Hyon-Seung Yi

Activation of toll-like receptor 3 attenuates alcoholic liver injury by stimulating Kupffer cells and stellate cells to produce interleukin-10 in mice

BACKGROUND & AIMS The important function of toll-like receptor (TLR) 4 in Kupffer cells and hepatic stellate cells (HSCs) has been well documented in alcoholic liver injury. However, little is known about the role of TLR3. Thus, we tested whether TLR3 activation in HSCs and Kupffer cells could attenuate alcoholic liver injury in vivo, and investigated its possible mechanism in vitro. METHODS Alcoholic liver injury was achieved by feeding wild type (WT), TLR3 knockout (TLR3(-/-)) and interleukin (IL)-10(-/-) mice with high-fat diet plus binge ethanol drinking for 2 weeks. To activate TLR3, polyinosinic-polycytidylic acid (poly I:C) was injected into mice. For in vitro studies, HSCs and Kupffer cells were isolated and treated with poly I:C. RESULTS In WT mice, poly I:C treatment reduced alcoholic liver injury and fat accumulation by suppressing nuclear factor-κB activation and sterol response element-binding protein 1c expression in the liver. In addition, freshly isolated HSCs and Kupffer cells from poly I:C-treated mice showed enhanced expression of IL-10 compared to controls. Infiltrated macrophage numbers and the expression of tumor necrosis factor-α, monocyte chemoattractant protein-1 and IL-6 on these cells were decreased after poly I:C treatment. In vitro, poly I:C treatment enhanced the expression of IL-10 via a TLR3-dependent mechanism in HSCs and Kupffer cells. Finally, the protective effects of poly I:C on alcoholic liver injury were diminished in TLR3(-/-) and IL-10(-/-) mice. CONCLUSIONS TLR3 activation ameliorates alcoholic liver injury via the stimulation of IL-10 production in HSCs and Kupffer cells. TLR3 could be a novel therapeutic target for the treatment of alcoholic liver injury.

Inhibiting poly ADP-ribosylation increases fatty acid oxidation and protects against fatty liver disease

BACKGROUND & AIMS To date, no pharmacological therapy has been approved for non-alcoholic fatty liver disease (NAFLD). The aim of the present study was to evaluate the therapeutic potential of poly ADP-ribose polymerase (PARP) inhibitors in mouse models of NAFLD. METHODS As poly ADP-ribosylation (PARylation) of proteins by PARPs consumes nicotinamide adenine dinucleotide (NAD+), we hypothesized that overactivation of PARPs drives NAD+ depletion in NAFLD. Therefore, we assessed the effectiveness of PARP inhibition to replenish NAD+ and activate NAD+-dependent sirtuins, hence improving hepatic fatty acid oxidation. To do this, we examined the preventive and therapeutic benefits of the PARP inhibitor (PARPi), olaparib, in different models of NAFLD. RESULTS The induction of NAFLD in C57BL/6J mice using a high-fat high-sucrose (HFHS)-diet increased PARylation of proteins by PARPs. As such, increased PARylation was associated with reduced NAD+ levels and mitochondrial function and content, which was concurrent with elevated hepatic lipid content. HFHS diet supplemented with PARPi reversed NAFLD through repletion of NAD+, increasing mitochondrial biogenesis and β-oxidation in liver. Furthermore, PARPi reduced reactive oxygen species, endoplasmic reticulum stress and fibrosis. The benefits of PARPi treatment were confirmed in mice fed with a methionine- and choline-deficient diet and in mice with lipopolysaccharide-induced hepatitis; PARP activation was attenuated and the development of hepatic injury was delayed in both models. Using Sirt1hep-/- mice, the beneficial effects of a PARPi-supplemented HFHS diet were found to be Sirt1-dependent. CONCLUSIONS Our study provides a novel and practical pharmacological approach for treating NAFLD, fueling optimism for potential clinical studies. LAY SUMMARY Non-alcoholic fatty liver disease (NAFLD) is now considered to be the most common liver disease in the Western world and has no approved pharmacological therapy. PARP inhibitors given as a treatment in two different mouse models of NAFLD confer a protection against its development. PARP inhibitors may therefore represent a novel and practical pharmacological approach for treating NAFLD.

