Action of guanosine 5'-[beta-thio]diphosphate on thrombin-induced activation and Ca2+ mobilization in saponin-permeabilized and intact human platelets

Evidence that inositol 1,4,5-trisphosphate 3-kinase and inositol 1,3,4,5-tetrakisphosphate are negative regulators of platelet function

Background

Inositol 1,3,4,5-tetrakisphosphate (IP4) is formed from inositol 1,4,5-trisphosphate (IP3) by IP3 3-kinase (ITPK) in most cells. Its function is unknown but has been suggested to be involved in Ca2+ entry, IP3 regulation, and phosphoinositide 3-kinase antagonism.

Objectives

To better elucidate a function for IP4, we tested a specific inhibitor of ITPK (GNF362) on platelets, the effects of IP4 directly in permeabilized platelets and its effect on phosphatidylinositol 3,4,5-trisphosphate (PIP3) binding to pleckstrin-homology (PH) domain-containing proteins in platelets.

Methods

Human platelets were utilized in whole blood for thrombus formation, in platelet-rich plasma and washed suspensions for aggregation, and for Ca2+ studies, or resuspended in high K+ and low Na+ buffers for permeabilization experiments. Phosphorylation of AKT-Ser473 and Rap1-GTP formation were measured by Western blotting and PIP3 binding using PIP3 beads.

Results

GNF362-enhanced platelet aggregation stimulated by low concentrations of ADP, collagen, thrombin, U46619, and thrombus formation in collagen-coated capillaries. GNF362 induced a transient elevation of Ca2+ concentration, elevated basal levels of IP3, and enhanced the peak height of Ca2+ elevated by agonists. In permeabilized platelets, IP4 inhibited GTPγS induced formation of AKT-Ser473 phosphorylation and platelet aggregation. IP4 reduced GTPγS-stimulated Rap1-GTP levels and potently reduced extraction of RASA3 and BTK by PIP3 beads.

Conclusion

ITPK and IP4 are negative regulators of platelet function. IP4 regulation of PH domain-containing proteins may represent a pathway by which platelet activation may be controlled during thrombosis.

Aquatic toxicity and biodegradability of a surfactant produced by Bacillus subtilis ICA56

In this work, the environmental compatibility of a biosurfactant produced by a Bacillus subtilis strain isolated from the soil of a Brazilian mangrove was investigated. The biosurfactant, identified as surfactin, is able to reduce surface tension (ST) to 31.5 ± 0.1 mN m^-1 and exhibits a lowcritical micelle concentration (CMC) value (0.015 ± 0.003 g L^-1). The highest crude biosurfactant concentration (224.3 ± 1.9 mg L^-1) was reached at 72 h of fermentation. Acute toxicity tests, carried out with Daphnia magna, Vibrio fischeri and Selenastrum capricornutum indicated that the toxicity of the biosurfactant is lower than that of its chemically derived counterparts. The results of the biodegradability tests demonstrated that the crude surfactin extract was degraded by both Pseudomonas putida and a mixed population from a sewage-treatment plant, in both cases the biodegradation efficiency being dependent on the initial concentration of the biosurfactant. Finally, as the biodegradation percentages obtained fall within the acceptance limits established by the Organization for Economic Co-operation and Development (Guidelines for Testing of Chemicals, OECD 301E), crude surfactin can be classified as a "readily" biodegradable compound.

Effects of Quillaja Saponin (Quillaja saponaria) on Early Embryonic Zebrafish (Danio rerio) Development

