Lipocalin-2 mediates non-alcoholic steatohepatitis by promoting neutrophil-macrophage crosstalk via the induction of CXCR2

A Central Role for Lipocalin-2 in the Adaptation to Short-Bowel Syndrome Through Down-Regulation of IL22 in Mice

BACKGROUND & AIMS

In short-bowel syndrome (SBS), inadequate intestinal adaptation is responsible for the majority of complications, including sepsis, liver failure, and death. In this study, we sought to further delineate the adaptive response to identify potential therapeutic targets.

METHODS

We performed a 75% small-bowel resection (SBR) or sham operation on C57Bl/6J wild-type (WT), lipocalin-2 (LCN2)-/-, and interleukin 22 (IL22)-/- mice. Exogenous IL22 was administered to SBR WT mice. Cecal fecal matter from SBR WT and SBR LCN2-/- mice were transplanted into germ-free mice. Intestinal permeability, inflammation, proliferation, and the microbiome were evaluated 1 week after surgery. CD4+IL22+ laminal propria lymphocytes were sorted by flow cytometry. Naïve T cells were polarized to T-helper cells with or without LCN2.

RESULTS

A 75% SBR in a mouse re-creates the increased intestinal permeability, enterocyte proliferation, and intestinal dysbiosis seen in SBS. LCN2 expression increases after 75% SBR, and this increase can be abrogated with broad-spectrum antibiotic treatment. LCN2-/- mice have less intestinal inflammation, increased IL22 expression, and greater adaptation as evidenced by less intestinal permeability, increased carbohydrate enzyme expression, less weight loss, and less dysbiosis after 75% SBR than WT mice. The proinflammatory and anti-adaptive effects of LCN2 can be transferred to germ-free mice via a fecal transplant. Administration of exogenous IL22 improves adaptation and restores the normal microbiome after 75% SBR in WT mice.

CONCLUSIONS

LCN2 promotes inflammation and slows intestinal adaptation through changes in the microbiome and IL22 inhibition in a mouse SBS model. Strategies to reduce LCN2 may offer novel therapeutic approaches to enhance adaptation in SBS.

Retinoic acid induced 16 deficiency exacerbates high-fat diet-induced steatohepatitis in mice

Non-alcoholic fatty liver disease (NAFLD) associated with obesity may progress to non-alcoholic steatohepatitis, cirrhosis and hepatocellular carcinoma (HCC). Retinoic acid induced 16 (RAI16) plays an important role in cell apoptosis and is also a potential marker for HCC. Here we aimed to test the effect of RAI16 deficiency on liver pathology in high-fat diet (HFD) fed mice. Wild type (WT) and RAI16 knockout (RAI16-/-) C57BL/6 mice were fed with HFD or chow for up to 12 months. With consumption of HFD diet, RAI16-/- mice on HFD developed much more excess fatty liver within 4 months than WT mice on HFD. The expressions of fatty acid synthesis associated molecules Ppar-γ, Srebp-1c and Fas were further increased in RAI16-/- mice compared with WT mice on HFD. Macrophage infiltration related molecules Mcp-1 and F4/80 and pro-inflammatory factor Lcn2 were significantly increased in RAI16-/- mice compared with WT mice on HFD. Conclusively, RAI16 deficiency exacerbated HFD-induced liver injury, associated with increased inflammation. These findings indicate that RAI16 plays an important role in HFD-induced liver pathology and might be considered as a target for treatment of NAFLD. SIGNIFICANCE: 1. RAI16-/- mice on HFD developed much more excess fatty liver. 2. RAI16-/- mice showed more macrophage infiltration and proinflammation.

Neutrophil chemoattractant receptors in health and disease: double-edged swords

Neutrophils are frontline cells of the innate immune system. These effector leukocytes are equipped with intriguing antimicrobial machinery and consequently display high cytotoxic potential. Accurate neutrophil recruitment is essential to combat microbes and to restore homeostasis, for inflammation modulation and resolution, wound healing and tissue repair. After fulfilling the appropriate effector functions, however, dampening neutrophil activation and infiltration is crucial to prevent damage to the host. In humans, chemoattractant molecules can be categorized into four biochemical families, i.e., chemotactic lipids, formyl peptides, complement anaphylatoxins and chemokines. They are critically involved in the tight regulation of neutrophil bone marrow storage and egress and in spatial and temporal neutrophil trafficking between organs. Chemoattractants function by activating dedicated heptahelical G protein-coupled receptors (GPCRs). In addition, emerging evidence suggests an important role for atypical chemoattractant receptors (ACKRs) that do not couple to G proteins in fine-tuning neutrophil migratory and functional responses. The expression levels of chemoattractant receptors are dependent on the level of neutrophil maturation and state of activation, with a pivotal modulatory role for the (inflammatory) environment. Here, we provide an overview of chemoattractant receptors expressed by neutrophils in health and disease. Depending on the (patho)physiological context, specific chemoattractant receptors may be up- or downregulated on distinct neutrophil subsets with beneficial or detrimental consequences, thus opening new windows for the identification of disease biomarkers and potential drug targets.

