title: Proteasome inhibitors activate autophagy as a cytoprotective response in human prostate cancer cells.
abstract: The ubiquitin-proteasome and lysosome-autophagy pathways are the two major intracellular protein degradation systems that work cooperatively to maintain homeostasis. Proteasome inhibitors (PIs) have clinical activity in hematological tumors, and inhibitors of autophagy are also being evaluated as potential antitumor therapies. In this study, we found that chemical PIs and small interfering RNA-mediated knockdown of the proteasome's enzymatic subunits promoted autophagosome formation, stimulated autophagic flux, and upregulated expression of the autophagy-specific genes (ATGs) (ATG5 and ATG7) in some human prostate cancer cells and immortalized mouse embryonic fibroblasts (MEFs). Upregulation of ATG5 and ATG7 only occurred in cells displaying PI-induced phosphorylation of the eukaryotic translation initiation factor 2 alpha (eIF2alpha), an important component of the unfolded protein responses. Furthermore, PIs did not induce autophagy or upregulate ATG5 in MEFs expressing a phosphorylation-deficient mutant form of eIF2alpha. Combined inhibition of autophagy and the proteasome induced an accumulation of intracellular protein aggregates reminiscent of neuronal inclusion bodies and caused more cancer cell death than blocking either degradation pathway alone. Overall, our data show that proteasome inhibition activates autophagy through a phospho-eIF2alpha-dependent mechanism to eliminate protein aggregates and alleviate proteotoxic stress.
authors.affiliation: Department of Cancer Biology, The University of Texas MD Anderson Cancer Center, Houston, TX 77030, USA.
authors.forename: K
authors.initials: K
authors.lastname: Zhu
authors.forename: K
authors.initials: K
authors.lastname: Dunner
authors.forename: D J
authors.initials: DJ
authors.lastname: McConkey
mesh_terms: D000818:Animals; D001343:Autophagy / Q000187:drug effects* / Q000235:genetics; D000071187:Autophagy-Related Protein 5; D000071193:Autophagy-Related Protein 7; D001897:Boronic Acids / Q000494:pharmacology; D000069286:Bortezomib; D002460:Cell Line; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000187:drug effects; D006801:Humans; D015151:Immunoblotting; D007783:Lactones / Q000494:pharmacology; D008247:Lysosomes / Q000378:metabolism / Q000648:ultrastructure; D008297:Male; D046529:Microscopy, Electron, Transmission; D008869:Microtubule-Associated Proteins / Q000235:genetics / Q000378:metabolism; D010588:Phagosomes / Q000378:metabolism* / Q000648:ultrastructure; D011471:Prostatic Neoplasms / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D011480:Protease Inhibitors / Q000494:pharmacology*; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D061988:Proteasome Inhibitors; D011719:Pyrazines / Q000494:pharmacology; D011758:Pyrroles / Q000494:pharmacology; D034622:RNA Interference; D020133:Reverse Transcriptase Polymerase Chain Reaction; D044764:Ubiquitin-Activating Enzymes / Q000235:genetics / Q000378:metabolism
chemical_list: C502778:ATG5 protein, human; C495773:Atg5 protein, mouse; C502661:Atg7 protein, mouse; D000071187:Autophagy-Related Protein 5; D001897:Boronic Acids; D007783:Lactones; D008869:Microtubule-Associated Proteins; D011480:Protease Inhibitors; D061988:Proteasome Inhibitors; D011719:Pyrazines; D011758:Pyrroles; D000069286:Bortezomib; C475865:marizomib; D046988:Proteasome Endopeptidase Complex; C509349:ATG7 protein, human; D000071193:Autophagy-Related Protein 7; D044764:Ubiquitin-Activating Enzymes