Autophagy-lysosomal pathway is involved in lipid degradation in rat liver

Effects of dietary chloroquine on fish growth, hepatic intermediary metabolism, antioxidant and inflammatory responses in turbot

Autophagy is a conserved cellular self-digestion process and is essential for individual growth, cellular metabolism and inflammatory responses. It was responsive to starvation, pathogens infection and environmental stress. However, the information on the regulation of autophagy in fish hepatic intermediary metabolism, antioxidant system, and immune responses were limited. In the present study, turbot with inhibited autophagy flux was built by dietary chloroquine. The hepatic metabolic response, antioxidant enzymes and immune responses were explored. Results showed that dietary chloroquine induced the expression of Beclin 1, SQSTM and LC-3II, and effectively inhibited autophagy flux. Autophagy dysfunction depressed fish growth and feed utilization, while it induced clusters of liver lipid droplets. The genes involved in lipolysis and fatty acid β-oxidation, as well as the lipogenesis-related genes in chloroquine group were depressed. The phosphorylation of AMPK was activated in chloroquine group, and the genes involved in glycolysis were induced. The hepatic content of malonyldialdehyde and the activities of SOD and CAT were induced when autophagy was inhibited. The content of Complement 3, Complement 4 and Immunoglobulin M, as well as the activity of lysozyme in plasma were depressed in chloroquine group. Dietary chloroquine induced the expression of toll-like receptors and stimulated the expression of myd88 and nf-κb p65, as well as the pro-inflammatory cytokines, such as tnf-α and il-1β. The expression of anti-inflammatory cytokine tgf-β was depressed in the chloroquine group. Our results would extend the knowledge on the role of autophagy in teleost and assist in improving fishery production.

Genetic Markers Predisposing to Nonalcoholic Steatohepatitis

The growing prevalence of nonalcoholic fatty liver disease (NAFLD) has sparked interest in understanding genetics and epigenetics associated with the development and progression of the disease. A better understanding of the genetic factors related to progression will be beneficial in the risk stratification of patients. These genetic markers can also serve as potential therapeutic targets in the future. In this review, we focus on the genetic markers associated with the progression and severity of NAFLD.

Pterostilbene alleviated NAFLD via AMPK/mTOR signaling pathways and autophagy by promoting Nrf2

BACKGROUND

NAFLD is a liver disease that is caused by liver damage or extreme lipid deposition but not alcohol. Nrf2 could mediate resistance to oxidative stress injury. Autophagy can degrade metabolic waste and accumulated toxic endogenous substances. Pterostilbene (PTE) is an active compound extracted from blueberry, and grape, that exhibits many biological effects, such as antiinflammation and antitumor.

PURPOSE

This study provides a mechanism of PTE affecting on oxidative stress and autophagy in NAFLD mice. Tyloxapol, oil acid (OA) and palmitic acid (PA) were used to induce lipid accumulation in mice and HepG2 cells.

METHODS

Western blotting, CRISPR/Cas 9 and other molecular biological approaches were applied to explore the mechanisms of PTE effected on NAFLD.

RESULTS

PTE pretreatment effectively reduced the lipid accumulation in OA and PA induced HepG2 cells and tyloxapol induced mice, and significantly promoted the expression of nNrf2, PPAR-α and HO-1, and AMPK activity, but inhibited the expression of mTORC 1 and SREBP-1c. PTE activated phosphatidylinositide 3-kinase (PI3K) and proteins in the autophagy-related gene (ATG) family, and promoted the transformation of LC3Ⅰ to LC3Ⅱ which indicated the activation of autophagy, however, these effects were abolished after Nrf2 knockout.

CONCLUSION

PTE effectively alleviated oxidative stress damage induced by excessive lipid accumulation in hepatocytes, thus promoting the metabolism and decomposition of fatty acids to improve NAFLD.

A Decade of Mighty Lipophagy: What We Know and What Facts We Need to Know?

Lipids are integral cellular components that act as substrates for energy provision, signaling molecules, and essential constituents of biological membranes along with a variety of other biological functions. Despite their significance, lipid accumulation may result in lipotoxicity, impair autophagy, and lysosomal function that may lead to certain diseases and metabolic syndromes like obesity and even cell death. Therefore, these lipids are continuously recycled and redistributed by the process of selective autophagy specifically termed as lipophagy. This selective form of autophagy employs lysosomes for the maintenance of cellular lipid homeostasis. In this review, we have reviewed the current literature about how lipid droplets (LDs) are recruited towards lysosomes, cross-talk between a variety of autophagy receptors present on LD surface and lysosomes, and lipid hydrolysis by lysosomal enzymes. In addition to it, we have tried to answer most of the possible questions related to lipophagy regulation at different levels. Moreover, in the last part of this review, we have discussed some of the pathological states due to the accumulation of these LDs and their possible treatments under the light of currently available findings.

Effect of a common missense variant in LIPA gene on fatty liver disease and lipid phenotype: New perspectives from a single-center observational study

Lysosomal acid lipase deficiency (LAL-D) is an autosomal recessive disease characterized by hypoalphalipoproteinemia, mixed hyperlipemia, and fatty liver (FL) due to mutations in LIPAse A, lysosomal acid type (LIPA) gene. The rs1051338 single-nucleotide polymorphism (SNP) in LIPA gene, in vitro, could adversely affect the LAL activity (LAL-A). Nonalcoholic fatty liver disease (NAFLD) is often associated with metabolic syndrome, and the diagnosis requires the exclusion of excess of alcohol intake and other causes of hepatic disease. The aim of the study was to evaluate the impact of rs1051338 rare allele on lipid phenotype, severity of FL, and LAL-A in patients suffering from dyslipidemia associated with NAFLD. We selected 74 subjects with hypoalphalipoproteinemia or mixed hyperlipemia and evaluated transaminases, liver assessment with controlled attenuation parameter (CAP), LAL-A, rs1051338 SNP genotype. The presence of rare allele caused higher levels of triglycerides and hepatic transaminase and lower levels of high-density lipoprotein cholesterol (HDL-C). Multivariate analysis highlighted independent association between rare allele and FL severity in subjects with NAFLD. The rs1051338 SNP may modulate FL severity and atherogenic dyslipidemia in patients suffering from NAFLD.

Autophagy is a major metabolic regulator involved in cancer therapy resistance

Autophagy sustains cellular homeostasis and metabolism in numerous diseases. By regulating cancer metabolism, both tumor and microenvironmental autophagy promote tumor growth. However, autophagy can support cancer progression through other biological functions such as immune response regulation or cytokine/growth factor secretion. Moreover, autophagy is induced in numerous tumor types as a resistance mechanism following therapy, highlighting autophagy inhibition as a promising target for anti-cancer therapy. Thus, better understanding the mechanisms involved in tumor growth and resistance regulation through autophagy, which are not fully understood, will provide insights into patient treatment.

Reduced Lysosomal Acid Lipase Activity in Blood and Platelets Is Associated With Nonalcoholic Fatty Liver Disease

To investigate whether blood total lysosomal acid lipase activity (BT-LAL) levels are uniquely associated with the noncirrhotic and cirrhotic stages of nonalcoholic fatty liver disease (NAFLD) and with protection from NAFLD in metabolically/genetically predisposed subjects and a normal liver. To clarify which enzyme-carrying circulating cells are involved in reduced BT-LAL of NAFLD.

