletal muscle.

Activated protein synthesis and suppressed protein breakdown signaling in skeletal muscle of critically ill patients

Background

Skeletal muscle mass is controlled by myostatin and Akt-dependent signaling on mammalian target of rapamycin (mTOR), glycogen synthase kinase 3β (GSK3β) and forkhead box O (FoxO) pathways, but it is unknown how these pathways are regulated in critically ill human muscle. To describe factors involved in muscle mass regulation, we investigated the phosphorylation and expression of key factors in these protein synthesis and breakdown signaling pathways in thigh skeletal muscle of critically ill intensive care unit (ICU) patients compared with healthy controls.

Methodology/Principal Findings

ICU patients were systemically inflamed, moderately hyperglycemic, received insulin therapy, and showed a tendency to lower plasma branched chain amino acids compared with controls. Using Western blotting we measured Akt, GSK3β, mTOR, ribosomal protein S6 kinase (S6k), eukaryotic translation initiation factor 4E binding protein 1 (4E-BP1), and muscle ring finger protein 1 (MuRF1); and by RT-PCR we determined mRNA expression of, among others, insulin-like growth factor 1 (IGF-1), FoxO 1, 3 and 4, atrogin1, MuRF1, interleukin-6 (IL-6), tumor necrosis factor α (TNF-α) and myostatin. Unexpectedly, in critically ill ICU patients Akt-mTOR-S6k signaling was substantially higher compared with controls. FoxO1 mRNA was higher in patients, whereas FoxO3, atrogin1 and myostatin mRNAs and MuRF1 protein were lower compared with controls. A moderate correlation (r² = 0.36, p<0.05) between insulin infusion dose and phosphorylated Akt was demonstrated.

Conclusions/Significance

We present for the first time muscle protein turnover signaling in critically ill ICU patients, and we show signaling pathway activity towards a stimulation of muscle protein synthesis and a somewhat inhibited proteolysis.

Ethyl pyruvate preserves IGF-I sensitivity toward mTOR substrates and protein synthesis in C2C12 myotubes

Bacterial infection decreases skeletal muscle protein synthesis via inhibition of the mammalian target of rapamycin (mTOR), a key regulator of translation initiation. To better define the mechanism by which muscle mTOR activity is decreased, we used an in vitro model of C2C12 myotubes treated with endotoxin [lipopolysaccharide (LPS)]and interferon (IFN)-γ to determine whether stable lipophilic pyruvate derivatives restore mTOR signaling. Myotubes treated with a combination of LPS and IFNγ down-regulated the phosphorylation of the mTOR substrates S6 kinase-1 and 4E binding protein-1. The phosphorylation of ribosomal protein S6 was decreased, whereas phosphorylation of elongation factor-2 was enhanced; all results consistent with defects in both translation initiation and elongation. LPS/IFNγ decreased protein synthesis 60% in myotubes. Treatment with methyl or ethyl pyruvate partially protected against the LPS/IFNγ-induced fall in mTOR signaling. The protective effect of ethyl and methyl pyruvate could not be replicated by an equimolar amount of sodium pyruvate. Although LPS/IFNγ treated myotubes were initially IGF-I responsive, prolonged exposure (≥ 17 h) resulted in IGF-I resistance at the level of mTOR despite normal IGF-I receptor phosphorylation. Ethyl pyruvate treatment restored IGF-I sensitivity as evidenced by the left shift in the IGF-I dose-response curve and maintained IGF-I responsiveness for a prolonged period of time. Ethyl pyruvate also restored IGF-I-stimulated protein synthesis in LPS/IFNγ-treated myotubes. Cotreatment with N-acetyl cysteine or ascorbic acid also preserved IGF-I sensitivity and mTOR activity. The data suggest that the combination of LPS and IFNγ inhibits mTOR activity and that prolonged exposure induces IGF-I resistance in myotubes. Lipophilic pyruvate derivatives and antioxidants show promise at rescuing mTOR activity and muscle protein synthesis by maintaining IGF-I sensitivity in this model.

Skeletal muscle catabolism in trinitrobenzene sulfonic acid-induced murine colitis

The present study determined whether the muscle atrophy produced by colitis is associated with altered rates of muscle protein synthesis or degradation, as well as the potential role of the local (eg, muscle) insulin-like growth factor (IGF) system and muscle-specific ubiquitin E3 ligases atrogin-1 and MuRF1 in mediating altered muscle protein balance. Colitis was induced in C57BL/6 mice by intrarectal administration of trinitrobenzene sulfonic acid (TNBS), and blood and tissues were collected on day 10. Mice with inflammatory bowel disease demonstrated reduced skeletal muscle mass and protein content, whereas colonic segment weight and gross damage score were both increased in mice with colitis, compared with time-matched control values. There was no change in muscle protein synthesis in mice with inflammatory bowel disease; but there was an increased protein breakdown (45%), proteasome activity (85%), and messenger RNA (mRNA) expression for atrogin-1 and MuRF1 (200%-300%) in muscle. These changes were associated with a reduction in liver (but not muscle) IGF-I mRNA as well as a reduction in both total and free IGF-I in the blood. Colitis decreased the hepatic content of IGF binding protein (IGFBP)-3 mRNA by 40% and increased IGFBP-1 mRNA by 100%. In contrast, colitis did alter IGFBP mRNAs in muscle. The tumor necrosis factor-α, interleukin-6, and nitric oxide synthase 2 mRNA content of both liver and skeletal muscle was increased in TNBS-treated mice; and plasma tumor necrosis factor-α and interleukin-6 concentrations were also elevated. These data suggest that TNBS-induced colitis is independent of a change in muscle protein synthesis but dependent on stimulation of protein degradation via increased expression of muscle-specific atrogenes, which may be mediated in part by the reduction in circulating concentration of IGF-I and the concomitant increase in inflammatory mediators observed in the blood and muscle per se.

