Immune complexes and IFN-gamma decrease cholesterol 27-hydroxylase in human arterial endothelium and macrophages.

The enzyme cholesterol 27-hydroxylase, expressed by arterial endothelium and monocytes/macrophages, is one of the first lines of defense against the development of atherosclerosis. By catalyzing the hydroxylation of cholesterol to 27-hydroxycholesterol, which is more soluble in aqueous medium, the enzyme promotes the removal of cholesterol from the arterial wall. Prior studies have suggested that immune reactants play a role in the pathogenesis of atherosclerosis; we report here that immune reactants, IFN-gamma and immune complexes bound to C1q, but not interleukin-1 and tumor necrosis factor, diminish the expression of cholesterol 27-hydroxylase in human aortic endothelial cells, peripheral blood mononuclear cells, monocyte-derived macrophages, and the human monocytoid cell line THP-1. In addition, our studies demonstrate that immune complexes down-regulate cholesterol 27-hydroxylase only after complement fixation via interaction with the 126-kD C1qRp protein on endothelial cells and THP-1 cells. These results are consistent with the prior demonstration that IFN-gamma contributes to the pathogenesis of atherosclerosis and suggest a role for C1q receptors in the atherogenic process. Moreover, these observations suggest that one mechanism by which immune reactants contribute to the development of atherosclerosis is by down-regulating the expression of the enzymes required to maintain cholesterol homeostasis in the arterial wall.

Cholesterol and hydroxycholesterol sulfotransferases: identification, distinction from dehydroepiandrosterone sulfotransferase, and differential tissue expression.

In humans, the biotransformation of cholesterol and its hydroxylated metabolites (oxysterols) by sulfonation is a fundamental process of great importance. Nevertheless, the sulfotransferase enzyme(s) that carries out this function has never been clearly identified. Cholesterol is a relatively poor substrate for the previously cloned hydroxysteroid sulfotransferase (HST), i.e. dehydroepiandrosterone (DHEA) sulfotransferase (HST1). Recently, cloning of a single human gene that encodes for two proteins related to HST1 was reported. These newly cloned sulfotransferases (HST2a and HST2b), while exhibiting sequence similarity to other members of the soluble sulfotransferase superfamily, also contain unique structural features. This latter aspect prompted an examination of their substrate specificity for comparison with HST1. Thus, HST1, HST2a, and HST2b were overexpressed as fusion proteins and purified. Furthermore, a novel procedure for the isolation of cholesterol and oxysterol sulfonates was developed that was used in association with HPLC to resolve specific sterol sulfonates. HST1 preferentially sulfonated DHEA and, to a lesser extent, oxysterols; whereas cholesterol was a negligible substrate. The reverse, however, was the case for the HST2 isoforms, particularly HST2b, which preferentially sulfonated cholesterol and oxysterols, in contrast to DHEA, which served as a poor substrate for this enzyme. RT-PCR analysis revealed distinct patterns of HST1, HST2a, and HST2b expression. It was particularly notable that both HST2 isoforms, but not HST1, were expressed in skin, a tissue where cholesterol sulfonation plays an important role in normal development of the skin barrier. In conclusion, substrate specificity and tissue distribution studies strongly suggest that HST2a and HST2b, in contrast to HST1, represent normal human cholesterol and oxysterol sulfotransferases. Furthermore, this study represents the first example of the sulfonation of oxysterols by a specific human HST.

Biologic role(s) of the 25(R),26-hydroxycholesterol metabolic pathway.

Expression of the gene coding for the synthesis of 25(R), 26-hydroxycholesterol in many tissues and the finding that this sterol can be the sole pathway for the production of bile acids have led to a renewed interest in this metabolic pathway. A further impetus for exploring the normal biologic roles that are served by expression of the CYP27A1 gene is the knowledge that mutations in humans are associated with accelerated atherosclerosis and with severe neurologic impairment. The molecular mechanisms governing these phenotypic expressions are not known but in light of the traditional role of steroids as ligands for receptors that regulate gene expression it seems likely that the intermediates in this pathway modulate a number of enzymatic activities that remain to be elucidated.

Oxysterols: 27-hydroxycholesterol and its radiolabeled analog.

We describe a convenient and stereoselective route to the synthesis of 27-hydroxycholesterol. Also its radiolabeled analog, 22, 23 di [(3)H]-27-hydroxycholesterol with high specific radioactivity (55 Ci/mmol) was synthesized by this method. Julia condensation of steroidal 22-sulfone with aldehyde, led to the addition of the 23-27 carbon side chain building block to the steroid backbone. Formed in this reaction beta-hydroxysulfone moiety was reduced by sodium amalgam generate 22-23 unsaturated bond. Further reduction either by hydrogen or tritium furnished substrates for the synthesis of title compounds.

