Alteration of fatty acid composition of LM cells by lipid supplementation and temperature

Does membrane curvature elastic energy play a role in mediating oxidative stress in lipid membranes?

The effects of oxidative stress on cells are associated with a wide range of pathologies. Oxidative stress is predominantly initiated by the action of reactive oxygen species and/or lipoxygenases on polyunsaturated fatty acid containing lipids. The downstream products are oxidised phospholipids, bioactive aldehydes and a range of Schiff base by-products between aldehydes and lipids, or other biomacromolecules. In this review we assess the impact of oxidative stress on lipid membranes, focusing on the changes that occur to the curvature preference (lipid spontaneous curvature) and elastic properties of membranes, since these biophysical properties modulate phospholipid homeostasis. Studies show that the lipid products of oxidative stress reduce stored curvature elastic energy in membranes. Based upon this observation, we hypothesize that the effects of oxidative stress on lipid membranes will be reduced by compounds that increase stored curvature elastic energy. We find a strong correlation appears across literature studies that we have reviewed, such that many compounds like vitamin E, Curcumin, Coenzyme Q10 and vitamin A show behaviour consistent with this hypothesis. Finally, we consider whether age-related changes in lipid composition represent the homeostatic response of cells to compensate for the accumulation of in vivo lipid oxidation products.

Mammalian phospholipid homeostasis: evidence that membrane curvature elastic stress drives homeoviscous adaptation in vivo

Several theories of phospholipid homeostasis have postulated that cells regulate the molecular composition of their bilayer membranes, such that a common biophysical membrane parameter is under homeostatic control. Two commonly cited theories are the intrinsic curvature hypothesis, which states that cells control membrane curvature elastic stress, and the theory of homeoviscous adaptation, which postulates cells control acyl chain packing order (membrane order). In this paper, we present evidence from data-driven modelling studies that these two theories correlate in vivo. We estimate the curvature elastic stress of mammalian cells to be 4-7 × 10(-12) N, a value high enough to suggest that in mammalian cells the preservation of membrane order arises through a mechanism where membrane curvature elastic stress is controlled. These results emerge from analysing the molecular contribution of individual phospholipids to both membrane order and curvature elastic stress in nearly 500 cellular compositionally diverse lipidomes. Our model suggests that the de novo synthesis of lipids is the dominant mechanism by which cells control curvature elastic stress and hence membrane order in vivo These results also suggest that cells can increase membrane curvature elastic stress disproportionately to membrane order by incorporating polyunsaturated fatty acids into lipids.

Dietary macronutrients and the aging liver sinusoidal endothelial cell

Fenestrations are pores within the liver sinusoidal endothelial cells (LSECs) that line the sinusoids of the highly vascularized liver. Fenestrations facilitate the transfer of substrates between blood and hepatocytes. With pseudocapillarization of the hepatic sinusoid in old age, there is a loss of fenestrations. LSECs are uniquely exposed to gut-derived dietary and microbial substrates delivered by the portal circulation to the liver. Here we studied the effect of 25 diets varying in content of macronutrients and energy on LSEC fenestrations using the Geometric Framework method in a large cohort of mice aged 15 mo. Macronutrient distribution rather than total food or energy intake was associated with changes in fenestrations. Porosity and frequency were inversely associated with dietary fat intake, while fenestration diameter was inversely associated with protein or carbohydrate intake. Fenestrations were also linked to diet-induced changes in gut microbiome, with increased fenestrations associated with higher abundance of Firmicutes and reduced abundance of Bacteroidetes Diet-induced changes in levels of several fatty acids (C16:0, C19:0, and C20:4) were also significantly inversely associated with fenestrations, suggesting a link between dietary fat and modulation of lipid rafts in the LSECs. Diet influences fenestrations and these data reflect both the key role of the LSECs in clearing gut-derived molecules from the vascular circulation and the impact these molecules have on LSEC morphology.

Nonesterified fatty acids modify inflammatory response and eicosanoid biosynthesis in bovine endothelial cells

Intense lipid mobilization during the transition period in dairy cows is associated with increased disease susceptibility. The potential impact of altered plasma nonesterified fatty acids (NEFA) concentrations and composition on host inflammatory responses that may contribute to disease incidence and severity are not known. The objective of this study was to evaluate if increased NEFA concentrations could modify vascular inflammatory responses in vitro by changing the expression of important inflammatory mediators that are important in the pathogenesis of infectious diseases of transition cows such as mastitis and metritis. Bovine aortic endothelial cells (BAEC) were cultured with different concentrations of a NEFA mixture that reflected the plasma NEFA composition during different stages of lactation. The expression of cytokines, adhesion molecules, and eicosanoids were measured to assess changes in BAEC inflammatory phenotype. Addition of NEFA mixtures altered the fatty acid profile of BAEC by increasing the concentration of stearic acid (C18:0) and decreasing the content of arachidonic acid (C20:4n6c) and other long-chain polyunsaturated fatty acids in the phospholipid fraction. A significant increase also occurred in mRNA expression of cytokine and adhesion molecules that are associated with increased inflammatory responses during the transition period. Expression of cyclooxygenase 2, an important enzyme associated with eicosanoid biosynthesis, was increased in a NEFA concentration-dependent manner. The production of linoleic acid-derived eicosanoids 9- and 13-hydroxyoctadecadienoic acids also was increased significantly after treatment with NEFA mixtures. This research described for the first time specific changes in vascular inflammatory response during in vitro exposure to NEFA mixtures that mimic the composition and concentration found in cows during the transition period. These findings could explain, in part, alterations in inflammatory responses observed during intense lipid mobilization stages such as in the transition period of dairy cows. Future studies should analyze specific mechanisms by which high NEFA concentrations induce a vascular proinflammatory phenotype including the effect of 9 and 13-hydroxyoctadecadienoic acids and other lipid mediators.

