Phosphate Groups in the Lipid A Moiety Determine the Effects of LPS on Hepatic Stellate Cells: A Role for LPS-Dephosphorylating Activity in Liver Fibrosis

Structural and Functional Integration of Tissue-Nonspecific Alkaline Phosphatase Within the Alkaline Phosphatase Superfamily: Evolutionary Insights and Functional Implications

Phosphatases are enzymes that catalyze the hydrolysis of phosphate esters. They play critical roles in diverse biological processes such as extracellular nucleotide homeostasis, transport of molecules across membranes, intracellular signaling pathways, or vertebrate mineralization. Among them, tissue-nonspecific alkaline phosphatase (TNAP) is today increasingly studied, due to its ubiquitous expression and its ability to dephosphorylate a very broad range of substrates and participate in several different biological functions. For instance, TNAP hydrolyzes inorganic pyrophosphate (PPi) to allow skeletal and dental mineralization. Additionally, TNAP hydrolyzes pyridoxal phosphate to allow cellular pyridoxal uptake, and stimulate vitamin B6-dependent reactions. Furthermore, TNAP has been identified as a key enzyme in non-shivering adaptive thermogenesis, by dephosphorylating phosphocreatine in the mitochondrial creatine futile cycle. This latter recent discovery and others suggest that the list of substrates and functions of TNAP may be much longer than previously thought. In the present review, we sought to examine TNAP within the alkaline phosphatase (AP) superfamily, comparing its sequence, structure, and evolutionary trajectory. The AP superfamily, characterized by a conserved central folding motif of a mixed beta-sheet flanked by alpha-helices, includes six subfamilies: AP, arylsulfatases (ARS), ectonucleotide pyrophosphatases/phosphodiesterases (ENPP), phosphoglycerate mutases (PGM), phosphonoacetate hydrolases, and phosphopentomutases. Interestingly, TNAP and several ENPP family members appear to participate in the same metabolic pathways and functions. For instance, extra-skeletal mineralization in vertebrates is inhibited by ENPP1-mediated ATP hydrolysis into the mineralization inhibitor PPi, which is hydrolyzed by TNAP expressed in the skeleton. Better understanding how TNAP and other AP family members differ structurally will be very useful to clarify their complementary functions. Structurally, TNAP shares the conserved catalytic core with other AP superfamily members but has unique features affecting substrate specificity and activity. The review also aims to highlight the importance of oligomerization in enzyme stability and function, and the role of conserved metal ion coordination, particularly magnesium, in APs. By exploring the structural and functional diversity within the AP superfamily, and discussing to which extent its members exert redundant, complementary, or specific functions, this review illuminates the evolutionary pressures shaping these enzymes and their broad physiological roles, offering insights into TNAP's multifunctionality and its implications for health and disease.

Necrotizing enterocolitis: a potential protective role for intestinal alkaline phosphatase as lipopolysaccharide detoxifying enzyme

Introduction

Necrotizing enterocolitis (NEC) is a life-threatening inflammatory disease. Its onset might be triggered by Toll-Like Receptor 4 (TLR4) activation via bacterial lipopolysaccharide (LPS). We hypothesize that a deficiency of intestinal alkaline phosphatase (IAP), an enzyme secreted by enterocytes that dephosphorylates LPS, may contribute to NEC development.

Methods

In this prospective pilot study, we analyzed intestinal resection specimens from surgical NEC patients, and from patients undergoing Roux-Y reconstruction for hepatobiliary disease as controls. We assessed IAP activity via enzymatic stainings and assays and explored IAP and TLR4 co-localization through immunofluorescence.

Results

The study population consisted of five NEC patients (two Bell's stage IIb and three-stage IIIb, median (IQR) gestational age 25 (24-28) weeks, postmenstrual age at diagnosis 28 (26-31) weeks) and 11 controls (unknown age). There was significantly lower IAP staining in NEC resection specimens [49 (41-50) U/g of protein] compared to controls [115 (76-144), P = 0.03]. LPS-dephosphorylating activity was also lower in NEC patients [0.06 (0-0.1)] than in controls [0.3 (0.2-0.5), P = 0.003]. Furthermore, we observed colocalization of IAP and TLR4 in NEC resection specimens.

Conclusion

This study suggests a significantly lower IAP level in resection specimens of NEC patients compared to controls. This lower IAP activity suggests a potential role of IAP as a protective agent in the gut, which needs further confirmation in larger cohorts.

