Down-regulation of the rat hepatic sterol 27-hydroxylase gene by bile acids in transfected primary hepatocytes: possible role of hepatic nuclear factor 1alpha

Bile Acids, Nuclear Receptors and Cytochrome P450

This review summarizes the importance of bile acids (BA) as important regulators of various homeostatic mechanisms with detailed focus on cytochrome P450 (CYP) enzymes. In the first part, synthesis, metabolism and circulation of BA is summarized and BA are reviewed as physiological ligands of nuclear receptors which regulate transcription of genes involved in their metabolism, transport and excretion. Notably, PXR, FXR and VDR are the most important nuclear receptors through which BA regulate transcription of CYP genes involved in the metabolism of both BA and xenobiotics. Therapeutic use of BA and their derivatives is also briefly reviewed. The physiological role of BA interaction with nuclear receptors is basically to decrease production of toxic non-polar BA and increase their metabolic turnover towards polar BA and thus decrease their toxicity. By this, the activity of some drug-metabolizing CYPs is also influenced what could have clinically relevant consequences in cholestatic diseases or during the treatment with BA or their derivatives.

FXR-induced secretion of FGF15/19 inhibits CYP27 expression in cholangiocytes through p38 kinase pathway

Cholangiocytes, bile duct lining cells, actively adjust the amount of cholesterol and bile acids in bile through expression of enzymes and channels involved in transportation and metabolism of the cholesterol and bile acids. Herein, we report molecular mechanisms regulating bile acid biosynthesis in cholangiocytes. Among the cytochrome p450 (Cyp) enzymes involved in bile acid biosynthesis, sterol 27-hydroxylase (Cyp27) that is the rate-limiting enzyme for the acidic pathway of bile acid biosynthesis expressed in cholangiocytes. Expression of other Cyp enzymes for the basic bile acid biosynthesis was hardly detected. The Cyp27 expression was negatively regulated by a hydrophobic bile acid through farnesoid X receptor (FXR), a nuclear receptor activated by bile acid ligands. Activated FXR exerted the negative effects by inducing an expression of fibroblast growth factor 15/19 (FGF15/19). Similar to its repressive function against cholesterol 7α-hydroxylase (Cyp7a1) expression in hepatocytes, secreted FGF15/19 triggered Cyp27 repression in cholangiocytes through interaction with its cognate receptor fibroblast growth factor receptor 4 (FGFR4). The involvements of FXR and FGFR4 for the bile acid-induced Cyp27 repression were confirmed in vivo using knockout mouse models. Different from the signaling in hepatocytes, wherein the FGF15/19-induced repression signaling is mediated by c-Jun N-terminal kinase (JNK), FGF15/19-induced Cyp27 repression in cholangiocytes was mediated by p38 kinase. Thus, the results collectively suggest that cholangiocytes may be able to actively regulate bile acid biosynthesis in cholangiocytes and even hepatocyte by secreting FGF15/19. We suggest the presence of cholangiocyte-mediated intrahepatic feedback loop in addition to the enterohepatic feedback loop against bile acid biosynthesis in the liver.

Increased cholesterol 7alpha-hydroxylase expression and size of the bile acid pool in the lactating rat

Maximal bile acid secretory rates and expression of bile acid transporters in liver and ileum are increased in lactation, possibly to facilitate increased enterohepatic recirculation of bile acids. We determined changes in the size and composition of the bile acid pool and key enzymes of the bile acid synthetic pathway [cholesterol 7alpha-hydroxylase (Cyp7a1), sterol 27-hydroxylase (Cyp27a1), and sterol 12alpha-hydroxylase (Cyp8b1)] in lactating rats relative to female virgin controls. The bile acid pool increased 1.9 to 2.5-fold [postpartum (PP) days 10, 14, and 19-23], compared with controls. A 1.5-fold increase in cholic acids and a 14 to 20% decrease in muricholic acids in lactation significantly increased the hydrophobicity index. In contrast, the hepatic concentration of bile acids and small heterodimer partner mRNA were unchanged in lactation. A 2.8-fold increase in Cyp7a1 mRNA expression at 16 h (10 h of light) demonstrated a shift in the diurnal rhythm at day 10 PP; Cyp7a1 protein expression and cholesterol 7alpha-hydroxylase activity were significantly increased at this time and remained elevated at day 14 PP but decreased to control levels by day 21 PP. There was an overall decrease in Cyp27a1 mRNA expression and a 20% decrease in Cyp27a1 protein expression, but there was no change in Cyp8b1 mRNA or protein expression at day 10 PP. The increase in Cyp7a1 expression PP provides a mechanism for the increase in the bile acid pool.

