CGI-58 facilitates the mobilization of cytoplasmic triglyceride for lipoprotein secretion in hepatoma cells

The ubiquitin E3 ligase BFAR promotes degradation of PNPLA3

A missense variant in patatin-like phospholipase domain-containing protein 3 [PNPLA3(I148M)] is the most impactful genetic risk factor for fatty liver disease (FLD). We previously showed that PNPLA3 is ubiquitylated and subsequently degraded by proteasomes and autophagosomes and that the PNPLA3(148M) variant interferes with this process. To define the machinery responsible for PNPLA3 turnover, we used small interfering (si)RNAs to inactivate components of the ubiquitin proteasome system. Inactivation of bifunctional apoptosis regulator (BFAR), a membrane-bound E3 ubiquitin ligase, reproducibly increased PNPLA3 levels in two lines of cultured hepatocytes. Conversely, overexpression of BFAR decreased levels of endogenous PNPLA3 in HuH7 cells. BFAR and PNPLA3 co-immunoprecipitated when co-expressed in cells. BFAR promoted ubiquitylation of PNPLA3 in vitro in a reconstitution assay using purified, epitope-tagged recombinant proteins. To confirm that BFAR targets PNPLA3, we inactivated Bfar in mice. Levels of PNPLA3 protein were increased twofold in hepatic lipid droplets of Bfar-/- mice with no associated increase in PNPLA3 mRNA levels. Taken together these data are consistent with a model in which BFAR plays a role in the post-translational degradation of PNPLA3. The identification of BFAR provides a potential target to enhance PNPLA3 turnover and prevent FLD.

Sustained release of therapeutic gene by injectable hydrogel for hepatocellular carcinoma

Gene therapy has shown remarkable effectiveness in the management of disease like cancer and inflammation as a revolutionary therapeutic. Nonetheless, therapeutic drug target discovery, efficient gene delivery, and gene delivery vehicles continue to be significant obstacles. Due to their effective gene transport capabilities and low immunogenicity, supramolecular polymers have garnered significant interest. Herein, ABHD5 is identified as a potential therapeutic target since it is dysregulated in hepatocellular carcinoma (HCC). Interestingly, the downregulation of ABHD5 could induce programmed death-ligand 1 (PD-L1) expression in liver cancer, which may contribute to the immunosuppression. To overcome the immunosuppression caused by PD-L1, an injectable hydrogel is designed to achieve efficient abhydrolase domain containing 5 (ABHD5) gene delivery via the host-guest interaction with branched polyethyleneimine-g-poly (ethylene glycol), poly (ethylene oxide) and poly (propylene oxide) block copolymers and α-CD (PPA/CD), demonstrating the capability for sustained gene release. The co-assembly hydrogel demonstrates good biocompatibility and enhanced gene transfection efficiency, efficiently triggering tumor cell apoptosis. Overall, the results of this study suggest that ABHD5 is a potential therapeutic target, and that a host-guest-based supramolecular hydrogel could serve as a promising platform for the inhibition of HCC.

ABHD5-A Regulator of Lipid Metabolism Essential for Diverse Cellular Functions

The α/β-Hydrolase domain-containing protein 5 (ABHD5; also known as comparative gene identification-58, or CGI-58) is the causative gene of the Chanarin-Dorfman syndrome (CDS), a disorder mainly characterized by systemic triacylglycerol accumulation and a severe defect in skin barrier function. The clinical phenotype of CDS patients and the characterization of global and tissue-specific ABHD5-deficient mouse strains have demonstrated that ABHD5 is a crucial regulator of lipid and energy homeostasis in various tissues. Although ABHD5 lacks intrinsic hydrolase activity, it functions as a co-activating enzyme of the patatin-like phospholipase domain-containing (PNPLA) protein family that is involved in triacylglycerol and glycerophospholipid, as well as sphingolipid and retinyl ester metabolism. Moreover, ABHD5 interacts with perilipins (PLINs) and fatty acid-binding proteins (FABPs), which are important regulators of lipid homeostasis in adipose and non-adipose tissues. This review focuses on the multifaceted role of ABHD5 in modulating the function of key enzymes in lipid metabolism.

Protective Effects of Tiaoganquzhi Decoction in Treating inflammatory Injury of Nonalcoholic Fatty liver Disease by Promoting CGI-58 and Inhibiting Expression of NLRP3 Inflammasome

Tiaoganquzhi Decoction (TGQZD) is a traditional Chinese herbal formulation demonstrated to be a clinically effective treatment for nonalcoholic fatty liver disease (NAFLD), although details concerning its clinical mechanism are poor. This study aimed to explore the mechanism of TGQZD on improvement of inflammatory damage and dyslipidemia caused by NAFLD through the CGI-58/ROS/NLRP3 inflammasome pathway. In our research, the in vivo protective effects of TGQZD on HFD-induced liver injury in rats and in vitro using lipopolysaccharide (LPS)+palmitate (PA)-stimulated HepG-2 cells model. Histological changes were evaluated by hematoxylin-eosin and Oil Red O staining. Inflammatory cytokines and protein expression were analyzed by ELISA, Real time PCR and western blotting. Liver function, blood lipids, free fatty acids (FFA), and reactive oxygen species (ROS) were determined by biochemical detection. Our results indicated that TGQZD exhibited anti-inflammatory activity, reduced the severity of NAFLD and ameliorated the pathological changes. Further, TGQZD improved liver function and lipid metabolism in NAFLD rats. TGQZD lowered serum aspartate aminotransferase, alanine aminotransferase, triglyceride, and total cholesterol levels. TGQZD suppressed the formulation of FFA and ROS. It also reduced the expression and release of the inflammatory cytokine interleukin-1β by promoting CGI-58 expression and inhibiting the expression of FFA, TNF-α, and the NLRP3 inflammasome induced by ROS. TGQZD exhibited anti-inflammatory effects via the CGI-58, ROS and NLRP3 inflammasome pathway in vivo and in vitro, respectively. Our findings demonstrated that TGQZD is a useful and effective therapeutic agent for treating NAFLD via promotion of CGI-58 to inhibit the expression of ROS-induced NLRP3 inflammasome.

Lipolysis: cellular mechanisms for lipid mobilization from fat stores

The perception that intracellular lipolysis is a straightforward process that releases fatty acids from fat stores in adipose tissue to generate energy has experienced major revisions over the last two decades. The discovery of new lipolytic enzymes and coregulators, the demonstration that lipophagy and lysosomal lipolysis contribute to the degradation of cellular lipid stores and the characterization of numerous factors and signalling pathways that regulate lipid hydrolysis on transcriptional and post-transcriptional levels have revolutionized our understanding of lipolysis. In this review, we focus on the mechanisms that facilitate intracellular fatty-acid mobilization, drawing on canonical and noncanonical enzymatic pathways. We summarize how intracellular lipolysis affects lipid-mediated signalling, metabolic regulation and energy homeostasis in multiple organs. Finally, we examine how these processes affect pathogenesis and how lipolysis may be targeted to potentially prevent or treat various diseases.

