Activation of nucleoside diphosphate kinase by mastoparan, a peptide isolated from wasp venom

Mastoparans: A Group of Multifunctional α-Helical Peptides With Promising Therapeutic Properties

Biologically active peptides have been attracting increasing attention, whether to improve the understanding of their mechanisms of action or in the search for new therapeutic drugs. Wasp venoms have been explored as a remarkable source for these molecules. In this review, the main findings on the group of wasp linear cationic α-helical peptides called mastoparans were discussed. These compounds have a wide variety of biological effects, including mast cell degranulation, activation of protein G, phospholipase A₂, C, and D activation, serotonin and insulin release, and antimicrobial, hemolytic, and anticancer activities, which could lead to the development of new therapeutic agents.

Emerging roles for protein histidine phosphorylation in cellular signal transduction: lessons from the islet beta-cell

Protein phosphorylation represents one of the key regulatory events in physiological insulin secretion from the islet β-cell. In this context, several classes of protein kinases (e.g. calcium-, cyclic nucleotide- and phospholipid-dependent protein kinases and tyrosine kinases) have been characterized in the β-cell. The majority of phosphorylated amino acids identified include phosphoserine, phosphothreonine and phosphotyrosine. Protein histidine phosphorylation has been implicated in the prokaryotic and eukaryotic cellular signal transduction. Most notably, phoshohistidine accounts for 6% of total protein phosphorylation in eukaryotes, which makes it nearly 100-fold more abundant than phosphotyrosine, but less abundant than phosphoserine and phosphothreonine. However, very little is known about the number of proteins with phosphohistidines, since they are highly labile and are rapidly lost during phosphoamino acid identification under standard experimental conditions. The overall objectives of this review are to: (i) summarize the existing evidence indicating the subcellular distribution and characterization of various histidine kinases in the islet β-cell, (ii) describe evidence for functional regulation of these kinases by agonists of insulin secretion, (iii) present a working model to implicate novel regulatory roles for histidine kinases in the receptor-independent activation, by glucose, of G-proteins endogenous to the β-cell, (iv) summarize evidence supporting the localization of protein histidine phosphatases in the islet β-cell and (v) highlight experimental evidence suggesting potential defects in the histidine kinase signalling cascade in islets derived from the Goto-Kakizaki (GK) rat, a model for type 2 diabetes. Potential avenues for future research to further decipher regulatory roles for protein histidine phosphorylation in physiological insulin secretion are also discussed.

Mammalian histidine kinases

Protein phosphorylation is one of the most ubiquitous and important types of post-translational modification for the regulation of cell function. The importance of two-component histidine kinases in bacteria, fungi and plants has long been recognised. In mammals, the regulatory roles of serine/threonine and tyrosine kinases have attracted most attention. However, the existence of histidine kinases in mammalian cells has been known for many years, although little is still understood about their biological roles by comparison with the hydroxyamino acid kinases. In addition, with the exception of NDP kinase, other mammalian histidine kinases remain to be identified and characterised. NDP kinase is a multifunctional enzyme that appears to act as a protein histidine kinase and as such, to regulate the activation of some G-proteins. Histone H4 histidine kinase activity has been shown to correlate with cellular proliferation and there is evidence that it is an oncodevelopmental marker in liver. This review mainly concentrates on describing recent research on these two types of histidine kinase. Developments in methods for the detection and assay of histidine kinases, including mass spectrometric methods for the detection of phosphohistidines in proteins and in-gel kinase assays for histone H4 histidine kinases, are described. Little is known about inhibitors of mammalian histidine kinases, although there is much interest in two-component histidine kinase inhibitors as potential antibiotics. The inhibition of a histone H4 histidine kinase by genistein is described and that of two-component histidine kinase inhibitors of structurally-related mammalian protein kinases. In addition, recent findings concerning mammalian protein histidine phosphatases are briefly described.

