7 alpha-Hydroxylation of 25-hydroxycholesterol and 27-hydroxycholesterol in human fibroblasts

Toxicity of (22R,23R)-22,23-dihydroxystigmastane derivatives to cultured cancer cells

Toxicity of eight 22,23-dihydroxystigmastane derivatives (four pairs of (22R,23R)- and (22S,23S)-isomers differing in steroid backbone structure) to human breast carcinoma MCF-7 cells was compared. For every pair of structurally related compounds, (22R,23R) isomer was found to be significantly more toxic than (22S,23S) isomer. Computational analysis showed that side chain of (22R,23R)-22,23-dihydroxystigmastane derivatives is rigid, whereas that of (22S,23S)-isomers is rather flexible. Structure of steroid backbone significantly affects cytotoxicity of (22R,23R)-22,23-dihydroxystigmastane derivatives to human breast carcinoma MCF-7 cells, human ovary carcinoma CaOv cells, and human prostate carcinoma LnCaP cells. (22R,23R)-3beta,22,23-trihydroxystigmast-5-ene and (22R,23R)-3beta,22,23-trihydroxystigmast-5-en-7-one, both comprising equatorial 3beta-hydroxyl group, exhibited the highest cytotoxicity, while the most polar 28-homobrassinolide and 28-homocastasterone, both comprising 2alpha,3alpha-dihydroxy groups, exhibited the lowest toxicity. Binding of (22R,23R)-22,23-dihydroxystigmastane derivatives to plasmatic membrane was suggested to be important for cytotoxicity.

Inhibition of acyl-coenzyme A:cholesterol acyltransferase stimulates cholesterol efflux from macrophages and stimulates farnesoid X receptor in hepatocytes

We investigated the mechanism of spontaneous cholesterol efflux induced by acyl-coenzyme A:cholesterol acyltransferase (ACAT) inhibition, and how an alteration of cholesterol metabolism in macrophages impacts on that in HepG2 cells. Oleic acid anilide (OAA), a known ACAT inhibitor reduced lipid storage substantially by promotion of cholesterol catabolism and repression of cholesteryl ester accumulation without further increase of cytotoxicity in acetylated low-density lipoprotein-loaded THP-1 macrophages. Analysis of expressed mRNA and protein revealed that cholesterol 7alpha-hydroxylase (CYP7A1), oxysterol 7alpha- hydroxylase (CYP7B1), and cholesterol 27-hydroxylase (CYP27) were highly induced by ACAT inhibition. The presence of a functional cytochrome P450 pathway was confirmed by quantification of the biliary cholesterol mass in cell monolayers and extracelluar medium. Notably, massively secreted biliary cholesterol from macrophages suppressed the expression of CYP7 proteins in a farnesoid X receptor (FXR)-dependent manner in HepG2 cells. The findings reported here provide new insight into mechanisms of spontaneous cholesterol efflux, and suggest that ACAT inhibition may stimulate cholesterol-catabolic (cytochrome P450) pathway in lesion-macrophages, in contrast, suppress it in hepatocyte via FXR induced by biliary cholesterol (BC).

The effect of 24S-hydroxycholesterol on cholesterol homeostasis in neurons: quantitative changes to the cortical neuron proteome

In humans, the brain represents only about 2% of the body's mass but contains about one-quarter of the body's free cholesterol. Cholesterol is synthesized de novo in brain and removed by metabolism to oxysterols. 24S-Hydoxycholesterol represents the major metabolic product of cholesterol in brain, being formed via the cytochrome P450 (CYP) enzyme CYP46A1. CYP46A1 is expressed exclusively in brain, normally by neurons. In this study, we investigated the effect of 24S-hydroxycholesterol on the proteome of rat cortical neurons. With the use of two-dimensional liquid chromatography linked to nanoelectrospray tandem mass spectrometry, over 1040 proteins were identified including members of the cholesterol, isoprenoid and fatty acid synthesis pathways. With the use of stable isotope labeling technology, the protein expression patterns of enzymes in these pathways were investigated. 24S-Hydroxycholesterol was found to down-regulate the expression of members of the cholesterol/isoprenoid synthesis pathways including 3-hydroxy-3-methylglutaryl-Coenzyme A synthase 1 (EC 2.3.3.10), diphosphomevalonate decarboxylase (EC 4.1.1.33), isopentenyl-diphosphate delta isomerase (EC 5.3.3.2), farnesyl-diphosphate synthase (Geranyl trans transferase, EC 2.5.1.10), and dedicated sterol synthesis enzymes, farnesyl-diphosphate farnesyltransferase 1 (squalene synthase, EC 2.5.1.21) and methylsterol monooxygenase (EC 1.14.13.72). The expression of many enzymes in the cholesterol/isoprenoid and fatty acid synthesis pathways are regulated by the membrane-bound transcription factors named sterol regulatory element-binding proteins (SREBPs), which themselves are both transcriptionally and post-transcriptionally regulated. The current proteomic data indicates that 24S-hydroxycholesterol down-regulates cholesterol synthesis in neurons, possibly, in a post-transcriptional manner through SREBP-2. In contrast to cholesterol metabolism, enzymes responsible for the synthesis of fatty acids were not found to be down-regulated in neurons treated with 24S-hydroxycholesterol, while apolipoprotein E (apo E), a cholesterol trafficking protein, was found to be up-regulated. Taken together, this data leads to the hypothesis that, in times of cholesterol excess, 24S-hydroxycholesterols signals down-regulation of cholesterol synthesis enzymes through SREBP-2, but up-regulates apo E synthesis (through the liver X receptor) leading to cholesterol storage and restoration of cholesterol balance.

