The phospholipid monolayer associated with perilipin-enriched lipid droplets is a highly organized rigid membrane structure

Viscosity-Sensitive Solvatochromic Fluorescent Probes for Lipid Droplets Staining

Lipid droplets (LDs) are simple intracellular storage sites for neutral lipids and exhibit important impact on many physiological processes. For example, the changes in the polar microenvironment inside LDs could affect physiological processes, such as lipid metabolism and storage, protein degradation, signal transduction, and enzyme catalysis. Herein, a new fluorescent chemo-sensor (Couoxo-LD) was formulated by our molecular design strategy. The probe could be applied to effectively label intracellular lipid droplets. Intriguingly, Couoxo-LD demonstrated positive sensitivity to both polarity and viscosity, which might be attributed to its D-π-A structure and the twisted rotational behavior of the carbon-carbon double bond (TICT). Additionally, Couoxo-LD was successfully implemented in cellular imaging due to its excellent selectivity, pH stability, and low biotoxicity. In HeLa cells, the co-localization curve between Couoxo-LD and commercial lipid droplet dyes overlapped at 0.93. The results indicated that the probe could selectively sense LDs in HeLa cells. Meanwhile, Couoxo-LD can be applied for in vivo imaging of zebrafish.

Monounsaturated fatty acids: key regulators of cell viability and intracellular signalling in cancer

Cancer cells feature increased macromolecular biosynthesis to support the formation of new organelles and membranes for cell division. In particular, lipids are key macromolecules that comprise cellular membrane components, substrates for energy generation and mediators of inter- and intracellular signalling. The emergence of more sensitive and accurate technology for profiling the "lipidome" of cancer cells has led to unprecedented leaps in understanding the complexity of cancer metabolism, but also highlighted promising therapeutic vulnerabilities. Notably, fatty acids, as lipid building blocks, are critical players in all stages of cancer development and progression and the importance of fatty acid desaturation and its impact on cancer cell biology has been well established. Recent years have seen the reports of new mechanistic insights into the role of monounsaturated fatty acids (MUFAs) in cancer, as regulators of cell death and lipid-related cellular signalling. This commentary aims to highlight these diverse roles of MUFAs in cancer cells which may yield new directions for therapeutic interventions involving these important fatty acids.

Perilipins at a glance

Lipid droplets (LDs) are ubiquitous organelles that store and supply lipids for energy metabolism, membrane synthesis and production of lipid-derived signaling molecules. While compositional differences in the phospholipid monolayer or neutral lipid core of LDs impact their metabolism and function, the proteome of LDs has emerged as a major influencer in all aspects of LD biology. The perilipins (PLINs) are the most studied and abundant proteins residing on the LD surface. This Cell Science at a Glance and the accompanying poster summarize our current knowledge of the common and unique features of the mammalian PLIN family of proteins, the mechanisms through which they affect cell metabolism and signaling, and their links to disease.

The C-Terminus of Perilipin 3 Shows Distinct Lipid Binding at Phospholipid-Oil-Aqueous Interfaces

Lipid droplets (LDs) are ubiquitously expressed organelles; the only intracellular organelles that contain a lipid monolayer rather than a bilayer. Proteins localize and bind to this monolayer as they do to intracellular lipid bilayers. The mechanism by which cytosolic LD binding proteins recognize, and bind, to this lipid interface remains poorly understood. Amphipathic α-helix bundles form a common motif that is shared between cytosolic LD binding proteins (e.g., perilipins 2, 3, and 5) and apolipoproteins, such as apoE and apoLp-III, found on lipoprotein particles. Here, we use pendant drop tensiometry to expand our previous work on the C-terminal α-helix bundle of perilipin 3 and the full-length protein. We measure the recruitment and insertion of perilipin 3 at mixed lipid monolayers at an aqueous-phospholipid-oil interface. We find that, compared to its C-terminus alone, the full-length perilipin 3 has a higher affinity for both a neat oil/aqueous interface and a phosphatidylcholine (PC) coated oil/aqueous interface. Both the full-length protein and the C-terminus show significantly more insertion into a fully unsaturated PC monolayer, contrary to our previous results at the air-aqueous interface. Additionally, the C-terminus shows a preference for lipid monolayers containing phosphatidylethanolamine (PE), whereas the full-length protein does not. These results strongly support a model whereby both the N-terminal 11-mer repeat region and C-terminal amphipathic α-helix bundle domains of perilipin 3 have distinct lipid binding, and potentially biological roles.

