Abstract. A cDNA encoding a cytoplasmic oxysterol binding protein was expressed at high levels by transfection in animal cells. This protein binds oxysterols such as 25-hydroxycholesterol that regulate sterol metabolism by transcriptional and posttranscriptional effects . In the transfected cells, some of the oxysterol binding protein (OSBP) was distributed diffusely in the cytoplasm, and some was bound to small vesicles near the nucleus, as revealed by indirect immunofluorescence. Upon addition of 25-hydroxycholesterol, most of the OSBP became concentrated in large perinuclear structures that stained with lentil lectin, a protein that stains the Golgi apparatus . The structures that contained OSBP were disrupted by brefeldin A,

Sphingomyelin (SM) and cholesterol are coregulated metabolically and associate physically in membrane microdomains involved in cargo sorting and signaling. One mechanism for regulation of this metabolic interface involves oxysterol binding protein (OSBP) via high-affinity binding to oxysterol regulators of cholesterol homeostasis and activation of SM synthesis at the Golgi apparatus. Here, we show that OSBP regulation of SM synthesis involves the endoplasmic reticulum (ER)-to-Golgi ceramide transport protein (CERT). RNA interference (RNAi) experiments in Chinese hamster ovary (CHO)-K1 cells revealed that OSBP and vesicle-associated membrane protein-associated protein (VAP) were required for stimulation of CERT-dependent ceramide transport and SM synthesis by 25-hydroxycholesterol and cholesterol depletion in response to cyclodextrin. Additional RNAi experiments in human embryonic kidney 293 cells supported OSBP involvement in oxysterol-activated SM synthesis and also revealed a role for OSBP in basal SM synthesis. Activation of ER-to-Golgi ceramide transport in CHO-K1 cells required interaction of OSBP with the ER and Golgi apparatus, OSBP-dependent Golgi translocation of CERT, and enhanced CERT-VAP interaction. Regulation of CERT by OSBP, sterols, and VAP reveals a novel mechanism for integrating sterol regulatory signals with ceramide transport and SM synthesis in the Golgi apparatus.

OSBP, the first member of the gene family to be characterized, was identified by Kandutsch and Shown (8) as a high affinity cytosolic receptor for a variety of oxysterol regulators of cholesterol synthesis, such as 25-hydroxycholesterol. Based on a correlation between the potency of oxysterols to suppress 3-hydroxy-3-methylglutaryl-CoA reductase activity and affinity for OSBP, it was proposed that OSBP played a role in mediating the effects of oxysterols on cholesterol homeostasis (9). Cloning and expression studies revealed that OSBP was a soluble protein that underwent translocation from a cytosolic/ vesicular compartment to the Golgi apparatus when cells were challenged with exogenous 25-hydroxycholesterol (10). Interaction with the Golgi apparatus has since been shown to involve the OSBP PH domain and is mediated by phosphatidylinositol 4,5-bisphosphate (PI-4,5-P 2 ) and possibly other unidentified protein or lipid factors (6, 11). The interaction of OSBP with the Golgi apparatus via the PH domain is important for function since overexpression of OSBP mutants with deletions of the PH domain did not cause alterations in cholesterol homeostasis that were evident with wild-type protein (6). In a model for oxysterol-mediated translocation, 25-hydroxycholesterol binding to the C-terminal region of OSBP could unmask the PH domain and thus facilitate binding to lipid or protein targets in the Golgi apparatus. Expression of the OSBP PH domain alone

Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) constitute a large gene family that differentially localize to organellar membranes, reflecting a functional role in sterol signaling and/or transport. OSBP partitions between the endoplasmic reticulum (ER) and Golgi apparatus where it imparts sterol-dependent regulation of ceramide transport and sphingomyelin synthesis. ORP9L also is localized to the ER-Golgi, but its role in secretion and lipid transport is unknown. Here we demonstrate that ORP9L partitioning between the trans-Golgi/trans-Golgi network (TGN), and the ER is mediated by a phosphatidylinositol 4-phosphate (PI-4P)-specific PH domain and VAMP-associated protein (VAP), respectively. In vitro, both OSBP and ORP9L mediated PI-4P-dependent cholesterol transport between liposomes, suggesting their primary in vivo function is sterol transfer between the Golgi and ER. Depletion of ORP9L by RNAi caused Golgi fragmentation, inhibition of vesicular somatitus virus glycoprotein transport from the ER and accumulation of cholesterol in endosomes/lysosomes. Complete cessation of protein transport and cell growth inhibition was achieved by inducible overexpression of ORP9S, a dominant negative variant lacking the PH domain. We conclude that ORP9 maintains the integrity of the early secretory pathway by mediating transport of sterols between the ER and trans-Golgi/TGN. INTRODUCTIONThe Golgi apparatus is a highly compartmentalized organelle that serves as a nexus for the processing, sorting, and vesicular transport of protein and lipid cargo between organelles. Regulation of vesicular transport is dependent on coat proteins and factors that control coat assembly/disassembly, as well as membrane dynamics and individual lipid regulators. A continuing challenge is the identification of the proteins responsible for spatial generation of Golgi lipid regulators by localized synthesis and/or transport (Huijbregts et al., 2000;De Matteis et al., 2007). In this study we investigated one such lipid-binding protein, oxysterol-binding protein (OSBP)-related protein 9 (ORP9), and its role in endoplasmic reticulum (ER)-to-Golgi transport.Lipid synthesis and transport to the Golgi apparatus regulates local and distal cellular functions. For example, synthesis of sphingomyelin (SM) and glycosphingolipids (Huitema et al., 2004) in combination with cholesterol is required for raft assembly in the Golgi and cargo delivery to the apical surface of polarized cells (Hoekstra et al., 2003). SM synthesis in the Golgi apparatus is dependent on delivery of ceramide from the ER by the ceramide transfer protein (CERT; Hanada et al., 2003). Ceramide transport by CERT involves binding of a phosphatidylinositol 4-phosphate (PI-4P)-specific pleckstrin homology (PH) domain to the Golgi apparatus and interaction with vesicle-associated membrane protein (VAMP)-associated-protein (VAP) by the "two phenylalanines in an acidic tract" (FFAT) motif (Hanada et al., 2003). In addition, CERT activates protein kinase D activity and secretory t...