Analysis of oxysterols by electrospray tandem mass spectrometry

Oxysterols are oxygenated derivatives of cholesterol. They are intermediates in cholesterol excretion pathways and may also be regarded as transport forms of cholesterol. The introduction of additional hydroxyl groups to the cholesterol skeleton facilitates the flux of oxysterols across the blood brain barrier, and oxysterols have been implicated in mediating a number of cholesterol-induced metabolic effects. Oxysterols are difficult to analyze by atmospheric pressure ionization mass spectrometry on account of the absence of basic or acidic functional groups in their structures. In this communication, we report a method for the derivatization and analysis of oxysterols by electrospray mass spectrometry. Oxysterols with a 3beta-hydroxy-Delta5 structure were converted by cholesterol oxidase to 3-oxo-Delta4 steroids and then derivatized with the Girard P reagent to give Girard P hydrazones, which were subsequently analyzed by tandem mass spectrometry. The improvement in sensitivity for the analysis of 25-hydroxycholesterol upon oxidation and derivatization was over 1000.

Cellular toxicity of oxycholesterols

Oxycholesterols (OS) are formed from cholesterol or its immediate precursors by enzymatic or free radical action in vivo, or they may be derived from food. OS exhibit a wide spectrum of biological activities. In OS cytotoxicity, several mechanisms seem to be involved: e.g. inhibition of HMG-CoA reductase activity, antiproliferative action, apoptosis induction, replacement of cholesterol by OS in membranes followed by changes in cellular membrane structure and functionality, and immune system functions alteration. Furthermore, OS may be mutagenic and carcinogenic and may serve as intracellular signaling or regulatory molecules. Here we review OS cellular activities with special attention to the cytotoxic action in vivo and in vitro using experimental models.

Synthesis of 7alpha-hydroxy-dehydroepiandrosterone and 7beta-hydroxy-dehydroepiandrosterone

The fermentation of dehydroepiandrosterone synthesized from the starting material diosgenin using Mucor racemosus produced 7alpha-hydroxy-dehydroepiandrosterone and 7beta-hydroxy-dehydroepiandrosterone. The bioactivity of the microbial metabolites is also discussed. The species M. racemosus was isolated by screening among stains from soil samples collected from various parts of China.

Severity of murine collagen-induced arthritis correlates with increased CYP7B activity: enhancement of dehydroepiandrosterone metabolism by interleukin-1beta

OBJECTIVE

The endogenous steroid dehydroepiandrosterone (DHEA) has been reported to play a role in rheumatoid arthritis (RA). DHEA is metabolized by the P450 enzyme CYP7B into 7alpha-OH-DHEA, which has immunostimulating properties. This study was undertaken to investigate the putative role of CYP7B in arthritis using murine collagen-induced arthritis (CIA), an interleukin-1beta (IL-1beta)-dependent model.

METHODS

DBA/1J mice were immunized and administered a booster with type II collagen. The presence of 7alpha-OH-DHEA was determined in both arthritic and nonarthritic joints and the serum of CIA mice by radioimmunoassay. CYP7B messenger RNA (mRNA) expression was analyzed in synovial biopsy samples, and in fibroblast-like synoviocytes (FLS) isolated from these synovial biopsy samples, by reverse transcriptase-polymerase chain reaction (RT-PCR). In addition, the regulatory role of IL-1beta on CYP7B activity in FLS was determined using RT-PCR, Western blotting, and high-performance liquid chromatography.

RESULTS

In knee joint synovial biopsy samples from arthritic mice, 7alpha-OH-DHEA levels were 5-fold higher than in nonarthritic mice. Elevated levels of 7alpha-OH-DHEA were accompanied by an increase in CYP7B mRNA expression and were positively correlated with disease severity. In serum, no differences in 7alpha-OH-DHEA levels were observed between arthritic and nonarthritic mice. Incubation of FLS with IL-1beta resulted in a dose-dependent increase in 7alpha-OH-DHEA formation. In addition, IL-1beta enhanced CYP7B mRNA and CYP7B protein levels in FLS.

CONCLUSION

Disease progression in CIA is correlated with enhanced CYP7B activity, which leads to locally enhanced 7alpha-OH-DHEA levels. Elevated IL-1beta levels within the arthritic joint may regulate this increase in CYP7B activity.

