27-Hydroxycholesterol: production rates in normal human subjects

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.

Cytochrome P450s in the synthesis of cholesterol and bile acids--from mouse models to human diseases

The present review describes the transgenic mouse models that have been designed to evaluate the functions of the cytochrome P450s involved in cholesterol and bile acid synthesis, as well as their link with disease. The knockout of cholesterogenic Cyp51 is embrionally lethal, with symptoms of Antley-Bixler syndrome occurring in mice, whereas the evidence for this association is conflicting in humans. Disruption of Cyp7a1 from classic bile acid synthesis in mice leads to either increased postnatal death or a milder phenotype with elevated serum cholesterol. The latter is similar to the case in humans, where CYP7A1 mutations associate with high plasma low-density lipoprotein and hepatic cholesterol content, as well as deficient bile acid excretion. Disruption of Cyp8b1 from an alternative bile acid pathway results in the absence of cholic acid and a reduced absorption of dietary lipids; however, the human CYP8B1 polymorphism fails to explain differences in bile acid composition. Unexpectedly, apparently normal Cyp27a1(-/-) mice still synthesize bile acids that originate from the compensatory pathway. In humans, CYP27A1 mutations cause cerebrotendinous xanthomatosis, suggesting that only mice can compensate for the loss of alternative bile acid synthesis. In line with this, Cyp7b1 knockouts are also apparently normal, whereas human CYP7B1 mutations lead to a congenital bile acid synthesis defect in children or spastic paraplegia in adults. Mouse knockouts of the brain-specific Cyp46a1 have reduced brain cholesterol excretion, whereas, in humans, CYP46A1 polymorphisms associate with cognitive impairment. At present, cytochrome P450 family 39 is poorly characterized. Despite important physiological differences between humans and mice, mouse models prove to be an invaluable tool for understanding the multifactorial facets of cholesterol and bile acid-related disorders.

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.

27-Hydroxycholesterol: the first identified endogenous SERM

The cholesterol metabolite 27-hydroxycholesterol (27OHC) classically delivers sterols from peripheral tissues to the liver and is a substrate for bile acid synthesis. Recent studies have revealed that 27OHC also binds to and modifies the function of estrogen receptors ERα and ERβ. Experiments in mice lacking the enzyme which synthesizes 27OHC, CYP27A1, or the enzyme which catabolizes 27OHC, CYP7B1, have demonstrated that 27OHC adversely affects estrogen-related cardiovascular protection and bone mineralization. Work in breast cancer cells further indicates that 27OHC alters ER target gene expression to promote cell growth. Therefore, 27OHC is the first identified endogenous selective estrogen receptor modulator (SERM) and could have an important impact upon the cardiovascular system, bone biology, and cancer.

Whole body cholesterol metabolism is impaired in Huntington's disease

We previously reported impaired cholesterol biosynthesis in rodent Huntington Disease (HD) models and HD patients' fibroblasts and post mortem brains. We also found that plasma levels of 24S-hydroxycholesterol (24OHC), the brain specific elimination product of cholesterol considered a marker of brain cholesterol turnover, were significantly reduced in HD patients at any disease stage. In the present study we analysed by mass spectrometry the fasting plasma levels of cholesterol, its biosynthetic precursors lanosterol and lathosterol, of the whole-body elimination products 27-hydroxycholesterol and of brain 24OHC in a cohort of premanifest and HD patients at different disease stages. We found that the cholesterol precursors lanosterol and lathosterol (both index of whole body cholesterol synthesis), the levels of the bile acid precursor 27-hydroxycholesterol, and of the brain specific 24OHC, were all significantly reduced in manifest HD patients, suggesting that whole-body and brain cholesterol homeostasis are both impaired in HD.

Developmental pattern of urinary bile acid profile in preterm infants

BACKGROUND

Bile acid metabolism in preterm infants is yet to be fully characterized. We compared the developmental pattern of urinary bile acid profiles in ten infants born at gestational ages from 25 to 33 weeks with previous data from full-term infants from birth to about 7 months of age.

METHODS

Gas chromatography-mass spectrometry was performed on serial samples.

RESULTS

Total urinary bile acid concentrations gradually increased until 1 to 2 months of age. After this peak of excretion (30 to 60 micromol/mmol creatinine), total urinary bile acid concentrations gradually decreased to less than 20 micromol/mmol creatinine. The percentage of usual bile acids (mainly cholic acid) relative to total urinary total bile acids gradually deceased from approximately 30% at birth to less than 15% at 7 months of age. On the other hand, 1beta-hydroxylated bile acids (mainly 1beta,3alpha,7alpha,12alpha-tetrahydroxy-5beta-cholan-24-oic acid) relative to total urinary bile acids were increased gradually from 60% at birth to reach 70% to 80% at 1 month of age. The percentage of 1beta-hydroxylated bile acids relative to total urinary bile acids then remained stable at a high percentage (70% to 90%) until the age of 7 months.

CONCLUSION

Physiological cholestasis in preterm infants persists longer than in full-term infants. Moreover, as large amounts of cholic and 1beta,3alpha,7alpha,12alpha-tetrahydroxy-5beta-cholan-24-oic acids were detected in urine from preterm infants during this study, the 25-hydroxylation pathway may be particularly important for bile acid synthesis in early preterm infants.

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

OBJECTIVE

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

MATERIAL AND METHODS

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

RESULTS

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

CONCLUSION

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

Altered bile composition after liver transplantation is associated with the development of nonanastomotic biliary strictures

BACKGROUND/AIMS

Nonanastomotic biliary strictures are troublesome complications after liver transplantation. The pathogenesis of NAS is not completely clear, but experimental studies suggest that bile salt toxicity is involved.

METHODS

In one hundred and eleven adult liver transplants, bile samples were collected daily posttransplantation for determination of bile composition. Expression of bile transporters was studied perioperatively.

RESULTS

Nonanastomotic biliary strictures were detected in 14 patients (13%) within one year after transplantation. Patient and donor characteristics and postoperative serum liver enzymes were similar between patients who developed nonanastomotic biliary strictures and those who did not. Secretions of bile salts, phospholipids and cholesterol were significantly lower in patients who developed strictures. In parallel, biliary phospholipids/bile salt ratio was lower in patients developing strictures, suggestive for increased bile cytotoxicity. There were no differences in bile salt pool composition or in hepatobiliary transporter expression.

CONCLUSIONS

Although patients who develop nonanastomotic biliary strictures are initially clinically indiscernible from patients who do not develop nonanastomotic biliary strictures, the biliary bile salts and phospholipids secretion, as well as biliary phospholipids/bile salt ratio in the first week after transplantation, was significantly lower in the former group. This supports the concept that bile cytotoxicity is involved in the pathogenesis of nonanastomotic biliary strictures.

Cholesterol-metabolizing cytochromes P450: implications for cholesterol lowering

Cardiovascular disease (CVD) continues to be a leading cause of death worldwide. Elevated serum cholesterol is one of the classical risk factors for CVD, which also include age, hypertension, smoking, diabetes mellitus, obesity and family history. Several therapeutic drug classes have been developed to treat hypercholesterolemia; yet, an important percentage of patients do not reach their treatment goals. Therefore, new cholesterol-lowering medications that have sites of action different from that of drugs available at present need to be developed. This review summarizes new information about cytochrome P450 enzymes 7A1, 27A1 and 46A1. These enzymes play key roles in cholesterol elimination and have the potential to serve as targets for cholesterol-lowering.

Crosstalk between cholesterol homeostasis and drug metabolism

Napjainkban a cardiovascularis megbetegedések vezető halálozási oknak számítanak világszerte. A szív- és érrendszeri megbetegedések kialakulásában jelentős szerepet játszik a magas szérumkoleszterin-szint, illetve az atherosclerosis. A vér koleszterinszintjének csökkentésével kedvezően befolyásolható a káros folyamatok kialakulása, és a már kialakult betegségekben is javulás érhető el. Az általánosan alkalmazott sztatinalapú gyógyszeres terápia ade novokoleszterin-bioszintézist gátolja a májban. Más hatóanyagok (például ezetimib) a koleszterin táplálékból történő felszívódását gátolják. A leghatékonyabb megoldást ezek kombinált alkalmazása jelentheti. A gyógyszeres terápia során azonban figyelembe kell venni, hogy számos vegyület (gyógyszer) képes specifikusan megváltoztatni – a koleszterinhomeosztázis fenntartásában szerepet játszó enzimek mellett – a gyógyszer-metabolizáló enzimek indukciójával a citokróm P450 enzimek mennyiségét is (például sztatinok), ami a terápia módosítását teszi szükségessé. A koleszterin-anyagcsere és a gyógyszer-metabolizmus szabályozásában ugyanis több kapcsolódási pont is található. A kapcsolat az úgynevezett nukleáris receptorokon keresztül valósul meg, ezért a koleszterinhomeosztázis és a gyógyszer-metabolizmus közti összefüggés megértése és ismereteink bővítése elengedhetetlen egy megfelelő terápiás stratégia kidolgozásához, illetve új gyógyszerek fejlesztéséhez.

Niemann-Pick C1 protects against atherosclerosis in mice via regulation of macrophage intracellular cholesterol trafficking

Niemann-Pick C1 (NPC1) is a key participant in cellular cholesterol trafficking. Loss of NPC1 function leads to defective suppression of SREBP-dependent gene expression and failure to appropriately activate liver X receptor-mediated (LXR-mediated) pathways, ultimately resulting in intracellular cholesterol accumulation. To determine whether NPC1 contributes to regulation of macrophage sterol homeostasis in vivo, we examined the effect of NPC1 deletion in BM-derived cells on atherosclerotic lesion development in the Ldlr-/- mouse model of atherosclerosis. High-fat diet-fed chimeric Npc1-/- mice reconstituted with Ldlr-/-Npc1-/- macrophages exhibited accelerated atherosclerosis despite lower serum cholesterol compared with mice reconstituted with wild-type macrophages. The discordance between the low serum lipoprotein levels and the presence of aortic atherosclerosis suggested that intrinsic alterations in macrophage sterol metabolism in the chimeric Npc1-/- mice played a greater role in atherosclerotic lesion formation than did serum lipoprotein levels. Macrophages from chimeric Npc1-/- mice showed decreased synthesis of 27-hydroxycholesterol (27-HC), an endogenous LXR ligand; decreased expression of LXR-regulated cholesterol transporters; and impaired cholesterol efflux. Lower 27-HC levels were associated with elevated cholesterol oxidation products in macrophages and plasma of chimeric Npc1-/- mice and with increased oxidative stress. Our results demonstrate that NPC1 serves an atheroprotective role in mice through regulation of LXR-dependent cholesterol efflux and mitigation of cholesterol-induced oxidative stress in macrophages.

