Cholesterol interaction with recombinant human sterol carrier protein-2

Role of STAR and SCP2/SCPx in the Transport of Cholesterol and Other Lipids

Cholesterol is a lipid molecule essential for several key cellular processes including steroidogenesis. As such, the trafficking and distribution of cholesterol is tightly regulated by various pathways that include vesicular and non-vesicular mechanisms. One non-vesicular mechanism is the binding of cholesterol to cholesterol transport proteins, which facilitate the movement of cholesterol between cellular membranes. Classic examples of cholesterol transport proteins are the steroidogenic acute regulatory protein (STAR; STARD1), which facilitates cholesterol transport for acute steroidogenesis in mitochondria, and sterol carrier protein 2/sterol carrier protein-x (SCP2/SCPx), which are non-specific lipid transfer proteins involved in the transport and metabolism of many lipids including cholesterol between several cellular compartments. This review discusses the roles of STAR and SCP2/SCPx in cholesterol transport as model cholesterol transport proteins, as well as more recent findings that support the role of these proteins in the transport and/or metabolism of other lipids.

Sterol carrier protein 2 in lipid metabolism and non-alcoholic fatty liver disease: Pathophysiology, molecular biology, and potential clinical implications

Non-alcoholic fatty liver disease (NAFLD) is considered as the most common chronic liver disease and has become a rapidly global public health problem. Sterol carrier protein 2 (SCP-2), also called non-specific lipid-transfer protein, is predominantly expressed by the liver. SCP-2 plays a key role in intracellular lipid transport and metabolism. SCP-2 has been closely implicated in the development of NAFLD-related metabolic disorders, such as obesity, atherosclerosis, Type 2 diabetes mellitus (T2DM), and gallstones. Recent studies indicate that SCP-2 plays a beneficial role in NAFLD by regulating cholesterol-, endocannabinoid-, and fatty acid-related aspects of lipid metabolism. Hence, in this paper, we summarize the latest findings about the roles of SCP-2 in hepatic steatosis and further describe its molecular function in the pathogenesis of NAFLD.

Changes in Porcine Corpus Luteum Proteome Associated with Development, Maintenance, Regression, and Rescue during Estrous Cycle and Early Pregnancy

Corpus luteum (CL), a transitory gland, undergoes rapid growth in a limited time to produce progesterone (P4) followed by its regression. A complex molecular signaling is involved in controlling luteal P4 production. In the present study, 2D gel electrophoresis-based proteomics and in silico functional analysis were used to identify changes in key proteins and pathways in CL along the different stages of the estrous cycle as its development progresses from early (Day 3) to mid-luteal phase (Day 9), effective functioning (Day 12) followed by regression (Day 15) or, in the case of pregnancy, rescue of function (Day 15). A total of 273 proteins were identified by MALDI-MS/MS analysis that showed significant changes in abundances at different stages of CL development or regression and rescue. Functional annotation of differentially abundant proteins suggested enrichment of several important pathways and functions during CL development and function maintenance including cell survival, endocytosis, oxidative stress response, estradiol metabolism, and angiogenesis. On the other hand, differentially abundant proteins during CL regression were associated with decreased steroid synthesis and metabolism and increased apoptosis, necrosis, and infiltration of immune cells. Establishment of pregnancy rescues CL from regression by maintaining the expression of proteins that support steroidogenesis as pathways such as the super-pathway of cholesterol biosynthesis, RhoA signaling, and functions such as fatty acid metabolism and sterol transport were enriched in CL of pregnancy. In this study, some novel proteins were identified along CL development that advances our understanding of CL survival and steroidogenesis.

NMR structure and function of Helicoverpa armigera sterol carrier protein-2, an important insecticidal target from the cotton bollworm

The cotton bollworm, Helicoverpa armigera, has developed strong resistance to many insecticides. Sterol Carrier Protein-2 (SCP-2) is an important non-specific lipid transfer protein in insects and appears to be a potential new target. In order to elucidate the structure and function of Helicoverpa armigera SCP-2 (HaSCP-2), NMR spectroscopy, docking simulations, mutagenesis and bioassays were performed. HaSCP-2 composed of five α-helices and four stranded β-sheets. The folds of α-helices and β-sheets interacted together to form a hydrophobic cavity with putative entrance and exit openings, which served as a tunnel for accommodating and transporting of lipids. Several sterols and fatty acids could interact with HaSCP-2 via important hydrophobic sites, which could be potential targets for insecticides. Mutagenesis experiments indicated Y51, F53, F89, F110, I117 and Q131 may be the key functional sites. HaSCP-2 showed high cholesterol binding activity and SCP-2 inhibitors (SCPIs) could inhibit the biological activity of HaSCP-2. SCPI-treated larvae at young stage showed a significant decrease of cholesterol uptake in vivo. Our study describes for the first time a NMR structure of SCP-2 in lepidopteran H. armigera and reveals its important function in cholesterol uptake, which facilitates the screening of effective insecticides targeting the insect cholesterol metabolism.

Impact of SCP-2/SCP-x gene ablation and dietary cholesterol on hepatic lipid accumulation

While a high-cholesterol diet induces hepatic steatosis, the role of intracellular sterol carrier protein-2/sterol carrier protein-x (SCP-2/SCP-x) proteins is unknown. We hypothesized that ablating SCP-2/SCP-x [double knockout (DKO)] would impact hepatic lipids (cholesterol and cholesteryl ester), especially in high-cholesterol-fed mice. DKO did not alter food consumption, and body weight (BW) gain decreased especially in females, concomitant with hepatic steatosis in females and less so in males. DKO-induced steatosis in control-fed wild-type (WT) mice was associated with 1) loss of SCP-2; 2) upregulation of liver fatty acid binding protein (L-FABP); 3) increased mRNA and/or protein levels of sterol regulatory element binding proteins (SREBP1 and SREBP2) as well as increased expression of target genes of cholesterol synthesis (Hmgcs1 and Hmgcr) and fatty acid synthesis (Acc1 and Fas); and 4) cholesteryl ester accumulation was also associated with increased acyl-CoA cholesterol acyltransferase-2 (ACAT2) in males. DKO exacerbated the high-cholesterol diet-induced hepatic cholesterol and glyceride accumulation, without further increasing SREBP1, SREBP2, or target genes. This exacerbation was associated both with loss of SCP-2 and concomitant downregulation of Ceh/Hsl, apolipoprotein B (ApoB), MTP, and/or L-FABP protein expression. DKO diminished the ability to secrete excess cholesterol into bile and oxidize cholesterol to bile acid for biliary excretion, especially in females. This suggested that SCP-2/SCP-x affects cholesterol transport to particular intracellular compartments, with ablation resulting in less to the endoplasmic reticulum for SREBP regulation, making more available for cholesteryl ester synthesis, for cholesteryl-ester storage in lipid droplets, and for bile salt synthesis and/or secretion. These alterations are significant findings, since they affect key processes in regulation of sterol metabolism.

