Impact of dietary phytol on lipid metabolism in SCP2/SCPX/L-FABP null mice

In vitro studies suggest that liver fatty acid binding protein (L-FABP) and sterol carrier protein-2/sterol carrier protein-x (SCP2/SCPx) gene products facilitate uptake and metabolism and detoxification of dietary-derived phytol in mammals. However, concomitant upregulation of L-FABP in SCP2/SCPx null mice complicates interpretation of their physiological phenotype. Therefore, the impact of ablating both the L-FABP gene and SCP2/SCPx gene (L-FABP/SCP2/SCPx null or TKO) was examined in phytol-fed female wild-type (WT) and TKO mice. TKO increased hepatic total lipid accumulation, primarily phospholipid, by mechanisms involving increased hepatic levels of proteins in the phospholipid synthetic pathway. Concomitantly, TKO reduced expression of proteins in targeting fatty acids towards the triacylglycerol synthetic pathway. Increased hepatic lipid accumulation was not associated with any concomitant upregulation of membrane fatty acid transport/translocase proteins involved in fatty acid uptake (FATP2, FATP4, FATP5 or GOT) or cytosolic proteins involved in fatty acid intracellular targeting (ACBP). In addition, TKO exacerbated dietary phytol-induced whole body weight loss, especially lean tissue mass. Since individually ablating SCPx or SCP2/SCPx elicited concomitant upregulation of L-FABP, these findings with TKO mice help to resolve the contributions of SCP2/SCPx gene ablation on dietary phytol-induced whole body and hepatic lipid phenotype independent of concomitant upregulation of L-FABP.     

Deficiencies in sex-regulated expression and levels of two hepatic sterol carrier proteins in a murine model of Niemann-Pick type C disease.

Hepatic sterol carrier protein-2 (SCP2) and sterol carrier protein-X (SCPx) levels in normal and in mutant Niemann-Pick Type C mice were determined by immunoblotting with antiserum against rat SCP2. A 14-kDa protein (SCP2) was detected in the cytosol fraction and a 58-kDa protein (SCPx) was found in both cytosolic and organellar fractions. Expression of hepatic SCPx protein was developmentally regulated in a sex-specific pattern. The amounts of organelle-associated SCPx increased 4-fold during sexual development of normal males but decreased dramatically during development of normal females. Levels of hepatic SCP2 increased much less dramatically during sexual maturation of normal males and females. Adult Niemann-Pick Type C mice were deficient in both hepatic SCPx and SCP2. The deficit in SCPx in affected males reflected a failure to increase hepatic SCPx levels during sexual maturation. In affected males SCPx remained at levels found in immature mice. Affected male and female mice were also unable to maintain levels of hepatic SCP2. The level of SCP2 was near normal in affected immature males and subnormal in affected immature females. During sexual maturation hepatic SCP2 declined in affected animals.     

Aberrant oxidation of the cholesterol side chain in bile acid synthesis of sterol carrier protein-2/sterol carrier protein-x knockout mice

Peroxisomal beta-oxidation plays an important role in the metabolism of a wide range of substrates, including various fatty acids and the steroid side chain in bile acid synthesis. Two distinct thiolases have been implicated to function in peroxisomal beta-oxidation: the long known 41-kDa beta-ketothiolase identified by Hashimoto and co-workers (Hijikata, M., Ishii, N., Kagamiyama, H., Osumi, T., and Hashimoto, T. (1987) J. Biol. Chem. 262, 8151-8158) and the recently discovered 60-kDa SCPx thiolase, that consists of an N-terminal domain with beta-ketothiolase activity and a C-terminal moiety of sterol carrier protein-2 (SCP2, a lipid carrier or transfer protein). Recently, gene targeting of the SCP2/SCPx gene has shown in mice that the SCPx beta-ketothiolase is involved in peroxisomal beta-oxidation of 2-methyl-branched chain fatty acids like pristanic acid. In our present work we have investigated bile acid synthesis in the SCP2/SCPx knockout mice. Specific inhibition of beta-oxidation at the thiolytic cleavage step in bile acid synthesis is supported by our finding of pronounced accumulation in bile and serum from the knockout mice of 3alpha,7alpha, 12alpha-trihydroxy-27-nor-5beta-cholestane-24-one (which is a known bile alcohol derivative of the cholic acid synthetic intermediate 3alpha,7alpha,12alpha-trihydroxy-24-keto-cholestano yl-coenzyme A). Moreover, these mice have elevated concentrations of bile acids with shortened side chains (i.e. 23-norcholic acid and 23-norchenodeoxycholic acid), which may be produced via alpha- rather than beta-oxidation. Our results demonstrate that the SCPx thiolase is critical for beta-oxidation of the steroid side chain in conversion of cholesterol into bile acids.     

