Skeletal Muscle Protein Balance Research Articles

Molecular mechanisms of post-burn muscle wasting and the therapeutic potential of physical exercise.

After a severe burn injury, a systemic stress response activates metabolic and inflammatory derangements that, among other, leads to muscle mass loss (muscle wasting). These negative effects on skeletal muscle continue for several months or years and are aggravated by short-term and long-term disuse. The dynamic balance between muscle protein synthesis and muscle protein breakdown (proteolysis) is regulated by complex signalling pathways that leads to an overall negative protein balance in skeletal muscle after a burn injury. Research concerning these molecular mechanisms is still scarce and inconclusive, understanding of which, if any, molecular mechanisms contribute to muscle wasting is of fundamental importance in designing of therapeutic interventions for burn patients as well. This review not only summarizes our present knowledge of the molecular mechanisms that underpin muscle protein balance but also summarizes the effects of exercise on muscle wasting post-burn as promising strategy to counteract the detrimental effects on skeletal muscle. Future research focusing on the pathways causing post-burn muscle wasting and the different effects of exercise on them is needed to confirm this hypothesis and to lay the foundation of therapeutic strategies.

Binge drinking effects on markers of skeletal muscle protein synthesis and degradation

Rates of binge drinking in the United States and around the world are increasing despite much data on the detrimental effects associated with this behavior. The deleterious effects of alcohol on brain, liver, and cardiac muscle have been well characterized, but there is still a gap in our understanding of the reduction in the net protein balance in skeletal muscle. Prior work has demonstrated dysregulation in muscle protein synthesis in response to chronic alcohol exposure; however, pathways involved in protein breakdown have been largely unstudied, and studies have been limited to work almost exclusively in males. The aim of the current study was to investigate the effects of binge drinking on markers of muscle protein synthesis and degradation, and to examine the role of exercise in this context. 36 female C57BL/6 mice (7‐weeks of age) were randomized to the following conditions: half of the mice were allowed access to a running wheel for 2.5 hours/day for 5 weeks (EX) and half remained sedentary (SED). During the final week, half of the mice from each condition underwent a binge drinking protocol, in which mice were allowed restricted alcohol access for 2 hours for 4 consecutive days and on the fifth day allowed to drink ad libitum for 4 hours. This experimental paradigm resulted in the following four groups: SED water, SED alcohol, EX water, and EX alcohol. Gastrocnemius gene expression of the muscle enriched ubiquitin ligases MuRF1 and MAFbx, was assessed via qPCR. Phosphorylation of p70S6K was examined by Western blot as an indicator of anabolic signaling. There were no differences in food intake or body weight among groups. MuRF1 was significantly decreased with exercise (p = 0.03), but unaffected by alcohol. There was a significant interaction between alcohol and exercise (p = 0.04), and a main effect of exercise (p < 0.001) on MuRF1. In sedentary mice, alcohol tended to increase MAFbx mRNA (p=0.07); whereas EX alcohol mice had reduced MAFbx compared to SED alcohol (p < 0.001). There was a tendency of exercise to increase phosphorylation of p70S6k (p = 0.065). Our results suggest this model of binge drinking may be useful for determining the effects of binge drinking on the skeletal muscle protein balance, and alcohol may promote expression of atrogenes. In addition, daily exercise could mitigate the dysregulation in skeletal muscle homeostasis caused by alcohol.

