Cholesterol-lowering drugs cause dissolution of cholesterol crystals and disperse Kupffer cell crown-like structures during resolution of NASH

Exploring the Role of Bempedoic Acid in Metabolic Dysfunction Associated Steatotic Liver Disease: Actual Evidence and Future Perspectives

Metabolic dysfunction-associated steatotic liver disease (MASLD) involves excessive lipid accumulation in hepatocytes, impacting global healthcare due to its high prevalence and risk of progression to severe liver conditions. Its pathogenesis involves genetic, metabolic, and inflammatory factors, with cardiovascular events as the leading cause of mortality. This review examines the role of lipid-lowering therapies in MASLD, with a particular focus on bempedoic acid, a recently approved cholesterol-lowering agent for hypercholesterolemia and high cardiovascular-risk patients. It explores its potential in liver disease by modulating lipid metabolism and inflammatory pathways based on the most recent studies available. Bempedoic acid inhibits ATP-citrate lyase, reducing cholesterol and fatty acid synthesis while activating AMP-activated protein kinase to suppress gluconeogenesis and lipogenesis. Animal studies indicate its efficacy in reducing hepatic steatosis, inflammation, and fibrosis. Bempedoic acid holds promise as a therapeutic for MASLD, offering dual benefits in lipid metabolism and inflammation. Further clinical trials are required to confirm its efficacy and safety in MASLD patients, potentially addressing the multifaceted nature of this disease.

Statins in Chronic Liver Disease: Review of the Literature and Future Role

Chronic liver disease (CLD) is a major contributor to global mortality, morbidity, and healthcare burden. Progress in pharmacotherapeutic for CLD management is lagging given its impact on the global population. While statins are indicated for the management of dyslipidemia and cardiovascular disease, their role in CLD prevention and treatment is emerging. Beyond their lipid-lowering effects, their liver-related mechanisms of action are multifactorial and include anti-inflammatory, antiproliferative, and immune-protective effects. In this review, we highlight what is known about the clinical benefits of statins in viral and nonviral etiologies of CLD and hepatocellular carcinoma (HCC), and explore key mechanisms and pathways targeted by statins. While their benefits may span the spectrum of CLD and potentially HCC treatment, their role in CLD chemoprevention is likely to have the largest impact. As emerging data suggest that genetic variants may impact their benefits, the role of statins in precision hepatology will need to be further explored.

An overview of the cholesterol metabolism and its proinflammatory role in the development of MASLD

Cholesterol is an essential lipid molecule in mammalian cells. It is not only involved in the formation of cell membranes but also serves as a raw material for the synthesis of bile acids, vitamin D, and steroid hormones. Additionally, it acts as a covalent modifier of proteins and plays a crucial role in numerous life processes. Generally, the metabolic processes of cholesterol absorption, synthesis, conversion, and efflux are strictly regulated. Excessive accumulation of cholesterol in the body is a risk factor for metabolic diseases such as cardiovascular disease, type 2 diabetes, and metabolic dysfunction-associated steatotic liver disease (MASLD). In this review, we first provide an overview of the discovery of cholesterol and the fundamental process of cholesterol metabolism. We then summarize the relationship between dietary cholesterol intake and the risk of developing MASLD, and also the animal models of MASLD specifically established with a cholesterol-containing diet. In the end, the role of cholesterol-induced inflammation in the initiation and development of MASLD is discussed.

A Systematic Review of Statins for the Treatment of Nonalcoholic Steatohepatitis: Safety, Efficacy, and Mechanism of Action

Nonalcoholic fatty liver disease (NAFLD) is the liver component of a cluster of conditions, while its subtype, nonalcoholic steatohepatitis (NASH), emerges as a potentially progressive liver disorder that harbors the risk of evolving into cirrhosis and culminating in hepatocellular carcinoma (HCC). NASH and cardiovascular disease (CVD) have common risk factors, but compared to liver-related causes, the most common cause of death in NASH patients is CVD. Within the pharmacological armamentarium, statins, celebrated for their lipid-modulating prowess, have now garnered attention for their expansive therapeutic potential in NASH. Evidence from a plethora of studies suggests that statins not only manifest anti-inflammatory and antifibrotic properties but also impart a multifaceted beneficial impact on hepatic health. In this review, we used "statin", "NAFLD", "NASH", and "CVD" as the major keywords and conducted a literature search using the PubMed and Web of Science databases to determine the safety and efficacy of statins in patients and animals with NASH and NAFLD, and the mechanism of statin therapy for NASH. Simultaneously, we reviewed the important role of the intestinal microbiota in statin therapy for NASH, as it is hoped that statins will provide new insights into modulating the harmful inflammatory microbiota in the gut and reducing systemic inflammation in NASH patients.

Statin therapy: a potential adjuvant to immunotherapies in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most prevalent cancers worldwide and accounts for more than 90% of primary liver cancer. The advent of immune checkpoint inhibitor (ICI)-related therapies combined with angiogenesis inhibition has revolutionized the treatment of HCC in late-stage and unresectable HCC, as ICIs alone were disappointing in treating HCC. In addition to the altered immune microenvironment, abnormal lipid metabolism in the liver has been extensively characterized in various types of HCC. Stains are known for their cholesterol-lowering properties and their long history of treating hypercholesterolemia and reducing cardiovascular disease risk. Apart from ICI and other conventional therapies, statins are frequently used by advanced HCC patients with dyslipidemia, which is often marked by the abnormal accumulation of cholesterol and fatty acids in the liver. Supported by a body of preclinical and clinical studies, statins may unexpectedly enhance the efficacy of ICI therapy in HCC patients through the regulation of inflammatory responses and the immune microenvironment. This review discusses the abnormal changes in lipid metabolism in HCC, summarizes the clinical evidence and benefits of stain use in HCC, and prospects the possible mechanistic actions of statins in transforming the immune microenvironment in HCC when combined with immunotherapies. Consequently, the use of statin therapy may emerge as a novel and valuable adjuvant for immunotherapies in HCC.

NLRP3 inflammasome in hepatic diseases: A pharmacological target

The NOD-like receptor family pyrin domain containing 3 (NLRP3) inflammasome pathway is mainly responsible for the activation and release of a cascade of proinflammatory mediators that contribute to the development of hepatic diseases. During alcoholic liver disease development, the NLRP3 inflammasome pathway contributes to the maturation of caspase-1, interleukin (IL)-1β, and IL-18, which induce a robust inflammatory response, leading to fibrosis by inducing profibrogenic hepatic stellate cell (HSC) activation. Substantial evidence demonstrates that nonalcoholic fatty liver disease (NAFLD) progresses to nonalcoholic steatohepatitis (NASH) via NLRP3 inflammasome activation, ultimately leading to fibrosis and hepatocellular carcinoma (HCC). Activation of the NLRP3 inflammasome in NASH can be attributed to several factors, such as reactive oxygen species (ROS), gut dysbiosis, leaky gut, which allow triggers such as cardiolipin, cholesterol crystals, endoplasmic reticulum stress, and uric acid to reach the liver. Because inflammation triggers HSC activation, the NLRP3 inflammasome pathway performs a central function in fibrogenesis regardless of the etiology. Chronic hepatic activation of the NLRP3 inflammasome can ultimately lead to HCC; however, inflammation also plays a role in decreasing tumor growth. Some data indicate that NLRP3 inflammasome activation plays an important role in autoimmune hepatitis, but the evidence is scarce. Most researchers have reported that NLRP3 inflammasome activation is essential in liver injury induced by a variety of drugs and hepatotropic virus infection; however, few reports indicate that this pathway can play a beneficial role by inducing liver regeneration. Modulation of the NLRP3 inflammasome appears to be a suitable strategy to treat liver diseases.