Pharmacological and nutritional modulation of autophagy in a rainbow trout (Oncorhynchus mykiss) gill cell line, RTgill-W1

Autophagy is involved in the modulation of nutrition, immunity, and disease in humans and animals. To understand the impact of autophagy modulation on a rainbow trout gill cell line, RTgill-W1, treatments including reduced nutrition (2% fetal bovine serum compared with 10% control), rapamycin, 3-methyladenine, deoxynivalenol, and chloroquine were tested. Western blot and immunofluorescence were used to detect microtubule-associated protein 1A/1B-light chain protein and quantitative polymerase chain reaction was used to detect the expression of 10 autophagy-related genes. At 3-d post-treatment, reduced nutrition significantly (p < 0.05) increased autophagy while deoxynivalenol significantly (p < 0.01) suppressed it compared to controls. These phenomena were confirmed by using immunofluorescence to detect the number of autophagosomes in RTgill-W1. Chloroquine is critical to allow observation of microtubule-associated protein 1A/1B-light chain protein in this model. The commonly used autophagy-modulating chemicals rapamycin and 3-methyladenine either activated or suppressed microtubule-associated protein 1A/1B-light chain protein, respectively, as expected from the literature, but did not act in a consistently significant manner. Expression of five of the 10 Atg genes, including lc3, gabarap, atg4, atg7, and atg12, were altered in a similar pattern to microtubule-associated protein 1A/1B-light chain protein. The consistent trend of autophagy-related gene upregulation including becn1, lc3, gabarap, and atg9 following treatment with 3-methyladenine and chloroquine is suggestive of a novel feedback regulation in the autophagy machinery.

Comparative Analysis of Metabolites in the Liver of Muscovy Ducks at Different Egg-Laying Stages Using Nontargeted Ultra-High-Performance Liquid Chromatography-Electrospray Mass Spectrometry-Based Metabolomics

Liver plays an important physiological function in the synthesis of yolk materials during egg laying in birds. Liver metabolite profiles of Muscovy ducks at different egg-laying stages from the perspective of nontargeted metabolomics were analyzed in this study. Twelve Muscovy ducks were selected at pre-laying (22 weeks, TT group), laying (40 weeks, FT group), and post-laying (60 weeks, ST group) stages, resulting in 36 hepatic metabolite profiles by using ultra-high-performance liquid chromatography-electrospray mass spectrometry. A total of 324 differential metabolites (156 increased and 168 decreased) in FT as compared to the TT (FT/TT) group and 332 differential metabolites (120 increased and 212 decreased) in ST as compared to the FT (ST/FT) group were screened out. Metabolic pathways enriched in FT/TT and ST/FT groups were mainly amino acid metabolism, glycerophospholipid metabolism, nucleotide metabolism, and vitamin metabolism. The amino acid metabolism pathways were upregulated in the FT/TT group and downregulated in the ST/FT group (P < 0.05). The glutathione and ascorbic acid abundances were downregulated, and the choline abundance was upregulated during egg laying (P < 0.05). The liver provides amino acids, lipids, nucleotides, vitamins, and choline, and so on, which are essential materials for yolk precursor synthesis. The decrease in the abundance of glutathione and ascorbic acid indicates that Muscovy ducks might be in a relatively stable physiological state during egg laying.

Liver-specific ceramide reduction alleviates steatosis and insulin resistance in alcohol-fed mice

Alcohol's impairment of both hepatic lipid metabolism and insulin resistance (IR) are key drivers of alcoholic steatosis, the initial stage of alcoholic liver disease (ALD). Pharmacologic reduction of lipotoxic ceramide prevents alcoholic steatosis and glucose intolerance in mice, but potential off-target effects limit its strategic utility. Here, we employed a hepatic-specific acid ceramidase (ASAH) overexpression model to reduce hepatic ceramides in a Lieber-DeCarli model of experimental alcoholic steatosis. We examined effects of alcohol on hepatic lipid metabolism, body composition, energy homeostasis, and insulin sensitivity as measured by hyperinsulinemic-euglycemic clamp. Our results demonstrate that hepatic ceramide reduction ameliorates the effects of alcohol on hepatic lipid droplet (LD) accumulation by promoting VLDL secretion and lipophagy, the latter of which involves ceramide cross-talk between the lysosomal and LD compartments. We additionally demonstrate that hepatic ceramide reduction prevents alcohol's inhibition of hepatic insulin signaling. These effects on the liver are associated with a reduction in oxidative stress markers and are relevant to humans, as we observe peri- LD ASAH expression in human ALD. Together, our results suggest a potential role for hepatic ceramide inhibition in preventing ALD.

Mitophagy in the Pathogenesis of Liver Diseases

Autophagy is a catabolic process involving vacuolar sequestration of intracellular components and their targeting to lysosomes for degradation, thus supporting nutrient recycling and energy regeneration. Accumulating evidence indicates that in addition to being a bulk, nonselective degradation mechanism, autophagy may selectively eliminate damaged mitochondria to promote mitochondrial turnover, a process termed “mitophagy”. Mitophagy sequesters dysfunctional mitochondria via ubiquitination and cargo receptor recognition and has emerged as an important event in the regulation of liver physiology. Recent studies have shown that mitophagy may participate in the pathogenesis of various liver diseases, such as liver injury, liver steatosis/fatty liver disease, hepatocellular carcinoma, viral hepatitis, and hepatic fibrosis. This review summarizes the current knowledge on the molecular regulations and functions of mitophagy in liver physiology and the roles of mitophagy in the development of liver-related diseases. Furthermore, the therapeutic implications of targeting hepatic mitophagy to design a new strategy to cure liver diseases are discussed.

The exchangeable apolipoproteins in lipid metabolism and obesity

Dyslipidemia, characterized by increased plasma levels of low-density lipoprotein cholesterol (LDL-C), very low-density lipoprotein cholesterol (VLDL-C), triglyceride (TG), and reduced plasma levels of high-density lipoprotein cholesterol (HDL-C), is confirmed as a hallmark of obesity and cardiovascular diseases (CVD), posing serious risks to the future health of humans. Thus, it is important to understand the molecular metabolism of dyslipidemia, which could help reduce the morbidity and mortality of obesity and CVD. Currently, several exchangeable apolipoproteins, such as apolipoprotein A1 (ApoA1), apolipoprotein A5 (ApoA5), apolipoprotein E (ApoE), and apolipoprotein C3 (ApoC3), have been verified to exert vital effects on modulating lipid metabolism and homeostasis both in plasma and in cells, which consequently affect dyslipidemia. In the present review, we summarize the findings of the effect of exchangeable apolipoproteins on affecting lipid metabolism in adipocytes and hepatocytes. Furthermore, we also provide new insights into the mechanisms by which the exchangeable apolipoproteins influence the pathogenesis of dyslipidemia and its related cardio-metabolic disorders.