Interaction of hepatic stellate cells with diverse types of immune cells: Foe or friend?

Activated hepatic stellate cells (HSCs) have been considered as a major type of cells in liver fibrosis by producing a huge amount of extracellular matrix, especially collagen fibers, and profibrotic mediators such as transforming growth factor-beta, interleukin-6 and monocyte chemoattractant protein-1. Recently, accumulated evidence suggests that the liver is an immunologic organ because of enrichment of diverse types of immune cells and that their interactions with HSCs are closely related with the progression of liver fibrosis. However, the underlying mechanisms of interaction between HSCs and immune cells remain largely unknown. Recently, several studies have demonstrated that natural killer cells, M2 macrophages, regulatory T cells, and bone marrow derived CD11b(+) Gr1(+) immature cells ameliorate liver fibrosis, whereas neutrophils, M1 macrophages, CD8 T cells, natural killer T cells and interleukin-17-producing cells accelerate liver fibrosis. However, there are still controversial issues about their functions during liver fibrogenesis. In this review, we summarize the diversity roles of immune cells (e.g. profibrotic/antifibrotic or both) in regulating the activation of HSCs during hepatic fibrogenesis, in which several producible mediators by HSCs play important roles in the interaction with them. Moreover, the current cell-based therapies using immune cells against liver fibrosis are discussed.

Exosome‐mediated activation of toll‐like receptor 3 in stellate cells stimulates interleukin‐17 production by γδ T cells in liver fibrosis

During liver injury, hepatocytes secrete exosomes that include diverse types of self‐RNAs. Recently, self‐noncoding RNA has been recognized as an activator of Toll‐like receptor 3 (TLR3). However, the roles of hepatic exosomes and TLR3 in liver fibrosis are not yet fully understood. Following acute liver injury and early‐stage liver fibrosis induced by a single or 2‐week injection of carbon tetrachloride (CCl4), increased interleukin (IL)‐17A production was detected primarily in hepatic γδ T cells in wild‐type (WT) mice. However, liver fibrosis and IL‐17A production by γδ T cells were both significantly attenuated in TLR3 knockout (KO) mice compared with WT mice. More interestingly, IL‐17A‐producing γδ T cells were in close contact with activated hepatic stellate cells (HSCs), suggesting a role for HSCs in IL‐17A production by γδ T cells. In vitro treatments with exosomes derived from CCl4‐treated hepatocytes significantly increased the expression of IL‐17A, IL‐1β, and IL‐23 in WT HSCs but not in TLR3 KO HSCs. Furthermore, IL‐17A production by γδ T cells was substantially increased upon coculturing with exosome‐treated WT HSCs or conditioned medium from TLR3‐activated WT HSCs. However, similar increases were not detected when γδ T cells were cocultured with exosome‐treated HSCs from IL‐17A KO or TLR3 KO mice. Using reciprocal bone marrow transplantation between WT and TLR3 KO mice, we found that TLR3 deficiency in HSCs contributed to decreased IL‐17A production by γδ T cells, as well as liver fibrosis. Conclusion: In liver injury, the exosome‐mediated activation of TLR3 in HSCs exacerbates liver fibrosis by enhancing IL‐17A production by γδ T cells, which might be associated with HSC stimulation by unknown self‐TLR3 ligands from damaged hepatocytes. Therefore, TLR3 might be a novel therapeutic target for liver fibrosis. (Hepatology 2016;64:616‐631)

Pro-inflammatory hepatic macrophages generate ROS through NADPH oxidase 2 via endocytosis of monomeric TLR4–MD2 complex