Oral Supplementation of Sodium Butyrate Attenuates the Progression of Non-Alcoholic Steatohepatitis

Sodium butyrate (SoB) supplementation has been suggested to attenuate the development of non-alcoholic fatty liver disease (NAFLD). Here, we determined the therapeutic potential of SoB on NAFLD progression and molecular mechanism involved. Eight-week old C57BL/6J mice were pair-fed a fat-, fructose- and cholesterol-rich diet (FFC) or control diet (C). After 8 weeks, some mice received 0.6g SoB/kg bw in their respective diets (C+SoB; FFC+SoB) or were maintained on C or FFC for the next 5 weeks of feeding. Liver damage, markers of glucose metabolism, inflammation, intestinal barrier function and melatonin metabolism were determined. FFC-fed mice progressed from simple steatosis to early non-alcoholic steatohepatitis, along with significantly higher TNFα and IL-6 protein levels in the liver and impaired glucose tolerance. In FFC+SoB-fed mice, disease was limited to steatosis associated with protection against the induction of Tlr4 mRNA and iNOS protein levels in livers. SoB supplementation had no effect on FFC-induced loss of tight junction proteins in the small intestine but was associated with protection against alterations in melatonin synthesis and receptor expression in the small intestine and livers of FFC-fed animals. Our results suggest that the oral supplementation of SoB may attenuate the progression of simple steatosis to steatohepatitis.

Prolyl endopeptidase gene disruption attenuates high fat diet-induced nonalcoholic fatty liver disease in mice by improving hepatic steatosis and inflammation

Background

Prolyl endopeptidase (PREP) is a serine endopeptidase that regulates inflammatory responses. PREP inhibitors can reduce hepatocyte lipid accumulation and may participate in the progression of nonalcoholic fatty liver disease (NAFLD). We investigated whether disruption of PREP regulates hepatic steatosis and inflammation in mice with NAFLD.

Methods

Wild-type and PREP gene disrupted mice were randomly divided into low-fat diet wild-type (LFD-WT), high-fat diet wild-type (HFD-WT), low-fat diet PREP disruption (LFD-PREP^gt), and high-fat diet PREP disruption (HFD-PREP^gt) groups. Animals were euthanized at the endpoint of 32 weeks. The NAFLD activity score and number of inflammatory cells were determined by hematoxylin-eosin staining and immunohistochemical staining of liver tissue. The expression levels of inflammation- and lipid metabolism-associated genes in the liver and serum were detected by quantitative reverse transcription PCR, mass spectrometry, or enzyme-linked immunosorbent assay.

Results

The body weight and epididymal fat tissue index of the HFD-PREP^gt mice were significantly decreased compared with that of the HFD-WT mice. Moreover, the NAFLD activity score and liver function were attenuated in the HFD-PREP^gt mice. Fat accumulation and the level of expression of mRNAs associated with lipid metabolism and proinflammatory responses were improved in the HFD-PREP^gt mice. The number of CD68-positive cells in liver tissue and the serum levels of inflammation-associated factors were significantly decreased in the HFD-PREP^gt mice compared with those in the HFD-WT mice. Further mechanistic investigations indicated that the protective effect of PREP disruption on liver inflammation was associated with the suppressed production of matrix metalloproteinases (MMPs) and proline-glycine-proline (PGP) and the inhibition of neutrophil infiltration.

Conclusions

Loss of PREP lowers the severity of hepatic steatosis and inflammatory responses in a high-fat diet-induced nonalcoholic steatohepatitis model. PREP inhibition may protect against NAFLD.

The role of CXCR2 in acute inflammatory responses and its antagonists as anti-inflammatory therapeutics

PURPOSE OF REVIEW

CXCR2 is key stimulant of immune cell migration and recruitment, especially of neutrophils. Alleviating excessive neutrophil accumulation and infiltration could prevent prolonged tissue damage in inflammatory disorders. This review focuses on recent advances in our understanding of the role of CXCR2 in regulating neutrophil migration and the use of CXCR2 antagonists for therapeutic benefit in inflammatory disorders.

RECENT FINDINGS

Recent studies have provided new insights into how CXCR2 signaling regulates hematopoietic cell mobilization and function in both health and disease. We also summarize several CXCR2 regulatory mechanisms during infection and inflammation such as via Wip1, T-bet, P-selectin glycoprotein ligand-1, granulocyte-colony-stimulating factor, and microbiome. Moreover, we provide an update of studies investigating CXCR2 blockade in the laboratory and in clinical trials.

SUMMARY

Neutrophil homeostasis, migration, and recruitment must be precisely regulated. The CXCR2 signaling pathway is a potential target for modifying neutrophil dynamics in inflammatory disorders. We discuss the recent clinical use of CXCR2 antagonists for controlling inflammation.

Samsum ant venom modulates the immune response and redox status at the acute toxic dose in vivo

BACKGROUND

Ant venoms express surface molecules that participate in antigen presentation involving pro- and anti-inflammatory cytokines. This work aims to investigate the expression of MHC-II, CD80 and CD86 on the polymorphonuclear cells (PMNs) in rats injected with samsum ant venom (SAV).

METHODS

Rats were divided into three groups - control, SAV-treated (intraperitoneal route, 600 μg/kg), and SAV-treated (subcutaneous route, 600 μg/kg). After five doses, animals were euthanized and samples collected for analysis.

