Cyclosporin a and enterohepatic circulation of bile salts in rats: decreased cholate synthesis but increased intestinal reabsorption

Gut microbiota inhibit Asbt-dependent intestinal bile acid reabsorption via Gata4

BACKGROUND & AIMS

Regulation of bile acid homeostasis in mammals is a complex process regulated via extensive cross-talk between liver, intestine and intestinal microbiota. Here we studied the effects of gut microbiota on bile acid homeostasis in mice.

METHODS

Bile acid homeostasis was assessed in four mouse models. Germfree mice, conventionally-raised mice, Asbt-KO mice and intestinal-specific Gata4-iKO mice were treated with antibiotics (bacitracin, neomycin and vancomycin; 100 mg/kg) for five days and subsequently compared with untreated mice.

RESULTS

Attenuation of the bacterial flora by antibiotics strongly reduced fecal excretion and synthesis of bile acids, but increased the expression of the bile acid synthesis enzyme CYP7A1. Similar effects were seen in germfree mice. Intestinal bile acid absorption was increased and accompanied by increases in plasma bile acid levels, biliary bile acid secretion and enterohepatic cycling of bile acids. In the absence of microbiota, the expression of the intestinal bile salt transporter Asbt was strongly increased in the ileum and was also expressed in more proximal parts of the small intestine. Most of the effects of antibiotic treatment on bile acid homeostasis could be prevented by genetic inactivation of either Asbt or the transcription factor Gata4.

CONCLUSIONS

Attenuation of gut microbiota alters Gata4-controlled expression of Asbt, increasing absorption and decreasing synthesis of bile acids. Our data support the concept that under physiological conditions microbiota stimulate Gata4, which suppresses Asbt expression, limiting the expression of this transporter to the terminal ileum. Our studies expand current knowledge on the bacterial control of bile acid homeostasis.

Cystic fibrosis related liver disease--another black box in hepatology

Due to improved medical care, life expectancy in patients with cystic fibrosis (CF) has veritably improved over the last decades. Importantly, cystic fibrosis related liver disease (CFLD) has become one of the leading causes of morbidity and mortality in CF patients. However, CFLD might be largely underdiagnosed and diagnostic criteria need to be refined. The underlying pathomechanisms are largely unknown, and treatment strategies with proven efficacy are lacking. This review focuses on current invasive and non-invasive diagnostic standards, the current knowledge on the pathophysiology of CFLD, treatment strategies, and possible future developments.

Applications of stable isotopes in clinical pharmacology

This review aims to present an overview of the application of stable isotope technology in clinical pharmacology. Three main categories of stable isotope technology can be distinguished in clinical pharmacology. Firstly, it is applied in the assessment of drug pharmacology to determine the pharmacokinetic profile or mode of action of a drug substance. Secondly, stable isotopes may be used for the assessment of drug products or drug delivery systems by determination of parameters such as the bioavailability or the release profile. Thirdly, patients may be assessed in relation to patient-specific drug treatment; this concept is often called personalized medicine. In this article, the application of stable isotope technology in the aforementioned three areas is reviewed, with emphasis on developments over the past 25 years. The applications are illustrated with examples from clinical studies in humans.

Sex-dependent programming of glucose and fatty acid metabolism in mouse offspring by maternal protein restriction

BACKGROUND

Nutritional conditions during fetal life influence the risk of the development of metabolic syndrome and cardiovascular diseases in adult life (metabolic programming). Impaired glucose tolerance and dysregulated fatty acid metabolism are hallmarks of metabolic syndrome.

OBJECTIVE

We aimed to establish a mouse model of metabolic programming focusing on the sex-specific effects of a maternal low-protein diet during gestation on glucose and lipid metabolism in the adult offspring.

METHODS

Pregnant C57BL/6 mice received a control or a low-protein diet (18% vs 9% casein) throughout gestation. Male and female offspring received a low-fat or a high-fat diet from 6 to 22 weeks of age.

RESULTS

Maternal low-protein diet during gestation led to deteriorated insulin sensitivity on high-fat feeding in female offspring, as determined by biochemical and microarray analyses. Female offspring of control diet-fed dams were relatively resistant to high-fat diet-induced metabolic dysregulation. In contrast, maternal low-protein diet did not specifically affect the metabolic parameters addressed in male offspring. In males, the high-fat diet led to insulin insensitivity regardless of the diet of the dam.

