Hepatic ATGL knockdown uncouples glucose intolerance from liver TAG accumulation

Acute sleep deprivation-induced hepatotoxicity and dyslipidemia in middle-aged female rats and its amelioration by butanol extract of Tinospora cordifolia

BACKGROUND

Sleep deprivation (SD) due to an unhealthy lifestyle poses an oxidative challenge and is closely associated with an increased risk and prevalence of different metabolic disorders. Although the negative consequences of SD are well reported on mental health little is known about its detrimental effects on liver function and lipid metabolism. Tinospora cordifolia is reported for its hepatoprotective activity in different pre-clinical model systems. The current study was designed to elucidate the cumulative effects of aging and acute SD on liver functions, oxidative stress, and lipid metabolism, and their management by butanol extract of T. cordifolia (B-TCE) using middle-aged female acyclic rats as the model system.

RESULTS

Rats were divided into 4 groups: (1) Vehicle-undisturbed (VUD) (2) Vehicle-sleep deprived (VSD) (3) B-TCE pre-treated sleep-deprived (TSD) (4) B-TCE pre-treated undisturbed sleep (TUD). TSD and TUD groups were given 35 mg/kg of B-TCE once daily for 15 days followed by 12 h of sleep deprivation (6 a.m.-6 p.m.) of VSD and TSD group animals using the gentle-handling method while VUD and TUD group animals were left undisturbed. SD of VSD group animals increased oxidative stress, liver function disruption, and dyslipidemia which were ameliorated by B-TCE pre-treatment. Further, B-TCE was observed to target AMPK and its downstream lipid metabolism pathways as well as the p-Akt/cyclinD1/p-bad pathway of cell survival as possible underlying mechanisms of its hepatoprotective activity.

CONCLUSIONS

These findings suggest that B-TCE being a multi-component extract may be a potential agent in curtailing sleep-related problems and preventing SD-associated hepatotoxicity and dyslipidemia in postmenopausal women.

Renal tubule-specific Atgl deletion links kidney lipid metabolism to glucagon-like peptide 1 and insulin secretion independent of renal inflammation or lipotoxicity

OBJECTIVE

Lipotoxic injury from renal lipid accumulation in obesity and type 2 diabetes (T2D) is implicated in associated kidney damage. However, models examining effects of renal ectopic lipid accumulation independent of obesity or T2D are lacking. We generated renal tubule-specific adipose triglyceride lipase knockout (RT-SAKO) mice to determine if this targeted triacylglycerol (TAG) over-storage affects glycemic control and kidney health.

METHODS

Male and female RT-SAKO mice and their control littermates were tested for changes in glycemic control at 10-12 and 16-18 weeks of age. Markers of kidney health and blood lipid and hormone concentrations were analyzed. Kidney and blood lysophosphatidic acid (LPA) levels were measured, and a role for LPA in mediating impaired glycemic control was evaluated using the LPA receptor 1/3 inhibitor Ki-16425.

RESULTS

All groups remained insulin sensitive, but 16- to 18-week-old male RT-SAKO mice became glucose intolerant, without developing kidney inflammation or fibrosis. Rather, these mice displayed lower circulating insulin and glucagon-like peptide 1 (GLP-1) levels. Impaired first-phase glucose-stimulated insulin secretion was detected and restored by Exendin-4. Kidney and blood LPA levels were elevated in older male but not female RT-SAKO mice, associated with increased kidney diacylglycerol kinase epsilon. Inhibition of LPA-mediated signaling restored serum GLP-1 levels, first-phase insulin secretion, and glucose tolerance.

CONCLUSIONS

TAG over-storage alone is insufficient to cause renal tubule lipotoxicity. This work is the first to show that endogenously derived LPA modulates GLP-1 levels in vivo, demonstrating a new mechanism of kidney-gut-pancreas crosstalk to regulate insulin secretion and glucose homeostasis.

Ablation of diacylglycerol kinase ε promotes whitening of brown adipose tissue under high fat diet feeding

Adipose tissue (AT) comprises distinct fat depots such as white AT and brown AT. White and brown adipocytes exhibit different morphological and physiological properties. White adipocytes containing large single lipid droplet (LD) provide energy on demand whereas brown adipocytes loaded with multilocular LDs consume energy to generate heat or dissipate excess energy. Recent studies have shown that multilocular brown-like cells emerge in white AT under certain conditions. These cells termed beige adipocytes participate in energy expenditure and heat generation. In the process of lipolysis, TG is broken down into free fatty acid and diacylglycerol (DG). In this regard, DG also serves as a signaling molecule activating some proteins such as protein kinase C. Therefore, DG kinase (DGK), an enzyme which phosphorylates DG into phosphatidic acid (PA), plays a pivotal role in integrating energy homeostasis and intracellular signaling. Recently, we described that DGKε-KO mice exhibit increased adiposity in visceral white AT accompanied with impaired glucose tolerance early (40 days) in the course of high fat diet (HFD) feeding, although these mice exhibit "browning or beiging" in visceral white AT associated with improved glucose tolerance after longer term HFD feeding (180 days). This study was conducted to understand the overall features of adipose tissues and investigate changes in subcutaneous (inguinal) white AT and interscapular brown AT of DGKε-KO mice during the course of HFD feeding. Results demonstrated that fat accumulation is promoted in all fat depots under 40 days of HFD feeding conditions. Remarkably, "whitening" of brown adipocytes was identified in DGKε-deficient brown AT during the course of HFD feeding, suggesting brown adipocyte dysfunction. In addition, insulin levels were considerably elevated in DGKε-KO mice under 180 days of HFD feeding conditions. Collectively, these findings suggest that brown adipocytes are dysfunctional in DGKε-KO mice, which promotes browning or beiging in visceral white AT. Beige adipocytes may take over energy disposal and contribute to improving glucose tolerance with the aid of high levels of insulin in DGKε-KO mice upon excess feeding.

