Adipocyte HSL is required for maintaining circulating vitamin A and RBP4 levels during fasting.

Vitamin A (retinol) is distributed via the blood bound to its specific carrier protein, retinol-binding protein 4 (RBP4). Retinol-loaded RBP4 is secreted into the circulation exclusively from hepatocytes, thereby mobilizing hepatic retinoid stores that represent the major vitamin A reserves in the body. The relevance of extrahepatic retinoid stores for circulating retinol and RBP4 levels that are usually kept within narrow physiological limits is unknown. Here, we show that fasting affects retinoid mobilization in a tissue-specific manner, and that hormone-sensitive lipase (HSL) in adipose tissue is required to maintain serum concentrations of retinol and RBP4 during fasting in mice. We found that extracellular retinol-free apo-RBP4 induces retinol release by adipocytes in an HSL-dependent manner. Consistently, global or adipocyte-specific HSL deficiency leads to an accumulation of retinoids in adipose tissue and a drop of serum retinol and RBP4 during fasting, which affects retinoid-responsive gene expression in eye and kidney and lowers renal retinoid content. These findings establish a novel crosstalk between liver and adipose tissue retinoid stores for the maintenance of systemic vitamin A homeostasis during fasting.

Phospholipase A2 group IVD mediates the transacylation of glycerophospholipids and acylglycerols.

In mammalian cells, glycerolipids are mainly synthesized using acyl-CoA-dependent mechanisms. The acyl-CoA-independent transfer of fatty acids between lipids, designated as transacylation reaction, represents an additional mechanism for lipid remodeling and synthesis pathways. Here, we demonstrate that human and mouse phospholipase A2 group IVD (PLA2G4D) catalyzes transacylase reactions using both phospholipids and acylglycerols as substrates. In the presence of mono- and diacylglycerol (MAG and DAG), purified PLA2G4D generates DAG and triacylglycerol (TAG), respectively. The enzyme also transfers fatty acids between phospholipids and from phospholipids to acylglycerols. Overexpression of PLA2G4D in COS7 cells enhances the incorporation of polyunsaturated fatty acids into TAG stores and induces the accumulation of lysophospholipids. In the presence of exogenously added MAG, the enzyme strongly increases cellular DAG formation, while MAG levels are decreased. PLA2G4D is not or poorly detectable in commonly used cell lines. It is expressed in keratinocytes, where it is strongly upregulated by proinflammatory cytokines. Pla2g4d-deficient mouse keratinocytes exhibit complex lipidomic changes in response to cytokine treatment, indicating that PLA2G4D is involved in the remodeling of the lipidome under inflammatory conditions. Transcriptomic analysis revealed that PLA2G4D modulates fundamental biological processes including cell proliferation, differentiation, and signaling. Together, our observations demonstrate that PLA2G4D has broad substrate specificity for fatty acid donor and acceptor lipids, allowing the acyl-CoA-independent synthesis of both phospholipids and acylglycerols. Loss-of-function studies indicate that PLA2G4D affects metabolic and signaling pathways in keratinocytes, which is associated with complex lipidomic and transcriptomic alterations.

KIAA1363 affects retinyl ester turnover in cultured murine and human hepatic stellate cells.

Large quantities of vitamin A are stored as retinyl esters (REs) in specialized liver cells, the hepatic stellate cells (HSCs). To date, the enzymes controlling RE degradation in HSCs are poorly understood. In this study, we identified KIAA1363 (also annotated as arylacetamide deacetylase 1 or neutral cholesterol ester hydrolase 1) as a novel RE hydrolase. We show that KIAA1363 is expressed in the liver, mainly in HSCs, and exhibits RE hydrolase activity at neutral pH. Accordingly, addition of the KIAA1363-specific inhibitor JW480 largely reduced RE hydrolase activity in lysates of cultured murine and human HSCs. Furthermore, cell fractionation experiments and confocal microscopy studies showed that KIAA1363 localizes to the endoplasmic reticulum. We demonstrate that overexpression of KIAA1363 in cells led to lower cellular RE content after a retinol loading period. Conversely, pharmacological inhibition or shRNA-mediated silencing of KIAA1363 expression in cultured murine and human HSCs attenuated RE degradation. Together, our data suggest that KIAA1363 affects vitamin A metabolism of HSCs by hydrolyzing REs at the endoplasmic reticulum, thereby counteracting retinol esterification and RE storage in lipid droplets.