Energy and Dynamics of Caveolae Trafficking

Caveolae are 70–100 nm diameter plasma membrane invaginations found in abundance in adipocytes, endothelial cells, myocytes, and fibroblasts. Their bulb-shaped membrane domain is characterized and formed by specific lipid binding proteins including Caveolins, Cavins, Pacsin2, and EHD2. Likewise, an enrichment of cholesterol and other lipids makes caveolae a distinct membrane environment that supports proteins involved in cell-type specific signaling pathways. Their ability to detach from the plasma membrane and move through the cytosol has been shown to be important for lipid trafficking and metabolism. Here, we review recent concepts in caveolae trafficking and dynamics. Second, we discuss how ATP and GTP-regulated proteins including dynamin and EHD2 control caveolae behavior. Throughout, we summarize the potential physiological and cell biological roles of caveolae internalization and trafficking and highlight open questions in the field and future directions for study.

Sizing lipid droplets from adult and geriatric mouse liver tissue via nanoparticle tracking analysis

The significance of lipid droplets in lipid metabolism, cell signaling, and regulating longevity is increasingly recognized, yet the lipid droplet's unique properties and architecture make it difficult to size and study using conventional methods. To begin to address this issue, we demonstrate the capabilities of nanoparticle tracking analysis (NTA) for sizing of lipid droplets. NTA was found to be adequate to assess lipid droplet stability over time, indicating that lipid droplet preparations are stable for up to 24 h. NTA had the ability to compare the size distributions of lipid droplets from adult and geriatric mouse liver tissue, suggesting an age-related decrease in lipid droplet size. This is the first report on the use of NTA to size intracellular organelles. Graphical Abstract Light scattering reveals the temporal positions of individual lipid droplets, which are recorded with a camera. The two-dimensional diffusion constant of each lipid droplet is extracted from the data set, which is then used to calculate a hydrodynamic radius using the Stokes-Einstein equation.

ATGL Promotes Autophagy/Lipophagy via SIRT1 to Control Hepatic Lipid Droplet Catabolism

Hepatic lipid droplet (LD) catabolism is thought to occur via cytosolic lipases such as adipose triglyceride lipase (ATGL) or through autophagy of LDs, a process known as lipophagy. We tested the potential interplay between these metabolic processes and its effects on hepatic lipid metabolism. We show that hepatic ATGL is both necessary and sufficient to induce both autophagy and lipophagy. Moreover, lipophagy is required for ATGL to promote LD catabolism and the subsequent oxidation of hydrolyzed fatty acids (FAs). Following previous work showing that ATGL promotes sirtuin 1 (SIRT1) activity, studies in liver-specific SIRT1^-/- mice and in primary hepatocytes reveal that SIRT1 is required for ATGL-mediated induction of autophagy and lipophagy. Taken together, these studies show that ATGL-mediated signaling via SIRT1 promotes autophagy/lipophagy as a primary means to control hepatic LD catabolism and FA oxidation.

Interaction of a model apolipoprotein, apoLp-III, with an oil-phospholipid interface

Lipid droplets are "small" organelles that play an important role in de novo synthesis of new membrane, and steroid hormones, as well as in energy storage. The way proteins interact specifically with the oil-(phospho-)lipid monolayer interface of lipid droplets is a relatively unexplored but crucial question. Here, we use our home built liquid droplet tensiometer to mimic intracellular lipid droplets and study protein-lipid interactions at this interface. As model neutral lipid binding protein, we use apoLp-III, an amphipathic α-helix bundle protein. This domain is also found in proteins from the perilipin family and in apoE. Protein binding to the monolayer is studied by the decrease in the oil/water surface tension. Previous work used POPC (one of the major lipids found on lipid droplets) to form the phospholipid monolayer on the triolein surface. Here we expand this work by incorporating other lipids with different physico-chemical properties to study the effect of charge and lipid head-group size. This study sheds light on the affinity of this important protein domain to interact with lipids.

