Perilipins are associated with cholesteryl ester droplets in steroidogenic adrenal cortical and Leydig cells

Leptin induces nitric oxide-mediated inhibition of lipolysis and glyceroneogenesis in rat white adipose tissue

Leptin is secreted by white adipose tissue (WAT) and induces lipolysis and nonesterified fatty acid (NEFA) oxidation. During lipolysis, NEFA efflux is the result of triglyceride breakdown, NEFA oxidation, and re-esterification via glyceroneogenesis. Leptin's effects on glyceroneogenesis remain unexplored. We investigated the effect of a long-term treatment with leptin at a physiological concentration (10 μg/L) on lipolysis and glyceroneogenesis in WAT explants and analyzed the underlying mechanisms. Exposure of rat WAT explants to leptin for 2 h resulted in increased NEFA and glycerol efflux. However, a longer treatment with leptin (18 h) did not affect NEFA release and reduced glycerol output. RT-qPCR showed that leptin significantly downregulated the hormone-sensitive lipase (HSL), cytosolic phosphoenolpyruvate carboxykinase (Pck1), and PPARγ genes. In agreement with its effect on mRNA, leptin also decreased the levels of PEPCK-C and HSL proteins. Glyceroneogenesis, monitored by [1-(14) C] pyruvate incorporation into lipids, was reduced. Because leptin increases nitric oxide (NO) production in adipocytes, we explored the role of NO in the leptin signaling pathway. Pretreatment of explants with the NO synthase inhibitor Nω-nitro-l-arginine methyl ester eliminated the effect of leptin on lipolysis, glyceroneogenesis, and expression of the HSL, Pck1, and PPARγ genes. The NO donor S-nitroso-N-acetyl-DL penicillamine mimicked leptin effects, thus demonstrating the role of NO in these pathways. The inverse time-dependent action of leptin on WAT is consistent with a process that limits NEFA re-esterification and energy storage while reducing glycerol release, thus preventing hypertriglyceridemia.

Identification of perilipin-2 as a lipid droplet protein regulated in oocytes during maturation

Lipid droplet proteins regulate the storage and utilisation of intracellular lipids. Evidence is emerging that oocyte lipid utilisation impacts embryo development, but lipid droplet proteins have not been studied in oocytes. The aim of the present study was to characterise the size and localisation of lipid droplets in mouse oocytes during the periovulatory period and to identify lipid droplet proteins as potential biomarkers of oocyte lipid content. Oocyte lipid droplets, visualised using a novel method of staining cumulus-oocyte complexes (COCs) with BODIPY 493/503, were small and diffuse in oocytes of preovulatory COCs, but larger and more centrally located after maturation in response to ovulatory human chorionic gonadotrophin (hCG) in vivo, or FSH + epidermal growth factor in vitro. Lipid droplet proteins perilipin, perilipin-2, cell death-inducing DNA fragmentation factor 45-like effector (CIDE)-A and CIDE-B were detected in the mouse ovary by immunohistochemistry, but only perilipin-2 was associated with lipid droplets in the oocyte. In COCs, perilipin-2 mRNA and protein increased in response to ovulatory hCG. IVM failed to induce perilipin-2 mRNA, yet oocyte lipid content was increased in this context, indicating that perilipin-2 is not necessarily reflective of relative oocyte lipid content. Thus, perilipin-2 is a lipid droplet protein in oocytes and its induction in the COC concurrent with dynamic reorganisation of lipid droplets suggests marked changes in lipid utilisation during oocyte maturation.

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.

Adoption of PERILIPIN as a unifying nomenclature for the mammalian PAT-family of intracellular lipid storage droplet proteins

The PAT family of proteins has been identified in eukaryotic species as diverse as vertebrates, insects, and amebazoa. These proteins share a highly conserved sequence organization and avidity for the surfaces of intracellular, neutral lipid storage droplets. The current nomenclature of the various members lacks consistency and precision, deriving more from historic context than from recognition of evolutionary relationship and shared function. In consultation with the Mouse Genomic Nomenclature Committee, the Human Genome Organization Genomic Nomenclature Committee, and conferees at the 2007 FASEB Conference on Lipid Droplets: Metabolic Consequences of the Storage of Neutral Lipids, we have established a unifying nomenclature for the gene and protein family members. Each gene member will incorporate the root term PERILIPIN (PLIN), the founding gene of the PAT family, with the different genes/proteins numbered sequentially.

Perilipin is present in islets of Langerhans and protects against lipotoxicity when overexpressed in the beta-cell line INS-1

Lipids have been shown to play a dual role in pancreatic beta-cells: a lipid-derived signal appears to be necessary for glucose-stimulated insulin secretion, whereas lipid accumulation causes impaired insulin secretion and apoptosis. The ability of the protein perilipin to regulate lipolysis prompted an investigation of the presence of perilipin in the islets of Langerhans. In this study evidence is presented for perilipin expression in rat, mouse, and human islets of Langerhans as well as the rat clonal beta-cell line INS-1. In rat and mouse islets, perilipin was verified to be present in beta-cells. To examine whether the development of lipotoxicity could be prevented by manipulating the conditions for lipid storage in the beta-cell, INS-1 cells with adenoviral-mediated overexpression of perilipin were exposed to lipotoxic conditions for 72 h. In cells exposed to palmitate, perilipin overexpression caused increased accumulation of triacylglycerols and decreased lipolysis compared with control cells. Whereas glucose-stimulated insulin secretion was retained after palmitate exposure in cells overexpressing perilipin, it was completely abolished in control beta-cells. Thus, overexpression of perilipin appears to confer protection against the development of beta-cell dysfunction after prolonged exposure to palmitate by promoting lipid storage and limiting lipolysis.

TIP47, a lipid cargo protein involved in macrophage triglyceride metabolism

OBJECTIVE

Uptake of lipids by macrophages (MPhi) leads to lipid droplet accumulation and foam cell formation. The PAT family proteins are implicated in lipid droplet formation, but the precise function of the 47-kDa tail interacting protein (TIP47), a member of this family, is poorly defined. The present study was performed to determine the function of TIP47 in MPhi lipid metabolism.

METHODS AND RESULTS

Freeze-fracture cytochemistry demonstrates that TIP47 is present in the plasma membrane of MPhi and is aggregated into clusters when the cells are incubated with oleate. Suppression of adipophilin levels using siRNA knockdown leads to migration of TIP47 from a cytoplasmic pool to the lipid droplet. Further, reduction of TIP47 decreases triglyceride levels, whereas raising TIP47 levels by expression of EGFP-TIP47 shows the opposite effect.

CONCLUSION

Our results show that the TIP47 protein levels directly correlate with triglyceride levels. We propose that TIP47 may act as a carrier protein for free fatty acids and in this way participates in conversion of MPhi into foam cells.

Expression of adipose differentiation-related protein (ADRP) and perilipin in macrophages infected with Mycobacterium leprae

Mycobacterium leprae survives and replicates within a lipid droplet stored in the enlarged phagosome of histiocytes, a typical feature of lepromatous leprosy that is thought to be an important nutrient source for the bacillus. However, the underlying mechanisms by which lipids accumulate within phagosomes remain unclear. Recently, it was revealed that the lipid droplet-associated proteins, including ADRP and perilipin, play essential roles in lipid accumulation in adipocytes or macrophages. Therefore, we attempted to examine the role of these proteins in leprosy pathogenesis. ADRP and perilipin localized to the phagosomal membrane, which contains M. leprae in skin biopsy specimens of lepromatous leprosy. ADRP expression was transiently increased after phagocytosis in THP-1 cells. However, high levels of ADRP expression persisted only when live M. leprae, but not dead bacilli or latex beads, was added. Furthermore, although peptidoglycan, a Toll-like receptor 2 ligand, suppressed the expression levels of ADRP and perilipin, M. leprae infection inhibited this suppression. These results suggest that live M. leprae has the ability to actively induce and support ADRP/perilipin expression to facilitate the accumulation of lipids within the phagosome and to further maintain a suitable environment for the intracellular survival within the macrophage.

