Hepatic ATGL mediates PPAR-α signaling and fatty acid channeling through an L-FABP independent mechanism

Prevention of high-fat-diet-induced obesity in mice by soluble dietary fiber from fermented and unfermented millet bran

Obesity-related diabetes, cardiovascular disease, and hypertension pose many risks to human health. Thus, mice on a high-fat diet were gavaged with millet bran (unfermented/fermented) soluble dietary fiber (RSDF/FSDF, 500 mg·kg-1) for 10 weeks in current research, and then evaluated the various biological indicators. These findings revealed that RSDF and FSDF supplements could prevent fat synthesis by inhibiting sterol regulatory element-binding protein-1c gene expression. The RSDF supplements can also accelerate fat catabolism through enhanced the mRNA expression levels of adipose triglyceride lipase and peroxisome proliferator-activated receptor α. FSDF supplements can prevent obesity by decreasing 3-hydroxy-3-methyl-glutaryl-CoA reductase expression and increasing cholesterol 7α-hydroxylase expression. Moreover, FSDF also controls obesity development by lowering total cholesterol and low-density lipoprotein cholesterol levels in the blood, triglyceride, total cholesterol, and bile acid levels in the liver. Notably, FSDF supplements can promote Bacteroides and Prevotella propagation; excretive propionic acid binds to free fatty acid receptor 2/3 and then stimulates intestinal epithelial cells to generate glucagon-like-peptide-1 and peptide YY, which can reduce food and energy intake and ultimately prevent obesity. All evidence suggests that FSDF supplements play a crucial role in preventing obesity.

Overexpression of ATGL impairs lipid droplet accumulation by accelerating lipolysis in goat mammary epithelial cells

Adipose triglyceride lipase (ATGL) is the key enzyme for the degradation of triacylglycerols (TAGs). It functions in concert with other enzymes to mobilize TAG and supply fatty acids (FAs) for energy production. Dysregulated lipolysis leads to excess concentrations of circulating FAs, which may lead to destructive and lipotoxic effects to the organism. To understand the role of ATGL in mammary lipid metabolism, ATGL was overexpressed in goat mammary epithelial cells (GMECs) by using a recombinant adenovirus system. ATGL overexpression decreased lipid droplet (LD) accumulation and cellular TG content (p < 0.05) along with a decrease in the expression of the key enzyme that catalyzes the final step of TG synthesis (DGAT). Significant increases were observed in the expression of genes related to lipolysis (hormone-sensitive lipase [HSL]) and FA desaturation (SCD) by ATGL overexpression. Genes responsible for FA oxidation (PPARα), LD formation and secretion (ADRP and BTN1A1), and long-chain FA uptake (CD36) were all decreased by ATGL overexpression (p < 0.05). The primary products of TAG lipolysis, free FAs (FFAs), were notably increased in the ATGL-overexpressing cells. Taken together, our results demonstrated that ATGL activation impairs lipid formation partially through accelerating lipolysis in GMECs.

The roles of DGAT1 and DGAT2 in human myotubes are dependent on donor patho-physiological background

The roles of DGAT1 and DGAT2 in lipid metabolism and insulin responsiveness of human skeletal muscle were studied using cryosections and myotubes prepared from muscle biopsies from control, athlete, and impaired glucose regulation (IGR) cohorts of men. The previously observed increases in intramuscular triacylglycerol (IMTG) in athletes and IGR were shown to be related to an increase in lipid droplet (LD) area in type I fibers in athletes but, conversely, in type II fibers in IGR subjects. Specific inhibition of both diacylglycerol acyltransferase (DGAT) 1 and 2 decreased fatty acid (FA) uptake by myotubes, whereas only DGAT2 inhibition also decreased fatty acid oxidation. Fatty acid uptake in myotubes was negatively correlated with the lactate thresholds of the respective donors. DGAT2 inhibition lowered acetate uptake and oxidation in myotubes from all cohorts whereas DGAT1 inhibition had no effect. A positive correlation between acetate oxidation in myotubes and resting metabolic rate (RMR) from fatty acid oxidation in vivo was observed. Myotubes from athletes and IGR had higher rates of de novo lipogenesis from acetate that were normalized by DGAT2 inhibition. Moreover, DGAT2 inhibition in myotubes also resulted in increased insulin-induced Akt phosphorylation. The differential effects of DGAT1 and DGAT2 inhibition suggest that the specialized role of DGAT2 in esterifying nascent diacylglycerols and de novo synthesized FA is associated with synthesis of a pool of triacylglycerol, which upon hydrolysis results in effectors that promote mitochondrial fatty acid oxidation but decrease insulin signaling in skeletal muscle cells.

Targeting PPARs for therapy of atherosclerosis: A review

Atherosclerosis, a chief pathogenic factor of cardiovascular disease, is associated with many factors including inflammation, dyslipidemia, and oxidative stress. Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors and are widely expressed with tissue- and cell-specificity. They control multiple genes that are involved in lipid metabolism, inflammatory response, and redox homeostasis. Given the diverse biological functions of PPARs, they have been extensively studied since their discovery in 1990s. Although controversies exist, accumulating evidence have demonstrated that PPAR activation attenuates atherosclerosis. Recent advances are valuable for understanding the mechanisms of action of PPAR activation. This article reviews the recent findings, mainly from the year of 2018 to present, including endogenous molecules in regulation of PPARs, roles of PPARs in atherosclerosis by focusing on lipid metabolism, inflammation, and oxidative stress, and synthesized PPAR modulators. This article provides information valuable for researchers in the field of basic cardiovascular research, for pharmacologists that are interested in developing novel PPAR agonists and antagonists with lower side effects as well as for clinicians.

