Molecular associations and surface-active properties of short- and long-N-acyl chain ceramides

Lipid raft composition modulates sphingomyelinase activity and ceramide-induced membrane physical alterations

Lipid rafts and ceramide (Cer)-platforms are membrane domains that play an important role in several biological processes. Cer-platforms are commonly formed in the plasma membrane by the action of sphingomyelinase (SMase) upon hydrolysis of sphingomyelin (SM) within lipid rafts. The interplay among SMase activity, initial membrane properties (i.e., phase behavior and lipid lateral organization) and lipid composition, and the amount of product (Cer) generated, and how it modulates membrane properties were studied using fluorescence methodologies in model membranes. The activity of SMase was evaluated by following the hydrolysis of radioactive SM. It was observed that 1), the enzyme activity and extent of hydrolysis are strongly dependent on membrane physical properties but not on substrate content, and are higher in raft-like mixtures, i.e., mixtures with liquid-disordered/liquid-ordered phase separation; and 2), Cer-induced alterations are also dependent on membrane composition, specifically the cholesterol (Chol) content. In the lowest-Chol range, Cer segregates together with SM into small ( approximately 8.5 nm) Cer/SM-gel domains. With increasing Chol, the ability of Cer to recruit SM and form gel domains strongly decreases. In the high-Chol range, a Chol-enriched/SM-depleted liquid-ordered phase predominates. Together, these data suggest that in biological membranes, Chol in particular and raft domains in general play an important role in modulating SMase activity and regulating membrane physical properties by restraining Cer-induced alterations.

Sphingolipids in inflammation: pathological implications and potential therapeutic targets

Sphingolipids are formed via the metabolism of sphingomyelin, a constituent of the plasma membrane, or by de novo synthesis. Enzymatic pathways result in the formation of several different lipid mediators, which are known to have important roles in many cellular processes, including proliferation, apoptosis and migration. Several studies now suggest that these sphingolipid mediators, including ceramide, ceramide 1-phosphate and sphingosine 1-phosphate (S1P), are likely to have an integral role in inflammation. This can involve, for example, activation of pro-inflammatory transcription factors in different cell types and induction of cyclooxygenase-2, leading to production of pro-inflammatory prostaglandins. The mode of action of each sphingolipid is different. Increased ceramide production leads to the formation of ceramide-rich areas of the membrane, which may assemble signalling complexes, whereas S1P acts via high-affinity G-protein-coupled S1P receptors on the plasma membrane. Recent studies have demonstrated that in vitro effects of sphingolipids on inflammation can translate into in vivo models. This review will highlight the areas of research where sphingolipids are involved in inflammation and the mechanisms of action of each mediator. In addition, the therapeutic potential of drugs that alter sphingolipid actions will be examined with reference to disease states, such as asthma and inflammatory bowel disease, which involve important inflammatory components. A significant body of research now indicates that sphingolipids are intimately involved in the inflammatory process and recent studies have demonstrated that these lipids, together with associated enzymes and receptors, can provide effective drug targets for the treatment of pathological inflammation.

Ceramides: branched alkyl chains in the sphingolipid siblings of diacylglycerol improve biological potency

The synthesis of a small number of ceramide analogues containing a combination of linear and highly branched alkyl chains on either the d-sphingosine or the N-acyl core of the molecule is reported. Regardless of location, the presence of the branched chain improves potency relative to the positive control, C2 ceramide; however, the most potent compound (4) has the branched side chain as part of the d-sphingosine core. The induction of apoptosis by 4 in terms of Annexin V binding and DiOC(6) labeling was superior to that achieved with C2 ceramide.

Effects of ceramide and other simple sphingolipids on membrane lateral structure

The available data concerning the ability of ceramide and other simple sphingolipids to segregate laterally into rigid, gel-like domains in a fluid bilayer has been reviewed. Ceramides give rise to rigid ceramide-enriched domains when their N-acyl chain is longer than C12. The high melting temperature of hydrated ceramides, revealing a tight intermolecular interaction, is probably responsible for their lateral segregation. Ceramides compete with cholesterol for the formation of domains with lipids such as sphingomyelin or saturated phosphatidylcholines; under these conditions displacement of cholesterol by ceramide involves a transition from a liquid-ordered to a gel-like phase in the domains involved. When ceramide is generated in situ by a sphingomyelinase, instead of being premixed with the other lipids, gel-like domain formation occurs as well, although the topology of the domains may not be the same, the enzyme causing clustering of domains that is not detected with premixed ceramide. Ceramide-1-phosphate is not likely to form domains in fluid bilayers, and the same is true of sphingosine and of sphingosine-1-phosphate. However, sphingosine does rigidify pre-existing gel domains in mixed bilayers.

Physicochemical and biological characterization of ceramide-containing liposomes: paving the way to ceramide therapeutic application

Ceramides mediate antiproliferative responses, and it has been proposed that increasing the level of ceramides in cancer cells may have a therapeutic antitumor effect. However, ceramides, because of their high "packing parameter" (PP), do not form lipid assemblies that can be dispersed in a form suitable for intravenous administration. We found that nanoliposomes containing short- or medium-chain ceramides are unstable because of their very high (>1.3) PP. To overcome this major obstacle, we included the lipopolymer 2kPEG-DSPE, which reduces the additive PP. The presence of PEG-DSPE allows the formation of highly stable (>1 year) ceramide (Cer)-containing nanoliposomes suitable for systemic administration. Using tumor cell lines, we found that the ceramide cytotoxicity was not impaired by their inclusion in nanoliposomes. The use of 14C-labeled ceramides shows that the C6Cer, but not C16Cer, was transferred from the nanoliposomes to the cells and metabolized efficiently. The difference between the two ceramides is related to the large difference between their critical aggregation concentration and was correlated with the much higher cytotoxity of liposomal C6Cer. The activity of 2kPEG-DSPE as a steric stabilizer (as previously shown for Doxil) was also confirmed for C6Cer-containing nanoliposomes. The 2kPEG-DSPE lipopolymer significantly reduced the desorption rate of the ceramide from the liposome bilayer, thereby allowing liposomes containing C6Cer to reach the tumor site and to demonstrate therapeutic efficacy.

Ceramide 1-phosphate stimulates macrophage proliferation through activation of the PI3-kinase/PKB, JNK and ERK1/2 pathways

Ceramide 1-phosphate (C1P) was first shown to be mitogenic for fibroblasts, but the mechanisms whereby it stimulated cell proliferation have remained largely unknown. Here we demonstrate that C1P stimulates DNA synthesis and cell division in murine bone marrow-derived macrophages. C1P caused rapid phosphorylation of protein kinase B (PKB, also known as Akt), a downstream target of phosphatidylinositol 3-kinase (PI3-K). Selective inhibition of PI3-K blocked both DNA synthesis and cell growth. C1P induced phosphorylation of GSK-3beta, which is a major target of PKB, and this effect was also abolished by inhibition of PI3-K. In addition, C1P upregulated the expression of cyclin D1 and c-Myc, two major targets of GSK-3beta, which are important regulators of cell proliferation. C1P stimulated the activity of NF-kappaB, and inhibitors of this transcription factor completely blocked macrophage proliferation. Lastly, C1P induced phosphorylation of the mitogen activated protein kinases (MAPK) extracellularly regulated kinases 1 and 2 (ERK1/2), and c-Jun N-terminal kinase (JNK). Inhibition of ERK1/2 and JNK also blocked C1P-induced macrophage proliferation. It can be concluded that C1P stimulates macrophage proliferation through activation of the PI3-K/PKB, ERK and JNK pathways, and that GSK-3beta, c-Myc, cyclin D1, and NF-kappaB are important downstream effectors in this action.

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.

Surface-active properties of the antitumour ether lipid 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (edelfosine)

The surface activity and interaction with lipid monolayers and bilayers of the antitumour ether lipid 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (edelfosine) have been studied. Edelfosine is a surface-active soluble amphiphile, with critical micellar concentrations at 3.5 microM and 19 microM in water. When the air-water interface is occupied by a phospholipid, edelfosine becomes inserted in the phospholipid monolayer, increasing surface pressure. This increase is dose-dependent, and reaches a plateau at ca. 2 microM edelfosine bulk concentration. The ether lipid can become inserted in phospholipid monolayers with initial surface pressures of up to 33 mN/m, which ensures its capacity to become inserted into cell membranes. Upon interaction with phospholipid vesicles, edelfosine exhibits a weak detergent activity, causing release of vesicle contents to a low extent (<5%), and a small proportion of lipid solubilization. The weak detergent properties of edelfosine can be related to its very low critical micellar concentrations. Its high affinity for lipid monolayers combined with low lytic properties support the use of edelfosine as a clinical drug. The surface-active properties of edelfosine are similar to those of other "single-chain" lipids, e.g. lysophosphatidylcholine, palmitoylcarnitine, or N-acetylsphingosine.

