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Biomedical subjects

D Papahadjopoulos

Publications and source records attributed to D Papahadjopoulos.

At least 163 records · Page 9Linked to original sources
PubMed1976

Phospholipid membrane stabilization by dimethylsulfoxide and other inducers of Friend leukemic cell differentiation.

A large number of low molecular weight polar cryoprotective agents have recently been found to induce erythroid differentiation of Friend leukemic cells in vitro. The effect of these agents on membrane fluidity in phospholipid vesicles was studied by determining the solid-to-liquid crystalline phase transition using differential scanning calorimetry. Some of the inducing agents studies were found to raise the normal transition temperature (Tc) by a few degrees. All of these agents were found to produce a separate transition at a much higher temperature. Changes in the head group of the phospholipid, the pH, the presence of divalent cations, and the addition of other membrane-active compounds were found to significantly influence the inducing agent's effects on the Tc of phospholipid membranes. The ability of the different agents to produce a new transition at a high temperature was found to correlate well with their ability to induce Friend leukemic cell differentiation. The possible mechansims of action of the chemical inducers, and the significance of the observed membrane effects on differentiation and malignancy are discussed. It is concluded that inducing agents decrease the fluidity and stabilize phospholipid membranes, and that their effects in cell differentiation might be initiated by a similar change in the properties of cell membranes.

Animals↗
PubMed1976

Studies on membrane fusion. II. Induction of fusion in pure phospholipid membranes by calcium ions and other divalent metals.

The effect of divalent metals on the interaction and mixing of membrane components in vesicles prepared from acidic phospholipids has been examined using freeze-fracture electron microscopy and differential scanning calorimetry. Ca2+, and to a certain extent Mg2+, induce extensive mixing of vesicle membrane components and drastic structural rearrangements to form new membranous structures. In contrast to the mixing of vesicle membrane components in the absence of Ca2+ described in the accompanying paper which occurs via diffusion of lipid molecules between vesicles, mixing of membrane components induced by Ca2+ or Mg2+ results from true fusion of entire vesicles. There appears to be a "threshold" concentration at which Ca2+ and Mg2+ become effective in inducing vesicle fusion and the threshold concentration varies for different acidic phospholipid species. Different phospholipids also vary markedly in their relative responsiveness to Ca2+ and Mg2+, with certain phospholipids being much more susceptible to fusion by Ca2+ than Mg2+. Vesicle fusion induced by divalent cations also requires that the lipids of the interacting membranes be in a "fluid" state (T greater than Tc). Fusion of vesicle membranes by Ca2+ and Mg2+ does not appear to be due to simple electrostatic charge neutralization. Rather the action of these cations in inducing fusion is related to their ability to induce isothermal phase transitions and phase separations in phospholipid membranes. It is suggested that under these conditions membranes become transiently susceptible to fusion as a result of changes in molecular packing and creation of new phase boundaries induced by Ca2+ (or Mg2+).

Calcium↗
PubMed1976

Studies on membrane fusion. I. Interactions of pure phospholipid membranes and the effect of myristic acid, lysolecithin, proteins and dimethylsulfoxide.

The interaction and mixing of membrane components in sonicated unilamellar vesicles and also non-sonicated multilamellar vesicles prepared from highly purified phospholipids suspended in NaCl solutions has been examined. Electron microscopy and differential scanning calorimetry were used to characterize the extent and kinetics of mixing of membrane components between different vesicle populations. No appreciable fusion was detected between populations of non-sonicated phospholipid vesicles incubated in aqueous salt (NaCl) solutions. Mixing of vesicle membrane components via diffusion of phospholipid molecules between vesicles was observed in populations of negatively charged phosphatidylglycerol vesicles but similar exchange diffusion was not detected in populations of neutral phosphatidylcholine vesicles. Incubation of sonicated vesicle populations at temperatures close to or above the phospholipid transition temperature resulted in an increase in vesicle size and mixing of vesicle membrane components as determined by a gradual change in the thermotropic properties of the mixed vesicle population. The interaction of purified phospholipid vesicles was also examined in the presence of myristic acid and lysolecithin. Our results indicate that while these agents enhance mixing of vesicle membrane components, in most cases mixing probably proceeds via diffusion of phospholipid molecules rather than by fusion of entire vesicles. Increased mixing of vesicle membrane components was also produced when vesicles were prepared containing a purified hydrophobic protein (myelin proteolipid apoprotein) or were incubated in the presence of dimethylsulfoxide. In these two systems, however, the evidence suggests that mixing of membrane components results from the fusion of entire vesicles.