Western diet induces severe nonalcoholic steatohepatitis, ductular reaction, and hepatic fibrosis in liver CGI-58 knockout mice

Humans and rodents with Comparative Gene Identification-58 (CGI-58) mutations manifest nonalcoholic fatty liver disease (NAFLD). Here we show that liver CGI-58 knockout (LivKO) mice fed a Western diet rapidly develop advanced NAFLD, including nonalcoholic steatohepatitis (NASH) and hepatic fibrosis. After 14 weeks of diet challenge, starting at 6 weeks of age, LivKO mice showed increased inflammatory cell infiltration and proinflammatory gene expression in the liver, which was associated with elevated plasma levels of aminotransferases. Hepatic ductular reactions, pericellular fibrosis, and bridging fibrosis were observed only in the LivKO mice. Consistently, the KO mice had a significant increase in hepatic mRNAs for fibrogenic genes. In addition, LivKO mice displayed massive accumulation of lipid droplets (LDs) in hepatocytes. LDs were also observed in the cholangiocytes of the LivKO mice, but not the floxed controls. Four of the five LD coat proteins, including perilipins 2, 3, 4, and 5, were increased in the CGI-58 KO liver. CRISPR/Cas9-mediated knockout of CGI-58 in Huh7 human hepatoma cells induced LD deposition and perilipin expression, suggesting a cell autonomous effect. Our findings establish the Western diet-fed LivKO mice as an animal model of NASH and hepatic fibrosis. These animals may facilitate preclinical screening of therapeutic agents that counter against NAFLD progression.

CGI-58: Versatile Regulator of Intracellular Lipid Droplet Homeostasis

Comparative gene identification-58 (CGI-58), also known as α/β-hydrolase domain-containing 5 (ABHD5), is a member of a large family of proteins containing an α/β-hydrolase-fold. CGI-58 is well-known as the co-activator of adipose triglyceride lipase (ATGL), which is a key enzyme initiating cytosolic lipid droplet lipolysis. Mutations in either the human CGI-58 or ATGL gene cause an autosomal recessive neutral lipid storage disease, characterized by the excessive accumulation of triglyceride (TAG)-rich lipid droplets in the cytoplasm of almost all cell types. CGI-58, however, has ATGL-independent functions. Distinct phenotypes associated with CGI-58 deficiency commonly include ichthyosis (scaly dry skin), nonalcoholic steatohepatitis, and hepatic fibrosis. Through regulated interactions with multiple protein families, CGI-58 controls many metabolic and signaling pathways, such as lipid and glucose metabolism, energy balance, insulin signaling, inflammatory responses, and thermogenesis. Recent studies have shown that CGI-58 regulates the pathogenesis of common metabolic diseases in a tissue-specific manner. Future studies are needed to molecularly define ATGL-independent functions of CGI-58, including the newly identified serine protease activity of CGI-58. Elucidation of these versatile functions of CGI-58 may uncover fundamental cellular processes governing lipid and energy homeostasis, which may help develop novel approaches that counter against obesity and its associated metabolic sequelae.

Novel Pharmacological Probes Reveal ABHD5 as a Locus of Lipolysis Control in White and Brown Adipocytes

Current knowledge regarding acute regulation of adipocyte lipolysis is largely based on receptor-mediated activation or inhibition of pathways that influence intracellular levels of cAMP, thereby affecting protein kinase A (PKA) activity. We recently identified synthetic ligands of α-β-hydrolase domain containing 5 (ABHD5) that directly activate adipose triglyceride lipase (ATGL) by dissociating ABHD5 from its inhibitory regulator, perilipin-1 (PLIN1). In the current study, we used these novel ligands to determine the direct contribution of ABHD5 to various aspects of lipolysis control in white (3T3-L1) and brown adipocytes. ABHD5 ligands stimulated adipocyte lipolysis without affecting PKA-dependent phosphorylation on consensus sites of PLIN1 or hormone-sensitive lipase (HSL). Cotreatment of adipocytes with synthetic ABHD5 ligands did not alter the potency or maximal lipolysis efficacy of the β-adrenergic receptor (ADRB) agonist isoproterenol (ISO), indicating that both target a common pool of ABHD5. Reducing ADRB/PKA signaling with insulin or desensitizing ADRB suppressed lipolysis responses to a subsequent challenge with ISO, but not to ABHD5 ligands. Lastly, despite strong treatment differences in PKA-dependent phosphorylation of HSL, we found that ligand-mediated activation of ABHD5 led to complete triglyceride hydrolysis, which predominantly involved ATGL, but also HSL. These results indicate that the overall pattern of lipolysis controlled by ABHD5 ligands is similar to that of isoproterenol, and that ABHD5 plays a central role in the regulation of adipocyte lipolysis. As lipolysis is critical for adaptive thermogenesis and in catabolic tissue remodeling, ABHD5 ligands may provide a means of activating these processes under conditions where receptor signaling is compromised.

Critical roles for α/β hydrolase domain 5 (ABHD5)/comparative gene identification-58 (CGI-58) at the lipid droplet interface and beyond

Mutations in the gene encoding comparative gene identification 58 (CGI-58), also known as α β hydrolase domain-containing 5 (ABHD5), cause neutral lipid storage disorder with ichthyosis (NLSDI). This inborn error in metabolism is characterized by ectopic accumulation of triacylglycerols (TAG) within cytoplasmic lipid droplets in multiple cell types. Studies over the past decade have clearly demonstrated that CGI-58 is a potent regulator of TAG hydrolysis in the disease-relevant cell types. However, despite the reproducible genetic link between CGI-58 mutations and TAG storage, the molecular mechanisms by which CGI-58 regulates TAG hydrolysis are still incompletely understood. It is clear that CGI-58 can regulate TAG hydrolysis by activating the major TAG hydrolase adipose triglyceride lipase (ATGL), yet CGI-58 can also regulate lipid metabolism via mechanisms that do not involve ATGL. This review highlights recent progress made in defining the physiologic and biochemical function of CGI-58, and its broader role in energy homeostasis. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

Regulation of Hepatic Triacylglycerol Metabolism by CGI-58 Does Not Require ATGL Co-activation

Adipose triglyceride lipase (ATGL) and comparative gene identification 58 (CGI-58) are critical regulators of triacylglycerol (TAG) turnover. CGI-58 is thought to regulate TAG mobilization by stimulating the enzymatic activity of ATGL. However, it is not known whether this coactivation function of CGI-58 occurs in vivo. Moreover, the phenotype of human CGI-58 mutations suggests ATGL-independent functions. Through direct comparison of mice with single or double deficiency of CGI-58 and ATGL, we show here that CGI-58 knockdown causes hepatic steatosis in both the presence and absence of ATGL. CGI-58 also regulates hepatic diacylglycerol (DAG) and inflammation in an ATGL-independent manner. Interestingly, ATGL deficiency, but not CGI-58 deficiency, results in suppression of the hepatic and adipose de novo lipogenic program. Collectively, these findings show that CGI-58 regulates hepatic neutral lipid storage and inflammation in the genetic absence of ATGL, demonstrating that mechanisms driving TAG lipolysis in hepatocytes differ significantly from those in adipocytes.

