- About this Journal ·
- Abstracting and Indexing ·
- Aims and Scope ·
- Annual Issues ·
- Article Processing Charges ·
- Author Guidelines ·
- Bibliographic Information ·
- Citations to this Journal ·
- Contact Information ·
- Editorial Board ·
- Editorial Workflow ·
- Free eTOC Alerts ·
- Publication Ethics ·
- Recently Accepted Articles ·
- Reviewers Acknowledgment ·
- Submit a Manuscript ·
- Subscription Information ·
- Table of Contents
BioMed Research International
Volume 2014 (2014), Article ID 120179, 13 pages
Autophagy and Non-Alcoholic Fatty Liver Disease
1INSERM, U1065, Bâtiment Universitaire Archimed, Team 8 “Hepatic Complications in Obesity”, 151 Route Saint Antoine de Ginestière, BP 2 3194, 06204 Nice Cedex 03, France
2Faculty of Medicine, University of Nice-Sophia-Antipolis, 06107 Nice Cedex 2, France
3Centre Hospitalier Universitaire of Nice, Digestive Center, 06202 Nice Cedex 3, France
Received 11 May 2014; Accepted 30 July 2014; Published 10 September 2014
Academic Editor: Ana-Maria Cuervo
Copyright © 2014 Vanessa J. Lavallard and Philippe Gual. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Autophagy, or cellular self-digestion, is a catabolic process that targets cell constituents including damaged organelles, unfolded proteins, and intracellular pathogens to lysosomes for degradation. Autophagy is crucial for development, differentiation, survival, and homeostasis. Important links between the regulation of autophagy and liver complications associated with obesity, non-alcoholic fatty liver disease (NAFLD), have been reported. The spectrum of these hepatic abnormalities extends from isolated steatosis to non-alcoholic steatohepatitis (NASH), steatofibrosis, which sometimes leads to cirrhosis, and hepatocellular carcinoma. NAFLD is one of the three main causes of cirrhosis and increases the risk of liver-related death and hepatocellular carcinoma. The pathophysiological mechanisms of the progression of a normal liver to steatosis and then more severe disease are complex and still unclear. The regulation of the autophagic flux, a dynamic response, and the knowledge of the role of autophagy in specific cells including hepatocytes, hepatic stellate cells, immune cells, and hepatic cancer cells have been extensively studied these last years. This review will provide insight into the current understanding of autophagy and its role in the evolution of the hepatic complications associated with obesity, from steatosis to hepatocellular carcinoma.
Autophagy, or cellular self-digestion, is a catabolic process that targets cell constituents, such as damaged organelles, unfolded proteins, and intracellular pathogens, to lysosomes for degradation [1, 2]. Under basal conditions, autophagy is involved in the degradation of long-lived proteins, whereas the ubiquitin-proteasome pathway, another catabolic process, is responsible for the degradation of short-lived proteins [3, 4]. In response to cellular stress such as nutrient deprivation, an increase in autophagic turnover maintains the cellular energy homeostasis. Three types of autophagy have been identified: macroautophagy, chaperone-mediated autophagy, and microautophagy. Macroautophagy, hereafter referred to as autophagy, involves the formation of a small vesicular sac called the isolation membrane or phagophore. The phagophore encloses a portion of cytoplasm resulting in the formation of a double-membraned structure termed an autophagosome. The autophagosome then fuses with a lysosome leading to the degradation of the cellular constituents sequestered into the autophagosome. Amino acids and other compounds generated by autophagic degradation of macromolecules are released into the cytoplasm for recycling or for energy production (Figure 1(a)) . The origin of the membranes involved in the formation of autophagosomes could be the endoplasmic reticulum (ER), mitochondria, and golgi [5–8]. However, it is still not clear which is/are the major contributor(s).
Microautophagy also involves the sequestration of cellular constituents within lysosomes but in this case through the invagination of the lysosomal membrane. Chaperone-mediated autophagy concerns the sequestration of polypeptides and soluble proteins containing a KFERQ motif in their amino acids sequence. These proteins are bound to a chaperone protein for translocation to lysosomes where binding to the lysosome-associated membrane protein type 2A receptor leads to protein internalization and degradation .
In autophagy, the formation of phagophores and autophagosomes requires 18 different autophagy-related proteins, Atg, which were initially identified in yeast . The process of autophagosome formation involves three major steps: initiation, nucleation, and elongation/enclosure (Figure 1(b)). The initiation step is controlled by the ULK1-Atg13-FIP200 complex [11, 12]. The serine/threonine kinase mammalian target of rapamycin, mTOR, a component of the mTORC1 complex, is the main inhibitor of autophagy. The nucleation step requires the Beclin-1-class III phosphatidylinositol 3-Kinase (PI3K) complex that includes Beclin-1, Vps34 (class III PI3K), Vps15, Atg14L/Barkor, and Ambra-1 . The involvement of mTOR and the Beclin-1-class III PI3K complex in the regulation of autophagy are discussed below. Two conjugation systems are involved in the elongation/enclosure step. The first is the conjugation of Atg12 to Atg5 mediated by two ligases Atg7 and Atg10. Atg5 also associates with Atg16 to form the Atg12-Atg5-Atg16 complex. The second involves the cleavage of LC3/Atg8 by Atg4 leading to the soluble form LC3-I, which is then conjugated to phosphatidylethanolamine, PE, via the participation of Atg7 and Atg3. This lipid conjugation forms the autophagic double-membrane-associated LC3-II protein allowing the closure of the autophagic vacuole [14, 15]. LC3-II is used as a marker of autophagosomes .
The last phases of autophagic process mediate the autophagy degradation. A large number of factors/actors regulate the autophagosome-lysosome fusion and the lysosomal biogenesis, activation, reformation, and turnover . In the autophagosome-lysosome fusion process, soluble N-ethylmaleimide-sensitive factor attachment protein, cytoskeleton proteins, and small GTPases are involved, for example. This mechanism is sensitive to changes in the membrane lipid composition of autophagosomes and lysosomes that can regulate their fusogenic capacity as it has been reported in lipid-enriched diets . Other important regulator of lysosomal biogenesis, function, and autophagy is the transcription factors EB, TFEB [19, 20]. TFEB coordinates the cellular responses to different stresses, including nutrient starvation, metabolic stress, and lysosomal stress, to maintain cellular homeostasis. Indeed, TFEB regulates the expression of genes involved in lipid metabolism and in the pathways of autophagy and lysosome [19, 20].
Autophagy is a cellular pathway that is crucial for the maintenance of cellular homeostasis, normal mammalian physiology and could play a protective or deleterious role in a range of diseases. Recent studies have reported its role and its regulation in the complications associated with obesity.
2. Hepatic Complications Associated with Obesity
The incidence of overweight and obesity is rapidly increasing in many Western countries. Obesity leads to numerous adverse metabolic disorders such as dyslipidemia, hypertension, reduced HDL cholesterol, and glucose intolerance. This cluster of metabolic abnormalities is grouped into the so-call metabolic syndrome that increases the risk of cardiovascular diseases, type 2 diabetes and liver complications, the non-alcoholic fatty liver disease (NAFLD). The spectrum of these hepatic abnormalities extends from isolated steatosis with triglyceride accumulation to non-alcoholic steatohepatitis (NASH), steatofibrosis, which sometimes leads to cirrhosis and hepatocellular carcinoma (Figure 2). NAFLD is one of the three main causes of cirrhosis . Despite this major public health concern, apart from lifestyle changes, NAFLD is still difficult to treat as no large study has shown any efficacy of pharmacological treatments for NAFLD.
