Alterations in plasma membrane lipid organization during lymphocyte differentiation

Potential of Cyanine Derived Dyes in Photodynamic Therapy

Photodynamic therapy (PDT) is a method of cancer treatment that leads to the disintegration of cancer cells and has developed significantly in recent years. The clinically used photosensitizers are primarily porphyrin, which absorbs light in the red spectrum and their absorbance maxima are relatively short. This review presents group of compounds and their derivatives that are considered to be potential photosensitizers in PDT. Cyanine dyes are compounds that typically absorb light in the visible to near-infrared-I (NIR-I) spectrum range (750-900 nm). This meta-analysis comprises the current studies on cyanine dye derivatives, such as indocyanine green (so far used solely as a diagnostic agent), heptamethine and pentamethine dyes, squaraine dyes, merocyanines and phthalocyanines. The wide array of the cyanine derivatives arises from their structural modifications (e.g., halogenation, incorporation of metal atoms or organic structures, or synthesis of lactosomes, emulsions or conjugation). All the following modifications aim to increase solubility in aqueous media, enhance phototoxicity, and decrease photobleaching. In addition, the changes introduce new features like pH-sensitivity. The cyanine dyes involved in photodynamic reactions could be incorporated into sets of PDT agents.

Inhibitory effects of merocyanine 540-mediated photodynamic therapy on cellular immune functions: A role in the prophylaxis of graft-versus-host disease?

Merocyanine 540-mediated photodynamic therapy (MC540-PDT) has been used in clinical trials for the purging of autologous hematopoietic stem cell grafts. When the same combinations of dye and light were applied to human peripheral blood lymphocytes, a broad range of T- and B-cell functions were impaired, prompting speculations about a potential role of MC540-PDT in the prophylaxis of graft-versus-host disease (GVHD). We here report on the effects of MC540-PDT on in vitro functions of murine lymphocytes as well as a preliminary evaluation of MC540-PDT for the prevention of GVHD in murine models of allogeneic bone marrow transplantation. Mixed lymphocyte reactions, proliferative responses to lectins, interleukin-2 and lipopolysaccharide, T-cell-mediated lysis, and NK activity were all inhibited by moderate doses of MC540-PDT. Whether MC540-PDT reduced the incidence and/or the severity of GVHD in murine models of allogeneic hematopoietic stem cell transplantation depended on the composition of the mismatched grafts and the intensity of the preparative regimen. MC540-PDT was only beneficial (i.e. reduced the incidence and/or severity of GVHD) when the spleen cell content of grafts was low and/or the radiation dose of the preparative regimen was not myeloablative, and, therefore, may have encouraged mixed chimerism.

Phosphatidylserine, a death knell

Virtually every cell in the body restricts phosphatidylserine (PS) to the inner leaflet of the plasma membrane by energy-dependent transport from the outer to the inner leaflet of the bilayer. Apoptotic cells of all types rapidly randomize the asymmetric distribution, bringing PS to the surface where it serves as a signal for phagocytosis. A myriad of phagocyte receptors have been implicated in the recognition of apoptotic cells, among them a PS receptor, yet few ligands other than PS have been identified on the apoptotic cell surface. Since apoptosis and the associated exposure of PS on the cell surface is probably over 600 million years old, it is not surprising that evolution has appropriated aspects of this process for specialized purposes such as blood coagulation, membrane fusion and erythrocyte differentiation. Failure to efficiently remove apoptotic cells may contribute to inflammatory responses and autoimmune diseases resulting from chronic, inappropriate exposure of PS.

Anomalous changes in forward scatter of lymphocytes with loosely packed membranes

BACKGROUND

Forward scatter (FSC) is generally associated with cell size and has been suggested as a way to differentiate apoptotic from viable cells. Among spleen cells cultured for 48 h, a population of cells (population B) was found to have decreased forward and increased side scatter relative to freshly purified cells (population A). Interestingly, population B was not present early in analysis; this report explores the change in FSC of population B.

METHODS

Using a Coulter (Hialeah, FL) Epics Elite ESP flow cytometer, changes in forward scatter and lipid packing of spleen cells were measured.

RESULTS

Over time, the FSC of unfixed cells in population B increased from that of the debris field, to reach a stable value by 30 sec (population A's FSC remained constant). When fixed, populations A and B exhibited constant FSC. Population B cells displayed altered lipid packing as reported by MC540, and the FSC changes were mimicked by Nonidet P-40 treatment of freshly purified spleen cells.

CONCLUSIONS

Data emphasize the importance of delaying measurements on unfixed cells until FSC readings have stabilized, and suggest that flow cytometry may be a useful tool in studying lipid packing.

