title: Suppression of BRCA1 sensitizes cells to proteasome inhibitors.
abstract: BRCA1 is a multifunctional protein best known for its role in DNA repair and association with breast and ovarian cancers. To uncover novel biologically significant molecular functions of BRCA1, we tested a panel of 198 approved and experimental drugs to inhibit growth of MDA-MB-231 breast cancer cells depleted for BRCA1 by siRNA. 26S proteasome inhibitors bortezomib and carfilzomib emerged as a new class of selective BRCA1-targeting agents. The effect was confirmed in HeLa and U2OS cancer cell lines using two independent siRNAs, and in mouse embryonic stem (ES) cells with inducible deletion of Brca1. Bortezomib treatment did not cause any increase in nuclear foci containing phosphorylated histone H2AX, and knockdown of BRCA2 did not entail sensitivity to bortezomib, suggesting that the DNA repair function of BRCA1 may not be directly involved. We found that a toxic effect of bortezomib on BRCA1-depleted cells is mostly due to deregulated cell cycle checkpoints mediated by RB1-E2F pathway and 53BP1. Similar to BRCA1, depletion of RB1 also conferred sensitivity to bortezomib, whereas suppression of E2F1 or 53BP1 together with BRCA1 reduced induction of apoptosis after bortezomib treatment. A gene expression microarray study identified additional genes activated by bortezomib treatment only in the context of inactivation of BRCA1 including a critical involvement of the ERN1-mediated unfolded protein response. Our data indicate that BRCA1 has a novel molecular function affecting cell cycle checkpoints in a manner dependent on the 26S proteasome activity.
authors.affiliation: Institute for Molecular Medicine Finland (FIMM), University of Helsinki, PO Box 20, Helsinki 00014, Finland.
authors.forename: Y
authors.initials: Y
authors.lastname: Gu
authors.affiliation: Division of Molecular Pathology and Cancer Genomics Centre, The Netherlands Cancer Institute, Amsterdam, The Netherlands.
authors.forename: P
authors.initials: P
authors.lastname: Bouwman
authors.affiliation: Unit of Systems Toxicology, Finnish Institute of Occupational Health (FIOH), Helsinki, Finland.
authors.forename: D
authors.initials: D
authors.lastname: Greco
authors.affiliation: Institute for Molecular Medicine Finland (FIMM), University of Helsinki, PO Box 20, Helsinki 00014, Finland.
authors.forename: J
authors.initials: J
authors.lastname: Saarela
authors.affiliation: Institute for Molecular Medicine Finland (FIMM), University of Helsinki, PO Box 20, Helsinki 00014, Finland.
authors.forename: B
authors.initials: B
authors.lastname: Yadav
authors.affiliation: Division of Molecular Pathology and Cancer Genomics Centre, The Netherlands Cancer Institute, Amsterdam, The Netherlands.
authors.forename: J
authors.initials: J
authors.lastname: Jonkers
authors.affiliation: Institute for Molecular Medicine Finland (FIMM), University of Helsinki, PO Box 20, Helsinki 00014, Finland.
authors.forename: S G
authors.initials: SG
authors.lastname: Kuznetsov
mesh_terms: D000818:Animals; D019313:BRCA1 Protein / Q000235:genetics* / Q000378:metabolism; D001897:Boronic Acids / Q000494:pharmacology*; D000069286:Bortezomib; D001943:Breast Neoplasms / Q000201:enzymology / Q000235:genetics* / Q000378:metabolism; D002453:Cell Cycle / Q000187:drug effects; D045744:Cell Line, Tumor; D015536:Down-Regulation; D005260:Female; D020868:Gene Silencing; D006801:Humans; D051379:Mice; D046988:Proteasome Endopeptidase Complex / Q000235:genetics / Q000378:metabolism; D061988:Proteasome Inhibitors / Q000494:pharmacology*; D011719:Pyrazines / Q000494:pharmacology*; D034741:RNA, Small Interfering / Q000235:genetics / Q000378:metabolism; D016160:Retinoblastoma Protein / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction
chemical_list: D019313:BRCA1 Protein; D001897:Boronic Acids; D061988:Proteasome Inhibitors; D011719:Pyrazines; D034741:RNA, Small Interfering; D016160:Retinoblastoma Protein; D000069286:Bortezomib; D046988:Proteasome Endopeptidase Complex; C061553:ATP dependent 26S protease