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Colorectal cancer (CRC) is the third most prevalent cancer in the United States and estimated to affect 151,030 people and kill 52,580 people in 2022. Although some populations are more susceptible for CRC due to inherited cancer-causing mutations or having family history of CRC, most CRC cases occur sporadically with accumulation of a series of somatic mutation in tumor suppressor genes, oncogenes, and DNA repair system in the colon. Mutations in the adenomatous polyposis coli (APC) and the Kirsten rat sarcoma viral oncogene homologue (KRAS) are known as cancer driving mutations which are most frequently identified genetic lesions at early stages of sporadic CRC development. To evaluate effective drugs for prevention and treatment, preclinical models of CRC that resemble key features of human CRCs are crucial. Genetically modified mouse models (GEMM) of CRC bearing loss of Apc with or without other mutations, such as oncogenic Kras have been valuable tools to study pathobiology, treatment, and prevention of CRC. However, a major limitation of most Apc-mutant GEMMs is the predominant distribution of tumors in the small intestine rather than in the colon. Previously, a murine model bearing colon-specific mutations in Apc and Kras has been reported to develop colon-specific tumorigenesis in the colon, utilizing the carbonic anhydrase 1 promoter/enhancer-Cre recombinase (CAC) in Cre-LoxP system to restrict Apc knockout and a latent expression of oncogenic Kras in the colon tissue. However, only limited features of this model, so called AKC mouse, have been characterized so far. The lack of in depth understanding of this model could potentially hamper its utility for cancer research. Therefore, in Chapter 2 of this dissertation, I first characterized key aspects of disease-related phenotypes including clinical and histopathological features, tumor-elicited inflammation, the transcriptomic profiles, the gut microbial profiles in the AKC mice. Further, comparative analysis has been made on the transcriptomic profiles between AKC mice and human colon cancers with mutations in APC and KRAS at cancer stage II or below to evaluate the utility of the mouse model for studying human CRCs.
Chemoprevention is the use of drugs or natural substances to inhibit initiation and delay of the progression of tumorigenesis, which could be a promising approach to reduce the incidence, mortality, and morbidity of CRC. Delta-tocotrienol (𝛿TE) is a natural analogue of vitamin E which has been shown to have antioxidant, anti-inflammatory and anticancer activities. Although its anticancer effects have been studied in different models of CRCs, including carcinogen-induced models, carcinogen-induced colitis-associated models, and colon cancer xenograft models, it has not been tested in a genetic model of sporadic CRC harboring Apc and Kras mutations. Therefore, in Chapter 3, the antitumor effects of 𝛿TE-rich tocotrienols (𝛿TE/gTE) and the potential mechanisms were investigated in AKC mice. 𝛿TE/𝛾TE-supplementation significantly improved the survival of AKC mice and suppressed tumorigenesis in association with inhibition of cell proliferation in the tumors. Further, the anti-tumor effects were correlated with reduction of pro-inflammatory and pro-tumorigenic cytokines, such as interleukin-1b and granulocyte-macrophage colony stimulating factor (GM-CSF), transcriptional enrichment of pathways involved in fatty acid metabolism, and reduction of diacylglycerol (DG) level in the colon tissue.
Finally, AKC mice were used for screening the efficacies of other potential chemoprevention candidates, including aspirin, sulindac, and resveratrol in Chapter 4. Aspirin and sulindac are nonsteroidal anti-inflammatory drugs (NSAIDs) and they are among the most studied chemoprevention agents for CRC. However, the long-term regular use of NSAIDs may cause bleedings in the gastrointestinal organ system or hemorrhagic stroke. For aspirin, although extensive studies have shown its beneficial role in the prevention of primary CRC, there are mixed results for its benefit and harms, which may require further identification of populations who will benefit from the regular use of aspirin for prevention of CRC. Resveratrol is a naturally derived polyphenol, which is known for its antioxidant, anti-inflammatory, and antimicrobial activities with generally good safety profile. In our study, when the dietary supplementation of three compounds alone or in combination with 𝛿TE/gTE were examined for its antineoplastic effects in the AKC mice, only aspirin significantly suppressed tumorigenesis with decreased pro-inflammatory and pro-tumorigenic cytokines in the colon without overt toxicity. We found that sulindac induced serious gastric lesions and potential liver toxicity with some animals died earlier than the rest over the study period. Additionally, in this model, resveratrol was not effective in reducing tumorigenesis, in contrast to a previous study where the workgroup used similar genetic model of CRC but different modality to induce the mutations. Our findings add lines of evidence that depending on the use of different models the test compounds, aspirin, sulindac, and resveratrol may exhibit varying cancer prevention effects. Further research is warranted to identify underlying mechanisms that could explain the heterogenous responses to the test compounds and to optimize the interventions.
- Doctor of Philosophy
- Nutrition Science
- West Lafayette