Binding and internalization of an ICAM-1 peptide by the surface receptors of T cells

The objective of this work was to evaluate the binding characteristics of a cyclic peptide, cyclo (1, 12)-Pen1-Pro2-Arg3-Gly4-Gly5-Ser6-Val7-Leu8-V al9-Thr10-Gly11-Cys12-OH (cIBR), to Molt-3 T cells. This cIBR peptide is derived from sequence numbers 11-20 of intercellular adhesion molecule-1 (ICAM-1). Binding studies were performed using a fluorescence-labeled peptide (FITC-cIBR) in which the fluorescence marker fluorescein 5-isothiocyanate (FITC) was conjugated to the N-terminal of the cIBR peptide. The binding affinity of the FITC-cIBR peptide to Molt-3 T cells was evaluated using a FACScan flow cytometer. The binding specificity of the FITC-cIBR peptide was also confirmed by inhibition of binding using unlabeled peptide (cIBR). The results show that FITC-cIBR binds to two populations of T cells with different affinities; population 1 has high cell numbers (75%) but low affinity, and population 2 has high binding affinity but low cell numbers (25%). Binding to both populations was saturable and could be inhibited by the unlabeled peptide (cIBR), suggesting a receptor-mediated binding process. In addition to binding, receptor-mediated internalization was also observed for population 2; this was confirmed by confocal microscopy and temperature-dependence studies at 37 degrees C and 4 degrees C. The binding and internalization of this peptide may be carried out by surface receptors on Molt-3 T cells such as LFA-1. In the future, the binding and internalization of cIBR peptide can be utilized as a method of targeted drug delivery to leukocytes for the treatment of leukocyte-related diseases.

Use of merocyanine 540 for the isolation of quiescent, primitive human bone marrow hematopoietic progenitor cells

Merocyanine 540 (MC540) is a membrane probe that inserts preferentially into loosely packed domains in the phospholipid bilayer of intact cells. Previous experiments have demonstrated that MC540 will bind to human bone marrow (BM) hematopoietic progenitor cells (HPC). Fractions of mononuclear BM cells expressing high MC540 fluorescence have been shown to be enriched for myeloid progenitors and cells residing in the S/G2 + M phases of the cell cycle. We rationalized that MC540 uptake could be used to distinguish between quiescent and metabolically active cells and, therefore, to fractionate normal and leukemic BM cells and normal mobilized peripheral blood (MPB) cells into functionally distinct groups of progenitors. BM and MPB cells were separated into fractions ranging in fluorescence from MC540Bright to MC540Dim. Cell cycle analysis of these fractions revealed that the MC540Dim fraction of normal and CML BM CD34+ cells constituted the most quiescent fraction, and the MC540Bright fractions from these cell types contained the most actively cycling cells. However, no differences in the percentage of cells in G/G1 were observed between MC540Bright and MC540Dim fractions of MPB CD34+ cells. To investigate if these cell cycle status differences translated into distinct functional properties, the hematopoietic potential of BM CD34+MC540Bright and CD34+MC540Dim cell fractions was analyzed in vitro in long-term BM cultures and limiting dilution analysis (LDA) assays. CD34+MC540Dim cells produced more total and committed progenitor cells in long-term cultures than did the CD34+MC540Bright fraction. The CD34+MC540Dim fraction also contained a 2-fold higher number of long-term hematopoietic culture-initiating cells (LTHCIC) than the CD34+MC540Bright fraction, as defined by LDA assays. These data demonstrate that MC540 can be a useful probe for the isolation of primitive HPC from some hematopoietic tissues and may assist in monitoring structural changes in the phospholipid bilayer during proliferation and differentiation of HPC.

Merocyanine 540 mediated photolysis of normal bone marrow, committed hemopoietic progenitors and neoplastic cells. implications for bone marrow purging

The effect of merocyanine 540 (Mc 540) mediated photoirradiation on both neoplastic and normal hemopoietic progenitor cells was studied. Bone marrow (BM) cells from children with acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML) at initial diagnosis, ALL in remission, neuroblastoma and normal children as well as cells of Reh-6 and HL-60 cell lines were incubated with Mc 540 in the presence of human albumin (HA) and exposed to different argon laser 514 nm doses. Cell survival was estimated using Trypan Blue supravital stain following a 24-h incubation and leukemic cell lines were studied in continuous cell cultures of 4 weeks duration. Our results showed that HA protects normal BM cells from Mc 540 mediated phototoxicity. A 99.9999% inhibition of Reh-6 and HL-60 was noted at irradiation doses where the corresponding mean survival of normal BM cells was 77.4 +/- 12 and 70.3 +/- 10%, respectively. BM leukemic cells from children with ALL and AML were also very sensitive to Mc 540 photoirradiation in contrast to neuroblastoma cells where only a three-fold reduction was observed. Finally, the survival of normal BM progenitors was 38% for colony forming unit erythroid CFU-E, 37% for burst forming unit erythroid BFU-E, 55% for CFU-GM and 29% for CFU-GEMM. In conclusion it seems that Mc 540 mediated photoirradiation in neoplastic cells exerts selective cytotoxicity and can be used in ex vivo purging of malignant cells in BM.

A study of the binding of merocyanine 540 to myeloid leukemia M1 cells using an intensified charge-coupled device for fluorescence imaging microscopy

The binding of merocyanine 540 (MC540) to murine myeloid leukemia (M1) cells and normal erythrocytes was measured by fluorescence digital imaging microscopy using an intensified charge-coupled device. It was found that, on average, about three times more MC540 were bound to a unit membrane area of M1 cells than erythrocytes, a result consistent with previous studies. However, it was shown for the first time that MC540 binding varied significantly from one M1 cell to the next, and about 15% of the sensitized M1 cells were as MC540-negative as normal erythrocytes. Using the leukemic inhibitory factor as a differentiation inducer, M1 cells were induced to differentiate into mature macrophage-like cells in vitro. Such treatment lowered the average MC540 binding by about one-third but did not affect the cell-to-cell variation significantly.

Flow cytometric ratio analysis of the Hoechst 33342 emission spectrum: multiparametric characterization of apoptotic lymphocytes

The apoptotic response to various stimuli is an important part of immune regulation, and the ability to identify apoptotic lymphocytes within a complex population is a prerequisite to a more detailed understanding of its role in vivo, We described a flow cytometric technique which utilizes viable cells and enables simultaneous identification of apoptotic cells and analyses of immunophenotype, cell cycle progression, membrane integrity and light scatter properties. It is based upon analysis of two regions of the emission spectrum of the DNA-binding vital dye hoechst 33342. We established a precise correlation between the ratio of red to blue fluorescence emission and apoptosis based upon nuclear morphology and the presence of characteristic DNA degradation patterns. In human peripheral blood lymphocytes (PBL) and mouse thymocytes we incorporated light scatter properties, cell cycle stage, relevant cell surface immunophenotypic markers (CD25 or CD4) and CD8) and a marker of plasma membrane integrity (merocyanine 540) to enable multiparametric phenotyping of apoptotic cells. We show that staurosporine-induced apoptosis of ConA-stimulated PBL is not correlated with cell cycle stage but is selective for activated cells since the frequency of large, CD25+ cells is decreased by staurosporine. Dexamethasone and ionomycin differ in their ability to induce apoptosis selectively in murine thymocyte subsets. Dexamethasone kills a broad spectrum of the CD4/8 immunophenotypes with no selectively for cell cycle stage. Ionomycin selectively deplete CD4+8+ cells, especially those in the Go/G1 region of the cell cycle, and spared CD4-8+ cells. This technique is broadly advantageous for in vitro and ex vivo models of apoptosis in that it interrogates individual viable cells and correlates membrane and nuclear apoptotic changes with standard flow cytometric immunophenotyping.

Merocyanine 540 mediated photoirradiation of leukemic cells. In vitro inference on cell survival

In order to evaluate the selective killing of merocyanine 540 (MC 540) mediated photoirradiation in neoplastic cells, bone narrow cells from children with leukaemia or neuroblastoma and normal children as well as peripheral blood cells and Reh-6 and HL-60 cell lines were studied. Cell suspensions were incubated with MC 540 and exposed to various argon laser 514 nm doses. Cell survival was estimated with trypan blue supravital stain following a 24 h incubation and has been followed in continuous cell cultures of 4 weeks duration. Our results showed that the inhibition of survival of neoplastic haemopoietic cells by laser in the presence of MC 540 is proportional to the MC 540 and photoirradiation doses. A 99.9999% inhibition of Reh-6 and HL-60 was noted at irradiation doses where the corresponding mean survival of normal bone narrow cells was (33.6 +/- 15.5)% and (50.6 +/- 10.7)% respectively. Peripheral blood mononuclear cells were not sensitive to MC 540 mediated photoirradiation. The inhibition of survival of bone marrow metastatic neuroblastoma cells was (69.9 +/- 4.1)%. In conclusion, it seems that MC 540 mediated photoirradiation in neoplastic cells exerts selective cytotoxicity and can be used in ex vivo purging of malignant cells in the bone marrow.

Geometry of domains and domain boundaries in monolayers and bilayers

Domains in lipid bilayers may exist when two or more phases are allowed. However, the geometry of domains are governed by additional constraints. The most direct way to study the geometry of domains is by microscopy. Electron diffraction patterns that are characteristic of particular molecular packings are used to enhance the contrast of domains by electron microscopy. Surface features associated with lipid phases are utilized in freeze-fracture electron microscopy to define domains. X-ray microanalysis helps to identify the composition of lipids in domains. Phase-specific fluorescence dyes such as merocyanine 540 (MC540) are useful in delineating domains in monolayers and bilayers. MC540 is also sensitive in detecting molecular packing stress below the onset of the bilayer-hexagonal phase transition. Domains in monolayers and bilayers, mapped by different microscopic techniques, are compared.

