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Evolution of recombinant lymphocytic choriomeningitis virus/Lassa virus in vivo highlights the importance of the GPC cytosolic tail in viral fitness. J Virol 2014; 88:8340-8. [PMID: 24829355 DOI: 10.1128/jvi.00236-14] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
UNLABELLED A key characteristic of arenaviruses is their ability to establish persistent infection in their natural host. Different factors like host age, viral dose strain, and route of infection may contribute to the establishment of persistence. However, the molecular mechanisms governing persistence are not fully understood. Here, we describe gain-of-function mutations of lymphocytic choriomeningitis virus (LCMV) expressing Lassa virus (LASV) GP, which can prolong viremia in mice depending on the sequences in the GP-2 cytoplasmic tail. The initial mutant variant (rLCMV/LASV mut GP) carried a point mutation in the cytosolic tail of the LASV glycoprotein GP corresponding to a K461G substitution. Unlike what occurred with the original rLCMV/LASV wild-type (wt) GP, infection of C57BL/6 mice with the mutated recombinant virus led to a detectable viremia of 2 weeks' duration. Further replacement of the entire sequence of the cytosolic tail from LASV to LCMV GP resulted in increased viral titers and delayed clearance of the viruses. Biosynthesis and cell surface localization of LASV wt and mut GPs were comparable. IMPORTANCE Starting from an emerging virus in a wild-type mouse, we engineered a panel of chimeric Lassa/lymphocytic choriomeningitis viruses. Mutants carrying a viral envelope with the cytosolic tail from the closely related mouse-adapted LCMV were able to achieve a productive viral infection lasting up to 27 days in wild-type mice. Biochemical assays showed a comparable biosynthesis and cell surface localization of LASV wt and mut GPs. These recombinant chimeric viruses could allow the study of immune responses and antivirals targeting the LASV GP.
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52
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Marinho HS, Real C, Cyrne L, Soares H, Antunes F. Hydrogen peroxide sensing, signaling and regulation of transcription factors. Redox Biol 2014; 2:535-62. [PMID: 24634836 PMCID: PMC3953959 DOI: 10.1016/j.redox.2014.02.006] [Citation(s) in RCA: 571] [Impact Index Per Article: 57.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/11/2014] [Revised: 02/19/2014] [Accepted: 02/21/2014] [Indexed: 12/12/2022] Open
Abstract
The regulatory mechanisms by which hydrogen peroxide (H2O2) modulates the activity of transcription factors in bacteria (OxyR and PerR), lower eukaryotes (Yap1, Maf1, Hsf1 and Msn2/4) and mammalian cells (AP-1, NRF2, CREB, HSF1, HIF-1, TP53, NF-κB, NOTCH, SP1 and SCREB-1) are reviewed. The complexity of regulatory networks increases throughout the phylogenetic tree, reaching a high level of complexity in mammalians. Multiple H2O2 sensors and pathways are triggered converging in the regulation of transcription factors at several levels: (1) synthesis of the transcription factor by upregulating transcription or increasing both mRNA stability and translation; (ii) stability of the transcription factor by decreasing its association with the ubiquitin E3 ligase complex or by inhibiting this complex; (iii) cytoplasm–nuclear traffic by exposing/masking nuclear localization signals, or by releasing the transcription factor from partners or from membrane anchors; and (iv) DNA binding and nuclear transactivation by modulating transcription factor affinity towards DNA, co-activators or repressors, and by targeting specific regions of chromatin to activate individual genes. We also discuss how H2O2 biological specificity results from diverse thiol protein sensors, with different reactivity of their sulfhydryl groups towards H2O2, being activated by different concentrations and times of exposure to H2O2. The specific regulation of local H2O2 concentrations is also crucial and results from H2O2 localized production and removal controlled by signals. Finally, we formulate equations to extract from typical experiments quantitative data concerning H2O2 reactivity with sensor molecules. Rate constants of 140 M−1 s−1 and ≥1.3 × 103 M−1 s−1 were estimated, respectively, for the reaction of H2O2 with KEAP1 and with an unknown target that mediates NRF2 protein synthesis. In conclusion, the multitude of H2O2 targets and mechanisms provides an opportunity for highly specific effects on gene regulation that depend on the cell type and on signals received from the cellular microenvironment. Complexity of redox regulation increases along the phylogenetic tree. Complex regulatory networks allow for a high degree of H2O2 biological plasticity. H2O2 modulates gene expression at all steps from transcription to protein synthesis. Fast response (s) is mediated by sensors with high H2O2 reactivity. Low reactivity H2O2 sensors may mediate slow (h) or localized H2O2 responses.
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Affiliation(s)
- H. Susana Marinho
- Departamento de Química e Bioquímica, Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Lisboa, Portugal
| | - Carla Real
- Departamento de Química e Bioquímica, Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Lisboa, Portugal
| | - Luísa Cyrne
- Departamento de Química e Bioquímica, Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Lisboa, Portugal
| | - Helena Soares
- Departamento de Química e Bioquímica, Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Lisboa, Portugal
- Instituto Gulbenkian de Ciência, Oeiras, Portugal
- Escola Superior de Tecnologia da Saúde de Lisboa, IPL, Lisboa, Portugal
| | - Fernando Antunes
- Departamento de Química e Bioquímica, Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Lisboa, Portugal
- Corresponding author.
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Shao W, Espenshade PJ. Sterol regulatory element-binding protein (SREBP) cleavage regulates Golgi-to-endoplasmic reticulum recycling of SREBP cleavage-activating protein (SCAP). J Biol Chem 2014; 289:7547-57. [PMID: 24478315 DOI: 10.1074/jbc.m113.545699] [Citation(s) in RCA: 57] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
Sterol regulatory element-binding protein (SREBP) transcription factors are central regulators of cellular lipogenesis. Release of membrane-bound SREBP requires SREBP cleavage-activating protein (SCAP) to escort SREBP from the endoplasmic reticulum (ER) to the Golgi for cleavage by site-1 and site-2 proteases. SCAP then recycles to the ER for additional rounds of SREBP binding and transport. Mechanisms regulating ER-to-Golgi transport of SCAP-SREBP are understood in molecular detail, but little is known about SCAP recycling. Here, we have demonstrated that SCAP Golgi-to-ER transport requires cleavage of SREBP at site-1. Reductions in SREBP cleavage lead to SCAP degradation in lysosomes, providing additional negative feedback control to the SREBP pathway. Current models suggest that SREBP plays a passive role prior to cleavage. However, we show that SREBP actively prevents premature recycling of SCAP-SREBP until initiation of SREBP cleavage. SREBP regulates SCAP in human cells and yeast, indicating that this is an ancient regulatory mechanism.
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Affiliation(s)
- Wei Shao
- From the Department of Cell Biology, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
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54
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García-Ruiz C, Baulies A, Mari M, García-Rovés PM, Fernandez-Checa JC. Mitochondrial dysfunction in non-alcoholic fatty liver disease and insulin resistance: Cause or consequence? Free Radic Res 2013; 47:854-68. [DOI: 10.3109/10715762.2013.830717] [Citation(s) in RCA: 70] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
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55
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Kim H, Zhang H, Meng D, Russell G, Lee JN, Ye J. UAS domain of Ubxd8 and FAF1 polymerizes upon interaction with long-chain unsaturated fatty acids. J Lipid Res 2013; 54:2144-2152. [PMID: 23720822 DOI: 10.1194/jlr.m037218] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
Ubxd8, a multidomain protein sensor for long-chain unsaturated fatty acids (FAs), plays a crucial role to maintain cellular homeostasis of FAs. Ubxd8 polymerizes upon interaction with long-chain unsaturated FAs, but the molecular mechanism involved in this polymerization remains unclear. Here we report that the UAS domain of Ubxd8 mediates this polymerization. We show that a positively charged surface area in the domain is required for the reaction. Mutations changing the positively charged residues in this area to glutamates prevented long-chain unsaturated FAs from inducing oligomerization of Ubxd8. Consequently, the mutant protein no longer responded to regulation by long-chain unsaturated FAs in cultured cells. Long-chain unsaturated FAs also induced polymerization of Fas-associated factor 1 (FAF1), the only other mammalian protein that contains a UAS domain homologous to that of Ubxd8. These results provide further insights into protein-FA interactions by identifying the UAS domain as a motif interacting with long-chain unsaturated FAs.
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Affiliation(s)
- Hyeonwoo Kim
- Departments of Molecular Genetics University of Texas Southwestern Medical Center, Dallas, TX 75390
| | - Hong Zhang
- Biophysics University of Texas Southwestern Medical Center, Dallas, TX 75390
| | - David Meng
- Departments of Molecular Genetics University of Texas Southwestern Medical Center, Dallas, TX 75390
| | - Geoffrey Russell
- Departments of Molecular Genetics University of Texas Southwestern Medical Center, Dallas, TX 75390
| | - Joon No Lee
- Departments of Molecular Genetics University of Texas Southwestern Medical Center, Dallas, TX 75390
| | - Jin Ye
- Departments of Molecular Genetics University of Texas Southwestern Medical Center, Dallas, TX 75390.
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Zhang Y, Motamed M, Seemann J, Brown MS, Goldstein JL. Point mutation in luminal loop 7 of Scap protein blocks interaction with loop 1 and abolishes movement to Golgi. J Biol Chem 2013; 288:14059-14067. [PMID: 23564452 DOI: 10.1074/jbc.m113.469528] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023] Open
Abstract
Scap is a polytopic protein of the endoplasmic reticulum (ER) that controls cholesterol homeostasis by transporting sterol regulatory element-binding proteins (SREBPs) from the ER to the Golgi complex. Scap has eight transmembrane helices (TM) joined by four small hydrophilic loops and three large loops. Two of the large loops (Loops 1 and 7) are in the ER lumen, and the other large loop (Loop 6) faces the cytosol where it binds COPII proteins that initiate transport to Golgi. Cholesterol binding to Loop 1 alters the configuration of Loop 6, precluding COPII binding and preventing the exit of Scap from the ER. Here, we create a point mutation (Y640S) in luminal Loop 7 that prevents Scap movement to Golgi. Trypsin cleavage assays show that Loop 6 of Scap(Y640S) is always in the configuration that precludes COPII binding, even in the absence of cholesterol. When expressed separately by co-transfection, the NH2-terminal portion of Scap (containing TM helices 1-6, including Loop 1) binds to the COOH-terminal portion (containing TM helices 7-8 and Loop 7) as determined by co-immunoprecipitation. This binding does not occur when Loop 7 contains the Y640S mutation. Co-immunoprecipitation is also abolished by a point mutation in Loop 1 (Y234A) that also prevents Scap movement. These data suggest that Scap Loop 1 must interact with Loop 7 to maintain Loop 6 in the configuration that permits COPII binding. These results help explain the operation of Scap as a sterol sensor.