Insulin resistance is at the core of the pathophysiology of the metabolic syndrome and type 2 diabetes. Insulin resistance is characterized by a decrease in insulin signaling and action. Adipose tissue plays a central role in the control of glucose and lipid metabolism through its ability to control glucose transport, lipid storage, and adipokines secretion. In obesity, the excessive gain of adipose tissue, in particular visceral adipose tissue, causes its dysfunction, which could participate in the development of insulin resistance and other obesity-linked complications such as the NAFLD. Adipose tissue expansion is associated with chronic low-grade inflammation with infiltration of the tissue by immune cells such as dendritic cells, macrophages, and T lymphocytes. Consequently, adipose tissue produces inflammatory cytokines and free fatty acids (FFA) in excess, and adipokine secretion is perturbed. Inflammatory cytokines and FFA antagonize local insulin signaling in adipocytes and also in muscles and liver. In liver, combined hyperglycemia and hyperinsulinemia promote de novo lipid synthesis and mitochondrial structural defects within hepatocytes. Moreover, insulin resistance of adipose tissue leads to an enhanced FFA flux to the liver, for example, in contribution to insulin resistance and steatosis. Oxidative and ER stresses play an important role in the alteration of insulin signaling and in the development of liver complications . NASH is characterized by a fatty liver, hepatic inflammation, and substantial death of hepatocytes. Hepatocyte apoptosis is important in the progression of the severity of liver complications. Apoptotic hepatocytes are engulfed by kupffer cells, which results in their activation and inflammation. The activation of stellate cells by apoptotic bodies or by TGFβ from activated kupffer cells then leads to liver fibrosis . Among the factors involved in this process, upregulation of cell death receptors, such as Fas and the TNFα receptor, and of TNFα has been reported in the NASH liver. Saturated FFA, sustained ER stress, cytokines, and adipokines could also be involved .
Interestingly, it has been recently reported that autophagy regulates food intake, adipose tissue development, hepatic complications, and insulin resistance and plays a protective role against lipotoxicity in β cells . Here, we will provide insight into the current understanding of the role of autophagy in the liver complications associated with obesity.
3. Hepatic Autophagy in Obesity
Under physiological conditions, autophagy participates in the basal turnover of lipids by engulfing and degrading lipid droplets. Autophagy is inhibited by the insulin-, amino acid-mTOR signaling pathway via both short- and long-term mechanisms of regulation. Short-term inhibition can be produced by the mTOR complex. Long-term regulation occurs via the transcription factors FoxO and TFEB, which control the transcription of autophagic genes and are inhibited by insulin-induced activation of Akt/PKB and mTOR, respectively . In obesity, the level of autophagy could be decreased in hepatocytes. Several mechanisms may account for this decline (Figure 3).(i)An obesity-induced increase in the calcium-dependent protease calpain-2 leads to the degradation of Atg7 and then to a defective autophagy. Acute inhibition of calpain is able to restore Atg7 expression . How obesity enhances the activity or expression of the hepatic calpain 2 has not yet been elucidated.(ii)In obese mice with hepatic steatosis, the autophagy inhibitor mTOR is overactivated in the liver, presumably as a result of an increased amino acid concentration following overnutrition. Indeed, it has been previously shown that the overactivation of mTOR by infusion of an amino acid mixture can result in liver and muscle insulin resistance because of phosphorylation and inhibition of IRS1 by S6 kinase, a downstream target of mTOR [25, 26].(iii)Although controversial, hyperinsulinemia may also contribute to downregulation of autophagy in obese mice. Indeed, Akt/PKB, a key molecule in the insulin pathway, decreases autophagy in the liver of obese mice . However, destruction of insulin production by β cells with streptozotocin does not increase autophagy in the liver of obese mice , in contrast to lean mice . The reasons for these discrepancies are unclear.(iv)A defect in lysosomal acidification and a reduction in cathepsin L that impaired substrate degradation in autolysosomes have also been reported for obese ob/ob mice. This is associated with an increased autophagosome number and normal fusion of autophagosomes to lysosomes . The same team recently reported that cathepsin B, D, and L expression was significantly decreased in the liver from NAFLD patients .(v)Defective autophagosome-lysosome fusion has also been reported in livers from high fat diet- (HFD-) induced obese mice. This defect was attributed to HFD-induced changes in the membrane lipid composition . A defect in hepatic autophagy and its associated decrease in the rate of lysosomal degradation contribute to a further increase in the ER stress. This could be induced by nutrient overload in an inflammatory milieu [24, 30]. Together, a decline in autophagy and an increase in ER stress lead to insulin resistance .
4. Hepatic Steatosis
In response to a moderate increase in lipid availability or during nutrient deprivation, hepatic autophagy degrades lipid droplets to provide FFA for ATP production. In contrast, a sustained availability of lipids, induced by a long-lasting HFD challenge, inhibits hepatic autophagic turnover . This ability of autophagy to degrade lipid droplets in hepatocytes has been termed lipophagy. Singh et al. reported for the first time that autophagy regulates lipid metabolism by eliminating triglycerides and by preventing development of steatosis. Inhibition of macroautophagy by genetic knockdown of the autophagy gene atg5, or pharmacological inhibition with 3-methyladenine in cultured hepatocytes challenged with a lipid load, significantly increased the cellular triglyceride content. Excessive triglycerides and cholesterol were retained in lipid droplets because of a decreased rate of lipolysis and the resultant reduction in fatty acid β-oxidation in cells in which macroautophagy was inhibited. Lipid movement through the autophagic pathway was confirmed by fluorescence microscopy demonstrating colocalization of lipid with autophagosomes and lysosomes, electron microscopic evidence of lipid in the autophagic vacuoles, and immunogold staining demonstrated an association of the autophagosome-associated protein microtubule-associated LC3 protein with lipid droplets. The LC3 protein directly interacts with lipid droplets before autophagosome formation. Lysosomes merge only with autophagosome-associated lipid droplets . These results have been confirmed .
Differences in the activation of autophagy, p53, damage-regulated autophagy modulator (DRAM), and BAX expression have also been observed in function to the severity of the hepatic steatosis. In the mouse model of mild (20 weeks of HFD) and severe hepatosteatosis (40 weeks of HFD), p53 expression increased in both mild and severe hepatic steatosis, and increased DRAM expression and autophagy were identified in mild hepatosteatosis, whereas higher BAX expression was observed in severe hepatosteatosis . From in vitro approaches, the authors proposed that mild steatosis induced autophagy and apoptosis mostly via a p53/DRAM pathway. In severe steatosis, apoptosis was mainly dependent on p53-induced expression of BAX, which also localized to mitochondria . Since discrepancies between in vivo and in vitro approaches exist, future investigations are necessary to confirm this potential mechanism.
