Activation and increase of radio-sensitive CD11b+ recruited Kupffer cells/macrophages in diet-induced steatohepatitis in FGF5 deficient mice. Sci Rep. 2016;6:34466. [PMID: 27708340 PMCID: PMC5052649 DOI: 10.1038/srep34466]

Single-cell immune profiling of mouse liver aging reveals Cxcl2+ macrophages recruit neutrophils to aggravate liver injury

BACKGROUND AND AIMS

Immune cells play a crucial role in liver aging. However, the impact of dynamic changes in the local immune microenvironment on age-related liver injury remains poorly understood. We aimed to characterize intrahepatic immune cells at different ages to investigate key mechanisms associated with liver aging.

APPROACH AND RESULTS

We carried out single-cell RNA sequencing on mouse liver tissues at 4 different ages, namely, the newborn, suckling, young, and aged stages. The transcriptomic landscape, cellular classification, and intercellular communication were analyzed. We confirmed the findings by multiplex immunofluorescence staining, flow cytometry, in vitro functional experiments, and chimeric animal models. Nine subsets of 89,542 immune cells with unique properties were identified, of which Cxcl2+ macrophages within the monocyte/macrophage subset were preferentially enriched in the aged liver. Cxcl2+ macrophages presented a senescence-associated secretory phenotype and recruited neutrophils to the aged liver through the CXCL2-CXCR2 axis. Through the secretion of IL-1β and TNF-α, Cxcl2+ macrophages stimulated neutrophil extracellular traps formation. Targeting the CXCL2-CXCR2 axis limited the neutrophils migration toward the liver and attenuated age-related liver injury. Moreover, the relationship between Cxcl2+ macrophages and neutrophils in age-related liver injury was further validated by human liver transplantation samples.

CONCLUSIONS

This in-depth study illustrates that the mechanism of Cxcl2+ macrophage-driven neutrophil activation involves the CXCL2-CXCR2 axis and provides a potential therapeutic strategy for age-related liver injury.

Caveolin-1 is critical for hepatic iron storage capacity in the development of nonalcoholic fatty liver disease

BACKGROUND

Nonalcoholic fatty liver disease (NAFLD) is associated with disordered lipid and iron metabolism. Our previous study has substantiated the pivotal role of Caveolin-1 (Cav-1) in protecting hepatocytes and mediating iron metabolism in the liver. This study aimed to explore the specific mechanisms underlying the regulation of iron metabolism by Cav-1 in NAFLD.

METHODS

Hepatocyte-specific Cav-1 overexpression mice and knockout mice were used in this study. Cav-1-knockdown of RAW264.7 cells and mouse primary hepatocytes were performed to verify the changes in vitro. Moreover, a high-fat diet and palmitic acid plus oleic acid treatment were utilized to construct a NAFLD model in vivo and in vitro, respectively, while a high-iron diet was used to construct an in vivo iron overload model. Besides, iron concentration, the expression of Cav-1 and iron metabolism-related proteins in liver tissue or serum were detected using iron assay kit, Prussian blue staining, Western blotting, immunofluorescence staining, immunohistochemical staining and ELISA. The related indicators of lipid metabolism and oxidative stress were evaluated by the corresponding reagent kit and staining.

RESULTS

Significant disorder of lipid and iron metabolism occurred in NAFLD. The expression of Cav-1 was decreased in NAFLD hepatocytes (P < 0.05), accompanied by iron metabolism disorder. Cav-1 enhanced the iron storage capacity of hepatocytes by activating the ferritin light chain/ferritin heavy chain pathway in NAFLD, subsequently alleviating the oxidative stress induced by excess ferrous ions in the liver. Further, CD68+CD163+ macrophages expressing Cav-1 were found to accelerate iron accumulation in the liver, which was contrary to the effect of Cav-1 in hepatocytes. Positive correlations were also observed between the serum Cav-1 concentration and the serum iron-related protein levels in NAFLD patients and healthy volunteers (P < 0.05).

CONCLUSIONS

These findings confirm that Cav-1 is an essential target protein that regulates iron and lipid metabolic homeostasis. It is a pivotal molecule for predicting and protecting against the development of NAFLD.

Novel phenotypical and functional sub-classification of liver macrophages highlights changes in population dynamics in experimental mouse models

Liver macrophages are critical components of systemic immune system defense mechanisms. F4/80high Kupffer cells (KCs) are the predominant liver-resident macrophages and the first immune cells to contact pathogens entering the liver. F4/80low monocyte-derived macrophages (MoMφs) are essential macrophages that modulate liver immune functions. Here we report a novel method of identifying subpopulations of these two populations using traditional flow cytometry and examine each subpopulation for its putative roles in the pathogenesis of an experimental non-alcoholic steatohepatitis model. Using male C57BL/6 mice, we isolated and analyzed liver non-parenchymal cells by flow cytometry. We identified F4/80high and F4/80low macrophage populations and characterized subpopulations using uniform manifold approximation and projection. We identified three subpopulations in F4/80high macrophages: CD163(+) KCs, CD163(-) KCs, and liver capsular macrophages. CD163(+) KCs had higher phagocytic and bactericidal activities and more complex cellular structures than CD163(-) KCs. We also identified four subpopulations of F4/80low MoMφs based on Ly6C and MHC class II expression: infiltrating monocytes, pro-inflammatory MoMφs, Ly6C(-) monocytes, and conventional dendritic cells. CCR2 knock-out mice expressed lower levels of these monocyte-derived cells, and the count varied by subpopulation. In high-fat- and cholesterol-diet-fed mice, only one subpopulation, pro-inflammatory MoMφs, significantly increased in count. This indicates that changes to this subpopulation is the first step in the progression to non-alcoholic steatohepatitis. The community can use our novel subpopulation and gating strategy to better understand complex immunological mechanisms in various liver disorders through detailed analysis of these subpopulations.