Alterations in target cell membrane phospholipids alter T cell but not NK cell killing

The ability of tumor cells to grow progressively in vivo despite the host immune response remains a major conundrum in tumor immunology. Various mechanisms have been proposed to explain how tumors evade immune destruction. The work presented herein shows that simple alterations in plasma membrane phospholipid composition can alter susceptibility to immune lysis. The phospholipid composition of target cells was specifically altered by growth in medium containing choline analogs. Manipulation of membrane phospholipids was observed to alter cell susceptibility to murine CTL but not NK cell lysis. The effects of such changes in phospholipid composition on CTL-mediated lysis appeared to occur during the recognition phase of lysis. This mechanism could be a means by which tumor cells, as well as other pathogenic organisms, escape immune detection and destruction.

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.

Environmental influences on signal transduction through membranes: a retrospective mini-review

This mini-review is addressed to the question how the membranous environment may affect traffic of receptors and signalling from membrane-anchored receptors on the outside of cells to transducers and targets on the inside. Signal transduction by membrane-anchored receptors to the interior of the cell and eventually to the genome is a central issue in cellular regulation. In this context the role of membrane fluidity and of the cytoskeleton in restricting the mobility of proteins are discussed and the evidence for a structural order in membranes which could limit the mobility of proteins is scrutinised.

Effect of trans fatty acids on calcium influx into human arterial endothelial cells

BACKGROUND

A recent task force of The American Society for Clinical Nutrition and American Society for Nutritional Sciences recommended in a position paper on trans fatty acids that models be developed to assess the effects of changes in fat intake on disease risk.

OBJECTIVE

The objective was to investigate, using human arterial endothelial cells as a model, the influence of trans fatty acids and magnesium on cell membrane composition and on calcium influx into arterial cells, a hallmark of atherosclerosis.

DESIGN

Endothelial cells were cultured for 3 d in media with high (adequate) or low (inadequate) amounts of magnesium plus various concentrations of trans,trans linoelaidic; cis,cis linoleic; trans elaidic; oleic; or stearic acids. The cells were then harvested and the fatty acid composition and the amount of (45)Ca(2+) incorporated into the cell was determined.

RESULTS

The percentage of fatty acids incorporated into the endothelial cells was proportional to the amount added to the culture medium. Adequate magnesium was crucial in preventing calcium influx into endothelial cells. Without an adequate amount of magnesium in the culture medium, linoelaidic and elaidic acids, even at low concentrations, increased the incorporation of (45)Ca(2+) into the cells, whereas stearic acid and oleic acid did not (P < 0.05).

CONCLUSION

Our model indicated that a diet inadequate in magnesium combined with trans fat may increase the risk of calcification of endothelial cells.

Thermotropic properties of phospholipid analogues

To understand the structural bases for the polymorphism of phospholipids, it is often essential to study the properties of "unnatural" phospholipid analogues with modified polar headgroups and or backbone structures. While the thermodynamic characteristics of the "classical" hydrated-gel-to-liquid-crystalline phase transition often appear surprisingly insensitive to these aspects of phospholipid structure, the rich and diverse solid-phase polymorphism of phospholipids is in fact exquisitely sensitive to the nature of both the polar headgroup and the backbone moieties. The tendencies of different phospholipids to form nonlamellar phases at higher temperatures also depend strongly (and in a sometimes surprising manner) on fine details of the headgroup and backbone structures. These points are illustrated by discussions of how the structures of headgroup- and backbone-modified phospholipid analogues influence their proclivities to form distinct types of hydrated solid phases, dehydrated "crystralline" phases and nonlamellar phases.

Inhibitory mechanism of cis-polyunsaturated fatty acids on platelet aggregation: the relation with their effects on Ca2+ mobilization, cyclic AMP levels and membrane fluidity

The in vitro inhibitory effects of cis-polyunsaturated fatty acids, linolenic (18:2 delta 9,12), alpha-linoleic (18:3 delta 9,12,15) and eicosatrienoic (20:3 delta 11,14,17) acid, on bovine platelet aggregation and their inhibitory mechanism were investigated. These fatty acids inhibited platelet aggregation induced by ADP and thrombin to similar extent. Fluorescence analyses with fura-2-loaded platelets showed that, in the concentration ranges that inhibited aggregation, they also inhibited agonist-induced increase in cytoplasmic Ca2+. According to radioimmunoassay study, addition of these fatty acids increased cyclic AMP contents in the presence of theophylline corresponded with their inhibitory effects on aggregation. These fatty acids induced a 1.6-1.8-fold increase over basal concentration of cyclic AMP in the concentration ranges that fully inhibited aggregation. On the other hand, saturated fatty acid, stearic acid, affected neither aggregation nor cyclic AMP levels. As reported previously [1985) Biochim. Biophys. Acta 818, 391), these unsaturated fatty acids induced increase in membrane fluidity in the same concentration range. These results suggest that inhibition of platelet aggregation by cis-polyunsaturated fatty acids is due to the increase in cyclic AMP levels. This increase seems to be due to stimulation of adenylate cyclase which is mediated by membrane perturbation.

Polyunsaturated fatty acid metabolism in cultured fish cells: Incorporation and metabolism of (n-3) and (n-6) series acids by Atlantic salmon (Salmo salar) cells

The incorporation and metabolism of (n-3) and (n-6) polyunsaturated fatty acids (PUFA) supplemented to growing cultures were studied in Atlantic salmon (AS) cells. A fatty acid concentration of 25 μM considerably altered the fatty acid composition of AS cells without increasing the neutral lipid content of the cells or inducing the production of cytoplasmic lipid droplets. Whereas Δ6 and Δ5 desaturase activities were significantly expressed in AS cells, Δ4 desaturase activity was very low. Both the Δ6 desaturase activity and the Δ5 desaturase activity showed some preference for (n-3) PUFA.

Role of polyunsaturated fatty acids and lipid peroxidation in LM fibroblast plasma membrane transbilayer structure

The effects of polyunsaturated fatty acids and lipid peroxidation on LM fibroblast plasma membrane individual leaflet sterol distribution and structural order were examined. The cytofacial (inner) leaflet was more rigid and contained more sterol than the exofacial (outer) leaflet. The static (limiting anisotropy) and dynamic (rotational relaxation time) structural components of diphenylhexatriene (DPH) motion in each leaflet were determined by phase and modulation fluorometry measurements combined with leaflet-specific quenching by trinitrophenyl groups. Polyunsaturated fatty acids, incorporated into the membrane phospholipids by culture medium supplementation, decreased the limiting anisotrophy of DPH in the cytofacial but not the exofacial leaflet thereby abolishing the transbilayer difference in fluidity. Peroxidation by Fe(II) + H2O2 resulted in a rigidification (increase in limiting anisotropy and rotational relaxation time) of the plasma membrane exofacial leaflet, regardless of whether the membranes contained saturated and monounsaturated fatty acids or were enriched in either linoleate or linolenate. The structure of the cytofacial leaflet reported by DPH was unaffected. Plasma membrane transbilayer sterol distribution, measured by leaflet-specific quenching of dehydroergosterol fluorescence, indicated that 20-28% of the sterol was localized in the exofacial leaflet. Polyunsaturated fatty acid supplementation of LM fibroblasts resulted in a complete reversal of plasma membrane transbilayer sterol distribution (72-76% exofacial leaflet). Sterol transbilayer distribution between the membrane leaflets was completely resistant to alteration by exposure to crosslinking agents and peroxidation in control plasma membranes and by peroxidation in linoleate- or linolenate-supplemented membranes.

The effects of decreased growth temperature on the cystine content of cystinotic fibroblasts

Cystinotic fibroblasts transferred from 37 degrees C to 28 degrees C accumulated additional cystine over the period from 4 to 7 days of incubation at 28 degrees C, after which the additional cystine was lost; warming (to 37 degrees C) of cells with elevated cystine stores led to rapid cystine loss. These results, taken together with previously published data showing cystine release from cystinotic fibroblasts incubated at above-normal temperature, are interpreted as indicating the presence in the cystinotic fibroblast lysosome membrane of a cystine-porter whose efficacy is increased by an increase in membrane fluidity. This porter may be the residual activity of the cystine porter that is known to be deficient in cystinosis, or it may be a second as yet unrecognized porter. It is further proposed that this porter is responsible for the presumed efflux of cystine from cystinotic lysosomes.

Polyunsaturated fatty acid metabolism in fish cells: differential metabolism of (n-3) and (n-6) series acids by cultured cells originating from a freshwater teleost fish and from a marine teleost fish

1. The incorporation and metabolism of (n-3) and (n-6) polyunsaturated fatty acids (PUFA) supplemented to growing cultures were studied in rainbow trout (RTG-2) and turbot (TF) cell lines. 2. A fatty acid concentration of 20 microM considerably altered the fatty acid composition of the cells without affecting lipid class composition or the appearance of cytoplasmic lipid droplets. 3. Both cell lines exhibited considerable delta 6 desaturase activities. 4. Whereas delta 5 desaturase activity was expressed in RTG-2 cells, delta 4 desaturase activity was absent and, conversely, delta 4 desaturase activity was expressed in TF cells, but there was an apparent deficiency in the C18 to C20 elongase multi-enzyme complex. 5. The delta 6 desaturase activity in both cell lines showed little preference between 18:2(n-6) and 18:3(n-3) but the delta 5 desaturase activity of RTG-2 cells and the delta 4 desaturase activity of TF cells showed a preference for (n-3)PUFA. 6. Two fish oil concentrates were assessed for their ability to generate fatty acid compositions in the cell lines more closely resembling those of intact fish tissues.

Fluorescence lifetime distributions of 1,6-diphenyl-1,3,5-hexatriene reveal the effect of cholesterol on the microheterogeneity of erythrocyte membrane

The fluorescence decay of 1,6 diphenyl-1,3,5-hexatriene (DPH) has been used to characterize aspects of the erythrocyte membrane structure related to the microheterogeneity of the lipid bilayer. The DPH decay has been studied using frequency domain fluorometry and the data analyzed either by a model of discrete exponential components or a model that assumes a continuous distribution of lifetime values. The main intensity fraction was associated with a lifetime value centered at about 11 ns in the erythrocyte membrane, but a short component of very low fractional intensity had to be considered to obtain a good fit to the data. The lifetime value of the long component was insensitive to temperature, while the width of the distribution decreased with increasing temperature. In multilamellar liposomes prepared from phospholipids extracted from the erythrocytes, the long lifetime component showed a temperature dependence. The depletion of 27% of the cholesterol in the erythrocyte membrane induced a broadening of the distribution, suggesting a homogenizing effect of cholesterol. This effect has also been detected in egg phosphatidylcholine at a very low cholesterol/phospholipid molar ratio. The role of cholesterol on membrane heterogeneity is discussed in relation to the effect of cholesterol on water penetration.

