Autophagy releases lipid that promotes fibrogenesis by activated hepatic stellate cells in mice and in human tissues

Contributions of metabolic dysregulation and inflammation to nonalcoholic steatohepatitis, hepatic fibrosis, and cancer

PURPOSE OF REVIEW

We review accumulating evidence that nonalcoholic steatohepatitis (NASH), a more advanced form of nonalcoholic fatty liver disease (NAFLD), predisposes patients to the risk of developing hepatocellular carcinoma (HCC), and we summarize recent advances in the elucidation of cancer-promoting pathways in NASH. We highlight the potential role of progenitor cells and hepatic stellate cells (HSCs) in promoting the early events that could culminate in cancer, as well as the emerging contribution of the gut-liver axis in promoting inflammation, senescence, and tumor growth in NASH and HCC. Finally, we review the role of bile acid receptors, vitamin D, and protective cellular pathways such as autophagy.

RECENT FINDINGS

Studies have recently uncovered roles for gut microbiota, bile acid receptors and vitamin D in regulating the progression from NAFLD to HCC. Intriguing findings linking senescence and autophagy in hepatic stellate cells to HCC have also been discovered, as well as a link between dysregulated progenitor cell regulation and HCC.

SUMMARY

NAFLD is the most common cause of chronic liver disease in the United States and Western Europe. The lack of definitive mechanisms underlying development of NASH among patients with NAFLD and its progression to HCC limit diagnosis and management, but new findings are paving the way for better biomarkers and therapies.

Transforming growth factor‑β1 reduces apoptosis via autophagy activation in hepatic stellate cells

Autophagy is a metabolic process that is important in fibrogenesis, in which cellular components are degraded by lysosomal machinery. Transforming growth factor β1 (TGF‑β1) is a potent fibrogenic cytokine involved in liver fibrosis; however, it remains elusive whether autophagy is regulated by TGF‑β1 in this process. In the present study, the function of TGF‑β1‑mediated autophagy in the proliferation and apoptosis of hepatic stellate cells (HSCs) was investigated. A rat HSC cell line (HSC‑T6) was incubated with or without TGF‑β1 followed by bafilomycin A1, and microtubule-associated proteins 1A/1B light chain 3 (LC3) small interfering (si)RNA was used to inhibit autophagy in order to assess the association between TGF‑β1 and autophagy. HSC‑T6 cell transient transfection was accomplished with a pLVX‑AcGFP‑N1‑rLC3B‑encoding plasmid. An MTS assay and flow cytometry were utilized to detect proliferation and apoptosis of HSC‑T6 cells. Quantitative polymerase chain reaction, immunofluorescence and western blot analysis were used to detect the presence of activation markers. Proliferation was increased and apoptosis was reduced in HSC‑T6 cells treated with TGF‑β1 compared with cells subjected to serum deprivation. However, when HSC‑T6 cells were treated with bafilomycin A1 and LC3 siRNA, increased apoptosis and reduced proliferation were observed. In addition, protein and mRNA expression levels of the autophagy marker LC3 were significantly increased. GFP‑LC3 punctate markings were more prolific following TGF‑β1 treatment of HSC‑T6 cells, indicating that TGF‑β1 may rescue HSC‑T6 cells from serum deprivation and reduce apoptosis via autophagy induction. The present study elucidated the possible functions of TGF‑β1‑mediated autophagy in the pathological process of liver fibrosis.

Fate tracing reveals hepatic stellate cells as dominant contributors to liver fibrosis independent of its aetiology

Although organ fibrosis causes significant morbidity and mortality in chronic diseases, the lack of detailed knowledge about specific cellular contributors mediating fibrogenesis hampers the design of effective antifibrotic therapies. Different cellular sources, including tissue-resident and bone marrow-derived fibroblasts, pericytes and epithelial cells, have been suggested to give rise to myofibroblasts, but their relative contributions remain controversial, with profound differences between organs and different diseases. Here we employ a novel Cre-transgenic mouse that marks 99% of hepatic stellate cells (HSCs), a liver-specific pericyte population, to demonstrate that HSCs give rise to 82-96% of myofibroblasts in models of toxic, cholestatic and fatty liver disease. Moreover, we exclude that HSCs function as facultative epithelial progenitor cells in the injured liver. On the basis these findings, HSCs should be considered the primary cellular target for antifibrotic therapies across all types of liver disease.

Autophagy and microRNA dysregulation in liver diseases

Autophagy is a catabolic process through which organelles and cellular components are sequestered into autophagosomes and degraded via fusion with lysosomes. Autophagy plays a role in many physiological processes, including stress responses, energy homeostasis, elimination of cellular organelles, and tissue remodeling. In addition, autophagy capacity changes in various disease states. A series of studies have shown that autophagy is strictly controlled to maintain homeostatic balance of energy metabolism and cellular organelle and protein turnover. These studies have also shown that this process is post-transcriptionally controlled by small noncoding microRNAs that regulate gene expression through complementary base pairing with mRNAs. Conversely, autophagy regulates the expression of microRNAs. Therefore, dysregulation of the link between autophagy and microRNA expression exacerbates the pathogenesis of various diseases. In this review, we summarize the roles of autophagy and microRNA dysregulation in the course of liver diseases, with the aim of understanding how microRNAs modify key autophagic effector molecules, and we discuss how this dysregulation affects both physiological and pathological conditions. This article may advance our understanding of the cellular and molecular bases of liver disease progression and promote the development of strategies for pharmacological intervention.

