Lipopolysaccharide impairs hepatocyte ureagenesis from ammonia: Involvement of mitochondrial aquaporin-8

The lipopolysaccharide-TLR4 axis regulates hepatic glutaminase 1 expression promoting liver ammonia build-up as steatotic liver disease progresses to steatohepatitis

INTRODUCTION

Ammonia is a pathogenic factor implicated in the progression of metabolic-associated steatotic liver disease (MASLD). The contribution of the glutaminase 1 (GLS) isoform, an enzyme converting glutamine to glutamate and ammonia, to hepatic ammonia build-up and the mechanisms underlying its upregulation in metabolic-associated steatohepatitis (MASH) remain elusive.

METHODS

Multiplex transcriptomics and targeted metabolomics analysis of liver biopsies in dietary mouse models representing the whole spectra of MASLD were carried out to characterize the relevance of hepatic GLS during disease pathological progression. In addition, the acute effect of liver-specific GLS inhibition in hepatic ammonia content was evaluated in cultured hepatocytes and in in vivo mouse models of diet-induced MASLD. Finally, the regulatory mechanisms of hepatic GLS overexpression related to the lipopolysaccharide (LPS)/Toll-like receptor 4 (TLR4) axis were explored in the context of MASH.

RESULTS

In mouse models of diet-induced MASLD, we found that augmented liver GLS expression is closely associated with the build-up of hepatic ammonia as the disease progresses from steatosis to steatohepatitis. Importantly, the acute silencing/pharmacological inhibition of GLS diminishes the ammonia burden in cultured primary mouse hepatocytes undergoing dedifferentiation, in steatotic hepatocytes, and in a mouse model of diet-induced steatohepatitis, irrespective of changes in ureagenesis and gut permeability. Under these conditions, GLS upregulation in the liver correlates positively with the hepatic expression of TLR4 that recognizes LPS. In agreement, the pharmacological inhibition of TLR4 reduces GLS and hepatic ammonia content in LPS-stimulated mouse hepatocytes and hyperammonemia animal models of endotoxemia.

CONCLUSIONS

Overall, our results suggest that the LPS/TLR4 axis regulates hepatic GLS expression promoting liver ammonia build-up as steatotic liver disease progresses to steatohepatitis.

Adenoviral Transfer of Human Aquaporin-8 Gene to Mouse Liver Improves Ammonia-Derived Ureagenesis

We previously reported that, in cultured hepatocytes, mitochondrial aquaporin-8 (AQP8) channels facilitate the conversion of ammonia to urea and that the expression of human AQP8 (hAQP8) enhances ammonia-derived ureagenesis. In this study, we evaluated whether hepatic gene transfer of hAQP8 improves detoxification of ammonia to urea in normal mice as well as in mice with impaired hepatocyte ammonia metabolism. A recombinant adenoviral (Ad) vector encoding hAQP8, AdhAQP8, or a control Ad vector was administered via retrograde infusion into the bile duct of the mice. Hepatocyte mitochondrial expression of hAQP8 was confirmed using confocal immunofluorescence and immunoblotting. The normal hAQP8-transduced mice showed decreased plasma ammonia and increased liver urea. Enhanced ureagenesis was confirmed via the NMR studies assessing the synthesis of 15N-labeled urea from 15N-labeled ammonia. In separate experiments, we made use of the model hepatotoxic agent, thioacetamide, to induce defective hepatic metabolism of ammonia in mice. The adenovirus-mediated mitochondrial expression of hAQP8 was able to restore normal ammonemia and ureagenesis in the liver of the mice. Our data suggest that hAQP8 gene transfer to mouse liver improves detoxification of ammonia to urea. This finding could help better understand and treat disorders with defective hepatic ammonia metabolism.

Secondary Dysfunction of the Intestinal Barrier in the Pathogenesis of Complications of Acute Poisoning

The last decade has been marked by an exponential increase in the number of publications on the physiological role of the normal human gut microbiota. The idea of a symbiotic relationship between the human organism and normal microbiota of its gastrointestinal tract has been firmly established as an integral part of the current biomedical paradigm. However, the type of this symbiosis varies from mutualism to parasitism and depends on the functional state of the host organism. Damage caused to the organism by external agents can lead to the emergence of conditionally pathogenic properties in the normal gut microbiota, mediated by humoral factors and affecting the outcome of exogenous exposure. Among the substances produced by symbiotic microbiota, there are an indefinite number of compounds with systemic toxicity. Some occur in the intestinal chyme in potentially lethal amounts in the case they enter the bloodstream quickly. The quick entry of potential toxicants is prevented by the intestinal barrier (IB), a set of structural elements separating the intestinal chyme from the blood. Hypothetically, severe damage to the IB caused by exogenous toxicants can trigger a leakage and subsequent systemic redistribution of toxic substances of bacterial origin. Until recently, the impact of such a redistribution on the outcome of acute exogenous poisoning remained outside the view of toxicology. The present review addresses causal relationships between the secondary dysfunction of the IB and complications of acute poisoning. We characterize acute systemic toxicity of such waste products of the normal gut microflora as ammonia and endotoxins, and demonstrate their involvement in the formation of such complications of acute poisoning as shock, sepsis, cerebral insufficiency and secondary lung injuries. The principles of assessing the functional state of the IB and the approaches to its protection in acute poisoning are briefly considered.

