Evidence that activation of phospholipase D can mediate secretion from permeabilized platelets

Phospholipase D1 produces phosphatidic acid at sites of secretory vesicle docking and fusion

Phospholipase D1 (PLD1) activity is essential for the stimulated exocytosis of secretory vesicles where it acts as a lipid-modifying enzyme to produces phosphatidic acid (PA). PLD1 localizes to the plasma membrane and secretory vesicles, and PLD1 inhibition or knockdowns reduce the rate of fusion. However, temporal data resolving when and where PLD1 and PA are required during exocytosis is lacking. In this work, PLD1 and production of PA are measured during the trafficking, docking, and fusion of secretory vesicles in PC12 cells. Using fluorescently tagged PLD1 and a PA-binding protein, cells were imaged using TIRF microscopy to monitor the presence of PLD1 and the formation of PA throughout the stages of exocytosis. Single docking and fusion events were imaged to measure the recruitment of PLD1 and the formation of PA. PLD1 is present on mobile, docking, and fusing vesicles and also colocalizes with Syx1a clusters. Treatment of cells with PLD inhibitors significantly reduces fusion, but not PLD1 localization to secretory vesicles. Inhibitors also alter the formation of PA; when PLD1 is active, PA slowly accumulates on docked vesicles. During fusion, PA is reduced in cells treated with PLD1 inhibitors, indicating that PLD1 produces PA during exocytosis.

Choline in acute coronary syndrome: an emerging biomarker with implications for the integrated assessment of plaque vulnerability

Whole-blood choline, plasma choline and serum choline are emerging biomarkers in acute coronary syndrome related to coronary plaque instability with platelet thrombus formation and ischemia. Whole-blood choline is an early predictor for cardiac events, which adds to troponins, natriuretic peptides and inflammatory markers. Serum choline is highly predictive for myocardial infarction and discriminates high- from low-risk subgroups in troponin-positive patients. Choline is a candidate marker to aid decision making in the emergency room in the upcoming era of sensitive troponin tests and the growing need to differentiate between ischemic and nonischemic etiologies of troponin elevations. The integrated approach of in vitro choline measurement in combination with advanced techniques of in vivo choline imaging represents a novel future strategy for detecting vulnerable plaques. This paper provides an up-to-date review of choline in acute coronary syndrome including key aspects of pathophysiology, analytical methods, clinical studies and implications for the integrated assessment of plaque vulnerability.

Phospholipase D in human platelets: presence of isoenzymes and participation of autocrine stimulation during thrombin activation

Phospholipase D (PLD), which hydrolyzes phosphatidylcholine to phosphatidic acid (PA) and choline, is present in human platelets. Thrombin and other agonists have been shown to activate PLD but the precise mechanisms of activation and PLDs role in platelet activation remains unclear. We measured thrombin-stimulated PLD activity in platelets as formation of phosphatidylethanol. Since no specific PLD inhibitors exist, we investigated possible roles for PLD in platelets by correlating PLD activity with platelet responses such as thrombin-mediated secretion and F-actin formation (part of platelet shape change). Extracellular Ca2+ potentiated thrombin-stimulated PLD, but did not stimulate PLD in the absence of thrombin. Thrombin-induced PLD activity was enhanced by secreted ADP and binding of fibrinogen to its receptors. In contrast to others, we also found a basal PLD activity. Comparison of time courses and dose responses of platelets with PLD showed many points of correlation between PLD activation and lysosomal secretion and F-actin formation. The finding of different PLD activities suggested that different PLD isoenzymes exist in platelets as reported for other cells. Here we present evidence for the presence of both PLD1 and PLD2 in platelets by use of specific antibodies with immunoblotting and immunohistochemistry. Both isoforms were randomly localized in resting platelets, but became rapidly translocated to the proximity of the plasma membrane upon thrombin stimulation, thus indicating a role for PLD in platelet activation.