BCATm deficiency ameliorates endotoxin-induced decrease in muscle protein synthesis and improves survival in septic mice

Endotoxin (LPS) and sepsis decrease mammalian target of rapamycin (mTOR) activity in skeletal muscle, thereby reducing protein synthesis. Our study tests the hypothesis that inhibition of branched-chain amino acid (BCAA) catabolism, which elevates circulating BCAA and stimulates mTOR, will blunt the LPS-induced decrease in muscle protein synthesis. Wild-type (WT) and mitochondrial branched-chain aminotransferase (BCATm) knockout mice were studied 4 h after Escherichia coli LPS or saline. Basal skeletal muscle protein synthesis was increased in knockout mice compared with WT, and this change was associated with increased eukaryotic initiation factor (eIF)-4E binding protein-1 (4E-BP1) phosphorylation, eIF4E.eIF4G binding, 4E-BP1.raptor binding, and eIF3.raptor binding without a change in the mTOR.raptor complex in muscle. LPS decreased muscle protein synthesis in WT mice, a change associated with decreased 4E-BP1 phosphorylation as well as decreased formation of eIF4E.eIF4G, 4E-BP1.raptor, and eIF3.raptor complexes. In BCATm knockout mice given LPS, muscle protein synthesis only decreased to values found in vehicle-treated WT control mice, and this ameliorated LPS effect was associated with a coordinate increase in 4E-BP1.raptor, eIF3.raptor, and 4E-BP1 phosphorylation. Additionally, the LPS-induced increase in muscle cytokines was blunted in BCATm knockout mice, compared with WT animals. In a separate study, 7-day survival and muscle mass were increased in BCATm knockout vs. WT mice after polymicrobial peritonitis. These data suggest that elevating blood BCAA is sufficient to ameliorate the catabolic effect of LPS on skeletal muscle protein synthesis via alterations in protein-protein interactions within mTOR complex-1, and this may provide a survival advantage in response to bacterial infection.

Skeletal muscle protein balance in mTOR heterozygous mice in response to inflammation and leucine

Sepsis and lipopolysaccharide (LPS) may decrease skeletal muscle protein synthesis by impairing mTOR (mammalian target of rapamycin) activity. The role of mTOR in regulating muscle protein synthesis was assessed in wild-type (WT) and mTOR heterozygous (+/-) mice under basal conditions and in response to LPS and/or leucine stimulation. No difference in body weight of mTOR(+/-) mice was observed compared with WT mice; whereas whole body lean body mass was reduced. Gastrocnemius weight was decreased in mTOR(+/-) mice, which was attributable in part to a reduced rate of basal protein synthesis. LPS decreased muscle protein synthesis in WT and mTOR(+/-) mice to the same extent. Reduced muscle protein synthesis in mTOR(+/-) mice under basal and LPS-stimulated conditions was associated with lower 4E-BP1 and S6K1 phosphorylation. LPS also decreased PRAS40 phosphorylation and increased phosphorylation of raptor and IRS-1 (Ser(307)) to the same extent in WT and mTOR(+/-) mice. Muscle atrogin-1 and MuRF1 mRNA content was elevated in mTOR(+/-) mice under basal conditions, implying increased ubiquitin-proteasome-mediated proteolysis, but the LPS-induced increase in these atrogenes was comparable between groups. Plasma insulin and IGF-I as well as tissue expression of TNFalpha, IL-6, or NOS2 did not differ between WT and mTOR(+/-) mice. Finally, whereas LPS impaired the ability of leucine to stimulate muscle protein synthesis and 4E-BP1 phosphorylation in WT mice, this inflammatory state rendered mTOR(+/-) mice leucine unresponsive. These data support the idea that the LPS-induced reduction in mTOR activity is relatively more important in regulating skeletal muscle mass in response to nutrient stimulation than under basal conditions.

Endotoxin and interferon-gamma inhibit translation in skeletal muscle cells by stimulating nitric oxide synthase activity

