Bile acids: regulation of synthesis

The use of stable and radioactive sterol tracers as a tool to investigate cholesterol degradation to bile acids in humans in vivo

Alterations of cholesterol homeostasis represent important risk factors for atherosclerosis and cardiovascular disease. Different clinical-experimental approaches have been devised to study the metabolism of cholesterol and particularly the synthesis of bile acids, its main catabolic products. Most evidence in humans has derived from studies utilizing the administration of labeled sterols; these have several advantages over in vitro assay of enzyme activity and expression, requiring an invasive procedure such as a liver biopsy, or the determination of fecal sterols, which is cumbersome and not commonly available. Pioneering evidence with administration of radioactive sterol derivatives has allowed to characterize the alterations of cholesterol metabolism and degradation in different situations, including spontaneous disease conditions, aging, and drug treatment. Along with the classical isotope dilution methodology, other approaches were proposed, among which isotope release following radioactive substrate administration. More recently, stable isotope studies have allowed to overcome radioactivity exposure. Isotope enrichment studies during tracer infusion has allowed to characterize changes in the degradation of cholesterol via the "classical" and the "alternative" pathways of bile acid synthesis. Evidence brought by tracer studies in vivo, summarized here, provides an exceptional tool for the investigation of sterol metabolism, and integrate the studies in vitro on human tissue.

Chitosan oligosaccharides promote reverse cholesterol transport and expression of scavenger receptor BI and CYP7A1 in mice

Chitosan oligosaccharides (COS) are beneficial in improving plasma lipids and diminishing atherosclerotic risks. In this study, we examined the effects of COS on reverse cholesterol transport (RCT) in C57BL/6 mice. (3)H-cholesterol-laden macrophages were injected intraperitoneally into mice fed with various dosage of COS (250, 500, 1000 mg/kg mouse weight, respectively) or vehicle by gastric gavages. Plasma lipid level was determined and (3)H-cholesterol was traced in plasma, liver, bile and feces. The effects of COS on hepatic cholesterol 7 alpha-hydroxylase (CYP7A1) and scavenger receptor BI (SR-BI) expression were also investigated. COS administration led to a significant decrease in plasma total cholesterol and low-density lipoprotein (LDL) cholesterol and a significant increase in peritoneal macrophage-derived (3)H-cholesterol in liver and bile as well as in feces. Liver protein expressions of CYP7A1, SR-BI and LDL receptor (LDL-R) were improved in a dosage-dependent manner in COS-administered mice. Our findings provide the first in vivo demonstration of a positive role for COS in RCT pathway and hepatic CYP7A1 and SR-BI expression in mice. Additionally, the LDL cholesterol lowering effect might be relative to hepatic LDL-R expression stimulated by COS in mice.

Antibody-mediated inhibition of fibroblast growth factor 19 results in increased bile acids synthesis and ileal malabsorption of bile acids in cynomolgus monkeys

Fibroblast growth factor 19 (FGF19) represses cholesterol 7α-hydroxylase (Cyp7α1) and inhibits bile acid synthesis in vitro and in vivo. Previous studies have shown that anti-FGF19 antibody treatment reduces growth of colon tumor xenografts and prevents hepatocellular carcinomas in FGF19 transgenic mice and thus may be a useful cancer target. In a repeat dose safety study in cynomolgus monkeys, anti-FGF19 treatment (3-100 mg/kg) demonstrated dose-related liver toxicity accompanied by severe diarrhea and low food consumption. The mechanism of anti-FGF19 toxicity was investigated using in vitro and in vivo approaches. Our results show that anti-FGF19 antibody had no direct cytotoxic effect on monkey hepatocytes. Anti-FGF19 increased Cyp7α1, as expected, but also increased bile acid efflux transporter gene (bile salt export pump, multidrug resistant protein 2 [MRP2], and MRP3) expression and reduced sodium taurocholate cotransporting polypeptide and organic anion transporter 2 expression in liver tissues from treated monkeys and in primary hepatocytes. In addition, anti-FGF19 treatment increased solute transporter gene (ileal bile acid-binding protein, organic solute transporter α [OST-α], and OST-β) expression in ileal tissues from treated monkeys but not in Caco-2 cells. However, deoxycholic acid (a secondary bile acid) increased expression of FGF19 and these solute transporter genes in Caco-2 cells. Gas chromatography-mass spectrometry analysis of monkey feces showed an increase in total bile acids and cholic acid derivatives. These findings suggest that high doses of anti-FGF19 increase Cyp7α1 expression and bile acid synthesis and alter the expression of bile transporters in the liver resulting in enhanced bile acid efflux and reduced uptake. Increased bile acids alter expression of solute transporters in the ileum causing diarrhea and the enhanced enterohepatic recirculation of bile acids leading to liver toxicity.

Hypolipidemic agent Z-guggulsterone: metabolism interplays with induction of carboxylesterase and bile salt export pump

Z-Guggulsterone is a major ingredient in the Indian traditional hypolipidemic remedy guggul. A study in mice has established that its hypolipidemic effect involves the farnesoid X receptor (FXR), presumably by acting as an antagonist of this receptor. It is generally assumed that the antagonism leads to induction of cytochrome P450 7A1 (CYP7A1), the rate-limiting enzyme converting free cholesterol to bile acids. In this study, we tested whether Z-guggulsterone indeed induces human CYP7A1. In addition, the expression of cholesteryl ester hydrolase CES1 and bile salt export pump (BSEP) was monitored. Contrary to the general assumption, Z-guggulsterone did not induce CYP7A1. Instead, this phytosterol significantly induced CES1 and BSEP through transactivation. Z-Guggulsterone underwent metabolism by CYP3A4, and the metabolites greatly increased the induction potency on BSEP but not on CES1. BSEP induction favors cholesterol elimination, whereas CES1 involves both elimination and retention (probably when excessively induced). Interestingly, clinical trials reported the hypolipidemic response rates from 18% to 80% and showed that higher dosages actually increased VLDL cholesterol. Our findings predict that better hypolipidemic outcomes likely occur in individuals who have a relatively higher capacity of metabolizing Z-guggulsterone with moderate CES1 induction, a scenario possibly achieved by lowering the dosing regimens.

Impaired generation of 12-hydroxylated bile acids links hepatic insulin signaling with dyslipidemia

The association of type 2 diabetes with elevated plasma triglyceride (TG) and very low-density lipoproteins (VLDL), and intrahepatic lipid accumulation represents a pathophysiological enigma and an unmet therapeutic challenge. Here, we uncover a link between insulin action through FoxO1, bile acid (BA) composition, and altered lipid homeostasis that brings new insight to this longstanding conundrum. FoxO1 ablation brings about two signature lipid abnormalities of diabetes and the metabolic syndrome, elevated liver and plasma TG. These changes are associated with deficiency of 12α-hydroxylated BAs and their synthetic enzyme, Cyp8b1, that hinders the TG-lowering effects of the BA receptor, Fxr. Accordingly, pharmacological activation of Fxr with GW4064 overcomes the BA imbalance, restoring hepatic and plasma TG levels of FoxO1-deficient mice to normal levels. We propose that generation of 12α-hydroxylated products of BA metabolism represents a signaling mechanism linking hepatic lipid abnormalities with type 2 diabetes, and a treatment target for this condition.

Enteral bile acid treatment improves parenteral nutrition-related liver disease and intestinal mucosal atrophy in neonatal pigs

Total parenteral nutrition (TPN) is essential for patients with impaired gut function but leads to parenteral nutrition-associated liver disease (PNALD). TPN disrupts the normal enterohepatic circulation of bile acids, and we hypothesized that it would decrease intestinal expression of the newly described metabolic hormone fibroblast growth factor-19 (FGF19) and also glucagon-like peptides-1 and -2 (GLP-1 and GLP-2). We tested the effects of restoring bile acids by treating a neonatal piglet PNALD model with chenodeoxycholic acid (CDCA). Neonatal pigs received enteral feeding (EN), TPN, or TPN + CDCA for 14 days, and responses were assessed by serum markers, histology, and levels of key regulatory peptides. Cholestasis and steatosis were demonstrated in the TPN group relative to EN controls by elevated levels of serum total and direct bilirubin and also bile acids and liver triglyceride (TG) content. CDCA treatment improved direct bilirubin levels by almost fourfold compared with the TPN group and also normalized serum bile acids and liver TG. FGF19, GLP-1, and GLP-2 were decreased in plasma of the TPN group compared with the EN group but were all induced by CDCA treatment. Intestinal mucosal growth marked by weight and villus/crypt ratio was significantly reduced in the TPN group compared with the EN group, and CDCA treatment increased both parameters. These results suggest that decreased circulating FGF19 during TPN may contribute to PNALD. Moreover, we show that enteral CDCA not only resolves PNALD but acts as a potent intestinal trophic agent and secretagogue for GLP-2.

