Comparison of phosphatidylcholines containing one or two docosahexaenoic acyl chains on properties of phospholipid monolayers and bilayers

Fundamentals about onset and progressive disease character of type 2 diabetes mellitus

ResearchGate is a world wide web for scientists and researchers to share papers, ask and answer questions, and find collaborators. As one of the more than 15 million members, the author uploads research output and reads and responds to some of the questions raised, which are related to type 2 diabetes. In that way, he noticed a serious gap of knowledge of this disease among medical professionals over recent decades. The main aim of the current study is to remedy this situation through providing a comprehensive review on recent developments in biochemistry and molecular biology, which can be helpful for the scientific understanding of the molecular nature of type 2 diabetes. To fill up the shortcomings in the curricula of medical education, and to familiarize the medical community with a new concept of the onset of type 2 diabetes, items are discussed like: Insulin resistance, glucose effectiveness, insulin sensitivity, cell membranes, membrane flexibility, unsaturation index (UI; number of carbon-carbon double bonds per 100 acyl chains of membrane phospholipids), slow-down principle, effects of temperature acclimation on phospholipid membrane composition, free fatty acids, energy transport, onset of type 2 diabetes, metformin, and exercise. Based on the reviewed data, a new model is presented with proposed steps in the development of type 2 diabetes, a disease arising as a result of a hypothetical hereditary anomaly, which causes hyperthermia in and around the mitochondria. Hyperthermia is counterbalanced by the slow-down principle, which lowers the amount of carbon-carbon double bonds of membrane phospholipid acyl chains. The accompanying reduction in the UI lowers membrane flexibility, promotes a redistribution of the lateral pressure in cell membranes, and thereby reduces the glucose transporter protein pore diameter of the transmembrane glucose transport channel of all Class I GLUT proteins. These events will set up a reduction in transmembrane glucose transport. So, a new blood glucose regulation system, effective in type 2 diabetes and its prediabetic phase, is based on variations in the acyl composition of phospholipids and operates independent of changes in insulin and glucose concentration. UI assessment is currently arising as a promising analytical technology for a membrane flexibility analysis. An increase in mitochondrial heat production plays a pivotal role in the existence of this regulation system.

Polyunsaturated Phospholipid Modified Membrane Degradation Catalyzed by a Secreted Phospholipase A2

To optimize the compositions of the lipid-based nanomedicine and to advance understanding of the roles of polyunsaturated phospholipids in biological membranes, this study examined the effects of polyunsaturated phospholipids on the degradation of giant unilamellar vesicles catalyzed by a secreted phospholipase A2 (sPLA₂) using fluorescence microscopy. Molecular interfacial packing, interaction, and degradation of the films containing various mixing ratios of saturated and polyunsaturated phospholipids were quantified using a Langmuir trough integrated with synchrotron X-ray surface scattering techniques. It was found that a high molar fraction (0.63 and above) of polyunsaturated phospholipids not only enhanced the rate of sPLA₂-catalyzed vesicle degradation but also changed the vesicle deformation process and degradation product morphology. Hydrolysis of the saturated phospholipids generated highly ordered liquid crystal domains, which was reduced or prohibited by the presence of the polyunsaturated phospholipids in the reactant film.

Alteration of mitochondrial phospholipid due to the PLA2 activation in rat brains under cadmium toxicity

Cadmium (Cd) is a heavy metal that has received considerable environmental and occupational concern. Cd causes toxic effects due to its accumulation in a variety of tissues, including the kidney, liver and the nervous system (CNS); however, the exact mechanism is poorly understood. In the present study, we tried to explore the impact of acute cadmium exposure on rat brain phospholipids (PLs). Cd exposure significantly reduced PLs in a time dependent manner and the reduction was due to the activation of the Phospholipase A2 enzymes (sPLA2, cPLA2). The release of arachidonic acid from PLs increased during inflammatory conditions by PLA2s. The mRNA expression of cyclooxygenase2 (COX2) and subsequently the pro-inflammatory cytokines, namely, Interleukin 1 (IL-1) and IL-6, were up regulated; however, the expression of anti-inflammatory cytokine IL-10 was reduced in a time dependent manner. The expression of the Tumor necrosis factor alpha (TNF-α), Inducible nitric oxide synthase (iNOS) and Interferon gamma (INF-γ) also experienced increases in the expression. Likewise the mRNA expression of the pro-apoptotic factor, Bcl-2-associated X protein (Bax), was elevated, whereas anti-apoptosis B-cell lymphoma 2 (Bcl2) was down regulated. This present study might help to decipher the effects of cadmium toxicity on rat brain.