The biology of VSIG4: Implications for the treatment of immune-mediated inflammatory diseases and cancer

V-set and immunoglobulin domain containing 4 (VSIG4), a type I transmembrane receptor exclusively expressed in a subset of tissue-resident macrophages, plays a pivotal role in clearing C3-opsonized pathogens and their byproducts from the circulation. VSIG4 maintains immune homeostasis by suppressing the activation of complement pathways or T cells and inducing regulatory T-cell differentiation, thereby inhibiting the development of immune-mediated inflammatory diseases but enhancing cancer progression. Consequently, VSIG4 exhibits a potential therapeutic effect for immune-mediated inflammatory diseases, but also is regarded as a novel target of immune checkpoint inhibition in cancer therapy. Recently, soluble VSIG4, the extracellular domain of VSIG4, shed from the surface of macrophages, has been found to be a biomarker to define macrophage activation-related diseases. This review mainly summarizes recent new findings of VSIG4 in macrophage phagocytosis and immune homeostasis, and discusses its potential diagnostic and therapeutic usage in infection, inflammation, and cancer.

Fibroblast growth factor 5 overexpression ameliorated lipopolysaccharide-induced apoptosis of hepatocytes through regulation of the phosphoinositide-3-kinase/protein kinase B pathway

BACKGROUND

Sepsis is a systemic inflammatory syndrome induced by several infectious agents. Multiple organs are affected by sepsis, including the liver, which plays an important role in metabolism and immune homeostasis. Fibroblast growth factors (FGFs) participate in several biological processes, although the role of FGF5 in sepsis is unclear.

METHODS

In this study, lipopolysaccharide (LPS) was administrated to mice to establish a sepsis-induced liver injury. A similar in vitro study was conducted using L-02 hepatocytes. Western blot and immunohistochemistry staining were performed to evaluate the FGF5 expression level in liver tissues and cells. Inflammatory cell infiltrations, cleaved-caspase-3 expressions, reactive oxygen species and levels of inflammatory cytokines were detected by immunofluorescence, dihydroethidium staining, and reverse transcription quantitative polymerase chain reaction analysis, respectively. Flow cytometry was used to detect the apoptosis level of cells. In addition, ribonucleic acid (RNA)-sequencing was applied to explore the possible mechanism by which FGF5 exerted effects.

RESULTS

LPS administration caused FGF5 down-regulation in the mouse liver as well as in L-02 hepatocytes. Additionally, with FGF5 overexpression, liver injury and the level of hepatocyte apoptosis were ameliorated. Further, RNA sequencing performed in hepatocytes revealed the phosphoinositide-3-kinase/protein kinase B (PI3K/AKT) pathway as a possible pathway regulated by FGF5 . This was supported using an inhibitor of the PI3K/AKT pathway, which abrogated the protective effect of FGF5 in LPS-induced hepatocyte injury.

CONCLUSION

The anti-apoptotic effect of FGF5 on hepatocytes suffering from LPS has been demonstrated and was dependent on the activation of the PI3K/AKT signaling pathway.

Suppressing circ_0008494 inhibits HSCs activation by regulating the miR-185-3p/Col1a1 axis

Background: Hepatic fibrosis (HF) is characterized by activation of hepatic stellate cells (HSCs) and extensive deposition of extracellular matrix components, especially collagens. However, effective antifibrotic therapies are still lacking. Recently, circular RNAs (circRNAs) have been identified as novel regulators of HF. Methods: circRNAs profile was screened by RNA sequencing and the location of circ_0008494 was confirmed by fluorescence in situ hybridization assay in human HF tissues. Bioinformatics analysis was used for result prediction and dual-luciferase reporter, together with AGO-RIP and biotin-coupled miRNA capture assays, were used to determine miR-185-3p/collagen type I alpha 1 chain (Col1a1) as the target of circ_0008494. A stable circ_0008494-interfering human HSCs cell line was constructed and used to determine the regulatory mechanism of circ_0008494/miR-185-3p/Col1a1 axis. Results: circ_0008494 was abundantly and significantly over-expressed in human HF tissues and located at the cytoplasm of HSCs. Together, dual-luciferase reporter, AGO-RIP and biotin-coupled miRNA capture assays confirmed that circ_0008494 acted as a sponge of miR-185-3p. Cell functional experiments and rescue assays demonstrated suppressing circ_0008494 could inhibit activation, proliferation, migration of HSCs and promote their apoptosis through miR-185-3p. In particular, the HF indicator, Col1a1, was validated as the direct target of miR-185-3p and the suppression of circ_0008494 inhibited the expression of Col1a1 by releasing miR-185-3p. Conclusion: Knocking down circ_0008494 inhibited HSCs activation through the miR-185-3p/Col1a1 axis. circ_0008494 could be a promising treatment target for HF.

FGF5 alleviated acute lung injury via AKT signal pathway in endothelial cells

Acute lung injury (ALI), with high morbidity and mortality, is mainly resulted by infectious or non-infectious inflammatory stimulators, and it will further evolve into acute respiratory distress syndrome if not controlled. Fibroblast growth factors (FGFs) consist of more than 23 kinds of members, which are involved in various pathophysiological processes of body. However, the effect of FGF5, one member of FGFs, is still not certain in lipopolysaccharide (LPS)-induced ALI. In this study, we explored the possible impacts of FGF5 in LPS-induced ALI and primarily focused on endothelial cell, which was one of the most vulnerable cells in septic ALI. In the mouse group of FGF5 overexpression, LPS-induced lung injuries were mitigated, as well as the pyroptosis levels of pulmonary vascular endothelial cells. Additionally, in vitro human umbilical vein endothelial cells (HUVECs), our results showed that the level of cell pyroptosis was ameliorated with FGF5 overexpression, and AKT signal was activated with the overexpression of FGF5, whereas after administration of MK2206, an inhibitor of AKT signal, the protection of FGF5 was inhibited. Therefore, these results suggested that FGF5 exerted protective effects in endothelial cells exposed to LPS, and this protection of FGF5 could be attributed to activated AKT signal.