Limited effects of bile acids and small heterodimer partner on hepatitis B virus biosynthesis in vivo

Multiple nuclear receptors, including hepatocyte nuclear factor 4α (HNF4α), retinoid X receptor α (RXRα) plus peroxisome proliferator-activated receptor α (PPARα), RXRα plus farnesoid X receptor α (FXRα), liver receptor homolog 1 (LRH1), and estrogen-related receptors (ERRs), have been shown to support efficient viral biosynthesis in nonhepatoma cells in the absence of additional liver-enriched transcription factors. Although HNF4α has been shown to be critical for the developmental expression of hepatitis B virus (HBV) biosynthesis in the liver, the relative importance of the various nuclear receptors capable of supporting viral transcription and replication in the adult in vivo has not been clearly established. To investigate the role of the nuclear receptor FXR and the corepressor small heterodimer partner (SHP) in viral biosynthesis in vivo, SHP-expressing and SHP-null HBV transgenic mice were fed a bile acid-supplemented diet. The increased FXR activity and SHP expression levels resulting from bile acid treatment did not greatly modulate HBV RNA and DNA synthesis. Therefore, FXR and SHP appear to play a limited role in modulating HBV biosynthesis, suggesting that alternative nuclear receptors are more critical determinants of viral transcription in the HBV transgenic mouse model of chronic viral infection. These observations suggest that hepatic bile acid levels or therapeutic agents targeting FXR may not greatly modulate viremia during natural infection.

Glucose and insulin induction of bile acid synthesis: mechanisms and implication in diabetes and obesity

Bile acids facilitate postprandial absorption of nutrients. Bile acids also activate the farnesoid X receptor (FXR) and the G protein-coupled receptor TGR5 and play a major role in regulating lipid, glucose, and energy metabolism. Transgenic expression of cholesterol 7α-hydroxylase (CYP7A1) prevented high fat diet-induced diabetes and obesity in mice. In this study, we investigated the nutrient effects on bile acid synthesis. Refeeding of a chow diet to fasted mice increased CYP7A1 expression, bile acid pool size, and serum bile acids in wild type and humanized CYP7A1-transgenic mice. Chromatin immunoprecipitation assays showed that glucose increased histone acetylation and decreased histone methylation on the CYP7A1 gene promoter. Refeeding also induced CYP7A1 in fxr-deficient mice, indicating that FXR signaling did not play a role in postprandial regulation of bile acid synthesis. In streptozocin-induced type I diabetic mice and genetically obese type II diabetic ob/ob mice, hyperglycemia increased histone acetylation status on the CYP7A1 gene promoter, leading to elevated basal Cyp7a1 expression and an enlarged bile acid pool with altered bile acid composition. However, refeeding did not further increase CYP7A1 expression in diabetic mice. In summary, this study demonstrates that glucose and insulin are major postprandial factors that induce CYP7A1 gene expression and bile acid synthesis. Glucose induces CYP7A1 gene expression mainly by epigenetic mechanisms. In diabetic mice, CYP7A1 chromatin is hyperacetylated, and fasting to refeeding response is impaired and may exacerbate metabolic disorders in diabetes.

Pronounced variation in bile acid synthesis in humans is related to gender, hypertriglyceridaemia and circulating levels of fibroblast growth factor 19

BACKGROUND

Bile acid (BA) synthesis is essential in cholesterol and lipid homoeostasis.

METHODS

Serum samples from 435 normal and 23 cholecystectomized subjects were obtained after overnight fasting and assayed for markers of BA and cholesterol synthesis, as well as cholesterol absorption. We determined whether BA synthesis was related to fibroblast growth factor 19 (FGF19; a circulating metabolic regulator that is thought to inhibit BA synthesis), gender, age and serum lipids.

RESULTS

Bile acid synthesis varied more than 9-fold in normal individuals and was 29% higher in men than in women. Whilst low-density lipoprotein cholesterol increased with age, BA and cholesterol synthesis were stable. BA production was positively correlated with serum triglycerides (TGs), and 35% of individuals with a high level (>95th percentile) of BA synthesis had hypertriglyceridaemia (HTG) (>95th percentile). Serum FGF19 levels varied by 7-fold in normal individuals and were related inversely to BA synthesis but were not related to gender, plasma lipids or history of cholecystectomy.

CONCLUSIONS

Bile acid synthesis has a wide inter-individual variation, is lower in women than in men and is correlated positively with serum TGs. High BA production is frequently linked to HTG. Age-related hypercholesterolaemia is not associated with changes in BA or cholesterol production, nor to an increase in cholesterol absorption. In humans, the circulating level of FGF19 may regulate hepatic BA production under fasting conditions.

Critical illness evokes elevated circulating bile acids related to altered hepatic transporter and nuclear receptor expression

Hyperbilirubinemia is common during critical illness and is associated with adverse outcome. Whether hyperbilirubinemia reflects intensive care unit (ICU) cholestasis is unclear. Therefore, the aim of this study was to analyze hyperbilirubinemia in conjunction with serum bile acids (BAs) and the key steps in BA synthesis, transport, and regulation by nuclear receptors (NRs). Serum BA and bilirubin levels were determined in 130 ICU and 20 control patients. In liver biopsies messenger RNA (mRNA) expression of BA synthesis enzymes, BA transporters, and NRs was assessed. In a subset (40 ICU / 10 controls) immunohistochemical staining of the transporters and receptors together with a histological evaluation of cholestasis was performed. BA levels were much more elevated than bilirubin in ICU patients. Conjugated cholic acid (CA) and chenodeoxycholic acid (CDCA) were elevated, with an increased CA/CDCA ratio. Unconjugated BA did not differ between controls and patients. Despite elevated serum BA levels, CYP7A1 protein, the rate-limiting enzyme in BA synthesis, was not lowered in ICU patients. Also, protein expression of the apical bile salt export pump (BSEP) was decreased, whereas multidrug resistance-associated protein (MRP) 3 was strongly increased at the basolateral side. This reversal of BA transport toward the sinusoidal blood compartment is in line with the increased serum conjugated BA levels. Immunostaining showed marked down-regulation of nuclear farnesoid X receptor, retinoid X receptor alpha, constitutive androstane receptor, and pregnane X receptor nuclear protein levels.

CONCLUSION

Failure to inhibit BA synthesis, up-regulate canalicular BA export, and localize pivotal NR in the hepatocytic nuclei may indicate dysfunctional feedback regulation by increased BA levels. Alternatively, critical illness may result in maintained BA synthesis (CYP7A1), reversal of normal BA transport (BSEP/MRP3), and inhibition of the BA sensor (FXR/RXRα) to increase serum BA levels.

Bile Acid signaling in liver metabolism and diseases

Obesity, diabetes, and metabolic syndromes are increasingly recognized as health concerns worldwide. Overnutrition and insulin resistance are the major causes of diabetic hyperglycemia and hyperlipidemia in humans. Studies in the past decade provide evidence that bile acids are not just biological detergents facilitating gut nutrient absorption, but also important metabolic regulators of glucose and lipid homeostasis. Pharmacological alteration of bile acid metabolism or bile acid signaling pathways such as using bile acid receptor agonists or bile acid binding resins may be a promising therapeutic strategy for the treatment of obesity and diabetes. On the other hand, bile acid signaling is complex, and the molecular mechanisms mediating the bile acid effects are still not completely understood. This paper will summarize recent advances in our understanding of bile acid signaling in regulation of glucose and lipid metabolism, and the potentials of developing novel therapeutic strategies that target bile acid metabolism for the treatment of metabolic disorders.

Microfluidics enables small-scale tissue-based drug metabolism studies with scarce human tissue

Early information on the metabolism and toxicity properties of new drug candidates is crucial for selecting the right candidates for further development. Preclinical trials rely on cell-based in vitro tests and animal studies to characterize the in vivo behavior of drug candidates, although neither are ideal predictors of drug behavior in humans. Improving in vitro systems for preclinical studies both from a technological and biological model standpoint thus remains a major challenge. This article describes how microfluidics can be exploited to come closer to this goal in combination with precision-cut liver slices (PCLS) as an improved organomimetic system. Recently, we developed a novel microfluidic-based system incorporating a microchamber for slice perifusion to perform drug metabolism studies with mammalian PCLS under continuous flow. In the present study, the viability and metabolism of human PCLS were assessed by the measurement of the leakage of liver-specific enzymes and metabolism of four different substrates: lidocaine, 7-hydroxycoumarin, 7-ethoxycoumarin, and testosterone. All experiments were verified with well plates, an excellent benchmark for these experiments. Clearly, however, human tissue is not readily available, and it is worth considering how to perform a maximum number of informative experiments with small amounts of material. In one approach, the microfluidic system was coupled to an HPLC system to allow on-line monitoring and immediate detection of unstable metabolites, something that is generally not possible with conventional well-plate systems. This novel microfluidic system also enables the in vitro measurement of interorgan interactions by connecting microchambers containing different organ slices in series for sequential perfusion. This versatile experimental system has the potential to yield more information about the metabolic profiles of new drug candidates in human and animal tissues in an early stage of development compared with well plates alone.

