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Sarcopenia from mechanism to diagnosis and treatment in liver disease

  • Srinivasan Dasarathy
    Correspondence
    Corresponding author. Address: Staff, Gastroenterology, Hepatology and Pathobiology, NE4 208, Lerner Research Institute, 9500 Euclid Avenue, Cleveland, OH 44195, United States. Tel.: +1 2164442980; fax: +1 2164453889.
    Affiliations
    Department of Gastroenterology, Hepatology and Pathobiology, Cleveland Clinic, United States
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  • Manuela Merli
    Affiliations
    Gastroenterology, Department of Clinical Medicine, Sapienza University of Rome, Rome, Italy
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Published:August 08, 2016DOI:https://doi.org/10.1016/j.jhep.2016.07.040

      Summary

      Sarcopenia or loss of skeletal muscle mass is the major component of malnutrition and is a frequent complication in cirrhosis that adversely affects clinical outcomes. These include survival, quality of life, development of other complications and post liver transplantation survival. Radiological image analysis is currently utilized to diagnose sarcopenia in cirrhosis. Nutrient supplementation and physical activity are used to counter sarcopenia but have not been consistently effective because the underlying molecular and metabolic abnormalities persist or are not influenced by these treatments. Even though alterations in food intake, hypermetabolism, alterations in amino acid profiles, endotoxemia, accelerated starvation and decreased mobility may all contribute to sarcopenia in cirrhosis, hyperammonemia has recently gained attention as a possible mediator of the liver-muscle axis. Increased muscle ammonia causes: cataplerosis of α-ketoglutarate, increased transport of leucine in exchange for glutamine, impaired signaling by leucine, increased expression of myostatin (a transforming growth factor beta superfamily member) and an increased phosphorylation of eukaryotic initiation factor 2α. In addition, mitochondrial dysfunction, increased reactive oxygen species that decrease protein synthesis and increased autophagy mediated proteolysis, also play a role. These molecular and metabolic alterations may contribute to the anabolic resistance and inadequate response to nutrient supplementation in cirrhosis. Central and skeletal muscle fatigue contributes to impaired exercise capacity and responses. Use of proteins with low ammoniagenic potential, leucine enriched amino acid supplementation, long-term ammonia lowering strategies and a combination of resistance and endurance exercise to increase muscle mass and function may target the molecular abnormalities in the muscle. Strategies targeting endotoxemia and the gut microbiome need further evaluation.

      Keywords

      Linked Article

      • Myokines: A promising therapeutic target for hepatic encephalopathy
        Journal of HepatologyVol. 66Issue 5
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          Recently, we read with great interest the review by Dasarathy and Merli [1] and the editorial by Rombouts et al. [2] regarding the effects of hyperammonemia on muscle activity and mass. These authors suggested a beneficial role of muscle in the prevention and treatment of hepatic encephalopathy (HE) by inhibiting wasting of muscle mass and facilitating muscle protein synthesis. We agree with their conclusion, and that this possibility provides a novel alternative for clinical treatment of HE.
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      • Reply to: “Myokines: a promising therapeutic target for hepatic encephalopathy”
        Journal of HepatologyVol. 66Issue 5
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          We read with great interest the letter by Yang and Luo on our work on hyperammonemia-mediated regulation of skeletal muscle mass and function [1]. We have previously reported that hyperammonemia transcriptionally upregulates myostatin in the skeletal muscle and that it results in impaired protein synthesis [2]. Myostatin is believed to be a myokine, and circulating myostatin levels are elevated in cirrhosis [2,3]. However, there is currently no evidence to support a direct cerebral effect of myostatin beyond that related to its effects on the skeletal muscle.
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      Introduction

      Sarcopenia is a frequent complication in cirrhosis. It is the major component of malnutrition and is not reversed after liver transplantation; in fact, it may worsen.
      Malnutrition in liver disease has been used for decades to describe the phenotype of skeletal muscle loss with or without loss of fat mass [
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      Malnutrition in cirrhosis: contribution and consequences of sarcopenia on metabolic and clinical responses.
      ]. The majority of “malnourished” patients with cirrhosis experience skeletal muscle wasting or sarcopenia, a major predictor of adverse clinical outcomes [
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      ]. Although alterations in body composition in cirrhosis have been reported using a number of methods, radiographic image analysis is believed to be the most precise technique to quantify muscle mass and define sarcopenia [
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      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.
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      ]. Over the past few years, a number of investigators have reported that sarcopenia occurs in 30–70% of cirrhotic patients [
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      Nutrition and survival in patients with liver cirrhosis.
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      • Meza-Junco J.
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      • Lieffers J.R.
      • Baracos V.E.
      • Bain V.G.
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      Muscle wasting is associated with mortality in patients with cirrhosis.
      ]. The clinical significance of sarcopenia in liver disease, primarily cirrhosis, is due to the high prevalence and adverse impact on clinical outcome measures including survival, quality of life, development of other complications of cirrhosis, and post liver transplant outcomes [
      • Periyalwar P.
      • Dasarathy S.
      Malnutrition in cirrhosis: contribution and consequences of sarcopenia on metabolic and clinical responses.
      ,
      • Merli M.
      • Riggio O.
      • Dally L.
      Does malnutrition affect survival in cirrhosis? PINC (Policentrica Italiana Nutrizione Cirrosi).
      ,
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      • Meza-Junco J.
      • Prado C.M.
      • Lieffers J.R.
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      Muscle wasting is associated with mortality in patients with cirrhosis.
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      • Merli M.
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      Cirrhotic patients are at risk for health care-associated bacterial infections.
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      • Merli M.
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      ]. Etiology and severity of the underlying liver disease, duration of illness, age and co-morbidities contribute to the severity of sarcopenia [
      • Periyalwar P.
      • Dasarathy S.
      Malnutrition in cirrhosis: contribution and consequences of sarcopenia on metabolic and clinical responses.
      ,
      • Merli M.
      • Riggio O.
      • Dally L.
      Does malnutrition affect survival in cirrhosis? PINC (Policentrica Italiana Nutrizione Cirrosi).
      ,
      • Merli M.
      • Giusto M.
      • Gentili F.
      • Novelli G.
      • Ferretti G.
      • Riggio O.
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      Nutritional status: its influence on the outcome of patients undergoing liver transplantation.
      ,
      • DiCecco S.R.
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      ]. Despite being widely recognized as a major complication of cirrhosis, most therapies to date are based on the principle of “deficiency replacement” rather than targeted treatments, and have generally been ineffective [
      • Dasarathy S.
      Consilience in sarcopenia of cirrhosis.
      ]. Nutritional supplementation has been a particular therapeutic focus because reduced dietary intake was believed to be the major cause of malnutrition and sarcopenia. However, these approaches have been frequently inadequate in improving survival [
      • Ney M.
      • Vandermeer B.
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      ]. An integrated metabolic-molecular approach in a comprehensive array of models has shown that hyperammonemia is a mediator of the liver-muscle axis [
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      ]. Physical activity has been suggested to improve functional capacity but the effect on skeletal muscle mass is still unclear [
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      Exercise capacity and muscle strength in patients with cirrhosis.
      ]. In recent years, a combination of sarcopenia with obesity has been increasingly recognized, especially in patients with non-alcoholic fatty liver disease (NAFLD) and post liver transplantation. Whether sarcopenia is mechanistically related to obesity and NAFLD, however, is still under debate [
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      ]. The major deficiency in the field of sarcopenia in cirrhosis is the lack of understanding of the mechanisms involved. A number of excellent recent reviews have described the clinical relevance of sarcopenia in cirrhosis but have not focused on the possible mechanisms and on the relevance of novel therapeutic targets that have the potential for clinical translation [
      • Periyalwar P.
      • Dasarathy S.
      Malnutrition in cirrhosis: contribution and consequences of sarcopenia on metabolic and clinical responses.
      ,
      • Dasarathy S.
      Consilience in sarcopenia of cirrhosis.
      ,
      • Dasarathy S.
      Treatment to improve nutrition and functional capacity evaluation in liver transplant candidates.
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      Clinical relevance of sarcopenia in patients with cirrhosis.
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      ].
      In the present review we will provide an overview of the clinical relevance of sarcopenia in liver cirrhosis, but the emphasis will be on the possible molecular and metabolic perturbations involved and the promising novel therapeutic approaches that could be made possible by these discoveries.

