Sphingomyelinase activity associated with human plasma low density lipoprotein

Identifying lipid traces of atherogenic mechanisms in human carotid plaque

BACKGROUND AND AIMS

Lipids play an important role in atherosclerotic plaque development and are interesting candidate predictive biomarkers. However, the link between circulating lipids, accumulating lipids in the vessel wall, and plaque destabilization processes in humans remains largely unknown. This study aims to provide new insights into the role of lipids in atherosclerosis using lipidomics and mass spectrometry imaging to investigate lipid signatures in advanced human carotid plaque and plasma samples.

METHODS

We used lipidomics and desorption electrospray ionization mass spectrometry imaging (DESI-MSI) to investigate lipid signatures of advanced human carotid plaque and plasma obtained from patients who underwent carotid endarterectomy (n = 14 out of 17 whose plaque samples were analyzed by DESI-MSI). Multivariate data analysis and unsupervised clustering were applied to identify lipids that were the most discriminative species between different patterns in plaque and plasma. These patterns were interpreted by quantitative comparison with conventional histology.

RESULTS

Lipidomics detected more than 300 lipid species in plasma and plaque, with markedly different relative abundances. DESI-MSI visualized the spatial distribution of 611 lipid-related m/z features in plaques, of which 330 m/z features could be assigned based on exact mass, comparison to the lipidomic data, and high mass resolution MSI. Matching spatial lipid patterns to histological areas of interest revealed several molecular species that were colocalized with pertinent disease processes in plaque including specific sphingomyelin and ceramide species with calcification, phospholipids and free fatty acids with inflammation, and triacylglycerols and phosphatidylinositols with fibrin-rich areas.

CONCLUSIONS

By comparing lipid species in plaque and plasma, we identified those circulating species that were also prominently present in plaque. Quantitative comparison of lipid spectral patterns with histology revealed the presence of specific lipid species in destabilized plaque areas, corroborating previous in vitro and animal studies.

Can Electronegative LDL Act as a Multienzymatic Complex?

Electronegative LDL (LDL(-)) is a minor form of LDL present in blood for which proportions are increased in pathologies with increased cardiovascular risk. In vitro studies have shown that LDL(-) presents pro-atherogenic properties, including a high susceptibility to aggregation, the ability to induce inflammation and apoptosis, and increased binding to arterial proteoglycans; however, it also shows some anti-atherogenic properties, which suggest a role in controlling the atherosclerotic process. One of the distinctive features of LDL(-) is that it has enzymatic activities with the ability to degrade different lipids. For example, LDL(-) transports platelet-activating factor acetylhydrolase (PAF-AH), which degrades oxidized phospholipids. In addition, two other enzymatic activities are exhibited by LDL(-). The first is type C phospholipase activity, which degrades both lysophosphatidylcholine (LysoPLC-like activity) and sphingomyelin (SMase-like activity). The second is ceramidase activity (CDase-like). Based on the complementarity of the products and substrates of these different activities, this review speculates on the possibility that LDL(-) may act as a sort of multienzymatic complex in which these enzymatic activities exert a concerted action. We hypothesize that LysoPLC/SMase and CDase activities could be generated by conformational changes in apoB-100 and that both activities occur in proximity to PAF-AH, making it feasible to discern a coordinated action among them.

Presence of Ceramidase Activity in Electronegative LDL

Electronegative low-density lipoprotein (LDL(-)) is a minor modified fraction of human plasma LDL with several atherogenic properties. Among them is increased bioactive lipid mediator content, such as lysophosphatidylcholine (LPC), non-esterified fatty acids (NEFA), ceramide (Cer), and sphingosine (Sph), which are related to the presence of some phospholipolytic activities, including platelet-activating factor acetylhydrolase (PAF-AH), phospholipase C (PLC), and sphingomyelinase (SMase), in LDL(-). However, these enzymes' activities do not explain the increased Sph content, which typically derives from Cer degradation. In the present study, we analyzed the putative presence of ceramidase (CDase) activity, which could explain the increased Sph content. Thin layer chromatography (TLC) and lipidomic analysis showed that Cer, Sph, and NEFA spontaneously increased in LDL(-) incubated alone at 37 °C, in contrast with native LDL(+). An inhibitor of neutral CDase prevented the formation of Sph and, in turn, increased Cer content in LDL(-). In addition, LDL(-) efficiently degraded fluorescently labeled Cer (NBD-Cer) to form Sph and NEFA. These observations defend the existence of the CDase-like activity's association with LDL(-). However, neither the proteomic analysis nor the Western blot detected the presence of an enzyme with known CDase activity. Further studies are thus warranted to define the origin of the CDase-like activity detected in LDL(-).

Gasdermins in Innate Host Defense Against Entamoeba histolytica and Other Protozoan Parasites

Gasdermins (GSDMs) are a group of proteins that are cleaved by inflammatory caspases to induce pore formation in the plasma membrane to cause membrane permeabilization and lytic cell death or pyroptosis. All GSDMs share a conserved structure, containing a cytotoxic N-terminal (NT) pore-forming domain and a C-terminal (CT) repressor domain. Entamoeba histolytica (Eh) in contact with macrophages, triggers outside-in signaling to activate inflammatory caspase-4/1 via the noncanonical and canonical pathway to promote cleavage of gasdermin D (GSDMD). Cleavage of GSDMD removes the auto-inhibition that masks the active pore-forming NT domain in the full-length protein by interactions with GSDM-CT. The cleaved NT-GSDMD monomers then oligomerize to form pores in the plasma membrane to facilitate the release of IL-1β and IL-18 with a measured amount of pyroptosis. Pyroptosis is an effective way to counteract intracellular parasites, which exploit replicative niche to avoid killing. To date, most GSDMs have been verified to perform pore-forming activity and GSDMD-induced pyroptosis is rapidly emerging as a mechanism of anti-microbial host defence. Here, we review our comprehensive and current knowledge on the expression, activation, biological functions, and regulation of GSDMD cleavage with emphases on physiological scenario and related dysfunctions of each GSDM member as executioner of cell death, cytokine secretion and inflammation against Eh and other protozoan parasitic infections.

The Complex Tail of Circulating Sphingolipids in Atherosclerosis and Cardiovascular Disease

Sphingolipids (SLs) are critical players in a number of cellular processes and have recently been implicated in a large number of human diseases, including atherosclerosis and cardiovascular disease (CVD). SLs are generated intracellularly in a stepwise manner, starting with the generation of the sphingoid long chain base (LCB), followed by N-acylation of the LCB to form ceramide, which can be subsequently metabolized to sphingomyelin and glycosphingolipids. Fatty acids, which are taken up by cells prior to their activation to fatty acyl-CoAs, are used in 2 of these enzymatic steps, including by ceramide synthases, which use fatty acyl-CoAs of different chain lengths to generate ceramides with different N-acyl chain lengths. Recently, alterations in plasma ceramides with specific N-acyl chain lengths and degrees of saturation have emerged as novel biomarkers for the prediction of atherosclerosis and overall cardiovascular risk in the general population. We briefly review the sources of plasma SLs in atherosclerosis, the roles of SLs in CVD, and the possible use of the "ceramide score" as a prognostic marker for CVD.