Reduced skeletal muscle protein balance in paediatric Crohn's disease

An inability to respond to nutrition could be implicated in low muscle mass in Crohn's disease. We aim to determine skeletal muscle metabolic response to feeding in Crohn's disease and healthy volunteers. Twenty asymptomatic Crohn's disease participants (15.6±0.5yrs; BMI 20.6±0.9kg/m2); 9 with active disease (faecal calprotectin, 808±225ug/g and C-reactive protein, 2.2±1.2mg/dl), 11 in deep remission (faecal calprotectin, 61±12ug/g and C-reactive protein, 0.3±0.2mg/dl) and 9 matched healthy volunteers (16.0±0.6yrs; BMI 20.7±0.6kg/m2) were recruited. Participants had a dual energy X-ray absorptiometry scan, handgrip dynamometer test, wore a pedometer and completed a food diary. Arterialised hand and venous forearm blood samples were collected concurrently and brachial artery blood flow measured at baseline and every 20min for 2hrs after the ingestion of a standardised liquid meal. Net balance of branched chain amino acids (BCAA) and glucose were derived. Controls had a positive mean BCAA balance. CD participants had an initial anabolic response to the meal, with increasing BCAA balance between t=0 & t=20, but returned to negative by t=60. This was associated with reduced FFM z-scores in CD but not with insulin resistance or disease activity. Exploratory analyses suggest that negative postprandial BCAA response seen in CD is predominant in males (p=0.049), with associated lower appendicular muscle mass (p=0.034), higher muscle fatigue (p=0.014) and reduced protein intake (p=0.026). The inability to sustain a positive protein balance postprandially could provide an explanation for the reduced muscle mass seen in CD. Further mechanistic studies will be needed to confirm these findings.

Beta2 -adrenoceptor agonist salbutamol increases protein turnover rates and alters signalling in skeletal muscle after resistance exercise in young men.

Animal models have shown that beta2 -adrenoceptor stimulation increases protein synthesis and attenuates breakdown processes in skeletal muscle. Thus, the beta2 -adrenoceptor is a potential target in the treatment of disuse-, disease- and age-related muscle atrophy. In the present study, we show that a few days of oral treatment with the commonly prescribed beta2 -adrenoceptor agonist, salbutamol, increased skeletal muscle protein synthesis and breakdown during the first 5h after resistance exercise in young men. Salbutamol also counteracted a negative net protein balance in skeletal muscle after resistance exercise. Changes in protein turnover rates induced by salbutamol were associated with protein kinase A-signalling, activation of Akt2 and modulation of mRNA levels of growth-regulating proteins in skeletal muscle. These findings indicate that protein turnover rates can be augmented by beta2 -adrenoceptor agonist treatment during recovery from resistance exercise in humans. The effect of beta2 -adrenoceptor stimulation on skeletal muscle protein turnover and intracellular signalling is insufficiently explored in humans, particularly in association with exercise. In a randomized, placebo-controlled, cross-over study investigating 12 trained men, the effects of beta2 -agonist (6×4mg oral salbutamol) on protein turnover rates, intracellular signalling and mRNA response in skeletal muscle were investigated 0.5-5h after quadriceps resistance exercise. Each trial was preceded by a 4-day lead-in treatment period. Leg protein turnover rates were assessed by infusion of [13 C6 ]-phenylalanine and sampling of arterial and venous blood, as well as vastus lateralis muscle biopsies 0.5 and 5h after exercise. Furthermore, myofibrillar fractional synthesis rate, intracellular signalling and mRNA response were measured in muscle biopsies. The mean (95% confidence interval) myofibrillar fractional synthesis rate was higher for salbutamol than placebo [0.079(95% CI, 0.064 to 0.093) vs. 0.066(95% CI, 0.056 to 0.075%)×h-1 ] (P<0.05). Mean net leg phenylalanine balance 0.5-5h after exercise was higher for salbutamol than placebo [3.6(95% CI, 1.0 to 6.2nmol)×min-1 ×100 gLeg Lean Mass-1 ] (P<0.01). Phosphorylation of Akt2, cAMP response element binding protein and PKA substrate 0.5 and 5h after exercise, as well as phosphorylation of eEF2 5h after exercise, was higher (P<0.05) for salbutamol than placebo. Calpain-1, Forkhead box protein O1, myostatin and Smad3 mRNA content was higher (P<0.01) for salbutamol than placebo 0.5h after exercise, as well as Forkhead box protein O1 and myostatin mRNA content 5h after exercise, whereas ActivinRIIB mRNA content was lower (P<0.01) for salbutamol 5h after exercise. These observations suggest that beta2 -agonist increases protein turnover rates in skeletal muscle after resistance exercise in humans, with concomitant cAMP/PKA and Akt2 signalling, as well as modulation of mRNA response of growth-regulating proteins.