Back and forth: the regulation and function of transbilayer phospholipid movement in eukaryotic cells

That some membranes restrict certain lipid species to one side of the bilayer and others to the opposite side has been known for two decades. However, how this asymmetric transbilayer distribution is generated and controlled, how many and what type of membranes are so structured, and even the reason for its existence is just now beginning to be understood. It has been a decade since the discovery of an activity which transports in an ATP-dependent manner only the aminophospholipids from the outer to the inner leaflet of the plasma membrane. This aminophospholipid translocase has yet to be isolated, reconstituted, and identified molecularly. Elevating intracellular Ca2+ allows all the major classes of phospholipids to move freely across the bilayer, scrambling lipids and dissipating asymmetry. The nature of this pathway and its mode of activation by Ca2+ remain to be determined. Though loss of transbilayer asymmetry by blood cells clearly produces a procoagulant surface and increases interactions with the reticuloendothelial system, it remains to be elucidated whether maintenance of blood homeostasis is just one expression of a more general raison d'être for lipid asymmetry. It is these persisting uncertainties and gaps in our knowledge which make the field such an interesting and exciting challenge at the present time.

Monoclonal antibodies directed against the E2 protein (MIC2 gene product) induce exposure of phosphatidylserine at the thymocyte cell surface

Monoclonal antibodies (mAbs) directed against E2, a 32-kDa transmembrane protein encoded by the MIC2 gene located in the pseudoautosomal region, induce a transbilayer movement of phosphatidylserine and, to a lesser extent, phosphatidylethanolamine in human thymocytes and a Jurkat T lymphocytes. The translocation of phosphatidylserine has been evidenced by using either derivatization of anionic phospholipids with trinitrobenzenesulfonate (TNBS) or cytofluorimetry after labeling of cells with antiphosphatidylserine antibodies. The perturbation of membrane phospholipids induced by anti-E2 mAbs was further evidenced by labeling the cells with merocyanine 540. The specificity of anti-E2-induced perturbations of membrane asymmetry was tested by using a number of mAbs able to activate T cells, including CD3 and CD2. The results strongly suggest that anti-E2-induced changes in PtdSer are related to cell aggregation since the same mAbs specifically induce the aggregation of both thymocytes and Jurkat cells and since the E2 molecule has been previously implicated in the adhesive properties of human T cells with erythrocytes.

Increased membrane permeability of apoptotic thymocytes: a flow cytometric study

We have recently developed a method for the separation and quantification of viable apoptotic cells without the need for permeabilisation or fixation of the cells. The method is based on the observation that apoptotic rat thymocytes fluoresce more brightly than normal cells after a brief incubation with the DNA binding dye, Hoechst 33342. In order to understand these differences, we have investigated the uptake of Hoechst 33342 by normal and apoptotic thymocytes. By staining with fluorescein diacetate, we have shown that the efflux of fluorescein from apoptotic cells is more rapid than that from normal thymocytes. This result demonstrated an increase in the permeability of the plasma membrane of the apoptotic thymocytes and it is this change which probably results in the more rapid uptake of Hoechst 33342. The data also revealed two populations of apoptotic thymocytes.

Plasma membrane lipid packing and leukocyte function-associated antigen-1-dependent aggregation of lymphocytes

A simple model system for study of adhesion mediated by leukocyte function-associated antigen-1 (LFA-1) is aggregation of lymphocytes stimulated in vitro. Although aggregation is blocked by monoclonal antibodies to LFA-1, not all lymphocytes expressing LFA-1 aggregate, indicating that LFA-1 is necessary but not sufficient for aggregation. To investigate whether the lipid bilayer plays a role in the functional activation of LFA-1, human peripheral blood lymphocytes and murine splenic lymphocytes were stimulated in culture, and measurements made of aggregation vs. packing of plasma membrane lipids. Progression of cells into aggregates was paralleled by a decrease in lipid packing of the population as a whole, as monitored by increased staining with the fluorescent probe merocyanine 540. Cells from aggregates stained more intensely than nonaggregated cells from the same population, indicating that aggregates are preferentially formed from cells in the population with the loosest packed membrane. In contrast, aggregated cells were found to express equivalent or even lower amounts of LFA-1 than nonaggregated cells. Looser lipid packing is therefore associated with the development of LFA-1-dependent aggregation, and might be involved in the functional activation of this cell adhesion molecule.

The response of human lymphocytes to phytohemagglutinin is impaired at different levels during aging

Several parameters generally believed to be necessary for the activation and progression of proliferation of human lymphocytes have been investigated and compared with special reference to aging. The responding capacity of plasma membrane potential to depolarizing and also repolarizing conditions induced by exposure to mitogens like PHA was lower in lymphocytes from old donors as compared to those of young ones. This indicates a significant age-dependent difference in the readiness to respond to channel-activating perturbations. As an early signal of activation, after one hour PHA stimulation the merocyanine 540 uptake by the lipid regions was chosen, based on the property of this fluorescent probe to bind to loosely packed lipids of the plasma membrane. The proteins encoded by the c-myc and c-myb genes were chosen as markers of the G0/G1 and G1/S phased transition, respectively. The mean number of cells that increased the uptake of MC 540 following mitogenic stimulation did not differ in young vs. old individuals. However, 4 samples out of 10 from the old population showed lower MC 540 fluorescence than the lowest signal from the young population. The number of responding cells was decreased during aging when the presence of the c-myc protein was taken as its measure; and this decrease was further accentuated, determining the expression of the c-myb protein. This frequently encountered age-dependent pattern, however, was not followed by the lymphocytes of all old donors. One example is reported in which the MC 540 uptake, the c-myc and c-myb expression in the cells from one old subject fell in the range of the young subjects. However, even in this case, the response of the lymphocytes as measured by 3H-thymidine incorporation was only 64% of that of young subjects. For this sample, we found an impairment of the response at the mitochondrial level. In addition to these parameters, the amount of 3H-thymidine incorporated by the cells expressing the c-myb protein was calculated. The values in old individuals were lower than those in the young, suggesting that not all the cells expressing the c-myb protein were able to synthesize DNA in lymphocyte populations from the elderly. Our data support the view that the age-dependent decline of lymphocyte responsiveness to mitogens can be accounted for by impairments at different levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Myogenic differentiation of the muscle clonal cell line BC3H-1 is accompanied by changes in its lipid composition

Phospholipid and neutral lipid composition was studied in the course of myogenic differentiation of the clonal cell line BC3H-1. Total phospholipid content increased during differentiation, predominantly in the major classes of choline and ethanolamine glycerophospholipids. The contents of other lipids, such as triacylglycerols, diminished more than 50% during this period. The content and distribution of fatty acids also underwent marked differentiation-dependent changes. The polyunsaturated (tetrapenta- and hexaenoic) fatty acid species of several phospholipid classes diminished during differentiation, especially those in choline, serine and inositol glycerophospholipids. Most noticeable were the changes in phosphatidylserine; long-chain fatty acids having 20 to 22 carbon atoms and 4 to 6 double bonds decreased from about 30 to about 10 mol%. Although increased levels of saturation in other phospholipid fatty acyl chains appear to accompany the myogenic changes of BC3H-1 cells, some unsaturated fatty acids, such as oleic acid (18:1), increased by as much as 80% during the same period, suggesting the activation of a delta 9 desaturase. Sphingomyelin contained only saturated and monoenoic fatty acids and exhibited a four- to five-fold decrease in its content of monoenoic acyl groups. Diacylglycerols became enriched in arachidonate and docosahexaenoate. The amount of cholesterol and its esters increased slightly during differentiation of BC3H-1 cells. The data show that several metabolic pathways change during myogenic differentiation of the BC3H-1 clonal cell line, particularly de novo biosynthetic pathways, elongation/desaturation reactions, and acyl chain turnover.(ABSTRACT TRUNCATED AT 250 WORDS)

Limited cell-cycle dependence of the merocyanine 540-sensitized photoinactivation of L1210 leukemia cells

L1210 leukemia cells were synchronized by a double thymidine block technique and then characterized with regard to their susceptibility to merocyanine 540 (MC540)-sensitized photoinactivation. Cells harvested 5 (G2/M phase) h after release from the second thymidine block were most susceptible to MC540-sensitized photoinactivation followed, in order of decreasing sensitivity, by cells harvested 2 (S phase) h and by cells harvested 7 (G1 phase) h after release from the second block. The expression of dye-binding sites changed very little during the cell cycle.

Stearoyl-CoA desaturase gene expression in lymphocytes

B lymphocytes from the spleens of normal (BALB/c) and autoimmune (MRL/lpr) strains of mice express the SCD-2 form of stearoyl-CoA desaturase as opposed to the SCD-1 form of the gene which is expressed in liver. However, whereas BALB/c T cells did not express SCD-1 or SCD-2, both BALB/c thymocytes and MRL/lpr T cells expressed SCD-2, suggesting a developmental down-regulation of SCD-2 within the T cell lineage. Northern analyses also revealed the expression of SCD-2 in the T cell lines BW5147, CTLL-2 and HT-2 and in BCL1, a B cell line. SCD-1 expression was not detected in any of the lymphoid cells tested. Finally, we show that SCD-2 gene expression is inhibited by arachidonic acid (20:4). These results demonstrate the complexity of SCD-2 regulation in lymphoid cells.