Biophysics of sphingolipids II. Glycosphingolipids: An assortment of multiple structural information transducers at the membrane surface

Glycosphingolipids are ubiquitous components of animal cell membranes. They are constituted by the basic structure of ceramide with its hydroxyl group linked to single carbohydrates or oligosaccharide chains of different complexity. The combination of the properties of their hydrocarbon moiety with those derived from the variety and complexity of their hydrophilic polar head groups confers to these lipids an extraordinary capacity for molecular-to-supramolecular transduction across the lateral/transverse planes in biomembranes and beyond. In our opinion, most of the advances made over the last decade on the biophysical behavior of glycosphingolipids can be organized into three related aspects of increasing structural complexity: (1) intrinsic codes: local molecular interactions of glycosphingolipids translated into structural self-organization. (2) Surface topography: projection of molecular shape and miscibility of glycosphingolipids into formation of coexisting membrane domains. (3) Beyond the membrane interface: glycosphingolipid as modulators of structural topology, bilayer recombination and surface biocatalysis.

Biophysics of sphingolipids I. Membrane properties of sphingosine, ceramides and other simple sphingolipids

Some of the simplest sphingolipids, namely sphingosine, ceramide, some closely related molecules (eicosasphingosine, phytosphingosine), and their phosphorylated compounds (sphingosine-1-phosphate, ceramide-1-phosphate), are potent metabolic regulators. Each of these lipids modifies in marked and specific ways the physical properties of the cell membranes, in what can be the basis for some of their physiological actions. This paper reviews the mechanisms by which these sphingolipid signals, sphingosine and ceramide in particular, are able to modify the properties of cell membranes.

Ceramides and other bioactive sphingolipid backbones in health and disease: lipidomic analysis, metabolism and roles in membrane structure, dynamics, signaling and autophagy

Sphingolipids are comprised of a backbone sphingoid base that may be phosphorylated, acylated, glycosylated, bridged to various headgroups through phosphodiester linkages, or otherwise modified. Organisms usually contain large numbers of sphingolipid subspecies and knowledge about the types and amounts is imperative because they influence membrane structure, interactions with the extracellular matrix and neighboring cells, vesicular traffic and the formation of specialized structures such as phagosomes and autophagosomes, as well as participate in intracellular and extracellular signaling. Fortunately, "sphingolipidomic" analysis is becoming feasible (at least for important subsets such as all of the backbone "signaling" subspecies: ceramides, ceramide 1-phosphates, sphingoid bases, sphingoid base 1-phosphates, inter alia) using mass spectrometry, and these profiles are revealing many surprises, such as that under certain conditions cells contain significant amounts of "unusual" species: N-mono-, di-, and tri-methyl-sphingoid bases (including N,N-dimethylsphingosine); 3-ketodihydroceramides; N-acetyl-sphingoid bases (C2-ceramides); and dihydroceramides, in the latter case, in very high proportions when cells are treated with the anticancer drug fenretinide (4-hydroxyphenylretinamide). The elevation of DHceramides by fenretinide is befuddling because the 4,5-trans-double bond of ceramide has been thought to be required for biological activity; however, DHceramides induce autophagy and may be important in the regulation of this important cellular process. The complexity of the sphingolipidome is hard to imagine, but one hopes that, when partnered with other systems biology approaches, the causes and consequences of the complexity will explain how these intriguing compounds are involved in almost every aspect of cell behavior and the malfunctions of many diseases.

Tailoring structure-function and targeting properties of ceramides by site-specific cationization

In the course of our studies on compartment-specific lipid-mediated cell regulation, we identified an intimate connection between ceramides (Cers) and the mitochondria-dependent death-signaling pathways. Here, we report on a new class of cationic Cer mimics, dubbed ceramidoids, designed to act as organelle-targeted sphingolipids (SPLs), based on conjugates of Cer and dihydroceramide (dhCer) with pyridinium salts (CCPS and dhCCPS, respectively). Ceramidoids having the pyridinium salt unit (PSU) placed internally (alpha and gamma- CCPS) or as a tether (omega-CCPS) in the N-acyl moiety were prepared by N-acylation of sphingoid bases with different omega-bromo acids or pyridine carboxylic acid chlorides following capping with respective pyridines or alkyl bromides. Consistent with their design, these analogs, showed a significantly improved solubility in water, well-resolved NMR spectra in D(2)O, broadly modified hydrophobicity, fast cellular uptake, and higher anticancer activities in cells in comparison to uncharged counterparts. Structure-activity relationship (SAR) studies in MCF-7 breast carcinoma cells revealed that the location of the PSU and its overall chain length affected markedly the cytotoxic effects of these ceramidoids. All omega-CCPSs were more potent (IC(50/48 h): 0.6-8.0 microM) than their alpha/gamma-CCPS (IC(50/48 h): 8-20 microM) or D-erythro-C6-Cer (IC(50/48 h): 15 microM) analogs. omega-DhCCPSs were also moderately potent (IC(50/48 h): 2.5-12.5 microM). Long-chain omega-dhCCPSs were rapidly and efficiently oxidized in cells to the corresponding omega-CCPSs, as established by LC-MS analysis. CCPS analogs also induced acute changes in the levels and composition of endogenous Cers (upregulation of C16-, C14-, and C18-Cers, and downregulation of C24:0- and C24:1-Cers). These novel ceramidoids illustrate the feasibility of compartment-targeted lipids, and they should be useful in cell-based studies as well as potential novel therapeutics.