HFD-fed mice (16 weeks) show impairment in the hepatic autophagic function, as demonstrated by the decreased mobilization of lipid into the autophagic compartment . Lipid accumulation altered the membrane structure, and a resultant decrease in the efficiency of fusion between autophagosomes and lysosomes may explain the inhibitory effect on macroautophagy of lipid accumulation induced by a HFD. However, in livers with a deficiency in autophagy, an alternative protective mechanism could then take place. Indeed, Kim et al. have recently reported that mice with a deficiency in hepatic autophagy displayed induction of fibroblast growth factor 21 (FGF21), resistance to diet-induced obesity, and amelioration in insulin resistance. The deficiency in autophagy and subsequent mitochondrial dysfunction could promote FGF21 expression, which in turn protects from diet-induced obesity and insulin resistance . In NAFLD patients, the serum level of FGF21 is also modified. In 146 overweight patients, it was reported that the serum levels of FGF21 were significantly higher in NASH patients. FGF21 also correlated positively with the triglyceride level, metabolic syndrome, steatosis grade, lobular inflammation, and fibrosis . Interestingly, the effects of LY2405319 (LY), an engineered FGF21 variant [36, 37], in a randomized, placebo-controlled, double-blind proof-of-concept trial on 46 patients with obesity and type 2 diabetes has been recently reported. Patients received placebo or 3, 10, or 20 mg of LY daily for 28 days. LY treatment produced a significant improvement in dyslipidemia. Favorable effects on body weight, fasting insulin, and adiponectin were also detected. While only a trend toward glucose lowering was observed, FGF21-based therapies may be effective for the treatment of selected metabolic disorders . Additional exploration would be necessary in order to assess the full range of LY effects and the potential to achieve significant antidiabetic efficacy. Further, the effect of LY administration on liver complications (hepatic steatosis) should be evaluated. The improvement of insulin sensitivity by LY could reduce the free fatty acid flux to the liver that contributes to steatosis.
5. Activation of Hepatic Autophagy Decreases Liver Steatosis
Activation of autophagy in hepatocytes could constitute a therapeutic approach against hepatic complications. To illustrate this, it has been reported that hepatic overexpression of Atg7 in HFD-fed mice or ob/ob mice improved the condition of the fatty liver and insulin resistance .
Starvation induces hepatic autophagy and increases delivery to the liver of FFA from adipose tissue. The liver of starved mice displayed an increase in the number of lipid droplets, autophagosomes, lysosomes, and autophagolysosomes . Hepatocyte-specific Atg7-deficient mice are characterized by hepatomegaly and accumulation of poly-ubiquitinylated proteins, as previously reported by Komatsu et al. . Hepatic triglycerides and the cholesterol content are also increased in these mice, which confirm the crucial role of autophagy in the regulation of lipid storage. The activation of autophagy by starvation is a complex mechanism. A new actor has been identified: acetyl-coenzyme A (AcCoA). AcCoA is a major integrator of the nutritional status at the crossroads of fat, sugar, and protein catabolism and cytosolic AcCoA functions as a central metabolic regulator of autophagy. Nutrient starvation causes rapid depletion of AcCoA and induction of autophagy via the reduction in the activity of acetyltransferase EP300, a suppressor of autophagy, by high AcCoA levels .
Enhancers of autophagy such as carbamazepine and rapamycin have been recently tested in HFD-obese mice and both had protective effects in reducing steatosis and in improving insulin sensitivity. The agents were given two or three times in the last week of a 12-week feeding scheme. This short-term treatment could significantly reduce hepatic steatosis and hepatic and blood triglyceride levels. The plasma ALT level was also noticeably, although not statistically significantly, reduced. Interestingly, insulin resistance was improved as evaluated by the level of blood glucose and insulin . While rapamycin and carbamazepine are already approved for other human clinical uses, the lack of specificity and the absence of organ or cell selectivity are the major limitations of these compounds.
The beneficial effects of coffee on hepatic steatosis and the link with autophagic flux have been recently tested. The recent evidence of the beneficial effects of coffee on the liver came from epidemiologic studies that revealed a strong association of drinking coffee with decreased serum hepatic enzymes, including GGT, AST, and ALT, in persons with a high risk of liver injury, such as in alcoholic, diabetic, and in viral infections . Recent epidemiologic studies further support the finding that drinking coffee reduces the risk for fatty liver, fibrosis, and hepatocellular carcinoma in NAFLD patients [43, 44]. Sinha et al. recently reported that mice given a HFD for 4 weeks, then continued HFD with 0.05% (w/v) caffeine in the drinking water for the next 4 weeks displayed an induction of hepatic autophagy (lipophagy) with a decrease in hepatic steatosis . This treatment was nevertheless associated with a decrease in HFD-induced obesity. Using genetic, pharmacological, and metabolomic approaches on hepatic cells and on the liver, the authors showed that caffeine induced lipophagy and mitochondria β-oxidation leading to a reduction in the intra-hepatic lipid content. Caffeine may inhibit PI3K-AKT and, in turn, inhibit mTOR to trigger autophagy by activating the ULK1 complex. The later includes ULK1, Atg13, FIP200, and Atg101. Autophagy selectively removes excess lipid droplets to generate FFA. Decreased mTOR levels induced TFEB nuclear translocation by decreasing TFEB phosphorylation. TFEB upregulates expression of autophagy and lysosomal genes, as well as PGC-1α and PPARα, which burn FFAs by increasing mitochondria β-oxidation. Thus, caffeine protects against fatty liver by coordinating the induction of lipophagy and mitochondrial β-oxidation [45, 46].
6. Hepatic Insulin Resistance
Recent studies have suggested that ER stress could be the link between obesity, insulin resistance, and type 2 diabetes [47–49]. The inhibition of hepatic ER stress reduced liver steatosis . Defective hepatic autophagy in obesity could promote ER stress and cause insulin resistance . Since autophagy is known to eliminate mis-/unfolded proteins, and impairment in hepatic autophagy could lead to accumulation of mis-/unfolded proteins and induction of ER stress. In the liver of obese mice, it has been reported that a decrease in autophagy promotes ER stress leading to insulin resistance [24, 51]. Overexpression of Atg7 in the liver of obese mice significantly reduced ER stress, decreased the triglyceride content, and improved glucose tolerance and insulin sensitivity . Thus, a vicious cycle takes place: hyper-insulinemia negatively regulates hepatic autophagy in the steatotic liver and the decline in hepatic autophagy enhances ER stress and insulin resistance (Figure 3).
7. Steatohepatitis (NASH)
The possible contribution of autophagy to the evolution of steatosis to NASH has not yet been fully elucidated . The diagnosis of NASH still requires a liver biopsy and is defined by histological features including steatosis, lobular inflammation, and hepatocellular ballooning [53, 54]. Evaluating hepatic fibrosis using a histopathological algorithm and scoring systems to diagnosis NASH is still under debate . Hepatic cell injury including hepatocyte death and hepatocellular ballooning are key features of NASH. Inhibition of autophagy and the accumulation of p62 could be involved in the formation of the Mallory-Denk bodies (MDB). MDB are mainly found in ballooned hepatocytes, one of the components used for NASH diagnosis and for the scoring system [53, 54]. In contrast, activation of autophagy by rapamycin leads to MDB resolution in mice .