CONCLUSIONS

Our findings show that fetal malnutrition has a limited impact on male mouse offspring, yet it does influence the metabolic response to a high-fat diet in females. These findings may have implications for future early diagnostics in metabolic syndrome and for the development of sex-specific treatment regimens.

Voluntary wheel running increases bile acid as well as cholesterol excretion and decreases atherosclerosis in hypercholesterolemic mice

OBJECTIVE

Regular physical activity decreases the risk for atherosclerosis but underlying mechanisms are not fully understood. We questioned whether voluntary wheel running provokes specific modulations in cholesterol turnover that translate into a decreased atherosclerotic burden in hypercholesterolemic mice.

METHODS

Male LDLR-deficient mice (8 weeks old) had either access to a voluntary running wheel for 12 weeks (RUN) or remained sedentary (CONTROL). Both groups were fed a western-type/high cholesterol diet. Running activity and food intake were recorded. At 12 weeks of intervention, feces, bile and plasma were collected to determine fecal, biliary and plasma parameters of cholesterol metabolism and plasma cytokines. Atherosclerotic lesion size was determined in the aortic root.

RESULTS

RUN weighed less (∼13%) while food consumption was increased by 17% (p=0.004). Plasma cholesterol levels were decreased by 12% (p=0.035) and plasma levels of pro-atherogenic lipoproteins decreased in RUN compared to control. Running modulated cholesterol catabolism by enhancing cholesterol turnover: RUN displayed an increased biliary bile acid secretion (68%, p=0.007) and increased fecal bile acid (93%, p=0.009) and neutral sterol (33%, p=0.002) outputs compared to control indicating that reverse cholesterol transport was increased in RUN. Importantly, aortic lesion size was decreased by ∼33% in RUN (p=0.033).

CONCLUSION

Voluntary wheel running reduces atherosclerotic burden in hypercholesterolemic mice. An increased cholesterol turnover, specifically its conversion into bile acids, may underlie the beneficial effect of voluntary exercise in mice.

Exercise enhances whole-body cholesterol turnover in mice

PURPOSE

Regular exercise reduces cardiovascular risk in humans by reducing cholesterol levels, but the underlying mechanisms have not been fully explored. Exercise might provoke changes in cholesterol and bile acid metabolism and thereby reduce cardiovascular risk. We examined whether voluntary wheel running in mice modulates cholesterol and bile acid metabolism.

METHODS

Male mice (10 wk old) were randomly assigned to have access to a voluntary running wheel for 2 wk (RUN group) or remained sedentary (SED group). Running wheel activity was recorded daily. In a first experiment, fecal sterol outputs, fecal bile acid profiles, plasma parameters, and expression levels of genes involved in cholesterol and bile acid metabolism were determined. In a second experiment, bile flow, biliary bile acid profile, and biliary secretion rates of cholesterol, phospholipids, and bile acids were determined.

RESULTS

The RUN group ran an average of 10 km.d and displayed lower plasma cholesterol compared with SED (P = 0.030). Fecal bile acid loss was induced by approximately 30% in running mice compared with SED (P = 0.0012). A approximately 30% increase in fecal cholesterol output in RUN (P = 0.014) was consistent with changes in parameters of cholesterol absorption, such as reduced plasma plant sterol-cholesterol ratio (P = 0.044) and decreased jejunal expression of Npc1l1 (P = 0.013). Supportive of an increased cholesterol synthesis to compensate for fecal sterol loss were increased hepatic mRNA levels of HMGCoA reductase (P = 0.006) and an increased plasma lathosterol-cholesterol ratio (P = 0.0011) in RUN.

CONCLUSIONS

Voluntary wheel running increased cholesterol turnover in healthy mice owing to an increased fecal bile acid excretion and a decreased intestinal cholesterol absorption. Enhanced cholesterol turnover may contribute to the established reduction of cardiovascular risk induced by regular exercise.