New aspects of sterol carrier protein 2 (Nonspecific lipid-transfer protein) in fusion proteins and in peroxisomes

Sterol carrier protein 2 (SCP2) is a 13-kDa peroxisomal protein, identical to nonspecific lipidtransfer protein, and stimulates various steps of cholesterol metabolism in vitro. Although the name is reminiscent of acyl carrier protein, which is involved in fatty acid synthesis, SCP2 does not bind to lipids specifically or stoichiometrically. This protein is expressed either as a small precursor or as a large fusion (termed SCPx) that carries at its C-terminal the complete sequence of SCP2. SCPx exhibits 3-oxoacyl-CoA thiolase activity, as well as sterol-carrier and lipid-transfer activities. The N- and C-terminal parts of SCPx are similar to the nematode protein P-44 and the yeast protein PXP-18, respectively. P-44, which has no SCP2 sequence, thiolytically cleaved the side chain of bile acid intermediate at a rate comparable to that of SCPx. This, together with the properties of other fusions with SCP2-like sequence, suggests that the SCP2 part of SCPx does not play a direct role in thiolase reaction. PXP-18, located predominantly inside peroxisomes, is similar to SCP2 in primary structure and lipid-transfer activity, and protects peroxisomal acyl-CoA oxidase from thermal denaturation. PXP-18 dimerized at a high temperature, formed an equimolar complex with the oxidase subunit, and released the active enzyme from the complex when the temperature went down. This article attempts to gain insight into the role of SPC2, and to present a model in which PXP-18, a member of the SCP2 family, functions as a molecular chaperone in peroxisomes.     

Lauric acid dependent enhancement in hepatic SCPx protein requires an insulin deficient environment

Sterol carrier protein X (SCPx) is a peroxisomal protein with both lipid transfer and thiolase activity. Treating with the fatty acid, lauric acid, induced SCPx mRNA levels in rat liver and in rat hepatoma H4IIE cells but enhanced protein levels of SCPx and the thiolase produced as a post-translational modification of SCPx were only seen in H4IIE cells. Further investigation revealed that the presence of insulin can mask lauric acid effects on the SCPx gene especially at the protein level. These data are in agreement with the findings that diabetes, a medical condition characterized by high levels of fatty acids in an insulin deficient environment, enhances the hepatic expression of SCPx.     

Sterol carrier protein-2

The compartmentalization of cholesterol metabolism implies target-specific cholesterol trafficking between the endoplasmic reticulum, plasma membrane, lysosomes, mitochondria and peroxisomes. One hypothesis has been that sterol carrier protein-2 (SCP2, also known as the non-specific lipid transfer protein) acts in cholesterol transport through the cytoplasm. Recent studies employing gene targeting in mice showed, however, that mice lacking SCP2 and the related putative sterol carrier known as SCPx, develop a defect in peroxisomal beta-oxidation. In addition, diminished peroxisomal alpha-oxidation of phytanic acid (3,7,11, 15-tetramethylhexadecanoic acid) in these null mice was attributed to the absence of SCP2 which has a number of properties supporting a function as carrier for fatty acyl-CoAs rather than for sterols.     

