Bile acid synthesis in man: metabolism of 7 -hydroxycholesterol- 14 C and 26-hydroxycholesterol- 3 H

Pulmonary primary oxysterol and bile acid synthesis as a predictor of outcomes in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a rare and fatal vascular disease with heterogeneous clinical manifestations. To date, molecular determinants underlying the development of PAH and related outcomes remain poorly understood. Herein, we identify pulmonary primary oxysterol and bile acid synthesis (PPOBAS) as a previously unrecognized pathway central to PAH pathophysiology. Mass spectrometry analysis of 2,756 individuals across five independent studies revealed 51 distinct circulating metabolites that predicted PAH-related mortality and were enriched within the PPOBAS pathway. Across independent single-center PAH studies, PPOBAS pathway metabolites were also associated with multiple cardiopulmonary measures of PAH-specific pathophysiology. Furthermore, PPOBAS metabolites were found to be increased in human and rodent PAH lung tissue and specifically produced by pulmonary endothelial cells, consistent with pulmonary origin. Finally, a poly-metabolite risk score comprising 13 PPOBAS molecules was found to not only predict PAH-related mortality but also outperform current clinical risk scores. This work identifies PPOBAS as specifically altered within PAH and establishes needed prognostic biomarkers for guiding therapy in PAH.

GC-MS analysis of oxysterols and their formation in cultivated liver cells (HepG2)

Oxysterols play a key role in many (patho)physiological processes and they are potential biomarkers for oxidative stress in several diseases. Here we developed a rapid gas chromatographic-mass spectrometry-based method for the separation and quantification of 11 biologically relevant oxysterols bearing hydroxy, epoxy, and dihydroxy groups. Efficient chromatographic separation (resolution ≥ 1.9) was achieved using a medium polarity 35%-diphenyl/65%-dimethyl polysiloxane stationary phase material (30 m × 0.25 mm inner diameter and 0.25 μm film thickness). Based on thorough analysis of the fragmentation during electron ionization we developed a strategy to deduce structural information of the oxysterols. Optimized sample preparation includes (i) extraction with a mixture of n-hexane/iso-propanol, (ii) removal of cholesterol by solid phase extraction with unmodified silica, and (iii) trimethylsilylation. The method was successfully applied on the analysis of brain samples, showing consistent results with previous studies and a good intra- and interday precision of ≤20%. Finally, we used the method for the investigation of oxysterol formation during oxidative stress in HepG2 cells. Incubation with tert-butyl hydroperoxide led to a massive increase in free radical formed oxysterols (7-keto-chol > 7β-OH-chol >> 7α-OH-chol), while 24 h incubation with the glutathione peroxidase 4 inhibitor RSL3 showed no increase in oxidative stress based on the oxysterol pattern. Overall, the new method described here enables the robust analysis of a biologically meaningful pattern of oxysterols with high sensitivity and precision allowing us to gain new insights in the biological formation and role of oxysterols.

Hydroxycholesterol substitution in ionizable lipid nanoparticles for mRNA delivery to T cells

Delivery of nucleic acids, such as mRNA, to immune cells has become a major focus in the past decade with ionizable lipid nanoparticles (LNPs) emerging as a clinically-validated delivery platform. LNPs-typically composed of ionizable lipids, cholesterol, phospholipids, and polyethylene glycol lipids -have been designed and optimized for a variety of applications including cancer therapies, vaccines, and gene editing. However, LNPs have only recently been investigated for delivery to T cells, which has various therapeutic applications including the engineering of T cell immunotherapies. While several LNP formulations have been evaluated for mRNA delivery, recent work has demonstrated that the utilization of cholesterol analogs may enhance mRNA delivery. Other studies have shown that cholesterols modified with hydroxyl groups can alter endocytic recycling mechanisms. Here, we engineered a library of LNPs incorporating hydroxycholesterols to evaluate their impact on mRNA delivery to T cells by leveraging endosomal trafficking mechanisms. Substitution of 25% and 50% 7α-hydroxycholesterol for cholesterol in LNPs enhanced mRNA delivery to primary human T cells ex vivo by 1.8-fold and 2.0-fold, respectively. Investigation of endosomal trafficking revealed that these modifications also increase late endosome production and reduce the presence of recycling endosomes. These results suggest that hydroxyl modification of cholesterol molecules incorporated into LNP formulations provides a mechanism for improving delivery of nucleic acid cargo to T cells for a range of immunotherapy applications.

Insulin Prevents Hypercholesterolemia by Suppressing 12a-Hydroxylated Bile Acid Production

Background: The risk of cardiovascular disease in type 1 diabetes remains extremely high, despite marked advances in blood glucose control and even the widespread use of cholesterol synthesis inhibitors. Thus, a deeper understanding of insulin regulation of cholesterol metabolism, and its disruption in type 1 diabetes, could reveal better treatment strategies. Methods: To define the mechanisms by which insulin controls plasma cholesterol levels, we knocked down the insulin receptor, FoxO1, and the key bile acid synthesis enzyme, CYP8B1. We measured bile acid composition, cholesterol absorption, and plasma cholesterol. In parallel, we measured markers of cholesterol absorption and synthesis in humans with type 1 diabetes treated with ezetimibe and statins in a double-blind crossover study. Results: Mice with hepatic deletion of the insulin receptor showed marked increases in 12α-hydroxylated bile acids (12HBAs), cholesterol absorption, and plasma cholesterol. This phenotype was entirely reversed by hepatic deletion of FoxO1. FoxO1 is inhibited by insulin, and required for the production of 12HBAs, which promote intestinal cholesterol absorption and suppress hepatic cholesterol synthesis. Knockdown of Cyp8b1 normalized 12HBA levels and completely prevented hypercholesterolemia in mice with hepatic deletion of the insulin receptor (n=5-30) as well as mouse models of type 1 diabetes (n=5-22). In parallel, the cholesterol absorption inhibitor, ezetimibe, normalized cholesterol absorption and LDL-cholesterol in patients with type 1 diabetes as well as, or better than, the cholesterol synthesis inhibitor, simvastatin (n=20). Conclusions: Insulin, by inhibiting FoxO1 in the liver, reduces 12HBAs, cholesterol absorption, and plasma cholesterol levels. Thus, type 1 diabetes leads to a unique set of derangements in cholesterol metabolism, with increased absorption rather than synthesis. These derangements are reversed by ezetimibe, but not statins, which are currently the first line of lipid-lowering treatment in type 1 diabetes. Taken together, these data suggest that a personalized approach to lipid lowering in type 1 diabetes may be more effective and highlight the need for further studies specifically in this group of patients.