Massive Ca-induced membrane fusion and phospholipid changes triggered by reverse Na/Ca exchange in BHK fibroblasts

Baby hamster kidney (BHK) fibroblasts increase their cell capacitance by 25-100% within 5 s upon activating maximal Ca influx via constitutively expressed cardiac Na/Ca exchangers (NCX1). Free Ca, measured with fluo-5N, transiently exceeds 0.2 mM with total Ca influx amounting to approximately 5 mmol/liter cell volume. Capacitance responses are half-maximal when NCX1 promotes a free cytoplasmic Ca of 0.12 mM (Hill coefficient approximately 2). Capacitance can return to baseline in 1-3 min, and responses can be repeated several times. The membrane tracer, FM 4-64, is taken up during recovery and can be released at a subsequent Ca influx episode. Given recent interest in signaling lipids in membrane fusion, we used green fluorescent protein (GFP) fusions with phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) and diacylglycerol (DAG) binding domains to analyze phospholipid changes in relation to these responses. PI(4,5)P(2) is rapidly cleaved upon activating Ca influx and recovers within 2 min. However, PI(4,5)P(2) depletion by activation of overexpressed hM1 muscarinic receptors causes only little membrane fusion, and subsequent fusion in response to Ca influx remains massive. Two results suggest that DAG may be generated from sources other than PI(4,5)P in these protocols. First, acylglycerols are generated in response to elevated Ca, even when PI(4,5)P(2) is metabolically depleted. Second, DAG-binding C1A-GFP domains, which are brought to the cell surface by exogenous ligands, translocate rapidly back to the cytoplasm in response to Ca influx. Nevertheless, inhibitors of PLCs and cPLA2, PI(4,5)P(2)-binding peptides, and PLD modification by butanol do not block membrane fusion. The cationic agents, FM 4-64 and heptalysine, bind profusely to the extracellular cell surface during membrane fusion. While this binding might reflect phosphatidylserine (PS) "scrambling" between monolayers, it is unaffected by a PS-binding protein, lactadherin, and by polylysine from the cytoplasmic side. Furthermore, the PS indicator, annexin-V, binds only slowly after fusion. Therefore, we suggest that the luminal surfaces of membrane vesicles that fuse to the plasmalemma may be rather anionic. In summary, our results provide no support for any regulatory or modulatory role of phospholipids in Ca-induced membrane fusion in fibroblasts.

Lipids and lipid modifications in the regulation of membrane traffic

Lipids play a multitude of roles in intracellular protein transport and membrane traffic. While a large body of data implicates phosphoinositides in these processes, much less is known about other glycerophospholipids such as phosphatidic acid, diacylglycerol, and phosphatidylserine. Growing evidence suggests that these lipids may also play an important role, either by mediating protein recruitment to membranes or by directly affecting membrane dynamics. Although membrane lipids are believed to be organized in microdomains, recent advances in cellular imaging methods paired with sophisticated reporters and proteomic analysis have led to the formulation of alternative ideas regarding the characteristics and putative functions of lipid microdomains and their associated proteins. In fact, the traditional view that membrane proteins may freely diffuse in a large 'sea of lipids' may need to be revised. Lastly, modifications of proteins by lipids or related derivatives have surprisingly complex roles on regulated intracellular transport of a wide range of molecules.

Whole blood choline and plasma choline in acute coronary syndromes: prognostic and pathophysiological implications

BACKGROUND

Whole blood choline (WBCHO) and plasma choline (PLCHO) concentrations increase rapidly after stimulation of phospholipase D in acute coronary syndromes (ACS). Early risk-stratification was analyzed in 217 patients with suspected ACS and a negative admission troponin T (<0.03 microg/L).

METHODS

WBCHO and PLCHO were measured using high-performance-liquid-chromatography mass spectrometry. Major cardiac events (MACE) were defined as cardiac death/arrest, coronary intervention or myocardial infarction (MI).

RESULTS

WBCHO (> or = 28.2 micromol/L) was predictive for MACE (hazard ratio [HR] 2.7; p<0.001), cardiac death/arrest (HR 4.2; p=0.015), heart failure (HR 2.8; p=0.003), coronary intervention (HR 2.1; p=0.01) and MI (HR 8.4; p=0.002) after 30 days. PLCHO (> or = 25.0 micromol/L) was predictive for MACE (HR 2.6; p=0.005), cardiac death/arrest (HR 15.7; p<0.001), heart failure (HR 6.0; p<0.001) but not for coronary intervention and MI. WBCHO and PLCHO were predictive for MACE in multivariate analysis (Odds ratio [OR] 2.7, p=0.009 and OR 3.3, p=0.03) independently of age, gender, prior MI, coronary risk factors and ECG.

