Regulation Of Muscle Protein Turnover Research Articles

Human pancreatic tumour organoid-derived factors enhance myogenic differentiation.

BackgroundMost patients with pancreatic cancer develop cachexia, which is characterized by progressive muscle loss. The mechanisms underlying muscle loss in cancer cachexia remain elusive. Pancreatic tumour organoids are 3D cell culture models that retain key characteristics of the parent tumour. We aimed to investigate the effect of pancreatic tumour organoid‐derived factors on processes that determine skeletal muscle mass, including the regulation of muscle protein turnover and myogenesis.MethodsConditioned medium (CM) was collected from human pancreatic cancer cell lines (PK‐45H, PANC‐1, PK‐1, and KLM‐1), pancreatic tumour organoid cultures from a severely cachectic (PANCO‐9a) and a non‐cachectic patient (PANCO‐12a), and a normal pancreas organoid culture. Differentiating C2C12 myoblasts and mature C2C12 myotubes were exposed to CM for 24 h or maintained in control medium. In myotubes, NF‐kB activation was monitored using a NF‐κB luciferase reporter construct, and mRNA expression of E3‐ubiquitin ligases and REDD1 was analysed by RT‐qPCR. C2C12 myoblast proliferation and differentiation were monitored by live cell imaging and myogenic markers and myosin heavy chain (MyHC) isoforms were assessed by RT‐qPCR.ResultsWhereas CM from PK‐1 and KLM‐1 cells significantly induced NF‐κB activation in C2C12 myotubes (PK‐1: 3.1‐fold, P < 0.001; KLM‐1: 2.1‐fold, P = 0.01), Atrogin‐1/MAFbx and MuRF1 mRNA were only minimally and inconsistently upregulated by the CM of pancreatic cancer cell lines. Similarly, E3‐ubiquitin ligases and REDD1 mRNA expression in myotubes were not altered by exposure to pancreatic tumour organoid CM. Compared with the control condition, CM from both PANCO‐9a and PANCO‐12a tumour organoids increased proliferation of myoblasts, which was accompanied by significant downregulation of the satellite cell marker paired‐box 7 (PAX7) (PANCO‐9a: −2.1‐fold, P < 0.001; PANCO‐12a: −2.0‐fold, P < 0.001) and myogenic factor 5 (MYF5) (PANCO‐9a: −2.1‐fold, P < 0.001; PANCO‐12a: −1.8‐fold, P < 0.001) after 48 h of differentiation. Live cell imaging revealed accelerated alignment and fusion of myoblasts exposed to CM from PANCO‐9a and PANCO‐12a, which was in line with significantly increased Myomaker mRNA expression levels (PANCO‐9a: 2.4‐fold, P = 0.001; PANCO‐12a: 2.2‐fold, P = 0.004). These morphological and transcriptional alterations were accompanied by increased expression of muscle differentiation markers such as MyHC‐IIB (PANCO‐9a: 2.5‐fold, P = 0.04; PANCO‐12a: 3.1‐fold, P = 0.006). Although the impact of organoid CM on myogenesis was not associated with the cachexia phenotype of the donor patients, it was specific for tumour organoids, as CM of control pancreas organoids did not modulate myogenic fusion.ConclusionsThese data show that pancreatic tumour organoid‐derived factors alter the kinetics of myogenesis, which may eventually contribute to impaired muscle mass maintenance in cancer cachexia.

A Muscle-Centric Perspective on Intermittent Fasting: A Suboptimal Dietary Strategy for Supporting Muscle Protein Remodeling and Muscle Mass?

Muscle protein is constantly “turning over” through the breakdown of old/damaged proteins and the resynthesis of new functional proteins, the algebraic difference determining net muscle gain, maintenance, or loss. This turnover, which is sensitive to the nutritional environment, ultimately determines the mass, quality, and health of skeletal muscle over time. Intermittent fasting has become a topic of interest in the health community as an avenue to improve health and body composition primarily via caloric deficiency as well as enhanced lipolysis and fat oxidation secondary to attenuated daily insulin response. However, this approach belies the established anti-catabolic effect of insulin on skeletal muscle. More importantly, muscle protein synthesis, which is the primary regulated turnover variable in healthy humans, is stimulated by the consumption of dietary amino acids, a process that is saturated at a moderate protein intake. While limited research has explored the effect of intermittent fasting on muscle-related outcomes, we propose that infrequent meal feeding and periods of prolonged fasting characteristic of models of intermittent fasting may be counter-productive to optimizing muscle protein turnover and net muscle protein balance. The present commentary will discuss the regulation of muscle protein turnover across fasted and fed cycles and contrast it with studies exploring how dietary manipulation alters the partitioning of fat and lean body mass. It is our position that intermittent fasting likely represents a suboptimal dietary approach to remodel skeletal muscle, which could impact the ability to maintain or enhance muscle mass and quality, especially during periods of reduced energy availability.

