Far from Inert: Membrane Lipids Possess Intrinsic Reactivity That Has Consequences for Cell Biology

In this article, it is hypothesized that a fundamental chemical reactivity exists between some non-lipid constituents of cellular membranes and ester-based lipids, the significance of which is not generally recognized. Many peptides and smaller organic molecules have now been shown to undergo lipidation reactions in model membranes in circumstances where direct reaction with the lipid is the only viable route for acyl transfer. Crucially, drugs like propranolol are lipidated in vivo with product profiles that are comparable to those produced in vitro. Some compounds have also been found to promote lipid hydrolysis. Drugs with high lytic activity in vivo tend to have higher toxicity in vitro. Deacylases and lipases are proposed as key enzymes that protect cells against the effects of intrinsic lipidation. The toxic effects of intrinsic lipidation are hypothesized to include a route by which nucleation can occur during the formation of amyloid fibrils.     

General aspects of peptide selectivity towards lipid bilayers and cell membranes studied by variation of the structural parameters of amphipathic helical model peptides

Model compounds of modified hydrophobicity (H), hydrophobic moment (μ) and angle subtended by charged residues (Φ) were synthesized to define the general roles of structural motifs of cationic helical peptides for membrane activity and selectivity. The peptide sets were based on a highly hydrophobic, non-selective KLA model peptide with high antimicrobial and hemolytic activity. Variation of the investigated parameters was found to be a suitable method for modifying peptide selectivity towards either neutral or highly negatively charged lipid bilayers. H and μ influenced selectivity preferentially via modification of activity on 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) bilayers, while the size of the polar/hydrophobic angle affected the activity against 1-palmitoyl-2-oleoylphosphatidyl-DL-glycerol (POPG). The influence of the parameters on the activity determining step was modest in both lipid systems and the activity profiles were the result of the parameters’ influence on the second less pronounced permeabilization step. Thus, the activity towards POPC vesicles was determined by the high permeabilizing efficiency, however, changes in the structural parameters preferentially influenced the relatively moderate affinity. In contrast, intensive peptide accumulation via electrostatic interactions was sufficient for the destabilization of highly negatively charged POPG lipid membranes, but changes in the activity profile, as revealed by the modification of Φ, seem to be preferentially caused by variation of the low permeabilizing efficiency. The parameters proved very effective also in modifying antimicrobial and hemolytic activity. However, their influence on cell selectivity was limited. A threshold value of hydrophobicity seems to exist which restricted the activity modifying potential of μ and Φ on both lipid bilayers and cell membranes.     

Derlin-1 and UBXD8 are engaged in dislocation and degradation of lipidated ApoB-100 at lipid droplets

Apolipoprotein B-100 (ApoB) is the principal component of very low density lipoprotein. Poorly lipidated nascent ApoB is extracted from the Sec61 translocon and degraded by proteasomes. ApoB lipidated in the endoplasmic reticulum (ER) lumen is also subjected to proteasomal degradation, but where and how it dislocates to the cytoplasm remain unknown. In the present study, we demonstrate that ApoB after lipidation is dislocated to the cytoplasmic surface of lipid droplets (LDs) and accumulates as ubiquitinated ApoB in Huh7 cells. Depletion of UBXD8, which is almost confined to LDs in this cell type, decreases recruitment of p97 to LDs and causes an increase of both ubiquitinated ApoB on the LD surface and lipidated ApoB in the ER lumen. In contrast, abrogation of Derlin-1 function induces an accumulation of lipidated ApoB in the ER lumen but does not increase ubiquitinated ApoB on the LD surface. UBXD8 and Derlin-1 bind with each other and with lipidated ApoB and show colocalization around LDs. These results indicate that ApoB after lipidation is dislocated from the ER lumen to the LD surface for proteasomal degradation and that Derlin-1 and UBXD8 are engaged in the predislocation and postdislocation steps, respectively.     

