Activation of farnesoid X receptor (Fxr, Nr1h4 ) is a major mechanism in suppressing bile-acid synthesis by reducing the expression levels of genes encoding key bile-acid synthetic enzymes (e.g., cytochrome P450 [ CYP]7A1/Cyp7a1 and CYP8B1/Cyp8b1 ). FXR-mediated induction of hepatic small heterodimer partner (SHP/Shp, Nr0b2 ) and intestinal fibroblast growth factor 15 (Fgf15; FGF19 in humans) has been shown to be responsible for this suppression. However, the exact contribution of Shp/Fgf15 to this suppression, and the associated cell-signaling pathway, is unclear. By using novel genetically modified mice, the current study showed that the intestinal Fxr/Fgf15 pathway was critical for suppressing both Cyp7a1 and Cyp8b1 gene expression, but the liver Fxr/Shp pathway was important for suppressing Cyp8b1 gene expression and had a minor role in suppressing Cyp7a1 gene expression. Furthermore, in vivo administration of Fgf15 protein to mice led to a strong activation of extracellular signal-related kinase (ERK) and, to a smaller degree, Jun N-terminal kinase (JNK) in the liver. In addition, deficiency of either the ERK or JNK pathway in mouse livers reduced the basal, but not the Fgf15-mediated, suppression of Cyp7a1 and Cyp8b1 gene expression. However, deficiency of both ERK and JNK pathways prevented Fgf15-mediated suppression of Cyp7a1 and Cyp8b1 gene expression. Conclusion: The current study clearly elucidates the underlying molecular mechanism of hepatic versus intestinal Fxr in regulating the expression of genes critical for bile-acid synthesis and hydrophobicity in the liver.

We reported previously that mice overexpressing cytochrome P450 7a1 (Cyp7a1; Cyp7a1-tg mice) are protected against high fat diet–induced hypercholesterolemia, obesity, and insulin resistance. Here, we investigated the underlying mechanism of bile acid signaling in maintaining cholesterol homeostasis in Cyp7a1-tg mice. Cyp7a1-tg mice had two-fold higher Cyp7a1 activity and bile acid pool than did wild-type mice. Gallbladder bile acid composition changed from predominantly cholic acid (57%) in wild-type to chenodeoxycholic acid (54%) in Cyp7a1-tg mice. Cyp7a1-tg mice had higher biliary and fecal cholesterol and bile acid secretion rates than did wild-type mice. Surprisingly, hepatic de novo cholesterol synthesis was markedly induced in Cyp7a1-tg mice but intestine fractional cholesterol absorption in Cyp7a1-tg mice remained the same as wild-type mice despite the presence of increased intestine bile acids. Interestingly, hepatic but not intestinal expression of several cholesterol (adenosine triphosphate–binding cassette G5/G8 [ABCG5/G8], scavenger receptor class B, member 1) and bile acid (ABCB11) transporters were significantly induced in Cyp7a1-tg mice. Treatment of mouse or human hepatocytes with a farnesoid X receptor (FXR) agonist GW4064 or bile acids induced hepatic Abcg5/g8 expression. A functional FXR binding site was identified in the Abcg5 gene promoter. Study of tissue-specific Fxr knockout mice demonstrated that loss of the Fxr , gene in the liver attenuated bile acid induction of hepatic Abcg5/g8 and gallbladder cholesterol content, suggesting a role of FXR in the regulation of cholesterol transport. Conclusion: This study revealed a new mechanism by which increased Cyp7a1 activity expands the hydrophobic bile acid pool, stimulating hepatic cholesterol synthesis and biliary cholesterol secretion without increasing intestinal cholesterol absorption. This study demonstrated that Cyp7a1 plays a critical role in maintaining cholesterol homeostasis and underscores the importance of bile acid signaling in regulating overall cholesterol homeostasis. (Hepatology 2011)

