b_leftb_right
This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

Editorial

Myostatin and beyond in cirrhosis: all roads lead to sarcopenia

Srinivasan Dasarathy*

Version of Record online: 23 NOV 2017

DOI: 10.1002/jcsm.12262


How to Cite

Dasarathy, S. (2017) Myostatin and beyond in cirrhosis: all roads lead to sarcopenia. Journal of Cachexia, Sarcopenia and Muscle, 8: 864–869. doi: 10.1002/jcsm.12262.

Author Information

Professor of Medicine, Cleveland Clinic Lerner College of Medicine; Director, Liver Metabolism Research; Staff, Departments of Gastroenterology, Hepatology and Pathobiology, Cleveland Clinic, Cleveland, OH, USA

*Correspondence to: Srinivasan Dasarathy, Cleveland Clinic Lerner College of Medicine; Director, Liver Metabolism Research; Staff, Departments of Gastroenterology, Hepatology and Pathobiology, Cleveland Clinic, Cleveland, OH, USA. Tel: 2164442980; Fax: 2164453889; Email: dasaras@ccf.org


Sarcopenia or loss of skeletal muscle mass is a major complication of cirrhosis and liver disease.[1-6] A large body of literature exists to support the prognostic significance of sarcopenia in cirrhosis.[1, 2, 5, 7] Independent clinical consequences of sarcopenia in cirrhosis include lower survival and quality of life, increases risk of complications including infections and encephalopathy, and lower post liver transplant survival.[1-5, 7-9] Interestingly, unlike other complications of cirrhosis, sarcopenia does not reverse and usually worsens after liver transplantation[10-12] that raises an important question of the utility of targeting sarcopenia for therapy before transplantation. Because the majority of patients with cirrhosis do not undergo transplantation and the window of opportunity is widest prior to transplantation,[7] the focus should be in trying to reduce the severity and frequency of sarcopenia in cirrhosis prior to transplantation. Even though clinicians nearly universally recognize the high clinical significance of sarcopenia in cirrhosis, there are no effective treatment options.[3, 7] The major reason for lack of effective therapies has generally been attributed to a limited understanding of the underlying mechanisms of sarcopenia in cirrhosis. However, other factors include the lack of precise measures of sarcopenia, absence of sensitive and specific biomarkers, and therapies that are based on deficiency replacement rather than mechanistic targets.[13]

The work by Nishikawa et al. in this issue goes towards addressing a number of these issues.[14] In a very elegant study, the investigators quantified serum myostatin in a large cohort of patients with cirrhosis. Subjects were stratified by gender and median concentration of serum myostatin. Serum myostatin was significantly higher in males than females. Myostatin concentrations were higher with worsening severity of liver disease measured by Child Pugh score, a standard clinical method to predict outcomes in cirrhosis. Higher myostatin concentration was an independent predictor of worse survival in both male and female patients with cirrhosis. Finally, serum myostatin concentrations were associated with lower muscle mass measured as psoas muscle index on computed tomography, serum ammonia concentration, serum albumin, and branched chain to tyrosine ratios. These studies complement an earlier brief report that serum myostatin was elevated in cirrhosis, but one must note that circulating myostatin concentrations are elevated in heart failure and COPD also.[15, 16] These studies are therefore of broad interest to investigators and physicians taking care of patients with other chronic diseases with sarcopenia. The present study also reiterates their data that circulating myostatin is inversely related to skeletal muscle mass but extends these data by demonstrating the prognostic significance and relate it to underlying pathophysiological perturbations. These investigators also report the use of psoas muscle index as a measure of muscle mass. This requires the use of imaging techniques while myostatin measurement is done in blood samples with lower costs and no risk of radiation exposure. Whether serial myostatin measurements will correlate with serial changes in muscle area or provide a better predictor of progressive muscle loss is an intriguing possibility because more rapid muscle loss worsens outcome in cirrhosis.[17]

