Role of GTP-binding protein in histamine release from mast cells

Tyrosine phosphorylation and inositol phosphate production: are early events in human eosinophil activation stimulated by immobilized secretory IgA and IgG?

Eosinophils are implicated as major inflammatory cells in parasite infection and allergic reactions. Among various mediators, eosinophil granule cationic proteins play an important role in the pathophysiology of diseases. However, little is known about the actual physiologic stimuli for eosinophil degranulation and the signaling events for triggering eosinophil degranulation. A series of in vivo and in vitro studies suggest that the interaction between antibody coated parasites and Fc receptors on eosinophils is one of the most effective triggers for eosinophil degranulation. Similarly, eosinophil degranulation can be induced in vitro by Sepharose beads coated with human sIgA or IgG. Eosinophil degranulation induced by these stimuli is mediated by PTX-sensitive membrane-bound heterotrimeric G protein(s), and is accompanied by the rapid turnover of inositol phosphates. The production of inositol phosphates is inhibited by PTX. Eosinophil activation by sIgA and IgG also involves tyrosine phosphorylation of several proteins and is inhibited by tyrosine kinase inhibitors. Thus phospholipase C-coupled G protein(s) and tyrosine kinases are key molecules in early signal transduction of eosinophil activation induced by sIgA and IgG. Although further studies are needed to identify which tyrosine kinase(s) is specifically involved in the eosinophil degranulation mechanism, these molecules could be a target for therapies of human diseases where eosinophils are involved.

Functional characterization of the Na(+)-H+ exchanger in rat mast cells: crosstalks between different kinase pathways

In our effort to understand the mechanisms by which rat mast cells regulate intracellular pH (pHi), we studied the effect of drugs acting on different transducting signals on the Na(+)-H+ antiport. We studied the activity of the antiporter in recovering pHi after an acid load with sodium propionate. The drugs used were okadaic acid, which inhibits the phosphatases 1 and 2A, pertussis toxin, which ADP-rybosylates the Gi-protein, cholera toxin, which ADP-rybosylates the Gs-protein, NaF which non-specifically activates G-proteins, and the phorbol esther 12-O-tetradecanoylphorbol 13-acetate (TPA) which specifically activates protein kinase C. The effect of TPA is a two-fold stimulation of the activity of the antiporter. A similar activation was observed with the combination okadaic acid plus cholera toxin. All the drugs alone did not modify the activity of the antiporter, and they all blocked the stimulatory activity of TPA. In a Ca(2+)-free medium, okadaic acid inhibits the activity of the antiporter. All the mechanisms affected by these drugs have some regulatory role on the Na(+)-H+ antiport. Our results indicate the great complexity of the crosstalks between the different signal transducing pathways.

Functional diversity of histamine and histamine receptors

In order to analyze the mechanisms by which a single biogenic amine like histamine is capable of inducing a wide variety of both physiologic and pathologic functions in various tissues/cells, histamine responses were dissected in detail from a biochemical and pharmacologic point of view. Histamine is synthesized by multiple isozymes of histidine decarboxylase, and catabolized by either diamine oxidase or histamine-N-methyltransferase. Synthesized intracellular histamine may play a role in cell proliferation, whereas released histamine binds to at least three different histamine-specific receptors, then activates various intracellular components, such as Ca++, cAMP, protein kinase, and ion channels. These second messenger pathways interact differentially with each other in various tissues/cells. Moreover, histamine not only activates its own receptors, but also activates other related receptors such as the serotonin 1c receptor. Therefore, to understand the complex actions of histamine, new approaches should be established, in which multiple phenomena can be monitored simultaneously.

