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Olkkonen VM, Koponen A, Arora A. OSBP-related protein 2 (ORP2): Unraveling its functions in cellular lipid/carbohydrate metabolism, signaling and F-actin regulation. J Steroid Biochem Mol Biol 2019; 192:105298. [PMID: 30716465 DOI: 10.1016/j.jsbmb.2019.01.016] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/20/2018] [Revised: 01/21/2019] [Accepted: 01/25/2019] [Indexed: 12/20/2022]
Abstract
Oxysterol-binding protein (OSBP)-related proteins (ORPs) constitute a family of intracellular lipid-binding/transport proteins (LTPs) in eukaryotes. They typically have a modular structure comprising a lipid-binding domain and membrane targeting determinants, being thus suited for function at membrane contact sites. Among the mammalian ORPs, ORP2/OSBPL2 is the only member that only exists as a 'short' variant lacking a membrane-targeting pleckstrin homology domain. ORP2 is expressed ubiquitously and has been assigned a multitude of functions. Its OSBP-related domain binds cholesterol, oxysterols, and phosphoinositides, and its overexpression enhances cellular cholesterol efflux. Consistently, the latest observations suggest a function of ORP2 in cholesterol transport to the plasma membrane (PM) in exchange for phosphatidylinositol 4,5-bisphosphate (PI4,5P2), with significant impacts on the concentrations of PM cholesterol and PI4,5P2. On the other hand, ORP2 localizes at the surface of cytoplasmic lipid droplets (LDs) and at endoplasmic-reticulum-LD contact sites, and its depletion modifies cellular triglyceride (TG) metabolism. Study in an adrenocortical cell line further suggested a function of ORP2 in the synthesis of steroid hormones. Our recent knock-out of ORP2 in human hepatoma cells revealed its function in hepatocellular PI3K/Akt signaling, glucose and triglyceride metabolism, as well as in actin cytoskeletal regulation, cell adhesion, migration and proliferation. ORP2 was shown to interact physically with F-actin regulators such as DIAPH1, ARHGAP12, SEPT9 and MLC12, as well as with IQGAP1 and the Cdc37-Hsp90 chaperone complex controlling the activity of Akt. Interestingly, mutations in OSBPL2 encoding ORP2 are associated with autosomal dominant non-syndromic hearing loss, and the protein was found to localize in cochlear hair cell stereocilia. The functions assigned to ORP2 suggest that this protein, in concert with other LTPs, controls the subcellular distribution of cholesterol in various cell types and steroid hormone synthesis in adrenocortical cells. However, it also impacts cellular TG and carbohydrate metabolism and F-actin-dependent functions, revealing a bewildering spectrum of activities.
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Affiliation(s)
- Vesa M Olkkonen
- Minerva Foundation Institute for Medical Research, Biomedicum 2U, FI-00290, Helsinki, Finland; Department of Anatomy, Faculty of Medicine, FI-00014, University of Helsinki, Finland.
