Doctor of Philosophy

dissertationThrough the production of milk, the mammary gland provides nutritional and immunological protection for newborns. However, lactation is required for only short periods of time in the adult animal, which allows the mammary gland to develop through a series of distinct stages. During embryogenesis and puberty, the gland establishes an extensive network of epithelial ducts, which then undergo widespread branching and differentiation during pregnancy to maximize milk production. Throughout this process, the mammary gland relies on coordination of major cellular processes, including proliferation, invasion, and differentiation. As many of these pathways become aberrantly regulated during breast cancer, understanding mechanisms that regulate mammary gland development has important disease implications. Although previous studies have characterized several systemic hormones and local factors central to mammary development, little is known about the downstream mediators of these pathways. To identify new factors, we established a three-dimensional model of mammary branching morphogenesis using primary mammary epithelial cells (MECs) stimulated with fibroblast growth factor-2 (FGF2). We performed a forward chemical genetic screen to identify compounds that modulate FGF2-induced branching and discovered a novel bis-aryloxadiazole, called 1023, which completely blocks branching through activation of the aryl hydrocarbon receptor (AHR). iv Using 1023 as a molecular probe, we found AHR activation blocks mammary branching through upregulation of desmosomal adhesion. These results identified desmosomes as a novel target of AHR signaling and suggested desmosomes are downregulated to facilitate mammary branching. Supporting this hypothesis, we found desmosomes absent in the mammary glands of pregnant mice in a cell-type specific manner. These results suggest desmosomes control initiation of mammary branching, and may also be targeted during breast cancer to promote cellular invasion. We also investigated mechanisms of AHR activation and the impact of AHR on mammary differentiation. We performed a structure activity relationship study of 1023 and defined moieties of the molecule critical for AHR stimulation. Moreover, we investigated the effect of AHR on mammary differentiation and elucidated a transcriptional mechanism through which the AHR pathway directly blocks lactation in MECs. Since several environmental pollutants stimulate AHR, these studies provide mechanistic insight for how toxins impair mammary function