Mastoparan promotes exocytosis and increases intracellular cyclic AMP in human platelets. Evidence for the existence of a Ge-like mechanism of secretion

Helminthes and insects: maladies or therapies

By definition, parasites cause harm to their hosts. But, considerable evidence from ancient traditional medicine has supported the theory of using parasites and their products in treating many diseases. Maggots have been used successfully to treat chronic, long-standing, infected wounds which failed to respond to conventional treatment by many beneficial effects on the wound including debridement, disinfection, and healing enhancement. Maggots are also applied in forensic medicine to estimate time between the death and discovery of a corpse and in entomotoxicology involving the potential use of insects as alternative samples for detecting drugs and toxins in death investigations. Leeches are segmented invertebrates, famous by their blood-feeding habits and used in phlebotomy to treat various ailments since ancient times. Leech therapy is experiencing resurgence nowadays in health care principally in plastic and reconstructive surgery. Earthworms provide a source of medicinally useful products with potential antimicrobial, antiviral, and anticancer properties. Lumbrokinases are a group of fibrinolytic enzymes isolated and purified from earthworms capable of degrading plasminogen-rich and plasminogen-free fibrin and so can be used to treat various conditions associated with thrombotic diseases. Helminth infection has been proved to have therapeutic effects in both animal and human clinical trials with promising evidence in treating many allergic diseases and can block the induction of or reduce the severity of some autoimmune disorders as Crohn's disease or ulcerative colitis. What is more, venomous arthropods such as scorpions, bees, wasps, spiders, ants, centipedes, snail, beetles, and caterpillars. The venoms and toxins from these arthropods provide a promising source of natural bioactive compounds which can be employed in the development of new drugs to treat diseases as cancer. The possibility of using these active molecules in biotechnological processes can make these venoms and toxins a valuable and promising source of natural bioactive compounds. The therapeutic use of helminthes and insects will be of great value in biomedicine and further studies on insect toxins will contribute extensively to the development of Biomedical Sciences.

Identification of the thiol isomerase-binding peptide, mastoparan, as a novel inhibitor of shear-induced transforming growth factor β1 (TGF-β1) activation

TGF-β1 is a disulfide-bonded homodimeric protein produced by platelets and other cells that plays a role in many physiologic and pathologic processes. TGF-β1 is secreted as an inactive large latent complex (LLC) comprised of TGF-β1, latency-associated peptide, and latent TGF-β binding protein 1. We previously demonstrated that shear force can activate LLC and that thiol-disulfide exchange contributes to the process. We have now investigated the role of thiol isomerases in the activation of LLC in platelet releasates (PR) and recombinant LLC. The wasp venom peptide mastoparan, which inhibits the chaperone activity of PDI, inhibited stirring- and shear-induced activation of latent TGF-β1 by 90 and 75% respectively. To identify the proteins that bind to mastoparan either directly or indirectly, PR were chromatographed on a mastoparan affinity column. Latent TGF-β binding protein 1, latency-associated peptide, TGF-β1, clusterin, von Willebrand factor, multimerin-1, protein disulfide isomerase (PDI), ERp5, ERp57, and ERp72 eluted specifically from the column. Anti-PDI RL90 attenuated the inhibitory effect of mastoparan on LLC activation. Furthermore, reduced PDI inhibited activation of PR LLC, whereas oxidized PDI had no effect. We conclude that thiol isomerases and thiol-disulfide exchange contribute to TGF-β1 activation and identify a number of molecules that may participate in the process.

Characterization of naturally occurring peptides in the skin secretion of Rana pipiens frog reveal pipinin-1 as the novel insulin-releasing agent

Naturally occurring insulinotropic peptides were isolated from the skin secretions of Rana pipiens frogs. Crude secretions (50 mg; 5-10 frogs) obtained by mild electrical stimulation of the dorsal skin surface were purified by reversed-phase high-performance liquid chromatography (HPLC) yielding 80 fractions. In acute incubations with glucose-responsive BRIN-BD11 cells, fractions 40-47 (band 1) and fractions 60-65 (band 2) showed significant 1.7-6.7-fold increases in insulin-releasing activity (P < 0.001) compared with 5.6 mm glucose alone. Pooled fractions in bands 1 and 2 were rechromatographed yielding a total of seven peaks capable of subsequent 1.2-1.8-fold stimulation of insulin release. Final purification by HPLC to single homogenous peaks revealed one prominent peptide (peak 4.1) with insulin-releasing activity which lacked effects on cell viability. Electrospray mass spectrometric analysis of this peptide indicated molecular mass of 2562.6 Da. Determination of the primary amino acid sequence of this peptide revealed a 24-amino acid sequence: FLPIIAGVAAKVFPKIFCAISKKC. Database search showed a 100% homology to histamine-releasing pipinin-1. In conclusion, this study revealed the skin secretions of Rana pipiens to be a rich source of insulin-releasing peptides. The discovery of insulinotropic activity for pipinin-1, initially characterized as an antimicrobial is interesting and merits further investigation.