The Perilipins: Major Cytosolic Lipid Droplet-Associated Proteins and Their Roles in Cellular Lipid Storage, Mobilization, and Systemic Homeostasis

The discovery by Dr. Constantine Londos of perilipin 1, the major scaffold protein at the surface of cytosolic lipid droplets in adipocytes, marked a fundamental conceptual change in the understanding of lipolytic regulation. Focus then shifted from the enzymatic activation of lipases to substrate accessibility, mediated by perilipin-dependent protein sequestration and recruitment. Consequently, the lipid droplet became recognized as a unique, metabolically active cellular organelle and its surface as the active site for novel protein-protein interactions. A new area of investigation emerged, centered on lipid droplets' biology and their role in energy homeostasis. The perilipin family is of ancient origin and has expanded to include five mammalian genes and a growing list of evolutionarily conserved members. Universally, the perilipins modulate cellular lipid storage. This review provides a summary that connects the perilipins to both cellular and whole-body homeostasis.

Neu1 sialidase interacts with perilipin 1 on lipid droplets and inhibits lipolysis in 3T3-L1 adipocytes

Fatty acids are stored within adipocytes in lipid droplets (LDs) as triacylglycerol (TG), which is converted to free fatty acid (FFA) and glycerol via lipolysis. Increased plasma FFA levels in obesity are associated with several clinical conditions. We previously found that Neu1 activity is aberrant in the epididymal fat and liver of obese and diabetic mice. Here, we examined involvement of Neu1 in lipolysis in 3T3-L1 adipocytes. Small interfering RNA against Neu1 was introduced into adipocytes, and glycerol concentrations were measured in the culture medium. We then assessed the effects of Neu1 knockdown on lipolytic protein expression and phosphorylation, as well as interactions between perilipin 1 (Plin1) and hormone-sensitive lipase (HSL) after isoproterenol (IS) stimulation. Interactions between Neu1 and Plin1 were analyzed by immunoprecipitation and immunofluorescent imaging using adipocytes transfected with pCMV6-mNeu1-myc-DYKDDDDK (mNeu1DDK). Neu1 knockdown increased glycerol concentrations in culture media and Plin1 phosphorylation in whole lysates of IS-stimulated cells. Neu1 knockdown increased interaction between Plin1 and HSL after IS stimulation whereas that between Neu1 and Plin1 on LD observed under basal conditions was lost. These results suggest that Neu1 inhibits lipolysis induced by β-adrenergic stimulation in adipocytes via interactions with Plin1 on LD.

Clues on the function of Manduca sexta perilipin 2 inferred from developmental and nutrition-dependent changes in its expression

Cellular triglycerides (TG) are stored in cytosolic lipid droplets (LDs). Perilipins (PLIN) are a group of LD-proteins that play important roles in the assembly and transport of LDs and in TG metabolism. Two members of the PLIN family are found in insects (PLIN1 & 2 or Lsd1 & 2). We have cloned and expressed Manduca sexta PLIN2 (MsPLIN2), and studied developmental and nutritional changes in the expression of PLIN2. Nutritional changes induced fast alterations in PLIN2 mRNA and protein levels in fat body and midgut of the feeding larvae. The relationship observed between PLIN2 expression and TG synthesis in both larval fat body and midgut suggests that PLIN2 is needed when tissues are accumulating TG. However, when the fat body was storing TG at maximal capacity, MsPLIN2 levels declined. This unexpected finding suggests the occurrence of alternative mechanism/s to shield TG from the action of lipases in M. sexta LDs. In addition, it implies that the cellular level of lipid storage could be modulating MsPLIN2 expression and/or degradation. The study also confirmed that MsPLIN2 was most abundant in the adult fat body, which is characterized by a high rate of TG hydrolysis and lipid mobilization. Whether MsPLIN2 is directly involved in lipolysis and/or the secretion of lipids in the fat body of adult of M. sexta is unknown at this time. Nonetheless, the coexistence of high PLIN2 and lipolysis levels suggests a complex role for MsPLIN2. Altogether, we found that MsPLIN2 is needed when the synthesis of glycerides, DG and TG, is active even if the insect is accumulating or consuming TG.

ATP Binding Cassette Transporter ABCA7 Regulates NKT Cell Development and Function by Controlling CD1d Expression and Lipid Raft Content

ABCA7 is an ABC transporter expressed on the plasma membrane, and actively exports phospholipid complexes from the cytoplasmic to the exocytoplasmic leaflet of membranes. Invariant NKT (iNKT) cells are a subpopulation of T lymphocytes that recognize glycolipid antigens in the context of CD1d-mediated antigen presentation. In this study, we demonstrate that ABCA7 regulates the development of NKT cells in a cell-extrinsic manner. We found that in Abca7^−/− mice there is reduced expression of CD1d accompanied by an alteration in lipid raft content on the plasma membrane of thymocytes and antigen presenting cells. Together, these alterations caused by absence of ABCA7 negatively affect NKT cell development and function.