Fluoride is a contractile agent of guinea pig airway smooth muscle

1. Guinea pig parenchymal lung strips and tracheal smooth muscle contract potently after NaF-addition. Maximal contractions of lung strips and tracheal rings induced by NaF were 208 +/- 17% (n = 6) and 151 +/- 8% (n = 4) of the maximal histamine response respectively. 2. The -log EC50-value for NaF on lung strips and tracheal rings was 2.38 +/- 0.01 (n = 6) and 2.28 +/- 0.01 (n = 4) respectively. 3. Contractions induced by NaF were augmented after Al3+ pretreatment, suggesting the involvement of a G-protein. NaF responses were not affected by blockade of H1-, muscarinic-, leukotriene C4- or leukotriene D4-receptors, indicating that mast cell degranulation or nerve activation is most probably not implicated. 4. Contractions after NaF-addition were relatively insensitive to removal of extracellular calcium and were reversed via cAMP- and cGMP-mediated pathways. 5. Relaxation studies with (-)isoprenaline and 8-bromo-cGMP on lung strips, precontracted to similar levels with either a H1-agonist, KCl or NaF, showed that the level of relaxation depends on the contractile agent that is used. 6. After precontraction with KCl (-)isoprenaline relaxes lung strips only to 58 +/- 9% (n = 5) of the initial contraction, whereas lung strips precontracted with NaF or a H1-agonist relax 114 +/- 8% (n = 4) and 120 +/- 7% (n = 5) respectively with (-)isoprenaline. 7. Similar results were obtained with relaxation induced with 8-bromo-cGMP. 8. These findings suggest that NaF-induced contractions are elicited via a mechanism, that is probably similar to that of the H1-receptor. The involvement of a G-protein in the observed NaF-responses is therefore likely.

Tyrosine phosphorylation coupled to IgE receptor-mediated signal transduction and histamine release

Antigen-induced cross-linking of IgE bound to its receptors at the surface of basophils or mast cells initiates a number of biochemical events culminating in the release of histamine-containing granules. In the present study, we investigated the possible involvement of tyrosine phosphorylation in signaling by the high-affinity IgE receptor (Fc epsilon RI). Cross-linking of Fc epsilon RI in rat basophilic leukemia cells (RBL-2H3) led to the phosphorylation of several proteins on tyrosine, the most prominent having a mass of 72 kDa. Tyrosine phosphorylation was rapid, detectable 1 min after stimulation, and correlated with both the time course and antigen dose for histamine release. Reversal of Fc epsilon RI cross-linking prevented continuation of the degranulation process and resulted in rapid loss of tyrosine phosphorylation. The receptor-mediated tyrosine phosphorylation was still induced in the absence of calcium in the medium. Depletion of protein kinase C with phorbol 12-myristate 13-acetate did not dramatically affect the tyrosine phosphorylation signal or the release of histamine. In contrast, the calcium ionophore A23187 induced histamine release in the absence of a perceptible increase in protein tyrosine phosphorylation. Thus, tyrosine phosphorylation is an early signal following Fc epsilon RI aggregation, independent of the exocytotic process itself. Taken together, our findings functionally link protein phosphorylation on tyrosine residues to Fc epsilon RI-mediated signal transduction leading to histamine release.

ATP-dependent and ATP-independent pathways of exocytosis revealed by interchanging glutamate and chloride as the major anion in permeabilized mast cells

Most investigations of the mechanism of regulated exocytosis have involved the use of secretory cells permeabilized in glutamate-based electrolyte solutions. In our previous work we have used NaCl-based electrolyte solutions. For secretion to occur from rat mast cells under these latter conditions, a dual effector system comprising Ca2+ and a guanine nucleotide are required; together they are sufficient. Here we compare the secretion from mast cells permeabilized in solutions of different electrolytes. Replacement of Na+ by K+ had little effect. Replacement of Cl- by Br-, SO4-, gluconate, isethionate, acetate, tartrate, succinate, etc. affected the maximal extent of secretion elicited by the dual effectors Ca2+ and guanosine-5'-O-(3-thiotriphosphate) (Ca2(+)-plus-GTP-gamma-S) but had little influence on the effective affinity for Ca2+. The dicarboxylic amino acids (L- and D-glutamate, and L-aspartate) permitted exocytosis to be elicited by Ca2+ or GTP-gamma-S alone. Secretion stimulated by GTP-gamma-S is strongly inhibited by Cl- (50% inhibition by 20 mM Cl-), whereas the extent of Ca2(+)-induced secretion is proportional to the concentration of glutamate in mixed electrolyte buffers. Unlike dual-effector stimulation, secretion due to the single effectors requires adenosine triphosphate (ATP) and is prevented by inhibitors of protein kinase C. These results point to the existence of two parallel pathways for control of exocytosis in permeabilized cells, one ATP dependent, the other ATP independent.