Differential properties of GTP- and Ca(2+)-stimulated exocytosis from large dense core vesicles

Many cells utilize a GTP-dependent pathway to trigger exocytosis in addition to Ca(2+)-triggered exocytosis. However, little is known about the mechanism by which GTP triggers exocytosis independent of Ca(2+). We used dual-color evanescent field microscopy to compare the motion and fusion of large dense core vesicles stimulated by either mastoparan (Mas) in Ca(2+)-free conditions or high K(+) in the presence of Ca(2+). We demonstrate that Mas is hardly effective in triggering the fusion of the predocked vesicles but predominantly mobilizes cytosolic vesicles. In contrast, Ca(2+)-dependent exocytosis is largely due to predocked vesicles. Fusion kinetics analysis and carbon-fiber amperometry reveal that Mas induces a brief 'kiss-and-run' fusion and releases only a small amount of the cargo, whereas Ca(2+) stimulates a more persistent opening of the fusion pore and larger release of the contents. Furthermore, we show that Mas-released vesicles require a much shorter time to reach fusion competence once they approach the plasma membrane. Our data suggest the involvement of different mechanisms not only in triggering and fusion but also in the docking and priming process for Ca(2+)- and GTP-dependent exocytosis.

An antimicrobial peptide tachyplesin acts as a secondary secretagogue and amplifies lipopolysaccharide-induced hemocyte exocytosis

In the horseshoe crab, bacterial lipopolysaccharide (LPS) induces exocytosis by granular hemocytes, resulting in the secretion of various defense molecules, such as lectins and antimicrobial peptides, via a G protein-mediating signaling pathway. This response is a key component of the horseshoe crab innate immune response against infectious microorganisms. Here, we report an endogenous amplification mechanism for LPS-induced hemocytes exocytosis. The concentration of LPS required for maximal secretion decreased in proportion to the density of hemocytes, suggesting the presence of a positive feedback mechanism for secretion via a mediator secreted from hemocytes. The exocytosed fluid of hemocytes was found able to induce hemocyte exocytosis in the absence of LPS. Furthermore, tachyplesin, a major antimicrobial peptide of hemocytes, was able to trigger exocytosis in an LPS-independent manner, which was inhibited by a phospholipase C inhibitor, U-73122, and a G protein inhibitor, pertussis toxin. Surface plasmon resonance analysis showed that tachyplesin directly interacts with bovine G protein. These findings suggest that the tachyplesin-induced hemocyte exocytosis also occurs via a G protein-mediating signaling pathway. We concluded that tachyplesin functions not only as an antimicrobial substance, but also as a secondary secretagogue of LPS-induced hemocyte exocytosis, leading to the amplification of the innate immune reaction at sites of injury.

Regulation of mastoparan-induced increase of paracellular permeability in T84 cells by RhoA and basolateral potassium channels

Mastoparan, a polypeptide known to activate heterotrimeric GTP-binding proteins, enhances the transport of Ca2+ and K+ across membranes. In the present study we investigated the influence of mastoparan on transepithelial resistance (TER) and on short circuit current (SCC) of the intestinal cell line T84. Mastoparan decreased the TER by 80% of baseline and induced a SCC of 8.34+/-1.38 microAcm(-2). The changes in paracellular conductance were estimated using the nystatin technique and showed that mastoparan increased the paracellular conductance 4-fold. Basolateral Cl(-)-free medium, or blockade of the basolateral Cl(-) uptake via the Na+/K+/2Cl(-) co-transporter with bumetanide, reduced SCC of T84 cells, but did not abolish the effect of mastoparan on the TER. Luminal addition of the Cl(-)-channel blocker DIDS or NPPB had no effect on the increase in SCC. In contrast, blocking the basolateral K(+)-channels by 2mM Ba2+ inhibited both the resistance decrease and elevation of the SCC, and further inhibited the mastoparan-induced increase in intracellular free Ca2. This indicates that mastoparan acts primarily via activating K+ channels with a secondary Cl(-) secretion and Ca2+ influx. Reduction of intracellular free Ca2+ did not alter the effect of mastoparan on TER. Stimulation with mastoparan led to a biphasic rearrangement of actin filaments and increased globular actin content in T84 cells. Depolymerization of actin filaments also correlated with inactivation of Rho-proteins, which are known regulators of the cytoskeleton. Mastoparan induced a 2-fold increase in GDI-complexed Rho. We conclude that mastoparan-induced changes in paracellular permeability are mediated via enhanced basolateral K+ conductance and Rho-protein inactivation. A secondary increase in intracellular Ca2+ or direct interaction of small GTPases with the cytoskeleton are likely mediators of the remodeling of the cytoskeleton with subsequent changes in paracellular permeability.

Subcellular distribution of 3 functional platelet SNARE proteins: human cellubrevin, SNAP-23, and syntaxin 2