A Low Protein Diet during Gestation and Lactation Increases Hepatic Lipid Accumulation through Autophagy and Histone Deacetylase

The present study examined the mechanism of a low protein (LP) diet on hepatic lipid metabolism during gestation and lactation. Timed-pregnant Sprague-Dawley rats were fed a control or an LP diet during gestation and lactation. LP dams had increased hepatic triglyceride accumulation and significantly higher aspartate/alanine transaminase ratio, accompanied by a decrease in circulating very low-density/low-density lipoprotein ratio. LC3B (Microtubule Associated Protein 1 Light Chain 3 Beta) expression was stimulated in LP dams along with increased histone acetylation. LP diet-induced co-localization of the LC3 binding motif-interacting proteins APOB or MTTP with LC3B, suggesting autophagic degradation. HDAC3 is found necessary to prevent lipid accumulation in response to amino acid deprivation in HepG2 cells. LC3B-mediated APOB protein degradation is related to increases in lipid accumulation. Conclusion: HDAC3 regulated LC3B-induced lipid accumulation potentially through autophagic degradation of APOB and MTTP in response to amino acid limitation caused by a low protein diet.

Selective autophagy, lipophagy and mitophagy, in the Harderian gland along the oestrous cycle: a potential retrieval effect of melatonin

Sexual dimorphism has been reported in many processes. However, sexual bias in favour of the use of males is very present in science. One of the main reasons is that the impact of hormones in diverse pathways and processes such as autophagy have not been properly addressed in vivo. The Harderian gland is a perfect model to study autophagic modulation as it exhibits important changes during the oestrous cycle. The aim of this study is to identify the main processes behind Harderian gland differences under oestrous cycle and their modulator. In the present study we show that redox-sensitive transcription factors have an essential role: NF-κB may activate SQSTM1/p62 in oestrus, promoting selective types of autophagy: mitophagy and lipophagy. Nrf2 activation in dioestrus, leads the retrieval phase and restoration of mitochondrial homeostasis. Melatonin’s receptors show higher expression in dioestrus, leading to decreases in pro-inflammatory mediators and enhanced Nrf2 expression. Consequently, autophagy is blocked, and porphyrin release is reduced. All these results point to melatonin as one of the main modulators of the changes in autophagy during the oestrous cycle.

In Utero Dexamethasone Exposure Exacerbates Hepatic Steatosis in Rats That Consume Fructose During Adulthood

Distinct environmental insults might interact with fructose consumption and contribute to the development of metabolic disorders. To address whether in utero glucocorticoid exposure and fructose intake modulate metabolic responses, adult female Wistar rats were exposed to dexamethasone (DEX) during pregnancy, and the offspring were administered fructose at a later time. Briefly, dams received DEX during the third period of pregnancy, while control dams remained untreated. Offspring born to control and DEX-treated mothers were defined as CTL-off and DEX-off, respectively, while untreated animals were designated CTL-off-CTL and DEX-off-CTL. CLT-off and DEX-off treated with 10% fructose in the drinking water for 8 weeks are referred to as CTL-off-FRU and DEX-off-FRU. We found that fructose promoted glucose intolerance and whole-body gluconeogenesis in both CTL-off-FRU and DEX-off-FRU animals. On the other hand, hepatic lipid accumulation was significantly stimulated in DEX-off-FRU rats when compared to the CTL-off-FRU group. The DEX-off-FRU group also displayed impaired very-low-density lipoprotein (VLDL) production and reduced hepatic expression of apoB, mttp, and sec22b. DEX-off-FRU has lower hepatic levels of autophagy markers. Taken together, our results support the unprecedented notion that in utero glucocorticoid exposure exacerbates hepatic steatosis caused by fructose consumption later in life.

Inhibited Lipophagy Suppresses Lipid Metabolism in Zebrafish Liver Cells

Lipophagy degrades lipid droplets (LDs) through the lysosomal degradative pathway, thus plays important roles in regulating lipid metabolism in mammals. However, information on the existence and functions of lipophagy in fish lipid metabolism is still limited. In the present study, we confirmed the existence of lipophagy by observing the structures of LDs sequestered in autophagic vacuoles in the zebrafish liver cell line (ZFL) via electronic microscopy. Moreover, starved cells increased the mRNA expression of the microtubule-associated protein 1A/1B light chain 3 beta (LC3), which is a marker protein for autophagy and protein conversion from LC3-I to LC3-II. Inhibiting autophagy with chloroquine increased significantly the LDs content and decreased fatty acid β-oxidation and esterification activities in the ZFL cells cultured in the fed state. Furthermore, inhibiting autophagy function downregulated the mRNA expression of the genes and their proteins related to lipid metabolism. Altogether, the present study verified the existence of lipophagy and its essential regulatory roles in lipid metabolism in fish cells.

ERβ promotes Aβ degradation via the modulation of autophagy

Alzheimer's Disease (AD) is the most common neurodegenerative disorder in the elderly. Beta-amyloid (Aβ) peptide accumulation is considered as a primary cause of AD pathogenesis, with defective autophagy in patients' brains. Enhanced autophagic activity has been reported to promote Aβ clearance in vitro and in vivo models. Meanwhile, there is growing evidence that estrogen receptor β (ERβ) is a viable therapeutic target that can ameliorate the pathological features associated with AD. Very little is known about the detailed molecular mechanisms underlying the relationship between ERβ, autophagy, and Aβ degradation in AD. This study aims to uncover whether ERβ participates in autophagy and promotes extracellular Aβ1-42 degradation through the autophagy-lysosome system. Here we find that overexpression of ERβ caused autophagic activation as seen by increased microtubule-associated protein 1 light chain 3-II (LC3-II), SQSTM1 (sequestosome 1) degradation, LC3 punctate distribution, autophagosome, and autolysosome accumulation. In addition, we show that ERβ could induce autophagy through direct protein-protein interaction with ATG7 (E1-like enzyme). Furthermore, ERβ-mediated decrease in Aβ1-42 was blocked by the autophagy inhibitor chloroquine (CQ) in SH-SY5Y cells and the HEK293T (AβPPsw) model. Aβ1-42 or CQ induced cytotoxicity was restored by a selective ERβ activator diarylpropionitrile (DPN). Collectively, these data indicate that overexpression of ERβ exerts a neuroprotective effect through interacting with ATG7 protein and further enhances autophagy-lysosomal activity for Aβ1-42 clearance at the cellular level.

DGAT1 deficiency disrupts lysosome function in enterocytes during dietary fat absorption

Enterocytes, the absorptive cells of the small intestine, mediate the process of dietary fat absorption by secreting triacylglycerol (TAG) into circulation. When levels of dietary fat are high, TAG is stored in cytoplasmic lipid droplets (CLDs) and sequentially hydrolyzed for ultimate secretion. Mice with deficiency in acyl CoA: diacylglycerol acyltransferase 1 (Dgat1^-/- mice) were previously reported to have a reduced rate of intestinal TAG secretion and abnormal TAG accumulation in enterocyte CLDs. This unique intestinal phenotype is critical to their resistance to diet-induced obesity; however, the underlying mechanism remains unclear. Emerging evidence shows that lysosomal TAG hydrolysis contributes to autophagy-mediated CLD mobilization termed lipophagy, and when disrupted results in CLD accumulation. In order to study how lipophagy contributes to the unique intestinal phenotype of Dgat1^-/- mice, enterocytes from wild-type (WT) and Dgat1^-/- mice were examined at 2 and 6 h after oral oil gavage. Through ultrastructural analysis we observed TAG present within autophagic vesicles (AVs) in mouse enterocytes, suggesting the role of lipophagy in intestinal CLD mobilization during dietary fat absorption. Furthermore, we found that Dgat1^-/- mice had abnormal TAG accumulation within AVs and less acidic lysosomes compared to WT mice. Together these findings suggest that the delayed dietary fat absorption seen in Dgat1^-/- mice is, in part, due to the dysregulated flux of autophagy-mediated CLD mobilization and impairment of lysosomal acidification in enterocytes. The present study highlights the critical role of lysosome in enterocyte CLD mobilization for proper dietary fat absorption.