Altered hepatic lipid droplet morphology and lipid metabolism in fasted Plin2-null mice

Perilipin 2 (Plin2) binds to the surface of hepatic lipid droplets (LDs) with expression levels that correlate with triacylglyceride (TAG) content. We investigated if Plin2 is important for hepatic LD storage in fasted or high-fat diet-induced obese Plin2+/+ and Plin2-/- mice. Plin2-/- mice had comparable body weights, metabolic phenotype, glucose tolerance, and circulating TAG and total cholesterol levels compared with Plin2+/+ mice, regardless of the dietary regime. Both fasted and high-fat fed Plin2-/- mice stored reduced levels of hepatic TAG compared with Plin2+/+ mice. Fasted Plin2-/- mice stored fewer but larger hepatic LDs compared with Plin2+/+ mice. Detailed hepatic lipid analysis showed substantial reductions in accumulated TAG species in fasted Plin2-/- mice compared with Plin2+/+ mice, whereas cholesteryl esters and phosphatidylcholines were increased. RNA-Seq revealed minor differences in hepatic gene expression between fed Plin2+/+ and Plin2-/- mice, in contrast to marked differences in gene expression between fasted Plin2+/+ and Plin2-/- mice. Our findings demonstrate that Plin2 is required to regulate hepatic LD size and storage of neutral lipid species in the fasted state, while its role in obesity-induced steatosis is less clear.

ACOT1 deficiency attenuates high-fat diet-induced fat mass gain by increasing energy expenditure

Acyl-CoA thioesterase 1 (ACOT1) catalyzes the hydrolysis of long-chain acyl-CoAs to free fatty acids and CoA and is typically upregulated in obesity. Whether targeting ACOT1 in the setting of high-fat diet-induced (HFD-induced) obesity would be metabolically beneficial is not known. Here we report that male and female ACOT1KO mice are partially protected from HFD-induced obesity, an effect associated with increased energy expenditure without alterations in physical activity or food intake. In males, ACOT1 deficiency increased mitochondrial uncoupling protein-2 (UCP2) protein abundance while reducing 4-hydroxynonenal, a marker of oxidative stress, in white adipose tissue and liver of HFD-fed mice. Moreover, concurrent knockdown (KD) of UCP2 with ACOT1 in hepatocytes prevented increases in oxygen consumption observed with ACOT1 KD during high lipid loading, suggesting that UCP2-induced uncoupling may increase energy expenditure to attenuate weight gain. Together, these data indicate that targeting ACOT1 may be effective for obesity prevention during caloric excess by increasing energy expenditure.

NAFLD: Mechanisms, Treatments, and Biomarkers

Nonalcoholic fatty liver disease (NAFLD), recently renamed metabolic-associated fatty liver disease (MAFLD), is one of the most common causes of liver diseases worldwide. NAFLD is growing in parallel with the obesity epidemic. No pharmacological treatment is available to treat NAFLD, specifically. The reason might be that NAFLD is a multi-factorial disease with an incomplete understanding of the mechanisms involved, an absence of accurate and inexpensive imaging tools, and lack of adequate non-invasive biomarkers. NAFLD consists of the accumulation of excess lipids in the liver, causing lipotoxicity that might progress to metabolic-associated steatohepatitis (NASH), liver fibrosis, and hepatocellular carcinoma. The mechanisms for the pathogenesis of NAFLD, current interventions in the management of the disease, and the role of sirtuins as potential targets for treatment are discussed here. In addition, the current diagnostic tools, and the role of non-coding RNAs as emerging diagnostic biomarkers are summarized. The availability of non-invasive biomarkers, and accurate and inexpensive non-invasive diagnosis tools are crucial in the detection of the early signs in the progression of NAFLD. This will expedite clinical trials and the validation of the emerging therapeutic treatments.

Adipose Triglyceride Lipase in Hepatic Physiology and Pathophysiology

The liver is extremely active in oxidizing triglycerides (TG) for energy production. An imbalance between TG synthesis and hydrolysis leads to metabolic disorders in the liver, including excessive lipid accumulation, oxidative stress, and ultimately liver damage. Adipose triglyceride lipase (ATGL) is the rate-limiting enzyme that catalyzes the first step of TG breakdown to glycerol and fatty acids. Although its role in controlling lipid homeostasis has been relatively well-studied in the adipose tissue, heart, and skeletal muscle, it remains largely unknown how and to what extent ATGL is regulated in the liver, responds to stimuli and regulators, and mediates disease progression. Therefore, in this review, we describe the current understanding of the structure–function relationship of ATGL, the molecular mechanisms of ATGL regulation at translational and post-translational levels, and—most importantly—its role in lipid and glucose homeostasis in health and disease with a focus on the liver. Advances in understanding the molecular mechanisms underlying hepatic lipid accumulation are crucial to the development of targeted therapies for treating hepatic metabolic disorders.

Microtubule affinity regulating kinase 4: A promising target in the pathogenesis of atherosclerosis

Microtubule affinity regulating kinase 4 (MARK4), an important member of the serine/threonine kinase family, regulates the phosphorylation of microtubule-associated proteins and thus modulates microtubule dynamics. In human atherosclerotic lesions, the expression of MARK4 is significantly increased. Recently, accumulating evidence suggests that MARK4 exerts a proatherogenic effect via regulation of lipid metabolism (cholesterol, fatty acid, and triglyceride), inflammation, cell cycle progression and proliferation, insulin signaling, and glucose homeostasis, white adipocyte browning, and oxidative stress. In this review, we summarize the latest findings regarding the role of MARK4 in the pathogenesis of atherosclerosis to provide a rationale for future investigation and therapeutic intervention.

Adipose triglyceride lipase promotes the proliferation of colorectal cancer cells via enhancing the lipolytic pathway

Abnormal lipid metabolism is the sign of tumour cells. Previous researches have revealed that the lipolytic pathway may contribute to the progression of colorectal cancer (CRC). However, adipose triglyceride lipase (ATGL) role in CRC cells remains unclear. Here, we find that elevated ATGL positively correlates with CRC clinical stages and negatively associates with overall survival. Overexpression of ATGL significantly promotes CRC cell proliferation, while knockdown of ATGL inhibits the proliferation and promotes the apoptosis of CRC cells in vitro. Moreover, in vivo experiments, ATGL promotes the growth of CRC cells. Mechanistically, ATGL enhances the carcinogenic function of CRC cells via promoting sphingolipid metabolism and CoA biosynthesis pathway‐related gene levels by degrading triglycerides, which provides adequate nutrition for the progression of CRC. Our researches clarify for the first time that ATGL is a novel oncogene in CRC and may provide an important prognostic factor and therapeutic target for CRC.

