Apoptosome-independent activation of the lysosomal cell death pathway by caspase-9.
The apoptosome, a heptameric complex of Apaf-1, cytochrome c, and caspase-9, has been considered indispensable for the activation of caspase-9 during apoptosis. By using a large panel of genetically modified murine embryonic fibroblasts, we show here that, in response to tumor necrosis factor (TNF), caspase-8 cleaves and activates caspase-9 in an apoptosome-independent manner. Interestingly, caspase-8-cleaved caspase-9 induced lysosomal membrane permeabilization but failed to activate the effector caspases whereas apoptosome-dependent activation of caspase-9 could trigger both events. Consistent with the ability of TNF to activate the intrinsic apoptosis pathway and the caspase-9-dependent lysosomal cell death pathway in parallel, their individual inhibition conferred only a modest delay in TNF-induced cell death whereas simultaneous inhibition of both pathways was required to achieve protection comparable to that observed in caspase-9-deficient cells. Taken together, the findings indicate that caspase-9 plays a dual role in cell death signaling, as an activator of effector caspases and lysosomal membrane permeabilization.
Gyrd-Hansen
Mads
M
Apoptosis Department and Centre for Genotoxic Stress, Institute for Cancer Biology, Danish Cancer Society, DK-2100 Copenhagen, Denmark.
Farkas
Thomas
T
Fehrenbacher
Nicole
N
Bastholm
Lone
L
Høyer-Hansen
Maria
M
Elling
Folmer
F
Wallach
David
D
Flavell
Richard
R
Kroemer
Guido
G
Nylandsted
Jesper
J
Jäättelä
Marja
M
D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000502:physiology*; D053477:Apoptotic Protease-Activating Factor 1 / Q000235:genetics / Q000378:metabolism*; D053181:Caspase 8 / Q000235:genetics / Q000378:metabolism; D053453:Caspase 9 / Q000235:genetics / Q000378:metabolism*; D002478:Cells, Cultured; D003513:Cycloheximide / Q000378:metabolism; D045304:Cytochromes c / Q000235:genetics / Q000378:metabolism*; D004789:Enzyme Activation; D005347:Fibroblasts / Q000166:cytology / Q000378:metabolism; D008247:Lysosomes / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D008928:Mitochondria / Q000378:metabolism; D011500:Protein Synthesis Inhibitors / Q000378:metabolism; D014409:Tumor Necrosis Factor-alpha / Q000235:genetics / Q000378:metabolism / Q000494:pharmacology
D053477:Apoptotic Protease-Activating Factor 1; D011500:Protein Synthesis Inhibitors; D014409:Tumor Necrosis Factor-alpha; D045304:Cytochromes c; D003513:Cycloheximide; D053181:Caspase 8; D053453:Caspase 9