Peroxisomal fatty acid oxidation disorders and 58 kDa sterol carrier protein X (SCPx). Activity measurements in liver and fibroblasts using a newly developed method

Sterol carrier protein X (SCPx) plays a crucial role in the peroxisomal oxidation of branched-chain fatty acids. To investigate whether patients with an unresolved defect in peroxisomal beta-oxidation are deficient for SCPx, we developed a novel and specific assay to measure the activity of SCPx in both liver and fibroblast homogenates. The substrate used in the assay, 3alpha, 7alpha,12alpha-trihydroxy-24-keto-5beta-cholestanoy l-CoA (24-keto-THC-CoA), is produced by preincubating the enoyl-CoA of the bile acid intermediate THCA with a lysate from the yeast Saccharomyces cerevisiae expressing human D-bifunctional protein. After the preincubation period, liver or fibroblast homogenate is added plus CoASH, and the production of choloyl-CoA is determined by HPLC. The specificity of the assay was demonstrated by the finding of a full deficiency in fibroblasts from an SCPx knock-out mouse. In addition to SCPx activity measurements in fibroblasts from patients with a defect in peroxisomal beta-oxidation of unresolved etiology, we studied the stability and activity of SCPx in fibroblasts from patients with Zellweger syndrome, which lack functional peroxisomes. We found that SCPx is not only stable in the cytosol, but displays a higher activity in fibroblasts from patients with Zellweger syndrome than in control fibroblasts. Furthermore, in all patients studied with a defect in peroxisomal beta-oxidation of unknown origin, SCPx was found to be normally active, indicating that human SCPx deficiency remains to be identified.     

Type-II 3-oxoacyl-CoA thiolase of the nematode Caenorhabditis elegans is located in peroxisomes, highly expressed during larval stages and induced by clofibrate.

We examined the expression and localization of type-II 3-oxoacyl-CoA thiolase in the nematode Caenorhabditis elegans. Type-II thiolase acts on 3-oxoacyl-CoA esters with a methyl group at the alpha carbon, whereas conventional thiolases do not. Mammalian type-II thiolase, which is also termed sterol carrier protein x (SCPx) or SCP2/3-oxoacyl-CoA thiolase, is located in the peroxisomes and involved in phytanic acid degradation and most probably in bile acid synthesis. The nematode enzyme lacks the SCP2 domain, which carries the peroxisomal-targeting signal, but produces bile acids in a cell-free system. Northern and Western blot analyses demonstrated that C. elegans expressed type-II thiolase throughout its life cycle, especially during the larval stages, and that the expression was significantly enhanced by the addition of clofibrate at 5 mM or more to the culture medium. Whole-mount in situ hybridization and immunostaining of L4 larvae revealed that the enzyme was mainly expressed in intestinal cells, which are multifunctional like many of the cell types in C. elegans. Subcellular fractionation and indirect immunoelectron microscopy of the nematode detected the enzyme in the matrix of peroxisomes. These results indicate the fundamental homology between mammalian SCPx and the nematode enzyme regardless of whether the SCP2 part is fused, suggesting their common physiological roles.     

Uncoupled respiration, ROS production, acute lipotoxicity and oxidative damage in isolated skeletal muscle mitochondria from UCP3-ablated mice

The function of uncoupling protein 3 (UCP3) is still not established. Mitochondrial uncoupling, control of ROS production, protection against lipotoxicity and protection against oxidative stress are functions classically discussed. To establish a role for UCP3 in these functions, we have here used UCP3 (-/-) mice, backcrossed for 10 generations on a C57Bl/6 background. In isolated skeletal muscle mitochondria, we examined uncoupled respiration, both unstimulated and in the presence of fatty acids. We did not observe any difference between mitochondria from wildtype and UCP3 (-/-) mice. We measured H(2)O(2) production rate and respiration rate under reactive oxygen species-generating conditions (succinate without rotenone) but found no effect of UCP3. We tested two models of acute lipotoxicity-fatty acid-induced oxidative inhibition and fatty acid-induced swelling-but did not observe any protective effect of UCP3. We examined oxidative stress by quantifying 4-hydroxynonenal protein adducts and protein carbonyls in the mitochondria-but did not observe any protective effect of UCP3. We conclude that under the experimental conditions tested here, we find no evidence for the function of UCP3 being basal or induced uncoupling, regulation of ROS production, protection against acute lipotoxicity or protection against oxidative damage.     