The diversity and breadth of cancer cell fatty acid metabolism

Tumor cellular metabolism exhibits distinguishing features that collectively enhance biomass synthesis while maintaining redox balance and cellular homeostasis. These attributes reflect the complex interactions between cell-intrinsic factors such as genomic-transcriptomic regulation and cell-extrinsic influences, including growth factor and nutrient availability. Alongside glucose and amino acid metabolism, fatty acid metabolism supports tumorigenesis and disease progression through a range of processes including membrane biosynthesis, energy storage and production, and generation of signaling intermediates. Here, we highlight the complexity of cellular fatty acid metabolism in cancer, the various inputs and outputs of the intracellular free fatty acid pool, and the numerous ways that these pathways influence disease behavior.

Hypoxia, hypoxia-inducible gene 2 (HIG2)/HILPDA, and intracellular lipolysis in cancer

Tumor tissues are chronically exposed to hypoxia owing to aberrant vascularity. Hypoxia induces metabolic alterations in cancer, thereby promoting aggressive malignancy and metastasis. While previous efforts largely focused on adaptive responses in glucose and glutamine metabolism, recent studies have begun to yield important insight into the hypoxic regulation of lipid metabolic reprogramming in cancer. Emerging evidence points to lipid droplet (LD) accumulation as a hallmark of hypoxic cancer cells. One critical underlying mechanism involves the inhibition of adipose triglyceride lipase (ATGL)-mediated intracellular lipolysis by a small protein encoded by hypoxia-inducible gene 2 (HIG2), also known as hypoxia inducible lipid droplet associated (HILPDA). In this review we summarize and discuss recent key findings on hypoxia-dependent regulation of metabolic adaptations especially lipolysis in cancer. We also pose several questions and hypotheses pertaining to the metabolic impact of lipolytic regulation in cancer under hypoxia and during hypoxia-reoxygenation transition.

The Role of Metabolic Lipases in the Pathogenesis and Management of Liver Disease

Intracellular lipolysis is an enzymatic pathway responsible for the catabolism of triglycerides (TGs) that is complemented by lipophagy as the autophagic breakdown of lipid droplets. The hydrolytic cleavage of TGs generates free fatty acids (FFAs), which can serve as energy substrates, precursors for lipid synthesis, and mediators in cell signaling. Despite the fundamental and physiological importance of FFAs, an oversupply can trigger lipotoxicity with impaired membrane function, endoplasmic reticulum stress, mitochondrial dysfunction, cell death, and inflammation. Conversely, impaired release of FFAs and other lipid mediators can also disrupt key cellular signaling functions that regulate metabolism and inflammatory processes. This review will focus on specific functions of intracellular lipases in lipid partitioning, covering basic and translational findings in the context of liver disease. In addition, the clinical relevance of genetic mutations in human disease and potential therapeutic opportunities will be discussed.

Adipose triglyceride lipase activity regulates cancer cell proliferation via AMP-kinase and mTOR signaling

Aberrant fatty acid (FA) metabolism is a hallmark of proliferating cells, including untransformed fibroblasts or cancer cells. Lipolysis of intracellular triglyceride (TG) stores by adipose triglyceride lipase (ATGL) provides an important source of FAs serving as energy substrates, signaling molecules, and precursors for membrane lipids. To investigate if ATGL-mediated lipolysis impacts cell proliferation, we modified ATGL activity in murine embryonic fibroblasts (MEFs) and in five different cancer cell lines to determine the consequences on cell growth and metabolism. Genetic or pharmacological inhibition of ATGL in MEFs causes impaired FA oxidation, decreased ROS production, and a substrate switch from FA to glucose leading to decreased AMPK-mTOR signaling and higher cell proliferation rates. ATGL expression in these cancer cells is low when compared to MEFs. Additional ATGL knockdown in cancer cells did not significantly affect cellular lipid metabolism or cell proliferation whereas the ectopic overexpression of ATGL increased lipolysis and reduced proliferation. In contrast to ATGL silencing, pharmacological inhibition of ATGL by Atglistatin© impeded the proliferation of diverse cancer cell lines, which points at an ATGL-independent effect. Our data indicate a crucial role of ATGL-mediated lipolysis in the regulation of cell proliferation. The observed low ATGL activity in cancer cells may represent an evolutionary selection process and mechanism to sustain high cell proliferation rates. As the increasing ATGL activity decelerates proliferation of five different cancer cell lines this may represent a novel therapeutic strategy to counteract uncontrolled cell growth.

Mice lacking DGKε show increased beige adipogenesis in visceral white adipose tissue after long-term high fat diet in a COX-2- dependent manner

Adipose tissue is a central site for energy storage in the form of triglyceride (TG). Under excess energy conditions, TG is synthesized by acylation of diacylglycerol (DG), whereas TG is broken down into DG and free fatty acid, which provide energy for mitochondrial lipid oxidation when needed. In this regard, DG is not merely an intermediate metabolite for TG metabolism; it also serves as a signaling molecule. DG kinase (DGK) phosphorylates DG to produce phosphatidic acid (PA). Consequently, DGK plays a pivotal role in the control of lipid metabolism and signal transduction pathway. Recently, a report has described that DGKε-knockout (KO) mice show expansion of epididymal white adipose tissue (WAT) together with the impairment of glucose clearance after short-term (40 days) high fat diet (HFD) feeding, an early presymptomatic phase of obesity in wild-type animals. Nevertheless, no report describes an investigation of their phenotype under long-term HFD feeding conditions. Remarkably, results obtained during long-term HFD feeding show that WAT mass is decreased significantly and that the blood glucose profile in response to glucose challenge is improved in DGKε-KO mice compared with wild-type, which contrast sharply against the phenotype shown for short-term HFD feeding. Morphological examination reveals that cyclooxygenase-2 (COX-2) expression and clusters of uncoupling protein 1 (UCP1)-positive multilocular brown-like ("beige") adipocyte are induced in DGKε-deficient WAT after long-term HFD feeding, suggesting that beige adipocytes facilitate energy expenditure during prolonged HFD feeding. Administration of celecoxib, a selective inhibitor of COX-2, abolishes the appearance of UCP1-positive beige adipocytes in DGKε-KO mice. These findings suggest that DGKε deficiency promotes visceral WAT remodeling in a COX-2-dependent manner under long-term HFD feeding conditions.