Loss of L-FABP, SCP-2/SCP-x, or both induces hepatic lipid accumulation in female mice

Although roles for both sterol carrier protein-2/sterol carrier protein-x (SCP-2/SCP-x) and liver fatty acid binding protein (L-FABP) have been proposed in hepatic lipid accumulation, individually ablating these genes has been complicated by concomitant alterations in the other gene product(s). For example, ablating SCP2/SCP-x induces upregulation of L-FABP in female mice. Therefore, the impact of ablating SCP-2/SCP-x (DKO) or L-FABP (LKO) individually or both together (TKO) was examined in female mice. Loss of SCP-2/SCP-x (DKO, TKO) more so than loss of L-FABP alone (LKO) increased hepatic total lipid and total cholesterol content, especially cholesteryl ester. Hepatic accumulation of nonesterified long chain fatty acids (LCFA) and phospholipids occurred only in DKO and TKO mice. Loss of SCP-2/SCP-x (DKO, TKO) increased serum total lipid primarily by increasing triglycerides. Altered hepatic level of proteins involved in cholesterol uptake, efflux, and/or secretion was observed, but did not compensate for the loss of L-FABP, SCP-2/SCP-x or both. However, synergistic responses were not seen with the combinatorial knock out animals-suggesting that inhibiting SCP-2/SCP-x is more correlative with hepatic dysfunction than L-FABP. The DKO- and TKO-induced hepatic accumulation of cholesterol and long chain fatty acids shared significant phenotypic similarities with non-alcoholic fatty liver disease (NAFLD).

Ablating L-FABP in SCP-2/SCP-x null mice impairs bile acid metabolism and biliary HDL-cholesterol secretion

On the basis of their abilities to bind bile acids and/or cholesterol, the physiological role(s) of liver fatty acid-binding protein (L-FABP) and sterol carrier protein (SCP) 2/SCP-x (SCP-2/SCP-x) gene products in biliary bile acid and cholesterol formation was examined in gene-ablated male mice. L-FABP (LKO) or L-FABP/SCP-2/SCP-x [triple-knockout (TKO)] ablation markedly decreased hepatic bile acid concentration, while SCP-2/SCP-x [double-knockout (DKO)] ablation alone had no effect. In contrast, LKO increased biliary bile acid, while DKO and TKO had no effect on biliary bile acid levels. LKO and DKO also altered biliary bile acid composition to increase bile acid hydrophobicity. Furthermore, LKO and TKO decreased hepatic uptake and biliary secretion of high-density lipoprotein (HDL)-derived 22-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-23,24-bisnor-5-cholen-3β-ol (NBD-cholesterol), while DKO alone had no effect. Finally, LKO and, to a lesser extent, DKO decreased most indexes contributing to cholesterol solubility in biliary bile. These results suggest different, but complementary, roles for L-FABP and SCP-2/SCP-x in biliary bile acid and cholesterol formation. L-FABP appears to function more in hepatic retention of bile acids as well as hepatic uptake and biliary secretion of HDL-cholesterol. Conversely, SCP-2/SCP-x may function more in formation and biliary secretion of bile acid, with less impact on hepatic uptake or biliary secretion of HDL-cholesterol.

Effects of mutations in Aedes aegypti sterol carrier protein-2 on the biological function of the protein

Sterol carrier protein-2 (SCP-2) is a nonspecific intracellular lipid carrier protein. However, the molecular mechanism of ligand selectivity and the in vivo function of SCP-2 remain unclear. In this study, we used site-directed mutagenesis to investigate the ligand selectivity and in vivo function of the yellow fever mosquito sterol carrier protein-2 protein (AeSCP-2). Mutations to amino acids in AeSCP-2 known to interact with bound ligand also weakened NBD-cholesterol binding. Substitution of amino acids in the ligand cavity changed the ligand specificity of mutant AeSCP-2. Overexpressing wild-type AeSCP-2 in the Aedes aegypti cultured Aag-2 cells resulted in an increase in the level of incorporation of [(3)H]cholesterol. However, overexpressing mutants that were deleterious to the binding of NBD-cholesterol in AeSCP-2 showed a loss of ability to enhance uptake of [(3)H]cholesterol in cultured cells. Interestingly, when [(3)H]palmitic acid was used as the substrate for incorporation in vivo, there was no change in the levels of incorporation with overexpression of wild-type protein or mutated AeSCP-2s. The in vivo data suggest that AeSCP-2 is involved in sterol uptake, but not fatty acid uptake. This is the first report that the cholesterol binding ability may directly correlate with AeSCP-2's in vivo function in aiding the uptake of cholesterol.

Sterol carrier protein-2 (SCP-2) involvement in cholesterol hydroperoxide cytotoxicity as revealed by SCP-2 inhibitor effects

Sterol carrier protein-2 (SCP-2) plays an important role in cholesterol trafficking and metabolism in mammalian cells. The purpose of this study was to determine whether SCP-2, under oxidative stress conditions, might also traffic hydroperoxides of cholesterol, thereby disseminating their cytotoxic effects. Two inhibitors, SCPI-1 and SCPI-3, known to block cholesterol binding by an insect SCP-2, were used to investigate this. A mouse fibroblast transfectant clone (SC2F) overexpressing SCP-2 was found to be substantially more sensitive to apoptotic killing induced by liposomal 7α-hydroperoxycholesterol (7α-OOH) than a wild-type control. 7α-OOH uptake by SC2F cells and resulting apoptosis were both inhibited by SCPI-1 or SCPI-3 at a subtoxic concentration. Preceding cell death, reactive oxidant accumulation and loss of mitochondrial membrane potential were also strongly inhibited. Similar SCPI protection against 7α-OOH was observed with two other types of SCP-2-expressing mammalian cells. In striking contrast, neither inhibitor had any effect on H(2)O(2)-induced cell killing. To learn whether 7α-OOH cytotoxicity is due to uptake/transport by SCP-2, we used a fluorescence-based competitive binding assay involving recombinant SCP-2, NBD-cholesterol, and SCPI-1/SCPI-3 or 7α-OOH. The results clearly showed that 7α-OOH binds to SCP-2 in SCPI-inhibitable fashion. Our findings suggest that cellular SCP-2 not only binds and translocates cholesterol but also cholesterol hydroperoxides, thus expanding their redox toxicity and signaling ranges under oxidative stress conditions.

Fluorescence techniques using dehydroergosterol to study cholesterol trafficking

Cholesterol itself has very few structural/chemical features suitable for real-time imaging in living cells. Thus, the advent of dehydroergosterol [ergosta-5,7,9(11),22-tetraen-3beta-ol, DHE] the fluorescent sterol most structurally and functionally similar to cholesterol to date, has proven to be a major asset for real-time probing/elucidating the sterol environment and intracellular sterol trafficking in living organisms. DHE is a naturally occurring, fluorescent sterol analog that faithfully mimics many of the properties of cholesterol. Because these properties are very sensitive to sterol structure and degradation, such studies require the use of extremely pure (>98%) quantities of fluorescent sterol. DHE is readily bound by cholesterol-binding proteins, is incorporated into lipoproteins (from the diet of animals or by exchange in vitro), and for real-time imaging studies is easily incorporated into cultured cells where it co-distributes with endogenous sterol. Incorporation from an ethanolic stock solution to cell culture media is effective, but this process forms an aqueous dispersion of DHE crystals which can result in endocytic cellular uptake and distribution into lysosomes which is problematic in imaging DHE at the plasma membrane of living cells. In contrast, monomeric DHE can be incorporated from unilamellar vesicles by exchange/fusion with the plasma membrane or from DHE-methyl-beta-cyclodextrin (DHE-MbetaCD) complexes by exchange with the plasma membrane. Both of the latter techniques can deliver large quantities of monomeric DHE with significant distribution into the plasma membrane. The properties and behavior of DHE in protein-binding, lipoproteins, model membranes, biological membranes, lipid rafts/caveolae, and real-time imaging in living cells indicate that this naturally occurring fluorescent sterol is a useful mimic for probing the properties of cholesterol in these systems.