Activation of the SCPx promoter in mouse adrenocortical Y1 cells

Sterol carrier protein X (SCPx) is a peroxisomal protein with both lipid transfer and thiolase activity. Treatment of mouse adrenal Y1 cells with cAMP for 24h caused a significant induction of SCPx mRNA levels. Reporter gene studies demonstrated that treatment with cAMP and SF-1 was able to activate the SCPx promoter. Sequence analysis revealed the presence of three putative steroidogenic factor-1 (SF-1) binding motifs (designated SFB1, SFB2, and SFB3) and one CRE. Only SFB1 and SFB3 were able to bind recombinant SF-1 protein in electrophoretic mobility shift assays. The CRE was able to form a DNA/protein complex in the presence of Y1 nuclear extracts. Mutational analysis studies demonstrated that SFB3 is required for full activation of the SCPx promoter by cAMP treatment. Regulation of the SCPx gene by SF-1 and cAMP is similar to the regulatory mechanisms observed for other steroidogenic genes.     

A yeast sterol carrier protein with fatty-acid and fatty-acyl-CoA binding activity

The 14-kDa sterol carrier protein 2 (SCP2) domain is present in Eukaria, Bacteria and Archaea, and has been implicated in the transport and metabolism of lipids. We report the cloning, expression, purification and physicochemical characterization of a SCP2 from the yeast Yarrowia lipolytica (YLSCP2). Analytical size-exclusion chromatography, circular dichroism and fluorescence spectra, indicate that recombinant YLSCP2 is a well-folded monomer. Thermal unfolding experiments show that SCP2 maximal stability is at pH 7.0-9.0. YLSCP2 binds cis-parinaric acid and palmitoyl-CoA with KD values of 81+/-40 nM and 73+/-33 nM, respectively, sustaining for the first time the binding of fatty acids and their CoA esters to a nonanimal SCP2. The role of yeast SCP2 and other lipid binding proteins in transport, storage and peroxisomal oxidation of fatty acids is discussed.     

Fatty Acid Transfer from Yarrowia lipolytica Sterol Carrier Protein 2 to Phospholipid Membranes

Sterol carrier protein 2 (SCP2) is an intracellular protein domain found in all forms of life. It was originally identified as a sterol transfer protein, but was recently shown to also bind phospholipids, fatty acids, and fatty-acyl-CoA with high affinity. Based on studies carried out in higher eukaryotes, it is believed that SCP2 targets its ligands to compartmentalized intracellular pools and participates in lipid traffic, signaling, and metabolism. However, the biological functions of SCP2 are incompletely characterized and may be different in microorganisms. Herein, we demonstrate the preferential localization of SCP2 of Yarrowia lipolytica (YLSCP2) in peroxisome-enriched fractions and examine the rate and mechanism of transfer of anthroyloxy fatty acid from YLSCP2 to a variety of phospholipid membranes using a fluorescence resonance energy transfer assay. The results show that fatty acids are transferred by a collision-mediated mechanism, and that negative charges on the membrane surface are important for establishing a “collisional complex”. Phospholipids, which are major constituents of peroxisome and mitochondria, induce special effects on the rates of transfer. In conclusion, YLSCP2 may function as a fatty acid transporter with some degree of specificity, and probably diverts fatty acids to the peroxisomal metabolism.     

Oxidative damage of retinal rod outer segment membranes and the role of vitamin E