PEDF promotes nuclear degradation of ATGL through COP1

Adipose triglyceride lipase (ATGL) plays a compelling role in hepatic lipid turnover and in the pathophysiology of non-alcoholic fatty liver disease. Hepatic ATGL is post-transcriptionally regulated by E3 ubiquitin ligase constitutive photomorphogenic1 (COP1) through polyubiquitylation and proteasomal degradation. However the physiological cue for COP1-mediated hepatocellular degradation of ATGL remained unknown. Here we checked for the role of pigment epithelium-derived factor (PEDF), a moonlighting hepatokine and the so-called ligand of ATGL for its stability in hepatocytes. We show that PEDF diminishes ATGL protein stability by promoting its proteasomal degradation in COP1-dependent manner. Despite being a secretory glycoprotein, PEDF is also sequestered in the nuclear compartment so as COP1. Interestingly, PEDF enhances nuclear import of predominantly cytosolic ATGL protein for its subsequent proteasomal degradation in the nucleus. PEDF also controls cell autonomous hepatocyte lipid accumulation and mobilization through COP1-ATGL axis, thereby unraveling a novel pathway for hepatic lipid metabolism.

Association of the PNPLA2, SCD1 and Leptin Expression with Fat Distribution in Liver and Adipose Tissue From Obese Subjects

The expansion of adipose tissue is regulated by insulin and leptin through sterol regulatory element-binding protein-1c (SREBP-1c), up-regulating lipogenesis in tissues by Stearoylcoenzyme A desaturase 1 (SCD1) enzyme, while adipose triglyceride lipase (ATGL) enzyme is key in lipolysis. The research objective was to evaluate the expression of Sterol Regulatory Element Binding Transcription Factor 1 (SREBF1), SCD1, Patatin Like Phospholipase Domain Containing 2 (PNPLA2), and leptin (LEP) genes in hepatic-adipose tissue, and related them with the increment and distribution of fat depots of individuals without insulin resistance. Thirty-eight subjects undergoing elective cholecystectomy with liver and adipose tissue biopsies (subcutaneous-omental) are included. Tissue gene expression was assessed by qPCR and biochemical parameters determined. Individuals are classified according to the body mass index, classified as lean (control group, n=12), overweight (n=11) and obesity (n=15). Abdominal adiposity was determined by anthropometric and histopathological study of the liver. Increased SCD1 expression in omental adipose tissue (p=0.005) and PNPLA2 in liver (p=0.01) were found in the obesity group. PNPLA2 decreased expression in subcutaneous adipose tissue was significant in individuals with abdominal adiposity (p=0.017). Anthropometric parameters positively correlated with liver PNPLA2 and the expression of liver PNPLA2 with serum leptin. SCD1 increased levels may represent lipid storage activity in omental adipose tissue. Liver PNPLA2 increased expression could function as a primary compensatory event of visceral fat deposits associated to the leptin hormone related to the increase of adipose tissue.

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.

Mechanisms of Insulin Action and Insulin Resistance

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have matured, coalescing into solid and fertile ground for clinical application; others remain incompletely investigated and scientifically controversial. Here, we attempt to synthesize this work to guide further mechanistic investigation and to inform the development of novel therapies for type 2 diabetes (T2D). The rational development of such therapies necessitates detailed knowledge of one of the key pathophysiological processes involved in T2D: insulin resistance. Understanding insulin resistance, in turn, requires knowledge of normal insulin action. In this review, both the physiology of insulin action and the pathophysiology of insulin resistance are described, focusing on three key insulin target tissues: skeletal muscle, liver, and white adipose tissue. We aim to develop an integrated physiological perspective, placing the intricate signaling effectors that carry out the cell-autonomous response to insulin in the context of the tissue-specific functions that generate the coordinated organismal response. First, in section II, the effectors and effects of direct, cell-autonomous insulin action in muscle, liver, and white adipose tissue are reviewed, beginning at the insulin receptor and working downstream. Section III considers the critical and underappreciated role of tissue crosstalk in whole body insulin action, especially the essential interaction between adipose lipolysis and hepatic gluconeogenesis. The pathophysiology of insulin resistance is then described in section IV. Special attention is given to which signaling pathways and functions become insulin resistant in the setting of chronic overnutrition, and an alternative explanation for the phenomenon of ‟selective hepatic insulin resistanceˮ is presented. Sections V, VI, and VII critically examine the evidence for and against several putative mediators of insulin resistance. Section V reviews work linking the bioactive lipids diacylglycerol, ceramide, and acylcarnitine to insulin resistance; section VI considers the impact of nutrient stresses in the endoplasmic reticulum and mitochondria on insulin resistance; and section VII discusses non-cell autonomous factors proposed to induce insulin resistance, including inflammatory mediators, branched-chain amino acids, adipokines, and hepatokines. Finally, in section VIII, we propose an integrated model of insulin resistance that links these mediators to final common pathways of metabolite-driven gluconeogenesis and ectopic lipid accumulation.

Fibroblast growth factor 21 induces lipolysis more efficiently than it suppresses lipogenesis in goat adipocytes

Fibroblast growth factor 21 (FGF21) potentially regulates glucose and lipid metabolism in energy homeostasis. We investigated dynamic changes in goat adipocytes treated with 75 nM FGF21 for 24, 36 and 48 h. Compared to controls, FGF21-treated adipocytes displayed smaller lipid droplets and altered levels of the mRNA transcripts encoding several lipolysis genes. The genes with elevated mRNA levels included: ATGL, HSL, CPT-1, and UCP1, and this was observed mainly at 24 and 36 h (P < 0.05). Some gene expression was attenuated including lipogenesis genes, such as SREBP1, PPARγ, C/EBPα, and ACC. This attenuation was observed mainly at 24 h (P < 0.05). Among the genes that were significantly induced or inhibited, ATGL, PGC1α, and C/EBPα were observed a significant effect at 48 h (P < 0.05). In addition, FGF21 treatment greatly increased number of mitochondria and the expression of genes implicated in mitochondrial biogenesis, such as PGC1α, NRF1, and TFAM. These results suggest that FGF21 treatment induced lipolysis more effectively than it suppressed lipogenesis in goat adipocytes, and that mitochondrial biogenesis plays an important role in these cells.

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.

Regulation of Lipolysis in Adipose Tissue and Clinical Significance

Lipolysis is a critical process to hydrolyze triglyceride in adipose tissue, thereby breaking down the stored lipid and maintaining energy homeostasis. Recent studies have made significant progress in understanding the steps of lipolysis. This chapter discusses the major pathways that regulate lipolysis in adipose tissue. Specifically we focus on the mechanisms by which the activities of critical lipolytic enzymes are regulated. We further discuss how the lipolysis is regulated by other factors, including insulin and neurotransmitters, in particular catecholamines and the role of sympathetic nervous system in the whole process. Finally we provide clinical perspectives about the novel therapeutic strategies to target or promote adipose tissue lipolysis for treatment/prevention of obesity and type 2 diabetes.

