Adipose triglyceride lipase is a major hepatic lipase that regulates triacylglycerol turnover and fatty acid signaling and partitioning

PNPLA2 influences secretion of triglyceride-rich lipoproteins by human hepatoma cells

Patatin-like phospholipase domain-containing proteins (PNPLAs) are involved in triglyceride hydrolysis and lipid-droplet homeostasis in mice, but the physiological significance of the PNPLAs for triglyceride metabolism in human hepatocytes is unclear. Here, we investigate the roles of PNPLA2, PNPLA3, and PNPLA4 in triglyceride metabolism of human Huh7 and HepG2 hepatoma cells using gene-specific inhibition methods. siRNA inhibition of PNPLA3 or PNPLA4 is not associated with changes in triglyceride hydrolysis, secretion of triglyceride-rich lipoproteins (TRLs), or triglyceride accumulation. However, PNPLA2 siRNA inhibition, both in the absence and presence of oleate-containing medium, or treatment with the PNPLA2 inhibitor Atglistatin reduced intracellular triglyceride hydrolysis and decreased TRL secretion. In contrast, PNPLA2 inhibition showed no effects on lipid-droplet homeostasis, which is the primary physiological function of PNPLA2 in nonhepatic tissues. Moreover, confocal microscopy analysis found no clear evidence for the localization of PNPLA2 around lipid droplets. However, significant colocalization of PNPLA2 with the endoplasmic reticulum marker protein disulfide-isomerase was found in HepG2 and Huh7 cells with Rcoloc values of 0.61 ± 0.06 and 0.81 ± 0.05, respectively. In conclusion, PNPLA2 influences TRL secretion, but is not involved in lipid-droplet homeostasis in human hepatoma cells, a physiological role that is quite distinct from the metabolic function of PNPLA2 in nonhepatic tissues.

HCV Pit Stop at the Lipid Droplet: Refuel Lipids and Put on a Lipoprotein Coat before Exit

The replication cycle of the liver-tropic hepatitis C virus (HCV) is tightly connected to the host lipid metabolism, during the virus entry, replication, assembly and egress stages, but also while the virus circulates in the bloodstream. This interplay coins viral particle properties, governs viral cell tropism, and facilitates immune evasion. This review summarizes our knowledge of these interactions focusing on the late steps of the virus replication cycle. It builds on our understanding of the cell biology of lipid droplets and the biosynthesis of liver lipoproteins and attempts to explain how HCV hijacks these organelles and pathways to assemble its lipo-viro-particles. In particular, this review describes (i) the mechanisms of viral protein translocation to and from the lipid droplet surface and the orchestration of an interface between replication and assembly complexes, (ii) the importance of the triglyceride mobilization from the lipid droplets for HCV assembly, (iii) the interplay between HCV and the lipoprotein synthesis pathway including the role played by apolipoproteins in virion assembly, and finally (iv) the consequences of these complex virus–host interactions on the virion composition and its biophysical properties. The wealth of data accumulated in the past years on the role of the lipid metabolism in HCV assembly and its imprint on the virion properties will guide vaccine design efforts and reinforce our understanding of the hepatic lipid metabolism in health and disease.

Naringenin Supplementation to a Chow Diet Enhances Energy Expenditure and Fatty Acid Oxidation, and Reduces Adiposity in Lean, Pair-Fed Ldlr-/- Mice

SCOPE

Naringenin is a citrus-derived flavonoid that has potent lipid-lowering and insulin-sensitizing effects in obese mouse models of metabolic dysfunction. However, in these models, a significant effect of naringenin supplementation is the prevention of weight gain, which in itself can confer metabolic protection. Therefore, in the present study, the effect of naringenin supplementation in lean, chow-fed Ldlr^-/- mice is investigated.

METHODS AND RESULTS

In Ldlr^-/- mice with isocaloric food consumption, treatment with naringenin for 8 weeks reduces body weight and adiposity compared to littermate controls pair-fed the chow diet alone. Furthermore, naringenin treatment reduces plasma lipids and enhances insulin sensitivity compared to chow-fed controls. Metabolic cage studies reveal that naringenin-treated mice have elevated energy expenditure with no change in ambulatory activity. Additionally, naringenin-treated mice have an increased respiratory exchange ratio and food consumption during the dark cycle. Treatment increases the expression of fatty acid oxidation genes in liver, and increased β-hydroxybutyrate concentrations in plasma, indicating that one mechanism through which naringenin mediates metabolic improvement is enhanced hepatic fatty acid oxidation.

CONCLUSIONS

These studies highlight the potential therapeutic utility of naringenin and suggest that this flavonoid maintains potent metabolic properties in the absence of obesity or a high-fat diet.

Polysaccharides derived from natural sources regulate triglyceride and cholesterol metabolism: a review of the mechanisms

This paper presents a comprehensive review of hypolipidemic mechanism of polysaccharides from natural sources.

Elevation of blood lipids in hepatocyte-specific fatty acid transport 4-deficient mice fed with high glucose diets

Fatty acid transport protein4 (FATP4) is upregulated in acquired and central obesity and its polymorphisms are associated with blood lipids and insulin resistance. Patients with FATP4 mutations and mice with global FATP4 deletion exhibit skin abnormalities characterized as ischthyosis prematurity syndrome (IPS). Cumulating data have shown that an absence of FATP4 increases the levels of cellular triglycerides (TG). However, FATP4 role and consequent lipid and TG metabolism in the hepatocyte is still elusive. Here, hepatocyte-specific FATP4 deficient (Fatp4^L-/-) mice were generated. When fed with chow, these mutant mice displayed no phenotypes regarding blood lipids. However when fed low-fat/high-sugar (HS) or high-fat/high-sugar (HFS) for 12 weeks, Fatp4^L-/- mice showed a significant increase of plasma TG, free fatty acids and glycerol when compared with diet-fed control mice. Interestingly, Fatp4^L-/- mice under HS diet had lower body and liver weights and they were not protected from HFS-induced body weight gain and hepatic steatosis. Male mutant mice were more sensitive to HFS diet than female mutant mice. Glucose intolerance was observed only in female Fatp4^L-/- mice fed with HS diet. Lipidomics analyses revealed that hepatic phospholipids were not disturbed in mutant mice under both diets. Thus, hepatic FATP4 deletion rendered an increase of blood lipids including glycerol indicating a preferential fatty-acid channeling to TG pools that are specifically available for lipolysis. Our results imply a possible risk of hyperlipidemia as a result of abnormal metabolism in liver in IPS patients with FATP4 mutations who consume high-sugar diets.