Establishment of Novel Mouse Model of Dietary NASH Rapidly Progressing into Liver Cirrhosis and Tumors

Non-alcoholic steatohepatitis (NASH), which is the most severe manifestation of non-alcoholic fatty liver disease (NAFLD), has been recognized as a major hepatocellular carcinoma (HCC) catalyst. However, the molecular mechanism of NASH-liver fibrosis-HCC sequence remains unclear and a specific and effective treatment for NASH has not yet been established. The progress in this field depends on the availability of reliable preclinical models which show the steady progression to NASH, liver cirrhosis, and HCC. However, most of the NASH mouse models that have been described to date develop NASH generally for more than 24 weeks and there is an uncertainty of HCC development. To overcome such shortcomings of experimental NASH studies, we established a novel NASH-HCC mouse model with very high reproducibility, generality, and convenience. We treated male C57BL/6J mice with a newly developed choline-deficient and methionine-restricted high-fat diet, named OYC-NASH2 diet, for 60 weeks. Treatment of OYC-NASH2 diet for 3 weeks revealed marked steatosis, lobular inflammation, and fibrosis, histologically diagnosed as NASH. Liver cirrhosis was observed in all mice with 48-week treatment. Liver nodules emerged at 12 weeks of the treatment, > 2 mm diameter liver tumors developed in all mice at 24 weeks of the treatment and HCC appeared after 36-week treatment. In conclusion, our rapidly progressive and highly reproducible NASH-liver cirrhosis-HCC model is helpful for preclinical development and research on the pathogenesis of human NAFLD-NASH-HCC. Our mouse model would be useful for the development of novel chemicals for NASH-HCC-targeted therapies.

Activation of Kupffer cells in NAFLD and NASH: mechanisms and therapeutic interventions

Non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) are emerging as the leading causes of liver disease worldwide. These conditions can lead to cirrhosis, liver cancer, liver failure, and other related ailments. At present, liver transplantation remains the sole treatment option for end-stage NASH, leading to a rapidly growing socioeconomic burden. Kupffer cells (KCs) are a dominant population of macrophages that reside in the liver, playing a crucial role in innate immunity. Their primary function includes phagocytosing exogenous substances, presenting antigens, and triggering immune responses. Moreover, they interact with other liver cells during the pathogenesis of NAFLD, and this crosstalk may either delay or exacerbate disease progression. Stimulation by endogenous signals triggers the activation of KCs, resulting in the expression of various inflammatory factors and chemokines, such as NLRP3, TNF-α, IL-1B, and IL-6, and contributing to the inflammatory cascade. In the past 5 years, significant advances have been made in understanding the biological properties and immune functions of KCs in NAFLD, including their interactions with tissue molecules, underlying molecular mechanisms, signaling pathways, and relevant therapeutic interventions. Having a comprehensive understanding of these mechanisms and characteristics can have enormous potential in guiding future strategies for the prevention and treatment of NAFLD.

Genetic deletion or pharmacologic inhibition of the Nlrp3 inflammasome did not ameliorate experimental NASH

It has been postulated that inflammasomes, in particular the NLRP3 (NLR family pyrin domain containing 3) inflammasome, mediate the necroinflammation and fibrosis that characterize nonalcoholic steatohepatitis (NASH) by engaging innate immune responses. We aimed to investigate the impact of genetic deletion or pharmacologic inhibition of the NLRP3 inflammasome on experimental steatohepatitis. Global Nlrp3 KO (expected to inhibit the NLRP3 inflammasome) or Casp1 KO (expected to inhibit all inflammasomes) mice were compared to wild type controls after 6 months on a high-fat, high-cholesterol (HFHC, 1% cholesterol) diet known to induce fibrosing steatohepatitis. Additionally, wildtype mice on a HFHC diet (0.75% or 0.5% cholesterol) for 6 months were either treated or not treated with an oral, pharmacologic inhibitor of Nlrp3 (MCC950) that was delivered in the drinking water (0.3 mg/ml). We found that genetic deletion or pharmacologic inhibition of the NLRP3 inflammasome did not ameliorate any of the histological components of fibrosing NASH in HFHC-fed mice. Collectively, these results do not support NLRP3 inhibition as a potential target for human NASH.

Targeting macrophagic 17β-HSD7 by fenretinide for the treatment of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disease worldwide and macrophage polarization plays an important role in its pathogenesis. However, which molecule regulates macrophage polarization in NAFLD remains unclear. Herein, we showed NAFLD mice exhibited increased 17β-hydroxysteroid dehydrogenase type 7 (17β-HSD7) expression in hepatic macrophages concomitantly with elevated M1 polarization. Single-cell RNA sequencing on hepatic non-parenchymal cells isolated from wild-type littermates and macrophage-17β-HSD7 knockout mice fed with high fat diet (HFD) for 6 weeks revealed that lipid metabolism pathways were notably changed. Furthermore, 17β-HSD7 deficiency in macrophages attenuated HFD-induced hepatic steatosis, insulin resistance and liver injury. Mechanistically, 17β-HSD7 triggered NLRP3 inflammasome activation by increasing free cholesterol content, thereby promoting M1 polarization of macrophages and the secretion of pro-inflammatory cytokines. In addition, to help demonstrate that 17β-HSD7 is a potential drug target for NAFLD, fenretinide was screened out from an FDA-approved drug library based on its 17β-HSD7 dehydrogenase inhibitory activity. Fenretinide dose-dependently abrogated macrophage polarization and pro-inflammatory cytokines production, and subsequently inhibited fat deposition in hepatocytes co-cultured with macrophages. In conclusion, our findings suggest that blockade of 17β-HSD7 signaling by fenretinide would be a drug repurposing strategy for NAFLD treatment.

Targeted therapeutics and novel signaling pathways in non-alcohol-associated fatty liver/steatohepatitis (NAFL/NASH)

Non-alcohol-associated fatty liver/steatohepatitis (NAFL/NASH) has become the leading cause of liver disease worldwide. NASH, an advanced form of NAFL, can be progressive and more susceptible to developing cirrhosis and hepatocellular carcinoma. Currently, lifestyle interventions are the most essential and effective strategies for preventing and controlling NAFL without the development of fibrosis. While there are still limited appropriate drugs specifically to treat NAFL/NASH, growing progress is being seen in elucidating the pathogenesis and identifying therapeutic targets. In this review, we discussed recent developments in etiology and prospective therapeutic targets, as well as pharmacological candidates in pre/clinical trials and patents, with a focus on diabetes, hepatic lipid metabolism, inflammation, and fibrosis. Importantly, growing evidence elucidates that the disruption of the gut-liver axis and microbe-derived metabolites drive the pathogenesis of NAFL/NASH. Extracellular vesicles (EVs) act as a signaling mediator, resulting in lipid accumulation, macrophage and hepatic stellate cell activation, further promoting inflammation and liver fibrosis progression during the development of NAFL/NASH. Targeting gut microbiota or EVs may serve as new strategies for the treatment of NAFL/NASH. Finally, other mechanisms, such as cell therapy and genetic approaches, also have enormous therapeutic potential. Incorporating drugs with different mechanisms and personalized medicine may improve the efficacy to better benefit patients with NAFL/NASH.

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.

Mitochondria and the NLRP3 Inflammasome in Alcoholic and Nonalcoholic Steatohepatitis

Alcoholic (ASH) and nonalcoholic steatohepatitis (NASH) are advanced stages of fatty liver disease and two of the most prevalent forms of chronic liver disease. ASH and NASH are associated with significant risk of further progression to cirrhosis and hepatocellular carcinoma (HCC), the most common type of liver cancer, and a major cause of cancer-related mortality. Despite extensive research and progress in the last decades to elucidate the mechanisms of the development of ASH and NASH, the pathogenesis of both diseases is still poorly understood. Mitochondrial damage and activation of inflammasome complexes have a role in inducing and sustaining liver damage. Mitochondrial dysfunction produces inflammatory factors that activate the inflammasome complexes. NLRP3 inflammasome (nucleotide-binding oligomerization domain-like receptor protein 3) is a multiprotein complex that activates caspase 1 and the release of pro-inflammatory cytokines, including interleukin-1β (IL-1β) and interleukin-18 (IL-18), and contributes to inflammatory pyroptotic cell death. The present review, which is part of the issue "Mitochondria in Liver Pathobiology", provides an overview of the role of mitochondrial dysfunction and NLRP3 activation in ASH and NASH.