A recent report has evaluated autophagic markers in obese patients with hepatic steatosis (), patients with steatosis with NASH, and fibrosis () versus normal liver (). A significant increase in the LC3-II/LC3-I ratio was detected in both patients with steatosis and NASH compared with subjects with a normal liver. A progressive increase in the amount of p62 was observed in obese patients with steatosis and in obese patients with NASH compared to control patients. The accumulation of p62 and the LC3II/LC3I ratio could indicate that autophagy was decreased in both patients with hepatic steatosis and NASH . The level of autophagic markers in patients with the same grade of steatosis and fibrosis with or without NASH remains to be investigated, to determine if a discrepancy exists between steatosis and NASH. In contrast, NASH patients had a significant increase in hepatic ER stress (ATF4 at the mRNA level; CHOP and GRP78 at the mRNA and protein levels) compared with patients with steatosis . In a mouse model of NASH (methionine/choline-deficient diet), the authors also demonstrated that the autophagic flux was impaired in the liver . We would like to underline the fact that the evaluation of autophagic flux in vivo in mice is still difficult and even more difficult in human samples. Further, assessing autophagy by the LC3-II/LC3-I ratio and p62 level need always caution. The levels of LC3-I can also vary. In addition, p62 levels are enhanced by other factors such as stress, or proteasome inhibition, which are mostly independent of changes in autophagy.
The immune system plays an important role in the evolution of NAFLD . For example, the dysregulation of the balance in M1 (pro-inflammatory) versus M2 (anti-inflammatory) macrophages is emerging as a central mechanism governing the pathogenesis of NAFLD [58, 59]. In the last few years, the regulation of the immune system by autophagy has been reported [60–63]. The evaluation of the autophagic flux and its role in specific cells such as macrophages, T cells, and neutrophils in the NAFLD liver remains to be fully investigated. To illustrate this, the role of autophagy in tumor-associated macrophages in hepatocellular carcinoma is discussed below.
8. Hepatic Fibrosis
Hepatocyte apoptosis and inflammation are key players in the progression of the severity of liver complications. With the aim of simplifying hepatic fibrogenesis , hepatic stellate cells (HSCs) in response to damage (hepatocyte death…) and inflammation (TGFβ) differentiate into myofibroblast-like cells, which produce most extracellular matrix components. Prevention of hepatocyte death can prevent fibrosis. For example, pan-caspase inhibitors reduced hepatic fibrosis in db/db mice feed with a methionine/choline-deficient diet . Circulating levels of markers of hepatocyte death and/or apoptosis also increased with the severity of hepatic fibrosis in obese or alcoholic patients [66, 67].
Since autophagy plays a hepatoprotective role, the steatotic liver with decreased hepatocyte autophagy is more vulnerable to injury induced by inflammation (TNFα, FASL) and stresses (ER and oxidative) and is therefore more prompted to develop fibrosis. Furthermore, activation of autophagy in HSCs has recently emerged as an additional mechanism involved in their activation (Figure 4) [64, 68, 69]. The increase in autophagic flux in activated HSCs leads to the loss of perinuclear lipid droplets (containing retinyl esters) associated with the transdifferentiation of quiescent HSC towards the activated phenotype [68, 69]. It has been hypothesized that the metabolism of lipid droplets mainly due to autophagy (probably by lipophagy) may provide the cellular energy that is critical for fueling catabolic pathways of HSCs activation. Under conditions of stress, ER stress (IRE1α-Xbp1-p38 pathway) could play an important role in the activation of autophagy and in turn fibrogenic activity of HSCs . In a mouse model of liver fibrosis (carbon tetrachloride or thioacetamide), hepatic autophagy is activated and the loss of autophagic function in HSCs (—glial fibrillary acidic protein—Cre mice) reduced their activation, fibrogenesis, and matrix accumulation. This inhibition of autophagy in HSCs also prevented the loss of perinuclear lipid droplets in vivo . Unfortunately, this has not yet been evaluated in a mouse model of NAFLD.
Thus, in addition to selective activation of autophagy in hepatocytes to decrease cell death, selective reduction of autophagic activity in fibrogenic cells in the liver unveils a potential new therapeutic strategy for liver fibrosis. It seems thus important to well targeting the liver cells and disease (severe fibrosis versus steatosis/NASH) for the treatment. Activation of autophagy in hepatocyte in early stages of NAFLD (steatosis/NASH) could prevent their progression to fibrosis. Indeed, limiting liver injury could be a therapeutic way to prevent the progression of hepatic complications . It has been reported that a pan-caspase inhibitor or overexpression of the anti-apoptotic Bcl2 protein reduced fibrosis in an animal model of NAFLD and fibrosis, respectively [65, 72]. Furthermore, two years of treatment with ursodeoxycholic in combination with vitamin E improved the level of hepatic enzymes AST/ALT and hepatic steatosis of patients with NASH . Ursodeoxycholic is commonly used in the treatment of cholestatic liver disorders but its potential ability to prevent hepatocyte apoptosis has been evaluated in NAFLD . Larger trials are warranted.
9. Hepatocellular Carcinoma, HCC
Autophagy is also involved in hepatocarcinogenesis, while its role remains controversial. Autophagy could play a dual role in cancer initiation and in cancer survival. First, autophagy eliminates senescent and injured cells, thereby limiting chromosomal instability and suppresses tumor initiation. Second, autophagy could provide energy by recycling damaged organelles, DNA, aggregated proteins, and pathogens to maintain energy balance, which promotes cancer cell survival . In addition, the regulation of autophagy in liver macrophages and more specifically in tumor-associated macrophages could also play an important role in the development of HCC.
To illustrate the tumor suppressor role of autophagy, it has been reported that mice with heterozygous disruption of beclin-1 have a high frequency of spontaneous hepatocellular carcinoma . Similarly, the deletion of Atg5 or Atg7 in liver resulted in the increasing incidence of benign liver adenomas . Further, the expression of several autophagy related genes (atg5, Atg7, and Atg6/beclin-1) and their corresponding autophagic activity is decreased in HCC cell lines compared to normal hepatic cell lines [77, 78]. Similarly, beclin-1 mRNA and protein levels are lower in HCC tissue samples than in adjacent nontumor tissues from the same patients . In 300 patients with HCC, the expression of Beclin-1 correlated with disease-free survival and overall survival only in the Bcl-xL+ patients. Multivariate analyses revealed that Beclin-1 expression was an independent predictor for disease-free survival and overall survival in Bcl-xL+ patients. Further, Beclin-1 expression correlated with tumor differentiation in Bcl-xL+ but not in Bcl-xL− HCC patients. These data suggest that a defect in autophagy synergizes with altered apoptotic activity and facilitates tumor progression and poor prognosis of HCC .
Since autophagy negatively regulates stresses and prevents cell death, its activation could also be involved in the survival of cancer. Indeed, LC3 was highly expressed in HCC compared with noncancerous tissues and correlated with tumor size. In addition, LC3 was an independent predictor of HCC recurrence after surgery only in the context of large tumors . Furthermore, autophagy induced by oncogenic K-Ras mediates functional loss of mitochondria during cell transformation to overcome an energy deficit resulting from glucose deficiency .
In addition, autophagy could also act via the regulation of the function of tumor-associated macrophages. It is well established that the tumor-associated macrophage density in human cancer correlates with poor prognosis in most human cancers and the inhibition or enhancement of the macrophage density in tumors by genetic and pharmacological approaches, respectively, inhibits or promotes tumor angiogenesis, growth, and progression [62, 81, 82]. Further, the polarization of macrophages into the M2 (anti-inflammatory) phenotype favors tumor progression, while M1 (proinflammatory) macrophages exert an anticancer activity. Tumor-associated macrophages could sense factors from the tumor microenvironment that lead to their polarization. Autophagy is involved in this process [62, 81]. For example, the TLR2 deficiency causes a reduction in macrophage infiltration but also induces significant suppression of autophagy associated with a decrease in the hepatic expression of tumor necrosis factor, interferon gamma, and [C-X-C motif] ligand 2. This enrichment in M2 macrophages in turn promotes hepatocarcinogenesis .
