- Email: [email protected]
Fibrinolytic capacity and risk of myocardial infarction
Fibrinolysis (1996) 10 Suppl 2, 9-12 Pearson Professional Ltd 1996
Fibrinolytic capacity and risk of myocardial infarction J.G. van der Born 1'2, M.L. Bots ~, F. Haverkate 2, D.E. Grobbee 1, C. Kluft 2 'Department of Epidemiology & Biostatistics, and the Netherlands Institute for Health Sciences, Erasmus University Medical School, Rotterdam, The Netherlands. 2Gaubius Laboratory,TNO-PG, Leiden, The Netherlands.
INTRODUCTION The clinical acceptance of thrombolytic treatment in evolving transmural myocardial infarction, improved image quality on coronary angiography, and the introduction of angioscopy have contributed to accept the significance of thrombosis in the pathogenesis of myocardial infarction, sudden death, and unstable angina pectoris. Simultaneously, hypercoagulability and impaired fibrinolytic function have been suggested to predispose to arterial thrombosis by promoting the formation of occlusive thrombi on fissured atherosclerotic plaques. In view of the role of thrombosis in the acute coronary syndrome the question arises whether an impaired fibrinolytic capacity could be identified as a risk factor in individuals. The fibrinolytic capacity of an individual is here defined by the ability to react to fibrin formation. It may play a role in the prevention of the development of an occluding thrombus in the coronary arteries. Several studies aimed at examining the association between fibrinolytic parameters and cardiovascular disease. We discuss the evidence available from longitudinal studies that have been performed to assess the association between fibrinolytic capacity and the risk of myocardial infarction and address some of the limitations and implications.
Longitudinal studies The studies aiming at examination of an association between fibrinolytic capacity and cardiovascular disease have focused on blood fibrinolytic status and indicate, of fibrin turn-over. Longitudinal studies have several advantages over those with a cross-sectional design. The determinant, fibrinolytic capacity, is measured prior to the event. Hence, fibrinolytic capacity is not yet influenced by Supported by a grant (92.398) from the Netherlands Heart Foundation.
Correspondence to: Johanna G. van der Bom, Department of Epidemiology & Biostatistics, Erasmus University Medical School, P.O. Box 1738, 3000 DR Rotterdam, The Netherlands. Tel +31-10-4087489; Fax +31-104365933. E-mail: [email protected].
the infarct. And, because non-fatal as well as fatal cases can be included into the study population, selection bias is minimized. Two types of observational longitudinal studies have been performed: the cohort study and the nested case control study. In a cohort study fibrinolytic capacity is measured in all subjects at baseline, and during a defined follow-up period the number of cases of myocardial infarction according to the different levels of fibrinolytic capacity are compared. In a nested case control study, blood samples from all members of a cohort are collected and stored. Subsequently, measurement of fibrinolytic capacity is limited to the cases of myocardial infarction that occur during the follow-up period and a sample of subjects that do not develop a myocardial infarction, the control subjects. In both studies the measure of association is essentially the same. The risk of myocardial infarction for different levels of fibrinolytic capacity is obtained, and relative risks can be estimated. In Table 1 the longitudinal studies published so far, in which the association between fibrinolytic capacity and the risk of myocardial infarction was studied are presented. Fibrinolytic capacity was approached by measurement of plasma levels or activity of fibrinolytic factors. Results from these studies suggested an association between plasma concentrations of fibrinolytic parameters and risk of myocardial infarction. In the Northwick Park Heart Study, a study in healthy men, an overall estimate of the blood fibrinolytic capacity was measured as lysis of clots from diluted blood.~ Low activity was associated with an increased risk of myocardial infarction. Further development of laboratory techniques allowed in later studies to study plasma levels of more specific components of the fibrinolytic system, such as tissue-type plasminogen activator (t-PA) and its inhibitor type 1 (PAl-l). A depression of active t-PA in patients with a history of myocardial infarction was associated with an increased risk of second myocardial infarction. 2-4 Increased concentrations of the antigen of t-PA were associated with an increased risk of myocardial infarction in subjects with
10
Van der Born et al
Table 1 Overview of the longitudinal studies on the association between fibrinolytic capacity and myocardial infarction. Study Hamsten, 19878
Design cohort
study population 109 MI survivors aged 45 years or less 16 patients had at least one reinfarction
follow-up 3 years
Gram, 19872
cohort
29 cases of myocardial infarction 9 patients had a reinfarction
4 years
Munkvad, 1990'
cohort