ABHD5/CGI-58, the Chanarin-Dorfman Syndrome Protein, Mobilises Lipid Stores for Hepatitis C Virus Production

Hepatitis C virus (HCV) particles closely mimic human very-low-density lipoproteins (VLDL) to evade humoral immunity and to facilitate cell entry. However, the principles that govern HCV association with VLDL components are poorly defined. Using an siRNA screen, we identified ABHD5 (α/β hydrolase domain containing protein 5, also known as CGI-58) as a new host factor promoting both virus assembly and release. ABHD5 associated with lipid droplets and triggered their hydrolysis. Importantly, ABHD5 Chanarin-Dorfman syndrome mutants responsible for a rare lipid storage disorder in humans were mislocalised, and unable to consume lipid droplets or support HCV production. Additional ABHD5 mutagenesis revealed a novel tribasic motif that does not influence subcellular localization but determines both ABHD5 lipolytic and proviral properties. These results indicate that HCV taps into the lipid droplet triglyceride reservoir usurping ABHD5 lipase cofactor function. They also suggest that the resulting lipid flux, normally devoted to VLDL synthesis, also participates in the assembly and release of the HCV lipo-viro-particle. Altogether, our study provides the first association between the Chanarin-Dorfman syndrome protein and an infectious disease and sheds light on the hepatic manifestations of this rare genetic disorder as well as on HCV morphogenesis.

HCV replication is linked to the host lipid metabolism, and virions are secreted as lipo-viro-particles whose density, size and biochemical content resemble VLDL. HCV assembles close to lipid droplets and is released via the secretory pathway, but it remains unclear how it accesses the VLDL assembly pathway. In this study, we identified ABHD5 as a new host factor supporting HCV assembly and release. ABHD5 is a lipid droplet-associated lipase cofactor. In hepatocytes, ABHD5 was proposed to promote the recruitment of triglycerides from cytosolic towards luminal lipid droplets by mediating a cycle of phospholipid hydrolysis/re-esterification. Our data suggest that this ABHD5-dependent lipid transfer is not only required for VLDL maturation, but also for HCV assembly and virion release, indicating that lipid remodelling impacts on both assembly and virus transport. Finally, ABHD5 is associated with the Chanarin-Dorfman syndrome, a rare human genetic lipid metabolism disorder. We found that the Chanarin-Dorfman syndrome mutants were unable to support HCV assembly, pointing at a new host polymorphism that could determine susceptibility to HCV infection. Altogether, our results establish a new link between HCV, VLDL assembly and lipid remodeling pathways and open new possibilities to study the etiology of the liver manifestations of the Chanarin-Dorfman syndrome.

Muscle-specific deletion of comparative gene identification-58 (CGI-58) causes muscle steatosis but improves insulin sensitivity in male mice

Intramyocellular accumulation of lipids is often associated with insulin resistance. Deficiency of comparative gene identification-58 (CGI-58) causes cytosolic deposition of triglyceride (TG)-rich lipid droplets in most cell types, including muscle due to defective TG hydrolysis. It was unclear, however, whether CGI-58 deficiency-induced lipid accumulation in muscle influences insulin sensitivity. Here we show that muscle-specific CGI-58 knockout mice relative to their controls have increased glucose tolerance and insulin sensitivity on a Western-type high-fat diet, despite TG accumulation in both heart and oxidative skeletal muscle and cholesterol deposition in heart. Although the intracardiomyocellular lipid deposition results in cardiac ventricular fibrosis and systolic dysfunction, muscle-specific CGI-58 knockout mice show increased glucose uptake in heart and soleus muscle, improved insulin signaling in insulin-sensitive tissues, and reduced plasma concentrations of glucose, insulin, and cholesterol. Hepatic contents of TG and cholesterol are also decreased in these animals. Cardiac steatosis is attributable, at least in part, to decreases in cardiac TG hydrolase activity and peroxisome proliferator-activated receptor-α/peroxisome proliferator-activated receptor-γ coactivator-1-dependent mitochondrial fatty acid oxidation. In conclusion, muscle CGI-58 deficiency causes cardiac dysfunction and fat deposition in oxidative muscles but induces a series of favorable metabolic changes in mice fed a high-fat diet.

Transgenic CGI-58 expression in macrophages alleviates the atherosclerotic lesion development in ApoE knockout mice

Comparative Gene Identification-58 (CGI-58), as an adipose triglyceride lipase (ATGL) activator, strongly in- creases ATGL-mediated triglyceride (TG) catabolism. Previous studies have shown that CGI-58 affects intestinal cholesterol homeostasis independently of ATGL activity. Therefore, we hypothesized that CGI-58 was involved in macrophage cholesterol metabolism and consequently atherosclerotic lesion formation. Here, we generated macrophage-specific CGI-58 transgenic mice (Mac-CGI-58 Tg) using an SRA promoter, which was further mated with ApoE-/- mice to create litters of CGI-58 Tg/ApoE-/- mice. These CGI-58 Tg/ApoE-/- mice exhibited an anti-atherosclerosis phenotype compared with wild type (WT) controls (CGI-58 WT/ApoE-/-), illustrated by less plaque area in aortic roots. Moreover, macrophage-specific CGI-58 overexpression in mice resulted in upregulated levels of plasma total cholesterol and HDL-cholesterol. Consequently, higher expression levels of PPARa, PPARγ, LXRα, ABCA1, and ABCG1 were detected in macrophages from CGI-58 Tg/ApoE-/- mice compared to CGI-58 WT/ApoE-/- counterparts, which were accompanied by elevated macrophage cholesterol efflux toward HDL and Apo A1. Nevertheless, serum levels of TNF-α and IL-6 were reduced by macrophage-specific CGI-58 overexpression. Finally, bone marrow (BM) transplantation experiments further revealed that ApoE-/- mice reconstituted with Mac-CGI-58 Tg BM cells (ApoE-/-Tg-BM chimera) displayed a significant reduction of atherosclerosis lesions compared with control mice reconstituted with Mac-CGI-58 WT BM cells (ApoE-/-/WT-BM chimera). Collectively, these data strongly suggest that CGI-58 overexpression in macrophages may protect against atherosclerosis development in mice.

Loss of abhd5 promotes colorectal tumor development and progression by inducing aerobic glycolysis and epithelial-mesenchymal transition

How cancer cells shift metabolism to aerobic glycolysis is largely unknown. Here, we show that deficiency of α/β-hydrolase domain-containing 5 (Abhd5), an intracellular lipolytic activator that is also known as comparative gene identification 58 (CGI-58), promotes this metabolic shift and enhances malignancies of colorectal carcinomas (CRCs). Silencing of Abhd5 in normal fibroblasts induces malignant transformation. Intestine-specific knockout of Abhd5 in Apc(Min/+) mice robustly increases tumorigenesis and malignant transformation of adenomatous polyps. In colon cancer cells, Abhd5 deficiency induces epithelial-mesenchymal transition by suppressing the AMPKα-p53 pathway, which is attributable to increased aerobic glycolysis. In human CRCs, Abhd5 expression falls substantially and correlates negatively with malignant features. Our findings link Abhd5 to CRC pathogenesis and suggest that cancer cells develop aerobic glycolysis by suppressing Abhd5-mediated intracellular lipolysis.

Distinct roles for alpha-beta hydrolase domain 5 (ABHD5/CGI-58) and adipose triglyceride lipase (ATGL/PNPLA2) in lipid metabolism and signaling

Catabolism of stored triacylglycerol (TAG) from cytoplasmic lipid droplets is critical for providing energy substrates, membrane building blocks and signaling lipids in most cells of the body. However, the lipolytic machinery dictating TAG hydrolysis varies greatly among different cell types. Within the adipocyte, TAG hydrolysis is dynamically regulated by hormones to ensure appropriate metabolic adaptation to nutritional and physiologic cues. In other cell types such as hepatocytes, myocytes and macrophages, mobilization of stored TAG is regulated quite differently. Within the last decade, mutations in two key genes involved in TAG hydrolysis, α-β hydrolase domain 5 (ABHD5/CGI-58) and adipose triglyceride lipase (ATGL/PNPLA2), were found to cause two distinct neutral lipid storage diseases (NLSD) in humans. These genetic links, along with supporting evidence in mouse models, have prompted a number of studies surrounding the biochemical function(s) of these proteins. Although both CGI-58 and ATGL have been clearly implicated in TAG hydrolysis in multiple tissues and have even been shown to physically interact with each other, recent evidence suggests that they may also have distinct roles. The purpose of this review is to summarize the most recent insights into how CGI-58 and ATGL regulate lipid metabolism and signaling.