The purpose of the present study was to test the hypothesis that endogenous NO negatively affects translation in skeletal muscle cells after exposure to a combination of endotoxin (LPS) and interferon-gamma (IFN-gamma). Individually, LPS and IFN-gamma did not alter protein synthesis, but in combination, they inhibited protein synthesis by 80% in C2C12 myotubes. The combination of LPS and IFN-gamma dramatically downregulated the autophosphorylation of the mammalian target of rapamycin and its substrates S6K1 and 4EBP-1. The phosphorylation of ribosomal protein S6 was decreased, whereas phosphorylation of elongation factor 2 and raptor was enhanced, consistent with defects in both translation initiation and elongation. Reduced S6 phosphorylation occurred 8 to 18 h after LPS/IFN-gamma and coincided with a prolonged upregulation of NOS2 messenger RNA and protein. NOS2 protein expression and the LPS/IFN-gamma-induced fall in phosphorylated S6 were prevented by the proteasome inhibitor MG-132. The general NOS inhibitor, L-NAME, and the specific NOS2 inhibitor, 1400W, also prevented the LPS/IFN-gamma-induced decrease in protein synthesis and restored translational signaling. LPS/IFN-gamma downregulated the phosphorylation of multiple Akt substrates, including the proline-rich Akt substrate 40, while enhancing the phosphorylation of raptor on a 5'-AMP-activated kinase (AMPK)-regulated site. The negative effects of LPS/IFN-gamma were blunted by the AMPK inhibitor compound C. The data suggest that, in combination, LPS and IFN-gamma induce a prolonged expression of NOS2 and excessive production of NO that reciprocally alter Akt and AMPK activity and consequently downregulate translation via reduced mammalian target of rapamycin signaling.

Castration differentially alters basal and leucine-stimulated tissue protein synthesis in skeletal muscle and adipose tissue

Reduced testosterone as a result of catabolic illness or aging is associated with loss of muscle and increased adiposity. We hypothesized that these changes in body composition occur because of altered rates of protein synthesis under basal and nutrient-stimulated conditions that are tissue specific. The present study investigated such mechanisms in castrated male rats (75% reduction in testosterone) with demonstrated glucose intolerance. Over 9 wk, castration impaired body weight gain, which resulted from a reduced lean body mass and preferential sparing of adipose tissue. Castration decreased gastrocnemius weight, but this atrophy was not associated with reduced basal muscle protein synthesis or differences in plasma IGF-I, insulin, or individual amino acids. However, oral leucine failed to normally stimulate muscle protein synthesis in castrated rats. In addition, castration-induced atrophy was associated with increased 3-methylhistidine excretion and in vitro-determined ubiquitin proteasome activity in skeletal muscle, changes that were associated with decreased atrogin-1 or MuRF1 mRNA expression. Castration decreased heart and kidney weight without reducing protein synthesis and did not alter either cardiac output or glomerular filtration. In contradistinction, the weight of the retroperitoneal fat depot was increased in castrated rats. This increase was associated with an elevated rate of basal protein synthesis, which was unresponsive to leucine stimulation. Castration also decreased whole body fat oxidation. Castration increased TNFα, IL-1α, IL-6, and NOS2 mRNA in fat but not muscle. In summary, the castration-induced muscle wasting results from an increased muscle protein breakdown and the inability of leucine to stimulate protein synthesis, whereas the expansion of the retroperitoneal fat depot appears mediated in part by an increased basal rate of protein synthesis-associated increased inflammatory cytokine expression.

Local insulin-like growth factor I prevents sepsis-induced muscle atrophy

The present study tests the hypotheses that local bioavailability of insulin-like growth factor I (IGF-I) is capable of regulating muscle protein balance and that muscle-directed IGF-I can selectively maintain muscle mass during bacterial infection. Initial studies in C57BL/6 mice demonstrated that increasing or decreasing bioavailable IGF-I within muscle by local administration of either Leu(24) Ala(31) IGF-I or IGF binding protein 1, respectively, produced proportional changes in surrogate markers (eg, phosphorylation of 4E-BP1 and S6K1) of protein synthesis. We next examined the ability of a sustained local administration of IGF-I to prevent sepsis-induced muscle atrophy over a 5-day period. At the time of cecal ligation and puncture or sham surgery, mice had a time-release pellet containing IGF-I implanted next to the gastrocnemius and a placebo pellet placed in the contralateral limb. Data indicated that IGF-I released locally only affected the adjacent muscle and was not released into the circulation. Gastrocnemius from septic mice containing the placebo pellet was atrophied and had a reduced IGF-I protein content. In contrast, locally directed IGF-I increased IGF-I protein within adjacent muscle to basal control levels. This change was associated with a proportional increase in muscle weight and protein, as well as increased phosphorylation of 4E-BP1 and the redistribution of eIF4E from the inactive eIF4E4EBP1 complex to the active eIF4EeIF4G complex. Local IGF-I also prevented the sepsis-induced increase in atrogin-1 messenger RNA in the exposed muscle. Finally, local IGF-I prevented the sepsis-induced increase in muscle interleukin-6 messenger RNA. Thus, muscle-directed IGF-I attenuates the sepsis-induced atrophic response apparently by increasing muscle protein synthesis and potentially decreasing proteolysis. Collectively, our data suggest that agents that increase the bioavailability of IGF-I within muscle per se might be effective in ameliorating the sepsis-induced loss of muscle mass without having undesirable effects on metabolic processes in distant organs.

