ATGL and DGAT1 are involved in the turnover of newly synthesized triacylglycerols in hepatic stellate cells

Early activation of hepatic stellate cells induces rapid initiation of retinyl ester breakdown while maintaining lecithin:retinol acyltransferase (LRAT) activity

Lecithin:retinol acyltransferase (LRAT) is the main enzyme producing retinyl esters (REs) in quiescent hepatic stellate cells (HSCs). When cultured on stiff plastic culture plates, quiescent HSCs activate and lose their RE stores in a process similar to that in the liver following tissue damage, leading to fibrosis. Here we validated HSC cultures in soft gels to study RE metabolism in stable quiescent HSCs and investigated RE synthesis and breakdown in activating HSCs. HSCs cultured in a soft gel maintained characteristics of quiescent HSCs, including the size, amount and composition of their characteristic large lipid droplets. Quiescent gel-cultured HSCs maintained high expression levels of Lrat and a RE storing phenotype with low levels of RE breakdown. Newly formed REs are highly enriched in retinyl palmitate (RP), similar to freshly isolated quiescent HSCs, which is associated with high LRAT activity. Comparison of these quiescent gel-cultured HSCs with activated plastic-cultured HSCs showed that although during early activation the total RE levels and RP-enrichment are reduced, levels of RE formation are maintained and mediated by LRAT. Loss of REs was caused by enhanced RE breakdown in activating HSCs. Upon prolonged culturing, activated HSCs have lost their LRAT activity and produce small amounts of REs by DGAT1. This study reveals unexpected dynamics in RE metabolism during early HSC activation, which might be important in liver disease as early stages are reversible. Soft gel cultures provide a promising model to study RE metabolism in quiescent HSCs, allowing detailed molecular investigations on the mechanisms for storage and release.

The effects of TGF-β-induced activation and starvation of vitamin A and palmitic acid on human stem cell-derived hepatic stellate cells

BACKGROUND

Hepatic stellate cells (HSC) have numerous critical roles in liver function and homeostasis, while they are also known for their importance during liver injury and fibrosis. There is therefore a need for relevant in vitro human HSC models to fill current knowledge gaps. In particular, the roles of vitamin A (VA), lipid droplets (LDs), and energy metabolism in human HSC activation are poorly understood.

METHODS

In this study, human pluripotent stem cell-derived HSCs (scHSCs), benchmarked to human primary HSC, were exposed to 48-hour starvation of retinol (ROL) and palmitic acid (PA) in the presence or absence of the potent HSC activator TGF-β. The interventions were studied by an extensive set of phenotypic and functional analyses, including transcriptomic analysis, measurement of activation-related proteins and cytokines, VA- and LD storage, and cell energy metabolism.

RESULTS

The results show that though the starvation of ROL and PA alone did not induce scHSC activation, the starvation amplified the TGF-β-induced activation-related transcriptome. However, TGF-β-induced activation alone did not lead to a reduction in VA or LD stores. Additionally, reduced glycolysis and increased mitochondrial fission were observed in response to TGF-β.

CONCLUSIONS

scHSCs are robust models for activation studies. The loss of VA and LDs is not sufficient for scHSC activation in vitro, but may amplify the TGF-β-induced activation response. Collectively, our work provides an extensive framework for studying human HSCs in healthy and diseased conditions.

An important role for triglyceride in regulating spermatogenesis

Drosophila is a powerful model to study how lipids affect spermatogenesis. Yet, the contribution of neutral lipids, a major lipid group which resides in organelles called lipid droplets (LD), to sperm development is largely unknown. Emerging evidence suggests LD are present in the testis and that loss of neutral lipid- and LD-associated genes causes subfertility; however, key regulators of testis neutral lipids and LD remain unclear. Here, we show LD are present in early-stage somatic and germline cells within the Drosophila testis. We identified a role for triglyceride lipase brummer (bmm) in regulating testis LD, and found that whole-body loss of bmm leads to defects in sperm development. Importantly, these represent cell-autonomous roles for bmm in regulating testis LD and spermatogenesis. Because lipidomic analysis of bmm mutants revealed excess triglyceride accumulation, and spermatogenic defects in bmm mutants were rescued by genetically blocking triglyceride synthesis, our data suggest that bmm-mediated regulation of triglyceride influences sperm development. This identifies triglyceride as an important neutral lipid that contributes to Drosophila sperm development, and reveals a key role for bmm in regulating testis triglyceride levels during spermatogenesis.

