Increasing dietary cholesterol induces different regulation of classic and alternative bile acid synthesis

Metabolic reprogramming of the intestinal microbiome with functional bile acid changes underlie the development of NAFLD

BACKGROUND AND AIMS

Bile acids are hepatic metabolites and have many properties considered to be relevant to the pathophysiology of NAFLD. Circulating levels of the intestinal microbiome-modified bile acid deoxycholate are increased in cirrhosis.

APPROACH AND RESULTS

To further elucidate the role of bile acids and intestinal microbiota linked to bile acids in progressively severe NAFLD, a multiomic study of feces including 16S rRNA sequencing, microbial transcriptomics and metabolomics was performed in a cohort with varying phenotypes of NAFLD. Several bile acids of microbial origin derived from deoxycholic acid (DCA) (glycodeoxycholate, 7-ketodeoxycholic acid, dehydrocholic acid) increased with disease activity and fibrosis stage. These were linked to increased expression of microbial bile salt hydrolase, bile acid operon (BaiCD) and hydroxysteroid dehydrogenases (hdhA) required for DCA and downstream metabolite synthesis providing a mechanistic basis for altered bile acid profiles with disease progression. Bacteroidetes and several genera of Lachnospiraceae family containing DCA generating genes increased with increasing disease severity, whereas several potentially beneficial microbes sensitive to antibacterial effects of DCA e.g., Ruminococcaceae were decreased. The clinical relevance of these data was confirmed in an independent cohort enrolled in a clinical trial for NASH where at entry DCA and its conjugates were associated with advanced fibrosis. In patients treated with placebo, DCA declined in those with fibrosis regression and increased in those with fibrosis progression. DCA rose further in those with compensated cirrhosis when they experienced decompensation.

CONCLUSIONS

These findings demonstrate a role for bile acids and the bile acid dependent microbiome in the development and progression of NAFLD and set the stage to leverage these findings for NASH biomarker development and for therapeutics.

Circulating Bile Acid Profiles: A Need for Further Examination

CONTEXT

Bile acids (BAs) are increasingly recognized as metabolic and chronobiologic integrators that synchronize the systemic metabolic response to nutrient availability. Alterations in the concentration and/or composition of circulating BAs are associated with a number of metabolic disorders, such as obesity, type 2 diabetes mellitus (T2DM), insulin resistance (IR), and metabolic associated fatty liver disease (MAFLD). This review summarizes recent evidence that links abnormal circulating BA profiles to multiple metabolic disorders, and discusses the possible mechanisms underlying the connections to determine the role of BA profiling as a novel biomarker for these abnormalities.

EVIDENCE ACQUISITION

The review is based on a collection of primary and review literature gathered from a PubMed search of BAs, T2DM, IR, and MAFLD, among other keywords.

EVIDENCE SYNTHESIS

Obese and IR subjects appear to have elevated fasting circulating BAs but lower postprandial increase when compared with controls. The possible underlying mechanisms are disruption in the synchronization between the feeding/fasting cycle and the properties of BA-regulated metabolic pathways. Whether BA alterations are associated per se with MAFLD remains inconclusive. However, increased fasting circulating BAs level was associated with higher risk of advanced fibrosis stage. Thus, for patients with MAFLD, dynamically monitoring the circulating BA profiles may be a promising tool for the stratification of MAFLD.

CONCLUSIONS

Alterations in the concentration, composition, and rhythm of circulating BAs are associated with adverse events in systemic metabolism. Subsequent investigations regarding these aspects of circulating BA kinetics may help predict future metabolic disorders and guide therapeutic interventions.

Metabolic characteristics of plasma bile acids in patients with intrahepatic cholestasis of pregnancy-mass spectrometric study

INTRODUCTION

Intrahepatic cholestasis of pregnancy (ICP) is one of the more common complications in the middle and late stages of pregnancy, which requires early detection and intervention.

OBJECTIVE

The aim of the study is to investigate the changes in the metabolic profile of bile acids (BAs) in plasma of pregnant women with ICP and to look biomarkers for the diagnosis and grading of ICP, and to explore the disease mechanism.

METHODS

The targeted metabolomics based on high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) was used to analyze plasma BAs.

RESULTS

Twenty-seven BAs can be quantified in all participants. Among them, 22 BAs were identified as differential BAs between ICP and control groups. Five BAs include 3β-CA, 3β-DCA, CDCA-3Gln, NCA, and Tβ-MCA, were found to be associated with ICP for the first time. Nine BAs include NCA, GCA, GCDCA, GHCA, GUDCA, HCA, TCA, TCDCA and THCA, can be used as possible ICP diagnostic biomarkers. Four BAs, i.e., GLCA, THCA, GHCA and TLCA-3S may be used as potential biomarkers for ICP grading.

CONCLUSION

There were significant differences in plasma BA profiles between ICP patients and the control. The BA profiles of mild ICP group and severe ICP group partially overlapped. Potential diagnostic and grading BA markers were identified. A significant characteristic of ICP group was the increase of conjugated BAs. A mechanism to sustain the equilibrium of BA metabolism and adaptive response has been developed in ICP patients to accelerate excretion and detoxification.

Cholestyramine alters bile acid amounts and the expression of cholesterol-related genes in rabbit intestinal and hepatic tissues

OBJECTIVE

Cholestyramine (CHO), as a bile acid sequestering exchange resin, has been widely used to treat hypercholesterolemia. The aim of this study was to explore how CHO regulated serum cholesterol amounts and bile acid levels in animal models.