Morphologic studies have demonstrated a process by which alpha-granule contents are released from platelets. Studies aimed at defining the molecular mechanisms of this release have demonstrated that SNARE proteins are required for alpha-granule secretion. These observations raise the possibility that morphologic features of alpha-granule secretion may be influenced by the subcellular distribution of SNARE proteins in the platelet. To evaluate this possibility, we analyzed the subcellular distribution of 3 functional platelet SNARE proteins-human cellubrevin, SNAP-23, and syntaxin 2. Exposure of streptolysin O-permeabilized platelets to antihuman cellubrevin antibody inhibited Ca(++)-induced alpha-granule secretion by approximately 50%. Inhibition of alpha-granule secretion by antihuman cellubrevin was reversed by a blocking peptide. Syntaxin 2 and SNAP-23 have previously been demonstrated to mediate platelet granule secretion. The subcellular localization of the 3 SNARE proteins was determined by ultrastructural studies, using a pre-embedding immunonanogold method, and by immunoblot analysis of subcellular fractions. Immunonanogold localization demonstrated that approximately 80% of human cellubrevin in resting platelets was localized to platelet granule membranes. In contrast, SNAP-23 localized predominantly to plasma membrane, whereas syntaxin 2 was more evenly distributed among membranes of alpha-granules, the open canalicular system, and plasma membrane. Thus, each of these SNARE proteins has a distinct subcellular distribution in platelets, and each of these membrane compartments demonstrates a unique SNARE protein composition. This distribution provides a basis for several characteristics of alpha-granule secretion that include homotypic alpha-granule fusion and the fusion of alpha-granules with the open canalicular system and plasma membrane.

Phosphatidylinositol 4,5-bisphosphate mediates Ca2+-induced platelet alpha-granule secretion: evidence for type II phosphatidylinositol 5-phosphate 4-kinase function

To understand the molecular basis of granule release from platelets, we examined the role of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) in alpha-granule secretion. Streptolysin O-permeabilized platelets synthesized PtdIns(4,5)P(2) when incubated in the presence of ATP. Incubation of streptolysin O-permeabilized platelets with phosphatidylinositol-specific phospholipase C reduced PtdIns(4,5)P(2) levels and resulted in a dose- and time-dependent inhibition of Ca(2+)-induced alpha-granule secretion. Exogenously added PtdIns(4,5)P(2) inhibited alpha-granule secretion, with 80% inhibition at 50 microm PtdIns(4,5)P(2). Nanomolar concentrations of wortmannin, 33.3 microm LY294002, and antibodies directed against PtdIns 3-kinase did not inhibit Ca(2+)-induced alpha-granule secretion, suggesting that PtdIns 3-kinase is not involved in alpha-granule secretion. However, micromolar concentrations of wortmannin inhibited both PtdIns(4,5)P(2) synthesis and alpha-granule secretion by approximately 50%. Antibodies directed against type II phosphatidylinositol-phosphate kinase (phosphatidylinositol 5-phosphate 4-kinase) also inhibited both PtdIns(4,5)P(2) synthesis and Ca(2+)-induced alpha-granule secretion by approximately 50%. These antibodies inhibited alpha-granule secretion only when added prior to ATP exposure and not when added following ATP exposure, prior to Ca(2+)-mediated triggering. The inhibitory effects of micromolar wortmannin and anti-type II phosphatidylinositol-phosphate kinase antibodies were additive. These results show that PtdIns(4,5)P(2) mediates platelet alpha-granule secretion and that PtdIns(4,5)P(2) synthesis required for Ca(2+)-induced alpha-granule secretion involves the type II phosphatidylinositol 5-phosphate 4-kinase-dependent pathway.

Hexamminecobalt(III) chloride inhibits glucose-induced insulin secretion at the exocytotic process

Hexamminecobalt(III) (HAC) chloride was found to have a potent inhibitory effect on glucose-induced insulin secretion from pancreatic islets. HAC at 2 mm inhibited the secretion in response to 22.2 mm glucose by 90% in mouse islets. Perifusion experiments revealed that the first phase of insulin secretion was severely suppressed and that the second phase of secretion was completely abrogated. Removal of HAC from the perifusate immediately restored insulin secretion with a transient overshooting above the normal level. However, HAC failed to affect glucose-induced changes in d-[6-(14)C]glucose oxidation, levels of reduced forms of NAD and NADP, mitochondrial membrane potential, ATP content, cytosolic calcium concentration, or calcium influx into mitochondria. Furthermore, HAC inhibited 50 mm potassium-stimulated insulin secretion by 77% and 10 microm mastoparan-stimulated insulin secretion in the absence of extracellular Ca(2+) by 80%. The results of a co-immunoprecipitation study of lysates from insulin-secreting betaHC9 cells using anti-syntaxin and anti-vesicle-associated membrane protein antibodies for immunoprecipitation or Western blotting suggested that HAC inhibited disruption of the SNARE complex, which is normally observed upon glucose challenge. These results suggest that the inhibitory effect of HAC on glucose-induced insulin secretion is exerted at a site(s) distal to the elevation of cytosolic [Ca(2+)], possibly in the exocytotic machinery per se; and thus, HAC may serve as a useful tool for dissecting the molecular mechanism of insulin exocytotic processes.

Mastoparan transiently permeabilizes Swiss 3T3 cells and induces c-fos proto-oncogene expression. Role of calcium and G protein activation

Mastoparan, a widely used tetradecapeptide activator of Gi/Go G proteins, has been reported to be a potent co-mitogen for Swiss 3T3 fibroblasts. However, we have previously shown that the peptide promotes the release of lactate dehydrogenase from Swiss 3T3 cells and evokes only a modest and delayed increase in DNA. We suggested that the ability of the peptide to permeabilise these cells may account for its mitogenic action. Here we show that mastoparan caused a rapid release of fluorescein from cells which had been pre-incubated with fluorescein diacetate, indicating that the peptide increases membrane permeability to small molecules. Furthermore, the release of lactate dehydrogenase evoked by mastoparan was lost after prolonged (24 h) incubation of cells with the peptide. Together, these data indicate that mastoparan-induced cell permeabilisation is both rapid and transient. We have also shown that mastoparan increased c-fos mRNA accumulation and that this response was not influenced by pertussis toxin or indomethacin. Although mastoparan increased the intracellular calcium concentration, the removal of extracellular calcium had no effect on mastoparan stimulated c-fos mRNA accumulation. These data show that mastoparan-induced c-fos mRNA accumulation is not mediated by activation of a G protein and subsequent activation of phospholipase D nor by a non-selective increase in calcium influx. The data have significance for the interpretation of studies in which mastoparan is, or has been, used as an activator of Gi/Go.

