Identification of a hormonal basis for gallbladder filling

Fifty years of advances in bile acid synthesis and metabolism

There are two major pathways that mammalian cells use to supply themselves with cholesterol, one involving the synthesis of sterols from acetyl-CoA and the other the metabolism of cholesterol-rich lipoprotein particles via receptor-mediated endocytosis. There also are several pathways that mammalian cells use to break down cholesterol, and these disposal pathways are equal in physiological importance to the supply pathways. A major catabolic route involves conversion of cholesterol into conjugated bile salts, a transformation mediated by 16 or more liver enzymes. This review highlights findings in cholesterol catabolism from the last five decades with special emphasis on advances in bile acid synthesis, transport, and regulation.

Targeting bile-acid signalling for metabolic diseases

Bile acids are increasingly being appreciated as complex metabolic integrators and signalling factors and not just as lipid solubilizers and simple regulators of bile-acid homeostasis. It is therefore not surprising that a number of bile-acid-activated signalling pathways have become attractive therapeutic targets for metabolic disorders. Here, we review how the signalling functions of bile acids can be exploited in the development of drugs for obesity, type 2 diabetes, hypertriglyceridaemia and atherosclerosis, as well as other associated chronic diseases such as non-alcoholic steatohepatitis.

Endocrine FGFs and Klothos: emerging concepts

Endocrine fibroblast growth factors (FGFs) control a variety of physiological processes including suppression of bile acid synthesis in hepatocytes, promotion of lipolysis in adipocytes, and inhibition of phosphate reabsorption and vitamin D biosynthesis in renal tubular cells. Endocrine FGFs require the Klotho gene family of transmembrane proteins as co-receptors to bind cognate FGF receptors. Importantly, expression of endocrine FGFs is regulated by nuclear receptors whose lipophilic ligands are generated under the control of these hormones in their target organs. Thus, novel endocrine axes have emerged that regulate diverse metabolic processes through feedback loops composed of the FGF, Klotho, and nuclear receptor gene families. This review covers roles of Klotho family proteins in the regulation of activity and expression of endocrine FGFs.

Coordinate regulation of gallbladder motor function in the gut-liver axis

Gallstones are one of the most common digestive diseases with an estimated prevalence of 10%-15% in adults living in the western world, where cholesterol-enriched gallstones represent 75%-80% of all gallstones. In cholesterol gallstone disease, the gallbladder becomes the target organ of a complex metabolic disease. Indeed, a fine coordinated hepatobiliary and gastrointestinal function, including gallbladder motility in the fasting and postprandial state, is of crucial importance to prevent crystallization and precipitation of excess cholesterol in gallbladder bile. Also, gallbladder itself plays a physiopathological role in biliary lipid absorption. Here, we present a comprehensive view on the regulation of gallbladder motor function by focusing on recent discoveries in animal and human studies, and we discuss the role of the gallbladder in the pathogenesis of gallstone formation.

Loss of orphan receptor small heterodimer partner sensitizes mice to liver injury from obstructive cholestasis

The orphan nuclear hormone receptor small heterodimer partner (SHP) regulates the expression of several genes involved in bile acid homeostasis in the liver. Because bile acid toxicity is a major source of liver injury in cholestatic disease, we explored the role of SHP in liver damage induced by common bile duct ligation (BDL). Shp(-/-) mice show increased sensitivity in this model of acute obstructive cholestasis, with greater numbers of bile infarcts and higher mortality than wild-type C57BL/6 mice. This increased sensitivity could not be accounted for by differences in expression of bile acid homeostatic genes 2 or 5 days after BDL. Instead, higher basal expression of such genes, including the key biosynthetic enzyme cholesterol 7alpha hydroxylase (Cyp7A1) and the bile salt export pump, is associated with both an increase in bile flow prior to BDL and an increase in acute liver damage at only 1.5 hours after BDL in Shp(-/-) mice, as shown by bile infarcts. At 3 hours, Cyp7A1 expression still remained elevated in Shp(-/-) with respect to wild-type mice, and the hepatic and serum bile acid levels and total hepatobiliary bile acid pool were significantly increased. The increased sensitivity of mice lacking SHP contrasts with the decreased sensitivity of mice lacking the farnesoid X receptor (FXR; nuclear receptor subfamily 1, group H, member 4) to BDL, which has been associated with decreased intraductal pressure and fewer bile infarcts.