A rotating light cycle promotes weight gain and hepatic lipid storage in mice

Processes involved in regulation of energy balance and intermediary metabolism are aligned to the light-dark cycle. Shift-work and high-fat diet (HFD)-induced obesity disrupt circadian rhythmicity and are associated with increased risk of nonalcoholic fatty liver disease. This study aimed to determine the effect of simulating shift work on hepatic lipid accumulation in lean and HFD mice. C57BL/6 mice fed a standard laboratory diet (SLD) or HFD for 4 wk were further allocated to a normal light (NL) cycle (lights on: 0600-1800) or rotating light (RL) cycle [3 days NL and 4 days reversed (lights on: 1800-0600) repeated] for 8 wk. Tissue was collected every 3 h beginning at 0600. HFD mice gained more weight than SLD mice, and RL mice gained more weight than NL mice. SLD-NL and HFD-NL mice, but not RL mice, were more active, had higher respiratory quotients, and consumed/expended more energy during the dark phase compared with the light phase. Blood glucose and plasma cholesterol and triglyceride concentrations were elevated in HFD and SLD-RL compared with SLD-NL mice. Hepatic glycogen was elevated in HFD compared with SLD mice. Hepatic triglycerides were elevated in SLD-RL and HFD mice compared with SLD-NL. Circadian rhythmicity of hepatic acetyl-CoA carboxylase (ACACA) mRNA was phase shifted in SLD-RL and HFD-NL and lost in HFD-RL mice. Hepatic ACACA protein was reduced in SLD-RL and HFD mice compared with SLD-NL mice. Hepatic adipose triglyceride lipase was elevated in HFD-NL compared with SLD-NL but lower in RL mice compared with NL mice irrespective of diet. In conclusion, an RL cycle model of shift work promotes weight gain and hepatic lipid storage even in lean conditions. NEW & NOTEWORTHY In this publication we describe the effects of a rotating light cycle model of shift work in lean and high-fat diet-induced obese mice on body mass, diurnal patterns of energy intake and expenditure, and hepatic lipid storage. The data indicate that modeling shift work, via a rotating light cycle, promotes weight gain and hepatic lipid accumulation even in mice on a standard laboratory diet.

MicroRNA-124 Regulates Fatty Acid and Triglyceride Homeostasis

MicroRNAs (miRNAs) are part of a complex regulatory network that modulates cellular lipid metabolism. Here, we identify miR-124 as a regulator of triglyceride (TG) metabolism. This study advances our knowledge of the role of miR-124 in human hepatoma cells. Transcriptional profiling of Huh7.5 cells overexpressing miR-124 reveals enrichment for host factors involved in fatty acid oxidation among repressed miRNA targets. In addition, miR-124 down-regulates arylacetamide deacetylase (AADAC) and adipose triglyceride lipase, lipases proposed to mediate breakdown of hepatic TG stores for lipoprotein assembly and mitochondrial β-oxidation. Consistent with the inhibition of TG and fatty acid catabolism, miR-124 expression promotes cellular TG accumulation. Interestingly, miR-124 inhibits the production of hepatitis C virus, a virus that hijacks lipid pathways during its life cycle. Antiviral activity of miR-124 is consistent with repression of AADAC, a pro-viral host factor. Overall, our data highlight miR-124 as a novel regulator of TG metabolism in human hepatoma cells.

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miR-124 regulates triglyceride and fatty acid metabolism

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miR-124 represses genes associated with fatty acid and triglyceride breakdown

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miR-124 promotes triglyceride accumulation in hepatoma cells

Gold nanoparticles as cell regulators: beneficial effects of gold nanoparticles on the metabolic profile of mice with pre-existing obesity

Background

We have previously shown that intraperitoneal injection of gold nanoparticles (AuNPs, 20–30 nm) into mice, decreases high-fat diet (HFD) induced weight gain and glucose intolerance, via suppression of inflammatory responses in both fat and liver tissues. This study investigates whether AuNPs provide similar benefit to mice with pre-existing obesity. Male C57BL/6 mice were fed a HFD for 15 weeks. AuNPs (OB-EAu 0.0785 μg/g/day, OB-LAu 0.785 μg/g/day, OB-HAu7.85 μg/g/day, ip) were administered to subgroups of HFD-fed mice over the last 5 weeks. Control group was fed standard chow and administered vehicle injection.

Results

Only the OB-LAu group demonstrated significant weight loss (12%), while all AuNP treated groups showed improved glycaemic control and reduced blood lipid levels. In the fat tissue, mRNA expression of pro-inflammatory markers were unchanged following AuNP treatment, while glucose and lipid metabolic markers were improved in OB-LAu and OB-HAu mice. In the liver, AuNP treatment downregulated inflammatory markers and improved lipid metabolic markers, with marked effects in OB-EAu and OB-LAu groups.

Conclusions

AuNP treatment can improve glucose and fat metabolism in mice with long-term obesity, however weight loss was only observed in a single specific dose regime. AuNP therapy is a promising new technology for managing metabolic disorders in the obese.

Electronic supplementary material

The online version of this article (10.1186/s12951-018-0414-6) contains supplementary material, which is available to authorized users.

Forcing ATGL expression in hepatocarcinoma cells imposes glycolytic rewiring through PPAR-α/p300-mediated acetylation of p53

Metabolic reprogramming is a typical feature of cancer cells aimed at sustaining high-energetic demand and proliferation rate. Here, we report clear-cut evidence for decreased expression of the adipose triglyceride lipase (ATGL), the first and rate-limiting enzyme of triglyceride hydrolysis, in both human and mouse-induced hepatocellular carcinoma (HCC). We identified metabolic rewiring as major outcome of ATGL overexpression in HCC-derived cell lines. Indeed, ATGL slackened both glucose uptake/utilization and cell proliferation in parallel with increased oxidative metabolism of fatty acids and enhanced mitochondria capacity. We ascribed these ATGL—downstream events to the activity of the tumor-suppressor p53, whose protein levels—but not transcript—were upregulated upon ATGL overexpression. The role of p53 was further assessed by abrogation of the ATGL-mediated effects upon p53 silencing or in p53-null hepatocarcinoma Hep3B cells. Furthermore, we provided insights on the molecular mechanisms governed by ATGL in HCC cells, identifying a new PPAR-α/p300 axis responsible for p53 acetylation/accumulation. Finally, we highlighted that ATGL levels confer different susceptibility of HCC cells to common therapeutic drugs, with ATGL overexpressing cells being more resistant to glycolysis inhibitors (e.g., 2-deoxyglucose and 3-bromopyruvate), compared to genotoxic compounds. Collectively, our data provide evidence for a previously uncovered tumor-suppressor function of ATGL in HCC, with the outlined molecular mechanisms shedding light on new potential targets for anticancer therapy.

Of mice and men: The physiological role of adipose triglyceride lipase (ATGL)

Adipose triglyceride lipase (ATGL) has been discovered 14 years ago and revised our view on intracellular triglyceride (TG) mobilization – a process termed lipolysis. ATGL initiates the hydrolysis of TGs to release fatty acids (FAs) that are crucial energy substrates, precursors for the synthesis of membrane lipids, and ligands of nuclear receptors. Thus, ATGL is a key enzyme in whole-body energy homeostasis. In this review, we give an update on how ATGL is regulated on the transcriptional and post-transcriptional level and how this affects the enzymes' activity in the context of neutral lipid catabolism. In depth, we highlight and discuss the numerous physiological functions of ATGL in lipid and energy metabolism. Over more than a decade, different genetic mouse models lacking or overexpressing ATGL in a cell- or tissue-specific manner have been generated and characterized. Moreover, pharmacological studies became available due to the development of a specific murine ATGL inhibitor (Atglistatin®). The identification of patients with mutations in the human gene encoding ATGL and their disease spectrum has underpinned the importance of ATGL in humans. Together, mouse models and human data have advanced our understanding of the physiological role of ATGL in lipid and energy metabolism in adipose and non-adipose tissues, and of the pathophysiological consequences of ATGL dysfunction in mice and men.

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Summary of mouse models with genetic or pharmacological manipulation of ATGL.

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Summary of patients with mutations in the human gene encoding ATGL.

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In depth discussion of the role of ATGL in numerous physiological processes in mice and men.

