Chapter 9 Mechanism of bile acid biosynthesis in mammalian liver

The enhanced replication of an S-intact PEDV during coinfection with an S1 NTD-del PEDV in piglets

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The replication of S-intact PEDV was enhanced during coinfection with an S1 NTD-del PEDV in neonatal pigs.

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The S1 NTD-del PEDV was unable to outcompete the S-intact PEDV during coinfection in piglets.

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Certain concentrations of mucin, bile and bile acids increased the replication of S-intact but not S1 NTD-del PEDV.

Porcine epidemic diarrhea virus (PEDV) variants having a large deletion in the N-terminal domain of the S1 subunit of spike (S) protein were designated as S1 NTD-del PEDVs. They replicate well in experimentally infected pigs. However, on farms they often co-infect pigs with the PEDV containing an intact S protein (S-intact PEDV). We aimed to characterize viral replication and pathogenesis in neonatal gnotobiotic pigs infected simultaneously with the two types of PEDV using two recombinant PEDVs: icPC22A and its S1 NTD-del form icPC22A-S1Δ197. Additionally, viral replication was compared in Vero and IPEC-DQ cells at the presence of bovine mucin (BM), porcine gastric mucin (PGM), swine bile and bile acids during inoculation. In the pigs coinfected with icPC22A and icPC22A-S1Δ197, icPC22A replicated to a higher peak titer than its infection of pigs without the presence of icPC22A-S1Δ197. The severity of diarrhea and intestinal atrophy were similar between icPC22A and the coinfection groups, but were significantly higher than icPC22A-S1Δ197 group. In Vero and IPEC-DQ cells, certain concentrations of BM, PGM, bile and bile acids increased significantly the infectivity of icPC22A but had no or negative effects on icPC22A-S1Δ197. These results indicated that the replication of the S-intact PEDV was enhanced during coinfection in piglets. This observation may be explained partially by the fact that mucin, bile and bile acids in gastrointestinal tract had facilitating effects on the infection of S-intact PEDV, but no/inhibition effects on S1 NTD-del PEDV.

Identification and Characterization of the Genes and Enzymes Belonging to the Bile Acid Catabolic Pathway in Pseudomonas

The study of the catabolic potential of microbial species isolated from different habitats has allowed the identification and characterization of bacteria able to assimilate bile acids and other steroids (e.g., testosterone and 4-androsten-3,17-dione). From soil samples, we have isolated several strains belonging to genus Pseudomonas that grow efficiently in chemical defined media containing some cyclopentane-perhydro-phenantrene derivatives as carbon sources. Genetic and biochemical studies performed with one of these bacteria (P. putida DOC21) allowed the identification of the genes and enzymes belonging to the 9,10-seco pathway, the route involved in the aerobic assimilation of steroids. In this manuscript, we describe the most relevant methods required for (1) isolation and characterization of these species; (2) determining the chromosomal location, nucleotide sequence, and functional analysis of the catabolic genes (or gene clusters) encoding the enzymes from this pathway; and (3) the tools employed to establish the role of some of the proteins that participate in this route.

Regulation of hepatic hydroxy methyl glutarate - CoA reductase for controlling hypercholesterolemia in rats

Hypercholesterolemia is a major risk factor upon developing cardiovascular diseases. This study is aiming to investigate the inhibition role of quercetin on hydroxy methyl glutarate - CoA reductase activity and its gene for attenuating hypercholesterolemia. The kinetic characteristics of HMG-CoA reductase activity were evaluated on extracellular rat liver microsomes. For studying the effect of quercetin by inducing hypercholesterolemia rats by Tyloxapol (i.v.). In addition, rats were treated with different doses of quercetin according to the inhibition constant of this inhibitor. Our results showed that in quercetin rats groups plasma cholesterol, triglycerides, LDL -cholesterol and total lipids levels and hepatic (TBARS) level were significantly decreased as compared with negative control. However, plasma HDL level, hepatic total thiol level, catalase activity and total protein level significantly increased groups as compared with negative control. In addition, HMG-CoA reductase activity was decreased in quercetin groups and this confirmed in gene expression that these groups caused downregulation for HMG-CoA reductase. However, LDL receptor (LDLr) gene expression was upregulated by quercetin. Moreover, histopathological examination of rat liver showed the ameliorative effect of quercetin on hypercholesterolemic effect of triton. In conclusion, quercetin may consider as a new saving candidate for the future development of hypocholesterolemia agents.

Blood lipid profiles and peripheral blood mononuclear cell cholesterol metabolism gene expression in patients with and without methotrexate treatment

BACKGROUND

Methotrexate (MTX) is the most commonly prescribed disease-modifying antirheumatic drug (DMARD) in rheumatoid arthritis. ATP-binding cassette transporter-A1 (ABCA1) and 27-Hydroxylase (HY27) are known antiatherogenic proteins that promote cellular cholesterol efflux. In THP-1 macrophages, MTX can promote the reversal of cholesterol transport, limit foam cell formation and also reverse COX-2 inhibitor-mediated downregulation of ABCA1. Despite its antiatherogenic potential in vitro, the impact of clinical use of low-dose MTX on cholesterol metabolism in humans is unknown. Objective of the study was to examine whether clinical MTX use is associated with altered blood lipids and/or ABCA1/HY27 expressions.

METHODS

In all, 100 rheumatoid arthritis subjects were recruited from a medical center in central Taiwan. Plasma lipid profiles and peripheral blood mononuclear cell HY27 and ABCA1 expressions were compared between subjects taking MTX (MTX+) and other disease-modifying antirheumatic drugs (DMARDs) (MTX-). Dietary intake was assessed by a registered dietician.

RESULTS

Though no difference observed in the blood lipids between MTX+ and MTX- subjects, the expressions of ABCA1 and HY27 were significantly elevated in MTX+ subjects (n = 67) compared to MTX- subjects (n = 32, p < 0.05). ABCA expression correlated with MTX doses (r = 0.205, p = 0.042), and MTX+ subjects are more likely to have increased HY27 compared to MTX- subjects (OR = 2.5, p = 0.038). Prevalence of dyslipidemia and overweight, and dietary fat/cholesterol intake were lower than that of the age-matched population. Although no differences were observed in the blood lipids, the potential impacts of MTX on cholesterol metabolism should not be overlooked and the atheroprotective effects from MTX induced HY27 and ABCA1 expressions may still be present in those persons with pre-existing dyslipidemia.

CONCLUSIONS

We demonstrated novel findings on the increased gene expressions of atheroprotective protein HY27 and ABCA1 in human peripheral blood mononuclear cells (PBMCs) with clinical use of low-dose MTX. Whether MTX induced HY27 and ABCA1 expressions can protect against cardiovascular disease in patients with chronic inflammation through the facilitation of cholesterol export remains to be established. Further studies on the impacts of low-dose MTX on hypercholesterolemic patients are underway.

Polymorphism in the coding part of the sterol 12α‐hydroxylase gene does not explain the marked differences in the ratio of cholic acid and chenodeoxycholic acid in human bile

OBJECTIVE

In humans, two primary bile acids are synthesized: cholic acid (CA) and chenodeoxycholic acid (CDCA), the first and rate-limiting enzyme being cholesterol 7alpha-hydroxylase (CYP7A1). CA has one more hydroxyl group at position 12alpha. This hydroxylation is carried out by the sterol 12alpha-hydroxylase (CYP8B1). Earlier, we and others have noticed a marked variation in the ratio between CA and CDCA in human bile. The aim of this study was to investigate whether this marked difference could be due to a genetic polymorphism in the gene of the CYP8B1.