Pcsk9 Deletion Promotes Murine Nonalcoholic Steatohepatitis and Hepatic Carcinogenesis: Role of Cholesterol

Proprotein convertase subtilisin/kexin type 9 (Pcsk9) binds to hepatic low-density lipoprotein receptor (LDLR) and induces its internalization and degradation. Pcsk9 inhibition increases LDLR expression by hepatocytes, which causes increased uptake of circulating LDL, thereby reducing plasma LDL-cholesterol. However, by increasing the uptake of LDL by the liver, Pcsk9 inhibition increases the exposure of the liver to cholesterol, which may result in higher risk of steatohepatitis and ever carcinogenesis. We compared Pcsk9-/- knockout (KO) mice and appropriate wild-type (WT) controls of the same strain assigned to a high-fat (15%, wt/wt) diet for 9 months supplemented with 0.25%, 0.5%, or 0.75% dietary cholesterol. Pcsk9 KO mice on a high-fat, high-cholesterol diet exhibited higher levels of hepatic free cholesterol loading and hepatic cholesterol crystallization than their WT counterparts. Pcsk9 KO mice developed crown-like structures of macrophages surrounding cholesterol crystal-containing lipid droplets and hepatocytes, exhibited higher levels of apoptosis, and developed significantly more hepatic inflammation and fibrosis consistent with fibrosing steatohepatitis, including 5-fold and 11-fold more fibrosis at 0.5% and 0.75% dietary cholesterol, respectively. When injected with diethylnitrosamine, a hepatic carcinogen, early-in-life Pcsk9 KO mice were more likely to develop liver cancer than WT mice. Conclusion: Pcsk9 KO mice on high-cholesterol diets developed increased hepatic free cholesterol and cholesterol crystals and fibrosing steatohepatitis with a higher predisposition to liver cancer compared with WT mice. Future studies should evaluate whether patients on long-term treatment with anti-PSCK9 monoclonal antibodies are at increased risk of hepatic steatosis, steatohepatitis or liver cancer, while accounting for concurrent use of statins.

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.

Is there a role of lipid-lowering therapies in the management of fatty liver disease?

Atherogenic dyslipidemia is characterized by increased triglyceride-rich lipoproteins and low high-density lipoprotein cholesterol concentrations. It is highly prevalent in non-alcoholic fatty liver disease (NAFLD) and contributes to the increased cardiovascular risk associated with this condition. Alongside insulin resistance it plays an important pathogenetic role in NAFLD/non-alcoholic steatohepatitis (NASH) development and progression. It has been shown that cholesterol-lowering reduces cardiovascular risk more in NAFLD vs non-NAFLD high-risk individuals. This evidence highlights the importance of effective lipid modulation in NAFLD. In this narrative review the effects of the most commonly used lipid-lowering therapies on liver outcomes alongside their therapeutic implications in NAFLD/NASH are critically discussed. Preclinical and clinical evidence suggests that statins reduce hepatic steatosis, inflammation and fibrosis in patients with NAFLD/NASH. Most data are derived from observational and small prospective clinical studies using changes in liver enzyme activities, steatosis/fibrosis scores, and imaging evidence of steatosis as surrogates. Also, relevant histologic benefits were noted in small biopsy studies. Atorvastatin and rosuvastatin showed greater benefits, whereas data for other statins are scarce and sometimes conflicting. Similar studies to those of statins showed efficacy of ezetimibe against hepatic steatosis. However, no significant anti-inflammatory and anti-fibrotic actions of ezetimibe have been shown. Preclinical studies showed that fibrates through peroxisome proliferator-activated receptor (PPAR)α activation may have a role in NAFLD prevention and management. Nevertheless, no relevant benefits have been noted in human studies. Species-related differences in PPARα expression and its activation responsiveness may help explain this discrepancy. Omega-3 fatty acids reduced hepatic steatosis in numerous heterogeneous studies, but their benefits on hepatic inflammation and fibrosis have not been established. Promising preliminary data for the highly purified eicosapentaenoic acid require further confirmation. Observational studies suggest that proprotein convertase subtilisin/kexin9 inhibitors may also have a role in the management of NAFLD, though this needs to be established by future prospective studies.

Hepatic Macrophage as a Key Player in Fatty Liver Disease

Fatty liver disease, characterized by excessive inflammation and lipid deposition, is becoming one of the most prevalent liver metabolic diseases worldwide owing to the increasing global incidence of obesity. However, the underlying mechanisms of fatty liver disease are poorly understood. Accumulating evidence suggests that hepatic macrophages, specifically Kupffer cells (KCs), act as key players in the progression of fatty liver disease. Thus, it is essential to examine the current evidence of the roles of hepatic macrophages (both KCs and monocyte-derived macrophages). In this review, we primarily address the heterogeneities and multiple patterns of hepatic macrophages participating in the pathogenesis of fatty liver disease, including Toll-like receptors (TLRs), NLRP3 inflammasome, lipotoxicity, glucotoxicity, metabolic reprogramming, interaction with surrounding cells in the liver, and iron poisoning. A better understanding of the diverse roles of hepatic macrophages in the development of fatty liver disease may provide a more specific and promising macrophage-targeting therapeutic strategy for inflammatory liver diseases.

Statins in Non-alcoholic Steatohepatitis

Non-alcoholic fatty liver disease (NAFLD) is the primary cause of chronic liver disease. The range is extensive, including hepatocellular carcinoma, cirrhosis, fibrosis, fatty liver, and non-alcoholic steatohepatitis (NASH). NASH is a condition related to obesity, overweight, metabolic syndrome, diabetes, and dyslipidemia. It is a dynamic condition that can regress to isolated steatosis or progress to fibrosis and cirrhosis. Statins exert anti-inflammatory, proapoptotic, and antifibrotic effects. It has been proposed that these drugs could have a relevant role in NASH. In this review, we provide an overview of current evidence, from mechanisms of statins involved in the modulation of NASH to human trials about the use of statins to treat or attenuate NASH.

Formation and Cellular Impact of Cholesterol Crystals in Health and Disease

Cholesterol crystals (CCs) were first discovered in atherosclerotic plaque tissue in the early 1900 and have since been observed and implicated in many diseases and conditions, including myocardial infarction, abdominal aortic aneurism, kidney disease, ocular diseases, and even central nervous system anomalies. Despite the widespread involvement of CCs in many pathologies, the mechanisms involved in their formation and their role in various diseases are still not fully understood. Current knowledge concerning the formation of CCs, as well as the molecular pathways activated upon cellular exposure to CCs, will be explored in this review. As CC formation is tightly associated with lipid metabolism, the role of cellular lipid homeostasis in the formation of CCs is highlighted, including the role of lysosomes. In addition, cellular pathways and processes known to be affected by CCs are described. In particular, CC-induced activation of the inflammasome and production of reactive oxygen species, along with the role of CCs in complement-mediated inflammation is discussed. Moreover, the clinical manifestation of embolized CCs is described with a focus on renal and skin diseases associated with CC embolism. Lastly, potential therapeutic measures that target either the formation of CCs or their impact on different cell types and tissues are highlighted.

Cholesterol-induced toxicity: An integrated view of the role of cholesterol in multiple diseases

High levels of cholesterol are generally considered to be associated with atherosclerosis. In the past two decades, however, a number of studies have shown that excess cholesterol accumulation in various tissues and organs plays a critical role in the pathogenesis of multiple diseases. Here, we summarize the effects of excess cholesterol on disease pathogenesis, including liver diseases, diabetes, chronic kidney disease, Alzheimer's disease, osteoporosis, osteoarthritis, pituitary-thyroid axis dysfunction, immune disorders, and COVID-19, while proposing that excess cholesterol-induced toxicity is ubiquitous. We believe this concept will help broaden the appreciation of the toxic effect of excess cholesterol, and thus potentially expand the therapeutic use of cholesterol-lowering medications.