Molybdate Attenuates Lipid Accumulation in the Livers of Mice Fed a Diet Deficient in Methionine and Choline

Both lipid accumulation and oxidative stress are major pathologic contributors to the development of hepatic steatosis. Treatment with molybdate reduces hepatic levels of lipids in diabetic rats. Potential activities of molybdate as an antioxidant have also been demonstrated in various animal models. In the present study, we evaluated the effects of sodium molybdate dihydrate (SM) on hepatic steatosis and associated disturbances in a widely used mouse model of the metabolic disease. Male C57Bl/6 mice at 10 weeks of age were fed a diet deficient in methionine and choline (MCD) and bottled water containing SM for four weeks. The SM treatment markedly attenuated MCD-induced accumulation of lipids, mainly triglycerides, in the liver. Lipid catabolic autophagic pathways were activated by SM in the MCD-fed mouse livers, as evidenced by a decreased level of p62 expression. MCD-induced oxidative damage, such as lipid and protein oxidation, was also alleviated by SM in the liver. However, the level of MCD-induced hepatocellular damage was not affected by SM. Taken together, these findings suggest that molybdate can be used in the treatment and prevention of hepatic steatosis without inducing adverse effects in the liver. To the best of our knowledge, this is the first experimental study to investigate the effects of molybdate in non-alcoholic fatty liver disease, and also the first that demonstrates molybdate-induced autophagy.

Genetically modified mouse models to study hepatic neutral lipid mobilization

Excessive accumulation of triacylglycerol is the common denominator of a wide range of clinical pathologies of liver diseases, termed non-alcoholic fatty liver disease. Such excessive triacylglycerol deposition in the liver is also referred to as hepatic steatosis. Although liver steatosis often resolves over time, it eventually progresses to steatohepatitis, liver fibrosis and cirrhosis, with associated complications, including liver failure, hepatocellular carcinoma and ultimately death of affected individuals. From the disease etiology it is obvious that a tight regulation between lipid uptake, triacylglycerol synthesis, hydrolysis, secretion and fatty acid oxidation is required to prevent triacylglycerol deposition in the liver. In addition to triacylglycerol, also a tight control of other neutral lipid ester classes, i.e. cholesteryl esters and retinyl esters, is crucial for the maintenance of a healthy liver. Excessive cholesteryl ester accumulation is a hallmark of cholesteryl ester storage disease or Wolman disease, which is associated with premature death. The loss of hepatic vitamin A stores (retinyl ester stores of hepatic stellate cells) is incidental to the onset of liver fibrosis. Importantly, this more advanced stage of liver disease usually does not resolve but progresses to life threatening stages, i.e. liver cirrhosis and cancer. Therefore, understanding the enzymes and pathways that mobilize hepatic neutral lipid esters is crucial for the development of strategies and therapies to ameliorate pathophysiological conditions associated with derangements of hepatic neutral lipid ester stores, including liver steatosis, steatohepatitis, and fibrosis. This review highlights the physiological roles of enzymes governing the mobilization of neutral lipid esters at different sites in liver cells, including cytosolic lipid droplets, endoplasmic reticulum, and lysosomes. This article is part of a Special Issue entitled Molecular Basis of Disease: Animal models in liver disease.

Comparative transcriptomics of hepatic differentiation of human pluripotent stem cells and adult human liver tissue

Hepatocytes derived from human pluripotent stem cells (hPSC-HEP) have the potential to replace presently used hepatocyte sources applied in liver disease treatment and models of drug discovery and development. Established hepatocyte differentiation protocols are effective and generate hepatocytes, which recapitulate some key features of their in vivo counterparts. However, generating mature hPSC-HEP remains a challenge. In this study, we applied transcriptomics to investigate the progress of in vitro hepatic differentiation of hPSCs at the developmental stages, definitive endoderm, hepatoblasts, early hPSC-HEP, and mature hPSC-HEP, to identify functional targets that enhance efficient hepatocyte differentiation. Using functional annotation, pathway and protein interaction network analyses, we observed the grouping of differentially expressed genes in specific clusters representing typical developmental stages of hepatic differentiation. In addition, we identified hub proteins and modules that were involved in the cell cycle process at early differentiation stages. We also identified hub proteins that differed in expression levels between hPSC-HEP and the liver tissue controls. Moreover, we identified a module of genes that were expressed at higher levels in the liver tissue samples than in the hPSC-HEP. Considering that hub proteins and modules generally are essential and have important roles in the protein-protein interactions, further investigation of these genes and their regulators may contribute to a better understanding of the differentiation process. This may suggest novel target pathways and molecules for improvement of hPSC-HEP functionality, having the potential to finally bring this technology to a wider use.

Complex role of autophagy in regulation of hepatic lipid and lipoprotein metabolism

Discovering new therapeutic interventions to treat lipid and lipoprotein disorders is of great interest and the discovery of autophagy as a regulator of lipid metabolism has opened up new avenues for targeting modulators of this pathway. Autophagy is a degradative process that targets cellular components to the lysosome and recent studies have indicated a role for autophagy in regulating hepatic lipid metabolism (known as lipophagy) as well as lipoprotein assembly. Autophagy directly targets apolipoprotein B-100 (apoB100), the structural protein component of very low-density lipoproteins (VLDLs), and further targets lipid droplets (LDs), the cellular storage for neutral lipids. Autophagy thus plays a complex and dual role in VLDL particle assembly by regulating apoB100 degradation as well as aiding the maturation of VLDL particles by hydrolyzing lipid from LDs. The purpose of this article is to review our current understanding of molecular and cellular mechanisms mediating authophagic control of hepatic lipid biogenesis and VLDL production as well as dysregulation in insulin resistance and dyslipidemia.

Lysosomal acid lipase regulates VLDL synthesis and insulin sensitivity in mice

AIMS/HYPOTHESIS

Lysosomal acid lipase (LAL) hydrolyses cholesteryl esters and triacylglycerols (TG) within lysosomes to mobilise NEFA and cholesterol. Since LAL-deficient (Lal (-/-) ) mice suffer from progressive loss of adipose tissue and severe accumulation of lipids in hepatic lysosomes, we hypothesised that LAL deficiency triggers alternative energy pathway(s).

METHODS

We studied metabolic adaptations in Lal (-/-) mice.

