101
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Llorente-Cortés V, Costales P, Bernués J, Camino-Lopez S, Badimon L. Sterol regulatory element-binding protein-2 negatively regulates low density lipoprotein receptor-related protein transcription. J Mol Biol 2006; 359:950-60. [PMID: 16697011 DOI: 10.1016/j.jmb.2006.04.008] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/17/2005] [Revised: 03/08/2006] [Accepted: 04/05/2006] [Indexed: 11/17/2022]
Abstract
Low density lipoprotein receptor-related protein (LRP1) binds aggregated LDL (agLDL) leading to a high intracellular cholesteryl ester (CE) accumulation. AgLDL up-regulates LRP1 expression concomitantly with an LDL receptor (LDLR) and sterol regulatory element binding protein (SREBP-2) down-regulation. The objectives were to investigate whether SREBP-2 regulates LRP1 transcription and determine the molecular mechanisms involved in the process. Down-regulation of active SREBP-2 by nLDL and agLDL led to LDLR down-regulation and LRP1 up-regulation. Enforced expression of an active form of SREBP-2 (SREBP-2-NT, amino acid residues 1-468) decreased LRP1 expression and LRP1 promoter (WT-LRP1) luciferase activity in a dose-dependent manner. LDL did not exert any significant effect on LRP1 promoter activity when a putative sterol regulatory element (SRE) (5-GTGGGGTGA-3'; +225 to +233) was mutated (SRE-MT-LRP1). SREBP-2 overexpression exerted stronger down-regulatory effects on WT-LRP1 than on SRE-MT-LRP1 promoter activity both in control, nLDL- and agLDL-exposed HeLa cells. Gel mobility shift assays showed that recombinant SREBP-2-NT protein (1-468) binds to a double-stranded LRP1 DNA fragment (215 to 245) containing a wild-type (wt) SRE sequence but not to a mutated SRE (mt) sequence (5-GAATTCGA-3'). Our results demonstrate that LDL stimulates LRP1 transcription and decreases SREBP-2 active form which negatively regulates LRP1 transcription. SRE sequence (+225 to +233) plays a pivotal role for the down-regulatory effect of SREBP-2 on LRP1 promoter activity.
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Affiliation(s)
- V Llorente-Cortés
- Cardiovascular Research Center, CSIC-ICCC, Hospital de la Santa Creu i Sant Pau, Barcelona, 08025, Spain
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102
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Zhou H, Pandak WM, Hylemon PB. Cellular mechanisms of lipodystrophy induction by HIV protease inhibitors. ACTA ACUST UNITED AC 2006. [DOI: 10.2217/17460875.1.2.163] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
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103
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Affiliation(s)
- Peter J Espenshade
- Johns Hopkins University School of Medicine, 725 N. Wolfe Street, Baltimore, MA 21205, USA.
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104
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Ozbay T, Rowan A, Leon A, Patel P, Sewer MB. Cyclic adenosine 5'-monophosphate-dependent sphingosine-1-phosphate biosynthesis induces human CYP17 gene transcription by activating cleavage of sterol regulatory element binding protein 1. Endocrinology 2006; 147:1427-37. [PMID: 16306078 DOI: 10.1210/en.2005-1091] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
In the human adrenal cortex, ACTH activates steroid hormone biosynthesis by acutely increasing cholesterol delivery to the mitochondrion and chronically increasing the transcription of steroidogenic genes (including CYP17) via a cAMP-dependent pathway. In the present study, we characterized the role of sphingolipids in ACTH-dependent steroidogenesis. H295R human adrenocortical cells were treated with ACTH or dibutyryl cAMP (Bt2cAMP) and the content of several sphingolipid species quantified by mass spectrometry. Both ACTH and Bt2cAMP decreased cellular amounts of several sphingolipids, including sphingomyelin, ceramides, and sphingosine and stimulating the activity of sphingosine kinase and increasing the release of sphingosine-1-phosphate (S1P) into the media. S1P increased CYP17 mRNA expression by promoting the cleavage and nuclear localization of sterol regulatory element binding protein (SREBP) 1. Chromatin immunoprecipitation assays revealed that Bt2cAMP and S1P increased acetylation of histone H3 and promoted binding of SREBP1 to the -520/-331 region of the CYP17 promoter. In summary, our studies demonstrate a role for sphingolipid metabolism and SREBP1 in ACTH-dependent CYP17 regulation and steroidogenesis.
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Affiliation(s)
- Tuba Ozbay
- School of Biology, Georgia Institute of Technology, 310 Ferst Drive, Atlanta, Georgia 30332-0230, USA
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105
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Martín de Llano JJ, Fuertes G, Andreu EJ, Puig O, Chaves FJ, Soutar AK, Armengod ME, Knecht E. A single point mutation in the low-density lipoprotein receptor switches the degradation of its mature protein from the proteasome to the lysosome. Int J Biochem Cell Biol 2006; 38:1340-51. [PMID: 16530458 DOI: 10.1016/j.biocel.2006.01.014] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2005] [Revised: 01/02/2006] [Accepted: 01/17/2006] [Indexed: 11/28/2022]
Abstract
Pathogenic mutations in the low-density lipoprotein receptor prevent cholesterol uptake and cause familial hypercholesterolemia. In comparison to the biogenesis and endocytic trafficking of this receptor and some of its mutants, their degradation mechanisms are not well understood. Therefore, to gain some insights into this aspect, we analyzed the effects of proteasomal and lysosomal inhibitors on the levels of the wild type low-density lipoprotein receptor and a mutant form, C358Y, which was prevalent in a sample of Spanish familial hypercholesterolemia patients. In transfected cells, the mutant C358Y exhibited lower activity than the wild type receptor, as well as retarded post-translational processing of its precursor to the mature form. Interestingly, about 30% of the mutant precursor was degraded by a lysosomal pathway. Moreover, its mature form was more rapidly degraded than the wild type receptor (half lives of 5.3 and 10.9 h, respectively) and its degradation was exclusively dependent on a lysosomal pathway. In contrast, the mature form of the wild type receptor was mainly degraded by proteasomes and, to a minor extent (30%), by lysosomes. We conclude that a single mutation in the low-density lipoprotein receptor switches the degradation of the mature receptor from a proteasomal to a lysosomal pathway which degrades the protein at a faster rate. This suggests cooperation of proteasomes and lysosomes in the degradation of the low-density lipoprotein receptor and adds an intriguing new aspect to our understanding of receptor-mediated endocytosis.
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Affiliation(s)
- José Javier Martín de Llano
- Laboratorio de Biología Celular, Centro de Investigación Príncipe Felipe, Avda. Autopista del Saler 16, 46013 Valencia, Spain
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106
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Balasubramanyam M, Sampathkumar R, Mohan V. Is insulin signaling molecules misguided in diabetes for ubiquitin-proteasome mediated degradation? Mol Cell Biochem 2006; 275:117-25. [PMID: 16335791 DOI: 10.1007/s11010-005-1083-y] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
Recent mining of the human and mouse genomes, use of yeast genetics, and detailed analyses of several biochemical pathways, have resulted in the identification of many new roles for ubiquitin-proteasome mediated degradation of proteins. In the context of last year's award of Noble Prize (Chemistry) work, the ubiquitin and ubiquitin-like modifications are increasingly recognized as key regulatory events in health and disease. Although the ATP-dependent ubiquitin-proteasome system has evolved as premier cellular proteolytic machinery, dysregulation of this system by several different mechanisms leads to inappropriate degradation of specific proteins and pathological consequences. While aberrations in the ubiquitin-proteasome pathway have been implicated in certain malignancies and neurodegenerative disorders, recent studies indicate a role for this system in the pathogenesis of diabetes and its complications. Inappropriate degradation of insulin signaling molecules such as insulin receptor substrates (IRS-1 and IRS-2) has been demonstrated in experimental diabetes, mediated in part through the up-regulation of suppressors of cytokine signaling (SOCS). It appears that altered ubiquitin-proteasome system might be one of the molecular mechanisms of insulin resistance in many pathological situations. Drugs that modulate the SOCS action and/or proteasomal degradation of proteins could become novel agents for the treatment of insulin resistance and Type 2 diabetes.