Translocation as a means of disseminating lipid hydroperoxide-induced oxidative damage and effector action

Lipid hydroperoxides (LOOHs) generated in cells and lipoproteins under oxidative pressure may induce waves of damaging chain lipid peroxidation near their sites of origin if O2 is readily available and antioxidant capacity is overwhelmed. However, recent studies have demonstrated that chain induction is not necessarily limited to a nascent LOOH's immediate surroundings but can extend to other cell membranes or lipoproteins by means of LOOH translocation through the aqueous phase. Mobilization and translocation can also extend the range of LOOHs as redox signaling molecules and in this sense they could act like the small, readily diffusible inorganic analogue H2O2, which has been studied much more extensively in this regard. In this article, basic mechanisms of free-radical- and singlet-oxygen-mediated LOOH formation and one-electron and two-electron LOOH reduction pathways and their biological consequences are reviewed. The first studies to document spontaneous and protein-assisted LOOH transfer in model systems and cells are described. Finally, LOOH translocation is discussed in the context of cytotoxicity vs detoxification and expanded effector action, i.e., redox signaling activity.

Activity and expression of different members of the caspase family in the rat corpus luteum during pregnancy and postpartum

Studies were designed to examine the expression and activity of four caspases that contribute to the initial (caspases-2, -8, and -9) and final (caspase-3) events in apoptosis in the rat corpus luteum (CL) during pregnancy (days 7, 17, 19, and 21 of gestation), postpartum (days 1 and 4), and after injection (0, 8, 16, 24, and 36 h) of the physiological luteolysin PGF2alpha. In addition, the temporal relationship of caspase expression/activity relative to steroid production and luteal regression was evaluated. During pregnancy, the activity of all four caspases was significantly greater on day 19, before a decline in CL progesterone (P) and CYP11A1 levels at day 21 of gestation. The levels of the caspase-3 active fragment (p17, measured by Western blot) also increased at days 19 and 21 of pregnancy. Immunohistochemical analyses detected specific staining for the caspases in luteal cells (large and small) as well as in endothelial cells. However, the percentage of apoptotic cells did not increase in the CL until postpartum. Following PGF2alpha injection, there was a significant decrease in CL P by 24 h, although the activity of all four caspases did not increase until 36 h posttreatment. The active p17 fragment of caspase-3 also significantly increased at 36 h post-PGF2alpha. These results suggest that an increase in the activity of caspases-2, -8, -9, and -3 is associated with the early events of natural luteolysis at the end of pregnancy. Also, the exogenous administration of the luteolysin PGF2alpha may regulate members of the caspase family.

Sterol carrier protein-2: new roles in regulating lipid rafts and signaling

Sterol carrier protein-2 (SCP-2) was independently discovered as a soluble protein that binds and transfers cholesterol as well as phospholipids (nonspecific lipid transfer protein, nsLTP) in vitro. Physiological functions of this protein are only now beginning to be resolved. The gene encoding SCP-2 also encodes sterol carrier protein-x (SCP-x) arising from an alternate transcription site. In vitro and in vivo SCP-x serves as a peroxisomal 3-ketoacyl-CoA thiolase in oxidation of branched-chain lipids (cholesterol to form bile acids; branched-chain fatty acid for detoxification). While peroxisomal SCP-2 facilitates branched-chain lipid oxidation, the role(s) of extraperoxisomal (up to 50% of total) are less clear. Studies using transfected fibroblasts overexpressing SCP-2 and hepatocytes from SCP-2/SCP-x gene-ablated mice reveal that SCP-2 selectively remodels the lipid composition, structure, and function of lipid rafts/caveolae. Studies of purified SCP-2 and in cells show that SCP-2 has high affinity for and selectively transfers many lipid species involved in intracellular signaling: fatty acids, fatty acyl CoAs, lysophosphatidic acid, phosphatidylinositols, and sphingolipids (sphingomyelin, ceramide, mono-di-and multi-hexosylceramides, gangliosides). SCP-2 selectively redistributes these signaling lipids between lipid rafts/caveolae and intracellular sites. These findings suggest SCP-2 serves not only in cholesterol and phospholipid transfer, but also in regulating multiple lipid signaling pathways in lipid raft/caveolae microdomains of the plasma membrane.

Lipid transfer protein binding of unmodified natural lipids as assessed by surface plasmon resonance methodology

A new approach for analyzing lipid-lipid transfer protein interactions is described. The transfer protein is genetically engineered for expression with a C-terminal biotinylated peptide extension (AviTag). This allows protein anchoring to a streptavidin-coated chip for surface plasmon resonance (SPR)-based assessment of lipid binding. Sterol carrier protein-2 (SCP-2), involved in the intracellular trafficking of cholesterol, fatty acids, and other lipids, was selected as the prototype. Biotinylated SCP-2 (bSCP-2) was expressed in Escherichia coli, purified to homogeneity by mutated streptavidin (SoftLink) affinity chromatography, and confirmed by mass spectrometry to contain one biotin group at the expected position. Intermembrane [(14)C]cholesterol transfer was strongly enhanced by bSCP-2, demonstrating that it was functional. Using bSCP-2 immobilized on a Biacore streptavidin chip, we determined on- and off-rate constants along with equilibrium dissociation constants for the following analytes: oleic acid, linoleic acid, cholesterol, and fluorophore (NBD)-derivatized cholesterol. The dissociation constant for NBD-cholesterol was similar to that determined by fluorescence titration for SCP-2 in solution, thereby validating the SPR approach. This method can be readily adapted to other transfer proteins and has several advantages over existing techniques for measuring lipid binding, including (i) the ability to study lipids in their natural states (i.e., without relatively large reporter groups) and (ii) the ability to measure on- and off- rate constants as well as equilibrium constants.

The molecular control of corpus luteum formation, function, and regression

The corpus luteum (CL) is one of the few endocrine glands that forms from the remains of another organ and whose function and survival are limited in scope and time. The CL is the site of rapid remodeling, growth, differentiation, and death of cells originating from granulosa, theca, capillaries, and fibroblasts. The apparent raison d'etre of the CL is the production of progesterone, and all the structural and functional features of this gland are geared toward this end. Because of its unique importance for successful pregnancies, the mammals have evolved a complex series of checks and balances that maintains progesterone at appropriate levels throughout gestation. The formation, maintenance, regression, and steroidogenesis of the CL are among the most significant and closely regulated events in mammalian reproduction. During pregnancy, the fate of the CL depends on the interplay of ovarian, pituitary, and placental regulators. At the end of its life span, the CL undergoes a process of regression leading to its disappearance from the ovary and allowing the initiation of a new cycle. The generation of transgenic, knockout and knockin mice and the development of innovative technologies have revealed a novel role of several molecules in the reprogramming of granulosa cells into luteal cells and in the hormonal and molecular control of the function and demise of the CL. The current review highlights our knowledge on these key molecular events in rodents.

