Changes in phospholipid composition of human aorta with age

Functions of cholesterol and the cholesterol bilayer domain specific to the fiber-cell plasma membrane of the eye lens

The most unique feature of the eye lens fiber-cell plasma membrane is its extremely high cholesterol content. Cholesterol saturates the bulk phospholipid bilayer and induces formation of immiscible cholesterol bilayer domains (CBDs) within the membrane. Our results (based on EPR spin-labeling experiments with lens-lipid membranes), along with a literature search, have allowed us to identify the significant functions of cholesterol specific to the fiber-cell plasma membrane, which are manifest through cholesterol-membrane interactions. The crucial role is played by the CBD. The presence of the CBD ensures that the surrounding phospholipid bilayer is saturated with cholesterol. The saturating cholesterol content in fiber-cell membranes keeps the bulk physical properties of lens-lipid membranes consistent and independent of changes in phospholipid composition. Thus, the CBD helps to maintain lens-membrane homeostasis when the membrane phospholipid composition changes significantly. The CBD raises the barrier for oxygen transport across the fiber-cell membrane, which should help to maintain a low oxygen concentration in the lens interior. It is hypothesized that the appearance of the CBD in the fiber-cell membrane is controlled by the phospholipid composition of the membrane. Saturation with cholesterol smoothes the phospholipid-bilayer surface, which should decrease light scattering and help to maintain lens transparency. Other functions of cholesterol include formation of hydrophobic and rigidity barriers across the bulk phospholipid-cholesterol domain and formation of hydrophobic channels in the central region of the membrane for transport of small, nonpolar molecules parallel to the membrane surface. In this review, we provide data supporting these hypotheses.

Lens lipids and maximum lifespan

Unlike in most organs, the lipid composition of lenses varies dramatically among species and with age. The focus of this study is to assess how these changes relate to lifespan. Studies on cataract suggest that the lens may serve as a window into the processes leading to accelerated mortality. As a first step toward elucidating cellular processes in the lens that may serve as markers for accelerated mortality, we examined the correlation between species-dependent and age-related lens lipid compositional differences and maximum life span. We included data from camels, which, even in old age, rarely develop cataracts although they live under adverse conditions. Camel lens lipids were mainly composed of sphingolipids (77%) and phosphatidylcholines (23%). Bovine lens lipid composition was comparable to a previous study, and both bovine lens sphingolipids, phosphatidylcholines and camel lens phosphatidylcholines content fit well (within the 95% confidence limits) in the curve obtained by plotting maximum life spans of other species with sphingolipids and phosphatidylcholines. Lifespan was directly related to lens sphingolipid content and indirectly related to lens phosphatidylcholine content. The camel lens sphingolipid value was significantly above the curve for other species. Except for the camel lens nucleus, lipid order and sphingolipid content were linearly related, p < 0.005 with a slope of 0.85+/-0.07, and intercept of 6.9+/-3.8. Lipid phase transition temperature and sphingolipid content were also linearly related, p = 0.01 with a slope of 0.20+/-0.07, and intercept of 21.7+/-5.3. Our data support the hypothesis that humans have adapted so that their lens membranes have a high sphingolipid content that confers resistance to oxidation, allowing these membranes to stay clear for a relatively longer time than is the case in many other species. Age-related changes in human lens lipid composition may serve as a marker for oxidative stress and may reflect systemic oxidative insult, providing a window into the health of an individual.

Age-related changes affecting atherosclerotic risk. Potential for pharmacological intervention

The incidence of cardiovascular diseases that are related to the atherosclerotic process increases exponentially with age. Organ lesions, the clinical manifestation of atherosclerotic disease, are late events due to complications in the plaque (ulceration, thrombosis, calcification) which are the result of an increased vulnerability to disruption of a previously stable plaque. The higher incidence of age-related clinical events could be explained by a rising sensitivity of plaques to destabilising factors, both parietal and humoral. The increased probability that a plaque in an elderly patient will became vulnerable could be related to those destabilising factors that significantly increase with aging, such as advanced glycation end-products. For these reasons, it seems most important that the analysis of these age-related destabilising factors, rather than those factors that promote the development of early atherosclerotic plaques, should be undertaken. Taking the point of view of a pharmacological intervention, this should eventually lead to a more complete understanding of this process.

