A critical review of the relationship between impaired fibrinolysis and myocardial infarction

Hypofibrinolysis is a risk factor for arterial thrombosis at young age

The relationship between defective fibrinolysis and arterial thrombosis is uncertain. The evaluation of the plasma fibrinolytic potential might provide stronger evidence linking fibrinolysis to arterial thrombosis than the evaluation of the individual fibrinolytic factors. We determined the plasma fibrinolytic potential of 335 young survivors of a first arterial thrombosis, including coronary artery disease (n = 198), ischaemic stroke (n = 103) and peripheral artery disease (n = 34), enrolled in a population-based case-control study and of 330 healthy individuals. Patients had significantly higher clot lysis times (CLTs) than the controls. Odds ratios (ORs) were calculated as a measure of relative risk. The OR for arterial thrombosis was determined in these subjects who had a CLT above the 60th, 70th, 80th, 90th and 95th percentiles of the values found in the control subjects. We found a progressive increase in risk of arterial thrombosis in subjects with hypofibrinolysis (OR: 1.7, 2.0, 2.3, 2.3 and 2.9, respectively). Relative risk estimates obtained in the whole group were comparable those obtained in the event-subgroups. In conclusion, a low plasma fibrinolytic potential, found in 10% of the population, increases the relative risk of arterial thrombosis twofold. This points to an important contribution of hypofibrinolysis to the burden of arterial thrombosis.

Pleiotropic functions of plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1), a 45-kDa serine proteinase inhibitor with reactive site peptide bond Arg345-Met346, is the main physiological plasminogen activator inhibitor. It occurs in human plasma at an antigen concentration of about 20 ng mL(-1). Besides the active inhibitory form of PAI-1 that spontaneously converts to a latent form, also a substrate form exists that is cleaved at the P1-P1' site by its target enzymes, but does not form stable complexes. Besides its role in regulating hemostasis, PAI-1 plays a role in several biological processes dependent on plasminogen activator or plasmin activity. Studies with transgenic mice have revealed a functional role for PAI-1 in wound healing, atherosclerosis, metabolic disturbances such as obesity and insulin resistance, tumor angiogenesis, chronic stress, bone remodeling, asthma, rheumatoid arthritis, fibrosis, glomerulonephritis and sepsis. It is not always clear if these functions depend on the antiproteolytic activity of PAI-1, on its binding to vitronectin or on its intereference with cellular migration or matrix binding.

Increased levels of endothelial haemostatic markers in patients with coronary heart disease

von Willebrand factor (vWF), thrombomodulin and tissue plasminogen activator antigen (tPAag) are regarded as markers of endothelial activation and/or damage. Elevated levels have been associated with atherosclerotic disease states. The aim of the present study was to compare the levels of vWF, thrombomodulin and tPAag in patients with coronary heart disease (CHD) and matched healthy individuals to see if they discriminated significantly between the study groups also after adjustment for established CHD risk factors. Patients (n=193) in various stages of CHD and matched controls (n=193) were included. To evaluate possible influence of acute phase reaction, reinvestigation was performed after 6 months. We observed elevated levels of vWF (P<.001) and tPAag (P<.001) but not thrombomodulin (P=.082) in CHD patients when compared to controls, still statistically significant after 6 months and also after adjustment for established risk factors. Our results indicate that vWF and tPAag but not thrombomodulin in the present population are independent markers of atherosclerosis.

Atheromas that cause fatal thrombosis are usually large and frequently accompanied by vessel enlargement

Several lines of clinical evidence show that AMI frequently occurs at sites with mild to moderate degree of coronary stenosis. The degree of luminal stenosis depends on plaque deposition and degree of vessel remodeling, features poorly assessed by coronary angiography. This postmortem study tested the hypothesis that the size of coronary atheroma and the type of remodeling distinguish culprit lesion responsible for fatal AMI from equi-stenotic nonculprit lesion in the same coronary tree. The main coronary branches from 36 consecutive patients with fatal AMI were studied. The culprit lesion (Group 1) and an equi-stenotic nonculprit segment (Group 2) obtained in measurements of another coronary branch from the same patient were compared. Morphometry and plaque composition was assessed in both groups. Compared to Group 2, Group 1 had larger areas of: plaque 9.6 vs. 4.7 mm(2), vessel 12.7 vs. 7.4 mm(2) and lumen 1.7 vs. 1.2 mm(2); (P< .01). Positive remodeling was more frequent in Group 1 than Group 2: 21/30 (70%) vs. 8/26 (31%). Plaque area correlated positively with lipid core and macrophages and negatively with fibrosis and smooth muscle cells. Atherosclerotic plaques that cause fatal thrombosis are more frequently positively remodeled and tend to be larger than nonculprit plaques with the same degree of cross-sectional stenosis. We tested whether arterial remodeling and plaque size vary between segments containing a fatal thrombosed plaque versus an equi-stenotic nonculprit plaque. Culprit vessel segments had higher cross-sectional areas of intimal plaque and of vessel wall than equi-stenotic nonculprit plaques. The cross-sectional area of the vessel correlated positively with both the lipid core area and CD68(+) macrophage content, and negatively with fibrosis area and smooth muscle cell content. These results add elements explaining limitations of angiography in identifying plaques and provide new insights into the role of remodeling in plaque instability.