Polyunsaturated fatty acids alter sterol transbilayer domains in LM fibroblast plasma membrane

Sterols are asymmetrically distributed between the leaflets of animal cell plasma membranes. Although transbilayer migration of sterols is extremely rapid, s to min, previous experimental manipulations have not altered their transmembrane steady-state distribution. However, the effect of polyunsaturated fatty acids has not been reported. When cultured in a lipid-free, chemically defined culture medium, LM fibroblasts do not synthesize polyunsaturated fatty acids but will incorporate polyunsaturated fatty acids into their plasma membranes if supplied in the medium. Sterol transbilayer distribution in LM plasma membranes was determined from quenching of fluorescence of dehydroergosterol by trinitrophenyl groups selectively attached to the exofacial leaflet. When cells are cultured in lipid-free media, 28.1% of the plasma membrane sterol is located in the exofacial (outside) leaflet. In contrast, when cells are cultured with linoleate- or linolenate-supplemented medium, 71.8% and 75.5% of the plasma membrane sterol is exofacial, respectively.

Effect of growth temperature on lipid composition and ultraviolet sensitivity of human cells

Human skin fibroblasts were incubated at either 25 or 37 degrees C before UV irradiation. Cells incubated at 25 degrees C were more resistant to near UV radiation than cells grown at 37 degrees C, but cells grown at the lower temperature were more sensitive to 254 nm radiation. Fatty acid analysis of membranes of cells showed that cells incubated at the lower temperature contained significantly higher amounts of linoleic acid (18:2) and linolenic acid (18:3) than cells incubated at 37 degrees C. To determine if this difference in fatty acid content of the membranes was responsible for the UV survival characteristics of cells incubated at different temperatures, cells were enriched with either linoleate or linolenate during a 37 degrees C incubation period. Gas chromatography revealed that cells incorporated the supplied fatty acid. Fatty acid enriched cells were then irradiated with near UV, and survival characteristics were compared to those obtained with cells grown at the lower incubation temperature. The results suggest that the different proportion of fatty acid content of the cells is not the cause of different UV sensitivities of cells grown at 25 degrees C compared to cells grown at 37 degrees C.

Interface between membrane biology and clinical medicine

Many enzymes that are embedded within membranes of cells are sensitive to the chemical and physical properties of the lipid components of the membrane. Because of this, the functions of these integral membrane-bound enzymes can be regulated to some extent by changes within the lipid portions of biologic membranes. That the functions of membrane-bound proteins can be manipulated by modifications of their intimate environment is not surprising. It is well known, for example, that the properties of the surrounding aqueous phase can modulate the function of proteins that are soluble in the cytosol of cells. In contrast, whereas significant changes in the chemical composition and physical properties of the aqueous portion of the cell (e.g., ionic strength and pH) are not allowed, normally tolerable fluctuations of diet appear to influence significantly the composition and properties of the lipid portions of intracellular membranes to the extent of altering the function of some membrane-bound enzymes. In addition, it appears that changes of this type can be induced by diseases that alter dietary intake and/or intermediary metabolism. In other words, it is likely that the functions of at least some integral membrane proteins can be manipulated in patients. Such manipulations may prove to be efficacious. Alternatively, the importance of diet and processes of intermediary metabolism for altering the course of certain diseases may not be fully appreciated. It is worthwhile to review, therefore, current ideas of how the lipid portion of a membrane interacts with integral proteins. The property of the lipids that appears to be most important in this regard is their viscosity. The types of manipulations of function of membrane-bound enzymes that can be achieved are illustrated by in vitro effects secondary to varying the lipids used to reconstitute pure, delipidated forms of these enzymes. The functions of pure delipidated enzymes are discussed for the hepatic drug-metabolizing enzyme uridine diphosphoglucuronyltransferase. In addition, data are presented to indicate that the function of this enzyme can be modified extensively in intact animals by changing the diet.

Effect of membrane fatty acid composition on radiosensitivity of V79 Chinese hamster cells

Exponentially growing V79-379A Chinese hamster fibroblasts were transferred to low-lipid medium enriched with a single fatty acid of the C18 series. After 24 h at 37 degrees C, the fatty acid composition was determined by gas chromatography of the corresponding methyl esters for the total lipid extracts of the cells and for the nuclear membrane fraction. Radiation survival curves, based upon a clonogenic assay, were obtained by irradiation with low dose-rate 60Co gamma rays at either 4 degrees C or room temperature. We observe no effect of fatty acid upon radiosensitivity of these cells at either temperature, in confirmation of published reports with other mammalian cell lines.

Biochemical studies on cell fusion. II. Control of fusion response by lipid alteration

The preceding communication (Roos, D.S. and P.W. Choppin, 1985, J. Cell Biol. 101:1578-1590) described the lipid composition of a series of mouse fibroblast cell lines which vary in susceptibility to the fusogenic effects of polyethylene glycol (PEG). Two alterations in lipid content were found to be directly correlated with resistance to PEG-induced cell fusion: increases in fatty acyl chain saturation, and the elevation of neutral glycerides, including an unusual ether-linked compound. In this study, we have probed the association between lipid composition and cell fusion through the use of fatty acid supplements to the cellular growth medium, and show that the fusibility of cells can be controlled by altering their acyl chain composition. The parental Clone 1D cells contain moderately unsaturated fatty acids with a ratio of saturates to polyunsaturates (S/P) approximately 1 and fuse virtually to completion following a standard PEG treatment. By contrast, the lipids of a highly fusion-resistant mutant cell line, F40, are highly saturated (S/P approximately 4). When the S/P ratio of Clone 1D cells was increased to approximate that normally found in F40 cells by growth in the presence of high concentrations of saturated fatty acids, they became highly resistant to PEG. Reduction of the S/P ratio of F40 cells by growth in cis-polyunsaturated fatty acids rendered them susceptible to fusion. Cell lines F8, F16, etc., which are normally intermediate between Clone 1D and F40 in both lipid composition and fusion response, can be altered in either direction (towards either increased or decreased susceptibility to fusion) by the addition of appropriate fatty acids to the growth medium. Although trans-unsaturated fatty acids have phase-transition temperatures roughly similar to saturated compounds, and might therefore be expected to affect membrane fluidity in a similar manner, trans-unsaturated fatty acids exerted the same effect as cis-unsaturates on the control of PEG-induced cell fusion. This observation suggests that the control of cell fusion by alteration of fatty acid content is not due to changes in membrane fluidity, and thus that the fatty acids are involved in some other way in the modulation of cell fusion.

Biosynthetic incorporation of fluorescent carbazolylundecanoic acid into membrane phospholipids of LM cells and determination of quenching constants and partition coefficients of hydrophobic quenchers

A fluorescence quenching method was developed for determining partition coefficients and diffusional rates of small molecules in cell membranes. This method involves quenching the fluorescence of carbazole-labeled membranes by hydrophobic molecules that partition into membranes. Cell membrane phospholipids of mouse LM cells in tissue culture were biosynthetically labeled with the carbazole moiety by supplementing the growth media with 11-(9-carbazolyl)undecanoic acid. Plasma membranes, microsomes, and mitochondria were isolated free of nonmembranous neutral lipids, and the incorporation of the fluorescent probe was characterized. Quenching studies of the carbazole moiety by a series of N-substituted picolinium perchlorate salts showed that the carbazole moiety was located in the hydrophobic interior of the membrane bilayer. The carbazole fluorescence also was quenched by the hydrophobic quenchers lindane, methoxychlor, and 1,1-dichloro-2,2-bis(rho-chlorophenyl)ethylene, indicating that these compounds partitioned into the membrane. Stern-Volmer quenching constants determined by fluorescence lifetime and intensity measurements were identical, as expected for dynamic quenching. The effects of different lipid compositions on quenching constants and partition coefficients were determined by comparing different membrane fractions. These parameters also were measured in membranes from cells in which the phospholipid composition was altered by substituting ethanolamine for choline in the growth medium. Changes in the lipid composition produced changes in the bimolecular quenching constants. For example, bimolecular quenching constants for 1,1-dichloro-2,2-bis(rho-chlorophenyl)ethylene were higher in mitochondrial membranes than in plasma membranes and microsomes. They were also higher in dispersions made from membrane phospholipids as compared with intact membranes or total lipid dispersion.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in serum influence the fatty acid composition of established cell lines

The fatty acid composition of different kinds of commercially available serum used to supplement cell culture media differs widely. As compared with fetal bovine serum, horse and bovine calf serum have a very high content of linoleic acid (18:2) and are low in arachidonic acid (20:4). (Fatty acids are abbreviated as number of carbon atoms:number of double bonds). Swine serum contains substantial amounts of both 18:2 and 20:4. Only fetal bovine serum contains more than 1% docosahexaenoic acid (22:6). Considerable differences in fatty acid composition occur when cells are grown in media containing any of these different serum supplements. The 18:2 and 20:4 content of 3T3 mouse fibroblast phospholipids is highest when the medium contains horse serum, intermediate with bovine calf serum, and lowest with swine or fetal bovine serum. Likewise, the highest phospholipid 18:2 content in Madin-Darby canine kidney cells (MDCK) occurs when the medium contains horse serum. With MDCK cells, however, growth in swine serum produces the highest 20:4 content. The 3T3 cell phospholipids accumulate more than 1% 22:6 only when the medium contains fetal bovine serum, whereas in no case do the MDCK cell phospholipids accumulate appreciable amounts of 22:6. The fact that the cellular fatty acid composition is likely to change should be taken into account when changes are contemplated in the serum used to grow established cell lines.

Electron-spin resonance studies of lipid-modified microsomes from Friend erythroleukemia cells

The fatty acid composition of cultured Friend erythroleukemia cells was modified by supplementation of the medium with oleic or linoleic acid. There was a 30% reduction in saturated and a 35% reduction in polyunsaturated fatty acids in microsomal phospholipids when the cells were grown in media supplemented with oleic acid, and a 3-fold increase in polyunsaturated fatty acids when the cells were grown in linoleic acid-supplemented media. Electron-spin resonance studies with the 5- nitroxystearate probe demonstrated that there was no appreciable change in microsomal lipid mobility as measured by the order parameters. In contrast, changes in lipid mobility were detected with the spin-label probe when microsomes were first isolated from Friend erythroleukemia cells and subsequently modified by incubation with liposomes composed of either dioleoyl- or dilinoleoylphosphatidylcholine plus bovine liver phospholipid-exchange protein. The fatty acid compositional changes produced in these microsomes were similar to those obtained when the intact cells were grown in media containing supplemental fatty acids. These findings indicate that the lipid mobility of Friend cell microsomes can be altered by phospholipid replacements in vitro, but that this does not occur when similar microsomal fatty acid modifications are produced during culture of the intact cell.

Abnormal insulin binding and membrane physical properties of a Friend erythroleukemia clone resistant to dimethylsulfoxide-induced differentiation

We have compared insulin binding, plasma membrane fluidity, and phospholipid composition of three different Friend erythroleukemia clones, a wild type (FLC) a mutant (R3) and the revertant to wild type F+. The R3 clone is a non-differentiating DMSO-resistant clone (R3) and has altered membrane fluidity and dramatically altered insulin-binding properties. The receptor of R3 bound insulin as if it possessed a single class of low affinity receptors that lacks the property of negative cooperativity. The Scatchard plot is linear and there is no ligand-induced acceleration of dissociation. The Hill coefficient for R3 is 1, implying 'no cooperativity', whereas the Hill coefficient for the two DMSO-inducible clones, (FLC and F+) is 0.3, implying 'negative cooperativity'. In addition, the insulin receptor of R3 has a decreased affinity for insulin, manifested as a 40-fold increase in the amount of insulin required to compete for half of the tracer binding (41 nM for R3 vs. 1 nM for FLC and F+). Computer-fitted Scatchard plots analyzed by the negative cooperativity model reveal that R3 has 95 000 receptor sites/cell, with a high affinity constant Ke of 0.016 nM-1, and a low affinity constant, Kf of 0.012 nM-1. Both DMSO-inducible clones have about 40 000 receptor sites/cell with Ke of 0.11 nM-1 and Kf of 0.02 nM-1. Electron spin resonance measurements with the 5-nitroxy stearate spin probe demonstrate that R3 had a more fluid plasma membrane than the FLC and F+ clones. The lipid composition of R3 is different from that of the DMSO-inducible clones. The weight ratio for unsaturated fatty acids to saturated fatty acids for R3 is 2.5, and the FLC clone has a lower ratio of 1.9. These results are consistent with our earlier findings in FLC that very high membrane fluidity is associated with alterations in the binding properties of the insulin receptor.