Cellular and molecular mechanisms in the pathogenesis of liver fibrosis: An update

There have been considerable recent advances towards a better understanding of the complex cellular and molecular network underlying liver fibrogenesis. Recent data indicate that the termination of fibrogenic processes and the restoration of deficient fibrolytic pathways may allow the reversal of advanced fibrosis and even cirrhosis. Therefore, efforts have been made to better clarify the cellular and molecular mechanisms that are involved in liver fibrosis. Activation of hepatic stellate cells (HSCs) remains a central event in fibrosis, complemented by other sources of matrix-producing cells, including portal fibroblasts, fibrocytes and bone marrow-derived myofibroblasts. These cells converge in a complex interaction with neighboring cells to provoke scarring in response to persistent injury. Defining the interaction of different cell types, revealing the effects of cytokines on these cells and characterizing the regulatory mechanisms that control gene expression in activated HSCs will enable the discovery of new therapeutic targets. Moreover, the characterization of different pathways associated with different etiologies aid in the development of disease-specific therapies. This article outlines recent advances regarding the cellular and molecular mechanisms involved in liver fibrosis that may be translated into future therapies. The pathogenesis of liver fibrosis associated with alcoholic liver disease, non-alcoholic fatty liver disease and viral hepatitis are also discussed to emphasize the various mechanisms involved in liver fibrosis.

Role of autophagy in the pathophysiology of nonalcoholic fatty liver disease: A controversial issue

Autophagy is a mechanism involved in cellular homeostasis under basal and stressed conditions delivering cytoplasmic content to the lysosomes for degradation to macronutrients. The potential role of autophagy in disease is increasingly recognised and investigated in the last decade. Nowadays it is commonly accepted that autophagy plays a role in the hepatic lipid metabolism. Hence, dysfunction of autophagy may be an underlying cause of non-alcoholic fatty liver disease. However, controversy of the exact role of autophagy in the lipid metabolism exists: some publications report a lipolytic function of autophagy, whereas others claim a lipogenic function. This review aims to give an update of the present knowledge on autophagy in the hepatic lipid metabolism, hepatic insulin resistance, steatohepatitis and hepatic fibrogenesis.

β-Adrenergic agonist and antagonist regulation of autophagy in HepG2 cells, primary mouse hepatocytes, and mouse liver

Autophagy recently has been shown to be involved in normal hepatic function and in pathological conditions such as non-alcoholic fatty liver disease. Adrenergic signalling also is an important regulator of hepatic metabolism and function. However, currently little is known about the potential role of adrenergic signaling on hepatic autophagy, and whether the β-adrenergic receptor itself may be a key regulator of autophagy. To address these issues, we investigated the actions of the β2-adrenergic receptor agonist, clenbuterol on hepatic autophagy. Surprisingly, we found that clenbuterol stimulated autophagy and autophagic flux in hepatoma cells, primary hepatocytes and in vivo. Similar effects also were observed with epinephrine treatment. Interestingly, propranolol caused a late block in autophagy in the absence and presence of clenbuterol, both in cell culture and in vivo. Thus, our results demonstrate that the β2-adrenergic receptor is a key regulator of hepatic autophagy, and that the β-blocker propranolol can independently induce a late block in autophagy.

Molecular mechanism and treatment of viral hepatitis-related liver fibrosis

Hepatic fibrosis is a wound-healing response to various chronic stimuli, including viral hepatitis B or C infection. Activated myofibroblasts, predominantly derived from the hepatic stellate cells (HSCs), regulate the balance between matrix metalloproteinases and their tissue inhibitors to maintain extracellular matrix homeostasis. Transforming growth factor-β and platelet-derived growth factor are classic profibrogenic signals that activate HSC proliferation. In addition, proinflammatory cytokines and chemokines coordinate macrophages, T cells, NK/NKT cells, and liver sinusoidal endothelial cells in complex fibrogenic and regression processes. In addition, fibrogenesis involves angiogenesis, metabolic reprogramming, autophagy, microRNA, and epigenetic regulations. Hepatic inflammation is the driving force behind liver fibrosis; however, host single nucleotide polymorphisms and viral factors, including the genotype, viral load, viral mutation, and viral proteins, have been associated with fibrosis progression. Eliminating the underlying etiology is the most crucial antifibrotic therapy. Growing evidence has indicated that persistent viral suppression with antiviral therapy can result in fibrosis regression, reduced liver disease progression, decreased hepatocellular carcinoma, and improved chances of survival. Preclinical studies and clinical trials are currently examining several investigational agents that target key fibrogenic pathways; the results are promising and shed light on this debilitating illness.

Autophagy--a key player in cellular and body metabolism

Knowledge gained over the past 10 years about the mechanisms that underpin autophagy has provided a universal framework for studies of diverse physiological and pathological processes. Of particular interest is the emerging role of autophagy in the maintenance of energy homeostasis, both at the cellular level and within the organism as a whole. Dysregulation of autophagy might contribute to the development of metabolic disorders, including insulin resistance, diabetes mellitus, obesity, atherosclerosis and osteoporosis. The authors of this Review highlight research findings on the regulation of cellular autophagy by nutrients. They also describe the role of autophagy in various tissues in the regulation of energy metabolism and the development of diseases related to altered metabolism. Finally, the potential of pharmacological modulation of autophagy as a treatment for human metabolic disorders is discussed.

Phospholipase D1 decreases type I collagen levels in hepatic stellate cells via induction of autophagy

Hepatic stellate cells (HSCs) are major players in liver fibrogenesis. Accumulating evidence shows that suppression of autophagy plays an important role in the development and progression of liver disease. Phospholipase D1 (PLD1), which catalyzes the hydrolysis of phosphatidylcholine to yield phosphatidic acid (PA) and choline, was recently shown to modulate autophagy. However, little is known about the effects of PLD1 on the production of type I collagen that characterizes liver fibrosis. Here, we examined whether PLD1 regulates type I collagen levels in HSCs through induction of autophagy. Adenovirus-mediated overexpression of PLD-1 (Ad-PLD1) reduced type I collagen levels in the activated human HSC lines, hTERT and LX2. Overexpression of PLD1 in HSCs led to induction of autophagy as demonstrated by increased LC3-II conversion and formation of LC3 puncta, and decreased p62 abundance. Moreover, inhibiting the induction of autophagy by treating cells with bafilomycin or a small interfering (si)RNA for ATG7 rescued Ad-PLD1-induced suppression of type I collagen accumulation in HSCs. The effects of PLD on type I collagen levels were not related to TGF-β/Smad signaling. Furthermore, treatment of cells with PA induced autophagy and inhibited type I collagen accumulation. The present study indicates that PLD1 plays a role in regulating type I collagen accumulation through induction of autophagy.