Perilipin A and the control of triacylglycerol metabolism

Perilipin A is the most abundant protein associated with the lipid droplets of adipocytes and functions to control both basal and stimulated lipolysis. Under basal or fed conditions, perilipin A shields stored triacylglycerols from cytosolic lipases, thus promoting triacylglycerol storage. When catecholamines bind to cell surface receptors to initiate signals that activate cAMP-dependent protein kinase (PKA), phosphorylated perilipin A facilitates maximal lipolysis. Mutagenesis studies have revealed that central sequences of moderately hydrophobic amino acids are required to target nascent perilipin A to lipid droplets and provide an anchor into the hydrophobic environment of lipid droplets. Sequences of amino acids in the unique carboxyl terminus of perilipin A and those in amino terminal sequences flanking the first hydrophobic stretch are required for the barrier function of perilipin A in promoting triacylglycerol storage. Site-directed mutagenesis studies of serine residues within six PKA consensus sites of perilipin A reveal functions for phosphorylation of at least three of the sites. Phosphorylation of one or more of the serines within three amino terminal PKA sites is required to facilitate hormone-sensitive lipase access to lipid substrates. Phosphorylation of serines within two carboxyl terminal sites is also required for maximal lipolysis. Phosphorylation of serine 492 (site 5) triggers a massive remodeling of lipid droplets, whereby large peri-nuclear lipid droplets fragment into myriad lipid micro-droplets that scatter throughout the cytoplasm. We hypothesize that perilipin A binds accessory proteins to provide assistance in carrying out these functions.

Identification of differentially expressed genes in a porcine in vivo model of adipogenesis using suppression subtractive hybridization

Although they provide valuable information, in vitro models of adipocyte development often require high doses of hormones and growth factors, which may influence gene expression and adipocyte differentiation patterns. To overcome these problems, a novel in vivo model of adipose tissue development was used to characterize genes involved in adipogenesis. The suppression subtractive hybridization technique was used to identify genes showing differential expression between the adipose tissue of a day 90 gestating sow, which is enriched in adipocytes, and day 90 fetal adipose tissue, which is enriched in preadipocytes. A total of 149 expressed sequence tags corresponding to identified genes and tentative consensus sequences emerged. Thirty-seven clones matched expressed sequence tags or genomic DNA sequences and six novel sequences were also identified. Adipogenesis-related genes were identified, many of which have never been reported to be expressed in mammalian adipose tissue, and may play a role in regulation of adipose tissue differentiation. Validation of differentially expressed genes was confirmed for perilipin, monocyte to macrophage differentiation-associated, myocilin, paraoxonase 3, stearoyl-CoA desaturase, angiotensinogen and adiponectin genes using real-time RT-PCR.

FSP27 contributes to efficient energy storage in murine white adipocytes by promoting the formation of unilocular lipid droplets

White adipocytes are unique in that they contain large unilocular lipid droplets that occupy most of the cytoplasm. To identify genes involved in the maintenance of mature adipocytes, we expressed dominant-negative PPARgamma in 3T3-L1 cells and performed a microarray screen. The fat-specific protein of 27 kDa (FSP27) was strongly downregulated in this context. FSP27 expression correlated with induction of differentiation in cultured preadipocytes, and the protein localized to lipid droplets in murine white adipocytes in vivo. Ablation of FSP27 in mice resulted in the formation of multilocular lipid droplets in these cells. Furthermore, FSP27-deficient mice were protected from diet-induced obesity and insulin resistance and displayed an increased metabolic rate due to increased mitochondrial biogenesis in white adipose tissue (WAT). Depletion of FSP27 by siRNA in murine cultured white adipocytes resulted in the formation of numerous small lipid droplets, increased lipolysis, and decreased triacylglycerol storage, while expression of FSP27 in COS cells promoted the formation of large lipid droplets. Our results suggest that FSP27 contributes to efficient energy storage in WAT by promoting the formation of unilocular lipid droplets, thereby restricting lipolysis. In addition, we found that the nature of lipid accumulation in WAT appears to be associated with maintenance of energy balance and insulin sensitivity.

Retinyl ester homeostasis in the adipose differentiation-related protein-deficient retina

The retinal pigmented epithelium (RPE) plays an essential role in vision, including storing and converting retinyl esters of the visual chromophore, 11-cis-retinal. Retinyl ester storage structures (RESTs), specialized lipid droplets within the RPE, take up retinyl esters synthesized in the endoplasmic reticulum. Here we report studies of mice lacking exons 2 and 3 of the gene encoding adipose differentiation-related protein (Adfp), a structural component of RESTs. We found that dark adaptation was slower in Adfp(Delta2-3/Delta2-3) than in Adfp+/+ mice and that Adfp(Delta2-3/Delta2-3) mice had consistently delayed clearances of all-trans-retinal and all-trans-retinol from rod photoreceptor cells. Two-photon microscopy revealed aberrant trafficking of all-trans-retinyl esters in the RPE of Adfp(Delta2-3/Delta2-3) mice, a problem caused by abnormal maintenance of RESTs in the dark-adapted state. Retinyl ester accumulation was also reduced in Adfp(Delta2-3/Delta2-3) as compared with Adfp+/+ mice. These observations suggest that Adfp plays a unique role in vision by maintaining proper storage and trafficking of retinoids within the eye.

Postprandial triacylglycerol metabolism is modified by the presence of genetic variation at the perilipin (PLIN) locus in 2 white populations

BACKGROUND

Several perilipin (PLIN) polymorphic sites have been studied for their potential use as markers for obesity and the metabolic syndrome.

OBJECTIVE

We aimed to examine whether the presence of polymorphisms at the perilipin (PLIN) locus (PLIN1, 6209T-->C; PLIN4, 11482G-->A; PLIN5, 13041A-->G; and PLIN6, 14995A-->T) influence postprandial lipoprotein metabolism in 2 white populations.

DESIGN

Eighty-eight healthy Spanish men and 271 healthy US subjects (men and women) underwent an oral-fat-load test in 2 independent studies. Blood samples were taken in the fasting state and during the postprandial phase at regular intervals. Total cholesterol and triacylglycerol and triacylglycerol in triacylglycerol-rich lipoproteins (TRL, large and small) were measured.

RESULTS

Carriers of the minor C allele at the PLIN1 variant displayed lower postprandial concentrations of large-TRL triacylglycerol (Spanish subjects: P = 0.024; US subjects: P = 0.005) than did subjects carrying the T/T genotype. The same pattern was observed in the Spanish population at the PLIN4 locus (P = 0.015), and both SNPs were in strong linkage disequilibrium. In both populations, subjects carrying the minor C and A alleles at PLIN1 and PLIN4, respectively, had significantly lower postprandial concentrations of plasma triacylglycerol (P < 0.05) and lower concentrations of small-TRL triacylglycerol than did those who were homozygous for the major alleles at PLIN1 and PLIN4 (Spanish subjects: P = 0.020 and 0.008, respectively; US subjects: P = 0.021 and 0.035, respectively).

