Quantification of the interactions among fatty acid, lysophosphatidylcholine, calcium, dimyristoylphosphatidylcholine vesicles, and phospholipase A2

Molecular interactions of phospholipid monolayers with a model phospholipase

The intrinsic overexpression of secretory phospholipase A2 (sPLA2) in various pro-inflammatory diseases and cancers has the potential to be exploited as a therapeutic strategy for diagnostics and treatment. To explore this potential and advance our knowledge of the role of sPLA2 in related diseases, it is necessary to systematically investigate the molecular interaction of the enzyme with lipids. By employing a Langmuir trough integrated with X-ray reflectivity and grazing incidence X-ray diffraction techniques, this study examined the molecular packing structure of 1,2-palmitoyl-sn-glycero-3-phosphocholine (DPPC) films before and after enzyme adsorption and enzyme-catalyzed degradation. Molecular interaction of sPLA2 (from bee venom) with the DPPC monolayer exhibited Ca2+ dependence. DPPC molecules at the interface without Ca2+ retained a monolayer organization; upon adsorption of sPLA2 to the monolayer the packing became tighter. In contrast, sPLA2-catalyzed degradation of DPPC occurred in the presence of Ca2+, leading to disruption of the ordered monolayer structure of DPPC. The interfacial film became a mixture of highly ordered multilayer domains of palmitic acid (PA) and loosely packed monolayer phase of 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (lysoPC) that potentially contained the remaining un-degraded DPPC. The redistribution of lipid degradation products into the third dimension, which produced multilayer PA domains, damaged the structural integrity of the original lipid layer and may explain the bursting of liposomes observed in other studies after a latency period of mixing liposomes with sPLA2. A quantitative understanding of the lipid packing and lipid-enzyme interaction provides an intuitive means of designing and optimizing lipid-related drug delivery systems.

Effects of cholesterol on physical properties of human erythrocyte membranes: impact on susceptibility to hydrolysis by secretory phospholipase A2

The ability of secretory phospholipase A(2) (sPLA(2)) to hydrolyze cell membranes is highly dependent on the physical properties of the membrane. The effects of cholesterol on these properties have been characterized in artificial bilayers and found to alter sPLA(2) activity significantly. It is hypothesized that the natural difference in cholesterol content between erythrocytes and leukocytes is in part responsible for their differing susceptibility to hydrolysis by sPLA(2). To test this hypothesis, defined amounts of cholesterol were removed from erythrocyte membranes using methyl-beta-cyclodextrin. Treatment of cells with methyl-beta-cyclodextrin increased the hydrolysis rate and total substrate hydrolyzed by sPLA(2). In general, this effect of cholesterol removal was more pronounced at higher temperatures. Comparison of the level of membrane order (assessed with the fluorescent probe laurdan) with hydrolysis rate revealed that sPLA(2) activity was greatly enhanced upon significant reductions in lipid order. Additional treatment of the cells with calcium ionophore further enhanced the hydrolysis rate and altered the relationship with membrane order. These data demonstrated that interactions with sPLA(2) observed in artificial bilayers apply to biological membranes. It is also proposed that the high level of cholesterol in erythrocyte membranes is a protective mechanism to guard against hydrolytic enzymes.

Ca2+-induced phase separation in the membrane of palmitate-containing liposomes and its possible relation to membrane permeabilization

A Ca(2+)-induced phase separation of palmitic acid (PA) in the membrane of azolectin unilamellar liposomes has been demonstrated with the fluorescent membrane probe nonyl acridine orange (NAO). It has been shown that NAO, whose fluorescence in liposomal membranes is quenched in a concentration-dependent way, can be used to monitor changes in the volume of lipid phase. The incorporation of PA into NAO-labeled liposomes increased fluorescence corresponding to the expansion of membrane. After subsequent addition of Ca(2+), fluorescence decreased, which indicated separation of PA/Ca(2+) complexes into distinct membrane domains. The Ca(2+)-induced phase separation of PA was further studied in relation to membrane permeabilization caused by Ca(2+) in the PA-containing liposomes. A supposition was made that the mechanism of PA/Ca(2+)-induced membrane permeabilization relates to the initial stage of Ca(2+)-induced phase separation of PA and can be considered as formation of fast-tightening lipid pores due to chemotropic phase transition in the lipid bilayer.

