Effects of Fenofibrate on CYP2D6 and Regulation of ANG1 and RNASE4 by the FXR Agonist Obeticholic Acid

CYP2D6 is a major drug metabolizing enzyme that exhibits large interindividual variability, in part due to differential transcriptional regulation of CYP2D6; however, the factors controlling transcriptional regulation of CYP2D6 remain poorly understood. SHP is a corepressor that represses CYP2D6 expression by binding and inhibiting HNF4α, and modulators of SHP have been shown to alter CYP2D6 expression. Fenofibrate, a drug commonly used to reduce triglyceride levels and total cholesterol levels in hypertriglycerideamic and hypercholesterolaemic patients, has been shown to induce SHP mRNA. Examination of the effect of fenofibrate on SHP and CYP2D6 can provide insight into the factors that control transcriptional regulation of CYP2D6. In chapter one, we investigated the effect of fenofibrate treatment on the mRNA and protein expression of SHP and CYP2D6 in mice and primary human hepatocytes by using real-time polymerase chain reaction (qRT-PCR) and western blot. We also examined the role of PPARα, a nuclear receptor induced by fenofibrate, in contributing to the regulation of CYP2D6 by transactivating the CYP2D6 promoter. The results showed that mice treatment with fenofibrate for 5 days had a 2-fold increase in SHP protein level. Importantly, despite the repressive role of SHP in CYP2D6 promoter activity, the mRNA and protein levels of CYP2D6 did not differ between the fenofibrate-treated and control mice. Similar findings were obtained in fenofibrate-treated human hepatocytes. These results indicate that drug-drug interaction between fenofibrate and CYP2D6 substrates appears unlikely. Antimicrobial peptides (AMPs) are innate antibiotic effector molecules that contribute to the control of commensal bacteria load as well as defense against intestinal pathogens in the small intestine. Accumulating evidence suggests that the farnesoid X receptor (FXR) has a critical role in protecting against bacterial overgrowth in the small intestine. Whether intestinal FXR regulates AMP expression remains unknown. In chapter two, we investigated whether FXR is capable of regulating AMP expression by treating mice and human hepatocytes with an FXR agonist. We also investigated whether FXR agonists can confer protection against pathogenic enteric bacterial infection by upregulating certain AMPs. To this end, we examined whether the FXR agonist obeticholic acid (OCA) effects the survival time of mice treated with Salmonella enterica. FXR activation increased Ang1 and Rnase4 ileal mRNA levels in mice and primary human hepatocytes, likely due to FXR directly transactivating the ANG1/RNASE4 promoter. However, treatment with the FXR agonist OCA was not sufficient to confer protection against S. enterica infection in mice and may not be a potential therapy for enteric pathogen infection.