Molecular mechanisms and new treatment strategies for non-alcoholic steatohepatitis (NASH)

Non-alcoholic steatohepatitis (NASH) is a severe form of non-alcoholic fatty liver disease (NAFLD), in which most patients exhibit non-progressive, non-alcoholic fatty liver (NAFL) attributable to simple steatosis. Multiple hits, including genetic differences, fat accumulation, insulin resistance and intestinal microbiota changes, account for the progression of NASH. NAFLD is strongly associated with obesity, which induces adipokine secretion, endoplasmic reticulum (ER) and oxidative stress at the cellular level, which in turn induces hepatic steatosis, inflammation and fibrosis. Among these factors, gut microbiota are acknowledged as having an important role in initiating this multifactorial disease. Oxidative stress is considered to be a key contributor in the progression from NAFL to NASH. Macrophage infiltration is apparent in NAFL and NASH, while T-cell infiltration is apparent in NASH. Although several clinical trials have shown that antioxidative therapy with vitamin E can effectively control hepatitis pathology in the short term, the long-term effects remain obscure and have often proved to be ineffective in many other diseases. Several long-term antioxidant protocols have failed to reduce mortality. New treatment modalities that incorporate current understanding of NAFLD molecular pathogenesis must be considered.

Protective effect of astaxanthin on liver fibrosis through modulation of TGF-β1 expression and autophagy

Liver fibrosis is a common pathway leading to cirrhosis and a worldwide clinical issue. Astaxanthin is a red carotenoid pigment with antioxidant, anticancer, and anti-inflammatory properties. The aim of this study was to investigate the effect of astaxanthin on liver fibrosis and its potential protective mechanisms. Liver fibrosis was induced in a mouse model using CCL4 (intraperitoneal injection, three times a week for 8 weeks), and astaxanthin was administered everyday at three doses (20, 40, and 80 mg/kg). Pathological results indicated that astaxanthin significantly improved the pathological lesions of liver fibrosis. The levels of alanine aminotransferase aspartate aminotransferase and hydroxyproline were also significantly decreased by astaxanthin. The same results were confirmed in bile duct liagtion, (BDL) model. In addition, astaxanthin inhibited hepatic stellate cells (HSCs) activation and formation of extracellular matrix (ECM) by decreasing the expression of NF-κB and TGF-β1 and maintaining the balance between MMP2 and TIMP1. In addition, astaxanthin reduced energy production in HSCs by downregulating the level of autophagy. These results were simultaneously confirmed in vivo and in vitro. In conclusion, our study showed that 80 mg/kg astaxanthin had a significant protective effect on liver fibrosis by suppressing multiple profibrogenic factors.

Autophagy in hepatic fibrosis

Hepatic fibrosis is a leading cause of morbidity and mortality worldwide. Hepatic fibrosis is usually associated with chronic liver diseases caused by infection, drugs, metabolic disorders, or autoimmune imbalances. Effective clinical therapies are still lacking. Autophagy is a cellular process that degrades damaged organelles or protein aggregation, which participates in many pathological processes including liver diseases. Autophagy participates in hepatic fibrosis by activating hepatic stellate cells and may participate as well through influencing other fibrogenic cells. Besides that, autophagy can induce some liver diseases to develop while it may play a protective role in hepatocellular abnormal aggregates related liver diseases and reduces fibrosis. With a better understanding of the potential effects of autophagy on hepatic fibrosis, targeting autophagy might be a novel therapeutic strategy for hepatic fibrosis in the near future.

Hepatic inflammation and progressive liver fibrosis in chronic liver disease

Chronic liver inflammation drives hepatic fibrosis, and current immunosuppressive, anti-inflammatory, and anti-viral therapies can weaken this driver. Hepatic fibrosis is reversed, stabilized, or prevented in 57%-79% of patients by conventional treatment regimens, mainly by their anti-inflammatory actions. Responses, however, are commonly incomplete and inconsistently achieved. The fibrotic mechanisms associated with liver inflammation have been clarified, and anti-fibrotic agents promise to improve outcomes as adjunctive therapies. Hepatitis C virus and immune-mediated responses can activate hepatic stellate cells by increasing oxidative stress within hepatocytes. Angiotensin can be synthesized by activated hepatic stellate cells and promote the production of reactive oxygen species. Anti-oxidants (N-acetylcysteine, S-adenosyl-L-methionine, and vitamin E) and angiotensin inhibitors (losartin) have had anti-fibrotic actions in preliminary human studies, and they may emerge as supplemental therapies. Anti-fibrotic agents presage a new era of supplemental treatment for chronic liver disease.

