Translocation of hormone-sensitive lipase and perilipin upon lipolytic stimulation of rat adipocytes

Adipocyte lipolysis was compared with hormone-sensitive lipase (HSL)/perilipin subcellular distribution and perilipin phosphorylation using Western blot analysis. Under basal conditions, HSL resided predominantly in the cytosol and unphosphorylated perilipin upon the lipid droplet. Upon lipolytic stimulation of adipocytes isolated from young rats with the beta-adrenergic agonist, isoproterenol, HSL translocated from the cytosol to the lipid droplet, but there was no movement of perilipin from the droplet to the cytosol; however, perilipin phosphorylation was observed. By contrast, upon lipolytic stimulation and perilipin phosphorylation in cells from more mature rats, there was no HSL translocation but a significant movement of perilipin away from the lipid droplet. Adipocytes from younger rats had markedly greater rates of lipolysis than those from the older rats. Thus high rates of lipolysis require translocation of HSL to the lipid droplet and translocation of HSL and perilipin can occur independently of each other. A loss of the ability to translocate HSL to the lipid droplet probably contributes to the diminished lipolytic response to catecholamines with age.     

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.     

Perilipin A is essential for the translocation of hormone-sensitive lipase during lipolytic activation

Akey step in lipolytic activation of adipocytes is the translocation of hormone-sensitive lipase (HSL) from the cytosol to the surface of the lipid storage droplet. Adipocytes from perilipin-null animals have an elevated basal rate of lipolysis compared with adipocytes from wild-type mice, but fail to respond maximally to lipolytic stimuli. This defect is downstream of the beta-adrenergic receptor-adenylyl cyclase complex. Now, we show that HSL is basally associated with lipid droplet surfaces at a low level in perilipin nulls, but that stimulated translocation from the cytosol to lipid droplets is absent in adipocytes derived from embryonic fibroblasts of perilipin-null mice. We have also reconstructed the HSL translocation reaction in the nonadipocyte Chinese hamster ovary cell line by introduction of GFP-tagged HSL with and without perilipin A. On activation of protein kinase A, HSL-GFP translocates to lipid droplets only in cells that express fully phosphorylatable perilipin A, confirming that perilipin is required to elicit the HSL translocation reaction. Moreover, in Chinese hamster ovary cells that express both HSL and perilipin A, these two proteins cooperate to produce a more rapidly accelerated lipolysis than do cells that express either of these proteins alone, indicating that lipolysis is a concerted reaction mediated by both protein kinase A-phosphorylated HSL and perilipin A.     

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.     

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.     

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.     

Functional studies on native and mutated forms of perilipins. A role in protein kinase A-mediated lipolysis of triacylglycerols

Perilipin A coats the lipid storage droplets in adipocytes and is polyphosphorylated by protein kinase A (PKA); the fact that PKA activates lipolysis in adipocytes suggests a role for perilipins in this process. To assess whether perilipins participate directly in PKA-mediated lipolysis, we have expressed constructs coding for native and mutated forms of the two major splice variants of the perilipin gene, perilipins A and B, in Chinese hamster ovary fibroblasts. Perilipins localize to lipid droplet surfaces and displace the adipose differentiation-related protein that normally coats the droplets in these cells. Perilipin A inhibits triacylglycerol hydrolysis by 87% when PKA is quiescent, but activation of PKA and phosphorylation of perilipin A engenders a 7-fold lipolytic activation. Mutation of PKA sites within the N-terminal region of perilipin abrogates the PKA-mediated lipolytic response. In contrast, perilipin B exerts only minimal protection against lipolysis and is unresponsive to PKA activation. Since Chinese hamster ovary cells contain no PKA-activated lipase, we conclude that the expression of perilipin A alone is sufficient to confer PKA-mediated lipolysis in these cells. Moreover, the data indicate that the unique C-terminal portion of perilipin A is responsible for its protection against lipolysis and that phosphorylation at the N-terminal PKA sites attenuates this protective effect.     

