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Caspase-8/FLICE functions as an executioner caspase in anticancer drug-induced apoptosis
I H Engels1, A Stepczynska, C Stroh
1Department of Immunology and Cell Biology, University of Münster, Germany.
Abstract:
Caspase-8 plays an essential role in apoptosis triggered by death receptors. Through the cleavage of Bid, a proapoptotic Bcl-2 member, it further activates the mitochondrial cytochrome c/Apaf-1 pathway. Because caspase-8 can be processed also by anticancer drugs independently of death receptors, we investigated its exact role and order in the caspase cascade. We show that in Jurkat cells either deficient for caspase-8 or overexpressing its inhibitor c-FLIP apoptosis mediated by CD95, but not by anticancer drugs was inhibited. In the absence of active caspase-8, anticancer drugs still induced the processing of caspase-9, -3 and Bid, indicating that Bid cleavage does not require caspase-8. Overexpression of Bcl-x(L) prevented the processing of caspase-8 as well as caspase-9, -6 and Bid in response to drugs, but was less effective in CD95-induced apoptosis. Similar responses were observed by overexpression of a dominant-negative caspase-9 mutant. To further determine the order of caspase-8 activation, we employed MCF7 cells lacking caspase-3. In contrast to caspase-9 that was cleaved in these cells, anticancer drugs induced caspase-8 activation only in caspase-3 transfected MCF7 cells. Thus, our data indicate that, unlike its proximal role in receptor signaling, in the mitochondrial pathway caspase-8 rather functions as an amplifying executioner caspase.
Insights
Anticancer drugs activate caspase-8 independently of death receptors, revealing its role as an amplifying executioner in the mitochondrial apoptosis pathway, not a proximal initiator.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Caspase-8 initiates apoptosis via death receptors and Bid cleavage, activating the mitochondrial pathway.
- Anticancer drugs can process caspase-8 independently of death receptors, prompting investigation into its role.
Purpose of the Study:
- To elucidate the precise role and sequential activation of caspase-8 in the caspase cascade.
- To differentiate caspase-8's function in death receptor-mediated versus drug-induced apoptosis.
Main Methods:
- Utilized Jurkat cells deficient in caspase-8 or overexpressing c-FLIP.
- Employed Bcl-x(L) and dominant-negative caspase-9 overexpression.
- Investigated caspase processing in MCF7 cells lacking caspase-3, with and without caspase-3 transfection.
Main Results:
- Anticancer drug-induced apoptosis was inhibited in caspase-8-deficient/c-FLIP-overexpressing cells, but CD95-mediated apoptosis was inhibited.
- Bid cleavage by anticancer drugs occurred independently of caspase-8.
- Bcl-x(L) and dominant-negative caspase-9 inhibited drug-induced caspase-8 and downstream processing, with less effect on CD95 signaling.
- Anticancer drugs induced caspase-8 activation in MCF7 cells only after caspase-3 transfection, unlike caspase-9.
Conclusions:
- Caspase-8 functions as an amplifying executioner caspase in the mitochondrial apoptosis pathway.
- Its role differs from its proximal function in death receptor-initiated signaling.