Source: Dissertation Abstracts International, Volume: 79-05(E), Section: B.

Adviser: Valerie Horsley.

Mature adipocytes comprise the majority of the mammary gland (MG) stroma and are required for proper development of the mammary epithelium. As the MG transitions from pregnancy to lactation, stromal adipocytes contract as epithelial cells differentiate into milk-producing alveoli that fill the area of the gland. Weaning triggers involution, during which adipocytes rapidly repopulate the stroma as the epithelial cells regress. It has been suggested that reversible transdifferentiation occurs between epithelial cells and adipocytes leading up to and following lactation. The two major goals of this work are to determine (1) the cellular mechanisms underlying mammary adipose size dynamics during lactation cycles, by evaluating the possible contribution of adipo-epithelial reversible transdifferentiation as well as additional mechanisms, and (2) the function of adipocyte lineage cells in the tissue remodeling process that follows lactation.

Employing genetic lineage tracing with several Cre recombinase mouse models, I find that in contrast to published reports, epithelial and adipocyte lineages are distinct during lactation cycles. Using in vivo analysis of adipocyte precursor cells (APs), I identify resident APs in the MG stroma in mice and humans. However, in vivo proliferation assays and long-term genetic lineage tracing of mature adipocytes throughout reproduction reveal that hypertrophy of existing adipocytes is the major mechanism of adipocyte repopulation during MG involution. I find that adipocyte hypertrophy during involution is not regulated hormonally, but rather by local signals within the gland, including macrophages. Further, I employed lipidomics analysis and developed an in vivo lipid tracking strategy to study lipid dynamics during involution, and identified that mature adipocytes fill with epithelial-derived milk lipid as they undergo hypertorphy during involution. Using a tissue and cell type-specific method that we designed to ablate MG adipocytes after lactation, I demonstrate that in the MG, mature adipocytes are necessary for lipid uptake into epithelial cells and proper epithelial alveolar regression during involution. Together, these findings reveal a long-lived population of mature adipocytes that undergo hypertrophy as they fill with epithelial-derived milk lipid to regenerate the MG stroma during involution, and elucidate a novel role for these cells in facilitating aspects of epithelial remodeling during involution. These findings have important implications for our understanding of MG physiology and the contribution of adipocytes to breast cancer pathogenesis.