Perspectives on the membrane fatty acid unsaturation/pacemaker hypotheses of metabolism and aging

The membrane pacemaker hypotheses of metabolism and aging are distinct, but interrelated hypotheses positing that increases in unsaturation of lipids within membranes are correlated with increasing basal metabolic rate and decreasing longevity, respectively. The two hypotheses each have evidence that either supports or contradicts them, but consensus has failed to emerge. In this review, we identify sources of weakness of previous studies supporting and contradicting these hypotheses and suggest different methods and lines of inquiry. The link between fatty acyl composition of membranes and membrane-bound protein activity is a central tenet of the membrane pacemaker hypothesis of metabolism, but the mechanism by which unsaturation would change protein activity is not well defined and, whereas fatty acid desaturases have been put forward by some as the mechanism behind evolutionary differences in fatty acyl composition of phospholipids among organisms, there have been no studies to differentiate whether desaturases have been more affected by natural selection on aging and metabolic rate than have elongases or acyltransferases. Past analyses have been hampered by potentially incorrect estimates of the peroxidizability of lipids and longevity of study animals, and by the confounding effect of phylogeny. According to some authors, body mass may also be a confounding effect that should be taken into account, though this is not universally accepted. Further research on this subject should focus more on mechanisms and take weaknesses of past studies into account.

Phospholipid peroxidation: lack of effect of fatty acid pairing

Phospholipids where both fatty acids are polyunsaturated are very rare. Most organisms prefer to couple their polyunsaturated fatty acids (PUFA) with either a saturated (SAT) or a monounsaturated (MUFA) fatty acid. This study examined if these natural couplings are there to protect PUFA from themselves. Specifically, does the coupling of PUFA to SAT or MUFA reduce the potential for increased rates of peroxidation by shrouding these highly peroxidisable fatty acids with less peroxidisable fatty acids? The influence of head group was examined by using the two most common phospholipids found in vertebrate membranes i.e. phosphatidylcholine and phosphatidylethanolamine species. Fatty acid pairings included 16:0/18:2 versus 18:2/18:2 and 16:0/22:6 versus 22:6/22:6. All phospholipids were incorporated into liposomes that were matched for their total PUFA content i.e. 25% PUFA/PUFA or 50% SAT/PUFA with phosphatidylcholine 16:0/16:0 used as the background phospholipid. An iron initiator (Fe²⁺/H₂O₂) was used to induce peroxidation and lipid hydroperoxide production was used to measure peroxidation. The results show that coupling of PUFA together on the same molecule does not increase peroxidation rates and therefore does not support the proposed hypothesis. The lower than expected levels of peroxidation measured for some phospholipid species (e.g. PtdEtn 22:6/22:6) is possibly due to the partitioning of these molecular species into the inner leaflet of the bilayer.

Visualization of the cell-selective distribution of PUFA-containing phosphatidylcholines in mouse brain by imaging mass spectrometry

Previous studies have shown that MALDI-imaging mass spectrometry (IMS) can be used to visualize the distribution of various biomolecules, especially lipids, in the cells and tissues. In this study, we report the cell-selective distribution of PUFA-containing glycerophospholipids (GPLs) in the mouse brain. We established a practical experimental procedure for the IMS of GPLs. We demonstrated that optimization of the composition of the matrix solution and spectrum normalization to the total ion current (TIC) is critical. Using our procedure, we simultaneously differentiated and visualized the localizations of specific molecular species of GPLs in mouse brain sections. The results showed that PUFA-containing phosphatidylcholines (PCs) were distributed in a cell-selective manner: arachidonic acid- and docosahexaenoic acid-containing PCs were seen in the hippocampal neurons and cerebellar Purkinje cells, respectively. Furthermore, these characteristic localizations of PUFA-PCs were formed during neuronal maturation. The phenomenon of brain cell-selective production of specific PUFA-GPLs will help elucidate the potential physiological functions of PUFAs in specific brain regions.

