THE ROLE OF EPIGENETICS IN TRANSCRIPTIONAL REGULATION OF FXR AND SILENCING FXR EXPRESSION IN HUMAN COLON CANCER

Abstract

Farnesoid X receptor (FXR) is a ligand activated transcription factor belonging to the nuclear receptor superfamily and bile acids are its endogenous ligands. FXR is a critical regulator of the enterohepatic circulation of bile acids, lipid homeostasis, glucose metabolism, and tumor suppression in liver and intestine. Consequently, FXR has become a very promising therapeutic target for the prevention and/or treatment of cholestasis, hyperlipidemic disorders, metabolic syndrome, and liver and colon cancer. Studies suggest epigenetic mechanisms are critical for proper transcriptional induction of nuclear receptors. Likewise, evidence shows epigenetic mechanisms are responsible for modulating the tissue/cell-specific FXR expression in human colon cancer. However, how epigenetic mechanisms are involved in FXR induced transcription or tissue-specific FXR expression remains elusive. Understanding these mechanisms is crucial for future development of pharmacological modulators of FXR as well as understanding the full physiological roles of FXR. This dissertation was designed to elucidate epigenetic mechanisms involved in tissue-specific FXR induced gene transcription, orphan nuclear receptors critical for regulating FXR function, and epigenetic mechanisms responsible for FXR silencing in colon cancer. In specific aim 1, a genome-wide FXR binding assay was done in mouse liver and intestine. Specific aim 2 focuses on the role of the orphan nuclear receptor hepatocyte nuclear factor 4fnalpha (HNF4&alpha) in regulating liver-specific functions of FXR. And finally, in specific aim 3, DNA methylation of FXR promoter was investigated as the mechanism responsible for FXR silencing in human colon cancer. In conclusion, genome-wide binding of FXR implicates novel epigenetic mechanisms and orphan nuclear receptors in regulating FXR function. Furthermore, this study indicates that HNF4&alpha is at least one orphan nuclear receptor capable of regulating FXR function in the liver. Findings from these first two aims succeeded in progressing drug development fields aimed at finding new FXR modulators for the treatment of multiple metabolic disorders by elucidating novel epigenetic mechanisms that may be investigated as therapeutic targets. Finally, FXR is at least partially down-regulated by DNA methylation in human colon cancer, suggesting a potential mechanism to be targeted for the prevention, treatment, and/or diagnosis of colon cancer.