Up to date on cholesterol 7 alpha-hydroxylase (CYP7A1) in bile acid synthesis

Cholesterol 7 alpha-hydroxylase (CYP7A1, EC1.14) is the first and rate-limiting enzyme in the classic bile acid synthesis pathway. Much progress has been made in understanding the transcriptional regulation of CYP7A1 gene expression and the underlying molecular mechanisms of bile acid feedback regulation of CYP7A1 and bile acid synthesis in the last three decades. Discovery of bile acid-activated receptors and their roles in the regulation of lipid, glucose and energy metabolism have been translated to the development of bile acid-based drug therapies for the treatment of liver-related metabolic diseases such as alcoholic and non-alcoholic fatty liver diseases, liver cirrhosis, diabetes, obesity and hepatocellular carcinoma. This review will provide an update on the advances in our understanding of the molecular biology and mechanistic insights of the regulation of CYP7A1 in bile acid synthesis in the last 40 years.

A Comprehensive Genome Survey Provides Novel Insights into Bile Salt Hydrolase (BSH) in Lactobacillaceae

Bile salt hydrolase (BSH) is a well-known enzyme that has been commonly characterized in probiotic bacteria, as it has cholesterol-lowering effects. However, its molecular investigations are scarce. Here, we build a local database of BSH sequences from Lactobacillaceae (BSH⁻SDL), and phylogenetic analysis and homology searches were employed to elucidate their comparability and distinctiveness among species. Evolutionary study demonstrates that BSH sequences in BSH⁻SDL are divided into five groups, named BSH A, B, C, D and E here, which can be the genetic basis for BSH classification and nomenclature. Sequence analysis suggests the differences between BSH-active and BSH-inactive proteins clearly, especially on site 82. In addition, a total of 551 BSHs from 107 species are identified from 451 genomes of 158 Lactobacillaceae species. Interestingly, those bacteria carrying various copies of BSH A or B can be predicted to be potential cholesterol-lowering probiotics, based on the results of phylogenetic analysis and the subtypes that those previously reported BSH-active probiotics possess. In summary, this study elaborates the molecular basis of BSH in Lactobacillaceae systematically, and provides a novel methodology as well as a consistent standard for the identification of the BSH subtype. We believe that high-throughput screening can be efficiently applied to the selection of promising candidate BSH-active probiotics, which will advance the development of healthcare products in cholesterol metabolism.

Bile acids in glucose metabolism in health and disease

Bile acids (BAs) are cholesterol-derived metabolites that facilitate the intestinal absorption and transport of dietary lipids. Recently, BAs also emerged as pivotal signaling molecules controlling glucose, lipid, and energy metabolism by binding to the nuclear hormone farnesoid X receptor (FXR) and Takeda G protein receptor 5 (TGR5) in multiple organs, leading to regulation of intestinal incretin secretion, hepatic gluconeogenesis, glycogen synthesis, energy expenditure, inflammation, and gut microbiome configuration. Alterations in BA metabolism and signaling are associated with obesity and type 2 diabetes mellitus (T2DM), whereas treatment of T2DM patients with BA sequestrants, or bariatric surgery in morbidly obese patients, results in a significant improvement in glycemic response that is associated with changes in the BA profile and signaling. Herein, we review the roles of BAs in glucose metabolism in health and disease; highlight the limitations, unknowns, and challenges in understanding the impact of BAs on the glycemic response; and discuss how this knowledge may be harnessed to develop innovative therapeutic approaches for the treatment of hyperglycemia and diabetes.

Novel bile acid therapeutics for the treatment of chronic liver diseases

Recent developments in understanding the role of bile acids (BAs) as signalling molecules in human metabolism and inflammation have opened new avenues in the field of hepatology research. BAs are no longer considered as simple molecules helping in fat digestion but as agents with real therapeutic value in treating complex autoimmune and metabolic liver diseases. BAs and their receptors such as farnesoid X receptor, transmembrane G protein-coupled receptor 5 and peroxisome proliferator-activated receptor have been identified as novel targets for drug development. Some of these novel pharmaceuticals are already in clinical evaluation with the most advanced drugs having reached phase III trials. Chronic liver diseases such as primary biliary cholangitis, primary sclerosing cholangitis and nonalcoholic fatty liver disease, for which there is no or limited pharmacotherapy, are most likely to gain from these developments. In this review we discuss recent and the most relevant basic and clinical research findings related to BAs and their implications for novel therapy for chronic liver diseases.

Breast cancer and (25R)-26-hydroxycholesterol

(25R)-26-Hydroxycholesterol (27-hydroxycholesterol) has been found to accumulate in breast tissue and to stimulate tumor growth via the estrogen receptor. Although most tissues express CYP27A1, the highest levels are in macrophages and most attention had been given to the production of 27-hydroxycholesterol in sub-endothelial macrophages as part of reverse cholesterol transport. In view of the newly identified biologic activity, it is important to consider the determinants of the levels of 27-hydroxycholesterol in macrophages that infiltrate breast tissue. Among these determinants are the oxysterol binding proteins expressed in macrophages, the level of expression of CYP7B1, the oxysterol 7 alpha hydroxylase that generates an inactive triol, and further oxidation of 27-hydroxycholestrol to the C27 acid by multifunctional CYP27A1. Transport of 27-hydroxycholesterol from macrophages to plasma is HDL-associated. In many tissues the ratio of 27-hydroxycholesterol to cholesterol (ng/μg) is higher than that in plasma. Tamoxifen, an effective estrogen receptor antagonist that prevents breast cancer, also has the biologic property of blocking several steps in the lanosterol to cholesterol metabolic pathway. In genetically disposed women, tamoxifen may increase the amount of 27-hydroxycholesterol in breast tissue.

Pleiotropic roles of bile acids in metabolism

Enzymatic oxidation of cholesterol generates numerous distinct bile acids that function both as detergents that facilitate digestion and absorption of dietary lipids, and as hormones that activate four distinct receptors. Activation of these receptors alters gene expression in multiple tissues, leading to changes not only in bile acid metabolism but also in glucose homeostasis, lipid and lipoprotein metabolism, energy expenditure, intestinal motility and bacterial growth, inflammation, liver regeneration, and hepatocarcinogenesis. This review covers the roles of specific bile acids, synthetic agonists, and their cognate receptors in controlling these diverse functions, as well as their current use in treating human diseases.