Cellular and molecular mechanisms in liver fibrogenesis

Liver fibrogenesis is a dynamic and highly integrated molecular, tissue and cellular process, potentially reversible, that drives the progression of chronic liver diseases (CLD) towards liver cirrhosis and hepatic failure. Hepatic myofibroblasts (MFs), the pro-fibrogenic effector cells, originate mainly from activation of hepatic stellate cells and portal fibroblasts being characterized by a proliferative and survival attitude. MFs also contract in response to vasoactive agents, sustain angiogenesis and recruit and modulate activity of cells of innate or adaptive immunity. Chronic activation of wound healing and oxidative stress as well as derangement of epithelial-mesenchymal interactions are "major" pro-fibrogenic mechanisms, whatever the etiology. However, literature has outlined a complex network of pro-fibrogenic factors and mediators proposed to modulate CLD progression, with some of them being at present highly debated in the field, including the role of epithelial to mesenchymal transition and Hedgehog signaling pathways. Hypoxia and angiogenesis as well as inflammasomes are recently emerged as ubiquitous pro-inflammatory and pro-fibrogenic determinants whereas adipokines are mostly involved in CLD related to metabolic disturbances (metabolic syndrome and/or obesity and type 2 diabetes). Finally, autophagy as well as natural killer and natural killer-T cells have been recently proposed to significantly affect fibrogenic CLD progression.

Cannabidiol protects liver from binge alcohol-induced steatosis by mechanisms including inhibition of oxidative stress and increase in autophagy

Acute alcohol drinking induces steatosis, and effective prevention of steatosis can protect liver from progressive damage caused by alcohol. Increased oxidative stress has been reported as one mechanism underlying alcohol-induced steatosis. We evaluated whether cannabidiol, which has been reported to function as an antioxidant, can protect the liver from alcohol-generated oxidative stress-induced steatosis. Cannabidiol can prevent acute alcohol-induced liver steatosis in mice, possibly by preventing the increase in oxidative stress and the activation of the JNK MAPK pathway. Cannabidiol per se can increase autophagy both in CYP2E1-expressing HepG2 cells and in mouse liver. Importantly, cannabidiol can prevent the decrease in autophagy induced by alcohol. In conclusion, these results show that cannabidiol protects mouse liver from acute alcohol-induced steatosis through multiple mechanisms including attenuation of alcohol-mediated oxidative stress, prevention of JNK MAPK activation, and increasing autophagy.

Liver autophagy: much more than just taking out the trash

Studies performed in the liver in the 1960s led to the identification of lysosomes and the discovery of autophagy, the process by which intracellular proteins and organelles are degraded in lysosomes. Early studies in hepatocytes also uncovered how nutritional status regulates autophagy and how various circulating hormones modulate the activity of this catabolic process in the liver. The intensive characterization of hepatic autophagy over the years has revealed that lysosome-mediated degradation is important not only for maintaining liver homeostasis in normal physiological conditions, but also for an adequate response of this organ to stressors such as proteotoxicity, metabolic dysregulation, infection and carcinogenesis. Autophagic malfunction has also been implicated in the pathogenesis of common liver diseases, suggesting that chemical manipulation of this process might hold potential therapeutic value. In this Review--intended as an introduction to the topic of hepatic autophagy for clinical scientists--we describe the different types of hepatic autophagy, their role in maintaining homeostasis in a healthy liver and the contribution of autophagic malfunction to liver disease.

Hepatic fibrosis and the microenvironment: fertile soil for hepatocellular carcinoma development

Hepatocellular carcinoma is an emerging worldwide health threat that has few curative treatment options and poor overall survival. Progressive hepatic fibrosis is a common pathway for all forms of chronic liver disease and is closely linked epidemiologically to hepatocellular carcinoma risk. However, the molecular events that predispose a fibrotic liver to cancer development remain elusive. Nonetheless, a permissive hepatic microenvironment provides fertile soil for transition of damaged hepatocytes into hepatocellular carcinoma. Key predisposing features include alterations in the extracellular matrix, bidirectional signaling pathways between parenchymal and nonparenchymal cells, and immune dysfunction. Emerging research into the contributions of autophagy, tumor-associated fibroblasts, and hepatocellular carcinoma progenitor cells to this dangerous milieu also provides new mechanistic underpinnings to explain the contribution of fibrosis to cancer. As effective antifibrotic therapies are developed, these approaches could attenuate the rising surge of hepatocellular carcinoma associated with chronic liver disease.

Emerging functions of autophagy in kidney transplantation

In response to ischemic, toxic or immunological insults, the more frequent injuries encountered by the kidney, cells must adapt to maintain vital metabolic functions and avoid cell death. Among the adaptive responses activated, autophagy emerges as an important integrator of various extracellular and intracellular triggers (often related to nutrients availability or immunological stimuli), which, as a consequence,may regulate cell viability, and also immune functions,both innate or adaptive. The aim of this review is to make the synthesis of the recent literature on the implications of autophagy in the kidney transplantation field and to discuss the future directions for research.

Emerging roles of autophagy in the stressed kidney allograft

The transplanted kidney integrates a considerable number of tissular stress that challenge cell viability and promote inflammation and fibrogenesis. These injuries ultimately may lead to structural deterioration (ie, interstitial fibrosis and tubular atrophy) and to loss of function. In response to ischemic, toxic, or immunologic insults, which are the most frequent injuries encountered by transplanted kidneys, cells must adapt to maintain vital metabolic functions and to avoid death. Among the adaptive responses activated, autophagy has emerged as an important integrator of various extracellular and intracellular triggers (often related to nutrient availability or immunologic stimuli), which may in turn regulate cell viability and both innate and adaptive immune functions. This review provides an overview of the recent literature on the implications of autophagy in the field of kidney transplantation and discusses future directions for research.