The genesis of atherosclerosis and risk factors: a review

Atherosclerosis constitutes the most common medical and surgical problem. This can be manifested clinically as stroke, coronary artery disease, or peripheral vascular disease. In the present review the microscopic appearance of the normal arterial wall, the definition of atherosclerosis and the five theories of atherogenesis are described. These are: the lipid theory, the hemodynamic theory, the fibrin incrustation theory, the nonspecific mesenchymal hypothesis and the response to injury hypothesis. Based on the above theories the sequence of events in atherogenesis is analyzed. The classification of the atherosclerotic lesions according to Stary (types I-VI) and their characteristics appear in a table. The epidemiology and the role of the following risk factors are presented in detail: age, sex, lipid abnormalities, cigarette smoking, hypertension, diabetes mellitus, physical inactivity, alcohol consumption, obesity, and hemostatic factors. In addition, less common genetically determined associations like homocystinuria, Tangier disease, Hutchinson-Gilford syndrome (progeria), Werner's syndrome, radiation induced atherosclerosis and the implications of Chlamydia pneumoniae on the arterial wall are discussed.

Premature atherosclerosis in systemic lupus erythematosus

SLE is a multifaceted disease; over the past 20 years, as survival has improved dramatically, new challenges have emerged. It is now clear from the results of studies at several centers that SLE is associated with at least a fivefold increased risk of CAD, which is accelerated at its onset and seems to abolish a female premenopausal protection against CAD. Several groups have also found by various techniques that subclinical disease occurs at a frequency of about 35% to 40%. The pathogenesis of atherosclerosis in this context seems to be a complex interaction of factors associated with the disease, its therapy, and traditional risk factors. Indeed, experimental models suggest a synergy of these different dimensions in plaque formation. Hypercholesterolemia has been identified as predictive of both future CAD events and sub-clinical disease. This is mainly the case in those patients in whom hypercholesterolemia is a sustained phenomenon. In addition, SLE itself seems to be a strong risk factor for CAD over and above the effects of the known traditional CAD risk factors. There is a lot that is still unknown about the pathogenesis of CAD in SLE. Current knowledge is sufficient to justify the belief that an aggressive approach to management of traditional CAD risk factors in patients with SLE is likely to have a major impact on morbidity and mortality in this population. For this to happen, patients must be educated about this issue and be encouraged to play an active role in lifestyle modifications. In addition, clinicians who care for patients with SLE need to assume a primary role in screening and coordinating the management of CAD risk factors in these high-risk patients.

Fibrinolytic potential is significantly increased by oestrogen treatment in postmenopausal women with mild dyslipidaemia

OBJECTIVE

To study the effects of oestrogen replacement treatment on fibrinolytic potential in postmenopausal women.

DESIGN

Randomised, double blind, placebo controlled trial of oral 17 beta-oestradiol.

SETTING

Subjects were evaluated in the outpatient setting.

PATIENTS

Nineteen postmenopausal women with mild dyslipidaemia, aged 44 to 69 years (mean (SD) 55.7 (6.7)).

MAIN OUTCOME MEASURES

Fibrinolytic activity (fibrin plate assay) and tissue plasminogen activator (t-PA) antigen were measured at baseline and after three, six, and nine weeks of each treatment.

RESULTS

After nine weeks of 2 mg oestradiol treatment, there was a significant increase in fibrinolytic potential compared with placebo, as indicated by an increase in fibrinolytic activity (mean (SEM), 80 (9) v 54 (5) mm2 of lysis in the fibrin plate, 2 mg v placebo, p = 0.002) and a decrease in t-PA antigen (5.8 (0.9) v 8.4 (1.2) ng/ml, 2 mg v placebo, p < 0.001). There was a similar trend with the 1 mg dose but the changes were less noticeable.

CONCLUSIONS

Hormone replacement treatment with 17 beta-oestradiol for nine weeks significantly increased fibrinolytic potential in postmenopausal women with mild dyslipidaemia. This suggests that the cardioprotective effect of oestrogen may be mediated, in part, by an increase in fibrinolytic potential.

