Markedly reduced bile acid synthesis but maintained levels of cholesterol and vitamin D metabolites in mice with disrupted sterol 27-hydroxylase gene

Regulation of cytochrome P450 (CYP) genes by nuclear receptors

Members of the nuclear-receptor superfamily mediate crucial physiological functions by regulating the synthesis of their target genes. Nuclear receptors are usually activated by ligand binding. Cytochrome P450 (CYP) isoforms often catalyse both formation and degradation of these ligands. CYPs also metabolize many exogenous compounds, some of which may act as activators of nuclear receptors and disruptors of endocrine and cellular homoeostasis. This review summarizes recent findings that indicate that major classes of CYP genes are selectively regulated by certain ligand-activated nuclear receptors, thus creating tightly controlled networks.

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

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

Disruption of the sterol 27-hydroxylase gene in mice results in hepatomegaly and hypertriglyceridemia. Reversal by cholic acid feeding

Sterol 27-hydroxylase (CYP27) participates in the conversion of cholesterol to bile acids. We examined lipid metabolism in mice lacking the Cyp27 gene. On normal rodent chow, Cyp27(-/-) mice have 40% larger livers, 45% larger adrenals, 2-fold higher hepatic and plasma triacylglycerol concentrations, a 70% higher rate of hepatic fatty acid synthesis, and a 70% increase in the ratio of oleic to stearic acid in the liver versus Cyp27(+/+) controls. In Cyp27(-/-) mice, cholesterol 7alpha-hydroxylase activity is increased 5-fold, but bile acid synthesis and pool size are 47 and 27%, respectively, of those in Cyp27(+/+) mice. Intestinal cholesterol absorption decreases from 54 to 4% in knockout mice, while fecal neutral sterol excretion increases 2.5-fold. A compensatory 2.5-fold increase in whole body cholesterol synthesis occurs in Cyp27(-/-) mice, principally in liver, adrenal, small intestine, lung, and spleen. The mRNA for the cholesterogenic transcription factor sterol regulatory element-binding protein-2 (SREBP-2) and mRNAs for SREBP-2-regulated cholesterol biosynthetic genes are elevated in livers of mutant mice. In addition, the mRNAs encoding the lipogenic transcription factor SREBP-1 and SREBP-1-regulated monounsaturated fatty acid biosynthetic enzymes are also increased. Hepatic synthesis of fatty acids and accumulation of triacylglycerols increases in Cyp27(-/-) mice and is associated with hypertriglyceridemia. Cholic acid feeding reverses hepatomegaly and hypertriglyceridemia but not adrenomegaly in Cyp27(-/-) mice. These studies confirm the importance of CYP27 in bile acid synthesis and they reveal an unexpected function of the enzyme in triacylglycerol metabolism.

Induction of bile acid synthesis by cholesterol and cholestyramine feeding is unimpaired in mice deficient in apolipoprotein AI

High density lipoprotein (HDL) cholesterol is believed to be preferentially utilized for bile acid synthesis and biliary secretion. In mice, the deletion of apolipoprotein AI (apo AI), the major apolipoprotein in HDL, results in very low plasma HDL-cholesterol levels. This article describes bile acid metabolism in apo AI-deficient (Apo AI(-/-)) mice and their C57BL/6 (Apo AI(+/+)) controls fed either a basal rodent diet alone or containing cholesterol or cholestyramine. Basal plasma HDL-cholesterol levels in the (-/-) mice (<10 mg/dL) were less than 20% of those in their (+/+) controls, but there were no phenotypic differences in either the relative cholesterol content of gallbladder bile, bile acid pool size and composition, fecal bile acid excretion or the activity of, or mRNA level for, cholesterol 7alpha-hydroxylase. However, compared with their (+/+) controls, the (-/-) mice absorbed more cholesterol (33 vs. 24%) and manifested lower rates of hepatic sterol synthesis (534 vs. 1,019 nmol/h per g). Cholesterol feeding increased hepatic cholesterol levels in the (+/+) animals from 2.7 to 4.4 mg/g and in the (-/-) mice from 2.6 to 8.1 mg/g. Bile acid synthesis increased 70% in both genotypes. Cholestyramine feeding stimulated bile acid synthesis 3.7 fold in both (-/-) and (+/+) mice. We conclude that the virtual loss of HDL-cholesterol from the circulation in apo AI deficiency has no impact on the ability of the hepatocyte to adapt its rate of bile acid synthesis in concert with the amount of cholesterol and bile acid returning to the liver from the small intestine.

