New targets for antithrombotic drugs

Annexin A5 as a novel player in prevention of atherothrombosis in SLE and in the general population

During recent years it has become evident that atherosclerosis is an inflammatory disease. Furthermore, immune reactions and especially autoimmunity, were demonstrated to modulate atherosclerosis in animal experiments. An interesting example of how autoimmune reactions can influence atherosclerosis and consequences thereafter, is systemic lupus erythematosus (SLE)-associated cardiovascular disease (CVD). Antithrombotic effect exerted by Annexin A5 (ANXA5) is thought to be mediated mainly by forming a mechanical shield over phospholipids (PLs) reducing availability of PLs for coagulation reactions. However, more specific properties of ANXA5 might be of importance for its antithrombotic function. Such examples include downregulation of surface-expressed tissue factor (TF), as well as upregulation of urokinase-type plasminogen activator (uPA) by ANXA5. Also, interaction of ANXA5 with ligands involved in hemostasis, such as sulfatide and heparin, has been demonstrated. We have recently described a novel mechanism potentially contributing to atherothrombosis in SLE, with ANXA5 binding to endothelium decreased in SLE, an effect caused by antiphospholipid antibodies (aPL). It may be hypothesized that ANXA5 can be effective as a treatment to prevent plaque rupture and atherothrombosis not only in SLE, but also in the general population prone to CVD. Antiatherothrombotic potential of ANXA5 deserves further attention and careful studies as the mechanism behind the majority of clinically significant cardiovascular ischemic disease is atherothrombosis, formed on an underlying vulnerable atherosclerotic lesion. It may be hypothesized that ANXA5 can be effective as a treatment to prevent plaque rupture and atherothrombosis not only in SLE, but also in a general population prone to CVD.

Antithrombotic action of annexin V proved as efficient as direct inhibition of tissue factor or thrombin

The role of phospholipid platelet membrane and tissue factor in thrombin generation and thrombus formation is accepted. In the present study we have explored antithrombotic action of strategies aimed to block exposure of negatively charged phospholipids and we compared effects with those obtained through tissue factor or a direct thrombin inhibition. Type III collagen was exposed to flowing blood (5 min, 300 s(-1)). Effects of inhibition of platelet deposition by annexin A5 (ANXA5), hirudin (HIR) or by an antibody against tissue factor (TF) were evaluated. Prothrombin fragment F1 + 2 (F1 + 2) was monitored. Pre-incubation of whole blood with HIR or ANXA5 resulted in a statistically significant reduction of platelet deposition (12.2 +/- 0.6% in control experiments vs. 8.3 +/- 0.4% and 8.5 +/- 0.5%, respectively, P < 0.05). A similar decrease was found when blood was incubated with an antibody against TF. Furthermore, ANXA5 and HIR inhibited the recruitment of platelets into forming aggregates. The height of platelet aggregates generated was decreased in the presence of HIR or ANXA5, but only incubation with both inhibitors reached levels of statistical significance. The presence of ANXA5 or HIR decreased levels of F1 + 2 suggesting a reduced activation of the coagulation system. In our experimental studies, the inhibitory potential of ANXA5 on platelet-thrombus formation was as effective as that of a direct thrombin inhibitor, as HIR, or an antibody against TF. Negatively charged phospholipids exposed on activated platelets potentiate the formation of platelet aggregates on a collagen surface and further suggest that inhibition of platelet procoagulant activity might be a specific target for antithrombotic drugs.