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Affiliation(s)
- Yinxin Zhang
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9046
| | - Massoud Motamed
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9046
| | - Joachim Seemann
- Department of Cell Biology, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9046
| | - Michael S Brown
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9046.
| | - Joseph L Goldstein
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9046.
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Watanabe M, Uesugi M. Small-molecule inhibitors of SREBP activation – potential for new treatment of metabolic disorders. MEDCHEMCOMM 2013. [DOI: 10.1039/c3md00177f] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/23/2023]
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58
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Dong XY, Tang SQ, Chen JD. Dual functions of Insig proteins in cholesterol homeostasis. Lipids Health Dis 2012; 11:173. [PMID: 23249523 PMCID: PMC3564778 DOI: 10.1186/1476-511x-11-173] [Citation(s) in RCA: 42] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/13/2012] [Accepted: 12/05/2012] [Indexed: 01/29/2023] Open
Abstract
The molecular mechanism of how cells maintain cholesterol homeostasis has become clearer for the understanding of complicated association between sterol regulatory element-binding proteins (SREBPs), SREBP cleavage-activating protein (SCAP), 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) and Insuin induced-genes (Insigs). The pioneering researches suggested that SREBP activated the transcription of genes encoding HMG-CoA reductase and all of the other enzymes involved in the synthesis of cholesterol and lipids. However, SREBPs can not exert their activities alone, they must form a complex with another protein, SCAP in the endoplasmic reticulum (ER) and translocate to Golgi. Insigs are sensors and mediators that regulate cholesterol homeostasis through binding to SCAP and HMG-CoA reductase in diverse tissues such as adipose tissue and liver, as well as the cultured cells. In this article, we aim to review on the dual functions of Insig protein family in cholesterol homeostasis.
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Affiliation(s)
- Xiao-Ying Dong
- College of Veterinary Medicine, South China Agricultural University, No,483 Wu Shan Road, Tian He District, Guangzhou, 510642, China.
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59
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Jo Y, Hartman IZ, DeBose-Boyd RA. Ancient ubiquitous protein-1 mediates sterol-induced ubiquitination of 3-hydroxy-3-methylglutaryl CoA reductase in lipid droplet-associated endoplasmic reticulum membranes. Mol Biol Cell 2012; 24:169-83. [PMID: 23223569 PMCID: PMC3564538 DOI: 10.1091/mbc.e12-07-0564] [Citation(s) in RCA: 64] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023] Open
Abstract
Sterol-induced binding to Insigs in endoplasmic reticulum (ER) membranes triggers ubiquitination of the cholesterol biosynthetic enzyme 3-hydroxy-3-methylglutaryl CoA reductase. This ubiquitination, which is mediated by Insig-associated ubiquitin ligases gp78 and Trc8, is obligatory for extraction of reductase from lipid droplet-associated ER membranes into the cytosol for proteasome-mediated, ER-associated degradation (ERAD). In this study, we identify lipid droplet-associated, ancient, ubiquitous protein-1 (Aup1) as one of several proteins that copurify with gp78. RNA interference (RNAi) studies show that Aup1 recruits the ubiquitin-conjugating enzyme Ubc7 to lipid droplets and facilitates its binding to both gp78 and Trc8. The functional significance of these interactions is revealed by the observation that RNAi-mediated knockdown of Aup1 blunts sterol-accelerated ubiquitination of reductase, which appears to occur in lipid droplet-associated membranes and subsequent ERAD of the enzyme. In addition, Aup1 knockdown inhibits ERAD of Insig-1, another substrate for gp78, as well as that of membrane-bound precursor forms of sterol-regulatory, element-binding protein-1 and -2, transcription factors that modulate expression of genes encoding enzymes required for cholesterol synthesis. Considered together, these findings not only implicate a role for Aup1 in maintenance of intracellular cholesterol homeostasis, but they also highlight the close connections among ERAD, lipid droplets, and lipid droplet-associated proteins.
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Affiliation(s)
- Youngah Jo
- Howard Hughes Medical Institute and Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75390-9046, USA
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60
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Cholesterol accumulation inhibits ER to Golgi transport and protein secretion: studies of apolipoprotein E and VSVGt. Biochem J 2012; 447:51-60. [PMID: 22747346 DOI: 10.1042/bj20111891] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Abstract
Cholesterol excess is typical of various diseases including atherosclerosis. We have investigated whether cholesterol accumulation in the ER (endoplasmic reticulum) can inhibit exit of vesicular cargo and secretion of proteins by studying apoE (apolipoprotein E), a significant glycoprotein in human health and disease. CHO (Chinese hamster ovary) cells expressing human apoE under a cholesterol-independent promoter incubated with cholesterol-cyclodextrin complexes showed increased levels of cellular free and esterified cholesterol, inhibition of SREBP-2 (sterol-regulatory-element-binding protein 2) processing, and a mild induction of ER stress, indicating significant accumulation of cholesterol in the ER. Secretion of apoE was markedly inhibited by cholesterol accumulation, and similar effects were observed in cells enriched with lipoprotein-derived cholesterol and in primary human macrophages. Removal of excess cholesterol by a cyclodextrin vehicle restored apoE secretion, indicating that the transport defect was reversible. That cholesterol impaired protein trafficking was supported by the cellular accumulation of less sialylated apoE glycoforms, and by direct visualization of altered ER to Golgi transport of thermo-reversible VSVG (vesicular stomatitis virus glycoprotein) linked to GFP (green fluorescent protein). We conclude that intracellular accumulation of cholesterol in the ER reversibly inhibits protein transport and secretion. Strategies to correct ER cholesterol may restore homoeostatic processes and intracellular protein transport in conditions characterized by cholesterol excess.
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61
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Chrétien M. My road to Damascus: how I converted to the prohormone theory and the proprotein convertases. Biochem Cell Biol 2012. [PMID: 23194189 DOI: 10.1139/o2012-031] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
My desire as a young endocrinologist to improve my clinical skills through a better knowledge of hormone chemistry led me to serendipitous discoveries and unexpected horizons. The first discovery, published in 1967, revealed that peptide hormones are derived from endoproteolytic cleavages of larger precursor polypeptides. It was the foundation of the prohormone theory. Initially thought to apply to a few hormones, the theory rapidly extended to many proteins, including neuropeptides, neurotrophins, growth and transcription factors, receptors, extracellular matrix proteins, bacterial toxins, and viral glycoproteins. Its endoproteolytic activation mechanism has become a fundamental cellular process, affecting many biological functions. It implied the existence of specific endoproteolytic enzymes. These proprotein convertases were discovered in 1990. They have been shown to play a wide range of important roles in health and disease. They have opened up novel therapeutic avenues. Inactivation of PCSK9 to reduce plasma cholesterol is currently the most promising. To make this good thing even better, I recently discovered in a French Canadian family a potent PCSK9 (Gln152His) mutation that significantly lowers plasma cholesterol and should confer cardiovascular longevity. The discovery helped me to complete the loop: "From the bedside to the bench and back to the bedside."
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Affiliation(s)
- Michel Chrétien
- Institut de recherches cliniques de Montréal, 110 avenue des Pins Ouest, Montréal, QC H2W 1R7, Canada.
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62
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Sterols regulate 3β-hydroxysterol Δ24-reductase (DHCR24) via dual sterol regulatory elements: cooperative induction of key enzymes in lipid synthesis by Sterol Regulatory Element Binding Proteins. Biochim Biophys Acta Mol Cell Biol Lipids 2012; 1821:1350-60. [PMID: 22809995 DOI: 10.1016/j.bbalip.2012.07.006] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/12/2012] [Revised: 06/25/2012] [Accepted: 07/09/2012] [Indexed: 01/28/2023]
Abstract
3β-Hydroxysterol Δ24-reductase (DHCR24) catalyzes a final step in cholesterol synthesis, and has been ascribed diverse functions, such as being anti-apoptotic and anti-inflammatory. How this enzyme is regulated transcriptionally by sterols is currently unclear. Some studies have suggested that its expression is regulated by Sterol Regulatory Element Binding Proteins (SREBPs) while another suggests it is through the Liver X Receptor (LXR). However, these transcription factors have opposing effects on cellular sterol levels, so it is likely that one predominates. Here we establish that sterol regulation of DHCR24 occurs predominantly through SREBP-2, and identify the particular region of the DHCR24 promoter to which SREBP-2 binds. We demonstrate that sterol regulation is mediated by two sterol regulatory elements (SREs) in the promoter of the gene, assisted by two nearby NF-Y binding sites. Moreover, we present evidence that the dual SREs work cooperatively to regulate DHCR24 expression by comparison to two known SREBP target genes, the LDL receptor with one SRE, and farnesyl-diphosphate farnesyltransferase 1, with two SREs.
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Abstract
This article reviews the current knowledge and experimental research about the mechanisms by which fatty acids and their derivatives control specific gene expression involved during carcinogenesis. Changes in dietary fatty acids, specifically the polyunsaturated fatty acids of the ω-3 and ω-6 families and some derived eicosanoids from lipoxygenases, cyclooxygenases, and cytochrome P-450, seem to control the activity of transcription factor families involved in cancer cell proliferation or cell death. Their regulation may be carried out either through direct binding to DNA as peroxisome proliferator-activated receptors or via modulation in an indirect manner of signaling pathway molecules (e.g., protein kinase C) and other transcription factors (nuclear factor kappa B and sterol regulatory element binding protein). Knowledge of the mechanisms by which fatty acids control specific gene expression may identify important risk factors for cancer and provide insight into the development of new therapeutic strategies for a better management of whole body lipid metabolism.