Highly purified bovine rod outer segment membranes show loss of structural integrity under an air atmosphere. Obvious ultrastructural changes are preceded by increases in absorbance below 400 nm. These changes are inhibited by Ar or N₂ atmospheres and appear to be due primarily to oxidative damage to the polyunsaturated fatty acids of the membrane lipids. Loss of polyunsaturated fatty acids, formation of malonaldehyde and fluorescent products characteristic of lipid oxidation accompany the spectral alterations. The elevated ultraviolet absorbance can largely be removed from the membranes by gentle extraction of the lipids using phospholipase C and hexane without changing the visible absorbance of rhodopsin.We have found a large seasonal variation in the endogenous level of α-tocopherol (vitamin E) in the bovine rod outer segment preparations. For much of the year we find that the rod outer segment membranes contain higher levels of α-tocopherol than have been previously reported in biological membranes. Rod outer segments which are low in endogenous tocopherol can be protected from oxygen damage by adding exogenous tocopherol. The rod outer segments are extremely susceptible to oxygen damage due to the unusually high content of polyunsaturated fatty acids in the membrane lipids. The presence of tocopherol inhibits oxygen damage but does not eliminate it. The tocopherol in the rod outer segments is consumed in air, thus complete protection from peroxidation in vitro requires an inert atmosphere as well as high levels of tocopherol.This work suggests that extensive precautions against oxidative degradation should also be employed in studies of other membrane systems where important deleterious effects of oxygen may be less obvious.     

Mutations in the gene encoding peroxisomal sterol carrier protein X (SCPx) cause leukencephalopathy with dystonia and motor neuropathy

In this report, we describe the first known patient with a deficiency of sterol carrier protein X (SCPx), a peroxisomal enzyme with thiolase activity, which is required for the breakdown of branched-chain fatty acids. The patient presented with torticollis and dystonic head tremor as well as slight cerebellar signs with intention tremor, nystagmus, hyposmia, and azoospermia. Magnetic resonance imaging showed leukencephalopathy and involvement of the thalamus and pons. Metabolite analyses of plasma revealed an accumulation of the branched-chain fatty acid pristanic acid, and abnormal bile alcohol glucuronides were excreted in urine. In cultured skin fibroblasts, the thiolytic activity of SCPx was deficient, and no SCPx protein could be detected by western blotting. Mutation analysis revealed a homozygous 1-nucleotide insertion, 545_546insA, leading to a frameshift and premature stop codon (I184fsX7).     

Oleic acid promotes adaptability against oxidative stress in 3T3-L1 cells through lipohormesis

Although fatty acids are important components of biological membranes, energy sources, and signal transducers or precursors of lipid mediators, excess intake of fatty acids and their accumulation cause obesity and metabolic syndrome. Thus, fatty acid quantity is known to be an important factor for obesity-related diseases, but the effects of different types of fatty acids (i.e., fatty acid quality) on human health are not completely understood. We here focused on the relationship between fatty acid quality and oxidative stress by investigating whether resistibility to tert-butyl hydrperoxide (t-BuOOH)-induced oxidative stress in 3T3-L1 cells varied according to the fatty acid type. Among eight fatty acids (both saturated and unsaturated) tested, oleic acid (OA) exerted the most pronounced cytoprotective effects, with efficacy over a wide range of concentrations. OA treatment markedly enhanced the intracellular levels of lipid peroxidation markers, including N ^ε-(hexanoyl)lysine, 4-hydroxy-2-nonenal, and acrolein. The levels of these markers in OA-treated cells were decreased after t-BuOOH exposure, whereas the levels in untreated control cells were notably increased after t-BuOOH exposure. Our results suggested that unsaturated fatty acids, particularly OA, could promote an adaptive response and enhance cell tolerance through increased cellular antioxidative capacity via OA-induced mild lipid peroxidation (lipohormesis), and thus protect cells against subsequent oxidative stress-related injury.     