Reactive oxygen species (ROS) contribute to the development of non-alcoholic fatty liver disease. ROS generation by infiltrating macrophages involves multiple mechanisms, including Toll-like receptor 4 (TLR4)-mediated NADPH oxidase (NOX) activation. Here, we show that palmitate-stimulated CD11b+F4/80low hepatic infiltrating macrophages, but not CD11b+F4/80high Kupffer cells, generate ROS via dynamin-mediated endocytosis of TLR4 and NOX2, independently from MyD88 and TRIF. We demonstrate that differently from LPS-mediated dimerization of the TLR4–MD2 complex, palmitate binds a monomeric TLR4–MD2 complex that triggers endocytosis, ROS generation and increases pro-interleukin-1β expression in macrophages. Palmitate-induced ROS generation in human CD68lowCD14high macrophages is strongly suppressed by inhibition of dynamin. Furthermore, Nox2-deficient mice are protected against high-fat diet-induced hepatic steatosis and insulin resistance. Therefore, endocytosis of TLR4 and NOX2 into macrophages might be a novel therapeutic target for non-alcoholic fatty liver disease.Reactive species of oxygen promote the development of hepatic steatosis. Here, Kim et al. demonstrate that palmitate stimulates macrophage infiltration and increases oxidative stress during steatosis by binding to the TLR4–MD2 complex, which results in the activation of NOX2.

Dysregulation of mitophagy in carcinogenesis and tumor progression

The mitochondrial role in carcinogenesis and cancer progression is an area of active research, with many unresolved questions. Various aspects of altered mitochondrial function have been implicated in tumorigenesis and tumor progression, including mitochondrial dysfunction, a metabolic switch to aerobic glycolysis, and dysregulation of mitophagy. Mitophagy is a highly specific quality control process which eliminates dysfunctional mitochondria and promotes mitochondrial turnover, and is involved in the adaptation to nutrient stress by controlling mitochondrial mass. The dysregulation of mitochondrial turnover has both a positive and negative role in cancer. This review will begin with a basic overview of the molecular mechanisms of mitophagy, and highlight recent trends in mitophagy from cancer studies. We will conclude this review by discussing areas of research in normal mitophagy that have yet to be explored in the context of cancer such as mitochondrial proteases, the mitochondrial unfolded protein response, and mitokine action. This article is part of a Special Issue entitled Mitochondria in Cancer, edited by Giuseppe Gasparre, Rodrigue Rossignol and Pierre Sonveaux.

GDF15 deficiency exacerbates chronic alcohol- and carbon tetrachloride-induced liver injury

Growth differentiation factor 15 (GDF15) has recently been shown to have an important role in the regulation of mitochondrial function and in the pathogenesis of complex human diseases. Nevertheless, the role of GDF15 in alcohol-induced or fibrotic liver diseases has yet to be determined. In this study, we demonstrate that alcohol- or carbon tetrachloride (CCl4)-mediated hepatic GDF15 production ameliorates liver inflammation and fibrosis. Alcohol directly enhanced GDF15 expression in primary hepatocytes, which led to increased oxygen consumption. Moreover, GDF15 reduced the expression of pro-inflammatory cytokines in liver-resident macrophages, leading to an improvement in inflammation and fibrosis in the liver. GDF15 knockout (KO) mice had more TNF-α-producing T cells and more activated CD4+ and CD8+ T cells in the liver than wild-type mice. Liver-infiltrating monocytes and neutrophils were also increased in the GDF15 KO mice during liver fibrogenesis. These changes in hepatic immune cells were associated with increased tissue inflammation and fibrosis. Finally, recombinant GDF15 decreased the expression of pro-inflammatory cytokines and fibrotic mediators and prevented the activation of T cells in the livers of mice with CCl4-induced liver fibrosis. These results suggest that GDF15 could be a potential therapeutic target for the treatment of alcohol-induced and fibrotic liver diseases.

Hepatic immunophenotyping for streptozotocin-induced hyperglycemia in mice

Emerging evidence revealed that diabetes induces abnormal immune responses that result in serious complications in organs. However, the effect of hyperglycemia on hepatic immunity remains obscure. We evaluated the population and function of hepatic immune cells in streptozotocin (STZ)-induced hyperglycemic mice. CC chemokine receptor 2 (CCR2)-knockout mice and mice with a depletion of regulatory T cells (DEREG) were used to investigate the migration and role of regulatory T cells (Tregs) in hyperglycemic mice. The inflammatory cytokines and hepatic transaminase levels were significantly increased in the hyperglycemic mice. The population and number of infiltrating monocytes, granulocytes, and Tregs were enhanced in the livers of the hyperglycemic mice. Hepatic monocytes other than macrophages showed the increased expression of inflammatory cytokines and chemokines in the hyperglycemic mice. The CCR2 knockout and DEREG chimeric mice exhibited increased populations of activated T cells and neutrophils compared to the WT chimeric mice, which promoted hepatic inflammation in the hyperglycemic mice. The migration of CCR2 knockout Tregs into the liver was significantly reduced compared to the WT Tregs. We demonstrated that hyperglycemia contributes to increase in infiltrating monocytes and Tregs, which are associated with hepatic immune dysfunction in mice. CCR2-mediated migration of Tregs regulates hyperglycemia-induced hepatic inflammation.