Macrophage CGI-58 deficiency activates ROS-inflammasome pathway to promote insulin resistance in mice

Overnutrition activates a proinflammatory program in macrophages to induce insulin resistance (IR), but its molecular mechanisms remain incompletely understood. Here, we show that saturated fatty acid and lipopolysaccharide, two factors implicated in high-fat diet (HFD)-induced IR, suppress macrophage CGI-58 expression. Macrophage-specific CGI-58 knockout (MaKO) in mice aggravates HFD-induced glucose intolerance and IR, which is associated with augmented systemic/tissue inflammation and proinflammatory activation of adipose tissue macrophages. CGI-58-deficient macrophages exhibit mitochondrial dysfunction due to defective peroxisome proliferator-activated receptor (PPAR)γ signaling. Consequently, they overproduce reactive oxygen species (ROS) to potentiate secretion of proinflammatory cytokines by activating NLRP3 inflammasome. Anti-ROS treatment or NLRP3 silencing prevents CGI-58-deficient macrophages from oversecreting proinflammatory cytokines and from inducing proinflammatory signaling and IR in the cocultured fat slices. Anti-ROS treatment also prevents exacerbation of inflammation and IR in HFD-fed MaKO mice. Our data thus establish CGI-58 as a suppressor of overnutrition-induced NLRP3 inflammasome activation in macrophages.

Comparative gene identification-58/α/β hydrolase domain 5: more than just an adipose triglyceride lipase activator?

PURPOSE OF REVIEW

Comparative gene identification-58 (CGI-58) is a lipid droplet-associated protein that controls intracellular triglyceride levels by its ability to activate adipose triglyceride lipase (ATGL). Additionally, CGI-58 was described to exhibit lysophosphatidic acid acyl transferase (LPAAT) activity. This review focuses on the significance of CGI-58 in energy metabolism in adipose and nonadipose tissue.

RECENT FINDINGS

Recent studies with transgenic and CGI-58-deficient mouse strains underscored the importance of CGI-58 as a regulator of intracellular energy homeostasis by modulating ATGL-driven triglyceride hydrolysis. In accordance with this function, mice and humans that lack CGI-58 accumulate triglyceride in multiple tissues. Additionally, CGI-58-deficient mice develop an ATGL-independent severe skin barrier defect and die soon after birth. Although the premature death prevented a phenotypical characterization of adult global CGI-58 knockout mice, the characterization of mice with tissue-specific CGI-58 deficiency revealed new insights into its role in neutral lipid and energy metabolism. Concerning the ATGL-independent function of CGI-58, a recently identified LPAAT activity for CGI-58 was shown to be involved in the generation of signaling molecules regulating inflammatory processes and insulin action.

SUMMARY

Although the function of CGI-58 in the catabolism of cellular triglyceride depots via ATGL is well established, further studies are required to consolidate the function of CGI-58 as LPAAT and to clarify the involvement of CGI-58 in the metabolism of skin lipids.

Intestinal Cgi-58 deficiency reduces postprandial lipid absorption

Comparative Gene Identification-58 (CGI-58), a lipid droplet (LD)-associated protein, promotes intracellular triglyceride (TG) hydrolysis in vitro. Mutations in human CGI-58 cause TG accumulation in numerous tissues including intestine. Enterocytes are thought not to store TG-rich LDs, but a fatty meal does induce temporary cytosolic accumulation of LDs. Accumulated LDs are eventually cleared out, implying existence of TG hydrolytic machinery in enterocytes. However, identities of proteins responsible for LD-TG hydrolysis remain unknown. Here we report that intestine-specific inactivation of CGI-58 in mice significantly reduces postprandial plasma TG concentrations and intestinal TG hydrolase activity, which is associated with a 4-fold increase in intestinal TG content and large cytosolic LD accumulation in absorptive enterocytes during the fasting state. Intestine-specific CGI-58 knockout mice also display mild yet significant decreases in intestinal fatty acid absorption and oxidation. Surprisingly, inactivation of CGI-58 in intestine significantly raises plasma and intestinal cholesterol, and reduces hepatic cholesterol, without altering intestinal cholesterol absorption and fecal neutral sterol excretion. In conclusion, intestinal CGI-58 is required for efficient postprandial lipoprotein-TG secretion and for maintaining hepatic and plasma lipid homeostasis. Our animal model will serve as a valuable tool to further define how intestinal fat metabolism influences the pathogenesis of metabolic disorders, such as obesity and type 2 diabetes.

Triglyceride-rich lipoproteins and cytosolic lipid droplets in enterocytes: key players in intestinal physiology and metabolic disorders

During the post-prandial phase, intestinal triglyceride-rich lipoproteins (TRL) i.e. chylomicrons are the main contributors to the serum lipid level, which is linked to coronary artery diseases. Hypertriglyceridemia can originate from decreased clearance or increased production of TRL. During lipid absorption, enterocytes produce and secrete chylomicrons and transiently store lipid droplets (LDs) in the cytosol. The dynamic fluctuation of triglycerides in cytosolic LDs suggests that they contribute to TRL production and may thus control the length and amplitude of the post-prandial hypertriglyceridemia. In this review, we will describe the recent advances in the characterization of enterocytic LDs. The role of LDs in chylomicron production and secretion as well as potential previously unsuspected functions in the metabolism of vitamins, steroids and prostaglandins and in viral infection will also be discussed.

Deficiency of liver Comparative Gene Identification-58 causes steatohepatitis and fibrosis in mice

Triglyceride (TG) accumulation in hepatocytes (hepatic steatosis) preludes the development of advanced nonalcoholic fatty liver diseases (NAFLDs) such as steatohepatitis, fibrosis, and cirrhosis. Mutations in human Comparative Gene Identification-58 (CGI-58) cause cytosolic TG-rich lipid droplets to accumulate in almost all cell types including hepatocytes. However, it is unclear if CGI-58 mutation causes hepatic steatosis locally or via altering lipid metabolism in other tissues. To directly address this question, we created liver-specific CGI-58 knockout (LivKO) mice. LivKO mice on standard chow diet displayed microvesicular and macrovesicular panlobular steatosis, and progressed to advanced NAFLD stages over time, including lobular inflammation and centrilobular fibrosis. Compared with CGI-58 floxed control littermates, LivKO mice showed 8-fold and 52-fold increases in hepatic TG content, which was associated with 40% and 58% decreases in hepatic TG hydrolase activity at 16 and 42 weeks, respectively. Hepatic cholesterol also increased significantly in LivKO mice. At 42 weeks, LivKO mice showed increased hepatic oxidative stress, plasma aminotransferases, and hepatic mRNAs for genes involved in fibrosis and inflammation, such as α-smooth muscle actin, collagen type 1 α1, tumor necrosis factor α, and interleukin-1β. In conclusion, CGI-58 deficiency in the liver directly causes not only hepatic steatosis but also steatohepatitis and fibrosis.