Novel features of amphiphilic peptide Mas7 in signalling via heterotrimeric G-proteins

Amphiphilic peptide Mas7, a structural analogue of mastoparan is a known activator of heterotrimeric Gi-proteins and its downstream effectors. This study investigated the functional interaction of Mas7 with a plasma membrane protein from CHO cells, the endogenous mono-ADP-ribosyltransferase. The substrate of endogenous mono-ADP-ribosyltransferase was the ADP-ribosylated protein with a molecular mass of 36 kDa, which corresponded to the beta subunit of heterotrimeric G-proteins. The effect of Mas7 on endogenous mono-ADP-ribosyltransferase activity was in the micromolar range with a maximal activation of 205% over the basal. In pertussis treated plasma membranes, it was found that the effect of Mas7 on endogenous mono-ADP-ribosyltransferase was partially blocked, which suggests the involvement of G-proteins, such as Gi or G0. In addition, an immunoassay was developed for the visualization of interaction between the a subunit and the betagamma dimer of G-protein on a Ni-NTA support. The physical interaction was tested of Mas7 with the heterotrimeric G-protein alphai2 subunit, which was overexpressed together with beta1gamma2-His6 subunits in sf9 cells. An interaction between Gi2 heterotrimer and Mas7 was not observed, which was not in accordance with previously reported results of mastoparan obtained for Gi-proteins from bovine brain. In conclusion, the signal is mediated from Mas7 to endogenous mono-ADP-ribosyltransferase via pertussis sensitive G-proteins. Furthermore, it is hypothesized that Gi2 G-proteins are not involved in the process.

Degradation of yolk platelets in the early amphibian embryo is regulated by fusion with late endosomes

The eggs of many animal species contain a large store of yolk platelets, lipid droplets and glycogen granules; these are consumed during early embryogenesis. However, the mechanisms by which degradation of these stored materials occurs during early embryogenesis are not clearly understood. The mechanisms underlying yolk degradation in amphibian (newt) embryos were investigated. Electron microscopy using an anion marker, cationic ferritin, revealed that yolk platelets were degraded after fusion with late endosomes containing primary lysosomes. Electron microscopy and the results of experiments using a number of reagents with selective effects on intracellular transport suggested that yolk degradation activity in early amphibian embryos may be regulated at the point of fusion between late endosomes and yolk platelets.

Nucleoside diphosphate kinase activity in soluble transducin preparations biochemical properties and possible role of transducin-beta as phosphorylated enzyme intermediate

Known nucleoside diphosphate kinases (NDPKs) are oligomers of 17-23-kDa subunits and catalyze the reaction N1TP + N2DP --> N1DP + N2TP via formation of a histidine-phosphorylated enzyme intermediate. NDPKs are involved in the activation of heterotrimeric GTP-binding proteins (G-proteins) by catalyzing the formation of GTP from GDP, but the properties of G-protein-associated NDPKs are still incompletely known. The aim of our present study was to characterize NDPK in soluble preparations of the retinal G-protein transducin. The NDPK is operationally referred to as transducin-NDPK. Like known NDPKs, transducin-NDPK utilizes NTPs and phosphorothioate analogs of NTPs as substrates. GDP was a more effective phosphoryl group acceptor at transducin-NDPK than ADP and CDP, and guanosine 5'-[gamma-thio]triphosphate (GTP[S]) was a more effective thiophosphoryl group donor than adenosine 5'-[gamma-thio]triphosphate (ATP[S]). In contrast with their action on known NDPKs, mastoparan and mastoparan 7 had no stimulatory effect on transducin-NDPK. Guanosine 5'-[beta, gamma-imido]triphosphate (p[NH]ppG) potentiated [3H]GTP[S] formation from [3H]GDP and ATP[S] but not [3H]GTP[S] formation from [3H]GDP and GTP[S]. Depending on the thiophosphoryl group acceptor and donor, [3H]NTP[S] formation was differentially regulated by Mg2+, Mn2+, Co2+, Ca2+ and Zn2+. [gamma-32P]ATP and [gamma-32P]GTP [32P]phosphorylated, and [35S]ATP[S] [35S]thiophosphorylated, a 36-kDa protein comigrating with transducin-beta. p[NH]ppG potentiated [35S]thiophosphorylation of the 36-kDa protein. 32P-labeling of the 36-kDa protein showed characteristics of histidine phosphorylation. There was no evidence for (thio)phosphorylation of 17-23-kDa proteins. Our data show the following: (a) soluble transducin preparations contain a GDP-prefering and guanine nucleotide-regulated NDPK; (b) transducin-beta may serve as a (thio)phosphorylated NDPK intermediate; (c) transducin-NDPK is distinct from known NDPKs and may consist of multiple kinases or a single kinase with multiple regulatory domains.

