BBA - Molecular and Cell Biology of Lipids (v.1781, #11-12)

Phase diagrams of lipid mixtures relevant to the study of membrane rafts by Félix M. Goñi; Alicia Alonso; Luis A. Bagatolli; Rhoderick E. Brown; Derek Marsh; Manuel Prieto; Jenifer L. Thewalt (665-684).
The present paper reviews the phase properties of phosphatidylcholine–sphingomyelin–cholesterol mixtures, that are often used as models for membrane “raft” microdomains. The available data based on X-ray, microscopic and spectroscopic observations, surface pressure and calorimetric measurements, and detergent solubilization assays, are critically evaluated and rationalized in terms of triangular phase diagrams. The remaining uncertainties are discussed specifically and separately from the data on which a consensus appears to exist.
Keywords: Membrane rafts; Lipid phase diagrams; Phosphatidylcholine; Sphingomyelin; Cholesterol;

Protective effect of eicosapentaenoic acid on palmitate-induced apoptosis in neonatal cardiomyocytes by Christine Leroy; Sabine Tricot; Bernard Lacour; Alain Grynberg (685-693).
Long chain polyunsaturated fatty acids (PUFAs) play an important role in cardioprotection. These effects have been largely attributed to membrane docosahexaenoic acid. Conversely, saturated fatty acids trigger apoptosis in cardiomyocytes, with modifications of mitochondrial properties including cardiolipin loss, cytochrome c release and caspase-3 activation. The purpose of this study was to investigate the chronic effect of eicosapentaenoic acid (EPA) on mitochondrial apoptosis induced by palmitate treatment and the associated signalling pathways. Confluent cultures of rat neonatal cardiomyocytes were treated for 2 days in media enriched with either EPA or arachidonic acid (AA) and then exposed to palmitate (0.5 mM) to induce apoptosis, in the absence of PUFA supplements. The EPA treatment resulted in significant membrane enrichment in n  − 3 PUFAs, especially in docosapentaenoic acid (DPA), and a large decrease in AA. Both AA and EPA treatments prevented caspase-3 activation, translocation of Bax to the mitochondria and release of cytochrome c induced by palmitate treatment. Furthermore, EPA, but not AA prevented the loss of mitochondrial cardiolipin due to apoptosis. These results suggest that EPA supplementation is able to protect cardiomyocytes against palmitate-induced apoptosis via an implication of different mitochondrial elements, possibly through its elongation to DPA, which is very efficient in cardiomyocytes.
Keywords: Polyunsaturated fatty acid; Cardiomyocyte; Apoptosis; Palmitate; Cardiolipin; Mitochondria;

Coordinate induction of PPARα and SREBP2 in multifunctional protein 2 deficient mice by Katrin Martens; Emiel Ver Loren van Themaat; Marinus F. van Batenburg; Merja Heinäniemi; Steven Huyghe; Paul Van Hummelen; Carsten Carlberg; Paul P. Van Veldhoven; Antoine Van Kampen; Myriam Baes (694-702).
Mice with inactivation of the D-specific multifunctional protein 2 (MFP2), a crucial enzyme of peroxisomal β-oxidation, develop multiple pathologies in diverse tissues already starting in the postnatal period. Gene expression profiling performed on liver of 2-day-old pups revealed up-regulation of PPARα responsive genes in knockout mice. Surprisingly, also genes involved in cholesterol biosynthesis were markedly induced. Real-time PCR confirmed the induction of PPARα target genes and of HMGCR and SREBP2, both involved in cholesterol synthesis, in lactating and in adult MFP2 knockout mice. In accordance, the rate of cholesterol biosynthesis was significantly increased in liver of knockout mice but the hepatic cholesterol concentration was unaltered. In MFP2/PPARα double knockout mice, up-regulations of SREBP2 and HMGCR were markedly attenuated. These data demonstrate a tight interrelationship between induction of PPARα by endogenous ligands and up-regulation of genes of cholesterol biosynthesis through increased expression of SREBP2.
Keywords: PPARα; Cholesterol synthesis; SREBP2; Peroxisome; Multifunctional protein 2;