Lumenal lipid metabolism: implications for lipoprotein assembly

Overproduction of apolipoprotein B (apoB)-containing lipoproteins by the liver and the intestine is 1 of the hallmarks of insulin resistance and type 2 diabetes and a well-established risk factor of cardiovascular disease. The assembly of apoB lipoproteins is regulated by the availability of lipids that form the neutral lipid core (triacylglycerol and cholesteryl ester) and the limiting lipoprotein monolayer (phospholipids and cholesterol). Although tremendous advances have been made over the past decade toward understanding neutral lipid and phospholipid biosynthesis and neutral lipid storage in cytosolic lipid droplets (LDs), little is known about the mechanisms that govern the transfer of lipids to the lumen of the endoplasmic reticulum for apoB lipidation. ApoB-synthesizing organs can deposit synthesized neutral lipids into at least 3 different types of LDs, each decorated with a subset of specific proteins: perilipin-decorated cytosolic LDs, and 2 types of LDs formed in the lumen of the endoplasmic reticulum, the secretion-destined LDs containing apoB, and resident lumenal LDs coated with microsomal triglyceride transfer protein and exchangeable apolipoproteins. This brief review will address the current knowledge of lumenal lipid metabolism in the context of apoB assembly and lipid storage.

The proteome of cytosolic lipid droplets isolated from differentiated Caco-2/TC7 enterocytes reveals cell-specific characteristics

Background information. Intestinal absorption of alimentary lipids is a complex process ensured by enterocytes and leading to TRL [TAG (triacylglycerol)-rich lipoprotein] assembly and secretion. The accumulation of circulating intestine-derived TRL is associated with atherosclerosis, stressing the importance of the control of postprandial hypertriglyceridaemia. During the postprandial period, TAGs are also transiently stored as CLDs (cytosolic lipid droplets) in enterocytes. As a first step for determining whether CLDs could play a role in the control of enterocyte TRL secretion, we analysed the protein endowment of CLDs isolated by sucrose-gradient centrifugation from differentiated Caco-2/TC7 enterocytes, the only human model able to secrete TRL in culture and to store transiently TAGs as CLDs when supplied with lipids. Cells were analysed after a 24 h incubation with lipid micelles and thus in a state of CLD-associated TAG mobilization.

Results. Among the 105 proteins identified in the CLD fraction by LC-MS/MS (liquid chromatography coupled with tandem MS), 27 were directly involved in lipid metabolism pathways potentially relevant to enterocyte-specific functions. The transient feature of CLDs was consistent with the presence of proteins necessary for fatty acid activation (acyl-CoA synthetases) and for TAG hydrolysis. In differentiated Caco-2/TC7 enterocytes, we identified for the first time LPCAT2 (lysophosphatidylcholine acyltransferase 2), involved in PC (phosphatidylcholine) synthesis, and 3BHS1 (3-β-hydroxysteroid dehydrogenase 1), involved in steroid metabolism, and confirmed their partial CLD localization by immunofluorescence. In enterocytes, LPCAT2 may provide an economical source of PC, necessary for membrane synthesis and lipoprotein assembly, from the lysoPC present in the intestinal lumen. We also identified proteins involved in lipoprotein metabolism, such as ApoA-IV (apolipoprotein A-IV), which is specifically expressed by enterocytes and has been proposed to play many functions in vivo, including the formation of lipoproteins and the control of their size. The association of ApoA-IV with CLD was confirmed by confocal and immunoelectron microscopy and validated in vivo in the jejunum of mice fed with a high-fat diet.

Conclusions. We report for the first time the protein endowment of Caco-2/TC7 enterocyte CLDs. Our results suggest that their formation and mobilization may participate in the control of enterocyte TRL secretion in a cell-specific manner.

CGI-58/ABHD5-derived signaling lipids regulate systemic inflammation and insulin action

Mutations of comparative gene identification 58 (CGI-58) in humans cause Chanarin-Dorfman syndrome, a rare autosomal recessive disease in which excess triacylglycerol (TAG) accumulates in multiple tissues. CGI-58 recently has been ascribed two distinct biochemical activities, including coactivation of adipose triglyceride lipase and acylation of lysophosphatidic acid (LPA). It is noteworthy that both the substrate (LPA) and the product (phosphatidic acid) of the LPA acyltransferase reaction are well-known signaling lipids. Therefore, we hypothesized that CGI-58 is involved in generating lipid mediators that regulate TAG metabolism and insulin sensitivity. Here, we show that CGI-58 is required for the generation of signaling lipids in response to inflammatory stimuli and that lipid second messengers generated by CGI-58 play a critical role in maintaining the balance between inflammation and insulin action. Furthermore, we show that CGI-58 is necessary for maximal TH1 cytokine signaling in the liver. This novel role for CGI-58 in cytokine signaling may explain why diminished CGI-58 expression causes severe hepatic lipid accumulation yet paradoxically improves hepatic insulin action. Collectively, these findings establish that CGI-58 provides a novel source of signaling lipids. These findings contribute insight into the basic mechanisms linking TH1 cytokine signaling to nutrient metabolism.

Adipose-selective overexpression of ABHD5/CGI-58 does not increase lipolysis or protect against diet-induced obesity

Adipose triglyceride lipase (ATGL) catalyzes the first step of triacylglycerol hydrolysis in adipocytes. Abhydrolase domain 5 (ABHD5) increases ATGL activity by an unknown mechanism. Prior studies have suggested that the expression of ABHD5 is limiting for lipolysis in adipocytes, as addition of recombinant ABHD5 increases in vitro TAG hydrolase activity of adipocyte lysates. To test this hypothesis in vivo, we generated transgenic mice that express 6-fold higher ABHD5 in adipose tissue relative to wild-type (WT) mice. In vivo lipolysis increased to a similar extent in ABHD5 transgenic and WT mice following an overnight fast or injection of either a β-adrenergic receptor agonist or lipopolysaccharide. Similarly, basal and β-adrenergic-stimulated lipolysis was comparable in adipocytes isolated from ABHD5 transgenic and WT mice. Although ABHD5 expression was elevated in thioglycolate-elicited macrophages from ABHD5 transgenic mice, Toll-like receptor 4 (TLR4) signaling was comparable in macrophages isolated from ABHD5 transgenic and WT mice. Overexpression of ABHD5 did not prevent the development of obesity in mice fed a high-fat diet, as shown by comparison of body weight, body fat percentage, and adipocyte hypertrophy of ABHD5 transgenic to WT mice. The expression of ABHD5 in mouse adipose tissue is not limiting for either basal or stimulated lipolysis.

Recent insights into the structure and function of comparative gene identification-58

PURPOSE OF REVIEW

Comparative gene identification-58 (CGI-58) is an important player in lipid metabolism. It acts as activator of triglyceride hydrolases and as acyl-CoA-dependent lysophosphatidic acid acyltransferase. This review aims at establishing a structure-function relationship of this still rather enigmatic protein based on recent studies characterizing different functions of CGI-58.

RECENT FINDINGS

Novel studies confirm the important regulatory role of CGI-58 as activator of the triglyceride hydrolase adipose triglyceride lipase. New evidence, corroborated by the characterization of a CGI-58 knockout mouse model, also suggests the existence of yet unknown lipases that are activated by CGI-58. Additionally, CGI-58 was identified to exert acyl-CoA-dependent lysophosphatidic acid acyltransferase activity, which implies possible roles in triglyceride or phospholipid synthesis or signaling processes. Unlike mammalian CGI-58 proteins, orthologs from plants and yeast additionally act as weak triglyceride and phospholipid hydrolases. A first three-dimensional model was calculated and allows preliminary structural considerations for the functions of CGI-58.