Macrophages, Low-Grade Inflammation, Insulin Resistance and Hyperinsulinemia: A Mutual Ambiguous Relationship in the Development of Metabolic Diseases

Metabolic derangement with poor glycemic control accompanying overweight and obesity is associated with chronic low-grade inflammation and hyperinsulinemia. Macrophages, which present a very heterogeneous population of cells, play a key role in the maintenance of normal tissue homeostasis, but functional alterations in the resident macrophage pool as well as newly recruited monocyte-derived macrophages are important drivers in the development of low-grade inflammation. While metabolic dysfunction, insulin resistance and tissue damage may trigger or advance pro-inflammatory responses in macrophages, the inflammation itself contributes to the development of insulin resistance and the resulting hyperinsulinemia. Macrophages express insulin receptors whose downstream signaling networks share a number of knots with the signaling pathways of pattern recognition and cytokine receptors, which shape macrophage polarity. The shared knots allow insulin to enhance or attenuate both pro-inflammatory and anti-inflammatory macrophage responses. This supposedly physiological function may be impaired by hyperinsulinemia or insulin resistance in macrophages. This review discusses the mutual ambiguous relationship of low-grade inflammation, insulin resistance, hyperinsulinemia and the insulin-dependent modulation of macrophage activity with a focus on adipose tissue and liver.

Effects of a CCR2 antagonist on macrophages and Toll-like receptor 9 expression in a mouse model of diabetic nephropathy

The pathogenesis of diabetic nephropathy (DN) is related to macrophage (Mφ) recruitment to the kidneys, tumor necrosis factor-α (TNF-α) production, and oxidative stress. Toll-like receptor 9 (TLR9) activation is reportedly involved in systemic inflammation, and it exacerbates this condition in metabolic syndrome. Therefore, we hypothesized that TLR9 plays a role in the pathogenesis of DN. Two subsets of kidney Mφs in DN model (db/db) mice were analyzed using flow cytometry to evaluate their distribution and TLR9 expression and function. Mice were administered the CCR2 antagonist INCB3344 for 8 wk; changes in Mφ distribution and function and its therapeutic effects on DN pathology were examined. Bone marrow-derived CD11bhigh (BM-Mφ) and tissue-resident CD11blow Mφs (Res-Mφ) were identified in the mouse kidneys. As DN progressed, the BM-Mφ number, TLR9 expression, and TNF-α production increased significantly. In Res-Mφs, reactive oxygen species (ROS) production and phagocytic activity were enhanced. INCB3344 decreased albuminuria, serum creatinine level, BM-Mφ abundance, TLR9 expression, and TNF-α production by BM-Mφs and ROS production by Res-Mφs. Both increased activation of BM-Mφ via TLR9 and TNF-α production and increased ROS production by Res-Mφs were involved in DN progression. Thus, inactivating Mφs and their TLR9 expression by INCB3344 is a potential therapeutic strategy for DN.NEW & NOTEWORTHY We classified kidney macrophages (Mφs) into bone marrow-derived Mφs (BM-Mφs) expressing high CD11b and tissue-specific resident Mφ (Res-Mφs) expressing low CD11b. In diabetic nephropathy (DN) model mice, Toll-like receptor 9 (TLR9) expression and TNF-α production via TLR9 activation in BM-Mφs and ROS production in Res-Mφs were enhanced. Furthermore, CCR2 antagonist suppressed the kidney infiltration of BM-Mφs and their function and the ROS production by Res-Mφs, with concomitant TLR9 suppression. Our study presents a new therapeutic strategy for DN.

Hepatocellular Carcinoma in Non-Alcoholic Fatty Liver Disease: From Epidemiology to Diagnostic Approach

Non-alcoholic fatty liver disease (NAFLD) is becoming the leading cause of liver morbidity worldwide and, as such, represents the pathogenic background for the increasing incidence of hepatocellular carcinoma (HCC). The annual incidence of NAFLD-related HCC is expected to increase by 45-130% by 2030. Diabetes mellitus is the most important risk factor for HCC development in NAFLD, with the risk further increased when associated with other metabolic traits, such as obesity, arterial hypertension and dyslipidemia. The highest risk of HCC exists in patients with advanced fibrosis or cirrhosis, although 20-50% of HCC cases arise in NAFLD patients with an absence of cirrhosis. This calls for further investigation of the pathogenic mechanisms that are involved in hepatocarcinogenesis, including genetics, metabolomics, the influence of the gut microbiota and immunological responses. Early identification of patients with or at risk of NAFLD is of utmost importance to improve outcomes. As NAFLD is highly prevalent in the community, the identification of cases should rely upon simple demographic and clinical characteristics. Once identified, these patients should then be evaluated for the presence of advanced fibrosis or cirrhosis and subsequently enter HCC surveillance programs if appropriate. A significant problem is the early recognition of non-cirrhotic NAFLD patients who will develop HCC, where new biomarkers and scores are potential solutions to tackle this issue.

