Spontaneous transfer of monoacyl amphiphiles between lipid and protein surfaces

Fatty acid uptake in Trypanosoma brucei: Host resources and possible mechanisms

Trypanosoma brucei spp. causes African Sleeping Sickness in humans and nagana, a wasting disease, in cattle. As T. brucei goes through its life cycle in its mammalian and insect vector hosts, it is exposed to distinct environments that differ in their nutrient resources. One such nutrient resource is fatty acids, which T. brucei uses to build complex lipids or as a potential carbon source for oxidative metabolism. Of note, fatty acids are the membrane anchoring moiety of the glycosylphosphatidylinositol (GPI)-anchors of the major surface proteins, Variant Surface Glycoprotein (VSG) and the Procyclins, which are implicated in parasite survival in the host. While T. brucei can synthesize fatty acids de novo, it also readily acquires fatty acids from its surroundings. The relative contribution of parasite-derived vs. host-derived fatty acids to T. brucei growth and survival is not known, nor have the molecular mechanisms of fatty acid uptake been defined. To facilitate experimental inquiry into these important aspects of T. brucei biology, we addressed two questions in this review: (1) What is known about the availability of fatty acids in different host tissues where T. brucei can live? (2) What is known about the molecular mechanisms mediating fatty acid uptake in T. brucei? Finally, based on existing biochemical and genomic data, we suggest a model for T. brucei fatty acid uptake that proposes two major routes of fatty acid uptake: diffusion across membranes followed by intracellular trapping, and endocytosis of host lipoproteins.

Fatty acid metabolism and acyl-CoA synthetases in the liver-gut axis

Fatty acids are energy substrates and cell components which participate in regulating signal transduction, transcription factor activity and secretion of bioactive lipid mediators. The acyl-CoA synthetases (ACSs) family containing 26 family members exhibits tissue-specific distribution, distinct fatty acid substrate preferences and diverse biological functions. Increasing evidence indicates that dysregulation of fatty acid metabolism in the liver-gut axis, designated as the bidirectional relationship between the gut, microbiome and liver, is closely associated with a range of human diseases including metabolic disorders, inflammatory disease and carcinoma in the gastrointestinal tract and liver. In this review, we depict the role of ACSs in fatty acid metabolism, possible molecular mechanisms through which they exert functions, and their involvement in hepatocellular and colorectal carcinoma, with particular attention paid to long-chain fatty acids and small-chain fatty acids. Additionally, the liver-gut communication and the liver and gut intersection with the microbiome as well as diseases related to microbiota imbalance in the liver-gut axis are addressed. Moreover, the development of potentially therapeutic small molecules, proteins and compounds targeting ACSs in cancer treatment is summarized.

Structure and Dynamics of Oxidized Lipoproteins In Vivo: Roles of High-Density Lipoprotein

Oxidative modification of lipoproteins is implicated in the occurrence and development of atherosclerotic lesions. Earlier studies have elucidated on the mechanisms of foam cell formation and lipid accumulation in these lesions, which is mediated by scavenger receptor-mediated endocytosis of oxidized low-density lipoprotein (oxLDL). Mounting clinical evidence has supported the involvement of oxLDL in cardiovascular diseases. High-density lipoprotein (HDL) is known as anti-atherogenic; however, recent studies have shown circulating oxidized HDL (oxHDL) is related to cardiovascular diseases. A modified structure of oxLDL, which was increased in the plasma of patients with acute myocardial infarction, was characterized. It had two unique features: (1) a fraction of oxLDL accompanied oxHDL, and (2) apoA1 was heavily modified, while modification of apoB, and the accumulation of oxidized phosphatidylcholine (oxPC) and lysophosphatidylcholine (lysoPC) was less pronounced. When LDL and HDL were present at the same time, oxidized lipoproteins actively interacted with each other, and oxPC and lysoPC were transferred to another lipoprotein particle and enzymatically metabolized rapidly. This brief review provides a novel view on the dynamics of oxLDL and oxHDL in circulation.

Physico-chemical and physiological determinants of lipo-nanoparticle stability

Liposome-based nanoparticles (NPs) comprised mostly of phospholipids (PLs) have been developed to deliver diagnostic and therapeutic agents. Whereas reassembled plasma lipoproteins have been tested as NP carriers of hydrophobic molecules, they are unstable because the components can spontaneously transfer to other PL surfaces-cell membranes and lipoproteins-and can be degraded by plasma lipases. Here we review two strategies for NP stabilization. One is to use PLs that contain long acyl-chains: according to a quantitative thermodynamic model and in vivo tests, increasing the chain length of a PL reduces the spontaneous transfer rate and increases plasma lifetime. A second strategy is to substitute ether for ester bonds which makes the PLs lipase resistant. We conclude with recommendations of simple ex vivo and in vitro tests of NP stability that should be conducted before in vivo tests are begun.

Advances in fatty acids nutrition in dairy cows: from gut to cells and effects on performance

High producing dairy cows generally receive in the diet up to 5-6% of fat. This is a relatively low amount of fat in the diet compared to diets in monogastrics; however, dietary fat is important for dairy cows as demonstrated by the benefits of supplementing cows with various fatty acids (FA). Several FA are highly bioactive, especially by affecting the transcriptome; thus, they have nutrigenomic effects. In the present review, we provide an up-to-date understanding of the utilization of FA by dairy cows including the main processes affecting FA in the rumen, molecular aspects of the absorption of FA by the gut, synthesis, secretion, and utilization of chylomicrons; uptake and metabolism of FA by peripheral tissues, with a main emphasis on the liver, and main transcription factors regulated by FA. Most of the advances in FA utilization by rumen microorganisms and intestinal absorption of FA in dairy cows were made before the end of the last century with little information generated afterwards. However, large advances on the molecular aspects of intestinal absorption and cellular uptake of FA were made on monogastric species in the last 20 years. We provide a model of FA utilization in dairy cows by using information generated in monogastrics and enriching it with data produced in dairy cows. We also reviewed the latest studies on the effects of dietary FA on milk yield, milk fatty acid composition, reproduction, and health in dairy cows. The reviewed data revealed a complex picture with the FA being active in each step of the way, starting from influencing rumen microbiota, regulating intestinal absorption, and affecting cellular uptake and utilization by peripheral tissues, making prediction on in vivo nutrigenomic effects of FA challenging.