CONCLUSION

We propose that differences in acute responses to BDL, particularly the early formation of bile infarcts, are a primary determinant of the differences in longer term sensitivity of the Fxr(-/-) and Shp(-/-) mice to acute obstructive cholestasis.

Actions and mode of actions of FGF19 subfamily members

Fibroblast growth factors (FGFs) are humoral factors with diverse biological functions. While most FGFs were shown to work as local factors regulating cell growth and differentiation, recent investigations indicated that FGF19 subfamily members, FGF15/19, FGF21 and FGF23 work as systemic factors. FGF15/19 produced by intestine inhibits bile acid synthesis and FGF21from liver is involved in carbohydrate and lipid metabolism. In addition, FGF23 was shown to be produced by bone and regulate phosphate and vitamin D metabolism. Furthermore, these FGFs require klotho or betaklotho for their actions in addition to canonical FGF receptors. It is possible that these FGFs together with their receptor systems might be targets for novel therapeutic measures in the future.

Look who's talking: nuclear receptors in the liver and gastrointestinal tract

The Nuclear Receptors in Liver and Digestive Diseases research workshop was held in Rockville, Maryland, on November 7 and 8, 2007, under the auspices of the National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health. Over 130 researchers from around the world gathered to assess the pathophysiology of nuclear receptors in the liver and gastrointestinal tract and to explore their potential use as therapeutic targets. This review covers some of the highlights of the meeting, including important areas of future investigation.

Mini-review: endocrine actions of fibroblast growth factor 19

Fibroblast growth factor (FGF) 19 is an atypical member of the fibroblast growth factor family of signaling molecules. FGF19, FGF21, and FGF23 comprise a phylogenetic subfamily with attributes that distinguish them from typical FGFs. The FGF19 subfamily has reduced heparin binding resulting from a disrupted beta-trefoil domain. Reduced heparin binding allows these FGFs to diffuse beyond their site of origin and act as endocrine hormones. This family of FGFs is regulated, at least in part, by nuclear hormone receptors. FGF19 expression is regulated by the farnesoid X receptor, a nuclear hormone receptor that is a key regulator of bile acid biosynthesis and transport. In line with its regulation by a bile acid receptor, FGF19 is involved in the regulation of bile acid biosynthesis and gallbladder filling. FGF19 originates from intestine and signals to liver via the portal circulation with a pronounced diurnal pattern. FGF19 is the only FGF to not have a closely related mouse homologue. The mouse homologue of FGF19, called FGF15, is only 53% identical to the human FGF19. FGF19 transgenic mice and mice administered exogenous FGF19 are resistant to the effects of a high fat diet, suggesting FGF19 may play a role in metabolic signaling pathways. Hepatocellular carcinoma is seen in mice, predominantly female mice, exposed to FGF19. Further investigation into the cellular mechanisms involved in these activities will allow better understanding of FGF19 biology in the context of human physiology.

Variation of the gene encoding the nuclear bile salt receptor FXR and gallstone susceptibility in mice and humans

BACKGROUND/AIMS

From quantitative trait locus mapping in inbred mice, we identified the Nr1h4 gene encoding the nuclear bile salt receptor FXR as a candidate gene for the cholesterol gallstone susceptibility locus Lith7. Here, we investigated further an association of the gene encoding FXR and gallstone susceptibility in mice and humans.

METHODS

The Nr1h4 gene was sequenced in inbred mouse strains with susceptible and resistant Lith7 alleles. Quantitative RT-PCR was employed to determine mRNA expression levels. Gallstone carriers and control subjects of three different populations comprising 1004 individuals were genotyped for polymorphisms of the orthologous human gene detected by sequencing.

