Deficiency of the Purinergic Receptor 2X7 Attenuates Nonalcoholic Steatohepatitis Induced by High-Fat Diet: Possible Role of the NLRP3 Inflammasome

Pharmacotherapy of Liver Fibrosis and Hepatitis: Recent Advances

Liver fibrosis is a progressive scarring process primarily caused by chronic inflammation and injury, often closely associated with viral hepatitis, alcoholic liver disease, metabolic dysfunction-associated steatotic liver disease (MASLD), drug-induced liver injury, and autoimmune liver disease (AILD). Currently, there are very few clinical antifibrotic drugs available, and effective targeted therapy is lacking. Recently, emerging antifibrotic drugs and immunomodulators have shown promising results in animal studies, and some have entered clinical research phases. This review aims to systematically review the molecular mechanisms underlying liver fibrosis, focusing on advancements in drug treatments for hepatic fibrosis. Furthermore, since liver fibrosis is a progression or endpoint of many diseases, it is crucial to address the etiological treatment and secondary prevention for liver fibrosis. We will also review the pharmacological treatments available for common hepatitis leading to liver fibrosis.

Mechanism of epigallocatechin gallate in treating non-alcoholic fatty liver disease: Insights from network pharmacology and experimental validation

To explore the therapeutic effects along with the molecular mechanisms of epigallocatechin gallate (EGCG) in non-alcoholic fatty liver disease (NAFLD) treatment using network pharmacology as well as animal experiments. Firstly, the Traditional Chinese Medicine (TCM) Systems Pharmacology Database was searched to identify the potential targets of EGCG. The DisGeNET Database was used to screen the potential targets of NAFLD. The GeneCards Database was searched to identify related genes involved in pyroptosis. Subsequently, the intersecting genes of EGCG targeting pyroptosis to regulate NAFLD were obtained using a Venn diagram. Simultaneously, the aforementioned intersecting genes were used to construct a drug-disease target protein-protein interaction (PPI) network. The DAVID database was adopted for Gene Ontology (GO) as well as Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses. The main pathway-target network was determined. Next, the potential mechanism of EGCG targeting pyroptosis to regulate NAFLD was investigated and validated through in vivo experiments. 626 potential targets of EGCG, 447 target genes of NAFLD, and 568 potential targets of pyroptosis were identified. The number of common targets between EGCG, NAFLD, and pyroptosis was 266. GO biological process items and 92 KEGG pathways were determined based on the analysis results. Animal experiments demonstrated that EGCG could ameliorate body weight, glucolipid metabolism, steatosis, and liver injury, enhance insulin sensitivity, and improve glucose tolerance in NAFLD mice through the classical pathway of pyroptosis. EGCG could effectively treat NAFLD through multiple targets and pathways. It was concluded that EGCG ameliorates hepatocyte steatosis, pyroptosis, dyslipidemia, and inflammation in NAFLD mice fed a high-fat diet (HFD), and the protective mechanism could be associated with the NLRP3-Caspase-1-GSDMD classical pyroptosis pathway.

Inflammasomes in chronic liver disease: Hepatic injury, fibrosis progression and systemic inflammation

Chronic liver disease leads to hepatocellular injury that triggers a pro-inflammatory state in several parenchymal and non-parenchymal hepatic cell types, ultimately resulting in liver fibrosis, cirrhosis, portal hypertension and liver failure. Thus, an improved understanding of inflammasomes - as key molecular drivers of liver injury - may result in the development of novel diagnostic or prognostic biomarkers and effective therapeutics. In liver disease, innate immune cells respond to hepatic insults by activating cell-intrinsic inflammasomes via toll-like receptors and NF-κB, and by releasing pro-inflammatory cytokines (such as IL-1β, IL-18, TNF-α and IL-6). Subsequently, cells of the adaptive immune system are recruited to fuel hepatic inflammation and hepatic parenchymal cells may undergo gasdermin D-mediated programmed cell death, termed pyroptosis. With liver disease progression, there is a shift towards a type 2 inflammatory response, which promotes tissue repair but also fibrogenesis. Inflammasome activation may also occur at extrahepatic sites, such as the white adipose tissue in MASH (metabolic dysfunction-associated steatohepatitis). In end-stage liver disease, flares of inflammation (e.g., in severe alcohol-related hepatitis) that spark on a dysfunctional immune system, contribute to inflammasome-mediated liver injury and potentially result in organ dysfunction/failure, as seen in ACLF (acute-on-chronic liver failure). This review provides an overview of current concepts regarding inflammasome activation in liver disease progression, with a focus on related biomarkers and therapeutic approaches that are being developed for patients with liver disease.