METHODS

New Zealand White rabbits were randomly assigned to the control (given distilled water) and CHO-treated (given CHO solution 1 g/kg per day for 2 weeks) groups. To assess bile acid pool size, bile fistulas were constructed in five rabbits in each group. Serum cholesterol levels and biliary and fecal bile outputs were determined. Liver cholesterol 7α-hydroxylase ( CYP7A1 ), small heterodimer partner ( SHP ), bile salt export pump ( BSEP ), ileal bile acid-binding protein ( IBABP ) and LDL receptor ( LDL-R ) mRNA expressions were assessed by real-time polymerase chain reaction. CYP7A1 activity was also determined.

RESULTS

CHO treatment decreased serum cholesterol levels by 12.1%. Although CHO did not change the bile acid pool size and biliary bile acid output, it significantly increased fecal bile acid output. Interestingly, CHO also significantly increased the expression and activity of CYP7A1, as well as IBABP and LDL-R mRNA expressions, but decreased hepatic SHP and BSEP gene expressions.

CONCLUSION

CHO markedly alters bile acid and cholesterol amounts in rabbit intestinal and liver tissues, downregulating genes responsible for cholesterol homeostasis.

Pleiotropic effects of antitumour alkylphospholipids on cholesterol transport and metabolism

BACKGROUND

Alkylphospholipid (APL) analogs are a new class of membrane-directed synthetic compounds with a variety of biological actions and clinical applications. In particular, these agents are promising candidates in cancer treatment. We have demonstrated that after prolonged treatment APLs alter intracellular cholesterol traffic and metabolism in human tumor-cell lines, leading to an accumulation of cholesterol inside the cell. After further investigation concerning the mode of action of APLs, we have explored the influence of several APLs on novel aspects of cholesterol and lipoprotein homeostasis using hepatoma HepG2 cells and THP1-derived macrophages.

METHODS

Quantitative real-time PCR analysis with a pathway-focused PCR array system was performed to measure relative changes in the mRNA expression of a number of genes related to cholesterol transport and metabolism. We compared the gene-expression profiles of HepG2 cells treated with miltefosine, edelfosine or perifosine for 6h and 24h with the profile of control cells. We also analysed particular genes of interest in both HepG2 and macrophage-like THP1 cells using specific PCR assays. Immunoblots were used to confirm protein-expression changes. Measurement of ABCA1-mediated cholesterol efflux was determined using apoA1 as cholesterol acceptor.

RESULTS

We found global changes in gene-expression patterns to maintain cholesterol homeostasis after exposure of cells to APLs. The pathways for cholesterol biosynthesis and LDL-cholesterol uptake were both transcriptionally upregulated by the three APLs assayed. Conversely, major pathways involved in the catabolism of cholesterol to bile acids and lipoprotein-associated cholesterol export were impaired after APL incubation, which may well contribute to the higher cell-cholesterol levels induced by these compounds.

CONCLUSION

Incubation of cells with different APLs stimulated cholesterol biosynthesis and uptake at the same time as it depressed common pathways for excess cholesterol removal in tumor cells, ultimately leading to altered cholesterol homeostasis.

A Diet-Sensitive BAF60a-Mediated Pathway Links Hepatic Bile Acid Metabolism to Cholesterol Absorption and Atherosclerosis

Dietary nutrients interact with gene networks to orchestrate adaptive responses during metabolic stress. Here, we identify Baf60a as a diet-sensitive subunit of the SWI/SNF chromatin-remodeling complexes in the mouse liver that links the consumption of fat- and cholesterol-rich diet to elevated plasma cholesterol levels. Baf60a expression was elevated in the liver following feeding with a western diet. Hepatocyte-specific inactivation of Baf60a reduced bile acid production and cholesterol absorption, and attenuated diet-induced hypercholesterolemia and atherosclerosis in mice. Baf60a stimulates expression of genes involved in bile acid synthesis, modification, and transport through a CAR/Baf60a feedforward regulatory loop. Baf60a is required for the recruitment of the SWI/SNF chromatin-remodeling complexes to facilitate an activating epigenetic switch on target genes. These studies elucidate a regulatory pathway that mediates the hyperlipidemic and atherogenic effects of western diet consumption.

Soya protein stimulates bile acid excretion by the liver and intestine through direct and indirect pathways influenced by the presence of dietary cholesterol

Several studies using different animal models have demonstrated that the consumption of soya protein (SP) reduces serum cholesterol concentrations by increasing the excretion of bile acids (BA). However, the mechanism by which SP enhances BA excretion is not fully understood. Therefore, the aim of the present study was to determine whether the consumption of SP regulates the expression of key enzymes involved in hepatic BA synthesis and the transporters involved in reverse cholesterol transport (RCT) via fibroblast growth factor 15 (FGF15) and/or small heterodimer protein (SHP) in rats. To achieve this aim, four groups of rats were fed experimental diets containing 20 % casein (C) or SP with or without the addition of 0·2 % cholesterol and the expression of hepatic genes involved in BA synthesis and the ileal and hepatic RCT was measured. Rats fed the SP diet had higher concentrations of ileal FGF15 and hepatic FGF15 receptor (FGFR4) and increased expression of SHP and liver receptor homolog 1 (LRH1) than those fed the C diet; as a result, the excretion of faecal BA was greater. The addition of cholesterol to the diet repressed the protein abundance of FGF15 and FGFR4; however, SP increased the expression of SHP and LRH1 to a lesser extent. Nonetheless, the expression of ABCG5/8 was increased in the intestine of rats fed the SP diet, and the effect was enhanced by the addition of cholesterol to the diet. In conclusion, SP in the presence of cholesterol increases BA synthesis via the repressions of FGF15 and SHP and accelerates BA excretion to prevent cholesterol overload in the enterocytes by increasing RCT.

