Outer Leaflet Of Plasma Membrane Research Articles

Water-Soluble Pristine Fullerenes C<SUB>60</SUB> Increase the Specific Conductivity and Capacity of Lipid Model Membrane and form the Channels in Cellular Plasma Membrane

The aim of the present work was the investigation of the interaction of water-soluble pristine fullerenes C60 with bimolecular lipid model membrane (BLM) and cellular plasma membrane (PM). The findings demonstrate that the fullerenes C60 at low concentrations, namely in the concentration range of 1.0 x 10(-4) - 1.0 mg x ml(-1), are preferentially incorporated into the hydrophobic region of BLM, increasing, thus, its specific conductivity and specific capacity. Furthermore, upon interaction of fullerenes C60 with PM, pores or other type of local defects are formed, which are wide enough in order to permit the passage of water molecules across the cell membrane resulting to the externalization of phosphatidylserine from the inner to the outer PM leaflet.

Aqueous fraction of Nephelium ramboutan-ake rind induces mitochondrial-mediated apoptosis in HT-29 human colorectal adenocarcinoma cells.

The aim of this study was to investigate the cytotoxic and apoptotic effects of Nephelium ramboutan-ake (pulasan) rind in selected human cancer cell lines. The crude ethanol extract and fractions (ethyl acetate and aqueous) of N. ramboutan-ake inhibited the growth of HT-29, HCT-116, MDA-MB-231, Ca Ski cells according to MTT assays. The N. ramboutan-ake aqueous fraction (NRAF) was found to exert the greatest cytotoxic effect against HT-29 in a dose-dependent manner. Evidence of apoptotic cell death was revealed by features such as chromatin condensation, nuclear fragmentation and apoptotic body formation. The result from a TUNEL assay strongly suggested that NRAF brings about DNA fragmentation in HT-29 cells. Phosphatidylserine (PS) externalization on the outer leaflet of plasma membranes was detected with annexin V-FITC/PI binding, confirming the early stage of apoptosis. The mitochondrial permeability transition is an important step in the induction of cellular apoptosis, and the results clearly suggested that NRAF led to collapse of mitochondrial transmembrane potential in HT-29 cells. This attenuation of mitochondrial membrane potential (Δψm) was accompanied by increased production of ROS and depletion of GSH, an increase of Bax protein expression, and induced-activation of caspase-3/7 and caspase-9. These combined results suggest that NRAF induces mitochondrial-mediated apoptosis.

Fighting Infection with Liposomes

Asymmetric engineered liposomes are antimicrobial and anti-inflammatory.

Effect of iron-doped multi-walled carbon nanotubes on lipid model and cellular plasma membranes

The aim of the present work was the study of the interaction of multi-walled carbon nanotubes filled with iron (Fe-MWCNTs) with bimolecular lipid model membrane (BLM) and cellular plasma membrane (PM). The findings demonstrate that the Fe-MWCNTs adsorb on the BLM surface with possible partial build up in the hydrophobic area of fatty acid residues of lipids and increase its specific conductivity and capacity. Furthermore, upon interaction with the PM, the Fe-MWCNTs form channels which allow the flow of water to the cells and the externalization of phosphatidylserine from the inner to the outer PM leaflet.

Milk Fermented by Propionibacterium freudenreichii Induces Apoptosis of HGT-1 Human Gastric Cancer Cells

BackgroundGastric cancer is one of the most common cancers in the world. The “economically developed countries” life style, including diet, constitutes a risk factor favoring this cancer. Diet modulation may lower digestive cancer incidence. Among promising food components, dairy propionibacteria were shown to trigger apoptosis of human colon cancer cells, via the release of short-chain fatty acids acetate and propionate.Methodology/Principal FindingsA fermented milk, exclusively fermented by P. freudenreichii, was recently designed. In this work, the pro-apoptotic potential of this new fermented milk was demonstrated on HGT-1 human gastric cancer cells. Fermented milk supernatant induced typical features of apoptosis including chromatin condensation, formation of apoptotic bodies, DNA laddering, cell cycle arrest and emergence of a subG1 population, phosphatidylserine exposure at the plasma membrane outer leaflet, reactive oxygen species accumulation, mitochondrial transmembrane potential disruption, caspase activation and cytochrome c release. Remarkably, this new fermented milk containing P. freudenreichii enhanced the cytotoxicity of camptothecin, a drug used in gastric cancer chemotherapy.Conclusions/SignificanceSuch new probiotic fermented milk may thus be useful as part of a preventive diet designed to prevent gastric cancer and/or as a food supplement to potentiate cancer therapeutic treatments.