Activation of AMP-activated protein kinase by 5-aminoimidazole-4-carboxamide-1-beta-D-ribonucleoside prevents leucine-stimulated protein synthesis in rat skeletal muscle

Several stress conditions are characterized by activation of 5'-AMP-activated protein kinase (AMPK) and the development of leucine resistance in skeletal muscle. In the present study, we determined whether direct activation of the AMPK by 5-aminoimidazole-4-carboxamide-1-beta-D-ribonucleoside (AICAR) prevents the characteristic leucine-induced increase in protein synthesis by altering mammalian target of rapamycin (mTOR) signal transduction. Rats were injected with AICAR or saline (Sal) and 1 h thereafter received an oral gavage of leucine (or Sal). Efficacy of AICAR was verified by increased AMPK phosphorylation. AICAR decreased basal in vivo muscle (gastrocnemius) protein synthesis and completely prevented the leucine-induced increase, independent of a change in muscle adenine nucleotide concentration. AICAR also prevented the hyperphosphorylation of eukaryotic initiation factor (eIF) 4E binding protein (4E-BP1), ribosomal protein S6 kinase (S6K1), S6, and eIF4G in response to leucine, suggesting a decrease in mTOR activity. Moreover, AICAR prevented the leucine-induced redistribution of eIF4E from the inactive eIF4E.4E-BP1 to the active eIF4E.eIF4G complex. This ability of AICAR to produce muscle leucine resistance could not be attributed to a change in phosphorylation of tuberous sclerosis complex (TSC)2, the formation of a TSC1.TSC2 complex, the binding of raptor with mTOR, or the phosphorylation of eukaryotic elongation factor-2. However, the inhibitory actions of AICAR were associated with reduced phosphorylation of proline-rich Akt substrate-40 and increased phosphorylation of raptor, which represent potential mechanisms by which AICAR might be expected to inhibit leucine-induced increases in mTOR activity and protein synthesis under in vivo conditions.

Beta-adrenergic blockade exacerbates sepsis-induced changes in tumor necrosis factor alpha and interleukin-6 in skeletal muscle and is associated with impaired translation initiation

BACKGROUND

Sepsis stimulates the sympathetic nervous system. The resultant elevation in plasma catecholamines, both norepinephrine and epinephrine (Epi), might be expected to alter the expression of inflammatory cytokines, which may directly or indirectly influence muscle protein balance. The purpose of this study was twofold: (1) determine whether Epi per se increases cytokine expression in skeletal muscle, and (2) determine whether beta-adrenergic blockade alters the sepsis-induced expression of inflammatory cytokines and mediators of protein balance in skeletal muscle.

METHODS

In the first study, rats were infused with Epi for 2 hour to increase the circulating Epi concentration to levels seen in septic animals. In the second study, sepsis was induced by cecal ligation and puncture and a nonspecific beta-adrenergic blockade produced with a continuous infusion of propranolol (PP). Tissues were obtained 24 after induction of sepsis and analyzed for tumor necrosis factor (TNF)-alpha interleukin (IL)-1beta, IL-6 mRNA and protein content. In addition, the tissue content of insulin-like growth factor (IGF)-I and various regulators of protein synthesis were assessed.

RESULTS

Epi acutely increased TNF-alpha IL-6 and IL-1beta mRNA content in muscle (3- to 40-fold). However, only the TNF-alpha and IL-6 protein content was increased in muscle by Epi. In the second study, beta-adrenergic blockade with PP exacerbated the sepsis-induced increase in muscle IL-6 and TNF-alpha mRNA but did not alter the increment in IL-1beta or HMGB1. Propranolol also accentuated the sepsis-induced increase in both IL-6 and TNF-alpha protein in muscle. The exaggerated muscle cytokine response in septic rats treated with PP was associated with a reduction in muscle IGF-I protein that was greater than detected in saline-infused septic rats. Finally, the combination of sepsis + PP also accentuated the sepsis-induced decrease in the phosphorylation of 4E-binding protein-1, ribosomal protein S6, and mTOR, which are key proteins controlling protein synthesis.

CONCLUSIONS

These results demonstrate that although Epi is capable of increasing tissue cytokines in naive rats, inhibition of the beta-adrenergic effects of catecholamines exacerbates the sepsis-induced increase of selected inflammatory cytokines. This exaggerated tissue response is associated with alterations in muscle IGF-I protein and translation initiation, which would be expected to impair tissue protein synthesis.

Regulation of muscle growth by pathogen-associated molecules

Skeletal muscle demonstrates great plasticity in response to environmental and hormonal factors including pathogen-associated molecules, inflammatory cytokines, and growth factors. These signals impinge on muscle by forcing individual muscle fibers to either grow or atrophy. We recently demonstrated that skeletal muscle cells express multiple Toll-like receptors (TLR) that recognize bacterial cell wall components, such as lipopolysaccharide (LPS). Exposure of myocytes to LPS and other TLR ligands initiates an inflammatory response culminating in the autocrine production of cytokines and NO by NO synthase (NOS)2. The TLR signal through protein kinases that phosphorylate and promote the degradation of an inhibitory protein that normally retains the transcription factor, nuclear factor kappaB (NFkappaB), in the cytoplasm. Phosphorylation and degradation of the inhibitor of NFkappaB allows for translocation of NFkappaB to the nucleus and activation of inflammatory genes. Overexpression of a constitutively active inhibitor of NFkappaB kinase in skeletal muscle causes severe wasting, and we found that inhibitors of either the phosphorylation of IkappaB or its proteolytic degradation prevent TLR ligand-induced expression of cytokines and NOS2. The combination of LPS and interferon gamma dramatically enhances the magnitude and duration of LPS-stimulated NOS2 expression and reduces protein translation. Lipopolysaccharide and interferon gamma also downregulates signaling from the mammalian target of rapamycin, a kinase that directs changes in cell size. Inhibitors of NOS block the fall in muscle cell protein synthesis and restore translational signaling, indicating that activation of the NOS2-NO pathway is responsible for the observed decrease in muscle protein synthesis. Our work provides a molecular explanation for reduced muscle growth during infection. Muscle is largely self-sufficient because it expresses receptors, signaling pathways, and effectors to regulate its own size. Prolonged activation of NFkappaB and NOS2 have emerged as detrimental facets of the immune response in muscle. The interplay between inflammatory components and growth factor signaling clearly places muscle at the interface between growth and immunity.