CONCLUSIONS

WBCHO and PLCHO are significant and independent predictors of major cardiac events in admission troponin T negative acute coronary syndromes. Both are predictive for events related to tissue ischemia and WBCHO is capable of detecting risks associated with coronary plaque instability.

Protease-activated receptors differentially regulate human platelet activation through a phosphatidic acid-dependent pathway

Pathological conditions such as coronary artery disease are clinically controlled via therapeutic regulation of platelet activity. Thrombin, through protease-activated receptor (PAR) 1 and PAR4, plays a central role in regulation of human platelet function in that it is known to be the most potent activator of human platelets. Currently, direct thrombin inhibitors used to block platelet activation result in unwanted side effects of excessive bleeding. An alternative therapeutic strategy would be to inhibit PAR-mediated intracellular platelet signaling pathways. To elucidate the best target, we are studying differences between the two platelet thrombin receptors, PAR1 and PAR4, in mediating thrombin's action. In this study, we show that platelet activation by PAR1-activating peptide (PAR1-AP) requires a phospholipase D (PLD)-mediated phosphatidic acid (PA) signaling pathway. We show that this PAR1-specific PA-mediated effect is not regulated through differential granule secretion after PAR-induced platelet activation. Perturbation of this signaling pathway via inhibition of lipid phosphate phosphatase-1 (LPP-1) by propranolol or inhibition of the phosphatidylcholine-derived phosphatidic acid (PA) formation by PLD with a primary alcohol significantly attenuated platelet activation by PAR1-AP. Platelet activation by thrombin or PAR4-AP was insensitive to these inhibitors. Furthermore, these inhibitors significantly attenuated activation of Rap1 after stimulation by PAR1-AP but not thrombin or PAR4-AP. Because PA metabolites such as diacylglycerol play an important role in intracellular signaling, identifying crucial differences in PA regulation of PAR-induced platelet activation may lead to a greater understanding of the role of PAR1 versus PAR4 in progression of thrombosis.

Platelet binding and phagocytosis by macrophages

BACKGROUND

Earlier it was reported that metabolic arrest followed by incubation at 4 degrees C reduces the platelet (PLT) storage defect. Here it is reported that this treatment also reduces binding and phagocytosis by macrophages.

STUDY DESIGN AND METHODS

Phagocytosis of mepacrine-labeled PLTs by macrophages changes the latter into bright fluorescent particles easily detected by fluorescence-activated cell sorting.

RESULTS

In combination with conventional binding analysis it was found that binding to phorbol 12-myristate 13-acetate-matured THP-1 cells is primarily regulated by PLT P-selectin expression and phagocytosis by combined phosphatidylserine (PS) exposure and glycoprotein (GP) Ibalpha clustering. It was found that trapping of PLT Ca2+ and raising cAMP reduces phagocytosis by lowering PS exposure. Chilling of PLTs leads to an increase in binding and PS- and GPIbalpha-mediated phagocytosis. Prior depletion of PLT energy stores prevents this increase by preserving low Ca2+ concentration, PS exposure, and PS-mediated phagocytosis.

CONCLUSION

These data characterize the individual factors that control PLT binding and phagocytosis and might help to define conditions that improve the survival of stored PLTs after transfusion.

A role for phospholipase D1 in neurotransmitter release

Phosphatidic acid produced by phospholipase D (PLD) as a result of signaling activity is thought to play a role in membrane vesicle trafficking, either as an intracellular messenger or as a cone-shaped lipid that promotes membrane fusion. We recently described that, in neuroendocrine cells, plasma membrane-associated PLD1 operates at a stage of Ca(2+)-dependent exocytosis subsequent to cytoskeletal-mediated recruitment of secretory granules to exocytotic sites. We show here that PLD1 also plays a crucial role in neurotransmitter release. Using purified rat brain synaptosomes subjected to hypotonic lysis and centrifugation, we found that PLD1 is associated with the particulate fraction containing the plasma membrane. Immunostaining of rat cerebellar granule cells confirmed localization of PLD1 at the neuronal plasma membrane in zones specialized for neurotransmitter release (axonal neurites, varicosities, and growth cone-like structures). To determine the potential involvement of PLD1 in neurotransmitter release, we microinjected catalytically inactive PLD1(K898R) into Aplysia neurons and analyzed its effects on evoked acetylcholine (ACh) release. PLD1(K898R) produced a fast and potent dose-dependent inhibition of ACh release. By analyzing paired-pulse facilitation and postsynaptic responses evoked by high-frequency stimulations, we found that the exocytotic inhibition caused by PLD1(K898R) was not the result of an alteration in stimulus-secretion coupling or in vesicular trafficking. Analysis of the fluctuations in amplitude of the postsynaptic responses revealed that the PLD1(K898R) blocked ACh release by reducing the number of active presynaptic-releasing sites. Our results provide evidence that PLD1 plays a major role in neurotransmission, most likely by controlling the fusogenic status of presynaptic release sites.

