Platelets are anticoagulants?

Effective control of blood coagulation requires that hemostasis proceed at the wound site without progressing to vessel occlusion. To accomplish this, it is presumably necessary for the anticoagulant mechanisms to respond more vigorously as the procoagulant stimulus increases. The protein C anti-coagulant pathway provides one such response since, from infusion studies, the generation of activated protein C (APC) appears to be roughly proportional to the thrombin concentration infused (Hanson et al, J Clin Invest. 1993;92:2003-2012), and the APC can then prevent clot extension. Protein C activation occurs when thrombin binds to thrombomodulin on the surface of the endothelium and then rapidly converts protein C bound to the endothelial cell protein C receptor to APC.

In this issue, Slungaard and colleagues (page 146) have identified a novel mechanism for further stimulation of protein C activation near platelet-rich thrombi. Through studies on both endothelial cells in culture and thrombin infusion experiments in monkeys, they demonstrate that platelet factor 4 (PF4) stimulates protein C activation substantially. In vivo, the coinfusion of PF4 and thrombin increased APC levels about 2-fold, compared with thrombin plus vehicle. Thus, the net effect of the PF4 is likely antithrombotic. This is quite surprising. PF4 is known to have many effects, but one dominant hypothesis was that it played a role in preventing heparin stimulation of antithrombin inhibition of thrombin. This would normally be considered procoagulant. But the inhibition of thrombin neutralization would lead to increased deposition of thrombin on thrombomodulin, thus increasing protein C activation. While this likely contributes to some of the increase in APC formation, another, possibly more important, mechanism involves direct stimulation of protein C activation by the thrombin-thrombomodulin complex.

The implications of the stimulated APC formation are many. APC has anti-inflammatory activity, reducing a variety of cytokines and adhesion molecules induced on cells primarily through NFκB-induced pathways. Stimulated APC formation in arterial beds could help decrease inflammation in atherosclerotic areas or following angioplasty. Consistent with a dominant role of the pathway in arterial injury, experiments from Woo's group (Waugh et al, Circulation. 2000;102:332-337) demonstrated that increasing thrombomodulin expression in injured arteries reduced thrombosis, leukocyte infiltration, and vessel thickening. Some of these activities may be due to direct anti-inflammatory activities of thrombomodulin itself (Conway et al, J Exp Med. 2002;196:565-577), but others are probably enhanced by the increased protein C activation, some of which would be further enhanced by PF4.

Another interesting finding from this paper was that the ability to enhance protein C activation varied markedly among cultured endothelium derived from different sources. Some of these differences are probably due to differences in attaching the chondroitin sulfate to thrombomodulin since, in purified systems, the chondroitin sulfate augments (but is not required for) the stimulation of protein C activation by PF4. Of major interest is the observation that PF4 has no effect on protein C activation by blood outgrowth endothelial cells. This suggests that there is yet another factor that participates in protein C activation and that it is missing from these endothelial cells. Since small changes in protein C activation can have a major effect on the coagulant and inflammatory responses, identification of this putative new member of the protein C activation complex could provide new insights into the regulation of both co-agulation and inflammation.