MATERIAL AND METHODS

Screening for genetic polymorphisms was carried out in a 2.4-kb-long area including the exon and part of the promoter region in subjects who had undergone cholecystectomy earlier, and where bile acid analysis had been performed. Among these subjects those with very high or low CA/CDCA ratios (ranging from 0.9 to 6.8) were investigated. The subjects were all female, normolipidaemic, having normal weight and a normal thyroid function.

RESULTS

No polymorphisms were found in the investigated sequence. However, a statistically significant correlation was found between the activity of the CYP7A1 and the ratio between CA and CDCA. The difference in ratio could, at least in part, be explained by the difference in rate of bile acid synthesis.

CONCLUSION

The difference in ratio between CA and CDCA cannot be explained by a polymorphism in the coding area of the CYP8B1.

Fifty years of advances in bile acid synthesis and metabolism

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

Species differences among various rodents in the conversion of 7alpha-hydroxycholesterol in liver microsomes

Our previous study demonstrated that there are species differences among vertebrates in their conversion of 7alpha-hydroxycholesterol (7HC) to 7-ketocholesterol (7KC). To examine this further, we investigated the differences in the products of 7alpha-hydroxycholesterol in various species of male muroid rodents. Adult male Syrian hamsters (Mesocricetus auratus), dwarf hamsters (Phodopus rovolovskii), Djungarian hamsters (Phodopus sungorus), Chinese hamsters (Cricetulus griseus), rat-like hamsters (Tscherskia triton), and hispid cotton rats (Sigmodon hispidus) were used. Microsomal fractions were prepared from their livers, and the activities of the enzymes that participate in the dehydrogenation of 7alpha-hydroxycholesterol were determined by measuring the products using high-performance liquid chromatography. 7alpha-hydroxycholesterol was converted to both 7alpha-hydroxy-4-cholesten-3-one (7HCO) and 7-ketocholesterol in all of the hamsters tested. However, in the rat-like hamster and the hispid cotton rat, 7alpha-hydroxycholesterol was converted mostly to 7alpha-hydroxy-4-cholesten-3-one, as also observed in the rat (Rattus norvegicus). The results suggest that microsomal enzyme activity in the conversion of 7alpha-hydroxycholesterol to 7-ketocholesterol varies considerably, even within the subfamily Cricetinae and the family Muridae.

Measurement of bile acid CoA esters by high-performance liquid chromatography-electrospray ionisation tandem mass spectrometry (HPLC-ESI-MS/MS)

The novel and rapid assay presented here combines high-performance liquid chromatography and electrospray ionisation tandem mass spectrometry (HPLC-ESI-MS/MS) to directly measure and quantify the CoA esters of 3alpha,7alpha,12alpha-trihydroxy- and 3alpha,7alpha-dihydroxy-5beta-cholestan-26-oic acid (THCA and DHCA). The latter are converted inside peroxisomes to the primary bile acids, cholic and chenodeoxycholic acids, respectively. Prior to MS/MS, esters were separated by reversed-phase HPLC on a C(18) column using an isocratic mobile phase (acetonitrile/water/2-propanol) and subsequently detected by multiple reaction monitoring. For quantification, the CoA ester of deuterium-labelled 3alpha,7alpha,12alpha-trihydroxy-5beta-cholan-24-oic acid (d(4)-CA) was used as internal standard. To complete an assay took less than 8 min. To verify the validity of the assay, the effect of peroxisomal proteins on the efficacy of extraction of the CoA esters was tested. To this end, variable amounts of the CoA esters were spiked with a fixed amount of either intact peroxisomes or peroxisomal matrix proteins and then extracted using a solid-phase extraction system. The CoA esters could be reproducibly recovered in the range of 0.1-4 micromol l(-1) (linear correlation coefficient R(2) > 0.99), with a detection limit of 0.1 micromol l(-1). In summary, electrospray ionization tandem mass spectrometry combined with HPLC as described here proved to be a rapid and versatile technique for the determination of bile acid CoA esters in a mixture with peroxisomal proteins. This suggests this technique to become a valuable tool in studies dealing with the multi-step biosynthesis of bile acids and its disturbances in disorders like the Zellweger syndrome.

Lithocholic acid decreases expression of UGT2B7 in Caco-2 cells: a potential role for a negative farnesoid X receptor response element

Human UDP-glucuronosyltransferase (UGT) 2B7 is the major isoform catalyzing the glucuronidation of a variety of endogenous compounds including bile acids. To determine the role of bile acids in the regulation of UGT2B7 expression, Caco-2 cells were incubated with the natural human farnesoid X receptor (hFXR) ligand, chenodeoxycholic acid, as well as the secondary bile acid, lithocholic acid, derived from chenodeoxycholic acid. Incubation of Caco-2 cells with lithocholic acid in the absence of exogenous hFXR resulted in a dose-dependent down-regulation of UGT2B7 mRNA levels, with an IC(50) of 13 microM. Similar down-regulation was also observed with chenodeoxycholic acid; however, much higher concentrations were required. Transient transfection of Caco-2 cells with hFXR suppressed UGT2B7 mRNA expression both in the absence and presence of ligand. UGT2B7 promoter transfection experiments and deletion/mutation analysis showed that lithocholic acid-activated hFXR decreased UGT2B7 promoter activity via a negative hFXR response element (NFRE) located between nucleotides -148 and -134. Cotransfection with hFXR and/or human retinoid X receptor further enhanced the repression. Electrophoretic mobility shift assays additionally confirmed the role of NFRE in UGT2B7 down-regulation by lithocholic acid. These findings suggest that lithocholic acid, an activator of nuclear hFXR, acts as a negative regulator of UGT2B7 expression, indicating that hFXR may play an essential role in lithocholic acid homeostasis through negative regulation of this UGT that is involved in lithocholic acid biotransformation. Therefore, it is postulated that lithocholic acid toxicity may be due to down-regulation of genes involved in its detoxification, including UGT2B7, leading to limited excretion of lithocholic acid from the body.

Role of farnesoid X receptor in the enhancement of canalicular bile acid output and excretion of unconjugated bile acids: a mechanism for protection against cholic acid-induced liver toxicity

Mice lacking the farnesoid X receptor (FXR) involved in the maintenance of hepatic bile acid levels are highly sensitive to cholic acid-induced liver toxicity. Serum aspartate aminotransferase (AST) activity was elevated 15.7-fold after feeding a 0.25% cholic acid diet, whereas only slight increases in serum AST (1.7- and 2.5-fold) were observed in wild-type mice fed 0.25 and 1% cholic acid diet, respectively. Bile salt export pump mRNA and protein levels were increased in wild-type mice fed 1% cholic acid diet (2.1- and 3.0-fold) but were decreased in FXR-null mice fed 0.25% cholic acid diet. The bile acid output rate was 2.0- and 3.7-fold higher after feeding of 0.25 and 1.0% cholic acid diet in wild-type mice, respectively. On the other hand, no significant increase in bile acid output rate was observed in FXR-null mice fed 0.25% cholic acid diet in contrast to a significant decrease observed in mice fed a 1.0% cholic acid diet in spite of the markedly higher levels of hepatic tauro-conjugated bile acids. Unconjugated cholic acid was not detected in the bile of wild-type mice fed a control diet, but it was readily detected in wild-type mice fed 1% cholic acid diet. The ratio of biliary unconjugated cholic acid to total cholic acid (unconjugated cholic acid and tauro-conjugated cholic acid) reached 30% under conditions of hepatic taurine depletion. These results suggest that the cholic acid-induced enhancement of canalicular bile acid output rates and excretion of unconjugated bile acids are involved in adaptive responses for prevention of cholic acid-induced toxicity.

