Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Pathology and Laboratory Medicine

Supervisor

Asfaha, Samuel

Abstract

Colorectal cancer (CRC) is the second leading cause of cancer death with a major risk factor being colitis. Indeed, patients with inflammatory bowel disease (IBD), characterized by chronic inflammation of the colon, are at increased risk of CRC. To better understand the mechanism by which colitis leads to CRC, we examined how colitis affects tuft cells during cancer initiation and investigated the role of nuclear factor kappa B (NF-κB) signaling in colitis-associated tumorigenesis.

Using two different murine models, we compared colonic tumorigenesis across five models of colitis. We discovered that tumorigenesis is associated with infiltration of a specific subset of macrophages marked by the expression of the cell surface markers F4/80+Ly6Chigh. Depletion of this macrophage population during acute colitis inhibited tumorigenesis, confirming that these cells are essential for inflammation-associated tumorigenesis. Consistent with these observations, we found that the macrophage-derived cytokines IL-1β, TNF-α, and IL-6 promoted stemness of tumor-initiating Dclk1+ tuft cells. Using the same mouse models of colitis-associated cancer (CAC), we additionally examined the role of Lgr5/R-spondin signaling in dedifferentiation of tuft cells and their subsequent tumor formation. Upon colitis induction, R-spondin 3 (RSPO3) levels were increased, and we detected rare Dclk1+ tuft cells that expressed the intestinal stem cell marker Lgr5. Blocking RSPO3 using an antibody significantly inhibited colonic tumorigenesis, suggesting that Lgr5/RSPO3 signaling is critical for colitis-associated tumor initiation. In separate studies, we also examined the role of canonical NF-κB signaling in tuft cell-derived tumorigenesis. Unexpectedly, we found that NF-κB activity in tuft cells inversely correlates with the severity of colitis and colonic tumorigenesis.

In summary, we show that the initiation of inflammation-associated cancer is affected by the type of inflammatory response and the capacity of specific epithelial cells to acquire stemness. Taken together, this work provides novel insight into the molecular mechanisms by which inflammation leads to the initiation of CAC, and these findings provide insight into potential new therapeutic targets for CAC prevention.

Summary for Lay Audience

Colorectal cancer (CRC) is the second leading cause of cancer death, and a major risk factor of CRC is prolonged inflammation of the colon. As such, patients with inflammatory bowel disease (IBD), a group of disorders that are characterized by chronic inflammation of the gastrointestinal (GI) tract, are at increased risk of CRC. Despite the clear link between inflammation and cancer, however, the mechanism by which inflammation leads to cancer is still unknown. Therefore, we aimed to study the changes that occur within the immune system and the epithelium to examine how colonic inflammation leads to colonic tumor formation.

In this study, we identified a unique subset of immune cells known as macrophages that are important for initiating inflammation-associated colon cancer. Furthermore, inflammatory cytokines known to be secreted by these cells, such as IL-1β, TNF-α, and IL-6, were found to promote stem cell properties in tuft cells, the cell-of-origin for colitis-associated tumors. In separate studies, we found that tuft cells express a previously identified marker of intestinal stem cells (i.e., Lgr5), consistent with the notion that tuft cells change to a stem cell state prior to cancer initiation. Colonic tumor formation was also associated with increased Lgr5/R-spondin-3 signaling, a pathway known to be important for stem cells. Accordingly, R-spondin-3 depletion inhibited colonic tumor formation. Finally, we demonstrate that the inflammation-related NF-κB pathway within tuft cells serves a protective role in colitis and subsequent tumorigenesis.

Overall, our data shows that colon cancer is regulated by multiple factors, including the type of inflammatory response as well as the intrinsic ability of epithelial cells to acquire stemness. Taken together, our work identifies ways by which inflammation leads to colon cancer.

Available for download on Sunday, December 31, 2023

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