Plasma membrane repair involvement in parasitic and other pathogen infections

Intracellular pathogens depend on specific mechanisms to be able to gain entry and survive into their host cells. For this, they subvert pathways involved in physiological cellular processes. Here we are going to focus on how two protozoan parasites, Trypanosoma cruzi and Leishmania sp, which may cause severe diseases in humans, use plasma membrane repair (PMR) mechanisms to gain entry in host intracellular environment. T. cruzi is the causative agent of Chagas disease, a disease originally endemic of central and South America, but that has become widespread around the globe. T. cruzi is able to invade any nucleated cell, but muscle cells are usually the main targets during chronic disease. During host cell contact, the parasite interacts with proteins at the host cell surface and may cause damage to their membrane, which has been shown to be responsible for inducing intracellular calcium increase and PMR-related events that culminate with parasite internalization. The same was recently observed for Leishmania sp, when infecting nonprofessional phagocytic cells, such as fibroblasts. Other pathogens, such as viruses or bacteria may also use PMR-related events for invasion and vacuole escape/maturation. In some cases, PMR may also be responsible to modulate pathogen intracellular development. These other PMR roles in pathogen infections will also be briefly discussed.

New evidence from plasma ceramides links apoE polymorphism to greater risk of coronary artery disease in Finnish adults

apoE, a key regulator of plasma lipids, mediates altered functionalities in lipoprotein metabolism and thus affects the risk of coronary artery disease (CAD). The significance of different apoE polymorphisms remains unclear; although the ε4 allele is clearly associated with increased cholesterol levels (which inform CAD risk), direct studies about apoE polymorphisms on CAD risk and development have yielded controversial results. Furthermore, certain species of ceramides-complex lipids abundant in plasma LDL-are markers of increased risk of myocardial infarction and cardiovascular death. Using a high-throughput MS approach, we quantified 30 molecular plasma ceramide species from a cohort of 2,160 apoE-genotyped (rs7412, rs429358) young adults enrolled in the population-based Cardiovascular Risk in Young Finns Study. We then searched this lipidome data set to identify new indications of pathways influenced by apoE polymorphisms and possibly related to CAD risk. This approach revealed a previously unreported association between apoE polymorphism and a consistently documented high-risk CAD marker, Cer(d18:1/16:0). Compared with the apoE ε3/3 reference group, plasma levels of apoE ε4 were elevated and those of apoE ε2 were lowered in all subjects without evidence of apoE-by-sex interactions. apoE associated with seven ceramides that are connected to atherogenically potent macrophages and/or lipoprotein particles; these associations could indicate a plausible linkage between apoE polymorphism and ceramide metabolism, leading to adverse plasma LDL metabolism and atherogenesis. In conclusion, new evidence from plasma ceramides links apoE polymorphism with an increased risk of CAD and extends our understanding of the role of apoE in health and disease.

Plasma membrane damage repair is mediated by an acid sphingomyelinase in Entamoeba histolytica

Entamoeba histolytica is a pathogen that during its infective process confronts the host defenses, which damages the amoebic plasma membrane (PM), resulting in the loss of viability. However, it is unknown whether amoebic trophozoites are able to repair their PM when it is damaged. Acid sphingomyelinases (aSMases) have been reported in mammalian cells to promote endocytosis and removal of PM lesions. In this work, six predicted amoebic genes encoding for aSMases were found to be transcribed in the HM1:IMSS strain, finding that the EhaSM6 gene is the most transcribed in basal growth conditions and rendered a functional protein. The secreted aSMase activity detected was stimulated by Mg⁺² and inhibited by Co⁺². Trophozoites that overexpress the EhaSM6 gene (HM1-SM6HA) exhibit an increase of 2-fold in the secreted aSMase activity. This transfectant trophozoites exposed to pore-forming molecules (SLO, Magainin, β-Defensin 2 and human complement) exhibited an increase from 6 to 25-fold in the secreted aSMase activity which correlated with higher amoebic viability in a Ca⁺² dependent process. However, other agents that affect the PM such as hydrogen peroxide also induced an increase of secreted aSMase, but to a lesser extent. The aSMase6 enzyme is N- and C-terminal processed. Confocal and transmission electron microscopy showed that trophozoites treated with SLO presented a migration of lysosomes containing the aSMase towards the PM, inducing the formation of membrane patches and endosomes in the control strain. These cellular structures were increased in the overexpressing strain, indicating the involvement of the aSMase6 in the PM injury repair. The pore-forming molecules induced an increase in the expression of EhaSM1, 2, 5 and 6 genes, meanwhile, hydrogen peroxide induced an increase in all of them. In all the conditions evaluated, the EhaSM6 gene exhibited the highest levels of induction. Overall, these novel findings show that the aSMase6 enzyme from E. histolytica promotes the repair of the PM damaged with pore-forming molecules to prevent losing cell integrity. This novel system could act when encountered with the lytic defense systems of the host.

The host-amoeba relationship is based on a series of interplays between host defense mechanisms and parasite survival strategies. While host cells elaborate diverse mechanisms for pathogen elimination, Entamoeba histolytica trophozoites have also developed complex strategies to counteract host immune response and facilitate its own survival while confronting host defenses. E. histolytica exposed to pore-forming proteins such as β-Defensin 2, human complement and Streptolysin O (SLO), increases the activity of secreted aSMase, which is related to greater amoebic viability. Other agents that affect plasma membrane (PM) may also increase secreted aSMase but to a lesser extent. SLO form pores in the PM of E. histolytica trophozoites that initiates the uncontrolled entry of Ca²⁺, recognized as the primary trigger for cell responses which favors the migration of the lysosomes to the periphery of the cell, fuses with the PM and release their content, including aSMase to the external side of the cell. The secreted aSMase favoring the internalization of the lesion for its degradation in phagolysosomes. During the early stages of PM damage, the pores are rapidly blocked by patch-like structures that prevent the lysis of the trophozoite and immediately begin internalizing the lesion. The aSMase6 overexpression favors the repair of the lesion and the survival of E. histolytica trophozoites. Pore-forming proteins induced an increase in the expression of EhaSM1, 2, 5 and 6 genes, meanwhile oxidative stress induced an increase in all of them. Here we report, for the first time, that E. histolytica possess a mechanism for PM damage repair mediated by aSMase similar to the system described in mammalian cells.

