title: Impairment of the ubiquitin-proteasome system by cellular FLIP.
abstract: Cellular FLIP (cFLIP) is a homologue of caspase-8 without protease activity that inhibits the apoptosis signaling initiated by death receptor ligation. We previously reported that a long form of cFLIP (cFLIP-L) inhibits ubiquitylation of beta-catenin and enhances Wnt signaling. Here we show that cFLIP-L impairs the function of the ubiquitin-proteasome system (UPS), and increases the accumulation of various short-lived proteins, such as GFP conjugated with destabilization sequence, beta-catenin and HIF1 alpha, that are subjected to rapid ubiquitylation and degradation by proteasomes. Accordingly, beta-catenin- and HIF1 alpha-mediated gene expressions are induced in the cFLIP-L-expressing cells. Exogenously expressed cFLIP-L accumulates in aggregates at the peri-nuclear region in the cells, and the cFLIP-L aggregates are refractory to solubilization. Like exogenously expressed cFLIP-L, the endogenous cFLIP in A549 lung cancer cells displays particulate distribution in the cells and more than 60% of cFLIP-L is refractory to solubilization. Down-regulation of cFLIP in A549 cells by RNA-mediated interference reduced beta-catenin- and HIF1 alpha-mediated gene expression. These results suggest that cFLIP-L is prone to aggregate and impairs UPS function, which could be involved in the pathological function of cFLIP-L expressed in certain cancer cells.
authors.affiliation: Institute of Molecular and Cellular Biosciences, The University of Tokyo, Bunkyo-ku, Tokyo 113-0032, Japan.
authors.forename: Toshiyasu
authors.initials: T
authors.lastname: Ishioka
authors.forename: Ryohei
authors.initials: R
authors.lastname: Katayama
authors.forename: Ryo
authors.initials: R
authors.lastname: Kikuchi
authors.forename: Michie
authors.initials: M
authors.lastname: Nishimoto
authors.forename: Shinji
authors.initials: S
authors.lastname: Takada
authors.forename: Ritsuko
authors.initials: R
authors.lastname: Takada
authors.forename: Shu-ichi
authors.initials: S
authors.lastname: Matsuzawa
authors.forename: John C
authors.initials: JC
authors.lastname: Reed
authors.forename: Takashi
authors.initials: T
authors.lastname: Tsuruo
authors.forename: Mikihiko
authors.initials: M
authors.lastname: Naito
mesh_terms: D017209:Apoptosis; D053446:CASP8 and FADD-Like Apoptosis Regulating Protein / Q000378:metabolism*; D002460:Cell Line; D045744:Cell Line, Tumor; D015536:Down-Regulation; D005786:Gene Expression Regulation; D015972:Gene Expression Regulation, Neoplastic; D049452:Green Fluorescent Proteins / Q000378:metabolism; D006801:Humans; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex / Q000737:chemistry*; D034622:RNA Interference; D015398:Signal Transduction; D025801:Ubiquitin / Q000737:chemistry* / Q000378:metabolism; D051176:beta Catenin / Q000378:metabolism
chemical_list: D053446:CASP8 and FADD-Like Apoptosis Regulating Protein; C497442:HIF1A protein, human; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D025801:Ubiquitin; D051176:beta Catenin; D049452:Green Fluorescent Proteins; D046988:Proteasome Endopeptidase Complex