Alpha-granule secretion from alpha-toxin permeabilized, MgATP-exposed platelets is induced independently by H+ and Ca2+

In order to better understand granule release from platelets, we developed an alpha-toxin permeabilized platelet model to study alpha-granule secretion. Secretion of alpha-granules was analyzed by flow cytometry using P-selectin as a marker for alpha-granule release. P-selectin surface expression occurred when platelets were permeabilized in the presence of Ca2+. Responsiveness to Ca2+ was lost 30 min after permeabilization but could be reconstituted with MgATP. Alpha-toxin-permeabilized, MgATP-exposed platelets also degranulated within a pH range of 5.4-5.9 without exposure to and independent of Ca2+. ATP, GTP, CTP, UTP, and ITP supported Ca2+-induced alpha-granule secretion, while H+-induced alpha-granule secretion occurred only with ATP and GTP. Both Ca2+- and H+-induced alpha-granule secretion required ATP hydrolysis. Kinase inhibitors blocked both Ca2+- and H+-induced secretion. These data suggest that alpha-granule secretion in this permeabilized platelet system shares many characteristics with granule secretion studied in other permeabilized cell models. Furthermore, these results show that H+ can trigger alpha-granule release independent of Ca2+.

Proteins of the exocytotic core complex mediate platelet alpha-granule secretion. Roles of vesicle-associated membrane protein, SNAP-23, and syntaxin 4

To understand the molecular basis of granule release from platelets, we examined the role of vesicle-associated membrane protein, SNAP-23, and syntaxin 4 in alpha-granule secretion. A vesicle-associated membrane protein, SNAP-23, and syntaxin 4 were detected in platelet lysate. These proteins form a SDS-resistant complex that disassembles upon platelet activation. To determine whether these proteins are involved in alpha-granule secretion, we developed a streptolysin O-permeabilized platelet model of alpha-granule secretion. Streptolysin O-permeabilized platelets released alpha-granules, as measured by surface expression of P-selectin, in response to Ca2+ up to 120 min after permeabilization. Incubation of streptolysin O-permeabilized platelets with an antibody directed against vesicle-associated membrane protein completely inhibited Ca2+-induced alpha-granule release. Tetanus toxin cleaved platelet vesicle-associated membrane protein and inhibited Ca2+-induced alpha-granule secretion from streptolysin O-permeabilized platelets. An antibody to syntaxin 4 also inhibited Ca2+-induced alpha-granule release by approximately 75% in this system. These results show that vesicle-associated membrane protein, SNAP-23, and syntaxin 4 form a heterotrimeric complex in platelets that disassembles with activation and demonstrate that alpha-granule release is dependent on vesicle SNAP receptor-target SNAP receptor (vSNARE-tSNARE) interactions.

Heterotrimeric G-protein candidates for Ge in the ACTH secretory pathway

The mouse AtT-20/D16-16 anterior pituitary tumour cell line was used to identify candidate heterotrimeric G-proteins for G-exocytosis (Ge) which mediates calcium ion-stimulated adrenocorticotrophin (ACTH) secretion in this cell line. AtT-20 cells express several heterotrimeric G-protein alpha subunits; Gs alpha, Gt alpha, Gq alpha, G11alpha, G12alpha, G13alpha, G14alpha, G15alpha, Gz alpha, Gi2alpha, Gi3alpha, and Go alpha and so heterotrimeric G-protein selective agents were used to differentiate between these candidates. Agents which stimulate ACTH secretion via Ge were not pertussis toxin (PTX)-sensitive nor was cholera toxin (CTX) able to stimulate ACTH secretion from permeabilised cells in the absence of calcium. G-protein antagonists which inhibit activation of Gs, Gi, and Gq subfamilies did not attenuate Ge-stimulated ACTH secretion from permeabilised AtT-20 cells. In AtT-20 cells the stimulatory G-protein involved in the late stages of the ACTH secretory pathway does not belong to the Gs, Gi (with the exception of Gz) or Gq subfamilies of heterotrimeric G-proteins leaving Gz, G12 or G13 as the strongest candidates for Ge.

Electrical permeabilization of rat luteal cells: in situ phosphorylation of endogenous protein

Progesterone synthesis in the corpus luteum is regulated primarily by luteinizing hormone which acts via the adenylate cyclase/cyclic AMP/protein kinase A signalling cascade. Protein phosphorylation therefore plays a key role in the regulation of steroidogenesis, but there are relatively few studies of the in situ phosphorylation of luteal cell substrates. This may in part reflect the difficulties inherent in measuring changes in protein phosphorylation in intact cells preloaded with 32P and difficulties in interpreting data obtained using broken cell preparations. We have now applied a method of stable permeabilization of luteal cell plasma membranes by exposure of cell populations to a high intensity electric field. Under optimum conditions (5 kV/cm, six discharges) electrical permeabilization reproducibly produced populations of luteal cells in which 70-80% of the cells were permeabilized, as assessed by Trypan blue exclusion and [14C] sucrose space measurements. Pores were stable for at least 1 h, and there were no ultrastructural changes to the cells that could be detected by transmission electron microscopy. Permeabilized cells showed rapid cyclic AMP-induced changes in phosphorylation of endogenous proteins when provided with [gamma - 32 P] ATP. Our results demonstrate that the electricity permeabilized luteal cell offers a useful model for studying intracellular events in steroidogenic stimulus-response coupling cascades.