Electronegative LDL: An Active Player in Atherogenesis or a By- Product of Atherosclerosis?

Low-density lipoproteins (LDLs) are the major plasma carriers of cholesterol. However, LDL particles must undergo various molecular modifications to promote the development of atherosclerotic lesions. Modified LDL can be generated by different mechanisms, but as a common trait, show an increased electronegative charge of the LDL particle. A subfraction of LDL with increased electronegative charge (LDL(-)), which can be isolated from blood, exhibits several pro-atherogenic characteristics. LDL(-) is heterogeneous, due to its multiple origins but is strongly related to the development of atherosclerosis. Nevertheless, the implication of LDL(-) in a broad array of pathologic conditions is complex and in some cases anti-atherogenic LDL(-) properties have been reported. In fact, several molecular modifications generating LDL(-) have been widely studied, but it remains unknown as to whether these different mechanisms are specific or common to different pathological disorders. In this review, we attempt to address these issues examining the most recent findings on the biology of LDL(-) and discussing the relationship between this LDL subfraction and the development of different diseases with increased cardiovascular risk. Finally, the review highlights the importance of minor apolipoproteins associated with LDL(-) which would play a crucial role in the different properties displayed by these modified LDL particles.

Circulating low-density lipoprotein ceramide concentrations increase in Holstein dairy cows transitioning from gestation to lactation

Low-density lipoprotein (LDL) ceramide causes insulin resistance in obese diabetic nonruminants. Because previous work suggests that liver-derived ceramide may impair insulin action in postpartum cows, our objectives were to characterize peripartal changes in lipoprotein ceramides. We further studied the effects of prepartum adiposity on lipoprotein ceramide levels. Twenty-eight pregnant Holstein cows (parity = 3.65 ± 1.62) with lean (body condition score, BCS = 2.97 ± 0.16; body weight, BW = 630 ± 55.2 kg; n = 15) or overweight (BCS = 3.93 ± 0.27; BW = 766 ± 46.1 kg; n = 13) body condition 28 d before expected parturition were evaluated. Sampling occurred on d -20.5 ± 1.74, -13.8 ± 1.71, -7.84 ± 4.07, -6.71 ± 1.00, -3.92 ± 0.64, and -1.28 ± 0.61 (before parturition); daily until d 8 postpartum; and on d 10, 12, 14, 21, and 28. Adipose tissue and liver were biopsied on d -7.84 ± 4.07 and 10. Postpartum insulin sensitivity was assessed using the hyperinsulinemic-euglycemic clamp. Lipoprotein fractions were isolated using liquid chromatography. Sphingolipids were quantified using mass spectrometry. Data were analyzed using a mixed model with repeated measures. Overweight cows had a higher BCS and BW at enrollment relative to lean cows, but BCS and BW were similar postpartum. Overweight cows lost more body condition (0.97 ± 0.36 vs. 0.55 ± 0.16 BCS units) and BW (291 ± 67.3 vs. 202 ± 54.5 kg) during transition relative to lean cows. Adipocyte volume and counts declined from prepartum to postpartum (50.4 and 13.7%, respectively), and adipocyte volume was greater (48.2%) in overweight cows prepartum relative to lean cows. Although DMI was comparable between BCS groups, milk yield tended to be greater in overweight cows. Plasma free fatty acid and β-hydroxybutyrate and liver lipid levels were 40, 16, and 37% greater, respectively, in overweight cows compared with lean cows. Glucose infusion rate during the hyperinsulinemic-euglycemic clamp tended to be lower in overweight cows. Ceramide levels within triacylglycerol-rich lipoprotein fractions declined postpartum, whereas LDL ceramide increased postpartum. Overweight cows had lower triacylglycerol-rich lipoprotein C16:0-ceramide levels relative to lean cows. Prepartum LDL C24:0-ceramide levels were greater in overweight cows relative to lean cows. Independent of prepartum adiposity, we concluded that serum LDL ceramide levels are elevated in early-lactation cows experiencing adipose tissue free fatty acid mobilization and hepatic steatosis.

Sphingolipid metabolism in non-alcoholic fatty liver diseases

Non-alcoholic fatty liver disease (NAFLD) involves a panel of pathologies starting with hepatic steatosis and continuing to irreversible and serious conditions like steatohepatitis (NASH) and hepatocarcinoma. NAFLD is multifactorial in origin and corresponds to abnormal fat deposition in liver. Even if triglycerides are mostly associated with these pathologies, other lipid moieties seem to be involved in the development and severity of NAFLD. That is the case with sphingolipids and more particularly ceramides. In this review, we explore the relationship between NAFLD and sphingolipid metabolism. After providing an analysis of complex sphingolipid metabolism, we focus on the potential involvement of sphingolipids in the different pathologies associated with NAFLD. An unbalanced ratio between ceramides and terminal metabolic products in the liver and plasma promotes weight gain, inflammation, and insulin resistance. In the etiology of NAFLD, some sphingolipid species such as ceramides may be potential biomarkers for NAFLD. We review the clinical relevance of sphingolipids in liver diseases.

Technical note: Characterization of ceramide in bovine lipoproteins

The hepatic synthesis and export of ceramide is enhanced in diabetic monogastrics. Moreover, ceramide in lipoproteins can mediate the development of insulin resistance. We have previously demonstrated that circulating ceramide increases during the progression of insulin resistance in postpartum dairy cows. Considering that the origins of circulating ceramide required investigation, our objective was to develop a method to characterize the ceramide profile of lipoprotein fractions collected from dairy cows. Serum was collected from 4 nonpregnant and nonlactating Holstein dairy cows. Serum lipoproteins were isolated using size exclusion chromatography by fast protein liquid chromatography (SEC-FPLC). Measurement of triacylglycerol (TAG), phospholipid, total cholesterol, and protein was performed using standard colorimetry practices. Following lipid extraction, fractions were analyzed using electrospray ionization tandem mass spectrometry. Data were analyzed as repeated measures using a mixed model. Lipoprotein isolation using SEC-FPLC and subsequent colorimetric analyses confirmed the presence of 4 distinct fractions: TAG-rich, low density (LDL), and large (buoyant) and small (dense) high density lipoprotein (HDL) subclasses. As expected, the fraction representing mixed very low density lipoproteins and chylomicrons primarily contained TAG. Low density lipoprotein fractions were equally enriched with cholesterol and phospholipid. Buoyant HDL contained elevated levels of cholesterol, phospholipid, and protein. In contrast, the fraction containing dense HDL primarily contained protein. Our method revealed that LDL are enriched with ceramides. Ceramides were also compartmentalized to a lesser extent within both HDL subclasses and TAG-rich lipoproteins. Comparable to whole serum, C16:0-ceramide was the predominant ceramide quantified in all lipoprotein subclasses. Interestingly, the proportion of C24:0-ceramide to total ceramide was elevated in TAG-rich lipoproteins, relative to all other lipoprotein subclasses. We conclude that SEC-FPLC coupled with mass spectrometry is a means to quantify ceramides in lipoprotein fractions. Moreover, ceramides are enriched within bovine LDL, and lipoprotein ceramide profiles reflect levels observed in whole serum. Future investigation will need to determine the biological importance of lipoprotein ceramides with distinct C-chains at amide residues.