CONCLUSION

These 2 studies suggest that the presence of the minor C and A alleles at PLIN1 and PLIN4, respectively, are associated with a lower postprandial response that may result in lower atherogenic risk for these persons.

Evidence of a possible role for Lys-gamma3-MSH in the regulation of adipocyte function

Lys-gamma3-MSH is a melanocortin peptide derived from the C-terminal of the 16 kDa fragment of POMC. The physiological role of Lys-gamma3-MSH is unclear, although it has previously been shown that, although not directly steroidogenic, it can act to potentiate the steroidogenic response of adrenal cortical cells to ACTH. This synergistic effect appears to be correlated with an ability to increase the activity of hormone sensitive lipase (HSL) and therefore the rate of cholesterol ester hydrolysis. Ligand binding studies have suggested that high-affinity binding sites for Lys-gamma3-MSH exist in the adrenal gland and a number of other rat tissues that express HSL, including adipose, skeletal muscle and testes. To investigate the hypothesis that Lys-gamma3-MSH may play a wider role in cholesterol and lipid metabolism, we tested the effect of Lys-gamma3-MSH on lipolysis, an HSL-mediated process, in 3T3-L1 adipocytes. In comparison with other melanocortin peptides, Lys-gamma3-MSH was found to be a potent stimulator of lipolysis. It was also able to phosphorylate HSL at key serine residues and stimulate the hyperphosphorylation of perilipin A. The receptor through which the lipolytic actions of Lys-gamma3-MSH are being mediated is not clear. Attempts to characterise this receptor suggest that either the pharmacology of the melanocortin receptor 5 in 3T3-L1 adipocytes is different from that described when expressed in heterologous systems or the possibility that a further, as yet uncharacterised, receptor exists.

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.

Adipocyte differentiation-related protein reduces the lipid droplet association of adipose triglyceride lipase and slows triacylglycerol turnover

Although neutral lipid storage droplets are ubiquitous in eukaryotic cells, very little is known about how their synthesis and turnover are controlled. Adipocyte differentiation-related protein (ADRP; also known as adipophilin) is found on the surface of lipid droplets in most mammalian cell types. To learn how ADRP affects lipid storage, we stably expressed the protein in human embryonic kidney 293 (HEK 293) cells, which express little endogenous ADRP. As expected, ADRP was targeted to the surface of lipid droplets and caused an increase in triacylglycerol (TAG) mass under both basal and oleate-supplemented conditions. At least part of the increased mass resulted from a 50% decrease in the rate of TAG hydrolysis in ADRP-expressing cells. Furthermore, ADRP expression increased the fraction of total cellular TAG that was stored in lipid droplets. ADRP expression induced a striking decrease in the association of adipose triglyceride lipase (ATGL) and mannose-6-phosphate receptor tail-interacting protein of 47 kDa with lipid droplets and also decreased the lipid droplet association of several other unknown proteins. Transient expression of ADRP in two other cell lines also reduced the lipid droplet association of catalytically inactive ATGL. We conclude that the reduced lipid droplet association of ATGL and/or other lipases may explain the decrease in TAG turnover observed in ADRP-expressing HEK 293 cells.

LSDP5 is a PAT protein specifically expressed in fatty acid oxidizing tissues

The PAT family (originally named for Perilipin, ADFP and TIP47) now includes four members: Perilipins, ADFP, TIP47 and S3-12. Significant primary sequence homology and the ability to associate with lipid storage droplets (LSDs) are well conserved within this family and across species. In this study, we have characterized a novel PAT protein, lipid storage droplet protein 5 (LSDP5) of 463 residues. A detailed sequence analysis of all murine PAT proteins reveals that LSDP5, TIP47 and ADFP share the highest order of sequence similarity, whereas perilipin and S3-12 have more divergent carboxyl- and amino-termini, respectively. Ectopically-expressed YFP-LSDP5 or flag-LSDP5 fusion proteins associate with LSDs. In accord with recent published data for perilipin, forced expression of LSDP5 in CHO cells inhibits lipolysis of intracellular LSDs. The LSDP5 gene is primarily transcribed in cells that actively oxidize fatty acids, such as heart, red muscle and liver. Expression of LSDP5 is stimulated by ligand activation of peroxisomal proliferator-activated receptor alpha (PPARalpha), and significantly reduced in liver and heart in the absence of this transcription factor. PPARalpha is generally required for regulation of fatty acid metabolism during fasting, but fasting induces LSDP5 mRNA in liver even in the absence of PPARalpha.

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.

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.

Association of PAT proteins with lipid storage droplets in term fetal membranes

As depots for neutral lipids, lipid storage droplets (LDs) accumulate with advancing gestation within the fetal membranes. Little is currently known about the proteins associated with the LDs of these cells. The PAT family [perilipin, adipose differentiation-related protein (ADRP), and tail-interacting protein of 47 kilodaltons (TIP47)] represents a unique group of proteins thought to contribute to LD formation and function. We examined the association of each of the PAT proteins with LDs of term fetal membranes. We found that large LDs of amnion epithelial cells were reactive for neutral lipid stains and simultaneously encoated with ADRP and TIP47, but not perilipin. Within the remaining cell types, LDs were frequently co-labeled with antibodies recognizing ADRP and TIP47; however, in cells harboring only small LDs, the majority of TIP47 labeling was cytoplasmic. Structures labeled with perilipin antibodies were present only in chorion laeve trophoblasts. Gene and protein expression analyses suggested this to be a small molecular weight perilipin isoform, such as that seen in steroidogenic cells. We conclude that LDs are heterogeneous among differing cell types of the fetal membranes. Subclassification of LDs based on associated proteins suggests that these organelles may serve specialized functions within individual cells.

Whole genome expression profiling of glucose-dependent insulinotropic peptide (GIP)- and adrenocorticotropin-dependent adrenal hyperplasias reveals novel targets for the study of GIP-dependent Cushing's syndrome

CONTEXT

The mechanisms responsible for the ectopic adrenal expression of glucose-dependent insulinotropic peptide (GIP) receptor (GIPR) in GIP-dependent Cushing's syndrome (CS) are unknown. Chronic adrenal stimulation by ACTH in Cushing's disease or GIP in GIP-dependent ACTH-independent macronodular adrenal hyperplasia both lead to the induction of genes implicated in adrenal proliferation and steroidogenesis.

OBJECTIVE

The objective of the study was to identify genes differentially expressed specifically in GIP-dependent CS that could be implicated in the ectopic expression of GIPR.

METHODS

We used the Affymetrix U133 plus 2.0 microarray oligochips to compare the whole genome expression profile of adrenal tissues from five cases of GIP-dependent bilateral ACTH-independent macronodular adrenal hyperplasia with CS, one case of GIP-dependent unilateral adenoma with CS, five cases of ACTH-dependent hyperplasias, and a pool of adrenals from 62 normal individuals.

RESULTS

After data normalization and statistical filtering, 723 genes with differential expression were identified, including 461 genes or sequences with a known functional implication, classified in eight dominant functional classes. Specific findings include repression of perilipin, the overexpression of 13 G protein-coupled receptors, and the potential involvement of Rho-GTPases. We also isolated 94 probe sets potentially linked to the formation of GIP-dependent nodules adjacent to the diffuse hyperplasia. These included probe sets related to the linker histone H1 and repression of RXRa and CCND2. The expression profiles for eight genes were confirmed by real-time RT-PCR.