Protective effects of an anti-4-HNE monoclonal antibody against liver injury and lethality of endotoxemia in mice

4-hydroxy-2-nonenal (4-HNE) is a lipid peroxidation product that is known to be elevated during oxidative stress. During systemic inflammation and endotoxemia, plasma levels of 4-HNE are elevated in response to lipopolysaccharide (LPS) stimulation. 4-HNE is a highly reactive molecule due to its generation of both Schiff bases and Michael adducts with proteins, which may result in modulation of inflammatory signaling pathways. In this study, we report the production of a 4-HNE adduct-specific monoclonal antibody (mAb) and the effectiveness of the intravenous injection of this mAb (1 mg/kg) in ameliorating LPS (10 mg/kg, i.v.)-induced endotoxemia and liver injury in mice. Endotoxic lethality in control mAb-treated group was suppressed by the administration of anti-4-HNE mAb (75 vs. 27%). After LPS injection, we observed a significant increase in the plasma levels of AST, ALT, IL-6, TNF-α and MCP-1, and elevated expressions of IL-6, IL-10 and TNF-α in the liver. All these elevations were inhibited by anti-4-HNE mAb treatment. As to the underlining mechanism, anti-4-HNE mAb inhibited the elevation of plasma high mobility group box-1 (HMGB1) levels, the translocation and release of HMGB1 in the liver and the formation of 4-HNE adducts themselves, suggesting a functional role of extracellular 4-HNE adducts in hypercytokinemia and liver injury associated with HMGB1 mobilization. In summary, this study reveals a novel therapeutic application of anti-4-HNE mAb for endotoxemia.

Gut Enterobacteriaceae and uraemic toxins - Perpetrators for ageing

Ageing is a complex process that is associated with changes in the composition and functions of gut microbiota. Reduction of gut commensals is the hallmarks of ageing, which favours the expansion of pathogens even in healthy centenarians. Interestingly, gut Enterobacteriaceae have been found to be increased with age and also consistently observed in the patients with metabolic diseases. Thus, they are associated with all-cause mortality, regardless of genetic origin, lifestyle, and fatality rate. Moreover, Enterobacteriaceae are also implicated in accelerating the ageing process through telomere attrition, cellular senescence, inflammasome activation and impairing the functions of mitochondria. However, acceleration of ageing is likely to be determined by intrinsic interactions between Enterobacteriaceae and other associated gut bacteria. Several studies suggested that Enterobacteriaceae possess genes for the synthesis of uraemic toxins. In addition to intestine, Enterobacteriaceae and their toxic metabolites have also been found in other organs, such as adipose tissue and liver and that are implicated in multiorgan dysfunction and age-related diseases. Therefore, targeting Enterobacteriaceae is a nuance approach for reducing inflammaging and enhancing the longevity of older people. This review is intended to highlight the current knowledge of Enterobacteriaceae-mediated acceleration of ageing process.

Inhibition of alkaline phosphatase impairs dyslipidemia and protects mice from atherosclerosis

Calcium accumulation in atherosclerotic plaques predicts cardiovascular mortality, but the mechanisms responsible for plaque calcification and how calcification impacts plaque stability remain debated. Tissue-nonspecific alkaline phosphatase (TNAP) recently emerged as a promising therapeutic target to block cardiovascular calcification. In this study, we sought to investigate the effect of the recently developed TNAP inhibitor SBI-425 on atherosclerosis plaque calcification and progression. TNAP levels were investigated in ApoE-deficient mice fed a high-fat diet from 10 weeks of age and in plaques from the human ECLAGEN biocollection (101 calcified and 14 non-calcified carotid plaques). TNAP was inhibited in mice using SBI-425 administered from 10 to 25 weeks of age, and in human vascular smooth muscle cells (VSMCs) with MLS-0038949. Plaque calcification was imaged in vivo with ¹⁸F-NaF-PET/CT, ex vivo with osteosense, and in vitro with alizarin red. Bone architecture was determined with µCT. TNAP activation preceded and predicted calcification in human and mouse plaques, and TNAP inhibition prevented calcification in human VSMCs and in ApoE-deficient mice. More unexpectedly, TNAP inhibition reduced the blood levels of cholesterol and triglycerides, and protected mice from atherosclerosis, without impacting the skeletal architecture. Metabolomics analysis of liver extracts identified phosphocholine as a substrate of liver TNAP, who's decreased dephosphorylation upon TNAP inhibition likely reduced the release of cholesterol and triglycerides into the blood. Systemic inhibition of TNAP protects from atherosclerosis, by ameliorating dyslipidemia, and preventing plaque calcification.