SUMMARY

Despite important progress concerning the different biochemical functions of CGI-58, the physiological importance of these activities requires better characterization. Furthermore, three-dimensional structural data for CGI-58 are required to unveil the molecular mechanism of how CGI-58 acts as activator of lipases and exerts its enzymatic functions.

A mutation in LIPN, encoding epidermal lipase N, causes a late-onset form of autosomal-recessive congenital ichthyosis

Autosomal-recessive congenital ichthyoses represent a large and heterogeneous group of disorders of epidermal cornification. Recent data suggest that most of these disorders might result from defective lipid transport and metabolism. In the present study, we describe a late-onset form of recessive ichthyosis in a large consanguineous pedigree. By using a combination of homozygosity mapping and positional candidate-gene screening, we identified a 2 bp deletion in LIPN that segregated with the disease phenotype throughout the family. LIPN encodes one of six acid lipases known to be involved in triglyceride metabolism in mammals . LIPN was found to be exclusively expressed in the epidermis and to be strongly induced during keratinocyte differentiation.

Adipose triacylglycerol lipase is a major regulator of hepatic lipid metabolism but not insulin sensitivity in mice

AIMS/HYPOTHESIS

Hepatic steatosis is characterised by excessive triacylglycerol accumulation and is strongly associated with insulin resistance. An inability to efficiently mobilise liver triacylglycerol may be a key event mediating hepatic steatosis. Adipose triacylglycerol lipase (ATGL) is a key triacylglycerol lipase in the liver and we hypothesised that liver-specific overproduction of ATGL would reduce steatosis and enhance insulin action in obese rodents.

METHODS

Studies of fatty acid metabolism were conducted in primary hepatocytes isolated from wild-type and Atgl (also known as Pnpla2)⁻(/)⁻ mice. An ATGL adenovirus was utilised to overproduce ATGL in the livers of obese insulin-resistant C57Bl/6 mice (Ad-ATGL). Blood chemistry, hepatic lipid content and insulin sensitivity were assessed in mice.

RESULTS

Triacylglycerol content was increased in Atgl⁻(/)⁻ hepatocytes and was associated with increased fatty acid uptake and impaired fatty acid oxidation. ATGL adenovirus administration in obese mice increased the production of hepatic ATGL protein and reduced triacylglycerol, diacylglycerol and ceramide content in the liver. Overproduction of ATGL improved insulin signal transduction in the liver but did not affect fasting glycaemia or insulinaemia. Inflammatory signalling was not suppressed by ATGL overproduction. While ATGL overproduction increased plasma non-esterified fatty acids, neither lipid deposition nor insulin-stimulated glucose uptake were affected in skeletal muscle.

CONCLUSIONS/INTERPRETATION

Liver ATGL overproduction decreases hepatic steatosis and mildly enhances liver insulin sensitivity. These effects are not sufficient to improve fasting glycaemia or insulinaemia in rodent obesity.

Crucial role of CGI-58/alpha/beta hydrolase domain-containing protein 5 in lipid metabolism

The surfaces of lipid droplets (LDs) constitute major sites of regulated accumulation and degradation of lipid in cells, and hence play important roles in lipid homeostasis of the whole body. CGI-58 (also called alpha/beta hydrolase domain-containing protein 5 (ABHD5)) is a member of the alpha/beta-hydrolase family of proteins and is a product of the causal gene of Chanarin-Dorfman syndrome (CDS), which is characterized by excessive storage of triacylglycerol (TG) in various tissues. CGI-58 is distributed predominantly on the surface of LDs and plays a crucial role in TG degradation in cells. In the process of lipolysis, CGI-58 coordinates with several proteins, including perilipin, a member of the PAT family of proteins, and adipose triglyceride lipase (ATGL), a putative rate-limiting enzyme for TG degradation in adipocytes. Besides its role in adipocytes, CGI-58 is involved in lipid degradation in various tissues, including those of skin and liver. This review focuses on the functions and protein interactions of CGI-58 on the surface of LDs in the regulation of fat mobilization in cells.

CGI-58 knockdown in mice causes hepatic steatosis but prevents diet-induced obesity and glucose intolerance

Mutations of Comparative Gene Identification-58 (CGI-58) in humans cause triglyceride (TG) accumulation in multiple tissues. Mice genetically lacking CGI-58 die shortly after birth due to a skin barrier defect. To study the role of CGI-58 in integrated lipid and energy metabolism, we utilized antisense oligonucleotides (ASOs) to inhibit CGI-58 expression in adult mice. Treatment with two distinct CGI-58-targeting ASOs resulted in ∼80-95% knockdown of CGI-58 protein expression in both liver and white adipose tissue. In chow-fed mice, ASO-mediated depletion of CGI-58 did not alter weight gain, plasma TG, or plasma glucose, yet raised hepatic TG levels ∼4-fold. When challenged with a high-fat diet (HFD), CGI-58 ASO-treated mice were protected against diet-induced obesity, but their hepatic contents of TG, diacylglycerols, and ceramides were all elevated, and intriguingly, their hepatic phosphatidylglycerol content was increased by 10-fold. These hepatic lipid alterations were associated with significant decreases in hepatic TG hydrolase activity, hepatic lipoprotein-TG secretion, and plasma concentrations of ketones, nonesterified fatty acids, and insulin. Additionally, HFD-fed CGI-58 ASO-treated mice were more glucose tolerant and insulin sensitive. Collectively, this work demonstrates that CGI-58 plays a critical role in limiting hepatic steatosis and maintaining hepatic glycerophospholipid homeostasis and has unmasked an unexpected role for CGI-58 in promoting HFD-induced obesity and insulin resistance.

Genetic variants in adipose triglyceride lipase influence lipid levels in familial combined hyperlipidemia

OBJECTIVE

Familial combined hyperlipidemia (FCHL) has been associated with abnormalities in fatty acid metabolism. The adipose triglyceride lipase (PNPLA2) plays a pivotal role in the turnover of fatty acids in adipose tissue and liver. This study was designed to evaluate whether selected PNPLA2 variants may influence the susceptibility to FCHL or its lipid-related traits.

METHODS

Four SNPs within the PNPLA2 gene (rs7925131, rs7942159, rs66460720 and the nonsynonymous P481L) were selected based on previous association with decreased plasma levels of free fatty acids (FFA) and total triglycerides (TG) and their high frequency (MAF>0.25). These SNPs were genotyped in 214 FCHL individuals from 83 families and in 103 controls and the corresponding haplotypes were reconstructed.

RESULTS

No association between individual SNPs and the FCHL trait was observed. However, two PNPLA2 haplotypes were associated with lower risk of FCHL (P<0.004 after Bonferroni's correction). Compared to the others, these haplotypes were related to lower TG (118.9 ± 66.8 vs. 197.1 ± 114.7 mg/dl; P=0.001) and higher HDL-C (62.3 ± 15.8 vs. 51.0 ± 15.0 mg/dl; P<0.005). In a subgroup of studied subjects (n=63) protective haplotypes were also associated with lower FFA levels (0.33 ± 0.11 vs. 0.46 ± 0.18 mEq/L; P<0.05). These effects were independent from age, BMI and HOMA(IR).

