Granzyme B induces BID-mediated cytochrome c release and mitochondrial permeability transition

Many cell death pathways converge at the mitochondria to induce release of apoptogenic proteins and permeability transition, resulting in the activation of effector caspases responsible for the biochemical and morphological alterations of apoptosis. The death receptor pathway has been described as a triphasic process initiated by the activation of apical caspases, a mitochondrial phase, and then the final phase of effector caspase activation. Granzyme B (GrB) activates apical and effector caspases as well as promotes cytochrome c (cyt c) release and loss of mitochondrial membrane potential. We investigated how GrB affects mitochondria utilizing an in vitro cell-free system and determined that cyt c release and permeability transition are initiated by distinct mechanisms. The cleavage of cytosolic BID by GrB results in truncated BID, initiating mitochondrial cyt c release. BID is the sole cytosolic protein responsible for this phenomenon in vitro, yet caspases were found to participate in cyt c release in some cells. On the other hand, GrB acts directly on mitochondria in the absence of cytosolic S100 proteins to open the permeability transition pore and to disrupt the proton electrochemical gradient. We suggest that GrB acts by two distinct mechanisms on mitochondria that ultimately lead to mitochondrial dysfunction and cellular demise.