Searching for the role of membrane sphingolipids in selectivity of antitumor ether lipid–edelfosine

Insight into the Molecular Mechanism of Surface Interactions of Phosphatidylcholines─Langmuir Monolayer Study Complemented with Molecular Dynamics Simulations

Mutual interactions between components of biological membranes are pivotal for maintaining their proper biophysical properties, such as stability, fluidity, or permeability. The main building blocks of biomembranes are lipids, among which the most important are phospholipids (mainly phosphatidylcholines (PCs)) and sterols (mainly cholesterol). Although there is a plethora of reports on interactions between PCs, as well as between PCs and cholesterol, their molecular mechanism has not yet been fully explained. Therefore, to resolve this issue, we carried out systematic investigations based on the classical Langmuir monolayer technique complemented with molecular dynamics simulations. The studies involved systems containing 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) analogues possessing in the structure one or two polar functional groups similar to those of DPPC. The interactions and rheological properties of binary mixtures of DPPC analogues with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and cholesterol were compared with reference systems (DPPC/POPC and DPPC/cholesterol). This pointed to the importance of the ternary amine group in PC/cholesterol interactions, while in PC mixtures, the phosphate group played a key role. In both cases, the esterified glycerol group had an effect on the magnitude of interactions. The obtained results are crucial for establishing structure-property relationships as well as for designing substitutes for natural lipids.

Interaction between Pharmaceutical Drugs and Polymer-Coated Fe3O4 Magnetic Nanoparticles with Langmuir Monolayers as Cellular Membrane Models

Surface modification of magnetic nanoparticles (MNPs) has been reported to play a significant role in determining their interactions with cell membranes. In this research, the interactions between polymer functionalized (chitosan, CHI or diethylamino-ethyl dextran, DEAE-D) Fe₃O₄ MNPs, pharmaceutical drugs and model cell membranes were investigated by Langmuir isotherms and adsorption measurements. In this study, 1,2-distearoyl-sn-glycerol-3-phosphate (DSPA) phospholipid monolayers were used as cell membrane models. Insertion experiments demonstrate that diclofenac (DCFN) is not absorbed at the air-water interface, whereas triflupromazine (TFPZ) has a MIP (maximum insertion pressure) of 35 m Nm^-1. The insertion of composites MNPs:TFPZ or DCFN has larger MIP values, indicating that the MNPs are adsorbed on the monolayer with the drugs. An Fe₃O₄@CHI:DCFN composite presented an MIP of 39 m Nm^-1 and Fe₃O₄@DEAE-D:DCFN presented an impressive MIP of 67 mNm^-1. In the case of TFPZ, the enhancement in the MIP values is also evident, being 42 mNm^-1 for Fe₃O₄@CHI:TFPZ and 40 mNm^-1 for Fe₃O₄@DEAE-D:DCFN composite. All MNPs:drugs composites have MIP values greater than commonly accepted membrane pressure values, indicating that MNPs:drugs can penetrate a cellular membrane. The fact that the composite MNPs:drugs present greater MIP values than separated compounds indicates that polymer-coated MNPs can act as good drug delivery systems.

Clusters of apoptotic signaling molecule-enriched rafts, CASMERs: membrane platforms for protein assembly in Fas/CD95 signaling and targets in cancer therapy

Mammalian cells show the ability to commit suicide through the activation of death receptors at the cell surface. Death receptors, among which Fas/CD95 is one of their most representative members, lack enzymatic activity, and depend on protein-protein interactions to signal apoptosis. Fas/CD95 death receptor-mediated apoptosis requires the formation of the so-called death-inducing signaling complex (DISC), bringing together Fas/CD95, Fas-associated death domain-containing protein and procaspase-8. In the last two decades, cholesterol-rich lipid raft platforms have emerged as scaffolds where Fas/CD95 can be recruited and clustered. The co-clustering of Fas/CD95 and rafts facilitates DISC formation, bringing procaspase-8 molecules to be bunched together in a limited membrane region, and leading to their autoproteolytic activation by oligomerization. Lipid raft platforms serve as a specific region for the clustering of Fas/CD95 and DISC, as well as for the recruitment of additional downstream signaling molecules, thus forming the so-called cluster of apoptotic signaling molecule-enriched rafts, or CASMER. These raft/CASMER structures float in the membrane like icebergs, in which the larger portion lies inside the cell and communicates with other subcellular structures to facilitate apoptotic signal transmission. This allows an efficient spatiotemporal compartmentalization of apoptosis signaling machinery during the triggering of cell death. This concept of proapoptotic raft platforms as a basic chemical-biological structure in the regulation of cell death has wide-ranging implications in human biology and disease, as well as in cancer therapy. Here, we discuss how these raft-centered proapoptotic hubs operate as a major linchpin for apoptosis signaling and as a promising target in cancer therapy.