20 cases of angina pectoris 8 patients had a myocardial infarction
6 years
Jansson, 1993 '
cohort
7 years
Meade, 1993'
cohort
Ridker, 19937
case control cohort
213 patients with severe angina pectoris 28 deaths due to cardiovascular disease 1382 'healthy' men 179 cases of ischemic heart disease 22071 US 'healthy' male physicians 231 cases of myocardial infarction 3043 patients with angina pectoris 106 coronary events
Thompson, 19958
coronary artery disease z'3'5'6, and in apparently healthy subjects. ~ Furthermore, increased PAI-1 levels have been associated with an increased risk of myocardial infarction. 8 Risk indicator or also risk factor The studies discussed show that subjects with increased levels of t-PA antigen, increased levels of PAI-1 antigen and activity or decreased levels of t-PA activity tend to have an increased risk to develop a myocardial infarction. Thus, the fibrinolytic parameters are risk indicators of myocardial infarction. However, adjustment for the established cardiovascular disease risk factors, such as smoking, body mass index, serum cholesterol and blood pressure, markedly decreases the strength of the association. 6'7 This may reflect confounding. The established risk factor, b y definition associated with cardiovascular disease, may also be associated with fibrinolytic parameters. Associations of t-PA and PAI-1 levels with cardiovascular disease risk factors have been described. 9 This may induce an artificial association between the disease and the fibrinolytic parameter, e.g. induce confounding, that disappears after adjustment for the established cardiovascular disease risk factor. It has been suggested that the elevation of PAl- 1 and t-PA antigen and the reduction of active t-PA are biologically related, and that a single metabolic change reflected in different analytes is observed. ~° Also, an established cardiovascular risk factor and the fibrinolytic parameter may both be part of the same causal chain. Fibrinolysis (1996) 10 Suppl 2, 9-12
16.1 years 5 years 2 years
determinant PAl-1 ag PA capacity (fibrin plate method) t-PA act (euglobulin fraction) t-PA ag PAl-1 ag plasminogen t-PA act (euglobulin fraction) t-PA ag PAl-1 ag PAl act t-PA ag PAl act
risk 1" 4, 4, 1" 1" ns -4, 1" 1" ns 1" ns 1" --
dilute clot lysis time
4,
t-PA antigen PAl-1 antigen t-PA antigen t-PA activity Euglobulin clot lysis time PAl-1 antigen PAl activity plasminogen
1" = 1` = = = = =
An additional problem is that not only the established cardiovascular disease risk factors, but also preclinical atherosclerosis seems to be associated with the fibrinolytic parameters. T M Cardiovascular disease risk factors cause atherosclerosis and in turn atherosclerosis may activate the fibrinolytic system. Consequently, adjustment for other cardiovascular disease risk factors will reduce the strength of the association between fibrinolytic activity and myocardial infarction. Finally, the association between a particular cardiovascular disease risk factor and myocardial infarction may be of such magnitude that inclusion of that risk factor into the model blurs the weaker associations between other determinants (fibrinolytic activity) and disease. Fibrinolytic c a p a c i t y The question addressed in the studies presented was, whether a reduced fibrinolytic capacity increases the risk of a myocardial infarction. In all studies blood variables of fibrinolysis were used to estimate fibrinolytic capacity. Whether these variables reflect local fibrinolytic capacity is unclear. Fibrinolysis is a process that is localized at sites of fibrin formation. During clot formation availability of t-PA is essential to reduce the size of a thrombus or even prevent its formation. When coagulation is stimulated, t-PA is acutely released from healthy endothelium 13 and exerts an anticoagulant function, t-PA incorporated in a thrombus is 100-1000 times more effective than t-PA provided from © Pearson Professional Ltd 1996
Fibrinolytic capacity and risk of myocardial infarction
outside the thrombus i.e., in blood 14. Furthermore, non healthy endothelium, an atherosclerotic plaque, may have a locally disturbed fibrinolytic capacity of which the effect may be too small to be reflected in peripheral plasma concentration. Inhibition of the fibrinolytic process by PAI-1 is also locally regulated. Next to plasma, PAI-1 is present in platelets. In thrombi platelets accumulate and the contribution of the platelet PAI-I, although largely in an inactive form, probably overrules that of plasma PAI-1.15 Circulating t-PA may, however, play a role in lysis of a thrombus as well. Firstly, for a thrombus located at the site of a compromised vascular wall the only source of t-PA may be that provided by the circulation• Secondly, the mechanism of lysis involves local accumulation of circulating plasminogen activators putting the low circulating levels into another perspective. The study ECAPTURE currently explores this concept for plasma urokinase-type plasminogen activator and tests whether moderate elevation of plasma levels by continuous infusion is preventive for