Mature human atherosclerotic plaque contains peroxidized phosphatidylcholine as a major lipid peroxide

The initial stage of atherosclerotic plaque formation involves oxidation of the phosphatidyl-choline moiety of low density lipoprotein (LDL) and subsequent uptake by macrophages. Ongoing uptake in developing plaque also may involve oxidized LDL and would require an oxidizing environment in plaque lipids. Atherosclerotic plaque lipids from 12 patients undergoing peripheral vascular procedures were extracted in chloroform: methanol (2:1). This extract was applied to a 25 cm 5 micron silica HPLC column and eluted with a ternary gradient mobile phase utilizing a laser light scattering (ELSD) mass detector. Individual lipid fractions were then analyzed. Cholesterol, both free and esterified, was the most prominent lipid in plaque (104 +/- 74 mg/gm tissue. However, lipid peroxides were present in much higher concentrations (3.52 +/- 2.84 FU X 10(4)/mg phospholipid) and overall level (21.27 +/- 10.10 FU X 10(4)/gm plaque) in the phospholipid component (*p< 0.05). Phosphatidyl-choline (PC) accounted for 63% of the total phospholipid peroxides recovered (6.31 +/- 5.09 mg/gm plaque; *p<0.05). PC and phosphatidylinositol (PI) content were linearly related to lipid peroxide fluorescence (PC; r=0.696; p=0.01) (PI; r=0.809; p=0.001). Lipid peroxides in human atherosclerotic plaque are present primarily in the phospholipid component and phosphatidyl-choline forms the bulk of these peroxides. PC may play an important role in ongoing plaque lipid accumulation.

Association of negatively-charged phospholipids with low-density lipoprotein (LDL) increases its uptake and the deposition of cholesteryl esters by macrophages

LDL, the major carrier of cholesterol in blood, is poorly metabolized by macrophages. In contrast, macrophages can recognize and endocytose anionic phospholipids such as phosphatidylserine, phosphatidylglycerol and cardiolipin. Since macrophages can take up large amounts of these phospholipids, experiments were performed to ascertain whether pre-incubation of native LDL with negatively-charged phospholipids would enhance the metabolism of LDL by macrophages. When 125I-LDL was incubated with cardiolipin liposomes for 18 h at 37 degrees C before addition to macrophages, an approx. 40-fold increase of LDL metabolism by these cells was observed. Similar results were found when LDL was pre-incubated with phosphatidylserine or phosphatidylglycerol; however, pre-incubation of LDL with phosphatidylcholine liposomes did not lead to an increase of LDL metabolism. The macrophage uptake of LDL pre-incubated with cardiolipin was reduced to approx. 40% of control values in the presence of dextran sulfate and fucoidin, inhibitors of anionic phospholipid uptake. Cytochalasin D, an inhibitor of phagocytosis, reduced the lysosomal degradation of LDL pre-incubated with cardiolipin to approx. 10% of control values. When the LDL-cardiolipin mixture was chromatographed on agarose gel, two peaks containing LDL were observed in the elution profile: the first peak appeared at the void volume and the second peak was detected just ahead of native LDL. The LDL in both peaks was much more extensively metabolized by macrophages than was native LDL; the LDL in the first peak was metabolized at a rate that was 8 times the second peak. The results demonstrate that negatively-charged phospholipids can form a complex with LDL which facilitates its phagocytosis by macrophages.

Phospholipid loss in dying platelets

The death of a cell results in a large amount of membrane lipid, predominantly phospholipids and cholesterol, that must be eliminated. In this study, we have examined what happens to phospholipids in dying rat platelets. Rat platelets were incubated for up to three days following their activation with thrombin. Platelet death occurred during the first day of incubation. This was indicated by a complete loss of platelet lactate dehydrogenase into the incubation medium. The platelets progressively lost over one-half of their phospholipid content during the three days of incubation. Cholesterol and sphingomyelin (the phospholipid with the highest affinity for cholesterol) were not lost during the same period. Our findings suggest that significant degradation of cellular non-sphingomyelin phospholipid can be triggered by cell death. The preservation of sphingomyelin in dying platelets, may be an adaptive response to maintain cholesterol in a solubilized state within dying cells.

