Evidence for the interaction of mast cell-degranulating peptide with pertussis toxin-sensitive G proteins in mast cells

Strategies towards Targeting Gαi/s Proteins: Scanning of Protein-Protein Interaction Sites To Overcome Inaccessibility

Heterotrimeric G proteins are classified into four subfamilies and play a key role in signal transduction. They transmit extracellular signals to intracellular effectors subsequent to the activation of G protein-coupled receptors (GPCRs), which are targeted by over 30 % of FDA-approved drugs. However, addressing G proteins as drug targets represents a compelling alternative, for example, when G proteins act independently of the corresponding GPCRs, or in cases of complex multifunctional diseases, when a large number of different GPCRs are involved. In contrast to Gαq, efforts to target Gαi/s by suitable chemical compounds has not been successful so far. Here, a comprehensive analysis was conducted examining the most important interface regions of Gαi/s with its upstream and downstream interaction partners. By assigning the existing compounds and the performed approaches to the respective interfaces, the druggability of the individual interfaces was ranked to provide perspectives for selective targeting of Gαi/s in the future.

Gangliosides inhibit bee venom melittin cytotoxicity but not phospholipase A(2)-induced degranulation in mast cells

Sting accident by honeybee causes severe pain, inflammation and allergic reaction through IgE-mediated anaphylaxis. In addition to this hypersensitivity, an anaphylactoid reaction occurs by toxic effects even in a non-allergic person via cytolysis followed by similar clinical manifestations. Auto-injectable epinephrine might be effective for bee stings, but cannot inhibit mast cell lysis and degranulation by venom toxins. We used connective tissue type canine mast cell line (CM-MC) for finding an effective measure that might inhibit bee venom toxicity. We evaluated degranulation and cytotoxicity by measurement of β-hexosaminidase release and MTT assay. Melittin and crude bee venom induced the degranulation and cytotoxicity, which were strongly inhibited by mono-sialoganglioside (G(M1)), di-sialoganglioside (G(D1a)) and tri-sialoganglioside (G(T1b)). In contrast, honeybee venom-derived phospholipase A(2) induced the net degranulation directly without cytotoxicity, which was not inhibited by G(M1), G(D1a) and G(T1b). For analysis of distribution of Gα(q) and Gα(i) protein by western blotting, lipid rafts were isolated by using discontinuous sucrose gradient centrifuge. Melittin disrupted the localization of Gα(q) and Gα(i) at lipid raft, but gangliosides stabilized the rafts. As a result from this cell-based study, bee venom-induced anaphylactoid reaction can be explained with melittin cytotoxicity and phospholipase A(2)-induced degranulation. Taken together, gangliosides inhibit the effect of melittin such as degranulation, cytotoxicity and lipid raft disruption but not phospholipase A(2)-induced degranulation in mast cells. Our study shows a potential of gangliosides as a therapeutic tool for anaphylactoid reaction by honeybee sting.

G protein-dependent activation of mast cell by peptides and basic secretagogues

Signaling pathways leading to exocytosis and arachidonate release from serosal mast cells by basic secretagogues, including cationic peptides, arise from the involvement of betagamma subunits from G(i2) and G(i3) GTP-binding proteins. The original concept that basic secretagogues directly interact with G proteins implicated the entry of secretagogues into mast cells. This has been demonstrated only for the neuropeptide substance P. Basic secretagogues might share a common mechanism of penetration with the newly described cell-penetrating peptides. The involvement of some membrane transporter or non-selective membrane receptor to basic secretagogues cannot be excluded.