β-Adrenergic induction of lipolysis in hepatocytes is inhibited by ethanol exposure

In liver steatosis (i.e. fatty liver), hepatocytes accumulate many large neutral lipid storage organelles known as lipid droplets (LDs). LDs are important in the maintenance of energy homeostasis, but the signaling mechanisms that stimulate LD metabolism in hepatocytes are poorly defined. In adipocytes, catecholamines target the β-adrenergic (β-AR)/cAMP pathway to activate cytosolic lipases and induce their recruitment to the LD surface. Therefore, the goal of this study was to determine whether hepatocytes, like adipocytes, also undergo cAMP-mediated lipolysis in response to β-AR stimulation. Using primary rat hepatocytes and human hepatoma cells, we found that treatment with the β-AR agent isoproterenol caused substantial LD loss via activation of cytosolic lipases adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL). β-Adrenergic stimulation rapidly activated PKA, which led to the phosphorylation of ATGL and HSL and their recruitment to the LD surface. To test whether this β-AR-dependent lipolysis pathway was altered in a model of alcoholic fatty liver, primary hepatocytes from rats fed a 6-week EtOH-containing Lieber-DeCarli diet were treated with cAMP agonists. Compared with controls, EtOH-exposed hepatocytes showed a drastic inhibition in β-AR/cAMP-induced LD breakdown and the phosphorylation of PKA substrates, including HSL. This observation was supported in VA-13 cells, an EtOH-metabolizing human hepatoma cell line, which displayed marked defects in both PKA activation and isoproterenol-induced ATGL translocation to the LD periphery. In summary, these findings suggest that β-AR stimulation mobilizes cytosolic lipases for LD breakdown in hepatocytes, and perturbation of this pathway could be a major consequence of chronic EtOH insult leading to fatty liver.

Ubiquitin Ligase COP1 Controls Hepatic Fat Metabolism by Targeting ATGL for Degradation

Optimal control of hepatic lipid metabolism is critical for organismal metabolic fitness. In liver, adipose triglyceride lipase (ATGL) serves as a major triacylglycerol (TAG) lipase and controls the bulk of intracellular lipid turnover. However, regulation of ATGL expression and its functional implications in hepatic lipid metabolism, particularly in the context of fatty liver disease, is unclear. We show that E3 ubiquitin ligase COP1 (also known as RFWD2) binds to the consensus VP motif of ATGL and targets it for proteasomal degradation by K-48 linked polyubiquitination, predominantly at the lysine 100 residue. COP1 thus serves as a critical regulator of hepatocyte TAG content, fatty acid mobilization, and oxidation. Moreover, COP1-mediated regulation of hepatic lipid metabolism requires optimum ATGL expression for its metabolic outcome. In vivo, adenovirus-mediated depletion of COP1 ameliorates high-fat diet-induced steatosis in mouse liver and improves liver function. Our study thus provides new insights into the regulation of hepatic lipid metabolism by the ubiquitin-proteasome system and suggests COP1 as a potential therapeutic target for nonalcoholic fatty liver disease.

Integrated Regulation of Hepatic Lipid and Glucose Metabolism by Adipose Triacylglycerol Lipase and FoxO Proteins

Metabolism is a highly integrated process that is coordinately regulated between tissues and within individual cells. FoxO proteins are major targets of insulin action and contribute to the regulation of gluconeogenesis, glycolysis, and lipogenesis in the liver. However, the mechanisms by which FoxO proteins exert these diverse effects in an integrated fashion remain poorly understood. We report that FoxO proteins also exert important effects on intrahepatic lipolysis and fatty acid oxidation via the regulation of adipose triacylglycerol lipase (ATGL), which mediates the first step in lipolysis, and its inhibitor, the G0/S1 switch 2 gene (G0S2). We also find that ATGL-dependent lipolysis plays a critical role in mediating diverse effects of FoxO proteins in the liver, including effects on gluconeogenic, glycolytic, and lipogenic gene expression and metabolism. These results indicate that intrahepatic lipolysis plays a critical role in mediating and integrating the regulation of glucose and lipid metabolism downstream of FoxO proteins.

Impact of Reduced ATGL-Mediated Adipocyte Lipolysis on Obesity-Associated Insulin Resistance and Inflammation in Male Mice

Emerging evidence suggests that impaired regulation of adipocyte lipolysis contributes to the proinflammatory immune cell infiltration of metabolic tissues in obesity, a process that is proposed to contribute to the development and exacerbation of insulin resistance. To test this hypothesis in vivo, we generated mice with adipocyte-specific deletion of adipose triglyceride lipase (ATGL), the rate-limiting enzyme catalyzing triacylglycerol hydrolysis. In contrast to previous models, adiponectin-driven Cre expression was used for targeted ATGL deletion. The resulting adipocyte-specific ATGL knockout (AAKO) mice were then characterized for metabolic and immune phenotypes. Lean and diet-induced obese AAKO mice had reduced adipocyte lipolysis, serum lipids, systemic lipid oxidation, and expression of peroxisome proliferator-activated receptor alpha target genes in adipose tissue (AT) and liver. These changes did not increase overall body weight or fat mass in AAKO mice by 24 weeks of age, in part due to reduced expression of genes involved in lipid uptake, synthesis, and adipogenesis. Systemic glucose and insulin tolerance were improved in AAKO mice, primarily due to enhanced hepatic insulin signaling, which was accompanied by marked reduction in diet-induced hepatic steatosis as well as hepatic immune cell infiltration and activation. In contrast, although adipocyte ATGL deletion reduced AT immune cell infiltration in response to an acute lipolytic stimulus, it was not sufficient to ameliorate, and may even exacerbate, chronic inflammatory changes that occur in AT in response to diet-induced obesity.