Secretion in alveolar type II cells at the interface of constitutive and regulated exocytosis

Long-term, simultaneous, measurements of cytoplasmic free Ca(2+) concentrations and single exocytotic fusion events in surfactant-secreting type II cells were performed. All fusion (constitutive, phorbol ester-induced, and agonist-induced) was Ca(2+)-dependent. Kinetic analysis revealed that agonist (adenosine triphosphate [ATP])-induced fusion exhibited a kinetic pattern that correlated well with the Ca(2+) signal. The effects of Ca(2+) release from intracellular stores (early) and Ca(2+) entry (late) could be demonstrated for the first time by dissecting the slow (10-to-15-min) fusion response to ATP into these two components. Bath Ba(2+) or Sr(2+) could replace Ca(2+) to elicit a fusion response in thapsigargin-pretreated cells lacking ATP-induced Ca(2+) release from stores. Although the late response was partially inhibited by interrupting the phospholipase D-protein kinase C axis, a high Ca(2+) dependence of the entire secretory course was demonstrated by a significant correlation between the integrated Ca(2+) signal and the fusion response. There was also a highly significant correlation between constitutive and ATP-stimulated fusion activity in individual cells. We propose a common mechanistic model for all types of fusion in this slow secretory cell, in which constitutive and regulated forms of exocytosis are subject to the same principles of regulation.

Phospholipase D1: a key factor for the exocytotic machinery in neuroendocrine cells

Phospholipase D (PLD) has been proposed to mediate cytoskeletal remodeling and vesicular trafficking along the secretory pathway. We recently described the activation of an ADP ribosylation factor-regulated PLD at the plasma membrane of chromaffin cells undergoing secretagogue-stimulated exocytosis. We show here that the isoform involved is PLD1b, and, using a real-time assay for individual cells, that PLD activation and exocytosis are closely correlated. Moreover, overexpressed PLD1, but not PLD2, increases stimulated exocytosis in a phosphatidylinositol 4,5-bisphosphate-dependent manner, whereas catalytically inactive PLD1 inhibits it. These results provide the first direct evidence that PLD1 is an important component of the exocytotic machinery in neuroendocrine cells.

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.

D609-phosphatidylcholine-specific phospholipase C inhibitor attenuates thapsigargin-induced sodium influx in human lymphocytes

Previously, we reported that the phosphatidylcholine-specific phospholipase C (PC-PLC) inhibitor tricyclodecan-9-yl xanthogenate (D609) potentiates thapsigargin-induced Ca(2+) influx in human lymphocytes. In the present study we examined the effect of D609 on the thapsigargin-induced Na(+) entry. We found that the early phase of the thapsigargin-induced increase in the intracellular Na(+) concentration (approx. 1-2 min after stimulation) was attenuated after preincubation of lymphocytes with D609. By contrast, thapsigargin-induced Na(+) influx was not affected in the presence butan-1-ol, which inhibits phosphatidylcholine-specific phospholipase D (PC-PLD). The thapsigargin-induced Na(+) influx could be mimicked by PC-PLC exogenously added to the lymphocyte suspension, whereas addition of PC-PLD had no effect. In addition, thapsigargin stimulated formation of the physiological PC-PLC products, diacylglycerol. Cell-permeable diacylglycerol analogue, dioctanoyl-glycerol (DOG), produced time- and concentration-dependent increase in the intracellular Na(+) concentration. Both thapsigargin- and DOG-induced Na(+) increases were not affected in the presence of Na(+)/H(+) antiport inhibitor, HOE609, or Na(+)/Ca(2+) antiport inhibitor, dimethylthiourea, as well as in the presence of Co(2+) and Ni(2+), which block store-operated Ca(2+) entry. By contrast, markedly reduced thapsigargin- and DOG-induced Na(+) influx were noted in the presence of flufenamic acid, which blocks the non-selective cation current (I(CRANC)). In conclusion, our results suggest that diacylglycerol released due to the PC-PLC activation contributes to the thapsigargin-induced Na(+) entry.

