Abstract 4216: Protein kinase C ≤ overexpression is associated with loss of membrane-associated E-cadherin and β-catenin in T47D xenograft breast tumors

Expression of protein kinase C ≤ (PKC∈) has been shown to be a clinically relevant indicator of tamoxifen response, recurrence and overall survival in breast cancer (Tonetti, 2003; Assender 2007). To study this association, we developed a model of PKC∈-overexpressing breast cancer by stable transfection of the prkca gene into T47D breast cancer cells (T47D/PKC∈, Tonetti, 2000). We have shown that PKC∈ induces a hormone-independent, tamoxifen-resistant phenotype in vitro and in vivo (Chisamore, 2001). This model is further characterized in vitro by increased migratory and invasive capability, expression of mesenchymal markers and reduced expression of adherens junctions and epithelial proteins (Perez White, AACR 2011). β-Catenin and E-cadherin are components of adherens junctions in normal epithelia and loss of E-cadherin is associated with worse outcome in breast cancer (Prat, 2010). Further, the reduction or cytoplasmic relocalization of β-catenin has been associated with poor prognosis (Lopez-Knowles, 2010; Dolled-Filhart, 2006) though the processes upstream or downstream of these changes are not well characterized in breast cancer. Therefore we hypothesized that xenograft tumors derived from T47D/PKC∈ cells would have loss of membrane-associated β-catenin and E-cadherin. Immunofluorescence staining showed cytoplasmic localization of β-catenin and E-cadherin in T47D/PKC∈ tumors in sharp contrast to membrane association in T47D/neo (vector control) tumors. Western blot showed reduction in the level of E-cadherin in T47D/PKC∈ tumor samples but no change in the level of β-catenin compared to T47D/neo tumors. Based on this, we next wanted evaluate the β-catenin degradation pathway mediated by glycogen synthase kinase-3β (GSK-3β). Cellular extracts show an increase in the levels of the serine 9 inhibitory phosphorylation of GSK-3β in T47D/PKC∈ cells but no increase in total protein compared to T47D/neo cells. Treatment with the proteasomal inhibitor MG132 (10 μM) for 2 h did not increase the levels of β-catenin in vitro in T47D/PKC∈ cells. Current data indicate a correlation between PKC∈ expression and loss of membrane-associated β-catenin and E-cadherin. Further, β-catenin is stabilized in the cytoplasm of T47D/PKC∈ cells possibly allowing for nuclear localization and downstream gene transcription. Taken together with previous data, PKC∈ correlates with markers of poor outcome in addition to hormone-independence and tamoxifen-resistance suggesting that PKC∈