Structural specificity in the suppression of HMG-CoA reductase in human fibroblasts by intermediates in bile acid biosynthesis

Neuro-oxysterols and neuro-sterols as ligands to nuclear receptors, GPCRs, ligand-gated ion channels and other protein receptors

The brain is the most cholesterol rich organ in the body containing about 25% of the body's free cholesterol. Cholesterol cannot pass the blood-brain barrier and be imported or exported; instead, it is synthesised in situ and metabolised to oxysterols, oxidised forms of cholesterol, which can pass the blood-brain barrier. 24S-Hydroxycholesterol is the dominant oxysterol in the brain after parturition, but during development, a myriad of other oxysterols are produced, which persist as minor oxysterols after birth. During both development and in later life, sterols and oxysterols interact with a variety of different receptors, including nuclear receptors, membrane bound GPCRs, the oxysterol/sterol sensing proteins INSIG and SCAP, and the ligand-gated ion channel NMDA receptors found in nerve cells. In this review, we summarise the different oxysterols and sterols found in the CNS whose biological activity is transmitted via these different classes of protein receptors. LINKED ARTICLES: This article is part of a themed issue on Oxysterols, Lifelong Health and Therapeutics. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.16/issuetoc.

Cholesterolomics: An update

Cholesterolomics can be regarded as the identification and quantification of cholesterol, its precursors post squalene, and metabolites of cholesterol and of its precursors, in a biological sample. These molecules include 1,25-dihydroxyvitamin D₃, steroid hormones and bile acids and intermediates in their respective biosynthetic pathways. In this short article we will concentrate our attention on intermediates in bile acid biosynthesis pathways, in particular oxysterols and cholestenoic acids. These molecular classes are implicated in the aetiology of a diverse array of diseases including autoimmune disease, Parkinson's disease, motor neuron disease, breast cancer, the lysosomal storage disease Niemann-Pick type C and the autosomal recessive disorder Smith-Lemli-Opitz syndrome. Mass spectrometry (MS) is the dominant technology for sterol analysis including both gas-chromatography (GC)-MS and liquid chromatography (LC)-MS and more recently matrix-assisted laser desorption/ionisation (MALDI)-MS for tissue imaging studies. Here we will discuss exciting biological findings and recent analytical improvements.

Optimization, validation and application of an assay for the activity of HMG-CoA reductase in vitro by LC-MS/MS

A stable HMG-CoA reductase (HMGR) reaction in vitro was developed by a sensitive, selective and precise liquid chromatography-tandem mass spectrometry (LC-MS/MS) method. The optimized enzyme reaction condition contained 1.5 μg of HMGR, 20 nM of NADPH with 50 min of reaction time. The method was validated by several intra- and inter-day assays. The production transitions of m/z 147.0/59.1 and m/z 154.0/59.1 were used to detect and quantify mevalonolactone (MVAL) and MVAL-D₇, respectively. The accuracy and precision of the method were evaluated over the concentration range of 0.005-1.000 μg/mL for MVAL and 0.010-0.500 μg/mL for lovastatin acid in three validation batch runs. The lower limit of quantitation was found to be 0.005 μg/mL for MVAL and 0.010 μg/mL for lovastatin acid. Intra-day and inter-day precision ranged from 0.95% to 2.39% and 2.26% to 3.38% for MVAL, 1.46% to 3.89% and 0.57% to 5.10% for lovastatin acid, respectively. The results showed that the active ingredients in Xuezhikang capsules were 12.2 and 14.5 mg/g, respectively. This assay method could be successfully applied to the quality control study of Xuezhikang capsule for the first time.

Characterization of oxysterols by electrospray ionization tandem mass spectrometry after one-step derivatization with dimethylglycine

We report a novel approach to derivatize the primary, secondary, and tertiary hydroxy group(s) of oxysterols with N,N-dimethylglycine (DMG) in the presence of both 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and 4-(N,N-dimethylamino)pyridine to yield their corresponding mono- or di-DMG esters. Eight oxysterols including 7-oxocholesterol, 5alpha,6alpha- and 5beta,6beta-epoxycholesterols, as well as 7alpha-, 7beta-, 24(S)-, 25-, and 27-hydroxycholesterols, were studied. Electrospray ionization tandem mass spectrometric characterization of these singly or doubly protonated derivatives demonstrates the presence of an informative fragmentation pattern for each oxysterol derivative. Potential dissociation pathways for the production of these unique fragmentation patterns are proposed and discussed. Collectively, these informative and unique fragmentation patterns allow rapid and direct discrimination of the identities of 7alpha-, 7beta-, 24(S)-, 25-, and 27-hydroxycholesterol isomers, as well as 5alpha,6alpha- and 5beta,6beta-epoxycholesterol isomers, thereby potentially providing a foundation for quantitative analysis of oxysterols in biological samples in combination with a chromatographic separation.