Targeting fatty acid metabolism to improve glucose metabolism

Disturbances in fatty acid metabolism in adipose tissue, liver, skeletal muscle, gut and pancreas play an important role in the development of insulin resistance, impaired glucose metabolism and type 2 diabetes mellitus. Alterations in diet composition may contribute to prevent and/or reverse these disturbances through modulation of fatty acid metabolism. Besides an increased fat mass, adipose tissue dysfunction, characterized by an altered capacity to store lipids and an altered secretion of adipokines, may result in lipid overflow, systemic inflammation and excessive lipid accumulation in non-adipose tissues like liver, skeletal muscle and the pancreas. These impairments together promote the development of impaired glucose metabolism, insulin resistance and type 2 diabetes mellitus. Furthermore, intrinsic functional impairments in either of these organs may contribute to lipotoxicity and insulin resistance. The present review provides an overview of fatty acid metabolism-related pathways in adipose tissue, liver, skeletal muscle, pancreas and gut, which can be targeted by diet or food components, thereby improving glucose metabolism.

Does Diacylglycerol Accumulation in Fatty Liver Disease Cause Hepatic Insulin Resistance?

Numerous studies conducted on obese humans and various rodent models of obesity have identified a correlation between hepatic lipid content and the development of insulin resistance in liver and other tissues. Despite a large body of the literature on this topic, the cause and effect relationship between hepatic steatosis and insulin resistance remains controversial. If, as many believe, lipid aggregation in liver drives insulin resistance and other metabolic abnormalities, there are significant unanswered questions as to which lipid mediators are causative in this cascade. Several published papers have now correlated levels of diacylglycerol (DAG), the penultimate intermediate in triglyceride synthesis, with development of insulin resistance and have postulated that this occurs via activation of protein kinase C signaling. Although many studies have confirmed this relationship, many others have reported a disconnect between DAG content and insulin resistance. It has been postulated that differences in methods for DAG measurement, DAG compartmentalization within the cell, or fatty acid composition of the DAG may explain these discrepancies. The purpose of this review is to compare and contrast some of the relevant findings in this area and to discuss a number of unanswered questions regarding the relationship between DAG and insulin resistance.

Physiological Consequences of Compartmentalized Acyl-CoA Metabolism

Meeting the complex physiological demands of mammalian life requires strict control of the metabolism of long-chain fatty acyl-CoAs because of the multiplicity of their cellular functions. Acyl-CoAs are substrates for energy production; stored within lipid droplets as triacylglycerol, cholesterol esters, and retinol esters; esterified to form membrane phospholipids; or used to activate transcriptional and signaling pathways. Indirect evidence suggests that acyl-CoAs do not wander freely within cells, but instead, are channeled into specific pathways. In this review, we will discuss the evidence for acyl-CoA compartmentalization, highlight the key modes of acyl-CoA regulation, and diagram potential mechanisms for controlling acyl-CoA partitioning.

PEDF and PEDF-derived peptide 44mer stimulate cardiac triglyceride degradation via ATGL

BACKGROUND

Pigment epithelium-derived factor (PEDF) is a 50-kDa secreted glycoprotein that is highly expressed in cardiomyocytes. A variety of peptides derived from PEDF exerts diverse physiological activities including anti-angiogenesis, antivasopermeability, and neurotrophic activities. Recent studies demonstrated that segmental functional peptides of PEDF, 44mer peptide (Val78-Thr121), show similar neurotrophic and cytoprotective effect to that of the holoprotein. We found that PEDF can reduce infarct size and protect cardiac function after acute myocardial infarction (AMI). However, the effects of PEDF on cardiac triglyceride (TG) accumulation after AMI remain unknown. The present study was performed to demonstrate the influence of PEDF and its functional peptides 44mer on TG degradation in AMI.

METHODS

The left ascending coronary artery (LAD) was ligated to induce AMI. PEDF-small interfering RNA (siRNA)-lentivirus (PEDF-RNAi-LV) or PEDF-LV was delivered to the ischemic myocardium in order to knock down or overexpress PEDF, respectively. Oil Red O staining and a TG assay kit were used to analyze the TG content in cardiomyocytes and infarcted areas.

RESULTS

The TG content significantly decreased in the PEDF-overexpressing heart compared to the sham group (P < 0.05). Both rPEDF and 44mer administration stimulate the TG degradation in cultured cardiomyocytes (P < 0.05). Adipose triglyceride lipase (ATGL)-specific inhibitor, atglistatin, attenuated the PEDF or 44mer-induced TG lipolysis activation of cardiomyocytes at 10 μmol/L. The effects of PEDF and 44mer on myocardial TG degradation were also abolished when ATGL was downregulated.

CONCLUSIONS

We conclude that PEDF and 44mer promote TG degradation in cardiomyocytes after AMI via ATGL. The substitution of PEDF and 44mer may be a novel therapeutic strategy for cardiac TG accumulation after AMI.

Effect of Global ATGL Knockout on Murine Fasting Glucose Kinetics

Mice deficient in adipose triglyceride lipase (ATGL(-/-)) present elevated ectopic lipid levels but are paradoxically glucose-tolerant. Measurement of endogenous glucose production (EGP) and Cori cycle activity provide insights into the maintenance of glycemic control in these animals. These parameters were determined in 7 wild-type (ATGL(+/-)) and 6 ATGL(-/-) mice by a primed-infusion of [U-(13)C6]glucose followed by LC-MS/MS targeted mass-isotopomer analysis of blood glucose. EGP was quantified by isotope dilution of [U-(13)C6]glucose while Cori cycling was estimated by analysis of glucose triose (13)C-isotopomers. Fasting plasma free fatty-acids were significantly lower in ATGL(-/-) versus control mice (0.43 ± 0.05 mM versus 0.73 ± 0.11 mM, P < 0.05). Six-hour fasting EGP rates were identical for both ATGL(-/-) and control mice (79 ± 11 versus 71 ± 7 μmol/kg/min, resp.). Peripheral glucose metabolism was dominated by Cori cycling (80 ± 2% and 82 ± 7% of glucose disposal for ATGL(-/-) and control mice, resp.) indicating that peripheral glucose oxidation was not significantly upregulated in ATGL(-/-) mice under these conditions. The glucose (13)C-isotopomer distributions in both ATGL(-/-) and control mice were consistent with extensive hepatic pyruvate recycling. This suggests that gluconeogenic outflow from the Krebs cycle was also well compensated in ATGL(-/-) mice.