Determination of key intermediates in cholesterol and bile acid biosynthesis by stable isotope dilution mass spectrometry

For more than a decade, we have developed stable isotope dilution mass spectrometry methods to quantify key intermediates in cholesterol and bile acid biosynthesis, mevalonate and oxysterols, respectively. The methods are more sensitive and reproducible than conventional radioisotope (RI), gas-chromatography (GC) or high-performance liquid chromatography (HPLC) methods, so that they are applicable not only to samples from experimental animals but also to small amounts of human specimens. In this paper, we review the development of stable isotope dilution mass spectrometry for quantifying mevalonate and oxysterols in biological materials, and demonstrate the usefulness of this technique.

Changes in classic and alternative pathways of bile acid synthesis in chronic liver disease

BACKGROUND

Cholesterol elimination occurs through bile acid synthesis that starts within the liver from 7alpha-hydroxylation or in extrahepatic tissues from 27-hydroxylation. This study was aimed at investigating in vivo these two pathways in patients with chronic liver disease.

METHODS

Serum concentrations of 7alpha- and 27-hydroxycholesterol were measured in 54 patients (29 with primary biliary cirrhosis and 25 with chronic hepatitis C) and 18 controls. The rate of oxysterol plasma appearance was calculated after intravenous infusions of deuterated 7alpha- and 27-hydroxycholesterol in patients (n=8) and control subjects (n=8) who gave consent. The expression of sterol 27-hydroxylase was evaluated in macrophages isolated from 20 subjects.

RESULTS

In patients with liver disease, the rate of plasma appearance of 7alpha-hydroxycholesterol was significantly reduced (1.44+/-0.96 vs. 2.75+/-1.43 mg/hour, p=0.03), the degree of reduction being related with the severity of the disease (p=0.01) whereas that of 27-hydroxycholesterol was unaffected. The rate of plasma appearance of 27-hydroxycholesterol was significantly related to its serum concentrations (r=0.54, p=0.03) and to its release from cultured macrophages ( r=0.85, p=0.03).

CONCLUSIONS

In liver disease 7alpha-hydroxylation of cholesterol seems to be impaired while 27-hydroxylation is unaffected. Serum concentrations of 27-hydroxycholesterol are useful to obtain information on the activity of this alternative pathway.

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.

Prevalence and clinical significance of sonographic detection of enlarged regional lymph nodes in Crohn's disease

OBJECTIVE

Reactive regional lymph node enlargement is a frequent ultrasonographic finding in patients with Crohn's disease. However, the prevalence of this condition and its clinical significance are unknown. This study assesses the prevalence of enlarged regional mesenteric lymph nodes and its clinical significance in Crohn's disease, and in particular whether there is a correlation between the sonographic detection of enlarged regional lymph nodes and the degree of clinical or biochemical activity of the disease.

MATERIAL AND METHODS

A total of 240 in- and outpatients with Crohn's disease underwent intestinal ultrasound to assess the presence of enlarged regional lymph nodes as well as the thickness and echopattern of the bowel wall, the site and extent of Crohn's disease and the presence of stenosis, fistulas and abscesses. Demographic, clinical and biochemical parameters were also collected for each patient. A multivariate model by means of multiple regression analysis was used to identify independent variables linked to regional lymph node enlargement.

RESULTS

Enlarged regional lymph nodes were detected ultrasonographically in 25.4% of Crohn's disease patients. The presence of regional lymph nodes showed a weak correlation with both clinical and biochemical Crohn's disease activity. Regional lymph nodes were found more frequently in young patients (50% of patients < 30 years, 18% of patients between 30 and 50 yrs, and 7% of patients > 50 yrs; p<0.0001) and in patients with a shorter disease duration. Enlarged regional lymph nodes were strongly correlated with internal fistulas and intra-abdominal abscesses. The multiple regression analysis showed that age, duration of disease and presence of internal fistulas were the best independent predictive factors linked to the presence of enlarged mesenteric lymph nodes.

CONCLUSIONS

The sonographic detection of enlarged regional lymph nodes is more frequent in young patients, which suggests an earlier phase of Crohn's disease and the presence of septic complications such as fistulas and abscesses, but this is of limited valued in assessing disease activity.

A minimally invasive technique for the evaluation of the regulatory steps of the two major pathways of bile acid synthesis

BACKGROUND

Bile acid synthesis accounts for more than 95% of total cholesterol catabolism per day. We have developed a minimally invasive technique in humans that quantifies the rates of plasma appearance of 7alpha- and 27-hydroxycholesterol, representing the first steps of the "classical" and "alternative" pathways of bile acid synthesis, respectively.

METHODS

For this purpose, during the intravenous infusion of synthetic deuterated isotopomers of 7alpha-hydroxycholesterol and 27-hydroxycholesterol plasma samples are collected and analysed by a GC-MS based method that allows to quantify the exogenous/natural isotopomer ratio of the two sterols. From this data, the rates of plasma appearance of 7alpha- and 27-hydroxycholesterol are calculated.

RESULTS

In a group of healthy individuals steady state kinetics are obtained during a 2 h period yielding mean values of 2.0+/-0.8 and 3.7+/-0.6 mg/h for 7alpha- and 27-hydroxycholesterol, respectively. The data are consistent with findings using older techniques that require studies over several days.

CONCLUSION

Considering that at steady state of the exogenous/natural isotopomer ratio the plasma appearance of the two regulatory hydroxysterols are related to the rate of bile acid synthesis via the "classical" and the "alternative" pathways, respectively, the proposed method could be used to evaluate the immediate effects of different diets and drugs and other determinants on cholesterol catabolism.