LDL electronegativity index: a potential novel index for predicting cardiovascular disease

High cardiovascular risk conditions are frequently associated with altered plasma lipoprotein profile, such as elevated low-density lipoprotein (LDL) and LDL cholesterol and decreased high-density lipoprotein. There is, however, accumulating evidence that specific subclasses of LDL may play an important role in cardiovascular disease development, and their relative concentration can be regarded as a more relevant risk factor. LDL particles undergo multiple modifications in plasma that can lead to the increase of their negative charge. The resulting electronegative LDL [LDL(–)] subfraction has been demonstrated to be especially atherogenic, and became a subject of numerous recent studies. In this review, we discuss the physicochemical properties of LDL(–), methods of its detection, atherogenic activity, and relevance of the LDL electronegativity index as a potential independent predictor of cardiovascular risk.

Ceramides contained in LDL are elevated in type 2 diabetes and promote inflammation and skeletal muscle insulin resistance

Dysregulated lipid metabolism and inflammation are linked to the development of insulin resistance in obesity, and the intracellular accumulation of the sphingolipid ceramide has been implicated in these processes. Here, we explored the role of circulating ceramide on the pathogenesis of insulin resistance. Ceramide transported in LDL is elevated in the plasma of obese patients with type 2 diabetes and correlated with insulin resistance but not with the degree of obesity. Treating cultured myotubes with LDL containing ceramide promoted ceramide accrual in cells and was accompanied by reduced insulin-stimulated glucose uptake, Akt phosphorylation, and GLUT4 translocation compared with LDL deficient in ceramide. LDL-ceramide induced a proinflammatory response in cultured macrophages via toll-like receptor-dependent and -independent mechanisms. Finally, infusing LDL-ceramide into lean mice reduced insulin-stimulated glucose uptake, and this was due to impaired insulin action specifically in skeletal muscle. These newly identified roles of LDL-ceramide suggest that strategies aimed at reducing hepatic ceramide production or reducing ceramide packaging into lipoproteins may improve skeletal muscle insulin action.

The Induction of Cytokine Release in Monocytes by Electronegative Low-Density Lipoprotein (LDL) Is Related to Its Higher Ceramide Content than Native LDL

Electronegative low-density lipoprotein (LDL(−)) is a minor modified LDL subfraction that is present in blood. LDL(−) promotes inflammation and is associated with the development of atherosclerosis. We previously reported that the increase of cytokine release promoted by this lipoprotein subfraction in monocytes is counteracted by high-density lipoprotein (HDL). HDL also inhibits a phospholipase C-like activity (PLC-like) intrinsic to LDL(−). The aim of this work was to assess whether the inhibition of the PLC-like activity by HDL could decrease the content of ceramide (CER) and diacylglycerol (DAG) generated in LDL(−). This knowledge would allow us to establish a relationship between these compounds and the inflammatory activity of LDL(−). LDL(−) incubated at 37 °C for 20 h increased its PLC-like activity and, subsequently, the amount of CER and DAG. We found that incubating LDL(−) with HDL decreased both products in LDL(−). Native LDL was modified by lipolysis with PLC or by incubation with CER-enriched or DAG-enriched liposomes. The increase of CER in native LDL significantly increased cytokine release, whereas the enrichment in DAG did not show these inflammatory properties. These data point to CER, a resultant product of the PLC-like activity, as a major determinant of the inflammatory activity induced by LDL(−) in monocytes.

Electronegative low-density lipoprotein. A link between apolipoprotein B misfolding, lipoprotein aggregation and proteoglycan binding

PURPOSE OF REVIEW

Subendothelial retention of lipoproteins is considered the first step in the development of atherosclerosis, but the molecular mechanisms involved are poorly understood. Recent findings on the atherogenic properties of a minor electronegative fraction of LDL (LDL(-)) could contribute to a better understanding of this process.

RECENT FINDINGS

Circular dichroism, Trp-fluorescence and two-dimensional nuclear magnetic resonance have shown that apolipoprotein B (apoB) in LDL(-) has an abnormal, misfolded conformation. Immunochemical analysis revealed a different conformation, mainly in the N-terminal and C-terminal extremes. These alterations contribute to the high susceptibility to aggregation of LDL(-). Moreover, LDL(-) can seed the aggregation of native LDL, suggesting an amyloidogenic character that has been attributed to the amphipathic helix cluster in the α2-domain. A phospholipase C (PLC)-like activity associated to LDL(-) seems to play a major role in the LDL(-)-induced aggregation. The aggregation of LDL(-) increases its binding to proteoglycans because of the abnormal conformation of the N-terminal extreme of apoB.

SUMMARY

LDL(-) could play a relevant role in atherogenesis by acting as a priming factor that stimulates lipoprotein aggregation. This process, which appears to be mediated by a PLC-like activity intrinsic to LDL(-), increases the binding of LDL to proteoglycans and could promote subendothelial retention of these lipoproteins.

Liquid chromatography-tandem mass spectrometry for the determination of sphingomyelin species from calf brain, ox liver, egg yolk, and krill oil

In this study, molecular species of sphingomyelin (SM) in egg yolk, calf brain, ox liver, and krill oil were investigated. Classes of phospholipids (PLs) were purified, identified, and quantified by normal phase semipreparative high-performance liquid chromatography (HPLC) combined with evaporative light scattering detectors (ELSD). For SM molecular species identification, pure SM collected through a flow splitter was loaded to HPLC-electrospray ionization-tandem mass spectrometry (LC-ESI-MS(2)), with 100% methanol containing 5 mM ammonium formate as mobile phase. In addition to classes of PLs, the used approach allowed the determination of profiles of SM species in egg yolk, ox liver, and calf brain, whereas krill oil turned out not to contain any SM. It also allowed the separation and identification of SM subclasses, as well as tentative identification of species with the same molecular mass, including isomers. The results showed that egg yolk contained the highest proportion of (d18:1-16:0)SM (94.1%). The major SM molecular species in ox liver were (d18:1-16:0)SM (25.5%), (d18:1-23:0)SM (19.7%), (d18:1-24:0)SM (13.2%), and (d18:1-22:0)SM (12.5%). Calf brain SM was rich in species such as (d18:1-18:0)SM (40.7%), (d18:1-24:1)SM (17.1%), and (d18:1-20:0)SM (10.8%).