ApoA-I induced CD31 in bone marrow-derived vascular progenitor cells increases adhesion: Implications for vascular repair by Karthikeyan Mythreye; Lisa L. Satterwhite; W. Sean Davidson; Pascal J. Goldschmidt-Clermont (703-709).
Transgenic over expression of apolipoprotein A-I (ApoA-I) the major structural apolipoprotein of HDL appears to convey the most consistent and strongest anti atherogenic effect observed in animal models so far. We tested the hypothesis that ApoA-I mediates its cardio protective effects additionally through ApoA-I induced differentiation of bone marrow-derived progenitor cells in vitro. This study demonstrates that lineage negative bone marrow cells (lin BMCs) alter and differentiate in response to free ApoA-I. We find that lin BMCs in culture treated with recombinant free ApoA-I at a concentration of 0.4 μM are twice as large in size and have altered cell morphology compared to untreated cells; untreated cells retain the original spheroid morphology. Further, the total number of CD31 positive cells in the ApoA-I treated population consistently increased by two fold. This phenotype was significantly reduced in untreated cells and points towards a novel ApoA-I dependent differentiation. A protein lacking its best lipid-binding region (ApoA-IΔ10) did not stimulate any changes in the linBMCs indicating that ApoA-I may mediate its effects by regulating cholesterol efflux. The increased CD31 correlates with an increased ability of the lin BMCs to adhere to both fibronectin and mouse brain endothelial cells. Our results provide the first evidence that exogenous free ApoA-I has the capacity to change the characteristics of progenitor cell populations and suggests a novel mechanism by which HDL may mediate its cardiovascular benefits.
Keywords: ApoA-I; Bone marrow cells (BMCs); CD31; Lineage minus; Vascular progenitor cell; Adhesion;

Amino acid residues crucial in pH regulation and proteolytic activation of N-acylethanolamine-hydrolyzing acid amidase by Jun Wang; Li-Ying Zhao; Toru Uyama; Kazuhito Tsuboi; Takeharu Tonai; Natsuo Ueda (710-717).
N-Acylethanolamine-hydrolyzing acid amidase (NAAA) is a lysosomal enzyme which hydrolyzes bioactive N-acylethanolamines, including anandamide and N-palmitoylethanolamine. NAAA shows acidic pH optimum in terms of both catalytic activity and maturation by specific proteolysis. However, molecular mechanism involved in this characteristic pH dependency remained unclear. Here we report the important role of Glu-195 of human NAAA by analyzing the mutants E195A and E195Q overexpressed in human embryonic kidney 293 cells. Concanamycin A, raising lysosomal pH, inhibited maturation of the wild-type, but not of the Glu-195 mutants. The purified precursors of the mutants, but not the wild-type, were proteolytically cleaved at pH 7.4 during 24-h incubation. Furthermore, when assayed for N-palmitoylethanolamine-hydrolyzing activity at different pH, the mutants did not exhibit a sharp peak around pH 4.5 in the pH-dependent activity profile. Mutants of other seven glutamic acid residues did not show such an abnormality. These results suggested a unique role of Glu-195 in the pH-dependent activity and proteolytic maturation. Moreover, Arg-142, Asp-145, and Asn-287 as well as previously identified Cys-126 were shown to be essential for the proteolytic activation. Since these residues were predicted to be catalytically important, the results strongly suggested that the proteolysis occurs through an autocatalytic mechanism.
Keywords: Acidic pH optimum; Anandamide; Endocannabinoid; Lysosomal enzyme; Ntn-hydrolase family;

Sterol 27-hydroxylase (CYP27A1) is required for the hepatic conversion of cholesterol into bile acids and for production of 27-hydroxycholesterol which affects cholesterol homeostasis in several ways. Dexamethasone increases hepatic bile acid biosynthesis and CYP27A1-mediated enzyme activity in HepG2 cells. This study examines the mechanism of the dexamethasone-induced effect on the human CYP27A1 promoter. Dexamethasone treatment of HepG2 cells overexpressed with glucocorticoid receptor α (GRα) increased the CYP27A1 promoter activity more than four-fold as compared with untreated cells. The GR-antagonist mifepristone almost completely abolished the dexamethasone-induced effect on the promoter activity. Progressive deletion analysis of the CYP27A1 promoter indicated that sequences involved in GR-mediated induction by dexamethasone are present in a region between -1094 and -792. Several putative GRE sites could be found in this region and EMSA experiments revealed that two of these could bind GR. Site-directed mutagenesis of GR-binding sequences in the CYP27A1 promoter identified a GRE at -824/-819 important for GR-mediated regulation of the transcriptional activity. Endogenous and pharmacological glucocorticoids may have a strong impact on several aspects of cholesterol homeostasis and other processes related to CYP27A1-mediated metabolism. The glucocorticoid-mediated induction of human CYP27A1 transcription is of particular interest due to the anti-atherogenic properties ascribed to this enzyme.
Keywords: Human CYP27A1; 27-Hydroxylation; Dexamethasone; Glucocorticoid receptor; Atherosclerosis; Transcriptional regulation;

