**4.4 p140Cap modulates Src activity and EGFR signalling in breast cancer cells**

The major function of the p140Cap adaptor is its ability to regulate Src kinase activation. In particular, in breast cancer cells, upon cell-matrix adhesion or EGF stimulation, p140Cap activates the Csk kinase, that phosphorylates the negative regulatory tyrosine 530 on the Cterminal domain of Src (Latour and Veillette, 2001) , resulting in inhibition of Src kinase. Consistently p140Cap silencing increases Src activation, leading to a fine tuning of integrin and growth factor receptor signalling (Figure 2) (Damiano *et al.,* 2010; Di Stefano *et al.*, 2007) As a consequence, in breast cancer cells expressing high levels of p140Cap, upon integrinmediated adhesion, the association between Src and Fak is impaired as well as integrindependent p130Cas phosphorylation (Figure 2). As described above p130Cas phosphorylation leads to the assembly of a p130Cas-Crk signalling complex that drives for cell migration and invasion through activation of Rac. Therefore elevated levels of p140Cap severely impair integrin-dependent Rac activity, while its down-regulation induces a sustained Rac activation (Di Stefano *et al.*, 2007).

In MCF7 breast cancer cells, p140Cap functionally interacts with E-cadherin and EGFR at the cell membrane, behaving as a new player in E-cadherin-dependent down-regulation of EGFR signalling. Indeed p140Cap-dependent inhibition of Src kinase activity results in Ecadherin immobilization at the cell membrane (Damiano *et al.,* 2010). E-cadherin is known to inhibit EGFR, either by interaction through the extracellular domains or by a beta catenindependent mechanism (Perrais *et al.*, 2007; Qian *et al.*, 2004; Takahashi and Suzuki, 1996). Consistently, EGFR activation, association and phosphorylation of Grb2 and Shc and Ras/Erk1/2 MAPK activities are profoundly impaired by p140Cap over-expression and enhanced by its silencing (Damiano *et al.,* 2010). Interestingly, rescue of Src activity and of Ecadherin mobility is sufficient to recover EGFR phosphorylation, but not Ras and Erk1/2 activation, that require an active RasV12, suggesting that p140Cap might regulate the Ras pathway through an additional mechanism. Therefore, in MCF7 cancer cells, p140Cap regulates EGFR signalling with dual mechanisms, involving both an E-cadherin-dependent inactivation of EGFR and a Ras-dependent inhibition of Erk1/2 activity (Damiano *et al.,*  2010).

Moreover, p140Cap expression also inhibits EGFR, Src and Erk phosphorylation in the highly aggressive MTLn3-EGFR breast cancer cells. Interestingly, in these cells, p140Cap affects also Cortactin phosphorylation in response to EGF (Damiano *et al.*, 2011).