Interleukin enhancer-binding factor 3 functions as a liver receptor homologue-1 co-activator in synergy with the nuclear receptor co-activators PRMT1 and PGC-1α

LRH-1 (liver receptor homologue-1), a transcription factor and member of the nuclear receptor superfamily, regulates the expression of its target genes, which are involved in bile acid and cholesterol homoeostasis. However, the molecular mechanisms of transcriptional control by LRH-1 are not completely understood. Previously, we identified Ku80 and Ku70 as LRH-1-binding proteins and reported that they function as co-repressors. In the present study, we identified an additional LRH-1-binding protein, ILF3 (interleukin enhancer-binding factor 3). ILF3 formed a complex with LRH-1 and the other two nuclear receptor co-activators PRMT1 (protein arginine methyltransferase 1) and PGC-1α (peroxisome proliferator-activated receptor γ co-activator-1α). We demonstrated that ILF3, PRMT1 and PGC-1α were recruited to the promoter region of the LRH-1-regulated SHP (small heterodimer partner) gene, encoding one of the nuclear receptors. ILF3 enhanced SHP gene expression in co-operation with PRMT1 and PGC-1α through the C-terminal region of ILF3. In addition, we found that the small interfering RNA-mediated down-regulation of ILF3 expression led to a reduction in the occupancy of PGC-1α at the SHP promoter and SHP expression. Taken together, our results suggest that ILF3 functions as a novel LRH-1 co-activator by acting synergistically with PRMT1 and PGC-1α, thereby promoting LRH-1-dependent gene expression.

Review article: colorectal neoplasia in patients with primary sclerosing cholangitis and inflammatory bowel disease

BACKGROUND

Primary sclerosing cholangitis (PSC) is a chronic cholestatic liver disease strongly associated with inflammatory bowel disease (IBD). IBD patients diagnosed with PSC have an increased risk of colorectal dysplasia and cancer.

AIMS

To review the available evidence regarding colorectal neoplasia epidemiology, preventive strategies and outcomes in patients with PSC and IBD, and to advance some hypotheses regarding possible mechanisms involved in cancer pathogenesis in these patients.

METHODS

A PubMed search was conducted for the English language publications with predetermined search criteria. Reference lists from studies selected were manually searched to identify further relevant reports. Relevant manuscripts considering colorectal neoplasia in patients with PSC-IBD were selected.

RESULTS

Primary sclerosing cholangitis increases the risk of colorectal neoplasia in patients with ulcerative colitis; fewer data are available for Crohn's disease. PSC-IBD patients tend to be younger at diagnosis of IBD and at diagnosis of colorectal cancer. Colorectal cancer in PSC-IBD patients predominates in the right colon. The increased risk of neoplasia is maintained after liver transplant and proctocolectomy. The role of ursodeoxycholic acid as a chemopreventive agent is controversial. The mechanisms underlying increased risk of colorectal neoplasia in these patients remain unknown.

CONCLUSIONS

A more comprehensive understanding of the mechanisms involved in colorectal neoplasia development in PSC-IBD patients is needed. Until then, early cancer detection through enrolment in surveillance programmes is the only available strategy to decrease cancer risk.

Mechanisms and genetic determinants regulating sterol absorption, circulating LDL levels, and sterol elimination: implications for classification and disease risk

This review integrates historical biochemical and modern genetic findings that underpin our understanding of the low-density lipoprotein (LDL) dyslipidemias that bear on human disease. These range from life-threatening conditions of infancy through severe coronary heart disease of young adulthood, to indolent disorders of middle- and old-age. We particularly focus on the biological aspects of those gene mutations and variants that impact on sterol absorption and hepatobiliary excretion via specific membrane transporter systems (NPC1L1, ABCG5/8); the incorporation of dietary sterols (MTP) and of de novo synthesized lipids (HMGCR, TRIB1) into apoB-containing lipoproteins (APOB) and their release into the circulation (ANGPTL3, SARA2, SORT1); and receptor-mediated uptake of LDL and of intestinal and hepatic-derived lipoprotein remnants (LDLR, APOB, APOE, LDLRAP1, PCSK9, IDOL). The insights gained from integrating the wealth of genetic data with biological processes have important implications for the classification of clinical and presymptomatic diagnoses of traditional LDL dyslipidemias, sitosterolemia, and newly emerging phenotypes, as well as their management through both nutritional and pharmaceutical means.

Tumor suppressor p53 regulates bile acid homeostasis via small heterodimer partner

Metabolic changes in cancer have been observed for almost a century. The mechanisms underlying these changes have begun to emerge from the recent studies implicating the tumor suppressor p53 in multiple metabolic pathways. The ability of p53 to regulate metabolism may also play important roles in the physiology of normal cells and organs. Here we demonstrate that p53 lowers bile acid (BA) levels under both normal and stressed conditions primarily through up-regulating expression of small heterodimer partner, a critical inhibitor of BA synthesis. Our results uncover a unique metabolic regulatory axis that unexpectedly couples p53 to BA homeostasis. Our results also warrant future studies to investigate a possible role of this axis in the tumor suppression by p53, because excessive quantities of BAs are cytotoxic and can cause liver damage and promote gastrointestinal cancers.

Bile acids and colon cancer: Solving the puzzle with nuclear receptors

Colorectal cancer is the third most common malignancy worldwide and is often linked to obesity, a sedentary lifestyle, carbohydrate- and fat-rich diets and elevated fecal excretion of secondary bile acids. Accumulation of toxic bile acids triggers oxidative damage, mitochondrial dysfunction and tumor progression. Nuclear receptors are transcription factors crucially involved in the regulation of bile acid metabolism and detoxification, and their activation may confer protection from bile acid tumor-promoting activity. In this review, we explore the tangled relationships among bile acids, nuclear receptors and the intestinal epithelium, with particular emphasis on the role of the farnesoid X receptor in colorectal cancer prevention and on novel nuclear receptor-based approaches to expand the portfolio of chemotherapeutic agents.

FXR an emerging therapeutic target for the treatment of atherosclerosis

Atherosclerosis is the leading cause of illness and death. Therapeutic strategies aimed at reducing cholesterol plasma levels have shown efficacy in either reducing progression of atherosclerotic plaques and atherosclerosis-related mortality. The farnesoid-X-receptor (FXR) is a member of metabolic nuclear receptors (NRs) superfamily activated by bile acids. In entero-hepatic tissues, FXR functions as a bile acid sensor regulating bile acid synthesis, detoxification and excretion. In the liver FXR induces the expression of an atypical NR, the small heterodimer partner, which subsequently inhibits the activity of hepatocyte nuclear factor 4α repressing the transcription of cholesterol 7a-hydroxylase, the critical regulatory gene in bile acid synthesis. In the intestine FXR induces the release of fibroblast growth factor 15 (FGF15) (or FGF19 in human), which activates hepatic FGF receptor 4 (FGFR4) signalling to inhibit bile acid synthesis. In rodents, FXR activation decreases bile acid synthesis and lipogenesis and increases lipoprotein clearance, and regulates glucose homeostasis by reducing liver gluconeogenesis. FXR exerts counter-regulatory effects on macrophages, vascular smooth muscle cells and endothelial cells. FXR deficiency in mice results in a pro-atherogenetic lipoproteins profile and insulin resistance but FXR^−/– mice fail to develop any detectable plaques on high-fat diet. Synthetic FXR agonists protect against development of aortic plaques formation in murine models characterized by pro-atherogenetic lipoprotein profile and accelerated atherosclerosis, but reduce HDL levels. Because human and mouse lipoprotein metabolism is modulated by different regulatory pathways the potential drawbacks of FXR ligands on HDL and bile acid synthesis need to addressed in relevant clinical settings.

A Klothoβ variant mediates protein stability and associates with colon transit in irritable bowel syndrome with diarrhea

BACKGROUND & AIMS

Bile acid (BA) malabsorption of moderate severity is reported in 32% of patients with chronic unexplained diarrhea, including diarrhea-predominant irritable bowel syndrome (IBS-D). We hypothesized that variants of genes regulating hepatic BA synthesis play a role in IBS-D.

METHODS

In 435 IBS and 279 healthy subjects, we tested individual associations of 15 common single nucleotide polymorphisms (SNPs) from 7 genes critical to BA homeostasis with symptom-based subgroups using dominant genetic models. In a subset of 238 participants, we tested association with colonic transit. SNP-SNP interactions were investigated based on known protein interactions in BA homeostasis. The function of SNP rs17618244 in Klothoβ (KLB) was evaluated using a protein stability assay in HEK293 cells.

