Loss of caspase-9 reveals its essential role for caspase-2 activation and mitochondrial membrane depolarization.
Caspase-9 plays an important role in apoptosis induced by genotoxic stress. Irradiation and anticancer drugs trigger mitochondrial outer membrane permeabilization, resulting in cytochrome c release and caspase-9 activation. Two highly contentious issues, however, remain: It is unclear whether the loss of the mitochondrial membrane potential DeltaPsi(M) contributes to cytochrome c release and whether caspases are involved. Moreover, an unresolved question is whether caspase-2 functions as an initiator in genotoxic stress-induced apoptosis. In the present study, we have identified a mutant Jurkat T-cell line that is deficient in caspase-9 and resistant to apoptosis. Anticancer drugs, however, could activate proapoptotic Bcl-2 proteins and cytochrome c release, similarly as in caspase-9-proficient cells. Interestingly, despite these alterations, the cells retained DeltaPsi(M). Furthermore, processing and enzyme activity of caspase-2 were not observed in the absence of caspase-9. Reconstitution of caspase-9 expression restored not only apoptosis but also the loss of DeltaPsi(M) and caspase-2 activity. Thus, we provide genetic evidence that caspase-9 is indispensable for drug-induced apoptosis in cancer cells. Moreover, loss of DeltaPsi(M) can be functionally separated from cytochrome c release. Caspase-9 is not only required for DeltaPsi(M) loss but also for caspase-2 activation, suggesting that these two events are downstream of the apoptosome.
Samraj
Ajoy K
AK
Institute of Molecular Medicine, University of Düsseldorf, Düsseldorf D-40225, Germany.
Sohn
Dennis
D
Schulze-Osthoff
Klaus
K
Schmitz
Ingo
I
D000970:Antineoplastic Agents / Q000494:pharmacology; D017209:Apoptosis / Q000187:drug effects; D053143:Caspase 2 / Q000378:metabolism*; D053453:Caspase 9 / Q000172:deficiency* / Q000378:metabolism*; D002843:Chromatin / Q000648:ultrastructure; D045304:Cytochromes c / Q000378:metabolism; D019008:Drug Resistance, Neoplasm; D004789:Enzyme Activation / Q000187:drug effects; D006801:Humans; D051034:Inhibitor of Apoptosis Proteins / Q000378:metabolism; D019169:Jurkat Cells; D053078:Membrane Potential, Mitochondrial / Q000187:drug effects / Q000502:physiology*; D051336:Mitochondrial Membranes / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D009153:Mutagens / Q000494:pharmacology; D019311:Staurosporine / Q000494:pharmacology; D014162:Transfection; D050998:bcl-2 Homologous Antagonist-Killer Protein / Q000378:metabolism
D000970:Antineoplastic Agents; D002843:Chromatin; D051034:Inhibitor of Apoptosis Proteins; D009153:Mutagens; D050998:bcl-2 Homologous Antagonist-Killer Protein; D045304:Cytochromes c; D053143:Caspase 2; D053453:Caspase 9; D019311:Staurosporine