Sterol homeostasis in the budding yeast, Saccharomyces cerevisiae

Lipid related diseases, such as obesity, type 2 diabetes, and atherosclerosis are epidemics in developed civilizations. A common underlying factor among these syndromes is excessive subcellular accumulation of lipids such as cholesterol and triglyceride. The homeostatic events that govern these metabolites are understood to varying degrees of sophistication. We describe here the utilization of a genetically powerful model organism, budding yeast, to identify and characterize novel aspects of sterol and lipid homeostasis.

A simple and rapid method to measure cholesterol binding to P450s and other proteins

Cholesterol plays an important role in cellular function and membrane compartmentalization and is involved in the interaction with more than a dozen of different proteins. Using three cholesterol-metabolizing cytochrome P450s (P450s 7A1, 46A1, and 11A1), we have developed a rapid and simple assay for measurements of nanomolar to micromolar cholesterol affinities. In this assay, the P450 is incubated with a fixed amount of radiolabeled cholesterol and varying concentrations of cold cholesterol followed by separation of free and protein-bound cholesterol via filtration through a membrane. Free cholesterol is found in the flow-through fraction, whereas P450 binds to the membrane. The radioactivity of the membranes is then measured, and a saturation curve is generated after correction for nonspecific binding of cholesterol to the filter. The validity of the filter assay was confirmed by spectral assay, a traditional method to evaluate the interaction of the P450 enzymes with their substrates. Two types of membranes, one binding positively charged proteins and another binding negatively charged proteins, were identified. These membranes were also found to hold proteins through hydrophobic interactions. Thus, the cholesterol binding properties of a wide variety of proteins could be characterized using this filter assay.

Sexually dimorphic metabolism of branched-chain lipids in C57BL/6J mice

Despite the importance of branched chain lipid oxidation in detoxification, almost nothing is known regarding factors regulating peroxisomal uptake, targeting, and metabolism. One peroxisomal protein, sterol carrier protein-x (SCP-x), is thought to catalyze a key thiolytic step in branched chain lipid oxidation. When mice with substantially lower hepatic levels of SCP-x were tested for susceptibility to dietary stress with phytol (a phytanic acid precursor and peroxisome proliferator), livers of phytol-fed female but not male mice i). accumulated phytol metabolites (phytanic acid, pristanic acid, and Delta-2,3-pristanic acid); ii). exhibited decreased fat tissue mass and increased liver mass/body mass; iii). displayed signs of histopathological lesions in the liver; and iv). demonstrated significant alterations in hepatic lipid distributions. Moreover, both male and female mice exhibited phytol-induced peroxisomal proliferation, as demonstrated by liver morphology and upregulation of the peroxisomal protein catalase. In addition, levels of liver fatty acid binding protein, along with SCP-2 and SCP-x, increased, suggesting upregulation mediated by phytanic acid, a known ligand agonist of the peroxisomal proliferator-activated receptor alpha. In summary, the present work establishes a role for SCP-x in branched chain lipid catabolism and demonstrates a sexual dimorphic response to phytol, a precursor of phytanic acid, in lipid parameters and hepatotoxicity.

The structural determination of an insect sterol carrier protein-2 with a ligand-bound C16 fatty acid at 1.35-A resolution

Yellow fever mosquito sterol carrier protein (SCP-2) is known to bind to cholesterol. We report here the three-dimensional structure of the complex of SCP-2 from Aedes aegypti with a C16 fatty acid to 1.35-A resolution. The protein fold is exceedingly similar to the human and rabbit proteins, which consist of a five-stranded beta-sheet that exhibits strand order 3-2-1-4-5 with an accompanying layer of four alpha-helices that cover the beta-sheet. A large cavity exists at the interface of the layer alpha-helices and the beta-sheet, which serves as the fatty acid binding site. The carboxylate moiety of the fatty acid is coordinated by a short loop that connects the first alpha-helix to the first beta-strand, whereas the acyl chain extends deep into the interior of the protein. Interestingly, the orientation of the fatty acid is opposite to the observed orientation for Triton X-100 in the SCP-2-like domain from the peroxisomal multifunctional enzyme (Haapalainen, A. M., van Aalten, D. M., Merilainen, G., Jalonen, J. E., Pirila, P., Wierenga, R. K., Hiltunen, J. K., and Glumoff, T. (2001) J. Mol. Biol. 313, 1127-1138). The present study suggests that the binding pocket in the SCP-2 family of proteins may exhibit conformational flexibility to allow coordination of a variety of lipids.

Isolation and expression of a sterol carrier protein-2 gene from the yellow fever mosquito, Aedes aegypti

Trafficking of cholesterol in insects is a very important process due to the fact that insects depend on dietary cholesterol to fulfil their physiological needs. We identified a putative mosquito sterol carrier protein-2 (SCP-2) cDNA from fourth instar subtracted cDNA library. The AeSCP-2 protein has high degree homology in the sterol transfer domain to both rat and human SCP-2. Transcripts of AeSCP-2 in fourth instars were detected strongly in the midgut, and weakly in the head and hindgut. In the early pupae, AeSCP-2 transcription was observed in the thorax, head and body wall of abdomen, but not in the gut. The interaction of mosquito sterol carrier protein-2 (AeSCP-2) with cholesterol was examined. The Kd of purified recombinant AeSCP-2 to cholesterol was 5.6 +/- 0.6 x 10-9 m using radiolabelled cholesterol-binding assay. The results suggest that AeSCP-2 has high affinity to cholesterol and may function as a carrier protein in mosquitoes.

Molecular and fluorescent sterol approaches to probing lysosomal membrane lipid dynamics

Although the most exogenous lipids enter the cell via the LDL-receptor pathway, the mechanism(s) whereby lipids leave the lysosome for transport to intracellular sites are not clearly resolved. As shown herein, expression of sterol carrier protein-2 (SCP-2) in transfected L-cells altered lysosomal membrane lipid distribution, dynamics, and response to lipid transfer proteins. SCP-2 expression decreased the mass of cholesterol and lyso-bis-phosphatidic acid [LBPA], as well as the ratios of cholesterol/phospholipid and polyunsaturated/monounsaturated fatty acids esterified to lysosomal membrane phospholipids. Concomitantly, a fluorescent sterol transfer assay showed that SCP-2 expression decreased the initial rates of spontaneous and SCP-2-mediated sterol transfer 5.5- and 3.8-fold, respectively, from lysosomal membranes isolated from SCP-2 expressing cells as compared to controls. SCP-2, sphingomyelinase, low density lipoprotein, and high density lipoprotein directly enhanced the initial rates of sterol transfer from isolated lysosomal membranes by 50-, 12-, 4-, and 5-fold, respectively. In contrast, albumin and cholesterol esterase had no effect on lysosomal sterol transfer. Spontaneous sterol was very slow, t(1/2)>4 days, regardless of the source of the lysosomal membrane, while SCP-2 added in vitro induced formation of rapid and slowly transferable sterol pools in lysosomal membranes of control cells. In contrast, SCP-2 did not induce formation of a rapidly transferable sterol domain in lysosomal membranes isolated from SCP-2 expressing cells. These data suggest that SCP-2 expression selectively shifted the distribution of lipids (cholesterol, LBPA, esterified polyunsaturated fatty acids) away from lysosomal membranes. Furthermore, the cholesterol depleted lysosomal membrane isolated from SCP-2 expressing cells was resistant to additional direct action of SCP-2 to further enhance sterol transfer and induce rapidly transferable sterol pools in the lysosomal membrane.

