Modulation of human type II secretory phospholipase A2 by sphingomyelin and annexin VI

Impact of Truncated Oxidized Phosphatidylcholines on Phospholipase A2 Activity in Mono- and Polyunsaturated Biomimetic Vesicles

The interplay between inflammatory and redox processes is a ubiquitous and critical phenomenon in cell biology that involves numerous biological factors. Among them, secretory phospholipases A₂ (sPLA₂) that catalyze the hydrolysis of the sn-2 ester bond of phospholipids are key players. They can interact or be modulated by the presence of truncated oxidized phosphatidylcholines (OxPCs) produced under oxidative stress from phosphatidylcholine (PC) species. The present study examined this important, but rarely considered, sPLA₂ modulation induced by the changes in biophysical properties of PC vesicles comprising various OxPC ratios in mono- or poly-unsaturated PCs. Being the most physiologically active OxPCs, 1-palmitoyl-2-(5'-oxo-valeroyl)-sn-glycero-3-phosphocholine (POVPC) and 1-palmitoyl-2-glutaryl-sn-glycero-3-phosphocholine (PGPC) have been selected for our study. Using fluorescence spectroscopy methods, we compared the effect of OxPCs on the lipid order as well as sPLA₂ activity in large unilamellar vesicles (LUVs) made of the heteroacid PC, either monounsaturated [1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)], or polyunsaturated [1-palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (PDPC)] at a physiological temperature. The effect of OxPCs on vesicle size was also assessed in both the mono- and polyunsaturated PC matrices. Results: OxPCs decrease the membrane lipid order of POPC and PDPC mixtures with PGPC inducing a much larger decrease in comparison with POVPC, indicative that the difference takes place at the glycerol level. Compared with POPC, PDPC was able to inhibit sPLA₂ activity showing a protective effect of PDPC against enzyme hydrolysis. Furthermore, sPLA₂ activity on its PC substrates was modulated by the OxPC membrane content. POVPC down-regulated sPLA₂ activity, suggesting anti-inflammatory properties of this truncated oxidized lipid. Interestingly, PGPC had a dual and opposite effect, either inhibitory or enhancing on sPLA₂ activity, depending on the protocol of lipid mixing. This difference may result from the chemical properties of the shortened sn-2-acyl chain residues (aldehyde group for POVPC, and carboxyl for PGPC), being, respectively, zwitterionic or anionic under hydration at physiological conditions.

Novel Lipids to Regulate Obesity and Brain Function: Comparing Available Evidence and Insights from QSAR In Silico Models

Lipid molecules, such as policosanol, ergosterol, sphingomyelin, omega 3 rich phosphatidylcholine, α-tocopherol, and sodium butyrate, have emerged as novel additions to the portfolio of bioactive lipids. In this state-of-the-art review, we discuss these lipids, and their activity against obesity and mental or neurological disorders, with a focus on their proposed cellular targets and the ways in which they produce their beneficial effects. Furthermore, this available information is compared with that provided by in silico Absorption, Distribution, Metabolism, Excretion, and Toxicity (ADMET) models in order to understand the usefulness of these tools for the discovery of new bioactive compounds. Accordingly, it was possible to highlight how these lipids interact with various cellular targets related to the molecule transportation and absorption (e.g., α-tocopherol transfer protein for α-Tocopherol, ATP-binding cassette ABC transporters or Apolipoprotein E for sphingomyelins and phospholipids) or other processes, such as the regulation of gene expression (involving Sterol Regulatory Element-Binding Proteins for ergosterol or Peroxisome Proliferator-Activated Receptors in the case of policosanol) and inflammation (the regulation of interleukins by sodium butyrate). When comparing the literature with in silico Quantitative Structure-Activity Relationship (QSAR) models, it was observed that although they are useful for selecting bioactive molecules when compared in batch, the information they provide does not coincide when assessed individually. Our review highlights the importance of considering a broad range of lipids as potential bioactives and the need for accurate prediction of ADMET parameters in the discovery of new biomolecules. The information presented here provides a useful resource for researchers interested in developing new strategies for the treatment of obesity and mental or neurological disorders.

Fluorescent Lipo-Beads for the Sensitive Detection of Phospholipase A2 and Its Inhibitors

Phospholipase A₂ (PLA₂) is a membrane lytic enzyme that is present in many organisms. Human PLA₂ has emerged as a potential biomarker as well as a therapeutic target for several diseases including cancer, cardiovascular diseases, and some inflammatory diseases. The current study focuses on the development of lipo-beads that are very reactive and highly sensitive to PLA₂. To develop the best supported lipid bilayer formulation, several lipid combinations were investigated using 10 μm porous silica beads. The reactivity of PLA₂ was monitored via the decrease in particle fluorescence because of the release of entrapped fluorescent dye from the particle pores or the disintegration of a fluorescent lipid constituted on the bilayer upon lipid hydrolysis using flow cytometry. The enzyme binding studies indicate that lipo-beads with bulky fluorescent tags in the lipid head group and anionic lipids produce a more pronounced response. The kinetic studies suggest that these lipo-beads are very reactive with PLA₂ and can generate a detectable signal in less than 5 min. The enzyme inhibition studies were also conducted with two known PLA₂ inhibitors, varespladib and quercetin. We find that quercetin can hydrolyze the supported membrane, and thus inhibition of PLA₂ is not observed; however, varespladib has shown significant PLA₂ inhibition on lipo-beads. We have demonstrated that our lipo-bead-based approach can detect annexin-3, a known disease biomarker, as low as 10 nM within 5 min after incubation.

Sphingolipids and glycerophospholipids - The "ying and yang" of lipotoxicity in metabolic diseases

Sphingolipids in general and ceramides in particular, contribute to pathophysiological mechanisms by modifying signalling and metabolic pathways. Here, we present the available evidence for a bidirectional homeostatic crosstalk between sphingolipids and glycerophospholipids, whose dysregulation contributes to lipotoxicity induced metabolic stress. The initial evidence for this crosstalk originates from simulated models designed to investigate the biophysical properties of sphingolipids in plasma membrane representations. In this review, we reinterpret some of the original findings and conceptualise them as a sort of "ying/yang" interaction model of opposed/complementary forces, which is consistent with the current knowledge of lipid homeostasis and pathophysiology. We also propose that the dysregulation of the balance between sphingolipids and glycerophospholipids results in a lipotoxic insult relevant in the pathophysiology of common metabolic diseases, typically characterised by their increased ceramide/sphingosine pools.