Preferred Endocytosis of Amyloid Precursor Protein from Cholesterol-Enriched Lipid Raft Microdomains

Amyloid precursor protein (APP) at the plasma membrane is internalized via endocytosis and delivered to endo/lysosomes, where neurotoxic amyloid-β (Aβ) is produced via β-, γ-secretases. Hence, endocytosis plays a key role in the processing of APP and subsequent Aβ generation. β-, γ-secretases as well as APP are localized in cholesterol-enriched lipid raft microdomains. However, it is still unclear whether lipid rafts are the site where APP undergoes endocytosis and whether cholesterol levels affect this process. In this study, we found that localization of APP in lipid rafts was increased by elevated cholesterol level. We also showed that increasing or decreasing cholesterol levels increased or decreased APP endocytosis, respectively. When we labeled cell surface APP, APP localized in lipid rafts preferentially underwent endocytosis compared to nonraft-localized APP. In addition, APP endocytosis from lipid rafts was regulated by cholesterol levels. Our results demonstrate for the first time that cholesterol levels regulate the localization of APP in lipid rafts affecting raft-dependent APP endocytosis. Thus, regulating the microdomain localization of APP could offer a new therapeutic strategy for Alzheimer’s disease.

Lipids and Membrane Microdomains: The Glycerolipid and Alkylphosphocholine Class of Cancer Chemotherapeutic Drugs

Synthetic antitumor lipids are metabolically stable lysophosphatidylcholine derivatives, encompassing a class of non-mutagenic drugs that selectively target cancerous cells. In this chapter we review the literature as relates to the clinical efficacy of these antitumor lipid drugs and how our understanding of their mode of action has evolved alongside key advances in our knowledge of membrane structure, organization, and function. First, the history of the development of this class of drugs is described, providing a summary of clinical outcomes of key members including edelfosine, miltefosine, perifosine, erufosine, and erucylphosphocholine. A detailed description of the biophysical properties of these drugs and specific drug-lipid interactions which may contribute to the selectivity of the antitumor lipids for cancer cells follows. An updated model on the mode of action of these lipid drugs as membrane disorganizing agents is presented. Membrane domain organization as opposed to targeting specific proteins on membranes is discussed. By altering membranes, these antitumor lipids inhibit many survival pathways while activating pro-apoptotic signals leading to cell demise.

Disruption of lipid domain organization in monolayers of complex yeast lipid extracts induced by the lysophosphatidylcholine analogue edelfosine in vivo

The lysophosphatidylcholine analogue edelfosine is a potent antitumor and antiparasitic drug that targets cell membranes. Previous studies have shown that edelfosine alters membrane domain organization inducing internalization of sterols and endocytosis of plasma membrane transporters. These early events affect signaling pathways that result in cell death. It has been shown that edelfosine preferentially partitions into more rigid lipid domains in mammalian as well as in yeast cells. In this work we aimed at investigating the effect of edelfosine on membrane domain organization using monolayers prepared from whole cell lipid extracts of cells treated with edelfosine compared to control conditions. In Langmuir monolayers we were able to detect important differences to the lipid packing of the membrane monofilm. Domain formation visualized by means of Brewster angle microscopy also showed major morphological changes between edelfosine treated versus control samples. Importantly, edelfosine resistant cells defective in drug uptake did not display the same differences. In addition, co-spread samples of control lipid extracts with edelfosine added post extraction did not fully mimic the results obtained with lipid extracts from treated cells. Altogether these results indicate that edelfosine induces changes in membrane domain organization and that these changes depend on drug uptake. Our work also validates the use of monolayers derived from complex cell lipid extracts combined with Brewster angle microscopy, as a sensitive approach to distinguish between conditions associated with susceptibility or resistance to lysophosphatidylcholine analogues.