Agmatine: a mastoparan-like activity related to direct activation of heterotrimeric G proteins

We examined agmatine and imidazoline derivatives as putative ligands of trimeric G protein in rat peritoneal mast cells. Agmatine induced a concentration-dependent and pertussis toxin-sensitive secretion of histamine (exocytosis) and arachidonate. Clonidine and idazoxan had no effect. Blockage of Gbetagamma dimers by a specific anti-Gbeta antibody inhibited exocytosis elicited by agmatine and mastoparan. The G protein antagonist [p-Glu(5),D-Trp(7,9,10)]substance P-(5-11) prevented both mastoparan- and agmatine-induced exocytosis when it was allowed to reach its intracellular targets by streptolysin-O permeabilisation. In intact cells, this response was prevented by both the removal of sialic acid residues by neuraminidase and by [D-Pro(4),D-Trp(7,9,10)]substance P-(4-11) acting at the mast cell surface. Exocytosis was restored by permeabilisation of the plasma membrane with streptolysin-O. These results suggest that agmatine might have several molecular targets, exerting its neurotransmitter function at low concentrations (i.e., with high affinity) through membrane receptors and at high concentrations (i.e., with weak affinity) through direct G protein activation.

Mast cell degranulating (MCD) peptide: a prototypic peptide in allergy and inflammation

The solid phase synthesis of mast degranulating peptide (MCD peptide) raised the possibility of preparing analogs and examining the pharmacology and the proposed role of this peptide as a potential agent in allergy and inflammation. MCD peptide, a cationic 22-amino acid residue peptide with two disulfide bridges, causes mast cell degranulation and histamine release at low concentrations and has anti-inflammatory activity at higher concentrations. Because of these unique immunologic properties, MCD peptide may serve as a useful tool for studying secretory mechanisms of inflammatory cells such as mast cells, basophils, and leukocytes, leading to the design of compounds with therapeutic potential.

Receptor-mediated and receptor-independent activation of G-proteins in rat brain membranes

High-affinity GTPase activity intrinsic to G-proteins, which serves as an index of G-protein activation elicited through agonist-stimulated receptors as well as by receptor-independent direct G-protein activators like mastoparan, was measured in rat brain membranes. Receptor-mediated high-affinity GTPase activity was detectable preferentially for the Gi subfamily associated with adenylyl cyclase inhibition mediated by group II metabotropic glutamate, pirenzepine-insensitive muscarinic acetylcholine, GABA(B), adenosine A1, dopamine D2-like (striatum), and serotonin 5-HT1A (hippocampus) receptors. The pharmacological characteristics of such receptor-mediated high-affinity GTPase activities were presented. Mastoparan, a tetradecapeptide from wasp venom which has been shown to directly activate Gi and Go, inhibited low-affinity GTP hydrolyzing activity, probably due to its activating effect on nucleoside diphosphokinase (NDPK). When NDPK activity was inhibited completely by UDP, mastoparan stimulated high-affinity GTPase activity in a concentration-dependent manner. There are many compounds other than mastoparan with apparently diverse structural properties which have been shown to directly activate G-proteins. The relevance and possible participation of receptor-independent mode of G-protein activation for some neuropeptides were discussed.

Drugs interacting with G protein alpha subunits: selectivity and perspectives

Extracellular signal molecules as diverse as hormones, neurotransmitters and photons use a signal transduction pathway involving a receptor, a G protein and effectors. Compounds that interact directly with G proteins can mimic the receptor-G protein interaction or can block the activation of G proteins by receptors. Several binding sites exist on the G alpha protein that may be exploited for the design of synthetic stimulatory or inhibitory ligands. The effector binding site is regulated by endogenous proteins and appears to be a target for selective exogenous ligands. The GTP binding site presents a large homology within the G protein families and therefore the nucleotide analogs might not be considered as a tool to discriminate between the G protein subclasses. In contrast, different experimental strategies have substantiated the specificity in the interaction between a receptor and a G protein, the receptor binding site of G proteins should be considered as potential drug targets. Drugs interfering with this site such as mastoparan and related peptides, GPAnt-2 and suramin, are lead compounds in the design of selective G protein antagonists. Benzalkonium chloride and methoctramine have agonist or antagonist properties, depending on G protein subtypes. Such compounds would be very useful to delineate the functions of G proteins and G protein-coupled receptors, to understand some side effects of drugs used in therapy and to develop new therapeutic agents.