Farnesoid X receptor (FXR) is a bile acid‐activated transcription factor belonging to the nuclear receptor superfamily. FXR is highly expressed in liver and intestine and crosstalk mediated by FXR in these two organs is critical in maintaining bile acid homeostasis. FXR deficiency has been implicated in many liver and intestine diseases. However, regulation of transcription by FXR at the genomic level is not known. This study analyzed genome‐wide FXR binding in liver and intestine of mice treated with a synthetic FXR ligand (GW4064) by chromatin immunoprecipitation coupled to massively parallel sequencing (ChIP‐seq). The results showed a large degree of tissue‐specific FXR binding, with only 11% of total sites shared between liver and intestine. The sites were widely distributed between intergenic, upstream, intragenic, and downstream of genes, with novel sites identified within even known FXR target genes. Motif analysis revealed a half nuclear receptor binding site, normally bound by a few orphan nuclear receptors, adjacent to the FXR response elements, indicating possible involvement of some orphan nuclear receptors in modulating FXR function. Furthermore, pathway analysis indicated that FXR may be extensively involved in multiple cellular metabolic pathways. Conclusion: This study reports genome‐wide FXR binding in vivo and the results clearly demonstrate tissue‐specific FXR/gene interaction. In addition, FXR may be involved in regulating broader biological pathways in maintaining hepatic and intestinal homeostasis. (HEPATOLOGY 2010.)

Previous studies suggest that the lipid-lowering effect of berberine (BBR) involves actions on the low-density lipoprotein receptor and the AMP-activated protein kinase signaling pathways. However, the implication of these mechanisms is unclear because of the low bioavailability of BBR. Because the main action site of BBR is the gut and intestinal farnesoid X receptor (FXR) plays a pivotal role in the regulation of lipid metabolism, we hypothesized that the effects of BBR on intestinal FXR signaling pathway might account for its pharmacological effectiveness. Using wild type (WT) and intestine-specific FXR knockout (FXR^int−/−) mice, we found that BBR prevented the development of high-fat-diet–induced obesity and ameliorated triglyceride accumulation in livers of WT, but not FXR^int−/− mice. BBR increased conjugated bile acids in serum and their excretion in feces. Furthermore, BBR inhibited bile salt hydrolase (BSH) activity in gut microbiota, and significantly increased the levels of tauro-conjugated bile acids, especially tauro-cholic acid(TCA), in the intestine. Both BBR and TCA treatment activated the intestinal FXR pathway and reduced the expression of fatty-acid translocase Cd36 in the liver. These results indicate that BBR may exert its lipid-lowering effect primarily in the gut by modulating the turnover of bile acids and subsequently the ileal FXR signaling pathway. In summary, we provide the first evidence to suggest a new mechanism of BBR action in the intestine that involves, sequentially, inhibiting BSH, elevating TCA, and activating FXR, which lead to the suppression of hepatic expression of Cd36 that results in reduced uptake of long-chain fatty acids in the liver.

Nonalcoholic steatohepatitis (NASH) comprises dysregulation of lipid metabolism and inflammation. Identification of the various genetic and environmental susceptibility factors for NASH may provide novel treatments to limit inflammation and fibrosis in patients. This study utilized a mouse model of hypercholesterolemia, low-density lipoprotein receptor knockout (LDLr^-/-) mice fed a high-fat diet for 5 months, to test the hypothesis that farnesoid X receptor (FXR) deficiency contributed to NASH development. Either the high-fat diet or FXR deficiency increased serum alanine aminotransferase activity, whereas only FXR deficiency increased bile acid and alkaline phosphatase levels. FXR deficiency and high-fat feeding increased serum cholesterol and triglycerides. Although high fat led to macrosteatosis and hepatocyte ballooning in livers of mice regardless of genotype, no inflammatory infiltrate was observed in the livers of LDLr^-/- mice. In contrast, in the livers of LDLr^-/-/FXR^-/- mice, foci of inflammatory cells were observed occasionally when fed the control diet and were greatly increased when fed the high-fat diet. Consistent with enhanced inflammatory cells, hepatic levels of tumor necrosis factor α and intercellular adhesion molecule-1 mRNA were increased by the high-fat diet in LDLr^-/-/FXR^-/- mice. In agreement with elevated levels of procollagen 1α1 and TGF-β mRNA, type 1 collagen protein levels were increased in livers of LDLr^-/-/FXR^-/- mice fed a high-fat diet. In conclusion, FXR deficiency induces pathologic manifestations required for NASH diagnosis in a mouse model of hypercholesterolemia, including macrosteatosis, hepatocyte ballooning, and inflammation, which suggest a combination of FXR deficiency and high-fat diet is a risk factor for NASH development, and activation of FXR may be a therapeutic intervention in the treatment of NASH.