Catecholamine-induced lipolysis causes mTOR complex dissociation and inhibits glucose uptake in adipocytes

Anabolic and catabolic signaling oppose one another in adipose tissue to maintain cellular and organismal homeostasis, but these pathways are often dysregulated in metabolic disorders. Although it has long been established that stimulation of the β-adrenergic receptor inhibits insulin-stimulated glucose uptake in adipocytes, the mechanism has remained unclear. Here we report that β-adrenergic-mediated inhibition of glucose uptake requires lipolysis. We also show that lipolysis suppresses glucose uptake by inhibiting the mammalian target of rapamycin (mTOR) complexes 1 and 2 through complex dissociation. In addition, we show that products of lipolysis inhibit mTOR through complex dissociation in vitro. These findings reveal a previously unrecognized intracellular signaling mechanism whereby lipolysis blocks the phosphoinositide 3-kinase-Akt-mTOR pathway, resulting in decreased glucose uptake. This previously unidentified mechanism of mTOR regulation likely contributes to the development of insulin resistance.

Role of metabolic lipases and lipolytic metabolites in the pathogenesis of NAFLD

Non-alcoholic fatty liver disease (NAFLD) is the most frequent chronic liver disease in Western countries, ranging from simple steatosis to steatohepatitis, cirrhosis, and hepatocellular cancer. Although the mechanisms underlying disease progression are incompletely understood, lipotoxic events in the liver resulting in inflammation and fibrosis appear to be central. Free fatty acids and their metabolites are potentially lipotoxic mediators triggering liver injury, suggesting a central role for metabolic lipases. These enzymes are major players in lipid partitioning between tissues and within cells, and provide ligands for nuclear receptors (NRs). We discuss the potential role of intracellular lipases and their lipolytic products in NAFLD. Because tissue-specific modulation of lipases is currently impossible, targeting NRs with ligands may open novel therapeutic perspectives.

High-viscosity dietary fibers reduce adiposity and decrease hepatic steatosis in rats fed a high-fat diet

Viscous dietary fiber consumption lowers the postprandial glucose curve and may decrease obesity and associated comorbidities such as insulin resistance and fatty liver. We determined the effect of 2 viscous fibers, one fermentable and one not, on the development of adiposity, fatty liver, and metabolic flexibility in a model of diet-induced obesity. Rats were fed a normal-fat (NF) diet (26% energy from fat), a high-fat diet (60% energy from fat), each containing 5% fiber as cellulose (CL; nonviscous and nonfermentable), or 5% of 1 of 2 highly viscous fibers-hydroxypropyl methylcellulose (HPMC; nonfermentable) or guar gum (GG; fermentable). After 10 wk, fat mass percentage in the NF (18.0%; P = 0.03) and GG groups (17.0%; P < 0.01) was lower than the CL group (20.7%). The epididymal fat pad weight of the NF (3.9 g; P = 0.04), HPMC (3.9 g; P = 0.03), and GG groups (3.6 g; P < 0.01) was also lower than the CL group (5.0 g). The HPMC (0.11 g/g liver) and GG (0.092 g/g liver) groups had lower liver lipid concentrations compared with the CL group (0.14 g/g liver). Fat mass percentage, epididymal fat pad weight, and liver lipid concentration were not different among the NF, HPMC, and GG groups. The respiratory quotient was higher during the transition from the diet-deprived to fed state in the GG group (P = 0.002) and tended to be higher in the HPMC group (P = 0.06) compared with the CL group, suggesting a quicker shift from fatty acid (FA) to carbohydrate oxidation. The HPMC group [15.1 nmol/(mg ⋅ h)] had higher ex vivo palmitate oxidation in muscle compared with the GG [11.7 nmol/(mg ⋅ h); P = 0.04] and CL groups [10.8 nmol/(mg ⋅ h); P < 0.01], implying a higher capacity to oxidize FAs. Viscous fibers can reduce the adiposity and hepatic steatosis that accompany a high-fat diet, and increase metabolic flexibility, regardless of fermentability.

Abrogating monoacylglycerol acyltransferase activity in liver improves glucose tolerance and hepatic insulin signaling in obese mice

Monoacylglycerol acyltransferase (MGAT) enzymes convert monoacylglycerol to diacylglycerol (DAG), a lipid that has been linked to the development of hepatic insulin resistance through activation of protein kinase C (PKC). The expression of genes that encode MGAT enzymes is induced in the livers of insulin-resistant human subjects with nonalcoholic fatty liver disease, but whether MGAT activation is causal of hepatic steatosis or insulin resistance is unknown. We show that the expression of Mogat1, which encodes MGAT1, and MGAT activity are also increased in diet-induced obese (DIO) and ob/obmice. To probe the metabolic effects of MGAT1 in the livers of obese mice, we administered antisense oligonucleotides (ASOs) against Mogat1 to DIO and ob/ob mice for 3 weeks. Knockdown of Mogat1 in liver, which reduced hepatic MGAT activity, did not affect hepatic triacylglycerol content and unexpectedly increased total DAG content. Mogat1 inhibition also increased both membrane and cytosolic compartment DAG levels. However, Mogat1 ASO treatment significantly improved glucose tolerance and hepatic insulin signaling in obese mice. In summary, inactivation of hepatic MGAT activity, which is markedly increased in obese mice, improved glucose tolerance and hepatic insulin signaling independent of changes in body weight, intrahepatic DAG and TAG content, and PKC signaling.

Targeting Hepatic Glycerolipid Synthesis and Turnover to Treat Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) encompasses a spectrum of metabolic abnormalities ranging from simple hepatic steatosis (accumulation of neutral lipid) to development of steatotic lesions, steatohepatitis, and cirrhosis. NAFLD is extremely prevalent in obese individuals and with the epidemic of obesity; nonalcoholic steatohepatitis (NASH) has become the most common cause of liver disease in the developed world. NASH is rapidly emerging as a prominent cause of liver failure and transplantation. Moreover, hepatic steatosis is tightly linked to risk of developing insulin resistance, diabetes, and cardiovascular disease. Abnormalities in hepatic lipid metabolism are part and parcel of the development of NAFLD and human genetic studies and work conducted in experimentally tractable systems have identified a number of enzymes involved in fat synthesis and degradation that are linked to NAFLD susceptibility as well as progression to NASH. The goal of this review is to summarize the current state of our knowledge on these pathways and focus on how they contribute to etiology of NAFLD and related metabolic diseases.

