title: Proteasome inhibition by quercetin triggers macroautophagy and blocks mTOR activity.
abstract: The bioflavonoid quercetin has long been known to exert anti-tumor effects, although the underlying mechanisms remain unknown. Investigation of the potential interference of this anti-oxidant with the efficacy of cell stress-inducing anti-cancer drugs revealed extensive intracellular vacuolation induced by quercetin in epithelial cancer cells that led to cell cycle arrest and ensuing apoptosis. Accumulation of biomarkers of autophagy, including fluorescent autophagy markers and acidotropic dyes characterized these vacuoles as phagolysosomes. Prior to the formation of autophagosomes, an immediate and pronounced inhibition of the autophagy-controlling mTOR activity in quercetin-treated cancer cells occurred, accompanied by a marked reduction in the phosphorylation of the mTOR substrates 4E-BP1 and p70S6 kinase. Assessment of cellular proteasome activity revealed an effective and immediate inhibition of the activity of the proteasome by quercetin in cancer cells. In addition to the formation of autophagosomes, accumulation of poly-ubiquitinated protein aggregates was observed. Thus, proteasome inhibition by quercetin can be regarded as a major cause of quercetin-induced cancer cell death. These results suggest potential new applications for quercetin in cancer science and identify quercetin as an easy-to-handle agent to study proteasome activity, mTOR signaling and autophagy in human cancer cells for cell biological purposes.
authors: Department of Obstetrics and Gynecology, Campus Innenstadt, Ludwig-Maximilians-University Munich, Munich, Germany.
authors: Anja K
authors: AK
authors: Klappan
authors: Stefanie
authors: S
authors: Hones
authors: Ioannis
authors: I
authors: Mylonas
authors: Ansgar
authors: A
authors: Brüning
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology*; D001343:Autophagy / Q000187:drug effects*; D002453:Cell Cycle / Q000187:drug effects; D016923:Cell Death / Q000187:drug effects; D049109:Cell Proliferation / Q000187:drug effects; D004354:Drug Screening Assays, Antitumor; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D006367:HeLa Cells; D006801:Humans; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D061988:Proteasome Inhibitors; D011794:Quercetin / Q000494:pharmacology*; D015398:Signal Transduction / Q000187:drug effects; D013329:Structure-Activity Relationship; D058570:TOR Serine-Threonine Kinases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D014407:Tumor Cells, Cultured
chemical_list: D000970:Antineoplastic Agents; D004791:Enzyme Inhibitors; D061988:Proteasome Inhibitors; D011794:Quercetin; C546842:MTOR protein, human; D058570:TOR Serine-Threonine Kinases; D046988:Proteasome Endopeptidase Complex