Phosphorylation of the ErbB3 binding protein Ebpl by p21 -activated kinase 1 in breast cancer cells

• Published in: British journal of cancer 2008-03, Vol.98 (6), p.1132-1140
• Main Authors: Akinmade, D, Talukder, AH, Zhang, Y, Luo, W-M, Kumar, R, Hamburger, A W
• Format: Article
• Language: English
• ISSN: 0007-0920
• DOI: 10.1038/sj.bjc.6604261

The ErbB3 binding protein (Ebpl) 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 Ebpl and transcriptional repression. The p21-activated serine/threonine kinase 1 (PAKI), 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 PAKI to phosphorylate and regulate the function of Ebpl. We found that PAKI phosphorylated Ebpl in vitro and mapped the phosphorylation site to threonine 261. Both HRG treatment and expression of a constitutively activated PAKI in MCF-7 breast cancer cells enhanced threonine phosphorylation of Ebpl. In MCF-7 cells, ectopically expressed Ebpl bound endogenous PAKI and this association was enhanced by treatment with HRG. Mutation of the PAKI phosphorylation site to glutamic acid, mimicking a phosphorylated state, completely abrogated the ability of Ebpl to repress transcription, inhibit growth of breast cancer cell lines and contribute to tamoxifen sensitivity. These studies demonstrate for the first time that Ebpl is a substrate of PAKI and the importance of the PAKI phosphorylation site for the functional activity of Ebpl in breast cancer cells.