CONCLUSION

This study identified an extensive series of potentially novel target candidate genes that could be implicated in the molecular mechanisms of ectopic expression of the GIPR as well as in the multistep progression of GIP-dependent CS.

Chronic ethanol feeding suppresses beta-adrenergic receptor-stimulated lipolysis in adipocytes isolated from epididymal fat

Chronic ethanol consumption disrupts G protein-dependent signaling pathways in rat adipocytes. Because lipolysis in adipocytes is regulated by G protein-mediated cAMP signal transduction, we hypothesized that cAMP-regulated lipolysis may be vulnerable to long-term ethanol exposure. Male Wistar rats were fed a liquid diet containing ethanol as 35% of total calories or pair-fed a control diet that isocalorically substituted maltose dextrins for ethanol for 4 wk. Lipolysis was measured by glycerol release over 1 h with or without agonists in adipocytes isolated from epididymal fat. Chronic ethanol feeding decreased beta-adrenergic receptor-stimulated lipolysis, but had no effect on basal lipolysis. In response to beta-adrenergic activation, the early peak of cAMP accumulation was suppressed after ethanol feeding, although the basal cAMP concentration in adipocytes did not differ between pair- and ethanol-fed rats. The suppression in cAMP accumulation caused by ethanol feeding was associated with increased activity of phosphodiesterase 4. Chronic ethanol feeding also decreased beta-adrenergic receptor-stimulated protein kinase A activation and phosphorylation of its downstream proteins, perilipin A and hormone-sensitive lipase, the primary lipase-mediating lipolysis. In conclusion, these data suggest that chronic ethanol feeding increased phosphodiesterase 4 activity in adipocytes, resulting in decreased accumulation of cAMP in response to beta-adrenergic activation and a suppression of beta-adrenergic stimulation of lipolysis.

Decrease in intramuscular lipid droplets and translocation of HSL in response to muscle contraction and epinephrine

A better understanding of skeletal muscle lipid metabolism is needed to identify the molecular mechanisms relating intramuscular triglyceride (IMTG) to muscle metabolism and insulin sensitivity. An increasing number of proteins have been reported to be associated with intracellular triglyceride (TG), among them the PAT family members: perilipin, ADRP (for adipocyte differentiation-related protein), and TIP47 (for tail-interacting protein of 47 kDa). Hormone-sensitive lipase (HSL) is thought to be the major enzyme responsible for IMTG hydrolysis in skeletal muscle. In adipocytes, regulation of HSL by intracellular redistribution has been demonstrated. The existence of such regulatory mechanisms in skeletal muscle has long been hypothesized but has never been demonstrated. The aim of this study was to characterize the PAT family proteins associated with IMTG and to investigate the effect of epinephrine stimulation or muscle contraction on skeletal muscle TG content and HSL intracellular distribution. Rat soleus muscles were either incubated with epinephrine or electrically stimulated for 15 min. Single muscle fibers were used for morphological analysis by confocal and transmission electron microscopy. We show a decrease in IMTG in response to both lipolytic stimuli. Furthermore, we identify two PAT family proteins, ADRP and TIP47, associated with IMTG. Finally, we demonstrate HSL translocation to IMTG and ADRP after stimulation with epinephrine or contraction.

Study of the impact of perilipin polymorphisms in a French population

Background

Perilipins are proteins localized at the surface of the lipid droplet in adipocytes, steroid-producing cells and ruptured atherosclerotic plaques playing a role in the regulation of triglyceride deposition and mobilization. We investigated whether perilipin gene polymorphisms were associated with obesity, type 2 diabetes, and their related variables (anthropometric variables, plasma leptin, lipids, glucose and insulin concentrations) in a cross-sectional random sample of 1120 French men and women aged 35 to 65 years old, including 227 obese (BMI ≥ 30 kg/m²) and 275 type 2 diabetes subjects.

Results

Among 7 perilipin polymorphisms tested, only 2 (rs4578621 and rs894160) of them were frequent enough to be fully investigated and we genotyped the sample using the PCR-RFLP method. No significant associations could be found between any of these polymorphisms and the studied phenotypes.

Conclusion

The rs4578621 and rs894160 polymorphisms of the perilipin gene are not major genetic determinants of obesity and type 2 diabetes-related phenotypes in a random sample of French men and women.

Identification and characterization of D-AKAP1 as a major adipocyte PKA and PP1 binding protein

Protein kinase A (PKA) plays an important role in the regulation of lipid metabolism in adipocytes. The activity of PKA is known to be modulated by its specific location in the cell, a process mediated by A-kinase anchoring proteins (AKAPs). In order to examine the subcellular localization of PKA in this tissue we performed a search for AKAP proteins in adipocytes. We purified a 120 kDa protein which can bind both the regulatory subunit of PKA as well as the catalytic subunit of protein phosphatase 1 (PP1). This protein was found to be enriched in the lipid droplet fraction of primary adipocytes and was identified as D-AKAP1. This protein may play an important role in the regulation of PKA in adipocytes.

Genetic variation at the perilipin locus is associated with changes in serum free fatty acids and abdominal fat following mild weight loss

OBJECTIVE

Perilipin (PLIN) is a class of protein-coating lipid droplets in adipocytes. We aimed to examine the association between common single-nucleotide polymorphisms (SNPs) at PLIN locus with circulating free fatty acid (FFA) and abdominal fat distribution in response to weight loss.

METHODS

Non-diabetic/overweight-obese Koreans (n=177) participated in a 12-week calorie restriction (-300kcal/day) program. Seven SNPs (6209T>C, 10076C>G, 10171A>T, 11482G>A, 13042A>G, 13048C>T and 14995A>T), abdominal fat areas (visceral/subcutaneous fat areas at 1st lumbar and 4th lumbar levels), serum lipids, glucose, insulin, FFA, oxidized low-density lipoprotein (LDL) and urinary 8-epi-prostaglandin F(2alpha) (PGF(2alpha)) were examined.

RESULTS

Single-nucleotide polymorphisms 10076C>G/10171A>T showed the strongest positive linkage disequilibrium (LD) (D'=0.923, R (2)=0.839, P<0.001) and SNPs11482G>A/14995A>T showed moderate positive LD (D'=0.824, R (2)=0.578, P<0.001). Calorie restriction induced 4.6% weight loss with significant abdominal fat reduction. In response to weight loss, subjects with nCA/nCA haplotypes at SNPs 10076C>G/10171A>T showed greater reduction in FFA levels than those with CA/CA haplotype (CA/CA: C/C at SNP 10076 and A/A at SNP 10171, nCA: non-CA haplotype carrier). On the other hand, subjects with nGA/nGA haplotype at SNPs 11482G>A/14995A>T had increased FFA levels with a rapid loss in abdominal fat, whereas GA/GA haplotype carriers had reduction in FFA levels. These results still remained significant after adjusting for age, gender and BMI. Prostaglandin F(2alpha) and oxidized LDL were also more reduced in GA/GA haplotype carriers than in nGA haplotype carriers. This effect remained significant after adjusting for baseline level, age, gender and BMI. Paradoxically, nGA haplotype carriers had increased levels of urinary PGF(2alpha) after weight reduction.

CONCLUSION

Fasting plasma FFA changes following a modest weight loss in overweight-obese subjects are influenced by the genetic variability at the PLIN locus. Furthermore, circulating FFA changes rather than body fat itself may determine changes in lipid peroxides such as urinary PGF(2alpha) and oxidized LDL.

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.

