Hepatic stellate cells orchestrate clearance of necrotic cells in a hypoxia-inducible factor-1α-dependent manner by modulating macrophage phenotype in mice

The evidence-based multifaceted roles of hepatic stellate cells in liver diseases: A concise review

Hepatic stellate cells (HSCs) play central roles in liver disease pathogenesis, spanning steatosis to cirrhosis and hepatocellular carcinoma. These cells, located in the liver's sinusoidal space of Disse, transition from a quiescent, vitamin A-rich state to an activated, myofibroblast-like phenotype in response to liver injury. This activation results from a complex interplay of cytokines, growth factors, and oxidative stress, leading to excessive collagen deposition and liver fibrosis, a hallmark of chronic liver diseases. Recently, HSCs have gained recognition for their dynamic, multifaceted roles in liver health and disease. Attention has shifted toward their involvement in various liver conditions, including acute liver injury, alcoholic and non-alcoholic fatty liver disease, and liver regeneration. This review aims to explore diverse functions of HSCs in these acute or chronic liver pathologies, with a focus on their roles beyond fibrogenesis. HSCs exhibit a wide range of actions, including lipid storage, immunomodulation, and interactions with other hepatic and extrahepatic cells, making them pivotal in the hepatic microenvironment. Understanding HSC involvement in the progression of liver diseases can offer novel insights into pathogenic mechanisms and guide targeted therapeutic strategies for various liver conditions.

Remodeling of the liver fibrosis microenvironment based on nilotinib-loaded multicatalytic nanozymes with boosted antifibrogenic activity

Liver fibrosis is a reversible pathological process caused by chronic liver damage and a major risk factor for hepatocellular carcinoma (HCC). Hepatic stellate cell (HSC) activation is considered the main target for liver fibrosis therapy. However, the efficiency of this strategy is limited due to the complex microenvironment of liver fibrosis, including excessive extracellular matrix (ECM) deposition and hypoxia-induced imbalanced ECM metabolism. Herein, nilotinib (NIL)-loaded hyaluronic acid (HA)-coated Ag@Pt nanotriangular nanozymes (APNH NTs) were developed to inhibit HSCs activation and remodel the microenvironment of liver fibrosis. APNH NTs efficiently eliminated intrahepatic reactive oxygen species (ROS) due to their inherent superoxide dismutase (SOD) and catalase (CAT) activities, thereby downregulating the expression of NADPH oxidase-4 (NOX-4) and inhibiting HSCs activation. Simultaneously, the oxygen produced by the APNH NTs further alleviated the hypoxic microenvironment. Importantly, the released NIL promoted collagen depletion by suppressing the expression of tissue inhibitor of metalloproteinase-1 (TIMP-1), thus synergistically remodeling the microenvironment of liver fibrosis. Notably, an in vivo study in CCl4-induced mice revealed that APNH NTs exhibited significant antifibrogenic effects without obvious long-term toxicity. Taken together, the data from this work suggest that treatment with the synthesized APNH NTs provides an enlightening strategy for remodeling the microenvironment of liver fibrosis with boosted antifibrogenic activity.

Interleukin-10 disrupts liver repair in acetaminophen-induced acute liver failure

Introduction

Systemic levels of the anti-inflammatory cytokine interleukin 10 (IL-10) are highest in acetaminophen (APAP)-induced acute liver failure (ALF) patients with the poorest prognosis. The mechanistic basis for this counterintuitive finding is not known, as induction of IL-10 is hypothesized to temper the pathological effects of immune cell activation. Aberrant production of IL-10 after severe liver injury could conceivably interfere with the beneficial, pro-reparative actions of immune cells, such as monocytes.

Methods

To test this possibility, we determined whether IL-10 levels are dysregulated in mice with APAP-induced ALF and further evaluated whether aberrant production of IL-10 prevents monocyte recruitment and/or the resolution of necrotic lesions by these cells.

Results

Our studies demonstrate that in mice challenged with 300 mg/kg acetaminophen (APAP), a hepatotoxic dose of APAP that fails to produce ALF (i.e., APAP-induced acute liver injury; AALI), Ly6Chi monocytes were recruited to the liver and infiltrated the necrotic lesions by 48 hours coincident with the clearance of dead cell debris. At 72 hours, IL-10 was upregulated, culminating in the resolution of hepatic inflammation. By contrast, in mice treated with 600 mg/kg APAP, a dose that produces clinical features of ALF (i.e., APAP-induced ALF; AALF), IL-10 levels were markedly elevated by 24 hours. Early induction of IL-10 was associated with a reduction in the hepatic numbers of Ly6Chi monocytes resulting in the persistence of dead cell debris. Inhibition of IL-10 in AALF mice, beginning at 24 hours after APAP treatment, increased the hepatic numbers of monocytes which coincided with a reduction in the necrotic area. Moreover, pharmacologic elevation of systemic IL-10 levels in AALI mice reduced hepatic myeloid cell numbers and increased the area of necrosis.

Discussion

Collectively, these results indicate that during ALF, aberrant production of IL-10 disrupts the hepatic recruitment of monocytes, which prevents the clearance of dead cell debris. These are the first studies to document a mechanistic basis for the link between high IL-10 levels and poor outcome in patients with ALF.

