title: Phosphorylation of the ErbB3 binding protein Ebp1 by p21-activated kinase 1 in breast cancer cells.
abstract: The ErbB3 binding protein (Ebp1) is a transcriptional corepressor that inhibits the activity of proliferation-associated genes and the growth of human breast cancer cell lines. Treatment of breast cancer cells with the ErbB3 ligand heregulin (HRG) results in increased phosphorylation of Ebp1 and transcriptional repression. The p21-activated serine/threonine kinase 1 (PAK1), which plays an important role in breast cancer progression and resistance to the anti-oestrogen tamoxifen, is also activated by HRG. We therefore examined the ability of PAK1 to phosphorylate and regulate the function of Ebp1. We found that PAK1 phosphorylated Ebp1 in vitro and mapped the phosphorylation site to threonine 261. Both HRG treatment and expression of a constitutively activated PAK1 in MCF-7 breast cancer cells enhanced threonine phosphorylation of Ebp1. In MCF-7 cells, ectopically expressed Ebp1 bound endogenous PAK1 and this association was enhanced by treatment with HRG. Mutation of the PAK1 phosphorylation site to glutamic acid, mimicking a phosphorylated state, completely abrogated the ability of Ebp1 to repress transcription, inhibit growth of breast cancer cell lines and contribute to tamoxifen sensitivity. These studies demonstrate for the first time that Ebp1 is a substrate of PAK1 and the importance of the PAK1 phosphorylation site for the functional activity of Ebp1 in breast cancer cells.
authors.affiliation: Department of Pathology, University of Maryland, Baltimore, MD, USA.
authors.forename: D
authors.initials: D
authors.lastname: Akinmade
authors.forename: A H
authors.initials: AH
authors.lastname: Talukder
authors.forename: Y
authors.initials: Y
authors.lastname: Zhang
authors.forename: W-M
authors.initials: WM
authors.lastname: Luo
authors.forename: R
authors.initials: R
authors.lastname: Kumar
authors.forename: A W
authors.initials: AW
authors.lastname: Hamburger
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics / Q000378:metabolism*; D001943:Breast Neoplasms / Q000378:metabolism*; D049109:Cell Proliferation / Q000187:drug effects; D006801:Humans; D009154:Mutation; D010766:Phosphorylation; D011485:Protein Binding; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D013379:Substrate Specificity; D013629:Tamoxifen / Q000494:pharmacology; D014162:Transfection; D014407:Tumor Cells, Cultured; D054462:p21-Activated Kinases / Q000378:metabolism*
chemical_list: D048868:Adaptor Proteins, Signal Transducing; C101633:PA2G4 protein, human; D016601:RNA-Binding Proteins; D013629:Tamoxifen; D054462:p21-Activated Kinases