Acute retinol mobilization by retinol-binding protein 4 in mouse liver induces fibroblast growth factor 21 expression.

Hepatocytes secrete retinol-binding protein 4 (RBP4) into circulation, thereby mobilizing vitamin A from the liver to provide retinol for extrahepatic tissues. Obesity and insulin resistance are associated with elevated RBP4 levels in the blood. However, in a previous study, we observed that chronically increased RBP4 by forced Rbp4 expression in the liver does not impair glucose homeostasis in mice. Here, we investigated the effects of an acute mobilization of hepatic vitamin A stores by hepatic overexpression of RBP4 in mice. We show that hepatic retinol mobilization decreases body fat content and enhances fat turnover. Mechanistically, we found that acute retinol mobilization increases hepatic expression and serum levels of fibroblast growth factor 21 (FGF21), which is regulated by retinol mobilization and retinoic acid in primary hepatocytes. Moreover, we provide evidence that the insulin-sensitizing effect of FGF21 is associated with organ-specific adaptations in retinoid homeostasis. Taken together, our findings identify a novel crosstalk between retinoid homeostasis and FGF21 in mice with acute RBP4-mediated retinol mobilization from the liver.

Small-Molecule Inhibitors Targeting Lipolysis in Human Adipocytes.

Chronically elevated circulating fatty acid levels promote lipid accumulation in nonadipose tissues and cause lipotoxicity. Adipose triglyceride lipase (ATGL) critically determines the release of fatty acids from white adipose tissue, and accumulating evidence suggests that inactivation of ATGL has beneficial effects on lipotoxicity-driven disorders including insulin resistance, steatohepatitis, and heart disease, classifying ATGL as a promising drug target. Here, we report on the development and biological characterization of the first small-molecule inhibitor of human ATGL. This inhibitor, designated NG-497, selectively inactivates human and nonhuman primate ATGL but not structurally and functionally related lipid hydrolases. We demonstrate that NG-497 abolishes lipolysis in human adipocytes in a dose-dependent and reversible manner. The combined analysis of mouse- and human-selective inhibitors, chimeric ATGL proteins, and homology models revealed detailed insights into enzyme-inhibitor interactions. NG-497 binds ATGL within a hydrophobic cavity near the active site. Therein, three amino acid residues determine inhibitor efficacy and species selectivity and thus provide the molecular scaffold for selective inhibition.

Metabolic regulation of the lysosomal cofactor bis(monoacylglycero)phosphate in mice.

Bis(monoacylglycero)phosphate (BMP), also known as lysobisphosphatidic acid, is a phospholipid that promotes lipid sorting in late endosomes/lysosomes by activating lipid hydrolases and lipid transfer proteins. Changes in the cellular BMP content therefore reflect an altered metabolic activity of the endolysosomal system. Surprisingly, little is known about the physiological regulation of BMP. In this study, we investigated the effects of nutritional and metabolic factors on BMP profiles of whole tissues and parenchymal and nonparenchymal cells. Tissue samples were obtained from fed, fasted, 2 h refed, and insulin-treated mice, as well as from mice housed at 5°C, 22°C, or 30°C. These tissues exhibited distinct BMP profiles that were regulated by the nutritional state in a tissue-specific manner. Insulin treatment was not sufficient to mimic refeeding-induced changes in tissue BMP levels, indicating that BMP metabolism is regulated by other hormonal or nutritional factors. Tissue fractionation experiments revealed that fasting drastically elevates BMP levels in hepatocytes and pancreatic cells. Furthermore, we observed that the BMP content in brown adipose tissue strongly depends on housing temperatures. In conclusion, our observations suggest that BMP concentrations adapt to the metabolic state in a tissue- and cell-type-specific manner in mice. Drastic changes observed in hepatocytes, pancreatic cells, and brown adipocytes suggest that BMP plays a role in the functional adaption to nutrient starvation and ambient temperature.

Hepatocyte-specific deletion of lysosomal acid lipase leads to cholesteryl ester but not triglyceride or retinyl ester accumulation.