Aspects of the regulatory mechanisms of PPAR functions: analysis of a bidirectional response element and regulation by sumoylation

Peroxisome proliferator-activated receptors (PPARs) constitute a subfamily of nuclear receptor superfamily. A wide variety of compounds including hypolipidemic agents, antidiabetic drugs, and long-chain fatty acids are the potential ligands of PPARs. To approach the regulatory mechanisms of PPARs, we studied on two subjects in this work. First, we identified a functional PPAR-binding site in the spacer region between the PEX11alpha and perilipin genes, which are arranged in tandem on the mouse genome. By gene reporter assays and in vivo as well as in vitro binding assays, we show that these genes are regulated tissue-selectively through this common binding site: The PEX11alpha gene is activated by PPARalpha in the liver, whereas the perilipin gene by PPARgamma in the adipose tissue. As the second subject, we found that PPARgamma2 is conjugated with small ubiquitin-related modifier (SUMO) at a specific lysine residue in the amino-terminal region. By site-directed mutagenesis combined with gene reporter assays and sumoylation analyses, we show that sumoylation represses the ligand-independent transactivating function carried by this region, and hence negatively regulates the whole transactivating competence of PPARgamma2. In addition, phosphorylation at a specific site in the amino-terminal region represses the transactivation by PPARgamma2 possibly through enhancing sumoylation.

Expression of adipose differentiation-related protein (ADRP) is conjointly regulated by PU.1 and AP-1 in macrophages

ADRP is associated with intracellular lipid droplets. We demonstrate the regulatory mechanism for ADRP expression in RAW264.7 macrophages. The ADRP mRNA expression was stimulated by PMA, and synergistically enhanced in association with its protein level in the presence of lipids. A proteasome inhibitor protected the protein from degradation under the lipid-free conditions. One of the possible sites of the PMA action was proved to be an Ets/AP-1 element in the promoter, since mutations of this site reduced the PMA-induced promoter activity, and ligation of this element led to a significant increase in the PMA-responsiveness of homologous or heterologous promoters. Mutations of this site diminished the synergistic effect on the promoter activity induced by PMA and oleic acid, suggesting a possible interaction between this site and the downstream PPARdelta site. EMSA revealed that PU.1 and AP-1 conjointly bound to this site. The juxtaposition of the two sequences was requisite for full activity, since spacer sequences between them decreased the PMA-induced activity. PI3 kinase inhibitor was found to reduce the PMA-induced mRNA expression and promoter activity in parallel with PU.1/AP-1 complex formation on EMSA. From these results, we concluded that the Ets/AP-1 site is an important cis-acting element that regulates the ADRP gene expression in macrophages.

Hormonal control of reversible translocation of perilipin B to the plasma membrane in primary human adipocytes

In adipocytes, perilipin coats and protects the central lipid droplet, which stores triacylglycerol. Alternative mRNA splicing gives rise to perilipin A and B. Hormones such as catecholamines and insulin regulate triacylglycerol metabolism through reversible serine phosphorylation of perilipin A. It was recently shown that perilipin was also located in triacylglycerol-synthesizing caveolae of the plasma membrane. We now report that perilipin at the plasma membrane of primary human adipocytes was phosphorylated on a cluster of threonine residues (299, 301, and 306) within an acidic domain that forms part of the lipid targeting domain. Perilipin B comprised <10% of total perilipin but was the major isoform associated with the plasma membrane of human adipocytes. This association was controlled by insulin and catecholamine: perilipin B was specifically depleted from the plasma membrane in response to the catecholamine isoproterenol, while insulin increased the amount of threonine phosphorylated perilipin at the plasma membrane. The reversible translocation of perilipin B to and from the plasma membrane in response to insulin and isoproterenol, respectively, suggests a specific function for perilipin B to protect newly synthesized triacylglycerol in the plasma membrane.

PPARalpha activators and fasting induce the expression of adipose differentiation-related protein in liver

The adipose differentiation-related protein (ADFP)/adipophilin belongs to a family of PAT (for perilipin, ADFP, and TIP47) proteins that associate on the surface of lipid droplets (LDs). Except for LD association, a clear role for ADFP has not been found. We demonstrate that ADFP is transcriptionally regulated by peroxisome proliferator-activated receptor alpha (PPARalpha) in mouse liver and rat and human hepatoma cells through a highly conserved direct repeat-1(DR-1) element. Although the ADFP mRNA is highly increased by a synthetic PPARalpha agonist, the ADFP protein is only substantially increased in cells containing LDs, such as hepatocytes incubated with fatty acids, and in livers of fasted mice. ADFP is induced by fasting even in the absence of a functional PPARalpha, in marked contrast to the PPARalpha target gene acyl-coenzyme A oxidase-1. Activation of LXRs, which stimulates LD formation through the activation of lipogenesis, does not affect ADFP mRNA levels. TIP47, another PAT member known to be expressed in liver, was unaffected by all treatments. This constitutively expressed PAT member seems to be less transcriptionally regulated than ADFP. These observations suggest that ADFP is primarily a fasting-induced protein in liver that coats the newly synthesized triacylglycerol-containing LDs formed during fasting.

Perilipin, a potential substitute for adipophilin in triglyceride storage in human macrophages

Abnormal lipid deposition in human arteries leads to the formation of fatty streaks due to the accumulation of a large number of macrophage derived-foam cells. The formation and catabolism of intracellular lipid droplets is regulated by droplet-associated proteins. Among such proteins, the role of perilipin in human macrophages was unknown. In this study, we first showed that perilipin expression was increased during differentiation of human monocytes to macrophages. Interestingly, cellular perilipin content was unaffected by treatment of cells with OxLDL, AcLDL, VLDL or sterol esters. Moreover, its expression was not dependent on the presence of adipophilin, another lipid droplet-associated protein, since it was not affected by transfection of macrophages with siRNA-adipophilin. Perilipin overexpression in macrophages with an expression vector resulted in significant lipid droplet formation and TG accumulation and this was unaffected by decreasing adipophilin levels using siRNA. Consequently, perilipin, like adipophilin, might play an important role in the conversion of macrophages into foam cells and contribute to lesion formation. Therefore, inhibition of adipophilin might not be sufficient to prevent lesion formation as previously suggested, and perilipin inhibition might be additionally required.

Degradation of perilipin is mediated through ubiquitination-proteasome pathway

Perilipin protein coats the surface of intracellular lipid droplets and plays fundamental roles in lipid droplet formation and triacylglycerol hydrolysis. Perilipin is transcriptionally regulated through peroxisome proliferator-activated receptor and post-translationally stabilized by stored intracellular neutral lipids. In this study, we show that perilipin protein accumulates in transfected Chinese hamster ovary cells cultured in the presence of fatty acids but in turn is destabilized when lipid precursors for triacylglycerol synthesis are removed from culture serum. Adding fatty acids in the culture medium prevents the degradation of perilipin. Moreover, specific proteasome inhibitors, MG132, lactacystin, and ALLN, block the degradation, whereas inhibitors of other proteases are ineffective. Pulse-chase experiments confirm that perilipin is degraded through proteasome, a process that is inhibited by MG132 or ALLN and blunted by the addition of oleic acid. We have detected the co-immunoprecipitation of perilipin and ubiquitin, thus confirming that perilipin is conjugated to poly-ubiquitin and targeted for proteasomal degradation. Treatment with MG132 increases the expression of perilipin associated with lipid droplets as well as modestly throughout the cytosol. We conclude that the degradation of perilipin is mediated through an ubiquitination-proteasome pathway, which suggests another mode for the post-translational regulation of perilipin.