Improved resolution of 31P nuclear magnetic resonance spectra of phospholipids from brain

A method is described wherein the resolution of 31P nuclear magnetic resonance spectra of the lipophilic fraction from a Bligh-Dyer extract of rat brain can be enhanced. The lipids are dispersed as micelles in aqueous solution with sodium deoxycholate, and spectral resolution is further optimized by adjusting the pH and by removing ions from the solution. The application of the method to the study of aging in rat brain serves as an example.

Cerebral microvascular and parenchymal phospholipid composition in the mouse

Cerebral microvessels consisting predominantly of capillaries and small arterioles (less than 30 micron dia.) were isolated from the cerebral cortex and cerebellum of 3-month-old mice. Lipids were extracted from both microvascular and brain parenchymal fractions and the major phospholipid classes (choline phosphoglyceride, ethanolamine phosphoglyceride, inositol phosphoglyceride, serine phosphoglyceride, and sphingomyelin) separated by 2-dimensional TLC. Comparison of mol % determined by phosphate analysis of each phospholipid revealed significant differences in membrane composition of ethanolamine phosphoglyceride, inositol phosphoglyceride, and sphingomyelin between microvascular and parenchymal components of the central nervous system. Moreover, the choline phosphoglyceride/sphingomyelin mol ratio, one of three determinants of membrane fluidity, is significantly lower for microvessel membrane than for membranes of the brain parenchyma.

Effect of post-mortem time on the biochemical composition of coronary arteries

The effects of post-mortem autolysis on the biochemical composition of coronary intima-medias were studied by keeping the coronary arteries of 25 slaughterhouse pigs at +4 degrees C for periods up to 7 days. The samples were analysed for DNA, total protein, collagen, glycosaminoglycans (GAGs), free and esterified cholesterol, triglycerides, phospholipid fractions, and fatty acid composition in various lipid classes. The content of sulphated GAGs decreased steadily, whereas that of hyaluronic acid remained unchanged. These alterations led to a significant decrease in the content of total GAGs and to an increase in the percentage of hyaluronic acid in total GAGs. The results indicate a continuous post-mortem degradation of arterial proteoglycans. After 7 days storage, there was a significant increase in the content of free fatty acids, and the composition of this fraction was changed by increases in the relative amounts of oleic and linoleic acids. These changes were accompanied by an increase in lysolecithin and a decrease in lecithin, which suggests a post-mortem action of arterial phospholipases.

Acyl chain order and lateral domain formation in mixed phosphatidylcholine--sphingomyelin multilamellar and unilamellar vesicles

The phase behavior of mixtures of dimyristoylphosphatidylcholine (DMPC) with N-palmitoylsphingosinephosphorylcholine (C16SHP) has been investigated in both small unilamellar and large multilamellar vesicles. The steady-state fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) has been used to detect temperature-induced structural changes in these membranes. In addition, electron microscopy has revealed vastly different fracture-face morphologies for large multilamellar vesicles "jet-frozen" from different temperatures. These data have been interpreted in terms of proposed phase diagrams for this lipid mixture. The shapes of the proposed phase diagrams have led us to conclude that phosphatidylcholine and sphingomyelin species of similar acyl chain length mix freely in both highly curved and uncurved bilayers, except at temperatures at which both lipids are in low-temperature, ordered phases. In addition, the similarity of these phase diagrams to phase diagrams for analogous mixtures of pure phosphatidylcholines suggested that sphingomyelin and phosphatidylcholine suggested that sphingomyelin and phosphatidylcholine species might substitute for each other in supporting the lamellar phase necessary for each other in supporting the lamellar phase necessary to cell membrane structure. Finally, the anisotropy of DPH fluorescence was found to be essentially invariant with sphingomyelin content at temperatures just above and below the solid--liquid phase separation in small unilamellar vesicles. This demonstrates that the sphingomyelin backbone, per se, does not order the membrane bilayer. These results are discussed in terms of the possible role of sphingomyelin in controlling acyl chain order within mammalian cell membranes.