Mammalian lipid droplets: structural, pathological, immunological and anti-toxicological roles

Mammalian lipid droplets (LDs) are specialized cytosolic organelles consisting of a neutral lipid core surrounded by a membrane made up of a phospholipid monolayer and a specific population of proteins that varies according to the location and function of each LD. Over the past decade, there have been significant advances in the understanding of LD biogenesis and functions. LDs are now recognized as dynamic organelles that participate in many aspects of cellular homeostasis plus other vital functions. LD biogenesis is a complex, highly-regulated process with assembly occurring on the endoplasmic reticulum although aspects of the underpinning molecular mechanisms remain elusive. For example, it is unclear how many enzymes participate in the biosynthesis of the neutral lipid components of LDs and how this process is coordinated in response to different metabolic cues to promote or suppress LD formation and turnover. In addition to enzymes involved in the biosynthesis of neutral lipids, various scaffolding proteins play roles in coordinating LD formation. Despite their lack of ultrastructural diversity, LDs in different mammalian cell types are involved in a wide range of biological functions. These include roles in membrane homeostasis, regulation of hypoxia, neoplastic inflammatory responses, cellular oxidative status, lipid peroxidation, and protection against potentially toxic intracellular fatty acids and lipophilic xenobiotics. Herein, the roles of mammalian LDs and their associated proteins are reviewed with a particular focus on their roles in pathological, immunological and anti-toxicological processes.

Lipidomic profiling of rat hepatic stellate cells during activation reveals a two-stage process accompanied by increased levels of lysosomal lipids

Hepatic stellate cells (HSCs) are liver-resident cells best known for their role in vitamin A storage under physiological conditions. Upon liver injury, HSCs activate into myofibroblast-like cells, a key process in the onset of liver fibrosis. Lipids play an important role during HSC activation. Here, we provide a comprehensive characterization of the lipidomes of primary rat HSCs during 17 days of activation in vitro. For lipidomic data interpretation, we expanded our previously described Lipid Ontology (LION) and associated web application (LION/Web) with the LION-PCA heatmap module, which generates heatmaps of the most typical LION-signatures in lipidomic datasets. Furthermore, we used LION to perform pathway analysis to determine the significant metabolic conversions in lipid pathways. Together, we identify two distinct stages of HSC activation. In the first stage, we observe a decrease of saturated phosphatidylcholine, sphingomyelin, and phosphatidic acid and an increase in phosphatidylserine and polyunsaturated bis(monoacylglycero)phosphate (BMP), a lipid class typically localized at endosomes and lysosomes. In the second activation stage, BMPs, hexosylceramides, and ether-linked phosphatidylcholines are elevated, resembling a lysosomal lipid storage disease profile. The presence of isomeric structures of BMP in HSCs was confirmed ex vivo in MS-imaging datasets of steatosed liver sections. Finally, treatment with pharmaceuticals targeting the lysosomal integrity led to cell death in primary HSCs but not in HeLa cells. In summary, our combined data suggest that lysosomes play a critical role during a two-stage activation process of HSCs.

Retinyl Ester Analysis by Orbitrap Mass Spectrometry

Retinoids are light-sensitive molecules that are normally detected by UV absorption techniques. Here we describe the identification and quantification of retinyl ester species by high-resolution mass spectrometry. Retinyl esters are extracted by the method of Bligh and Dyer and subsequently separated by HPLC in runs of 40 min. The retinyl esters are identified and quantified by mass spectrometry analysis. This procedure enables the highly sensitive detection and characterization of retinyl esters in biological samples such as hepatic stellate cells.

Lipid droplets, the Holy Grail of hepatic stellate cells: In health and hepatic fibrosis

Lipid droplets (LDs) are distinct morphological markers of hepatic stellate cells (HSCs). They are composed of a core of predominantly retinyl esters and triacylglycerols surrounded by a phospholipid layer; the latter harbors perilipins 2, 3, and 5, which help control LD lipolysis. Electron microscopy distinguishes between Types I and II LDs. Type I LDs are surrounded by acid phosphatase-positive lysosomes, which likely digest LDs. LD count and retinoid concentration are modulated by vitamin A intake. Alcohol consumption depletes hepatic retinoids and HSC LDs, with concomitant transformation of HSCs to fibrogenic myofibroblast-like cells. LD loss and accompanying HSC activation occur in HSC cell culture models. Loss of LDs is a consequence of and not a prerequisite for HSC activation. LDs are endowed with enzymes for synthesizing retinyl esters and triacylglycerols as well as neutral lipases and lysosomal acid lipase for breaking down LDs. HSCs have two distinct metabolic LD pools: an "original" pool in quiescent HSCs and a "new" pool emerging in HSC activation; this two-pool model provides a platform for analyzing LD dynamics in HSC activation. Besides lipolysis, LDs are degraded by lipophagy; however, the coordination between and relative contributions of these two pathways to LD removal are unclear. While induction of autophagy accelerates LD loss in quiescent HSCs and promotes HSC activation, blocking autophagy impairs LD degradation and inhibits HSC activation and fibrosis. This article is a critique of five decades of investigations into the morphology, molecular structure, synthesis, and degradation of LDs associated with HSC activation and fibrosis.