HDL and electronegative LDL exchange anti- and pro-inflammatory properties

Electronegative LDL [LDL(-)] is a minor modified LDL subfraction present in blood with inflammatory effects. One of the antiatherogenic properties of HDL is the inhibition of the deleterious effects of in vitro modified LDL. However, the effect of HDL on the inflammatory activity of LDL(-) isolated from plasma is unknown. We aimed to assess the putative protective role of HDL against the cytokine released induced in monocytes by LDL(-). Our results showed that LDL(-) cytokine release was inhibited when LDL(-) was coincubated with HDL and human monocytes and also when LDL(-) was preincubated with HDL and reisolated prior to cell incubation. The addition of apoliprotein (apo)AI instead of HDL reproduced the protective behavior of HDL. HDL preincubated with LDL(-) promoted greater cytokine release than native HDL. Incubation of LDL(-) with HDL decreased the electronegative charge, phospholipase C-like activity, susceptibility to aggregation and nonesterified fatty acid (NEFA) content of LDL(-), whereas these properties increased in HDL. NEFA content in LDL appeared to be related to cytokine production because NEFA-enriched LDL induced cytokine release. HDL, at least in part through apoAI, inhibits phospholipase-C activity and cytokine release in monocytes, thereby counteracting the inflammatory effect of LDL(-). In turn, HDL acquires these properties and becomes inflammatory.

Oxidized LDL: diversity, patterns of recognition, and pathophysiology

Oxidative modification of LDL is known to elicit an array of pro-atherogenic responses, but it is generally underappreciated that oxidized LDL (OxLDL) exists in multiple forms, characterized by different degrees of oxidation and different mixtures of bioactive components. The variable effects of OxLDL reported in the literature can be attributed in large part to the heterogeneous nature of the preparations employed. In this review, we first describe the various subclasses and molecular composition of OxLDL, including the variety of minimally modified LDL preparations. We then describe multiple receptors that recognize various species of OxLDL and discuss the mechanisms responsible for the recognition by specific receptors. Furthermore, we discuss the contentious issues such as the nature of OxLDL in vivo and the physiological oxidizing agents, whether oxidation of LDL is a prerequisite for atherogenesis, whether OxLDL is the major source of lipids in foam cells, whether in some cases it actually induces cholesterol depletion, and finally the Janus-like nature of OxLDL in having both pro- and anti-inflammatory effects. Lastly, we extend our review to discuss the role of LDL oxidation in diseases other than atherosclerosis, including diabetes mellitus, and several autoimmune diseases, such as lupus erythematosus, anti-phospholipid syndrome, and rheumatoid arthritis.

Effect of sphingomyelinase-mediated generation of ceramide on aggregation of low-density lipoprotein

This study addresses the response-to-retention hypothesis, which states that the subendothelial retention of atherogenic lipoproteins is the necessary and sufficient condition for the initiation of atherosclerosis. Here we focus on the relationship between the generation of ceramide in the low-density lipoprotein (LDL) phospholipid monolayer and the resulting aggregation of LDL particles. This study provides the first measurement of neutral, Mg (2+)-dependent Sphingomyelinase (Smase)-mediated ceramide formation from LDL-sphingomyelin and does so for a range of enzyme concentrations (0-0.22 units Smase/mL). The kinetics of ceramide generation was measured using a fluorescence assay for the above enzyme concentrations with a fixed substrate concentration (0.33 mg LDL/mL). The kinetics of LDL aggregate formation was measured by dynamic light scattering (DLS, method of cumulants) for identical enzyme concentrations. Ceramide concentration profiles were fit with a modification of the Michaelis-Menten model ( k a = 1.11 x 10 (-1) microM (-1) min (-1), k -a = 6.54 x 10 (2) microM (-1) min (-1), k 1 = 3.33 x 10 (1) microM (-1) min (-1), k -1 = 1.41 x 10 (-2) min (-1), k cat = 8.05 x 10 (1) min (-1), K M = 2.418 microM, k deact = 4.66 x 10 (-2) microM (-1) min (-1)) that accounts for the effects of enzyme attachment to the LDL monolayer and for deactivation of Smase due to product inhibition. LDL aggregation is described by a mass action model as explained in previous studies. A key result of this work is the finding that LDL aggregate size depends directly on ceramide concentration and is independent of enzyme concentration. This study demonstrates how principles of colloid science are relevant to important biomedical problems.

Novel phospholipolytic activities associated with electronegative low-density lipoprotein are involved in increased self-aggregation

Electronegative low-density lipoprotein (LDL(-)) is a minor LDL subfraction present in plasma with increased platelet-activating factor acetylhydrolase (PAF-AH) activity. This activity could be involved in the proinflammatory effects of LDL(-). Our aim was to study the presence of additional phospholipolytic activities in LDL(-). Total LDL was fractionated into electropositive (LDL(+)) and LDL(-) by anion-exchange chromatography, and phospholipolytic activities were measured by fluorometric methods. Phospholipolytic activity was absent in LDL(+) whereas LDL(-) presented activity against lysophosphatidylcholine (LPC, 82.4 +/- 34.9 milliunits/mg of apoB), sphingomyelin (SM, 53.3 +/- 22.5 milliunits/mg of apoB), and phosphatidylcholine (PC, 25.7 +/- 4.3 milliunits/mg of apoB). LDL(-), but not LDL(+), presented spontaneous self-aggregation at 37 degrees C in parallel to phospholipid degradation. This was observed in the absence of lipid peroxidation and suggests the involvement of phospholipolytic activity in self-aggregation of LDL(-). Phospholipolytic activity was not due to PAF-AH, apoE, or apoC-III and was not increased in LDL(+) modified by Cu (2+) oxidation, acetylation, or secretory phospholipase A 2 (PLA 2). However, LDL(-) efficiently degraded phospholipids of lipoproteins enriched in LPC, such as oxidized LDL or PLA 2-LDL, but not native or acetylated LDL. This finding supports that LPC is the best substrate for LDL(-)-associated phospholipolytic activity. These results reveal novel properties of LDL(-) that could play a significant role in its atherogenic properties.

