Role of acid sphingomyelinase of Kupffer cells in cholestatic liver injury in mice

Protaetia brevitarsis larvae extract protects against lipopolysaccharides-induced ferroptosis and inflammation by inhibiting acid sphingomyelinase

BACKGROUND/OBJECTIVES

Inflammation and ferroptosis are implicated in various diseases and lipopolysaccharides (LPS) have been linked with these disorders. Recently, many edible insects, such as Gryllus bimaculatus, Protaetia brevitarsis larvae (PB) and Tenebrio molitor larvae, have been recommended as alternative foods because they contain lots of nutritional sources. In this study, we explored the potential of PB extract in preventing LPS-induced inflammation and ferroptosis in Hep3B cells.

MATERIALS/METHODS

PB powder was extracted using 70% ethanol and applied to Hep3B cells. Co-treatment with LPS was conducted to induce ferroptosis and inflammation. The anti-inflammatory and anti-ferroptosis mechanisms of the PB extract were confirmed using Western blot, enzyme-linked immunosorbent assay, and real-time polymerase chain reaction analysis.

RESULTS

PB extract effectively prevented LPS-induced cell death and restored LPS-induced inflammatory cytokine production, NF-κB signaling, endoplasmic reticulum (ER) stress and ferroptosis. Interestingly, PB extract reduced LPS-induced ceramide increase and acid sphingomyelinase (ASMase) expression. The use of the ASMase inhibitor, desipramine, also demonstrated a reduction in these pathways, highlighting the pivotal role of ASMase in inflammation and ferroptosis. Treatment with each inhibitor revealed that ferroptosis causes ER stress and that NF-κB and MAP kinase pathways are involved in inflammation.

CONCLUSION

PB emerges as a potential functional food with inhibitory effects on LPS-induced inflammation and ferroptosis, making it a promising candidate for nutritional interventions.

Comparing animal well-being between bile duct ligation models

A prevailing animal model currently used to study severe human diseases like obstructive cholestasis, primary biliary or sclerosing cholangitis, biliary atresia, and acute liver injury is the common bile duct ligation (cBDL). Modifications of this model include ligation of the left hepatic bile duct (pBDL) or ligation of the left bile duct with the corresponding left hepatic artery (pBDL+pAL). Both modifications induce cholestasis only in the left liver lobe. After induction of total or partial cholestasis in mice, the well-being of these animals was evaluated by assessing burrowing behavior, body weight, and a distress score. To compare the pathological features of these animal models, plasma levels of liver enzymes, bile acids, bilirubin, and within the liver tissue, necrosis, fibrosis, inflammation, as well as expression of genes involved in the synthesis or transport of bile acids were assessed. The survival rate of the animals and their well-being was comparable between pBDL+pAL and pBDL. However, surgical intervention by pBDL+pAL caused confluent necrosis and collagen depositions at the edge of necrotic tissue, whereas pBDL caused focal necrosis and fibrosis in between portal areas. Interestingly, pBDL animals had a higher survival rate and their well-being was significantly improved compared to cBDL animals. On day 14 after cBDL liver aspartate, as well as alanine aminotransferase, alkaline phosphatase, glutamate dehydrogenase, bile acids, and bilirubin were significantly elevated, but only glutamate dehydrogenase activity was increased after pBDL. Thus, pBDL may be primarily used to evaluate local features such as inflammation and fibrosis or regulation of genes involved in bile acid synthesis or transport but does not allow to study all systemic features of cholestasis. The pBDL model also has the advantage that fewer mice are needed, because of its high survival rate, and that the well-being of the animals is improved compared to the cBDL animal model.

Limitations of PLX3397 as a microglial investigational tool: peripheral and off-target effects dictate the response to inflammation

Microglia, the resident macrophages of the central nervous system (CNS), play a critical role in CNS homeostasis and neuroinflammation. Pexidartinib (PLX3397), a colony-stimulating factor 1 (CSF1) receptor inhibitor, is widely used to deplete microglia, offering flexible options for both long-term depletion and highly versatile depletion-repopulation cycles. However, the potential impact of PLX3397 on peripheral (immune) cells remains controversial. Until now, the microglia-specificity of this type of compounds has not been thoroughly evaluated, particularly in the context of peripherally derived neuroinflammation. Our study addresses this gap by examining the effects of PLX3397 on immune cells in the brain, liver, circulation and bone marrow, both in homeostasis and systemic inflammation models. Intriguingly, we demonstrate that PLX3397 treatment not only influences the levels of tissue-resident macrophages, but also affects circulating and bone marrow immune cells beyond the mononuclear phagocyte system (MPS). These alterations in peripheral immune cells disrupt the response to systemic inflammation, consequently impacting the phenotype irrespective of microglial depletion. Furthermore, we observed that a lower dose of PLX3397, which does not deplete microglia, demonstrates similar (non-)MPS effects, both in the periphery and the brain, but fails to fully replicate the peripheral alterations seen in the higher doses, questioning lower doses as a 'peripheral control' strategy. Overall, our data highlight the need for caution when interpreting studies employing this compound, as it may not be suitable for specific investigation of microglial function in the presence of systemic inflammation.

Hepatocytic AP-1 and STAT3 contribute to chemotaxis in alphanaphthylisothiocyanate-induced cholestatic liver injury

The development of cholestatic liver injury (CLI) involves inflammation, but the dominant pathway mediating the chemotaxis is not yet established. This work explored key signaling pathway mediating chemotaxis in CLI and the role of Kupffer cells in the inflammatory liver injury. Probe inhibitors T-5224 (100 mg/kg) for AP-1 and C188-9 (100 mg/kg) for STAT3 were used to validate key inflammatory pathways in alpha-naphthylisothiocyanate (ANIT, 100 mg/kg)-induced CLI. Two doses of GdCl₃ (10 mg/kg and 40 mg/kg) were used to delete Kupffer cells and explore their role in CLI. Serum and liver samples were collected for biochemical and mechanism analysis. The liver injury in ANIT-treated mice were significantly increased supported by biochemical and histopathological changes, and neutrophils gathering around the necrotic loci. Inhibitor treatments down-regulated liver injury biomarkers except the level of total bile acid. The chemokine Ccl2 increased by 170-fold and to a less degree Cxcl2 by 45-fold after the ANIT treatment. p-c-Jun and p-STAT3 were activated in the group A but inhibited by the inhibitors in western blot analysis. The immunofluorescence results showed AP-1 not STAT3 responded to inhibitors in ANIT-induced CLI. With or without GdCl₃, there was no significant difference in liver injury among the CLI groups. In necrotic loci in CLI, CXCL2 colocalized with hepatocyte biomarker Albumin, not with the F4/80 in Kupffer cells. Conclusively, AP-1 played a more critical role in the inflammation cascade than STAT3 in ANIT-induced CLI. Hepatocytes, not the Kupffer cells released chemotactic factors mediating the chemotaxis in CLI.