Lysosomal acid lipase (LAL) hydrolyzes cholesteryl ester (CE) and retinyl ester (RE) and triglyceride (TG). Mice globally lacking LAL accumulate CE most prominently in the liver. The severity of the CE accumulation phenotype progresses with age and is accompanied by hepatomegaly and hepatic cholesterol crystal deposition. In contrast, hepatic TG accumulation is much less pronounced in these mice, and hepatic RE levels are even decreased. To dissect the functional role of LAL for neutral lipid ester mobilization in the liver, we generated mice specifically lacking LAL in hepatocytes (hep-LAL-ko). On a standard chow diet, hep-LAL-ko mice exhibited increased hepatic CE accumulation but unaltered TG and RE levels. Feeding the hep-LAL-ko mice a vitamin A excess/high-fat diet (VitA/HFD) further increased hepatic cholesterol levels, but hepatic TG and RE levels in these mice were lower than in control mice. Performing in vitro activity assays with lysosome-enriched fractions from livers of mice globally lacking LAL, we detected residual acid hydrolytic activities against TG and RE. Interestingly, this non-LAL acid TG hydrolytic activity was elevated in lysosome-enriched fractions from livers of hep-LAL-ko mice upon VitA/HFD feeding. In conclusion, the neutral lipid ester phenotype in livers from hep-LAL-ko mice indicates that LAL is limiting for CE turnover, but not for TG and RE turnovers. Furthermore, in vitro hydrolase activity assays revealed the existence of non-LAL acid hydrolytic activities for TG and RE. The corresponding acid lipase(s) catalyzing these reactions remains to be identified.

Comparative transcriptomics reveals candidate carotenoid color genes in an East African cichlid fish.

BACKGROUND: Carotenoids contribute significantly to animal body coloration, including the spectacular color pattern diversity among fishes. Fish, as other animals, derive carotenoids from their diet. Following uptake, transport and metabolic conversion, carotenoids allocated to body coloration are deposited in the chromatophore cells of the integument. The genes involved in these processes are largely unknown. Using RNA-Sequencing, we tested for differential gene expression between carotenoid-colored and white skin regions of a cichlid fish, Tropheus duboisi "Maswa", to identify genes associated with carotenoid-based integumentary coloration. To control for positional gene expression differences that were independent of the presence/absence of carotenoid coloration, we conducted the same analyses in a closely related population, in which both body regions are white. RESULTS: A larger number of genes (n = 50) showed higher expression in the yellow compared to the white skin tissue than vice versa (n = 9). Of particular interest was the elevated expression level of bco2a in the white skin samples, as the enzyme encoded by this gene catalyzes the cleavage of carotenoids into colorless derivatives. The set of genes with higher expression levels in the yellow region included genes involved in xanthophore formation (e.g., pax7 and sox10), intracellular pigment mobilization (e.g., tubb, vim, kif5b), as well as uptake (e.g., scarb1) and storage (e.g., plin6) of carotenoids, and metabolic conversion of lipids and retinoids (e.g., dgat2, pnpla2, akr1b1, dhrs). Triglyceride concentrations were similar in the yellow and white skin regions. Extracts of integumentary carotenoids contained zeaxanthin, lutein and beta-cryptoxanthin as well as unidentified carotenoid structures. CONCLUSION: Our results suggest a role of carotenoid cleavage by Bco2 in fish integumentary coloration, analogous to previous findings in birds. The elevated expression of genes in carotenoid-rich skin regions with functions in retinol and lipid metabolism supports hypotheses concerning analogies and shared mechanisms between these metabolic pathways. Overlaps in the sets of differentially expressed genes (including dgat2, bscl2, faxdc2 and retsatl) between the present study and previous, comparable studies in other fish species provide useful hints to potential carotenoid color candidate genes.

Amyloid-beta impairs insulin signaling by accelerating autophagy-lysosomal degradation of LRP-1 and IR-ß in blood-brain barrier endothelial cells in vitro and in 3XTg-AD mice.