Coordination Among Lipid Droplets, Peroxisomes, and Mitochondria Regulates Energy Expenditure Through the CIDE-ATGL-PPARα Pathway in Adipocytes

Metabolic homeostasis is maintained by an interplay among tissues, organs, intracellular organelles, and molecules. Cidea and Cidec are lipid droplet (LD)-associated proteins that promote lipid storage in brown adipose tissue (BAT) and white adipose tissue (WAT). Using ob/ob/Cidea^-/- , ob/ob/Cidec^-/- , and ob/ob/Cidea^-/-/Cidec^-/- mouse models and CIDE-deficient cells, we studied metabolic regulation during severe obesity to identify ways to maintain metabolic homeostasis and promote antiobesity effects. The phenotype of ob/ob/Cidea^-/- mice was similar to that of ob/ob mice in terms of serum parameters, adipose tissues, lipid storage, and gene expression. Typical lipodystrophy accompanied by insulin resistance occurred in ob/ob/Cidec^-/- mice, with ectopic storage of lipids in the BAT and liver. Interestingly, double deficiency of Cidea and Cidec activated both WAT and BAT to consume more energy and to increase insulin sensitivity compared with their behavior in the other three mouse models. Increased lipolysis, which occurred on the LD surfaces and released fatty acids, led to activated β-oxidation and oxidative phosphorylation in peroxisomes and mitochondria in CIDE-deficient adipocytes. The coordination among LDs, peroxisomes, and mitochondria was regulated by adipocyte triglyceride lipase (ATGL)-peroxisome proliferator-activated receptor α (PPARα). Double deficiency of Cidea and Cidec activated energy consumption in both WAT and BAT, which provided new insights into therapeutic approaches for obesity and diabetes.

Ex situ machine perfusion as a tool to recondition steatotic donor livers: Troublesome features of fatty livers and the role of defatting therapies. A systematic review

Long-standing research has shown that increased lipid content in donor livers is associated with inferior graft outcomes posttransplant. The global epidemic that is obesity has increased the prevalence of steatosis in organ donors, to the extent that it has become one of the main reasons for declining livers for transplantation. Consequently, it is one of the major culprits behind the discrepancy between the number of donor livers offered for transplantation and those that go on to be transplanted. Steatotic livers are characterized by poor microcirculation, depleted energy stores because of an impaired capacity for mitochondrial recovery, and a propensity for an exaggerated inflammatory response following reperfusion injury culminating in poorer graft function postoperatively. Ex situ machine perfusion, currently a novel method in graft preservation, is showing great promise in providing a tool for the recovery and reconditioning of marginal livers. Hence, reconditioning these steatotic livers using machine perfusion has the potential to increase the number of liver transplants performed. In this review, we consider the problematic issues associated with fatty livers in the realm of transplantation and discuss pharmacological and nonpharmacological options that are being developed to enhance recovery of these organs using machine perfusion and defatting strategies.

GCN2 deficiency protects against high fat diet induced hepatic steatosis and insulin resistance in mice

Nonalcoholic fatty liver disease (NAFLD) is characterized by hepatic lipid deposition and oxidative stress. It has been demonstrated that general control nonderepressible 2 (GCN2) is required to maintain hepatic fatty acid homeostasis under conditions of amino acid deprivation. However, the impact of GCN2 on the development of NAFLD has not been investigated. In this study, we used Gcn2^-/- mice to investigate the effect of GCN2 on high fat diet (HFD)-induced hepatic steatosis. After HFD feeding for 12 weeks, Gcn2^-/- mice were less obese than wild-type (WT) mice, and Gcn2^-/- significantly attenuated HFD-induced liver dysfunction, hepatic steatosis and insulin resistance. In the livers of the HFD-fed mice, GCN2 deficiency resulted in higher levels of lipolysis genes, lower expression of genes related to FA synthesis, transport and lipogenesis, and less induction of oxidative stress. Furthermore, we found that knockdown of GCN2 attenuated, whereas overexpression of GCN2 exacerbated, palmitic acid-induced steatosis, oxidative & ER stress, and changes of peroxisome proliferator-activated receptor gamma (PPARγ), fatty acid synthase (FAS) and metallothionein (MT) expression in HepG2 cells. Collectively, our data provide evidences that GCN2 deficiency protects against HFD-induced hepatic steatosis by inhibiting lipogenesis and reducing oxidative stress. Our findings suggest that strategies to inhibit GCN2 activity in the liver may provide a novel approach to attenuate NAFLD development.

LncRNA SRA deregulation contributes to the development of atherosclerosis by causing dysfunction of endothelial cells through repressing the expression of adipose triglyceride lipase

It has been well established that long non‑coding RNAs (lncRNAs) are crucial mediators in a diverse range of diseases, including atherosclerosis. The present study aimed to examine the molecular mechanisms underlying the association between steroid receptor RNA activator (SRA) and atherosclerosis. Reverse transcription‑quantitative polymerase chain reaction analysis, western blot analysis and luciferase assays were performed to examine interactions among SRA, adipose triglyceride lipase (ATGL) and peroxisome proliferator‑activated receptor (PPARγ), and the effect of resveratrol (RSV) on the levels of SRA, ATGL and PPARγ. ELISA was performed to determine the effects of SRA and RSV on the production of inflammatory‑associated cytokines. The results showed that knockdown of the expression of SRA by transfecting HUVECs with short hairpin RNA‑SRA inhibited the production of ATGL and PPARγ. A plasmid coding SRA RNA, but not the SRAP protein, attenuated the luciferase activity of the ATGL promoter. PPARγ had no effect on the luciferase activity driven by the ATGL promoter in the absence of rosiglitazone, whereas the luciferase activity of the ATGL promoter was elevated in the presence of rosiglitazone. This effect was eliminated by SRA. SRA enhanced the production of inflammatory‑associated cytokines, including tumor necrosis factor‑α, interleukin‑6, monocyte chemotactic protein‑1 and intercellular adhesion molecule‑1; however, the promoting effect of SRA was eliminated by RSV. RSV increased the expression of ATGL and PPARγ, but not that of SRA. RSV distinctly and concentration‑dependently upregulated the luciferase activity of ATGL, compared with that in the cells without RSV treatment, whereas treating with rosiglitazone inhibited the effect of RSV on the luciferase activity of ATGL. The present study examined the roles of SRA in atherosclerosis, and the effects of changes in SRA and ATGL on inflammatory cytokines and HUVEC dysfunction.

Emerging roles of long non-coding RNAs in tumor metabolism

Compared with normal cells, tumor cells display distinct metabolic characteristics. Long non-coding RNAs (lncRNAs), a large class of regulatory RNA molecules with limited or no protein-coding capacity, play key roles in tumorigenesis and progression. Recent advances have revealed that lncRNAs play a vital role in cell metabolism by regulating the reprogramming of the metabolic pathways in cancer cells. LncRNAs could regulate various metabolic enzymes that integrate cell malignant transformation and metabolic reprogramming. In addition to the known functions of lncRNAs in regulating glycolysis and glucose homeostasis, recent studies also implicate lncRNAs in amino acid and lipid metabolism. These observations reveal the high complexity of the malignant metabolism. Elucidating the metabolic-related functions of lncRNAs will provide a better understanding of the regulatory mechanisms of metabolism and thus may provide insights for the clinical development of cancer diagnostics, prognostics and therapeutics.

