Phospholipase A2: its role in ADP- and thrombin-induced platelet activation mechanisms

Agomelatine inhibits platelet aggregation through melatonin receptor-dependent and independent mechanisms

AIMS

Melatonin is known to inhibit platelet aggregation induced by arachidonic acid (AA). In the present study we investigated whether agomelatine (Ago), an antidepressant with agonist activity at melatonin receptor 1 (MT1) and MT2 could reduce platelets aggregation and adhesion.

MAIN METHODS

Human platelets from healthy donors were used to test the in vitro effects of Ago in the presence of different platelet activators. We performed aggregation and adhesion assays, thromboxane B₂ (TxB₂), cAMP and cGMP measurements, intra-platelet calcium registration and flow cytometry assays.

KEY FINDINGS

Our data revealed that different concentrations of Ago reduced AA- and collagen-induced human platelet aggregation in vitro. Ago also reduced AA-induced increase in thromboxane B₂ (TxB₂) production, intracellular calcium levels and P-selectin expression at plasma membrane. The effects of Ago in AA-activated platelets were likely dependent on MT1 as they were blocked by luzindole (a MT1/MT2 antagonist) and mimicked by the MT1 agonist UCM871 in a luzindole-sensitive manner. The MT2 agonist UCM924 was also able to inhibit platelet aggregation, but this response was not affected by luzindole. On the other hand, although UCM871 and UCM924 reduced collagen-induced platelet aggregation and adhesion, inhibition of collagen-induced platelet aggregation by Ago was not mediated by melatonin receptors because it was not affected by luzindole.

SIGNIFICANCE

The present data show that Ago suppresses human platelet aggregation and suggest that this antidepressant may have the potential to prevent atherothrombotic ischemic events by reducing thrombus formation and vessel occlusion.

How not to discover a drug - integrins

INTRODUCTION

Integrins are a family of 24 cell adhesion receptors that play a role in the biggest unmet needs in medicine - cardiovascular disease, immunology and cancer. Their discovery promised huge potential for the pharmaceutical industry. Areas covered. Over 35-years since their discovery, there is little to show for the hundreds of billions of dollars of investment in anti-integrin drug discovery programmes. In this review the author discusses the reasons for the failure of this promising class of drugs and the future for this class of drugs. Expert opinion. Within 10-years, there was a plethora of potent, specific anti-integrin molecules and since their discovery, many of these agents have entered clinical trials. The success in discovering these agents was due to recently discovered monoclonal antibody technology. The integrin-recognition domain Arg-Gly-Asp (RGD) provided the basis for discovering small molecule inhibitors to integrins - both cyclic peptides and peptidomimetics. Most agents failed in the Phase III clinical trials and those agents that did make it to the market were plagued with issues of toxicity and limited efficacy and were soon replaced with non-integrin targeting agents. Their failure was due to a combination of poor pharmacokinetics and pharmacodynamics, complicated by the complex pathophysiology of integrins.

Factors influencing the eicosanoids synthesis in vivo

External factors activate a sequence of reactions involving the reception, transduction, and transmission of signals to effector cells. There are two main phases of the body's reaction to harmful factors: the first aims to neutralize the harmful factor, while in the second the inflammatory process is reduced in size and resolved. Secondary messengers such as eicosanoids are active in both phases. The discovery of lipoxins and epi-lipoxins demonstrated that not all arachidonic acid (AA) derivatives have proinflammatory activity. It was also revealed that metabolites of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) such as resolvins, protectins, and maresins also take part in the resolution of inflammation. Knowledge of the above properties has stimulated several clinical trials on the influence of EPA and DHA supplementation on various diseases. However, the equivocal results of those trials prevent the formulation of guidelines on EPA and DHA supplementation. Prescription drugs are among the substances with the strongest influence on the profile and quantity of the synthesized eicosanoids. The lack of knowledge about their influence on the conversion of EPA and DHA into eicosanoids may lead to erroneous conclusions from clinical trials.