Functional Roles of Gangliosides in Neurodevelopment: An Overview of Recent Advances

Gangliosides are sialic acid-containing glycosphingolipids that are most abundant in the nervous system. They are localized primarily in the outer leaflets of plasma membranes and participated in cell-cell recognition, adhesion, and signal transduction and are integral components of cell surface microdomains or lipid rafts along with proteins, sphingomyelin and cholesterol. Ganglioside-rich lipid rafts play an important role in signaling events affecting neural development and the pathogenesis of certain diseases. Disruption of gangloside synthase genes in mice induces developmental defects and neural degeneration. Targeting ganglioside metabolism may represent a novel therapeutic strategy for intervention in certain diseases. In this review, we focus on recent advances on metabolic and functional studies of gangliosides in normal brain development and in certain neurological disorders.

Role of thiol pathways in TF procoagulant regulation

The generation of procoagulant Tissue Factor (TF) is crucial for thrombosis. TF contains a surface exposed allosteric disulfide bond that stabilizes the carboxyl-terminal domain involved in ligand interactions with coagulation factors VIIa and X. TF procoagulant activation typically occurs following cellular perturbations that also cause the appearance of procoagulant phosphatidylserine in the outer leaflet of cell membranes. However, thiol modifying agents, without suppressing phosphatidylserine exposure, can prevent TF activation, implicating thiol-disulfide exchange reactions in the regulation of TF procoagulant activity of primary cells. Protein disulfide isomerase (PDI), a regulator of extracellular thiol exchange, is associated with cell surface TF and required for TF-dependent thrombosis in vivo. PDI regulates the thiol-dependent biogenesis of procoagulant microparticles that are released from myeloid cells and smooth muscle cells following activation of the purinergic P2X7 receptor. Genetic deletion of P2X7 signaling attenuates FeCl3-induced carotid artery thrombosis in mice, indicating that TF prothrombotic activity is regulated by specific cell signaling pathways in vivo.

Relative ion yields in mammalian cell components using C60 SIMS.

Time of flight secondary ion mass spectrometry has been used to better understand the influence of molecular environment on the relative ion yields of membrane lipid molecules found in high abundance in a model mammalian cell line, RAW264.7. Control lipid mixtures were prepared to simulate lipid-lipid interactions in the inner and outer leaflet of cell membranes. Compared with its pure film, the molecular ion yields of 1,2-dioleoyl-sn-glycero-3-phosphocholine and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine are suppressed when mixed with 2-dipalmitoyl-sn-glycero-3-phosphocholine. In the mixture, proton competition between 1,2-dioleoyl-sn-glycero-3-phosphocholine, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, and 2-dipalmitoyl-sn-glycero-3-phosphocholine led to lower ionization efficiency. The possible mechanism for ion suppression was also investigated with 1H and 13C nuclear magnetic resonance spectroscopy. The formation of a hydroxyl bond in lipid mixtures confirms the mechanism involving proton exchange with the surrounding environment. Similar effects were observed for lipid mixtures mimicking the composition of the inner leaflet of cell membranes. The secondary molecular ion yield of 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine was observed to be enhanced in the presence of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine.

Interaction of Hexadecylbetainate Chloride with Biological Relevant Lipids

The present work investigates the interaction of hexadecylbetainate chloride (C(16)BC), a glycine betaine-based ester with palmitoyl-oleoyl-phosphatidylcholine (POPC), sphingomyelin (SM), and cholesterol (CHOL), three biological relevant lipids present in the outer leaflet of the mammalian plasma membrane. The binding affinity and the mixing behavior between the lipids and C(16)BC are discussed based on experimental (isothermal titration calorimetry (ITC) and Langmuir film balance) and molecular modeling studies. The results show that the interaction between C(16)BC and each lipid is thermodynamically favorable and does not affect the integrity of the lipid vesicles. The primary adsorption of C(16)BC into the lipid film is mainly governed by a hydrophobic effect. Once C(16)BC is inserted in the lipid film, the polar component of the interaction energy between C(16)BC and the lipid becomes predominant. Presence of CHOL increases the affinity of C(16)BC for membrane. This result can be explained by the optimal matching between C(16)BC and CHOL within the film rather by a change of membrane fluidity due to the presence of CHOL. The interaction between C(16)BC and SM is also favorable and gives rise to highly stable monolayers probably due to hydrogen bonds between their hydrophilic groups. The interaction of C(16)BC with POPC is less favorable but does not destabilize the mixed monolayer from a thermodynamic point of view. Interestingly, for all the monolayers investigated, the exclusion surface pressures are above the presumed lateral pressure of the plasma membranes suggesting that C(16)BC would be able to penetrate into mammalian plasma membranes in vivo. These results may serve as a useful basis in understanding the interaction of C(16)BC with real membranes.