Cholesterol binding to cytochrome P450 7A1, a key enzyme in bile acid biosynthesis

The conversion of cholesterol to 7alpha-hydroxycholesterol catalyzed by cytochrome P450 7A1 (CYP7A1) initiates the major pathway for cholesterol elimination in mammals. In the present work we focused on identification of determinants of the CYP7A1 substrate specificity inside the active site using a homology model with a novel P450-fold, site-directed mutagenesis, and substrate-binding and kinetic studies. Forty-one mutants, encompassing twenty-six amino acid residues, were generated and characterized, and of these, seven residues appear to determine cholesterol binding in the active site. In addition, four cholesterol derivatives were used as active site probes in the wild type and the seven mutant enzymes, and the spectral binding constants and products were analyzed. It was concluded that Asn288 in the I helix plays a key role in the P450-cholesterol contacts by hydrogen bonding to the steroid 3beta-hydroxyl, while Val280 and Ala284 are beside and the Trp283 is above the steroid nucleus orienting the cholesterol molecule. Leu360 and Ala358 between the K helix and the beta1-4 strand and Leu485 in the beta4 sheet-turn appear to define the size of the active site over the heme pyrrole ring A, thus limiting the orientation and size of the substrate at the steroid A ring. Additionally, the A358V mutant was found to form two new products, one being 7beta-hydroxycholesterol. Our data indicate that a tight fit of cholesterol in the enzyme active site is in part responsible for the high efficiency of cholesterol turnover by CYP7A1.

Clemmensen reduction of diosgenin and kryptogenin: synthesis of [16,16,22,22,23,23-(2)H(6)]-(25R)-26-hydroxycholesterol

A new experimental protocol has been established for the Clemmensen reduction of diosgenin and kryptogenin with the aim to prepare deuterated isotopomers of (25R)-26-hydroxycholesterol. Uncontrolled deuteration has been achieved from diosgenin, whereas [16,16,22,22,23,23-(2)H(6)]-(25R)-26-hydroxycholesterol (1) can be synthesized from kryptogenin.

Overexpression of cholesterol transporter StAR increases in vivo rates of bile acid synthesis in the rat and mouse

Bile acid synthesis (BAS) occurs mainly via two pathways: the "neutral" pathway, which is initiated by highly regulated microsomal CYP7A1, and an "acidic" pathway, which is initiated by mitochondrial CYP27A1. Previously, we have shown that overexpression of the steroidogenic acute regulatory protein (StAR), a mitochondrial cholesterol transport protein, increases bile acid biosynthesis more than 5-fold via the acidic pathway in primary rat hepatocytes. This observation suggests that mitochondrial cholesterol transport is the rate-limiting step of BAS via this pathway. The objective of this study was to determine the effect of increased StAR on rates of BAS in vivo. Overexpression of StAR and CYP7A1 were mediated via infection with recombinant adenoviruses. BAS rates were determined in chronic biliary-diverted rats and mice, and in mice with an intact enterohepatic circulation. The protein/messenger RNA levels of StAR and CYP7A1 increased dramatically following overexpression. Overexpression of StAR or CYP7A1 led to a similar 2-fold (P <.01) increase in BAS over up-regulated (approximately 2-fold) 3-day chronic biliary-diverted control rats. Additionally, overexpression of StAR led to more than 3- and 6-fold increases over controls in the rates of BAS in biliary-diverted and intact mice, respectively (P <.01). In conclusion, in both rats and mice in vivo, overexpression of StAR led to a marked increase in the rates of BAS initiated by delivery of cholesterol to mitochondria containing CYP27A1.

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.

Cholic acid supplementation enhances cholesterol absorption in humans

BACKGROUND & AIMS

Qualitative and quantitative changes in intralumenal bile acid composition may alter cholesterol absorption and synthesis and low-density lipoprotein (LDL) receptor expression. The role of cholic acid (CA) in cholesterol absorption in humans remains unclear and, thus, was examined in the current study.

METHODS

In a crossover design outpatient study, 12 adults aged 24-36 years took 15 mg/kg/day (CA) or no bile acid supplement (control) while being fed a controlled diet (AHA heart-healthy diet). A liquid meal of defined composition was given on day 14 of the diet, and lumenal samples were collected. Thereafter, cholesterol absorption and cholesterol fractional synthetic rate (FSR) were assessed by stable isotopic methods from days 16 to 20.

RESULTS

With CA treatment, bile was enriched significantly with CA (P < 0.0004) to 60.2% +/- 2.4% (mean +/- SEM) compared with 43.3% +/- 2.4% for controls. CA plus diet treatment significantly increased (P = 0.013) cholesterol absorption (72.6% +/- 2.9%) compared with diet treatment alone (60.4% +/- 2.9%). Percentage micellar cholesterol was increased by CA plus diet treatment vs. diet alone after meal ingestion (P = 0.004). Plasma total and high-density lipoprotein (HDL) and LDL cholesterol was unchanged with CA treatment.

CONCLUSIONS

Thus, enrichment in lumenal bile with CA results in an increase in cholesterol absorption, an effect potentially mediated by enhanced cholesterol solubilization in micelles.