RESULTS

Despite loss of adipose tissue, Lal (-/-) mice show enhanced glucose clearance during insulin and glucose tolerance tests and have increased uptake of [(3)H]2-deoxy-D-glucose into skeletal muscle compared with wild-type mice. In agreement, fasted Lal (-/-) mice exhibit reduced glucose and glycogen levels in skeletal muscle. We observed 84% decreased plasma leptin levels and significantly reduced hepatic ATP, glucose, glycogen and glutamine concentrations in fed Lal (-/-) mice. Markedly reduced hepatic acyl-CoA concentrations decrease the expression of peroxisome proliferator-activated receptor α (PPARα) target genes. However, treatment of Lal (-/-) mice with the PPARα agonist fenofibrate further decreased plasma TG (and hepatic glucose and glycogen) concentrations in Lal (-/-) mice. Depletion of hepatic nuclear factor 4α and forkhead box protein a2 in fasted Lal (-/-) mice might be responsible for reduced expression of microsomal TG transfer protein, defective VLDL synthesis and drastically reduced plasma TG levels.

CONCLUSIONS/INTERPRETATION

Our findings indicate that neither activation nor inactivation of PPARα per se but rather the availability of hepatic acyl-CoA concentrations regulates VLDL synthesis and subsequent metabolic adaptations in Lal (-/-) mice. We conclude that decreased plasma VLDL production enhances glucose uptake into skeletal muscle to compensate for the lack of energy supply.

Reduced lysosomal acid lipase activity - A potential role in the pathogenesis of non alcoholic fatty liver disease in pediatric patients

BACKGROUND

Within the spectrum of nonalcoholic fatty liver disease (NAFLD), recent evidence suggests that adult patients with nonalcoholic steatohepatitis (NASH) have significantly lower blood lysosomal acid lipase (LAL) activity than those with steatosis. This has not been studied in pediatric patients with NAFLD.

AIM

Investigate blood LAL activity in pediatric patients with NAFLD and assess its correlation with histological severity.

METHODS

We collected data on consecutive children with biopsy-proven NAFLD including demographics, anthropometrics, and routine laboratory tests. The histological features were graded according to the NAFLD activity scoring proposed by Kleiner et al. Blood LAL activity was measured prospectively using Lalistat 2.

RESULTS

A total of 168 children were included for analysis. Mean age was 12.6±8.5 years, 60.1% were males and 52.4% had NASH. Children with significant fibrosis (stage 2-3, n=64) had a significantly lower LAL activity compared to those with mild fibrosis (stage 0-1, n=104). There was no significant difference in LAL activity between children with NASH compared to those without NASH.

CONCLUSION

Reduced blood LAL activity correlates with severity of liver fibrosis in children with NAFLD indicating a potential role of reduced LAL activity in the pathogenesis of NAFLD-induced fibrosis.

Lipids, lysosomes, and autophagy

Lipids are essential components of a cell providing energy substrates for cellular processes, signaling intermediates, and building blocks for biological membranes. Lipids are constantly recycled and redistributed within a cell. Lysosomes play an important role in this recycling process that involves the recruitment of lipids to lysosomes via autophagy or endocytosis for their degradation by lysosomal hydrolases. The catabolites produced are redistributed to various cellular compartments to support basic cellular function. Several studies demonstrated a bidirectional relationship between lipids and lysosomes that regulate autophagy. While lysosomal degradation pathways regulate cellular lipid metabolism, lipids also regulate lysosome function and autophagy. In this review, we focus on this bidirectional relationship in the context of dietary lipids and provide an overview of recent evidence of how lipid-overload lipotoxicity, as observed in obesity and metabolic syndrome, impairs lysosomal function and autophagy that may eventually lead to cellular dysfunction or cell death.

Jujuboside A Protects H9C2 Cells from Isoproterenol-Induced Injury via Activating PI3K/Akt/mTOR Signaling Pathway

Jujuboside A is a kind of the saponins isolated from the seeds of Ziziphus jujuba, which possesses multiple biological effects, such as antianxiety, antioxidant, and anti-inflammatory effects; however, its mediatory effect on isoproterenol-stimulated cardiomyocytes has not been investigated yet. In this study, we tried to detect the protective effect and potential mechanism of JUA on ISO-induced cardiomyocytes injury. H9C2 cells were treated with ISO to induce cell damage. Cells were pretreated with JUA to investigate the effects on the cell viability, morphological changes, light chain 3 conversion, and the activation of PI3K/Akt/mTOR signaling pathway. Results showed that ISO significantly inhibited the cell viability in a time- and dose-dependent manner. JUA pretreatment could reverse the reduction of cell viability and better the injury of H9C2 cells induced by ISO. Western blot analysis showed that JUA could accelerate the phosphorylation of PI3K, Akt, and mTOR. Results also indicated that JUA could significantly decrease the ratio of microtubule-associated protein LC3-II/I in H9C2 cells. Taken together, our research showed that JUA could notably reduce the damage cause by ISO via promoting the phosphorylation of PI3K, Akt, and mTOR and inhibiting LC3 conversion, which may be a potential choice for the treatment of heart diseases.

TFE3 Alleviates Hepatic Steatosis through Autophagy-Induced Lipophagy and PGC1α-Mediated Fatty Acid β-Oxidation

Autophagy flux deficiency is closely related to the development of hepatic steatosis. Transcription factor E3 (TFE3) is reported to be a crucial gene that regulates autophagy flux and lysosome function. Therefore, we investigated the role of TFE3 in a cell model of hepatic steatosis. We constructed L02 hepatocyte lines that stably over-expressed or knocked down the expression of TFE3. Subsequently, the effects of TFE3 on hepatocellular lipid metabolism were determined by autophagy flux assay, lipid oil red O (ORO) staining, immunofluorescence staining, and mitochondrial β-oxidation assessment. Finally, we analyzed whether peroxisome proliferative activated receptor gamma coactivator 1α (PGC1α) was the potential target gene of TFE3 in the regulation of hepatic steatosis using a chromatin immunoprecipitation (CHIP) assay and a luciferase reporter system. We found that overexpression of TFE3 markedly alleviated hepatocellular steatosis. On the contrary, downregulation of TFE3 resulted in an aggravated steatosis. The mechanistic studies revealed that the TFE3-manipulated regulatory effects on hepatocellular steatosis are dependent on autophagy-induced lipophagy and PGC1α-mediated fatty acid β-oxidation because blocking these pathways with an Atg5 small interfering RNA (siRNA) or PGC1α siRNA dramatically blunted the TFE3-mediated regulation of steatosis. In conclusion, TFE3 gene provides a novel insight into the treatment of hepatic steatosis and other metabolic disease.

Advanced Glycation End Products (AGE) Potently Induce Autophagy through Activation of RAF Protein Kinase and Nuclear Factor κB (NF-κB)

Advanced glycation end products (AGE) accumulate in diabetic patients and aging people because of high amounts of three- or four-carbon sugars derived from glucose, thereby causing multiple consequences, including inflammation, apoptosis, obesity, and age-related disorders. It is important to understand the mechanism of AGE-mediated signaling leading to the activation of autophagy (self-eating) that might result in obesity. We detected AGE as one of the potent inducers of autophagy compared with doxorubicin and TNF. AGE-mediated autophagy is inhibited by suppression of PI3K and potentiated by the autophagosome maturation blocker bafilomycin. It increases autophagy in different cell types, and that correlates with the expression of its receptor, receptor for AGE. LC3B, the marker for autophagosomes, is shown to increase upon AGE stimulation. AGE-mediated autophagy is partially suppressed by inhibitor of NF-κB, PKC, or ERK alone and significantly in combination. AGE increases sterol regulatory element binding protein activity, which leads to an increase in lipogenesis. Although AGE-mediated lipogenesis is affected by autophagy inhibitors, AGE-mediated autophagy is not influenced by lipogenesis inhibitors, suggesting that the turnover of lipid droplets overcomes the autophagic clearance. For the first time, we provide data showing that AGE induces several cell signaling cascades, like NF-κB, PKC, ERK, and MAPK, that are involved in autophagy and simultaneously help with the accumulation of lipid droplets that are not cleared effectively by autophagy, therefore causing obesity.