Fifty years with bile acids and steroids in health and disease

Cholesterol and its metabolites, e.g., steroid hormones and bile acids, constitute a class of compounds of great biological importance. Their chemistry, biochemistry, and regulation in the body have been intensely studied for more than two centuries. The author has studied aspects of the biochemistry and clinical chemistry of steroids and bile acids for more than 50 years, and this paper, which is an extended version of the Schroepfer Medal Award lecture, reviews and discusses part of this work. Development and application of analytical methods based on chromatography and mass spectrometry (MS) have been a central part of many projects, aiming at detailed characterization and quantification of metabolic profiles of steroids and bile acids under different conditions. In present terminology, much of the work may be termed steroidomics and cholanoidomics. Topics discussed are bile acids in human bile and feces, bile acid production, bacterial dehydroxylation of bile acids and steroids during the enterohepatic circulation, profiles of steroid sulfates in plasma of humans and other primates, development of neutral and ion-exchanging lipophilic derivatives of Sephadex for sample preparation and group separation of steroid and bile acid conjugates, profiles of steroids and bile acids in human urine under different conditions, hydroxylation of bile acids in liver disease, effects of alcohol-induced redox changes on steroid synthesis and metabolism, alcohol-induced changes of bile acid biosynthesis, compartmentation of bile acid synthesis studied with 3H-labeled ethanol, formation and metabolism of sulfated metabolites of progesterone in human pregnancy, abnormal patterns of these in patients with intrahepatic cholestasis of pregnancy corrected by ursodeoxycholic acid, inherited and acquired defects of bile acid biosynthesis and their treatment, conjugation of bile acids and steroids with N-acetylglucosamine, sulfate-glucuronide double conjugates of hydroxycholesterols, extrahepatic 7alpha-hydroxylation and 3-dehydrogenation of hydroxycholesterols, and extrahepatic formation of C27 bile acids. The final part discusses analysis of free and sulfated steroids in brain tissue by capillary liquid chromatography-electrospray MS and suggests a need for reevaluation of the function of steroid sulfates in rat brain.

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

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

Steroid and sterol 7-hydroxylation: ancient pathways

B-ring hydroxylation is a major metabolic pathway for cholesterols and some steroids. In liver, 7 alpha-hydroxylation of cholesterols, mediated by CYP7A and CYP39A1, is the rate-limiting step of bile acid synthesis and metabolic elimination. In brain and other tissues, both sterols and some steroids including dehydroepiandrosterone (DHEA) are prominently 7 alpha-hydroxylated by CYP7B. The function of extra-hepatic steroid and sterol 7-hydroxylation is unknown. Nevertheless, 7-oxygenated cholesterols are potent regulators of cell proliferation and apoptosis; 7-oxygenated derivatives of DHEA, pregnenolone, and androstenediol can have major effects in the brain and in the immune system. The receptor targets involved remain obscure. It is argued that B-ring modification predated steroid evolution: non-enzymatic oxidation of membrane sterols primarily results in 7-oxygenation. Such molecules may have provided early growth and stress signals; a relic may be found in hydroxylation at the symmetrical 11-position of glucocorticoids. Early receptor targets probably included intracellular sterol sites, some modern steroids may continue to act at these targets. 7-Hydroxylation of DHEA may reflect conservation of an early signaling pathway.

Neurosteroid hydroxylase CYP7B: vivid reporter activity in dentate gyrus of gene-targeted mice and abolition of a widespread pathway of steroid and oxysterol hydroxylation

The major adrenal steroid dehydroepiandrosterone (DHEA) enhances memory and immune function but has no known dedicated receptor; local metabolism may govern its activity. We described a cytochrome P450 expressed in brain and other tissues, CYP7B, that catalyzes the 7alpha-hydroxylation of oxysterols and 3beta-hydroxysteroids including DHEA. We report here that CYP7B mRNA and 7alpha-hydroxylation activity are widespread in rat tissues. However, steroids related to DHEA are reported to be modified at positions other than 7alpha, exemplified by prominent 6alpha-hydroxylation of 5alpha-androstane-3beta,17beta-diol (A/anediol) in some rodent tissues including brain. To determine whether CYP7B is responsible for these and other activities we disrupted the mouse Cyp7b gene by targeted insertion of an IRES-lacZ reporter cassette, placing reporter enzyme activity (beta-galactosidase) under Cyp7b promoter control. In heterozygous mouse brain, chromogenic detection of reporter activity was strikingly restricted to the dentate gyrus. Staining did not exactly reproduce the in situ hybridization expression pattern; post-transcriptional control is inferred. Lower level staining was detected in cerebellum, liver, and kidney, and which largely paralleled mRNA distribution. Liver and kidney expression was sexually dimorphic. Mice homozygous for the insertion are viable and superficially normal, but ex vivo metabolism of DHEA to 7alpha-hydroxy-DHEA was abolished in brain, spleen, thymus, heart, lung, prostate, uterus, and mammary gland; lower abundance metabolites were also eliminated. 7alpha-Hydroxylation of 25-hydroxycholesterol and related substrates was also abolished, as was presumed 6alpha-hydroxylation of A/anediol. These different enzyme activities therefore derive from the Cyp7b gene. CYP7B is thus a major extrahepatic steroid and oxysterol hydroxylase and provides the predominant route for local metabolism of DHEA and related molecules in brain and other tissues.

