title: Chemical blockage of the proteasome inhibitory function of bortezomib: impact on tumor cell death.
abstract: The proteasome inhibitor bortezomib is emerging as a potent anti-cancer agent. Still, recent clinical trials have revealed a significant secondary toxicity of bortezomib. Consequently, there is much interest in dissecting the mechanism of action of this compound to rationally improve its therapeutic index. The cytotoxic effect of bortezomib is frequently characterized by interfering with downstream events derived from the accumulation of proteasomal targets. Here we identify the first chemical agent able to act upstream of the proteasome to prevent cell killing by bortezomib. Specifically, we show that the polyhydroxyl compound Tiron can function as a competitive inhibitor of bortezomib. This effect of Tiron was surprising, since it is a classical radical spin trap and was expected to scavenge reactive oxygen species produced as a consequence of bortezomib action. The inhibitory effect of Tiron against bortezomib was selective, since it was not shared by other antioxidants, such as vitamin E, MnTBAP, L-N-acetyl-cysteine, and FK-506. Comparative analyses with nonboronated proteasome inhibitors (i.e. MG132) revealed a specificity of Tiron for bortezomib. We exploited this novel feature of Tiron to define the "point of no return" of proteasome inhibition in melanoma cells and to block cell death in a three-dimensional model of human skin. Cells from T-cell lymphoma, breast carcinoma, and non-small cell lung cancer were also responsive to Tiron, suggesting a broad impact of this agent as a bortezomib blocker. These results may have important implications for the analysis of bortezomib in vivo and for the design of drug mixtures containing proteasome inhibitors.
authors.affiliation: Comprehensive Cancer Center and the Department of Dermatology, University of Michigan, Ann Arbor, 48109, USA.
authors.forename: Yolanda
authors.initials: Y
authors.lastname: Fernández
authors.forename: Thomas P
authors.initials: TP
authors.lastname: Miller
authors.forename: Christophe
authors.initials: C
authors.lastname: Denoyelle
authors.forename: Jose A
authors.initials: JA
authors.lastname: Esteban
authors.forename: Wen-Hua
authors.initials: WH
authors.lastname: Tang
authors.forename: Audrey L
authors.initials: AL
authors.lastname: Bengston
authors.forename: María S
authors.initials: MS
authors.lastname: Soengas
mesh_terms: D014013:1,2-Dihydroxybenzene-3,5-Disulfonic Acid Disodium Salt / Q000737:chemistry / Q000494:pharmacology*; D000970:Antineoplastic Agents / Q000494:pharmacology; D000975:Antioxidants / Q000737:chemistry; D001667:Binding, Competitive; D001897:Boronic Acids / Q000494:pharmacology*; D000069286:Bortezomib; D001943:Breast Neoplasms / Q000378:metabolism; D016923:Cell Death / Q000187:drug effects*; D002460:Cell Line; D045744:Cell Line, Tumor; D002470:Cell Survival; D004336:Drug Antagonism; D005347:Fibroblasts / Q000378:metabolism; D005609:Free Radicals; D006801:Humans; D015151:Immunoblotting; D007700:Kinetics; D007976:Leupeptins / Q000494:pharmacology; D008544:Melanocytes / Q000378:metabolism; D008545:Melanoma / Q000188:drug therapy; D008564:Membrane Potentials; D008954:Models, Biological; D011480:Protease Inhibitors / Q000494:pharmacology; D061988:Proteasome Inhibitors; D011719:Pyrazines / Q000494:pharmacology*; D017382:Reactive Oxygen Species; D012867:Skin / Q000473:pathology; D013997:Time Factors
chemical_list: D000970:Antineoplastic Agents; D000975:Antioxidants; D001897:Boronic Acids; D005609:Free Radicals; D007976:Leupeptins; D011480:Protease Inhibitors; D061988:Proteasome Inhibitors; D011719:Pyrazines; D017382:Reactive Oxygen Species; D014013:1,2-Dihydroxybenzene-3,5-Disulfonic Acid Disodium Salt; D000069286:Bortezomib; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde