Changes in plasma membrane phospholipids inhibit antibody-mediated lysis

A variety of mechanisms have been proposed to explain how tumors evade immune destruction. This work has identified one such mechanism that determines susceptibility to immune lysis; membrane phospholipid composition altered susceptibility to antibody plus complement (Ab+C)-mediated lysis. Effects on antibody plus complement-mediated lysis were correlated with levels of major histocompatibility complex (MHC) molecules but not inherent resistance to complement damage. This cellular mechanism could be a means by which tumor cells escape immune detection and destruction.     

Changes in erythrocyte membrane proteins in patients with cirrhosis]

The serum and erythrocyte membrane proteins of patients with cirrhosis were studied. Recent work on abnormalities of the erythrocyte membrane resulted in the identification of several types of membrane skeleton lesions. The different techniques for separation of membrane proteins have been revised and compared. These were studied on slabs using the SDS-polyacrylamide gel and discontinuous buffer system with a linear concentration gradient of 5-15% (w/v). Serum proteins were separated by using continuous buffer system with a linear concentration gradient of 4-30%. A decreased in serum and red cell membrane proteins in those patients, were observed.     

Essential fatty acids in erythrocyte phospholipids during pregnancy and at delivery in mothers and their neonates: comparison with plasma phospholipids

Evidence that the essential fatty acid (EFA) status during pregnancy and at birth may not be optimal is mainly based on fatty acid profiles of maternal and neonatal plasma phospholipids. However, erythrocyte phospholipids may be more reliable than plasma phospholipids to reflect the EFA status of an individual. Therefore, the present study compares the levels of EFA and of their derivatives (LCPUFA) in erythrocyte and plasma phospholipids collected during pregnancy and at delivery of 184 women and of their infants at birth. In general, the relative concentrations of erythrocyte and plasma phospholipid fatty acids (% of total fatty acids) were strongly correlated, but not at early pregnancy. The overall changes in fatty acid concentrations during pregnancy were qualitatively comparable between erythrocytes and plasma, although the comparability became less towards the end of pregnancy. The changes in absolute amounts (mg/l) of fatty acids in erythrocyte and plasma phospholipids also compare quite well till 32 weeks of gestation, but not thereafter. Most maternal-neonatal differences in relative fatty acid concentrations are qualitatively comparable for erythrocyte and plasma phospholipids. However, significant differences were observed for the absolute amounts of arachidonic and docosahexaenoic acids. No matter these differences, plasma and erythrocyte phospholipids seem equally suitable to reliably quantify the more functional EFA and LCPUFA status based on fatty acid ratios. Correlations between neonatal and maternal fatty acid values at delivery/birth are highly significant in erythrocyte as well as plasma phospholipids. Neonatal erythrocyte (but not plasma) values also correlated strongly with maternal values at early pregnancy. Therefore, the neonatal EFA and LCPUFA status might be predicted on the basis of EFA and LCPUFA concentrations of maternal erythrocyte phospholipids at early pregnancy.     

Levels and distributions of phospholipids and cholesterol in the plasma membrane of neuroblastoma cells.

Murine neuroblastoma cells (clone N-2A) grown in suspension (spinner cells) or attached on a plastic surface (monolayer cells) were used in studies of the phospholipid and cholesterol composition of whole cells, primary plasma membranes, plasma membranes internalized during phagocytosis of polystyrene latex beads, mitochondria and microsomes. Monolayer cells contained higher concentrations of total phospholipid, phosphatidylserine and phosphatidylcholine, and lower concentration of phosphatidylethanolamine than spinner cells. The cholesterol levels and the relative proportions of the various phospholipids were similar in both cell types except phosphatidylethanolamine and sphingomyelin whose proportions were lower in monolayer cells. The primary plasma membranes of the two cell types differed significantly in the relative proportions of all phospholipids, except sphingomyelin, and the phospholipid to protein and the cholesterol to protein ratios were all higher in the membranes of spinner cells. In contrast to these results, all the phospholipid to protein and the cholesterol to protein ratios of the internalized plasma membranes were higher in monolayer than in spinner cells, and the proportions of all phospholipids, except phosphatidylethanolamine, were similar in both cell types. The membrane distributions of individual phospholipids and cholesterol were inferred from comparison of the phospholipid and cholesterol compositions of primary plasma membranes and plasma membranes internalized during phagocytosis of polystyrene beads. The results are consistent with a non-random distribution of most phospholipids in both spinner and monolayer cells, but the patterns of these distributions were different in the two cell types. With regard to cholesterol the results are compatible with a random or a heterogeneous distribution. All the phospholipid to protein ratios of the mitochondrial fraction of both cell types were lower than those of the plasma membranes. However, these ratios of the microsomal fraction were higher than those of the plasma membranes of monolayer cells, whereas they were comparable, with a few exceptions, to those of spinner cell membranes. The cholesterol to phospholipid molar ratios of plasma membranes were 6.4 and 4.3 fold greater than those of the mitochondrial and microsomal fractions, respectively.     

The exchange of erythrocyte membrane phospholipids with rat liver extracts in vitro

Intact rat or human erythrocytes and their isolated (ghost) membranes were incubated with the high speed supernatant fraction of homogenates derived from ³²P-labeled rat livers. Phospholipid molecules were transferred between the red cell membranes and the liver extracts, as reflected by the convergence of their specific radioactivities with time. Whereas ghosts usually approached isotopic equilibrium with the liver supernatant fraction during a few hours of incubation at 37° C, the exchange of phospholipids by intact cells was no more than one-half, even after 18 hr. Phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine and sphingomyelin were all exchanged in both intact cells and ghosts, albeit to different extents. (A control experiment, incubating ³²P-labeled rat erythrocytes or ghosts with unlabeled rat liver extracts, also demonstrated the exchange of all four major phospholipids.) These data may signify that the phospholipids on the cytoplasmic side of the membrane of intact erythrocytes do not exchange with the phospholipids in exogenous liver extracts. If so, all four major phospholipid classes would appear to be present to some extent at both membrane surfaces. The first inference is in agreement with several other studies on this membrane, while the second inference is not.     