Insertion of perilipin 3 into a glycero(phospho)lipid monolayer depends on lipid headgroup and acyl chain species

Lipid droplets (LDs) are organelles that contribute to various cellular functions that are vital for life. Aside from acting as a neutral lipid storage depot, they are also involved in building new membranes, synthesis of steroid hormones, and cell signaling. Many aspects of LD structure and function are not yet well-understood. Here we investigate the interaction of perilipin 3, a member of the perilipin family of LD binding proteins, and three N-terminal truncation mutants with lipid monolayers. The interaction is studied as a function of surface pressure for a series of systematically chosen lipids. We find that the C terminus of perilipin 3 has different insertion behavior from that of the longer truncation mutants and the full-length protein. Inclusion of N-terminal sequences with the C terminus decreases the ability of the protein construct to insert in lipid monolayers. Coupling of anionic lipids to negative spontaneous curvature facilitates protein interaction and insertion. The C terminus shows strong preference for lipids with more saturated fatty acids. This work sheds light on the LD binding properties and function of the different domains of perilipin 3.

Synaptogyrin-2 Promotes Replication of a Novel Tick-borne Bunyavirus through Interacting with Viral Nonstructural Protein NSs

Synaptogyrin-2 is a non-neuronal member of the synaptogyrin family involved in synaptic vesicle biogenesis and trafficking. Little is known about the function of synaptogyrin-2. Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease characterized by high fever, thrombocytopenia, and leukocytopenia with high mortality, caused by a novel tick-borne phlebovirus in the family Bunyaviridae. Our previous studies have shown that the viral nonstructural protein NSs forms inclusion bodies (IBs) that are involved in viral immune evasion, as well as viral RNA replication. In this study, we sought to elucidate the mechanism by which NSs formed the IBs, a lipid droplet-based structure confirmed by NSs co-localization with perilipin A and adipose differentiation-related protein (ADRP). Through a high throughput screening, we identified synaptogyrin-2 to be highly up-regulated in response to SFTS bunyavirus (SFTSV) infection and to be a promoter of viral replication. We demonstrated that synaptogyrin-2 interacted with NSs and was translocated into the IBs, which were reconstructed from lipid droplets into large structures in infection. Viral RNA replication decreased, and infectious virus titers were lowered significantly when synaptogyrin-2 was silenced in specific shRNA-expressing cells, which correlated with the reduced number of the large IBs restructured from regular lipid droplets. We hypothesize that synaptogyrin-2 is essential to promoting the formation of the IBs to become virus factories for viral RNA replication through its interaction with NSs. These findings unveil the function of synaptogyrin-2 as an enhancer in viral infection.

Genetics of Lipid-Storage Management in Caenorhabditis elegans Embryos

Lipids play a pivotal role in embryogenesis as structural components of cellular membranes, as a source of energy, and as signaling molecules. On the basis of a collection of temperature-sensitive embryonic lethal mutants, a systematic database search, and a subsequent microscopic analysis of >300 interference RNA (RNAi)-treated/mutant worms, we identified a couple of evolutionary conserved genes associated with lipid storage in Caenorhabditis elegans embryos. The genes include cpl-1 (cathepsin L-like cysteine protease), ccz-1 (guanine nucleotide exchange factor subunit), and asm-3 (acid sphingomyelinase), which is closely related to the human Niemann-Pick disease-causing gene SMPD1. The respective mutant embryos accumulate enlarged droplets of neutral lipids (cpl-1) and yolk-containing lipid droplets (ccz-1) or have larger genuine lipid droplets (asm-3). The asm-3 mutant embryos additionally showed an enhanced resistance against C band ultraviolet (UV-C) light. Herein we propose that cpl-1, ccz-1, and asm-3 are genes required for the processing of lipid-containing droplets in C. elegans embryos. Owing to the high levels of conservation, the identified genes are also useful in studies of embryonic lipid storage in other organisms.