P019 Paediatric Crohn’s disease patients in remission have a reduced skeletal muscle protein balance after feeding

P019 Paediatric Crohn’s disease patients in remission have a reduced skeletal muscle protein balance after feeding

Systemic inflammation is associated with exaggerated skeletal muscle protein catabolism in maintenance hemodialysis patients.

Systemic inflammation and muscle wasting are highly prevalent and coexist in patients on maintenance hemodialysis (MHD). We aimed to determine the effects of systemic inflammation on skeletal muscle protein metabolism in MHD patients. Whole body and skeletal muscle protein turnover were assessed by stable isotope kinetic studies. We incorporated expressions of E1, E214K, E3αI, E3αII, MuRF-1, and atrogin-1 in skeletal muscle tissue from integrin β1 gene KO CKD mice models. Among 129 patients with mean (± SD) age 47 ± 12 years, 74% were African American, 73% were male, and 22% had diabetes mellitus. Median high-sensitivity C-reactive protein (hs-CRP) concentration was 13 (interquartile range 0.8, 33) mg/l. There were statistically significant associations between hs-CRP and forearm skeletal muscle protein synthesis, degradation, and net forearm skeletal muscle protein balance (P < 0.001 for all). The associations remained statistically significant after adjustment for clinical and demographic confounders, as well as in sensitivity analysis, excluding patients with diabetes mellitus. In attempting to identify potential mechanisms involved in this correlation, we show increased expressions of E1, E214K, E3αI, E3αII, MuRF-1, and atrogin-1 in skeletal muscle tissue obtained from an animal model of chronic kidney disease. These data suggest that systemic inflammation is a strong and independent determinant of skeletal muscle protein homeostasis in MHD patients, providing rationale for further studies using anticytokine therapies in patients with underlying systemic inflammation. This study was in part supported by NIH grants R01 DK45604 and 1K24 DK62849, the Clinical Translational Science Award UL1-TR000445 from the National Center for Advancing Translational Sciences, the Veterans Administration Merit Award I01 CX000414, the SatelliteHealth Normon Coplon Extramural Grant Program, and the FDA grant 000943.

Restorative Mechanisms Regulating Protein Balance in Skeletal Muscle During Recovery From Sepsis

Muscle deconditioning is commonly observed in patients surviving sepsis. Little is known regarding the molecular mechanisms regulating muscle protein homeostasis during the recovery or convalescence phase. We adapted a sepsis-recovery mouse model that uses cecal ligation and puncture (CLP), followed 24 h later by cecal resection and antibiotic treatment, to identify putative cellular pathways regulating protein synthesis and breakdown in skeletal muscle. Ten days after CLP, body weight and food consumption did not differ between control and sepsis-recovery mice, but gastrocnemius weight was reduced. During sepsis-recovery, muscle protein synthesis was increased 2-fold and associated with enhanced mTOR kinase activity (4E-BP1 and S6K1 phosphorylation). The sepsis-induced increase in 4E-BP1 was associated with enhanced formation of the eIF4E-eIF4G active cap-dependent complex, while the increased S6K1 was associated with increased phosphorylation of downstream targets S6 and eIF4B. Proximal to mTOR, sepsis-recovery increased Akt and TSC2 phosphorylation, did not alter AMPK phosphorylation, and decreased REDD1 protein content. Despite the decreased mRNA content for the E3 ubiquitin ligases atrogin-1 and muscle RING-finger 1, proteasomal activity was increased 50%. In contrast, sepsis-recovery was associated with an apparent decrease in autophagy (e.g., increased ULK-1 phosphorylation, decreased LCB3-II, and increased p62). The mRNA content for IL-1β, IL-18, TNFα, and IL-6 in muscle was elevated in sepsis-recovery. During recovery after sepsis skeletal muscle responds with an increase in Akt-TSC2-mTOR-dependent protein synthesis and decreased autophagy, but full restoration of muscle protein content may be slowed by the continued stimulation of ubiquitin-proteasome activity.

Alterations in protein and amino acid metabolism in rats fed a branched-chain amino acid- or leucine-enriched diet during postprandial and postabsorptive states.

BackgroundMany people believe in favourable effects of branched-chain amino acids (BCAAs; valine, leucine, and isoleucine), especially leucine, on muscle protein balance and consume BCAAs for many years. We determined the effects of the chronic intake of a BCAA- or leucine-enriched diet on protein and amino acid metabolism in fed and postabsorptive states.MethodsRats were fed a standard diet, a diet with a high content of valine, leucine, and isoleucine (HVLID), or a high content of leucine (HLD) for 2 months. Half of the animals in each group were sacrificed in the fed state on the last day, and the other half were sacrificed after overnight fast. Protein synthesis was assessed using the flooding dose method (L-[3,4,5-3H]phenylalanine), proteolysis on the basis of chymotrypsin-like activity (CHTLA) of proteasome and cathepsin B and L activities.ResultsChronic intake of HVLID or HLD enhanced plasma levels of urea, alanine and glutamine. HVLID also increased levels of all three BCAA and branched-chain keto acids (BCKA), HLD increased leucine, ketoisocaproate and alanine aminotransferase and decreased valine, ketovaline, isoleucine, ketoisoleucine, and LDL cholesterol. Tissue weight and protein content were lower in extensor digitorum longus muscles in the HLD group and higher in kidneys in the HVLID and HLD groups. Muscle protein synthesis in postprandial state was higher in the HVLID group, and CHTLA was lower in muscles of the HVLID and HLD groups compared to controls. Overnight starvation enhanced alanine aminotransferase activity in muscles, and decreased protein synthesis in gastrocnemius (in HVLID group) and extensor digitorum longus (in HLD group) muscles more than in controls. Effect of HVLID and HLD on CHTLA in muscles in postabsorptive state was insignificant.ConclusionsThe results failed to demonstrate positive effects of the chronic consumption of a BCAA-enriched diet on protein balance in skeletal muscle and indicate rather negative effects from a leucine-enriched diet. The primary effects of both diets are an activated catabolism of BCAAs, which leads to an enhanced production of BCKA, alanine and glutamine and their utilization in visceral tissues and an impaired protein synthesis in postabsorptive state, particularly in fast-twitch (white) muscles.

Dysregulation of skeletal muscle protein metabolism by alcohol.

Alcohol abuse, either by acute intoxication or prolonged excessive consumption, leads to pathological changes in many organs and tissues including skeletal muscle. As muscle protein serves not only a contractile function but also as a metabolic reserve for amino acids, which are used to support the energy needs of other tissues, its content is tightly regulated and dynamic. This review focuses on the etiology by which alcohol perturbs skeletal muscle protein balance and thereby over time produces muscle wasting and weakness. The preponderance of data suggest that alcohol primarily impairs global protein synthesis, under basal conditions as well as in response to several anabolic stimuli including growth factors, nutrients, and muscle contraction. This inhibitory effect of alcohol is mediated, at least in part, by a reduction in mTOR kinase activity via a mechanism that remains poorly defined but likely involves altered protein-protein interactions within mTOR complex 1. Furthermore, alcohol can exacerbate the decrement in mTOR and/or muscle protein synthesis present in other catabolic states. In contrast, alcohol-induced changes in muscle protein degradation, either global or via specific modulation of the ubiquitin-proteasome or autophagy pathways, are relatively inconsistent and may be model dependent. Herein, changes produced by acute intoxication versus chronic ingestion are contrasted in relation to skeletal muscle metabolism, and limitations as well as opportunities for future research are discussed. As the proportion of more economically developed countries ages and chronic illness becomes more prevalent, a better understanding of the etiology of biomedical consequences of alcohol use disorders is warranted.