Site-activity relationship of nitroxide radical's antioxidative effect

A relatively new strategy in preventing oxidative damage employs cyclic nitroxides. These stable radicals have been widely used as biophysical probes, spin labels, and are currently tested as contrast agents for nuclear magnetic resonance imaging. Nitroxides were found to protect cells, organs, and whole animals against diverse oxidative insults. The present study concentrated on comparing the antioxidative activity of nitroxides against oxidative damage, initiated either in the lipid or aqueous phase, to egg phosphatidylcholine acyl chains (13.4% polyunsaturated fatty acids) in small unilamellar vesicles. We determined the lipophilicity and liposome-membrane/aqueous-medium partition coefficient for several nitroxides and compared their specific protective effects. The aim was to study the relation between nitroxides' concentration, location in the lipid bilayer, and their protection against oxidative damage. Both 6-membered- and 5-membered-ring nitroxides were studied for: (i) partitioning between the lipid bilayer and the aqueous phase (nitroxides were quantified using EPR spectroscopy); (ii) the intrabilayer distribution, using three different fluorescent probes of known location of their fluorophors in the lipid bilayer; and (iii) the specific antioxidative effect (protection per concentration) against radicals formed in a liposomal dispersion. The radicals were generated using the thermolabile, radical-generating compounds 2,2'-azobis (2-amidinopropane) dihydrochloride (AAPH) in the aqueous phase, and 2,2'-azobis (2,4-dimethyl-valeronitrile) (AMVN) in the lipid phase. The results show that nitroxides react, in a concentration-dependent manner, with deleterious species at their formation sites, both in the aqueous and the lipid phase, and that their specific protective effects for the lipophilic target, the lipid bilayer, are similar for both the lipophilic and the hydrophilic nitroxides.     

The crystal structure of sterol carrier protein 2 from Yarrowia lipolytica and the evolutionary conservation of a large, non-specific lipid-binding cavity

Sterol carrier protein 2 (SCP2), a small intracellular domain present in all forms of life, binds with high affinity a broad spectrum of lipids. Due to its involvement in the metabolism of long-chain fatty acids and cholesterol uptake, it has been the focus of intense research in mammals and insects; much less characterized are SCP2 from other eukaryotic cells and microorganisms. We report here the X-ray structure of Yarrowia lipolytica SCP2 (YLSCP2) at 2.2 Å resolution in complex with palmitic acid. This is the first fungal SCP2 structure solved, and it consists of the canonical five-stranded β-sheet covered on the internal face by a layer of five α-helices. The overall fold is conserved among the SCP2 family, however, YLSCP2 is most similar to the SCP2 domain of human MFE-2, a bifunctional enzyme acting on peroxisomal β-oxidation. We have identified the common structural elements defining the shape and volume of the large binding cavity in all species characterized. Moreover, we found that the cavity of the SCP2 domains is distinctly formed by carbon atoms, containing neither organized water nor rigid polar interactions with the ligand. These features are in contrast with those of fatty acid binding proteins, whose internal cavities are more polar and contain bound water. The results will help to design experiments to unveil the SCP2 function in very different cellular contexts and metabolic conditions.     

MFE1, a member of the peroxisomal hydroxyacyl coenzyme A dehydrogenase family,affects fatty acid metabolism necessary for morphogenesis in Dictyostelium spp

Beta-oxidation of long-chain fatty acids and branched-chain fatty acids is carried out in mammalian peroxisomes by a multifunctional enzyme (MFE) or D-bifunctional protein, with separate domains for hydroxyacyl coenzyme A (CoA) dehydrogenase, enoyl-CoA hydratase, and steroid carrier protein SCP2. We have found that Dictyostelium has a gene, mfeA, encoding MFE1 with homology to the hydroxyacyl-CoA dehydrogenase and SCP2 domains. A separate gene, mfeB, encodes MFE2 with homology to the enoyl-CoA hydratase domain. When grown on a diet of bacteria, Dictyostelium cells in which mfeA is disrupted accumulate excess cyclopropane fatty acids and are unable to develop beyond early aggregation. Axenically grown mutant cells, however, developed into normal fruiting bodies composed of spores and stalk cells. Comparative analysis of whole-cell lipid compositions revealed that bacterially grown mutant cells accumulated cyclopropane fatty acids that remained throughout the developmental stages. Such a persistent accumulation was not detected in wild-type cells or axenically grown mutant cells. Bacterial phosphatidylethanolamine that contains abundant cyclopropane fatty acids inhibited the development of even axenically grown mutant cells, while dipalmitoyl phosphatidylethanolamine did not. These results suggest that MFE1 protects the cells from the increase of the harmful xenobiotic fatty acids incorporated from their diets and optimizes cellular lipid composition for proper development. Hence, we propose that this enzyme plays an irreplaceable role in the survival strategy of Dictyostelium cells to form spores for their efficient dispersal in nature.