Dysregulated bile acid synthesis, metabolism and excretion in a high fat-cholesterol diet-induced fibrotic steatohepatitis in rats

BACKGROUND AND AIMS

Cholesterol over-intake is involved in the onset of nonalcoholic steatohepatitis (NASH), and hepatocellular bile acid (BA) accumulation correlates with liver injuries. However, how dietary cholesterol influences cholesterol and BA kinetics in NASH liver remains ambiguous and needs to be clarified.

METHODS

Molecular markers involved in cholesterol and BA kinetics were investigated at protein and mRNA levels in an already-established stroke-prone spontaneously hypertensive 5/Dmcr rat model with fibrotic steatohepatitis, by feeding a high fat-cholesterol (HFC) diet.

RESULTS

Unlike the control diet, the HFC diet deposited cholesterol greatly in rat livers, where 3-hydroxy-3-methylglutaryl CoA reductase, low-density lipoprotein (LDL) receptor and LDL receptor-related protein-1 were expectedly downregulated, especially at 8 and 14 weeks, suggesting that cholesterol synthesis and uptake in response to cholesterol accumulation may not be disorganized. The HFC diet did not upregulate liver X receptor-α, conversely, it enhanced classic BA synthesis by upregulating cholesterol 7α-hydroxylase but downregulating sterol 12α-hydroxylase, and influenced alternative synthesis by downregulating sterol 27-hydroxylase but upregulating oxysterol 7α-hydroxylase, mainly at 8 and 14 weeks, indicating that there were different productions of primary BA species. Unexpectedly, no feedback inhibition of BA synthesis by farnesoid X receptor occurred. Additionally, the HFC diet impaired BA detoxification by UDP-glucuronosyltransferase and sulfotransferase 2A1, and decreased excretion by bile salt export pump at 8 and 14 weeks, although it induced compensatory export by multidrug resistance-associated protein-3. The disturbed BA detoxification may correlate with suppressed pregnane X receptor and constitutive androstane receptor.

CONCLUSIONS

The HFC diet may accumulate BA in rat livers, which influences fibrotic steatohepatitis progression.

Oxysterols in bile acid metabolism

Increasing body of evidence is available indicating that oxysterols are more much than intermediates of metabolic pathways. Oxysterols play a role in the regulation of cholesterol synthesis, transport and efflux. A scavenger effect of cholesterol 27-hydroxylase on elevated serum cholesterol levels is well demonstrated. Bile acid synthesis occurs through two main pathways, the classic and the alternative ones. Since plasma concentrations of 27-hydroxycholesterol were clearly shown to reflect its production rate the alternative pathway of bile acid synthesis can be easily explored. Conversely this was not true for 7α-hydroxycholesterol and also the direct evaluation of the classic pathway by kinetic studies is more difficult since the rate of plasma appearance during continuous infusion of deuterated isotopomers may not exactly measure its production rate. Hepatic cholesterol 7alpha-hydroxylase activity is absent during fetal life in humans and upregulates after birth. Both the classic and alternative pathways become mature after the age of 4 years. It has been clearly demonstrated that in patients with liver disease the classic pathway is impaired while the alternative one is preserved. Conversely, in obese patients, preliminary data suggest an increase of the production rate of 27-hydroxycholesterol, a possible mechanism to counteract the increase of atherosclerotic risk.

Exploitation of bile acid transport systems in prodrug design

The enterohepatic circulation of bile acids is one of the most efficient recycling routes in the human body. It is a complex process involving numerous transport proteins, which serve to transport bile acids from the small intestine into portal circulation, from the portal circulation into the hepatocyte, from the hepatocyte into the bile, and from the gall bladder to the small intestine. The tremendous transport capacity and organ specificity of enterohepatic circulation combined with versatile derivatization possibilities, rigid steroidal backbone, enantiomeric purity, availability, and low cost have made bile acids attractive tools in designing pharmacological hybrid molecules and prodrugs with the view of improving intestinal absorption, increasing the metabolic stability of pharmaceuticals, specifically targeting drugs to organs involved in enterohepatic circulation, as well as sustaining therapeutically reasonable systemic concentrations of active agents. This article briefly describes bile acid transport proteins involved in enterohepatic circulation, summarizes the key factors affecting on the transport by these proteins, and reviews the use of bile acids and their derivatives in designing prodrugs capable of exploiting the bile acid transport system.

Biliary lipids, cholesterol and bile synthesis: different adaptive mechanisms to dietary cholesterol in lean and obese subjects

BACKGROUND

Increased biliary cholesterol secretion together with elevated cholesterol synthesis may predispose obese subjects to cholesterol gallstone formation.

AIM

To investigate whether processing of dietary cholesterol is altered in obesity, we enrolled eight lean and seven obese subjects in a double-blind crossover study.

METHODS

Cholesterol consumption was 300 mg/day on low and 1300 mg/day on high cholesterol diet. After 3 weeks on either diet, hepatic bile was collected to determine biliary lipid secretion, and bile salt composition by high-performance liquid chromatography and cholesterol saturation index was calculated. Cholesterol synthesis was measured employing mass isotopomer distribution analysis. Bile acid synthesis via neutral and acidic pathway was assessed by serum levels of 7alpha-hydroxy-4-cholesten-3-one and 27-hydroxycholesterol.

RESULTS

Cholesterol synthesis was increased in obese compared with lean and feedback inhibited only in obese. On low cholesterol diet, cholesterol secretion was doubled in obese but bile acid composition and synthesis was similar between the two groups. After high cholesterol diet, cholesterol saturation index and bile secretion were unchanged. In contrast to obese, lean increased bile acid synthesis only via the acidic pathway.

CONCLUSIONS

Dietary cholesterol appears to preferentially induce bile acid synthesis via the acidic pathway in lean, whereas cholesterol synthesis was inhibited in obese. Thus, stable cholesterol saturation index may be achieved by different mechanisms.

Evaluation of 3-hydroxy-3-methylglutaryl-COA-reductase, cholesterol-7?-hydroxylase and acyl-COA:cholesterol acyltransferase activities: alternative chromatographic methods to separate metabolites

Alternative HPLC and solid-phase extraction column methods were developed to separate metabolites of enzymes involved in cholesterol metabolism in rabbit liver microsomes: hydroxyl-methylglutaryl-CoA reductase, cholesterol-7alpha-hydroxylase and acyl-CoA:cholesterol acyltransferase. A comparison method of thin-layer chromatography and solid-phase extraction column were assayed to separate substrate and metabolite of hydroxy-methylglutaryl-CoA reductase, whereas for cholesterol-7alpha-hydroxylase and acyl-CoA:cholesterol acyltransferase, this comparison was done between thin layer chromatography and HPLC. The results obtained by the new analytical chromatographic methods are not significantly different than those observed in literature. Moreover a larger percentage recovery was obtained for analysed metabolites. Our results demonstrate the reliability of these alternative chromatographic techniques and showed that they are valuable tools to precisely and rapidly measure the activity of those enzymes.

Inhibition of ileal bile acid transport lowers plasma cholesterol levels by inactivating hepatic farnesoid X receptor and stimulating cholesterol 7 alpha-hydroxylase

We investigated the effect of SC-435, a competitive inhibitor of ileal apical sodium-dependent bile acid cotransporter (ASBT) on ileal bile acid absorption and the hepatic nuclear receptor FXR (farnesoid X receptor), which regulates cholesterol 7 alpha-hydroxylase (CYP7A1) activity and mRNA levels. Eighteen New Zealand White (NZW) rabbits were divided into 2 groups: controls (n = 10) and fed SC-435 125 mg/kg/d for 1 week (n = 8). In rabbits treated with SC-435, fecal bile acid outputs increased by more than 8 times, reflecting substantial bile acid malabsorption. Plasma cholesterol levels decreased 26%, while bile acid pool sizes and biliary bile acid outputs did not change after treatment. CYP7A1 activity increased 64% and mRNA rose by 4 times after treatment. The expression of FXR target genes in the liver, short heterodimer partner (SHP) and bile salt export pump (BSEP), decreased 11.6 and 2.6 times, respectively, after treatment, which indicates inactivation of hepatic FXR. However, the mRNA levels of ileal bile acid binding protein (IBABP) did not change significantly, while ileal ASBT mRNA expression increased by 2.4 times after treatment. Rabbits treated with SC-435 developed ileal bile acid malabsorption, which decreased the return of bile acids (FXR ligands) to the liver to inactivate hepatic FXR, which upregulated CYP7A1 and lowered plasma cholesterol levels. Although fecal bile acid malabsorption was substantial, increased bile acid production from hepatic cholesterol kept biliary bile acid outputs intact. Thus, a new balance was reached in the liver, where increased bile acid synthesis compensated for diminished ileal bile acid absorption to maintain the circulating enterohepatic bile acid pool.

Dietary cholesterol stimulates CYP7A1 in rats because farnesoid X receptor is not activated

Cholesterol feeding upregulates CYP7A1 in rats but downregulates CYP7A1 in rabbits. To clarify the mechanism responsible for the upregulation of CYP7A1 in cholesterol-fed rats, the effects of dietary cholesterol (Ch) and cholic acid (CA) on the activation of the nuclear receptors, liver X-receptor (LXR-alpha) and farsenoid X-receptor (FXR), which positively and negatively regulate CYP7A1, were investigated in rats. Studies were carried out in four groups (n = 12/group) of male Sprague-Dawley rats fed regular chow (control), 2% Ch, 2% Ch + 1% CA, and 1% CA alone for 1 wk. Changes in mRNA expression of short heterodimer partner (SHP) and bile salt export pump (BSEP), target genes for FXR, were determined to indicate FXR activation, whereas the expression of ABCA1 and lipoprotein lipase (LPL), target genes for LXR-alpha, reflected activation. CYP7A1 mRNA and activity increased twofold and 70%, respectively, in rats fed Ch alone when the bile acid pool size was stable but decreased 43 and 49%, respectively, after CA was added to the Ch diet, which expanded the bile acid pool 3.4-fold. SHP and BSEP mRNA levels did not change after feeding Ch but increased 88 and 37% in rats fed Ch + CA. This indicated that FXR was activated by the expanded bile acid pool. When Ch or Ch + CA were fed, hepatic concentrations of oxysterols, ligands for LXR-alpha increased to activate LXR-alpha, as evidenced by increased mRNA levels of ABCA1 and LPL. Feeding CA alone enlarged the bile acid pool threefold and increased the expression of both SHP and BSEP. These results suggest that LXR-alpha was activated in rats fed both Ch or Ch + CA, whereas CYP7A1 mRNA and activity were induced only in Ch-fed rats where the bile acid pool was not enlarged such that FXR was not activated. In rats fed Ch + CA, the bile acid pool expanded, which activated FXR to offset the stimulatory effects of LXR-alpha on CYP7A1.