Minireview. Roles of turnover and repair of macromolecules and supramolecular structural components

Macromolecules and supramolecular structural components that are incorrectly synthesized or are damaged by radiation or by reactive chemicals are either repaired or selectively degraded and resynthesized. In addition, turnover rates for macromolecules and supramolecular structures can be elevated by alternation of fasting and feeding periods and can be influenced by metabolic regulatory mechanisms which are governed by steady-state concentrations of labile macromolecules.

Binding of [3H]flunitrazepam to the LM cell, a transformed murine fibroblast

LM cells have a saturable, high affinity binding site for [3H]flunitrazepam with a KD of 13 nM and a Bmax of 19 pmoles/mg protein. The IC50 values for Ro 5-4864, flunitrazepam and clonazepam against [3H]flunitrazepam were 6, 23 and 2800 nM, respectively, indicating that this receptor is of the peripheral type. A decrease of 37, 26 and 26% in Bmax was associated with substituting dimethylethanolamine, monomethylethanolamine or ethanolamine, respectively, for choline in the cell culture medium. These treatments did not change either the KD of [3H]flunitrazepam binding or the IC50 values of the different benzodiazepine drugs. Metastatic cell lines of the LM cell obtained from either athymic or C3H/Hef mice exhibited alterations in the binding parameters of [3H]flunitrazepam. There was a reduction in the Bmax values of the athymic (34%) and the C3H/Hef (44%) cell lines compared to the LM cell. In both groups there was a 90% increase in the KD. In the C6 astrocytoma, the peripheral type receptor appears to regulate plasma membrane mediated synthesis of phosphatidylcholine from phosphatidylethanolamine. However, this was not observed in the LM cell. Nor did it modulate cyclic AMP metabolism as assessed by measurement of cyclic AMP levels in whole cells after drug treatment.

Effect of changes in the phospholipid composition on the enzymatic activity of D-beta-hydroxybutyrate dehydrogenase in rat hepatocytes

The phospholipid composition of primary rat hepatocytes was manipulated by supplementing the medium with choline analogues. The unnatural analogue l-2-amino-1-butanol was incorporated into membrane phospholipids to the largest extent, whereas the natural choline analogues ethanolamine, N-methylethanolamine, and N,N-dimethyl-ethanolamine were methylated to yield phosphatidylcholine. When cells were supplemented with [14C]ethanolamine, greater than 25% of the total phosphatidylcholine contained radiolabel in the polar head group after 2 days of supplementation. The extent of phospholipid methylation was reduced by depriving the cells of serine and methionine. Under these conditions, N-methylethanolamine and N,N-dimethylethanolamine were incorporated into phospholipids and were not further metabolized to phosphatidylcholine. After 3 days of supplementation with N-methylethanolamine, the content of phosphatidyl-methylethanolamine went from essentially 0 to 40% of the total phospholipids and surpassed the extent of incorporation of all other analogues. The formation of the new phospholipid species was primarily at the expense of phosphatidylcholine and phosphatidylethanolamine. D-beta-Hydroxybutyrate dehydrogenase, which requires phosphatidylcholine for activity, was assayed in submitochondrial membranes isolated from supplemented cells. For cells supplemented with either l-2-amino-1-butanol or N-methylethanolamine, the Km for NADH increased relative to choline-supplemented cells while the Km for acetoacetate remained the same. For example, after 3 days of supplementation with N-methylethanolamine, the Km for NADH was 3-fold higher than the value for the choline-supplemented control cells. The change in the Km was due to the change in the lipid environment with no alteration in the enzyme itself. The results suggest that the phosphatidylcholine molecules necessary to activate the enzyme exchange with the other phospholipids in the membrane so that the Km of the enzyme reflects the overall content of phosphatidylcholine as well as other properties of the membrane phospholipids.

Lipid composition alters phagocytosis of fluorescent latex beads

A new assay using fluoresbrite microspheres was developed to determine phagocytic rate in LM fibroblasts grown in a variety of culture conditions. Fluoresbrite beads of diameter 0.86 microns or greater were taken up by the cells at a linear rate for 60 min over a wide range of bead/cell ratios. Phagocytosis was measured as the difference in fluorescence of cells exposed to beads at 37 degrees C and at 4 degrees C, to correct for adsorbance of beads to the cells. Fluoresbrite bead phagocytosis was zero at zero time and was saturable. LM fibroblasts cultured in a serum-free, chemically-defined medium were supplemented with choline analogues or fatty acids to alter plasma membrane lipid composition. Choline analogue supplementation (N,N'-dimethylethanolamine, N-monomethylethanolamine, or ethanolamine) altered the plasma membrane phospholipid polar head group composition and dramatically decreased the phagocytic rate as compared with choline fed cells. Supplementation with polyunsaturated and monounsaturated fatty acids, but not saturated fatty acids, increased the phagocytic rate. The phagocytic rate was correlated with the plasma membrane phospholipid fatty acid index of unsaturation.