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+.

Activation of the small GTPase Ral in platelets

Ral is a ubiquitously expressed Ras-like small GTPase which is abundantly present in human platelets. The biological function of Ral and the signaling pathway in which Ral is involved are largely unknown. Here we describe a novel method to measure Ral activation utilizing the Ral binding domain of the putative Ral effector RLIP76 as an activation-specific probe. With this assay we investigated the signaling pathway that leads to Ral activation in human platelets. We found that Ral is rapidly activated after stimulation with various platelet agonists, including alpha-thrombin. In contrast, the platelet antagonist prostaglandin I2 inhibited alpha-thrombin-induced Ral activation. Activation of Ral by alpha-thrombin could be inhibited by depletion of intracellular Ca2+, whereas the induction of intracellular Ca2+ resulted in the activation of Ral. Our results show that Ral can be activated by extracellular stimuli. Furthermore, we show that increased levels of intracellular Ca2+ are sufficient for Ral activation in platelets. This activation mechanism correlates with the activation mechanism of the small GTPase Rap1, a putative upstream regulator of Ral guanine nucleotide exchange factors.

Phosphatidylcholine-specific phospholipase C regulates thapsigargin-induced calcium influx in human lymphocytes

The involvement of phosphatidylcholine-specific phospholipase C (PC-PLC) and D (PC-PLD) in the regulation of the thapsigargin-induced Ca2+ increase was investigated. Pretreatment of human lymphocytes with the PC-PLC inhibitors D609 or U73122 enhanced the thapsigargin-induced Ca2+ influx. By contrast, no effect was observed in the presence of phospholipase D inhibitor butanol. Addition of exogenous PC-PLC but not PC-PLD to lymphocytes prestimulated with thapsigargin led to a decrease of intracellular Ca2+. In addition, thapsigargin was shown to release diacylglycerol (DAG) from cellular phosphatidylcholine pools. The thapsigargin-induced DAG formation was inhibited by U73122 and D609 but not by butanol. Moreover, no formation of the PC-PLD activity marker phosphatidylbutanol was detected. Thapsigargin-induced DAG formation was dependent on the Ca2+ entry, as it was abolished in the absence of extracellular Ca2+ or in the presence of Ni2+. Further investigations demonstrated that the inhibition of the cellular DAG target, protein kinase C (PKC), enhanced thapsigargin-induced Ca2+ increase, whereas direct PKC activation had an inhibitory effect. Taken together, our results reveal the involvement of PC-PLC in the regulation of the thapsigargin-induced Ca2+ increase and point to the existence of a physiologic feedback mechanism activated by Ca2+ influx and acting via consecutive activation of PC-PLC and PKC to limit the rise of intracellular Ca2+.

Protein kinase C-dependent and Ca2+-dependent mechanisms of secretion from streptolysin O-permeabilized platelets: effects of leakage of cytosolic proteins