Purification and characterization of 7beta-hydroxysteroid dehydrogenase from rabbit liver microsomes

7beta-Hydroxysteroid dehydrogenase (7beta-HSD), a specific enzyme active in the metabolization of 7beta-hydroxycholesterol, was purified about 300-fold from male rabbit liver microsomes using ion exchange, hydroxylapatite, 2'5'ADP Sepharose 4B, and high-performance liquid chromatography on the basis of its catalytic activity. The specific activity of the purified enzyme was 276 nmol/min/mg protein. The molecular weight of the purified enzyme was 34,000. The preferred coenzyme was beta-NADP+. The optimum pH for oxidation was around 7.7 in potassium phosphate buffer, and 11.0 in glycine-NaOH buffer. The purified enzyme catalyzed the synthesis of not only 7beta-hydroxycholesterol but also corticosterone and hydrocortisone. Enzyme activities toward these three substrates accompanied all purification steps of 7beta-HSD. The amino acid sequence of the N-terminal of the purified enzyme showed that 7beta-HSD had sequence similarity to rabbit type I 11beta-hydroxysteroid dehydrogenase (11beta-HSD), indicating that 7beta-HSD may belong to the rabbit type I 11beta-HSD family and may play the same role in the metabolism of 11-hydroxysteroids and 7-hydroxysterols.

Species difference in cholesterol 7alpha-hydroxylase expression of rabbit and rat liver microsomes after bile duct ligation

BACKGROUND

Bile duct ligation (BDL) produces a good animal model for investigation of the metabolic changes in obstructive jaundice. The aim of this study was to investigate the species difference in expression of cholesterol 7alpha-hydroxylase (7alpha-hydroxylase) in rabbits and rats after BDL.

MATERIALS AND METHODS

Ten male New Zealand white rabbits weighing 2.5-3 kg and 12 male Wistar strain rats weighing 250-300 g were used. Half the animals underwent BDL, and half were sham operated (Sham). The animals were sacrificed on day 5 after operation. The livers were harvested, and levels of mRNA and 7alpha-hydroxylase activity were determined. Concentrations of serum bilirubin and bile acids were also measured.

RESULTS

In BDL rats, the levels of mRNA were increased 30%, and 7alpha-hydroxylase activity was three times that of the Sham group. In BDL rabbits, however, these values were approximately 60 and 50% lower than the Sham group, respectively. Serum bile acid concentrations increased up to 13 times in BDL rabbits and 70 times in BDL rats over that of the Sham groups. Serum cholesterol and serum total bilirubin concentration also increased after BDL in both animals.

CONCLUSION

These results suggest that there is a species difference in the expression of 7alpha-hydroxylase after BDL in rabbits and rats and the reason for this difference is most likely pretranslational regulation.

Definition of a novel growth factor-dependent signal cascade for the suppression of bile acid biosynthesis

The nuclear bile acid receptor FXR has been proposed to play a central role in the feedback repression of the gene encoding cholesterol 7 alpha-hydroxylase (CYP7A1), the first and rate-limiting step in the biosynthesis of bile acids. We demonstrate that FXR directly regulates expression of fibroblast growth factor-19 (FGF-19), a secreted growth factor that signals through the FGFR4 cell-surface receptor tyrosine kinase. In turn, FGF-19 strongly suppresses expression of CYP7A1 in primary cultures of human hepatocytes and mouse liver through a c-Jun N-terminal kinase (JNK)-dependent pathway. This signaling cascade defines a novel mechanism for feedback repression of bile acid biosynthesis and underscores the vital role of FXR in the regulation of multiple pathways of cholesterol catabolism in the liver.

Naringenin 7-O-cetyl ether as inhibitor of HMG-CoA reductase and modulator of plasma and hepatic lipids in high cholesterol-fed rats

Numerous studies in vitro have shown a close relationship between the chemical structure and biologic activity of flavonoids, whereby their basic structure is modified to increase or decrease their biologic activity. The effects of naringenin (1) and its synthetic derivative, naringenin 7-O-cetyl ether (2), on the lipid profile, the cholesterol-regulating enzyme activity and the excretion of sterol were compared in rats fed a high-cholesterol (1% wt/wt) diet. Either 1 or 2 was supplemented with a high-cholesterol diet for 6 weeks at a dose of 0.073 mmol/100g diet. The supplementation of 1 or 2 significantly lowered the levels (mean+/-SE) of the plasma total cholesterol (4.93+/-0.19 and 4.75+/-0.16 mmol/L vs 5.87+/-0.36 mmol/L, p<0.05) and hepatic triglyceride (0.12+/-0.01 and 0.11+/-0.01 mmol/g vs 0.18+/-0.01 mmol/g, p<0.05) and cholesterol (0.23+/-0.01 and 0.21+/-0.01 mmol/g vs 0.31+/-0.01 mmol/g, p<0.05) compared to those of the control. The compound 1 or 2 supplementation appeared to decrease the excretion of neutral sterols. The plasma HDL-cholesterol concentration and ratio of HDL to total cholesterol were significantly higher in 1 and 2 groups than in control group. Although the biological effect of 2 on inhibiting hepatic HMG-CoA reductase and ACAT activities was only significant compared to the control group, both compounds exhibited a significant hypocholesterolemic effect in rats fed a high-cholesterol diet. The results suggest that cholesterol biosynthesis and esterification were concomitantly reduced by 2, as indicated by the decreased HMG-CoA reductase and ACAT activities.

Dual action of neutral sphingomyelinase on rat hepatocytes: activation of cholesteryl ester metabolism and biliary cholesterol secretion and inhibition of VLDL secretion

To address the role of cell membrane neutral sphingomyelinase (EC 3.1.4.12; SMase) in the regulation of cholesterol metabolism in the liver parenchymal cell, we examined the effect of exogenous neutral SMase on the metabolism of cholesteryl esters and the secretion of VLDL and biliary lipids in isolated rat hepatocytes. We show that treatment of hepatocytes with SMase (20 mU/mL) resulted in the intracellular buildup of cholesteryl esters, increased ACAT (EC 2.3.1.26) activity without affecting the ACAT2 mRNA level, and increased cytosolic and microsomal cholesteryl ester hydrolase (EC 3.1.1.13) activity. This was accompanied by increases in the secretion of biliary bile acid, phospholipid, and cholesterol and in increased cholesterol 7alpha-hydroxylase (EC 1.14.13.17) activity and levels of mRNA, as well as decreased levels of apoB mRNA and a decreased secretion of VLDL apoB (apoB-48, approximately 45%; apoB-100, approximately 32%) and lipids (approximately 55%). Moreover, the VLDL particles secreted had an abnormal size and lipid composition; they were larger than controls, were relatively enriched in cholesteryl ester, and depleted in TG and cholesterol. Cell-permeable ceramides did not replicate any of the reported effects. These findings demonstrate that the increased cholesteryl ester turnover, oversecretion of biliary cholesterol and bile acids, and undersecretion of VLDL cholesterol and particles are concerted responses of the primary hepatocytes to exogenous neutral SMase brought about by regulation at several levels. We suggest that plasma membrane neutral SMase may have a specific, ceramide-independent effect in the regulation of cholesterol output pathways in hepatocytes.