thrombosis. ~7 Besides t-PA and PAI-1, also other factors may play a role in reduction of fibrinolytic activity. Factor XIII and blood platelets are also important determinants of the dilute blood clot lysis time. Fibrinolytic capacity is also reflected in end-products of fibrinolysis, such as fibrin degradation products. Plasma concentrations of these products are determined by two main factors: the degree of fibrin formation providing the amount of substrate for formation of degradation products and the degree of activity of the fibrinolytic system. Increased levels of D-dimer have been associated with an increased risk of myocardial infarction in patients with peripheral atherosclerosis 18 and in apparently healthy men. J9 These increased levels of D-dimer apparently contradict with the reduced blood fibrinolytic activity discussed above. Indeed increased fibrinolytic action must be assumed, but we have to interpret this in relation to the degree of fibrin formation. The fibrinolytic capacity may be insufficient and not provide a quantitative adequate response to coagulation. Lowe et al studied markers of both fibrin formation and degradation in patients with established cardiovascular risk factors and the findings suggested an imbalance in favour of coagulation. Similarly, in post-operative patients, reduced blood fibrinolytic activity, known as post-operative shut-down, and increased levels of fibrin degradation products are found. In the early post-operative phase also an imbalance in favour of coagulation is suggested by increases in thrombin related products (Fibrin peptide A) not associated with comparable increases in plasmin related products (fibrinopeptide BI3 15-42 and plasmin-antiplasmin complex). 21 •
•
20
© Pearson Professional Ltd 1996
16
11
Increased levels of fibrin degradation products in patients with atherosclerosis suggest that the pathogenetic process is associated with increased fibrin formation and turn-over. It is suspected that fibrinolysis may have a sub-normal counteracting activity and levels of D-dimer should have been higher. Further analysis of the balance between coagulation and fibrinolysis is indicated. An interesting new variable that provides a message about this balance is circulating or soluble fibrin. 22
The prospect of genetic analyses The assessment of genetic parameters has the advantage that intersubject variability can be measured without disturbance by (preclinical) atherosclerosis or cardiovascular disease risk factors. Recent studies on the association between genetic polymorphism in the genes coding for the fibrinolytic factors and the risk of myocardial infarction provided new insights. For PAI-1 a polymorphism in the promotor region has been identified and associated with risk of myocardial infarction. 23 Similarly, a polymorphism in the gene for t-PA was associated with cardiovascular risk. 24 The polymorphism in the PAI-1 gene has been associated with plasma PAI-1 concentrations and an association between the allele coding for higher PAI-1 concentrations and an increased risk of myocardial infarction has been described. For the t-PA polymorphism such a relationship has not yet been established. It was not associated with t-PA production in cell culture. 25 Studies on the association between a polymorphism and disease present a powerful tool of identification of risk factors playing a causal role. Before being able to definitely conclude about the identity of the factor it should be verified by detailed studies with multiple markers in the gene, that we are dealing with the right gene and that the studied polymorphism is not a marker for an adjacent gene. Even when a plausible association as for PAI-1 is found, such certainty need to be obtained. The pathophysiological mechanism of the association may be more difficult to reveal. The polymorphism may be associated with changes in basal blood concentrations and/ or with changes in induction pattern following stimuli and/or with local functioning in tissue remodelling. Which of these aspects or other unknown ones are essential is difficult to decide from the genetic population data only. To further identify the mechanisms involved we might learn from rare genetic defects in specific functions of a molecule or create such specific defects in transgenic animals. From structure function analysis of the protein and the promotor region abnormalities in function and regulation can be recognised and tested for relevance•
Fibrinolysis (1996) 10 Suppl 2, 9-12
12
Van der Bom et al
CONCLUSIONS From the longitudinal studies that have been performed so far, it may be concluded that increased plasma levels o f t-PA and PAI-1 are indicators o f increased risk o f myocardial infarction. Illuminating whether these indicators are solely a marker o f presence o f atherosclerosis or whether they also have a causal role in the atherosclerotic process and in the acute myocardial infarction is a challenge for future research. Other measures potentially associated with fibrinolytic capacity, such as genetic markers, may help to answer the question.
