Role of adipose differentiation-related protein in lung surfactant production: a reassessment

Based on data developed with the use of isolated lipid droplets from neonatal rat lung lipofibroblasts, we speculated previously that the droplet coat protein, adipose differentiation-related protein (ADFP), mediated the transfer of lipids into type 2 lung epithelial cells for the production of surfactant phospholipids. The present studies were designed to test the role of ADFP in this transfer with the use of ADFP-coated lipid droplets from CHO fibroblast cells and a cultured human lung epithelial cell line. We found no role for ADFP in the lipid transfer and conclude that a lipase associated with the lipid droplets hydrolyzes their core triacylglycerols, releasing fatty acids that are taken up by the epithelial cells.     

Protection against fatty liver but normal adipogenesis in mice lacking adipose differentiation-related protein

Adipose differentiation-related protein (ADFP; also known as ADRP or adipophilin), is a lipid droplet (LD) protein found in most cells and tissues. ADFP expression is strongly induced in cells with increased lipid load. We have inactivated the Adfp gene in mice to better understand its role in lipid accumulation. The Adfp-deficient mice have unaltered adipose differentiation or lipolysis in vitro or in vivo. Importantly, they display a 60% reduction in hepatic triglyceride (TG) and are resistant to diet-induced fatty liver. To determine the mechanism for the reduced hepatic TG content, we measured hepatic lipogenesis, very-low-density lipoprotein (VLDL) secretion, and lipid uptake and utilization, all of which parameters were shown to be similar between mutant and wild-type mice. The finding of similar VLDL output in the presence of a reduction in total TG in the Adfp-deficient liver is explained by the retention of TG in the microsomes where VLDL is assembled. Given that lipid droplets are thought to form from the outer leaflet of the microsomal membrane, the reduction of TG in the cytosol with concomitant accumulation of TG in the microsome of Adfp-/- cells suggests that ADFP may facilitate the formation of new LDs. In the absence of ADFP, impairment of LD formation is associated with the accumulation of microsomal TG but a reduction in TG in other subcellular compartments.     

Activation of Hormone-sensitive Lipase Requires Two Steps, Protein Phosphorylation and Binding to the PAT-1 Domain of Lipid Droplet Coat Proteins

Lipolysis is an important metabolic pathway controlling energy homeostasis through degradation of triglycerides stored in lipid droplets and release of fatty acids. Lipid droplets of mammalian cells are coated with one or more members of the PAT protein family, which serve important functions in regulating lipolysis. In this study, we investigate the mechanisms by which PAT family members, perilipin A, adipose differentiation-related protein (ADFP), and LSDP5, control lipolysis catalyzed by hormone-sensitive lipase (HSL), a major lipase in adipocytes and several non-adipose cells. We applied fluorescence microscopic tools to analyze proteins in situ in cultured Chinese hamster ovary cells using fluorescence recovery after photobleaching and anisotropy Forster resonance energy transfer. Fluorescence recovery after photobleaching data show that ADFP and LSDP5 exchange between lipid droplet and cytoplasmic pools, whereas perilipin A does not. Differences in protein mobility do not correlate with PAT protein-mediated control of lipolysis catalyzed by HSL or endogenous lipases. Forster resonance energy transfer and co-immunoprecipitation experiments reveal that each of the three PAT proteins bind HSL through interaction of the lipase with amino acids within the highly conserved amino-terminal PAT-1 domain. ADFP and LSDP5 bind HSL under basal conditions, whereas phosphorylation of serine residues within three amino-terminal protein kinase A consensus sequences of perilipin A is required for HSL binding and maximal lipolysis. Finally, protein kinase A-mediated phosphorylation of HSL increases lipolysis in cells expressing ADFP or LSDP5; in contrast, phosphorylation of perilipin A exerts the major control over HSL-mediated lipolysis when perilipin is the main lipid droplet protein.     