RESULTS

The subcutaneous SAV-trated rats presented decreased levels of glutathione with increased cholesterol and triglyceride levels. Intraperitoneal SAV-treated animals displayed significantly reduced concentrations of both IFN-γ and IL-17 in comparison with the control group. However, intraperitoneal and subcutaneous SAV-treated rats were able to upregulate the expressions of MHC-II, CD80 and CD86 on PMNs in comparison with the control respectively. The histological examination showed severe lymphocyte depletion in the splenic white pulp of the intraperitoneal SAV-injected rats.

CONCLUSION

Stimulation of PMNs by SAV leads to upregulation of MHC-II, CD 80, and CD 86, which plays critical roles in antigen presentation and consequently proliferation of T-cells. Subcutaneous route was more efficient than intraperitoneal by elevating MHC-II, CD80 and CD86 expression, disturbing oxidative stability and increasing lipogram concentration.

Lipocalin 2 Protects Against Escherichia coli Infection by Modulating Neutrophil and Macrophage Function

Lipocalin 2 (Lcn2) is an essential component of the antimicrobial innate immune system. It attenuates bacterial growth by binding and sequestering the iron-scavenging siderophores to prevent bacterial iron acquisition. Whereas, the ability of Lcn2 to sequester iron is well-described, the role of Lcn2 in regulating immune cells during bacterial infection remains unclear. In this study, we showed that upon infection with Escherichia coli (O157:H7), Lcn2-deficient (Lcn2 ^-/-) mice carried more bacteria in blood and liver, and the acute-phase sera lost their antibacterial activity in vitro. Neutrophils from Lcn2 ^-/- mice were defective in homeostasis and morphological development. E. coli O157:H7 infection of Lcn2 ^-/- mice resulted in a reduced neutrophil migration capacity, with 30% reduction of extravasated neutrophils, and impaired chemotaxis, as shown by a reduction in the secretion of chemoattractants, such as tumor necrosis factor (TNF)-α, monocyte chemoattractant protein (MCP)-1, and macrophage inflammatory protein (MIP)-2, which are instrumental in eliciting a neutrophil response. We also found that some secreted cytokines [interleukin (IL)-6, IL-1β, and TNF-α] were decreased. Transcripts of inflammatory cytokines (IL-6, IL-1β, TNF-α, and IL-10), chemokines (MIP-2 and MCP-1), and iNOS production were all strongly repressed in Lcn2 ^-/- macrophages. Furthermore, Lcn2 could induce the production of chemokines and promote the migration and phagocytosis of macrophages. Thus, Lcn2 deficiency could impair the migration and chemotaxis ability of neutrophils and disturb the normal secretion of inflammatory cytokines of macrophages. Therefore, the heightened sensitivity of Lcn2 ^-/- mice to E. coli O157:H7 is not only due to the antibacterial function of Lcn2 but also a consequence of impaired functions of immune cells, including neutrophils and macrophages.

The role of osteoblasts in energy homeostasis

Osteoblasts are specialized mesenchymal cells that synthesize bone matrix and coordinate the mineralization of the skeleton. These cells work in harmony with osteoclasts, which resorb bone, in a continuous cycle that occurs throughout life. The unique function of osteoblasts requires substantial amounts of energy production, particularly during states of new bone formation and remodelling. Over the last 15 years, studies have shown that osteoblasts secrete endocrine factors that integrate the metabolic requirements of bone formation with global energy balance through the regulation of insulin production, feeding behaviour and adipose tissue metabolism. In this article, we summarize the current understanding of three osteoblast-derived metabolic hormones (osteocalcin, lipocalin and sclerostin) and the clinical evidence that suggests the relevance of these pathways in humans, while also discussing the necessity of specific energy substrates (glucose, fatty acids and amino acids) to fuel bone formation and promote osteoblast differentiation.

Lipocalin-2-The myth of its expression and function

Lipocalin-2 is a functional biomarker for acute and chronic kidney diseases, heart failure and obesity-related medical complications. It is rapidly induced in epithelial cells under stress conditions, but constitutively produced from pre-adipocytes and mature adipocytes. Measuring the lipocalin-2 levels represents an effective approach for risk prediction, patient stratification and disease management. Nevertheless, due to ligand-binding, post-translational modification and protein-protein interaction, lipocalin-2 exists as multiple variants that elicit different pathophysiological functions. To characterize the specific structure-functional relationships of lipocalin-2 variants is critical for the development of biomarker assays with sufficient precision and reliability. Moreover, identifying the pathological forms of lipocalin-2 will provide new therapeutic targets and treatment approaches for obesity-related complications.

Beyond adiponectin and leptin: adipose tissue-derived mediators of inter-organ communication

The breakthrough discoveries of leptin and adiponectin more than two decades ago led to a widespread recognition of adipose tissue as an endocrine organ. Many more adipose tissue-secreted signaling mediators (adipokines) have been identified since then, and much has been learned about how adipose tissue communicates with other organs of the body to maintain systemic homeostasis. Beyond proteins, additional factors, such as lipids, metabolites, noncoding RNAs, and extracellular vesicles (EVs), released by adipose tissue participate in this process. Here, we review the diverse signaling mediators and mechanisms adipose tissue utilizes to relay information to other organs. We discuss recently identified adipokines (proteins, lipids, and metabolites) and briefly outline the contributions of noncoding RNAs and EVs to the ever-increasing complexities of adipose tissue inter-organ communication. We conclude by reflecting on central aspects of adipokine biology, namely, the contribution of distinct adipose tissue depots and cell types to adipokine secretion, the phenomenon of adipokine resistance, and the capacity of adipose tissue to act both as a source and sink of signaling mediators.