Vesicular and non-vesicular sterol transport in living cells. The endocytic recycling compartment is a major sterol storage organelle

We examined the intracellular transport of sterol in living cells using a naturally fluorescent cholesterol analog, dehydroergosterol (DHE), which has been shown to mimic many of the properties of cholesterol. By using DHE loaded on methyl-beta-cyclodextrin, we followed this cholesterol analog in pulse-chase studies. At steady state, DHE co-localizes extensively with transferrin (Tf), a marker for the endocytic recycling compartment (ERC), and redistributes with Tf in cells with altered ERC morphology. Expression of a dominant-negative mutation of an ERC-associated protein, mRme-1 (G429R), results in the slowing of both DHE and Tf receptor return to the cell surface. [3H]Cholesterol is found in the same fraction as 125I-Tf on sucrose density gradients, and this fraction can be specifically shifted to a higher density based on the presence of horseradish peroxidase-conjugated Tf in the same organelle. Whereas vesicular transport of Tf and efflux of DHE from the ERC are entirely blocked in energy-depleted cells, delivery of DHE to the ERC from the plasma membrane is only slightly affected. Biochemical studies performed using [3H]cholesterol show that the energy dependence of cholesterol transport to and from the ERC is similar to DHE transport. We propose that a large portion of intracellular cholesterol is localized in the ERC, and this pool might be important in maintaining cellular cholesterol homeostasis.

Gene structure, intracellular localization, and functional roles of sterol carrier protein-2

Since its discovery three decades ago, sterol carrier protein-2 (SCP-2) has remained a fascinating protein whose physiological function in lipid metabolism remains an enigma. Its multiple proposed functions arise from its complex gene structure, post-translational processing, intracellular localization, and ligand specificity. The SCP-2 gene has two initiation sites coding for proteins that share a common 13 kDa SCP-2 C-terminus: (1) One site codes for 58 kDa SCP-x which is partially post-translationally cleaved to 13 kDa SCP-2 and a 45 kDa protein. (2) A second site codes for 15 kDa pro-SCP-2 which is completely post-translationally cleaved to 13 kDa SCP-2. Very little is yet known regarding how the relative proportions of the two transcripts are regulated. Although all three proteins contain a C-terminal SKL peroxisomal targeting sequence, it is unclear why all three proteins are not exclusively localized in peroxisomes. However, the recent demonstration that the SCP-2 N-terminal presequence in pro-SCP-2 dramatically modulated the intracellular targeting coded by the C-terminal peroxisomal targeting sequence may account for the observation that as much as half of total SCP-2 is localized outside the peroxisome. The tertiary and secondary structure of the 13 kDa SCP-2, but not that of 15 kDa pro-SCP-2 and 58 kDa SCP-x, are now resolved. Increasing evidence suggests that the 58 kDa SCP-x and 45 kDa proteins are peroxisomal 3-ketoacyl-CoA-thiolases involved in the oxidation of branched chain fatty acids. Since 15 kDa pro-SCP-2 is post-translationally completely cleaved to 13 kDa SCP-2, relatively little attention has been focused on this protein. Finally, although the 13 kDa SCP-2 is the most studied of these proteins, because it exhibits diversity of its ligand partners (fatty acids, fatty acyl CoAs, cholesterol, phospholipids), new potential physiological function(s) are still being proposed and questions regarding potential compensation by other proteins with overlapping specificity are only beginning to be resolved.

Sterol carrier protein-2 expression modulates protein and lipid composition of lipid droplets

Despite the critical role lipid droplets play in maintaining energy reserves and lipid stores for the cell, little is known about the regulation of the lipid or protein components within the lipid droplet. Although immunofluorescence of intact cells as well as Western analysis of isolated lipid droplets revealed that sterol carrier protein-2 (SCP-2) was not associated with lipid droplets, SCP-2 expression significantly altered the structure of the lipid droplet. First, the targeting of fatty acid and cholesterol to the lipid droplets was significantly decreased. Second, the content of several proteins important for lipid droplet function was differentially increased (perilipin A), reduced severalfold (adipose differentiation-related protein (ADRP), vimentin), or almost completely eliminated (hormone-sensitive lipase and proteins >93 kDa) in the isolated lipid droplet. Third, the distribution of lipids within the lipid droplets was significantly altered. Double labeling of cells with 12-(N-methyl)-N-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl) amino]-octadecanoic acid (NBD-stearic acid) and antisera to ADRP showed that 70, 24, and 13% of lipid droplets contained ADRP, NBD-stearic acid, or both, respectively. SCP-2 expression decreased the level of ADRP in the lipid droplet but increased the proportion wherein ADRP and NBD-stearic acid colocalized by 3-fold. SCP-2 expression also decreased the lipid droplet fatty acid and cholesterol mass (nmol/mg protein) by 5.2- and 6.6-fold, respectively. Finally, SCP-2 expression selectively altered the pattern of esterified fatty acids in favor of polyunsaturated fatty acids within the lipid droplet. Displacement studies showed differential binding affinity of ADRP for cholesterol and fatty acids. These data suggested that SCP-2 and ADRP play a significant role in regulating fatty acid and cholesterol targeting to lipid droplets as well as in determining their lipid and protein components.

Sterol carrier protein-2 localization in endoplasmic reticulum and role in phospholipid formation

Although sterol carrier protein-2 (SCP-2; also called nonspecific lipid transfer protein) binds fatty acids and fatty acyl-CoAs, its role in fatty acid metabolism is not fully understood. L-cell fibroblasts stably expressing SCP-2 were used to resolve the relationship between SCP-2 intracellular location and fatty acid transacylation in the endoplasmic reticulum. Indirect immunofluorescence double labeling and laser scanning confocal microscopy detected SCP-2 in peroxisomes > endoplasmic reticulum > mitochondria > lysosomes. SCP-2 enhanced incorporation of exogenous [(3)H]oleic acid into phospholipids and triacylglycerols of overexpressing cells 1.6- and 2.5-fold, respectively, stimulated microsomal incorporation of [1-(14)C]oleoyl-CoA into phosphatidic acid in vitro 13-fold, and exhibited higher specificity for unsaturated versus saturated fatty acyl-CoA. SCP-2 enhanced the rate-limiting step in microsomal phosphatidic acid biosynthesis mediated by glycerol-3-phosphate acyltransferase. SCP-2 also enhanced microsomal acyl-chain remodeling of phosphatidylethanolamine up to fivefold and phosphatidylserine twofold, depending on the specific fatty acyl-CoA, but had no effect on other phospholipid classes. In summary, these results were consistent with a role for SCP-2 in phospholipid synthesis in the endoplasmic reticulum.

High density lipoprotein-mediated cholesterol uptake and targeting to lipid droplets in intact L-cell fibroblasts. A single- and multiphoton fluorescence approach

Fluorescent sterols, dehydroergosterol and NBD-cholesterol, were used to examine high density lipoprotein-mediated cholesterol uptake and intracellular targeting in L-cell fibroblasts. The uptake, but not esterification or targeting to lipid droplets, of these sterols differed >100-fold, suggesting significant differences in uptake pathways. NBD-cholesterol uptake kinetics and lipoprotein specificity reflected high density lipoprotein-mediated sterol uptake via the scavenger receptor B1. Fluorescence energy transfer showed an average intermolecular distance of 26 A between the two fluorescent sterols in L-cells. Indirect immunofluorescence revealed that both fluorescent sterols localized to L-cell lipid droplets, the surface of which contained adipose differentiation-related protein. This lipid droplet-specific protein specifically bound NBD-cholesterol with high affinity (K(d) = 2 nM) at a single site. Thus, NBD-cholesterol and dehydroergosterol were useful fluorescent probes of sterol uptake and intracellular sterol targeting. NBD-cholesterol more selectively probed high density lipoprotein-mediated uptake and rapid intracellular targeting of sterol to lipid droplets. Targeting of sterol to lipid droplets was correlated with the presence of adipose differentiation related protein, a lipid droplet-specific protein shown for the first time to bind unesterified sterol with high affinity.