Endurance exercise-mediated metabolic reshuffle attenuates high-caloric diet-induced non-alcoholic fatty liver disease

INTRODUCTION AND AIM

Non-alcoholic fatty liver disease (NAFLD) is one of the most common diseases in the United States. Metabolic distress (obese diabetes) is the main causative element of NAFLD. While there is no cure for NAFLD, endurance exercise (EEx) has emerged as a therapeutic strategy against NAFLD. However, mechanisms of EXE-induced hepatic protection especially in female subjects remain unidentified. Thus, the aim of the study is to examine molecular mechanisms of EXE-induced hepatic protection against diet-induced NAFLD in female mice.

MATERIAL AND METHODS

Nine-week-old female C57BL/6J mice were randomly divided into three groups: normal-diet control group (CON, n=11); high-fat diet/high-fructose group (HFD/HF, n=11); and HFD/HF+EEx group (HFD/HF+EEx, n=11). The mice assigned to HFD/HF and HFD/HF+EEx groups were fed with HFD/HF for 12 weeks, after which the mice assigned to the EEx group began treadmill exercise for 12 weeks, with HFD/HF continued.

RESULTS

EEx attenuated hepatic steatosis, reduced de novo lipogenesis (reduction in ATP-Citrate- Lyase and diacylglycerol-O-acyltransferase 1), and enhanced mitochondrial biogenesis and fatty-acid activation (oxidative phosphorylation enzymes and Acyl-CoA synthetase1). Also, EEx prevented upregulation of gluconeogenic proteins (glyceraldehyde-3-phosphate dehydrogenase, glucose-6-phosphatase, and phosphoenolpyruvate-carboxykinase1), premature senescence (suppression of p53, p22, and p16, tumor-necrosis-factor-α, and interleukin-1β, and oxidative stress), and autophagy deficiency. Furthermore, EXE reversed apoptosis arrest (cleaved cysteine-dependent-aspartate-directed protease3 and Poly-(ADP-ribose)-polymerase1).

CONCLUSION

EEx-mediated reparations of metabolic and redox imbalance (utilization of pentose phosphate pathway), and autophagy deficiency caused by metabolic distress critically contribute to preventing/delaying severe progression of NAFLD. Also, EEx-induced anti-senescence and cell turnover are crucial protective mechanisms against NAFLD.

Transcriptome Sequencing to Identify Important Genes and lncRNAs Regulating Abdominal Fat Deposition in Ducks

Simple Summary

Abdominal fat deposition affects the quality of duck meat and the feed conversion ratio. Here, we performed transcriptome sequencing of the abdominal adipose tissue of ducks with high and low abdominal fat rate by RNA sequencing, exploring the key regulatory genes and lncRNAs related to abdominal fat deposition. As a result, several candidate genes, lncRNAs, and pathways related to abdominal fat deposition in ducks were retrieved. This study lays the foundations for exploring molecular mechanisms underlying the regulation of abdominal fat deposition in ducks, providing a theoretical reference for breeding high-quality meat-producing ducks.

Abstract

Abdominal fat deposition is an important trait in meat-producing ducks. F2 generations of 304 Cherry Valley and Runzhou Crested White ducks were studied to identify genes and lncRNAs affecting abdominal fat deposition. RNA sequencing was used to study abdominal fat tissue of four ducks each with high or low abdominal fat rates. In all, 336 upregulated and 297 downregulated mRNAs, and 95 upregulated and 119 downregulated lncRNAs were identified. Target gene prediction of differentially expressed lncRNAs identified 602 genes that were further subjected to Gene Ontology and KEGG pathway analysis. The target genes were enriched in pathways associated with fat synthesis and metabolism and participated in biological processes, including Linoleic acid metabolism, lipid storage, and fat cell differentiation, indicating that these lncRNAs play an important role in abdominal fat deposition. PPAPA, FOXO3, FASN, PNPLA2, FKBP5, TCF7L2, BMP2, FGF2, LIFR, ZBTB16, SIRT, GYG2, NCOR1, and NR3C1 were involved in the regulation of abdominal fat deposition. PNPLA2, TCF7L2, FGF2, LIFR, BMP2, FKBP5, GYG2, and ZBTB16 were regulated by the lncRNAs TCONS_00038080, TCONS_0033547, TCONS_00066773, XR_001190174.3, XR_003492471.1, XR_003493494.1, XR_001192142.3, XR_002405656.2, XR_002401822.2, XR_003497063.1, and so on. This study lays foundations for exploring molecular mechanisms underlying the regulation of abdominal fat deposition in ducks and provides a theoretical basis for breeding high-quality meat-producing ducks.