Fecal metabolome of the Hadza hunter-gatherers: a host-microbiome integrative view

The recent characterization of the gut microbiome of traditional rural and foraging societies allowed us to appreciate the essential co-adaptive role of the microbiome in complementing our physiology, opening up significant questions on how the microbiota changes that have occurred in industrialized urban populations may have altered the microbiota-host co-metabolic network, contributing to the growing list of Western diseases. Here, we applied a targeted metabolomics approach to profile the fecal metabolome of the Hadza of Tanzania, one of the world's few remaining foraging populations, and compared them to the profiles of urban living Italians, as representative of people in the post-industrialized West. Data analysis shows that during the rainy season, when the diet is primarily plant-based, Hadza are characterized by a distinctive enrichment in hexoses, glycerophospholipids, sphingolipids, and acylcarnitines, while deplete in the most common natural amino acids and derivatives. Complementary to the documented unique metagenomic features of their gut microbiome, our findings on the Hadza metabolome lend support to the notion of an alternate microbiome configuration befitting of a nomadic forager lifestyle, which helps maintain metabolic homeostasis through an overall scarcity of inflammatory factors, which are instead highly represented in the Italian metabolome.

Phospholipase A2-Induced Remodeling Processes on Liquid-Ordered/Liquid-Disordered Membranes Containing Docosahexaenoic or Oleic Acid: A Comparison Study

Vesicle cycling, which is an important biological event, involves the interplay between membrane lipids and proteins, among which the enzyme phospholipase A2 (PLA2) plays a critical role. The capacity of PLA2 to trigger the budding and fission of liquid-ordered (L(o)) domains has been examined in palmitoyl-docosahexaenoylphosphatidylcholine (PDPC) and palmitoyl-oleoylphosphatidylcholine (POPC)/sphingomyelin/cholesterol membranes. They both exhibited a L(o)/liquid-disordered (L(d)) phase separation. We demonstrated that PLA2 was able to trigger budding in PDPC-containing vesicles but not POPC ones. The enzymatic activity, line tension, and elasticity of the membrane surrounding the L(o) domains are critical for budding. The higher line tension of Lo domains in PDPC mixtures was assigned to the greater difference in order parameters of the coexisting phases. The higher amount of lysophosphatidylcholine generated by PLA2 in the PDPC-containing mixtures led to a less-rigid membrane, compared to POPC. The more elastic L(d) membranes in PDPC mixtures exert a lower counteracting force against the L(o) domain bending.

Inhibition of endothelial lipase activity by sphingomyelin in the lipoproteins

Endothelial lipase (EL) is a major determinant of plasma HDL concentration, its activity being inversely proportional to HDL levels. Although it is known that it preferentially acts on HDL compared to LDL and VLDL, the basis for this specificity is not known. Here we tested the hypothesis that sphingomyelin, a major phospholipid in lipoproteins is a physiological inhibitor of EL, and that the preference of the enzyme for HDL may be due to low sphingomyelin/phosphatidylcholine (PtdCho) ratio in HDL, compared to other lipoproteins. Using recombinant human EL, we showed that sphingomyelin inhibits the hydrolysis of PtdCho in the liposomes in a concentration-dependent manner. While the enzyme showed lower hydrolysis of LDL PtdCho, compared to HDL PtdCho, this difference disappeared after the degradation of lipoprotein sphingomyelin by bacterial sphingomyelinase. Analysis of molecular species of PtdCho hydrolyzed by EL in the lipoproteins showed that the enzyme preferentially hydrolyzed PtdCho containing polyunsaturated fatty acids (PUFA) such as 22:6, 20:5, 20:4 at the sn-2 position, generating the corresponding PUFA-lyso PtdCho. This specificity for PUFA-PtdCho species was not observed after depletion of sphingomyelin by sphingomyelinase. These results show that sphingomyelin not only plays a role in regulating EL activity, but also influences its specificity towards PtdCho species.

Dietary sphingomyelin lowers hepatic lipid levels and inhibits intestinal cholesterol absorption in high-fat-fed mice

Controlling intestinal lipid absorption is an important strategy for maintaining lipid homeostasis. Accumulation of lipids in the liver is a major risk factor for metabolic syndrome and nonalcoholic fatty liver disease. It is well-known that sphingomyelin (SM) can inhibit intestinal cholesterol absorption. It is, however, unclear if dietary SM also lowers liver lipid levels. In the present study (i) the effect of pure dietary egg SM on hepatic lipid metabolism and intestinal cholesterol absorption was measured with [¹⁴C]cholesterol and [³H]sitostanol in male C57BL/6 mice fed a high-fat (HF) diet with or without 0.6% wt/wt SM for 18 days; and (ii) hepatic lipid levels and gene expression were determined in mice given a HF diet with or without egg SM (0.3, 0.6 or 1.2% wt/wt) for 4 weeks. Mice supplemented with SM (0.6% wt/wt) had significantly increased fecal lipid and cholesterol output and reduced hepatic [¹⁴C]cholesterol levels after 18 days. Relative to HF-fed mice, SM-supplemented HF-fed mice had significantly lower intestinal cholesterol absorption (−30%). Liver weight was significantly lower in the 1.2% wt/wt SM-supplemented mice (−18%). Total liver lipid (mg/organ) was significantly reduced in the SM-supplemented mice (−33% and −40% in 0.6% wt/wt and 1.2% wt/wt SM, respectively), as were triglyceride and cholesterol levels. The reduction in liver triglycerides was due to inactivation of the LXR-SREBP-1c pathway. In conclusion, dietary egg SM has pronounced hepatic lipid-lowering properties in mice maintained on an obesogenic diet.

A lytic mechanism based on soluble phospholypases A2 (sPLA2) and β-galactoside specific lectins is exerted by Ciona intestinalis (ascidian) unilocular refractile hemocytes against K562 cell line and mammalian erythrocytes

Hemocytes from the ascidian Ciona intestinalis exert in vitro Ca²+-dependent cytotoxic activity toward mammalian erythrocytes and K562 cells. To examine the lytic mechanism, hemocyte populations were separated (B1-B6 bands) through a Percoll discontinuous density gradient, the hemocyte cytotoxic activity (HCA) and the lytic activity of the hemocyte lysate supernatant (HLS) were assayed. In addition the separated hemocytes were cultured and the cell-free culture medium (CFM) assayed after 3 h culture. Results support that unilocular refractile hemocytes (URGs), enriched in B5, are cytotoxic. The B5-HLS contains lysins and the activity of B5-CFM shows that lysins can be released into a culture medium. The B5 activity was blocked by D-galactose, α-lactose, lactulose, LacNAc, thiodigalactoside (TDG), L-fucose, D-mannose, D-glucose, sphingomyelin (SM), and soluble phospholipase A2 (sPLA2) inhibitors (dibucain, quinacrine). Accordingly, HLS chemico-physical properties (alkaline medium, high thermostability, Ca²+-dependence, trypsin treatment, protease inhibitors) and SEM observations of the affected targets suggested that sPLA2 could be responsible for changes and large alterations of the target cell membrane. An apoptotic activity, as recorded by a caspase 3, 7 assay, was found by treating K562 cells with very diluted HLS. A lytic mechanism involving sPLA2 and lectins promptly released by URGs and morula cells respectively is suggested, whereas target cell membrane SM could be a modulator of the enzyme activity.