Bioactive dietary long-chain fatty acids: emerging mechanisms of action

The plasma membranes of all eukaryotic cells contain heterogeneous self-organising intrinsically unstable liquid ordered domains or lipid assemblies in which key signal transduction proteins are localised. These assemblies are classified as 'lipid rafts' (10-200 nm), which are composed mostly of cholesterol and sphingolipid microdomains and therefore do not integrate well into the fluid phospholipid bilayers. In addition, caveolae represent a subtype of lipid raft macrodomain that form flask-shaped membrane invaginations containing structural proteins, i.e. caveolins. With respect to the diverse biological effects of long-chain PUFA, increasing evidence suggests that n-3 PUFA and perhaps conjugated fatty acids uniquely alter the basic properties of cell membranes. Because of its polyunsaturation, DHA and possibly conjugated linoleic acid are sterically incompatible with sphingolipid and cholesterol and, therefore, appear to alter lipid raft behaviour and protein function. The present review examines the evidence indicating that dietary sources of n-3 PUFA can profoundly alter the biochemical make up of lipid rafts/caveolae microdomains, thereby influencing cell signalling, protein trafficking and cell cytokinetics.

Seasonal acclimatization of brain lipidome in a eurythermal fish (Carassius carassius) is mainly determined by temperature

Crucian carp ( Carassius carassius) is an excellent vertebrate model for studies on temperature adaptation in biological excitable membranes, since the species can tolerate temperatures from 0 to +36°C. To determine how temperature affects the lipid composition of brain, the fish were acclimated for 4 wk at +30, +16, or +4°C in the laboratory, or seasonally acclimatized individuals were captured from the wild throughout the year (temperature = +1 to +23°C), and the brain glycerophospholipid and sphingolipid compositions were analyzed in detail by electrospray-ionization mass spectrometry. Numerous significant temperature-related changes were found in the molecular species composition of the membrane lipids. The most notable and novel finding was a large (∼3-fold) increase of the di-22:6n-3 phosphatidylserine and phosphatidylethanolamine species in the cold. Since the increase of 22:6n-3 in the total fatty acyl pool of the brain was small, the formation of di-22:6n-3 aminophospholipid species appears to be a specific adaptation to low temperature. Such highly unsaturated species could be needed to maintain adequate membrane fluidity in the vicinity of transporters and other integral membrane proteins. Plasmalogens increased somewhat at higher temperatures, possibly to protect membranes against oxidation. The modifications of brain lipidome during the 4-wk laboratory acclimation were, in many respects, similar to those found in the wild, which indicates that the seasonal changes observed in the wild are temperature dependent rather than induced by other environmental factors.

Oleic and docosahexaenoic acid differentially phase separate from lipid raft molecules: a comparative NMR, DSC, AFM, and detergent extraction study

We have previously suggested that the omega-3 polyunsaturated fatty acid, docosahexaenoic acid (DHA) may in part function by enhancing membrane lipid phase separation into lipid rafts. Here we further tested for differences in the molecular interactions of an oleic (OA) versus DHA-containing phospholipid with sphingomyelin (SM) and cholesterol (CHOL) utilizing (2)H NMR spectroscopy, differential scanning calorimetry, atomic force microscopy, and detergent extractions in model bilayer membranes. (2)H NMR and DSC (differential scanning calorimetry) established the phase behavior of the OA-containing 1-[(2)H(31)]palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (16:0-18:1PE-d(31))/SM (1:1) and the DHA-containing 1-[(2)H(31)]palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphoethanolamine (16:0-22:6PE-d(31))/SM (1:1) in the absence and presence of equimolar CHOL. CHOL was observed to affect the OA-containing phosphatidylethanolamine (PE) more than the DHA-containing PE, as exemplified by >2 x greater increase in order measured for the perdeuterated palmitic chain in 16:0-18:1PE-d(31)/SM (1:1) compared to 16:0-22:6PE-d(31)/SM (1:1) bilayers in the liquid crystalline phase. Atomic force microscopy (AFM) experiments showed less lateral phase separation between 16:0-18:1PE-rich and SM/CHOL-rich raft domains in 16:0-18:1PE/SM/CHOL (1:1:1) bilayers than was observed when 16:0-22:6PE replaced 16:0-18:1PE. Differences in the molecular interaction of 16:0-18:1PE and 16:0-22:6PE with SM/CHOL were also found using biochemical detergent extractions. In the presence of equimolar SM/CHOL, 16:0-18:1PE showed decreased solubilization in comparison to 16:0-22:6PE, indicating greater phase separation with the DHA-PE. Detergent experiments were also conducted with cardiomyocytes fed radiolabeled OA or DHA. Although both OA and DHA were found to be largely detergent solubilized, the amount of OA that was found to be associated with raft-rich detergent-resistant membranes exceeded DHA by almost a factor of 2. We conclude that the OA-PE phase separates from rafts far less than DHA-PE, which may have implications for cellular signaling.