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Affiliation(s)
- Muthuswamy Balasubramanyam
- Department of Cell and Molecular Biology, Madras Diabetes Research Foundation, 6B, Conran Smith Road, Gopalapuram, Chennai 600 086, India.
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107
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Logette E, Le Jossic-Corcos C, Masson D, Solier S, Sequeira-Legrand A, Dugail I, Lemaire-Ewing S, Desoche L, Solary E, Corcos L. Caspase-2, a novel lipid sensor under the control of sterol regulatory element binding protein 2. Mol Cell Biol 2005; 25:9621-31. [PMID: 16227610 PMCID: PMC1265809 DOI: 10.1128/mcb.25.21.9621-9631.2005] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
Caspases play important roles in apoptotic cell death and in some other functions, such as cytokine maturation, inflammation, or differentiation. We show here that the 5'-flanking region of the human CASP-2 gene contains three functional response elements for sterol regulatory element binding proteins (SREBPs), proteins that mediate the transcriptional activation of genes involved in cholesterol, triacylglycerol, and fatty acid synthesis. Exposure of several human cell lines to statins, lipid-lowering drugs that drive SREBP proteolytic activation, induced the CASP-2 gene to an extent similar to that for known targets of SREBP proteins. Adenoviral vector-mediated transfer of active SREBP-2 also induced expression of the CASP-2 gene and the caspase-2 protein and increased the cholesterol and triacylglycerol cellular content. These rises in lipids were strongly impaired following small interfering RNA-mediated silencing of the CASP-2 gene. Taken together, our results identify the human CASP-2 gene as a member of the SREBP-responsive gene battery that senses lipid levels in cells and raise the possibility that caspase-2 participates in the control of cholesterol and triacylglycerol levels.
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Affiliation(s)
- E Logette
- INSERM 517, IFR100, Faculté de Médecine, 7 boulevard Jeanne of Arc, 21000 Dijon, France
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108
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Nakahara M, Furuya N, Takagaki K, Sugaya T, Hirota K, Fukamizu A, Kanda T, Fujii H, Sato R. Ileal Bile Acid-binding Protein, Functionally Associated with the Farnesoid X Receptor or the Ileal Bile Acid Transporter, Regulates Bile Acid Activity in the Small Intestine. J Biol Chem 2005; 280:42283-9. [PMID: 16230354 DOI: 10.1074/jbc.m507454200] [Citation(s) in RCA: 61] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Bile acids secreted in the small intestine are reabsorbed in the ileum where they activate the nuclear farnesoid X receptor (FXR), which in turn stimulates expression of the ileal bile acid-binding protein (I-BABP). We first hypothesized that I-BABP may negatively regulate the FXR activity by competing for the ligands, bile acids. Reporter assays using stable HEK293 cell lines expressing I-BABP revealed that I-BABP enhances rather than attenuates FXR activity. In these cells I-BABP localizes predominantly in the cytosol and partially in the nucleus, a distribution that does not shift in response to FXR expression. In vitro binding assays reveal that recombinant I-BABP is able to bind 35S-labeled FXR and that chenodeoxycholic acid (CDCA) stimulates this interaction modestly. When FLAG-tagged FXR was expressed in stable cells, the FXR.I-BABP complex in the nuclear extracts was more efficiently immunoprecipitable with anti-FLAG antibodies in the presence of CDCA. These results indicate that I-BABP stimulates FXR activity through a mutual interaction augmented by bile acids. When stable cells were transfected with an expression plasmid of the ileal bile acid transporter 14(IBAT) essential for the reabsorption of conjugated bile acids, the C-labeled conjugated bile acid, glycocholic acid, was more efficiently imported via IBAT in the presence than absence of I-BABP, whereas no change was observed in 14C-labeled CDCA uptake, which is independent of IBAT. Immunofluorescent staining analysis revealed that these two proteins co-localize in the vicinity of the plasma membrane in stable cells. Taken together, the current data provide the first evidence that I-BABP is functionally associated with FXR and IBAT in the nucleus and on the membrane, respectively, stimulating FXR transcriptional activity and the conjugated bile acid uptake mediated by IBAT in the ileum.
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Affiliation(s)
- Mayuko Nakahara
- Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, University of Tokyo, Tokyo 113-8657
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109
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Kong WJ, Liu J, Jiang JD. Human low-density lipoprotein receptor gene and its regulation. J Mol Med (Berl) 2005; 84:29-36. [PMID: 16292665 DOI: 10.1007/s00109-005-0717-6] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/25/2005] [Accepted: 08/05/2005] [Indexed: 01/23/2023]
Abstract
The low-density lipoprotein (LDL) receptor is a transmembrane glycoprotein that mediates the binding and endocytosis of lipoproteins containing apolipoprotein B and E, especially the cholesterol-rich LDL. Mutations in the LDL receptor gene can produce dysfunctional LDL receptors and cause familial hypercholesterolemia. The expression of the LDL receptor gene is under an intriguing regulation by sterol and nonsterol mediators either at the transcriptional level or at the posttranscriptional level, both of which are linked to cell signaling pathways. Upregulation of liver LDL receptor expression is effective in treating hypercholesterolemia. In this review, we focus on the latest progress on the mechanisms and regulation of the LDL receptor gene expression.
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Affiliation(s)
- Wei-Jia Kong
- Institute of Medicinal Biotechnology, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing 100050, People's Republic of China
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110
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Jump DB, Botolin D, Wang Y, Xu J, Christian B, Demeure O. Fatty acid regulation of hepatic gene transcription. J Nutr 2005; 135:2503-6. [PMID: 16251601 DOI: 10.1093/jn/135.11.2503] [Citation(s) in RCA: 331] [Impact Index Per Article: 17.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Dietary fat regulates gene expression by controlling the activity or abundance of key transcription factors. In vitro binding and cell culture studies have identified many transcription factors as prospective targets for fatty acid regulation, including peroxisome proliferator-activated receptors (PPARalpha, beta, gamma1, and gamma2), sterol regulatory element binding protein-1c (SREBP-1c), hepatic nuclear factors (HNF-4alpha and gamma), retinoid X receptor (RXRalpha), liver X receptor (LXRalpha), and others. In vivo studies established that PPARalpha- and SREBP-1c-regulated genes are key targets for PUFA control of hepatic gene expression. PUFA activate PPARalpha by direct binding, leading to the induction of hepatic fatty acid oxidation. PUFA inhibit hepatic fatty acid synthesis by suppressing SREBP-1c nuclear abundance through several mechanisms, including suppression of SREBP-1c gene transcription and enhancement of proteasomal degradation and mRNA(SREBP1c) decay. Changes in intracellular nonesterified fatty acids (NEFA) correlate well with changes in PPARalpha activity and mRNA(SREBP-1c) abundance. Several mechanisms regulate intracellular NEFA composition, including fatty acid transport, acyl CoA synthetases and thioesterases, fatty acid elongases and desaturases, neutral and polar lipid lipases, and fatty acid oxidation. Many of these mechanisms are regulated by PPARalpha or SREBP-1c. Together, these mechanisms control hepatic lipid composition and affect whole-body lipid composition.