CONCLUSION

These data demonstrate that variants within PNPLA2 may modulate the TG component of FCHL trait, thus implicating PNPLA2 as modifier gene in this lipid disorder. They also suggest a potential role of PNPLA2 in the metabolism of TG-rich lipoproteins.

Triacylglycerol lipases and metabolic control: implications for health and disease

Fatty acids derived from the hydrolysis of adipose tissue and skeletal muscle triacylglycerol (TG) are an important energy substrate at rest and during physical activity. This review outlines the identification of the new TG lipase, adipose triglyceride lipase, the current understanding of how cellular TG lipases are regulated, and the implications for understanding the integrated control of TG lipolysis. Furthermore, this review outlines recent advances that propose a "revised" role for TG lipases in cellular function, metabolic homeostasis, and disease prevention.

Growth retardation, impaired triacylglycerol catabolism, hepatic steatosis, and lethal skin barrier defect in mice lacking comparative gene identification-58 (CGI-58)

Comparative gene identification-58 (CGI-58), also designated as alpha/beta-hydrolase domain containing-5 (ABHD-5), is a lipid droplet-associated protein that activates adipose triglyceride lipase (ATGL) and acylates lysophosphatidic acid. Activation of ATGL initiates the hydrolytic catabolism of cellular triacylglycerol (TG) stores to glycerol and nonesterified fatty acids. Mutations in both ATGL and CGI-58 cause "neutral lipid storage disease" characterized by massive accumulation of TG in various tissues. The analysis of CGI-58-deficient (Cgi-58(-/-)) mice, presented in this study, reveals a dual function of CGI-58 in lipid metabolism. First, systemic TG accumulation and severe hepatic steatosis in newborn Cgi-58(-/-) mice establish a limiting role for CGI-58 in ATGL-mediated TG hydrolysis and supply of nonesterified fatty acids as energy substrate. Second, a severe skin permeability barrier defect uncovers an essential ATGL-independent role of CGI-58 in skin lipid metabolism. The neonatal lethal skin barrier defect is linked to an impaired hydrolysis of epidermal TG. As a consequence, sequestration of fatty acids in TG prevents the synthesis of acylceramides, which are essential lipid precursors for the formation of a functional skin permeability barrier. This mechanism may also underlie the pathogenesis of ichthyosis in neutral lipid storage disease patients lacking functional CGI-58.

The N-terminal region of comparative gene identification-58 (CGI-58) is important for lipid droplet binding and activation of adipose triglyceride lipase

In mammals, excess energy is stored in the form of triacylglycerol primarily in lipid droplets of white adipose tissue. The first step of lipolysis (i.e. the mobilization of fat stores) is catalyzed by adipose triglyceride lipase (ATGL). The enzymatic activity of ATGL is strongly enhanced by CGI-58 (comparative gene identification-58), and the loss of either ATGL or CGI-58 function causes systemic triglyceride accumulation in humans and mice. However, the mechanism by which CGI-58 stimulates ATGL activity is unknown. To gain insight into CGI-58 function using structural features of the protein, we generated a three-dimensional homology model based on sequence similarity with other proteins. Interestingly, the model of CGI-58 revealed that the N terminus forms an extension of the otherwise compact structure of the protein. This N-terminal region (amino acids 1-30) harbors a lipophilic tryptophan-rich stretch, which affects the localization of the protein. (1)H NMR experiments revealed strong interaction between the N-terminal peptide and dodecylphosphocholine micelles as a lipid droplet-mimicking system. A role for this N-terminal region of CGI-58 in lipid droplet binding was further strengthened by localization studies in cultured cells. Although wild-type CGI-58 localizes to the lipid droplet, the N-terminally truncated fragments of CGI-58 are dispersed in the cytoplasm. Moreover, CGI-58 lacking the N-terminal extension loses the ability to stimulate ATGL, implying that the ability of CGI-58 to activate ATGL is linked to correct localization. In summary, our study shows that the N-terminal, Trp-rich region of CGI-58 is essential for correct localization and ATGL-activating function of CGI-58.

CGI-58/ABHD5 is a coenzyme A-dependent lysophosphatidic acid acyltransferase

Mutations in human CGI-58/ABHD5 cause Chanarin-Dorfman syndrome (CDS), characterized by excessive storage of triacylglycerol in tissues. CGI-58 is an alpha/beta-hydrolase fold enzyme expressed in all vertebrates. The carboxyl terminus includes a highly conserved consensus sequence (HXXXXD) for acyltransferase activity. Mouse CGI-58 was expressed in Escherichia coli as a fusion protein with two amino terminal 6-histidine tags. Recombinant CGI-58 displayed acyl-CoA-dependent acyltransferase activity to lysophosphatidic acid, but not to other lysophospholipid or neutral glycerolipid acceptors. Production of phosphatidic acid increased with time and increasing concentrations of recombinant CGI-58 and was optimal between pH 7.0 and 8.5. The enzyme showed saturation kinetics with respect to 1-oleoyl-lysophosphatidic acid and oleoyl-CoA and preference for arachidonoyl-CoA and oleoyl-CoA. The enzyme showed slight preference for 1-oleoyl lysophosphatidic acid over 1-palmitoyl, 1-stearoyl, or 1-arachidonoyl lysophosphatidic acid. Recombinant CGI-58 showed intrinsic fluorescence for tryptophan that was quenched by the addition of 1-oleoyl-lysophosphatidic acid, oleoyl-CoA, arachidonoyl-CoA, and palmitoyl-CoA, but not by lysophosphatidyl choline. Expression of CGI-58 in fibroblasts from humans with CDS increased the incorporation of radiolabeled fatty acids released from the lipolysis of stored triacylglycerols into phospholipids. CGI-58 is a CoA-dependent lysophosphatidic acid acyltransferase that channels fatty acids released from the hydrolysis of stored triacylglycerols into phospholipids.

Arylacetamide deacetylase attenuates fatty-acid-induced triacylglycerol accumulation in rat hepatoma cells

Mobilization of hepatic triacylglycerol stores provides substrates for mitochondrial beta-oxidation and assembly of VLDLs; however, the identity of lipolytic enzymes involved in the regulation of this process remains largely unknown. Arylacetamide deacetylase (AADA) shares homology with hormone-sensitive lipase and therefore could potentially participate in hepatic lipid metabolism, including the regulation of hepatic triacylglycerol levels. We have established McArdle-RH7777 (rat hepatoma) cell lines stably expressing mouse AADA cDNA and performed metabolic labeling as well as lipid mass analyses. Expression of AADA cDNA in McArdle-RH7777 cells significantly reduced intracellular triacylglycerol levels and apolipoprotein B secretion and increased fatty acid oxidation. These results suggest that fatty acids released by AADA-mediated hydrolysis of lipids are channeled for -oxidation rather than for the assembly of lipoproteins.

Neutral lipid storage disease: genetic disorders caused by mutations in adipose triglyceride lipase/PNPLA2 or CGI-58/ABHD5

Neutral lipid storage disease (NLSD) is a group of autosomal recessive disorders characterized by the excessive accumulation of neutral lipids in multiple tissues. Recently, two genes, adipose triglyceride lipase (ATGL/PNPLA2) and comparative gene identification-58 (CGI-58/ABHD5), have been shown to cause NLSD. ATGL specifically hydrolyzes the first fatty acid from triacylglycerols (TG) and CGI-58/ABHD5 stimulates ATGL activity by a currently unknown mechanism. Mutations in both the ATGL and the CGI-58 genes are associated with systemic TG accumulation, yet the resulting clinical manifestations are not identical. Patients with defective ATGL function suffer from more severe myopathy (NLSDM) than patients with defective CGI-58 function. On the other hand, CGI-58 mutations are always associated with ichthyosis (NLSDI), which was not observed in patients with defective ATGL function. These observations indicate an ATGL-independent function of CGI-58. This review summarizes recent findings with the goal of relating structural variants of ATGL and CGI-58 to functional consequences in lipid metabolism.

