The interferon-induced protein kinase (PKR), triggers apoptosis through FADD-mediated activation of caspase 8 in a manner independent of Fas and TNF-alpha receptors.
The interferon-induced dsRNA-dependent protein kinase (PKR) induces apoptosis of mammalian cells. Apoptosis induction by PKR involves phosphorylation of the translational factor eIF-2alpha and activation of the transcriptional factor NF-kappaB, but caspase pathways activated by PKR are not known. Upregulation of Fas mRNA by PKR has been suggested to play a role in PKR-induced apoptosis. To learn how PKR induces apoptosis, we have analysed the role of molecules in death receptor pathways. We showed the involvement of the FADD-caspase 8 pathway on PKR-induced apoptosis based on four experimental findings: upregulation of caspase 8 activity during PKR-induced apoptosis, blocking of PKR-induced apoptosis by the use of a chemical inhibitor of caspase 8, and inhibition of PKR-induced apoptosis by expression of both a FADD dominant negative or a viral FLIP molecule. Significantly, despite the PKR-mediated upregulation of Fas mRNA expression, the Fas receptor-ligand pathway is not needed for PKR-induced apoptosis. Antibodies that inhibit TNFalpha-TNFR1 or Fas-FasL interactions were not able to block PKR-induced apoptosis. Taken together, our observations establish the involvement of caspase 8 in PKR-induced apoptosis and suggest that death receptors other than Fas or TNFR1 or, alternatively, a novel mechanism involving FADD independently of death receptors, are responsible for PKR-induced apoptosis.
Gil
J
J
Department of Molecular and Cellular Biology, Centro Nacional de Biotecnología, Consejo Superior de Investigaciones Científicas, Madrid, Spain.
Esteban
M
M
D016475:3T3 Cells; D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D015703:Antigens, CD / Q000378:metabolism*; D017209:Apoptosis; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D053148:Caspase 3; D053181:Caspase 8; D053453:Caspase 9; D061945:Caspase Inhibitors; D020169:Caspases / Q000378:metabolism*; D002460:Cell Line; D002522:Chlorocebus aethiops; D004789:Enzyme Activation; D053222:Fas Ligand Protein; D053200:Fas-Associated Death Domain Protein; D015870:Gene Expression; D006367:HeLa Cells; D006801:Humans; D007372:Interferons / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000378:metabolism; D051379:Mice; D012333:RNA, Messenger; D018124:Receptors, Tumor Necrosis Factor / Q000378:metabolism*; D047888:Receptors, Tumor Necrosis Factor, Type I; D014158:Transcription, Genetic; D014409:Tumor Necrosis Factor-alpha / Q000378:metabolism; D015854:Up-Regulation; D019892:eIF-2 Kinase / Q000235:genetics / Q000378:metabolism*; D019014:fas Receptor / Q000235:genetics / Q000378:metabolism*
D048868:Adaptor Proteins, Signal Transducing; D015703:Antigens, CD; D002352:Carrier Proteins; D061945:Caspase Inhibitors; C505533:FADD protein, human; C505661:FASLG protein, human; C505534:Fadd protein, mouse; D053222:Fas Ligand Protein; D053200:Fas-Associated Death Domain Protein; C505658:Fasl protein, mouse; D008562:Membrane Glycoproteins; D012333:RNA, Messenger; D018124:Receptors, Tumor Necrosis Factor; D047888:Receptors, Tumor Necrosis Factor, Type I; D014409:Tumor Necrosis Factor-alpha; D019014:fas Receptor; D007372:Interferons; D019892:eIF-2 Kinase; C505407:CASP3 protein, human; C505530:CASP8 protein, human; C506433:CASP9 protein, human; C505409:Casp3 protein, mouse; C505526:Casp8 protein, mouse; C506434:Casp9 protein, mouse; D053148:Caspase 3; D053181:Caspase 8; D053453:Caspase 9; D020169:Caspases