Deconvoluting the biology and druggability of protein lipidation using chemical proteomics

Lipids are indispensable cellular building blocks, and their post-translational attachment to proteins makes them important regulators of many biological processes. Dysfunction of protein lipidation is also implicated in many pathological states, yet its systematic analysis presents significant challenges. Thanks to innovations in chemical proteomics, lipidation can now be readily studied by metabolic tagging using functionalized lipid analogs, enabling global profiling of lipidated substrates using mass spectrometry. This has spearheaded the first deconvolution of their full scope in a range of contexts, from cells to pathogens and multicellular organisms. Protein N-myristoylation, S-acylation, and S-prenylation are the most well-studied lipid post-translational modifications because of their extensive contribution to the regulation of diverse cellular processes. In this review, we focus on recent advances in the study of these post-translational modifications, with an emphasis on how novel mass spectrometry methods have elucidated their roles in fundamental biological processes.     

Role of ceramide/sphingomyelin (SM) balance regulated through “SM cycle” in cancer

Sphingomyelin synthase (SMS), which comprises of two isozymes, SMS1 and SMS2, is the only enzyme that generates sphingomyelin (SM) by transferring phosphocholine of phosphatidylcholine to ceramide in mammals. Conversely, ceramide is generated from SM hydrolysis via sphingomyelinases (SMases), ceramide de novo synthesis, and the salvage pathway. The biosynthetic pathway for SM and ceramide content by SMS and SMase, respectively, is called “SM cycle.” SM forms a SM-rich microdomain on the cell membrane to regulate signal transduction, such as proliferation/survival, migration, and inflammation. On the other hand, ceramide acts as a lipid mediator by forming a ceramide-rich platform on the membrane, and ceramide exhibits physiological actions such as cell death, cell cycle arrest, and autophagy induction. Therefore, the regulation of ceramide/SM balance by SMS and SMase is responsible for diverse cell functions not only in physiological cells but also in cancer cells. This review outlines the implications of ceramide/SM balance through “SM cycle” in cancer progression and prevention. In addition, the possible involvement of “SM cycle” is introduced in anti-cancer tumor immunity, which has become a hot topic to innovate a more effective and safer way to conquer cancer in recent years.     

Influence of the Lipid Anchor Motif of N-Ras on the Interaction with Lipid Membranes: A Surface Plasmon Resonance Study

Ras GTPases play a crucial role in signal transduction cascades involved in cell differentiation and proliferation, and membrane binding is essential for their proper function. To determine the influence of the nature of the lipid anchor motif and the difference between the active (GTP) and inactive (GDP) forms of N-Ras on partitioning and localization in the lipid membrane, five different N-Ras constructs with different lipid anchors and nucleotide loading (Far/Far (GDP), HD/Far (GDP), HD/HD (GDP), Far (GDP), and HD/Far (GppNHp)) were synthesized. Using the surface plasmon resonance technique, we were able to follow the insertion and dissociation process of the lipidated proteins into and out of model membranes consisting of pure liquid-ordered (l_o) or liquid-disordered (l_d) phase and a heterogeneous two-phase mixture, i.e., a raft mixture with l_o + l_d phase coexistence. In addition, we examined the influence of negatively charged headgroups and stored curvature elastic stress on the binding properties of the lipidated N-Ras proteins. In most cases, significant differences were found for the various anchor motifs. In general, N-Ras proteins insert preferentially into a fluidlike, rather than a rigid, ordered lipid bilayer environment. Electrostatic interactions with lipid headgroups or stored curvature elastic stress of the membrane seem to have no drastic effect on the binding and dissociation processes of the lipidated proteins. The monofarnesylated N-Ras exhibits generally the highest association rate and fastest dissociation process in fluidlike membranes. Double lipidation, especially including farnesylation, of the protein leads to drastically reduced initial binding rates but strong final association. The change in the nucleotide loading of the natural N-Ras HD/Far induces a slightly different binding and dissociation kinetics, as well as stability of association, and seems to influence the tendency to segregate laterally in the membrane plane. The GDP-bound inactive form of N-Ras with an HD/Far anchor shows stronger membrane association, which might be due to a more pronounced tendency to self-assemble in the membrane matrix than is seen with the active GTP-bound form.     