Although much attention has focused on environmental contamination by heavy metals, pesticides, and polychlorinated biphenyls, potential deleterious effects of naturally occurring organic compounds have received much less consideration. Saponins, which are glycosides found in many plants, are important, environmentally ubiquitous organic compounds. Saponins have both beneficial and deleterious effects in adults, but little is known about how saponins effect early vertebrate embryonic development. The authors tested the toxicity of quillaja saponin using a zebrafish embryo assay. Quillaja saponin, extracted from bark of the tree, Quillaja saponaria, is a common foaming agent used in foods and beverages. At 6 h post fertilization, zebrafish embryos were exposed to five concentrations (0 [negative control], 1, 5, 10 or 20 μg) of quillaja saponin per milliliter of medium. Zebrafish embryos exposed to 2% ethanol were positive controls (100% embryonic death). Embryos were assessed at 30, 54, and 72 h post fertilization for changes in embryonic development, mortality, time of hatching, and morphological deformities. Embryos exposed to 1 and 5 μg saponin were healthy, showed no obvious deformities, but exhibited shrinkage of the chorion. Hatching time for zebrafish embryos exposed to 1 and 5 μg/ml saponin decreased by 18 h compared to unexposed embryos. Zebrafish embryos treated with 5 μg/ml saponin responded less to touch than embryos treated with 1 μg/ml saponin or controls. Zebrafish embryos exposed to more than 5 μg/ml saponin exhibited 100% embryonic mortality. These results indicate that exposure to 5 μg/ml or less of quillaja saponin acts as a growth promoter, whereas concentrations of 10 μg/ml or greater are lethal.

Effects of plasma membrane Ca(2+) -ATPase tyrosine phosphorylation on human platelet function

BACKGROUND

The plasma membrane Ca(2+)-ATPase (PMCA) plays an essential role in maintaining low intracellular Ca(2+) ([Ca(2+)](i)) in resting platelets. Earlier studies demonstrated that platelet activation by thrombin results in tyrosine phosphorylation of PMCA, which inhibits pump activity.

OBJECTIVES

The objective was to determine the functional consequences of PMCA tyrosine phosphorylation.

METHODS

A decapeptide including the tyrosine phosphorylation site of PMCA and a scrambled version were synthesized and introduced into human platelets using saponin. Fura-2 calcium monitoring and aggregometry were used to characterize the effects of inhibition of tyrosine phosphorylation.

RESULTS

Western blot analysis of immunoprecipitates showed that introduction of the inhibitory peptide decreased tyrosine phosphorylation of PMCA by nearly 60% in saponin-permeabilized, thrombin-treated platelets as compared with the scrambled control peptide. Concomitant with inhibition of PMCA tyrosine phosphorylation was a significant decrease in [Ca(2+)](i) during thrombin-mediated platelet activation. The functional consequence of reduced PMCA tyrosine phosphorylation and decreased [Ca(2+)](i) was a significant delay in the onset of thrombin-mediated platelet aggregation.

CONCLUSIONS

The results demonstrate that PMCA tyrosine phosphorylation regulates [Ca(2+)](i) during platelet activation, which affects downstream events in the activation process. Moreover, PMCA tyrosine phosphorylation and resultant inhibition of PMCA activity produces a positive feedback loop mechanism by enhancing the increase in [Ca(2+)](i) accompanying platelet activation.

A role for Sec1/Munc18 proteins in platelet exocytosis

A critical aspect of haemostasis is the release of clot-forming components from the three intra-platelet stores: dense-core granules, alpha granules and lysosomes. Exocytosis from these granules is mediated by soluble proteins [N-ethylmaleimide-sensitive fusion protein (NSF) and soluble NSF attachment proteins (SNAPs)] and integral membrane proteins [vesicle and target SNAP receptors (v- and t-SNAREs)]. Three Sec1/Munc18 proteins (SM proteins) are present in platelets (Munc18a, Munc18b and Munc18c) and they bind to and potentially regulate specific syntaxin t-SNAREs. In resting platelets, these SM proteins associate with granules and open canalicular system membranes predominantly but not with the plasma membrane. Munc18a binds to syntaxin 2 alone and does not associate with other members of the core SNARE complex. Munc18b associates with a larger complex that contains synaptosome-associated protein of 23 kDa (SNAP-23) and cellubrevin/vesicle-associated membrane protein 3. Munc18c associates with both syntaxins 2 and 4, with synaptosome-associated protein of 23 kDa (SNAP-23) and with a v-SNARE. On stimulation, most of the platelet SM proteins are still found in membrane fractions. Phosphorylation of each Munc18 increases in thrombin-treated cells and phosphorylated Munc18c remains associated with syntaxins 2 and 4, but its affinity for the SNAREs appears to be reduced. To determine the functional role of the platelet SM proteins, we examined the effects of Munc18-based peptides (Munc18a peptide 3 and Munc18c peptide 3). Addition of the peptides to permeabilized platelets inhibits secretion from all three platelet granules. These peptides also inhibit agonist-induced aggregation in saponin-permeabilized platelets. These studies demonstrate a clear role for SM proteins in platelet exocytosis and aggregation and suggest a dominant role for Munc18c in all three granule-release events.

