Packing constraints and electrostatic surface potentials determine transmembrane asymmetry of phosphatidylethanol

Biophysics of cell membrane lipids in cancer drug resistance: Implications for drug transport and drug delivery with nanoparticles

In this review, we focus on the biophysics of cell membrane lipids, particularly when cancers develop acquired drug resistance, and how biophysical changes in resistant cell membrane influence drug transport and nanoparticle-mediated drug delivery. Recent advances in membrane lipid research show the varied roles of lipids in regulating membrane P-glycoprotein function, membrane trafficking, apoptotic pathways, drug transport, and endocytic functions, particularly endocytosis, the primary mechanism of cellular uptake of nanoparticle-based drug delivery systems. Since acquired drug resistance alters lipid biosynthesis, understanding the role of lipids in cell membrane biophysics and its effect on drug transport is critical for developing effective therapeutic and drug delivery approaches to overcome drug resistance. Here we discuss novel strategies for (a) modulating the biophysical properties of membrane lipids of resistant cells to facilitate drug transport and regain endocytic function and (b) developing effective nanoparticles based on their biophysical interactions with membrane lipids to enhance drug delivery and overcome drug resistance.

Toward a mechanical control of drug delivery. On the relationship between Lipinski's 2nd rule and cytosolic pH changes in doxorubicin resistance levels in cancer cells: a comparison to published data

Based on molecular and physiological resemblance, the mechanism that controls drug bioavailability and toxicity also shares strong similarities to the one that controls drug resistance. In both cases, this mechanism relies on the expression of drug transporters and the physico-chemical properties of drugs, which together alter the intracellular accumulation of chemicals in cells or tissues. However, a parameter that is central and has received great attention in the field of bioavailability, but almost none in the field of drug resistance, is the molecular weight of drugs. In the former area, it is well known that to achieve a reasonable bioavailability, drugs must have-among other properties-a molecular weight less than 500, known as Lipinski's 2nd rule. Accordingly, it is worth questioning whether a similar rule exists in the field of drug resistance and what subsequent mechanism would control the membrane permeability to drugs as a function of their molecular weight. I demonstrate here that cytosolic pH fixes the molecular weight of drugs entering cells, by altering the cell membrane mechanical properties and that, both cytosolic pH and membrane mechanical properties are needed and sufficient to explain doxorubicin resistance levels in different cancerous cell lines. Finally, I discuss the efficiency of a drug handling activity by transporters in MDR and suggest ways to control drug delivery mechanically. In addition, and for the first time, the literal expression of a Law similar to Lipinski's 2nd rule will be described as a function of cytosolic pH and lipid number asymmetry.

Phosphatidic acid regulation of phosphatidylinositol 4-phosphate 5-kinases

Phosphatidic acid (PA) production by receptor-stimulated phospholipase D is believed to play an important role in the regulation of cell function. The second messenger function of PA remains to be elucidated. PA can bind and affect the activities of different enzymes and here we summarise the current status of activation of Type I phosphatidylinositol 4-phosphate 5-kinase by PA. Type 1 phosphatidylinositol 4-phosphate 5-kinase is also regulated by ARF proteins as is phospholipase D and we discuss the contributions of ARF and PA towards phosphatidylinositol(4,5)bisphosphate synthesis at the plasma membrane.

On the relationship between drug's size, cell membrane mechanical properties and high levels of multi drug resistance: a comparison to published data

Multi drug resistance (MDR) or cross resistance to drugs was initially explained on the basis that MDR cells express drug transporters that expel membrane-embedded drugs. However, it is now clear that transporters are a single piece from a complex puzzle. An issue that has been solved recently is, given that these transporters have to handle drugs, why should a membrane-embedded drug and a transporter meet? To solve this problem, a theory has been suggested considering the interaction between the cell membrane mechanical properties and the size of drugs. In simple terms, this theory proposes that an excess in the packing of lipid in the inner leaflet of the membrane of MDR cells is responsible for blocking drugs mechanically as a function of their sizes at the membrane level, thus impairing their flux into the cytosol. In turn it is expected that this would allow any membrane embedded drug to diffuse toward transporters. The study concluded that the size of drugs is necessarily important regarding the mechanical interaction between the drug and the membrane, and likely to be central to MDR. Remarkably, an experimental study based on MDR and published years ago concluded that the molecular weight (MW) of drugs was central to MDR (Biedler and Riehm in Cancer Res 30:1174-1184, 1970). Given that size and MW are linked together, I have compared the former theory to the latter experimental data and demonstrate that, indeed, basic membrane mechanics is involved in high levels of cross resistance to drugs in Pgp expressing cells.

