The Gab2 docking protein is a target of several oncogenic protein tyrosine kinases and potentiates activation of the Ras/extracellular signal regulated kinase and phosphatidylinositol 3-kinase (PI3-kinase) pathways. The prototypical member of the Src family of protein tyrosine kinases, c-Src, phosphorylates Gab2 and both proteins are overexpressed in breast cancers. However, whether overexpression of these two proteins contributes to mammary tumourigenesis had not been previously investigated. Pharmacological inhibition of c-Src in breast cancer cell lines reduced Gab2 tyrosine phosphorylation while overexpression of these two proteins increased this effect, demonstrating a contribution of c-Src to Gab2 tyrosine phosphorylation in breast cancer cells. The biological effects of Gab2 and c-Src overexpression were determined in a three-dimensional cell culture model using the human mammary epithelial cell line MCF-10A. When cultured on a basement membrane, MCF10A cells form acini that model mammary lobules in vivo. Overexpression of Gab2 in MCF10As conferred increased acinar size and independence of the morphogenetic program from exogenous EGF. While overexpression of c-Src alone did not affect acinar morphogenesis, it potentiated the EGF-independent acinar growth induced by Gab2 overexpression. As enhanced c-Src kinase activity is often observed in breast cancer, the effect of Gab2 co-expression with active Src constructs was next determined. Expression of v-Src or c-SrcY527F altered acinar morphology and the resulting structures were categorised as spheroidal, discohesive or dispersed, according to the degree of phenotypic disruption. Gab2 co-expression shifted the proportion of structures towards the dispersed phenotype. This shift reflects a negative role for Gab2 at adherens junctions in the context of active Src expression, as in monolayer cells Gab2 significantly decreased E-cadherin-based adhesive strength without altering the surface expression of this adhesion molecule. Furthermore, Gab2 associated with the E-cadherin complex. The ability of Gab2 to weaken the strength of cell-cell contacts in active Src-expressing cells may be due to enhanced activation of PI3-kinase signalling at adherens junctions, as the potentiating effects of Gab2 in both monolayer and three-dimensional cultures were dependent upon Gab2 recruitment of the p85 subunit of PI3-kinase. Finally, Gab2 increased migration and invasion of v-Src-expressing cells in transwell assays, however these effects were p85-independent. This is the first study to demonstrate Gab2 co-operation with various forms of Src to augment proliferative, invasive and migratory signals, as well as revealing a novel mechanism whereby Gab2 may promote metastatic spread. This study thus demonstrates multiple roles for Gab2 in contributing to breast cancer progression.
| Identifer | oai:union.ndltd.org:ADTP/242588 |
| Date | January 2008 |
| Creators | Bennett, Haley Lorraine, Garvan Institute of Medical Research, Faculty of Medicine, UNSW |
| Publisher | Awarded by:University of New South Wales. |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Bennett Haley Lorraine., http://unsworks.unsw.edu.au/copyright |
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