Human platelets containing dense granules labelled with 5-hydroxy[14C]tryptamine ([14C]5-HT) were permeabilized by exposure to streptolysin O (SLO) in the presence of 4 mM [gamma-32P]ATP. Addition of either 100 nM phorbol 12-myristate 13-acetate (PMA) or of Ca2+ (pCa 5) at the same time as SLO induced secretion of dense-granule [14C]5-HT and the phosphorylation of pleckstrin by protein kinase C (PKC). Ca2+ also induced phosphorylation of myosin P-light chains. Guanosine 5'-[gamma-thio]triphosphate (GTP[S], 100 microM) did not stimulate secretion from SLO-permeabilized platelets in the absence of Ca2+ (pCa>9), but greatly potentiated secretion in the presence of low PMA (10 nM) or low Ca2+ (pCa 6). However, GTP[S] did stimulate myosin P-light-chain phosphorylation in the absence of Ca2+, an effect that was associated with morphological changes, including granule centralization. Inhibition of PKC and of pleckstrin phosphorylation by Ro 31-8220 blocked secretion induced by PMA or by GTP[S] and PMA in the absence of Ca2+, but did not prevent the GTP[S]-induced phosphorylation of myosin P-light chains or secretion induced by Ca2+ at pCa 5. When the time period between exposure of platelets to SLO and challenge at pCa>9 with PMA or with GTP[S] and PMA was increased, there were rapid and parallel decreases in the secretion and pleckstrin phosphorylation responses, which were lost after 3-5 min. In contrast, the responsiveness of secretion to Ca2+ (pCa 5) or to GTP[S] and Ca2+ (pCa 6) persisted for at least 10 min after exposure of platelets to SLO, although the ability of pleckstrin to undergo phosphorylation was still lost after 3-5 min. Both PKC and pleckstrin were undetectable within platelets after 5 min exposure to SLO. The results suggest that the loss of responsiveness to PMA or to GTP[S] and PMA is attributable to the leakage of PKC (and possibly pleckstrin) from the platelets, whereas secretion stimulated by Ca2+ or by GTP[S] and Ca2+ utilizes membrane-associated Ca2+- and GTP-binding proteins and occurs independently of PKC activation.

Characteristics of protein kinase C-independent exocytosis in human platelets

We evaluated the characteristics of the protein kinase C (PKC)-independent mechanism for ATP release in platelet-rich plasma. When ADP (10 microM) and U46619 (1 microM) were both added as agonists, a significant release was observed immediately after stimulation. The PKC inhibitor, Ro-31-7549 (10 microM), or a cyclooxygenase inhibitor, aspirin (400 microM) or indomethacin (20 microM), partially inhibited ATP release with little effect on platelet aggregation. The ATP release observed in the presence of Ro-31-7549 was abolished by a cyclooxygenase inhibitor or by preventing aggregation without stirring. In the nonstirred condition, thromboxane B2 formation was reduced by 93%. When sodium arachidonate (1 mM) rather than U46619 was used with ADP, ATP release in the presence of Ro-31-7549 was abolished by stopping the stirring with no effect on thromboxane B2 formation. In contrast, ADP/U46619-induced ATP release observed in the presence of aspirin was only partially inhibited when the stirring was stopped. This release was also inhibited dose-dependently by Ro-31-7549 at concentrations between 1 and 10 microM. These results suggest that PKC-independent ATP-release in this system requires aggregation and is inhibited by a cyclooxygenase inhibitor, while PKC-dependent exocytosis is insensitive to aggregation and a cyclooxygenase inhibitor.

Induction of cytosolic phospholipase A2 activity by phosphatidic acid and diglycerides in permeabilized human neutrophils: interrelationship between phospholipases D and A2

Relationships between phospholipases are poorly understood, but phosphatidic acid (PA) and diglycerides (DGs), produced by phospholipase D (PLD) and phosphatidate phosphohydrolase actions, might function as second messengers coupling cell stimulation to cellular responses. This study investigates the role of PLD-mediated PA and DG formation in inducing phospholipase A2 (PLA2) activity in intact human neutrophils (PMNs) and in PMNs permeabilized with Staphylococcus aureus alpha-toxin. PMNs were labelled with [3H]arachidonic acid (AA) to assess AA release and metabolism and diacylglycerol formation, or with [3H]1-O-hexadecyl-2-lyso-glycerophosphatidylcholine for the determination of platelet-activating factor (PAF), PA and alkylacylglycerol production. In intact PMNs primed with tumour necrosis factor alpha before stimulation with N-formyl-Met-Leu-Phe, AA release and metabolism and PAF formation increased in parallel with enhanced PA and DG formation, and inhibition of PA and DG production led to a decrease in both AA release and PAF accumulation. In alpha-toxin-permeabilized PMNs, AA release and PAF production result from the specific activation of cytosolic PLA2 (cPLA2). In this system, PA and DG formation were always present when cPLA2 activation occurred; blocking PA and DG production inhibited AA release and PAF accumulation. Adding either PA or DG back to permeabilized cells (with endogenous PA and DG formation blocked) led to a partial restoration of AA release and PAF formation; a combination of PA and DGs reconstituted full cPLA2 activity. These results strongly suggest that products of PLD participate in activating cPLA2 in PMNs.