Autophagy and immune response in kidneys

Autophagy is a ubiquitous intracellular catabolic process that contributes to homeostatic maintenance and regulates the balance between health and disease. Emerging evidence from both the immunology and renal literature suggests that important relationships exist between the immune system and renal autophagy that may have significant implications for our understanding of the pathogenesis of kidney diseases. Autophagic flux in renal parenchymal cells can protect against acute and chronic kidney injury and can be stimulated via activation of innate immune receptors, cytokine secretion, and/or direct contact by immune cells. Conversely, modulation of autophagy in renal cells may influence both adaptive and innate immune cell responses. Autophagy can promote the ability of renal epithelial cells, which can act as antigen-presenting cells, to process and present self-antigen to immune cells. In addition, autophagic control of inflammasome function can modify the intrarenal inflammatory milieu, thereby preventing immune cell infiltration. Because autophagy and immune responses may promote or protect against kidney injury, further research is needed to better understand how interactions between renal parenchymal cells and the immune system are altered by autophagy. Novel agents are being developed that promote or inhibit various steps of the autophagy pathway, and it is likely that whether such agents are beneficial or harmful in the context of kidney disease will depend, at least in part, on whether and how they influence the relationship between autophagy and the immune response in the kidney.

Regulation of autophagy by TGF-β: emerging role in kidney fibrosis

Autophagy is a highly conserved homoeostatic mechanism for cell survival under conditions of stress, and is widely implicated as an important pathway in many biological processes and diseases. In progressive kidney diseases, fibrosis represents the common pathway to end-stage kidney failure. Transforming growth factor-β1 (TGF-β1) is a pleiotropic cytokine that has been established as a central mediator of kidney fibrosis. A recently emerging body of evidence from studies in renal cells in culture and experimental animal models suggests that TGF-β1 regulates autophagy and that autophagy regulates many critical aspects of normal and disease conditions associated with kidney fibrosis, such as tubulointerstitial fibrosis, glomerulosclerosis, and diabetic nephropathy. Here, we review the recent advances exploring the process of autophagy, its regulation by TGF-β1, and the implication in the pathogenesis of progressive kidney fibrosis and injury responses. Understanding the cellular and molecular bases of this process is crucial for identifying potential new diagnostic and therapeutic targets of kidney fibrosis.

Multiple hits, including oxidative stress, as pathogenesis and treatment target in non-alcoholic steatohepatitis (NASH)

Multiple parallel hits, including genetic differences, insulin resistance and intestinal microbiota, account for the progression of non-alcoholic steatohepatitis (NASH). Multiple hits induce adipokine secretion, endoplasmic reticulum (ER) and oxidative stress at the cellular level that subsequently induce hepatic steatosis, inflammation and fibrosis, among which oxidative stress is considered a key contributor to progression from simple fatty liver to NASH. Although several clinical trials have shown that anti-oxidative therapy can effectively control hepatitis activities in the short term, the long-term effect remains obscure. Several trials of long-term anti-oxidant protocols aimed at treating cerebrovascular diseases or cancer development have failed to produce a benefit. This might be explained by the non-selective anti-oxidative properties of these drugs. Molecular hydrogen is an effective antioxidant that reduces only cytotoxic reactive oxygen species (ROS) and several diseases associated with oxidative stress are sensitive to hydrogen. The progress of NASH to hepatocellular carcinoma can be controlled using hydrogen-rich water. Thus, targeting mitochondrial oxidative stress might be a good candidate for NASH treatment. Long term clinical intervention is needed to control this complex lifestyle-related disease.

Experimental liver fibrosis research: update on animal models, legal issues and translational aspects

Liver fibrosis is defined as excessive extracellular matrix deposition and is based on complex interactions between matrix-producing hepatic stellate cells and an abundance of liver-resident and infiltrating cells. Investigation of these processes requires in vitro and in vivo experimental work in animals. However, the use of animals in translational research will be increasingly challenged, at least in countries of the European Union, because of the adoption of new animal welfare rules in 2013. These rules will create an urgent need for optimized standard operating procedures regarding animal experimentation and improved international communication in the liver fibrosis community. This review gives an update on current animal models, techniques and underlying pathomechanisms with the aim of fostering a critical discussion of the limitations and potential of up-to-date animal experimentation. We discuss potential complications in experimental liver fibrosis and provide examples of how the findings of studies in which these models are used can be translated to human disease and therapy. In this review, we want to motivate the international community to design more standardized animal models which might help to address the legally requested replacement, refinement and reduction of animals in fibrosis research.

Effects of indoleamine 2,3-dioxygenase deficiency on high-fat diet-induced hepatic inflammation

Hepatic immune regulation is associated with the progression from simple steatosis to non-alcoholic steatohepatitis, a severe condition of inflamed fatty liver. Indoleamine 2,3-dioxygenase (IDO), an intracellular enzyme that mediates the catabolism of L-tryptophan to L-kynurenine, plays an important role in hepatic immune regulation. In the present study, we examined the effects of IDO gene silencing on high-fat diet (HFD)-induced liver inflammation and fibrosis in mice. After being fed a HFD for 26 weeks, the IDO-knockout (KO) mice showed a marked infiltration of inflammatory cells, especially macrophages and T lymphocytes, in the liver. The expression levels of F4/80, IFNγ, IL-1β, and IL-6 mRNA in the liver and the expression levels of F4/80 and TNF-α mRNA in the white adipose tissue were significantly increased in IDO-KO mice, although hepatic steatosis, the accumulation of intrahepatic triglycerides, and the amount of oxidative stress were lower than those in IDO-wild-type mice. IDO-KO mice also developed marked pericellular fibrosis in the liver, accumulated hepatic hydroxyproline, and exhibited increased expression levels of hepatic TGF-β1 mRNA. These findings suggest that IDO-KO renders the mice more susceptible to HFD-induced hepatic inflammation and fibrosis. Therefore, IDO may have a protective effect against hepatic fibrosis, at least in this HFD-induced liver injury model.