Pyroptosis in Steatohepatitis and Liver Diseases

Pyroptosis is an inflammatory form of regulated cell death, which functions in the clearance of intracellularly replicating pathogens by cell lysis in order to induce further immune response. Since the discovery of the gasdermin (GSDM) family, pyroptosis has attracted attention in a wide range of inflammatory diseases such as nonalcoholic steatohepatitis and other liver diseases. Due to the cleavage of GSDMs by different caspases, the amino-terminal GSDM fragments form membrane pores essential for pyroptosis that facilitate the release of inflammatory cytokines by loss of ionic gradient and membrane rupture. In this review, we address the key molecular and cellular processes that induce pyroptosis in the liver and its significance in the pathogenesis of common liver diseases in different human and experimental mice studies.

Role of Cholesterol-Associated Steatohepatitis in the Development of NASH

The rising prevalence of nonalcoholic fatty liver disease (NAFLD) and NAFLD-related cirrhosis in the United States and globally highlights the need to better understand the mechanisms causing progression of hepatic steatosis to fibrosing steatohepatitis and cirrhosis in a small proportion of patients with NAFLD. Accumulating evidence suggests that lipotoxicity mediated by hepatic free cholesterol (FC) overload is a mechanistic driver for necroinflammation and fibrosis, characteristic of nonalcoholic steatohepatitis (NASH), in many animal models and also in some patients with NASH. Diet, lifestyle, obesity, key genetic polymorphisms, and hyperinsulinemia secondary to insulin resistance are pivotal drivers leading to aberrant cholesterol signaling, which leads to accumulation of FC within hepatocytes. FC overload in hepatocytes can lead to ER stress, mitochondrial dysfunction, development of toxic oxysterols, and cholesterol crystallization in lipid droplets, which in turn lead to hepatocyte apoptosis, necrosis, or pyroptosis. Activation of Kupffer cells and hepatic stellate cells by hepatocyte signaling and cholesterol loading contributes to this inflammation and leads to hepatic fibrosis. Cholesterol accumulation in hepatocytes can be readily prevented or reversed by statins. Observational studies suggest that use of statins in NASH not only decreases the substantially increased cardiovascular risk, but may ameliorate liver pathology. Conclusion: Hepatic FC loading may result in cholesterol-associated steatohepatitis and play an important role in the development and progression of NASH. Statins appear to provide significant benefit in preventing progression to NASH and NASH-cirrhosis. Randomized controlled trials are needed to demonstrate whether statins or statin/ezetimibe combination can effectively reverse steatohepatitis and liver fibrosis in patients with NASH.

The Role of Innate Immune Cells in Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is a very common hepatic pathology featuring steatosis and is linked to obesity and related conditions, such as the metabolic syndrome. When hepatic steatosis is accompanied by inflammation, the disorder is defined as nonalcoholic steatohepatitis (NASH), which in turn can progress toward fibrosis development that can ultimately result in cirrhosis. Cells of innate immunity, such as neutrophils or macrophages, are central regulators of NASH-related inflammation. Recent studies utilizing new experimental technologies, such as single-cell RNA sequencing, have revealed substantial heterogeneity within the macrophage populations of the liver, suggesting distinct functions of liver-resident Kupffer cells and recruited monocyte-derived macrophages with regards to regulation of liver inflammation and progression of NASH pathogenesis. Herein, we discuss recent developments concerning the function of innate immune cell subsets in NAFLD and NASH.

Atorvastatin Modulates Bile Acid Homeostasis in Mice with Diet-Induced Nonalcoholic Steatohepatitis

Bile acids (BA) play a significant role in the pathophysiology of nonalcoholic steatohepatitis (NASH). The present study evaluates the modulation of bile acid metabolomics by atorvastatin, a cholesterol-lowering agent commonly used to treat cardiovascular complications accompanying NASH. NASH was induced in mice by 24 weeks of consuming a high–saturated fat, high-fructose, and high-cholesterol diet (F), with atorvastatin administered orally (20 mg/kg/day) during the last three weeks. Biochemical and histological analyses confirmed the effectiveness of the F diet in inducing NASH. Untreated NASH animals had significantly reduced biliary secretion of BA and increased fecal excretion of BA via decreased apical sodium-dependent bile salt transporter (Asbt)-mediated reabsorption. Atorvastatin decreased liver steatosis and inflammation in NASH animals consistently with a reduction in crucial lipogenic enzyme stearoyl–coenzyme A (CoA) desaturase-1 and nuclear factor kappa light chain enhancer of activated B-cell pro-inflammatory signaling, respectively. In this group, atorvastatin also uniformly enhanced plasma concentration, biliary secretion and fecal excretion of the secondary BA, deoxycholic acid (DCA). However, in the chow diet–fed animals, atorvastatin decreased plasma concentrations of BA, and reduced BA biliary secretions. These changes stemmed primarily from the increased fecal excretion of BA resulting from the reduced Asbt-mediated BA reabsorption in the ileum and suppression of synthesis in the liver. In conclusion, our results reveal that atorvastatin significantly modulates BA metabolomics by altering their intestinal processing and liver synthesis in control and NASH mice.

Dynamic Shifts in the Composition of Resident and Recruited Macrophages Influence Tissue Remodeling in NASH

Macrophage-mediated inflammation is critical in the pathogenesis of non-alcoholic steatohepatitis (NASH). Here, we describe that, with high-fat, high-sucrose-diet feeding, mature TIM4^pos Kupffer cells (KCs) decrease in number, while monocyte-derived Tim4^neg macrophages accumulate. In concert, monocyte-derived infiltrating macrophages enter the liver and consist of a transitional subset that expresses Cx3cr1/Ccr2 and a second subset characterized by expression of Trem2, Cd63, Cd9, and Gpmnb; markers ascribed to lipid-associated macrophages (LAMs). The Cx3cr1/Ccr2-expressing macrophages, referred to as C-LAMs, localize to macrophage aggregates and hepatic crown-like structures (hCLSs) in the steatotic liver. In C-motif chemokine receptor 2 (Ccr2)-deficient mice, C-LAMs fail to appear in the liver, and this prevents hCLS formation, reduces LAM numbers, and increases liver fibrosis. Taken together, our data reveal dynamic changes in liver macrophage subsets during the pathogenesis of NASH and link these shifts to pathologic tissue remodeling.

Strategies for Targeting the NLRP3 Inflammasome in the Clinical and Preclinical Space

Inhibiting the NLRP3 inflammasome mediates inflammation in an extensive number of preclinical models. As excitement in this field has grown, several companies have recently initiated testing of direct NLRP3 inhibitors in the clinic. At the same time, the NLRP3 inflammasome is part of a larger pro-inflammatory pathway, whose modulation is also being explored. Multiple targets in this pathway are already impinged upon by molecules that have been through clinical trials. These data, informed by the growing mechanistic understanding of the NLRP3 inflammasome in the preclinical space, provide a rich backdrop to assess the current state of the field. Here we explore attempts to inhibit the NLRP3 inflammasome in light of clinical and preclinical data around efficacy and safety.

Danger signals in liver injury and restoration of homeostasis

Damage-associated molecular patterns are signalling molecules involved in inflammatory responses and restoration of homeostasis. Chronic release of these molecules can also promote inflammation in the context of liver disease. Herein, we provide a comprehensive summary of the role of damage-associated molecular patterns as danger signals in liver injury. We consider the role of reactive oxygen species and reactive nitrogen species as inducers of damage-associated molecular patterns, as well as how specific damage-associated molecular patterns participate in the pathogenesis of chronic liver diseases such as alcohol-related liver disease, non-alcoholic steatohepatitis, liver fibrosis and liver cancer. In addition, we discuss the role of damage-associated molecular patterns in ischaemia reperfusion injury and liver transplantation and highlight current studies in which blockade of specific damage-associated molecular patterns has proven beneficial in humans and mice.