Changes in the content of various types of cholesterol and phospholipids in rat tissues during cold acclimation

Acclimation of rats to cooling (10 and 3 degrees C) for 2 weeks is accompanied by changes in the content of cholesterol, various forms of phospholipids and the degree of saturation of their fatty acids. No changes in the lipid composition of tissues were noted which might indicate pathological condition in animals.     

The reactivity of plasma phospholipids with lecithin:cholesterol acyltransferase is decreased in fish oil-fed monkeys

The size of low density lipoproteins (LDL) is strongly correlated with LDL cholesteryl ester (CE) content and coronary artery atherosclerosis in monkeys fed cholesterol and saturated fat. African green monkeys fed 11% (weight) fish oil diets have smaller LDL and less CE per LDL particle than lard-fed animals. We hypothesized that this might be due to a lower plasma lecithin:cholesterol acyltransferase (LCAT) activity in fish oil-fed animals. Using recombinant particles made of egg yolk lecithin-[14C]cholesterol-apoA-I as exogenous substrate, we found no difference in plasma LCAT activity (27 versus 28 nmol CE formed per h/ml) of fish oil- versus lard-fed animals, respectively; furthermore, no diet-induced difference in immunodetectable LCAT was found. However, plasma phospholipids from fish oil-fed animals were over 4-fold enriched in n-3 fatty acids in the sn-2 position compared to those of lard-fed animals. Additionally, the proportion of n-3 fatty acid-containing CE products formed by LCAT, relative to the available n-3 fatty acid in the sn-2 position of phospholipids, was less than one-tenth of that for linoleic acid. The overall rate of LCAT-catalyzed CE formation with phospholipid substrates from fish oil-fed animals was lower (5-50%) than with phospholipid substrates from lard-fed animals. These data show that n-3 fatty acids in phospholipids are not readily utilized by LCAT for formation of CE; rather, LCAT preferentially utilizes linoleic acid for CE formation. The amount of linoleic acid in the sn-2 position of plasma phospholipids is reduced and replaced with n-3 fatty acids in fish oil-fed animals. As a result, LCAT-catalyzed plasma CE formation in vivo is likely reduced in fish oil-fed animals contributing to the decreased cholesteryl ester content and smaller size of LDL particles in the animals of this diet group.     

Influence of the bovine seminal plasma protein PDC-109 on cholesterol in the presence of phospholipids

The interaction of the major bovine seminal plasma protein PDC-109 with cholesterol was studied by employing spin-labelled analogues. It could be shown that PDC-109 does not interact directly with cholesterol molecules. However, in the presence of phospholipids we found a strong reduction of cholesterol motion by PDC-109. The fraction of immobilized cholesterol was largest for phosphorylcholine-containing lipids. This is consistent with the preferential interaction between PDC-109 and phosphatidylcholine. It is concluded that a stronger association and interaction of PDC-109 with phosphatidylcholine leads to an enhanced fraction of immobilized cholesterol analogues, but not to a phospholipid-dependent specific interaction between the protein and cholesterol. Moreover, the interaction of PDC-109 with various spin-labelled analogues of phosphatidylcholine (lysoPC, diacylPC) was investigated. In membranes of lipid vesicles the protein caused an immobilization of the phosphatidylcholine analogues mainly in the outer membrane leaflet, with no differences between diacylPC and lysoPC. The results are of relevance for understanding the physiological role of PDC-109 in the genesis of sperm cells.     

Changes in phospholipids, cholesterol and protein content of oviduct fluid of cows during the oestrous cycle

The oviducts of 4 cows were cannulated and oviduct fluid was collected daily from the exteriorized cannulas for a total of 5 oestrous cycles. Daily serum samples were assayed for oestradiol-17 beta and progesterone to monitor the oestrous cycle. Data for each cycle were compared for oviduct fluid collected during the non-luteal phase (serum progesterone less than or equal to 1.5 ng/ml) and the luteal phase (serum progesterone greater than 1.5 ng/ml). During the non-luteal phase oviduct fluid volume was higher and the osmolality was lower than during the luteal phase. Total protein, cholesterol and phospholipid secreted daily was greater during the non-luteal phase. Cholesterol and protein concentrations were generally lower during the non-luteal phase, but phospholipid concentrations were generally higher. About 40% of the phospholipid in oviduct fluid was phosphatidylcholine and lysophosphatidylcholine, while phosphatidylinositol and lysophosphatidylinositol accounted for 20%. The ratio of 1-acyl-phospholipid to diacylphospholipid increased during the non-luteal phase. An increased cholesterol to phospholipid ratio, and a decreased cholesterol to protein ratio in oviduct fluid also were associated with the non-luteal phase. Changes in the lipid composition of oviduct fluid during the oestrous cycle may play a role in the preparation of gametes for fertilization.     

Condensed complexes of cholesterol and phospholipids

There is overwhelming evidence that lipid bilayer regions of animal cell membranes are in a liquid state. Quantitative models of these bilayer regions must then be models of liquids. These liquids are highly non-ideal. For example, it has been known for more than 75 years that mixtures of cholesterol and certain phospholipids undergo an area contraction or condensation in lipid monolayers at the air-water interface. In the past 3 years, a thermodynamic model of “condensed complexes” has been proposed to account for this non-ideal behavior. Here we give an overview of the model, its relation to other models, and to modern views of the properties of animal cell membranes.