Anti-β2GP1 antibodies have variable effects on platelet aggregation

AIM

To investigate the effects of affinity-purified rabbit anti-β2GP1, and anti-β2GP1 purified from patients with systemic lupus erythematosus (SLE), on adenosine diphosphate (ADP)-induced aggregation.

METHODS

Whole blood was collected and processed to obtain platelet poor plasma (PPP) from normal controls (n = 15) and SLE patients (n = 15). Using PPP, anti-β2GP1 titres were determined using an ELISA and IgG fractions isolated using a HiTrap protein G column. Anti-β2GP1 was purified from two SLE patients using purified β2GP1 coupled to a HiTrap NHS-activated HP column.

RESULTS

The effect of rabbit and human derived anti-β2GP1 (0-100 μg/mL), on ADP (2.5, 5 μM) induced platelet aggregation were investigated using light transmission aggregometry. Rabbit anti-β2GP1 significantly inhibited all parameters of 5 μM ADP-induced platelet aggregation; %Max (p = 0.028), %AUC (p = 0.014) and slope (p < 0.001). In contrast, anti-β2GP1 purified from SLE patients significantly enhanced the %Max (p = 0.031) and %AUC (p = 0.007) in a concentration dependent manner, but inhibited the slope (p < 0.05) of 5 μM ADP-induced platelet aggregation.

CONCLUSION

Our data suggest anti-β2GP1 purified from different species have variable effects on in vitro platelet aggregation. The disparity between rabbit and human anti-β2GP1 may be due to the heterogeneous nature of anti-β2GP1, varying avidity or different antibody binding specificities between species.

Molecular basis of anti-inflammatory action of platelet-rich plasma on human chondrocytes: mechanisms of NF-κB inhibition via HGF

Loss of articular cartilage through injury or disease presents major clinical challenges also because cartilage has very poor regenerative capacity, giving rise to the development of biological approaches. As autologous blood product, platelet-rich plasma (PRP) provides a promising alternative to surgery by promoting safe and natural healing. Here we tested the possibility that PRP might be effective as an anti-inflammatory agent, providing an attractive basis for regeneration of articular cartilage, and two principal observations were done. First, activated PRP in chondrocytes reduced the transactivating activity of NF-κB, critical regulator of the inflammatory process, and decreased the expression of COX-2 and CXCR4 target genes. By analyzing a panel of cytokines with different biological significance, in activated PRP we observed increases in hepatocyte growth factor (HGF), interleukin-4 and tumor necrosis factor-α (TNF-α). HGF and TNF-α, by disrupting NF-κB-transactivating activity, were important for the anti-inflammatory function of activated PRP. The key molecular mechanisms involved in PRP-inhibitory effects on NF-κB activity were for HGF the enhanced cellular IkBα expression, that contributed to NF-κB-p65 subunit retention in the cytosol and nucleo-cytoplasmic shuttling, and for TNF-α the p50/50 DNA-binding causing inhibition of target-gene expression. Second, activated PRP in U937-monocytic cells reduced chemotaxis by inhibiting chemokine transactivation and CXCR4-receptor expression, thus possibly controlling local inflammation in cartilage. In conclusion, activated PRP is a promising biological therapeutic agent, as a scaffold in micro-invasive articular cartilage regeneration, not only for its content of proliferative/differentiative growth factors, but also for the presence of anti-inflammatory agents including HGF.

Surfactin C inhibits platelet aggregation

This study was designed to investigate the effect of surfactin C, which is derived from Bacillus subtilis, on platelet aggregation and homotypic leucocyte aggregation. Surfactin C strongly and dose-dependently inhibited platelet aggregation, which was stimulated both by thrombin (0.1 U mL−1), a potent agonist that activates the G protein-coupled protease receptor, and by collagen (5 μg mL−1), a potent ligand that activates αIIbβ3 with IC50 values (concentration inhibiting platelet aggregation by 50%) of 10.9 and 17.0 μM, respectively. Moreover, surfactin C significantly suppressed the intracellular Ca2+ mobilization in thrombin-activated platelets. Surfactin C, however, did not affect various integrin-mediated U937 cell aggregation, implying that the anti-platelet activity of surfactin C was not due to its detergent effect but by its action on the downstream signalling pathway. Therefore, the results suggest that surfactin C may have a beneficial therapeutic effect on aberrant platelet aggregation-mediated cardiovascular diseases.