Since the discovery of myostatin, the number of publications has increased exponentially with a detailed characterization of its biological properties.[18, 19] Even though myostatin is consistently expressed in skeletal muscle, other tissues also express and possibly secrete myostatin.[18] Signalling and functional responses to myostatin have focused on a paracrine effect even though there is increasing interest in myostatin as an endocrine factor or myokine. Consistent with its being a member of the TGFβ superfamily, myostatin binds to its receptor, activin IIB receptor, a type 2 transmembrane protein.[19-21] Upon binding with myostatin, activin IIBR then heterodimerizes with a type 1 receptor, activin-like kinase 4 or 5 in a context dependent manner, and this complex functions as a serine threonine kinase to phosphorylate Smad2/3 that in turn transcriptionally regulates target genes.[22] Myostatin has also been reported to regulate a number of other signalling proteins and transcription factors including β catenin, forkhead box, and 5' adenosine monophosphate-activated protein kinase.[23-25] Unlike the extensive work done on the downstream signalling responses to myostatin, there is more limited data on the upstream regulation and the mechanisms of increased myostatin in disease states.[20, 24, 26-36] A number of promoter analyses of myostatin in different species have been reported and targeted studies on specific transcription factors have been published.[20, 26, 32, 33, 37-39] However, the mechanisms of context specificity have yet to be determined including the possible differential response to exogenous and endogenous myostatin.[28] Most studies that have dissected the biological consequences of myostatin have used exogenous administration or delivery of myostatin with few studies on the response to endogenously stimulated myostatin, and these are of interest to not only translational scientist but also to clinical investigators because of the implications for therapy.

In addition to the novel observation by Nishikawa et al. that circulating myostatin was correlated with overall survival, another important observation was that these concentrations related to ammonia concentrations.[14] Ammonia has been reported to transcriptionally upregulate myostatin via an NFkB dependent mechanism in the skeletal muscle but whether such a mechanism is relevant in other tissues is currently unknown.[40] Ammonia is a cytotoxic molecule generated by a variety of physiological processes including amino acid and purine catabolism and gut microbial metabolism.[40, 41] The hepatocyte is the only cell that is capable of metabolizing ammonia to urea, a relatively non-toxic metabolite that is excreted by the kidneys. In liver disease, due to a combination of hepatocellular dysfunction and portosystemic shunting, circulating ammonia concentrations are increased, and the major clinical consequences are noted in the brain with the development of encephalopathy or coma. One protective mechanism is the skeletal muscle uptake of ammonia, and this was has been reported by three independent groups, but it was always believed that the skeletal muscle functioned as a metabolic sink and converted the ammonia to glutamine.[40, 42, 43] Glutamine has cytoregulatory properties, and different cells use circulating glutamine as an anaplerotic substrate to regenerate ammonia that again needs to be removed by the hepatocytes.[44-47] Because ureagenesis is impaired in cirrhosis, there is no permanent disposal of ammonia, and the circulating glutamine serves as a source of ammoniagenesis in those tissues that utilize glutamine, maintaining hyperammonemia that is taken up again by the skeletal muscle. This pathway thus essentially transfers the carbon skeleton from the tricarboxylic acid (TCA) cycle in the muscle to other tissues resulting in skeletal muscle bioenergetics dysfunction and consequent impaired proteostasis and sarcopenia.

In the skeletal muscle, muscle, ammonia enters the myotubes, most likely by the RhBG class of ammonia transporters.[48] Other perturbations in cirrhosis can also activate myostatin and include a reduction in growth hormone, testosterone, and increased tumour necrosis factor α. However, whether reversing these abnormalities can reverse myostatin is not known.[13] In contrast, increased myostatin expression in response to hyperammonemia was reversed in response to ammonia withdrawal in myotubes in vitro or ammonia lowering measures in the portacaval anastomosis rat.[49]