Responses of hepatic sinusoidal cells to liver ischemia–reperfusion injury

The liver displays a remarkable regenerative capacity in response to acute liver injury. In addition to the proliferation of hepatocytes during liver regeneration, non-parenchymal cells, including liver macrophages, liver sinusoidal endothelial cells (LSECs), and hepatic stellate cells (HSCs) play critical roles in liver repair and regeneration. Liver ischemia–reperfusion injury (IRI) is a major cause of increased liver damage during liver resection, transplantation, and trauma. Impaired liver repair increases postoperative morbidity and mortality of patients who underwent liver surgery. Successful liver repair and regeneration after liver IRI requires coordinated interplay and synergic actions between hepatic resident cells and recruited cell components. However, the underlying mechanisms of liver repair after liver IRI are not well understood. Recent technological advances have revealed the heterogeneity of each liver cell component in the steady state and diseased livers. In this review, we describe the progress in the biology of liver non-parenchymal cells obtained from novel technological advances. We address the functional role of each cell component in response to liver IRI and the interactions between diverse immune repertoires and non-hematopoietic cell populations during the course of liver repair after liver IRI. We also discuss how these findings can help in the design of novel therapeutic approaches. Growing insights into the cellular interactions during liver IRI would enhance the pathology of liver IRI understanding comprehensively and further develop the strategies for improvement of liver repair.

Hypoxia-induced factor and its role in liver fibrosis

Liver fibrosis develops as a result of severe liver damage and is considered a major clinical concern throughout the world. Many factors are crucial for liver fibrosis progression. While advancements have been made to understand this disease, no effective pharmacological drug and treatment strategies have been established that can effectively prevent liver fibrosis or even could halt the fibrotic process. Most of those advances in curing liver fibrosis have been aimed towards mitigating the causes of fibrosis, including the development of potent antivirals to inhibit the hepatitis virus. It is not practicable for many individuals; however, a liver transplant becomes the only suitable alternative. A liver transplant is an expensive procedure. Thus, there is a significant need to identify potential targets of liver fibrosis and the development of such agents that can effectively treat or reverse liver fibrosis by targeting them. Researchers have identified hypoxia-inducible factors (HIFs) in the last 16 years as important transcription factors driving several facets of liver fibrosis, making them possible therapeutic targets. The latest knowledge on HIFs and their possible role in liver fibrosis, along with the cell-specific activities of such transcription factors that how they play role in liver fibrosis progression, is discussed in this review.

Targeting the Hepatic Microenvironment to Improve Ischemia/Reperfusion Injury: New Insights into the Immune and Metabolic Compartments

Hepatic ischemia/reperfusion injury (IRI) is mainly characterized by high activation of immune inflammatory responses and metabolic responses. Understanding the molecular and metabolic mechanisms underlying development of hepatic IRI is critical for developing effective therapies for hepatic IRI. Recent advances in research have improved our understanding of the pathogenesis of IRI. During IRI, hepatocyte injury and inflammatory responses are mediated by crosstalk between the immune cells and metabolic components. This crosstalk can be targeted to treat or reverse hepatic IRI. Thus, a deep understanding of hepatic microenvironment, especially the immune and metabolic responses, can reveal new therapeutic opportunities for hepatic IRI. In this review, we describe important cells in the liver microenvironment (especially non-parenchymal cells) that regulate immune inflammatory responses. The role of metabolic components in the diagnosis and prevention of hepatic IRI are discussed. Furthermore, recent updated therapeutic strategies based on the hepatic microenvironment, including immune cells and metabolic components, are highlighted.

Comparison of DLin-MC3-DMA and ALC-0315 for siRNA Delivery to Hepatocytes and Hepatic Stellate Cells

Ionizable cationic lipids are essential for efficient in vivo delivery of RNA by lipid nanoparticles (LNPs). DLin-MC3-DMA (MC3), ALC-0315, and SM-102 are the only ionizable cationic lipids currently clinically approved for RNA therapies. ALC-0315 and SM-102 are structurally similar lipids used in SARS-CoV-2 mRNA vaccines, while MC3 is used in siRNA therapy to knock down transthyretin in hepatocytes. Hepatocytes and hepatic stellate cells (HSCs) are particularly attractive targets for RNA therapy because they synthesize many plasma proteins, including those that influence blood coagulation. While LNPs preferentially accumulate in the liver, evaluating the ability of different ionizable cationic lipids to deliver RNA cargo into distinct cell populations is important for designing RNA-LNP therapies with minimal hepatotoxicity. Here, we directly compared LNPs containing either ALC-0315 or MC3 to knock-down coagulation factor VII (FVII) in hepatocytes and ADAMTS13 in HSCs. At a dose of 1 mg/kg siRNA in mice, LNPs with ALC-0315 achieved a 2- and 10-fold greater knockdown of FVII and ADAMTS13, respectively, compared to LNPs with MC3. At a high dose (5 mg/kg), ALC-0315 LNPs increased markers of liver toxicity (ALT and bile acids), while the same dose of MC3 LNPs did not. These results demonstrate that ALC-0315 LNPs achieves potent siRNA-mediated knockdown of target proteins in hepatocytes and HSCs, in mice, though markers of liver toxicity can be observed after a high dose. This study provides an initial comparison that may inform the development of ionizable cationic LNP therapeutics with maximal efficacy and limited toxicity.