Recent insights into the molecular pathophysiology of lipid droplet formation in hepatocytes

Triacyglycerols are a major energy reserve of the body and are normally stored in adipose tissue as lipid droplets (LDs). The liver, however, stores energy as glycogen and digested triglycerides in the form of fatty acids. In stressed condition such as obesity, imbalanced nutrition and drug induced liver injury hepatocytes accumulate excess lipids in the form of LDs whose prolonged storage leads to disease conditions most notably non-alcoholic fatty liver disease (NAFLD). Fatty liver disease has become a major health burden with more than 90% of obese, nearly 70% of overweight and about 25% of normal weight patients being affected. Notably, research in recent years has shown LD as highly dynamic organelles for maintaining lipid homeostasis through fat storage, protein sorting and other molecular events studied in adipocytes and other cells of living organisms. This review focuses on the molecular events of LD formation in hepatocytes and the importance of cross talk between different cell types and their signalling in NAFLD as to provide a perspective on molecular mechanisms as well as possibilities for different therapeutic intervention strategies.

Targeted disruption of G0/G1 switch gene 2 enhances adipose lipolysis, alters hepatic energy balance, and alleviates high-fat diet-induced liver steatosis

Recent biochemical and cell-based studies identified G0/G1 switch gene 2 (G0S2) as an inhibitor of adipose triglyceride lipase (ATGL), a key mediator of intracellular triacylglycerol (TG) mobilization. Here, we show that upon fasting, G0S2 protein expression exhibits an increase in liver and a decrease in adipose tissue. Global knockout of G0S2 in mice enhanced adipose lipolysis and attenuated gain of body weight and adiposity. More strikingly, G0S2 knockout mice displayed a drastic decrease in hepatic TG content and were resistant to high-fat diet (HFD)-induced liver steatosis, both of which were reproduced by liver-specific G0S2 knockdown. Mice with hepatic G0S2 knockdown also showed increased ketogenesis, accelerated gluconeogenesis, and decelerated glycogenolysis. Conversely, overexpression of G0S2 inhibited fatty acid oxidation in mouse primary hepatocytes and caused sustained steatosis in liver accompanied by deficient TG clearance during the fasting-refeeding transition. In response to HFD, there was a profound increase in hepatic G0S2 expression in the fed state. Global and hepatic ablation of G0S2 both led to improved insulin sensitivity in HFD-fed mice. Our findings implicate a physiological role for G0S2 in the control of adaptive energy response to fasting and as a contributor to obesity-associated liver steatosis.

Pleiotropic regulation of mitochondrial function by adipose triglyceride lipase-mediated lipolysis

Lipolysis is defined as the catabolism of triacylglycerols (TGs) stored in cellular lipid droplets. Recent discoveries of essential lipolytic enzymes and characterization of numerous regulatory proteins and mechanisms have fundamentally changed our perception of lipolysis and its impact on cellular metabolism. Adipose triglyceride lipase (ATGL) is the rate-limiting enzyme for TG catabolism in most cells and tissues. This review focuses on recent advances in understanding the (patho)physiological impact due to defective lipolysis by ATGL deficiency on mitochondrial (dys)function. Depending on the type of cells and tissues investigated, absence of ATGL has pleiotropic roles in mitochondrial function.

Development of small-molecule inhibitors targeting adipose triglyceride lipase

Adipose triglyceride lipase (ATGL) is rate limiting in the mobilization of fatty acids from cellular triglyceride stores. This central role in lipolysis marks ATGL as an interesting pharmacological target as deregulated fatty acid metabolism is closely linked to dyslipidemic and metabolic disorders. Here we report on the development and characterization of a small-molecule inhibitor of ATGL. Atglistatin is selective for ATGL and reduces fatty acid mobilization in vitro and in vivo.

The g0/g1 switch gene 2 is an important regulator of hepatic triglyceride metabolism

Nonalcoholic fatty liver disease is associated with obesity and insulin resistance. Factors that regulate the disposal of hepatic triglycerides contribute to the development of hepatic steatosis. G₀/G₁ switch gene 2 (G0S2) is a target of peroxisome proliferator-activated receptors and plays an important role in regulating lipolysis in adipocytes. Therefore, we investigated whether G0S2 plays a role in hepatic lipid metabolism. Adenovirus-mediated expression of G0S2 (Ad-G0S2) potently induced fatty liver in mice. The liver mass of Ad-G0S2-infected mice was markedly increased with excess triglyceride content compared to the control mice. G0S2 did not change cellular cholesterol levels in hepatocytes. G0S2 was found to be co-localized with adipose triglyceride lipase at the surface of lipid droplets. Hepatic G0S2 overexpression resulted in an increase in plasma Low-density lipoprotein (LDL)/Very-Low-density (VLDL) lipoprotein cholesterol level. Plasma High-density lipoprotein (HDL) cholesterol and ketone body levels were slightly decreased in Ad-G0S2 injected mice. G0S2 also increased the accumulation of neutral lipids in cultured HepG2 and L02 cells. However, G0S2 overexpression in the liver significantly improved glucose tolerance in mice. Livers expressing G0S2 exhibited increased 6-(N-(7-nitrobenz-2-oxa-1-3-diazol-4-yl) amino)-6-deoxyglucose uptake compared with livers transfected with control adenovirus. Taken together, our results provide evidence supporting an important role for G0S2 as a regulator of triglyceride content in the liver and suggest that G0S2 may be a molecular target for the treatment of insulin resistance and other obesity-related metabolic disorders.

The impact of genetic stress by ATGL deficiency on the lipidome of lipid droplets from murine hepatocytes

We showed earlier that nutritional stress like starvation or high-fat diet resulted in phenotypic changes in the lipidomes of hepatocyte lipid droplets (LDs), representative for the pathophysiological status of the mouse model. Here we extend our former study by adding genetic stress due to knockout (KO) of adipocyte triglyceride lipase (ATGL), the rate limiting enzyme in LD lipolysis. An intervention trial for 6 weeks with male wild-type (WT) and ATGL-KO mice was carried out; both genotypes were fed lab chow or were exposed to short-time starvation. Isolated LDs were analyzed by LC-MS/MS. Triacylglycerol, diacylglycerol, and phosphatidylcholine lipidomes, in that order, provided the best phenotypic signatures characteristic for respective stresses applied to the animals. This was evidenced at lipid species level by principal component analysis, calculation of average values for chain-lengths and numbers of double bonds, and by visualization in heat maps. Structural backgrounds for analyses and metabolic relationships were elaborated at lipid molecular species level. Relating our lipidomic data to nonalcoholic fatty liver diseases of nutritional and genetic etiologies with or without accompanying insulin resistance, phenotypic distinction in hepatocyte LDs dependent on insulin status emerged. Taken together, lipidomes of hepatocyte LDs are sensitive responders to nutritional and genetic stress.