Synergistic Cooperation of Snail and Beta-catenin in the Initiation of Breast Cancer Metastasis

Breast cancer is the most common cancer in women. The clinical symptoms and outcome of breast cancer depend largely on whether it is confined to the breast or has spread to adjacent or distant parts of the body. Despite more than 3 decades of research, approximately 90% of breast cancer deaths are caused by local invasion and distant metastasis of tumor cells, and the average time to live after documentation of metastasis is approximately 2 years. Thus, novel treatment strategies based on the biology of how breast cancer metastasizes are urgently needed to combat this life-threatening disease. The process of metastasis involves multiple steps. At the initial step, tumor cells at the tumor-host boundary detach from the primary tumor and start to invade the adjacent tissue. This early event is crucial for the entire metastatic process. The major challenge in breast cancer research is to identify the factors within the cell and the signals outside the cell that initiate this early event. The increased motility and invasiveness of tumor cells at the initial step of metastasis are similar to the occurrence of the epithelial-mesenchymal transition (EMT) which is required for embryonic development, tissue remodeling, and wound healing. In EMT, cells change from “static” to “motile” in appearance and start to migrate, this is because the cell-cell adhesion molecule E-cadherin is lost, and tumor cells break away from one other and start to migrate and invade the surrounding tissue. At the same time the tumor cells lose E-cadherin, they activate a survival mechanism called the beta-catenin pathway to protect them from death during the odyssey of invasion and metastasis. The signal that triggers the EMT process and the activation of the beta-catenin pathway in breast cancer remains a mystery. Interestingly, under normal physiological condition, EMT takes place at the edge of the site of injury during wound healing. Similarly, in many tumors, EMT occurs at the invasive front (the tumor–host boundary) of metastatic cancers. As cancer has been called as “a wound that never heals,” these observations suggest that EMT is triggered by inflammation that occurs at the wound or at the tumor-host boundary of cancer. Consistent with our hypothesis, we recently found that the amount of Snail, a protein that acts as a master switch at the nucleus to control EMT and the mobility of the cell, is dramatically increased by inflammation. When too much Snail protein is generated, as occurs in many cases of metastatic breast cancer, it suppresses the expression of E-cadherin and induces the activation of the “survival” beta-catenin pathway. In fact, high levels of Snail have been linked to metastasis, tumor cell survival, and recurrence of breast cancer and predict a poor clinical outcome. Based on our novel findings, we propose that an inflammatory microenvironment at the tumor-host boundary induces the production of Snail in the tumor cells, which results in the suppression of E-cadherin and the induction of EMT, and consequently leads to the activation of beta-catenin pathway. We hypothesize that Snail and beta-catenin work cooperatively to initiate breast cancer metastasis. We will test this hypothesis by determining (1) how Snail stabilization and EMT are induced by inflammation; (2) the synergistic effect of Snail and beta-catenin on cell migration and invasion; and (3) the role of Snail and beta-catenin in breast cancer metastasis in vivo. We will accomplish our goal by utilizing contemporary methods such as, co-culture experiment with macrophages (inflammatory cells), siRNA techniques, 3D cultures, animal models, and analyses of human breast cancer specimens in our study. In addition, we have designed a 3D macrophage co-culture experiment that is built to mimic the invasive front of breast cancer in vivo. By using multiple and complementary approaches, we will carefully examine the dialogue or signaling between breast cancer cells and inflammatory cells (macrophages). Furthermore, we will validate our finding by using breast cancer metastasis model in mice that have human breast tumors growing in their mammary glands. We will induce an inflammation in these mice and examine the signaling of Snail and beta-catenin during the development of breast cancer metastasis. Finally, we will compare and evaluate the inflammation status and the expression of these molecules in 120 tumor samples from patient with breast cancer. We believe that our systematic study will not only enhance our knowledge of how breast cancer metastasis develops but also lead to the development of new treatment strategies that will be tested in clinical trials for the benefit of patients with metastatic breast cancer.