In vivo effects of chronic contamination with depleted uranium on vitamin D3 metabolism in rat

The extensive use of depleted uranium (DU) in today's society results in the increase of the number of human population exposed to this radionuclide. The aim of this work was to investigate in vivo the effects of a chronic exposure to DU on vitamin D(3) metabolism, a hormone essential in mineral and bone homeostasis. The experiments were carried out in rats after a chronic contamination for 9 months by DU through drinking water at 40 mg/L (1 mg/rat/day). This dose corresponds to the double of highest concentration found naturally in Finland. In DU-exposed rats, the active vitamin D (1,25(OH)(2)D(3)) plasma level was significantly decreased. In kidney, a decreased gene expression was observed for cyp24a1, as well as for vdr and rxralpha, the principal regulators of CYP24A1. Similarly, mRNA levels of vitamin D target genes ecac1, cabp-d28k and ncx-1, involved in renal calcium transport were decreased in kidney. In the brain lower levels of messengers were observed for cyp27a1 as well as for lxrbeta, involved in its regulation. In conclusion, this study showed for the first time that DU affects both the vitamin D active form (1,25(OH)(2)D(3)) level and the vitamin D receptor expression, and consequently could modulate the expression of cyp24a1 and vitamin D target genes involved in calcium homeostasis.

Hepatic gene expression profiling using Genechips in zebrafish exposed to 17alpha-ethynylestradiol

Genomic, proteomic, and metabolomic technologies continue to receive increasing interest from environmental toxicologists. This interest is due to the great potential of these technologies to identify detailed modes of action and to provide assistance in the evaluation of a contaminant's risk to aquatic organisms. Our experimental model is the zebrafish (Danio rerio) exposed to reference endocrine disrupting compounds in order to investigate compound-induced changes in gene transcript profiles. Adult, female zebrafish were exposed to 0, 15, 40, and 100ng/L of 17alpha-ethynylestradiol (EE2) and concentration and time-dependent changes in hepatic gene expression were examined using Affymetrix GeneChip Zebrafish Genome Microarrays. At 24, 48, and 168h, fish were sacrificed and liver mRNA was extracted for gene expression analysis (24 and 168h only). In an effort to link gene expression changes to effects on higher levels of biological organization, body and ovary weights were measured and blood was collected for measurement of plasma steroid hormones (17beta-estradiol (E2), testosterone (T)) and vitellogenin (VTG) using ELISA. EE2 exposure significantly affected gene expression, GSI, E2, T, and VTG. We observed 1622 genes that were significantly affected (p< or =0.001) in a concentration-dependent manner by EE2 exposure at either 24 or 168h. Gene ontology (GO) analysis revealed that EE2 exposure affected genes involved in hormone metabolism, vitamin A metabolism, steroid binding, sterol metabolism, and cell growth. Plasma VTG was significantly increased at 24, 48, and 168h (p< or =0.05) at 40 and 100ng/L and at 15ng/L at 168h. E2 and T were significantly reduced following EE2 exposure at 48 and 168h. GSI was decreased in a concentration-dependent manner at 168h. In this study, we identified genes involved in a variety of biological processes that have the potential to be used as markers of exposure to estrogenic substances. Future work will evaluate the use of these genes in zebrafish exposed to weak estrogens to determine if these genes are indicative of exposure to estrogens with varying potencies.

Effects of depleted uranium after short-term exposure on vitamin D metabolism in rat

Uranium is a natural radioactive heavy metal. Its toxicity has been demonstrated for different organs, including bone, kidney, liver and brain. Effects of an acute contamination by depleted uranium (DU) were investigated in vivo on vitamin D(3) biosynthetic pathway. Rats received an intragastric administration of DU (204 mg/kg) and various parameters were studied either on day 1 or day 3 after contamination. Cytochrome P450 (CYP27A1, CYP2R1, CYP27B1, CYP24A1) enzymes involved in vitamin D metabolism and two vitamin D(3)-target genes (ECaC1, CaBP-D9K) were assessed by real time RT-PCR in liver and kidneys. CYP27A1 activity was measured in liver and vitamin D and parathyroid hormone (PTH) level were measured in plasma. In acute treated-rats, vitamin D level was increased by 62% and decreased by 68% in plasma, respectively at day 1 and at day 3, which paralleled with a concomitant decrease of PTH level (90%) at day 3. In liver, cyp2r1 mRNA level was increased at day 3. Cyp27a1 activity decreased at day 1 and increased markedly at day 3. In kidney, cyp27b1 mRNA was increased at days 1 and 3 (11- and 4-fold respectively). Moreover, ecac1 and cabp-d9k mRNA levels were increased at day 1 and decreased at day 3. This work shows for the first time that DU acute contamination modulates both activity and expression of CYP enzymes involved in vitamin D metabolism in liver and kidney, and consequently affects vitamin D target genes levels.