Functionalization of peptide nucleolipid bioconjugates and their structure anti-cancer activity relationship studies

In the search for more potent peptide-based anti-cancer conjugates the generation of new, functionally diverse nucleolipid derived D-(KLAKLAK)₂-AK sequences has enabled a structure and anti-cancer activity relationship study. A reductive amination approach was key for the synthesis of alkylamine, diamine and polyamine derived nucleolipids as well as those incorporating heterocyclic functionality. The carboxy-derived nucleolipids were then coupled to the C-terminus of the D-(KLAKLAK)₂-AK killer peptide sequence and produced with and without the FITC fluorophore for investigating biological activity in cancer cells. The amphiphilic, α-helical peptide-nucleolipid bioconjugates were found to exhibit variable effects on the viability of MM.1S cells, with the histamine derived nucleolipid peptide bioconjugate displaying the most significant anti-cancer effects. Thus, functionally diverse nucleolipids have been developed to fine-tune the structure and anti-cancer properties of killer peptide sequences, such as D-(KLAKLAK)₂-AK.     

Development of novel membrane active lipidated peptidomimetics active against drug resistant clinical isolates

A new series of small cationic lipidated peptidomimetics have been synthesized and found to be highly active against several susceptible as well as drug resistant clinical isolates of bacteria and fungi. All lipidated peptidomimetics do not cause significant lysis of human erythrocytes (HC₅₀ > 200 μg/mL). Calcein dye leakage experiment revealed membranolytic effect of LPEP08 which was further confirmed by scanning electron microscopy (SEM). The involvement of intracellular targets as an alternate mode of action was precluded by DNA retardation assay. Additionally, LPEP08 exhibit high proteolytic stability and dose not elicit resistance against drug resistant clinical isolate of Staphylococcus aureus, even after 16 rounds of passaging. These results demonstrate the potential of lipidated peptidomimetics as biocompatible anti-infective therapeutics.     

Novel properties of antimicrobial peptide anoplin

Antimicrobial decapeptide anoplin was tested for its antifungal activity against plant pathogen Leptosphaeria maculans and protection of Brassica napus plants from disease. To reveal the mode of action of the peptide, a natural form of anoplin amidated on C-terminus (ANP-NH₂), and its carboxylated analog (ANP-OH) were used in the study. We demonstrated strong antifungal activity of anoplin in vitro regardless C-terminus modification. In addition we show that both ANP-NH₂ and ANP-OH induce expression of defence genes in B. napus and protects plants from L. maculans infection. The results indicate that the amidation of anoplin is not essential for its antifungal and plant defence stimulating activities.     

Enhancing of GM3 synthase expression during differentiation of human blood monocytes into macrophages as in vitro model of GM3 accumulation in atherosclerotic lesion

Several lines of evidence suggest that dysregulated lipid metabolism may participate in the pathogenesis of Alzheimer’s disease (AD). Epidemiologic studies suggest that elevated mid-life plasma cholesterol levels may be associated with an increased risk of AD and that statin use may reduce the prevalence of AD. Cellular studies have shown that the levels and distribution of intracellular cholesterol markedly affect the processing of amyloid precursor protein into Aβ peptides, which are the toxic species that accumulate as amyloid plaques in the AD brain. Most importantly, genetic evidence identifies apolipoprotein E, the major cholesterol carrier in the central nervous system, as the primary genetic risk factor for sporadic AD. In humans, apoE exists as three major alleles (apoE2, apoE3, and apoE4), and inheritance of the apoE4 allele increases the risk of developing AD at an earlier age. However, exactly how apoE functions in the pathogenesis of AD remains to be fully determined. Our studies have identified that the cholesterol transporter ABCA1 is a crucial regulator of apoE levels and lipidation in the brain. Deficiency of ABCA1 leads to the loss of approximately 80% of apoE in the brain, and the residual 20% that remains is poorly lipidated. Several independent studies have shown this poorly lipidated apoE increases amyloid burden in mouse models of AD, demonstrating that apoE lipidation by ABCA1 affects key steps in amyloid deposition or clearance. Conversely, robust overexpression of ABCA1 in the brain promotes apoE lipidation and nearly eliminates the formation of mature amyloid plaques. These studies show that the lipid binding capacity of apoE is a major mechanism of its function in the pathogenesis of AD, and suggest that increasing apoE lipidation may be of therapeutic importance for this devastating disease.     

