Xaf1 can cooperate with TNFalpha in the induction of apoptosis, independently of interaction with XIAP.
XIAP-associated factor 1 (Xaf1) binds XIAP and re-localizes it to the nucleus, thus inhibiting XIAP activity and enhancing apoptosis [1]. Xaf1 expression is reduced or absent in tumor samples and cell lines suggesting it may function as a tumor suppressor [2-5]. To further study Xaf1 function we generated Xaf1 inducible cells in the osteosarcoma cell line Saos-2. Despite Xaf1 inducing apoptosis that is dramatically enhanced by TNFalpha we find no evidence for an interaction between Xaf1 and XIAP. Furthermore, Xaf1 expression sensitized XIAP-/- fibroblasts to TNFalpha, demonstrating the existence of a novel mechanism of Xaf1 induced apoptosis distinct from antagonizing XIAP. Xaf1 expression promotes cytochrome c release that cannot be blocked by inhibition of caspase activity. This implicates a role for the mitochondrial apoptotic pathway, consistent with the ability of Bcl2 to block Xaf1 induced apoptosis. The data indicate that in Saos2 cells Xaf1 activates the mitochondrial apoptotic pathway to facilitate cytochrome c release, thus amplifying apoptotic signals from death receptors.
Xia
Yan
Y
Medical College of Georgia, Institute of Molecular Medicine and Genetics, CB2803, 1120 15th Street, Augusta, GA 30912, USA.
Novak
Rachel
R
Lewis
Jennifer
J
Duckett
Colin S
CS
Phillips
Andrew C
AC
D048868:Adaptor Proteins, Signal Transducing; D000590:Amino Acid Chloromethyl Ketones / Q000494:pharmacology; D000818:Animals; D017209:Apoptosis / Q000187:drug effects* / Q000502:physiology; D051017:Apoptosis Regulatory Proteins; D061945:Caspase Inhibitors; D045744:Cell Line, Tumor; D002478:Cells, Cultured; D003513:Cycloheximide / Q000494:pharmacology; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D045304:Cytochromes c / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004318:Doxycycline / Q000494:pharmacology; D005347:Fibroblasts / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D005434:Flow Cytometry; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins; D051379:Mice; D008928:Mitochondria / Q000187:drug effects / Q000502:physiology; D009154:Mutation / Q000235:genetics; D009363:Neoplasm Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology*; D051636:X-Linked Inhibitor of Apoptosis Protein / Q000235:genetics / Q000378:metabolism*
D048868:Adaptor Proteins, Signal Transducing; D000590:Amino Acid Chloromethyl Ketones; D051017:Apoptosis Regulatory Proteins; D061945:Caspase Inhibitors; D015853:Cysteine Proteinase Inhibitors; D047908:Intracellular Signaling Peptides and Proteins; D009363:Neoplasm Proteins; D014409:Tumor Necrosis Factor-alpha; D051636:X-Linked Inhibitor of Apoptosis Protein; C417551:XAF1 protein, human; C096713:benzyloxycarbonylvalyl-alanyl-aspartyl fluoromethyl ketone; D045304:Cytochromes c; D003513:Cycloheximide; D004318:Doxycycline