Treating sarcopenia in older and oldest old.

The presence of sarcopenia is not only rapidly rising in geriatric clinical practice and research, but is also becoming a significant concept in numerous medical specialties. This rapidly rising concept has encouraged the need to identify methods for treating sarcopenia. Physical activity measures using resistance training exercise, combined with nutritional interventions (protein and amino acid supplementation) have shown to significantly improve muscle mass and strength in older persons. Moreover, resistance training may improve muscle strength and mass by improving protein synthesis in skeletal muscle cells. Aerobic exercise has also shown to hold beneficial impacts on sarcopenia by improving insulin sensitivity. At the moment, the literature indicates that most significant improvement in sarcopenia is based on exercise programs. Thus, this type of intervention should be implemented in a persistent manner over time in elders, with or at risk of muscle loss. At the same time, physical training exercise should include correcting nutritional deficits with supplementation methods. For example, in older sarcopenic patients with adequate renal function, daily protein intake should be increased to >1. 0 grams of protein per kilogram of body weight. In particular, leucine, - hydroxy β-methylbutyrate (HMB), creatine and some milk-based proteins have been shown to improve skeletal muscle protein balance. In addition, it is also recommended for adjustment of for vitamin D deficiency, if present, considering the crucial role of vitamin D in the skeletal muscle. In this review, we provide evidence regarding the effects of different physical exercise protocols, specific nutritional intervention, and some new metabolic agents (HMB, citrulline malate, ornithine, and others) on clinical outcomes related to sarcopenia in older adults.

Paraplegia increases skeletal muscle autophagy

Paraplegia results in significant skeletal muscle atrophy through increases in skeletal muscle protein breakdown. Recent work has identified a novel SIRT1-p53 pathway that is capable of regulating autophagy and protein breakdown. Soleus muscle was collected from 6 male Sprague-Dawley rats 10 weeks after complete T4-5 spinal cord transection (paraplegia group) and 6 male sham-operated rats (control group). We utilized immunoblotting methods to measure intracellular proteins and quantitative real-time polymerase chain reaction to measure the expression of skeletal muscle microRNAs. SIRT1 protein expression was 37% lower, and p53 acetylation (LYS379) was increased in the paraplegic rats (P < 0.05). Atg7 and Beclin-1, markers of autophagy induction, were elevated in the paraplegia group compared with controls (P < 0.05). Severe muscle atrophy resulting from chronic paraplegia appears to increase skeletal muscle autophagy independent of SIRT1 signaling. We conclude that chronic paraplegia may cause an increase in autophagic cell death and negatively impact skeletal muscle protein balance.

Skeletal muscle protein balance and metabolism in the elderly.

The loss of lean muscle mass occurring with advancing age is termed sarcopenia. This condition often leads to a concomitant loss of strength, increased frailty and risk of falls and an overall loss of functional independence in the elderly. Muscle protein metabolism is a dynamic process characterized by the balance between the synthesis and breakdown of muscle proteins. A disturbance of this equilibrium can lead to the loss of muscle mass, and a perturbation of muscle protein turnover with aging has been proposed to play a role in the development of sarcopenia. However, basal muscle protein synthesis and breakdown rates do not differ between young and old adults, which has led to the hypothesis that older adults are resistant to anabolic stimuli. In support of this hypothesis, older adults have either no response or a blunted response to nutrients, insulin and resistance exercise, and this anabolic resistance is likely a key factor in the loss of skeletal muscle mass with aging. Recent studies have investigated potential interventions to overcome this anabolic resistance. In particular, combining resistance exercise with essential amino acid supplementation restores the muscle protein anabolic response in older men. The novel rehabilitation technique of performing light resistance exercise during blood flow restriction was also successful in overcoming the anabolic resistance to exercise. Future research is needed to determine whether these novel interventions will be successful in preventing sarcopenia and improving muscle strength and function in older adults.