Human sterol 27-hydroxylase (CYP27) overexpressor transgenic mouse model. Evidence against 27-hydroxycholesterol as a critical regulator of cholesterol homeostasis

CYP27-overexpressed transgenic mice were generated with the use of a human full-length CYP27 coding region cloned into a ubiquitous expression vector. Positive transgenic mice were identified by tail DNA genotyping and high fecal 27-hydroxycholesterol content. The levels of 27-hydroxycholesterol were found to be 3-5 times higher in the circulation and the tissues of the overexpressed mice when compared with littermate controls. There were no gross morphological differences between the overexpressed mice and their controls. Total cholesterol and triglyceride levels were not affected by overexpression of CYP27. Serum lathosterol was also normal, suggesting a normal rate of cholesterol synthesis. Serum levels of 7alpha-hydroxycholesterol were unaffected, suggesting a normal rate of bile acid formation in the pathway involving cholesterol 7alpha-hydroxylase. Biliary bile acid composition was slightly affected by CYP27 overexpression in female but not in male mice. Fecal levels of neutral steroids were slightly but significantly increased in overexpressor female mice but not in male mice. Levels of 24-hydroxycholesterol in the circulation were significantly reduced in the overexpressed mice, probably as a consequence of a recently described catabolic pathway involving CYP27. Combined with the results of our previous work on mice with a disruption of the CYP27 gene, the present results suggest that the levels of 27-hydroxycholesterol are not of critical importance for cholesterol homeostasis in mice.

Absorption of intestinal free cholesterol is lowered by supplementation of Areca catechu L. extract in rats

Areca extracts exhibiting a strong inhibitory activity against pancreatic cholesterol esterase (pCEase) in vitro were previously found to lower the absorption of dietary cholesteryl ester. Therefore, to determine whether a combined Areca extract also affects the absorption of intestinal free cholesterol, male rats were fed a diet containing free cholesterol (1%, w/w) either with or without an Areca nut extract supplement (0.5%, w/w). The Areca extract supplement significantly lowered the plasma cholesterol concentration by 25% without any change in the plasma triglyceride concentration, when compared to the control group. The supplement also significantly lowered the small intestinal pCEase activity by 39.1% compared to that of the control group. As regards the hepatic and intestinal ACAT activities, only the intestinal enzyme activity was significantly lowered by the supplement, when compared to the control group. The absorbed cholesterol that appeared in the blood after an oral dose of [1,2(n)-3H] free cholesterol was significantly lower in the rats supplemented with the Areca nut extract, compared with the control group. These results suggest that the inhibition of intestinal ACAT and possibly pCEase may facilitate the metabolic efficiency of the Areca nut extract as regards the absorption of intestinal free cholesterol. The structure and chemical properties of the active compound in the water-soluble Areca extract remain to be elucidated.

Lipid-lowering and antioxidative activities of 3,4-di(OH)-cinnamate and 3,4-di(OH)-hydrocinnamate in cholesterol-fed rats

BACKGROUND

Polyphenols appear to have antioxidant activities and may mediate lipid lowering.

METHODS

Four groups of rats, a high-cholesterol control (HC), HC+lovastatin, HC+3,4-di(OH)-cinnamate, and HC+3,4-di(OH)-hydrocinnamate, were given a semi-synthetic diet. The cinnamate derivative or lovastatin (0.1 g/100 g) supplements were given for 6 weeks.

RESULTS

The plasma total cholesterol concentration was significantly lowered by the 3,4-di(OH)-cinnamate supplement compared to the control or lovastatin group. The 3,4-di(OH)-cinnamate and 3,4-di(OH)-hydrocinnamate supplements significantly lowered both the hepatic cholesterol and triglyceride levels, while lovastatin only lowered the hepatic cholesterol. The hepatic HMG-CoA reductase activities were significantly lower in the 3,4-di(OH)-cinnamate and 3,4-di(OH)-hydrocinnamate groups than in the control or lovastatin group. The ACAT activity was only significantly lower in the lovastatin group compared to the other groups. With regards the hepatic antioxidant enzyme system, the CAT activity was significantly higher in the 3,4-di(OH)-cinnamate and 3,4-di(OH)-hydrocinnamate groups compared to the control or lovastatin group. The two cinnamate derivatives resulted in an increased hepatic GSH-Px activity. Meanwhile, all the supplements significantly lowered the hepatic thiobarbituric acid reactive substances (TBARS) content. However, the 3,4-di(OH)-cinnamate and 3,4-di(OH)-hydrocinnamate supplements did not alter the neutral sterol and total fecal sterol.

CONCLUSIONS

Both cinnamate derivatives were potent in lipid-lowering and altering the antioxidative enzyme. Furthermore, these results also suggest that 3,4-di(OH)-cinnamate is more effective than 3,4-di(OH)-hydrocinnamate in its lipid-lowering action.

Fibrates suppress bile acid synthesis via peroxisome proliferator-activated receptor-alpha-mediated downregulation of cholesterol 7alpha-hydroxylase and sterol 27-hydroxylase expression

Fibrates are hypolipidemic drugs that affect the expression of genes involved in lipid metabolism by activating peroxisome proliferator-activated receptors (PPARs). Fibrate treatment causes adverse changes in biliary lipid composition and decreases bile acid excretion, leading to an increased incidence of cholesterol gallstones. In this study, we investigated the effect of fibrates on bile acid synthesis. Ciprofibrate and the PPARalpha agonist Wy14,643 decreased bile acid synthesis in cultured rat hepatocytes and suppressed cholesterol 7alpha-hydroxylase and sterol 27-hydroxylase activities, paralleled by a similar reduction of the respective mRNAs. Treatment of rats with 0.05% (wt/wt) ciprofibrate decreased cholesterol 7alpha-hydroxylase enzyme activity and mRNA. The functional involvement of PPARalpha in the suppression of both enzymes was proven with the use of PPARalpha-null mice. In wild-type mice, ciprofibrate reduced cholesterol 7alpha-hydroxylase and sterol 27-hydroxylase enzyme activities and mRNA. The decrease in mRNA of both enzymes is regulated transcriptionally and posttranscriptionally, respectively, resulting in a decline in the output of fecal bile acids (-45%) and a 3-fold increase in fecal cholesterol secretion. These effects were completely abolished in PPARalpha-null mice. A decreased bile acid production by PPARalpha-mediated downregulation of cholesterol 7alpha-hydroxylase and sterol 27-hydroxylase may contribute to the increased risk of gallstone formation after fibrate treatment.

Synthesis of coenzyme A esters of 3alpha,7alpha,12alpha-trihydroxy- and 3alpha,7alpha-dihydroxy-24-oxo-5beta-cholestan-26-oic acids for the study of beta-oxidation in bile acid biosynthesis

3alpha,7alpha,12alpha-Trihydroxy- and 3alpha,7alpha-dihydroxy-24-oxo-5beta-cholestan-26-oyl CoAs were chemically synthesized by the conventional method for the study of side chain cleavage in bile acid biosynthesis. 3alpha,7alpha,12alpha-Triformyloxy- and 3alpha,7alpha-diformyloxy-5beta-cholan-24-als were initially subjected to the Reformatsky reaction with methyl alpha-bromopropionate, and the products were then converted into methyl 3alpha,7alpha,12alpha-triformyloxy- and 3alpha,7alpha-diformyloxy-24-oxo-5beta-cholestan-26-oates. Protection by acetalization of the 24-oxo-group of these methyl esters with ethylene glycol, followed by alkaline hydrolysis, gave 3alpha,7alpha,12alpha-trihydroxy- and 3alpha,7alpha-dihydroxy-24,24-ethylenedioxy-5beta-cholestan-26-oic acids. These acids were condensed with coenzyme A by a mixed anhydride method, and the resulting CoA esters were treated with 4M-hydrocholic acid to remove the protecting group to give 24-oxo-5beta-cholestanoic acid CoA esters. The chromatographic behaviors of these CoA esters were also investigated.