Direct demonstration of involvement of protein kinase Calpha in the Ca2+-induced platelet aggregation

Platelets play critical roles in hemostasis and thrombosis through their aggregation following activation of integrin alphaIIbbeta3. However, the molecular mechanism of the integrin activation inside platelets remains largely unknown. Pharmacological experiments have demonstrated that protein kinase C (PKC) plays an important role in platelet aggregation. Because PKC inhibitors can have multiple substrates and given that non-PKC-phorbol ester-binding signaling molecules have been demonstrated to play important roles, the precise involvement of PKC in cellular functions requires re-evaluation. Here, we have established an assay for analyzing the Ca2+-induced aggregation of permeabilized platelets. The aggregation of platelets was inhibited by the addition of the arginine-glycine-aspartate-serine peptide, an integrin-binding peptide inhibitor of alphaIIbbeta3, suggesting that the aggregation was mediated by the integrin. The aggregation was also dependent on exogenous ATP and platelet cytosol, indicating the existence of essential cytosolic factors required for the aggregation. To examine the role of PKC in the aggregation assay, we immunodepleted PKCalpha and beta from the cytosol. The PKC-depleted cytosol lost the aggregation-supporting activity, which was recovered by the addition of purified PKCalpha. Furthermore, the addition of purified PKCalpha in the absence of cytosol did not support the aggregation, whereas the cytosol containing less PKC supported it efficiently, suggesting that additional factors besides PKC would also be required. Thus, we directly demonstrated that PKCalpha is involved in the regulation of Ca2+-induced platelet aggregation.

The biological action of saponins in animal systems: a review

Saponins are steroid or triterpenoid glycosides, common in a large number of plants and plant products that are important in human and animal nutrition. Several biological effects have been ascribed to saponins. Extensive research has been carried out into the membrane-permeabilising, immunostimulant, hypocholesterolaemic and anticarcinogenic properties of saponins and they have also been found to significantly affect growth, feed intake and reproduction in animals. These structurally diverse compounds have also been observed to kill protozoans and molluscs, to be antioxidants, to impair the digestion of protein and the uptake of vitamins and minerals in the gut, to cause hypoglycaemia, and to act as antifungal and antiviral agents. These compounds can thus affect animals in a host of different ways both positive and negative.

Plasma membrane Ca(2+)-ATPase associates with the cytoskeleton in activated platelets through a PDZ-binding domain

The plasma membrane Ca(2+)-ATPase (PMCA) plays an essential role in maintaining low cytosolic Ca(2+) in resting platelets. During platelet activation PMCA is phosphorylated transiently on tyrosine residues resulting in inhibition of the pump that enhances elevation of Ca(2+). Tyrosine phosphorylation of many proteins during platelet activation results in their association with the cytoskeleton. Consequently, in the present study we asked if PMCA interacts with the platelet cytoskeleton. We observed that very little PMCA is associated with the cytoskeleton in resting platelets but that approximately 80% of total PMCA (PMCA1b + PMCA4b) is redistributed to the cytoskeleton upon activation with thrombin. Tyrosine phosphorylation of PMCA during activation was not associated with the redistribution because tyrosine-phosphorylated PMCA was not translocated specifically to the cytoskeleton. Because PMCA b-splice isoforms have C-terminal PSD-95/Dlg/ZO-1 homology domain (PDZ)-binding domains, a C-terminal peptide was used to disrupt potential PDZ domain interactions. Activation of saponin-permeabilized platelets in the presence of the peptide led to a significant decrease of PMCA in the cytoskeleton. PMCA associated with the cytoskeleton retained Ca(2+)-ATPase activity. These results suggest that during activation active PMCA is recruited to the cytoskeleton by interaction with PDZ domains and that this association provides a microenvironment with a reduced Ca(2+) concentration.