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.

Effects of the wasp venom peptide, mastoparan, on GTP hydrolysis in rat brain membranes

1. The effects of mastoparan, a wasp venom toxin, on GTP hydrolyzing activity were examined in rat brain membranes. 2. Mastoparan inhibited the low-affinity GTPase activity, defined as the amount of 32Pi released from 0.3 microm [gamma-32P]-GTP in the presence of 100 microM unlabelled GTP, in a concentration-dependent manner. This inhibitory effect of mastoparan on low-affinity GTPase activity was diminished by increasing concentrations of UDP and was completely attenuated at 20 mM, indicating that activation of nucleoside diphosphokinase (NDPK) is inolved in the phenomenon. 3. In the presence of 20 mM UDP, mastoparan stimulated the high-affinity GTPase activity by increasing the Vmax value without affecting the apparent K(M) for GTP. Mastoparan-stimulated high-affinity GTPase activity was apparent at concentrations higher than 1 microM, in a concentration-dependent manner, but without saturation even at 100 microM. 4. Mastoparan-induced high-affinity GTPase activity showed a characteristic sensitivity to MgCl2, quite different from that seen in L-glutamate-stimulated activity, a representative of receptor-mediated G-protein activation. 5. There appeared to be a simple additive interaction between mastoparan- and L-glutamate-stimulated high-affinity GTPase activities, indicting that distinct pools of G-proteins are involved in receptor-independent and receptor-mediated G-protein activation. 6. These results suggest that G-proteins in brain membranes are functionally altered by mastoparan through multiple mechanisms of action and that the mastoparan-induced, direct G-protein activating process lacks a synergistic or antagonistic interaction with an agonist-induced, receptor-mediated activation of G-proteins.

Induction of cytosolic Ca2+ elevation mediated by Mas-7 occurs through membrane pore formation

Mas-7, a mastoparan derivative, induces elevation of intracellular free Ca2+ concentration ([Ca2+]i) along two independent pathways. The minor contribution occurs via phospholipase C activation and is negatively regulated by treatment with phorbol 12-myristate 13-acetate, a protein kinase C activator. The major contribution involves plasma membrane pores allowing not only Ca2+, Mn2+, and Na+ to enter but also the uptake of ethidium bromide (314 Da) and lucifer yellow (457 Da), but not fura-2 (831 Da), Evans blue (961 Da), and fluorescein-conjugate phalloidin (1,175 Da). Mas-7-induced current, as measured in planar lipid bilayers, reveals that Mas-7-induced pores have two slope conductances, 290 and 94 pS, and that the pores are nonselective for cations. The results also indicate that Mas-7 can produce pores by direct interaction with the plasma membrane without the involvement of membrane proteins and cytosolic factors. Besides in human neuroblastoma cells, similar Mas-7 effects were also observed in other cell lines such as HL-60, 1321N1 human astrocytoma, and bovine chromaffin cells. The data suggest that the Mas-7-induced [Ca2+]i elevation is the combined result of Ca2+ release from stores via phosphoinositide turnover and prolonged Ca2+ influx through membrane pores.

Pertussis toxin-insensitive effects of mastoparan, a wasp venom peptide, in PC12 cells