Conjugation reactions catalyzed by bifunctional proteins related to beta-oxidation in bile acid biosynthesis

The conjugation reactions of hydration and dehydrogenation catalyzed by the dehydratase and dehydrogenase activities of D-3-hydroxyacyl-CoA dehydratase/D-3-hydroxyacyl-CoA dehydrogenase bifunctional protein (DBP) and enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase bifunctional protein (LBP) in the side chain degradation step of bile acid biosynthesis were investigated using chemically synthesized C27-bile acid CoA esters as substrates. The hydration catalyzed by DBP showed high diastereoselectivity for (24E)-3alpha,7alpha,12alpha-trihydroxy- and (24E)-3alpha,7alpha-dihydroxy-5beta-cholest-24-en-26-oyl CoA to give (24R,25R)-3alpha,7alpha,12alpha,24-tetrahydroxy- and (24R,25R)-3alpha,7alpha,24-trihydroxy-5beta-cholestan-26-oyl CoAs, respectively, and the dehydrogenation catalyzed by DBP also showed high stereospecificity for the above (24R,25R)-isomers to give 3alpha,7alpha,12alpha-trihydroxy- and 3alpha,7alpha-dihydroxy-24-oxo-5beta-cholestan-26-oyl CoAs, respectively. On the other hand, the dehydratase activity of LBP displayed a different diastereoselectivity producing the (24S,25S)-isomer, and dehydrogenase activity of LBP was stereospecific for the (24S,25R)-isomer to give the above 24-oxo-derivative. The hydration and dehydrogenation reactions catalyzed by DBP were effectively conjugated to convert (24E)-5beta-cholestenoyl CoA to 24-oxo-5beta-cholestanoyl CoA. However, the reactions catalyzed by LBP were not conjugated. These results indicate that DBP plays an important role in the biosynthesis of bile acid.

Expression of CYP27A, a gene encoding a vitamin D-25 hydroxylase in human liver and kidney

Vitamin D3 (D3) is not active but must be hydroxylated at C-25 in liver before acquiring its hormonal potential in the kidney. The sterol-27 hydroxylase (gene symbol: CYP27A) catalyses the oxidation of sterol side chain in bile acid synthesis but the enzyme is also known as a D3-25 hydroxylase. The study examined the expression of the gene encoding CYP27A in adult and fetal human livers and kidneys. Thirty-nine adults (18 men and 21 women; mean age 58 years in men and 57 years in women) and three normal fetuses gestational age 17-19 weeks were studied. Tissue CYP27A mRNA and serum 25OHD concentrations were measured. Normal specimens: CYP27A transcript was found to be higher in adult than in fetal livers but its expression was similar in adult and fetal kidneys. In fetuses, no difference was observed between CYP27A levels in livers and kidneys. In adult livers CYP27A levels were higher in women than in men. Hepatic CYP27A mRNA and serum 25OHD concentrations were both found to be higher in summer than in winter. Multiple linear regression analyses indicate that the season of the year and the serum 25OHD concentrations (but not 1,25(OH)2D concentrations) are the best predictors of CYP27A mRNA abundance in normal adult livers. In situ hybridization illustrates a clear label in hepatocytes which increases in intensity in the perivenous region of the hepatic acinus. Pathological specimens: In one man with an hepatic carcinoma there was a very large increase in CYP27A (> 1000 fold) compared to the level found in the normal liver. In that patient, serum 25OHD concentrations were found to be high considering the level of CYP27A mRNA in the normal hepatic area suggesting that the neoplastic tissue contributed to the C-25 hydroxylation of vitamin D. Specimens obtained from two patients suffering from focal hepatic hyperplasia indicate that in one case the level of CYP27A mRNA was twice as high in the pathological than in the normal area while in the other its levels were similar in both areas. No difference in the CYP27A transcript was observed between specimens obtained from normal areas and those obtained form either an hepatic adenoma or from two intrahepatic colonic metastases. CYP27A is present not only in the human adult liver but also in the adult kidney, and in the fetal liver and kidney. Our findings illustrate that CYP27A can be significantly upregulated in certain pathological situations such as in hepatic carcinoma and that the neoplastic tissue could contribute to the circulating concentration of 25OHD.

Bile acid synthesis in cultured human hepatocytes: support for an alternative biosynthetic pathway to cholic acid

The biosynthesis of bile acids by primary cultures of normal human hepatocytes has been investigated. A general and sensitive method for the isolation and analysis of sterols and bile acids was used, based on anion exchange chromatography and gas chromatography-mass spectrometry (GC/MS). Following incubation for 5 days, 8 oxysterols and 8 C(27)- or C(24)-bile acids were identified in media and cells. Cholic and chenodeoxycholic acids conjugated with glycine or taurine were by far the major steroids found, accounting for 70% and 24% of the total, respectively, being consistent with bile acid synthesis in human liver. Small amounts of sulfated 3beta-hydroxy-5-cholenoic acid and 3beta,7alpha-dihydroxy-5beta-cholanoic acid were also detected. Nine steroids were potential bile acid precursors (2% of total), the major precursors being 7alpha, 12alpha-dihydroxy-3-oxo-4-cholenoic acid and its 5beta-reduced form. These 2 and 5 other intermediates formed a complete metabolic sequence from cholesterol to cholic acid (CA). This starts with 7alpha-hydroxylation of cholesterol, followed by oxidation to 7alpha-hydroxy-4-cholesten-3-one and 12alpha-hydroxylation. Notably, 27-hydroxylation of the product 7alpha, 12alpha-dihydroxy-4-cholesten-3-one and further oxidation and cleavage of the side chain precede A-ring reduction. A-Ring reduction may also occur before side-chain cleavage, but after 27-hydroxylation, yielding 3alpha,7alpha, 12alpha-trihydroxy-5beta-cholestanoic acid as an intermediate. The amounts of the intermediates increased in parallel to those of CA during 4 days of incubation. Suppressing 27-hydroxylation with cyclosporin A (CsA) resulted in a 10-fold accumulation of 7alpha, 12alpha-dihydroxy-4-cholesten-3-one and a decrease of the production of CA and its acidic precursors. These results suggest that the observed intermediates reflect an alternative biosynthetic pathway to CA, which may be quantitatively significant in the cells.

Feedback and hormonal regulation of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase: the concept of cholesterol buffering capacity

Regulation of the expression of hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase by the major end product of the biosynthetic pathway, cholesterol, and by various hormones is critical to maintaining constant serum and tissue cholesterol levels in the face of an ever-changing external environment. The ability to downregulate this enzyme provides a means to buffer the body against the serum cholesterol-raising action of dietary cholesterol. The higher the basal expression of hepatic HMG-CoA reductase, the greater the "cholesterol buffering capacity" and the greater the resistance to dietary cholesterol. This review focuses on the mechanisms of feedback and hormonal regulation of HMG-CoA reductase in intact animals rather than in cultured cells and presents the evidence that leads to the proposal that regulation of hepatic HMG-CoA reductase acts as a cholesterol buffer. Recent studies with animals have shown that feedback regulation of hepatic HMG-CoA reductase occurs at the level of translation in addition to transcription. The translational efficiency of HMG-CoA reductase mRNA is diminished through the action of dietary cholesterol. Oxylanosterols appear to be involved in this translational regulation. Feedback regulation by dietary cholesterol does not appear to involve changes in the state of phosphorylation of hepatic HMG-CoA reductase or in the rate of degradation of this enzyme. Several hormones act to alter the expression of hepatic HMG-CoA reductase in animals. These include insulin, glucagon, glucocorticoids, thyroid hormone and estrogen. Insulin stimulates HMG-CoA reductase activity likely by increasing the rate of transcription, whereas glucagon acts by opposing this effect. Hepatic HMG-CoA reductase activity undergoes a significant diurnal variation due to changes in the level of immunoreactive protein primarily mediated by changes in insulin and glucagon levels. Thyroid hormone increases hepatic HMG-CoA reductase levels by acting to increase both transcription and stability of the mRNA. Glucocorticoids act to decrease hepatic HMG-CoA reductase expression by destabilizing reductase mRNA. Estrogen acts to increase hepatic HMG-CoA reductase activity primarily by stabilizing the mRNA. Deficiencies in those hormones that act to increase hepatic HMG-CoA reductase gene expression lead to elevations in serum cholesterol levels. High basal expression of hepatic HMG-CoA reductase, whether due to genetic or hormonal factors, appears to result in greater cholesterol buffering capacity and thus increased resistance to dietary cholesterol.