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64
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Moon YA, Liang G, Xie X, Frank-Kamenetsky M, Fitzgerald K, Koteliansky V, Brown MS, Goldstein JL, Horton JD. The Scap/SREBP pathway is essential for developing diabetic fatty liver and carbohydrate-induced hypertriglyceridemia in animals. Cell Metab 2012; 15:240-6. [PMID: 22326225 PMCID: PMC3662050 DOI: 10.1016/j.cmet.2011.12.017] [Citation(s) in RCA: 252] [Impact Index Per Article: 21.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/17/2011] [Revised: 11/28/2011] [Accepted: 12/24/2011] [Indexed: 12/19/2022]
Abstract
Insulin resistance leads to hypertriglyceridemia and hepatic steatosis and is associated with increased SREBP-1c, a transcription factor that activates fatty acid synthesis. Here, we show that steatosis in insulin-resistant ob/ob mice was abolished by deletion of Scap, an escort protein necessary for generating nuclear isoforms of all three SREBPs. Scap deletion reduced lipid synthesis and prevented fatty livers despite persistent obesity, hyperinsulinemia, and hyperglycemia. Scap deficiency also prevented steatosis in mice fed high-fat diets. Steatosis was also prevented when siRNAs were used to silence Scap in livers of sucrose-fed hamsters, a model of diet-induced steatosis and hypertriglyceridemia. This silencing reduced all three nuclear SREBPs, decreasing lipid biosynthesis and abolishing sucrose-induced hypertriglyceridemia. These results demonstrate that SREBP activation is essential for development of diabetic hepatic steatosis and carbohydrate-induced hypertriglyceridemia, but not insulin resistance. Inhibition of SREBP activation has therapeutic potential for treatment of hypertriglyceridemia and fatty liver disease.
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Affiliation(s)
- Young-Ah Moon
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75390-9046, USA
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65
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Miller WL, Bose HS. Early steps in steroidogenesis: intracellular cholesterol trafficking. J Lipid Res 2011; 52:2111-2135. [PMID: 21976778 DOI: 10.1194/jlr.r016675] [Citation(s) in RCA: 361] [Impact Index Per Article: 27.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
Steroid hormones are made from cholesterol, primarily derived from lipoproteins that enter cells via receptor-mediated endocytosis. In endo-lysosomes, cholesterol is released from cholesterol esters by lysosomal acid lipase (LAL; disordered in Wolman disease) and exported via Niemann-Pick type C (NPC) proteins (disordered in NPC disease). These diseases are characterized by accumulated cholesterol and cholesterol esters in most cell types. Mechanisms for trans-cytoplasmic cholesterol transport, membrane insertion, and retrieval from membranes are less clear. Cholesterol esters and "free" cholesterol are enzymatically interconverted in lipid droplets. Cholesterol transport to the cholesterol-poor outer mitochondrial membrane (OMM) appears to involve cholesterol transport proteins. Cytochrome P450scc (CYP11A1) then initiates steroidogenesis by converting cholesterol to pregnenolone on the inner mitochondrial membrane (IMM). Acute steroidogenic responses are regulated by cholesterol delivery from OMM to IMM, triggered by the steroidogenic acute regulatory protein (StAR). Chronic steroidogenic capacity is determined by CYP11A1 gene transcription. StAR mutations cause congenital lipoid adrenal hyperplasia, with absent steroidogenesis, potentially lethal salt loss, and 46,XY sex reversal. StAR mutations initially destroy most, but not all steroidogenesis; low levels of StAR-independent steroidogenesis are lost later due to cellular damage, explaining the clinical findings. Rare P450scc mutations cause a similar syndrome. This review addresses these early steps in steroid biosynthesis.
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Affiliation(s)
- Walter L Miller
- Department of Pediatrics, School of Medicine, University of California, San Francisco, CA 94143; UCSF Benioff Children's Hospital, San Francisco, CA 94143.
| | - Himangshu S Bose
- Department of Biochemistry, Mercer University School of Medicine, Savannah, GA 31404; and; Memorial University Medical Center, Savannah, GA 31404
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66
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Motamed M, Zhang Y, Wang ML, Seemann J, Kwon HJ, Goldstein JL, Brown MS. Identification of luminal Loop 1 of Scap protein as the sterol sensor that maintains cholesterol homeostasis. J Biol Chem 2011; 286:18002-12. [PMID: 21454655 DOI: 10.1074/jbc.m111.238311] [Citation(s) in RCA: 71] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Cellular cholesterol homeostasis is maintained by Scap, an endoplasmic reticulum (ER) protein with eight transmembrane helices. In cholesterol-depleted cells, Scap transports sterol regulatory element-binding proteins (SREBPs) to the Golgi, where the active fragment of SREBP is liberated by proteases so that it can activate genes for cholesterol synthesis. When ER cholesterol increases, Scap binds cholesterol, and this changes the conformation of cytosolic Loop 6, which contains the binding site for COPII proteins. The altered conformation precludes COPII binding, abrogating movement to the Golgi. Consequently, cholesterol synthesis declines. Here, we identify the cholesterol-binding site on Scap as Loop 1, a 245-amino acid sequence that projects into the ER lumen. Recombinant Loop 1 binds sterols with a specificity identical to that of the entire Scap membrane domain. When tyrosine 234 in Loop 1 is mutated to alanine, Loop 6 assumes the cholesterol-bound conformation, even in sterol-depleted cells. As a result, full-length Scap(Y234A) cannot mediate SREBP processing in transfected cells. These results indicate that luminal Loop 1 of Scap controls the conformation of cytosolic Loop 6, thereby determining whether cells produce cholesterol.
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Affiliation(s)
- Massoud Motamed
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9046, USA
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67
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Gill S, Stevenson J, Kristiana I, Brown AJ. Cholesterol-dependent degradation of squalene monooxygenase, a control point in cholesterol synthesis beyond HMG-CoA reductase. Cell Metab 2011; 13:260-73. [PMID: 21356516 DOI: 10.1016/j.cmet.2011.01.015] [Citation(s) in RCA: 211] [Impact Index Per Article: 16.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/24/2010] [Revised: 10/22/2010] [Accepted: 01/20/2011] [Indexed: 10/18/2022]
Abstract
Exquisite control of cholesterol synthesis is crucial for maintaining homeostasis of this vital yet potentially toxic lipid. Squalene monooxygenase (SM) catalyzes the first oxygenation step in cholesterol synthesis, acting on squalene before cyclization into the basic steroid structure. Using model cell systems, we found that cholesterol caused the accumulation of the substrate squalene, suggesting that SM may serve as a flux-controlling enzyme beyond 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR, considered as rate limiting). Cholesterol accelerated the proteasomal degradation of SM which required the N-terminal domain, partially conserved in vertebrates but not in lower organisms. Unlike HMGR, SM degradation is not mediated by Insig, 24,25-dihydrolanosterol, or side-chain oxysterols, but rather by cholesterol itself. Importantly, SM's N-terminal domain conferred cholesterol-regulated turnover on heterologous fusion proteins. Furthermore, proteasomal inhibition almost totally eliminated squalene accumulation, highlighting the importance of this degradation mechanism for the control of SM and suggesting this as a possible control point in cholesterol synthesis.
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Affiliation(s)
- Saloni Gill
- School of Biotechnology and Biomolecular Sciences, University of New South Wales, Sydney NSW 2052, Australia
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68
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Identification of Ubxd8 protein as a sensor for unsaturated fatty acids and regulator of triglyceride synthesis. Proc Natl Acad Sci U S A 2010; 107:21424-9. [PMID: 21115839 DOI: 10.1073/pnas.1011859107] [Citation(s) in RCA: 74] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023] Open
Abstract
Fatty acids (FAs) are essential for cell survival, yet their overaccumulation causes lipotoxicity. To prevent lipotoxicity, cells store excess FAs as triglycerides (TGs). In cultured cells TG synthesis is activated by excess unsaturated but not saturated FAs. Here, we identify Ubxd8 as a sensor for unsaturated FAs and regulator of TG synthesis. In cultured cells depleted of FAs, Ubxd8 inhibits TG synthesis by blocking conversion of diacylglycerols (DAGs) to TGs. Excess unsaturated but not saturated FAs relieve this inhibition. As a result, unsaturated FAs are incorporated into TGs, whereas saturated FAs are incorporated into DAGs. In vitro, unsaturated but not saturated FAs alter the structure of purified recombinant Ubxd8 as monitored by changes in its thermal stability, trypsin cleavage pattern, and oligomerization. These results suggest that Ubxd8 acts as a brake that limits TG synthesis, and this brake is released when its structure is altered by exposure to unsaturated FAs.
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69
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Wang ML, Motamed M, Infante RE, Abi-Mosleh L, Kwon HJ, Brown MS, Goldstein JL. Identification of surface residues on Niemann-Pick C2 essential for hydrophobic handoff of cholesterol to NPC1 in lysosomes. Cell Metab 2010; 12:166-73. [PMID: 20674861 PMCID: PMC3034247 DOI: 10.1016/j.cmet.2010.05.016] [Citation(s) in RCA: 177] [Impact Index Per Article: 12.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/03/2010] [Revised: 04/20/2010] [Accepted: 05/27/2010] [Indexed: 10/19/2022]
Abstract
Water-soluble Niemann-Pick C2 (NPC2) and membrane-bound NPC1 are cholesterol-binding lysosomal proteins required for export of lipoprotein-derived cholesterol from lysosomes. The binding site in NPC1 is located in its N-terminal domain (NTD), which projects into the lysosomal lumen. Here we perform alanine-scanning mutagenesis to identify residues in NPC2 that are essential for transfer of cholesterol to NPC1(NTD). Transfer requires three residues that form a patch on the surface of NPC2. We previously identified a patch of residues on the surface of NPC1(NTD) that are required for transfer. We present a model in which these two surface patches on NPC2 and NPC1(NTD) interact, thereby opening an entry pore on NPC1(NTD) and allowing cholesterol to transfer without passing through the water phase. We refer to this transfer as a hydrophobic handoff and hypothesize that this handoff is essential for cholesterol export from lysosomes.
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Affiliation(s)
- Michael L Wang
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390, USA
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70
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Miao H, Jiang W, Ge L, Li B, Song B. Tetra-glutamic acid residues adjacent to Lys248 in HMG-CoA reductase are critical for the ubiquitination mediated by gp78 and UBE2G2. Acta Biochim Biophys Sin (Shanghai) 2010; 42:303-10. [PMID: 20458442 DOI: 10.1093/abbs/gmq022] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Sterol-regulated degradation of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) is a rapid feedback regulatory mechanism by which cells employ to control the cholesterol biosynthesis. This process is initiated by the sterol-induced interaction between HMGCR and Insig-1/ gp78, a membrane-bound ubiquitin ligase complex. There are two Lys residues (Lys89 and Lys248) facing cytosol in the membrane domain of HMGCR, and Lys248 is the major ubiquitination site. In this study, we investigated the mechanism of ubiquitination site selection in HMGCR. We find that the distance of Lys248 to membrane is dispensable for its ubiquitination. However, the conserved tetra-glutamic acid residues adjacent to Lys248 in HMGCR are essential. Replacement of these negatively charged residues with tetraarginine causes the resistance of HMGCR to sterol-induced ubiquitination and degradation, albeit this mutant HMGCR can still binds to Insig-1. We further find that the tetra-glutamic acid residues are necessary but not sufficient for the modification on their adjacent Lys, since they are not functional on Lys89 of HMGCR or in SCAP. UBE2G2, a previously known E2 of gp78, is demonstrated to be involved in the sterol-regulated ubiquitination and degradation of HMGCR. In summary, these results identify the tetraglutamic acid residues as a critical motif in HMGCR for the ubiquitination reaction mediated by gp78 and UBE2G2.