Mechanisms of Feedback Regulation of Vitamin A Metabolism

Vitamin A is an essential nutrient required throughout life. Through its various metabolites, vitamin A sustains fetal development, immunity, vision, and the maintenance, regulation, and repair of adult tissues. Abnormal tissue levels of the vitamin A metabolite, retinoic acid, can result in detrimental effects which can include congenital defects, immune deficiencies, proliferative defects, and toxicity. For this reason, intricate feedback mechanisms have evolved to allow tissues to generate appropriate levels of active retinoid metabolites despite variations in the level and format, or in the absorption and conversion efficiency of dietary vitamin A precursors. Here, we review basic mechanisms that govern vitamin A signaling and metabolism, and we focus on retinoic acid-controlled feedback mechanisms that contribute to vitamin A homeostasis. Several approaches to investigate mechanistic details of the vitamin A homeostatic regulation using genomic, gene editing, and chromatin capture technologies are also discussed.

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.

Retinyl esters form lipid droplets independently of triacylglycerol and seipin

Lipid droplets store neutral lipids, primarily triacylglycerol and steryl esters. Seipin plays a role in lipid droplet biogenesis and is thought to determine the site of lipid droplet biogenesis and the size of newly formed lipid droplets. Here we show a seipin-independent pathway of lipid droplet biogenesis. In silico and in vitro experiments reveal that retinyl esters have the intrinsic propensity to sequester and nucleate in lipid bilayers. Production of retinyl esters in mammalian and yeast cells that do not normally produce retinyl esters causes the formation of lipid droplets, even in a yeast strain that produces only retinyl esters and no other neutral lipids. Seipin does not determine the size or biogenesis site of lipid droplets composed of only retinyl esters or steryl esters. These findings indicate that the role of seipin in lipid droplet biogenesis depends on the type of neutral lipid stored in forming droplets.

Lipid droplet: A functionally active organelle in monocyte to macrophage differentiation and its inflammatory properties

Lipid droplets (LDs) perform several important functions like inflammatory responses, membrane trafficking, acts as secondary messengers, etc. rather than simply working as an energy reservoir. LDs have been implicated as a controlling factor in the progression of atherosclerosis followed by foam cell formation that derives from macrophages during the differentiation process. However, the role of LDs in monocyte differentiation or its further immunological function is still an area that mandates in-depth investigation. We report that LD dynamics is important for differentiation of monocytes and is absolutely required for sustained and prolonged functional activity of differentiated macrophages. In THP-1 cell line model system, we elucidated that increase in total LD content in monocyte by external lipid supplements, can induce monocyte differentiation independent of classical stimuli, PMA. Differential expression of PLIN2 and ATGL during the event, together with abrogation of de novo lipogenesis further confirmed the fact. Besides, an increase in LD content by free fatty acid supplement was able to exert a synergistic effect with PMA on differentiation and phagocytic activity compared to when they are used alone. Additionally, we have shown Rab5a to play a vital role in LDs biosynthesis/maturation in monocytes and thereby directly affecting differentiation of monocytes into macrophages via AKT pathway. Thus our study reveals the multi-faceted function of LDs during the process of monocyte to macrophage differentiation and thereby helping to maintain the functional activity.

Lipophagy-derived fatty acids undergo extracellular efflux via lysosomal exocytosis

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

Playing Jekyll and Hyde-The Dual Role of Lipids in Fatty Liver Disease

Lipids play Jekyll and Hyde in the liver. On the one hand, the lipid-laden status of hepatic stellate cells is a hallmark of healthy liver. On the other hand, the opposite is true for lipid-laden hepatocytes—they obstruct liver function. Neglected lipid accumulation in hepatocytes can progress into hepatic fibrosis, a condition induced by the activation of stellate cells. In their resting state, these cells store substantial quantities of fat-soluble vitamin A (retinyl esters) in large lipid droplets. During activation, these lipid organelles are gradually degraded. Hence, treatment of fatty liver disease is treading a tightrope—unsophisticated targeting of hepatic lipid accumulation might trigger problematic side effects on stellate cells. Therefore, it is of great importance to gain more insight into the highly dynamic lipid metabolism of hepatocytes and stellate cells in both quiescent and activated states. In this review, part of the special issue entitled “Cellular and Molecular Mechanisms underlying the Pathogenesis of Hepatic Fibrosis 2020”, we discuss current and highly versatile aspects of neutral lipid metabolism in the pathogenesis of non-alcoholic fatty liver disease (NAFLD).

Silencing of tissue factor by antisense deoxyoligonucleotide mitigates thioacetamide-induced liver injury

BACKGROUND

Retinoid receptors (RRs), RAR-α and RXR-α, work as transcription factors that regulate cell growth, differentiation, survival, and death. Hepatic stellate cells (HSCs) store retinoid and release its RRs as lipid droplets upon their activation.

PURPOSE

We test the hypothesis that loss of retinoid receptors RAR-α and RXR-α from HSCs is dependent on tissue factor (TF) during thioacetamide (TAA)-induced liver injury.

METHODS

Liver toxicity markers, TF, fibrin, cleaved caspase-3, and cyclin D1 as well as histopathology were investigated.