Intestinal fatty acid binding protein: A rising therapeutic target in lipid metabolism

Fatty acid binding proteins (FABPs) are key proteins in lipid transport, and the isoforms are segregated according to their tissue origins. Several isoforms, such as adipose-FABP and epidermal-FABP, have been shown to participate in multiple pathologic processes due to their ubiquitous expression. Intestinal fatty acid binding protein, also termed FABP2 or I-FABP, is specifically expressed in the small intestine. FABP2 can traffic lipids from the intestinal lumen to enterocytes and bind superfluous fatty acids to maintain a steady pool of fatty acids in the epithelium. As a lipid chaperone, FABP2 can also carry lipophilic drugs to facilitate targeted transport. When the integrity of the intestinal epithelium is disrupted, FABP2 is released into the circulation. Thus, it can potentially serve as a clinical biomarker. In this review, we discuss the pivotal role of FABP2 in intestinal lipid metabolism. We also summarize the molecular interactions that have been reported to date, highlighting the clinical prospects of FABP2 research.

PPAR Alpha as a Metabolic Modulator of the Liver: Role in the Pathogenesis of Nonalcoholic Steatohepatitis (NASH)

Simple Summary

In the context of liver disease, one of the more growing public health problems is the transition from simple steatosis to non-alcoholic steatohepatitis. Profound metabolic dysregulations linked to inflammation and hepatic injury are features of non-alcoholic steatohepatitis. Since the peroxisomal-proliferator-activated receptor alpha has long been considered one of the key transcriptional factors in hepatic metabolism, its role in the pathogenesis of non-alcoholic steatohepatitis is discussed in this review.

Abstract

The strong relationship between metabolic alterations and non-alcoholic steatohepatitis (NASH) suggests a pathogenic interplay. However, many aspects have not yet been fully clarified. Nowadays, NASH is becoming the main cause of liver-associated morbidity and mortality. Therefore, an effort to understand the mechanisms underlying the pathogenesis of NASH is critical. Among the nuclear receptor transcription factors, peroxisome-proliferator-activated receptor alpha (PPARα) is highly expressed in the liver, where it works as a pivotal transcriptional regulator of the intermediary metabolism. In this context, PPARα’s function in regulating the lipid metabolism is essential for proper liver functioning. Here, we review metabolic liver genes under the control of PPARα and discuss how this aspect can impact the inflammatory condition and pathogenesis of NASH.

Fatty acid binding protein 5 regulates lipogenesis and tumor growth in lung adenocarcinoma

AIMS

Lung cancer is the leading cause of cancer-related death. Unfortunately, targeted-therapies have been unsuccessful for most patients with lung adenocarcinoma (LUAD). Thus, new early biomarkers and treatment options are a pressing need. Fatty acid binding protein 5 (FABP5) has been associated with various types of cancers. Its contribution to LUAD onset, progression and metabolic reprogramming is, however, not fully understood. In this study we assessed the importance of FABP5 in LUAD and its role in cancer lipid metabolism.

MAIN METHODS

By radioactive labeling and metabolite quantification, we studied the function of FABP5 in fatty acid metabolism using genetic/pharmacologic inhibition and overexpression models in LUAD cell lines. Flow cytometry, heterologous transplantation and bioinformatic analysis were used, in combination with other methodologies, to assess the importance of FABP5 for cellular proliferation in vitro and in vivo and in patient survival.

KEY FINDINGS

We show that high expression of FABP5 is associated with poor prognosis in patients with LUAD. FABP5 regulates lipid metabolism, diverting fatty acids towards complex lipid synthesis, whereas it does not affect their catabolism in vitro. Moreover, FABP5 is required for de novo fatty acid synthesis and regulates the expression of enzymes involved in the pathway (including FASN and SCD1). Consistently with the changes in lipid metabolism, FABP5 is required for cell cycle progression, migration and in vivo tumor growth.

SIGNIFICANCE

Our results suggest that FABP5 is a regulatory hub of lipid metabolism and tumor progression in LUAD, placing it as a new putative therapeutic target for this disease.

Early Lipid Metabolic Effects of the Anti-Psychotic Drug Olanzapine on Weight Gain and the Associated Gene Expression

BACKGROUND

Atypical antipsychotics such as olanzapine often cause metabolic side effects such as obesity and diabetes, leading to an increased risk of nonalcoholic fatty liver disease. The aim of the present study was to investigate the effects of olanzapine treatment on hepatic lipid metabolism and its possible relationship with adipose tissue status.

METHODS

Using a female rat model, we investigated the effects of chronic olanzapine administration on the regulation of carbohydrate and lipid metabolism including lipid biosynthesis, oxidation, efflux, and lipolysis in liver and adipose tissue.

RESULTS

The body weight, liver mass and visceral adiposity after olanzapine treatment (2 mg/kg) for five weeks were not significantly different compared with vehicle controls. The serum level of triglycerides was higher in the vehicle controls than in olanzapine-treated rats. Unexpectedly, olanzapine treatment did not reduce glucose tolerance in our model. The expression of functional thermogenic protein uncoupling protein 1 (UCP1) was increased in brown adipose tissue (BAT) of the olanzapine group. Additionally, olanzapine treatment also reduced adipose inflammation in white adipose tissue (WAT). The transcription factor sterol regulatory element-binding protein (SREBP)-1c, a key early regulator of lipogenesis, was downregulated following olanzapine treatment. The expression of genes related to the triglycerides synthesis apparatus in the liver was upregulated in the olanzapine group. Olanzapine treatment induced genes involved in PPAR-α signaling and mitochondrial fatty acid oxidation in response to increased ATGL-mediated lipolysis in the liver.

CONCLUSION

Together, our findings suggest a complicated link between olanzapine therapy and metabolic disturbance and may garner interest in assessing the action of antipsychotic-induced metabolic disturbances.