Cholesterol feeding of mice expressing cholesterol 7alpha-hydroxylase increases bile acid pool size despite decreased enzyme activity

Dietary cholesterol regulation of cholesterol 7alpha-hydroxylase (Cyp7a1), the rate-limiting enzyme in the classical pathway of bile acid synthesis, has been implicated in plasma cholesterol responsiveness. In the current study, the effects of 0.0% and 0.5% cholesterol diets were examined in Cyp7a1 knockout (KO), heterozygous Cyp7a1 KO (Het), and human Cyp7a1 transgenic mice on the mouse Cyp7a1 KO background (Tg+KO). We confirmed previous findings that dietary cholesterol increased mouse Cyp7a1 activity in Het mice but decreased human Cyp7a1 activity in Tg+KO mice. However, in both Het and Tg+KO mice, dietary cholesterol increased bile acid pool size (36% and 72%, respectively) and fecal bile acid excretion (2.2- and 3.6-fold, respectively). The expression of cholesterol 27-hydroxylase (Cyp27), the major enzyme of the alternative pathway of bile acid synthesis, was not significantly different in cholesterol-fed KO, Het, or Tg+KO mice. Furthermore, dietary cholesterol had comparable effects on total plasma cholesterol and non-high-density lipoprotein cholesterol in KO, Het, and Tg+KO mice. Thus, in Tg+KO mice, dietary cholesterol regulates bile acid pool size, fecal bile acid excretion, and plasma cholesterol independently of Cyp7a1 activity. These results challenge the notion that dietary cholesterol regulation of Cyp7a1 is a major determinant of plasma cholesterol responsiveness.

FXR-mediated down-regulation of CYP7A1 dominates LXRalpha in long-term cholesterol-fed NZW rabbits

We investigated how cholesterol feeding regulates cholesterol 7alpha-hydroxylase (CYP7A1) via the nuclear receptors farnesoid X receptor (FXR) and liver X receptor alpha (LXRalpha) in New Zealand white rabbits. After 1 day of 2% cholesterol feeding, when the bile acid pool size had not expanded, mRNA levels of the FXR target genes short-heterodimer partner (SHP) and sterol 12alpha-hydroxylase (CYP8B) were unchanged, indicating that FXR activation remained constant. In contrast, the mRNA levels of the LXRalpha target genes ATP binding cassette transporter A1 (ABCA1) and cholesteryl ester transfer protein (CETP) increased 5-fold and 2.3-fold, respectively, associated with significant increases in hepatic concentrations of oxysterols. Activity and mRNA levels of CYP7A1 increased 2.4 times and 2.2 times, respectively. After 10 days of cholesterol feeding, the bile acid pool size increased nearly 2-fold. SHP mRNA levels increased 4.1-fold while CYP8B declined 64%. ABCA1 mRNA rose 8-fold and CETP mRNA remained elevated. Activity and mRNA of CYP7A1 decreased 60% and 90%, respectively. Feeding cholesterol for 1 day did not enlarge the ligand pool size or change FXR activation, while LXRalpha was activated highly secondary to increased hepatic oxysterols. As a result, CYP7A1 was up-regulated. After 10 days of cholesterol feeding, the bile acid (FXR ligand) pool size increased, which activated FXR and inhibited CYP7A1 despite continued activation of LXRalpha. Thus, in rabbits, when FXR and LXRalpha are activated simultaneously, the inhibitory effect of FXR overrides the stimulatory effect of LXRalpha to suppress CYP7A1 mRNA expression.

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.

Impact of an enriched-cholesterol diet on enzymatic cholesterol metabolism during rabbit gestation

An appropriate cholesterol homeostasis is vital for the maintenance and the optimal fetal development. The cholesterol is essential for the synthesis of progesterone and 17beta-estradiol, hormones that actively participate to sustain gestation. However, the administration of 0.2% enriched cholesterol diet (ECD) during rabbit gestation significantly increased the cholesterol blood profile (total-cholesterol, LDL, HDL, esterified-cholesterol and free-cholesterol) of dams and offspring, and induced a reduction of the offspring weight of 15% as compared to the control group. Enzymes involved in cholesterol metabolism (ACAT, HMG-CoA-reductase and cholesterol-7alpha-hydroxylase) are greatly influenced by cholesterol profile. We hypothesized that the administration of an ECD during rabbit gestation modifies the activity of those enzymes. Female rabbits (pregnant or not) were fed with a standard diet or an ECD. At term, livers (dams and offspring) and placentas were collected and ACAT, HMG-CoA-reductase and cholesterol-7alpha-hydroxylase activities were assayed. Our results demonstrate that gestation induced a reduction of ACAT activity (48.9%) in dam's liver and, an augmentation of HMG-CoA-reductase activity (142.4%) whereas it has no effect on cholesterol-7alpha-hydroxylase activity. The administration of the ECD has no additive effect on ACAT, but significantly reduced the HMG-CoA-reductase activity and cholesterol-7alpha-hydroxylase activity as compared with the pregnant control group. In placentas the ECD supplementation has an influence for HMG-CoA-reductase activity, where a 43% increased in observed. Any ACAT activity was detected in placenta and the ECD has no influence on the cholesterol-7alpha-hydroxylase activity. Whereas their offspring's liver present a reduction of ACAT and HMG-CoA-reductase activity. Gestation associated with ECD reduces significantly the HMG-CoA-reductase activity, decreasing the cholesterol synthesis, but placenta seems to compensate this effect by increasing its HMG-CoA-reductase activity.

