Carcinogenesis Advance Access originally published online on January 3, 2008
Carcinogenesis 2008 29(2):244-251; doi:10.1093/carcin/bgm245
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Grb2 binding to Tyr284 in TβR-II is essential for mammary tumor growth and metastasis stimulated by TGF-β
Department of Pharmacology, University of Colorado Health Sciences Center, Aurora, CO 80045, USA
* To whom correspondence should be addressed at Department of Pharmacology, University of Colorado Health Sciences Center, RC1 South Tower, Room L18-6110, 12801 East 17th Avenue, PO Box 6511, Aurora, Colorado 80045, USA. Tel: +1-303-724-1541; Fax: +1-303-724-3663; Email: bill.schiemann{at}uchsc.edu
We demonstrated previously that growth factor receptor-bound protein 2 (Grb2) associates with the transforming growth factor-β (TGF-β) type II receptor [TβR-II] upon its phosphorylation on Tyr284 by Src. Although this phosphotransferase reaction is critical in mediating TGF-β stimulation of epithelial-mesenchymal transition (EMT) and invasion in mammary epithelial cells (MECs), the necessity of Grb2 in promoting these TGF-β-dependent events remain purely correlative. Herein, we further evaluated the role of Grb2 in mediating the oncogenic activities of TGF-β and show that the binding of Grb2 to TβR-II paralleled the induction of EMT in MECs stimulated by TGF-β. Introducing siRNAs against Grb2 or expression of a TβR-II mutant that cannot bind Grb2 (i.e. Y284F-TβR-II) had no effect on the ability of TGF-β to activate Smad3, but significantly impaired its stimulation of p38 mitogen-activated protein kinase (MAPK) in MECs. Importantly, these same cellular conditions also prevented the ability of MECs to undergo EMT in response to TGF-β, and to invade synthetic basement membranes when stimulated by β3 integrin and TGF-β. Finally, we show that the ability of TGF-β to stimulate breast cancer growth and pulmonary metastasis in mice required TβR-II to be phosphorylated on Tyr284, which activated p38 MAPK in developing and progressing mammary tumors. Collectively, our findings have established the necessity of Grb2 in mediating TGF-β stimulation of EMT and invasion in MECs, as well as demonstrated the essential function of the 
vβ3 integrin:Src:phospho-Y284-TβR-II:Grb2:p38 MAPK signaling axis to promote breast cancer growth and metastasis in vivo.
Abbreviations: bFGF, basic fibroblast growth factor; EMT, epithelial-mesenchymal transition; GFP, green fluorescent protein; Grb2, growth factor receptor-bound protein 2; MAPK, mitogen-activated protein kinase; MEC, mammary epithelial cell; RTK, receptor tyrosine kinase; TGF-β, transforming growth factor-β; TNF-, tumor necrosis factor-alpha; TβR-II, transforming growth factor-β type II receptor; VEGF, vascular endothelial growth factor; WT, wild-type
Received August 18, 2007; revised October 24, 2007; accepted October 26, 2007.