Dynamic activity of lipid droplets: protein phosphorylation and GTP-mediated protein translocation

Lipid droplet is a cellular organelle with a neutral lipid core surrounded by a phospholipid monolayer and coated with structural as well as functional proteins. The determination of these proteins, especially their functional regulations and dynamic movement on and off droplets, holds a key to resolving the biological functions of the cellular organelle. To address this, we carried out a comprehensive proteomic study that includes a complete proteomic, a phosphoprotein proteomic, and a comparative proteomic analysis using purified lipid droplets and mass spectrometry techniques. The complete proteome identified 125 proteins of which 70 proteins had not been identified on droplets of mammalian cells previously. In phosphoprotein proteomic analysis, 7 functional lipid droplet proteins were determined to be phosphorylated, including adipose differentiation related protein (ADRP/ADFP), two Rab proteins, and four lipid metabolism enzymes, including adipose triglyceride lipase (ATGL). To understand the dynamics of lipid droplets, GTP-dependent protein recruitment was analyzed by comparative proteomics. Arf1 and some of its coatomers, three other Arfs, several other small G-proteins including 3 Rabs, and several lipid synthetic enzymes were recruited from cytosol to purified droplets. Together, the present study suggests that lipid droplet is an active and dynamic cellular organelle that governs lipid homeostasis and intracellular trafficking through protein phosphorylation as well as GTP-regulated protein translocation.     

Functional compensation for adipose differentiation-related protein (ADFP) by Tip47 in an ADFP null embryonic cell line

Ectopic accumulation of lipid droplets in non-adipose tissues correlates with the degree of insulin resistance in these tissues. Emerging evidence indicates that lipid droplets are specialized organelles that participate in lipid metabolism and intracellular trafficking. These properties are thought to derive from the lipid droplet-associated PAT protein family (perilipin, ADFP, and Tip47). The functions of the ubiquitously distributed adipose differentiation-related protein (ADFP) and Tip47 remain unknown. To evaluate the roles of ADFP and Tip47 in lipid biogenesis and metabolism, ADFP null and wild type (wt) clonal cell lines were established from ADFP null and wt mice, respectively. In ADFP null cells, Tip47 was identified as the sole lipid droplet-associated protein from the PAT family by mass spectroscopy, which was further confirmed by immunoblotting and immunocytochemistry. Following incubation with oleic acid, ADFP null cells were able to form lipid droplets to the same extent as wt cells. No statistical differences between the two cell types were observed in NEFA uptake or lipolysis. Small interference RNAs (siRNAs) against Tip47 were found to down-regulate protein levels for Tip47 by 85%. ADFP null cells treated with Tip47 siRNA retained the ability to form lipid droplets but to a lesser extent and shunted the utilization of exogenously added NEFA from triglycerides to phospholipids. These data support the hypothesis that Tip47 plays an important role in lipid metabolism. Tip47 and ADFP in peripheral tissues may play a critical role in regulating the formation and turnover, and hence metabolic consequences, of ectopic fat.     

Lipid droplets lighting up: Insights from live microscopy

Lipid droplets emerge as important intracellular organelles relevant for lipid homeostasis and the pathophysiology of metabolic diseases. Here, we present a personal view on the current knowledge about the biogenesis of mammalian cytoplasmic lipid droplets, with a focus on microscopy and especially live imaging. We also discuss difficulties related to the lipid droplet proteome, contentious views on lipid droplet growth, and last but not least the evidence for the heterogeneity of lipid droplets within a single cell. We conclude with an outline of the most important future challenges.     

Microalgal lipids biochemistry and biotechnological perspectives

In the last few years, there has been an intense interest in using microalgal lipids in food, chemical and pharmaceutical industries and cosmetology, while a noteworthy research has been performed focusing on all aspects of microalgal lipid production. This includes basic research on the pathways of solar energy conversion and on lipid biosynthesis and catabolism, and applied research dealing with the various biological and technical bottlenecks of the lipid production process. In here, we review the current knowledge in microalgal lipids with respect to their metabolism and various biotechnological applications, and we discuss potential future perspectives.     