Delphinidin Ameliorates Hepatic Triglyceride Accumulation in Human HepG2 Cells, but Not in Diet-Induced Obese Mice

Anthocyanin consumption is linked to benefits in obesity-related metabolic alterations and non-alcoholic fatty liver disease (NAFLD), though the functional role of delphinidin (Dp) is yet to be established. Therefore, this study examined the effects of Dp on metabolic alterations associated with NAFLD, and molecular mechanisms in HepG2 cells and diet-induced obese mice. Cells incubated with palmitate to induce lipid accumulation, concomitantly treated with Dp, reduced triglyceride accumulation by ~53%, and downregulated gene expression of CPT1A, SREBF1, and FASN without modifying AMP-activated protein kinase (AMPK) levels. C57BL/6Nhsd mice were fed a standard diet (control) or a high-fat/high-carbohydrate diet (HFHC) for 16 weeks. Mice in the HFHC group were subdivided and treated with Dp (HFHC-Dp, 15 mg/kg body weight/day) or a vehicle for four weeks. Dp did not affect body weight, energy intake, hyperglycemia, insulin resistance, or histological abnormalities elicited by the HFHC diet. Furthermore, the messenger RNA (mRNA) expressions of Acaca, and Fasn in hepatic or epididymal adipose tissue, and the hepatic sirtuin 1 (SIRT1)/liver kinase B1 (LKB1)/AMPK and proliferator-activated receptor alpha (PPARα) signaling axis did not significantly change due to the HFHC diet or Dp. In summary, Dp effectively reduced triglyceride accumulation in vitro through the modulation of lipid metabolic gene expression. However, a dose of Dp administrated in mice simulating the total daily anthocyanin intake in humans had no effect on either metabolic alterations or histological abnormalities associated with HFHC diets.

Diet-Induced Circadian Enhancer Remodeling Synchronizes Opposing Hepatic Lipid Metabolic Processes

Overnutrition disrupts circadian metabolic rhythms by mechanisms that are not well understood. Here, we show that diet-induced obesity (DIO) causes massive remodeling of circadian enhancer activity in mouse liver, triggering synchronous high-amplitude circadian rhythms of both fatty acid (FA) synthesis and oxidation. SREBP expression was rhythmically induced by DIO, leading to circadian FA synthesis and, surprisingly, FA oxidation (FAO). DIO similarly caused a high-amplitude circadian rhythm of PPARα, which was also required for FAO. Provision of a pharmacological activator of PPARα abrogated the requirement of SREBP for FAO (but not FA synthesis), suggesting that SREBP indirectly controls FAO via production of endogenous PPARα ligands. The high-amplitude rhythm of PPARα imparted time-of-day-dependent responsiveness to lipid-lowering drugs. Thus, acquisition of rhythmicity for non-core clock components PPARα and SREBP1 remodels metabolic gene transcription in response to overnutrition and enables a chronopharmacological approach to metabolic disorders.

Lipid Droplets in Cancer: Guardians of Fat in a Stressful World

Cancer cells possess remarkable abilities to adapt to adverse environmental conditions. Their survival during severe nutrient and oxidative stress depends on their capacity to acquire extracellular lipids and the plasticity of their mechanisms for intracellular lipid synthesis, mobilisation, and recycling. Lipid droplets, cytosolic fat storage organelles present in most cells from yeast to men, are emerging as major regulators of lipid metabolism, trafficking, and signalling in various cells and tissues exposed to stress. Their biogenesis is induced by nutrient and oxidative stress and they accumulate in various cancers. Lipid droplets act as switches that coordinate lipid trafficking and consumption for different purposes in the cell, such as energy production, protection against oxidative stress or membrane biogenesis during rapid cell growth. They sequester toxic lipids, such as fatty acids, cholesterol and ceramides, thereby preventing lipotoxic cell damage and engage in a complex relationship with autophagy. Here, we focus on the emerging mechanisms of stress-induced lipid droplet biogenesis; their roles during nutrient, lipotoxic, and oxidative stress; and the relationship between lipid droplets and autophagy. The recently discovered principles of lipid droplet biology can improve our understanding of the mechanisms that govern cancer cell adaptability and resilience to stress.

Molecular cloning of adipose triglyceride lipase (ATGL) gene from blunt snout bream and its expression after LPS-induced TNF-α factor

The aims of the present study were to clone the full-length cDNA of adipose triglyceridelipase (ATGL) and to analyze its expression after lipopolysaccharide (LPS)-induced tumor necrosis factor alpha (TNF-α). The cDNA obtained covered 1801 bp with an open reading frame of 1500 bp encoding 499 amino acids. Sequence alignment and phylogenetic analysis show the best identity with Cyprinus carpio (86%). The ATGL protein shared a highly conserved 169-amino acid patatin domain, containing a glycine-rich motif, an active serine hydrolase motif, and an aspartic active site. The highest ATGL expression was observed in the liver followed by muscle, whereas relatively low values were detected in the brain and adipose. TNF-α is regarded as an important factor in regulating fat metabolism. Here, LPS was used to induce TNF-α in vivo to verify whether TNF-α can affect ATGL expression. TNF-α expression in liver and muscle is increased and remains unchanged in adipose tissue and brain. The variation of ATGL activity is consistent with that of TNF-α gene expression. Next, we explored the mechanism by which LPS-induced TNF-α mediates the mRNA expression of ATGL in the liver and muscle. For liver, the mRNA levels of c-Jun N-terminal kinase (JNK), nuclear factor kappa B (NF-κB), Sirtuin 1 (SIRT1), and AMP-activated protein kinase (AMPK) were increased by LPS-induced TNF-α. Differencing from the situation in the liver, there was a near-significant decrease trend in the expression of SIRT1 in muscle. Those results indicated that the ATGL gene of blunt snout bream shared a high similarity with the other vertebrates. The expression level of ATGL in tissues with high-fat content was intended to be high. LPS can induce ATGL expression perhaps related to TNF-α.

An effective protocol for pharmacological defatting of primary human hepatocytes which is non-toxic to cholangiocytes or intrahepatic endothelial cells

Introduction

Pharmacological defatting of rat hepatocytes and hepatoma cell lines suggests that the same method could be used to ameliorate macrovesicular steatosis in moderate to severely fatty livers. However there is no data assessing the effects of those drugs on primary human liver cells. We aimed to determine the effectiveness of a pharmacological cocktail in reducing the in vitro lipid content of primary human hepatocytes (PHH). In addition we sought to determine the cytotoxicity of the cocktail towards non-parenchymal liver cells.

Methods

Steatosis was induced in PHH by supplementation with a combination of saturated and unsaturated free fatty acids. This was followed by addition of a defatting drug cocktail for up to 48 hours. The same experimental method was used with human intra-hepatic endothelial cells (HIEC) and human cholangiocytes. MTT assay was used to assess cell viability, triglyceride quantification and oil red O staining were used to determine intracellular lipids content whilst ketone bodies were measured in the supernatants following experimentation.

Results

Incubation of fat loaded PHH with the drugs over 48 hours reduced the intracellular lipid area by 54%, from 12.85% to 5.99% (p = 0.002) (percentage of total oil red O area), and intracellular triglyceride by 35%, from 28.24 to 18.30 nmol/million of cells (p<0.001). Total supernatant ketone bodies increased 1.4-fold over 48 hours in the defatted PHH compared with vehicle controls (p = 0.002). Moreover incubation with the drugs for 48 hours increased the viability of PHH by 11%, cholangiocytes by 25% whilst having no cytotoxic effects on HIEC.

Conclusion

These data demonstrate that pharmacological intervention can significantly decrease intracellular lipid content of PHH, increase fatty acids β-oxidation whilst being non-toxic to PHH, HIEC or cholangiocytes.

