Proteomic profiling of lipid droplet-associated proteins in primary adipocytes of normal and obese mouse

LET-767 determines lipid droplet protein targeting and lipid homeostasis

Lipid droplets (LDs) are composed of a core of neutral lipids wrapped by a phospholipid (PL) monolayer containing several hundred proteins that vary between different cells or organisms. How LD proteins target to LDs is still largely unknown. Here, we show that RNAi knockdown or gene mutation of let-767, encoding a member of hydroxysteroid dehydrogenase (HSD), displaced the LD localization of three well-known LD proteins: DHS-3 (dehydrogenase/reductase), PLIN-1 (perilipin), and DGAT-2 (diacylglycerol O-acyltransferase 2), and also prevented LD growth in Caenorhabditis elegans. LET-767 interacts with ARF-1 (ADP-ribosylation factor 1) to prevent ARF-1 LD translocation for appropriate LD protein targeting and lipid homeostasis. Deficiency of LET-767 leads to the release of ARF-1, which further recruits and promotes translocation of ATGL-1 (adipose triglyceride lipase) to LDs for lipolysis. The displacement of LD proteins caused by LET-767 deficiency could be reversed by inhibition of either ARF-1 or ATGL-1. Our work uncovers a unique LET-767 for determining LD protein targeting and maintaining lipid homeostasis.

Senp7 deficiency impairs lipid droplets maturation in white adipose tissues via Plin4 deSUMOylation

Lipid metabolism is important for the maintenance of physiological homeostasis. Several members of the small ubiquitin-like modifier (SUMO)-specific protease (SENP) family have been reported as the regulators of lipid homeostasis. However, the function of Senp7 in lipid metabolism remains unclear. In this study, we generated both conventional and adipocyte-specific Senp7 KO mice to characterize the role of Senp7 in lipid metabolism homeostasis. Both Senp7-deficient mice displayed reduced white adipose tissue mass and decreased size of adipocytes. By analyzing the lipid droplet morphology, we demonstrated that the lipid droplet size was significantly smaller in Senp7-deficient adipocytes. Mechanistically, Senp7 could deSUMOylate the perilipin family protein Plin4 to promote the lipid droplet localization of Plin4. Our results reveal an important role of Senp7 in the maturation of lipid droplets via Plin4 deSUMOylation.

Imetelstat-mediated alterations in fatty acid metabolism to induce ferroptosis as a therapeutic strategy for acute myeloid leukemia

Telomerase enables replicative immortality in most cancers including acute myeloid leukemia (AML). Imetelstat is a first-in-class telomerase inhibitor with clinical efficacy in myelofibrosis and myelodysplastic syndromes. Here, we develop an AML patient-derived xenograft resource and perform integrated genomics, transcriptomics and lipidomics analyses combined with functional genetics to identify key mediators of imetelstat efficacy. In a randomized phase II-like preclinical trial in patient-derived xenografts, imetelstat effectively diminishes AML burden and preferentially targets subgroups containing mutant NRAS and oxidative stress-associated gene expression signatures. Unbiased, genome-wide CRISPR/Cas9 editing identifies ferroptosis regulators as key mediators of imetelstat efficacy. Imetelstat promotes the formation of polyunsaturated fatty acid-containing phospholipids, causing excessive levels of lipid peroxidation and oxidative stress. Pharmacological inhibition of ferroptosis diminishes imetelstat efficacy. We leverage these mechanistic insights to develop an optimized therapeutic strategy using oxidative stress-inducing chemotherapy to sensitize patient samples to imetelstat causing substantial disease control in AML.

Luteal Lipid Droplets: A Novel Platform for Steroid Synthesis

Progesterone is an essential steroid hormone that is required to initiate and maintain pregnancy in mammals and serves as a metabolic intermediate in the synthesis of endogenously produced steroids, including sex hormones and corticosteroids. Steroidogenic luteal cells of the corpus luteum have the tremendous capacity to synthesize progesterone. These specialized cells are highly enriched with lipid droplets that store lipid substrate, which can be used for the synthesis of steroids. We recently reported that hormone-stimulated progesterone synthesis by luteal cells requires protein kinase A-dependent mobilization of cholesterol substrate from lipid droplets to mitochondria. We hypothesize that luteal lipid droplets are enriched with steroidogenic enzymes and facilitate the synthesis of steroids in the corpus luteum. In the present study, we analyzed the lipid droplet proteome, conducted the first proteomic analysis of lipid droplets under acute cyclic adenosine monophosphate (cAMP)-stimulated conditions, and determined how specific lipid droplet proteins affect steroidogenesis. Steroidogenic enzymes, cytochrome P450 family 11 subfamily A member 1 and 3 beta-hydroxysteroid dehydrogenase (HSD3B), were highly abundant on lipid droplets of the bovine corpus luteum. High-resolution confocal microscopy confirmed the presence of active HSD3B on the surface of luteal lipid droplets. We report that luteal lipid droplets have the capacity to synthesize progesterone from pregnenolone. Lastly, we analyzed the lipid droplet proteome following acute stimulation with cAMP analog, 8-Br-cAMP, and report increased association of HSD3B with luteal lipid droplets following stimulation. These findings provide novel insights into the role of luteal lipid droplets in steroid synthesis.

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.

Isolation and proteomic study of fish liver lipid droplets

Lipid droplets (LDs) are a neutral lipid storage organelle that is conserved in almost all species. Excessive storage of neutral lipids in LDs is directly associated with many metabolic syndromes. Zebrafish is a better model animal for the study of LD biology due to its transparent embryonic stage compared to other organisms. However, the study of LDs in fish has been difficult due to the lack of specific LD marker proteins and the limitation of purification technology. In this paper, the purification and proteomic analysis of liver LDs of fish including zebrafish and Carassius auratus were performed for the first time. 259 and 267 proteins were identified respectively. Besides most of the identified proteins were reported in previous LD proteomes of mammals, indicating the similarity between mammal and fish LDs. We also identified many unique proteins of liver LDs in fish that are involved in the regulation of LD dynamics. Through morphological and biochemical analysis, we found that the marker protein Plin2 of zebrafish LD was located on LDs in Huh7 cells. These results will facilitate further study of LDs in fish and liver metabolic diseases using fish as a model animal.

Dysregulation of Lipid Droplet Protein Expression in Adipose Tissues and Association with Metabolic Risk Factors in Adult Females with Obesity and Type 2 Diabetes

BACKGROUND

Adipocyte dysregulation of lipid droplet (LD) metabolism caused by altered expression of LD proteins contributes to obesity-related metabolic diseases.

OBJECTIVES

We aimed to investigate whether expression levels of PLIN1, CIDEA, and CIDEC were altered in adipose tissues of women with obesity and type 2 diabetes and whether their alterations were associated with metabolic risk factors.

METHODS

Normal-weight (NW; 18.5 kg/m² < BMI ≤ 25 kg/m²; n = 43), nondiabetic obese (OB; BMI > 30 kg/m²; n = 38), and diabetic obese (OB/DM; BMI > 30 kg/m², fasting glucose ≥ 126 mg/dL, HbA1c ≥ 6.5%; n = 22) women were recruited. Metabolic parameters were measured, and expressions of PLIN1, CIDEA, CIDEC, and obesity-related genes were quantified in abdominal subcutaneous (SAT) and visceral adipose tissues (VAT). Effects of proinflammatory cytokines, endoplasmic reticulum (ER) stress inducers, and metabolic improvement agents on LD protein gene expressions were investigated in human adipocytes.

RESULTS

PLIN1, CIDEA, and CIDEC expressions were lower in SAT and higher in VAT in OB subjects relative to NW subjects; however, they were suppressed in both fat depots in OB/DM subjects relative to OB (P < 0.05). Across the entire cohort, whereas VAT PLIN1 (r = 0.349) and CIDEC expressions (r = 0.282) were positively associated with BMI (P < 0.05), SAT PLIN1 (r = -0.390) and CIDEA expressions (r = -0.565) were inversely associated. After adjustment for BMI, some or all of the adipose LD protein gene expressions were negatively associated with fasting glucose (r = -0.259 or higher) and triglyceride levels (r = -0.284 or higher) and positively associated with UCP1 expression (r = 0.353 or higher) (P < 0.05). In adipocytes, LD protein gene expressions were 55-70% downregulated by increased proinflammatory cytokines and ER stress but 2-4-fold upregulated by the metabolic improvement agents exendin-4 and dapagliflozin (P < 0.05).