Heterotrimeric G-protein Gq/11 localized on pancreatic zymogen granules is involved in calcium-regulated amylase secretion

The heterotrimeric G-protein Gq/11 was identified on pancreatic acinar zymogen granules and its function in calcium-regulated exocytosis was examined. Western blotting showed alphaq/11, but not alphas or alphao, to be localized to the zymogen granule membrane along with G-protein beta-subunit; all three alpha subunits were present in a plasma membrane fraction and the alphaq/11 signal was 30-fold more enriched in the plasma membrane as compared with granule membrane. Neither CCK receptors nor alpha subunits of the sodium pump, both plasma membrane markers were present on granule membranes. Immunohistochemistry of pancreatic lobules showed that alphaq/11 localized to the zymogen granule-rich apical region of acinar cells together with a much stronger signal at the basolateral plasma membrane. When the substance-P-related peptide GPAnt-2a, an antagonist of Gq/11, was introduced into streptolysin-O permeabilized acini to bypass the plasma membrane, the amylase release induced by 10 microM free calcium was potentiated in a concentration-dependent manner. By contrast, another substance-P-related peptide, GPAnt-1, an antagonist of Go and Gi, showed no effect on calcium-induced amylase release from permeabilized acini. GPAnt-2a peptide also exerted an inhibitory effect on the total GTPase activity of the purified zymogen granules and a larger inhibitory effect on the GTPase activity of the Gq/11 protein immunopurified from zymogen granules. GPAnt-1, however, did not inhibit GTPase activity of either zymogen granules or immunopurified Gq/11. These results suggest that GPAnt-2a peptide augmented calcium-induced amylase release from permeabilized acini by inhibiting GTPase activity of the Gq/11 protein on zymogen granules. We conclude that Gq/11 protein on zymogen granules plays a tonic inhibitory role in calcium-regulated amylase secretion from pancreatic acini.

Mastoparan-stimulated prolactin secretion in rat pituitary GH3 cells involves activation of Gq/11 proteins

Mastoparan has been reported to induce a wide variety of cellular actions by activating GTP-binding proteins (G proteins) in various cells. Here, we demonstrate that mastoparan is able to stimulate the secretion of PRL from rat anterior pituitary tumor GH3 cells in dose- and time-dependent manners. Mastoparan had no effect on the accumulation of intracellular cAMP; however, it induced a rapid increase in the intracellular Ca2+ concentration in GH3 cells. Extracellular Ca2+ was required for mastoparan-induced PRL secretion, which was inhibited by nifedipine, an L-type Ca2+ channel blocker. Incubation of mastoparan with myo-[3H]inositol-labeled GH3 cells also resulted in the increased formation of inositol phosphates (InsPs) compared with control cells. Neomycin sulfate and U73122, both phospholipase C inhibitors, suppressed mastoparan-induced PRL secretion. Guanosine 5'-1beta-thioldiphosphate (GDPbetaS) encapsulated in GH3 cells by reversible electropermeabilization suppressed the response to mastoparan. However, pretreatment with pertussis toxin had no effect on the stimulation of PRL secretion by mastoparan, and both Mas7 (a highly active analogue of mastoparan) and Mas17 (an inactive analogue) enhanced the secretion of PRL to a similar level to that of mastoparan-induced GH3 cells. In contrast, the substance P-related peptide GPant-2A, a Gq antagonist, inhibited mastoparan-induced PRL release, whereas GPant-2, a G(i/o) antagonist, did not in electropermeabilized GH3 cells. Moreover, a specific G(q/11) antibody against the carboxyl terminus of the G(q/11) alpha-subunit blocked the stimulatory effect of mastoparan on secretion and mastoparan-stimulated InsPs production in digitonin-permeabilized GH3 cells. These results indicate that mastoparan induces the Ca2+-regulated secretion of PRL from GH3 cells by activating G(q/11) and the phospholipase C pathway.

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.

Mastoparan causes cell permeabilisation and delayed activation of DNA synthesis in Swiss 3T3 fibroblasts

Mastoparan, a tetradecapeptide from wasp venom, preferentially activates the heterotrimeric G proteins, Go and Gi by promoting GDP/GTP exchange. The peptide was originally identified as a potent secretagogue and has since been shown to promote DNA synthesis in Swiss 3T3 fibroblasts. Here, we have shown that mastoparan (10-20 microM), either alone or in combination with the co-mitogen insulin, had no effect on DNA synthesis when incubated with cells for 24 h. However, in the presence of insulin, the peptide evoked a small increase in DNA synthesis after incubation for 40 h. Thus, unlike other mitogens, mastoparan caused a delayed activation of DNA synthesis. At concentrations of mastoparan (15-17.5 microM) which promoted DNA synthesis, the peptide caused a rapid release of lactate dehydrogenase from the cells. These data suggest that the mitogenic action of mastoparan occurs by a mechanism distinct from that of physiological mitogens.