Edelfosine and miltefosine effects on lipid raft properties: membrane biophysics in cell death by antitumor lipids

Edelfosine (1-O-octadecyl-2-O-methyl-sn-glycero-phosphocholine) and miltefosine (hexadecylphosphocholine) are synthetic alkylphospholipids (ALPs) that are reported to selectively accumulate in tumor cell membranes, inducing Fas clustering and activation on lipid rafts, triggering apoptosis. However, the exact mechanism by which these lipids elicit these events is still not fully understood. Recent studies propose that their mode of action might be related with alterations of lipid rafts biophysical properties caused by these lipid drugs. To achieve a clear understanding of this mechanism, we studied the effects of pharmacologically relevant amounts of edelfosine and miltefosine in the properties of model and cellular membranes. The influence of these molecules on membrane order, lateral organization, and lipid rafts molar fraction and size were studied by steady-state and time-resolved fluorescence methods, Förster resonance energy transfer (FRET), confocal and fluorescence lifetime imaging microscopy (FLIM). We found that the global membrane and lipid rafts biophysical properties of both model and cellular membranes were not significantly affected by both the ALPs. Nonetheless, in model membranes, a mild increase in membrane fluidity induced by both alkyl lipids was detected, although this effect was more noticeable for edelfosine than miltefosine. This absence of drastic alterations shows for the first time that ALPs mode of action is unlikely to be directly linked to alterations of lipid rafts biophysical properties caused by these drugs. The biological implications of this result are discussed in the context of ALPs effects on lipid metabolism, mitochondria homeostasis modulation, and their relationship with tumor cell death.

Behavior of platelet activating factor in membrane-mimicking environment. Langmuir monolayer study complemented with grazing incidence X-ray diffraction and Brewster angle microscopy

1-O-octadecyl-2-acetyl-sn-glycero-3-phosphocholine (PAF) belonging to the class of single-chained ether phospholipids is widely known from its essential biological activities. There is a growing body of evidence that some significant aspects of PAF actions are connected with its capability to direct intercalation into biomembranes' environment. Although this mechanism is of great importance in the perspective of understanding PAF implications in various physiological processes, in the literature, there is a lack of studies devoted to this subject. It is still unknown which is the exact influence of membrane composition, molecular organization, and its other properties on the PAF impact on cells and tissues. Unfortunately, the biological studies carried out on cell cultures do not provide satisfactory results, mainly because of the complexity of natural systems. In order to obtain insight into the behavior of PAF in a lipid environment at the molecular level, the application of appropriate model systems is required. Among them, Langmuir monolayers are very often applied as a simple but very efficient platform for studies of the interactions between membrane lipids. In the present paper, special attention is focused on the issue concerning the interactions between PAF and two representatives of membrane components occurring mainly in the outer leaflet of natural bilayers, namely, cholesterol and DPPC. The application of Langmuir monolayers enabled us to construct the effective model mimicking the exogenous incorporation of PAF into membrane environment. On the basis of the obtained results, a thorough discussion was carried out and the conclusions derived from the traditional thermodynamic analysis were confronted with microscopic analysis of surface domains and the GIXD results. The selection of experimental techniques enables us to obtain information regarding the miscibility and interactions in the binary mixed films as well as the molecular organization of film-forming molecules on water surface. The experiments revealed that the addition of the investigated single-chained ether phospholipid into both cholesterol and DPPC monolayers causes a considerable decrease of monolayer condensation. On the basis of thermodynamic analysis, it was found that PAF mixes and consequently interacts strongly with cholesterol, whereas its interactions with DPPC are thermodynamically unfavorable. Differences between the PAF influence on cholesterol and DPPC monolayer found its corroboration in the results obtained with the GIXD technique. Namely, the monolayer of DPPC can incorporate more PAF than the model membrane containing cholesterol. The obtained results indicate that short chained sn-2 ether phospholipid is able to modify model membrane properties in a concentration-dependent way.