Plasma 11-dehydrothromboxane B2: a reliable indicator of platelet hyperfunction in patients with ischemic stroke

Relationship between the Thromboxane A2 Receptor Gene and Susceptibility to Cerebral Infarction

The risk of cerebral infarction (CI) in an individual is dependent on the interplay between genetic risk factors and environmental influences. Binding of thromboxane A2 (TXA2) to its receptor (TP) modulates thrombosis/hemostasis and plays a significant role in the pathogenesis of CI. The aim of the present study was to investigate the relationship between human TP gene single nucleotide polymorphisms (SNPs) and haplotypes and CI in a Japanese population. A genetic association study was performed in 194 CI patients and 365 non-CI subjects by specifically characterizing 6 SNPs in the human TP gene (rs2271875, rs768963, rs2238634, rs11085026, rs4523 and rs4806942). Analysis demonstrated that there were significant differences in the overall distribution of genotypes and dominant or recessive models of rs2271875 and rs768963 between the CI and the non-CI groups. Multiple logistic regression analysis revealed that the C allele of rs768963 was significantly associated with CI (p = 0.029), even after adjusting for confounding factors (odds ratio: 2.41). Further, the C-T-C haplotype of rs768963-rs2238634-rs4806942 was significantly more frequent in the CI group (23.0%) than in the non-CI group (17.7%). These results suggest that specific SNPs and haplotypes may have utility as genetic markers for the risk of CI and that TP or a neighboring gene is associated with the increased susceptibility to CI.

Platelets and stroke

Platelets are anucleate cells with little or no capacity for de novo protein synthesis. Their potential haemostatic reactivity is established at or before thrombopoiesis by their precursor cell, the bone marrow megakaryocyte. In some pathologic conditions, the megakaryocyte-platelet-haemostatic axis (MPHA) becomes perturbed, resulting in the formation of hyperfunctional platelets which may contribute to the development of vascular disease or an acute thrombotic event such as ischaemic stroke or myocardial infarction. Laboratory measurements of platelet function have established that platelet reactivity is accentuated in acute ischaemic stroke, particularly following cortical rather than lacunar infarction. Whether accentuated platelet function is a cause or a consequence of stroke is not yet clear, but it is likely that patients with certain risk factor profiles have some degree of platelet activation preceding the stroke. Further work into the MPHA is required to establish whether enhanced post-stroke platelet reactivity can be referred to the megakaryocyte. The antiplatelet agents tested to date are effective in secondary but not primary prevention of stroke. This probably reflects the diverse pathophysiology of stroke: accentuated platelet function is only likely to be a significant factor in cortical stroke.

Kinetic and structural evidence for the identification of 11-hydroxythromboxane B2 dehydrogenase as cytosolic aldehyde dehydrogenase

We have recently purified 11-hydroxythromboxane B2 dehydrogenase from porcine kidney and identified it as cytosolic aldehyde dehydrogenase (EC 1.2.1.3) based on amino acid analysis and other protein characteristics. In the present paper we have studied the catalytic interaction of thromboxane B2 (TXB2) with different aldehyde substrates and a potent aldehyde dehydrogenase inhibitor, disulfiram. TXB2 was a competitive inhibitor of the aldehyde dehydrogenase reaction in assays with 3,4-dihydroxyphenylacetaldehyde, a high affinity substrate. The conversion of TXB2 to 11-dehydro-TXB2 was also inhibited by propanal and disulfiram. The protein characteristics of the enzyme have also been further studied. The native enzyme is a tetramer and has an isoelectric point of 7.0 which is comparable with that of cytosolic aldehyde dehydrogenases from other species. Taken together the present data further indicate that 11-hydroxythromboxane B2 dehydrogenase is identical with cytosolic aldehyde dehydrogenase and that substrates and inhibitors of aldehyde dehydrogenase interact with thromboxane metabolism in vitro.

Pyrazolopyridine derivative acts as a novel cyclooxygenase inhibitor: antiplatelet effect in aged patients with ischemic stroke

OBJECTIVE

To examine the antiplatelet effect of a novel pyrazolopyridine derivative (KC-764) in geriatric patients with ischemic stroke.

DESIGN

Randomized clinical trial of three graded dose levels.

SETTING

A geriatric clinic attached to a nursing home.

PATIENTS

Fifteen patients with a history of cerebral infarction with a mean age of 75 +/- 5 years (range, 65-83). Patients were divided into three groups and administered 10, 20, or 40 mg/day KC-764 for 8 weeks.

MEASUREMENTS

Platelet aggregation induced by arachidonate, ADP, collagen and platelet-activating factor. Plasma or serum levels of thromboxane B2 and 6-ketoprostaglandin F1 alpha.

MAIN RESULTS

Platelet aggregation was inhibited by KC-764 administration and returned to the control level after discontinuation. Although plasma thromboxane B2 levels were markedly decreased, plasma 6-ketoprostaglandin F1 alpha was not affected. However, the dose of 10 mg/day was not sufficient to maintain an effective plasma level of KC-764. There were no side effects or changes in laboratory findings.

CONCLUSIONS

We confirmed that KC-764 at a dose of 20 to 40 mg/day is an effective antiplatelet agent and a good candidate for a trial to see if it is feasible for long-term use for the prevention of ischemic stroke in high-risk patients.