CONCLUSIONS

The findings suggest that reduction of adipose LD protein expression is involved in the pathogenesis of metabolic disorders in women with obesity and type 2 diabetes and that increasing LD protein expression in adipocytes could control development of metabolic disorders.

Annexin A6 Polymorphism Is Associated with Pro-atherogenic Lipid Profiles and with the Downregulation of Methotrexate on Anti-Atherogenic Lipid Profiles in Psoriasis

BACKGROUND

Annexin A6 (AnxA6) is a lipid-binding protein that regulates cholesterol homeostasis and secretory pathways. However, the correlation of AnxA6 polymorphism with lipometabolism has never been studied in psoriasis.

OBJECTIVES

To investigate the impact of AnxA6 polymorphism on lipid profiles and the expression of AnxA6 protein in both peripheral blood mononuclear cells (PBMCs) and lipometabolism in psoriasis.

METHODS

A total of 265 psoriatic patients received methotrexate (MTX) treatment for 12 weeks, after which their lipid profiles were determined by measuring total cholesterol (TC), triglycerides (TGs), lipoprotein (a) [LP(a)], high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein (LDL), apolipoprotein (a)1 (ApoA1), and apolipoprotein B (ApoB). In addition, AnxA6 (rs11960458) was genotyped in 262 patients and the expression of AnxA6 in PBMCs was measured by Western blotting at baseline and week 8 post-MTX treatment.

RESULTS

The CC genotype carriers of rs11960458 had a lower expression of AnxA6 and lower levels of the pro-atherogenic lipids TC, LDL, and ApoB compared to TC genotype carriers. MTX significantly downregulated the levels of the anti-atherogenic lipids HDL-C and ApoA1 and the level of AnxA6 in TC genotype carriers, as well as the level of TGs in CC genotype carriers.

CONCLUSIONS

The polymorphism of AnxA6, rs11960458, was statistically associated with the levels of pro-atherogenic lipids and with the downregulation of MTX on the levels of anti-atherogenic lipids and TGs in psoriasis.

Recruitment of Peroxin14 to lipid droplets affects lipid storage in Drosophila

Both peroxisomes and lipid droplets regulate cellular lipid homeostasis. Direct inter-organellar contacts as well as novel roles for proteins associated with peroxisome or lipid droplets occur when cells are induced to liberate fatty acids from lipid droplets. We have shown a non-canonical role for as subset of peroxisome-assembly (Peroxin) proteins in this process. Transmembrane proteins Peroxin3, Peroxin13 and Peroxin14 surround newly formed lipid droplets. Trafficking of Peroxin14 to lipid droplets was enhanced by loss of Peroxin19, which directs insertion of transmembrane proteins like Peroxin14 into the peroxisome bilayer membrane. Accumulation of Peroxin14 around lipid droplets did not induce changes to peroxisome size or number, nor was co-recruitment of the remaining Peroxins needed to assemble peroxisomes observed. Increasing the relative level of Peroxin14 surrounding lipid droplets affected recruitment of Hsl lipase. Fat-body specific reduction of these lipid droplet-associated Peroxins causes a unique effect on larval fat body development and affected their survival on lipid-enriched or minimal diets.

Lipid droplets in skeletal muscle during grass snake (Natrix natrix L.) development

Lipid droplets (LDs) are common organelles observed in Eucaryota. They are multifunctional organelles (involved in lipid storage, metabolism, and trafficking) that originate from endoplasmic reticulum (ER). LDs consist of a neutral lipid core, made up of diacyl- and triacylglycerols (DAGs and TAGs) and cholesterol esters (CEs), surrounded by a phospholipid monolayer and proteins, which are necessary for their structure and dynamics. Here, we report the protein and lipid composition as well as characterization and dynamics of grass snake (Natrix natrix) skeletal muscle LDs at different developmental stages. In the present study, we used detailed morphometric, LC-MS, quantitative lipidomic analyses of LDs isolated from the skeletal muscles of the snake embryos, immunofluorescence, and TEM. Our study also provides a valuable insight concerning the LDs' multifunctionality and ability to interact with a variety of organelles. These LD features are reflected in their proteome composition, which contains scaffold proteins, metabolic enzymes signalling polypeptides, proteins necessary for the formation of docking sites, and many others. We also provide insights into the biogenesis and growth of muscle LDs goes beyond the conventional mechanism based on the synthesis and incorporation of TAGs and LD fusion. We assume that the formation and functioning of grass snake muscle LDs are based on additional mechanisms that have not yet been identified, which could be related to the unique features of reptiles that are manifested in the after-hatching period of life, such as a reptile-specific strategy for energy saving during hibernation.

Protein Quality Control and Lipid Droplet Metabolism

Lipid droplets (LDs) are endoplasmic reticulum-derived organelles that consist of a core of neutral lipids encircled by a phospholipid monolayer decorated with proteins. As hubs of cellular lipid and energy metabolism, LDs are inherently involved in the etiology of prevalent metabolic diseases such as obesity and nonalcoholic fatty liver disease. The functions of LDs are regulated by a unique set of associated proteins, the LD proteome, which includes integral membrane and peripheral proteins. These proteins control key activities of LDs such as triacylglycerol synthesis and breakdown, nutrient sensing and signal integration, and interactions with other organelles. Here we review the mechanisms that regulate the composition of the LD proteome, such as pathways that mediate selective and bulk LD protein degradation and potential connections between LDs and cellular protein quality control.

Annexin Animal Models-From Fundamental Principles to Translational Research

Routine manipulation of the mouse genome has become a landmark in biomedical research. Traits that are only associated with advanced developmental stages can now be investigated within a living organism, and the in vivo analysis of corresponding phenotypes and functions advances the translation into the clinical setting. The annexins, a family of closely related calcium (Ca²⁺)- and lipid-binding proteins, are found at various intra- and extracellular locations, and interact with a broad range of membrane lipids and proteins. Their impacts on cellular functions has been extensively assessed in vitro, yet annexin-deficient mouse models generally develop normally and do not display obvious phenotypes. Only in recent years, studies examining genetically modified annexin mouse models which were exposed to stress conditions mimicking human disease often revealed striking phenotypes. This review is the first comprehensive overview of annexin-related research using animal models and their exciting future use for relevant issues in biology and experimental medicine.

Galectin-1 accelerates high-fat diet-induced obesity by activation of peroxisome proliferator-activated receptor gamma (PPARγ) in mice

Galectin-1 contains a carbohydrate-recognition domain (CRD) as a member of the lectin family. Here, we investigated whether galectin-1 regulates adipogenesis and lipid accumulation. Galectin-1 mRNA is highly expressed in metabolic tissues such as the muscle and adipose tissues. Higher mRNA expression of galectin-1 was detected in white adipose tissues (WATs) of mice that were fed a high-fat diet (HFD) than in those of mice fed a normal-fat diet (NFD). Protein expression of galectin-1 also increased during adipocyte differentiation. Galectin-1 silencing inhibited the differentiation of 3T3-L1 cells and the expression of lipogenic factors, such as PPARγ, C/EBPα, FABP4, and FASN at both mRNA and protein levels. Lactose, an inhibitor by the binding with CRD of galectin-1 in extracellular matrix, did not affect adipocyte differentiation. Galectin-1 is localized in multiple cellular compartments in 3T3-L1 cells. However, we found that DMI (dexamethasone, methylisobutylxanthine, insulin) treatment increased its nuclear localization. Interestingly, galectin-1 interacted with PPARγ. Galectin-1 overexpression resulted in increased PPARγ expression and transcriptional activity. Furthermore, we prepared galectin-1-knockout (Lgals1^−/−) mice and fed a 60% HFD. After 10 weeks, Lgals1^−/− mice exhibited lower body weight and gonadal WAT (gWAT) mass than wild-type mice. Fasting glucose level was also lower in Lgals1^−/−mice than that in wild-type mice. Moreover, lipogenic genes were significantly downregulated in the gWATs and liver tissues from Lgals1^−/− mice. Pro-inflammatory cytokines, such as CCL2, CCL3, TNFα, and F4/80, as well as macrophage markers, were also drastically downregulated in the gWATs and liver tissues of Lgals1^−/− mice. In addition, Lgals1^−/−mice showed elevated expression of genes involved in thermogenesis in the brown adipose tissue. Collectively, galectin-1 exacerbates obesity of mice fed HFD by increment of PPARγ expression and activation. Our findings suggest that galectin-1 could be a potential therapeutic target for obesity and needed further study for clinical application.