Plasma oxysterols and tocopherol in patients with diabetes mellitus and hyperlipidemia

The plasma levels of free oxysterols (7-ketocholesterol; 7alpha-hydroxy-, 7beta-hydroxy-, 25-hydroxy-, and 27-hydroxycholesterol; and 5alpha,6alpha-epoxycholestanol) in patients with diabetes mellitus and hypercholesterolemia were determined using gas chromatography-mass spectrometry with selective ion monitoring. We studied 39 patients with diabetes mellitus, 20 nondiabetic patients with hypercholesterolemia, and 37 normal controls. Plasma cholesterol levels in diabetic and hypercholesterolemic patients showed no statistical difference. Plasma 7-ketocholesterol was significantly higher in patients with diabetes (31.6+/-2.8 ng/mL) or hypercholesterolemia (52.3+/-5.9) than in the control group (22.4+/-1.2). The increased plasma cholesterol can be regarded as an oxidation substrate for the oxidant stress and the higher absolute levels of oxysterols in hypercholesterolemic plasma compared with the control plasma. This difference disappeared when 7-ketocholesterol was expressed in proportion to total cholesterol. The oxidizability of plasma cholesterol was evaluated by comparing the increased ratio of 7-ketocholesterol after CuSO4 oxidation to the ratio before. We demonstrated that the patients with diabetes showed increased oxidizability (77.5%) compared with the control (36.6%) or hyperlipemic group (45.3%), which is likely due to the lower amounts of alpha-tocopherol in the diabetics. Measurement of oxysterols may serve as a marker for in vivo oxidized lipoproteins in diabetes and hyperlipemia.

Acyl-coenzyme A:cholesterol acyltransferase inhibitor, avasimibe, stimulates bile acid synthesis and cholesterol 7alpha-hydroxylase in cultured rat hepatocytes and in vivo in the rat

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) inhibitors are currently in clinical development as potential lipid-lowering and antiatherosclerotic agents. We investigated the effect of avasimibe (Cl- 1011), a novel ACAT inhibitor, on bile acid synthesis and cholesterol 7alpha-hydroxylase in cultured rat hepatocytes and rats fed different diets. Avasimibe dose-dependently decreased ACAT activity in rat hepatocytes in the presence and absence of beta-migrating very low-density lipoproteins (betaVLDL) (by 93% and 75% at 10 micromol/L) and reduced intracellular storage of cholesteryl esters. Avasimibe (3 micromol/L) increased bile acid synthesis (2.9-fold) after preincubation with betaVLDL and cholesterol 7alpha-hydroxylase activity (1.7- and 2.6-fold, with or without betaVLDL), the latter paralleled by a similar induction of its messenger RNA (mRNA). Hepatocytes treated with avasimibe showed a shift from storage and secretion of cholesteryl esters to conversion of cholesterol into bile acids. In rats fed diets containing different amounts of cholesterol and cholate, avasimibe reduced plasma cholesterol (by 52% to 71%) and triglyceride levels (by 28% to 62%). Avasimibe did not further increase cholesterol 7alpha-hydroxylase activity and mRNA in cholesterol-fed rats, but prevented down-regulation by cholate. Avasimibe did not affect sterol 27-hydroxylase and oxysterol 7alpha-hydroxylase, 2 enzymes in the alternative pathway in bile acid synthesis. No increase in the ratio of biliary excreted cholesterol to bile acids was found, indicating that ACAT inhibition does not result in a more lithogenic bile. Avasimibe increases bile acid synthesis in cultured hepatocytes by enhancing the supply of free cholesterol both as substrate and inducer of cholesterol 7alpha-hydroxylase. These effects may partially explain the potent cholesterol-lowering effects of avasimibe in the rat.

Cafestol, the cholesterol-raising factor in boiled coffee, suppresses bile acid synthesis by downregulation of cholesterol 7 alpha-hydroxylase and sterol 27-hydroxylase in rat hepatocytes

Consumption of boiled coffee raises serum cholesterol levels in humans. The diterpenes cafestol and kahweol in boiled coffee have been found to be responsible for the increase. To investigate the biochemical background of this effect, we studied the effects of cafestol and a mixture of cafestol/kahweol/isokahweol (48:47:5 w/w) on bile acid synthesis and cholesterol 7 alpha-hydroxylase and sterol 27-hydroxylase in cultured rat hepatocytes. Dose-dependent decreases of bile acid mass production and cholesterol 7 alpha-hydroxylase and sterol 27-hydroxylase activity were found, showing a maximal reduction of -91%, -79%, and -49% respectively, at a concentration of 20 micrograms/mL cafestol. The decrease in 7 alpha-hydroxylase and 27-hydroxylase activity paralleled well the suppression of the respective mRNAs, being -79% and -77%, and -49% and -46%, respectively, at 20 micrograms/mL cafestol. Run-on data showed a reduction in 7 alpha-hydroxylase and 27-hydroxylase gene transcriptional activity after incubation with cafestol. The mixture of cafestol/kahweol/isokahweol was less potent in suppression of bile acid synthesis and cholesterol 7 alpha-hydroxylase. Cafestol (20 micrograms/mL) had no effect on lithocholic acid 6 beta-hydroxylase mRNA, another enzyme involved in bile acid synthesis. LDL-receptor, HMG-CoA reductase, and HMG-CoA synthase mRNAs were significantly decreased by cafestol (-18%, -20%, and -43%, respectively). We conclude that cafestol suppresses bile acid synthesis by downregulation of cholesterol 7 alpha-hydroxylase and of, to a lesser extent, sterol 27-hydroxylase in cultured rat hepatocytes, whereas kahweol and isokahweol are less active. We suggest that suppression of bile acid synthesis may provide an explanation for the cholesterol-raising effect of cafestol in humans.

Molecular cloning and expression of cDNA encoding 3alpha,7alpha,12alpha-trihydroxy-5beta-chole stanoyl-CoA oxidase from rabbit liver

The steroid side chain cleavage in bile acid formation is catalyzed by liver peroxisomal enzymes (Pedersen, J. I. and Gustafsson, J. (1980) FEBS Lett. 121, 345-348; Kase, F., Björkhem, I., and Pedersen, J. I. (1983) J. Lipid Res. 24, 1560-1567). We here describe the cloning and sequencing of a cDNA coding the first of these enzymes, a 3alpha,7alpha,12alpha-trihydroxy-5beta-choles tanoyl-CoA oxidase (THCA-CoA oxidase) from rabbit liver peroxisomes. After tryptic digestion of purified protein in a polyacrylamide gel, five peptides were isolated and sequenced. Using two oligonucleotides deduced from the amino acid sequence data, two overlappping clones were isolated from a rabbit liver cDNA library, which together made up a unique cDNA sequence of 2139 base pairs. It contained an open reading frame of 2046 base pairs encoding a protein of 681 amino acids with a molecular mass of 76,209 daltons. All five peptides could be localized within the sequence. Transfection of COS cells with the coding part of the cDNA resulted in a significant expression of THCA-CoA oxidase activity. We were not able to demonstrate 3alpha, 7alpha-dihydroxy-5beta-cholestanoyl-CoA oxidase activity under the same conditions. The obtained sequence showed 73.6% similarity with a proposed rat THCA-CoA oxidase and 81% similarity with a recently reported human branched chain acyl-CoA oxidase, indicating that these three proteins represent the same enzyme. The similarity with rat palmitoyl-CoA oxidase was 41.8%. The C-terminal tripeptide of the protein was SNL, a previously undescribed variant of the main class of peroxisomal targeting signals. Northern blot analysis revealed that the gene is transcribed in liver and kidney, and the major mRNA fraction had a size of approximately 2.6 kilobase pairs.