Transfer and Enzyme-Mediated Metabolism of Oxidized Phosphatidylcholine and Lysophosphatidylcholine between Low- and High-Density Lipoproteins

Oxidized low-density lipoprotein (oxLDL) and oxidized high-density lipoprotein (oxHDL), known as risk factors for cardiovascular disease, have been observed in plasma and atheromatous plaques. In a previous study, the content of oxidized phosphatidylcholine (oxPC) and lysophosphatidylcholine (lysoPC) species stayed constant in isolated in vivo oxLDL but increased in copper-induced oxLDL in vitro. In this study, we prepared synthetic deuterium-labeled 1-palmitoyl lysoPC and palmitoyl-glutaroyl PC (PGPC), a short chain-oxPC to elucidate the metabolic fate of oxPC and lysoPC in oxLDL in the presence of HDL. When LDL preloaded with d₁₃-lysoPC was mixed with HDL, d₁₃-lysoPC was recovered in both the LDL and HDL fractions equally. d₁₃-LysoPC decreased by 50% after 4 h of incubation, while d₁₃-PC increased in both fractions. Diacyl-PC production was abolished by an inhibitor of lecithin-cholesterol acyltransferase (LCAT). When d₁₃-PGPC-preloaded LDL was incubated with HDL, d₁₃-PGPC was transferred to HDL in a dose-dependent manner when both LCAT and lipoprotein-associated phospholipase A₂ (Lp-PLA₂) were inhibited. Lp-PLA₂ in both HDL and LDL was responsible for the hydrolysis of d₁₃-PGPC. These results suggest that short chain-oxPC and lysoPC can transfer between lipoproteins quickly and can be enzymatically converted from oxPC to lysoPC and from lysoPC to diacyl-PC in the presence of HDL.

Lipid Droplets Mediate Salt Stress Tolerance in Parachlorella kessleri

Microalgae are known to respond to salinity stress via mechanisms that include accumulation of compatible solutes and synthesis of antioxidants. Here, we describe a salinity-tolerance mechanism mediated by lipid droplets (LDs). In the alga Parachlorella kessleri grown under salt-stress conditions, we observed significant increases in cell size and LD content. LDs that were closely grouped along the plasma membrane shrank as the plasma membrane expanded, and some LDs were engulfed by vacuoles. Transcriptome analysis showed that genes encoding lysophospholipid acyltransferases (LPLATs) and phospholipase A2 were significantly up-regulated following salt stress. Diacylglycerol kinase and LPLAT were identified in the proteome of salt-induced LDs, alongside vesicle trafficking and plastidial proteins and histone H2B. Analysis of fatty acid composition revealed an enrichment of C18:1 and C18:2 at the expense of C18:3 in response to salt stress. Pulse-chase experiments further suggested that variations of fatty acid composition were associated with LDs. Acetate stimulation research further confirmed a positive role of LDs in cell growth under salt stress. These results suggest that LDs play important roles in salt-stress tolerance, through harboring proteins, participating in cytoplasmic component recycling, and providing materials and enzymes for membrane modification and expansion.

Hyperpolarised NMR to follow water proton transport through membrane channels via exchange with biomolecules

Water uptake in vesicles and the subsequent exchange between water protons and amide -NH protons in amino acids can be followed by a new, highly sensitive, type of magnetic resonance spectroscopy: dynamic nuclear polarisation (DNP)-enhanced NMR in the liquid state. Water hydrogen atoms are detected prior to and after their transfer to molecular sites in peptides and proteins featuring highly-accessible proton-exchangeable groups, as is the case for the -NH groups of intrinsically disordered proteins. The detected rates for amide proton-water proton exchange can be modulated by membrane-crossing rates, when a membrane channel is interposed. We hyperpolarised water proton spins via dynamic nuclear polarisation followed by sample dissolution (d-DNP) and transferred the created polarisation to -NH groups with high solvent accessibility in an intrinsically disordered protein domain. This domain is the membrane anchor of c-Src kinase, whose activity controls cell proliferation. The hindrance of effective water proton transfer rate constants observed in free solvent when a membrane-crossing step is involved is discussed. This study aims to assess the feasibility of recently-introduced hyperpolarised (DNP-enhanced) NMR to assess water membrane crossing dynamics.

Progress in synthesizing protocells

IMPACT STATEMENT

Advances in the understanding of the biophysics of membranes, the nonenzymatic and enzymatic polymerization of RNA, and in the design of complex chemical reaction networks have led to a new, integrated way of viewing the shared chemistry needed to sustain life. Although a protocell capable of Darwinian evolution has yet to be built, the seemingly disparate pieces are beginning to fit together. At the very least, better cellular mimics are on the horizon that will likely teach us much about the physicochemical underpinnings of cellular life.

Rethinking reverse cholesterol transport and dysfunctional high-density lipoproteins

Human plasma high-density lipoprotein cholesterol concentrations are a negative risk factor for atherosclerosis-linked cardiovascular disease. Pharmacological attempts to reduce atherosclerotic cardiovascular disease by increasing plasma high-density lipoprotein cholesterol have been disappointing so that recent research has shifted from HDL quantity to HDL quality, that is, functional vs dysfunctional HDL. HDL has varying degrees of dysfunction reflected in impaired reverse cholesterol transport (RCT). In the context of atheroprotection, RCT occurs by 2 mechanisms: one is the well-known trans-hepatic pathway comprising macrophage free cholesterol (FC) efflux, which produces early forms of FC-rich nascent HDL (nHDL). Lecithin:cholesterol acyltransferase converts HDL-FC to HDL-cholesteryl ester while converting nHDL from a disc to a mature spherical HDL, which transfers its cholesteryl ester to the hepatic HDL receptor, scavenger receptor B1 for uptake, conversion to bile salts, or transfer to the intestine for excretion. Although widely cited, current evidence suggests that this is a minor pathway and that most HDL-FC and nHDL-FC rapidly transfer directly to the liver independent of lecithin:cholesterol acyltransferase activity. A small fraction of plasma HDL-FC enters the trans-intestinal efflux pathway comprising direct FC transfer to the intestine. SR-B1^-/- mice, which have impaired trans-hepatic FC transport, are characterized by high plasma levels of a dysfunctional FC-rich HDL that increases plasma FC bioavailability in a way that produces whole-body hypercholesterolemia and multiple pathologies. The design of future therapeutic strategies to improve RCT will have to be formulated in the context of these dual RCT mechanisms and the role of FC bioavailability.

Brain docosahexaenoic acid uptake and metabolism

Docosahexaenoic acid (DHA) is the most abundant n-3 polyunsaturated fatty acid in the brain where it serves to regulate several important processes and, in addition, serves as a precursor to bioactive mediators. Given that the capacity of the brain to synthesize DHA locally is appreciably low, the uptake of DHA from circulating lipid pools is essential to maintaining homeostatic levels. Although, several plasma pools have been proposed to supply the brain with DHA, recent evidence suggests non-esterified-DHA and lysophosphatidylcholine-DHA are the primary sources. The uptake of DHA into the brain appears to be regulated by a number of complementary pathways associated with the activation and metabolism of DHA, and may provide mechanisms for enrichment of DHA within the brain. Following entry into the brain, DHA is esterified into and recycled amongst membrane phospholipids contributing the distribution of DHA in brain phospholipids. During neurotransmission and following brain injury, DHA is released from membrane phospholipids and converted to bioactive mediators which regulate signaling pathways important to synaptogenesis, cell survival, and neuroinflammation, and may be relevant to treating neurological diseases. In the present review, we provide a comprehensive overview of brain DHA metabolism, encompassing many of the pathways and key enzymatic regulators governing brain DHA uptake and metabolism. In addition, we focus on the release of non-esterified DHA and subsequent production of bioactive mediators and the evidence of their proposed activity within the brain. We also provide a brief review of the evidence from post-mortem brain analyses investigating DHA levels in the context of neurological disease and mood disorder, highlighting the current disparities within the field.