RESULTS

SNP rs17618244 (Arg728Gln in KLB) is associated with colonic transit at 24 hours. G allele (Arg728) compared with A allele (Gln728) is associated with accelerated colonic transit (P=.0007) in the overall cohort; this association was restricted to IBS-D (P=.0018). Interaction tests of KLB rs17618244 with 3 nonsynonymous SNPs of fibroblast growth factor receptor 4 (FGFR4) revealed that rs1966265 (Val10Ile) and rs351855 (Gly388Arg) modulate rs1768244's association with colonic transit in IBS-D (P=.0025 and P=.0023, respectively). KLB Arg728 significantly reduced protein stability compared with KLB Gln728, demonstrating KLB rs17618244's functional significance. No significant associations with symptom-based subgroups of IBS were detected.

CONCLUSIONS

A functional KLB gene variant mediating protein stability associates with colonic transit in IBS-D. This association is modulated by 2 genetic variants in FGFR4. The FGF19-FGFR4-KLB pathway links regulation of BA synthesis to colonic transit in IBS-D.

Enhanced in vitro refolding of fibroblast growth factor 15 with the assistance of SUMO fusion partner

Fibroblast growth factor 15 (Fgf15) is the mouse orthologue of human FGF19. Fgf15 is highly expressed in the ileum and functions as an endocrine signal to regulate liver function, including bile acid synthesis, hepatocyte proliferation and insulin sensitivity. In order to fully understand the function of Fgf15, methods are needed to produce pure Fgf15 protein in the prokaryotic system. However, when expressed in Escherichia coli (E. coli), the recombinant Fgf15 protein was insoluble and found only in inclusion bodies. In the current study, we report a method to produce recombinant Fgf15 protein in E. coli through the use of small ubiquitin-related modifier (SUMO) fusion tag. Even though the SUMO has been shown to strongly improve protein solubility and expression levels, our studies suggest that the SUMO does not improve Fgf15 protein solubility. Instead, proper refolding of Fgf15 protein was achieved when Fgf15 was expressed as a partner protein of the fusion tag SUMO, followed by in vitro dialysis refolding. After refolding, the N-terminal SUMO tag was cleaved from the recombinant Fgf15 fusion protein by ScUlp1 (Ubiquitin-Like Protein-Specific Protease 1 from S. cerevisiae). With or without the SUMO tag, the refolded Fgf15 protein was biologically active, as revealed by its ability to reduce hepatic Cyp7a1 mRNA levels in mice. In addition, recombinant Fgf15 protein suppressed Cyp7a1 mRNA levels in a dose-dependent manner. In summary, we have developed a successful method to express functional Fgf15 protein in prokaryotic cells.

Bile acid retention and activation of endogenous hepatic farnesoid-X-receptor in the pathogenesis of fatty liver disease in ob/ob-mice

The nuclear bile acid receptor FXR (farnesoid-X-receptor) has recently been implicated in the pathophysiology of non-alcoholic fatty liver disease because selective FXR-agonists improve glucose and lipid metabolism in rodent models of obesity. However, the regulation of FXR and other relevant nuclear receptors as well as their lipogenic target genes in fatty liver is still not revealed in detail. Livers were harvested from 14-week-old male ob/ob mice and wild-type controls. Serum bile acids were quantified by radioimmunoassay. mRNA and protein expression of transporters and nuclear receptors was analyzed by reverse transcriptase-polymerase chain reaction and Western blotting, whereas DNA binding to the IR-1 element was examined by electrophoretic mobility shift assay. In this study we show: (i) bile acid retention in ob/ob mice, (ii) a resulting FXR upregulation and binding to the IR-1 element in ob/ob animals and (iii) concomitant activation of the fatty acid synthase as a potential lipogenic FXR target gene in vivo. The present study suggests a potential role of hepatic bile acid retention and FXR activation in the induction of lipogenic target genes. Differences between intestinal and hepatic FXR could explain apparent contradictory information regarding its effects on fatty liver disease.

Colonization-induced host-gut microbial metabolic interaction

The gut microbiota enhances the host’s metabolic capacity for processing nutrients and drugs and modulate the activities of multiple pathways in a variety of organ systems. We have probed the systemic metabolic adaptation to gut colonization for 20 days following exposure of axenic mice (n = 35) to a typical environmental microbial background using high-resolution ¹H nuclear magnetic resonance (NMR) spectroscopy to analyze urine, plasma, liver, kidney, and colon (5 time points) metabolic profiles. Acquisition of the gut microbiota was associated with rapid increase in body weight (4%) over the first 5 days of colonization with parallel changes in multiple pathways in all compartments analyzed. The colonization process stimulated glycogenesis in the liver prior to triggering increases in hepatic triglyceride synthesis. These changes were associated with modifications of hepatic Cyp8b1 expression and the subsequent alteration of bile acid metabolites, including taurocholate and tauromuricholate, which are essential regulators of lipid absorption. Expression and activity of major drug-metabolizing enzymes (Cyp3a11 and Cyp2c29) were also significantly stimulated. Remarkably, statistical modeling of the interactions between hepatic metabolic profiles and microbial composition analyzed by 16S rRNA gene pyrosequencing revealed strong associations of the Coriobacteriaceae family with both the hepatic triglyceride, glucose, and glycogen levels and the metabolism of xenobiotics. These data demonstrate the importance of microbial activity in metabolic phenotype development, indicating that microbiota manipulation is a useful tool for beneficially modulating xenobiotic metabolism and pharmacokinetics in personalized health care.

Gut bacteria have been associated with various essential biological functions in humans such as energy harvest and regulation of blood pressure. Furthermore, gut microbial colonization occurs after birth in parallel with other critical processes such as immune and cognitive development. Thus, it is essential to understand the bidirectional interaction between the host metabolism and its symbionts. Here, we describe the first evidence of an in vivo association between a family of bacteria and hepatic lipid metabolism. These results provide new insights into the fundamental mechanisms that regulate host-gut microbiota interactions and are thus of wide interest to microbiological, nutrition, metabolic, systems biology, and pharmaceutical research communities. This work will also contribute to developing novel strategies in the alteration of host-gut microbiota relationships which can in turn beneficially modulate the host metabolism.

Overexpression of cholesterol 7α-hydroxylase promotes hepatic bile acid synthesis and secretion and maintains cholesterol homeostasis

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.

Combination of retinoic acid and ursodeoxycholic acid attenuates liver injury in bile duct-ligated rats and human hepatic cells

Cholestasis leads to liver cell death, fibrosis, cirrhosis, and eventually liver failure. Despite limited benefits, ursodeoxycholic acid (UDCA) is the only Food and Drug Administration-approved treatment for cholestatic disorders. Retinoic acid (RA) is a ligand for nuclear receptors that modulate bile salt homeostasis. RA also possesses immunomodulatory effects and is used to treat acute promyelocytic leukemia and inflammatory disorders such as psoriasis, acne, and rheumatoid arthritis. To test whether the supplementation of RA with UDCA is superior to UDCA alone for treating cholestasis, male Sprague-Dawley rats underwent common bile duct ligation (BDL) for 14 days and were treated with phosphate-buffered saline (PBS), UDCA, all-trans retinoic acid (atRA), or UDCA and atRA by gavage. Treatment with UDCA and atRA substantially improved animal growth rates, significantly reduced liver fibrosis and bile duct proliferation, and nearly eliminated liver necrosis after BDL. Reductions in the bile salt pool size and liver hydroxyproline content were also seen with treatment with atRA or atRA and UDCA versus PBS and UDCA. Furthermore, atRA and UDCA significantly reduced liver messenger RNA and/or protein expression of transforming growth factor β1 (Tgf-β1), collagen 1a1 (Col1A1), matrix metalloproteinase 2 (Mmp2), cytokeratin 19, α-smooth muscle actin (α-SMA), cytochrome P450 7A1 (Cyp7a1), tumor necrosis factor α, and interleukin-β1. The molecular mechanisms of this treatment were also assessed in human hepatocytes, hepatic stellate cells, and LX-2 cells. atRA alone or in combination with UDCA greatly repressed CYP7A1 expression in human hepatocytes and significantly inhibited COL1A1, MMP2, and α-SMA expression and/or activity in primary human hepatic stellate cells and LX-2 cells. Furthermore, atRA reduced TGF-β1-induced Smad2 phosphorylation in LX-2 cells.

CONCLUSION

Our findings indicate that the addition of RA to UDCA reduces the bile salt pool size and liver fibrosis and might be an effective supplemental therapy with UDCA for cholestatic diseases.

APOBEC-1-mediated RNA editing

RNA editing defines a molecular process by which a nucleotide sequence is modified in the RNA transcript and results in an amino acid change in the recoded message from that specified in the gene. We will restrict our attention to the type of RNA editing peculiar to mammals, i.e., nuclear C to U RNA editing. This category of RNA editing contrasts with RNA modifications described in plants, i.e., organellar RNA editing (reviewed in Ref 1). Mammalian RNA editing is genetically and biochemically classified into two groups, namely insertion-deletional and substitutional. Substitutional RNA editing is exclusive to mammals, again with two types reported, namely adenosine to inosine and cytosine to uracil (C to U). This review will examine mammalian C to U RNA editing of apolipoproteinB (apoB) RNA and the role of the catalytic deaminase Apobec-1. We will speculate on the functions of Apobec-1 beyond C to U RNA editing as implied from its ability to bind AU-rich RNAs and discuss evidence that dysregulation of Apobec-1 expression might be associated with carcinogenesis through aberrant RNA editing or altered RNA stability.