In addition to the myostatin mediated signalling perturbations during hyperammonemia, ammonia is converted to glutamate in the mitochondria by cataplerosis of the critical TCA cycle intermediate, α ketoglutarate, and subsequent conversion of glutamate to glutamine in the skeletal muscle that is then exchanged for leucine by SLC7A5[47, 50, 51] (Figure 1). These reactions can explain elevated circulating glutamine in cirrhosis. Both hyperammonemia and loss of α ketoglutarate contribute to the loss of muscle mass and mitochondrial dysfunction and reduced adenosine triphosphate content with impaired contractile function.[52] Even though contractile function was not measured in these subjects, deconditioning or frailty is being increasingly recognized as an independent adverse prognostic indicator in cirrhosis.[53, 54] Even though contractile function and muscle mass are not necessarily related, it is, however, possible that the underlying mechanisms that result in these clinical manifestations may be common including reduced bioenergetics as has been reported in the past.[52, 55] Recent data also show post-translational modifications of proteins may be responsible for impaired muscle strength and consequent frailty.[52] This is important because even though myostatin depletion results in greater muscle mass, over time, muscle strength is not consistently maintained.[56-58]

Figure 1.

Figure 1.

Of the various metabolic, hormonal and cytokine abnormalities in cirrhosis, hyperammonemia perturbs a number of signalling and molecular pathways. Myostatin is transcriptionally upregulated in the muscle that impairs mammalian target of rapamycin complex 1 signalling that decreases protein synthesis and increases autophagy. As a metabolic response, ammonia disposal occurs via glutamine synthesis that is in turn exchanged for leucine (and potentially other branched chain amino acid) that enter the muscle cell providing an explanation for decreased plasma branched chain amino acid in cirrhosis. An additional cellular response via the general control nondepressible 2-eukaryotic initiation factor 2α axis impairs protein synthesis. There are a number of potential points of cross talk between these metabolic and molecular responses to hyperammonemia, all of which contribute to dysregulated proteostasis and sarcopenia.

Another interesting observation reported by Nishikawa is the relation between myostatin and serum albumin and tyrosine to branched chain amino acid (BCAA) ratios. Even though these have been considered as measures of ‘nutritional status’ in the past,[59] it is increasingly recognized that the term ‘malnutrition’ in cirrhosis needs to be replaced by more specific terms.[7] Two major components of ‘malnutrition’ in adult patients are being recognized: loss of skeletal muscle mass or sarcopenia and alteration in energy metabolism.[3] Even though these seem disparate, in metabolic terms, these are interrelated. Sarcopenia was initially used by Rosenberg to refer to the progressive loss of skeletal muscle with weakness that occurs with aging.[60] However, the term sarcopenia is translated to loss of skeletal muscle mass (sarcos, flesh; penia, deficiency) and is now used to refer to muscle loss in chronic diseases.[57, 61] In contrast, serum albumin is believed to be a measure hepatocyte synthetic capacity. Current data supports the role of myostatin primarily in the skeletal muscle.[36] However, albumin synthesis requires essential amino acids that are derived from dietary sources or endogenous proteolysis.[62] However, since cirrhosis is a state of accelerated starvation,[63] it is possible that the muscle protein synthesis is restricted to divert amino acids for synthesis of critical proteins including albumin in the hepatocytes. This hypothesis this needs to be explored in metabolic studies using tracer techniques.

The tyrosine to BCAA ratio is another measure that the authors have used as a measure of hepatic protein synthesis but is truly reflects the severity of liver disease and is due to skeletal muscle proteolysis and BCAA utilization.[47, 64, 65] It is also recognized that BCAA are a metabolized primarily in the skeletal muscle as a source of energy and for potential detoxification of ammonia via anaplerotic influx into the TCA cycle (Figure 1).[47, 51, 65] BCAA especially leucine and isoleucine can also function as a source of acetyl coenzyme A (CoA) independent of pyruvate because ammonia inhibits pyruvate dehydrogenase.[66-68] These provide a mechanistic basis for low plasma BCAA in cirrhosis. Interestingly, L-leucine also activates mammalian target of rapamycin complex 1 that increases protein synthesis and decreases autophagy that restores proteostasis or protein homeostasis and reverse sarcopenia.[51, 69]

In addition to myostatin dependent dysregulated proteostasis and sarcopenia, cellular stress pathways are activated during hyperammonemia.[51] Unlike canonical stress pathways mediated via a number of eukaryotic initiation factor 2α kinases including general control non-derepressed 2 that is activated in response to amino acid deficiency and during protein kinase R-like endoplasmic reticulum kinase that is activated during unfolded or misfolded proteins.[70-72] During hyperammonemia, a novel stress response has been reported that results in phosphorylation of the α subunit of the eukaryotic initiation factor with inhibition of protein synthesis.[51] Even though hyperammonemia activates both myostatin and the HASR, the crosstalk between these pathways needs investigation (Figure 1).

