Cadherina

Molecular Biology of the Cell Vol. 18, 3214 –3223, August 2007

E-Cadherin Adhesion Activates c-Src Signaling at Cell–Cell Contacts
Robert W. McLachlan,* Astrid Kraemer,* Falak M. Helwani,* Eva M. Kovacs,† and Alpha S. Yap*
*Division of Molecular Cell Biology, Institute for Molecular Bioscience, and †School for Biomedical Science, The University of Queensland, St. Lucia, Brisbane,Queensland, Australia 4072
Submitted December 27, 2006; Revised May 4, 2007; Accepted May 29, 2007 Monitoring Editor: Keith Mostov

Cadherin-based cell– cell contacts are prominent sites for phosphotyrosine signaling, being enriched in tyrosine-phosphorylated proteins and tyrosine kinases and phosphatases. The functional interplay between cadherin adhesion and tyrosine kinase signaling, however, iscomplex and incompletely understood. In this report we tested the hypothesis that cadherin adhesion activates c-Src signaling and sought to assess its impact on cadherin function. We identified c-Src as part of a cadherin-activated cell signaling pathway that is stimulated by ligation of the adhesion receptor. However, c-Src has a biphasic impact on cadherin function, exerting a positive supportiverole at lower signal strengths, but inhibiting function at high signal strengths. Inhibiting c-Src under circumstances when it is activated by cadherin adhesion decreased several measures of cadherin function. This suggests that the cadherin-activated c-Src signaling pathway serves positively to support cadherin function. Finally, our data implicate PI3-kinase signaling as a target forcadherin-activated c-Src signaling that contributes to its positive impact on cadherin function. We conclude that E-cadherin signaling is an important activator of c-Src at cell– cell contacts, providing a key input into a signaling pathway where quantitative changes in signal strength may result in qualitative differences in functional outcome.

INTRODUCTION Classical cadherin cell adhesion molecules arefundamental determinants of tissue organization whose biological function is intimately linked to cell signaling. Diverse signaling molecules are found at cell– cell contacts and many are activated in a cadherin-dependent manner when cells adhere to one another (Yap and Kovacs, 2003). Among the relevant signals, it has long been recognized that cadherin adhesions are major sites for proteintyrosine phosphorylation. Many tyrosine-phosphorylated proteins are found at adherens junctions, which are, indeed, conspicuous sites for phosphotyrosine staining in cells (Takata and Singer, 1988; Tsukita et al., 1991; Calautti et al., 1998). Moreover, a wide range of tyrosine kinases are found at cell– cell contacts, including both receptor tyrosine kinases and cytoplasmic kinases (Tsukita et al.,1991; Thomas and Brugge, 1997; Calautti et al., 1998). However, the precise relationship between cadherin adhesion and tyrosine kinase signaling remains poorly understood. This is exemplified by c-Src and other Src-family kinases (SFKs). Members of this family of cytoplasmic tyrosine kinases are often found at cadherin-based cell– cell contacts (Tsukita et al., 1991; Calautti et al., 1998). Avariety of studies further reported that expression of v-Src or constitutively active (CA) Src perturbs the integrity of cell– cell interactions in epithelia (Warren and Nelson, 1987; Volberg et al., 1991). This was accompanied by tyThis article was published online ahead of print in MBC in Press (http://www.molbiolcell.org/cgi/doi/10.1091/mbc.E06 –12–1154) on June 6, 2007. Address correspondence to:Alpha S. Yap (a.yap@imb.uq.edu.au). 3214

rosine phosphorylation of components of the cadherin/ catenin complex, notably -catenin (Matsuyoshi et al., 1992; Behrens et al., 1993). Together these have led to the common notion that SFK signaling negatively regulates cadherin adhesion. In contrast, other studies that typically utilized loss-of-function approaches reported the capacity for SFKs to...