Porcine microsomal vitamin D(3) 25-hydroxylase (CYP2D25). Catalytic properties, tissue distribution, and comparison with human CYP2D6

The metabolic activation of the prohormone vitamin D(3) requires a 25-hydroxylation that has been reported to be catalyzed by both mitochondrial CYP27A and a microsomal vitamin D(3) 25-hydroxylase in the liver. CYP27A has been extensively studied, but its role as a physiologically important vitamin D(3) 25-hydroxylase has been questioned. The present paper reports that the microsomal vitamin D(3) 25-hydroxylase, purified from pig liver, converted vitamin D(3) into 25-hydroxyvitamin D(3) in substrate concentrations which are within the physiological range (apparent K(m) = 0.1 microm). The enzyme 25-hydroxylated vitamin D(3), 1 alpha-hydroxyvitamin D(3) and vitamin D(2) and also converted tolterodine, a substrate for human CYP2D6, into its 5-hydroxymethyl metabolite. Tolterodine inhibited the microsomal 25-hydroxylation, whereas quinidine, an inhibitor of CYP2D6, did not markedly inhibit the reaction. The primary structure of the microsomal vitamin D(3) 25-hydroxylase, designated CYP2D25, shows 77% identity with that of human CYP2D6. Northern blot and reverse transcription-polymerase chain reaction experiments revealed that CYP2D25 mRNA is expressed in higher levels in liver than in kidney and in small amounts in adrenals, brain, heart, intestine, lung, muscle, spleen, and thymus. Experiments with human liver microsomes and recombinantly expressed CYP2D6 strongly indicate that the microsomal 25-hydroxylation of vitamin D(3) in human liver is catalyzed by an enzyme different from CYP2D6.

Cholestenoic acid is a naturally occurring ligand for liver X receptor alpha

Excessive cholesterol is eliminated from extrahepatic cells by reverse cholesterol transport, a process by which neutral sterols are transferred to extracellular acceptor lipoproteins for further transport to the liver. Another process independent of lipoproteins involves excretion of 3beta-hydroxy-5-cholesten-25(R)-26-carboxylic (cholestenoic) acid, a metabolite of 27-hydroxycholesterol. Physiological concentrations of cholestenoic acid activated the nuclear receptor liver X receptor alpha (LXR alpha; NR1H3), but not other oxysterol receptors. As a ligand, cholestenoic acid modulated interaction of LXR alpha with the nuclear receptor coactivator Grip-1. Cholestenoic acid, therefore, may function as a signaling molecule for regulation of lipid metabolism via LXR alpha.

The bile acid synthetic gene 3beta-hydroxy-Delta(5)-C(27)-steroid oxidoreductase is mutated in progressive intrahepatic cholestasis

We used expression cloning to isolate cDNAs encoding a microsomal 3beta-hydroxy-Delta(5)-C(27)-steroid oxidoreductase (C(27) 3beta-HSD) that is expressed predominantly in the liver. The predicted product shares 34% sequence identity with the C(19) and C(21) 3beta-HSD enzymes, which participate in steroid hormone metabolism. When transfected into cultured cells, the cloned C(27) 3beta-HSD cDNA encodes an enzyme that is active against four 7alpha-hydroxylated sterols, indicating that a single C(27) 3beta-HSD enzyme can participate in all known pathways of bile acid synthesis. The expressed enzyme did not metabolize several different C(19/21) steroids as substrates. The levels of hepatic C(27) 3beta-HSD mRNA in the mouse are not sexually dimorphic and do not change in response to dietary cholesterol or to changes in bile acid pool size. The corresponding human gene on chromosome 16p11.2-12 contains six exons and spans 3 kb of DNA, and we identified a 2-bp deletion in the C27 3beta-HSD gene of a patient with neonatal progressive intrahepatic cholestasis. This mutation eliminates the activity of the enzyme in transfected cells. These findings establish the central role of C(27) 3beta-HSD in the biosynthesis of bile acids and provide molecular tools for the diagnosis of a third type of neonatal progressive intrahepatic cholestasis associated with impaired bile acid synthesis.