Fatty acid signaling: the new function of intracellular lipases

Until recently, intracellular triacylglycerols (TAG) stored in the form of cytoplasmic lipid droplets have been considered to be only passive “energy conserves”. Nevertheless, degradation of TAG gives rise to a pleiotropic spectrum of bioactive intermediates, which may function as potent co-factors of transcription factors or enzymes and contribute to the regulation of numerous cellular processes. From this point of view, the process of lipolysis not only provides energy-rich equivalents but also acquires a new regulatory function. In this review, we will concentrate on the role that fatty acids liberated from intracellular TAG stores play as signaling molecules. The first part provides an overview of the transcription factors, which are regulated by fatty acids derived from intracellular stores. The second part is devoted to the role of fatty acid signaling in different organs/tissues. The specific contribution of free fatty acids released by particular lipases, hormone-sensitive lipase, adipose triacylglycerol lipase and lysosomal lipase will also be discussed.

Comparative analysis of lipotoxicity induced by endocrine, pharmacological, and innate immune stimuli in rat basophilic leukemia cells

Cellular lipotoxicity manifests as the steatotic accumulation of lipid droplets or lipid bodies, and/or induction of phospholipidosis. Lipotoxicity can be induced by hyperinsulinemia/nutrient overload, cationic amphiphilic drugs (CAD), and innate immunological stimuli, all of which are stimuli relevant to mast cell physiology. Hyper-accumulation of mast cell lipid bodies in response to hyperinsulinemia has been documented, but lipotoxicity in response to CAD or innate immunologic stimuli has not been analysed comparatively. Moreover, gaps in our understanding of this steatosis remain, specifically as to whether hyperinsulinemia-driven steatosis in these cells attains lipotoxic levels or is accompanied by phospholipidosis. To compare endocrine, pharmacological, and innate immunological stimuli for their ability to induce steatosis and phospholipidosis in a rat basophilic leukemia mast cell model (RBL2H3), differential fluorescence microscopy staining and quantitation of phospholipidosis and steatosis in the RBL2H3 cell line was examined. The three classes of stimuli differentially induced phospholipidosis and steatosis. PPARγ up-regulation was not uniformly associated with the expansion of the lipid body population. Fluorescence imaging of lipid-enriched structures generated in response to lipotoxic cationic amphiphilic drugs, chronic insulin exposure, and TLR2/4 ligands revealed differential staining patterns when visualized using lipophilic dyes. It is concluded that lipotoxicity-inducing pathways in this model mast cell system are diverse, and include steatotic responses to an endocrine stimulus, as well as phospholipidosis responses to cationic lipophilic drugs not previously described in this cell type.

Metabolic syndrome in pediatrics

The ever growing prevalence of childhood obesity is being accompanied by an increase in the pediatric population of diseases once believed to be exclusive of the adulthood such as the metabolic syndrome (MS). The MS has been defined as the link between insulin resistance, hypertension, dyslipidemia, impaired glucose tolerance, and other metabolic abnormalities associated with an increased risk of atherosclerotic cardiovascular diseases in adults. In this review, we will discuss the peculiar aspects of the pediatric MS and the role of novel molecules and biomarkers in its pathogenesis.

GRAF1a is a brain-specific protein that promotes lipid droplet clustering and growth, and is enriched at lipid droplet junctions

Lipid droplets are found in all cell types. Normally present at low levels in the brain, they accumulate in tumours and are associated with neurodegenerative diseases. However, little is known about the mechanisms controlling their homeostasis in the brain. We found that GRAF1a, the longest GRAF1 isoform (GRAF1 is also known as ARHGAP26), was enriched in the brains of neonates. Endogenous GRAF1a was found on lipid droplets in oleic-acid-fed primary glial cells. Exclusive localization required a GRAF1a-specific hydrophobic segment and two membrane-binding regions, a BAR and a PH domain. Overexpression of GRAF1a promoted lipid droplet clustering, inhibited droplet mobility and severely perturbed lipolysis following the chase of cells overloaded with fatty acids. Under these conditions, GRAF1a concentrated at the interface between lipid droplets. Although GRAF1-knockout mice did not show any gross abnormal phenotype, the total lipid droplet volume that accumulated in GRAF1(-/-) primary glia upon incubation with fatty acids was reduced compared to GRAF1(+/+) cells. These results provide additional insights into the mechanisms contributing to lipid droplet growth in non-adipocyte cells, and suggest that proteins with membrane sculpting BAR domains play a role in droplet homeostasis.