Membranes: a meeting point for lipids, proteins and therapies

Membranes constitute a meeting point for lipids and proteins. Not only do they define the entity of cells and cytosolic organelles but they also display a wide variety of important functions previously ascribed to the activity of proteins alone. Indeed, lipids have commonly been considered a mere support for the transient or permanent association of membrane proteins, while acting as a selective cell/organelle barrier. However, mounting evidence demonstrates that lipids themselves regulate the location and activity of many membrane proteins, as well as defining membrane microdomains that serve as spatio-temporal platforms for interacting signalling proteins. Membrane lipids are crucial in the fission and fusion of lipid bilayers and they also act as sensors to control environmental or physiological conditions. Lipids and lipid structures participate directly as messengers or regulators of signal transduction. Moreover, their alteration has been associated with the development of numerous diseases. Proteins can interact with membranes through lipid co-/post-translational modifications, and electrostatic and hydrophobic interactions, van der Waals forces and hydrogen bonding are all involved in the associations among membrane proteins and lipids. The present study reviews these interactions from the molecular and biomedical point of view, and the effects of their modulation on the physiological activity of cells, the aetiology of human diseases and the design of clinical drugs. In fact, the influence of lipids on protein function is reflected in the possibility to use these molecular species as targets for therapies against cancer, obesity, neurodegenerative disorders, cardiovascular pathologies and other diseases, using a new approach called membrane-lipid therapy.

Lipid second messengers and cell signaling in vascular wall

Agonists of cellular receptors, such as receptor tyrosine kinases, G protein-coupled receptors, cytokine receptors, etc., activate phospholipases (C(gamma), C(beta), A(2), D), sphingomyelinase, and phosphatidylinositol-3-kinase. This produces active lipid metabolites, some of which are second messengers: inositol trisphosphate, diacylglycerides, ceramide, and phosphatidylinositol 3,4,5-trisphosphate. These universal mechanisms are involved in signal transduction to maintain blood vessel functions: regulation of vasodilation and vasoconstriction, mechanical stress resistance, and anticoagulant properties of the vessel lumen surface. Different signaling pathways realized through lipid second messengers interact to one another and modulate intracellular events. In early stages of atherogenesis, namely, accumulation of low density lipoproteins in the vascular wall, cascades of pro-atherogenic signal transduction are triggered through lipid second messengers. This leads to atherosclerosis, the general immuno-inflammatory disease of the vascular system.

Composition-driven surface domain structuring mediated by sphingolipids and membrane-active proteins. Above the nano- but under the micro-scale: mesoscopic biochemical/structural cross-talk in biomembranes

Biomembranes contain a wide variety of lipids and proteins within an essentially two-dimensional structure. The coexistence of such a large number of molecular species causes local tensions that frequently relax into a phase or compositional immiscibility along the lateral and transverse planes of the interface. As a consequence, a substantial microheterogeneity of the surface topography develops and that depends not only on the lipid-protein composition, but also on the lateral and transverse tensions generated as a consequence of molecular interactions. The presence of proteins, and immiscibility among lipids, constitute major perturbing factors for the membrane sculpturing both in terms of its surface topography and dynamics. In this work, we will summarize some recent evidences for the involvement of membrane-associated, both extrinsic and amphitropic, proteins as well as membrane-active phosphohydrolytic enzymes and sphingolipids in driving lateral segregation of phase domains thus determining long-range surface topography.

Preferential Sphingosine-1-Phosphate Enrichment and Sphingomyelin Depletion Are Key Features of Small Dense HDL3 Particles

Objective— The purpose of this study was to define heterogeneity in the molecular profile of lipids, including sphingomyelin and sphingosine-1-phosphate, among physicochemically-defined HDL subpopulations and potential relevance to antiatherogenic biological activities of dense HDL3.

Methods and Results— The molecular profile of lipids (cholesteryl esters, phospholipids, sphingomyelin, and sphingosine-1-phosphate) in physicochemically-defined normolipidemic HDL subpopulations was determined by high-performance liquid chromatography and gas chromatography. As HDL particle size and molecular weight decreased with increment in density, molar lipid content diminished concomitantly. On a % basis, sphingomyelin abundance diminished in parallel with progressive increase in HDL density from HDL2b (12.8%) to HDL3c (6.2%; P <0.001); in contrast, sphingosine-1-phosphate was preferentially enriched in small HDL3 (40 to 50 mmol/mol HDL) versus large HDL2 (15 to 20 mmol/mol HDL; P <0.01). Small HDL3c was equally enriched in LpA-I particles relative to LpA-I:A-II. The sphingosine-1-phosphate/sphingomyelin ratio correlated positively with the capacities of HDL subspecies to attenuate apoptosis in endothelial cells ( r =0.73, P <0.001) and to retard LDL oxidation ( r =0.58, P <0.01).

Conclusions— An elevated sphingosine-1-phosphate/sphingomyelin ratio is an integral feature of small dense HDL3, reflecting enrichment in sphingosine-1-phosphate, a key antiapoptotic molecule, and depletion of sphingomyelin, a structural lipid with negative impact on surface fluidity and LCAT activity. These findings further distinguish the structure and antiatherogenic activities of small, dense HDL.

Gene expression alterations in the sphingolipid metabolism pathways during progression of dementia and Alzheimer's disease: a shift toward ceramide accumulation at the earliest recognizable stages of Alzheimer's disease?

There is mounting evidence linking Abeta42 generation in Alzheimer's disease (AD) with sphingomyelin catabolism. Using microarray technology to study 17 brain regions from subjects with varying severity of AD and dementia we detected multiple gene expression abnormalities of the key enzymes that control sphingolipid metabolism. These changes were correlated with the progression of clinical dementia. The upregulation of gene expression of the enzymes controlling synthesis de novo of Cer and the downregulation of the enzymes involved in glycosphingolipid synthesis was evident as early in disease progression as in mild dementia. Together these changes suggest a shift in sphingolipid metabolism towards accumulation of Cer, depletion of glycosphingolipids and the reduction of synthesis of the anti-apoptosis signaling lipid-sphingosine 1-phosphate as a function of disease progression. This disrupted balance within the sphingolipid metabolism may trigger signaling events promoting neurodegeneration across cortical regions. This potential mechanism may provide a link between lipid metabolism disturbance and AD.

Regulation of the activity and fatty acid specificity of lecithin-cholesterol acyltransferase by sphingomyelin and its metabolites, ceramide and ceramide phosphate

Sphingomyelin (SM), the second most abundant phospholipid in plasma lipoproteins, was previously shown to be a physiological inhibitor of the lecithin-cholesterol acyltransferase (LCAT) reaction. In this study, we investigated the effects of its metabolites, ceramide and ceramide phosphate, on the activity and fatty acid specificity of LCAT in vitro. Treatment of SM-containing substrate with SMase C, which hydrolyzes SM to ceramide, abolished the inhibitory effect of SM, whereas treatment with SMase D, which hydrolyzes it to ceramide phosphate, increased the level of inhibition. Although incorporation of ceramide into the substrate in the absence of SM activated the LCAT reaction only modestly, its co-incorporation with SM neutralized the inhibitory effect of SM. Ceramide phosphate, on the other hand, inhibited the LCAT reaction more strongly than SM. The effects of the sphingolipids on the phospholipase A and cholesterol esterification reactions of the enzyme were similar, indicating that they regulate the binding of phosphatidylcholine (PC) to the active site, rather than the esterification step. Incorporation of ceramide into the substrate stimulated the synthesis of unsaturated cholesteryl esters at the expense of saturated esters. However, these effects on fatty acid specificity disappeared when the PC substrates were incorporated into an inert diether PC matrix, suggesting that ceramide increases the availability of polyunsaturated PCs to the enzyme by altering the macromolecular structure of the substrate particle. Since the plasma ceramide levels are increased during inflammation, these results indicate that the activity and fatty acid specificity of LCAT may be altered during the inflammatory response.