Previously, using an oxysterol to induce cholesterol trafficking to the Endoplasmic Reticulum (ER), we reported a dissociation between cholesterol transport to two important cholesterol regulatory components in the ER: the cholesterol esterifying enzyme ACAT (Acyl CoA:Cholesterol Acyltransferase) and the membrane-bound transcription factor SREBP (Sterol Regulatory Element Binding Protein) (X. Du, Y.H. Pham and A.J. Brown, Effects of 25-hydroxycholesterol on cholesterol esterification and SREBP processing are dissociable: implications for cholesterol movement to the regulatory pool in the endoplasmic reticulum, J. Biol Chem. 279 (2004) 47010–47016). Here, we employed low-density lipoprotein (LDL) as a more physiologically-relevant mode of cholesterol delivery, and compared cholesterol transport to ACAT (determined by esterification) and SREBP (assessed by processing) in mutant Chinese Hamster Ovary cells that have cholesterol-trafficking defects (including Niemann–Pick type C). We showed clear differences in kinetics between the two, with impaired cholesterol trafficking to SREBP being resolved more rapidly than to ACAT. This is unlikely to be due to a reduced threshold of cholesterol sensed by the SREBP system relative to ACAT, since both responded to LDL-derived cholesterol within 2 h whereas the divergence observed between the two was prolonged (> 20 h). Furthermore, ACAT inhibition did not expand the ER regulatory pool of cholesterol as judged by unaltered sensitivity of SREBP processing to LDL. Collectively, our data favor the contention that there are different cholesterol pools and/or transport pathways which feed ACAT and SREBP within the ER.
Keywords: SREBP; ACAT; Niemann–Pick type C; 2-2 cell; 3-6 cell; U18666A;

Phosphatidate phosphatases, PAPs, are key enzymes in lipid biosynthesis and signaling. Type I PAP enzymes participate in de-novo phospholipid biosynthesis, whereas type II PAP enzymes have an established role in lipid signaling. To identify novel human type II PAPs potentially involved in de-novo phospholipid synthesis we used bioinformatics to screen for enzymes with an active site exposed to the cytosolic side of membranes. Two related enzymes, a novel lipid phosphatase related protein (LPRP-A) and a presqualene diphosphate phosphatase (PA-PSP) met this criterion. PA-PSP and LPRP-A have differential tissue and subcellular distribution, and novel yet differential roles in lipid metabolism. Specifically, PA-PSP, but not LPRP-A, was a potent Mg2+-independent, NEM-insensitive type II PAP. Subcellular fractionation detection indicated that both proteins were associated with membranes, while immunofluorescent deconvolution imaging revealed that these membranes were exclusively from the nuclear envelope and the endoplasmic reticulum. PA-PSP overexpression, but not LPRP-A, accelerated the synthesis of phosphatidylcholine and caused accumulation of triacylglycerol with concomitant decrease in the rate of phosphatidylinositol synthesis. Coexpression of human CTP:phosphocholine cytidylyltransferase-α with PA-PSP enhanced the effect of PA-PSP on phosphatidylcholine levels, yet attenuated its effect on triacylglycerol. Taken together, our studies provide the first evidence that the eukaryotic, ER-resident PA-PSP is a bifunctional enzyme with specific type II PAP activity, and regulates, in addition to type I PAPs, the de-novo biosynthesis of phospholipids and triacylglycerols.
Keywords: Lipid metabolism; Phosphatidate phosphatase; Phosphatidic acid; Tricylglycerol; Bifunctional enzyme;