Small GTPase Rho regulates thrombin-induced platelet aggregation

Platelets play essential roles in hemostasis and thrombosis by aggregating with each other. However, the molecular mechanism governing platelet aggregation is not yet fully understood. Here, we established an assay system using platelets permeabilized with streptolysin-O to analyze mechanism of the thrombin-induced aggregation, focusing upon a controversial issue in the field whether small GTPase Rho regulates the aggregation. Incubation of the permeabilized platelets with Rho GDP-dissociation inhibitor, an inhibitory regulator for Rho family GTPases, extracted Rho family proteins extensively from the plasma and intracellular membranes, and inhibited the thrombin-induced aggregation. Incubation of the permeabilized platelets with botulinum exoenzyme C3, which specifically inhibits Rho function by ADP-ribosylating it, abolished the thrombin-induced aggregation. Thus, Rho is involved in thrombin-induced aggregation of platelets.

Alpha 2A-adrenergic receptors activate protein kinase C in human platelets via a pertussis toxin-sensitive G-protein

4,4'-Diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS) stimulates human platelets via alpha 2A-adrenergic receptor-mediated activation of protein kinase C (PKC) independent of the phospholipase C pathway. Here we show, that in permeabilized platelets activation of PKC by DIDS (20 microM), measured as 32P incorporation in pleckstrin, is completely inhibited by guanosine 5'-(2-O-thio)diphosphate (200 microM), an inhibitor of heterotrimeric G-proteins. Also pertussin toxin (4 micrograms/ml), which ADP-ribosylates the alpha-subunits of Gi's and Go, prevents pleckstrin phosphorylation by DIDS. N-Ethylmaleimide (50 microM), which uncouples Gi from alpha 2A-adrenoceptors, inhibits pleckstrin phosphorylation by DIDS in intact platelets. Activation of PKC by 55 nM phorbol 12-myristate 13-acetate and 500 nM platelet-activating factor are not disturbed by NEM. DIDS inhibits by 40 +/- 5% (n = 4) the pertussis toxin-catalyzed [32P]ADP-ribosylation of a 41 kDa protein fraction previously shown to contain the alpha-subunits of Gi alpha-1, Gi alpha-2 and Gi alpha-3. Thus, the alpha 2A-adrenergic receptor activates PKC via a G-protein of the Gi-family.

Ca2+ release from platelet intracellular stores by thapsigargin and 2,5-di-(t-butyl)-1,4-benzohydroquinone: relationship to Ca2+ pools and relevance in platelet activation

The effects of the Ca(2+)-ATPase inhibitors thapsigargin (Tg) and 2,5-di-(t-butyl)-1,4-benzohydroquinone (tBuBHQ) were examined by using Ca(2+)-regulatory systems of platelet mixed membranes, saponin-permeabilized and intact platelets. Both agents inhibit Ca(2+)-ATPase activities of platelet mixed membranes, without any effect on the basal Mg(2+)-ATPase activity. Tg is more effective (EC50 = 35 nM) than tBuBHQ (EC50 = 580 nM). The effect of the two inhibitors on 45Ca2+ release from saponin-permeabilized platelets has also been characterized. 45Ca2+ uptake into non-mitochondrial intracellular stores occurs via an ATP-dependent mechanism, and if added at equilibrium the second messenger Ins(1,4,5)P3 releases 50% of the accumulated 45Ca2+. Maximally effective concentrations of Tg (1 microM) and tBuBHQ (50 microM) release 77% and 68% of the accumulated 45Ca2+. Addition of Ins(1,4,5)P3 together with either Tg or tBuBHQ resulted in a non-additive release which was the same as with either Tg or tBuBHQ alone, indicating that the Ins(1,4,5)P3-sensitive Ca2+ pool was a subset of the pool that is sensitive to the Ca(2+)-ATPase inhibitors. Release of 45Ca2+ by either Tg or tBuBHQ was not affected by heparin, which totally blocked Ins(1,4,5)P3-induced Ca2+ release, and Tg was found not to affect [32P]Ins(1,4,5)P3 binding to its receptor on mixed membranes. Thus both Tg and tBuBHQ release Ca2+ from a pool that totally overlaps the Ins(1,4,5)P3-sensitive pool without affecting Ins(1,4,5)P3 function. In intact indomethacin-treated Fura 2-loaded platelets, Tg and tBuBHQ cause Ca2+ elevation, arising from release from intracellular stores and influx from the outside. Both Tg and tBuBHQ elevated Ca2+ to similar levels, which were less and slower than those observed with thrombin. Addition of thrombin to cells already treated with Tg or tBuBHQ produced further elevation of Ca2+, indicating agonist utilization of a Ca(2+)-ATPase inhibitor-insensitive pool. In aggregation experiments Tg and tBuBHQ showed different functional effects. In indomethacin-treated cells Tg induces slow aggregation and secretion responses, whereas tBuBHQ only induces shape change. Both agents show synergistic secretory responses with the protein kinase C activator dioctanoylglycerol (DiC8). Tg also showed greater ability than tBuBHQ to release [3H]arachidonic acid (AA) from [3H]AA-labelled platelets. Additionally, in [32P]Pi-labelled platelets both Tg and tBuBHQ induced phosphorylation of myosin light chain, a 27 kDa protein and the 45 kDa protein pleckstrin, but Tg showed a greater ability than tBuBHQ to cause phosphorylation of pleckstrin. These studies indicate that Tg and tBuBHQ are effective in releasing the Ins(1,4,5)P3-sensitive Ca2+ pool in platelets.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of a glucocorticosteroid-induced inhibitor of interferon-gamma induction of HLA-DR expression

