Platelet aggregability to platelet activating factor at rest and after exercise in patients with coronary artery disease

Platelet activating factor levels and metabolism in Tangier disease: a case study

Background

Tangier disease (TD) is a phenotypic expression of rare familial syndrome with mutations in the ABCA1 transporter. The risk of coronary artery disease in patients with TD is variable. On the other hand the pivotal role of Platelet-Activating Factor (PAF) mediator in atheromatosis was found. Plasma lipoproteins are transporters of the PAF acetylhydrolase (PAF-AH) in cells and known as lipoprotein-phospholipase A₂ (Lp-PLA₂) in plasma and regulators of PAF levels in blood. In addition, PAF can be biosynthesized from the remodeling and the de novo pathways in which Lyso-platelet activating factor-acetyltransferase (Lyso-PAF-AT) and platelet activating factor-cholinephosphotransferase (PAF-CPT) are the regulatory enzymes. The aim of this study is to investigate in a TD patient with a unique mutation (C2033A), the concentration of PAF in blood, the Equivalent Concentration for 50% aggregation (EC₅₀) values of platelet rich plasma (PRP) toward PAF, adenosine diphosphate (ADP) and thrombin, and the activities of PAF metabolic enzymes Lp-PLA₂, PAF-AH, Lyso-PAF-AT and PAF-CPT.

Methods

The EC₅₀ value of PRP was measured by an aggregometer. The determination of the specific activity of PAF-CPT and Lyso-PAF-AT was made after in vitro enzymatic assay, chromatographic separation and measurement of the produced PAF in a biological assay with washed rabbit platelets. The determination of PAF-AH and Lp-PLA₂ was made after an in vitro enzymatic assay from the decay of radioactive PAF.

Results

The TD patient had lower bound-PAF values in blood, decreased specific activity of PAF-CPT and Lyso-PAF-AT, increased specific activity of PAF-AH in platelets and leukocytes and Lp-PLA₂ activity in plasma compared to healthy women. The EC₅₀ of PAF and Thrombin were higher compared to healthy women.

Conclusion

The increased Lp-PLA₂ activity, as well as, the decreased activities of PAF-CPT and Lyso-PAF-AT, explain the decreased bound-PAF level in TD patient and the EC₅₀ of PAF. However, total PAF is in a normal range and this probably can explain one of the reasons this TD patient has no CAD.

Effect of amlodipine on exercise-induced platelet activation in patients affected by chronic stable angina

BACKGROUND

Literature concerning exercise-induced platelet activation in chronic stable angina is somewhat confusing. The reason lies in the type of exercise as well as in methodological problems. A powerful, recently introduced procedure to detect platelet activation is flow cytometry. Platelet response to activating factors is mediated by calcium uptake; however, calcium antagonist effect on platelet activity is still unclear.

HYPOTHESIS

The study was undertaken to investigate exercise-induced platelet activation before and after treatment with amlodipine in chronic stable angina.

METHODS

Twenty patients with chronic stable angina were entered into the study. Each subject underwent a symptom-limited cycloergometer stress test following a washout period of 2 weeks. Blood samples were collected before and immediately after exercise. All subjects were then randomized into two groups of 10 patients each, with Group 1 and Group 2 taking amlodipine 10 mg/day, and placebo for 4 weeks, respectively. They subsequently underwent a second exercise stress test, and blood samples were obtained before and immediately after exercise. Flow-cytometric evaluation of platelet activity was performed in order to recognize GMP-140 expression on platelet membrane.

RESULTS

Strenuous exercise induced a significant increase in platelet activation in all subjects prior to therapy. No significant differences were observed in platelet activity at rest between Groups 1 and 2, whereas a significant decrease in exercise-induced platelet activation was demonstrated in Group 1 compared with Group 2.

CONCLUSION

Our data provide evidence of the favorable effect of amlodipine on exercise-induced platelet activation in patients affected by chronic stable angina.