Hepatic Macrophages Express Melanoma Differentiation-Associated Gene 5 in Nonalcoholic Steatohepatitis

The activation of innate immune system is essential for the pathogenesis of nonalcoholic steatohepatitis (NASH). Among pattern recognition receptors, it is well-characterized that toll-like receptors (TLRs) are deeply involved in the development of NASH to reflect exposure of the liver to gut-driven endotoxins. In contrast, it has not been elucidated whether retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs) are similarly implicated in the disease progression. In the present study, we examined the expression of melanoma differentiation-associated antigen 5 (MDA5), known to be a member of RLRs, in a diet-induced murine model of NASH. The liver tissues were collected from C57BL/6 J mice at 1, 3, and 6 weeks after choline-deficient L-amino acid-defined high-fat diet (CDAHFD), and the expression of MDA5 was analyzed by western blotting, immunofluorescence (IF), and real-time quantitative PCR (qPCR). The results of western blotting showed that hepatic expression of MDA5 was increased at 3 and 6 weeks. In IF, MDA5-positive cells co-expressed F4/80 and CD11b, indicating they were activated macrophages, and these cells began to appear at 1 week after CDAHFD. The mRNA expression of MDA5 was significantly upregulated at 1 week. Additionally, we performed IF using liver biopsy specimens collected from 11 patients with nonalcoholic fatty liver diseases (NAFLD), and found that MDA5-positive macrophages were detected in eight out of eleven patients. In an in vitro study, MDA5 was induced upon stimulation with lipopolysaccharide in murine bone marrow-derived macrophages and THP-1 cells. Our findings suggest that MDA5 may be involved in the inflammation of NASH.

Contrasting functional responses of resident Kupffer cells and recruited liver macrophages to irradiation and liver X receptor stimulation

In the murine liver, there are two major macrophage populations, namely resident Kupffer cells (resKCs) with phagocytic activity and recruited macrophages (recMφs) with cytokine-producing capacity. This study was performed to clarify the functional differences between these two populations, focusing on their susceptibility to radiation and response to stimulation via liver X receptors (LXRs), which are implicated in cholesterol metabolism and immune regulation. Liver mononuclear cells (MNCs) were obtained from C57BL/6 (WT) mice with or without 2 Gy irradiation, and the phagocytic activity against Escherichia coli (E. coli) as well as TNF-α production were compared between the two macrophage populations. To assess LXR functions, phagocytosis, TNF-α production, and endocytosis of acetylated low-density lipoprotein (LDL) were compared after synthetic LXR ligand stimulation. Furthermore, LXRα/β knockout (KO) mice and LXRα KO mice were compared with WT mice. Irradiation decreased intracellular TNF-α production by recMφs but did not affect the phagocytic activity of resKCs. In vitro LXR stimulation enhanced E. coli phagocytosis by resKCs but decreased E. coli-stimulated TNF-α production by recMφs. Phagocytic activity and acetylated LDL endocytosis were decreased in both LXRα/β KO mice and LXRα KO mice, with serum TNF-α levels after E. coli injection in the former being higher than those in WT mice. In conclusion, resKCs and recMφs exhibited different functional features in response to radiation and LXR stimulation, highlighting their distinct roles liver immunity and lipid metabolism.

Role of fibroblast growth factor signalling in hepatic fibrosis

Fibrotic remodelling is a highly conserved protective response to tissue injury and it is essential for the maintenance of structural and functional tissue integrity. Also hepatic fibrosis can be considered as a wound-healing response to liver injury, reflecting a balance between liver repair and scar formation. In contrast, pathological fibrosis corresponds to impaired wound healing. Usually, the liver regenerates after acute injury. However, if the damaging mechanisms persist, the liver reacts with progressive and uncontrolled accumulation of extracellular matrix proteins. Eventually, excessive fibrosis can lead to cirrhosis and hepatic failure. Furthermore, cirrhosis is the major risk factor for the development of hepatocellular cancer (HCC). Therefore, hepatic fibrosis is the most critical pathological factor that determines the morbidity and mortality of patients with chronic liver disease. Still, no effective anti-fibrogenic therapies exist, despite the very high medical need. The regulation of fibroblast growth factor (FGF) signalling is a prerequisite for adequate wound healing, repair and homeostasis in various tissues and organs. The FGF family comprises 22 proteins that can be classified into paracrine, intracrine and endocrine factors. Most FGFs signal through transmembrane tyrosine kinase FGF receptors (FGFRs). Although FGFRs are promising targets for the treatment of HCC, the expression and function of FGFR-ligands in hepatic fibrosis is still poorly understood. This review summarizes the latest advances in our understanding of FGF signalling in hepatic fibrosis. Furthermore, the potential of FGFs as targets for the treatment of hepatic fibrosis and remaining challenges for the field are discussed.

Hydrogen-rich water protects against liver injury in nonalcoholic steatohepatitis through HO-1 enhancement via IL-10 and Sirt 1 signaling

Nonalcoholic steatohepatitis (NASH) could progress to hepatic fibrosis in the absence of effective control. The purpose of our experiment was to investigate the protective effect of drinking water with a high concentration of hydrogen, namely, hydrogen-rich water (HRW), on mice with nonalcoholic fatty liver disease to elucidate the mechanism underlying the therapeutic action of molecular hydrogen. The choline-supplemented, l-amino acid-defined (CSAA) or the choline-deficient, l-amino acid-defined (CDAA) diet for 20 wk was used to induce NASH and fibrosis in the mice model and simultaneously treated with the high-concentration 7-ppm HRW for different periods (4 wk, 8  wk, and 20 wk). Primary hepatocytes were stimulated by palmitate to mimic liver lipid metabolism during fatty liver formation. Primary hepatocytes were cultured in a closed vessel filled with 21% O₂ + 5% CO₂ + 3.8% H₂ and N₂ as the base gas to verify the response of primary hepatocytes in a high concentration of hydrogen gas in vitro. Mice in the CSAA + HRW group had lower serum levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) and milder histological damage. The inflammatory cytokines were expressed at lower levels in the HRW group than in the CSAA group. Importantly, HRW reversed hepatocyte fatty acid oxidation and lipogenesis as well as hepatic inflammation and fibrosis in preexisting hepatic fibrosis specimens. Molecular hydrogen inhibits the lipopolysaccharide-induced production of inflammation cytokines through increasing heme oxygenase-1 (HO-1) expression. Furthermore, HRW improved hepatic steatosis in the CSAA + HRW group. Sirtuin 1 (Sirt1) induction by molecular hydrogen via the HO-1/adenosine monophosphate activated protein kinase (AMPK)/peroxisome proliferator-activated receptor α (PPARα)/peroxisome proliferator-activated receptor γ (PPAR-γ) pathway suppresses palmitate-mediated abnormal fat metabolism. Orally administered HRW suppressed steatosis induced by CSAA and attenuated fibrosis induced by CDAA, possibly by reducing oxidative stress and the inflammation response.NEW & NOTEWORTHY The mRNA expression of inflammatory cytokines in the HRW group was lower than in the CSAA group. HRW reversed hepatocyte apoptosis as well as hepatic inflammation and fibrosis in NASH specimens. Molecular hydrogen inhibits LPS-induced inflammation via an HO-1/interleukin 10 (IL-10)-independent pathway. HRW improved hepatic steatosis in the CSAA + HRW group. Sirt1 induction by molecular hydrogen via the HO-1/AMPK/PPARα/PPARγ pathway suppresses palmitate-mediated abnormal fat metabolism.

