Exocytosis in mast cells by basic secretagogues: evidence for direct activation of GTP-binding proteins.

Abstract

Abstract. Histamine release induced by the introduction of a nonhydrolyzable analogue of GTP, GTP-~/-S, into ATP-permeabilized mast cells, is associated with phosphoinositide breakdown, as evidenced by the production of phosphatidic acid (PA) in a neomycinsensitive process. The dependency of both PA formation and histamine secretion on GTP-3,-S concentrations is bell shaped. Whereas concentrations of up to 0.1 mM GTP-3,-S stimulate both processes, at higher concentrations the cells' responsiveness is inhibited. At a concentration of 1 mM, GTP-'r-S self-inhibits both PA formation and histamine secretion. Inhibition of secretion can, however, be overcome by the basic secretagogues compound 48/80 and mastoparan that in suboptimal doses synergize with 1 mM GTP-3,-S to potentiate secretion. Secretion under these conditions is not accompanied by PA formation and is resistant txoth to depletion of Ca 2+ from internal stores and to pertussis toxin (PtX) treatment. In addition, 48/80, like mastoparan, is capable of directly stimulating the GTPase activity of G-proteins in a cell-free system. Together, our results are consistent with a model in which the continuous activation of a phosphoinositidehydrolyzing phospholipase C (PLC) by a stimulatory G-protein suffices to trigger histamine secretion. Basic secretagogues of mast ceils, such as compound 48/80 and mastoparan, are capable of inducing secretion in a mechanism that bypasses PLC by directly activating a G-protein that is presumably located downstream from PLC (GE). Thereby, these secretagogues induce histamine secretion in a receptor-independent manner.

Activation of Mast Cells by Trimeric G Protein Gi3; Coupling to the A3 Adenosine Receptor Directly and upon T Cell Contact

“…A decade later, we identified the trimeric G-protein Gi3 as an essential element in the trigger of exocytosis by basic secretagogues (7). However, basic secretagogues are believed to function as receptor mimetic agents that directly activate Gi proteins (3,5,6), whereas the physiological GPCR that couples to Gi3 and its role in the pathophysiology of mast cells have remained obscure. Our recent development of a cell permeable peptide (ALL1) that selectively inhibits Gi3 function in intact cells (8), provided us with a powerful tool to begin dissecting signaling pathways initiated by various mast cell stimuli, and identifying those that are mediated by Gi3.…”

“…The large repertoire of molecules, including neuropeptides, opiates and the synthetic polyamine compound 48/80 (c48/80), which constitute this family of stimuli (1), their common structural moieties and the fact that micromolar concentrations are required to evoke their biological activity, have suggested that members of this family trigger mast cell activation in a receptor-independent manner (2,3). Consistent with this notion, in vitro studies have demonstrated the ability of members of this family to activate directly Gi proteins (3)(4)(5)(6). We have previously shown that rat peritoneal mast cells (RPMCs) express only two Ptx-sensitive G-proteins, Gi 2 and Gi 3 (7).…”

Atrial natriuretic peptide induces rat peritoneal mast cell activation by cGMP-independent and calcium uptake-dependent mechanism

“…It is commonly accepted that noncytotoxic secretagogue agents act on mast cells by binding to cell surface receptors and induce, through a variable sequence of steps, a change in free cytoplasmic calcium concentration, the signal that invokes the final common pathway culminating in exocytosis of the histamine-containing granules (Raison et al, 1999). In the mast cell exocytosis, heterodimeric GTP-binding proteins (G proteins) are involved in the stimulation of mast cell degranulation by basic peptides (Lorenz et al, 1998;Aridor et al, 1990). Thus, application of compound 48/80 and other cationic secretagogues transiently increase intracellular Ca 2+ concentrations through G protein-mediated activation of phospholipase C. The peptide-induced mast cell degranulation assumes a direct interaction of peptides with G protein a subunits subsequent to their translocation across the plasma membrane (Lorenz et al 1998).…”

Actin filament organization in activated mast cells is regulated by heterotrimeric and small GTP-binding proteins.

“…Extensive studies of permeabilized mast cells led to a postulation of a GTP-binding protein, GE, which acts downstream to second messenger production and may be an integral part of the membrane fusion mechanism (Cockcrofi et al, 1987). Its nature, however is still unclear and it is now becoming apparent that a number of GTPases, both heterotrimeric (Lillie and Gomperts, 1993;Aridor et al, 1993) and small (Bar-Sagi and Gomperts, 1988;Oberhauser et al, 1992), may be involved.In this paper, we examine the response of the actin cytoskeleton to stimulation of intact mast cells with compound 48/80, a basic secretagogue that probably activates GTP-binding proteins directly, in a receptor-independent manner (Mousli et al, 1990;Aridor et al, 1990). We compare these responses with those of streptolysin-O (SL-O) 1…”