Immune and Metabolic Alterations in Liver Fibrosis: A Disruption of Oxygen Homeostasis?

According to the WHO, “cirrhosis of the liver” was the 11th leading cause of death globally in 2019. Many kinds of liver diseases can develop into liver cirrhosis, and liver fibrosis is the main pathological presentation of different aetiologies, including toxic damage, viral infection, and metabolic and genetic diseases. It is characterized by excessive synthesis and decreased decomposition of extracellular matrix (ECM). Hepatocyte cell death, hepatic stellate cell (HSC) activation, and inflammation are crucial incidences of liver fibrosis. The process of fibrosis is also closely related to metabolic and immune disorders, which are usually induced by the destruction of oxygen homeostasis, including mitochondrial dysfunction, oxidative stress, and hypoxia pathway activation. Mitochondria are important organelles in energy generation and metabolism. Hypoxia-inducible factors (HIFs) are key factors activated when hypoxia occurs. Both are considered essential factors of liver fibrosis. In this review, the authors highlight the impact of oxygen imbalance on metabolism and immunity in liver fibrosis as well as potential novel targets for antifibrotic therapies.

High salt exacerbates acute kidney injury by disturbing the activation of CD5L/apoptosis inhibitor of macrophage (AIM) protein

The influence of excess salt intake on acute kidney injury (AKI) has not been examined precisely except for some clinical data, unlike in chronic kidney disease. Here, we addressed the influence of high salt (HS) on AKI and its underlying mechanisms in terms of the activity of circulating apoptosis inhibitor of macrophage (AIM, also called CD5L) protein, a facilitator of AKI repair. HS loading in mice subjected to ischemia/reperfusion (IR) resulted in high mortality with advanced renal tubular obstruction and marked exacerbation in biomarkers of proximal renal tubular damage. This AKI exacerbation appeared to be caused mainly by the reduced AIM dissociation from IgM pentamer in serum, as IgM-free AIM is indispensable for the removal of intratubular debris to facilitate AKI repair. Injection of recombinant AIM (rAIM) ameliorated the AKI induced by IR/HS, dramatically improving the tubular damage and mouse survival. The repair of lethal AKI by AIM was dependent on AIM/ kidney injury molecule-1 (KIM-1) axis, as rAIM injection was not effective in KIM-1 deficient mice. Our results demonstrate that the inhibition of AIM dissociation from IgM is an important reason for the exacerbation of AKI by HS, that AIM is a strong therapeutic tool for severe AKI.

Hypoxia, Hypoxia-Inducible Factors and Liver Fibrosis

Liver fibrosis is a potentially reversible pathophysiological event, leading to excess deposition of extracellular matrix (ECM) components and taking place as the net result of liver fibrogenesis, a dynamic and highly integrated process occurring during chronic liver injury of any etiology. Liver fibrogenesis and fibrosis, together with chronic inflammatory response, are primarily involved in the progression of chronic liver diseases (CLD). As is well known, a major role in fibrogenesis and fibrosis is played by activated myofibroblasts (MFs), as well as by macrophages and other hepatic cell populations involved in CLD progression. In the present review, we will focus the attention on the emerging pathogenic role of hypoxia, hypoxia-inducible factors (HIFs) and related mediators in the fibrogenic progression of CLD.

Melatonin and its protective role in attenuating warm or cold hepatic ischaemia/reperfusion injury

Although the liver is the only organ with regenerative capacity, various injury factors induce irreversible liver dysfunction and end‐stage liver disease. Liver resection and liver transplantation (LT) are effective treatments for individuals with liver failure, liver cirrhosis and liver cancers. The remnant or transplanted liver tissues will undergo hepatic ischaemia/reperfusion (IR), which leads to oxidative stress, inflammation, immune injury and liver damage. Moreover, systemic ischaemia induced by trauma, stroke, myocardial ischaemia, haemorrhagic shock and other injury factors also induces liver ischaemia/reperfusion injury (IRI) in individuals. Hepatic IRI can be divided into warm IRI, which is induced by liver surgery and systemic ischaemia, and cold IRI, which is induced by LT. Multiple studies have shown that melatonin (MT) acts as an endogenous free radical scavenger with antioxidant capacity and is also able to attenuate hepatic IRI via its anti‐inflammatory and antiapoptotic capacities. In this review, we discuss the potential mechanisms and current strategies of MT administration in liver surgery for protecting against warm or cold hepatic IRI. We highlight strategies to improve the efficacy and safety of MT for attenuating hepatic IRI in different conditions. After the potential mechanisms underlying the interactions between MT and other important cellular processes during hepatic IR are clarified, more opportunities will be available to use MT to treat liver diseases in the future.

