Docosahexaenoic acid promotes micron scale liquid-ordered domains. A comparison study of docosahexaenoic versus oleic acid containing phosphatidylcholine in raft-like mixtures

The understanding of the functional role of the lipid diversity in biological membranes is a major challenge. Lipid models have been developed to address this issue by using lipid mixtures generating liquid-ordered (Lo)/liquid-disordered (Ld) immiscibility. The present study examined mixtures comprising Egg sphingomyelin (SM), cholesterol (chol) and phosphatidylcholine (PC) either containing docosahexaenoic (PDPC) or oleic acid (POPC). The mixtures were examined in terms of their capability to induce phase separation at the micron- and nano-scales. Fluorescence microscopy, electron spin resonance (ESR), X-ray diffraction (XRD) and calorimetry methods were used to analyze the lateral organization of the mixtures. Fluorescence microscopy of giant vesicles could show that the temperature of the micron-scale Lo/Ld miscibility is higher for PDPC than for POPC ternary mixtures. At 37°C, no micron-scale Lo/Ld phase separation could be identified in the POPC containing mixtures while it was evident for PDPC. In contrast, a phase separation was distinguished for both PC mixtures by ESR and XRD, indicative that PDPC and POPC mixtures differed in micron vs nano domain organization. Compared to POPC, the higher line tension of the Lo domains observed in PDPC mixtures is assumed to result from the higher difference in Lo/Ld order parameter rather than hydrophobic mismatch.

Cell proliferation in in vivo-like three-dimensional cell culture is regulated by sequestration of ERK1/2 to lipid rafts

OBJECTIVES

Regulatory mechanisms of cell proliferation have been extensively studied as they represent major challenges when dealing with pathologies such as fibrosis, tumourigenesis or tissue regeneration. Numerous in vitro studies still exploit conventional, two-dimensional cell cultures where cells are forced to adhere to unnaturally stiff and flat surfaces of culture dishes. In the living organism, however, each cell is in contact with components of the extracellular matrix and/or neighbouring cells, thus creating a complex three-dimensional (3D) tissue structure. The current paper describes a native 3D culture of cells, based on the GD25β1 fibroblast cell line, and its use for investigating cell proliferation in in vivo-like conditions.

MATERIALS AND METHODS

Four-day post-confluent culture of GD25β1 fibroblasts resulted in formation of a 3D system of cells embedded in naturally synthesized extracellular matrix. Morphological characterization of the culture was performed by histochemistry, immunohistochemistry and immunofluorescence. Viability/proliferation was assayed by MTT testing, FACS analysis and Western blotting for determination of expression levels and activation status of the relevant signalling molecules.

RESULTS

GD25b1 fibroblasts, grown as 3D culture, gave rise to tissue-like structures characterized by low level of apoptosis, low senescence and development of 3D matrix adhesions, typical of living tissues. Transition to three-dimensionality led to a switch from exponential to linear culture growth, accompanied by accumulation of activated ERK1/2 into caveolin-containing raft domains. Disruption of raft domains as well as reverse transition from 3D back to monolayer culture led to release of phosphorylated ERK1/2 from rafts, activation of cyclin D1 expression and increase in proliferation levels.

CONCLUSIONS

These results imply that under in vivo-like conditions, cells might achieve reduction of their proliferation level by sequestering activated ERK1/2 to lipid rafts.

Influence of membrane phospholipid composition and structural organization on spontaneous lipid transfer between membranes

Investigations were carried out on the influence of phospholipid composition of model membranes on the processes of spontaneous lipid transfer between membranes. Acceptor vesicles were prepared from phospholipids extracted from plasma membranes of control and ras-transformed fibroblasts. Acceptor model membranes with manipulated levels of phosphatidylethanolamine (PE), sphingomyelin and phosphatidic acid were also used in the studies. Donor vesicles were prepared of phosphatidylcholine (PC) and contained two fluorescent lipid analogues, NBD-PC and N-Rh-PE, at a self-quenching concentration. Lipid transfer rate was assessed by measuring the increase of fluorescence in acceptor membranes due to transfer of fluorescent lipid analogues from quenched donor to unquenched acceptor vesicles. The results showed that spontaneous NBD-PC transfer increased upon fluidization of acceptor vesicles. In addition, elevation of PE concentration in model membranes was also accompanied by an increase of lipid transfer to all series of acceptor vesicles. The results are discussed with respect to the role of lipid composition and structural order of cellular plasma membranes in the processes of spontaneous lipid exchange between membrane bilayers.