The role of bacterial translocation in sepsis: a new target for therapy

Sepsis is a leading cause of death in critically ill patients, primarily due to multiple organ failures. It is associated with a systemic inflammatory response that plays a role in the pathogenesis of the disease. Intestinal barrier dysfunction and bacterial translocation (BT) play pivotal roles in the pathogenesis of sepsis and associated organ failure. In this review, we describe recent advances in understanding the mechanisms by which the gut microbiome and BT contribute to the pathogenesis of sepsis. We also discuss several potential treatment modalities that target the microbiome as therapeutic tools for patients with sepsis.

Enhanced ammonia detoxification to urea in hepatocytes transduced with human aquaporin-8 gene

Hepatic ammonia detoxification to urea is critical for the prevention of hyperammonemia and neurological damage. Hepatocyte mitochondrial aquaporin-8 (AQP8) channels have been involved in ammonia-derived ureagenesis. Herein, we studied whether the adenoviral gene transfer of human AQP8 (hAQP8) to hepatocyte mitochondria enhances ammonia conversion to urea. Using primary cultured rat hepatocytes, we first confirmed the mitochondrial expression of hAQP8 and then, using unlabeled or ¹⁵ N-labeled ammonia, we demonstrated that the urea synthesis was significantly enhanced in hAQP8-transduced hepatocytes. Studies using isolated hAQP8-expressing mitochondria also showed an increased ammonia metabolism. hAQP8 transduction was able to recover the impaired ammonia-derived ureagenesis in hepatotoxin-treated hepatocytes. Our data suggest that mitochondrially-expressed hAQP8 enhances and improves hepatocyte ammonia conversion to urea, a finding with potential therapeutic implications for liver disease with impaired ammonia detoxification.

Data of ureagenesis from ammonia, glutamine and alanine, and mitochondrial aquaporin-8 expression in thioacetamide-treated hepatocytes

We present data about the synthesis of urea from different substrates, i.e., free ammonia, glutamine and alanine in primary cultured rat hepatocytes treated or untreated with the model hepatotoxic agent thioacetamide (TAA). We also provide data about the expression of mitochondrial aquaporin-8 (mtAQP8), a hepatocyte channel protein which facilitates ammonia diffusion into mitochondria to supply the urea cycle. Ammonia-derived ureagenesis was significantly inhibited by about 30% while that from the both amino acids resulted unaffected in TAA-treated hepatocytes. Protein expression of mtAQP8 was decreased by about 80% after TAA treatment. These data can be useful for the understanding of the mechanisms of drug-induced hepatic dysfunction.

Acetylation of Phenylalanine Hydroxylase and Tryptophan 2,3-Dioxygenase Alters Hepatic Aromatic Amino Acid Metabolism in Weaned Piglets

Weaning significantly alters hepatic aromatic amino acid (AAA) metabolism and physiological functions. However, less is known about the regulating mechanism of hepatic AAA metabolism after weaning. A total of 200 21-day-old piglets (Duroc × Landrace) were assigned randomly to the control group and the weaning group. In this study, weaning significantly decreased the concentration of phenylalanine, tryptophan, and tyrosine in piglet livers (p < 0.05). Additionally, through the detection of liver AAA metabolites and metabolic enzyme activity, it was observed that hepatic tryptophan catabolism was enhanced, while that of phenylalanine was weakened (p < 0.05). Intriguingly, acetyl-proteome profiling of liver from weaned piglets showed that weaning exacerbated the acetylation of phenylalanine hydroxylase (PAH) and the deacetylation of tryptophan 2,3-dioxygenase (TDO). Analysis of PAH and TDO acetylation in Chang liver cells showed that acetylation decreased the PAH activity, while deacetylation increased the TDO activity (p < 0.05). Furthermore, metabolites of AAAs and the acetylation statuses of PAH and TDO in primary hepatocytes from weaned piglets were consistent with the results in vivo. These findings indicated that weaning altered the PAH and TDO activity by affecting the acetylation state of the enzyme in piglets’’ livers. Lysine acetylation may be a potential regulatory mechanism for AAA metabolism in response to weaning.