Snx14 proximity labeling reveals a role in saturated fatty acid metabolism and ER homeostasis defective in SCAR20 disease

Fatty acids (FAs) are central cellular metabolites that contribute to lipid synthesis, and can be stored or harvested for metabolic energy. Dysregulation in FA processing and storage causes toxic FA accumulation or altered membrane compositions and contributes to metabolic and neurological disorders. Saturated lipids are particularly detrimental to cells, but how lipid saturation levels are maintained remains poorly understood. Here, we identify the cerebellar ataxia spinocerebellar ataxia, autosomal recessive 20 (SCAR20)-associated protein Snx14, an endoplasmic reticulum (ER)-lipid droplet (LD) tethering protein, as a factor required to maintain the lipid saturation balance of cell membranes. We show that following saturated FA (SFA) treatment, the ER integrity of SNX14 ^KO cells is compromised, and both SNX14 ^KO cells and SCAR20 disease patient-derived cells are hypersensitive to SFA-mediated lipotoxic cell death. Using APEX2-based proximity labeling, we reveal the protein composition of Snx14-associated ER-LD contacts and define a functional interaction between Snx14 and Δ-9 FA desaturase SCD1. Lipidomic profiling reveals that SNX14 ^KO cells increase membrane lipid saturation following exposure to palmitate, phenocopying cells with perturbed SCD1 activity. In line with this, SNX14 ^KO cells manifest delayed FA processing and lipotoxicity, which can be rescued by SCD1 overexpression. Altogether, these mechanistic insights reveal a role for Snx14 in FA and ER homeostasis, defects in which may underlie the neuropathology of SCAR20.

Influence of Nutritional Intake of Carbohydrates on Mitochondrial Structure, Dynamics, and Functions during Adipogenesis

Obesity is an alarming yet increasing phenomenon worldwide, and more effective obesity management strategies have become essential. In addition to the numerous anti-adipogenic treatments promising a restauration of a healthy white adipose tissue (WAT) function, numerous studies reported on the critical role of nutritional parameters in obesity development. In a metabolic disorder context, a better control of nutrient intake is a key step in slowing down adipogenesis and therefore obesity. Of interest, the effect on WAT remodeling deserves deeper investigations. Among the different actors of WAT plasticity, the mitochondrial network plays a central role due to its dynamics and essential cellular functions. Hence, the present in vitro study, conducted on the 3T3-L1 cell line, aimed at evaluating the incidence of modulating the carbohydrates intake on adipogenesis through an integrated assessment of mitochondrial structure, dynamics, and functions-correlated changes. For this purpose, our experimental strategy was to compare the occurrence of adipogenesis in 3T3-L1 cells cultured either in a high-glucose (HG) medium (25 mM) or in a low-glucose (LG) medium (5 mM) supplemented with equivalent galactose (GAL) levels (20 mM). The present LG-GAL condition was associated, in differentiating adipocytes, to a reduced lipid droplet network, lower expressions of early and late adipogenic genes and proteins, an increased mitochondrial network with higher biogenesis marker expression, an equilibrium in the mitochondrial fusion/fission pattern, and a decreased expression of mitochondrial metabolic overload protein markers. Therefore, those main findings show a clear effect of modulating glucose accessibility on 3T3-L1 adipogenesis through a combined effect of adipogenesis modulation and overall improvement of the mitochondrial health status. This nutritional approach offers promising opportunities in the control and prevention of obesity.

2-Hydroxypropyl-β-cyclodextrin reduces retinal cholesterol in wild-type and Cyp27a1-/- Cyp46a1-/- mice with deficiency in the oxysterol production

BACKGROUND AND PURPOSE

2-Hydroxypropyl-β-cyclodextrin (HPCD) is an FDA approved vehicle for drug delivery and an efficient cholesterol-lowering agent. HPCD was proposed to lower tissue cholesterol via multiple mechanisms including those mediated by oxysterols. CYP27A1 and CYP46A1 are the major oxysterol-producing enzymes in the retina that convert cholesterol to 27- and 24-hydroxycholesterol, respectively. We investigated whether HPCD treatments affected the retina of wild-type and Cyp27a1^-/- Cyp46a1^-/- mice that do not produce the major retinal oxysterols.

EXPERIMENTAL APPROACH

HPCD administration was either by i.p., p.o. or s.c. Delivery to the retina was confirmed by angiography using the fluorescently labelled HPCD. Effects on the levels of retinal sterols, mRNA and proteins were evaluated by GC-MS, qRT-PCR and label-free approach, respectively.

KEY RESULTS

In both wild-type and Cyp27a1^-/- Cyp46a1^-/- mice, HPCD crossed the blood-retinal barrier when delivered i.p. and lowered the retinal cholesterol content when administered p.o. and s.c. In both genotypes, oral HPCD treatment affected the expression of cholesterol-related genes as well as the proteins involved in endocytosis, lysosomal function and lipid homeostasis. Mechanistically, liver X receptors and the altered expression of Lipe (hormone-sensitive lipase), Nceh1 (neutral cholesterol ester hydrolase 1) and NLTP (non-specific lipid-transfer protein) could mediate some of the HPCD effects.

CONCLUSIONS AND IMPLICATIONS

HPCD treatment altered retinal cholesterol homeostasis and is a potential therapeutic approach for the reduction of drusen and subretinal drusenoid deposits, cholesterol-rich lesions and hallmarks of age-related macular degeneration.

LINKED ARTICLES

This article is part of a themed issue on Oxysterols, Lifelong Health and Therapeutics. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.16/issuetoc.

Trafficking of cholesterol from lipid droplets to mitochondria in bovine luteal cells: Acute control of progesterone synthesis

The corpus luteum is a transient endocrine gland that synthesizes and secretes the steroid hormone, progesterone, which is vital for establishment and maintenance of pregnancy. Luteinizing hormone (LH) via activation of protein kinase A (PKA) acutely stimulates luteal progesterone synthesis via a complex process, converting cholesterol via a series of enzymatic reactions, into progesterone. Lipid droplets in steroidogenic luteal cells store cholesterol in the form of cholesterol esters, which are postulated to provide substrate for steroidogenesis. Early enzymatic studies showed that hormone sensitive lipase (HSL) hydrolyzes luteal cholesterol esters. In this study, we tested whether HSL is a critical mediator of the acute actions of LH on luteal progesterone production. Using LH-responsive bovine small luteal cells our results reveal that LH, forskolin, and 8-Br cAMP-induced PKA-dependent phosphorylation of HSL at Ser563 and Ser660, events known to promote HSL activity. Small molecule inhibition of HSL activity and siRNA-mediated knock down of HSL abrogated LH-induced progesterone production. Moreover, western blotting and confocal microscopy revealed that LH stimulates phosphorylation and translocation of HSL to lipid droplets. Furthermore, LH increased trafficking of cholesterol from the lipid droplets to the mitochondria, which was dependent on both PKA and HSL activation. Taken together, these findings identify a PKA/HSL signaling pathway in luteal cells in response to LH and demonstrate the dynamic relationship between PKA, HSL, and lipid droplets in luteal progesterone synthesis.