Features of the retinal environment which affect the activities and product profile of cholesterol-metabolizing cytochromes P450 CYP27A1 and CYP11A1

The retina is the sensory organ in the back of the eye which absorbs and converts light to electrochemical impulses transferred to the brain. Herein, we studied how retinal environment affects enzyme-mediated cholesterol removal. We focused on two mitochondrial cytochrome P450 enzymes, CYPs 27A1 and 11A1, which catalyze the first steps in metabolism of cholesterol in the retina and other tissues. Phospholipids (PL) from mitochondria of bovine neural retina, retinal pigment epithelium, liver and adrenal cortex were isolated and compared for the effect on kinetic properties of purified recombinant CYPs in the reconstituted system in vitro. The four studied tissues were also evaluated for the mitochondrial PL and cholesterol content and levels of CYPs 27A1, 11A1 and their redox partners. The data obtained were used for modeling the retinal environment in the in vitro enzyme assays in which we detected the P450 metabolites, 22R-hydroxycholesterol and 5-cholestenoic acid, unexpectedly found by us in the retina in our previous studies. The effect of the by-product of the visual cycle pyridinium bis-retinoid A2E on kinetics of CYP27A1-mediated cholesterol metabolism was also investigated. The results provide insight into the retina's regulation of the enzyme-mediated cholesterol removal.

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.

Effects of dietary oxysterols on coronary arteries in hyperlipidaemic hamsters

The aim of this study was to evaluate the effect of dietary oxysterols on coronary atherosclerosis and vasospasm. Golden Syrian hamsters were fed three diets with different lipid contents for 3 months: (1) a normolipidaemic diet containing 25 g corn oil–fish oil (4:1, w/w)/kg (group Low L); (2) a hyperlipidaemic diet composed of the normolipidaemic diet supplemented with 150 g lard+30 g cholesterol/kg (group High L); (3) a third diet, similar to the hyperlipidaemic diet, in which 4 g cholesterol/kg was replaced by a mixture of oxysterols (group High L+OS). The oxysterol mixture contained (g/kg): 5,6α-epoxycholesterol 211, 5,6β-epoxycholesterol 179, 7α-hydroxycholesterol 67, 7β-hydroxycholesterol (7βOH) 185, 7-ketocholesterol (7 K) 235; and trace amounts of 7-hydroperoxycholesterols (approximately 30 g/kg). Atherosclerosis was evaluated by measuring myocardial Ca, oxysterols and acyl-CoA cholesterol acyl transferase (ACAT) activity; furthermore, coronary reactivity to sodium nitroprusside (5×10-6 m) was measured and the morphology of coronary arteries was visualized by transmission electron microscopy. Coronary spasm was determined by evaluating reactivity to serotonin (5×10-6 m). Feeding the high-lipid diet (group High L) increased the plasma level of 7βOH, 7 K and cholestanetriol. The presence of oxysterols in the diet (group High L+OS) further increased the concentrations of 7βOH and 7 K in the plasma. However, as evidenced by myocardial Ca, ACAT activity and coronary reactivity to sodium nitroprusside, severe atherosclerosis did not develop during the 3-month diet. 7 K was increased in myocardial lipids of groups High L and High L+OS. Electron microscopy did not show the development of atherosclerosis in group High L, whereas vascular wall thickening, endothelial damage and smooth muscle cell proliferation and migration occurred when oxysterols were present in the food. Serotonin (5×10-6 m) induced exacerbated coronary vasoconstriction in group High L that was completely reversed by dietary oxysterols. In conclusion, dietary oxysterols exhibit anti-spasmodic properties, but they cannot be used as agents against excess dietary lipid-induced coronary spasm because of their atherogenic properties.

Cytochrome P450s and cholesterol homeostasis

By participating in pathways of cholesterol biosynthesis and elimination, different cytochrome P450 (P450 or CYP) enzymes play an important role in maintenance of cholesterol homeostasis. CYP51 is involved in cholesterol biosynthesis, whereas CYP 7A1, 27A1, 46A1, 7B1, 39A1, and 8B1 are the key enzymes in cholesterol catabolism to bile acids, the major route of cholesterol elimination in mammals. Cholesterol transformations to steroid hormones are also initiated by the P450 enzyme CYP11A1. Finally, one of the major drug-metabolizing P450s CYP3A4 seems to contribute to bile acid biosynthesis as well. The 9 P450s will be the focus of this review and assessed as drug targets for cholesterol lowering.

CHOLESTEROL-METABOLIZING CYTOCHROMES P450

By catalyzing the first steps in different pathways of cholesterol degradation, cytochromes P450 (P450s) 7A1, 27A1, 11A1, and 46A1 play key roles in cholesterol homeostasis. CYP7A1 is a microsomal liver-specific enzyme that converts cholesterol to 7alpha-hydroxycholesterol. CYP27A1 is a ubiquitously expressed mitochondrial P450 that metabolizes cholesterol to 27-hydroxycholesterol. CYP11A1 also resides in mitochondria but is expressed mainly in steroidogenic tissues, where it catalyzes the conversion of cholesterol to pregnenolone. Finally, CYP46A1 is a brain-selective microsomal monooxygenase producing 24S-hydroxycholesterol from cholesterol. Catalytic efficiencies of cholesterol-metabolizing P450s vary significantly and probably reflect physiological requirements of different organs for the rate of cholesterol turnover. P450s 7A1, 27A1, 11A1, and 46A1 represent a unique system for elucidation of how different enzymes have adapted to fit their specific roles in cholesterol elimination. Studies of cholesterol-metabolizing P450s suggest that their activities could be modulated post-translationally and that they should also be considered as targets for regulation of cholesterol homeostasis.