Aberrant insulin signaling constitutes an early change in Alzheimer's disease (AD). Insulin receptors (IR) and low-density lipoprotein receptor-related protein-1 (LRP-1) are expressed in brain capillary endothelial cells (BCEC) forming the blood-brain barrier (BBB). There, insulin may regulate the function of LRP-1 in Aß clearance from the brain. Changes in IR-ß and LRP-1 and insulin signaling at the BBB in AD are not well understood. Herein, we identified a reduction in cerebral and cerebrovascular IR-ß levels in 9-month-old male and female 3XTg-AD (PS1M146V, APPSwe, and tauP301L) as compared to NTg mice, which is important in insulin mediated signaling responses. Reduced cerebral IR-ß levels corresponded to impaired insulin signaling and LRP-1 levels in brain. Reduced cerebral and cerebrovascular IR-ß and LRP-1 levels in 3XTg-AD mice correlated with elevated levels of autophagy marker LC3B. In both genotypes, high-fat diet (HFD) feeding decreased cerebral and hepatic LRP-1 expression and elevated cerebral Aß burden without affecting cerebrovascular LRP-1 and IR-ß levels. In vitro studies using primary porcine (p)BCEC revealed that Aß peptides 1-40 or 1-42 (240 nM) reduced cellular levels and interaction of LRP-1 and IR-ß thereby perturbing insulin-mediated signaling. Further mechanistic investigation revealed that Aß treatment accelerated the autophagy-lysosomal degradation of IR-ß and LRP-1 in pBCEC. LRP-1 silencing in pBCEC decreased IR-ß levels through post-translational pathways further deteriorating insulin-mediated responses at the BBB. Our findings indicate that LRP-1 proves important for insulin signaling at the BBB. Cerebral Aß burden in AD may accelerate LRP-1 and IR-ß degradation in BCEC thereby contributing to impaired cerebral and cerebromicrovascular insulin effects.

Metabolic disease and ABHD6 alter the circulating bis(monoacylglycerol)phosphate profile in mice and humans.

Bis(monoacylglycerol)phosphate (BMP) is a phospholipid that is crucial for lipid degradation and sorting in acidic organelles. Genetic and drug-induced lysosomal storage disorders (LSDs) are associated with increased BMP concentrations in tissues and in the circulation. Data on BMP in disorders other than LSDs, however, are scarce, and key enzymes regulating BMP metabolism remain elusive. Here, we demonstrate that common metabolic disorders and the intracellular BMP hydrolase α/ß-hydrolase domain-containing 6 (ABHD6) affect BMP metabolism in mice and humans. In mice, dietary lipid overload strongly affects BMP concentration and FA composition in the liver and plasma, similar to what has been observed in LSDs. Notably, distinct changes in the BMP FA profile enable a clear distinction between lipid overload and drug-induced LSDs. Global deletion of ABHD6 increases circulating BMP concentrations but does not cause LSDs. In humans, nonalcoholic fatty liver disease and liver cirrhosis affect the serum BMP FA composition and concentration. Furthermore, we identified a patient with a loss-of-function mutation in the ABHD6 gene, leading to an altered circulating BMP profile. In conclusion, our results suggest that common metabolic diseases and ABHD6 affect BMP metabolism in mice and humans.

ABHD5 stimulates PNPLA1-mediated ω-O-acylceramide biosynthesis essential for a functional skin permeability barrier.

Mutations in the genes coding for patatin-like phospholipase domain-containing 1 (PNPLA1) and α/ß-hydrolase domain-containing 5 (ABHD5), also known as comparative gene identification 58, are causative for ichthyosis, a severe skin barrier disorder. Individuals with mutations in either of these genes show a defect in epidermal ω-O-acylceramide (AcylCer) biosynthesis, suggesting that PNPLA1 and ABHD5 act in the same metabolic pathway. In this report, we identified ABHD5 as a coactivator of PNPLA1 that stimulates the esterification of ω-hydroxy ceramides with linoleic acid for AcylCer biosynthesis. ABHD5 interacts with PNPLA1 and recruits the enzyme to its putative triacylglycerol substrate onto cytosolic lipid droplets. Conversely, alleles of ABHD5 carrying point mutations associated with ichthyosis in humans failed to accelerate PNPLA1-mediated AcylCer biosynthesis. Our findings establish an important biochemical function of ABHD5 in interacting with PNPLA1 to synthesize crucial epidermal lipids, emphasizing the significance of these proteins in the formation of a functional skin permeability barrier.