Sex-specific metabolic functions of adipose Lipocalin-2

OBJECTIVE

Lipocalin-2 (LCN2) is a secreted protein involved in innate immunity and has also been associated with several cardiometabolic traits in both mouse and human studies. However, the causal relationship of LCN2 to these traits is unclear, and most studies have examined only males.

METHODS

Using adeno-associated viral vectors we expressed LCN2 in either adipose or liver in a tissue specific manner on the background of a whole-body Lcn2 knockout or wildtype mice. Metabolic phenotypes including body weight, body composition, plasma and liver lipids, glucose homeostasis, insulin resistance, mitochondrial phenotyping, and metabolic cage studies were monitored.

RESULTS

We studied the genetics of LCN2 expression and associated clinical traits in both males and females in a panel of 100 inbred strains of mice (HMDP). The natural variation in Lcn2 expression across the HMDP exhibits high heritability, and genetic mapping suggests that it is regulated in part by Lipin1 gene variation. The correlation analyses revealed striking tissue dependent sex differences in obesity, insulin resistance, hepatic steatosis, and dyslipidemia. To understand the causal relationships, we examined the effects of expression of LCN2 selectively in liver or adipose. On a Lcn2-null background, LCN2 expression in white adipose promoted metabolic disturbances in females but not males. It acted in an autocrine/paracrine manner, resulting in mitochondrial dysfunction and an upregulation of inflammatory and fibrotic genes. On the other hand, on a null background, expression of LCN2 in liver had no discernible impact on the traits examined despite increasing the levels of circulating LCN2 more than adipose LCN2 expression. The mechanisms underlying the sex-specific action of LCN2 are unclear, but our results indicate that adipose LCN2 negatively regulates its receptor, LRP2 (or megalin), and its repressor, ERα, in a female-specific manner and that the effects of LCN2 on metabolic traits are mediated in part by LRP2.

CONCLUSIONS

Following up on our population-based studies, we demonstrate that LCN2 acts in a highly sex- and tissue-specific manner in mice. Our results have important implications for human studies, emphasizing the importance of sex and the tissue source of LCN2.

Circulating levels of lipocalin-2 are associated with fatty pancreas but not fatty liver

Lipocalin-2 (LCN-2), a peptide with diverse expression pattern, has been identified as a biomarker of various diseases as well as a factor contributing to inflammatory responses associated with excess adiposity and ensuing metabolic disorders. Although the inter-relationship between LCN-2 and excess adiposity is increasingly recognized, little is known about the inter-relationship between LCN-2 and ectopic fat deposition. The present study aimed to investigate the associations between LCN-2 and fatty pancreas as well as fatty liver. In addition, the associations between LCN-2 and pro-inflammatory cytokines were studied. Magnetic resonance imaging was used to quantify intra-pancreatic fat deposition and visceral-to-subcutaneous fat volume ratio whereas magnetic resonance spectroscopy was used to quantify liver fat deposition. Fasting venous blood was analyzed for LCN-2, C-C motif chemokine ligand 2, interleukin-6, leptin, tumor necrosis factor-α, glycated hemoglobin, glucose, and insulin. Binary logistic regression and linear regression analyses were conducted. Three statistical models were built to adjust for demographics, comorbidities, levels of glycated hemoglobin, insulin resistance, and abdominal fat distribution. A total of 79 individuals were studied, of whom 20 had fatty pancreas, 14 had fatty liver, and 4 had both. Lipocalin-2 was significantly associated with fatty pancreas in all the adjusted models (p = 0.014 in the most adjusted model) but was not significantly associated with fatty liver in any of the studied models. Lipocalin-2 was significantly associated with interleukin-6 and tumor necrosis factor-α, in both the unadjusted and adjusted models. Leptin and C-C motif chemokine ligand 2 were not significantly associated with LCN-2 in any of the studied models. These findings suggest that LCN-2 is a potential biomarker of fatty pancreas, independent of abdominal fat distribution, insulin resistance, and other covariates. The role of LCN-2 in intra-pancreatic fat deposition and related low-grade inflammation warrants further investigations.

Paradoxical role of lipocalin-2 in metabolic disorders and neurological complications

Lipocalin-2 (LCN2), also known as 24p3 and neutrophil gelatinase-associated lipocalin (NGAL), is a 25-kDa secreted protein implicated in various metabolic and inflammatory diseases. Early studies suggest the protective function of LCN2 in which it acts as a bacteriostatic agent that competes with bacteria for iron-bound siderophores. However, both detrimental and beneficial roles of LCN2 have recently been documented in metabolic and neuroinflammatory diseases. Metabolic inflammation, as observed in diabetes and obesity, has been closely associated with the upregulation of LCN2 in blood plasma and several tissues in both humans and rodents, suggesting its pro-diabetic and pro-obesogenic role. On the contrary, other studies imply an anti-diabetic and anti-obesogenic role of LCN2 whereby a deficiency in the Lcn2 gene results in the impairment of insulin sensitivity and enhances the high-fat-diet-induced expansion of fat. A similar dual role of LCN2 has also been reported in various animal models for neurological disorders. In the midst of these mixed findings, there is no experimental evidence to explain why LCN2 shows such a contrasting role in the various studies. This debate needs to be resolved (or reconciled) and an integrated view on the topic is desirable. Herein, we attempt to address this issue by reviewing the recent findings on LCN2 in metabolic disorders and assess the potential cellular or molecular mechanisms underlying the dual role of LCN2. We further discuss the possibilities and challenges of targeting LCN2 as a potential therapeutic strategy for metabolic disorders and neurological complications.

