Thrombin-triggered platelet apoptosis.

Abstract

Thrombin is primarily known as a coagulation factor and as an inducer of platelet activation and aggregation. It has been reported that thrombin modulates apoptosis of nucleated cells. The current study investigated whether thrombin can affect apoptosis in anucleated human platelets. Using flow cytometry, we studied platelet apoptosis at the single-cell level, analyzing markers of mitochondrial and cytoplasmic apoptosis. Western blotting was also employed, in addition to flow cytometry, for determining the expression of Bcl-2 family proteins. We found that human alpha-thrombin induced four key manifestations of apoptosis in human platelets: (i) mitochondrial inner transmembrane potential (DeltaPsi m) depolarization; (ii) strong expression of pro-apoptotic Bax and Bak proteins but only weak expression of anti-apoptotic Bcl-2 protein; (iii) caspase-3 activation; and (iv) phosphatidylserine (PS) exposure. This study demonstrates that, aside from its 'classical' function as an inducer of platelet activation, thrombin can trigger platelet apoptosis, where it acts as a death ligand. These data indicate that thrombin triggers platelet apoptosis by impacting on several intracellular apoptotic targets, including shifting the balance between Bcl-2 regulatory proteins in a pro-apoptotic direction, depolarizing the inner mitochondrial membrane, activating the executioner caspase-3, and stimulating aberrant exposure of PS on the platelet surface.