Investigation of the role of adjacent amino acids to the 313-320 sequence of the alphaIIb subunit on platelet activation and fibrinogen binding to alphaIIbbeta3

The platelet integrin receptor alphaIIbbeta3 plays a critical role in thrombosis and haemostasis by mediating interactions between platelets and several ligands, primarily fibrinogen. We have previously shown that the synthetic peptide YMESRADRKLAEVGRVYLFL corresponding to residues 313-332 of alphaIIb, is a potent inhibitor of platelet aggregation and fibrinogen binding to alphaIIbbeta3, interacting with fibrinogen rather than the receptor. Furthermore, we have demonstrated that the biological activities of the above peptide are due to the sequence YMESRADR, which corresponds to residues 313-320. By using new synthetic peptide analogues we investigated the structural characteristics responsible for the biological activity of YMESRADR as well the possible influence of the adjacent amino acids on the peptide's biological potency. According to our results, the synthetic octapeptide YMESRADR, is a potent inhibitor of platelet aggregation and P-selectin expression. Furthermore, YMESRADR inhibits fibrinogen binding but it does not significantly influence the binding of PAC-1 to ADP-activated platelets. The inhibitory potency of YMESRADR was gradually diminished by deleting the YMES sequence from the amino terminus and prolonging the carboxyl terminus of this peptide with the KLAE sequence. Extension of YMESRADR towards the amino terminus with the GAPL sequence (GAPLYMESRADR) does not modify the biological activity of YMESRADR. Furthermore, extension of GAPLYMESRADR at its carboxy terminus with the KLAE sequence (GAPLYMESRADRKLAE) significantly diminished its biological potency. Substitution of E315 with D significantly enhances antiaggregatory potency and completely abolishes the inhibitory effect on P-selectin expression. Importantly, the D315-containing peptides inhibit to a similar extent both fibrinogen and PAC-1 binding to activated alphaIIbbeta3 in contrast to the E315-containing peptide which only inhibits fibrinogen binding. In conclusion, the present study suggests that the YMESRADR sequence 313-320 of alphaIIb, is an important functional region of the insert connecting the beta2 and beta3 antiparallel beta-strands of the W5 blade of the alphaIIb subunit. Structural changes significantly modify the biological properties of this region.

The inhibitory potency of clopidogrel on ADP-induced platelet activation is not attenuated when it is co-administered with atorvastatin (20 mg/day) for 5 weeks in patients with acute coronary syndromes

The antiplatelet potency of clopidogrel may be attenuated by short-term co-administration of lipophilic statins metabolized through the cytochrome P-450, isoform 3A4. We investigated whether the co-administration of atorvastatin (20?mg/day) for 5 weeks, in patients with acute coronary syndromes (ACS) could affect the antiplatelet activity of clopidogrel. Fifty-one patients with the first episode of an ACS were included in the study. All patients underwent percutaneous coronary intervention (PCI) and received a loading dose of 375 mg of clopidogrel, followed by 75 mg/day for at least 3 months. Twenty-six of them presented with low density lipoprotein (LDL) cholesterol levels >100?mg/dl (2.6 mmol/l) (measured within 24 h from the onset of symptoms) and received daily 20 mg/day of atorvastatin. The ADP- or TRAP-induced platelet aggregation, as well as P-selectin and CD40L surface expression, were studied at baseline (within 30 min after admission) and 5 weeks afterwards. Atorvastatin did not influence either the clopidogrel-induced inhibition of platelet aggregation initiated by 5 or 10 microM ADP or the clopidogrel-induced reduction of the membrane expression of P-selectin and CD40L induced by ADP. In conclusion, atorvastatin, even at a dose of 20 mg/day does not affect the antiplatelet efficacy of clopidogrel when co-administered for 5 weeks in ACS patients.

Effects of eccentric exercise-induced muscle injury on blood levels of platelet activating factor (PAF) and other inflammatory markers

It has been reported that exercise with eccentric contractions can induce damage and inflammation in human muscle tissue, the severity of which depends on the duration and the intensity of exercise. Platelet activating factor (PAF) is a potent inflammatory mediator implicated in a series of pathophysiological conditions. We sought to investigate the relationship between PAF and eccentric exercise induced muscle damage. Thirteen healthy, recreationally active male subjects (27.5+/-3.78 year) performed 36 maximum voluntary eccentric contractions on a motorized muscle dynamometer using the elbow flexor muscles of the non-dominant arm. Venous blood samples were collected immediately before and after exercise at 2, 24, 48, 72 and 96 h. PAF was isolated, purified and determined by a platelet aggregation assay. Serum levels of creatine kinase (CK), lactate dehydrogenase (LDH), C-reactive protein (CRP) and complement C3 (C3), plasma level of fibrinogen and whole blood level of leukocytes (and their subsets) were determined. Established indicators of muscle damage as maximum isometric torque (MIT), range of motion (ROM), relaxed arm angle (RANG), flexed arm angle (FANG), arm circumference (CIRC) and muscle soreness were also measured at the same time points. PAF, leukocytes, CK and LDH were elevated after exercise, while other biochemical parameters such as CRP, C3 and fibrinogen were unchanged. We also observed an inverse association between PAF and MIT and ROM, as well as a positive association with other markers of muscle injury, i.e. CK, LDH, FANG and CIRC (all P's<0.05). Our findings may imply a role for PAF in the mechanism of eccentric exercise induced muscle damage.

