Caspase inhibition decreases both platelet phosphatidylserine exposure and aggregation - Caspase inhibition of platelets

Apoptosis of nucleated cells is regulated by caspases, a group of cysteine proteases, and is characterized by phosphatidylserine expression on the outer leaflet of the plasma membrane. Reports indicate that platelets contain caspases. However, the role of caspases in platelet function is not welt understood. When platelets become activated, they express phosphatidylserine (PS) on the outer leaftet of the plasma membrane. In addition, platelets aggregate when activated. The aims of this study were to determine if caspase inhibition (using the pan-caspase inhibitor zVAD-fmk): (1) decreased PS expression and (2) decreased platelet aggregation following activation. Flow cytometry was used to determine PS expression and a platelet aggregometer was used to assess aggregation. We found that platelets treated with zVAD-fmk significantly decreased both A23187-induced PS exposure (total fluorescence index, TFI: A23187 = 791.42 +/- 174; zVAD+A23187 = 92.97 +/- 57, p less than or equal to 0.05) and ADP-induced PS exposure (TFI: ADP = 669.24 +/- 145, zVAD+ADP=174.61 +/- 51, p less than or equal to 0.05). Further, treatment with zVAD-fmk significantly decreased ADP-induced platelet aggregation (%: untreated = 80 +/- 1.5, zVAD treated = 69 +/- 3.0, p less than or equal to 0.05). These results indicate that caspases play a rote in platelet activation, suggesting a unique physiologic role for these proteases. (C) 2004 Elsevier Ltd. All rights reserved.