Tripartite Motif-Containing 27 Attenuates Liver Ischemia/Reperfusion Injury by Suppressing Transforming Growth Factor β-Activated Kinase 1 (TAK1) by TAK1 Binding Protein 2/3 Degradation

BACKGROUND AND AIMS

Hepatic ischemia-reperfusion (I/R) injury, which mainly involves inflammatory responses and apoptosis, is a common cause of organ dysfunction in liver transplantation (LT). As a critical mediator of inflammation and apoptosis in various cell types, the role of tripartite motif-containing (TRIM) 27 in hepatic I/R injury remains worthy of study.

APPROACH AND RESULTS

This study systemically evaluated the putative role of TRIM27/transforming growth factor β-activated kinase 1 (TAK1)/JNK (c-Jun N-terminal kinase)/p38 signaling in hepatic I/R injury. TRIM27 expression was significantly down-regulated in liver tissue from LT patients, mice subjected to hepatic I/R surgery, and hepatocytes challenged by hypoxia/reoxygenation (H/R) treatment. Subsequently, using global Trim27 knockout mice (Trim27-KO mice) and hepatocyte-specific Trim27 transgenic mice (Trim27-HTG mice), TRIM27 functions to ameliorate liver damage, reduce the inflammatory response, and prevent cell apoptosis. In parallel in vitro studies, activating TRIM27 also prevented H/R-induced hepatocyte inflammation and apoptosis. Mechanistically, TRIM27 constitutively interacted with the critical components, TAK1 and TAK1 binding protein 2/3 (TAB2/3), and promoted the degradation of TAB2/3, leading to inactivation of TAK1 and the subsequent suppression of downstream JNK/p38 signaling.

CONCLUSIONS

TRIM27 is a key regulator of hepatic I/R injury by mediating the degradation of TAB2/3 and suppression of downstream TAK1-JNK/p38 signaling. TRIM27 may be a promising approach to protect the liver against I/R-mediated hepatocellular damage in transplant recipients.

Trans-portal hepatic infusion of cultured bone marrow-derived mesenchymal stem cells in a steatohepatitis murine model

The incidence of nonalcoholic steatohepatitis-related liver cirrhosis is increasing. We used a steatohepatitis murine model fed a choline-deficient, l-amino acid-defined (CDAA) diet with a single injection of carbon tetrachloride (CCl₄) to evaluate the efficacy of trans-portal hepatic infusion of bone marrow-derived mesenchymal stem cells (BMSCs) for liver fibrosis, liver steatosis, and oxidative stress. Mice were fed a CDAA diet and injected with a single intraperitoneal dose of CCl₄ (0.5 ml/kg) after 4 weeks of CDAA diet. After 12 weeks of CDAA diet, 1 × 10⁶ luciferase-positive syngeneic BMSCs (Luc-BMSCs) were infused into the animal spleen. An in vivo imaging system was used to confirm Luc-BMSC accumulation in the liver via the portal vein, and at 4 weeks after infusion, we compared liver fibrosis, liver steatosis, and oxidative stress. After the BMSC-infusion, serum albumin and serum total bilirubin were significantly improved. Liver fibrosis assessed by Sirius red staining, α-smooth muscle actin protein, and collagen 1A1 mRNA expression was significantly suppressed. Furthermore, liver steatosis area was significantly lower, the 8-hydroxy-2'-deoxyguanosine-positive cells were significantly fewer, and superoxide dismutase 2 protein expression of the liver was significantly increased. In conclusion, our data confirmed the efficacy of trans-portal hepatic infusion of BMSCs in a steatohepatitis murine model.

Loss of Hepatic Carcinoembryonic Antigen-Related Cell Adhesion Molecule 1 Links Nonalcoholic Steatohepatitis to Atherosclerosis

Patients with nonalcoholic fatty liver disease/steatohepatitis (NAFLD/NASH) commonly develop atherosclerosis through a mechanism that is not well delineated. These diseases are associated with steatosis, inflammation, oxidative stress, and fibrosis. The role of insulin resistance in their pathogenesis remains controversial. Albumin (Alb)Cre⁺Cc1^flox(fl)/fl mice with the liver‐specific null deletion of the carcinoembryonic antigen‐related cell adhesion molecule 1 (Ceacam1; alias Cc1) gene display hyperinsulinemia resulting from impaired insulin clearance followed by hepatic insulin resistance, elevated de novo lipogenesis, and ultimately visceral obesity and systemic insulin resistance. We therefore tested whether this mutation causes NAFLD/NASH and atherosclerosis. To this end, mice were propagated on a low‐density lipoprotein receptor (Ldlr)^−/− background and at 4 months of age were fed a high‐cholesterol diet for 2 months. We then assessed the biochemical and histopathologic changes in liver and aortae. Ldlr^−/−AlbCre⁺Cc1^fl/fl mice developed chronic hyperinsulinemia with proatherogenic hypercholesterolemia, a robust proinflammatory state associated with visceral obesity, elevated oxidative stress (reduced NO production), and an increase in plasma and tissue endothelin‐1 levels. In parallel, they developed NASH (steatohepatitis, apoptosis, and fibrosis) and atherosclerotic plaque lesions. Mechanistically, hyperinsulinemia caused down‐regulation of the insulin receptor followed by inactivation of the insulin receptor substrate 1–protein kinase B–endothelial NO synthase pathway in aortae, lowering the NO level. This also limited CEACAM1 phosphorylation and its sequestration of Shc‐transforming protein (Shc), activating the Shc–mitogen‐activated protein kinase–nuclear factor kappa B pathway and stimulating endothelin‐1 production. Thus, in the presence of proatherogenic dyslipidemia, hyperinsulinemia and hepatic insulin resistance driven by liver‐specific deletion of Ceacam1 caused metabolic and vascular alterations reminiscent of NASH and atherosclerosis. Conclusion: Altered CEACAM1‐dependent hepatic insulin clearance pathways constitute a molecular link between NASH and atherosclerosis.