Signal transduction during exocytosis in Limulus polyphemus granulocytes

Bacterial lipopolysaccharide (LPS)-induced exocytosis is one of the primary immune responses of the Limulus granulocyte (GR). Exocytosis can be mediated by guanine nucleotide-binding protein (G-protein)-linked surface receptors that activate phospholipase C (PLC) to produce inositol 1,4,5-triphosphate (IP3) and diacylglycerol (DAG). IP3 mobilizes intracellular Ca2+ ([Ca2+]i), which can lead to exocytosis. We used activators and inhibitors of known signal transduction pathways to investigate the signaling pathway responsible for LPS-induced exocytosis in the GR. These compounds have been shown to similarly effect pathways in vertebrate and invertebrate systems and this assumption is made here. Pretreatment of GRs with cholera and pertussis toxins, which modulate G-proteins, and U73122, which inhibits PLC, inhibited LPS-induced exocytosis, but pretreatment with the tyrosine kinase inhibitor herbimycin did not. In contrast, exocytosis was induced with fluoride (a G-protein activator) and thapsigargin with Mg2+ (an inhibitor of endomembranous Ca(2+)-ATPase). Exocytosis was not induced by phorbol ester, which mimics DAG to activate protein kinase C (PKC) and it was not effected by ethanol or chelerythrine, which inhibit phospholipase D and PKC, respectively. Microinjection of GRs with different concentrations of IP3, an IP3 analog (DL-2,3,6,trideoxy-myo-inositol 1,4,5-triphosphate), Mg2+, or Ca2+ induced different percentages of exocytosis in individual cells, while HEPES buffer did not. Microfluorometric analysis of intracellular Mg2+ ([Mg2+]i) and [Ca2+]i, using the dyes Mag Fura-2AM and Calcium Green 5N, respectively, revealed [Mg2+]i and [Ca2+]i fluxes during LPS-induced exocytosis. This study suggests that LPS induces exocytosis in the Limulus GR through activation of G-protein-coupled receptors, which stimulate the IP3 signaling pathway to induce both [Ca2+]i and [Mg2+]i fluxes to facilitate vesicular and plasma membrane fusion. This is the first demonstration of the signal transduction pathway responsible for the primary immune response of the GR.

Rho is only ARF the story. Phospholipid signalling

The GTP-binding proteins ARF and Rho control a number of important cellular processes, such as protein traffic and cell morphology; there is increasing evidence that phospholipase D is a key mediator of these ARF/Rho-regulated events.

Differential effects of G-protein activators on 5-hydroxytryptamine and platelet-derived growth factor release from streptolysin-O-permeabilized human platelets

In this paper we have used streptolysin O (SLO)-permeabilized human platelets to examine the G-protein(s) that control Ca2+-independent secretion from alpha and dense-core granules. As shown for electropermeabilized platelets, Ca2+ alone stimulated a concentration-dependent increase in 5-hydroxytryptamine (5-HT) (dense-core-granule marker) and platelet-derived growth factor (PDGF) (alpha-granule marker) release from the SLO-permeabilized cells. The EC50 values of Ca2+-dependent 5-HT and PDGF release were 5 microM and 10 microM respectively. Guanosine 5'-[gamma-thio]triphosphate (GTP[S]) (100 microM) stimulated Ca2+-independent release from both alpha and dense-core granules. In contrast, AlF4- had no effect on Ca2+-independent release from either alpha or dense-core granules. Neither GTP[S] nor AlF4- appeared to have a significant effect on Ca2+-dependent release from alpha and dense-core granules. GTP[S] can activate both heterotrimeric and low-molecular-mass G-proteins, whereas AlF4- activates only heterotrimeric G-proteins. Our results, therefore suggest that secretion in the human platelet is regulated by a small G-protein. Both GTP[S]- and Ca2+-dependent secretion were effected by extending the time between permeabilization with SLO and stimulation of secretion. GTP[S]-stimulated secretion from alpha and dense-core granules decreased rapidly after permeabilization. In contrast, Ca2+-dependent 5-HT and PDGF release ran down at a much lower rate. These observations indicate that GTP[S] and Ca2+ act through parallel pathways to stimulate secretion from SLO-permeabilized platelets.