A specific enhancement in autophagy in tumor-associated macrophages could enhance polarization into the M1 phenotype and could have a beneficial effect. The activation of the mTOR-TSC2 pathway, a key negative regulator of autophagy, is critical for macrophage polarization toward the M2 phenotype to promote tumor angiogenesis and growth in mouse hepatocellular carcinoma models. In contrast, inhibition of this pathway by for example rapamycin/silorimus exerts the opposite effects . Interestingly, the use of rapamycin/silorimus as an immunosuppressor displays beneficial responses in patients. Indeed, survival after liver transplantation has been evaluated according to the immunosuppression protocol applied to 2491 adult recipients of isolated liver transplantation for HCC and 12,167 for non-HCC. These patients remained on stable maintenance of immunosuppression protocols for at least 6 months after transplant. Treatment with rapamycin was associated with improved survival after transplantation for HCC. Interestingly, rapamycin showed a trend toward lower rates of survival in non-HCC recipients, confirming the specificity of its beneficial impact for cancer patients .
Activation of autophagy in HCC targeting tumor-associated macrophages may represent a promising and effective strategy for liver cancer therapy. In contrast, the inhibition of autophagy in malignant cells may be a novel strategy to improve the efficacy of anticancer therapy. However, further investigations are required to determine the role of autophagy as a function of the type of tumor, stage, and genetic context.
The prevalence of type 2 diabetes and liver complications increases with age and it is well established that autophagy and chaperone-mediated autophagy are impaired in aging [86, 87]. This decline is associated with an increase in the lipid content in different organs such as the liver. As seen above, excessive accumulation of lipids alters further autophagic turnover and its protective role against accumulation of lipid droplets and insulin resistance. A link between aging, loss of mitochondrial function, and ROS production has also been reported [86, 88]. Calorie restriction enhances longevity and this may be due, at least in part, to activation of autophagy, in particular lipophagy and mitophagy leading to a decrease in the lipid content and in oxidative stress [89–91]. Aging, dysfunctional mitochondria, and oxidative stress contribute to the development of type 2 diabetes and liver complications [92, 93]. The increase in autophagic turnover could enhance mitochondrial turnover and biogenesis, and, consequently, decrease ROS production. This may contribute to life span extension and prevent or delay the development of complications associated with obesity . The development of NAFLD has recently been examined in mice of different ages (2, 8, and 18 months) in response to a fixed period 16 weeks of HFD. Weight gain, insulin resistance, and hepatic steatosis were equivalent for the three ages. In contrast, liver injury occurred exclusively in the two older ages and older mice had an elevated innate immune response with more M1 (proinflammatory) macrophages. Aged hepatocytes were further selectively sensitized to the Fas death pathway in vitro. Aging thus leads to increased hepatocellular injury and inflammation upon HFD challenge . Unfortunately, the level of autophagy was not evaluated in this study.
Autophagy is a crucial physiological process in providing nutrients to maintain vital cellular functions during fasting, but also to purge the cell of superfluous or damaged organelles, lipids, and misfolded proteins in obesity. Obesity is a complex multifactorial chronic disease affecting multiorgans and physiological responses . It is clear that modifications in the autophagic turnover during obesity mediate protective or deleterious responses depending on the cell/organ . This review summarized the knowledge of the role of autophagy in specific cells, including hepatocytes, macrophages, HSCs and cancer cells, and liver diseases from steatosis to HCC in the context of obesity (Figure 4). Some drugs that modulate autophagy are already approved for other human clinical: rapamycin, carbamazepine, cisplatin, and metformin promote autophagy in some cells, while the antimalaria compound chloroquine (which increases the intralysosomal pH) inhibits autophagic turnover. However, the lack of specificity and the absence of organ or cell selectivity are the major limitations of these compounds. Better understanding of the molecular mechanisms of autophagy and its implication in specific liver cells, such as hepatocyte, HSCs, macrophages but also endothelial and other immune cells, during the evolution of the liver complications will highlight new potential therapeutic targets.
|Akt/PKB:||Protein kinase B|
|ATF4:||Activating transcription factor 4|
|BAX:||Bcl-2-associated X protein|
|Bcl-2:||B-cell lymphoma 2|
|Bcl-xL:||B-cell lymphoma extra-large|
|DRAM:||Damage-regulated autophagy modulator|
|FFA:||Free fatty acids|
|FGF21:||Fibroblast growth factor 21|
|FIP200:||Focal adhesion kinase family interacting protein of 200 kD|
|FoxO:||Forkhead box protein O|
|GRP78:||Glucose regulate protein 78|
|HDL:||High density lipoprotein|
|HFD:||High fat diet|
|HSCs:||Hepatic stellate cells|
|IRE1:||Inositol requiring enzyme 1|
|LAMP-2A:||Lysosome-associated membrane protein type 2A|
|LC3:||Microtubule-associated protein 1A/1B-light chain 3|
|mTOR:||Mammalian target of rapamycin|
|NAFLD:||Nonalcoholic fatty liver disease|
|PGC-1:||Peroxisome proliferator-activated receptor gamma coactivator 1|
|PPAR:||Peroxisome proliferator-activated receptor|
|TFEB:||Transcription factor EB|
|TGF:||Transforming growth factor|
|TNF:||Tumor necrosis factor|
|Xbp1:||X-box binding protein 1.|
Conflict of Interests
Authors declare no conflict of interests.
The authors thank Dr. M. C. Brahimi-Horn for editorial correction. The current address of Vanessa J. Lavallard is Cell Isolation and Transplantation Center, Department of Surgery, Geneva University Hospitals and University of Geneva, Geneva, Switzerland. This work was supported by grants from INSERM (France), the University of Nice, the Programme Hospitalier de Recherche Clinique (Centre Hospitalier Universitaire of Nice), and charities (Association Française pour l’Etude du Foie (AFEF)/LFB to Philippe Gual, Société Francophone du Diabète (SFD) and SFD/Roche Pharma to Philippe Gual, European Foundation for the study of Diabetes/Lilly European Diabetes Research Programme to Philippe Gual). This work was also funded by the French Government (National Research Agency, ANR) through the “Investments for the Future” LABEX SIGNALIFE: program reference no. ANR-11-LABX-0028-01. Vanessa J. Lavallard was supported by the Programme Hospitalier de Recherche Clinique (Centre Hospitalier Universitaire of Nice) and the Association pour la Recherche sur le Cancer (France).
- T. Yorimitsu and D. J. Klionsky, “Autophagy: molecular machinery for self-eating,” Cell Death and Differentiation, vol. 12, supplement 2, pp. 1542–1552, 2005.
- P. Codogno and A. J. Meijer, “Autophagy and signaling: their role in cell survival and cell death,” Cell Death and Differentiation, vol. 12, no. 2, pp. 1509–1518, 2005.
- A. Ciechanover, A. Orian, and A. L. Schwartz, “Ubiquitin-mediated proteolysis: biological regulation via destruction,” Bioessays, vol. 22, pp. 442–451, 2000.
- N. B. Nedelsky, P. K. Todd, and J. P. Taylor, “Autophagy and the ubiquitin-proteasome system: collaborators in neuroprotection,” Biochimica et Biophysica Acta: Molecular Basis of Disease, vol. 1782, no. 12, pp. 691–699, 2008.