Evidence for a correlation between swimming velocity and membrane fluidity of Tetrahymena cells

The influence of the physical state of the membrane on the swimming behaviour of Tetrahymena pyriformis was studied in cells with lipid-modified membranes. When the growth temperature of Tetrahymena cells was increased from 15 degrees C to 34 degrees C or decreased from 39 degrees C to 15 degrees C, their swimming velocity changed gradually in a similar to the adaptive change in membrane lipid composition. Therefore, such adaptive changes in swimming velocity were not observed during short exposures to a different environment. Tetrahymena cells adapted to 34 degrees C swam at 570 microns/s. On incubation at 15 degrees C these cells swam at 100 microns/s. When the temperature was increased to 34 degrees C after a 90-min incubation at 15 degrees C, the initial velocity was immediately recovered. On replacement of tetrahymanol with ergosterol, the swimming velocity of 34 degrees C-grown cells decreased to 210 microns/s, and the cells ceased to move when the temperature was decreased to 15 degrees C. To investigate the influence of the physical state of the membrane on the swimming velocity, total phospholipids were prepared from Tetrahymena cells grown under these different conditions. The fluidities of liposomes of these phospholipid were measured using stearate spin probe. The membrane fluidity of the cells cooled to 15 degrees C increased gradually during incubation at 15 degrees C. On the other hand, the fluidity of the heated cell decreased during incubation at 34 degrees C. Replacement of tetrahymanol with ergosterol decreased the membrane fluidity markedly. Consequently, a good correlation was observed between swimming velocity and membrane fluidity; as the membrane fluidity increased, the swimming velocity increased linearly up to 600 microns/s. These results provide evidence for the regulation of the swimming behaviour by physical properties of the membrane.

Development and characterization of a tissue culture cell line with essential fatty acid deficiency

We have developed an essential fatty acid-deficient cell line from a parental cell line, HSDM1C1, which metabolizes arachidonic acid to prostaglandin E2 (PGE2). This cell line, designated EFD-1, is depleted of arachidonate, is unable to synthesize PGE2 in response to bradykinin, and has changes in fatty acid composition characteristic of tissues from animals with essential fatty acid deficiency. Within 15 min of repletion by arachidonate, the ability to synthesize PGE2 is restored. Linoleate also is able to restore PGE2 synthesis, indicating that deficient cells contain both the rate-limiting delta 6 desaturase enzyme and the delta 5 desaturase enzyme, which are required to form arachidonate. When parental cells are incubated in lipid-free medium, there is rapid induction of the ability to convert linoleate to arachidonate. Arachidonate prevents this induction, suggesting that icosanoid precursor availability controls the rate of arachidonate formation.

Uptake of arachidonic acid into membrane phospholipids: effect on chloride transport across cornea

We demonstrate that arachidonic acid (AA) stimulation of chloride transport across frog cornea is mediated via two independent pathways: (1) stimulation of prostaglandins and cAMP synthesis, and (2) a direct physical change in the membrane produced by substitution of different phospholipid acyl chains. AA is well known as a precursor in the synthesis of prostaglandins, which have been shown to stimulate cAMP synthesis and chloride transport in frog cornea. We show that frog cornea can convert exogenous AA to PGE2, but that in the presence of 10(-5) M indomethacin both the conversion to PGE2 and stimulation of cAMP are completely blocked. However, with indomethacin the action of AA to stimulate chloride transport (as measured by SCC) remains, but peak height of the response is reduced to 57% of that found when AA alone is given. Similarly, we show that propranolol completely blocks cAMP stimulation, but stimulation of SCC is reduced to 45% of the original response. Therefore, cAMP appears to be responsible for roughly half of the observed stimulation in SCC. By gas chromatographic analysis we show that significant quantities of AA can rapidly substitute into membrane phospholipids of corneal epithelium and L929 cells following the addition of AA to the medium. Modification of membrane phospholipid structure can affect membrane viscosity, membrane-bound enzyme activity, and the distribution and lateral mobility of integral proteins. It seems likely that such alterations in the properties of the membrane may modulate the rate of chloride transport, and this may constitute the second mechanism. Upon addition of AA, both mechanisms appear to stimulate chloride transport simultaneously, and are apparently additive. We show that prolonged exposure to AA results in a large incorporation of AA into phospholipid and consequently, a perturbation in the ratio of unsaturated to saturated fatty acids. We also find evidence of a compensatory cellular mechanism that alters the ratio of endogenously synthesized fatty acids and tends to reduce the membrane-perturbing effect of AA.U

Age-dependent alterations in lipids and function of rat heart sarcolemma

The objective of the present study was to examine the effect of age on heart sarcolemma structure and function. Sarcolemmal fractions were prepared from hearts of young (1-1.5 months) and adult (10-12 months) rats and assayed for marker enzyme activities. The membrane fractions were found to be devoid of other cellular organelles upon examination by electron microscopy. They were enriched with 5'-nucleotidase and devoid of succinate dehydrogenase activity. The only age-related lipid compositional changes noted in these membranes were changes in the fatty acid composition of membrane phospholipids with increasing age. Most changes were detected in phosphatidylcholine and phosphatidylethanolamine with very little alteration of sphingomyelin and phosphatidylserine plus phosphatidylinositol fatty acids. Polyunsaturated fatty acids, especially 18:2 and 20:4, were decreased with saturated fatty acids increased in membrane phosphatidylcholine and phosphatidylethanolamine fractions as the animal develops. There was a decrease in the specific activities of (Na+ + K+)-ATPase and 5'-nucleotidase of these membranes with age. On the other hand, membrane (K+)-rho-nitrophenylphosphatase was not affected by age.