Autophagy and Lipid Droplets in the Liver

Autophagy is a conserved quality-control pathway that degrades cytoplasmic contents in lysosomes. Autophagy degrades lipid droplets through a process termed lipophagy. Starvation and an acute lipid stimulus increase autophagic sequestration of lipid droplets and their degradation in lysosomes. Accordingly, liver-specific deletion of the autophagy gene Atg7 increases hepatic fat content, mimicking the human condition termed nonalcoholic fatty liver disease. In this review, we provide insights into the molecular regulation of lipophagy, discuss fundamental questions related to the mechanisms by which autophagosomes recognize lipid droplets and how ATG proteins regulate membrane curvature for lipid droplet sequestration, and comment on the possibility of cross talk between lipophagy and cytosolic lipases in lipid mobilization. Finally, we discuss the contribution of lipophagy to the pathophysiology of human fatty liver disease. Understanding how lipophagy clears hepatocellular lipid droplets could provide new ways to prevent fatty liver disease, a major epidemic in developed nations.

Effect of animal mixing as a stressor on biomarkers of autophagy and oxidative stress during pig muscle maturation

The objective of this work was to study the postmortem evolution of potential biomarkers of autophagy (Beclin 1, LC3-II/LC3-I ratio) and oxidative stress (total antioxidant activity, TAA; superoxide dismutase activity, SOD and catalase activity, CAT) in the Longissimus dorsi muscle of entire male ((Large White×Landrace)×Duroc) pigs subjected to different management treatments that may promote stress, such as mixing unfamiliar animals at the farm and/or during transport and lairage before slaughter. During the rearing period at the farm, five animals were never mixed after the initial formation of the experimental groups (unmixed group at the farm, UF), whereas 10 animals were subjected to a common routine of being mixed with unfamiliar animals (mixed group at the farm, MF). Furthermore, two different treatments were used during the transport and lairage before slaughter: 10 pigs were not mixed (unmixed group during transport and lairage, UTL), whereas five pigs were mixed with unfamiliar animals on the lorry and during lairage (mixed group during transport and lairage, MTL). These mixing treatments were then combined into three pre-slaughter treatments - namely, UF-UTL, MF-UTL and MF-MTL. The results show that MF-UTL and MF-MTL increased significantly the muscle antioxidant defense (TAA, SOD and CAT) at short postmortem times (4 and 8 h; P<0.001), followed by an earlier depletion of the antioxidant activity at 24 h postmortem (P<0.05). We also found that mixing unfamiliar animals, both at the farm and during transport and lairage, triggers postmortem muscle autophagy, which showed an earlier activation (higher expression of Beclin 1 and LC3-II/LC3-I ratio at 4 h postmortem followed by a decreasing pattern of this ratio along first 24 h postmortem) in the muscle tissues of animals from the MF-UTL and MF-MTL groups, as an adaptive strategy of the muscle cells for counteracting induced stress. From these results, we propose that monitoring the evolution of the main biomarkers of autophagy (Beclin 1, LC3-II/LC3-I ratio) and muscle antioxidant defense (TAA, SOD, CAT) in the muscle tissue within the first 24 h postmortem may help the detection of animal stress and its potential effect on the postmortem muscle metabolism.

Beneficial mechanisms of aerobic exercise on hepatic lipid metabolism in non-alcoholic fatty liver disease

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) refers to any fatty liver disease that is not due to excessive use of alcohol. NAFLD probably results from abnormal hepatic lipid metabolism and insulin resistance. Aerobic exercise is shown to improve NAFLD. This review aimed to evaluate the molecular mechanisms involved in the beneficial effects of aerobic exercise on NAFLD.

DATA SOURCE

We searched articles in English on the role of aerobic exercise in NAFLD therapy in PubMed.

RESULTS

The mechanisms of chronic aerobic exercise in regulating the outcome of NAFLD include: (i) reducing intrahepatic fat content by down-regulating sterol regulatory element-binding protein-1c and up-regulating peroxisome proliferator-activated receptor gamma expression levels; (ii) decreasing hepatic oxidative stress through modulating the reactive oxygen species, and enhancing antioxidant enzymes such as catalase and glutathione peroxidase; (iii) ameliorating hepatic inflammation via the inhibition of pro-inflammatory mediators such as tumor necrosis factor-alpha and interleukin-1 beta; (iv) attenuating mitochondrial dependent apoptosis by reducing cytochrome C released from the mitochondria to the cytosol; and (v) inducing hepato-protective autophagy.

CONCLUSION

Aerobic exercise, via different mechanisms, significantly decreases the fat content of the liver and improves the outcomes of patients with NAFLD.

Carnitine supplementation alleviates lipid metabolism derangements and protects against oxidative stress in non-obese hereditary hypertriglyceridemic rats

The aim of this study was to estimate the effect of carnitine supplementation on lipid disorders and peripheral tissue insulin sensitivity in a non-obese animal model of insulin resistance, the hereditary hypertriglyceridemic (HHTg) rat. Male HHTg rats were fed a standard diet, and half of them received daily doses of carnitine (500 mg·kg−1body weight) for 8 weeks. Rats of the original Wistar strain were used for comparison. HHTg rats exhibited increased urinary excretion of free carnitine and reduced carnitine content in the liver and blood. Carnitine supplementation compensated for this shortage and promoted urinary excretion of acetylcarnitine without any signs of (acyl)carnitine accumulation in skeletal muscle. Compared with their untreated littermates, carnitine-treated HHTg rats exhibited lower weight gain, reduced liver steatosis, lower fasting triglyceridemia, and greater reduction of serum free fatty acid content after glucose load. Carnitine treatment was associated with increased mitochondrial biogenesis and oxidative capacity for fatty acids, amelioration of oxidative stress, and restored substrate switching in the liver. In skeletal muscle (diaphragm), carnitine supplementation was associated with significantly higher palmitate oxidation and a more favorable complete to incomplete oxidation products ratio. Carnitine supplementation further enhanced insulin sensitivity ex vivo. No effects on whole-body glucose tolerance were observed. Our data suggest that some metabolic syndrome-related disorders, particularly fatty acid oxidation, steatosis, and oxidative stress in the liver, could be attenuated by carnitine supplementation. The effect of carnitine could be explained, at least partly, by enhanced substrate oxidation and increased fatty acid transport from tissues in the form of short-chain acylcarnitines.