Apolipoproteins A-I,A-II and E are independently distributed among intracellular and newly secreted HDL of human hepatoma cells

Whereas hepatocytes secrete the major human plasma high density lipoproteins (HDL)-protein, apo A-I, as lipid-free and lipidated species, the biogenic itineraries of apo A-II and apo E are unknown. Human plasma and HepG2 cell-derived apo A-II and apo E occur as monomers, homodimers and heterodimers. Dimerization of apo A-II, which is more lipophilic than apo A-I, is catalyzed by lipid surfaces. Thus, we hypothesized that lipidation of intracellular and secreted apo A-II exceeds that of apo A-I, and once lipidated, apo A-II dimerizes. Fractionation of HepG2 cell lysate and media by size exclusion chromatography showed that intracellular apo A-II and apo E are fully lipidated and occur on nascent HDL and VLDL respectively, while only 45% of intracellular apo A-I is lipidated. Secreted apo A-II and apo E occur on small HDL and on LDL and large HDL respectively. HDL particles containing both apo A-II and apo A-I form only after secretion from both HepG2 and Huh7 hepatoma cells. Apo A-II dimerizes intracellularly while intracellular apo E is monomeric but after secretion associates with HDL and subsequently dimerizes. Thus, HDL apolipoproteins A-I, A-II and E have distinct intracellular and post-secretory pathways of hepatic lipidation and dimerization in the process of HDL formation. These early forms of HDL are expected to follow different apolipoprotein-specific pathways through plasma remodeling and reverse cholesterol transport.     

Apolipoprotein B100 exit from the endoplasmic reticulum (ER) is COPII-dependent,and its lipidation to very low density lipoprotein occurs post-ER

Hepatic apolipoprotein B100 (apoB100) associates with lipids to form dense lipoprotein particles in the endoplasmic reticulum (ER) and is further lipidated to very low density lipoproteins (VLDL). Because the VLDL diameter can exceed 200 nm, classical ER-derived vesicles may be unable to accommodate VLDLs. Using hepatic membranes and cytosol to reconstitute ER budding, apoB100-containing vesicles sedimented distinct from those harboring more typical cargo but contained Sec23. Moreover, ER exit of apoB was inhibited by dominant-negative Sar1. Budding required Sar1 regardless of whether oleic acid (OA) was added to stimulate apoB lipidation; therefore, either large apoB100-lipoproteins reside in secretory vesicles, or full lipidation occurs post-ER. Using membranes from cells incubated in the presence or absence of OA, we determined that apoB100-lipoproteins in ER vesicles had not become lipidated to VLDLs. VLDL particles resided in the Golgi, but not the ER, after fractionation of OA-treated cells. We conclude that apoB100-lipoproteins exit the ER in COPII vesicles, but under conditions favorable for VLDL formation final lipid loading occurs post-ER.     