Purification and preliminary crystallographic analysis of a new Lys49-PLA2 from B. Jararacussu

BjVIII is a new myotoxic Lys49-PLA2 isolated from Bothrops jararacussu venom that exhibits atypical effects on human platelet aggregation. To better understand the mode of action of BjVIII, crystallographic studies were initiated. Two crystal forms were obtained, both containing two molecules in the asymmetric unit (ASU). Synchrotron radiation diffraction data were collected to 2.0 A resolution and 1.9 A resolution for crystals belonging to the space group P2(1)2(1)2(1) (a = 48.4 A, b = 65.3 A, c = 84.3 A) and space group P3(1)21 (a = b = 55.7 A, c = 127.9 A), respectively. Refinement is currently in progress and the refined structures are expected to shed light on the unusual platelet aggregation activity observed for BjVIII.

Carane derivative stereoisomers of different local anaesthetic and antiplatelet activity similarly potentiate forskolin-stimulated cyclic AMP response and bind to beta-adrenoceptors in the rat brain cortex

A carane derivative, KP-23 [RS](-)-4-(2-hydroxy-3)N-isopropylamino)-propoxyimino)-cis-carane, was earlier described as a potential local anaesthetic and antiplatelet agent, and the following studies revealed that its R and S stereoisomers, KP-23R and KP-23S, have different potencies in the infiltration anaesthesia and platelet aggregation tests. The effects of these stereoisomers on the cyclic AMP (cAMP) generating system and the displacement of [(3)H]CGP 12177 (a beta-adrenoceptor ligand) from its binding sites in the rat cerebral cortical tissue were investigated. The stereoisomers did not affect the basal cAMP level, but, at concentrations between 10(-4) and 10(-3) M, they elevated the forskolin-induced accumulation of cAMP with similar potency. The compounds displaced [(3)H]CGP 12177, however the stereoisomer R was less potent than the racemic KP-23 and the S form (K(i) = 64.1 +/- 5.9 nM, 161.1 +/- 10 nM and 62.1 +/- 5.6 nM for KP-23, KP-23R and KP-23S, respectively). The fact that the stereoisomers differed in both tests only slightly, if at all, suggests that their pharmacological effects are not related to the action on the beta-adrenoceptor/adenylate cyclase system.

Compounds stimulating cytosolic phospholipase A2 activity with a combinational action mode

The screening of small synthetic compound libraries is a useful means of identifying molecules that modulate various cellular responses. We screened more than 10,000 different small compounds and identified three synthetic compounds that stimulate arachidonic acid (AA) release in a combinational manner in neutrophil-like differentiated HL60 cells. These three compounds were designated as AARIC-1, -2, and -3, representing AA release inducing compounds-1, -2, and -3. Although AA release was not induced by any single one of these compounds, it was dramatically stimulated by the three compounds in combination. Moreover, the effect of combined treatment by these compounds on AA release was completely abolished by MAFP and AACOCF(3), specific cytosolic phospholipase A(2) inhibitors. Furthermore, we found that AARIC-3 stimulates cytosolic calcium influx, while AARIC-1 induces ERK activation. Taken together, we demonstrate a useful approach to the study of complicated and nonlinear intracellular signaling networks using small synthetic compounds in combination.