Hypoxia-inducible factors and innate immunity in liver cancer

The liver has strong innate immunity to counteract pathogens from the gastrointestinal tract. During the development of liver cancer, which is typically driven by chronic inflammation, the composition and biological roles of the innate immune cells are extensively altered. Hypoxia is a common finding in all stages of liver cancer development. Hypoxia drives the stabilization of hypoxia-inducible factors (HIFs), which act as central regulators to dampen the innate immunity of liver cancer. HIF signaling in innate immune cells and liver cancer cells together favors the recruitment and maintenance of pro-tumorigenic immune cells and the inhibition of anti-tumorigenic immune cells, promoting immune evasion. HIFs represent attractive therapeutic targets to inhibit the formation of an immunosuppressive microenvironment and growth of liver cancer.

Liver fibrosis is driven by protease-activated receptor-1 expressed by hepatic stellate cells in experimental chronic liver injury

BACKGROUND

Blood coagulation protease activity is proposed to drive hepatic fibrosis through activation of protease-activated receptors (PARs). Whole-body PAR-1 deficiency reduces experimental hepatic fibrosis, and in vitro studies suggest a potential contribution by PAR-1 expressed by hepatic stellate cells. However, owing to a lack of specific tools, the cell-specific role of PAR-1 in experimental hepatic fibrosis has never been formally investigated. Using a novel mouse expressing a conditional PAR-1 allele, we tested the hypothesis that PAR-1 expressed by hepatic stellate cells contributes to hepatic fibrosis.

METHODS

PAR-1flox/flox mice were crossed with mice expressing Cre recombinase controlled by the lecithin retinol acyltransferase (LRAT) promoter, which induces recombination in hepatic stellate cells. Male PAR-1flox/flox/LRATCre and PAR-1flox/flox mice were challenged twice weekly with carbon tetrachloride (CCl4, 1 mL/kg i.p.) for 6 weeks to induce liver fibrosis.

RESULTS

PAR-1 mRNA levels were reduced (>95%) in hepatic stellate cells isolated from PAR-1flox/flox/LRATCre mice. Hepatic stellate cell activation was evident in CCl4-challenged PAR-1flox/flox mice, indicated by increased α-smooth muscle actin labeling and induction of several profibrogenic genes. CCl4-challenged PAR-1flox/flox mice displayed robust hepatic collagen deposition, indicated by picrosirius red staining and type I collagen immunolabeling. Notably, stellate cell activation and collagen deposition were significantly reduced (>30%) in PAR-1flox/flox/LRATCre mice. Importantly, the reduction in liver fibrosis was not a consequence of reduced acute CCl4 hepatotoxicity in PAR-1flox/flox/LRATCre mice.

CONCLUSIONS

The results constitute the first direct experimental evidence that PAR-1 expressed by stellate cells directly promotes their profibrogenic phenotype and hepatic fibrosis in vivo.

Genetically Encoded Tools for Research of Cell Signaling and Metabolism under Brain Hypoxia

Hypoxia is characterized by low oxygen content in the tissues. The central nervous system (CNS) is highly vulnerable to a lack of oxygen. Prolonged hypoxia leads to the death of brain cells, which underlies the development of many pathological conditions. Despite the relevance of the topic, different approaches used to study the molecular mechanisms of hypoxia have many limitations. One promising lead is the use of various genetically encoded tools that allow for the observation of intracellular parameters in living systems. In the first part of this review, we provide the classification of oxygen/hypoxia reporters as well as describe other genetically encoded reporters for various metabolic and redox parameters that could be implemented in hypoxia studies. In the second part, we discuss the advantages and disadvantages of the primary hypoxia model systems and highlight inspiring examples of research in which these experimental settings were combined with genetically encoded reporters.

Hepatic Macrophages in Liver Injury

Ample evidence suggests that hepatic macrophages play key roles in the injury and repair mechanisms during liver disease progression. There are two major populations of hepatic macrophages: the liver resident Kupffer cells and the monocyte-derived macrophages, which rapidly infiltrate the liver during injury. Under different disease conditions, the tissue microenvironmental cues of the liver critically influence the phenotypes and functions of hepatic macrophages. Furthermore, hepatic macrophages interact with multiple cells types in the liver, such as hepatocytes, neutrophils, endothelial cells, and platelets. These crosstalk interactions are of paramount importance in regulating the extents of liver injury, repair, and ultimately liver disease progression. In this review, we summarize the novel findings highlighting the impact of injury-induced microenvironmental signals that determine the phenotype and function of hepatic macrophages. Moreover, we discuss the role of hepatic macrophages in homeostasis and pathological conditions through crosstalk interactions with other cells of the liver.