Developments in bile acid kinetic measurements using (13)C and (2)H: 10(5) times improved sensitivity during the last 40 years

Bile acid kinetics involve the measurement of pool sizes and turnover rates of individual bile acids. The technique is based on isotope dilution and was first described in the 1950s using radioactive (14)C-labelled cholic acid (CA). It took until the 1970s before stable isotopes were introduced for this purpose ((13)C, (2)H) and isotope analysis methods were developed for CA and chenodeoxycholic acid (CDCA) applying gas chromatography/electron impact mass spectrometry. Until the 1980s, the isotope enrichment measurements were performed in bile samples aspirated from the duodenum. Thereafter, methodology became available allowing measurements to be performed in blood requiring at least 2 ml serum samples. Simultaneous measurement of kinetics of metabolically dependent CA and deoxycholic acid using (13)C and (2)H labels was introduced. Until the 1990s, this technique was only possible in adult humans due to the large sample sizes. Introduction of pentafluorobenzyl bromide derivatisation and electron capture negative ion mass spectrometry (GC/ECN-MS) reduced the sample volume to 50 microl serum. This allowed isotope abundance measurement of CA in rats and in mice. However, repetitive collection of 100 microl blood samples in mice is too invasive (collection via the orbita) and exhaustive. Therefore, the method development is now focussing on enhanced sensitivity and reduction of blank effects originating from the sample preparation. The final goal is to determine CA isotope enrichments in 20 microl mouse blood obtained from the tail vein. This paper shows the feasibility of reaching this goal.

Effects of essential fatty acid deficiency on enterohepatic circulation of bile salts in mice

Essential fatty acid (EFA) deficiency in mice has been associated with increased bile production, which is mainly determined by the enterohepatic circulation (EHC) of bile salts. To establish the mechanism underlying the increased bile production, we characterized in detail the EHC of bile salts in EFA-deficient mice using stable isotope technique, without interrupting the normal EHC. Farnesoid X receptor (FXR) has been proposed as an important regulator of bile salt synthesis and homeostasis. In Fxr(-/-) mice we additionally investigated to what extent alterations in bile production during EFA deficiency were FXR dependent. Furthermore, we tested in differentiating Caco-2 cells the effects of EFA deficiency on expression of FXR-target genes relevant for feedback regulation of bile salt synthesis. EFA deficiency-enhanced bile flow and biliary bile salt secretion were associated with elevated bile salt pool size and synthesis rate (+146 and +42%, respectively, P < 0.05), despite increased ileal bile salt reabsorption (+228%, P < 0.05). Cyp7a1 mRNA expression was unaffected in EFA-deficient mice. However, ileal mRNA expression of Fgf15 (inhibitor of bile salt synthesis) was significantly reduced, in agreement with absent inhibition of the hepatic bile salt synthesis. Bile flow and biliary secretion were enhanced to the same extent in EFA-deficient wild-type and Fxr(-/-) mice, indicating contribution of other factors besides FXR in regulation of EHC during EFA deficiency. In vitro experiments show reduced induction of mRNA expression of relevant genes upon chenodeoxycholic acid and a selective FXR agonist GW4064 stimulation in EFA-deficient Caco-2 cells. In conclusion, our data indicate that EFA deficiency is associated with interrupted negative feedback of bile salt synthesis, possibly because of reduced ileal Fgf15 expression.

Role of bile acids and bile acid receptors in metabolic regulation

The incidence of the metabolic syndrome has taken epidemic proportions in the past decades, contributing to an increased risk of cardiovascular disease and diabetes. The metabolic syndrome can be defined as a cluster of cardiovascular disease risk factors including visceral obesity, insulin resistance, dyslipidemia, increased blood pressure, and hypercoagulability. The farnesoid X receptor (FXR) belongs to the superfamily of ligand-activated nuclear receptor transcription factors. FXR is activated by bile acids, and FXR-deficient (FXR(-/-)) mice display elevated serum levels of triglycerides and high-density lipoprotein cholesterol, demonstrating a critical role of FXR in lipid metabolism. In an opposite manner, activation of FXR by bile acids (BAs) or nonsteroidal synthetic FXR agonists lowers plasma triglycerides by a mechanism that may involve the repression of hepatic SREBP-1c expression and/or the modulation of glucose-induced lipogenic genes. A cross-talk between BA and glucose metabolism was recently identified, implicating both FXR-dependent and FXR-independent pathways. The first indication for a potential role of FXR in diabetes came from the observation that hepatic FXR expression is reduced in animal models of diabetes. While FXR(-/-) mice display both impaired glucose tolerance and decreased insulin sensitivity, activation of FXR improves hyperglycemia and dyslipidemia in vivo in diabetic mice. Finally, a recent report also indicates that BA may regulate energy expenditure in a FXR-independent manner in mice, via activation of the G protein-coupled receptor TGR5. Taken together, these findings suggest that modulation of FXR activity and BA metabolism may open new attractive pharmacological approaches for the treatment of the metabolic syndrome and type 2 diabetes.