Morphine-induced venodilation in humans

Morphine has been extensively used in the treatment of pulmonary edema, and its action is believed to be mediated in part by its ability to produce peripheral venodilation. This study investigated whether opiates produce venodilation in human hand veins and explored the underlying mechanism(s). Fifteen healthy volunteers (11 men and four women) were studied with use of the dorsal hand vein compliance technique. After preconstriction with the selective alpha 1-adrenergic receptor agonist phenylephrine, dose-response curves were constructed to (1) opiate receptor agonists morphine (1 to 30 micrograms/min) or fentanyl (0.07 to 1 microgram/min), (2) a combination of morphine and the mu-opiate receptor antagonist naloxone, and (3) morphine and a combination of histamine (H1 and H2) receptor antagonists. Infusion of morphine caused venodilation in a dose-dependent manner, whereas fentanyl did not produce venodilation. Coinfusion of naloxone and morphine impaired the venodilation only slightly. Coinfusion of the H1- and H2-antagonists completely abolished the venodilatory effect of morphine. These results suggest that the venodilatory effect of morphine is mediated through histamine release and that mu-opiate receptors have little or no involvement in this process.

Human and rat cutaneous mast cells: involvement of a G protein in the response to peptidergic stimuli

Recent evidence suggests that peptides induce the release of mediators from rat peritoneal mast cell by means of a receptor-independent mechanism, possibly involving an interaction with sialic acid residues at the cell surface followed by the activation of a guanine nucleotide binding protein (G protein). We have now examined the potential involvement of sialic acid residues and of G protein stimulation in the activation of both human and rat cutaneous mast cells by neuropeptide Y, its C-terminal fragments and the wasp venom peptide, mastoparan. Neuropeptide Y-(18-36) was the most effective histamine releaser of the fragments tested, the order of potency being neuropeptide Y-(18-36) > neuropeptide Y-(22-36) > neuropeptide Y-(1-36). This order of potency suggests that the effects of the peptides are not mediated through classical NPY receptors. The hydrolysis of sialic acid residues by neuraminidase and the inhibition of G proteins by benzalkonium chloride or pertussis toxin significantly inhibited the secretory response of cutaneous mast cells to neuropeptide Y-(18-36) and mastoparan. These results demonstrate that the peptidergic pathway described for the activation of peritoneal rat mast cells is also involved in the response of cutaneous human and rat mast cells to peptides.

Effect of ion composition on the changes in membrane potential induced with several stimuli in rat mast cells

We studied, in different ionic conditions, the effect of various agents on the membrane potential of rat peritoneal mast cells using the fluorescent probe bisoxonol. Ouabain and ionophore A23187 lead to a fast depolarization of the plasma membrane of mast cells, while compound 48/80 and thapsigargin induced membrane hyperpolarization, which was more pronounced in the case of compound 48/80. When using compound 48/80, the amount of gramicidin necessary to depolarize the cells was twice the amount required in resting cells, which indicates that compound 48/80 increases considerably the activity of the Na+/K+ pump. On the other hand, the ionophore A23187 elicited a clear depolarization which was oblated in the absence of intracellular calcium. The increase in the osmolarity of the medium causes a depolarization in the plasma membrane of mast cells. Hypertonicity-stimulated depolarization is inhibited by removing sodium and potassium.