      Diagnosis of sarcopenia in cirrhosis

      Most studies to date have used the term “malnutrition” to identify primarily skeletal muscle loss determined by one or more criteria that are not always uniform or precise and an alteration in energy metabolism and potentially fat mass depletion. The diagnosis of skeletal muscle loss requires analysis of the body composition using one or more of a number of available techniques (Table 1) as well as the normal values to define the appropriate cut-off values for sarcopenia [
      • Merli M.
      • Romiti A.
      • Riggio O.
      • Capocaccia L.
      Optimal nutritional indexes in chronic liver disease.
      ,
      • Dasarathy J.
      • Alkhouri N.
      • Dasarathy S.
      Changes in body composition after transjugular intrahepatic portosystemic stent in cirrhosis: a critical review of literature.
      ,
      • Kallwitz E.R.
      Sarcopenia and liver transplant: The relevance of too little muscle mass.
      ]. Even though few studies have directly compared different methods, computed tomography (CT; Supplementary Fig. 1) with one of the image analysis programs is being increasingly used since skeletal muscle can be directly viewed and quantified [
      • Giusto 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.
      ,
      • Montano-Loza A.J.
      • Meza-Junco J.
      • Prado C.M.
      • Lieffers J.R.
      • Baracos V.E.
      • Bain V.G.
      • et al.
      Muscle wasting is associated with mortality in patients with cirrhosis.
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      • Glass C.
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      • Tsien C.
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      Sarcopenia and a physiologically low respiratory quotient in patients with cirrhosis: a prospective controlled study.
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      Reversal of sarcopenia predicts survival after a transjugular intrahepatic portosystemic stent.
      ,
      • Tsien C.
      • Garber A.
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      • Shah S.N.
      • Barnes D.
      • Eghtesad B.
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      Post-liver transplantation sarcopenia in cirrhosis: a prospective evaluation.
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      • Durand F.
      • Buyse S.
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      Prognostic value of muscle atrophy in cirrhosis using psoas muscle thickness on computed tomography.
      ]. Magnetic Resonance Imaging (MRI) has also been proposed as a valuable method although objective data in cirrhosis are scarce [
      • Cruz-Jentoft A.J.
      • Baeyens J.P.
      • Bauer J.M.
      • Boirie Y.
      • Cederholm T.
      • Landi F.
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      ]. Abdominal CT and MRI scans would be difficult to justify for quantifying muscle mass due to the cost and/or radiation exposure. However, most cirrhotic patients have surveillance scans for focal liver lesions, hepatocellular carcinoma, vascular disease and pre-transplant evaluation.
      Table 1Methods to quantify skeletal muscle evaluation in cirrhosis.
      CT, computed tomography; DEXA, dual energy X-ray absorptiometry; MRI, magnetic resonance imaging.
      Muscle mass depends on gender (lower in females) and age (lower with increasing age), and cut-off values for gender and age have been recently reported [
      • Tsien C.
      • Garber A.
      • Narayanan A.
      • Shah S.N.
      • Barnes D.
      • Eghtesad B.
      • et al.
      Post-liver transplantation sarcopenia in cirrhosis: a prospective evaluation.
      ]. Handgrip strength (a measure of muscle function) has been utilized in cirrhotic patients, but it may not be accurate when normalized to body mass index in cirrhosis due to fluctuations body water content.
      Quantifying muscle mass by measurements in a single anatomic area like the limb or abdominal muscles are believed to provide a reasonably accurate measure of whole body muscle mass [
      • Shen W.
      • Punyanitya M.
      • Wang Z.
      • Gallagher D.
      • St-Onge M.P.
      • Albu J.
      • et al.
      Total body skeletal muscle and adipose tissue volumes: estimation from a single abdominal cross-sectional image.
      ]. In cirrhosis, as in most chronic diseases, a preferential loss of type II or fast fibers is expected but in vivo measurements of fiber type loss in cirrhotic patients is still lacking. Appendicular muscle mass (limb muscles) is strongly influenced by the activity level. Measurements of psoas and abdominal muscle mass on CT images at L3 or L4 vertebra are used due to their relative independence from the activity level. However these muscles contain both type I and type IIA fibers [
      • Schiaffino S.
      • Reggiani C.
      Fiber types in mammalian skeletal muscles.
      ], which also needs to be considered. Another consideration is the quality of skeletal muscle that has been reported based on the CT attenuation that is lower in the muscles of cirrhotics compared to controls [
      • Tsien C.
      • Shah S.N.
      • McCullough A.J.
      • Dasarathy S.
      Reversal of sarcopenia predicts survival after a transjugular intrahepatic portosystemic stent.
      ] and is indicative of fatty infiltration with adverse clinical outcomes [
      • Fujiwara N.
      • Nakagawa H.
      • Kudo Y.
      • Tateishi R.
      • Taguri M.
      • Watadani T.
      • et al.
      Sarcopenia, intramuscular fat deposition, and visceral adiposity independently predict the outcomes of hepatocellular carcinoma.
      ,
      • Hamaguchi Y.
      • Kaido T.
      • Okumura S.
      • Kobayashi A.
      • Fujimoto Y.
      • Ogawa K.
      • et al.
      Muscle steatosis is an independent predictor of postoperative complications in patients with hepatocellular carcinoma.
      ]. Whether muscle quality can be determined by measuring contractile function or by the CT attenuation values needs to be ascertained (Table 2). The possible impact of these parameters on clinical outcomes has not been systematically evaluated.
      Table 2Sarcopenia adversely impacts outcome in cirrhosis.
      CT, computed tomography; HR, hazard ratio; ICU, intensive care unit; MAMA, mid arm muscle area; OR, odd ratio; SPPB, short physical performance battery; TSF, triceps skinfold thickness.

      Clinical impact of sarcopenia in cirrhosis

      A number of cross sectional and longitudinal studies using different methods to quantify muscle mass have reported that median survival and probability of survival are lower in patients who have cirrhosis with sarcopenia than those without sarcopenia (Table 2) [
      • Tandon P.
      • Ney M.
      • Irwin I.
      • Ma M.M.
      • Gramlich L.
      • Bain V.G.
      • et al.
      Severe muscle depletion in patients on the liver transplant wait list: its prevalence and independent prognostic value.
      ,
      • Merli M.
      • Giusto M.
      • Gentili F.
      • Novelli G.
      • Ferretti G.
      • Riggio O.
      • et al.
      Nutritional status: its influence on the outcome of patients undergoing liver transplantation.
      ,
      • Montano-Loza A.J.
      • Meza-Junco J.
      • Prado C.M.
      • Lieffers J.R.
      • Baracos V.E.
      • Bain V.G.
      • et al.
      Muscle wasting is associated with mortality in patients with cirrhosis.
      ,
      • Durand F.
      • Buyse S.
      • Francoz C.
      • Laouenan C.
      • Bruno O.
      • Belghiti J.
      • et al.
      Prognostic value of muscle atrophy in cirrhosis using psoas muscle thickness on computed tomography.
      ,
      • Wang C.W.
      • Feng S.
      • Covinsky K.E.
      • Hayssen H.
      • Zhou L.Q.
      • Yeh B.M.
      • et al.
      A comparison of muscle function, mass, and quality in liver transplant candidates: results from the functional assessment in liver transplantation study.
      ,
      • Kalafateli M.
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      • Choi Yau Y.
      • Mohammad A.O.
      • Arora S.
      • Rodrigues S.
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      Malnutrition and sarcopenia predict post-liver transplantation outcomes independently of the Model for End-stage Liver Disease score.
      ,
      • Hanai 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.
      ,
      • Kim T.Y.
      • Kim M.Y.
      • Sohn J.H.
      • Kim S.M.
      • Ryu J.A.
      • Lim S.
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      Sarcopenia as a useful predictor for long-term mortality in cirrhotic patients with ascites.
      ,
      • Masuda T.
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      • Ikegami T.
      • Harimoto N.
      • Yoshizumi T.
      • Soejima Y.
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      Sarcopenia is a prognostic factor in living donor liver transplantation.
      ,
      • DiMartini A.
      • Cruz Jr., R.J.
      • Dew M.A.
      • Myaskovsky L.
      • Goodpaster B.
      • Fox K.
      • et al.
      Muscle mass predicts outcomes following liver transplantation.
      ,
      • Englesbe M.J.
      • Patel S.P.
      • He K.
      • Lynch R.J.
      • Schaubel D.E.
      • Harbaugh C.
      • et al.
      Sarcopenia and mortality after liver transplantation.
      ,
      • Hamaguchi Y.
      • Kaido T.
      • Okumura S.
      • Fujimoto Y.
      • Ogawa K.
      • Mori A.
      • et al.
      Impact of quality as well as quantity of skeletal muscle on outcomes after liver transplantation.
      ,
      • Hara N.
      • Iwasa M.
      • Sugimoto R.
      • Mifuji-Moroka R.
      • Yoshikawa K.
      • Terasaka E.
      • et al.
      Sarcopenia and sarcopenic obesity are prognostic factors for overall survival in patients with cirrhosis.
      ,
      • Kaido T.
      • Ogawa K.
      • Fujimoto Y.
      • Ogura Y.
      • Hata K.
      • Ito T.
      • et al.
      Impact of sarcopenia on survival in patients undergoing living donor liver transplantation.
      ,
      • Shahid M.
      • Johnson J.
      • Nightingale P.
      • Neuberger J.
      Nutritional markers in liver allograft recipients.
      ,
      • Lai J.C.
      • Feng S.
      • Terrault N.A.
      • Lizaola B.
      • Hayssen H.
      • Covinsky K.
      Frailty predicts waitlist mortality in liver transplant candidates.
      ,
      • Carey E.J.
      • Steidley D.E.
      • Aqel B.A.
      • Byrne T.J.
      • Mekeel K.L.
      • Rakela J.
      • et al.
      Six-minute walk distance predicts mortality in liver transplant candidates.
      ,
      • Alvares-da-Silva M.R.
      • Reverbel da Silveira T.
      Comparison between handgrip strength, subjective global assessment, and prognostic nutritional index in assessing malnutrition and predicting clinical outcome in cirrhotic outpatients.
      ]. Some of these reports suggest that sarcopenia adds to the prognostic value of the model for end-stage liver disease (MELD) scoring system [
      • Kalafateli M.
      • Mantzoukis K.
      • Choi Yau Y.
      • Mohammad A.O.
      • Arora S.
      • Rodrigues S.
      • et al.
      Malnutrition and sarcopenia predict post-liver transplantation outcomes independently of the Model for End-stage Liver Disease score.
      ,
      • Montano-Loza A.J.
      • Duarte-Rojo A.
      • Meza-Junco J.
      • Baracos V.E.
      • Sawyer M.B.
      • Pang J.X.
      • et al.
      Inclusion of sarcopenia within MELD (MELD-Sarcopenia) and the prediction of mortality in patients with cirrhosis.
      ]. The cause(s) of higher mortality is however not as evident though both increased risk of infection and encephalopathy may be contributory factors [
      • van Vugt J.L.
      • Levolger S.
      • de Bruin R.W.
      • van Rosmalen J.
      • Metselaar H.J.
      • Metselaar H.J.
      • IJzermans J.N.
      Systematic review and meta-analysis of the impact of computed tomography assessed skeletal muscle mass on outcome in patients awaiting or undergoing liver transplantation.
      ]. Sarcopenia may also impair diaphragmatic work due to reduced muscle mass and this event may favor pulmonary complications especially in the context of surgery (liver resection or liver transplantation).
      Sepsis related mortality is higher in sarcopenic than non-sarcopenic cirrhosis [
      • Montano-Loza A.J.
      • Meza-Junco J.
      • Prado C.M.
      • Lieffers J.R.
      • Baracos V.E.
      • Bain V.G.
      • et al.
      Muscle wasting is associated with mortality in patients with cirrhosis.
      ,
      • Merli M.
      • Lucidi C.
      • Giannelli V.
      • Giusto M.
      • Riggio O.
      • Falcone M.
      • et al.
      Cirrhotic patients are at risk for health care-associated bacterial infections.
      ,
      • Caregaro L.
      • Alberino F.
      • Amodio P.
      • Merkel C.
      • Bolognesi M.
      • Angeli P.
      • et al.
      Malnutrition in alcoholic and virus-related cirrhosis.
      ]. For appropriate antibody and cytokine responses, adequate amino acid supply is necessary that is impaired when skeletal muscle mass is decreased but a direct causal or mechanistic link between sarcopenia and impaired immune function has not been shown [
      • Roubenoff R.
      Sarcopenia: effects on body composition and function.
      ]. Furthermore, it is also possible that factors that cause sarcopenia, including hormonal and biochemical alterations as well as circulating endotoxins, also contribute to the impaired immune function and increase the risk of infection. Lack of mobility or frailty in sarcopenia may also play a role [
      • Yende S.
      • Iwashyna T.J.
      • Angus D.C.
      Interplay between sepsis and chronic health.
      ]. Interestingly, cirrhotic patients with refractory ascites seem particularly prone to malnutrition and sarcopenia. Ascites is known to increase resting energy expenditure [
      • Dolz C.
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      • Obrador A.
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      • Gaya J.
      Ascites increases the resting energy expenditure in liver cirrhosis.
      ] while food intake is decreased due to raised abdominal pressure and early satiety. Treating refractory ascites by transjugular intrahepatic portosystemic shunt has been shown to improve body composition in malnourished cirrhotic patients [
      • Dasarathy J.
      • Alkhouri N.
      • Dasarathy S.
      Changes in body composition after transjugular intrahepatic portosystemic stent in cirrhosis: a critical review of literature.
      ,
      • Tsien C.
      • Shah S.N.
      • McCullough A.J.
      • Dasarathy S.
      Reversal of sarcopenia predicts survival after a transjugular intrahepatic portosystemic stent.
      ].
      Quality of life is lower in sarcopenic cirrhosis patients, but it is unclear whether this is due to the loss of muscle mass or impaired contractile function and subsequent limited mobility, or increased risk of other complications. This is still a field that needs well-designed studies [
      • Periyalwar P.
      • Dasarathy S.
      Malnutrition in cirrhosis: contribution and consequences of sarcopenia on metabolic and clinical responses.
      ,
      • Shiraki M.
      • Nishiguchi S.
      • Saito M.
      • Fukuzawa Y.
      • Mizuta T.
      • Kaibori M.
      • et al.
      Nutritional status and quality of life in current patients with liver cirrhosis as assessed in 2007–2011.
      ,
      • Huisman E.J.
      • Trip E.J.
      • Siersema P.D.
      • van Hoek B.
      • van Erpecum K.J.
      Protein energy malnutrition predicts complications in liver cirrhosis.
      ]. All domains of the quality of life are lower in malnourished patients when measures that primarily quantify skeletal muscle mass are utilized [
      • Periyalwar P.
      • Dasarathy S.
      Malnutrition in cirrhosis: contribution and consequences of sarcopenia on metabolic and clinical responses.
      ].
      Hepatocellular carcinoma (HCC) is a frequent complication in the natural history of chronic liver disease and recent studies have reported that sarcopenia is an independent prognostic factor decreasing survival and increasing treatment related mortality in patients with HCC [
      • Fujiwara N.
      • Nakagawa H.
      • Kudo Y.
      • Tateishi R.
      • Taguri M.
      • Watadani T.
      • et al.
      Sarcopenia, intramuscular fat deposition, and visceral adiposity independently predict the outcomes of hepatocellular carcinoma.
      ,
      • Meza-Junco J.
      • Montano-Loza A.J.
      • Baracos V.E.
      • Prado C.M.
      • Bain V.G.
      • Beaumont C.
      • et al.
      Sarcopenia as a prognostic index of nutritional status in concurrent cirrhosis and hepatocellular carcinoma.
      ].
      Liver transplantation is currently the definitive therapy to cure end-stage liver disease and sarcopenia adversely impacts outcomes in patients on the transplant list, in the peri-transplant period and post transplantation [
      • Tandon P.
      • Ney M.
      • Irwin I.
      • Ma M.M.
      • Gramlich L.
      • Bain V.G.
      • et al.
      Severe muscle depletion in patients on the liver transplant wait list: its prevalence and independent prognostic value.
      ,
      • Merli M.
      • Giusto M.
      • Gentili F.
      • Novelli G.
      • Ferretti G.
      • Riggio O.
      • et al.
      Nutritional status: its influence on the outcome of patients undergoing liver transplantation.
      ,
      • Englesbe M.J.
      • Patel S.P.
      • He K.
      • Lynch R.J.
      • Schaubel D.E.
      • Harbaugh C.
      • et al.
      Sarcopenia and mortality after liver transplantation.
      ,
      • Merli M.
      • Giusto M.
      • Giannelli V.
      • Lucidi C.
      • Riggio O.
      Nutritional status and liver transplantation.
      ]. Survival is lower in sarcopenic cirrhotic patients before liver transplantation while increased length of hospitalization, prolonged intensive care unit stay, and longer time of intubation have been reported after transplantation compared to patients without sarcopenia [
      • Merli M.
      • Giusto M.
      • Gentili F.
      • Novelli G.
      • Ferretti G.
      • Riggio O.
      • et al.
      Nutritional status: its influence on the outcome of patients undergoing liver transplantation.
      ,
      • Montano-Loza A.J.
      Clinical relevance of sarcopenia in patients with cirrhosis.
      ,
      • Englesbe M.J.
      • Patel S.P.
      • He K.
      • Lynch R.J.
      • Schaubel D.E.
      • Harbaugh C.
      • et al.
      Sarcopenia and mortality after liver transplantation.
      ].
      It is important to emphasize that clinical outcomes also depend on other factors, but sarcopenia is recognized as a major contributor to adverse outcomes in the management of the cirrhotic patient undergoing liver transplantation.