A new hat for an old enzyme: Waste management

The history of research regarding secretory phospholipase A(2) (sPLA(2)) has often focused in one of two directions. Originally, the enzyme was studied biophysically in terms of its fundamental structure, enzymology, and the relationship between membrane physics and catalytic activity. More recently, a large and growing body of information has accumulated concerning regulatory factors, tissue distribution, and physiological/pathological roles of sPLA(2). Evidence is presented that suggests an additional function for the protein in which it helps to clear dead and damaged cells while avoiding digestion of those that are healthy. Apparently, the ability of the enzyme to discriminate between susceptible and resistant cells depends on physical properties of membrane lipids related to order, distribution, and neighbor/neighbor interactions. Investigations into this action of the enzyme offer the rare opportunity to apply biophysical approaches and principles to a physiological setting.

A two-photon view of an enzyme at work: Crotalus atrox venom PLA2 interaction with single-lipid and mixed-lipid giant unilamellar vesicles

We describe the interaction of Crotalus atrox-secreted phospholipase A2 (sPLA2) with giant unilamellar vesicles (GUVs) composed of single and binary phospholipid mixtures visualized through two-photon excitation fluorescent microscopy. The GUV lipid compositions that we examined included 1-palmitoyl-2-oleoyl-phosphatidylcholine, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) (above their gel-liquid crystal transition temperatures) and two well characterized lipid mixtures, 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE):DMPC (7:3) and 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC)/1,2-diarachidoyl-sn-glycero-3-phosphocholine (DAPC) (1:1) equilibrated at their phase-coexistence temperature regime. The membrane fluorescence probes, 6-lauroyl-2-(dimethylamino) napthalene, 6-propionyl-2-(dimethylamino) naphthalene, and rhodamine-phosphatidylethanolamine, were used to assess the state of the membrane and specifically mark the phospholipid domains. Independent of their lipid composition, all GUVs were reduced in size as sPLA2-dependent lipid hydrolysis proceeded. The binding of sPLA2 was monitored using a fluorescein-sPLA2 conjugate. The sPLA2 was observed to associate with the entire surface of the liquid phase in the single phospholipid GUVs. In the mixed-lipid GUV's, at temperatures promoting domain coexistence, a preferential binding of the enzyme to the liquid regions was also found. The lipid phase of the GUV protein binding region was verified by the introduction of 6-propionyl-2-(dimethylamino) naphthalene, which partitions quickly into the lipid fluid phase. Preferential hydrolysis of the liquid domains supported the conclusions based on the binding studies. sPLA2 hydrolyzes the liquid domains in the binary lipid mixtures DLPC:DAPC and DMPC:DMPE, indicating that the solid-phase packing of DAPC and DMPE interferes with sPLA2 binding, irrespective of the phospholipid headgroup. These studies emphasize the importance of lateral packing of the lipids in C. atrox sPLA2 enzymatic hydrolysis of a membrane surface.

Changes in a phospholipid bilayer induced by the hydrolysis of a phospholipase A2 enzyme: a molecular dynamics simulation study

Phospholipase A2 (PLA2) enzymes are important in numerous physiological processes. Their function at lipid-water interfaces is also used as a biophysical model for protein-membrane interactions. These enzymes catalyze the hydrolysis of the sn-2 bonds of various phospholipids and the hydrolysis products are known to increase the activity of the enzymes. Here, we have applied molecular dynamics (MD) simulations to study the membrane properties in three compositionally different systems that relate to PLA2 enzyme action. One-nanosecond simulations were performed for a 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphatidylcholine (PLPC) bilayer and for two of its PLA2-hydrolyzed versions, i.e., bilayers consisting of lysophospholipids and of either free charged linoleate or free uncharged linoleic acid molecules. The results revealed loosening of the structure in the hydrolyzed bilayer due to increased mobility of the molecules in the direction normal to the bilayer. This loss of integrity due to the hydrolysis products is in accord with observations that not only the presence of hydrolysis products, but also a variety of other perturbations of the membrane may activate PLA2. Additionally, changes were observed in other structural parameters and in the electrostatic potential across the membrane-water interface. These changes are discussed in relation to the simulation methodology and the experimental observations of PLA2-hydrolyzed membranes.