RESULTS

Increased TF, fibrin, cleaved caspase-3, and cyclin D1 protein expression is seen in zone of central vein after TAA injection compared with vehicle-treated mice. A strong downregulation of RAR-α and RXR-α is seen in TAA-induced liver injury. In addition, histopathological obliteration and pericentral expression of cleaved caspase 3 and cyclin D1 are observed after TAA injection compared with the normal vehicle-treated mice. No changes have been seen in TAA/TF-sense (SC) in whole parameters compared with TAA-treated animals. TAA/TF-antisense (AS)-treated mice show normal expression of all parameters and normal histopathological features when compared with the control mice. In conclusion, this study declares that the strong downregulation of RAR-α and RXR-α may cause liver injury and particularly activation of HSCs in TAA-induced toxicity. TF-AS treatment not only downregulates TF protein expression but also alleviates loss of liver RAR-α and RXR-α and suppresses the activated apoptosis signals in TAA-induced liver toxicity. Finally, TF and RAR-α/RXR-α are important regulatory molecules in TAA induced acute liver injury.

Lysosomal acid lipase is the major acid retinyl ester hydrolase in cultured human hepatic stellate cells but not essential for retinyl ester degradation

Vitamin A is stored as retinyl esters (REs) in lipid droplets of hepatic stellate cells (HSCs). To date, two different pathways are known to facilitate the breakdown of REs: (i) Hydrolysis of REs by neutral lipases, and (ii) whole lipid droplet degradation in autolysosomes by acid hydrolysis. In this study, we evaluated the contribution of neutral and acid RE hydrolases to the breakdown of REs in human HSCs. (R)-Bromoenol lactone (R-BEL), inhibitor of adipose triglyceride lipase (ATGL) and patatin-like phospholipase domain-containing 3 (PNPLA3), the hormone-sensitive lipase (HSL) inhibitor 76-0079, as well as the serine-hydrolase inhibitor Orlistat reduced neutral RE hydrolase activity of LX-2 cell-lysates between 20 and 50%. Interestingly, in pulse-chase experiments, R-BEL, 76-0079, as well as Orlistat exerted little to no effect on cellular RE breakdown of LX-2 cells as well as primary human HSCs. In contrast, Lalistat2, a specific lysosomal acid lipase (LAL) inhibitor, virtually blunted acid in vitro RE hydrolase activity of LX-2 cells. Accordingly, HSCs isolated from LAL-deficient mice showed RE accumulation and were virtually devoid of acidic RE hydrolase activity. In pulse-chase experiments however, LAL-deficient HSCs, similar to LX-2 cells and primary human HSCs, were not defective in degrading REs. In summary, results demonstrate that ATGL, PNPLA3, and HSL contribute to neutral RE hydrolysis of human HSCs. LAL is the major acid RE hydrolase in HSCs. Yet, LAL is not limiting for RE degradation under serum-starvation. Together, results suggest that RE breakdown of HSCs is facilitated by (a) so far unknown, non-Orlistat inhibitable RE-hydrolase(s).

Retinoids in health and disease: A role for hepatic stellate cells in affecting retinoid levels

Vitamin A (retinol) is important for normal growth, vision and reproduction. It has a role in the immune response and the development of metabolic syndrome. Most of the retinol present in the body is stored as retinyl esters within lipid droplets in hepatic stellate cells (HSCs). In case of liver damage, HSCs release large amounts of stored retinol, which is partially converted to retinoic acid (RA). This surge of RA can mediate the immune response and enhance the regeneration of the liver. If the damage persists activated HSCs change into myofibroblast-like cells producing extracellular matrix, which increases the chance of tumorigenesis to occur. RA has been shown to decrease proliferation and metastasis of hepatocellular carcinoma. The levels of RA and RA signaling are influenced by the possibility to esterify retinol towards retinyl esters. This suggests a complex regulation between different retinoids, with an important regulatory role for HSCs.

Lipid Droplets and the Management of Cellular Stress

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

Cellular and molecular effects of Baccharis dracunculifolia D.C. and Plectranthus barbatus Andrews medicinal plant extracts on retinoid metabolism in the human hepatic stellate cell LX-2

Background

Chronic hepatic diseases are serious problems worldwide, which may lead to the development of fibrosis and eventually cirrhosis. Despite the significant number of people affected by hepatic fibrosis, no effective treatment is available. In the liver, hepatic stellate cells are the major fibrogenic cell type that play a relevant function in chronic liver diseases. Thus, the characterization of components that control the fibrogenesis in the hepatic stellate cells is relevant in supporting the development of innovative therapies to treat and/or control liver fibrosis. The present study investigated the effects of Baccharis dracunculifolia D.C. and Plectranthus barbatus Andrews medicinal plant extracts in LX-2 transdifferentiation.

Methods

LX-2 is a human immortalized hepatic stellate cell that can transdifferentiate in vitro from a quiescent-like phenotype to a more proliferative and activated behavior, and it provides a useful platform to assess antifibrotic drugs. Then, the antifibrotic effects of hydroalcoholic extracts of Baccharis dracunculifolia and Plectranthus barbatus medicinal plants on LX-2 were evaluated.