Organic anion-transporting polypeptide 1b2 (Oatp1b2) is important for the hepatic uptake of unconjugated bile acids: Studies in Oatp1b2-null mice

The organic anion-transporting polypeptide 1b family (Oatp1b2 in rodents and OATP1B1/1B3 in humans) is liver-specific and transports various chemicals into the liver. However, the role of the Oatp1b family in the hepatic uptake of bile acids (BAs) into the liver is unknown. Therefore, in Oatp1b2-null mice, the concentrations of BAs in plasma, liver, and bile were compared with wild-type (WT) mice. It was first determined that livers of the Oatp1b2-null mice were not compensated by altered expression of other hepatic transporters. However, the messenger RNA of Cyp7a1 was 70% lower in the Oatp1b2-null mice. Increased expression of fibroblast growth factor 15 in intestines of Oatp1b2-null mice might be responsible for decreased hepatic expression of Cyp7a1 in Oatp1b2-null mice. The hepatic concentration and biliary excretion of conjugated and unconjugated BAs were essentially the same in Oatp1b2-null and WT mice. The serum concentration of taurine-conjugated BAs was essentially the same in the two genotypes. In contrast, the serum concentrations of unconjugated BAs were 3-45 times higher in Oatp1b2-null than WT mice. After intravenous administration of cholate to Oatp1b2-null mice, its clearance was 50% lower than in WT mice, but the clearance of taurocholate was similar in the two genotypes.

CONCLUSION

This study indicates that Oatp1b2 has a major role in the hepatic uptake of unconjugated BAs.

A chronic high-cholesterol diet paradoxically suppresses hepatic CYP7A1 expression in FVB/NJ mice

Cholesterol 7α-hydroxylase (CYP7A1) encodes for the rate-limiting step in the conversion of cholesterol to bile acids in the liver. In response to acute cholesterol feeding, mice upregulate CYP7A1 via stimulation of the liver X receptor (LXR) α. However, the effect of a chronic high-cholesterol diet on hepatic CYP7A1 expression in mice is unknown. We demonstrate that chronic cholesterol feeding (0.2% or 1.25% w/w cholesterol for 12 weeks) in FVB/NJ mice results in a >60% suppression of hepatic CYP7A1 expression associated with a >2-fold increase in hepatic cholesterol content. In contrast, acute cholesterol feeding induces a >3-fold upregulation of hepatic CYP7A1 expression. We show that chronic, but not acute, cholesterol feeding increases the expression of hepatic inflammatory cytokines, tumor necrosis factor (TNF)α, and interleukin (IL)-1β, which are known to suppress hepatic CYP7A1 expression. Chronic cholesterol feeding also results in activation of the mitogen activated protein (MAP) kinases, c-Jun N-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK). Furthermore, we demonstrate in vitro that suppression of CYP7A1 by TNFα and IL-1β is dependent on JNK and ERK signaling. We conclude that chronic high-cholesterol feeding suppresses CYP7A1 expression in mice. We propose that chronic cholesterol feeding induces inflammatory cytokine activation and liver damage, which leads to suppression of CYP7A1 via activation of JNK and ERK signaling pathways.

A microfluidic approach for in vitro assessment of interorgan interactions in drug metabolism using intestinal and liver slices

Over the past two decades, it has become increasingly clear that the intestine, in addition to the liver, plays an important role in the metabolism of xenobiotics. Previously, we developed a microfluidic-based in vitro system for the perifusion of precision-cut liver slices for metabolism studies. In the present study, the applicability of this system for the perifusion of precision-cut intestinal slices, and for the sequential perifusion of intestinal and liver slices, all from rat, was tested to mimic the in vivo first pass situation. Intestinal and liver slices, exposed to the substrates 7-ethoxycoumarin (7-EC), 7-hydroxycoumarin (7-HC) and lidocaine (Li), exhibited similar metabolic rates in the biochip and in the well plates for periods of at least 3 h. The metabolic rate remained the same when two slices were placed in adjacent microchambers and perifused sequentially. In addition, the system has been adapted to sequentially perifuse intestinal and liver tissue slices in a two-compartment co-culture perfusion system with a continuous flow of medium. It becomes possible to direct metabolites or other excreted compounds formed by an intestinal slice in the first compartment to the second compartment containing a liver slice. The intestine does not influence liver metabolism for these substrates. The interplay between these two organs was demonstrated by exposing the slices to the primary bile acid, chenodeoxycholic acid (CDCA). CDCA induced the expression of fibroblast growth factor 15 (FGF15) in the intestinal slice, which resulted in a stronger down-regulation of the enzyme, cytochrome P450 7A1 (CYP7A1), in the liver slice in the second compartment than when the liver slice was exposed to CDCA in a single-microchamber biochip. We thus demonstrate in this paper that intestinal slices, in addition to liver slices, remain functional in the biochip under flow conditions, and that the two-microchamber biochip has great potential for the study of interorgan effects. This is the first example of the incorporation of both liver and intestinal slices in a microfluidic device. Use of this microfluidic system will improve our insight into interorgan interactions and elucidate as yet unknown mechanisms involved in toxicity, gene regulation and drug-drug interactions.

Mechanisms underlying the cholesterol-lowering properties of soluble dietary fibre polysaccharides

A number of studies have shown a positive relationship between diets rich in soluble dietary fibres (SDF) such as β-glucan, pectin, guar gum and psyllium, and reduced serum cholesterol and thus a decreased risk of cardiovascular disease (CVD). Three major biological mechanisms have been proposed to explain the cholesterol-reducing effects of SDF: prevention of bile salt (BS) re-absorption from the small intestine leading to an excess faecal BS excretion; reduced glycemic response leading to lower insulin stimulation of hepatic cholesterol synthesis; and physiological effects of fermentation products of SDF, mainly propionate. Evidence for the latter mechanism is inconclusive, whereas in vivo, ex vivo and in vitro experiments suggest that BS micelles "bind" to SDF preventing their re-absorption. Whereas, glycemic responses to SDF have been studied extensively, the nature of interactions between bile salt micelles and SDF that lead to incomplete BS re-absorption are poorly defined. Three potential physicochemical mechanisms are proposed together with suggestions for in vitro experiments to test them.

Interference with bile salt export pump function is a susceptibility factor for human liver injury in drug development

The bile salt export pump (BSEP) is an efflux transporter, driving the elimination of endobiotic and xenobiotic substrates from hepatocytes into the bile. More specifically, it is responsible for the elimination of monovalent, conjugated bile salts, with little or no assistance from other apical transporters. Disruption of BSEP activity through genetic disorders is known to manifest in clinical liver injury such as progressive familial intrahepatic cholestasis type 2. Drug-induced disruption of BSEP is hypothesized to play a role in the development of liver injury for several marketed or withdrawn therapeutics. Unfortunately, preclinical animal models have been poor predictors of the liver injury associated with BSEP interference observed for humans, possibly because of interspecies differences in bile acid composition, differences in hepatobiliary transporter modulation or constitutive expression, as well as other mechanisms. Thus, a BSEP-mediated liver liability may go undetected until the later stages of drug development, such as during clinical trials or even postlicensing. In the absence of a relevant preclinical test system for BSEP-mediated liver injury, the toxicological relevance of available in vitro models to human health rely on the use of benchmark compounds with known clinical outcomes, such as marketed or withdrawn drugs. In this study, membrane vesicles harvested from BSEP-transfected insect cells were used to assess the activity of more than 200 benchmark compounds to thoroughly investigate the relationship between interference with BSEP function and liver injury. The data suggest a relatively strong association between the pharmacological interference with BSEP function and human hepatotoxicity. Although the most accurate translation of risk would incorporate pharmacological potency, pharmacokinetics, clearance mechanisms, tissue distribution, physicochemical properties, indication, and other drug attributes, the additional understanding of a compound's potency for BSEP interference should help to limit or avoid BSEP-related liver liabilities in humans that are not often detected by standard preclinical animal models.

Getting the mOST from OST: Role of organic solute transporter, OSTalpha-OSTbeta, in bile acid and steroid metabolism

The organic solute transporter (OST)(alpha)-OST(beta) is an unusual heteromeric carrier expressed in a variety of tissues including the small intestine, colon, liver, biliary tract, kidney, and adrenal gland. In polarized epithelial cells, OSTalpha-OSTbeta protein is localized on the basolateral membrane and functions in the export or uptake of bile acids and steroids. This article reviews recent results including studies of knockout mouse models that provide new insights to the role of OSTalpha-OSTbeta in the compartmentalization and metabolism of these important lipids.

