BH3-mimetic ABT-737 induces strong mitochondrial membrane depolarization in platelets but only weakly stimulates apoptotic morphological changes, platelet shrinkage and microparticle formation

Background: Depolarization of mitochondrial inner transmembrane potential (Delta psi m) is a key biochemical manifestation of the intrinsic apoptosis pathway in anucleate platelets. Little is known, however, about the relationship between Delta psi m depolarization and downstream morphological manifestations of platelet apoptosis, cell shrinkage and microparticle (MP) formation. Objectives: To elucidate this relationship in human platelets. Materials and Methods: Using flow cytometry, we analyzed Delta psi m depolarization, platelet shrinkage and MP formation in platelets treated with BH3-mimetic ABT-737 and calcium ionophore A23187, well-known inducers of intrinsic platelet apoptosis. Results: We found that at optimal treatment conditions (90 min, 37 degrees C) both ABT-737 and A23187 induce Delta psi m depolarization in the majority (88-94%) of platelets and strongly increase intracellular free calcium. In contrast, effects of A23187 and ABT-737 on platelet shrinkage and MP formation are quite different. A23187 strongly stimulates cell shrinkage and MP formation, whereas ABT-737 only weakly induces these events (10-20% of the effect seen with A23187, P < 0.0001). Conclusions: These data indicate that a high level of Delta psi m depolarization and intracellular free calcium does not obligatorily ensure strong platelet shrinkage and MP formation. Since ABT-737 efficiently induces clearance of platelets from the circulation, our results suggest that platelet clearance may occur in the absence of the morphological manifestations of apoptosis. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.