Signaling pathways initiated by beta-hydroxy-beta-methylbutyrate to attenuate the depression of protein synthesis in skeletal muscle in response to cachectic stimuli

To investigate the mechanism by which beta-hydroxy-beta-methylbutyrate (HMB) attenuates the depression of protein synthesis in the skeletal muscle of cachectic mice, a study has been carried out in murine myotubes in the presence of proteolysis-inducing factor (PIF). PIF inhibited protein synthesis by 50% within 4 h, and this was effectively attenuated by HMB (25-50 muM). HMB (50 muM) alone stimulated protein synthesis, and this was attenuated by rapamycin (27 nM), an inhibitor of mammalian target of rapamycin (mTOR). Further evidence for an involvement of this pathway was shown by an increased phosphorylation of mTOR, the 70-kDa ribosomal S6 kinase (p70(S6k)), and initiation factor 4E-binding protein (4E-BP1) and an increased association of eukaryotic initiation factor 2 (eIF4E) with eIF4G. PIF alone induced a transient (1-2 h) stimulation of phosphorylation of mTOR and p70(S6k). However, in the presence of HMB, phosphorylation of mTOR, p70(S6k), and 4E-BP1 was increased, and inactive 4E-BP1-eIF4E complex was reduced, whereas the active eIF4G.eIF4E complex was increased, suggesting continual stimulation of protein synthesis. HMB alone reduced phosphorylation of elongation factor 2, but this effect was not seen in the presence of PIF. PIF induced autophosphorylation of the double-strand RNA-dependent protein kinase (PKR), leading to phosphorylation of eIF2 on the alpha-subunit, which would inhibit protein synthesis. However, in the presence of HMB, phosphorylation of PKR and eIF2alpha was attenuated, and this was also observed in skeletal muscle of cachectic mice administered HMB (0.25 g/kg). These results suggest that HMB attenuates the depression of protein synthesis by PIF in myotubes through multiple mechanisms.

Acute oral leucine administration stimulates protein synthesis during chronic sepsis through enhanced association of eukaryotic initiation factor 4G with eukaryotic initiation factor 4E in rats

Sepsis induces the loss of muscle proteins by impairing skeletal muscle protein synthesis through an inhibition of messenger RNA (mRNA) translation initiation. Amino acids and Leu (Leu) in particular stimulate mRNA translation initiation. The experiments were designed to test the effects of Leu on potential signal transduction pathways that may be important in accelerating mRNA translation initiation in skeletal muscle of rats with chronic (5-6 d) septic intra-abdominal abscess. Gastrocnemius from male Sprague Dawley rats gavaged with Leu or water were sampled 5-6 d following development of an intra-abdominal sterile or septic abscess. Gavage with Leu stimulated protein synthesis and enhanced the assembly of the active eukaryotic initiation factor (eIF)4G-eIF4E complex. Increased assembly of the active eIF4G-eIF4E complex was associated with a robust rise in phosphorylation of eIF4G(Ser(1108)) and a decreased assembly of inactive eIF4E binding protein-1 (4E-BP1)-eIF4E complex in both sterile inflammatory and septic rats. The reduced assembly of 4E-BP1-eIF4E complex was associated with an increase in phosphorylation of 4E-BP1 in the gamma-form following Leu gavage. Phosphorylation of 70-kDa ribosomal protein S6 kinase on Thr(389) was also increased following Leu gavage, as well as the phosphorylation of mammalian target of rapamycin on Ser(2448) or Ser(2481). In contrast, phosphorylation of protein kinase B (PKB) on Thr(308) or Ser(473) was not augmented following Leu gavage in septic rats. We conclude that Leu stimulates a PKB-independent signal pathway elevating the eIF4G-eIF4E complex assembly through increased phosphorylation of eIF4G and decreased association of 4E-BP1 with eIF4E in skeletal muscle during sepsis.