Liraglutide reduces lipid accumulation in steatotic L‑02 cells by enhancing autophagy

Simple hepatic steatosis is the early stage of non-alcoholic fatty liver disease and is recognized as a benign process. Previous studies show that glucagon-like peptide-1 has great potential in improving hepatic steatosis. Recent data have revealed that inhibiting autophagy exacerbates lipid accumulation in hepatocytes. Therefore, the present study aimed to determine the effects of liraglutide (LG) on simple hepatic steatosis and the possible role of autophagy. Firstly, steatotic L-02 cells were induced by incubating L-02 cells with 1 mmol/l free fatty acid (FFA) mixture (oleic acid and palmitic acid at a molar ratio of 2:1) for 24 h. Intracellular lipid accumulation, cell viability, oxidative stress and apoptosis were evaluated. Secondly, steatotic L-02 cells were treated with 10 or 100 nmol/l LG, 100 nmol/l LG plus 3-methyladenine (3-MA), or rapamycin for 24 h, and then lipid accumulation was measured. Next, the degree of lipid accumulation and the intensity of autophagy were assessed. Oil red O staining and triglyceride quantification demonstrated notable steatosis in L-02 cells following exposure to 1 mmol/l FFA mixture for 24 h. There was no significant cytotoxicity, oxidative stress or apoptosis in steatotic L-02 cells. Treatment with 100 nmol/l LG reduced lipid accumulation in steatotic L-02 cells and increased the mRNA levels of microtubule-associated protein 1 light chain 3B. Additionally, it enhanced the autophagic flux in steatotic L-02 cells, as measured by western blot analysis and shown by electron microscopy. Additionally, 3-MA weakened the ability of LG to improve hepatic steatosis and enhance autophagy. Our data indicate that LG reduces the lipid accumulation in steatotic L-02 cells, and the activation of autophagy plays a significant role in this process.

Liver autophagy in anorexia nervosa and acute liver injury

Autophagy, a lysosomal catabolic pathway for long-lived proteins and damaged organelles, is crucial for cell homeostasis, and survival under stressful conditions. During starvation, autophagy is induced in numerous organisms ranging from yeast to mammals, and promotes survival by supplying nutrients and energy. In the early neonatal period, when transplacental nutrients supply is interrupted, starvation-induced autophagy is crucial for neonates' survival. In adult animals, autophagy provides amino acids and participates in glucose metabolism following starvation. In patients with anorexia nervosa, autophagy appears initially protective, allowing cells to copes with nutrient deprivation. However, when starvation is critically prolonged and when body mass index reaches 13 kg/m(2) or lower, acute liver insufficiency occurs with features of autophagic cell death, which can be observed by electron microscopy analysis of liver biopsy samples. In acetaminophen overdose, a classic cause of severe liver injury, autophagy is induced as a protective mechanism. Pharmacological enhancement of autophagy protects against acetaminophen-induced necrosis. Autophagy is also activated as a rescue mechanism in response to Efavirenz-induced mitochondrial dysfunction. However, Efavirenz overdose blocks autophagy leading to liver cell death. In conclusion, in acute liver injury, autophagy appears as a protective mechanism that can be however blocked or overwhelmed.

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.

Important role of autophagy in regulation of metabolic processes in health, disease and aging

Autophagy is the basic catabolic mechanism that involves degradation of dysfunctional cellular components through the action of lysosome as well as supplying energy and compounds for the synthesis of essential biomacromolecules. This process enables cells to survive stress from the external environment like nutrient deprivation. Autophagy is important in the breakdown of proteins, carbohydrates and lipids as well. Furthermore, recent studies have shown that autophagy is critical in wide range of normal human physiological processes, and defective autophagy is associated with diverse diseases, including lysosomal storage disease, myopathies, neurodegeneration and various metabolic disorders. This review summarizes the most up-to-date findings on what role autophagy plays in metabolism.

Caffeine stimulates hepatic lipid metabolism by the autophagy-lysosomal pathway in mice

Caffeine is one of the world's most consumed drugs. Recently, several studies showed that its consumption is associated with lower risk for nonalcoholic fatty liver disease (NAFLD), an obesity-related condition that recently has become the major cause of liver disease worldwide. Although caffeine is known to stimulate hepatic fat oxidation, its mechanism of action on lipid metabolism is still not clear. Here, we show that caffeine surprisingly is a potent stimulator of hepatic autophagic flux. Using genetic, pharmacological, and metabolomic approaches, we demonstrate that caffeine reduces intrahepatic lipid content and stimulates β-oxidation in hepatic cells and liver by an autophagy-lysosomal pathway. Furthermore, caffeine-induced autophagy involved down-regulation of mammalian target of rapamycin signaling and alteration in hepatic amino acids and sphingolipid levels. In mice fed a high-fat diet, caffeine markedly reduces hepatosteatosis and concomitantly increases autophagy and lipid uptake in lysosomes.

CONCLUSION

These results provide novel insight into caffeine's lipolytic actions through autophagy in mammalian liver and its potential beneficial effects in NAFLD.

Melatonin administration decreases adipogenesis in the liver of ob/ob mice through autophagy modulation

Despite efforts to curb the incidence of obesity and its comorbidities, this condition remains the fifth leading cause of death worldwide. To identify ways to reduce this global effect, we investigated the actions of daily melatonin administration on oxidative stress parameters and autophagic processes as a possible treatment of obesity in ob/ob mice. The involvement of melatonin in many physiological functions, such as the regulation of seasonal body weight variation, glucose uptake, or adiposity, and the role of this indoleamine as an essential antioxidant, has become the focus of numerous anti-obesity studies. Here, we examined the oxidative status in the livers of obese melatonin-treated and untreated mice, observing a decrease in the oxidative stress levels through elevated catalase activity. ROS-mediated autophagy was downregulated in the liver of melatonin-treated animals and was accompanied by significant accumulation of p62. Autophagy is closely associated with adipogenesis; in this study, we report that melatonin-treated obese mice also showed reduced adiposity, as demonstrated by diminished body weight and reduced peroxisome proliferator-activated receptor gamma expression. Based on these factors, it is reasonable to assume that oxidative stress and autophagy play important roles in obesity, and therefore, melatonin could be an interesting target molecule for the development of a potential therapeutic agent to curb body weight.

Two-Generation Diet-Induced Obesity Model Producing Mice With Increased Amount of Body Fat in Early Adulthood

The aim of our study was to develop a model producing obese mice in early adulthood (4-6 weeks) based on their over-nutrition during fetal and early postnatal development. The fertilized dams of the parental generation were fed the standard diet supplemented with high-energy nutritional product Ensure Plus during gestation and lactation. Delivered weanlings were then fed with standard or supplemented diet and assessed for body fat deposits using EchoMRI at the time of early and late adulthood. Maternal over-feeding during the period before weaning had the most significant effect on obesity development in the filial generation. In weanlings, significantly higher body fat deposits and average body weight were recorded. Later, further significant increase in percentage of body fat in both male and female mice was observed. Withdrawal of the Ensure Plus supplement caused a decrease in the percentage of body fat in part of the filial generation. In offspring fed the standard diet, higher fat deposits persisted till the time of late adulthood. We conclude that this diet-induced obesity model might be used in exploration of the effects of elevated body fat on physiological functions of various organ systems during juvenile and early adulthood periods of life of a human being.