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

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

Expression cloning of an oxysterol 7alpha-hydroxylase selective for 24-hydroxycholesterol

The synthesis of 7alpha-hydroxylated bile acids from oxysterols requires an oxysterol 7alpha-hydroxylase encoded by the Cyp7b1 locus. As expected, mice deficient in this enzyme have elevated plasma and tissue levels of 25- and 27-hydroxycholesterol; however, levels of another major oxysterol, 24-hydroxycholesterol, are not increased in these mice, suggesting the presence of another oxysterol 7alpha-hydroxylase. Here, we describe the cloning and characterization of murine and human cDNAs and genes that encode a second oxysterol 7alpha-hydroxylase. The genes contain 12 exons and are located on chromosome 6 in the human (CYP39A1 locus) and in a syntenic position on chromosome 17 in the mouse (Cyp39a1 locus). CYP39A1 is a microsomal cytochrome P450 enzyme that has preference for 24-hydroxycholesterol and is expressed in the liver. The levels of hepatic CYP39A1 mRNA do not change in response to dietary cholesterol, bile acids, or a bile acid-binding resin, unlike those encoding other sterol 7alpha-hydroxylases. Hepatic CYP39A1 expression is sexually dimorphic (female > male), which is opposite that of CYP7B1 (male > female). We conclude that oxysterol 7alpha-hydroxylases with different substrate specificities exist in mice and humans and that sexually dimorphic expression patterns of these enzymes in the mouse may underlie differences in bile acid metabolism between the sexes.

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.

Progesterone metabolism in human fibroblasts is independent of P-glycoprotein levels and Niemann-Pick type C disease

Progesterone inhibits intracellular transport of lysosomal cholesterol in cultured cells, and thus at least in part mimics the biochemical phenotype of Niemann-Pick type C disease (NPC) in human fibroblasts. The goal of this study was to determine whether metabolism of progesterone to other steroids is affected by the NPC mutation or by P-glycoprotein (a known progesterone target). We found that human fibroblasts metabolize progesterone in three steps: rapid conversion to 5alpha-pregnane-3,20-dione, which is then reduced to 5alpha-pregnane-3beta(alpha)-ol-20-one with subsequent 6alpha-hydroxylation. The pattern and rates of progesterone metabolism were not significantly different in a variety of fibroblasts from normal individuals, NPC patients, and obligate heterozygotes. Inhibition of steroid 5alpha-reductase with finasteride completely blocked metabolism of progesterone but had no effect on inhibition of LDL-stimulated cholesterol esterification (IC50 = 10 microM). Progesterone also partially inhibited 25-hydroxycholesterol-induced cholesterol esterification, with similar dose-dependence in normal and NPC fibroblasts. P-glycoprotein levels varied significantly among the various fibroblasts tested, but no correlation with NPC phenotype or rate of progesterone metabolism was noted, and P-glycoprotein inhibitors did not affect conversion of progesterone to products. These results indicate that metabolism of progesterone in human fibroblasts is largely independent of its ability to interfere with cholesterol traffic and P-glycoprotein function.

Distribution of exogenous 25-hydroxycholesterol in Hep G2 cells between two different pools

Binding of [26,27-(3)H]25-hydroxycholesterol (25HC) to human hepatoma Hep G2 cells was saturated within 120 min. Two intracellular pools of 25HC were identified in a pulse-chase experiment: (i) an exchangeable pool which was in dynamic equilibrium with 25HC in the medium (t(1/2) of reversible exchange 15 min) and (ii) an unexchangeable pool which remained in cells during incubation in medium containing LPDS. 25HC from the exchangeable pool inhibits cholesterol biosynthesis, decreases the HMG CoA reductase mRNA level and stimulates cholesterol acylation. 25HC from the unexchangeable pool was partially bound to cytosolic proteins and apparently utilized for metabolic transformation. Incubation of Hep G2 cells with [26,27-(3)H]25HC in the presence of a 30-fold molar excess of 3beta-hydroxy-5alpha-cholest-8(14)-en-15-one was found to cause (i) 2-fold decrease in the binding of [26,27-(3)H]25HC to cytosolic proteins (sedimentation constant of radioactive complex was 4-5 S) and (ii) the 35% inhibition of 25HC transformation to polar metabolites.