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Affiliation(s)
- Honghua Miao
- The State Key Laboratory of Molecular Biology, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200031, China
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71
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High density lipoprotein inhibits the activation of sterol regulatory element-binding protein-1 in cultured cells. FEBS Lett 2010; 584:1217-22. [PMID: 20171215 DOI: 10.1016/j.febslet.2010.02.034] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2010] [Revised: 02/02/2010] [Accepted: 02/12/2010] [Indexed: 11/21/2022]
Abstract
A link between cellular uptake of high density lipoprotein (HDL) and regulation of sterol regulatory element-binding protein-1 (SREBP-1) was investigated in vitro. HDL decreased nuclear SREBP-1 levels as well as SREBP-1 target gene expression in HepG2 and HEK293 cells. However, HDL did not repress an exogenously expressed, constitutively active form of SREBP-1. HDL increased cellular cholesterol levels, and cellular cholesterol depletion by methyl-beta-cyclodextrin abolished the effects of HDL. These results suggest that HDL inhibits the activation of SREBP-1 through a cholesterol-dependent mechanism, which may play an important role in regulating lipid synthetic pathways mediated by SREBP-1.
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72
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Lee PCW, DeBose-Boyd RA. Intramembrane glycine mediates multimerization of Insig-2, a requirement for sterol regulation in Chinese hamster ovary cells. J Lipid Res 2010; 51:192-201. [PMID: 19617589 DOI: 10.1194/jlr.m900336-jlr200] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Sterol-induced binding of endoplasmic reticulum (ER) membrane proteins Insig-1 and Insig-2 to SREBP cleavage-activating protein (Scap) and HMG-CoA reductase triggers regulatory events that limit cholesterol synthesis in animal cells. Binding of Insigs to Scap prevents proteolytic activation of sterol-regulatory element binding proteins (SREBPs), membrane-bound transcription factors that enhance cholesterol synthesis, by trapping Scap-SREBP complexes in the ER. Insig binding to reductase causes ubiquitination and subsequent proteasome-mediated degradation of the enzyme from ER membranes, slowing a rate-limiting step in cholesterol synthesis. Here, we report the characterization of mutant Chinese hamster ovary cells, designated SRD-20, that are resistant to 25-hydroxycholesterol, which potently inhibits SREBP activation and stimulates degradation of reductase. SRD-20 cells were produced by mutagenesis of Insig-1-deficient SRD-14 cells, followed by selection in 25-hydroxycholesterol. DNA sequencing reveals that SRD-20 cells harbor a point mutation in one Insig-2 allele that results in production of a truncated, nonfunctional protein, whereas the other allele contains a point mutation that results in substitution of glutamic acid for glycine-39. This glycine residue localizes to the first membrane-spanning segment of Insig-2 and is also present in the corresponding region of Insig-1. Mutant forms of Insig-1 and Insig-2 containing the Glu-to-Gly substitution fail to confer sterol regulation upon overexpressed Scap and reductase. These studies identify the intramembrane glycine as a key residue for normal sterol regulation in animal cells.
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Affiliation(s)
- Peter C W Lee
- Howard Hughes Medical Institute, University of Texas Southwestern Medical Center Dallas TX 75390-9046, USA
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73
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Sterol regulatory element binding proteins in fungi: hypoxic transcription factors linked to pathogenesis. EUKARYOTIC CELL 2010; 9:352-9. [PMID: 20118213 DOI: 10.1128/ec.00358-09] [Citation(s) in RCA: 115] [Impact Index Per Article: 8.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
Sterol regulatory element binding proteins (SREBPs) are membrane-bound transcription factors whose proteolytic activation is controlled by the cellular sterol concentration. Mammalian SREBPs are activated in cholesterol-depleted cells and serve to regulate cellular lipid homeostasis. Recent work demonstrates that SREBP is functionally conserved in fungi. While the ability to respond to sterols is conserved, fungal SREBPs are hypoxic transcription factors required for adaptation to a low-oxygen environment. In the fission yeast Schizosaccharomyces pombe, oxygen regulates the SREBP homolog Sre1 by independently controlling both its proteolytic activation and its degradation. SREBP is also required for adaptation to hypoxia in the human pathogens Cryptococcus neoformans and Aspergillus fumigatus. In these organisms, SREBP is required for virulence and resistance to antifungal drugs, making the SREBP pathway a potential target for antifungal therapy.
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74
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Abstract
Bilayer synthesis during membrane biogenesis involves the concerted assembly of multiple lipid species, requiring coordination of the level of lipid synthesis, uptake, turnover, and subcellular distribution. In this review, we discuss some of the salient conclusions regarding the coordination of lipid synthesis that have emerged from work in mammalian and yeast cells. The principal instruments of global control are a small number of transcription factors that target a wide range of genes encoding enzymes that operate in a given metabolic pathway. Critical in mammalian cells are sterol regulatory element binding proteins (SREBPs) that stimulate expression of genes for the uptake and synthesis of cholesterol and fatty acids. From work with Saccharomyces cerevisiae, much has been learned about glycerophospholipid and ergosterol regulation through Ino2p/Ino4p and Upc2p transcription factors, respectively. Lipid supply is fine-tuned through a multitude of negative feedback circuits initiated by both end products and intermediates of lipid synthesis pathways. Moreover, there is evidence that the diversity of membrane lipids is maintained through cross-regulatory effects, whereby classes of lipids activate the activity of enzymes operating in another metabolic branch.
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Affiliation(s)
- Axel Nohturfft
- Molecular and Metabolic Signalling Centre, Division of Basic Medical Sciences, St. George's University of London, London, SW17 0RE United Kingdom.
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75
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Ikeda Y, Demartino GN, Brown MS, Lee JN, Goldstein JL, Ye J. Regulated endoplasmic reticulum-associated degradation of a polytopic protein: p97 recruits proteasomes to Insig-1 before extraction from membranes. J Biol Chem 2009; 284:34889-900. [PMID: 19815544 DOI: 10.1074/jbc.m109.044875] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Polytopic membrane proteins subjected to endoplasmic reticulum (ER)-associated degradation are extracted from membranes and targeted to proteasomes for destruction. The extraction mechanism is poorly understood. One polytopic ER protein subjected to ER-associated degradation is Insig-1, a negative regulator of cholesterol synthesis. Insig-1 is rapidly degraded by proteasomes when cells are depleted of cholesterol, and its degradation is inhibited when sterols accumulate in cells. Insig-2, a functional homologue of Insig-1, is degraded slowly, and its degradation is not regulated by sterols. Here, we report that a single amino acid substitution in Insig-2, Insig-2(L210A), causes Insig-2 to be degraded in an accelerated and sterol-regulated manner similar to Insig-1. In seeking an explanation for the accelerated degradation, we found that proteasomes bind to wild type Insig-1 and mutant Insig-2(L210A) but not to wild type Insig-2, whereas the proteins are still embedded in cell membranes. This binding depends on at least two factors, ubiquitination of Insig and association with the ATPase p97/VCP complex. These data suggest that p97 recruits proteasomes to polytopic ER proteins even before they are extracted from membranes.
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Affiliation(s)
- Yukio Ikeda
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9046, USA
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76
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Irisawa M, Inoue J, Ozawa N, Mori K, Sato R. The sterol-sensing endoplasmic reticulum (ER) membrane protein TRC8 hampers ER to Golgi transport of sterol regulatory element-binding protein-2 (SREBP-2)/SREBP cleavage-activated protein and reduces SREBP-2 cleavage. J Biol Chem 2009; 284:28995-9004. [PMID: 19706601 DOI: 10.1074/jbc.m109.041376] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
TRC8 (translocation in renal cancer from chromosome 8) is an intrinsic protein of the endoplasmic reticulum that contains a sterol-sensing domain and a RING finger motif encoding an E3 ubiquitin ligase. Here we show that TRC8 overexpression hinders sterol regulatory element-binding protein-2 (SREBP-2) processing, thereby reducing SREBP-2 target gene expression, TRC8 depletion has the opposite effect. Mutation analyses of TRC8 reveal that the ubiquitin ligase activity is dispensable for these effects. Activating transcription factor 6 (ATF6) is also processed in the Golgi by the same two proteases as those for SREBP, but ATF6 processing is not affected by TRC8. TRC8 is capable of binding both SREBP-2 and SREBP cleavage-activated protein (SCAP), thereby forming a TRC8.SREBP-2.SCAP complex. This complex formation hampers the interaction between SCAP and Sec24, one of the COPII proteins that are involved in SREBP-2 transport to the Golgi, thereby reducing SREBP-2 cleavage. TRC8 conjugated by ubiquitin is unstable, whereas the mutant TRC8, lacking the E3 ubiquitin ligase activity and only slightly modified by ubiquitin, is quite stable. TRC8 becomes stable when cells are cultured with a proteasome inhibitor or under a lipoprotein-depleted condition. Lipoprotein depletion impairs ubiquitination of TRC8. Taken together, TRC8 is a novel sterol-sensing endoplasmic reticulum membrane protein that hinders SREBP-2 processing through interaction with SREBP-2 and SCAP, regulating its own turnover rate by means of its E3 ubiquitin ligase activity.