Understanding the Impact of Dietary Cholesterol on Chronic Metabolic Diseases through Studies in Rodent Models

The development of certain chronic metabolic diseases has been attributed to elevated levels of dietary cholesterol. However, decades of research in animal models and humans have demonstrated a high complexity with respect to the impact of dietary cholesterol on the progression of these diseases. Thus, recent investigations in non-alcoholic fatty liver disease (NAFLD) point to dietary cholesterol as a key factor for the activation of inflammatory pathways underlying the transition from NAFLD to non-alcoholic steatohepatitis (NASH) and to hepatic carcinoma. Dietary cholesterol was initially thought to be the key factor for cardiovascular disease development, but its impact on the disease depends partly on the capacity to modulate plasmatic circulating low-density lipoprotein (LDL) cholesterol levels. These studies evidence a complex relationship between these chronic metabolic diseases and dietary cholesterol, which, in certain conditions, might promote metabolic complications. In this review, we summarize rodent studies that evaluate the impact of dietary cholesterol on these two prevalent chronic diseases and their relevance to human pathology.

Food Restriction is Required to Preserve the Antisteatotic Effects of Central Leptin in the Liver of Middle-Aged Rats

OBJECTIVE

Aging is a significant risk factor for the development of obesity and hepatic steatosis associated with insulin and leptin resistance. Food restriction (FR) is commonly used for reducing body weight (BW), adiposity, and liver steatosis. Thus, this study aimed to determine whether FR in middle-aged rats can recover the central leptin antisteatotic effects observed in the liver in young animals.

METHODS

Two groups of 4-month-old Wistar rats were fed ad libitum (AL) or were on FR for 3 months. At 7 months of age, both groups were centrally treated with rat leptin (0.2 μg/d, 7 days) or saline.

RESULTS

Central leptin reduced food intake and BW, but not the hepatic triglyceride content, in 7-month-old rats fed AL. However, in 7-month-old FR rats, leptin did not affect BW but markedly reduced serum leptin, serum and hepatic triglyceride levels, and the expression of hepatic lipogenic genes. In addition, central leptin decreased serum and hepatic endogenous norepinephrine levels of FR rats, exerting a homeostatic effect beyond its antisteatotic actions.

CONCLUSIONS

These findings suggest that in middle-aged rats, moderate FR is required for both preserving the antisteatotic actions of central leptin and avoiding excessive weight loss.

The Nuclear Orphan Receptor Nur77 Alleviates Palmitate-induced Fat Accumulation by Down-regulating G0S2 in HepG2 Cells

Excessive triglyceride accumulation in hepatocytes is the hallmark of obesity-associated nonalcoholic fatty liver disease (NAFLD). Elevated levels of the saturated free fatty acid palmitate in obesity are a major contributor to excessive hepatic lipid accumulation. The nuclear orphan receptor Nur77 is a transcriptional regulator and a lipotoxicity sensor. Using human HepG2 hepatoma cells, this study aimed to investigate the functional role of Nur77 in palmitate-induced hepatic steatosis. The results revealed that palmitate significantly induced lipid accumulation and suppressed lipolysis in hepatocytes. In addition, palmitate significantly suppressed Nur77 expression and stimulated the expression of peroxisome proliferator-activated receptor γ (PPARγ) and its target genes. Nur77 overexpression significantly reduced palmitate-induced expression of PPARγ and its target genes. Moreover, Nur77 overexpression attenuated lipid accumulation and augmented lipolysis in palmitate-treated hepatocytes. Importantly, G0S2 knockdown significantly attenuated lipid accumulation and augmented lipolysis in palmitate-treated hepatocytes, whereas G0S2 knockdown had no effect on the palmitate-induced expression of Nur77, PPARγ, or PPARγ target genes. In summary, palmitate suppresses Nur77 expression in HepG2 cells, and Nur77 overexpression alleviates palmitate-induced hepatic fat accumulation by down-regulating G0S2. These results display a novel molecular mechanism linking Nur77-regulated G0S2 expression to palmitate-induced hepatic steatosis.

Deletion of Adipose Triglyceride Lipase Links Triacylglycerol Accumulation to a More-Aggressive Phenotype in A549 Lung Carcinoma Cells

Adipose triglyceride lipase (ATGL) catalyzes the rate limiting step in triacylglycerol breakdown in adipocytes but is expressed in most tissues. The enzyme was shown to be lost in many human tumors, and its loss may play a role in early stages of cancer development. Here, we report that loss of ATGL supports a more-aggressive cancer phenotype in a model system in which ATGL was deleted in A549 lung cancer cells by CRISPR/Cas9. We observed that loss of ATGL led to triacylglycerol accumulation in lipid droplets and higher levels of cellular phospholipid and bioactive lipid species (lyso- and ether-phospholipids). Label-free quantitative proteomics revealed elevated expression of the pro-oncogene SRC kinase in ATGL depleted cells, which was also found on mRNA level and confirmed on protein level by Western blot. Consistently, higher expression of phosphorylated (active) SRC (Y416 phospho-SRC) was observed in ATGL-KO cells. Cells depleted of ATGL migrated faster, which was dependent on SRC kinase activity. We propose that loss of ATGL may thus increase cancer aggressiveness by activation of pro-oncogenic signaling via SRC kinase and increased levels of bioactive lipids.

Hints on ATGL implications in cancer: beyond bioenergetic clues

Among metabolic rearrangements occurring in cancer cells, lipid metabolism alteration has become a hallmark, aimed at sustaining accelerated proliferation. In particular, fatty acids (FAs) are dramatically required by cancer cells as signalling molecules and membrane building blocks, beyond bioenergetics. Along with de novo biosynthesis, free FAs derive from dietary sources or from intracellular lipid droplets, which represent the storage of triacylglycerols (TAGs). Adipose triglyceride lipase (ATGL) is the rate-limiting enzyme of lipolysis, catalysing the first step of intracellular TAGs hydrolysis in several tissues. However, the roles of ATGL in cancer are still neglected though a putative tumour suppressor function of ATGL has been envisaged, as its expression is frequently reduced in different human cancers (e.g., lung, muscle, and pancreas). In this review, we will introduce lipid metabolism focusing on ATGL functions and regulation in normal cell physiology providing also speculative perspectives on potential non-energetic functions of ATGL in cancer. In particular, we will discuss how ATGL is implicated, mainly through the peroxisome proliferator-activated receptor-α (PPAR-α) signalling, in inflammation, redox homoeostasis and autophagy, which are well-known processes deregulated during cancer formation and/or progression.

Pathophysiology of Nonalcoholic Fatty Liver Disease/Nonalcoholic Steatohepatitis

Nonalcoholic fatty liver disease (NAFLD) encompasses a spectrum of liver disorders ranging from hepatic steatosis to nonalcoholic steatohepatitis (NASH) and ultimately may lead to cirrhosis. Hepatic steatosis or fatty liver is defined as increased accumulation of lipids in hepatocytes and results from increased production or reduced clearance of hepatic triglycerides or fatty acids. Fatty liver can progress to NASH in a significant proportion of subjects. NASH is a necroinflammatory liver disease governed by multiple pathways that are not completely elucidated. This review describes the main mechanisms that have been reported to contribute to the pathophysiology of NAFLD and NASH.