Purinergic Signaling in Non-Parenchymal Liver Cells

Purinergic signaling has emerged as an important paracrine-autocrine intercellular system that regulates physiological and pathological processes in practically all organs of the body. Although this system has been thoroughly defined since the nineties, recent research has made substantial advances regarding its role in aspects of liver physiology. However, most studies have mainly targeted the entire organ, 70% of which is made up of parenchymal cells or hepatocytes. Because of its physiological role, the liver is exposed to toxic metabolites, such as xenobiotics, drugs, and fatty acids, as well as to pathogens such as viruses and bacteria. Under injury conditions, all cell types within the liver undergo adaptive changes. In this context, the concentration of extracellular ATP has the potential to increase dramatically. Indeed, this purinergic response has not been studied in sufficient detail in non-parenchymal liver cells. In the present review, we systematize the physiopathological adaptations related to the purinergic system in chronic liver diseases of non-parenchymal liver cells, such as hepatic stellate cells, Kupffer cells, sinusoidal endothelial cells, and cholangiocytes. The role played by non-parenchymal liver cells in these circumstances will undoubtedly be strategic in understanding the regenerative activities that support the viability of this organ under stressful conditions.

Hepatic extracellular ATP/adenosine dynamics in zebrafish models of alcoholic and metabolic steatotic liver disease

Steatotic liver disease (SLD) is a burgeoning health problem predominantly associated with excessive alcohol consumption, which causes alcohol-related liver disease (ALD), and high caloric intake, which results in metabolic dysfunction-associated SLD (MASLD). The pathogenesis of ALD and MASLD, which can progress from steatohepatitis to more severe conditions such as liver fibrosis, cirrhosis, and hepatocellular carcinoma, is complicated by several factors. Recently, extracellular ATP and adenosine (Ado), as damage-associated molecular patterns, were reported to promote inflammation and liver fibrosis, contributing to SLD pathogenesis. Here, we explored the in vivo dynamics of hepatic extracellular ATP and Ado during the progression of steatohepatitis using a genetically encoded GPCR-activation-based sensor (GRAB) in zebrafish models. We established hepatocyte-specific GRABATP and GRABAdo in zebrafish and investigated the changes in in vivo hepatic extracellular ATP and Ado levels under ALD or MASLD conditions. Disease-specific changes in hepatocyte extracellular ATP and Ado levels were observed, clearly indicating a correlation between hepatocyte extracellular ATP/Ado dynamics and disease progression. Furthermore, clodronate, a vesicular nucleotide transporter inhibitor, alleviated the MASLD phenotype by reducing the hepatic extracellular ATP and Ado content. These findings provide deep insights into extracellular ATP/Ado dynamics in disease progression, suggesting therapeutic potential for ALD and MASLD.

JT002, a small molecule inhibitor of the NLRP3 inflammasome for the treatment of autoinflammatory disorders

The NLRP3 inflammasome is an intracellular, multiprotein complex that promotes the auto-catalytic activation of caspase-1 and the subsequent maturation and secretion of the pro-inflammatory cytokines, IL-1β and IL-18. Persistent activation of the NLRP3 inflammasome has been implicated in the pathophysiology of a number of inflammatory and autoimmune diseases, including neuroinflammation, cardiovascular disease, non-alcoholic steatohepatitis, lupus nephritis and severe asthma. Here we describe the preclinical profile of JT002, a novel small molecule inhibitor of the NLRP3 inflammasome. JT002 potently reduced NLRP3-dependent proinflammatory cytokine production across a number of cellular assays and prevented pyroptosis, an inflammatory form of cell death triggered by active caspase-1. JT002 demonstrated in vivo target engagement at therapeutically relevant concentrations when orally dosed in mice and prevented body weight loss and improved inflammatory and fibrotic endpoints in a model of Muckle-Wells syndrome (MWS). In two distinct models of neutrophilic airway inflammation, JT002 treatment significantly reduced airway hyperresponsiveness and airway neutrophilia. These results provide a rationale for the therapeutic targeting of the NLRP3 inflammasome in severe asthma and point to the use of JT002 in a variety of inflammatory disorders.

Chrysin prevents inflammation-coinciding liver steatosis via AMPK signalling

OBJECTIVES

We aimed to elucidate the therapeutic potential of Chrysin (CN) against the high-fat diet (HFD) induced non-alcoholic fatty liver disease (NAFLD) and its mechanism.

METHODS

To assess the hypothesis, NAFLD was induced in C57BL/6 mice by feeding a high-fat diet for up to two months, followed by CN administration (for three months). Liver injury/toxicity, lipid deposition, inflammation and fibrosis were detected via molecular and biochemical analysis, including blood chemistry, immunoimaging and immunoblotting. Moreover, we performed proteomic analysis to illuminate Chrysin's therapeutic effects further.

KEY FINDINGS

CN treatment significantly reduced liver-fat accumulation and inflammation, ultimately improving obesity and liver injury in NAFLD mice. Proteomic analysis showed that CN modified the protein expression profiles in the liver, particularly improving the expression of proteins related to energy, metabolism and inflammation. Mechanistically, CN treatment increased AMP-activated protein and phosphorylated CoA (P-ACC). Concurrently, it reduced inflammation and inflammation activation by inhibiting NLRP3 expression.