Regulation of bile acid biosynthesis

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

Hepatic and extrahepatic dehydrogenation/isomerization of 5-cholestene-3 beta,7 alpha-diol: localization of 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase in pig tissues and subcellular fractions

Conversion of 5-cholestene-3 beta,7 alpha-diol (7 alpha-hydroxycholesterol) into 7 alpha-hydroxy-4-cholesten-3-one was studied with microsomes from different pig tissues and with liver subcellular fractions. Dehydrogenase/isomerase activity was efficient in microsomes from liver, ovary and lung, but less efficient in microsomes from adrenal gland and kidney. Microsomes from these tissues, with the exception of lung, were also active in dehydrogenation/isomerization of dehydroepiandrosterone and pregnenolone. Inhibition studies were carried out with trilostane, a competitive inhibitor of 3 beta-hydroxysteroid dehydrogenases active in steroid hormone biosynthesis (C19/C21-dehydrogenases), and a monoclonal antibody raised against a purified hepatic 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase. The results showed that the C27-dehydrogenase activity in the tissues was not dependent on the C19/C21 dehydrogenases, but was dependent on the 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase. Liver mitochondria, cytosol and peroxisomes lacked dehydrogenase/isomerase activity towards 7 alpha-hydroxycholesterol when microsomal contamination was taken into account. Immunoblotting experiments with monoclonal antibodies raised against the 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase showed immunoreactivity only with protein in liver microsomes. Immunohistochemical studies showed localization of the 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase in the bile duct epithelium. It is concluded that 7 alpha-hydroxycholesterol is converted into 7 alpha-hydroxy-4-cholesten-3-one by the microsomal 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase in liver and extrahepatic tissues.

Identification of a new inborn error in bile acid synthesis: mutation of the oxysterol 7alpha-hydroxylase gene causes severe neonatal liver disease

We describe a metabolic defect in bile acid synthesis involving a deficiency in 7alpha-hydroxylation due to a mutation in the gene for the microsomal oxysterol 7alpha-hydroxylase enzyme, active in the acidic pathway for bile acid synthesis. The defect, identified in a 10-wk-old boy presenting with severe cholestasis, cirrhosis, and liver synthetic failure, was established by fast atom bombardment ionization-mass spectrometry, which revealed elevated urinary bile acid excretion, a mass spectrum with intense ions at m/z 453 and m/z 510 corresponding to sulfate and glycosulfate conjugates of unsaturated monohydroxy-cholenoic acids, and an absence of primary bile acids. Gas chromatography-mass spectrometric analysis confirmed the major products of hepatic synthesis to be 3beta-hydroxy-5-cholenoic and 3beta-hydroxy-5-cholestenoic acids, which accounted for 96% of the total serum bile acids. Levels of 27-hydroxycholesterol were > 4,500 times normal. The biochemical findings were consistent with a deficiency in 7alpha-hydroxylation, leading to the accumulation of hepatotoxic unsaturated monohydroxy bile acids. Hepatic microsomal oxysterol 7alpha-hydroxylase activity was undetectable in the patient. Gene analysis revealed a cytosine to thymidine transition mutation in exon 5 that converts an arginine codon at position 388 to a stop codon. The truncated protein was inactive when expressed in 293 cells. These findings indicate the quantitative importance of the acidic pathway in early life in humans and define a further inborn error in bile acid synthesis as a metabolic cause of severe cholestatic liver disease.

Two 7 alpha-hydroxylase enzymes in bile acid biosynthesis

The addition of a 7-hydroxyl group is an early and often rate-limiting step in the synthesis of bile acids. This reaction is catalysed by two cytochrome P450 enzymes known as cholesterol 7 alpha-hydroxylase and oxysterol 7 alpha-hydroxylase. cDNAs encoding these proteins have been isolated and used to define two evolutionarily conserved pathways that produce 7 alpha-hydroxylated bile acids.

Biochemical characterization of the 7alpha-hydroxylase activities towards 27-hydroxycholesterol and dehydroepiandrosterone in pig liver microsomes

Microsomal cytochrome P-450 catalyzing the 7alpha-hydroxylation of 27-hydroxycholesterol and dehydroepiandrosterone was partially purified from pig liver. This enzyme fraction also catalyzed 7alpha-hydroxylation of 25-hydroxycholesterol and pregnenolone but did not 7alpha-hydroxylate cholesterol or testosterone. Studies with extrahepatic tissues have suggested the possibility of one common enzyme responsible for the 7alpha-hydroxylation of 27-hydroxycholesterol and dehydroepiandrosterone. A series of experiments was performed to study if there are one or several enzymes 7alpha-hydroxylating these steroids in the liver. The activities towards the two substrates copurified but the ratio between 27-hydroxycholesterol and dehydroepiandrosterone 7alpha-hydroxylation varied considerably in different purification steps and between different preparations. The enzyme inhibitors disulfiram, N-bromosuccinimide, ketoconazole, metyrapone and alpha-naphthoflavone affected the activities in a similar way. Dehydroepiandrosterone inhibited 27-hydroxycholesterol 7alpha-hydroxylation whereas 27-hydroxycholesterol had almost no inhibitory effect on dehydroepiandrosterone 7alpha-hydroxylation. Experiments to examine the nature of inhibition by dehydroepiandrosterone indicated that the two steroids did not compete for the same active site. The results of this study do not rule out the possibility of one single enzyme catalyzing 7alpha-hydroxylation of the two steroids. However, taken together the data suggest that hepatic microsomal 7alpha-hydroxylation of 27-hydroxycholesterol and dehydroepiandrosterone involves at least two, probably closely related, enzymes. (c) 1998 Elsevier Science B. V.