ADFP promotes cell proliferation in lung adenocarcinoma via Akt phosphorylation

Previously, we identified differentially expressed proteins, including ADFP, between lung adenocarcinoma (LAC) tissue and paired normal bronchioloalveolar epithelium. In this study, we investigated the role of ADFP in LAC. ADFP levels in the serum of patients with lung cancer and benign diseases were measured by enzyme-linked immunosorbent assays (ELISA). shRNA was used to knock-down or overexpress ADFP in A549 and NCI-H1299 cells. The biological function of ADFP and its underlying mechanisms was evaluated in vivo and in vitro. ADFP was highly expressed in the serum of lung cancer patients, especially those with LAC. ADFP promoted cell proliferation and up-regulated the p-Akt/Akt ratio in A549 and NCI-H1299 cells in vitro. Furthermore, in nude mice, ADFP promoted tumour formation with high levels of p-Akt/Akt, Ki67 and proliferating cell nuclear antigen (PCNA). Similar to the effect of ADFP knock-down, MK-2206 (a phosphorylation inhibitor of Akt) reduced A549 and NCI-H1299 cell proliferation. In ADFP-overexpressing A549 and NCI-H1299 cells, proliferation was suppressed by MK-2206 and returned to the control level. ADFP did not regulate invasion, migration or adhesion in LAC cells. Together, these results suggest that ADFP promotes LAC cell proliferation in vitro and in vivo by increasing Akt phosphorylation level.     

Re-evaluating lipotoxic triggers in skeletal muscle: relating intramyocellular lipid metabolism to insulin sensitivity

Ectopic fat accumulation has been linked to lipotoxic events, including the development of insulin resistance in skeletal muscle. Indeed, intramyocellular lipid storage is strongly associated with the development of type 2 diabetes. Research during the last two decades has provided evidence for a role of lipid intermediates like diacylglycerol and ceramide in the induction of lipid-induced insulin resistance. However, recently novel data has been gathered that suggest that the relation between lipid intermediates and insulin resistance is less straightforward than has been previously suggested, and that there are several routes towards lipid-induced insulin resistance. For example, research in this field has shifted towards imbalances in lipid metabolism and lipid droplet dynamics. Next to imbalances in key lipogenic and lipolytic proteins, lipid droplet coat proteins appear to be essential for proper intramyocellular lipid storage, turnover and protection against lipid-induced insulin resistance.Here, we discuss the current knowledge on lipid-induced insulin resistance in skeletal muscle with a focus on the evidence from human studies. Furthermore, we discuss the available data that provides supporting mechanistic information.     

Conserved Amphipathic Helices Mediate Lipid Droplet Targeting of Perilipins 1–3

Perilipins (PLINs) play a key role in energy storage by orchestrating the activity of lipases on the surface of lipid droplets. Failure of this activity results in severe metabolic disease in humans. Unlike all other lipid droplet-associated proteins, PLINs localize almost exclusively to the phospholipid monolayer surrounding the droplet. To understand how they sense and associate with the unique topology of the droplet surface, we studied the localization of human PLINs in Saccharomyces cerevisiae, demonstrating that the targeting mechanism is highly conserved and that 11-mer repeat regions are sufficient for droplet targeting. Mutations designed to disrupt folding of this region into amphipathic helices (AHs) significantly decreased lipid droplet targeting in vivo and in vitro. Finally, we demonstrated a substantial increase in the helicity of this region in the presence of detergent micelles, which was prevented by an AH-disrupting missense mutation. We conclude that highly conserved 11-mer repeat regions of PLINs target lipid droplets by folding into AHs on the droplet surface, thus enabling PLINs to regulate the interface between the hydrophobic lipid core and its surrounding hydrophilic environment.     

Fixation methods for the study of lipid droplets by immunofluorescence microscopy.

The study of proteins associated with lipid droplets in adipocytes and many other cells is a rapidly developing area of inquiry. Although lipid droplets are easily visible by light microscopy, few standardized microscopy methods have been developed. Several methods of chemical fixation have recently been used to preserve cell structure before visualization of lipid droplets by light microscopy. We tested the most commonly used methods to compare the effects of the fixatives on cellular lipid content and lipid droplet structure. Cold methanol fixation has traditionally been used before visualization of cytoskeletal elements. We found this method unacceptable for study of lipid droplets because it extracted the majority of cellular phospholipids and promoted fusion of lipid droplets. Cold acetone fixation is similarly unacceptable because the total cellular lipids are extracted, causing collapse of the shell of lipid droplet-associated proteins. Fixation of cells with paraformaldehyde is the method of choice, because the cells retain their lipid content and lipid droplet structure is unaffected. As more lipid droplet-associated proteins are discovered and studied, it is critical to use appropriate methods to avoid studying artifacts.     

