Pharmacokinetics/pharmacodynamics of L-ornithine phenylacetate in overt hepatic encephalopathy and the effect of plasma ammonia concentration reduction on clinical outcomes

Attenuating hyperammonemia preserves protein synthesis and muscle mass via restoration of perturbed metabolic pathways in bile duct-ligated rats

Sarcopenia and hepatic encephalopathy (HE) are complications of chronic liver disease (CLD), which negatively impact clinical outcomes. Hyperammonemia is considered to be the central component in the pathogenesis of HE, however ammonia's toxic effects have also been shown to impinge on extracerebral organs including the muscle. Our aim was to investigate the effect of attenuating hyperammonemia with ornithine phenylacetate (OP) on muscle mass loss and associated molecular mechanisms in rats with CLD. Six-week bile duct-ligated (BDL) rats and Sham-operated controls were treated with OP (1 g/kg, oral) for 5 weeks. Body composition, assessed by EchoMRI, and muscle protein fractional synthesis rate were evaluated. Signalling mechanisms regulating protein homeostasis, ATP content and metabolic intermediates in the tricarboxylic acid cycle (TCA) in skeletal muscle were quantified. OP treatment attenuated hyperammonemia, prevented brain edema and improved locomotor activity in BDL rats. Increased muscle ammonia, reduction in lean body mass, decreased muscle protein synthesis rate and ATP content were restored in OP-treated versus saline-treated BDL rats. TCA cycle intermediary metabolite, α-ketoglutarate, alterations of molecular markers regulating protein homeostasis including mTOR signalling and autophagy, were also preserved in muscle of OP-treated BDL rats. OP attenuated hyperammonemia, preserved muscle protein synthesis and prevented muscle mass loss in a preclinical model of CLD through restoration of perturbed signalling responses and altered TCA intermediary metabolites. Ammonia-lowering strategies have the potential for rapid clinical translation for simultaneous neuroprotection and sarcopenia prevention in patients with CLD.

Ammonia-induced stress response in liver disease progression and hepatic encephalopathy

Ammonia levels are orchestrated by a series of complex interrelated pathways in which the urea cycle has a central role. Liver dysfunction leads to an accumulation of ammonia, which is toxic and is strongly associated with disruption of potassium homeostasis, mitochondrial dysfunction, oxidative stress, inflammation, hypoxaemia and dysregulation of neurotransmission. Hyperammonaemia is a hallmark of hepatic encephalopathy and has been strongly associated with liver-related outcomes in patients with cirrhosis and liver failure. In addition to the established role of ammonia as a neurotoxin in the pathogenesis of hepatic encephalopathy, an increasing number of studies suggest that it can lead to hepatic fibrosis progression, sarcopenia, immune dysfunction and cancer. However, elevated systemic ammonia levels are uncommon in patients with metabolic dysfunction-associated steatotic liver disease. A clear causal relationship between ammonia-induced immune dysfunction and risk of infection has not yet been definitively proven. In this Review, we discuss the mechanisms by which ammonia produces its diverse deleterious effects and their clinical relevance in liver diseases, the importance of measuring ammonia levels for the diagnosis of hepatic encephalopathy, the prognosis of patients with cirrhosis and liver failure, and how our knowledge of inter-organ ammonia metabolism is leading to the development of novel therapeutic approaches.

Cerebral Aspects of Portal Hypertension: Hepatic Encephalopathy

Portal hypertension has cerebral consequences via its causes and complications, namely hepatic encephalopathy (HE), a common and devastating brain disturbance caused by liver insufficiency and portosystemic shunting. The pathogenesis involves hyperammonemia and systemic inflammation. Symptoms are disturbed personality and reduced attention. HE is minimal or grades I to IV (coma). Bouts of HE are episodic and often recurrent. Initial treatment is of events that precipitated the episode and exclusion of nonhepatic causes. Specific anti-HE treatment is lactulose. By recurrence, rifaximin is add-on. Anti-HE treatment is efficacious also for prophylaxis, but emergence of HE marks advanced liver disease and a dismal prognosis.

NHH promotes Sepsis-associated Encephalopathy with the expression of AQP4 in astrocytes through the gut-brain Axis

Sepsis-associated encephalopathy (SAE) is a significant cause of mortality in patients with sepsis. Despite extensive research, its exact cause remains unclear. Our previous research indicated a relationship between non-hepatic hyperammonemia (NHH) and SAE. This study aimed to investigate the relationship between NHH and SAE and the potential mechanisms causing cognitive impairment. In the in vivo experimental results, there were no significant abnormalities in the livers of mice with moderate cecal ligation and perforation (CLP); however, ammonia levels were elevated in the hippocampal tissue and serum. The ELISA study suggest that fecal microbiota transplantation in CLP mice can reduce ammonia levels. Reduction in ammonia levels improved cognitive dysfunction and neurological impairment in CLP mice through behavioral, neuroimaging, and molecular biology studies. Further studies have shown that ammonia enters the brain to regulate the expression of aquaporins-4 (AQP4) in astrocytes, which may be the mechanism underlying brain dysfunction in CLP mice. The results of the in vitro experiments showed that ammonia up-regulated AQP4 expression in astrocytes, resulting in astrocyte damage. The results of this study suggest that ammonia up-regulates astrocyte AQP4 expression through the gut-brain axis, which may be a potential mechanism for the occurrence of SAE.