A potential role for sterol carrier protein-2 in cholesterol transfer to mitochondria

Mitochondrial cholesterol oxidation rapidly depletes cholesterol from the relatively cholesterol-poor mitochondrial membranes. However, almost nothing is known regarding potential mechanism(s) whereby the mitochondrial cholesterol pool is restored. Since most exogenous cholesterol enters the cell via the lysosomal pathway, this could be a source of mitochondrial cholesterol. In the present study, an in vitro fluorescent sterol transfer assay was used to examine whether the lysosomal membrane could be a putative cholesterol donor to mitochondria. First, it was shown that spontaneous sterol transfer from lysosomal to mitochondrial membranes was very slow (initial rate, 0.316 +/- 0.032 pmol/min). This was due, in part, to the fact that 90% of the lysosomal membrane sterol was not exchangeable, while the remaining 10% also had a relatively long half-time of exchange t(1/2) = 202 +/- 19 min. Second, the intracellular sterol carrier protein-2 (SCP-2) and its precursor (pro-SCP-2) increased the initial rate of sterol transfer from the lysosomal to mitochondrial membrane by 5.2- and 2.0-fold, respectively, but not in the reverse direction. The enhanced sterol transfer was due to a 3.5-fold increase in exchangeable sterol pool size and to induction of a very rapidly (t(1/2) = 4.1 +/- 0.6 min) exchangeable sterol pool. Confocal fluorescence imaging and indirect immunocytochemistry colocalized significant amounts of SCP-2 with the mitochondrial marker enzyme cytochrome oxidase in transfected L-cells overexpressing SCP-2. In summary, SCP-2 and pro-SCP-2 both stimulated molecular sterol transfer from lysosomal to mitochondrial membranes, suggesting a potential mechanism for replenishing mitochondrial cholesterol pools depleted by cholesterol oxidation.

Holo-sterol carrier protein-2. (13)C NMR investigation of cholesterol and fatty acid binding sites

Although sterol carrier protein-2 (SCP-2) stimulates sterol transfer in vitro, almost nothing is known regarding the identity of the putative cholesterol binding site. Furthermore, the interrelationship(s) between this SCP-2 ligand binding site and the recently reported SCP-2 long chain fatty acid (LCFA) and long chain fatty acyl-CoA (LCFA-CoA) binding site(s) remains to be established. In the present work, two SCP-2 ligand binding sites were identified. First, both [4-(13)C]cholesterol and 22-(N-(7-nitrobenz-2-oxa-1, 3-diazol-4-yl)amino)-23,24-bisnor-5-cholen-3beta-ol (NBD-cholesterol) binding assays were consistent with a single cholesterol binding site in SCP-2. This ligand binding site had high affinity for NBD-cholesterol, K(d) = 4.15 +/- 0.71 nM. (13)C NMR-labeled ligand competition studies demonstrated that the SCP-2 high affinity cholesterol binding site also bound LCFA or LCFA-CoA. However, only the LCFA-CoA was able to effectively displace the SCP-2-bound [4-(13)C]cholesterol. Thus, the ligand affinities at this SCP-2 binding site were in the relative order cholesterol = LCFA-CoA > LCFA. Second, (13)C NMR studies demonstrated the presence of another ligand binding site on SCP-2 that bound either LCFA or LCFA-CoA but not cholesterol. Photon correlation spectroscopy was consistent with SCP-2 being monomeric in both liganded and unliganded states. In summary, both (13)C NMR and fluorescence techniques demonstrated for the first time that SCP-2 had a single high affinity binding site that bound cholesterol, LCFA, or LCFA-CoA. Furthermore, results with (13)C NMR supported the presence of a second SCP-2 ligand binding site that bound either LCFA or LCFA-CoA but not cholesterol. These data contribute to our understanding of a role for SCP-2 in both cellular cholesterol and LCFA metabolism.

Microsomal long chain fatty acyl-CoA transacylation: differential effect of sterol carrier protein-2

The recent discovery that sterol carrier protein-2 (SCP-2) binds long chain++ (LCFA-CoA) with high affinity (A. Frolov et al., J. Biol. Chem. 271 (1997) 31878-31884) suggests new possible functions of this protein in LCFA-CoA metabolism. The purpose of the present investigation was to determine whether SCP-2 differentially modulated microsomal LCFA-CoA transacylation to cholesteryl esters, triacylglycerols, and phospholipids in vitro. Microsomal acyl-CoA:cholesterol acyltransferase (ACAT) activity measured with liposomal membrane cholesterol donors depended on substrate LCFA-CoA level, mol% cholesterol in the liposomal membrane, and total amount of liposomal cholesterol. As compared to basal activity without liposomes, microsomal ACAT was inhibited 30-50% in the presence of cholesterol poor (1.4 mol%) liposomes. In contrast, cholesterol rich (>25 mol%) liposomes stimulated ACAT up to 6.4-fold compared to basal activity without liposomes and nearly 10-fold as compared to cholesterol pool (1.4 mol%) liposomes. Increasing oleoyl-CoA reversed the inhibition of microsomal ACAT by cholesterol poor (1.4 mol%) liposomes, but did not further stimulate ACAT in the presence of cholesterol rich (35 mol%) liposomes. In contrast, high (100 microM) oleoyl-CoA inhibited ACAT nearly 3-fold. This inhibition was reversed by LCFA-CoA binding proteins, bovine serum albumin (BSA) and SCP-2. SCP-2 was 10-fold more effective (mole for mole) than BSA in reversing LCFA-CoA inhibited microsomal ACAT. Concomitantly, under conditions in which SCP-2 stimulated ACAT it equally enhanced transacylation of oleoyl-CoA into phospholipids, and 5.2-fold enhanced oleoyl-CoA transacylation to triacylglycerols. In summary, SCP-2 appeared to exert its greatest effects on microsomal transacylation in vitro by reversing LCFA-CoA inhibition of ACAT and by differentially targeting LCFA-CoA to triacylglycerols. These data suggest that the high affinity interaction of SCP-2s with LCFA-CoA may be physiologically important in microsomal transacylation reactions.