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。     

The phosphorylation of serine 492 of perilipin a directs lipid droplet fragmentation and dispersion

Perilipin A is a key regulator of triacylglycerol storage and hydrolysis in adipocytes; phosphorylation of perilipin A by protein kinase A facilitates maximal lipolysis. Chronic stimulation of lipolysis in 3T3-L1 adipocytes causes large perinuclear lipid droplets to fragment into myriad dispersed perilipin A-covered microlipid droplets. In cultured fibroblasts stably expressing ectopic perilipin A, clustered lipid droplets disperse throughout the cytoplasm upon incubation of the cells with forskolin and isobutylmethylxanthine (IBMX) to elevate levels of cAMP and activate protein kinase A, mirroring events observed in adipocytes. Furthermore, diethylum-belliferyl phosphate inhibits stimulated lipolysis but not the dispersion of lipid droplets, suggesting that products of lipolysis are not required for this remodeling process. We hypothesized that protein kinase A-mediated phosphorylation of perilipin A triggers the remodeling of lipid droplets. The mutation of serine 492 of perilipin A to alanine prevented the dispersion of clustered lipid droplets in fibroblasts stably expressing the mutated perilipin upon incubation with forskolin and IBMX. In contrast, the substitution of serines 81, 222, 276, or 433 with alanine, either singly or in combinations, did not affect the protein kinase A-mediated remodeling of lipid droplets. Interestingly, substitution of serines 433, 492, and 517 of perilipin A with glutamic acid residues blocked the dispersion of clustered lipid droplets in cells incubated with forskolin and IBMX, indicating that the addition of a negative charge does not mimic a phosphate group. We conclude that protein kinase A-mediated phosphorylation of serine 492 of perilipin A drives the fragmentation and dispersion of lipid droplets.     

CGI-58 facilitates lipolysis on lipid droplets but is not involved in the vesiculation of lipid droplets caused by hormonal stimulation

A lipid droplet (LD)-associated protein, perilipin, is a critical regulator of lipolysis in adipocytes. We previously showed that Comparative Gene Identification-58 (CGI-58), a product of the causal gene of Chanarin-Dorfman syndrome, interacts with perilipin on LDs. In this study, we investigated the function of CGI-58 using RNA interference. Notably, CGI-58 knockdown caused an abnormal accumulation of LDs in both 3T3-L1 preadipocytes and Hepa1 hepatoma cells. CGI-58 knockdown did not influence the differentiation of 3T3-L1 adipocytes but reduced the activity of both basal and cAMP-dependent protein kinase-stimulated lipolysis. In vitro studies showed that CGI-58 itself does not have lipase/esterase activity, but it enhanced the activity of adipose triglyceride lipase. Upon lipolytic stimulation, endogenous CGI-58 was rapidly dispersed from LDs into the cytosol along with small particulate structures. This shift in localization depends on the phosphorylation of perilipin, because phosphorylated perilipin lost the ability to bind CGI-58. During lipolytic activation, LDs in adipocytes vesiculate into micro-LDs. Using coherent anti-Stokes Raman scattering microscopy, we pursued the formation of micro-LDs in single cells, which seemed to occur in cytoplasmic regions distant from the large central LDs. CGI-58 is not required for this process. Thus, CGI-58 facilitates lipolysis in cooperation with perilipin and other factors, including lipases.     

Calyculin and okadaic acid promote perilipin phosphorylation and increase lipolysis in primary rat adipocytes

Lipolysis is primarily regulated by protein kinase A (PKA), which phosphorylates perilipin and hormone-sensitive lipase (HSL), and causes translocation of HSL from cytosol to lipid droplets in adipocytes. Perilipin coats lipid droplet surface and assumes to prevent lipase access to triacylglycerols, thus inhibiting basal lipolysis; phosphorylated perilipin facilitates lipolysis on PKA activation. Here, we induced lipolysis in primary rat adipocytes by inhibiting protein serine/threonine phosphatase with specific inhibitors, okadaic acid and calyculin. The incubation with calyculin promotes incorporation of 32Pi into perilipins, thus, confirming that perilipin is hyperphosphorylated. The lipolysis response to calyculin is gradually accompanied by increased accumulation of phosphorylated perilipin A in a concentration- and time-responsive manner. When perilipin phosphorylation is abrogated by the addition of N-ethylmaleimide, lipolysis ceases. Different from a considerable translocation of HSL upon PKA activation with isoproterenol, calyculin does not alter HSL redistribution in primary or differentiated adipocytes, as confirmed by both immunostaining and immunoblotting. Thus, we suggest that inhibition of the phosphatase by calyculin activates lipolysis via promoting perilipin phosphorylation rather than eliciting HSL translocation in adipocytes. Further, we show that when the endogenous phosphatase is inhibited by calyculin, simultaneous PKA activation with isoproterenol converts most of the perilipin to the hyperphosphorylated species, and induces enhanced lipolysis. Apparently, as PKA phosphorylates perilipin and stimulates lipolysis, the phosphatase acts to dephosphorylate perilipin and attenuate lipolysis. This suggests a two-step strategy governed by a kinase and a phosphatase to modulate the steady state of perilipin phosphorylation and hence the lipolysis response to hormonal stimulation.     