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

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

Sterols and isoprenoids: signaling molecules derived from the cholesterol biosynthetic pathway

Compounds derived from the isoprenoid/cholesterol biosynthetic pathway have recently been shown to have novel biological activities. These compounds include certain sterols, oxysterols, farnesol, and geranylgeraniol, as well as the diphosphate derivatives of isopentenyl, geranyl, farnesyl, geranylgeranyl, and presqualene. They regulate transcriptional and post-transcriptional events that in turn affect lipid synthesis, meiosis, apoptosis, developmental patterning, protein cleavage, and protein degradation.

Sterol 27-hydroxylase deficiency: a rare cause of xanthomas in normocholesterolemic humans

Cerebrotendinous xanthomatosis is characterized by the accumulation of cholestanol and cholesterol in xanthomas and brain causing a number of severe symptoms. More than 20 different mutations have been identified in the gene encoding sterol 27-hydroxylase. Defects in the gene lead to reduced bile acid biosynthesis, with accumulation of 7 alpha-hydroxylated intermediates, one of which is a precursor to cholestanol. The disease can be treated successfully with chenodeoxycholic acid, which reduces the upregulation of cholesterol 7 alpha-hydroxylase and, therefore, the formation of cholestanol. Disruption of the gene encoding sterol 27-hydroxylase in mice does not have the same metabolic consequences as in humans.

Intracellular transport of bile acids

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

Disruption of the oxysterol 7alpha-hydroxylase gene in mice

Mice without oxysterol 7alpha-hydroxylase, an enzyme of the alternate bile acid synthesis pathway with a sexually dimorphic expression pattern, were constructed by the introduction of a null mutation at the Cyp7b1 locus. Animals heterozygous (Cyp7b1(+/-)) and homozygous (Cyp7b1(-/-)) for this mutation were grossly indistinguishable from wild-type mice. Plasma and tissue levels of 25- and 27-hydroxycholesterol, two oxysterol substrates of this enzyme with potent regulatory actions in cultured cells, were markedly elevated in Cyp7b1(-/-) knockout animals. Parameters of bile acid metabolism as well as plasma cholesterol and triglyceride levels in male and female Cyp7b1(-/-) mice were normal. The cholesterol contents of major tissues were not altered. In vivo sterol biosynthetic rates were unaffected in multiple tissues with the exception of the male kidney, which showed a approximately 40% decrease in de novo synthesis versus controls. We conclude that the major physiological role of the CYP7B1 oxysterol 7alpha-hydroxylase is to metabolize 25- and 27-hydroxycholesterol and that loss of this enzyme in the liver is compensated for by increases in the synthesis of bile acids by other pathways. A failure to catabolize oxysterols in the male kidney may lead to a decrease in de novo sterol synthesis.

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

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

Kinetics and plasma concentrations of 26-hydroxycholesterol in baboons

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

Inactivation of the peroxisomal multifunctional protein-2 in mice impedes the degradation of not only 2-methyl-branched fatty acids and bile acid intermediates but also of very long chain fatty acids

According to current views, peroxisomal beta-oxidation is organized as two parallel pathways: the classical pathway that is responsible for the degradation of straight chain fatty acids and a more recently identified pathway that degrades branched chain fatty acids and bile acid intermediates. Multifunctional protein-2 (MFP-2), also called d-bifunctional protein, catalyzes the second (hydration) and third (dehydrogenation) reactions of the latter pathway. In order to further clarify the physiological role of this enzyme in the degradation of fatty carboxylates, MFP-2 knockout mice were generated. MFP-2 deficiency caused a severe growth retardation during the first weeks of life, resulting in the premature death of one-third of the MFP-2(-/-) mice. Furthermore, MFP-2-deficient mice accumulated VLCFA in brain and liver phospholipids, immature C(27) bile acids in bile, and, after supplementation with phytol, pristanic and phytanic acid in liver triacylglycerols. These changes correlated with a severe impairment of peroxisomal beta-oxidation of very long straight chain fatty acids (C(24)), 2-methyl-branched chain fatty acids, and the bile acid intermediate trihydroxycoprostanic acid in fibroblast cultures or liver homogenates derived from the MFP-2 knockout mice. In contrast, peroxisomal beta-oxidation of long straight chain fatty acids (C(16)) was enhanced in liver tissue from MFP-2(-/-) mice, due to the up-regulation of the enzymes of the classical peroxisomal beta-oxidation pathway. The present data indicate that MFP-2 is not only essential for the degradation of 2-methyl-branched fatty acids and the bile acid intermediates di- and trihydroxycoprostanic acid but also for the breakdown of very long chain fatty acids.