Dexamethasone exposure of neonatal rats modulates biliary lipid secretion and hepatic expression of genes controlling bile acid metabolism in adulthood without interfering with primary bile acid kinetics

Literature suggests that glucocorticoid (GC) exposure during early life may have long-term consequences into adult life. GCs are known to influence hepatic bile acid synthesis and their transport within the enterohepatic circulation. This study addresses effects of early postnatal exposure to GC on hepatic expression of key genes in bile acid metabolism and bile acid kinetics in adult rats. Male rats were treated with either dexamethasone (DEX) or saline at days 1-3 d after birth. Liver tissue and blood were collected from 2 d to 50 wk of age. Bile acid kinetics were determined at week 8. DEX acutely induced hepatic mRNA levels of cholesterol 7alpha-hydroxylase (Cyp7a1), cholesterol 27-hydroxylase (Cyp27), and in particular sterol 12alpha-hydroxylase (Cyp8b1), whereas expression of the bile acid transporters bile salt export pump (Bsep) and sodium taurocholate cotransporting polypeptide (Ntcp) was moderately affected. Neonatal DEX administration led to increased bilary lipid secretion, decreased Cyp8B1 mRNA expression and a 3-fold higher Cyp7a1/Cyp8b1 mRNA ratio in rats at week 8 compared with age-matched controls without alterations in bile acid kinetics. Therefore, neonatal DEX administration causes altered gene expressions later in life that are not translated into quantitative changes in bile acid kinetics.

Monitoring of blood cyclosporine concentration in steroid-resistant nephrotic syndrome

OBJECTIVE

Cyclosporine has been used for patients with nephrotic syndrome. Because of substantial inter- and intra-patient variability and a narrow therapeutic window, drug monitoring of cyclosporine is mandatory. To confirm the therapeutic effects of a cyclosporine microemulsion (CSAME), the absorption profile of the agent after preprandial administration was determined in steroid-resistant patients with refractory nephrotic syndrome.

METHODS

Fourteen patients were enrolled into the study (mean age, 31.2+/-12; 6 men, 8 women). The patients received 1.5 mg/kg of cyclosporine 30 minutes before breakfast for 6 months. Blood cyclosporine concentration was measured 5 times serially: before administration (C0) and at 1-hour intervals until 4 hours after administration of cyclosporine (C1-C4). In addition, area under the concentration-time curve from 0-4 hours (AUC0-4) was calculated.

RESULTS

After 6 months, CSAME showed marked improvement in proteinuria levels (8.3+/-4.8 g/day vs 0.8+/-0.4 g/day, p<0.001). No changes in serum creatinine and urea nitrogen levels were observed. In 83% of the patients, the CSAME peak concentration appeared within 1 hour after administration (C1). A strong positive correlation was noted between AUC0-4 and C1 (R2=0.90312) and C2 (R2=0.78431). The mean steroid (prednisolone) dose was 40 mg/day when CSAME treatment was started, but a lowering of the dose to 17.5 mg/day (p<0.001) was achieved at 6 months after CSAME therapy.

CONCLUSION

Preprandial administration of CSAME is effective in steroid-resistant patients with refractory nephrotic syndrome. C1 or C2, but not C0, was a good clinical marker for CSAME exposure.

Intestinal capacity to digest and absorb carbohydrates is maintained in a rat model of cholestasis

Cholestasis is associated with systemic accumulation of bile salts and with deficiency of bile in the intestinal lumen. During the past years bile salts have been identified as signaling molecules that regulate lipid, glucose, and energy metabolism. Bile salts have also been shown to activate signaling routes leading to proliferation, apoptosis, or differentiation. It is unclear, however, whether cholestasis affects the constitution and absorptive capacity of the intestinal epithelium in vivo. We studied small intestinal morphology, proliferation, apoptosis, expression of intestine-specific genes, and carbohydrate absorption in cholestatic (1 wk bile duct ligation), bile-deficient (1 wk bile diversion), and control (sham) rats. Absorptive capacity was assessed by determination of plasma [(2)H]- and [(13)C]glucose concentrations after intraduodenal administration of [(2)H]glucose and naturally enriched [(13)C]sucrose, respectively. Small intestinal morphology, proliferation, apoptosis, and gene expression of intestinal transcription factors (mRNA levels of Cdx-2, Gata-4, and Hnf-1alpha, and Cdx-2 protein levels) were similar in cholestatic, bile-deficient, and control rats. The (unlabeled) blood glucose response after intraduodenal administration was delayed in cholestatic animals, but the absorption over 180 min was quantitatively similar between the groups. Plasma concentrations of [(2)H]glucose and [(13)C]glucose peaked to similar extents in all groups within 7.5 and 30 min, respectively. Absorption of [(2)H]glucose and [(13)C]glucose in plasma was similar in all groups. The present data indicate that neither accumulation of bile salts in the body, nor their intestinal deficiency, two characteristic features of cholestasis, affect rat small intestinal proliferation, differentiation, apoptosis, or its capacity to digest and absorb carbohydrates.