Identification and analysis of lipid droplet-related proteome in the adipose tissue of grass carp (Ctenopharyngodon idella) under fed and starved conditions

Fat accumulation in the mesenteric adipose tissue is a serious problem in grass carp (Ctenopharyngodon idella) culture. Lipid droplet-related proteins (LDRPs) are involved in the formation, degradation, and biological functions of lipid droplets. In this study, we aimed to provide reference proteomics data to study lipid droplet regulation in fish. We isolated LDRPs from the mesenteric adipose tissue of grass carp (1-year-old) after normal feeding and 7 days of starvation, and identified and analysed them using isobaric tags for relative and absolute quantitation (iTRAQ) technology. Short-term starvation had no significant effect on the body weight, condition factor, visceral index, hepatopancreas index, intraperitoneal fat index, adipose tissue triglyceride content, and adipocyte size of grass carp. Nine hundred and fifty proteins were identified and annotated using the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases; they are involved in a variety of metabolic and signalling pathways, including amino acid, lipid, and carbohydrate metabolism, and the PI3K-Akt signalling pathway. There were 296 differentially expressed proteins (DEPs), with 143 up-regulated and 153 down-regulated proteins. Three proteins involved in triglyceride and fatty acid syntheses and two proteins involved in autophagy were up-regulated, and six proteins involved in lipid catabolism were down-regulated. These results indicate that under short-term starvation, lipid droplets in the adipose tissue of grass carp may maintain their shape by promoting fat production and inhibiting lipolysis, and autophagy may be one of the main strategies for coping with short-term energy deprivation.

Formation and characterization of lipid droplets of the bovine corpus luteum

Establishment and maintenance of pregnancy depends on progesterone synthesized by luteal tissue in the ovary. Our objective was to identify the characteristics of lipid droplets (LDs) in ovarian steroidogenic cells. We hypothesized that LDs are a major feature of steroidogenic luteal cells and store cholesteryl esters. Whole bovine tissues, isolated ovarian steroidogenic cells (granulosa, theca, small luteal, and large luteal), and isolated luteal LDs were assessed for LD content, LD-associated proteins and lipid analyses. Bovine luteal tissue contained abundant lipid droplets, LD-associated perilipins 2/3/5, hormone-sensitive lipase, and 1-acylglycerol-3-phosphate O-acyltransferase ABHD5. Luteal tissue was enriched in triglycerides (TGs) compared to other tissues, except for adipose tissue. Luteal cells were distinguishable from follicular cells by the presence of LDs, LD-associated proteins, and increased TGs. Furthermore, LDs from large luteal cells were numerous and small; whereas, LDs from small luteal cells were large and less numerous. Isolated LDs contained nearly all of the TGs and cholesteryl esters present in luteal tissue. Isolated luteal LDs were composed primarily of TG, with lesser amounts of cholesteryl esters, diglyceride and other phospholipids. Bovine luteal LDs are distinct from LDs in other bovine tissues, including follicular steroidogenic cells.

Selenium and Selenoproteins in Adipose Tissue Physiology and Obesity

Selenium (Se) homeostasis is tightly related to carbohydrate and lipid metabolism, but its possible roles in obesity development and in adipocyte metabolism are unclear. The objective of the present study is to review the current data on Se status in obesity and to discuss the interference between Se and selenoprotein metabolism in adipocyte physiology and obesity pathogenesis. The overview and meta-analysis of the studies on blood Se and selenoprotein P (SELENOP) levels, as well as glutathione peroxidase (GPX) activity in obese subjects, have yielded heterogenous and even conflicting results. Laboratory studies demonstrate that Se may modulate preadipocyte proliferation and adipogenic differentiation, and also interfere with insulin signaling, and regulate lipolysis. Knockout models have demonstrated that the selenoprotein machinery, including endoplasmic reticulum-resident selenoproteins together with GPXs and thioredoxin reductases (TXNRDs), are tightly related to adipocyte development and functioning. In conclusion, Se and selenoproteins appear to play an essential role in adipose tissue physiology, although human data are inconsistent. Taken together, these findings do not support the utility of Se supplementation to prevent or alleviate obesity in humans. Further human and laboratory studies are required to elucidate associations between Se metabolism and obesity.

The Puzzling Conservation and Diversification of Lipid Droplets from Bacteria to Eukaryotes

Membrane compartments are amongst the most fascinating markers of cell evolution from prokaryotes to eukaryotes, some being conserved and the others having emerged via a series of primary and secondary endosymbiosis events. Membrane compartments comprise the system limiting cells (one or two membranes in bacteria, a unique plasma membrane in eukaryotes) and a variety of internal vesicular, subspherical, tubular, or reticulated organelles. In eukaryotes, the internal membranes comprise on the one hand the general endomembrane system, a dynamic network including organelles like the endoplasmic reticulum, the Golgi apparatus, the nuclear envelope, etc. and also the plasma membrane, which are linked via direct lateral connectivity (e.g. between the endoplasmic reticulum and the nuclear outer envelope membrane) or indirectly via vesicular trafficking. On the other hand, semi-autonomous organelles, i.e. mitochondria and chloroplasts, are disconnected from the endomembrane system and request vertical transmission following cell division. Membranes are organized as lipid bilayers in which proteins are embedded. The budding of some of these membranes, leading to the formation of the so-called lipid droplets (LDs) loaded with hydrophobic molecules, most notably triacylglycerol, is conserved in all clades. The evolution of eukaryotes is marked by the acquisition of mitochondria and simple plastids from Gram-positive bacteria by primary endosymbiosis events and the emergence of extremely complex plastids, collectively called secondary plastids, bounded by three to four membranes, following multiple and independent secondary endosymbiosis events. There is currently no consensus view of the evolution of LDs in the Tree of Life. Some features are conserved; others show a striking level of diversification. Here, we summarize the current knowledge on the architecture, dynamics, and multitude of functions of the lipid droplets in prokaryotes and in eukaryotes deriving from primary and secondary endosymbiosis events.

TMT-labeled quantitative proteomic analyses on the longissimus dorsi to identify the proteins underlying intramuscular fat content in pigs

The Laiwu pig is famous for its excessively extremely high level of intramuscular fat content (IMF), however, the exact regulatory mechanism underlying intramuscular fat deposition in skeletal muscle is still unknown. As an economically important trait in pigs, IMF is controlled by multiple genes and biological pathways. In this study, we performed an integrated transcriptome-assisted TMT-labeled quantitative proteomic analysis of the longissimus dorsi (LD) muscle in Laiwu pigs at the fastest IMF deposition stage and identified 5074 unique proteins and 52 differentially abundant proteins (DAPs) (>1.5-fold cutoff, p < .05). These DAPs were hierarchically clustered in the LD muscle over two developmental stages from 120 d to 240 d. A comparison between transcriptomic (mRNA) and proteomic data revealed two differentially expressed genes corresponding to the DAPs. Changes in the levels of the nine proteins were further analyzed using RT-qPCR and parallel reaction monitoring (PRM). The proteins identified in this study could serve as candidates for elucidating the molecular mechanism of IMF deposition in pigs. SIGNIFICANCE: The intramuscular fat content (IMF) refers to the amount of fat within muscles and plays an important role in meat quality by affecting meat quality-related traits, such as tenderness, juiciness and flavor. Using the integrated transcriptome-assisted TMT-labeled quantitative proteomic approach to characterize changes in the proteomic profile of the longissimus dorsi muscle, we identified differentially abundant proteins, such as ALDH1B1, OTX2, AnxA6 and Zfp512, that are associated with intramuscular fat deposition and fat biosynthesis in pigs. These proteins could serve as candidates for elucidating the molecular mechanism of IMF deposition in pigs.