Results

The results in our cellular analyses, under the investigated concentrations of the plant extracts, indicate no deleterious effects on LX-2 metabolism, such as toxicity, genotoxicity, or apoptosis. Moreover, the extracts induced changes in actin filament distribution of activated LX-2, despite not affecting the cellular markers of transdifferentiation. Consistent effects in cellular retinoid metabolism were observed, supporting the presumed activity of the plant extracts in hepatic lipids metabolism, which corroborated the traditional knowledge about their uses for liver dysfunction.

Conclusion

The combined results suggested a potential hepatoprotective effect of the investigated plant extracts reinforcing their safe use as coadjuvants in treating imbalanced liver lipid metabolism.

Recent insights on the role and regulation of retinoic acid signaling during epicardial development

The vitamin A metabolite, retinoic acid, carries out essential and conserved roles in vertebrate heart development. Retinoic acid signals via retinoic acid receptors (RAR)/retinoid X receptors (RXRs) heterodimers to induce the expression of genes that control cell fate specification, proliferation, and differentiation. Alterations in retinoic acid levels are often associated with congenital heart defects. Therefore, embryonic levels of retinoic acid need to be carefully regulated through the activity of enzymes, binding proteins and transporters involved in vitamin A metabolism. Here, we review evidence of the complex mechanisms that control the fetal uptake and synthesis of retinoic acid from vitamin A precursors. Next, we highlight recent evidence of the role of retinoic acid in orchestrating myocardial compact zone growth and coronary vascular development.

Hormone-sensitive lipase is a retinyl ester hydrolase in human and rat quiescent hepatic stellate cells

Hepatic stellate cells (HSC) store vitamin A as retinyl esters and control circulating retinol levels. Upon liver injury, quiescent (q)HSC lose their vitamin A and transdifferentiate to myofibroblasts, e.g. activated (a)HSC, which promote fibrosis by producing excessive extracellular matrix. Adipose triglyceride lipase/patatin-like phospholipase domain-containing protein 2 (ATGL/PNPLA2) and adiponutrin (ADPN/PNPLA3) have so far been shown to mobilize retinol from retinyl esters in HSC. Here, we studied the putative role of hormone-sensitive lipase (HSL/LIPE) in HSC, as it is the major retinyl ester hydrolase (REH) in adipose tissue. Lipe/HSL expression was analyzed in rat liver and primary human and rat qHSC and culture-activated aHSC. Retinyl hydrolysis was analyzed after Isoproterenol-mediated phosphorylation/activation of HSL. Primary human HSC contain 2.5-fold higher LIPE mRNA levels compared to hepatocytes. Healthy rat liver contains significant mRNA and protein levels of HSL/Lipe, which predominates in qHSC and cells of the portal tree. Q-PCR comparison indicates that Lipe mRNA levels in qHSC are dominant over Pnpla2 and Pnpla3. HSL is mostly phosphorylated/activated in qHSC and partly colocalizes with vitamin A-containing lipid droplets. Lipe/HSL and Pnpla3 expression is rapidly lost during HSC culture-activation, while Pnpla2 expression is maintained. HSL super-activation by isoproterenol accelerates loss of lipid droplets and retinyl palmitate from HSC, which coincided with a small, but significant reduction in HSC proliferation and suppression of Collagen1A1 mRNA and protein levels. In conclusion, HSL participates in vitamin A metabolism in qHSC. Equivalent activities of ATGL and ADPN provide the healthy liver with multiple routes to control circulating retinol levels.

Hepatic stellate cell activation: A source for bioactive lipids

Hepatic stellate cells (HSCs) are non-parenchymal liver cells that characteristically contain multiple retinoid (vitamin A)-containing lipid droplets. In this study, we addressed the metabolic fate of non-retinoid lipids originating from lipid droplet loss during HSCs activation. UPLC/MS/MS and qRT-PCR were used to monitor the lipid composition and mRNA expression of selected genes regulating lipid metabolism in freshly isolated, overnight-, 3- and 7-day cultures or primary mouse HSCs. A preferential accumulation of specific C20-C24 fatty acid species, especially arachidonic (C20:4) and docosahexaenoic acids (C22:6), was revealed in culture-activated HSCs along with an upregulation of transcription of fatty acid desaturases (Scd1, Scd2) and elongases (Elovl5, Elovl6). This was accompanied with an enrichment of activated HSCs with 36:4 and 38:4 phosphatidylcholine species containing polyunsaturated fatty acids and associated accumulation of selective lipid mediators, including endocannabinoids and related N-acylethanolamides, as well as ceramides. An increase in 2-arachidonoylglycerol and N-arachydonoylethanolamide concentrations was observed along with an upregulation of Daglα mRNA expression in HSCs during culture activation. N-palmitoylethanolamide was identified as the most abundant endocannabinoid-like species in activated HSCs. An increase in total ceramide levels and enrichment with N-palmitoyl (C16:0), N-tetracosenoyl (C24:1), N-tetracosanoyl (C24:0) and N-docosanoyl (C22:0) ceramides was detected in activated HSC cultures and was preceded by increased mRNA expression of ceramide synthesizing enzymes (CerS2, CerS5 and Smpd1). Our data suggest an active redistribution of non-retinoid lipids in HSCs underlying the formation of low abundance, highly bioactive lipid species that may affect signaling during HSC activation, as well as extracellularly within the liver.