The role of platelet mediated thromboinflammation in acute liver injury

Acute liver injuries have wide and varied etiologies and they occur both in patients with and without pre-existent chronic liver disease. Whilst the pathophysiological mechanisms remain distinct, both acute and acute-on-chronic liver injury is typified by deranged serum transaminase levels and if severe or persistent can result in liver failure manifest by a combination of jaundice, coagulopathy and encephalopathy. It is well established that platelets exhibit diverse functions as immune cells and are active participants in inflammation through processes including immunothrombosis or thromboinflammation. Growing evidence suggests platelets play a dualistic role in liver inflammation, shaping the immune response through direct interactions and release of soluble mediators modulating function of liver sinusoidal endothelial cells, stromal cells as well as migrating and tissue-resident leucocytes. Elucidating the pathways involved in initiation, propagation and resolution of the immune response are of interest to identify therapeutic targets. In this review the provocative role of platelets is outlined, highlighting beneficial and detrimental effects in a spatial, temporal and disease-specific manner.

Heterogeneity and Function of Kupffer Cells in Liver Injury

Kupffer cells (KCs) are key regulators of liver immunity composing the principal part of hepatic macrophages even body tissue macrophages. They reside in liver sinusoids towards portal vein. The micro-environment shapes KCs unique immunosuppressive features and functions. KCs express specific surface markers that distinguish from other liver macrophages. By engulfing gut-derived foreign products and apoptotic cells without triggering excessive inflammation, KCs maintain homeostasis of liver and body. Heterogeneity of KCs has been identified in different studies. In terms of the origin, adult KCs are derived from progenitors of both embryo and adult bone marrow. Embryo-derived KCs compose the majority of KCs in healthy and maintain by self-renewal. Bone marrow monocytes replenish massively when embryo-derived KC proliferation are impaired. The phenotype of KCs is also beyond the traditional dogma of M1-M2. Functionally, KCs play central roles in pathogenesis of acute and chronic liver injury. They contribute to each pathological stage of liver disease. By initiating inflammation, regulating fibrosis, cirrhosis and tumor cell proliferation, KCs contribute to the resolution of liver injury and restoration of tissue architecture. The underlying mechanism varied by damage factors and pathology. Understanding the characteristics and functions of KCs may provide opportunities for the therapy of liver injury. Herein, we attempt to afford insights on heterogeneity and functions of KCs in liver injury using the existing findings.

TGR5 protects against cholestatic liver disease via suppressing the NF-κB pathway and activating the Nrf2/HO-1 pathway

Background

Characterized by the presence of inflammation, fibrosis, and bile duct proliferation, cholestatic liver disease (CLD) affects people of all age groups. Takeda G-protein-coupled receptor (TGR5) has been implicated in the suppression of inflammation via toll-like receptor 4 (TLR4) and nuclear factor kappa B (NF-κB). Kupffer cells and their M1 polarization play important roles in inflammation and cholestatic liver injury via production of pro-inflammatory cytokines. Nevertheless, the function of TGR5 signaling in CLD is largely unknown.

Methods

We conducted liver tissue experiments, animal experiments, serum marker testing, liver histology analysis, Kupffer cell experiments, RNA extraction and Real-time PCR, western blotting, evaluation of ROS production by flow cytometry and statistical differences were analyzed by student t-test using GraphPad Prism.

Results

We found that serum bile acid (BA) and TGR5 levels were elevated in patients with cholestasis cirrhosis. Knockout of TGR5 in animals significantly increased bile duct ligation (BDL)-caused liver injury through increasing oxidative stress, promoting M1-predominant polarization of Kupffer cells, and elevating the serum levels of inflammatory cytokines. In contrast, TGR5 activation inhibited ROS production, secretion of pro-inflammatory cytokines, and M1-predominant polarization of Kupffer cells. Moreover, results showed that TGR5 exerted its effects via suppressing NF-κB signaling and activating nuclear factor 2 (Nrf2)/HO-1 signaling. Finally, the effect of TGR5 on cholestatic liver damage was also confirmed in vivo.

Conclusions

TGR5 activation protected against BDL-induced CLD by both suppressing inflammation via inhibiting the NF-κB pathway and reducing ROS production via activation of Nrf2/HO-1 signaling. These findings show the importance of TGR5 in CLD and provide new insight into therapeutic strategies for CLD.

Hepatic macrophages in liver homeostasis and diseases-diversity, plasticity and therapeutic opportunities

Macrophages, which are key cellular components of the liver, have emerged as essential players in the maintenance of hepatic homeostasis and in injury and repair processes in acute and chronic liver diseases. Upon liver injury, resident Kupffer cells (KCs) sense disturbances in homeostasis, interact with hepatic cell populations and release chemokines to recruit circulating leukocytes, including monocytes, which subsequently differentiate into monocyte-derived macrophages (MoMϕs) in the liver. Both KCs and MoMϕs contribute to both the progression and resolution of tissue inflammation and injury in various liver diseases. The diversity of hepatic macrophage subsets and their plasticity explain their different functional responses in distinct liver diseases. In this review, we highlight novel findings regarding the origins and functions of hepatic macrophages and discuss the potential of targeting macrophages as a therapeutic strategy for liver disease.

Macrophage MerTK Promotes Liver Fibrosis in Nonalcoholic Steatohepatitis

Nonalcoholic steatohepatitis (NASH) is emerging as a leading cause of chronic liver disease. However, therapeutic options are limited by incomplete understanding of the mechanisms of NASH fibrosis, which is mediated by activation of hepatic stellate cells (HSCs). In humans, human genetic studies have shown that hypomorphic variations in MERTK, encoding the macrophage c-mer tyrosine kinase (MerTK) receptor, provide protection against liver fibrosis, but the mechanisms remain unknown. We now show that holo- or myeloid-specific Mertk targeting in NASH mice decreases liver fibrosis, congruent with the human genetic data. Furthermore, ADAM metallopeptidase domain 17 (ADAM17)-mediated MerTK cleavage in liver macrophages decreases during steatosis to NASH transition, and mice with a cleavage-resistant MerTK mutant have increased NASH fibrosis. Macrophage MerTK promotes an ERK-TGFβ1 pathway that activates HSCs and induces liver fibrosis. These data provide insights into the role of liver macrophages in NASH fibrosis and provide a plausible mechanism underlying MERTK as a genetic risk factor for NASH fibrosis.