Surface features of the lipid droplet mediate perilipin 2 localization

All eukaryotic organisms store excess lipid in intracellular lipid droplets. These dynamic structures are associated with and regulated by numerous proteins. Perilipin 2, an abundant protein on most lipid droplets, promotes neutral lipid accumulation in lipid droplets. However, the mechanism by which perilipin 2 binds to and remains anchored on the lipid droplet surface is unknown. Here we identify features of the lipid droplet surface that influence perilipin 2 localization. We show that perilipin 2 binding to the lipid droplet surface requires both hydrophobic and electrostatic interactions. Reagents that disrupt these interactions also decrease binding. Moreover, perilipin 2 binding does not depend on other lipid droplet-associated proteins but is influenced by the lipid composition of the surface. Perilipin 2 binds to synthetic vesicles composed of dioleoylphosphatidylcholine, a phospholipid with unsaturated acyl chains. Decreasing the temperature of the binding reaction, or introducing phospholipids with saturated acyl chains, decreases binding. We therefore demonstrate a role for surface lipids and acyl chain packing in perilipin 2 binding to lipid droplets. The ability of the lipid droplet phospholipid composition to impact protein binding may link changes in nutrient availability to lipid droplet homeostasis.

Lipid Droplets as Signaling Platforms Linking Metabolic and Cellular Functions

The main cells of the adipose tissue of animals, adipocytes, are characterized by the presence of large cytosolic lipid droplets (LDs) that store triglyceride (TG) and cholesterol. However, most cells have LDs and the ability to store lipids. LDs have a well-known central role in storage and provision of fatty acids and cholesterol. However, the complexity of the regulation of lipid metabolism on the surface of the LDs is still a matter of intense study. Beyond this role, a number of recent studies have suggested that LDs have major functions in other cellular processes, such as protein storage and degradation, infection, and immunity. Thus, our perception of LDs has been radically transformed from simple globules of fat to highly dynamic organelles of unexpected complexity. Here, we compiled some recent evidence supporting the emerging view that LDs act as platforms connecting a number of relevant metabolic and cellular functions.

Modulation of adipose tissue lipolysis and body weight by high-density lipoproteins in mice

BACKGROUND

Obesity is associated with reduced levels of circulating high-density lipoproteins (HDLs) and its major protein, apolipoprotein (apo) A-I. As a result of the role of HDL and apoA-I in cellular lipid transport, low HDL and apoA-I may contribute directly to establishing or maintaining the obese condition.

METHODS

To test this, male C57BL/6 wild-type (WT), apoA-I deficient (apoA-I(-/-)) and apoA-I transgenic (apoA-I(tg/tg)) mice were fed obesogenic diets (ODs) and monitored for several clinical parameters. We also performed cell culture studies.

RESULTS

ApoA-I(-/-) mice gained significantly more body weight and body fat than WT mice over 20 weeks despite their reduced food intake. During a caloric restriction regime imposed on OD-fed mice, apoA-I deficiency significantly inhibited the loss of body fat as compared with WT mice. Reduced body fat loss with caloric restriction in apoA-I(-/-) mice was associated with blunted stimulated adipose tissue lipolysis as verified by decreased levels of phosphorylated hormone-sensitive lipase (p-HSL) and lipolytic enzyme mRNA. In contrast to apoA-I(-/-) mice, apoA-I(tg/tg) mice gained relatively less weight than WT mice, consistent with other reports. ApoA-I(tg/tg) mice showed increased adipose tissue lipolysis, verified by increased levels of p-HSL and lipolytic enzyme mRNA. In cell culture studies, HDL and apoA-I specifically increased catecholamine-induced lipolysis possibly through modulating the adipocyte plasma membrane cholesterol content.

CONCLUSIONS

Thus, apoA-I and HDL contribute to modulating body fat content by controlling the extent of lipolysis. ApoA-I and HDL are key components of lipid metabolism in adipose tissue and constitute new therapeutic targets in obesity.