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Affiliation(s)
- Masato Irisawa
- Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan
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77
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Hughes BT, Nwosu CC, Espenshade PJ. Degradation of sterol regulatory element-binding protein precursor requires the endoplasmic reticulum-associated degradation components Ubc7 and Hrd1 in fission yeast. J Biol Chem 2009; 284:20512-21. [PMID: 19520858 DOI: 10.1074/jbc.m109.002436] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
Sre1, the fission yeast sterol regulatory element-binding protein (SREBP), is an endoplasmic reticulum (ER) membrane-bound transcription factor that is a principal regulator of hypoxic gene expression. Under low oxygen, Sre1 is cleaved from its inactive ER precursor form to generate an active nuclear transcription factor that up-regulates genes required for low oxygen growth. To maintain a constant supply of Sre1, Sre1 precursor synthesis must be regulated to replenish Sre1 precursor lost to proteolytic cleavage under low oxygen. In this study, we investigated the mechanisms controlling Sre1 precursor levels. We found that positive feedback regulation at the sre1(+) promoter increases the synthesis of the Sre1 precursor under low oxygen and that this regulation is required for maximal Sre1 activation and target gene expression. We also demonstrate that the Sre1 precursor is rapidly degraded by the proteasome in the absence of its binding partner Scp1, which is required for oxygen-regulated Sre1 cleavage. Degradation of Sre1 in the absence of Scp1 requires the ER-associated degradation (ERAD) components Ubc7, an E2 ubiquitin conjugating enzyme, and Hrd1, an E3 ubiquitin ligase. We conclude that positive feedback regulation to up-regulate Sre1 precursor synthesis under low oxygen is essential for Sre1 function and propose that excess Sre1 precursor is removed by ERAD to ensure complex formation between Sre1 and its binding partner Scp1. Thus, Sre1 is a new example of an endogenous ERAD substrate, establishing fission yeast as an organism for the study of this important degradative pathway.
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Affiliation(s)
- Bridget T Hughes
- Department of Cell Biology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA
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78
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Chatterjee S, Szustakowski JD, Nanguneri NR, Mickanin C, Labow MA, Nohturfft A, Dev KK, Sivasankaran R. Identification of novel genes and pathways regulating SREBP transcriptional activity. PLoS One 2009; 4:e5197. [PMID: 19381295 PMCID: PMC2668173 DOI: 10.1371/journal.pone.0005197] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/18/2008] [Accepted: 02/05/2009] [Indexed: 02/07/2023] Open
Abstract
BACKGROUND Lipid metabolism in mammals is orchestrated by a family of transcription factors called sterol regulatory element-binding proteins (SREBPs) that control the expression of genes required for the uptake and synthesis of cholesterol, fatty acids, and triglycerides. SREBPs are thus essential for insulin-induced lipogenesis and for cellular membrane homeostasis and biogenesis. Although multiple players have been identified that control the expression and activation of SREBPs, gaps remain in our understanding of how SREBPs are coordinated with other physiological pathways. METHODOLOGY To identify novel regulators of SREBPs, we performed a genome-wide cDNA over-expression screen to identify proteins that might modulate the transcription of a luciferase gene driven from an SREBP-specific promoter. The results were verified through secondary biological assays and expression data were analyzed by a novel application of the Gene Set Enrichment Analysis (GSEA) method. CONCLUSIONS/SIGNIFICANCE We screened 10,000 different cDNAs and identified a number of genes and pathways that have previously not been implicated in SREBP control and cellular cholesterol homeostasis. These findings further our understanding of lipid biology and should lead to new insights into lipid associated disorders.
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Affiliation(s)
- Sandipan Chatterjee
- Developmental and Molecular Pathways, Novartis Institutes for BioMedical Research, Cambridge, Massachusetts, United States of America
| | - Joseph D. Szustakowski
- Developmental and Molecular Pathways, Novartis Institutes for BioMedical Research, Cambridge, Massachusetts, United States of America
| | - Nirmala R. Nanguneri
- Developmental and Molecular Pathways, Novartis Institutes for BioMedical Research, Cambridge, Massachusetts, United States of America
| | - Craig Mickanin
- Developmental and Molecular Pathways, Novartis Institutes for BioMedical Research, Cambridge, Massachusetts, United States of America
| | - Mark A. Labow
- Developmental and Molecular Pathways, Novartis Institutes for BioMedical Research, Cambridge, Massachusetts, United States of America
| | - Axel Nohturfft
- Division of Basic Medical Sciences, St. George's University of London, London, United Kingdom
| | - Kumlesh K. Dev
- Department of Anatomy, University College Cork, Cork, Ireland
- * E-mail: (KKD); (RS)
| | - Rajeev Sivasankaran
- Developmental and Molecular Pathways, Novartis Institutes for BioMedical Research, Cambridge, Massachusetts, United States of America
- * E-mail: (KKD); (RS)
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79
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Lee JN, Zhang X, Feramisco JD, Gong Y, Ye J. Unsaturated fatty acids inhibit proteasomal degradation of Insig-1 at a postubiquitination step. J Biol Chem 2008; 283:33772-83. [PMID: 18835813 DOI: 10.1074/jbc.m806108200] [Citation(s) in RCA: 73] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023] Open
Abstract
Proteasomes mediate the regulated degradation of Insig-1, a membrane protein of the endoplasmic reticulum (ER) that plays a crucial role in lipid metabolism. We showed previously that sterols inhibit this degradation by blocking ubiquitination of Insig-1. Here we show that unsaturated fatty acids stabilize Insig-1 without affecting its ubiquitination. Instead unsaturated fatty acids inhibit extraction of ubiquitinated Insig-1 from membranes, a process known to be mediated by valosin-containing protein and necessary for ER-associated degradation. Valosin-containing protein is recruited to Insig-1 through the action of another protein, Ubxd8. Unsaturated fatty acids block the binding between Ubxd8 and Insig-1, thereby abrogating the membrane extraction of Insig-1. Unsaturated fatty acid-mediated stabilization of Insig-1 enhances the ability of sterols to inhibit proteolytic activation of SREBP-1, which activates transcription of genes involved in fatty acid synthesis. The current study provides a molecular mechanism for regulation of proteasome-mediated ER protein degradation at a postubiquitination step.
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Affiliation(s)
- Joon No Lee
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas 75390, USA
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80
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Pallottini V, Guantario B, Martini C, Totta P, Filippi I, Carraro F, Trentalance A. Regulation of HMG-CoA reductase expression by hypoxia. J Cell Biochem 2008; 104:701-9. [PMID: 18459144 DOI: 10.1002/jcb.21757] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
The ability to maintain O(2) homeostasis is essential to the survival of all invertebrate and vertebrate species. The transcriptional factor, hypoxia inducible factor 1 (HIF-1), is the principal regulator of oxygen homeostasis. Under hypoxic condition HIF-1 induces the transcription of several hypoxia-responsive genes by binding to hypoxia-response elements (HRE) in their promoters. In recent years it has been demonstrated that hypoxia could be related to metabolic variations such as hyper-cholesterolemia in mouse models. On the basis of this observation, the present study was performed to verify the involvement of HIF-1, and in particular the effect of chemical and environmental induction of HIF-1alpha (the oxygen sensitive isoform) accumulation in 3-hydroxy 3-methylglutaryl coenzyme A reductase (HMG-CoAR, the key and rate limiting enzyme of cholesterol biosynthetic pathway) regulation. Our results show that HIF-1alpha accumulation is able to increase level and activity of HMG-CoAR by stimulating its transcription. The raised transcription of the reductase could be related to an induction by HIF-1alpha even if a parallel action of SREBP-2 actively translocated to nucleus by the increased level of SCAP cannot be excluded.
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Affiliation(s)
- Valentina Pallottini
- Department of Biology, University of Roma Tre, Viale Marconi, 446-00146 Rome, Italy.
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81
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DeBose-Boyd RA. Feedback regulation of cholesterol synthesis: sterol-accelerated ubiquitination and degradation of HMG CoA reductase. Cell Res 2008; 18:609-21. [PMID: 18504457 DOI: 10.1038/cr.2008.61] [Citation(s) in RCA: 260] [Impact Index Per Article: 16.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023] Open
Abstract
3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase produces mevalonate, an important intermediate in the synthesis of cholesterol and essential nonsterol isoprenoids. The reductase is subject to an exorbitant amount of feedback control through multiple mechanisms that are mediated by sterol and nonsterol end-products of mevalonate metabolism. Here, I will discuss recent advances that shed light on one mechanism for control of reductase, which involves rapid degradation of the enzyme. Accumulation of certain sterols triggers binding of reductase to endoplasmic reticulum (ER) membrane proteins called Insig-1 and Insig-2. Reductase-Insig binding results in recruitment of a membrane-associated ubiquitin ligase called gp78, which initiates ubiquitination of reductase. This ubiquitination is an obligatory reaction for recognition and degradation of reductase from ER membranes by cytosolic 26S proteasomes. Thus, sterol-accelerated degradation of reductase represents an example of how a general cellular process (ER-associated degradation) is used to control an important metabolic pathway (cholesterol synthesis).
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Affiliation(s)
- Russell A DeBose-Boyd
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75390-9046, USA.
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82
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Abstract
Cholesterol is an essential component of mammalian cell membranes and is required for proper membrane permeability, fluidity, organelle identity, and protein function. Cells maintain sterol homeostasis by multiple feedback controls that act through transcriptional and posttranscriptional mechanisms. The membrane-bound transcription factor sterol regulatory element binding protein (SREBP) is the principal regulator of both sterol synthesis and uptake. In mammalian cells, the ER membrane protein Insig has emerged as a key component of homeostatic regulation by controlling both the activity of SREBP and the sterol-dependent degradation of the biosynthetic enzyme HMG-CoA reductase. In this review, we focus on recent advances in our understanding of the molecular mechanisms of the regulation of sterol synthesis. A comparative analysis of SREBP and HMG-CoA reductase regulation in mammals, yeast, and flies points toward an equilibrium model for how lipid signals regulate the activity of sterol-sensing proteins and their downstream effectors.
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Affiliation(s)
- Peter J Espenshade
- Department of Cell Biology, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
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83
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Hughes AL, Stewart EV, Espenshade PJ. Identification of twenty-three mutations in fission yeast Scap that constitutively activate SREBP. J Lipid Res 2008; 49:2001-12. [PMID: 18503029 DOI: 10.1194/jlr.m800207-jlr200] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
The endoplasmic reticulum membrane protein SREBP cleavage-activating protein (Scap) senses sterols and regulates activation of sterol-regulatory element binding proteins (SREBPs), membrane-bound transcription factors that control lipid homeostasis in fission yeast and mammals. Transmembrane segments 2-6 of Scap function as a sterol-sensing domain (SSD) that recognizes changes in cellular sterols and facilitates activation of SREBP. Previous studies identified conserved mutations Y298C, L315F, and D443N in the SSD of mammalian Scap and fission yeast Scap (Scp1) that render cells insensitive to sterols and cause constitutive SREBP activation. In this study, we utilized fission yeast genetics to identify additional functionally important residues in the SSD of Scp1 and Scap. Using a site-directed mutagenesis selection, we sampled all possible amino acid substitutions at 50 conserved residues in the SSD of Scp1 for their effects on yeast SREBP (Sre1) activation. We found mutations at 23 different amino acids in Scp1 that rendered Scp1 insensitive to sterols and caused constitutive activation of Sre1. To our surprise, the majority of the homologous Scap mutants displayed wild-type function, and only one mutation, V439G, caused constitutive activation of SREBP in mammals. These results suggest that the sterol-sensing mechanism of Scap and the functional requirements for SREBP activation are different between fission yeast and mammals.