Fibrotic liver has prompt recovery after ischemia-reperfusion injury

Hepatic ischemia-reperfusion (I/R) is a major complication of liver resection, trauma, and liver transplantation; however, liver repair after I/R in diseased liver has not been studied. The present study sought to determine the manner in which the fibrotic liver repairs itself after I/R. Liver fibrosis was established in mice by CCl₄ administration for 6 wk, and then liver I/R was performed to investigate liver injury and subsequent liver repair in fibrotic and control livers. After I/R, fibrotic liver had more injury compared with nonfibrotic, control liver; however, fibrotic liver showed rapid resolution of liver necrosis and reconstruction of liver parenchyma. Marked accumulation of hepatic stellate cells and macrophages were observed specifically in the fibrotic septa in early reparative phase. Fibrotic liver had higher numbers of hepatic stellate cells, macrophages, and hepatic progenitor cells during liver recovery after I/R than did control liver, but hepatocyte proliferation was unchanged. Fibrotic liver also had significantly greater number of phagocytic macrophages than control liver. Clodronate liposome injection into fibrotic mice after I/R caused decreased macrophage accumulation and delay of liver recovery. Conversely, CSF1-Fc injection into normal mice after I/R resulted in increased macrophage accumulation and concomitant decrease in necrotic tissue during liver recovery. In conclusion, fibrotic liver clears necrotic areas and restores normal parenchyma faster than normal liver after I/R. This beneficial response appears to be directly related to the increased numbers of nonparenchymal cells, particularly phagocytic macrophages, in the fibrotic liver.NEW & NOTEWORTHY This study is the first to reveal how diseased liver recovers after ischemia-reperfusion (I/R) injury. Although it was not completely unexpected that fibrotic liver had increased hepatic injury after I/R, a novel finding was that fibrotic liver had accelerated recovery and repair compared with normal liver. Enhanced repair after I/R in fibrotic liver was associated with increased expansion of phagocytic macrophages, hepatic stellate cells, and progenitor cells.

Dichotomous Role of Plasmin in Regulation of Macrophage Function after Acetaminophen Overdose

Kupffer cells and monocyte-derived macrophages are critical for liver repair after acetaminophen (APAP) overdose. These cells produce promitogenic cytokines and growth factors, and they phagocytose dead cell debris, a process that is critical for resolution of inflammation. The factors that regulate these dynamic functions of macrophages after APAP overdose, however, are not fully understood. We tested the hypothesis that the fibrinolytic enzyme, plasmin, is a key regulator of macrophage function after APAP-induced liver injury. In these studies, inhibition of plasmin in mice with tranexamic acid delayed up-regulation of proinflammatory cytokines after APAP overdose. In culture, plasmin directly, and in synergy with high-mobility group B1, stimulated Kupffer cells and bone marrow-derived macrophages to produce cytokines by a mechanism that required NF-κB. Inhibition of plasmin in vivo also prevented trafficking of monocyte-derived macrophages into necrotic lesions after APAP overdose. This prevented phagocytic removal of dead cells, prevented maturation of monocyte-derived macrophages into F4/80-expressing macrophages, and prevented termination of proinflammatory cytokine production. Our studies reveal further that phagocytosis is an important stimulus for cessation of proinflammatory cytokine production as treatment of proinflammatory, monocyte-derived macrophages, isolated from APAP-treated mice, with necrotic hepatocytes decreased expression of proinflammatory cytokines. Collectively, these studies demonstrate that plasmin is an important regulator of macrophage function after APAP overdose.

Differential Sensitivity of Kupffer Cells and Hepatic Monocyte-Derived Macrophages to Bacterial Lipopolysaccharide

The liver contains two distinct populations of macrophages, monocyte-derived macrophages (MDMs), which primarily reside proximal to the Glisson's capsule and Kupffer cells, which reside within the sinusoids. Kupffer cells infiltrate the liver during embryogenesis and are replenished from local proliferation of mature Kupffer cells. By contrast MDMs arise from hematopoietic stem cells in the bone marrow and are replenishedfrom circulating monocytes. Studies have revealed that these two hepatic macrophage populations possess distinct transcriptomic profiles, suggesting that they may be functionally distinct. In the present study, we tested the hypothesis that MDMs and Kupffer cells are differentially sensitive to bacterial lipopolysaccharide (LPS). MDMs and Kupffer cells were purified to greater than 90% from the livers of mice by using magnetic beads labeled with Cx3cr1 antibody for MDMs and F4/80 antibody for Kupffer cells. Basal levels of tumor necrosis factor-α (TNF-α) mRNA were higher in MDMs when compared to Kupffer cells. After treatment with LPS, mRNA levels of TNF-α, Cxcll, and Cxcl2 were increased to a greater extent in MDMs when compared to Kupffer cells. To confirm these findings, Kupffer cells and MDMs were isolated from mice in which bone marrow transplantation was used to selectively tag cells arising from hematopoietic stem cells in adult mice. Similar to above, treatment of MDMs with LPS increased TNF-α, Cxcll, and Cxcl2 to a greater extent when compared to Kupffer cells. Collectively, these results indicate that MDMs exhibit a greater pro-inflammatory phenotype in the liver when exposed to LPS.

Dabigatran Reduces Liver Fibrosis in Thioacetamide-Injured Rats

BACKGROUND

Liver fibrosis can progress to cirrhosis, hepatocellular carcinoma, or liver failure. Unfortunately, the antifibrotic agents are limited. Thrombin activates hepatic stellate cells (HSCs). Therefore, we investigated the effects of a direct thrombin inhibitor, dabigatran, on liver fibrosis.

METHODS

Adult male Sprague-Dawley rats were injected intraperitoneally with thioacetamide (TAA, 200 mg/kg twice per week) for 8 or 12 weeks to induce liver fibrosis. The injured rats were assigned an oral gavage of dabigatran etexilate (30 mg/kg/day) or vehicle in the last 4 weeks of TAA administration. Rats receiving an injection of normal saline and subsequent oral gavage of dabigatran etexilate or vehicle served as controls.