Fibroblast growth factor signaling in non-alcoholic fatty liver disease and non-alcoholic steatohepatitis: Paving the way to hepatocellular carcinoma

Metabolic disorders are increasingly leading to non-alcoholic fatty liver disease, subsequent steatohepatitis, cirrhosis and hepatocellular carcinoma. Fibroblast growth factors and their receptors play an important role in maintaining metabolic homeostasis also in the liver and disorders in signaling have been identified to contribute to those pathophysiologic conditions leading to hepatic lipid accumulation and chronic inflammation. While specific and well tolerated inhibitors of fibroblast growth factor receptor activity are currently developed for (non-liver) cancer therapy, treatment of non-alcoholic fatty liver disease and non-alcoholic steatohepatitis is still limited. Fibroblast growth factor-mimicking or restoring approaches have recently evolved as a novel therapeutic option and the impact of such interactions with the fibroblast growth factor receptor signaling network during non-alcoholic fatty liver disease/non-alcoholic steatohepatitis development is reviewed here.

Inflammation and Ectopic Fat Deposition in the Aging Murine Liver Is Influenced by CCR2

Aging is associated with inflammation and metabolic syndrome, which manifests in the liver as nonalcoholic fatty liver disease (NAFLD). NAFLD can range in severity from steatosis to fibrotic steatohepatitis and is a major cause of hepatic morbidity. However, the pathogenesis of NAFLD in naturally aged animals is unclear. Herein, we performed a comprehensive study of lipid content and inflammatory signature of livers in 19-month-old aged female mice. These animals exhibited increased body and liver weight, hepatic triglycerides, and inflammatory gene expression compared with 3-month-old young controls. The aged mice also had a significant increase in F4/80⁺ hepatic macrophages, which coexpressed CD11b, suggesting a circulating monocyte origin. A global knockout of the receptor for monocyte chemoattractant protein (CCR2) prevented excess steatosis and inflammation in aging livers but did not reduce the number of CD11b⁺ macrophages, suggesting changes in macrophage accumulation precede or are independent from chemokine (C-C motif) ligand-CCR2 signaling in the development of age-related NAFLD. RNA sequencing further elucidated complex changes in inflammatory and metabolic gene expression in the aging liver. In conclusion, we report a previously unknown accumulation of CD11b⁺ macrophages in aged livers with robust inflammatory and metabolic transcriptomic changes. A better understanding of the hallmarks of aging in the liver will be crucial in the development of preventive measures and treatments for end-stage liver disease in elderly patients.

Pioglitazone improves phagocytic activity of liver recruited macrophages in elderly mice possibly by promoting glucose catabolism

Recent studies have revealed that the immunological function of leukocytes is dependent on their cellular metabolism, and some researchers have advocated the beneficial effects of pioglitazone against sepsis in young mice, although bacterial infections are more prevalent in elderly hosts. Here, we investigated pioglitazone’s preventative effect against sepsis induced by intravenous injection of a lethal dose of Escherichia coli in elderly mice (50–60 wk old) and examined its immunological and metabolic effects on liver leukocytes. Pioglitazone improved bacterial elimination in the peripheral blood, lowered serum pro-inflammatory cytokines (TNF-α, IL-12, IFN-γ), and prevented septic death. It also enhanced bacterial elimination in the liver, by increasing the phagocytic and bactericidal activities of liver F4/80+CD11b+ recruited macrophages (Mφ), their CD206 expression and reactive oxygen species production. Quantitative PCR revealed that pioglitazone treatment enhanced gene expression of rate-limiting enzymes for glycolysis in hepatic CD11b+ cells (including neutrophils and recruited Mφ), and their improved phagocytic and bactericidal activities were abolished by glycolysis inhibiting reagents. These findings present the possibility that pioglitazone strengthens the phagocytic and bactericidal activities of liver recruited Mφ and that these immunological activities are closely associated with their glucose catabolism.

Frontline Science: Concanavalin A-induced acute hepatitis is attenuated in vitamin D receptor knockout mice with decreased immune cell function

The vitamin D receptor (VDR) is a nuclear receptor for the active form of vitamin D, 1α,25-dihydroxyvitamin D₃ , and regulates various physiologic processes, such as bone and calcium metabolism, cellular proliferation and differentiation, and immunity. VDR is highly expressed in the intestine, kidney, bone, and macrophages, but is expressed at a low level in the liver. The liver is a major metabolic organ and also acts as an immune gateway for dietary nutrients and xenobiotics. In this study, we investigated the function of VDR in hepatic immune cells, such as Kupffer cells/macrophages, utilizing VDR knockout (KO) mice. We showed that VDR is functionally expressed in hepatic mononuclear cells, specifically resident Kupffer cells. We examined the role of VDR in acute hepatitis induced by concanavalin A (Con-A) and found that Con-A-induced hepatitis is attenuated in VDR-KO mice compared to wild-type (WT) mice. Con-A-induced hepatitis is known to be mediated by NKT cell activation, cytokine production, and reactive oxygen species (ROS) production in Kupffer cells/macrophages. However, the proportions of Kupffer cells/macrophages and the NKT cell activation were similar in the liver of WT and VDR-KO mice and inflammatory cytokine gene expression was increased in VDR-KO mice. On the other hand, plasma and hepatic ROS levels were decreased in the liver of VDR-KO mice compared to WT mice. The phagocytic activity of resident Kupffer cells and hepatic neutrophils were also decreased in VDR-KO mice. Therefore, VDR is necessary for Con-A-induced acute hepatitis and plays an important role in hepatic immune cell functions.