Activation of the lipid droplet controls the rate of lipolysis of triglycerides in the insect fat body

The hydrolysis of triglyceride (TG) stored in the lipid droplets of the insect fat body is under hormonal regulation by the adipokinetic hormone (AKH), which triggers a rapid activation cAMP-dependent kinase cascade (protein kinase A (PKA)). The role of phosphorylation on two components of the lipolytic process, the TG-lipase and the lipid droplet, was investigated in fat body adipocytes. The activity of purified TG-lipase determined using in vivo TG-radiolabeled lipid droplets was unaffected by the phosphorylation of the lipase. However, the activity of purified lipase was 2.4-fold higher against lipid droplets isolated from hormone-stimulated fat bodies than against lipid droplets isolated from unstimulated tissue. In vivo stimulation of lipolysis promotes a rapid phosphorylation of a lipid droplet protein with an apparent mass of 42-44 kDa. This protein was identified as "Lipid Storage Droplet Protein 1" (Lsdp1). In vivo phosphorylation of this protein reached a peak approximately 10 min after the injection of AKH. Supporting a role of Lsdp1 in lipolysis, maximum TG-lipase activity was also observed with lipid droplets isolated 10 min after hormonal stimulation. The activation of lipolysis was reconstituted in vitro using purified insect PKA and TG-lipase and lipid droplets. In vitro phosphorylation of lipid droplets catalyzed by PKA enhanced the phosphorylation of Lsdp1 and the lipolytic rate of the lipase, demonstrating a prominent role PKA and protein phosphorylation on the activation of the lipid droplets. AKH-induced changes in the properties of the substrate do not promote a tight association of the lipase with the lipid droplets. It is concluded that the lipolysis in fat body adipocytes is controlled by the activation of the lipid droplet. This activation is achieved by PKA-mediated phosphorylation of the lipid droplet. Lsdp1 is the main target of PKA, suggesting that this protein is a major player in the activation of lipolysis in insects.     

Perilipin targets a novel pool of lipid droplets for lipolytic attack by hormone-sensitive lipase

Adipocytes serve as the principal energy reservoir of the body; however, the subcellular organization of the machinery regulating lipid trafficking and metabolism is poorly understood. Mobilization of stored triglyceride is thought be controlled by interactions among intracellular lipases and proteins that coat lipid storage droplets. A major limitation of previous studies of hormone-mediated lipolysis, however, is the use of cultured model adipocytes whose three-dimensional architectures do not resemble those in real adipose tissue. To address this limitation, we investigated the intracellular targeting of perilipin, a major lipid coat protein, and hormone-sensitive lipase in three preparations that exhibit more appropriate morphologies: 3T3-L1 adipocytes grown in three-dimensional matrix, dissociated mature adipocytes from mouse adipose tissue, and adipocytes within intact fat pads. High resolution imaging of native and fluorescently tagged proteins indicate that: 1) perilipin preferentially targets a special class of peripheral lipid storage droplets, but not the major or central lipid storage droplets, 2) the peripheral droplets are the sites of attack by hormone-sensitive lipase, and 3) perilipin and hormone-sensitive lipase are continuously colocalized following lipolytic activation. These results indicate that in white adipose tissue, lipolysis takes place in a specialized subcellular domain that is distinct from the major lipid storage site and is defined by perilipin.     

