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.     

PAT家族蛋白在细胞内脂滴代谢过程中的作用

哺乳动物细胞内的甘油三酯是以脂滴的形式贮存的,现在有很多证据表明,脂滴参与多种代谢过程,因而被看作胞内有功能的细胞器。脂滴含有甘油三酯构成的脂质核心,脂核表面覆盖有单层磷脂,在单层磷脂内镶嵌着在结构上具有相关性的PAT家族蛋白,包括perilipin、ADRP、TIP47和S3—12。本文就这些蛋白在甘油三酯水解和脂滴合成中的调节作用加以综述。     

Distinct cellular pools of perilipin 5 point to roles in lipid trafficking

The PAT family of lipid storage droplet proteins comprised five members, each of which has become an established regulator of cellular neutral lipid metabolism. Perilipin 5 (also known as lsdp-5, MLDP, PAT-1, and OXPAT), the most recently discovered member of the family, has been shown to localize to two distinct intracellular pools: the lipid storage droplet (LD), and a poorly characterized cytosolic fraction. We have characterized the denser of these intracellular pools and find that a population of perilipin 5 not associated with large LDs resides in complexes with a discrete density (~ 1.15 g/ml) and size (~ 575 kDa). Using immunofluorescence, western blotting of isolated sucrose density fractions, native gradient gel electrophoresis, and co-immunoprecipitation, we have shown that these small (~ 15 nm), perilipin 5-encoated structures do not contain the PAT protein perilipin 2 (ADRP), but do contain perilipin 3 and several other as of yet uncharacterized proteins. The size and density of these particles as well as their susceptibility to degradation by lipases suggest that like larger LDs, they have a neutral lipid rich core. When treated with oleic acid to promote neutral lipid deposition, cells ectopically expressing perilipin 5 experienced a reorganization of LDs in the cell, resulting in fewer, larger droplets at the expense of smaller ones. Collectively, these data demonstrate that a portion of cytosolic perilipin 5 resides in high density lipid droplet complexes that participate in cellular neutral lipid accumulation.     

Lipid droplets gain PAT family proteins by interaction with specialized plasma membrane domains

Proteins of the PAT family, named after perilipin, adipophilin, and TIP47 (tail-interacting protein of 47 kDa), are associated with lipid droplets and have previously been localized by immunofluorescence microscopy exclusively to the droplet surface. These proteins are considered not to be present in any other subcellular compartment. By applying the high resolution technique of freeze-fracture electron microscopy combined with immunogold labeling, we now demonstrate that in macrophages and adipocytes PAT family proteins are, first, distributed not only in the surface but also throughout the lipid droplet core and, second, are integral components of the plasma membrane. Under normal culture conditions these proteins are dispersed in the cytoplasmic leaflet of the plasma membrane. Stimulation of lipid droplet formation by incubation of the cells with acetylated low density lipoprotein leads to clustering of the PAT family proteins in raised plasma membrane domains. Fractures penetrating beneath the plasma membrane demonstrate that lipid droplets are closely apposed to these domains. A similar distribution pattern of labeling in the form of linear aggregates within the clusters is apparent in the cytoplasmic monolayer of the plasma membrane and the immediately adjacent outer monolayer of the lipid droplet. The aggregation of the PAT family proteins into such assemblies may facilitate carrier-mediated lipid influx from the extracellular environment into the lipid droplet. Lipid droplets appear to acquire their PAT proteins by interaction with plasma membrane domains enriched in these proteins.     