Crosstalk Between Acid Sphingomyelinase and Inflammasome Signaling and Their Emerging Roles in Tissue Injury and Fibrosis

Inflammasomes are a group of protein complexes that are assembled by pattern recognition receptors following the recognition of invading pathogens or host-derived danger signals. Inflammasomes such as NLRP3 mediate the activation of caspase-1 and the production of the proinflammatory cytokines IL-18 and IL-1β. Regulation of inflammasome signaling is critical for host defense against infections and maintenance of cellular homeostasis upon exposure to multiple harmful stimuli. Recent studies have highlighted an important role of acid sphingomyelinase (ASM) in regulating inflammasome activation. ASM hydrolyzes sphingomyelin to ceramide, which further fuses to large ceramide-enriched platforms functioning in stabilizing and amplifying molecules and receptors. Here, we will discuss the current understanding of the ASM-ceramide system in inflammasome activation, and how it contributes to multiple diseases. Insights into such mechanisms would pave the way for further exploration of novel diagnostic, preventative, and therapeutic targets against tissue injury and fibrosis.

Acid sphingomyelinase deficiency exacerbates LPS-induced experimental periodontitis

BACKGROUND

Mutation of the gene for acid sphingomyelinase (ASMase) causes Niemann-Pick disease. However, the effect of ASMase deficiency on periodontal health is unknown. Periodontal disease is a disease resulting from infection and inflammation of periodontal tissue and alveolar bone that support the teeth. The goal of this study was to determine the role of ASMase deficiency in periodontal inflammation and alveolar bone loss.

METHODS

We induced periodontitis in wild-type and ASMase-deficient (ASMase^-/- ) mice with periodontal lipopolysaccharide (LPS) injection and compared the alveolar bone loss and periodontal inflammation between these mice.

RESULTS

Results showed that ASMase deficiency did not significantly change metabolic parameters, but exacerbated LPS-induced alveolar bone loss, osteoclastogenesis, and periodontal tissue inflammation. To understand the mechanisms by which ASMase deficiency aggravates LPS-induced periodontitis, we analyzed sphingolipids in periodontal tissues. Results showed that ASMase deficiency led to increases in not only sphingomyelin, but also ceramide (CER), a bioactive sphingolipid known to promote inflammation. Results further showed that ASMase deficiency increased CER de novo synthesis.

CONCLUSION

ASMase deficiency exacerbated LPS-induced alveolar bone loss and periodontal inflammation. ASMase deficiency leads to an unexpected CER increase by stimulating de novo synthesis CER, which is likely to be involved in the ASMase deficiency-exacerbated periodontitis.

Bile salts regulate CYP7A1 expression and elicit a fibrotic response and abnormal lipid production in 3D liver microtissues

Disrupted regulation and accumulation of bile salts (BS) in the liver can contribute towards progressive liver damage and fibrosis. Here, we investigated the role of BS in the progression of cholestatic injury and liver fibrosis using 3D scaffold-free multicellular human liver microtissues (MTs) comprising the cell lines HepaRG, THP-1 and hTERT-HSCs. This in vitro model has been shown to recapitulate cellular events leading to fibrosis including hepatocellular injury, inflammation and activation of HSCs, ultimately leading to increased deposition of extracellular matrix (ECM). In order to better differentiate the contribution of individual cells during cholestasis, the effects of BS were evaluated either on each of the three cell types individually or on the multicellular MTs. Our data corroborate the toxic effects of BS on HepaRG cells and indicate that BS exposure elicited a slight increase in cytokines without causing stellate cell activation. Contrarily, using the MTs, we could demonstrate that low concentrations of BS led to cellular damage and triggered a fibrotic response. This indicates that cellular interplay is required to achieve BS-triggered activation of HSC. Moreover, BS were capable of down-regulating CYP7A1 expression in MTs and elicited abnormal lipid production (accumulation) concordant with clinical cases where chronic cholestasis results in hypercholesterolemia.

Liver Macrophages: Old Dogmas and New Insights

Inflammation is a hallmark of virtually all liver diseases, such as liver cancer, fibrosis, nonalcoholic steatohepatitis, alcoholic liver disease, and cholangiopathies. Liver macrophages have been thoroughly studied in human disease and mouse models, unravelling that the hepatic mononuclear phagocyte system is more versatile and complex than previously believed. Liver macrophages mainly consist of liver‐resident phagocytes, or Kupffer cells (KCs), and bone marrow‐derived recruited monocytes. Although both cell populations in the liver demonstrate principal functions of macrophages, such as phagocytosis, danger signal recognition, cytokine release, antigen processing, and the ability to orchestrate immune responses, KCs and recruited monocytes retain characteristic ontogeny markers and remain remarkably distinct on several functional aspects. While KCs dominate the hepatic macrophage pool in homeostasis (“sentinel function”), monocyte‐derived macrophages prevail in acute or chronic injury (“emergency response team”), making them an interesting target for novel therapeutic approaches in liver disease. In addition, recent data acquired by unbiased large‐scale techniques, such as single‐cell RNA sequencing, unraveled a previously unrecognized complexity of human and murine macrophage polarization abilities, far beyond the old dogma of inflammatory (M1) and anti‐inflammatory (M2) macrophages. Despite tremendous progress, numerous challenges remain in deciphering the full spectrum of macrophage activation and its implication in either promoting liver disease progression or repairing injured liver tissue. Being aware of such heterogeneity in cell origin and function is of crucial importance when studying liver diseases, developing novel therapeutic interventions, defining macrophage‐based prognostic biomarkers, or designing clinical trials. Growing knowledge in gene expression modulation and emerging technologies in drug delivery may soon allow shaping macrophage populations toward orchestrating beneficial rather than detrimental inflammatory responses.

Role of macrophages in experimental liver injury and repair in mice

Liver macrophages make up the largest proportion of tissue macrophages in the host and consist of two dissimilar groups: Kupffer cells (KCs) and monocyte-derived macrophages (MoMø). As the liver is injured, KCs sense the injury and initiate inflammatory cascades mediated by the release of inflammatory cytokines and chemokines. Subsequently, inflammatory monocytes accumulate in the liver via chemokine-chemokine receptor interactions, resulting in massive inflammatory MoMø infiltration. When live r injury ceases, restorative macrophages, derived from recruited inflammatory monocytes (lymphocyte antigen 6 complex, locus C^hi monocytes), promote the resolution of hepatic damage and fibrosis. Consequently, a large number of studies have assessed the mechanisms by which liver macrophages exert their opposing functions at different time-points during liver injury. The present review primarily focuses on the diverse functions of macrophages in experimental liver injury, fibrosis and repair in mice and illustrates how macrophages may be targeted to treat liver disease.