Mitochondrial aquaporin-8 is involved in SREBP-controlled hepatocyte cholesterol biosynthesis

Cholesterol, via sterol regulatory element-binding protein (SREBP) transcription factors, activates or represses genes involved in its hepatic biosynthetic pathway, and also modulates the expression of hepatocyte mitochondrial aquaporin-8 (mtAQP8), a channel that can function as peroxiporin by facilitating the transmembrane diffusion of H₂O₂. Here we tested the hypothesis that mtAQP8 is involved in the SREBP-mediated regulation of hepatocyte cholesterol biosynthesis. Using human hepatocyte-derived Huh-7 cells and primary rat hepatocytes, we found that mtAQP8 knockdown significantly downregulated de novo cholesterol synthesis as well as protein expressions of SREBP-2 and its target gene, a rate-limiting enzyme in cholesterol synthesis 3-Hydroxy-3-Methylglutaryl-CoA Reductase (HMGCR). In contrast, adenovirus-mediated human AQP8 mitochondrial expression significantly increased de novo cholesterol synthesis and protein expressions of SREBP-2 and HMGCR. In mtAQP8-overexpressed hepatocytes, mitochondrial H₂O₂ release was found to be increased; and a mitochondria-targeted antioxidant prevented the upregulation of mitochondrial H₂O₂ release and that of cholesterol synthesis. Our results suggest that peroxiporin mtAQP8 plays a role in the SREBP-controlled hepatocyte cholesterogenesis, a finding that might be relevant to cholesterol-related metabolic disorders.

Acetyl-CoA from inflammation-induced fatty acids oxidation promotes hepatic malate-aspartate shuttle activity and glycolysis

Hepatic metabolic syndrome is associated with inflammation, as inflammation stimulates the reprogramming of nutrient metabolism and hepatic mitochondria-generated acetyl-CoA, but how acetyl-CoA affects the reprogramming of nutrient metabolism, especially glucose and fatty acids, in the condition of inflammation is still unclear. Here, we used an acute inflammation model in which pigs were injected with lipopolysaccharide (LPS) and found that hepatic glycolysis and fatty acid oxidation are both promoted. Acetyl-proteome profiling of LPS-infected pigs liver showed that inflammatory stress exacerbates the acetylation of mitochondrial proteins. Both mitochondrial glutamate oxaloacetate transaminase 2 (GOT2) and malate dehydrogenase 2 (MDH2) were acetylated, and the malate-aspartate shuttle (MAS) activity was stimulated to maintain glycolysis. With the use of ¹³C-carbon tracing in vitro, acetyl-CoA was found to be mainly supplied by lipid-derived fatty acid oxidation rather than glucose-derived pyruvate oxidative decarboxylation, while glucose was mainly used for lactate production in response to inflammatory stress. The results of the mitochondrial experiment showed that acetyl-CoA directly increases MDH2 and, in turn, the GOT2 acetylation level affects MAS activity. Treatment with palmitate in primary hepatocytes from LPS-injected pigs increased the hepatic production of acetyl-CoA, pyruvate, and lactate; MAS activity; and hepatic MDH2 and GOT2 hyperacetylation, while the deficiency of long-chain acetyl-CoA dehydrogenase resulted in the stabilization of these parameters. These observations suggest that acetyl-CoA produced by fatty acid oxidation promotes MAS activity and glycolysis via nonenzymatic acetylation during the inflammatory stress response.

Function of aquaporins in sepsis: a systematic review

BACKGROUND

Sepsis is a common cause of death in intensive care units worldwide. Due to the high complexity of this immunological syndrome development of novel therapeutic strategies is urgent. Promising drug targets or biomarkers may depict aquaporins (AQPs) as they regulate crucial key mechanisms of sepsis.

MAIN BODY

Here we report on base of the current literature that several AQPs are involved in different physiological processes of sepsis. In immune system mainly AQPs 3, 5 and 9 seem to be important, as they regulate the migration of different immune cells. Several studies showed that AQP3 is essential for T cell function and macrophage migration and that AQP5 and AQP9 regulate neutrophil cell migration and impact sepsis survival. Additionally, to the function in immune system AQPs 1 and 5 play a role in sepsis induced lung injury and their downregulation after inflammatory stimuli impair lung injury. By contrast, AQP4 expression is up-regulated during brain inflammation and aggravates brain edema in sepsis. In kidney AQP2 expression is downregulated during sepsis and can cause renal failure. Some studies also suggest a role of AQP1 in cardiac function.

CONCLUSION

In conclusion, AQPs are involved in many physiological dysfunctions in sepsis and their expressions are differently regulated. Additional research on the regulatory mechanisms of aquaporins may identify potential therapeutic targets.

