Apoptosis can be initiated by damaging mitochondria and inducing the mitochondrial permeability transition.  However, while this may be sufficient to initiate the process, this is not a necessary condition. Finucane et al. (1999) and Krohn et al., (1999) demonstrated cytochrome c release from the intermembrane space and caspase activation without depolarization of the inner membrane. Often inner membrane depolarization follows cytochrome c release and the potential can be restored in permeabilized cells by adding exogenous cytochrome c.  Eventually, inner membrane depolarization does occur, but this may be the result of degredative pathways that lead to cellular fragmentation.  Thus, changes in the inner membrane may be secondary or subsequent to changes in the outer membrane (unless the event initiating the apoptotic process specifically targeted components of the inner membrane/matrix).  Indeed, strong evidence indicates that when IL-3 is removed from the medium of a cell line that depends on this growth factor for growth and survival, prior to cytochrome c release the permeability of the outer membrane to metabolites is actually greatly reduced (Bialik et al., 1999; Vander Heiden et al., 1999).  It is this reduction in permeability that leads to changes that eventually result in the release of the contents of the intermembrane space.  At this state, cells can be rescued by restoring IL-3 and thus reversing the drop in outer membrane permeability and preventing progression to apoptosis (Vander Heiden et al., 2000).
The reduction in metabolite permeability is likely the result of closure of VDAC channels as these are the major permeability pathways through the outer membrane. Different groups have reported that the anti-apoptotic protein, Bcl-xL, can interact with and modulate the activity of VDAC channels from the mitochondrial outer membrane.  However, whether Bcl-xL promotes the open or closed conformation of VDAC is a controversial issue.  Recent publications by the Tsujimoto laboratory (Shimizu et al., 1999, 2000) contradict those obtained in the Colombini and Thompson laboratories (J. Biol. Chem. in press).   The latter report that Bcl-xL promotes the maintenance of VDAC in the open configuration.  According to these results, Bcl-xL exerts its protective role by restoring metabolic exchange across the outer membrane without inducing the loss of intermembrane space proteins such as cytochrome c.
 

References

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Finucane, D.M., Waterhouse, N.J., Amarante-Mendes, G.P., Cotter, T.G., and Green, D.R. (1999). Collapse of the inner mitochondrial transmembrane potential is not required for apoptosis of HL60 cells. Exp. Cell Res. 251(1): 166-74
Krohn, A.J., Wahlbrink, T., and Prehn, J.H. (1999). Mitochondrial depolarization is not required for neuronal apoptosis. J. Neurosci. 19(17): 7394-7404
Shimizu, S., Konishi, A., Kodama, T., and Tsujimoto, Y. (2000). BH4 domain of antiapoptotic Bcl-2 family members closes voltage-dependent anion channel and inhibits apoptotic mitochondrial changes and cell death. Proc. Natl. Acad. Sci. USA. 97: 3100-3105
Shimizu, S., Narita, M., and Tsujimoto, Y. (1999). Bcl-2 family proteins regulate the release of apoptogenic cytochrome c by the mitochondrial channel VDAC. Nature. 399: 483-487
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Vander Heiden, M.G., Chandel, N.S., Li, X.X., Schumacker, P.T., Colombini, M., and Thompson, C.B. (2000). Outer mitochondrial membrane permeability can regulate coupled respiration and cell survival. Proc. Natl. Acad. Sci. USA. 97: 4666-4671
Vander Heiden,M.G., Li, X.X. , Gottleib, E. , Hill, R.B., Thompson, C.B., and Colombini, M. 2001. Bcl-xL Promotes the Open Configuration of VDAC and Metabolite Passage through the Mitochondrial Outer Membrane. Journal of Biological Chemistry 276: 19414-9