Structure and functions of human oxysterol 7alpha-hydroxylase cDNAs and gene CYP7B1.

Oxysterol 7alpha-hydroxylase has broad substrate specificity for sterol metabolites and may be involved in many metabolic processes including bile acid synthesis and neurosteroid metabolism. The cloned human oxysterol 7alpha-hydroxylase (CYP7B1) cDNA encodes a polypeptide of 506 amino acid residues that shares 40% sequence identity to human cholesterol 7alpha-hydroxylase (CYP7A1), the rate-limiting enzyme in the conversion of cholesterol to bile acids in the liver. In contrast to the liver-specific expression of CYP7A1, CYP7B1 mRNA transcripts were detected in human tissues involved in steroid genesis (brain, testes, ovary, and prostate) and in bile acid synthesis (liver) and reabsorption (colon, kidney, and small intestine). The human oxysterol 7alpha-hydroxylase transiently expressed in 293/T cells was able to catalyze 7alpha-hydroxylation of 27-hydroxycholesterol and dehydroepiandrosterone (DHEA). The human CYP7A1 and CYP7B1 both contain six exons and five introns. However, CYP7B1 spans at least 65 kb of the genome and is about 6-fold longer than CYP7A1. The transcription start site (+1) was localized 204 bp upstream of the initiation codon. No TATA box-like sequence was found near the transcription start site. Transient transfection assays of CYP7B1 promoter/luciferase reporter constructs in HepG2 cells revealed that the promoter was highly active. The 5' upstream region from nt -83 to +189 is the core promoter of the gene.

27-hydroxycholesterol: production rates in normal human subjects.

We attempted to quantitate production of bile acid via the 27-hydroxylation pathway in six human subjects. After bolus intravenous injection of known amounts of [24-14C]cholic acid and [24-14C]chenodeoxycholic acid, each subject underwent a constant intravenous infusion of a mixture of [22, 23-3H]-27-hydroxycholesterol and [2H]-27-hydroxycholesterol for 6;-10 h. Production rate of 27-hydroxycholesterol was calculated from the infusion rate of [2H]-27-hydroxycholesterol and the serum ratio of deuterated/protium 27-hydroxycholesterol, which reached a plateau level by 4 h of infusion. Conversion of 27-hydroxycholesterol to cholic and chenodeoxycholic acids was determined from the 3H/14C ratio of these two bile acids in bile samples obtained the day after infusion. In five of the six subjects, independent measurement of bile acid synthesis by fecal acidic sterol output was available from previous studies. Endogenous production of 27-hydroxycholesterol averaged 17.6 mg/day and ranged from 5.0 to 28.2 mg/day, which amounted to 8.7% (range 3.0;-17.9%) of total bile acid synthesis. On average 66% of infused 27-hydroxycholesterol was converted to bile acid, of which 72.6% was chenodeoxycholic acid. These data suggest that relatively little bile acid synthesis takes place via the 27-hydroxylation pathway in healthy humans. Nevertheless, even this amount, occurring predominantly in vascular endothelium and macrophages, could represent an important means for removal of cholesterol deposited in endothelium.

Oxysterol regulation of steroidogenic acute regulatory protein gene expression. Structural specificity and transcriptional and posttranscriptional actions.