Molecular cloning and expression of rabbit sterol 12alpha-hydroxylase

Sterol 12alpha-hydroxylase is an important enzyme in bile acid biosynthesis, responsible for the balance between formation of cholic acid and chenodeoxycholic acid. The enzyme has been purified to apparent homogeneity from rabbit liver (Ishida, H., Noshiro, M., Okuda, K., and Coon, M. J. (1992) J. Biol. Chem. 267, 21319-21323), and we here describe the cloning and sequencing of a cDNA coding for this enzyme. After tryptic digestion of purified protein in a polyacrylamide gel, eight different peptides were isolated and sequenced. Using oligonucleotides deduced from the amino acid sequences, clones were isolated from a rabbit liver cDNA library. In addition to several overlapping clones, one full-length clone was obtained that coded for a polypeptide of 500 amino acids, corresponding to a molecular mass of 57 kDa. All of the eight peptides and the reported NH2-terminal amino acid sequence were matched against the sequence. The peptide sequence showed a 39% similarity with human prostacyclin synthase (CYP8) and 31% similarity with the rate-limiting enzyme in over-all synthesis of bile acids, the cholesterol 7alpha-hydroxylase (CYP7) of the rabbit. The similarity with most other sterol cytochrome P-450 hydroxylases was less. Thus, this species of cytochrome P-450 should belong to a group of its own, here denoted CYP12. Transfection of COS cells with the coding part of the cDNA resulted in a significant expression of sterol 12alpha-hydroxylase activity toward 7alpha-hydroxy-4-cholesten-3-one. Northern blotting showed that the enzyme was exclusively expressed in the liver. The major mRNA fraction in rabbit liver had a size of approximately 2.9 kilobases, and those found in rat and human liver were about 2.5 and 4.5 kilobases, respectively. Fasting of rats and mice led to a severalfold increase in both enzyme activity and mRNA levels. In contrast, starvation of rabbits had little or no stimulatory effect on enzyme activity and mRNA levels.

Purification of a 3beta-hydroxy-delta5-C27-steroid dehydrogenase from pig liver microsomes active in major and alternative pathways of bile acid biosynthesis

A 3beta-hydroxy-Delta5-C27-steroid dehydrogenase active in bile acid biosynthesis was purified from pig liver microsomes by solubilization with sodium cholate and by chromatography on DEAE-Sepharose, aminohexyl-Sepharose, and blue Sepharose. The last step in the purification procedure was preparative isoelectric focusing in a Rotofor cell. The final enzyme preparation showed only one protein band upon SDS-polyacrylamide gel electrophoresis. The isoelectric point was estimated to about 7.0 and the apparent Mr was 36,000. The purified enzyme catalyzed the conversion of 7alpha-hydroxycholesterol, 7alpha,25-dihydroxycholesterol, 7alpha, 27-dihydroxycholesterol, and 3beta,7alpha-dihydroxy-5-cholestenoic acid into the corresponding 3-oxo-Delta4 compounds. The enzyme was inactive with C19 and C21 steroids as substrates. The enzyme was also inactive with C27 steroids having the 7-hydroxy group in beta- instead of alpha-position. The Km was found to be 0.30 and 0.32 microM with 7alpha-hydroxycholesterol and 7alpha, 27-dihydroxycholesterol as substrates, respectively. NAD+ was the preferred cofactor. A monoclonal antibody raised against the 3beta-hydroxy-delta5-C27-steroid dehydrogenase was prepared. After coupling to Sepharose, the antibody was able to bind the dehydrogenase and to decrease the conversion of 7alpha-hydroxycholesterol into 7alpha-hydroxy-4-cholest-3-one by more than 90%. The N-terminal amino acid sequence was determined and found to be similar but not identical with those of known 3beta-hydroxy-Delta5-steroid dehydrogenases active in steroid hormone biosynthesis. Thus, the purified enzyme active toward C27 steroids in bile acid biosynthesis appears to represent a novel type of 3beta-hydroxy-delta5-steroid dehydrogenase.

Relationship between the serum concentration of 7 alpha-hydroxycholesterol and fecal bile acid excretion in humans

BACKGROUND

Serum levels of 7 alpha-hydroxycholesterol have been shown to reflect the activity of cholesterol 7 alpha-hydroxylase, the key enzyme of bite acid synthesis in the liver, but a comparison with direct measurements of bile acid synthesis rates has never been performed.

METHODS

7 alpha-Hydroxycholesterol was measured by gas-liquid chromatography/mass spectrometry and bile acid synthesis by the fecal balance method in 35 subjects.

RESULTS

A significant correlation was found between 7 alpha-hydroxycholesterol concentration in serum and bile acid synthesis (r = 0.863, p < 0.001). Serum levels of 7 alpha-hydroxycholesterol in 20 patients treated with a 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor did not differ from levels obtained in healthy volunteers (78 +/- 7 ng/ml versus 63 +/- 5 ng/ml; NS). Treatment with fenofibrate reduced 7 alpha-hydroxycholesterol concentrations in six patients from 107 +/- 47 ng/ml to 61 +/- 12 ng/ml (p < 0.05).

CONCLUSIONS

We conclude that the concentration of 7 alpha-hydroxycholesterol in serum is an indicator of bile acid synthesis and that serum levels of 7 alpha-hydroxycholesterol are not affected in patients treated with HMG-CoA reductase inhibitors but are affected in those treated with fenofibrate.

Importance of a novel oxidative mechanism for elimination of intracellular cholesterol in humans

We have recently demonstrated that cultured human alveolar macrophages efficiently convert cholesterol into excretable 27-oxygenated products. We show here that increasing the intracellular concentration of cholesterol by a factor of 10 leads to about a twofold increase in the excretion of 27-oxygenated products from cultured macrophages. Inhibition of the sterol 27-hydroxylase caused a significant intracellular accumulation of cholesterol. A direct comparison was made between flux of cholesterol and 27-oxygenated products from macrophages preloaded with [4-14C]cholesterol. Under the specific conditions employed with fetal calf serum in the culture medium, the flux of 27-oxygenated products was about 10% of that of cholesterol. Since the sterol 27-hydroxylase, which converts cholesterol to 27-oxygenated products, is present in many cell types, we suggest that 27-oxygenation is a general mechanism for removal of intracellular cholesterol. To evaluate this hypothesis, we measured the net uptake by the human liver of circulating 27-oxygenated products, which was found to be about 20 mg/24 h. This uptake corresponds to approximately 4% of the bile acid production, assuming quantitative conversion into bile acids. It is concluded that the 27-hydroxylase pathway is of significance for elimination of extrahepatic cholesterol.

Effect of etofibrate on bile production in the normolipidemic rat

1. The effect of etofibrate, the ethandiol-1,2 diester of nicotinic and clofibric acids on bile production was studied in male rats that received a daily dose of 300 mg of etofibrate/kg body weight by stomach tube for 10 days and were compared with control rats receiving the medium. 2. The bile duct was cannulated, animals were intravenously given 1 microCi (4-14C)-cholesterol/100 b.w. and bile was collected at different intervals for a total of 4 hr. 3. Etofibrate treatment decreased plasma cholesterol and triglyceride concentrations and increased the bile flow. The cummulative amount of both bile volume and total bile radioactivity secreted increased linearly in all the animals; the respective slopes being higher in etofibrate treated rats than in controls. 4. The main labelled component found in the bile was always bile acids rather than cholesterol and the proportion of each of these compounds was similar in both groups. Neither was any difference between the groups found in the concentration of bile acids, cholesterol and phospholipids nor in the cholesterol/(bile+phospholipid) ratio. 5. Besides other factors, the present results indicate that an increase in bile flow and biliary cholesterol excretion in its free form and after its conversion into bile acids should contribute to the hypocholesterolemic effect of etofibrate.