Pleiotropic Effects of Statins in the Light of Non-Alcoholic Fatty Liver Disease and Non-Alcoholic Steatohepatitis

Statins, the lipid-lowering drugs, and non-alcoholic fatty liver disease (NAFLD)/non-alcoholic steatohepatitis (NASH), a lipid-related pathology, share a complex relationship, one known to be hepatotoxic and other being hepatic injury. NASH is an unresolved mystery in terms of treatment. Could statins prove to be a promising solution due to their pleiotropic properties in addition to the cholesterol-lowering effect? This study aims to find statin effectiveness in NAFLD/NASH treatment and prevention of associated adverse outcomes. An extensive data search was done to identify the studies assessing statin effect on NAFLD/NASH and then analyzed to establish the relationship. Several studies demonstrated a reduction in NAFLD/NASH-associated inflammation and fibrosis with statin treatment. These anti-inflammatory and anti-fibrotic effects were through their pleiotropic properties, which were in addition to their cholesterol-lowering effect. In various animal studies, statins were found to improve hepatic lipotoxicity, oxidative stress, inflammatory responses, and fibrosis associated with NASH through multiple pathways. Statins exert these protective effects by recovering the gene expression level of peroxisomal proliferator-activated receptor alpha (PPARα) and therefore restore the mitochondrial and peroxisomal fatty acid oxidation (FAO). Statin treatment also increased the levels of paraoxonase 1 (PON1), an antioxidant and antiatherogenic enzyme that is reduced in NAFLD as well as encounter the hepatic lipotoxicity by resolving cholesterol crystals and Kupffer cells (KCs) with crown-like structures (CLSs). They exhibited antitumor properties by inhibiting proinflammatory cytokines and vascular proliferative factors. Moreover, they restored a healthy liver sinusoidal endothelial cell (LSEC) and hepatic stellate cells (HSC) along with inhibiting the activation of HSC via modulating inducible nitric oxide synthase (iNOS) and expressions of endothelial nitric oxide synthase (eNOS). Besides, they were protective against cardiovascular disease (CVD)-related morbidity and mortality, hepatocellular carcinoma (HCC), and metabolic syndrome (MS) associated with NAFLD/NASH. NASH and its precursor, NAFLD, could be treated and prevented with statins owing to their pleiotropic properties. This study helps to prove this by looking back at different literature and has successfully enlightened the point. Once proved through large clinical trials on humans, it could revolutionize the NASH therapy. 

Automated computerized image analysis for the user-independent evaluation of disease severity in preclinical models of NAFLD/NASH

Pathologists use a semiquantitative scoring system (NAS or SAF score) to facilitate the reporting of disease severity and evolution. Similar scores are applied for the same purposes in rodents. Histological scores have inherent inter- and intra-observer variability and yield discrete and not continuous values. Here we performed an automatic numerical quantification of NASH features on liver sections in common preclinical NAFLD/NASH models. High-fat diet-fed foz/foz mice (Foz HF) or wild-type mice (WT HF) known to develop progressive NASH or an uncomplicated steatosis, respectively, and C57Bl6 mice fed a choline-deficient high-fat diet (CDAA) to induce steatohepatitis were analyzed at various time points. Automated software image analysis of steatosis, inflammation, and fibrosis was performed on digital images from entire liver sections. Data obtained were compared with the NAS score, biochemical quantification, and gene expression. As histologically assessed, WT HF mice had normal liver up to week 34 when they harbor mild steatosis with if any, little inflammation. Foz HF mice exhibited grade 2 steatosis as early as week 4, grade 3 steatosis at week 12 up to week 34; inflammation and ballooning increased gradually with time. Automated measurement of steatosis (macrovesicular steatosis area) revealed a strong correlation with steatosis scores (r = 0.89), micro-CT liver density, liver lipid content (r = 0.89), and gene expression of CD36 (r = 0.87). Automatic assessment of the number of F4/80-immunolabelled crown-like structures strongly correlated with conventional inflammatory scores (r = 0.79). In Foz HF mice, collagen deposition, evident at week 20 and progressing at week 34, was automatically quantified on picrosirius red-stained entire liver sections. The automated procedure also faithfully captured and quantitated macrovesicular steatosis, mixed inflammation, and pericellular fibrosis in CDAA-induced steatohepatitis. In conclusion, the automatic numerical analysis represents a promising quantitative method to rapidly monitor NAFLD activity with high-throughput in large preclinical studies and for accurate monitoring of disease evolution.

Self-Micellizing Technology Improves the Properties of Ezetimibe and Increases Its Effect on Hyperlipidemic Rats

The aim of this work was to develop ezetimibe self-micellizing solid dispersions using Kolliphor^® RH40 (MS-K) as a surfactant incorporating ezetimibe (EZ) into the croscarmellose hydrophilic carrier. Different ezetimibe:Kolliphor^® ratios were studied to select micellar systems that improve the dissolution properties of ezetimibe. The different formulations were characterized by means of solid state analysis by SEM, powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC), and dissolution studies. These physicochemical studies showed a decrease from the crystalline structure of ezetimibe (EZ) to its amorphous state in the micellar systems (MS-K). A rapid dissolution profile was observed in these micellar systems compared to the drug raw material and physical mixture. Efficacy studies were conducted using a high-fat diet that induced hyperlipidemic rats. The micellar system selected (MS-K 1:0.75) revealed a significant improvement in serum levels of total cholesterol (TC), low-density lipoproteins (LDL), and triglycerides (TG) compared to ezetimibe raw material. The histopathological examination of liver tissue also showed that this micellar system exhibited more beneficial effects on liver steatosis compared to ezetimibe raw material (EZ-RM) and the high-fat diet group (HFD). This study suggests that EZ micellar systems using Kolliphor^® RH40 could enhance the antihyperlipidemic effect of ezetimibe and reduce liver steatosis.

Longitudinal characterization of diet-induced genetic murine models of non-alcoholic steatohepatitis with metabolic, histological, and transcriptomic hallmarks of human patients

Non-alcoholic steatohepatitis (NASH) is a fast-growing liver disease in the Western world. Currently, only a few animal models show both the metabolic and histological features of human NASH. We aimed to explore murine NASH models in a time dependent manner that exhibit metabolic, histological and transcriptomic hallmarks of human NASH. For this, the murine strains C57BL/6J, ob/ob, and KK-A^y were used and three types of nutritional regimes were administered: normal chow diet (NCD); high-fat, high-fructose, and high-cholesterol diet (fast food diet; FFD); or choline-deficient, L-amino acid-defined, high-fat diet (CDAHFD), for 2, 4, 8, 12, 18, 24, and 30 weeks. All strains under the FFD and CDAHFD regimes developed steatohepatitis. Among the strains treated with FFD, the non-alcoholic fatty liver disease (NAFLD) activity score, fibrosis progression and metabolic abnormalities such as hyperinsulinemia and obesity were more pronounced in ob/ob mice than in C57BL/6J and KK-A^y mice. In ob/ob mice fed FFD, the development of hepatic crown-like structures was confirmed. Furthermore, molecular pathways involved in steatohepatitis and fibrosis showed significant changes from as early as 2 weeks of starting the FFD regime. Ob/ob mice fed FFD showed metabolic, histological, and transcriptomic dysfunctions similar to human NASH, suggesting their potential as an experimental model to discover novel drugs for NASH.

Summary: We revealed that ob/ob mice under fast food diet (FFD) regime develop metabolic, histological and transcriptomic dysfunctions similar to human non-alcoholic steatohepatitis (NASH).

