脂滴——细胞脂类代谢的细胞器

脂滴是细胞内中性脂贮存的主要场所,由极性单磷脂层包裹疏水核心组成。近年来的蛋白质组学研究表明,脂滴表面还存在着许多功能蛋白,进一步揭示了脂滴可能参与细胞内物质的代谢和转运,以及细胞信号传导等过程,是一个活动旺盛的多功能细胞器。实验结果还证明,脂滴不但是甘油三酯贮存和分解、花生四烯酸代谢和前列腺素合成的主要场所,脂滴还具有合成甘油三酯和磷酯的功能。由此可见,脂滴可能是细胞内参与脂类合成代谢的细胞器。     

骨骼肌的脂质代谢及其与胰岛素抵抗的研究进展

一系列的研究表明骨骼肌细胞内甘油三酯(intramyocellular triacyglycerol,IMTG)和胰岛素抵抗之间有密切联系。许多因素可调控IMTG的动态变化,如甘油三酯水解酶、激素敏感性脂肪酶、甘油一酯脂肪酶和脂滴结合蛋白等。而IMTG的代谢中间产物甘油二酯、神经酰胺和脂肪酸在骨骼肌中的聚集也与胰岛素抵抗密切相关。脂滴和线粒体对于细胞内甘油三酯及其代谢产物的周转速度起关键作用,对胰岛素抵抗也起着重要的作用。     

一种调控脂解的重要蛋白——围脂滴蛋白(Perilipin)

围脂滴蛋白（perilipin）是脂滴相关蛋白家族的核心成员之一,是定位于脂滴表面的高磷酸化的蛋白,对脂肪组织中甘油三酯的代谢有双重调节作用,既可通过阻止脂肪酶接近脂滴降低基础状态下的脂解,又可促进激素刺激的脂肪分解.Perilipin在脂代谢中发挥重要作用,其表达调控可能与肥胖及其相关代谢疾病如糖尿病、胰岛素抵抗等有重要关系.本文主要介绍了perilipin的发现、命名、结构特征以及激素和转录因子对perilipin的调控,并阐述了其与相关脂肪酶间的相互作用.目前的研究主要集中于围脂滴蛋白（perilipin）和激素敏感脂肪酶（HSL）之间,与新近发现的脂肪酶脂肪三酰甘油脂酶（ATGL）的相互作用则有待于进一步研究.     

脂滴与心力衰竭的研究进展

脂滴(LDs)是脂肪组织的基本单位,广泛存在于真核生物胞浆中,其表面覆盖以Perilipin 1(Plin1)为主的脂滴包被蛋白,在脂滴表面发挥屏障作用,保护脂滴内部甘油三酯及胆固醇酯免受胞浆中脂肪酶分解。近年来研究表明脂滴与心力衰竭关联密切,脂滴包被蛋白缺失可引起机体脂质代谢异常,进而引起心肌细胞结构改变,最终导致心力衰竭发生。这些研究为认识脂滴与心力衰竭间关系提供了依据,本文将脂滴与心力衰竭的研究进展进行归纳。     

脂肪组织甘油三酯水解酶参与脂肪分解调控

循环中游离脂肪酸增高与肥胖、胰岛素抵抗和2型糖尿病密切相关,其主要来源于脂肪细胞内甘油三酯水解.调控脂肪分解的脂肪酶主要包括激素敏感脂肪酶(hormone-sensitive lipase,HSL)和最近发现的脂肪组织甘油三酯水解酶(adipose triglyceride lipase,ATGL),后者主要分布在脂肪组织,特异水解甘油三酯为甘油二酯,其转录水平受多种因素调控.CGI-58(属于α/β水解酶家族蛋白),可以活化ATGL,基础条件下该蛋白和脂滴包被蛋白(perilipin)紧密结合于脂滴表面,蛋白激酶A激活刺激脂肪分解时,CGI-58与perilipin分离,进而活化ATGL.     

脂滴包被蛋白(perilipin)调控脂肪分解

脂滴包被蛋白（perilipin）包被在脂肪细胞和甾体生成细胞脂滴表面。基础状态下perilipin可减少甘油三酯水解,使其贮备增加;脂肪分解时磷酸化的perilipin能促进甘油三酯水解,而且该蛋白对激素敏感脂酶从胞浆向脂滴转位是必需的。据推测,perilipin可能在脂肪分解调控中起到“分子开关”的作用。蛋白激酶A（PKA）、细胞外信号调节激酶（ERK）等信号转导通路参与了脂肪分解。肿瘤坏死因子仅（TNFα）、过氧化物酶体增殖物激活受体γ（PPAγ）激动剂、瘦素（leptin）均可以影响perilipin的表达。新近研究表明,perilipin可通过蛋白酶体途径来调节其蛋白量的表达。脂肪分解调控中的关键蛋白perilipin可以和2型糖尿病、肥胖、动脉粥样硬化等多种代谢性疾病及心血管疾病联系起来。     