Links Between Testosterone, Oestrogen, and the Growth Hormone/Insulin-Like Growth Factor Axis and Resistance Exercise Muscle Adaptations.

Maintenance of skeletal muscle mass throughout the life course is key for the regulation of health, with physical activity a critical component of this, in part, due to its influence upon key hormones such as testosterone, estrogen, growth hormone (GH), and insulin-like growth factor (IGF). Despite the importance of these hormones for the regulation of skeletal muscle mass in response to different types of exercise, their interaction with the processes controlling muscle mass remain unclear. This review presents evidence on the importance of these hormones in the regulation of skeletal muscle mass and their responses, and involvement in muscle adaptation to resistance exercise. Highlighting the key role testosterone plays as a primary anabolic hormone in muscle adaptation following exercise training, through its interaction with anabolic signaling pathways and other hormones via the androgen receptor (AR), this review also describes the potential importance of fluctuations in other hormones such as GH and IGF-1 in concert with dietary amino acid availability; and the role of estrogen, under the influence of the menstrual cycle and menopause, being especially important in adaptive exercise responses in women. Finally, the downstream mechanisms by which these hormones impact regulation of muscle protein turnover (synthesis and breakdown), and thus muscle mass are discussed. Advances in our understanding of hormones that impact protein turnover throughout life offers great relevance, not just for athletes, but also for the general and clinical populations alike.

The Effect of Estradiol Administration on Muscle Mass Loss and Cachexia Progression in Female Apc Min/+ Mice.

Cancer cachexia is a multifactorial muscle wasting condition characterized by severe body weight and muscle mass loss which is secondary to chronic disease. The mechanistic examination of cachexia has predominately focused on the male phenotype and created significant gaps in understanding cachexia progression in the female. Female hypogonadism can accompany cancer cachexia and is characterized by reduced circulating 17ß-estradiol and uterine atrophy. Estrogen has known functions in skeletal muscle homeostasis involving the regulation of muscle protein turnover, cellular stressors, and oxidative metabolism. However, 17ß-estradiol's ability to regulate cachexia progression in the female is not known. The purpose of this study was to determine the effect of gonadal function and estradiol administration on muscle mass loss and cachexia progression in female ApcMin/+ mice.Methods: Female C57BL/6 (B6; N = 82) and ApcMin/+ (MIN; N = 88) mice were used in two separate experiments. In experiment 1, mice were sacrificed at either 12 (N = 20) or 20 (N = 41) weeks of age. Body weight and estrous cycle presence was determined weekly. In experiment 2, B6 and MIN mice were randomly allocated to: Control (N = 17), received E2 pellet (E2, N = 18), ovariectomy surgery (OVX; N = 19) or ovariectomy surgery with E2 pellet (OVX + E2; N = 21). 17ß-estradiol was administered through an implanted slow-releasing pellet (0.1 mg). In estrogen and ovariectomy experiments, food intake, and functional outcomes were recorded 1 week prior to sacrifice.Results: We report that E2 administration prevented body weight loss, muscle mass loss, cage inactivity, and grip strength loss associated with cachexia. In skeletal muscle, E2 reduced skeletal muscle AMPK phosphorylation, improved mTORC1 signaling, and prevented mitochondrial dysfunction.Conclusion: Our results demonstrate a role for 17ß-estradiol for the prevention of skeletal muscle mass loss in female tumor bearing mice. Furthermore, 17ß-estradiol prevented cachexia's disruption in skeletal muscle signaling involving AMPK and mTORC1, in addition to improving mitochondrial function in female tumor bearing mice.

Muscle Protein Turnover and the Molecular Regulation of Muscle Mass during Hypoxia.