Regulation of phosphatidic Acid metabolism by sphingolipids in the central nervous system

This paper explores the way ceramide, sphingosine, ceramide 1-phosphate, and sphingosine 1-phosphate modulate the generation of second lipid messengers from phosphatidic acid in two experimental models of the central nervous system: in vertebrate rod outer segments prepared from dark-adapted retinas as well as in rod outer segments prepared from light-adapted retinas and in rat cerebral cortex synaptosomes under physiological aging conditions. Particular attention is paid to lipid phosphate phosphatase, diacylglycerol lipase, and monoacylglycerol lipase. Based on the findings reported in this paper, it can be concluded that proteins related to phototransduction phenomena are involved in the effects derived from sphingosine 1-phosphate/sphingosine or ceramide 1-phosphate/ceramide and that age-related changes occur in the metabolism of phosphatidic acid from cerebral cortex synaptosomes in the presence of either sphingosine 1-phosphate/sphingosine or ceramide 1-phosphate/ceramide. The present paper demonstrates, in two different models of central nervous system, how sphingolipids influence phosphatidic acid metabolism under different physiological conditions such as light and aging.

Sphingolipids and the orchestration of endothelium-derived vasoactive factors: when endothelial function demands greasing

Vasomotor tone is regulated by a complex interplay of a variety of extrinsic neurohumoral and intrinsic factors. It is the endothelium that has a major influence on smooth muscle cell tone via the release of intrinsic vasoactive factors and is therefore an important regulator of vasomotor tone. Sphingolipids are an emerging class of lipid mediators with important physiological properties. In the last two decades it has not only become increasingly clear that sphingolipid signaling plays a pivotal role in immune function, but also its role in the vascular system is now becoming more recognized. In this mini-review we will highlight the possible cross-talk between sphingolipids and intrinsic vasoactive factors released by the endothelium. Via this cross-talk sphingolipids can orchestrate vasomotor tone and may therefore also be involved in the pathophysiology of disease states associated with endothelial dysfunction.

Modulation of the activity of cytosolic phospholipase A2alpha (cPLA2alpha) by cellular sphingolipids and inhibition of cPLA2alpha by sphingomyelin

We examined the effect of the cellular sphingolipid level on the release of arachidonic acid (AA) and activity of cytosolic phospholipase A2alpha (cPLA2alpha) using two Chinese hamster ovary (CHO)-K1-derived mutants deficient in sphingolipid synthesis: LY-B cells defective in the LCB1 subunit of serine palmitoyltransferase for de novo synthesis of sphingolipid species, and LY-A cells defective in the ceramide transfer protein CERT for SM synthesis. When LY-B and LY-A cells were cultured in Nutridoma medium and the sphingolipid level was reduced, the release of AA stimulated by the Ca(2+) ionophore A23187 increased 2-fold and 1.7-fold, respectively, compared with that from control cells. The enhancement in LY-B cells was decreased by adding sphingosine and treatment with the cPLA2alpha inhibitor. When CHO cells were treated with an acid sphingomyelinase inhibitor to increase the cellular SM level, the release of AA induced by A23187 or PAF was decreased. In vitro studies were then conducted to test whether SM interacts directly with cPLA2alpha. Phosphatidylcholine vesicles containing SM reduced cPLA2alpha activity. Furthermore, SM disturbed the binding of cPLA2alpha to glycerophospholipids. These results suggest that SM at the biomembrane plays important roles in regulating the cPLA2alpha-dependent release of AA by inhibiting the binding of cPLA2alpha to glycerophospholipids.

The capacity of group V sPLA2 to increase atherogenicity of ApoE-/- and LDLR-/- mouse LDL in vitro predicts its atherogenic role in vivo

OBJECTIVE

In vitro data indicate that human LDL modified by Group V secretory phospholipase A(2) (GV sPLA(2)) is proatherogenic. Consistent with this, gain and loss of function studies demonstrated that GV sPLA(2) promotes atherosclerosis in LDLR(-/-) mice. The current study investigates whether GV sPLA(2) promotes atherosclerotic processes in apoE(-/-) mice.

METHODS AND RESULTS

LDL (d=1.019 to 1.063) from apoE(-/-) and LDLR(-/-) mice fed chow or Western diet were hydrolyzed by GV sPLA(2). Phosphatidylcholine on LDL from LDLR(-/-) mice fed either a chow or Western diet was hydrolyzed to a greater extent (61.1+/-0.4% and 45.3+/-4.6%) than the corresponding fractions from apoE(-/-) mice (41.7+/-3.6% and 39.4+/-1.2%). ApoE(-/-) LDL induced macrophage foam cell formation in vitro without modification by GV sPLA(2), whereas hydrolysis of LDLR(-/-) LDL was a prerequisite for foam cell formation. In contrast to findings in LDLR(-/-) mice, GV sPLA(2) deficiency did not significantly reduce atherosclerosis in apoE(-/-) mice, although collagen content was significantly reduced in lesions of apoE(-/-) mice lacking GV sPLA(2).

CONCLUSIONS

The ability of GV sPLA(2) to promote atherosclerotic lipid deposition in apoE(-/-) and LDLR(-/-) mice may be related to its ability to increase the atherogenic potential of LDL from these mice as assessed in vitro.

Effect of double bond geometry in sphingosine base on the antioxidant function of sphingomyelin

We previously showed that sphingomyelin (SM) inhibits peroxidation of phosphatidylcholine (PC) and cholesterol. Since SM uniquely has a trans unsaturation in its sphingosine base, we investigated whether this feature is important for its antioxidant function. Substitution of the natural trans Delta(4)-double bond with a cis double bond (cis-SM), however, increased SM's ability to inhibit Cu(2+)-mediated 16:0-18:2 PC oxidation by up to eightfold. Dihydro-SM, which lacks the double bond, was equally effective as trans-SM. In contrast to its effect in the sphingosine base, the presence of a cis double bond in the N-acyl group of trans-SM was not protective. cis-SM also inhibited the oxidation of cholesterol by FeSO_(4)/ascorbate more efficiently than the trans isomer. The enhanced protective effect of cis-SM is selective for metal ion-promoted oxidation, and appears to arise from a decrease in the effective concentration of metal ions. These studies show that the trans double bond of SM is not essential for its antioxidant effects.