Palmitoleic acid (n-7) increases white adipocyte lipolysis and lipase content in a PPARα-dependent manner

We investigated whether palmitoleic acid, a fatty acid that enhances whole body glucose disposal and suppresses hepatic steatosis, modulates triacylglycerol (TAG) metabolism in adipocytes. For this, both differentiated 3T3-L1 cells treated with either palmitoleic acid (16:1n7, 200 μM) or palmitic acid (16:0, 200 μM) for 24 h and primary adipocytes from wild-type or PPARα-deficient mice treated with 16:1n7 (300 mg·kg(-1)·day(-1)) or oleic acid (18:1n9, 300 mg·kg(-1)·day(-1)) by gavage for 10 days were evaluated for lipolysis, TAG, and glycerol 3-phosphate synthesis and gene and protein expression profile. Treatment of differentiated 3T3-L1 cells with 16:1n7, but not 16:0, increased basal and isoproterenol-stimulated lipolysis, mRNA levels of adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) and protein content of ATGL and pSer(660)-HSL. Such increase in lipolysis induced by 16:1n7, which can be prevented by pharmacological inhibition of PPARα, was associated with higher rates of PPARα binding to DNA. In contrast to lipolysis, both 16:1n7 and 16:0 increased fatty acid incorporation into TAG and glycerol 3-phosphate synthesis from glucose without affecting glyceroneogenesis and glycerokinase expression. Corroborating in vitro findings, treatment of wild-type but not PPARα-deficient mice with 16:1n7 increased primary adipocyte basal and stimulated lipolysis and ATGL and HSL mRNA levels. In contrast to lipolysis, however, 16:1n7 treatment increased fatty acid incorporation into TAG and glycerol 3-phosphate synthesis from glucose in both wild-type and PPARα-deficient mice. In conclusion, palmitoleic acid increases adipocyte lipolysis and lipases by a mechanism that requires a functional PPARα.

Insertion of apoLp-III into a lipid monolayer is more favorable for saturated, more ordered, acyl-chains

Neutral lipid transport in mammals is complicated involving many types of apolipoprotein. The exchangeable apolipoproteins mediate the transfer of hydrophobic lipids between tissues and particles, and bind to cell surface receptors. Amphipathic α-helices form a common structural motif that facilitates their lipid binding and exchangeability. ApoLp-III, the only exchangeable apolipoprotein found in insects, is a model amphipathic α-helix bundle protein and its three dimensional structure and function mimics that of the mammalian proteins apoE and apoAI. Even the intracellular exchangeable lipid droplet protein TIP47/perilipin 3 contains an α-helix bundle domain with high structural similarity to that of apoE and apoLp-III. Here, we investigated the interaction of apoLp-III from Locusta migratoria with lipid monolayers. Consistent with earlier work we find that insertion of apoLp-III into fluid lipid monolayers is highest for diacylglycerol. We observe a preference for saturated and more highly ordered lipids, suggesting a new mode of interaction for amphipathic α-helix bundles. X-ray reflectivity shows that apoLp-III unfolds at a hydrophobic interface and flexible loops connecting the amphipathic α-helices stay in solution. X-ray diffraction indicates that apoLp-III insertion into diacylglycerol monolayers induces additional ordering of saturated acyl-chains. These results thus shed important new insight into the protein-lipid interactions of a model exchangeable apolipoprotein with significant implications for its mammalian counterparts.

Mitochondrial dysfunction induces formation of lipid droplets as a generalized response to stress

Lipid droplet (LD) formation is a hallmark of cellular stress. Cells attempt to combat noxious stimuli by switching their metabolism from oxidative phosphorylation to glycolysis, sparing resources in LDs for generating cellular reducing power and for anabolic biosynthesis. Membrane phospholipids are also a source of LDs. To elucidate the formation of LDs, we exposed mice to hyperoxia, hypoxia, myocardial ischemia, and sepsis induced by cecal ligation and puncture (CLP). All the above-mentioned stressors enhanced the formation of LDs, as assessed by transmission electron microscopy, with severe mitochondrial swelling. Disruption of mitochondria by depleting mitochondrial DNA ( ρ 0 cells) significantly augmented the formation of LDs, causing transcriptional activation of fatty acid biosynthesis and metabolic reprogramming to glycolysis. Heme oxygenase (HO)-1 counteracts CLP-mediated septic shock in mouse models. In HO-1-deficient mice, LD formation was not observed upon CLP, but a concomitant decrease in "LD-decorating proteins" was observed, implying a link between LDs and cytoprotective activity. Collectively, LD biogenesis during stress can trigger adaptive LD formation, which is dependent on mitochondrial integrity and HO-1 activity; this may be a cellular survival strategy, apportioning energy-generating substrates to cellular defense.

