title: Proteasome inhibition can induce an autophagy-dependent apical activation of caspase-8.
abstract: Antiapoptotic Bcl-2 family proteins are often highly expressed in chemotherapy-resistant cancers and impair mitochondrial outer membrane permeabilisation (MOMP), an important requirement for caspase activation via the intrinsic apoptosis pathway. Interestingly, although Bcl-2 overexpression in HeLa cervical cancer cells abrogated caspase processing in response to intrinsic apoptosis induction by staurosporine, tunicamycin or etoposide, residual caspase processing was observed following proteasome inhibition by bortezomib ([(1R)-3-methyl-1-({(2S)-3-phenyl-2-[(pyrazin-2-ylcarbonyl)amino]propanoyl}amino)butyl]boronic acid), epoxomicin (N-acetyl-N-methyl-lisoleucyl-L-isoleucyl-N-[(1S)-3-methyl-1-[[(2R)-2-methyloxiranyl]carbonyl]butyl]-L-threoninamide) or MG-132 (N-(benzyloxycarbonyl)leucinylleucinylleucinal). Similar responses were found in Bcl-2-overexpressing H460 NSCLC cells and Bax/Bak-deficient mouse embyronic fibroblasts. Mild caspase processing resulted in low DEVDase activities, which were MOMP independent and persisted for long periods without evoking immediate cell death. Surprisingly, depletion of caspase-3 and experiments in caspase-7-depleted MCF-7-Bcl-2 cells indicated that the DEVDase activity did not originate from effector caspases. Instead, Fas-associated death domain (FADD)-dependent caspase-8 activation was the major contributor to the slow, incomplete substrate cleavage. Caspase-8 activation was independent of death ligands, but required the induction of autophagy and the presence of Atg5. Depletion of XIAP or addition of XIAP-antagonising peptides resulted in a switch towards efficient apoptosis execution, suggesting that the requirement for MOMP was bypassed by activating the caspase-8/caspase-3 axis. Combination treatments of proteasome inhibitors and XIAP antagonists therefore represent a promising strategy to eliminate highly resistant cancer cells, which overexpress antiapoptotic Bcl-2 family members.
authors: Department of Physiology and Medical Physics, Royal College of Surgeons in Ireland, Dublin, Ireland.
authors: M A
authors: MA
authors: Laussmann
authors: E
authors: E
authors: Passante
authors: H
authors: H
authors: Düssmann
authors: J A
authors: JA
authors: Rauen
authors: M L
authors: ML
authors: Würstle
authors: M E
authors: ME
authors: Delgado
authors: M
authors: M
authors: Devocelle
authors: J H M
authors: JH
authors: Prehn
authors: M
authors: M
authors: Rehm
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000235:genetics / Q000502:physiology; D001343:Autophagy / Q000235:genetics / Q000502:physiology*; D000071187:Autophagy-Related Protein 5; D001897:Boronic Acids / Q000494:pharmacology; D000069286:Bortezomib; D053148:Caspase 3 / Q000378:metabolism; D053179:Caspase 7 / Q000378:metabolism; D053181:Caspase 8 / Q000235:genetics / Q000378:metabolism*; D053200:Fas-Associated Death Domain Protein / Q000378:metabolism; D005434:Flow Cytometry; D031541:Fluorescence Resonance Energy Transfer; D006367:HeLa Cells; D006801:Humans; D015151:Immunoblotting; D051379:Mice; D008817:Mice, Mutant Strains; D008856:Microscopy, Fluorescence; D008869:Microtubule-Associated Proteins; D010447:Peptide Hydrolases / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex / Q000187:drug effects / Q000378:metabolism*; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000235:genetics / Q000378:metabolism*; D011719:Pyrazines / Q000494:pharmacology; D051636:X-Linked Inhibitor of Apoptosis Protein / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism
chemical_list: C502778:ATG5 protein, human; D000071187:Autophagy-Related Protein 5; D001897:Boronic Acids; C505533:FADD protein, human; D053200:Fas-Associated Death Domain Protein; D008869:Microtubule-Associated Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D011719:Pyrazines; D051636:X-Linked Inhibitor of Apoptosis Protein; C496866:XIAP protein, human; D000069286:Bortezomib; D010447:Peptide Hydrolases; D053148:Caspase 3; D053179:Caspase 7; D053181:Caspase 8; D046988:Proteasome Endopeptidase Complex; C108711:DEVDase