Interactions of fatty acids with phosphatidylethanolamine membranes: X-ray diffraction and molecular dynamics studies

Anticancer drugs tamoxifen and 4hydroxytamoxifen as effectors of phosphatidylethanolamine lipid polymorphism

The interaction of tamoxifen (TMX) and its metabolite 4-hydroxytamoxifen (HTMX) with a biomimetic membrane model system composed of 1,2-dielaidoylphosphatidylethanolamine (DEPE) has been studied using a biophysical approach. Incorporation of TMX into DEPE bilayers gives rise to a progressive broadening of the L_β/L_α phase transition and a downward temperature shift. The L_β/L_α phase transition presents multiple endotherms, indicating a lateral segregation of TMX/DEPE domains within the plane of the bilayer. TMX and HTMX also widen and shift the L_α to hexagonal-H_II transition toward lower values, the phase diagrams showing that both compounds facilitate formation of the H_II phase. TMX increases motional disorder of DEPE acyl chains in the L_β, L_α and H_II phases, whereas the effect of HTMX is clearly different. In addition, neither TMX nor HTMX significantly perturb the hydration state of the polar headgroup region of DEPE. Molecular dynamics (MD) simulations indicate that these drugs do not affect membrane thickness, area per lipid, or the conformation of DEPE molecules. As a general rule, the interaction of HTMX with DEPE is qualitatively similar to TMX but less intense. However, a significant difference shown by MD is that HTMX is mainly placed around the center of each monolayer while TMX is located mainly at the center of the membrane, also having a greater tendency to cluster formation. These results are discussed to understand the modulation of phosphatidylethanolamine lipid polymorphism carried out by these drugs, which could be of relevance to explain their effects on enzyme activity or membrane permeabilization.

The molecular-level understanding of the uptake of PFOS and its alternatives (6:2 Cl-PFESA and OBS) into phospholipid bilayers

Bioaccumulation of perfluoroalkyl and polyfluoroalkyl substances (PFASs) is an important indicator of their hazard. Partitioning to membrane phospholipids is one of the pathways for their bioaccumulation. However, the molecular mechanism on PFASs uptake into membrane phospholipids is not yet to be fully understood. In this work, we used molecular dynamics (MD) simulations to study the uptake processes of PFOS and its alternatives (6:2 Cl-PFESA and OBS) into DPPC bilayers, and to evaluate their interaction with DPPC bilayers and their effect on properties of DPPC bilayers. The result of free energy changes shows that a barrier of 2-3 kcal mol^-1 exists when these adsorbed PFASs on the surface are absorbed into DPPC bilayers. After incorporating into DPPC bilayers, three DPPC molecules interact with and thus stabilize a PFOS (or 6:2 Cl-PFESA or OBS) molecule. And another role of the three DPPC molecules is to shield these PFASs from exposure to water environment. These PFASs have the similar condensing effect on the model membrane. The molecular-level study is beneficial for understanding the bioaccumulation and toxicity of PFOS and its alternatives.

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.

The Influence of Fatty Acid Methyl Esters (FAMEs) in the Biochemistry and the Na(+)/K(+)-ATPase Activity of Culex quinquefasciatus Larvae

Culex quinquefasciatus is the main vector of lymphatic filariasis and combating this insect is of great importance to public health. There are reports of insects that are resistant to the products currently used to control this vector, and therefore, the search for new products has increased. In the present study, we have evaluated the effects of fatty acid methyl esters (FAMEs) that showed larvicidal activity against C. quinquefasciatus, on glucose, total protein, and triacylglycerol contents and Na(+)/K(+)-ATPase activity in mosquito larvae. The exposure of the fourth instar larvae to the compounds caused a decrease in the total protein content and an increase in the activity of the Na(+)/K(+)-ATPase. Furthermore, the direct effect of FAMEs on cell membrane was assessed on purified pig kidney Na(+)/K(+)-ATPase membranes, erythrocyte ghost membranes, and larvae membrane preparation. No modifications on total phospholipids and cholesterol content were found after FAMEs 20 min treatment on larvae membrane preparation, but only 360 µg/mL FAME 2 was able to decrease total phospholipid of erythrocyte ghost membrane. Moreover, only 60 and 360 µg/mL FAME 3 caused an activation of purified Na(+)/K(+)-ATPase, that was an opposite effect of FAMEs treatment in larvae membrane preparation, and caused an inhibition of the pump activity. These data together suggest that maybe FAMEs can modulate the Na(+)/K(+)-ATPase on intact larvae for such mechanisms and not for a direct effect, one time that the direct effect of FAMEs in membrane preparation decreased the activity of Na(+)/K(+)-ATPase. The biochemical changes caused by the compounds were significant and may negatively influence the development and survival of C. quinquefasciatus larvae.