Identification of 14-3-3 proteins in human platelets: effects of synthetic peptides on protein kinase C activation

The 14-3-3 proteins inhibit protein kinase C (PKC) activity in vitro and contain conserved sequences that resemble the pseudosubstrate domain of PKC and the C-terminus of the annexins. In the present study we have identified the isoforms of 14-3-3 in human platelets and used synthetic peptides derived from the regions with similarity to PKC and annexins to examine the potential role of 14-3-3 in regulating platelet activity. Immunoblotting studies with isoform-specific antisera raised against the acetylated peptides corresponding to the N-termini of 14-3-3 showed that these cells express high levels of the beta, gamma and zeta 14-3-3 isoforms. In addition, low levels of the epsilon and eta 14-3-3 isoforms were detected. In washed, saponin-permeabilized platelets incubated with [gamma-32P]ATP, thrombin- and phorbol 12-myristate 13-acetate (PMA)-induced phosphorylation of several proteins (66, 45, and 20kDa) was inhibited by preincubation with AS peptide (KNVVGARRSSWRVISSIEQK) based on the pseudosubstrate-like region of the 14-3-3 family. A control peptide of similar size had no effect on PKC-mediated phosphorylation. PMA caused a rapid translocation of PKC activity from the cytosol to the particulate fraction of saponin-permeabilized platelets that was unaffected by either the AS peptide or a peptide derived from the annexin-like 14-3-3 domain (MKGDYYRYLAEVATGDD). These results suggest that isoforms of the 14-3-3 family may play an important physiological role as inhibitors of PKC activity in human platelets but are unlikely to be involved in controlling association of PKC with the membrane.

Drugs activating G proteins disturb cycling of ADH-dependent water channels in toad urinary bladder

In the toad urinary bladder, antidiuretic hormone (ADH)-mediated changes in water permeability depend on exocytic insertion and endocytic retrieval of water channels into and from the apical membrane, respectively. Because GTP-binding proteins (G proteins) are well-recognized regulators of vesicular trafficking throughout the cell, we tested the hypothesis that drugs interfering with G protein would modify the hydrosmotic response to ADH and the ADH-regulated formation of endosomes, as assessed by luminal incorporation of a fluid-phase marker [fluorescein isothiocyanate (FITC)-dextran, 70 kDa]. Mastoparan (4 microM) and compound 48/80 (poly-p-methoxyphenylethylmethylamine; 50 micrograms/ml), added to the luminal side of the toad urinary bladder, as well as AlF3 added to the serosal side (400 microM), inhibited ADH- and 8-bromoadenosine 3',5'-cyclic monophosphate-induced transepithelial water flow by > 50% and simultaneously enhanced cellular incorporation of FITC-dextran by > 200%. The pattern of FITC-dextran uptake observed using fluorescence microscopy both in scraped cells and in the intact bladder was granular, suggesting fluid-phase endocytosis. Mastoparan and AlF3, which are both probes of G proteins, increased FITC-dextran uptake only in the presence of ADH and a transepithelial osmotic gradient, i.e., under conditions where water channel-carrying endosomes presumably cycle. Therefore, we suggest that the ADH-dependent cycling of water channels could be controlled by one or more G proteins associated with the apical membrane and/or the water channel-carrying vesicles.

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.

The heterotrimeric G-protein Gi is localized to the insulin secretory granules of beta-cells and is involved in insulin exocytosis

Mastoparan, a tetradecapeptide found in wasp venom that stimulates G-proteins, increases insulin secretion from beta-cells. In this study, we have examined the role of heterotrimeric G-proteins in mastoparan-induced insulin secretion from the insulin-secreting beta-cell line beta-TC3. Mastoparan stimulated insulin secretion in a dose-dependent manner from digitonin-permeabilized beta-TC3 cells. Active mastoparan analogues mastoparan 7, mastoparan 8, and mastoparan X also stimulated secretion. Mastoparan 17, an inactive analogue of mastoparan, did not increase insulin secretion from permeabilized beta-TC3 cells. Mastoparan-induced insulin secretion from permeabilized beta-TC3 cells was inhibited by pretreatment of the cells with pertussis toxin, suggesting that mastoparan-induced insulin secretion is mediated through a pertussis toxin-sensitive G-protein present distally in exocytosis. Enriched insulin secretory granules (ISG) were prepared by sucrose/nycodenz ultracentrifugation. Western immunoblotting performed on beta-TC3 homogenate and ISG demonstrated that G alpha i was dramatically enriched in ISG. Levels of G alpha o and G alpha q were comparable in homogenate and ISG. Mastoparan stimulated ISG GTPase activity in a pertussis toxin-sensitive manner. Mastoparan 7 and mastoparan 8 also stimulated GTPase activity in the ISG, while the inactive analogue mastoparan 17 had no effect. Selective localization of G alpha i to ISG was confirmed with electron microscopic immunocytochemistry in beta-TC3 cells and beta-cells from rat pancreas. In contrast to G alpha o and G alpha q, G alpha was clearly localized to the ISG. Together, these data suggest that mastoparan may act through the heterotrimeric G-protein G alpha i located in the ISG of beta-cells to stimulate insulin secretion.