Phenotypic switch of human and mouse macrophages and resultant effects on apoptosis resistance in hepatocytes

Acute-on-chronic liver failure (ACLF) carries a significant burden on critical care services and health care resources. However, the exact pathogenesis of ACLF remains to be elucidated, and novel treatments are desperately required. In our previous work, we utilized mice subjected to acute insult in the context of hepatic fibrosis to simulate the development of ACLF and documented the favorable hepatoprotection conferred by M2-like macrophages in vivo and in vitro. In the present study, we focused on the phenotypic switch of human and mouse macrophages and assessed the effects of this switch on apoptosis resistance in hepatocytes. For this purpose, human and mouse macrophages were isolated and polarized into M0, M(IFN-γ), M(IFN-γ→IL-4), M(IL-4) or M(IL-4→IFN-γ) subsets. Conditioned media (CM) from these subsets were applied to human and mouse hepatocytes followed by apoptosis induction. Cell apoptosis was evaluated by immunostaining for cleaved caspase-3. As a result, M(IFN-γ) or M(IL-4) macrophages switched their phenotype into M(IFN-γ→IL-4) or M(IL-4→IFN-γ) through reprogramming with IL-4 or IFN-γ, respectively. Importantly, hepatocytes pre-treated with M(IFN-γ→IL-4) CMs exhibited much weaker expression of cleaved caspase-3, compared to those pre-treated with M(IFN-γ) CM, and vice versa. Together, phenotypic switch of macrophages toward M(IL-4) phenotype confers hepatocytes enhanced resistance to apoptosis.

A new perspective on acute-on-chronic liver failure: Liver fibrosis and injury resistance

Acute-on-chronic liver failure (ACLF) is an increasingly recognized entity encompassing an acute deterioration of liver function in patients with pre-existing chronic liver diseases, which is usually associated with a precipitating event. Compared to acute liver failure, ACLF patients exhibit relatively slow disease progression and prolonged survival. Recent studies show that patients without previous decompensation have higher short-term mortality than those with prior hepatic decompensation. These interesting and important facts motivate clinicians and researchers to dissect the underlying mechanisms of ACLF from a new perspective, namely, the correlation between chronic liver diseases and injury resistance. In this review, we will make a comment on the phenomena as well as cellular and molecular mechanisms behind injury resistance in the setting of hepatic fibrosis (simulating the development of ACLF), in hopes of providing novel insights into the pathogenesis and therapy of ACLF.

Wnt/β-Catenin Signaling as a Potential Target for the Treatment of Liver Cirrhosis Using Antifibrotic Drugs

Cirrhosis is a form of liver fibrosis resulting from chronic hepatitis and caused by various liver diseases, including viral hepatitis, alcoholic liver damage, nonalcoholic steatohepatitis, and autoimmune liver disease. Cirrhosis leads to various complications, resulting in poor prognoses; therefore, it is important to develop novel antifibrotic therapies to counter liver cirrhosis. Wnt/β-catenin signaling is associated with the development of tissue fibrosis, making it a major therapeutic target for treating liver fibrosis. In this review, we present recent insights into the correlation between Wnt/β-catenin signaling and liver fibrosis and discuss the antifibrotic effects of the cAMP-response element binding protein/β-catenin inhibitor PRI-724.

Cellular Mechanisms of Hepatoprotection Mediated by M2-Like Macrophages

Background

Acute liver injury in the setting of hepatic fibrosis is an intriguing and still unsettled issue. We previously have demonstrated the protective effects conferred by M2-like macrophages in the fibrotic liver. In the present work, we further decipher the cellular mechanisms governing this hepatoprotection.

Material/Methods

Macrophages were isolated from control mice (M0 macrophages), then polarized into M1 or M2 phenotype using IFN-γ or IL-4, respectively. Conditioned media (CM) from M0, M1, and M2 macrophages were harvested and applied to M1 macrophages. Cell apoptosis was evaluated by immunostaining and real-time PCR. Similarly, human monocyte-derived macrophages were isolated and polarized, then M0, M1, and M2 CM were applied to HL-7702 or HepG2 cells followed by apoptosis induction. Cell apoptosis was assessed by flow cytometry.

Results

For the mouse conditioned medium experiment, stronger expression of cleaved caspase 3 and higher Bax/Bcl-2 mRNA ratio were found in M1 macrophages pretreated with M2 CM compared to those in M1 macrophages pretreated with M0 or M1 CM. Similarly, exposure of HL-7702 and HepG2 cells to either M0 or M1 CM had no significant effect on cell apoptosis. Nevertheless, the frequency of hepatocyte apoptosis was substantially reduced in HL-7702 (from 32.23±2.99 to 15.37±0.69 for Annexin V+/PI+ staining, p<0.01) and HepG2 cells (from 36.1±7.26 to 15.2±1.2 for Annexin V+/PI+ staining, p<0.01) with M2 CM pretreatment.

Conclusions

M2-like macrophages exert their hepatoprotective effect by promoting M1-like macrophage apoptosis but protecting against hepatocyte apoptosis.

Tumor necrosis factor-α-mediated hepatocyte apoptosis stimulates fibrosis in the steatotic liver in mice

Hepatocyte apoptosis has been implicated in the progression of nonalcoholic steatohepatitis. However, it is unclear whether the induction of tumor necrosis factor (TNF)‐α‐mediated hepatocyte apoptosis in the simple fatty liver triggers liver fibrosis. To address this question, high‐fat diet‐fed mice were repeatedly administered D‐galactosamine, which increases the sensitivity of hepatocytes to TNF‐α‐mediated apoptosis. In mice treated with a high‐fat diet plus D‐galactosamine, hepatocyte apoptosis and liver fibrosis were induced, whereas both apoptosis and fibrosis were inhibited in these mice following gut sterilization with antimicrobials or knockout of TNF‐α. Furthermore, liver fibrosis was diminished when hepatocyte apoptosis was inhibited by expressing a constitutively active inhibitor of nuclear factor κB kinase subunit β. Thus, hepatocyte apoptosis induced by intestinal dysbiosis or TNF‐α up‐regulation in the steatotic liver caused fibrosis. Organ fibrosis, including liver fibrosis, involves the interaction of cyclic adenosine monophosphate‐response element‐binding protein‐binding protein (CBP) and β‐catenin. Here, hepatocyte‐specific CBP‐knockout mice showed reduced liver fibrosis accompanied by hepatocyte apoptosis diminution; notably, liver fibrosis was also decreased in mice in which CBP was specifically knocked out in collagen‐producing cells because the activation of these cells was now suppressed. Conclusion: TNF‐α‐mediated hepatocyte apoptosis induced fibrosis in the steatotic liver, and inhibition of CBP/β‐catenin signaling attenuated the liver fibrosis due to the reduction of hepatocyte apoptosis and suppression of the activation of collagen‐producing cells. Thus, targeting CBP/β‐catenin may represent a new therapeutic strategy for treating fibrosis in nonalcoholic steatohepatitis. (Hepatology Communications 2018;2:407‐420)

M2-like macrophages in the fibrotic liver protect mice against lethal insults through conferring apoptosis resistance to hepatocytes