CONCLUSIONS

In summary, CN treatment reduced lipid metabolism by AMPK and inflammasome activation by NLRP3 inhibition, ultimately improving NAFLD progression. These findings suggest that CN could be a potential treatment candidate for the NFLAD condition.

Research and progress of inflammasomes in nonalcoholic fatty liver disease

With the development of the social economy, unhealthy living habits and eating styles are gradually affecting people's health in recent years. As a chronic liver disease, NAFLD is deeply affected by unhealthy living habits and eating styles and has gradually become an increasingly serious public health problem. As a protein complex in clinical research, the inflammasomes play a crucial role in the development of NAFLD, atherosclerosis, and other diseases. This paper reviews the types, composition, characteristics of inflammasomes, and molecular mechanism of the inflammasome in NAFLD. Meanwhile, the paper reviews the drugs and non-drugs that target NLRP3 inflammasome in the treatment of NAFLD in the past decades. we also analyzed and summarized the related experimental models, mechanisms, and results of NAFLD. Although current therapeutic strategies for NAFLD are not effective, we expect that we will be able to find an appropriate treatment to address this problem in the future with further research on inflammasome.

Research progress on the P2X7 receptor in liver injury and hepatocellular carcinoma

Purinergic ligand-gated ion channel 7 receptor (P2X7 receptor) is an adenosine triphosphate (ATP)-gated ion channel that is widely distributed on the surfaces of immune cells and tissues such as those in the liver, kidney, lung, intestine, and nervous system. Hepatocellular carcinoma (HCC) is one of the most common malignancies with increasing incidence and mortality. Although many treatments for liver cancer have been studied, the prognosis for liver cancer is still very poor. Therefore, new liver cancer treatments are urgently needed. P2X7 receptor activation can secrete proinflammatory factors through the P2X7 receptor-NLRP3 signaling pathway, thereby affecting the progression of liver injury. The P2X7 receptor may be a target for growth inhibition of HCC cells and may affect the invasion and migration of HCC cells through the PI3K/AKT and AMPK signaling pathways. In recent years, P2X7 receptor antagonists or inhibitors have attracted widespread attention as therapeutic targets for hepatocellular carcinoma and liver injury. Therefore, this review covers the basic concepts of the P2X7 receptor and role of the P2X7 receptor in liver cancer and liver injury, providing new potential therapeutic targets for hepatocellular carcinoma and liver injury.

Jinlida granules ameliorate the high-fat-diet induced liver injury in mice by antagonising hepatocytes pyroptosis

CONTEXT

Jinlida (JLD) as a traditional Chinese medicine formula has been used to treat type 2 diabetes mellitus (T2DM) and studies have shown its anti-obesity effect.

OBJECTIVE

To investigate the therapeutic effects of JLD in a mouse model of non-alcoholic fatty liver (NAFL).

MATERIALS AND METHODS

C57BL/6J mice were divided into three groups and fed a low-diet diet (LFD), high-fat diet (HFD), or HFD + JLD (3.8 g/kg) for 16 weeks, respectively. The free fatty acids-induced lipotoxicity in HepG2 cells were used to evaluate the anti-pyroptotic effects of JLD. The pharmacological effects of JLD on NAFL were investigated by pathological examination, intraperitoneal glucose and insulin tolerance tests, western blotting, and quantitative real-time PCR.

RESULTS

In vivo studies showed that JLD ameliorated HFD-induced liver injury, significantly decreased body weight and enhanced insulin sensitivity and improved glucose tolerance. Furthermore, JLD suppressed both the mRNA expression of caspase-1 (1.58 vs. 2.90), IL-1β (0.93 vs. 3.44) and IL-18 (1.34 vs. 1.60) and protein expression of NLRP3 (2.04 vs. 5.71), pro-caspase-1 (2.68 vs. 4.92) and IL-1β (1.61 vs. 2.60). In vitro, JLD inhibited the formation of lipid droplets induced by 2 mM FFA (IC₅₀ = 2.727 mM), reduced the protein expression of NLRP3 (0.74 vs. 2.27), caspase-1 (0.57 vs. 2.68), p20 (1.67 vs. 3.33), and IL-1β (1.44 vs. 2.41), and lowered the ratio of p-IKB-α/IKB-α (0.47 vs. 2.19).

CONCLUSION

JLD has a protective effect against NAFLD, which may be related to its anti-pyroptosis, suggesting that JLD has the potential as a novel agent in the treatment of NAFLD.