REFERENCES 1. Meade TW, Ruddock V, Stirling Y, Chakrabarti R, Miller GJ. Fibrinolytic activity, clotting factors, and long-term incidence of ischemic heart disease in the Northwick Park Heart Study. Lancet 1993;342:1076-79. 2. Gram J, Jespersen J. A selective depression of tissue plasminogen activator (t-PA) activity in euglobulin characterizes a risk group among young survivors of acute myocardial infarction. Thromb Haemostas 1987;57:137-9. 3. Gram J, Jespersen J, Kluft C, Rijken DC. On the usefulness of fibrinolysis variables in the characterization of a risk group for myocardial reinfarction. Acta Med Scand 1987;221:149-53. 4. Munkvad S, Gram J, Jespersen J. A depression of active tissue plasminogen activator in plasma characterizes patients with unstable angina pectoris who develop myocardial infarction. Eur Heart J 1990;6:525-8. 5. Jansson JH, Olofsson BO, Nilsson TK. Predictive value of tissue plasminogen activator mass concentration on long-term mortality in patients with coronary artery disease. Circulation
1993;88:2030-4. 6. Thompson SG, Kienast J, Pyke SDM, Haverkate F, van de Loo JCW for the European Concerted Action on Thrombosis and Disabilities Angina Pectoris Study Group. Hemostatic factors and risk of myocardial infarction or sudden death in patients with angina pectoris. N Engl J Med 1995;332:635-41. 7. Ridker PM, Vaughan DE, Stampfer MJ, Manson JE, Hennekens CH. Endogenous tissue-type plasminogen activator and risk of myocardial infarction. Lancet 1993;341:1165-8. 8. Hamsten A, de Faire U, Walldius G, Dahlen G, Szamosi A, Landou C, Blomback M, Wiman B. Plasminogen Activator Inhibitor in plasma: risk factor for recurrent myocardial infarction. Lancet 1987;4:3-9. 9. Geppert A, Backmann R, Graf S, Hornykeywycz S, Lang I, Schuster E, Binder BR, Huber K. Tissue type plasminogen activator and type 1 plasminogen activator inhibitor inpatients with coronary artery disease - relations to clinical variables and cardiovascular disease. Fibrinolysis 1995;9 (Suppl 2): 109-13. 10. Kluft C. Constitutive synthesis of tissue-type plasminogen activator (t-PA) and plasminogen activator inhibitor type 1 (PAl-l): Conditions and therapeutic targets. Fibrinolysis 1994; 8, suppl 2: 1-7.