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.

Lowering apolipoprotein CIII protects against high-fat diet-induced metabolic derangements

Increased levels of apolipoprotein CIII (apoCIII), a key regulator of lipid metabolism, result in obesity-related metabolic derangements. We investigated mechanistically whether lowering or preventing high-fat diet (HFD)–induced increase in apoCIII protects against the detrimental metabolic consequences. Mice, first fed HFD for 10 weeks and thereafter also given an antisense (ASO) to lower apoCIII, already showed reduced levels of apoCIII and metabolic improvements after 4 weeks, despite maintained obesity. Prolonged ASO treatment reversed the metabolic phenotype due to increased lipase activity and receptor-mediated hepatic uptake of lipids. Fatty acids were transferred to the ketogenic pathway, and ketones were used in brown adipose tissue (BAT). This resulted in no fat accumulation and preserved morphology and function of liver and BAT. If ASO treatment started simultaneously with the HFD, mice remained lean and metabolically healthy. Thus, lowering apoCIII protects against and reverses the HFD-induced metabolic phenotype by promoting physiological insulin sensitivity.

Lipophagy-derived fatty acids undergo extracellular efflux via lysosomal exocytosis

The autophagic degradation of lipid droplets (LDs), termed lipophagy, is a major mechanism that contributes to lipid turnover in numerous cell types. While numerous factors, including nutrient deprivation or overexpression of PNPLA2/ATGL (patatin-like phospholipase domain containing 2) drive lipophagy, the trafficking of fatty acids (FAs) produced from this pathway is largely unknown. Herein, we show that PNPLA2 and nutrient deprivation promoted the extracellular efflux of FAs. Inhibition of autophagy or lysosomal lipid degradation attenuated FA efflux highlighting a critical role for lipophagy in this process. Rather than direct transport of FAs across the lysosomal membrane, lipophagy-derived FA efflux requires lysosomal fusion to the plasma membrane. The lysosomal Ca2+ channel protein MCOLN1/TRPML1 (mucolipin 1) regulates lysosomal-plasma membrane fusion and its overexpression increased, while inhibition blocked FA efflux. In addition, inhibition of autophagy/lipophagy or MCOLN1, or sequestration of extracellular FAs with BSA attenuated the oxidation and re-esterification of lipophagy-derived FAs. Overall, these studies show that the well-established pathway of lysosomal fusion to the plasma membrane is the primary route for the disposal of FAs derived from lipophagy. Moreover, the efflux of FAs and their reuptake or subsequent extracellular trafficking to adjacent cells may play an important role in cell-to-cell lipid exchange and signaling.Abbreviations: ACTB: beta actin; ADRA1A: adrenergic receptor alpha, 1a; ALB: albumin; ATG5: autophagy related 5; ATG7: autophagy related 7; BafA1: bafilomycin A1; BECN1: beclin 1; BHBA: beta-hydroxybutyrate; BSA: bovine serum albumin; CDH1: e-cadherin; CQ: chloroquine; CTSB: cathepsin B; DGAT: diacylglycerol O-acyltransferase; FA: fatty acid; HFD: high-fat diet; LAMP1: lysosomal-associated membrane protein 1; LD: lipid droplet; LIPA/LAL: lysosomal acid lipase A; LLME: Leu-Leu methyl ester hydrobromide; MAP1LC3B/LC3: microtubule associated protein 1 light chain 3 beta; MCOLN1/TRPML1: mucolipin 1; MEF: mouse embryo fibroblast; PBS: phosphate-buffered saline; PIK3C3/VPS34: phosphatidylinositol 3-kinase catalytic subunit type 3; PLIN: perilipin; PNPLA2/ATGL patatin-like phospholipase domain containing 2; RUBCN (rubicon autophagy regulator); SM: sphingomyelin; TAG: triacylglycerol; TMEM192: transmembrane protein 192; VLDL: very low density lipoprotein.

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.

Hepatic lipid droplets: A balancing act between energy storage and metabolic dysfunction in NAFLD

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is defined by the abundance of lipid droplets (LDs) in hepatocytes. While historically considered simply depots for energy storage, LDs are increasingly recognized to impact a wide range of biological processes that influence cellular metabolism, signaling, and function. While progress has been made toward understanding the factors leading to LD accumulation (i.e. steatosis) and its progression to advanced stages of NAFLD and/or systemic metabolic dysfunction, much remains to be resolved.

SCOPE OF REVIEW

This review covers many facets of LD biology. We provide a brief overview of the major pathways of lipid accretion and degradation that contribute to steatosis and how they are altered in NAFLD. The major focus is on the relationship between LDs and cell function and the detailed mechanisms that couple or uncouple steatosis from the severity and progression of NAFLD and systemic comorbidities. The importance of specific lipids and proteins within or on LDs as key components that determine whether LD accumulation is linked to cellular and metabolic dysfunction is presented. We discuss emerging areas of LD biology and future research directions that are needed to advance our understanding of the role of LDs in NAFLD etiology.

MAJOR CONCLUSIONS

Impairments in LD breakdown appear to contribute to disease progression, but inefficient incorporation of fatty acids (FAs) into LD-containing triacylglycerol (TAG) and the consequential changes in FA partitioning also affect NAFLD etiology. Increased LD abundance in hepatocytes does not necessarily equate to cellular dysfunction. While LD accumulation is the prerequisite step for most NAFLD cases, the protein and lipid composition of LDs are critical factors in determining the progression from simple steatosis. Further defining the detailed molecular mechanisms linking LDs to metabolic dysfunction is important for designing effective therapeutic approaches targeting NAFLD and its comorbidities.