Hepatic fatty acid trafficking: multiple forks in the road

The liver plays a unique, central role in regulating lipid metabolism. In addition to influencing hepatic function and disease, changes in specific pathways of fatty acid (FA) metabolism have wide-ranging effects on the metabolism of other nutrients, extra-hepatic physiology, and the development of metabolic diseases. The high prevalence of nonalcoholic fatty liver disease (NAFLD) has led to increased efforts to characterize the underlying biology of hepatic energy metabolism and FA trafficking that leads to disease development. Recent advances have uncovered novel roles of metabolic pathways and specific enzymes in generating lipids important for cellular processes such as signal transduction and transcriptional activation. These studies have also advanced our understanding of key branch points involving FA partitioning between metabolic pathways and have identified new roles for lipid droplets in these events. This review covers recent advances in our understanding of FA trafficking and its regulation. An emphasis will be placed on branch points in these pathways and how alterations in FA trafficking contribute to NAFLD and related comorbidities.

Autophagosomes contribute to intracellular lipid distribution in enterocytes

Delivery of alimentary lipids induces immediate autophagic response in enterocytes. Forming autophagosomes are recruited to the ER membrane, where they capture nascent lipid droplets and later fuse with lysosomes, illustrating for the first time the role of autophagy in neutral-lipid distribution in enterocytes.

Enterocytes, the intestinal absorptive cells, have to deal with massive alimentary lipids upon food consumption. They orchestrate complex lipid-trafficking events that lead to the secretion of triglyceride-rich lipoproteins and/or the intracellular transient storage of lipids as lipid droplets (LDs). LDs originate from the endoplasmic reticulum (ER) membrane and are mainly composed of a triglyceride (TG) and cholesterol-ester core surrounded by a phospholipid and cholesterol monolayer and specific coat proteins. The pivotal role of LDs in cellular lipid homeostasis is clearly established, but processes regulating LD dynamics in enterocytes are poorly understood. Here we show that delivery of alimentary lipid micelles to polarized human enterocytes induces an immediate autophagic response, accompanied by phosphatidylinositol-3-phosphate appearance at the ER membrane. We observe a specific and rapid capture of newly synthesized LD at the ER membrane by nascent autophagosomal structures. By combining pharmacological and genetic approaches, we demonstrate that autophagy is a key player in TG targeting to lysosomes. Our results highlight the yet-unraveled role of autophagy in the regulation of TG distribution, trafficking, and turnover in human enterocytes.

Autophagy in obesity and atherosclerosis: Interrelationships between cholesterol homeostasis, lipoprotein metabolism and autophagy in macrophages and other systems

The incidence of diseases characterized by a dysregulation of lipid metabolism such as obesity, diabetes and atherosclerosis is rising at alarming rates, driving research to uncover new therapies to manage dyslipidemias and resolve the metabolic syndrome conundrum. Autophagy and lipid homeostasis - both ancient cellular pathways - have seemingly co-evolved to share common regulatory elements, and autophagy has emerged as a prominent mechanism involved in the regulation of lipid metabolism. This review highlights recent findings on the role of autophagy in the regulation of cellular cholesterol homeostasis and lipoprotein metabolism, with special emphasis on macrophages. From modulation of inflammation to regulation of cellular cholesterol levels, a protective role for autophagy in atherosclerosis is emerging. The manipulation of autophagic activity represents a new possible therapeutic approach for the treatment complex metabolic disorders such as obesity and the metabolic syndrome.

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.

The opposite effects of high-sucrose and high-fat diet on Fatty Acid oxidation and very low density lipoprotein secretion in rat model of metabolic syndrome

Aims. To determine the effect of two different diets (high-sucrose (HS) and high-fat (HF)) on the main metabolic pathways potentially contributing to the development of steatosis: (1) activity of the liver lysosomal and heparin-releasable lipases; (2) fatty acid (FFA) oxidation; (3) FFA synthesis de novo; (4) VLDL output in vivo in a rat model of metabolic syndrome (MetS), hereditary hypertriglyceridemic (HHTg) rats fed HS or HF diets. Results. Both diets resulted in triacylglycerol (TAG) accumulation in the liver (HF > HS). The intracellular TAG lipolysis by lysosomal lipase was increased in both groups and positively correlated with the liver TAG content. Diet type significantly affected partitioning of intracellular TAG-derived fatty acids among FFA-utilizing metabolic pathways as HS feeding accentuated VLDL secretion and downregulated FFA oxidation while the HF diet had an entirely opposite effect. FFA de novo synthesis from glucose was significantly enhanced in the HS group (fed ≫ fasted) while being completely eradicated in the HF group. Conclusions. We found that in rats prone to the development of MetS associated diseases dietary-induced steatosis is not simply a result of impaired TAG degradation but that it depends on other mechanisms (elevated FFA synthesis or attenuated VLDL secretion) that are specific according to diet composition.

The autophagy-related gene 14 (Atg14) is regulated by forkhead box O transcription factors and circadian rhythms and plays a critical role in hepatic autophagy and lipid metabolism

Autophagy plays a critical role in cell survival from prolonged starvation and recycling of aggregated proteins and damaged organelles. One of the essential genes involved in the autophagic initiation is autophagy-related 14 (Atg14), also called Barkor for Beclin 1-associated autophagy-related key regulator. Although its crucial role in the autophagic process has been reported, the gene regulation of Atg14 and its metabolic functions remain unclear. In this work we have identified that the Atg14 gene is regulated by forkhead box O (FoxO) transcription factors and circadian rhythms in the mouse liver. Luciferase reporter analyses and chromatin immunoprecipitation assays have revealed well conserved cis-elements for FoxOs and Clock/Bmal1 in the proximal promoter of the Atg14 gene. To examine the functions of hepatic Atg14, we have performed the gene knockdown and overexpression in the mouse livers. Remarkably, knockdown of Atg14 leads to elevated levels of triglycerides in the liver and serum as well. Conversely, overexpression of Atg14 improves hypertriglyceridemia in both high fat diet-treated wild-type mice and FoxO1/3/4 liver-specific knock-out mice. In summary, our data suggest that Atg14 is a new target gene of FoxOs and the core clock machinery, and this gene plays an important role in hepatic lipid metabolism.

Involvement of autophagy in ovarian cancer: a working hypothesis

Autophagy is a lysosomal-driven catabolic process that contributes to preserve cell and tissue homeostases through the regular elimination of damaged, aged and redundant self-constituents. In normal cells, autophagy protects from DNA mutation and carcinogenesis by preventive elimination of pro-oxidative mitochondria and protein aggregates. Mutations in oncogenes and oncosuppressor genes dysregulate autophagy. Up-regulated autophagy may confer chemo- and radio-resistance to cancer cells, and also a pro-survival advantage in cancer cells experiencing oxygen and nutrient shortage. This fact is the rationale for using autophagy inhibitors along with anti-neoplastic therapies. Yet, aberrant hyper-induction of autophagy can lead to cell death, and this phenomenon could also be exploited for cancer therapy. The actual level of autophagy in the cancer cell is greatly affected by vascularization, inflammation, and stromal cell infiltration. In addition, small non-coding microRNAs have recently emerged as important epigenetic modulators of autophagy. The present review focuses on the potential involvement of macroautophagy, and on its genetic and epigenetic regulation, in ovarian cancer pathogenesis and progression.