Stability and binding properties of wild-type and c17s mutated human sterol carrier protein 2

The temperature- and solvent-induced denaturation of both the SCP2 wild-type and the mutated protein c71s were studied by CD measurements at 222 nm. The temperature-induced transition curves were deconvoluted according to a two-state mechanism resulting in a transition temperature of 70.5 degrees C and 59.9 degrees C for the wild-type and the c71s, respectively, with corresponding values of the van't Hoff enthalpies of 183 and 164 kJ/mol. Stability parameters characterizing the guanidine hydrochloride denaturation curves were also calculated on the basis of a two-state transition. The transitions of the wild-type occurs at 0.82 M GdnHCl and that of the c71s mutant at 0.55 M GdnHCl. These differences in the half denaturation concentration of GdnHCl reflect already the significant stability differences between the two proteins. A quantitative measure are the Gibbs energies DeltaG(0)(D)(buffer) at 25 degrees C of 15.5 kJ/mol for the wild-type and 8.0 kJ/mol for the mutant. We characterized also the alkyl chain binding properties of the two proteins by measuring the interaction parameters for the complex formation with 1-O-Decanyl-beta-D-glucoside using isothermal titration microcalorimetry. The dissociation constants, K(d), for wild-type SCP2 are 335 microM at 25 degrees C and 1.3 mM at 35 degrees C. The corresponding binding enthalpies, DeltaH(b), are -21. 5 kJ/mol at 25 degrees C and 72.2 kJ/mol at 35 degrees C. The parameters for the c71s mutant at 25 degrees C are K(d)=413 microM and DeltaH(b)=16.6 kJ/mol. These results suggest that both SCP2 wild-type and the c71s mutant bind the hydrophobic compound with moderate affinity.

Sterol carrier protein-2 expression increases NBD-stearate uptake and cytoplasmic diffusion in L cells

The effects of sterol carrier protein-2 (SCP-2) expression on fatty acid uptake and cytoplasmic diffusion were determined using L cell fibroblasts transfected with cDNA encoding either the 15- or 13. 2-kDa form of SCP-2. Cis-parinarate and 12-N-methyl-(7-nitrobenz-2-oxa-1,3-diazol)aminostearate (NBD-stearate) were used as nonesterifiable fluorescent fatty acid probes. NBD-stearate and cis-parinarate uptake was rapid and saturable. In 15-kDa SCP-2-expressing cells, the extent of NBD-stearate and cis-parinarate uptake was increased 1.4- and 1. 2-fold, respectively, compared with control. In the 13.2-kDa SCP-2-expressing cells, the extent of NBD-stearate and cis-parinarate uptake was increased 1.3- and 1.1-fold, respectively, compared with control cells. NBD-stearate cytoplasmic diffusion was increased 1.5-fold in 15-kDa SCP-2-expressing cells, but not in 13. 2-kDa SCP-2-expressing cells, compared with control cells. After incubation with NBD-stearate for 30 min at 37 degrees C, fluorescence imaging indicated that NBD-stearate was localized primarily in lipid droplets in all cell lines. These results suggest that SCP-2 may be involved not only in fatty acid uptake but also in intracellular fatty acid trafficking.

The effect of chronic luteinizing hormone treatment on adult rat Leydig cells

We investigated the chronic effects of luteinizing hormone (LH) treatment on adult rat Leydig cell structure and function. Two groups of sexually mature male Sprague-Dawley rats were used; controls and rats implanted subdermally with LH-filled Alzet miniosmotic pumps (delivers 24 micrograms of LH per day). After 2 weeks of LH treatment, testes of these rats were fixed by 2.5% glutaraldehyde in cacodylate buffer and processed and embedded in epon-araldite for light and electron microscopy and electron microscopic immunocytochemistry. Using light microscopic stereology, Leydig cell volume density, number of Leydig cells per testis, and the average volume of a Leydig cell were determined. Additionally, the organelle volumes per Leydig cells were quantified by electron microscopic stereology. Sterol carrier protein-2 (SCP2) and catalase in Leydig cells were immunolocalized via the Protein A gold method. Isolated and purified Leydig cells were used to determine the LH-stimulated (100 ng/ml) testosterone secretory capacity per Leydig cell in vitro and to compare the SCP2 and catalase content in equal numbers of Leydig cells using immunoblot analysis. After 2 weeks of LH-treatment, Leydig cell number per testis and the average volume showed a two-fold increase. All organelles tested, except the lipid droplets, were significantly (P < 0.05) increased two-fold in volume per Leydig cell. Testosterone secretory capacity per Leydig cell was increased approximately six-fold in the LH-treated group. Immunolabeling studies showed that the intraperoxisomal SCP2 content was significantly greater (P < 0.05) and the catalase content was significantly lower (P < 0.05) in LH-treated rats compared to to controls. Immunoblots showed that the total SCP2 content per cell is greater and the catalase content per cell is similar in Leydig cells of LH-treated rats compared to controls. In summary, chronic LH treatment produced hyperplasia, hypertrophy and increased testosterone secretory capacity in leydig cells of adult rats. However, the increase in the testosterone secretory capacity per Leydig cell exceeds the degree of Leydig cell hypertrophy, which cannot be explained by a generalized increase in volumes of all Leydig cell organelles in the LH-treated rats. These results also suggested that chronic LH treatment induces differential synthesis of peroxisomal proteins, i.e. an increase in SCP2 synthesis and no change in catalase synthesis. This resulted in peroxisomes rich in SCP2 and lower in catalase. Significance of these effects in relation to the increased steroidogenic capacity of Leydig cells remains to be determined.

Lipid specificity and location of the sterol carrier protein-2 fatty acid-binding site: a fluorescence displacement and energy transfer study

Although it was recently recognized that sterol carrier protein-2 (SCP-2) interacts with fatty acids, little is known regarding the specificity of SCP-2 for long-chain fatty acids or branched-chain fatty-acid-like molecules. Likewise the location of the fatty-acid binding site within SCP-2 is unresolved. A fluorescent cis-parinaric acid displacement assay was used to show that SCP-2 optimally interacted with 14-22 carbon chain lipidic molecules: polyunsaturated fatty acids > monounsaturated, saturated > branched-chain isoprenoids > branched-chain phytol-derived fatty acids. In contrast, the other major fatty-acid binding protein in liver, fatty-acid binding protein (L-FABP), displayed a much narrower carbon chain preference in general: polyunsaturated fatty acids > branched-chain phytol-derived fatty acids > 14- and 16-carbon saturated > branched-chain isoprenoids. However, both SCP-2 and L-FABP displayed a very similar unsaturated fatty-acid specificity profile. The presence and location of the SCP-2 lipid binding site were investigated by fluorescence energy transfer. The distance between the SCP-2 Trp50 and bound cis-parinaric acid was determined to be 40 A. Thus, the SCP-2 fatty-acid binding site appeared to be located on the opposite side of the SCP-2 Trp50. These findings not only contribute to our understanding of the SCP-2 ligand binding site but also provide evidence suggesting a potential role for SCP-2 and/or L-FABP in metabolism of branched-chain fatty acids and isoprenoids.

Phospholipid transfer proteins revisited

Phosphatidylinositol transfer protein (PI-TP) and the non-specific lipid transfer protein (nsL-TP) (identical with sterol carrier protein 2) belong to the large and diverse family of intracellular lipid-binding proteins. Although these two proteins may express a comparable phospholipid transfer activity in vitro, recent studies in yeast and mammalian cells have indicated that they serve completely different functions. PI-TP (identical with yeast SEC14p) plays an important role in vesicle flow both in the budding reaction from the trans-Golgi network and in the fusion reaction with the plasma membrane. In yeast, vesicle budding is linked to PI-TP regulating Golgi phosphatidylcholine (PC) biosynthesis with the apparent purpose of maintaining an optimal PI/PC ratio of the Golgi complex. In mammalian cells, vesicle flow appears to be dependent on PI-TP stimulating phosphatidylinositol 4,5-bisphosphate (PIP2) synthesis. This latter process may also be linked to the ability of PI-TP to reconstitute the receptor-controlled PIP2-specific phospholipase C activity. The nsL-TP is a peroxisomal protein which, by its ability to bind fatty acyl-CoAs, is most likely involved in the beta-oxidation of fatty acids in this organelle. This protein constitutes the N-terminus of the 58 kDa protein which is one of the peroxisomal 3-oxo-acyl-CoA thiolases. Further studies on these and other known phospholipid transfer proteins are bound to reveal new insights in their important role as mediators between lipid metabolism and cell functions.