Lipid Droplet-Derived Monounsaturated Fatty Acids Traffic via PLIN5 to Allosterically Activate SIRT1

Lipid droplets (LDs) provide a reservoir for triacylglycerol storage and are a central hub for fatty acid trafficking and signaling in cells. Lipolysis promotes mitochondrial biogenesis and oxidative metabolism via a SIRT1/PGC-1α/PPARα-dependent pathway through an unknown mechanism. Herein, we identify that monounsaturated fatty acids (MUFAs) allosterically activate SIRT1 toward select peptide-substrates such as PGC-1α. MUFAs enhance PGC-1α/PPARα signaling and promote oxidative metabolism in cells and animal models in a SIRT1-dependent manner. Moreover, we characterize the LD protein perilipin 5 (PLIN5), which is known to enhance mitochondrial biogenesis and function, to be a fatty-acid-binding protein that preferentially binds LD-derived monounsaturated fatty acids and traffics them to the nucleus following cAMP/PKA-mediated lipolytic stimulation. Thus, these studies identify the first-known endogenous allosteric modulators of SIRT1 and characterize a LD-nuclear signaling axis that underlies the known metabolic benefits of MUFAs and PLIN5.

Lipid Droplets and the Management of Cellular Stress

Lipid droplets are cytosolic fat storage organelles present in most eukaryotic cells. Long regarded merely as inert fat reservoirs, they are now emerging as major regulators of cellular metabolism. They act as hubs that coordinate the pathways of lipid uptake, distribution, storage, and use in the cell. Recent studies have revealed that they are also essential components of the cellular stress response. One of the hallmark characteristics of lipid droplets is their capacity to buffer excess lipids and to finely tune their subsequent release based on specific cellular requirements. This simple feature of lipid droplet biology, buffering and delayed release of lipids, forms the basis for their pleiotropic roles in the cellular stress response. In stressed cells, lipid droplets maintain energy and redox homeostasis and protect against lipotoxicity by sequestering toxic lipids into their neutral lipid core. Their mobility and dynamic interactions with mitochondria enable an efficient delivery of fatty acids for optimal energy production. Lipid droplets are also involved in the maintenance of membrane and organelle homeostasis by regulating membrane composition, preventing lipid peroxidation and removing damaged proteins and lipids. Finally, they also engage in a symbiotic relationship with autophagy and act as reservoirs of bioactive lipids that regulate inflammation and immunity. Thus, lipid droplets are central managers of lipid metabolism that function as safeguards against various types of cellular stress.

Long non-coding RNA NEAT1-modulated abnormal lipolysis via ATGL drives hepatocellular carcinoma proliferation

BACKGROUND

Abnormal metabolism, including abnormal lipid metabolism, is a hallmark of cancer cells. Some studies have demonstrated that the lipogenic pathway might promote the development of hepatocellular carcinoma (HCC). However, the role of the lipolytic pathway in HCC has not been elucidated.

METHODS

We compared levels of adipose triglyceride lipase (ATGL) in human HCC and healthy liver tissues by real time PCR, western blot and immunohistochemistry. We measured diacylglycerol(DAG) and free fatty acid (FFA) levels in HCC cells driven by the NEAT1-ATGL axis and in HCC tissues. We also assessed the effects of ATGL, DAG, FFA, and NEAT1 on HCC cells proliferation in vitro and in an orthotopic xenograft HCC mouse model. We also performed a luciferase reporter assay to investigate the interaction between NEAT1/ATGL and miR-124-3p.

RESULTS

We found that the lipolytic enzyme, ATGL is highly expressed in human HCC tissues and predicts poor prognosis. We also found that high levels of DAG and FFA are present in HCC tissues. Furthermore, the lncRNA-NEAT1 was found to modulate ATGL expression and disrupt lipolysis in HCC cells via ATGL. Notably, ATGL and its products, DAG and FFA, were shown to be responsible for NEAT1-mediated HCC cell growth. NEAT1 regulated ATGL expression by binding miR-124-3p. Additionally, NEAT1 knockdown attenuated HCC cell growth through miR-124-3p/ATGL/DAG+FFA/PPARα signaling.

CONCLUSION

Our results reveal that NEAT1-modulates abnormal lipolysis via ATGL to drive HCC proliferation.

Genistein and daidzein induce apoptosis of colon cancer cells by inhibiting the accumulation of lipid droplets

Aim

The purpose of this study was to investigate the possible mechanisms of genistein (GEN) and daidzein (DAI) in inducing apoptosis of colon cancer cells by inhibition of lipid droplets (LDs) accumulation.

Methods

HT-29 cells were used and treated by GEN or DAI in this paper. LDs accumulation was induced and inhibited by oleic acid (OA) and C75, respectively. The expression changes of LDs-related markers were confirmed by semiquantitative real time-PCR (RT–PCR), Western blotting, and immunofluorescence staining.

Results

GEN and DAI effectively reduced the LDs accumulation and downregulated the expression of Perilipin-1, ADRP and Tip-47 family proteins and vimentin levels. GEN and DAI significantly induced the mRNA expression of PPAR-γ, Fas, FABP, glycerol-3-phosphate acyltransferase (GPAT3), and microsomal TG transfer protein (MTTP), and reduced the mRNA expression of UCP2. Furthermore, the results showed a decrease of PI3K expression by GEN and DAI when compared with OA treatment, and both GEN and DAI can increase the expression of FOXO3a and caspase-8 significantly when these proteins were decreased by OA treatment. GEN is more effective than DAI in inducing cell apoptosis.

Conclusion

Our results demonstrated that GEN and DAI inhibit the accumulation of LDs by regulating LDs-related factors and lead to a final apoptosis of colon cancer cells. These results may provide important new insights into the possible molecular mechanisms of isoflavones in anti-obesity and anti-tumor functions.