Recent studies have shown that mastoparan, an amphiphilic peptide derived from wasp venom, modifies the secretion of neurotransmitters and hormones from a variety of cell types. Mastoparan interacts with heterotrimeric guanine nucleotide-binding proteins (G proteins) such as Gi and G(o), which are ADP-ribosylated by pertussis toxin (PTX) and thereby uncoupled from receptors. Previously, some of the effects of mastoparan including secretion were reported to be modified selectively by PTX but not by cholera toxin (CTX). In the present study, we examined the influence of bacterial toxins on the effects of mastoparan in PC12 cells. Mastoparan stimulated [3H]noradrenaline (NA) release from prelabeled PC12 cells in the absence of CaCl2, although high K+ or ATP-stimulated the release in a Ca(2+)-dependent manner. Pretreatment with CTX, not PTX, for 24 h inhibited mastoparan-stimulated [3H]NA release. Mastoparan inhibited forskolin-stimulated cyclic AMP accumulation in a dose-dependent manner, although mastoparan had no effect by itself. Pretreatment with PTX completely abolished the inhibitory effect of carbachol via Gi on cyclic AMP accumulation and partially reduced the effect of mastoparan. However, the inhibitory effect of 20 microM mastoparan was not modified by pretreatment with PTX. Thus, we investigated the effect of mastoparan on CTX-catalyzed [32P]ADP-ribosylation of proteins in PC12 cells. A subunit of CTX (CTX-A) catalyzed [32P]ADP-ribosylation of many proteins in the cytosolic fraction of PC12 cells. One of these was a 20 kDa protein, named ADP-ribosylating factor (ARF). The addition of mastoparan to assay mixtures inhibited ADP-ribosylation of many proteins including ARF and CTX-A in the presence of the cytosolic fraction. In the absence of the cytosolic fraction, however, mastoparan slightly enhanced ADP-ribosylation of bovine serum albumin and auto-ADP-ribosylation by CTX-A. Mastoparan did not inhibit ADP-ribosylation of the alpha subunit of Gs in the membrane fraction. These findings suggest that 1) mastoparan interacts with PTX-insensitive and CTX-sensitive factor(s) to stimulate NA release, and 2) mastoparan interacts with ARF inhibiting its activity to enhance the ADP-ribosylation reaction by CTX. ARF may be an exocytosis-linked G protein.

Participation of nucleoside-diphosphate kinase in muscarinic K+ channel activation does not involve GTP formation

Agonist-bound muscarinic receptors open atrial K+ channels through a GTP-dependent pathway mediated by the G protein Gk. However, nucleotides other than GTP are also able to support channel activity, even in the absence of agonists. This process was proposed to be mediated by nucleoside-diphosphate (NDP) kinase, which would transfer phosphate from nucleotide triphosphates to the GDP bound to Gk, producing Gk-GTP without the need for receptor-induced GDP-GTP exchange. We examined the effect of antibodies to NDP kinase on the ATP-supported activity of atrial muscarinic K+ channels and the corresponding GIRK1/CIR channels expressed in HEK 293 cells. Inhibitory antibodies reduced ATP-induced channel openings, but this effect displayed an absolute requirement for agonist and was also seen with antibodies that do not inhibit the enzyme. Both types of antibodies also reduced agonist-dependent channel activity in the presence of GTP, ruling out a role for NDP kinase in GDP rephosphorylation. Channel activity was not affected by the antibodies in preparations where ATP-induced muscarinic channels are not under tight receptor control, namely pertussis toxin-treated atrial patches and membranes from cells expressing KACh channel subunits. Thus, participation of NDP kinase in this pathway requires activated receptors and has a function distinct from phosphate transfer between nucleotides.

Association of nucleoside diphosphate kinase with pancreatic zymogen granules: effects of local GTP generation on granule membrane characteristics

It is well established that both GTP-binding proteins and phosphoproteins are involved in the control of exocytosis in the exocrine pancreas. Exocytotic membrane fusion is stimulated by guanosine 5'-[gamma-thio]triphosphate, and the phosphorylation states of several proteins, including at least one on the zymogen granule membrane, are known to change during exocytosis. We show here that a nucleoside diphosphate kinase is associated with the cytoplasmic face of pancreatic zymogen granules. This enzyme behaves as a phosphoprotein of apparent molecular mass 21 kDa on SDS/polyacrylamide gels, and is able to produce GTP by using ATP to phosphorylate endogenous GDP. GTP production by nucleoside diphosphate kinase is stimulated by the wasp venom peptide mastoparan, both through a direct action on the enzyme and through its ability to increase the availability of endogenous GDP. Two effects of the GTP produced by nucleoside diphosphate kinase are demonstrated: phosphorylation of a 37 kDa zymogen granule protein on histidine residues, and stimulation of the fusion of zymogen granules with pancreatic plasma membranes in vitro. These results suggest that granule-associated nucleoside diphosphate kinase is able to maintain local GTP concentrations, and raise the possibility that it might be involved in the control of exocytosis in the pancreatic acinar cell.