Cholesterol Crystals in Hepatocyte Lipid Droplets Are Strongly Associated With Human Nonalcoholic Steatohepatitis

It is unclear what drives the development of fibrosing nonalcoholic steatohepatitis (NASH). We aimed to determine whether cholesterol crystallization within hepatocyte lipid droplets (LDs) distinguishes patients with fibrosing NASH from patients with isolated hepatic steatosis and to study pathways leading to cholesterol accumulation in hepatocyte LDs. Patients with fibrosing NASH (n = 16) were compared to patients with isolated steatosis (n = 14). Almost all patients with fibrosing NASH had free cholesterol staining by filipin (16/16) and cholesterol crystals (15/16) in hepatocyte LDs, mostly in association with the LD membrane, compared to only 3/14 with cholesterol crystals and 3/14 with faint filipin staining in patients with isolated steatosis (P < 0.05). We were unable to identify significant differences in the expression of genes in liver tissue related to cholesterol homeostasis or LD proteins between patients with fibrosing NASH and isolated steatosis. Human hepatoma cell line (HepG2) cells were supplemented with low-density lipoprotein (LDL)-cholesterol and oleic acid to develop large LDs, similar to those observed in patients with NASH. Fluorescent markers were used to track the uptake and intracellular trafficking of LDL-cholesterol. LDL-cholesterol was taken up by HepG2 cells and transported through the endosomal-lysosomal compartment directly to LDs, suggesting direct contact sites between late endosomes and LDs. Exposure of HepG2 cells to LDL-cholesterol resulted in a high concentration of cholesterol and cholesterol crystallization in LDs. Conclusion: Excess cholesterol is stored in the liver primarily within hepatocyte LDs where it can crystallize. Our findings are best explained by direct transport of cholesterol from late endosomes/lysosomes to LDs in hepatocytes. We found a strong association between the presence of LD cholesterol crystals and the development of fibrosing NASH in humans, suggesting a causal relationship.

Freshwater clam extract reduces liver injury by lowering cholesterol accumulation, improving dysregulated cholesterol synthesis and alleviating inflammation in high-fat, high-cholesterol and cholic acid diet-induced steatohepatitis in mice

Freshwater clam (Corbicula fluminea) is a traditional liver-protective food in Asia. Recent studies have renewed attention on high cholesterol accumulation and dysregulated cholesterol synthesis in the liver as a critical factor in the progression of nonalcoholic fatty liver disease (NAFLD) to nonalcoholic steatohepatitis (NASH). In this study, we investigated the protective effects of freshwater clam extract (FCE) and its fat fraction (FCE oil) on high-fat, high-cholesterol and cholic acid (HFHC) diet-induced lean steatohepatitis in mice. Mice were fed a HFHC diet containing FCE or FCE oil for 6 weeks. FCE, but not FCE oil, feeding reduced liver injury as indicated by decreased plasma alanine aminotransferase activity. Liver total cholesterol accumulation was reduced after FCE and FCE oil treatment. Accumulation of squalene and desmosterol, the precursors of cholesterol, in the liver was reduced by FCE but not by FCE oil. The caspase-1 (p10) and interleukin (IL)-1β (p17) protein expressions in the liver were suppressed by both FCE and FCE oil. Therefore, FCE may act as functional food that can reduce steatohepatitis and liver injury by reducing cholesterol accumulation, improving dysregulated cholesterol synthesis and attenuating inflammation.

Adapted Immune Responses of Myeloid-Derived Cells in Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD) is considered to be one of the most frequent chronic liver diseases worldwide and is associated with an increased risk of developing liver cirrhosis and hepatocellular carcinoma. Hepatic macrophages, mainly comprising monocyte derived macrophages and tissue resident Kupffer cells, are characterized by a high diversity and plasticity and act as key regulators during NAFLD progression, in conjunction with other infiltrating myeloid cells like neutrophils or dendritic cells. The activation and polarization of myeloid immune cells is influenced by dietary components, inflammatory signals like danger-associated molecular patterns (DAMPs) or cytokines as well as gut-derived inflammatory factors such as pathogen-associated molecular patterns (PAMPs). The functionality of myeloid leukocytes in the liver is directly linked to their inflammatory polarization, which is shaped by local and systemic inflammatory mediators such as cytokines, chemokines, PAMPs, and DAMPs. These environmental signals provoke intracellular adaptations in myeloid cells, including inflammasome and transcription factor activation, inflammatory signaling pathways, or switches in cellular metabolism. Dietary changes and obesity also promote a dysbalance in intestinal microbiota, which can facilitate intestinal permeability and bacterial translocation. The aim of this review is to highlight recent findings on the activating pathways of innate immune cells during the progression of NAFLD, dissecting local hepatic and systemic signals, dietary and metabolic factors as well as pathways of the gut-liver axis. Understanding the mechanism by which plasticity of myeloid-derived leukocytes is related to metabolic changes and NAFLD progression may provide options for new therapeutic approaches.

Role of Innate Immune Signaling in Non-Alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD) has become the most epidemic liver disease worldwide owing to rapid changes in lifestyle over the past few decades. This chronic condition intertwines with low-grade inflammation and metabolic disequilibrium, and potentiates the onset and progression of devastating hepatic and extrahepatic complications. In addition to an integral role in promoting host defense, recent studies also implicate innate immune signaling in a multitude of processes that control the progression of NAFLD. The focus of this review is to highlight emerging evidence regarding the role of innate immunity in NAFLD and the integration of different pathways that affect both inflammation and metabolism across the spectrum of this liver morbidity.

NLRP3 Inflammasome and IL-33: Novel Players in Sterile Liver Inflammation

In sterile liver inflammation, danger signals are released in response to tissue injury to alert the immune system; e.g., by activation of the NLRP3 inflammasome. Recently, IL-33 has been identified as a novel type of danger signal or “alarmin”, which is released from damaged and necrotic cells. IL-33 is a pleiotropic cytokine that targets a broad range of immune cells and exhibits pro- and anti-inflammatory properties dependent on the disease. This review summarizes the immunomodulatory roles of the NLRP3 inflammasome and IL-33 in sterile liver inflammation and highlights potential therapeutic strategies targeting these pathways in liver disease.

Macrophages in Nonalcoholic Steatohepatitis: Friend or Foe?

Nonalcoholic steatohepatitis (NASH) is a subtype of nonalcoholic fatty liver disease that is characterised by steatosis, chronic inflammation, and hepatocellular injury with or without fibrosis. The role and activation of macrophages in the pathogenesis of NASH is complex and is being studied for possible therapeutic options to help the millions of people diagnosed with the disease. The purpose of this review is to discuss the pathogenesis of NASH through the activation and role of Kupffer cells and other macrophages in causing inflammation and progression of NASH. Furthermore, this review aims to outline some of the current therapeutic options targeting the pathogenesis of NASH.

Role of Inflammasomes in the Development of Gastrointestinal Diseases

Many diseases of the gastrointestinal tract have been attributed to chronic inflammation, and a few have identified the role of inflammasomes in their pathogenesis. Inflammasomes are a group of protein complexes comprising of several intracellular proteins that link the sensing of microbial products and metabolic stress to the proteolytic activation of the proinflammatory cytokines. Recent studies have implicated activation of several families of NOD-like receptors (NLRs) which are major components of inflammasomes in the development and exacerbation of many diseases of human systems. In this chapter, we discuss the role of inflammasomes in some of the most prevalent diseases of the gastrointestinal tract and highlight potential targets for treatment.

