E. Grandone

Abnormally high thromboxane biosynthesis in homozygous homocystinuria. Evidence for platelet involvement and probucol-sensitive mechanism.

Homocystinuria due to homozygous cystathionine beta-synthase deficiency is an inborn error of metabolism characterized by a high incidence of thrombosis and premature atherosclerosis. We evaluated TXA2 biosynthesis in vivo and several in vitro tests of platelet function in 11 homocystinuric patients and 12 healthy controls. In vitro, patients platelet aggregation was within control values as were TXB2 formation, fibrinogen binding, and ATP secretion in response to thrombin. In contrast, the urinary excretion of 11-dehydro-TXB2, a major enzymatic derivative of TXA2, was > 2 SD of controls in all patients (1,724 +/- 828 pg/mg creatinine, mean +/- SD, in patients vs. 345 +/- 136 in controls, P < 0.001). The administration to four patients of low-dose aspirin (50 mg/d for 1 wk) reduced metabolite excretion by > 80%. The recovery of 11-dehydro-TXB2 excretion over the 10 d that followed aspirin cessation occurred with a pattern consistent with the entry into the circulation of platelets with intact cyclooxygenase activity. Prolonged partial reduction in the abnormally high excretion of both 11-dehydro-TXB2 and 2,3-dinor-TXB2, was also observed in seven patients who ingested 500 mg daily for 3 wk of the antioxidant drug probucol. These results provide evidence for enhanced thromboxane biosynthesis in homocystinuria and for its partial dependence on probucol-sensitive mechanisms. Furthermore, the elevated TXA2 formation in homocystinuria is likely to reflect, at least in part, in vivo platelet activation.

Abnormally high circulation levels of tissue plasminogen activator and plasminogen activator inhibitor-1 in patients with a history of ischemic stroke.

We evaluated 106 subjects with and 109 subjects without a history of ischemic stroke. All were attending a metabolic ward. The two groups were compared for major risk factors for ischemic events. A positive family history for ischemic complications of atherosclerosis was more common in subjects with a history of stroke than in those without; moreover, plasma levels of plasminogen activator inhibitor-1 (PAI-1) and tissue-type plasminogen activator (TPA) were higher in patients with documented previous events. A strong positive significant correlation was found between TPA and PAI-1 levels, and an interaction between age and TPA was observed when the sample was stratified according to ages being above or below 70 years. When the patient population was analyzed according to the number of ischemic events, it was found that 62 of the 106 subjects with a history of stroke had experienced more than one ischemic event. Under these conditions, the levels of TPA and PAI-1 still correlated with the occurrence of previous ischemic episodes. As in the whole patient sample, TPA was the strongest discriminator. We conclude that in subjects attending a metabolic ward, TPA and PAI-1 levels consistently help identify subjects with a history of cerebral ischemic episodes and that TPA is the strongest discriminator.

Anticardiolipin antibodies in patients with liver disease.

Objective:Our aim was to test the hypothesis that anticardiolipin antibodies (aCL) may cause an antiphospholipid syndrome and thrombotic events in patients with liver disease.Methods:aCL were measured in 116 healthy controls and 372 patients with liver disease of different stage and etiology: 136 cases secondary to hepatitis C virus (HCV) infection, 139 due to hepatitis B virus (HBV) infection, 69 with alcoholic liver damage, and 28 cryptogenic in origin. Prior thrombotic events were recorded. The results were related to age, gender, stage, severity, and etiology of the liver disease, as well as to the occurrence of organ- and nonorgan-specific autoantibodies.Results:aCL were positive in 4.4% of controls and in 18.8% of patients (p < 0.0002). Patients with aCL were more frequently men with an advanced cirrhosis and simultaneous occurrence of anti–smooth-muscle antibodies (ASMA) in serum (p < 0.0006); their liver damage was often secondary to HBV (37.3%) or alcohol abuse (18.5%). At conditional logistic regression analysis, only the presence of ASMA (odds ratio [OR] = 3.02, 95% confidence interval [CI] 1.7–5.5, p= 0.0003), HBV (OR = 3.4, 95% CI 1.6–7.2, p= 0.0013), or alcoholic liver disease (OR = 5.3, 95% CI 2.3–12.2, p= 0.0001) were independently associated with aCL. Thrombosis was encountered in 24 patients (6.4%). At conditional logistic regression analysis, thrombosis was significantly associated with advanced age (OR = 1.07, 95% CI 1.0–1.1, p= 0.0094), development of hepatocellular carcinoma (OR = 17.8, 95% CI 1.6–196.0, p= 0.01), HBV etiology (OR = 6.3, 95% CI, 1.6–24.6, p= 0.0076), or cryptogenic liver disease (OR = 54.8, 95% CI 5–599.9, p= 0.001). Of the five patients with newly documented portal thrombosis during the follow-up, only one tested positive for aCL.Conclusion:In patients with nonautoimmune liver disease, aCL production is an epiphenomenon of the liver damage and is not associated with thrombotic complications. These data do not support the hypothesis that HCV is a cause of the antiphospholipid syndrome.