Demonstration of covalent binding of lipoprotein(a) [Lp(a)] to fibrin and endothelial cells

It has been well documented that Lp(a) binds noncovalently to fibrin or human umbilical vein endothelial cells. This binding is to lysines and is inhibited by lysine analogues such as epsilon-aminocaproic acid (EACA). In the present study, Lp(a) (0.006-0.6 microM) binding to immobilized fibrin and endothelial cells was evaluated by ELISA with an anti-Lp(a) antibody. A significant portion (approximately 65%) of the Lp(a) was found to resist dissociation by EACA (0.2 M). The EACA resistant binding of Lp(a) was time and concentration dependent. The addition of EDTA to the incubation mixture had no effect, thereby excluding cross-linking by transglutaminase as a mechanism. This portion of Lp(a) was also resistant to dissociation by acid (0.1 N HCl), 0.1% SDS, 1 M benzamidine, Tris-HCl (1 M, pH 12), or DTT (5 mM), but it was washed off by 0.1 N NaOH (which did not remove the immobilized fibrin). This suggested that the Lp(a) was covalently linked by an ester bond. Covalent binding was inhibited when Lp(a) was mildly oxidized by BioRad Enzymobeads, which may explain why it escaped recognition in experiments with radiolabeled Lp(a). Covalent binding was attenuated when Lp(a) was pretreated with DFP suggesting that the serine residue in the pseudo active site of Lp(a) was involved. Lp(a) also bound covalently to immobilized BSA, indicating some nonspecificity. However, binding to BSA was almost 3-fold less than to fibrin, suggesting that lysine binding may facilitate covalent binding. A similar proportion of EACA resistant binding of Lp(a) was found with endothelial cells. In conclusion, the findings demonstrate a novel, covalent binding by Lp(a) which is kringle independent and is postulated to involve the pseudo protease domain of Lp(a). This property may contribute to the deposition of Lp(a) on endothelial surfaces and its colocalization with fibrin in atheromas.

Physical exertion induces thrombin formation and fibrin degradation in patients with peripheral atherosclerosis

Sudden extreme physical stress is associated with an increased risk of myocardial infarction mainly in people with preexisting atherosclerosis. In this study we compared the effect of submaximal exercise on coagulation and fibrinolysis in patients with peripheral arterial occlusive disease (PAOD) with that in healthy control subjects. Fifteen PAOD) patients with intermittent claudication and 15 healthy control subjects, matched for age, sex, medication use, smoking habit, and conditioning, were studied. Thrombin-antithrombin III complex (TAT), D-dimer, tissue plasminogen activator (t-PA) and plasminogen activator inhibitor (PAI)-1 antigens (Ag), t-PA activity, and plasmin-alpha2-antiplasmin complex (PAP), as well as plasma catecholamines, were measured before and after a treadmill exercise test. At rest, fibrinogen (3.3+/-0.5 versus 2.9+/-0.5 g/L [mean+/-SD]; P<.05), D-dimer (392+/-128 versus 271+/-113 ng/mL; P<.05), t-PA Ag (9.1+/-5.1 versus 5.5+/-1.2 ng/mL; P<.02), and PAI-1 Ag (14.9+/-7.1 versus 7.6+/-3.8 ng/mL; P<.002) levels in plasma were markedly higher in the patient group than in the control group. In patients but not in control subjects, exercise of similar intensity elevated circulating concentrations of TAT (from 3.43+/-1.45 to 4.83+/-2.27 ng/mL; P<.05). Exercise caused a parallel increase in D-dimer, t-PA Ag, t-PA activity, PAP, and catecholamines in both groups, whereas PAI-1 Ag remained stable. Plasma lactic acid was significantly higher in patients after exercise and was associated with lower-limb ischemia. Compared with healthy control subjects, patients with PAOD showed higher t-PA Ag, PAI-1 Ag, and D-dimer levels both at rest and after exercise. Notably, submaximal exercise on a treadmill enhanced thrombin formation in patients with PAOD but not in the control subjects. Sudden catecholamine release and local ischemia during exercise may accelerate the preexisting prothrombotic potential of the atherosclerotic vessel wall.