Post-translational regulation of adipose differentiation-related protein by the ubiquitin/proteasome pathway

Adipose differentiation-related protein (ADRP) is localized to lipid droplets in most mammalian cells. ADRP, proposed to regulate fatty acid mobilization and lipid droplet formation, is linked to lipid accumulation in foam cells of human atherosclerotic lesions. In this report, we show that ADRP protein accumulates in Chinese hamster ovary fibroblastic cells cultured in the presence of oleic acid but is destabilized when fatty acid sources are removed from culture serum. The latter effect was blocked by the proteasome inhibitor MG132, whereas inhibitors of other proteolytic processes were ineffective. Pulse-chase experiments confirmed that ADRP degradation is inhibited by MG132. Conditions that stimulate ADRP degradation also promoted the covalent modification of ADRP by ubiquitin, whereas the addition of oleic acid to culture media, which promotes triacylglycerol deposition, blunted the appearance of ubiquitinated-ADRP. Treatment with MG132 increased the levels of ADRP associated with lipid droplets, as well as throughout the cytosol. Finally, we demonstrate that the disappearance of ADRP protein after the onset of perilipin expression during adipocyte differentiation is due to degradation by proteasomes Thus, proteolytic degradation of ADRP mediated through the ubiquitin/proteasome pathway appears to be a major mode for the post-translational regulation of ADRP.

Expression of perilipin A on the surface of lipid droplets increases along with the differentiation of hamster sebocytes in vivo and in vitro

To clarify the involvement of perilipin, a lipid-droplet-surface protein associated with adipocytes and steroidogenic cells, in the differentiation of sebocytes, we investigated the expression of perilipin in sebaceous glands in vivo and in vitro. Perilipin was expressed in sebaceous glands of the hamster auricle in vivo and was localized at the surface of intracellular lipid droplets in differentiated hamster sebocytes in vitro. Western blot analysis showed that perilipin with a molecular weight of approximately 57 kDa, which was identical to that in differentiated mouse 3T3-L1 adipocytes, was detected in cultured sebocytes, indicating that sebaceous glands expressed perilipin A. In addition, the production of perilipin A in cultured sebocytes was transcriptionally augmented by sebocytic-lipogenesis stimulators, insulin, and 5alpha-dihydrotestosterone, whereas it was decreased by a suppressor of sebocytic differentiation, epidermal growth factor. Furthermore, hamster sebocytes were found to express peroxisome proliferation-activating receptor alpha and gamma1, the activation of which by WY14643 and troglitazone, respectively, caused the transcriptional augmentation of perilipin A expression along with an increase in levels of triacylglycerols in lipid droplets in sebocytes. Therefore, these results provide novel evidence that the expression of perilipin A increases on the surface of intracellular lipid droplets augmented along with the differentiation of hamster sebocytes.

Characterization of big stanniocalcin variants in mammalian adipocytes and adrenocortical cells

The hormone stanniocalcin (STC) is widely distributed, and in rodents the highest levels of expression are in the ovaries. In both cows and rodents, ovarian STC consists of three high-molecular-weight variants collectively known as big STC. In the ovary, big STC is made by theca cells and interstitial cells and is targeted to lipid storage droplets of nearby luteal cells to inhibit progesterone release. An endocrine pathway is operative during pregnancy and lactation. Whether or not big STC is made by tissues other than ovary has never been addressed. Therefore, the purpose of this study was to determine via a detailed characterization of adrenal glands and adipocytes whether big STC is present in other cells that store and release lipids. The results showed that STC was made in bovine and mouse adrenals, mainly in steroidogenic, adrenocortical cells. The majority of ligand and receptor were likewise confined to cortical zone cells. As in luteal cells, high levels of ligand and receptor were found in the adrenocortical cell lipid droplet fraction. However, adrenals made only the largest (135 kDa) of the three big STC variants. Nonetheless, adrenal STC had much greater receptor affinity than a mixture of the three big STC variants. Adipocytes contained all three big STC variants, and both ligand and receptor were heavily concentrated on the lipid droplets. Moreover, adipocyte lipid storage droplets had 50-fold more receptors than those in steroidogenic cells, indicating that big STC is heavily targeted to adipose cells. The findings collectively support the hypothesis that big STC is not unique to ovarian steroidogenic cells but is in fact common to cells with a role in lipid storage and release.

Redistribution of macrophage cholesteryl ester hydrolase from cytoplasm to lipid droplets upon lipid loading

Hydrolysis of intracellular cholesteryl esters (CEs) represents the first step in the removal of cholesterol from lipid-laden foam cells associated with atherosclerotic lesions. Neutral cholesteryl ester hydrolase (CEH) catalyzes this reaction, and we recently cloned the cDNA for the human macrophage CEH and demonstrated increased mobilization of intracellular CE droplets by CEH overexpression. The present study was undertaken to test the hypothesis that for CE hydrolysis, CEH must become associated with the surface of the cytoplasmic lipid droplets. Our data show the redistribution of CEH from cytosol to lipid droplets upon lipid loading of human THP-1 macrophages. Depletion of triacylglycerol (TG) by incubation with the acyl-CoA synthetase inhibitor Triacsin D had no effect on CEH association with the lipid droplets, suggesting that CEH associates with mixed (CE + TG) as well as TG-depleted CE droplets. However, CEH had 2.5-fold higher activity when mixed droplets were used as substrate in an in vitro assay, consistent with the reported higher cholesterol efflux from cells containing mixed isotropic droplets. Perilipin as well as adipophilin, two lipid droplet-associated proteins, were also present on the lipid droplets in THP-1 macrophages. In conclusion, CEH associates with its intracellular substrate (lipid droplets) and hydrolyzes CE more efficiently from mixed droplets.

Role of PAT proteins in lipid metabolism

One of the central reactions in bodily energy metabolism is lipolysis in adipocytes, the reaction that governs the release of stored fatty acids from the adipocyte triacylglycerol pool, which constitutes the major energy reserve in animals. These fatty acids are then transported by serum albumin to various tissues to supply their energy requirements. This reaction was previously thought to result from phosphorylation and activation of hormone-sensitive lipase by protein kinase A (PKA) but is now known to be governed by a translocation of the lipase from the cytosol to the surface of the intracellular lipid droplet that houses the reservoir of TAG. This droplet is coated with perilipin A, which is also phosphorylated by PKA in response to lipolytic stimuli, and phosphorylation of perilipin A is essential for HSL translocation and stimulated lipolysis.

Effects of the human immunodeficiency virus-protease inhibitor, ritonavir, on basal and catecholamine-stimulated lipolysis

Several of the aspartic acid protease inhibitors used to treat HIV infection increase basal lipolysis in adipocytes, but the cellular mechanisms leading to this augmentation are not well understood. We therefore studied the effects of chronic exposure to the HIV protease inhibitor, ritonavir, on the lipolytic cascade in 3T3-L1 adipocytes. Treatment of 3T3-L1 adipocytes with ritonavir for 14 d (during and after differentiation) enhanced basal, isoproterenol (Iso)-stimulated, and cAMP analog-stimulated lipolysis. Enhancement of lipolysis was observed after Iso at concentrations between 0.1 and 10 mum. Despite a significant decrease in cyclic nucleotide phosphodiesterase (PDE)3B activity and protein levels, there were no changes in Iso-stimulated intracellular cAMP, protein kinase A (PKA) expression, or PKA activity. Ritonavir-augmented lipolysis was also observed under conditions that reversed the effect on PDE3B activity via preincubation with 1 mum (-)-N(6)-(2-phenylisopropyl)adenosine. In ritonavir-treated cells, protein expression of the lipid droplet-protective protein, perilipin, was significantly decreased, whereas there was no change in hormone-sensitive lipase. Activation of ERK1/2 by Iso did not play a role in the augmentation. We conclude that ritonavir decreases PDE3B and perilipin protein expression and affects both basal and catecholamine-stimulated lipolysis in 3T3-L1 adipocytes primarily through actions at sites downstream of PKA.