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.

Mitochondrial cholesterol transport: A possible target in the management of hyperlipidemia

Sterol 27-hydroxylase (CYP27A1) may defend cells against accumulation of excess cholesterol, making this enzyme a possible target in the management of hyperlipidemia. The study objective was to analyze cholesterol homeostatic responses to increases in CYP27A1 activity in HepG2 cells and primary human hepatocytes. Increasing CYP27A1 activity by increasing enzyme expression led to significant increases in bile acid synthesis with compensatory increases in HMG-CoA reductase (HMGR) activity/protein, LDL receptor (LDLR) mRNA, and LDLR-mediated cholesterol uptake. Under these conditions, only a small increase in cellular 27-hydroxycholesterol (27OH-Chol) concentration was observed. No changes were detected in mature sterol regulatory element-binding proteins (SREBP) 1 or 2. Increasing CYP27A1 activity by increasing mitochondrial cholesterol transport (i.e., substrate availability) led to greater increases in bile acid synthesis with significant increases in cellular 27OH-Chol concentration. Mature SREBP 2 protein decreased significantly with compensatory decreases in HMGR protein. No change was detected in mature SREBP 1 protein. Despite increasing 27OH-Chol and lowering SREBP 2 protein concentrations, LDLR mRNA increased significantly, suggesting alternative mechanisms of LDLR transcriptional regulation. These findings suggest that regulation of liver mitochondrial cholesterol transport represents a potential therapeutic strategy in the treatment of hyperlipidemia and atherosclerosis.

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.

Regulation of oxysterol 7alpha-hydroxylase (CYP7B1) in the rat

Cholesterol metabolized to 7alpha-hydroxylated bile acids is a principle pathway of cholesterol degradation. Cholesterol 7alpha-hydroxylase (CYP7A1) is the initial and rate-determining enzyme in the "classic pathway" of bile acid synthesis. An "alternative" pathway of bile acid synthesis begins with 27-hydroxylation of cholesterol by 27-hydroxylase (CYP27), followed by 7alpha-hydroxylation by oxysterol 7alpha-hydroxylase (CYP7B1). The aim of the current study was to investigate the regulation of CYP7B1 by bile acids, cholesterol, and thyroid hormone in a previously well-studied in vivo model of bile acid synthesis, and to compare its regulation to that of CYP7A1. Three study groups were examined. In the first, male Sprague-Dawley rats with intact enterohepatic circulations were fed normal chow (controls), cholestyramine (CT), cholic acid (CA), chenodeoxycholic acid (CDCA), deoxycholic acid (DCA), or cholesterol (Chol). In the second group, taurocholate (TCA) was continuously intraduodenally infused for 48 hours to chronic biliary diverted rats. In a third set of studies, squalestatin, an inhibitor of cholesterol synthesis, was intravenously infused for 48 hours. In a fourth set of studies, the diurnal variation in CYP7B1 was compared to that of CYP7A1. At the end of each study livers were harvested, and CYP7B1 and CYP7A1 activities and mRNA levels were determined. Complete biliary diversion significantly increased the specific activity (SA) of both CYP7B1 ( upward arrow 212%; P <.002) and CYP7A1 ( upward arrow 212%; P <.007). Intraduodenal infusion of TCA to rats with biliary diversion decreased SA of both CYP7B1 ( downward arrow 29%; P <.001) and CYP7A1 ( downward arrow 46%; P <.01). The addition of CA, CDCA, or DCA to rat chow led to downregulation of CYP7B1 SAs by 42% (P <.003), 51% (P <.009), and 47% (P <.003), and CYP7A1 SAs by 32% +/- 6% (P <.003), 73% +/- 9% (P <.002), and 60% +/- 13% (P <.004), respectively. CT feeding upregulated both CYP7B1 ( upward arrow 136%; P <.004) and CYP7A1 ( upward arrow 216%; P <.001) SAs. While Chol feeding significantly upregulated CYP7A1 SA, no significant increase in CYP7B1 SA was found. Conversely, as previously shown in vitro, inhibition of cholesterol synthesis significantly suppressed both CYP7A1 and CYP7B1 activity and mRNA levels. Both CYP7B1 and CYP7A1 underwent diurnal variation, with peak and trough values for CYP7B1 lagging approximately 6 hours behind CYP7A1. We conclude that, in the rat, like CYP7A1, CYP7B1 demonstrates diurnal rhythm and is regulated by bile acids and cholesterol.