Fibrinolysis (1996) 10 Suppl 2, 9-12
11. Salomaa V, Stinson V, Kark JD, Folsom AR, Davis CE, Wu • KK. Associations offibrinolytic parameters with early atherosclerosis. Circulation 1995;91:284-90. 12. Smith FB, Lee AJ, Rumley A, Fowkes FGR, Lowe GDO. Tissue-plasminogen activator, plasminogen activator inhibitor and risk of peripheral arterial disease. 1995; I 15:35-43. 13. Emeis JJ. Mechanisms involved in short-term changes in blood levels of t-PA. In: Kluft C, ed. Tissue-type plasminogen activator (t-PA): physiological and clinical aspects. Florida: CRC-Press, 1988:21-35 14. Brommer EJP. The level of extrinsic plasminogen activator (tPA) during clotting as a determinant of the rate of fibrinolysis; inefficiency of activators added afterwards. Thromb Res 1984; 34: 109-115. 15. Potter van Loon B J, Rijken DC, Brommer EJP, van der Maas APC. The amount of plasminogen, tissue-type plasminogen activator and plasminogen activator inhibitor type I in human thrombi and the relation to ex-vivo. Thromb Haemostas 1992; 67:101-105 16. Sakharov DV, Nagelkerke JF, Rijken DC. Rearrangement of the fibrin network and spatial distribution of fibrinolytic components during plasma clot lysis. J Biol Chem 1996; 271: 2133-2138 17. Dooijewaard G, Kluft C. Stimulation of indigenous fibrinolysis: a novel approach to prevention of thrombosis. In: P. Glas-Greenwalt ed. Fibrinolysis in disease: the malignant process, interventions in thrombogenic mechanisms and novel treatment modalities. New York: CRC-Press, 1995:233-236 18. Fowkes FG, Lowe GD, Housley E, Rattray A, Rumley A, Elton RA, MacGregor IR, Dawes J. Cross-linked fibrin degradation products, progression of peripheral arterial disease and risk of coronary heart disease. Lancet
1993;342:84-6. 19. Ridker PM, Hennekens CH, Cerskus A, Stampfer MJ. Plasma concentration of cross-linked fibrin degradation product (Ddimer) and the risk of future myocardial infarction among apparently healthy men. Circulation 1994;90"2236-40. 20. Lowe GDO, Wood DA, Douglas JT et al. Relationships of plasma viscosity, coagulation and fibrinolysis to coronary risk factors and angina. Thromb Haemostat 1991; 65:339-343 21. Kluft C. Fibrinolytic shut-down after surgery. In: Fibrinolysis and the central nervous system. Sawaya R., ed. Hanley and Belfus, inc. Philadelphia, 1990: 127-140. 22. Nieuwenhuizen W. Soluble fibrin as a molecular marker for a pre-thrombotic state: a mini-review. Blood Coagul Fibrinol. 1993; 4: 93-96. 23. Eriksson P, Kallin B, van't Hooft FM, Baveholm P, Hamsten A. Allelle-specific increase in basal transcription of the plasminogen-activator inhibitor 1 gene is associated with myocardial infarction. Proc Natl Acad Sci USA 1995;92:1851-55. 24. van der Bom JG, de KnijffP, Bots ML, Haverkate F, Hofman A, Kluft C, Grobbee DE. Polymorphism of the blood clot lysis enzyme tissue type plasminogen activator is associated with myocardial infarction. Circulation 1995;92(Suppl 2):I30. 25. van den Eijnden-Schrauwen Y, Lakenberg N, Emeis JJ, de Knijff P. Alu-repeat polymorphism in the tissue-type plasminogen activator gene does not affect basal endothelial tPA synthesis. (Letter) Thromb Haemostas 1995;74:1202. © Pearson Professional Ltd 1996
Fibrinolytic capacity and risk of myocardial infarction J.G. van der Born 1'2, M.L. Bots ~, F. Haverkate 2, D.E. Grobbee 1, C. Kluft 2 'Department of Epidemiology & Biostatistics, and the Netherlands Institute for Health Sciences, Erasmus University Medical School, Rotterdam, The Netherlands. 2Gaubius Laboratory,TNO-PG, Leiden, The Netherlands.
INTRODUCTION The clinical acceptance of thrombolytic treatment in evolving transmural myocardial infarction, improved image quality on coronary angiography, and the introduction of angioscopy have contributed to accept the significance of thrombosis in the pathogenesis of myocardial infarction, sudden death, and unstable angina pectoris. Simultaneously, hypercoagulability and impaired fibrinolytic function have been suggested to predispose to arterial thrombosis by promoting the formation of occlusive thrombi on fissured atherosclerotic plaques. In view of the role of thrombosis in the acute coronary syndrome the question arises whether an impaired fibrinolytic capacity could be identified as a risk factor in individuals. The fibrinolytic capacity of an individual is here defined by the ability to react to fibrin formation. It may play a role in the prevention of the development of an occluding thrombus in the coronary arteries. Several studies aimed at examining the association between fibrinolytic parameters and cardiovascular disease. We discuss the evidence available from longitudinal studies that have been performed to assess the association between fibrinolytic capacity and the risk of myocardial infarction and address some of the limitations and implications.
