Adipocyte Differentiation-related Protein Research Articles

ADRP is dissociated from lipid droplets by ARF1-dependent mechanism

Adipocyte differentiation-related protein (ADRP) is a member of PAT proteins existing in lipid droplets (LDs). By yeast two-hybrid screening, we identified ADP-ribosylation factor 1 (ARF1) as a binding partner of ADRP. The interaction of ADRP and ARF1 was verified by GST pull-down and co-immunoprecipitation experiments. Interestingly, ADRP precipitated the GDP-bound ARF1 preferentially to the GTP-bound ARF1. Consistent with this, either brefeldin A (BFA), a fungal metabolite to inhibit ARF-GEF, or a dominant-negative mutant of ARF1 caused dissociation of ADRP from LD. On the other hand, overexpression of wild-type ARF1 did not promote the ADRP dissociation or new LD formation. By using deletion mutants, a central domain of ADRP, which is dispensable for LD binding, was shown to bind to ARF1. The present study showed that the GDP-bound ARF1 induces dissociation of ADRP from the LD surface, and that LD is a target of BFA action.

The lipid droplet-associated protein adipophilin is expressed in human trophoblasts and is regulated by peroxisomal proliferator-activated receptor-gamma/retinoid X receptor.

Uptake and transplacental transfer of fatty acids is essential for fetal development. Human adipophilin and its murine ortholog adipocyte differentiation-related protein are lipid droplet-associated proteins that are implicated in cellular fatty acid uptake in adipocytes. The nuclear receptor peroxisome proliferator-activated receptor-gamma (PPARgamma) stimulates lipid uptake by adipocytes and enhances differentiation of placental trophoblasts. We therefore hypothesized that adipophilin is expressed in human trophoblasts and that its expression is regulated by PPARgamma. We initially determined that adipophilin is expressed in human villous trophoblasts and that adipophilin expression is enhanced during differentiation of cultured primary term human trophoblasts. We also found that exposure of cultured human trophoblasts to the PPARgamma ligand troglitazone resulted in a concentration-dependent increase in adipophilin expression. We observed a similar increase with LG268, a ligand for retinoid X receptor (RXR), the heterodimeric partner of PPARgamma. Lastly, we demonstrated that ligand-activated PPARgamma and RXR stimulated the transcriptional activity of adipophilin promoter in CV-1 cells and in the placental JEG3 cell line. We conclude that the expression of adipophilin is enhanced during trophoblast differentiation and is up-regulated by ligand-activated PPARgamma/RXR. Enhanced adipophilin expression may contribute to fatty acid uptake by the placenta.

A Phospholipase D-dependent Process Forms Lipid Droplets Containing Caveolin, Adipocyte Differentiation-related Protein, and Vimentin in a Cell-free System

We developed a microsome-based, cell-free system that assembles newly formed triglyceride (TG) into spherical lipid droplets. These droplets were recovered in the d </= 1.055 g/ml fraction by gradient ultracentrifugation and were similar in size and appearance to those isolated from rat adipocytes and 3T3-L1 cells. Caveolin 1 and 2, vimentin, adipocyte differentiation-related protein, and the 78-kDa glucose regulatory protein were identified on the droplets from the cell-free system. The caveolin was soluble in 1% Triton X-100, as was the caveolin on lipid droplets from 3T3-L1 cells. The lipid droplets from the cell-free system, like those from 3T3-L1 cells, contained TG, diacylglycerol, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine. The assembly of these TG-containing structures was dependent on the rate of TG biosynthesis and required an activator present in the 160,000 x g supernatant from homogenized rat adipocytes. The activator induced phospholipase D (PLD) activity, and its effect on the release of the TG-containing structures from the microsomes was inhibited by 1-butanol (but not 2-butanol) or 2,3-diphosphoglycerate. The activator could be replaced by a constitutively active PLD or phosphatidic acid. These results indicate that PLD and the formation of phosphatidic acid are important in the assembly of the TG-containing structures.

Sigma-1 receptors (sigma(1) binding sites) form raft-like microdomains and target lipid droplets on the endoplasmic reticulum: roles in endoplasmic reticulum lipid compartmentalization and export.