Regulation of sterol 27-hydroxylase in human monocyte-derived macrophages: up-regulation by transforming growth factor β1

Regulatory mechanisms for human CYP27A1 enzyme have not yet been fully investigated. Our approach was to add different hormones and cytokines to cultured human monocyte-derived macrophages, and assess the effects on the CYP27A1 by measuring the production of 27-hydroxylated cholesterol in the media. Of the different hormones and cytokines tested, only transforming growth factor beta1 (TGF-beta1) had a clear effect on CYP27A1. Further experiments showed a significant increase in 27-hydroxylated cholesterol products (27-hydroxycholesterol and 3beta-hydroxy-5-cholestenoic acid). A concomitant increase in CYP27A1 mRNA levels was also seen and this positive effect was confirmed using a human CYP27A1 luciferase reporter gene expressed in HepG2 cells. Experiments with progressive deletion/luciferase reporter gene constructs indicated that a TGF-beta1 responsive sequence might be localized in a region about 400 bp upstream of the CYP27A1 translation start. The possibility is discussed that induction of CYP27A1 by TGF-beta1 may be responsible for some of the anti-atherogenic properties of this cytokine.

Fatal olanzapine-induced hyperglycemic ketoacidosis

Olanzapine is an antipsychotic medication linked to the development, or exacerbation of, type 2 diabetes mellitus. This report describes 3 patients being treated with olanzapine who died suddenly and unexpectedly with hyperglycemic ketoacidosis. All had olanzapine concentrations within the therapeutic range. Vitreous glucose concentrations ranged from 640 mg/dL to 833 mg/dL, and blood acetone concentrations from 25.6 mg/dL to 57.6 mg/dL. Beta-hydroxybutyrate concentrations in blood were from 55.2 mg/dL to 110 mg/dL. Low levels of isopropanol were also detected. None had a history or family history of diabetes mellitus. Glycolated (A1C) hemoglobin in 2 cases was 14.3% and 14.7%. No predisposing factors to olanzapine-induced diabetes were identified. It is recommended that chemical testing of patients dying suddenly while being treated with antipsychotic drugs include vitreous glucose and blood acetone determinations to elucidate the cause and mechanism of death in these patients. Warnings concerning this potentially fatal complication of olanzapine therapy should be included in standard pharmaceutical and prescription references.

Hepatic expression of cytochrome P450s in hepatocyte nuclear factor 1-alpha (HNF1alpha)-deficient mice

Hepatocyte nuclear factor 1 alpha (HNF1alpha) is a liver enriched homeodomain-containing transcription factor that has been shown to transactivate the promoters of several cytochrome P450 (CYP) genes, including CYP2E1, CYP1A2, CYP7A1, and CYP27, in vitro. In humans, mutations in HNF1alpha are linked to the occurrence of maturity onset diabetes of the young type 3, an autosomal dominant form of non-insulin-dependent diabetes mellitus in which afflicted subjects generally develop hyperglycemia before 25 years of age. Mice lacking HNF1alpha also develop similar phenotypes reminiscent of non-insulin-dependent diabetes mellitus. To investigate a potential role for HNF1alpha in the regulation of CYPs in vivo, the expression of major CYP genes from each family was examined in the livers of mice lacking HNF1alpha. Analysis of CYP gene expression revealed marked reductions in expression of Cyp1a2, Cyp2c29 and Cyp2e1, and a moderate reduction of Cyp3a11. In contrast Cyp2a5, Cyp2b10 and Cyp2d9 expression were elevated. There are also significant changes in the expression of genes encoding CYPs involved in fatty acid and bile acid metabolism characterized by a reduction in the expression of Cyp7b1, and Cyp27 as well as elevations in Cyp4a1/3, Cyp7a1, Cyp8b1, and Cyp39a1 expression. These results point to a critical role for HNF1alpha in the regulation of CYPs in vivo and suggest that this transcription factor may have an important influence on drug metabolism as well as lipid and bile acid homeostasis in maturity onset diabetes of the young type 3 diabetics.

Regulation of human sterol 27-hydroxylase gene (CYP27A1) by bile acids and hepatocyte nuclear factor 4alpha (HNF4alpha)

Mitochondrial sterol 27-hydroxylase (CYP27A1) catalyses sterol side-chain oxidation of bile acid synthesis from cholesterol, and the first reaction of the acidic bile acid biosynthetic pathway. Hydrophobic bile acids suppress human CYP27A1 gene reporter activity when assayed in human hepatocellular blastoma HepG2 cells. Bile acids also inhibit CYP27A1 reporter activity in human embryonic kidney 293 cells. A putative bile acid response element (BARE) was mapped to a region downstream of nt -147 of the human CYP27A1 gene, within which a binding site for a liver-specific nuclear receptor, HNF4alpha, is identified. HNF4alpha strongly stimulates CYP27A1 gene transcription and mutation of its binding site markedly reduced promoter activity. Results suggest that human CYP27A1 gene transcription is suppressed by bile acids and HNF4alpha plays a pivotal role in transcriptional regulation of this gene.