7alpha-Hydroxylation and 3-dehydrogenation abolish the ability of 25-hydroxycholesterol and 27-hydroxycholesterol to induce apoptosis in thymocytes

Oxygenated derivatives of sterols (oxysterols), including 25-hydroxycholesterol and 27-hydroxycholesterol, have immunosuppressive effects. Oxysterols can directly induce apoptosis in immature thymocytes, cells which are inherently sensitive to induction of programmed cell death. For that reason, the metabolism of 25-hydroxycholesterol and 27-hydroxycholesterol in mouse thymus has been studied. When incubated with thymic tissue, both oxysterols were found to be 7alpha-hydroxylated with subsequent oxidation to 7alpha-hydroxy-3-oxo-delta4 steroids. A minor fraction of 27-hydroxycholesterol was also metabolised to 3beta-hydroxy-5-cholestenoic, 3beta,7alpha-dihydroxy-5-cholestenoic and 7alpha-hydroxy-3-oxo-4-cholestenoic acids. The 7alpha-hydroxylase was found to be localised to the thymic epithelial cells and the reaction was stimulated by interleukin-1beta and inhibited by metyrapone and RU486. In contrast to 25-hydroxycholesterol and 27-hydroxycholesterol, the 7alpha-hydroxylated metabolites, 7alpha,25-dihydroxycholesterol, 7alpha,25-dihydroxy-4-cholesten-3-one and 7alpha,27-dihydroxy-4-cholesten-3-one did not induce thymocyte apoptosis. The results suggest that 7alpha-hydroxylation may be of regulatory importance, possibly by protecting the developing thymocytes against toxic effects by oxysterols.

Cyp7b, a novel brain cytochrome P450, catalyzes the synthesis of neurosteroids 7alpha-hydroxy dehydroepiandrosterone and 7alpha-hydroxy pregnenolone

Steroids produced locally in brain (neurosteroids), including dehydroepiandrosterone (DHEA), influence cognition and behavior. We previously described a novel cytochrome P450, Cyp7b, strongly expressed in rat and mouse brain, particularly in hippocampus. Cyp7b is most similar to steroidogenic P450s and potentially could play a role in neurosteroid metabolism. To examine the catalytic activity of the enzyme mouse Cyp7b cDNA was introduced into a vaccinia virus vector. Extracts from cells infected with the recombinant showed NADPH-dependent conversion of DHEA (Km, 13.6 microM) and pregnenolone (Km, 4.0 microM) to slower migrating forms on thin layer chromatography. The expressed enzyme was less active against 25-hydroxycholesterol, 17beta-estradiol and 5alpha-androstane-3beta,17beta-diol, with low to undetectable activity against progesterone, corticosterone, and testosterone. On gas chromatography and mass spectrometry of the Cyp7b metabolite of DHEA the retention time and fragmentation patterns were identical to those obtained with authentic 7alpha-hydroxy DHEA. The reaction product also comigrated on thin layer chromatography with 7alpha-hydroxy DHEA but not with 7beta-hydroxy DHEA; when [7alpha-3H]pregnenolone was incubated with Cyp7b extracts the extent of release of radioactivity into the medium suggested that hydroxylation was preferentially at the 7alpha position. Brain extracts also efficiently liberated tritium from [7alpha-3H]pregnenolone and converted DHEA to a product with a chromatographic mobility indistinguishable from 7alpha-hydroxy DHEA. We conclude that Cyp7b is a 7alpha-hydroxylase participating in the synthesis, in brain, of neurosteroids 7alpha-hydroxy DHEA, and 7alpha-hydroxy pregnenolone.

Studies on the relationships between 7 alpha-hydroxylation and the ability of 25- and 27-hydroxycholesterol to suppress the activity of HMG-CoA reductase