: Effects of environmental hypoxia on fat-free mass are well studied. Negative energy balance, increased nitrogen excretion, and fat-free mass loss are commonly observed in lowlanders sojourning at high altitude. Reductions in fat-free mass can be minimized if energy consumption matches energy expenditure. However, in nonresearch settings, achieving energy balance during high-altitude sojourns is unlikely, and myofibrillar protein mass is usually lost, but the mechanisms accounting for the loss of muscle mass are not clear. At sea level, negative energy balance reduces basal and blunts postprandial muscle protein synthesis, with no relevant change in muscle protein breakdown. Downregulations in muscle protein synthesis and loss of fat-free mass during energy deficit at sea level are largely overcome by consuming at least twice the recommended dietary allowance for protein. Hypoxia may increase or not affect resting muscle protein synthesis, blunt postexercise muscle protein synthesis, and markedly increase proteolysis independent of energy status. Hypoxia-induced mTORC1 dysregulation and an upregulation in calpain- and ubiquitin proteasome-mediated proteolysis may drive catabolism in lowlanders sojourning at high altitude. However, the combined effects of energy deficit, exercise, and dietary protein manipulations on the regulation of muscle protein turnover have never been studied at high altitude. This article reviews the available literature related to the effects of high altitude on fat-free mass, highlighting contemporary studies that assessed the influence of altitude exposure (or hypoxia) on muscle protein turnover and intramuscular regulation of muscle mass. Knowledge gaps are addressed, and studies to identify effective and feasible countermeasures to hypoxia-induced muscle loss are discussed.

Differential regulation of muscle protein turnover in response to emphysema and acute pulmonary inflammation

BackgroundExacerbations in COPD are often accompanied by pulmonary and systemic inflammation, and associated with increased susceptibility to and prevalence of weight loss and muscle wasting. Muscle mass loss during disease exacerbations may contribute to emphysema-associated muscle atrophy. However, whether pulmonary inflammation in presence of emphysema differentially affects skeletal muscle, including protein synthesis and degradation signaling pathways has not previously been addressed. The aims of this study were to 1) develop a mouse model of disease exacerbation-associated muscle wasting, 2) evaluate whether emphysema and muscle wasting can be monitored non-invasively and 3) assess alterations in muscle protein turnover regulation.MethodsEmphysema was induced by three, weekly intra-tracheal (IT) elastase (E) or vehicle control (vc) instillations, followed by one single IT-LPS bolus (L) or vc instillation to mimic pulmonary inflammation-driven disease exacerbation. Consequently, four experimental groups were defined: vc/vc (‘C’), E/vc (‘E’), vc/LPS (‘L’), E/LPS (‘E + L’). Using micro cone-beam CT-scans, emphysema development and muscle mass changes were monitored, and correlated to muscle weight 48 h after LPS instillation. Protein turnover signaling was assessed in muscle tissue collected 24 h post LPS instillation.ResultsMicro-CT imaging correlated strongly with established invasive measurements of emphysema and muscle atrophy. Pulmonary inflammation following LPS instillation developed irrespective of emphysema and body and muscle weight were similarly reduced in the ‘L’ and ‘E + L’ groups. Accordingly, mRNA and protein expression levels of genes of the ubiquitin-proteasome pathway (UPS) and the autophagy-lysosomal pathway (ALP) were upregulated in skeletal muscle following IT-LPS (‘L’ and ‘E + L’). In contrast, mTOR signaling, which controls ALP and protein synthesis, was reduced by pulmonary inflammation (‘L’ and ‘E + L’) as well as emphysema as a single insult (‘E’) compared to control.ConclusionChanges in lung tissue density and muscle mass can be monitored non-invasively to evaluate emphysema and muscle atrophy longitudinally. Acute loss of muscle mass evoked by pulmonary inflammation is similar in control and emphysematous mice. Although muscle atrophy cues in response to pulmonary inflammation are not altered by emphysema, emphysema itself affects protein synthesis and ALP signaling, which may interfere with muscle mass recovery and impair maintenance of muscle mass in emphysema.

Insulin and IGF-1 receptors regulate FoxO-mediated signaling in muscle proteostasis.