Transactivation of rat apical sodium-dependent bile acid transporter and increased bile acid transport by 1alpha,25-dihydroxyvitamin D3 via the vitamin D receptor

Transactivation of the rat apical sodium-dependent bile acid transporter (ASBT; Slc10a2) by 1alpha,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] via the vitamin D receptor (VDR), was studied. Levels of ASBT protein and mRNA were low in the duodenum and high in the ileum, and both were induced by 1,25(OH)(2)D(3). The nuclear receptor protein, VDR, was present uniformly in the duodenum, jejunum, and ileum of the rat small intestine. The physiological relevance of ASBT induction by 1,25(OH)(2)D(3) was assessed by measuring absorption of cholylsarcosine, a non-metabolized synthetic bile acid analog, from duodenal or ileal closed loops of the perfused rat small intestine preparation. Absorption of cholylsarcosine was much greater from the ileal segment (28-fold that of the duodenum under control conditions) and was enhanced with 1,25(OH)(2)D(3) treatment. Transient transfection analysis of the rat ASBT promoter in Caco-2 cells revealed concentration-dependent enhancement of luciferase reporter activity after treatment with 1,25(OH)(2)D(3). The activation by 1,25(OH)(2)D(3) was abrogated after site-directed mutagenesis or deletion of the vitamin D response element (VDRE) in the ASBT promoter. Gel-shift mobility assays of nuclear extracts from rat ileum showed that both rat retinoid X receptor and VDR were bound to the VDRE. The results indicate that rat ASBT gene expression is activated by 1,25(OH)(2)D(3) by specific binding to the VDRE and that such activation enhances ileal bile acid transport. Human ABST mRNA and promoter activity were also increased in Caco-2 cells treated with 1,25(OH)(2)D(3), suggesting a physiological role of VDR in human ileal bile acid homeostasis.

Cyclosporine A versus tacrolimus monotherapy. Comparison on bile lipids in the first 3 months after liver transplant in humans

Biliary lipids output is reduced after liver transplantation and tends to normalize thereafter. Cyclosporine A (CyA) is reported to interfere with the normal bile-restoring process after liver grafting, but data are inconclusive, in particular regarding the comparison with the other widely used calcineurin inhibitor tacrolimus (TCR). Furthermore, previous researches were conducted in patients taking multiple immunosuppressive therapies and with a short follow up. In this study we readdressed this issue by comparing biliary lipids in the first 3 months after liver transplant, in 20 patients randomized to receive immunosuppression with CyA or TCR monotherapy. Bile samples, harvested through a T-tube at days 1, 3, 7, 15, 30, 60 and 90 were assessed for cholesterol, phospholipids, and total and individual concentrations of bile acids (BA). Liver and kidney function tests were evaluated as well. We found no differences between CyA and TCR in biochemical findings or in total biliary BAs, cholesterol, and phospholipids. However, CyA-treated patients showed lower levels of glycochenodeoxycholic acid at day 15, compared to those treated with TCR (P < 0.04). This difference normalized thereafter, without any biochemical or clinical effect at 3-month follow up.

Bile duct proliferation associated with bile salt-induced hypercholeresis in Mdr2 P-glycoprotein-deficient mice

BACKGROUND/AIMS

Bile flow consists of bile salt-dependent bile flow (BSDF), generated by canalicular secretion of bile salts, and bile salt-independent flow (BSIF), probably of combined canalicular and ductular origin. Bile salt transport proteins have been identified in cholangiocytes, suggesting a role in control of BSDF and/or in control of bile salt synthesis through cholehepatic shunting.

METHODS

We studied effects of bile duct proliferation under non-cholestatic conditions in multidrug resistance-2 P-glycoprotein (Abcb4)-deficient multidrug resistance gene-2 (Mdr2(-/-)) mice. BSDF and BSIF were determined in wild-type and Mdr2(-/-) mice during infusion of step-wise increasing dosages of tauroursodeoxycholate (TUDC). Cholate synthesis rate was determined by 2H4-cholate dilution. Results were related to expression of transport proteins in liver and intestine.

