title: Endoplasmic reticulum vacuolization and valosin-containing protein relocalization result from simultaneous hsp90 inhibition by geldanamycin and proteasome inhibition by velcade.
abstract: Geldanamycin and Velcade, new anticancer drugs with novel mechanisms of action, are currently undergoing extensive clinical trials. Geldanamycin interrupts Hsp90 chaperone activity and causes down-regulation of its many client proteins by the ubiquitin-proteasome pathway; Velcade is a specific proteasome inhibitor. Misfolded Hsp90 clients within the endoplasmic reticulum (ER) lumen are cleared by ER--associated protein degradation, a sequential process requiring valosin-containing protein (VCP)-dependent retrotranslocation followed by ubiquitination and proteasomal proteolysis. Cotreatment of cells with geldanamycin and Velcade prevents destruction of destabilized, ubiquitinated Hsp90 client proteins, causing them to accumulate. Here, we report that misfolded protein accumulation within the ER resulting from geldanamycin and Velcade exposure overwhelms the ability of the VCP--centered machine to maintain the ER secretory pathway, causing the ER to distend into conspicuous vacuoles. Overexpression of dominant-negative VCP or the "small VCP--interacting protein" exactly recapitulated the vacuolated phenotype provoked by the drugs, associating loss of VCP function with ER vacuolization. In cells transfected with a VCP--enhanced yellow fluorescent protein fluorescent construct, geldanamycin plus Velcade treatment redistributed VCP--enhanced yellow fluorescent protein from the cytoplasm and ER into perinuclear aggresomes. In further support of the view that compromise of VCP function is responsible for ER vacuolization, small interfering RNA interference of VCP expression induced ER vacuolization that was markedly increased by Velcade. VCP knockdown by small interfering RNA eventually deconstructed both the ER and Golgi and interdicted protein trafficking through the secretory pathway to the plasma membrane. Thus, simultaneous geldanamycin and Velcade treatment has far-reaching secondary cytotoxic consequences that likely contribute to the cytotoxic activity of this anticancer drug combination.
authors.affiliation: Urologic Oncology Branch, Center for Cancer Research, National Cancer Institute, NIH, 9000 Rockville Pike, Building 10, Room 1-5940, Bethesda, MD 20892-1107, USA.
authors.forename: Edward G
authors.initials: EG
authors.lastname: Mimnaugh
authors.forename: Wanping
authors.initials: W
authors.lastname: Xu
authors.forename: Michele
authors.initials: M
authors.lastname: Vos
authors.forename: Xitong
authors.initials: X
authors.lastname: Yuan
authors.forename: Len
authors.initials: L
authors.lastname: Neckers
mesh_terms: D000251:Adenosine Triphosphatases; D000818:Animals; D016227:Benzoquinones / Q000494:pharmacology*; D001897:Boronic Acids / Q000494:pharmacology*; D000069286:Bortezomib; D019556:COS Cells; D018797:Cell Cycle Proteins / Q000096:biosynthesis / Q000378:metabolism*; D002522:Chlorocebus aethiops; D004721:Endoplasmic Reticulum / Q000187:drug effects* / Q000378:metabolism*; D018734:Genes, erbB-2; D018841:HSP90 Heat-Shock Proteins / Q000037:antagonists & inhibitors*; D047029:Lactams, Macrocyclic / Q000494:pharmacology*; D011480:Protease Inhibitors / Q000494:pharmacology; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D061988:Proteasome Inhibitors; D021381:Protein Transport / Q000187:drug effects / Q000502:physiology; D011719:Pyrazines / Q000494:pharmacology*; D034741:RNA, Small Interfering / Q000235:genetics; D014415:Tunicamycin / Q000494:pharmacology; D014452:Ubiquitins / Q000378:metabolism; D014617:Vacuoles / Q000187:drug effects / Q000378:metabolism*; D000074405:Valosin Containing Protein
chemical_list: D016227:Benzoquinones; D001897:Boronic Acids; D018797:Cell Cycle Proteins; D018841:HSP90 Heat-Shock Proteins; D047029:Lactams, Macrocyclic; D011480:Protease Inhibitors; D061988:Proteasome Inhibitors; D011719:Pyrazines; D034741:RNA, Small Interfering; D014452:Ubiquitins; D014415:Tunicamycin; D000069286:Bortezomib; D046988:Proteasome Endopeptidase Complex; D000251:Adenosine Triphosphatases; D000074405:Valosin Containing Protein; C001277:geldanamycin