Merocyanine 540 as a probe to monitor the molecular packing of phosphatidylcholine: a monolayer epifluorescence microscopy and spectroscopy study

The characteristics of the fluorescent dye, merocyanine 540 (MC-540), incorporated in monolayers of 1,2-dipalmitoyl-phosphatidylcholine (DPPC), and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) were studied in different states of molecular packing. Conditions for phase separation in these monolayers were defined by their pressure/area (pi-A) isotherms. Within the liquid expanded (LE) and the liquid condensed (LC) coexisting phases of DPPC monolayers, low light level epifluorescence microscopy revealed 'dark' discoid domains embedded in a 'bright' matrix. Under the same conditions, and at temperatures as low as 12 degrees C, the pi-A isotherms of POPC demonstrate the existence of a single phase, and no fluorescent domains were observed. Fluorescence spectra of MC-540 labelled monolayers, recorded in different structural states, reveal three distinct emission peaks: a 572 nm peak, present for monolayer packing conditions at low surface pressures; a 585 nm peak, similar to that obtained from dye molecules in fluid phase lipid bilayers, and observed here within the respective area/molecule ranges of 54-62 A2 and 62-69 A2 for monolayers of DPPC and POPC with diminishing intensity at increasing surface pressure; and finally, a peak at 560 nm, which predominates in densely packed POPC monolayers. Our results are interpreted on the basis of dye partitioning between monolayer and subphase, and different orientations of the dye with respect to the monolayer in various structural states. The usefulness of MC-540 to differentiate lipid packing in cell membranes is discussed.

Phytohemagglutinin induced changes of membrane lipid packing, c-myc and c-myb encoded protein expression in human lymphocytes during aging

Three parameters which signal different stages of cell activation were analyzed in lymphocytes from young and old subjects. Merocyanine 540 (MC-540) incorporation into the membrane lipid phase was used as a very early marker of activation and was measured after 1 h of phytohemagglutinin (PHA) stimulation. The proteins coded by c-myc and c-myb protooncogenes were determined by appropriate antibodies and were taken as markers of the G0/G1 and G1/S phase transition, respectively. The number of cells which increased the uptake of MC-540 following PHA stimulation did not differ when comparing young and old individuals. Both the number of the responding cells and the size of the response were decreased during aging when the presence of the c-myc protein was taken into account. A consistent decrease of the percentage of lymphocytes able to express the c-myb protein was observed in the cells from old donors as compared to those from the young ones, but the amount of detectable protein per cell remained unchanged. Our data suggest that the deficiency of responsiveness which accompanies aging is due to impairments at different points of the cell cycle. The very low number of cells expressing the c-myb protein is likely the result of step by step elimination of those cells not able to fulfill the requirements to progress along the cell cycle.

Oxysterols: biological activities and physicochemical studies

To improve the understanding of the various biological activities of oxysterols (oxygenated derivatives of cholesterol), studies of their physicochemical properties have been undertaken. Oxysterols modify membrane dynamic properties which consequently trigger several biological effects. Despite the presence of at least one oxygenated group in addition to the C3 beta-hydroxyl, oxysterols insert perfectly into the lipidic bilayer of the membrane inducing a condensing effect similar to, but less potent than, that of cholesterol. In biological membranes oxysterols probably interact with membrane components as they are not easily exchanged after their incorporation into the cell membrane. These lipid-protein interactions are probably crucial for the expression of the biological activities of the oxysterols.

An electron spin resonance study of merocyanine 540-mediated type I reactions in liposomes

The merocyanine 540 (MC540)-mediated reduction of nitroxide spin labels in a liposomal system was examined using electron spin resonance (ESR) spectroscopy. Spin label reduction was light driven, and occurred in liposomes composed of both fully-saturated (dimyristoyl) and mono-unsaturated (1-palmitoyl-2-oleoyl) phosphatidylcholine. Loss of the nitroxide ESR signal was enhanced by the physiological electron donors glutathione, cysteine, and NADPH; and was strongly inhibited by the presence of molecular oxygen. Nitroxides reduced in the presence of MC540 alone could be regenerated either by purging the sample with air or by the addition of ferricyanide, indicating that the ESR signal loss was due to reduction to the corresponding hydroxylamines. Only partial regeneration was attained for nitroxides reduced in the presence of glutathione, cysteine, or NADPH. Reduction rates for the lipophilic spin labels, 5-, 12-, and 16-doxyl stearic acid, were not influenced by the position of the nitroxide moiety along the alkyl chain, however reduction of spin labels occupying primarily the aqueous phase was much slower. These studies demonstrate that MC540 can initiate oxidation/reduction (Type I) reactions. Such Type I processes may augment the effects of singlet oxygen in MC540-mediated photodynamic therapy.