Ceramide 1-phosphate/ceramide, a switch between life and death

Ceramide is a well-characterized sphingolipid metabolite and second messenger that participates in numerous biological processes. In addition to serving as a precursor to complex sphingolipids, ceramide is a potent signaling molecule capable of regulating vital cellular functions. Perhaps its major role in signal transduction is to induce cell cycle arrest, and promote apoptosis. In contrast, little is known about the metabolic or signaling pathways that are regulated by the phosphorylated form of ceramide. It was first demonstrated that ceramide-1-phosphate (C1P) had mitogenic properties, and more recently it has been described as potent inhibitor of apoptosis and inducer of cell survival. C1P and ceramide are antagonistic molecules that can be interconverted in cells by kinase and phosphatase activities. An appropriate balance between the levels of these two metabolites seems to be crucial for cell and tissue homeostasis. Switching this balance towards accumulation of one or the other may result in metabolic dysfunction, or disease. Therefore, the activity of the enzymes that are involved in C1P and ceramide metabolism must be efficiently coordinated to ensure normal cell functioning.

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.

Sphingosine increases the permeability of model and cell membranes

Sphingosine, at 5-15 mol % total lipids, remarkably increases the permeability to aqueous solutes of liposomal and erythrocyte ghost membranes. The increased permeability cannot be interpreted in terms of leakage occurring at the early stages of a putative membrane solubilization by sphingosine, nor is it due to a sphingosine-induced generation of nonlamellar structures, or flip-flop lipid movement. Instead, sphingosine stabilizes (rigidifies) gel domains in membranes, raising their melting temperatures and increasing the transition cooperativity. Structural defects originating during the lateral phase separation of the "more rigid" and "less rigid" domains are likely sites for the leakage of aqueous solutes to the extravesicular medium. The presence of coexisting domains in the plasma membrane makes it a target for sphingosine permeabilization. The sphingosine-induced increase in rigidity and breakdown of the plasma membrane permeability barrier could be responsible for some of the physiological effects of sphingosine.

Acyl chain length affects ceramide action on sterol/sphingomyelin-rich domains

The effects of ceramides with varying saturated N-linked acyl chains (C2-C14) on cholesterol displacement from sphingomyelin-rich domains and on the stability of ordered domains were studied. The bilayers examined were made from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), D-erythro-N-palmitoyl-sphingomyelin (PSM), D-erythro-N-acyl-sphingosine, and cholesterol (60:15:15:10 mol%, respectively). Cholestatrienol (CTL) or D-erythro-N-trans-parinoyl-sphingomyelin (tParSM) were used as reporter molecules (at 1 mol%) for the ordered domains, and 1-palmitoyl-2-stearoyl-(7-doxyl)-sn-glycero-3-phosphocholine (7SLPC) as a fluorescence quencher (30 mol%, replacing POPC) in the liquid-disordered phase. The results indicate that the ceramide had to have an N-linked acyl chain with at least 8 methylene units in order for it to displace cholesterol from the sphingomyelin-rich domains at the concentration used. The melting of the sphingomyelin-rich domain shifted to higher temperatures (compared to the ceramide-free control) with C2, C12 and longer chain ceramides, whereas C4-C10 ceramides led to domain melting at lower temperatures than control. This study shows that short-chain ceramides do not have the same effects on sterol- and sphingomyelin-rich domains as naturally occurring longer-chain ceramides have.