Hormonal regulation of the human sterol 27-hydroxylase gene CYP27A1

The nucleotide sequence data reported in this paper will appear in EMBL Nucleotide Sequence Database under the accession number AJ 544720. The mitochondrial sterol 27-hydroxylase (CYP27A1) is a multifunctional cytochrome P450 enzyme that catalyses important hydroxylations in the biosynthesis of bile acids and bioactivation of vitamin D(3). Previous results [Babiker, Andersson, Lund, Xiu, Deeb, Reshef, Leitersdorf, Diczfalusy and Bj örkhem (1997) J. Biol. Chem. 272, 26253-26261] suggest that CYP27A1 plays an important role in cholesterol homoeostasis and affects atherogenesis. In the present study, the regulation of the human CYP27A1 gene by growth hormone (GH), insulin-like growth factor-1 (IGF-1), dexamethasone, thyroid hormones and PMA was studied. HepG2 cells were transfected transiently with luciferase reporter gene constructs containing DNA fragments flanking the 5'-region of the human CYP27A1 gene. GH, IGF-1 and dexamethasone increased the promoter activity by 2-3-fold, whereas thyroxine (T(4)) and PMA repressed the activity significantly when measured with luciferase activity expressed in the cells. The endogenous CYP27A1 enzyme activity in the cells was stimulated by GH, IGF-1 and dexamethasone, whereas T(4) and PMA inhibited the activity. Experiments with progressive deletion/luciferase reporter gene constructs indicated that the response elements for GH may be localized in a region upstream to position -1094 bp. The putative response elements for dexamethasone were mapped to positions between -792 and -1095 bp. The -451 bp fragment of the human CYP27A1 gene was found to confer the activation by IGF-1, and the inhibition by T(4) and PMA. Results of the present study suggest that CYP27A1 is regulated in human cells by hormones and signal-transduction pathways.

Hepatocyte nuclear factor 1 alpha: a key mediator of the effect of bile acids on gene expression

Bile acids regulate the expression of genes involved in cholesterol homeostasis. They are ligands of the farnesoid X receptor, which induces small heterodimer partner (SHP)-1, a transcriptional repressor of bile acid synthetic enzymes. In cholestatic liver disease, hepatic bile acid concentrations are elevated and expression of the major Na+-independent bile acid uptake system, organic anion transporting polypeptide (OATP)-C (solute carrier gene family SLC21A6), is markedly decreased. Because the OATP-C gene is transcriptionally dependent on the hepatocyte nuclear factor (HNF) 1 alpha, we hypothesized that bile acids decrease OATP-C expression through direct repression of HNF1 alpha. To test this hypothesis, we studied the regulation of the human HNF1 alpha gene by bile acids. HNF1 alpha expression in cultured hepatoma cells was decreased approximately 50% after 12 hours' exposure to 100 micromol/L chenodeoxycholic acid (CDCA). Characterization of the human HNF1 alpha gene promoter identified a consensus bile acid response element that binds and is activated by HNF4 alpha. Mutagenesis of the HNF4 alpha site abolished baseline HNF1 alpha promoter activity. The central mechanism by which bile acids repress HNF1 alpha is decreased activation by HNF4 alpha. SHP directly inhibits HNF4 alpha-mediated transactivation of the HNF1 alpha promoter in cotransfection assays. In addition, HNF4 alpha nuclear binding activity is decreased by CDCA and the human HNF4 alpha gene promoter is repressed by CDCA through an SHP-independent mechanism. In conclusion, we show that repression of HNF1 alpha is an important new mechanism by which bile acids regulate the expression of HNF1 alpha-dependent genes in man. This explains the suppressive effect of bile acids on the OATP-C gene promoter, leading to decreased expression in cholestatic liver disease.

Regulation of P450 genes by liver-enriched transcription factors and nuclear receptors

Cytochrome P450s (P450s) constitute a superfamily of heme-proteins that play an important role in the activation of chemical carcinogens, detoxification of numerous xenobiotics as well as in the oxidative metabolism of endogenous compounds such as steroids, fatty acids, prostaglandins, and leukotrienes. In addition, some P450s have important roles in physiological processes, such as steroidogenesis and the maintenance of bile acid and cholesterol homeostasis. Given their importance, the molecular mechanisms of P450 gene regulation have been intensely studied. Direct interactions between transcription factors, including nuclear receptors, with the promoters of P450 genes represent one of the primary means by which the expression of these genes is controlled. In this review, several liver-enriched transcription factors that play a role in the tissue-specific, developmental, and temporal regulation of P450s are discussed. In addition, the nuclear receptors that play a role in the fine control of cholesterol and bile acid homeostasis, in part, through their modulation of specific P450s, are discussed.

