Phosphorylation of myosin light chain in resting platelets from NIDDM patients is enhanced: correlation with spontaneous aggregation

Measurement of soluble C-type lectin-like receptor 2 in human plasma

Detection of platelet activation in vivo is useful to identify patients at risk of thrombotic diseases. Platelet factor 4 (PF4) and β-thromboglobulin (β-TG) are used for this purpose; however, they are easily released upon the minimal platelet activation that occurs during sampling. Soluble forms of several platelet membrane proteins are released upon platelet activation; however, the soluble form of C-type lectin-like receptor 2 (sCLEC-2) has not yet been fully investigated. Western blotting with an anti-CLEC-2 antibody showed that sCLEC-2 was released from washed human platelets stimulated with collagen mimetics. To detect sCLEC-2 in plasma, we established a sandwich enzyme-linked immunosorbent assay (ELISA) using F(ab')2 anti-CLEC-2 monoclonal antibodies. Although plasma mixed with citrate, adenosine, theophylline and adenosine (CTAD) is needed for the PF4 and β-TG assays, effects of anti-coagulants (EDTA, citrate and CTAD) on the sCLEC-2 ELISA were negligible. Moreover, while special techniques are required for blood sampling and sample preparation for PF4 and β-TG assay, the standard blood collections procedures used in daily clinical laboratory tests have shown to suffice for sCLEC-2 analysis. In this study, we found that two forms of sCLEC-2 are released after platelet activation: a shed fragment and a microparticle-bound full-length protein, both of which are detected by the sCLEC-2 ELISA. The average concentration of sCLEC-2 in the plasma of 10 healthy individuals was 97 ± 55 pg/ml, whereas that in the plasma of 25 patients with diabetes mellitus (DM) was 149 ± 260 pg/ml. A trend towards an increase in sCLEC-2 concentration in the DM patients may reflect in vivo platelet activation in the patients, suggesting that sCLEC-2 may have clinical significance as a biomarker of in vivo platelet activation.

Assessment of spontaneous platelet aggregation using laser light scattering in healthy subjects: an attempt to standardize

INTRODUCTION

When measuring platelet aggregation using laser light scattering, small aggregates forming without the addition of agonists may be observed. This event is called 'spontaneous platelet aggregation (SPA)'. The platelet hyperactivity observed in arterial thrombotic diseases can be detected with relative ease by measuring SPA. Standardization is urgently needed because of differences between measurement conditions among various laboratories.

METHODS

We conducted a systematic study of factors that affect SPA measurement, compared SPA results to flow cytometry detection of surface antigens expressed on activated platelet membranes (P-selectin, activated glycoprotein IIb/IIIa), and determined conditions that yield stable measurements.

RESULTS AND CONCLUSIONS

We evaluated results from 125 healthy volunteers and established conditions for a stable measurement of SPA. As the occurrence of SPA tended to increase with age, we determined conditions valid for subjects aged 20-60 years. Blood should be collected using a syringe, and the sample should be prepared after allowing the whole blood to rest for 30 min after collection. To isolate platelet-rich plasma, a 2-mL tube should be used and centrifuged at 150 g. The sample should be stored at room temperature, the platelet count of the sample should be (250 ± 10) × 10(9) /L, and the measurement should be completed within 90 min of blood collection.

Insensitivity to the α2-adrenergic receptor blocker yohimbine hydrochloride and occurrence of spontaneous platelet macroaggregation (SPMA) in diabetes

We report here a study of platelet aggregation in diabetes, induced by epinephrine and its inhibition by yohimbine hydrochloride (YH), an alpha(2)-adrenergic receptor-blocking agent. Interestingly, emergence of spontaneous platelet macroaggregation (SPMA) was observed in six out of 75 cases in the absence of any agonist. The SPMA cases were strongly associated with insensitivity to YH (in contrast with non-SPMA cases) when epinephrine was used as an agonist. We suggest that the observed correlation is a result of over expression of platelet alpha(2)-adrenoceptors in such subjects. The quantitative nature of the effect is supported by the observation that addition of YH at higher concentration (more than 5 microM) led to restoration of the adrenergic receptor-blocking activity of the said agent. Eventually for non-SPMA subjects YH exhibited blocking activity even at lower concentration. The aggregation profile and the platelet morphology of the SPMA cases had distinctive features as compared to microaggregates formed in other diabetic subjects (non-SPMA cases).