Expression of fatty acyl-CoA binding proteins in colon cells: response to butyrate and transformation

Fatty acyl-CoA affect many cellular functions as well as serving as cellular building blocks. Several families of cytosolic fatty acyl-CoA binding proteins may modulate the activities of fatty acyl-CoA. Intestinal enterocytes contain at least three unique families of cytosolic proteins that bind fatty acyl-CoA: acyl-CoA binding protein (ACBP), fatty acid binding proteins (including the liver, L-FABP and intestinal, I-FABP), and sterol carrier protein-2 (SCP-2). Immortalized rat colon epithelial cell lines expressed only ACBP and SCP-2 at levels of 0.75 +/- 0.13 and 0.42 +/- 0.02 ng/microgram protein. Ras and src transformation increased colon cell density and differentially altered ACBP and SCP-2 expression without affecting I-FABP or L-FABP levels. ACBP levels were 1.8-fold and 1.5-fold increased in ras- and src-transformed cells, respectively. In contrast, SCP-2 expression was significantly decreased 55 and 67% in ras- and src-transformed cells, respectively. Butyrate treatment of ras- and src-transformed cells decreased cell proliferation up to 60-85% as compared to 25-30% in control cells. Butyrate treatment decreased ACBP expression in all cell lines but had no effect on the levels of SCP-2, I-FABP, or L-FABP. These studies suggest that the differential expression of ACBP and SCP-2 in rat colonic cell lines, as well as their modulation by butyrate, may be altered by cell transformation.

Sterol carrier protein-2 mediated cholesterol esterification in transfected L-cell fibroblasts

The relative function of the 15 and 13.2 kDa forms of SCP-2 in cholesterol trafficking and metabolism was assessed using L-cell fibroblasts permanently transfected with the cDNA encoding for either the mouse 15 kDa or 13.2 kDa SCP-2. Expression of the 15 kDa, but not the 13.2 kDa SCP-2 increased [3H]cholesteryl ester formation from medium derived cholesterol by 30% compared to control cells. In both SCP-2 expressing cell lines, sphingomyelinase treatment increased the initial rate of [3 H]cholesteryl ester formation from plasma membrane derived cholesterol more than 11-fold and elevated [3H]cholesteryl ester levels 1.5-fold compared to control cells. Expression of both proteins resulted in nearly a 1.5-fold increase in [3H]oleic acid esterification into cholesteryl esters, although [3H]oleic acid esterification into triacylglycerols was also increased in the 13.2 kDa SCP-2 expressing cells relative to control. In both transfected cell lines, the cholesteryl ester mass was increased nearly 2-fold compared to control cells, consistent with increased cholesteryl ester synthesis. Similarly, triacylglycerol levels were increased 1.3-fold in the 13.2 kDa SCP-2 expressing cells which is consistent with the increased [3H]oleic acid esterification into triacylglycerol. In the 15 kDa SCP-2 expressing cells, triacylglycerol levels were decreased 60%, but free cholesterol levels were increased 1.2-fold relative to control cells. Thus, only the 15 kDa expression product, containing the putative targeting sequence, specifically enhanced cholesteryl ester formation from either plasma membrane or medium-derived cholesterol. In contrast, the 13.2 kDa expression product, lacking the putative targeting sequence, stimulated an increase in [3H]oleic acid esterification into both cholesterol and triacylglycerol pools, suggesting a non-specific stimulation of fatty acid esterification.

The sterol carrier protein-2 fatty acid binding site: an NMR, circular dichroic, and fluorescence spectroscopic determination

The interaction and orientation of fatty acids with recombinant human sterol carrier protein-2 (SCP-2) were examined by nuclear magnetic resonance (NMR), circular dichroism (CD), and fluorescence techniques. 13C-NMR spectroscopy of stearic acid and oleic acid as well as fluorescence spectroscopy of cis-parinaric acid demonstrated that SCP-2 bound naturally occurring fatty acids with near 1:1 stoichiometry. Several findings indicated that the fatty acid was oriented in the binding site with its methyl end buried in the protein interior and its carboxylate exposed at the surface: the chemical shift of bound [18-13C]-stearate; dicarboxylic/monocarboxylic acid cis-parinaric acid displacement; complete ionization of the carboxylate group of SCP-2 bound [1-13C]stearate at neutral pH; lack of electrostatic interactions between 13C-fatty acids with SCP-2 cationic residues: pH titratability of the SCP-2 bound [1-13C]stearate carboxylate group. SCP-2 did not undergo global structural changes upon ligand binding or pH decrease as indicated by the absence of significant changes in NMR and only small alterations in time resolved fluorescence parameters. However, SCP-2 did undergo secondary structural changes detected by CD in the pH range 5-6. While these changes in secondary structure did not alter the fatty acid:SCP-2 binding stoichiometry, the affinity for fatty acid was increased severalfold at lower pH. In summary, 13C-NMR, CD, and fluorescence spectroscopy provided a detailed understanding of the interaction of fatty acids with SCP-2 and further showed for the first time the orientation of the fatty acid within the binding site. The pH-induced changes in SCP-2 secondary structure and ligand binding activity may be important to the mechanism whereby this protein interacts with membrane surfaces to enhance lipid binding/transfer.

Acyl-CoA binding proteins: multiplicity and function

The physiological role of long-chain fatty acyl-CoA is thought to be primarily in intermediary metabolism of fatty acids. However, recent data show that nM to microM levels of these lipophilic molecules are potent regulators of cell functions in vitro. Although long-chain fatty acyl-CoA are present at several hundred microM concentration in the cell, very little long-chain fatty acyl-CoA actually exists as free or unbound molecules, but rather is bound with high affinity to membrane lipids and/or proteins. Recently, there is growing awareness that cytosol contains nonenzymatic proteins also capable of binding long-chain fatty acyl-CoA with high affinity. Although the identity of the cytosolic long-chain fatty acyl-CoA binding protein(s) has been the subject of some controversy, there is growing evidence that several diverse nonenzymatic cytosolic proteins will bind long-chain fatty acyl-CoA. Not only does acyl-CoA binding protein specifically bind medium and long-chain fatty acyl-CoA (LCFA-CoA), but ubiquitous proteins with multiple ligand specificities such as the fatty acid binding proteins and sterol carrier protein-2 also bind LCFA-CoA with high affinity. The potential of these acyl-CoA binding proteins to influence the level of free LCFA-CoA and thereby the amount of LCFA-CoA bound to regulatory sites in proteins and enzymes is only now being examined in detail. The purpose of this article is to explore the identity, nature, function, and pathobiology of these fascinating newly discovered long-chain fatty acyl-CoA binding proteins. The relative contributions of these three different protein families to LCFA-CoA utilization and/or regulation of cellular activities are the focus of new directions in this field.