Abstract 993: A novel cadherin-dependent Rac/Stat3 pathway in invasive breast cancer

Stat (signal transducer and activator of transcription) 3 is a transcription factor involved in a number of signaling pathways activated by receptor and non-receptor tyrosine kinases. Constitutively active Stat3 alone is sufficient to induce tumorigenesis. We have previously shown that Stat3 can also be activated by engagement of the epithelial adhesion molecule E-cadherin (MCR 7:1310-27, 2009). An additional novel finding in this study was that levels of total and activated Rac1 proteins increase dramatically, thereby causing the Stat3 stimulation. Interestingly, Stat3 expression was independently found to downregulate p53 expression in several epithelial and fibroblast cell lines. These findings suggest a novel Cadherin/Rac1/Stat3-dependent pathway that causes sustained downregulation of p53, resulting in uncontrolled growth and survival. As a first step to test this hypothesis in breast cancer, we are assessing the expression profiles and localization of these molecules in mouse breast epithelial and carcinoma cell lines. The results revealed that at low cell density in 2-D culture conditions, E-cadherins and b-catenin showed strong cytoplasmic and perinuclear localization, respectively. In contrast, at high cell density both markers showed strong localization to the plasma membrane. On the other hand, a breast carcinoma cell line showed cytoplasmic staining of pan-cadherin and nuclear staining of β-catenin under both low and high cell density conditions. Western blotting revealed that levels of pan and activated (pY705) Stat3, and Rac1 were increased at high compared to low cell densities, consistent with a cadherin-dependent activation of this pathway. We are currently extrapolating these findings to a 3-D Matrigel culture model in which mammary epithelial cells (EPH4) form acini-like spheroids with hollow. Under these conditions, total Stat3 levels are sustained for 4-6 days followed by a transient increase in cyclin D1 expression. We are also performing immunohistochemical analyses of these molecular biomarkers on a cohort of human breast cancer tissues (n=59) to test associations of protein expression profiles in human cancer. Preliminary results have shown increased expression of active Stat3 in breast tumours compared to normal mammoplasty tissues. In addition, increased expression of p53 was observed in a subset (17%) of these breast cancers. Detailed ARIOL imaging analysis is currently in progress to evaluate relative expression levels and associations of Stat3 and p53 in breast cancer tissues. Our findings suggest a novel pathway by which Stat3 activation causes a dramatic decrease in p53 levels, resulting in increased proliferation and survival of cancer cells. (Supported by CIHR and CBCRA). Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 993.