Maspin is a unique serpin involved in the suppression of tumor growth and metastasis. To investigate whether increased levels of maspin protect against tumor progression in vivo, we established a transgenic model in which maspin is targeted to mammary epithelial cells by the Whey Acidic Protein (WAP) promoter for overexpression. We crossed these WAP-maspin transgenic mice with the WAP-TAg mouse model of tumor progression. Maspin overexpression increased the rate of apoptosis of both preneoplastic and carcinomatous mammary epithelial cells. Maspin reduced tumor growth through a combination of reduced angiogenesis and increased apoptosis. The number of pulmonary metastases was reduced in the presence of maspin overexpression. These data demonstrate that targeted overexpression of maspin can inhibit tumor progression in vivo, likely through a combination of increased apoptosis, decreased angiogenesis, and inhibition of tumor cell migration. Oncogene (2000) 19, 6053 ± 6058.

Recent data suggest the existence of a unique subset of breast cancer cells capable of initiating tumor growth and giving rise to all other cells characteristic of a given tumor. These cells have been termed “cancer stem cells” or “tumor-initiating cells”. Breast cancer stem cells appear to be resistant to chemo- and radiotherapies and may be responsible for recurrence and metastasis in breast cancer patients. Identifying the cellular signaling pathways responsible for breast cancer stem cell maintenance and self-renewal represents a critical hurdle for developing effective therapeutics. The Stat3 pathway is a critical regulator of the function of normal stem cells, and shows altered expression in human breast cancers [4]. Moreover, IL-6, the Stat3 signaling agonist, is required for breast cancer stem cell function in human breast cancer cell lines. All these suggest an important role of Stat3 signaling in breast cancer stem cells. However, due to lack of method for pathway-activity-based live cell separation, whether Stat3 functions in the cancer stem cells themselves or whether it may function in surrounding niche cells remain unknown. We have constructed a lentiviral fluorescent reporter for Stat3 signaling which contains four copies of M67 Stat3 binding sites upstream of enhanced Green Fluorescent Protein. This reporter system enables FACS-sorting of cells with active Stat3 signaling and in vivo/in situ localization of Stat3 responsive cells. In addition, C188-9, the first Stat3 specific competitive inhibitor, has been developed in our collaborator's lab, which provides us an advanced tool in studying Stat3 function. We hypothesize that Stat3 signaling is preferentially active in stemlike subpopulation and depend on non-stem cancer cells to maintain its activation. Three aims to test this hypothesis include: 1. To test whether cancer cells with activated Stat3 signaling are enriched for breast cancer stem cells or whether they may serve as niche cells. 2. To test whether small molecule antagonists of Stat3 signaling can inhibit cancer stem cell function. 3. To identify novel targets of Stat3 signaling that may serve as indicators of responsiveness to Stat3 antagonist or predict treatment response. So far, our GFP reporters for Stat3 signaling effectively report Stat3 activity in both patient xenografts and human breast cancer cell lines in vivo and in vitro, which enables effective separation of Stat3+/Stat3- cells for functional Study. MDA231 and SUM159 tumors with reporters have been established. Tumor cells will be sorted according to Stat3 activity and perform limiting dilution and MSFE assays. Tumors with reporters will also be treated by C188-9, Chemo, or in combination to study synergistic effect of Stat3 inhibitors on tumor growth. Preliminary data based on inhibitor and limiting dilution studies suggested that Stat3 signaling is required for stem cell function. However, neither Stat3+ nor Stat3- cells are self-sufficient in performing these functions, indicating communications between subpopulations. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr PD08-05.