| | - Annika Koponen
- Minerva Foundation Institute for Medical Research, Biomedicum 2U, FI-00290, Helsinki, Finland; Department of Anatomy, Faculty of Medicine, FI-00014, University of Helsinki, Finland
| | - Amita Arora
- Minerva Foundation Institute for Medical Research, Biomedicum 2U, FI-00290, Helsinki, Finland; Department of Anatomy, Faculty of Medicine, FI-00014, University of Helsinki, Finland
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Wang H, Lin C, Yao J, Shi H, Zhang C, Wei Q, Lu Y, Chen Z, Xing G, Cao X. Deletion of OSBPL2 in auditory cells increases cholesterol biosynthesis and drives reactive oxygen species production by inhibiting AMPK activity. Cell Death Dis 2019; 10:627. [PMID: 31427568 PMCID: PMC6700064 DOI: 10.1038/s41419-019-1858-9] [Citation(s) in RCA: 21] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2019] [Revised: 07/25/2019] [Accepted: 07/30/2019] [Indexed: 02/07/2023]
Abstract
Oxysterol-binding protein like 2 (OSBPL2) was identified as a novel causal gene for autosomal dominant nonsyndromic hearing loss. However, the pathogenesis of OSBPL2 deficits in ADNSHL was still unclear. The function of OSBPL2 as a lipid-sensing regulator in multiple cellular processes suggested that OSBPL2 might play an important role in the regulation of cholesterol-homeostasis, which was essential for inner ear. In this study the potential roles of OSBPL2 in cholesterol biosynthesis and ROS production were investigated in Osbpl2-KO OC1 cells and osbpl2b-KO zebrafish. RNA-seq-based analysis suggested that OSBPL2 was implicated in cholesterol biosynthesis and AMPK signaling pathway. Furthermore, Osbpl2/osbpl2b-KO resulted in a reduction of AMPK activity and up-regulation of Srebp2/srebp2, Hmgcr/hmgcr and Hmgcs1/hmgcs1, key genes in the sterol biosynthetic pathway and associated with AMPK signaling. In addition, OSBPL2 was also found to interact with ATIC, key activator of AMPK. The levels of total cholesterol and ROS in OC1 cells or zebrafish inner ear were both increased in Osbpl2/osbpl2b-KO mutants and the mitochondrial damage was detected in Osbpl2-KO OC1 cells. This study uncovered the regulatory roles of OSBPL2 in cellular cholesterol biosynthesis and ROS production. These founds might contribute to the deep understanding of the pathogenesis of OSBPL2 mutation in ADNSHL.
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Affiliation(s)
- Hongshun Wang
- Department of Medical Genetics, School of Basic Medical Science, Nanjing Medical University, Nanjing, China
| | - Changsong Lin
- Department of Medical Genetics, School of Basic Medical Science, Nanjing Medical University, Nanjing, China
| | - Jun Yao
- Department of Medical Genetics, School of Basic Medical Science, Nanjing Medical University, Nanjing, China.,Jiangsu Key Laboratory of Xenotransplantation, Nanjing Medical University, Nanjing, China
| | - Hairong Shi
- Department of Medical Genetics, School of Basic Medical Science, Nanjing Medical University, Nanjing, China
| | - Cui Zhang
- Department of Medical Genetics, School of Basic Medical Science, Nanjing Medical University, Nanjing, China
| | - Qinjun Wei
- Department of Medical Genetics, School of Basic Medical Science, Nanjing Medical University, Nanjing, China.,Jiangsu Key Laboratory of Xenotransplantation, Nanjing Medical University, Nanjing, China.,The Laboratory Center for Basic Medical Sciences, Nanjing Medical University, Nanjing, China
| | - Yajie Lu
- Department of Medical Genetics, School of Basic Medical Science, Nanjing Medical University, Nanjing, China.,Jiangsu Key Laboratory of Xenotransplantation, Nanjing Medical University, Nanjing, China
| | - Zhibin Chen
- Department of Otolaryngology, The First Affiliated Hospital of Nanjing Medical University, Nanjing, China
| | - Guangqian Xing
- Department of Otolaryngology, The First Affiliated Hospital of Nanjing Medical University, Nanjing, China
| | - Xin Cao
- Department of Medical Genetics, School of Basic Medical Science, Nanjing Medical University, Nanjing, China. .,Jiangsu Key Laboratory of Xenotransplantation, Nanjing Medical University, Nanjing, China. .,The Laboratory Center for Basic Medical Sciences, Nanjing Medical University, Nanjing, China.