The P2X7R-NLRP3 and AIM2 Inflammasome Platforms Mark the Complexity/Severity of Viral or Metabolic Liver Damage

P2X7R-NLRP3 and AIM2 inflammasomes activate caspase-1 and the release of cytokines involved in viral-related liver disease. Little is known about their role in non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steato-hepatitis (NASH). We characterized the role of inflammasomes in NAFLD, NASH, and HCV. Gene expression and subcellular localization of P2X7R/P2X4R-NLRP3 and AIM2 inflammasome components were examined in histopathological preparations of 46 patients with biopsy-proven viral and metabolic liver disease using real-time PCR and immunofluorescence. P2X7R, P2X4R, and Caspase-1 are two- to five-fold more expressed in patients with NAFLD/NASH associated with chronic HCV infection than those with metabolic damage only (p ≤ 0.01 for all comparisons). The AIM2 inflammasome is 4.4 times more expressed in patients with chronic HCV infection, regardless of coexistent metabolic abnormalities (p = 0.0006). IL-2, a cytokine playing a pivotal role during chronic HCV infection, showed a similar expression in HCV and NASH patients (p = 0.77) but was virtually absent in NAFLD. The P2X7R-NLRP3 complex prevailed in infiltrating macrophages, while AIM2 was localized in Kupffer cells. Caspase-1 expression correlated with elastography-based liver fibrosis (r = 0.35, p = 0.02), whereas P2X7R, P2X4R, NRLP3, Caspase-1, and IL-2 expression correlated with circulating markers of disease severity. P2X7R and P2X4R play a major role in liver inflammation accompanying chronic HCV infection, especially when combined with metabolic damage, while AIM2 is specifically expressed in chronic viral hepatitis. We describe for the first time the hepatic expression of IL-2 in NASH, so far considered a peculiarity of HCV-related liver damage.

Inflammasomes and Pyroptosis of Liver Cells in Liver Fibrosis

Inflammasomes are multiprotein complexes that can sense danger signals and activate caspase-1 to mediate pro-inflammatory cytokines release and pyroptotic cell death. There are two main canonical and non-canonical signaling pathways that trigger inflammasome activation. Inflammasomes are expressed and assembled in parenchymal and nonparenchymal cells in response to liver injury in the liver. Additionally, the hepatocytes, biliary epithelial cells (cholangiocytes), hepatic stellate cells (HSCs), hepatic macrophages, and liver sinusoidal endothelial cells (LSECs) contribute to liver fibrosis via different mechanisms. However, the underlying mechanism of the inflammasome and pyroptosis in these liver cells in liver fibrosis remains elusive. This review summarizes the activation and function of inflammasome complexes and then discusses the association between inflammasomes, pyroptosis, and liver fibrosis. Unlike other similar reviewers, we will focus on the effect of inflammasome activation and pyroptosis in the various liver cells during the development of liver fibrosis. We will also highlight the latest progress of pharmacological intervention in inflammasome-mediated liver fibrosis.

P2X7 Activation Enhances Lipid Accumulation During Adipocytes Differentiation Through Suppressing the Expression of Sirtuin-3, Sirtuin-5, and Browning Genes

P2X7 signaling has been explored in adipose tissue because of its potential to promote ATP-activated inflammatory cascades during obesogenic environments. However, limited literature has investigated the role of the P2X7 receptor in lipid metabolism during adipocyte differentiation. This study sought to explore the regulatory roles of P2X7 in adipocytes. This study utilized the in vitro 3T3-L1 differentiation model. Lipid accumulation, intracellular triglyceride, and extracellular glycerol were determined. The selective P2X7 agonist BzATP and antagonist A438079 were administered to investigate the functions of P2X7. We found that the expression of P2X7 and the lipid accumulation increased during adipocyte differentiation from D0 to D4. When administered at D0/D2, A438079 attenuated, while BzATP enhanced the degree of lipid accumulation during adipocyte differentiation. Neither did BzATP and A438079 administration affect the expression of PPARγ and C/EBPα genes that increased at D4. In addition, both intracellular triglyceride and extracellular glycerol levels at D4 were reduced by A438079 treatment and enhanced by BzATP administration. When administered at stage 2 of adipocyte differentiation, BzATP consistently enhanced lipid accumulation and intracellular triglyceride and extracellular glycerol levels without affecting mRNA and protein levels of PPARγ and C/EBPα that increased at D4. However, treating A438079 or BzATP at D4 did not affect intracellular triglyceride formation and extracellular glycerol release in differentiated adipocytes at D7. Notably, BzATP administration at stage 2 exerted a concentration-dependent inhibition on the enhanced expression of PRDM16, PGC-1α, and UCP-1 at D4. Furthermore, BzATP administration at D0/D2 inhibited the protein and mRNA levels of sirtuin-3/5 at D4. BzATP treatment at stage 2 also suppressed the mRNA levels of sirtuin-3/5 genes upregulated by insulin. In conclusion, this study demonstrated P2X7 enhances lipid accumulation during adipogenesis by suppressing the expression of sirtuin-3/5 and the browning genes.

