Author ORCID Identifier
0000-0002-3741-9003
Date of Graduation
5-2025
Document Type
Dissertation (PhD)
Program Affiliation
Cancer Biology
Degree Name
Doctor of Philosophy (PhD)
Advisor/Committee Chair
Florencia McAllister
Committee Member
Shin-heng Chiou
Committee Member
Michael Kim
Committee Member
Greg Lizee
Committee Member
Guillermina Lozano
Committee Member
Paul Scheet
Abstract
Cancer is a leading cause of death worldwide. Despite recent advances in diagnostic and therapeutic modalities, pancreatic ductal adenocarcinoma (PDAC) is an aggressive cancer that remains highly lethal. Efforts have been made to increase the understanding of the underlying mechanisms driving early stages of tumor development, when the cancer is most susceptible to intervention. To this end, genetically engineered mouse models (GEMMs) have been developed with the mutations most commonly seen in the patient population. Expression of these mutations in select cellular compartments has resulted in the ability to recapitulate characteristics of human pancreatic cancer such as the development of precursor lesions, metastatic activity, and cytokine signaling. These models represent convenient and cost-effective tools to investigate various aspects of pancreatic cancer. The utility of these genetic models, however, may be stymied by inherent variation between research institutions, including commensal bacteria present at individual facilities that has been shown to influence the host immune system. The microbiota, or the community of microbes, present in the gut has gained recent attention for the pivotal role it plays in myriad aspects of host physiology. The relationship between microbes and cancer is complex. Recent studies have shown that PDAC patients harbor distinct gut microbiomes relative to healthy patients, and that transplantation of gut microbes from patients into mice can differentially alter the rate of tumor progression in murine models of PDAC. The potential mere association between pancreatic cancer and microbiome changes vs the potential role of microbiome in cancer initiation remains not fully elucidated.
To this end, we serendipitously identified two cohorts of mice housed at geographically distinct vivaria that exhibited significantly different rates of pancreatic premalignancy despite possessing a shared lineage. After ruling out potential differences in food and water sources as well as handling protocols, we examined composition of the gut microbiomes in both cohorts. These analyses revealed that in addition to possessing significantly different rates of tumorigenesis, mice harbor distinct gut microbiomes at the respective geographically distant vivarium. Fecal microbiota transplantation (FMT) experiments consist of transplanting fecal bacteria and other microbes from a donor directly into the gastrointestinal tract of the recipient and have been used clinically to treat conditions as recurrent Clostridioides difficile infections and inflammatory bowel disease. We assessed the ability of FMT to influence the rate of pancreatic tumorigenesis in mice, which significantly accelerated the rate of premalignancy in the mice housed in the vivaria characterized by slow phenotype.
To interrogate the effect of downstream effects elicited by differences in the gut microbiome, we interrogated the metabolome of mice housed at the separate institutions. These analyses identified an abundance of succinic acid in the plasma and pancreata of mice displaying the accelerated phenotype. We utilized an independent transgenic mouse model in which PDAC driver mutations are expressed pancreas-wide, which developed a wide array of pancreatic dysplasia. These data suggest that this microbially dependent protective effect is specific to the cells of acinar origin. Furthermore, this independent mouse model demonstrated an increase in plasma levels of succinic acid in relation to pancreatic dysplasia, validating our identification of succinic acid as a metabolite of interest.
To better understand the mechanisms by which bacterially mediated succinic acid may promote the progression of pancreatic premalignancy, we examined the rate of transformation of pancreatic explanted acinar cells to ductal-like structures. Following the addition of succinic acid or succinic acid producing bacteria, we observed a significant increase in morphological transformation. This increase in transformation was also reflected by an increased expression of ductal genes. Due to previous reports that abundant succinic acid stabilizes HIF1α, we sought to determine the role of HIF1α signaling in our model. We observed that pre-treatment of acinar explant cells with a commercially available HIF1α inhibitor abrogated the pro-tumorigenic effects of succinic acid and succinic acid producing bacteria. Furthermore, we identified increased expression of HIF1α and target genes in cells exposed to succinic acid in vitro.
In addition to microbially mediated mechanisms that promote the early development of pancreatic cancer, we also investigated immune signaling capable of mediating tumorigenesis. By generating transgenic animals lacking IL-17 receptor A (IL-17RA) in epithelial compartments, we observed delayed initiation and progression of premalignant lesions. We identified that B7-H4, a known inhibitor of T cell activation was highly upregulated via IL-17 during the early stages of tumorigenesis. In addition to increased expression of B7-H4, we identified increased levels of T cell exhaustion marker, Eomes. Histological staining confirmed increased expression of Eomes. Taken together these data suggest that within the pancreatic epithelium, IL-17 signaling regulates B7-H4 which can promote T cells exhaustion.
In this dissertation we provide novel insights into the mechanisms, both microbial and immune, that drive the early development of pancreatic cancer and identify innovative opportunities for potential cancer interception and therapy.
Keywords
microbes, metabolites, PDAC, microbiome
Included in
Biology Commons, Other Immunology and Infectious Disease Commons, Other Microbiology Commons