Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

OSBP and OSBPL1–11/ORP1–11

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101732

Synonyms

Historical Background

During early studies on the regulation of cellular cholesterol homeostasis, oxysterols such as 25-hydroxycholesterol (25-OHC) were found to potently reduce the activity of HMGCoA reductase, a rate-limiting enzyme in cholesterol biosynthesis. These findings motivated search for proteins mediating the effects of oxysterols on cholesterol homeostatic machinery, resulting in the isolation of protein fractions with oxysterol-binding activity. Taylor, Kandutsch, and coworkers identified in the 1980s a cytosolic oxysterol-binding protein (OSBP), which was purified (Taylor et al. 1984; Dawson et al. 1989), and cDNAs were cloned from different species. Discovery of the sterol regulatory element binding proteins (SREBPs) and the liver X receptors (LXRs) turned major interest in the field of cholesterol homeostasis research away from OSBP. However, the study of OSBP function continued in the laboratory of N. Ridgway at Dalhousie University, Halifax, Canada. After some years, families of genes/proteins related to OSBP were discovered in eukaryotic organisms from yeast to man, which evoked new interest in these gene products and their functions in lipid metabolism, cell signaling, and vesicle transport in eukaryotes. These proteins are called either OSBP-like (OSBPL) or OSBP-related proteins (ORPs; for a review see Olkkonen and Li 2013).

Structural Features and Subcellular Targeting of OSBP-Like Proteins

OSBP consists of a carboxy-terminal ligand-binding domain (designated OSBP-related domain, ORD) and an amino-terminal region that contains a pleckstrin homology (PH) domain interacting with phosphatidylinositol-4-phosphate (PI4P) in the trans-Golgi membranes. Between these domains, there is a dimerization motif, which mediates homodimer formation and heterodimerization with the closely related ORP4L/OSBP2, as well as a sequence motif (two phenylalanines in an acidic tract, FFAT) that interacts with ER VAMP-associated proteins (Loewen et al. 2003). X-ray crystallography studies of yeast ORPs, designated OSBP homologues (Osh), revealed a β-barrel resembling fold of the ORD with a lipid-binding cavity, in which bound sterol or the fatty acid moieties of bound glycerophospholipid are accommodated. The ligand cavity is covered by a flexible lid that, in the case of sterols, interacts with their side chain, while the 3β-hydroxyl group of sterols is oriented toward the bottom of the cavity (reviewed by Olkkonen and Li 2013). Instead of a FFAT motif for ER targeting, two of the mammalian ORPs, ORP5 and ORP8, are anchored to ER membranes through a transmembrane segment located at the carboxy-terminus of the proteins. Of the 7 yeast Osh proteins, 3 have an amino-terminal PH domain, thus representing a “long” ORP subtype, while 4 (“short” subtype) lack this feature. Among the 12 mammalian OSBPLs, 11 encode “long” variants. In addition, OSBPL1A, OSBPL4, and OSBPL9 encode “short” variants. Of note, the mammalian OSBPL2 gene only encodes a protein (ORP2) of the “short” subtype (Fig. 1).
OSBP and OSBPL1–11/ORP1–11, Fig. 1

A schematic image of mammalian OSBPL/ORP protein structures. The major structural elements are identified. The roman numerals on the right indicate subfamilies of closely related proteins. The abbreviations are PH pleckstrin homology domain, D dimerization domain, L leucine repeat motif, FFAT two phenylalanines in an acidic tract, OF oxysterol-binding protein “fingerprint” (consensus EQVSHHPP), ORD OSBP-related (ligand-binding) domain, ANK ankyrin repeats, TM transmembrane segment. L and S indicate “long” and “short” variants, respectively

OSBP-Like Proteins as Lipid Transporters Over Membrane Contact Sites

Importantly, the ORD of OSBPL proteins can, in addition to sterols, also bind glycerophospholipids, and some family members may in fact not bind sterols at all. The first structural evidence for the binding of a glycerophospholipid by an ORP was reported by De Saint-Jean et al. (2011), who determined the structure of yeast Osh4p crystallized with PI4P within the ligand cavity. Furthermore, they demonstrated that a bound sterol was readily exchanged for the PI4P. The authors suggested that the two lipids could be transported by Osh4p in opposite directions, sterol from the ER to the trans-Golgi/PM, and PI4P in the opposite direction. In addition to a lipid transporter function, yeast Osh3p has been demonstrated to regulate the activity of lipid metabolizing enzymes at ER-plasma membrane (PM) contact sites (Stefan et al. 2011; Tavassoli et al. 2013).

