Hydroxylations in biosynthesis of bile acids. Isolation of a cytochrome P-450 from rabbit liver mitochondria catalyzing 26-hydroxylation of C27-steroids

CYP27A1-27-hydroxycholesterol axis in the respiratory system contributes to house dust mite-induced allergic airway inflammation

BACKGROUND

27-Hydroxycholesterol (27-HC) derived from sterol 27-hydroxylase (CYP27A1) has pro-inflammatory biological activity and is associated with oxidative stress and chronic inflammation in COPD. However, the role of regulation of CYP27A1- 27-HC axis in asthma is unclear. This study aimed to elucidate the contribution of the axis to the pathophysiology of asthma.

METHODS

House dust mite (HDM) extract was intranasally administered to C57BL/6 mice and the expression of CYP27A1 in the airways was analyzed by immunostaining. The effect of pre-treatment with PBS or CYP27A1 inhibitors on the cell fraction in the bronchoalveolar lavage fluid (BALF) was analyzed in the murine model. In vitro, BEAS-2B cells were treated with HDM and the levels of CYP27A1 expression were examined. Furthermore, the effect of 27-HC on the expressions of E-cadherin and ZO-1 in the cells was analyzed. The amounts of RANTES and eotaxin from the 27-HC-treated cells were analyzed by ELISA.

RESULTS

The administration of HDM increased the expression of CYP27A1 in the airways of mice as well as the number of eosinophils in the BALF. CYP27A1 inhibitors ameliorated the HDM-induced increase in the number of eosinophils in the BALF. Treatment with HDM increased the expression of CYP27A1 in BEAS-2B cells. The administration of 27-HC to BEAS-2B cells suppressed the expression of E-cadherin and ZO-1, and augmented the production of RANTES and eotaxin.

CONCLUSIONS

The results of this study suggest that aeroallergen could enhance the induction of CYP27A1, leading to allergic airway inflammation and disruption of the airway epithelial tight junction through 27-HC production.

Identification and in vivo detection of side-chain hydroxylated metabolites of 4β-hydroxycholesterol

Oxysterols are oxidized derivatives of cholesterol that are formed by enzymatic processes or through the action of reactive oxygen species. Several of these bioactive lipids have been shown to be affected and/or play a role in inflammatory processes. 4β-hydroxycholesterol is one of the major oxysterols in mice and humans and its levels are affected by inflammatory diseases. However, apart from its long half-life, little is known about its catabolism. By incubating 4β-hydroxycholesterol with mouse mitochondria-enriched liver fractions, as well as 25-hydroxycholesterol and 27-hydroxycholesterol with recombinant CYP3A4, we identified 4β,25-dihydroxycholesterol and 4β,27-dihydroxycholesterol as 4β-hydroxycholesterol metabolites. Supporting the biological relevance of this metabolism, we detected both metabolites after incubation of J774, primary mouse peritoneal macrophages and PMA-differentiated THP-1 cells with 4β-hydroxycholesterol. Across our experiments, the incubation of cells with lipopolysaccharides differentially affected the levels of the 25- and 27-hydroxylated metabolites of 4β-hydroxycholesterol. Finally, 4β,27-dihydroxycholesterol was also detected in mice liver and plasma after intraperitoneal administration of 4β-hydroxycholesterol. To our knowledge, this is the first report of the in vitro and in vivo detection and quantification of 4β-hydroxycholesterol metabolites.

Challenges and Opportunities in P450 Research on the Eye

Of the 57 cytochrome P450 enzymes found in humans, at least 30 have ocular tissues as an expression site. Yet knowledge of the roles of these P450s in the eye is limited, in part because only very few P450 laboratories expanded their research interests to studies of the eye. Hence the goal of this review is to bring attention of the P450 community to the eye and encourage more ocular studies. This review is also intended to be educational for eye researchers and encourage their collaborations with P450 experts. The review starts with a description of the eye, a fascinating sensory organ, and is followed by sections on ocular P450 localizations, specifics of drug delivery to the eye, and individual P450s, which are grouped and presented based on their substrate preferences. In sections describing individual P450s, available eye-relevant information is summarized and concluded by the suggestions on the opportunities in ocular studies of the discussed enzymes. Potential challenges are addressed as well. The conclusion section outlines several practical suggestions on how to initiate eye-related research. SIGNIFICANCE STATEMENT: This review focuses on the cytochrome P450 enzymes in the eye to encourage their ocular investigations and collaborations between P450 and eye researchers.