Evidence for leaflet-dependent redistribution of charged molecules in fluid supported phospholipid bilayers

The asymmetric distribution of charged molecules between the leaflets of solid-substrate-supported phospholipid bilayers is studied using imaging ellipsometry, fluorescence microscopy, and numerical solutions of the Poisson-Boltzmann equation. Experiments are facilitated by the use of patterned substrates that allow for side-by-side comparison of lipid monolayers and supported bilayers. On silica surfaces, negatively charged lipid components are shown to be enriched in the outer leaflet of a supported bilayer system at modest salt concentrations. The approaches developed provide a general means for determining asymmetries of charged components in supported lipid bilayers.

Bilayer edge and curvature effects on partitioning of lipids by tail length: atomistic simulations

The partitioning of lipids among different microenvironments in a bilayer is of considerable relevance to characterization of composition variations in biomembranes. Atomistic simulation has been ill-suited to model equilibrated lipid mixtures because the time required for diffusive exchange of lipids among microenvironments exceeds typical submicrosecond molecular dynamics trajectories. A method to facilitate local composition fluctuations, using Monte Carlo mutations to change lipid structures within the semigrand-canonical ensemble (at a fixed difference in component chemical potentials, Deltamu), was recently implemented to address this challenge. This technique was applied here to mixtures of dimyristoylphosphatidylcholine and a shorter-tail lipid, either symmetric (didecanoylphosphatidylcholine (DDPC)) or asymmetric (hexanoyl-myristoylphosphatidylcholine), arranged in two types of structure: bilayer ribbons and buckled bilayers. In ribbons, the shorter-tail component showed a clear enrichment at the highly curved rim, more so for hexanoyl-myristoylphosphatidylcholine than for DDPC. Results on buckled bilayers were variable. Overall, the DDPC content of buckled bilayers tended to exceed by several percent the DDPC content of flat ones simulated at the same Deltamu, but only for mixtures with low overall DDPC content. Within the buckled bilayer structure, no correlation could be resolved between the sign or magnitude of the local curvature of a leaflet and the mean local lipid composition. Results are discussed in terms of packing constraints, surface area/volume ratios, and curvature elasticity.

The 3-hydroxy group and 4,5-trans double bond of sphingomyelin are essential for modulation of galactosylceramide transmembrane asymmetry

The structural features of SPM that control the transbilayer distribution of beta-GalCer in POPC vesicles were investigated by (13)C- and (31)P-NMR spectroscopy using lipid analogs that share physical similarities with GalCer or SPM. The SPM analogs included N-palmitoyl-4,5-dihydro-SPM, 3-deoxy-SPM, 1-alkyl-2-amidophosphatidylcholine, and dipalmitoylphosphatidylcholine, a popular model "raft lipid". The transbilayer distributions of the SPM analogs and SPM in POPC vesicles were similar by (31)P-NMR. To observe the dramatic change in GalCer transbilayer distribution that occurs when SPM is included in POPC vesicles, the 3-OH group, 4,5-trans double bond, and amide linkage all were required in SPM. However, inclusion of 2 and 10 mol % dihydroSPM in SPM/POPC (1:1) vesicles mitigated and completely abrogated the effect of SPM on the transbilayer distribution of GalCer. Despite sharing some structural features with GalCer and localizing preferentially to the inner leaflet of POPC vesicles, dimyristoylphosphatidylethanolamine did not undergo a change in transbilayer distribution when SPM was incorporated into the vesicles. The results support the hypothesis that specific interactions may be favored among select sphingolipids in curvature-stressed membranes and emphasize the potential importance of the SPM-dihydroSPM ratio in membrane fission and fusion processes associated with vesicle biogenesis and trafficking.

Phosphatidylethanol stimulates calcium-dependent cytosolic phospholipase A(2) activity of a macrophage cell line (RAW 264.7)

The synthesis of inflammation mediators produced from arachidonic acid is regulated primarily by the cellular concentration of free arachidonic acid. Since intracellular arachidonic acid is almost totally present as phospholipid esters, the concentration of intracellular arachidonic acid is primarily dependent on the balance between the release of arachidonic acid from membrane phospholipids and the uptake of arachidonic acid into membrane phospholipids. Cytosolic phospholipase A(2) is a calciumdependent enzyme that catalyzes the stimulus-coupled hydrolysis of arachidonic acid from membrane phospholipids. Following exposure of macrophages to various foreign or endogenous stimulants, cytosolic phospholipase A(2) is activated. Treatment with these compounds may also stimulate phospholipase D activity, and, in the presence of ethanol, phospholipase D catalyzes the synthesis of phosphatidylethanol. A cell-free system was used to evaluate the effect of phosphatidylethanol on cytosolic phospholipase A(2) activity. Phosphatidylethanol (0.5 microM) added to 1-stearoyl-2-[(3)H]-arachidonoyl-sn-glycero-3-phosphocholine vesicles stimulated cytosolic phospholipase A(2) activity. However, high concentrations (20-100 microM) of phosphatidylethanol inhibited cytosolic phospholipase A(2) activity. Phosphatidic acid, the normal phospholipase D product, also stimulated cytosolic phospholipase A(2) activity at 0.5 microM, but had an inhibitory effect on cytosolic phospholipase A(2) activity at concentrations of 50 and 100 microM. Ethanol (20-200 mM), the precursor of phosphatidylethanol, added directly to the assay did not alter cytosolic phospholipase A(2) activity. These results suggest that phosphatidylethanol alters the physical properties of the substrate, and at lower concentrations of anionic phospholipids the substrate is more susceptible to hydrolysis. However, at high concentrations, phosphatidylethanol either reverses the alterations in physical properties of the substrate or phosphatidylethanol may be competing as the substrate. Both interactions may result in lower cytosolic phospholipase A(2) activity.