Regulation of muscle protein synthesis during sepsis and inflammation

Prolonged sepsis and exposure to an inflammatory milieu decreases muscle protein synthesis and reduces muscle mass. As a result of its ability to integrate diverse signals, including hormones and nutrients, the mammalian target of rapamycin (mTOR) is a dominant regulator in the translational control of protein synthesis. Under postabsorptive conditions, sepsis decreases mTOR kinase activity in muscle, as evidenced by reduced phosphorylation of both eukaryotic initiation factor (eIF)4E-binding protein (BP)-1 and ribosomal S6 kinase (S6K)1. These sepsis-induced changes, along with the redistribution of eIF4E from the active eIF4E.eIF4G complex to the inactive eIF4E.4E-BP1 complex, are preventable by neutralization of tumor necrosis factor (TNF)-alpha but not by antagonizing glucocorticoid action. Although the ability of mTOR to respond to insulin-like growth factor (IGF)-I is not disrupted by sepsis, the ability of leucine to increase 4E-BP1 and S6K1 phosphorylation is greatly attenuated. This "leucine resistance" results from a cooperative interaction between both TNF-alpha and glucocorticoids. Finally, although septic animals are not IGF-I resistant, the anabolic actions of IGF-I are nonetheless reduced because of the development of growth hormone resistance, which decreases both circulating and muscle IGF-I. Herein, we highlight recent advances in the mTOR signaling network and emphasize their connection to the atrophic response observed in skeletal muscle during sepsis. Although many unanswered questions remain, understanding the cellular basis of the sepsis-induced decrease in translational activity will contribute to the rational development of therapeutic interventions and thereby minimize the debilitating affects of the atrophic response that impairs patient recovery.

Glucocorticoids and TNFalpha interact cooperatively to mediate sepsis-induced leucine resistance in skeletal muscle

Sepsis blunts the ability of nutrient signaling by leucine to stimulate skeletal muscle protein synthesis by impairing translation initiation. The present study tested the hypothesis that overproduction of either tumor necrosis factor (TNF)-alpha or glucocorticoids mediate the sepsis-induced leucine resistance. Prior to producing peritonitis, rats received either vehicle, TNF binding protein (TNF(BP)) to inhibit endogenous TNFalpha action, and/or the glucocorticoid receptor antagonist RU486. Leucine was orally administered to all rats 24 h thereafter and the gastrocnemius removed 20 min later to assess protein synthesis and signaling components important in controlling peptide-chain initiation. Muscle protein synthesis was 65% lower in septic rats administered leucine than in leucine-treated control animals. This reduction was not prevented by either TNF(BP) or RU486 alone, but was completely reversed by the combination. This sepsis-induced leucine resistance was associated with an 80% reduction in the amount of active eIF4E.eIF4G complex, a 5-fold increase in the formation of the inactive eIF4E.4E-BP1 complex as well as markedly reduced (at least 70%) phosphorylation of 4E-BP1, eIF4G, S6K1, S6, and mTOR. Pretreatment of septic rats with either TNF(BP) or RU486 individually only nominally improved the leucine action as assessed by the above-mentioned endpoints. In contrast, when TNF(BP) and RU486 were co-administered, the ability of sepsis to impair the leucine-stimulated phosphorylation of 4E-BP1, eIF4G, S6K1, and S6 as well as the redistribution of eIF4E was essentially prevented. No differences in the total amount or phosphorylation of eIF2alpha and eIF2Bepsilon were detected between the different groups, and changes could not be attributed to differences in the prevailing plasma concentration of insulin or leucine. Our data demonstrate the sepsis-induced leucine resistance in skeletal muscle results from the cooperative interaction of both TNFalpha and glucocorticoids.

Protein kinase B/Akt: a nexus of growth factor and cytokine signaling in determining muscle mass

Although the boundaries of skeletal muscle size are fundamentally determined by genetics, this dynamic tissue also demonstrates great plasticity in response to environmental and hormonal factors. Recent work indicates that contractile activity, nutrients, growth factors, and cytokines all contribute to determining muscle mass. Muscle responds not only to endocrine hormones but also to the autocrine production of growth factors and cytokines. Skeletal muscle synthesizes anabolic growth factors such as insulin-like growth factor (IGF)-I and potentially inhibitory cytokines such as interleukin (IL)-6, tumor necrosis factor (TNF)-alpha, and myostatin. These self-regulating inputs in turn influence muscle metabolism, including the use of nutrients such as glucose and amino acids. These changes are principally achieved by altering the activity of the protein kinase known as protein kinase B or Akt. Akt plays a central role in integrating anabolic and catabolic responses by transducing growth factor and cytokine signals via changes in the phosphorylation of its numerous substrates. Activation of Akt stimulates muscle hypertrophy and antagonizes the loss of muscle protein. Here we review the many signals that funnel through Akt to alter muscle mass.

Hormone, cytokine, and nutritional regulation of sepsis-induced increases in atrogin-1 and MuRF1 in skeletal muscle

Various atrophic stimuli increase two muscle-specific E3 ligases, muscle RING finger 1 (MuRF1) and atrogin-1, and knockout mice for these "atrogenes" display resistance to denervation-induced atrophy. The present study determined whether increased atrogin-1 and MuRF1 mRNA are mediated by overproduction of endogenous glucocorticoids or inflammatory cytokines in adult rats and whether atrogene expression can be downregulated by anabolic agents such as insulin-like growth factor (IGF)-I and the nutrient-signaling amino acid leucine. Both atrogin-1 and MuRF1 mRNA in gastrocnemius was upregulated dose and time dependently by endotoxin. Additionally, peritonitis produced by cecal ligation and puncture increased atrogin-1 and MuRF1 mRNA in gastrocnemius (but not soleus or heart) by 8 h, which was sustained for 72 and 24 h, respectively. Whereas the sepsis-induced increase in atrogin-1 expression was completely prevented by IGF-I, the increased MuRF1 was not altered. In contrast to the IGF-I effect, the sepsis-induced increased mRNA of both atrogenes was unresponsive to either acute or repetitive administration of leucine. Whereas exogenous infusion of TNF-alpha increased atrogin-1 and MuRF1 in gastrocnemius, pretreatment of septic rats with the TNF antagonist TNF-binding protein did not prevent increased expression of either atrogene. Similarly, whereas dexamethasone increased atrogene expression, pretreatment with the glucocorticoid receptor antagonist RU-486 failed to ameliorate the sepsis-induced increase in atrogin-1 and MuRF1. Thus, under in vivo conditions in mature adult rats, the sepsis-induced increase in muscle atrogin-1 and MuRF1 mRNA appears both glucocorticoid and TNF independent and is unresponsive to leucine.