Critical role of antioxidant programs in enzalutamide-resistant prostate cancer

Therapy resistance to second-generation androgen receptor (AR) antagonists, such as enzalutamide, is common in patients with advanced prostate cancer (PCa). To understand the metabolic alterations involved in enzalutamide resistance, we performed metabolomic, transcriptomic, and cistromic analyses of enzalutamide-sensitive and -resistant PCa cells, xenografts, patient-derived organoids, patient-derived explants, and tumors. We noted dramatically higher basal and inducible levels of reactive oxygen species (ROS) in enzalutamide-resistant PCa and castration-resistant PCa (CRPC), in comparison to enzalutamide-sensitive PCa cells or primary therapy-naive tumors respectively. Unbiased metabolomic evaluation identified that glutamine metabolism was consistently upregulated in enzalutamide-resistant PCa cells and CRPC tumors. Stable isotope tracing studies suggest that this enhanced glutamine metabolism drives an antioxidant program that allows these cells to tolerate higher basal levels of ROS. Inhibition of glutamine metabolism with either a small-molecule glutaminase inhibitor or genetic knockout of glutaminase enhanced ROS levels, and blocked the growth of enzalutamide-resistant PCa. The critical role of compensatory antioxidant pathways in maintaining enzalutamide-resistant PCa cells was validated by targeting another antioxidant program driver, ferredoxin 1. Taken together, our data identify a metabolic need to maintain antioxidant programs and a potentially targetable metabolic vulnerability in enzalutamide-resistant PCa.

Enzymatic activation in vitamin D signaling - Past, present and future

Vitamin D signaling is important in regulating calcium homeostasis essential for bone health but also displays other functions in cells of several tissues. Disturbed vitamin D signaling is linked to a large number of diseases. The multiple cytochrome P450 (CYP) enzymes catalyzing the different hydroxylations in bioactivation of vitamin D₃ are crucial for vitamin D signaling and function. This review is focused on the progress achieved in identification of the bioactivating enzymes and their genes in production of 1α,25-dihydroxyvitamin D₃ and other active metabolites. Results obtained on species- and tissue-specific expression, catalytic reactions, substrate specificity, enzyme kinetics, and consequences of gene mutations are evaluated. Matters of incomplete understanding regarding the physiological roles of some vitamin D hydroxylases are critically discussed and the authors will give their view of the importance of each enzyme for vitamin D signaling. Roles of different vitamin D receptors and an alternative bioactivation pathway, leading to 20-hydroxylated vitamin D₃ metabolites, are also discussed. Considerable progress has been achieved in knowledge of the vitamin D₃ bioactivating enzymes. Nevertheless, several intriguing areas deserve further attention to understand the pleiotropic and diverse activities elicited by vitamin D signaling and the mechanisms of enzymatic activation necessary for vitamin D-induced responses.

Bile acids and their receptors in regulation of gut health and diseases

It is well established that bile acids play important roles in lipid metabolism. In recent decades, bile acids have also been shown to function as signaling molecules via interacting with various receptors. Bile acids circulate continuously through the enterohepatic circulation and go through microbial transformation by gut microbes, and thus bile acids metabolism has profound effects on the liver and intestinal tissues as well as the gut microbiota. Farnesoid X receptor and G protein-coupled bile acid receptor 1 are two pivotal bile acid receptors that highly expressed in the intestinal tissues, and they have emerged as pivotal regulators in bile acids metabolism, innate immunity and inflammatory responses. There is considerable interest in manipulating the metabolism of bile acids and the expression of bile acid receptors as this may be a promising strategy to regulate intestinal health and disease. This review aims to summarize the roles of bile acids and their receptors in regulation of gut health and diseases.

Polygonum multiflorum Thunb. Induces hepatotoxicity in SD rats and hepatocyte spheroids by Disrupting the metabolism of bilirubin and bile acid

ETHNOPHARMACOLOGICAL RELEVANCE

The liver damage associated with Polygonum multiflorum Thunb. (P. multiflorum) and its preparations have aroused widespread concern. Opinions on the toxicity mechanisms and targets of P. multiflorum vary, and the toxic components are even more controversial. However, based on the current research results, we believed that any single component in P. multiflorum could not directly lead to liver injury, but may be the synergistic effect of multiple components. In addition, the toxicity mechanism also involved multiple targets.

AIM OF THE STUDY

This study aimed to elucidate the mechanism and target of the hepatotoxicity of P. multiflorum.