      Mechanisms of skeletal muscle loss in cirrhosis

      Alterations in protein turnover, energy disposal and metabolic changes induce muscle depletion in cirrhotic patients

      As seen above, a number of studies and reviews have provided descriptive data on the high prevalence and adverse clinical impact of sarcopenia in cirrhosis [
      • Periyalwar P.
      • Dasarathy S.
      Malnutrition in cirrhosis: contribution and consequences of sarcopenia on metabolic and clinical responses.
      ,
      • Merli M.
      • Riggio O.
      • Dally L.
      Does malnutrition affect survival in cirrhosis? PINC (Policentrica Italiana Nutrizione Cirrosi).
      ,
      • Tandon P.
      • Ney M.
      • Irwin I.
      • Ma M.M.
      • Gramlich L.
      • Bain V.G.
      • et al.
      Severe muscle depletion in patients on the liver transplant wait list: its prevalence and independent prognostic value.
      ,
      • Montano-Loza A.J.
      • Meza-Junco J.
      • Prado C.M.
      • Lieffers J.R.
      • Baracos V.E.
      • Bain V.G.
      • et al.
      Muscle wasting is associated with mortality in patients with cirrhosis.
      ,
      • Dasarathy S.
      Treatment to improve nutrition and functional capacity evaluation in liver transplant candidates.
      ,
      • Merli M.
      • Giusto M.
      • Giannelli V.
      • Lucidi C.
      • Riggio O.
      Nutritional status and liver transplantation.
      ]. Skeletal muscle is the major protein store in the human body [
      • Daniel P.M.
      The metabolic homoeostatic role of muscle and its function as a store of protein.
      ]. Skeletal muscle mass is maintained by a balance between protein synthesis, protein breakdown and regenerative capacity regulated by muscle satellite cell function [
      • Periyalwar P.
      • Dasarathy S.
      Malnutrition in cirrhosis: contribution and consequences of sarcopenia on metabolic and clinical responses.
      ]. Broadly, two types of studies have contributed to the current understanding of the pathogenesis of sarcopenia in cirrhosis: metabolic-tracer kinetics and molecular signaling pathway studies [
      • Qiu J.
      • Tsien C.
      • Thapalaya S.
      • Narayanan A.
      • Weihl C.C.
      • Ching J.K.
      • et al.
      Hyperammonemia-mediated autophagy in skeletal muscle contributes to sarcopenia of cirrhosis.
      ,
      • Qiu J.
      • Thapaliya S.
      • Runkana A.
      • Yang Y.
      • Tsien C.
      • Mohan M.L.
      • et al.
      Hyperammonemia in cirrhosis induces transcriptional regulation of myostatin by an NF-kappaB-mediated mechanism.
      ,
      • Tessari P.
      • Kiwanuka E.
      • Vettore M.
      • Barazzoni R.
      • Zanetti M.
      • Cecchet D.
      • et al.
      Phenylalanine and tyrosine kinetics in compensated liver cirrhosis: effects of meal ingestion.
      ,
      • Tessari P.
      • Vettore M.
      • Millioni R.
      • Puricelli L.
      • Orlando R.
      Effect of liver cirrhosis on phenylalanine and tyrosine metabolism.
      ,
      • McCullough A.J.
      • Mullen K.D.
      • Tavill A.S.
      • Kalhan S.C.
      In vivo differences between the turnover rates of leucine and leucine’s ketoacid in stable cirrhosis.
      ,
      • Thapaliya S.
      • Runkana A.
      • McMullen M.R.
      • Nagy L.E.
      • McDonald C.
      • Naga Prasad S.V.
      • et al.
      Alcohol-induced autophagy contributes to loss in skeletal muscle mass.
      ,
      • Tsien C.
      • Davuluri G.
      • Singh D.
      • Allawy A.
      • Ten Have G.A.
      • Thapaliya S.
      • et al.
      Metabolic and molecular responses to leucine-enriched branched chain amino acid supplementation in the skeletal muscle of alcoholic cirrhosis.
      ]. An integrated approach using both strategies to examine how metabolic perturbations alter molecular signaling and vice-versa has allowed identification of novel potential therapeutic targets.
      Whole body turnover studies using labeled phenylalanine and leucine as primed constant infusion have yielded conflicting results with unaltered, increased or decreased protein breakdown and protein synthesis [
      • Tessari P.
      • Kiwanuka E.
      • Vettore M.
      • Barazzoni R.
      • Zanetti M.
      • Cecchet D.
      • et al.
      Phenylalanine and tyrosine kinetics in compensated liver cirrhosis: effects of meal ingestion.
      ,
      • Tessari P.
      • Vettore M.
      • Millioni R.
      • Puricelli L.
      • Orlando R.
      Effect of liver cirrhosis on phenylalanine and tyrosine metabolism.
      ,
      • McCullough A.J.
      • Mullen K.D.
      • Tavill A.S.
      • Kalhan S.C.
      In vivo differences between the turnover rates of leucine and leucine’s ketoacid in stable cirrhosis.
      ]. Arteriovenous difference studies and release of 3-methylhistidine to quantify protein synthesis and breakdown suggest impaired skeletal muscle protein synthesis [
      • Morrison W.L.
      • Bouchier I.A.
      • Gibson J.N.
      • Rennie M.J.
      Skeletal muscle and whole-body protein turnover in cirrhosis.
      ]. Explanations for these conflicting observations included heterogeneity in etiology, duration, age, and severity of liver disease. Heterogeneity in methods used to determine protein turnover and in the contribution of different organs to whole body turnover also explain these differences. Whole body substrate utilization studies using indirect calorimetry have shown that cirrhosis is a state of accelerated starvation because fatty acid oxidation and gluconeogenesis are increased early in the postabsorptive or fasting state [
      • Glass C.
      • Hipskind P.
      • Tsien C.
      • Malin S.K.
      • Kasumov T.
      • Shah S.N.
      • et al.
      Sarcopenia and a physiologically low respiratory quotient in patients with cirrhosis: a prospective controlled study.
      ,
      • Tsien C.D.
      • McCullough A.J.
      • Dasarathy S.
      Late evening snack: exploiting a period of anabolic opportunity in cirrhosis.
      ,
      • Merli M.
      • Eriksson L.S.
      • Hagenfeldt L.
      • Wahren J.
      Splanchnic and leg exchange of free fatty acids in patients with liver cirrhosis.
      ]. Since glucose is a preferred substrate in many tissues, and fatty acid carbon cannot be used for gluconeogenesis, amino acids are used for gluconeogenesis [
      • Chen X.
      • Iqbal N.
      • Boden G.
      The effects of free fatty acids on gluconeogenesis and glycogenolysis in normal subjects.
      ]. The primary source of amino acids for gluconeogenesis is proteolysis in the skeletal muscle that generates both aromatic and branched chain amino acids (BCAA). Only BCAA are catabolized in the skeletal muscle due to the localization of the branched chain ketodehydrogenase and oxidation of the carbon skeleton as an energy source [
      • Blonde-Cynober F.
      • Aussel C.
      • Cynober L.
      Abnormalities in branched-chain amino acid metabolism in cirrhosis: influence of hormonal and nutritional factors and directions for future research.
      ]. As a consequence, plasma BCAA concentrations are lower in cirrhotic patients. In contrast, aromatic amino acids are primarily metabolized in the liver but due to both portosystemic shunting and hepatocellular dysfunction, their plasma concentrations are increased in chronic liver disease [
      • Tessari P.
      • Kiwanuka E.
      • Vettore M.
      • Barazzoni R.
      • Zanetti M.
      • Cecchet D.
      • et al.
      Phenylalanine and tyrosine kinetics in compensated liver cirrhosis: effects of meal ingestion.
      ,
      • Bembr A.
      The amino acid composition of animal tissue protein.
      ,
      • Iob V.
      • Coon W.W.
      • Sloan M.
      Free amino acids in liver, plasma, and muscle of patients with cirrhosis of the liver.
      ,
      • Montanari A.
      • Simoni I.
      • Vallisa D.
      • Trifiro A.
      • Colla R.
      • Abbiati R.
      • et al.
      Free amino acids in plasma and skeletal muscle of patients with liver cirrhosis.
      ,
      • Plauth M.
      • Egberts E.H.
      • Abele R.
      • Muller P.H.
      • Furst P.
      Characteristic pattern of free amino acids in plasma and skeletal muscle in stable hepatic cirrhosis.
      ]. This interpretation that accelerated starvation and increased gluconeogenesis are bioenergetics perturbations in cirrhosis is supported by the low respiratory quotient in sarcopenia cirrhotics [
      • Glass C.
      • Hipskind P.
      • Tsien C.
      • Malin S.K.
      • Kasumov T.
      • Shah S.N.
      • et al.
      Sarcopenia and a physiologically low respiratory quotient in patients with cirrhosis: a prospective controlled study.
      ]. Most therapies to date have focused on treating the amino acid imbalance rather than targeting the mechanisms that contribute to these alterations that finally result in sarcopenia.

