The fibrinolytic enzyme system : new markers of potential interest in cardiovascular disease

Abstract

Impaired fibrinolytic function, mainly due to elevated plasma levels of PAI-1, is a common finding in patients with coronary heart disease. Determination or evaluation of the plasma PAI-1 concentrations are complicated, due to a natural variation in blood plasma or due to other reasons, such as a poor stability under physiological conditions.

A polymorphism is located 675 bp upstream of the transcriptional start site in the promoter region of the PAI-1 gene, consisting of a single guanosine (G) insertion/deletion variation, and resulting in two alleles containing either 4 or 5 G in a row. Individuals who are homozygous for the 4 G allele have increased PAI-1 concentrations in plasma. An increased risk for cardiovascular disease has also been hypothesized. We therefore elaborated and characterized a rapid and reliable allele specific PCR method to determine this polymorphism. The content of PAI-1 in platelets is quite high. Because of the rapid conversion of active PAI-1 to the latent form under physiological conditions and due to that the normal turnover of platelets in the circulation is 5-10 days, it would be expected that less than 1 % of PAI-1 in platelets would be active. We found that platelet PAI-1 in healthy individuals was about 10 % active, which is much higher than expected. The reason for this is not understood, but it may indeed explain why platelet rich thrombi are difficult to lyse by tPA.

The tPA activity constitutes a very small portion of tPA antigen in plasma. The major portion is inactive and constitutes complexes with different serpins (antiplasmin, Cl -inhibitor or PAI-1). A method to specifically measure the complex between tPA and PAI-1 in plasma samples was developed. It was found that the concentration of this complex correlated well with PAI-1 activity or with tPA antigen in plasma. However, no correlation was found between IPA activity and tPA antigen in the plasma samples. It was considered important to further evaluate measurement of this complex, especially in connection with myocardial infarction (MI).

In several prospective studies it has been found that elevated plasma PAI-1 or tPA antigen concentrations in patients with manifest coronary heart disease are linked to future cardiovascular events such as MI. Distribution, correlation and interaction of plasma concentrations of tPA/PAI-1 complex were analyzed in 886 MI patients and 1198 matched control individuals in the Stockholm Heart Epidemiological Program (SHEEP). Analysis of coagulation, other fibrinolytic, lipoprotein and inflammatory compounds were also performed. The data clearly demonstrated that the plasma concentration of tPA/PAI-1 complex was significantly associated with the risk of MI, in both genders. Synergistic interactions were observed in men for the co-exposure to high plasma concentrations of tPA/PAI-1 complex in combination with smoking (OR=4.6) or with diabetes mellitus (OR=7.9).

Using a cohort within the SHEEP material, constituting the control individuals in this study, the tPA/PAI-1 complex was prospectively evaluated as a predictor of MI. During a follow-up period of 6-8 years, 58 out of the 1611 controls had suffered a first myocardial infarction. High plasma concentrations of tPA/PAI-1 complex or of PAI-1 significantly predicted a forthcoming MI. In conclusion, an impaired fibrinolytic function seems to be important in cardiovascular disease.