Diabetes strongly impacts protein metabolism, particularly in skeletal muscle. Insulin and IGF-1 enhance muscle protein synthesis through their receptors, but the relative roles of each in muscle proteostasis have not been fully elucidated. Using mice with muscle-specific deletion of the insulin receptor (M-IR-/- mice), the IGF-1 receptor (M-IGF1R-/- mice), or both (MIGIRKO mice), we assessed the relative contributions of IR and IGF1R signaling to muscle proteostasis. In differentiated muscle, IR expression predominated over IGF1R expression, and correspondingly, M-IR-/- mice displayed a moderate reduction in muscle mass whereas M-IGF1R-/- mice did not. However, these receptors serve complementary roles, such that double-knockout MIGIRKO mice displayed a marked reduction in muscle mass that was linked to increases in proteasomal and autophagy-lysosomal degradation, accompanied by a high-protein-turnover state. Combined muscle-specific deletion of FoxO1, FoxO3, and FoxO4 in MIGIRKO mice reversed increased autophagy and completely rescued muscle mass without changing proteasomal activity. These data indicate that signaling via IR is more important than IGF1R in controlling proteostasis in differentiated muscle. Nonetheless, the overlap of IR and IGF1R signaling is critical to the regulation of muscle protein turnover, and this regulation depends on suppression of FoxO-regulated, autophagy-mediated protein degradation.

Expression of genes related to the regulation of muscle protein turnover in Angus and Nellore bulls.

We aimed to evaluate the expression of genes related to the regulation of muscle protein turnover in the longissimus dorsi (LD) muscle of Angus and Nellore bulls and to estimate the within-breed correlations of gene expression and performance traits. Thirteen genes related to the IGF-1 and myostatin pathways were studied. Thirteen animals, with an initial average BW of 381.2 ± 11.8 kg, from each breed were used in a completely randomized 2 × 2 factorial design (2 breeds and 2 feeding levels). The diet consisted of corn silage and a corn-soybean meal concentrate in a roughage-to-concentrate ratio of 30:70. Cattle were fed ad libitum (with 9 animals from each breed) or feed restricted (a 55% restriction of total DMI of ad libitum-fed animals, calculated as percentage of metabolic BW, with 4 animals of each breed). The experimental period lasted for 82 d and it was preceded by a 28-d adaptation period. The performance traits evaluated were slaughter body weight, total ADG (from d 1 to 82 of the trial), initial ADG (from d 1 to 41 of the trial), final ADG (from d 42 to 82 of the trial), total DMI (from d 1 to 82 of the trial), initial DMI (from d 1 to 41 of the trial), final DMI (from d 42 to 82 of the trial), HCW, LD weight (LDW), and rib eye area (REA). After slaughter, samples were taken from the LD muscle between the 12th and 13th ribs for gene expression analysis by quantitative reverse transcription PCR. There was no difference ( > 0.05) in the expression of any of the genes studied between ad libitum-fed Angus and ad libitum-fed Nellore, whereas feed restriction increased the expression of (; < 0.001), (; = 0.05), and (; = 0.04) and decreased the expression of ( < 0.01). The REA was negatively correlated to (; = 0.01), (; = 0.02), and ( = 0.05). The HCW was negatively correlated to ( = 0.01) and ( = 0.01) and tended to be negatively correlated to ( = 0.07), whereas the LDW tended to be negatively correlated to ( = 0.08). The genes , , and seem to be important for muscle growth and may be worthy of further investigation as future strategies for increasing muscle in livestock.

Skeletal muscle glycoprotein 130's role in Lewis lung carcinoma–induced cachexia

Chronic inflammation is associated with cachexia-induced skeletal muscle mass loss in cancer. Levels of IL-6 cytokine family members are increased during cancer-related cachexia and induce intracellular signaling through glycoprotein130 (gp130). Although muscle STAT3 and circulating IL-6 are implicated in cancer-induced muscle wasting, there is limited understanding of muscle gp130's role in this process. Therefore, we investigated the role of skeletal muscle gp130 (skm-gp130) in cancer-induced alterations in the regulation of muscle protein turnover. Lewis lung carcinoma (LLC) cells were injected into 8-wk-old skm-gp130-knockout (KO) mice or wild-type mice. Skeletal muscle loss was attenuated by 16% in gp130-KO mice, which coincided with attenuated LLC-induced phosphorylation of muscle STAT3, p38, and FOXO3. gp130 KO did not restore mTOR inhibition or alter AMP-activated protein kinase (AMPK) expression. The induction of atrogin expression and p38 phosphorylation in C2C12 myotubes exposed to LLC-treated medium was attenuated by gp130 inhibition, but mTOR inhibition was not restored. STAT signaling inhibition in LLC-treated myotubes did not attenuate the induction of p38 or AMPK phosphorylation. We concluded that, during LLC-induced cachexia, skm-gp130 regulates muscle mass signaling through STAT3 and p38 for the activation of FOXO3 and atrogin, but does not directly regulate the suppression of mTOR.