Elevated cholesterol metabolism and bile acid synthesis in mice lacking membrane tyrosine kinase receptor FGFR4

Heparan sulfate-regulated transmembrane tyrosine kinase receptor FGFR4 is the major FGFR isotype in mature hepatocytes. Fibroblast growth factor has been implicated in the definition of liver from foregut endoderm where FGFR4 is expressed and stimulation of hepatocyte DNA synthesis in vitro. Here we show that livers of mice lacking FGFR4 exhibited normal morphology and regenerated normally in response to partial hepatectomy. However, the FGFR4 (-/-) mice exhibited depleted gallbladders, an elevated bile acid pool and elevated excretion of bile acids. Cholesterol- and bile acid-controlled liver cholesterol 7alpha-hydroxylase, the limiting enzyme for bile acid synthesis, was elevated, unresponsive to dietary cholesterol, but repressed normally by dietary cholate. Expression pattern and cholate-dependent, cholesterol-induced hepatomegaly in the FGFR4 (-/-) mice suggested that activation of receptor interacting protein 140, a co-repressor of feed-forward activator liver X receptor alpha, may mediate the negative regulation of cholesterol- and bile acid-controlled liver cholesterol 7alpha-hydroxylase transcription by FGFR4 and cholate. The results demonstrate that transmembrane sensors interface with metabolite-controlled transcription networks and suggest that pericellular matrix-controlled liver FGFR4 in particular may ensure adequate cholesterol for cell structures and signal transduction.

Clinical and molecular genetic characteristics of patients with cerebrotendinous xanthomatosis

Cerebrotendinous xanthomatosis (CTX) is a lipid storage disease caused by a deficiency of the mitochondrial enzyme 27-sterol hydroxylase (CYP 27), due to mutations in its gene. In this study we report on mutations in 58 patients with CTX out of 32 unrelated families. Eight of these were novel mutations, two of which were found together with two already known pathogenic mutations. Twelve mutations found in this patient group have been described in the literature. In the patients from 31 families, mutations were found in both alleles. In the literature, 28 mutations in 67 patients with CTX out of 44 families have been described. Pooling our patient group and the patients from the literature together, 37 different mutations in 125 patients out of 74 families were obtained. Identical mutations have been found in families from different ethnic backgrounds. In 41% of all the patients, CYP 27 gene mutations are found in the region of exons 6-8. This region encodes for adrenodoxin and haem binding sites of the protein. Of these 125 patients, a genotype-phenotype analysis was done for 79 homozygous patients harbouring 23 different mutations, out of 45 families. The patients with compound heterozygous mutations were left out of the genotype-phenotype analysis. The genotype-phenotype analysis did not reveal any correlation.

Regulation of cytochrome P450 (CYP) genes by nuclear receptors

Members of the nuclear-receptor superfamily mediate crucial physiological functions by regulating the synthesis of their target genes. Nuclear receptors are usually activated by ligand binding. Cytochrome P450 (CYP) isoforms often catalyse both formation and degradation of these ligands. CYPs also metabolize many exogenous compounds, some of which may act as activators of nuclear receptors and disruptors of endocrine and cellular homoeostasis. This review summarizes recent findings that indicate that major classes of CYP genes are selectively regulated by certain ligand-activated nuclear receptors, thus creating tightly controlled networks.