Significance of plasma 7alpha-hydroxy-4-cholesten-3-one and 27-hydroxycholesterol concentrations as markers for hepatic bile acid synthesis in cholesterol-fed rabbits

Plasma 7alpha-hydroxy-4-cholesten-3-one has been used as an index of hepatic bile acid synthesis. The aim of the current study was to ascertain whether the level of this oxysterol reflects hepatic cholesterol 7alpha-hydroxylase activity when plasma cholesterol concentrations are markedly changed. In addition, the relationship of hepatic sterol 27-hydroxylase activity with plasma concentrations of 27-hydroxycholesterol and 3beta-hydroxy-5-cholestenoic acid was studied. We used New Zealand white rabbits fed 2% cholesterol for 5 or 10 days and/or constructed bile fistula. Feeding cholesterol markedly increased and bile drainage reduced plasma cholesterol concentrations. Initially, in these models there was no correlation between plasma 7alpha-hydroxy-4-cholesten-3-one concentrations and hepatic cholesterol 7alpha-hydroxylase activities (r = -0.24, n = 10). Cholesterol feeding was associated with downregulated 7alpha-hydroxylase activities, while plasma 7alpha-hydroxy-4-cholesten-3-one concentrations were elevated in the presence of increased plasma cholesterol levels. However, this discrepancy was overcome and significant correlation was observed (r = 0.73, P <.05, n = 10) by expressing 7alpha-hydroxy-4-cholesten-3-one levels relative to cholesterol. In contrast, hepatic sterol 27-hydroxylase activities were not significantly correlated with plasma absolute (r = 0.23, difference not significant [NS], n = 10) nor cholesterol-related levels of 27-hydroxycholesterol (r = -0.13, NS, n = 10), or 3beta-hydroxy-5-cholestenoic acid concentrations (r = 0.30, NS, n = 10). In conclusion, plasma 7alpha-hydroxy-4-cholesten-3-one concentrations reflected hepatic cholesterol 7alpha-hydroxylase activities when the sterol levels were adjusted to plasma cholesterol concentrations in rabbits with hypercholesterolemia. The results suggest that plasma 7alpha-hydroxy-4-cholesten-3-one relative to cholesterol is a better marker for hepatic cholesterol 7alpha-hydroxylase activity than the absolute concentration when hypercholesterolemia is present. In contrast, 27-hydroxycholesterol and 3beta-hydroxy-5-cholestenoic acid levels in plasma did not reflect hepatic sterol 27-hydroxylase activities even if the levels were adjusted to plasma cholesterol concentrations.

The enzymes, regulation, and genetics of bile acid synthesis

The synthesis and excretion of bile acids comprise the major pathway of cholesterol catabolism in mammals. Synthesis provides a direct means of converting cholesterol, which is both hydrophobic and insoluble, into a water-soluble and readily excreted molecule, the bile acid. The biosynthetic steps that accomplish this transformation also confer detergent properties to the bile acid, which are exploited by the body to facilitate the secretion of cholesterol from the liver. This role in the elimination of cholesterol is counterbalanced by the ability of bile acids to solubilize dietary cholesterol and essential nutrients and to promote their delivery to the liver. The synthesis of a full complement of bile acids requires 17 enzymes. The expression of selected enzymes in the pathway is tightly regulated by nuclear hormone receptors and other transcription factors, which ensure a constant supply of bile acids in an ever changing metabolic environment. Inherited mutations that impair bile acid synthesis cause a spectrum of human disease; this ranges from liver failure in early childhood to progressive neuropathy in adults.

Programming of initial steps in bile acid synthesis by breat-feeding vs. Formula-feeding in the baboon

We tested the hypothesis that breast- vs. formula-feeding differentially affects the enzymatic activity of three sterol hydroxylases critical in the initial steps of bile acid formation. Thirty baboons were either breast-fed or formula-fed for the first 14 wk of life before weaning to baboon chow. At 14 and 34 wk of age, liver biopsies were assayed for cholesterol 7alpha-hydroxylase (CYP7A1), 27-hydroxycholesterol-7alpha-hydroxylase (CYP7B1), and cholesterol 27-hydroxylase (CYP27A1). We also determined the kinetics of 3H-27-hydroxycholesterol (27-OHC) turnover in vivo at both ages. At 14 wk of age, hepatic CYP7A1 activity was low but sevenfold higher among formula-fed vs. breast-fed baboons. By 34 wk, CYP7A1 activity had increased nearly 10-fold in both infant diet groups, and the sevenfold difference in CYP7A1 between previously breast- and formula-fed animals persisted. There were no differences in CYP7B1 activities between infant diet groups at either 14 or 34 wk of age although the activity increased in both groups by about 50% from 14 to 34 wk. CYP27A1 activity also increased between 14 and 34 wk of age, and, compared with CYP7A1, relatively small differences in CYP27A1 activity due to infant diet were observed at each age. Plasma 27-OHC turnover had a half-time of 2-4 min. We had previously reported that after weaning, the total bile acid synthesis rate was higher among baboons that were formula-fed than among breast-fed animals. The present results suggest that this difference is most likely due to significantly higher CYP7A1 activity among formula-fed vs. breast-fed animals.

On the origin of the cholestenoic acids in human circulation

3 Beta-hydroxy-5-cholestenoic acid, 3 beta,7 alpha-dihydroxy-5-cholestenoic acid, and 7 alpha-hydroxy-3-oxo-4-cholestenoic acid are metabolites of cholesterol present at significant concentrations (40-80 ng/ml) in human circulation. The 7 alpha-hydroxylated acids may be formed from cholesterol via two major pathways initiated by oxidations at either the 7 alpha- or 27-positions. In an attempt to clarify the origin and possible precursor-product relationships between these cholestenoic acids, we measured their deuterium enrichment in a unique experiment, after infusion of 10 g of [2H(6)]-cholesterol to a healthy volunteer. The observed extent and time-course of deuterium enrichment of circulating 3 beta-hydroxy-5-cholestenoic and 3 beta,7 alpha-dihydroxy-5-cholestenoic acid were almost identical, while different from that of cholesterol and 7 alpha-hydroxycholesterol. Notably, the deuterium enrichment of 7 alpha-hydroxy-3-oxo-4-cholestenoic acid was similar to that of 7 alpha-hydroxycholesterol (and its metabolite 7 alpha-hydroxy-4-cholesten-3-one), though distinct from the other cholestenoic acids. Finally, the enrichment of unesterified 27-hydroxycholesterol followed a similar, though less pronounced, time curve to the delta(5)-cholestenoic acids. In conclusion, these results suggest that plasma 3 beta-hydroxy-5-cholestenoic acid is formed from a pool of cholesterol distinct from that used for the formation of the bulk of 27-hydroxycholesterol. The results are also in accordance with a formation of 3 beta,7 alpha-dihydroxy-5-cholestenoic acid directly from 3 beta-hydroxy-5-cholestenoic acid, and a formation of most of the circulating 7 alpha-hydroxy-4-cholesten-3-one from 7 alpha-hydroxycholesterol. These results are consistent with a flux of 7 alpha-hydroxycholesterol from the liver into the circulation, and an extrahepatic metabolism of this steroid into 7 alpha-hydroxy-3-oxo-4-cholestenoic acid.