RESULTS

In the 8-week TAA-injured rats, dabigatran ameliorated fibrosis, fibrin deposition, and phosphorylated ERK1/2 in liver, without altering the transcript expression of thrombin receptor protease-activated receptor-1. In vitro, dabigatran inhibited thrombin-induced HSC activation. Furthermore, dabigatran reduced intrahepatic angiogenesis and portal hypertension in TAA-injured rats. Similarly, in the 12-week TAA-injured rats, a 4-week treatment with dabigatran reduced liver fibrosis and portal hypertension.

CONCLUSIONS

By inhibiting thrombin action, dabigatran reduced liver fibrosis and intrahepatic angiogenesis. Dabigatran may be a promising therapeutic agent for treatment of liver fibrosis.

YAP Activation Drives Liver Regeneration after Cholestatic Damage Induced by Rbpj Deletion

Liver cholestasis is a chronic liver disease and a major health problem worldwide. Cholestasis is characterised by a decrease in bile flow due to impaired secretion by hepatocytes or by obstruction of bile flow through intra- or extrahepatic bile ducts. Thereby cholestasis can induce ductal proliferation, hepatocyte injury and liver fibrosis. Notch signalling promotes the formation and maturation of bile duct structures. Here we investigated the liver regeneration process in the context of cholestasis induced by disruption of the Notch signalling pathway. Liver-specific deletion of recombination signal binding protein for immunoglobulin kappa j region (Rbpj), which represents a key regulator of Notch signalling, induces severe cholestasis through impaired intra-hepatic bile duct (IHBD) maturation, severe necrosis and increased lethality. Deregulation of the biliary compartment and cholestasis are associated with the change of several signalling pathways including a Kyoto Encyclopedia of Genes and Genomes (KEGG) gene set representing the Hippo pathway, further yes-associated protein (YAP) activation and upregulation of SRY (sex determining region Y)-box 9 (SOX9), which is associated with transdifferentiation of hepatocytes. SOX9 upregulation in cholestatic liver injury in vitro is independent of Notch signalling. We could comprehensively address that in vivo Rbpj depletion is followed by YAP activation, which influences the transdifferentiation of hepatocytes and thereby contributing to liver regeneration.

Protective role of heme oxygenase-1 in fatty liver ischemia-reperfusion injury

Ischemia–reperfusion (IR) injury is a kind of injury resulting from the restoration of the blood supply after blood vessel closure during liver transplantation and is the main cause of graft failure. The pathophysiological mechanisms of hepatic IR include a variety of oxidative stress responses. Hepatic IR is characterized by ischemia and hypoxia inducing oxidative stress, immune response and apoptosis. Fat-denatured livers are also used as donors due to the lack of liver donors. Fatty liver is less tolerant to IR than normal liver. Heme oxygenase (HO) is an enzyme that breaks down hemoglobin to bilirubin, ferrous iron and carbon monoxide (CO). Inducible HO subtype HO-1 is an important protective molecule in mammalian cells used to improve acute and chronic liver injury owing to its characteristic anti-inflammatory and anti-apoptotic qualities. HO-1 degrades heme, and its reaction product CO has been shown to reduce hepatic IR injury and increase the survival rate of grafts. As an induced form of HO, HO-1 also exerts a protective effect against liver IR injury and may be useful as a new strategy of ameliorating this kind of damage. This review summarizes the protective effects of HO-1 in liver IR injury, especially in fatty liver.

Crosstalk between mesenchymal stem cells and T regulatory cells is crucially important for the attenuation of acute liver injury

One of the therapeutic options for the treatment of fulminant hepatitis is repopulation of intrahepatic regulatory cells because their pool is significantly reduced during acute liver failure. Although it is known that mesenchymal stem cells (MSCs), which have beneficent effects in the therapy of fulminant hepatitis, may promote expansion of regulatory T cells (Tregs) and regulatory B cells (Bregs), the role of these regulatory cells in MSC-mediated attenuation of acute liver injury is unknown. Herewith, we described the molecular mechanisms involved in the crosstalk between MSCs and liver regulatory cells and analyzed the potential of MSC-based therapy for the expansion of intrahepatic regulatory cells in mouse model of acute liver failure. MSC-dependent attenuation of α-galactosylceramide (α-GalCer)-induced acute liver injury in mice was accompanied with an increased presence of interleukin (IL) 10-producing CD4⁺ CD25⁺ forkhead box P3⁺ Tregs and IL10- and transforming growth factor β-producing marginal zone-like Bregs in the liver. Depletion of Bregs did not alter MSC-based alleviation of acute liver failure, whereas depletion of Tregs completely abrogated hepatoprotective effects of MSCs and inhibited their capacity to attenuate hepatotoxicity of liver natural killer T cells (NKTs), indicating that Tregs, and not Bregs, were critically involved in MSC-based modulation of acute liver inflammation. MSCs, in a paracrine, indoleamine 2,3-dioxygenase-dependent manner, significantly increased the capacity of Tregs to produce immunosuppressive IL10 and to suppress hepatotoxicity of liver NKTs. Accordingly, adoptive transfer of MSC-primed Tregs resulted in the complete attenuation of α-GalCer-induced acute liver failure. In conclusion, our findings highlighted the crucial importance of Tregs for MSC-based attenuation of acute liver failure and indicated the significance of MSC-mediated priming of Tregs as a new therapeutic approach in Treg-based therapy of acute liver injury. Liver Transplantation 24 687-702 2018 AASLD.