      Potential mediators of the liver – muscle axis in cirrhosis

      Other perturbations that contribute to sarcopenia include endotoxemia, increased aromatase activity to lower testosterone, and mitochondrial dysfunction.
      One of the major reasons for the very limited understanding of sarcopenia in cirrhosis has been the difficulty in identifying the mediator(s) of the liver-muscle axis. A number of potential mediators have been proposed including increased ammonia, decreased testosterone and growth hormone, and endotoxemia [
      • Qiu J.
      • Tsien C.
      • Thapalaya S.
      • Narayanan A.
      • Weihl C.C.
      • Ching J.K.
      • et al.
      Hyperammonemia-mediated autophagy in skeletal muscle contributes to sarcopenia of cirrhosis.
      ,
      • Qiu J.
      • Thapaliya S.
      • Runkana A.
      • Yang Y.
      • Tsien C.
      • Mohan M.L.
      • et al.
      Hyperammonemia in cirrhosis induces transcriptional regulation of myostatin by an NF-kappaB-mediated mechanism.
      ,
      • Mowat N.A.
      • Edwards C.R.
      • Fisher R.
      • McNeilly A.S.
      • Green J.R.
      • Dawson A.M.
      Hypothalamic-pituitary-gonadal function in men with cirrhosis of the liver.
      ,
      • Kovarik M.
      • Muthny T.
      • Sispera L.
      • Holecek M.
      The dose-dependent effects of endotoxin on protein metabolism in two types of rat skeletal muscle.
      ]. Even though there is evidence to support each of these potential mediators, hyperammonemia has been studied most extensively [
      • Dasarathy S.
      Consilience in sarcopenia of cirrhosis.
      ,
      • Qiu J.
      • Tsien C.
      • Thapalaya S.
      • Narayanan A.
      • Weihl C.C.
      • Ching J.K.
      • et al.
      Hyperammonemia-mediated autophagy in skeletal muscle contributes to sarcopenia of cirrhosis.
      ,
      • Qiu J.
      • Thapaliya S.
      • Runkana A.
      • Yang Y.
      • Tsien C.
      • Mohan M.L.
      • et al.
      Hyperammonemia in cirrhosis induces transcriptional regulation of myostatin by an NF-kappaB-mediated mechanism.
      ,
      • McDaniel J.
      • Davuluri G.
      • Hill E.A.
      • Moyer M.
      • Runkana A.
      • Prayson R.
      • et al.
      Hyperammonemia results in reduced muscle function independent of muscle mass.
      ].
      Of the hepatic metabolic functions, ammonia disposal by ureagenesis is critical. Both hepatocellular dysfunction and portosystemic shunting that are components of the pathophysiological changes in cirrhosis contribute to impaired ureagenesis [
      • Shangraw R.E.
      • Jahoor F.
      Effect of liver disease and transplantation on urea synthesis in humans: relationship to acid-base status.
      ]. Ammonia is generated by a number of mechanisms including amino acid metabolism, purine metabolism, enterocyte glutaminase activity and urealysis in the gut [
      • Olde Damink S.W.
      • Jalan R.
      • Dejong C.H.
      Interorgan ammonia trafficking in liver disease.
      ]. Neurotoxicity is the best-studied cytotoxic effect of ammonia [
      • Olde Damink S.W.
      • Jalan R.
      • Dejong C.H.
      Interorgan ammonia trafficking in liver disease.
      ,
      • Hadjihambi A.
      • Rose C.F.
      • Jalan R.
      Novel insights into ammonia-mediated neurotoxicity pointing to potential new therapeutic strategies.
      ]. Independent investigators have reported increased skeletal muscle ammonia uptake and conversion to glutamate and glutamine in patients and models of liver disease [
      • Dam G.
      • Ott P.
      • Aagaard N.K.
      • Vilstrup H.
      Branched-chain amino acids and muscle ammonia detoxification in cirrhosis.
      ,
      • Holecek M.
      Evidence of a vicious cycle in glutamine synthesis and breakdown in pathogenesis of hepatic encephalopathy-therapeutic perspectives.
      ,
      • Lockwood A.H.
      • McDonald J.M.
      • Reiman R.E.
      • Gelbard A.S.
      • Laughlin J.S.
      • Duffy T.E.
      • et al.
      The dynamics of ammonia metabolism in man. Effects of liver disease and hyperammonemia.
      ,
      • Ganda O.P.
      • Ruderman N.B.
      Muscle nitrogen metabolism in chronic hepatic insufficiency.
      ]. Despite the well recognized cytotoxic effects of ammonia in the neurons and astrocytes, skeletal muscle effects have only been recently reported [
      • Qiu J.
      • Tsien C.
      • Thapalaya S.
      • Narayanan A.
      • Weihl C.C.
      • Ching J.K.
      • et al.
      Hyperammonemia-mediated autophagy in skeletal muscle contributes to sarcopenia of cirrhosis.
      ,
      • Qiu J.
      • Thapaliya S.
      • Runkana A.
      • Yang Y.
      • Tsien C.
      • Mohan M.L.
      • et al.
      Hyperammonemia in cirrhosis induces transcriptional regulation of myostatin by an NF-kappaB-mediated mechanism.
      ,
      • Dasarathy S.
      • Muc S.
      • Hisamuddin K.
      • Edmison J.M.
      • Dodig M.
      • McCullough A.J.
      • et al.
      Altered expression of genes regulating skeletal muscle mass in the portacaval anastomosis rat.
      ,
      • Dasarathy S.
      • McCullough A.J.
      • Muc S.
      • Schneyer A.
      • Bennett C.D.
      • Dodig M.
      • et al.
      Sarcopenia associated with portosystemic shunting is reversed by follistatin.
      ]. Studies in human skeletal muscle, the hyperammonemic portacaval anastomosis (PCA) rat, mice during hyperammonemia and in vitro studies in myotubes cultures suggest that ammonia accumulates in the skeletal muscle and activates a program of molecular alterations that contribute to sarcopenia [
      • Qiu J.
      • Tsien C.
      • Thapalaya S.
      • Narayanan A.
      • Weihl C.C.
      • Ching J.K.
      • et al.
      Hyperammonemia-mediated autophagy in skeletal muscle contributes to sarcopenia of cirrhosis.
      ,
      • Qiu J.
      • Thapaliya S.
      • Runkana A.
      • Yang Y.
      • Tsien C.
      • Mohan M.L.
      • et al.
      Hyperammonemia in cirrhosis induces transcriptional regulation of myostatin by an NF-kappaB-mediated mechanism.
      ,
      • Dasarathy S.
      • Muc S.
      • Hisamuddin K.
      • Edmison J.M.
      • Dodig M.
      • McCullough A.J.
      • et al.
      Altered expression of genes regulating skeletal muscle mass in the portacaval anastomosis rat.
      ,
      • Dasarathy S.
      • McCullough A.J.
      • Muc S.
      • Schneyer A.
      • Bennett C.D.
      • Dodig M.
      • et al.
      Sarcopenia associated with portosystemic shunting is reversed by follistatin.
      ]. Even though the mechanism of entry of ammonia into the skeletal muscle has not been well studied, ammonia transporters including the Rh B and C proteins are expressed in the muscle [
      • Takeda K.
      • Takemasa T.
      Expression of ammonia transporters Rhbg and Rhcg in mouse skeletal muscle and the effect of 6-week training on these proteins.
      ]. Following entry, ammonia activates a series of signaling responses whose exact mechanisms are as yet unclear.

      Hyperammonemia contributes to muscle depletion: intracellular signaling

      Hyperammonemia mediated upregulation of myostatin is believed to be a mechanism of impaired protein synthesis and increased autophagy, that contribute to sarcopenia.
      In murine myotubes and murine cells cultures, the response to hyperammonemia-mediated activation of p65-NF-κB is an increased expression of myostatin, a TGFβ superfamily member (Fig. 1) [
      • Qiu J.
      • Thapaliya S.
      • Runkana A.
      • Yang Y.
      • Tsien C.
      • Mohan M.L.
      • et al.
      Hyperammonemia in cirrhosis induces transcriptional regulation of myostatin by an NF-kappaB-mediated mechanism.
      ,
      • Garcia P.S.
      • Cabbabe A.
      • Kambadur R.
      • Nicholas G.
      • Csete M.
      Brief-reports: elevated myostatin levels in patients with liver disease: a potential contributor to skeletal muscle wasting.
      ]. Increased expression of myostatin in the skeletal muscle and plasma of cirrhotic patients has been reported [
      • Garcia P.S.
      • Cabbabe A.
      • Kambadur R.
      • Nicholas G.
      • Csete M.
      Brief-reports: elevated myostatin levels in patients with liver disease: a potential contributor to skeletal muscle wasting.
      ,
      • Merli M.
      • Giusto M.
      • Molfino A.
      • Bonetto A.
      • Rossi M.
      • Ginanni Corradini S.
      • et al.
      MuRF-1 and p-GSK3beta expression in muscle atrophy of cirrhosis.
      ] and these results should be confirmed in future studies. Myostatin is a known inhibitor of protein synthesis and potentially activates the ubiquitin proteasome and autophagy mediated proteolysis [
      • Qiu J.
      • Tsien C.
      • Thapalaya S.
      • Narayanan A.
      • Weihl C.C.
      • Ching J.K.
      • et al.
      Hyperammonemia-mediated autophagy in skeletal muscle contributes to sarcopenia of cirrhosis.
      ,
      • Qiu J.
      • Thapaliya S.
      • Runkana A.
      • Yang Y.
      • Tsien C.
      • Mohan M.L.
      • et al.
      Hyperammonemia in cirrhosis induces transcriptional regulation of myostatin by an NF-kappaB-mediated mechanism.
      ,
      • Han H.Q.
      • Zhou X.
      • Mitch W.E.
      • Goldberg A.L.
      Myostatin/activin pathway antagonism: molecular basis and therapeutic potential.
      ]. The ubiquitin-mediated proteolysis is not activated but autophagy has been found to be increased in muscle in experimental models of cirrhosis or during hyperammonemia [
      • Qiu J.
      • Tsien C.
      • Thapalaya S.
      • Narayanan A.
      • Weihl C.C.
      • Ching J.K.
      • et al.
      Hyperammonemia-mediated autophagy in skeletal muscle contributes to sarcopenia of cirrhosis.
      ,
      • Qiu J.
      • Thapaliya S.
      • Runkana A.
      • Yang Y.
      • Tsien C.
      • Mohan M.L.
      • et al.
      Hyperammonemia in cirrhosis induces transcriptional regulation of myostatin by an NF-kappaB-mediated mechanism.
      ]. Other potential mechanisms for activation of autophagy include ammonia mediated mitochondrial dysfunction and generation of reactive oxygen species [
      • Kosenko E.
      • Venediktova N.
      • Kaminsky Y.
      • Montoliu C.
      • Felipo V.
      Sources of oxygen radicals in brain in acute ammonia intoxication in vivo.
      ]. Even though these molecular signaling responses have been reported only in neural tissue, similar perturbations may also occur in the skeletal muscle [
      • Davuluri G.
      • Krokowski D.
      • Guan B.J.
      • Kumar A.
      • Thapaliya S.
      • Singh D.
      • et al.
      Metabolic adaptation of skeletal muscle to hyperammonemia drives the beneficial effects of L-leucine in cirrhosis.
      ].
      Figure thumbnail gr1
      Fig. 1Myostatin is transcriptionally upregulated by hyperammonemia in the skeletal muscle. Ammonia enters the skeletal muscle via the transport proteins Rh B and G. In the muscle, ammonia activates transforming growth factor β activated kinase 1 (TAK1) that activates TRAF6. Activated TRAF6 (k63 polyubiquitination) activates inhibitor of kappa B (IκB) kinase (IKK) that in turn phosphorylates nuclear factor kappa B (NF-κB) inhibitor protein IκB. Phospho IκB is degraded via a proteasome pathway releasing p65-NF-κB that enters the nucleus and transcriptionally upregulates myostatin.
      Interestingly, skeletal muscle metabolic responses to hyperammonemia are being increasingly recognized albeit in preliminary data [
      • Davuluri G.
      • Krokowski D.
      • Guan B.J.
      • Kumar A.
      • Thapaliya S.
      • Singh D.
      • et al.
      Metabolic adaptation of skeletal muscle to hyperammonemia drives the beneficial effects of L-leucine in cirrhosis.
      ]. Physiologically, glutamine and glutamate serve as anaplerotic substrates to generate α ketoglutarate (αKG) and ammonia in most tissues to maintain sufficient concentrations of the tricarboxylic acid (TCA) cycle intermediates [
      • Owen O.E.
      • Kalhan S.C.
      • Hanson R.W.
      The key role of anaplerosis and cataplerosis for citric acid cycle function.
      ]. This reaction is catalyzed by the bidirectional enzyme, glutamate dehydrogenase (GDH). The reaction preferentially occurs in the direction generating αKG, because the GDH Km for ammonia is very high (∼1 mM), a value that is significantly supraphysiological [
      • Ertan H.
      Some properties of glutamate dehydrogenase, glutamine synthetase and glutamate synthase from Corynebacterium callunae.
      ]. However, in cirrhosis, due to impaired ureagenesis and decreased hepatic ammonia disposal, the skeletal muscle functions as a metabolic partner for the liver and skeletal muscle ammonia concentrations are much higher potentially favoring cataplerosis or loss of critical TCA cycle intermediate, αKG [
      • Qiu J.
      • Thapaliya S.
      • Runkana A.
      • Yang Y.
      • Tsien C.
      • Mohan M.L.
      • et al.
      Hyperammonemia in cirrhosis induces transcriptional regulation of myostatin by an NF-kappaB-mediated mechanism.
      ]. This results in a number of potential consequences including lower flux of the TCA cycle, impaired mitochondrial function and decreased adenosine triphosphate (ATP) synthesis. Since protein synthesis, especially translation initiation, is an energy intense process, low ATP concentrations may also cause reduced protein synthesis. Another consequence of hyperammonemia that can explain a number of clinical observations is that oxodehydrogenases are inhibited by ammonia in a tissue specific manner [
      • Lai J.C.
      • Cooper A.J.
      Neurotoxicity of ammonia and fatty acids: differential inhibition of mitochondrial dehydrogenases by ammonia and fatty acyl coenzyme A derivatives.
      ]. These include pyruvate dehydrogenase, that catalyzes the conversion of pyruvate to acetyl coenzyme A (CoA), and αKG dehydrogenase that catalyzes conversion of αKG to succinyl CoA. An overview of these pathways is shown in Fig. 2. A number of clinical studies and meta-analyses have failed to show significant benefit of nutritional supplementation in malnourished cirrhotic patients [
      • Ney M.
      • Vandermeer B.
      • van Zanten S.J.
      • Ma M.M.
      • Gramlich L.
      • Tandon P.
      Meta-analysis: oral or enteral nutritional supplementation in cirrhosis.
      ,
      • Koretz R.L.
      • Avenell A.
      • Lipman T.O.
      Nutritional support for liver disease.
      ,
      • Antar R.
      • Wong P.
      • Ghali P.
      A meta-analysis of nutritional supplementation for management of hospitalized alcoholic hepatitis.
      ,
      • Dasarathy S.
      Treatment to improve nutrition and functional capacity evaluation in liver transplant candidates.
      ]. This may be due to the impaired acetyl CoA generation that necessitates formation of acetyl CoA from non-pyruvate sources including amino acids and fatty acids. Continued mitochondrial dysfunction, generation of reactive oxygen species, and impaired bioenergetics in the skeletal muscle all contribute to impaired protein synthesis and activate a metabolic, adaptive response, autophagy.
      Figure thumbnail gr2
      Fig. 2Biochemical abnormalities in the skeletal muscle that contribute impaired protein synthesis and increased autophagy with consequent sarcopenia. Metabolic and molecular perturbations that can be potentially reversed by intervention at targeted sites. 1. Long-term ammonia lowering strategies. 2. Myostatin blocking agent including antagomirs. 3. L-leucine provides acetyl CoA, activates mTORC1 and protein synthesis. 4. Glucogenic amino acids can be a source of anaplerotic input to provide succinyl CoA replacing the loss of (cataplerosis) of αKG that is converted to glutamate during hyperammonemia (since skeletal muscle cannot generate urea). 5. Cell permeable esters of αKG are a potential strategy to reverse cataplerosis and a novel method to increase muscle ammonia disposal. 6. Physical activity stimulates mTORC1 via phosphatidic acid.
      Reduced ATP in the muscle, impaired mitochondrial function, low concentrations of TCA cycle intermediates, increased gluconeogenesis and an increased fatty acid oxidation in the skeletal muscle during hyperammonemia suggest a bioenergetics crisis with a starvation like response. Decreased cellular ATP is consistent with activation of the cellular energy sensor, 5’ adenosine monophosphate-activated protein kinase (AMPK) and impaired mTORC1 signaling [
      • Tsien C.
      • Davuluri G.
      • Singh D.
      • Allawy A.
      • Ten Have G.A.
      • Thapaliya S.
      • et al.
      Metabolic and molecular responses to leucine-enriched branched chain amino acid supplementation in the skeletal muscle of alcoholic cirrhosis.
      ].
      Increased cataplerosis and muscle catabolism of branched chain amino acids as a source of energy may be responsible for low circulating branched chain amino acids with skeletal muscle concentrations of BCAA expected to be decreased in the muscle of cirrhotics due to increased utilization. Reduced cellular amino acid concentrations activate adaptive responses that include increased skeletal muscle autophagy that has been reported in both cirrhosis and hyperammonemia in myotubes. Another response to intracellular amino acid deficiency is the integrated stress response mediated by activation of amino acid deficiency sensor, general control non-depressed 2 (GCN2) via phosphorylation of eukaryotic initiation factor 2 that are increased during hyperammonemia and cirrhosis [
      • Davuluri G.
      • Krokowski D.
      • Guan B.J.
      • Kumar A.
      • Thapaliya S.
      • Singh D.
      • et al.
      Metabolic adaptation of skeletal muscle to hyperammonemia drives the beneficial effects of L-leucine in cirrhosis.
      ]. Surprisingly, skeletal muscle concentrations of branched chain have been mostly reported to be unaltered except for a single study that reported lower muscle concentrations of BCAA [
      • Iob V.
      • Coon W.W.
      • Sloan M.
      Free amino acids in liver, plasma, and muscle of patients with cirrhosis of the liver.
      ,
      • Montanari A.
      • Simoni I.
      • Vallisa D.
      • Trifiro A.
      • Colla R.
      • Abbiati R.
      • et al.
      Free amino acids in plasma and skeletal muscle of patients with liver cirrhosis.
      ,
      • Plauth M.
      • Egberts E.H.
      • Abele R.
      • Muller P.H.
      • Furst P.
      Characteristic pattern of free amino acids in plasma and skeletal muscle in stable hepatic cirrhosis.
      ]. Preliminary studies in hyperammonemic myotubes increased cellular transport and concentrations of leucine despite which supplementation with leucine enriched BCAA resulted in reversal of GCN2 activation. This rescued impaired mTORC1 signaling in patients with cirrhosis [
      • Tsien C.
      • Davuluri G.
      • Singh D.
      • Allawy A.
      • Ten Have G.A.
      • Thapaliya S.
      • et al.
      Metabolic and molecular responses to leucine-enriched branched chain amino acid supplementation in the skeletal muscle of alcoholic cirrhosis.
      ] and in myotubes during hyperammonemia. Other amino acids with therapeutic potential include L citrulline that is a precursor for L arginine and stimulates mTORC1 and protein synthesis [
      • Breuillard C.
      • Cynober L.
      • Moinard C.
      Citrulline and nitrogen homeostasis: an overview.
      ]. The beneficial effects of citrulline are believed to be due to decreased ureagenesis resulting in amino acid sparing, but it is not known if impaired ureagenesis will aggravate hyperammonemia and its consequences in cirrhosis and need to be studied systematically.
      Published data suggest that hyperammonemia is a mediator of the liver-muscle axis and the skeletal muscle does not function only as a metabolic sink for ammonia [
      • Qiu J.
      • Thapaliya S.
      • Runkana A.
      • Yang Y.
      • Tsien C.
      • Mohan M.L.
      • et al.
      Hyperammonemia in cirrhosis induces transcriptional regulation of myostatin by an NF-kappaB-mediated mechanism.
      ]. Ammonia uptake and disposal via glutamine synthesis in the muscle and transport into the circulation may be involved in sarcopenia. At the same time, if there is low muscle mass, non-hepatic disposal of ammonia is impaired which may cause further adverse effects. Consistently, some investigators have reported that encephalopathy is more frequent in sarcopenic than non-sarcopenic cirrhotics [
      • Huisman E.J.
      • Trip E.J.
      • Siersema P.D.
      • van Hoek B.
      • van Erpecum K.J.
      Protein energy malnutrition predicts complications in liver cirrhosis.
      ,
      • Merli 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.
      ].