The metabolism of 25-hydroxycholesterol in different cell types was studied and the role of 7 alpha-hydroxylation for the effect of 25-hydroxycholesterol on the activity of HMG-CoA reductase was determined. Human diploid fibroblasts (HDF) and the human melanoma cell line SK-MEL-2 converted 25-hydroxycholesterol into 7 alpha,25-dihydroxycholesterol and 7 alpha,25-dihydroxy-4-cholesten-3-one while the virus-transformed fibroblast line 90VA-VI, the colon carcinoma cell line WiDr and the breast cancer cell line MDA-231 did not express 7 alpha-hydroxylase activity. The 7 alpha-hydroxylation of 25-hydroxycholesterol in HDF could be stimulated by dexamethasone and cortisol and inhibited by metyrapone. An unidentified, possibly 4-hydroxylated, metabolite was formed by 90VA-VI cells and a polar, probably conjugated, metabolite was formed by WiDr cells. The 7 alpha-hydroxylated metabolites of 25-hydroxycholesterol suppressed the activity of HMG-CoA reductase to a similar extent as 25-hydroxycholesterol in HDF but not in 90VA-VI cells, while the 7 alpha-hydroxylated metabolites of 27-hydroxycholesterol suppressed the activity of HMG-CoA reductase also in 90VA-VI cells. The suppression of HMG-CoA reductase activity by 25- and 27-hydroxycholesterol was decreased or abolished by dehydroepiandrosterone or pregnenolone which have little or no effect on the 7 alpha-hydroxylation. The results indicate that 7 alpha-hydroxylation is not directly involved, positively or negatively, in the action of 25- or 27-hydroxycholesterol as suppressors of HMG-CoA reductase activity.

Metabolism of 27-, 25- and 24-hydroxycholesterol in rat glial cells and neurons

The metabolism of 27-, 25- and 24-hydroxycholesterol in cultures of rat astrocytes, Schwann cells and neurons was studied. 27- and 25-Hydroxycholesterol, but not 24-hydroxycholesterol, underwent 7 alpha-hydroxylation with subsequent oxidation to 7 alpha-hydroxy-3-oxo-delta 4 steroids in all three cell types. When cells were incubated for 24 h with 0.28 nmol of 27-hydroxycholesterol in 10 ml of medium, the rates of conversion into 7 alpha-hydroxylated metabolites were 0.21, 0.12 and 0.02 nmol/24 h per 10(6) cells in the media of astrocytes, Schwann cells and neurons respectively. The corresponding values for 25-hydroxycholesterol were 0.26, 0.16 and 0.04. A minor fraction of 27-hydroxycholesterol and its 7 alpha-hydroxylated metabolites was oxidized to 3 beta-hydroxy-5-cholestenoic acid. 3 beta, 7 alpha-dihydroxy-5-cholestenoic acid and 7 alpha-hydroxy-3-oxo-4-cholestenoic acid. In addition to the two hydroxycholesterols, other 3 beta-hydroxy-delta 4 steroids, dehydro-epiandrosterone, pregnenolone, 3 beta-hydroxy-5-cholestenoic acid and 3 beta-hydroxy-5-cholenoic acid underwent 7 alpha-hydroxylation. Competitive experiments did not distinguish between the presence of one or several 7 alpha-hydroxylases. In astrocyte incubations, 27-hydroxycholesterol also underwent 25-hydroxylation, and 12% of its metabolites carried a 25-hydroxy group. 25-Hydroxylation of added 24-hydroxycholesterol was also observed in the astrocyte incubations, as was the formation of 7 alpha, 25-dihydroxy-4-cholesten-3-one, 25-hydroxycholesterol and 7 alpha, 25-dihydroxycholesterol from endogenous precursor(s). Our study indicates that side-chain oxygenated cholesterol can undergo metabolic transformations that may be of importance for cholesterol homoeostasis in the brain.

Activation of acyl-CoA: cholesterol acyltransferase in rat liver microsomes by 25-hydroxycholesterol

25-Hydroxycholesterol stimulated acyl-CoA:cholesterol acyltransferase (ACAT) activity in rat liver microsomes in vitro with half-maximal stimulation at 16.8 microM oxysterol and a maximal activity that was three times that in its absence. The current study was conducted to determine the effect of 25-hydroxycholesterol on rates and extent of intervesicular cholesterol transfers within microsomes and to determine whether this activation of ACAT could be accounted for on the basis of increased cholesterol availability for the enzyme. Cholesterol transfer kinetics were assessed in systems that either enriched or depleted microsomal cholesterol. Incubation of microsomes at 37 degrees C with phosphatidylcholine:cholesterol liposomes or purified plasma membranes resulted in enrichment of microsomal cholesterol. Incubation of microsomes with just phosphatidylcholine liposomes resulted in depletion of cholesterol. The extent of cholesterol enrichment or depletion depended on incubation time and the initial concentration of cholesterol in donor and acceptor vesicles. The rate and extent of cholesterol transfer from liposomes to microsomes were slightly increased when 25-hydroxycholesterol was present during the transfer process. Irrespective of the treatment, 25-hydroxycholesterol continued to stimulate the ACAT activity of the treated microsomes. Microsomes that were enriched or depleted of cholesterol in the absence of 25-hydroxycholesterol yielded as much enzyme activities when assayed in the presence of 25-hydroxycholesterol as with the systems that contained 25-hydroxycholesterol during both the transfer process and enzyme assays. The results suggest that a major part of the activation of microsomal ACAT by 25-hydroxycholesterol is not ascribable to increased substrate availability for the enzyme.