Treatment with 4-Methylpyrazole Modulated Stellate Cells and Natural Killer Cells and Ameliorated Liver Fibrosis in Mice

Background & Aims Accumulating evidence suggests that retinol and its metabolites are closely associated with liver fibrogenesis. Recently, we demonstrated that genetic ablation of alcohol dehydrogenase 3 (ADH3), a retinol metabolizing gene that is expressed in hepatic stellate cells (HSCs) and natural killer (NK) cells, attenuated liver fibrosis in mice. In the current study, we investigated whether pharmacological ablation of ADH3 has therapeutic effects on experimentally induced liver fibrosis in mice. Methods Liver fibrosis was induced by intraperitoneal injections of carbon tetrachloride (CCl4) or bile duct ligation (BDL) for two weeks. To inhibit ADH3-mediated retinol metabolism, 10 μg 4-methylpyrazole (4-MP)/g of body weight was administered to mice treated with CCl4 or subjected to BDL. The mice were sacrificed at week 2 to evaluate the regression of liver fibrosis. Liver sections were stained for collagen and α-smooth muscle actin (α-SMA). In addition, HSCs and NK cells were isolated from control and treated mice livers for molecular and immunological studies. Results Treatment with 4-MP attenuated CCl4- and BDL-induced liver fibrosis in mice, without any adverse effects. HSCs from 4-MP treated mice depicted decreased levels of retinoic acids and increased retinol content than HSCs from control mice. In addition, the expression of α-SMA, transforming growth factor-β1 (TGF-β1), and type I collagen α1 was significantly reduced in the HSCs of 4-MP treated mice compared to the HSCs from control mice. Furthermore, inhibition of retinol metabolism by 4-MP increased interferon-γ production in NK cells, resulting in increased apoptosis of activated HSCs. Conclusions Based on our data, we conclude that inhibition of retinol metabolism by 4-MP ameliorates liver fibrosis in mice through activation of NK cells and suppression of HSCs. Therefore, retinol and its metabolizing enzyme, ADH3, might be potential targets for therapeutic intervention of liver fibrosis.

Growth Differentiation Factor 15 Mediates Systemic Glucose Regulatory Action of T-Helper Type 2 Cytokines

T-helper type 2 (Th2) cytokines, including interleukin (IL)-13 and IL-4, produced in adipose tissue, are critical regulators of intra-adipose and systemic lipid and glucose metabolism. Furthermore, IL-13 is a potential therapy for insulin resistance in obese mouse models. Here, we examined mediators produced by adipocytes that are responsible for regulating systemic glucose homeostasis in response to Th2 cytokines. We used RNA sequencing data analysis of cultured adipocytes to screen factors secreted in response to recombinant IL-13. Recombinant IL-13 induced expression of growth differentiation factor 15 (GDF15) via the Janus kinase-activated STAT6 pathway. In vivo administration of α-galactosylceramide or IL-33 increased IL-4 and IL-13 production, thereby increasing GDF15 levels in adipose tissue and in plasma of mice; however, these responses were abrogated in STAT6 knockout mice. Moreover, administration of recombinant IL-13 to wild-type mice fed a high-fat diet (HFD) improved glucose intolerance; this was not the case for GDF15 knockout mice fed the HFD. Taken together, these data suggest that GDF15 is required for IL-13–induced improvement of glucose intolerance in mice fed an HFD. Thus, beneficial effects of Th2 cytokines on systemic glucose metabolism and insulin sensitivity are mediated by GDF15. These findings open up a potential pharmacological route for reversing insulin resistance associated with obesity.