Adverse effects of chronic intake of glutamine-supplemented diet on amino acid concentrations and protein metabolism in rat: Effect of short-term starvation

Summary Background & aims Glutamine supplementation is recommended in severe illness, preterm infants, and athletes. However, the information about side effects of chronic intake of glutamine-supplemented diet is limited. Methods Male Wistar rats were fed with glutamine-supplemented diet (GlnD). The controls were fed by standard or high-protein diets. After three months, the effect of GlnD on amino acid concentration in body fluids and protein metabolism was examined in fed and in 24 h-fasted animals. Protein synthesis was examined by the “flooding method” using phenylalanine; proteolysis on the basis of chymotrypsin-like activity (CHTLA) of proteasome and cathepsin B and L activities; amino acid concentrations were estimated using HPLC. Student’s t -tests with Bonferroni correction were used for statistical analysis. Results In the blood of GlnD fed animals sacrificed in fed state we found higher concentrations of glutamine, urea, and histidine, and lower concentration of creatinine and of a number of amino acids than in controls. The main alterations in intracellular free amino acid concentration were increased concentration of glutamine, increased concentration of histidine in skeletal muscle, and decreased concentrations of glycine, serine, and threonine in several tissues. In GlnD fed animals we found enhanced amount and concentration of protein in the kidneys, decreased protein content in tibialis muscle, and enhanced CHTLA in soleus muscle. We did not find significant effect on protein synthesis in any tissue. Starvation of animals previously fed by GlnD led to a marked decrease in glutamine concentration and an increase in glycine, serine, and threonine in plasma and in most of examined tissues, and in more significant decrease in protein synthesis in the liver, jejunum, colon, and spleen than in animals fed before starvation by standard diet. Conclusions It is concluded that chronic intake of GlnD has negative effect on protein balance in skeletal muscle, significantly alters amino acid concentrations in plasma and tissues, and results in impaired response to starvation, particularly to the rapid development of glutamine deficiency in body fluids and impaired protein synthesis in visceral tissues.

Intracellular signaling pathways regulating net protein balance following diaphragm muscle denervation

Unilateral denervation (DNV) of rat diaphragm muscle increases protein synthesis at 3 days after DNV (DNV-3D) and degradation at DNV-5D, such that net protein breakdown is evident by DNV-5D. On the basis of existing models of protein balance, we examined DNV-induced changes in Akt, AMP-activated protein kinase (AMPK), and ERK&frac12; activation, which can lead to increased protein synthesis via mammalian target of rapamycin (mTOR)/p70S6 kinase (p70S6K), glycogen synthase kinase-3β (GSK3β), or eukaryotic initiation factor 4E (eIF4E), and increased protein degradation via forkhead box protein O (FoxO). Protein phosphorylation was measured using Western analyses through DNV-5D. Akt phosphorylation decreased at 1 h and 6 h after DNV compared with sham despite decreased AMPK phosphorylation. Both Akt and AMPK phosphorylation returned to sham levels by DNV-1D. Phosphorylation of their downstream effector mTOR (Ser2481) did not change at any time point after DNV, and phosphorylated p70S6K and eIF4E-binding protein 1 (4EBP1) increased only by DNV-5D. In contrast, ERK&frac12; phosphorylation and its downstream effector eIF4E increased 1.7-fold at DNV-1D and phosphorylated GSK3β increased 1.5-fold at DNV-3D (P < 0.05 for both comparisons). Thus, following DNV there are differential effects on protein synthetic pathways with preferential activation of GSK3β and eIF4E over p70S6K. FoxO1 nuclear translocation occurred by DNV-1D, consistent with its role in increasing expression of atrogenes necessary for subsequent ubiquitin-proteasome activation evident by DNV-5D. On the basis of our results, increased protein synthesis following DNV is associated with changes in ERK&frac12;-dependent pathways, but protein degradation results from downregulation of Akt and nuclear translocation of FoxO1. No single trigger is responsible for protein balance following DNV. Protein balance in skeletal muscle depends on multiple synthetic/degradation pathways that should be studied in concert.