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Affiliation(s)
- Adam L Hughes
- Department of Cell Biology, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA
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84
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Wong J, Quinn CM, Guillemin G, Brown AJ. Primary human astrocytes produce 24(S),25-epoxycholesterol with implications for brain cholesterol homeostasis. J Neurochem 2007; 103:1764-73. [DOI: 10.1111/j.1471-4159.2007.04913.x] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
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85
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Villagra A, Ulloa N, Zhang X, Yuan Z, Sotomayor E, Seto E. Histone Deacetylase 3 Down-regulates Cholesterol Synthesis through Repression of Lanosterol Synthase Gene Expression. J Biol Chem 2007; 282:35457-70. [DOI: 10.1074/jbc.m701719200] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
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86
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Enhanced sterol response element-binding protein in postintervention restenotic blood vessels plays an important role in vascular smooth muscle proliferation. Life Sci 2007; 82:174-81. [PMID: 18068195 DOI: 10.1016/j.lfs.2007.10.025] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/20/2007] [Revised: 10/30/2007] [Accepted: 10/31/2007] [Indexed: 01/01/2023]
Abstract
Postintervention restenosis (PIRS) after balloon angioplasty or stent implantation is a limitation for these interventional procedures even with the advent of new drug-eluting stents. Sterol regulatory element-binding proteins (SREBP) are transcription factors governing cellular lipid biosynthesis and thus critical in the regulation of the lipid-rich cell membranes. PIRS following injury results partially from newly proliferating cells expressing vascular smooth muscle cell (VSMC) markers. Platelet-derived growth factor (PDGF), lysophosphatidic acid (LPA) and alpha(1)-adrenergic receptor stimulation are well recognized diverse mitogens for VSMC activation in PIRS. We examined whether PDGF, LPA and alpha(1)-adrenergic receptor stimulation with phenylephrine (PE) regulate SREBP expression and subsequently, VSMC proliferation. Our results show that PDGF, LPA and PE upregulate SREBP-1 in a time- and dose-dependent manner. PDGF, LPA and PE-mediated proliferation is dependent on SREBP since inhibition of SREBP expression using targeted knockdown of the SREBP precursor SREBP activating protein (SCAP) by siRNA led to an attenuation of SREBP expression and decreased PDGF, LPA and PE induced proliferation. In two different in vivo PIRS models we found that SREBP-1 was enhanced in the injured blood vessel wall, especially within the neointima and co-localized with alpha-smooth muscle actin positive cells. Thus, SREBP is enhanced in the vessel wall following PIRS and is important in the regulation of pro-hyperplasia molecular signaling. SREBP inhibition may be a powerful tool to limit PIRS.
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87
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Infante RE, Abi-Mosleh L, Radhakrishnan A, Dale JD, Brown MS, Goldstein JL. Purified NPC1 protein. I. Binding of cholesterol and oxysterols to a 1278-amino acid membrane protein. J Biol Chem 2007; 283:1052-63. [PMID: 17989073 DOI: 10.1074/jbc.m707943200] [Citation(s) in RCA: 162] [Impact Index Per Article: 9.5] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The Niemann-Pick, Type C1 protein (NPC1) is required for the transport of lipoprotein-derived cholesterol from lysosomes to endoplasmic reticulum. The 1278-amino acid, polytopic membrane protein has not been purified, and its mechanism of action is unknown. Unexpectedly, we encountered NPC1 in a search for a membrane protein that binds 25-hydroxycholesterol (25-HC) and other oxysterols. A 25-HC-binding protein was purified more than 14,000-fold from rabbit liver membranes and identified as NPC1 by mass spectroscopy. We prepared recombinant human NPC1 and confirmed its ability to bind oxysterols, including those with a hydroxyl group on the 24, 25, or 27 positions. Hydroxyl groups on the 7, 19, or 20 positions failed to confer binding. Recombinant human NPC1 also bound [(3)H]cholesterol in a reaction inhibited by Nonidet P-40 above its critical micellar concentration. Low concentrations of unlabeled 25-HC abolished binding of [(3)H]cholesterol, but the converse was not true, i.e. unlabeled cholesterol, even at high concentrations, did not abolish binding of [(3)H]25-HC. NPC1 is not required for the known regulatory actions of oxysterols. Thus, in NPC1-deficient fibroblasts 25-HC blocked the processing of sterol regulatory element-binding proteins and activated acyl-CoA:cholesterol acyltransferase in a normal fashion. The availability of assays to measure NPC1 binding in vitro may further the understanding of ways in which oxysterols regulate intracellular lipid transport.
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Affiliation(s)
- Rodney E Infante
- Department of Molecular Genetics, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75390, USA
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88
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Klewitz C, Klenk HD, Ter Meulen J. Amino acids from both N-terminal hydrophobic regions of the Lassa virus envelope glycoprotein GP-2 are critical for pH-dependent membrane fusion and infectivity. J Gen Virol 2007; 88:2320-2328. [PMID: 17622638 DOI: 10.1099/vir.0.82950-0] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Lassa virus glycoprotein 2 (LASV GP-2) belongs to the class I fusion protein family. Its N terminus contains two stretches of highly conserved hydrophobic amino acids (residues 260-266 and 276-298) that have been proposed as N-terminal or internal fusion peptide segments (N-FPS, I-FPS) by analogy with similar sequences of other viral glycoproteins or based on experimental data obtained with synthetic peptides, respectively. By using a pH-dependent, recombinant LASV glycoprotein mediated cell-cell fusion assay and a retroviral pseudotype infectivity assay, an alanine scan of all hydrophobic amino acids within both proposed FPSs was performed. Fusogenicity and infectivity were correlated, both requiring correct processing of the glycoprotein precursor. Most point mutations in either FPS accounted for reduced or abolished fusion or infection, respectively. Some mutations also had an effect on pre-fusion steps of virus entry, possibly by inducing structural changes in the glycoprotein. The data demonstrate that several amino acids from both hydrophobic regions of the N terminus, some of which (W264, G277, Y278 and L280) are 100 % conserved in all arenaviruses, are involved in fusogenicity and infectivity of LASV GP-2.
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Affiliation(s)
- Christian Klewitz
- The Institute for Virology, Philipps University, Hans-Meerwein-Straße 3, 35043 Marburg, Germany
| | - Hans-Dieter Klenk
- The Institute for Virology, Philipps University, Hans-Meerwein-Straße 3, 35043 Marburg, Germany
| | - Jan Ter Meulen
- Crucell Holland BV, PO Box 2048, 2301 CA Leiden, The Netherlands
- The Institute for Virology, Philipps University, Hans-Meerwein-Straße 3, 35043 Marburg, Germany
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89
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Nguyen AD, McDonald JG, Bruick RK, DeBose-Boyd RA. Hypoxia Stimulates Degradation of 3-Hydroxy-3-methylglutaryl-coenzyme A Reductase through Accumulation of Lanosterol and Hypoxia-Inducible Factor-mediated Induction of Insigs. J Biol Chem 2007; 282:27436-27446. [PMID: 17635920 DOI: 10.1074/jbc.m704976200] [Citation(s) in RCA: 96] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022] Open
Abstract
Endoplasmic reticulum-associated degradation of the enzyme 3-hydroxy-3-methylglutaryl-CoA reductase represents one mechanism by which cholesterol synthesis is controlled in mammalian cells. The key reaction in this degradation is binding of reductase to Insig proteins in the endoplasmic reticulum, which is stimulated by the cholesterol precursor lanosterol. Conversion of lanosterol to cholesterol requires removal of three methyl groups, which consumes nine molecules of dioxygen. Here, we report that oxygen deprivation (hypoxia) slows demethylation of lanosterol and its metabolite 24,25-dihydrolanosterol, causing both sterols to accumulate in cells. In addition, hypoxia increases the amount of Insig-1 and Insig-2 in a response mediated by hypoxia-inducible factor (HIF)-1alpha. Accumulation of lanosterol together with increased Insigs accelerates degradation of reductase, which ultimately slows a rate-determining step in cholesterol synthesis. These results define a novel oxygen-sensing mechanism mediated by the combined actions of methylated intermediates in cholesterol synthesis and the hypoxia-activated transcription factor HIF-1alpha.
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Affiliation(s)
- Andrew D Nguyen
- Departments of Molecular Genetics and University of Texas Southwestern Medical Center, Dallas, Texas 75390-9046
| | - Jeffrey G McDonald
- Departments of Molecular Genetics and University of Texas Southwestern Medical Center, Dallas, Texas 75390-9046
| | - Richard K Bruick
- Departments of Biochemistry, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9046
| | - Russell A DeBose-Boyd
- Departments of Molecular Genetics and University of Texas Southwestern Medical Center, Dallas, Texas 75390-9046.
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90
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Lee H, Bien CM, Hughes AL, Espenshade PJ, Kwon-Chung KJ, Chang YC. Cobalt chloride, a hypoxia-mimicking agent, targets sterol synthesis in the pathogenic fungus Cryptococcus neoformans. Mol Microbiol 2007; 65:1018-33. [PMID: 17645443 DOI: 10.1111/j.1365-2958.2007.05844.x] [Citation(s) in RCA: 67] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
We investigated the effects of the hypoxia-mimetic CoCl2 in the pathogenic fungus Cryptococcus neoformans and demonstrated that CoCl2 leads to defects in several enzymatic steps in ergosterol biosynthesis. Sterol defects were amplified in cells lacking components of the Sre1p-mediated oxygen-sensing pathway. Consequently, Sre1p and its binding partner Scp1p were essential for growth in the presence of CoCl2. Interestingly, high copies of a single gene involved in ergosterol biosynthesis, ERG25, rescued this growth defect. We show that the inhibitory effect of CoCl2 on scp1Delta and sre1Delta cells likely resulted from either an accumulation of non-viable methylated sterols or a decrease in the amount of ergosterol. Similar findings were also observed in the ascomycetous yeast, Schizosaccharomyces pombe, suggesting that the effects of CoCl2 on the Sre1p-mediated response are conserved in fungi. In addition, gene expression analysis revealed limited overlap between Sre1p-dependant gene activation in the presence of CoCl2 and low oxygen. The majority of genes similarly affected by both CoCl2 and low oxygen were involved in ergosterol synthesis and in iron/copper transport. This article identifies the Sre1p pathway as a common mechanism by which yeast cells sense and adapt to changes in both CoCl2 concentrations and oxygen levels.
