Bid-induced Cytochrome c Release Is Mediated by a Pathway Independent of Mitochondrial Permeability Transition Pore and Bax

Bid, a pro-apoptosis "BH3-only" member of the Bcl-2 family, can be cleaved by caspase-8 after Fas/TNF-R1 engagement. The p15 form of truncated Bid (tBid) translocates to mitochondria and induces cytochrome c release, leading to the activation of downstream caspases and apoptosis. In the current study, we investigated the mechanism by which tBid regulated cytochrome c release in terms of its relationship to mitochondrial permeability transition and Bax, another Bcl-2 family protein. We employed an in vitro reconstitution system as well as cell cultures and an animal model to reflect the physiological environment where Bid could be functional. We found that induction of cytochrome c release by tBid was not accompanied by a permeability transition even at high doses. Indeed, inhibition of permeability transition did not suppress the activity of tBid in vitro nor could they block Fas activation-induced, Bid-dependent hepatocyte apoptosis in cultures. Furthermore, Mg(2+), although inhibiting permeability transition, actually enhanced the ability of tBid to induce cytochrome c release. We also found that tBid did not require Bax to induce cytochrome c release in vitro. In addition, mice deficient in bax were still highly susceptible to anti-Fas-induced hepatocyte apoptosis, in which cytochrome c release was unaffected. Moreover, although Bax-induced cytochrome c release was not dependent on tBid, the two proteins could function synergistically. We conclude that Bid possesses the biochemical activity to induce cytochrome c release through a mechanism independent of mitochondrial permeability transition pore and Bax.