Apoptosis induction by caspase-8 is amplified through the mitochondrial release of cytochrome c.
Apoptosis often involves the release of cytochrome c from mitochondria, leading to caspase activation. However, in apoptosis mediated by CD95 (Fas/APO-1), caspase-8 (FLICE/MACH/Mch5) is immediately activated and, in principle, could process other caspases directly. To investigate whether caspase-8 could also act through mitochondria, we added active caspase-8 to a Xenopus cell-free system requiring these organelles. Caspase-8 rapidly promoted the apoptotic program, culminating in fragmentation of chromatin and the nuclear membrane. In extracts devoid of mitochondria, caspase-8 produced DNA degradation, but left nuclear membranes intact. Thus, mitochondria were required for complete engagement of the apoptotic machinery. In the absence of mitochondria, high concentrations of caspase-8 were required to activate downstream caspases. However, when mitochondria were present, the effects of low concentrations of caspase-8 were vastly amplified through cytochrome c-dependent caspase activation. Caspase-8 promoted cytochrome c release indirectly, by cleaving at least one cytosolic substrate. Bcl-2 blocked apoptosis only at the lowest caspase-8 concentrations, potentially explaining why CD95-induced apoptosis can often evade inhibition by Bcl-2.
Kuwana
T
T
La Jolla Institute for Allergy and Immunology, San Diego, California 92121, USA.
Smith
J J
JJ
Muzio
M
M
Dixit
V
V
Newmeyer
D D
DD
Kornbluth
S
S
D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D053178:Caspase 6; D053181:Caspase 8; D053453:Caspase 9; D020169:Caspases; D002474:Cell-Free System; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D003574:Cytochrome c Group / Q000378:metabolism*; D008928:Mitochondria / Q000378:metabolism*; D010447:Peptide Hydrolases / Q000378:metabolism; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000494:pharmacology; D014981:Xenopus
D015853:Cysteine Proteinase Inhibitors; D003574:Cytochrome c Group; D019253:Proto-Oncogene Proteins c-bcl-2; D010447:Peptide Hydrolases; D053178:Caspase 6; D053181:Caspase 8; D053453:Caspase 9; D020169:Caspases; D003546:Cysteine Endopeptidases; C108711:DEVDase