      Other potential mediators of the liver – muscle axis in cirrhosis: testosterone, growth hormone

      Other mediators of the liver-muscle axis include the low testosterone due to increased aromatase activity in liver disease [
      • Dasarathy S.
      • Mullen K.D.
      • Dodig M.
      • Donofrio B.
      • McCullough A.J.
      Inhibition of aromatase improves nutritional status following portacaval anastomosis in male rats.
      ]. Decreased growth hormone concentrations or impaired growth hormone response in the muscle are also likely contributors to sarcopenia in cirrhosis [
      • Bucuvalas J.C.
      • Cutfield W.
      • Horn J.
      • Sperling M.A.
      • Heubi J.E.
      • Campaigne B.
      • et al.
      Resistance to the growth-promoting and metabolic effects of growth hormone in children with chronic liver disease.
      ,
      • Moller S.
      • Becker U.
      • Gronbaek M.
      • Juul A.
      • Winkler K.
      • Skakkebaek N.E.
      Short-term effect of recombinant human growth hormone in patients with alcoholic cirrhosis.
      ]. Both growth hormone and testosterone are known to inhibit myostatin expression and signaling responses [
      • Liu W.
      • Thomas S.G.
      • Asa S.L.
      • Gonzalez-Cadavid N.
      • Bhasin S.
      • Ezzat S.
      Myostatin is a skeletal muscle target of growth hormone anabolic action.
      ,
      • Lakshman K.M.
      • Bhasin S.
      • Corcoran C.
      • Collins-Racie L.A.
      • Tchistiakova L.
      • Forlow S.B.
      • et al.
      Measurement of myostatin concentrations in human serum: Circulating concentrations in young and older men and effects of testosterone administration.
      ] but it is not known if these hormonal alterations of cirrhosis also contribute to the impaired protein synthesis and increased myostatin expression in cirrhosis. A recent randomized trial showed that testosterone supplementation in male cirrhotics did result in an increase in lean body mass but not survival [
      • Sinclair M.
      • Grossmann M.
      • Hoermann R.
      • Angus P.W.
      • Gow P.J.
      Testosterone therapy increases muscle mass in men with cirrhosis and low testosterone: A randomised controlled trial.
      ].
      Hepatocellular and immune dysfunction as well as portosystemic shunting worsen the endotoxemia due to impaired gut barrier function and potentially altered gut microbiome in cirrhosis [
      • Bajaj J.S.
      • Heuman D.M.
      • Hylemon P.B.
      • Sanyal A.J.
      • White M.B.
      • Monteith P.
      • et al.
      Altered profile of human gut microbiome is associated with cirrhosis and its complications.
      ]. Endotoxemia via tumour necrosis factor (TNF)α dependent and potentially TNF independent pathways may also impair protein synthesis and potentially activate autophagy [
      • Lang C.H.
      • Frost R.A.
      • Nairn A.C.
      • MacLean D.A.
      • Vary T.C.
      TNF-alpha impairs heart and skeletal muscle protein synthesis by altering translation initiation.
      ,
      • Keller C.W.
      • Fokken C.
      • Turville S.G.
      • Lunemann A.
      • Schmidt J.
      • Munz C.
      • et al.
      TNF-alpha induces macroautophagy and regulates MHC class II expression in human skeletal muscle cells.
      ]. Careful molecular studies on these mediators are not available and the cross talk between hyperammonemia and other putative mediators such as those described above are not presently known. The next decade is likely to see major advances in our understanding of the molecular-metabolic interaction and how it contributes to or causes sarcopenia in liver disease.
      Finally, sarcopenic obesity has been reported in patients with NAFLD and after liver transplantation [
      • Hong H.C.
      • Hwang S.Y.
      • Choi H.Y.
      • Yoo H.J.
      • Seo J.A.
      • Kim S.G.
      • et al.
      Relationship between sarcopenia and nonalcoholic fatty liver disease: the Korean Sarcopenic Obesity Study.
      ,
      • Choudhary N.S.
      • Saigal S.
      • Saraf N.
      • Mohanka R.
      • Rastogi A.
      • Goja S.
      • et al.
      Sarcopenic obesity with metabolic syndrome: a newly recognized entity following living donor liver transplantation.
      ,
      • Carias S.
      • Castellanos A.L.
      • Vilchez V.
      • Nair R.
      • Dela Cruz A.C.
      • Watkins J.
      • et al.
      Nonalcoholic steatohepatitis is strongly associated with sarcopenic obesity in patients with cirrhosis undergoing liver transplant evaluation.
      ]. It is possible that the combination of skeletal muscle loss and increased fat mass may contribute to the development of metabolic components including insulin resistance, diabetes mellitus, hyperlipidemia and possibly NAFLD but whether there is a common underlying mechanism for both sarcopenia and obesity is still not known [
      • Merli M.
      • Iebba V.
      • Giusto M.
      What is new about diet in hepatic encephalopathy.
      ].

      Management strategies

      There is compelling evidence that sarcopenia is associated with adverse consequences while there are limited data showing that increasing muscle mass improves survival in the non-transplanted and post liver transplant population of cirrhotics [
      • Tsien C.
      • Shah S.N.
      • McCullough A.J.
      • Dasarathy S.
      Reversal of sarcopenia predicts survival after a transjugular intrahepatic portosystemic stent.
      ,
      • Tsien C.
      • Garber A.
      • Narayanan A.
      • Shah S.N.
      • Barnes D.
      • Eghtesad B.
      • et al.
      Post-liver transplantation sarcopenia in cirrhosis: a prospective evaluation.
      ]. Therefore, reversing muscle mass is a priority area for therapeutic interventions in cirrhotic patients (Fig. 3). Interventions that focus only on deficiency replacement have generally been ineffective while targeted therapies have the potential to reverse muscle loss [
      • Periyalwar P.
      • Dasarathy S.
      Malnutrition in cirrhosis: contribution and consequences of sarcopenia on metabolic and clinical responses.
      ,
      • Ney M.
      • Vandermeer B.
      • van Zanten S.J.
      • Ma M.M.
      • Gramlich L.
      • Tandon P.
      Meta-analysis: oral or enteral nutritional supplementation in cirrhosis.
      ,
      • Koretz R.L.
      • Avenell A.
      • Lipman T.O.
      Nutritional support for liver disease.
      ,
      • Antar R.
      • Wong P.
      • Ghali P.
      A meta-analysis of nutritional supplementation for management of hospitalized alcoholic hepatitis.
      ,
      • Dasarathy S.
      Treatment to improve nutrition and functional capacity evaluation in liver transplant candidates.
      ,
      • Tsien C.
      • Davuluri G.
      • Singh D.
      • Allawy A.
      • Ten Have G.A.
      • Thapaliya S.
      • et al.
      Metabolic and molecular responses to leucine-enriched branched chain amino acid supplementation in the skeletal muscle of alcoholic cirrhosis.
      ,
      • Dasarathy S.
      • McCullough A.J.
      • Muc S.
      • Schneyer A.
      • Bennett C.D.
      • Dodig M.
      • et al.
      Sarcopenia associated with portosystemic shunting is reversed by follistatin.
      ]. The major strategies that have been used to improve muscle mass include supplemental calorie and protein intake, increased physical activity, supplemental hormone therapy, and mechanistic targeted treatments [
      • Dasarathy S.
      Consilience in sarcopenia of cirrhosis.
      ,
      • Dasarathy S.
      Treatment to improve nutrition and functional capacity evaluation in liver transplant candidates.
      ,
      • Dasarathy S.
      Cause and management of muscle wasting in chronic liver disease.
      ,
      • Sinclair M.
      • Gow P.J.
      • Grossmann M.
      • Angus P.W.
      Review article: sarcopenia in cirrhosis–aetiology, implications and potential therapeutic interventions.
      ,
      • Williams T.J.
      • McKenna M.J.
      Exercise limitation following transplantation.
      ]. The critical outcome measures include survival, hospitalization, quality of life, development of and recovery from other complications of cirrhosis. It is not clear if the improved clinical outcomes are due to an increase in muscle mass, amelioration in skeletal muscle contractile dysfunction or a combination of the two. Despite the current focus being on reversing sarcopenia, it is also important to take into consideration skeletal muscle function that include maximum contractile strength, maintenance of contraction, and muscle fatigue in response to persistent and repetitive contraction [
      • McDaniel J.
      • Davuluri G.
      • Hill E.A.
      • Moyer M.
      • Runkana A.
      • Prayson R.
      • et al.
      Hyperammonemia results in reduced muscle function independent of muscle mass.
      ].
      Figure thumbnail gr3
      Fig. 3Overview of strategies to reverse sarcopenia and potentially contractile dysfunction in cirrhosis. Molecular targets are depicted in blue boxes and putative interventions are outside the boxes. Modified from
      [
      • Dasarathy S.
      Cause and management of muscle wasting in chronic liver disease.
      ]
      with permission.

