Apoptosis Bcl-2 Finds a New Way to Save Cells

Proteins in the Bcl-2 family play opposing roles in regulating apoptosis, the process of programmed cell death. Bcl-2 inhibits apoptosis through a mechanism widely believed to involve inhibition of cytochrome c release from mitochondria and consequently of cytochrome c-dependent activation of caspase-9 by apoptosis protease activating factor-1 (Apaf-1). By comparing the behavior of cells lacking Apaf-1 or caspase-9 to that of cells overexpressing Bcl-2 or defective in the pro-apoptotic Bcl-2 protein Bim, Marsden et al. demonstrated that Bcl-2 can inhibit apoptosis by a mechanism independent of the cytochrome c/Apaf-1/caspase-9 pathway. Cultured fetal liver cells from Apaf-1^-/- or caspase-9^-/- transgenic mice died after cytokine withdrawal at a rate similar to that for wild-type cells, whereas cells overexpressing Bcl-2 were protected from cell death. Irradiated mice that were hematopoetically reconstituted with fetal stem cells from wild-type, Apaf-1^-/-, or caspase-9^-/- mice had comparable numbers of lymphoid cell subtypes, and lymphoid cells from these mice displayed similar vulnerability to cytokine withdrawal, -irradiation, etoposide and dexamethasone. Bcl-2 overexpression, however, was associated with increased numbers of splenic lymphocytes, and protected against apoptotic stimuli. Bim^-/- cells were protected from cytokine withdrawal, but not dexamethasone or etoposide. Dying Apaf-1- or caspase-9-deficient lymphocytes displayed such hallmarks of apoptosis as cell-surface exposure of phosphatidylserine, DNA laddering, loss of mitochondrial membrane potential, and terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling. Caspase activity was apparent at a reduced level in -irradiated cells lacking Apaf-1 and caspase-9, as determined by substrate cleavage, and both apoptosis and cytochrome c release were blocked by a caspase inhibitor. The authors conclude that Bcl-2 inhibits apoptosis through a caspase-mediated pathway independent of Apaf-1 and caspase-9.

V. S. Marsden, L. O'Connor, L. A. O'Reilly, J. Silke, D. Metcalf, P. G. Ekert, D. C. S. Huang, F. Cecconi, K. Kuida, K. J. Tomaselli, S. Roy, D. W. Nicholson, D. L. Vaux, P. Bouillet, J. M. Adams, A. Strasser, Apoptosis initiated by Bcl-2-regulated caspase activation independently of the cytochrome c/Apaf-1/caspase-9 apoptosome. Nature 419,634-637 (2002). [Online Journal]