Thematic review series: adipocyte biology. The perilipin family of structural lipid droplet proteins: stabilization of lipid droplets and control of lipolysis

The majority of eukaryotic cells synthesize neutral lipids and package them into cytosolic lipid droplets. In vertebrates, triacylglycerol-rich lipid droplets of adipocytes provide a major energy storage depot for the body, whereas cholesteryl ester-rich droplets of many other cells provide building materials for local membrane synthesis and repair. These lipid droplets are coated with one or more of five members of the perilipin family of proteins: adipophilin, TIP47, OXPAT/MLDP, S3-12, and perilipin. Members of this family share varying levels of sequence similarity, lipid droplet association, and functions in stabilizing lipid droplets. The most highly studied member of the family, perilipin, is the most abundant protein on the surfaces of adipocyte lipid droplets, and the major substrate for cAMP-dependent protein kinase [protein kinase A (PKA)] in lipolytically stimulated adipocytes. Perilipin serves important functions in the regulation of basal and hormonally stimulated lipolysis. Under basal conditions, perilipin restricts the access of cytosolic lipases to lipid droplets and thus promotes triacylglycerol storage. In times of energy deficit, perilipin is phosphorylated by PKA and facilitates maximal lipolysis by hormone-sensitive lipase and adipose triglyceride lipase. A model is discussed whereby perilipin serves as a dynamic scaffold to coordinate the access of enzymes to the lipid droplet in a manner that is responsive to the metabolic status of the adipocyte.     

Distinct cellular pools of perilipin 5 point to roles in lipid trafficking

The PAT family of lipid storage droplet proteins comprised five members, each of which has become an established regulator of cellular neutral lipid metabolism. Perilipin 5 (also known as lsdp-5, MLDP, PAT-1, and OXPAT), the most recently discovered member of the family, has been shown to localize to two distinct intracellular pools: the lipid storage droplet (LD), and a poorly characterized cytosolic fraction. We have characterized the denser of these intracellular pools and find that a population of perilipin 5 not associated with large LDs resides in complexes with a discrete density (~1.15 g/ml) and size (~575 kDa). Using immunofluorescence, western blotting of isolated sucrose density fractions, native gradient gel electrophoresis, and co-immunoprecipitation, we have shown that these small (~15 nm), perilipin 5-encoated structures do not contain the PAT protein perilipin 2 (ADRP), but do contain perilipin 3 and several other as of yet uncharacterized proteins. The size and density of these particles as well as their susceptibility to degradation by lipases suggest that like larger LDs, they have a neutral lipid rich core. When treated with oleic acid to promote neutral lipid deposition, cells ectopically expressing perilipin 5 experienced a reorganization of LDs in the cell, resulting in fewer, larger droplets at the expense of smaller ones. Collectively, these data demonstrate that a portion of cytosolic perilipin 5 resides in high density lipid droplet complexes that participate in cellular neutral lipid accumulation.     

Temporal and spatial assembly of lipid droplet-associated proteins in 3T3-L1 preadipocytes

This study aimed to investigate the relationship between newly formed lipid droplets and lipid droplet surface proteins, including perilipin, adipose differentiation related protein (ADRP), and p200 kDa protein (p200) in 3T3-L1 preadipocytes, during lipogenesis. Sterol ester was used to induce nascent lipid droplets in 3T3-L1 preadipocytes and the sequence of lipids and lipid droplet surface proteins was studied using a combination of immunohistochemistry and Nile red staining/Oil red O. We demonstrated that, although most growing lipid droplets appeared to have a lipid core surrounded by a fluorescent rim of ADRP, perilipin, and p200, tiny protein aggregates of ADRP, perilipin, or p200 could also be found to occur in the absence of lipid accumulation. In addition, ADRP associated with nascent lipid droplets prior to that of perilipin or p200. We provide evidence that lipid droplet surface proteins, especially ADRP and perilipin, are important in serving as a nucleation center for the assembly of lipid to form nascent lipid droplets.     

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.     

Lipid droplet associated proteins: an emerging role in atherogenesis

Coronary heart disease and stroke, caused by rupture of atherosclerotic plaques in the arterial wall, are the major causes of death in industrialized countries. A key event in the pathogenesis of atherosclerosis is the transformation of smooth muscle cells and in particular of macrophages into foam cells, a result of massive accumulation of lipid droplets. It is well known that the formation of these lipid droplets is a result of the uninhibited uptake of modified lipoproteins by scavenger receptors. However, only more recently has it become apparent that a special set of lipid droplet associated proteins - the PAT protein family (perilipin, adipophilin, TIP47, S3-12 and OXPAT) - is fundamental to the formation, growth, stabilization and functions of lipid droplets. Here we review recent findings and assess the current state of knowledge on lipid droplets and their PAT proteins in atherogenesis.     