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Affiliation(s)
- Donald B Jump
- Departments of Physiology, Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824, USA.
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111
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Botolin D, Wang Y, Christian B, Jump DB. Docosahexaneoic acid (22:6,n-3) regulates rat hepatocyte SREBP-1 nuclear abundance by Erk- and 26S proteasome-dependent pathways. J Lipid Res 2005; 47:181-92. [PMID: 16222032 PMCID: PMC2764363 DOI: 10.1194/jlr.m500365-jlr200] [Citation(s) in RCA: 121] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
Insulin induces and dietary n-3 PUFAs suppress hepatic de novo lipogenesis by controlling sterol-regulatory element binding protein-1 nuclear abundance (nSREBP-1). Our goal was to define the mechanisms involved in this regulatory process. Insulin treatment of rat primary hepatocytes rapidly augments nSREBP-1 and mRNA(SREBP-1c) while suppressing mRNA(Insig-2) but not mRNA(Insig-1). These events are preceded by rapid but transient increases in Akt and Erk phosphorylation. Removal of insulin from hepatocytes leads to a rapid decline in nSREBP-1 [half-time (T1/2) approximately 10 h] that is abrogated by inhibitors of 26S proteasomal degradation. 22:6,n-3, the major n-3 PUFA accumulating in livers of fish oil-fed rats, suppresses hepatocyte levels of nSREBP-1, mRNA(SREBP-1c), and mRNA(Insig-2) but modestly and transiently induces mRNA(Insig-1). More importantly, 22:6,n-3 accelerates the disappearance of hepatocyte nSREBP-1 (T1/2 approximately 4 h) through a 26S proteasome-dependent process. 22:6,n-3 has minimal effects on microsomal SREBP-1 and sterol-regulatory element binding protein cleavage-activating protein or nuclear SREBP-2. 22:6,n-3 transiently inhibits insulin-induced Akt phosphorylation but induces Erk phosphorylation. Inhibitors of Erk phosphorylation, but not overexpressed constitutively active Akt, rapidly attenuate 22:6,n-3 suppression of nSREBP-1. Thus, 22:6,n-3 suppresses hepatocyte nSREBP-1 through 26S proteasome- and Erk-dependent pathways. These studies reveal a novel mechanism for n-3 PUFA regulation of hepatocyte nSREBP-1 and lipid metabolism.
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112
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Yellaturu CR, Deng X, Cagen LM, Wilcox HG, Park EA, Raghow R, Elam MB. Posttranslational processing of SREBP-1 in rat hepatocytes is regulated by insulin and cAMP. Biochem Biophys Res Commun 2005; 332:174-80. [PMID: 15896314 DOI: 10.1016/j.bbrc.2005.04.112] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/20/2005] [Accepted: 04/22/2005] [Indexed: 10/25/2022]
Abstract
Insulin and cAMP have opposing effects on de novo fatty acid synthesis in liver and in cultured hepatocytes mediated by sterol-regulatory element binding protein (SREBP). To determine whether these agents regulate the cleavage of full-length SREBP to generate the transcriptionally active N-terminal fragment (nSREBP) in primary rat hepatocytes, an adenoviral vector (Ad-SREBP-1a) was constructed to constitutively express full-length SREBP-1a. Insulin increased, and dibutyryl (db)-cAMP inhibited, generation of nSREBP-1a from its full-length precursor. Insulin stimulated processing of SREBP-1a within 1h, and the effect was sustained for at least 24h. The initial stimulation of SREBP processing by insulin preceded measurable reduction in Insig-2 mRNA levels. Rat hepatocytes were also infected with an adenovirus expressing the nuclear form of SREBP-1c (Ad-nSREBP-1c). Insulin increased the half-life of constitutively expressed nSREBP-1c, and this effect of insulin was also inhibited by db-cAMP.
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113
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Cagen LM, Deng X, Wilcox HG, Park EA, Raghow R, Elam MB. Insulin activates the rat sterol-regulatory-element-binding protein 1c (SREBP-1c) promoter through the combinatorial actions of SREBP, LXR, Sp-1 and NF-Y cis-acting elements. Biochem J 2005; 385:207-16. [PMID: 15330762 PMCID: PMC1134689 DOI: 10.1042/bj20040162] [Citation(s) in RCA: 118] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
The enhanced synthesis of fatty acids in the liver and adipose tissue in response to insulin is critically dependent on the transcription factor SREBP-1c (sterol-regulatory-element-binding protein 1c). Insulin increases the expression of the SREBP-1c gene in intact liver and in hepatocytes cultured in vitro. To learn the mechanism of this stimulation, we analysed the activation of the rat SREBP-1c promoter and its truncated or mutated congeners driving a luciferase reporter gene in transiently transfected rat hepatocytes. The rat SREBP-1c promoter contains binding sites for LXR (liver X receptor), Sp1, NF-Y (nuclear factor-Y) and SREBP itself. We have found that each of these sites is required for the full stimulatory response of the SREBP-1c promoter to insulin. Mutation of either the putative LXREs (LXR response elements) or the SRE (sterol response element) in the proximal SREBP-1c promoter reduced the stimulatory effect of insulin by about 50%. Insulin and the LXR agonist TO901317 increased the association of SREBP-1 with the SREBP-1c promoter. Ectopic expression of LXRalpha or SREBP-1c increased activity of the SREBP-1c promoter, and this effect is further enhanced by insulin. The Sp1 and NF-Y sites adjacent to the SRE are also required for full activation of the SREBP-1c promoter by insulin. We propose that the combined actions of the SRE, LXREs, Sp1 and NF-Y elements constitute an insulin-responsive cis-acting unit of the SREBP-1c gene in the liver.
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Affiliation(s)
- Lauren M Cagen
- Department of Pharmacology, The University of Tennessee Health Science Center, 874 Union Avenue, Memphis, TN 38163, USA.
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114
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Sundqvist A, Bengoechea-Alonso MT, Ye X, Lukiyanchuk V, Jin J, Harper JW, Ericsson J. Control of lipid metabolism by phosphorylation-dependent degradation of the SREBP family of transcription factors by SCF(Fbw7). Cell Metab 2005; 1:379-91. [PMID: 16054087 DOI: 10.1016/j.cmet.2005.04.010] [Citation(s) in RCA: 340] [Impact Index Per Article: 17.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/13/2004] [Revised: 04/08/2005] [Accepted: 04/27/2005] [Indexed: 12/11/2022]
Abstract
The sterol regulatory element binding protein (SREBP) family of transcription factors controls cholesterol and lipid metabolism. The nuclear forms of these proteins are rapidly degraded by the ubiquitin-proteasome pathway, but the signals and factors required for this are unknown. Here, we identify a phosphodegron in SREBP1a that serves as a recognition motif for the SCF(Fbw7) ubiquitin ligase. Fbw7 interacts with nuclear SREBP1a and enhances its ubiquitination and degradation in a manner dependent on the phosphorylation of T426 and S430 by GSK-3. Fbw7 also degrades nuclear SREBP1c and SREBP2, and inactivation of endogenous Fbw7 results in stabilization of nuclear SREBP1 and -2, enhanced expression of SREBP target genes, enhanced synthesis of cholesterol and fatty acids, and enhanced receptor-mediated uptake of LDL. Thus, our results suggest that Fbw7 may be a major regulator of lipid metabolism through control of the phosphorylation-dependent degradation of the SREBP family of transcription factors.