Effect of Acyl Chain Length on the Rate of Phospholipid Flip-Flop and Intermembrane Transfer

The rate at which phospholipids equilibrate between different membranes and between the non-polar environments in biological fluids is of high importance in the understanding of biomembrane diversity, as well as in the development of liposomes for drug delivery. In this work, we characterize the rate of insertion into and desorption from POPC bilayers for a homologous series of amphiphiles with the fluorescent NBD group attached to phosphoethanolamines of different acyl chain lengths, NBD-diC _n -PE with n = 6, 8, 10, and 12. The rate of translocation between bilayer leaflets was also characterized, providing all the relevant parameters for their interaction with lipid bilayers. The results are complemented with data for NBD-diC₁₄-PE obtained from literature (Abreu et al. Biophys J 87:353-365, 2004; Moreno et al. Biophys J 91:873-881, 2006). The rate of translocation between the POPC leaflets is not dependent on the length of the acyl chains, while this affects strongly the rate of desorption from the bilayer. Insertion in the POPC bilayer is not diffusion controlled showing a significant dependence on the acyl chain length and on temperature. The results obtained are compared with those previously reported for NBD-LysoC₁₄-PE (Sampaio et al. Biophys J 88:4064-4071, 2005), and with the homologous series of single chain amphiphiles NBD-C _n (Cardoso et al. J Phys Chem B 114:16337-16346, 2010; J Phys Chem B 115:10098-10108, 2011). This allows the establishment of important relations between the rate constants for interaction with the lipid bilayers and the structural properties of the amphiphiles, namely the total surface and the cross-section of their non-polar region.

Native and Reconstituted Plasma Lipoproteins in Nanomedicine: Physicochemical Determinants of Nanoparticle Structure, Stability, and Metabolism

Although many acute and chronic diseases are managed via pharmacological means, challenges remain regarding appropriate drug targeting and maintenance of therapeutic levels within target tissues. Advances in nanotechnology will overcome these challenges through the development of lipidic particles, including liposomes, lipoproteins, and reconstituted high-density lipoproteins (rHDL) that are potential carriers of water-soluble, hydrophobic, and amphiphilic molecules. Herein we summarize the properties of human plasma lipoproteins and rHDL, identify the physicochemical determinants of lipid transfer between phospholipid surfaces, and discuss strategies for increasing the plasma half-life of lipoprotein- and liposome-associated molecules.

Lipid droplet growth: regulation of a dynamic organelle

Intracellular lipid droplets (LDs) are remarkably dynamic and complex organelles that enact regulated storage and release of lipids to fulfil their fundamental roles in energy metabolism, membrane synthesis and provision of lipid-derived signaling molecules. Although small LDs are observed in all types of eukaryotic cells, it is adipocytes that present the widest range of sizes up to the massive unilocular droplet of a white adipocyte. Our knowledge of the proteins and associated processes that control LD dynamics is improving. The dynamic expression of LD-associated proteins is vital for controlling LD biology and is most apparent during adipocyte differentiation. Recent findings on the molecular mechanisms of lipid droplet enlargement reveal the importance of distinct functional groups of proteins and phospholipids.

Lipid-free antigen B subunits from echinococcus granulosus: oligomerization, ligand binding, and membrane interaction properties

Background

The hydatid disease parasite Echinococcus granulosus has a restricted lipid metabolism, and needs to harvest essential lipids from the host. Antigen B (EgAgB), an abundant lipoprotein of the larval stage (hydatid cyst), is thought to be important in lipid storage and transport. It contains a wide variety of lipid classes, from highly hydrophobic compounds to phospholipids. Its protein component belongs to the cestode-specific Hydrophobic Ligand Binding Protein family, which includes five 8-kDa isoforms encoded by a multigene family (EgAgB1-EgAgB5). How lipid and protein components are assembled into EgAgB particles remains unknown. EgAgB apolipoproteins self-associate into large oligomers, but the functional contribution of lipids to oligomerization is uncertain. Furthermore, binding of fatty acids to some EgAgB subunits has been reported, but their ability to bind other lipids and transfer them to acceptor membranes has not been studied.

Methodology/Principal Findings

Lipid-free EgAgB subunits obtained by reverse-phase HPLC were used to analyse their oligomerization, ligand binding and membrane interaction properties. Size exclusion chromatography and cross-linking experiments showed that EgAgB8/2 and EgAgB8/3 can self-associate, suggesting that lipids are not required for oligomerization. Furthermore, using fluorescent probes, both subunits were found to bind fatty acids, but not cholesterol analogues. Analysis of fatty acid transfer to phospholipid vesicles demonstrated that EgAgB8/2 and EgAgB8/3 are potentially capable of transferring fatty acids to membranes, and that the efficiency of transfer is dependent on the surface charge of the vesicles.

Conclusions/Significance

We show that EgAgB apolipoproteins can oligomerize in the absence of lipids, and can bind and transfer fatty acids to phospholipid membranes. Since imported fatty acids are essential for Echinococcus granulosus, these findings provide a mechanism whereby EgAgB could engage in lipid acquisition and/or transport between parasite tissues. These results may therefore indicate vulnerabilities open to targeting by new types of drugs for hydatidosis therapy.

Echinococcus granulosus is a causative agent of hydatidosis, a parasitic disease that affects humans and livestock with significant economic and public health impact worldwide. Antigen B (EgAgB), an abundant product of E. granulosus larvae, is a lipoprotein that carries a wide variety of lipids, including fatty acids and cholesterol. As E. granulosus is unable to synthesize these lipids, EgAgB likely plays an important role in parasite metabolism, participating in both the acquisition of host lipids and their distribution between parasite tissues. The protein component of EgAgB consists of 8 kDa subunits encoded by separate genes. However, the biochemical properties of EgAgB subunits, particularly their ability to bind and transfer lipids, are poorly known. Herein, using in vitro assays, we found that EgAgB subunits were capable of oligomerizing in the absence of lipids and to bind fatty acids, but not cholesterol. Moreover, EgAgB subunits showed the ability to transfer fatty acids to artificial phospholipid membranes. These results indicate new points of attack at which the parasite might be vulnerable to drugs.

Characterization of fatty acid binding and transfer from Δ98Δ, a functional all-β abridged form of IFABP

Intestinal fatty acid binding protein (IFABP) is an intracellular lipid binding protein whose specific functions within the cell are still uncertain. An abbreviated version of IFABP encompassing residues 29-126, dubbed Δ98Δ is a stable product of limited proteolysis with clostripain of holo-IFABP. Cumulative evidence shows that Δ98Δ adopts a stable, monomeric and functional fold, with compact core and loose periphery. In agreement with previous results, this abridged variant indicates that the helical domain is-not necessary to preserve the general topology of IFABP's β-barrel and that the helix-turn-helix motif is a fundamental element of the portal region involved in ligand binding and protein-membrane interactions. Results presented here suggest that Δ98Δ binds fatty acids with affinities lower than IFABP but higher than those shown by previous helix-less variants, shows a 'diffusional' fatty acid transfer mechanism and it interacts with artificial membranes. This work highlights the importance of the β-barrel of IFABP for its specific functions.