The implications of the report by Nishikawa et al. for developing treatment options cannot be overemphasized.[14] Currently, the major approach to therapy in medicine is based on targeting deficiency rather than focusing on the mechanisms.[13] Their report shows that myostatin and hyperammonemia are potential mechanistic treatment targets. Unfortunately, myostatin antagonists have not yet become clinically available and ammonia-lowering therapies have been used in human subjects only to reverse hepatic encephalopathy, the best-known consequence of hyperammonemia.[36, 49] However, as mentioned above, preclinical data do support the use of long-term ammonia lowering as a potential treatment option that should be evaluated in randomized trials with serum myostatin as a measure of therapeutic response. BCAA have been used to treat the consequences of hyperammonemia in cirrhosis with limited benefit. One potential reason may be the selective partitioning into the mitochondria to provide the carbon skeletons for anaplerosis as well as acetyl-CoA as a TCA cycle substrate (Figure 2).[51] These molecular and metabolic alterations formed the rationale for a high-dose leucine supplementation to satisfy the mitochondrial metabolic demand during hyperammonemia so that leucine in the cytoplasm can activate mTORC1 to restore proteostasis. Data from preclinical and clinical studies have supported such a beneficial mechanism and hold potential for long-term treatment with such supplements.[51, 73] However, since leucine supplementation did not lower blood ammonia, myostatin expression was not altered but mTORC1, the direct target of leucine was activated with restoration of proteostasis.[73] The reasons for the high significance of the study by Nishikawa et al. is that in addition to providing a compelling rationale for the use of serum myostatin as a potential biomarker for muscle loss and prognosis in cirrhosis, they also lay the foundation for the use of serial measurement of circulating myostatin as a potential strategy to evaluate response to interventions targeting sarcopenia in cirrhosis and possibly other chronic diseases. Currently, there are no non-invasive circulating biomarkers to determine response to treatments to prevent or reverse sarcopenia in liver and chronic diseases and if serum myostatin is indeed such a marker, it will fill a longstanding need in the field of muscle loss.

Figure 2.

Figure 2.

Leucine and potentially isoleucine and valine are selectively partitioned to the mitochondria to provide a source of acetyl coenzyme A as well as an anaplerotic substrate during hyperammonemia. This may explain the impaired mammalian target of rapamycin complex 1 signalling that is responsive to a high dose of leucine supplementation.

Acknowledgements

The authors certify that they comply with the ethical guidelines for authorship and publishing of the Journal of Cachexia, Sarcopenia and Muscle.[74] The author has received grant support from the National Institutes of Health and has served as a consultant to Fresenius Kabi GmBH (NIH grants R21 AA22742; P50AA024333 8236; RO1 GM119174; R21 AR071046; and UO1 DK061732).