Electric field strength of membrane lipids from vertebrate species: membrane lipid composition and Na+-K+-ATPase molecular activity

Intramembrane electric field strength is a very likely determinant of the activity of ion-transporting membrane proteins in living cells. In the absence of any transmembrane electrical potential or surface potential, its magnitude is determined by the dipole potential of the membrane's lipid components and their associated water of hydration. Here we have used a fluorometric method to quantify the dipole potential of vesicles formed from lipids extracted from kidney and brain of 11 different animal species from four different vertebrate classes. The dipole potential was compared with the fatty acid composition and with the Na(+)-K(+)-ATPase molecular activity of each preparation. The magnitude of the dipole potential was found to be relatively constant across all animal species, i.e., 236-334 mV for vesicles prepared from the total membrane lipids and 223-256 mV for phospholipids alone. The significantly lower value for phospholipids alone is potentially related to the removal of cholesterol and/or other common soluble lipid molecules from the membrane. Surprisingly, no significant dependence of the dipole potential on fatty acid composition was found. This may, however, be due to concomitant compensatory variations in lipid head group composition. The molecular activity of the Na(+)-K(+)-ATPase was found to increase with increasing dipole potential. The fact that the dipole potential is maintained at a relatively constant value over a wide range of animal species suggests that it may play a fundamental role in ensuring correct ion pump conformation and function within the membrane.

Packing and electrostatic behavior of sn-2-docosahexaenoyl and -arachidonoyl phosphoglycerides

Mammalian synaptic membranes appear to contain high proportions of specific, sn-1-stearoyl-2-docosahexaenoyl- and sn-1-stearoyl-2-arachidonoyl phosphoglycerides, but the structural significance of this is unclear. Here we used a standardized approach to compare the properties of homogeneous monolayers of the corresponding phosphatidylcholines, phosphatidylethanolamines, phosphatidylserines, and phosphatidic acids with those of control monolayers of sn-1-stearoyl-2-oleoyl- and sn-1-palmitoyl-2-oleoyl phosphoglycerides. Major findings were: 1), that the presence of an sn-2-docosahexaenoyl group or an sn-2-arachidonoyl group increases the molecular areas of phosphoglycerides by 3.8 A(2) (7%) relative to the presence of an sn-2-oleoyl group; 2), that the phosphorylcholine headgroup independently increases molecular areas by a larger amount, 7.1 A(2) (13%); and 3), that the dipole moments of species having an arachidonoyl moiety or an oleoyl moiety are 83 mD (19%) higher than those of comparable docosahexaenoic acid-containing phosphoglycerides. These and other results provide new information about the molecular packing properties of polyenoic phosphoglycerides and raise important questions about the role of these phosphoglycerides in synapses.

Docosahexaenoic acid: membrane properties of a unique fatty acid

Docosahexaenoic acid (DHA) with 22-carbons and 6 double bonds is the extreme example of an omega-3 polyunsaturated fatty acid (PUFA). DHA has strong medical implications since its dietary presence has been positively linked to the prevention of numerous human afflictions including cancer and heart disease. The PUFA, moreover, is essential to neurological function. It is remarkable that one simple molecule has been reported to affect so many seemingly unrelated biological processes. Although details of a molecular mode of action remain elusive, DHA must be acting at a fundamental level common to many tissues that is related to the high degree of conformational flexibility that the multiple double bonds have been identified to confer. One likely target for DHA action is at the cell membrane where the fatty acid is known to readily incorporate into membrane phospholipids. Once esterified into phospholipids DHA has been demonstrated to significantly alter many basic properties of membranes including acyl chain order and "fluidity", phase behavior, elastic compressibility, permeability, fusion, flip-flop and protein activity. It is concluded that DHA's interaction with other membrane lipids, particularly cholesterol, may play a prominent role in modulating the local structure and function of cell membranes.

Docosahexaenoic acid (DHA) content of membranes determines molecular activity of the sodium pump: implications for disease states and metabolism

The omega-3 polyunsaturate, docosahexaenoic acid (DHA), plays a number of biologically important roles, particularly in the nervous system, where it is found in very high concentrations in cell membranes. In infants DHA is required for the growth and functional development of the brain, with a deficiency resulting in a variety of learning and cognitive disorders. During adulthood DHA maintains normal brain function and recent evidence suggests that reduced DHA intake in adults is linked with a number of neurological disorders including schizophrenia and depression. Here we report a high positive correlation between the molecular activity (ATP min(-1)) of individual Na(+)K(+)ATPase units and the content of DHA in the surrounding membrane bilayer. This represents a fundamental relationship underlying metabolic activity, but may also represent a link between reduced levels of DHA and neurological dysfunction, as up to 60% of energy consumption in the brain is linked to the Na(+)K(+)ATPase enzyme.