IFABP portal region insertion during membrane interaction depends on phospholipid composition

Intestinal fatty acid-binding protein (IFABP) is highly expressed in the intestinal epithelium and it belongs to the family of soluble lipid binding proteins. These proteins are thought to participate in most aspects of the biology of lipids, regulating its availability for specific metabolic pathways, targeting and vectorial trafficking of lipids to specific subcellular compartments. The present study is based on the ability of IFABP to interact with phospholipid membranes, and we characterized its immersion into the bilayer's hydrophobic central region occupied by the acyl-chains. We constructed a series of Trp-mutants of IFABP to selectively probe the interaction of different regions of the protein, particularly the elements forming the portal domain that is proposed to regulate the exit and entry of ligands to/from the binding cavity. We employed several fluorescent techniques based on selective quenching induced by soluble or membrane confined agents. The results indicate that the portal region of IFABP penetrates deeply into the phospholipid bilayer, especially when CL-containing vesicles are employed. The orientation of the protein and the degree of penetration were highly dependent on the lipid composition, the superficial net charge and the ionic strength of the medium. These results may be relevant to understand the mechanism of ligand transfer and the specificity responsible for the unique functions of each member of the FABP family.

Role of Phospholipids in Endocytosis, Phagocytosis, and Macropinocytosis

Endocytosis, phagocytosis, and macropinocytosis are fundamental processes that enable cells to sample their environment, eliminate pathogens and apoptotic bodies, and regulate the expression of surface components. While a great deal of effort has been devoted over many years to understanding the proteins involved in these processes, the important contribution of phospholipids has only recently been appreciated. This review is an attempt to collate and analyze the rapidly emerging evidence documenting the role of phospholipids in clathrin-mediated endocytosis, phagocytosis, and macropinocytosis. A primer on phospholipid biosynthesis, catabolism, subcellular distribution, and transport is presented initially, for reference, together with general considerations of the effects of phospholipids on membrane curvature and charge. This is followed by a detailed analysis of the critical functions of phospholipids in the internalization processes and in the maturation of the resulting vesicles and vacuoles as they progress along the endo-lysosomal pathway.

Phospholipids and lipid droplets

Lipid droplets are ubiquitous cellular organelles that allow cells to store large amounts of neutral lipids for membrane synthesis and energy supply in times of starvation. Compared to other cellular organelles, lipid droplets are structurally unique as they are made of a hydrophobic core of neutral lipids and are separated to the cytosol only by a surrounding phospholipid monolayer. This phospholipid monolayer consists of over a hundred different phospholipid molecular species of which phosphatidylcholine is the most abundant lipid class. However, lipid droplets lack some indispensable activities of the phosphatidylcholine biogenic pathways suggesting that they partially depend on other organelles for phosphatidylcholine synthesis. Here, we discuss very recent data on the composition, origin, transport and function of the phospholipid monolayer with a particular emphasis on the phosphatidylcholine metabolism on and for lipid droplets. In addition, we highlight two very important quantitative aspects: (i) The amount of phospholipid required for lipid droplet monolayer expansion is remarkably small and (ii) to maintain the invariably round shape of lipid droplets, a cell must have a highly sensitive but so far unknown mechanism that regulates the ratio of phospholipid to neutral lipid in lipid droplets. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Direct interaction between EgFABP1, a fatty acid binding protein from Echinococcus granulosus, and phospholipid membranes

Background

Growth and maintenance of hydatid cysts produced by Echinococcus granulosus have a high requirement for host lipids for biosynthetic processes, membrane building and possibly cellular and developmental signalling. This requires a high degree of lipid trafficking facilitated by lipid transporter proteins. Members of the fatty acid binding protein (FABP) family have been identified in Echinococcus granulosus, one of which, EgFABP1 is expressed at the tegumental level in the protoscoleces, but it has also been described in both hydatid cyst fluid and secretions of protoscoleces. In spite of a considerable amount of structural and biophysical information on the FABPs in general, their specific functions remain mysterious.

Methodology/Principal Findings

We have investigated the way in which EgFABP1 may interact with membranes using a variety of fluorescence-based techniques and artificial small unilamellar vesicles. We first found that bacterial recombinant EgFABP1 is loaded with fatty acids from the synthesising bacteria, and that fatty acid binding increases its resistance to proteinases, possibly due to subtle conformational changes induced on EgFABP1. By manipulating the composition of lipid vesicles and the ionic environment, we found that EgFABP1 interacts with membranes in a direct contact, collisional, manner to exchange ligand, involving both ionic and hydrophobic interactions. Moreover, we observed that the protein can compete with cytochrome c for association with the surface of small unilamellar vesicles (SUVs).

Conclusions/Significance

This work constitutes a first approach to the understanding of protein-membrane interactions of EgFABP1. The results suggest that this protein may be actively involved in the exchange and transport of fatty acids between different membranes and cellular compartments within the parasite.

Echinococcus granulosus is the causative agent of hydatidosis, a zoonotic infection that affects humans and livestock, representing a public health and economic burden in many countries. Since the parasites are unable to synthesise most of their lipids de novo, they must acquire them from the host and then deliver them by carrier proteins to specific destinations. E. granulosus produces in abundance proteins of the fatty acid binding protein (FABP) family, one of which, EgFABP1 has been characterised at the structural and ligand binding levels, but it has not been studied in terms of the mechanism of its interaction with membranes. We have investigated the lipid transport properties and protein-membrane interaction characteristics of EgFABP1 by applying biophysical techniques. We found that EgFABP1 interacts with membranes by a mechanism which involves direct contact with them to exchange their cargo. Given that the protein has been found in the secretions of the parasite, the implications of its direct interactions with host membranes should be considered.

Exogenous ether lipids predominantly target mitochondria

Ether lipids are ubiquitous constituents of cellular membranes with no discrete cell biological function assigned yet. Using fluorescent polyene-ether lipids we analyzed their intracellular distribution in living cells by microscopy. Mitochondria and the endoplasmic reticulum accumulated high amounts of ether-phosphatidylcholine and ether-phosphatidylethanolamine. Both lipids were specifically labeled using the corresponding lyso-ether lipids, which we established as supreme precursors for lipid tagging. Polyfosine, a fluorescent analogue of the anti-neoplastic ether lipid edelfosine, accumulated to mitochondria and induced morphological changes and cellular apoptosis. These data indicate that edelfosine could exert its pro-apoptotic power by targeting and damaging mitochondria and thereby inducing cellular apoptosis. In general, this study implies an important role of mitochondria in ether lipid metabolism and intracellular ether lipid trafficking.

Interaction of 7-nitrobenz-2-oxa-1,3-diazol-4-yl-labeled fatty amines with 1-palmitoyl, 2-oleoyl-sn-glycero-3-phosphocholine bilayers: a molecular dynamics study

A complete homologous series of fluorescent 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-labeled fatty amines of varying alkyl chain length, NBD-C(n), inserted in 1-palmitoyl, 2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers, was studied using atomistic molecular dynamics (MD) simulations. For all amphiphiles, the NBD fluorophore locates near the glycerol backbone/carbonyl region of POPC and establishes stable hydrogen bonding with POPC ester oxygen atoms. Small differences observed in the transverse location of the fluorophore correlate with other calculated parameters and with small discrepancies recently measured in the photophysical properties of the molecules. The longer-chained NBD-C(n) amphiphiles show significant mass density near the bilayer midplane, and the chains of these derivatives interdigitate to some extent the opposite bilayer leaflet. This phenomenon leads to a slower lateral diffusion for the longer-chained derivatives (n > 12). Effects of these amphiphiles on the structure and dynamics of the host lipid were found to be relatively mild, in comparison with acyl-chain-labeled NBD probes. The molecular details obtained by this work allow the rationalization of the nonmonotonic behavior, recently obtained experimentally, for the photophysical parameters of the amphiphiles and the kinetic and thermodynamic parameters for their interaction with the POPC membranes.

Chain-length dependence of insertion, desorption, and translocation of a homologous series of 7-nitrobenz-2-oxa-1,3-diazol-4-yl-labeled aliphatic amines in membranes

We present a complete characterization of the kinetics of interaction between the homologous series of fluorescent fatty amines with the fluorescent moiety 7-nitrobenz-2-oxa-1,3-diazol-4-yl covalently bound to the amine group, NBD-C(n) (n = 8-16), and a lipid bilayer in the liquid disordered phase. The insertion into and the desorption from the lipid bilayer, as well as the rate of translocation across the two bilayer leaflets, has been measured at different temperatures, allowing an estimation of the thermodynamic parameters in the formation of the transition state. This is the first report on the complete characterization of the kinetics of the interaction of a large series of structurally homologous amphiphiles. In a recent paper from this research group, the equilibrium interaction of NBD-C(n) (n = 4-10) with POPC bilayers and serum albumin was reported. This information allows the calculation of the equilibrium distribution of the amphiphiles among the aqueous phase, serum proteins, and biomembranes. The data presented in this manuscript complement its characterization with information on the kinetics of the interactions, making possible the quantitative evaluation of their pharmacokinetics. The rate of translocation is shown to decrease with increasing alkyl chain length up to n = 12, becoming relatively insensitive to further increases in n. The Gibbs free energy variation associated with the rate of desorption from the lipid bilayer increased linearly with n, with ΔΔG(‡o) = 3.4 ± 0.5 kJ mol(-1) per methylene group. It was also found that the process of insertion in the lipid bilayer is not diffusion-limited, although it is close to this limit for the smaller amphiphiles in the homologous series at high temperatures.

Human lysophosphatidylcholine acyltransferases 1 and 2 are located in lipid droplets where they catalyze the formation of phosphatidylcholine

Phosphatidylcholine (PC) is synthesized by two different pathways, the Lands cycle and the Kennedy pathway. The recently identified key enzymes of the Lands cycle, lysophosphatidylcholine acyltransferase 1 and 2 (LPCAT1 and -2), were reported to localize to the endoplasmic reticulum and to function in lung surfactant production and in inflammation response. Here, we show in various mammalian cell lines that both enzymes additionally localize to lipid droplets (LDs), which consist of a core of neutral lipids surrounded by a monolayer of phospholipid, mainly PC. This dual localization is enabled by the monotopic topology of these enzymes demonstrated in this study. Furthermore, we show that LDs have the ability to locally synthesize PC and that this activity correlates with the LPCAT1 and -2 expression level. This suggests that LPCAT1 and -2 have, in addition to their known function in specialized cells, a ubiquitous role in LD-associated lipid metabolism.

Natural ligand binding and transfer from liver fatty acid binding protein (LFABP) to membranes

Liver fatty acid-binding protein (LFABP) is distinctive among fatty acid-binding proteins because it binds more than one molecule of long-chain fatty acid and a variety of diverse ligands. Also, the transfer of fluorescent fatty acid analogues to model membranes under physiological ionic strength follows a different mechanism compared to most of the members of this family of intracellular lipid binding proteins. Tryptophan insertion mutants sensitive to ligand binding have allowed us to directly measure the binding affinity, ligand partitioning and transfer to model membranes of natural ligands. Binding of fatty acids shows a cooperative mechanism, while acyl-CoAs binding presents a hyperbolic behavior. Saturated fatty acids seem to have a stronger partition to protein vs. membranes, compared to unsaturated fatty acids. Natural ligand transfer rates are more than 200-fold higher compared to fluorescently-labeled analogues. Interestingly, oleoyl-CoA presents a markedly different transfer behavior compared to the rest of the ligands tested, probably indicating the possibility of specific targeting of ligands to different metabolic fates.

VLDL lipolysis products increase VLDL fluidity and convert apolipoprotein E4 into a more expanded conformation

Our previous work indicated that apolipoprotein (apo) E4 assumes a more expanded conformation in the postprandial period. The postprandial state is characterized by increased VLDL lipolysis. In this article, we tested the hypothesis that VLDL lipolysis products increase VLDL particle fluidity, which mediates expansion of apoE4 on the VLDL particle. Plasma from healthy subjects was collected before and after a moderately high-fat meal and incubated with nitroxyl-spin labeled apoE. ApoE conformation was examined by electron paramagnetic resonance spectroscopy using targeted spin probes on cysteines introduced in the N-terminal (S76C) and C-terminal (A241C) domains. Further, we synthesized a novel nitroxyl spin-labeled cholesterol analog, which gave insight into lipoprotein particle fluidity. Our data revealed that the order of lipoprotein fluidity was HDL approximately LDL<VLDL<VLDL+lipoprotein lipase. Moreover, the conformation of apoE4 depended on the lipoprotein fraction: VLDL-associated apoE4 had a more linear conformation than apoE4 associated with LDL or HDL. Further, by changing VLDL fluidity, VLDL lipolysis products significantly altered apoE4 into a more expanded conformation. Our studies indicate that after every meal, VLDL fluidity is increased causing apoE4 associated with VLDL to assume a more expanded conformation, potentially enhancing the pathogenicity of apoE4 in vascular tissue.

Albumin as fatty acid transporter

SUMMARY

Fatty acids play critical roles in mammalian energy metabolism. Moreover, they are important substrates for the synthesis of membrane phospholipids and biologically active compounds like eicosanoids and leukotrienes. Because of their low solubility in aqueous solutions such as blood plasma and interstitial fluid, fatty acids are in need of binding proteins to increase their concentration in vascular and interstitial compartments. Albumin acts as main fatty acid binding protein in extracellular fluids. Plasma albumin possesses about 7 binding sites for fatty acids with moderate to high affinity, enhancing the concentration of fatty acids by a several orders of magnitude. Despite the high affinity of albumin for fatty acids, uptake of fatty acids by parenchymal cells such as skeletal and cardiac myocytes seems not to be hampered by albumin. In contrast, experimental findings suggest that albumin may facilitate the uptake of fatty acids by organs in need of these substrates. In the present overview the following issues will be briefly discussed: (i) transport and storage of fatty acids in the mammalian body, (ii) biosynthesis of albumin in the liver, (iii) localization and concentration of albumin in body fluids, (iv) interactions between albumin and fatty acids, (v) albumin structure and fatty acid binding sites, (vi) uptake of fatty acids by organs and roles for plasma albumin and (vii) lessons from patients and experimental animals lacking plasma albumin.

