Mechanism of the concanavalin A-induced change of membrane fluidity of chicken erythrocytes

Concanavalin A amplifies both beta-adrenergic and muscarinic cholinergic receptor-adenylate cyclase-linked pathways in cardiac myocytes

Concanavalin A (Con A) is a tetrameric plant lectin that disrupts plasma membrane-cytoskeletal interactions and alters plasma membrane fluidity. We used Con A as a probe to explore beta-adrenergic and muscarinic cholinergic receptor-mediated regulation of cAMP in intact neonatal rat ventricular myocytes. Preincubation with Con A, 0.5 micrograms/ml, attenuated 1 microM (-)-norepinephrine (NE)-induced downregulation of beta-adrenergic receptors and resulted in a 50% augmentation of cAMP accumulation stimulated by 1 microM NE. Con A also augmented forskolin (1-10 microM)-stimulated cAMP accumulation by an average of 37% (P less than 0.05); however, Con A preincubation had no effect on basal or cholera toxin-stimulated cAMP content. The muscarinic cholinergic agonist carbachol (1-100 microM) decreased 1 microM NE-stimulated cAMP generation by an average of 32% (n = 7, P less than 0.05); preincubation with Con A further enhanced the inhibitory effect of carbachol by 18% (n = 7, P less than 0.05). Carbachol (1 microM) for 2 h decreased muscarinic cholinergic receptor density in whole cells by 33%; preincubation with Con A prevented this receptor downregulation. Con A pretreatment did not affect (-)-isoproterenol- or forskolin-stimulated adenylate cyclase activity in cell homogenates, suggesting that an intact cytoarchitecture is necessary for Con A to augment cAMP formation. We conclude that Con A, through its modulation of beta-adrenergic and muscarinic cholinergic receptor signaling, amplifies both stimulatory and inhibitory adenylate cyclase-linked pathways in intact neonatal ventricular myocytes. These data suggest the possibility that plasma membrane-cytoskeletal interaction is an important regulator of transmembrane signaling because interference with this interaction results in alterations in cAMP accumulation mediated by both beta-adrenergic- and muscarinic cholinergic-adenylate cyclase pathways.

Specific stimulation of heart sarcolemmal Ca2+/Mg2+ ATPase by concanavalin A

The effects of concanavalin A (Con A) on membrane Ca2+/Mg2+ ATPase activities as well as the characteristics of Con A binding were examined by employing rat heart sarcolemmal preparations. Con A stimulated the Ca2+ ATPase and Mg2+ ATPase activities in sarcolemma; maximal stimulation in these parameters was seen at a concentration of 10 micrograms/ml. The observed effects of Con A were blocked by alpha-methylmannoside. Sarcolemmal Na+-K+ ATPase and Ca2+-stimulated ATPase were not affected by Con A. Likewise, Con A did not alter the mitochondrial, sarcoplasmic reticular, and myofibrillar ATPase activities. Con A was found to bind to sarcolemma; alpha-methylmannoside prevented this binding. The Scatchard plot analysis of the data on specific Con A binding showed a straight line with a Kd of about 530 nM and a Bmax of 235 pmol/mg protein, thus indicating that there was only one kind of binding site for Con A in sarcolemma. These results suggest that Con A is a specific activator of the low affinity Ca2+/Mg2+ ATPase system in the heart sarcolemmal membrane.

Stimulation of heart sarcolemmal Na+-Ca2+ exchange by concanavalin A

The effects of Concanavalin A (Con A) on membrane Ca2+ translocation activities were examined by employing rat heart sarcolemmal preparations. Con A stimulated Na+-dependent Ca2+ uptake and ATP-dependent Ca2+ uptake activities in the sarcolemmal vesicles; maximal stimulation was seen at a concentration of 10 microgram/ml. These effects of Con A were blocked by alpha-metylmannoside. Sarcolemmal Na+-induced Ca2+ release was not affected by Con A. It is suggested that Con A-like substances may play a regulatory role in Ca2+-translocation processes of heart sarcolemma.