PPARgamma2 regulates lipogenesis and lipid accumulation in steatotic hepatocytes

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is considered to be one of the master regulators of adipocyte differentiation. PPARgamma2 is abundantly expressed in mature adipocytes and is elevated in the livers of animals that develop fatty livers. The aim of this study was to determine the ability of PPARgamma2 to induce lipid accumulation in hepatocytes and to delineate molecular mechanisms driving this process. The hepatic cell line AML-12 was used to generate a cell line stably expressing PPARgamma2. Oil Red O staining revealed that PPARgamma2 induces lipid accumulation in hepatocytes. This phenotype is accompanied by a selective upregulation of several adipogenic and lipogenic genes including adipose differentiation-related protein (ADRP), adipocyte fatty acid-binding protein 4, sterol regulatory element-binding protein-1 (SREBP-1), fatty acid synthase (FAS), and acetyl-CoA carboxylase, genes whose expression levels are known to increase in steatotic livers of ob/ob mice. Furthermore, the PPARgamma2-regulated induction of both SREBP-1 and FAS parallels an increase in de novo triacylglycerol synthesis in hepatocytes. Triacylglycerol synthesis and lipid accumulation are further enhanced by culturing hepatocytes with troglitazone in the absence of exogenous lipids. These results correspond with an increase in the lipid droplet protein, ADRP, and the data demonstrate that ADRP functions to coat lipid droplets in hepatocytes as observed by confocal microscopy. Taken together, these observations propose a role for PPARgamma2 as an inducer of steatosis in hepatocytes and suggest that this phenomenon occurs through an induction of pathways regulating de novo lipid synthesis.

Magnolol induces the distributional changes of p160 and adipose differentiation-related protein in adrenal cells

Magnolol stimulates adrenal steroidogenesis and induces the distributional changes of p160 and adipose differentiation-related protein (ADRP) in rat adrenal cells. This study investigated the underlying signaling mechanisms involved in these processes. Magnolol (30 microM) caused a time-dependent increase in the phosphorylation of extracellular signal-related kinase (ERK) in cultured adrenal cells. The following evidence supports a link between ERK activation and p160 translocation. First, the magnolol-induced redistribution of p160 from the lipid droplet surface to the cytosol, resulting in the decrease in the percentages of p160-positive cells, and this decrease in p160-positive cells was completely blocked by pretreatment with either of the MAPK-ERK kinase (MEK) inhibitors PD98059 or U0126. Second, magnolol did not significantly decrease total p160 protein levels but caused an increase in threonine phosphorylation of p160, which reached a maximum after 5 min of magnolol treatment, and this magnolol-induced phosphorylation of p160 was prevented by pretreatment with U0126, suggesting the involvement of ERK. In addition, magnolol decreased both ADRP immunostaining intensity at the lipid droplet surface and the percentage of ADRP-positive cells. This was further confirmed biochemically by the decrease in ADRP levels in total cell homogenates and in lipid droplet fractions. Magnolol-induced decrease in ADRP staining at the lipid droplet surface was not affected by pretreatment with PD98059 or U0126, indicating that ERK signaling was not involved in this event. Furthermore, treatment with 30 microM magnolol for 6 h resulted in about 50% decrease in ADRP protein level. Therefore, decreased protein levels of p160 and ADRP at the lipid droplet surface induced by magnolol were mediated via two different mechanisms: phosphorylation of p160 and downregulation of ADRP expression, respectively.

Gender-specific association of a perilipin gene haplotype with obesity risk in a white population

OBJECTIVE

Perilipin is a class of protein-coating lipid droplets in adipocytes and steroidogenic cells. Our purpose was to examine the association between common single-nucleotide polymorphisms (SNPs) at the perilipin (PLIN) locus and obesity, as well as related phenotypes, in unrelated American adults.

RESEARCH METHODS AND PROCEDURES

Four PLIN SNPs (PLIN 6209T>C, 11482G>A, 13041A>G, and 14995A>T) were typed in 734 white subjects (373 men and 361 women) attending a residential lifestyle intervention program. The baseline anthropometric and biochemical measures were used. Obesity was defined as BMI > or = 30 kg/m(2).

RESULTS

Multivariate analysis demonstrated that, in women, two of the SNPs (13041A>G, and 14995A>T) were significantly associated with percentage body fat (p = 0.016 for 13041A>G and p = 0.010 for 14995A>T) and waist circumference (p = 0.020 for 13041A>G and p = 0.045 for 14995A>T). Moreover, haplotype analysis using these two SNPs indicated that haplotypes A/T and G/T were both associated with significantly increased obesity risk (odds ratio = 1.76, 95% confidence interval 1.07 to 2.90 for haplotype A/T, and odds ratio = 1.73, 95% confidence interval 1.06 to 2.82 for haplotype G/T) when compared with haplotype A/A. No significant associations between PLIN variations and obesity were found in men.

DISCUSSION

Our data support the hypothesis that the PLIN locus may be a significant genetic determinant for obesity risk in whites and that women are more sensitive to the genetic effects of perilipin than men.

Intragenic linkage disequilibrium structure of the human perilipin gene (PLIN) and haplotype association with increased obesity risk in a multiethnic Asian population

Perilipin is a lipid droplet surface protein present in adipocytes and steroidogenic cells. We examined five common single nucleotide polymorphisms (SNPs) at the perilipin (PLIN) locus (PLIN 6209C>T, 10171A>T, 11482G>A, 13041A>G, and 14995A>T) to investigate their association with obesity risk. The study population included 4,131 subjects of three ethnic groups (Chinese, Malay, and Indian) from Singapore. The prevalence of obesity in Malays and Indians was much higher than in Chinese. Moreover, in these groups the prevalence of obesity was three times higher in women than in men. Crude analysis indicated that haplotype 11212 (CAAAT) is shared by Malays and Indians and is significantly associated with increased obesity risk as compared to the most common haplotype 21111 (TAGAA): OR 1.65 (95% CI 1.11-2.46) in Malays and 1.94 (95% CI 1.06-3.53) in Indians. No associations between PLIN haplotypes and obesity risk were found in Chinese. To simplify the haplotype analyses we used a subgroup of three SNPs (11482G>A, 13041A>G, and 14995A>T) in positive linkage disequilibrium. These analyses revealed similar associations, showing that haplotypes XX212 (XXAAT) and XX222 (XXAGT) are associated with increased obesity risk in Malays OR 2.04 (95% CI 1.28-3.25) and 2.05 (95% CI 1.35-3.12) respectively, and that haplotype XXX212 (XXAAT) is significantly associated with increased obesity risk in Indians OR 2.16 (95% CI 1.10-4.26) after adjusting for covariates including age, sex, smoking, alcohol consumption, exercise, and diabetes status. Moreover, individual SNP analyses demonstrated that the PLIN 14995A>T SNP is the most informative single genetic marker for the observed haplotype association, being significantly associated with increased obesity risk in both Malays OR 2.28 (95% CI 1.45-3.57) and Indians OR 2.04 (95% CI 1.08-3.64). These results support the role of the PLIN locus as an ethnically dependent modulator of obesity risk in humans.

