Regulation of hepatic drug-metabolizing enzymes by the bile acid-FXR-FGF15/19 pathway
Creators: name: Rizzolo, Daniel
Publisher: name: No Publisher Supplied
Year: 2022

description: Introduction: The nuclear receptor farnesoid X receptor (FXR) is the master regulator of bile acid (BA) homeostasis. The activation of intestinal FXR by BAs results in an induction of fibroblast growth factor 19 (FGF19; mouse ortholog FGF15). FGF15/19 are secreted into portal circulation to specifically activate fibroblast growth factor receptor 4 (FGFR4) on hepatocytes. Activation of FGFR4 suppresses BA production by downregulating the expression of genes critical in BA synthesis. In addition to regulating BA levels, FGF15/19 have been found to have positive effects on aspects of metabolic syndrome and non-alcoholic steatohepatitis (NASH), including increased energy metabolism, reduced lipogenesis, and decreased adiposity. For this reason, the pharmaceutical sector has been actively investigating FXR and FGF19 as potential targets for therapeutic intervention in cholestasis and NASH. We sought to determine the impact of chronic elevations of FGF15/19 on liver drug-metabolizing enzymes (DMEs) .Methods: To determine the impact of low BA levels and/or sustained elevations of FGF15 levels on DMEs, three aims were developed. In aim 1, we created a mouse model lacking key enzymes involved in BA synthesis, which may allow us to determine the role of individual BA species in vivo. In aim 2, we used Fgf15 transgenic mice (Fgf15 Tg) to quantify the impact that the overexpression of Fgf15 had on DME concentrations and functions. In aim 3, we used constitutive androstane receptor (CAR) knockout mice (CAR-/-) to determine to what degree CAR deficiency affects the alteration in DMEs caused by FGF15 overexpression. Hypothesis: Our central hypothesis was that chronic exposure to elevated levels of FGF15/19 causes an alteration of expression of genes encoding DMEs by increased CAR activation. To test this hypothesis, we used in vivo and in vitro mechanistic approaches: a tet-off Fgf15 Tg mouse model that continually overexpresses Fgf15, CAR-/- mice, and human cell models HepG2, HepaRG, and primary human hepatocytes (PHH) treated with recombinant FGF19 protein. Results: Transcriptomic analysis showed that the overexpression of Fgf15 caused alterations of DME mRNA expression in the livers of Fgf15 Tg mice. This induction was also manifested at protein levels revealed by Western blot and LCMS protein profiling of cytochrome p450 isoforms. Using hepatic microsomes and an in vivo pharmacokinetic study, we found that the overexpression of Fgf15 resulted in an increase in the metabolism of CYP2B substrates in mice. Using CAR-/- mice, we found that CAR played a minor role in the FGF15 overexpression induced alterations to DME gene expression. Additionally, we found that the overexpression of Fgf15 led to a male-to-female switch in sexually dimorphic hepatic gene expression, signifying there are additional mechanisms at play. Some preliminary data presented in this dissertation suggest there may be a reduction in growth hormone (GH) signaling, which is in line with mechanisms known to influence sexually dimorphic hepatic gene expressions. Conclusion: Through the integration of three specific aims in this dissertation, we have shown that chronic exposure of mice to elevated levels of FGF15 causes an induction of DMEs at the mRNA, protein, and functional levels. Additionally, we have demonstrated that the overexpression of Fgf15 led to a male-to-female switch of the expression of genes encoding DMEs in the liver. The mechanism responsible for this switch is unclear; however, initial data from our studies suggest that reduced GH signaling and increased CAR activation are associated with the alterations in DME gene expression and sex-related expression patterns in male mice overexpressing Fgf15.