Dynamic changes of the chicken erythrocyte membranes induced by N-methyl-N-aryl-N-nitrosoureas and their tumorigenicity

The dynamic changes of chicken erythrocyte membranes induced by a series of N-methyl-N'-aryl-N-nitrosoureas (I X) were investigated in comparison with those of N-methyl-N-nitrosourea (MNU) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) by electron spin resonance spectroscopy. Both I X and MNNG increased the intensity of the less mobile lipid component (h1) during the incubation while MNU decreased it. The more mobile lipid component (h2), however, was not much influenced by these agents. A positive correlation was observed between the degree of the spectral alterations (h2/h1) caused by I-X compounds and their tumorigenic potency on mouse skin with the exception of the chloride derivative (I-C1). The present results suggest that the reactions of ultimate carcinogens with cell membranes may play an important role in the tumor promoting process.

Phospholipase A2 of chicken erythrocyte membranes

Chicken erythrocytes have been found to have at least two kinds of phospholipase A2. The first is a soluble enzyme from the cytosole fraction and has no calcium sensitivity. The second can be extracted from the plasma membrane fraction with the nonionic detergent Triton X-100. In this study the membrane-bound enzyme was partially purified by affinity chromatography on phosphatidylcholine-Sepharose, and its specific activity was increased 1100-fold compared with that of the cell homogenate without nuclei. It has an optimum pH of 8.5 and required calcium for maximum activity. It showed the specificity for both phosphatidylcholine and phosphatidylethanolamine, but reacted preferentially on the former substrate. Analysis by concanavalin A-Sepharose affinity chromatography revealed that the membrane-bound phospholipase A2 was retained on the resin and could be eluted specifically with a haptenic sugar, methyl alpha-D-mannopyranoside. The enzyme seems to be either a concanavalin A-binding glycoprotein or a part of a complex with certain concanavalin A-binding glycoproteins.

Induction of increased calcium uptake in liposomes having membrane proteins of chicken erythrocytes by S-adenosylmethionine

Liposomes having membrane proteins of chicken erythrocytes were prepared and the effect of S-adenosylmethionine on 45Ca2+ uptake into the liposomes was investigated. S-Adenosylmethionine, a donor of methyl groups in enzymatic methylation, induced an increase of 45Ca2+ uptake into the proteoliposomes with membrane proteins but not into the liposomes without membrane proteins. Increased release of 45Ca2+ from the inside of the proteoliposomes was also induced by S-adenosylmethionine. These increases of uptake and release of 45Ca2+ were inhibited by S-adenosylhomocystein, an inhibitor of enzymatic methylation. Furthermore, membrane proteins from chicken erythrocytes showed protein and phospholipid methyltransferase activities. The uptake of other materials, 3-0-[methyl-3H]glucose, alpha-[1-14C]aminoisobutyric acid, 42K+ and 54Mn2+, into the proteoliposomes was not increased by S-adenosylmethionine. These results suggest that enzymatic methylation of membrane components may have an important role in the regulation of calcium transport in the chicken erythrocyte membrane and this regulation is rather specific for calcium.

Concanavalin A binding sites on the erythrocytes of normal and genetically dystrophic chickens

Red blood cells (RBCs) were obtained from genetically dystrophic chickens (Dy) and age-matched controls (C). Dy-RBCs had a lower titer of agglutination to concanavalin A (Con A) compared to C-RBCs. In order to ascertain the difference in agglutination, Con A binding on RBCs was studied, using 125I-labeled Con A ([125I]Con A) and ferritin conjugate to Con A (Fer-Con A). Kinetic analysis of [125I]Con A binding to Dy-RBCs showed a reduction of major binding sites of Con A. There was no difference in the apparent association constant for the major binding sites of Con A between Dy-RBCs and C-RBCs. Quantitative analysis of Con A binding site distribution on RBCs using Fer-Con A showed a remarkable diminution of ferritin particles tagged on the surface of Dy-RBCs. There was no significant difference in the distribution pattern of ferritin particles between Dy-RBCs and C-RBCs.

Studies on endogenous phosphorylation of chicken erythrocyte membranes. Calcium-dependent phosphorylation of specific proteins

In this work, endogenous phosphorylation of chicken erythrocyte membranes was investigated. The membrane proteins were rapidly phosphorylated endogenously (half maximum time was 30 s) in the presence of millimolar concentration of Mg2+ under physiological conditions. As an exogenous substrate, protamine was phosphorylated most rapidly of those tested, and histone, casein and bovine serum albumin were rather poor substrates. Cyclic nucleotides had no effect on the endogenous membrane phosphorylation. EGTA inhibited the phosphorylation of a membrane protein having an approximate molecular weight of 43000, and this inhibition was reversed by the addition of a stoichiometric amount of Ca2+. Furthermore, trifluoperazine, an inhibitor of calmodulin, was found to have the same effect as that of EGTA. The phosphorylated 43 kDa protein could be extracted from the membranes under high salt conditions, and was precipitated specifically with anti-actin antibody. These results suggest that the phosphorylation of a peripheral membrane protein (which has an approximate molecular weight of 43000) of chicken erythrocytes by membranous protein kinase(s) depends on Ca2+ and possibly on calmodulin.