The brain sigma-1 receptors can bind neurosteroids and psychotropic drugs, including neuroleptics and cocaine and are implicated in schizophrenia, depression, and drug dependence. In this study, we found that sigma-1 receptors specifically target lipid storage sites (lipid droplets) on the endoplasmic reticulum by forming a distinct class of lipid microdomains. Both endogenously expressing sigma-1 receptors and transfected C-terminally enhanced yellow fluorescent protein (EYFP)-tagged sigma-1 receptors (Sig-1R-EYFP) target unique "ring-like" structures associated with endoplasmic reticulum reticular networks in NG108-15 cells. The ring-like structures contain neutral lipids and are enlarged by the oleate treatment, indicating that they are endoplasmic reticulum-associated lipid droplets (ER-LDs). sigma-1 receptors colocalize with caveolin-2, a cholesterol-binding protein in lipid rafts on the ER-LDs, but not with adipocyte differentiation-related protein (ADRP), a cytosolic lipid droplet (c-LD)-specific protein. When the double-arginine ER retention signal on the N terminus of sigma-1 receptors is truncated, sigma-1 receptors no longer exist on ER-LDs, but predominantly target c-LDs, which contain ADRP. sigma-1 receptors on ER-LDs form detergent-resistant raft-like lipid microdomains, the buoyancy of which is different from that of plasma membrane lipid rafts. (+)-Pentazocine causes sigma-1 receptors to disappear from the microdomains. N-Terminally EYFP-tagged sigma-1 receptors (EYFP-Sig-1R) failed to target ER-LDs. EYFP-Sig-1R-transfected cells showed an unrestricted distribution of neutral lipids all over the endoplasmic reticulum network, decreases in c-LDs and cholesterol in plasma membranes, and the bulbous aggregation of endoplasmic reticulum. Thus, sigma-1 receptors are unique endoplasmic reticulum proteins that regulate the compartmentalization of lipids on the endoplasmic reticulum and their export from the endoplasmic reticulum to plasma membrane and c-LDs.

Live Cell Analysis and Targeting of the Lipid Droplet-binding Adipocyte Differentiation-related Protein

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

Induction of endogenous genes by peroxisome proliferator activated receptor alpha ligands in a human kidney cell line and in vivo

The peroxisome proliferator activated receptor alpha (PPARα) plays a key role in regulating fatty acid metabolism by regulating expression of genes involved in fatty acid oxidation. To identify endogenous transcripts that could be used as surrogate markers for on-target activity of PPARα agonists, we employed a global profiling approach using DNA microarrays. The HK-2 cell line derived from proximal tubules of the human kidney, showed induction of several genes, including pyruvate dehydrogenase kinase 4 (PDK-4) and adipocyte differentiation related protein (ADRP) by PPARα ligands. HK-2 cells express detectable levels of PPARα and its dimerization partner the retinoid X receptor (RXRα) proteins. Induction of PDK-4 in these cells correlates with induction of PDK-4 in the liver of fat-fed hamsters. The magnitude of fibrate induction of PDK-4 in the liver also mirrors the decrease in serum triglyceride levels. Thus, induction of PDK-4 by PPARα agonists in the HK-2 cell model closely correlates with its induction in vivo and may represent an early marker for PPARα agonist action.

소 Adipocyte Differentiation Related Protein (ADRP) 유전자의 Genomic Organization 및 Promoter Region의 특성 규명