Role of lipid droplet proteins in liver steatosis

Five proteins of the perilipin (Plin) family such as Plin1 (perilipin) Plin2 (adipose differentiation-related protein), Plin3 (tail-interacting protein of 47 kDa), Plin4 (S3-12), and Plin5 (myocardial lipid droplet protein) are characterized as lipid droplet (LD) proteins in adipocytes. Recent reports have demonstrated that fat-specific protein 27 (FSP27) and hypoxia-inducible protein 2 (HIG2) are also thought to be novel LD proteins in addition to proteins of the Plin family. Growing evidence have shown that LD proteins play a role in the pathophysiology in the fatty liver disease which is characterized by hepatocytes containing LD with excessive neutral lipid. Studies showed LD proteins such as Plin1, Plin2, Plin3, Plin5, FSP27, and HIG2 are expressed in the liver steatosis. Among them, a high fat diet increases expression of Plin2 and/or FSP27 through activation of peroxisome proliferator-activated receptor γ to develop fatty liver. In this article, recent advances on the role of LD proteins in pathophysiology of fatty liver diseases are summarized.     

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.     

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.     

Adipocyte protein S3-12 coats nascent lipid droplets

Most animals store lipid intracellularly in protein-coated droplets. The protein coat usually contains at least one member of the PAT (perilipin, adipose differentiation-related protein, and TIP47) family. Evidence suggests that PAT proteins control access to the lipid they enclose. The protein S3-12, which has sequence similarity to the PAT proteins, was found in a screen for adipocyte-specific proteins. The adipocyte expression of S3-12 and its similarity to the PAT proteins suggest that S3-12 is involved in adipocyte lipid storage. To test this hypothesis, we supplemented 3T3-L1 adipocytes with fatty acids and assessed the distribution of S3-12 by immunofluorescence microscopy. Prior to fatty acid incubation, S3-12 was distributed diffusely throughout the cytoplasm on punctate structures of heterogeneous size. After 10 min of lipid loading, S3-12 localized to 500-nm structures concentrated at the adipocyte periphery. After longer incubations, S3-12 coated the surface of lipid droplets up to several micrometers in diameter. Initially, these droplets were distinct from those droplets surrounded by perilipin; but by 240 min, most perilipin-coated droplets had some S3-12 on the surface as well. We additionally report that the formation of S3-12-coated droplets 1) required glucose and fatty acids that can be incorporated into triacylglycerol, 2) was blocked by an inhibitor of triacylglycerol synthesis, and 3) was insulin-dependent. This study reports for the first time the early morphological events in the genesis and maturation of adipocyte lipid droplets.     

Structure of a lipid droplet protein; the PAT family member TIP47

The perilipin/ADRP/TIP47 (PAT) proteins localize to the surface of intracellular neutral lipid droplets. Perilipin is essential for lipid storage and hormone regulated lipolysis in adipocytes, and perilipin null mice exhibit a dramatic reduction in adipocyte lipid stores. A significant fraction of the approximately 200 amino acid N-terminal region of the PAT proteins consists of 11-mer helical repeats that are also found in apolipoproteins and other lipid-associated proteins. The C-terminal 60% of TIP47, a representative PAT protein, comprises a monomeric and independently folded unit. The crystal structure of the C-terminal portion of TIP47 was determined and refined at 2.8 A resolution. The structure consists of an alpha/beta domain of novel topology and a four-helix bundle resembling the LDL receptor binding domain of apolipoprotein E. The structure suggests an analogy between PAT proteins and apolipoproteins in which helical repeats interact with lipid while the ordered C-terminal region is involved in protein:protein interactions.     