A spin-label study of the correlation between stomatocyte formation and membrane fluidization of erythrocytes

Change in the membrane fluidity of human erythrocytes on transformation to stomatocytes was observed by ESR spectroscopy using 12-doxyl stearic acid or its methyl ester as a probe. When the transformation to stomatocytes was induced by four qualitatively different methods, i.e. (a) addition of cationic amphiphilic agents such as chloropromazine, tetracaine, chloroquine or primaquine, (b) addition of Triton X-100, a non-ionic detergent, (c) lowering the pH, and (d) depleting membrane cholesterol, membrane fluidization was always observed. This indicates the existence of a close correlation between stomatocyte formation and increase in the membrane fluidity. Furthermore, since the stomatocytes fixed by diamide treatment exhibited membrane fluidization only in the presence of the reagent, the enhanced membrane fluidity was a direct consequence of the reagent interacting with, and changing the state of, the lipid bilayer itself, and not through the influence of some structural alteration of spectrin. These results provide experimental support for the theoretical prediction made by Brailsford et al. [J. theoret. Biol. 86, 531 (1980)]. Plausible mechanisms for the discocyte-stomatocyte transformation are discussed.

Phospholipid metabolism, calcium flux, and the receptor-mediated induction of chemotaxis in rabbit neutrophils

Rabbit neutrophils were stimulated with the chemotactic peptide fMet-Leu-Phe in the presence of the methyltransferase inhibitors homocysteine (HCYS) and 3-deazaadenosine (3-DZA). HCYS and 3-DZA inhibited chemotaxis, phospholipid methylation, and protein carboxymethylation in a dose-dependent manner. The chemotactic peptide-stimulated release of [14C]arachidonic acid previously incorporated into phospholipid was also partially blocked by the methyltransferase inhibitors. Stimulation by fMet-Leu-Phe or the calcium ionophore A23187 caused release of arachidonic acid but not of previously incorporated [14C]-labeled linoleic, oleic, or stearic acids. Unlike the arachidonic acid release caused by fMet-Leu-Phe, release stimulated by the ionophore could not be inhibited by HCYS and 3-DZA, suggesting that the release was caused by a different mechanism or by stimulating a step after methylation in the pathway from receptor activation to arachidonic acid release. Extracellular calcium was required for arachidonic acid release, and methyltransferase inhibitors were found to partially inhibit chemotactic peptide-stimulated calcium influx. These results suggest that methylation pathways may be associated with the chemotactic peptide receptor stimulation of calcium influx and activation of a phospholipase A2 specific for cleaving arachidonic acid from phospholipids.

Rat basophilic leukemia cell lines defective in phospholipid methyltransferase enzymes, Ca2+ influx, and histamine release: reconstitution by hybridization

Variants of the rat basophilic leukemia (RBL) cell line were isolated and screened for phospholipid methyltransferase I and II activities, enzymes that convert phosphatidylethanolamine to phosphatidylcholine. Two variants were found that had decreased phospholipid methyltransferase enzyme levels and were unable to cause an influx of Ca2+ or release histamine in an IgE-mediated reaction. However, these cells were able to release histamine through an ionophore-induced reaction, indicating that the releasing mechanism distal to the Ca2+ channel was intact. One cell line, 1C1.B1, had low specific activity for phospholipid methyltransferase I. A second variant, 2H3.B6, had reduced phospholipid methyltransferase II activity. Although both variants were unable to incorporate label from [methyl-3H]methionine or [3H]serine into phosphatidylcholine, they were able to incorporate [methyl-3H]choline and myo-[2-3H(N)]inositol into phospholipids. Fusion of the two cell lines and isolation on selective media resulted in the growth of eight independent hybrids. All eight had an increased number of chromosomes and normal phospholipid methyltransferase activities. Stimulation of the hybrids with IgE resulted in CA2+ influx and histamine release. These results indicate that phospholipid methylation precedes and is necessary for Ca2+ influx, and they further support the hypothesis that methylation is a necessary early step in the IgE-mediated histamine release reaction in RBL cells.