References


1Montano-Loza AJ, Meza-Junco J, Prado CM, Lieffers JR, Baracos VE, Bain VG, et al. Muscle wasting is associated with mortality in patients with cirrhosis. Clin Gastroenterol Hepatol 2012;10:166–73, 73 e1.
2Tandon P, Ney M, Irwin I, Ma MM, Gramlich L, Bain VG, et al. Severe muscle depletion in patients on the liver transplant wait list: its prevalence and independent prognostic value. Liver Transpl 2012;18:1209–1216.
3Periyalwar P, Dasarathy S. Malnutrition in cirrhosis: contribution and consequences of sarcopenia on metabolic and clinical responses. Clin Liver Dis 2012;16:95–131.
4Merli M, Giusto M, Lucidi C, Giannelli V, Pentassuglio I, Di Gregorio V, et al. Muscle depletion increases the risk of overt and minimal hepatic encephalopathy: results of a prospective study. Metab Brain Dis 2013;28:281–284.
5Hanai T, Shiraki M, Nishimura K, Ohnishi S, Imai K, Suetsugu A, et al. Sarcopenia impairs prognosis of patients with liver cirrhosis. Nutrition 2015;31:193–199.
6Giusto M, Lattanzi B, Albanese C, Galtieri A, Farcomeni A, Giannelli V, et al. Sarcopenia in liver cirrhosis: the role of computed tomography scan for the assessment of muscle mass compared with dual-energy X-ray absorptiometry and anthropometry. Eur J Gastroenterol Hepatol 2015;27:328–334.
7Dasarathy S. Consilience in sarcopenia of cirrhosis. J Cachexia Sarcopenia Muscle 2012;3:225–237.
8Krell RW, Kaul DR, Martin AR, Englesbe MJ, Sonnenday CJ, Cai S, et al. Association between sarcopenia and the risk of serious infection among adults undergoing liver transplantation. Liver Transpl 2013;19:1396–1402.
9van Vugt JL, Levolger S, de Bruin RW, van Rosmalen J, Metselaar HJ, JN IJ. Systematic review and meta-analysis of the impact of computed tomography-assessed skeletal muscle mass on outcome in patients awaiting or undergoing liver transplantation. Am J Transplant 2016;16:2277–2292.
10Dasarathy S. Posttransplant sarcopenia: an underrecognized early consequence of liver transplantation. Dig Dis Sci 2013;58:3103–3111.
11Tsien C, Garber A, Narayanan A, Shah SN, Barnes D, Eghtesad B, et al. Post-liver transplantation sarcopenia in cirrhosis: a prospective evaluation. J Gastroenterol Hepatol 2014;29:1250–1257.
12Hamaguchi Y, Kaido T, Okumura S, Kobayashi A, Shirai H, Yagi S, et al. Impact of skeletal muscle mass index, intramuscular adipose tissue content, and visceral to subcutaneous adipose tissue area ratio on early mortality of living donor liver transplantation. Transplantation 2017;101:565–574.
13Dasarathy S, Merli M. Sarcopenia from mechanism to diagnosis and treatment in liver disease. J Hepatol 2016;65:1232–1244.
14Nishikawa H, Enomoto H, Ishii A, Iwata Y, Miyamoto Y, Ishii N, et al. Elevated serum myostatin level is associated with worse survival in patients with liver cirrhosis. J Cachexia Sarcopenia Muscle 2017;https://doi.org/10.1002/jcsm.12212.
15Furihata T, Kinugawa S, Fukushima A, Takada S, Homma T, Masaki Y, et al. Serum myostatin levels are independently associated with skeletal muscle wasting in patients with heart failure. Int J Cardiol 2016;220:483–487.
16Ju CR, Chen RC. Serum myostatin levels and skeletal muscle wasting in chronic obstructive pulmonary disease. Respir Med 2012;106:102–108.
17Hanai T, Shiraki M, Ohnishi S, Miyazaki T, Ideta T, Kochi T, et al. Rapid skeletal muscle wasting predicts worse survival in patients with liver cirrhosis. Hepatol Res 2016;46:743–751.
18McPherron AC, Lawler AM, Lee SJ. Regulation of skeletal muscle mass in mice by a new TGF-beta superfamily member. Nature 1997;387:83–90.
