title: Ubiquitin-mediated proteasomal degradation of non-synonymous SNP variants of human ABC transporter ABCG2.
abstract: Clinical relevance is implicated between the genetic polymorphisms of the ABC (ATP-binding cassette) transporter ABCG2 (ABC subfamily G, member 2) and the individual differences in drug response. We expressed a total of seven non-synonymous SNP (single nucleotide polymorphism) variants in Flp-In-293 cells by using the Flp (flippase) recombinase system. Of these, ABCG2 F208S and S441N variants were found to be expressed at markedly low levels, whereas their mRNA levels were equal to those of the other SNP variants and ABCG2 WT (wild-type). Interestingly, protein expression levels of the ABCG2 F208S and S441N variants increased 6- to 12-fold when Flp-In-293 cells were treated with MG132, a proteasome inhibitor. Immunoprecipitation followed by immunoblot analysis showed that the ABCG2 F208S and S441N variant proteins were endogenously ubiquitinated in Flp-In-293 cells, and treatment with MG132 significantly enhanced the level of these ubiquitinated variants. Immunofluorescence microscopy demonstrated that MG132 greatly affected the ABCG2 F208S and S441N variants in terms of both protein levels and intracellular distribution. Immunoblot analysis revealed that those variants were N-glycosylated; however, their oligosaccharides were immature compared with those present on ABCG2 WT. The ABCG2 F208S and S441N variant proteins do not appear to be processed in the Golgi apparatus, but undergo ubiquitin-mediated protein degradation in proteasomes, whereas ABCG2 WT is sorted to the plasma membrane and then degraded via the lysosomal pathway. The present study provides the first evidence that certain genetic polymorphisms can affect the protein stability of ABCG2. Control of proteasomal degradation of ABCG2 would provide a novel approach in cancer chemotherapy to circumvent multidrug resistance of human cancers.
authors.affiliation: Department of Biomolecular Engineering, Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, Yokohama 226-8501, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Nakagawa
authors.forename: Ai
authors.initials: A
authors.lastname: Tamura
authors.forename: Kanako
authors.initials: K
authors.lastname: Wakabayashi
authors.forename: Kazuyuki
authors.initials: K
authors.lastname: Hoshijima
authors.forename: Masayuki
authors.initials: M
authors.lastname: Komada
authors.forename: Takashi
authors.initials: T
authors.lastname: Yoshida
authors.forename: Satoshi
authors.initials: S
authors.lastname: Kometani
authors.forename: Takayoshi
authors.initials: T
authors.lastname: Matsubara
authors.forename: Kenta
authors.initials: K
authors.lastname: Mikuriya
authors.forename: Toshihisa
authors.initials: T
authors.lastname: Ishikawa
mesh_terms: D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters / Q000235:genetics* / Q000378:metabolism*; D002460:Cell Line; D005786:Gene Expression Regulation / Q000187:drug effects; D006031:Glycosylation / Q000187:drug effects; D006801:Humans; D007976:Leupeptins / Q000494:pharmacology; D018942:Macrolides / Q000494:pharmacology; D009363:Neoplasm Proteins / Q000235:genetics* / Q000378:metabolism*; D020641:Polymorphism, Single Nucleotide / Q000235:genetics*; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D025801:Ubiquitin / Q000378:metabolism*
chemical_list: C116614:ABCG2 protein, human; D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters; D007976:Leupeptins; D018942:Macrolides; D009363:Neoplasm Proteins; D025801:Ubiquitin; C040929:bafilomycin A1; D046988:Proteasome Endopeptidase Complex; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde