ELUCIDATING THE FUNCTIONAL ROLES OF PEROXISOME PROLIFERATOR-ACTIVATED RECEPTOR β/δ IN HUMAN COLON CANCER CELLS

Open Access
- Author:
- Wang, Xiaohan
- Graduate Program:
- Biochemistry, Microbiology, and Molecular Biology
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- April 09, 2019
- Committee Members:
- Jeffrey M. Peters, Thesis Advisor/Co-Advisor
Gary H. Perdew, Committee Member
Troy L. Ott, Committee Member - Keywords:
- Peroxisome proliferator-activated receptor
PPAR
colon cancer - Abstract:
- Peroxisome proliferator-activated receptor β/δ (PPARβ/δ) is an important regulator in various physiological processes, including lipid metabolism and glucose homeostasis. However, its role in cancer remains controversial. Although PPARβ/δ is highly expressed in the intestines of normal adults, it has been reported to be up- or down-regulated during colon tumorigenesis. Researchers have not reached a consensus for whether PPARβ/δ is beneficial, detrimental, or unrelated to colon cancer initiation, survival, growth, and metastasis, in mouse or and human cancer models. One of the first mechanisms described that PPARβ/δ promotes carcinogenesis was the hypothesis that PPARβ/δ is a target gene of the oncogenic APC/β-CATENIN pathway, a major pathway that is activated by mutations in colon cancer. However, subsequent studies did not observe a correlation between PPARβ/δ expression and β-CATENIN activation, and questioned whether PPARD (gene coding for PPARβ/δ) is a bona fide APC/β-CATENIN target protein. Moreover, the functionality of PPARβ/δ as influenced by the APC/β-CATENIN pathway, has not been critically examined to date. Therefore, in the first part of this thesis, the hypothesis that PPARβ/δ is functionally regulated by the APC/β-CATENIN pathway as a tumor-promoting protein was tested. We first investigated whether mutations of the APC/CTNNB1 (β-CATENIN) genes or overexpression of functional β-CATENIN modulate PPARβ/δ cellular retention and its response to ligand activation in human colon cancer cell lines. We further examined the effect of ligand activation of PPARβ/δ using a classic agonist, as well as selective repression of PPARβ/δ using ligands that stimulate its transcriptional repression activity, on the growth of colon cancer cells with wild-type or mutant APC/CTNNB1. We observed that cytosol and nuclear retention of PPARβ/δ, with or without ligand activation, were not different between cell lines with wild-type or mutant APC/CTNNB1 (gene coding for β-CATENIN). Second, target gene activation of PPARβ/δ following ligand activation occurred faster in cell lines with mutant APC/CTNNB1 compared to a non-mutant cell line, although this difference was not observed with transient overexpression of β-CATENIN. Third, ligand activation and selective repression of PPARβ/δ inhibited growth in several APC/CTNNB1 mutant cell lines but had no effect on the non-mutant cell line. These results suggest that cellular retention and transcriptional activity of PPARβ/δ are not directly regulated by the APC/β-CATENIN pathway. However, the results also suggest that PPARβ/δ may be enhanced by the presence of APC/CTNNB1 mutations in human colon cancer cell lines. The role of PPARβ/δ in colon cancer invasion and metastasis also remains elusive. In the second part of this thesis, the influence of PPARβ/δ activation on malignancy-related features of colon cancer was examined. We hypothesized that ligand activation or selective repression of PPARβ/δ would inhibit anchorage-independent growth, migration, invasion, epithelial to mesenchymal transition (EMT), and metalloprotease (MMP) activity. Results, some preliminary in nature, showed that selective repression of PPARβ/δ reduced anchorage-independent growth by inducing apoptosis, inhibited migration, and reduced EMT marker expression, but did not change TNFα/TGFβ-induced MMP activity. By contrast, ligand activation of PPARβ/δ reduced migration and TNFα/TGFβ-induced MMP activity, but did not affect anchorage-independent growth and EMT marker expression. These results suggest that both ligand activation and selective repression of PPARβ/δ reduce the malignant potential of colon cancer, although the underlying mechanisms could be different. Combined, results from this study indicate that PPARβ/δ is not functionally regulated by the APC/β-CATENIN pathway. Further, ligand activation or selective repression of PPARβ/δ using synthetic ligands may modulate colon cancer growth and malignancy-related features, in particular in cells with APC/CTNNB1 mutations.