Stearoyl coenzyme A desaturase 1 is associated with hepatitis C virus replication complex and regulates viral replication

The hepatitis C virus (HCV) life cycle is tightly regulated by lipid metabolism of host cells. In order to identify host factors involved in HCV propagation, we have recently screened a small interfering RNA (siRNA) library targeting host genes that control lipid metabolism and lipid droplet formation using cell culture-grown HCV (HCVcc)-infected cells. We selected and characterized the gene encoding stearoyl coenzyme A (CoA) desaturase 1 (SCD1). siRNA-mediated knockdown or pharmacological inhibition of SCD1 abrogated HCV replication in both subgenomic replicon and Jc1-infected cells, while exogenous supplementation of either oleate or palmitoleate, products of SCD1 activity, resurrected HCV replication in SCD1 knockdown cells. SCD1 was coimmunoprecipitated with HCV nonstructural proteins and colocalized with both double-stranded RNA (dsRNA) and HCV nonstructural proteins, indicating that SCD1 is associated with HCV replication complex. Moreover, SCD1 was fractionated and enriched with HCV nonstructural proteins at detergent-resistant membrane. Electron microscopy data showed that SCD1 is required for NS4B-mediated intracellular membrane rearrangement. These data further support the idea that SCD1 is associated with HCV replication complex and that its products may contribute to the proper formation and maintenance of membranous web structures in HCV replication complex. Collectively, these data suggest that manipulation of SCD1 activity may represent a novel host-targeted antiviral strategy for the treatment of HCV infection.

IMPORTANCE

Stearoyl coenzyme A (CoA) desaturase 1 (SCD1), a liver-specific enzyme, regulates hepatitis C virus (HCV) replication through its enzyme activity. HCV nonstructural proteins are associated with SCD1 at detergent-resistant membranes, and SCD1 is enriched on the lipid raft by HCV infection. Therein, SCD1 supports NS4B-mediated membrane rearrangement to provide a suitable microenvironment for HCV replication. We demonstrated that either genetic or chemical knockdown of SCD1 abrogated HCV replication in both replicon cells and HCV-infected cells. These findings provide novel mechanistic insights into the roles of SCD1 in HCV replication.

Differential effect of 2-hydroxyoleic acid enantiomers on protein (sphingomyelin synthase) and lipid (membrane) targets

The complex dual mechanism of action of 2-hydroxyoleic acid (2OHOA), a potent anti-tumor compound used in membrane lipid therapy (MLT), has yet to be fully elucidated. It has been demonstrated that 2OHOA increases the sphingomyelin (SM) cell content via SM synthase (SGMS) activation. Its presence in membranes provokes changes in the membrane lipid structure that induce the translocation of PKC to the membrane and the subsequent overexpression of CDK inhibitor proteins (e.g., p21(Cip1)). In addition, 2OHOA also induces the translocation of Ras to the cytoplasm, provoking the silencing of MAPK and its related pathways. These two differential modes of action are triggered by the interactions of 2OHOA with either lipids or proteins. To investigate the molecular basis of the different interactions of 2OHOA with membrane lipids and proteins, we synthesized the R and S enantiomers of this compound. A molecular dynamics study indicated that both enantiomers interact similarly with lipid bilayers, which was further confirmed by X-ray diffraction studies. By contrast, only the S enantiomer was able to activate SMS in human glioma U118 cells. Moreover, the anti-tumor efficacy of the S enantiomer was greater than that of the R enantiomer, as the former can act through both MLT mechanisms. The present study provides additional information on this novel therapeutic approach and on the magnitude of the therapeutic effects of type-1 and type-2 MLT approaches. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

2-Hydroxyoleic acid induces ER stress and autophagy in various human glioma cell lines

Background

2-Hydroxyoleic acid is a synthetic fatty acid with potent anti-cancer activity which does not induce undesired side effects. However, the molecular and cellular mechanisms by which this compound selectively kills human glioma cancer cells without killing normal cells is not fully understood. The present study was designed to determine the molecular bases underlying the potency against 1321N1, SF-767 and U118 human glioma cell lines growth without affecting non cancer MRC-5 cells.

