My laboratory conducts basic research investigating the molecular mechanisms that regulate cell growth and survival, specifically in mammary epithelial and breast tumor cells. We use mouse models and cells derived from these animals to characterize the molecular signaling pathways in tumor cells and in normal mammary epithelial cells within the mammary gland life cycle. We also study signaling pathways in human breast tumor cells in vitro, and we analyze patient samples to determine the clinical relevance of our observations. Our investigations revealed that C/EBPdelta can be both a tumor suppressor as well as a tumor promoter, depending on the cell type, cellular environment, and stage of tumor development. Gene expression analyses in human tumors and genetic studies with cell culture lines have implicated C/EBPdelta as a tumor suppressor gene. In support of this notion, we found that C/EBPdelta induces the expression of the Cdc27/APC3 subunit of the anaphase promoting complex/cyclosome (APC/C), which in turn results in degradation of the cell cycle regulator cyclin D1 (Pawar et al., 2010, PNAS). Furthermore, we have shown that C/EBPdelta can augment genomic stability (Huang et al., 2004, Oncogene) and promote DNA repair by catalyzing the nuclear translocation of FANCD2 (Wang et al., PNAS 2010). In the mouse mammary gland, C/EBPdelta promotes cell death of mammary epithelial cells during postlactational involution (Thangaraju et al., 2005, Development). Specifically in this reporting period, we have shown that in human breast epithelial cell lines, the C/EBPdelta protein is targeted for degradation by the SIAH2 E3 ligase in a manner, which requires Src tyrosine kinase activity. We identified a novel pathway of Src kinase signaling which contributes to activation of the SIAH2 E3 ligase and targets the transcription factor C/EBPdelta for degradation. The Src family of tyrosine kinases is both upstream and downstream of many tumor promoting signaling pathways, and this study provides molecular insights into its mechanisms of action and those of Src-kinase inhibitors, which are in clinical trials for breast cancer.Consistent with a tumor-suppressor function, downregulation of C/EBPdelta was found to augment breast epithelial cell transformation by Src, whereas stabilization of C/EBPdelta promoted the response of tumor cell lines to Src-kinase inhibitors in vitro. This study provided new insights into the regulation of C/EBPdelta expression and the mechanisms by which Src kinase promotes transformed features in breast epithelial cell lines (Sarkar et al., 2012, MCB). We have evaluated the role of C/EBPdelta in vivo with the transgenic MMTV-Neu mouse model of mammary tumorigenesis. C/EBPdelta null mice exhibited increased tumor incidence, indicating that C/EBPdelta indeed acts as a tumor suppressor in this in vivo system. However, these mice also had reduced distant metastases, demonstrating that C/EBPdelta may have additional roles in tumor progression. Our mechanistic analyses revealed that C/EBPdelta expression is induced by hypoxia and in turn inhibits the expression of the tumor suppressor FBXW7. Thereby, C/EBPdelta augments the mTOR/AKT/S6K/HIF-1 pathway, which is critical for the adaptation to hypoxia, a metastasis-promoting condition. Our findings have defined a novel role for C/EBPdelta in this pathway (Balamurugan et al., 2010, EMBO J.). In addition, we found that a role of C/EBPdelta in macrophage activation and the innate immune response may underly in part both its role as a tumor suppressor as well as in promoting metastasis (Balamurugan et al., in revision). Building on our previous discoveries, the group's current and future investigations have two main goals: (1) Further characterization of the role and mechanisms of C/EBPdelta in signaling pathways within breast epithelial cells and (2) Characterization of cell type specific functions of C/EBPdelta that influence mammary tumor development. Specifically, we are investigating the molecular mechanism(s) that determine whether C/EBPdelta acts as a tumor suppressor or tumor promoter within mammary epithelial cells, and the contribution of C/EBPdelta functions in the tumor microenvironment on mammary tumor development. Towards this goal, we have recently developed mice with a conditional knockout allele of C/EBPdelta.Collectively, these approaches will provide deeper insight into the molecular mechanisms that modulate mammary epithelial cell biology and may allow us to better understand the complexity and diversity of cellular processes in normal development and breast cancer.
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