Lipid droplet-membrane contact sites - from protein binding to function

In the general context of an increasing prevalence of obesity-associated diseases, which follows changing paradigms in food consumption and worldwide use of industry-transformed foodstuffs, much attention has been given to the consequences of excessive fattening on health. Highly related to this clinical problem, studies at the cellular and molecular level are focused on the fundamental mechanism of lipid handling in dedicated lipid droplet (LD) organelles. This Review briefly summarizes how views on LD functions have evolved from those of a specialized intracellular compartment dedicated to lipid storage to exerting a more generalized role in the stress response. We focus on the current understanding of how proteins bind to LDs and determine their function, and on the new paradigms that have emerged from the discoveries of the multiple contact sites formed by LDs. We argue that elucidating the important roles of LD tethering to other cellular organelles allows for a better understanding of LD diversity and dynamics.

Diet Induced Obesity Alters Intestinal Cytoplasmic Lipid Droplet Morphology and Proteome in the Postprandial Response to Dietary Fat

Dietary fat absorption by the small intestine is an efficient, multistep process that regulates the uptake and delivery of essential nutrients and energy. Fatty acids taken up by enterocytes, the absorptive cells of the small intestine, are resynthesized into triacylglycerol (TAG) and either secreted in chylomicrons or temporarily stored in cytoplasmic lipid droplets (CLDs). Proteins that associate with CLDs are thought to regulate the dynamics of TAG storage and mobilization. It is currently unclear what effect diet induced obesity (DIO) has on the balance between dietary fat storage and secretion. Specifically, there is limited knowledge of how DIO affects the level and diversity of proteins that associate with CLDs and regulate CLD dynamics. In the current study, we characterize CLDs from lean and DIO mice through histological and proteomic analyses. We demonstrate that DIO mice have larger intestinal CLDs compared to lean mice in response to dietary fat. Additionally, we identified 375 proteins in the CLD fraction isolated from enterocytes of lean and DIO mice. We identified a subgroup of lipid related proteins that are either increased or unique to the DIO CLD proteome. These proteins are involved in steroid synthesis, TAG synthesis, and lipolysis. This analysis expands our knowledge of the effect of DIO on the process of dietary fat absorption in the small intestine (D’Aquila, 2016).

Altered hepatic glucose homeostasis in AnxA6-KO mice fed a high-fat diet

Annexin A6 (AnxA6) controls cholesterol and membrane transport in endo- and exocytosis, and modulates triglyceride accumulation and storage. In addition, AnxA6 acts as a scaffolding protein for negative regulators of growth factor receptors and their effector pathways in many different cell types. Here we investigated the role of AnxA6 in the regulation of whole body lipid metabolism and insulin-regulated glucose homeostasis. Therefore, wildtype (WT) and AnxA6-knockout (KO) mice were fed a high-fat diet (HFD) for 17 weeks. During the course of HFD feeding, AnxA6-KO mice gained less weight compared to controls, which correlated with reduced adiposity. Systemic triglyceride and cholesterol levels of HFD-fed control and AnxA6-KO mice were comparable, with slightly elevated high density lipoprotein (HDL) and reduced triglyceride-rich lipoprotein (TRL) levels in AnxA6-KO mice. AnxA6-KO mice displayed a trend towards improved insulin sensitivity in oral glucose and insulin tolerance tests (OGTT, ITT), which correlated with increased insulin-inducible phosphorylation of protein kinase B (Akt) and ribosomal protein S6 kinase (S6) in liver extracts. However, HFD-fed AnxA6-KO mice failed to downregulate hepatic gluconeogenesis, despite similar insulin levels and insulin signaling activity, as well as expression profiles of insulin-sensitive transcription factors to controls. In addition, increased glycogen storage in livers of HFD- and chow-fed AnxA6-KO animals was observed. Together with an inability to reduce glucose production upon insulin exposure in AnxA6-depleted HuH7 hepatocytes, this implicates AnxA6 contributing to the fine-tuning of hepatic glucose metabolism with potential consequences for the systemic control of glucose in health and disease.

The utrophin-beta 2 syntrophin complex regulates adipocyte lipid droplet size independent of adipogenesis

Utrophin is a widely expressed cytoskeleton protein and is associated with lipid droplets (LDs) in adipocytes. The scaffold protein beta 2 syntrophin (SNTB2) controls signaling events by recruiting distinct membrane and cytoskeletal proteins, and binds to utrophin. Here we show that SNTB2 forms a complex with utrophin in adipocytes. SNTB2 protein is strongly diminished when utrophin is low. Of note, knock-down of utrophin or SNTB2 enhances LD growth during adipogenesis. SNTB2 reduction has no effect on basal and induced lipolysis, and insulin-stimulated phosphorylation of Akt is normal. The antilipolytic activity of insulin is enhanced in adipocytes with low SNTB2, while knock-down of utrophin has no effect. Uptake of exogenously supplied oleate and linoleate is comparable in scrambled and SNTB2 siRNA-treated cells. In the fibroblasts, diminished SNTB2 is associated with lower proliferation. CCAAT/enhancer-binding protein alpha and sterol regulatory element-binding proteins which are critical transcription factors for adipogenesis are normally expressed. Consequently, maturation of cells with SNTB2 knock-down is not grossly impaired. In fibroblasts, SNTB2 is localized to filamentous and vesicular structures which are distinct from beta actin, alpha tubulin, endoplasmic reticulum, early endosomes, lysosomes and mitochondria. Collectively, our data provide evidence that the utrophin-SNTB2 complex regulates LD size without affecting adipogenesis.

Triglyceride Metabolism in the Liver

Triglyceride molecules represent the major form of storage and transport of fatty acids within cells and in the plasma. The liver is the central organ for fatty acid metabolism. Fatty acids accrue in liver by hepatocellular uptake from the plasma and by de novo biosynthesis. Fatty acids are eliminated by oxidation within the cell or by secretion into the plasma within triglyceride-rich very low-density lipoproteins. Notwithstanding high fluxes through these pathways, under normal circumstances the liver stores only small amounts of fatty acids as triglycerides. In the setting of overnutrition and obesity, hepatic fatty acid metabolism is altered, commonly leading to the accumulation of triglycerides within hepatocytes, and to a clinical condition known as nonalcoholic fatty liver disease (NAFLD). In this review, we describe the current understanding of fatty acid and triglyceride metabolism in the liver and its regulation in health and disease, identifying potential directions for future research. Advances in understanding the molecular mechanisms underlying the hepatic fat accumulation are critical to the development of targeted therapies for NAFLD. © 2018 American Physiological Society. Compr Physiol 8:1-22, 2018.

Fat Cell and Fatty Acid Turnover in Obesity

The ratio of free fatty acid (FFA) turnover decreases significantly with the expansion of white adipose tissue. Adipose tissue and dietary saturated fatty acid levels significantly correlate with an increase in fat cell size and number. Inhibition of adipose triglyceride lipase leads to an accumulation of triglyceride, whereas inhibition of hormone-sensitive lipase leads to the accumulation of diacylglycerol. The G0/G1 switch gene 2 increases lipid content in adipocytes and promotes adipocyte hypertrophy through the restriction of triglyceride turnover. Excess triacylglycerols (TAGs), sterols and sterol esters are surrounded by the phospholipid monolayer surface and form lipid droplets. Following the release of lipid droplets from endoplasmic reticulum, cytoplasmic lipid droplets increase their volume either by local TAG synthesis or by homotypic fusion. The number and the size of lipid droplet distribution is correlated with obesity. Obesity-associated adipocyte death exhibits feature of necrosis-like programmed cell death. NOD-like receptors family pyrin domain containing 3 (NLRP3) inflammasome-dependent caspase-1 activation in hypertrophic adipocytes induces obese adipocyte death by pyroptosis. Actually adipocyte death may be a prerequisite for the transition from hypertrophic to hyperplastic obesity. Major transcriptional factors, CCAAT/enhancer-binding proteins beta and delta, play a central role in the subsequent induction of critical regulators, peroxisome-proliferator-activated receptor gamma, CCAAT/enhancer-binding protein alpha and sterol regulatory element-binding protein 1, in the transcriptional control of adipogenesis in obesity.Collectively, in this chapter the concept of adipose tissue remodeling in response to adipocyte death or adipogenesis, and the complexity of lipid droplet interactions with the other cellular organelles are reviewed. Furthermore, in addition to lipid droplet growth, the functional link between the adipocyte-specific lipid droplet-associated protein and fatty acid turn-over is also debated.