Genetic background of cholesterol gallstone disease

Cholesterol gallstone formation is a multifactorial process involving a multitude of metabolic pathways. The primary pathogenic factor is hypersecretion of free cholesterol into bile. For people living in the Western Hemisphere, this is almost a normal condition, certainly in the elderly, which explains the very high incidence of gallstone disease. It is probably because the multifactorial background genes responsible for the high incidence have not yet been identified, despite the fact that genetic factors clearly play a role. Analysis of the many pathways involved in biliary cholesterol secretion reveals many potential candidates and considering the progress in unraveling the regulatory mechanisms of the responsible genes, identification of the primary gallstone genes will be successful in the near future.

Regulation of oxysterol 7alpha-hydroxylase (CYP7B1) in primary cultures of rat hepatocytes

Conversion of cholesterol into 7alpha-hydroxylated bile acids is a principal pathway of cholesterol disposal. Cholesterol 7alpha-hydroxylase (CYP7A1) is the initial and rate-determining enzyme in the "classic" pathway of bile acid synthesis. An "alternative" pathway of bile acid synthesis is initiated by sterol 27-hydroxylase (CYP27) with subsequent 7alpha-hydroxylation of 27-hydroxycholesterol by oxysterol 7alpha-hydroxylase (CYP7B1). The regulation of CYP7B1, possibly a rate-determining enzyme in the alternative pathway, has not been thoroughly studied. The aims of this study were to (1) study the regulation of liver CYP7B1 by bile acids, cholesterol, adenosine 3', 5'-cyclic monophosphate (cAMP), and phorbol myristate acetate (PMA) in primary rat hepatocytes and (2) determine the effect of CYP7B1 overexpression on rates of bile acid synthesis. The effects of different bile acids (3-150 micromol/L), cAMP (50 micromol/L), PMA (100 nmol/L; protein kinase C stimulator), cholesterol (200 micromol/L), and squalestatin (1 micromol/L; cholesterol synthesis inhibitor) on CYP7B1 expression in primary rat hepatocytes were studied. Taurocholic acid and taurodeoxycholic acid decreased CYP7B1 activity by 45% +/- 10% and 36% +/- 7%, respectively. Tauroursodeoxycholic acid and taurochenodeoxycholic acid did not alter CYP7B1 activity. Inhibition of cholesterol synthesis with squalestatin decreased CYP7B1 activity by 35%, whereas addition of cholesterol increased activity by 39%. Both PMA and cAMP decreased CYP7B1 activity by 60% and 34%, respectively, in a time-dependent fashion. Changes in CYP7B1 messenger RNA (mRNA) levels correlated with changes in specific activities. Overexpression of CYP7B1 led to a marked increase in CYP7B1 mRNA levels and specific activity but no change in rates of bile acid synthesis. In conclusion, in the rat, CYP7B1 specific activity is highly regulated but does not seem to be rate limiting for bile acid synthesis.

Overexpression of CYP27 in hepatic and extrahepatic cells: role in the regulation of cholesterol homeostasis

