Metabolic Reprogramming of the Irradiated Mammary Gland Microenvironment in Breast Cancer Recurrence

Abstract

Triple-negative breast cancer (TNBC) is a highly aggressive and metastatic subtype of breast cancer that cannot be targeted with traditional hormone therapies. Radiation therapy (RT) is routinely used in the clinic to reduce the incidence of recurrent disease; however, patients with TNBC still experience disproportionately high locoregional recurrence rates compared to other breast cancer subtypes. Pre-clinical models have suggested that locoregional recurrence may be due to the interactions of RT-resistant cancer cells or circulating tumor cells with the RT-damaged mammary gland tissue following surgical resection of the tumor. RT-resistant and circulating tumor cells are known to have altered metabolic profiles that aid in their survival following treatment; however, little is known about how RT influences the metabolic profiles of stromal cells of the mammary gland such as fibroblasts, adipocytes, and endothelial cells. Understanding metabolic alterations in these cells following RT could provide insights into TNBC recurrence mechanisms. In this dissertation, I demonstrate that irradiated fibroblasts undergo a dramatic metabolic shift that is characterized by lipid accumulation, changes in mitochondrial morphology, and high levels of mitochondrial respiration that is supported by an increase in fatty acid oxidation. These processes are accompanied by an increase in autophagic flux which regulates the fibroblast’s metabolic response to radiation damage. I then demonstrate that irradiated fibroblasts undergo a mitochondrial stress response over time that induces a complex secretory profile of lactate, mitokines, and cytokines that is altered through autophagy inhibition. These changes induce an aggressive phenotype in TNBC cells, as demonstrated through increased migration and tumorsphere growth that can be mitigated when autophagy is blocked in irradiated fibroblasts. Finally, I demonstrate how adipocytes and endothelial cells, other important mammary gland stromal cells, interact with each other following RT to influence the wound healing response. These observations lead to the development of a novel in vitro hydrogel system in which adipocyte spheroids are co-cultured with endothelial cells and fibroblasts, enabling the study of stromal-immune cell interactions post-RT. Ultimately, the work in this dissertation provides important mechanistic insight into metabolic pathways that can be targeted in combination with RT to reduce recurrent disease for TNBC patients.