Deletion of Monoglyceride Lipase in Astrocytes Attenuates Lipopolysaccharide-induced Neuroinflammation.

Monoglyceride lipase (MGL) is required for efficient hydrolysis of the endocannabinoid 2-arachidonoylglyerol (2-AG) in the brain generating arachidonic acid (AA) and glycerol. This metabolic function makes MGL an interesting target for the treatment of neuroinflammation, since 2-AG exhibits anti-inflammatory properties and AA is a precursor for pro-inflammatory prostaglandins. Astrocytes are an important source of AA and 2-AG, and highly express MGL. In the present study, we dissected the distinct contribution of MGL in astrocytes on brain 2-AG and AA metabolism by generating a mouse model with genetic deletion of MGL specifically in astrocytes (MKO(GFAP)). MKO(GFAP) mice exhibit moderately increased 2-AG and reduced AA levels in brain. Minor accumulation of 2-AG in the brain of MKO(GFAP) mice does not cause cannabinoid receptor desensitization as previously observed in mice globally lacking MGL. Importantly, MKO(GFAP) mice exhibit reduced brain prostaglandin E2 and pro-inflammatory cytokine levels upon peripheral lipopolysaccharide (LPS) administration. These observations indicate that MGL-mediated degradation of 2-AG in astrocytes provides AA for prostaglandin synthesis promoting LPS-induced neuroinflammation. The beneficial effect of astrocyte-specific MGL-deficiency is not fully abrogated by the inverse cannabinoid receptor 1 agonist SR141716 (Rimonabant) suggesting that the anti-inflammatory effects are rather caused by reduced prostaglandin synthesis than by activation of cannabinoid receptors. In conclusion, our data demonstrate that MGL in astrocytes is an important regulator of 2-AG levels, AA availability, and neuroinflammation.

Lysosomal Acid Lipase Hydrolyzes Retinyl Ester and Affects Retinoid Turnover.

Lysosomal acid lipase (LAL) is essential for the clearance of endocytosed cholesteryl ester and triglyceride-rich chylomicron remnants. Humans and mice with defective or absent LAL activity accumulate large amounts of cholesteryl esters and triglycerides in multiple tissues. Although chylomicrons also contain retinyl esters (REs), a role of LAL in the clearance of endocytosed REs has not been reported. In this study, we found that murine LAL exhibits RE hydrolase activity. Pharmacological inhibition of LAL in the human hepatocyte cell line HepG2, incubated with chylomicrons, led to increased accumulation of REs in endosomal/lysosomal fractions. Furthermore, pharmacological inhibition or genetic ablation of LAL in murine liver largely reduced in vitro acid RE hydrolase activity. Interestingly, LAL-deficient mice exhibited increased RE content in the duodenum and jejunum but decreased RE content in the liver. Furthermore, LAL-deficient mice challenged with RE gavage exhibited largely reduced post-prandial circulating RE content, indicating that LAL is required for efficient nutritional vitamin A availability. In summary, our results indicate that LAL is the major acid RE hydrolase and required for functional retinoid homeostasis.

The interplay of protein kinase A and perilipin 5 regulates cardiac lipolysis.

Defective lipolysis in mice lacking adipose triglyceride lipase provokes severe cardiac steatosis and heart dysfunction, markedly shortening life span. Similarly, cardiac muscle (CM)-specific Plin5 overexpression (CM-Plin5) leads to severe triglyceride (TG) accumulation in cardiomyocytes via impairing TG breakdown. Interestingly, cardiac steatosis due to overexpression of Plin5 is compatible with normal heart function and life span indicating a more moderate impact of Plin5 overexpression on cardiac lipolysis and energy metabolism. We hypothesized that cardiac Plin5 overexpression does not constantly impair cardiac lipolysis. In line with this assumption, TG levels decreased in CM of fasted compared with nonfasted CM-Plin5 mice indicating that fasting may lead to a diminished barrier function of Plin5. Recent studies demonstrated that Plin5 is phosphorylated, and activation of adenylyl cyclase leads to phosphorylation of Plin5, suggesting that Plin5 is a substrate for PKA. Furthermore, any significance of Plin5 phosphorylation by PKA in the regulation of TG mobilization from lipid droplets (LDs) is unknown. Here, we show that the lipolytic barrier of Plin5-enriched LDs, either prepared from cardiac tissue of CM-Plin5 mice or Plin5-transfected cells, is abrogated by incubation with PKA. Notably, PKA-induced lipolysis of LDs enriched with Plin5 carrying a single mutation at serine 155 (PlinS155A) of the putative PKA phosphorylation site was substantially impaired revealing a critical role for PKA in Plin5-regulated lipolysis. The strong increase in protein levels of phosphorylated PKA in CM of Plin5 transgenic mice may partially restore fatty acid release from Plin5-enriched LDs, rendering these hearts compatible with normal heart function despite massive steatosis.