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Affiliation(s)
- Anders Sundqvist
- Ludwig Institute for Cancer Research, Box 595, Husargatan 3, S-751 24 Uppsala, Sweden
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115
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Parker RA, Flint OP, Mulvey R, Elosua C, Wang F, Fenderson W, Wang S, Yang WP, Noor MA. Endoplasmic reticulum stress links dyslipidemia to inhibition of proteasome activity and glucose transport by HIV protease inhibitors. Mol Pharmacol 2005; 67:1909-19. [PMID: 15755908 DOI: 10.1124/mol.104.010165] [Citation(s) in RCA: 84] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The lipid and metabolic disturbances associated with human immunodeficiency virus (HIV) protease inhibitor therapy in AIDS have stimulated interest in developing new agents that minimize these side effects in the clinic. The underlying explanation of mechanism remains enigmatic, but a recently described link between endoplasmic reticulum (ER) stress and dysregulation of lipid metabolism suggests a provocative integration of existing and emerging data. We provide new evidence from in vitro models indicating that proteasome inhibition and differential glucose transport blockade by protease inhibitors are proximal events eliciting an ER stress transcriptional response that can regulate lipogenic pathways in hepatocytes or adipocytes. Proteasome activity was inhibited in vitro by several protease inhibitors at clinically relevant (micromolar) levels. In the intact cells, protease inhibitors rapidly elicited a pattern of gene expression diagnostic of intracellular proteasome inhibition and activation of an ER stress response. This included induction of transcription factors GADD153, ATF4, and ATF3; amino acid metabolic enzymes; proteasome components; and certain ER chaperones. In hepatocyte lines, the ER stress response was closely linked to moderate increases in lipogenic and cholesterogenic gene expression. However, in adipocytes where GLUT4 was directly inhibited by some protease inhibitors, time-dependent suppression of lipogenic genes and triglyceride synthesis was observed in coordination with the ER stress response. These results further link ER stress to dyslipidemia and contribute to a unifying mechanism for the pathophysiology of protease inhibitor-associated lipodystrophy, helping explain differences in clinical metabolic profiles among protease inhibitors.
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Affiliation(s)
- Rex A Parker
- Metabolic and Cardiovascular Discovery Biology, Bristol-Myers Squibb Pharmaceutical Research Institute, 311 Pennington-Rocky Hill Road, Pennington, NJ 08534, USA.
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116
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Hegarty BD, Bobard A, Hainault I, Ferré P, Bossard P, Foufelle F. Distinct roles of insulin and liver X receptor in the induction and cleavage of sterol regulatory element-binding protein-1c. Proc Natl Acad Sci U S A 2005; 102:791-6. [PMID: 15637161 PMCID: PMC545517 DOI: 10.1073/pnas.0405067102] [Citation(s) in RCA: 155] [Impact Index Per Article: 8.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Sterol regulatory element-binding proteins (SREBPs) are transcription factors central to the regulation of lipid metabolism. The SREBPs are synthesized as precursor proteins that require proteolytic processing to become transcriptionally active. Whereas the regulation of SREBP-1a and -2 cleavage by cellular sterol content is well defined, much less is known about the regulation of SREBP-1c, the predominant SREBP isoform in the liver. Both insulin and liver X receptor alpha (LXRalpha) induce SREBP-1c transcription; however, the respective roles of these factors and the mechanism responsible for proteolytic cleavage of this SREBP isoform are not known. In this study, we compare the effects of insulin and LXR agonist TO-901317 on SREBP-1c expression and transcriptional activity in isolated rat hepatocytes. We report that full induction of the mature and transcriptionally active form of SREBP-1c protein requires insulin. Although activation of LXR leads to the induction of SREBP-1c gene expression and precursor protein, it has a very poor effect in inducing the mature nuclear form of the transcription factor. This may be due to the induction of insulin-induced gene-2a mRNA and protein by LXR activation. The LXR-induced SREBP-1c precursor, however, is rapidly cleaved on acute exposure to insulin via a phosphatidylinositol 3-kinase-dependent mechanism. Finally, we show through experiments in suckling mice that this acute action of insulin to stimulate the proteolytic processing of SREBP-1c is functional in vivo.
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Affiliation(s)
- Bronwyn D Hegarty
- Institut National de la Santé et de la Recherche Médicale Unit 465, Université Paris 6, 15, Rue de l'Ecole de Médecine, 75006 Paris, France
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117
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Eberlé D, Hegarty B, Bossard P, Ferré P, Foufelle F. SREBP transcription factors: master regulators of lipid homeostasis. Biochimie 2004; 86:839-48. [PMID: 15589694 DOI: 10.1016/j.biochi.2004.09.018] [Citation(s) in RCA: 1062] [Impact Index Per Article: 53.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/01/2004] [Accepted: 09/27/2004] [Indexed: 02/08/2023]
Abstract
Sterol regulatory element binding proteins (SREBPs) are a family of transcription factors that regulate lipid homeostasis by controlling the expression of a range of enzymes required for endogenous cholesterol, fatty acid (FA), triacylglycerol and phospholipid synthesis. The three SREBP isoforms, SREBP-1a, SREBP-1c and SREBP-2, have different roles in lipid synthesis. In vivo studies using transgenic and knockout mice suggest that SREBP-1c is involved in FA synthesis and insulin induced glucose metabolism (particularly in lipogenesis), whereas SREBP-2 is relatively specific to cholesterol synthesis. The SREBP-1a isoform seems to be implicated in both pathways. SREBP transcription factors are synthetized as inactive precursors bound to the endoplasmic reticulum (ER) membranes. Upon activation, the precursor undergoes a sequential two-step cleavage process to release the NH(2)-terminal active domain in the nucleus (designated nSREBPs). SREBP processing is mainly controlled by cellular sterol content. When sterol levels decrease, the precursor is cleaved to activate cholesterogenic genes and maintain cholesterol homeostasis. This sterol-sensitive process appears to be a major point of regulation for the SREBP-1a and SREBP-2 isoforms but not for SREBP-1c. Moreover, the SREBP-1c isoform seems to be mainly regulated at the transcriptional level by insulin. The unique regulation and activation properties of each SREBP isoform facilitate the co-ordinate regulation of lipid metabolism; however, further studies are needed to understand the detailed regulation pathways that specifically regulate each SREBP isoform.