Superiority of plasma 11-dehydro-TXB2 to TXB2 as an index of in vivo TX formation in rabbits after dosing of CS-518, a TX synthase inhibitor

We compared plasma 11-dhTXB2 and TXB2 for their ability to reflect in vivo TX formation in rabbits treated with AA and CS-518, a TX synthase inhibitor. The average plasma level of TXB2 in rabbits was much higher than that of 11-dhTXB2, probably because of artificial formation of TXB2 during blood sampling. CS-518 (1 mg/kg, p.o.) caused a long-lasting suppression of the 11-dhTXB2 level, and its inhibitory effect on 11-dhTXB2 was much more extensive than that on TXB2. AA-infusion for 5 min resulted in transient and remarkable increases of both TXs, and prevention of such increases by CS-518 pretreatment (1 mg/kg, i.v.) was shown: inhibitions of 11-dhTXB2 and TXB2 were 85% and 40%, respectively. The inhibitory effect caused by CS-518, which was more clearly observed on plasma 11-dhTXB2 than on TXB2, was due to not only the completely inhibited levels without artificial formation but also the durable high levels based on the long half-life of 11-dhTXB2 in AA-infused rabbits. CS-518 injection during sustained AA-infusion also resulted in a 2-fold faster disappearance of plasma 11-dhTXB2 than was seen without CS-518, despite its long half-life. Considering the absence of artificial formation, the long half-life, and the good response to change of TX formation, plasma 11-dhTXB2 is superior to TXB2 as an index for monitoring in vivo TX synthase activity and its pharmacological modification with AA and CS-518.

Purification and some properties of 11-hydroxythromboxane B2 dehydrogenase from porcine kidney

A protein with NAD-dependent 11-hydroxythromboxane B2 dehydrogenase activity was purified to apparent homogeneity from porcine kidney using a relatively simple purification procedure, involving precipitation, anion-exchange chromatography (diethylaminoethyl-cellulose), affinity chromatography (5'-AMP-Sepharose) and gel-filtration chromatography (Protein Pak 125). The dehydrogenase was found to have a molecular mass of 50-55 kDa as determined by comparison with standards on SDS/PAGE. The molecular mass on gel-filtration chromatography was dependent on the ionic strength of the buffer. The apparent Km and Vmax values for thromboxane B2 were also dependent of the ionic strength with a Vmax of 214 nmol min-1 mg-1 using 250 mM Tris/HCl, pH 8.0, and a corresponding Km of 2.9 mM. The enzyme was NAD dependent and was clearly separated from the proteins with 15-hydroxyprostaglandin dehydrogenase activity also present in the kidney. Furthermore, it was found that 11-hydroxythromboxane B2 dehydrogenase did not utilize prostaglandin D2, prostaglandin E2, prostaglandin F2 alpha or cholic acid as substrate, and that the enzyme did not catalyse the reverse reaction, conversion of 11-dehydrothromboxane B2 to thromboxane B2.

High-performance liquid chromatographic assay of platelet-produced thromboxane B2

A method for the routine determination of platelet-produced thromboxane B2 (TXB2) from human serum is presented. To induce the secretion of thromboxane A2 from the platelets, blood is kept at 37 degrees C for 30 min before serum is separated. Serum is prepurified through small reversed-phase columns and TXB2 is analysed by reversed-phase high-performance liquid chromatography. A column-switching technique is used to remove the interfering compounds present in serum. The detection limit with standard solution is 30 ng per injection. The method was applied to the measurement of platelet-produced TXB2 serum from 1040 men. The mean TXB2 was 247 +/- 134 ng/ml in the serum of men who had not used prostaglandin inhibitors, and 208 +/- 123 ng/ml in the serum of men who had used a prostaglandin inhibitor during a two-week period before blood sampling.

Effects of low-to-high doses of aspirin on platelet aggregability and metabolites of thromboxane A2 and prostacyclin

BACKGROUND AND PURPOSE

The purpose of this study was to compare the effects of low-to-high doses of aspirin on platelet aggregability determined by different methods and on the metabolism of thromboxane A2 and prostacyclin.

METHODS

We administered increasing doses (40, 320, and 1,280 mg/day) of aspirin to 19 poststroke patients and studied the differences in 1) the changes in platelet aggregability depending on the methods of evaluation and 2) the concentrations of prostaglandin metabolites in the blood and urine.

RESULTS

Aggregation of platelet-rich plasma induced by a strong stimulus (10 microM ADP) was significantly reduced after 40 mg/day aspirin (p less than 0.005), and this reduction was similar to that after higher aspirin doses. In contrast, aggregation of platelet-rich plasma induced by weaker stimuli (1 and 5 microM ADP) decreased less significantly after 40 mg/day aspirin compared with that after higher aspirin doses. The serum thromboxane B2 generated after ex vivo incubation was reduced significantly (by 85%) after 40 mg/day aspirin and decreased further after 320 mg/day (by 96%) and 1,280 mg/day (by greater than 99%) of aspirin. The urinary 11-dehydro-thromboxane B2 concentration decreased less significantly after 40 mg/day aspirin (by 42%) compared with that after 320 mg/day (by 78%) and 1,280 mg/day (by 91%) aspirin doses. The urinary concentration of 2,3-dinor-6-keto-prostaglandin F1 alpha did not decrease after 40 mg/day aspirin but decreased significantly after higher doses of aspirin.

CONCLUSIONS

These findings suggest that different doses of aspirin may be necessary to prevent thrombogenesis induced by different triggers of different strengths and that 40 mg/day aspirin is able to inhibit a large proportion of maximum thromboxane A2 release provoked acutely, with the prostaglandin I2 synthesis being little affected; however, higher doses of aspirin are required to attain further inhibition.