Activation of phospholipase D in human platelets by collagen and thrombin and its relationship to platelet aggregation

Antimicrobial agent triclosan suppresses mast cell signaling via phospholipase D inhibition

Humans are exposed to the antimicrobial agent triclosan (TCS) through use of TCS-containing products. Exposed tissues contain mast cells, which are involved in numerous biological functions and diseases by secreting various chemical mediators through a process termed degranulation. We previously demonstrated that TCS inhibits both Ca²⁺ influx into antigen-stimulated mast cells and subsequent degranulation. To determine the mechanism linking the TCS cytosolic Ca²⁺ depression to inhibited degranulation, we investigated the effects of TCS on crucial signaling enzymes activated downstream of the Ca²⁺ rise: protein kinase C (PKC; activated by Ca²⁺ and reactive oxygen species [ROS]) and phospholipase D (PLD). We found that TCS strongly inhibits PLD activity within 15 minutes post-antigen, a key mechanism of TCS mast cell inhibition. In addition, experiments using fluorescent constructs and confocal microscopy indicate that TCS delays antigen-induced translocations of PKCβII, PKCδ and PKC substrate myristoylated alanine-rich C-kinase. Surprisingly, TCS does not inhibit PKC activity or overall ability to translocate, and TCS actually increases PKC activity by 45 minutes post-antigen; these results are explained by the timing of both TCS inhibition of cytosolic Ca²⁺ (~15+ minutes post-antigen) and TCS stimulation of ROS (~45 minutes post-antigen). These findings demonstrate that it is incorrect to assume that all Ca²⁺ -dependent processes will be synchronously inhibited when cytosolic Ca²⁺ is inhibited by a toxicant or drug. The results offer molecular predictions of the effects of TCS on other mammalian cell types, which share these crucial signal transduction elements and provide biochemical information that may underlie recent epidemiological findings implicating TCS in human health problems.

The effects of an inhibitor of diglyceride lipase on collagen-induced platelet activation

Human platelet activation by collagen occurs in a dose-dependent manner. High concentrations of collagen bind to a pair of receptors, the α2β1 integrin and glycoprotein (GP)VI/Fc-receptor γ-chain (FcRγ), which stimulate a cascade of events including Syk, LAT, Btk, Gads, and phospholipase Cγ2, leading to calcium release and protein kinase C (PKC) activation. Calcium and PKC are responsible for a range of platelet responses including exocytosis and aggregation, as well as the cytosolic phospholipase A2 (cPLA2)-mediated release of arachidonic acid, which is converted to thromboxane (Tx)A2. In contrast, low concentrations of collagen are acutely aspirin-sensitive, and calcium release and aggregation are TxA2-dependent. Under these conditions, cPLA2 is not involved and it has been suggested that phospholipase C generates 1,2-diacylglycerol (DG) from which arachidonic acid is liberated by diglyceride lipase (DGL). Here a novel DGL blocker (OMDM-188) inhibited collagen-, but not arachidonic acid-induced aggregation and TxA2 synthesis. Furthermore, OMDM-188 inhibited collagen-induced arachidonic acid release. Finally OMDM-188 inhibited collagen-induced p38(MAPK) phosphorylation, but not extracellular signal-regulated kinase (ERK) phosphorylation, with no effect on the phosphorylation of either enzyme in response to arachidonic acid. Taken together, these data suggest a role for a pathway involving phospholipase C liberating DG from membrane phospholipids in response to minimally activating concentrations of collagen. The DG serves as a substrate for DGL, potentially under the regulations of p38(MAPK), to release arachidonic acid, which is subsequently converted to TxA2, which mediates the final platelet response.