Ceramides reduce CD36 cell surface expression and oxidised LDL uptake by monocytes and macrophages

Oxidised LDL accumulates in macrophages following scavenger receptor (SR) uptake. The expression of the SR, CD36, is increased by oxidised LDL. The signalling molecule, ceramide, can modulate intracellular peroxides and increase lipid peroxidation. Ceramide also accumulates in atherosclerotic plaques. Thus, we have examined whether ceramide can modulate CD36 expression and function in human monocyte/macrophages. Addition of synthetic short chain ceramides or the action of sphingomyelinase to generate physiological long chain ceramides in situ caused significant reductions in CD36 expression by monocytes/macrophages which was not due to inhibition of mRNA expression. Inhibition of proteasomal degradation using lactacystin had no effect on CD36 expression, however, flow cytometric analysis of permeabilised cells suggested an intracellular trafficking blockade. Ceramide treated monocytes/macrophages showed dose dependent reduction in oxidised LDL uptake. Taken together, it is suggested that ceramide blocks the transport of CD36 to the membrane of monocytes/macrophages, thereby preventing uptake of oxidised LDL.

Formation of ceramide-enriched domains in lipid particles enhances the binding of apolipoprotein E

We investigated the interaction between apolipoprotein E (apoE) and ceramide (CER)-enriched domains on the particles, by using lipid emulsions containing sphingomyelin (SM) or CER as model particles of lipoproteins. The sphingomyelinase (SMase)-induced aggregation of emulsion particles was prevented by apoE. CER increased the amount of apoE bound to emulsion particles. The confocal images of CER-containing large emulsions with two fluorescent probes showed three-dimensional microdomains enriched in CER. SMase also induced the formation of CER-enriched domains. We propose apoE prefers to bind on CER-enriched domains exposed on particle surface, and thus inhibits the aggregation or fusion of the particles.

Asymmetric addition of ceramides but not dihydroceramides promotes transbilayer (flip-flop) lipid motion in membranes

Transbilayer lipid motion in membranes may be important in certain physiological events, such as ceramide signaling. In this study, the transbilayer redistribution of lipids induced either by ceramide addition or by enzymatic ceramide generation at one side of the membrane has been monitored using pyrene-labeled phospholipid analogs. When added in organic solution to preformed liposomes, egg ceramide induced transbilayer lipid motion in a dose-dependent way. Short-chain (C6 and C2) ceramides were less active than egg ceramide, whereas dihydroceramides or dioleoylglycerol were virtually inactive in promoting flip-flop. The same results (either positive or negative) were obtained when ceramides, dihydroceramides, or diacylglycerols were generated in situ through the action of a sphingomyelinase or of a phospholipase C. The phenomenon was dependent on the bilayer lipid composition, being faster in the presence of lipids that promote inverted phase formation, e.g., phosphatidylethanolamine and cholesterol; and, conversely, slower in the presence of lysophosphatidylcholine, which inhibits inverted phase formation. Transbilayer motion was almost undetectable in bilayers composed of pure phosphatidylcholine or pure sphingomyelin. The use of pyrene-phosphatidylserine allowed detection of flip-flop movement induced by egg ceramide in human red blood cell membranes at a rate comparable to that observed in model membranes. The data suggest that when one membrane leaflet becomes enriched in ceramides, they diffuse toward the other leaflet. This is counterbalanced by lipid movement in the opposite direction, so that net mass transfer between monolayers is avoided. These observations may be relevant to the physiological mechanism of transmembrane signaling via ceramides.

Interactions of Histone H1 with Phospholipids and Comparison of Its Binding to Giant Liposomes and Human Leukemic T Cells†

Due to its net positive charge histone H1 readily associates with liposomes containing acidic phospholipids, such as phosphatidylserine (PS). Interestingly, circular dichroism reveals that while histone H1 in aqueous solutions appears as a random coil, its binding to liposomes containing PS is associated with a pronounced increase in alpha-helicity and beta-sheet content, estimated at 7% and 24%, respectively. This interaction further results in vesicle aggregation and lipid mixing. Fluorescence microscopy revealed rapid binding of Texas Red-labeled H1 (TR-H1) to giant liposomes composed of phosphatidylcholine and PS (SOPC/brain PS, 9/1 molar ratio), followed by lateral segregation and subsequent translocation of the membrane-bound H1 into the giant liposome. The above processes in giant liposomes did depend on the presence of the negatively charged PS. Comparison of the behavior of H1 in giant liposomes to that in cultured leukemic T cells demonstrated very similar patterns. More specifically, fluorescence microscopy revealed binding of TR-H1 to the plasma membrane as lateral segregated microdomains, followed by translocation into the cell. H1 also triggered membrane blebbing and fragmentation of the nuclei of these cells, thus suggesting induction of apoptosis. Our findings indicate that histone H1 and acidic phospholipids form supramolecular aggregates in the plasma membrane of T cells, subsequently resulting in major rearrangements of cellular membranes. Our results allow us to conclude that the minimal requirement for the interaction of histone H1 with the leukemia cell plasma membrane is reproduced by giant liposomes composed of unsaturated phosphatidylcholine and phosphatidylserine, the latter being mandatory for the observed changes in the secondary structure of H1 as well as the macroscopic consequences of the H1-PS interactions.

Lysenin, a unique sphingomyelin-binding protein

Sphingomyelin plays complex structural and signaling functions in the plasma membrane. Of special interest is that hydrolysis of sphingomyelin to ceramide can modulate dynamics of membrane rafts, which serve as signaling platforms for various receptors. This review is focused on a recently discovered sphingomyelin-binding protein, lysenin, which can be used as a unique probe to trace distribution and turnover of sphingomyelin in cellular membranes. We analyze the primary and secondary structures of lysenin with respect to its interaction with the plasma membrane. The specificity of lysenin binding to sphingomyelin, revealed by both biochemical and cytochemical approaches, is discussed.