Effects of dietary 27-hydroxycholesterol on cholesterol metabolism and bile acid biosynthesis in the hamster

27-hydroxycholesterol (27OH-Chol) is an important endogenous oxysterol resulting from the action of sterol 27-hydroxylase (CYP27A1) on cholesterol in the liver and numerous extrahepatic tissues. It may act as a modulator of cholesterol and bile acid metabolism. The effects of 27OH-Chol on the main enzymes and receptors of cholesterol metabolism were investigated by feeding male hamsters a diet supplemented with 27OH-Chol (0.1% w/w) for 1 week. Intestinal scavenger class B, type I (SR-BI) protein level was decreased (–65%), but hepatic expression was increased (+34%). Liver 3β-hydroxy-3β-methyl glutaryl coenzyme A reductase (–58%), cholesterol 7α-hydroxylase (–54%), oxysterol 7α-hydroxylase (–44%), and sterol 12α-hydroxylase (–70%) activities were all decreased. Bile acid composition was changed (fourfold increase in the chenodeoxycholic/cholic acid ratio). This study demonstrates that dietary 27OH-Chol modulates major enzymes of cholesterol metabolism and alters the biliary bile acid profile, making it more hydrophobic, at least at this level of intake. Its effects on SR-BI protein levels are organ dependent. The properties of 27OH-Chol or its metabolites on cholesterol metabolism probably result from the activation of specific transcription factors. Key words: cholesterol 7α-hydroxylase (CYP7A1), sterol 12α-hydroxylase (CYP8B1), sterol 27-hydroxylase (CYP27A1), 3β-hydroxy-3β-methyl glutaryl coenzyme A reductase (HMGCoAR), scavenger receptor class B type I (SR-BI).

Membrane-protein interactions contribute to efficient 27-hydroxylation of cholesterol by mitochondrial cytochrome P450 27A1

Mitochondrial cytochrome P450 27A1 (P450 27A1) catalyzes 27-hydroxylation of cholesterol, the first step in the alternative bile acid biosynthetic pathway. Although several crystal structures of P450s are known, no structural information is available for the mammalian, membrane-bound enzymes involved in the removal of cholesterol from the body. We prepared a three-dimensional model of P450 27A1 based on the structure of P450 BM-3. Conservative and non-conservative mutations were introduced at hydrophobic and positively charged residues in the putative F-G loop and the adjacent helix G (positions 219-237). Subcellular distribution of the mutant P450s expressed in Escherichia coli was used as a measure of membrane-protein interactions. Conservative substitutions of residues located on the surface, according to our model, L219V, L219I, Y220F, F223Y, L224I, R229K, V231L, F234Y, K236R, and R237K, weakened the association of the mutant P450s with the membrane and led to the appearance of up to 21% of P450 27A1 in the bacterial cytosol. It is likely that the mutated side chains are involved in binding to membrane phospholipids. Substitutions in the F-G loop did not significantly affect the K(m) value for cholesterol hydroxylation. However, non-conservative mutants, L219N, Y220A, Y220S, F223A, K226R, and R229A, had significantly impaired catalytic properties, indicating strict requirements for the size and polarity of the side chains at these positions for the catalysis. The results provide insight into the membrane topology of mitochondrial P450s and indicate the importance of membrane-protein interactions in the efficiency of reactions catalyzed by P450 27A1.

Cholesterol, hydroxycholesterols, and bile acids

Although a variety of oxidation products of cholesterol occur in vitro, enzyme-catalyzed oxidations can occur at only 5 sites on the cholesterol molecule: C7alpha, C22R, C24S, C25, and C27. The genes coding for the synthesis of these enzymes were cloned, the tissue expressions of the mRNAs were identified, and the enzymes were characterized. The biologic properties of the hydroxycholesterol molecules that are initially generated and their metabolites are under study. Downregulation of cholesterol synthesis via the SREBP/SCAP regulatory pathway is common to the initial hydroxycholesterols, but more variations exist with respect to these intermediates functioning as ligands for the nuclear receptor LXRalpha. Because this receptor regulates the expression of cholesterol 7alpha-hydroxylase and ABC transporter proteins, hydroxycholesterols and their intermediate steroid metabolites modulate a number of biologic processes. Metabolism of 22S-hydroxycholesterol to steroid hormones differs from that of the other hydroxycholesterols which form mostly steroid acidic products, otherwise known as bile acids. In vivo estimates of their production rates in intact humans indicate that 24S and 25-hydroxycholesterol account for no more than 7% of total bile acid production per day. Current evidence indicates that cholesterol 7alpha-hydroxycholesterol generated in the liver is the major source of bile acids in older adults. It is also known that the cholesterol 27-hydroxylation pathway is the only one expressed in fetal and neonatal life. Precisely when the proportions contributed by these two metabolic pathways to bile acid synthesis begin to shift and the role of the cholesterol 27-hydroxylase pathway in reverse cholesterol transport mandate further study.

Conversion of 7 alpha-hydroxycholesterol to bile acid in human subjects: is there an alternate pathway favoring cholic acid synthesis?

Despite the fact that most human subjects synthesize about twice as much cholic acid as chenodeoxycholic acid, available evidence suggests that 7 alpha-hydroxycholesterol, the first intermediate in the major pathway for bile acid synthesis, is converted about equally to these two bile acids. Synthesis through the main alternate pathway can not explain this discrepancy because 27-hydroxycholesterol, the first intermediate in that pathway, is converted preferentially to chenodeoxycholic acid. To examine the validity of these contradictory observations, we administered (24-(14)C)-cholic acid and (24-(14)C)-chenodeoxycholic acid together with (7 beta-(3)H)-7 alpha-hydroxycholesterol on one occasion and (22,23-(3)H)-27-hydroxycholesterol on a separate occasion to eight normal human subjects. Synthesis of the two primary bile acids was determined by means of standard isotope dilution kinetics of the carbon 14-specific activities of biliary bile acids. Conversion of (7 beta-(3)H)-7 alpha-hydroxycholesterol and (22,23-(3)H)-27-hydroxycholesterol to bile acid was calculated from the tritium/carbon 14 ratio in cholic and chenodeoxycholic acid. For synthesis, the mean +/- SEM cholic/chenodeoxycholic ratio was 1.82 +/- 0.26. For apparent conversion of (7 beta-(3)H)-7 alpha-hydroxycholesterol to bile acid, the mean +/- SEM cholic/ chenodeoxycholic ratio was 1.02 +/- 0.09, whereas for (22,23(3)H)-27-hydroxycholesterol, the mean +/- SEM cholic/chenodeoxycholic ratio was 0.38 +/- 0.03. These data imply that, on average, more than 40% of cholic acid in these subjects was synthesized through a pathway that bypassed initial 7 alpha-hydroxylation. However, consideration of all potential candidates for such a pathway raises doubts that any of them contributes substantially to bile acid synthesis.