Distinct responses of protein turnover regulatory pathways in hypoxia- and semistarvation-induced muscle atrophy

The balance of muscle protein synthesis and degradation determines skeletal muscle mass. We hypothesized that hypoxia-induced muscle atrophy and alterations in the regulation of muscle protein turnover include a hypoxia-specific component, in addition to the observed effects of reduction in food intake in response to hypoxia. Mice were subjected to normoxic, hypoxic (8% oxygen), or pair-fed conditions for 2, 4, and 21 days. Cell-autonomous effects of hypoxia on skeletal muscle were also assessed in differentiated C2C12 myotubes. Hypoxia induced an initial rapid loss of body and muscle weight, which remained decreased during chronic hypoxia and could only in part be explained by the hypoxia-induced reduction of food intake (semistarvation). Regulatory steps of protein synthesis (unfolded protein response and mammal target of rapamycin signaling) remained active in response to acute and sustained hypoxia but not to semistarvation. Activation of regulatory signals for protein degradation, including increased expression of Murf1, Atrogin-1, Bnip3, and Map1lc3b mRNAs, was observed in response to acute hypoxia and to a lesser extent following semistarvation. Conversely, the sustained elevation of Atrogin-1, Bnip3, and Map1lc3b mRNAs and the increased activity of their upstream transcriptional regulator Forkhead box O1 were specific to chronic hypoxia because they were not observed in response to reduced food intake. In conclusion, altered regulation of protein turnover during hypoxia-induced muscle atrophy resulted from an interaction of semistarvation and a hypoxia-specific component. The finding that food restriction but not hypoxia-induced semistarvation inhibited regulatory steps in protein synthesis suggests a hypoxia-specific impairment of the coordination between protein-synthesis signaling and protein-degradation signaling in skeletal muscle.

Pulmonary inflammation‐induced muscle atrophy involves a GSK‐3beta‐GR signaling axis

Skeletal muscle atrophy is induced by raised glucocorticoid (GC) levels and reduced insulin/IGF‐1‐Akt signaling. Glycogen synthase kinase‐3 beta (GSK‐3b) is a downstream kinase of Akt involved in regulation of muscle protein turnover.We hypothesized that muscle GC receptor (GR) and GSK‐3b are required for muscle atrophy in response to pulmonary inflammation.Control, muscle specific‐GR (mGRKO) or ‐GSK‐3b (mGSK‐3bKO) mice were subjected to intra‐tracheal LPS instillation to induce acute pulmonary inflammation.Increases in circulating corticosterone levels following IT‐LPS were similar in control, mGRKO and mGSK‐3KO mice, but accompanied by substantial muscle atrophy in control mice only. Accordingly, increases in mRNA transcripts encoding effectors of the Ub 26S‐proteasome (MuRF1 and Atrogin‐1) and autophagy‐lysosomal (Bnip3, LC3b) systems were strongly attenuated in muscle of mGRKO or mGSK‐3KO mice. Interestingly, increased expression of GC‐sensitive genes (Glul, KLF15 and REDD1) and FOXO1 and ‐3a following IT‐LPS, was suppressed in skeletal muscle of GRKO and GSK‐3KO mice. This dependency of GR signaling on GSK‐3b was confirmed in cultured muscle cells.We conclude that pulmonary inflammation‐induced muscle atrophy is dependent on an intact GSK‐3b‐GR signaling axis.Support: Dutch Astma Foundation (3.2.07.017) and Dutch Top Institute Pharma (T1–201)