Rapid flip-flop in polyunsaturated (docosahexaenoate) phospholipid membranes

The transbilayer movement (flip-flop) of 7-nitrobenz-2-oxa-1,3-diazol-4-yl phosphatidylethanolamine (NBD-PE) in phosphatidylcholine (PC) membranes containing various acyl chains was measured by dithionite quenching of NBD fluorescence. Of specific interest was docosahexaenoic acid (DHA), the longest and most unsaturated acyl chain commonly found in membranes. This molecule represents the extreme example of a family of important fatty acids known as omega-3s and has been clearly demonstrated to alter membrane structure and function. One important property that has yet to be reported is the effect of DHA on membrane phospholipid flip-flop. This study demonstrates that as the number of double bonds in the fatty acyl chains comprising the membrane increases, so does the rate of flip-flop of the NBD-PE probe. The increase is particularly marked in the presence of DHA. Half-lives t(1/2) of 0.29 and 0.086 h describe the process in 1-stearoyl-2-docosahexaenoylphosphatidylcholine and 1,2-didocosahexaenoylphosphatidylcholine, respectively, whereas in 1-stearoyl-2-oleoylphosphatidylcholine t(1/2)=11.5h. Enhanced permeability to dithionite with increasing unsaturation was also indicated by our results. We conclude that PC membranes containing DHA support faster flip-flop and permeability rates than those measured for other less-unsaturated PCs.

The acyl composition of mammalian phospholipids: an allometric analysis

Data concerning the acyl composition of tissue phospholipids from mammal species, ranging in size from the shrew (7 g) to cattle (370 kg), has been collated from the literature and analysed allometrically. Phospholipids from heart, skeletal muscle, liver and kidney exhibited similar allometric trends whereby phospholipids had a significant decrease in unsaturation index (number of double bonds per 100 acyl chains) as species body size increased whilst there was no change in the percent of unsaturated acyl chains. Whilst total polyunsaturate content did not change with body mass, both heart and skeletal muscle phospholipids showed a significant allometric decrease in the omega-3 polyunsaturate content. The content of the highly polyunsaturated docosahexaenoic acid (22:6 n-3) in phospholipids showed significant and substantial allometric decline with increasing body mass in all four tissues (exponents ranged from -0.19 in liver to -0.40 in skeletal muscle). Brain phospholipids showed no allometric trends in acyl composition and were highly polyunsaturated in all species. These trends are discussed in light of the hypothesis that the relative content of polyunsaturated acyl chains in membranes, and especially docosahexaenoate (22:6 n-3), can act as a membrane pacemaker for metabolic activity.

Synthesis of a novel phosphatidylcholine conjugated to docosahexaenoic acid and methotrexate that inhibits cell proliferation

Here we report the synthesis and characterization of a lipophilic phosphatidylcholine containing the omega-3 fatty acid docosahexaenoic acid (DHA) and the cytotoxic drug methotrexate (MTX). This novel phospholipid combines the fatty acid's and the drug's anticancer activities in a molecule amenable to a liposome bilayer for safe, simultaneous delivery of the two agents. Two phosphatidylcholines were synthesized, from 1-stearoyl or 1-docosahexaenoyl, 2-hydroxy-sn-glycero-3-phosphocholine, to contain MTX in the sn-2 position and either stearic acid or DHA in the sn-1 position. The products contain fatty acid, MTX and phosphorus (1:1:1), and the MTX was released by phospholipase A(2), consistent with the proposed phospholipid structure. The predominant product linked MTX to the glycerol moiety through MTX's gamma-carboxyl group. Liposomes composed of 1-stearoyl, 2-oleoyl phosphatidylcholine plus 1-stearoyl, 2-oleoyl phosphatidylethanolamine and various concentrations of the novel phospholipids caused dose-dependent inhibition of murine leukemia cell proliferation in culture. The DHA- and MTX-containing phosphatidylcholine was more effective than that containing stearic acid, and DHA appeared to synergize with MTX when they were added as free agents or covalently linked in the phospholipid. These data show the feasibility of synthesizing, and the inhibitory activity of phosphatidylcholine with DHA in the sn-1 position and MTX in the sn-2 position, and suggest the compound's potential use in cancer chemotherapy.