Disturbed Vitamin A Metabolism in Non-Alcoholic Fatty Liver Disease (NAFLD)

Vitamin A is required for important physiological processes, including embryogenesis, vision, cell proliferation and differentiation, immune regulation, and glucose and lipid metabolism. Many of vitamin A’s functions are executed through retinoic acids that activate transcriptional networks controlled by retinoic acid receptors (RARs) and retinoid X receptors (RXRs).The liver plays a central role in vitamin A metabolism: (1) it produces bile supporting efficient intestinal absorption of fat-soluble nutrients like vitamin A; (2) it produces retinol binding protein 4 (RBP4) that distributes vitamin A, as retinol, to peripheral tissues; and (3) it harbors the largest body supply of vitamin A, mostly as retinyl esters, in hepatic stellate cells (HSCs). In times of inadequate dietary intake, the liver maintains stable circulating retinol levels of approximately 2 μmol/L, sufficient to provide the body with this vitamin for months. Liver diseases, in particular those leading to fibrosis and cirrhosis, are associated with impaired vitamin A homeostasis and may lead to vitamin A deficiency. Liver injury triggers HSCs to transdifferentiate to myofibroblasts that produce excessive amounts of extracellular matrix, leading to fibrosis. HSCs lose the retinyl ester stores in this process, ultimately leading to vitamin A deficiency. Non-alcoholic fatty liver disease (NAFLD) is the hepatic manifestation of metabolic syndrome and is a spectrum of conditions ranging from benign hepatic steatosis to non-alcoholic steatohepatitis (NASH); it may progress to cirrhosis and liver cancer. NASH is projected to be the main cause of liver failure in the near future. Retinoic acids are key regulators of glucose and lipid metabolism in the liver and adipose tissue, but it is unknown whether impaired vitamin A homeostasis contributes to or suppresses the development of NAFLD. A genetic variant of patatin-like phospholipase domain-containing 3 (PNPLA3-I148M) is the most prominent heritable factor associated with NAFLD. Interestingly, PNPLA3 harbors retinyl ester hydrolase activity and PNPLA3-I148M is associated with low serum retinol level, but enhanced retinyl esters in the liver of NAFLD patients. Low circulating retinol in NAFLD may therefore not reflect true “vitamin A deficiency”, but rather disturbed vitamin A metabolism. Here, we summarize current knowledge about vitamin A metabolism in NAFLD and its putative role in the progression of liver disease, as well as the therapeutic potential of vitamin A metabolites.

Inhibition of intracellular lipolysis promotes human cancer cell adaptation to hypoxia

Tumor tissues are chronically exposed to hypoxia owing to aberrant vascularity. Lipid droplet (LD) accumulation is a hallmark of hypoxic cancer cells, yet how LDs form and function during hypoxia remains poorly understood. Herein, we report that in various cancer cells upon oxygen deprivation, HIF-1 activation down-modulates LD catabolism mediated by adipose triglyceride lipase (ATGL), the key enzyme for intracellular lipolysis. Proteomics and functional analyses identified hypoxia-inducible gene 2 (HIG2), a HIF-1 target, as a new inhibitor of ATGL. Knockout of HIG2 enhanced LD breakdown and fatty acid (FA) oxidation, leading to increased ROS production and apoptosis in hypoxic cancer cells as well as impaired growth of tumor xenografts. All of these effects were reversed by co-ablation of ATGL. Thus, by inhibiting ATGL, HIG2 acts downstream of HIF-1 to sequester FAs in LDs away from the mitochondrial pathways for oxidation and ROS generation, thereby sustaining cancer cell survival in hypoxia.

ATGL Promotes Autophagy/Lipophagy via SIRT1 to Control Hepatic Lipid Droplet Catabolism

Hepatic lipid droplet (LD) catabolism is thought to occur via cytosolic lipases such as adipose triglyceride lipase (ATGL) or through autophagy of LDs, a process known as lipophagy. We tested the potential interplay between these metabolic processes and its effects on hepatic lipid metabolism. We show that hepatic ATGL is both necessary and sufficient to induce both autophagy and lipophagy. Moreover, lipophagy is required for ATGL to promote LD catabolism and the subsequent oxidation of hydrolyzed fatty acids (FAs). Following previous work showing that ATGL promotes sirtuin 1 (SIRT1) activity, studies in liver-specific SIRT1^-/- mice and in primary hepatocytes reveal that SIRT1 is required for ATGL-mediated induction of autophagy and lipophagy. Taken together, these studies show that ATGL-mediated signaling via SIRT1 promotes autophagy/lipophagy as a primary means to control hepatic LD catabolism and FA oxidation.

Triglyceride Metabolism in the Liver

Triglyceride molecules represent the major form of storage and transport of fatty acids within cells and in the plasma. The liver is the central organ for fatty acid metabolism. Fatty acids accrue in liver by hepatocellular uptake from the plasma and by de novo biosynthesis. Fatty acids are eliminated by oxidation within the cell or by secretion into the plasma within triglyceride-rich very low-density lipoproteins. Notwithstanding high fluxes through these pathways, under normal circumstances the liver stores only small amounts of fatty acids as triglycerides. In the setting of overnutrition and obesity, hepatic fatty acid metabolism is altered, commonly leading to the accumulation of triglycerides within hepatocytes, and to a clinical condition known as nonalcoholic fatty liver disease (NAFLD). In this review, we describe the current understanding of fatty acid and triglyceride metabolism in the liver and its regulation in health and disease, identifying potential directions for future research. Advances in understanding the molecular mechanisms underlying the hepatic fat accumulation are critical to the development of targeted therapies for NAFLD. © 2018 American Physiological Society. Compr Physiol 8:1-22, 2018.

Adipose tissue deficiency of hormone-sensitive lipase causes fatty liver in mice

Fatty liver is a major health problem worldwide. People with hereditary deficiency of hormone-sensitive lipase (HSL) are reported to develop fatty liver. In this study, systemic and tissue-specific HSL-deficient mice were used as models to explore the underlying mechanism of this association. We found that systemic HSL deficient mice developed fatty liver in an age-dependent fashion between 3 and 8 months of age. To further explore the mechanism of fatty liver in HSL deficiency, liver-specific HSL knockout mice were created. Surprisingly, liver HSL deficiency did not influence liver fat content, suggesting that fatty liver in HSL deficiency is not liver autonomous. Given the importance of adipose tissue in systemic triglyceride metabolism, we created adipose-specific HSL knockout mice and found that adipose HSL deficiency, to a similar extent as systemic HSL deficiency, causes age-dependent fatty liver in mice. Mechanistic study revealed that deficiency of HSL in adipose tissue caused inflammatory macrophage infiltrates, progressive lipodystrophy, abnormal adipokine secretion and systemic insulin resistance. These changes in adipose tissue were associated with a constellation of changes in liver: low levels of fatty acid oxidation, of very low density lipoprotein secretion and of triglyceride hydrolase activity, each favoring the development of hepatic steatosis. In conclusion, HSL-deficient mice revealed a complex interorgan interaction between adipose tissue and liver: the role of HSL in the liver is minimal but adipose tissue deficiency of HSL can cause age-dependent hepatic steatosis. Adipose tissue is a potential target for treating the hepatic steatosis of HSL deficiency.