Acute injury in the setting of liver fibrosis is an interesting and still unsettled issue. Most recently, several prominent studies have indicated the favourable effects of liver fibrosis against acute insults. Nevertheless, the underlying mechanisms governing this hepatoprotection remain obscure. In the present study, we hypothesized that macrophages and their M1/M2 activation critically involve in the hepatoprotection conferred by liver fibrosis. Our findings demonstrated that liver fibrosis manifested a beneficial role for host survival and apoptosis resistance. Hepatoprotection in the fibrotic liver was tightly related to innate immune tolerance. Macrophages undertook crucial but divergent roles in homeostasis and fibrosis: depleting macrophages in control mice protected from acute insult; conversely, depleting macrophages in fibrotic liver weakened the hepatoprotection and gave rise to exacerbated liver injury upon insult. The contradictory effects of macrophages can be ascribed, to a great extent, to the heterogeneity in macrophage activation. Macrophages in fibrotic mice exhibited M2-preponderant activation, which was not the case in acutely injured liver. Adoptive transfer of M2-like macrophages conferred control mice conspicuous protection against insult. In vitro, M2-polarized macrophages protected hepatocytes against apoptosis. Together, M2-like macrophages in fibrotic liver exert the protective effects against lethal insults through conferring apoptosis resistance to hepatocytes.

Mechanisms of bile acid mediated inflammation in the liver

Bile acids are synthesized in the liver and are the major component in bile. Impaired bile flow leads to cholestasis that is characterized by elevated levels of bile acid in the liver and serum, followed by hepatocyte and biliary injury. Although the causes of cholestasis have been extensively studied, the molecular mechanisms as to how bile acids initiate liver injury remain controversial. In this chapter, we summarize recent advances in the pathogenesis of bile acid induced liver injury. These include bile acid signaling pathways in hepatocytes as well as the response of cholangiocytes and innate immune cells in the liver in both patients with cholestasis and cholestatic animal models. We focus on how bile acids trigger the production of molecular mediators of neutrophil recruitment and the role of the inflammatory response in this pathological process. These advances point to a number of novel targets where drugs might be judged to be effective therapies for cholestatic liver injury.

M2-like Kupffer cells in fibrotic liver may protect against acute insult

AIM

To investigate the mechanism of hepatoprotection conferred by liver fibrosis through evaluating the activation phenotype of kupffer cells.

METHODS

Control and fibrotic mice were challenged with a lethal dose of D-GalN/lipopolysaccharide (LPS), and hepatic damage was assessed by histology, serum alanine transferase (ALT) levels, and hepatic expression of HMGB1, a potent pro-inflammatory mediator. The localization of F4/80 (a surrogate marker of KCs), HMGB1, and type I collagen (Col-1) was determined by immunofluorescence staining. The phenotype of KCs was characterized by real-time PCR. KCs isolated from control or fibrotic mice were challenged with LPS or HMGB1 peptide, and HMGB1 translocation was analyzed.

RESULTS

Liver fibrosis protected mice against D-GalN/LPS challenge, as shown by improved hepatic histology and reduced elevation of ALT compared with the normal mice treated in the same way. This hepatoprotection was also accompanied by inhibition of HMGB1 expression in the liver. Co-localization of F4/80, HMGB1, and Col-1 was found in fibrotic livers, indicating the close relationship between KCs, HMGB1 and liver fibrosis. KCs isolated from fibrotic mice predominantly exhibited an M2-like phenotype. In vitro experiments showed that HMGB1 was localized in the nucleus of the majority of M2-like KCs and that the translocation of HMGB1 was inhibited following stimulation with LPS or HMGB1 peptide, while both LPS and HMGB1 peptide elicited translocation of intranuclear HMGB1 in KCs isolated from the control mice.

CONCLUSION

M2-like Kupffer cells in fibrotic liver may exert a protective effect against acute insult by inhibiting the translocation of HMGB1.

Isofraxidin, a coumarin component improves high-fat diet induced hepatic lipid homeostasis disorder and macrophage inflammation in mice

Isofraxidin (IF) is a coumarin compound produced in the functional foodsSiberian ginsengandApium graveolens.

Protective effect of liver fibrosis against acute liver injury

Liver fibrosis is the excessive accumulation of extracellular matrix proteins in liver tissue. Liver fibrosis as the characteristic change of chronic liver injury has the potential to develop into liver cirrhosis, liver failure and hepatic carcinoma, and is considered a devastating pathologic process. However, recent studies demonstrate that liver fibrosis is not only reversible, but also can protect the liver from acute injury. Currently, the mechanisms of hepatoprotective effect of liver fibrosis have become a hot research area, which include promoted regeneration of the remaining normal liver cells and apoptotic resistance. In the present article, we will review the hepatoprotective effect of liver fibrosis and the underlying molecular mechanisms, aiming to provide a theoretical basis for understanding the pathogenesis of acute-on-chronic liver failure and provide new therapeutic targets for this disease.

Pathogenesis of Kupffer Cells in Cholestatic Liver Injury

Kupffer cells are the resident macrophages in the liver. They are located in hepatic sinusoid, which allows them to remove foreign materials, pathogens, and apoptotic cells efficiently. Activated Kupffer cells secrete various mediators, including cytokines and chemokines, to initiate immune responses, inflammation, or recruitment of other liver cells. Bile duct ligation (BDL) surgery in rodents is often studied as an animal model of cholestatic liver disease, characterized by obstruction of bile flow. BDL mice show altered functional activities of Kupffer cells compared with sham-operated mice, including elevated cytokine secretion and impaired bacterial clearance. Various mediators produced by other liver cells can regulate Kupffer cell activation, which suggest that Kupffer cells orchestrate with other liver cells to relay inflammatory signals and to maintain liver homeostasis during BDL-induced liver injury. Blocking or depletion of Kupffer cells, an approach for the treatment of liver diseases, has shown controversial implications. Procedures in Kupffer cell research have limitations and may produce various results in Kupffer cell research. It is important, however, to reveal underlying mechanisms of activation and functions of Kupffer cells, followed by hepatic inflammation and fibrosis. This review summarizes present Kupffer cell studies in cholestatic liver injury.