- E. L. Axe, S. A. Walker, M. Manifava et al., “Autophagosome formation from membrane compartments enriched in phosphatidylinositol 3-phosphate and dynamically connected to the endoplasmic reticulum,” Journal of Cell Biology, vol. 182, no. 4, pp. 685–701, 2008.
- M. Hayashi-Nishino, N. Fujita, T. Noda, A. Yamaguchi, T. Yoshimori, and A. Yamamoto, “A subdomain of the endoplasmic reticulum forms a cradle for autophagosome formation,” Nature Cell Biology, vol. 11, no. 12, pp. 1433–1437, 2009.
- P. Ylä-Anttila, H. Vihinen, E. Jokitalo, and E.-L. Eskelinen, “3D tomography reveals connections between the phagophore and endoplasmic reticulum,” Autophagy, vol. 5, no. 8, pp. 1180–1185, 2009.
- M. Mari, S. A. Tooze, and F. Reggiori, “The puzzling origin of the autophagosomal membrane,” F1000 Biology Reports, vol. 3, p. 25, 2011.
- E. Arias and A. M. Cuervo, “Chaperone-mediated autophagy in protein quality control,” Current Opinion in Cell Biology, vol. 23, no. 2, pp. 184–189, 2011.
- H. Nakatogawa, K. Suzuki, Y. Kamada, and Y. Ohsumi, “Dynamics and diversity in autophagy mechanisms: lessons from yeast,” Nature Reviews Molecular Cell Biology, vol. 10, no. 7, pp. 458–467, 2009.
- E. Y. W. Chan, A. Longatti, N. C. McKnight, and S. A. Tooze, “Kinase-inactivated ULK proteins inhibit autophagy via their conserved C-terminal domains using an Atg13-independent mechanism,” Molecular and Cellular Biology, vol. 29, no. 1, pp. 157–171, 2009.
- E. Y. Chan, “MTORC1 phosphorylates the ULK1-mAtg13-FIP200 autophagy regulatory complex,” Science Signaling, vol. 2, no. 84, article pe51, 2009.
- N. Mizushima and M. Komatsu, “Autophagy: renovation of cells and tissues,” Cell, vol. 147, no. 4, pp. 728–741, 2011.
- Y. Ohsumi, “Molecular dissection of autophagy: two ubiquitin-like systems,” Nature Reviews Molecular Cell Biology, vol. 2, no. 3, pp. 211–216, 2001.
- Y. Kabeya, N. Mizushima, A. Yamamoto, S. Oshitani-Okamoto, Y. Ohsumi, and T. Yoshimori, “LC3, GABARAP and GATE16 localize to autophagosomal membrane depending on form-II formation,” Journal of Cell Science, vol. 117, no. 13, pp. 2805–2812, 2004.
- D. J. Klionsky, F. C. Abdalla, H. Abeliovich, et al., “Guidelines for the use and interpretation of assays for monitoring autophagy,” Autophagy, vol. 8, no. 4, pp. 445–544, 2012.
- H. M. Shen and N. Mizushima, “At the end of the autophagic road: an emerging understanding of lysosomal functions in autophagy,” Trends in Biochemical Sciences, vol. 39, pp. 61–71, 2014.
- H. Koga, S. Kaushik, and A. M. Cuervo, “Altered lipid content inhibits autophagic vesicular fusion,” The FASEB Journal, vol. 24, no. 8, pp. 3052–3065, 2010.
- C. Settembre, C. Di Malta, V. A. Polito et al., “TFEB links autophagy to lysosomal biogenesis,” Science, vol. 332, no. 6036, pp. 1429–1433, 2011.
- C. Settembre, R. De Cegli, G. Mansueto et al., “TFEB controls cellular lipid metabolism through a starvation-induced autoregulatory loop,” Nature Cell Biology, vol. 15, no. 8, pp. 647–658, 2013.
- O. T. Hardy, M. P. Czech, and S. Corvera, “What causes the insulin resistance underlying obesity?” Current Opinion in Endocrinology, Diabetes and Obesity, vol. 19, no. 2, pp. 81–87, 2012.
- A. Tran and P. Gual, “Non-alcoholic steatohepatitis in morbidly obese patients,” Clinics and Research in Hepatology and Gastroenterology, vol. 37, no. 1, pp. 17–29, 2013.
- V. J. Lavallard, A. J. Meijer, P. Codogno, and P. Gual, “Autophagy, signaling and obesity,” Pharmacological Research, vol. 66, no. 6, pp. 513–525, 2012.
- L. Yang, P. Li, S. Fu, E. S. Calay, and G. S. Hotamisligil, “Defective hepatic autophagy in obesity promotes ER stress and causes insulin resistance,” Cell Metabolism, vol. 11, no. 6, pp. 467–478, 2010.
- P. Gual, Y. le Marchand-Brustel, and J. F. Tanti, “Positive and negative regulation of insulin signaling through IRS-1 phosphorylation,” Biochimie, vol. 87, no. 1, pp. 99–109, 2005.
- F. Tremblay, M. Krebs, L. Dombrowski et al., “Overactivation of S6 kinase 1 as a cause of human insulin resistance during increased amino acid availability,” Diabetes, vol. 54, no. 9, pp. 2674–2684, 2005.
- H. Y. Liu, J. Han, S. Y. Cao et al., “Hepatic autophagy is suppressed in the presence of insulin resistance and hyperinsulinemia. Inhibition of FoxO1-dependent expression of key autophagy genes by insulin,” Journal of Biological Chemistry, vol. 284, no. 45, pp. 31484–31492, 2009.
- Y. Inami, S. Yamashina, K. Izumi et al., “Hepatic steatosis inhibits autophagic proteolysis via impairment of autophagosomal acidification and cathepsin expression,” Biochemical and Biophysical Research Communications, vol. 412, no. 4, pp. 618–625, 2011.
- Y. Fukuo, S. Yamashina, H. Sonoue, et al., “Abnormality of autophagic function and cathepsin expression in the liver from patients with non-alcoholic fatty liver disease,” Hepatology Research, 2014.
- G. S. Hotamisligil, “Endoplasmic reticulum stress and the inflammatory basis of metabolic disease,” Cell, vol. 140, no. 6, pp. 900–917, 2010.
- R. Singh, S. Kaushik, Y. Wang et al., “Autophagy regulates lipid metabolism,” Nature, vol. 458, no. 7242, pp. 1131–1135, 2009.
- M. Shibata, K. Yoshimura, N. Furuya et al., “The MAP1-LC3 conjugation system is involved in lipid droplet formation,” Biochemical and Biophysical Research Communications, vol. 382, no. 2, pp. 419–423, 2009.
- K. Liu, J. Lou, T. Wen et al., “Depending on the stage of hepatosteatosis, p53 causes apoptosis primarily through either DRAM-induced autophagy or BAX,” Liver International, vol. 33, pp. 1566–1574, 2013.
- K. H. Kim, Y. T. Jeong, H. Oh et al., “Autophagy deficiency leads to protection from obesity and insulin resistance by inducing Fgf21 as a mitokine,” Nature Medicine, vol. 19, no. 1, pp. 83–92, 2013.
- J. Shen, H. L.-Y. Chan, G. L.-H. Wong et al., “Non-invasive diagnosis of non-alcoholic steatohepatitis by combined serum biomarkers,” Journal of Hepatology, vol. 56, no. 6, pp. 1363–1370, 2012.
