title: Caspase-3 feeds back on caspase-8, Bid and XIAP in type I Fas signaling in primary mouse hepatocytes.
abstract: The TNF-R1 like receptor Fas is highly expressed on the plasma membrane of hepatocytes and plays an essential role in liver homeostasis. We recently showed that in collagen-cultured primary mouse hepatocytes, Fas stimulation triggers apoptosis via the so-called type I extrinsic signaling pathway. Central to this pathway is the direct caspase-8-mediated cleavage and activation of caspase-3 as compared to the type II pathway which first requires caspase-8-mediated Bid cleavage to trigger mitochondrial cytochrome c release for caspase-3 activation. Mathematical modeling can be used to understand complex signaling systems such as crosstalks and feedback or feedforward loops. A previously published model predicted a positive feedback loop between active caspases-3 and -8 in both type I and type II FasL signaling in lymphocytes and Hela cells, respectively. Here we experimentally tested this hypothesis in our hepatocytic type I Fas signaling pathway by using wild-type and XIAP-deficient primary hepatocytes and two recently characterized, selective caspase-3/-7 inhibitors (AB06 and AB13). Caspase-3/-7 activity assays and quantitative western blotting confirmed that fully processed, active p17 caspase-3 feeds back on caspase-8 by cleaving its partially processed p43 form into the fully processed p18 species. Our data do not discriminate if p18 positively or negatively influences FasL-induced apoptosis or is responsible for non-apoptotic aspects of FasL signaling. However, we found that caspase-3 also feeds back on Bid and degrades its own inhibitor XIAP, both events that may enhance caspase-3 activity and apoptosis. Thus, potent, selective caspase-3 inhibitors are useful tools to understand complex signaling circuitries in apoptosis.
authors.affiliation: Institute of Molecular Medicine and Cell Research, University of Freiburg, Freiburg, Germany.
authors.forename: Karine Sá
authors.initials: KS
authors.lastname: Ferreira
authors.forename: Clemens
authors.initials: C
authors.lastname: Kreutz
authors.forename: Sabine
authors.initials: S
authors.lastname: Macnelly
authors.forename: Karin
authors.initials: K
authors.lastname: Neubert
authors.forename: Angelika
authors.initials: A
authors.lastname: Haber
authors.forename: Matthew
authors.initials: M
authors.lastname: Bogyo
authors.forename: Jens
authors.initials: J
authors.lastname: Timmer
authors.forename: Christoph
authors.initials: C
authors.lastname: Borner
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D051023:BH3 Interacting Domain Death Agonist Protein / Q000378:metabolism*; D053148:Caspase 3 / Q000378:metabolism*; D053181:Caspase 8 / Q000378:metabolism*; D061945:Caspase Inhibitors; D002478:Cells, Cultured; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D004789:Enzyme Activation; D053222:Fas Ligand Protein / Q000502:physiology; D025461:Feedback, Physiological; D055786:Gene Knockout Techniques; D022781:Hepatocytes / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D000071137:Poly (ADP-Ribose) Polymerase-1; D011065:Poly(ADP-ribose) Polymerases / Q000378:metabolism; D061251:Primary Cell Culture; D015398:Signal Transduction; D051636:X-Linked Inhibitor of Apoptosis Protein / Q000235:genetics / Q000378:metabolism*; D019014:fas Receptor / Q000378:metabolism*
chemical_list: D051023:BH3 Interacting Domain Death Agonist Protein; C494815:Bid protein, mouse; D061945:Caspase Inhibitors; D015853:Cysteine Proteinase Inhibitors; D053222:Fas Ligand Protein; C491899:Fas protein, mouse; C505658:Fasl protein, mouse; D051636:X-Linked Inhibitor of Apoptosis Protein; D019014:fas Receptor; C499321:Parp1 protein, mouse; D000071137:Poly (ADP-Ribose) Polymerase-1; D011065:Poly(ADP-ribose) Polymerases; C505409:Casp3 protein, mouse; C505526:Casp8 protein, mouse; D053148:Caspase 3; D053181:Caspase 8