Hyperammonemia and proteostasis in cirrhosis

Purpose of review Skeletal muscle loss or sarcopenia is a frequent complication of cirrhosis that adversely affects clinical outcomes. As skeletal muscle is the largest store of proteins in the body, proteostasis or protein homeostasis is required for maintenance of muscle mass. This review will focus on disordered skeletal muscle proteostasis in liver disease. Recent findings Increased skeletal muscle uptake of ammonia initiates responses that result in disordered proteostasis including impaired protein synthesis and increased autophagy. The cellular response to the stress of hyperammonemia (hyperammonemic stress response, HASR) involves the coordinated action of diverse signaling pathways targeting the molecular mechanisms of regulation of protein synthesis. Transcriptional upregulation of myostatin, a TGF beta superfamily member, results in impaired mTORC1 signaling. Phosphorylation of the eukaryotic translation initiation factor 2 alpha (eIF2 alpha) also relates to decreased global protein synthesis rates and mTORC1 signaling. Ammonia also causes mitochondrial and bioenergetic dysfunction because of cataplerosis of alpha-ketoglutarate. Lowering ammonia, targeting components of HASR and regulating cellular amino acid levels can potentially restore proteostasis. Summary Signaling via myostatin and eIF2 alpha phosphorylation causes decreases in protein synthesis and mTORC1 activity with a parallel mitochondrial dysfunction and increased autophagy contributing to proteostasis perturbations during skeletal muscle hyperammonemia of liver disease.