19Lee YS, Huynh TV, Lee SJ. Paracrine and endocrine modes of myostatin action. J Appl Physiol 1985). 2016;120:592–598.
20Lee SJ, McPherron AC. Regulation of myostatin activity and muscle growth. Proc Natl Acad Sci U S A 2001;98:9306–9311.
21Han HQ, Mitch WE. Targeting the myostatin signaling pathway to treat muscle wasting diseases. Curr Opin Support Palliat Care 2011;5:334–341.
22Kemaladewi DU, de Gorter DJ, Aartsma-Rus A, van Ommen GJ, ten Dijke P, 't Hoen PA et al. Cell-type specific regulation of myostatin signaling. FASEB J 2012;26(4):1462–72
23Shan T, Liang X, Bi P, Kuang S. Myostatin knockout drives browning of white adipose tissue through activating the AMPK-PGC1alpha-Fndc5 pathway in muscle. FASEB J 2013;27:1981–1989.
24Bernardi H, Gay S, Fedon Y, Vernus B, Bonnieu A, Bacou F. Wnt4 activates the canonical beta-catenin pathway and regulates negatively myostatin: functional implication in myogenesis. Am J Physiol Cell Physiol 2011;300:C1122–C1138.
25Steelman CA, Recknor JC, Nettleton D, Reecy JM. Transcriptional profiling of myostatin-knockout mice implicates Wnt signaling in postnatal skeletal muscle growth and hypertrophy. FASEB J 2006;20:580–582.
26Ma K, Mallidis C, Artaza J, Taylor W, Gonzalez-Cadavid N, Bhasin S. Characterization of 5′-regulatory region of human myostatin gene: regulation by dexamethasone in vitro. Am J Physiol Endocrinol Metab 2001;281:E1128–E1136.
27Liu W, Thomas SG, Asa SL, Gonzalez-Cadavid N, Bhasin S, Ezzat S. Myostatin is a skeletal muscle target of growth hormone anabolic action. J Clin Endocrinol Metab 2003;88:5490–5496.
28Rios R, Fernandez-Nocelos S, Carneiro I, Arce VM, Devesa J. Differential response to exogenous and endogenous myostatin in myoblasts suggests that myostatin acts as an autocrine factor in vivo. Endocrinology 2004;145:2795–2803.
29McFarlane C, Plummer E, Thomas M, Hennebry A, Ashby M, Ling N, et al. Myostatin induces cachexia by activating the ubiquitin proteolytic system through an NF-kappaB-independent, FoxO1-dependent mechanism. J Cell Physiol 2006;209:501–514.
30Wang BW, Chang H, Kuan P, Shyu KG. Angiotensin II activates myostatin expression in cultured rat neonatal cardiomyocytes via p38 MAP kinase and myocyte enhance factor 2 pathway. J Endocrinol 2008;197:85–93.
31Oldham JM, Osepchook CC, Jeanplong F, Falconer SJ, Matthews KG, Conaglen JV, et al. The decrease in mature myostatin protein in male skeletal muscle is developmentally regulated by growth hormone. J Physiol 2009;587:669–677.
32Zhang L, Pan J, Dong Y, Tweardy DJ, Dong Y, Garibotto G, et al. Stat3 activation links a C/EBPdelta to myostatin pathway to stimulate loss of muscle mass. Cell Metab 2013;18:368–379.
33Qin J, Du R, Yang YQ, Zhang HQ, Li Q, Liu L, et al. Dexamethasone-induced skeletal muscle atrophy was associated with upregulation of myostatin promoter activity. Res Vet Sci 2013;94:84–89.
34De Naeyer H, Lamon S, Russell AP, Everaert I, De Spaey A, Vanheel B, et al. Androgenic and estrogenic regulation of Atrogin-1, MuRF1 and myostatin expression in different muscle types of male mice. Eur J Appl Physiol 2014;114:751–761.
35Sharma M, McFarlane C, Kambadur R, Kukreti H, Bonala S, Srinivasan S. Myostatin: expanding horizons. IUBMB Life 2015;67:589–600.
36Chen PR, Lee K. Invited review: inhibitors of myostatin as methods of enhancing muscle growth and development. J Anim Sci 2016;94:3125–3134.
37Song XC, Xu C, Yue ZG, Wang L, Wang GW, Yang FH. Bioinformatic analysis based on the complete coding region of the MSTN gene within and among different species. Genet Mol Res 2016;15:https://doi.org/10.4238/gmr.15025031.