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.

Regulation of the farnesoid X receptor (FXR) by bile acid flux in rabbits

We investigated the roles of hydrophobic deoxycholic acid (DCA) and hydrophilic ursocholic acid (UCA) in the regulation of the orphan nuclear farnesoid X receptor (FXR) in vivo. Rabbits with bile fistula drainage (removal of the endogenous bile acid pool), rabbits with bile fistula drainage and replacement with either DCA or UCA, and intact rabbits fed 0.5% cholic acid (CA) (enlarged endogenous bile acid pool) were studied. After bile fistula drainage, cholesterol 7alpha-hydroxylase (CYP7A1) mRNA and activity levels increased, FXR-mediated transcription was decreased, and FXR mRNA and nuclear protein levels declined. Replacing the enterohepatic bile acid pool with DCA restored FXR mRNA and nuclear protein levels and activated FXR-mediated transcription as evidenced by the increased expression of its target genes, SHP and BSEP, and decreased CYP7A1 mRNA level and activity. Replacing the bile acid pool with UCA also restored FXR mRNA and nuclear protein levels but did not activate FXR-mediated transcription, because the SHP mRNA level and CYP7A1 mRNA level and activity were unchanged. Feeding CA to intact rabbits expanded the bile acid pool enriched with the FXR high affinity ligand, DCA. FXR-mediated transcription became activated as shown by increased SHP and BSEP mRNA levels and decreased CYP7A1 mRNA level and activity but did not change FXR mRNA or nuclear protein levels. Thus, both hydrophobic and hydrophilic bile acids are effective in maintaining FXR mRNA and nuclear protein levels. However, the activating ligand (DCA) in the enterohepatic flux is necessary for FXR-mediated transcriptional regulation, which leads to down-regulation of CYP7A1.

Analysis of the effect of intestinal resection on rat ileal bile Acid transporter expression and on bile Acid and cholesterol homeostasis

Ileal reclamation of bile salts is mediated in large part by an apical sodium-dependent bile acid transporter (ASBT) located in the terminal ileum. The following studies were performed to elucidate the adaptive response of ASBT to intestinal resection. Two separate series of intestinal resections were performed: 1) limited (25%) ileal and 2) massive (70%) intestinal resection. The boundaries of the resections were varied to examine differences in compensation when variable amounts of endogenous transporter activity were resected. Previously demonstrated supraphysiologic expression of ASBT, which was seen after proximal ileal resection, led to a contraction in the bile acid pool size and a paradoxical reduction in bile acid (cholesterol 7alpha-hydroxylase and sterol 27-hydroxylase) and cholesterol (hydroxymethylglutaryl coenzyme A reductase) biosynthetic enzyme activities. Massive intestinal resection resulted in ileal hypertrophy and an apparently maladaptive specific down-regulation in ASBT protein expression. In this model bile acid pool size correlated with the amount of residual ASBT-expressing terminal ileum. Cholesterol and bile acid biosynthetic enzyme activities were inversely related to bile acid pool size. Adaptive changes in ASBT expression and alterations in bile acid and cholesterol homeostasis are dependent on the type and location of intestinal resection.

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.

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.

Potency of select statin drugs in a new mouse model of hyperlipidemia and atherosclerosis