Effects of calcium and thrombin on growth factor release from platelet concentrates: kinetics and regulation of endothelial cell proliferation

Platelet concentrates (PCs) constitute new biological mediators used in osseous reconstructive surgery. In this study, we assessed (i) the effects of various concentrations of calcium and thrombin on the kinetics of platelet-derived growth factor (PDGF-BB), transforming growth factor-beta1(TGF-beta 1), basic fibroblast growth factor (bFGF), and vascular endothelial growth factor (VEGF) release by PCs and (ii) the contribution of PC supernatants to endothelial cell proliferation. Our results indicate that high concentrations of calcium (Ca) and thrombin (Thr) trigger an immediate and significant increase in bFGF, TGF-beta 1 and PDGF-BB concentrations. Thereafter, PDGF-BB, VEGF and TGF-beta 1 levels remained generally constant over a 6-day period while a decrease in bFGF concentrations was noted after 24h. Lower Ca and Thr concentrations tended to reduce and delay growth factors release from PCs. Endothelial cell proliferation was greatly enhanced with PC supernatants (mean: 20-fold increase). This was especially evident when endothelial cells were treated with supernatants harvested early after PC treatment with high concentrations of Ca and Thr or later after PC treatment with low Ca and Thr concentrations. Additional research aiming to measure the effects of Ca and Thr on bone formation in vivo is needed.

Systemic Treatment of Cerebral Cortex Lesions in Rats with a New Secreted Phospholipase A2 Inhibitor

An internal fragment of the human neuroprotective polypeptide DSEP (Diffusible Survival Evasion Peptide) was delivered at 0.4 mg/kg (subcutaneously) 20-30 min after stab wound lesions in the parietal cortex of anesthetized rats. The peptide, CHEASAAQC or CHEC-9, inhibited the inflammatory response to the lesion and the degeneration of neurons adjacent to the wound. Four days after surgery, peptide-treated animals (n = 6) had 75% fewer reactive ameboid microglia/brain macrophages in the cortical parenchyma surrounding the lesion compared to vehicle-injected control rats (n = 6, p = 0.004). The cortical laminae in area 2 adjacent to the lesion were completely obscured in controls because of the increase in inflammatory cells and frank degeneration of neurons, while there was preservation of the neurons and cytoarchitecture after peptide treatment. In parallel experiments, CHEC-9 was found to inhibit the enzymatic activity of secreted phospholipase A2 (sPLA2), including activity present in the serum of peptide-injected rats. Kinetic analysis revealed the peptide increased the average Km for serum by 318% when tested 45 min after treatment (peptide-treated, n = 6; control-treated, n = 6; p = 0.0087), suggesting the principal effect of the peptide was to lower the affinity of serum sPLA2 for substrate. The sPLA2 inhibition by this particular peptide sequence appeared to be highly specific since inversion of a single pair of amino acids eliminated the inhibitory effect. Phorbol-12-myristate-13-acetate stimulated platelet aggregation, a PLA2-regulated activity, was also inhibited by the peptide. The discovery of CHEC-9 makes it possible to study in vivo the long appreciated contribution made by PLA2-directed inflammation to both acute and chronic neurodegeneration and may be helpful in designing therapies to limit neuron death in these conditions.

Identifying pathways involved in leptin-dependent aggregation of human platelets

OBJECTIVE

To investigate the role of phospholipase C (PLC), phospholipase A(2) (PLA(2)), calcium, and protein kinase C (PKC) in mediating leptin-enhanced aggregation of human platelets.

DESIGN

In vitro, ex vivo study.

SETTING

Outpatient's Service for Prevention and Treatment of Obesity at the University Hospital of Messina, Italy.

SUBJECTS

In total, 14 healthy normal-weight male (age 31.4+/-1.9 y; body mass index 22.7+/-0.6 kg/m2) subjects.

MEASUREMENTS

Adenosine diphosphate-(ADP-) induced platelet aggregation and platelet free calcium were measured after incubation of platelets with leptin alone (5-500 ng/ml), or leptin (50 and 100 ng/ml) in combination with anti-human leptin receptor long form antibody (anti-ObRb-Ab, 1:800-1:100 dilutions), PLC inhibitor U73122 (3.125-25 microM), PLA(2) inhibitor AACOCF3 (1.25-10 microM), or PKC inhibitor Ro31-8220 (1.25-10 microM).