Relationship between membrane physical properties and secretory phospholipase A2 hydrolysis kinetics in S49 cells during ionophore-induced apoptosis

During apoptosis, changes occur in lymphocyte membranes that render them susceptible to hydrolysis by secretory phospholipase A(2) (sPLA(2)). To study the relevant mechanisms, a simplified model of apoptosis using a calcium ionophore was applied. Kinetic and flow cytometry experiments provided key observations regarding ionophore treatment: the initial rate of hydrolysis was elevated at all enzyme concentrations, the total amount of reaction product was increased fourfold, and adsorption of the enzyme to the membrane surface was unaltered. Analysis of these results suggested that susceptibility during calcium-induced apoptosis is limited by availability of substrate rather than adsorption of enzyme. Fluorescence experiments identified three membrane alterations during apoptosis that might affect substrate access to the sPLA(2) active site. First, intercalation of merocyanine 540 into the membrane was improved, suggesting an increase in lipid spacing. Second, laurdan detected increased solvation of the lower headgroup region of the membrane. Third, the rate at which fluorescent lipids could be removed from the membrane by albumin was enhanced, implying greater vertical mobility of phospholipids. Thus, it is proposed that the membranes of apoptotic cells become susceptible to sPLA(2) through a reduction in lipid-neighbor interactions that facilitates migration of phospholipids into the enzyme active site.

A new hat for an old enzyme: Waste management

The history of research regarding secretory phospholipase A(2) (sPLA(2)) has often focused in one of two directions. Originally, the enzyme was studied biophysically in terms of its fundamental structure, enzymology, and the relationship between membrane physics and catalytic activity. More recently, a large and growing body of information has accumulated concerning regulatory factors, tissue distribution, and physiological/pathological roles of sPLA(2). Evidence is presented that suggests an additional function for the protein in which it helps to clear dead and damaged cells while avoiding digestion of those that are healthy. Apparently, the ability of the enzyme to discriminate between susceptible and resistant cells depends on physical properties of membrane lipids related to order, distribution, and neighbor/neighbor interactions. Investigations into this action of the enzyme offer the rare opportunity to apply biophysical approaches and principles to a physiological setting.

Interplay between lipoproteins and bee venom phospholipase A2 in relation to their anti-plasmodium toxicity

We previously showed that the in vitro intraerythrocytic development of the malarial agent Plasmodium falciparum is strongly inhibited by secreted phospholipases A(2) (sPLA(2)s) from animal venoms. Inhibition is dependent on enzymatic activity and requires the presence of serum lipoproteins in the parasite culture medium. To evaluate the potential involvement of host lipoproteins and sPLA(2)s in malaria, we investigated the interactions between bee venom phospholipase A(2) (bvPLA(2)), human triglyceride-rich lipoproteins, and infected erythrocytes. Even at high enzyme concentration (100x IC(50)), bvPLA(2) binding to Plasmodium-infected or normal erythrocytes was not detected. On the contrary, tight association with lipoproteins was observed through the formation of buoyant bvPLA(2)/lipoprotein complexes. Direct involvement of the hydrolysis lipid products in toxicity was demonstrated. Arachidonic acid (C20:4), linoleic acid (C18:2), and, to a lesser extent, docosahexaenoic acid (C22:6) appeared as the main actors in toxicity. Minimal oxidation of lipoproteins enhanced toxicity of the lipolyzed particles and induced their interaction with infected or normal erythrocytes. Fresh or oxidized lipolyzed lipoproteins induced the parasite degeneration without host cell membrane disruption, ruling out a possible membranolytic action of fatty acids or peroxidation products in the death process. In conclusion, our data enlighten on the capability of secreted PLA(2)s to exert cytotoxicity via the extracellular generation of toxic lipids, and raise the question of whether such mechanisms could be at play in pathophysiological situations such as malaria.

Dietary ganglioside decreases cholesterol content, caveolin expression and inflammatory mediators in rat intestinal microdomains

Membrane microdomains rich in cholesterol and sphingolipids, including gangliosides (GGs), are known to be important regions for cell signaling and binding sites for various pathogens. Cholesterol depletion inhibits the cellular entry of pathogens and also reduces inflammatory signals by disrupting microdomain structure. Our previous study showed that dietary gangliosides increased total ganglioside incorporation while decreasing cholesterol in the intestinal mucosa. We hypothesized that diet-induced reduction in cholesterol content in the intestinal mucosa disrupts microdomain structure resulting in reduced pro-inflammatory signals. Male weanling Sprague-Dawley rats were fed semipurified diets for 2 weeks. Experimental diets were formulated to include either ganglioside-enriched lipid (GG diet, 0.02% gangliosides [w/w of diet] ) or polyunsaturated fatty acid (PUFA diet, 1% arachidonic acid and 0.5% docosahexaenoic acid, w/w of total fat), in a control diet containing 20% fat. Levels of cholesterol, GG, caveolin, platelet activating factor (PAF), and diglyceride (DG) were measured in the microdomain isolated from the intestinal brush border. The GG diet increased total gangliosides by 50% with a relative increase in GD3 and a relative decrease in GM3. Cholesterol content was also reduced by 23% in the intestinal microdomain. These changes resulted in a significant decrease in the ratio of cholesterol to ganglioside. The GG diet and the PUFA diet were both associated with reduction in caveolin, PAF, and DG content in microdomains, whereas no change occurred in the ganglioside profile of animals fed the PUFA diet. Dietary gangliosides decrease the cholesterol/ganglioside ratio, caveolin, PAF and DG content in microdomains thus exerting a potential anti-inflammatory effect during gut development.