Pathogenesis of NASH: How Metabolic Complications of Overnutrition Favour Lipotoxicity and Pro-Inflammatory Fatty Liver Disease

Overnutrition, usually with obesity and genetic predisposition, lead to insulin resistance, which is an invariable accompaniment of nonalcoholic fatty liver disease (NAFLD). The associated metabolic abnormalities, pre- or established diabetes, hypertension and atherogenic dyslipidemia (clustered as metabolic syndrome) tend to be worse for nonalcoholic steatohepatitis (NASH), revealing it as part of a continuum of metabolic pathogenesis. The origins of hepatocellular injury and lobular inflammation which distinguish NASH from simple steatosis have intrigued investigators, but it is now widely accepted that NASH results from liver lipotoxicity. The key issue is not the quantity of liver fat but the type(s) of lipid molecules that accumulate, and how they are "packaged" to avoid subcellular injury. Possible lipotoxic mediators include free (unesterified) cholesterol, saturated free fatty acids, diacylglycerols, lysophosphatidyl-choline, sphingolipids and ceramide. Lipid droplets are intracellular storage organelles for non-structural lipid whose regulation is influenced by genetic polymorphisms, such as PNPLA3. Cells unable to sequester chemically reactive lipid molecules undergo mitochondrial injury, endoplasmic reticulum (ER) stress and autophagy, all processes of interest for NASH pathogenesis. Lipotoxicity kills hepatocytes by apoptosis, a highly regulated, non-inflammatory form of cell death, but also by necrosis, necroptosis and pyroptosis; the latter involve mitochondrial injury, oxidative stress, activation of c-Jun N-terminal kinase (JNK) and release of danger-associated molecular patterns (DAMPs). DAMPs stimulate innate immunity by binding pattern recognition receptors, such as Toll-like receptor 4 (TLR4) and the NOD-like receptor protein 3 (NLRP3) inflammasome, which release a cascade of pro-inflammatory chemokines and cytokines. Thus, lipotoxic hepatocellular injury attracts inflammatory cells, particularly activated macrophages which surround ballooned hepatocytes as crown-like structures. In both experimental and human NASH, livers contain cholesterol crystals which are a second signal for NLRP3 activation; this causes interleukin (IL)-1β and IL18 secretion to attract and activate macrophages and neutrophils. Injured hepatocytes also liberate plasma membrane-derived extracellular vesicles; these have been shown to circulate in NASH and to be pro-inflammatory. The way metabolic dysfunction leads to lipotoxicity, innate immune responses and the resultant pattern of cellular inflammation in the liver are likely also relevant to hepatic fibrogenesis and hepatocarcinogenesis. Pinpointing the key molecules involved pharmacologically should eventually lead to effective pharmacotherapy against NASH.

A longitudinal study of whole body, tissue, and cellular physiology in a mouse model of fibrosing NASH with high fidelity to the human condition

The sequence of events that lead to inflammation and fibrosing nonalcoholic steatohepatitis (NASH) is incompletely understood. Hence, we investigated the chronology of whole body, tissue, and cellular events that occur during the evolution of diet-induced NASH. Male C57Bl/6 mice were assigned to a fast-food (FF; high calorie, high cholesterol, high fructose) or standard-chow (SC) diet over a period of 36 wk. Liver histology, body composition, mitochondrial respiration, metabolic rate, gene expression, and hepatic lipid content were analyzed. Insulin resistance [homeostasis model assessment-insulin resistance (HOMA-IR)] increased 10-fold after 4 wk. Fibrosing NASH was fully established by 16 wk. Total hepatic lipids increased by 4 wk and remained two- to threefold increased throughout. Hepatic triglycerides declined from sixfold increase at 8 wk to threefold increase by 36 wk. In contrast, hepatic cholesterol levels steadily increased from baseline at 8 wk to twofold by 36 wk. The hepatic immune cell population altered over time with macrophages persisting beyond 16 wk. Mitochondrial oxygen flux rates of FF mice diet were uniformly lower with all the tested substrates (13-276 pmol·s^-1·ml^-1 per unit citrate synthase) than SC mice (17-394 pmol·s^-1·ml^-1 per unit citrate synthase) and was accompanied by decreased mitochondrial:nuclear gene copy number ratios after 4 wk. Metabolic rate was lower in FF mice. Mitochondrial glutathione was significantly decreased at 24 wk in FF mice. Expression of dismutases and catalase was also decreased in FF mice. The evolution of NASH in the FF diet-induced model is multiphasic, particularly in terms of hepatic lipid composition. Insulin resistance precedes hepatic inflammation and fibrosis. Mitochondrial dysfunction and depletion occur after the histological features of NASH are apparent. Collectively, these observations provide a unique overview of the sequence of changes that coevolve with the histological evolution of NASH.NEW & NOTEWORTHY This study demonstrates in a first of kind longitudinal analysis, the evolution of nonalcoholic steatohepatitis (NASH) on a fast-food diet-induced model. Key findings include 1) hepatic lipid composition changes in a multiphasic fashion as NASH evolves; 2) insulin resistance precedes hepatic inflammation and fibrosis, answering a longstanding chicken-and-egg question regarding the relationship of insulin resistance to liver histology in NASH; and 3) mitochondrial dysfunction and depletion occur after the histological features of NASH are apparent.

NLRP3 inflammasome blockade reduces liver inflammation and fibrosis in experimental NASH in mice

BACKGROUND & AIMS

NOD-like receptor protein 3 (NLRP3) inflammasome activation occurs in Non-alcoholic fatty liver disease (NAFLD). We used the first small molecule NLRP3 inhibitor, MCC950, to test whether inflammasome blockade alters inflammatory recruitment and liver fibrosis in two murine models of steatohepatitis.

METHODS

We fed foz/foz and wild-type mice an atherogenic diet for 16weeks, gavaged MCC950 or vehicle until 24weeks, then determined NAFLD phenotype. In mice fed an methionine/choline deficient (MCD) diet, we gavaged MCC950 or vehicle for 6weeks and determined the effects on liver fibrosis.

RESULTS

In vehicle-treated foz/foz mice, hepatic expression of NLRP3, pro-IL-1β, active caspase-1 and IL-1β increased at 24weeks, in association with cholesterol crystal formation and NASH pathology; plasma IL-1β, IL-6, MCP-1, ALT/AST all increased. MCC950 treatment normalized hepatic caspase 1 and IL-1β expression, plasma IL-1β, MCP-1 and IL-6, lowered ALT/AST, and reduced the severity of liver inflammation including designation as NASH pathology, and liver fibrosis. In vitro, cholesterol crystals activated Kupffer cells and macrophages to release IL-1β; MCC950 abolished this, and the associated neutrophil migration. MCD diet-fed mice developed fibrotic steatohepatitis; MCC950 suppressed the increase in hepatic caspase 1 and IL-1β, lowered numbers of macrophages and neutrophils in the liver, and improved liver fibrosis.

CONCLUSION

MCC950, an NLRP3 selective inhibitor, improved NAFLD pathology and fibrosis in obese diabetic mice. This is potentially attributable to the blockade of cholesterol crystal-mediated NLRP3 activation in myeloid cells. MCC950 reduced liver fibrosis in MCD-fed mice. Targeting NLRP3 is a logical direction in pharmacotherapy of NASH.

LAY SUMMARY

Fatty liver disease caused by being overweight with diabetes and a high risk of heart attack, termed non-alcoholic steatohepatitis (NASH), is the most common serious liver disease with no current treatment. There could be several causes of inflammation in NASH, but activation of a protein scaffold within cells termed the inflammasome (NLRP3) has been suggested to play a role. Here we show that cholesterol crystals could be one pathway to activate the inflammasome in NASH. We used a drug called MCC950, which has already been shown to block NLRP3 activation, in an attempt to reduce liver injury in NASH. This drug partly reversed liver inflammation, particularly in obese diabetic mice that most closely resembles the human context of NASH. In addition, such dampening of liver inflammation in NASH achieved with MCC950 partly reversed liver scarring, the process that links NASH to the development of cirrhosis.