RESULTS

Platelet stimulation with leptin leads to a significant and dose-dependent increase in ADP-induced platelet aggregation and platelet free calcium concentrations. Leptin effects on both platelet aggregation and calcium mobilization were completely abated by the co-incubation with leptin and anti-ObRb-Ab. Leptin-induced platelet aggregation was dose-dependently inhibited by U73122, AACOCF3, or Ro31-8220. The effect of leptin on intracellular calcium was inhibited in a dose-dependent manner by incubation with U73122 and AACOCF3, but not with Ro31-8220.

CONCLUSIONS

Our study confirms that leptin is able to enhance ADP-induced aggregation of human platelets, and raise the possibility that PLC, PKC, PLA(2), and calcium could play a relevant role in mediating the proaggregating action of leptin.

Novel chemoattractant peptides for human leukocytes

Phospholipase A(2) plays a key role in phagocytic cell functions. By screening a synthetic hexapeptide combinatorial library, we identified 24 novel peptides based on their ability to stimulate arachidonic acid release associated with cytosolic phospholipase A(2) activity in differentiated HL60 cells. The identified peptides, that contain the consensus sequence (K/R/M)KYY(P/V/Y)M, also induce intracellular calcium release in a pertussis toxin-sensitive manner showing specific action on phagocytic leukocytes, but not on other cells. Functionally, the peptides stimulate superoxide generation and chemotactic migration in human neutrophils and monocytes. Four of the tested active peptides were ligands for formyl peptide receptor like 1. Among these, two peptides with the consensus sequence (R/M)KYYYM can induce intracellular calcium release in undifferentiated HL60 cells that do not express formyl peptide receptor like 1, indicating usage of other receptor(s). A study of intracellular signaling in differentiated HL60 cells induced by the peptides has revealed that four of the novel peptides can induce extracellular signal-regulated protein kinase activation via shared and distinct signaling pathways, based on their dependence of phospatidylinositol-3-kinase, protein kinase C, and MEK. These peptides provide previously unavailable tools for study of differential signaling in leukocytes.

Platelet Concentrates: Effects of Calcium and Thrombin on Endothelial Cell Proliferation and Growth Factor Release

BACKGROUND

Clinical evidence suggests that platelet concentrate (PC) could have beneficial therapeutic effects on hard and soft tissue healing, due to the contents of growth factors (GFs) stored in the platelets. The objectives of this study were: 1) to determine the concentrations of platelet-derived growth factor-BB (PDGF-BB), transforming growth factor-beta1 (TGF-beta1), vascular endothelial growth factor (VEGF), and basic fibroblast growth factor (bFGF) released from PCs and whole blood (WB), before and after the addition of various concentrations of calcium and thrombin, and 2) to assess the physiological importance of the released GFs on angiogenesis.

METHODS

WB and PCs were harvested and prepared from three healthy volunteers. Enzyme-linked immunosorbent assay tests, specific for PDGF-BB, TGF-beta1, VEGF, and bFGF, were performed on WB and PC supernatants, collected before and 30 minutes after the addition of various concentrations of calcium and thrombin. The supernatants were also added to human umbilical vein endothelial cell (HUVEC) cultures in order to measure their effects on endothelial cell proliferation.

RESULTS

Growth factor concentrations detected in PC supernatants were significantly greater (280% to 800% increase) than concentrations present in WB supernatants. Calcium and thrombin induced immediate GF release from PCs in a dose-dependent fashion. Furthermore, PC supernatants led to greater HUVEC proliferation rates than WB supernatants. However, there was no correlation between the concentrations of specific GFs and HUVEC proliferation rates.

CONCLUSION

These results suggest that PCs could stimulate blood vessel formation. They also reinforce the relevance for using PCs in regenerative therapies.