Milk sphingomyelin is more effective than egg sphingomyelin in inhibiting intestinal absorption of cholesterol and fat in rats

We reported previously that egg sphingomyelin (SM) inhibits the intestinal absorption of cholesterol and fat in rats. This study was conducted to compare the relative efficiencies of milk and egg SM in inhibiting intestinal absorption of cholesterol and other lipids. Adult male rats with lymph cannulae were infused at 3.0 mL/h for 8 h via a duodenal catheter with a lipid emulsion (451.7 micromol triolein, 20.7 micromol cholesterol, 33.3 kBq (14)C-cholesterol, 3.1 micromol alpha-tocopherol, and 396.0 micromol sodium taurocholate in 24 mL PBS, pH, 6.5), without SM (controls), or with 80.0 micromol egg SM or milk SM. The lymphatic absorptions of (14)C-cholesterol were significantly lower in rats infused with milk SM (19.5 +/- 1.4% dose) and egg SM (24.4 +/- 1.9% dose) than in those infused with no SM (37.6 +/- 1.8% dose). In addition, the lymphatic outputs of fatty acids and phospholipid were significantly lowered by milk and egg SM. Similarly, the absorption of alpha-tocopherol also was decreased by milk SM (13.6 +/- 1.7% dose) and egg SM (18.3 +/- 2.4% dose) compared with controls (27.0 +/- 1.8% dose). Total lymphatic SM output was not affected by egg SM, but markedly decreased by milk SM, relative to controls. The results indicate that both milk and egg SM markedly inhibit the absorption of cholesterol, fat, and other lipids. However, milk SM is a more potent inhibitor than egg SM. The strong inhibitory effect of milk SM may be associated with the higher degree of saturation and longer chain length of its fatty acyl groups, which may slow the rate of luminal lipolysis, micellar solubilization, and transfer of micellar lipids to the enterocyte.

Egg sphingomyelin lowers the lymphatic absorption of cholesterol and alpha-tocopherol in rats

Evidence indicates that phosphatidylcholine (PC) inhibits the intestinal absorption of cholesterol (CH) in rats. This study was designed to determine whether sphingomyelin (SM), structurally similar to PC, also inhibits the lymphatic absorption of CH. Sprague-Dawley rats with lymph cannulae were infused at 3.0 mL/h for 8 h via a duodenal catheter with a lipid emulsion [33.3 kBq 14C-CH, 20.7 micromol CH, 451.7 micromol triolein, 3.1 micromol alpha-tocopherol (alphaTP), 75.4 nmol retinol and 396.0 micromol sodium taurocholate in 24 mL of PBS (pH, 6.5)], without egg SM (SM0) as control, or with 5.0 micromol/h (SM5) or 10.0 micromol/h (SM10). Egg SM lowered the lymphatic absorption of 14C-CH in a dose dependent manner. Likewise, SM lowered the lymphatic absorptions of alphaTP and fatty acid (oleic acid), whereas it had no effect on retinol absorption. SM at a high dose (SM10) lowered the lymphatic outputs of both PC and SM, whereas there was no such effect at a lower dose (SM5). These results indicate that luminal egg SM has an inhibitory effect on the intestinal absorption of CH and other lipids of relatively high hydrophobicity. Our findings suggest that SM, if ingested in sufficient amounts, may inhibit the intestinal absorption of dietary lipids including cholesterol and alphaTP.

Membrane Lipid Homeostasis

The lipid matrix of biological membranes is composed of a complex mixture of polar lipids. It has been estimated that more than 600 distinct molecular species of lipid are constituents of biological membranes. This rather remarkable feature raises the questions of why such complexity is required when barrier properties and many protein functions can be reconstituted with relatively simple lipid systems. Secondly, the molecular species composition of morphologically distinct membranes appears to be preserved within fairly narrow limits. The biochemical mechanism(s) responsible for this homeostasis are not fully understood. This review examines the origin of membrane lipid complexity, the methods that are currently employed to measure and detect lipid molecular species and the biochemical reactions associated with the turnover of membrane lipids in resting and stimulated cells.

Phospholipase A(2) isoforms: a perspective

Several new PLA(2)s have been identified based on their nucleotide gene sequences. They were classified mainly into three groups: cytosolic PLA(2) (cPLA(2)), secretary PLA(2) (sPLA(2)), and intracellular PLA(2) (iPLA(2)). They differ from each other in terms of substrate specificity, Ca(2+) requirement and lipid modification. The questions that still remain to be addressed are the subcellular localization and differential regulation of the isoforms in various cell types and under different physiological conditions. It is required to identify the downstream events that occur upon PLA(2) activation, particularly target protein or metabolic pathway for liberated arachidonic acid or other fatty acids. Understanding the same will greatly help in the development of potent and specific pharmacological modulators that can be used for basic research and clinical applications. The information of the human and other genomes of PLA(2)s, combined with the use of proteomics and genetically manipulated mouse models of different diseases, will illuminate us about the specific and potentially overlapping roles of individual phospholipases as mediators of physiological and pathological processes. Hopefully, such understanding will enable the development of specific agents aimed at decreasing the potential contribution of individual secretary phospholipases to vascular diseases. The signaling cascades involved in the activation of cPLA(2) by mitogen activated protein kinases (MAPKs) is now evident. It has been demonstrated that p44 MAPK phosphorylates cPLA(2) and increases its activity in cells and tissues. The phosphorylation of cPLA(2) at ser505 occurs before the increase in intracellular Ca(2+) that facilitate the binding of the lipid binding domain of cPLA(2) to phospholipids, promoting its translocation to cellular membranes and AA release. Recently, a negative feed back loop for cPLA(2) activation by MAPK has been proposed. If PLA(2) activation in a given model depends on PKC, PKA, cAMP, or MAPK then inhibition of these phosphorylating enzymes may alter activities of PLA(2) isoforms during cellular injury. Understanding the signaling pathways involved in the activation/deactivation of PLA(2) during cellular injury will point to key events that can be used to prevent the cellular injury. Furthermore, to date, there is limited information available regarding the regulation of iPLA(2) or sPLA(2) by these pathways.

On the Binding Preference of Human Groups IIA and X Phospholipases A2 for Membranes with Anionic Phospholipids

Mammals contain 9-10 secreted phospholipases A(2) (sPLA(2)s) that display widely different affinities for membranes, depending on the phospholipid composition. The much higher enzymatic activity of human group X sPLA(2) (hGX) compared with human group IIA sPLA(2) (hGIIA) on phosphatidylcholine (PC)-rich vesicles is due in large part to the higher affinity of the former enzyme for such vesicles; this result also holds when vesicles contain cholesterol and sphingomyelin. The inclusion of anionic phosphatidylserine in PC vesicles dramatically enhances interfacial binding and catalysis of hGIIA but not of hGX. This is the result of the large number of lysine and arginine residues scattered over the entire surface of hGIIA, which cause the enzyme to form a supramolecular aggregate with multiple vesicles. Thus, high affinity binding of hGIIA to anionic vesicles is a complex process and cannot be attributed to a few basic residues on its interfacial binding surface, as is also evident from mutagenesis studies. The main reason hGIIA binds poorly to PC-rich vesicles is that it lacks a tryptophan residue on its interfacial binding surface, a residue that contributes to the high affinity binding of hGX to PC-rich vesicles. Results show that the lag in the onset of hydrolysis of PC vesicles by hGIIA is due in part to the poor affinity of this enzyme for these vesicles. Binding affinity of hGIIA, hGX, and their mutants to PC-rich vesicles is well correlated to the ability of these enzymes to act on the PC-rich outer plasma membrane of mammalian cells.