ER stress contributes to alpha-naphthyl isothiocyanate-induced liver injury with cholestasis in mice

Endoplasmic reticulum (ER) stress is involved in the development of several liver diseases and tumors. This study investigated the underlying mechanisms of α-naphthyl isothiocyanate (ANIT)-induced liver injury with cholestasis in mice and found ER stress contributes to the injury. All animals were randomly divided into three groups. In the ANIT-intoxicated group, mice were intragastrically given 100mg/kg ANIT (dissolved in corn oil), while the other groups received an equal volume of vehicle as control. After 24 and 48h of ANIT administration, blood samples and liver tissues of all animals were collected for serum biochemistry and hepatic histopathological examinations to evaluate liver injuries with cholestasis. Hepatocellular apoptosis was assessed by the terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assay. The expression of hepatic ER stress-related markers was determined by real-time PCR, immunohistochemical assay and Western blot. ANIT was found to significantly induce liver injury with cholestasis compared with control mice as evidenced by the increase of serum transaminases and total bilirubin (TBil), and histopathological changes in mice. ANIT remarkably induced hepatocellular apoptosis, upregulated the expression of caspase-9 and cytochrome c, and inhibited the gene and protein expression of proliferating cell nuclear antigen (PCNA). The gene expression of ER stress-related markers, including glucose-regulated protein 78 (GRP78), protein kinase R-like ER kinase (PERK), eukaryotic initiation factor 2α (eIF2α), inositol requiring enzyme-1α (IRE-1α) and activating transcription factor 6 (ATF6) was upregulated by ANIT in mice. ANIT also upregulated the protein expression of GRP78 and activated the phosphorylation of IRE1. These results suggested that ANIT induced liver injury with cholestasis partly due to its ability to activate the ER stress pathway.

β-Arrestin 2 Promotes Hepatocyte Apoptosis by Inhibiting Akt Protein

Recent studies reveal that multifunctional protein β-arrestin 2 (Arrb2) modulates cell apoptosis. Survival and various aspects of liver injury were investigated in WT and Arrb2 KO mice after bile duct ligation (BDL). We found that deficiency of Arrb2 enhances survival and attenuates hepatic injury and fibrosis. Following BDL, Arrb2-deficient mice as compared with WT controls displayed a significant reduction of hepatocyte apoptosis as demonstrated by the TUNEL assay. Following BDL, the levels of phospho-Akt and phospho-glycogen synthase kinase 3β (GSK3β) in the livers were significantly increased in Arrb2 KO compared with WT mice, although p-p38 increased in WT but not in Arrb2-deficient mice. Inhibition of GSK3β following BDL decreases hepatic apoptosis and decreased p-p38 in WT mice but not in Arrb2 KO mice. Activation of Fas receptor with Jo2 reduces phospho-Akt and increases apoptosis in WT cells and WT mice but not in Arrb2-deficient cells and Arrb2-deficient mice. Consistent with direct interaction of Arrb2 with and regulating Akt phosphorylation, the expression of a full-length or N terminus but not the C terminus of Arrb2 reduces Akt phosphorylation and coimmunoprecipates with Akt. These results reveal that the protective effect of deficiency of Arrb2 is due to loss of negative regulation of Akt due to BDL and decreased downstream GSK3β and p38 MAPK signaling pathways.

Inhibition of Cyclic Adenosine Monophosphate (cAMP)-response Element-binding Protein (CREB)-binding Protein (CBP)/β-Catenin Reduces Liver Fibrosis in Mice

Wnt/β-catenin is involved in every aspect of embryonic development and in the pathogenesis of many human diseases, and is also implicated in organ fibrosis. However, the role of β-catenin-mediated signaling on liver fibrosis remains unclear. To explore this issue, the effects of PRI-724, a selective inhibitor of the cAMP-response element-binding protein-binding protein (CBP)/β-catenin interaction, on liver fibrosis were examined using carbon tetrachloride (CCl4)- or bile duct ligation (BDL)-induced mouse liver fibrosis models. Following repetitive CCl4 administrations, the nuclear translocation of β-catenin was observed only in the non-parenchymal cells in the liver. PRI-724 treatment reduced the fibrosis induced by CCl4 or BDL. C-82, an active form of PRI-724, inhibited the activation of isolated primary mouse quiescent hepatic stellate cells (HSCs) and promoted cell death in culture-activated HSCs. During the fibrosis resolution period, an increase in F4/80(+) CD11b(+) and Ly6C(low) CD11b(+) macrophages was induced by CCl4 and was sustained for two weeks thereafter, even after having stopped CCl4 treatment. PRI-724 accelerated the resolution of CCl4-induced liver fibrosis, and this was accompanied by increased matrix metalloproteinase (MMP)-9, MMP-2, and MMP-8 expression in intrahepatic leukocytes. In conclusion, targeting the CBP/β-catenin interaction may become a new therapeutic strategy in treating liver fibrosis.

Liver fibrosis protects mice against lethal injury induced by D-GalN/LPS

AIM: To assess the tolerance of mice with carbon tetrachloride (CCl₄)-induced fibrosis to a lethal dose of D-galactosamine/lipopolysaccharide (D-GalN/LPS).

METHODS: A mouse model of hepatic fibrosis was established by intraperitoneal injection of CCl₄ (in mineral oil), twice a week for 6 wk. At the end of fibrosis induction, mice were challenged intraperitoneally with D-GalN (700 mg/kg)/LPS (50 μg/kg). Normal mice treated in the same way were used as controls. Mice were sacrificed 24 h after acute insult. Sera and liver tissues were harvested for analyses. To evaluate the tolerance of normal and fibrotic mice to a lethal dose of D-GalN/LPS, survival rate, serum alanine aminotransferase (sALT) levels and histological changes of the liver were compared between before and after acute challenge.

RESULTS: The survival rate of fibrotic mice subjected to a lethal dose of D-GalN/LPS was significantly higher than that of normal mice treated in the same way (100% vs 20%). After challenged by D-GalN/LPS, sALT in normal mice increased by 455.9 folds (49.2 U/L ± 12.9 U/L vs 22429 U/L ± 5446 U/L, P < 0.01), which was significantly higher than that in fibrotic mice (14.3 folds) [(463.7 U/L ± 109.0 U/L vs 6630 U/L ± 1675 U/L, P < 0.01). The tolerance of fibrotic mice to D-GalN/LPS was confirmed by well-preserved liver architecture as compared with controls.

CONCLUSION: CCl₄-induced liver fibrosis protects mice against a lethal dose of D-GalN/LPS.

Serum acid sphingomyelinase is upregulated in chronic hepatitis C infection and non alcoholic fatty liver disease

Sphingolipids constitute bioactive molecules with functional implications in homeostasis and pathogenesis of various diseases. However, the role of sphingolipids as possible disease biomarkers in chronic liver disease remains largely unexplored. In the present study we used mass spectrometry and spectrofluorometry methods in order to quantify various sphingolipid metabolites and also assess the activity of an important corresponding regulating enzyme in the serum of 72 healthy volunteers as compared to 69 patients with non-alcoholic fatty liver disease and 69 patients with chronic hepatitis C virus infection. Our results reveal a significant upregulation of acid sphingomyelinase in the serum of patients with chronic liver disease as compared to healthy individuals (p<0.001). Especially in chronic hepatitis C infection acid sphingomyelinase activity correlated significantly with markers of hepatic injury (r=0.312, p=0.009) and showed a high discriminative power. Accumulation of various (dihydro-) ceramide species was identified in the serum of patients with non-alcoholic fatty liver disease (p<0.001) and correlated significantly to cholesterol (r=0.448, p<0.001) but showed a significant accumulation in patients with normal cholesterol values as well (p<0.001). Sphingosine, a further bioactive metabolite, was also upregulated in chronic liver disease (p<0.001). However, no significant correlation to markers of hepatic injury was identified.