Perinatal bile acid metabolism: analysis of urinary unsaturated ketonic bile acids in preterm and full-term infants

AIM

To compare urinary concentrations of unsaturated ketonic bile acids in preterm and full-term infants.

METHODS

Urinary unsaturated ketonic bile acids were determined using gas chromatography-mass spectrometry.

RESULTS

Urinary concentrations of total bile acids in early preterm infants (of less than 29wk gestational age) exceeded concentrations in late preterm (between 30 and 37 wk) and full-term infants (between 38 and 41 wk; p < 0.01). The percentage of ketonic bile acids (7alpha, 12alpha-dihydroxy-3-oxo-4-cholenoic acid and 7alpha-hydroxy-3-oxo-4-cholenoic acid) among total urinary bile acids in full-term infants (20.2 +/- 14.1%) was higher than that in early preterm infants (8.94 +/- 8.1%; p < 0.05). The percentage of unsaturated bile acids (3beta-hydroxy-delta5-bile acids) among total bile acids in urine did not differ greatly between groups.

CONCLUSION

The percentage of 3-oxo-delta4 bile acids among total bile acids in urine gradually increased from early to late preterm infants, while healthy full-term infants excreted large amounts of 3-oxo-delta4 bile acids in urine at delivery.

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.

Bile acid regulation of hepatic physiology: III. Regulation of bile acid synthesis: past progress and future challenges

Bile acids, amphipathic detergent-like molecules synthesized from cholesterol, are highly conserved by means of enterohepatic circulation. They participate in the generation of bile flow and biliary lipid secretion and also promote absorption of fat-soluble vitamins and lipids. Conversion of cholesterol to bile acids represents a quantitatively important route to eliminate cholesterol from the body. Regulation of bile acid synthesis involves a complex and interrelated group of transcription regulators that link bile acid synthesis to cholesterol and fatty acid metabolism. Targeting key steps of bile acid synthetic pathways as well as the metabolic network that maintains homeostatic levels of lipids should provide exciting novel opportunities for the treatment of cardiovascular and liver diseases.

Synthesis of (20S)-[7,7,21,21-2H4]-3beta-(tert-butyldimethylsilanyloxy)-20-methyl-pregn-5-en-21-ol, a useful intermediate for the preparation of deuterated isotopomers of sterols

(20S)-[7,7,21,21-2H(4)]-3beta-(tert-Butyldimethylsilanyloxy)-20-methyl-pregn-5-en-21-ol, an intermediate for the preparation of deuterated isotopomers of sterols to be used as standards for biomedical studies, was prepared by reduction with dichloroaluminum deuteride of ethyl (20S)-3beta-(tert-butyldimethylsilanyloxy)-7-oxo-pregn-5-en-20-carboxylate. Using controlled experimental conditions, it has also been shown that the dichloroaluminum hydride reduction of a 7-keto steroid affords the corresponding 7beta-hydroxy derivative in a highly stereoselective manner.

Transport of cholesterol into mitochondria is rate-limiting for bile acid synthesis via the alternative pathway in primary rat hepatocytes

Bile acid synthesis occurs mainly via two pathways: the "classic" pathway, initiated by microsomal cholesterol 7alpha-hydroxylase (CYP7A1), and an "alternative" (acidic) pathway, initiated by sterol 27-hydroxylase (CYP27). CYP27 is located in the inner mitochondrial membrane, where cholesterol content is very low. We hypothesized that cholesterol transport into mitochondria may be rate-limiting for bile acid synthesis via the "alternative" pathway. Overexpression of the gene encoding steroidogenic acute regulatory (StAR) protein, a known mitochondrial cholesterol transport protein, led to a 5-fold increase in bile acid synthesis. An increase in StAR protein coincided with an increase in bile acid synthesis. CYP27 overexpression increased bile acid synthesis by <2-fold. The rates of bile acid synthesis following a combination of StAR plus CYP27 overexpression were similar to those obtained with StAR alone. TLC analysis of (14)C-labeled bile acids synthesized in cells overexpressing StAR showed a 5-fold increase in muricholic acid; in chloroform-extractable products, a dramatic increase was seen in bile acid biosynthesis intermediates (27- and 7,27-hydroxycholesterol). High-performance liquid chromatography analysis showed that 27-hydroxycholesterol accumulated in the mitochondria of StAR-overexpressing cells only. These findings suggest that cholesterol delivery to the inner mitochondrial membrane is the predominant rate-determining step for bile acid synthesis via the alternative pathway.