Phosphotransfer reactions as a means of G protein activation

Heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins) serve to transduce information from agonist-bound receptors to effector enzymes or ion channels. Current models of G protein activation-deactivation indicate that the oligomeric GDP-bound form must undergo release of GDP, bind GTP and undergo subunit dissociation, in order to be in active form (GTP bound alpha subunits and free beta gamma dimers) and to regulate effectors. The effect of receptor occupation by an agonist is generally accepted to be promotion of guanine nucleotide exchange thus allowing activation of the G protein. Recent studies indicate that transphosphorylation leading to the formation of GTP from GDP and ATP in the close vicinity, or even at the G protein, catalysed by membrane-associated nucleoside diphosphate kinase, may further activate G proteins. This activation is demonstrated by a decreased affinity of G protein-coupled receptors for agonists and an increased response of G protein coupled effectors. In addition, a phosphorylation of G protein beta subunits and consequent phosphate transfer reaction resulting in G protein activation has also been demonstrated. Finally, endogenously formed GTP was preferentially effective in activating some G proteins compared to exogenous GTP. The aim of this report is to present an overview of the evidence to date for a transphosphorylation as a means of G protein activation (see also refs [1 and 2] for reviews).

Activation of GTP formation and high-affinity GTP hydrolysis by mastoparan in various cell membranes. G-protein activation via nucleoside diphosphate kinase, a possible general mechanism of mastoparan action

The wasp venom, mastoparan (MP), is a direct activator of reconstituted pertussis toxin-sensitive G-proteins and of purified nucleoside diphosphate kinase (NDPK) [E.C. 2.6.4.6.]. In HL-60 membranes, MP activates high-affinity GTPase [E.C. 3.6.1.-] and NDPK-catalyzed GTP formation, but not photolabeling of G-protein alpha-subunits with GTP azidoanilide; this suggests that the venom activates G-proteins in this system indirectly via stimulation of NDPK. Moreover, the MP analogue, mastoparan 7 (MP 7), is a much more effective activator of reconstituted G-proteins than MP, whereas with regard to NDPK and GTPase in HL-60 membranes, the two peptides are similarly effective. In our present study, we investigated NDPK- and G-protein activation by MP in membranes of the human neuroblastoma cell line, SH-SY5Y, the human erythroleukemia cell line, HEL, the rat basophilic leukemia cell line, RBL 2H3, and the hamster ductus deferens smooth muscle cell line, DDT1MF-2. All these membranes exhibited high NDPK activities that were increased by MP. Compared to basal GTP formation rates, basal rates of high-affinity GTP hydrolysis in cell membranes were low. MP activated high-affinity GTP hydrolysis in cell membranes but did not enhance incorporation of GTP azidoanilide into G-protein alpha-subunits. As with HL-60 membranes, MP and MP 7 were similarly effective activators of NDPK and GTPase in SH-SY5Y membranes. Pertussis toxin inhibited MP-stimulated GTP hydrolyses in SH-SY5Y- and HEL membranes, whereas NDPK activations by MP were pertussis toxin-insensitive. Our data suggest that indirect G-protein activation via NDPK is not restricted to HL-60 membranes but is a more general mechanism of MP action in cell membranes. Pertussis toxin-catalyzed ADP-ribosylation of alpha-subunits may inhibit the transfer of GTP from NDPK to G-proteins. NDPK may play a much more important role in transmembrane signal transduction than was previously appreciated and, moreover, the GTPase of G-protein alpha-subunits may serve as GDP-synthase for NDPK.

Mastoparan-induced phosphatidylcholine hydrolysis by phospholipase D activation in human astrocytoma cells