Mesmin et al. (2013) provided evidence that OSBP can tether ER and Golgi membranes and mediates the transport of cholesterol against its concentration gradient from the ER to trans-Golgi, driven by the retrograde transport of PI4P generated by PI-4-kinases in Golgi membranes (Fig. 2). The fact that PI4P, which is retrogradely transported by the ORD of OSBP, also acts as a ligand for the pleckstrin homology domain of OSBP in Golgi membranes provides an obvious mechanism of autoregulation of the lipid transport activity of OSBP. The high-affinity ligand of OSBP, 25-OHC, was shown to lock OSBP at Golgi membranes thus inhibiting the transport function. In addition to the direct cholesterol/PI4P carrier function suggested above, OSBP was reported to promote recruitment of the ceramide transporter (CERT) to ER-Golgi contact sites, thus promoting the conversion of ceramides to sphingomyelins at Golgi membranes (Perry and Ridgway 2006). An interesting extension of the above findings is the essential role of OSBP in the replication of enteroviruses and hepatitis C virus. A number of studies suggest that lipid transport over MCSs by OSBP is required for generation and/or function of the viral replication membranes, the subverted membrane structures at which the viral RNA replication takes place (reviewed by Nchoutmboube et al. 2015). This important finding brings up the possibility of employing OSBP/ORPs as targets for the development of new antiviral therapies.
OSBP and OSBPL1–11/ORP1–11, Fig. 2

Models for the function of ORPs at membrane contact sites (MSCs). (a) OSBP transfers cholesterol (C) within its ligand-binding domain (ORD) over MCSs from the ER to trans-Golgi and PI4P in the opposite direction. The phosphatidylinositol 4-phosphate (PI4P) retrogradely transported to the ER is hydrolyzed by the phosphatase Sac1 to phosphatidylinositol (PI) and inorganic phosphate. The PI4P gradient between Golgi and ER thus drives the forward transport of cholesterol against its concentration gradient. “Two phenylalanines in an acidic tract” (FFAT) motif mediating the interaction of OSBP with the VAMP-associated proteins (VAP) in the ER; pleckstrin homology domain that interacts with PI4P in Golgi membranes (PH). (b) Models for ORP function as regulators of enzymatic or signaling processes at MCSs. An ORP binds a lipidous ligand (L) which regulates the conformation of the protein, allowing it to stimulate at an ER-plasma membrane contact (a) the activity of an enzyme (E) that is anchored in ER membranes and modifies in trans the structure of lipid substrates in the plasma membrane, or (b) the activation of a signaling component, e.g., a small GTPase (G). X, an activator of the GTPase. The inactive enzyme and GTPase are depicted in green and the active ones in orange (Reproduced from Olkkonen VM, Lipid Insights. 2015;Suppl. 1:1–9)

Maeda et al. (2013) demonstrated that the yeast ORPs Osh6p and Osh7p, which localize at the cortical ER, are instrumental for the transport of the glycerophospholipid phosphatidylserine (PS) from the ER to the PM. Moser von Filseck et al. (2015) further reported that Osh6p can transport PS and PI4P in an exchange-type fashion, in analogy with the cholesterol/PI4P transport activity of OSBP (see above), while ORP5 and ORP8 were found to execute a similar PS/PI4P exchange function at the ER-PM contacts of mammalian cells (Chung et al. 2015). These observations raised the question of whether OSBPL family members could more generally function as lipid transporters over membrane contact sites (MCSs).

Function of OSBP-Like Proteins in Cell Signaling

One can envision that inter-organelle lipid transfer, which maintains the specific lipid compositions of organelle membranes, may modify signaling processes via indirect mechanisms. As a result, it is not easy to interpret the signaling-related outcomes of studies in which ORP expression or function has been targeted. However, a number of reports have suggested direct roles of ORPs in cell signaling.