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Pietrangelo A, Ridgway ND. Bridging the molecular and biological functions of the oxysterol-binding protein family. Cell Mol Life Sci 2018; 75:3079-3098. [PMID: 29536114 PMCID: PMC11105248 DOI: 10.1007/s00018-018-2795-y] [Citation(s) in RCA: 65] [Impact Index Per Article: 10.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/20/2017] [Revised: 02/18/2018] [Accepted: 03/07/2018] [Indexed: 12/19/2022]
Abstract
Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) constitute a large eukaryotic gene family that transports and regulates the metabolism of sterols and phospholipids. The original classification of the family based on oxysterol-binding activity belies the complex dual lipid-binding specificity of the conserved OSBP homology domain (OHD). Additional protein- and membrane-interacting modules mediate the targeting of select OSBP/ORPs to membrane contact sites between organelles, thus positioning the OHD between opposing membranes for lipid transfer and metabolic regulation. This unique subcellular location, coupled with diverse ligand preferences and tissue distribution, has identified OSBP/ORPs as key arbiters of membrane composition and function. Here, we will review how molecular models of OSBP/ORP-mediated intracellular lipid transport and regulation at membrane contact sites relate to their emerging roles in cellular and organismal functions.
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Affiliation(s)
- Antonietta Pietrangelo
- Atlantic Research Center, C306 CRC Bldg, Department of Pediatrics, and Biochemistry and Molecular Biology, Dalhousie University, 5849 University Av., Halifax, NS, B3H4R2, Canada
| | - Neale D Ridgway
- Atlantic Research Center, C306 CRC Bldg, Department of Pediatrics, and Biochemistry and Molecular Biology, Dalhousie University, 5849 University Av., Halifax, NS, B3H4R2, Canada.
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Escajadillo T, Wang H, Li L, Li D, Sewer MB. Oxysterol-related-binding-protein related Protein-2 (ORP2) regulates cortisol biosynthesis and cholesterol homeostasis. Mol Cell Endocrinol 2016; 427:73-85. [PMID: 26992564 PMCID: PMC4833515 DOI: 10.1016/j.mce.2016.03.006] [Citation(s) in RCA: 34] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/09/2015] [Revised: 03/04/2016] [Accepted: 03/04/2016] [Indexed: 12/30/2022]
Abstract
Oxysterol binding protein-related protein 2 (ORP2) is a lipid binding protein that has been implicated in various cellular processes, including lipid sensing, cholesterol efflux, and endocytosis. We recently identified ORP2 as a member of a protein complex that regulates glucocorticoid biosynthesis. Herein, we examine the effect of silencing ORP2 on adrenocortical function and show that the ORP2 knockdown cells exhibit reduced amounts of multiple steroid metabolites, including progesterone, 11-deoxycortisol, and cortisol, but have increased concentrations of androgens, and estrogens. Moreover, silencing ORP2 suppresses the expression of most proteins required for cortisol production and reduces the expression of steroidogenic factor 1 (SF1). ORP2 silencing also increases cellular cholesterol, concomitant with decreased amounts of 22-hydroxycholesterol and 7-ketocholesterol, two molecules that have been shown to bind to ORP2. Further, we show that ORP2 binds to liver X receptor (LXR) and is required for nuclear LXR expression. LXR and ORP2 are recruited to the CYP11B1 promoter in response to cAMP signaling. Additionally, ORP2 is required for the expression of other LXR target genes, including ABCA1 and the LDL receptor (LDLR). In summary, we establish a novel role for ORP2 in regulating steroidogenic capacity and cholesterol homeostasis in the adrenal cortex.
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Affiliation(s)
- Tamara Escajadillo
- Skaggs School of Pharmacy & Pharmaceutical Sciences, University of California San Diego, La Jolla, CA, USA.