TIP47 associates with lipid droplets

Most mammalian cells package neutral lipids into droplets that are surrounded by a monolayer of phospholipids and a specific set of proteins including the adipose differentiation-related protein (ADRP; also called adipophilin), which is found in a wide array of cell types, and the perilipins, which are restricted to adipocytes and steroidogenic cells. TIP47 was initially identified in a yeast two-hybrid screen for proteins that interact with the cytoplasmic tail of the mannose 6-phosphate receptor, yet its sequence is highly similar to the lipid droplet protein, ADRP, and more distantly related to perilipins. Hence, we hypothesized that TIP47 might be associated with lipid droplets. In HeLa cells grown in standard low lipid-containing culture media, immunofluorescence microscopy revealed that the cells had few lipid droplets; however, TIP47 and ADRP were found on the surfaces of the small lipid droplets present. When the cells were grown in media supplemented with physiological levels of fatty acids, the amount of neutral lipid stored in lipid droplets increased dramatically, as did the staining of TIP47 and ADRP surrounding these droplets. TIP47 was found primarily in the cytosolic fractions of HeLa cells and murine MA10 Leydig cells grown in low lipid-containing culture medium, while ADRP was undetectable in these fractionated cell homogenates. When HeLa and MA10 Leydig cells were lipid-loaded, significant levels of ADRP were found in the floating lipid droplet fractions and TIP47 levels remained constant, but the distribution of a significant portion of TIP47 shifted from the cytosolic fractions to the lipid droplet fractions. Thus, we conclude that TIP47 associates with nascent lipid droplets and can be classified as a lipid droplet-associated protein.     

PERILIPIN-dependent control of lipid droplet structure and fat storage in Drosophila

Lipid droplets are intracellular organelles enriched in adipose tissue that govern the body fat stores of animals. In mammals, members of the evolutionarily conserved PERILIPIN protein family are associated with the lipid droplet surface and participate in lipid homeostasis. Here, we show that Drosophila mutants lacking the PERILIPIN PLIN1 are hyperphagic and suffer from adult-onset obesity. PLIN1 is a central and Janus-faced component of fat metabolism. It provides barrier function to storage lipid breakdown and acts as a key factor of stimulated lipolysis by modulating the access of proteins to the lipid droplet surface. It also shapes lipid droplet structure, transforming unilocular into multilocular fat cells. We generated flies devoid of all PERILIPIN family members and show that they exhibit impaired yet functional body fat regulation. Our data reveal the existence of a basal and possibly ancient lipid homeostasis system.     

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.     

Retinyl esters form lipid droplets independently of triacylglycerol and seipin

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

Fat-specific protein 27, a novel lipid droplet protein that enhances triglyceride storage

Fat-specific protein (FSP)27/Cidec is most highly expressed in white and brown adipose tissues and increases in abundance by over 50-fold during adipogenesis. However, its function in adipocytes has remained elusive since its discovery over 15 years ago. Here we demonstrate that FSP27/Cidec localizes to lipid droplets in cultured adipocytes and functions to promote lipid accumulation. Ectopically expressed FSP27-GFP surrounds lipid droplets in 3T3-L1 adipocytes and colocalizes with the known lipid droplet protein perilipin. Immunostaining of endogenous FSP27 in 3T3-L1 adipocytes also confirmed its presence on lipid droplets. FSP27-GFP expression also markedly increases lipid droplet size and enhances accumulation of total neutral lipids in 3T3-L1 preadipocytes as well as other cell types such as COS cells. Conversely, RNA interference-based FSP27/Cidec depletion in mature adipocytes significantly stimulates lipolysis and reduces the size of lipid droplets. These data reveal FSP27/Cidec as a novel adipocyte lipid droplet protein that negatively regulates lipolysis and promotes triglyceride accumulation.     

Osmotic stress activates distinct lipid and MAPK signalling pathways in plants.

Plants are continuously exposed to all kinds of water stress such as drought and salinity. In order to survive and adapt, they have developed survival strategies that have been well studied, but little is known about the early mechanisms by which the osmotic stress is perceived and transduced into these responses. During the last few years, however, a variety of reports suggest that specific lipid and MAPK pathways are involved. This review briefly summarises them and presents a model showing that osmotic stress is transmitted by multiple signalling pathways.