Testosterone regulation of Akt/mTORC1/FoxO3a signaling in skeletal muscle

Low endogenous testosterone production, known as hypogonadism is commonly associated with conditions inducing muscle wasting. Akt signaling can control skeletal muscle mass through mTOR regulation of protein synthesis and FoxO regulation of protein degradation, and this pathway has been previously identified as a target of androgen signaling. However, the testosterone sensitivity of Akt/mTOR signaling requires further understanding in order to grasp the significance of varied testosterone levels seen with wasting disease on muscle protein turnover regulation. Therefore, the purpose of this study is to determine the effect of androgen availability on muscle Akt/mTORC1/FoxO3a regulation in skeletal muscle and cultured C2C12 myotubes. C57BL/6 mice were either castrated for 42days or castrated and treated with the nandrolone decanoate (ND) (6mg/kg bw/wk). Testosterone loss (TL) significantly decreased volitional grip strength, body weight, and gastrocnemius (GAS) muscle mass, and ND reversed these changes. Related to muscle mass regulation, TL decreased muscle IGF-1 mRNA, the rate of myofibrillar protein synthesis, Akt phosphorylation, and the phosphorylation of Akt targets, GSK3β, PRAS40 and FoxO3a. TL induced expression of FoxO transcriptional targets, MuRF1, atrogin1 and REDD1. Muscle AMPK and raptor phosphorylation, mTOR inhibitors, were not altered by low testosterone. ND restored IGF-1 expression and Akt/mTORC1 signaling while repressing expression of FoxO transcriptional targets. Testosterone (T) sensitivity of Akt/mTORC1 signaling was examined in C2C12 myotubes, and mTOR phosphorylation was induced independent of Akt activation at low T concentrations, while a higher T concentration was required to activate Akt signaling. Interestingly, low concentration T was sufficient to amplify myotube mTOR and Akt signaling after 24h of T withdrawal, demonstrating the potential in cultured myotubes for a T initiated positive feedback mechanism to amplify Akt/mTOR signaling. In summary, androgen withdrawal decreases muscle myofibrillar protein synthesis through Akt/mTORC1 signaling, which is independent of AMPK activation, and readily reversible by anabolic steroid administration. Acute Akt activation in C2C12 myotubes is sensitive to a high concentration of testosterone, and low concentrations of testosterone can activate mTOR signaling independent of Akt.

Exercise and Amino Acid Anabolic Cell Signaling and the Regulation of Skeletal Muscle Mass

A series of complex intracellular networks influence the regulation of skeletal muscle protein turnover. In recent years, studies have examined how cellular regulators of muscle protein turnover modulate metabolic mechanisms contributing to the loss, gain, or conservation of skeletal muscle mass. Exercise and amino acids both stimulate anabolic signaling potentially through several intracellular pathways including the mammalian target of rapamycin complex 1 and the mitogen activated protein kinase cell signaling cascades. As novel molecular regulators of muscle integrity continue to be explored, a contemporary analysis of the literature is required to understand the metabolic mechanisms by which contractile forces and amino acids affect cellular process that contribute to long-term adaptations and preservation of muscle mass. This article reviews the literature related to how exercise and amino acid availability affect cellular regulators of skeletal muscle mass, especially highlighting recent investigations that have identified mechanisms by which contractile forces and amino acids modulate muscle health. Furthermore, this review will explore integrated exercise and nutrition strategies that promote the maintenance of muscle health by optimizing exercise, and amino acid-induced cell signaling in aging adults susceptible to muscle loss.

The Regulation of Skeletal Muscle Protein Turnover during the Progression of Cancer Cachexia in the ApcMin/+ Mouse

Muscle wasting that occurs with cancer cachexia is caused by an imbalance in the rates of muscle protein synthesis and degradation. The ApcMin/+ mouse is a model of colorectal cancer that develops cachexia that is dependent on circulating IL-6. However, the IL-6 regulation of muscle protein turnover during the initiation and progression of cachexia in the ApcMin/+ mouse is not known. Cachexia progression was studied in ApcMin/+ mice that were either weight stable (WS) or had initial (≤5%), intermediate (6–19%), or extreme (≥20%) body weight loss. The initiation of cachexia reduced %MPS 19% and a further ∼50% with additional weight loss. Muscle IGF-1 mRNA expression and mTOR targets were suppressed with the progression of body weight loss, while muscle AMPK phosphorylation (Thr 172), AMPK activity, and raptor phosphorylation (Ser 792) were not increased with the initiation of weight loss, but were induced as cachexia progressed. ATP dependent protein degradation increased during the initiation and progression of cachexia. However, ATP independent protein degradation was not increased until cachexia had progressed beyond the initial phase. IL-6 receptor antibody administration prevented body weight loss and suppressed muscle protein degradation, without any effect on muscle %MPS or IGF-1 associated signaling. In summary, the %MPS reduction during the initiation of cachexia is associated with IGF-1/mTOR signaling repression, while muscle AMPK activation and activation of ATP independent protein degradation occur later in the progression of cachexia. IL-6 receptor antibody treatment blocked cachexia progression through the suppression of muscle protein degradation, while not rescuing the suppression of muscle protein synthesis. Attenuation of IL-6 signaling was effective in blocking the progression of cachexia, but not sufficient to reverse the process.