Bimodal regulatory effect of melittin and phospholipase A2-activating protein on human type II secretory phospholipase A2

Melittin and phospholipase A2-activating protein (PLAP) are known as efficient activators of secretory phospholipase A2(sPLA2) types I, II, and III when phospholipid liposomes are used as substrate. The present study demonstrates that both peptides can either inhibit or activate sPLA2 depending on the peptide/phospholipid ratio when erythrocyte membranes serve as a biologically relevant substrate. Low concentrations of melittin and PLAP were observed to inhibit sPLA2-triggered release of fatty acids from erythrocyte membranes. The inhibition was reversed at melittin concentrations above 1 microM. PLAP-induced inhibition of sPLA2 persisted steadily throughout the used concentration range (0-150 nM). The two peptides induced a dose-dependent activation of sPLA2 at low concentrations, followed by inhibition when model membranes were used as substrate. This opposite modulatory effect on biological membranes and model membranes is discussed with respect to different mechanisms the interaction of the regulatory peptides with the enzyme molecules and the substrate vesicles.

Stimulated nonspecific transport of phospholipids results in elevated external appearance of phosphatidylserine in ras-transformed fibroblasts

The content of phosphatidylserine (PS) was found to be increased three times in the plasma membrane outer leaflet of ras-transformed fibroblasts compared to their nontransformed counterparts. In an attempt to determine the mechanisms responsible for the enhanced external appearance of PS, we investigated the activities of aminophospholipid translocase and the nonspecific lipid scramblase. Both transport systems could separately or in combination contribute to PS accumulation in the extracellular leaflet. Aminophospholipid transfer was assessed by measuring the rate of NBD-PS internalization, and scramblase activity was estimated from the internalization of NBD-PC. The results showed that the aminophospholipid transport was inhibited and the nonspecific transport was stimulated in ras-transformed cells. To assess which of these two transport systems was related to elevation of PS external appearance, each of them was submitted to reversible alterations and the content of PS was measured simultaneously. Aminophospholipid translocase activity was inhibited by pyridyldithioethylamine treatment and reversed by reduction with dithiothreitol. Scramblase activity was modulated by a calcium repletion-depletion procedure. Calcium depletion was performed by cell incubation with BAPTA-AM and EGTA as Ca2+ intracellular and extracellular chelators. Restoration of the intracellular Ca2+ was achieved by cell incubation with Ca2+ and Ca2+-ionophore A23187. The results showed that the changes in PS outer appearance did not correlate with the uptake of NBD-PS but were closely related to NBD-PC internalization, suggesting that the nonspecific bidirectional lipid transfer was the major transport system translocating PS to the outer leaflet in ras-transformed cells.

Ha-ras-transformation alters the metabolism of phosphatidylethanolamine and phosphatidylcholine in NIH 3T3 fibroblasts

Cultured NIH 3T3 fibroblasts were employed to investigate the changes in the phospholipid metabolism induced by Ha-ras transformation. All phospholipid fractions were reduced in ras-transformed fibroblasts except phosphatidylethanolamine (PE). The incorporation of labeled choline and ethanolamine into phosphatidylcholine (PC), PE and their corresponding metabolites were elevated in a similar manner in the transformed cells. The enhanced uptake of choline and ethanolamine correlated with the activation of choline kinase and ethanolamine kinase. Similarly, the uptake of arachidonic, oleic and palmitic acids by PC and PE was higher in ras-cells. Acyl-CoA synthetases, which esterify fatty acid before their incorporation into lysophospholipids, were also activated. However, both CTP:phosphocholine-cytidylyltransferase and CTP:phosphoethanolamine-chytidyltransferase were inhibited in the transformed cells. This fact, taken together with the observed activation of choline- and ethanolamine kinases, led to accumulation of phosphocholine and phosphoethanolamine, which have been presumed to participate in the processes of tumor development. PC biosynthesis seemed to be carried out through the CDP-choline pathway, which was stimulated in the oncogenic cells, whereas PE was more likely, a product of phosphatidylserine decarboxylation rather than the CDP-ethanolamine pathway.