Forskolin引起大鼠脂肪细胞内脂滴重塑(英文)

脂肪组织是人体的主要能量储库,甘油三酯贮存在细胞内脂滴(lipid droplets, LDs)中,越来越多的研究表明脂滴是细胞内代谢活跃的细胞器。本研究旨在探讨forskolin长时间刺激脂肪分解过程中脂滴形态和脂滴表面perilipin家族蛋白的改变。以Sprague-Dawley (SD)大鼠附睾脂肪垫来源的分化脂肪细胞为研究对象,给予1μmol/L forskolin慢性刺激24 h,采用比色方法测定培养基中甘油的浓度;采用尼罗红染色观察细胞内脂滴形态的变化;采用荧光定量PCR检测perilipin家族蛋白mRNA水平的改变;采用免疫印迹和免疫荧光染色观察蛋白水平以及蛋白的亚细胞定位。结果表明,1μmol/L forskolin孵育24 h可以持续刺激脂肪分解。伴随着脂肪分解的进行,细胞内脂滴形态逐渐发生改变,细胞内聚集存在的大脂滴逐渐减少,位于细胞周边的小脂滴逐渐增加,最终细胞内大脂滴全部消失,取而代之的是在细胞质中弥散存在的微小脂滴。在脂肪分解过程中,perilipin家族蛋白水平也发生明显变化。分化成熟的脂肪细胞几乎没有Plin2蛋白表达,而forskolin慢性刺激可以显著增加Plin2蛋白以及mRNA的水平,增加的Plin2蛋白特异性结合在脂滴表面。Forskolin慢性刺激对Plin3的mRNA水平无显著影响,但可以显著降低Plin1、Plin4和Plin5的mRNA水平。以上结果提示,在forskolin慢性持续刺激脂肪分解过程中,脂滴形态和perilipin家族蛋白均发生显著改变。     

雷帕霉素对小鼠原代肝细胞脂滴形态和脂滴表面蛋白表达的影响

目的:探讨雷帕霉素(Rapamycin)对小鼠原代肝细胞脂滴形态和脂滴表面蛋白表达的影响。方法:采用胶原酶灌注方法分离和培养小鼠原代肝细胞,采用100μM油酸诱导肝细胞内脂肪的合成。采用0、10、20、50μM的雷帕霉素处理肝细胞12 hr后,利用中性脂肪染料Bodipy493/503对肝细胞内的脂滴进行染色,荧光显微镜下观察细胞脂滴形态和数量。定量试剂盒检测细胞内甘油三酯(TG)的含量利用Western blot检测不同浓度雷帕霉素处理的小鼠原代肝细胞脂滴表面蛋白ADRP的表达水平。结果:成功分离和培养了小鼠原代肝细胞,使用油酸处理能够明显增加原代肝细胞内脂滴的数量。随着体外雷帕霉素处理浓度的增加,荧光显微镜下观察发现原代肝细胞内脂滴的数量呈现明显的下降趋势,甘油三酯的含量也呈见明确的下降趋势,在20μM浓度下就表现出显著性差异。Western blot结果显示雷帕霉素能够在抑制肝细胞内脂肪储积的同时降低脂滴表面蛋白ADRP的表达水平,并且随着雷帕霉素处理浓度的增加,其对ADRP表达的抑制越明显。结论:雷帕霉素能够抑制肝细胞内中性脂肪的储积,同时降低脂滴表面蛋白ADRP的表达水平。也间接说明了mTOR信号通路能够影响肝细胞内脂肪的储积,也为脂肪肝的防治提供了一个新的实验基础。     

脂滴与合成类固醇的细胞

正脂滴(LD)存在于的合成类固醇组织由磷脂单分子层及中性脂构成的疏水核心构成,并且表面分布有很多蛋白。与其他组织相比,他们往往体积较小,其数量巨大。这些LD上面包被着大量的PLIN1c、PLIN2和PLIN3。在小鼠卵巢发现了诱导细胞凋亡的因子A和B。由于激素刺激来自于脂滴(LD)中的游离胆固醇(FC)是类固醇合成的主要的底物来源,同时激素敏感     