Role of hepatic Annexin A6 in fatty acid-induced lipid droplet formation

Annexin A6 (AnxA6) has been implicated in the regulation of endo-/exocytic pathways, cholesterol transport, and the formation of multifactorial signaling complexes in many different cell types. More recently, AnxA6 has also been linked to triglyceride storage in adipocytes. Here we investigated the potential role of AnxA6 in fatty acid (FA) - induced lipid droplet (LD) formation in hepatocytes. AnxA6 was associated with LD from rat liver and HuH7 hepatocytes. In oleic acid (OA) -loaded HuH7 cells, substantial amounts of AnxA6 bound to LD in a Ca²⁺-independent manner. Remarkably, stable or transient AnxA6 overexpression in HuH7 cells led to elevated LD numbers/size and neutral lipid staining under control conditions as well as after OA loading compared to controls. In contrast, overexpression of AnxA1, AnxA2 and AnxA8 did not impact on OA-induced lipid accumulation. On the other hand, incubation of AnxA6-depleted HuH7 cells or primary hepatocytes from AnxA6 KO-mice with OA led to reduced FA accumulation and LD numbers. Furthermore, morphological analysis of liver sections from A6-KO mice revealed significantly lower LD numbers compared to wildtype animals. Interestingly, pharmacological inhibition of cytoplasmic phospholipase A2α (cPLA₂α)-dependent LD formation was ineffective in AnxA6-depleted HuH7 cells. We conclude that cPLA₂α-dependent pathways contribute to the novel regulatory role of hepatic AnxA6 in LD formation.

Establishing the lipid droplet proteome: Mechanisms of lipid droplet protein targeting and degradation

Lipid droplets (LDs) are ubiquitous, endoplasmic reticulum (ER)-derived organelles that mediate the sequestration of neutral lipids (e.g. triacylglycerol and sterol esters), providing a dynamic cellular storage depot for rapid lipid mobilization in response to increased cellular demands. LDs have a unique ultrastructure, consisting of a core of neutral lipids encircled by a phospholipid monolayer that is decorated with integral and peripheral proteins. The LD proteome contains numerous lipid metabolic enzymes, regulatory scaffold proteins, proteins involved in LD clustering and fusion, and other proteins of unknown functions. The cellular role of LDs is inherently determined by the composition of its proteome and alteration of the LD protein coat provides a powerful mechanism to adapt LDs to fluctuating metabolic states. Here, we review the current understanding of the molecular mechanisms that govern LD protein targeting and degradation. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

Dgat1 and Dgat2 regulate enterocyte triacylglycerol distribution and alter proteins associated with cytoplasmic lipid droplets in response to dietary fat

Enterocytes, the absorptive cells of the small intestine, mediate efficient absorption of dietary fat (triacylglycerol, TAG). The digestive products of dietary fat are taken up by enterocytes, re-esterified into TAG, and packaged on chylomicrons (CMs) for secretion into blood or temporarily stored within cytoplasmic lipid droplets (CLDs). Altered enterocyte TAG distribution impacts susceptibility to high fat diet associated diseases, but molecular mechanisms directing TAG toward these fates are unclear. Two enzymes, acyl CoA: diacylglycerol acyltransferase 1 (Dgat1) and Dgat2, catalyze the final, committed step of TAG synthesis within enterocytes. Mice with intestine-specific overexpression of Dgat1 (Dgat1Int) or Dgat2 (Dgat2Int), or lack of Dgat1 (Dgat1-/-), were previously found to have altered intestinal TAG secretion and storage. We hypothesized that varying intestinal Dgat1 and Dgat2 levels alters TAG distribution in subcellular pools for CM synthesis as well as the morphology and proteome of CLDs. To test this we used ultrastructural and proteomic methods to investigate intracellular TAG distribution and CLD-associated proteins in enterocytes from Dgat1Int, Dgat2Int, and Dgat1-/- mice 2h after a 200μl oral olive oil gavage. We found that varying levels of intestinal Dgat1 and Dgat2 altered TAG pools involved in CM assembly and secretion, the number or size of CLDs present in enterocytes, and the enterocyte CLD proteome. Overall, these results support a model where Dgat1 and Dgat2 function coordinately to regulate the process of dietary fat absorption by preferentially synthesizing TAG for incorporation into distinct subcellular TAG pools in enterocytes.

The protein and neutral lipid composition of lipid droplets isolated from the fission yeast, Schizosaccharomyces pombe

Lipid droplets consist of a core of neutral lipids surrounded by a phospholipid monolayer with bound proteins. Much of the information on lipid droplet function comes from proteomic and lipodomic studies that identify the components of droplets isolated from organisms throughout the phylogenetic tree. Here, we add to that important inventory by reporting lipid droplet factors from the fission yeast, Schizosaccharomyces pombe. Unique to this study was the fact that cells were cultured in three different environments: 1) late log growth phase in glucose-based media, 2) stationary phase in glucosebased media, and 3) late log growth phase in media containing oleic acid. We confirmed colocalization of major factors with lipid droplets using live-cell fluorescent microscopy. We also analyzed droplets from each of the three conditions for sterol ester (SE) and triacylglycerol (TAG) content, along with their respective fatty acid compositions. We identified a previously undiscovered lipid droplet protein, Vip1p, which affects droplet size distribution. The results provide further insight into the workings of these ubiquitous organelles.

A Luciferase-fragment Complementation Assay to Detect Lipid Droplet-associated Protein-Protein Interactions

A critical challenge for all organisms is to carefully control the amount of lipids they store. An important node for this regulation is the protein coat present at the surface of lipid droplets (LDs), the intracellular organelles dedicated to lipid storage. Only limited aspects of this regulation are understood so far. For the probably best characterized case, the regulation of lipolysis in mammals, some of the major protein players have been identified, and it has been established that this process crucially depends on an orchestrated set of protein-protein interactions. Proteomic analysis has revealed that LDs are associated with dozens, if not hundreds, of different proteins, most of them poorly characterized, with even fewer data regarding which of them might physically interact. To comprehensively understand the mechanism of lipid storage regulation, it will likely be essential to define the interactome of LD-associated proteins.Previous studies of such interactions were hampered by technical limitations. Therefore, we have developed a split-luciferase based protein-protein interaction assay and test for interactions among 47 proteins from Drosophila and from mouse. We confirmed previously described interactions and identified many new ones. In 1561 complementation tests, we assayed for interactions among 487 protein pairs of which 92 (19%) resulted in a successful luciferase complementation. These results suggest that a prominent fraction of the LD-associated proteome participates in protein-protein interactions.In targeted experiments, we analyzed the two proteins Jabba and CG9186 in greater detail. Jabba mediates the sequestration of histones to LDs. We successfully applied our split luciferase complementation assay to learn more about this function as we were e.g. able to map the interaction between Jabba and histones. For CG9186, expression levels affect the positioning of LDs. Here, we reveal the ubiquitination of CG9186, and link this posttranslational modification to LD cluster induction.

High-fat diet feeding promotes stemness and precancerous changes in murine gastric mucosa mediated by leptin receptor signaling pathway

Obesity increases the risk for gastric cancers. However, the occurrence and mechanisms of precancerous atrophic gastritis induced by high-fat diet (HFD) remain unclear. Here, we show that HFD-associated lipotoxicity induces precancerous lesions that are accompanied by the disruption of organelle homeostasis, tissue integrity, and deregulated expression of stemness genes in the gastric epithelium mediated by leptin receptor (ObR) signaling. Following HFD feeding, ectopic fat accumulated and expression of LAMP2A in lysosome and COX IV in mitochondria increased in the gastric mucosa. HFD feeding also led to enhanced expression of activated-Notch1 and stem cell markers Lgr5, CD44, and EpCAM. In addition, HFD-fed mice showed intracellular β-catenin accumulation in the gastric mucosa with increased expression of its target genes, Nanog, Oct4, and c-Myc. These observations were abrogated in the leptin-deficient ob/ob mice and ObR-mutated db/db mice, indicating that these HFD-induced changes were responsible for effects downstream of the ObR. Consistent with this, the expression of the Class IA and III PI3Ks was increased following ObR activation in the gastric mucosa of HFD-fed mice. Together, these results suggest that HFD-induced lipotoxicity and deregulated organelle biosynthesis confer cancer stem cell-like properties to the gastric mucosa via signaling pathway mediated by leptin, PI3K and β-catenin.