Secretory Phospholipase A2 Inhibits Epidermal Growth Factor-Induced Receptor Activation

Secretory phospholipase A(2) (sPLA(2)) plays important roles in mediating various cellular processes, including cell proliferation, differentiation, apoptosis, and inflammatory response. In this study, we demonstrated that a basic sPLA(2) inhibits epidermal growth factor (EGF)-induced EGF receptor activation, as determined by autophosphorylation of EGF receptor, EGF-activated phospholipase D (PLD) activity, and phospholipase C-gamma(1) (PLC-gamma(1)) tyrosine phosphorylation in a human epidermoid carcinoma cell line, A-431. Treatment of cells with exogenous neutral sphingomyelinase (SMase) or a cell permeable ceramide analog, C(2)-ceramide, also caused similar inhibitory effects on EGF-induced activation of EGF receptor, tyrosine phosphorylation of PLC-gamma(1), and the activation of PLD. sPLA(2)-induced inhibition of EGF receptor was associated with arachidonic acid release, which was followed by an increase in intracellular ceramide formation. Both sPLA(2) and exogenous C(2)-ceramide are able to inhibit the proliferation of A-431. The data presented indicate for the first time that sPLA(2) downregulates the EGF receptor-mediated intracellular signal transduction that may be mediated by arachidonic acid and/or ceramide.

Phospholipase A2 enzymes

Phospholipase A2 (PLA2) catalyzes the hydrolysis of the sn-2 position of membrane glycerophospholipids to liberate arachidonic acid (AA), a precursor of eicosanoids including prostaglandins and leukotrienes. The same reaction also produces lysophosholipids, which represent another class of lipid mediators. So far, at least 19 enzymes that possess PLA2 activity have been identified and cloned in mammals. The secretory PLA2 (sPLA2) family, in which 10 isozymes have been identified, consists of low-molecular weight, Ca2+-requiring secretory enzymes that have been implicated in a number of biological processes, such as modification of eicosanoid generation, inflammation, and host defense. The cytosolic PLA2 (cPLA2) family consists of three enzymes, among which cPLA2alpha has been paid much attention by researchers as an essential component of the initiation of AA metabolism. The activation of cPLA2alpha is tightly regulated by Ca2+ and phosphorylation. The Ca2+-independent PLA2 (iPLA2) family contains two enzymes and may play a major role in phospholipid remodeling. The platelet-activating factor (PAF) acetylhydrolase (PAF-AH) family contains four enzymes that exhibit unique substrate specificity toward PAF and/or oxidized phospholipids. Degradation of these bioactive phospholipids by PAF-AHs may lead to the termination of inflammatory reaction and atherosclerosis.

Ceramides increase the activity of the secretory phospholipase A2 and alter its fatty acid specificity

Modulation of human recombinant secretory type II phospholipase A(2) activity by ceramide and cholesterol was investigated using model glycerophospholipid substrates composed of phosphatidylethanolamine and phosphatidylserine dispersed in aqueous medium. Enzyme activity was monitored by measurement of released fatty acids using capillary GC-MS. Fatty acids from the sn-2 position of the phospholipids were hydrolysed by the enzyme in proportion to the relative abundance of the phospholipid in the substrate. Addition of increasing amounts of ceramide to the substrate progressively enhanced phospholipase activity. The increased activity was accomplished largely by preferential hydrolysis of polyunsaturated fatty acids, particularly arachidonic acid, derived from phosphatidylethanolamine. The addition of sphingomyelin to the substrate glycerophospholipids inhibited phospholipase activity but its progressive substitution by ceramide, so as to mimic sphingomyelinase activity, counteracted the inhibition. The presence of cholesterol in dispersions of glycerophospholipid-substrate-containing ceramides suppressed activation of the enzyme resulting from the presence of ceramide. The molecular basis of enzyme modulation was investigated by analysis of the phase structure of the dispersed lipid substrate during temperature scans from 46 to 20 degrees C using small-angle synchrotron X-ray diffraction. These studies indicated that intermediate structures created after ceramide-dependent phase separation of hexagonal and lamellar phases represent the most susceptible form of the substrate for enzyme hydrolysis.

Role of group IIa and group V secretory phospholipases A(2) in the metabolism of lipoproteins. Substrate specificities of the enzymes and the regulation of their activities by sphingomyelin

Although many isoforms of secretory phospholipases A(2) (sPLA(2)) are known to be secreted by various inflammatory cells, and are present in plasma, their role in lipoprotein metabolism is unknown. We studied the in vitro hydrolysis of lipoprotein phospholipids by group IIa and group V sPLA(2), two structurally related enzymes with differing phospholipid specificities. The group V sPLA(2) was about 30 times more efficient than the group IIa enzyme in the hydrolysis of lipoprotein phosphatidylcholine (PC), and both enzymes were more active on high density liporotein (HDL) than on low density lipoprotein (LDL). The lower activity on LDL appears to be due to the higher sphingomyelin (SPH) concentration in this lipoprotein. PC hydrolysis in lipoproteins was stimulated significantly by enzymatic depletion of their SPH. The hydrolysis of PC in liposomes was inhibited by the incorporation of SPH, and this inhibition was reversed by treatment with sphingomyelinase. The incorporation of ceramide, on the other hand, stimulated the sPLA(2) activity significantly. Unlike most sPLA(2), which show no fatty acid preference, group V sPLA(2) released disproportionately more linoleate, and less arachidonate from lipoproteins. These studies show that group V sPLA(2) is physiologically more important than group IIa enzyme in lipoprotein metabolism, that the sPLA(2) activities are regulated by sphingomyelin and ceramide, and that the pathological effects of sPLA(2) may not be mediated through stimulation of eicosanoid synthesis.

Secretory phospholipase A(2) induces apoptosis via a mechanism involving ceramide generation

Secretory phospholipase A(2) (sPLA(2)) plays important roles in cellular signaling and various biological events. In this study, we examined the biological effects and the potential signaling mechanism of purified sPLA(2) in MV1Lu cells. Three types of snake venom sPLA(2) were purified and their enzymatic activities were characterized by using various lipid substrates prepared from [3H]-myristate-labeled cells and by determining their effects on the induction of arachidonic acid (AA) release. The purified sPLA(2) induced apoptosis in Mv1Lu cells in a dose- and time-dependent manner, and was associated with a rapid increase in the intracellular ceramide level. Similar apoptotic effects were observed in Mv1Lu cells treated with exogenous ceramide analog, C(2)- and C(8)-ceramide. Moreover, treatment of cells with sphingomyelinase (SMase), which reduced the intracellular SM level, enhanced the apoptotic response to sPLA(2)s. sPLA(2)s also displayed an inhibitory effect on bradykinin-induced phospholipase D (PLD) activity, which can be imitated by exogenous ceramide. Our data indicate that sPLA(2) induces cell apoptosis via a mechanism involving increased ceramide generation.