Triglycerides and gallstone formation

Changes in bile acid (BA) metabolism and gallbladder function are critical factors in the pathogenesis of gallstones. Patients with hypertriglyceridemia (HTG) - often overweight and insulin resistant - are at risk for gallstone disease. The question arises whether HTG itself contributes to gallstone formation or whether gallstone disease only associates with this disorder. Triglycerides are formed in response to fluxes of non-esterified fatty acids and glucose. Hypertriglyceridemia results from either overproduction of triglycerides by the liver, impaired lipolysis or a combination of both. Hyperinsulinemia, as observed in the insulin resistant state, stimulates very low-density lipoprotein (VLDL)-triglyceride synthesis. Peroxisome proliferator-activated receptors (PPARs), liver X receptors (LXRs), farnesoid X receptor (FXR) and hepatocyte nuclear factor 4α (HNF4α) are the nuclear receptors involved in the regulation of lipogenesis. Microsomal triglyceride transfer protein (MTP) is involved in the production of VLDL and its activation is also under control of transcription factors as FXR and Forkhead box-O1 (FoxO1). Triglyceride and BA metabolism are linked. There is an inverse relationship between bile acid fluxes and pool size and VLDL production and SHP (small heterodimer partner) and FXR are the link between BAs and TG metabolism. BAs are also ligands for FXR and G-protein-coupled receptors, such as TGR5. FXR activation by BAs suppresses the expression of MTP, transcription factor sterol regulatory element binding protein (SREBP)-1c and other lipogenic genes. LXRs stimulate lipogenesis whereas FXRs inhibit the metabolic process. Synthesis of BAs from cholesterol occurs either via the classical pathway (7α-hydroxylation of cholesterol; CYP7A1) or via the alternate pathway (CYP39A1 or CYP7B1). BAs induce FXR, which inhibits CYP7A1 transcription by activation of SHP and inhibition of HNF4α transactivation. Bile composition (supersaturation with cholesterol), gallbladder dysmotility, inflammation, hypersecretion of mucin gel in the gallbladder and slow large intestinal motility and increased intestinal cholesterol absorption may contribute to the pathogenesis of cholesterol gallstones. In HTG patients supersaturated bile may be related to the presence of obesity rather than to HTG itself. Contraction and relaxation of the gallbladder are regulated by neuronal, hormonal and paracrine factors. Postprandial gallbladder emptying is regulated by cholecystokinin (CCK). Poor postprandial gallbladder contraction may be due to the magnitude of the CCK response and to the amount of CCK receptors in the gallbladder smooth muscle cells. In the fasting state gallbladder motility is associated with the intestinal migrating motor complex (MMC) activity and with elevated plasma motilin levels. Fibroblast growth factor (FGF19), produced on arrival of bile acids in the ileum, is also important for gallbladder motility. Gallbladder motility is impaired in HTG patients compared to BMI matched controls. There is evidence that the gallbladder in HTG is less sensitive to CCK and that this sensitivity improves after reversal of high serum TG levels by use of TG lowering agents. In hypertriglyceridemia TG lowering therapy (fibrates or fish-oil) is essential to prevent cardiovascular disease and pancreatitis. Fibrates, however, also increase the risk for cholelithiasis by increasing biliary cholesterol saturation and by reduction of bile acid synthesis. On the other hand fish-oil decreases biliary cholesterol saturation. Fish-oil may increase bile acid synthesis by activation of 7alpha-hydroxylase and may inhibit VLDL production and secretion through activation of nuclear factors and increased apoB degradation. In HTG patients, gallbladder motility improves during bezafibrate as well as during fish-oil therapy. The question remains whether improvement of gallbladder motility and increased lithogenicity of bile by bezafibrate therapy counteract each other or still result in gallstone formation in HTG patients.

Transgenic expression of cholesterol 7alpha-hydroxylase in the liver prevents high-fat diet-induced obesity and insulin resistance in mice

Cholesterol 7alpha-hydroxylase (CYP7A1) is the rate-limiting enzyme in the bile acid biosynthetic pathway that converts cholesterol into bile acids in the liver. Recent studies have shown that bile acids may play an important role in maintaining lipid, glucose, and energy homeostasis. However, the role of CYP7A1 in the development of obesity and diabetes is currently unclear. In this study, we demonstrated that transgenic mice overexpressing Cyp7a1 in the liver [i.e., Cyp7a1 transgenic (Cyp7a1-tg) mice] were resistant to high-fat diet (HFD)-induced obesity, fatty liver, and insulin resistance. Cyp7a1-tg mice showed increased hepatic cholesterol catabolism and an increased bile acid pool. Cyp7a1-tg mice had increased secretion of hepatic very low density lipoprotein but maintained plasma triglyceride homeostasis. Gene expression analysis showed that the hepatic messenger RNA expression levels of several critical lipogenic and gluconeogenic genes were significantly decreased in HFD-fed Cyp7a1-tg mice. HFD-fed Cyp7a1-tg mice had increased whole body energy expenditure and induction of fatty acid oxidation genes in the brown adipose tissue.

CONCLUSION

This study shows that Cyp7a1 plays a critical role in maintaining whole body lipid, glucose, and energy homeostasis. The induction of CYP7A1 expression with the expansion of the hydrophobic bile acid pool may be a potential therapeutic strategy for treating metabolic disorders such as fatty liver diseases, obesity, and diabetes in humans.

Retinoic acid represses CYP7A1 expression in human hepatocytes and HepG2 cells by FXR/RXR-dependent and independent mechanisms

Cholesterol 7alpha-hydroxylase (CYP7A1) plays a key role in maintaining lipid and bile salt homeostasis as it is the rate-limiting enzyme converting cholesterol to bile acids. Deficiency of CYP7A1 leads to hyperlipidemia in man and mouse. Hyperlipidemia is often seen in patients when treated with high-dose retinoic acid (RA), but the molecular mechanisms remain elusive. Our present study revealed that CYP7A1 mRNA expression is greatly repressed by RA in both human hepatocytes and HepG2 cells where increased fibroblast growth factor 19 (FGF19) and small heterodimer partner (SHP) expressions were also observed, suggesting farnesoid X receptor (FXR) and retinoid X receptor (RXR) were activated. Promoter reporter assays demonstrate that all-trans RA (atRA) specifically activated FXR/RXR. However, detailed molecular analyses indicate that this activation is through RXR, whose ligand is 9-cis RA. Knocking down of FXR or RXRalpha by small interference RNA (siRNA) in human hepatocytes increased CYP7A1 basal expression, but the repressive effect of atRA persisted, suggesting there are also FXR/RXR-independent mechanisms mediating atRA repression of CYP7A1 expression. Chromatin immunoprecipitation (ChIP) assay and cell transfection results indicate that PGC-1alpha plays a role in the FXR/RXR-independent mechanism. Our findings may provide a potential explanation for hyperlipidemic side effects observed in some patients treated with high-dose RA.

Hypocholesterolemic effects of hydroxypropyl methylcellulose are mediated by altered gene expression in hepatic bile and cholesterol pathways of male hamsters

Hydroxypropyl methylcellulose (HPMC), a semisynthetic, nonfermentable soluble dietary fiber, is not absorbed by the body, but its presence in the intestinal lumen increases fecal fat, sterol, and bile acid excretions and decreases intestinal cholesterol absorption, all of which may indirectly affect hepatic lipid metabolism. We measured the expression of hepatic genes involved in cholesterol, bile acid, and fatty acid metabolism in hamsters fed diets containing 39% of energy as fat and 5% of weight as HPMC or microcrystalline cellulose (control) for 4 wk. HPMC-fed hamsters gained significantly less body weight than the control group but did not differ in food intake. They had significantly lower plasma triglyceride and total-, VLDL-, HDL-, and LDL-cholesterol concentrations and hepatic total lipid, total and free cholesterol and triglyceride concentrations than controls. Compared with controls, HPMC-fed hamsters had greater levels of mRNA for CYP7A1 (cytochrome P450 7A1; 8-fold of control; P < 0.05), CYP51 (lanosterol 14alpha-demethylase; 5.3-fold of control; P < 0.05), and HMG-CoAR (3-hydroxy-3-methylglutaryl CoA reductase; 1.8-fold of control; P < 0.05). The plasma total cholesterol concentrations from both the control and HPMC groups were inversely correlated with expression of hepatic CYP7A1 (r = -0.54; P < 0.05), CYP51 (r = -0.79; P < 0.005), and HMG-CoAR (r = -0.75; P < 0.005) genes. This suggests that HPMC supplementation affected both cholesterol and bile acid synthesis. Our data confirm that altered hepatic expression of lipid metabolism-related genes, possibly due to modulation of fecal bile acid excretion and intestinal cholesterol absorption, contributes to the lipid-lowering effects of HPMC.