The NLRP3 Inflammasome in Non-Alcoholic Fatty Liver Disease and Steatohepatitis: Therapeutic Targets and Treatment

Non-alcoholic fatty liver disease (NAFLD) is among the most prevalent primary liver diseases worldwide and can develop into various conditions, ranging from simple steatosis, through non-alcoholic steatohepatitis (NASH), to fibrosis, and eventually cirrhosis and hepatocellular carcinoma. Nevertheless, there is no effective treatment for NAFLD due to the complicated etiology. Recently, activation of the NLPR3 inflammasome has been demonstrated to be a contributing factor in the development of NAFLD, particularly as a modulator of progression from initial hepatic steatosis to NASH. NLRP3 inflammasome, as a caspase-1 activation platform, is critical for processing key pro-inflammatory cytokines and pyroptosis. Various stimuli involved in NAFLD can activate the NLRP3 inflammasome, depending on the diverse cellular stresses that they cause. NLRP3 inflammasome-related inhibitors and agents for NAFLD treatment have been tested and demonstrated positive effects in experimental models. Meanwhile, some drugs have been applied in clinical studies, supporting this therapeutic approach. In this review, we discuss the activation, biological functions, and treatment targeting the NLRP3 inflammasome in the context of NAFLD progression. Specifically, we focus on the different types of therapeutic agents that can inhibit the NLRP3 inflammasome and summarize their pharmacological effectiveness for NAFLD treatment.

Signalling by extracellular nucleotides in health and disease

Nucleotides are released from all cells through regulated pathways or as a result of plasma membrane damage or cell death. Outside the cell, nucleotides act as signalling molecules triggering multiple responses via specific plasma membrane receptors of the P2 family. In the nervous system, purinergic signalling has a key function in neurotransmission. Outside the nervous system, purinergic signalling is one of the major modulators of basal tissue homeostasis, while its dysregulation contributes to the pathogenesis of various disease, including inflammation and cancer. Pre-clinical and clinical evidence shows that selective P2 agonists or antagonists are effective treatments for many pathologies, thus highlighting the relevance of extracellular nucleotides and P2 receptors as therapeutic targets.

Red yeast rice ameliorates non-alcoholic fatty liver disease through inhibiting lipid synthesis and NF-κB/NLRP3 inflammasome-mediated hepatic inflammation in mice

BACKGROUND

Red yeast rice (RYR), a nutraceutical with a profound cholesterol-lowering effect, was found to attenuate non-alcoholic fatty liver disease (NAFLD) in mice. Despite monacolin K in RYR being a specific inhibitor of hydroxymethylglutaryl-coenzyme A reductase (HMCGR), the mechanisms underlying the protective effects of RYR against NAFLD are not fully elucidated.

METHODS

Using a mouse model of high-fat diet (HFD) feeding and a cellular model of HepG2 cells challenged by lipopolysaccharide (LPS) and palmitic acid (PA), the possible molecular mechanisms were exploited in the aspects of NF-κB/NLRP3 inflammasome and mTORC1-SREBPs signaling pathways by examining the relevant gene/protein expressions. Subsequently, the correlation between these two signals was also verified using cellular experiments.

RESULTS

RYR ameliorated lipid accumulation and hepatic inflammation in vivo and in vitro. RYR improved lipid metabolism through modulating mTORC1-SREBPs and their target genes related to triglyceride and cholesterol synthesis. Furthermore, RYR suppressed hepatic inflammation by inhibiting the NF-κB/NLRP3 inflammasome signaling. Interestingly, the treatment with RYR or MCC950, a specific NLRP3 inhibitor, resulted in the reduced lipid accumulation in HepG2 cells challenged by LPS plus PA, suggesting that the inhibitory effects of RYR on NLRP3 inflammasome-mediated hepatic inflammation may partially, in turn, contribute to the lipid-lowering effect of RYR.

CONCLUSIONS

The modulation of NF-κB/NLRP3 inflammasome and lipid synthesis may contribute to the ameliorative effects of RYR against HFD-induced NAFLD.

Role of the Inflammasome in Liver Disease

The involvement of inflammasomes in the proinflammatory response observed in chronic liver diseases, such as alcohol-associated liver disease (ALD) and nonalcoholic fatty liver disease (NAFLD), is widely recognized. Although there are different types of inflammasomes, most studies to date have given attention to NLRP3 (nucleotide-binding oligomerization domain-like receptor family, pyrin domain containing 3) in the pathogenesis of ALD, NAFLD/nonalcoholic steatohepatitis, and fibrosis. Canonical inflammasomes are intracellular multiprotein complexes that are assembled after the sensing of danger signals and activate caspase-1, which matures interleukin (IL)-1β, IL-18, and IL-37 and also induces a form of cell death called pyroptosis. Noncanonical inflammasomes activate caspase-11 to induce pyroptosis. We discuss the different types of inflammasomes involved in liver diseases with a focus on (a) signals and mechanisms of inflammasome activation, (b) the role of different types of inflammasomes and their products in the pathogenesis of liver diseases, and (c) potential therapeutic strategies targeting components of the inflammasomes or cytokines produced upon inflammasome activation.