Longitudinal studies The studies aiming at examination of an association between fibrinolytic capacity and cardiovascular disease have focused on blood fibrinolytic status and indicate, of fibrin turn-over. Longitudinal studies have several advantages over those with a cross-sectional design. The determinant, fibrinolytic capacity, is measured prior to the event. Hence, fibrinolytic capacity is not yet influenced by Supported by a grant (92.398) from the Netherlands Heart Foundation.
Correspondence to: Johanna G. van der Bom, Department of Epidemiology & Biostatistics, Erasmus University Medical School, P.O. Box 1738, 3000 DR Rotterdam, The Netherlands. Tel +31-10-4087489; Fax +31-104365933. E-mail: [email protected].
the infarct. And, because non-fatal as well as fatal cases can be included into the study population, selection bias is minimized. Two types of observational longitudinal studies have been performed: the cohort study and the nested case control study. In a cohort study fibrinolytic capacity is measured in all subjects at baseline, and during a defined follow-up period the number of cases of myocardial infarction according to the different levels of fibrinolytic capacity are compared. In a nested case control study, blood samples from all members of a cohort are collected and stored. Subsequently, measurement of fibrinolytic capacity is limited to the cases of myocardial infarction that occur during the follow-up period and a sample of subjects that do not develop a myocardial infarction, the control subjects. In both studies the measure of association is essentially the same. The risk of myocardial infarction for different levels of fibrinolytic capacity is obtained, and relative risks can be estimated. In Table 1 the longitudinal studies published so far, in which the association between fibrinolytic capacity and the risk of myocardial infarction was studied are presented. Fibrinolytic capacity was approached by measurement of plasma levels or activity of fibrinolytic factors. Results from these studies suggested an association between plasma concentrations of fibrinolytic parameters and risk of myocardial infarction. In the Northwick Park Heart Study, a study in healthy men, an overall estimate of the blood fibrinolytic capacity was measured as lysis of clots from diluted blood.~ Low activity was associated with an increased risk of myocardial infarction. Further development of laboratory techniques allowed in later studies to study plasma levels of more specific components of the fibrinolytic system, such as tissue-type plasminogen activator (t-PA) and its inhibitor type 1 (PAl-l). A depression of active t-PA in patients with a history of myocardial infarction was associated with an increased risk of second myocardial infarction. 2-4 Increased concentrations of the antigen of t-PA were associated with an increased risk of myocardial infarction in subjects with
10
Van der Born et al
Table 1 Overview of the longitudinal studies on the association between fibrinolytic capacity and myocardial infarction. Study Hamsten, 19878
Design cohort
study population 109 MI survivors aged 45 years or less 16 patients had at least one reinfarction
follow-up 3 years
Gram, 19872
cohort
29 cases of myocardial infarction 9 patients had a reinfarction
4 years
Munkvad, 1990'
cohort
20 cases of angina pectoris 8 patients had a myocardial infarction
6 years
Jansson, 1993 '
cohort
7 years
Meade, 1993'
cohort
Ridker, 19937
case control cohort
213 patients with severe angina pectoris 28 deaths due to cardiovascular disease 1382 'healthy' men 179 cases of ischemic heart disease 22071 US 'healthy' male physicians 231 cases of myocardial infarction 3043 patients with angina pectoris 106 coronary events
Thompson, 19958
coronary artery disease z'3'5'6, and in apparently healthy subjects. ~ Furthermore, increased PAI-1 levels have been associated with an increased risk of myocardial infarction. 8 Risk indicator or also risk factor The studies discussed show that subjects with increased levels of t-PA antigen, increased levels of PAI-1 antigen and activity or decreased levels of t-PA activity tend to have an increased risk to develop a myocardial infarction. Thus, the fibrinolytic parameters are risk indicators of myocardial infarction. However, adjustment for the established cardiovascular disease risk factors, such as smoking, body mass index, serum cholesterol and blood pressure, markedly decreases the strength of the association. 6'7 This may reflect confounding. The established risk factor, b y definition associated with cardiovascular disease, may also be associated with fibrinolytic parameters. Associations of t-PA and PAI-1 levels with cardiovascular disease risk factors have been described. 9 This may induce an artificial association between the disease and the fibrinolytic parameter, e.g. induce confounding, that disappears after adjustment for the established cardiovascular disease risk factor. It has been suggested that the elevation of PAl- 1 and t-PA antigen and the reduction of active t-PA are biologically related, and that a single metabolic change reflected in different analytes is observed. ~° Also, an established cardiovascular risk factor and the fibrinolytic parameter may both be part of the same causal chain. Fibrinolysis (1996) 10 Suppl 2, 9-12