Increased expression of procoagulant activity on the surface of human platelets exposed to heavy-metal compounds

One of the essential roles for platelets in haemostasis is in the potentiation of blood clotting due to the contribution of anionic phospholipid from the surface of the cells, as an essential cofactor to the proteolytic reactions of coagulation (platelet procoagulant activity). Only a limited number of agonists are known to initiate platelet procoagulant activity. In this study the rate of thrombin formation on the platelet surface was observed to increase in a dose-dependent manner upon treatment of washed platelets with heavy-metal compounds. Unlike the immediate increase observed upon treatment of platelets with calcium ionophore, A23187, the change due to these agents was progressive, approaching a maximum after 10 min. The maximum-fold acceleration of the rate of thrombin formation compared with control platelets was calculated for HgCl2 (56-fold), AgNO3 (42-fold) phenylmercuriacetate (24-fold) and thimerosal (14-fold), compared with 70-fold observed for calcium ionophore. The increase in procoagulant activity due to HgCl2 coincided with a large increase in intracellular calcium and phosphorylation of 22 and 45 kDa proteins. It is considered that the mechanism responsible for the increase in procoagulant activity is exposure of anionic phospholipids. This was detected by a 2-fold increase in the binding of 125I-annexin V upon addition of HgCl2, compared with resting platelets (3-fold on treatment of platelets with calcium ionophore). In contrast to the generation of activity by A23187 and other known agonists of this reaction, heavy-metal compounds appeared to cause little or no release of microparticles from the platelet surface. Since HgCl2 did not cause aggregation of platelets or significant release of serotinin, these findings may give further support to the need for exposure and ligation of glycoprotein IIb:IIIa for vesiculization to occur. Treatment of platelets with heavy metals may constitute a new approach to investigating the early changes in the cell membrane which lead to increased expression of anionic phospholipid.

Ca(2+)-independent fusion of secretory granules with phospholipase A2-treated plasma membranes in vitro

The fusion of secretory granules with plasma membranes prepared from rat parotid gland was studied in vitro to clarify the mechanism of exocytosis. Fusion of the granules with plasma membranes was measured by a fluorescence-dequenching assay with octadecyl rhodamine B, and release of amylase was also measured to confirm the fusion as a final step of the secretory process. Plasma membranes that had been pretreated with porcine phospholipase A2 (PLA2) in the presence of 20 microM Ca2+ fused with the granules within 30 s, and induced amylase release by reacting with the membranes of granules, whereas without this pretreatment they had no significant effect. The fusion process accompanied by amylase release was induced in the presence of 10 mM EGTA, and therefore was apparently Ca(2+)-independent. On the other hand, the presence of EGTA or 100 microM quinacrine, an inhibitor of PLA2, during treatment of plasma membranes with PLA2 inhibited their fusogenic activity, suggesting the importance of activation of PLA2. Arachidonic acid and linoleic acid were released from the plasma membranes during the PLA2 treatment. The presence of albumin, an adsorbent of fatty acids, during the treatment also inhibited the activity. Pretreatment of the membranes with arachidonic acid or linoleic acid did not have any effect, but the presence of exogenously added arachidonic acid during PLA2 treatment enhanced the membrane-fusion-inducing effect of PLA2. Pretreatment of the membranes with lysophosphatidylcholine induced fusogenic activity. These findings suggest that the conformational change in the plasma-membrane phospholipids induced by PLA2 and the presence of arachidonic acid or linoleic acid produced by PLA2 are important in the process of fusion of secretory granules with the plasma membranes of rat parotid acinar cells and that the fusion process itself is independent of Ca2+.

The peptide mastoparan is a potent facilitator of the mitochondrial permeability transition

Mastoparan facilitates opening of the mitochondrial permeability transition pore through an apparent bimodal mechanism of action. In the submicromolar concentration range, the action of mastoparan is dependent upon the medium Ca2+ and phosphate concentration and is subject to inhibition by cyclosporin A. At concentrations above 1 microM, pore induction by mastoparan occurs without an apparent Ca2+ requirement and in a cyclosporin A insensitive manner. Studies utilizing phospholipid vesicles show that mastoparan perturbs bilayer membranes across both concentration ranges, through a mechanism which is strongly dependent upon transmembrane potential. However, solute size exclusion studies suggest that the pores formed in mitochondria in response to both low and high concentrations of mastoparan are the permeability transition pore. It is proposed that low concentrations of mastoparan influence the pore per se, with higher concentrations having the additional effect of depolarizing the mitochondrial inner membrane through an action exerted upon the lipid phase. It may be the combination of these effects which allow pore opening in the absence of Ca2+ and in the presence of cyclosporin A, although other interpretations remain viable. A comparison of the activities of mastoparan and its analog, MP14, on mitochondria and phospholipid vesicles provides an initial indication that a G-protein may participate in regulation of the permeability transition pore. These studies draw attention to peptides, in a broad sense, as potential pore regulators in cells, under both physiological and pathological conditions.