Cellular mechanisms of tissue fibrosis. 5. Novel insights into liver fibrosis

Liver fibrosis is the common scarring reaction associated with chronic liver injury that results from prolonged parenchymal cell injury and/or inflammation. The fibrogenic response is characterized by progressive accumulation of extracellular matrix components enriched in fibrillar collagens and a failure of matrix turnover. This process is driven by a heterogeneous population of hepatic myofibroblasts, which mainly derive from hepatic stellate cells and portal fibroblasts. Regression of fibrosis can be achieved by the successful control of chronic liver injury, owing to termination of the fibrogenic reaction following clearance of hepatic myofibroblasts and restoration of fibrolytic pathways. Understanding of the complex network underlying liver fibrogenesis has allowed the identification of a large number of antifibrotic targets, but no antifibrotic drug has as yet been approved. This review will highlight recent advances regarding the mechanisms that regulate liver fibrogenesis and fibrosis regression, with special focus on novel signaling pathways and the role of inflammatory cells. Translation of these findings to therapies will require continued efforts to develop multitarget therapeutic approaches that will improve the grim prognosis of liver cirrhosis.

Endoplasmic reticulum stress induces fibrogenic activity in hepatic stellate cells through autophagy

BACKGROUND & AIMS

Metabolic stress during liver injury enhances autophagy and provokes stellate cell activation, with secretion of scar matrix. Conditions that augment protein synthesis increase demands on the endoplasmic reticulum (ER) folding capacity and trigger the unfolded protein response (UPR) to cope with resulting ER stress. Generation of reactive oxygen species (ROS) is a common feature of hepatic fibrogenesis, and crosstalk between oxidant stress and ER stress has been proposed. The aim of our study was to determine the impact of oxidant and ER stress on stellate cell activation.

METHODS

Oxidant stress was induced in hepatic stellate cells using H2O2 in culture or by ethanol feeding in vivo, and the UPR was analyzed. Because the branch of the UPR mainly affected was IREα, we blocked this pathway in stellate cells and analyzed the fibrogenic response, together with autophagy and downstream MAPK signaling. The Nrf2 antioxidant response was also evaluated in stellate cells under oxidant stress conditions.

RESULTS

H2O2 treatment in culture or ethanol feeding in vivo increased the UPR based on splicing of XBP1 mRNA, which triggered autophagy. The Nrf2-mediated antioxidant response, as measured by qRT-PCR of its target genes was also induced under ER stress conditions. Conversely, blockade of the IRE1α pathway in stellate cells significantly decreased both their activation and autophagic activity in a p38 MAPK-dependent manner, leading to a reduced fibrogenic response.

CONCLUSIONS

These data implicate mechanisms underlying protein folding quality control in regulating the fibrogenic response in hepatic stellate cells.

Liver Injury and the Activation of the Hepatic Myofibroblasts

Liver fibrosis is a wound healing process, the end result of chronic liver injury elicited by different noxious stimuli. Activated hepatic stellate cells or myofibroblasts and portal myofibroblasts are considered as the main producers of the extracellular matrix in the liver. Upon liver injury the quiescent stellate cells transdifferentiate into myofibroblasts a process highlighted by the loss of vitamin A stores, upregulation of interstitial type collagens, smooth muscle α actin, matrix metalloproteinases, proteoglycans, and the induction of cell survival pathways. Activation of hepatic stellate cells is a result of a complex interplay between the parenchymal cells, immune cells, extracellular matrix mechanics and extrahepatic milieu such as the gut microbiome. In this review we will focus on the pathomechanism of stellate cell activation following chronic liver injury; with the aim of identifying possible treatment targets for anti-fibrogenic agents.

Functions of autophagy in normal and diseased liver

Autophagy has emerged as a critical lysosomal pathway that maintains cell function and survival through the degradation of cellular components such as organelles and proteins. Investigations specifically employing the liver or hepatocytes as experimental models have contributed significantly to our current knowledge of autophagic regulation and function. The diverse cellular functions of autophagy, along with unique features of the liver and its principal cell type the hepatocyte, suggest that the liver is highly dependent on autophagy for both normal function and to prevent the development of disease states. However, instances have also been identified in which autophagy promotes pathological changes such as the development of hepatic fibrosis. Considerable evidence has accumulated that alterations in autophagy are an underlying mechanism of a number of common hepatic diseases including toxin-, drug- and ischemia/reperfusion-induced liver injury, fatty liver, viral hepatitis and hepatocellular carcinoma. This review summarizes recent advances in understanding the roles that autophagy plays in normal hepatic physiology and pathophysiology with the intent of furthering the development of autophagy-based therapies for human liver diseases.

Nilotinib induces apoptosis and autophagic cell death of activated hepatic stellate cells via inhibition of histone deacetylases

Increasing hepatic stellate cell (HSC) death is a very attractive approach for limiting liver fibrosis. Tyrosine kinase inhibitors have been shown to have anti-fibrotic properties, but the mechanisms are poorly understood. Here, we identified the mechanism of action of the second-generation tyrosine kinase inhibitor nilotinib in inducing HSC death. Human HSC line (LX-2) and rat HSCs were treated with nilotinib and its predecessor, imatinib, in the absence or presence of various blockers, known to interfere with death signaling pathways. Nilotinib, but not imatinib, induced progressive cell death of activated, but not quiescent, HSCs in a dose-dependent manner. Activated HSCs died through apoptosis, as denoted by increased DNA fragmentation and caspase activation, and through autophagy, as indicated by the accumulation of autophagic markers, light chain (LC)3A-II and LC3B-II. Although inhibition of caspases with Z-VAD-FMK suppressed nilotinib-induced HSCs' apoptosis, there was no increase in HSCs' survival, because autophagy was exacerbated. However, blocking the mitochondrial permeability transition pore (mPTP) opening with cyclosporin A completely abolished both apoptosis and autophagy due to nilotinib. Moreover, nilotinib treatment decreased the protein expression of histone deacetylases 1, 2 and 4. Interestingly, pretreament with C646, a selective p300/CBP histone acetyl transferase inhibitor, resulted in diverting nilotinib-induced apoptosis and autophagy towards necrosis. In conclusion, the identification of mPTP as a target of nilotinib in activated HSCs suggests coordination with histone deacetylases inhibition to induce apoptosis and autophagy. Thus, our study provides novel insights into the anti-fibrotic effects of nilotinib.