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.

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.

MicroRNA profiles of fibroblasts derived from in vivo fertilized and fat-1 transgenic cattle

Fat-1 transgenic cattle have high levels of ω-3 fatty acids, which regulate several genes in fatty acid metabolism. In the current study, fibroblasts derived from in vivo fertilized (Ferti) and fat-1 transgenic (TG) Luxi cattle (Bos taurus), a local breed in China, were cultured and their miRNA expression was characterized. Expression of 352 known miRNAs differed in cells from Ferti and TG cattle: 83 miRNAs were found to be specifically expressed in cells from Ferti cattle while 23 miRNAs were found to be specifically expressed in cells from TG cattle. Novel differences in miRNA expression were also found in cells from Ferti and TG cattle. The identity of seven differentially expressed miRNAs was verified using quantitative real-time PCR, and target genes were identified computationally. GO and KEGG analysis revealed that these miRNAs were involved in seven major biological pathways, including metabolism, MAPK signaling, calcium signaling, purine metabolism, ubiquitin mediated proteolysis, pyrimidine metabolism, and the cell cycle. Overexpression of one of these miRNAs, miR-21-5p, was found to suppress expression of fibroblast growth factor 10 (FGF10) and adipose triglyceride lipase (ATGL) in fibroblasts from TG cattle and 3T3-L1 pre-adipocytes. Conversely, knockdown of miR-21-5p stimulated expression. Together, these results suggest that miRNAs potentially play a role in expression of lipogenic and lipolytic genes as well as in synthesis of ω-3 fatty acids facilitated by fat-1.

Silencing of FABP1 ameliorates hepatic steatosis, inflammation, and oxidative stress in mice with nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is increasing in prevalence worldwide and has been identified as a risk factor for cirrhosis and hepatocellular carcinoma. However, there is no effective pharmacologic treatment for NAFLD. FABP1 is a liver‐specific fatty acid‐binding protein (FABP) that plays important roles in intracellular lipid metabolism in the liver. We investigated the effect of repression of FABP1 expression on NAFLD, using adenovirus‐mediated silencing of FABP1. FABP1 knockdown in the liver decreased the liver weight and hepatic triglyceride (TG) accumulation. The expression of inflammatory and oxidative stress markers in the liver was also reduced. The level of thiobarbituric acid‐reactive substances, a marker of lipid peroxidation, in the liver of FABP1 knockdown mice was significantly decreased. These results suggest that FABP1 reduction in the liver is an effective approach against NAFLD.

Lipid Droplet-Associated Hydrolase Promotes Lipid Droplet Fusion and Enhances ATGL Degradation and Triglyceride Accumulation

Lipid droplet (LD)-associated hydrolase (LDAH) is a newly identified LD protein abundantly expressed in tissues that predominantly store triacylglycerol (TAG). However, how LDAH regulates TAG metabolism remains unknown. We found that upon oleic acid loading LDAH translocalizes from the ER to newly formed LDs, and induces LD coalescence in a tubulin-dependent manner. LDAH overexpression and downregulation in HEK293 cells increase and decrease, respectively, TAG levels. Pulse and chase experiments show that LDAH enhances TAG biogenesis, but also decreases TAG turnover and fatty acid release from cells. Mutations in predicted catalytic and acyltransferase motifs do not influence TAG levels, suggesting that the effect is independent of LDAH’s enzymatic activity. However, a LDAH alternative-splicing variant missing 90 amino acids at C-terminus does not promote LD fusion or TAG accumulation, while it still localizes to LDs. Interestingly, LDAH enhances polyubiquitination and proteasomal degradation of adipose triglyceride lipase (ATGL), a rate limiting enzyme of TAG hydrolysis. Co-expression of ATGL reverses the changes in LD phenotype induced by LDAH, and both proteins counterbalance their effects on TAG stores. Together, these studies support that under conditions of TAG storage in LDs LDAH plays a primarily lipogenic role, inducing LD growth and enhancing degradation of ATGL.

A Coupled Approach Utilizing Immunohistochemistry and Immunocytochemistry to Visualize Cellular Lipophagy

Aberrant liver function as a consequence of excessive fat accumulation as observed in non-alcoholic fatty liver disease (NAFLD) is responsible for a plethora of disorders leading to the incidence of cardiovascular diseases. In most cases, the cause of NAFLD is not completely understood but has been primarily linked to the occurrence of insulin resistance that often accompanies obesity, dyslipidemia, and Type 2 Diabetes. Hence, it is of paramount importance to elucidate the molecular mechanisms of lipid metabolism underlying the pathogenesis of NAFLD and its comorbidities. Recent evidence indicates the role of lipid turnover in the pathophysiology of NAFLD via autophagy, namely lipophagy. Moreover, certain features of the liver such as its regenerative capacity make it an autophagy-dependent organ. This paper describes several autophagy molecular imaging tools that can be employed to visualize hepatic lipid turnover to elucidate the importance of this signaling cascade in pathological conditions, especially NAFLD.