Fatty acid transfer from Yarrowia lipolytica sterol carrier protein 2 to phospholipid membranes

Sterol carrier protein 2 (SCP2) is an intracellular protein domain found in all forms of life. It was originally identified as a sterol transfer protein, but was recently shown to also bind phospholipids, fatty acids, and fatty-acyl-CoA with high affinity. Based on studies carried out in higher eukaryotes, it is believed that SCP2 targets its ligands to compartmentalized intracellular pools and participates in lipid traffic, signaling, and metabolism. However, the biological functions of SCP2 are incompletely characterized and may be different in microorganisms. Herein, we demonstrate the preferential localization of SCP2 of Yarrowia lipolytica (YLSCP2) in peroxisome-enriched fractions and examine the rate and mechanism of transfer of anthroyloxy fatty acid from YLSCP2 to a variety of phospholipid membranes using a fluorescence resonance energy transfer assay. The results show that fatty acids are transferred by a collision-mediated mechanism, and that negative charges on the membrane surface are important for establishing a "collisional complex". Phospholipids, which are major constituents of peroxisome and mitochondria, induce special effects on the rates of transfer. In conclusion, YLSCP2 may function as a fatty acid transporter with some degree of specificity, and probably diverts fatty acids to the peroxisomal metabolism.

Effect of bilayer phospholipid composition and curvature on ligand transfer by the alpha-tocopherol transfer protein

We report here our preliminary investigations on the mechanism of alpha-TTP-mediated ligand transfer as assessed using fluorescence resonance energy transfer (FRET) assays. These assays monitor the movement of the model alpha-tocopherol fluorescent derivative ((R)-2,5,7,8-tetramethyl-chroman-2-[9-(7-nitro-benzo[1,2,5]oxadiazol-4-yl amino)-nonyl]-chroman-6-ol; NBD-Toc) from protein to acceptor vesicles containing the fluorescence quencher TRITC-PE. We have found that alpha-TTP utilizes a collisional mechanism of ligand transfer requiring direct protein-membrane contact, that rates of ligand transfer are greater to more highly curved lipid vesicles, and that such rates are insensitive to the presence of anionic phospholipids in the acceptor membrane. These results point to hydrophobic features of alpha-TTP dominating the binding energy between protein and membrane.

Measuring the adsorption of Fatty acids to phospholipid vesicles by multiple fluorescence probes

Fatty acids (FA) are important nutrients that the body uses to regulate the storage and use of energy resources. The predominant mechanism by which long-chain fatty acids enter cells is still debated widely as it is unclear whether long-chain fatty acids require protein transporters to catalyze their transmembrane movement. We use stopped-flow fluorescence (millisecond time resolution) with three fluorescent probes to monitor different aspects of FA binding to phospholipid vesicles. In addition to acrylodan-labeled fatty acid binding protein, a probe that detects unbound FA in equilibrium with the lipid bilayer, and cis-parinaric acid, which detects the insertion of the FA acyl chain into the membrane, we introduce fluorescein-labeled phosphatidylethanolamine as a new probe to measure the binding of FA anions to the outer membrane leaflet. We combined these three approaches with measurement of intravesicular pH to show very fast FA binding and translocation in the same experiment. We validated quantitative predictions of our flip-flop model by measuring the number of H(+) delivered across the membrane by a single dose of FA with the probe 6-methoxy-N-(3-sulfopropyl) quinolinium. These studies provide a framework and basis for evaluation of the potential roles of proteins in binding and transport of FA in biological membranes.

The integrity of the alpha-helical domain of intestinal fatty acid binding protein is essential for the collision-mediated transfer of fatty acids to phospholipid membranes

Intestinal FABP (IFABP) and liver FABP (LFABP), homologous proteins expressed at high levels in intestinal absorptive cells, employ markedly different mechanisms of fatty acid transfer to acceptor model membranes. Transfer from IFABP occurs during protein-membrane collisional interactions, while for LFABP transfer occurs by diffusion through the aqueous phase. In addition, transfer from IFABP is markedly faster than from LFABP. The overall goal of this study was to further explore the structural differences between IFABP and LFABP which underlie their large functional differences in ligand transport. In particular, we addressed the role of the alphaI-helix domain in the unique transport properties of intestinal FABP. A chimeric protein was engineered with the 'body' (ligand binding domain) of IFABP and the alphaI-helix of LFABP (alpha(I)LbetaIFABP), and the fatty acid transfer properties of the chimeric FABP were examined using a fluorescence resonance energy transfer assay. The results showed a significant decrease in the absolute rate of FA transfer from alpha(I)LbetaIFABP compared to IFABP. The results indicate that the alphaI-helix is crucial for IFABP collisional FA transfer, and further indicate the participation of the alphaII-helix in the formation of a protein-membrane "collisional complex". Photo-crosslinking experiments with a photoactivable reagent demonstrated the direct interaction of IFABP with membranes and further support the importance of the alphaI helix of IFABP in its physical interaction with membranes.

New insights into the roles of proteins and lipids in membrane transport of fatty acids

Recent calculations of the apparent permeability coefficients for long-chain fatty acids (LCFA) in phospholipid bilayers provide a new perspective on their transport in a membrane. LCFA have permeabilities that are many orders of magnitude higher than glucose, amino acids, and ions. Transport of LCFA through membranes must therefore be considered to be much different from these nutrients, and there is no a priori requirement for catalysis by a membrane protein. New evidence indicates that the plasma membrane proteins postulated as catalysts for transporting LCFA into the cell fall into three categories. Some act as enzymes, mainly for the activation of LCFA to the acyl CoA, which is required for subsequent intracellular metabolism of LCFA. Other proteins appear to participate in sequestering and trafficking of LCFA. Finally, some proteins have undefined mechanisms. The established mechanisms are consistent with biophysical properties of LCFA in membranes, including fast free diffusion by "flip-flop" in the phospholipid bilayer.

A Model for Fatty Acid Transport into the Brain

A key function of fatty acid (FA) transport into the brain is to supply polyunsaturated fatty acids (PUFA) that are not synthesized in brain cells but are essential signaling molecules and components of the phospholipid membrane. In addition, common dietary FAs such as palmitic acid are also rapidly taken up by the brain and esterified to phospholipids or oxidized to provide cellular energy. Most evidence shows that FA crossing the blood brain barrier (BBB) is derived mainly from FA/albumin complexes and, to a lesser extent, from circulating lipoproteins. Our model proposes that FA diffuse across the lipid bilayer of the BBB without specific transporters to reach brain cells. They cross the luminal and transluminal leaflets of the endothelial cells and the plasma membrane of neural cells by reversible flip-flop. Acyl-CoA synthetases trap FA by forming acyl-CoA, which cannot diffuse out of the cell. Selection of FA is controlled largely by enzymes in the pathways of intracellular metabolism, beginning with the acyl-CoA synthetase.