Influence of acetaminophen and ibuprofen on skeletal muscle adaptations to resistance exercise in older adults

Evidence suggests that consumption of over-the-counter cyclooxygenase (COX) inhibitors may interfere with the positive effects that resistance exercise training has on reversing sarcopenia in older adults. This study examined the influence of acetaminophen or ibuprofen consumption on muscle mass and strength during 12 wk of knee extensor progressive resistance exercise training in older adults. Thirty-six individuals were randomly assigned to one of three groups and consumed the COX-inhibiting drugs in double-blind placebo-controlled fashion: placebo (67 ± 2 yr; n = 12), acetaminophen (64 ± 1 yr; n = 11; 4 g/day), and ibuprofen (64 ± 1 yr; n = 13; 1.2 g/day). Compliance with the resistance training program (100%) and drug consumption (via digital video observation, 94%), and resistance training intensity were similar (P > 0.05) for all three groups. Drug consumption unexpectedly increased muscle volume (acetaminophen: 109 ± 14 cm(3), 12.5%; ibuprofen: 84 ± 10 cm(3), 10.9%) and muscle strength (acetaminophen: 19 ± 2 kg; ibuprofen: 19 ± 2 kg) to a greater extent (P < 0.05) than placebo (muscle volume: 69 ± 12 cm(3), 8.6%; muscle strength: 15 ± 2 kg), when controlling for initial muscle size and strength. Follow-up analysis of muscle biopsies taken from the vastus lateralis before and after training showed muscle protein content, muscle water content, and myosin heavy chain distribution were not influenced (P > 0.05) by drug consumption. Similarly, muscle content of the two known enzymes potentially targeted by the drugs, COX-1 and -2, was not influenced (P > 0.05) by drug consumption, although resistance training did result in a drug-independent increase in COX-1 (32 ± 8%; P < 0.05). Drug consumption did not influence the size of the nonresistance-trained hamstring muscles (P > 0.05). Over-the-counter doses of acetaminophen or ibuprofen, when consumed in combination with resistance training, do not inhibit and appear to enhance muscle hypertrophy and strength gains in older adults. The present findings coupled with previous short-term exercise studies provide convincing evidence that the COX pathway(s) are involved in the regulation of muscle protein turnover and muscle mass in humans.

MAFbx/Atrogin-1 expression is a poor index of muscle proteolysis

MAFbx/Atrogin-1 expression is a poor index of muscle proteolysis

Exercise intensity matters for both young and old muscles

Exercise intensity matters for both young and old muscles

Whole-body resting and exercise-induced lipolysis in sarcopaenic patients with COPD

Impaired beta-adrenoceptor-mediated lipolysis has been reported in sarcopenic [corrected] chronic obstructive pulmonary disease (COPD) patients. This could play a role in the shift in body composition towards decreased fat-free mass (FFM) and relative maintenance of fat mass (FM). Lipolysis could be affected by chronic treatment with beta(2)-agonists or disease-related factors. Therefore, whole-body resting and exercise-induced lipolysis were investigated in sarcopenic [corrected] COPD patients with moderate disease severity. Seven sarcopenic [corrected] COPD patients (mean+/-sem forced expiratory volume in one second (FEV(1)) 53+/-5% of the predicted value; body mass index (BMI) 27.5+/-0.9 kg x m(-2)) and seven controls matched for age, sex and BMI were studied. In addition, six underweight COPD patients (FEV(1) 51+/-5% pred; BMI 20.6+/-0.7 kg x m(-2)) matched for disease severity were recruited. Lipolysis and plasma levels of catecholamines were assessed during infusion of [(2)H(5)]glycerol at rest and during submaximal cycling exercise. The proportional FM was comparable between sarcopenic [corrected] patients and controls, whereas the FFM index was significantly reduced in patients. At rest, the rate of appearance (R(a)) of glycerol (4.1+/-0.6 and 3.3+/-0.2 micromol x kg FFM(-1) x min(-1), respectively) did not differ significantly. In underweight patients, glycerol R(a) (4.3+/-0.5 micromol x kg FFM(-1) x min(-1)) was also comparable. End-of-exercise lipolytic rates did not differ significantly between groups. Glycerol R(a) was not related to FM. Resting adrenalin levels were significantly increased in underweight COPD patients and were related to resting lipolysis. Sarcopenia [corrected] in chronic obstructive pulmonary disease patients with moderate disease severity is not characterised by an abnormal lipolytic rate. Altered regulation of muscle protein turnover seems to be the trigger in the body compositional shift observed in these patients.