Involvement of Src kinases and PLCgamma2 in clot retraction

The integrin α_IIbβ₃ plays a critical role in mediating clot retraction by platelets which is important in vivo in consolidating thrombus formation. Actin–myosin interaction is essential for clot retraction. In the present study, we demonstrate that the structurally distinct Src kinase inhibitors, PP2 and PD173952, significantly reduced the rate of clot retraction, but did not prevent it reaching completion. This effect was accompanied by abolition of α_IIbβ₃-dependent protein tyrosine phosphorylation, including PLCγ2. A role for PLCγ2 in mediating clot retraction was demonstrated using PLCγ2-deficient murine platelets. Furthermore, platelet adhesion to fibrinogen leads to MLC phosphorylation through a pathway that is inhibited by PP2 and by the PLC inhibitor, U73122. These results demonstrate a partial role for Src kinase-dependent activation of PLCγ2 and MLC phosphorylation in mediating clot retraction downstream of integrin α_IIbβ₃.

Aspirin resistance detected with aggregometry cannot be explained by cyclooxygenase activity: involvement of other signaling pathway(s) in cardiovascular events of aspirin-treated patients

OBJECTIVES

Although the concept of aspirin resistance is extensively reported in medical literature, its precise mechanisms and clinical outcomes are largely unknown. In this study, we examined individual thromboxane biosynthesis and platelet aggregation in aspirin-treated patients, and whether the results of a platelet aggregation test influenced clinical outcomes.

RESULTS

Subjects taking 81 mg of aspirin (n = 50) and controls (n = 38) were evaluated for platelet aggregation and platelet cyclooxygenase-1 (COX-1) activity by measuring collagen-induced thromboxane B2 production. For aggregometry, both light transmission (LT) and laser-light scattering methods were employed to quantitatively evaluate aggregate sizes and numbers. Aspirin treatment resulted in the inhibition of collagen-induced platelet aggregation, particularly the transition from small to large platelet aggregates. Although platelet COX-1 activity seemed to be uniformly inhibited in all patients, platelet aggregation studies showed great inter-individual differences; variation in platelet COX-1 activity only accounted for 6-20% of the individual aggregations. Factor analysis revealed the existence of a common factor (other than platelet COX-1) that explained 48.4% of the variations in platelet aggregation induced by collagen, adenosine diphosphate (ADP), and collagen-related peptide. We then prospectively enrolled 136 aspirin-treated patients in our study, and we found that being in the upper quartile level of LT, or with large aggregate formation induced by collagen, was an independent risk factor for developing cardiovascular events within 12 months [hazard ratio (HR) = 7.98, P = 0.008 for LT; HR = 7.76, P = 0.007 for large aggregates]. On the other hand, the existence of diabetes mellitus was an independent risk factor for overall outcomes (HR 1.30-11.9, P = 0.015-0.033).

CONCLUSIONS

Aspirin resistance expressed as unsuppressed platelet COX-1 activity is a rare condition in an out-patient population. Other factor(s) affecting collagen-induced platelet aggregation may influence early outcomes in aspirin-treated patients.

Pathobiology and cell interactions of platelets in diabetes

Diabetes is a well-recognised risk factor for atherosclerotic cardiovascular disease and in fact most diabetic patients die from vascular complications. The Diabetes Control and Complications Trial (DCCT) and the U.K. Prospective Diabetes Study (UKPDS) indicate a consistent relationship between hyperglycaemia and the incidence of chronic vascular complications in patients with diabetes. Platelets are essential for haemostasis, and abnormalities of platelet function may cause vascular disease in diabetes. Diabetic patients have hyperreactive platelets with exaggerated adhesion, aggregation and thrombin generation. In summary, the entire coagulation cascade is dysfunctional in diabetes. This review provides a comprehensive overview of the physiological role of platelets in maintaining haemostasis and of the pathophysiological processes that contribute to platelet dysfunction in diabetes and associated cardiovascular diseases, with special emphasis on proteomic approaches and leukocyte-platelet cross-talk.

Platelet aggregates in various stages of diabetic retinopathy: evaluation using the particle-counting light-scattering method

AIMS

To investigate the possible correlation between platelet aggregation and the severity of diabetic retinopathy using the light-scattering method.

METHODS

Using a light-scattering platelet aggregometer, we measured spontaneous platelet aggregation in 86 diabetics with retinopathy of varying severity and 30 healthy volunteers (controls). Platelet aggregates were classified as small, medium, and large according to their light intensity; patients were grouped based on the severity of retinopathy. In each patient group, we recorded for 10 min the total light intensities emitted by each aggregate size in the area under the curve (AUC). Then, we compared the AUC of each level of retinopathy severity with the controls and determined the correlation between the AUC of each aggregate size and each severity level.

RESULTS

Of the 86 diabetics, 22 had no apparent retinopathy (NAR), 13 had mild nonproliferative diabetic retinopathy (NPDR), 17 had moderate NPDR, 12 had severe NPDR, and 22 had proliferative diabetic retinopathy (PDR). While the NAR group manifested significantly more small aggregates than the controls (20.5 x 10(6) versus 8.3 x 10(6) a.u., p=0.024), none of the other groups showed a significant increase in aggregates of any size. In the AUC of large aggregates, there was a weak-positive correlation with the severity of retinopathy (r=0.255, p=0.018); in the AUC of small and medium aggregates, there was no correlation.

CONCLUSION

Although we did not find a significant correlation between platelet aggregation and the severity of diabetic retinopathy, our pilot study did detect some tendencies. Further studies on larger populations are underway to determine whether these tendencies are real.

Comparative effect of antiplatelet therapy in retinal vein occlusion evaluated by the particle-counting method using light scattering

PURPOSE

To evaluate platelet aggregation in patients with retinal vein occlusion (RVO) by the light-scattering method and compare the effects of three antiplatelet drugs on aggregate formation.

DESIGN

Prospective, nonrandomized, interventional clinical trial.

METHODS

(1) Platelet aggregation was measured in 42 patients with untreated branch RVO (BRVO), 26 patients with central RVO (CRVO), and 30 healthy control subjects using a light-scattering platelet aggregometer. Platelet aggregates were classified as small, medium, and large according to light intensity. Total light intensities of each aggregate size were compared between BRVO, CRVO, and control subjects. (2) In 33 patients with RVO, platelet aggregation before and 2 weeks after the administration of ticlopidine, beraprost, or aspirin were compared.

RESULTS

(1) There was a statistically significant difference (P = 0.0073) between the control subjects (8.3 x 10(6) a.u.) and CRVO patients (22.9 x 10(6) a.u.) with respect to small aggregates. There was no statistically significant difference with respect to medium and large aggregate formation between the control subjects and either patient group. (2) Compared with aggregates formed in the absence of antiplatelet drugs, ticlopidine significantly inhibited only the formation of small aggregates and beraprost that of all sizes; aspirin did not significantly inhibit the formation of any aggregate sizes.

CONCLUSIONS

Increase in small platelet aggregates may be attributable to RVO pathogenesis. Beraprost and ticlopidine appear to inhibit small aggregate formation in RVO patients and may represent effective antiplatelet treatments. The light-scattering method is useful to investigate the pathogenesis of RVO and the effects of antiplatelet drugs.