Recent discoveries on absorption of dietary fat: Presence, synthesis, and metabolism of cytoplasmic lipid droplets within enterocytes

Dietary fat provides essential nutrients, contributes to energy balance, and regulates blood lipid concentrations. These functions are important to health, but can also become dysregulated and contribute to diseases such as obesity, diabetes, cardiovascular disease, and cancer. Within enterocytes, the digestive products of dietary fat are re-synthesized into triacylglycerol, which is either secreted on chylomicrons or stored within cytoplasmic lipid droplets (CLDs). CLDs were originally thought to be inert stores of neutral lipids, but are now recognized as dynamic organelles that function in multiple cellular processes in addition to lipid metabolism. This review will highlight recent discoveries related to dietary fat absorption with an emphasis on the presence, synthesis, and metabolism of CLDs within this process.

Monoacylglycerol Lipases Act as Evolutionarily Conserved Regulators of Non-oxidative Ethanol Metabolism

Fatty acid ethyl esters (FAEEs) are non-oxidative metabolites of ethanol that accumulate in human tissues upon ethanol intake. Although FAEEs are considered as toxic metabolites causing cellular dysfunction and tissue damage, the enzymology of FAEE metabolism remains poorly understood. In this study, we used a biochemical screen in Saccharomyces cerevisiae to identify and characterize putative hydrolases involved in FAEE catabolism. We found that Yju3p, the functional orthologue of mammalian monoacylglycerol lipase (MGL), contributes >90% of cellular FAEE hydrolase activity, and its loss leads to the accumulation of FAEE. Heterologous expression of mammalian MGL in yju3Δ mutants restored cellular FAEE hydrolase activity and FAEE catabolism. Moreover, overexpression or pharmacological inhibition of MGL in mouse AML-12 hepatocytes decreased or increased FAEE levels, respectively. FAEEs were transiently incorporated into lipid droplets (LDs) and both Yju3p and MGL co-localized with these organelles. We conclude that the storage of FAEE in inert LDs and their mobilization by LD-resident FAEE hydrolases facilitate a controlled metabolism of these potentially toxic lipid metabolites.

Lipid droplets, lipophagy, and beyond

Lipids are essential components for life. Their various structural and physical properties influence diverse cellular processes and, thereby, human health. Lipids are not genetically encoded but are synthesized and modified by complex metabolic pathways, supplying energy, membranes, signaling molecules, and hormones to affect growth, physiology, and response to environmental insults. Lipid homeostasis is crucial, such that excess fatty acids (FAs) can be harmful to cells. To prevent such lipotoxicity, cells convert excess FAs into neutral lipids for storage in organelles called lipid droplets (LDs). These organelles do not simply manage lipid storage and metabolism but also are involved in protein quality management, pathogenesis, immune responses, and, potentially, neurodegeneration. In recent years, a major trend in LD biology has centered around the physiology of lipid mobilization via lipophagy of fat stored within LDs. This review summarizes key findings in LD biology and lipophagy, offering novel insights into this rapidly growing field. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

Biology and pathobiology of lipid droplets and their potential role in the protection of the organ of Corti

The current review article seeks to extend our understanding on the role of lipid droplets within the organ of Corti. In addition to presenting an overview of the current information about the origin, structure and function of lipid droplets we draw inferences from the collective body of knowledge about this cellular organelle to build a conceptual framework to better understanding their role in auditory function. This conceptual model considers that lipid droplets play a significant role in the synthesis, storage, and release of lipids and proteins for energetic use and/or modulating cell signaling pathways. We describe the role and mechanism by which LD play a role in human diseases, and we also review emerging data from our laboratory revealing the potential role of lipid droplets from Hensen cells in the auditory organ. We suggest that lipid droplets might help to develop rapidly and efficiently the resolution phase of inflammatory responses in the mammalian cochlea, preventing inflammatory damage of the delicate inner ear structures and, consequently, sensorineural hearing loss.

Characteristics and functions of lipid droplets and associated proteins in enterocytes

Cytosolic lipid droplets (LDs) are observed in enterocytes of jejunum during lipid absorption. One important function of the intestine is to secrete chylomicrons, which provide dietary lipids throughout the body, from digested lipids in meals. The current hypothesis is that cytosolic LDs in enterocytes constitute a transient pool of stored lipids that provides lipids during interprandial period while lowering chylomicron production during the post-prandial phase. This smoothens the magnitude of peaks of hypertriglyceridemia. Here, we review the composition and functions of lipids and associated proteins of enterocyte LDs, the known physiological functions of LDs as well as the role of LDs in pathological processes in the context of the intestine.

Turnover of the actomyosin complex in zebrafish embryos directs geometric remodelling and the recruitment of lipid droplets

Lipid droplets (LDs), reservoirs of cholesterols and fats, are organelles that hydrolyse lipids in the cell. In zebrafish embryos, the actomyosin complex and filamentous microtubules control the periodic regulation of the LD geometry. Contrary to the existing hypothesis that LD transport involves the kinesin-microtubule system, we find that their recruitment to the blastodisc depends on the actomyosin turnover and is independent of the microtubules. For the first time we report the existence of two distinct states of LDs, an inactive and an active state, that occur periodically, coupled weakly to the cleavage cycles. LDs are bigger, more circular and more stable in the inactive state in which the geometry of the LDs is maintained by actomyosin as well as microtubules. The active state has smaller and irregularly shaped LDs that show shape fluctuations that are linked to actin depolymerization. Because most functions of LDs employ surface interactions, our findings on the LD geometry and its regulation bring new insights to the mechanisms associated with specific functions of LDs, such as their storage capacity for fats or proteins, lipolysis etc.

Microsomal Triglyceride Transfer Protein (MTP) Associates with Cytosolic Lipid Droplets in 3T3-L1 Adipocytes

Lipid droplets are intracellular energy storage organelles composed of a hydrophobic core of neutral lipid, surrounded by a monolayer of phospholipid and a diverse array of proteins. The function of the vast majority of these proteins with regard to the formation and/or turnover of lipid droplets is unknown. Our laboratory was the first to report that microsomal triglyceride transfer protein (MTP), a lipid transfer protein essential for the assembly of triglyceride-rich lipoproteins, was expressed in adipose tissue of humans and mice. In addition, our studies suggested that MTP was associated with lipid droplets in both brown and white fat. Our observations led us to hypothesize that MTP plays a key role in lipid droplet formation and/or turnover. The objective of these studies was to gain insight into the function of MTP in adipocytes. Using molecular, biochemical, and morphologic approaches we have shown: 1) MTP protein levels increase nearly five-fold as 3T3-L1 cells differentiate into adipocytes. 2) As 3T3-L1 cells undergo differentiation, MTP moves from the juxtanuclear region of the cell to the surface of lipid droplets. MTP and perilipin 2, a major lipid droplet surface protein, are found on the same droplets; however, MTP does not co-localize with perilipin 2. 3) Inhibition of MTP activity has no effect on the movement of triglyceride out of the cell either as a lipid complex or via lipolysis. 4) MTP is found associated with lipid droplets within hepatocytes from human fatty livers, suggesting that association of MTP with lipid droplets is not restricted to adipocytes. In summary, our data demonstrate that MTP is a lipid droplet-associated protein. Its location on the surface of the droplet in adipocytes and hepatocytes, coupled with its known function as a lipid transfer protein and its increased expression during adipocyte differentiation suggest a role in lipid droplet biology.