38Rossi G, Antonini S, Bonfanti C, Monteverde S, Vezzali C, Tajbakhsh S, et al. Nfix regulates temporal progression of muscle regeneration through modulation of myostatin expression. Cell Rep 2016;14:2238–2249.
39Jia Y, Gao G, Song H, Cai D, Yang X, Zhao R. Low-protein diet fed to crossbred sows during pregnancy and lactation enhances myostatin gene expression through epigenetic regulation in skeletal muscle of weaning piglets. Eur J Nutr 2016;55:1307–1314.
40Qiu J, Thapaliya S, Runkana A, Yang Y, Tsien C, Mohan ML, et al. Hyperammonemia in cirrhosis induces transcriptional regulation of myostatin by an NF-kappaB-mediated mechanism. Proc Natl Acad Sci U S A 2013;110:18162–18167.
41Dasarathy S, Mookerjee RP, Rackayova V, Rangroo Thrane V, Vairappan B, Ott P, et al. Ammonia toxicity: from head to toe? Metab Brain Dis 2017;32:529–538.
42Lockwood AH, McDonald JM, Reiman RE, Gelbard AS, Laughlin JS, Duffy TE, et al. The dynamics of ammonia metabolism in man. Effects of liver disease and hyperammonemia. J Clin Invest 1979;63:449–460.
43Ganda OP, Ruderman NB. Muscle nitrogen metabolism in chronic hepatic insufficiency. Metabolism 1976;25:427–435.
44Mates JM, Segura JA, Campos-Sandoval JA, Lobo C, Alonso L, Alonso FJ, et al. Glutamine homeostasis and mitochondrial dynamics. Int J Biochem Cell Biol 2009;41:2051–2061.
45Wang B, Wu G, Zhou Z, Dai Z, Sun Y, Ji Y, et al. Glutamine and intestinal barrier function. Amino Acids 2015;47:2143–2154.
46Scalise M, Pochini L, Galluccio M, Indiveri C. Glutamine transport. From energy supply to sensing and beyond. Biochim Biophys Acta 2016;1857:1147–1157.
47Holecek M. Branched-chain amino acid supplementation in treatment of liver cirrhosis: Updated views on how to attenuate their harmful effects on cataplerosis and ammonia formation. Nutrition 2017;41:80–85.
48Takeda K, Takemasa T. Expression of ammonia transporters Rhbg and Rhcg in mouse skeletal muscle and the effect of 6-week training on these proteins. Physiol Rep 2015;3:https://doi.org/10.14814/phy2.12596.
49Kumar A, Davuluri G, Silva RNE, Engelen M, Ten Have GAM, Prayson R, et al. Ammonia lowering reverses sarcopenia of cirrhosis by restoring skeletal muscle proteostasis. Hepatology 2017;65:2045–2058.
50Davuluri G, Allawy A, Thapaliya S, Rennison JH, Singh D, Kumar A, et al. Hyperammonaemia-induced skeletal muscle mitochondrial dysfunction results in cataplerosis and oxidative stress. J Physiol 2016;594:7341–7360.
51Davuluri G, Krokowski D, Guan BJ, Kumar A, Thapaliya S, Singh D, et al. Metabolic adaptation of skeletal muscle to hyperammonemia drives the beneficial effects of l-leucine in cirrhosis. J Hepatol 2016;65:929–937.
52McDaniel J, Davuluri G, Hill EA, Moyer M, Runkana A, Prayson R, et al. Hyperammonemia results in reduced muscle function independent of muscle mass. Am J Physiol Gastrointest Liver Physiol 2016;310:G163–G170.
53Lai JC, Covinsky KE, Dodge JL, Boscardin WJ, Segev DL, Roberts JP, et al. Development of a novel frailty index to predict mortality in patients with end-stage liver disease. Hepatology 2017;66:564–574.
54Dunn MA, Josbeno DA, Tevar AD, Rachakonda V, Ganesh SR, Schmotzer AR, et al. Frailty as Tested by Gait Speed is an Independent Risk Factor for Cirrhosis Complications that Require Hospitalization. Am J Gastroenterol 2016;111:1768–1775.
55Jacobsen EB, Hamberg O, Quistorff B, Ott P. Reduced mitochondrial adenosine triphosphate synthesis in skeletal muscle in patients with Child-Pugh class B and C cirrhosis. Hepatology 2001;34:7–12.