Perilipins: lipid droplet coat proteins adapted for tissue-specific energy storage and utilization, and lipid cytoprotection

Cytosolic lipid storage droplets are primary functional organelles that regulate cellular lipid metabolism and homeostasis. Paradoxically, excess lipid stores are linked to both adaptive (fasting and chronic exercise) and mal-adaptive (obesity and related health complications) conditions. Thus, collective metabolic and physiological processes must balance lipid storage and utilization with prevention of lipocytotoxicity and compounding tissue dysfunctions, urging the need to further define the connection of mammalian lipid droplet function and lipid homeostasis. The perilipins are a multi-protein family that targets lipid droplet surfaces and regulates lipid storage and hydrolysis. Study of perilipin functions has provided insight into the physiological roles of cytosolic lipid droplets and their relationship with obesity-related pathologies. Here, we review the current knowledge of the multiple perilipin proteins in regulating tissue-specific lipid droplets and associations with tissue and systemic energetics.

Role of adipose specific lipid droplet proteins in maintaining whole body energy homeostasis

Excess or insufficient lipid storage in white adipose tissue lipid droplets is associated with dyslipidemia, insulin resistance and increased risk for diabetes type 2. Thus, maintenance of adipose lipid droplet growth and function is critical to preserve whole body insulin sensitivity and energy homeostasis. Progress in understanding biology of lipid droplets has underscored the role of proteins that interact with lipid droplets. Here, we review the current knowledge of adipose specific lipid droplet proteins, which share unique functions controlling adipocyte lipid storage, limiting lipid spill-over and lipotoxic effects thought to contribute to disease. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.

Phospholipids and lipid droplets

Lipid droplets are ubiquitous cellular organelles that allow cells to store large amounts of neutral lipids for membrane synthesis and energy supply in times of starvation. Compared to other cellular organelles, lipid droplets are structurally unique as they are made of a hydrophobic core of neutral lipids and are separated to the cytosol only by a surrounding phospholipid monolayer. This phospholipid monolayer consists of over a hundred different phospholipid molecular species of which phosphatidylcholine is the most abundant lipid class. However, lipid droplets lack some indispensable activities of the phosphatidylcholine biogenic pathways suggesting that they partially depend on other organelles for phosphatidylcholine synthesis. Here, we discuss very recent data on the composition, origin, transport and function of the phospholipid monolayer with a particular emphasis on the phosphatidylcholine metabolism on and for lipid droplets. In addition, we highlight two very important quantitative aspects: (i) The amount of phospholipid required for lipid droplet monolayer expansion is remarkably small and (ii) to maintain the invariably round shape of lipid droplets, a cell must have a highly sensitive but so far unknown mechanism that regulates the ratio of phospholipid to neutral lipid in lipid droplets. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Direct interaction of Plin2 with lipids on the surface of lipid droplets: a live cell FRET analysis

Despite increasing awareness of the health risks associated with excess lipid storage in cells and tissues, knowledge of events governing lipid exchange at the surface of lipid droplets remains unclear. To address this issue, fluorescence resonance energy transfer (FRET) was performed to examine live cell interactions of Plin2 with lipids involved in maintaining lipid droplet structure and function. FRET efficiencies (E) between CFP-labeled Plin2 and fluorescently labeled phosphatidylcholine, sphingomyelin, stearic acid, and cholesterol were quantitated on a pixel-by-pixel basis to generate FRET image maps that specified areas with high E (>60%) in lipid droplets. The mean E and the distance R between the probes indicated a high yield of energy transfer and demonstrated molecular distances on the order of 44-57 Å, in keeping with direct molecular contact. In contrast, FRET between CFP-Plin2 and Nile red was not detected, indicating that the CFP-Plin2/Nile red interaction was beyond FRET proximity (>100 Å). An examination of the effect of Plin2 on cellular metabolism revealed that triacylglycerol, fatty acid, and cholesteryl ester content increased while diacylglycerol remained constant in CFP-Plin2-overexpressing cells. Total phospholipids also increased, reflecting increased phosphatidylcholine and sphingomyelin. Consistent with these results, expression levels of enzymes involved in triacylglycerol, cholesteryl ester, and phospholipid synthesis were significantly upregulated in CFP-Plin2-expressing cells while those associated with lipolysis either decreased or were unaffected. Taken together, these data show for the first time that Plin2 interacts directly with lipids on the surface of lipid droplets and influences levels of key enzymes and lipids involved in maintaining lipid droplet structure and function.