Orientational order in the choline headgroup of sphingomyelin: A 14N-NMR study

An aqueous dispersion of fully hydrated bovine sphingomyelin was studied using 14N-NMR spectroscopy. Spectra were obtained as a function of temperature over the range 15-80 degrees C, in both the liquid crystal and gel phases. In the liquid crystal phase, powder pattern lineshapes were obtained, whose quadrupolar splitting slowly decreases with increasing temperature. The spectra are increasingly broadened as the temperature is lowered through the phase transition into the gel phase. The linewidths and the second moments of these spectra indicate that the onset of a broad phase transition occurs at approx. 35 degrees C, in agreement with previous calorimetric and 31 P-NMR measurements. There is no evidence from the lineshapes for an hexagonal phase in this system, and this conclusion is supported by X-ray diffraction measurements carried out on aqueous dispersions of sphingomyelin in both phases. Assuming that the static nitrogen quadrupole coupling constant is the same for both sphingomyelin and dipalmitoyl-L-alpha-phosphatidylcholine (DPPC), the decrease observed in the quadrupolar splitting of sphingomyelin compared to that of DPPC indicates that the orientational order of the choline headgroup in liquid crystalline sphingomyelin is not the same as that of its counterpart in DPPC. Preliminary relaxation time measurements of T1 and T2 are presented which suggest that there are also dynamic differences between sphingomyelin and DPPC in the choline headgroup.

A survey on spontaneous atherosclerosis of camels (Camelus dromedarius) in Iran

In a survey carried out on 200 camels (Camelus dromedarius) from different parts of Iran, three cases were diagnosed positive for atherosclerosis on gross and microscopic examination. A sex and age predilection is suspected. The complications of atherosclerosis were not present. To the best of the authors' knowledge this is the first report of atherosclerosis in camels.

Phospholipid composition of hibernating ground squirrel (Citellus lateralis) kidney and low-temperature membrane function

1. Phospholipid analysis of kidney lipids from active and hibernating ground squirrels (Citellus lateralis) indicate that molar quantities of phosphatidyl choline increase, while sphingomyelin decreases in hibernating animals. 2. Both of these changes are in such a direction as to enhance membrane fluidity and possibly contribute to low-temperature membrane function in these organisms.

Age related changes in the lipids of the microsomal and the mitochondrial membranes of rat liver and kidney

The lipid contents of the microsomal and mitochondrial membrane fractions of liver and kidney were determined in 6 and 24 month old rats. A significant age related increase in the molar ratio of cholesterol/phospholipid was observed in all membrane fractions. A significant age related reduction of phospholipid was noted in the microsomal fractions of liver and kidney. The relative amount of phosphatidylethanolamine was found to decrease in all membrane fractions during aging. Membrane glyceride content, however, remained relatively constant with age. Significant increase in oleic acid was seen in the neutral lipid of both liver and kidney and in the polar lipid of kidney. Significant increase in docosahexaenoic acid and significant decrease in linoleic acid were seen in the polar lipid of the liver membrane fractions. Possible alterations in membrane physiochemical properties and in membrane function due to these age related lipid changes are discussed.

Turnover and aortic uptake of very low density lipoproteins (VLDL) from hypercholesteremic rabbits as a model for testing antiatherosclerotic compounds

VLDL from hypercholesteremic (HC) rabbits display features which are suggestive of inherent atherogenicity. The lipid composition, compared to that of control VLDL, shows an enrichment of cholesterol esters, which have a very high 18:1/18:2 ratio in their fatty acids, and an increased sphingomyelin content, with decreased PC/Sph ratio. This lipid composition is very similar to that of the atherosclerotic plaqua. Apoprotein peptides of HC VLDL show a predominance of arg-rich proteins, similar to human conditions (Type III hyperlipoproteinemia and hypothyroidism) characterized by early and severe atherosclerosis. Turnover of 125I-labelled HC VLDL is significantly slower than that of control VLDL, both when the lipoprotein is injected into the donor animals and into controls. Conversion of HC VLDL into lipoproteins of higher density is also very small, as compared to control VLDL. Uptake of radioactivity into the aortic wall after injection is about doubled, as compared to control VLDL, when HC rabbits receive HC VLDL. This experimental model suggests that structural modifications of the HC VLDL make them poorly metabolizable, and possible more akin to the recently described arterial lipoprotein complexing factor (ALCF). Metformin was selected as the test compound, because it has been shown to decrease aortic and liver lipid accumulation in cholesterol fed rabbits, while only slightly affecting plasma cholesterol levels. VLDL from rabbits fed cholesterol and metformin (HC+Met), while still enriched in cholesterol esters, have a higher protein content, less sphingomyelin and more phosphatidylethanolamine and phosphatidylinositol than HC VLDL, while fatty acid composition of cholesterol esters does not differ. Turnover of HC+Met VLDL is extremely rapid, with a t1/2 even shorter than that of control VLDL.