STIMULATION OF FIBRINOLYSIS BY ACTIVATED PROTEIN C (APC)
Authors: de Four N J , Bertina R M , Havgrkate F 
Published: August 2018

In 1960 Mammen and Seegers reported the discovery of a new protein (autoprothrombin II-A, APC) with both anticoagulant and profibrinolytic activity. They found that APC accelerated clot lysis in vitro and proposed that this was due to a reduction of plasmin - inhibitory activity. Many years later Comp et al (J Clin Inv 68: 1221) reported that the infusion of APC into dogs resulted in an increase in circulating plasminogen activator activity. This observation stimulated more extensive studies of the profibrinolytic effects of APC.

In our laboratories we have studied the effect of human APC on clot lysis both in whole blood (human) and in a system of purified human proteins. In these systems 125I-labelled fibrinogen was incorporated in a clot formed after the addition of Jombin (complete clot formation within 5 min) and the subsequent lysis of this clot was followed by measuring the release of I-labelled fibrin degradation products (FDP) into the supernatant. Human t-PA was added to the system to achieve complete lysis of the clot within a few hours.

When APC was added to citrated whole blood before clot formation, it was found to accelerate clot lysis in a dose dependent way. This effeg| was specific for APC and dependent on an intact active site, on the presence of protein S (the protein cofactor of APC) and Ca . The presence of APC did not influence the composition of the FDP formed, as analysed by means of SDS-polyacry-1 amide gel electroforesis, and its effect was found to be independent of the presence or absence of a.-antiplasmin.

Subsequently we developped a clot lysis system using the purified human proteins of the fibrinolytic system: fibrinogen, FXIII, t-PA, PAI-1 (from human endothelial cells), glu-plasminogen and a -antiplasmin. In this system clot lysis was dependent on the concentrations of plasminogen, -antiplasmin, t-PA and PAI-1, but independent on the thrombin concentration and the presence or absence of phospholipids (purified from human brain). In the absence of PAI-1, no effect of APC on clot lysis was observed. However, in the presence of PAI-1, APC accelerated clot lysis. This effect was independent of the presence or absence of phospholipids and/or protein S and could be explained by the observation that APC can form a complex with PAI-1 (~ 95 kd) and under certain conditions even can convert active PAI-1 (~ 46 kd) into an inactive degradation product (~ 42 kd). However, complex formation is relatively slow anti high PAI-1 concentrations are needed to observe the reaction. The addition of protein S or phospholipids in the presence of Ca did not stimulate complex formation. Therefore, it seems highly unlikely that neutralization of PAI-1 by APC is responsible for the profibrinolytic effect of APC in the whole blood clot lysis.

A completely different explanation for the profibrinolytic effect of APC was suggested by the observation that the addition of blood-platelets to the system of purified fibrinolytic components introduced a dependence of the clot lysis rate on the thrombin concentration (decrease in clot lysis at increasing thrombin concentration). This finding opened the possibility that APC stimulated fibrinolysis by reducing the effective thrombin concentration. Subsequent experiments using the whole blood clot lysis system revealed that in the presence of anti-FX antibodies clot lysis was no longer accelerated by APC, while the actual rate of clot lysis depended on the concentration of thrombin added.

We like to propose, that in a blood clot lysis system APC most likely accelerates fibrinolysis by reducing the effective thrombin concentration; if at all, neutralization of PAI-1 may play only a minor role.