Expression of cholesterol 7alpha-hydroxylase restores bile acid synthesis in McArdle RH7777 cells

Bile acid synthesis involves several enzymes and occurs only in liver cells. The first and rate-determining step is catalyzed by cholesterol 7alpha-hydroxylase (cyp7a). McArdle RH7777 hepatoma cells do not synthesize bile acids and do not express the cyp7a gene. A synthetic cyp7a gene was stably expressed in this cell line to determine if restoration of cyp7a activity is sufficient to reconstitute the bile acid synthetic pathway. The transfected cells contained the recombinant cyp7a mRNA and the corresponding protein. Microsomes from recombinant cells converted cholesterol into 7alpha-hydroxycholesterol, indicating that the recombinant enzyme was active. Radiolabeled bile acids, originated from exogenously supplied radiolabeled cholesterol, were detected in the culture medium of recombinant cells. Thus, expression of cyp7a is sufficient in restoring bile acid synthesis in McArdle RH7777 cells. The results also show that the additional complement of enzymatic activities required to convert cholesterol into bile acids has remained active in this cell line.

Intracellular transport of bile acids

Bile acids originate from the liver and are transported via bile to the intestines where they perform an important role in the absorption of lipids and lipid-soluble nutrients. Most of the bile acids are reclaimed from the terminal ileum and returned to the liver via portal blood for reuse. The transport of bile acids is vectorial in both liver and intestinal cells, originating and terminating at opposite poles. Bile acids enter through the basolateral pole in liver cells, and through the apical pole in intestinal cells. During the past decade, much has been learned about the mechanisms by which bile acids enter and exit liver and intestinal cells. By contrast, the mechanisms by which bile acids are transported across cells remain poorly understood. The current body of evidence suggests that bile acids do not traverse the cell by vesicular transport. Although a carrier-mediated mechanism is a likely alternative, only a handful of intracellular proteins capable of binding bile acids have been described. The significance of these proteins in the intracellular transport of bile acids remains to be tested.

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.

Kinetics and plasma concentrations of 26-hydroxycholesterol in baboons

26-Hydroxycholesterol (26OHC), a major oxysterol in human blood, is believed to play an important role in reverse cholesterol transport, bile acid formation, and regulation of various cellular processes. Using isotope dilution mass spectrometry, we measured plasma 26OHC concentrations in baboons fed either a high cholesterol/saturated fat (HC-SF) or normal chow diet. Plasma 26OHC levels in baboons were comparable to those reported for humans and were positively correlated with plasma cholesterol concentrations. Animals on the HC-SF diet had significantly higher 26OHC levels (0.274+/-0.058 microM, mean+/-S.D.) than those on the chow diet (0.156+/-0.046 microM). In separate experiments, [(3)H]26OHC was injected into four tethered baboons, and multiple blood samples drawn over a 1-h period were analyzed for [(3)H]26OHC and 26OHC. Fitting the specific radioactivity data to a two-pool compartmental model indicated a rapidly turning over plasma compartment (t(1/2) 2.9-6.0 min) and a second compartment with slow turnover (t(1/2) 76-333 min). The calculated 26OHC production rate was 2.5 micromol/kg body weight/day. Assuming all 26OHC is converted to bile acids, the 26OHC production rate corresponds to about 10% of total bile acid production in adult baboons. These results indicate that rapid turnover of plasma 26OHC at submicromolar concentrations could significantly contribute to bile acid synthesis.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

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.

Lipoprotein cholesterol uptake mediates up-regulation of bile-acid synthesis by increasing cholesterol 7alpha-hydroxylase but not sterol 27-hydroxylase gene expression in cultured rat hepatocytes

Lipoproteins may supply substrate for the formation of bile acids, and the amount of hepatic cholesterol can regulate bile-acid synthesis and increase cholesterol 7alpha-hydroxylase expression. However, the effect of lipoprotein cholesterol on sterol 27-hydroxylase expression and the role of different lipoproteins in regulating both enzymes are not well established. We studied the effect of different rabbit lipoproteins on cholesterol 7alpha-hydroxylase and sterol 27-hydroxylase in cultured rat hepatocytes. beta-Migrating very-low-density lipoprotein (betaVLDL) and intermediate-density lipoprotein (IDL) caused a significant increase in the intracellular cholesteryl ester content of cells (2. 3- and 2-fold, respectively) at a concentration of 200 microgram of cholesterol/ml, whereas high-density lipoprotein (HDL, 50% v/v), containing no apolipoprotein E (apo E), showed no effect after a 24-h incubation. betaVLDL and IDL increased bile-acid synthesis (1. 9- and 1.6-fold, respectively) by up-regulation of cholesterol 7alpha-hydroxylase activity (1.7- and 1.5-fold, respectively). Dose- and time-dependent changes in cholesterol 7alpha-hydroxylase mRNA levels and gene expression underlie the increase in enzyme activity. Incubation of cells with HDL showed no effect. Sterol 27-hydroxylase gene expression was not affected by any of the lipoproteins added. Transient-expression experiments in hepatocytes, transfected with a promoter-reporter construct containing the proximal 348 nucleotides of the rat cholesterol 7alpha-hydroxylase promoter, showed an enhanced gene transcription (2-fold) with betaVLDL, indicating that a sequence important for a cholesterol-induced transcriptional response is located in this part of the cholesterol 7alpha-hydroxylase gene. The extent of stimulation of cholesterol 7alpha-hydroxylase is associated with the apo E content of the lipoprotein particle, which is important in the uptake of lipoprotein cholesterol. We conclude that physiological concentrations of cholesterol in apo E-containing lipoproteins increase bile-acid synthesis by stimulating cholesterol 7alpha-hydroxylase gene transcription, whereas HDL has no effect and sterol 27-hydroxylase is not affected.