α/ß Hydrolase Domain-containing 6 (ABHD6) Degrades the Late Endosomal/Lysosomal Lipid Bis(monoacylglycero)phosphate.

α/ß Hydrolase domain-containing 6 (ABHD6) can act as monoacylglycerol hydrolase and is believed to play a role in endocannabinoid signaling as well as in the pathogenesis of obesity and liver steatosis. However, the mechanistic link between gene function and disease is incompletely understood. Here we aimed to further characterize the role of ABHD6 in lipid metabolism. We show that mouse and human ABHD6 degrade bis(monoacylglycero)phosphate (BMP) with high specific activity. BMP, also known as lysobisphosphatidic acid, is enriched in late endosomes/lysosomes, where it plays a key role in the formation of intraluminal vesicles and in lipid sorting. Up to now, little has been known about the catabolism of this lipid. Our data demonstrate that ABHD6 is responsible for ∼ 90% of the BMP hydrolase activity detected in the liver and that knockdown of ABHD6 increases hepatic BMP levels. Tissue fractionation and live-cell imaging experiments revealed that ABHD6 co-localizes with late endosomes/lysosomes. The enzyme is active at cytosolic pH and lacks acid hydrolase activity, implying that it degrades BMP exported from acidic organelles or de novo-formed BMP. In conclusion, our data suggest that ABHD6 controls BMP catabolism and is therefore part of the late endosomal/lysosomal lipid-sorting machinery.

Fatty Acid-binding Proteins Interact with Comparative Gene Identification-58 Linking Lipolysis with Lipid Ligand Shuttling.

The coordinated breakdown of intracellular triglyceride (TG) stores requires the exquisitely regulated interaction of lipolytic enzymes with regulatory, accessory, and scaffolding proteins. Together they form a dynamic multiprotein network designated as the "lipolysome." Adipose triglyceride lipase (Atgl) catalyzes the initiating step of TG hydrolysis and requires comparative gene identification-58 (Cgi-58) as a potent activator of enzyme activity. Here, we identify adipocyte-type fatty acid-binding protein (A-Fabp) and other members of the fatty acid-binding protein (Fabp) family as interaction partners of Cgi-58. Co-immunoprecipitation, microscale thermophoresis, and solid phase assays proved direct protein/protein interaction between A-Fabp and Cgi-58. Using nuclear magnetic resonance titration experiments and site-directed mutagenesis, we located a potential contact region on A-Fabp. In functional terms, A-Fabp stimulates Atgl-catalyzed TG hydrolysis in a Cgi-58-dependent manner. Additionally, transcriptional transactivation assays with a luciferase reporter system revealed that Fabps enhance the ability of Atgl/Cgi-58-mediated lipolysis to induce the activity of peroxisome proliferator-activated receptors. Our studies identify Fabps as crucial structural and functional components of the lipolysome.

Adipose triglyceride lipase is involved in the mobilization of triglyceride and retinoid stores of hepatic stellate cells.