      Supplemental nutrition

      Since caloric and protein intake are frequently decreased in cirrhosis, Guidelines and Consensus papers have consistently recommended to provide adequate amounts of calories and proteins either by frequent feeding, through oral dietary supplementation or when indicated, by enteral or parenteral nutrition [
      • Plauth M.
      • Merli M.
      • Kondrup J.
      • Weimann A.
      • Ferenci P.
      • Muller M.J.
      • et al.
      ESPEN guidelines for nutrition in liver disease and transplantation.
      ,
      • Plauth M.
      • Cabre E.
      • Riggio O.
      • Assis-Camilo M.
      • Pirlich M.
      • Kondrup J.
      • et al.
      ESPEN guidelines on enteral nutrition: liver disease.
      ,
      • Plank L.D.
      • Mathur S.
      • Gane E.J.
      • Peng S.L.
      • Gillanders L.K.
      • McIlroy K.
      • et al.
      Perioperative immunonutrition in patients undergoing liver transplantation: a randomized double-blind trial.
      ,
      • Amodio P.
      • Bemeur C.
      • Butterworth R.
      • Cordoba J.
      • Kato A.
      • Montagnese S.
      • et al.
      The nutritional management of hepatic encephalopathy in patients with cirrhosis: International Society for Hepatic Encephalopathy and Nitrogen Metabolism Consensus.
      ]. Regimens providing extra calories via high caloric feeding, and/or enteral feeding have been extensively studied (Table 3) [
      • Plank L.D.
      • Mathur S.
      • Gane E.J.
      • Peng S.L.
      • Gillanders L.K.
      • McIlroy K.
      • et al.
      Perioperative immunonutrition in patients undergoing liver transplantation: a randomized double-blind trial.
      ,
      • Hirsch S.
      • Bunout D.
      • de la Maza P.
      • Iturriaga H.
      • Petermann M.
      • Icazar G.
      • et al.
      Controlled trial on nutrition supplementation in outpatients with symptomatic alcoholic cirrhosis.
      ,
      • de Ledinghen V.
      • Beau P.
      • Mannant P.R.
      • Borderie C.
      • Ripault M.P.
      • Silvain C.
      • et al.
      Early feeding or enteral nutrition in patients with cirrhosis after bleeding from esophageal varices? A randomized controlled study.
      ,
      • Le Cornu K.A.
      • McKiernan F.J.
      • Kapadia S.A.
      • Neuberger J.M.
      A prospective randomized study of preoperative nutritional supplementation in patients awaiting elective orthotopic liver transplantation.
      ,
      • Marchesini G.
      • Bianchi G.
      • Merli M.
      • Amodio P.
      • Panella C.
      • Loguercio C.
      • et al.
      Nutritional supplementation with branched-chain amino acids in advanced cirrhosis: a double-blind, randomized trial.
      ,
      • Hu Q.G.
      • Zheng Q.C.
      The influence of Enteral Nutrition in postoperative patients with poor liver function.
      ,
      • Les I.
      • Doval E.
      • Garcia-Martinez R.
      • Planas M.
      • Cardenas G.
      • Gomez P.
      • et al.
      Effects of branched-chain amino acids supplementation in patients with cirrhosis and a previous episode of hepatic encephalopathy: a randomized study.
      ,
      • Dupont B.
      • Dao T.
      • Joubert C.
      • Dupont-Lucas C.
      • Gloro R.
      • Nguyen-Khac E.
      • et al.
      Randomised clinical trial: enteral nutrition does not improve the long-term outcome of alcoholic cirrhotic patients with jaundice.
      ,
      • Sorrentino P.
      • Castaldo G.
      • Tarantino L.
      • Bracigliano A.
      • Perrella A.
      • Perrella O.
      • et al.
      Preservation of nutritional-status in patients with refractory ascites due to hepatic cirrhosis who are undergoing repeated paracentesis.
      ]. Interestingly, few studies suggest improvement in nitrogen retention or nutritional status using very heterogeneous criteria that measure primarily fat and non-fat mass [
      • Dasarathy S.
      Consilience in sarcopenia of cirrhosis.
      ,
      • Koretz R.L.
      • Avenell A.
      • Lipman T.O.
      Nutritional support for liver disease.
      ,
      • Antar R.
      • Wong P.
      • Ghali P.
      A meta-analysis of nutritional supplementation for management of hospitalized alcoholic hepatitis.
      ,
      • Dupont B.
      • Dao T.
      • Joubert C.
      • Dupont-Lucas C.
      • Gloro R.
      • Nguyen-Khac E.
      • et al.
      Randomised clinical trial: enteral nutrition does not improve the long-term outcome of alcoholic cirrhotic patients with jaundice.
      ,
      • Matsuoka S.
      • Tamura A.
      • Nakagawara H.
      • Moriyama M.
      Improvement in the nutritional status and clinical conditions of patients with liver failure using a liver diet combined with a branched chain amino acids-enriched elemental diet.
      ,
      • Fialla A.D.
      • Israelsen M.
      • Hamberg O.
      • Krag A.
      • Gluud L.L.
      Nutritional therapy in cirrhosis or alcoholic hepatitis: a systematic review and meta-analysis.
      ]. On the other hand, a recent randomized controlled trial measured total body protein utilizing perioperative immunonutrition enriched in n-3 fatty acids, arginine, and nucleotides vs. an isocaloric diet in patients undergoing liver transplantation. Protein content was measured by neutron activation analysis, from study entry until immediately prior to LT but did not find any change in total body protein. Postoperative outcomes were also not influenced by the nutritional supplementation [
      • Plank L.D.
      • Mathur S.
      • Gane E.J.
      • Peng S.L.
      • Gillanders L.K.
      • McIlroy K.
      • et al.
      Perioperative immunonutrition in patients undergoing liver transplantation: a randomized double-blind trial.
      ].
      Table 3Studies about nutritional intervention in adult liver cirrhosis reporting data about changes in parameters dealing with muscle mass.
      OLT, orthotopic liver transplantation; PN, parenteral nutrition.
      Another approach has been to shorten the duration of post-absorptive or fasting state in cirrhosis because of the accelerated starvation that results in proteolysis, because after food intake, recovery of muscle mass is incomplete [
      • Tsien C.D.
      • McCullough A.J.
      • Dasarathy S.
      Late evening snack: exploiting a period of anabolic opportunity in cirrhosis.
      ]. Daytime and nocturnal feeding have been evaluated and there is evidence that a late evening snack has the most beneficial effects and it is currently believed that a late evening and an early morning protein supplement are likely to have the greatest benefit on preventing continued muscle loss in cirrhosis [
      • Tsien C.D.
      • McCullough A.J.
      • Dasarathy S.
      Late evening snack: exploiting a period of anabolic opportunity in cirrhosis.
      ,
      • Okuda H.
      • Shiratori K.
      Long-term nutritional assessment and quality of life in patients with cirrhosis taking a late evening snack.
      ]. Meta analyses of supplemental nutrition in patients with alcoholic hepatitis and those with cirrhosis were disappointing, however, as nutritional supplementation by various routes did not improve survival [
      • Ney M.
      • Vandermeer B.
      • van Zanten S.J.
      • Ma M.M.
      • Gramlich L.
      • Tandon P.
      Meta-analysis: oral or enteral nutritional supplementation in cirrhosis.
      ,
      • Koretz R.L.
      • Avenell A.
      • Lipman T.O.
      Nutritional support for liver disease.
      ,
      • Antar R.
      • Wong P.
      • Ghali P.
      A meta-analysis of nutritional supplementation for management of hospitalized alcoholic hepatitis.
      ]. Even though the exact reason for very limited improvement in sarcopenia with nutritional supplementation is not yet clear, cirrhosis can be seen as a state of anabolic resistance and caloric supplementation alone seems to be inadequate. As mentioned earlier, despite providing calories, impaired mitochondrial function and bioenergetics in combination with impaired molecular responses to nutrient administration in muscle are potential reasons for lack of benefit. Whether other outcomes including encephalopathy, sepsis and quality of life improve with reversal of sarcopenia are currently unknown.
      Protein supplementation is another alternative to improve the availability of essential amino acids. However, cirrhosis and hyperammonemia may accelerate amino acids catabolism with further generation of skeletal muscle ammonia that can impair protein synthesis and increase autophagy further with little or no benefit in reversing sarcopenia. Animal proteins have the added disadvantage of being rich in aromatic amino acids that are not metabolized by the skeletal muscle and may worsen encephalopathy [
      • Bembr A.
      The amino acid composition of animal tissue protein.
      ,
      • Nguyen D.L.
      • Morgan T.
      Protein restriction in hepatic encephalopathy is appropriate for selected patients: a point of view.
      ]. Vegetable proteins are rich in BCAA and may have a beneficial effect by removing one mole of ammonia per mole of BCAA via the αKG → glutamate → glutamine pathway. Therefore, instead of protein supplementation, BCAA have been used in the past as treatment for hepatic encephalopathy in a number of acute and long-term studies [
      • Metcalfe E.L.
      • Avenell A.
      • Fraser A.
      Branched-chain amino acid supplementation in adults with cirrhosis and porto-systemic encephalopathy: systematic review.
      ,
      • Alexander W.F.
      • Spindel E.
      • Harty R.F.
      • Cerda J.J.
      The usefulness of branched chain amino acids in patients with acute or chronic hepatic encephalopathy.
      ,
      • Gluud L.L.
      • Dam G.
      • Borre M.
      • Les I.
      • Cordoba J.
      • Marchesini G.
      • et al.
      Oral branched-chain amino acids have a beneficial effect on manifestations of hepatic encephalopathy in a systematic review with meta-analyses of randomized controlled trials.
      ]. A recent Cochrane review suggested benefit in the primary outcome, hepatic encephalopathy but not on survival, quality of life or nutritional parameters [
      • Gluud L.L.
      • Dam G.
      • Les I.
      • Cordoba J.
      • Marchesini G.
      • Borre M.
      • et al.
      Branched-chain amino acids for people with hepatic encephalopathy.
      ]. Lack of benefit in nutritional parameters was counter to expected outcomes, since BCAA provide a source of energy to the muscle in addition to being substrates for protein synthesis. Another mechanism by which BCAA may function is by inhibiting the amino acid deficiency sensor, GCN2 and reversing eIF2α phosphorylation [
      • Zhang P.
      • McGrath B.C.
      • Reinert J.
      • Olsen D.S.
      • Lei L.
      • Gill S.
      • et al.
      The GCN2 eIF2alpha kinase is required for adaptation to amino acid deprivation in mice.
      ], impaired protein synthesis and improve muscle mass. Finally, leucine directly activates mTORC1 that stimulates protein synthesis and decreases autophagy [
      • Carroll B.
      • Korolchuk V.I.
      • Sarkar S.
      Amino acids and autophagy: cross-talk and co-operation to control cellular homeostasis.
      ], both of which have the potential to improve muscle mass. A recent study in human cirrhosis reported that a leucine enriched BCAA mixture was able to reverse the molecular perturbations in the skeletal muscle downstream of myostatin in cirrhotic patients [
      • Tsien C.
      • Davuluri G.
      • Singh D.
      • Allawy A.
      • Ten Have G.A.
      • Thapaliya S.
      • et al.
      Metabolic and molecular responses to leucine-enriched branched chain amino acid supplementation in the skeletal muscle of alcoholic cirrhosis.
      ]. Tracer kinetic studies with direct quantification of muscle protein synthesis showed similar rates of protein synthesis in response to a single oral dose of leucine enriched BCAA mixture did reverse the GCN2-eIF2α mediated impaired protein synthesis and increased mTORC1 signaling [
      • Tsien C.
      • Davuluri G.
      • Singh D.
      • Allawy A.
      • Ten Have G.A.
      • Thapaliya S.
      • et al.
      Metabolic and molecular responses to leucine-enriched branched chain amino acid supplementation in the skeletal muscle of alcoholic cirrhosis.
      ]. These data provide the first direct evidence of molecular perturbations in the skeletal muscle in cirrhosis and in combination with animal and in vitro cell culture data support the role of hyperammonemia as a mediator of the liver-muscle axis.