Sphingomyelinases: enzymology and membrane activity

This paper reviews our present knowledge of sphingomyelinases as enzymes, and as enzymes acting on a membrane constituent lipid, sphingomyelin. Six types of sphingomyelinases are considered, namely acidic, secretory, Mg(2+)-dependent neutral, Mg(2+)-independent neutral, alkaline, and bacterial enzymes with both phospholipase C and sphingomyelinase activity. Sphingomyelinase assay methods and specific inhibitors are reviewed. Kinetic and mechanistic studies are summarized, a kinetic model and a general-base catalytic mechanism are proposed. Sphingomyelinase-membrane interactions are considered from the point of view of the influence of lipids on the enzyme activity. Moreover, effects of sphingomyelinase activity on membrane architecture (increased membrane permeability, membrane aggregation and fusion) are described. Finally, a number of open questions on the above topics are enunciated.

Observation of topical catalysis by sphingomyelinase coupled to microspheres

Sphingomyelinase, SMase (EC 3.1.4.12), was coupled onto amino-derivatized acrylate microspheres and was shown to retain its catalytic activity. The immobilized enzyme allows one to carry out topical enzymatic reaction in a controlled manner. Accordingly, these spheres were held with a micropipet and using micromanipulator brought into contact with a giant liposome membrane composed of phosphatidylcholine and sphingomyelin (SOPC/C16:0-SM, 0.75:0.25, molar ratio), representing the substrate for the immobilized enzyme. The macroscopic consequences of the enzyme reaction were visualized using fluorescence microscopy as well as differential interference contrast microscopy. The surface contact of the giant vesicle and immobilized enzyme causes membrane microdomain formation and domain clustering (capping) in the membrane and subsequent shedding of small vesicles from the membrane into the interior of the giant liposome. The method described represents a novel approach to study enzymatic reactions and allows manipulating giant vesicles as well as cultured cells in a spatially controlled manner.

Macroscopic consequences of the action of phospholipase C on giant unilamellar liposomes

Macroscopic consequences of the formation of diacylglycerol by phospholipase C (PC-PLC) in giant 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC) unilamellar vesicles (GUVs, diameter 10-100 microm) were studied by phase contrast and fluorescence microscopy. PC-PLC caused a series of fast stepwise shrinkages of fluid SOPC GUVs, continuing until the vesicle disappeared beyond the optical resolution of the microscope. The presence of N-palmitoyl-sphingomyelin (mole fraction X = 0.25) in the GUVs did not affect the outcome of the PC-PLC reaction. In addition to hydrolysis, PC-PLC induced adhesion of vicinal vesicles. When multilamellar SOPC vesicles were used only a minor decrease in their diameter was evident suggesting that PC-PLC can exert its hydrolytic activity only in the outer monolayer. A series of stepwise shrinkages was observed also for 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) GUVs above their main phase transition temperature, T(m), i.e., when the bilayer is in the liquid crystalline state. However, this process was not observed for DMPC GUVs in the gel state, below T(m). These results are supported by the enhanced activity of PC-PLC upon exceeding T(m) of DMPC large unilamellar vesicles (diameter approximately 0.1 microm) used as a substrate. Studies on SOPC monolayers revealed that PC-PLC can exert its hydrolytic activity only at surface pressures below approximately 30 mN/m. Accordingly, the lack of changes in the gel state DMPC GUVs could be explained by the equilibrium lateral pressure in these vesicles exceeding this critical value.

Interactions of the antimicrobial peptides temporins with model biomembranes. Comparison of temporins B and L

Temporins are short (10-13 amino acids) and linear antimicrobial peptides first isolated from the skin of the European red frog, Rana temporaria, and are effective against Gram-positive bacteria and Candida albicans. To get insight into their mechanism(s) of action, we compared the effects on model membranes exerted by two members of this family, viz., temporin B (LLPIVGNLLKSLL-NH(2)) and temporin L (FVQWFSKFLGRIL-NH(2)). More specifically, we measured their insertion into lipid monolayers as well as their effects on the structural dynamics of liposomal bilayers as revealed by diphenylhexatriene (DPH)- and pyrene-labeled phospholipids. We also observed the impact of these peptides on the topology of giant vesicles. Both temporins readily penetrate into lipid monolayers, their intercalation being enhanced in the presence of the common bacterial negatively charged phospholipid phosphatidylglycerol. Instead, the eukaryotic lipid cholesterol did to some extent counteract their penetration into the lipid films. Both temporin B and temporin L caused an enrichment of phospholipids in the bilayers, and in the presence of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG), these peptides increased acyl chain order. Temporin B had practically no effect on giant liposomes composed of 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), whereas rapid vesiculation was observed when POPG was present. In contrast, temporin L induced vesiculation of both SOPC and SOPC/POPG giant vesicles while the presence of cholesterol in SOPC giant vesicles attenuated this effect.

Sphingomyelinase cleavage of sphingomyelin in pure and mixed lipid membranes. Influence of the physical state of the sphingolipid

Sphingomyelin hydrolysis by sphingomyelinase is essential in regulating membrane levels of ceramide, a well-known metabolic signal. Since natural sphingomyelins have a gel-to-fluid transition temperature in the range of the physiological temperatures of mammals and birds, it is important to understand the influence of the physical state of the lipid on the enzyme activity. With that aim, large unilamellar vesicles consisting of pure egg sphingomyelin (gel-to-fluid crystalline transition temperature ca. 39 degrees C) were treated with sphingomyelinase in the temperature range 10-70 degrees C. The vesicles were also examined by differential scanning calorimetry (DSC). Shingomyelinase was active on pure sphingomyelin bilayers, leading to concomitant lipid hydrolysis, vesicle aggregation, and leakage of aqueous liposomal contents. Enzyme activity was found to be much higher when the substrate was in the fluid than when it was in the gel state. Sphingomyelinase activity was found to exhibit lag times, followed by bursts of activity. Lag times decreased markedly when the substrate went from the gel to the fluid state. When egg phosphatidylcholine, or egg phosphatidylethanolamine were included in the bilayer composition together with sphingomyelin, sphingomyelinase activity at 37 degrees C, that was negligible for the pure sphingolipid bilayers, was seen to increase with the proportion of glycerophospholipid, while the latency times became progressively shorter. A DSC study of the mixed-lipid vesicles revealed that both phosphatidylcholine and phosphatidyletanolamine decreased in a dose-dependent way the transition temperature of sphingomyelin. Thus, as those glycerophospholipids were added to the membrane composition, the proportion of sphingomyelin in the fluid state at 37 degrees C increased accordingly, in this way becoming amenable to rapid hydrolysis by the enzyme. Thus sphingomyelinase requires the substrate in bilayer form to be in the fluid state, irrespective of whether this is achieved through a thermotropic transition or by modulating bilayer composition.