Characterization of lipid composition in stimulated human lymphocytes by 1H-NMR

Recent in vivo NMR studies have raised interest in the structural changes of cellular lipids during proliferative activity. We investigated the changes in plasma membrane lipid and total cell lipid during mitogenically-stimulated proliferation of human peripheral blood lymphocytes by extraction of lipids and assay by 500 MHz 1H-NMR. Resonances were assigned using one- and two-dimensional spectroscopic techniques, and signals unique to certain species of lipid were identified. Choline and ethanolamine-containing lipids, glycerophospholipid backbones, sphingolipids, cholesterol, plasmalogens and triacylglycerols were readily detected. Resolution of a number of lipid species was not possible, despite the use of high-resolution techniques. NMR values for proliferation-induced changes in the most easily determined parameters, namely the total cholesterol to total phospholipid molar ratio, and phosphatidylcholine, phosphatidylethanolamine and sphingolipid composition, were found to agree with traditional methods. Differences in phospholipid and fatty acid profiles were found between plasma membranes and total cell lipid for resting values and for response to mitogen.

Dietary fish oil and olive oil supplementation in patients with rheumatoid arthritis. Clinical and immunologic effects

Forty-nine patients with active rheumatoid arthritis completed a 24-week, prospective, double-blind, randomized study of dietary supplementation with 2 different dosages of fish oil and 1 dosage of olive oil. Clinical evaluations were performed at baseline and every 6 weeks thereafter, and immunologic variables were measured at baseline and after 24 weeks of study. The 3 groups of patients were matched for age, sex, disease severity, and use of disease-modifying antirheumatic drugs (DMARDs). Subjects continued receiving DMARDs and other background medications without change during the study. Twenty patients consumed daily dietary supplements of n3 fatty acids containing 27 mg/kg eicosapentaenoic acid (EPA) and 18 mg/kg docosahexaenoic acid (DHA) (low dose), 17 patients ingested 54 mg/kg EPA and 36 mg/kg DHA (high dose), and 12 patients ingested olive oil capsules containing 6.8 gm of oleic acid. Significant improvements from baseline in the number of tender joints were noted in the low-dose group at week 24 (P = 0.05) and in the high-dose group at week 18 (P = 0.04) and 24 (P = 0.02). Significant decreases from baseline in the number of swollen joints were noted in the low-dose group at weeks 12 (P = 0.003), 18 (P = 0.002), and 24 (P = 0.001) and in the high-dose group at weeks 12 (P = 0.0001), 18 (P = 0.008), and 24 (P = 0.02). A total of 5 of 45 clinical measures were significantly changed from baseline in the olive oil group, 8 of 45 in the low-dose fish oil group, and 21 of 45 in the high-dose fish oil group during the study (P = 0.0002). Neutrophil leukotriene B4 production decreased by 19% from baseline in the low-dose fish oil group (P = 0.0003) and 20% in the high-dose group (P = 0.03), while macrophage interleukin-1 production decreased by 38.5% in the olive oil group (P not significant), 40.6% in the low-dose group (P = 0.06), and 54.7% in the high-dose group (P = 0.0005). Tritiated thymidine incorporation in peripheral blood mononuclear cells after stimulation with concanavalin A increased significantly in all 3 groups after 24 weeks, compared with baseline values. We conclude that the clinical benefits of dietary supplementation with omega-3 fatty acids are more commonly observed in patients consuming higher dosages of fish oil for time intervals that are longer than those previously studied. Dietary supplementation with olive oil is also associated with certain changes in immune function, which require further investigation.

Plasma membrane lipid organization and the adherence of differentiating lymphocytes to macrophages

Changes in the packing of phospholipids in the plasma membrane of lymphocytes occur during differentiation within primary and secondary lymphoid organs. As they differentiate, lymphocytes interact with a variety of reticuloendothelial cells, including macrophages. To investigate a possible relation between these two phenomena, the strength of the interactions between lymphocytes and macrophages was measured in vitro as a function of the tightness of packing of phospholipids on the lymphocyte surface. Strength of adherence was measured by the ability of lymphocytes to remain adherent to macrophages when subjected to increasing centrifugal forces. Phospholipid packing was assessed using the fluorescent lipophilic probe merocyanine 540 (MC540), which preferentially binds to bilayers in which the lipids are more loosely packed. Three subpopulations of murine thymocytes were resolved with respect to strength of adherence to peritoneal or thymic macrophages. To determine whether these subpopulations corresponded with the three classes of cells distinguishable by MC540 fluorescence, populations enriched for staining or non-staining cells, and cells sorted on the basis of MC540 fluorescence intensity, were examined. The least fluorescent cells were the least strongly adherent; the most fluorescent cells were the most strongly adherent; and cells of intermediate fluorescence had intermediate adherence. When splenic lymphocytes were examined with respect to adherence to peritoneal or splenic macrophages, similar patterns of fluorescence and adherence were seen. These results suggest that the organization of the plasma membrane lipid bilayer of lymphocytes may be involved in their interactions with macrophages during primary and secondary differentiation. The adherence signal for lymphocytes thus may be similar to that proposed for other blood cells.