The first evidence for ORP involvement cell signaling was reported by Sugawara et al. (2001), who found that a C. elegans ORP designated bone morphogenetic protein receptor-associated molecule (BRAM)-interacting protein, BIP, operates in transforming growth factor-β (TGF-β) signaling and body length regulation in the worm. Later on, the study of Wang et al. (2005) suggested that mammalian OSBP acts as a scaffold for PP2A and HePTP, two protein phosphatases that control the activity of the extracellular signal-regulated kinases (ERKs), components of the mitogen-activated protein kinase (MAPK) signaling pathways. While the cholesterol-bound form of OSBP associated with the active phosphatases, removal of cholesterol or addition of 25-OHC dissociated the complex. Furthermore, Romeo and Kazlauskas (2008) found that 7-ketocholesterol-induced upregulation of profilin-1, an actin-binding protein implicated in endothelial dysfunction and atherosclerosis, is mediated by OSBP. This process apparently involves interaction of the OSBP-7KC complex with JAK-2, a tyrosine kinase that phosphorylates Tyr394 on OSBP, resulting in the activation of STAT3 and induction of profilin. The association of OSBP with Golgi membranes is intimately connected with regulatory signals from protein kinase D (PKD) and the Golgi PI-4-kinase IIαs, and activation of OSBP reorganizes the Golgi PI4P pools with key impacts on lipid transfer and metabolism (Goto et al. 2016). Lessman et al. (2007) demonstrated that ORP9, a protein also implicated as a regulator of Golgi apparatus structure and function, contains a phosphoinositide-dependent kinase-2 (PDK-2) phosphorylation site, the phosphorylation of which depends on PKC-β or mTOR. ORP9 was suggested to interact with these kinases to dampen phosphorylation of the PDK-2 site of Akt, a major controller of cell survival, cell cycle progression, and glucose metabolism.

A further interesting example of a signaling function of ORPs is ORP1L, which binds to the small GTPase Rab7 on late endosomes/lysosomes (LE/lys) and controls, as part of a Rab7 effector protein complex, in a sterol-dependent manner the motility, tethering, and fusion of LE/lys, as well as autophagy (Wijdeven et al. 2016). However, ORP1L was also reported to play a role in cholesterol transport from the ER to endosomes over MCSs (Eden et al. 2016). Another example is ORP3, which was shown to physically interact with the small GTPase R-Ras, a regulator of cell adhesion and migration, and to control the activity of the GTPase, apparently at sites of ER-PM contact (Weber-Boyvat et al. 2015). The latest study by the group of D. Yan (Jinan University, Guangzhou, China) revealed that ORP4L/OSBP2, not present in normal T cells, is strongly induced in acute T-lymphoblastic leukemia (T-ALL) cells and generates there a new G protein coupled signaling route that maintains IP3 production by phospholipase C β3, Ca2+ release from the ER, robust oxidative phosphorylation, and cell viability (Zhong et al. 2016). A key implication of these findings is that a number of ORPs act as lipid sensors with scaffolding functions in cell signaling and organelle motility/dynamics.

ORP Inhibitors as Antiproliferative Agents

Molecular profiling of tumors or cancerous cells has revealed altered quantities of ORP mRNAs or proteins, indicating that aberrant ORP expression or function may be associated with malignant growth. This idea gained further support when Burgett et al. (2011) identified OSBP and its closest homologue, ORP4L/OSBP2, as targets of the antiproliferative natural products cephalostatin 1, OSW-1, ritterazine B, and schweinfurthin A, which the authors collectively named ORPphilins. The above results revealed an essential function of OSBP and ORP4L/OSBP2 as signaling factors controlling cell proliferation. Consistently, the group of N. Ridgway found that ORP4L/OSBP2 knockdown by RNAi resulted in growth arrest of cancer cells and apoptosis of non-transformed cells. The above observations suggest that OSBP and/or ORP4L/OSBP2 could in the future be employed as targets for the development of new tumor selective therapeutics.

Summary

Several OSBPLs have the capacity to control the formation of membrane contact sites, to transfer lipids over MCSs, and to regulate the activity of enzymatic machineries at these sites. Thereby, ORPs affect organelle membrane lipid compositions, with impacts on cell signaling and vesicle transport. In addition, ORPs execute scaffolding functions of protein complexes that control cell signaling and organelle dynamics. A number of studies have revealed clues of the complex and versatile regulatory functions of ORPs as sterol and glycerophospholipid sensors and transporters with downstream impacts on central regulatory machineries of cellular metabolism and proliferation. Moreover, ORPs have been shown to play key roles in the replication of pathogenic viruses. These and other recent observations have brought up the possibility of employing ORPs as therapy targets and motivate intense research efforts for deepening our understanding of their function.

References

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Minerva Foundation Institute for Medical ResearchHelsinkiFinland