CONCLUSION

Chronic hepatitis C virus infection and non-alcoholic fatty liver disease induce a significant upregulation of serum acid sphingomyelinase which appears as a novel biomarker in chronic hepatopathies. Further studies are required to elucidate the potential of the sphingolipid signaling pathway as putative therapeutic target in chronic liver disease.

Tumor necrosis factor-α promotes cholestasis-induced liver fibrosis in the mouse through tissue inhibitor of metalloproteinase-1 production in hepatic stellate cells

Tumor necrosis factor (TNF)-α, which is a mediator of hepatotoxicity, has been implicated in liver fibrosis. However, the roles of TNF-α on hepatic stellate cell (HSC) activation and liver fibrosis are complicated and remain controversial. To explore this issue, the role of TNF-α in cholestasis-induced liver fibrosis was examined by comparing between TNF-α(-/-) mice and TNF-α(+/+) mice after bile duct ligation (BDL). Serum TNF-α levels in mice were increased by common BDL combined with cystic duct ligation (CBDL+CDL). TNF-α deficiency reduced liver fibrosis without affecting liver injury, inflammatory cell infiltration, and liver regeneration after CBDL+CDL. Increased expression levels of collagen α1(I) mRNA, transforming growth factor (TGF)-β mRNA, and α-smooth muscle actin (αSMA) protein by CBDL+CDL in the livers of TNF-α(-/-) mice were comparable to those in TNF-α(+/+) mice. Exogenous administration of TNF-α decreased collagen α1(I) mRNA expression in isolated rat HSCs. These results suggest that the reduced fibrosis in TNF-α(-/-) mice is regulated in post-transcriptional level. Tissue inhibitor of metalloproteinase (TIMP)-1 plays a crucial role in the pathogenesis of liver fibrosis. TIMP-1 expression in HSCs in the liver was increased by CBDL+CDL, and the induction was lower in TNF-α(-/-) mice than in TNF-α(+/+) mice. Fibrosis in the lobe of TIMP-1(-/-) mice with partial BDL was also reduced. These findings indicate that TNF-α produced by cholestasis can promote liver fibrosis via TIMP-1 production from HSCs. Thus, targeting TNF-α and TIMP-1 may become a new therapeutic strategy for treating liver fibrosis in cholestatic liver injury.

Contrasting responses of Kupffer cells and inflammatory mononuclear phagocytes to biliary obstruction in a mouse model of cholestatic liver injury

BACKGROUND

Biliary obstruction and cholestasis are serious complications of many liver diseases. Although resident hepatic macrophages (Kupffer cells) are frequently implicated in disease progression, most studies fail to differentiate the contribution of Kupffer cells and inflammatory mononuclear phagocytes (iMNPs) that infiltrate the liver subsequent to obstruction.

AIM

This study was undertaken to examine the roles and potential interactions of these two disparate mononuclear phagocyte populations in hepatic injury attending cholestasis.

METHODS

Female, C57Bl/6 mice were injected with magnetic beads on day 3 prior to sham operation or bile duct ligation (BDL) to facilitate subsequent Kupffer cell isolation. Three days post-surgery, animals were euthanized, and bead-containing Kupffer cells and iMNPs were separated, purified and analysed. To examine the ability of Kupffer cells to modulate iMNP activity, iMNPs were isolated from the livers of intact and Kupffer cell-depleted mice on day 3 post-surgery and compared.

RESULTS

Purified Kupffer cells and iMNP populations obtained from BDL mice exhibited heterogeneous morphologies rendering them visually indistinguishable. iMNPs, however, were characterized by the increased expression of Ly-6C and CD11b and the elevated production of chemokines/cytokines characteristic of inflammatory cells. In the absence of Kupffer cells, iMNPs immigrating to the liver following BDL exhibited significant decreases in CD11b and Ly-6C expression, and in pro-inflammatory chemokine/cytokine production.

CONCLUSIONS

Kupffer cells and iMNPs exhibit disparate biological responses to biliary obstruction and cholestasis. Kupffer cells play a key role in regulating iMNP influx and activity.

Liver acid sphingomyelinase inhibits growth of metastatic colon cancer

Acid sphingomyelinase (ASM) regulates the homeostasis of sphingolipids, including ceramides and sphingosine-1-phosphate (S1P). These sphingolipids regulate carcinogenesis and proliferation, survival, and apoptosis of cancer cells. However, the role of ASM in host defense against liver metastasis remains unclear. In this study, the involvement of ASM in liver metastasis of colon cancer was examined using Asm-/- and Asm+/+ mice that were inoculated with SL4 colon cancer cells to produce metastatic liver tumors. Asm-/- mice demonstrated enhanced tumor growth and reduced macrophage accumulation in the tumor, accompanied by decreased numbers of hepatic myofibroblasts (hMFs), which express tissue inhibitor of metalloproteinase 1 (TIMP1), around the tumor margin. Tumor growth was increased by macrophage depletion or by Timp1 deficiency, but was decreased by hepatocyte-specific ASM overexpression, which was associated with increased S1P production. S1P stimulated macrophage migration and TIMP1 expression in hMFs in vitro. These findings indicate that ASM in the liver inhibits tumor growth through cytotoxic macrophage accumulation and TIMP1 production by hMFs in response to S1P. Targeting ASM may represent a new therapeutic strategy for treating liver metastasis of colon cancer.

Hepatitis B virus particles preferably induce Kupffer cells to produce TGF-β1 over pro-inflammatory cytokines

BACKGROUND

Kupffer cells and related cytokines are thought to play a critical role in liver fibrosis; however, the role played by Kupffer cells in hepatitis B virus-related fibrogenesis is unknown.

METHODS

Primary rat Kupffer cells were cultured with different titres of hepatitis B virus particles and the concentrations of transforming growth factor (TGF)-β1, interleukin (IL)-1, IL-6 and tumour necrosis factor (TNF)-α in the culture supernatant were measured every 24h for 7 days. The mRNA and protein levels of these cytokines in Kupffer cells were also analysed using quantitative real-time polymerase chain reaction and western blotting, respectively.

RESULTS

Kupffer cells maintained normal morphology and function throughout the 7-day exposure to hepatitis B virus. The concentration of TGF-β1 secreted by hepatitis B virus-stimulated Kupffer cells (6 log IU/ml hepatitis B virus) increased 5.38- and 7.75-fold by Days 3 and 7, respectively (p<0.01). Western blotting showed that TGF-β1 expression in Kupffer cells exposed to high titres of hepatitis B virus increased 1.80- and 2.42-fold by Days 3 and 7, respectively (p<0.01). In contrast, Kupffer cell expression and secretion of pro-inflammatory cytokines (IL-6, IL-1 and TNF-α) was unchanged throughout the experiment.