Elongation Factor 1A-1 Is a Mediator of Hepatocyte Lipotoxicity Partly through Its Canonical Function in Protein Synthesis

Elongation factor 1A-1 (eEF1A-1) has non-canonical functions in regulation of the actin cytoskeleton and apoptosis. It was previously identified through a promoter-trap screen as a mediator of fatty acid-induced cell death (lipotoxicity), and was found to participate in this process downstream of ER stress. Since ER stress is implicated in the pathogenesis of nonalcoholic fatty liver disease (NAFLD), we investigated the mechanism of action of eEF1A-1 in hepatocyte lipotoxicity. HepG2 cells were exposed to excess fatty acids, followed by assessments of ER stress, subcellular localization of eEF1A-1, and cell death. A specific inhibitor of eEF1A-1 elongation activity, didemnin B, was used to determine whether its function in protein synthesis is involved in lipotoxicity. Within 6 h, eEF1A-1 protein was modestly induced by high palmitate, and partially re-localized from its predominant location at the ER to polymerized actin at the cell periphery. This early induction and subcellular redistribution of eEF1A-1 coincided with the onset of ER stress, and was later followed by cell death. Didemnin B did not prevent the initiation of ER stress by high palmitate, as indicated by eIF2α phosphorylation. However, consistent with sustained inhibition of eEF1A-1-dependent elongation activity, didemnin B prevented the recovery of protein synthesis and increase in GRP78 protein that are normally associated with later phases of the response to ongoing ER stress. This resulted in decreased palmitate-induced cell death. Our data implicate eEF1A-1, and its function in protein synthesis, in hepatocyte lipotoxicity.

Characterization of the proteome of cytoplasmic lipid droplets in mouse enterocytes after a dietary fat challenge

Dietary fat absorption by the small intestine is a multistep process that regulates the uptake and delivery of essential nutrients and energy. One step of this process is the temporary storage of dietary fat in cytoplasmic lipid droplets (CLDs). The storage and mobilization of dietary fat is thought to be regulated by proteins that associate with the CLD; however, mechanistic details of this process are currently unknown. In this study we analyzed the proteome of CLDs isolated from enterocytes harvested from the small intestine of mice following a dietary fat challenge. In this analysis we identified 181 proteins associated with the CLD fraction, of which 37 are associated with known lipid related metabolic pathways. We confirmed the localization of several of these proteins on or around the CLD through confocal and electron microscopy, including perilipin 3, apolipoprotein A-IV, and acyl-CoA synthetase long-chain family member 5. The identification of the enterocyte CLD proteome provides new insight into potential regulators of CLD metabolism and the process of dietary fat absorption.

Chlamydia trachomatis Infection Leads to Defined Alterations to the Lipid Droplet Proteome in Epithelial Cells

The obligate intracellular bacterium Chlamydia trachomatis is a major human pathogen and a main cause of genital and ocular diseases. During its intracellular cycle, C. trachomatis replicates inside a membrane-bound vacuole termed an “inclusion”. Acquisition of lipids (and other nutrients) from the host cell is a critical step in chlamydial replication. Lipid droplets (LD) are ubiquitous, ER-derived neutral lipid-rich storage organelles surrounded by a phospholipids monolayer and associated proteins. Previous studies have shown that LDs accumulate at the periphery of, and eventually translocate into, the chlamydial inclusion. These observations point out to Chlamydia-mediated manipulation of LDs in infected cells, which may impact the function and thereby the protein composition of these organelles. By means of a label-free quantitative mass spectrometry approach we found that the LD proteome is modified in the context of C. trachomatis infection. We determined that LDs isolated from C. trachomatis-infected cells were enriched in proteins related to lipid metabolism, biosynthesis and LD-specific functions. Interestingly, consistent with the observation that LDs intimately associate with the inclusion, a subset of inclusion membrane proteins co-purified with LD protein extracts. Finally, genetic ablation of LDs negatively affected generation of C. trachomatis infectious progeny, consistent with a role for LD biogenesis in optimal chlamydial growth.

Spastin binds to lipid droplets and affects lipid metabolism

Mutations in SPAST, encoding spastin, are the most common cause of autosomal dominant hereditary spastic paraplegia (HSP). HSP is characterized by weakness and spasticity of the lower limbs, owing to progressive retrograde degeneration of the long corticospinal axons. Spastin is a conserved microtubule (MT)-severing protein, involved in processes requiring rearrangement of the cytoskeleton in concert to membrane remodeling, such as neurite branching, axonal growth, midbody abscission, and endosome tubulation. Two isoforms of spastin are synthesized from alternative initiation codons (M1 and M87). We now show that spastin-M1 can sort from the endoplasmic reticulum (ER) to pre- and mature lipid droplets (LDs). A hydrophobic motif comprised of amino acids 57 through 86 of spastin was sufficient to direct a reporter protein to LDs, while mutation of arginine 65 to glycine abolished LD targeting. Increased levels of spastin-M1 expression reduced the number but increased the size of LDs. Expression of a mutant unable to bind and sever MTs caused clustering of LDs. Consistent with these findings, ubiquitous overexpression of Dspastin in Drosophila led to bigger and less numerous LDs in the fat bodies and increased triacylglycerol levels. In contrast, Dspastin overexpression increased LD number when expressed specifically in skeletal muscles or nerves. Downregulation of Dspastin and expression of a dominant-negative variant decreased LD number in Drosophila nerves, skeletal muscle and fat bodies, and reduced triacylglycerol levels in the larvae. Moreover, we found reduced amount of fat stores in intestinal cells of worms in which the spas-1 homologue was either depleted by RNA interference or deleted. Taken together, our data uncovers an evolutionarily conserved role of spastin as a positive regulator of LD metabolism and open up the possibility that dysfunction of LDs in axons may contribute to the pathogenesis of HSP.

Hereditary spastic paraplegia (HSP) is a genetically heterogeneous neurological disease characterized by weakness and spasticity of the lower limbs, caused by progressive retrograde degeneration of the corticospinal axons, the longest in the central nervous system. The most commonly mutated gene in autosomal dominant forms of HSP, SPAST, encodes for spastin, a microtubule-severing protein. Spastin has been implicated in several processes involving remodeling of membrane structures. We now show that the longest spastin form, spastin-M1, harbors a lipid droplet targeting sequence, which allows targeting of the protein to the surface of lipid droplets, the organelles where cells store neutral lipids. Furthermore, we demonstrate that depletion of the homologous spastin proteins in both flies and worms affects lipid droplet number and triacylglycerol content. Our study adds to recent discoveries that implicate other HSP proteins in lipid droplet and lipid metabolism, and strongly suggests that lipid droplet dysfunction in neurons should be investigated to understand pathogenesis of HSP.

Characterization of the sebocyte lipid droplet proteome reveals novel potential regulators of sebaceous lipogenesis

Lipid metabolism depends on lipid droplets (LD), cytoplasmic structures surrounded by a protein-rich phospholipid monolayer. Although lipid synthesis is the hallmark of sebaceous gland cell differentiation, the LD-associated proteins of sebocytes have not been evaluated systematically. The LD fraction of SZ95 sebocytes was collected by density gradient centrifugation and associated proteins were analyzed by nanoliquid chromatography/tandem mass spectrometry. 54 proteins were significantly enriched in LD fractions, and 6 of them have not been detected previously in LDs. LD fractions contained high levels of typical LD-associated proteins as PLIN2/PLIN3, and most proteins belonged to functional categories characteristic for LD-associated proteins, indicating a reliable dataset. After confirming expression of transcripts encoding the six previously unidentified proteins by qRT-PCR in SZ95 sebocytes and in another sebocyte line (SebE6E7), we focused on two of these proteins, ALDH1A3 and EPHX4. While EPHX4 was localized almost exclusively on the surface of LDs, ALDH1A3 showed a more widespread localization that included additional cytoplasmic structures. siRNA-mediated downregulation revealed that depletion of EPHX4 increases LD size and sebaceous lipogenesis. Further studies on the roles of these proteins in sebocyte physiology and sebaceous lipogenesis may indicate novel strategies for the therapy of sebaceous gland-associated diseases such as acne.