Structure and Activity Analysis of Inauhzin Analogs as Novel Antitumor Compounds That Induce p53 and Inhibit Cell Growth

Identifying effective small molecules that specifically target the p53 pathway in cancer has been an exciting, though challenging, approach for the development of anti-cancer therapy. We recently identified Inauhzin (INZ) as a novel p53 activator, selectively and efficiently suppressing tumor growth without displaying genotoxicity and with little toxicity to normal cells. In order to reveal the structural features essential for anti-cancer activity of this small molecule, we have synthesized a panel of INZ analogs and evaluated their ability to induce cellular p53 and to inhibit cell growth in cell-based assays. This study as described here leads to the discovery of INZ analog 37 that displays much better potency than INZ in both of p53 activation and cell growth inhibition in several human cancer cell lines including H460 and HCT116^+/+ cells. This INZ analog exhibited much less effect on p53-null H1299 cells and HCT116^−/− cells, and importantly no toxicity on normal human p53-containing WI-38 cells. Hence, our results not only unveil key chemical features for INZ activity, but also identify the newly synthesized INZ analog 37 as a better small molecule for further development of anti-cancer therapy.     

Role of the polypeptide region of a 33kDa mycobacterial lipoprotein for efficient IL-12 production

Mycobacterium leprae lipoprotein, LpK, induced IL-12 production from human monocytes. To determine the components essential for cytokine production and the relative role of lipidation in the activation process, we produced lipidated and non-lipidated truncated forms of LpK. While 0.5nM of lipidated LpK-a having N-terminal 60 amino acids of LpK produced more than 700pg/ml IL-12 p40, the non-lipidated LpK-b having the same amino acids as that of LpK-a required more than 20nM of the protein to produce an equivalent dose of cytokine. Truncated protein having the C-terminal 192 amino acids of LpK did not induce any cytokine production. Fifty nanomolar of the synthetic lipopeptide of LpK produced only about 200pg/ml IL-12. Among the truncated LpK, only LpK-a and lipopeptide stimulated NF-kB-dependent reporter activity in TLR-2 transfectant. However, when monocytes were stimulated with lipopeptide in the presence of non-lipidated protein, they produced IL-12 synergistically. Therefore, both peptide regions of LpK and lipid residues are necessary for efficient IL-12 production.     

Identification of vibsanin A analog as a novel HSP90 inhibitor

Vibsanin A is the first natural product isolated from Viburnum awabuki and has several biological activities. We have reported that a vibsanin A analog, obtained from process of total synthesis of vibsanin A, has anti-proliferative activity against human cancer cell lines. In this study, we evaluated anti-proliferative effect of the vibsanin A analogs against various human cancer cell lines and examined molecular target of the analog in human cells. Among the vibsanin A analogs, vibsanin A analog C (VAC) showed anti-proliferative effect against various cancer cell lines, and the anti-proliferative activity was strongest among the vibsanin A analogs. Additionally, VAC fluctuated amounts of HSP90-related proteins in cells and inhibited HSP90-mediated protein refolding of luciferase in vitro. These results suggest that the anti-proliferative activity of VAC is due to HSP90 inhibition, and VAC has a potential as novel anti-cancer drug as HSP90 inhibitor.     

Fluoro-modified chemotactic peptides: fMLF analogues.

A small library of peptide analogues of the chemotactic tripeptide For-Met-Leu-Phe-NH2 modified by substitution of Leu at position 2 by three different fluorinated amino acids varying in content of fluorine, length of the fluorinated side chain, and alkylation degree at the alpha-carbon atom was synthesized. The influence of the fluorine substitution on the biological activity was investigated by measuring the oxidative activity of neutrophils using a luminol-dependent chemiluminescence assay.     

Lysylated phospholipids stabilize models of bacterial lipid bilayers and protect against antimicrobial peptides