Liver Fibrosis-From Mechanisms of Injury to Modulation of Disease

The Transregional Collaborative Research Center "Organ Fibrosis: From Mechanisms of Injury to Modulation of Disease" (referred to as SFB/TRR57) was funded for 13 years (2009-2021) by the German Research Council (DFG). This consortium was hosted by the Medical Schools of the RWTH Aachen University and Bonn University in Germany. The SFB/TRR57 implemented combined basic and clinical research to achieve detailed knowledge in three selected key questions: (i) What are the relevant mechanisms and signal pathways required for initiating organ fibrosis? (ii) Which immunological mechanisms and molecules contribute to organ fibrosis? and (iii) How can organ fibrosis be modulated, e.g., by interventional strategies including imaging and pharmacological approaches? In this review we will summarize the liver-related key findings of this consortium gained within the last 12 years on these three aspects of liver fibrogenesis. We will highlight the role of cell death and cell cycle pathways as well as nutritional and iron-related mechanisms for liver fibrosis initiation. Moreover, we will define and characterize the major immune cell compartments relevant for liver fibrogenesis, and finally point to potential signaling pathways and pharmacological targets that turned out to be suitable to develop novel approaches for improved therapy and diagnosis of liver fibrosis. In summary, this review will provide a comprehensive overview about the knowledge on liver fibrogenesis and its potential therapy gained by the SFB/TRR57 consortium within the last decade. The kidney-related research results obtained by the same consortium are highlighted in an article published back-to-back in Frontiers in Medicine.

High extracellular ATP levels released through pannexin-1 channels mediate inflammation and insulin resistance in skeletal muscle fibres of diet-induced obese mice

AIMS/HYPOTHESIS

Skeletal muscle is a key target organ for insulin's actions and is the main regulator of blood glucose. In obese individuals and animal models, there is a chronic low-grade inflammatory state affecting highly metabolic organs, leading to insulin resistance. We have described that adult skeletal muscle fibres can release ATP to the extracellular medium through pannexin-1 (PANX1) channels. Besides, it is known that high extracellular ATP concentrations can act as an inflammatory signal. Here, we propose that skeletal muscle fibres from obese mice release high levels of ATP, through PANX1 channels, promoting inflammation and insulin resistance in muscle cells.

METHODS

C57BL/6J mice were fed with normal control diet (NCD) or high-fat diet (HFD) for 8 weeks. Muscle fibres were isolated from flexor digitorum brevis (FDB) muscle. PANX1-knockdown FDB fibres were obtained by in vivo electroporation of a short hairpin RNA Panx1 plasmid. We analysed extracellular ATP levels in a luciferin/luciferase assay. Gene expression was studied with quantitative real-time PCR (qPCR). Protein levels were evaluated by immunoblots, ELISA and immunofluorescence. Insulin sensitivity was analysed in a 2-NBDG (fluorescent glucose analogue) uptake assay, immunoblots and IPGTT.

RESULTS

HFD-fed mice showed significant weight gain and insulin resistance compared with NCD-fed mice. IL-6, IL-1β and TNF-α protein levels were increased in FDB muscle from obese mice. We observed high levels of extracellular ATP in muscle fibres from obese mice (197 ± 55 pmol ATP/μg RNA) compared with controls (32 ± 10 pmol ATP/μg RNA). ATP release in obese mice fibres was reduced by application of 100 μmol/l oleamide (OLE) and 5 μmol/l carbenoxolone (CBX), both PANX1 blockers. mRNA levels of genes linked to inflammation were reduced using OLE, CBX or 2 U/ml ATPase apyrase in muscle fibres from HFD-fed mice. In fibres from mice with pannexin-1 knockdown, we observed diminished extracellular ATP levels (78 ± 10 pmol ATP/μg RNA vs 252 ± 37 pmol ATP/μg RNA in control mice) and a lower expression of inflammatory markers. Moreover, a single pulse of 300 μmol/l ATP to fibres from control mice reduced insulin-mediated 2-NBDG uptake and promoted an elevation in mRNA levels of inflammatory markers. PANX-1 protein levels were increased two- to threefold in skeletal muscle from obese mice compared with control mice. Incubation with CBX increased Akt activation and 2-NBDG uptake in HFD fibres after insulin stimulation, rescuing the insulin resistance condition. Finally, in vivo treatment of HFD-fed mice with CBX (i.p. injection of 10 mg/kg each day) for 14 days, compared with PBS, reduced extracellular ATP levels in skeletal muscle fibres (51 ± 10 pmol ATP/μg RNA vs 222 ± 28 pmol ATP/μg RNA in PBS-treated mice), diminished inflammation and improved glycaemic management.