In the liver, sterol 27-hydroxylase (CYP27) participates in the classic and alternative pathways of bile acid biosynthesis from cholesterol (Chol). In extrahepatic tissues, CYP27 converts intracellular Chol to 27-hydroxycholesterol (27OH-Chol), which may regulate the activity of 3-hydroxy-3-methylglutaryl CoA reductase (HMG-CoA-R). This study attempts to better define the role of CYP27 in the maintenance of Chol homeostasis in hepatic and extrahepatic cells by overexpressing CYP27 in Hep G2 cells and Chinese hamster ovary (CHO) cells through infection with a replication-defective recombinant adenovirus encoding for CMV-CYP27. After infection, CYP27 mRNA and protein levels increased dramatically. CYP27 specific activity also increased two- to fourfold in infected cells (P < or = 0.02), with a marked increase in conversion of [(14)C]Chol to [(14)C]27OH-Chol (approximately 150%; P < or = 0.01). Accumulation of 27OH-Chol in CHO cells was associated with a 50% decrease in HMG-CoA-R specific activity (P < or = 0.02). In infected Hep G2 cells, the significant increase in bile acid synthesis (46%; P < or = 0.006), which prevented the accumulation of intracellular 27OH-Chol, resulted in increased HMG-CoA-R activity (183%; P < or = 0.02). Overexpression of CYP27 in Hep G2 cells also increased acyl CoA-cholesterol acyltransferase (71%, P < or = 0.02) and decreased cholesteryl ester hydrolase (55%, P < or = 0.02). In conclusion, CYP27 generates different physiological responses depending on cell type and presence or absence of bile acid biosynthetic pathways.

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.

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.

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

In vitro and in vivo studies have shown that the sterol 27-hydroxylase (CYP27) gene is transcriptionally repressed by hydrophobic bile acids. The molecular mechanism(s) of repression of CYP27 by bile acids is unknown. To identify the bile acid responsive element (BARE) and transcription factor(s) that mediate the repression of CYP27 by bile acids, constructs of the CYP27 5'-flanking DNA were linked to either the CAT or luciferase reporter gene and transiently transfected into primary rat hepatocytes. Taurocholate (TCA), taurodeoxycholate (TDCA) and taurochenodeoxycholate (TCDCA) significantly reduced CAT activities of the -840/+23, -329/+23, and -195/+23 mCAT constructs. A -76/+23 construct showed no regulation by bile acids. When a DNA fragment (-110/-86) from this region was cloned in front of an SV 40 promoter it showed down-regulation by TDCA. 'Super'-electrophoretic mobility shift assays (EMSA) indicated that both HNF1alpha and C/EBP bind to the -110 to -86 bp DNA fragment. Recombinant rat HNF1alpha and C/EBPalpha competitively bound to this DNA fragment. 'Super'-EMSA showed that TDCA addition to hepatocytes in culture decreased HNF1alpha, but not C/EBP, binding to the -110/-86 bp DNA fragment. A four base pair substitution mutation (-103 to -99) in this sequence eliminated TCA and TDCA regulation of the (-840/+23) construct. The substitution mutation also eliminated (>95%) HNF1alpha, but not C/EBP, binding to this DNA fragment. We conclude that bile acids repress CYP27 transcription through a putative BARE located between -110 and -86 bp of the CYP27 promoter. The data suggest that bile acids repress CYP27 transcriptional activity by decreasing HNF1alpha binding to the CYP27 promoter.

Sterol synthesis. Preparation and characterization of fluorinated and deuterated analogs of oxygenated derivatives of cholesterol

Oxygenated sterols, including both autoxidation products and sterol metabolites, have many important biological activities. Identification and quantitation of oxysterols by chromatographic and spectroscopic methods is greatly facilitated by the availability of authentic standards, and deuterated and fluorinated analogs are valuable as internal standards for quantitation. We describe the preparation, purification and characterization of 43 oxygenated sterols, including the 4 beta-hydroxy, 7 alpha-hydroxy, 7 beta-hydroxy, 7-keto, and 19-hydroxy derivatives of cholesterol and their analogs with 25,26,26,26,27,27,27-heptafluoro (F7) and 26,26,26,27,27,27-hexadeuterio (d6) substitution. The 7 alpha-hydroxy, 7 beta-hydroxy, and 7-keto derivatives of (25R)-cholest-5-ene-3 beta, 26-diol (1d) and their 16,16-dideuterio analogs were also prepared. These d2-26-hydroxysterols and [16,16-2H2]-(25R)-cholest-5-ene-3 beta, 26-diol (1e) were synthesized from [16,16-2H2]-(25R)-cholest-5-ene-3 beta, 26-diol diacetate (2e), which can be prepared from diosgenin. The highly specific deuterium incorporation at C-16 in 1e and 2e should be useful in mass spectral analysis of 26-hydroxycholesterol samples by isotope dilution methods. The delta 5-3 beta, 7 alpha, 26- and delta 5-3 beta, 7 beta, 26-triols were regioselectively oxidized/isomerized to the corresponding delta 4-3-ketosteroids with cholesterol oxidase. Also described are 5,6 alpha-epoxy-5 alpha-cholestan-3 beta-ol, its 5 beta,6 beta-isomer, cholestane-3 beta, 5 alpha,6 beta-triol, their F7 and d6 derivatives, and d3-25-hydroxycholesterol, which was prepared from 3 beta-acetoxy-27-norcholest-5-en-25-one (30). The 43 oxysterols and most synthetic intermediates were isolated in high purity and characterized by chromatographic and spectroscopic methods, including mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. Detailed mass spectral assignments are presented, and 1H NMR stereochemical assignments are derived for the C-19 protons of 19-hydroxysterols and for the side-chain protons of 30.

