Sarcopenia in cirrhosis is mediated by a hyperammonemic stress response

摘要 在肝硬变中，高氨血症是一种持续的异常，这是由于肝脏尿失禁和 门体分流术。骨质疏松症或骨骼肌块丢失是 肝硬变高氨血症与门体分流术。尽管几乎所有人都承认 关于骨质疏松症的发病率和不良临床后果，目前还没有有效的治疗方法，因为 在肝硬变中肌肉丢失的机制还不清楚。肌肉质量的丧失是由于 调节失调的蛋白质稳态或蛋白质稳态受损的蛋白质合成和蛋白分解增加 自噬。细胞应激过程中的蛋白质动态平衡是通过激活综合应激反应来实现的 (Isr)通过激活转录因子4反应eif2α的磷酸化。氨是一种细胞 在肠道内氨基酸分解代谢、嘌呤代谢和合成过程中产生的应激源。我们 在骨骼肌中发现了一种独特的细胞应激反应，我们称之为高氨血症 应激反应(HASR)。在hasr期间，我们观察到eif2α的磷酸化和激活增加。 激酶和氨基酸缺乏传感器，由L逆转的一般控制的非降压性2(GCN2)- 补充亮氨酸。这些扰动类似于氨基酸缺乏反应，尽管增加了 高氨血症时细胞内L亮氨酸浓度的变化。有趣的是，我们还观察到只有一个 未折叠蛋白反应(UPR)的3个组成部分IRE1α在HASR过程中被激活。有趣的是， UPR的另外两个分支：经典的内质网应激调节因子PERK和ATF6 在HASR期间未激活。与eIF2α磷酸化的细胞应激反应不同，整合的 诱导ATF4及其支持翻译恢复的靶点的应激反应也没有 在HASR期间观察到的。这些观察结果表明，hasr具有一些氨基酸特征。 虚弱反应(无虚弱)和普遍定期审议的一些特征。我们的初步数据和公布的数据 暗示了一种浓度和时间相关的初始适应，这种适应发展到了不适应阶段 骨骼肌导致骨质疏松症。我们假设HASR被激活是为了响应 高氨血症，涉及GCN2/mTORC1轴，抑制蛋白质合成，诱导 氨基酸转运体对氨基酸摄取增加和蛋白平衡调控的适应性反应 SLC7A5。我们还假设，在HASR期间，只有IRE1α/XBP1被激活，自噬增加 以及通过RIDD(受调节的IRE1依赖α的衰退)降解。HASR和HASR的作用机制 可以增加肌管对高氨血症的保护性适应的干预措施将在一项 人骨骼肌和高氨血症的细胞和啮齿动物模型的综合阵列 3个特定目标中的肝硬化症。在每个目标中，都将测试特定的分子治疗干预措施 快速临床翻译的潜力，以逆转和潜在地预防肝脏疾病中的石棺减少。

ABSTRACT In cirrhosis, hyperammonemia is a consistent abnormality due to impaired hepatic ureagenesis and portosystemic shunting. Sarcopenia or loss of skeletal muscle mass is a major complication of hyperammonemia in cirrhosis and portosystemic shunting. Despite nearly universal recognition of the prevalence and adverse clinical consequences of sarcopenia, there are no effective therapies because the mechanisms of muscle loss in cirrhosis are not well understood. Loss of muscle mass occurs due to dysregulated protein homeostasis or proteostasis with impaired protein synthesis and increased proteolysis by autophagy. Protein homeostasis during cellular stress is achieved by activating an integrated stress response (ISR) in response to eIF2α phosphorylation via the activating transcription factor 4. Ammonia is a cellular stressor that is generated during amino acid catabolism, purine metabolism and synthesis in the gut. We identified a unique cellular stress response in the skeletal muscle that we have termed the hyperammonemic stress response (HASR). During HASR, we observed an increased phosphorylation and activation of the eIF2α kinase and amino acid deficiency sensor, general control nonderepressed 2 (GCN2) that is reversed by L- leucine supplementation. These perturbations resemble an amino acid deficiency response despite increased cellular L-leucine concentrations during hyperammonemia. Interestingly, we also observed that only one of the 3 components of the unfolded protein response (UPR), IRE1α, is activated during HASR. Interestingly, the other 2 limbs of the UPR: PERK, the classical mediator of Endoplasmic Reticulum (ER) stress and ATF6 were not activated during HASR. Unlike the cellular stress responses with eIF2α phosphorylation, the integrated stress response with induction of ATF4 and its targets that support translational recovery were also not observed during HASR. These observations show that HASR shares some characteristics of amino acid deficiency response (without deficiency) and some features of the UPR. Our preliminary and published data suggest a concentration and time dependent initial adaptive that progresses to a maladaptive phase in the skeletal muscle results in sarcopenia. We hypothesized that HASR is activated in response to hyperammonemia and involves a GCN2/mTORC1 axis that represses protein synthesis and induces an adaptive response of increased amino acid uptake and proteostasis control via the amino acid transporter SLC7A5. We also hypothesize that during HASR, only the IRE1α/XBP1s is activated with increased autophagy and mRNA degradation via RIDD (Regulated IRE1α dependent decay). The mechanisms of HASR and interventions that can increase protective adaptations to hyperammonemia in myotubes will be studied in a comprehensive array of cellular and rodent models of hyperammonemia and in the skeletal muscle of human cirrhotics in 3 specific aims. In each aim, a specific molecular therapeutic intervention will be tested with the potential for rapid clinical translation to reverse and potentially prevent sarcopenia in liver disease.