Protective effect of three diallyl sulphides against glucose-induced erythrocyte and platelet oxidation, and ADP-induced platelet aggregation

Isolated human erythrocyte membrane and platelet were used to study the antioxidative and anti-aggregative effects of three diallyl sulphides (diallyl sulphide, DAS; diallyl disulphide, DADS; diallyl trisulphide, DAT) against glucose-induced oxidation and adenosine 5'-diphosphate (ADP)-induced platelet aggregation. Three sulphide agents showed dose-dependent antioxidative protection against glucose-induced erythrocyte membrane oxidation (p<.05), and these agents at 10 microM significantly increased the retention of alpha-tocopherol in erythrocyte membrane (p<.05), in which DAT was the most effective agent (p<.05). Three sulphide agents significantly inhibited 30 and 50 mM of glucose-induced platelet oxidation (p<.05). The anti-aggregative activity of each sulphide agent was dose dependent (p<.05), in which DAT showed the greatest inhibitory effect on platelet aggregation than DADS, followed by DAS (p<.05). After ADP stimulation, the malondialdehyde (MDA) formation in platelets treated with sulphide agents was significantly less (p<.05), in which DADS and DAT showed similar antioxidative activities (p>.05). These results suggested that DADS and DAT could be considered as strong antioxidative and antithrombotic agents to prevent or control oxidative damage and platelet hyperactivity.

Involvement of GD3 in platelet activation. A novel association with Fcgamma receptor

Gangliosides are sialic acid-containing glycosphingolipids present in the outer leaflet of the plasma membrane of many cell types where they modulate adhesive processes. The main population of glycolipids in resting platelets is represented by ganglioside M3 (GM3). It has been demonstrated that following platelet activation ganglioside D3 (GD3) is rapidly formed from the GM3 pool. The present study was designed to evaluate the link between platelet activation and GD3 expression and to verify whether this ganglioside might play a role in modulating signal transduction events. Our results suggest that following platelet activation, GD3 is rapidly expressed on the platelet surface and internalised to the cytoskeleton where it transiently associates first with the Src family tyrosine kinase Lyn then with the Fc receptor gamma chain. This sequence of events ultimately leads to an enhanced CD32 (the Fc receptor isoform present in platelets) expression on the platelet membrane. These data drive us to speculate that GD3 might act as second messenger in the activatory cascade, which leads to CD32 expression and triggers platelet adhesion and spreading to the subendothelial matrix.

ADP-induced platelet aggregation and inhibition of adenylyl cyclase activity stimulated by prostaglandins: signal transduction mechanisms

ADP is the oldest and one of the most important agonists of platelet activation. ADP induces platelet shape change, exposure of fibrinogen binding sites, aggregation, and influx and intracellular mobilization of Ca2+. ADP-induced platelet aggregation is important for maintaining normal hemostasis, but aberrant platelet aggregation manifests itself pathophysiologically in myocardial ischemia, stroke, and atherosclerosis. Another important aspect of ADP-induced platelet activation is the ability of ADP to antagonize adenylyl cyclase activated by prostaglandins. ADP-induced inhibition of the stimulated adenylyl cyclase activity does not appear to play a role in ADP-induced platelet aggregation in vitro or in vivo. It is believed that a single ADP receptor mediates the above two ADP-induced platelet responses in platelets. The ADP receptor mediating ADP-induced platelet aggregation and inhibition of the stimulated adenylyl cyclase activity has not been purified. Therefore, the nature of molecular mechanisms underlying the two seemingly unrelated ADP-induced platelet responses remains either unclear or less well understood. The purpose of this commentary is to examine and make suggestions concerning the role of phospholipases and G-proteins in the molecular mechanisms of signal transduction underlying the two ADP-induced platelet responses. It is hoped that such discussion would stimulate thinking and invite future debates on this subject, and energize investigators in their efforts to advance our knowledge of the details of the molecular mechanisms of ADP-induced platelet activation.