Thrombosis, antithrombotic agents, and the antithrombotic approach in cardiac disease

To develop a rational approach to antithrombotic therapy, in cardiac disease, a sound understanding is required (1) of the hemostatic processes leading to thrombosis, (2) of the various antithrombotic agents, and (3) of the relative risks of thrombosis and thromboembolism in the various cardiac disease entities. With the understanding of pathogenesis and risk of thrombus formation, a rational approach to the use of antiplatelet and anticoagulant agents can be formulated. Those at high risk of thrombus formation should generally receive a high degree of antithrombotics and, depending on the pathophysiology of the thrombus, may benefit from the concomitant use of antiplatelet and anticoagulant agents. Those with a medium risk of thrombus formation may benefit with the use of an antiplatelet agent alone or anticoagulants alone. Patients at low risk of thrombus formation should not receive antithrombotics. Such rational approach to antithrombotic therapy serves as the basis of this article.

Coagulation activation in patients undergoing directional coronary atherectomy

Restenosis is a major problem of percutaneous transluminal coronary angioplasty (PTCA) and related procedures. To better understand the underlying pathophysiologic mechanisms, coagulation and fibrinolytic variables were analysed prospectively in 35 patients after directional coronary atherectomy (DCA) and in 20 control patients undergoing diagnostic heart catheterisation and coronary angiography. Blood samples were taken before and 1 h, 24 h and 48 h after the procedure. No subacute thrombosis or unstable angina were documented in any patient. In 8 out of these 35 patients late restenosis was diagnosed during follow-up angiography 3-6 months after DCA. In these 8 patients prothrombin fragments (F1 + 2) rose from 0.7 to 0.9 nmol/l (P < 0.01) and thrombin-antithrombin III complexes (TAT) from 2.9 to 6.0 micrograms/l (P < 0.01), but not significantly in 27 patients without restenosis and in the control patients. In patients with late restenosis plasminogen activator inhibitor (PAI-1) also increased from 2.4 to 4.9 U/ml (P < 0.05) 24 h after DCA while there were no significant changes in patients without restenosis and in control patients. D-Dimer/TAT ratio reflecting the balance between clotting activation and fibrinolysis was significantly lower after 24 h in restenosis patients. The findings suggest that coagulation activation and hypofibrinolysis during 48 h after DCA might be associated with the development of late restenosis.

Hypobetalipoproteinemia is associated with low levels of hemostatic risk factors in the Framingham offspring population

BACKGROUND

Given the importance of thrombosis in causation of acute coronary syndromes, it is possible that the beneficial effect of low lipid levels on the risk of coronary events is achieved by lowering thrombotic potential of the blood. Hypobetalipoproteinemia is characterized by plasma concentrations of apolipoprotein B and LDL cholesterol that are one third of those observed in the general population. The aim of this study was to utilize subjects with hypobetalipoproteinemia to examine the relation between thrombotic potential and low levels of LDL cholesterol.

METHODS AND RESULTS

Hemostatic risk factors were measured in 1878 individuals (1003 women and 875 men) participating in cycle 5 of the Framingham Offspring Study. The subjects were divided into five groups on the basis of LDL cholesterol level. Subjects with hypobetalipoproteinemia (LDL cholesterol < 70 mg/dL) had the lowest levels of fibrinogen, plasminogen activator inhibitor-1 antigen, and tissue plasminogen activator antigen. As LDL cholesterol increased, there was a significant increase in the levels of the hemostatic risk factors, with the exception of von Willebrand factor antigen. Adjustment with multivariate regression analyses for the covariates age, sex, body mass index, diabetes mellitus, smoking, alcohol intake, triglyceride level, and use of antihypertensive medication did not materially alter the results.

CONCLUSIONS

Decreasing levels of LDL cholesterol are associated with decreasing levels of hemostatic risk factors. Subjects with hypobetalipoproteinemia have the lowest levels of hemostatic risk factors and may be protected against thrombotic complications of atherosclerotic cardiovascular disease because of reduced thrombotic potential. One mechanism by which lipid-lowering therapy may decrease clinical cardiac events is through a reduction in thrombotic tendency.

Haemostatic parameters and vascular risk

Associations between increased plasma concentrations of the three haemostatic factors--fibrinogen, Factor VII and plasminogen activator inhibitor 1 (PAI-1)--and cardiovascular disease (CVD) have recently been demonstrated in epidemiological studies. The relative risk of CVD is increased by 2-3 times in subjects whose levels of fibrinogen fall within the upper third of the general distribution in comparison to those subjects who fall into the lower third; a positive correlation exists between increased Factor VII activity and cardiovascular mortality. Many studies have demonstrated that elevated levels of PAI-1 and tPA antigen are associated with CVD. Several studies have also shown that there are important associations between conventional risk factors for atherothrombosis and haemostatic factors. Fibrinogen levels are strongly linked with smoking, and Factor VII levels are linked with cholesterol, triglycerides and fat intake. PAI-1 and tPA antigen levels, on the other hand, are closely associated with insulin resistance. This paper discusses the principal genetic and environmental determinants of the plasma levels of each of these three haemostatic factors and relates them to the risk of developing CVD.