27-Oxygenation of C27-sterols and 25-hydroxylation of vitamin D3 in kidney: cloning, structure and expression of pig kidney CYP27A

This paper describes the molecular cloning of a cytochrome P450 enzyme in pig kidney that catalyses the hydroxylations of vitamin D(3) (cholecalciferol) and C(27)-sterols. DNA sequence analysis of the cDNA revealed that the enzyme belongs to the CYP27 family. The first 36 amino acids have many hallmarks of a mitochondrial signal sequence. The mature pig kidney CYP27 protein contains 498 amino acids. The M(r) of the mature protein was calculated to be 56607. The structure of pig kidney CYP27, as deduced by DNA sequence analysis, shows 77-83% identity with CYP27A in rat, rabbit and human liver. Transfection of the renal CYP27A cDNA into simian COS cells resulted in the synthesis of an enzyme that catalysed the 25-hydroxylation of vitamin D(3) and the 27-hydroxylation of 5beta-cholestane-3alpha,7alpha,12alpha-triol, and the further oxidation of the product into the corresponding C(27)-acid 3alpha,7alpha,12alpha-trihydroxy-5beta-cholestanoic acid. As part of these studies, the enzymic activities of cultured human embryonic kidney cells were examined using vitamin D(3) and C(27)-sterols as substrates. The cells were found to express CYP27A mRNA and to convert the respective substrates into the same products as recombinantly expressed CYP27A, i.e. 25-hydroxyvitamin D(3) and 27-oxygenated C(27)-sterols. The results of the present study describing the structure and expression of CYP27A in kidney suggest that this enzyme is involved in the renal metabolism of vitamin D(3) and that the kidney plays a role in the metabolism of cholesterol and other C(27)-sterols.

Apolipoprotein E regulates dietary cholesterol absorption and biliary cholesterol excretion: studies in C57BL/6 apolipoprotein E knockout mice

The present study examined the role of apolipoprotein E (apoE) in the regulation of dietary cholesterol absorption and biliary cholesterol excretion. Increasing dietary cholesterol from 0.02% to 0.5% in C57BL/6 wild-type mice decreased the percentage of dietary cholesterol that is absorbed by 25%, and this decrease was associated with a 2-fold increase in gallbladder biliary cholesterol concentration. In contrast, increasing dietary cholesterol from 0. 02% to 0.5% in C57BL/6 apoE knockout mice produced no significant suppression of the percentage dietary cholesterol absorption and increased gallbladder biliary cholesterol concentration only 16%. Whereas in wild-type mice, the increase in dietary cholesterol increased the hepatic excretion of biliary cholesterol 4-fold, there was only a 2-fold increase in apoE knockout mice. On both the low- and the high-cholesterol diets, whole liver and isolated hepatocyte cholesterol content was higher in the apoE knockout mice. These results suggest that, in response to dietary cholesterol, apoE may play a critical role in decreasing the percentage absorption of dietary cholesterol and increasing biliary cholesterol excretion. These observations suggest a mechanism whereby the absence of apoE contributes to the propensity for tissue cholesterol deposition and accelerated atherogenesis in apoE knockout mice.

Apolipoprotein E regulates dietary cholesterol absorption and biliary cholesterol excretion: Studies in C57BL/6 apolipoprotein E knockout mice

The present study examined the role of apolipoprotein E (apoE) in the regulation of dietary cholesterol absorption and biliary cholesterol excretion. Increasing dietary cholesterol from 0.02% to 0.5% in C57BL/6 wild-type mice decreased the percentage of dietary cholesterol that is absorbed by 25%, and this decrease was associated with a 2-fold increase in gallbladder biliary cholesterol concentration. In contrast, increasing dietary cholesterol from 0. 02% to 0.5% in C57BL/6 apoE knockout mice produced no significant suppression of the percentage dietary cholesterol absorption and increased gallbladder biliary cholesterol concentration only 16%. Whereas in wild-type mice, the increase in dietary cholesterol increased the hepatic excretion of biliary cholesterol 4-fold, there was only a 2-fold increase in apoE knockout mice. On both the low- and the high-cholesterol diets, whole liver and isolated hepatocyte cholesterol content was higher in the apoE knockout mice. These results suggest that, in response to dietary cholesterol, apoE may play a critical role in decreasing the percentage absorption of dietary cholesterol and increasing biliary cholesterol excretion. These observations suggest a mechanism whereby the absence of apoE contributes to the propensity for tissue cholesterol deposition and accelerated atherogenesis in apoE knockout mice.

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.