Ceramides increase the activity of the secretory phospholipase A2 and alter its fatty acid specificity

Modulation of human recombinant secretory type II phospholipase A(2) activity by ceramide and cholesterol was investigated using model glycerophospholipid substrates composed of phosphatidylethanolamine and phosphatidylserine dispersed in aqueous medium. Enzyme activity was monitored by measurement of released fatty acids using capillary GC-MS. Fatty acids from the sn-2 position of the phospholipids were hydrolysed by the enzyme in proportion to the relative abundance of the phospholipid in the substrate. Addition of increasing amounts of ceramide to the substrate progressively enhanced phospholipase activity. The increased activity was accomplished largely by preferential hydrolysis of polyunsaturated fatty acids, particularly arachidonic acid, derived from phosphatidylethanolamine. The addition of sphingomyelin to the substrate glycerophospholipids inhibited phospholipase activity but its progressive substitution by ceramide, so as to mimic sphingomyelinase activity, counteracted the inhibition. The presence of cholesterol in dispersions of glycerophospholipid-substrate-containing ceramides suppressed activation of the enzyme resulting from the presence of ceramide. The molecular basis of enzyme modulation was investigated by analysis of the phase structure of the dispersed lipid substrate during temperature scans from 46 to 20 degrees C using small-angle synchrotron X-ray diffraction. These studies indicated that intermediate structures created after ceramide-dependent phase separation of hexagonal and lamellar phases represent the most susceptible form of the substrate for enzyme hydrolysis.

Sphingomyelin: a natural modulator of membrane homeostasis and inflammation

Although membrane sphingomyelin (SPH) serves as the precursor for many signaling molecules, its presence in large amounts, and its specific localization in the outer monolayer of the plasma membrane suggest that it may have a cytoprotective function. We propose that SPH helps maintain the integrity of the plasma membrane by protecting phosphatidylcholine (PC) against oxidative damage and phospholipase degradation. Since it contains mostly saturated longchain hydrocarbon groups, we postulate that SPH impedes the lateral propagation of the lipid peroxides by decreasing membrane fluidity, while also acting as an 'insulating' molecule. By virtue of its structural similarity to PC, it acts as a competitive inhibitor of the phospholipases, which may otherwise hydrolyze PC excessively. Because phospholipase reaction is the rate-limiting step in eicosanoid synthesis, SPH may serve as an endogenous anti-inflammatory molecule.

Effect of a moderately energy-restricted diet on obese patients with fatty liver

The effects of a moderately energy-restricted (25 kcal/kg) diet on liver-function tests, anthropometric measurements, mononuclear-cell phospholipid fatty acid, lymphocyte blastogenesis, and plasma prostaglandin E2 and alpha-tocopherol levels were observed at weeks 0, 8, and 24 in 14 obese patients with fatty liver. Serum aminotransferase levels were improved significantly, with decreases in the body mass index and waist circumference. Decreases in energy intake from carbohydrate and increases in intake of vitamin A, vitamin C, and vegetables were observed at week 24. In mononuclear-cell phospholipids, linoleic acid (18:2omega 6), which was significantly lower in patients than in controls at week 0, was increased at week 24. In contrast, arachidonic acid was decreased. Plasma prostaglandin E2 levels were significantly lower in patients than in controls at week 0 and increased at week 24. The mononuclear-cell response for phytohemagglutinin correlated with 18:2omega 6 in mononuclear-cell phospholipids (r = 0.692, P < 0.01). Improvement of the serum alanine-aminotransferase level correlated with an increase in the plasma alpha-tocopherol level (r = -0.667, P < 0.01) and increases in consumption of vitamin A, omega 3 polyunsaturated fatty acids, and vegetables. These findings suggest that a hypoenergetic diet rich in omega 3 polyunsaturated fatty acids and antioxidants might be beneficial for obese patients with fatty liver.

Interleukin-1beta-induced type IIA secreted phospholipase A2 gene expression and extracellular activity in rat vascular endothelial cells

Two phospholipase A2 (PLA2) isoforms, secretory and cytosolic, have been implicated in inflammation. Secretory type IIA PLA2 (sPLA2-IIA), which hydrolyzes fatty acids bound at the sn-2 position of glycerophospholipids, has been detected universally in a variety of mammalian tissues and cells. The expression of the sPLA2-IIA gene and its extracellular activity were shown to be regulated by different factors such as hypoxia, cytokines and phorbol esters. In the present study, we examined the effects of interleukin-1beta (IL-1beta) on the expression of the 14kDa sPLA2-IIA, determined using reverse transcription polymerase chain reaction and radiometric Escherichia coli enzyme assay in primary cultures of rat endothelial cells and in two different rat endothelial cell lines (SVAREC and RBE4). These experiments revealed that IL-1beta induces sPLA2-IIa gene expression and secretion of the enzyme in endothelial cells in a dose- and time-dependent manner. The cAMP-elevator forskolin did not augment the cytokine-induced elevation of sPLA2-IIa enzyme activity but significantly increased the IL-1beta-stimulated sPLA2-IIa mRNA contents in endothelial cells.

Cholesterol favors phase separation of sphingomyelin

The phase behavior of mixed lipid dispersions representing the inner leaflet of the cell membrane has been characterized by X-ray diffraction. Aqueous dispersions of phosphatidylethanolamine:phosphatidylserine (4:1 mole/mole) have a heterogeneous structure comprising an inverted hexagonal phase H(II) and a lamellar phase. Both phases coexist in the temperature range 20-45 degrees C. The fluid-to-gel mid-transition temperature of the lamellar phase assigned to phosphatidylserine is decreased from 27 to 24 degrees C in the presence of calcium. Addition of sphingomyelin to phosphatidylethanolamine/phosphatidylserine prevents phase separation of the hexagonal H(II) phase of phosphatidylethanolamine but the ternary mixture phase separates into two lamellar phases of periodcity 6.2 and 5.6 nm, respectively. The 6.2-nm periodicity is assigned to the gel phase enriched in sphingomyelin of molecular species comprising predominantly long saturated hydrocarbon chains because it undergoes a gel-to-fluid phase transition above 40 degrees C. The coexisting fluid phase we assign to phosphatidylethanolamine and phosphatidylserine and low melting point molecular species of sphingomyelin which suppresses the tendency of phosphatidylethanolamine to phase-separate into hexagonal H(II) structure. There is evidence for considerable hysteresis in the separation of lamellar fluid and gel phases during cooling. The addition of cholesterol prevents phase separation of the gel phase of high melting point sphingomyelin in mixtures with phosphatidylserine and phosphatidylethanolamine. In the quaternary mixture the lamellar fluid phase, however, is phase separated into two lamellar phases of periodicities of 6.3 and 5.6 nm (20 degrees C), respectively. The lamellar phase of periodicity 5.6 nm is assigned to a phase enriched in aminoglycerophospholipids and the periodicity 6.3 nm to a liquid-ordered phase formed from cholesterol and high melting point molecular species of sphingomyelin characterized previously by ESR. Substituting 7-dehydrocholesterol for cholesterol did not result in evidence for lamellar phase separation in the mixture within the temperature range 20-40 degrees C. The specificity of cholesterol in creation of liquid-ordered lamellar phase is inferred.