Stearoyl-CoA desaturase 1 deficiency reduces lipid accumulation in the heart by activating lipolysis independently of peroxisome proliferator-activated receptor α

Stearoyl-CoA desaturase 1 (SCD1) has recently been shown to be a critical control point in the regulation of cardiac metabolism and function. Peroxisome proliferator-activated receptor α (PPARα) is an important regulator of myocardial fatty acid uptake and utilization. The present study used SCD1 and PPARα double knockout (SCD1^-/-/PPARα^-/-) mice to test the hypothesis that PPARα is involved in metabolic changes in the heart that are caused by SCD1 downregulation/inhibition. SCD1 deficiency decreased the intracellular content of free fatty acids, triglycerides, and ceramide in the heart of SCD1^-/- and SCD1^-/-/PPARα^-/- mice. SCD1 ablation in PPARα^-/- mice decreased diacylglycerol content in cardiomyocytes. These results indicate that the reduction of fat accumulation in the heart associated with SCD1 deficiency occurs independently of the PPARα pathway. To elucidate the mechanism of the observed changes, we treated HL-1 cardiomyocytes with the SCD1 inhibitor A939572 and/or PPARα inhibitor GW6471. SCD1 inhibition decreased the level of lipogenic proteins and increased lipolysis, reflected by a decrease in the content of adipose triglyceride lipase inhibitor G0S2 and a decrease in the ratio of phosphorylated hormone-sensitive lipase (HSL) at Ser565 to HSL (pHSL[Ser565]/HSL). PPARα inhibition alone did not affect the aforementioned protein levels. Finally, PPARα inhibition decreased the phosphorylation level of 5'-adenosine monophosphate-activated protein kinase, indicating lower mitochondrial fatty acid oxidation. In summary, SCD1 ablation/inhibition decreased cardiac lipid content independently of the action of PPARα by reducing lipogenesis and activating lipolysis. The present data suggest that SCD1 is an important component in maintaining proper cardiac lipid metabolism.

Structural characterization and hypolipidemic effect of Cyclocarya paliurus polysaccharide in rat

Polysaccharide is one of the important active ingredients of Cyclocarya paliurus (Batal.) Iljinskaja leaves. The aims of this work were to analyze the structure of the polysaccharide of Cyclocarya paliurus (Batal.) Iljinskaja leaves (CPP), and to investigate the antihyperlipidemic effect of CPP on high-fat emulsion (HFE)-induced hyperlipidaemic rats. CPP, comprised of two polysaccharides with average molecular weight (Mw) of 1.35×10(5)Da and 9.34×10(3)Da, was consisted of rhamnose, arabinose, xylose, mannose, glucose and galactose in the molar ratio of 1.00:2.23:0.64:0.49:0.63:4.16. Oral administration of CPP could significantly decrease levels of serum total cholesterol (TC), triglycerides (TG) and low-density lipoprotein cholesterol (LDL-C), increase high density lipoprotein (HDL-C) in hyperlipidemic rats. CPP exerts therapeutic effects on hyperlipidaemic rats, by up-regulating expressions of adipose triglyceride lipase (ATGL) and peroxisome proliferator-activated receptor alpha (PPARα), via down-regulating fatty acid synthase (FAS) and hydroxy methylglutaryl coenzyme A reductase (HMG-CoA). This study demonstrates that CPP may be beneficial for the treatment of hyperlipidemia.

Perinatal maternal high-fat diet promotes alterations in hepatic lipid metabolism and resistance to the hypolipidemic effect of fish oil in adolescent rat offspring

SCOPE

Maternal high-fat diet (HFD) promotes obesity and metabolic disturbances in offspring at weaning and adult life. We investigated metabolic consequences of maternal HFD in adolescent rat offspring and the potential benefic effects of fish oil (FO) (n-3 polyunsaturated fatty acid source).

METHODS AND RESULTS

Female rats received isocaloric, standard diet (STD: 9% fat) or HFD (28.6%) before mating, and throughout pregnancy and lactation. After weaning, male offspring received standard diet and, from 25th to 45th day, received oral administration of soybean oil (SO) or FO. HFD offspring showed higher body weight and adiposity, which was not attenuated by FO. In STD offspring, FO reduced serum triglyceride and cholesterol, as expected, but not in HFD offspring. Liver of HFD offspring groups showed increased free cholesterol and FO-treated HFD group showed lower expression of Abcg8, suggesting decreased cholesterol biliary excretion. HFD offspring presented higher hepatic expression of lipogenic markers, Srebf1 mRNA and acetyl CoA carboxylase (ACC). Serum n-3 PUFA were decreased in FO-treated HFD compared to FO-treated STD offspring, which may explain the reduced hypolipidemic FO effect.

CONCLUSION

Maternal HFD impaired the ability of FO to reduce adiposity and serum lipids in adolescent offspring, suggesting a potential predisposition to future development of metabolic disorders.

The role of mTOR in lipid homeostasis and diabetes progression

PURPOSE OF REVIEW

Metabolic diseases, such as type 2 diabetes, cardiac dysfunction, hypertension, and hepatic steatosis, share one critical causative factor: abnormal lipid partitioning, that redistribution of triglycerides from adipocytes to nonadipose peripheral tissues. Lipid overload of these tissues causes a number of pathological effects collectively known as lipotoxicity. If we find the way to correct lipid partitioning, we will restrain metabolic diseases, improve life quality and life expectancy and radically reduce healthcare costs.

RECENT FINDINGS

Lipid partitioning in the body is maintained by tightly regulated and balanced rates of de novo lipogenesis, lipolysis, adipogenesis, and mitochondrial oxidation primarily in fat and liver. Recent studies highlighted in this review have established mTOR as a central regulator of lipid storage and metabolism.

SUMMARY

Increased activity of mTOR in obesity may compensate for the negative consequences of overnutrition, whereas dysregulation of mTOR may lead to abnormal lipid partitioning and metabolic disease.

