Summary
Although platelets possess the hallmarks of apoptosis such as activation of caspases,
cytochrome c release and depolarisation of the mitochondrial transmembrane potential
(ΔΨm), their entire apoptotic-signalling pathway is not totally understood. Therefore
we studied the expression of various apoptotic proteins and found that platelets contain
the pro-apoptotic proteins Omi/HtrA2 and Smac/Diablo, as well as their target the
X-linked inhibitor of apoptosis XIAP. Omi/HtrA2 and Smac/Diablo were released from
mitochondria into the platelet cytosol together with cytochrome c after induction
of apoptosis by the Ca2+ ionophore A23187 or the BH3 mimetic ABT-737, and to a lesser extent, after platelet
stimulation with collagen and thrombin. Inhibition of Omi/HtrA2 led to decreased levels
of activated caspase-3/7 and caspase-9, but did not abolish loss of ΔΨm or prevent
release of Omi/HtrA2 from mitochondria. These results indicate that platelets have
a functional intrinsic apoptotic-signalling pathway including the pro-apoptotic protease
Omi/HtrA2 and its target protein XIAP.
Keywords
Apoptosis - caspase-3 - platelets - Omi/HtrA2 - XIAP