ADRP 유전자가 24개월령 한우 등심조직에서 발현량이 급격히 증가하여 30개월령 등심조직에서는 발현량이 다소 감소하는 발현양상 분석결과로부터 이전 연구에서는 ADRP 유전자를 한우 성장단계 특이발현 유전자로 선정하였다. 본 연구에서는 ADRP 유전자의 발현조절 기작을 분석하기 위하여 promoter 영역을 포함하는 ADRP 유전자 전체영역을 cloning하였으며, 구조를 분석하고 promoter의 특성을 조사하였다. 한우 ADRP cDNA 단편을 probe로 합성하여 Southern blot 분석을 실시한 결과로부터 ADRP 유전자가 한우 genome 상에서 single copy로 존재하고 크기는 대략 12 kb에 해당하는 것을 확인하였다. Genomic DNA library screening을 실시하여 promoter 영역을 포함하는 ADRP 전체 유전자에 해당하는 clone을 확보하고 HwADRPg-1으로 명명한 후, 염기서열을 결정하고 분석하였다. 한우 ADRP 유전자, HwADRPg-1은 8개의 exon과 7개의 intron으로 구성되어 있으며 모든 exon-intron 경계는 GT/AG 원칙을 따르고 있었고, coding 영역은 7,633 bp로서 6개의 intron에 의해 7개의 exon으로 나누어져 있었다. HwADRPg-1의 promoter 영역에서는 TATAA box는 발견되지 않았으며, -70 위치에 근육 특이적 transcription activator인 Myo G 서열이 존재하였고, -629 위치에는 지방세포의 분화를 유도하는 것으로 알려진 C/EBP (CCAAT/enhancer binding protein) 서열이 존재하였다. HwADRPg-1의 조절영역에 있는 Myo G factor가 근육조직에서 ADRP 유전자가 발현될 수 있도록 하며, 근육의 발달정도를 신호로써 감지하여 근육조직에서 성장단계에 따른 ADRP 유전자의 발현량을 조절할 것으로 추정되고, 다른 종류의 지방세포 특이적인 전사인자 및 지방세포의 분화정도를 신호로 인식하는 전사단계 조절인자를 조사하기 위하여 promoter 영역의 추가분석이 이루어져야 할 것으로 사료된다. To understand the structure and regulation of bovine ADRP (Adipocyte Differentiation Related Protein) gene, we have isolated the genomic clone of bovine ADRP and determined its sequence. A genomic Southern blot analysis confirmed that ADRP gene is present as a single copy in bovine genome and the ADRP gene spans 12 kb. Bovine ADRP genomic clone, HwADRPg-1, had 8 exons and 7 introns, and all splicing sites conformed to the GT/AG rule with the exon-intron boundaries located exactly. Analysis of the upstream 649 bp of the sequence of HwADRPg-1 showed that it does not contain any canonical TATAA boxes; however Sp1 binding sites and CAAT boxes are found. The promoter contained potential binding sites for AP-1, AP-2 and several putative transcription factor binding sites. The 5'-flanking region of HwADRPg-1 contained muscle specific transcription activator Myo G and C/EBP (CCAAT/ enhancer binding protein) recognizing site. These results suppose that the Myo G transcription activator regulate the transcription of bovine ADRP gene in muscular tissue and its transcriptional activity was triggered by degree of muscular development. Our results provide the necessary analysis for other flanking sequences are needed in addition to the proximal cis elements of this promoter to confer adipocyte differentiation-dependent or growth-dependent transcriptional control.

한우 성장단계 특이발현 유전자의 발현양상 분석

We have investigated the expression patterns of candidates for growth stage specifically expressed genes. The expression patterns of the EPV20, aldolase A, Translationally Controlled Tumor Protein (TCTP) and Adipocyte Differentiation Related Protein (ADRP) were examined by semiquantitative RT-PCR and northern blot analysis in skeletal muscle tissues of Hanwoo, especially in the longissimus dorsi at various growth stages. The EPV20 mRNA was expressed in longissimus dorsi tissue of Hanwoo, but there was no difference of expression levels during growth stages. Though the aldolase A gene was reported to be muscle-specific and regulated at developmental stages, the expression levels of aldolase A mRNA in the longissimus dorsi tissues showed little differences at various growth stages. The expression levels of TCTP which was reported as growth-related protein regulated at translation step were gradually increased during growth of Hanwoo. The expression levels of ADRP mRNA were rapidly increased at 24-month-old longissimus dorsi tissue of Hanwoo, and decreased at 30-month-old. Our data suggest that the ADRP gene show as growth-stage dependent expression and is related to fat deposition within muscular tissue.