- A. C. Adams, C. A. Halstead, B. C. Hansen et al., “LY2405319, an engineered FGF21 variant, improves the metabolic status of diabetic monkeys,” PLoS ONE, vol. 8, no. 6, Article ID e65763, 2013.
- A. Kharitonenkov, J. M. Beals, R. Micanovic et al., “Rational design of a fibroblast growth factor 21-based clinical candidate, LY2405319,” PLoS ONE, vol. 8, no. 3, Article ID e58575, 2013.
- G. Gaich, J. Y. Chien, and H. Fu, “The effects of LY2405319, an FGF21 analog, in obese human subjects with type 2 diabetes,” Cell Metabolism, vol. 18, pp. 333–340, 2013.
- M. Komatsu, S. Waguri, T. Ueno et al., “Impairment of starvation-induced and constitutive autophagy in Atg7-deficient mice,” Journal of Cell Biology, vol. 169, no. 3, pp. 425–434, 2005.
- G. Mariño, F. Pietrocola, T. Eisenberg, et al., “Regulation of autophagy by cytosolic acetyl-coenzyme A,” Molecular Cell, vol. 53, no. 5, pp. 710–725, 2014.
- C.-W. Lin, H. Zhang, M. Li et al., “Pharmacological promotion of autophagy alleviates steatosis and injury in alcoholic and non-alcoholic fatty liver conditions in mice,” Journal of Hepatology, vol. 58, no. 5, pp. 993–999, 2013.
- C. E. Ruhl and J. E. Everhart, “Coffee and caffeine consumption reduce the risk of elevated serum alanine aminotransferase activity in the United States,” Gastroenterology, vol. 128, no. 1, pp. 24–32, 2005.
- J. W. Molloy, C. J. Calcagno, C. D. Williams, F. J. Jones, D. M. Torres, and S. A. Harrison, “Association of coffee and caffeine consumption with fatty liver disease, nonalcoholic steatohepatitis, and degree of hepatic fibrosis,” Hepatology, vol. 55, no. 2, pp. 429–436, 2012.
- R. Anty, S. Marjoux, A. Iannelli et al., “Regular coffee but not espresso drinking is protective against fibrosis in a cohort mainly composed of morbidly obese European women with NAFLD undergoing bariatric surgery,” Journal of Hepatology, vol. 57, no. 5, pp. 1090–1096, 2012.
- R. A. Sinha, B. L. Farah, B. K. Singh, et al., “Caffeine stimulates hepatic lipid metabolism by the autophagy-lysosomal pathway in mice,” Hepatology, vol. 59, pp. 1366–1380, 2014.
- W. X. Ding, “Drinking coffee burns hepatic fat by inducing lipophagy coupled with mitochondrial beta-oxidation,” Hepatology, vol. 59, pp. 1235–1238, 2014.
- P. Codogno and A. J. Meijer, “Autophagy: a potential link between obesity and insulin resistance,” Cell Metabolism, vol. 11, no. 6, pp. 449–451, 2010.
- H. P. Harding, H. Zeng, Y. Zhang et al., “Diabetes mellitus and exocrine pancreatic dysfunction in perk-/- mice reveals a role for translational control in secretory cell survival,” Molecular Cell, vol. 7, no. 6, pp. 1153–1163, 2001.
- H. Kaneto, T.-A. Matsuoka, Y. Nakatani et al., “Oxidative stress, ER stress, and the JNK pathway in type 2 diabetes,” Journal of Molecular Medicine, vol. 83, no. 6, pp. 429–439, 2005.
- H. L. Kammoun, H. Chabanon, I. Hainault et al., “GRP78 expression inhibits insulin and ER stress-induced SREBP-1c activation and reduces hepatic steatosis in mice,” Journal of Clinical Investigation, vol. 119, no. 5, pp. 1201–1215, 2009.
- U. Özcan, Q. Cao, E. Yilmaz et al., “Endoplasmic reticulum stress links obesity, insulin action, and type 2 diabetes,” Science, vol. 306, no. 5695, pp. 457–461, 2004.
- M. Amir and M. J. Czaja, “Autophagy in nonalcoholic steatohepatitis,” Expert Review of Gastroenterology and Hepatology, vol. 5, no. 2, pp. 159–166, 2011.
- E. M. Brunt, C. G. Janney, A. M. Di Bisceglie, B. A. Neuschwander-Tetri, and B. R. Bacon, “Nonalcoholic steatohepatitis: a proposal for grading and staging the histological lesions,” The American Journal of Gastroenterology, vol. 94, no. 9, pp. 2467–2474, 1999.
- D. E. Kleiner, E. M. Brunt, M. Van Natta et al., “Design and validation of a histological scoring system for nonalcoholic fatty liver disease,” Hepatology, vol. 41, no. 6, pp. 1313–1321, 2005.
- P. Bedossa, C. Poitou, N. Veyrie et al., “Histopathological algorithm and scoring system for evaluation of liver lesions in morbidly obese patients,” Hepatology, vol. 56, no. 5, pp. 1751–1759, 2012.
- M. Harada, S. Hanada, D. M. Toivola, N. Ghori, and M. B. Omary, “Autophagy activation by rapamycin eliminates mouse Mallory-Denk bodies and blocks their proteasome inhibitor-mediated formation,” Hepatology, vol. 47, no. 6, pp. 2026–2035, 2008.
- A. Gonzalez-Rodriguez, R. Mayoral, N. Agra, et al., “Impaired autophagic flux is associated with increased endoplasmic reticulum stress during the development of NAFLD,” Cell Death & Disease, vol. 5, Article ID e1179, 2014.
- A. Sica, P. Invernizzi, and A. Mantovani, “Macrophage plasticity and polarization in liver homeostasis and pathology,” Hepatology, vol. 59, no. 5, pp. 2034–2042, 2014.
- A. Sica and A. Mantovani, “Macrophage plasticity and polarization: in vivo veritas,” The Journal of Clinical Investigation, vol. 122, no. 3, pp. 787–795, 2012.
- C. C. Mihalache and H. U. Simon, “Autophagy regulation in macrophages and neutrophils,” Experimental Cell Research, vol. 318, no. 11, pp. 1187–1192, 2012.
- D. J. Puleston and A. K. Simon, “Autophagy in the immune system,” Immunology, vol. 141, no. 1, pp. 1–8, 2014.
- P. Chen, M. Cescon, and P. Bonaldo, “Autophagy-mediated regulation of macrophages and its applications for cancer,” Autophagy, vol. 10, no. 2, pp. 192–200, 2014.
- C. S. Shi, K. Shenderov, N. N. Huang et al., “Activation of autophagy by inflammatory signals limits IL-1β production by targeting ubiquitinated inflammasomes for destruction,” Nature Immunology, vol. 13, no. 3, pp. 255–263, 2012.
- E. Novo, S. Cannito, C. Paternostro, C. Bocca, A. Miglietta, and M. Parola, “Cellular and molecular mechanisms in liver fibrogenesis,” Archives of Biochemistry and Biophysics, vol. 548, pp. 20–37, 2014.
- R. P. Witek, W. C. Stone, F. G. Karaca et al., “Pan-caspase inhibitor VX-166 reduces fibrosis in an animal model of nonalcoholic steatohepatitis,” Hepatology, vol. 50, no. 5, pp. 1421–1430, 2009.