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Affiliation(s)
- Delphine Eberlé
- Inserm U465, Université Pierre et Marie Curie, 15, rue de l'Ecole de médecine, 75270 Paris cedex 06, France
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118
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Toth JI, Datta S, Athanikar JN, Freedman LP, Osborne TF. Selective coactivator interactions in gene activation by SREBP-1a and -1c. Mol Cell Biol 2004; 24:8288-300. [PMID: 15340088 PMCID: PMC515064 DOI: 10.1128/mcb.24.18.8288-8300.2004] [Citation(s) in RCA: 98] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
Requisite levels of intracellular cholesterol and fatty acids are maintained in part by the sterol regulatory element binding proteins (SREBPs). Three major SREBP isoforms exist; SREBP-1a and SREBP-1c are expressed from overlapping mRNAs, whereas SREBP-2 is encoded by a separate gene. The active forms of SREBP-1a and SREBP-1c differ only at their extreme N termini; SREBP-1c lacks 28 aa present in SREBP-1a and instead contains 4 unique aa of its own. While the SREBP-1a and -1c isoforms differentially activate transcription, the molecular basis of this difference is unknown. Here we define the differences between these proteins that confer the enhanced activity of SREBP-1a and demonstrate that this enhancement is a direct result of its avid binding to the coactivator CREB binding protein (CBP) and the mammalian mediator complex. While previous work determined that the C/H1 zinc finger and KIX domains of CBP bind to SREBP-1a, we provide evidence that the interaction with C/H1 is important for gene activation. We further show that the association between the activation domain of SREBP-1 and mediator is through aa 500 to 824 of DRIP150. Finally, we demonstrate the recruitment of mediator to an SREBP-responsive promoter in a sterol-dependent manner.
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Affiliation(s)
- Julia I Toth
- Molecular Biology and Biochemistry, University of California, Irvine, Irvine, CA 92612-3900, USA
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119
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Zhang Y, Hillgartner FB. Starvation and feeding a high-carbohydrate, low-fat diet regulate the expression sterol regulatory element-binding protein-1 in chickens. J Nutr 2004; 134:2205-10. [PMID: 15333705 DOI: 10.1093/jn/134.9.2205] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
In mammalian liver, the mature form of sterol regulatory element-binding protein-1c (SREBP-1c) is an important activator of a wide array of genes involved in triacylglycerol biosynthesis. Starvation and feeding a high-carbohydrate, low-fat diet modulate the concentration of mature SREBP-1c primarily by a pretranslational mechanism. It is not known whether alterations in nutritional status regulate the concentration of SREBPs in nonmammalian species. In this study, we found that in previously starved chicks, feeding a high-carbohydrate, low-fat diet stimulated a robust increase (14-fold at 5 h of feeding) in the concentration of mature SREBP-1 in liver. Feeding a high-carbohydrate, low-fat diet also increased the concentration of precursor SREBP-1 and SREBP-1 messenger RNA in chick liver; however, the magnitude of this effect was substantially lower than that observed for mature SREBP-1. DNA binding experiments demonstrated that 3 protein complexes containing SREBP bound the acetyl-CoA carboxylase-alpha (ACCalpha) sterol regulatory element (SRE) in chick liver and that the binding activity of 2 of these complexes was increased by consumption of a high-carbohydrate, low-fat diet. Additional analyses showed that feeding a high-carbohydrate, low-fat diet had no effect on the concentration of mature SREBP-2 and the binding of SREBP-2 to the ACCalpha SRE in chick liver. These results indicate that alterations in the concentration of mature SREBP-1 play a role in mediating the effects of starvation and feeding a high-carbohydrate, low-fat diet on ACCalpha transcription in chick liver and that diet-induced changes in mature SREBP-1 concentration in chick liver are mediated primarily by a posttranslational mechanism.
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Affiliation(s)
- Yanqiao Zhang
- Department of Biochemistry and Molecular Pharmacology, School of Medicine, West Virginia University, Morgantown, WV 26506, USA
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120
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Hirokane H, Nakahara M, Tachibana S, Shimizu M, Sato R. Bile acid reduces the secretion of very low density lipoprotein by repressing microsomal triglyceride transfer protein gene expression mediated by hepatocyte nuclear factor-4. J Biol Chem 2004; 279:45685-92. [PMID: 15337761 DOI: 10.1074/jbc.m404255200] [Citation(s) in RCA: 125] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
Microsomal triglyceride transfer protein (MTP) is involved in the transfer of triglycerides, cholesterol esters, and phospholipids to newly synthesized apolipoprotein (apo) B. It is therefore essential for lipoprotein synthesis and secretion in the liver and the small intestine. Although several recent experiments have revealed the transcriptional regulation of the MTP gene, little has been revealed to date about hepatocyte nuclear factor-4 (HNF-4)-dependent regulation. We here report that the human MTP gene promoter contains a pair of functional responsive elements for HNF-4 and HNF-1, the latter of which is another target gene of HNF-4. Chromatin immunoprecipitation assays provide evidence that endogenous HNF-4 and HNF-1 can bind these elements in chromatin. In Hep G2 cells overexpression of either a dominant negative form of HNF-4 or small interfering RNAs (siRNAs) against HNF-4 dramatically reduces the activities of both the wild type and the HNF-4 site mutant MTP promoter. This suggests that HNF-4 regulates MTP gene expression either directly or indirectly through elevated HNF-1 levels. When Hep G2 cells were cultured with chenodeoxycholic acid (CDCA), a ligand for the farnesoid X receptor (FXR), mRNA levels for MTP and apo B were reduced because of increased expression of the factor small heterodimer partner (SHP), which factor suppresses HNF-4 activities. Chenodeoxycholic acid, but not a synthetic FXR ligand, attenuated expression of HNF-4, bringing about a further suppression of MTP gene expression. Over time the intracellular MTP protein levels and apo B secretion in the culture medium significantly declined. These results indicate that two nuclear receptors, HNF-4 and FXR, are closely involved in MTP gene expression, and the results provide evidence for a novel interaction between bile acids and lipoprotein metabolism.
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Affiliation(s)
- Hisako Hirokane
- Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan
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121
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Kuhn DJ, Burns AC, Kazi A, Dou QP. Direct inhibition of the ubiquitin–proteasome pathway by ester bond-containing green tea polyphenols is associated with increased expression of sterol regulatory element-binding protein 2 and LDL receptor. Biochim Biophys Acta Mol Cell Biol Lipids 2004; 1682:1-10. [PMID: 15158750 DOI: 10.1016/j.bbalip.2003.12.006] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2003] [Revised: 12/12/2003] [Accepted: 12/12/2003] [Indexed: 01/08/2023]
Abstract
Green tea has been shown to lower plasma cholesterol, associated with up-regulation of the low-density lipoprotein receptor (LDLR) although the responsible molecular mechanism is unknown. Previously, we reported that ester bond-containing green tea polyphenols (GTPs), such as (-)-epigallocatechin-3-gallate [(-)-EGCG], potently inhibit the tumor cellular proteasome activity, which may contribute to the cancer-preventative effect of green tea. In the current study, we hypothesize that the proteasome is a heart disease-associated molecular target of GTPs. We have shown that ester bond-containing GTPs, including (-)-EGCG, potently inhibit the proteasomal activity in intact hepatocellular carcinoma HepG2 and cervical carcinoma HeLa cells, as evident by accumulation of ubiquitinated proteins and three natural proteasome targets (p27, IkappaB-alpha and Bax). (-)-EGCG selectively inhibits the chymotrypsin-like, but not trypsin-like, activity of the proteasome. Associated with proteasome inhibition by ester bond-containing GTPs, there was a significant, time- and concentration-dependent increase in levels of the cleaved, activated, but not the precursor, form of sterol regulatory element-binding protein 2 (SREBP-2), an essential factor for LDLR transcription. Subsequently, LDL receptor expression was increased dramatically in HepG2 and HeLa cells treated with (-)-EGCG. Our results suggest that ester bond-containing GTPs inhibit ubiquitin/proteasome-mediated degradation of the active SREBP-2, resulting in up-regulation of LDLR. This identified molecular mechanism may be related to the previously reported cholesterol-lowering and heart disease-preventative effects of green tea.