RESULTS

During TUDC infusion, BSDF was increased by approximately 50% and BSIF by approximately 100% in Mdr2(-/-) mice compared with controls. Cholate synthesis rate was unaffected in Mdr2(-/-) mice. Hepatic expression of the apical sodium-dependent bile salt transporter (Asbt), its truncated form (tAsbt) and the multidrug resistance-related protein 3 were upregulated in Mdr2(-/-) mice.

CONCLUSIONS

Bile duct proliferation in Mdr2(-/-) mice enhances cholehepatic shunting of bile salts, which is associated with a disproportionally high bile flow but does not affect bile salt synthesis.

Cyclosporine A-induced reduction of bile salt synthesis associated with increased plasma lipids in children after liver transplantation

Hyperlipidemia is a common side effect of cyclosporine A (CsA) after solid organ transplantation. CsA also markedly reduces the synthesis rate of bile salts in rats and can inhibit biliary bile salt secretion. It is not known, however, whether CsA inhibits the synthesis of bile salts in humans, and whether the hyperlipidemic effects of CsA are related to bile salt metabolism. Our objective was to assess the effects of CsA on the synthesis rate of bile salts and on plasma triglycerides and cholesterol levels in pediatric liver transplant patients. Before and after discontinuation of CsA treatment after liver transplantation, synthesis rate and pool size of the primary bile salts cholate and chenodeoxycholate were determined using a stable isotope dilution technique and related to plasma lipids. In 6 children (age: 3-16 years) CsA treatment was discontinued at 2 years (median 2.3 years) after liver transplantation. Discontinuation of CsA increased synthesis rate of chenodeoxycholate (+38%, P <.001) and cholate (+21%, P <.05) and the pool size of chenodeoxycholate (+54%, P <.001). Discontinuation of CsA decreased plasma levels of cholesterol (-18%, P <.05) and triglycerides (-23%, P <.05). Bile salt synthesis rate appeared to be inversely correlated with plasma cholesterol (Spearman rank correlation coefficient [r(s)] = -0.82, P <.01) and plasma triglyceride levels (r(s) = -0.62, P <.05). In conclusion, CsA inhibits bile salt synthesis and increases plasma concentration of cholesterol and triglycerides in pediatric liver transplant patients. Suppression of bile salt synthesis by long-term CsA treatment may contribute to hyperlipidemia and thus to increased risk for cardiovascular disease.

Enterohepatic circulation of bile salts in farnesoid X receptor-deficient mice: efficient intestinal bile salt absorption in the absence of ileal bile acid-binding protein

The bile salt-activated farnesoid X receptor (FXR; NR1H4) controls expression of several genes considered crucial in maintenance of bile salt homeostasis. We evaluated the physiological consequences of FXR deficiency on bile formation and on the kinetics of the enterohepatic circulation of cholate, the major bile salt species in mice. The pool size, fractional turnover rate, synthesis rate, and intestinal absorption of cholate were determined by stable isotope dilution and were related to expression of relevant transporters in the livers and intestines of FXR-deficient (Fxr-/-) mice. Fxr-/- mice showed only mildly elevated plasma bile salt concentrations associated with a 2.4-fold higher biliary bile salt output, whereas hepatic mRNA levels of the bile salt export pump were decreased. Cholate pool size and total bile salt pool size were increased by 67 and 39%, respectively, in Fxr-/- mice compared with wild-type mice. The cholate synthesis rate was increased by 85% in Fxr-/- mice, coinciding with a 2.5-fold increase in cholesterol 7alpha-hydroxylase (Cyp7a1) and unchanged sterol 12alpha-hydroxylase (Cyp8b1) expression in the liver. Despite a complete absence of ileal bile acid-binding protein mRNA and protein, the fractional turnover rate and cycling time of the cholate pool were not affected. The calculated amount of cholate reabsorbed from the intestine per day was approximately 2-fold higher in Fxr-/- mice than in wild-type mice. Thus, the absence of FXR in mice is associated with defective feedback inhibition of hepatic cholate synthesis, which leads to enlargement of the circulating cholate pool with an unaltered fractional turnover rate. The absence of ileal bile acid-binding protein does not negatively interfere with the enterohepatic circulation of cholate in mice.