Oxysterols exert a major influence over cellular cholesterol homeostasis. We examined the effects of oxysterols on the expression of steroidogenic acute regulatory protein (StAR), which increases the delivery of cholesterol to sterol-metabolizing P450s in the mitochondria. 22(R)-hydroxycholesterol (22(R)-OHC), 25-OHC, and 27-OHC each increased steroidogenic factor-1 (SF-1)-mediated StAR gene transactivation by approximately 2-fold in CV-1 cells. In contrast, cholesterol, progesterone, and the 27-OHC metabolites, 27-OHC-5beta-3-one and 7alpha,27-OHC, had no effect. Unlike our findings in CV-1 cells, SF-1-dependent StAR promoter activity was not augmented by 27-OHC in COS-1 cells, Y-1 cells, BeWo choriocarcinoma cells, Chinese hamster ovary (CHO) cells, and human granulosa cells. Studies examining the metabolism of 27-OHC indicated that CV-1 cells formed a single polar metabolite, 3beta-OH-5-cholestenoic acid from radiolabeled 27-OHC. However, this metabolite inhibited StAR promoter activity in CV-1, COS-1 and CHO cells. Because 7alpha,27-OHC was unable to increase SF-1-dependent StAR promoter activity, we examined 27-OHC 7alpha-hydroxylase in COS-1 and CHO cells. COS-1 cells contained high 7alpha-hydroxylase activity, whereas the enzyme was undetectable in CHO cells. The hypothesis that oxysterols act in CV-1 cells to increase StAR promoter activity by reducing nuclear levels of sterol regulatory element binding protein was tested. This notion was refuted when it was discovered that sterol regulatory element binding protein-1a is a potent activator of the StAR promoter in CV-1, COS-1, and human granulosa cells. Human granulosa and theca cells, which express endogenous SF-1, contained more than 5-fold more StAR protein following addition of 27-OHC, whereas StAR mRNA levels remained unchanged. We conclude that 1) there are cell-specific effects of oxysterols on SF-1-dependent transactivation; 2) the ability to increase transactivation is limited to certain oxysterols; 3) there are cell-specific pathways of oxysterol metabolism; and 4) oxysterols elevate StAR protein levels through posttranscriptional actions.

Cholesterol 7-hydroxylase knockout mouse: a model for monohydroxy bile acid-related neonatal cholestasis.

BACKGROUND & AIMS: Cyp 7-/- mice lack a functional cholesterol 7alpha-hydroxylase enzyme and develop cholestasis before up-regulation of 27-hydroxycholesterol 7alpha-hydroxylase activity. Because 7alpha-hydroxylation is not the initial step in this metabolic pathway, we tested the hypothesis that cholesterol 27-hydroxylase is expressed at an earlier step and leads to the production of monohydroxy bile acids. METHODS: Polymerase chain reaction with specific oligonucleotides was used to detect messenger RNA (mRNA) coding for cholesterol 27-hydroxylase in 5-day-old normal and Cyp 7-/- mice. Gas-liquid chromatography-mass spectrometry and reverse isotope dilution were used to identify intermediates in the cholesterol 27-hydroxylase metabolic pathway. Light and electron microscopy were used to evaluate the morphological appearance of the liver. RESULTS: mRNA for cholesterol 27-hydroxylase was identified in the liver and spleen. The monohydroxy bile acids 3beta-hydroxy-5-cholenoate and 3alpha-hydroxy-5beta-cholanoate together with their precursor, 27-hydroxycholesterol, were identified in liver homogenates. Cholestasis, present focally, was manifested as dilated bile canaliculi, partial loss of microvilli, and retention of electron-dense biliary material. CONCLUSIONS: The cholesterol 27-hydroxylase metabolic pathway of bile acid synthesis is expressed in neonatal life. The absence of 7alpha-hydroxylase activities unmasks the cholestatic potential of monohydroxy bile acids. The Cyp 7-/- knockout mouse mimics cholestatic events known to occur in humans and provides a unique opportunity for studying regulatory determinants.

Bile acid analysis in biological fluids: a novel approach.

In contrast to current methods of bile acid analysis that require the separation of bile acids into different groups prior to their analysis, the HPLC method using a reverse phase column and gradient elution that we developed permits the separation and detection of nonconjugated, glycine-conjugated, and esterified bile acids as their fluorescent dimethoxycoumarin esters. The mild conditions for ester formation make possible the identification of allylic bile acids characteristic of metabolic errors in bile acid synthesis. Quantification is obtained using 7 alpha,12 alpha-dihydroxy-5 beta-cholanoic acid as an internal standard. In addition to identification based on retention time, peak-shift strategy is used by treatment of aliquots with cholyglycine hydrolase and/or solvolysis. Loss of the parent peak and appearance of the derivative provide further assurance of the identity of each bile acid in biologic fluids that contain other organic acids.

Sterol 27-hydroxylase: expression in human arterial endothelium.

Human endothelium obtained from both the aorta and the pulmonary artery has been evaluated for the presence of the messenger RNA coding for the expression of sterol 27-hydroxylase. Unique oligomers were designed to detect the mRNA by reverse transcription followed by the polymerase chain reaction. The amplified product was sequenced and was found to be identical to the published sequence for nucleotides 491 to 802 of the human sterol 27-hydroxylase cDNA. Northern blot analysis confirmed the presence of 27-hydroxylase mRNA in pulmonary artery and aortic endothelium. As part of these studies, enzymatic activity was assayed in cultured arterial endothelium using cholesterol as a substrate and isotope ratio gas-liquid chromatography-mass spectrometry to identify the metabolites, 27-hydroxycholesterol and 3 beta-hydroxy-5-cholestenoic acid, in the medium. Localization of sterol 27-hydroxylase to vascular endothelium indicates intracellular production of the biologically active metabolite 27-hydroxycholesterol.

7 alpha-hydroxylation of 27-hydroxycholesterol: biologic role in the regulation of cholesterol synthesis.

The report of a novel cytochrome P450 enzyme in mouse hippocampus (cyp7b) with close homology to cholesterol 7 alpha-hydroxylase led us to determine the substrate specificity with respect to 27-hydroxycholesterol, known to be a potent inhibitor of cholesterol synthesis. Transfection of 293/T-cells with PcDNA3.1(+)-mcyp7b was followed by metabolism of 2.5 microM 27-hydroxycholesterol to the 7 alpha-hydroxy intermediate, cholest-5-ene,3 beta,7 alpha,27-triol, with complete loss of down-regulation of cholesterol synthesis. Addition of 5 microM and 10 microM concentrations of the triol to HepG2 and CHO cells, respectively, also did not reduce cholesterol synthesis. The contrast between the biologic effect on cholesterol synthesis by these two C27 hydroxysterols and the wide tissue distribution of both cholesterol 27-hydroxylase and hydroxysterol 7 alpha-hydroxylase implies local regulatory effects prior to their further catabolism in the liver to chenodeoxycholic and cholic acids.

Bile acid synthesis: 7 alpha-hydroxylation of intermediates in the sterol 27-hydroxylase metabolic pathway.

The recognition that the 7 alpha-hydroxylation of 27-hydroxycholesterol is catalyzed by an enzyme that is different from cholesterol 7 alpha-hydroxylase raises the question as to the number of similar enzymes that may be present in liver and subserve bile acid synthesis. Thus, both 3 beta-hydroxy-5-cholestenoic acid and 3 beta-hydroxy-5-cholenoic acid, further oxidation products derived from 27-hydroxycholesterol, are also 7 alpha-hydroxylated during their metabolism to chenodeoxycholic acid. Using a microsomal fraction of hamster liver and competition plot analysis, we found that the 7 alpha-hydroxylase activity for the acid substrates was approximately one-tenth that found for 27-hydroxycholesterol. Mixtures of the different substrates did not depress the total rate of 7 alpha-hydroxylation. The evidence supports the view that these substrates share the same catalytic site on a single enzyme.

Structure of the human steroidogenic acute regulatory protein (StAR) gene: StAR stimulates mitochondrial cholesterol 27-hydroxylase activity.

Steroidogenic acute regulatory protein (StAR) plays a key role in steroid hormone synthesis by enhancing the metabolism of cholesterol into pregnenolone. We determined the organization of the StAR structural gene, mapped to 8p11.2. The gene spans 8 kb and consists of seven exons interrupted by six introns. The 1.3 kb of DNA upstream from the transcription start site directed expression of a luciferase reporter gene in mouse Y-1 adrenal cortical tumor cells but not in BeWo choriocarcinoma cells. Reporter gene expression in the Y-1 cells was increased more than 2-fold by 8-Br-cAMP, indicating that the 1.3 kb DNA fragment contains sequences that confer tissue-specific expression and cAMP regulation. The sequence of a related StAR pseudogene, mapped to chromosome 13, lacks introns and has an insertion, numerous substitutions, and deletions. Expression of StAR in COS-1 cells cotransfected with cholesterol 27-hydroxylase (P450c27) and adrenodoxin resulted in a 6-fold increase in formation of 3 beta-hydroxy-5-cholestenoic acid, demonstrating that StAR's actions are not specific to steroidogenesis but extend to other mitochondrial cholesterol-metabolizing enzymes.

Bile acid synthesis from cholesterol: regulatory and auxiliary pathways.