An additional electrostatic interaction between adrenodoxin and P450c27 (CYP27A1) results in tighter binding than between adrenodoxin and p450scc (CYP11A1)

Mitochondrial cytochrome P450c27 (product of the CYP27A1 gene) is found to have significantly higher affinity for the common redox partner adrenodoxin than another mitochondrial P450, P450scc (product of the CYP11A1 gene). To investigate the basis of the approximately 30-fold difference in adrenodoxin binding, two sets of P450c27 mutants were generated, expressed in Escherichia coli, and purified. Mutations of one set were within the putative adrenodoxin-binding site containing conserved lysine residues also crucial in P450scc for binding adrenodoxin. The second set included mutations within a sequence aligning with the "meander region" of P450BM-3 proposed to be a site of redox-partner interactions in P450s (Hasemann, C. A., Kurumbail, R. G., Boddupalli, S. S., Peterson, J. A., and Deisenhofer, J. (1995) Structure 3, 41-62). Mutation of the P450c27 conserved lysines (K354A and K358A) led to a approximately 20-fold increase in apparent Ks for adrenodoxin, confirming that these two positively charged residues conserved in mitochondrial P450s are important for adrenodoxin binding. Mutation of Arg-418, conserved in the CYP27A1 family, to serine also decreased the affinity for adrenodoxin approximately 20-fold. This residue is predicted to be located in the meander region. A triple K354A/K358A/R418S mutation profoundly reduced adrenodoxin binding. Thus, in contrast to P450scc, where mutation of the two conserved positively charged residues results in virtually complete inhibition of adrenodoxin binding, in P450c27 there are three of such residues (Lys-354, Lys-358, and Arg-418) important for adrenodoxin interaction.

Activities of recombinant human cytochrome P450c27 (CYP27) which produce intermediates of alternative bile acid biosynthetic pathways

The primary physiological significance of cytochrome P450c27 (CYP27) has been associated with its role in the degradation of the side chain of C27 steroids in the hepatic bile acid biosynthesis pathway, which begins with 7alpha-hydroxylation of cholesterol in liver. However, recognition that in humans P450c27 is a widely or ubiquitously expressed mitochondrial P450, and that there are alternative pathways of bile acid synthesis which begin with 27-hydroxylation of cholesterol catalyzed by P450c27, suggests the need to reevaluate the role of this enzyme and its catalytic properties. 27-Hydroxycholesterol was thought to be the only product formed upon reaction of P450c27 with cholesterol. However, the present study demonstrates that recombinant human P450c27 is also able to further oxidize 27-hydroxycholesterol giving first an aldehyde and then 3beta-hydroxy-5-cholestenoic acid. Kinetic data indicate that in a reconstituted system, after 27-hydroxycholesterol is formed from cholesterol, it is released from the P450 and then competes with cholesterol for reentry the enzyme active site for further oxidation. Under subsaturating substrate concentrations, the efficiencies of oxidation of 27-hydroxycholesterol and 3beta-hydroxy-5-cholestenal to the acid by human P450c27 are greater than the efficiency of hydroxylation of cholesterol to 27-hydroxycholesterol indicating that the first hydroxylation step in the overall conversion of cholesterol into 3beta-hydroxy-5-cholestenoic acid is rate-limiting. Interestingly, 3beta-hydroxy-5-cholestenoic acid was found to be further metabolized by the recombinant human P450c27, giving two monohydroxylated products with the hydroxyl group introduced at different positions on the steroid nucleus.

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.

Formation of cholic acid and chenodeoxycholic acid from 7 alpha-hydroxycholesterol and 27-hydroxycholesterol by primary cultures of human hepatocytes

It has been suggested that chenodeoxycholic acid is preferentially formed by the alternative or 'acidic' pathway of bile acid biosynthesis starting with 27-hydroxylation of cholesterol, while cholic acid is derived from 7 alpha-hydroxycholesterol which initiates the 'neutral' pathway. We have studied bile acid formation from each of these precursors using human hepatocytes cultured in a novel sandwich collagen configuration. Culture supernatants were analyzed using capillary gas chromatography and gas chromatography-mass spectrometry. 27-Hydroxycholesterol and 7 alpha-hydroxycholesterol were both found to be efficiently converted to cholic acid as well as chenodeoxycholic acid. Analysis of acidic intermediates after addition of 7 alpha-hydroxycholesterol to the cultures revealed a significant increase of side-chain oxygenated C24- and C27-steroids with a 3-oxo-7 alpha-hydroxy-delta 4-ring structure. These data indicate that (i) the 'neutral' pathway is connected to the 'acidic' pathway by side-chain oxidation of C27-steroids with a 3-oxo-7 alpha-hydroxy-delta 4-ring structure and that (ii) the relative formation of cholic acid and chenodeoxycholic acid is regulated by metabolic events distal to the initial hydroxylation at either position 7 or position 27 of the cholesterol molecule.

Regulation of sterol 27-hydroxylase and an alternative pathway of bile acid biosynthesis in primary cultures of rat hepatocytes

In man, hepatic mitochondrial sterol 27-hydroxylase and microsomal cholesterol 7alpha-hydroxylase initiate distinct pathways of bile acid biosynthesis from cholesterol, the "acidic" and "neutral" pathways, respectively. A similar acidic pathway in the rat has been hypothesized, but its quantitative importance and ability to be regulated at the level of sterol 27-hydroxylase are uncertain. In this study, we explored the molecular regulation of sterol 27-hydroxylase and the acidic pathway of bile acid biosynthesis in primary cultures of adult rat hepatocytes. mRNA and protein turnover rates were approximately 10-fold slower for sterol 27-hydroxylase than for cholesterol 7alpha-hydroxylase. Sterol 27-hydroxylase mRNA was not spontaneously expressed in culture. The sole requirement for preserving sterol 27-hydroxylase mRNA at the level of freshly isolated hepatocytes (0 h) after 72 h was the addition of dexamethasone (0.1 microM; > 7-fold induction). Sterol 27-hydroxylase mRNA, mass and specific activity were not affected by thyroxine (1.0 microM), dibutyryl-cAMP (5O microM), nor squalestatin 1 (15O nM-1.0 microM), an inhibitor of cholesterol biosynthesis. Taurocholate (50 microM), however, repressed sterol 27-hydroxylase mRNA levels by 55%. Sterol 27-hydroxylase specific activity in isolated mitochondria was increased > 10-fold by the addition of 2-hydroxypropyl-beta-cyclodextrin. Under culture conditions designed to maximally repress cholesterol 7alpha-hydroxylase and bile acid synthesis from the neutral pathway but maintain sterol 27-hydroxylase mRNA and activity near 0 h levels, bile acid synthesis from [14C]cholesterol remained relatively high and consisted of beta-muricholate, the product of chenodeoxycholate in the rat. We conclude that rat liver harbors a quantitatively important alternative pathway of bile acid biosynthesis and that its initiating enzyme, sterol 27-hydroxylase, may be slowly regulated by glucocorticoids and bile acids.