Lipocalin-2 Protects Against Diet-Induced Nonalcoholic Fatty Liver Disease by Targeting Hepatocytes

Hepatocytes are the major source of hepatic lipocalin-2 (LCN2), which is up-regulated in response to inflammation, injury, or metabolic stress. So far, the role of hepatocyte-derived LCN2 in the development of nonalcoholic fatty liver disease (NAFLD) remains unknown. Herein we show that overexpression of human LCN2 in hepatocytes protects against high fat/high cholesterol/high fructose (HFCF) diet-induced liver steatosis and nonalcoholic steatohepatitis by promoting lipolysis and fatty acid oxidation (FAO) and inhibiting de novo lipogenesis (DNL), lipid peroxidation, and apoptosis. LCN2 fails to reduce triglyceride accumulation in hepatocytes lacking sterol regulatory element-binding protein 1. In contrast, Lcn2-/- mice have defective lipolysis, increased lipid peroxidation and apoptosis, and exacerbated NAFLD after being fed an HFCF diet. In primary hepatocytes, Lcn2 deficiency stimulates de novo lipogenesis but inhibits FAO. Conclusion: The current study indicates that hepatocyte LCN2 protects against diet-induced NAFLD by regulating lipolysis, FAO, DNL, lipid peroxidation, and apoptosis. Targeting hepatocyte LCN2 may be useful for treatment of NAFLD.

HSPA12A Is a Novel Player in Nonalcoholic Steatohepatitis via Promoting Nuclear PKM2-Mediated M1 Macrophage Polarization

Nonalcoholic steatohepatitis (NASH) is the most prevalent cause of chronic liver disease worldwide. Macrophage-mediated inflammation plays a critical role in NASH pathogenesis; however, optimum therapies for macrophage activation and NASH remain elusive. HSPA12A encodes a novel member of the HSP70 family. Here, we report that NASH patients showed increased hepatic HSPA12A expression and serum HSPA12A contents. Intriguingly, knockout of HSPA12A (Hspa12a-/- ) in mice attenuated high-fat diet (HFD)-induced hepatic steatosis and injury. HFD-induced macrophage polarization toward an M1 phenotype and inflammatory responses in the liver of Hspa12a-/- mice were also attenuated. Loss- and gain-of-function studies revealed that the de novo lipogenesis in hepatocytes was regulated by the paracrine effects of macrophage HSPA12A rather than by hepatocyte HSPA12A. In-depth molecular analysis revealed that HSPA12A interacted with the M2 isoform of pyruvate kinase (PKM2) in macrophages and increased its nuclear translocation, thereby promoting M1 polarization and secretion of proinflammatory M1 cytokines; this led, ultimately, to hepatocyte steatosis via paracrine effects. Taken together, these findings show that HSPA12A acts as a novel regulator of M1 macrophage polarization and NASH pathogenesis by increasing nuclear PKM2. Strategies that inhibit macrophage HSPA12A might be a potential therapeutic intervention for NASH.

Chemokines and Chemokine Receptors in the Development of NAFLD

Chemokines are chemo-attractants for leukocyte trafficking, growth, and activation in injured and inflammatory tissues. The chemokine system is comprised of 50 chemokine ligands and 20 cognate chemokine receptors. In the context of liver diseases, leukocytes, hepatocytes, hepatic stellate cells, endothelial cells, and vascular smooth muscle cells are capable of producing chemokines. Chemokine receptors are typically expressed in various leukocyte subsets. Given that inflammation is a critical factor for the transition from simple steatosis to non-alcoholic steatohepatitis (NASH), and fibrosis, the chemokine system may play a prominent role in the pathogenesis of non-alcoholic fatty liver disease (NAFLD). Indeed, accumulating evidence shows elevated expression of chemokines and their receptors in the livers of obese patients with advanced steatosis and NASH. This chapter will discuss the underlying molecular mechanisms and the therapeutic potential of the chemokine systems in the pathogenesis of NAFLD. Among chemokines, we will highlight CCL2, CCL5, CXCL8-10, CX3CL1, and CXCL16 as pivotal mediators in the development of steatosis, NASH, and fibrosis.

