OSBP and OSBPL1–11/ORP1–11
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-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).
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.
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.