| | - Hongxia Wang
- Skaggs School of Pharmacy & Pharmaceutical Sciences, University of California San Diego, La Jolla, CA, USA
| | - Linda Li
- Skaggs School of Pharmacy & Pharmaceutical Sciences, University of California San Diego, La Jolla, CA, USA
| | - Donghui Li
- Skaggs School of Pharmacy & Pharmaceutical Sciences, University of California San Diego, La Jolla, CA, USA
| | - Marion B Sewer
- Skaggs School of Pharmacy & Pharmaceutical Sciences, University of California San Diego, La Jolla, CA, USA
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Identification of OSBPL2 as a novel candidate gene for progressive nonsyndromic hearing loss by whole-exome sequencing. Genet Med 2014; 17:210-8. [PMID: 25077649 DOI: 10.1038/gim.2014.90] [Citation(s) in RCA: 46] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/30/2014] [Accepted: 06/11/2014] [Indexed: 01/10/2023] Open
Abstract
PURPOSE Various forms of hearing loss have genetic causes, but many of the responsible genes have not yet been identified. Here, we describe a large seven-generation Chinese family with autosomal dominant nonsyndromic hearing loss that has been excluded as being caused by known deafness gene mutations associated with autosomal dominant nonsyndromic hearing loss with the aim of identifying a novel causative gene involved in deafness. METHODS Whole-exome sequencing was conducted in three affected family members, and cosegregation analysis was performed on other members of the family. RESULTS Whole-exome sequencing and subsequent segregation analysis identified a heterozygous frameshift mutation (c.153_154delCT, p.Gln53Argfs*100) in the oxysterol binding protein-like 2 (OSBPL2) gene in 25 affected family members. The deletion mutation is predicted to lead to premature truncation of the OSBPL2 protein. Modeling and structure-based analysis support the theory that this gene deletion is functionally deleterious. Our finding was further confirmed by the detection of another missense mutation, a c.583C>A transversion (p.Leu195Met) in exon 7 of OSBPL2, in an additional sporadic case of deafness. CONCLUSION Based on this study, OSBPL2 was identified as an excellent novel candidate gene for autosomal dominant nonsyndromic hearing loss; this study is the first to implicate OSBPL2 mutations in autosomal dominant nonsyndromic hearing loss.
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Abstract
In eukaryotic cells, membranes of the late secretory pathway contain a disproportionally large amount of cholesterol in relation to the endoplasmic reticulum, nuclear envelope and mitochondria. At one extreme, enrichment of the plasma membrane with cholesterol and sphingolipids is crucial for formation of liquid ordered domains (rafts) involved in cell communication and transport. On the other hand, regulatory machinery in the endoplasmic reticulum is maintained in a relatively cholesterol-poor environment, to ensure appropriate rapid responses to fluctuations in cellular sterol levels. Thus, cholesterol homeostasis is absolutely dependent on its distribution along an intracellular gradient. It is apparent that this gradient is maintained by a combination of sterol-lipid interactions, vesicular transport and sterol-binding/transport proteins. Evidence for rapid, energy-independent transport between organelles has implicated transport proteins, in particular the eukaryotic oxysterol binding protein (OSBP) family. Since the founding member of this family was identified more than 25 years ago, accumulated evidence implicates the 12-member family of OSBP and OSBP-related proteins (ORPs) in sterol signalling and/or sterol transport functions. The OSBP/ORP gene family is characterized by a conserved beta-barrel sterol-binding fold but is differentiated from other sterol-binding proteins by the presence of additional domains that target multiple organelle membranes. Here we will discuss the functional and structural characteristics of the mammalian OSBP/ORP family that support a 'dual-targeting' model for sterol transport between membranes.
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Affiliation(s)
- Neale D Ridgway
- Department of Pediatrics, Dalhousie University, Halifax, Nova Scotia, B3H 4H7, Canada.
