Smac/DIABLO release from mitochondria and XIAP inhibition are essential to limit clonogenicity of Type I tumor cells after TRAIL receptor stimulation.
Death receptors, such as Fas/CD95 and TRAIL receptors, engage the extrinsic pathway for caspase activation, but also couple to the intrinsic mitochondrial route. In so-called Type II cells, death receptors require the mitochondrial pathway for apoptotic execution, whereas in Type I cells they reportedly do not. For established tumor cell lines, the Type I/Type II distinction is based on short-term apoptosis assays. We report here that the mitochondrial pathway is essential for apoptotic execution of Type I tumor cells by death receptors, when long-term clonogenicity is taken into account. A blockade of the mitochondrial pathway in Type I tumor cells - by RNA interference for Bid or Bcl-2 overexpression - reduced effector caspase activity and mediated significant clonogenic resistance to TRAIL. Downstream from the mitochondria, Caspase-9 did not contribute to clonogenic death of TRAIL-treated Type I cells. Rather, the release of Smac/DIABLO and the inhibition of XIAP activity proved to be crucial for full effector caspase activity and clonogenic execution. Thus, in Type I cells the intrinsic pathway downstream from death receptors is not redundant, but limits clonogenicity by virtue of Smac/DIABLO release and XIAP inhibition. This finding is relevant for cancer therapy using death receptor agonists.
Maas
C
C
Division of Immunology, The Netherlands Cancer Institute, Amsterdam, The Netherlands.
Verbrugge
I
I
de Vries
E
E
Savich
G
G
van de Kooij
L W
LW
Tait
S W G
SW
Borst
J
J
D017209:Apoptosis; D051017:Apoptosis Regulatory Proteins; D051023:BH3 Interacting Domain Death Agonist Protein / Q000235:genetics / Q000378:metabolism; D053453:Caspase 9 / Q000378:metabolism; D045744:Cell Line, Tumor; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism*; D008928:Mitochondria / Q000378:metabolism*; D024101:Mitochondrial Proteins / Q000378:metabolism*; D009369:Neoplasms / Q000378:metabolism* / Q000473:pathology; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000235:genetics / Q000378:metabolism; D034622:RNA Interference; D053220:Receptors, TNF-Related Apoptosis-Inducing Ligand / Q000378:metabolism*; D053221:TNF-Related Apoptosis-Inducing Ligand / Q000494:pharmacology; D051636:X-Linked Inhibitor of Apoptosis Protein / Q000037:antagonists & inhibitors* / Q000378:metabolism
D051017:Apoptosis Regulatory Proteins; D051023:BH3 Interacting Domain Death Agonist Protein; C412403:DIABLO protein, human; D047908:Intracellular Signaling Peptides and Proteins; D024101:Mitochondrial Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D053220:Receptors, TNF-Related Apoptosis-Inducing Ligand; D053221:TNF-Related Apoptosis-Inducing Ligand; D051636:X-Linked Inhibitor of Apoptosis Protein; D053453:Caspase 9