title: Down-regulation of BRCA1-BARD1 ubiquitin ligase by CDK2.
abstract: BRCA1, a breast and ovarian tumor suppressor, is a phosphoprotein whose cellular expression level is regulated in a cell cycle-dependent manner. BRCA1 interacts with BARD1 to generate significant ubiquitin ligase activity which catalyzes nontraditional Lys-6-linked polyubiquitin chains. However, it is not clear how the activity is regulated and how this affects BRCA1's multiple cellular functions. Here we show that the ubiquitin ligase activity of BRCA1-BARD1 is down-regulated by CDK2. During the cell cycle, BARD1 expression can largely be categorized into three patterns: moderately expressed in a predominantly unphosphorylated form in early G(1) phase, expressed at low levels in both phosphorylated and unphosphorylated forms during late G(1) and S phases, and highly expressed in its phosphorylated form during mitosis coinciding with BRCA1 expression. CDK2-cyclin A1/E1 and CDK1-cyclin B1 phosphorylate BARD1 on its NH(2) terminus in vivo and in vitro. Intriguingly, the BRCA1-BARD1-mediated in vivo ubiquitination of nucleophosmin/B23 (NPM) and autoubiquitination of BRCA1 are dramatically disrupted by coexpression of CDK2-cyclin A1/E1, but not by CDK1-cyclin B1. The inhibition of ubiquitin ligase activity is not due to the direct effect of the kinases on BARD1 because an unphosphorylatable mutant of BARD1, S148A/S251A/S288A/T299A, is still inhibited by CDK2-cyclin E1. Alternatively, BRCA1 and BARD1 are likely exported to the cytoplasm and their expressions are remarkably reduced by CDK2-cyclin E1 coexpression. Recognizing the importance of cyclin E1 overexpression in breast cancer development, these results suggest a CDK2-BRCA1-NPM pathway that coordinately functions in cell growth and tumor progression pathways.
authors.affiliation: Division of Breast and Endocrine Surgery, St. Marianna University School of Medicine, Kawasaki, Japan.
authors.forename: Ryosuke
authors.initials: R
authors.lastname: Hayami
authors.forename: Ko
authors.initials: K
authors.lastname: Sato
authors.forename: Wenwen
authors.initials: W
authors.lastname: Wu
authors.forename: Toru
authors.initials: T
authors.lastname: Nishikawa
authors.forename: Junya
authors.initials: J
authors.lastname: Hiroi
authors.forename: Ritsuko
authors.initials: R
authors.lastname: Ohtani-Kaneko
authors.forename: Mamoru
authors.initials: M
authors.lastname: Fukuda
authors.forename: Tomohiko
authors.initials: T
authors.lastname: Ohta
mesh_terms: D019313:BRCA1 Protein / Q000235:genetics* / Q000378:metabolism; D001943:Breast Neoplasms; D042846:CDC2-CDC28 Kinases / Q000378:metabolism*; D019927:Cyclin E; D051357:Cyclin-Dependent Kinase 2; D005260:Female; D006801:Humans; D016297:Mutagenesis, Site-Directed; D015513:Oncogene Proteins / Q000378:metabolism; D010051:Ovarian Neoplasms; D010766:Phosphorylation; D017354:Point Mutation; D011994:Recombinant Proteins / Q000378:metabolism; D014162:Transfection; D025521:Tumor Suppressor Proteins / Q000037:antagonists & inhibitors / Q000235:genetics* / Q000378:metabolism; D044767:Ubiquitin-Protein Ligases / Q000037:antagonists & inhibitors / Q000235:genetics* / Q000378:metabolism
chemical_list: D019313:BRCA1 Protein; C475820:CCNE1 protein, human; D019927:Cyclin E; D015513:Oncogene Proteins; D011994:Recombinant Proteins; D025521:Tumor Suppressor Proteins; C102449:BARD1 protein, human; D044767:Ubiquitin-Protein Ligases; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; D051357:Cyclin-Dependent Kinase 2