Oat bran stimulates bile acid synthesis within 8 h as measured by 7alpha-hydroxy-4-cholesten-3-one

BACKGROUND

Oat bran contains soluble fibers, such as beta-glucan, that increase bile acid excretion and thus decrease serum cholesterol. Bile acid synthesis correlates with serum concentrations of the metabolite 7alpha-hydroxy-4-cholesten-3-one (alpha-HC).

OBJECTIVE

The objective was to investigate whether consumption of beta-glucan from oat bran increases bile acid synthesis, as measured by the serum alpha-HC concentration, within hours after consumption in response to the loss of bile acids from the liver.

DESIGN

In a randomized, single-blind, wheat bran-controlled study with crossover design, 8 subjects were served a controlled diet during 2 periods of 3 d each, with an 11-d washout between the periods. Breakfast included either 75 g extruded oat bran, of which 11 g was beta-glucan, or 75 g wheat bran, of which 1 g was beta-glucan. Alpha-HC was measured by HPLC on each day at 0, 12, and 24 h after breakfast and also at 8 h after breakfast on the first day.

RESULTS

After 8 and 12 h of the oat bran diet period, the serum alpha-HC concentration was 84% (P = 0.012) and 92% (P = 0.017) higher, respectively, than that before breakfast. Serum concentrations returned to the baseline value after 24 h. Wheat bran did not influence serum alpha-HC concentrations.

CONCLUSIONS

Consumption of beta-glucan from oat bran nearly doubled the serum alpha-HC concentration within 8 h, indicating increased bile acid synthesis. alpha-HC in serum could be used as a marker of increased bile acid excretion induced by the diet.

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

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

Bile acid regulation of gene expression: roles of nuclear hormone receptors

Bile acids derived from cholesterol and oxysterols derived from cholesterol and bile acid synthesis pathways are signaling molecules that regulate cholesterol homeostasis in mammals. Many nuclear receptors play pivotal roles in the regulation of bile acid and cholesterol metabolism. Bile acids activate the farnesoid X receptor (FXR) to inhibit transcription of the gene for cholesterol 7alpha-hydroxylase, and stimulate excretion and transport of bile acids. Therefore, FXR is a bile acid sensor that protects liver from accumulation of toxic bile acids and xenobiotics. Oxysterols activate the liver orphan receptors (LXR) to induce cholesterol 7alpha-hydroxylase and ATP-binding cassette family of transporters and thus promote reverse cholesterol transport from the peripheral tissues to the liver for degradation to bile acids. LXR also induces the sterol response element binding protein-1c that regulates lipogenesis. Therefore, FXR and LXR play critical roles in coordinate control of bile acid, cholesterol, and triglyceride metabolism to maintain lipid homeostasis. Nuclear receptors and bile acid/oxysterol-regulated genes are potential targets for developing drug therapies for lowering serum cholesterol and triglycerides and treating cardiovascular and liver diseases.

Differences in the regulation of the classical and the alternative pathway for bile acid synthesis in human liver. No coordinate regulation of CYP7A1 and CYP27A1

It has been reported that there is a coordinate regulation of sterol 27-hydroxylase (CYP27A1) and cholesterol 7alpha-hydroxylase (CYP7A1) in rats. Thus, the levels of the mRNA corresponding to these two enzymes were found to change in the same direction in rat liver and in isolated rat hepatocytes. In contrast, other groups have not seen such regulation of CYP27A1 in rabbit liver or in rat liver when using an activity assay. In the present work, the effect of bile acid treatment on human CYP27A1/luciferase reporter activity was studied in a transient transfection assay in human liver-derived HepG2 cells. Neither the endogenous 27-hydroxylase activity nor the CYP27A1/luciferase reporter activity were down-regulated by treatment of HepG2 cells with chenodeoxycholic acid or taurochenodeoxycholic acid. We also measured CYP27A1 mRNA and CYP7A1 mRNA in liver of humans subjected to treatment with chenodeoxycholic acid, ursodeoxycholic acid, hydroxymethylglutaryl (HMG)-CoA reductase inhibitor and a combination of HMG-CoA reductase inhibitor and cholestyramine. There was a 60-fold variation in the levels of CYP7A1 mRNA but only a 5-fold variation in the levels of CYP27A1 mRNA. There was no correlation between the two mRNA species. It is concluded that, in humans, there is little or no coordinate regulation of CYP7A1 and CYP27A1 at the transcriptional level, and that CYP27A1 is not subject to a negative feedback control by bile acids. The results underline that marked species differences may exist in mechanisms for control of synthesis of bile acids and cholesterol homeostasis.

Oxysterols in human circulation: which role do they have?

Oxysterols are oxygenated derivatives of cholesterol that are intermediates in cholesterol excretion pathways. They may also be regarded as transport forms of cholesterol and introduction of an additional hydroxyl group facilitates flux of cholesterol across cell membranes and the blood-brain barrier. According to current concepts, oxysterols are also mediating a number of cholesterol-induced metabolic effects. The recent discovery of nuclear receptors with an affinity for oxysterols has given support to this concept. Nuclear receptors such as liver X receptor alpha do have a role in cholesterol homeostasis, but there is still only indirect evidence that oxysterols are the physiological ligands. In this overview we report some recent advancements in our knowledge about the origin and metabolic fate of the quantitatively most important oxysterols occurring in the circulation. In addition, we discuss the possibility that some of these oxysterols may activate liver X receptors and regulate cholesterol homeostasis.

Oxysterols: friends, foes, or just fellow passengers?