Cellular components that functionally interact with signaling phospholipase A(2)s

Accumulating evidence has suggested that cytosolic phospholipase A(2) (cPLA(2)) and several secretory PLA(2) (sPLA(2)) isozymes are signaling PLA(2)s that are functionally coupled with downstream cyclooxygenase (COX) isozymes for prostaglandin (PG) biosynthesis. Arachidonic acid (AA) released by cPLA(2) and sPLA(2)s is supplied to both COX-1 and COX-2 in the immediate, and predominantly to COX-2 in the delayed, PG-biosynthetic responses. Vimentin, an intermediate filament component, acts as a functional perinuclear adapter for cPLA(2), in which the C2 domain of cPLA(2) associates with the head domain of vimentin in a Ca(2+)-sensitive manner. The heparin-binding signaling sPLA(2)-IIA, IID and V bind the glycosylphosphatidylinositol-anchored heparan sulfate proteoglycan glypican, which plays a role in sorting of these isozymes into caveolae and perinuclear compartments. Phospholipid scramblase, which facilitates transbilayer movement of anionic phospholipids, renders the cellular membranes more susceptible to signaling sPLA(2)s. There is functional cooperation between cPLA(2) and signaling sPLA(2)s in that prior activation of cPLA(2) is required for the signaling sPLA(2)s to act properly. cPLA(2)-derived AA is oxidized by 12/15-lipoxygenase, the products of which not only augment the induction of sPLA(2) expression, but also cause membrane perturbation, leading to increased cellular susceptibility to the signaling sPLA(2)s. sPLA(2)-X, a heparin-non-binding sPLA(2) isozyme, is capable of releasing AA from intact cells in the absence of cofactors. This property is attributed to its ability to avidly hydrolyze zwitterionic phosphatidylcholine, a major phospholipid in the outer plasma membrane. sPLA(2)-V can also utilize this route in several cell types. Taken together, the AA-releasing function of sPLA(2)s depends on the presence of regulatory cofactors and interfacial binding to membrane phospholipids, which differ according to cell type, stimuli, secretory processes, and subcellular distributions.

Transcriptional regulation of inflammatory secreted phospholipases A(2)

Secreted phospholipases A(2) is a family of small molecular weight and calcium-dependent enzymes of which the members list is presently growing. Among these enzymes, the synovial type IIA and the type V phospholipases A(2) are involved in inflammation. Although their actual mechanism is still a subject of debate, new therapeutic strategies can result from the knowledge of the regulations of their gene expression. The human genes of the type IIA and type V phospholipases A(2) are located on the chromosome 1 at close positions and transcribed in reverse orientations. These genes can therefore be regulated by common elements but only the regulation of the type IIA phospholipase A(2) gene expression has been extensively studied. Pro-inflammatory cytokines upregulate while the growth factors downregulate the type IIA phospholipase A(2) gene expression. Interleukin-6 and interleukin-1beta exert their effects at least partially at the transcriptional level. The transcriptional regulation of the type IIA phospholipase A(2) gene is cell- and species-specific. The activity of the human promoter is controlled by the CAAT-enhancer binding protein (C/EBP) factors while that of the rat promoter is regulated by nuclear factor kappaB (NF-kappaB) and C/EBPs. Furthermore, the human promoter is constitutively repressed in hepatocytes by single strand DNA binding proteins whose effects are relieved by C/EBP factors while the glucocorticoid receptor interacts with C/EBPs in chondrocytes to achieve full basal and interleukin-1beta-stimulated transcription activity. Other factors like CTF/NF1 and Sp1 might be involved in the regulation of both the rat and human promoter. Peroxisome proliferator-activated receptors could contribute to the stimulation of the rat promoter by NF-kappaB in vascular smooth muscle cells. The study of the coactivators and coinhibitors associated to these transcription factors will give a better understanding of the diversity and complexity of the transcriptional regulations of the type IIA phospholipase A(2) gene.

Novel physiological function of sphingomyelin in plasma. Inhibition of lipid peroxidation in low density lipoproteins

Although sphingomyelin (SPH) is a major constituent of all lipoproteins, its physiological function in plasma is not known. In this study, we tested the hypothesis that SPH inhibits lipid peroxidation in low density lipoproteins (LDL) because of its effects on surface fluidity and packing density and that the relative resistance of the buoyant LDL to oxidation, compared with the dense LDL, is partly due to their higher SPH content. Depletion of SPH by treatment with SPHase resulted in shortened lag times and increased rates of oxidation in both LDL subfractions, as measured by the conjugated diene formation in the presence of Cu(2+). Oxidation of LDL by soybean lipoxygenase was similarly stimulated by the degradation of SPH. Oxidation-induced fluorescence decay of diphenylhexatriene-labeled phosphatidylcholine (PC), equilibrated with LDL-PC, was accelerated significantly by the enzymatic depletion of SPH from the lipoprotein. Oxidation of 16:0-18:2 PC in the proteoliposomes was inhibited progressively by the incorporation of increasing amounts of egg SPH into the liposomes. Treatment of SPH-containing proteoliposomes with SPHase reversed the effect of SPH, showing that the presence of intact SPH is necessary for the inhibition of oxidation. Although the incorporation of SPH into the same liposome as the PC (intrinsic SPH) protected the PC against oxidation, the addition of SPH liposomes to PC liposomes (extrinsic SPH) was not effective. Oxidation of 16:0-18:2 PC in liposomes was also inhibited by the incorporation of dipalmitoyl-PC, but not by free cholesterol. These results suggest that SPH acts as a physiological inhibitor of lipoprotein oxidation, possibly by modifying the fluidity of the phospholipid monolayer and thereby inhibiting the lateral propagation of the lipid peroxy radicals.