Bcl-2 and Bcl-xL inhibit CD95-mediated apoptosis by preventing mitochondrial release of Smac/DIABLO and subsequent inactivation of X-linked inhibitor-of-apoptosis protein.
Bcl-2 and Bcl-x(L) are reported to inhibit CD95-mediated apoptosis in "type II" but not in "type I" cells. In the present studies, we found that stimulation of CD95 receptors, with either agonistic antibody or CD95 ligand, resulted in the activation of caspase-8, which in turn processed caspase-3 between its large and small subunits. However, in contrast to control cells, those overexpressing either Bcl-2 or Bcl-x(L) displayed a distinctive pattern of caspase-3 processing. Indeed, the resulting p20/p12 caspase-3 was not active and did not undergo normal autocatalytic processing to form p17/p12 caspase-3, because it was bound to and inhibited by endogenous X-linked inhibitor-of-apoptosis protein (XIAP). Importantly, Bcl-2 and Bcl-x(L) inhibited the release of both cytochrome c and Smac from mitochondria. However, since Smac alone was sufficient to promote caspase-3 activity in vitro by inactivating XIAP, we proposed the existence of a death receptor-induced, Smac-dependent and apoptosome-independent pathway. This type II pathway was subsequently reconstituted in vitro using purified recombinant proteins at endogenous concentrations. Thus, mitochondria and associated Bcl-2 and Bcl-x(L) proteins may play a functional role in death receptor-induced apoptosis by modulating the release of Smac. Our data strongly suggest that the relative ratios of XIAP (and other inhibitor-of-apoptosis proteins) to active caspase-3 and Smac may dictate, in part, whether a cell exhibits a type I or type II phenotype.
Sun
Xiao-Ming
XM
Medical Research Council Toxicology Unit, Hodgkin Building, University of Leicester, P. O. Box 138, Lancaster Road, Leicester LE1 9HN, United Kingdom.
Bratton
Shawn B
SB
Butterworth
Michael
M
MacFarlane
Marion
M
Cohen
Gerald M
GM
D017209:Apoptosis / Q000502:physiology*; D051017:Apoptosis Regulatory Proteins; D002352:Carrier Proteins / Q000378:metabolism*; D008040:Genetic Linkage; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins; D019169:Jurkat Cells; D008928:Mitochondria / Q000378:metabolism*; D024101:Mitochondrial Proteins / Q000378:metabolism*; D011506:Proteins / Q000037:antagonists & inhibitors* / Q000235:genetics; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000502:physiology*; D051636:X-Linked Inhibitor of Apoptosis Protein; D051020:bcl-X Protein; D019014:fas Receptor / Q000502:physiology*
D051017:Apoptosis Regulatory Proteins; C494811:BCL2L1 protein, human; D002352:Carrier Proteins; C412403:DIABLO protein, human; D047908:Intracellular Signaling Peptides and Proteins; D024101:Mitochondrial Proteins; D011506:Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D051636:X-Linked Inhibitor of Apoptosis Protein; C496866:XIAP protein, human; D051020:bcl-X Protein; D019014:fas Receptor