Fatty liver is a major complication of obesity and of type 2 diabetes mellitus. It carries a high risk of cirrhosis and liver cancer. In fatty liver, triglycerides accumulate to high levels in the cytoplasm of hepatocytes. Triglycerides are degraded by lipolysis, which has been most studied in fat cells where its three steps are catalyzed by different enzymes. The second step, hydrolysis of diglyceride to a monoglyceride, can be mediated by hormone-sensitive lipase (HSL). Patients with genetic deficiency of HSL have fatty liver. In this study, we found that systemic HSL deficient mice developed fatty liver with aging. To study the mechanism of steatosis, we made liver-specific HSL-deficient mice. Surprisingly, these mice had normal liver fat content. We then studied mice with HSL deficiency in adipose tissue. Adipose HSL-deficient mice developed hepatic steatosis to a similar extent as mice with systemic HSL deficiency, showing that adipose HSL deficiency is sufficient to cause fatty liver. Furthermore, like reported HSL-deficient humans, mice with adipose HSL deficiency had systemic insulin resistance, reduced fat mass and inflammation in fat tissue. Each of these is known to promote hepatic steatosis. Livers of adipose HSL-deficient mice showed low levels of hepatic fatty acid (FA) oxidation, of very low density lipoprotein (VLDL) secretion and of triglycerides (TG) hydrolase activity, each of which could contribute to fat accumulation in liver. Tissue-selective genetic alterations may help in identifying and understanding the tissues responsible for complex metabolic phenotypes like fatty liver. Our data suggest that at least in mice, strategies for treatment of fatty liver related to HSL deficiency should concentrate on adipose tissue.

Peroxisome Proliferator-Activated Receptors Associated with Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is rapidly becoming a major cause of chronic liver disease worldwide. Concurrent to an increase in NAFLD prevalence, there is an increase in the obesity epidemic and the correlated insulin-resistant state. It is a challenge to diagnose NAFLD because many patients are asymptomatic until the later stages of disease. The most common symptoms include fatigue, malaise, and discomfort in the right upper quadrant. The major and most accurate tool to clinically diagnose NAFLD is a liver biopsy, followed by histological analysis. However, this procedure is invasive and often carries a high risk of complications. Currently, there are no officially approved medications for the treatment of NAFLD. Although lifestyle modifications with proper diet and exercise have been shown to be beneficial, this has been difficult to achieve and sustain for many patients. Effective pharmacological treatments are still lacking; therefore, additional research to identify novel drugs is clearly warranted. PPARs are promising drug targets for the management of NAFLD and its related conditions of type 2 diabetes mellitus and cardiovascular disease. In this review, we provide an overview of recent studies on the association of PPARs and NAFLD.

Palmitate induces fat accumulation by activating C/EBPβ-mediated G0S2 expression in HepG2 cells

AIM

To determine the role of G0/G1 switch gene 2 (G0S2) and its transcriptional regulation in palmitate-induced hepatic lipid accumulation.

METHODS

HepG2 cells were treated with palmitate, or palmitate in combination with CCAAT/enhancer binding protein (C/EBP)β siRNA or G0S2 siRNA. The mRNA expression of C/EBPβ, peroxisome proliferator-activated receptor (PPAR)γ and PPARγ target genes (G0S2, GPR81, GPR109A and Adipoq) was examined by qPCR. The protein expression of C/EBPβ, PPARγ, and G0S2 was determined by Western blotting. Lipid accumulation was detected with Oil Red O staining and quantified by absorbance value of the extracted Oil Red O dye. Lipolysis was evaluated by measuring the amount of glycerol released into the medium.

RESULTS

Palmitate caused a dose-dependent increase in lipid accumulation and a dose-dependent decrease in lipolysis in HepG2 cells. In addition, palmitate increased the mRNA expression of C/EBPβ, PPARγ, and PPARγ target genes (G0S2, GPR81, GPR109A, and Adipoq) and the protein expression of C/EBPβ, PPARγ, and G0S2 in a dose-dependent manner. Knockdown of C/EBPβ decreased palmitate-induced PPARγ and its target genes (G0S2, GPR81, GPR109A, and Adipoq) mRNA expression and palmitate-induced PPARγ and G0S2 protein expression in HepG2 cells. Knockdown of C/EBPβ also attenuated lipid accumulation and augmented lipolysis in palmitate-treated HepG2 cells. G0S2 knockdown attenuated lipid accumulation and augmented lipolysis, while G0S2 knockdown had no effects on the mRNA expression of C/EBPβ, PPARγ, and PPARγ target genes (GPR81, GPR109A and Adipoq) in palmitate-treated HepG2 cells.

CONCLUSION

Palmitate can induce lipid accumulation in HepG2 cells by activating C/EBPβ-mediated G0S2 expression.

Grape seed procyanidin extract ameliorates lead-induced liver injury via miRNA153 and AKT/GSK-3β/Fyn-mediated Nrf2 activation

Lead-induced hepatotoxicity is characterized by an extensive oxidative stress. Grape seed procyanidin extract (GSPE) possesses abundant biological activities. Herein, we investigated the protective role of GSPE against lead-induced liver injury and determined the potential molecular mechanisms. In vivo, rats were treated with/without lead acetate (PbAc) (0.05%, w/v) in the presence/absence of GSPE (200 mg/kg). In vitro, hepatocytes were pretreated with/without GSPE (100 μg/ml) in the presence/absence of PbAc (100 μM). PbAc administration to rats resulted in anemia, liver dysfunction, lead accumulation in the bone and liver, oxidative stress, DNA damage and apoptosis. GSPE significantly attenuated these adverse effects, except lead accumulation in liver. GSPE also decreased the expression of miRNA153 and increased the translocation of nuclear factor erythroid 2-related factor 2 (Nrf2) and levels of its downstream protein, and protein kinase B (AKT) phosphorylation in PbAc-induced liver injury. In primary hepatocytes treated with PbAc, GSPE increased hepatocyte viability and decreased lactate dehydrogenase release and reactive oxygen species levels. Dietary GSPE attenuated PbAc-induced liver injury in rats via an integrated mechanism associated with the miRNA153 and AKT/glycogen synthase kinase 3 beta/Fyn-mediated Nrf2 activation.

TM5441, a plasminogen activator inhibitor-1 inhibitor, protects against high fat diet-induced non-alcoholic fatty liver disease

Recent evidences showed that elevation of plasminogen activator inhibitor 1 (PAI-1) was responsible in mediating obesity-induced non-alcoholic fatty liver disease (NAFLD) and metabolic disorders. Here, we investigated the effect of TM5441, an oral PAI-1 inhibitor that lacks of bleeding risk, on high-fat diet (HFD)-induced NAFLD. HFD-fed C57BL/6J mice was daily treated with 20 mg/kg TM5441. To examine the preventive effect, 10-week-treatment was started along with initiation of HFD; alternatively, 4-week-treatment was started in mice with glucose intolerance in the interventional strategy. In vivo study showed that early and delayed treatment decreased hepatic steatosis. Particularly, early treatment prevented the progression of hepatic inflammation and fibrosis in HFD mice. Interestingly, both strategies abrogated hepatic insulin resistance and mitochondrial dysfunction, presented by enhanced p-Akt and p-GSK3β, reduced p-JNK signaling, along with p-AMPK and PGC-1α activation. Consistently, TM5441 treatment in the presence of either PAI-1 exposure or TNF-α stimulated-PAI-1 activity showed a restoration of mitochondrial biogenesis related genes expression on HepG2 cells. Thus, improvement of insulin sensitivity and mitochondrial function was imperative to partially explain the therapeutic effects of TM5441, a novel agent targeting HFD-induced NAFLD.