RESULTS

Expression and sequence analyses in inbred mice were consistent with Nr1h4 underlying Lith7. In the human populations, we identified three frequent haplotypes that accounted for > 95% of all haplotypes observed. In a Mexican population, the most common haplotype NR1H4_1 was associated with gallstone prevalence. In contrast, NR1H4_1 displayed no association with gallstone prevalence in a German population, whereas in a Chilean population we observed a trend towards a protective effect of NR1H4_1.

CONCLUSIONS

Our study in an inbred mouse model and in three ethnically distinct populations indicates complex interactions of NR1H4 alleles and other risk factors for the development of cholelithiasis.

FXR agonists and FGF15 reduce fecal bile acid excretion in a mouse model of bile acid malabsorption

Bile acid malabsorption, which in patients leads to excessive fecal bile acid excretion and diarrhea, is characterized by a vicious cycle in which the feedback regulation of bile acid synthesis is interrupted, resulting in additional bile acid production. Feedback regulation of bile acid synthesis is under the control of an endocrine pathway wherein activation of the nuclear bile acid receptor, farnesoid X receptor (FXR), induces enteric expression of the hormone, fibroblast growth factor 15 (FGF15). In liver, FGF15 acts together with FXR-mediated expression of small heterodimer partner to repress bile acid synthesis. Here, we show that the FXR-FGF15 pathway is disrupted in mice lacking apical ileal bile acid transporter, a model of bile acid malabsorption. Treatment of Asbt-/- mice with either a synthetic FXR agonist or FGF15 downregulates hepatic cholesterol 7alpha-hydroxylase mRNA levels, decreases bile acid pool size, and reduces fecal bile acid excretion. These findings suggest that FXR agonists or FGF15 could be used therapeutically to interrupt the cycle of excessive bile acid production in patients with bile acid malabsorption.

FXR signaling in metabolic disease

Farnesoid X receptor (FXR), a member of the nuclear receptor superfamily, has been shown to be important in controlling numerous metabolic pathways; these include roles in maintaining bile acid, lipid and glucose homeostasis, in preventing intestinal bacterial infection and gallstone formation and in modulating liver regeneration and tumorigenesis. The accumulating data suggest that FXR may be a pharmaceutical target for the treatment of certain metabolic diseases.

Co-receptor Requirements for Fibroblast Growth Factor-19 Signaling

FGF19 is a unique member of the fibroblast growth factor (FGF) family of secreted proteins that regulates bile acid homeostasis and metabolic state in an endocrine fashion. Here we investigate the cell surface receptors required for signaling by FGF19. We show that betaKlotho, a single-pass transmembrane protein highly expressed in liver and fat, induced ERK1/2 phosphorylation in response to FGF19 treatment and significantly increased the interactions between FGF19 and FGFR4. Interestingly, our results show that alphaKlotho, another Klotho family protein related to betaKlotho, also induced ERK1/2 phosphorylation in response to FGF19 treatment and increased FGF19-FGFR4 interactions in vitro, similar to the effects of betaKlotho. In addition, heparin further enhanced the effects of both alphaKlotho and betaKlotho in FGF19 signaling and interaction experiments. These results suggest that a functional FGF19 receptor may consist of FGF receptor (FGFR) and heparan sulfate complexed with either alphaKlotho or betaKlotho.

Tissue-specific expression of betaKlotho and fibroblast growth factor (FGF) receptor isoforms determines metabolic activity of FGF19 and FGF21

The fibroblast growth factor (FGF) 19 subfamily of ligands, FGF19, FGF21, and FGF23, function as hormones that regulate bile acid, fatty acid, glucose, and phosphate metabolism in target organs through activating FGF receptors (FGFR1-4). We demonstrated that Klotho and betaKlotho, homologous single-pass transmembrane proteins that bind to FGFRs, are required for metabolic activity of FGF23 and FGF21, respectively. Here we show that, like FGF21, FGF19 also requires betaKlotho. Both FGF19 and FGF21 can signal through FGFR1-3 bound by betaKlotho and increase glucose uptake in adipocytes expressing FGFR1. Additionally, both FGF19 and FGF21 bind to the betaKlotho-FGFR4 complex; however, only FGF19 signals efficiently through FGFR4. Accordingly, FGF19, but not FGF21, activates FGF signaling in hepatocytes that primarily express FGFR4 and reduces transcription of CYP7A1 that encodes the rate-limiting enzyme for bile acid synthesis. We conclude that the expression of betaKlotho, in combination with particular FGFR isoforms, determines the tissue-specific metabolic activities of FGF19 and FGF21.