Obesity Drives STAT-1-Dependent NASH and STAT-3-Dependent HCC

Obesity is a major driver of cancer, especially hepatocellular carcinoma (HCC). The prevailing view is that non-alcoholic steatohepatitis (NASH) and fibrosis or cirrhosis are required for HCC in obesity. Here, we report that NASH and fibrosis and HCC in obesity can be dissociated. We show that the oxidative hepatic environment in obesity inactivates the STAT-1 and STAT-3 phosphatase T cell protein tyrosine phosphatase (TCPTP) and increases STAT-1 and STAT-3 signaling. TCPTP deletion in hepatocytes promoted T cell recruitment and ensuing NASH and fibrosis as well as HCC in obese C57BL/6 mice that normally do not develop NASH and fibrosis or HCC. Attenuating the enhanced STAT-1 signaling prevented T cell recruitment and NASH and fibrosis but did not prevent HCC. By contrast, correcting STAT-3 signaling prevented HCC without affecting NASH and fibrosis. TCPTP-deletion in hepatocytes also markedly accelerated HCC in mice treated with a chemical carcinogen that promotes HCC without NASH and fibrosis. Our studies reveal how obesity-associated hepatic oxidative stress can independently contribute to the pathogenesis of NASH, fibrosis, and HCC.

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Obesity promotes hepatic STAT-1 and STAT-3 signaling

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Obesity promotes STAT-1-dependent T cell-infiltration, NASH, and fibrosis

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Obesity promotes NASH-independent STAT-3-dependent HCC

Neutrophil extracellular traps promote inflammation and development of hepatocellular carcinoma in nonalcoholic steatohepatitis

Nonalcoholic steatohepatitis (NASH) is a progressive, inflammatory form of fatty liver disease. It is the most rapidly rising risk factor for the development of hepatocellular carcinoma (HCC), which can arise in NASH with or without cirrhosis. The inflammatory signals promoting the progression of NASH to HCC remain largely unknown. The propensity of neutrophils to expel decondensed chromatin embedded with inflammatory proteins, known as neutrophil extracellular traps (NETs), has been shown to be important in chronic inflammatory conditions and in cancer progression. In this study, we asked whether NET formation occurs in NASH and contributes to the progression of HCC. We found elevated levels of a NET marker in serum of patients with NASH. In livers from STAM mice (NASH induced by neonatal streptozotocin and high-fat diet), early neutrophil infiltration and NET formation were seen, followed by an influx of monocyte-derived macrophages, production of inflammatory cytokines, and progression of HCC. Inhibiting NET formation, through treatment with deoxyribonuclease (DNase) or using mice knocked out for peptidyl arginine deaminase type IV (PAD4^-/- ), did not affect the development of a fatty liver but altered the consequent pattern of liver inflammation, which ultimately resulted in decreased tumor growth. Mechanistically, we found that commonly elevated free fatty acids stimulate NET formation in vitro.

CONCLUSION

Our findings implicate NETs in the protumorigenic inflammatory environment in NASH, suggesting that their elimination may reduce the progression of liver cancer in NASH. (Hepatology 2018).

Hepatocytes and neutrophils cooperatively suppress bacterial infection by differentially regulating lipocalin-2 and neutrophil extracellular traps

Lipocalin-2 (LCN2), also known as neutrophil gelatinase-associated lipocalin (NGAL), a key antibacterial protein, is highly elevated in patients with end-stage liver disease that is often associated with bacterial infection. LCN2 is expressed at high levels in both hepatocytes and neutrophils; however, how hepatocyte-derived and neutrophil-derived LCN2 cooperate to combat bacterial infection remains unclear. Here, by studying hepatocyte-specific and myeloid-specific Lcn2 knockout mice in two models of systemic and local Klebsiella pneumoniae infections, we demonstrated that hepatocytes played a critical role in controlling systemic infection by secreting LCN2 protein into the circulation following intraperitoneal injection of bacteria, whereas neutrophils were more important in combating local lung infection by carrying LCN2 in their specific granules to the local infection site following intratracheal intubation of bacteria. Both hepatocyte-derived and myeloid cell-derived LCN2 were required against bacterial infection in the peritoneal cavity and liver necrotic areas following intraperitoneal injection of Klebsiella pneumoniae. LCN2/NGAL protein was detected in neutrophil extracellular traps (NETs) in activated neutrophils from mice and humans. Disruption of the Lcn2 gene in neutrophils abolished LCN2 on NETs, whereas deletion of this gene in hepatocytes did not affect LCN2 protein on NETs. Genetic deletion of the Lcn2 gene globally or specifically in neutrophils did not affect NET formation but reduced the bactericidal effect of NETs in vitro. Finally, NGAL-positive NETs were detected in the liver from patients with various types of liver diseases.

CONCLUSION

Both hepatocytes and neutrophils combat bacterial infection through the production of LCN2; extracellular LCN2 secreted by hepatocytes limits systemic bacterial infection, whereas neutrophils carry LCN2 protein to the local site and against local bacterial infection through NETs. (Hepatology 2018).

Lipocalin-2 contributes to experimental atherosclerosis in a stage-dependent manner

BACKGROUND AND AIMS

Lipocalin-2 (Lcn2) is a glycoprotein which can be secreted by immune cells. Several studies in humans have suggested Lcn2 can be used as a biomarker for the detection of unstable atherosclerotic lesions, partly as it is known to interact with MMP-9.

METHODS

In this study, we generated Ldlr^-/-Lcn2^-/- mice to assess the functional role of Lcn2 in different stages of atherosclerosis. Atherosclerotic lesions were characterized through histological analysis and myeloid cell populations were examined using flow cytometry.