Lipid phase separation in phospholipid bilayers and monolayers modeling the plasma membrane

It is postulated that biological membrane lipids are heterogeneously distributed into lipid microdomains. Recent evidence indicates that docosahexaenoic acid-containing phospholipids may be involved in biologically important lipid phase separations. Here we investigate the elastic and thermal properties of a model plasma membrane composed of egg sphingomyelin (SM), cholesterol and 1-stearoyl-2-docosahexaenoyl-sn-glycerophosphoethanolamine (SDPE). Two techniques are employed, pressure-area isotherms on monolayers to examine condensation and interfacial elasticity behavior, and differential scanning calorimetry (DSC) on bilayers to evaluate phase separations. Significant levels of condensation are observed for mixtures of SM and cholesterol. Surface elasticity measurements indicate that cholesterol decreases and SDPE increases the in-plane elasticity of SM monolayers. At X(SDPE)> or =0.15 in SM, a more horizontal region emerges in the pressure-area isotherms indicating 'squeeze out' of SDPE from the monolayers. Addition of cholesterol to equimolar amounts of SM and SDPE further increases the amount of 'squeeze out', supporting the concept of phase separation into a cholesterol- and SM-rich liquid ordered phase and a SDPE-rich liquid disordered phase. This conclusion is corroborated by DSC studies where as little as X(Chol)=0.0025 induces a phase separation between the two lipids.

Docosahexaenoic acid reverses cyclosporin A-induced changes in membrane structure and function

The use of a fish oil vehicle for cyclosporin A (CsA) can decrease the toxic effects of CsA but the mechanism is unclear. Here we examine the mechanism by which docosahexaenoic acid (DHA), a fish oil-derived polyunsaturated fatty acid, can alter the toxic effects of CsA on mouse organ function, endothelial macromolecular permeability, and membrane bilayer function. Mice given CsA and fish oil showed increased liver toxicity, kidney toxicity, incorporation of DHA, and evidence of oxidized fatty acids compared to control animals. We hypothesized that the toxic effects of CsA were primarily a result of membrane perturbation, which could be decreased if DHA were not oxidized. The presence of CsA (10 mol%) alone increased dipalmitoylphosphatidylcholine membrane permeability by seven fold over control (no CsA, no DHA). However, if non-oxidized DHA (15 mol%) and CsA were added to the membrane, the permeability returned to control levels. Interestingly, if the DHA was oxidized, the antagonistic effect of DHA on CsA was completely lost. While CsA alone increased endothelial permeability to albumin, the combination of non-oxidized DHA and CsA had no effect on endothelial macromolecular permeability. However the combination of oxidized DHA and CsA was no different than the effects of CsA only. CsA increased the fluorescence anisotropy of DPH in the liquid crystalline state of DPPC, while DHA decreased fluorescence anisotropy. However the combination of CsA and DHA was no different than DHA alone. We conclude that non-oxidized DHA can reverse the membrane perturbing effects of CsA, and the increases in endothelial macromolecular permeability, which may explain how fish oil is capable of decreasing the toxicity of CsA.

Molecular organization of cholesterol in polyunsaturated phospholipid membranes: a solid state 2H NMR investigation

We compared the molecular organization of equimolar [3alpha-2H1]cholesterol in 18:0-18:1PC (1-stearoyl-2-oleoylphosphatidylcholine), 18:0-22:6PC (1-stearoyl-2-docosahexaenoylphosphatidylcholine), 18:0-20:4PC (1-stearoyl-2-arachidonylphosphatidylcholine) and 20:4-20:4PC (1,2-diarachidonylphosphatidylcholine) bilayers by solid state 2H NMR. Essentially identical quadrupolar splittings (delta v(r) = 45 +/- 1 kHz) corresponding to the same molecular orientation characterized by tilt angle alpha0 = 16 +/- 1 degrees were measured in 18:0-18:1PC, 18:0-22:6PC and 18:0-20:4PC. A profound difference in molecular interaction with dipolyunsaturated 20:4-20:4PC, in contrast, is indicated for the sterol. Specifically, the tilt angle alpha0 = 22 +/- 1 degrees (derived from delta v(r) = 37 +/- 1 kHz) is greater and its membrane intercalation is only 15 mol%.