Disruption of cholesterol 7alpha-hydroxylase gene in mice. II. Bile acid deficiency is overcome by induction of oxysterol 7alpha-hydroxylase

Past experiments and current paradigms of cholesterol homeostasis suggest that cholesterol 7alpha-hydroxylase plays a crucial role in sterol metabolism by controlling the conversion of cholesterol into bile acids. Consistent with this conclusion, we show in the accompanying paper that mice deficient in cholesterol 7alpha-hydroxylase (Cyp7-/- mice) exhibit a complex phenotype consisting of abnormal lipid excretion, skin pathologies, and behavioral irregularities (Ishibashi, S., Schwarz, M., Frykman, P. K. , Herz, J., and Russell, D. W.(1996) J. Biol. Chem. 261, 18017-18023). Aspects of lipid metabolism in the Cyp7-/- mice are characterized here to deduce the physiological basis of this phenotype. Serum lipid, cholesterol, and lipoprotein contents are indistinguishable between wild-type and Cyp7-/- mice. Vitamin D3 and E levels are low to undetectable in knockout animals. Stool fat content is significantly elevated in newborn Cyp7-/- mice and gradually declines to wild-type levels at 28 days of age. Several species of 7alpha-hydroxylated bile acids are detected in the bile and stool of adult Cyp7-/- animals. A hepatic oxysterol 7alpha-hydroxylase enzyme activity that may account for the 7alpha-hydroxylated bile acids is induced between days 21 and 30 in both wild-type and deficient mice. An anomalous oily coat in the Cyp7-/- animals is due to the presence of excess monoglyceride esters in the fur. These data show that 7alpha-hydroxylase and the pathway of bile acid synthesis initiated by this enzyme are essential for proper absorption of dietary lipids and fat-soluble vitamins in newborn mice, but not for the maintenance of serum cholesterol and lipid levels. In older animals, an alternate pathway of bile acid synthesis involving an inducible oxysterol 7alpha-hydroxylase plays a crucial role in lipid and bile acid metabolism.

27-hydroxylated low density lipoprotein (LDL) cholesterol can be converted to 7alpha,27-dihydroxy-4-cholesten-3-one (cytosterone) before suppressing cholesterol production in normal human fibroblasts. Evidence that an altered metabolism of ldl cholesterol can underlie a defective feedback control in malignant cells

The formation of oxysterols in cultured human fibroblasts and their physiological roles as intracellular regulators of cholesterol production have been investigated. In the presence of low density lipoproteins (LDL), normal fibroblasts converted LDL cholesterol to 27hydroxycholesterol, which was further metabolized to 7alpha, 27-dihydroxycholesterol, 7alpha,27-dihydroxy-4-cholesten-3-one, and 7alpha-hydroxy-3-oxo-4-cholestenoic acid. Autooxidation products of cholesterol contaminating the lipoproteins were also metabolized in the cells. 7alpha-Hydroxycholesterol was converted to 7alpha-hydroxy-4-cholesten-3-one prior to 27-hydroxylation and further oxidation to 7alpha-hydroxy-3-oxo-4-cholestenoic acid. 7beta-Hydroxycholesterol and 7-oxocholesterol were 27-hydroxylated and then oxidized to C27-acids. Oxidation of the 7beta-hydroxy group also occurred. 25-Hydroxycholesterol was 7alpha-hydroxylated and further oxidized to 7alpha,25-dihydroxy-4-cholesten-3-one. 25-Hydroxylation of sterols was observed only under specific conditions. In contrast, only small amounts of oxysterols were formed in virus-transformed human fibroblasts when incubated with lipoproteins. This was due to very low activities of the 27- and 7alpha-hydroxylating enzymes. The rate of oxidation at C-3 was also decreased moderately. A defective suppression of 3-hydroxy-3-methylglutaryl coenzyme A reductase by LDL and autooxidation products of cholesterol observed in the transformed fibroblasts could be caused by the deficiencies of the sterol-metabolizing enzymes, since these cells responded normally to the sterol metabolites 7alpha,27-dihydroxy-4-cholesten-3-one, 7alpha, 25-dihydroxy-4-cholesten-3-one, and 27-hydroxy-7-oxo-cholesterol. These metabolites, which all possessed an oxo group with a conjugated double bond in the steroid nucleus and a hydroxyl group in the side chain, did not seem to require further metabolism in order to be active. An impaired response to LDL was also seen in other human tumor cells, including breast carcinoma, colonic carcinoma, and malignant melanoma cells. Common to all the malignant cells was an intracellular shortage of 7alpha, 27-dihydroxy-4-cholesten-3-one caused by a decreased formation or an increased metabolism.

Review of progress in sterol oxidations: 1987-1995

Material dealing with the chemistry, biochemistry, and biological activities of oxysterols is reviewed for the period 1987-1995. Particular attention is paid to the presence of oxysterols in tissues and foods and to their physiological relevance.