RESULTS

We show that Ldlr^-/-Lcn2^-/- mice developed larger atherosclerotic lesions during earlier stages of atherosclerosis and had increased circulating Ly6C^hi inflammatory monocytes compared to Ldlr^-/- mice. Advanced atherosclerotic lesions from Ldlr^-/-Lcn2^-/- mice had decreased necrotic core area, suggesting Lcn2 deficiency may affect lesion stability. Furthermore, MMP-9 activity was diminished in plaques from Ldlr^-/-Lcn2^-/- mice.

CONCLUSIONS

Altogether, these findings suggest that Lcn2 deficiency promotes lesion growth in earlier stages of the disease while it decreases MMP-9 activity and necrotic core size in advanced atherosclerosis.

Protective effect of genetic deletion of pannexin1 in experimental mouse models of acute and chronic liver disease

Pannexins are transmembrane proteins that form communication channels connecting the cytosol of an individual cell with its extracellular environment. A number of studies have documented the presence of pannexin1 in liver as well as its involvement in inflammatory responses. In this study, it was investigated whether pannexin1 plays a role in acute liver failure and non-alcoholic steatohepatitis, being prototypical acute and chronic liver pathologies, respectively, both featured by liver damage, oxidative stress and inflammation. To this end, wild-type and pannexin1^-/- mice were overdosed with acetaminophen for 1, 6, 24 or 48h or were fed a choline-deficient high-fat diet for 8weeks. Evaluation of the effects of genetic pannexin1 deletion was based on a number of clinically relevant read-outs, including markers of liver damage, histopathological analysis, lipid accumulation, protein adduct formation, oxidative stress and inflammation. In parallel, in order to elucidate molecular pathways affected by pannexin1 deletion as well as to mechanistically anchor the clinical observations, whole transcriptome analysis of liver tissue was performed. The results of this study show that pannexin1^-/- diseased mice present less liver damage and oxidative stress, while inflammation was only decreased in pannexin1^-/- mice in which non-alcoholic steatohepatitis was induced. A multitude of genes related to inflammation, oxidative stress and xenobiotic metabolism were differentially modulated in both liver disease models in wild-type and in pannexin1^-/- mice. Overall, the results of this study suggest that pannexin1 may play a role in the pathogenesis of liver disease.

The deubiquitinating enzyme TNFAIP3 mediates inactivation of hepatic ASK1 and ameliorates nonalcoholic steatohepatitis

Activation of apoptosis signal-regulating kinase 1 (ASK1) in hepatocytes is a key process in the progression of nonalcoholic steatohepatitis (NASH) and a promising target for treatment of the condition. However, the mechanism underlying ASK1 activation is still unclear, and thus the endogenous regulators of this kinase remain open to be exploited as potential therapeutic targets. In screening for proteins that interact with ASK1 in the context of NASH, we identified the deubiquitinase tumor necrosis factor alpha-induced protein 3 (TNFAIP3) as a key endogenous suppressor of ASK1 activation, and we found that TNFAIP3 directly interacts with and deubiquitinates ASK1 in hepatocytes. Hepatocyte-specific ablation of Tnfaip3 exacerbated nonalcoholic fatty liver disease- and NASH-related phenotypes in mice, including glucose metabolism disorders, lipid accumulation and enhanced inflammation, in an ASK1-dependent manner. In contrast, transgenic or adeno-associated virus-mediated TNFAIP3 gene delivery in the liver in both mouse and nonhuman primate models of NASH substantially blocked the onset and progression of the disease. These results implicate TNFAIP3 as a functionally important endogenous suppressor of ASK1 hyperactivation in the pathogenesis of NASH and identify it as a potential new molecular target for NASH therapy.

Targeting inflammation for the treatment of alcoholic liver disease

Alcoholic liver disease (ALD) is a leading cause of chronic liver disease with a wide spectrum of manifestations including simple steatosis to steatohepatitis, cirrhosis, and hepatocellular carcinoma. Liver injury in ALD is caused by chronic inflammation, which has been actively investigated as a therapeutic target for the treatment of ALD for over the last four decades. In this review, we summarize a wide variety of inflammatory mediators that have been shown to contribute to the pathogenesis of ALD, and discuss the therapeutic potential of these mediators for the treatment of ALD.

Role of human lipocalin proteins in abdominal obesity after acute pancreatitis

Lipocalin proteins are small regulatory peptides implicated in metabolism, inflammation, and immunity. Although lipocalin proteins have been linked to various clinical conditions, their role in the acute inflammatory setting, such as acute pancreatitis (AP), has only been sparsely investigated. Two members of the lipocalin family, lipocalin-2 (LCN-2) and retinol binding protein -4 (RBP-4), play an important role in obesity and insulin resistance. In this study, we analysed circulating levels of LCN-2 and RBP-4 in 92 individuals after AP, of whom 41 individuals had abdominal obesity and 51 did not. Binary logistic regression analyses were performed to determine whether abdominal obesity was associated with the two lipocalin proteins. Lipocalin-2 was significantly associated with abdominal obesity in the unadjusted model (Odds ratio (OR)=1.014 [95% confidence interval (CI): 1.000, 1.028], P=0.05) and after adjusting for patient related (age, ethnicity, and diabetes mellitus) and pancreatitis related (aetiology, severity, recurrence, and duration of AP) characteristics (OR=1.018 [95% CI: 1.001, 1.036], p=0.04). Further, the association of LCN-2 with waist circumference was significant in individuals with alcohol aetiology of AP (β=1.082 [95% CI: 1.011, 1.158], p=0.02]. The association between RBP-4 and abdominal obesity was not significant in both unadjusted and adjusted models. These findings indicate that circulating levels of LCN-2 in patients after AP may play a role in chronic low grade inflammation associated with abdominal adiposity and that alcohol consumption may further exacerbate adipose tissue dysfunction.