Impact of SCP-2/SCP-x gene ablation and dietary cholesterol on hepatic lipid accumulation

While a high-cholesterol diet induces hepatic steatosis, the role of intracellular sterol carrier protein-2/sterol carrier protein-x (SCP-2/SCP-x) proteins is unknown. We hypothesized that ablating SCP-2/SCP-x [double knockout (DKO)] would impact hepatic lipids (cholesterol and cholesteryl ester), especially in high-cholesterol-fed mice. DKO did not alter food consumption, and body weight (BW) gain decreased especially in females, concomitant with hepatic steatosis in females and less so in males. DKO-induced steatosis in control-fed wild-type (WT) mice was associated with 1) loss of SCP-2; 2) upregulation of liver fatty acid binding protein (L-FABP); 3) increased mRNA and/or protein levels of sterol regulatory element binding proteins (SREBP1 and SREBP2) as well as increased expression of target genes of cholesterol synthesis (Hmgcs1 and Hmgcr) and fatty acid synthesis (Acc1 and Fas); and 4) cholesteryl ester accumulation was also associated with increased acyl-CoA cholesterol acyltransferase-2 (ACAT2) in males. DKO exacerbated the high-cholesterol diet-induced hepatic cholesterol and glyceride accumulation, without further increasing SREBP1, SREBP2, or target genes. This exacerbation was associated both with loss of SCP-2 and concomitant downregulation of Ceh/Hsl, apolipoprotein B (ApoB), MTP, and/or L-FABP protein expression. DKO diminished the ability to secrete excess cholesterol into bile and oxidize cholesterol to bile acid for biliary excretion, especially in females. This suggested that SCP-2/SCP-x affects cholesterol transport to particular intracellular compartments, with ablation resulting in less to the endoplasmic reticulum for SREBP regulation, making more available for cholesteryl ester synthesis, for cholesteryl-ester storage in lipid droplets, and for bile salt synthesis and/or secretion. These alterations are significant findings, since they affect key processes in regulation of sterol metabolism.

Liver Perilipin 5 Expression Worsens Hepatosteatosis But Not Insulin Resistance in High Fat-Fed Mice

Perilipin 5 (PLIN5) is a lipid droplet (LD) protein highly expressed in oxidative tissues, including the fasted liver. However, its expression also increases in nonalcoholic fatty liver. To determine whether PLIN5 regulates metabolic phenotypes of hepatosteatosis under nutritional excess, liver targeted overexpression of PLIN5 was achieved using adenoviral vector (Ad-PLIN5) in male C57BL/6J mice fed high-fat diet. Mice treated with adenovirus expressing green fluorescent protein (GFP) (Ad-GFP) served as control. Ad-PLIN5 livers increased LD in the liver section, and liquid chromatography with tandem mass spectrometry revealed increases in lipid classes associated with LD, including triacylglycerol, cholesterol ester, and phospholipid classes, compared with Ad-GFP liver. Lipids commonly associated with hepatic lipotoxicity, diacylglycerol, and ceramides, were also increased in Ad-PLIN5 liver. The expression of genes in lipid metabolism regulated by peroxisome proliferator-activated receptor-α was reduced suggestive of slower mobilization of stored lipids in Ad-PLIN5 mice. However, the increase of hepatosteatosis by PLIN5 overexpression did not worsen glucose homeostasis. Rather, serum insulin levels were decreased, indicating better insulin sensitivity in Ad-PLIN5 mice. Moreover, genes associated with liver injury were unaltered in Ad-PLIN5 steatotic liver compared with Ad-GFP control. Phosphorylation of protein kinase B was increased in Ad-PLIN5-transduced AML12 hepatocyte despite of the promotion of fatty acid incorporation to triacylglycerol as well. Collectively, our data indicates that the increase in liver PLIN5 during hepatosteatosis drives further lipid accumulation but does not adversely affect hepatic health or insulin sensitivity.

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.

Naringenin prevents obesity, hepatic steatosis, and glucose intolerance in male mice independent of fibroblast growth factor 21

The molecular mechanisms and metabolic pathways whereby the citrus flavonoid, naringenin, reduces dyslipidemia and improves glucose tolerance were investigated in C57BL6/J wild-type mice and fibroblast growth factor 21 (FGF21) null (Fgf21(-/-)) mice. FGF21 regulates energy homeostasis and the metabolic adaptation to fasting. One avenue of this regulation is through induction of peroxisome proliferator-activated receptor-γ coactivator-1α (Pgc1a), a regulator of hepatic fatty acid oxidation and ketogenesis. Because naringenin is a potent activator of hepatic FA oxidation, we hypothesized that induction of FGF21 might be an integral part of naringenin's mechanism of action. Furthermore, we predicted that FGF21 deficiency would potentiate high-fat diet (HFD)-induced metabolic dysregulation and compromise metabolic protection by naringenin. The absence of FGF21 exacerbated the response to a HFD. Interestingly, naringenin supplementation to the HFD robustly prevented obesity in both genotypes. Gene expression analysis suggested that naringenin was not primarily targeting fatty acid metabolism in white adipose tissue. Naringenin corrected hepatic triglyceride concentrations and normalized hepatic expression of Pgc1a, Cpt1a, and Srebf1c in both wild-type and Fgf21(-/-) mice. HFD-fed Fgf21(-/-) mice displayed greater muscle triglyceride deposition, hyperinsulinemia, and impaired glucose tolerance as compared with wild-type mice, confirming the role of FGF21 in insulin sensitivity; however, naringenin supplementation improved these metabolic parameters in both genotypes. We conclude that FGF21 deficiency exacerbates HFD-induced obesity, hepatic steatosis, and insulin resistance. Furthermore, FGF21 is not required for naringenin to protect mice from HFD-induced metabolic dysregulation. Collectively these studies support the concept that naringenin has potent lipid-lowering effects and may act as an insulin sensitizer in vivo.