Selective inhibition of CYP27A1 and of chenodeoxycholic acid synthesis in cholestatic hamster liver

The aim of this study was to explore the regulation of serum cholic acid (CA)/chenodeoxycholic acid (CDCA) ratio in cholestatic hamster induced by ligation of the common bile duct for 48 h. The serum concentration of total bile acids and CA/CDCA ratio were significantly elevated, and the serum proportion of unconjugated bile acids to total bile acids was reduced in the cholestatic hamster similar to that in patients with obstructive jaundice. The hepatic CA/CDCA ratio increased from 3.6 to 11.0 (P<0.05) along with a 2.9-fold elevation in CA concentration (P<0.05) while the CDCA level remained unchanged. The hepatic mRNA and protein level as well as microsomal activity of the cholesterol 7alpha-hydroxylase, 7alpha-hydroxy-4-cholesten-3-one 12alpha-hydroxylase and 5beta-cholestane-3alpha,7alpha,12alpha-triol 25-hydroxylase were not significantly affected in cholestatic hamsters. In contrast, the mitochondrial activity and enzyme mass of the sterol 27-hydroxylase were significantly reduced, while its mRNA levels remained normal in bile duct-ligated hamster. In conclusion, bile acid biosynthetic pathway via mitochondrial sterol 27-hydroxylase was preferentially inhibited in bile duct-ligated hamsters. The suppression of CYP27A1 is, at least in part, responsible for the relative decreased production of CDCA and increased CA/CDCA ratio in the liver, bile and serum of cholestatic hamsters.

Differences in the regulation of the classical and the alternative pathway for bile acid synthesis in human liver. No coordinate regulation of CYP7A1 and CYP27A1

It has been reported that there is a coordinate regulation of sterol 27-hydroxylase (CYP27A1) and cholesterol 7alpha-hydroxylase (CYP7A1) in rats. Thus, the levels of the mRNA corresponding to these two enzymes were found to change in the same direction in rat liver and in isolated rat hepatocytes. In contrast, other groups have not seen such regulation of CYP27A1 in rabbit liver or in rat liver when using an activity assay. In the present work, the effect of bile acid treatment on human CYP27A1/luciferase reporter activity was studied in a transient transfection assay in human liver-derived HepG2 cells. Neither the endogenous 27-hydroxylase activity nor the CYP27A1/luciferase reporter activity were down-regulated by treatment of HepG2 cells with chenodeoxycholic acid or taurochenodeoxycholic acid. We also measured CYP27A1 mRNA and CYP7A1 mRNA in liver of humans subjected to treatment with chenodeoxycholic acid, ursodeoxycholic acid, hydroxymethylglutaryl (HMG)-CoA reductase inhibitor and a combination of HMG-CoA reductase inhibitor and cholestyramine. There was a 60-fold variation in the levels of CYP7A1 mRNA but only a 5-fold variation in the levels of CYP27A1 mRNA. There was no correlation between the two mRNA species. It is concluded that, in humans, there is little or no coordinate regulation of CYP7A1 and CYP27A1 at the transcriptional level, and that CYP27A1 is not subject to a negative feedback control by bile acids. The results underline that marked species differences may exist in mechanisms for control of synthesis of bile acids and cholesterol homeostasis.

Oxysterols: friends, foes, or just fellow passengers?

Oxysterols are oxygenated derivatives of cholesterol that are intermediates or even end products in cholesterol excretion pathways. Because of their ability to pass cell membranes and the blood-brain barrier at a faster rate than cholesterol itself, they are also important as transport forms of cholesterol. In addition, oxysterols have been ascribed a number of important roles in connection with cholesterol turnover, atherosclerosis, apoptosis, necrosis, inflammation, immunosuppression, and the development of gallstones. According to current concepts, oxysterols are physiological mediators in connection with a number of cholesterol-induced metabolic effects. However, most of the evidence for this is still indirect, and there is a discrepancy between the documented potent effects of oxysterols under in vitro conditions and the studies demonstrating that they are of physiological importance in vivo. Oxysterol-binding proteins, such as liver X receptor-alpha (a nuclear receptor), do have a regulatory role in cholesterol turnover, but the physiological ligand of the protein has not yet been defined with certainty. Recently developed genetically engineered mouse models with markedly reduced or increased concentration of some of the oxysterols have exhibited surprisingly small changes in cholesterol turnover and homeostasis. The present review is a critical evaluation of the literature on oxysterols, in particular, the in vivo evidence for a role of oxysterols as physiological regulators of cholesterol homeostasis and as atherogenic factors.