Cholesterol crystallization within hepatocyte lipid droplets and its role in murine NASH

We recently reported that cholesterol crystals form in hepatocyte lipid droplets (LDs) in human and experimental nonalcoholic steatohepatitis. Herein, we assigned WT C57BL/6J mice to a high-fat (15%) diet for 6 months, supplemented with 0%, 0.25%, 0.5%, 0.75%, or 1% dietary cholesterol. Increasing dietary cholesterol led to cholesterol loading of the liver, but not of adipose tissue, resulting in fibrosing steatohepatitis at a dietary cholesterol concentration of ≥0.5%, whereas mice on lower-cholesterol diets developed only simple steatosis. Hepatic cholesterol crystals and crown-like structures also developed at a dietary cholesterol concentration ≥0.5%. Crown-like structures consisted of activated Kupffer cells (KCs) staining positive for NLRP3 and activated caspase 1, which surrounded and processed cholesterol crystal-containing remnant LDs of dead hepatocytes. The KCs processed LDs at the center of crown-like structures in the extracellular space by lysosomal enzymes, ultimately transforming into lipid-laden foam cells. When HepG2 cells were exposed to LDL cholesterol, they developed cholesterol crystals in LD membranes, which caused activation of THP1 cells (macrophages) grown in coculture; upregulation of TNF-alpha, NLRP3, and interleukin 1beta (IL1β) mRNA; and secretion of IL-1beta. In conclusion, cholesterol crystals form on the LD membrane of hepatocytes and cause activation and cholesterol loading of KCs that surround and process these LDs by lysosomal enzymes.

The effect of cannabinoid receptor 1 blockade on hepatic free fatty acid profile in mice with nonalcoholic fatty liver disease

We used rimonabant to investigate the role of CB1 receptor on hepatic FFAs profile during NAFLD. Male mice C57BL/6 were divided into: control group fed with control diet 20 weeks (C; n=6); group fed with HFD 20 weeks (HF; n=6); group fed with control diet and treated with rimonabant after 18 weeks (R; n=9); group fed with HFD and treated with rimonabant after 18 weeks (HFR; n=10). Rimonabant (10mg/kg) was administered daily to HFR and R group by oral gavage. Rimonabant decreased liver palmitic acid proportion in HFR group compared to HF group (p<0.05). Liver stearic and oleic acid proportions were decreased in R group compared to control (p<0.01 respectively). Rimonabant increased liver linoleic and arachidonic acid proportions in HFR group compared to HF group (p<0.01 respectively). CB1 blockade may be useful in the treatment of HFD-induced NAFLD due to modulation of plasma lipid and hepatic FFA profile.

FOXO transcription factors protect against the diet-induced fatty liver disease

Forkhead O transcription factors (FOXOs) have been implicated in glucose and lipid homeostasis; however, the role of FOXOs in the development of nonalcoholic fatty liver disease (NAFLD) is not well understood. In this study, we designed experiments to examine the effects of two different diets-very high fat diet (HFD) and moderately high fat plus cholesterol diet (HFC)-on wildtype (WT) and liver-specific Foxo1/3/4 triple knockout mice (LTKO). Both diets induced severe hepatic steatosis in the LTKO mice as compared to WT controls. However, the HFC diet led to more severe liver injury and fibrosis compared to the HFD diet. At the molecular levels, hepatic Foxo1/3/4 deficiency triggered a significant increase in the expression of inflammatory and fibrotic genes including Emr1, Ccl2, Col1a1, Tgfb, Pdgfrb, and Timp1. Thus, our data suggest that FOXO transcription factors play a salutary role in the protection against the diet-induced fatty liver disease.

Key Events Participating in the Pathogenesis of Alcoholic Liver Disease

Alcoholic liver disease (ALD) is a leading cause of morbidity and mortality worldwide. It ranges from fatty liver to steatohepatitis, fibrosis, cirrhosis and hepatocellular carcinoma. The most prevalent forms of ALD are alcoholic fatty liver, alcoholic hepatitis (AH) and alcoholic cirrhosis, which frequently progress as people continue drinking. ALD refers to a number of symptoms/deficits that contribute to liver injury. These include steatosis, inflammation, fibrosis and cirrhosis, which, when taken together, sequentially or simultaneously lead to significant disease progression. The pathogenesis of ALD, influenced by host and environmental factors, is currently only partially understood. To date, lipopolysaccharide (LPS) translocation from the gut to the portal blood, aging, gender, increased infiltration and activation of neutrophils and bone marrow-derived macrophages along with alcohol plus iron metabolism, with its associated increase in reactive oxygen species (ROS), are all key events contributing to the pathogenesis of ALD. This review aims to introduce the reader to the concept of alcohol-mediated liver damage and the mechanisms driving injury.

The Role of Tissue Macrophage-Mediated Inflammation on NAFLD Pathogenesis and Its Clinical Implications

The obese phenotype is characterized by a state of chronic low-grade systemic inflammation that contributes to the development of comorbidities, including nonalcoholic fatty liver disease (NAFLD). In fact, NAFLD is often associated with adipocyte enlargement and consequent macrophage recruitment and inflammation. Macrophage polarization is often associated with the proinflammatory state in adipose tissue. In particular, an increase of M1 macrophages number or of M1/M2 ratio triggers the production and secretion of various proinflammatory signals (i.e., adipocytokines). Next, these inflammatory factors may reach the liver leading to local M1/M2 macrophage polarization and consequent onset of the histological damage characteristic of NAFLD. Thus, the role of macrophage polarization and inflammatory signals appears to be central for pathogenesis and progression of NAFLD, even if the heterogeneity of macrophages and molecular mechanisms that govern their phenotype switch remain incompletely understood. In this review, we discuss the role of adipose and liver tissue macrophage-mediated inflammation in experimental and human NAFLD. This focus is relevant because it may help researchers that approach clinical and experimental studies on this disease advancing the knowledge of mechanisms that could be targeted in order to revert NAFLD-related fibrosis.

SREBP-2/PNPLA8 axis improves non-alcoholic fatty liver disease through activation of autophagy

Dysregulated autophagy is associated with steatosis and non-alcoholic fatty liver disease (NAFLD), however the mechanisms connecting them remain poorly understand. Here, we show that co-administration of lovastatin and ezetimibe (L/E) significantly reverses hepatic triglyceride accumulation concomitant with an increase in SREBP-2 driven autophagy in mice fed a high-fat diet (HFD). We further show that the statin mediated increase in SREBP-2 directly activates expression of patatin-like phospholipase domain-containing enzyme 8 (PNPLA8) gene, and PNPLA8 associates with autophagosomes and is associated with a decrease in cellular triglyceride. Moreover, we show that over-expression of PNPLA8 dramatically decreases hepatic steatosis through increased autophagy in hepatocytes of HFD-fed mice. Live-cell imaging analyses also reveal that PNPLA8 dynamically interacts with LC3 and we suggest that the SREBP-2/PNPLA8 axis represents a novel regulatory mechanism for lipid homeostasis. These data provide a possible mechanism for the reported beneficial effects of statins for decreasing hepatic triglyceride levels in NAFLD patients.

Intracellular cholesterol transport proteins: roles in health and disease

Effective cholesterol homoeostasis is essential in maintaining cellular function, and this is achieved by a network of lipid-responsive nuclear transcription factors, and enzymes, receptors and transporters subject to post-transcriptional and post-translational regulation, whereas loss of these elegant, tightly regulated homoeostatic responses is integral to disease pathologies. Recent data suggest that sterol-binding sensors, exchangers and transporters contribute to regulation of cellular cholesterol homoeostasis and that genetic overexpression or deletion, or mutations, in a number of these proteins are linked with diseases, including atherosclerosis, dyslipidaemia, diabetes, congenital lipoid adrenal hyperplasia, cancer, autosomal dominant hearing loss and male infertility. This review focuses on current evidence exploring the function of members of the ‘START’ (steroidogenic acute regulatory protein-related lipid transfer) and ‘ORP’ (oxysterol-binding protein-related proteins) families of sterol-binding proteins in sterol homoeostasis in eukaryotic cells, and the evidence that they represent valid therapeutic targets to alleviate human disease.