Fibrinogen and mechanisms of thrombosis. A difficult link

Epidemiological observations indicate that high plasma fibrinogen levels are strongly correlated to the frequency of two major thrombotic complications of atherosclerosis: stroke and myocardial infarction. Thrombosis is increasingly recognized as a central mechanism in stroke and myocardial infartion, and fibrinogen is involved in events thought to play a major role in thrombosis. Therefore, elucidation of the relationship between fibrinogen and thrombosis may strengthen the predictive value of this protein and define new interventions against stroke and myocardial infarction. In addition, advances in the understanding of the atherogenic potential of several risk factors of coronary heart disease took advantage of information emerging from the measurement of the factor in population-based studies. Thus, it is conceivable that measuring plasma fibrinogen to predict stroke and myocardial infarction is a major direction to be followed to gain insight into the thrombogenic potential on this protein and inspire new strategies against thrombotic complications of atherosclerosis.

Plasma predictors of ischemic complications of atherosclerosis: open issues

In western countries, acute myocardial infarction is the commonest cause of morbidity and mortality [19]. An occlusive coronary thrombus on an ulcerated atherosclerotic plaque in the coronary arteries is the etiological event in more than 90% of patients with Q-wave myocardial infarction [38]. The underlying abnormality in non-Q-wave myocardial infarction is often a ruptured atherosclerotic plaque, which acts as a nidus for the deposition and activation of platelets. In this case, thrombosis occurs, but may not be totally occlusive, or an early spontaneous recanalization may occur. On the other hand, some clinical trials showed that a prolonged treatment with antiplatelet drugs significantly reduces the recurrence of coronary ischemia. Thus, atherosclerosis is a necessary condition for myocardial infarction, but it is not sufficient in that it usually needs the occurrence of thrombosis. However, only 25-30% of these thrombotic events are prevented by the administration of antiplatelets drugs. In recent years, epidemiological studies identified some hemostatic parameters whose abnormalities may help predict the risk of ischemic events: fibrinogen [14], plasminogen activator inhibitor-1 (PAI-1) [3], lipoprotein(a) [46], anticardiolipin antibodies (ACA) and lupus anticoagulant (LA) [10], leukocyte count [34], blood viscosity [34]. Some of these, such as fibronogen and PAI-1 are acute-phase proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Drugs affecting plasma fibrinogen levels. Implications for new antithrombotic strategies

Current evidence indicates that plasma fibrinogen is synthesized by the liver; that genetic and environmental factors regulate plasma fibrinogen levels; that interleukin-6 (IL-6) affects the synthesis of plasma fibrinogen by mechanisms involving protein kinase C, and that during the acute-phase response, monocytes generate a variety of monokines including IL-6. Certain drugs and nutrients have been reported to lower plasma fibrinogen levels. The mechanism(s) involved in this effect is poorly understood. However, since most of these substances quantitatively and/or qualitatively affect monocytes, the possibility that these drugs affect plasma fibrinogen levels via these cells should be considered. In addition to fibrinogen, IL-6 also regulates the synthesis of other acute-phase proteins. Especially when combined, major risk factors for atherosclerosis cause vascular injury that triggers inflammatory events. This raises the issue of whether high plasma fibrinogen levels are just the epiphenomenon of as yet unknown events in thrombosis and atherosclerosis. Thus, the issue to be addressed is whether high plasma fibrinogen concentrations should be lowered or should they serve to suggest strong interventions on established risk factors. As for other risk factors, fibrinogen measurements in population-based studies, in parallel with measurements of established risk factors will help define appropriate directions to be followed to gain insight into the issue and define new antithrombotic strategies.

Genetic Polymorphisms and Arterial Thrombosis

In Western countries, acute myocardial infarction and stroke, are major causes of morbidity and mortality (1). An occlusive coronary thrombus on an ulcerated atherosclerotic plaque in the coronary arteries is the central event in more than 90% of patients with Q-wave myocardial infarction (2). The underlying abnormality in non-Q-wave infarctions is a ruptured atherosclerotic plaque, which acts as a nidus for the deposition and activation of platelets. Atherosclerotic changes in the carotid and vertebrobasilar arteries resemble those of coronary arteries (3). These lesions may progress to local occlusions, but most often do not become symptomatic until embolization to more distal arteries takes place (ischemic stroke). In the cerebral arteries, vessel wall and lumen may be normal until the artery is occluded by an embolus from a more proximal arterial lesion or from the heart (4). Rather than to emboli, occlusion of the small penetrating arteries is often due to degenerative changes of the vessels or to extracranial large-artery disease (5).