**3. The ErbB2 oncogene in breast cancer**

The ErbB2 oncogene is a member of the Epidermal Growth Factor Receptor (EGFR) family of receptor tyrosine kinases (RTKs). This family comprises four related members: EGFR, ErbB2 (also known as Neu, HER-2), ErbB3 (HER-3), and ErbB4 (HER-4) (Holbro *et al.*, 2003). Over-expressed and mutated ErbB2 has been found in human tumors and cancer cell lines (Mukohara; Yarden *et al.*, 2004). In addition, several studies have shown a strong correlation of ErbB2 over-expression with a negative clinical prognosis in breast cancer (Choi *et al.*, 2009; Mukohara). Significantly, ErbB-2 may be useful not only as a prognostic marker but

Hyper-phosphorylation or over-expression of p130Cas has been implicated in transformation induced by several oncogenes. For example, p130Cas involvement in c-Srcmediated tumourigenesis has been demonstrated by the inability of c-Src to transform p130Cas-null MEFs (Honda *et al.*, 1998). The C-terminal region of p130Cas containing the Src binding domain is sufficient to recover the ability of Src to promote anchorage-independent growth. In breast carcinoma cells p130Cas over-expression accelerates and up-regulates Src activity (Cabodi *et al.*, 2004) as well as increases tyrosine phosphorylation of multiple endogenous cellular proteins (Brabek *et al.*, 2004; Burnham *et al.*, 1996; Cabodi *et al.*, 2004). It was recently reported that bosutinib, a novel Src inhibitor, derived from breast cancer patients, inhibits cell spreading, migration, and invasion of human cancer cells, derived from breast cancer patients by stabilizing cell-to-cell adhesions and membrane localization of beta-catenin. These effects are dependent on the inhibition of the Src/Fak/p130Cas signaling pathway (Buettner *et al.*, 2008). It has been recently reported that Fak promotes mammary tumorigenesis by enabling Src-mediated phosphorylation of p130Cas. Consistently, knock-down of p130Cas causes proliferative arrest in breast cancer cell lines harbouring oncogenic mutations in K-Ras, B-Raf, PTEN and PIK3CA (Pylayeva *et al.*, 2009), underlying a role for p130Cas as a general regulator of breast cancer cell growth induced by

Transforming growth factor-beta (TGF-beta) is a powerful suppressor of mammary tumorigenesis because of its ability to repress mammary epithelial cell proliferation, as well as through its creation of cell microenvironments that inhibit mammary epithelial cells (MECs) motility, invasion, and metastasis. Yet, paradoxically, cancer cells elicit mechanisms that subvert the tumour suppressing functions of TGF-beta, and in doing so, confer oncogenic and metastatic activities upon this multifunctional cytokine (Massague, 2008). In epithelial cells, integrin beta1 suppresses apoptosis and growth inhibition induced by TGFbeta (Zhang *et al.*, 2003). In this context p130Cas has been shown to be a crucial player by binding to Smad3, and preventing its phosphorylation by TGF-beta receptor. As a consequence, the transcription of the cyclin-dependent kinase inhibitors p15 and p21 is inhibited, resulting in cell cycle progression (Kim *et al.*, 2008). Recently, it has been reported that p130Cas over-expression in MECs shifts TGF-beta signalling from Smad2/SMAD3 phosphorylation to p38 MAPK activation, rendering MECs resistant to TGF-beta -induced growth arrest and enhancing their metastatic potential (Wendt *et al.*, 2009). Overall, p130Cas can act as a molecular rheostat that switches the tumour suppressor function of TGF-beta to

The ErbB2 oncogene is a member of the Epidermal Growth Factor Receptor (EGFR) family of receptor tyrosine kinases (RTKs). This family comprises four related members: EGFR, ErbB2 (also known as Neu, HER-2), ErbB3 (HER-3), and ErbB4 (HER-4) (Holbro *et al.*, 2003). Over-expressed and mutated ErbB2 has been found in human tumors and cancer cell lines (Mukohara; Yarden *et al.*, 2004). In addition, several studies have shown a strong correlation of ErbB2 over-expression with a negative clinical prognosis in breast cancer (Choi *et al.*, 2009; Mukohara). Significantly, ErbB-2 may be useful not only as a prognostic marker but

**2.4 Role of p130Cas in c-Src dependent cell transformation** 

**2.5 Role of p130Cas in TGF-beta signalling in breast cancer cells** 

a pro-metastatic role during breast cancer progression.

**3. The ErbB2 oncogene in breast cancer** 

different oncogenes.

also as a predictive marker, given that its elevated expression predicts tamoxifen resistance of the primary tumor and the response to anti-HER2 targeted therapy such as the monoclonal antibody Herceptin.

Further understanding of the mechanisms by which ErbB2 leads to tumorigenesis in the mammary gland comes from studies of ErbB2 mouse models. Expression of Neu mutation that promotes spontaneous receptor dimerization (NeuT), under the MMTV promoter, or more recently under the ErbB2 endogenous promoter (ErbB2/KI model), leads to the formation of mammary adenocarcinomas (Andrechek *et al.*, 2000; Muller *et al.*, 1998). Interestingly, the expression of the ErbB2 protooncogene in a MMTV-transgenic mice show late tumor latency with a low penetrance of lung metastasis, suggesting that gene amplification of the wild type receptor may be the main mechanism implicated in ErbB2 mediated tumorigenesis. Indeed, elevated protein and mRNA ErbB2 levels in the ErbB2/KI model also correlated with selective genomic amplification of the activated ErbB2 allele (Andrechek and Muller, 2000; Hodgson *et al.*, 2005; Montagna *et al.*, 2002). One of the most significant effects associated with ErbB2 activation is enhanced and sustained signal transduction cascades leading to the regulation a variety of cellular processes, including proliferation, apoptosis, cell polarity, migration and invasion (Feigin and Muthuswamy, 2009). Activation of specific ErbB homo- or heterodimer pairs leads to initiation of the mitogen activated protein kinase (MAPK) cascade, activation of phospholipase C gamma (PLCγ) and phosphatidylinositol 3 kinase (PI3K), as well as induction of the small GTPases Rho, Rac and Cdc42, among many other effectors (Hynes and MacDonald, 2009; Kurebayashi, 2001). Several reports have demonstrated a role for these pathways in ErbBinduced cell migration.