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Hynynen R, Suchanek M, Spandl J, Bäck N, Thiele C, Olkkonen VM. OSBP-related protein 2 is a sterol receptor on lipid droplets that regulates the metabolism of neutral lipids. J Lipid Res 2009; 50:1305-15. [PMID: 19224871 DOI: 10.1194/jlr.m800661-jlr200] [Citation(s) in RCA: 88] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023] Open
Abstract
Oxysterol binding protein-related protein 2 (ORP2) is a member of the oxysterol binding protein family, previously shown to bind 25-hydroxycholesterol and implicated in cellular cholesterol metabolism. We show here that ORP2 also binds 22(R)-hydroxycholesterol [22(R)OHC], 7-ketocholesterol, and cholesterol, with 22(R)OHC being the highest affinity ligand of ORP2 (K(d) 1.4 x 10(-8) M). We report the localization of ORP2 on cytoplasmic lipid droplets (LDs) and its function in neutral lipid metabolism using the human A431 cell line as a model. The ORP2 LD association depends on sterol binding: Treatment with 5 microM 22(R)OHC inhibits the LD association, while a mutant defective in sterol binding is constitutively LD bound. Silencing of ORP2 using RNA interference slows down cellular triglyceride hydrolysis. Furthermore, ORP2 silencing increases the amount of [(14)C]cholesteryl esters but only under conditions in which lipogenesis and LD formation are enhanced by treatment with oleic acid. The results identify ORP2 as a sterol receptor present on LD and provide evidence for its role in the regulation of neutral lipid metabolism, possibly as a factor that integrates the cellular metabolism of triglycerides with that of cholesterol.
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Affiliation(s)
- Riikka Hynynen
- National Institute for Health and Welfare, University of Helsinki, Helsinki, Finland
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Yan D, Olkkonen VM. Characteristics of oxysterol binding proteins. INTERNATIONAL REVIEW OF CYTOLOGY 2008; 265:253-85. [PMID: 18275891 DOI: 10.1016/s0074-7696(07)65007-4] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Protein families characterized by a ligand binding domain related to that of oxysterol binding protein (OSBP) have been identified in eukaryotic species from yeast to humans. These proteins, designated OSBP-related (ORP) or OSBP-like (OSBPL) proteins, have been implicated in various cellular functions. However, the detailed mechanisms of their action have remained elusive. Data from our and other laboratories suggest that binding of sterol ligands may be a unifying theme. Work with Saccharomyces cerevisiae ORPs suggests a function of these proteins in the nonvesicular intracellular transport of sterols, in secretory vesicle transport from the Golgi complex, and in the establishment of cell polarity. Mammals have more ORP genes, and differential splicing substantially increases the complexity of the encoded protein family. Functional studies on mammalian ORPs point in different directions: integration of sterol and sphingomyelin metabolism, sterol transport, regulation of neutral lipid metabolism, control of the microtubule-dependent motility of endosomes/lysosomes, and regulation of signaling cascades. We envision that during evolution, the functions of ORPs have diverged from an ancestral one in sterol transport, to meet the increasing demand of the regulatory potential in multicellular organisms. Our working hypothesis is that mammalian ORPs mainly act as sterol sensors that relay information to a spectrum of different cellular processes.
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Affiliation(s)
- Daoguang Yan
- Department of Molecular Medicine, National Public Health Institute, Biomedicum, FI-00290 Helsinki, Finland
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Suchanek M, Hynynen R, Wohlfahrt G, Lehto M, Johansson M, Saarinen H, Radzikowska A, Thiele C, Olkkonen V. The mammalian oxysterol-binding protein-related proteins (ORPs) bind 25-hydroxycholesterol in an evolutionarily conserved pocket. Biochem J 2007; 405:473-80. [PMID: 17428193 PMCID: PMC2267293 DOI: 10.1042/bj20070176] [Citation(s) in RCA: 116] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
