Effect of homocysteine on platelet activation induced by collagen

The binding of autotaxin to integrins mediates hyperhomocysteinemia-potentiated platelet activation and thrombosis in mice and humans

Hcy increases integrin αIIbβ3 activation by promoting phospholipid hydrolysis and ATX interaction in platelets.

Targeting ATX-mediated integrin αIIbβ3 activation alleviates HHcy-potentiated thrombosis.

Hyperhomocysteinemia (HHcy) is associated with an exaggerated platelet thrombotic response at sites of vascular injury. In this study, human medical examination showed that elevated human plasma Hcy levels correlated positively with enhanced blood coagulation and platelet activity, suggesting that humans with HHcy are more prone to thrombus formation at the sites of vascular injury. Accordingly, we observed accelerated platelet activation, primary hemostasis, and thrombus formation in apolipoprotein E-deficient (ApoE^−/−) mice with acute or chronic HHcy. Upon homocysteine (Hcy) administration in C57BL/6J mice, platelet aggregation, spreading and clot retraction were markedly induced. More important, Hcy increased the affinity of platelet integrin αIIbβ3 with ligands and enhanced integrin outside-in signaling by promoting membrane phosphatidylserine exposure in vitro. Mechanistically, lipidomics analysis showed that lysophosphatidylcholines were the primary metabolites leading to clustering of HHcy-stimulated platelets. Cytosolic phospholipase A2 (cPLA2) activity and autotaxin (ATX, a secreted lysophospholipase D) secretion were upregulated by Hcy, leading to membrane phospholipid hydrolysis and PS exposure. Moreover, secreted ATX directly interacted with integrin β3. Inhibitors of cPLA2 and ATX activity blocked integrin αIIbβ3 outside-in signaling and thrombosis in HHcy ApoE^−/− mice. In this study, we identified a novel mechanism by which HHcy promotes platelet membrane phospholipid catabolism and extracellular ATX secretion to activate integrin outside-in signaling, consequently exacerbating thrombosis and the results revealed an innovative approach to treating HHcy-related thrombotic diseases.

Effects of Hyperhomocysteinemia on the Platelet-Driven Contraction of Blood Clots

Hyperhomocysteinemia (HHcy) is associated with thrombosis, but the mechanistic links between them are not understood. We studied effects of homocysteine (Hcy) on clot contraction in vitro and in a rat model of HHcy. Incubation of blood with exogenous Hcy for 1 min enhanced clot contraction, while 15-min incubation led to a dose-dependent suppression of contraction. These effects were likely due to direct Hcy-induced platelet activation followed by exhaustion, as revealed by an increase in fibrinogen-binding capacity and P-selectin expression determined by flow cytometry. In the blood of rats with HHcy, clot contraction was enhanced at moderately elevated Hcy levels (10–50 μM), while at higher Hcy levels (>50 μM), the onset of clot contraction was delayed. HHcy was associated with thrombocytosis combined with a reduced erythrocyte count and hypofibrinogenemia. These data suggest that in HHcy, platelets get activated directly and indirectly, leading to enhanced clot contraction that is facilitated by the reduced content and resilience of fibrin and erythrocytes in the clot. The excessive platelet activation can lead to exhaustion and impaired contractility, which makes clots larger and more obstructive. In conclusion, HHcy modulates blood clot contraction, which may comprise an underappreciated pro- or antithrombotic mechanism.

Homocysteine Level Predicts Response to Dual Antiplatelet in Women With Minor Stroke or Transient Ischemic Attack: Subanalysis of the CHANCE Trial

OBJECTIVES

To investigate the relationship of homocysteine levels with the efficacy and safety of dual antiplatelet therapy in female and male patients. Approach and Results: The CHANCE trial (Clopidogrel in High-Risk Patients With Acute Nondisabling Cerebrovascular Events) randomized patients with acute minor ischemic stroke or high-risk transient ischemic attack to clopidogrel plus aspirin or aspirin alone from October 1, 2009, to July 30, 2012, in China. A subgroup of 3044 consecutive patients with baseline homocysteine levels from 73 (64%) prespecified clinical sites was analyzed. Participants were grouped by sex. Primary outcome was stroke recurrence within 90 days. Secondary outcomes consisted of composite vascular events and independent living or death. Safety outcome was any bleeding. Cox proportional-hazards models were used to assess the interaction of homocysteine levels with randomized antiplatelet therapy on efficacy and safety outcomes. A significant interaction between homocysteine levels and the randomized antiplatelet therapies was found on recurrent stroke after adjustment for confounding factors in women (P=0.010) but not in men (P=0.595). Compared with aspirin alone, clopidogrel plus aspirin significantly reduced the risk of recurrent stroke in women without elevated homocysteine levels (adjusted hazard ratio, 0.459 [95% CI, 0.271-0.776]; P=0.004). Such benefit disappeared in female patients with increased homocysteine level. No significant interaction on functional outcome or bleeding rate was observed.