Calorimetry↗
PubMed1976

Membrane action of DMSO and other chemical inducers of Friend leukaemic cell differentiation.

DMSO and other cryoprotective agents produce a pronounced increase on the phase transition temperature of phospholipid membranes, indicating an increased stability. The effects of DMSO and other cryoprotective agents, divalent cations, and local anaesthetics on the transition temperature of phospholipid membranes seem to correlate with their effects on the differentiation of Friend leukaemic cells in vitro. These studies suggest that the induction of differentiation by cryoprotective agents may be the result of the interaction of these agents with cell membranes.

Anesthetics, Local↗
PubMed1976

Effect of a phase transition on the binding of 1-anilino-8-naphthalenesulfonate to phospholipid membranes.

The binding of 1-anilino-8-naphthalenesulfonate (ANS) to dipalmitoyl-phosphatidycholine (DPPC)-sonicated vesicles was measured by a fluorimetric method in the vicinity of the gel-to-liquid crystalline phase transition temperature (Tm). A similar measurement was performed on large multimellar DPPC vesicles through equilibrium dialysis. Both measurements demonstrated anomalous dye binding in the temperature region of the Tm and slightly above (prefreezing region). The amount of ANS bound at this temperature region was in excess of what would be expected based on extrapolation of the high temperature binding data; just below the Tm, the amount of bound dye decreased abruptly. The fluorimetric studies on vesicles also indicated that inner monolayer binding of ANS was markedly inhibited below the Tm. The possibility that the increase in bound dye in the prefreezing region was caused by enhanced lateral compressibility, density fluctuations, or additional binding sites at the boundary of transient gel-like clusters is discussed and the general topic of anomalous increases in various membrane processes in the vicinity of a phase transition is briefly reviewed.

Anilino Naphthalenesulfonates↗
PubMed1976

Lipid vesicles as carriers for introducing materials into cultured cells: influence of vesicle lipid composition on mechanism(s) of vesicle incorporation into cells.

The mechanisms involved in the uptake of uni- and multi-lamellar lipid vesicles by BALB/c mouse 3T3 cells have been investigated. Vesicles are incorporated into cells both by endocytosis and by a nonendocytotic mechanism which we propose involves fusion of vesicles with the plasma membrane. The nonendocytotic pathway predominates in the uptake of negatively charged vesicles composed of phospholipids that are "fluid" (phosphatidylserine/phosphatidylcholine) at 37 degrees. Neutral fluid vesicles (phosphatidylcholine) and negatively charged vesicles prepared from "solid" phospholipids (phosphatidylserine/distearylphosphatidylcholine/dipalmitoylphosphatidylcholine) are instead incorporated largely by endocytosis. Uptake of the latter classes of vesicle is reduced (80-90% inhibition) by inhibitors of cellular energy metabolism and by cytochalasin B.

Azides↗
PubMed1976

Inhibition of tumor cell growth in vitro and in vivo by 1-beta-D-arabinofuranosylcytosine entrapped within phospholipid vesicles.

Phospholipid vesicles have been used as a carrier vehicle to enhance the cytotoxic activity of 1-beta-D-arabinofuranosyl-cytosine (ara-C) and 1-beta-D-arabinofuranosylcytosine 5'-triphosphate against several tumor cell lines. The activity of both compounds in free solution or entrapped within phospholipid vesicles was compared against L1210 cells, Ehrlich ascites cells, and SV40-transformed 3T3 cells in vitro. In addition, the activity of vesicle-entrapped ara-C against L1210 cells was also studied in vivo. The results obtained in vitro with ara-C indicated no difference in the concentration needed to inhibit growth of cells by 50% between free ara-C and vesicle-entrapped ara-C. In contrast, 1-beta-D-arabinofuranosylcytosine 5'-triphosphate entrapped in phospholipid vesicles was a more potent inhibitor of L1210 in culture (ID50, 2 X 10(-8) M) compared to the relatively inactive free 1-beta-D-arabinofuranosylcytosine 5'-triphosphate (id50 greater than 10(-7) M). Experiments carried out with L1210 cells in mice showed that, after a single i.p. dose (10 mg/kg) of vesicle-entrapped ara-C, the average survival times of mice inoculated with 10(5) L1210 cells were increased by over 90%. In control experiments, free ara-C or vesicles plus free ara-C (10 mg/kg) did not prolong survival of mice.

Animals↗
PubMed1976

Use of lipid vesicles as carriers to introduce actinomycin D into resistant tumor cells.

Unilamellar lipid vesicles have been used as a carrier vehicle to enhance the uptake of actinomycin D into an actinomycin D-resistant Chinese hamster tumor cell line (DC-3F/ADX). The DC-3F/ADX cell line is resistant to actinomycin D as a result of its decreased capacity to transport actinomycin D across the plasma membrane and is able to grow in the presence of concentrations of actinomycin D that are cytotoxic for the sensitive parent cell line (DC-3F). Incubation of resistant DC-3F/ADX cells with actinomycin D-containing vesicles produced a 5-fold increase in intracellular drug concentration over that achieved by exposure to identical concentrations of the drug added to the culture medium. Vesicle-mediated uptake of actinomycin D into resistant cells produced inhibition of cellular RNA synthesis and cell growth at actinomycin D concentrations that had no inhibitory effect when added as free drug in the culture medium. Dose-response measurements established that direct introduction of actinomycin D into resistant cells by means of lipid vesicles resulted in a 200-fold reduction in the concentration of actinomycin D required to inhibit cellular RNA synthesis and a 120-fold reduction in the dose of actinomycin D required to produce a 50% inhibition of cell growth. These results lend strong support to the hypothesis that cellular resistant to actinomycin D is due to a lower capacity to take up actinomycin D as a result of a reduction in the permeability of the cellular plasma membrane to this drug. The potential value of lipid vesicles for introducing other classes of drugs into cultured cells and their possible use in chemotherapy are also discussed.

Animals↗
PubMed1975

Effects of proteins on thermotropic phase transitions of phospholipid membranes.

A variety of proteins have been studied for their ability to interact and alter the thermotropic properties of phospholipid bilayer membranes as detected by differential scanning calorimeter. The proteins studied included: basic myelin protein (A1 protein), cytochrome c, major apoprotein of myelin proteolipid (N-2 apoprotein), gramicidin A, polylysine, ribonuclease and hemoglobin. The lipids used for the interactions were dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylglycerol. The interactions were grouped in three catagories each having very different effects on the phospholipid phase transition from solid to liquid crystalline. The calorimetric studies were also correlated with data from vesicle permeability and monolayer expansion. Ribonuclease and polylysine which exemplify group 1 interactions, show strong dependence on electrostatic binding. Their effects on lipid bilayers include an increase in the enthalpy of transition (deltaH) accompanied by either an increase or no change in the temperature of transition (Tc). In addition, they show minimal effects on vesicle permeability and monolayer expansion. It was concluded that these interactions represent simple surface binding of the protein on the lipid bilayer without penetration into the hydrocarbon region. Cytochrome c and A1 protein, which exemplify group 2 interactions, also show a strong dependence on the presence of net negative charges on the lipid bilayers for their binding. In contrast to the first group, however, they induce a drastic decrease in both Tc and deltaH of the lipid phase transition. Furthermore, they induce a large increase in the permeability of vesicles and a substantial expansion in area of closely packed monolayers at the air-water interface. It was concluded that group 2 interactions represent surface binding followed by partial penetration and/or deformation of the bilayer. Group 3 interactions, shown by proteolipid apoprotein and gramicidin A, were primarily non-polar in character, not requiring electrostatic charges and not inhibited by salt and pH changes. They had no appreciable effect on the Tc but did induce a linear decrease in the magnitude of the deltaH, proportional to the percentage of protein by weight. Membranes containing 50% proteolipid protein still exhibited a thermotropic transition with a deltaH one half that of the pure lipid, and only a small diminution of the size of the cooperative unit. It was concluded that in this case the protein was embedded within the bilayer, associating with a limited number of molecules via non-polar interactions, while the rest of the bilayer was largely unperturbed.

Animals↗
PubMed1975

Effects of local anesthetics on membrane properties. I. Changes in the fluidity of phospholipid bilayers.