Changes in the pool of bile acids in hepatocyte nuclei during rat liver regeneration

BACKGROUND/AIMS

To investigate changes in nuclear bile acids (BAs) during rat liver regeneration.

METHODS

Nuclei were isolated from control rat livers and after two-thirds partial hepatectomy (PH). BAs in bile, liver homogenate and nuclei were measured by gas chromatography-mass spectrometry. Nuclear translocation of radiolabeled BAs was determined using fresh isolated hepatocytes from control donors.

RESULTS

Liver BA concentrations were transiently reduced after PH. Relative increases in: -MCA at 1 day, deoxycholic acid at 7 days and cholic acid (CA) at 3 and 14 days were found. Nuclear BAs accounted for <0.5% of liver BAs. Contamination with cytosolic BAs during nuclei isolation was <4%. Unconjugated- and conjugated-CA were able to reach the nucleus with similar efficiency. The pattern of nuclear BAs--CA (80%) and ursodeoxycholic acid (UDCA) (8.5%) being the most abundant--did not match that found in liver or bile. A transient decrease in CA/UDCA ratio, in absence of significant change in total BA content, was observed in nuclei after PH. "Flat" BA species were only detected in homogenate, but not in nuclei, at 1 day after PH.

CONCLUSIONS

BA pool in nuclei of rat hepatocytes, whose composition is different to that of total liver BA pool, undergoes important changes during liver regeneration.

Functional analysis of the promoter of human sterol 27-hydroxylase gene in HepG2 cells

Human sterol 27-hydroxylase catalyses the first step in the alternative pathway of bile acids biosynthesis in hepatocytes. However the gene encoding this enzyme (CYP27 gene) is expressed in every tissue and some evidence suggests that this enzyme plays a role in cholesterol homeostasis. Although modulation of CYP27 expression has been reported, the mechanisms underlying the regulation of this gene in human tissues is still poorly understood. To elucidate the mechanism governing CYP27 expression we cloned a 4.3 kb fragment of the 5' flanking region of the human CYP27 gene and constructed deletion mutants which were transfected into HepG2 cells. Functional assays showed that the -217/-10 nucleotide region from the translation start site (minimal promoter), devoid of TATA and CAAT boxes, contains all the elements for basal transcription. Foot-printing analysis of minimal promoter showed four protected regions (A-D). Regions A, B and D each contain one Sp1 binding site, and region C contains a HNF4 site. Electrophoretic mobility shift assays demonstrated that Sp1, Sp3 and HNF4 transcription factors bind these sites. Mutagenesis of any of these sites resulted in the loss of promoter activity. Co-transfection of the minimal promoter with Sp1 and Sp3 expression vectors transactivated CYP27 gene promoter in Drosophila SL2 cells, which lack endogenous Sp proteins. Transactivation of the minimal promoter was also observed in HeLa cells co-transfected with HNF4 expression vector. Therefore, Sp1, Sp3 and HNF4 co-operate in the expression of the human CYP27 gene in HepG2 cells.

In vivo and in vitro regulation of sterol 27-hydroxylase in the liver during the acute phase response. potential role of hepatocyte nuclear factor-1

The host response to infection is associated with several alterations in lipid metabolism that promote lipoprotein production. These changes can be reproduced by lipopolysaccharide (LPS) administration. LPS stimulates hepatic cholesterol synthesis and suppresses the conversion of cholesterol to bile acids. LPS down-regulates hepatic cholesterol 7alpha-hydroxylase, the rate-limiting enzyme in the classic pathway of bile acid synthesis. We now demonstrate that LPS markedly decreases the activity of sterol 27-hydroxylase, the rate-limiting enzyme in the alternate pathway of bile acid synthesis, in the liver of Syrian hamsters. Moreover, LPS progressively decreases hepatic sterol 27-hydroxylase mRNA levels by 75% compared with controls over a 24-h treatment period. LPS also decreases oxysterol 7alpha-hydroxylase mRNA levels in mouse liver. In vitro studies in HepG2 cells demonstrate that tumor necrosis factor and interleukin (IL)-1 decrease sterol 27-hydroxylase mRNA levels by 48 and 80%, respectively, whereas IL-6 has no such effect. The IL-1-induced decrease in sterol 27-hydroxylase mRNA expression occurs early, is sustained for 48 h, and requires very low doses. In vivo IL-1 treatment also lowers hepatic sterol 27-hydroxylase mRNA levels in Syrian hamsters. Studies investigating the molecular mechanisms of LPS-induced decrease in sterol 27-hydroxylase show that LPS markedly decreases mRNA and protein levels of hepatocyte nuclear factor-1 (HNF-1), a transcription factor that regulates sterol 27-hydroxylase, in the liver. Moreover, LPS decreases the binding activity of HNF-1 by 70% in nuclear extracts in hamster liver, suggesting that LPS may down-regulate sterol 27-hydroxylase by decreasing the binding of HNF-1 to its promoter. Coupled with our earlier studies on cholesterol 7alpha-hydroxylase, these data indicate that LPS suppresses both the classic and alternate pathways of bile acid synthesis. A decrease in bile acid synthesis in liver would reduce cholesterol catabolism and thereby contribute to the increase in hepatic lipoprotein production that is induced by LPS and cytokines.