- D. L. Diab, L. Yerian, P. Schauer et al., “Cytokeratin 18 fragment levels as a noninvasive biomarker for nonalcoholic steatohepatitis in bariatric surgery patients,” Clinical Gastroenterology and Hepatology, vol. 6, no. 11, pp. 1249–1254, 2008.
- V. J. Lavallard, S. Bonnafous, S. Patouraux et al., “Serum markers of hepatocyte death and apoptosis are non invasive biomarkers of severe fibrosis in patients with alcoholic liver disease,” PLoS ONE, vol. 6, no. 3, Article ID e17599, 2011.
- V. Hernndezgea, Z. Ghiassinejad, R. Rozenfeld et al., “Autophagy releases lipid that promotes fibrogenesis by activated hepatic stellate cells in mice and in human tissues,” Gastroenterology, vol. 142, no. 4, pp. 938–946, 2012.
- L. F. Thoen, E. L. Guimaraes, L. Dolle et al., “A role for autophagy during hepatic stellate cell activation,” Journal of Hepatology, vol. 55, no. 6, pp. 1353–1360, 2011.
- V. Hernández-Gea, M. Hilscher, R. Rozenfeld et al., “Endoplasmic reticulum stress induces fibrogenic activity in hepatic stellate cells through autophagy,” Journal of Hepatology, vol. 59, no. 1, pp. 98–104, 2013.
- H. Malhi and G. J. Gores, “Cellular and molecular mechanisms of liver injury,” Gastroenterology, vol. 134, no. 6, pp. 1641–1654, 2008.
- C. Mitchell, M.-A. Robin, A. Mayeuf et al., “Protection against hepatocyte mitochondrial dysfunction delays fibrosis progression in mice,” American Journal of Pathology, vol. 175, no. 5, pp. 1929–1937, 2009.
- J.-F. Dufour, C. M. Oneta, J.-J. Gonvers et al., “Randomized placebo-controlled trial of ursodeoxycholic acid with vitamin e in nonalcoholic steatohepatitis,” Clinical Gastroenterology and Hepatology, vol. 4, no. 12, pp. 1537–1543, 2006.
- M. Haedrich and J.-F. Dufour, “UDCA for NASH: end of the story?” Journal of Hepatology, vol. 54, no. 5, pp. 856–858, 2011.
- Z. Wang, W. Han, X. Sui, Y. Fang, and H. Pan, “Autophagy: a novel therapeutic target for hepatocarcinoma (review),” Oncology Letters, vol. 7, pp. 1345–1351, 2014.
- X. Qu, J. Yu, G. Bhagat et al., “Promotion of tumorigenesis by heterozygous disruption of the beclin 1 autophagy gene,” Journal of Clinical Investigation, vol. 112, no. 12, pp. 1809–1820, 2003.
- A. Takamura, M. Komatsu, T. Hara et al., “Autophagy-deficient mice develop multiple liver tumors,” Genes and Development, vol. 25, no. 8, pp. 795–800, 2011.
- Z.-B. Ding, Y.-H. Shi, J. Zhou et al., “Association of autophagy defect with a malignant phenotype and poor prognosis of hepatocellular carcinoma,” Cancer Research, vol. 68, no. 22, pp. 9167–9175, 2008.
- J. D. Yang, S.-Y. Seol, S.-H. Leem et al., “Genes associated with recurrence of hepatocellular carcinoma: integrated analysis by gene expression and methylation profiling,” Journal of Korean Medical Science, vol. 26, no. 11, pp. 1428–1438, 2011.
- J.-H. Kim, H. Y. Kim, Y.-K. Lee et al., “Involvement of mitophagy in oncogenic K-Ras-induced transformation: overcoming a cellular energy deficit from glucose deficiency,” Autophagy, vol. 7, no. 10, pp. 1187–1198, 2011.
- P. Chen and P. Bonaldo, “Role of macrophage polarization in tumor angiogenesis and vessel normalization: implications for new anticancer therapies,” International Review of Cell and Molecular Biology, vol. 301, pp. 1–35, 2013.
- B.-Z. Qian and J. W. Pollard, “Macrophage diversity enhances tumor progression and metastasis,” Cell, vol. 141, no. 1, pp. 39–51, 2010.
- H. Lin, J. Yan, Z. Wang et al., “Loss of immunity-supported senescence enhances susceptibility to hepatocellular carcinogenesis and progression in Toll-like receptor 2-deficient mice,” Hepatology, vol. 57, no. 1, pp. 171–182, 2013.
- W. Chen, T. Ma, X.-N. Shen et al., “Macrophage-induced tumor angiogenesis is regulated by the TSC2-mTOR pathway,” Cancer Research, vol. 72, no. 6, pp. 1363–1372, 2012.
- C. Toso, S. Merani, D. L. Bigam, A. M. J. Shapiro, and N. M. Kneteman, “Sirolimus-based immunosuppression is associated with increased survival after liver transplantation for hepatocellular carcinoma,” Hepatology, vol. 51, no. 4, pp. 1237–1243, 2010.
- A. M. Cuervo, E. Bergamini, U. T. Brunk, W. Dröge, M. Ffrench, and A. Terman, “Autophagy and aging: the importance of maintaining “clean” cells,” Autophagy, vol. 1, no. 3, pp. 131–140, 2005.
- W.-L. Yen and D. J. Klionsky, “How to live long and prosper: autophagy, mitochondria, and aging,” Physiology, vol. 23, no. 5, pp. 248–262, 2008.
- M. K. Shigenaga, T. M. Hagen, and B. N. Ames, “Oxidative damage and mitochondrial decay in aging,” Proceedings of the National Academy of Sciences of the United States of America, vol. 91, no. 23, pp. 10771–10778, 1994.
- E. Bergamini, G. Cavallini, A. Donati, and Z. Gori, “The role of autophagy in aging: its essential part in the anti-aging mechanism of caloric restriction,” Annals of the New York Academy of Sciences, vol. 1114, pp. 69–78, 2007.
- S. Miwa, C. Lawless, and T. von Zglinicki, “Mitochondrial turnover in liver is fast in vivo and is accelerated by dietary restriction: application of a simple dynamic model,” Aging Cell, vol. 7, no. 6, pp. 920–923, 2008.
- A. J. Meijer and P. Codogno, “Autophagy: regulation and role in disease,” Critical Reviews in Clinical Laboratory Sciences, vol. 46, no. 4, pp. 210–240, 2009.
- J. L. Evans, I. D. Goldfine, B. A. Maddux, and G. M. Grodsky, “Oxidative stress and stress-activated signaling pathways: a unifying hypothesis of type 2 diabetes,” Endocrine Reviews, vol. 23, no. 5, pp. 599–622, 2002.
- B. B. Lowell and G. I. Shulman, “Mitochondrial dysfunction and type 2 diabetes,” Science, vol. 307, no. 5708, pp. 384–387, 2005.
- L. Guarente, “Mitochondria-a nexus for aging, calorie restriction, and sirtuins?” Cell, vol. 132, no. 2, pp. 171–176, 2008.
- L. Fontana, E. Zhao, M. Amir, H. Dong, K. Tanaka, and M. J. Czaja, “Aging promotes the development of diet-induced murine steatohepatitis but not steatosis,” Hepatology, vol. 57, no. 3, pp. 995–1004, 2013.