Pathophysiology of plaque rupture and the concept of plaque stabilization

Atherosclerosis complicated by plaque rupture or disruption and thrombosis is primarily responsible for the potentially lethal acute coronary syndromes. Plaques with a large extracellular lipid-rich core, thin fibrous cap with reduced collagen content and smooth muscle density, and increased numbers of activated macrophages and mast cells appear to be most vulnerable to rupture. Plaque disruption tends to occur at points at which the plaque surface is weakest and most vulnerable, which coincide with points at which stresses, resulting from biomechanical and hemodynamic forces acting on plaques, are concentrated. Reduced matrix synthesis as well as increased matrix degradation may predispose fibrous caps to rupture spontaneously or in response to extrinsic mechanical or hemodynamic stresses. Modification of endothelial dysfunction and reduction of vulnerability to plaque rupture and thrombosis may lead to plaque stabilization resulting in reduction of the frequency of acute coronary syndromes. This putative concept of plaque stabilization, although attractive, has not yet been rigorously validated in humans. Indirect data from clinical trials involving lipid lowering/modification and lifestyle/risk factor modification, however, provide strong support for this new paradigm. Thus, plaque stabilization may prove to be an important modality for reduction of lethal consequences of coronary atherosclerosis.

Fibrinolytic abnormalities in two different cutaneous manifestations of venous disease

BACKGROUND

Chronic venous insufficiency may be associated with lipodermatosclerosis or atrophie blanche. Coagulation abnormalities may be related to these cutaneous disorders.

OBJECTIVE

Our purpose was to determine whether fibrinolytic abnormalities exist in patients with lipodermatosclerosis or atrophie blanche.

METHODS

A case control study of patients with venous disease and atrophie blanche or lipodermatosclerosis was performed. Plasma levels of tissue plasminogen activator (t-PA) and plasminogen activator inhibitor-1 (PAI-1) in a resting and venous occluded state were measured.

RESULTS

Plasma levels of PAI-1 were different between the two groups of patients. The lipodermatosclerosis group had significantly higher levels of PAI-1 in both the resting and venous occluded states (p < 0.001). Patients with atrophie blanche had milder elevations of PAI-1 in the resting and venous occluded state (p = 0.06).

CONCLUSION

Fibrinolytic abnormalities are present in patients with venous disease. These abnormalities are different between patients with lipodermatosclerosis and patients with atrophie blanche.

Levels of factor VIIc associated with decreased tissue factor pathway inhibitor and increased plasminogen activator inhibitor-1 in dyslipidemias

Tissue factor pathway inhibitor (TFPI), a kunitztype inhibitor of the extrinsic coagulation pathway, factor VII coagulant (FVIIc), FVIIa, and the fibrinolytic factors plasminogen activator inhibitor-1 (PA1-1) and tissue plasminogen activator (TPA) have been studied in various hyperlipidemias. Compared with a normal lipidic group, mean TFPI activity was 70% higher (P < .001) and 36% higher (P < .001) in type IIa and IIb hyperlipidemias, respectively, and was lower by 13% in type IV hyperlipidemia (P = .05). TFPI was correlated with LDL cholesterol (P < .001), total cholesterol (P < .001), HDL cholesterol (P < .01), apolipoproteins (apo) AI (P < .001) and B (P < .001) and lipoprotein a (P < .01). TFPI was negatively correlated with the triglyceride level (P < .05); the correlation was dependent on LDL cholesterol and HDL cholesterol levels, which were decreased in type IV hyperlipidemia. FVIIc activity (P < .001) was increased by 30% in both type IV and type IIb hyperlipidemia and was correlated with triglyceride levels. FVIIa was not significantly increased in any group compared with control group. FVIIc was correlated with triglyceride level (P < .001), while FVIIa was not. Interestingly, FVIIa was correlated with FVIIc (r = .5, P < .001) in the control group as well as in the hyperlipidemic groups (r = .32, P < .01). These results favor the hypothesis that higher FVIIc concentrations in hyperlipidemic patients are likely due to enhancement of synthesis of FVII and that a part of this FVII circulates in an activated chemical form. Compared with the control group, PAI-1 activity was twofold higher (P < .08) in type IIa hyperlipidemia, threefold higher (P < .001) in type IIb hyperlipidemia, and fourfold higher in type IV hyperlipidemia (P < .001). PAI-1 activity correlated with triglyceride levels (P < .001), apoB levels (P < .001) and total cholesterol levels (P < .05). These correlations were dependent on apoB and probably reflect the correlation between PAI-1 and VLDL. In contrast, TPA level was normal in the different hyperlipidemias. No correlation was found between TFPI, FVIIc, and PAI-1. Variation of TFPI activity appears to be related to the variations of its main lipoprotein carriers: LDL, HDL, and Lp (a). The association in hypertriglycemic patients of hypercoagulability (increased FVIIc and decreased TFPI) and hypofibrinolysis (increased PAI-1) may explain thrombosis predisposition of some of these patients. However, it would be interesting to study the increased levels of endothelium-derived TFPI in plasma induced by the injection of heparin.