Docosahexaenoic acid in phosphatidylcholine mediates cytotoxicity more effectively than other omega-3 and omega-6 fatty acids

We reported previously that docosahexaenoic acid (22:6)-containing phosphatidylcholine (PC), but not oleic acid-containing PC nor 22:6-containing phosphatidylethanolamine, is toxic to tumor cells in vitro. To test whether other polyunsaturated fatty acids share 22:6's cytotoxic activity, we treated cultured T27A murine leukemia cells with PC liposomes composed of stearic acid in the sn-1 position and alpha-linolenic acid (alpha-18:3), arachidonic acid (20:4), or eicosapentaenoic acid (20:5) in the sn-2 position. PC containing 22:6 in both positions was also tested. Following treatment, the cells were monitored for fatty acid composition, liposome uptake and viability. Here we demonstrate that cytotoxicity is unique to 22:6-containing PCs and is not shared by PCs with other polyunsaturated omega-3 and omega-6 fatty acids. Because PCs with fatty acids other than 22:6 were taken up by cells but did not kill the cells, we propose that 22:6-containing PCs incorporated into cellular membranes produce unique changes in the membrane structure incompatible with cell survival. PC liposomes containing 22:6 are potential drug delivery vehicles that may, by virtue of their cytotoxicity, serve concomitantly as adjunct cancer therapy.

Effect of cholesterol on molecular order and dynamics in highly polyunsaturated phospholipid bilayers

The effect of cholesterol on phospholipid acyl chain packing in bilayers consisting of highly unsaturated acyl chains in the liquid crystalline phase was examined for a series of symmetrically and asymmetrically substituted phosphatidylcholines (PCs). The time-resolved fluorescence emission and decay of fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) was used to characterize equilibrium and dynamic structural properties of bilayers containing 30 mol % cholesterol. The bilayers were composed of symmetrically substituted PCs with acyl chains of 14:0, 18:1n9, 20:4n6, or 22:6n3, containing 0, 1, 4, or 6 double bonds, respectively, and mixed-chain PCs with a saturated 16:0 sn-1 chain and 1, 4, or 6 double bonds in the sn-2 chain. DPH excited-state lifetime was fit to a Lorentzian lifetime distribution, the center of which was increased 1-2 ns by 30 mol % cholesterol relative to the cholesterol-free bilayers. Lifetime distributions were dramatically narrowed by the addition of cholesterol in all bilayers except the two consisting of dipolyunsaturated PCs. DPH anisotropy decay was interpreted in terms of the Brownian rotational diffusion model. The effect of cholesterol on both the perpendicular diffusion coefficient D perpendicular and the orientational distribution function f(theta) varied with acyl chain unsaturation. In all bilayers, except the two dipolyunsaturated PCs, 30 mol % cholesterol dramatically slowed DPH rotational motion and restricted DPH orientational freedom. The effect of cholesterol was especially diminished in di-22:6n3 PC, suggesting that this phospholipid may be particularly effective at promoting lateral domains, which are cholesterol-rich and unsaturation-rich, respectively. The results are discussed in terms of a model for lipid packing in membranes containing cholesterol and PCs with highly unsaturated acyl chains.

Docosahexaenoic acid (DHA) alters the structure and composition of membranous vesicles exfoliated from the surface of a murine leukemia cell line

Membrane lipid microdomains are regions of the membrane thought to be functionally important, but which have remained poorly characterized because they have proven to be difficult to isolate. The exfoliation of small membranous vesicles from the cell surface is a continuous and normal activity in many cells. If microdomains are relatively large or stable, they may influence the structure and composition of exfoliated vesicles, which are easy to isolate. We tested the ability of docosahexaenoic acid (DHA), a fatty acid proposed to alter the structure of microdomains, to change the structure and composition of vesicles exfoliated from a murine leukemia cell line. Cells were cultured in normal and DHA-enriched media for 72 h, then washed and given a 15-h exfoliation period. Afterwards, the pooled vesicles and their parent plasma membrane were collected and analyzed. Vesicles and plasma membrane from cells grown in normal culture medium had similar fatty acid compositions, including equal, and low, proportions of DHA, but the vesicles had much more cholesterol and displayed higher anisotropy than the plasma membrane. When cells were grown in DHA-enriched medium, both the plasma membrane and exfoliated vesicles had 10-fold elevated levels of DHA in their phospholipids, with the DHA displacing other polyunsaturates. These cells released vesicles having significantly reduced levels of cholesterol and monoenoic fatty acids than those in normal culture. The anisotropy of these vesicles was also dramatically reduced. These data are consistent with DHA altering the structure and composition of membrane microdomains on the cell surface, and suggest that exfoliated vesicles may prove useful in the further study of membrane microdomains.