The effect of the local anesthetic dibucaine on the solid to liquid-crystalline phase transition in phospholipid vesicles was studied by calorimetry and fluorescence polarization. The partition coefficient (greater than 3000) of dibucaine in the membranes of vesicles prepared from acidic phospholipids was more than 20 times higher than in neutral phospholipid membranes under the same conditions. Calorimetric measurements on vesicles prepared form acidic phospholipids (bovine brain phosphatidylserine; dipalmitoylphosphatidylglycerol) showed that dibucaine (1 with 10(-4) M) produced a significant reduction in the gel-liquid crystalline transition temperature (Tc). This fluidizing effect of dibucaine on acidic phospholipid membranes was even more marked in the presence of Ca2+. In contrast, dibucaine at the same concentration did not alter the Tc of neutral phospholipids (dipalmitoylphosphatidylcholine). Significant increase in the fluidity of neutral phospholipid membranes occurred only at higher dibucaine concentrations (2 with 10(-3) M). Measurements of the fluorescence polarization and lifetime of the probe, 1,6-diphenylhexatriene, in acidic phospholipid vesicles revealed that dibucaine (1 with 10(-4) M) caused an increase in the probe rotation rate indicating an increase in the fluidity of the phospholipid membranes. A good correlation was obtained between fluorescence polarization data on dibucaine-induced changes in membrane fluidity and calorimetric measurements on vesicles of the same type.

Anesthetics, Local↗
PubMed1975

Effects of local anesthetics on membrane properties. II. Enhancement of the susceptibility of mammalian cells to agglutination by plant lectins.

Treatment of untransformed mouse and hamster cells with the tertiary amine local anesthetics dibucaine, tetracaine and procaine increases their susceptibility to agglutination by low doses of the plant lectin concanavalin A. Agglutination of anesthetic-treated untransformed cells by low doses of concanavalin A is accompanied by redistribution of concanavalin A receptors on the cell surface to form patches, similar to that occurring in spontaneous agglutination of virus-transformed cells by concanavalin A. Immunofluorescence and freeze-fracture electronmicroscopic observations indicate that local anesthetics per se do not induce this redistribution of concanavalin A receptors but modify the plasma membrane so that receptor redistribution is facilitated on binding of concanavalin A to the cell surface. Fluorescence polarization measurements on the rotational freedom of the membrane-associated probe, diphenylhexatriene, indicate that local anesthetics produce a small increase in the fluidity of membrane lipids. Spontaneous agglutination of transformed cells by low doses of concanavalin A is inhibited by colchicine and vinblastine but these alkaloids have no effect on concanavalin A agglutination of anesthetic-treated cells. Evidence is presented which suggests that local anesthetics may impair membrane peripheral proteins sensitive to colchicine (microtubules) and cytochalasin-B (microfilaments). Combined treatment of untransformed 3T3 cells with colchicine and cytochalasin B mimics the effect of local anesthetics in enhancing susceptibility to agglutination by low doses of concanavalin A. A hypothesis is presented on the respective roles of colchicine-sensitive and cytochalasin B-sensitive peripheral membrane proteins in controlling the topographical distribution of lectin receptors on the cell surface.

Agglutination↗
PubMed1975

Cochleate lipid cylinders: formation by fusion of unilamellar lipid vesicles.

Freeze-fracture electron microscopy was used to study the morphological changes occurring following the addition of Ca-2+ to sonicated preparations of phosphatidylserine in aqueous NaCl buffer. Before the addition of Ca-2+, preparations contained only small (200-500 A diameter) spheroidal vesicles. After the addition of Ca-2+ (10 mM) and incubation for 1 h at 37 degrees C preparations contained only large (2000-10 000 A) apparently multilamellar structures many of which were cylindrical in shape. The lamellae in these cylinders appear to be folded in a spiral configuration. Addition of EDTA to these preparations produced large, closed, spherical, unilamellar vesicles. We suggest the name cochleate lipid cylinders for the spiral structures and propose that they are formed by fusion of unilamellar vesicles into large sheets which fold spirally to form cylinders.

Calcium↗
PubMed1975

Electron diffraction study of hydrated phospholipid single bilayers. Effects of temperature hydration and surface pressure of the "precursor" monolayer.

The molecular packing and phase transition of hydrated dipalmitoylphosphatidylcholine single bilayers are studied by electron diffraction, using an electron microscope equipped with a hydration stage. The phase transition and area per molecule are measured as functions of temperature, hydration and the surface pressure of the monolayer from which the bilayer is formed. The transition temperature of a bilayer agrees with calorimetric measurements on bulk lipid/water mixtures. The molecular packing of a bilayer corresponds to that of the precursor monolyer at a surface pressure of 47 dyne/cm.

Binding Sites↗