Phosphoinositide Binding Differentially Regulates NHE1 Na+/H+ Exchanger-dependent Proximal Tubule Cell Survival

Tubular atrophy predicts chronic kidney disease progression, and is caused by proximal tubular epithelial cellcaused by proximal tubular epithelial cell (PTC) apoptosis. The normally quiescent Na(+)/H(+) exchanger-1 (NHE1) defends against PTC apoptosis, and is regulated by PI(4,5)P(2) binding. Because of the vast array of plasma membrane lipids, we hypothesized that NHE1-mediated cell survival is dynamically regulated by multiple anionic inner leaflet phospholipids. In membrane overlay and surface plasmon resonance assays, the NHE1 C terminus bound phospholipids with low affinity and according to valence (PIP(3) > PIP(2) > PIP = PA > PS). NHE1-phosphoinositide binding was enhanced by acidic pH, and abolished by NHE1 Arg/Lys to Ala mutations within two juxtamembrane domains, consistent with electrostatic interactions. PI(4,5)P(2)-incorporated vesicles were distributed to apical and lateral PTC domains, increased NHE1-regulated Na(+)/H(+) exchange, and blunted apoptosis, whereas NHE1 activity was decreased in cells enriched with PI(3,4,5)P(3), which localized to basolateral membranes. Divergent PI(4,5)P(2) and PI(3,4,5)P(3) effects on NHE1-dependent Na(+)/H(+) exchange and apoptosis were confirmed by selective phosphoinositide sequestration with pleckstrin homology domain-containing phospholipase Cδ and Akt peptides, PI 3-kinase, and Akt inhibition in wild-type and NHE1-null PTCs. The results reveal an on-off switch model, whereby NHE1 toggles between weak interactions with PI(4,5)P(2) and PI(3,4,5)P(3). In response to apoptotic stress, NHE1 is stimulated by PI(4,5)P(2), which leads to PI 3-kinase activation, and PI(4,5)P(2) phosphorylation. The resulting PI(3,4,5)P(3) dually stimulates sustained, downstream Akt survival signaling, and dampens NHE1 activity through competitive inhibition and depletion of PI(4,5)P(2).

Effect of the MACS technique on rabbit sperm motility

Abstract Magnetic-activated cell sorting (MACS) separates apoptotic spermatozoa by the use of annexin V-conjugated nanoparticles which bind to phosphatidylserine that is externalized on the outer leaflet of the sperm plasma membrane. This technique yields two fractions: annexin V-negative (AnV−) and annexin V-positive (AnV+). The aim of the study was to evaluate the effect of MACS application on the motility parameters of rabbit spermatozoa. Rabbit semen samples collected separately from 4 bucks (I, II, III, and IV) were filtered and separated in a MACS system. The semen samples from a control (untreated) group, AnV− and AnV+ fraction were evaluated using CASA system. The experiment was replicated 4 times for each buck. The AnV+ sperm had significantly lower concentration than the AnV− fractions and the control samples (P<0.05 for bucks I, II, III, but not IV). We observed that the proportion of apoptotic spermatozoa in the semen of NZW bucks is about 20%. There was no significant difference in the percentage of motile and progressively motile spermatozoa between the AnVfractions and control samples. In conclusion, the MACS technique has no harmful effect on the rabbit sperm concentration and motility.