Purification and characterization of a new anticoagulant protein, PP27, from placenta

It has long been known that water extracts of placenta hominis (Jahage in Korean) are effective for treating immunological and vascular diseases and is a major constituent of traditional oriental medicines. We report herein on the isolation and purification of a new type of anticoagulant protein, PP27, from human placenta. PP27 ran as a single band on SDS-PAGE with a molecular mass (Mr) of 27 kDa under denaturing conditions and chromatography on a calibrated Sepharose 4B column indicated a molecular mass of 23 kDa, a value that is similar to those of other PP4 enzymes reported to date. The isoelectric point of PP27 was pI 5.2. PP27, at doses higher than 10 microg/ml, inhibited platelet activating factor (PAF)-induced platelet activation in a dose-dependent manner. The protein was found to inhibit the coagulation time in a concentration-dependent manner. PP27, which acts as a vascular anticoagulant of annexin type, inhibits the blood clotting process by virtue of its binding of essential lipids, which is dependent on the presence of Ca2+ ions. In the presence of Ca2+ ions, PP27 combines with platelet membranes and neutralizes their procoagulant effect. Coagulation, triggered by the addition of thromboplastin/lipid mixtures, is extinguished by PP27.

Effect of synthetic peptides corresponding to residues 313-332 of the alphaIIb subunit on platelet activation and fibrinogen binding to alphaIIbbeta3

The platelet integrin receptor alphaIIbbeta3 plays a critical role in thrombosis and haemostasis by mediating interactions between platelets and several ligands but primarily fibrinogen. It has been shown previously that the YMESRADR KLAEVGRVYLFL (313-332) sequence of the alphaIIb subunit plays an important role in platelet activation, fibrinogen binding and alphaIIbbeta3-mediated outside-in signalling. Furthermore, we recently showed that the 20-residue peptide (20-mer) alphaIIb 313-332, is a potent inhibitor of platelet aggregation and fibrinogen binding to alphaIIbbeta3, interacting with fibrinogen rather than the receptor. In an effort to determine the sequence and the minimum length required for the biological activity of the above 20-mer, we synthesized seven octapeptides, each overlapping by six residues, covering the entire sequence and studied their effect on platelet activation as well as fibrinogen binding to activated platelets. We show for the first time that octapeptides containing the RAD sequence are capable of inhibiting platelet aggregation and secretion as well as fibrinogen binding to the activated alphaIIbbeta3, possibly interacting with the ligand rather than the receptor. This suggests that the RAD sequence, common to all the inhibitory peptides, is critical for their biological activity. However, the presence of the YMES sequence, adjacent to RAD, significantly increases the peptide's biological potency. The development of such inhibitors derived from the 313-332 region of the alphaIIb subunit may be advantageous against the RGD-like antagonists as they could inhibit platelet activation without interacting with alphaIIbbeta3, thus failing to further induce alphaIIbbeta3-mediated outside-in signalling.

Atorvastatin Does Not Affect the Antiplatelet Potency of Clopidogrel When It Is Administered Concomitantly for 5 Weeks in Patients With Acute Coronary Syndromes

Background— The antiplatelet effect of clopidogrel may be attenuated by short-term coadministration of lipophilic statins. We investigated whether the coadministration of atorvastatin for 5 weeks in patients with acute coronary syndromes (ACS) could affect the antiplatelet potency of clopidogrel.