Benznidazole, the trypanocidal drug used for Chagas disease, induces hepatic NRF2 activation and attenuates the inflammatory response in a murine model of sepsis

Molecular mechanisms on sepsis progression are linked to the imbalance between reactive oxygen species (ROS) production and cellular antioxidant capacity. Previous studies demonstrated that benznidazole (BZL), known for its antiparasitic action on Trypanosoma cruzi, has immunomodulatory effects, increasing survival in C57BL/6 mice in a model of polymicrobial sepsis induced by cecal ligation and puncture (CLP). The mechanism by which BZL inhibits inflammatory response in sepsis is poorly understood. Also, our group recently reported that BZL is able to activate the nuclear factor erytroide-derived 2-Like 2 (NRF2) in vitro. The aim of the present work was to delineate the beneficial role of BZL during sepsis, analyzing its effects on the cellular redox status and the possible link to the innate immunity receptor TLR4. Specifically, we analyzed the effect of BZL on Nrf2 regulation and TLR4 expression in liver of mice 24hours post-CLP. BZL was able to induce NRF2 nuclear protein localization in CLP mice. Also, we found that protein kinase C (PKC) is involved in the NRF2 nuclear accumulation and induction of its target genes. In addition, BZL prompted a reduction in hepatic CLP-induced TLR4 protein membrane localization, evidencing its immunomodulatory effects. Together, our results demonstrate that BZL induces hepatic NRF2 activation with the concomitant increase in the antioxidant defenses, and the attenuation of inflammatory response, in part, by inhibiting TLR4 expression in a murine model of sepsis.

Hepatocyte and Sertoli Cell Aquaporins, Recent Advances and Research Trends

Aquaporins (AQPs) are proteinaceous channels widespread in nature where they allow facilitated permeation of water and uncharged through cellular membranes. AQPs play a number of important roles in both health and disease. This review focuses on the most recent advances and research trends regarding the expression and modulation, as well as physiological and pathophysiological functions of AQPs in hepatocytes and Sertoli cells (SCs). Besides their involvement in bile formation, hepatocyte AQPs are involved in maintaining energy balance acting in hepatic gluconeogenesis and lipid metabolism, and in critical processes such as ammonia detoxification and mitochondrial output of hydrogen peroxide. Roles are played in clinical disorders including fatty liver disease, diabetes, obesity, cholestasis, hepatic cirrhosis and hepatocarcinoma. In the seminiferous tubules, particularly in SCs, AQPs are also widely expressed and seem to be implicated in the various stages of spermatogenesis. Like in hepatocytes, AQPs may be involved in maintaining energy homeostasis in these cells and have a major role in the metabolic cooperation established in the testicular tissue. Altogether, this information represents the mainstay of current and future investigation in an expanding field.

Sickness-Associated Anorexia: Mother Nature's Idea of Immunonutrition?

During an infection, expansion of immune cells, assembly of antibodies, and the induction of a febrile response collectively place continual metabolic strain on the host. These considerations also provide a rationale for nutritional support in critically ill patients. Yet, results from clinical and preclinical studies indicate that aggressive nutritional support does not always benefit patients and may occasionally be detrimental. Moreover, both vertebrates and invertebrates exhibit a decrease in appetite during an infection, indicating that such sickness-associated anorexia (SAA) is evolutionarily conserved. It also suggests that SAA performs a vital function during an infection. We review evidence signifying that SAA may present a mechanism by which autophagic flux is upregulated systemically. A decrease in serum amino acids during an infection promotes autophagy not only in immune cells, but also in nonimmune cells. Similarly, bile acids reabsorbed postprandially inhibit hepatic autophagy by binding to farnesoid X receptors, indicating that SAA may be an attempt to conserve autophagy. In addition, augmented autophagic responses may play a critical role in clearing pathogens (xenophagy), in the presentation of epitopes in nonprovisional antigen presenting cells and the removal of damaged proteins and organelles. Collectively, these observations suggest that some patients might benefit from permissive underfeeding.

Ouabain-induced cytoplasmic vesicles and their role in cell volume maintenance

Cellular swelling is controlled by an active mechanism of cell volume regulation driven by a Na(+)/K(+)-dependent ATPase and by aquaporins which translocate water along the osmotic gradient. Na(+)/K(+)-pump may be blocked by ouabain, a digitalic derivative, by inhibition of ATP, or by drastic ion alterations of extracellular fluid. However, it has been observed that some tissues are still able to control their volume despite the presence of ouabain, suggesting the existence of other mechanisms of cell volume control. In 1977, by correlating electron microscopy observation with ion and water composition of liver slices incubated in different metabolic conditions in the presence or absence of ouabain, we observed that hepatocytes were able to control their volume extruding water and recovering ion composition in the presence of ouabain. In particular, hepatocytes were able to sequester ions and water in intracellular vesicles and then secrete them at the bile canaliculus pole. We named this "vesicular mechanism of cell volume control." Afterward, this mechanism has been confirmed by us and other laboratories in several mammalian tissues. This review summarizes evidences regarding this mechanism, problems that are still pending, and questions that need to be answered. Finally, we shortly review the importance of cell volume control in some human pathological conditions.