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.

Effect of dietary lipids on hepatic and extrahepatic sterol 27-hydroxylase activity in high- and low-responding baboons

Our previous studies found that low low-density lipoprotein (LDL)-responding baboons compared with high LDL-responding baboons have higher hepatic sterol 27-hydroxylase activity when consuming a high-cholesterol and high-fat (HCHF) diet. The present studies were conducted to determine whether the extrahepatic activity of sterol 27-hydroxylase is also higher in low-responding baboons and to assess whether the enzyme is regulated at the protein level. We measured the hepatic sterol 27-hydroxylase activity and protein level and plasma 27-hydroxycholesterol concentration in six low- and six high-responding baboons on both the basal and the HCHF diet. We also compared the sterol 27-hydroxylase activity in the adrenal gland and 27-hydroxycholesterol concentration in blood lymphocytes from high- and low-responding baboons consuming the HCHF diet. With the HCHF diet, the plasma 27-hydroxycholesterol concentration and hepatic sterol 27-hydroxylase activity and protein level increased rapidly in low responders, but not in high responders. Blood lymphocytes of low-responding baboons cultured in the presence of lipoprotein-deficient serum (LPDS) had lower cholesterol concentrations than those from high-responding baboons. Addition of exogenous 27-hydroxycholesterol to the culture medium of blood lymphocytes decreased the cellular cholesterol concentration. Plasma 27-hydroxycholesterol and hepatic sterol 27-hydroxylase activity and protein levels were negatively correlated with the plasma VLDL + LDL cholesterol concentration and VLDL + LDL/HDL cholesterol ratio after 6 weeks on the HCHF diet, but not on the chow diet. The results suggest that sterol 27-hydroxylase activity in both hepatic and extrahepatic tissues attenuates the dietary responsiveness in baboons, and the enzyme activity is not regulated by the specific activity of the protein.

Effect of naturally reduced ovarian function on plasma lipoprotein and 27-hydroxycholesterol levels in baboons (Papio sp.)

Female baboons over 15 years of age develop irregular menstrual cycles, an indication of declining ovarian function similar to that occurring in perimenopausal women. To determine the effect of declining ovarian function on plasma lipoprotein metabolism and plasma oxysterols, we measured plasma lipoprotein and 27-hydroxycholesterol levels in 86 female baboons from 15-28 years of age with regular (n = 51) and irregular (n = 35) menstrual cycles. We sampled blood and liver while they were consuming a basal diet and after consuming a high cholesterol and high fat diet for 7 weeks. On the basal diet, baboons with irregular cycles had higher VLDL + LDL/HDL cholesterol ratios (P = 0.034). After consuming the HCHF diet for 7 weeks, total plasma (P < 0.001) and VLDL + LDL (P < 0.001) cholesterol concentrations and VLDL + LDL/HDL sterol ratios (P < 0.001) increased in both cycle groups; whereas HDL cholesterol concentrations increased only in baboons with regular cycles (P = 0.009). As a result, HDL cholesterol concentrations (P = 0.006) were lower and VLDL + LDL/HDL cholesterol ratios (P = 0.002) were higher in baboons with irregular cycles on the HCHF diet. Plasma 27-hydroxycholesterol concentrations were higher in baboons with regular cycles than in those with irregular cycles on both basal (P = 0.018) and HCHF (P = 0.037) diets and were positively correlated (P < 0.001) with hepatic sterol 27-hydroxylase activities on both diets. Hepatic sterol 27-hydroxylase activities were negatively correlated with the VLDL + LDL/HDL cholesterol ratios on the HCHF diet (r = -0.342, P = 0.033). These results suggest that declining ovarian function changes the plasma lipoprotein pattern to one that is more atherogenic. Ovarian failure is also associated with decreased concentrations of plasma 27-hydroxycholesterol (the major oxysterol of plasma), and the decrease in plasma 27-hydroxycholesterol concentration was due to the decrease in hepatic sterol 27-hydroxylase activity. The effects of ovarian failure on plasma lipoprotein metabolism and plasma 27-hydroxycholesterol may be mediated by the decreased production of estrogen in perimenopausal baboons. Thus, the perimenopausal baboon is an excellent model for menopause and can be used for studies that cannot be conducted in women.