Measurement of Lipid Nanodomain (Raft) Formation and Size in Sphingomyelin/POPC/Cholesterol Vesicles Shows TX-100 and Transmembrane Helices Increase Domain Size by Coalescing Preexisting Nanodomains But Do Not Induce Domain Formation

Mixtures of unsaturated lipids, sphingolipids, and cholesterol form coexisting liquid-disordered and sphingolipid and cholesterol-rich liquid-ordered (Lo) phases in water. The detergent Triton X-100 does not readily solubilize Lo domains, but does solubilize liquid-disordered domains, and is commonly used to prepare detergent-resistant membranes from cells and model membranes. However, it has been proposed that in membranes with mixtures of sphingomyelin (SM), 1-palmitoyl 2-oleoyl phosphatidylcholine (POPC), and cholesterol Triton X-100 may induce Lo domain formation, and therefore detergent-resistant membranes may not reflect the presence of preexisting domains. To examine this hypothesis, the effect of Triton on Lo domain formation was measured in SM/POPC/cholesterol vesicles. Nitroxide quenching methods that can detect ordered nanodomains with radii >12 Å showed that in the absence of Triton X-100 this mixture formed ordered state domains that melt with a midpoint (= Tmid) at ∼45°C. However, Tmid was lower when detected using various fluorescence resonance energy transfer (FRET) pairs. Furthermore, the Tmid value was Ro dependent, and decreased as Ro increased. Because FRET can only readily detect domains with radii >Ro, this result can be explained by domain radii that are close to Ro and decrease as temperature increases. An analysis of FRET and quenching data suggests that nanodomain radius gradually decreases from ≥150 Å to <40 Å as temperature increases from 10 to 45°C. Interestingly, the presence of Triton X-100 or a transmembrane-type peptide did not stabilize ordered state formation when detected by nitroxide quenching, i.e., did not increase Tmid. However, FRET-detected Tmid did increase in the presence of Triton X-100 or a transmembrane peptide, indicating that both increased domain size. Controls showed that the results could not be accounted for by probe-induced perturbations. Thus, SM/POPC/cholesterol, a mixture similar to that in the outer leaflet of plasma membranes, forms nanodomains at physiological temperatures, and TX-100 does not induce domain formation or increase the fraction of the bilayer in the ordered state, although it does increase domain size by coalescing preexisting domains.

Phosphatidylserine directly and positively regulates fusion of myoblasts into myotubes

Cell membrane consists of various lipids such as phosphatidylserine (PS), phosphatidylcholine (PC), and phosphatidylethanolamine (PE). Among them, PS is a molecular marker of apoptosis, because it is located to the inner leaflet of plasma membrane generally but it is moved to the outer leaflet during programmed cell death. The process of apoptosis has been implicated in the fusion of muscle progenitor cells, myoblasts, into myotubes. However, it remained unclear whether PS regulates muscle cell differentiation directly. In this paper, localization of PS to the outer leaflet of plasma membrane in proliferating primary myoblasts and during fusion of these myoblasts into myotubes is validated using Annexin V. Moreover, we show the presence of PS clusters at the cell–cell contact points, suggesting the importance of membrane ruffling and PS exposure for the myogenic cell fusion. Confirming this conclusion, experimentally constructed PS, but not PC liposomes dramatically enhance the formation of myotubes from myoblasts, thus demonstrating a direct positive effect of PS on the muscle cell fusion. In contrast, myoblasts exposed to PC liposomes produce long myotubes with low numbers of myonuclei. Moreover, pharmacological masking of PS on the myoblast surface inhibits fusion of these cells into myotubes in a dose-dependent manner.

Thiol pathways in the regulation of tissue factor prothrombotic activity

The transmembrane protein tissue factor (TF), the initiator of the extrinsic coagulation cascade and a risk factor for thrombotic disease, largely resides on the cell surface in an inactive, encrypted conformation. Whereas TF procoagulant activation, or decryption, is closely associated with the exposure of procoagulant phosphatidylserine in the outer leaflet of cell membranes, thiol pathways and protein disulfide isomerase (PDI) play increasingly recognized roles in regulating TF prothrombotic pathways. TF contains a solvent-exposed allosteric disulfide bond that stabilizes the carboxyl-terminal domain involved in ligand interactions with coagulation factors VIIa and X. PDI is a prime candidate to modify the allosteric disulfide by reduction, S-nitrosylation and glutathionation, implicated as regulators of TF procoagulant activity. In-vivo studies localize PDI to sites of thrombus formation and inhibition of PDI attenuates arterial thrombosis. PDI also contributes to the biogenesis of procoagulant microparticles that are released in the context of TF decryption following ATP-mediated activation of the purinergic P2X7 receptor. Genetic deletion of P2X7 signaling attenuates TF and PDI-dependent thrombosis, identifying a new connection between prothrombotic and proinflammatory pathways. Although the precise biochemical events of TF encryption and decryption require further study, PDI and thiol pathways emerge as important regulators of vascular thrombotic diseases.