Suppression of sterol 27-hydroxylase mRNA and transcriptional activity by bile acids in cultured rat hepatocytes

In previous work we have demonstrated suppression of cholesterol 7 alpha-hydroxylase by bile acids at the level of mRNA and transcription, resulting in a similar decline in bile acid synthesis in cultured rat hepatocytes [Twisk, Lehmann and Princen (1993) Biochem. J. 290, 685-691]. In view of the substantial contribution of the 'alternative' or '27-hydroxylase' route to total bile acid synthesis, as demonstrated in cultured rat hepatocytes and in vivo in humans, we here evaluate the effects of various bile acids commonly found in bile of rats on the regulation of sterol 27-hydroxylase in cultured rat hepatocytes. Addition of taurocholic acid, the predominant bile acid in rat bile, to the culture medium of rat hepatocytes resulted in a 72% inhibition of sterol 27-hydroxylase activity. The effect was exerted at the level of sterol 27-hydroxylase mRNA, showing a time- and dose-dependent decline with a maximal suppression (-75%) at 50 microM taurocholic acid after 24 h of culture. The decline in mRNA followed first-order kinetics with an apparent half-life of 13 h. Under these conditions cholesterol 7 alpha-hydroxylase mRNA (-91%) and bile acid synthesis (i.e. chenodeoxycholic and beta-muricholic acid, -81%) were also maximally suppressed. In contrast, no change was found in the level of lithocholic acid 6 beta-hydroxylase mRNA. Assessment of the transcriptional activity of a number of genes involved in routing of cholesterol towards bile acids showed similar suppressive effects of taurocholate on expression of the sterol 27-hydroxylase and cholesterol 7 alpha-hydroxylase genes (-43% and -42% respectively), whereas expression of the lithocholic 6 beta-hydroxylase gene was not affected. Taurocholic acid and unconjugated cholic acid were equally as effective in suppressing sterol 27-hydroxylase mRNA. The more hydrophobic bile acids, chenodeoxycholic acid and deoxycholic acid, also produced a strong inhibition of 57% and 76% respectively, whereas the hydrophilic beta-muricholic acid was not active. We conclude that (1) a number of bile acids, at physiological concentrations, suppress sterol 27-hydroxylase by down-regulation of sterol 27-hydroxylase mRNA and transcriptional activity and (2) co-ordinated suppression of both sterol 27-hydroxylase and cholesterol 7 alpha-hydroxylase results in inhibition of bile acid synthesis in cultured rat hepatocytes.

Atherosclerosis and sterol 27-hydroxylase: evidence for a role of this enzyme in elimination of cholesterol from human macrophages

27-Hydroxycholesterol was found in surprisingly high amounts in atherosclerotic human femoral arteries. When human macrophages were cultured in a medium containing serum, there was a significant transfer of 27-hydroxy-cholesterol and 3 beta-hydroxy-5-cholestenoic acid from the cells into the medium. Sterol 27-hydroxylase (EC 1.14.13.15) is likely to be responsible for formation of the two products as shown by use of immunoblotting, a specific inhibitor, and the 18O-labeling technique. Sterol 27-hydroxylase has the unusual ability to hydroxylate the same methyl group three times to give a carboxylic acid; thus, 3 beta-hydroxy-5-cholestenoic acid is likely to be a direct product of the enzyme. The production of these steroids increased after addition of cholesterol to the culture medium. By using deuterium-labeled cholesterol, it was ascertained that most of the oxidized products were formed from exogenous cholesterol taken up by the cells. 27-Hydroxycholesterol and 3 beta-hydroxy-5-cholestenoic acid are present in the circulation and are efficiently converted into bile acids in human liver. It is suggested that conversion of cholesterol into 27-hydroxycholesterol and 3 beta-hydroxy-5-cholestenoic acid represents a general defence mechanism for macrophages and possibly also other peripheral cells exposed to cholesterol. Absence of this defence mechanism may contribute to the premature atherosclerosis known to occur in patients with sterol 27-hydroxylase deficiency (cerebrotendinous xanthomatosis).

Apical excrescences in the gallbladder epithelium of the female Syrian hamster in response to medroxyprogesterone

All the intact female Syrian hamsters treated with medroxyprogesterone (MP) for a one-month period, without dietary manipulation, display gallbladder surface epithelial changes, and intraluminal deposits. These changes include excrescences in various stages, bulging, and extrusion of material from the epithelial cells. The most striking scanning electron microscopic observations are the dramatic events, comparable to apocrine-like secretory events observed in another related study using oophorectomized hamsters. Since the hamster gallbladder does not possess mucous goblet cells, it appears that this phenomenon could be a response to the MP treatment, thus providing a larger amount of mucous product than usual with cellular material, in addition to the possible alteration in the quality of the bile following this treatment. As a result of MP treatment, intraluminal deposits were also confirmed by using light and transmission electron microscopy. In control hamsters these events were not observed, however, small blebs outlining surface epithelial cells are seen. The results in this report complement the previous studies using the male and oophorectomized Syrian hamster model subjected to similar experimental conditions.

Selective inhibition of mitochondrial 27-hydroxylation of bile acid intermediates and 25-hydroxylation of vitamin D3 by cyclosporin A

It was demonstrated recently that cyclosporin A blocks bile acid synthesis in cultured rat and human hepatocytes by specific inhibition of chenodeoxycholic acid formation. The site of inhibition was found to be the 27-hydroxylation of cholesterol catalysed by the liver mitochondrial 27-hydroxylase [Princen, Meijer, Wolthers, Vonk and Kuipers (1991) Biochem J. 275, 501-505]. In this paper the mechanism by which cyclosporin A blocks mitochondrial 27-hydroxylation was further investigated. It is shown that cyclosporin A inhibited 27-hydroxylation of bile acid intermediates, depending on their polarity. In isolated rat liver mitochondria, 27-hydroxylation of cholesterol was dose-dependently blocked by the drug, giving half-maximal inhibition at 4 microM, whereas 27-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol was not affected. A similar observation was made using electrophoretically homogeneous cytochrome P-450(27) isolated from rabbit liver mitochondria, excluding the possibility that cyclosporin A interfered with transport of substrates into the mitochondrion. Kinetic studies showed that inhibition of the 27-hydroxylation of cholesterol by cyclosporin A was of a non-competitive type. The drug also inhibited the 25-hydroxylase activity towards vitamin D3, catalysed by the same enzyme preparation, to the same extent as 27-hydroxylation of cholesterol. These results suggest that cyclosporin A may interfere with binding of cholesterol, but not of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol, to the active site of the enzyme. These data provide an explanation for the selective inhibition of chenodeoxycholic acid synthesis.