16.1 years 5 years 2 years
determinant PAl-1 ag PA capacity (fibrin plate method) t-PA act (euglobulin fraction) t-PA ag PAl-1 ag plasminogen t-PA act (euglobulin fraction) t-PA ag PAl-1 ag PAl act t-PA ag PAl act
risk 1" 4, 4, 1" 1" ns -4, 1" 1" ns 1" ns 1" --
dilute clot lysis time
4,
t-PA antigen PAl-1 antigen t-PA antigen t-PA activity Euglobulin clot lysis time PAl-1 antigen PAl activity plasminogen
1" = 1` = = = = =
An additional problem is that not only the established cardiovascular disease risk factors, but also preclinical atherosclerosis seems to be associated with the fibrinolytic parameters. T M Cardiovascular disease risk factors cause atherosclerosis and in turn atherosclerosis may activate the fibrinolytic system. Consequently, adjustment for other cardiovascular disease risk factors will reduce the strength of the association between fibrinolytic activity and myocardial infarction. Finally, the association between a particular cardiovascular disease risk factor and myocardial infarction may be of such magnitude that inclusion of that risk factor into the model blurs the weaker associations between other determinants (fibrinolytic activity) and disease. Fibrinolytic c a p a c i t y The question addressed in the studies presented was, whether a reduced fibrinolytic capacity increases the risk of a myocardial infarction. In all studies blood variables of fibrinolysis were used to estimate fibrinolytic capacity. Whether these variables reflect local fibrinolytic capacity is unclear. Fibrinolysis is a process that is localized at sites of fibrin formation. During clot formation availability of t-PA is essential to reduce the size of a thrombus or even prevent its formation. When coagulation is stimulated, t-PA is acutely released from healthy endothelium 13 and exerts an anticoagulant function, t-PA incorporated in a thrombus is 100-1000 times more effective than t-PA provided from © Pearson Professional Ltd 1996
Fibrinolytic capacity and risk of myocardial infarction
outside the thrombus i.e., in blood 14. Furthermore, non healthy endothelium, an atherosclerotic plaque, may have a locally disturbed fibrinolytic capacity of which the effect may be too small to be reflected in peripheral plasma concentration. Inhibition of the fibrinolytic process by PAI-1 is also locally regulated. Next to plasma, PAI-1 is present in platelets. In thrombi platelets accumulate and the contribution of the platelet PAI-I, although largely in an inactive form, probably overrules that of plasma PAI-1.15 Circulating t-PA may, however, play a role in lysis of a thrombus as well. Firstly, for a thrombus located at the site of a compromised vascular wall the only source of t-PA may be that provided by the circulation• Secondly, the mechanism of lysis involves local accumulation of circulating plasminogen activators putting the low circulating levels into another perspective. The study ECAPTURE currently explores this concept for plasma urokinase-type plasminogen activator and tests whether moderate elevation of plasma levels by continuous infusion is preventive for thrombosis. ~7 Besides t-PA and PAI-1, also other factors may play a role in reduction of fibrinolytic activity. Factor XIII and blood platelets are also important determinants of the dilute blood clot lysis time. Fibrinolytic capacity is also reflected in end-products of fibrinolysis, such as fibrin degradation products. Plasma concentrations of these products are determined by two main factors: the degree of fibrin formation providing the amount of substrate for formation of degradation products and the degree of activity of the fibrinolytic system. Increased levels of D-dimer have been associated with an increased risk of myocardial infarction in patients with peripheral atherosclerosis 18 and in apparently healthy men. J9 These increased levels of D-dimer apparently contradict with the reduced blood fibrinolytic activity discussed above. Indeed increased fibrinolytic action must be assumed, but we have to interpret this in relation to the degree of fibrin formation. The fibrinolytic capacity may be insufficient and not provide a quantitative adequate response to coagulation. Lowe et al studied markers of both fibrin formation and degradation in patients with established cardiovascular risk factors and the findings suggested an imbalance in favour of coagulation. Similarly, in post-operative patients, reduced blood fibrinolytic activity, known as post-operative shut-down, and increased levels of fibrin degradation products are found. In the early post-operative phase also an imbalance in favour of coagulation is suggested by increases in thrombin related products (Fibrin peptide A) not associated with comparable increases in plasmin related products (fibrinopeptide BI3 15-42 and plasmin-antiplasmin complex). 21 •
