Expression of a phosphorylated p130^Cas substrate domain attenuates the phosphatidylinositol 3-kinase/Akt survival pathway in tamoxifen resistant breast cancer cells

Elevated expression of p130^Cas/BCAR1 (breast cancer anti estrogen resistance 1) in human breast tumors is a marker of poor prognosis and poor overall survival. Specifically, p130^Cas signaling has been associated with antiestrogen resistance, for which the mechanism is currently unknown. TAM-R cells, which were established by long-term exposure of estrogen (E₂)-dependent MCF-7 cells to tamoxifen, displayed elevated levels of total and activated p130^Cas. Here we have investigated the effects of p130^Cas inhibition on growth factor signaling in tamoxifen resistance. To inhibit p130^Cas, a phosphorylated substrate domain of p130^Cas, that acts as a dominant-negative (DN) p130^Cas molecule by blocking signal transduction downstream of the p130^Cas substrate domain, as well as knockdown by siRNA was employed. Interference with p130^Cas signaling/expression induced morphological changes, which were consistent with a more epithelial-like phenotype. The phenotypic reversion was accompanied by reduced migration, attenuation of the ERK and phosphatidylinositol 3-kinase/Akt pathways, and induction of apoptosis. Apoptosis was accompanied by downregulation of the expression of the anti-apoptotic protein Bcl-2. Importantly, these changes re-sensitized TAM-R cells to tamoxifen treatment by inducing cell death. Therefore, our findings suggest that targeting the product of the BCAR1 gene by a peptide which mimics the phosphorylated substrate domain may provide a new molecular avenue for treatment of antiestrogen resistant breast cancers.