1. The effect of mastoparan on phosphatidylcholine hydrolysis was examined in 1321N1 human astrocytoma cells. Mastoparan (3-30 microM) caused an accumulation of diacylglycerol (DG) and phosphatidic acd (PA) accompanied by choline release in a concentration- and time-dependent manner. 2. In the presence of 2% n-butanol, mastoparan (3-100 microM) induced phosphatidylbutanol (PBut) accumulation in a concentration- and time-dependent manner, suggesting that mastoparan activates phospholipase D (PLD). Propranolol (30-300 microM), a phosphatidate phosphohydrolase inhibitor, inhibited DG accumulation induced by mastoparan, supporting this idea. 3. Depletion of extracellular free calcium ion did not alter the effect of mastoparan on PLD activity. 4. A protein kinase C (PKC) inhibitor, calphostin C (1 microM), did not inhibit mastoparan-induce PLD activation but the ability of mastoparan to stimulate phospholipase D activity was decreased in the PKC down regulated cells. 5. PLD activity stimulated by mastoparan was not prevented by pretreatment of the cells with pertussis toxin (PT) or C3 ADP-ribosyltransferase. Furthermore, guanine nucleotides did not affect PLD activity stimulation by mastoparan in membrane preparations. 6. Mastoparan stimulated PLD in several cell lines such as RBL-2H3, RBL-1, HL-60, P388, endothelial cells, as well as 1321N1 human astrocytoma cells. 7. These results suggest that mastoparan induces phosphatidylcholine (PC) hydrolysis by activation of PLD, not by activation of phosphatidylcholine-specific phospholipase C (PC-PLC); mastoparan-induced PLD activation is not mediated by G proteins.

Direct and indirect receptor-independent G-protein activation by cationic-amphiphilic substances. Studies with mast cells, HL-60 human leukemic cells and purified G-proteins

Studies from several laboratories have revealed that structurally diverse substances including the wasp venom, mastoparan (MP), activate purified regulatory heterotrimeric guanine nucleotide-binding proteins (G-proteins) in a receptor-independent manner, presumably by mimicking the effects of heptahelical receptors. Mast cells and differentiated HL-60 human leukemic cells are useful model systems for the analysis of receptor-independent G-protein activation. We compared the effects of 2-phenylhistamines which are cationic-amphiphilic, too, and of MP on G-protein activation in dibutyryl cAMP-differentiated HL-60 cells and in the rat basophilic leukemia cell line, RBL 2H3. In HL-60 cells, 2-phenylhistamines show stimulatory effects which resemble those of formyl peptide receptor agonists but which cannot be attributed to agonism at classical receptors. 2-phenylhistamines do not, however, activate RBL 2H3 cells and various other myeloid cell types, pointing to cell type-specificity of receptor-independent G-protein activation. In HL-60 cells, MP shows effects on G-protein activation which differ substantially from those of formyl peptides. In RBL 2H3 membranes, MP shows similar effects on G-protein activation as in HL-60 membranes. We develop a model according to which receptor-independent G-protein activation can be subdivided into direct and indirect receptor-independent G-protein activation. In case of the former mechanism, substances like 2-phenylhistamines interact with G-protein alpha-subunits and in case of the latter mechanism, substances like MP interact with nucleoside diphosphate kinase which catalyzes the formation of GTP. This newly formed GTP is then transferred to, and cleaved by, G-protein alpha-subunits.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of mastoparan upon the late stages of the ACTH secretory pathway of AtT-20 cells