Synthesis of potential C27-intermediates in bile acid biosynthesis and their deuterium-labeled analogs

In connection with studies of alternative pathways in bile acid biosynthesis, potential intermediates in a pathway starting with 27-hydroxylation of cholesterol have been prepared in natural and deuterated forms. Established methods were used to prepare 27-hydroxycholesterol and 3 beta-hydroxy-5-cholestenoic acid. Clemmensen reduction of kryptogenin in unlabeled and deuterated solvents yielded 27-hydroxy-cholesterol and 16-oxo-5-cholestene-3 beta,27-diol, which were separated by adsorption chromatography on Unisil. The labeled 27-hydroxycholesterol and 3 beta-hydroxy-5-cholestenoic acid derived from it consisted of molecules with seven (50%), six (20%), and eight (20%) deuterium atoms, and unlabeled molecules were not detected. The acetates of 27-hydroxycholesterol and methyl 3 beta-hydroxy-5-cholestenoate were 7 alpha-hydroxylated in a copper-catalyzed reaction with tert-butylperbenzoate, and the products were purified by chromatography on Unisil. The 7 beta-epimers were obtained as side products. Labeled 3 beta,7 alpha-dihydroxy-5-cholenoic acid was prepared in the same way from 3 beta-hydroxy-5-[2,2,4,4,23-2H5]-cholenoic acid. The 3-oxo-delta 4 analogs of the 3 beta-hydroxy-delta 5 compounds were prepared by oxidation with cholesterol oxidase. The labeled products had the same isotopic composition as the starting materials. Gas chromatographic retention indices and mass spectral characteristics of the trimethylsilyl ether derivatives of the neutral steroids and the methylated acids are given for all compounds.

7 alpha-hydroxylation of 27-hydroxycholesterol in human liver microsomes

Human liver microsomes were found to catalyze 7 alpha-hydroxylation of 27-hydroxycholesterol at a rate of up to about 0.2 nmol/mg protein per min. The product of the reaction, 5-cholestene-3 beta, 7 alpha, 27-triol, was identified by means of combined gas chromatography-mass spectrometry. Liver microsomes from two patients with an upregulated cholesterol 7 alpha-hydroxylase, did not have higher 7 alpha-hydroxylase activity towards 27-hydroxycholesterol than those from untreated patients, suggesting that the 7 alpha-hydroxylase active towards 27-hydroxycholesterol is not the same as that active towards cholesterol. The mitochondrial fraction of liver from untreated patients and patients treated with cholestyramine, had negligible 7 alpha-hydroxylase activity towards 27-hydroxycholesterol less than 0.01 nmol/mg protein per min). The results are in accord with the possibility that there is a pathway to bile acids in human liver in which the first step is a 27-hydroxylation of cholesterol.

Tight junction permeability and liver plasma membrane fluidity in lithocholate-induced cholestasis

The present study correlated the reversibility of bile flow (BF) impairment with biochemical and morphological changes in the liver after injection of a cholestatic dose (12 mumole/100 g body weight) of lithocholic acid (LCA). BF declined maximally at 60 min but recovered totally at 210 min after LCA treatment. During the cholestatic period, there was an increase in tight junction permeability as measured by the bile to plasma (B/P) ratio of inulin and using lanthanum as a tracer. Cholesterol content and the cholesterol/phospholipid ratio in liver plasma membranes (LPM) were augmented while the fluidity of bile canalicular membranes (BCM) was decreased at 30 and 60 min after LCA injection. These changes in cholesterol content and membrane fluidity seemed to be correlated with LCA incorporation in LPM; their reversal at 120 min preceded the recovery of BF (210 min). Some biochemical disorders were evident after LCA injection, but they did not correlate with the variation in BF. These data suggest that increased tight junction permeability and decreased BCM fluidity are important pathogenic steps in LCA-induced cholestasis.

Identification of 7 alpha-hydroxy-3-oxo-4-cholestenoic acid in chronic subdural hematoma

We detected a novel kind of bile acid in the content of chronic subdural hematoma. This substance was specifically found in chronic subdural hematoma, and not in subdural hygroma, which is pathologically similar except for the lack of capsular membrane. The compound was identified as 7 alpha-hydroxy-3-oxo-4-cholestenoic acid by high performance liquid chromatography, gas chromatography-mass spectrometry, and nuclear magnetic resonance spectrometry. The structure was confirmed by the comparison with the chemically synthesized compound. The average contents in chronic subdural hematoma were 658.09 +/- 137.53 ng/ml, while those in normal human plasma were 126.27 +/- 17.73 ng/ml. It was not detected in normal cerebrospinal fluid. The higher level in chronic subdural hematoma than human plasma strongly suggests the local, extrahepatic production of this type of C27 bile acids.

7 alpha-hydroxylation of 26-hydroxycholesterol, 3 beta-hydroxy-5-cholestenoic acid and 3 beta-hydroxy-5-cholenoic acid by cytochrome P-450 in pig liver microsomes

Pig liver microsomes were found to catalyze the 7 alpha-hydroxylation of several potential bile acid precursors besides cholesterol. 26-Hydroxycholesterol, 3 beta-hydroxy-5-cholestenoic acid and 3 beta-hydroxy-5-cholenoic acid were all efficiently converted into the 7 alpha-hydroxylated products. Two cytochrome P-450 fractions showing 7 alpha-hydroxylase activity could be isolated. One fraction catalyzed 7 alpha-hydroxylation of 26-hydroxycholesterol, 3 beta-hydroxy-5-cholestenoic acid and 3 beta-hydroxy-5-cholenoic acid but was inactive towards cholesterol. The other fraction catalyzed 7 alpha-hydroxylation of cholesterol in addition to the other substrates. 26-Hydroxycholesterol in equimolar concentration did not inhibit the cholesterol 7 alpha-hydroxylase activity of this fraction. It is concluded that liver microsomes contain a cytochrome P-450 catalyzing 7 alpha-hydroxylation of 26-hydroxycholesterol, 3 beta-hydroxy-5-cholestenoic acid and 3 beta-hydroxy-5-cholenoic acid. The results indicate that this cytochrome P-450 is different from that catalyzing 7 alpha-hydroxylation of cholesterol.