Molecular order and dynamics in bilayers consisting of highly polyunsaturated phospholipids

The time-resolved fluorescence emission and decay of fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) was used to characterize equilibrium and dynamic bilayer structural properties of symmetrically substituted phosphatidylcholines (PCs) with acyl chains containing no, one, four, or six double bonds and mixed-chain phosphatidylcholines with a saturated sn-1 chain and one, four, or six double bonds in the sn-2 chain. Both the Brownian rotational diffusion (BRD) model and the wobble-in-cone model were fit to all differential polarization data, and the descriptions of the data provided by the BRD model were found to be statistically superior. Global analysis of differential polarization data revealed two statistically equivalent solutions. The solution corresponding to a bimodal orientational distribution function, f(theta), was selected based on the effects of temperature on f(theta) and previous measurements on fixed, oriented bilayers. The overall equilibrium acyl chain order in these bilayers was analyzed by comparing the orientational probability distribution for DPH, f(theta) sin theta, with a random orientational distribution. Orientational order decreased and probe dynamics increased in mixed-chain species as the unsaturation of the sn-2 chain was increased. The degree of orientational order dropped dramatically in the dipolyunsaturated species compared with the mixed-chain phosphatidylcholines, which contained a polyunsaturated sn-2 chain. In terms of both orientational order and probe dynamics, the differences between the highly polyunsaturated species and the monounsaturated species were much greater than the differences between the monounsaturated species and a disaturated PC.

Can docosahexaenoic acid inhibit metastasis by decreasing deformability of the tumor cell plasma membrane?

Murine leukemia cells were fused with small unilamellar vesicles composed of 1-stearoyl, 2-docosahexaenoylphosphatidylcholine. The docosahexaenoic acid (DHA)-modified cells were tested for deformability by forcing them through 5.0-microm Nucleopore filters. As the cellular DHA content increased, the cells passed through the filters with more difficulty. Furthermore, cells that passed through the filters had less DHA than cells that did not. Monitored by steady-state fluorescence polarization of membrane interior and surface probes, DHA reduced the membrane order in the hydrophobic interior while increasing the order at the aqueous interface. We attribute DHA's anti-metastatic properties in part to effects on membrane structure that reduce cell deformability.

Phosphatidylcholine acyl unsaturation modulates the decrease in interfacial elasticity induced by cholesterol

The effect of cholesterol on the interfacial elastic packing interactions of various molecular species of phosphatidylcholines (PCs) has been investigated by using a Langmuir-type film balance and analyzing the elastic area compressibility moduli (Cs(-1)) as a function of average cross-sectional molecular area. Emphasis was on the high surface pressure regions (pi > or = 30 mN/m) which are thought to mimic biomembrane conditions. Increasing levels of cholesterol generally caused the in-plane elasticity of the mixed monolayers to decrease. Yet, the magnitude of the cholesterol-induced changes was markedly dependent upon PC hydrocarbon structure. Among PC species with a saturated sn-1 chain but different sn-2 chain cis unsaturation levels [e.g., myristate (14:0), oleate (18:1delta9(c), linoleate (18:2delta9,12(c), arachidonate (20:4delta5,8,11,14(c), or docosahexenoate (22:6delta4,7,10,13,16,19(c)], the in-plane elasticity moduli of PC species with higher sn-2 unsaturation levels were less affected by high cholesterol mol fractions (e.g., >30 mol %) than were the more saturated PC species. The largest cholesterol-induced decreases in the in-plane elasticity were observed when both chains of PC were saturated (e.g., di-14:0 PC). When both acyl chains were identically unsaturated, the resulting PCs were 20-25% more elastic in the presence of cholesterol than when their sn-1 chains were long and saturated (e.g., palmitate). The mixing of cholesterol with PC was found to diminish the in-plane elasticity of the films beyond what was predicted from the additive behavior of the individual lipid components apportioned by mole and area fraction. Deviations from additivity were greatest for di-14:0 PC and were least for diarachidonoyl PC and didocosahexenoyl PC. In contrast to Cs(-1) analyses, sterol-induced area condensations were relatively unresponsive to subtle structural differences in the PCs at high surface pressures. Cs(-1) versus average area plots also indicated the presence of cholesterol concentration-dependent, low-pressure (<14 mN/m) phase boundaries that became more prominent as PC acyl chain unsaturation increased. Hence, area condensations measured at low surface pressures often do not accurately portray which lipid structural features are important in the lipid-sterol interactions that occur at high membrane-like surface pressures.