Disabled‐2 modulates homotypic and heterotypic platelet interactions by binding to sulfatides

Disabled-2 (Dab2) inhibits platelet aggregation by competing with fibrinogen for binding to the α(IIb) β(3) integrin receptor, an interaction that is modulated by Dab2 binding to sulfatides at the outer leaflet of the platelet plasma membrane. The disaggregatory function of Dab2 has been mapped to its N-terminus phosphotyrosine-binding (N-PTB) domain. Our data show that the surface levels of P-selectin, a platelet transmembrane protein known to bind sulfatides and promote cell-cell interactions, are reduced by Dab2 N-PTB, an event that is reversed in the presence of a mutant form of the protein that is deficient in sulfatide but not in integrin binding. Importantly, Dab2 N-PTB, but not its sulfatide binding-deficient form, was able to prevent sulfatide-induced platelet aggregation when tested under haemodynamic conditions in microfluidic devices at flow rates with shear stress levels corresponding to those found in vein microcirculation. Moreover, the regulatory role of Dab2 N-PTB extends to platelet-leucocyte adhesion and aggregation events, suggesting a multi-target role for Dab2 in haemostasis.

Labeling Membrane Glycoproteins or Glycolipids with Fluorescent Wheat Germ Agglutinin

Glycoprotein and glycolipids are found as part of the outer leaflet of cellular plasma membranes. Those glycoproteins or glycolipids that contain sialic acid and N-acetylglucosamine residues can be labeled with wheat germ agglutinin (WGA), a plant lectin that exists as a dimer (mwt ~36,000) and is normally cationic. Fluorescently labeled WGA is commercially available with fluorescein, Alexa Fluor, or rhodamine moieties. Fluorescent WGA can also be used to stain the Golgi (the trans-Golgi) in fixed cells. Fluorescently labeled concanavalin A (Con A), which selectively binds to α-mannopyranosyl and α-glucopyranosyl residues, can also be used to label plasma membranes. This protocol describes the labeling of membrane glycoproteins or glycolipids with fluorescent WGA.

Novel applications for glycosylphosphatidylinositol-anchored proteins in pharmaceutical and industrial biotechnology

Glycosylphosphatidylinositol (GPI)-anchored proteins have been regarded as typical cell surface proteins found in most eukaryotic cells from yeast to man. They are embedded in the outer plasma membrane leaflet via a carboxy-terminally linked complex glycolipid GPI structure. The amphiphilic nature of the GPI anchor, its compatibility with the function of the attached protein moiety and the capability of GPI-anchored proteins for spontaneous insertion into and transfer between artificial and cellular membranes initially suggested their potential for biotechnological applications. However, these expectations have been hardly fulfilled so far. Recent developments fuel novel hopes with regard to: (i) Automated online expression, extraction and purification of therapeutic proteins as GPI-anchored proteins based on their preferred accumulation in plasma membrane lipid rafts, (ii) multiplex custom-made protein chips based on GPI-anchored cell wall proteins in yeast, (iii) biomaterials and biosensors with films consisting of sets of distinct GPI-anchored binding-proteins or enzymes for sequential or combinatorial catalysis, and (iv) transport of therapeutic proteins across or into relevant tissue cells, e.g., enterocytes or adipocytes. Latter expectations are based on the demonstrated translocation of GPI-anchored proteins from plasma membrane lipid rafts to cytoplasmic lipid droplets and eventually further into microvesicles which upon release from donor cells transfer their GPI-anchored proteins to acceptor cells. The value of these technologies, which are all based on the interaction of GPI-anchored proteins with membranes and surfaces, for the engineering, production and targeted delivery of biomolecules for a huge variety of therapeutic and biotechnological purposes should become apparent in the near future.