重组蛋白转导域-神经肽Y融合蛋白对体外培养大鼠脂肪细胞的影响

目的探讨重组蛋白转导域-神经肽Y融合蛋白对体外培养大鼠脂肪细胞的影响。方法采用剪切消化法分离大鼠前脂肪细胞,培养液中添加重组PTD-NPY融合蛋白,检测前脂肪细胞和成熟脂肪细胞的形态学变化、细胞中甘油三酯含量和甘油磷酸脱氢酶(GPDH)活性。结果重组PTD-NPY融合蛋白处理大鼠前脂肪细胞72 h,细胞体积增大,细胞数量增加,细胞甘油三酯含量和GPDH活性升高;重组PTD-NPY融合蛋白处理成熟脂肪细胞48h后,细胞体积明显增大,细胞内脂肪滴数量增加并融合成较大的脂滴,甘油三酯含量和GPDH活性均显著升高。结论重组PTD-NPY融合蛋白明显促进前脂肪细胞的分化,促进脂肪细胞中甘油三酯的合成与沉积。为重组PTD-NPY融合蛋白在动物生产及人类疾病治疗中的实际应用奠定理论基础。     

Characterization of the Drosophila lipid droplet subproteome

Lipid storage droplets are universal organelles essential for the cellular and organismal lipometabolism including energy homeostasis. Despite their apparently simple design they are proposed to participate in a growing number of cellular processes, raising the question to what extent the functional multifariousness is reflected by a complex organellar proteome composition. Here we present 248 proteins identified in a subproteome analysis using lipid storage droplets of Drosophila melanogaster fat body tissue. In addition to previously known lipid droplet-associated PAT (Perilipin, ADRP, and TIP47) domain proteins and homologues of several mammalian lipid droplet proteins, this study identified a number of proteins of diverse biological function, including intracellular trafficking supportive of the dynamic and multifaceted character of these organelles. We performed intracellular localization studies on selected newly identified subproteome members both in tissue culture cells and in fat body cells directly. The results suggest that the lipid droplets of fat body cells are of combinatorial protein composition. We propose that subsets of lipid droplets within single cells are characterized by a protein "zip code," which reflects functional differences or specific metabolic states.     

Functional conservation for lipid storage droplet association among Perilipin,ADRP, and TIP47 (PAT)-related proteins in mammals,Drosophila, and Dictyostelium

Intracellular neutral lipid storage droplets are essential organelles of eukaryotic cells, yet little is known about the proteins at their surfaces or about the amino acid sequences that target proteins to these storage droplets. The mammalian proteins Perilipin, ADRP, and TIP47 share extensive amino acid sequence similarity, suggesting a common function. However, while Perilipin and ADRP localize exclusively to neutral lipid storage droplets, an association of TIP47 with intracellular lipid droplets has been controversial. We now show that GFP-tagged TIP47 co-localizes with isolated intracellular lipid droplets. We have also detected a close juxtaposition of TIP47 with the surfaces of lipid storage droplets using antibodies that specifically recognize TIP47, further indicating that TIP47 associates with intracellular lipid storage droplets. Finally, we show that related proteins from species as diverse as Drosophila and Dictyostelium can also target mammalian or Drosophila lipid droplet surfaces in vivo. Thus, sequence and/or structural elements within this evolutionarily ancient protein family are necessary and sufficient to direct association to heterologous intracellular lipid droplet surfaces, strongly indicating that they have a common function for lipid deposition and/or mobilization.     

Live cell analysis and targeting of the lipid droplet-binding adipocyte differentiation-related protein

Neutral lipid is stored in spherical organelles called lipid droplets that are bounded by a coat of proteins. The protein that is most frequently found at the surface of lipid droplets is adipocyte differentiation-related protein (ADRP). In this study, we demonstrate that fusion of either the human or mouse ADRP coding sequences to green fluorescent protein (GFP) does not disrupt the ability of the protein to associate with lipid droplets. Using this system to identify targeting elements, discontinuous segments within the coding region were required for directing ADRP to lipid droplets. GFP-tagged protein was employed also to examine the behavior of lipid droplets in live cells. Time lapse microscopy demonstrated that in HuH-7 cells, which are derived from a human hepatoma, a small number of lipid droplets could move rapidly, indicating transient association with intracellular transport pathways. Most lipid droplets did not show such movement but oscillated within a confined area; these droplets were in close association with the endoplasmic reticulum membrane and moved in concert with the endoplasmic reticulum. Fluorescence recovery analysis of GFP-tagged ADRP in live cells revealed that surface proteins do not rapidly diffuse between lipid droplets, even in conditions where they are closely packed. This system provides new insights into the properties of lipid droplets and their interaction with cellular processes.     

Perilipins, ADRP, and other proteins that associate with intracellular neutral lipid droplets in animal cells.

Although all animal cells package and store neutral lipids in discrete intracellular storage droplets, there is little information on the molecular processes that govern either the deposition or catabolism of the stored lipid components. Studies on adipocytes have uncovered the perilipins and ADRP, related proteins that appear to be intrinsic to the surfaces of intracellular lipid storage droplets. We discuss the properties, distribution, localization, and potential functions of these proteins, as well as those of vimentin and the recently-described 'capsular' proteins, in lipid storage and metabolism.     

