Clot lysis: role of plasminogen activator inhibitors in haemostasis and therapy

An unimpeded circulation of blood depends on the concerted activation of coagulation and fibrinolytic factors. The latter entails the controlled, localised conversion of plasma zymogen plasminogen to the active enzyme plasmin mediated by tissue-type plasminogen activator (tPA). Bulk of tPA activity is in the proximity of the endogenous plasminogen activator inhibitor (PAI) as an active complex. The advent of molecular biology techniques has enabled isolation of cDNA for the inhibitors PAI-1, PAI-2 and PAI-3 and data indicate that these belong to the serine protease inhibitor (Serpine) family with arginine as its active site but immunologically distinct from each other. Enhanced tPA or PAI-1 forms one of the risk factors related to cardiac diseases and thrombotic disorders. A line of therapy entails lowering of PAIs with concomitant increase in tPA levels leading to net enhancement in fibrinolytic activity. In as much as plasminogen activators exert their action extracellularly, they are accessible to inhibitors and therefore PAIs could have a therapeutic potential and serve as prognostic indicators in cancer. Documented findings related to the biochemical characteristics and therapeutic potential of PAIs are presented and discussed in the review.