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Affiliation(s)
- Deborah J Kuhn
- Drug Discovery Program, H Lee Moffitt Cancer Center and Research Institute, University of South FLorida, Tampa, FL 33612, USA
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122
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Rome S, Meugnier E, Vidal H. The ubiquitin-proteasome pathway is a new partner for the control of insulin signaling. Curr Opin Clin Nutr Metab Care 2004; 7:249-54. [PMID: 15075914 DOI: 10.1097/00075197-200405000-00002] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
PURPOSE OF REVIEW Insulin signaling is a transitory effect that has to be tightly controlled in magnitude and duration in order to maintain cell homeostasis. Recent reports have demonstrated that members of the ubiquitin-proteasome pathway represent new partners that have to be taken into account for the regulation of insulin action. RECENT FINDINGS The protein amounts of the different signaling molecules involved in insulin action are regulated by their rates of synthesis and degradation. The ubiquitin-proteasome system is involved in the internalization of the insulin receptor, in the control of the amount of insulin receptor substrates 1 and 2, and in insulin degradation. Finally, ubiquitination and sumoylation regulate transcription factors and nuclear receptors that mediate insulin-induced gene expression. SUMMARY It is well known from transgenic models that inappropriate levels of signaling molecules strongly affect insulin action. In humans also, several reports have provided evidence of altered levels of key proteins involved in insulin action in pathologies such as type 2 diabetes. The relationship between these abnormalities and the ubiquitin-proteasome pathway has yet to be clarified, but clarifying the role of ubiquitination in insulin action will certainly lead to a better understanding of insulin resistance.
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Affiliation(s)
- Sophie Rome
- UMR INRA1235-INSERM 449, Mécanismes moléculaires du diabète, Faculté de Médecine Laënnec, 69008 Lyon, France.
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123
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Arimura N, Horiba T, Imagawa M, Shimizu M, Sato R. The Peroxisome Proliferator-activated Receptor γ Regulates Expression of the Perilipin Gene in Adipocytes. J Biol Chem 2004; 279:10070-6. [PMID: 14704148 DOI: 10.1074/jbc.m308522200] [Citation(s) in RCA: 159] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Recent studies have shown that lipid droplets are covered with a proteinaceous coat, although the functions and identities of the component proteins have not yet been well elucidated. The first identified lipid droplet-specific proteins are the perilipins, a family of proteins coating the surfaces of lipid droplets of adipocytes. The generation of perilipin-null mice has revealed that although they consume more food than control mice, they have normal body weight and are resistant to diet-induced obesity. In one study (Martinez-Botas, J., Anderson, J. B., Tessier, D., Lapillonne, A., Chang, B. H. J., Quast, M. J., Gorenstein, D., Chen, K. H., and Chan, L. (2000) Nat. Genet. 26, 474-479) it was reported that in an animal model obesity was reversible by breeding perilipin -/- alleles into Lepr db/db obese mice, ostensibly by increasing the metabolic rate of the mice. To understand the exact mechanisms that drive the exclusive expression of the perilipin gene in adipocytes, we analyzed the 5'-flanking region of the mouse gene. Treatment of differentiating 3T3-L1 adipocytes with an agonist of proliferator-activated receptor (PPAR) gamma, the putative "master regulator" of adipocyte differentiation, significantly augmented perilipin gene expression. Reporter assays using the -2.0-kb promoter revealed that this region contains a functional PPARgamma-responsive element. Gel mobility shift and chromatin immunoprecipitation assays showed that endogenous PPARgamma protein binds to the perilipin promoter. PPARgamma2, an isoform exclusively expressed in adipocytes, was found to be the most potent regulator from among the PPAR family members including PPARalpha and PPARgamma1. These results make evident the fact that perilipin gene expression in differentiating adipocytes is crucially regulated by PPARgamma2, providing new insights into the adipogenic action of PPARgamma2 and adipose-specific gene expression, as well as potential anti-obesity pharmaceutical agents targeted to a reduction of the perilipin gene product.
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Affiliation(s)
- Naoto Arimura
- Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Japan
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124
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Li Y, Lu W, Schwartz AL, Bu G. Degradation of the LDL receptor class 2 mutants is mediated by a proteasome-dependent pathway. J Lipid Res 2004; 45:1084-91. [PMID: 14993243 DOI: 10.1194/jlr.m300482-jlr200] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
Familial hypercholesterolemia is a genetic disorder that results from various gene mutations, primarily within the LDL receptor (LDLR). Approximately 50% of the LDLR mutations are defined as class 2 mutations, with the mutant proteins partially or entirely retained in the endoplasmic reticulum. To determine the degradation pathway of the LDLR class 2 mutants, we examined the effects of inhibition of several potential pathways on the levels of the wild-type LDLR and its four representative class 2 mutants (S156L, C176Y, E207K, and C646Y) stably expressed in Chinese hamster ovary (CHO) cells. We found that proteasome inhibitors MG132 and lactacystin blocked the degradation of the LDLR mutants, but not that of the wild-type LDLR. Treatment of CHO cells with these proteasome inhibitors led to a significant accumulation of the mutants at steady state. Furthermore, cell surface levels of the LDLR mutants were significantly increased upon inhibition of the proteasome degradation pathway. In contrast to the proteasome inhibitors, inhibitors of trypsin-like proteases, chymotrypsin-like proteases, and lysosomal pathway inhibitors did not affect the levels of the LDLR mutants. Taken together, these data demonstrate that the proteasome is the principal degradation pathway for LDLR class 2 mutants.
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Affiliation(s)
- Yonghe Li
- Department of Pediatrics, St. Louis Children's Hospital, Washington University School of Medicine, St. Louis, MO 63110, USA.
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125
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Sundqvist A, Ericsson J. Transcription-dependent degradation controls the stability of the SREBP family of transcription factors. Proc Natl Acad Sci U S A 2003; 100:13833-8. [PMID: 14615581 PMCID: PMC283507 DOI: 10.1073/pnas.2335135100] [Citation(s) in RCA: 65] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/06/2003] [Indexed: 01/12/2023] Open
Abstract
Cholesterol metabolism is tightly controlled by members of the sterol regulatory element-binding protein (SREBP) family of transcription factors. Here we demonstrate that the ubiquitination and degradation of SREBPs depend on their transcriptional activity. Mutations in the transactivation or DNA-binding domains of SREBPs inhibit their transcriptional activity and stabilize the proteins. The transcriptional activity and degradation of these mutants are restored when fused to heterologous transactivation or DNA-binding domains. When SREBP1a was fused to the DBD of Gal4, the ubiquitination and degradation of the fusion protein depended on coexpression of a promoter-reporter gene containing Gal4-binding sites. In addition, disruption of the interaction between WT SREBP and endogenous p300/CBP resulted in inhibition of SREBP-dependent transcription and stabilization of SREBP. Chemical inhibitors of transcription reduced the degradation of transcriptionally active SREBP1a, whereas they had no effect on the stability of transcriptionally inactive mutants, demonstrating that transcriptional activation plays an important role in the degradation of SREBPs. Thus, transcription-dependent degradation of SREBP constitutes a feedback mechanism to regulate the expression of genes involved in cholesterol metabolism and may represent a general mechanism to regulate the duration of transcriptional responses.