1. The mouse AtT-20/D16-16 anterior pituitary tumour cell line was used as a model system for the study of the effects of mastoparan upon the late stages of the adrenocorticotrophin (ACTH) secretory pathway. 2. Mastoparan (10(-8)-10(-5) M), an activator of heterotrimeric guanosine 5'-triphosphate binding proteins (G-proteins), stimulated ACTH secretion from electrically-permeabilized AtT-20 cells in a concentration-dependent manner in the effective absence of calcium ions with a threshold of 10(-6) M. Guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) (10(-8)-10(-4) M) also stimulated ACTH secretion from electrically-permeabilized AtT-20 cells in a concentration-dependent manner in the effective absence of calcium ions with a threshold of 10(-6) M. This GTP-gamma-S-evoked secretion is consistent with previous studies which demonstrated that a G-protein, termed GE, mediates calcium evoked ACTH secretion from AtT-20 cells. GTP-gamma-S-evoked secretion however was not as great as that obtained in response to mastoparan. 3. Both mastoparan (10(-5) M) and GTP-gamma-S (10(-4) M) stimulated ACTH secretion from electrically-permeabilized AtT20 cells in a time-dependent manner. A time of 30 min was adopted as the standard incubation period for the study of both mastoparan and GTP-gamma-S-stimulated ACTH secretion from permeabilized AtT-20 cells. 4. Mastoparan (10(-8)-10(-5) M) stimulated ACTH secretion from permeabilized AtT-20 cells to the same extent in the presence and absence of the protein kinase C (PKC) inhibitor, chelerythrine chloride (10(-5) M). 5. Mastoparan (10-8 10-5 M)-stimulated ACTH secretion from permeabilized AtT-20 cells was significantly reduced in the presence of guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S, 10-4 M).6. The mastoparan analogue, Mas-7 (10-8-10-5 M) stimulated ACTH secretion from permeabilized AtT-20 cells to a greater extent than mastoparan (10-8 10-5 M) however, the mastoparan analogue Mas-17 (10-8- 10-5 M) had no effect upon ACTH secretion from permeabilized AtT-20 cells.7. Mastoparan (10-8-10-5 M) stimulated ACTH secretion from permeabilized AtT-20 cells in the presence and absence of ATP, normally present in the standard permeabilization medium at a concentration of 5 mM. Mastoparan (10-8- 10-5 M)-stimulated ACTH secretion as well as control secretion was reduced when ATP was omitted.8. The results of the present study demonstrate that mastoparan stimulated ACTH secretion from permeabilized AtT-20 cells and displayed characteristics consistent with calcium ion- and GTP-y-gamma-S-stimulated ACTH secretion from permeabilized AtT-20 cells. This suggests that in permeabilized AtT-20 cells, mastoparan directly activates GE and that this G-protein may be a heterotrimeric G-protein. This study also suggests mastoparan may be a useful alternative to GTP-gamma-S as a means of directly activating GE.

Non-specific stimulatory effects of mastoparan on pancreatic islet nucleoside diphosphokinase activity: dissociation from insulin secretion

We examined whether mastoparan (MAS)-induced insulin secretion might involve the activation of nucleoside diphosphokinase (NDP kinase), which catalyzes the conversion of GDP to GTP, a known permissive factor for insulin secretion. MAS and MAS 7 (which activate GTP-binding proteins), but not MAS 17 (an inactive analog), stimulated insulin secretion from normal rat islets. In contrast to their specific effects on insulin secretion, MAS, MAS 7 and MAS 17 each stimulated formation of the phosphoenzyme-intermediate of NDP kinase, as well as its catalytic activity. These effects were mimicked by several cationic drugs. Thus, caution is indicated in using MAS to study cellular regulation, since some of its effects appear to be non-specific, and may be due, in part, to its amphiphilic, cationic nature.

Mastoparan may activate GTP hydrolysis by Gi-proteins in HL-60 membranes indirectly through interaction with nucleoside diphosphate kinase

The wasp venom, mastoparan (MP), activates reconstituted pertussis toxin (PTX)-sensitive G-proteins in a receptor-independent manner. We studied the effects of MP and its analogue, mastoparan 7 (MP 7), on G-protein activation in HL-60 cells and a reconstituted system and on nucleoside diphosphate kinase (NDPK)-catalysed GTP formation. MP activated high-affinity GTP hydrolysis in HL-60 membranes with an EC50 of 1-2 microM and a maximum at 10 microM. Unlike the effects of the formyl peptide receptor agonist, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMet-Leu-Phe), on GTPase, those of MP were only partially PTX-sensitive. MP-induced rises in cytosolic Ca2+ concentration and superoxide-anion formation in intact HL-60 cells were also only incompletely PTX-sensitive. N-Ethylmaleimide inhibited MP-stimulated GTP hydrolysis to a greater extent than that stimulated by fMet-Leu-Phe. Unlike the latter, MP did not enhance incorporation of GTP azidoanilide into, and cholera toxin-catalysed ADP-ribosylation of, Gi-protein alpha-subunits in HL-60 membranes. By contrast to fMet-Leu-Phe, MP did not or only weakly stimulated binding of guanosine 5'-[gamma-thio]triphosphate to Gi-protein alpha-subunits. MP 7 was considerably more effective than MP at activating the GTPase of reconstituted Gi/G(o)-proteins, whereas in HL-60 membranes, MP and MP 7 were similarly effective. MP and MP 7 were similarly effective at activating [3H]GTP formation from [3H]GDP and GTP in HL-60 membranes and by NDPK purified from bovine liver mitochondria. Our data suggest the following: (1) MP activates Gi-proteins in HL-60 cells, but (2) the venom does not simply mimic receptor activation. (3) MP and MP 7 may activate GTP hydrolysis in HL-60 membranes indirectly through interaction with NDPK. (4) MP 7 is a more effective direct activator of PTX-sensitive G-proteins than MP, whereas with regard to NDPK, MP and MP 7 are similarly effective.