Activity of phosphatidylinositol transfer protein is sensitive to ethanol and membrane curvature

Phosphatidylinositol transfer protein (PITP) is critical for many cellular signalling and trafficking events that are influenced by ethanol. The influence of ethanol and membrane curvature on the activity of recombinant mouse PITP-alpha in vitro is evaluated by monitoring the transfer of phosphatidylinositol (PtdIns) from rat hepatic microsomes to unilamellar vesicles. Acute exposure to pharmacological levels of ethanol enhanced the function of PITP. Chloroform shared a similar ability to enhance function when both drug concentrations were normalized to their respective octanol/water partition coefficients, indicating that the effect is not unique to ethanol and might be common to hydrophobic solutes. Neither the PITP activity nor its ethanol enhancement was altered by using thermally pretreated (denatured) or protease-treated microsomes, indicating that the native microsomal protein structure was unlikely to be a determinant of transfer. Kinetic analyses indicated that ethanol acted by increasing the PITP-mediated flux of PtdIns from both microsomal and liposomal surfaces. The activity of PITP was strongly dependent on the lipid structure, with a steep dependence on the expressed curvature of the membrane. Activity was greatest for small, highly curved sonicated vesicles and decreased markedly for large, locally planar unilamellar vesicles. Ethanol enhanced PITP-mediated PtdIns transfer to all vesicles, but its effect was much smaller than the enhancement due to curvature, which is consistent with ethanol's comparatively modest ability to perturb membrane lipids. The ethanol efficacy observed is as pronounced as any previously described lipid-mediated ethanol action. In addition, these observations raise the possibility that PITP specifically delivers PtdIns to metabolically active membrane domains of convex curvature and/or low surface densities of lipid.

The electrostatics of lipid surfaces

Charged lipids constitute a substantial fraction of all membrane lipids. Their charges vary in quantity and distribution within their headgroup regions. In long range interactions, their charges' value and electrostatic potential in the vicinity of the membrane surface can be approximated by the Guy-Chapman theory. This theory treats the interface as a charged structureless plain surrounded by uniform environments. However, if one considers intermolecular interactions, such assumptions need to be revised. The interface is in reality a thick region containing the residual charges of lipid headgroups. Their arrangement depends on the type of lipid present in the membrane. The variety of lipids and their biological functions suggests that charge distribution determines the extent and type of interaction with surface associated molecules. Numerous examples show that protein behavior at the lipid bilayer surface is determined by the type of lipid present, indicating protein specificity towards certain surface locations and local properties (determined by lipid composition) of a particular type. Such specificity is achieved by a combination of electrostatic, hydrophobic and enthropic effects. Comparing lipid biological activity, it can be stated that residual charge distribution is one of the factors of intermolecular recognition leading to the specific interaction of lipid molecules and selected proteins in various processes, particularly those involved with signal transduction pathways. Such specificity enables a variety of processes occurring simultaneously on the same membrane surface to function without cross-reaction interference.

Tracking phospholipid populations in polymorphism by sideband analyses of 31P magic angle spinning NMR

A method was developed to track the distributional preferences of phospholipids in polymorphism based on sideband analyses of the 31P magic angle spinning nuclear magnetic resonance spectra. The method was applied to lipid mixtures containing phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn) and either cholesterol (Chol) or tetradecane, as well as mixtures containing the anionic phosphatidylmethanol, phosphatidylethanolamine, and diolein. The phospholipid composition of coexisting lamellar (Lalpha) and inverted hexagonal (HII) phases remained constant throughout the Lalpha --> HII transition in all mixtures, except those that contained saturated PtdCho and unsaturated PtdEtn in the presence of cholesterol-mixtures that are known to be microimmiscible because of favored associations between Chol and saturated acyl chains. In the latter mixture, saturated PtdCho was enriched in the planar bilayer structure, and unsaturated PtdEtn was enriched in the highly curved HII structure. This enrichment was coincident with an increase in the transition width. When compositional heterogeneity among coexisting phases was observed, it appeared that preexisting lateral microheterogeneities led to compositionally distinct transitional clusters, such that the distributional preferences that resulted were not those of the individual phospholipids.