Peptidergic pathway in human skin and rat peritoneal mast cell activation

The common pathway of heterogenous mast cell activation as mediated by antigens is through the cross-linking of IgE bound to Fc epsilon RI receptors. The peptidergic pathway of mast cell activation, achieved by cationic secretagogues, is restricted to "serosal" mast cells, the experimental models being rat peritoneal and human skin mast cells. Cationic secretagogues include positively charged peptides but also various amines such as compound 48/80 and natural polyamines. An early intracellular event of this pathway is the activation of pertussis toxin-sensitive G proteins. The correlation observed between the ability of basic compounds to trigger mast cell exocytosis and their potency to activate purified G proteins strongly suggests that cationic compounds activate mast cell G proteins via a receptor-independent but membrane-assisted process. In this paper, alternative mechanisms are discussed. The consequence of G protein stimulation is the activation of phospholipase C with an increase in inositol triphosphates. Natural polyamines are relatively poor triggers of mast cells (10(-4) to 10(-2) M). Neuropeptides such as substance P, neuropeptide Y or vasoactive intestinal peptide, peptidic hormones such as kinins, and venoms such as mastoparan and mast cell degranulating peptide, are all active in a concentration range from 10(-7) to 10(-4) M. The cationic anaphylatoxin C3a also stimulates mast cells at concentrations below precursor complement C3 blood levels. The component C3 of the complement system is one of only a few plasma proteins having activation fragments (i.e. C3a) that can be generated at micromolar levels. The effects of basic secretagogues defines a peptidergic pathway of mast cell activation, which represents a potentially toxic process considering the tissue effects caused by exogenous basic compounds such as venom peptides and certain amine containing drugs. Peptidergic activation of mast cells may also be a pathophysiological process having an important role in neurogenic inflammation and in diseases involving extensive activation of the blood complement cascade.

Structure-activity studies of bradykinin analogues on rat mast cell histamine release

Bradykinin (BK), kallidin (KD), and various analogues induced histamine release from rat mast cells. The results obtained with substituted analogues of BK indicated that: 1) the presence of both Arg residues at position 1 and 9 of kinins was favorable to confer histamine-releasing activity, 2) acetylation of the N-terminal amino acid residue led to a drastic reduction of this activity, 3) addition of a D-Arg residue at the N-terminus reduced their activity, as well as trans-4-hydroxyproline (Hyp) substitutions at position 2 or 3,4) D-Arg0 addition and Hyp3 substitution were synergistic in lowering activity, and 5) D-Phe7 substitution led to enhanced histamine-releasing activity.

The mechanism of inhibition of alkylamines on the mast-cell peptidergic pathway

GTP-binding proteins are known to play an important role in controlling mast-cell exocytosis and are described as the primary targets of peptidic mast-cell histamine releasers. The mechanism of inhibition of the mast-cell peptidergic pathway by alkylamines, which are selective inhibitors of this pathway, was investigated using intact or permeabilized rat peritoneal mast cells. Histamine release induced by GTP gamma S and by mastoparan (a venom peptide activating G proteins) was inhibited by pretreating mast cells with 0.1 to 3 micrograms/ml of a mixture of benzalkonium chloride containing in majority a twelve-carbon-atom aliphatic chain (BAC(C approximately 12)). Pure benzalkonium chloride, with a fourteen-carbon-atom aliphatic chain (BAC (C14)), at 5 to 10 microM also inhibited histamine release induced by GTP gamma S and mastoparan. The dose-response curve of mastoparan-induced histamine release from intact mast cells was shifted to the right by various concentrations of BAC (C14). Similar results were obtained with another alkylamine differing from BAC (C14) by the absence of the benzene ring, tetradecyltrimethylammonium bromide, TAB (C14). This illustrates that the presence of the phenyl radical is not required for the inhibitory effect of benzalkonium chloride. BAC (C approximately 12) and BAC (C14) inhibited the generation of inositol polyphosphates induced by GTP gamma S. BAC (C approximately 12) and TAB (C14) inhibited the mastoparan-stimulated GTPase activity from mast-cell Gi-like proteins. These results suggest that alkylamines exert selectively their inhibitory effect via an interaction with mast-cell Gi-like proteins coupled to phospholipase C, i.e., at an early stage in the stimulus-secretion coupling process.