Transcriptional regulation of the human sterol 27-hydroxylase gene (CYP27) and promoter mapping

Recent evidence suggests that sterol 27-hydroxylase may play a role in cholesterol homeostasis and affect atherogenesis. The major objective of the study was to map and characterize the sterol 27-hydroxylase (CYP27) promoter region. Here we show that CYP27 gene has a TATA-less promoter and transcription initiates at a cluster of sites. The basic promoter is located between -166 and -187 bp from the translation initiation site. Possible positive transcription regulation sites are located at position -187 to -320 and -857 to -1087 bp. A negative transcription regulator site is located in position -320 to -413 bp. An enhancer sequence is located upstream to position -1087. CYP27 is upregulated by dexamethasone and downregulated by cyclosporin A and cholic acid. The dexamethasone responsive element is located between 1087 and 678 bp upstream to the putative ATG. Cyclosporin A affects bile acid metabolism by repressing CYP27 at the transcriptional level. The cyclosporin A- responsive element is mapped to between 1087 and 4000 bp upstream of the ATG. Cholic acid represses sterol 27-hydroxylase mRNA level by affecting the stability of its mRNA. The results obtained here imply that CYP27 has a potentially important role in cholesterol homeostasis in human cells, and is regulated by several substances that were previously shown to affect bile acid metabolism.

Physical interaction and functional synergy between glucocorticoid receptor and Ets2 proteins for transcription activation of the rat cytochrome P-450c27 promoter

We demonstrate that dexamethasone-mediated transcription activation of the cytochrome P-450c27 promoter involves a physical interaction and functional synergy between glucocorticoid receptor (GR) and Ets2 factor. Ets2 protein binding to a "weak" Ets-like site of the promoter is dependent on GR bound to the adjacent cryptic glucocorticoid response element. Coimmunoprecipitation and chemical cross-linking experiments show physical interaction between GR and Ets2 proteins. Mutational analyses show synergistic effects of Ets2 and GR in dexamethasone-mediated activation of the cytochrome P-450c27 promoter. The DNA-binding domain of GR, lacking the transcription activation and ligand-binding domains, was fully active in synergistic activation of the promoter with intact Ets2. The DNA-binding domain of Ets2 lacking the transcription activation domain showed a dominant negative effect on the transcription activity. Finally, a fusion protein consisting of the GR DNA-binding domain and the transcription activation domain of Ets2 fully supported the transcription activity, suggesting a novel synergy between the two proteins, which does not require the transactivation domain of GR. Our results also provide new insights on the role of putative weak consensus Ets sites in transcription activation, possibly through synergistic interaction with other gene-specific transcription activators.

Transport of bile acids in hepatic and non-hepatic tissues

Bile acids are steroidal amphipathic molecules derived from the catabolism of cholesterol. They modulate bile flow and lipid secretion, are essential for the absorption of dietary fats and vitamins, and have been implicated in the regulation of all the key enzymes involved in cholesterol homeostasis. Bile acids recirculate through the liver, bile ducts, small intestine and portal vein to form an enterohepatic circuit. They exist as anions at physiological pH and, consequently, require a carrier for transport across the membranes of the enterohepatic tissues. Individual bile acid carriers have now been cloned from several species. Na(+)-dependent transporters that mediate uptake into hepatocytes and reabsorption from the intestine and biliary epithelium and an ATP-dependent transporter that pumps bile acids into bile comprise the classes of transporter that are specific for bile acids. In addition, at least four human and five rat genes that code for Na(+)-independent organic anion carriers with broad multi-substrate specificities that include bile acids have been discovered. Studies concerning the regulation of these carriers have permitted identification of molecular signals that dictate eventual changes in the uptake or excretion of bile acids, which in turn have profound physiological implications. This overview summarizes and compares all known bile acid transporters and highlights findings that have identified diseases linked to molecular defects in these carriers. Recent advances that have fostered a more complete appreciation for the elaborate disposition of bile acids in humans are emphasized.