Bile acid synthesis from cholesterol can occur via two pathways, one initiated by sterol 27-hydroxylase activity or one initiated by that of cholesterol 7 alpha-hydroxylase. In contrast to cholesterol 7 alpha-hydroxylase, which is found in the liver, sterol 27-hydroxylase is a widely distributed mitochondrial enzyme with high activity in vascular endothelial cells. Although both pathways lead to the production of chenodeoxycholic and cholic acids, the key step, 7 alpha-hydroxylation, is governed by two different enzymes. Both 27-hydroxycholesterol and 3 beta-hydroxy-5-cholestenoic acid, the metabolites of cholesterol occurring via sterol 27-hydroxylase activity, normally circulate in plasma. After their uptake by the liver they are metabolized mostly to chenodeoxycholic acid, which down-regulates the activity of cholesterol 7 alpha-hydroxylase, the rate-limiting step for the production of bile acids in the liver. Because of this relationship and also in view of the accelerated atherosclerosis and cholesterol deposition in tissues that occur as a consequence of genetically determined sterol 27-hydroxylase deficiency and of the potent biologic effect of 27-hydroxycholesterol in cell culture, it is proposed that this metabolic pathway serves a regulatory function. The pathway beginning with cholesterol 7 alpha-hydroxylation is modulated by genetic, hormonal, and probably dietary factors, and becomes most prominent with the interruption of the enterohepatic circulation of bile acids.

Bile acid synthesis in HepG2 cells: effect of cyclosporin.

The hypothesis that cyclosporin specifically affects the pathway of bile acid synthesis that begins with 27-hydroxylation of cholesterol was evaluated in HepG2 cells, which synthesize chenodeoxycholic acid and cholic acid from endogenous 7 alpha-hydroxycholesterol. At a concentration in the medium of 8.3 microM cyclosporin, the proportion of cholic acid increased from 29 +/- 7% to 44 +/- 6% (P < 0.001) with no major change in total bile acid production. Chenodeoxycholic acid synthesis was enhanced by the addition of either 7 alpha-hydroxycholesterol or 5 beta-cholestane-3 alpha,7 alpha-diol to the medium and cholic acid synthesis was enhanced by the addition of 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol to the medium. Cyclosporin significantly inhibited only enhanced chenodeoxycholic acid synthesis, indicating a selective interference in mitochondrial side chain oxidation of less polar intermediates in bile acid synthesis derived from either initial 7 alpha- or initial 27-hydroxylation of cholesterol. The increase in the proportion of cholic acid that occurs in the presence of cyclosporin mimics that occurring in genetically determined sterol 27-hydroxylase deficiency (cerebrotendinous xanthomatosis). Cyclosporin is useful for dissecting the subcellular pathways of bile acid synthesis.

Sterol 27-hydroxylase: high levels of activity in vascular endothelium.

Sterol 27-hydroxylase activity in bovine aortic endothelial (BAE) cells in culture has been compared with that in HepG2 cells and in Chinese hamster ovary (CHO) cells using identical culture conditions. The total enzyme activity of BAE cells (3.0 nmol/72 h per mg cell protein) was comparable with that of HepG2 cells (4.0 nmol/72 h per mg protein) and both values were significantly greater than that in CHO cells (0.002 nmol/72 h per mg protein). The enzyme was identified in the mitochondria extracted from BAE cells by Western blotting using an antibody of proven specificity, and its metabolites 27-hydroxycholesterol and 3 beta-hydroxy-5-cholestenoic acid were identified by mass spectrum analysis. The presence of the enzyme in endothelium provides a mechanism for preventing accumulation of intracellular cholesterol by initiating a pathway of bile acid synthesis different from that initiated by 7 alpha-hydroxylation of cholesterol in the liver.

Breast-gut connection: origin of chenodeoxycholic acid in breast cyst fluid.

The notion that a breast-gut connection might modulate the microenvironment of breast tissue was supported by the finding that breast cyst fluid contains bile acids that are characteristically found in the intestines. To establish that the gut, rather than circulating steroid precursors, is the source of bile acids in breast cyst fluid, we gave two patients deuterium-labelled chenodeoxycholic acid (three 200 mg doses by mouth), starting 9 days before aspiration of breast cysts. The chenodeoxycholic acid concentration of seven samples of aspirated cyst fluid ranged from 42 to 94 mumol/L. The corresponding serum concentrations of chenodeoxycholic acid on the same day were 0.8 and 2.9 mumol/L, of which the labelled compound comprised 13.0% (0.38 mumol/L) and 28.2% (0.23 mumol/L). The deuterated chenodeoxycholic acid concentrations in cyst fluid were 0.79 and 1.26 mumol/L in two samples from patient 1 and 3.22 mumol/L in patient 2; these values are equivalent to 11-17% of the serum concentrations [corrected]. This study shows that intestinal bile acids rapidly gain access to cyst fluid. Further studies should investigate the mechanisms that govern the exchange processes and the maintenance of the high cyst fluid to plasma concentration gradients, and the biological half-lives of individual constituents.