The effects of mastoparan on the carrot cell plasma membrane polyphosphoinositide phospholipase C

When [3H]inositol-labeled carrot (Daucus carota L.) cells were treated with 10 or 25 microM wasp venom peptide mastoparan or the active analog Mas-7 there was a rapid loss of more than 70% of [3H]phosphatidylinositol-4-monophosphate (PIP) and [3H]phosphatidylinositol-4,5-bisphosphate (PIP2) and a 3- and 4-fold increase in [3H]inositol-1,4-P2 and [3H]inositol-1,4,5-P3, respectively. The identity of [3H]inositol-1,4,5-P3 was confirmed by phosphorylation with inositol-1,4,5-P3 3-kinase and co-migration with inositol-1,3,4,5-P4. The changes in phosphoinositides were evident within 1 min. The loss of [3H]PIP was evident only when cells were treated with the higher concentrations (10 and 25 microM) of mastoparan or Mas-7. At 1 microM Mas-7, [3H]PIP increased. The inactive mastoparan analog Mas-17 had little or no effect on [3H]PIP or [3H]PIP2 hydrolysis in vivo. Neomycin (100 microM) inhibited the uptake of Mas-7 and thereby inhibited the Mas-7-stimulated hydrolysis of [3H]PIP and [3H]PIP2. Plasma membranes isolated from mastoparan-treated cells had increased PIP-phospholipase C (PLC) activity. However, when Mas-7 was added to isolated plasma membranes from control cells, it had no effect on PIP-PLC activity at low concentrations and inhibited PIP-PLC at concentrations greater than 10 microM. In addition, guanosine-5'-O-(3-thiotriphosphate) had no effect on the PIP-PLC activity when added to plasma membranes isolated from either the Mas-7-treated or control cells. The fact that Mas-7 did not stimulate PIP-PLC activity in vitro indicated that the Mas-7-induced increase in PIP-PLC in vivo required a factor that was lost from the membrane during isolation.

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.

Staurosporine induces hydrolysis of phosphatidyl inositol 4,5-bisphosphate in human platelets

Staurosporine in the micromolar range raised inositol trisphosphate in intact human platelets to levels comparable to that mediated by thrombin. This response was inhibited by neomycin, a phospholipase C antagonist. Staurosporine alone induced a weak, transient rise in cytosolic free calcium levels ([Ca2+]i) from release of internal Ca2+ stores but potentiated the effect induced by thrombin. Therefore, it is unlikely that this alkaloid suppressed inositol trisphosphate mobilization of Ca2+. Additional studies show that staurosporine, 0.5-5 microM, stimulated GTPase activity in platelet membranes while 2 microM K252a and 20 microM H7 were inactive. Present results suggest that staurosporine may activate platelet phospholipase C at the level of G proteins or receptors.

Correlation between secretion and phospholipase D activation in differentiated HL60 cells

Receptor-directed agonists including N-formylmethionyl-leucyl-phenylalanine (fMetLeuPhe), C5a, ATP and UTP all activate phospholipase D (PLD), which is accompanied by secretion in differentiated HL60 cells. Interference in the production of phosphatidase (PA) by the PLD pathway by diverting it towards the production of phosphatidylethanol (PEt) in the presence of ethanol leads to near-total inhibition of the secretion evoked by ATP and UTP and a partial inhibition of that evoked by fMetLeuPhe and C5a. In streptolysin-O-permeabilized cells, fMetLeuPhe is able to activate PLD, and this is dependent on the presence of a low concentration of guanosine 5'-[gamma-thio]-triphosphate (GTP[S]). Ca2+ (10 microM) and GTP[S] individually or in combination are also able to activate PLD and secretion. The stimulation of secretion in permeabilized cells stimulated by Ca2+ alone or fMetLeuPhe or GTP[S] is also abrogated when the production of PA is diverted to PEt by the presence of ethanol. Activation of PLD by GTP[S] or fMetLeuPhe is decreased if the cells are permeabilized first and GTP[S] or fMetLeuPhe is added subsequently. This corresponds well with the loss of the secretory response. We conclude that the ability of GTP[S] or fMetLeuPhe to stimulate secretion from permeabilized cells is dependent on a prior activation of the PLD signalling pathway. PA, generated as a consequence of PLD activation, acts as second messenger that can provide an initiating signal for secretion and is not required for exocytosis itself.

Mastoparan stimulates insulin secretion from pancreatic beta-cells by effects at a late stage in the secretory pathway

Mastoparan (MP) is a component of wasp venom which stimulates secretion from a number of cell types. We have used intact and electrically permeabilised islets of Langerhans to investigate the mechanisms through which MP stimulates insulin secretion from pancreatic beta-cells. MP caused a temperature-dependent and dose-related stimulation of insulin secretion from intact islets at a substimulatory concentration (2 mM) of glucose, which was not dependent upon the presence of extracellular Ca2+. MP also stimulated ATP-independent insulin secretion from electrically permeabilised islets in which intracellular Ca2+ was clamped at a substimulatory concentration (50 nM). MP-induced insulin secretion was not inhibited by down-regulation of islet protein kinase C, nor by the protein kinase inhibitor staurosporine, nor by the cyclic AMP antagonist Rp-adenosine 3',5'-cyclic phosphorothioate. However, MP-induced secretion from permeabilised islets was inhibited by the presence of guanosine 5'-O-2-thiodiphosphate. These results suggest that MP stimulates insulin secretion by a mechanism that is independent of changes in cytosolic Ca2+ or protein kinase activation, but which is dependent, at least in part, upon activation of a GTP-binding protein at a late stage in the secretory process.