CONCLUSIONS

Homocysteine could be a potential biomarker to discriminate the effects of dual and single antiplatelet therapy in female patients with minor ischemic stroke or high-risk transient ischemic attack.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifier: NCT00979589.

Association of plasma homocysteine level with vaso-occlusive crisis in sickle cell anemia patients of Odisha, India

Vascular complications of sickle cell anemia (SCA) are influenced by many factors. Elevated plasma homocysteine (Hcy) is supposed to be an independent risk factor and is either genetic or nutritional origin. The present study evaluated the plasma Hcy level, MTHFR C677T gene polymorphism, effect of folic acid (FA) supplementation' and hemato-biochemical parameters in SCA and their effect on the vaso-occlusive crisis (VOC) in SCA patients of an Asian-Indian haplotype population. One hundred twenty cases of SCA (HbSS) and 50 controls with normal hemoglobin(HbAA) were studied. It was found that the plasma Hcy level is significantly higher (p < 0.0001) in patients with SCA (22.41 ± 7.8 μmol/L) compared to controls (13.2 ± 4.4 μmol/L). Moreover, patients without FA supplementation had a significantly (p < 0.001) higher Hcy level (27 ± 7 μmol/L) compared to those with supplementation (17.75 ± 5.7 μmol/L). Turkey-Kramer multiple comparison tests show that there is a significant difference (p < 0.05) in HbF percent, hemoglobin (Hb), platelet count, serum bilirubin (direct:Bil-D and total:Bil-T), aspartate transaminase (AST), lactate dehydrogenase (LDH), and plasma Hcy levels between mild and severe VOC. Between moderate VOC and severe VOC, there was a significant difference (p < 0.05) in HbF%, Bil-D, AST, Hcy. Pearson correlation revealed that plasma Hcy had a significantly (p < 0.05) positive correlation with AST, serum bilirubin (indirect and total), LDH, jaundice, stroke, VOC per year, and hospitalization per year whereas it was inversely correlated with HbF percentage, Hb level, and FA treatment. In the study population, increased plasma Hcy level, hemolysis, and platelet activation were found to influence VOC in SCA.

Caffeic acid inhibits acute hyperhomocysteinemia-induced leukocyte rolling and adhesion in mouse cerebral venules

OBJECTIVE

To investigate the effects and possible mechanisms of CA on acute HHcy-induced leukocyte rolling and adhesion in mouse cerebral venules.

METHODS

Male C57 BL/6J mice were injected with DL-Hcy (50 mg/kg) and CA (10 mg/kg). The effect of CA on HHcy-induced leukocyte rolling and adhesion in cerebral vessels was assessed using intravital microscopy. Plasma cytokines and chemokines were evaluated by cytometric bead array. ROS production in HUVECs and adhesion molecule expression on leukocytes were determined by flow cytometry. E-selectin and ICAM-1 expression in cerebrovascular endothelium was detected by immunohistochemistry. CD18 phosphorylation and the Src/PI3K/Akt pathway in leukocytes were determined by confocal microscopy and Western blot.

RESULTS

CA inhibited HHcy-elicited leukocyte rolling and adhesion, decreased ROS production in HUVECs, and reduced plasma KC, MIP-2, and MCP-1 levels. CA reduced the E-selectin and ICAM-1 expression on cerebrovascular endothelium and CD11b/CD18 on leukocytes caused by HHcy. Of notice, CA depressed CD18 phosphorylation and the Src/PI3K/Akt pathway in leukocytes.

CONCLUSIONS

CA inhibited HHcy-provoked leukocyte rolling and adhesion in cerebral venules, ameliorating adhesion molecule expression and activation, which is related to the suppression of the Src/PI3K/Akt pathway in leukocytes.