Immunocytochemical studies on lipid droplet-surface proteins in adrenal cells

Perilipin and ADRP, located on the surface of intracellular lipid droplets, are proposed to be involved in adipocyte lipid metabolism. The aim of the present study was to investigate the effect of PKA and PKC activities on the distribution of perilipin and ADRP in primary cultured adrenal cells, and the role of ERK in PMA- and calphostin C-induced steroidogenesis. Immunofluorescence staining indicated that in addition to p160, a capsular protein of steroidogenic lipid droplets, perilipin and ADRP were localized on the lipid droplet surface. Stimuli such as activation of PKA by db cAMP or inhibition of PKC by calphostin C, which increase corticosterone synthesis in various magnitudes, caused detachment of p160 and perilipin, but not ADRP, from the lipid droplet surface. Activation of PKC by PMA induced increase in corticosterone synthesis, however, it did not affect the distribution of perilipin, p160, or ADRP on the lipid droplet surface, suggesting the presence of mechanisms for promoting sterodiogensis other than causing detachment of lipid droplet surface proteins. We further demonstrated that ERK pathway was involved in PMA-induced steroidogenesis, since PD98059, specific inhibitor of MEK, blocked the increases in steroidogenesis and phosphorylation of ERK caused by PMA, but not by cAMP-PKA. These data indicate that p160, perilipin, and ADRP were all located on the lipid droplet surface in rat adrenal cells. On the basis of its non-responsiveness to lipolytic stimulation, ADRP may be a structural protein of the lipid droplet surface, whereas their immediate response to lipolytic stimuli suggest that perilipin and p160 are functional proteins. PKC regulates adrenal steroidogenesis through ERK cascade, whereas PKA pathway does not involve ERK.     

Heat shock protein 70 is translocated to lipid droplets in rat adipocytes upon heat stimulation

In mammalian cells, lipid storage droplets contain a triacylglycerol and cholesterol ester core surrounded by a phospholipid monolayer into which a number of proteins are imbedded. These proteins are thought to be involved in modulating the formation and metabolic functions of the lipid droplet. In this study, we show that heat stress upregulates several heat shock proteins (Hsps), including Hsp27, Hsp60, Hsp70, Hsp90, and Grp78, in primary and differentiated adipocytes. Immunostaining and immunoblotting data indicate that among the Hsps examined, only Hsp70 is induced to redirect to the lipid droplet surface in heat-stressed adipocytes. The thermal induction of Hsp70 translocation to lipid droplet does not typically happen in a temperature- or time-dependent manner and occurs abruptly at 30-40 min and rapidly achieves a steady state within 60 min after 40 degrees C stress of adipocytes. Though Hsp70 is co-localized with perilipin on the lipid droplets in stressed adipocytes, immunoprecipitation experiments suggest that Hsp70 does not directly interact with perilipin. Alkaline treatments indicate that Hsp70 associates with the droplet surface through non-hydrophobic interactions. We speculate that Hsp70 might noncovalently associate with monolayer microdomains of the lipid droplet in a manner similar to its interaction with lipid bilayer moieties composed of specific fatty acids. As an acute and specific cellular response to the heat stimulation, accumulation of Hsp70 on adipocytes lipid droplets might be involved in stabilizing the droplet monolayer, transferring nascent proteins to the lipid droplets, or chaperoning denatured proteins on the droplet for subsequent refolding.     

Proteomic analysis of proteins associated with lipid droplets of basal and lipolytically stimulated 3T3-L1 adipocytes

Adipocytes hold the body's major energy reserve as triacylglycerols packaged in large lipid droplets. Perilipins, the most abundant proteins on these lipid droplets, play a critical role in facilitating both triacylglycerol storage and hydrolysis. The stimulation of lipolysis by beta-adrenergic agonists triggers rapid phosphorylation of perilipin and translocation of hormone-sensitive lipase to the surfaces of lipid droplets and more gradual fragmentation and dispersion of micro-lipid droplets. Because few lipid droplet-associated proteins have been identified in adipocytes, we isolated lipid droplets from basal and lipolytically stimulated 3T3-L1 adipocytes and identified the component proteins by mass spectrometry. Structural proteins identified in both preparations include perilipin, S3-12, vimentin, and TIP47; in contrast, adipophilin, caveolin-1, and tubulin selectively localized to droplets in lipolytically stimulated cells. Lipid metabolic enzymes identified in both preparations include hormone-sensitive lipase, lanosterol synthase, NAD(P)-dependent steroid dehydrogenase-like protein, acyl-CoA synthetase, long chain family member (ACSL) 1, and CGI-58. 17-beta-Hydroxysteroid dehydrogenase, type 7, was identified only in basal preparations, whereas ACSL3 and 4 and two short-chain reductase/dehydrogenases were identified on droplets from lipolytically stimulated cells. Additionally, both preparations contained FSP27, ribophorin I, EHD2, diaphorase I, and ancient ubiquitous protein. Basal preparations contained CGI-49, whereas lipid droplets from lipolytically stimulated cells contained several Rab GTPases and tumor protein D54. A close association of mitochondria with lipid droplets was suggested by the identification of pyruvate carboxylase, prohibitin, and a subunit of ATP synthase in the preparations. Thus, adipocyte lipid droplets contain specific structural proteins as well as lipid metabolic enzymes; the structural reorganization of lipid droplets in response to the hormonal stimulation of lipolysis is accompanied by increases in the relative mass of several proteins and the recruitment of additional proteins.     

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

Lipid droplets in adipocytes serve as the principal long-term energy storage depot of animals. There is increasing recognition that lipid droplets are not merely a static neutral lipid storage site, but in fact dynamic and multi-functional organelles. Structurally, lipid droplet consists of a neutral lipid core surrounded by a phospholipid monolayer and proteins embedded in or bound to the phospholipid layer. Proteins on the surface of lipid droplets are crucial to droplet structure and dynamics. To understand the lipid droplet-associated proteome of primary adipocyte with a large central lipid droplet, lipid droplets of white adipose tissue from C57BL/6 mice were isolated. And the proteins were extracted and analyzed by liquid chromatography coupled with tandem mass spectrometry. A total of 193 proteins including 73 previously unreported proteins were identified. Furthermore, the isotope-coded affinity tags (ICAT) was used to compare the difference of lipid droplet-associated proteomes between the normal lean and the high-fat diet-induced obese C57BL/6 mice. Of 23 proteins quantified by ICAT analysis, 3 proteins were up-regulated and 4 proteins were down-regulated in the lipid droplets of adipose tissue from the obese mice. Importantly, two structural proteins of lipid droplets, perilipin A and vimentin, were greatly reduced in the lipid droplets of the adipose tissue from the obese mice, implicating reduced protein machinery for lipid droplet stability.     

MLDP, a novel PAT family protein localized to lipid droplets and enriched in the heart, is regulated by peroxisome proliferator-activated receptor alpha

Cytosolic lipid droplets (LDs) are multifunctional organelles that exist in all types of eukaryotic cells and control lipid homeostasis. In mammalian cells LDs contain a class of proteins in their surface layers that share a homologous sequence called the PAT domain, including perilipin, adipose differentiation-related protein (ADRP), a tail-interacting protein of 47 kDa (TIP47), and S3-12, which are distributed tissue- or cell type-selectively. Expression in some cases is regulated by peroxisome proliferator-activated receptors (PPARs). In this study we identified a new PAT family member named MLDP (myocardial LD protein) in a murine cDNA data base and showed the mRNA and protein to be highly enriched in the heart and also expressed at lower levels in the liver and adrenals. Upon subcellular fractionation, a substantial amount of MLDP was detected in the top fraction enriched with LDs. Furthermore, overexpressed MLDP tagged with green fluorescent protein accumulated at the surfaces of LDs and co-localized with perilipin and ADRP. Deletion analysis demonstrated the N-terminal region containing a PAT-1 domain and the following 33-mer domain to be required for targeting of MLDP to LDs. MLDP was found to be up-regulated at both mRNA and protein levels in the heart and liver by a selective ligand for PPARalpha, Wy14,643, but not in PPARalpha knock-out mice. MLDP expression was also increased upon fasting in parallel with ADRP. These results indicate that MLDP is a bona fide new PAT family member localized in LDs. Its expression depends on the physiological conditions and the action of PPARalpha.