Arachidonoyl-CoA:lysophosphatidylcholine acyltransferase activity in ras-transformed NIH 3T3 fibroblasts depends on the membrane composition

Investigations were performed on the influence of membrane lipids on arachidonoyl-CoA:lysophosphatidylcholine acyltransferase in microsomal membranes from control and ras-transformed NIH 3T3 fibroblasts. Of all the tested phospholipids only sphingomyelin induced activation of acyltransferase in membranes from ras-transformed cells. No specific phospholipid effect on the acyltransferase was observed in microsomal membranes from control fibroblasts. Diacylglycerol was found to inhibit acyltransferase in both cell lines, whereas ceramide accumulation induced inhibition only in membranes from the transformed cells. The effects of diacylglycerol, ceramide, sphingomyelin and sphingomyelinase are discussed with respect to their putative roles in the signal transduction pathways in oncogene-expressing cells.

Phosphatidylethanolamine and phosphatidylcholine are sources of diacylglycerol in ras-transformed NIH 3T3 fibroblasts

Ras-transformation of cells is accompanied by an increase of the level of diacylglycerol (DAG), which participates in the signal transduction pathways. DAG could be generated from phospholipids either by activation of phospholipase C or by a more complex pathway involving phospholipase D and phosphatidate phosphohydrolase. To clarify which phospholipids produce DAG and which pathways are involved, we examined the DAG generating enzyme activities, using phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylinositol (PI) as substrates. The study showed that the breakdown of PC and more markedly of PE by phospholipases C and D was stimulated in membranes from ras-transformed cells. Phosphatidate phosphohydrolase activity was also elevated in oncogene-expressing cells. The increase in glycerol uptake was most pronounced in cells given PE, followed by PC. The fatty acid analysis revealed apparent similarities between the acyl chains of PE and DAG only in the transformed cells. These findings suggest that PE is a source of DAG in ras-fibroblasts but does not rule out the role of PC in DAG production, due to the activation of the PC-specific phospholipases C and D.

Phospholipid dependence of membrane-bound phospholipase A2 in ras-transformed NIH 3T3 fibroblasts

Although the activation of phospholipase A2 (PLA2) in ras-transformed cells has been well documented, the mechanisms underlying this activation are poorly understood. In this study we tried to elucidate whether the membrane phospholipid composition and physical state influence the activity of membrane-associated PLA2 in ras-transformed fibroblasts. For this purpose membranes from non-transfected and ras-transfected NIH 3T3 fibroblasts were enriched with different phospholipids by the aid of partially purified lipid transfer protein. The results showed that of all tested phospholipids only phosphatidylcholine (PC) increased PLA2 activity in the control cells, whereas in their transformed counterparts both PC and phosphatidic acid (PA) induced such effect. Further we investigated whether the activatory effect was due only to the polar head of these phospholipids, or if it was also related to their acyl chain composition. The results demonstrated that the arachidonic acid-containing PC and PA molecules induced a more pronounced increase of membrane-associated PLA2 activity in ras-transformed cells compared to the corresponding palmitate-stearate- or oleate- containing molecular species. However, we did not observe any specific effect of the phospholipid fatty acid composition in non-transformed NIH 3T3 fibroblasts. In ras-transformed cells incubated with increasing concentrations of arachidonic acid, PLA2 activity was altered in parallel with the changes of the cellular content of this fatty acid. The role of phosphatidic and arachidonic acids as specific activators of PLA2 in ras-transformed cells is discussed with respect to their possible role in the signal transduction pathways as well as in the processes of malignant transformation of cells.

Influence of carbicron (O-[(2-butenoic acid)-N,N-dimethylamide-3-yl] O,O-dimethylphosphate) on some biochemical and biophysical parameters of rat liver membranes

1. Treatment of rats with carbicron induced a reduction of the phospholipids in both microsomal and plasma membranes. 2. A decrease of the structural order parameter (SDPH) and an increase of the pyrene excimer-to-monomer fluorescence ratio (IE/IM) was also observed, indicating membrane fluidization. 3. The specific activity of membrane-bound phospholipase A2 and phospholipase C were decreased in both types of membranes, whereas acyl-CoA:lysophosphatidylcholine acyltransferase activity was augmented due to carbicron treatment.