The reorganization of metabolic pathways in cancer facilitates the flux of carbon and reducing equivalents into anabolic pathways at the expense of oxidative phosphorylation. This provides rapidly dividing cells with the necessary precursors for membrane, protein and nucleic acid synthesis. A fundamental metabolic perturbation in cancer is the enhanced synthesis of fatty acids by channeling glucose and/or glutamine into cytosolic acetyl-CoA and upregulation of key biosynthetic genes. This lipogenic phenotype also extends to the production of complex lipids involved in membrane synthesis and lipid-based signaling. Cancer cells display sensitivity to ablation of fatty acid synthesis possibly as a result of diminished capacity to synthesize complex lipids involved in signaling or growth pathways. Evidence has accrued that phosphatidylcholine, the major phospholipid component of eukaryotic membranes, as well as choline metabolites derived from its synthesis and catabolism, contribute to both proliferative growth and programmed cell death. This review will detail our current understanding of how coordinated changes in substrate availability, gene expression and enzyme activity lead to altered phosphatidylcholine synthesis in cancer, and how these changes contribute directly or indirectly to malignant growth. Conversely, apoptosis targets key steps in phosphatidylcholine synthesis and degradation that are linked to disruption of cell cycle regulation, reinforcing the central role that phosphatidylcholine and its metabolites in determining cell fate.

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The endoplasmic reticulum (ER) is an interconnected network of tubular and planar membranes that supports the synthesis and export of proteins, carbohydrates and lipids. Phospholipids, in particular phosphatidylcholine (PC), are synthesized in the ER where they have essential functions including provision of membranes required for protein synthesis and export, cholesterol homeostasis, and triacylglycerol storage and secretion. Coordination of these biological processes is essential, as highlighted by findings that link phospholipid metabolism in the ER with perturbations in lipid storage/secretion and stress responses, ultimately contributing to obesity/diabetes, atherosclerosis and neurological disorders. Phospholipid synthesis is not uniformly distributed in the ER but is localized at membrane interfaces or contact zones with other organelles, and in dynamic, proliferating ER membranes. The topology of phospholipid synthesis is an important consideration when establishing the etiology of diseases that arise from ER dysfunction. This review will highlight our current understanding of the contribution of phospholipid synthesis to proper ER function, and how alterations contribute to aberrant stress responses and disease. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Oxysterol-binding protein (OSBP) is a high-affinity receptor for a variety of oxysterols, such as 25-hydroxycholesterol, that down-regulate cholesterol synthesis and stimulate cholesterol esterification. To examine a potential role for OSBP in regulating cholesterol metabolism, we stably overexpressed this protein in Chinese-hamster ovary (CHO)-K1 cells. Compared with mock-transfected controls, several cell lines overexpressing wild-type OSBP (CHO-OSBP) displayed a 50% decrease in cholesteryl ester synthesis when cultured in medium with delipidated serum, 25-hydroxycholesterol or low-density lipoprotein (LDL). CHO-OSBP cells showed a 40-60% decrease in acyl-CoA:cholesterol acyltransferase activity and mRNA, a 50% elevation in mRNA for three sterol-regulated genes [LDL receptor, 3-hydroxy-3-methylgluraryl (HMG)-CoA reductase and HMG-CoA synthase], and an 80% increase in [14C]acetate incorporation into cholesterol. CHO-K1 cells overexpressing two OSBP mutants with a complete or N-terminal deletion of the pleckstrin homology (PH) domain had cholesterol esterification and synthesis rates that were similar to those shown by mock-transfected controls. Unlike wild-type OSBP, both PH domain mutants displayed diffuse cytoplasmic immunofluorescence staining and did not translocate to the Golgi apparatus in the presence of 25-hydroxycholesterol. CHO-K1 cells overexpressing OSBP have pronounced alterations in cholesterol esterification and synthesis, indicating a potential role for this receptor in cholesterol homoeostasis. The phenotype observed in cells overexpressing OSBP is dependent on the PH domain, which appears to be necessary for ligand-dependent localization of OSBP to the Golgi apparatus.