Role of adipocyte differentiation-related protein in surfactant phospholipid synthesis by type II cells

Adipocyte differentiation-related protein (ADrP) is an intrinsic lipid storage droplet protein that is highly expressed in lung. ADrP localizes to lipid storage droplets within lipofibroblasts, pulmonary cells characterized by high triacylglycerol, which is a precursor for surfactant phospholipid synthesis by alveolar type II epithelial (EPII) cells. The developmental pattern of ADrP mRNA and protein expression in lung tissue parallels triacylglycerol accumulation in rat lung. ADrP mRNA levels are relatively high in isolated lipofibroblasts, accounting for the high ADrP expression in lung. Isolated EPII cells, which do not store neutral lipids but derive them from lipofibroblasts, have low levels of ADrP mRNA expression. ADrP is found around lipid droplets in cultured lipofibroblasts, but not in EPII cells isolated from developing rat lung. After coculture with lipofibroblasts, EPII cells acquired ADrP, which associates with lipid droplets. Furthermore, (3)H-labeled triolein in isolated ADrP-coated lipid droplets is a tenfold better substrate for surfactant phospholipid synthesis by cultured EPII cells than (3)H-labeled synthetic triolein alone. Antibodies to ADrP block transfer of neutral lipid. These data suggest a role for ADrP in this novel mechanism for the transfer of lipid between lipofibroblasts and EPII cells.

Isolation of Adipophilin and Butyrophilin from Bovine Milk and Characterization of a cDNA Encoding Adipophilin

The milk fat globule membrane-associated proteins adipophilin (alias adipocyte differentiation-related protein) and butyrophilin were purified from bovine milk by reverse-phase chromatography. The nucleotide sequence of bovine adipophilin was obtained via peptide mapping and sequencing of a mammary gland cDNA clone, which comprises 1841 nucleotides and has an open reading frame of 450 amino acids. By peptide mapping, 19% of the amino acid sequence was confirmed. The obtained amino acid sequence has 87 and 80% identical residues with human and mouse adipophilin, respectively. Alignment with the proteins perilipin and TIP47 revealed two highly conserved segments, which may assemble into amphipathic α-helices.

Adipocyte differentiation-related protein is secreted into milk as a constituent of milk lipid globule membrane.

Milk lipid globules from humans, cows and rats contained a protein identified as adipocyte differentiation-related protein (ADRP) associated with the globule surface membrane material. This protein, previously believed to be specific to adipocytes, was a major constituent of the globule surface and was present in a detergent-insoluble complex that contained stoichiometric amounts of butyrophilin and xanthine oxidase. Identification of ADRP was by sequence similarity of tryptic peptides from cow and human proteins with the sequence inferred from the cDNA for mouse ADRP. The putative ADRP of lipid globules from cow, human and rat milk was recognized specifically by antisera raised against a peptide synthesized to duplicate the N-terminal 26 residues of the mouse protein. In homogenates of lactating mammary gland, ADRP was found only in endoplasmic reticulum and in lipid droplet fractions. ADRP was modified, apparently post-translationally, and one modification apparently was acylation, primarily with C14, C16 and C18 fatty acids. Two isoelectric variants of ADRP were present in cow globule membrane material. In vitro, ADRP served as a substrate for protein kinases associated with milk lipid globule membrane, but this protein did not seem to become phosphorylated intracellularly.

Mapping genes in the mouse using single-strand conformation polymorphism analysis of recombinant inbred strains and interspecific crosses.

We have utilized a PCR-based analysis of single-strand conformation polymorphisms to identify polymorphisms that can be used for mapping cloned DNA sequences in the mouse. We have found that single-strand conformation polymorphism analysis of sequences that are potentially less subject to conservation (i.e., intron and 3' untranslated regions) is a relatively efficient means of detecting polymorphisms between inbred strains. Fifty percent of the tested primer pairs were polymorphic between inbred strains and 90% were polymorphic between mouse species, which is a frequency comparable to that found for microsatellite repeat sequences. We have found that this technique can be readily used to determine the strain distribution pattern in a recombinant inbred series and is a simple and rapid means to obtain a map position for cloned sequences. When this strategy was tested on a number of previously mapped cloned genes, the strain distribution patterns obtained were consistent with that to be expected on the basis of the known map position. We also tested the utility of this approach for characterizing genes that have not been previously mapped. Dvl, the mouse homolog of the putative Drosophila dishevelled gene, and Adfp, encoding an adipocyte differentiation-related protein, were found to map to chromosome 4. These results were confirmed using single-strand conformation polymorphism analysis of an interspecific backcross.