Structural aspects of bile acids involved in the regulation of cholesterol 7 alpha-hydroxylase and sterol 27-hydroxylase

We have recently reported that coordinate down-regulation of cholesterol 7 alpha-hydroxylase and sterol 27-hydroxylase by bile acids results in suppression of bile acid synthesis in cultured rat hepatocytes [Twisk, J., De Wit, E. & Princen, H. M. G. (1995) Biochem. J. 305, 505-511]. In the current study, we have assessed the effects of a large group of different bile acids, both naturally occurring and synthetic, on these two key enzymes, to elucidate structural features which render bile acids potent as regulators of bile acid synthesis. Addition of 50 microM deoxycholate or cholate, two relatively hydrophobic bile acids, to the culture medium of hepatocytes resulted in strong suppression of cholesterol 7 alpha-hydroxylase (suppression of 75%) and 88%, respectively) and sterol 27-hydroxylase activity (suppression of 76% and 72%, respectively). These effects were also reflected in the mRNA levels and the transcriptional activities of the two enzymes, showing a parallel suppression of both parameters in response to cholate (suppression of 78% and 43% for cholesterol 7 alpha-hydroxylase mRNA and transcription, respectively, and suppression of 76% and 42% for sterol 27-hydroxylase mRNA and transcription, respectively). In contrast, no effects were observed with the two hydrophilic bile acids, beta-muricholate and ursocholate. Transient expression analysis in cultured rat hepatocytes, using a promoter-reporter construct containing the proximal part of the cholesterol 7 alpha-hydroxylase promoter, demonstrated a reduction of transcriptional activity by cholate (reduction of 72%), but not by ursocholate. Assessment of the effects of 27 different bile acids, varying in the number, position and orientation (alpha/beta) of hydroxyl groups on the steroid nucleus of the molecule, on cholesterol 7 alpha-hydroxylase mRNA showed only a moderate correlation with the hydrophobicity index of the bile acid involved (r = 0.61; P < 0.0001). Analysis of the three-dimensional structure of a number of these bile acids suggests that hydroxyl groups situated in close proximity to each other within the molecule, creating a hydrophilic environment, as in the case of cholate, may be a prerequisite for a strong inhibitory potency. Deviation from this situation leads to a markedly lesser effect on suppression of cholesterol 7 alpha-hydroxylase and sterol 27-hydroxylase.

Cloning and expression of cDNA of human delta 4-3-oxosteroid 5 beta-reductase and substrate specificity of the expressed enzyme

The enzyme delta 4-3-oxosteroid 5 beta-reductase (3-oxo-5 beta-steroid: NADP+ oxidoreductase and 4,5 beta-dihydrocortisone: NADP+ delta 4-oxidoreductase) catalyzes the reduction of the delta 4 double bond of bile acid intermediates and steroid hormones carrying the delta 4-3-one structure in the A/B cis configuration. Human delta 4-3-oxosteroid 5 beta-reductase cDNA was isolated from a liver cDNA library by cross hybridization with a previously cloned rat cDNA, which was used as a probe [Onishi, Y. Noshiro, M., Shimosato, T. & Okuda, K.-I. (1991) FEBS Lett. 283, 215-218]. DNA sequence analysis of a hybridization-positive clone predicted the human delta 4-3-oxosteroid 5 beta-reductase to contain 326 amino acids. The amino acid sequence of the human delta 4-3-oxosteroid 5 beta-reductase had 79% overall identity to the rat enzyme sequence. It also showed 54% and 50% overall identity with rat 3 alpha-hydroxysteroid dehydrogenase and human aldose reductase, respectively. RNA blotting analysis demonstrated the existence of a single delta 4-3-oxosteroid 5 beta-reductase mRNA of approximately 2.7 kb in human liver. Transfection of the cDNA into COS cells resulted in the expression of an active enzyme with a high activity toward the bile acid intermediates 7 alpha,12 alpha-dihydroxy-4-cholesten-3-one and 7 alpha-hydroxy-4-cholesten-3-one. In addition, the expressed enzyme showed a small but significant 5 beta-reduction activity toward 11 beta,17 alpha,21-trihydroxy-delta 4-pregnene-3,20-dione (cortisol) and 17 beta-hydroxy-delta 4-androsten-3-one (testosterone) whereas no activity was observed toward delta 4-pregnene-3,20-dione (progesterone) or delta 4-androstene-3-17-dione (androstenedione). The substrate specificity of the human enzyme is considerably narrower than that of the rat enzyme, and the enzyme seems to be more important for bile acid biosynthesis than for metabolism of steroid hormones.

Inborn errors of metabolism with consequences for bile acid biosynthesis. A minireview

Five inborn errors with consequences for bile acid biosynthesis have been described: 7-dehydrocholesterol 7-reductase deficiency, 3 beta-hydroxysteroid delta 5-oxidoreductase/isomerase deficiency, 3-oxo-delta 4-steroid 5 beta-reductase deficiency, sterol 27-hydroxylase deficiency (cerebrotendinous xanthomatosis), and peroxisomal disease(s) with absence of peroxisomes. Diagnosis and treatment of these very rare disorders are discussed. Bile acid therapy is important in most of these disorders and in the case of 3 beta-hydroxysteroid delta 5-oxidoreductase/isomerase deficiency and 3-oxo-delta 4-steroid 5 beta-reductase deficiency such therapy may save the life of the affected cholestatic infant. In the case of sterol 27-hydroxylase deficiency, early treatment with chenodeoxycholic acid may prevent the development of progressive neurological dysfunction, dementia, and ataxia. In the latter three cases early diagnosis and treatment is of utmost importance.

Hepatic cholesterol and bile acid synthesis in Japanese patients with cholesterol gallstones

In Japan the composition of gallstones is changing rapidly from the once-predominant brown-pigment stones to cholesterol ones. The present work was undertaken to clarify the mechanism of cholesterol supersaturated bile production in Japanese patients with cholesterol gallstones. In 26 non-obese and normolipidemic patients (11 with cholesterol gallstones, 8 with black- or brown-pigment gallstones, 7 without gallstones) a liver biopsy and hepatic bile were surgically obtained under standardized conditions. The cholesterol saturation of hepatic bile was significantly higher in cholesterol gallstone patients than in gallstone-free controls (195 +/- 10 vs. 146 +/- 8%, respectively; P < 0.01). The microsomal activities of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-limiting enzyme for cholesterol synthesis, cholesterol 7 alpha-hydroxylase, the rate-limiting enzyme for bile acid synthesis, and 7 alpha-hydroxy-4-cholesten-3-one 12 alpha-hydroxylase (12 alpha-hydroxylase), the rate-limiting enzyme for cholic acid synthesis, were assayed simultaneously in the same subjects. There were positive correlations between HMG-CoA reductase and cholesterol 7 alpha-hydroxylase activities (Rs = 0.62, P < 0.005), and between cholesterol 7 alpha-hydroxylase and 12 alpha-hydroxylase activities (Rs = 0.44, P < 0.05) in all subjects, irrespective of the existence of gallstones. The activities of the three rate-limiting enzymes did not differ significantly among the three groups (cholesterol stone, pigment stone and stone-free). In conclusion, the cholesterol supersaturation of hepatic bile in nonobese and normolipidemic Japanese patients with cholesterol gallstones does not result from an increased hepatic cholesterol synthesis or a decreased bile acid synthesis.