Methods and Results— Forty-five hypercholesterolemic patients with the first episode of an ACS were included in the study. Patients were randomized to receive daily either 10 mg of atorvastatin (n=21) or 40 mg of pravastatin (n=24). Thirty patients who underwent percutaneous coronary intervention (PCI) received a loading dose of 375 mg of clopidogrel, followed by 75 mg/d for at least 3 months. In the remaining 15 patients who refused to undergo PCI, clopidogrel therapy was not administered. Eight normolipidemic patients with the first episode of an ACS were also included and received only clopidogrel. The serum levels of soluble CD40L and the adenosine 5′-diphosphate– or thrombin receptor activating peptide-14–induced platelet aggregation, as well as P-selectin and CD40L surface expression, were studied at baseline (within 30 minutes after admission) and 5 weeks later. Neither atorvastatin nor pravastatin significantly influenced the clopidogrel-induced inhibition of platelet activation, nor did clopidogrel influence the therapeutic efficacy of atorvastatin.

Conclusions— Atorvastatin does not affect the antiplatelet potency of clopidogrel when coadministered for 5 weeks in ACS patients.

Mapping the binding domains of the alpha(IIb) subunit. A study performed on the activated form of the platelet integrin alpha(IIb)beta(3)

alpha(IIb)beta(3), a member of the integrin family of adhesive protein receptors, is the most abundant glycoprotein on platelet plasma-membranes and binds to adhesive proteins via the recognition of short amino acid sequences, for example the ubiquitous RGD motif. However, elucidation of the ligand-binding domains of the receptor remains controversial, mainly owing to the fact that integrins are conformationally labile during purification and storage. In this study, a detailed mapping of the extracellular region of the alpha(IIb) subunit is presented, using overlapping 20-peptides, in order to identify the binding sites of alpha(IIb) potentially involved in the platelet-aggregation event. Regions alpha(IIb) 313-332, alpha(IIb) 265-284 and alpha(IIb) 57-64 of alpha(IIb)beta(3) were identified as putative fibrinogen-binding domains because the corresponding peptides inhibited platelet aggregation and antagonized fibrinogen association, possibly by interacting with this ligand. The latter is further supported by the finding that the above peptides did not interfere with the binding of PAC-1 to the activated form of alpha(IIb)beta(3). Furthermore, alpha(IIb) 313-332 was found to bind to fibrinogen in a solid-phase binding assay. It should be emphasized that all the experiments in this study were carried out on activated platelets and consequently on the activated form of this integrin receptor. We hypothesize that RAD and RAE adhesive motifs, encompassed in alpha(IIb) 313-332, 265-284 and 57-64, are capable of recognizing complementary domains of fibrinogen, thus inhibiting the binding of this ligand to platelets.

Platelet hyperaggregability to platelet activating factor (PAF) in non-ST elevation acute coronary syndromes

It is known that myocardial ischaemia increases platelet aggregatory response to various agonists, ex vivo. We investigated the platelet aggregatory response to platelet activating factor (PAF), ex vivo, in patients with non-ST elevation acute coronary syndromes and determined the specificity and sensitivity of this response. Thirty-two consecutive patients with non-ST elevation acute coronary syndromes and 20 healthy volunteers were studied. Platelet aggregation in platelet-rich plasma was studied on the day of admission. The maximal aggregation achieved within 2 min after the addition of PAF (100 nM) was expressed as a percentage of 100% light transmission. PAF EC50 values were defined as the concentration that induces 50% of maximal aggregation. The PAF EC50 values of the non-ST elevation acute coronary syndromes patients were significantly lower compared to those of the controls (p < 0.0001). The maximal percentage of aggregation was also significantly higher (p < 0.0005). Ninety-one per cent of the patients were correctly classified using PAF EC50 values (specificity 90.0% and sensitivity 91.2%); the corresponding results using the maximal percentage of aggregation were 80% (specificity 70.0% and sensitivity 87.5%). The estimated values used as thresholds were 22.47 nM and 17.97 for the PAF EC50 and the maximal percentage of aggregation, respectively. The results of the present study suggest that platelet hyperaggregability to PAF, ex vivo, in non-ST elevation acute coronary syndromes is characterised by a high specificity and sensitivity, and thus it may represent a mechanism contributing to the pathophysiology of acute coronary syndromes.