Smase D Activates Voltage-Gated Cation Channels

Voltage-gated ion channels generate electric impulses in nerve, muscle and endocrine cells. These channels open in response to membrane depolarization. Previously studies from our group show that extracellular application of sphingomyelinase (SMase) D shifts the G-V curve of voltage-gated K+ channels in the hyperpolarized direction, thereby activating the channels in hyperpolarized potentials where they otherwise remain deactivated. SMase D hydrolyzes the phospholipid sphingomyelin which is primarily present in the outer leaflet of cell membranes. In doing so, it removes the positively charged choline group of sphingomyelin. Enzymatic removal of the positively charged choline groups makes it energetically easier for the positively charged voltage sensor to move outwardly, assuming an activated conformation. We have recently found that extracellular application of SMase D also shifts the conductance-voltage (G-V) relation of voltage-gated Na+ and Ca2+ channels in the hyperpolarized direction. Thus, SMase D apparently activates all three major classes of voltage-gated cation channels at hyperpolarized potentials where they otherwise remain closed.

Structures, Biosynthesis, and Functions of Gangliosides-an Overview

Gangliosides are sialic acid-containing glycosphingolipids that are most abundant in the nervous system. Heterogeneity and diversity of the structures in their carbohydrate chains are characteristic hallmarks of these lipids; so far, 188 gangliosides with different carbohydrate structures have been identified in vertebrates. The molecular structural complexity increases manifold if one considers heterogeneity in the lipophilic components. The expression levels and patterns of brain gangliosides are known to change drastically during development. In cells, gangliosides are primarily, but not exclusively, localized in the outer leaflets of plasma membranes and are integral components of cell surface microdomains with sphingomyelin and cholesterol from which they participate in cell-cell recognition, adhesion, and signal transduction. In this brief review, we discuss the structures, metabolism and functions of gangliosides.

Human Monocytes and Monocyte-Like Cells Are Highly Sensitive to Plitidepsin-Induced Cell Death In In Vitro Assays

AbstractAbstract 4942Plitidepsin (Aplidin®) is a marine-derived cyclic depsipeptide with strong cytotoxic effects against a variety of cancer cell types. It is currently in Phase II/III clinical trials for solid and hematologic malignancies. We evaluated the cytotoxic effects of Plitidepsin on lymphoid and monocyte cells from healthy donors and analysed the mechanisms involved. We found that monocyte cells showed a markedly higher sensitivity to Plitidepsin compared to lymphoid subsets as determined by incorporation of 7-AAD and flow cytometry analysis. Plitidepsin induced a dose- and time-dependent cell death on freshly isolated monocytes (7-AAD+ cells at 24 h: control: 3±1% versus Plitidepsin 10 nM: 47±5%, mean±SEM, n=6) and on macrophages (5±2 vs 49±3, n=5) and dendritic cells (4±1 vs 21±4, n=5) differentiated in vitro. By contrast, resting or mitogen-activated lymphocytes were not affected by Plitidepsin at 10 nM. The mechanisms of induced cell death in monocytes involved the early exposure of phosphatidylserine to the outer leaflet of plasma membrane, activation of caspase-3 and subsequent PARP fragmentation, indicative of death via apoptosis. Incubation of monocytes with Plitidepsin (10 or 100 nM) for 30 minuntes induced ROS production. On the other hand, the cytotoxic effect of Plitidepsin was significantly diminished when monocytes were pre-incubated with the antioxidant reagent Ebselen, which acts as an scavenger of peroxynitrite. We also determined Plitidepsin activity on malignant cells from chronic myelomonocytic leukemia (CMML) and chronic lymphocytic leukemia (CLL) patients. Monocyte-like cells obtained from 3 CMML samples resulted sensitive to Plitidepsin, though to a different extent (viable CD14+ cells at 24 h: control 100%, incubated with Plitidepsin 10 nM: sample 1: 79%, sample 2: 82%, sample 3: 29%). Finally we evaluated the effects of Plitidepsin on nurse-like cells (NLC) from CLL patients. These cells represent a subset differentiated from monocytes that favours leukemic cell progression through pro-survival signals. NLC were very sensitive to Plitidepsin and, more importantly, their death indirectly decreased neoplasic clone viability (7-AAD+ leukemic B cells co-cultured with control NLC: 36±10 versus 7-AAD+ leukemic B cells co-cultured with NLC exposed to Plitidepsin 100 nM for 24 h: 47±6, n=5, p<0.05). Myeloid cells are present in tumor microenvironment and actively affect the malignant cells, both directly and indirectly via the suppression of host immunity. Given their relevance in supporting tumor growth, the sensitivity of monocyte-like cells to Plitidepsin may contribute to its antitumoral effects. Disclosures:Galmarini: PharmaMar: Employment, Equity Ownership. Giordano: PharmaMar: Research Funding.