GCN2 kinase is a key regulator of fibrogenesis and acute and chronic liver injury induced by carbon tetrachloride in mice

General control nonderepresible 2 (GCN2) is a highly conserved cytosolic kinase that modulates a complex response for coping with the stress owing to lack of amino acids. GCN2 has been recently shown to be involved in the regulation of metabolic balance and lipid degradation rate in the liver. We hypothesized that GCN2 could have a role in in hepatic fibrogenesis and in the response to acute or chronic liver injury. Activation of GCN2 in primary or immortalized human hepatic stellate cells by incubation with medium lacking the essential amino acid histidine correlated with decreased levels of collagen type I protein and mRNA, suggesting an antifibrogenic effect of GCN2. In vivo studies with Gcn2 knock-out mice (Gcn2(-/-)) showed increased susceptibility to both acute or chronic liver damage induced by CCl(4), as shown by higher alanine aminotransferase and aspartate aminotransferase activities, increased necrosis and higher inflammatory infiltrates compared with wild-type mice (WT). Chronic CCl(4) treatment increased deposition of interstitial collagen type I more in Gcn2(-/-) mice than in WT mice. Col1a1 and col1a2 mRNA levels also increased in CCl(4)-treated Gcn2(-/-) mice compared with WT mice. These results suggest that GCN2 is a key regulator of the fibrogenic response to liver injury.

The role of autophagy in liver cancer: molecular mechanisms and potential therapeutic targets

Autophagy is an evolutionarily conserved pathway for degradation of cytoplasmic proteins and organelles via lysosome. Proteins coded by the autophagy-related genes (Atgs) are the core molecular machinery in control of autophagy. Among the various biological functions of autophagy identified so far, the link between autophagy and cancer is probably among the most extensively studied and is often viewed as controversial. Autophagy might exert a dual role in cancer development: autophagy can serve as an anti-tumor mechanism, as defective autophagy (e.g., heterozygous knockdown Beclin 1 and Atg7 in mice) promotes the malignant transformation and spontaneous tumors. On the other hand, autophagy functions as a protective or survival mechanism in cancer cells against cellular stress (e.g., nutrient deprivation, hypoxia and DNA damage) and hence promotes tumorigenesis and causes resistance to therapeutic agents. Liver cancer is one of the common cancers with well-established etiological factors including hepatitis virus infection and environmental carcinogens such as aflatoxin and alcohol exposure. In recent years, the involvement of autophagy in liver cancer has been increasingly studied. Here, we aim to provide a systematic review on the close cross-talks between autophagy and liver cancer, and summarize the current status in development of novel liver cancer therapeutic approaches by targeting autophagy. It is believed that understanding the molecular mechanisms underlying the autophagy modulation and liver cancer development may provoke the translational studies that ultimately lead to new therapeutic strategies for liver cancer.

Autophagy: Emerging roles in lipid homeostasis and metabolic control

Current evidence implicates autophagy in the regulation of lipid stores within the two main organs involved in maintaining lipid homeostasis, the liver and adipose tissue. Critical to this role in hepatocytes is the breakdown of cytoplasmic lipid droplets, a process referred to as lipophagy. Conversely, autophagy is required for adipocyte differentiation and the concurrent accumulation of lipid droplets. Autophagy also affects lipid metabolism through contributions to lipoprotein assembly. A number of reports have now implicated autophagy in the degradation of apolipoprotein B, the main structural protein of very-low-density-lipoprotein. Aberrant autophagy may also be involved in conditions of deregulated lipid homeostasis in metabolic disorders such as the metabolic syndrome. First, insulin signalling and autophagy activity appear to diverge in a mechanism of reciprocal regulation, suggesting a role for autophagy in insulin resistance. Secondly, upregulation of autophagy may lead to conversion of white adipose tissue into brown adipose tissue, thus regulating energy expenditure and obesity. Thirdly, upregulation of autophagy in hepatocytes could increase breakdown of lipid stores controlling triglyceride homeostasis and fatty liver. Taken together, autophagy appears to play a very complex role in lipid homeostasis, affecting lipid stores differently depending on the tissue, as well as contributing to pathways of lipoprotein metabolism.

Fibroblast autophagy in fibrotic disorders

Fibrotic disorders are multistage progressive processes that often arise from different causes and are commonly associated with chronic inflammation. Excessive deposition of extracellular matrix is the hallmark of many fibrotic diseases. This may be due to an excess of fibroblast recruitment and activation, as well as to their differentiation in myofibroblasts. These events may be triggered by cytokines, chemokines and growth factors released by lymphocytes or macrophages. The excessive production of extracellular matrix is apparently due to alterations of metabolic pathways in activated fibroblasts. It has been suggested that a defective autophagy, an important subcellular pathway with multiple homeostatic roles, also recognized as a key component of both innate and acquired immunity, could play a role. In this review we illustrate recent insights in the field, suggesting the possible implication of the immune system in orchestrating the fibrotic response via the modulation of autophagic pathways.

Autophagy and mesenchymal cell fibrogenesis

Autophagy is a catabolic pathway essential for cellular energy homeostasis that involves the self-degradation of intracellular components in lysosomes. This process has been implicated in the pathophysiology of many human disorders, including infection, cancer, and fibrosis. Autophagy is also recognized as a mediator of survival and proliferation, and multiple pathways induce autophagy under conditions of cellular stress, including nutrient and energy depletion. High autophagic activity has been detected in fibrogenic cells from several tissues; however the role of autophagy in fibrogenesis and mesenchymal cells varies greatly in different tissues and settings, with contributions uncovered to energy metabolism and collagen turnover by fibrogenic cells. Because several chemical modulators of autophagy have already been identified, autophagy regulation constitutes a potential target for antifibrotic therapy. This article is part of a Special Issue entitled: Fibrosis: Translation of basic research to human disease.