•
20
© Pearson Professional Ltd 1996
16
11
Increased levels of fibrin degradation products in patients with atherosclerosis suggest that the pathogenetic process is associated with increased fibrin formation and turn-over. It is suspected that fibrinolysis may have a sub-normal counteracting activity and levels of D-dimer should have been higher. Further analysis of the balance between coagulation and fibrinolysis is indicated. An interesting new variable that provides a message about this balance is circulating or soluble fibrin. 22
The prospect of genetic analyses The assessment of genetic parameters has the advantage that intersubject variability can be measured without disturbance by (preclinical) atherosclerosis or cardiovascular disease risk factors. Recent studies on the association between genetic polymorphism in the genes coding for the fibrinolytic factors and the risk of myocardial infarction provided new insights. For PAI-1 a polymorphism in the promotor region has been identified and associated with risk of myocardial infarction. 23 Similarly, a polymorphism in the gene for t-PA was associated with cardiovascular risk. 24 The polymorphism in the PAI-1 gene has been associated with plasma PAI-1 concentrations and an association between the allele coding for higher PAI-1 concentrations and an increased risk of myocardial infarction has been described. For the t-PA polymorphism such a relationship has not yet been established. It was not associated with t-PA production in cell culture. 25 Studies on the association between a polymorphism and disease present a powerful tool of identification of risk factors playing a causal role. Before being able to definitely conclude about the identity of the factor it should be verified by detailed studies with multiple markers in the gene, that we are dealing with the right gene and that the studied polymorphism is not a marker for an adjacent gene. Even when a plausible association as for PAI-1 is found, such certainty need to be obtained. The pathophysiological mechanism of the association may be more difficult to reveal. The polymorphism may be associated with changes in basal blood concentrations and/ or with changes in induction pattern following stimuli and/or with local functioning in tissue remodelling. Which of these aspects or other unknown ones are essential is difficult to decide from the genetic population data only. To further identify the mechanisms involved we might learn from rare genetic defects in specific functions of a molecule or create such specific defects in transgenic animals. From structure function analysis of the protein and the promotor region abnormalities in function and regulation can be recognised and tested for relevance•
Fibrinolysis (1996) 10 Suppl 2, 9-12
12
Van der Bom et al
CONCLUSIONS From the longitudinal studies that have been performed so far, it may be concluded that increased plasma levels o f t-PA and PAI-1 are indicators o f increased risk o f myocardial infarction. Illuminating whether these indicators are solely a marker o f presence o f atherosclerosis or whether they also have a causal role in the atherosclerotic process and in the acute myocardial infarction is a challenge for future research. Other measures potentially associated with fibrinolytic capacity, such as genetic markers, may help to answer the question.
REFERENCES 1. Meade TW, Ruddock V, Stirling Y, Chakrabarti R, Miller GJ. Fibrinolytic activity, clotting factors, and long-term incidence of ischemic heart disease in the Northwick Park Heart Study. Lancet 1993;342:1076-79. 2. Gram J, Jespersen J. A selective depression of tissue plasminogen activator (t-PA) activity in euglobulin characterizes a risk group among young survivors of acute myocardial infarction. Thromb Haemostas 1987;57:137-9. 3. Gram J, Jespersen J, Kluft C, Rijken DC. On the usefulness of fibrinolysis variables in the characterization of a risk group for myocardial reinfarction. Acta Med Scand 1987;221:149-53. 4. Munkvad S, Gram J, Jespersen J. A depression of active tissue plasminogen activator in plasma characterizes patients with unstable angina pectoris who develop myocardial infarction. Eur Heart J 1990;6:525-8. 5. Jansson JH, Olofsson BO, Nilsson TK. Predictive value of tissue plasminogen activator mass concentration on long-term mortality in patients with coronary artery disease. Circulation
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