Poloxamer-407 (P-407) is a nonionic surfactant that induces atheroma formation in the aortas of C57BL/6 mice with long-term (14 weeks) administration. The objectives of the present study were to determine the mechanism(s) responsible for the induction of hypercholesterolemia as well as to determine whether this animal model may be of potential use in rank ordering the efficacy (lipid lowering) of various statin drugs. The effect of long-term (16 weeks) administration of P-407 on the catalytic activities of rate-limiting enzymes of cholesterol biosynthesis [HMG-CoA reductase (HMGR)] and catabolism [microsomal cholesterol 7alpha-hydroxylase (C7alphaH) and mitochondrial sterol 27 hydroxylase (S27H)] was assessed in C57BL/6 mice. Effects of P-407 on these enzymes were compared in mice fed an atheroma-inducing diet (high-cholesterol, supplemented with cholic acid) and animals maintained on a basal diet and injected with saline (controls) after 16 weeks. The mean value for the activities of C7alphaH in P-407-injected mice was 24.3+/-3.8 pmol min(-1) mg(-1) and was significantly (P<0.05) less than the mean value determined for sham-injected control animals (37.0+/-14.3 pmol min(-1) mg(-1)). In contrast, the mean values for the catalytic activities of S27H and HMGR did not change with P-407 administration. Neither C7alphaH nor S27H activity in mice fed the high-cholesterol diet differed from values for control animals, whereas the mean HMGR activity was drastically reduced (-94%, P<0.05). The hypercholesterolemic effect of P-407 is not due to altered cholesterol biosynthesis, but is mediated by reduced cholesterol catabolism due to decreased activity of the rate limiting enzyme (C7alphaH) in the classic bile acid synthetic pathway. Plasma triglyceride lowering resulting from the oral administration of equal doses of various statin drugs appeared, in general, to be positively correlated with their relative aqueous solubility and paralleled the efficacy of these agents to lower low-density-lipoprotein-associated cholesterol (LDL-C) in humans. The plasma triglyceride lowering effect of the five statin drugs tested produced the following rank order; pravastatin sodium (-44%)>atorvastatin calcium (-36%)>simvastatin (-33%)>lovastatin (-25%)>fluvastatin sodium (-19%). While reductions in plasma total cholesterol following administration of the statin drugs was not as profound as that observed with triglycerides, the relative rank order or trend was preserved. The percent reduction in plasma triglycerides in the present model appears to be a useful parameter with which to predict the relative reduction in plasma LDL-C expected for these agents in humans.

High-fat, high-cholesterol diet increases the incidence of gastritis in LDL receptor-negative mice

Transgenic and knockout mice are widely used as models for atherogenesis studies. While developing a Helicobacter infection model in LDL receptor-negative (LDLR(-/-)) mice, we noticed that mice fed a high-fat, high-cholesterol diet often contracted gastritis independent of infection. To further investigate this finding, we studied 27 male and 18 female LDLR(-/-) mice fed high-fat, 1% or 1.25% cholesterol diets for 3 to 4 months. The extent of atherosclerosis was morphometrically analyzed in the whole aorta, and the degree of gastric inflammation was scored histologically in hematoxylin-eosin-stained stomach sections. The autoantibody titers to epitopes of oxidized LDL were also measured. Mice fed high-fat, high-cholesterol diets had a significantly higher incidence of gastritis than mice fed normal chow, 62% versus 5%, respectively (P<0.0001). This effect was specific for LDLR(-/-) mice, because no difference in gastritis was found in wild-type mice fed either diet. Animals with gastritis showed slightly more atherosclerosis than animals without gastritis: 16.3+/-6.4% versus 12.8+/-3.4% in males and 9.4+/-3.5% versus 6.5+/-3.3% in females. Cholesterol-fed mice also had significantly higher IgG autoantibody titers against modified LDL than normal chow-fed animals, but no difference was seen between the gastritis and nongastritis groups. We conclude that the standard high-fat, high-cholesterol diet commonly used in many murine models to induce atherosclerosis increased the incidence of gastritis significantly in LDLR(-/-) mice.

Hamsters predisposed to sucrose-induced cholesterol gallstones (LPN strain) are more resistant to excess dietary cholesterol than hamsters that are not sensitive to cholelithiasis induction

We compared the effects of cholesterol feeding in male hamsters from two strains with different propensities to sucrose-induced cholelithiasis; Laboratoire de Physiologie de la Nutrition (LPN) hamsters are predisposed to developing biliary cholesterol gallstones, whereas Janvier (JAN) hamsters are not. When fed a basal control diet, LPN hamsters had a lower cholesterolemia (-21%, P = 0.01) than JAN hamsters, and a higher activity of 3-hydroxy-3-methyl glutaryl coenzyme A reductase in liver (+148%, P = 0.018) and intestine (+281%, P < 0.0001). After feeding the same diet enriched with 0.3% cholesterol for 5 wk, cholesterolemia increased more dramatically in JAN hamsters (+235%, P < 0.001) than in LPN hamsters (+108%, P < 0.001), as did the liver concentration of cholesterol, which reached 152.30 +/- 13.00 and 44.41 +/- 9.06 micromol/g, respectively. Only JAN hamsters displayed hepatomegaly, with an increased cholesterol saturation index of the gallbladder bile (+100%, P < 0.01), due to the cholesterol challenge. In liver, cholesterol feeding reduced cholesterol 7alpha-hydroxylase activity and mRNA level, and stimulated sterol 27-hydroxylase and oxysterol 7alpha-hydroxylase activities. Hepatic levels of LDL receptor decreased by approximately 60% in both strains, whereas HDL receptor scavenger class B type 1 (SR-BI) levels were unaffected by dietary cholesterol. The greater resistance of LPN hamsters to the hypercholesterolemic diet can be explained by a lower capacity to store cholesterol in the liver and greater efficiency in reducing the activity of 3-hydroxy-3-methyl glutaryl coenzyme A reductase in response to cholesterol feeding [from 11263 to 261 pmol/(min x organ) in LPN hamsters and from 4530 to 694 pmol/(min x organ) in JAN hamsters]. These results highlight the usefulness of this two-strain model, which offers some analogy with the inverse association between the predisposition to cholelithiasis and the risk of atherosclerosis in humans.

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.