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。     

Sterol carrier protein-2 expression modulates protein and lipid composition of lipid droplets

Despite the critical role lipid droplets play in maintaining energy reserves and lipid stores for the cell, little is known about the regulation of the lipid or protein components within the lipid droplet. Although immunofluorescence of intact cells as well as Western analysis of isolated lipid droplets revealed that sterol carrier protein-2 (SCP-2) was not associated with lipid droplets, SCP-2 expression significantly altered the structure of the lipid droplet. First, the targeting of fatty acid and cholesterol to the lipid droplets was significantly decreased. Second, the content of several proteins important for lipid droplet function was differentially increased (perilipin A), reduced severalfold (adipose differentiation-related protein (ADRP), vimentin), or almost completely eliminated (hormone-sensitive lipase and proteins >93 kDa) in the isolated lipid droplet. Third, the distribution of lipids within the lipid droplets was significantly altered. Double labeling of cells with 12-(N-methyl)-N-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl) amino]-octadecanoic acid (NBD-stearic acid) and antisera to ADRP showed that 70, 24, and 13% of lipid droplets contained ADRP, NBD-stearic acid, or both, respectively. SCP-2 expression decreased the level of ADRP in the lipid droplet but increased the proportion wherein ADRP and NBD-stearic acid colocalized by 3-fold. SCP-2 expression also decreased the lipid droplet fatty acid and cholesterol mass (nmol/mg protein) by 5.2- and 6.6-fold, respectively. Finally, SCP-2 expression selectively altered the pattern of esterified fatty acids in favor of polyunsaturated fatty acids within the lipid droplet. Displacement studies showed differential binding affinity of ADRP for cholesterol and fatty acids. These data suggested that SCP-2 and ADRP play a significant role in regulating fatty acid and cholesterol targeting to lipid droplets as well as in determining their lipid and protein components.     

Optic atrophy 1 is an A-kinase anchoring protein on lipid droplets that mediates adrenergic control of lipolysis

Adrenergic stimulation of adipocytes yields a cAMP signal that activates protein kinase A (PKA). PKA phosphorylates perilipin, a protein localized on the surface of lipid droplets that serves as a gatekeeper to regulate access of lipases converting stored triglycerides to free fatty acids and glycerol in a phosphorylation-dependent manner. Here, we report a new function for optic atrophy 1 (OPA1), a protein known to regulate mitochondrial dynamics, as a dual-specificity A-kinase anchoring protein associated with lipid droplets. By a variety of protein interaction assays, immunoprecipitation and immunolocalization experiments, we show that OPA1 organizes a supramolecular complex containing both PKA and perilipin. Furthermore, by a combination of siRNA-mediated knockdown, reconstitution experiments using full-length OPA1 with or without the ability to bind PKA or truncated OPA1 fused to a lipid droplet targeting domain and cellular delivery of PKA anchoring disruptor peptides, we demonstrate that OPA1 targeting of PKA to lipid droplets is necessary for hormonal control of perilipin phosphorylation and lipolysis.     

SREBP Activation by Antipsychotic- and Antidepressant-Drugs in Cultured Human Liver Cells: Relevance for Metabolic Side-Effects?

Despite the critical roles of intracellular lipid droplets (LDs) in lipid storage and metabolism, little is known about the molecular mechanisms of their functions. Several protein components associated with the surface of LDs have been identified. A major one is perilipin in adipocytes and steroidogenic cells, whereas ADRP in most other cell types. They are loosely grouped as a small protein family sharing a common N-terminal motif, called the PAT domain. Perilipin regulates the breakdown of triacylglycerol in LDs via its phosphorylation. ADRP is characterized as a fatty acid binding protein and involved in lipid uptake and LD formation. For examining the functions of perilipin and ADRP at the molecular level, we performed yeast two-hybrid screening in this study, to find their functional partners. We identified CGI-58, a product of the causal gene of Chanarin-Dorfman syndrome (CDS), as an interactor for both perilipin and ADRP. Specific interaction between CGI-58 and perilipin was confirmed in a GST-pulldown assay and supported by fluorescence microscopic analyses. We further demonstrated that CGI-58 is principally located at the surface of LDs in 3T3-L1 cells, together with perilipin, and its expression is upregulated upon stimulation for adipocyte differentiation. Other than CGI-58, we also identified in yeast two-hybrid screening HSP86 and D52 tumor proteins as binding partners of perilipin, and IRG-47 of ADRP. These factors might be cooperated with perilipin and ADRP, and hence involved in membrane dynamics of LDs as well as the regulation of lipolysis on the surface of LDs.     