Fibrin generation and digestion in patients with angina pectoris

Fibrin generation and lysis were studied in 28 patients with angina pectoris (14 with active disease and 14 with inactive disease) and in 14 normal controls. The fibrinolytic response was evaluated by comparing the ratio between the plasma levels of fibrinopeptide A and fibrin degradation products. Levels of both were higher in patients than in controls (P < 0.001), with higher levels in active than in inactive disease (P < 0.001). The fibrinopeptide A/fibrin degradation products ratio was much higher (P < 0.001) in the active group than in other groups. Thus, in patients with angina pectoris, especially in the active state, the increased thrombin generation is not paralleled by an equivalent increase in fibrinolytic activity.

Low proteins C and S and activation of fibrinolysis in treated essential thrombocythemia

The aim of this study was to investigate whether abnormalities in the fibrinolytic system and in the naturally occurring anticoagulant proteins could contribute to the thrombotic risk in essential thrombocythemia. Euglobulin lysis time, fibrin plate lysis area, tissue plasminogen activator antigen, and activity and plasminogen activator inhibitor antigen were measured before and after venous occlusion in a group of 16 patients with essential thrombocythemia and in 16 healthy age and sex matched controls. In addition, resting levels of antithrombin III, D-dimer, prothrombin fragment 1 + 2, and protein C and S were assessed. The results were related to the presence or absence of a thrombotic history. The results demonstrated that the patients had a significantly elevated fibrin plate lysis area and significantly decreased plasminogen activator antigen, both at baseline and after venous occlusion. They also had significantly decreased levels of plasma protein C and total protein S. There was a modest, non-significant elevation in the plasma concentration of D-Dimer and F 1 + 2. Those patients with a history of thrombosis had significantly lower protein C levels compared with individuals without a thrombotic history. We conclude that patients with essential thrombocythemia have evidence of activated fibrinolysis in the resting state and after stimulation. This, and the decreased levels of protein C and total protein S, may be secondary to chronic clinically occult thrombosis occurring in myeloproliferative disorders.

Pathophysiology of fibrinolysis

The observed association between abnormal fibrinolysis and a tendency toward bleeding or thrombosis demonstrates the (patho)physiological importance of the fibrinolytic system. Deficient fibrinolysis represents an important haemostatic abnormality associated with thrombosis. A decreased fibrinolytic activity may be due to a defective synthesis and/or release of tissue-type plasminogen activator (t-PA) from the vessel wall, to a deficiency or functional defect in the plasminogen or fibrinogen molecule or more frequently to increased levels of t-PA inhibitor. Alternatively, excessive fibrinolysis due to increased levels of t-PA, to alpha 2-antiplasmin deficiency or to plasminogen activator inhibitor-1 (PAI-1) deficiency, may result in bleeding tendency.

Association between increased estrogen status and increased fibrinolytic potential in the Framingham Offspring Study

BACKGROUND

Although extensive evidence indicates that estrogen is responsible for the markedly decreased cardiovascular risk of premenopausal women, the mechanism through which estrogen might exert its protective effect has not been adequately explained. Since thrombosis is now recognized to play an important role in the onset of cardiovascular disease, we investigated the relation between estrogen status and fibrinolytic potential, a determinant of thrombotic risk.

METHODS AND RESULTS

We determined levels of plasminogen activator inhibitor (PAI-1) antigen and tissue plasminogen activator (TPA) antigen in 1431 subjects from the Framingham Offspring Study. Fibrinolytic potential was compared between subjects with high estrogen status (premenopausal women and postmenopausal women receiving hormone replacement therapy) and low estrogen status (men and postmenopausal women not receiving hormone replacement therapy). In all comparisons, subjects with high estrogen status had greater fibrinolytic potential (lower PAI-1 levels) than subjects with low estrogen status. First, postmenopausal women receiving estrogen replacement therapy had lower levels of PAI-1 than those not receiving therapy (13.0 +/- 0.5 versus 19.5 +/- 1.0 ng/mL, P < .001). Second, premenopausal women had lower levels of PAI-1 than men of a similar age (14.8 +/- 0.6 versus 20.3 +/- 0.8 ng/mL, P < .001); this sex difference diminished when postmenopausal women not receiving hormone replacement therapy were compared with men of a similar age (19.6 +/- 0.7 versus 21.1 +/- 0.7 ng/mL, P = .089). Third, premenopausal women had markedly lower levels of PAI-1 antigen than postmenopausal women not receiving estrogen therapy (14.8 +/- 0.6 versus 19.5 +/- 1.0 ng/mL, P < .001). The between-group differences observed for TPA antigen were similar to those for PAI-1 antigen.