Alterations in rat alveolar surfactant system induced by treatment with carbicron (O-[(2-butenoic acid)-N,N-dimethylamide-3-yl]-O,O-dimethylphosphate)

1. Intoxication of rats with carbicron (O-([2-butenoic acid)-N,N-dimethylamide-3-yl]-O,O-dimethylphosphate) induced a reduction of the total phospholipids and phosphatidylcholine in lung alveolar surfactant. 2. The lipid transfer protein activity was inhibited due to carbicron treatment. 3. No alterations were observed in phospholipase A2 activity in the alveolar surfactant of intoxicated animals. The structural order parameter (SDPH) of bilayer liposomes, prepared from surfactant phospholipids of carbicron-treated rats also remained unchanged.

Rat liver microsomal phospholipase A2 and membrane fluidity

The influence of the physical state and the lipid composition on microsomal membrane-bound phospholipase A2 activity has been investigated. It was established that the decrease of membrane fluidity expressed by the alterations of the steady-state fluorescent anisotropy (rs) and the structural order parameter for DPH (SDPH) leads to augmentation of phospholipase A2 specific activity. Phosphatidylinositol is the only phospholipid having a specific activating effect on microsomal membrane-bound phospholipase A2.

Sensitivity of 5'-nucleotidase and phospholipase A2 towards liver plasma membranes modifications

The changes in the phospholipid and fatty acid composition of liver plasma membranes isolated from rats, fed two different diets, containing either saturated or unsaturated fatty acids, were investigated. We established that dietary treatment can considerably modify the fatty acid as well as the phospholipid composition of liver plasma membranes. Lipid transfer proteins were used for enrichment of liver plasma membranes with sphingomyelin, dioleoylphosphatidylcholine, dipalmitoylphosphatidylcholine, and phosphatidylinositol. A marked sphingomyelin and membrane fluidity dependence of the membrane-bound 5'-nucleotidase and phospholipase A2 was observed.

Maternal diabetes mellitus and changes in neonatal rat lung and alveolar surfactant phospholipids

We investigated the extent of the influence of maternal diabetes on the phospholipid composition and exchange activity of the neonatal lung alveolar surfactant. The results show that each phospholipid fraction (as well as the total phospholipid content) of the surfactant of neonates with diabetic mothers are decreased to about 30% of the control values. Phosphatidylcholine and phosphatidylglycerol, which are the most important surface active phospholipid fractions, were decreased to 27% and 34% respectively. In lung tissue of the neonates with diabetic mothers, all phospholipid fractions were increased. We found that the phosphatidylcholine-exchange activity in the alveolar surfactant does not exist in neonates with diabetic mothers. This inhibited phospholipid-exchange activity may be the reason for the decrease in the surfactant phospholipids and their increase in the lungs of neonates with diabetic mothers. The cholesterol content in the surfactant of such neonates decreased by almost half in comparison with the controls, while in lung tissue it remained unchanged. Producing an experimental respiratory distress syndrome could permit to study more deeply the causes which provoke it and the accompanying metabolic changes.

Phospholipid composition of subcellular fractions and phospholipid-exchange activity in chicken liver and MC-29 hepatoma

The phospholipid composition of mitochondria, microsomes and plasma membranes from liver and MC-29 hepatoma from White Leghorn chickens has been investigated. It was established that all mitochondria and microsome phospholipid fractions obtained from MC-29 hepatoma are increased strongly compared to those from liver. The sphingomyelin augmentation was particularly great. In hepatoma plasma membranes only the sphingomyelin quantity was increased. Sphingomyelin- and phosphatidylcholine-exchange activities were observed in avian liver for the first time. These two activities were increased in MC-29 hepatoma cells. Three phospholipid-exchange proteins have been established in chicken liver 105000 X g supernatant. One of them specifically transports phosphatidylcholine, the second one is non-specific for phosphatidylcholine and sphingomyelin, and the third one is specific only for sphingomyelin. In hepatoma cells only a non-specific phosphatidylcholine- and sphingomyelin-exchange protein was found.