How fatty acids of different chain length enter and leave cells by free diffusion

Opposing views exist as to how unesterified fatty acids (FA) enter and leave cells. It is commonly believed that for short- and medium-chain FA free diffusion suffices whereas it is questioned whether proteins are required to facilitate transport of long-chain fatty acid (LCFA). Furthermore, it is unclear whether these proteins facilitate binding to the plasma membrane, trans-membrane movement, dissociation into the cytosol and/or transport in the cytosol. In this mini-review we approach the controversy from a different point of view by focusing on the membrane permeability constant (P) of FA with different chain length. We compare experimentally derived values of the P of short and medium-chain FA with values of apparent permeability coefficients for LCFA calculated from their dissociation rate constant (k(off)), flip-flop rate constant (k(flip)) and partition coefficient (Kp) in phospholipid bilayers. It was found that Overton's rule is valid as long as k(flip)<<k(off). With increasing chain length, the permeability increases according to increasing Kp and reaches a maximum for LCFA with chain length of 18 carbons or longer. For fast flip-flop (e.g. k(flip)=15s(-1)), the apparent permeability constant for palmitic acid is very high (P(app)=1.61 cm/s). Even for a slow flip-flop rate constant (e.g. k(flip)=0.3s(-1)), the permeability constant of LCFA is still several orders of magnitude larger than the P of water and other small non-electrolytes. Since polyunsaturated FA have basically the same physico-chemical properties as LCFA, they have similar membrane permeabilities. The implications for theories involving proteins to facilitate uptake of FA are discussed.

Different Mechanisms of Free Fatty Acid Flip-Flop and Dissociation Revealed by Temperature and Molecular Species Dependence of Transport across Lipid Vesicles

The mechanism of free fatty acid (FFA) transport across membranes is a subject of intense investigation. We have demonstrated recently that flip-flop is the rate-limiting step for transport of oleic acid across phospholipid vesicles (Cupp, D., Kampf, J. P., and Kleinfeld, A. M. (2004) Biochemistry 43, 4473-4481). To better understand the nature of the flip-flop barrier, we measured the temperature dependence of a series of saturated and monounsaturated FFA. We determined the rate constants for flip-flop and dissociation for small (SUV), large (LUV), and giant (GUV) unilamellar vesicles composed of egg phosphatidylcholine. For all FFA and vesicle types, dissociation was faster than flip-flop, and for all FFA, flip-flop and dissociation were faster in SUV than in LUV or GUV. Rate constants for both flip-flop and dissociation decreased exponentially with increasing FFA size. However, only the flip-flop rate constants increased significantly with temperature; the barrier to flip-flop was virtually entirely due to an enthalpic activation free energy. The barrier to dissociation was primarily entropic. Analysis in terms of a simple free volume (V(f)) model revealed V(f) values for flip-flop that ranged between approximately 12 and 15 Angstroms(3), with larger values for SUV than for LUV or GUV. V(f) values increased with temperature, and this temperature dependence generated the enthalpic barrier to flip-flop. The barrier for dissociation and its size dependence primarily reflect the aqueous solubility of FFA. These are the first results to distinguish the energetics of flipflop and dissociation. This should lead to a better understanding of the mechanisms governing FFA transport across biological membranes.

Protein-membrane interaction and fatty acid transfer from intestinal fatty acid-binding protein to membranes. Support for a multistep process

Fatty acid transfer from intestinal fatty acid-binding protein (IFABP) to phospholipid membranes occurs during protein-membrane collisions. Electrostatic interactions involving the alpha-helical "portal" region of the protein have been shown to be of great importance. In the present study, the role of specific lysine residues in the alpha-helical region of IFABP was directly examined. A series of point mutants in rat IFABP was engineered in which the lysine positive charges in this domain were eliminated or reversed. Using a fluorescence resonance energy transfer assay, we analyzed the rates and mechanism of fatty acid transfer from wild type and mutant proteins to acceptor membranes. Most of the alpha-helical domain mutants showed slower absolute fatty acid transfer rates to zwitterionic membranes, with substitution of one of the lysines of the alpha2 helix, Lys27, resulting in a particularly dramatic decrease in the fatty acid transfer rate. Sensitivity to negatively charged phospholipid membranes was also reduced, with charge reversal mutants in the alpha2 helix the most affected. The results support the hypothesis that the portal region undergoes a conformational change during protein-membrane interaction, which leads to release of the bound fatty acid to the membrane and that the alpha2 segment is of particular importance in the establishment of charge-charge interactions between IFABP and membranes. Cross-linking experiments with a phospholipid-photoactivable reagent underscored the importance of charge-charge interactions, showing that the physical interaction between wild-type intestinal fatty acid-binding protein and phospholipid membranes is enhanced by electrostatic interactions. Protein-membrane interactions were also found to be enhanced by the presence of ligand, suggesting different collisional complex structures for holo- and apo-IFABP.

Kinetics and thermodynamics of lipid amphiphile exchange between lipoproteins and albumin in serum

We have examined the kinetics and thermodynamics of the exchange of a fluorescent amphiphile derived from a phospholipid, NBD-DMPE, between serum albumin and the serum lipoproteins of high density (HDL2 and HDL3), LDL, and VLDL. Binding of the fluorescent lipid amphiphile to bovine serum albumin is characterized, at 35 degrees C, by an equilibrium binding constant of approximately 3 x 10(6) M(-1) and a characteristic time < or = 0.1 s. Association of NBD-DMPE with the lipoprotein particles, if considered as a partitioning of amphiphile monomers between the aqueous phase and the lipoprotein particles, is characterized by an equilibrium partition coefficient between 10(5) and 10(6), being highest for LDL and lowest for HDL. The association of NBD-DMPE monomers with lipoprotein particles can be described by insertion rate constants on the order of 10(5) M(-1) s(-1) for VLDL and LDL and 10(4) M(-1) s(-1) for HDL. The desorption rate constants are on the order of 10(-5) s(-1) for all particles. The study was performed as a function of temperature between 15 and 35 degrees C. This permitted the calculation of the equilibrium thermodynamic parameters (deltaG(o), deltaH(o), and deltaS(o)) as well as the activation parameters (deltaG++(o), deltaH++(o), and deltaS++(o)) for the insertion and desorption processes. The association equilibrium is dominated by the entropic contribution to the free energy in all cases. The results are discussed in relation to phospholipid and amphiphile exchange phenomena involving the lipoproteins.

Fatty acid-albumin complexes and the determination of the transport of long chain free fatty acids across membranes

Understanding the mechanism that governs the transport of long chain free fatty acids (FFA) across lipid bilayers is critical for understanding transport across cell membranes. Conflicting results have been reported for lipid vesicles; most investigators report that flip-flop occurs within the resolution time of the method (<5 ms) and that dissociation from the membrane is rate limiting, while other studies find that flip-flop is rate limiting and on the order of seconds. We have reinvestigated this problem and find that the methods used in studies reporting rapid flip-flop have not been interpreted correctly. We find that accurate information about transport of FFA across lipid vesicles requires that FFA be delivered to the vesicles as complexes with albumin (BSA). For example, we find that stopped-flow mixing of uncomplexed FFA with small unilamellar vesicles (SUV) containing pyranine yields the very fast influx rates reported previously (>100 s(-1)). However, these influx rates increase linearly with lipid vesicle concentration and can therefore not, as previously interpreted, represent flip-flop. In contrast, measurements of influx rates in SUV and giant unilamellar vesicles performed with oleate-BSA complexes reveal no dependence on vesicle concentration and yield influx rate constants of approximately 4 and approximately 0.5 s(-1), respectively. Rate constants for efflux and dissociation were determined from the transfer of oleate from vesicles to BSA and reveal similar influx and efflux but dissociation rate constants that are approximately 5-10-fold greater. We conclude that flip-flop is rate limiting for transport of FFA across lipid vesicles and slows with an increasing radius of curvature. These results, in contrast to those reporting that flip-flop is extremely fast, indicate that the lipid bilayer portion of biological membranes may present a significant barrier to transport of FFA across cell membranes.