Sepsis-induced suppression of skeletal muscle translation initiation mediated by tumor necrosis factor alpha

Inhibition of translational efficiency is responsible at least in part for the sepsis-induced decrease in protein synthesis observed in skeletal muscle. Moreover, infusion of the inflammatory cytokine tumor necrosis factor alpha (TNF-alpha) into naive rats produces a comparable decrement. Therefore, the purpose of the present study was to determine whether inhibition of TNF action under in vivo conditions could prevent the sepsis-induced decrease in translation initiation observed in the postabsorptive state. To address this aim, sepsis was produced by cecal ligation and puncture (CLP) and rats were studied in the fasted condition 20 to 24 hours thereafter. Both septic and time-matched nonseptic control rats were pretreated with TNF-binding protein (TNF(BP)) before CLP or sham surgery to neutralize endogenously produced TNF. Sepsis altered the distribution of eukaryotic initiation factor 4E (eIF4E) in the gastrocnemius by increasing the amount associated with 4E-BP1 (inactive complex) and decreasing the amount bound to eIF4G (active complex). This change in eIF4E availability was associated with a decreased phosphorylation of 4E-BP1. Furthermore, the phosphorylation of ribosomal protein S6 and mammalian target of rapamycin (mTOR) was also decreased in the gastrocnemius from septic rats. Pretreatment of septic rats with TNF(BP) largely ameliorated the altered distribution of eIF4E as well as the reduced phosphorylation of 4E-BP1, S6, and mTOR. In contrast, sepsis did not change either the total amount or the phosphorylation state of eIF2alpha or eIF2Bepsilon. Furthermore, no sepsis-induced change in eIFs was detected in the slow-twitch soleus muscle. The ability of TNF(BP) to prevent the sepsis-induced alterations in translation initiation was independent of change in plasma insulin and proportional to the insulinlike growth factor I content in blood and muscle but was associated with a reduction in plasma corticosterone. Hence, the decreased constitutive protein synthesis observed in fast-twitch skeletal muscle in response to peritonitis is mediated by a TNF-dependent mechanism affecting mTOR regulation of translation initiation.

Elevated plasma free fatty acids decrease basal protein synthesis, but not the anabolic effect of leucine, in skeletal muscle

Elevations in free fatty acids (FFAs) impair glucose uptake in skeletal muscle. However, there is no information pertaining to the effect of elevated circulating lipids on either basal protein synthesis or the anabolic effects of leucine and insulin-like growth factor I (IGF-I). In chronically catheterized conscious rats, the short-term elevation of plasma FFAs by the 5-h infusion of heparin plus Intralipid decreased muscle protein synthesis by approximately 25% under basal conditions. Lipid infusion was associated with a redistribution of eukaryotic initiation factor (eIF)4E from the active eIF4E.eIF4G complex to the inactive eIF4E.4E-BP1 complex. This shift was associated with a decreased phosphorylation of eIF4G but not 4E-BP1. Lipid infusion did not significantly alter either the total amount or phosphorylation state of mTOR, TSC2, S6K1, or the ribosomal protein S6 under basal conditions. In control rats, oral leucine increased muscle protein synthesis. This anabolic response was not impaired by lipid infusion, and no defects in signal transduction pathways regulating translation initiation were detected. In separate rats that received a bolus injection of IGF-I, lipid infusion attenuated the normal redistribution of eIF4E from the active to inactive complex and largely prevented the increased phosphorylation of 4E-BP1, eIF4G, S6K1, and S6. This IGF-I resistance was associated with enhanced Ser(307) phosphorylation of insulin receptor substrate-1 (IRS-1). These data indicate that the short-term elevation of plasma FFAs impairs basal protein synthesis in muscle by altering eIF4E availability, and this defect may be related to impaired phosphorylation of eIF4G, not 4E-BP1. Moreover, hyperlipidemia impairs IGF-I action but does not produce leucine resistance in skeletal muscle.