Macrophages in the Aging Liver and Age-Related Liver Disease

The number of individuals aged 65 or older is projected to increase globally from 524 million in 2010 to nearly 1. 5 billion in 2050. Aged individuals are particularly at risk for developing chronic illness, while being less able to regenerate healthy tissue and tolerate whole organ transplantation procedures. In the liver, these age-related diseases include non-alcoholic fatty liver disease, alcoholic liver disease, hepatitis, fibrosis, and cirrhosis. Hepatic macrophages, a population comprised of both Kupffer cells and infiltrating monocyte derived macrophages, are implicated in several chronic liver diseases and also play important roles in the homeostatic functions of the liver. The effects of aging on hepatic macrophage population dynamics, polarization, and function are not well understood. Studies performed on macrophages derived from other aged sources, such as the bone marrow, peritoneal cavity, lungs, and brain, demonstrate general reductions in autophagy and phagocytosis, dysfunction in cytokine signaling, and altered morphology and distribution, likely mediated by epigenetic changes and mitochondrial defects, that may be applicable to hepatic macrophages. This review highlights recent findings in macrophage developmental biology and function, particularly in the liver, and discusses the role of macrophages in various age-related liver diseases. A better understanding of the biology of aging that influences hepatic macrophages and thus the progression of chronic liver disease will be crucial in order to develop new interventions and treatments for liver disease in aging populations.

Dysregulation of Kupffer Cells/Macrophages and Natural Killer T Cells in Steatohepatitis in LXRα Knockout Male Mice

Liver X receptor (LXR) α expression is mainly localized to metabolic tissues, such as the liver, whereas LXRβ is ubiquitously expressed. LXRα is activated by oxysterols and plays an important role in the regulation of lipid metabolism in metabolic tissues. In macrophages, LXRs stimulate reverse cholesterol transport and regulate immune responses. Although a high-cholesterol diet induces severe steatohepatitis in LXRα-knockout (KO) mice, the underlying mechanisms linking lipid metabolism and immune responses remain largely unknown. In this study, we investigated the role of LXRα in the pathogenesis of steatohepatitis by assessing the effects of a high-fat and high-cholesterol diet (HFCD) on hepatic immune cell proportion and function as well as lipid metabolism in wild-type (WT) and LXRα-KO mice. HFCD feeding induced severe steatohepatitis in LXRα-KO mice compared with WT mice. These mice had higher cholesterol levels in the plasma and the liver and dysregulated expression of LXR target and proinflammatory genes in both whole liver samples and isolated hepatic mononuclear cells. Flow cytometry showed an increase in CD68+CD11b+ Kupffer cells/macrophages and a decrease in invariant natural killer T cells in the liver of HFCD-fed LXRα-KO mice. These mice were more susceptible to lipopolysaccharide-induced liver injury and resistant to inflammatory responses against α-galactosylceramide or concanavalin-A treatment. The findings provide evidence for activation of bone marrow-derived Kupffer cells/macrophages and dysfunction of invariant natural killer T cells in LXRα-KO mouse liver. These findings indicate that LXRα regulates hepatic immune function along with lipid metabolism and protects against the pathogenesis of nonalcoholic steatohepatitis.

Non-alcoholic steatohepatitis induces transient changes within the liver macrophage pool

Kupffer cells (KCs) and monocyte-derived macrophages are implicated in non-alcoholic steatohepatitis (NASH) pathogenesis but their functions remain unclear due to the lack of specific markers to distinguish between the different cell types. Additionally, it is unclear if multiple subsets of KCs are present during NASH. Here, we characterized the liver macrophage subsets during methionine/choline deficient (MCD) diet-induced NASH and recovery. We observed a significant reduced contribution of Ly6C^loClec4F⁺Tim4⁺KCs to the hepatic macrophage pool in MCD fed mice, which normalized during recovery. Ly6C^loClec4F^-Tim4^- monocyte-derived macrophages increased during MCD feeding and returned to baseline during recovery. Ly6C^loClec4F⁺Tim4^- monocyte-derived KCs developed during initial recovery but did not self-renew as their numbers were reduced after full recovery. Initial recovery from MCD diet feeding was further characterized by increased proportions of Ki-67⁺ proliferating KCs. In conclusion, the hepatic macrophage pool undergoes substantial albeit transient changes during NASH and recovery, with the KC pool being maintained by proliferation and differentiation of short-lived monocyte-derived KCs.