Aminoacylated phosphatidylglycerols are common lipids in bacterial cytoplasmic membranes. Their presence in Staphylococcus aureus has been linked to increased resistance to a number of antibacterial agents, including antimicrobial peptides. Most commonly, the phosphatidylglycerol headgroup is esterified to lysine, which converts anionic phosphatidylglycerol into a cationic lipid with a considerably increased headgroup size. In the present work, we investigated the interactions of two well-studied antimicrobial peptides, cecropin A and mastoparan X, with lipid vesicles composed of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) and 1-palmitoyl-2-oleoyl-phosphatidylglycerol (POPG), containing varying fractions of an aminoacylated phosphatidylethanolamine, a stable analog of the corresponding phosphatidylglycerol-derivative. To differentiate between the effects of headgroup size and charge on peptide–lipid interactions, we synthesized two different derivatives. In one, the headgroup was modified by the addition of lysine, and in the other, by glutamine. The modification by glutamine results in a phospholipid with a headgroup size comparable to that of the lysylated version. However, whereas lysyl-phosphatidylethanolamine (Lys-PE) is cationic, glutaminyl-phosphatidylethanolamine (Gln-PE) is zwitterionic. We found that binding of mastoparan X and cecropin A was not significantly altered if the content of aminoacylated phosphatidylethanolamines did not exceed 20 mol.%, which is the concentration found in bacterial membranes. However, a lysyl-phosphatidylethanolamine content of 20 mol% significantly inhibits dye release from lipid vesicles, to a degree that depends on the peptide. In the case of mastoparan X, dye release is essentially abolished at 20 mol.% lysyl-phosphatidylethanolamine, whereas cecropin A is less sensitive to the presence of lysyl-phosphatidylethanolamine. These observations are understood through the complex interplay between peptide binding and membrane stabilization as a function of the aminoacylated lipid content. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.     

Molecular dynamics simulation of the membrane binding and disruption mechanisms by antimicrobial scorpion venom-derived peptides

Pandinin 2 (Pin2) is an alpha-helical polycationic peptide, identified and characterized from venom of the African scorpion Pandinus imperator with high antimicrobial activity against Gram-positive bacteria and less active against Gram-negative bacteria, however it has demonstrated strong hemolytic activity against sheep red blood cells. In the chemically synthesized Pin2GVG analog, the GVG motif grants it low hemolytic activity while keeping its antimicrobial activity. In this work, we performed 12 μs all-atom molecular dynamics simulation of the antimicrobial peptides (AMPs) Pin2 and Pin2GVG to explore their adsorption mechanism and the role of their constituent amino acid residues when interacting with pure POPC and pure POPG membrane bilayers. Starting from an α-helical conformation, both AMPs are attracted at different rates to the POPC and POPG bilayer surfaces due to the electrostatic interaction between the positively charged amino acid residues and the charged moieties of the membranes. Since POPG is an anionic membrane, the PAMs adhesion is stronger to the POPG membrane than to the POPC membrane and they are stabilized more rapidly. This study reveals that, before the insertion begins, Pin2 and Pin2GVG remained partially folded in the POPC surface during the first 300 and 600 ns, respectively, while they are mostly unfolded in the POPG surface during most of the simulation time. The unfolded structures provide for a large number of intermolecular hydrogen bonds and stronger electrostatic interactions with the POPG surface. The results show that the aromatic residues at the N-terminus of Pin2 initiate the insertion process in both POPC and POPG bilayers. As for Pin2GVG in POPC the C-terminus residues seem to initiate the insertion process while in POPG this process seems to be slowed down due to a strong electrostatic attraction. The membrane conformational effects upon PAMs binding are measured in terms of the area per lipid and the contact surface area. Several replicas of the systems lead to the same observations.     

Truncated and constrained helical analogs of antimicrobial esculentin-2EM

Esculentin-2EM is a 37-residue, cationic, amphipathic, α-helical antimicrobial peptide isolated from a Korean frog, Glandirama emeljanovi. Many studies revealed that truncation of this peptide results in substantial decreases in its antimicrobial activity. Lee and his colleagues have recently reported that a 23-residue esculentin-2EM analog containing a tryptophanyl substitution at position 16 showed a significant recovery of the antimicrobial activity of the parent peptide. Here we report a new series of 15-residue esculentin-2EM analogs which are constrained into an α-helical conformation via an oct-4-enyl cross-link. The resulting ‘stapled’ derivatives displayed remarkable increases not only in antimicrobial activity but also in helical content and protease resistance compared to Lee’s original 23-residue esculentin-2EM analog. The preliminary data obtained in this work strongly supports the potential of our strategy for the development of a new class of peptide antibiotics.