Hyperammonemia reduces skeletal muscle protein synthesis via a beta-catenin-cMyc mediated impaired ribosomal biogenesis

点击翻译按钮获取中文摘要

ABSTRACT There are over 2.5 million patients with cirrhosis, with an annual incidence of 40,000 and about 27,000 deaths per year. Most patients do not get transplanted and management of complications remains the mainstay of therapy for cirrhosis. Skeletal muscle loss is the most frequent complication in cirrhosis and results in reduced quality of life, increased morbidity and mortality. Despite the high clinical significance of muscle loss in cirrhosis, there are no established therapies because the underlying mechanisms are not known. Identifying the mechanisms of skeletal muscle loss in cirrhosis is therefore of high clinical significance. Hyperammonemia is a consistent abnormality in cirrhosis because of reduced hepatic ureagenesis and portosystemic shunting. We have previously reported that ammonia decreases muscle protein synthesis. In preliminary studies, we show that hyperammonemia results in impaired β-catenin signaling and decreased expression of its target, c- MYC. Canonical regulation of β-catenin is mediated by GSK3β mediated phosphorylation. Interestingly, we noted that ammonia activates IKKβ and decreases β-catenin expression and transcriptional activity independent of GSK3β. We also showed that ammonia inhibits β-catenin by a novel, non-canonical IKKβ dependent mechanism. In the muscle, cMYC increases protein synthesis and muscle hypertrophy via activation of ribosomal biogenesis. However, whether lower β-catenin and consequent reduced cMYC expression and activity result in muscle loss is not known. The studies proposed in this application will aim to identify the molecular mechanisms by which ammonia impairs β-catenin signaling and the perturbations in the ribosomal biogenesis pathways. Based on compelling preliminary data generated in a comprehensive array of models with muscle hyperammonemia including human cirrhosis, portacaval anastamosis (PCA) rat and C2C12 myotube cultures, we hypothesize that reduced skeletal muscle ribosomal biogenesis and protein synthesis during hyperammonemia are mediated by a non-canonical IKKβ dependent impaired β-catenin signaling. We will examine this hypothesis by loss and gain of function studies in rodent and cell culture models by the following aims: First we will identify the mechanism by which hyperammonemia impairs β- catenin signaling by a non-canonical IKKβ-mediated mechanism. In-vivo silencing of IKKβ in the PCA rat and molecular studies in myotubes will be used to dissect the mechanisms of inactivation of β-catenin. Second, we will determine the mechanism by which hyperammonemia decreases ribosomal biogenesis, the critical step in protein synthesis, via the c-MYC transcriptional complex of ribosomal proteins. We will determine the mechanism by which ammonia inhibits the β-catenin-cMYC-ribosome biogenesis in murine myotubes and C2C12 myotubes by loss and gain in function studies. Our studies will determine the molecular mechanisms responsible for impaired muscle protein synthesis and provide the basis for developing novel interventions to reverse muscle loss in cirrhosis and other chronic diseases with hyperammonemia including heart failure.