Detergent-resistant, ceramide-enriched domains in sphingomyelin/ceramide bilayers

When cell membranes are treated with Triton X-100 or other detergents at 4 degrees C, a nonsolubilized fraction can often be recovered, the "detergent-resistant membranes", that is not found when detergent treatment takes place at 37 degrees C. Detergent-resistant membranes may be related in some cases to membrane "rafts". However, several basic aspects of the formation of detergent-resistant membranes are poorly understood. To answer some of the relevant questions, a simple bilayer composition that would mimic detergent-resistant membranes was required. The screening of multiple lipid compositions has shown that the binary mixture egg sphingomyelin/egg ceramide (SM/Cer) exhibits the required detergent resistance. In detergent-free membranes composed of different mixtures of SM and Cer (5-30 mol % of Cer) differential scanning calorimetry, fluorescence spectroscopy, and fluorescence microscopy experiments reveal the presence of discrete, Cer-enriched gel domains in a broad temperature range. In particular, at temperatures below SM phase transition ( approximately 40 degrees C) two gel (respectively Cer-rich and SM-rich) phases are directly observed using fluorescence microscopy. Although pure SM membranes are fully solubilized by Triton X-100 at room temperature, 5 mol % Cer is also enough to induce detergent resistance, even with a large detergent excess and lengthy equilibration times. Short-chain Cers do not give rise to detergent resistance. SM/Cer mixtures containing up to 30 mol % Cer become fully soluble at approximately 50 degrees C, i.e., well above the gel-fluid transition temperature of SM. The combined results of temperature-dependent solubilization and differential scanning calorimetry reveal that SM-rich domains are preferentially solubilized over the Cer-rich ones as soon as the former melt (i.e., at approximately 40 degrees C). As a consequence, at temperatures allowing only partial solubilization, the nonsolubilized residue is enriched in Cer with respect to the original bilayer composition. Fluorescence microscopy of giant unilamellar vesicles at room temperature clearly shows that SM-rich domains are preferentially solubilized over the Cer-rich ones and that the latter become more rigid and extensive as a consequence of the detergent effects. These observations may be relevant to the phenomena of sphingomyelinase-dependent signaling, generation of "raft platforms", and detergent-resistant cell membranes.

New ceramides from Rantherium suaveolens

A mixture of five new ceramides was isolated from the aerial parts of Rantherium suaveolens and characterized by spectroscopic and chemical methods. Their structures were elucidated by spectroscopic and chemical methods as (2S,3S, 4R,2'R, 14E)-2-(2'-hydroxydocosanoylamino) - 14 - octadecene -1,3,4-triol (1), (2S,3S,4R,2'R, 14E)-2-(2'-hydroxytricosanoylamino)-14-octadecene-1,3,4-triol (2), (2S,3S,4R,2'R, 14E)-2-(2'-hydroxytetracosanoylamino)- 14 - octadecene - 1,3,4 - triol (3), (2S, 3S,4R,2'R, 14E) - 2 - (2'- hydroxypentacosanoylamino) - 14 - octadecene - 1,3,4-triol (4), and (2S,3S,4R,2'R,14E)-2-(2'-hydroxyhexacosanoylamino)-14-octadecene-1,3,4-triol (5).

Different effects of long- and short-chain ceramides on the gel-fluid and lamellar-hexagonal transitions of phospholipids: a calorimetric, NMR, and x-ray diffraction study

The effects on dielaidoylphosphatidylethanolamine (DEPE) bilayers of ceramides containing different N-acyl chains have been studied by differential scanning calorimetry small angle x-ray diffraction and (31)P-NMR spectroscopy. N-palmitoyl (Cer16), N-hexanoyl (Cer6), and N-acetyl (Cer2) sphingosines have been used. Both the gel-fluid and the lamellar-inverted hexagonal transitions of DEPE have been examined in the presence of the various ceramides in the 0-25 mol % concentration range. Pure hydrated ceramides exhibit cooperative endothermic order-disorder transitions at 93 degrees C (Cer16), 60 degrees C (Cer6), and 54 degrees C (Cer2). In DEPE bilayers, Cer16 does not mix with the phospholipid in the gel phase, giving rise to high-melting ceramide-rich domains. Cer16 favors the lamellar-hexagonal transition of DEPE, decreasing the transition temperature. Cer2, on the other hand, is soluble in the gel phase of DEPE, decreasing the gel-fluid and increasing the lamellar-hexagonal transition temperatures, thus effectively stabilizing the lamellar fluid phase. In addition, Cer2 was peculiar in that no equilibrium could be reached for the Cer2-DEPE mixture above 60 degrees C, the lamellar-hexagonal transition shifting with time to temperatures beyond the instrumental range. The properties of Cer6 are intermediate between those of the other two, this ceramide decreasing both the gel-fluid and lamellar-hexagonal transition temperatures. Temperature-composition diagrams have been constructed for the mixtures of DEPE with each of the three ceramides. The different behavior of the long- and short-chain ceramides can be rationalized in terms of their different molecular geometries, Cer16 favoring negative curvature in the monolayers, thus inverted phases, and the opposite being true of the micelle-forming Cer2. These differences may be at the origin of the different physiological effects that are sometimes observed for the long- and short-chain ceramides.