Leucine and Calcium Regulate Fat Metabolism and Energy Partitioning in Murine Adipocytes and Muscle Cells

Dietary calcium modulation of adiposity is mediated, in part, by suppression of calcitriol, while the additional effect of dairy products is mediated by additional components; these include the high concentration of leucine, a key factor in the regulation of muscle protein turnover. We investigated the effect of leucine, calcitriol and calcium on energy metabolism in murine adipocytes and muscle cells and on energy partitioning between adipocytes and skeletal muscle. Leucine induced a marked increase in fatty acid oxidation in C2C12 muscle cells (P<0.001) and decreased FAS expression by 66% (P<0.001) in 3T3-L1 adipocytes. Calcitriol decreased muscle cell fatty acid oxidation by 37% (P<0.001) and increased adipocyte FAS gene expression by threefold (P<0.05); these effects were partially reversed by either leucine or calcium channel antagonism with nifedipine. Co-culture of muscle cells with adipocytes or incubation with 48-h adipocyte conditioned medium decreased muscle fatty acid oxidation by 62% (P<0.001), but treating adipocytes with leucine and/or nifedipine attenuated this effect. Leucine, nifedipine and calcitriol also modulated adiponectin production and thereby exerted additional indirect effects on fatty acid oxidation in C2C12 myotubes. Adiponectin increased IL-15 and IL-6 release by myotubes and partially reversed the inhibitory effects of calcitriol. Comparable effects of leucine, calcitriol and adiponectin were found in myotubes treated with conditioned medium derived from adipocytes or co-cultured with adipocytes. These data suggest that leucine and nifedipine promote energy partitioning from adipocytes to muscle cells, resulting in decreased energy storage in adipocytes and increasing fatty acid utilization in muscle.

The metabolic role of branched-chain amino acids

14 C] leucine entering the tissue is decarboxylated in the diaphragm and that a considerable proportion is degraded even further. The percentages of decarboxylation for isoleucine (58%) and valine (31%) also were substantial. Addition of insulin brought about a small but consistent stimulation of leucine oxidation. Moreover, the fraction of isotope taken up by the diaphragm appearing as 14 CO2 did not decline markedly as the concentration of amino acids supplied rose, in contrast with the amount of isotope incorporated into protein, which dropped steadily as the specific activity fell. 3 Further extensive work exploring the possibility that oxidation might be a major metabolic pathway for leucine (and possibly all three BCAAs) in rat diaphragm and in soleus and extensor digitorm longus muscles was published in 1972 by Odessey and Goldberg.4 They determined that the diaphragm posseses a marked ability to degrade leucine 1- 14 Ct o 14 CO2. The metabolism of leucine by the diaphragm depended on the concentration, and leucine uptake and oxidation increased with increasing external concentrations. Further, the amount of leucine degradation relative to its incorporation into protein increased 10-fold as its external concentration was raised. The red soleus and pale extensor degitorum longus muscles exhibited a similar marked capacity for leucine oxidation. The rates of CO2 production, incorporation into protein, and total uptake in these two muscles were lower than those in the diaphragm. The skeletal muscle mass, which constitutes 43% of the body mass, is the major site for leucine oxidation in the body, marking leucine and possibly the other BCAAs as significant energy sources for the muscle. Complete oxidation of leucine in the muscle yields more adenosine triphosphate molecules on a molar basis than complete oxidation of glucose. Further, this inherent ability of muscle to oxidize leucine increases under certain physiologic states such as food deprivation. 5 Later work determined that the rate of oxidation of BCAA in muscle is under metabolic and hormonal regulation. Buse and Reid6 in skeletal muscle and Chua et al.7 in cardiac muscle suggested that leucine also regulates the turnover of protein in muscle cells by inhibiting protein degradation and enhancing protein synthesis. This regulation of muscle protein turnover by leucine influences the transition to negative