Interferon gamma (IFN-gamma) induces human leukocyte antigen (HLA)-DR antigen expression on a variety of cell types, and in human skin cells this induction is inhibited by trypsin inhibitors. Recently a trypsin-like protease was characterized whose activity is required for HLA-DR induction in a hybrid epidermal cell line. Glucocorticosteroids also inhibit IFN-gamma-induced HLA-DR expression, and similarities have been noted between the inhibition by trypsin inhibitors and by glucocorticosteroids. To assess the possibility that glucocorticosteroid inhibition of IFN-gamma-induced HLA-DR expression might be due to induction of an inhibitor of trypsin activity that is re-expression, we examined culture medium supernates (CM) of glucocorticosteroid-treated cells for HLA-DR- and trypsin-inhibitory activities. We report here that CM of glucocorticosteroid-treated H12 cells contain inhibitors of HLA-DR expression and of trypsin activity, but that the two inhibitors are not identical. H12 cells constitutively secrete a greater than 30,000 MW, acid- and heat-stable trypsin inhibitor, whose expression is not modulated by glucocorticosteroid or IFN-gamma, and that does not inhibit IFN-gamma-induced HLA-DR expression. The HLA-DR inhibitor, on the other hand, is present only in CM of glucocorticosteroid-treated cells, is distinct from glucocorticosteroid itself, of a MW less than 500 and does not inhibit trypsin. We conclude, therefore, that the glucocorticosteroid inhibition of IFN-gamma-induced HLA-DR expression is by a mechanism other than secretion of a trypsin inhibitor.

Guanosine 5'-0-(3-thiotriphosphate) induced calcium release in human platelets is mediated by inositol 1,4,5-triphosphate

Guanosine 5'-triphosphate (GTP) and its nonhydrolyzable analogs, such as guanosine 5'-0-(3-thiotriphosphate) (GTP gamma S), induce several responses in platelets including secretion, production of inositol 1,4,5-triphosphate (IP3) and mobilization of Ca2+ from intracellular sites. Because IP3 is well established as a second messenger in mobilizing Ca2+ from intracellular stores it has been generally assumed that Ca2+ release by GTP/GTP gamma S in platelets is mediated by IP3. However, studies in neuronal, hepatic and smooth muscle cells have suggested that IP3 and GTP/GTP gamma S activate Ca2+ release by distinct mechanisms and that IP3-independent mechanisms mediate GTP/GTP gamma S-induced Ca2+ release. In several tissues heparin inhibits binding of IP3 and blocks IP3-stimulated Ca2+ release in a competitive and specific manner. In the present studies, IP3 and GTP gamma S induced Ca2+ release and their relationship was examined in human platelets using heparin as a probe. In saponin permeabilized platelets, IP3 (0.05-5 microM) induced a prompt, dose-dependent release of Ca2+ (EC50 0.5 microM). GTP gamma S (1-50 microM) released Ca2+ in a dose-dependent manner with EC50 of 2 microM but with a time lag of 30-90 seconds. Exposure of platelets to 1 microM IP3 following a submaximal response with GTP gamma S (1 microM) resulted in a further increase in Ca2+ release but no further increase was noted on adding 1 microM IP3 following a maximal response with GTP gamma S (10 microM); similar findings were noted on reversing the order of addition of GTP gamma S and IP3 suggesting that these effectors release Ca2+ from the same source. IP3 (0.5 microM) induced Ca2+ release was blocked by low molecular weight (4000-6000) heparin (IC50 30 micrograms/ml). More importantly, heparin abolished GTP gamma S (2.5 microM) induced Ca2+ release (IC50 10 micrograms/ml). These results indicate that, in contrast to the findings in some other cells, in human platelets GTP gamma S-induced Ca2+ release is mediated largely by a mechanism involving IP3.