The pharmacological exploitation of cholesterol 7alpha-hydroxylase, the key enzyme in bile acid synthesis: from binding resins to chromatin remodelling to reduce plasma cholesterol

Mammals dispose of cholesterol mainly through 7alpha-hydroxylated bile acids, and the enzyme catalyzing the 7alpha-hydroxylation, cholesterol 7alpha-hydroxylase (CYP7A1), has a deep impact on cholesterol homeostasis. In this review, we present the study of regulation of CYP7A1 as a good exemplification of the extraordinary contribution of molecular biology to the advancement of our understanding of metabolic pathways that has taken place in the last 2 decades. Since the cloning of the gene from different species, experimental evidence has accumulated, indicating that the enzyme is mainly regulated at the transcriptional level and that bile acids are the most important physiological inhibitors of CYP7A1 transcription. Multiple mechanisms are involved in the control of CYP7A1 transcription and a variety of transcription factors and nuclear receptors participate in sophisticated regulatory networks. A higher order of transcriptional regulation, stemming from the so-called histone code, also applies to CYP7A1, and recent findings clearly indicate that chromatin remodelling events have profound effects on its expression. CYP7A1 also acts as a sensor of signals coming from the gut, thus representing another line of defence against the toxic effects of bile acids and a downstream target of agents acting at the intestinal level. From the pharmacological point of view, bile acid binding resins were the first primitive approach targeting the negative feed-back regulation of CYP7A1 to reduce plasma cholesterol. In recent years, new drugs have been designed based on recent discoveries of the regulatory network, thus confirming the position of CYP7A1 as a focus for innovative pharmacological intervention.

Differential regulation of bile acid homeostasis by the farnesoid X receptor in liver and intestine

Bile acid concentrations are controlled by a feedback regulatory pathway whereby activation of the farnesoid X receptor (FXR) represses transcription of both the CYP7A1 gene, encoding the rate-limiting enzyme in the classic bile acid synthesis pathway, and the CYP8B1 gene, required for synthesis of cholic acid. The tissue-specific roles of FXR were examined using liver- and intestine-specific FXR-null models. FXR deficiency in either liver (Fxr DeltaL) or intestine (Fxr DeltaIE) increased bile acid pool size. Treatment with the FXR-selective agonist GW4064 significantly repressed CYP7A1 in Fxr DeltaL mice but not Fxr DeltaIE mice, demonstrating that activation of FXR in intestine but not liver is required for short-term repression of CYP7A1 in liver. This intestinal-specific effect of FXR is likely mediated through induction of the hormone FGF15, which suppresses CYP7A1. In comparison to CYP7A1, FXR-mediated repression of CYP8B1 was more dependent on the presence of FXR in liver and less dependent on its presence in intestine. Consistent with these findings, recombinant FGF15 repressed CYP7A1 mRNA levels without affecting CYP8B1 expression. These data provide evidence that FXR-mediated repression of bile acid synthesis requires the complementary actions of FXR in both liver and intestine and reveal mechanistic differences in feedback repression of CYP7A1 and CYP8B1.

Lithocholic acid induction of the FGF19 promoter in intestinal cells is mediated by PXR

AIM: To study the effect of the toxic secondary bile acid lithocholic acid (LCA) on the expression of fibroblast growth factor 19 (FGF19) in intestinal cells and to characterize the pregnane-X-receptor (PXR) response of the FGF19 promoter region.