Genes involved in initial steps of bile acid synthesis

The mechanism and regulation of the degradation of cholesterol into bile acids has attracted increased interest, in particular after the recent discovery that nuclear receptors (farnesoid X receptor and liver X receptor) are involved in the regulation of bile acid synthesis. Recently, it has also been shown that the biosynthesis of bile acids is not exclusively restricted to the liver, and that degradation may start by a hydroxylation of cholesterol in the brain or in other extrahepatic organs. During the past 2 years the genes coding for three of the six enzymes catalysing the first steps in bile acid biosynthesis have been cloned and characterized. These genes and their gene products will be described here.

Role of FXR and FTF in bile acid-mediated suppression of cholesterol 7alpha-hydroxylase transcription

Bile acid biosynthesis is subjected to feedback regulation whereby bile acids down-regulate their own synthesis. The major point of this regulation is at the level of cholesterol 7alpha-hydroxylase (7alpha-hydroxylase), which controls bile acid output from the classic pathway. This regulation is at the level of transcription of the gene. Two bile acid response elements have been localized within the 5'-flanking region of the rat gene and these elements overlap three nuclear receptor binding sites for hepatocyte nuclear factor (HNF-4), liver X receptor (LXR) and alpha(1)-fetoprotein transcription factor (FTF). Recently it has been shown that bile acids are physiological ligands for the farnesyl X receptor (FXR), which suggested that FXR could function by binding to one of the three nuclear receptor sites to mediate regulation of 7alpha-hydroxylase transcription by bile acids. In this study we show that FXR is indeed a crucial factor for bile acid-mediated regulation, but that it functions without binding to DNA. Furthermore, we also demonstrate that neither the LXR nor the HNF-4 sites are involved in bile acid-mediated regulation of 7alpha-hydroxylase transcription. Most importantly, we show that the FTF site is essential for regulation of 7alpha-hydroxylase by bile acids, similar to what we have recently demonstrated for another gene of the bile acid biosynthetic pathway, the sterol 12alpha-hydroxylase gene. These studies demonstrate the crucial role of FTF in the expression and regulation of a critical gene in the bile acid biosynthetic pathways.

Targeted disruption of the nuclear receptor FXR/BAR impairs bile acid and lipid homeostasis

Mice lacking the nuclear bile acid receptor FXR/BAR developed normally and were outwardly identical to wild-type littermates. FXR/BAR null mice were distinguished from wild-type mice by elevated serum bile acid, cholesterol, and triglycerides, increased hepatic cholesterol and triglycerides, and a proatherogenic serum lipoprotein profile. FXR/BAR null mice also had reduced bile acid pools and reduced fecal bile acid excretion due to decreased expression of the major hepatic canalicular bile acid transport protein. Bile acid repression and induction of cholesterol 7alpha-hydroxylase and the ileal bile acid binding protein, respectively, did not occur in FXR/BAR null mice, establishing the regulatory role of FXR/BAR for the expression of these genes in vivo. These data demonstrate that FXR/BAR is critical for bile acid and lipid homeostasis by virtue of its role as an intracellular bile acid sensor.

Alpha 1-fetoprotein transcription factor is required for the expression of sterol 12alpha -hydroxylase, the specific enzyme for cholic acid synthesis. Potential role in the bile acid-mediated regulation of gene transcription

Cholesterol conversion to bile acids occurs via the "classic" (neutral) or the "alternative" (acidic) bile acid biosynthesis pathways. Sterol 12alpha-hydroxylase/CYP8b1 is the specific enzyme required for cholic acid synthesis. The levels of this enzyme determine the ratio of cholic acid to chenodeoxycholic acid and thus the hydrophobicity of the circulating bile acid pool. Expression of the 12alpha-hydroxylase gene is tightly down-regulated by hydrophobic bile acids. In this study, we report the characterization of two DNA elements that are required for both the 12alpha-hydroxylase promoter activity and bile acid-mediated regulation. Mutation of these elements suppresses 12alpha-hydroxylase promoter activity. Mutations of any other part of the promoter do not alter substantially the promoter activity or alter regulation by bile acids relative to the wild type promoter. These two DNA elements bind alpha(1)-fetoprotein transcription factor (FTF), a member of the nuclear receptor family. We also show that overexpression of FTF in a non-liver cell line activates the sterol 12alpha-hydroxylase promoter. These studies demonstrate the crucial role of FTF for the expression and regulation of a critical gene in the bile acid biosynthetic pathways.

Regulation of bile acid biosynthesis

This article provides a review of the pathways through which cholesterol is degraded to bile acids. Regulation of key enzymes in the bile acid biosynthestic pathways is discussed. The important role of these pathways in the maintenance of cholesterol homeostasis and the possible therapeutic implications for the treatment of hypercholesterolemia are emphasized.