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.

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.

Effect of estrogen and progesterone on the expression of hepatic and extrahepatic sterol 27-hydroxylase in baboons (Papio sp)

Sterol 27-hydroxylase plays an important role in cholesterol metabolism in hepatic and extrahepatic tissues. To determine whether female sex steroid hormones influence its expression, we measured plasma and hepatic 27-hydroxycholesterol, hepatic mRNA levels, activity of sterol 27-hydroxylase, and adrenal mRNA levels of this enzyme in baboons (n = 6 per group) treated with placebo, estrogen, estrogen + progesterone, and progesterone. We also measured hepatic cholesterol concentration and hepatic acyl coenzyme A:cholesterol acyltransferase (ACAT) activity to determine their relationship with hepatic sterol 27-hydroxylase activity. Plasma 27-hydroxycholesterol concentration was increased by estrogen and estrogen + progesterone and was negatively correlated with plasma (P = .090) and LDL (P = .026) cholesterol concentrations. Similarly, hepatic sterol 27-hydroxylase activity was increased by estrogen and estrogen + progesterone and was negatively correlated with plasma (P = .056) and LDL (P = .052) cholesterol concentrations but was positively correlated with hepatic and plasma 27-hydroxycholesterol concentrations (P < .001). Hepatic ACAT activity was increased by progesterone (P < .004) and was positively correlated with plasma (P = .002) and LDL (P = .009) cholesterol concentrations but was negatively correlated with hepatic sterol 27-hydroxylase activity (P = .035). Hepatic and adrenal gland mRNA levels for sterol 27-hydroxylase were increased by estrogen alone or in combination with progesterone (P < .05). Hepatic sterol 27-hydroxylase activity was positively correlated with hepatic mRNA levels (P < .001), an observation suggesting that estrogen increases the activity of sterol 27-hydroxylase by increasing its synthesis. Hepatic cholesterol concentration was not influenced by the hormone treatment. These observations suggest that estrogen alone or in combination with progesterone increases the synthesis of sterol 27-hydroxylase in hepatic and extrahepatic tissues, and the increased activity of hepatic sterol 27-hydroxylase resulting from the increased synthesis is associated with a hypolipidemic effect on plasma LDL levels. Furthermore, progesterone alone increases the hepatic ACAT activity, but given in combination with estrogen progesterone does not have the same effect on hepatic ACAT activity. The effect of estrogen on hepatic ACAT activity may be mediated by sterol 27-hydroxylase and its effect on cholesterol metabolism (decreased cholesterol synthesis and increased output of cholesterol in the bile) in liver.

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.

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

The metabolism of 27-hydroxycholesterol and 25-hydroxycholesterol was studied in cultures of human diploid fibroblasts. Both steroids underwent 7 alpha-hydroxylation with subsequent oxidation to 7 alpha-hydroxy-3-oxo-delta 4 steroids. A minor fraction of the 27-hydroxysteroids was oxidized to acids. Competition experiments indicated that both hydroxycholesterols were hydroxylated by the same enzyme, different from cholesterol 7 alpha-hydroxylase. 7 alpha,25-Dihydroxycholesterol suppressed the activity of HMG-CoA reductase at least as effectively as 25-hydroxycholesterol whereas 7 alpha,25-dihydroxy-4-cholesten-3-one was a less effective suppressor. The results suggest that cholesterol might be converted to 7 alpha-hydroxylated bile acid precursors in extrahepatic tissues in vivo and that the regulation of the activity of HMG-CoA reductase by oxysterols might be modulated by 7 alpha-hydroxylation and subsequent oxidation by 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase/isomerase.