Dependence of stimulus-transcription coupling on phospholipase D in agonist-stimulated pituitary cells

Stimulation of phospholipase D activity is frequently observed during agonist activation of Ca(2+)-mobilizing receptors, but the cellular functions of this signaling pathway are not well defined. Pituitary gonadotrophs express Ca(2+)-mobilizing receptors for gonadotropin-releasing hormone (GnRH) and endothelin (ET), activation of which stimulates luteinizing hormone secretion and transient expression of c-fos. In pituitary cells and alpha T3-1 gonadotrophs, GnRH action was associated with both initial and sustained diacylglycerol (DG) production, whereas ET-1 induced only a transient DG response. Also, phospholipase D activity, estimated by the production of phosphatidylethanol from phosphatidylcholine in the presence of ethanol, was stimulated by GnRH but not ET-1. Such formation of phosphatidylethanol at the expense of phosphatidic acid (PA) during GnRH-induced activation of phospholipase D significantly reduced the production of PA, DG, and cytidine diphosphate diacylglycerol. Inhibition of PA-phosphohydrolase activity by propranolol also decreased GnRH-induced DG production and, in contrast to ethanol, increased PA and cytidine diphosphate diacylglycerol levels. The fall in DG production caused by ethanol and propranolol was accompanied by inhibition of GnRH-induced c-fos expression, whereas agonist-induced luteinizing hormone release was not affected. In contrast to their inhibitory actions on GnRH-induced early gene expression, neither ethanol nor propranolol affected ET-1-induced c-fos expression, or GnRH- and ET-1-induced inositol trisphosphate/Ca2+ signaling. These findings demonstrate that phospholipase D participates in stimulus-transcription but not stimulus-secretion coupling, and indicate that DG is the primary signal for this action.

Stimulation of phospholipase D in rabbit platelet membranes by nucleoside triphosphates and by phosphocreatine: roles of membrane-bound GDP, nucleoside diphosphate kinase and creatine kinase

Previous work has shown that guanosine 5'-[gamma-thio]triphosphate (GTP[S]) and GTP stimulate phospholipase D (PLD) in rabbit platelet membranes and that these effects are greatly enhanced by pretreatment of platelets with phorbol esters that activate protein kinase C [Van der Meulen and Haslam (1990), Biochem. J. 271, 693-700]. In the present study, the effects of Mg2+, various nucleoside triphosphates and phosphocreatine (PCr) were investigated. Platelet membranes containing phospholipids labelled with [3H]glycerol were assayed for PLD in the presence of an optimal Mg2+ concentration (10 mM) by measuring [3H]phosphatidylethanol formation in incubations that included 300 mM ethanol. In membranes from phorbolester-treated platelets, the same maximal increases in PLD activity (5-fold) were seen with 1 microM GTP[S]), and 100 microM GTP. Addition of adenosine 5'-[gamma-thio]triphosphate (ATP[S]), ITP, XTP, UTP and CTP had similar stimulatory effects, but only at > or = 1 mM. In contrast, ATP had a biphasic action, causing a maximal (2-fold) stimulation at 10 microM and smaller effects at higher concentrations; the inhibitory component of the action of ATP was blocked by 2 microM staurosporine. Guanosine 5'-[beta-thio]diphosphate decreased the stimulatory effects of ATP and ATP[S]. UDP, which can inhibit nucleoside diphosphate kinase (NDPK), decreased the activation of PLD by ATP[S], ATP, XTP, CTP and to a lesser extent ITP, but had no effect on the actions of GTP[S] and GTP. Rabbit platelet membranes contained NDPK and addition of [gamma-32P]ATP led to the formation of [32P]GTP in amounts sufficient to explain most or all of the activation of PLD; UDP prevented GTP formation. PCr (0.04-1 mM) also stimulated membrane PLD activity, an effect that was dependent on endogenous membrane-bound creatine kinase (CK). UDP and guanosine 5'-[beta-thio]diphosphate each inhibited this effect of PCr. The results show that in rabbit platelet membranes, CK, NDPK and the GTP-binding protein that activates PLD can be functionally coupled. However, assay of membrane preparations at increasing dilutions showed that stimulation of PLD by the compounds studied, with the partial exception of ATP[S], involved diffusible rather than protein-bound intermediates.