Metabolism of lipids in human white adipocyte

Adipose tissue is considered as the body's largest storage organ for energy in the form of triacylglycerols, which are mobilized through lipolysis process, to provide fuel to other organs and to deliver substrates to liver for gluconeogenesis (glycerol) and lipoprotein synthesis (free fatty acids). The release of glycerol and free fatty acids from human adipose tissue is mainly dependent on hormone-sensitive lipase which is intensively regulated by hormones and agents, such as insulin (inhibition of lipolysis) and catecholamines (stimulation of lipolysis). A special attention is paid to the recently discovered perilipins which could regulate the activity of the lipase hormono-sensible. Most of the plasma triacylglycerols are provided by dietary lipids, secreted from the intestine in the form of chylomicron or from the liver in the form of VLDL. Released into circulation as non-esterified fatty acids by lipoprotein lipase, those are taken up by adipose tissue via specific plasma fatty acid transporters (CD36, FATP, FABPpm) and used for triacylglycerol synthesis. A small part of triacylglycerols is synthesized into adipocytes from carbohydrates (lipogenesis) but its regulation is still debated in human. Physiological factors such as dieting/fasting regulate all these metabolic pathways, which are also modified in pathological conditions e.g. obesity.

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.

Metabolic Adaptations in the Absence of Perilipin

Targeted disruption of the lipid droplet protein, perilipin, in mice leads to constitutional lipolysis associated with marked reduction in white adipose tissue as a result of unbridled lipolysis. To investigate the metabolic adaptations in response to the constitutive lipolysis, we studied perilipin-null (plin(-/-)) mice in terms of their fatty acid oxidation and glycerol and glucose metabolism homeostasis by using dynamic biochemical testing and clamp and tracer infusion methods. plin(-/-) mice showed increased beta-oxidation in muscle, liver, and adipose tissue resulting from a coordinated regulation of the enzymes and proteins involved in beta-oxidation. The increased beta-oxidation helped remove the extra free fatty acids created by the constitutive lipolysis. An increase in the expression of the transcripts for uncoupling proteins-2 and -3 also accompanied this increase in fatty acid oxidation. Adult plin(-/-) mice had normal plasma glucose but a reduced basal hepatic glucose production (46% that of plin(+/+)). Insulin infusion during low dose hyperinsulinemic-euglycemic clamp further lowered the glucose production in plin(-/-) mice, but plin(-/-) mice also showed a 36% decrease (p < 0.007) in glucose disposal rate during the low dose insulin clamp, indicating peripheral insulin resistance. However, compared with plin(+/+) mice, 14-week-old plin(-/-) mice showed no significant difference in glucose disposal rate during the high dose hyperinsulinemic clamp, whereas 42-week-old plin(-/-) mice displayed significant insulin resistance on high dose hyperinsulinemic clamp. Despite increasing insulin resistance with age, plin(-/-) mice at different ages maintained a normal glucose response during an intraperitoneal glucose tolerance curve, being compensated by the increased beta-oxidation and reduced hepatic glucose production. These experiments uncover the metabolic adaptations associated with the constitutional lipolysis in plin(-/-) mice that allowed the mice to continue to exhibit normal glucose tolerance in the presence of peripheral insulin resistance.

Hormone-sensitive lipase--new roles for an old enzyme

Although described initially as an intracellular adipocyte-specific triacylglycerol lipase, it is now clear that HSL (hormone-sensitive lipase) is expressed in multiple tissues and plays a number of roles in lipid metabolism, including that of a neutral cholesteryl ester hydrolase. The major isoform is a single polypeptide with a molecular mass of approx. 84 kDa and which comprises three major domains: a catalytic domain, a regulatory domain encoding several phosphorylation sites and an N-terminal domain involved in protein-protein and protein-lipid interactions. The activity of HSL is regulated acutely by several mechanisms, including reversible phosphorylation by a number of different protein kinases, translocation to different sites within the cell and interaction with a number of proteins, some of which may serve to direct the inhibitory products of HSL away from the protein. It is also apparent from work with HSL null mice that more than one enzyme species may be classified as a hormone-sensitive lipase. The possible presence of HSL in macrophages remains controversial, and the role of the protein in pancreatic beta-cells has yet to be fully elucidated. Altered expression of HSL in different cell types may be associated with a number of pathological states, including obesity, atherosclerosis and Type II diabetes.

CGI-58 interacts with perilipin and is localized to lipid droplets. Possible involvement of CGI-58 mislocalization in Chanarin-Dorfman syndrome

Lipid droplets (LDs) are a class of ubiquitous cellular organelles that are involved in lipid storage and metabolism. Although the mechanisms of the biogenesis of LDs are still unclear, a set of proteins called the PAT domain family have been characterized as factors associating with LDs. Perilipin, a member of this family, is expressed exclusively in the adipose tissue and regulates the breakdown of triacylglycerol in LDs via its phosphorylation. In this study, we used a yeast two-hybrid system to examine the potential function of perilipin. We found direct interaction between perilipin and CGI-58, a deficiency of which correlated with the pathogenesis of Chanarin-Dorfman syndrome (CDS). Endogenous CGI-58 was distributed predominantly on the surface of LDs in differentiated 3T3-L1 cells, and its expression increased during adipocyte differentiation. Overexpressed CGI-58 tagged with GFP gathered at the surface of LDs and colocalized with perilipin. This interaction seems physiologically important because CGI-58 mutants carrying an amino acid substitution identical to that found in CDS lost the ability to be recruited to LDs. These mutations significantly weakened the binding of CGI-58 with perilipin, indicating that the loss of this interaction is involved in the etiology of CDS. Furthermore, we identified CGI-58 as a binding partner of ADRP, another PAT domain protein expressed ubiquitously, by yeast two-hybrid assay. GFP-CGI-58 expressed in non-differentiated 3T3-L1 or CHO-K1 cells was colocalized with ADRP, and the CGI-58 mutants were not recruited to LDs carrying ADRP, indicating that CGI-58 may also cooperate with ADRP.

Role of caveolin-1 in the modulation of lipolysis and lipid droplet formation

Recently, it was shown that caveolin-1 can be redirected from the cell surface to intracellular lipid droplets in a variety of cell types. Here, we directly address the role of caveolin-1 in lipid droplet formation and breakdown, showing that caveolin-1 null mice exhibit markedly attenuated lipolytic activity. Mechanistically, although the activity of protein kinase A (PKA) was greatly increased in caveolin-1 null adipocytes, the phosphorylation of perilipin was dramatically reduced, indicating that caveolin-1 may facilitate the PKA-mediated phosphorylation of perilipin. In support of this hypothesis, coimmunoprecipitation experiments revealed that treatment with a beta(3)-adrenergic receptor agonist resulted in ligand-induced complex formation between perilipin, caveolin-1, and the catalytic subunit of PKA in wild-type but not in caveolin-1 null fat pads. We also show that caveolin-1 expression is important for efficient lipid droplet formation because caveolin-1 null embryonic fibroblasts stably transfected with perilipin accumulated approximately 4.5-fold less lipid than perilipin-transfected wild-type cells. Finally, high-pressure freeze-substitution electron microscopy of adipose tissue revealed dramatic perturbations in the architecture of the "lipid droplet cortex" (the interface between the lipid droplet surface and the cytoplasm) in caveolin-1 null perigonadal adipocytes. Taken together, our data provide the first molecular genetic evidence that caveolin-1 plays a critical functional and structural role in the modulation of both lipid droplet biogenesis and metabolism in vivo.