Platelet-associated and secreted PAF-acetylhydrolase activity in patients with stable angina: sequential changes of the enzyme activity after angioplasty

BACKGROUND

Platelet-activating factor (PAF), the lipid mediator of inflammation and potent platelet agonist, can be hydrolysed and inactivated by PAF-acetylhydrolase (PAF-AH). We investigated the PAF-AH activity in relation to PAF formation in platelets from patients with stable angina undergoing elective percutaneous transluminal coronary angioplasty (PTCA).

DESIGN

Twenty-seven patients with stable angina, undergoing PTCA, and 30 age- and sex-matched controls were studied. The platelet-associated and secreted PAF-AH activity was measured, before PTCA, as well as at 4 h, 48 h and 6 months afterwards. PAF formation by thrombin-stimulated platelets and the platelet aggregation responses to PAF and ADP were also determined.

RESULTS

The PAF-AH activity secreted by thrombin-stimulated platelets before PTCA (in pmol/10(9) cells/h) was significantly higher compared to controls (892 +/- 222 vs. 624 +/- 144, P < 0.001). The enzyme activity was not altered at 4 h after PTCA, but was significantly increased at 48 h (1284 +/- 312, P < 0.005) to return to the levels observed in the control group 6 months afterwards. Detectable levels of PAF in thrombin-stimulated platelets were found only at 6 months after PTCA. Furthermore, the cell-associated enzyme activity in resting platelets before PTCA was significantly lower compared with controls. Unlike in controls, the platelet-associated enzyme activity in the patient group was not increased after stimulation with thrombin and it was associated by a platelet hyperaggregability to PAF. Both the intact cell-associated activity and the platelet hyper-reactivity to PAF were restored at 6 months after PTCA.

CONCLUSIONS

Alterations in the platelet PAF-AH activity, which affect the PAF formation in thrombin-stimulated platelets and are associated by an increased aggregatory response to PAF, are observed in patients with stable angina and are completely restored after PTCA.

Regulation of platelet-activating factor (PAF) activity in human diseases by phospholipase A2 inhibitors, PAF acetylhydrolases, PAF receptor antagonists and free radical scavengers

The aim of this review is to present recent findings indicating the likely involvement of platelet-activating factor (PAF) in human diseases, and possible ways of alleviating its harmful effects. PAF is a potent proinflammatory mediator and promotes adhesive interactions between leukocytes and endothelial cells, leading to transendothelial migration of leukocytes, by a process of juxtacrine intercellular signalling. This process leads to activation of leukocytes and the release of reactive oxygen radicals, lipid mediators, cytokines and enzymes. These reaction products subsequently contribute to the pathological features of various inflammatory diseases. The reactive oxygen radicals cause low density lipoprotein (LDL) oxidation which mediates the development of atherosclerosis. Oxidized LDL may damage cellular and subcellular membranes, leading to tissue injury and cell death. Among the therapeutic approaches considered are agents that inhibit/degrade proinflammatory mediators and thereby have anti-inflammatory and/or anti-atherogenic potential. These include inhibitors of phospholipase A2 activity, PAF-acetylhydrolases, PAF antagonists and free radical scavengers/antioxidants, the latter protecting against oxidized LDL-induced cytotoxicity.

Plasma platelet-activating factor acetylhydrolase deficiency in Japanese patients with asthma

Platelet-activating factor (PAF), a phospholipid with a wide range of proinflammatory actions, is immediately degraded and inactivated in vivo by PAF acetylhydrolase (PAF-AH). Surprisingly, 4% of the Japanese population lacks the extracellular isoform of this enzyme, plasma PAF-AH, due to a genetic missense (V279F) mutation. We studied the association of this mutation with asthma prevalence and phenotypes in the Japanese adult population. The allele frequency of V279F mutation was 18.6% in 279 patients with asthma (28.7% heterozygotes and 4.3% homozygotes) and 21.7% in 217 healthy subjects (32.3% heterozygotes and 5.5% homozygotes). V279F mutant allele prevalence was consistent regardless of asthma type (16.3% in atopic [n = 156] and 21.6% in nonatopic [n = 123]), or the severity of disease (21.7% in patients with mild [n = 97], 17.5% in those with moderate [n = 131], and 15.8% in those with severe [n = 51] asthma). Plasma PAF-AH activity was inversely proportional to the number of mutant alleles, and did not correlate with asthma prevalence, type, or severity. We concluded that plasma PAF-AH deficiency due to V279F mutation is not essential to the pathophysiology of asthma in the Japanese adult population.