ABCG5/ABCG8-independent biliary cholesterol excretion in lactating rats

Lactation is associated with increased expression of bile acid transporters and an increased size and hydrophobicity of the bile acid pool in rats. ATP-binding cassette (ABC) transporters multidrug resistance protein 2 (Mdr2), Abcb11 [bile salt export pump (Bsep)], and Abcg5/Abcg8 heterodimers are essential for the biliary secretion of phospholipids, bile acids, and cholesterol, respectively. We investigated the expression of these transporters and secretion of their substrates in female control and lactating Sprague Dawley rats and C57BL/6 mice. Expression of Abcg5/Abcg8 mRNA was decreased by 97 and 60% by midlactation in rats and mice, respectively; protein levels of Abcg8 were below detection limits in lactating rats. Mdr2 mRNA expression was decreased in lactating rats and mice by 47 and 59%, respectively. Despite these changes in transporter expression, basal concentrations of cholesterol and phospholipid in bile were unchanged in rats and mice, whereas increased Bsep mRNA expression in early lactation coincided with an increased basal biliary bile acid concentration in lactating mice. Following taurocholate infusion, coupling of phospholipid and taurocholate secretion in bile of lactating mice was significantly impaired relative to control mice, with no significant changes in maximal secretion of cholesterol or bile acids. In rats, taurocholate infusion revealed a significantly impaired coupling of cholesterol to taurocholate secretion in bile in lactating vs. control animals. These data reveal marked utilization of an Abcg5/Abcg8-independent mechanism for basal biliary cholesterol secretion in rats during lactation, but a dependence on Abcg5/g8 for maximal biliary cholesterol secretion.

Analytical strategies for characterization of bile acid and oxysterol metabolomes

Cholesterol is the precursor of many compounds with functions in the physiology and metabolism of the organism. Methods for the multicomponent analysis of these compounds and their metabolites (metabolomics) are needed to improve our understanding of their roles in different species, organs, cells and metabolic situations and to clarify structure/activity relationships. This review discusses methods based on combinations of ion exchange and reversed-phase separations for sample preparation with derivatization and "charge-tagging" for chromatography-mass spectrometry in qualitative and quantitative characterizations of oxysterol, bile alcohol, bile acid, and steroid hormone metabolomes. Advantages, disadvantages and potential improvements for high-throughput applications are briefly discussed.

A novel bile acid-activated vitamin D receptor signaling in human hepatocytes

Vitamin D receptor (VDR) is activated by natural ligands, 1alpha, 25-dihydroxy-vitamin D(3) [1alpha,25(OH)(2)-D(3)] and lithocholic acid (LCA). Our previous study shows that VDR is expressed in human hepatocytes, and VDR ligands inhibit bile acid synthesis and transcription of the gene encoding cholesterol 7alpha-hydroxylase (CYP7A1). Primary human hepatocytes were used to study LCA and 1alpha,25(OH)(2)-D(3) activation of VDR signaling. Confocal immunofluorescent microscopy imaging and immunoblot analysis showed that LCA and 1alpha, 25(OH)(2)-D(3) induced intracellular translocation of VDR from the cytosol to the nucleus and also plasma membrane where VDR colocalized with caveolin-1. VDR ligands induced tyrosine phosphorylation of c-Src and VDR and their interaction. Inhibition of c-Src abrogated VDR ligand-dependent inhibition of CYP7A1 mRNA expression. Kinase assays showed that VDR ligands specifically activated the c-Raf/MEK1/2/extracellular signal-regulated kinase (ERK) 1/2 pathway, which stimulates serine phosphorylation of VDR and hepatocyte nuclear factor-4alpha, and their interaction. Mammalian two-hybrid assays showed a VDR ligand-dependent interaction of nuclear receptor corepressor-1 and silencing mediator of retinoid and thyroid with VDR/retinoid X receptor-alpha (RXRalpha). Chromatin immunoprecipitation assays revealed that an ERK1/2 inhibitor reversed VDR ligand-induced recruitment of VDR, RXRalpha, and corepressors to human CYP7A1 promoter. In conclusion, VDR ligands activate membrane VDR signaling to activate the MEK1/2/ERK1/2 pathway, which stimulates nuclear VDR/RXRalpha recruitment of corepressors to inhibit CYP7A1 gene transcription in human hepatocytes. This membrane VDR-signaling pathway may be activated by bile acids to inhibit bile acid synthesis as a rapid response to protect hepatocytes from cholestatic liver injury.

Unexpected effects of fasting on murine lipid homeostasis--transcriptomic and lipid profiling

BACKGROUND & AIMS

Starvation induces massive perturbations in metabolic pathways involved in energy metabolism, but its effect on the metabolism of lipids, particularly cholesterol, is little understood.

METHODS

A comparative genomic analysis of the gut and the liver in response to fasting was performed, with intestinal perfusion and lipid profiling of the plasma, bile, liver, intestinal tissue, perfusate, and faeces in FVB mice.

RESULTS

The expression profiles suggested increased cholesterol trafficking in the liver and decreased trafficking in the small intestine. Plasma cholesterol concentrations significantly increased, and triglycerides decreased in fasting. Surprisingly, in prolonged fasting, the biliary bile salt and lipid output rates increased, with increased hepatic and intestinal lipid turnover, and enhanced trans-intestinal cholesterol excretion. In contrast, faecal sterol loss declined sharply. To investigate whether the increased biliary phospholipid secretion could nourish the intestinal epithelium, we studied the histology of the small intestines upon fasting in multidrug resistant protein 2 deficient mice with scarce biliary phospholipids. Their adaptive biliary response to fasting was lost, while the shortage of biliary phospholipids strongly induced apoptosis and proliferation in the small intestine and increased the number of mucin-producing cells.

CONCLUSION

Even with no dietary fat, lipid levels remain remarkably constant in the murine liver and intestines during prolonged fasting. The biliary system, always assumed to be coupled to the postprandial response, shows a paradoxical increase in activity. We hypothesise that biliary lipids are mobilised to supply the enterocytes with luminal fuel and to stabilise transport systems in the intestine for ensuring a rapid recovery when the food supply resumes.

Nuclear receptors as drug targets in cholestasis and drug-induced hepatotoxicity

Nuclear receptors are key regulators of various processes including reproduction, development, and metabolism of xeno- and endobiotics such as bile acids and drugs. Research in the last two decades provided researchers and clinicians with a detailed understanding of the regulation of these processes and, most importantly, also prompted the development of novel drugs specifically targeting nuclear receptors for the treatment of a variety of diseases. Some nuclear receptor agonists are already used in daily clinical practice but many more are currently designed or tested for the treatment of diabetes, dyslipidemia, fatty liver disease, cancer, drug hepatotoxicity and cholestasis. The hydrophilic bile acid ursodeoxycholic acid is currently the only available drug to treat cholestasis but its efficacy is limited. Therefore, development of novel treatments represents a major goal for both pharmaceutical industry and academic researchers. Targeting nuclear receptors in cholestasis is an intriguing approach since these receptors are critically involved in regulation of bile acid homeostasis. This review will discuss the general role of nuclear receptors in regulation of transporters and other enzymes maintaining bile acid homeostasis and will review the role of individual receptors as therapeutic targets. In addition, the central role of nuclear receptors and other transcription factors such as the aryl hydrocarbon receptor (AhR) and the nuclear factor-E2-related factor (Nrf2) in mediating drug disposition and their potential therapeutic role in drug-induced liver disease will be covered.

A putative role of micro RNA in regulation of cholesterol 7alpha-hydroxylase expression in human hepatocytes

Cholesterol 7alpha-hydroxylase (CYP7A1) plays a critical role in regulation of bile acid synthesis in the liver. CYP7A1 mRNAs have very short half-lives, and bile acids destabilize CYP7A1 mRNA via the 3'-untranslated region (3'-UTR). However, the underlying mechanism of translational regulation of CYP7A1 mRNA remains unknown. Screening of a human micro RNA (miRNA) microarray has identified five differentially expressed miRNAs in human primary hepatocytes treated with chenodeoxycholic acid, GW4064, or fibroblast growth factor (FGF)19. These compounds also significantly induced the expression of miR-122a, a liver-specific and the predominant miRNA in human hepatocytes. The putative recognition sequences for miR-122a and miR-422a were localized in the 3'-UTR of human CYP7A1 mRNA. The miR-122a and miR-422a mimics inhibited, whereas their inhibitors stimulated CYP7A1 mRNA expression. These miRNAs specifically inhibited the activity of the CYP7A1-3'-UTR reporter plasmids, and mutations of miRNA binding sites in 3'-UTR abrogated miRNA inhibition of reporter activity. These results suggest that miR-122a and miR-422a may destabilize CYP7A1 mRNA to inhibit CYP7A1 expression. However, these miRNAs did not play a role in mediating FGF19 inhibition of CYP7A1 transcription. Under certain conditions, miRNA may reduce CYP7A1 mRNA stability to inhibit bile acid synthesis, and the miR-122a antagomirs may stimulate bile acid synthesis to reduce serum cholesterol and triglycerides.