Targeting autophagy for the treatment of liver diseases

Autophagy is a lysosomal degradation pathway that can degrade bulk cytoplasm and superfluous or damaged organelles, such as mitochondria, to maintain cellular homeostasis. It is now known that dysregulation of autophagy can cause pathogenesis of numerous human diseases. Here, we discuss the critical roles that autophagy plays in the pathogenesis of liver diseases such as non-alcoholic and alcoholic fatty liver, drug-induced liver injury, protein aggregate-related liver diseases, viral hepatitis, fibrosis, aging and liver cancer. In particular, we discuss the emerging therapeutic potential by pharmacological modulation of autophagy for these liver diseases.

Regulation of lipid stores and metabolism by lipophagy

Intracellular lipids are stored in lipid droplets (LDs) and metabolized by cytoplasmic neutral hydrolases to supply lipids for cell use. Recently, an alternative pathway of lipid metabolism through the lysosomal degradative pathway of autophagy has been described and termed lipophagy. In this form of lipid metabolism, LD triglycerides (TGs) and cholesterol are taken up by autophagosomes and delivered to lysosomes for degradation by acidic hydrolases. Free fatty acids generated by lipophagy from the breakdown of TGs fuel cellular rates of mitochondrial β-oxidation. Lipophagy therefore functions to regulate intracellular lipid stores, cellular levels of free lipids such as fatty acids and energy homeostasis. The amount of lipid metabolized by lipophagy varies in response to the extracellular supply of nutrients. The ability of the cell to alter the amount of lipid targeted for autophagic degradation depending on nutritional status demonstrates that this process is selective. Intracellular lipids themselves regulate levels of autophagy by unclear mechanisms. Impaired lipophagy can lead to excessive tissue lipid accumulation such as hepatic steatosis, alter hypothalamic neuropeptide release to affect body mass, block cellular transdifferentiation and sensitize cells to death stimuli. Future studies will likely identify additional mechanisms by which lipophagy regulates cellular physiology, making this pathway a potential therapeutic target in a variety of diseases.

Replacement of retinyl esters by polyunsaturated triacylglycerol species in lipid droplets of hepatic stellate cells during activation

Activation of hepatic stellate cells has been recognized as one of the first steps in liver injury and repair. During activation, hepatic stellate cells transform into myofibroblasts with concomitant loss of their lipid droplets (LDs) and production of excessive extracellular matrix. Here we aimed to obtain more insight in the dynamics and mechanism of LD loss. We have investigated the LD degradation processes in rat hepatic stellate cells in vitro with a combined approach of confocal Raman microspectroscopy and mass spectrometric analysis of lipids (lipidomics). Upon activation of the hepatic stellate cells, LDs reduce in size, but increase in number during the first 7 days, but the total volume of neutral lipids did not decrease. The LDs also migrate to cellular extensions in the first 7 days, before they disappear. In individual hepatic stellate cells. all LDs have a similar Raman spectrum, suggesting a similar lipid profile. However, Raman studies also showed that the retinyl esters are degraded more rapidly than the triacylglycerols upon activation. Lipidomic analyses confirmed that after 7 days in culture hepatic stellate cells have lost most of their retinyl esters, but not their triacylglycerols and cholesterol esters. Furthermore, we specifically observed a large increase in triacylglycerol-species containing polyunsaturated fatty acids, partly caused by an enhanced incorporation of exogenous arachidonic acid. These results reveal that lipid droplet degradation in activated hepatic stellate cells is a highly dynamic and regulated process. The rapid replacement of retinyl esters by polyunsaturated fatty acids in LDs suggests a role for both lipids or their derivatives like eicosanoids during hepatic stellate cell activation.

Autophagy protects against necrotic renal epithelial cell-induced death of renal interstitial fibroblasts

We recently reported that necrotic renal proximal tubular cells (RPTC) can induce the death of renal interstitial fibroblasts. Since autophagy plays either cytoprotective or cytodestructive roles depending on the experimental condition, the present study was carried out to investigate whether necrotic RPTC would induce autophagy of renal interstitial fibroblasts and, if so, whether autophagy would contribute to cell death or exert a protective effect. Exposure of necrotic RPTC supernatant (RPTC-Sup) induced autophagy in renal interstitial fibroblast cells (NRK-49F) in a time- and dose-dependent manner, and its induction was earlier than caspase-3 activation. Inhibition of autophagy with 3-methyladenine (3-MA) or knockdown of Beclin-1, a molecule involved in the initiation of autophagosome formation, with small interference RNA (siRNA) significantly enhanced necrotic RPTC-Sup-induced cell death. Necrotic RPTC-Sup induced phosphorylation of extracellular signal-regulated kinases (ERK1/2), p38, c-Jun NH(2)-terminal kinases (JNKs), and AKT. Treatment with an ERK1/2 pathway inhibitor, but not with specific inhibitors for p38, JNKs, or AKT pathways, blocked NRK-49F autophagy and cell death upon exposure to necrotic RPTC-Sup. Furthermore, knockdown of MEK1 with siRNA also reduced autophagy along with cell death in NRK-49F exposed to necrotic RPTC-Sup. In contrast, overexpression of MEK1/2 increased RPTC-Sup-induced fibroblast cell death without enhancing autophagy. Collectively, this study demonstrates that necrotic RPTC induce both autophagy and cell death and that autophagy plays a cytoprotective or prosurvival role in renal fibroblasts. Furthermore, necrotic RPTC-induced autophagy and cell death in renal fibroblasts is mediated by the activation of the MEK1-ERK1/2 signaling pathway.