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.

Neuregulin reduces protein degradation in skeletal muscle in a PI3 kinase/Akt and MAP kinase dependent manner

Skeletal muscle growth is achieved by a net increase in muscle protein synthesis, decreased protein degradation or both. The Akt and mTOR signaling transduction pathways regulate both protein synthesis and degradation. We previously reported that the nerve‐derived growth factor neuregulin (NRG) increases muscle protein synthesis (~20%) in a PI3 kinase (PI3K)/Akt‐dependent manner. In the present study, we report that basal protein degradation in the diaphragm muscle is also significantly reduced by NRG treatment (23%). The NRG‐induced reduction of basal protein degradation is abrogated by pretreatment with inhibitors of PI3K (LY294002, 50 μM), mTOR (rapamycin, 100 nM) or MEK (PD98059, 50 μM). In fact, inhibition of any of these pathways alone is sufficient to increase basal protein degradation by greater than 30%. Importantly, NRG treatment nullifies the increase in protein degradation induced by PI3K or MEK inhibition, but not that induced by mTOR inhibition. Thus, unlike NRG effects on protein synthesis, the NRG‐induced decrease in protein degradation is abrogated not only by inhibition of the PI3K/Akt pathway, but also by inhibition of the MAP kinase. Based on these results, we suggest that both the PI3K/Akt and MAP kinase pathways are important for NRG effects on protein degradation, and that mTOR is a critical regulator of NRG effects on protein balance in skeletal muscle. Supported by NIH grant AR51173.

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

We assessed the role of mTOR in regulating muscle protein synthesis in wild‐type (WT) and mTOR heterozygous (+/−) mice under basal conditions, and in response to endotoxin (LPS) and/or leucine. Lean body mass was decreased in mTOR+/− vs WT mice. Gastrocnemius weight was decreased in mTOR+/− mice due in part to a reduced rate of basal protein synthesis. LPS decreased muscle protein synthesis in WT and mTOR+/− mice to the same extent. Changes in protein synthesis in mTOR+/− mice under basal and LPS‐stimulated conditions were associated with reduced 4E‐BP1 and S6K1 phosphorylation. LPS also decreased PRSA40 phosphorylation and increased phosphorylation of raptor and IRS‐1 (Ser307) to the same extent in WT and mTOR+/− mice. Muscle atrogin‐1 and MuRF1 mRNAs were elevated in mTOR+/− mice under basal conditions implying increased proteolysis, and the LPS‐induced increase in these atrogenes was comparable between groups. Differences in basal muscle protein balance were not due to differences in plasma insulin or IGF‐I, nor to tissue TNFα, IL‐6 or NOS2. LPS impaired leucine's ability to increase muscle 4E‐BP1 and S6K1 phosphorylation in WT mice and this leucine resistance was exacerbated in mTOR+/− mice. Our data support the contention that sepsis‐induced reductions in mTOR activity are central in regulating skeletal muscle mass under basal conditions and especially in response to nutrient stimulation. (GM38032)

Hibernating black bears ( Ursus americanus) experience skeletal muscle protein balance during winter anorexia

Black bears spend four to seven months every winter confined to their den and anorexic. Despite potential for skeletal muscle atrophy and protein loss, bears appear to retain muscle integrity throughout winter dormancy. Other authors have suggested that bears are capable of net protein anabolism during this time. The present study was performed to test this hypothesis by directly measuring skeletal muscle protein metabolism during the summer, as well as early and late hibernation periods. Muscle biopsies were taken from the vastus lateralis of six free-ranging bears in the summer, and from six others early in hibernation and again in late winter. Protein synthesis and breakdown were measured on biopsies using 14C-phenylalanine as a tracer. Muscle protein, nitrogen, and nucleic acid content, as well as nitrogen stable isotope enrichment, were also measured. Protein synthesis was greater than breakdown in summer bears, suggesting that they accumulate muscle protein during periods of seasonal food availability. Protein synthesis and breakdown were both lower in winter compared to summer but were equal during both early and late denning, indicating that bears are in protein balance during hibernation. Protein and nitrogen content, nucleic acid, and stable isotope enrichment measurements of the biopsies support this conclusion.