Biphasic regulation of Fc-receptor mediated phagocytosis of rabbit alveolar macrophages by surfactant phospholipids

Dipalmitoyl phosphatidylcholine (DPPC) is a major phospholipid constituent in the pulmonary surfactant, whereas lysophosphatidylcholine (Lyso-PC) is a minor constituent, this membrane phospholipid being produced at inflammatory sites in association with activation of phospholipase A2. To determine the role of these two different forms of phospholipids in the phagocytic function of alveolar macrophages (AM), we examined the effects of DPPC or Lyso-PC on Fc-mediated phagocytosis. We demonstrated a significant decrease of the ingestion activity of AM for anti-sheep erythrocyte immunoglobulin G-coated sheep erythrocytes (EA: IgG) by DPPC. On the other hand, Lyso-PC caused significantly increased ingestion of EA: IgG by AM. These data indicate that increase of Lyso-PC due to the hydrolysis of DPPC through activation of phospholipase A, may up-regulate AM-mediated phagocytic functions in the alveolar milieu associated with infections and inflammation. DPPC may suppress and stabilize the AM-mediated phagocytosis in the normal alveolar environment.

Lysophosphatidylcholine stimulates the release of arachidonic acid in human endothelial cells

Lysophosphatidylcholine (lyso-PC) is a product of phosphatidylcholine hydrolysis by phospholipase A2 (PLA2) and is present in cell membranes, oxidized lipoproteins, and atherosclerotic tissues. It has the ability to alter endothelial functions and is regarded as a causal agent in atherogenesis. In this study, the modulation of arachidonate release by lyso-PC in human umbilical vein endothelial cells was examined. Incubation of endothelial cells with lyso-PC resulted in an enhanced release of arachidonate in a time- and concentration-dependent manner. Maximum arachidonate release was observed at 10 min of incubation with 50 microM lyso-PC. Lyso-PC species containing palmitoyl (C16:0) or stearoyl (C18:0) groups elicited the enhancement of arachidonate release, while other lysolipids such as lysophosphatidylethanolamine, lysophosphatidylserine, lysophosphatidylinositol, or lysophosphatidate were relatively ineffective. Lyso-PC-induced arachidonate release was decreased by treatment of cells with PLA2 inhibitors such as para-bromophenacyl bromide and arachidonoyl trifluoromethyl ketone. Furthermore, arachidonate release was attenuated in cells grown in the presence of antisense oligodeoxynucleotides that specifically bind cytosolic PLA2 mRNA. Treatment of cells with lyso-PC resulted in a translocation of PLA2 activity from the cytosolic to the membrane fractions of cells. Lyso-PC induced a rapid influx of Ca2+ from the medium into the cells, with a simultaneous enhancement of protein kinase C (PKC) activity in the membrane fractions. The lyso-PC-induced arachidonate release was attenuated when cells were preincubated with specific inhibitors of PKC (staurosporine and Ro31-8220) or a specific inhibitor of mitogen-activated protein kinase/extracellular regulated kinase kinase (PD098059). Taken together, the results of this study show that lyso-PC caused the elevation of cellular Ca2+ and the activation of PKC, which stimulated cytosolic PLA2 in an indirect manner and resulted in an enhanced release of arachidonate.

Enhancement of Agkistrodon piscivorus piscivorus venom phospholipase A2 activity toward phosphatidylcholine vesicles by lysolecithin and palmitic acid: studies with fluorescent probes of membrane structure

The activity of phospholipase A2 from snake venom to hydrolyze bilayers of phosphatidylcholines is greatly enhanced by the presence of the hydrolysis products, lysolecithin and fatty acid, in the bilayer. The fluorescence of several probes of membrane structure was used to monitor changes in bilayer physical properties during vesicle hydrolysis. These changes were compared to emission spectra and fluorescence polarization results occurring upon direct addition of lysolecithin and/or fatty acid to the bilayer. The excimer to monomer ratio of 1,3-bis(1-pyrene)propane was insensitive to vesicle hydrolysis, suggesting that changes in the order of the phospholipid chains were not relevant to the effect of the hydrolysis products on phospholipase activity. The fluorescence of 6-propionyl-2-(dimethylamino)-naphthalene (Prodan) suggested that the polarity of the bilayer in the region of the phospholipid head groups increases as the hydrolysis products accumulate in the bilayer. The fluorescence of 6-dodecanoyl-2-(dimethylamino)naphthalene (Laurdan) confirmed that such effects were restricted to the bilayer surface. Furthermore, the lysolecithin appeared to be the product most responsible for these changes. These results suggested that lysolecithin increases the activity of phospholipase A2 during vesicle hydrolysis by disrupting the bilayer surface, making the phospholipid molecules more accessible to the enzyme active site.