Lipophilic beta-adrenoceptor antagonists and local anesthetics are effective direct activators of G-proteins

We studied the effects of various beta-adrenoceptor (beta AR) antagonists and local anesthetics (LAs), i.e. substances possessing one basic and one lipophilic domain each, on activation of regulatory heterotrimeric guanine nucleotide-binding proteins (G-proteins). In membranes of differentiated HL-60 cells, propranolol activated high-affinity GTP hydrolysis with a half-maximal effect at 0.19 mM and a maximum at 1 mM. There was a close correlation between the log Q values (logarithm of the octanol: water partition coefficient) of beta AR antagonists and the logarithm of their effectiveness at activating GTPase (EC 3.6.1.-) in HL-60 membranes. The lipophilic LA, tetracaine, was also an effective activator of GTPase in HL-60 membranes, whereas more hydrophilic LAs were less stimulatory (bupivacaine and lidocaine) or even inhibitory (procaine). Propranolol and tetracaine also stimulated binding of guanosine 5'-O-[3-thio]triphosphate (GTP[gamma S]) to HL-60 membranes, but their stimulatory effects on GTP[gamma S] binding were smaller than on GTP hydrolysis. The stimulatory effects of propranolol and tetracaine on GTPase and GTP[gamma S] binding were inhibited by pertussis toxin. Propranolol and tetracaine effectively activated GTP hydrolysis of a reconstituted mixture of bovine brain Gi/Go-proteins, but the concentrations of substances needed for GTPase activation were higher than in HL-60 membranes. Procaine showed stimulatory effects on the GTPase of Gi/Go-proteins. Our data show that beta AR antagonists and LAs activate pertussis toxin-sensitive G-proteins, presumably through interaction with the C-terminus of their alpha-subunits. Apparently, the lipophilic domain of beta AR antagonists and LAs is more important for G-protein activation than the basic domain. We discuss the possibility that activation of nucleoside diphosphate kinase by beta AR antagonists and LAs contributes to their stimulatory effects on GTP hydrolysis in HL-60 membranes.

Dual effects of mastoparan on intracellular free Ca2+ concentrations in human astrocytoma cells

1. The effect of mastoparan, a wasp venom toxin, on intracellular free Ca2+ concentration ([Ca2+]i) was examined in human astrocytoma cells. Mastoparan inhibited [Ca2+]i induced by carbachol (100 microM) in a concentration-dependent manner in the absence of extracellular Ca2+, consistent with our previous results showing that mastoparan inhibits phosphoinositide hydrolysis in human astrocytoma cells. 2. In contrast, mastoparan itself increased [Ca2+]i and augmented carbachol-induced increase in the [Ca2+]i in the presence of extracellular Ca2+, suggesting that mastoparan elicited Ca2+ influx from the extracellular medium. The increase appeared to be maximum at extracellular Ca2+ concentrations of 0.1-0.2 mM. The higher concentrations of extracellular Ca2+ depressed the influx. 3. Pertussis toxin did not affect mastoparan-induced inhibition of [Ca2+]i in the absence of extracellular Ca2+, consistent with the previous results that pertussis toxin did not affect mastoparan-induced inhibition of phosphoinositide hydrolysis. 4. Pertussis toxin augmented mastoparan-induced increase in [Ca2+]i in the presence of extracellular Ca2+, suggesting that pertussis toxin substrate(s) seems to be inhibitory for Ca2+ influx induced by mastoparan. 5. Verapamil, nifedipine and diltiazem (each 10 microM), L-type Ca2+ antagonists, did not affect mastoparan-induced Ca2+ influx. However, verapamil (10 microM) slightly inhibited the increase in [Ca2+]i induced by carbachol in the presence of mastoparan. 6. The results obtained in the present study indicate that mastoparan has two opposite effects on [Ca2+]i in human astrocytoma cells and possibly has at least two sites of action.