CONCLUSIONS/INTERPRETATION

In this work, we propose a novel mechanism for the development of inflammation and insulin resistance in the skeletal muscle of obese mice. We found that high extracellular ATP levels, released by overexpressed PANX1 channels, lead to an inflammatory state and insulin resistance in skeletal muscle fibres of obese mice.

Clodronate, an inhibitor of the vesicular nucleotide transporter, ameliorates steatohepatitis and acute liver injury

The vesicular nucleotide transporter (VNUT) is responsible for the vesicular storage and release of ATP from various ATP-secreting cells, and it plays an essential role in purinergic signaling. Although extracellular ATP and its degradation products are known to mediate various inflammatory responses via purinoceptors, whether vesicular ATP release affects steatohepatitis and acute liver injury is far less understood. In the present study, we investigated the effects of clodronate, a potent and selective VNUT inhibitor, on acute and chronic liver inflammation in mice. In a model of methionine/choline-deficient diet-induced non-alcoholic steatohepatitis (NASH), the administration of clodronate reduced hepatic inflammation, fibrosis, and triglyceride accumulation. Clodronate also protected mice against high-fat/high-cholesterol diet-induced steatohepatitis. Moreover, prophylactic administration of clodronate prevented d-galactosamine and lipopolysaccharide-induced acute liver injury by reducing inflammatory cytokines and hepatocellular apoptosis. In vitro, clodronate inhibited glucose-induced vesicular ATP release mediated by VNUT and reduced the intracellular level and secretion of triglycerides in isolated hepatocytes. These results suggest that VNUT-dependent vesicular ATP release plays a crucial role in the recruitment of immune cells, cytokine production, and the aggravation of steatosis in the liver. Pharmacological inhibition of VNUT may provide therapeutic benefits in liver inflammatory disorders, including NASH and acute toxin-induced injury.

[VNUT Is a Therapeutic Target for Type 2 Diabetes and NASH]

ATP, used in cells as an energy currency, also acts as an extracellular signaling molecule. Studies of purinergic receptor subtypes have revealed that purinergic chemical transmission plays important roles in various cell types. The vesicular nucleotide transporter (VNUT), the ninth transporter in the SLC17 organic anion transporter family, is essential for vesicular ATP storage and its subsequent release. The VNUT is localized on the membrane of secretory vesicles and actively transports ATP into vesicles using an electrochemical gradient of protons supplied by vacuolar proton ATPase (V-ATPase) as a driving force. ATP acts as a damage-associated molecular pattern (DAMPs), contributing to the persistence of chronic inflammation. Chronic inflammation induces systemic insulin resistance, which is the underlying pathology of type 2 diabetes and non-alcoholic fatty liver disease (NAFLD), ranging from simple steatosis to non-alcoholic steatohepatitis (NASH). We previously demonstrated that ATP transported in insulin granules via the VNUT negatively regulates insulin secretion. We also found that hepatocytes release ATP in a VNUT-dependent manner, which elicits hepatic insulin resistance and inflammation. VNUT-knockout mice exhibited improved glucose tolerance and were resistant to the development of high fat diet-induced NAFLD. In this article, we summarize recent advances in our understanding of the mechanism of the VNUT, the development of inhibitors, and its pathological involvement in type 2 diabetes and NAFLD. The pharmacological inhibition of the VNUT may represent a potential therapeutic approach for the treatment of metabolic diseases.

Immunological mechanisms and therapeutic targets of fatty liver diseases

Alcoholic liver disease (ALD) and nonalcoholic fatty liver disease (NAFLD) are the two major types of chronic liver disease worldwide. Inflammatory processes play key roles in the pathogeneses of fatty liver diseases, and continuous inflammation promotes the progression of alcoholic steatohepatitis (ASH) and nonalcoholic steatohepatitis (NASH). Although both ALD and NAFLD are closely related to inflammation, their respective developmental mechanisms differ to some extent. Here, we review the roles of multiple immunological mechanisms and therapeutic targets related to the inflammation associated with fatty liver diseases and the differences in the progression of ASH and NASH. Multiple cell types in the liver, including macrophages, neutrophils, other immune cell types and hepatocytes, are involved in fatty liver disease inflammation. In addition, microRNAs (miRNAs), extracellular vesicles (EVs), and complement also contribute to the inflammatory process, as does intertissue crosstalk between the liver and the intestine, adipose tissue, and the nervous system. We point out that inflammation also plays important roles in promoting liver repair and controlling bacterial infections. Understanding the complex regulatory process of disrupted homeostasis during the development of fatty liver diseases may lead to the development of improved targeted therapeutic intervention strategies.