OSBP (oxysterol-binding protein) homologues, ORPs (OSBP-related proteins), constitute a 12-member family in mammals. We employed an in vitro [3H]25OH (25-hydroxycholesterol)-binding assay with purified recombinant proteins as well as live cell photo-cross-linking with [3H]photo-25OH and [3H]photoCH (photo-cholesterol), to investigate sterol binding by the mammalian ORPs. ORP1 and ORP2 [a short ORP consisting of an ORD (OSBP-related ligand-binding domain) only] were in vitro shown to bind 25OH. GST (glutathione S-transferase) fusions of the ORP1L [long variant with an N-terminal extension that carries ankyrin repeats and a PH domain (pleckstrin homology domain)] and ORP1S (short variant consisting of an ORD only) variants bound 25OH with similar affinity (ORP1L, K(d)=9.7x10(-8) M; ORP1S, K(d)=8.4 x10(-8) M), while the affinity of GST-ORP2 for 25OH was lower (K(d)=3.9x10(-6) M). Molecular modelling suggested that ORP2 has a sterol-binding pocket similar to that of Saccharomyces cerevisiae Osh4p. This was confirmed by site-directed mutagenesis of residues in proximity of the bound sterol in the structural model. Substitution of Ile249 by tryptophan or Lys150 by alanine markedly inhibited 25OH binding by ORP2. In agreement with the in vitro data, ORP1L, ORP1S, and ORP2 were cross-linked with photo-25OH in live COS7 cells. Furthermore, in experiments with either truncated cDNAs encoding the OSBP-related ligand-binding domains of the ORPs or the full-length proteins, photo-25OH was bound to OSBP, ORP3, ORP4, ORP5, ORP6, ORP7, ORP8, ORP10 and ORP11. In addition, the ORP1L variant and ORP3, ORP5, and ORP8 were cross-linked with photoCH. The present study identifies ORP1 and ORP2 as OSBPs and suggests that most of the mammalian ORPs are able to bind sterols.
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Affiliation(s)
- Monika Suchanek
- *Max-Planck-Institute of Molecular Cell Biology and Genetics, Pfotenhauerstrasse 108, D-01307 Dresden, Germany
| | - Riikka Hynynen
- †Department of Molecular Medicine, National Public Health Institute, Biomedicum, P.O. Box 104, FIN-00251 Helsinki, Finland
| | - Gerd Wohlfahrt
- ‡Orion Pharma, Computer-Aided Drug Design, P.O. Box 65, FIN-02101 Espoo, Finland
| | - Markku Lehto
- †Department of Molecular Medicine, National Public Health Institute, Biomedicum, P.O. Box 104, FIN-00251 Helsinki, Finland
| | - Marie Johansson
- †Department of Molecular Medicine, National Public Health Institute, Biomedicum, P.O. Box 104, FIN-00251 Helsinki, Finland
| | - Hannu Saarinen
- †Department of Molecular Medicine, National Public Health Institute, Biomedicum, P.O. Box 104, FIN-00251 Helsinki, Finland
| | - Anna Radzikowska
- *Max-Planck-Institute of Molecular Cell Biology and Genetics, Pfotenhauerstrasse 108, D-01307 Dresden, Germany
| | - Christoph Thiele
- *Max-Planck-Institute of Molecular Cell Biology and Genetics, Pfotenhauerstrasse 108, D-01307 Dresden, Germany
- Correspondence may be addressed to either of the authors (email or )
| | - Vesa M. Olkkonen
- †Department of Molecular Medicine, National Public Health Institute, Biomedicum, P.O. Box 104, FIN-00251 Helsinki, Finland
- Correspondence may be addressed to either of the authors (email or )
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Olkkonen VM, Johansson M, Suchanek M, Yan D, Hynynen R, Ehnholm C, Jauhiainen M, Thiele C, Lehto M. The OSBP-related proteins (ORPs): global sterol sensors for co-ordination of cellular lipid metabolism, membrane trafficking and signalling processes? Biochem Soc Trans 2006; 34:389-91. [PMID: 16709169 DOI: 10.1042/bst0340389] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
Abstract
Protein families related to OSBP (oxysterol-binding protein) are present in eukaryotes from yeast to human. The functions of the ORPs (OSBP-related proteins) have remained largely enigmatic. Even though they have been implicated in the function of ERJs (endoplasmic reticulum junctions), it is evident that any single model for their mechanism of action is insufficient. The existing evidence points in many different directions, such as integration of sterol and sphingomyelin metabolism, regulation of neutral lipid metabolism, control of signalling cascades, regulation of secretory vesicle generation, and function in the microtubule-based motility of endo/lysosomes. Some of these functions could involve ERJ and non-vesicular transport of lipids, but this is unlikely to be the unifying feature. We believe, rather, that the common denominator for ORP function is acting as sterol sensors that relay information to a spectrum of cellular processes.