Hepatic stellate cells (HSCs) store triglycerides (TGs) and retinyl ester (RE) in cytosolic lipid droplets. RE stores are degraded following retinoid starvation or in response to pathogenic stimuli resulting in HSC activation. At present, the major enzymes catalyzing lipid degradation in HSCs are unknown. In this study, we investigated whether adipose triglyceride lipase (ATGL) is involved in RE catabolism of HSCs. Additionally, we compared the effects of ATGL deficiency and hormone-sensitive lipase (HSL) deficiency, a known RE hydrolase (REH), on RE stores in liver and adipose tissue. We show that ATGL degrades RE even in the presence of TGs, implicating that these substrates compete for ATGL binding. REH activity was stimulated and inhibited by comparative gene identification-58 and G0/G1 switch gene-2, respectively, the physiological regulators of ATGL activity. In cultured primary murine HSCs, pharmacological inhibition of ATGL, but not HSL, increased RE accumulation. In mice globally lacking ATGL or HSL, RE contents in white adipose tissue were decreased or increased, respectively, while plasma retinol and liver RE levels remained unchanged. In conclusion, our study shows that ATGL acts as REH in HSCs promoting the degradation of RE stores in addition to its established function as TG lipase. HSL is the predominant REH in adipocytes but does not affect lipid mobilization in HSCs.

DAG tales: the multiple faces of diacylglycerol--stereochemistry, metabolism, and signaling.

The neutral lipids diacylglycerols (DAGs) are involved in a plethora of metabolic pathways. They function as components of cellular membranes, as building blocks for glycero(phospho)lipids, and as lipid second messengers. Considering their central role in multiple metabolic processes and signaling pathways, cellular DAG levels require a tight regulation to ensure a constant and controlled availability. Interestingly, DAG species are versatile in their chemical structure. Besides the different fatty acid species esterified to the glycerol backbone, DAGs can occur in three different stereo/regioisoforms, each with unique biological properties. Recent scientific advances have revealed that DAG metabolizing enzymes generate and distinguish different DAG isoforms, and that only one DAG isoform holds signaling properties. Herein, we review the current knowledge of DAG stereochemistry and their impact on cellular metabolism and signaling. Further, we describe intracellular DAG turnover and its stereochemistry in a 3-pool model to illustrate the spatial and stereochemical separation and hereby the diversity of cellular DAG metabolism.

ATGL and CGI-58 are lipid droplet proteins of the hepatic stellate cell line HSC-T6.

Lipid droplets (LDs) of hepatic stellate cells (HSCs) contain large amounts of vitamin A [in the form of retinyl esters (REs)] as well as other neutral lipids such as TGs. During times of insufficient vitamin A availability, RE stores are mobilized to ensure a constant supply to the body. To date, little is known about the enzymes responsible for the hydrolysis of neutral lipid esters, in particular of REs, in HSCs. In this study, we aimed to identify LD-associated neutral lipid hydrolases by a proteomic approach using the rat stellate cell line HSC-T6. First, we loaded cells with retinol and FAs to promote lipid synthesis and deposition within LDs. Then, LDs were isolated and lipid composition and the LD proteome were analyzed. Among other proteins, we found perilipin 2, adipose TG lipase (ATGL), and comparative gene identification-58 (CGI-58), known and established LD proteins. Bioinformatic search of the LD proteome for α/ß-hydrolase fold-containing proteins revealed no yet uncharacterized neutral lipid hydrolases. In in vitro activity assays, we show that rat (r)ATGL, coactivated by rat (r)CGI-58, efficiently hydrolyzes TGs and REs. These findings suggest that rATGL and rCGI-58 are LD-resident proteins in HSCs and participate in the mobilization of both REs and TGs.

A peptide derived from G0/G1 switch gene 2 acts as noncompetitive inhibitor of adipose triglyceride lipase.

The protein G0/G1 switch gene 2 (G0S2) is a small basic protein that functions as an endogenous inhibitor of adipose triglyceride lipase (ATGL), a key enzyme in intracellular lipolysis. In this study, we identified a short sequence covering residues Lys-20 to Ala-52 in G0S2 that is still fully capable of inhibiting mouse and human ATGL. We found that a synthetic peptide corresponding to this region inhibits ATGL in a noncompetitive manner in the nanomolar range. This peptide is highly selective for ATGL and does not inhibit other lipases, including hormone-sensitive lipase, monoacylglycerol lipase, lipoprotein lipase, and patatin domain-containing phospholipases 6 and 7. Because increased lipolysis is linked to the development of metabolic disorders, the inhibition of ATGL by G0S2-derived peptides may represent a novel therapeutic tool to modulate lipolysis.