Relation between the organization of spectrin and of membrane lipids in lymphocytes

In lymphocytes, the cytoskeletal protein spectrin exhibits two organizational states. Because the plasma membrane lipids of lymphocytes also display two organizational states, it was asked whether there is a relation between the organization of spectrin and of membrane lipids. When mouse thymocytes were stained with merocyanine 540 (MC540), a fluorescent lipophilic probe that binds preferentially to loosely packed, disorganized lipid bilayers, some cells fluoresced brightly and some only dimly or not at all. When the same population was stained for spectrin by indirect immunofluorescence, the spectrin in some cells was uniformly distributed, while in others it was concentrated in a unipolar aggregate. Techniques enriching for mature thymocytes selected for cells displaying low MC540 fluorescence and aggregated spectrin, the same characteristics found in peripheral blood lymphocytes. Flow cytometric sorting of thymocytes based on MC540 phenotype simultaneously sorted them by spectrin phenotype. Finally, treatment with agents that alter the distribution of spectrin caused mature lymphocytes to display high MC540 fluorescence and uniform spectrin. Thus, a relation exists between the organizational states of spectrin and of membrane lipids in lymphocytes: aggregated spectrin is found in cells with tightly organized membrane lipids, uniform spectrin in those with loosely organized lipids. Spectrin may thus be involved in modulating membrane lipid organization in lymphocytes as it is in erythrocytes. Since loosely organized lipids may promote adhesion of blood cells to reticuloendothelial cells, spectrin may thereby be involved in transducing an internally generated adhesion signal to the lymphocyte surface.

Maintenance of membrane phospholipid asymmetry. Lipid-cytoskeletal interactions or lipid pump?

Two models for the mechanism of maintenance of lipid asymmetry in erythrocytes are considered: binding of internal lipids to cytoskeletal proteins, and pumping of internal lipids from the outside to the inside of the cell. Analysis of the kinetics of lipid internalization suggests that the first model is more likely, and that the apparent pumping of lipids represents the activity of an ATP-dependent lipid flip/flop catalyst.

Perfluorinated fatty acids alter merocyanine 540 dye binding to plasma membranes

We have evaluated the effect of the perfluorinated fatty acids pentadecafluoro-n-octanoic acid (PFOA) and nonadecafluoro-n-decanoic acid (NDFDA) on the ability of a human B-lymphoblastoid cell line to bind the lipid-binding, membrane-impermeant, fluorescent dye merocyanine 540 (MC540). Subtoxic concentrations of perfluorinated fatty acids (0.9 mM PFOA; 0.5 mM NDFDA) greatly diminish binding of MC540 by normal plasma membranes, as determined by fluorescence flow cytometry. When perfluorinated fatty acids are added to cells at toxic or lethal concentrations (1.2 mM PFOA; 0.75 mM NDFDA), MC540 binding increases dramatically, with entrance of dye to internal membrane domains. Neither perfluorinated fatty acid molecule reduces the ability of surface immunoglobulin to migrate laterally and cap on cells. Our data suggest that perfluorinated fatty acids either interact directly with lipid binding sites for MC540, and thereby inhibit dye intercalation, or alter membrane lipid architecture and lipid packing to diminish MC540 binding. Both possibilities support a direct, physical, membrane-altering mechanism for perfluorinated fatty acid toxicity on mammalian cells.

Membrane phospholipid asymmetry as a determinant of erythrocyte recognition by macrophages

To assess the role of transbilayer phospholipid distribution in the recognition and phagocytosis of erythrocytes by macrophages, human erythrocytes with either a symmetric or asymmetric distribution of membrane phospholipids were prepared by hypotonic hemolysis and then incubated with cultures of human monocyte-derived macrophages. Erythrocytes with an abnormal, symmetric distribution were phagocytosed 4 times more readily than their counterparts with an asymmetric distribution or than normal, asymmetric intact erythrocytes. This enhanced phagocytosis correlated with two biophysical properties of the membrane: the spacing of phospholipids, as assessed by binding of the dye merocyanine 540, and the relative hydrophobicity, as measured by aqueous two-phase polymer partitioning. These results suggest a mechanism by which loss of membrane asymmetry is translated into recognition by macrophages and provide guidelines in loading erythrocytes that may be useful in manipulating the mode of delivery when erythrocytes are used as drug carriers in vivo.