Polysaccharide of Atractylodes macrocephala Koidz alleviate lipopolysaccharide-stimulated liver inflammation injury of goslings through miR-223/NLRP3 axis

Lipopolysaccharide (LPS) infection could cause severe liver inflammation and lead to liver damage, even death. Previous studies have shown that polysaccharide of Atractylodes macrocephala Koidz (PAMK) could protect liver from inflammation caused by LPS in mice. However, whether PAMK could alleviate liver inflammatory injury in other animals with LPS is still unknown. For evaluating whether PAMK could alleviate liver inflammatory injury in goslings with LPS, a total of 80 healthy 1-day old Magang goslings were randomly divided into 4 groups (control group, PAMK group, LPS group, and PAMK+LPS group). Goslings in control group and LPS group were fed with basal diet, and goslings in PAMK group and PAMK+LPS group were fed basal diet supplemented with 400 mg/kg PAMK to the end of trial. On 24 d of age, goslings in the control group and PAMK group were intraperitoneal injected 0.5 mL normal saline, and goslings in LPS and PAMK+LPS groups were intraperitoneal injected with LPS at 5 mg/kg BW. The serum and liver samples were collected for further analysis after treatment of LPS at 6, 12, 24, and 48 h. Furthermore, the hepatocytes were extracted from goose embryo to measure the expression of the key genes of miR-223/NLRP3 axis. The results showed that PAMK pretreatment could maintain normal cell morphology of liver, alleviate the enhanced levels of biochemical indexes ALT and AST, decrease the levels of IL-1β and IL-18, increase the relative mRNA expression of miR-223, and decrease the expression of NLRP3, Caspase-1, and cleaved Caspase-1 in liver and hepatocytes of goslings induced by LPS. These results indicated that PAMK could relieve inflammatory liver tissue damage after LPS treatment and downregulate the level of inflammation factors via miR-223/NLRP3 axis, thus playing a liver protective role in liver inflammation injury in goslings.

Rifaximin Improves Spatial Learning and Memory Impairment in Rats with Liver Damage-Associated Neuroinflammation

Patients with non-alcoholic fatty liver disease (NAFLD) may show mild cognitive impairment. Neuroinflammation in the hippocampus mediates cognitive impairment in rat models of minimal hepatic encephalopathy (MHE). Treatment with rifaximin reverses cognitive impairment in a large proportion of cirrhotic patients with MHE. However, the underlying mechanisms remain unclear. The aims of this work were to assess if rats with mild liver damage, as a model of NAFLD, show neuroinflammation in the hippocampus and impaired cognitive function, if treatment with rifaximin reverses it, and to study the underlying mechanisms. Mild liver damage was induced with carbon-tetrachloride. Infiltration of immune cells, glial activation, and cytokine expression, as well as glutamate receptors expression in the hippocampus and cognitive function were assessed. We assessed the effects of daily treatment with rifaximin on the alterations showed by these rats. Rats with mild liver damage showed hippocampal neuroinflammation, reduced membrane expression of glutamate N-methyl-D-aspartate (NMDA) receptor subunits, and impaired spatial memory. Increased C-C Motif Chemokine Ligand 2 (CCL2), infiltration of monocytes, microglia activation, and increased tumor necrosis factor α (TNFα) were reversed by rifaximin, that normalized NMDA receptor expression and improved spatial memory. Thus, rifaximin reduces neuroinflammation and improves cognitive function in rats with mild liver damage, being a promising therapy for patients with NAFLD showing mild cognitive impairment.

TNAP: A New Multitask Enzyme in Energy Metabolism

Tissue-nonspecific alkaline phosphatase (TNAP) is mainly known for its necessary role in skeletal and dental mineralization, which relies on the hydrolysis of the mineralization inhibitor inorganic pyrophosphate (PP_i). Mutations in the gene encoding TNAP leading to severe hypophosphatasia result in strongly reduced mineralization and perinatal death. Fortunately, the relatively recent development of a recombinant TNAP with a bone anchor has allowed to correct the bone defects and prolong the life of affected babies and children. Researches on TNAP must however not be slowed down, because accumulating evidence indicates that TNAP activation in individuals with metabolic syndrome (MetS) is associated with enhanced cardiovascular mortality, presumably in relation with cardiovascular calcification. On the other hand, TNAP appears to be necessary to prevent the development of steatohepatitis in mice, suggesting that TNAP plays protective roles. The aim of the present review is to highlight the known or suspected functions of TNAP in energy metabolism that may be associated with the development of MetS. The location of TNAP in liver and its function in bile excretion, lipopolysaccharide (LPS) detoxification and fatty acid transport will be presented. The expression and function of TNAP in adipocyte differentiation and thermogenesis will also be discussed. Given that TNAP is a tissue- and substrate-nonspecific phosphatase, we believe that it exerts several crucial pathophysiological functions that are just beginning to be discovered.