Student Theses and Dissertations

Date of Award

2021

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Abstract

Overweight and obesity affect more than 70% of American adults and are major risk factors for the development of comorbidities, including cancer. Breast cancer is the most commonly diagnosed malignancy in American women: 1 in 8 women will develop breast cancer in their lifetimes, and more than 40,000 women die from breast cancer each year. Obesity is associated with increased incidence and worse prognosis in breast cancer, including aggressive triple-negative breast cancer, which has a particularly poor prognosis with few treatment options. The goal of this thesis was to elucidate the mechanisms by which obesity promotes breast cancer progression. To that end, we used an orthotopic model of triple-negative murine mammary carcinoma in a diet-induced model of obesity. We found that obesity significantly accelerated tumor growth, which was due to increased proliferation. Upon performing whole-tissue RNA sequencing of tumors isolated from lean or obese animals, we found that many of the transcriptional programs differentially regulated in obesity were immune-related. Since tumors are cellularly heterogeneous and contain tumor-associated cell types, such as immune cells, adipocytes, endothelial cells, and fibroblasts, we devised a fluorescence-based approach to specifically isolate cancer cells prior to analysis. To achieve this, we produced a stable-mCherry expressing breast cancer cell line and utilized fluorescence-activated cell sorting to isolate cancer cells from heterogeneous tumor tissue. RNA sequencing of this pure cancer cell population identified the acyl-CoA synthetase, Acsbg1, as robustly upregulated in cancer cells isolated from tumors in obese compared to lean animals. Overexpression of Acsbg1 in tumor cells further enhanced tumor growth in an obesity-specific manner, which required intact creatine transport. Inhibition of acyl-CoA synthetase activity, on the other hand, significantly attenuated tumor growth in obese animals. This led us to hypothesize that Acsbg1 may reprogram the breast cancer cell metabolome. We performed targeted metabolomic analysis on tumors from lean and obese animals and identified a role for Acsbg1 in promoting mitochondrial ATP production, which was confirmed with plate-based respirometry. We used untargeted lipidomic analysis to identify the lipid substrate and products of Acsbg1 activity and found that phospholipids containing 20:4 and 22:4 side chains were more abundant in Acsbg1-overexpressing tumors in obesity. The most common 20:4 and 22:4 lipid species are arachidonic and adrenic acid, respectively. Since arachidonic and adrenic acid are products of sequential steps of linoleic acid metabolism, an essential fatty acid, this suggests a role for dietary lipids in the pathogenesis of obesity-driven breast cancer progression. These findings further suggest that Acsbg1 supports obesity-related tumor progression through both catabolic ATP generation and anabolic processes to build biomass. Finally, we analyzed a dataset of human tumor gene expression and found that Acsbg1 levels are associated with worse tumor grade and with aggressive, basal-like cancers in overweight and obese individuals. Overall, these studies identified an undescribed, obesityspecific role for Acsbg1 in promoting tumor progression and provide the foundation for further studies investigating Acsbg1 and acyl-coA synthetase activity as a possible target for therapeutic intervention in breast cancer and other obesity-driven cancer types.

Comments

A Thesis Presented to the Faculty of The Rockefeller University in Partial Fulfillment of the Requirements for the degree of Doctor of Philosophy

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