Role of Metabolism in Hepatic Stellate Cell Activation and Fibrogenesis

Activation of hepatic stellate cell (HSC) involves the transition from a quiescent to a proliferative, migratory, and fibrogenic phenotype (i.e., myofibroblast), which is characteristic of liver fibrogenesis. Multiple cellular and molecular signals which contribute to HSC activation have been identified. This review specially focuses on the metabolic changes which impact on HSC activation and fibrogenesis.

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

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

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

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

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

Retinoids Issued from Hepatic Stellate Cell Lipid Droplet Loss as Potential Signaling Molecules Orchestrating a Multicellular Liver Injury Response

Hepatic stellate cells (HSCs) serve as the main body storage compartment for vitamin A through retinyl ester (RE)-filled lipid droplets (LDs). Upon liver injury, HSCs adopt a myofibroblastic phenotype characterized by an elevated expression of extracellular matrix proteins and a concomitant loss of LDs. On the one hand, LD breakdown has been suggested to provide the energy required for HSC activation into myofibroblast-like cells. On the other hand, this process could mitigate HSC activation following the transformation of released REs into retinoic acids (RAs), ligands for nuclear receptors exerting antifibrotic transcriptional regulatory activities in HSCs. Importantly, RAs may also constitute a means for HSCs to orchestrate the liver response to injury by triggering transcriptional effects in multiple additional surrounding liver cell populations. We envision that new approaches, such as single-cell technologies, will allow to better define how RAs are issued from LD loss in HSCs exert a multicellular control of the liver (patho)physiology.

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

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

Oleic Acid and Hydroxytyrosol Inhibit Cholesterol and Fatty Acid Synthesis in C6 Glioma Cells

Recently, the discovery of natural compounds capable of modulating nervous system function has revealed new perspectives for a healthier brain. Here, we investigated the effects of oleic acid (OA) and hydroxytyrosol (HTyr), two important extra virgin olive oil compounds, on lipid synthesis in C6 glioma cells. OA and HTyr inhibited both de novo fatty acid and cholesterol syntheses without affecting cell viability. The inhibitory effect of the individual compounds was more pronounced if OA and HTyr were administered in combination. A reduction of polar lipid biosynthesis was also detected, while triglyceride synthesis was marginally affected. To clarify the lipid-lowering mechanism of these compounds, their effects on the activity of key enzymes of fatty acid biosynthesis (acetyl-CoA carboxylase-ACC and fatty acid synthase-FAS) and cholesterologenesis (3-hydroxy-3-methylglutaryl-CoA reductase-HMGCR) were investigated in situ by using digitonin-permeabilized C6 cells. ACC and HMGCR activities were especially reduced after 4 h of 25 μM OA and HTyr treatment. No change in FAS activity was observed. Inhibition of ACC and HMGCR activities is corroborated by the decrease of their mRNA abundance and protein level. Our results indicate a direct and rapid downregulatory effect of the two olive oil compounds on lipid synthesis in C6 cells.

Some Lipid Droplets Are More Equal Than Others: Different Metabolic Lipid Droplet Pools in Hepatic Stellate Cells

Hepatic stellate cells (HSCs) are professional lipid-storing cells and are unique in their property to store most of the retinol (vitamin A) as retinyl esters in large-sized lipid droplets. Hepatic stellate cell activation is a critical step in the development of chronic liver disease, as activated HSCs cause fibrosis. During activation, HSCs lose their lipid droplets containing triacylglycerols, cholesteryl esters, and retinyl esters. Lipidomic analysis revealed that the dynamics of disappearance of these different classes of neutral lipids are, however, very different from each other. Although retinyl esters steadily decrease during HSC activation, triacylglycerols have multiple pools one of which becomes transiently enriched in polyunsaturated fatty acids before disappearing. These observations are consistent with the existence of preexisting “original” lipid droplets with relatively slow turnover and rapidly recycling lipid droplets that transiently appear during activation of HSCs. Elucidation of the molecular machinery involved in the regulation of these distinct lipid droplet pools may open new avenues for the treatment of liver fibrosis.