Insulin resistance is associated with skeletal muscle protein breakdown in non-diabetic chronic hemodialysis patients

Deranged protein metabolism is known to complicate uremia. Insulin resistance is evident in chronic hemodialysis (CHD) patients. We hypothesized that the degree of insulin resistance would predict protein catabolism in non-diabetic CHD patients. We examined the relationship between Homeostasis Model Assessment (HOMA) and fasting whole-body and skeletal muscle protein turnover in 18 non-diabetic CHD patients using primed-constant infusions of L-(1-(13)C) leucine and L-(ring-(2)H(5)) phenylalanine. Mean+/-s.d. fasting glucose and body mass index were 80.6+/-9.8 mg/dl and 25.4+/-4.4 kg/m(2), respectively. Median (interquartile range) HOMA was 1.6 (1.4, 3.9). Mean+/-s.e.m. skeletal muscle protein synthesis, breakdown, and net balance were 89.57+/-11.67, 97.02+/-13.3, and -7.44+/-7.14 microg/100 ml/min, respectively. Using linear regression, a positive correlation was observed between HOMA and skeletal muscle protein synthesis (R(2)=0.28; P=0.024), and breakdown (R(2)=0.49; P=0.001). An inverse association between net skeletal muscle protein balance and HOMA was also noted (R(2)=0.20; P=0.066). After adjustment for C-reactive protein, only the relationship between HOMA and skeletal muscle protein breakdown persisted (R(2)=0.49; P=0.006). There were no significant associations between components of whole-body protein turnover and HOMA. This study demonstrates that insulin resistance is evident in non-diabetic dialysis patients, is associated with skeletal muscle protein breakdown, and represents a novel target for intervention in uremic wasting.

Intradialytic Oral Nutrition Improves Protein Homeostasis in Chronic Hemodialysis Patients with Deranged Nutritional Status

Decreased dietary protein intake and hemodialysis (HD)-associated protein catabolism predispose chronic HD (CHD) patients to deranged nutritional status, which is associated with poor clinical outcome in this population. Intradialytic parenteral nutrition (IDPN) reverses the net negative whole-body and skeletal muscle protein balance during HD. IDPN is costly and restricted by Medicare and other payers. Oral supplementation (PO) is a more promising, physiologic, and affordable intervention in CHD patients. Protein turnover studies were performed by primed-constant infusion of L-(1-(13)C) leucine and L-(ring-(2)H(5)) phenylalanine in eight CHD patients with deranged nutritional status before, during, and after HD on three separate occasions: (1) with IDPN infusion, (2) with PO administration, and (3) with no intervention (control). Results showed highly positive whole-body net balance during HD for both IDPN and PO (4.43 +/- 0.7 and 5.71 +/- 1.2 mg/kg fat-free mass per min, respectively), compared with a neutral balance with control (0.25 +/- 0.5 mg/kg fat-free mass per min; P = 0.002 and <0.001 for IDPN versus control and PO versus control, respectively). Skeletal muscle protein homeostasis during HD also improved with both IDPN and PO (50 +/- 19 and 42 +/- 17 microg/100 ml per min) versus control (-27 +/- 13 microg/100 ml per min; P = 0.005 and 0.009 for IDPN versus control and PO versus control, respectively). PO resulted in persistent anabolic benefits in the post-HD phase for muscle protein metabolism, when anabolic benefits of IDPN dissipated (-53 +/- 25 microg/100 ml per min for control, 47 +/- 41 microg/100 ml per min for PO [P = 0.039 versus control], and -53 +/- 24 microg/100 ml per min for IDPN [P = 1.000 versus control and 0.039 versus PO]). Long-term studies using intradialytic oral supplementation are needed for CHD patients with deranged nutritional status.