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.     

PAT family proteins pervade lipid droplet cores

The PAT family proteins, named after perilipin, adipophilin, and the tail-interacting protein of 47 kDa (TIP47), are implicated in intracellular lipid metabolism. They associate with lipid droplets, but how is completely unclear. From immunofluorescence studies, they are reported to be restricted to the outer membrane monolayer enveloping the lipid droplet and not to enter the core. Recently, we found another kind of lipid droplet-associated protein, caveolin-1, inside lipid droplets. Using freeze-fracture immunocytochemistry and electron microscopy, we now describe the distributions of perilipin and caveolin-1 and of adipophilin and TIP47 in lipid droplets of adipocytes and macrophages. All of these lipid droplet-associated proteins pervade the lipid droplet core and hence are not restricted to the droplet surface. Moreover, lipid droplets are surprisingly heterogeneous with respect to their complements and their distribution of lipid droplet-associated proteins. Whereas caveolin-1 is synthesized in the endoplasmic reticulum and is transferred to the lipid droplet core by inundating lipids during droplet budding, the PAT proteins, which are synthesized on free ribosomes in the cytoplasm, evidently target to the lipid droplet after it has formed. How the polar lipid droplet-associated proteins are accommodated among the essentially hydrophobic neutral lipids of the lipid droplet core remains to be determined.     

Perilipin and adipophilin expression in lipid loaded macrophages

Lipid-filled macrophages (foam cells) are a defining feature of atherosclerotic plaques. Foam cells contain lipid droplet-associated proteins that in other cell types regulate lipid turnover. In foam cell such proteins may directly affect lipid droplet formation and lipid efflux. Differentiated primary human monocytes or THP-1 cells were lipid loaded by incubation with aggregated low density lipoproteins (AgLDL) or VLDL resulting in macrophage foam cells with predominantly cholesterol ester or triglyceride-rich lipid droplets, respectively. Lipid droplets were isolated and major proteins identified by mass spectrometry, among them the apolipoprotein B-48 receptor that has not previously been recognized in this context. Expression of two proteins, perilipin and adipophilin, was quantified by Western blots of cell lysates. Perilipin content decreased and adipophilin increased with lipoprotein lipid loading regardless of intracellular neutral lipid composition. This protein expression pattern may hinder lipid turnover in macrophage foam cells, thereby increasing lipid content of atherosclerotic plaques.     

Rab32 is important for autophagy and lipid storage in Drosophila

Lipids are essential components of all organisms. Within cells, lipids are mainly stored in a specific type of organelle, called the lipid droplet. The molecular mechanisms governing the dynamics of lipid droplets have been little explored. The protein composition of lipid droplets has been analyzed in numerous proteomic studies, and a large number of lipid droplet-associated proteins have been identified, including Rab small GTPases. Rab proteins are known to participate in many intracellular membranous events; however, their exact role in lipid droplets is largely unexplored. Here we systematically investigate the roles of Drosophila Rab family proteins in lipid storage in the larval adipose tissue, fat body. Rab32 and several other Rabs were found to affect the size of lipid droplets as well as lipid levels. Further studies showed that Rab32 and Rab32 GEF/Claret may be involved in autophagy, consequently affecting lipid storage. Loss-of-function mutants of several components in the autophagy pathway result in similar effects on lipid storage. These results highlight the potential functions of Rabs in regulating lipid metabolism.     

Intracellular origin and secretion of milk fat globules

The cream or fat fraction of milk consists of fat droplets composed primarily of triacylglycerols that are surrounded by cellular membranes. In this review we discuss what is known about how these droplets are formed in and secreted by mammary epithelial cells during lactation. This secretion mechanism, which appears to be unique, is unlike the exocytotic mechanism used by other cell types to secrete lipids. Milk fat globules originate as small, triacylglycerol-rich, droplets that are formed on or in endoplasmic reticulum membranes. These droplets are released from endoplasmic reticulum into the cytosol as microlipid droplets coated by proteins and polar lipids. Microlipid droplets can fuse with each other to form larger cytoplasmic lipid droplets. Droplets of all sizes appear to be unidirectionally transported to apical cell regions by as yet unknown mechanisms that may involve cytoskeletal elements. These lipid droplets appear to be secreted from the cell in which they were formed by being progressively enveloped in differentiated regions of apical plasma membrane. While plasma membrane envelopment appears to be the primary mechanism by which lipid droplets are released from the cell, a mechanism involving exocytosis of lipid droplets from cytoplasmic vacuoles also has been described. As discussed herein, while we have a general overview of the steps leading to the fat globules of milk, virtually nothing is known about the molecular mechanisms involved in milk fat globule formation, intracellular transit, and secretion.