A unifying mathematical model of lipid droplet metabolism reveals key molecular players in the development of hepatic steatosis

The liver responds to elevated plasma concentrations of free fatty acids (FFAs) with an enhanced uptake of FFAs and their esterification to triacylglycerol (TAG). On the long term, this may result in massive hepatic TAG accumulation called steatosis hepatitis. In hepatocytes, the poor water-soluble TAG is packed in specialized organelles: Lipid droplets (LDs) serving as transient cellular deposit and lipoproteins (LPs) transporting TAG and cholesterol esters to extra-hepatic tissues. The dynamics of these organelles is controlled by a variety of regulatory surface proteins (RSPs). Assembly and export of VLDLs are mainly regulated by the microsomal transfer protein (MTP) and apoprotein B100. Formation and lipolysis of LDs are regulated by several RSPs. The best studied regulators belong to the PAT (Perilipin/Adipophilin/TIP47) and CIDE families. Knockdown or overexpression of SRPs may significantly affect the total number and size distribution of LDs. Intriguingly, a large cell-to-cell heterogeneity with respect to the number and size of LDs has been found in various cell types including hepatocytes. These findings suggest that the extent of cellular lipid accumulation is determined not only by the imbalance between lipid supply and utilization but also by variations in the expression of RSPs and metabolic enzymes. To better understand the relative regulatory impact of individual processes involved in the cellular TAG turnover, we developed a comprehensive kinetic model encompassing the pathways of the fatty acid and triglyceride metabolism and the main molecular processes governing the dynamics of LDs. The model was parametrized such that a large number of experimental in vitro and in vivo findings are correctly recapitulated. A control analysis of the model revealed that variations in the activity of FFA uptake, diacylglycerol acyltransferase (DGAT) 2, and adipose triglyceride lipase (ATGL) have the strongest influence on the cellular TAG level. We used the model to simulate LD size distributions in human hepatoma cells and hepatocytes exposed to a challenge with FFAs. A random fold change by a factor of about two in the activity of RSPs was sufficient to reproduce the large diversity of droplet size distributions observed in individual cells. Under the premise that the same extent of variability of RSPs holds for the intact organ, our model predicts variations in the TAG content of individual hepatocytes by a factor of about 3-6 depending on the nutritional regime. Taken together, our modeling approach integrates numerous experimental findings on individual processes in the cellular TAG metabolism and LD dynamics metabolism to a consistent state-of-the-art dynamic network model that can be used to study how changes in the external conditions or systemic parameters will affect the TAG content of hepatocytes.

Maternal Prenatal Folic Acid Supplementation Programs Offspring Lipid Metabolism by Aberrant DNA Methylation in Hepatic ATGL and Adipose LPL in Rats

The effects of maternal prenatal folic acid supplementation (FAS) on offspring lipid metabolism in adulthood remains unclear, although prenatal FAS is compulsively suggested in many countries. Female Sprague-Dawley rats were fed with control (CON) or FAS diets before and during pregnancy. Male offspring of CON and FAS dams were further divided into two groups at seven weeks for CON and high-fat (HF) diet interventions for eight weeks in adulthood (n = 10). The interactive effects of maternal prenatal FAS and offspring HF in adulthood on lipid metabolism and DNA methylation of genes involved in lipids metabolism were assessed. The male offspring of FAS dams had elevated serum and liver triglyceride level when fed with HF compared to the male offspring of CON dams. The mRNA and protein expression levels of hepatic ATGL and adipose LPL were significantly decreased in offspring of FAS dams than in offspring of CON dams. Furthermore, maternal prenatal FAS resulted in elevated DNA methylation levels in the promoter and first exon region of hepatic ATGL and adipose LPL in offspring. Maternal FAS exacerbated the adverse effects of HF on lipid metabolism in offspring through inducing aberrant DNA methylation levels of hepatic ATGL and adipose LPL.

Acyl-CoA Thioesterase 1 (ACOT1) Regulates PPARα to Couple Fatty Acid Flux With Oxidative Capacity During Fasting

Hepatic acyl-CoA thioesterase 1 (ACOT1) catalyzes the conversion of acyl-CoAs to fatty acids (FAs) and CoA. We sought to determine the role of ACOT1 in hepatic lipid metabolism in C57Bl/6J male mice 1 week after adenovirus-mediated Acot1 knockdown. Acot1 knockdown reduced liver triglyceride (TG) as a result of enhanced TG hydrolysis and subsequent FA oxidation. In vitro experiments demonstrated that Acot1 knockdown led to greater TG turnover and FA oxidation, suggesting that ACOT1 is important for controlling the rate of FA oxidation. Despite increased FA oxidation, Acot1 knockdown reduced the expression of peroxisome proliferator-activated receptor α (PPARα) target genes, whereas overexpression increased PPARα reporter activity, suggesting ACOT1 regulates PPARα by producing FA ligands. Moreover, ACOT1 exhibited partial nuclear localization during fasting and cAMP/cAMP-dependent protein kinase signaling, suggesting local regulation of PPARα. As a consequence of increased FA oxidation and reduced PPARα activity, Acot1 knockdown enhanced hepatic oxidative stress and inflammation. The effects of Acot1 knockdown on PPARα activity, oxidative stress, and inflammation were rescued by supplementation with Wy-14643, a synthetic PPARα ligand. We demonstrate through these results that ACOT1 regulates fasting hepatic FA metabolism by balancing oxidative flux and capacity.

Liver-specific G0 /G1 switch gene 2 (G0s2) expression promotes hepatic insulin resistance by exacerbating hepatic steatosis in male Wistar rats

BACKGROUND

Hepatic steatosis is strongly associated with insulin resistance. It has been reported that G₀ /G₁ switch gene 2 (G0s2) inhibits the lipolytic activity of adipose triglyceride lipase, which is a major lipase in the liver as well as in adipocytes. Moreover, G0s2 protein content is increased in the livers of high-fat diet (HFD)-fed rats. In the present study, we investigated the effect of hepatic G0s2 on insulin sensitivity in male Wistar rats.

METHODS

Male Wistar rats were fed a 60% HFD for 4 weeks. After 3 weeks of feeding, rats were injected with adenovirus-expressing green fluorescent protein (Ad-GFP; control) or adenovirus-expressing mouse G0s2 (Ad-G0s2). On Day 7 after injection, a euglycemic-hyperinsulinemic clamp study was performed in rats fasted for 8 h.

RESULTS

Body weight and fasting glucose levels were not significantly different between the Ad-GFP and Ad-G0s2 groups. During the clamp study, the glucose infusion rate required for euglycemia decreased significantly by 16% in the Ad-G0s2 compared with Ad-GFP group. The insulin-suppressed hepatic glucose output increased significantly in the Ad-G0s2 group, but the insulin-stimulated glucose disposal rate was not significantly different between the two groups. Consistent with the clamp data, insulin-stimulated phosphorylation of Akt decreased significantly in livers of rats injected with Ad-G0s2. Furthermore, Oil Red O-staining indicated that overexpression of G0s2 protein in the liver promoted hepatic steatosis by 2.5-fold in HFD-fed rats.

CONCLUSION

The results of the present study indicate that hepatic G0s2 protein may promote hepatic insulin resistance by exacerbating hepatic steatosis.

Lipolysis and lipases in white adipose tissue - An update

Lipolysis is defined as the sequential hydrolysis of triacylglycerol (TAG) stored in cell lipid droplets. For many years, it was believed that hormone-sensitive lipase (HSL) and monoacylglycerol lipase (MGL) were the main enzymes catalyzing lipolysis in the white adipose tissue. Since the discovery of adipose triglyceride lipase (ATGL) in 2004, many studies were performed to investigate and characterize the actions of this lipase, as well as of other proteins and possible regulatory mechanisms involved, which reformulated the concept of lipolysis. Novel findings from these studies include the identification of lipolytic products as signaling molecules regulating important metabolic processes in many non-adipose tissues, unveiling a previously underestimated aspect of lipolysis. Thus, we present here an updated review of concepts and regulation of white adipocyte lipolysis with a special emphasis in its role in metabolism homeostasis and as a source of important signaling molecules.