AIM associated with the IgM pentamer: attackers on stand-by at aircraft carrier

Circulating immunoglobulin M (IgM) exists in a pentameric form, possessing a polyreactive nature that responds not only to foreign antigens but also to autoantigens; thus, it is involved in both beneficial and detrimental immune responses, including protection from infection and the progression of autoimmunity. On the other hand, IgM also behaves as a carrier of the apoptosis inhibitor of macrophage (AIM) protein, storing a large amount of the inactivated form of AIM in the blood through this association. Under different disease conditions, AIM can dissociate from IgM locally or systemically to exert its function, inducing the removal of various biological debris such as excess fat, bacteria, cancer cells or dead cell debris. Most typically, upon induction of acute kidney injury (AKI), IgM-free AIM is filtered by the glomerulus in the kidney, which stimulates the clearance of intraluminal dead cells debris at the obstructed proximal tubules, thereby facilitating the repair of kidney injury. Interestingly, cats exhibit a deficiency in AIM release from IgM, which may increase their susceptibility to renal failure. Conversely, association with AIM inhibits IgM binding to the Fcα/μ receptor on follicular dendritic cells at the splenic germinal center, thereby protecting the IgM immune complex from Fcα/μ receptor-mediated internalization, which supports IgM-dependent antigen presentation to B cells and stimulates high-affinity IgG antibody production. The regulation of AIM-IgM binding, resulting from the discovery of reciprocal actions between AIM and IgM, could lead to the development of novel therapies against different diseases.

Hepatic Ischemia/Reperfusion: Mechanisms of Tissue Injury, Repair, and Regeneration

Hepatic ischemia/reperfusion (I/R) injury is a major complication of liver surgery, including liver resection, liver transplantation, and trauma surgery. Much has been learned about the inflammatory injury response induced by I/R, including the cascade of proinflammatory mediators and recruitment of activated leukocytes. In this review, we discuss the complex network of events that culminate in liver injury after I/R, including cellular, protein, and molecular mechanisms. In addition, we address the known endogenous regulatory mediators that function to maintain homeostasis and resolve injury. Finally, we cover more recent insights into how the liver repairs and regenerates after I/R injury, a setting in which physical mass remains unchanged, but functional liver mass is greatly reduced. In this regard, we focus on recent work highlighting a novel role of CXC chemokines as important regulators of hepatocyte proliferation and liver regeneration after I/R injury.

Immunofluorescent characterization of non-myelinating Schwann cells and their interactions with immune cells in mouse mesenteric lymph node

The central nervous system (CNS) influences the immune system in a general fashion by regulating the systemic concentration of humoral substances, whereas the autonomic nervous system communicates specifically with the immune system according to local interactions. Data concerning the mechanisms of this bidirectional crosstalk of the peripheral nervous system (PNS) and immune system remain limited. To gain a better understanding of local interactions of the PNS and immune system, we have used immunofluorescent staining of glial fibrillary acidic protein (GFAP), coupled with confocal microscopy, to investigate the non-myelinating Schwann cell (NMSC)-immune cell interactions in mouse mesenteric lymph nodes. Our results demonstrate i) the presence of extensive NMSC processes and even of cell bodies in each compartment of the mouse mesenteric lymph node; ii) close associations/interactions of NMSC processes with blood vessels (including high endothelial venules) and the lymphatic vessel/sinus; iii) close contacts/associations of NMSC processes with various subsets of dendritic cells (such as CD4+CD11c+, CD8+CD11c+ dendritic cells), macrophages (F4/80+ and CD11b+ macrophages), and lymphocytes. Our novel findings concerning the distribution of NMSCs and NMSC-immune cell interactions inside the mouse lymph node should help to elucidate the mechanisms through which the PNS affects cellular- and humoral-mediated immune responses or vice versa in health and disease.

Apoptosis inhibitor of macrophage ameliorates fungus-induced peritoneal injury model in mice

Fungal peritonitis in a patient on peritoneal dialysis (PD) is a refractory injury accompanied by severe inflammation, predisposing patients to a poor prognosis. Defective clearance of necrotic tissue interferes with amelioration of tissue injury and induces abnormal tissue remodeling. In the recent reports, apoptosis inhibitor of macrophage (AIM, also called CD5L) prevents obesity, hepatocellular carcinoma and acute kidney injury. Here, we investigated potential roles of AIM in prevention of progression of fungal peritonitis models. AIM^−/− mice subjected to zymosan-induced peritonitis exhibited progressive inflammation and sustained peritoneal necrosis tissue on day 28 after the disease induction, whereas there was an improvement in AIM^+/+ mice. This appeared to be caused by deposition of AIM at the necrotic peritoneum in AIM^+/+ mice. In vitro, AIM enhanced the engulfment of necrotic debris by macrophages derived from zymosan-induced peritonitis, M1- and M2a-like bone marrow derived macrophages, as well as by mesothelial cells. In addition, administration of recombinant AIM dramatically ameliorated severe inflammation associated with necrosis in zymosan-induced peritonitis of AIM^−/− mice. Our observations suggest that AIM appears to be involved in the repair process of zymosan-induced peritonitis, and thus, could be the basis of development of new therapeutic strategies for PD-related fungal peritonitis.