CONCLUSIONS

Each of these comparisons indicates that the cardioprotective effect of estrogen may be mediated, in part, by an increase in fibrinolytic potential. These findings might provide at least a partial explanation for the protection against cardiovascular disease experienced by premenopausal women, and the loss of that protection following menopause.

Thrombolysis and reocclusion in experimental jugular vein and coronary artery thrombosis. Effects of a plasminogen activator inhibitor type 1-neutralizing monoclonal antibody

BACKGROUND

Thrombolytic therapy for acute myocardial infarction is often complicated by reocclusion of the initially reperfused artery. Platelets have been shown to play an important role in this process. We determined the contribution of plasminogen activator inhibitor type 1 (PAI-1), stored in the alpha-granules of platelets, to thrombolysis resistance and to reocclusion.

METHODS AND RESULTS

In a rabbit jugular vein thrombosis model, the effect of a PAI-1-neutralizing monoclonal antibody (CLB-2C8) on thrombolysis and thrombus growth was assessed. The effect on reperfusion, reocclusion, and duration of vessel patency was studied in a canine model of coronary artery thrombosis superimposed on a high-grade stenosis and endothelial damage. In the rabbit jugular vein model, the intravenous administration of 1 mg/kg anti-PAI-1 antibody significantly enhanced the endogenous thrombolysis from 5.5 +/- 1.3% in the animals treated with a nonspecific monoclonal antibody (control) to 13.7 +/- 2.6% in the animals treated with the anti-PAI-1 antibody. Thrombus growth was reduced significantly, from 41.3 +/- 2.6% in the control animals to 22.8 +/- 2.8% in the animals treated with the anti-PAI-1 antibody. In combination with a single bolus injection of recombinant tissue-type plasminogen activator (rTPA; 0.25 mg/kg), the anti-PAI-1 antibody reduced thrombus growth significantly, from 21.5 +/- 2.7% in the animals treated with rTPA alone to 12.2 +/- 2.6% in the animals treated with rTPA and the antibody. No additional effect of the anti-PAI-1 antibody was observed on rTPA-induced thrombolysis. In the canine coronary artery thrombosis model, the administration of a suboptimal dose of rTPA (0.45 mg/kg) induced reperfusion in 7 of the 8 dogs after 19.5 +/- 8.2 minutes. Reperfusion was followed by reocclusion in all animals after 3.3 +/- 2.6 minutes. Administration of the anti-PAI-1 antibody in combination with rTPA significantly reduced time to reperfusion (8.1 +/- 5.2 minutes) and delayed the occurrence of reocclusion to 11.6 +/- 12.5 minutes.

CONCLUSIONS

Administration of the anti-PAI-1 antibody (CLB-2C8) results in increased endogenous thrombolysis and inhibition of thrombus growth in a venous thrombosis model in rabbits and facilitated reperfusion and reduction of reocclusion in a canine model of coronary artery thrombosis.

Fibrinolysis and thrombosis

In conclusion, venous and arterial thrombi contain significant amounts of fibrin. There is evidence that defective fibrinolytic mechanisms may participate in their genesis or persistence in the body. Some evidence suggests that defective fibrinolysis may presage and predict both arterial and venous thrombi; this is stronger evidence of a causal link with thrombosis. Thrombi protect themselves from lysis by incorporating inhibitors (plasminogen activator inhibitor-1 and alpha 2-antiplasmin) in their structure. Fibrinolytic mechanisms may play a role in the genesis or development of atherosclerotic lesions, in addition to thrombosis. Thrombolytic agents lyse both arterial and venous thrombi when administered shortly after their development. If it proves possible in future to enhance natural fibrinolytic mechanisms and to sustain this enhancement by pharmacological or other means, the interesting prospect of harnessing the fibrinolytic system to prevent thrombus formation, as well as to treat it, will arise.

