1. ¹Key Laboratory of Proteomics, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai, China
2. ²SHARF Laboratory, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai, China
3. ³Graduate School of the Chinese Academy of Sciences, Max-Planck-Institute of Biochemistry, Martinsried, Germany
4. ⁴Department of Molecular Biology, Max-Planck-Institute of Biochemistry, Martinsried, Germany

Correspondence: Dr Z Chen, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Room 210, Building 23, 320 Yue Yang Road, Shanghai 200031, PR China. E-mail: zjchen@sibs.ac.cn

Received 9 November 2005; Revised 24 February 2006; Accepted 24 February 2006; Published online 24 April 2006.

Abstract

Epidermal growth factor receptor (EGFR) and Src tyrosine kinase cooperate in regulating EGFR-mediated cell signaling and promoting cell transformation and tumorigenesis in pathological conditions. Activation of Src is tightly regulated by the C-terminal Src kinase (Csk). The Csk-binding protein (Cbp) is a ubiquitously expressed transmembrane protein. Its functions include suppression of T-cell receptor activation through recruiting Csk and inhibiting Src family kinase (SFK). However, a potential role of Cbp in EGF-induced cell activities has not been investigated. Here, we report that EGF-stimulation-induced Cbp tyrosine phosphorylation followed by Cbp–Csk association, in a SFK-dependent manner. Expression of wild-type (wt) Cbp remarkably suppressed EGF-induced activation of Src, ERK1/2, and Akt-1 enzymes, and NIH3T3 cell transformation, as well as colony formation of a breast cancer cell line (MDA-MB-468) in soft agar. In contrast, expression of CbpY317F or knockdown endogenous Cbp in NIH3T3 cells by RNA interference significantly enhanced EGF-induced activation of these enzymes and cell transformation. In addition, overexpression of multiple receptor tyrosine kinases (RTKs)-induced Cbp tyrosine phosphorylation. These results demonstrate that Cbp functions as a negative regulator of cell transformation and tumor cell growth through downregulation of Src activation, suggesting that Cbp might be broadly involved in RTKs-activated signaling pathways and tumorigenesis.