Characterization of individual bile acids in vivo utilizing a novel low bile acid mouse model.

Bile acids (BAs) are signaling molecules synthesized in the liver initially by CYP7A1 and CYP27A1 in the classical and alternative pathways, respectively. BAs are essential for cholesterol clearance, intestinal absorption of lipids, and endogenous modulators of farnesoid x receptor (FXR). FXR is critical in maintaining BA homeostasis and gut-liver crosstalk. Complex reactions in vivo and the lack of suitable animal models impede our understanding of the functions of individual BAs. In this study, we characterized the in vivo effects of three-day feeding of cholic acid (CA), deoxycholic acid (DCA), or ursodeoxycholic acid (UDCA) at physiological/non-hepatotoxic concentrations in a novel low-BA mouse model (Cyp7a1  -/-/Cyp27a1  -/-, DKO). Liver injury, BA levels and composition and BA signaling by the FXR-fibroblast growth factor 15 (FGF15) axis were determined. Overall, higher basal inflammation and altered lipid metabolism in DKO mice might be associated with low BAs. CA, DCA and UDCA feeding activated FXR signals with tissue specificity. Dietary CA and DCA similarly altered tissue BA profiles to be less hydrophobic, while UDCA promoted a more hydrophobic tissue BA pool with the profiles shifted towards non-12α-OH BAs and secondary BAs. However, UDCA did not offer any overt protective effects as expected. These findings allow us to determine the precise effects of individual BAs in vivo on BA-FXR signaling and overall BA homeostasis in liver physiology and pathologies.

FXR and NASH: an avenue for tissue-specific regulation.

NASH is within the spectrum of NAFLD, a liver condition encompassing liver steatosis, inflammation, hepatocyte injury, and fibrosis. The prevalence of NASH-induced cirrhosis is rapidly rising and has become the leading indicator for liver transplantation in the US. There is no Food and Drug Administration (FDA)-approved pharmacological intervention for NASH. The farnesoid X receptor (FXR) is essential in regulating bile acid homeostasis, and dysregulation of bile acids has been implicated in the pathogenesis of NASH. As a result, modulators of FXR that show desirable effects in mitigating key characteristics of NASH have been developed as promising therapeutic approaches. However, global FXR activation causes adverse effects such as cholesterol homeostasis imbalance and pruritus. The development of targeted FXR modulation is necessary for ideal NASH therapeutics, but information regarding tissue-specific and cell-specific FXR functionality is limited. In this review, we highlight FXR activation in the regulation of bile acid homeostasis and NASH development, examine the current literature on tissue-specific regulation of nuclear receptors, and speculate on how FXR regulation will be beneficial in the treatment of NASH.

Myclobutanil-mediated alteration of liver-gut FXR signaling in mice.

The effects of exposure to Myclobutanil, a triazole fungicide, on the development and progression of nonalcoholic fatty liver disease (NAFLD) are unclear, but activation of nuclear receptors (NRs) is a known mechanism of azole-induced liver toxicity. Farnesoid X receptor (FXR) is a NR and is highly expressed in the liver and intestine. Activation of FXR tightly regulates bile acid (BA), lipid and glucose homeostasis, and inflammation partly through the induction of fibroblast growth factor 15 (FGF15; human ortholog FGF19). FXR activation is downregulated during NAFLD and agonists are currently being explored as potential therapeutic strategy. In this study, we aimed to clarify the effects of Myclobutanil exposure on FXR activation and NAFLD development. Reporter assay showed Myclobutanil treatment, following FXR activation with potent FXR agonist (GW4064), resulted in a dose-dependent decrease of FXR activity. Furthermore, a 10-day study in male mice demonstrated that cotreatment with Myclobutanil led to an 80% reduction of GW4064-induced ileal expression of Fgf15. In a diet-induced NAFLD study, low-fat diet (LFD) fed mice administered myclobutanil displayed decreased FXR activity in the liver and ileum, while high-fat-high-sugar-diet (HFHSD) fed mice showed an increase in hepatic FXR activity and an induction of target genes regulated by constitutive androstane receptor and/or pregnane X receptor. Our work demonstrates Myclobutanil inhibits FXR activity and modulates FXR activity differentially in mice fed LFD or HFHSD. Our studies suggest the importance of understanding how Myclobutanil could contribute to BA dysregulation in disease states such as NAFLD.