Calcium-independent phospholipases A2 and their roles in biological processes and diseases

Among the family of phospholipases A2 (PLA2s) are the Ca(2+)-independent PLA2s (iPLA2s) and they are designated group VI iPLA2s. In relation to secretory and cytosolic PLA2s, the iPLA2s are more recently described and details of their expression and roles in biological functions are rapidly emerging. The iPLA2s or patatin-like phospholipases (PNPLAs) are intracellular enzymes that do not require Ca(2+) for activity, and contain lipase (GXSXG) and nucleotide-binding (GXGXXG) consensus sequences. Though nine PNPLAs have been recognized, PNPLA8 (membrane-associated iPLA2γ) and PNPLA9 (cytosol-associated iPLA2β) are the most widely studied and understood. The iPLA2s manifest a variety of activities in addition to phospholipase, are ubiquitously expressed, and participate in a multitude of biological processes, including fat catabolism, cell differentiation, maintenance of mitochondrial integrity, phospholipid remodeling, cell proliferation, signal transduction, and cell death. As might be expected, increased or decreased expression of iPLA2s can have profound effects on the metabolic state, CNS function, cardiovascular performance, and cell survival; therefore, dysregulation of iPLA2s can be a critical factor in the development of many diseases. This review is aimed at providing a general framework of the current understanding of the iPLA2s and discussion of the potential mechanisms of action of the iPLA2s and related involved lipid mediators.

ATGL-catalyzed lipolysis regulates SIRT1 to control PGC-1α/PPAR-α signaling

Sirtuin 1 (SIRT1), an NAD(+)-dependent protein deacetylase, regulates a host of target proteins, including peroxisome proliferator-activated receptor (PPAR)-γ coactivator-1α (PGC-1α), a transcriptional coregulator that binds to numerous transcription factors in response to deacetylation to promote mitochondrial biogenesis and oxidative metabolism. Our laboratory and others have shown that adipose triglyceride lipase (ATGL) increases the activity of the nuclear receptor PPAR-α, a PGC-1α binding partner, to promote fatty acid oxidation. Fatty acids bind and activate PPAR-α; therefore, it has been presumed that fatty acids derived from ATGL-catalyzed lipolysis act as PPAR-α ligands. We provide an alternate mechanism that links ATGL to PPAR-α signaling. We show that SIRT1 deacetylase activity is positively regulated by ATGL to promote PGC-1α signaling. In addition, ATGL mediates the effects of β-adrenergic signaling on SIRT1 activity, and PGC-1α and PPAR-α target gene expression independent of changes in NAD(+). Moreover, SIRT1 is required for the induction of PGC-1α/PPAR-α target genes and oxidative metabolism in response to increased ATGL-mediated lipolysis. Taken together, this work identifies SIRT1 as a critical node that links β-adrenergic signaling and lipolysis to changes in the transcriptional regulation of oxidative metabolism.

Adipose triglyceride lipase regulates lipid metabolism in dairy goat mammary epithelial cells

Adipose triglyceride lipase (ATGL) catalyzes the initial step in the lipid lipolysis process, hydrolyzing triglyceride (TG) to produce diacylglycerol (DG) and free fatty acids (FFA). In addition, ATGL regulates lipid storage and release in adipocyte cells. However, its role in mammary gland tissue remains unclear. To assess the role of the ATGL gene in the goat mammary gland, this study analyzed the tissue distribution and expression of key genes together with lipid accumulation after knockdown of the ATGL gene. The mRNA of ATGL was highly expressed in subcutaneous adipose tissue, the lung and the mammary gland with a significant increase in expression during the lactation period compared with the dry period of the mammary gland. Knockdown of the ATGL gene in goat mammary epithelial cells (GMECs) using siRNA resulted in a significant decrease in both ATGL mRNA and protein levels. Silencing of the ATGL gene markedly increased lipid droplet accumulation and intracellular TG concentration (P<0.05), while it reduced FFA levels in GMECs (P<0.05). Additionally, the expression of HSL for lipolysis, FABP3 for fatty acid transport, PPARα for fatty acid oxidation, ADFP, BTN1A1, and XDH for milk fat formation and secretion was down-regulated (P<0.05) after knockdown of the ATGL gene, with increased expression of CD36 for fatty acid uptake (P<0.05). In conclusion, these data suggest that the ATGL gene plays an important role in triglyceride lipolysis in GMECs and provides the first experimental evidence that ATGL may be involved in lipid metabolism during lactation.

Fatty acid elongase-5 (Elovl5) regulates hepatic triglyceride catabolism in obese C57BL/6J mice

Nonalcoholic fatty liver disease is a major public health concern in the obese and type 2 diabetic populations. The high-fat lard diet induces obesity and fatty liver in C57BL/6J mice and suppresses expression of the PPAR-target gene, FA elongase 5 (Elovl5). Elovl5 plays a key role in MUFA and PUFA synthesis. Increasing hepatic Elovl5 activity in obese mice lowered hepatic TGs and endoplasmic reticulum stress markers (X-box binding protein 1 and cAMP-dependent transcription factor 6α) and increased TG catabolism and fatty acyl carnitines. Increased hepatic Elovl5 activity did not increase hepatic capacity for β-oxidation. Elovl5 effects on hepatic TG catabolism were linked to increased protein levels of adipocyte TG lipase (ATGL) and comparative gene identification 58 (CGI58). Elevated hepatic Elovl5 activity also induced the expression of some (pyruvate dehydrogenase kinase 4 and fibroblast growth factor 21), but not other cytochrome P450 4A10 (CYP4A10), PPAR-target genes. FA products of Elovl5 activity increased ATGL, but not CGI58, mRNA through PPARβ-dependent mechanisms in human HepG2 cells. Treatment of mouse AML12 hepatocytes with the PPARβ agonist (GW0742) decreased (14)C-18:2,n-6 in TGs but did not affect β-oxidation. These studies establish that Elovl5 activity regulates hepatic levels of FAs controlling PPARβ activity, ATGL expression, and TG catabolism, but not FA oxidation.