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

Recent studies have shown that mastoparan, an amphiphilic peptide derived from wasp venom, accelerates guanine nucleotide exchange and GTPase activity of purified GTP-binding proteins. In the present study we have examined the functional consequences of exposure of intact human platelets to mastoparan. Mastoparan promoted rapid (less than or equal to 1 min) dose-dependent increases in 5-hydroxy[14C]tryptamine and beta-thromboglobulin release from dense-granule and alpha-granule populations respectively. The exocytotic response did not result from a lytic effect of mastoparan and occurred in the complete absence of platelet shape change and aggregation. Liberation of [3H]arachidonate and increases in cytosolic [Ca2+] (detected with fura 2) were not observed in platelets stimulated with mastoparan. Similarly, in platelets preloaded with [3H]inositol during reversible electroporation, mastoparan did not cause the accumulation of [3H]inositol phosphates. Mastoparan-induced secretion was unaffected by preincubation with either the protein kinase C inhibitor staurosporine (10 nM-10 microM) or prostacyclin (PGI2; 100 ng/ml) and was not accompanied by phosphorylation of the 45 kDa protein kinase C substrate or the 20 kDa protein normally associated with platelet activation. The G-protein inhibitor guanosine 5'-[beta-thio]diphosphate (GDP[S]; 1 mM) attenuated the secretion induced by mastoparan in both intact and saponin-permeabilized platelets. Encapsulation of GDP[S] during reversible permeabilization inhibited mastoparan-induced secretion, providing evidence for an intracellular action of GDP[S]. In all these studies thrombin (0.05-0.2 unit/ml) elicited characteristic responses, and thrombin-induced secretion was inhibited by staurosporine, PGI2 and GDP[S]. Mastoparan also increased intra-platelet cyclic AMP in a dose-dependent manner. Mastoparan and PGI2 increased 32P incorporation into a protein of approx. 24 kDa, whereas phosphorylation of a 50 kDa substrate was only seen in PGI2-stimulated platelets. These results indicate that mastoparan promotes secretion by a mechanism which does not involve stimulation of phospholipase C and suggest that the secretory event may result either from a direct fusogenic action of mastoparan and/or from stimulation of the putative exocytosis-linked G-protein, Ge.

Influence of heparins on inositol 1,4,5-trisphosphate-induced calcium mobilization in permeabilized human platelets

Heparin has been shown to prevent inositol 1,4,5-trisphosphate (IP3) binding to its receptor and to inhibit IP3-induced calcium mobilization in a variety of cells. Heparin added to whole blood at a concentration of 1 U/ml prevented thrombin-induced secretion of granule contents and irreversible aggregation of platelets. Heparin (2-15 kDa) had no inhibitory effect on IP3-induced calcium mobilization in Fura 2-loaded, saponin (10-15 micrograms/ml)-permeabilized platelets. None of the commercially available heparin preparations can induce inhibition of agonist-induced calcium mobilization in intact platelets because they are not cell permeant. Mild saponin treatment makes the membrane permeable to IP3, but restricts the action of heparins. Recent observations suggesting heparin's affinity to IP3 binding sites will be of clinical interest if effective cell permeant analogs can be developed.