The relationship between plasma membrane lipid composition and physical-chemical properties. III. Detailed physical and biochemical analysis of fatty acid-substituted EL4 plasma membranes

Murine leukemia EL4 cells were modified by supplementation of culture media with fatty acids for 24 h. A plasma membrane-enriched fraction was prepared from substituted and normal cells. Analyses were performed to determine fatty acyl composition, phospholipid headgroup composition and cholesterol content. The two major membrane phospholipids, phosphatidylethanolamine (PE) and phosphatidylcholine (PC) were isolated by thin-layer chromatography and ESR measurements were done on liposomes prepared from these lipids as well as on the intact plasma membrane preparations. Slight perturbations in overall plasma membrane lipid composition were observed when EL4 cells were supplemented with a single exogenous fatty acid. This may be consistent with the idea that the incorporation of exogenous fatty acid induces compensatory changes in membrane lipid composition. On the other hand, we observed no significant difference in two ESR motional parameters between the unsubstituted control and various fatty acid-substituted plasma membranes. ESR measurements carried out on PE and PC liposomes derived from 17:0- and 18:2c-substituted membranes also failed to detect major differences between these liposomes and those made from normal EL4 phospholipids. In the case of liposomes prepared from 18:2t,-substituted membranes, the order parameter was significantly changed from the normal. However, the change was in opposite directions in PE and PC, perhaps accounting for the fact that no change parameter is seen in intact 18:2t-substituted plasma membrane. Measurements of order parameter (S) in mixed lipid vesicles showed that at up to 50 mol% mixture of a synthetic PC with plasma membrane PC, the value of S was only marginally different from that of the plasma membrane PC vesicles. We interpret these data as an indication that the two ESR parameters used are not sufficiently sensitive to detect changes due to modifications of the acyl chain composition of a complex biological membrane.

Accumulation of N-3 polyunsaturated fatty acids cultured human Y79 retinoblastoma cells

The metabolism of the n-3 class of polyunsaturated fatty acids, which occur in relatively high quantities in neural tissues, was studied in human Y79 retinoblastoma cells. These cells contained low levels of n-3 polyunsaturates when grown in culture media supplemented with fetal bovine serum. The cells readily incorporated performed docosahexaenoic acid (22.6 n-3) into phospholipids, but human skin fibroblasts did this to a similar extent. When 10 to 30 mumol/ml linolenic acid (18:3 n-3) was added, the cell also accumulated 22:6 in phospholipids. The capacity to convert appreciable amounts of 18:3 to 22:6 appears to be a unique property of the retinoblastoma cells as compared with other continuously cultured cell lines. More 18:3 than linoleic acid (18:2 n-6) was incorporated into phospholipids by the retinoblastoma cultures, and 18:3 was channeled to a larger extent into the ethanolamine glycerophospholipid fraction. These findings indicate that retinoblastoma cells handle n-3 polyunsaturated fatty acids in a manner very similar to neural tissue in vivo. Based on the results obtained with this model system, it appears that three processes may contribute to the accumulation of 22:6 in retina and neural tissue: increased ability to incorporate 18:3, the capacity to convert 18:3 to 22:6, and channeling of 18:3 and its metabolites into ethanolamine glycerophospholipids.

The relationship between plasma membrane lipid composition and physical-chemical properties. I. Fluorescence polarization studies of fatty acid-altered EL4 tumor cell membranes

EL4 cels were cultured with exogenous fatty acids under conditions that resulted in their incorporation into membranes phospholipids. The behavior of the fluorescent lipid probes diphenylhexatriene and perylene was monitored in intact EL4 cells and in isolated EL4 plasma membranes. In whole cells substituted with unsaturated fatty acids, there was always a marked decrease in the P value of both probes compared to the P value of the probes in unsubstituted cells. In whole cells substituted with saturated fatty acids, on the other hand, P values for both probes were unchanged compared to unsubstituted cells. In plasma membrane isolated from EL4 cells, no difference in P values for either probe was observed among membranes from unsubstituted, saturated fatty acid substituted or unsaturated fatty acid substituted cells, even when the degree of fatty acid substitution was quite substantial. Most of the fluorescent signal for both probes in whole cells appeared to come from cytoplasmic lipid droplets. The value of techniques such as fluorescent polarization from monitoring physical properties of membranes (such as 'fluidity') is discussed.

Temperature-induced homeoviscous adaptation of Chinese hamster ovary cells

Exponential and plateau phase Chinese hamster ovary cells were maintained for 3 days at 32, 37, 39 or 41 degrees C. The effect of growth temperature on the fluidity and composition of the cellular membranes, and on the ability of the cells to resist a subsequent heat treatment at 43 degrees C, was measured. Cells grown at temperatures above 37 degrees C displayed increased resistance or tolerance to a 43 degree C heat treatment, whereas cells grown at 32 degrees C were sensitized to heat. Extensive cell division was not required for expression of heat tolerance. Membrane fluidity, as determined by the degree of rotational mobility of the fluorescent probe diphenylhexatriene, decreased with increasing growth temperatures, but the relationship did not hold in exponential phase cells grown at 32 degrees C. The cholesterol : phospholipid molar ratio correlated with the fluorescence polarization values, suggesting that the cells are able to adjust membrane fluidity by varying the concentration of cholesterol. The results are compatible with the concept of homeoviscous adaptation: that organisms strive to maintain an optimal level of membrane fluidity and when grown at a different temperature will alter the lipid composition in order to maintain this level. Up until now, cholesterol has not been implicated in this process.

Chaulmoogric acid: assimilation into the complex lipids of mycobacteria

Lipid analysis of Mycobacterium vaccae, grown in the presence of chaulmoogric acid, demonstrates that this cyclopentenyl fatty acid is taken up by the organism and incorporated into cellular phospholipids and triacylglycerols. As cell growth is retarded by the addition of chaulmoogric acid to the growth medium, it is possible that the antimicrobial properties of this compound results from a perturbation of membrane processes.