Mesangial cell filamentous actin disassembly and hypocontractility in high glucose are mediated by PKC-zeta

In high glucose (HG), mesangial cells (MCs) lose their contractile response to endothelin-1 (ET-1) coincidently with filamentous (F)-actin disassembly. We postulated that these MC phenotypic changes are mediated by altered protein kinase C (PKC) isozyme activity, myosin light chain (MLC(20)) phosphorylation, or Ca(2+) signaling. MCs were growth arrested for 24 h in 0.5% fetal bovine serum (FBS)-DMEM in 5.6 (normal glucose; NG) or 30 mM glucose (high glucose; HG). In HG, the planar area was reduced [2,608 +/- 135 vs. 3,952 +/- 225 (SE) microm(2) in NG, P < 0.01, n = 31] with no contractile response to 0.1 microM ET-1. Mannitol did not affect cell size or ET-1 response. Confocal imaging of fluo 3- loaded cells revealed that the peak intensity of ET-1-induced Ca(2+) signaling was not altered in HG vs. NG. Immunoblotting of phosphorylated MLC(20) showed that HG increased mono- and decreased unphosphorylated MLC(20) (42 +/- 16 and 49 +/- 15 vs. 13 +/- 3 and 80 +/- 4% of total in NG, P < 0.05, n = 3), but the peak phosphorylation responses to ET-1 were identical in NG and HG. ET-1 stimulated translocation of PKC-delta and -epsilon from cytosolic to membrane and particulate fractions identically in NG and HG but did not cause PKC-zeta translocation. In HG, membrane accumulation of PKC-zeta was observed. Membrane PKC-zeta activity measured by immunoprecipitation and (32)P phosphorylation of PKC-epsilon pseudosubstrate peptide was 190 +/- 18% of NG (P < 0.01, n = 4), which was completely inhibited by pretreatment with a myristoylated peptide inhibitor (ZI). In HG, pretreatment with ZI for 24 h restored normal MC size and contractile and F-actin disassembly responses to ET-1. In conclusion, in HG, decreased MC size is due to decreased F-actin assembly, and loss of contractile response to ET-1 occurs in the presence of normal Ca(2+) signaling and normal MLC(20) phosphorylation. In HG, altered F-actin and contractile functions in MCs are mediated by PKC-zeta.

Vascular complications in diabetes and their prevention

Diabetes mellitus is increasing throughout the world. Cardiovascular disease (CVD) accounts for up to 80% of excess mortality in this high-risk population. Patients with diabetes have the same CVD risk factors as those people without diabetes. However, these risk factors are much more powerful in diabetic patients. CVD risk is especially high for diabetic women, and premenopausal diabetic women lose all the protection normally afforded to them by female sex hormones. Controlled clinical trials have clearly demonstrated that rigorous treatment of blood pressure, dyslipidemia and platelet hyperaggrebility strikingly reduces CVD risk in diabetic patients. Strategies directed at interrupting the renin-angiotensin system (both tissue and systemic systems) and the use of 3-hydroxy-3-methyl-glutaryl coenzyme A (HMG-CoA) reductase inhibitors have proven to be especially beneficial for this high-risk population.

The prognostic value of small-sized platelet aggregates in unstable angina: detection by a novel laser-light scattering method

Platelet activation plays a pivotal role in the pathogenesis of acute coronary syndromes. This study was designed to evaluate the platelet aggregability in patients with unstable angina using a new aggregometer with laser-light scattering. We also examined whether there was a relationship between these platelet aggregabilities and unfavorable outcome during in-hospital stay. We measured platelet aggregability, in particular small-sized platelet aggregates in 31 patients with unstable angina, 31 patients with stable exertional angina, and 30 patients with chest pain syndrome. The patients with unstable angina were divided into two groups by their cardiac events during in-hospital stay, cardiac events (+)(n=11) group and cardiac events (-)(n=20) group. On admission, the number of small-sized platelet aggregates (V) was higher in patients with unstable angina (3.0+/-0.5x10(4)) than in those with stable exertional angina (1.4+/-0.3x10(4), P=.017) and chest pain syndrome (0.7+/-0.2x10(4), P=.0003). The number of small-sized platelet aggregates was higher in the cardiac events (+) group than in the cardiac events (-) group (5.5+/-0.9x10(4) vs. 1.6+/-0.4x10(4), P=.0001). A previous study elucidated that small-sized platelet aggregates ultimately developed into medium-sized and large-sized aggregates as platelet aggregation proceeds. Therefore, the production of small-sized platelet aggregates is more sensitive for hyperaggregability. Furthermore, the production of small-sized platelet aggregates increased significantly in patients with unstable angina than in those with stable exertional angina and chest pain syndrome. These findings suggest that a tendency toward thrombus formation increases markedly in patients with unstable angina and increased number of small-sized platelet aggregates on admission predicts poor prognosis during in-hospital stay in patients with unstable angina.