Fatty Acid Transport in Adipocytes Monitored by Imaging Intracellular Free Fatty Acid Levels

Transport of free fatty acids (FFA) across the adipocyte plasma membrane is critical for maintaining homeostasis. To determine the membrane's role in regulating transport we describe here the first measurements of the intracellular (unbound) FFA concentration ([FFA(i)]) and their use in monitoring transport of FFA across 3T3F442A adipocytes. [FFA(i)] was measured by microinjecting cells with ADIFAB, a fluorescently labeled fatty acid-binding protein that is used to measure unbound FFA levels. We used ratio fluorescence microscopy of intracellular ADIFAB to image unbound FFA levels and determined the time course of [FFA(i)] in response to changing the extracellular unbound FFA concentration ([FFA(o)]). [FFA(o)] was clamped at defined levels using complexes of FFA and bovine serum albumin. We show that FFA influx is slow, requiring about 300 s to reach steady state (rate constant approximately 0.02 s(-1)) and saturable (K(o) approximately 200 nm). Efflux is twice as fast as influx, for zero [FFA(o)], but decreases with increasing [FFA(o)]. Surprisingly, at steady state [FFA(i)] is 2-5-fold (average 2-fold) greater than [FFA(o)] and this [FFA(i)]/[FFA(o)] gradient is abolished by depleting cellular ATP. Our results indicate that FFA transport across adipocyte membranes is highly regulated, involving an ATP-driven pump and a mechanism for gating efflux that is sensitive to [FFA(o)]. These characteristics are well described by a membrane carrier model but are not consistent with FFA transport across the membrane's lipid phase. We suggest that these characteristics are important in regulating circulating FFA levels by the adipocyte.

Energy translocation across cell membranes and membrane models

Fatty acid transport is an important process in cellular energy distribution and storage in both normal and pathological states, especially obesity-linked type 2 diabetes mellitus. Fatty acid transport has been studied by the complementary approaches of cell biology and biophysics. According to the latter approach, specific proteins that enhance the uptake and storage of fatty acids are posited as fatty acid translocases, which facilitate fatty acid movement from the outer to inner leaflets of the plasma membrane. According to biophysical studies conducted in vitro, fatty acid translocation occurs by a rapid diffusive process that does not require a protein. Herein, we critically review these two mechanisms and their importance in the regulation of fatty acid uptake in vivo.

Structural and biochemical characterization of toad liver fatty acid-binding protein

Two paralogous groups of fatty acid-binding proteins (FABPs) have been described in vertebrate liver: liver FABP (L-FABP) type, extensively characterized in mammals, and liver basic FABP (Lb-FABP) found in fish, amphibians, reptiles, and birds. We describe here the toad Lb-FABP complete amino acid sequence, its X-ray structure to 2.5 A resolution, ligand-binding properties, and mechanism of fatty acid transfer to phospholipid membranes. Alignment of the amino acid sequence of toad Lb-FABP with known L-FABPs and Lb-FABPs shows that it is more closely related to the other Lb-FABPs. Toad Lb-FABP conserves the 12 characteristic residues present in all Lb-FABPs and absent in L-FABPs and presents the canonical fold characteristic of all the members of this protein family. Eight out of the 12 conserved residues point to the lipid-binding cavity of the molecule. In contrast, most of the 25 L-FABP conserved residues are in clusters on the surface of the molecule. The helix-turn-helix motif shows both a negative and positive electrostatic potential surface as in rat L-FABP, and in contrast with the other FABP types. The mechanism of anthroyloxy-labeled fatty acids transfer from Lb-FABP to phospholipid membranes occurs by a diffusion-mediated process, as previously shown for L-FABP, but the rate of transfer is 1 order of magnitude faster. Toad Lb-FABP can bind two cis-parinaric acid molecules but only one trans-parinaric acid molecule while L-FABP binds two molecules of both parinaric acid isomers. Although toad Lb-FABP shares with L-FABP a broad ligand-binding specificity, the relative affinity is different.

Fast flip-flop of cholesterol and fatty acids in membranes: implications for membrane transport proteins

PURPOSE OF REVIEW

The rates by which unesterified fatty acids and cholesterol move through and desorb from membranes have been difficult to measure, in part because of the simple structures of these lipids but also because methods have generally not clearly distinguished the two steps of membrane transport. Lack of definitive knowledge has given rise to speculation about the mechanism(s) of membrane 'transport' proteins for fatty acids and cholesterol.

RECENT FINDINGS

New biophysical and biochemical approaches have provided evidence that fatty acids and cholesterol exhibit rapid diffusion (flip-flop), as fast as milliseconds, across both protein-free phospholipid bilayers and cell membranes. In contrast, desorption of the cholesterol molecule from a membrane surface (hours) is much slower than that of common dietary fatty acids (milliseconds to seconds).

SUMMARY

Knowledge of these properties provides a framework for understanding transport and metabolism of cholesterol and fatty acids and how their putative membrane and intracellular transporters might function.

Rapid flip-flop of oleic acid across the plasma membrane of adipocytes

Nonesterified long-chain fatty acids may enter cells by free diffusion or by membrane protein transporters. A requirement for proteins to transport fatty acids across the plasma membrane would imply low partitioning of fatty acids into the membrane lipids, and/or a slower rate of diffusion (flip-flop) through the lipid domains compared to the rates of intracellular metabolism of fatty acids. We used both vesicles of the plasma membrane of adipocytes and intact adipocytes to study transmembrane fluxes of externally added oleic acid at concentrations below its solubility limit at pH 7.4. Binding of oleic acid to the plasma membrane was determined by measuring the fluorescent fatty acid-binding protein ADIFAB added to the external medium. Changes in internal pH caused by flip-flop and metabolism were measured by trapping a fluorescent pH indicator in the cells. The metabolic end products of oleic acid were evaluated over the time interval required for the return of intracellular pH to its initial value. The primary findings were that (i) oleic acid rapidly binds with high avidity in the lipid domains of the plasma membrane with an apparent partition coefficient similar to that of protein-free phospholipid bilayers; (ii) oleic acid rapidly crosses the plasma membrane by the flip-flop mechanism (both events occur within 5 s); and (iii) the kinetics of esterification of oleic acid closely follow the time dependence of the recovery of intracellular pH. Any postulated transport mechanism for facilitating translocation of fatty acid across the plasma membrane of adipocytes, including a protein transporter, would have to compete with the highly effective flip-flop mechanism.