Methodology/Principal Findings

The cellular levels of endoplasmic reticulum (ER) stress, unfolded protein response (UPR) and autophagy markers were determined by quantitative RT-PCR and immunoblotting on 1321N1, SF-767 and U118 human glioma cells and non-tumor MRC-5 cells incubated in the presence or absence of 2OHOA or the ER stress/autophagy inducer, palmitate. The cellular response to these agents was evaluated by fluorescence microscopy, electron microscopy and flow cytometry. We have observed that 2OHOA treatments induced augments in the expression of important ER stress/UPR markers, such as phosphorylated eIF2α, IRE1α, CHOP, ATF4 and the spliced form of XBP1 in human glioma cells. Concomitantly, 2OHOA led to the arrest of 1321N1 cells in the G₂/M phase of the cell cycle, with down-regulation of cyclin B1 and Cdk1/Cdc2 proteins in the three glioma cell lines studied. Finally, 2OHOA induced autophagy in 1321N1, SF-767 and U118 cells, with the appearance of autophagic vesicles and the up-regulation of LC3BI, LC3BII and ATG7 in 1321N1 cells, increases of LC3BI, LC3BII and ATG5 in SF-767 cells and up-regulation of LC3BI and LC3BII in U118 cells. Importantly, 2OHOA failed to induce such changes in non-tumor MRC-5 cells.

Conclusion/Significance

The present results demonstrate that 2OHOA induces ER stress/UPR and autophagy in human glioma (1321N1, SF-767 and U118 cell lines) but not normal (MRC-5) cells, unraveling the molecular bases underlying the efficacy and lack of toxicity of this compound.

Self-assembled nanostructures of fully hydrated monoelaidin-elaidic acid and monoelaidin-oleic acid systems

In recent years, there has been a surge of interest in exploring the effect of trans-fatty acids (TFAs) on biological membrane properties. The research studies are motivated by an increasing body of evidence suggesting that the consumption of TFAs increases the risk of developing negative health effects such as coronary heart disease and cancer. The ultimate goal of studying the lipid-fatty acid interactions at the molecular level is to predict the biological role of fatty acids in cells. In this regard, it is interesting to elucidate the effect of loading TFAs and their counterpart cis-fatty acids (CFAs) on the physical properties of lipid model membranes. Here, the present study focuses on discussing the following: (1) the effect of mixing monoelaidin (ME, TFA-containing lipid) with its counterpart monoolein (MO, CFA-containing lipid) on modulating the fully hydrated self-assembled structure, and (2) the influence of solubilizing oleic acid (OA) and its trans counterpart elaidic acid (EA) on the fully hydrated ME system. The ME model membrane was selected due to its sensitivity to variations in lipid composition and temperature. Synchrotron small-angle X-ray scattering (SAXS) was applied for studying the temperature-dependent structural behavior of the fully hydrated ME/MO-based system prepared with an equal ME/MO weight ratio and also for characterizing the fully hydrated OA- and EA-loaded ME systems. Wide-angle X-ray (WAXS) experiments were also performed for characterizing the formed crystalline lamellar phases at ambient temperatures. The results demonstrate the significant influence of the partial replacement of ME by MO on the phase behavior. The addition of MO induces the lamellar-nonlamellar phase transitions at ambient temperatures and promotes the formation of the inverted type hexagonal (H(2)) phase above 72 °C. The fully hydrated ME/EA and ME/OA systems with their rich polymorphism exhibit an interesting temperature-dependent complex behavior. The experimental findings show that the temperature-induced phase transitions are dictated by the solubilized fatty acid concentration and its configuration. Both EA and OA have a significant impact on the fully hydrated ME system. Similar to previous published studies, OA induces a significantly stronger mean negative membrane curvature as compared to EA. The two phase diagrams are discussed in terms of water-lipid and lipid-fatty acid interactions, membrane bending, and lipid packing concepts. A newly observed interesting epitaxial relationship for the lamellar-hexagonal phase transition in the EA-loaded ME system is illustrated and discussed in detail.

Practical considerations for building GROMOS-compatible small-molecule topologies

Molecular dynamics simulations are being applied to increasingly complex systems, including those involving small endogenous compounds and drug molecules. In order to obtain meaningful and accurate data from these simulations, high-quality topologies for small molecules must be generated in a manner that is consistent with the derivation of the force field applied to the system. Often, force fields are designed for use with macromolecules such as proteins, making their transferability to other species challenging. Investigators are increasingly attracted to automated topology generation programs, although the quality of the resulting topologies remains unknown. Here we assess the applicability of the popular PRODRG server that generates small-molecule topologies for use with the GROMOS family of force fields. We find that PRODRG does not reproduce topologies for even the most well-characterized species in the force field due to inconsistent charges and charge groups. We assessed the effects of PRODRG-derived charges on several systems: pure liquids, amino acids at a hydrophobic-hydrophilic interface, and an enzyme-cofactor complex. We found that partial atomic charges generated by PRODRG are largely incompatible with GROMOS force fields, and the behavior of these systems deviates substantially from that of simulations using GROMOS parameters. We conclude by proposing several points as "best practices" for parametrization of small molecules under the GROMOS force fields.