Reconciling the evidence on serum homocysteine and ischaemic heart disease: a meta-analysis

Background

Results from genetic epidemiological studies suggest that raised serum homocysteine is a cause of ischaemic heart disease, but the results of randomised trials suggest otherwise. We aimed to update meta-analyses on each type of study using the latest published data and test a hypothesis based on antiplatelet therapy use in the trials to explain the discrepancy.

Methods and Findings

Meta-analyses of ischaemic heart disease using (i) 75 studies in which the prevalence of a mutation (CT) in the MTHFR gene (which increases homocysteine) was determined in cases (22,068) and controls (23,618), and (ii) 14 randomised trials (39,597 participants) of homocysteine lowering and ischaemic heart disease events. The summary estimates from the two analyses were compared. Meta-analysis of the MTHFR studies showed a statistically significantly increased risk of ischaemic heart disease in TT compared with CC homozygotes; odds ratio 1.16 (1.04 to 1.29) for a 1.9 µmol/L homocysteine difference (TT minus CC). Meta-analysis of randomised trials showed no significant reduction in IHD risk from folic acid; relative risk 1.00 (0.93 to 1.08), despite a reduction in homocysteine of 3.3 µmol/L. There was a statistically significant difference in risk reduction between the 5 trials with the lowest prevalence of antiplatelet therapy (60% on average, usually aspirin), RR 0.93 (0.84 to 1.05) and the 5 trials with the highest prevalence (91% on average), RR 1.09 (1.00 to 1.19), p = 0.037 for the difference.

Conclusion

Discordant results from MTHFR studies and randomised trials could be explained by aspirin reducing or negating the anti-platelet effect of lowering homocysteine. On this basis, folic acid would have a role in the primary prevention of ischaemic heart disease, when aspirin is not taken routinely, but not in secondary prevention, when it is routine.

Homocysteine decreases extracellular nucleotide hydrolysis in rat platelets

Hyperhomocysteinemia is an independent risk factor for atherothrombotic disease. Platelets play an important role in cardiovascular disease and release pro-aggregates mediators when activated, such as ADP, a physiological agonist involved in normal hemostasis and thrombosis. NTPDases and 5'-nucleotidase are ecto-enzymes that hydrolyze ATP, ADP and AMP to adenosine playing an important role on blood flow and thrombogenesis by regulating ADP catabolism. The aim of the present study was evaluate extracellular adenine nucleotide hydrolysis of rat platelets exposed to homocysteine in vitro and in vivo. In vitro homocysteine (Hcy) in the concentration range of 20 to 500 microM caused a significant decrease on ATP (around 30%) and ADP (around 45%) hydrolysis, respectively, while AMP hydrolysis was not altered. Hcy was not able to inhibit the hydrolysis of ATP and ADP catalyzed by purified apyrase at the same concentrations tested in vitro on platelets, suggesting an indirect effect. The inhibitory effect of Hcy on platelets was prevented by antioxidants agents in vitro and in vivo. Furthermore homocysteine treatment increased platelet aggregation induced by ADP. Based on the results presented herein, we propose that inhibition of extracellular ATP and ADP hydrolysis caused by homocysteine was probably due oxidative stress, since antioxidants prevented such effects. These findings may contribute to an increase platelet response to ADP and consequence development of thrombotic risk attributed to hyperhomocysteinemia.

Homocysteine harasses the imprinting expression of IGF2 and H19 by demethylation of differentially methylated region between IGF2/H19 genes

Homocysteine (Hcy) can induce proliferation of vascular smooth muscle cells (VSMCs), which is a key event in the genesis of the lesions of atherosclerosis. Insulinlike growth factor 2 (IGF2) and H19 are two important regulating molecules of cell proliferation. The role of Hcy in the proliferation of smooth muscle cell by regulating IGF2 and H19 has not been shown or analyzed so far. This study aims to investigate the potential impact of Hcy on gene imprinting of IGF2 and H19. Cultured human umbilical VSMCs were treated with different concentrations of Hcy. The DNA methylation status of VSMCs was assayed by nested methylationspecific polymerase chain reaction (PCR). The mRNA levels of H19, IGF2, and CCCTC-binding factor (CTCF) were detected by reverse transcription PCR, and the protein expression of IGF2 by Western blotting. The results showed that the Hcy treatment resulted in hypomethylation of the sixth CTCF-binding site upstream of H19 of VSMCs. The expression of H19 was increased, whereas the IGF2 mRNA and protein were decreased, the CTCF expression increased with the increase in Hcy concentration. These data indicated that Hcy could induce hypomethylation of the sixth CTCF-binding sites upstream of H19, which is an important regulating area for the imprinting expression of IGF2 and H19. The increased CTCF expression may be a potential mechanism for the demethylation modification of DNA, which resulted from the Hcy treatment.