Lipocalin-2: A Master Mediator of Intestinal and Metabolic Inflammation

Lipocalin-2 (LCN2), also known as neutrophil gelatinase-associated lipocalin (NGAL), is released by various cell types and is an attractive biomarker of inflammation, ischemia, infection, and kidney damage. Both intestinal and metabolic inflammation, as observed in obesity and related disorders, are associated with increased LCN2 synthesis. While LCN2 in the intestinal tract regulates the composition of the gut microbiota and shows anti-inflammatory activities, it also exhibits proinflammatory activities in other experimental settings. In animal models of metabolic inflammation, type 2 diabetes mellitus (T2DM), or nonalcoholic steatohepatitis (NASH), increased LCN2 expression favors inflammation via the recruitment of inflammatory cells, such as neutrophils, and the induction of proinflammatory cytokines. A better understanding of this crucial marker of innate immunity might pave the way for targeting this pathway in future therapies.

Altered mitochondrial and peroxisomal integrity in lipocalin-2-deficient mice with hepatic steatosis

Lipocalin-2 (LCN2) is a secreted adipokine that transports small hydrophobic molecules such as fatty acids and steroids. LCN2 limits bacterial growth by sequestering iron-containing siderophores and in mammalian liver protects against inflammation, infection, injury and other stressors. Because LCN2 modulates hepatic fat metabolism and homeostasis, we performed a comparative profiling of proteins and lipids of wild type (WT) and Lcn2-deficient mice fed either standard chow or a methionine- and choline-deficient (MCD) diet. Label-free proteomics and 2D-DIGE protein expression profiling revealed differential expression of BRIT1/MCPH1, FABP5, HMGB1, HBB2, and L-FABP, results confirmed by Western blotting. Gene ontology enrichment analysis identified enrichment for genes associated with mitochondrial membrane permeabilization and metabolic processes involving carboxylic acid. Measurements of mitochondrial membrane potential, mitochondrial chelatable iron pool, intracellular lipid peroxidation, and peroxisome numbers in primary hepatocytes confirmed that LCN2 regulates mitochondrial and peroxisomal integrity. Matrix-Assisted Laser Desorption/Ionization Time-Of-Flight (MALDI-TOF) mass spectrometry imaging identified significant changes to sphingomyelins, triglycerides, and glycerophospholipids in livers of mice fed an MCD diet regardless of LCN2 status. However, two arachidonic acid-containing glycerophospholipids were increased in Lcn2-deficient livers. Thus, LCN2 influences peroxisomal and mitochondrial biology in the liver to maintain triglyceride balance, handle oxidative stress, and control apoptosis.

Role of FXR in Liver Inflammation during Nonalcoholic Steatohepatitis

PURPOSE OF REVIEW

About 15-25% of patients with simple steatosis of non-alcoholic fatty liver disease progresses to non-alcoholic steatohepatitis (NASH), and the underlying mechanism for this progression has not been elucidated. NASH ultimately could progress to cirrhosis, an irreversible condition.

RECENT FINDINGS

Farnesoid X receptor (FXR) has been studied for its role in modulating inflammation, and the expression of FXR is down-regulated during NASH development. FXR deficiency has shown to progress and exacerbate NASH development, and FXR activation has been protective against liver inflammation associated with NASH. The expression of factors in both the adaptive and innate immune response in the liver are regulated in a FXR-dependent and -independent manner.

SUMMARY

Therefore, understanding key signaling pathways of liver inflammation in NASH is important to determine essential components that predispose, progress, or exacerbate NASH. FXR has been identified as a therapeutic target for NASH to prevent liver inflammation.

Lipocalin 2 (LCN2) Expression in Hepatic Malfunction and Therapy

Lipocalin 2 (LCN2) is a secreted protein that belongs to the Lipocalins, a group of transporters of small lipophilic molecules such as steroids, lipopolysaccharides, iron, and fatty acids in circulation. Two decades after its discovery and after a high variety of published findings, LCN2's altered expression has been assigned to critical roles in several pathological organ conditions, including liver injury and steatosis, renal damage, brain injury, cardiomyopathies, muscle-skeletal disorders, lung infection, and cancer in several organs. The significance of this 25-kDa lipocalin molecule has been impressively increased during the last years. Data from several studies indicate the role of LCN2 in physiological conditions as well as in response to cellular stress and injury. LCN2 in the liver shows a protective role in acute and chronic injury models where its expression is highly elevated. Moreover, LCN2 expression is being considered as a potential strong biomarker for pathological conditions, including rheumatic diseases, cancer in human organs, hepatic steatosis, hepatic damage, and inflammation. In this review, we summarize experimental and clinical findings linking LCN2 to the pathogenesis of liver disease.