Mammalian phospholipase D physiological and pathological roles

Phospholipase D (PLD), a superfamily of signalling enzymes that most commonly generate the lipid second messenger phosphatidic acid, is found in diverse organisms from bacteria to humans and functions in multiple cellular pathways. Since the early 1980s when mammalian PLD activities were first described, most of the important insights concerning PLD function have been gained from studies on cellular models. Reports on physiological and pathophysiological roles for members of the mammalian PLD superfamily are now starting to emerge from genetic models. In this review, we summarize recent findings on PLD functions in these model systems, highlighting newly appreciated connections of the superfamily to cancer, neuronal pathophysiology, cardiovascular topics, spermatogenesis and infectious diseases.

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.

Present and Future Biochemical Markers for Detection of Acute Coronary Syndrome

The use of biochemical markers in the diagnosis and management of patients with acute coronary syndrome has increased continually in recent decades. The development of highly sensitive and cardiac-specific troponin assays has changed the view on diagnosis of myocardial infarction and also extended the role of biochemical markers of necrosis into risk stratification and guidance for treatment. The consensus definition of myocardial infarction places increased emphasis on cardiac marker testing, with cardiac troponin replacing creatine kinase MB as the "gold standard" for diagnosis of myocardial infarction. Along with advances in the use of more cardiac-specific markers of myocardial necrosis, biochemical markers that are involved in the progression of atherosclerotic plaques to the vulnerable state or that signal the presence of vulnerable plaques have recently been identified. These markers have variable abilities to predict the risk of an individual for acute coronary syndrome. The aim of this review is to provide an overview of the well-established markers of myocardial necrosis, with a special focus on cardiac troponin I, together with a summary of some of the potential future markers of inflammation, plaque instability, and ischemia.

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.

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.

Prognostic implications of elevated whole blood choline levels in acute coronary syndromes

Troponins I and T represent the current biomarker standard for diagnosis of myocardial infarction. Even small increases of cardiac troponins have prognostic implications, but not all patients at risk are correctly classified, particularly at admission. We identified elevated whole-blood choline as a promising marker and performed a prospective study of 327 patients with a suspected acute coronary syndrome that focused on the analysis of troponin-negative patients. Diagnostic classification of patients and the definition of troponin cutoffs were performed according to the new European Society of Cardiology/American College of Cardiology criteria. Blood was sampled serially and choline was measured using high-performance liquid chromatography mass spectrometry in whole blood. Patients were followed for 30 days. In patients with negative troponin I test results at admission (n = 250), choline was a predictor of cardiac death and nonfatal cardiac arrest (hazard ratio 6.0, p = 0.003), life-threatening arrhythmias (hazard ratio 3.75, p = 0.004), heart failure (hazard ratio 2.87, p = 0.002), and coronary angioplasty (hazard ratio 2.57, p = 0.001). In multivariate analysis of troponin-negative patients, choline was the strongest predictor of cardiac death or arrest (odds ratio 6.05, p = 0.01). Choline was not a marker for myocardial necrosis but indicated high-risk unstable angina in patients without acute myocardial infarction (sensitivity 86.4%, specificity 86.2%). Thus, an increased concentration of choline at hospital admission is a predictor of adverse cardiac events in patients with suspected acute coronary syndromes. Whole blood choline may be useful for early risk stratification of these patients, particularly if troponin results are negative on admission.

Phospholipase D in platelets and megakaryocytic cells

Phospholipase D (PLD) is stimulated in platelets by various agents. Phosphatidylcholine is the major substrate for PLD. This enzymatic pathway generates phosphatidic acid selectively. Guanine nucleotides also stimulate PLD in platelet membranes. Furthermore, tyrosine kinase may also be involved in platelet PLD regulation. It appears that multiple signals acting sequentially or in parallel converge on PLD. Among others, PLD has been proposed to play a role in platelet secretion and PLA2 regulation. PLD is also present in platelet percursor megakaryocytric cells and can be activated by platelet agonists. In these cells both PKC and G-proteins (e.g. Rho) may regulate PLD activity. The significance of PLD in megakaryocytes awaits investigation. These recent developments offer new avenues of research to further elucidate the biochemistry of platelet and megakaryocyte function.