The NLRP3 Inflammasome in Alcoholic and Nonalcoholic Steatohepatitis

Nonalcoholic steatohepatitis (NASH) and alcoholic hepatitis (ASH) are advanced forms of fatty liver diseases that are associated with a high morbidity and mortality worldwide. Patients with ASH or NASH are more susceptible to the progression of fibrosis and cirrhosis up to the development of hepatocellular carcinoma. Currently, there are limited medical therapies available. Accompanied by the asymptomatic disease progression, the demand for liver transplants is high. This review provides an overview about the growing evidence for a central role of NLR family pyrin domain containing 3 (NLRP3) inflammasome, a multiprotein complex that acts as a central driver of inflammation via activation of caspase 1, maturation and release of pro-inflammatory cytokines including interleukin-1β, and trigger of inflammatory pyroptotic cell death in both NASH and ASH. We also discuss potential therapeutic approaches targeting NLRP3 inflammasome and related upstream and downstream pathways to develop prognostic biomarkers and medical treatments for both liver diseases.

The Inflammasome in Chronic Complications of Diabetes and Related Metabolic Disorders

Diabetes mellitus (DM) ranks seventh as a cause of death worldwide. Chronic complications, including cardiovascular, renal, and eye disease, as well as DM-associated non-alcoholic fatty liver disease (NAFLD) account for most of the morbidity and premature mortality in DM. Despite continuous improvements in the management of late complications of DM, significant gaps remain. Therefore, searching for additional strategies to prevent these serious DM-related conditions is of the utmost importance. DM is characterized by a state of low-grade chronic inflammation, which is critical in the progression of complications. Recent clinical trials indicate that targeting the prototypic pro-inflammatory cytokine interleukin-1β (IL-1 β) improves the outcomes of cardiovascular disease, which is the first cause of death in DM patients. Together with IL-18, IL-1β is processed and secreted by the inflammasomes, a class of multiprotein complexes that coordinate inflammatory responses. Several DM-related metabolic factors, including reactive oxygen species, glyco/lipoxidation end products, and cholesterol crystals, have been involved in the pathogenesis of diabetic kidney disease, and diabetic retinopathy, and in the promoting effect of DM on the onset and progression of atherosclerosis and NAFLD. These metabolic factors are also well-established danger signals capable of regulating inflammasome activity. In addition to presenting the current state of knowledge, this review discusses how the mechanistic understanding of inflammasome regulation by metabolic danger signals may hopefully lead to novel therapeutic strategies targeting inflammation for a more effective treatment of diabetic complications.

The P2X7 Receptor and NLRP3 Axis in Non-Alcoholic Fatty Liver Disease: A Brief Review

Non-alcoholic fatty liver disease (NAFLD) is the most common liver disease worldwide, and its prevalence is reaching epidemic characteristics both in adults and in children. The increase of NAFLD prevalence parallels that of obesity, now representing the major cause of liver inflammation, increasing the risk of cirrhosis and hepatocarcinoma. Furthermore, NAFLD is a risk factor for cardiovascular diseases and type 2 diabetes, two of the major leading causes of morbidity and mortality in western countries. Thus a significant amount of studies have dealt with the evaluation of the possible molecular mechanisms leading to NAFLD and its inflammatory consequences within the liver, the non-alcoholic steatohepatitis, and cirrhosis. The inflammasome is a key player in the inflammation and fibrogenic responses in many different tissues, including the liver. The activation of the NLRP3 inflammasome requires the activation by extracellular adenosine tri-phosphate (ATP) of a specific purinergic receptor named P2X7 located in the target cells, although other pathways have been described. To this regard, extracellular ATP acts as an internal danger signal coming from damaged cells participating in the activation of the inflammatory process, a signaling pathway common to many different tissues. Here, we briefly review the involvement of the P2X7 receptor/inflammasome NLRP3 axis in the pathophysiological events leading to NAFLD and its inflammatory and fibrotic evolutions, reporting the possible therapeutical strategies targeting the P2X7 receptor/NLRP3 inflammasome.

Role of the P2X7 receptor in the pathogenesis of type 2 diabetes and its microvascular complications

P2X7 receptors can be found in many tissues and organs, where they mediate several biological functions. This review summarizes the current knowledge about the role of this receptor in the pathogenesis of type 2 diabetes, in which the key clinical features are impaired insulin secretion and sensitivity, hyperglycemia, coexistence of other cardiovascular risk factors such as dyslipidemia and hypertension, and subclinical inflammation. The receptor modulates crucial pathways in the pancreatic islets (where it can either exert a trophic or detrimental action on β cells), and in the liver, in the adipose tissue and in the skeletal muscle, which are main sites of insulin resistance. P2X7 receptors also modulate a series of inflammatory responses that participate in the development of the microvascular complications of the disease. Potent and selective P2X7R blockers are available to be tested in Phase I/II clinical studies for the treatment of several chronic diseases, and it might be worthwhile to consider inclusion of patients with type 2 diabetes and its complications.