Mitochondrial targets in hyperammonemia: Addressing urea cycle function to improve drug therapies

The urea cycle (UC) is a critically important metabolic process for the disposal of nitrogen (ammonia) produced by amino acids catabolism. The impairment of this liver-specific pathway induced either by primary genetic defects or by secondary causes, namely those associated with hepatic disease or drug administration, may result in serious clinical consequences. Urea cycle disorders (UCD) and certain organic acidurias are the major groups of inherited rare diseases manifested with hyperammonemia (HA) with UC dysregulation. Importantly, several commonly prescribed drugs, including antiepileptics in monotherapy or polytherapy from carbamazepine to valproic acid or specific antineoplastic agents such as asparaginase or 5-fluorouracil may be associated with HA by mechanisms not fully elucidated. HA, disclosing an imbalance between ammoniagenesis and ammonia disposal via the UC, can evolve to encephalopathy which may lead to significant morbidity and central nervous system damage. This review will focus on biochemical mechanisms related with HA emphasizing some poorly understood perspectives behind the disruption of the UC and mitochondrial energy metabolism, namely: i) changes in acetyl-CoA or NAD+ levels in subcellular compartments; ii) post-translational modifications of key UC-related enzymes, namely acetylation, potentially affecting their catalytic activity; iii) the mitochondrial sirtuins-mediated role in ureagenesis. Moreover, the main UCD associated with HA will be summarized to highlight the relevance of investigating possible genetic mutations to account for unexpected HA during certain pharmacological therapies. The ammonia-induced effects should be avoided or overcome as part of safer therapeutic strategies to protect patients under treatment with drugs that may be potentially associated with HA.

Gut microbiota-dependent phenylacetylglutamine in cardiovascular disease: current knowledge and new insights

Phenylacetylglutamine (PAGln) is an amino acid derivate that comes from the amino acid phenylalanine. There are increasing studies showing that the level of PAGln is associated with the risk of different cardiovascular diseases. In this review, we discussed the metabolic pathway of PAGln production and the quantitative measurement methods of PAGln. We summarized the epidemiological evidence to show the role of PAGln in diagnostic and prognostic value in several cardiovascular diseases, such as heart failure, coronary heart disease/atherosclerosis, and cardiac arrhythmia. The underlying mechanism of PAGln is now considered to be related to the thrombotic potential of platelets via adrenergic receptors. Besides, other possible mechanisms such as inflammatory response and oxidative stress could also be induced by PAGln. Moreover, since PAGln is produced across different organs including the intestine, liver, and kidney, the cross-talk among multiple organs focused on the function of this uremic toxic metabolite. Finally, the prognostic value of PAGln compared to the classical biomarker was discussed and we also highlighted important gaps in knowledge and areas requiring future investigation of PAGln in cardiovascular diseases.

Role of ammonia in NAFLD: An unusual suspect

Mechanistically, the symptomatology and disease progression of non-alcoholic fatty liver disease (NAFLD) remain poorly understood, which makes therapeutic progress difficult. In this review, we focus on the potential importance of decreased urea cycle activity as a pathogenic mechanism. Urea synthesis is an exclusive hepatic function and is the body's only on-demand and definitive pathway to remove toxic ammonia. The compromised urea cycle activity in NAFLD is likely caused by epigenetic damage to urea cycle enzyme genes and increased hepatocyte senescence. When the urea cycle is dysfunctional, ammonia accumulates in liver tissue and blood, as has been demonstrated in both animal models and patients with NAFLD. The problem may be augmented by parallel changes in the glutamine/glutamate system. In the liver, the accumulation of ammonia leads to inflammation, stellate cell activation and fibrogenesis, which is partially reversible. This may be an important mechanism for the transition of bland steatosis to steatohepatitis and further to cirrhosis and hepatocellular carcinoma. Systemic hyperammonaemia has widespread negative effects on other organs. Best known are the cerebral consequences that manifest as cognitive disturbances, which are prevalent in patients with NAFLD. Furthermore, high ammonia levels induce a negative muscle protein balance leading to sarcopenia, compromised immune function and increased risk of liver cancer. There is currently no rational way to reverse reduced urea cycle activity but there are promising animal and human reports of ammonia-lowering strategies correcting several of the mentioned untoward aspects of NAFLD. In conclusion, the ability of ammonia-lowering strategies to control the symptoms and prevent the progression of NAFLD should be explored in clinical trials.

Hepatic encephalopathy

Hepatic encephalopathy (HE) is a prognostically relevant neuropsychiatric syndrome that occurs in the course of acute or chronic liver disease. Besides ascites and variceal bleeding, it is the most serious complication of decompensated liver cirrhosis. Ammonia and inflammation are major triggers for the appearance of HE, which in patients with liver cirrhosis involves pathophysiologically low-grade cerebral oedema with oxidative/nitrosative stress, inflammation and disturbances of oscillatory networks in the brain. Severity classification and diagnostic approaches regarding mild forms of HE are still a matter of debate. Current medical treatment predominantly involves lactulose and rifaximin following rigorous treatment of so-called known HE precipitating factors. New treatments based on an improved pathophysiological understanding are emerging.