PROJECT SUMMARY The mammary epithelium is composed of diverse cell populations that are responsible for regulating key processes in mammary gland development, especially lactation. Alveolar progenitor (AP) cells are partially differentiated stem cells that produce alveolar/milk-producing cells and these cells have recently been implicated as cells of origin in breast cancer. Through analysis of published single cell RNA-seq data, integrin β4, a protein known to function primarily in basal mammary epithelial cells to maintain structural integrity of the gland, was found to be expressed in the AP population. This unexpected result suggests that β4 plays a novel role in the AP population and may regulate dynamic processes within the developing mammary gland. This goal of this proposal is to understand the functional role of β4 in the AP cells of the nulliparous and lactating mammary gland and elucidate the mechanism by which β4 regulates this population. Preliminary studies have revealed that β4 expression is necessary to promote progenitor potential of the AP cells and identified LacdiNAc, a novel glycosylation modification, on β4 that we hypothesize is essential for its localization in lipid rafts that promotes its function in the AP population. Using an AP cell culture model as well as a Cre-lox mouse model to conditionally knock out β4 from the AP population, the function of β4 in the AP population will be assessed in vitro and in the virgin and lactating mammary gland. Also, glycomics analyses and biochemical approaches will identify novel glycans on β4 and help in understanding how LacdiNAc affects function of β4 in the AP population. This approach will further our understanding of the novel role of β4 in the AP population and a new mechanism involving LacdiNAc-β4 localization to lipid rafts to regulate alveolar differentiation. These studies have to potential to define novel mechanisms that regulate the AP population during normal mammary gland development as well as breast cancer progression.

PROJECT NARRATIVE This proposal seeks to better understand the cell populations and mechanisms that regulate normal mammary gland development. These populations and pathways are important for regulating processes in the mammary gland during pregnancy and lactation. The results of these findings will have a major impact on our understanding of mammary biology and have the potential to reveal new mechanisms that used or altered by breast cancer cells.