C57BL/6 Substrains Exhibit Different Responses to Acute Carbon Tetrachloride Exposure: Implications for Work Involving Transgenic Mice

Biological differences exist between strains of laboratory mice, and it is becoming increasingly evident that there are differences between substrains. In the C57BL/6 mouse, the primary substrains are called 6J and 6N. Previous studies have demonstrated that 6J and 6N mice differ in response to many experimental models of human disease. The aim of our study was to determine if differences exist between 6J and 6N mice in terms of their response to acute carbon tetrachloride (CCl4) exposure. Mice were given CCl4 once and were euthanized 12 to 96 h later. Relative to 6J mice, we found that 6N mice had increased liver injury but more rapid repair. This was because of the increased speed with which necrotic hepatocytes were removed in 6N mice and was directly related to increased recruitment of macrophages to the liver. In parallel, enhanced liver regeneration was observed in 6N relative to 6J mice. Hepatic stellate cell activation occurred earlier in 6N mice, but there was no difference in matrix metabolism between substrains. Taken together, these data demonstrate specific and significant differences in how the C57BL/6 substrains respond to acute CCl4, which has important implications for all mouse studies utilizing this model.

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.

Glucocorticoids Have Opposing Effects on Liver Fibrosis in Hepatic Stellate and Immune Cells

Liver fibrosis is a reversible wound-healing process that is protective in the short term, but prolonged fibrotic responses lead to excessive accumulation of extracellular matrix components that suppresses hepatocyte regeneration, resulting in permanent liver damage. Upon liver damage, nonparenchymal cells including immune cells and hepatic stellate cells (HSCs) have crucial roles in the progression and regression of liver fibrosis. Here, we report differential roles of the glucocorticoid receptor (GR), acting in immune cells and HSCs, in liver fibrosis. In the carbon tetrachloride hepatotoxin-induced fibrosis model, both steroidal and nonsteroidal GR ligands suppressed expression of fibrotic genes and decreased extracellular matrix deposition but also inhibited immune cell infiltration and exacerbated liver injury. These counteracting effects of GR ligands were dissociated in mice with conditional GR knockout in immune cells (GR(LysM)) or HSC (GR(hGFAP)): the impacts of dexamethasone on immune cell infiltration and liver injury were totally blunted in GR(LysM) mice, whereas the suppression of fibrotic gene expression was diminished in GR(hGFAP) mice. The effect of GR activation in HSC was further confirmed in the LX-2 HSC cell line, in which antifibrotic effects were mediated by GR ligand inhibition of Sma and mad-related protein 3 (SMAD3) expression. We conclude that GR has differential roles in immune cells and HSCs to modulate liver injury and liver fibrosis. Specific activation of HSC-GR without alteration of GR activity in immune cells provides a potential therapeutic approach to treatment of hepatic fibrosis.

Apoptosis inhibitor of macrophage protein enhances intraluminal debris clearance and ameliorates acute kidney injury in mice

Acute kidney injury (AKI) is associated with prolonged hospitalization and high mortality, and it predisposes individuals to chronic kidney disease. To date, no effective AKI treatments have been established. Here we show that the apoptosis inhibitor of macrophage (AIM) protein on intraluminal debris interacts with kidney injury molecule (KIM)-1 and promotes recovery from AKI. During AKI, the concentration of AIM increases in the urine, and AIM accumulates on necrotic cell debris within the kidney proximal tubules. The AIM present in this cellular debris binds to KIM-1, which is expressed on injured tubular epithelial cells, and enhances the phagocytic removal of the debris by the epithelial cells, thus contributing to kidney tissue repair. When subjected to ischemia-reperfusion (IR)-induced AKI, AIM-deficient mice exhibited abrogated debris clearance and persistent renal inflammation, resulting in higher mortality than wild-type (WT) mice due to progressive renal dysfunction. Treatment of mice with IR-induced AKI using recombinant AIM resulted in the removal of the debris, thereby ameliorating renal pathology. We observed this effect in both AIM-deficient and WT mice, but not in KIM-1-deficient mice. Our findings provide a basis for the development of potentially novel therapies for AKI.

Role of Hypoxia-Inducible Factors in the Development of Liver Fibrosis

Liver fibrosis remains a significant clinical problem in the United States and throughout the world. Although important advances in the understanding of this disease have been made, no effective pharmacologic agents have been developed that directly prevent or reverse the fibrotic process. Many of the successes in liver fibrosis treatment have been targeted toward treating the cause of fibrosis, such as the development of new antivirals that eradicate hepatitis virus. For many patients, however, this is not feasible, so a liver transplant remains the only viable option. Thus, there is a critical need to identify new therapeutic targets that will slow or reverse the progression of fibrosis in such patients. Research over the last 16 years has identified hypoxia-inducible factors (HIFs) as key transcription factors that drive many aspects of liver fibrosis, making them potential targets of therapy. In this review, we discuss the latest work on HIFs and liver fibrosis, including the cell-specific functions of these transcription factors in the development of liver fibrosis.