Aberrant oxidation of the cholesterol side chain in bile acid synthesis of sterol carrier protein-2/sterol carrier protein-x knockout mice

Peroxisomal beta-oxidation plays an important role in the metabolism of a wide range of substrates, including various fatty acids and the steroid side chain in bile acid synthesis. Two distinct thiolases have been implicated to function in peroxisomal beta-oxidation: the long known 41-kDa beta-ketothiolase identified by Hashimoto and co-workers (Hijikata, M., Ishii, N., Kagamiyama, H., Osumi, T., and Hashimoto, T. (1987) J. Biol. Chem. 262, 8151-8158) and the recently discovered 60-kDa SCPx thiolase, that consists of an N-terminal domain with beta-ketothiolase activity and a C-terminal moiety of sterol carrier protein-2 (SCP2, a lipid carrier or transfer protein). Recently, gene targeting of the SCP2/SCPx gene has shown in mice that the SCPx beta-ketothiolase is involved in peroxisomal beta-oxidation of 2-methyl-branched chain fatty acids like pristanic acid. In our present work we have investigated bile acid synthesis in the SCP2/SCPx knockout mice. Specific inhibition of beta-oxidation at the thiolytic cleavage step in bile acid synthesis is supported by our finding of pronounced accumulation in bile and serum from the knockout mice of 3alpha,7alpha, 12alpha-trihydroxy-27-nor-5beta-cholestane-24-one (which is a known bile alcohol derivative of the cholic acid synthetic intermediate 3alpha,7alpha,12alpha-trihydroxy-24-keto-cholestano yl-coenzyme A). Moreover, these mice have elevated concentrations of bile acids with shortened side chains (i.e. 23-norcholic acid and 23-norchenodeoxycholic acid), which may be produced via alpha- rather than beta-oxidation. Our results demonstrate that the SCPx thiolase is critical for beta-oxidation of the steroid side chain in conversion of cholesterol into bile acids.

Bile acids and lipoprotein metabolism: a renaissance for bile acids in the post-statin era?

Based on an improved molecular understanding of how bile acid metabolism is regulated, an exciting period of research developments can be expected. By new ways of stimulating cholesterol breakdown to bile acids, novel therapeutic principles can be forseen which will further improve our potential for treating and preventing atherosclerosis.

Cholestatic syndromes

Recent advances in cholestatic liver disease have occurred in several areas. Molecular cloning of hepatobiliary transport systems has resulted in the identification of the molecular basis of hereditary and acquired cholestatic syndromes. Apoptosis has been identified as an important mechanism of cholestatic liver injury and bile duct loss. New insights into the pathogenesis of pruritus and fatigue have resulted in new treatment strategies for these debilitating symptoms. Important new studies have been published about pathogenesis, clinical features, and treatment of primary biliary cirrhosis, primary sclerosing cholangitis, cholestasis of pregnancy, and drug-induced cholestasis.

Removal of cholesterol from extrahepatic sources by oxidative mechanisms

Sterol 27-hydroxylase is an evolutionarily old cytochrome P450 species that is critical for oxidation of the side chain of cholesterol in connection with bile acid biosynthesis in the liver. The wide tissue and organ distribution of the enzyme suggests that it may also have other functions. It was recently shown that some cells (e.g. macrophages) have a high capacity to convert cholesterol into both 27-hydroxycholesterol and cholestenoic acid and that there is a significant flux of these steroids from extrahepatic sources to the liver where they are further oxidized into bile acids. The magnitude of this flux is such that it may be of importance for overall homeostasis of cholesterol. Very recently it was shown that the brain utilizes a similar mechanism for removal of cholesterol. A unique brain-specific 24S-hydroxylase converts cholesterol into 24S-hydroxycholesterol that is transported over the blood-brain barrier much more rapidly than unmetabolized cholestero. When 24S-hydroxycholesterol has reached the circulation it is taken up by the liver and further metabolized, most probably into bile acids. This flux is likely to be of importance for cholesterol homeostasis in the brain. This review summarizes our current knowledge regarding oxidative mechanisms for removal of extrahepatic cholesterol. It is evident that some cells utilize these mechanisms as alternatives or complements to the classical HDL-dependent reverse cholesterol transport.