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Affiliation(s)
- Hyeseung Lee
- Laboratory of Clinical Infectious Diseases, NIAID, NIH, Bethesda, MD 20892, USA
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91
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Lee PCW, Liu P, Li WP, Debose-Boyd RA. Amplification of the gene for SCAP, coupled with Insig-1 deficiency, confers sterol resistance in mutant Chinese hamster ovary cells. J Lipid Res 2007; 48:1944-54. [PMID: 17586788 DOI: 10.1194/jlr.m700225-jlr200] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The endoplasmic reticulum membrane proteins Insig-1 and Insig-2 limit cholesterol synthesis, in part through their sterol-dependent binding to sterol-regulatory element binding protein (SREBP) cleavage-activating protein (SCAP). This binding prevents proteolytic processing of SREBPs, membrane-bound transcription factors that enhance cholesterol synthesis. We report here the characterization of mutant Chinese hamster ovary (CHO) cells, designated SRD-19, that are resistant to 25-hydroxycholesterol, a potent inhibitor of SREBP processing. SRD-19 cells were produced by mutagenesis of Insig-1-deficient SRD-14 cells, followed by selection in high levels of 25-hydroxycholesterol. 25-Hydroxycholesterol fails to suppress SREBP processing in SRD-19, even though they express normal levels of Insig-2. The number of copies of the gene encoding SCAP was found to be increased by 4-fold in SRD-19 cells compared with wild-type CHO cells, leading to the overproduction of SCAP mRNA and protein. Our data indicate that overproduced SCAP saturates the remaining Insig-2 in SRD-19 cells, thus explaining their resistance to 25-hydroxycholesterol. Consistent with this conclusion, regulated SREBP processing is restored in SRD-19 cells upon transfection of plasmids encoding either Insig-1 or Insig-2. These results highlight the importance of SCAP/Insig ratios in normal sterol-regulated processing of SREBPs in cultured cells.
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Affiliation(s)
- Peter C W Lee
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75390-9046, USA
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92
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Radhakrishnan A, Ikeda Y, Kwon HJ, Brown MS, Goldstein JL. Sterol-regulated transport of SREBPs from endoplasmic reticulum to Golgi: oxysterols block transport by binding to Insig. Proc Natl Acad Sci U S A 2007; 104:6511-8. [PMID: 17428920 PMCID: PMC1851665 DOI: 10.1073/pnas.0700899104] [Citation(s) in RCA: 445] [Impact Index Per Article: 26.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Cholesterol synthesis in animals is controlled by the regulated transport of sterol regulatory element-binding proteins (SREBPs) from the endoplasmic reticulum to the Golgi, where the transcription factors are processed proteolytically to release active fragments. Transport is inhibited by either cholesterol or oxysterols, blocking cholesterol synthesis. Cholesterol acts by binding to the SREBP-escort protein Scap, thereby causing Scap to bind to anchor proteins called Insigs. Here, we show that oxysterols act by binding to Insigs, causing Insigs to bind to Scap. Mutational analysis of the six transmembrane helices of Insigs reveals that the third and fourth are important for Insig's binding to oxysterols and to Scap. These studies define Insigs as oxysterol-binding proteins, explaining the long-known ability of oxysterols to inhibit cholesterol synthesis in animal cells.
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Affiliation(s)
| | | | - Hyock Joo Kwon
- Biochemistry, University of Texas Southwestern Medical Center, Dallas, TX 75390
| | - Michael S. Brown
- Departments of *Molecular Genetics and
- To whom correspondence may be addressed. E-mail: or
| | - Joseph L. Goldstein
- Departments of *Molecular Genetics and
- To whom correspondence may be addressed. E-mail: or
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93
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Sun LP, Seemann J, Goldstein JL, Brown MS. Sterol-regulated transport of SREBPs from endoplasmic reticulum to Golgi: Insig renders sorting signal in Scap inaccessible to COPII proteins. Proc Natl Acad Sci U S A 2007; 104:6519-26. [PMID: 17428919 PMCID: PMC1851663 DOI: 10.1073/pnas.0700907104] [Citation(s) in RCA: 219] [Impact Index Per Article: 12.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Two classes of sterols, cholesterol and oxysterols, block export of sterol regulatory element-binding proteins (SREBPs) from the endoplasmic reticulum (ER) to the Golgi by preventing the binding of COPII-coated proteins to a hexapeptide sorting signal (MELADL) in Scap, the SREBP-escort protein. Here, we show that anti-MELADL blocks COPII binding in vitro, and microinjection of Fab anti-MELADL blocks Scap.SREBP movement in cells. Cholesterol and oxysterols block COPII binding to MELADL by binding to different intracellular receptors, cholesterol to Scap and oxysterols to Insig. Cysteine labeling shows that both binding events produce a conformational change near the MELADL sequence, abrogating COPII binding but not anti-MELADL binding. Mutagenesis experiments raise the possibility that the distance of MELADL from the ER membrane is crucial for COPII binding, and we speculate that sterols and Insig block SREBP transport by altering the location of MELADL with respect to the membrane, rendering it inaccessible to COPII proteins.
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Affiliation(s)
| | - Joachim Seemann
- Cell Biology, University of Texas Southwestern Medical Center, Dallas, TX 75390
| | - Joseph L. Goldstein
- Departments of *Molecular Genetics and
- To whom correspondence may be addressed. E-mail: or
| | - Michael S. Brown
- Departments of *Molecular Genetics and
- To whom correspondence may be addressed. E-mail: or
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94
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Wong J, Quinn C, Brown A. SREBP-2 positively regulates transcription of the cholesterol efflux gene, ABCA1, by generating oxysterol ligands for LXR. Biochem J 2006; 400:485-91. [PMID: 16901265 PMCID: PMC1698594 DOI: 10.1042/bj20060914] [Citation(s) in RCA: 104] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Cholesterol accumulation and removal are regulated by two different transcription factors. SREBP-2 (sterol-regulatory-element-binding protein-2) is best known to up-regulate genes involved in cholesterol biosynthesis and uptake, whereas LXR (liver X receptor) is best known for up-regulating cholesterol efflux genes. An important cholesterol efflux gene that is regulated by LXR is the ATP-binding cassette transporter, ABCA1 (ATP-binding cassette transporter-A1). We have previously shown that statin treatment down-regulated ABCA1 expression in human macrophages, probably by inhibiting synthesis of the LXR ligand 24(S),25-epoxycholesterol. However, it was subsequently reported that ABCA1 expression is down-regulated by SREBP-2 through binding of SREBP-2 to an E-box element in ABCA1's proximal promoter. As statin treatment induces SREBP-2 activation, this may provide an alternative explanation for the statin-mediated down-regulation of ABCA1. In the present study, we employed a set of CHO (Chinese-hamster ovary) mutant cell lines to investigate the role of SREBP-2 in the regulation of ABCA1. We observed increased ABCA1 mRNA levels in SREBP-2-overexpressing cells and decreased levels in cells lacking a functional SREBP-2 pathway, which were restored when the SREBP-2 pathway was reinstated. Moreover, ABCA1 gene expression was positively associated with synthesis of 24(S),25-epoxycholesterol in these cell lines. In studies using a human ABCA1 promoter reporter assay, mutation of the E-box motif had a similar response as the wild-type construct to either statin treatment or addition of 24(S),25-epoxycholesterol. By contrast, these responses were completely ablated when the DR4 element to which LXR binds was mutated. These results support the idea that 24(S),25-epoxycholesterol and statin treatment influence ABCA1 transcription via supply of an LXR ligand and not through an SREBP-2/E-box-related mechanism. In addition, our results indicate a critical role of SREBP-2 as a positive regulator of ABCA1 gene expression by enabling the generation of oxysterol ligands for LXR.
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Affiliation(s)
- Jenny Wong
- *School of Biotechnology and Biomolecular Sciences, Biosciences Building D26, The University of New South Wales, Sydney, NSW 2052, Australia
| | - Carmel M. Quinn
- †Centre for Vascular Research, School of Medical Sciences, University of New South Wales, Sydney, NSW 2052, Australia
- ‡Centre for Vascular Research, Department of Haematology, Prince of Wales Hospital, Sydney, NSW 2031, Australia
| | - Andrew J. Brown
- *School of Biotechnology and Biomolecular Sciences, Biosciences Building D26, The University of New South Wales, Sydney, NSW 2052, Australia
- To whom correspondence should be addressed (email )
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95
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Lee JN, Song B, DeBose-Boyd RA, Ye J. Sterol-regulated degradation of Insig-1 mediated by the membrane-bound ubiquitin ligase gp78. J Biol Chem 2006; 281:39308-15. [PMID: 17043353 DOI: 10.1074/jbc.m608999200] [Citation(s) in RCA: 124] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
Insig-1 and Insig-2, closely related endoplasmic reticulum membrane proteins, mediate transcriptional and post-transcriptional mechanisms that assure cholesterol homeostasis through their sterol-induced binding to Scap (SREBP cleavage-activating protein) and 3-hydroxy-3-methylglutaryl coenzyme A reductase. Recent studies show that Insig-1 (but not Insig-2) is ubiquitinated and rapidly degraded when cells are depleted of sterols. Conversely, ubiquitination of Insig-1 is blocked, and the protein is stabilized when intracellular sterols accumulate. Here, we report that the ubiquitin ligase gp78, which binds with much higher affinity to Insig-1 than Insig-2, is required for ubiquitination and degradation of Insig-1 in sterol-depleted cells. Sterols prevent Insig-1 ubiquitination and degradation by displacing gp78 from Insig-1, an event that results from sterol-induced binding of Scap to Insig-1. In addition to providing a mechanism for sterol-regulated degradation of Insig-1, these results help to explain why Scap is subject to endoplasmic reticulum retention upon Insig-1 binding, whereas 3-hydroxy-3-methylglutaryl coenzyme A reductase is ubiquitinated and degraded.