Lipid droplet functions beyond energy storage

Lipid droplets are cytoplasmic organelles that store neutral lipids and are critically important for energy metabolism. Their function in energy storage is firmly established and increasingly well characterized. However, emerging evidence indicates that lipid droplets also play important and diverse roles in the cellular handling of lipids and proteins that may not be directly related to energy homeostasis. Lipid handling roles of droplets include the storage of hydrophobic vitamin and signaling precursors, and the management of endoplasmic reticulum and oxidative stress. Roles of lipid droplets in protein handling encompass functions in the maturation, storage, and turnover of cellular and viral polypeptides. Other potential roles of lipid droplets may be connected with their intracellular motility and, in some cases, their nuclear localization. This diversity highlights that lipid droplets are very adaptable organelles, performing different functions in different biological contexts. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

Lysosome-mediated degradation of a distinct pool of lipid droplets during hepatic stellate cell activation

Activation of hepatic stellate cells (HSCs) is a critical step in the development of liver fibrosis. During activation, HSCs lose their lipid droplets (LDs) containing triacylglycerols (TAGs), cholesteryl esters, and retinyl esters (REs). We previously provided evidence for the presence of two distinct LD pools, a preexisting and a dynamic LD pool. Here we investigate the mechanisms of neutral lipid metabolism in the preexisting LD pool. To investigate the involvement of lysosomal degradation of neutral lipids, we studied the effect of lalistat, a specific lysosomal acid lipase (LAL/Lipa) inhibitor on LD degradation in HSCs during activation in vitro The LAL inhibitor increased the levels of TAG, cholesteryl ester, and RE in both rat and mouse HSCs. Lalistat was less potent in inhibiting the degradation of newly synthesized TAG species as compared with a more general lipase inhibitor orlistat. Lalistat also induced the presence of RE-containing LDs in an acidic compartment. However, targeted deletion of the Lipa gene in mice decreased the liver levels of RE, most likely as the result of a gradual disappearance of HSCs in livers of Lipa^-/- mice. Lalistat partially inhibited the induction of activation marker α-smooth muscle actin (α-SMA) in rat and mouse HSCs. Our data suggest that LAL/Lipa is involved in the degradation of a specific preexisting pool of LDs and that inhibition of this pathway attenuates HSC activation.

Hepatic Retinyl Ester Hydrolases and the Mobilization of Retinyl Ester Stores

For mammals, vitamin A (retinol and metabolites) is an essential micronutrient that is required for the maintenance of life. Mammals cannot synthesize vitamin A but have to obtain it from their diet. Resorbed dietary vitamin A is stored in large quantities in the form of retinyl esters (REs) in cytosolic lipid droplets of cells to ensure a constant supply of the body. The largest quantities of REs are stored in the liver, comprising around 80% of the body’s total vitamin A content. These hepatic vitamin A stores are known to be mobilized under times of insufficient dietary vitamin A intake but also under pathological conditions such as chronic alcohol consumption and different forms of liver diseases. The mobilization of REs requires the activity of RE hydrolases. It is astounding that despite their physiological significance little is known about their identities as well as about factors or stimuli which lead to their activation and consequently to the mobilization of hepatic RE stores. In this review, we focus on the recent advances for the understanding of hepatic RE hydrolases and discuss pathological conditions which lead to the mobilization of hepatic RE stores.

Hepatic stellate cells retain the capacity to synthesize retinyl esters and to store neutral lipids in small lipid droplets in the absence of LRAT

Hepatic stellate cells (HSCs) play an important role in liver physiology and under healthy conditions they have a quiescent and lipid-storing phenotype. Upon liver injury, HSCs are activated and rapidly lose their retinyl ester-containing lipid droplets. To investigate the role of lecithin:retinol acyltransferase (LRAT) and acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1) in retinyl ester synthesis and lipid droplet dynamics, we modified LC-MS/MS procedures by including multiple reaction monitoring allowing unambiguous identification and quantification of all major retinyl ester species. Quiescent primary HSCs contain predominantly retinyl palmitate. Exogenous fatty acids are a major determinant in the retinyl ester species synthesized by activated HSCs and LX-2 cells, indicating that HSCs shift their retinyl ester synthesizing capacity from LRAT to DGAT1 during activation. Quiescent LRAT^-/- HSCs retain the capacity to synthesize retinyl esters and to store neutral lipids in lipid droplets ex vivo. The median lipid droplet size in LRAT^-/- HSCs (1080nm) is significantly smaller than in wild type HSCs (1618nm). This is a consequence of an altered lipid droplet size distribution with 50.5±9.0% small (≤700nm) lipid droplets in LRAT^-/- HSCs and 25.6±1.4% large (1400-2100nm) lipid droplets in wild type HSC cells. Upon prolonged (24h) incubation, the amounts of small (≤700nm) lipid droplets strongly increased both in wild type and in LRAT^-/- HSCs, indicating a dynamic behavior in both cell types. The absence of retinyl esters and reduced number of lipid droplets in LRAT-deficient HSCs in vivo will be discussed.