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Affiliation(s)
- Anders Sundqvist
- Ludwig Institute for Cancer Research, Biomedical Center, Box 595, Husargatan 3, S-751 24 Uppsala, Sweden
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126
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Roitelman J, Masson D, Avner R, Ammon-Zufferey C, Perez A, Guyon-Gellin Y, Bentzen CL, Niesor EJ. Apomine, a novel hypocholesterolemic agent, accelerates degradation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase and stimulates low density lipoprotein receptor activity. J Biol Chem 2003; 279:6465-73. [PMID: 14627708 DOI: 10.1074/jbc.m308094200] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Apomine, a novel 1,1-bisphosphonate ester, has been shown to lower plasma cholesterol concentration in several species. Here we show that Apomine reduced the levels of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR), the rate-limiting enzyme in the mevalonate pathway, both in rat liver and in cultured cells. Apomine resembles sterols such as 25-hydroxycholesterol in its ability to potently accelerate the rate of HMGR degradation by the ubiquitin-proteasome pathway, a process that depends on the transmembrane domain of the enzyme. The similarity between Apomine and sterols in promoting rapid HMGR degradation extends to its acute requirements for ongoing protein synthesis and mevalonate-derived non-sterol product(s) as a co-regulator. Yet, at suboptimal concentrations, sterols potentiated the effect of Apomine in stimulating HMGR degradation, indicating that these agents act via distinct modes. Furthermore, unlike sterols, Apomine inhibited the activity of acyl-CoA:cholesterol acyltransferase in intact cells but not in cell-free extracts. Apomine stimulated the cleavage of the precursor of sterol-regulatory element-binding protein-2 and increased the activity of low density lipoprotein receptor pathway. This Apomine-enhanced activation of sterol-regulatory element-binding protein-2 was prevented by sterols or mevalonate. Taken together, our results provide a molecular mechanism for the hypocholesterolemic activity of Apomine.
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Affiliation(s)
- Joseph Roitelman
- Institute of Lipid and Atherosclerosis Research, Sheba Medical Center, Tel Hashomer 52621, Israel
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127
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Misawa K, Horiba T, Arimura N, Hirano Y, Inoue J, Emoto N, Shimano H, Shimizu M, Sato R. Sterol regulatory element-binding protein-2 interacts with hepatocyte nuclear factor-4 to enhance sterol isomerase gene expression in hepatocytes. J Biol Chem 2003; 278:36176-82. [PMID: 12855700 DOI: 10.1074/jbc.m302387200] [Citation(s) in RCA: 58] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
In the course of an effort to identify unknown targets genes for sterol regulatory element-binding proteins (SREBPs) by PCR, the gene for ATP citrate-lyase was determined to be one such gene. (Sato, R., Okamoto, A., Inoue, J., Miyamoto, W., Sakai, Y., Emoto, N., Shimano, H., and Maeda, M. (2000) J. Biol. Chem. 275, 12497-12502). We here report that gene expression of sterol Delta8-isomerase (SI), which catalyzes the conversion of the 8-ene isomer into the 7-ene isomer in the last steps of the cholesterol biosynthetic pathway, is regulated by SREBPs, mainly by SREBP-2. Luciferase assays using the promoter of the human SI gene revealed that a 200-base pair segment upstream region from the transcription start site contains functional elements required for the activity of the SREBPs, Sp1 and NF-Y. Interestingly, SI gene expression was well regulated by sterols in Caco-2 and HepG2 cells, in contrast with HEK293 and HeLa cells. Overexpression of hepatocyte nuclear factor (HNF)-4 in HEK293 cells augmented expression of SREBP-responsive genes including the SI gene, whereas inactivation of HNF-4 by small interfering RNAs in HepG2 cells reduced the SI gene promoter activity. The in vitro pull-down and in vivo co-immunoprecipitation experiments showed the direct interaction between SREBP-2 and HNF-4. These data provide a novel pathway by which HNF-4 potentiates the SREBP functions and stimulates expression of SREBP-responsive genes in enterohepatic cells.
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Affiliation(s)
- Koichi Misawa
- Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan
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128
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Bagutti C, Forro G, Ferralli J, Rubin B, Chiquet-Ehrismann R. The intracellular domain of teneurin-2 has a nuclear function and represses zic-1-mediated transcription. J Cell Sci 2003; 116:2957-66. [PMID: 12783990 DOI: 10.1242/jcs.00603] [Citation(s) in RCA: 74] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Teneurin-2, a vertebrate homologue of the Drosophila pair-rule gene ten-m/odz, is revealed to be a membrane-bound transcription regulator. In the nucleus, the intracellular domain of teneurin-2 colocalizes with promyelocytic leukemia (PML) protein in nuclear bodies implicated in transcription control. Since Drosophila ten-m acts epistatically to another pair-rule gene opa, we investigated whether gene regulation by the mammalian opa homologue zic-1 was influenced by the intracellular domain of teneurin-2. We found that zic-mediated transcription from the apolipoprotein E promoter was inhibited. Release of the intracellular domain of teneurin-2 could be stimulated by homophilic interaction of the extracellular domain, and the intracellular domain was stabilized by proteasome inhibitors. We have previously shown that teneurin-2 is expressed by neurons belonging to the same functional circuit. Therefore, we hypothesize that homophilic interaction enables neurons to identify their targets and that the release of the intracellular domain of teneurin-2 provides them with a signal to switch their gene expression program from growth towards differentiation once the proper contact has been made.
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Affiliation(s)
- Claudia Bagutti
- Friedrich Miescher Institute, Novartis Forschungsstiftung, PO Box 2543, CH-4002 Basel, Switzerland
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129
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Rome S, Clément K, Rabasa-Lhoret R, Loizon E, Poitou C, Barsh GS, Riou JP, Laville M, Vidal H. Microarray profiling of human skeletal muscle reveals that insulin regulates approximately 800 genes during a hyperinsulinemic clamp. J Biol Chem 2003; 278:18063-8. [PMID: 12621037 DOI: 10.1074/jbc.m300293200] [Citation(s) in RCA: 153] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Insulin action in target tissues involved precise regulation of gene expression. To define the set of insulin-regulated genes in human skeletal muscle, we analyzed the global changes in mRNA levels during a 3-h hyperinsulinemic euglycemic clamp in vastus lateralis muscle of six healthy subjects. Using 29,308 cDNA element microarrays, we found that the mRNA expression of 762 genes, including 353 expressed sequence tags, was significantly modified during insulin infusion. 478 were up-regulated and 284 down-regulated. Most of the genes with known function are novel targets of insulin. They are involved in the transcriptional and translational regulation (29%), intermediary and energy metabolisms (14%), intracellular signaling (12%), and cytoskeleton and vesicle traffic (9%). Other categories consisted of genes coding for receptors, carriers, and transporters (8%), components of the ubiquitin/proteasome pathways (7%) and elements of the immune response (5.5%). These results thus define a transcriptional signature of insulin action in human skeletal muscle. They will help to better define the mechanisms involved in the reduction of insulin effectiveness in pathologies such as type 2 diabetes mellitus, a disease characterized by defective regulation of gene expression in response to insulin.
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Affiliation(s)
- Sophie Rome
- INSERM U.449 and Human Nutrition Research Center of Lyon, Faculty of Medicine R. Laennec, Lyon Cédex 08, France.