METHODS: The intestinal cell line LS174T was stimulated with various concentrations of chenodeoxy-cholic acid and lithocholic acid for several time points. FGF19 mRNA levels were determined with quantitative realtime RT-PCR. FGF19 deletion promoter constructs were generated and the LCA response was analzyed in reporter assays. Co-transfections with PXR and RXR were carried out to study FGF19 regulation by these factors.

RESULTS: LCA and CDCA strongly up-regulate FGF19 mRNA expression in LS174T cells in a time and dose dependent manner. Using reporter gene assays with several deletion constructs we found that the LCA responsive element in the human FGF19 promoter maps to the proximal regulatory region containing two potential binding sites for PXR. Overexpression of PXR and its dimerization partner retinoid X receptor (RXR) and stimulation with LCA or the potent PXR ligand rifampicin leads to a significant induction of FGF19 promoter activity in intestinal cells.

CONCLUSION: LCA induced feedback inhibition of bile acid synthesis in the liver is likely to be regulated by PXR inducing intestinal FGF19 expression.

Liver-specific activities of FGF19 require Klotho beta

Hepatocyte function is regulated by members of the fibroblast growth factor (FGF) family of proteins, but little is known about the specific molecular mechanisms of this endocrine pathway. FGF19 regulates bile acid homeostasis and gall bladder filling; FGF19 binds only to FGF receptor 4 (FGFR4), but its liver-specific activity cannot be explained solely by the distribution of this receptor. Although it has been suggested that Klotho beta (KLB) may have a role in mediating FGF19 activity, we have provided for the first time definitive evidence that KLB is required for FGF19 binding to FGFR4, intracellular signaling, and downstream modulation of gene expression. We have shown that FGFR4 is widely distributed in mouse, whereas KLB distribution is more restricted. Liver was the only organ in which both genes were abundantly expressed. We show that in mice, FGF19 injection triggers liver-specific induction of c-Fos and repression of CYP7A1. The tissue-specific activity of FGF19 supports the unique intersection of KLB and FGFR4 distribution in liver. These studies define KLB as a novel FGFR4 coreceptor required for FGF19 liver specific functions.

Bile acids, farnesoid X receptor, atherosclerosis and metabolic control

PURPOSE OF REVIEW

Bile acids are amphiphilic molecules synthesized from cholesterol exclusively in the liver that are essential for effective absorption of dietary fat. In addition to this 'classical role', bile acids act as signalling molecules that control their own metabolism by activating the nuclear receptor, farnesoid X receptor.

RECENT FINDINGS

Recent work demonstrates that farnesoid X receptor exerts metabolic control beyond bile acid homeostasis, notably effects on HDL, triglyceride and glucose metabolism. Farnesoid X receptor influences insulin sensitivity of tissues that are not part of the enterohepatic circulation, for example, adipose tissue. Certain metabolic effects in the liver appear to be mediated via farnesoid X receptor-stimulated release of an intestinal growth factor. In addition, novel signalling pathways independent of farnesoid X receptor have been identified that may contribute to bile acid-mediated metabolic regulation.

SUMMARY

Farnesoid X receptor represents a potentially attractive target for treatment of various aspects of the metabolic syndrome and for prevention of atherosclerosis. Yet, in view of its pleiotropic effects and apparent species-specificity, it is evident that successful interference of the farnesoid X receptor signalling system will require the development of gene-specific and/or organ-specific farnesoid X receptor modulators and extensive testing in human models of disease.

Gallstone disease

Gallstone disease is one of the most prevalent gastrointestinal diseases with a substantial burden to health care systems that is supposed to increase in ageing populations at risk. Aetiology and pathogenesis of cholesterol gallstones still are not well defined, and strategies for prevention and efficient nonsurgical therapies are missing. This review summarizes current concepts on the pathogenesis of cholesterol gallstones with focus on the uptake and secretion of biliary lipids and special emphasis on recent studies into the genetic background.

Homing in on bile acid physiology

Fibroblast growth factor 15 (Fgf15), a hormone made by the terminal ileum in response to bile acids, represses hepatic bile acid biosynthesis. demonstrate that Fgf15 is also required for gallbladder filling.