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Affiliation(s)
- Joon No Lee
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas 75390, USA
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96
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Yang C, McDonald JG, Patel A, Zhang Y, Umetani M, Xu F, Westover EJ, Covey DF, Mangelsdorf DJ, Cohen JC, Hobbs HH. Sterol intermediates from cholesterol biosynthetic pathway as liver X receptor ligands. J Biol Chem 2006; 281:27816-26. [PMID: 16857673 DOI: 10.1074/jbc.m603781200] [Citation(s) in RCA: 212] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The liver X receptors (LXRs) are ligand-activated transcription factors that regulate the expression of genes controlling lipid metabolism. Oxysterols bind LXRs with high affinity in vitro and are implicated as ligands for the receptor. We showed previously that accumulation of selected dietary sterols, in particular stigmasterol, is associated with activation of LXR in vivo. In the course of the defining of structural features of stigmasterol that confer LXR agonist activity, we determined that the presence of an unsaturated bond in the side chain of the sterol was necessary and sufficient for activity, with the C-24 unsaturated cholesterol precursor sterols desmosterol and zymosterol exerting the largest effects. Desmosterol failed to increase expression of the LXR target gene, ABCA1, in LXRalpha/beta-deficient mouse fibroblasts, but was fully active in cells lacking cholesterol 24-, 25-, and 27-hydroxylase; thus, the effect of desmosterol was LXR-dependent and did not require conversion to a side chain oxysterol. Desmosterol bound to purified LXRalpha and LXRbeta in vitro and supported the recruitment of steroid receptor coactivator 1. Desmosterol also inhibited processing of the sterol response element-binding protein-2 and reduced expression of hydroxymethylglutaryl-CoA reductase. These observations are consistent with specific intermediates in the cholesterol biosynthetic pathway regulating lipid homeostasis through both the LXR and sterol response element-binding protein pathways.
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Affiliation(s)
- Chendong Yang
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, 75390, USA
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97
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Qiu H, Xia T, Chen X, Zhao X, Gan L, Feng S, Lei T, Yang Z. Cloning, comparative characterization of porcine SCAP gene, and identification of its two splice variants. Mol Genet Genomics 2006; 276:187-96. [PMID: 16705418 DOI: 10.1007/s00438-006-0134-8] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/2005] [Accepted: 04/21/2006] [Indexed: 10/24/2022]
Abstract
Sterol responsive element binding protein (SREBP) cleavage-activating protein (SCAP) is the key regulator of activation of SREBPs, which stimulate most enzymes in cholesterol and lipid synthesis. In order to investigate the molecular basis of lipid metabolism in the pig, a unique model for fat deposition, we isolated and characterized the porcine SCAP. The 4,096-bp full-length porcine SCAP cDNA contains an open reading frame of 3,840 bp. The predicted SCAP protein consists of 1,280 amino acids of 55-92% identity with its vertebrate counterparts. The porcine SCAP gene consists of at least 19 exons and 18 introns, which span over 13 kb of the genome. The porcine SCAP gene was mapped to chromosome 13q21-22 using a porcine-rodent somatic cell hybrid panel. Comparison of SCAP genomic structures from various species revealed intron losses in porcine, Tetraodon and fugu SCAP, and intron gains in cow and chicken SCAP. Moreover, we isolated two novel splicing SCAP variants with 193-bp (variant 2) in-frame deletion from testis and a variant with 291-bp (variant 3) in-frame deletion from liver and muscle, which may affect the function of the porcine SCAP. In conclusion, the intron gains and losses appear to have contributed to the shape of the modern SCAP family. The splice variants detected, first to be reported in any species, may be involved in the particulars of the fat metabolism in the pig. Our data lay foundation for further study of SCAP function in this species.
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Affiliation(s)
- Huan Qiu
- Lab of Biochemistry, College of Life Science and Technology, Key Laboratory of Agricultural Animal Genetics, Breeding and Reproduction of Ministry of Education, Huazhong Agricultural University, Wuhan, 430070, People's Republic of China
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98
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Gong Y, Lee JN, Brown MS, Goldstein JL, Ye J. Juxtamembranous aspartic acid in Insig-1 and Insig-2 is required for cholesterol homeostasis. Proc Natl Acad Sci U S A 2006; 103:6154-9. [PMID: 16606821 PMCID: PMC1435366 DOI: 10.1073/pnas.0601923103] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Insig-1 and Insig-2 are closely related proteins of the endoplasmic reticulum (ER) that mediate feedback control of cholesterol synthesis by sterol-dependent binding to the following two membrane proteins: the escort protein Scap, thus preventing proteolytic processing of sterol regulatory element-binding proteins; and the cholesterol biosynthetic enzyme 3-hydroxy-3-methylglutaryl CoA reductase, thus inducing the ubiquitination and ER-associated degradation of the enzyme. Here, we report that the conserved Asp-205 in Insig-1, which abuts the fourth transmembrane helix at the cytosolic side of the ER membrane, is essential for its dual function. When Asp-205 was mutated to alanine, the mutant Insig-1 lost the ability to bind to Scap and, thus, was unable to suppress the cleavage of sterol regulatory element-binding proteins. The mutant Insig-1 was ineffective also in accelerating sterol-stimulated degradation of 3-hydroxy-3-methylglutaryl CoA reductase. Alanine substitution of the corresponding aspartic acid in Insig-2 produced the same dual defects. These studies identify a single amino acid residue that is crucial for the function of Insig proteins in regulating cholesterol homeostasis in mammalian cells.
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Affiliation(s)
- Yi Gong
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75390-9046
| | - Joon No Lee
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75390-9046
| | - Michael S. Brown
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75390-9046
- *To whom correspondence may be addressed at:
Department of Molecular Genetics, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Room L5.238, Dallas, TX 75390-9046. E-mail:
or
| | - Joseph L. Goldstein
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75390-9046
- *To whom correspondence may be addressed at:
Department of Molecular Genetics, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Room L5.238, Dallas, TX 75390-9046. E-mail:
or
| | - Jin Ye
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75390-9046
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99
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Du X, Kristiana I, Wong J, Brown AJ. Involvement of Akt in ER-to-Golgi transport of SCAP/SREBP: a link between a key cell proliferative pathway and membrane synthesis. Mol Biol Cell 2006; 17:2735-45. [PMID: 16571675 PMCID: PMC1474807 DOI: 10.1091/mbc.e05-11-1094] [Citation(s) in RCA: 134] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
Akt is a critical regulator of cell growth, proliferation, and survival that is activated by phosphatidylinositol 3-kinase (PI3K). We investigated the effect of PI3K inhibition on activation of sterol regulatory element binding protein-2 (SREBP-2), a master regulator of cholesterol homeostasis. SREBP-2 processing increased in response to various cholesterol depletion approaches (including statin treatment) and this increase was blunted by treatment with a potent and specific inhibitor of PI3K, LY294002, or when a plasmid encoding a dominant-negative form of Akt (DN-Akt) was expressed. LY294002 also suppressed SREBP-2 processing induced by insulin-like growth factor-1. Furthermore, LY294002 treatment down-regulated SREBP-2 or -1c gene targets and decreased cholesterol and fatty acid synthesis. Fluorescence microscopy studies indicated that LY294002 disrupts transport of the SREBP escort protein, SCAP, from the endoplasmic reticulum to the Golgi. This disruption was also shown by immunofluorescence staining when DN-Akt was expressed. Taken together, our studies indicate that the PI3K/Akt pathway is involved in SREBP-2 transport to the Golgi, contributing to the control of SREBP-2 activation. Our results provide a crucial mechanistic link between the SREBP and PI3K/Akt pathways that may be reconciled teleologically because synthesis of new membrane is an absolute requirement for cell growth and proliferation.
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Affiliation(s)
- Ximing Du
- School of Biotechnology and Biomolecular Sciences, University of New South Wales, Sydney 2052, Australia
| | - Ika Kristiana
- School of Biotechnology and Biomolecular Sciences, University of New South Wales, Sydney 2052, Australia
| | - Jenny Wong
- School of Biotechnology and Biomolecular Sciences, University of New South Wales, Sydney 2052, Australia
| | - Andrew J. Brown
- School of Biotechnology and Biomolecular Sciences, University of New South Wales, Sydney 2052, Australia
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Durst R, Jansen A, Erez G, Bravdo R, Butbul E, Ben Avi L, Shpitzen S, Lotan C, Leitersdorf E, Defesche J, Friedlander Y, Meiner V, Miserez AR. The discrete and combined effect of SREBP-2 and SCAP isoforms in the control of plasma lipids among familial hypercholesterolaemia patients. Atherosclerosis 2006; 189:443-50. [PMID: 16466730 DOI: 10.1016/j.atherosclerosis.2006.01.001] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/02/2005] [Revised: 12/07/2005] [Accepted: 01/09/2006] [Indexed: 02/06/2023]
Abstract
BACKGROUND AND AIM Hypercholesterolaemia is a major risk factor for atherosclerosis. Cholesterol is modulated by genetic and environmental factors. An important regulatory pathway is controlled by the sterol-regulatory element-binding proteins (SREBPs) and the SREBP cleavage-activating protein (SCAP). Both SREBP-2 and SCAP are candidates to contribute to the development of atherosclerosis. We investigated the possible effects of the variability of proteins involved in this regulatory pathway on plasma lipids among familial hypercholesterolaemia patients. METHODS AND RESULTS Single nucleotide polymorphisms (SNPs) in the genes encoding SREBP-2 and SCAP causing amino acid changes at positions 595 (595G/A) and 796 (796I/V), respectively, were genotyped in 801 FH individuals originating from Israel, The Netherlands, and Switzerland. A linear regression model to examine the associations between SREBP-2 and SCAP isoforms and lipid and lipoprotein levels was used. In females, homozygosity either for the SREBP-2-595A or for the SCAP-796I isoform was associated with higher LDL-cholesterol plasma concentrations (14.7 mg/dl and 20.3 mg/dl, respectively). Surprisingly, heterozygosity for the combination SREBP-2-595A/SCAP-796I was associated with a decrease of 30.28 mg/dl in LDL-C (p-value for gene-gene interaction=0.09). No such effect was observed among FH males. Subgroup analysis considering the most frequent (N>/=24) LDL receptor mutations (del191-2, ins313+1-2, C660X, E207K, S285L) revealed further gene-dosage- and gender-dependent effects of the SCAP mutations on LDL-cholesterol concentrations (p=0.0345). These effects were, however, not present when less frequent LDL receptor mutations were investigated. CONCLUSIONS These results suggest a possible gene-gene interaction between the genes encoding SREBP-2 and SCAP that modulate plasma lipids in a strictly gender-specific fashion. Further investigation is needed to confirm this effect. A study in a larger FH group or in non-FH hypercholesterolaemic subjects may further define the role of this regulatory mechanism in determining plasma lipid concentration.
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Affiliation(s)
- R Durst
- Internal Medicine Division, Hadassah Hebrew University Medical Center, Jerusalem, Israel.
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