      Exercise and physical activity

      Therapies including nutrient supplementation and exercise are not consistently effective since they target replacing deficiency rather than the underlying mechanisms.
      The type of exercise determines the muscle related outcomes [
      • Fyfe J.J.
      • Bishop D.J.
      • Stepto N.K.
      Interference between concurrent resistance and endurance exercise: molecular bases and the role of individual training variables.
      ]. Resistance exercise increases skeletal muscle mass by inducing muscle injury and regeneration and protein synthesis [
      • Damas F.
      • Phillips S.
      • Vechin F.C.
      • Ugrinowitsch C.
      A review of resistance training-induced changes in skeletal muscle protein synthesis and their contribution to hypertrophy.
      ]. Endurance exercise improves functional capacity but does not necessarily reverse sarcopenia. A combination of resistance and endurance exercise have the potential to improve muscle mass and functional capacity but such studies have not been performed in cirrhosis. Randomized studies have reported improvement in short-term outcomes in response to exercise in cirrhotics [
      • Jones J.C.
      • Coombes J.S.
      • Macdonald G.A.
      Exercise capacity and muscle strength in patients with cirrhosis.
      ]. Since direct comparisons of outcomes in healthy subjects and cirrhotic patients in response to exercise have not been reported, it is not possible to determine if the anabolic resistance to nutrients is also observed with exercise. There is evidence that protein kinase Cζ –phosphatidic acid mediates signal transduction of mechanical activity to signaling responses by activating mTORC1 signaling and protein synthesis [
      • Hornberger T.A.
      • Chu W.K.
      • Mak Y.W.
      • Hsiung J.W.
      • Huang S.A.
      • Chien S.
      The role of phospholipase D and phosphatidic acid in the mechanical activation of mTOR signaling in skeletal muscle.
      ]. However, it is not known if these physiological responses are blunted in cirrhosis and if ammonia is the mediator of such blunted responses. A recent study in a comprehensive array of models including hyperammonemic rats, human subjects and ex vivo muscle preparations does suggest that hyperammonemia also alters contractile response and increases fatigue in cirrhosis [
      • McDaniel J.
      • Davuluri G.
      • Hill E.A.
      • Moyer M.
      • Runkana A.
      • Prayson R.
      • et al.
      Hyperammonemia results in reduced muscle function independent of muscle mass.
      ]. Whether immobilization and injury responses in the cirrhotic skeletal muscle are altered has not been studied but may explain the rapid deconditioning observed during hospitalization.

      Anabolic hormones

      Testosterone and growth hormone have been used in the past to improve nutritional status and, potentially, muscle mass in cirrhosis but have not been consistently beneficial [
      • Bucuvalas J.C.
      • Cutfield W.
      • Horn J.
      • Sperling M.A.
      • Heubi J.E.
      • Campaigne B.
      • et al.
      Resistance to the growth-promoting and metabolic effects of growth hormone in children with chronic liver disease.
      ,
      • Moller S.
      • Becker U.
      • Gronbaek M.
      • Juul A.
      • Winkler K.
      • Skakkebaek N.E.
      Short-term effect of recombinant human growth hormone in patients with alcoholic cirrhosis.
      ,
      • Sinclair M.
      • Grossmann M.
      • Hoermann R.
      • Angus P.W.
      • Gow P.J.
      Testosterone therapy increases muscle mass in men with cirrhosis and low testosterone: A randomised controlled trial.
      ,
      • Orr R.
      • Fiatarone Singh M.
      The anabolic androgenic steroid oxandrolone in the treatment of wasting and catabolic disorders: review of efficacy and safety.
      ,
      • Rambaldi A.
      • Iaquinto G.
      • Gluud C.
      Anabolic-androgenic steroids for alcoholic liver disease: a Cochrane review.
      ]. Despite adverse effects, these therapies are not effective in reversing nutritional status or sarcopenia. Increased aromatase activity contributes to conversion of testosterone to estradiol that blunts its effect [
      • Dasarathy S.
      • Mullen K.D.
      • Dodig M.
      • Donofrio B.
      • McCullough A.J.
      Inhibition of aromatase improves nutritional status following portacaval anastomosis in male rats.
      ]. Aromatase resistant androgens like oxandrolone may therefore be beneficial but have not been borne out in clinical practice [
      • Orr R.
      • Fiatarone Singh M.
      The anabolic androgenic steroid oxandrolone in the treatment of wasting and catabolic disorders: review of efficacy and safety.
      ]. Lack of therapeutic benefit with hormone replacement may also be due to impaired signaling responses including mTORC1 response downstream of androgen and growth hormone receptors may be responsible for failure of these therapies. Increasing the understanding of molecular and metabolic perturbations in the skeletal muscle not only provides explanations for the lack of clinical benefit of standard therapies but also is likely to help identify novel, specific therapeutic targets for reversing sarcopenia.

      Ammonia lowering strategies

      Current methods to lower ammonia include non-absorbable disaccharides and antibiotics to prevent gut generation of ammonia [
      • Rose C.F.
      Ammonia-lowering strategies for the treatment of hepatic encephalopathy.
      ]. The primary outcomes of these treatments are reversal of encephalopathy and lowering of blood ammonia concentrations. It is, however, well known that blood ammonia concentrations do not always correlate with the severity of encephalopathy, the most studied response to hyperammonemia [
      • Lockwood A.H.
      Blood ammonia levels and hepatic encephalopathy.
      ]. Skeletal muscle turnover is a slow process and lowering ammonia transiently may not necessarily lower muscle ammonia concentrations or reverse the ongoing metabolic and molecular perturbations rapidly. Studies on long-term ammonia lowering strategies, quantifying muscle ammonia concentrations and signaling responses to these interventions are necessary before such an approach can be used to reverse muscle loss and impaired contractile function. Novel and potential methods to lower muscle ammonia include the use of cell permeable esters of αKG that can provide a direct anaplerotic influx with removal of ammonia as glutamine. However, glutamine disposal will then become limiting and strategies for long-term ammonia disposal to protect the skeletal muscle are necessary. Isoleucine and valine as anaplerotic substrates have been suggested because they can remove one mole of ammonia per mole of amino acid but the molecular and functional responses to these interventions have not been evaluated in preclinical or clinical studies to lower muscle ammonia or reverse sarcopenia [
      • Dam G.
      • Ott P.
      • Aagaard N.K.
      • Vilstrup H.
      Branched-chain amino acids and muscle ammonia detoxification in cirrhosis.
      ,
      • Holecek M.
      Evidence of a vicious cycle in glutamine synthesis and breakdown in pathogenesis of hepatic encephalopathy-therapeutic perspectives.
      ].

      Novel molecular targeted strategies

      Myostatin antagonists [
      • Han H.Q.
      • Zhou X.
      • Mitch W.E.
      • Goldberg A.L.
      Myostatin/activin pathway antagonism: molecular basis and therapeutic potential.
      ], direct mTORC1 activators [
      • Tsien C.
      • Davuluri G.
      • Singh D.
      • Allawy A.
      • Ten Have G.A.
      • Thapaliya S.
      • et al.
      Metabolic and molecular responses to leucine-enriched branched chain amino acid supplementation in the skeletal muscle of alcoholic cirrhosis.
      ,
      • Carroll B.
      • Korolchuk V.I.
      • Sarkar S.
      Amino acids and autophagy: cross-talk and co-operation to control cellular homeostasis.
      ], antioxidants, and mitochondrial protective agents have the potential to benefit skeletal muscle protein turnover but have not been adequately evaluated. Careful mechanistic studies are necessary with preclinical testing before these interventions can be translated to clinical practice.

      Post liver transplantation sarcopenia

      Therapies targeting mitochondrial function, including: mitochondrial antioxidants, mTORC1 signaling, and myostatin, hold promise for the future.
      The underlying molecular mechanisms and mediators need to be ascertained before therapies can be recommended. Direct mTORC1 inhibitors that block protein synthesis responses and accelerate autophagy are largely used after liver transplantation, at least in the United States [
      • Dasarathy S.
      Consilience in sarcopenia of cirrhosis.
      ]. Calcineurin inhibits muscle growth and hypertrophy [
      • Semsarian C.
      • Wu M.J.
      • Ju Y.K.
      • Marciniec T.
      • Yeoh T.
      • Allen D.G.
      • et al.
      Skeletal muscle hypertrophy is mediated by a Ca2+-dependent calcineurin signalling pathway.
      ] and calcineurin inhibitors are used in the vast majority of post transplant patients. The contribution of these medications to post transplant sarcopenia and sarcopenic obesity needs to be evaluated. Whether anabolic resistance of cirrhosis is reversed by liver transplantation is not known and integrated metabolic-molecular studies with muscle biopsies are needed before specific therapies and preventive measures can be developed. Finally, the reversibility of hyperammonemia induced signaling responses and impaired protein synthesis after liver transplantation is not known. It is possible that epigenetic changes in the regulatory molecules result in long-term or persistent sarcopenia even after transplantation or ammonia lowering therapies.

      Conclusion

      In summary, there is compelling evidence to show that sarcopenia is the major complication of cirrhosis and adversely affects outcomes during the entire course of a cirrhotic patient’s life. Evidence to show that sarcopenia can be reversed is much more limited and it is not clear if reversing sarcopenia will indeed improve outcomes as expected. Nutritional supplementation is not consistently effective in improving outcomes but long-term BCAA with leucine are promising therapies to prevent and treat sarcopenia in cirrhosis. Long-term reduction of muscle ammonia, novel approaches to enhance muscle ammonia disposal, and strategies to block myostatin hold potential for the future. Identification of molecular and metabolic perturbations in the cirrhotic skeletal muscle will allow development of targeted therapies that focus in reversing the anabolic resistance in these patients.

      Financial support

      Funded in part by NIH RO1 DK83414 , R21 AA 022742 , UO1 DK 061732 , UO1 AA021893 and P50 AA024333-01 8236 to SD.

      Conflict of interest

      Dr. Dasarathy reports grants from National Institutes of Health, during the conduct of the study. These include grants RO1 DK83414, R21 AA 022742, UO1 DK 061732, UO1 AA021893 and P50 AA024333-01 8236. Dr. Merli declared that she does not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.

      Authors’ contributions

      Dr. Dasarathy generated the initial draft, edited the manuscript, generated the figures and tables and approved the final draft. Dr Merli assisted with the initial draft, edited the draft, edited the figures and approved the final manuscript.

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