Regulation of cytochrome P450 (CYP) genes by nuclear receptors

Members of the nuclear-receptor superfamily mediate crucial physiological functions by regulating the synthesis of their target genes. Nuclear receptors are usually activated by ligand binding. Cytochrome P450 (CYP) isoforms often catalyse both formation and degradation of these ligands. CYPs also metabolize many exogenous compounds, some of which may act as activators of nuclear receptors and disruptors of endocrine and cellular homoeostasis. This review summarizes recent findings that indicate that major classes of CYP genes are selectively regulated by certain ligand-activated nuclear receptors, thus creating tightly controlled networks.

Variable association between genetic variation in the CYP7 gene promoter and plasma lipoproteins in three Canadian populations

The promoter sequence variant -278A in the CYP7 gene, which encodes cholesterol 7-alpha hydroxylase, was previously reported to be associated with reduced plasma low density lipoprotein (LDL) cholesterol concentration. We tested for association of CYP7-278A with plasma lipoprotein traits in samples taken from three distinct Canadian populations: 594 Alberta Hutterites, 325 Ontario Oji-Cree and 190 Keewatin Inuit. The CYP7-278A allele frequencies in these three groups were 0.708, 0.466 and 0.490, respectively. The frequencies of CYP7-278A/A homozygotes were 0.481, 0.215 and 0.247, respectively. In the Hutterites, CYP7-278A was associated with reduced plasma HDL-cholesterol and apolipoprotein AI concentration. In the Oji-Cree, CYP7-278A was not significantly associated with any plasma lipoprotein trait. In the Inuit CYP7-278A was associated with elevated plasma total and LDL-cholesterol. There was no consistent relationship between the population mean plasma LDL-cholesterol concentration and the population CYP7-278A frequency. Our findings suggest that the common -278A promoter variant of CYP7 was inconsistently associated with variation in plasma LDL- and HDL-cholesterol in samples from three independent populations. The inconsistencies could be due to differences in genetic background or to unspecified environmental or genetic factors.

Assay of microsomal oxysterol 7alpha-hydroxylase activity in the hamster liver by a sensitive method: in vitro modulation by oxysterols

A method of assaying hepatic cytochrome P-450, oxysterol 7alpha-hydroxylase (CYP7B), was developed by combining the use of 25-[26,27-(3)H]hydroxycholesterol as a substrate and hydroxypropyl-beta-cyclodextrin as a substrate vehicle. When these assay conditions were tested, an undesirable transformation was observed of the reaction product, 7alpha,25-dihydroxycholesterol, into 3-oxo-7alpha,25-dihydroxy-4-cholesten by the activity of 3beta-hydroxy-Delta(5)-C(27) steroid oxydoreductase, a microsomal NAD(+) and NADP(+) dependent enzyme of bile acid metabolism. A great improvement was reached by using a continuous NADPH generating system which constantly re-transforms NADP(+) into NADPH, thus inhibiting this activity. This improved CYP7B assay, comparable to our previously described assay for cholesterol 7alpha-hydroxylase (CYP7A), allowed a 3-fold increase of the apparent enzyme activity. The possibility to simultaneously measure CYP7A and CYP7B activities on the same microsomal preparation was investigated. A marked decrease (-33%) in the CYP7B activity was noticed, while that of CYP7A remained unchanged. The CYP7B activity was observed to be inhibited by cholesterol (-30%) and also by the oxysterols 7alpha-hydroxycholesterol (-21%), 7beta-hydroxycholesterol (-25%) and epicoprostanol (-20%), and by cyclosporin A (-26%). It can be concluded that this sensible and easy to perform CYP7B assay allows to observe, at least in vitro, a modulation of the enzyme activity by oxysterols.

Disruption of the oxysterol 7alpha-hydroxylase gene in mice

Mice without oxysterol 7alpha-hydroxylase, an enzyme of the alternate bile acid synthesis pathway with a sexually dimorphic expression pattern, were constructed by the introduction of a null mutation at the Cyp7b1 locus. Animals heterozygous (Cyp7b1(+/-)) and homozygous (Cyp7b1(-/-)) for this mutation were grossly indistinguishable from wild-type mice. Plasma and tissue levels of 25- and 27-hydroxycholesterol, two oxysterol substrates of this enzyme with potent regulatory actions in cultured cells, were markedly elevated in Cyp7b1(-/-) knockout animals. Parameters of bile acid metabolism as well as plasma cholesterol and triglyceride levels in male and female Cyp7b1(-/-) mice were normal. The cholesterol contents of major tissues were not altered. In vivo sterol biosynthetic rates were unaffected in multiple tissues with the exception of the male kidney, which showed a approximately 40% decrease in de novo synthesis versus controls. We conclude that the major physiological role of the CYP7B1 oxysterol 7alpha-hydroxylase is to metabolize 25- and 27-hydroxycholesterol and that loss of this enzyme in the liver is compensated for by increases in the synthesis of bile acids by other pathways. A failure to catabolize oxysterols in the male kidney may lead to a decrease in de novo sterol synthesis.

Regulation of cytochrome P450 (CYP) genes by nuclear receptors

Members of the nuclear-receptor superfamily mediate crucial physiological functions by regulating the synthesis of their target genes. Nuclear receptors are usually activated by ligand binding. Cytochrome P450 (CYP) isoforms often catalyse both formation and degradation of these ligands. CYPs also metabolize many exogenous compounds, some of which may act as activators of nuclear receptors and disruptors of endocrine and cellular homoeostasis. This review summarizes recent findings that indicate that major classes of CYP genes are selectively regulated by certain ligand-activated nuclear receptors, thus creating tightly controlled networks.