Functional dissociation of ΔΨm and cytochrome c release defines the contribution of mitochondria upstream of caspase activation during granzyme B-induced apoptosis

Loss of Bid confers clonogenic survival to granzyme B-treated cells, however the exact role of Bid-induced mitochondrial damage - upstream or downstream of caspases - remains controversial. Here we show that direct cleavage of Bid by granzyme B, but not caspases, was required for granzyme B-induced apoptosis. Release of cytochrome c and SMAC, but not AIF or endonuclease G, occurred in the absence of caspase activity and correlated with the onset of apoptosis and loss of clonogenic potential. Loss of mitochondrial trans-membrane potential (Delta psi m) was also caspase independent, however if caspase activity was blocked the mitochondria regenerated their Delta psi m. Loss of Delta psi m was not required for rapid granzyme B-induced apoptosis and regeneration of Delta psi m following cytochrome c release did not confer clonogenic survival. This functional dissociation of cytochrome c and SMAC release from loss of Delta psi m demonstrates the essential contribution of Bid upstream of caspase activation during granzyme B-induced apoptosis.