Regulation of bile acid synthesis by fat-soluble vitamins A and D

Bile acids are required for proper absorption of dietary lipids, including fat-soluble vitamins. Here, we show that the dietary vitamins A and D inhibit bile acid synthesis by repressing hepatic expression of the rate-limiting enzyme CYP7A1. Receptors for vitamin A and D induced expression of Fgf15, an intestine-derived hormone that acts on liver to inhibit Cyp7a1. These effects were mediated through distinct cis-acting response elements in the promoter and intron of Fgf15. Interestingly, transactivation of both response elements appears to be required to maintain basal Fgf15 expression levels in vivo. Furthermore, whereas induction of Fgf15 by vitamin D is mediated through its receptor, the induction of Fgf15 by vitamin A is mediated through the retinoid X receptor/farnesoid X receptor heterodimer and is independent of bile acids, suggesting that this heterodimer functions as a distinct dietary vitamin A sensor. Notably, vitamin A treatment reversed the effects of the bile acid sequestrant cholestyramine on Fgf15, Shp, and Cyp7a1 expression, suggesting a potential therapeutic benefit of vitamin A under conditions of bile acid malabsorption. These results reveal an unexpected link between the intake of fat-soluble vitamins A and D and bile acid metabolism, which may have evolved as a means for these dietary vitamins to regulate their own absorption.

Sodium taurocholate inhibits intestinal adenoma formation in APCMin/+ mice, potentially through activation of the farnesoid X receptor

In light of clinical and biological evidence that bile constituents exert preventive effects against colorectal cancer, we evaluated the influence of oral bilirubin and sodium taurocholate (NaTC) on intestinal tumor formation in APC(Min/+) mice. Mice received bilirubin and/or bovine serum albumin (BSA) and NaTC in the drinking water for 8 weeks, after which the number, size and location of intestinal adenomas were determined. Tissue specimens were analyzed by light microscopy, TUNEL staining, immunohistochemistry for beta-catenin and Ki-67 and quantitative polymerase chain reaction for farnesoid X receptor (FXR)-dependent gene expression. Colon tumor formation also was assessed in azoxymethane (AOM)-treated hyperbilirubinemic Gunn (j/j) and wild-type (+/+) rats. Compared with untreated APC(Min/+) mice, the mean number of intestinal adenomas was markedly lower in both bilirubin (10.5 +/- 0.9 versus 37.0 +/- 5.2; +/-SEM; P < 0.001) and NaTC plus BSA (14.3 +/- 5.4; P = 0.01)-treated animals. Both treatment groups exhibited reduced levels of cellular proliferation in the ileum (by Ki-67 staining), but no differences in TUNEL staining or the percentage of beta-catenin-positive crypts. Bilirubin feeding reduced intestinal inducible nitric oxide synthase expression, but did not alter adenoma multiplicity in APC(Min/+) mice or in AOM-treated j/j versus +/+ rats. Mice receiving NaTC manifested increased intestinal expression of the FXR-regulated genes, Shp, FGF15 and IBABP, and a concomitant decrease in cyclin D1 message. Administering NaTC to APC(Min/+) mice causes a marked reduction in intestinal adenomas. We postulate that this effect is mediated through activation of FXR, leading to increased Shp expression and consequent downregulation of cyclin D1.

The hepatic response to FGF19 is impaired in patients with nonalcoholic fatty liver disease and insulin resistance

Intestinal FGF19 has emerged as a novel endocrine regulator of hepatic bile salt and lipid metabolism. In patients with nonalcoholic fatty liver disease (NAFLD) hepatic lipid metabolism is deranged. A possible role of FGF19 in NAFLD has not been reported yet. In this study, we assessed intestinal FGF19 production and the hepatic response to FGF19 in NAFLD patients with and without insulin resistance [homeostasis model of assessment (HOMA) score > or =2.5 (n = 12) and HOMA score <2.5 (n = 8), respectively]. To this end, NAFLD patients received a standardized oral fat challenge. Postprandial excursions of triglycerides, bile salts, and FGF19 were monitored, and plasma levels of a marker for bile salt synthesis (7alpha-hydroxy-4-cholesten-3-one) were determined. Fasted FGF19 levels were comparable in a control group of healthy volunteers (n = 15) and in NAFLD patients (0.26 +/- 0.28 vs. 0.18 +/- 0.09 ng/ml, respectively, P = 0.94). Postprandial FGF19 levels in both controls and NAFLD patients peaked between 3-4 h and were three times higher than baseline levels. The areas under the postprandial FGF19 curve were similar in controls and in the HOMA score-based NAFLD subgroups. In NAFLD patients with HOMA score <2.5, the postprandial increase in plasma FGF19 was accompanied by a lowering of plasma levels of 7alpha-hydroxy-4-cholesten-3-one (-30%, P = 0.015). This anticipated decline was not observed in insulin-resistant NAFLD patients (+10%, P = 0.22). In conclusion, patients with NAFLD show an unimpaired intestinal FGF19 production. However, the hepatic response to FGF19 is impaired in NAFLD patients with insulin resistance (HOMA score > or =2.5). This impaired hepatic response to FGF19 may contribute to the dysregulation of lipid homeostasis in NAFLD.

Thyroid hormone crosstalk with nuclear receptor signaling in metabolic regulation

Thyroid hormone influences diverse metabolic pathways important in lipid and glucose metabolism, lipolysis and regulation of body weight. Recently, it has been recognized that thyroid hormone receptor interacts with transcription factors that predominantly respond to nutrient signals including the peroxisome proliferator-activated receptors, liver X receptor and others. Crosstalk between thyroid hormone signaling and these nutrient responsive factors occurs through a variety of mechanisms: competition for retinoid X receptor heterodimer partners, DNA binding sites and transcriptional cofactors. This review focuses on the mechanisms of interaction of thyroid hormone signaling with other metabolic pathways and the importance of understanding these interactions to develop therapeutic agents for treatment of metabolic disorders, such as dyslipidemias, obesity and diabetes.

Bile salt sequestration induces hepatic de novo lipogenesis through farnesoid X receptor- and liver X receptor alpha-controlled metabolic pathways in mice

Diabetes is characterized by high blood glucose levels and dyslipidemia. Bile salt sequestration has been found to improve both plasma glycemic control and cholesterol profiles in diabetic patients. Yet bile salt sequestration is also known to affect triglyceride (TG) metabolism, possibly through signaling pathways involving farnesoid X receptor (FXR) and liver X receptor alpha (LXRalpha). We quantitatively assessed kinetic parameters of bile salt metabolism in lean C57Bl/6J and in obese, diabetic db/db mice upon bile salt sequestration using colesevelam HCl (2% wt/wt in diet) and related these to quantitative changes in hepatic lipid metabolism. As expected, bile salt sequestration reduced intestinal bile salt reabsorption. Importantly, bile salt pool size and biliary bile salt secretion remained unchanged upon sequestrant treatment due to compensation by de novo bile salt synthesis in both models. Nevertheless, lean and db/db mice showed increased, mainly periportally confined, hepatic TG contents, increased expression of lipogenic genes, and increased fractional contributions of newly synthesized fatty acids. Lipogenic gene expression was not induced in sequestrant-treated Fxr(-/-) and Lxralpha(-/-) mice compared with wild-type littermates, in line with reports indicating a regulatory role of FXR and LXRalpha in bile salt-mediated regulation of hepatic lipid metabolism.

CONCLUSION

Bile salt sequestration by colesevelam induces the lipogenic pathway in an FXR- and LXRalpha-dependent manner without affecting the total pool size of bile salts in mice. We speculate that a shift from intestinal reabsorption to de novo synthesis as source of bile salts upon bile salt sequestration affects zonation of metabolic processes within the liver acinus.

Modulation of retinoic acid receptor-related orphan receptor alpha and gamma activity by 7-oxygenated sterol ligands

The retinoic acid receptor-related orphan receptors alpha and gamma (RORalpha (NR1F1) and RORgamma (NR1F3)) are orphan nuclear receptors and perform critical roles in regulation of development, metabolism, and immune function. Cholesterol and cholesterol sulfate have been suggested to be RORalpha ligands, but the physiological significance is unclear. To date, no endogenous RORgamma ligands have been described. Here, we demonstrate that 7-oxygenated sterols function as high affinity ligands for both RORalpha and RORgamma by directly binding to their ligand-binding domains (K(i) approximately 20 nM), modulating coactivator binding, and suppressing the transcriptional activity of the receptors. One of the 7-oxygenated sterols, 7alpha-hydroxycholesterol (7alpha-OHC), serves as a key intermediate in bile acid metabolism, and we show that 7alpha-OHC modulates the expression of ROR target genes, including Glc-6-Pase and phosphoenolpyruvate carboxykinase, in an ROR-dependent manner. Furthermore, glucose output from hepatocytes is suppressed by 7alpha-OHC functioning as an RORalpha/gamma ligand. Thus, RORalpha and RORgamma are ligand-regulated members of the NR superfamily and may serve as sensors for 7-oxygenated sterols.