Sepsis and inflammatory insults downregulate IGFBP-5, but not IGFBP-4, in skeletal muscle via a TNF-dependent mechanism

The purpose of the present study was to determine whether catabolic stimuli that induce muscle atrophy alter the muscle mRNA abundance of insulin-like growth factor binding protein (IGFBP)-4 and -5, and if so determine the physiological mechanism for such a change. Catabolic insults produced by endotoxin (LPS) and sepsis decreased IGFBP-5 mRNA time- and dose-dependently in gastrocnemius muscle. This reduction did not result from muscle disuse because hindlimb immobilization increased IGFBP-5. Continuous infusion of a nonlethal dose of tumor necrosis factor-α (TNF-α) decreased IGFBP-5 mRNA 70%, whereas pretreatment of septic rats with a neutralizing TNF binding protein completely prevented the reduction in muscle IGFBP-5. The addition of LPS or TNF-α to cultured C2C12myoblasts also decreased IGFBP-5 expression. Although exogenously administered growth hormone (GH) increased IGFBP-5 mRNA 2-fold in muscle from control rats, muscle from septic animals was GH resistant and no such elevation was detected. In contrast, exogenous administration of IGF-I as part of a binary complex composed of IGF-I/IGFBP-3 produced comparable increases in IGFBP-5 mRNA in both control and septic muscle. Concomitant determinations of IGF-I mRNA content revealed a positive linear relationship between IGF-I and IGFBP-5 mRNA in the same muscle in response to LPS, sepsis, TNF-α, and GH treatment. Although dexamethasone decreased muscle IGFBP-5, pretreatment of rats with the glucocorticoid receptor antagonist RU486 did not prevent the sepsis-induced decrease in IGFBP-5 mRNA. In contrast, muscle IGFBP-4 mRNA abundance was not significantly altered by LPS, sepsis, or hindlimb immobilization. In summary, these data demonstrate that various inflammatory insults decrease muscle IGFBP-5 mRNA, without altering IGFBP-4, by a TNF-dependent glucocorticoid-independent mechanism. Finally, IGF-I appears to be a dominant positive regulator of IGFBP-5 gene expression in muscle under both normal and catabolic conditions.

Indinavir alters regulators of protein anabolism and catabolism in skeletal muscle

The HIV protease inhibitor indinavir adversely impairs carbohydrate and lipid metabolism, whereas its influence on protein metabolism under in vivo conditions remains unknown. The present study tested the hypothesis that indinavir also decreases basal protein synthesis and impairs the anabolic response to insulin in skeletal muscle. Indinavir was infused intravenously for 4 h into conscious rats, at which time the homeostasis model assessment of insulin resistance was increased. Indinavir decreased muscle protein synthesis by 30%, and this reduction was due to impaired translational efficiency. To identify potential mechanisms responsible for regulating mRNA translation, several eukaryotic initiation factors (eIFs) were examined. Under basal fasted conditions, there was a redistribution of eIF4E from the active eIF4E.eIF4G complex to the inactive eIF4E.4E-BP1 complex, and this change was associated with a marked decrease in the phosphorylation of 4E-BP1 in muscle. Likewise, indinavir decreased constitutive phosphorylation of eIF4G and mTOR in muscle, but not S6K1 or the ribosomal protein S6. In contrast, the ability of a maximally stimulating dose of insulin to increase the phosphorylation of PKB, 4E-BP1, S6K1, or mTOR was not altered 20 min after intravenous injection. Indinavir increased mRNA expression of the ubiquitin ligase MuRF1, but the plasma concentration of 3-methylhistidine remained unaltered. These indinavir-induced changes were associated with a marked reduction in the plasma testosterone concentration but were independent of changes in plasma levels of IGF-I, corticosterone, TNF-alpha, or IL-6. In conclusion, indinavir acutely impairs basal protein synthesis and translation initiation in skeletal muscle but, in contrast to muscle glucose uptake, does not impair insulin-stimulated signaling of protein synthetic pathways.

TNFalpha mediates sepsis-induced impairment of basal and leucine-stimulated signaling via S6K1 and eIF4E in cardiac muscle

Decreased translation initiation adversely impacts protein synthesis and contributes to the myocardial dysfunction produced by sepsis. Therefore, the purpose of the present study was to identify sepsis-induced changes in signal transduction pathways known to regulate translation initiation in cardiac muscle and to determine whether the stimulatory effects of leucine can reverse the observed defects. To address this aim, sepsis was produced by cecal ligation and puncture (CLP) in anesthetized rats and the animals studied in the fasted condition 24 h later. Separate groups of septic and time-matched control rats also received an oral gavage of leucine. To identify potential mechanisms responsible for regulating cap-dependent mRNA translation in cardiac muscle, several eukaryotic initiation factors (eIFs) were examined. Under basal conditions, hearts from septic rats demonstrated a redistribution of the rate-limiting factor eIF4E due to increased binding of the translational repressor 4E-BP1 with eIF4E. However, this change was independent of an alteration in the phosphorylation state of 4E-BP1. The phosphorylation of mTOR, S6K1, the ribosomal protein (rp) S6, and eIF4G was not altered in hearts from septic rats under basal conditions. In control rats, leucine failed to alter eIF4E distribution but increased the phosphorylation of S6K1 and S6. In contrast, in hearts from septic rats leucine acutely reversed the alterations in eIF4E distribution. However, the ability of leucine to increase S6K1 and rpS6 phosphorylation in septic hearts was blunted. Sepsis increased the content of tumor necrosis factor (TNF)-alpha in heart and pre-treatment of rats with a TNF antagonist prevented the above-mentioned sepsis-induced changes. These data indicate that oral administration of leucine acutely reverses sepsis-induced alterations eIF4E distribution observed under basal conditions but the anabolic actions of this amino acid on S6K1 and rpS6 phosphorylation remain blunted, providing evidence for a leucine resistance. Finally, TNFalpha, either directly or indirectly, appears to mediate the sepsis-induced defects in myocardial translation initiation.