Comparative studies on homocysteine and its metabolite-homocysteine thiolactone action in blood platelets in vitro

Homocysteine (Hcy), an intermediate formed during the catabolism of the essential dietary amino acid methionine, and its cyclic thioester, homocysteine thiolactone (TL) formed from Hcy in plasma, may be implicated in pathological haemostasis and atherosclerosis. The mechanism by which TL exerts the prothrombotic effect and influences blood platelets remains unclear. Activation of blood platelets plays an important role in prothrombotic events. The aim of our study was to establish and compare the influence of a reduced form of homocysteine (at final doses of 10-100 microM) and its cyclic thioester, homocysteine thiolactone (0.1-1 microM), on platelet activation induced by thrombin (platelet aggregation), on platelet protein modifications (determined by parameters such as level of protein carbonyl groups, 3-nitrotyrosine residues in proteins) and on superoxide anion radicals ( O2-*) generation using the model system in vitro. We have observed that TL, like its precursor, Hcy, stimulates the generation of O2* in platelets and causes an augmentation of platelet aggregation induced by thrombin. Our present results in vitro also demonstrate that Hcy (10-100 microM) and TL at lower doses than Hcy (0.1-1 microM) cause modification of platelet proteins: diminished formation of carbonyl groups and distinctly decreased tyrosine nitration in platelet proteins after thrombin stimulation, but increased platelet aggregation induced by thrombin. TL like Hcy (at concentrations corresponding to concentrations in blood during hyperhomocysteinemia) modifies platelet responses to an important physiological agonist--thrombin.

Hyperhomocysteinemia and venous thrombosis

It has been recognized, since the first description of the disease, that arterial and venous thrombosis are common in patients with homocysteinuria. Interest in the condition increased with reports from a large number of mainly retrospective studies showing that mildly elevated homocysteine levels are also associated with venous thromboembolism (VTE), thrombotic stroke, and peripheral vascular disease. This association is less strong when populations are studied prospectively. Vitamin supplementation, primarily with folic acid, and to a lesser degree with pyridoxine and vitamin B(12), is effective in reducing elevated levels of plasma homocysteine. Surprisingly, however, recent prospective intervention studies showed that despite lowering of the homocysteine level with such treatment, there was no impact on the risk of recurrence of venous or arterial disease.

Vitamin B6 suppresses apoptosis of NM-1 bovine endothelial cells induced by homocysteine and copper

Hyperhomocysteinemia is an important risk factor for atherosclerosis. We previously reported that formation of early atherosclerosis in the rat aorta was associated with hyperhomocysteinemia and reduction of antioxidant activity caused by low concentration of vitamin B(6)in vivo. In the present study, we examined effects of vitamin B(6) on apoptosis of bovine endothelial cells (NM-1 cells) treated with homocysteine and copper. Homocysteine and copper induced extracellular hydrogen peroxide, intracellular ROS and cellular lipid peroxide levels. Cell viability was reduced to 30% compared to that of control cells. On the other hand, pyridoxal treatment as well as EDTA treatment increased viability of NM-1 cells treated with homocysteine and copper to about 60%, and significantly decreased extracellular hydrogen peroxide, intracellular ROS and cellular lipid peroxide levels. The treatment of catalase recovered cell viability and reduced the level of extracellular hydrogen peroxide and intracellular ROS. Cell death by homocysteine and copper was confirmed to be due to apoptosis by evaluation of DNA fragmentation and by TUNEL assay. However, apoptosis of NM-1 cells induced by homocysteine and copper was due to a caspase-independent pathway as it was not inhibited by the caspase inhibitor, Z-VAD-fmk. Apoptosis of NM-1 cells induced by homocysteine and copper accompanied with mitochondrial permeability but not cytochrome c release. These results suggest that pyridoxal treatment suppresses apoptosis of NM-1 cells induced by homocysteine and copper, most likely through antioxidant effects.