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 well understood. When platelets become activated, they express phosphatidylserine (PS) on the outer leaflet 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≤0.05) and ADP-induced PS exposure (TFI: ADP=669.24±145, zVAD+ADP=174.6±151, p≤0.05). Further, treatment with zVAD-fmk significantly decreased ADP-induced platelet aggregation (%: UNTREATED=80±1.5, zVAD TREATED=69±3.0, p≤0.05). These results indicate that caspases play a role in platelet activation, suggesting a unique physiologic role for these proteases.