Oxysterols are oxygenated derivatives of cholesterol that are intermediates or even end products in cholesterol excretion pathways. Because of their ability to pass cell membranes and the blood-brain barrier at a faster rate than cholesterol itself, they are also important as transport forms of cholesterol. In addition, oxysterols have been ascribed a number of important roles in connection with cholesterol turnover, atherosclerosis, apoptosis, necrosis, inflammation, immunosuppression, and the development of gallstones. According to current concepts, oxysterols are physiological mediators in connection with a number of cholesterol-induced metabolic effects. However, most of the evidence for this is still indirect, and there is a discrepancy between the documented potent effects of oxysterols under in vitro conditions and the studies demonstrating that they are of physiological importance in vivo. Oxysterol-binding proteins, such as liver X receptor-alpha (a nuclear receptor), do have a regulatory role in cholesterol turnover, but the physiological ligand of the protein has not yet been defined with certainty. Recently developed genetically engineered mouse models with markedly reduced or increased concentration of some of the oxysterols have exhibited surprisingly small changes in cholesterol turnover and homeostasis. The present review is a critical evaluation of the literature on oxysterols, in particular, the in vivo evidence for a role of oxysterols as physiological regulators of cholesterol homeostasis and as atherogenic factors.

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.

Effects of CYP7A1 overexpression on cholesterol and bile acid homeostasis

The initial and rate-limiting step in the classic pathway of bile acid biosynthesis is 7alpha-hydroxylation of cholesterol, a reaction catalyzed by cholesterol 7alpha-hydroxylase (CYP7A1). The effect of CYP7A1 overexpression on cholesterol homeostasis in human liver cells has not been examined. The specific aim of this study was to determine the effects of overexpression of CYP7A1 on key regulatory steps involved in hepatocellular cholesterol homeostasis, using primary human hepatocytes (PHH) and HepG2 cells. Overexpression of CYP7A1 in HepG2 cells and PHH was accomplished by using a recombinant adenovirus encoding a CYP7A1 cDNA (AdCMV-CYP7A1). CYP7A1 overexpression resulted in a marked activation of the classic pathway of bile acid biosynthesis in both PHH and HepG2 cells. In response, there was decreased HMG-CoA-reductase (HMGR) activity, decreased acyl CoA:cholesterol acyltransferase (ACAT) activity, increased cholesteryl ester hydrolase (CEH) activity, and increased low-density lipoprotein receptor (LDLR) mRNA expression. Changes observed in HMGR, ACAT, and CEH mRNA levels paralleled changes in enzyme specific activities. More specifically, LDLR expression, ACAT activity, and CEH activity appeared responsive to an increase in cholesterol degradation after increased CYP7A1 expression. Conversely, accumulation of the oxysterol 7alpha-hydroxycholesterol in the microsomes after CYP7A1 overexpression was correlated with a decrease in HMGR activity.

Cholesterol catabolism in patients with acute myelogenous leukemia and hypocholesterolemia: suppressed levels of a circulating marker for bile acid synthesis

Hypocholesterolemia is a frequent finding in patients with acute myelogenous leukemia (AML) and in other types of malignancies. Since bile acids are major excretion products of cholesterol, the hepatic degradation of cholesterol to bile acids was investigated in AML patients by analyzing a circulating marker for bile acid synthesis. In addition, plasma levels of a marker for cholesterol synthesis were determined. The plasma levels of 7alpha-hydroxy-4-cholesten-3-one, reflecting bile acid production, were markedly lower in patients with AML than in healthy controls. The median levels were 3.3 and 18.5ng/ml (P<0.0001) in the AML patients (n=29) and the healthy subjects (n=16), respectively. The plasma levels of 7-dehydrocholesterol, reflecting hepatic cholesterol synthesis, were similar for the AML patients and the controls. The results show that the conversion of cholesterol to bile acids was suppressed in AML patients, a phenomenon that may result in a decreased intestinal absorption of cholesterol and subsequent hypocholesterolemia.

Expression of sterol 12alpha-hydroxylase alters bile acid pool composition in primary rat hepatocytes and in vivo

BACKGROUND & AIMS

The rate of 12alpha-hydroxylation of bile acid intermediates is believed to determine the ratio of cholic acid (CA) to chenodeoxycholic acid (CDCA) biosynthesis and the overall hydrophobicity of the bile acid pool. The aim of this study was to determine the effects of the level of expression of sterol 12alpha-hydroxylase (CYP8b1) and cholesterol 7alpha-hydroxylase (CYP7a1) on rates of CA biosynthesis and bile acid pool composition.

METHODS

Expression of CYP8b1 and CYP7a1 was accomplished through infection of primary rat hepatocytes (PRH) or intact male SD rats with replication-defective recombinant adenoviruses encoding either CYP8b1 or CYP7a1.

RESULTS

Increased expression of CYP7a1 over basal levels in PRH dramatically increased bile acid biosynthesis (586% +/- 82%, P < 0.001) but did not alter the ratio of CA to CDCA. Conversely, increased expression of CYP8b1 in vitro had no significant effect on the rates of total bile acid synthesis but significantly increased (4.1-fold) the rates of CA biosynthesis, resulting in an increase in the CA-CDCA ratio from 1:6.6 to 2.8:1. In whole rats, increased CYP8b1 expression over basal levels markedly increased the CA in the bile acid pool from 36% +/- 3.4% to 50% +/- 2.9% in 5 days. CDCA and its muricholic acid derivatives decreased from 64% +/- 3.4% to 50% +/- 2.9%.

CONCLUSIONS

Increased expression of CYP8b1 led to a marked increase in CA biosynthesis both in PRH and in whole animals. CYP8b1 is capable of 12alpha-hydroxylating bile acid intermediates from both the classic and acidic pathways.