Effects of contraceptive steroids on cardiovascular risk factors in women with insulin-dependent diabetes mellitus

OBJECTIVE

We evaluated established cardiovascular risk factors within lipoprotein metabolism, hemostasis, and endothelial function in women with insulin-dependent diabetes mellitus who were using oral contraceptives.

STUDY DESIGN

Twenty-five women with uncomplicated insulin-dependent diabetes mellitus, allocated to treatment with a monophasic combination of 30 micrograms ethinyl estradiol and 75 micrograms gestodene (treatment group, n = 12) or with nonhormonal contraception (control group, n = 13), were prospectively followed up for 12 months. Nonparametric methods were used for statistical evaluation.

RESULTS

No statistical differences in the biochemical risk markers were noted between the two groups at the start of the study. In the treatment group serum levels of low-density lipoprotein cholesterol decreased, whereas the concentrations of total cholesterol, high-density lipoprotein cholesterol, very-low-density lipoprotein cholesterol, and triglycerides were unchanged. Within the coagulation system factor VII coagulant activity increased, while fibrinogen levels were unchanged. In the fibrinolytic system we found unchanged activities but decreased antigen concentrations of tissue plasminogen activator and plasminogen activator inhibitor. The concentration of von Willebrand factor increased, but no change in albumin excretion rates were found. In the control group no changes in any of the variables were observed.

CONCLUSION

Intake of modern oral contraceptives does not deteriorate the cardiovascular risk profile in women with insulin-dependent diabetes mellitus, but our study indicates a risk of disturbances of the endothelial integrity, which needs further investigation.

Toward the quiescent coronary plaque

The treatment of coronary atherosclerosis requires an understanding of the pathophysiology of plaque rupture. The rupture of lipid-laden, macrophage-rich plaques initiates unstable angina, acute myocardial infarction and sudden cardiac death. Plaque rupture occurs when the circumferential tension on a plaque exceeds its tensile strength, an event that cannot be predicted by coronary angiography. The incidence of plaque rupture appears to be reduced in patients receiving cholesterol-lowering therapy, beta-adrenergic blocking agents and, possibly, angiotensin-converting enzyme inhibitors and antioxidants. Not all ruptured coronary plaques produce an acute coronary syndrome. The consequences of plaque rupture depend on the extent of thrombus formation over the fissured plaque. This is determined by flow characteristics within the vessel as well as the activity of the thrombotic and fibrinolytic systems. Recent advances in cardiovascular molecular biology, coronary diagnostic techniques and cardiac therapeutics have opened windows of opportunity to study and modify the factors leading to plaque rupture. The local modification of gene expression to alter plaque composition and to elucidate and subsequently inhibit the prothrombotic and fibrinolytic defects that promote coronary thrombosis may, in future, prevent plaque rupture and its consequences. The application of such a concerted interdisciplinary approach promises a paradigm shift in the management of coronary artery disease toward the prevention of plaque rupture and its sequelae.

Lipoprotein(a): a kinetic study of its influence on fibrin-dependent plasminogen activation by prourokinase or tissue plasminogen activator

Lipoprotein(a) [Lp(a)] has been postulated to inhibit fibrinolysis due to its structural homology to plasminogen. Indeed, it has been reported that Lp(a) competitively inhibits the promotion by fibrin of tissue plasminogen activator (t-PA)-catalyzed plasminogen activation. However, it has also been reported that this inhibition is uncompetitive. No studies have been published, to our knowledge, of the effect of Lp(a) on prourokinase (pro-UK)-catalyzed plasminogen activation. Plasminogen activation by pro-UK or a plasmin-resistant mutant pro-UK was previously shown to be promoted by fibrin fragment E2, whereas that by t-PA is promoted by fragment D. Therefore, the influence of Lp(a) on the kinetics of these two reactions was examined. When Lp(a) was added (90-600 nM), no change in the rate of plasmin generation by Ala158-pro-UK was observed. Consistent with this, immobilized Lp(a) also failed to bind to fragment E2, whereas it did bind to D dimer. When t-PA-catalyzed plasminogen activation in the presence of D dimer was measured, uncompetitive inhibition by Lp(a) was found, but only at low concentrations of D dimer (< 0.5 microM) or t-PA (0.05 nM). At higher concentrations of D dimer and t-PA, instead of inhibition, Lp(a) induced a 2.4-fold promotion of plasminogen activation. Similarly, Lp(a) enhanced (up to 2.5-fold) plasminogen binding to immobilized fibrin in both buffer and plasma milieus at the physiological concentration of plasminogen (2.0 microM). In conclusion, Lp(a) had no effect on plasminogen activation by pro-UK and induced only limited inhibition of activation by t-PA.(ABSTRACT TRUNCATED AT 250 WORDS)