56Amthor H, Macharia R, Navarrete R, Schuelke M, Brown SC, Otto A, et al. Lack of myostatin results in excessive muscle growth but impaired force generation. Proc Natl Acad Sci USA 2007;104:1835–1840.
57Dasarathy S, McCullough AJ, Muc S, Schneyer A, Bennett CD, Dodig M, et al. Sarcopenia associated with portosystemic shunting is reversed by follistatin. J Hepatol 2011;54:915–921.
58Murphy KT, Chee A, Gleeson BG, Naim T, Swiderski K, Koopman R, et al. Antibody-directed myostatin inhibition enhances muscle mass and function in tumor-bearing mice. Am J Physiol Regul Integr Comp Physiol 2011;301:R716–R726.
59Caregaro L, Alberino F, Amodio P, Merkel C, Bolognesi M, Angeli P, et al. Malnutrition in alcoholic and virus-related cirrhosis. Am J Clin Nutr 1996;63:602–609.
60Rosenberg IH. Sarcopenia: origins and clinical relevance. J Nutr 1997;127:990S–991S.
61Collamati A, Marzetti E, Calvani R, Tosato M, D'Angelo E, Sisto AN, et al. Sarcopenia in heart failure: mechanisms and therapeutic strategies. J Geriatr Cardiol 2016;13:615–624.
62Young VR. Adult amino acid requirements: the case for a major revision in current recommendations. J Nutr 1994;124:1517S–1523S.
63Glass C, Hipskind P, Tsien C, Malin SK, Kasumov T, Shah SN, et al. Sarcopenia and a physiologically low respiratory quotient in patients with cirrhosis: a prospective controlled study. J Appl Physiol 1985). 2013;114:559–565.
64Hayashi M, Ohnishi H, Kawade Y, Muto Y, Takahashi Y. Augmented utilization of branched-chain amino acids by skeletal muscle in decompensated liver cirrhosis in special relation to ammonia detoxication. Gastroenterol Jpn 1981;16:64–70.
65Dam G, Ott P, Aagaard NK, Vilstrup H. Branched-chain amino acids and muscle ammonia detoxification in cirrhosis. Metab Brain Dis 2013;28:217–220.
66Cooper AJ, Lai JC. Cerebral ammonia metabolism in normal and hyperammonemic rats. Neurochem Pathol 1987;6:67–95.
67Haussinger D, Gerok W, Sies H. Inhibition of pyruvate dehydrogenase during the metabolism of glutamine and proline in hemoglobin-free perfused rat liver. Eur J Biochem 1982;126:69–76.
68Zwingmann C, Chatauret N, Leibfritz D, Butterworth RF. Selective increase of brain lactate synthesis in experimental acute liver failure: results of a [H-C] nuclear magnetic resonance study. Hepatology 2003;37:420–428.
69Efeyan A, Zoncu R, Chang S, Gumper I, Snitkin H, Wolfson RL, et al. Regulation of mTORC1 by the Rag GTPases is necessary for neonatal autophagy and survival. Nature 2013;493:679–683.
70Walter P, Ron D. The unfolded protein response: from stress pathway to homeostatic regulation. Science 2011;334:1081–1086.
71Hinnebusch AG. Evidence for translational regulation of the activator of general amino acid control in yeast. Proc Natl Acad Sci USA 1984;81:6442–6446.
72Ramirez M, Wek RC. Vazquez de Aldana CR, Jackson BM, Freeman B, Hinnebusch AG. Mutations activating the yeast eIF-2 alpha kinase GCN2: isolation of alleles altering the domain related to histidyl-tRNA synthetases. Mol Cell Biol 1992;12:5801–5815.
73Tsien C, Davuluri G, Singh D, Allawy A, Ten Have GA, Thapaliya S, et al. Metabolic and molecular responses to leucine-enriched branched chain amino acid supplementation in the skeletal muscle of alcoholic cirrhosis. Hepatology 2015;61:2018–2029.
74von Haehling S, Morley JE, Coats AJS, Anker SD. Ethical guidelines for publishing in the Journal of Cachexia, Sarcopenia and Muscle: update 2015. J Cachexia Sarcopenia Muscle 2015;6:315–316.