Inhibition of connexin hemichannels alleviates non-alcoholic steatohepatitis in mice

While gap junctions mediate intercellular communication and support liver homeostasis, connexin hemichannels are preferentially opened by pathological stimuli, including inflammation and oxidative stress. The latter are essential features of non-alcoholic steatohepatitis. In this study, it was investigated whether connexin32 and connexin43 hemichannels play a role in non-alcoholic steatohepatitis. Mice were fed a choline-deficient high-fat diet or normal diet for 8 weeks. Thereafter, TAT-Gap24 or TAT-Gap19, specific inhibitors of hemichannels composed of connexin32 and connexin43, respectively, were administered for 2 weeks. Subsequently, histopathological examination was carried out and various indicators of inflammation, liver damage and oxidative stress were tested. In addition, whole transcriptome microarray analysis of liver tissue was performed. Channel specificity of TAT-Gap24 and TAT-Gap19 was examined in vitro by fluorescence recovery after photobleaching analysis and measurement of extracellular release of adenosine triphosphate. TAT-Gap24 and TAT-Gap19 were shown to be hemichannel-specific in cultured primary hepatocytes. Diet-fed animals treated with TAT-Gap24 or TAT-Gap19 displayed decreased amounts of liver lipids and inflammatory markers, and augmented levels of superoxide dismutase, which was supported by the microarray results. These findings show the involvement of connexin32 and connexin43 hemichannels in non-alcoholic steatohepatitis and, simultaneously, suggest a role as potential drug targets in non-alcoholic steatohepatitis.

In vivo Lipopolysaccharide Tolerance Recruits CD11b+ Macrophages to the Liver with Enhanced Bactericidal Activity and Low Tumor Necrosis Factor-Releasing Capability, Resulting in Drastic Resistance to Lethal Septicemia

OBJECTIVES

In vivo lipopolysaccharide (LPS) tolerance on bacterial infection was investigated, focusing on liver macrophages.

METHODS

LPS tolerance was induced by intraperitoneal injections with 5 μg/kg of LPS for 3 consecutive days, and then mice were intravenously infected with Escherichia coli.

RESULTS

All LPS-primed mice survived lethal bacterial infection. Drastic enhancement of bactericidal activity of liver macrophages strongly contributed to bacterial clearance. Although LPS-primed mice produced substantial amounts of tumor necrosis factor (TNF) inside the liver, TNF efflux into the systemic circulation was markedly suppressed. These mice showed a dramatic increase in CD11b+ monocyte- derived macrophages in the liver. The CD11b+ macrophages that increased in LPS-primed mice were those with strong phagocytic/bactericidal activity and an upregulated expression of Fcγ receptor I, but the subfraction with a potent TNF-producing capacity and poor phagocytic activity diminished. The adoptive transfer of CD11b+ macrophages from LPS-primed mice to control mice increased survival after bacterial infection and reduced the elevation of plasma TNF. LPS priming did not affect the CD68+ resident Kupffer cells, and CD68+ Kupffer cell-depleted mice still exhibited LPS tolerance with strong resistance to bacteremia.

CONCLUSIONS

LPS tolerance recruits CD11b+ macrophages to the liver with enhanced bactericidal activity, which plays a central role in resistance to lethal bacteremia.

Targeting hepatic macrophages to treat liver diseases

Our view on liver macrophages in the context of health and disease has been reformed by the recognition of a remarkable heterogeneity of phagocytes in the liver. Liver macrophages consist of ontogenically distinct populations termed Kupffer cells and monocyte-derived macrophages. Kupffer cells are self-renewing, resident and principally non-migratory phagocytes, serving as sentinels for liver homeostasis. Liver injury triggers Kupffer cell activation, leading to inflammatory cytokine and chemokine release. This fosters the infiltration of monocytes into the liver, which give rise to large numbers of inflammatory monocyte-derived macrophages. Liver macrophages are very plastic and adapt their phenotype according to signals derived from the hepatic microenvironment (e.g. danger signals, fatty acids, phagocytosis of cellular debris), which explains their manifold and even opposing functions during disease. These central functions include the perpetuation of inflammation and hepatocyte injury, activation of hepatic stellate cells with subsequent fibrogenesis, and support of tumor development by angiogenesis and T cell suppression. If liver injury ceases, specific molecular signals trigger hepatic macrophages to switch their phenotype towards reparative phagocytes that promote tissue repair and regression of fibrosis. Novel strategies to treat liver disease aim at targeting macrophages. These interventions modulate Kupffer cell activation (e.g. via gut-liver axis or inflammasome formation), monocyte recruitment (e.g. via inhibiting chemokine pathways like CCR2 or CCL2) or macrophage polarization and differentiation (e.g. by nanoparticles). Evidence from mouse models and early clinical studies in patients with non-alcoholic steatohepatitis and fibrosis support the notion that pathogenic macrophage subsets can be successfully translated into novel treatment options for patients with liver disease.

LAY SUMMARY

Macrophages (Greek for "big eaters") are a frequent non-parenchymal cell type of the liver that ensures homeostasis, antimicrobial defense and proper metabolism. However, liver macrophages consist of different subtypes regarding their ontogeny (developmental origin), differentiation and function. Understanding this heterogeneity and the critical regulation of inflammation, fibrosis and cancer by macrophage subsets opens promising new options for treating liver diseases.

Natural killer cell activation contributes to hepatitis B viral control in a mouse model

The roles of CD4 + T cells and CD8 + T cells in hepatitis B virus (HBV) infection have been well documented. However, the role of innate immunity in HBV infection remains obscure. Here we examined the effect of activation of innate immunity by polyinosinic: polycytidylic acid (PolyI:C) on HBV infection. A chronic HBV replication mouse model was established by hydrodynamical injection of pAAV/HBV1.2 plasmid into C57BL/6 mice. We found that HBV did not seem to induce an active NK-cell response in the mouse model. Early PolyI:C treatment markedly decreased serum HBV levels and led to HBV clearance. Following PolyI:C injection, NK cells were activated and accumulated in the liver. Depletion of NK cells markedly attenuated the anti-HBV activity of PolyI:C. Moreover, we found that IFN-γ production from NK cells was essential for the antiviral effect of PolyI:C in the model. Importantly, activation of NK cells by PolyI:C could also lead to HBV suppression in HBV-tolerant mice and HBV-transgenic mice. These results suggest that activated NK cells might suppress HBV and contribute to HBV clearance during natural HBV infection. In addition, therapeutic activation of NK cells may represent a new strategy for the treatment of chronic HBV infection.