Increased rate of formation of small-sized platelet aggregates in patients with acute coronary syndromes

Coronary thrombosis has been implicated in the pathogenesis of acute coronary syndromes, and platelet activation plays a pivotal role in the pathogenesis of coronary thrombus. A new platelet aggregometer using a laserlight scattering beam was trialled for assessment of platelet aggregation. Platelet aggregability, especially small-sized platelet aggregates, was investigated on admission using the laser-light scattering method and again after treatment in 23 patients with acute coronary syndromes. The platelet aggregability in 14 patients with stable exertional angina and in 14 control subjects was also examined. On admission, the number of small- and medium-sized platelet aggregates in the acute coronary syndromes group was significantly greater than in the stable exertional angina group or control group. However, the number of large-sized platelet aggregates on admission was not increased in the acute coronary syndromes group. Furthermore, the number of small- and medium-sized platelet aggregates decreased significantly after treatment in the acute coronary syndromes group. The increased number of small-sized platelet aggregates may sensitively reflect attacks of thrombosis in patients suffering acute coronary syndromes.

Comorbidity of hypertension and diabetes: the fosinopril versus amlodipine cardiovascular events trial (FACET)

Macrovascular disease is the major cause of mortality in persons with type 2 diabetes mellitus, and hypertension is an important factor contributing to this high prevalence. High blood pressure is about twice as common in persons with diabetes mellitus as in those without. Up to 75% of diabetes-related cardiovascular complications are attributed to hypertension. These observations are part of the rationale for recommendations for more aggressive lowering of blood pressure (to < 130/85 mm Hg) in persons with coexistent diabetes and hypertension. This may require therapy with a combination of antihypertensive agents. The Fosinopril versus Amlodipine Cardiovascular Events Trial (FACET), discussed herein, supports the case for combination therapy with an angiotensin-converting enzyme (ACE) inhibitor and a calcium antagonist in diabetic patients with hypertension.

A cross-sectional evaluation of spontaneous platelet aggregation in relation to complications in patients with type II diabetes mellitus

To clarify the relationship between platelet function and diabetic complications, we investigated spontaneous platelet aggregation (SPA) and agonist-induced platelet aggregation by a particle counting method using light scattering (LS) and by a conventional light transmission method (LT) in 23 age- and sex-matched control subjects and 74 patients with type II diabetes mellitus. We also observed platelets using the FIC-2 (TOA Medical Electronics, Kobe, Japan) flow cytometer and imaging device. Observation by the FIC-2 device showed microaggregates of platelets in samples with increased SPA-LS. SPA-LS was significantly elevated in patients with type II diabetes mellitus as a whole compared with control subjects. SPA-LS also showed significant differences between control subjects and three diabetic patient subgroups with a varying severity of retinopathy, nephropathy, or neuropathy, and the mean values increased along with the increasing severity of complications. On the other hand, although SPA-LT also showed significant differences between these groups, the absolute values were all less than 10%, which we believe does not warrant quantitative analysis. Adenosine-5'-diphosphate (ADP)-induced platelet aggregation failed to show significant differences between controls and subjects with a varying severity of retinopathy by either LS or LT, which indicates that SPA is more sensitive than agonist-induced platelet aggregation in relation to diabetic complications. We observed significant correlations between SPA-LS and the patients' age, hemoglobin A1c (HbA1c) level, plasma fibrinogen level, or 6-keto-PGF1alpha (6KF) to 11-dehydro-thromboxane B2 (TXB2) ratio. Our study demonstrated a close relationship between platelet hyperaggregability and diabetic complications, and a longitudinal prospective study of SPA-LS in diabetic patients is warranted to clarify cause-and-effect relationships.