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130
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Hirano Y, Murata S, Tanaka K, Shimizu M, Sato R. Sterol regulatory element-binding proteins are negatively regulated through SUMO-1 modification independent of the ubiquitin/26 S proteasome pathway. J Biol Chem 2003; 278:16809-19. [PMID: 12615929 DOI: 10.1074/jbc.m212448200] [Citation(s) in RCA: 91] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023] Open
Abstract
Sterol regulatory element-binding proteins (SREBPs) are major transcription factors that activate the genes involved in cholesterol and fatty acid biosynthesis. We here report that the nuclear forms of SREBPs are modified by the small ubiquitin-related modifier (SUMO)-1. Mutational analyses identified two major sumoylation sites (Lys(123) and Lys(418)) in SREBP-1a and a single site (Lys(464)) in SREBP-2. Mutant SREBPs lacking one or two sumoylation sites exhibited increased transactivation capacity on an SREBP-responsive promoter. Overexpression of SUMO-1 reduced whereas its dominant negative form increased mRNA levels of SREBP-responsive genes. Nuclear SREBPs interacted with the SUMO-1-conjugating enzyme Ubc9, and overexpression of a dominant negative form of Ubc9 increased the mRNA levels of SREBP-responsive genes. Pulse-chase experiments revealed that sumoylation did not affect the degradation of SREBPs through the ubiquitin-proteasome pathway. In vitro ubiquitylation assay showed no competition between ubiquitin and SUMO-1 for the same lysine. Considered together, our results indicate that SUMO-1 modification suppresses the transactivation capacity of nuclear SREBPs in a manner different from the negative regulatory mechanism mediated by proteolysis.
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Affiliation(s)
- Yuko Hirano
- Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, the University of Tokyo, Tokyo 113-8657, Japan
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131
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Giandomenico V, Simonsson M, Grönroos E, Ericsson J. Coactivator-dependent acetylation stabilizes members of the SREBP family of transcription factors. Mol Cell Biol 2003; 23:2587-99. [PMID: 12640139 PMCID: PMC150731 DOI: 10.1128/mcb.23.7.2587-2599.2003] [Citation(s) in RCA: 185] [Impact Index Per Article: 8.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Members of the SREBP family of transcription factors control cholesterol and lipid homeostasis and play important roles during adipocyte differentiation. The transcriptional activity of SREBPs is dependent on the coactivators p300 and CBP. We now present evidence that SREBPs are acetylated by the intrinsic acetyltransferase activity of p300 and CBP. In SREBP1a, the acetylated lysine residue resides in the DNA-binding domain of the protein. Coexpression with p300 dramatically increases the expression of both SREBP1a and SREBP2, and this effect is dependent on the acetyltransferase activity of p300, indicating that acetylation of SREBPs regulates their stability. Indeed, acetylation or mutation of the acetylated lysine residue in SREBP1a stabilizes the protein. We demonstrate that the acetylated residue in SREBP1a is also targeted by ubiquitination and that acetylation inhibits this process. Thus, our studies define acetylation-dependent stabilization of transcription factors as a novel mechanism for coactivators to regulate gene expression.
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Nakahara M, Fujii H, Maloney PR, Shimizu M, Sato R. Bile acids enhance low density lipoprotein receptor gene expression via a MAPK cascade-mediated stabilization of mRNA. J Biol Chem 2002; 277:37229-34. [PMID: 12149270 DOI: 10.1074/jbc.m206749200] [Citation(s) in RCA: 73] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Recent studies have indicated that bile acids regulate the expression of several genes involved in bile acid and lipid metabolism as ligands for the farnesoid X receptor (FXR). We report here that bile acids are directly able to govern cholesterol metabolism by a novel mechanism. We show that chenodeoxycholic acid (CDCA) enhances low density lipoprotein (LDL) receptor gene expression in human cultured cell lines (HeLa, Hep G2, and Caco-2). The proteolytic activation of sterol regulatory element-binding protein-2 (SREBP-2), a major regulator for LDL receptor gene expression, is not affected by CDCA. Both deoxycholic acid and lithocholic acid as well as CDCA, but not ursodeoxycholic acid, increase the mRNA level for the LDL receptor, even when Hep G2 cells are cultured with 25-hydroxycholesterol, a potent suppressor of gene expression for the LDL receptor. Although it seems possible that FXR might be involved in genetic regulation, both reporter assays with a reporter gene containing the LDL receptor promoter as well as Northern blot analysis reveal that FXR is not involved in the process. On the other hand, inhibition of mitogen-activated protein (MAP) kinase activities, which are found to be induced by CDCA, abolishes the CDCA-mediated up-regulation of LDL receptor gene expression. We further demonstrate that CDCA stabilizes LDL receptor mRNA and that the MAP kinase inhibitors accelerate its turnover. Taken together, these results indicate that bile acids increase LDL uptake and the intracellular cholesterol levels through the activation of MAP kinase cascades in conjunction with a down-regulation of bile acid biosynthesis by FXR. This work opens up a new avenue for developing pharmaceutical interventions that lower plasma LDL by stabilizing LDL receptor mRNA.
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Affiliation(s)
- Mayuko Nakahara
- Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan
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Bastard JP, Caron M, Vidal H, Jan V, Auclair M, Vigouroux C, Luboinski J, Laville M, Maachi M, Girard PM, Rozenbaum W, Levan P, Capeau J. Association between altered expression of adipogenic factor SREBP1 in lipoatrophic adipose tissue from HIV-1-infected patients and abnormal adipocyte differentiation and insulin resistance. Lancet 2002; 359:1026-31. [PMID: 11937183 DOI: 10.1016/s0140-6736(02)08094-7] [Citation(s) in RCA: 313] [Impact Index Per Article: 14.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/12/2023]
Abstract
BACKGROUND Lipodystrophy is a major side-effect of antiretroviral therapy but its pathophysiology remains elusive. In-vitro studies show that HIV-1-protease inhibitors affect adipocyte differentiation at an early step involving sterol-regulatory-element-binding-protein-1 (SREBP1), but in-vivo studies are lacking. METHODS We compared fat morphology and mRNA and protein expression of major adipocyte differentiation markers and cytokines in subcutaneous abdominal adipose tissue from 26 HIV-1-infected patients who developed peripheral lipoatrophy while on protease inhibitors and from 18 HIV-1-seronegative healthy controls. FINDINGS Patients' fat contained a higher proportion of small adipocytes than control fat, together with lower mRNA concentrations of the adipogenic differentiation factors CCAAT-enhancer binding protein (C/EBP) beta and alpha, peroxisome proliferator-activated receptor (PPAR) gamma, and the 1c isoform of SREBP1, with a median decrease of 93% in the latter. The SREBP1 protein concentration was increased 2.6-fold, whereas the PPARgamma protein concentration was decreased by 70%. The expression of adipocyte-specific markers, including leptin, was lower in fat from patients than in fat from controls, whereas expression of tumour necrosis factor (TNF) alpha was higher and correlated negatively with the expression of SREBP1c and downstream adipogenic factors. SREBP1c mRNA concentrations correlated negatively, and TNFalpha mRNA concentrations positively, with glycaemia and insulin resistance, but did not correlate with lipid variables. INTERPRETATION The altered differentiation status of peripheral adipocytes in HIV-1-infected patients with antiretroviral-induced lipoatrophy is associated with greatly reduced SREBP1c expression. Since the differentiation factor SREBP1 is rapidly targeted by protease inhibitors in vitro, our results suggest that SREBP1c could be an important mediator of peripheral lipoatrophy in this setting, leading to metabolic alterations such as insulin resistance.
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