Bax-induced caspase activation and apoptosis via cytochrome c release frommitochondria is inhibitable by Bcl-xL

A growing body of evidence supports a role for mitochondria and mitochondri a-derived factors in the cell death process. In particular, much attention has focused on cytochrome c, a key component of the electron transport chai n, that has been reported to translocate from the mitochondria to the cytos ol in cells undergoing apoptosis. The mechanism for this release is, as yet , unknown. Here we report that ectopic expression of Bax induces apoptosis with an early release of cytochrome c preceding many apoptosis-associated m orphological alterations as well as caspase activation and subsequent subst rate proteolysis, A loss of mitochondrial transmembrane potential was detec ted in vivo, although no mitochondrial swelling or loss of transmembrane po tential was observed in isolated mitochondria treated with Bax in vitro. Ca spase inhibitors, such as endogenous XIAP and synthetic peptide benzyloxyca rbonyl-Val-Ala-Asp-fluoromethyl ketone (zVAD-fmk), although capable of alte ring the kinetics and perhaps mode of cell death, had no influence on this release, suggesting that if cytochrome c plays a role in caspase activation it must precede this step in the apoptotic process. Mitochondrial permeabi lity transition was also shown to be significantly prevented by caspase inh ibition, indicating that the translocation of cytochrome c from mitochondri a to cytosol is not a consequence of events requiring mitochondrial membran e depolarization, In contrast, Bcl-xL was capable of preventing cytochrome c release while also significantly inhibiting cell death. It would therefor e appear that the mitochondrial release of factors such as cytochrome c rep resents a critical step in committing a cell to death, and this release is independent of permeability transition and caspase activation but is inhibi ted by Bcl-xL.