Mammary Branching Morphogenesis
During the process of branching morphogenesis, the mammary gland undergoes distinct phases of remodeling to form an elaborate ductal network that ultimately produces and delivers milk to newborn animals. These developmental events rely on tight regulation of critical cellular pathways, many of which are likely disrupted during initiation and progression of breast cancer. Traditionally, genetic mouse models have been used to identify molecular pathways important for mammary gland development. Although highly informative, these models require significant time and resources to generate and are difficult to manipulate. We desired a more rapid and accessible model to study the dynamics of cell signaling that occur during mammary branching morphogenesis. Accordingly, we developed an in vitro model of mammary morphogenesis using purified primary mammary epithelial cells (MECs) stimulated with fibroblast growth factor-2 (FGF2). We then employed a forward chemical genetic approach to identify modulators of this process. In our screen, we identified a number of lead compounds that potently block branching, including an agonist of the aryl hydrocarbon receptor (AHR). Using these small molecules as a tool, we have identified desmosomal adhesion as a novel target of AHR signaling and shown desmosomes are critical for AHR agonists to block branching. We are now focused on understanding the role of desmosomes during normal mammary gland development.
Stimulation of primary mammary epithelial cells with FGF2 in Matrigel over time.
Breast cancer can be categorized into distinct subtypes based on the pattern of genes expressed by the tumor. These gene expression signatures correlate with clinical outcome and provide insight into the complex molecular pathways driving the disease. The question remains whether specific molecular pathways establish the subtype or whether different cell types within the mammary hierarchy become transformed and give rise to each tumor subtype. These questions are difficult to address using existing models of breast cancer, since cell lines may not retain the same cell population as the original tumor, and transgenic mouse models of breast cancer utilize promoters that are preferentially expressed in more differentiated cells. Therefore, our laboratory is developing new model systems that more appropriately mimic the cellular heterogeneity observed in breast cancer. Using these breast cancer models we are performing the following projects:
Determination of the cellular origin of breast cancer
Breast cancer is a disease with significant diversity in histology, cellular composition and clinical outcome. This diversity may result from either specific genetic mutations or transformation of different cell populations. We are performing experiments to determine whether oncogenes targeted to different mammary epithelial cells can affect the cellular composition and pathology of tumors.
Chemical library screen to identify cancer-subtype-specific therapeutics
The heterogeneous nature of breast cancer may reflect both its cellular origin and activation of specific signaling pathways. The complex etiology of breast cancer is not appropriately reproduced with established cell lines, making them of limited use in drug discovery screens. Therefore, we have developed a chemical library screen using primary tumor cells that embody the diversity of the signaling pathways activated, and the cellular variation of the original tumor. Primary tumor cells, representing both basal and luminal cancer subtypes, have been generated using an oncogene-induced mouse model of breast cancer. Tumor cells were pre-screened by both gene expression profiling and histology to identify the cancer subtype that they represent. The tumor cells representing specific subtypes were grown as 3D organoids within a laminin-rich extracellular matrix and screened using different chemical libraries. From this work, we have identified a number of lead compounds, which we are actively pursuing in order to better understand the molecular pathways that drive specific breast cancer subtypes.