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Affiliation(s)
- V M Olkkonen
- Department of Molecular Medicine, National Public Health Institute, Biomedicum, Helsinki, Finland.
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Hynynen R, Laitinen S, Käkelä R, Tanhuanpää K, Lusa S, Ehnholm C, Somerharju P, Ikonen E, Olkkonen V. Overexpression of OSBP-related protein 2 (ORP2) induces changes in cellular cholesterol metabolism and enhances endocytosis. Biochem J 2005; 390:273-83. [PMID: 15859942 PMCID: PMC1184581 DOI: 10.1042/bj20042082] [Citation(s) in RCA: 67] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
ORP2 [OSBP (oxysterol-binding protein)-related protein 2] belongs to the 12-member mammalian ORP gene/protein family. We characterize in the present study the effects of inducible ORP2 overexpression on cellular cholesterol metabolism in HeLa cells and compare the results with those obtained for CHO cells (Chinese-hamster ovary cells) that express ORP2 constitutively. In both cell systems, the prominent phenotype is enhancement of [14C]cholesterol efflux to all extracellular acceptors, which results in a reduction of cellular free cholesterol. No change was observed in the plasma membrane cholesterol content or distribution between raft and non-raft domains upon ORP2 expression. However, elevated HMG-CoA (3-hydroxy-3-methylglutaryl-CoA) reductase activity and LDL (low-density lipoprotein) receptor expression, as well as enhanced transport of newly synthesized cholesterol to a cyclodextrin-accessible pool, suggest that the ORP2 expression stimulates transport of cholesterol out of the endoplasmic reticulum. In contrast with ORP2/CHO cells, the inducible ORP2/HeLa cells do not show down-regulation of cholesterol esterification, suggesting that this effect represents an adaptive response to long-term cholesterol depletion in the CHO cell model. Finally, we provide evidence that ORP2 binds PtdIns(3,4,5)P(3) and enhances endocytosis, phenomena that are probably interconnected. Our results suggest a function of ORP2 in both cholesterol trafficking and control of endocytic membrane transport.
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Affiliation(s)
- Riikka Hynynen
- *Department of Molecular Medicine, National Public Health Institute, Biomedicum, P.O. Box 104, Helsinki FI-00251, Finland
| | - Saara Laitinen
- *Department of Molecular Medicine, National Public Health Institute, Biomedicum, P.O. Box 104, Helsinki FI-00251, Finland
| | - Reijo Käkelä
- †Department of Biology, University of Joensuu, P.O. Box 111, Joensuu FI-80101, Finland
| | - Kimmo Tanhuanpää
- ‡Institute of Biotechnology, University of Helsinki, Viikinkaari 9, P.O. Box 56, Helsinki FI-00014, Finland
| | - Sari Lusa
- ‡Institute of Biotechnology, University of Helsinki, Viikinkaari 9, P.O. Box 56, Helsinki FI-00014, Finland
| | - Christian Ehnholm
- *Department of Molecular Medicine, National Public Health Institute, Biomedicum, P.O. Box 104, Helsinki FI-00251, Finland
| | - Pentti Somerharju
- §Department of Biochemistry, Institute of Biomedicine, University of Helsinki, Biomedicum, P.O. Box 63, Helsinki FI-00014, Finland
| | - Elina Ikonen
- ‡Institute of Biotechnology, University of Helsinki, Viikinkaari 9, P.O. Box 56, Helsinki FI-00014, Finland
| | - Vesa M. Olkkonen
- *Department of Molecular Medicine, National Public Health Institute, Biomedicum, P.O. Box 104, Helsinki FI-00251, Finland
- To whom correspondence should be addressed (email )
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