Proteasome inhibition can induce an autophagy-dependent apical activation of caspase-8

Cell Death Differ. 2011 Oct;18(10):1584-97. doi: 10.1038/cdd.2011.27. Epub 2011 Apr 1.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Apoptosis / genetics
  • Apoptosis / physiology
  • Autophagy / genetics
  • Autophagy / physiology*
  • Autophagy-Related Protein 5
  • Boronic Acids / pharmacology
  • Bortezomib
  • Caspase 3 / metabolism
  • Caspase 7 / metabolism
  • Caspase 8 / genetics
  • Caspase 8 / metabolism*
  • Fas-Associated Death Domain Protein / metabolism
  • Flow Cytometry
  • Fluorescence Resonance Energy Transfer
  • HeLa Cells
  • Humans
  • Immunoblotting
  • Mice
  • Mice, Mutant Strains
  • Microscopy, Fluorescence
  • Microtubule-Associated Proteins
  • Peptide Hydrolases / metabolism
  • Proteasome Endopeptidase Complex / drug effects
  • Proteasome Endopeptidase Complex / metabolism*
  • Proto-Oncogene Proteins c-bcl-2 / genetics
  • Proto-Oncogene Proteins c-bcl-2 / metabolism*
  • Pyrazines / pharmacology
  • X-Linked Inhibitor of Apoptosis Protein / antagonists & inhibitors
  • X-Linked Inhibitor of Apoptosis Protein / genetics
  • X-Linked Inhibitor of Apoptosis Protein / metabolism


  • ATG5 protein, human
  • Autophagy-Related Protein 5
  • Boronic Acids
  • FADD protein, human
  • Fas-Associated Death Domain Protein
  • Microtubule-Associated Proteins
  • Proto-Oncogene Proteins c-bcl-2
  • Pyrazines
  • X-Linked Inhibitor of Apoptosis Protein
  • XIAP protein, human
  • Bortezomib
  • Peptide Hydrolases
  • Caspase 3
  • Caspase 7
  • Caspase 8
  • Proteasome Endopeptidase Complex
  • DEVDase