Unique regulation of adipose triglyceride lipase (ATGL) by perilipin 5, a lipid droplet-associated protein

Lipolysis is a critical metabolic pathway contributing to energy homeostasis through degradation of triacylglycerides stored in lipid droplets (LDs), releasing fatty acids. Neutral lipid lipases act at the oil/water interface. In mammalian cells, LD surfaces are coated with one or more members of the perilipin protein family, which serve important functions in regulating lipolysis. We investigated mechanisms by which three perilipin proteins control lipolysis by adipocyte triglyceride lipase (ATGL), a key lipase in adipocytes and non-adipose cells. Using a cell culture model, we examined interactions of ATGL and its co-lipase CGI-58 with perilipin 1 (perilipin A), perilipin 2 (adipose differentiation-related protein), and perilipin 5 (LSDP5) using multiple techniques as follows: anisotropy Forster resonance energy transfer, co-immunoprecipitation, [(32)P]orthophosphate radiolabeling, and measurement of lipolysis. The results show that ATGL interacts with CGI-58 and perilipin 5; the latter is selectively expressed in oxidative tissues. Both proteins independently recruited ATGL to the LD surface, but with opposite effects; interaction of ATGL with CGI-58 increased lipolysis, whereas interaction of ATGL with perilipin 5 decreased lipolysis. In contrast, neither perilipin 1 nor 2 interacted directly with ATGL. Activation of protein kinase A (PKA) increased [(32)P]orthophosphate incorporation into perilipin 5 by 2-fold, whereas neither ATGL nor CGI-58 was labeled under the incubation conditions. Cells expressing both ectopic perilipin 5 and ATGL showed a 3-fold increase in lipolysis following activation of PKA. Our studies establish perilipin 5 as a novel ATGL partner and provide evidence that the protein composition of perilipins at the LD surface regulates lipolytic activity of ATGL.     

Perilipin A mediates the reversible binding of CGI-58 to lipid droplets in 3T3-L1 adipocytes

Perilipins, the major structural proteins coating the surfaces of mature lipid droplets of adipocytes, play an important role in the regulation of triacylglycerol storage and hydrolysis. We have used proteomic analysis to identify CGI-58, a member of the alpha/beta-hydrolase fold family of enzymes, as a component of lipid droplets of 3T3-L1 adipocytes. CGI-58 mRNA is highly expressed in adipose tissue and testes, tissues that also express perilipins, and at lower levels in liver, skin, kidney, and heart. Both endogenous CGI-58 and an ectopic CGI-58-GFP chimera show diffuse cytoplasmic localization in 3T3-L1 preadipocytes, but localize almost exclusively to the surfaces of lipid droplets in differentiated 3T3-L1 adipocytes. The localization of endogenous CGI-58 was investigated in 3T3-L1 cells stably expressing mutated forms of perilipin using microscopy. CGI-58 binds to lipid droplets coated with perilipin A or mutated forms of perilipin with an intact C-terminal sequence from amino acid 382 to 429, but not to lipid droplets coated with perilipin B or mutated perilipin A lacking this sequence. Immunoprecipitation studies confirmed these findings, but also showed co-precipitation of perilipin B and CGI-58. Remarkably, activation of cAMP-dependent protein kinase by the incubation of 3T3-L1 adipocytes with isoproterenol and isobutylmethylxanthine disperses CGI-58 from the surfaces of lipid droplets to a cytoplasmic distribution. This shift in subcellular localization can be reversed by the addition of propanolol to the culture medium. Thus, CGI-58 binds to perilipin A-coated lipid droplets in a manner that is dependent upon the metabolic status of the adipocyte and the activity of cAMP-dependent protein kinase.