ABHD5/CGI-58 facilitates the assembly and secretion of apolipoprotein B lipoproteins by McA RH7777 rat hepatoma cells

Lipolysis of stored triacylglycerols provides lipid precursors for the assembly of apolipoprotein B (apoB) lipoproteins in hepatocytes. Abhydrolase domain containing 5 (ABHD5) is expressed in liver and facilitates the lipolysis of triacylglycerols. To study the function of ABHD5 in lipoprotein secretion, we silenced the expression of ABHD5 in McA RH7777 cells using RNA interference and studied the metabolism of lipids and secretion of apoB lipoproteins. McA RH7777 cells deficient in ABHD5 secreted reduced amounts of apoB, triacylglycerols, and cholesterol esters. Detailed analysis of liquid chromatography-mass spectrometry data for the molecular species of secreted triacylglycerols revealed that deficiency of ABHD5 significantly reduced secretion of triacylglycerols containing oleate, even when oleate was supplied in the culture medium; the ABHD5-deficient cells partially compensated by secreting higher levels of triacylglycerols containing saturated fatty acids. In experiments tracking the metabolism of [(14)C]oleate, silencing of ABHD5 reduced lipolysis of cellular triacylglycerols and incorporation of intermediates derived from stored lipids into secreted triacylglycerols and cholesterol esters. In contrast, the incorporation of exogenous oleate into secreted triacylglycerols and cholesterol esters was unaffected by deficiency of ABHD5. These findings suggest that ABHD5 facilitates the use of lipid intermediates derived from lipolysis of stored triacylglycerols for the assembly of lipoproteins.

Adipose triglyceride lipase and the lipolytic catabolism of cellular fat stores

Fatty acids (FAs) are essential components of all lipid classes and pivotal substrates for energy production in all vertebrates. Additionally, they act directly or indirectly as signaling molecules and, when bonded to amino acid side chains of peptides, anchor proteins in biological membranes. In vertebrates, FAs are predominantly stored in the form of triacylglycerol (TG) within lipid droplets of white adipose tissue. Lipid droplet-associated TGs are also found in most nonadipose tissues, including liver, cardiac muscle, and skeletal muscle. The mobilization of FAs from all fat depots depends on the activity of TG hydrolases. Currently, three enzymes are known to hydrolyze TG, the well-studied hormone-sensitive lipase (HSL) and monoglyceride lipase (MGL), discovered more than 40 years ago, as well as the relatively recently identified adipose triglyceride lipase (ATGL). The phenotype of HSL- and ATGL-deficient mice, as well as the disease pattern of patients with defective ATGL activity (due to mutation in ATGL or in the enzyme's activator, CGI-58), suggest that the consecutive action of ATGL, HSL, and MGL is responsible for the complete hydrolysis of a TG molecule. The complex regulation of these enzymes by numerous, partially uncharacterized effectors creates the "lipolysome," a complex metabolic network that contributes to the control of lipid and energy homeostasis. This review focuses on the structure, function, and regulation of lipolytic enzymes with a special emphasis on ATGL.

Regulation and function of triacylglycerol lipases in cellular metabolism

The ability to store energy in the form of energy-dense TAG (triacylglycerol) and to mobilize these stores rapidly during times of low carbohydrate availability (fasting or famine) or during heightened metabolic demand (exercise or cold-stress) is a highly conserved process essential for survival. Today, in the presence of nutrient excess and sedentary lifestyles, the regulation of this pathway is viewed as an important therapeutic target for disease prevention, as elevated circulating fatty acids in obesity contribute to many aspects of the metabolic syndrome including hepatic steatosis, atherosclerosis and insulin resistance. In the present review, we discuss the metabolic regulation and function of TAG lipases with a focus on HSL (hormone-sensitive lipase), ATGL (adipose triacylglycerol lipase) and newly identified members of the lipolytic proteome.

CGI-58, the causative gene for Chanarin-Dorfman syndrome, mediates acylation of lysophosphatidic acid

cgi-58 (comparative gene identification-58) is a member of alpha/beta-hydrolase family of proteins. Mutations in CGI-58 are shown to be responsible for a rare genetic disorder known as Chanarin-Dorfman syndrome, characterized by an excessive accumulation of triacylglycerol in several tissues and ichthyosis. We have earlier reported that YLR099c encoding Ict1p in Saccharomyces cerevisiae can acylate lysophosphatidic acid to phosphatidic acid. Here we report that human CGI-58 is closely related to ICT1. To understand the biochemical function of cgi-58, the gene was overexpressed in Escherichia coli, and the purified recombinant protein was found to specifically acylate lysophosphatidic acid in an acyl-CoA-dependent manner. Overexpression of CGI-58 in S. cerevisiae showed an increase in the formation of phosphatidic acid resulting in an overall increase in the total phospholipids. However, the triacylglycerol level was found to be significantly reduced. In addition, the physiological significance of cgi-58 in mice white adipose tissue was studied. We found soluble lysophosphatidic acid acyltransferase activity in mouse white adipose tissue. Immunoblot analysis using anti-Ict1p antibodies followed by mass spectrometry of the immunocross-reactive protein in lipid droplets revealed its identity as cgi-58. These observations suggest the existence of an alternate cytosolic phosphatidic acid biosynthetic pathway in the white adipose tissue. Collectively, these results reveal the role of cgi-58 as an acyltransferase.

The patatin-like lipase family in Gallus gallus

Background

In oviparous species, genes encoding proteins with functions in lipid remodeling, such as specialized lipases, may have evolved to facilitate the assembly and utilization of yolk lipids by the embryo. The mammalian gene family of patatin-like phospholipases (PNPLAs) has received significant attention, but studies in other vertebrates are lacking; thus, we have begun investigations of PNPLA genes in the chicken (Gallus gallus).

Results

We scanned the draft chicken genome using human PNPLA sequences, and performed PCR to amplify and sequence orthologous cDNAs. Full-length cDNA sequences of galline PNPLA2/ATGL, PNPLA4, -7, -8, -9, and the activator protein CGI-58, as well as partial cDNA sequences of avian PNPLA1, -3, and -6 were obtained. The high degree of sequence identities (~50 to 80%) between the avian and human orthologs suggests conservation of important enzymatic functions. Quantitation by qPCR of the transcript levels of PNPLAs and CGI-58 in 21 tissues indicates that expression patterns and levels diverge greatly between species. A particularly interesting tissue in which certain PNPLAs may contribute to physiological specialization is the extraembryonic yolk sac.

Conclusion

Knowledge about the exact in-vivo functions of PNPLAs in any system is still sparse. Thus, studies about the temporal expression patterns and functions of the enzymes identified here, and of other already known extracellular lipases and co-factors, in the yolk sac and embryonic tissues during embryogenesis are called for. Based on the information obtained, further studies are anticipated to provide important insights of the roles of PNPLAs in the yolk sac and embryo development.