CONCLUSION

Hepatitis B virus preferentially stimulates Kupffer cells to produce the pro-fibrogenic/anti-inflammatory cytokine TGF-β1 rather than the pro-inflammatory cytokines IL-6, IL-1 and TNF-α. This may partly explain why overt liver fibrosis still presents in cases of chronic hepatitis B virus infection with minimal (or no) necro-inflammation.

Kupffer cells influence parenchymal invasion and phenotypic orientation, but not the proliferation, of liver progenitor cells in a murine model of liver injury

Activation of myofibroblasts (MF) and extracellular matrix (ECM) deposition predispose the expansion and differentiation of liver progenitor cells (LPC) during chronic liver injury. Because Kupffer cells (KC) are active modulators of tissue response and fibrosis, we analyzed their role in a model of LPC proliferation. A choline-deficient diet, supplemented by ethionine (CDE) was administrated to C57Bl/6J mice that were depleted of KC by repeated injections of clodronate (CLO) and compared to PBS-injected mice. On CDE, massive KC activation was observed in the PBS group, but this was blunted in CLO-treated mice. The depletion of KC did not influence LPC proliferation but reduced their invasive behavior. Instead of being found far into the parenchyma, as was found in the PBS group (mean distance from portal vein: 209 μm), LPC of CLO mice remained closer to the portal area (138 μm), forming aggregates and phenotypically resembling cells of biliary lineage. Notably, removal of KC was also associated with a significant decrease in amount of MF and ECM and in the expression of profibrotic factors. Thus, besides ECM and MF, KC are also a significant component of the microenvironmental changes preceding LPC expansion. Depletion of KC may limit the LPC parenchymal invasion through a deficiency in chemoattracting factors, reduced activation of MF, and/or a paucity of the ECM framework necessary for cell motility.

L-tryptophan-mediated enhancement of susceptibility to nonalcoholic fatty liver disease is dependent on the mammalian target of rapamycin

Nonalcoholic fatty liver disease is one of the most common liver diseases. l-Tryptophan and its metabolite serotonin are involved in hepatic lipid metabolism and inflammation. However, it is unclear whether l-tryptophan promotes hepatic steatosis. To explore this issue, we examined the role of l-tryptophan in mouse hepatic steatosis by using a high fat and high fructose diet (HFHFD) model. l-Tryptophan treatment in combination with an HFHFD exacerbated hepatic steatosis, expression of HNE-modified proteins, hydroxyproline content, and serum alanine aminotransaminase levels, whereas l-tryptophan alone did not result in these effects. We also found that l-tryptophan treatment increases serum serotonin levels. The introduction of adenoviral aromatic amino acid decarboxylase, which stimulates the serotonin synthesis from l-tryptophan, aggravated hepatic steatosis induced by the HFHFD. The fatty acid-induced accumulation of lipid was further increased by serotonin treatment in cultured hepatocytes. These results suggest that l-tryptophan increases the sensitivity to hepatic steatosis through serotonin production. Furthermore, l-tryptophan treatment, adenoviral AADC introduction, and serotonin treatment induced phosphorylation of the mammalian target of rapamycin (mTOR), and a potent mTOR inhibitor rapamycin attenuated hepatocyte lipid accumulation induced by fatty acid with serotonin. These results suggest the importance of mTOR activation for the exacerbation of hepatic steatosis. In conclusion, l-tryptophan exacerbates hepatic steatosis induced by HFHFD through serotonin-mediated activation of mTOR.

Lithocholic acid disrupts phospholipid and sphingolipid homeostasis leading to cholestasis in mice

Lithocholic acid (LCA) is an endogenous compound associated with hepatic toxicity during cholestasis. LCA exposure in mice resulted in decreased serum lysophosphatidylcholine (LPC) and sphingomyelin levels due to elevated lysophosphatidylcholine acyltransferase (LPCAT) and sphingomyelin phosphodiesterase (SMPD) expression. Global metabolome analysis indicated significant decreases in serum palmitoyl-, stearoyl-, oleoyl-, and linoleoyl-LPC levels after LCA exposure. LCA treatment also resulted in decreased serum sphingomyelin levels and increased hepatic ceramide levels, and induction of LPCAT and SMPD messenger RNAs (mRNAs). Transforming growth factor-β (TGF-β) induced Lpcat2/4 and Smpd3 gene expression in primary hepatocytes and the induction was diminished by pretreatment with the SMAD3 inhibitor SIS3. Furthermore, alteration of the LPCs and Lpcat1/2/4 and Smpd3 expression was attenuated in LCA-treated farnesoid X receptor-null mice that are resistant to LCA-induced intrahepatic cholestasis.

CONCLUSION

This study revealed that LCA induced disruption of phospholipid/sphingolipid homeostasis through TGF-β signaling and that serum LPC is a biomarker for biliary injury.

Acidic sphingomyelinase controls hepatic stellate cell activation and in vivo liver fibrogenesis

The mechanisms linking hepatocellular death, hepatic stellate cell (HSC) activation, and liver fibrosis are largely unknown. Here, we investigate whether acidic sphingomyelinase (ASMase), a known regulator of death receptor and stress-induced hepatocyte apoptosis, plays a role in liver fibrogenesis. We show that selective stimulation of ASMase (up to sixfold), but not neutral sphingomyelinase, occurs during the transdifferentiation/activation of primary mouse HSCs into myofibroblast-like cells, coinciding with cathepsin B (CtsB) and D (CtsD) processing. ASMase inhibition or genetic down-regulation by small interfering RNA blunted CtsB/D processing, preventing the activation and proliferation of mouse and human HSCs (LX2 cells). In accordance, HSCs from heterozygous ASMase mice exhibited decreased CtsB/D processing, as well as lower levels of alpha-smooth muscle actin expression and proliferation. Moreover, pharmacological CtsB inhibition reproduced the antagonism of ASMase in preventing the fibrogenic properties of HSCs, without affecting ASMase activity. Interestingly, liver fibrosis induced by bile duct ligation or carbon tetrachloride administration was reduced in heterozygous ASMase mice compared with that in wild-type animals, regardless of their sensitivity to liver injury in either model. To provide further evidence for the ASMase-CtsB pathway in hepatic fibrosis, liver samples from patients with nonalcoholic steatohepatitis were studied. CtsB and ASMase mRNA levels increased eight- and threefold, respectively, in patients compared with healthy controls. These findings illustrate a novel role of ASMase in HSC biology and liver fibrogenesis by regulating its downstream effectors CtsB/D.