MATERIALS AND METHODS

In this study, the manifestations of liver injury triggered by P. multiflorum and the associated metabolic enzymes/transporters in the metabolic pathways of bilirubin and bile acid were investigated to elucidate the mechanism and target of the hepatotoxicity of P. multiflorum and related components. First, the hepatotoxicity and potential effect of P. multiflorum on both metabolic pathways were studied in rats administered P. multiflorum extracts (in 70% ethanol) for 42 days. Then, in vitro cultured hepatocyte spheroids were used to determine the hepatotoxicity of monomer components.

RESULTS

This revealed that P. multiflorum could simultaneously block bilirubin(BIL) and bile acid(BA) metabolism pathways, subsequently leading to liver damage. The targets and modes of action include reducing the activity of UGT1A1, the only metabolic enzyme of BIL, downregulating BIL and BA uptake transporters NTCP, OATP1B1, OATP1B3, efflux transporters MRP2, and BSEP, and upregulating efflux transporter MRP3. Furthermore, our data indicated that 2,3,5,4'-tetrahydroxystilbene-2-O-β-glucoside (TSG) and emodin-8-O-β-D-glucoside (EG) are the main toxic components in P. multiflorum. TSG accounts for 3.71% of the total content of P. multiflorum. In addition to markedly downregulating UGT1A1, TSG can upregulate OATP1B1/3 and promote the uptakes of bilirubin and bile acid, producing synergistic toxicity. EG accounts for 0.29% of the total content and demonstrates direct hepatotoxicity and extensive substrate overlap with bilirubin and bile acids. It can affect these two metabolic pathways simultaneously, promoting the accumulation of both bilirubin and bile acid for further toxic effects. Emodin is other major component, accounting for 0.01% of the total content, and its hepatotoxicity mechanisms include direct toxicity and inhibitory effects on bilirubin metabolizing enzymes. However, emodin is mainly distributed in the kidneys, so its hepatotoxicity risk is relatively low.

CONCLUSION

The simultaneous blockade of bilirubin and bile acid metabolic pathways as the critical toxic mechanism of P. multiflorum-induced liver injury, and potential toxic components were TSG and EG.

Clinical and Genetic Characteristics of Splicing Variant in CYP27A1 in an Iranian Family with Cerebrotendinous Xanthomatosis

Background:

CTX is a rare congenital lipid-storage disorder, leading to a progressive multisystem disease. CTX with autosomal recessive inheritance is caused by a defect in the CYP27A1 gene. Chronic diarrhea, tendon xanthomas, neurologic impairment, and bilateral cataracts are common symptoms of the disease.

Methods:

Three affected siblings with an initial diagnosis of non-syndromic intellectual disability were recruited for further molecular investigations. To identify the possible genetic cause(s), WES was performed on the proband. Sanger sequencing was applied to confirm the final variant. The clinical and molecular genetic features of the three siblings from the new CTX family and other patients with the same mutations, as previously reported, were analyzed. The CYP27A1 gene was also studied for the number of pathogenic variants and their location.

Results:

We found a homozygous splicing mutation, NM_000784: exon6: c.1184+1G>A, in CYP27A1 gene, which was confirmed by Sanger sequencing. Among the detected pathogenic variants, the splice site mutation had the highest prevalence, and the mutations were mostly found in exon 4.

Conclusion:

This study is the first to report the c.1184+1G>A mutation in Iran. Our findings highlight the other feature of the disease, which is the lack of relationship between phenotype and genotype. Due to nonspecific symptoms and delay in diagnosis, CYP27A1 genetic analysis should be the definitive method for CTX diagnosis.

CYP46A1-dependent and independent effects of efavirenz treatment

Cholesterol excess in the brain is mainly disposed via cholesterol 24-hydroxylation catalysed by cytochrome P450 46A1, a CNS-specific enzyme. Cytochrome P450 46A1 is emerging as a promising therapeutic target for various brain diseases with both enzyme activation and inhibition having therapeutic potential. The rate of cholesterol 24-hydroxylation determines the rate of brain cholesterol turnover and the rate of sterol flux through the plasma membranes. The latter was shown to affect membrane properties and thereby membrane proteins and membrane-dependent processes. Previously we found that treatment of 5XFAD mice, an Alzheimer's disease model, with a small dose of anti-HIV drug efavirenz allosterically activated cytochrome P450 46A1 in the brain and mitigated several disease manifestations. Herein, we generated Cyp46a1-/- 5XFAD mice and treated them, along with 5XFAD animals, with efavirenz to ascertain cytochrome P450 46A1-dependent and independent drug effects. Efavirenz-treated versus control Cyp46a1-/- 5XFAD and 5XFAD mice were compared for the brain sterol and steroid hormone content, amyloid β burden, protein and mRNA expression as well as synaptic ultrastructure. We found that the cytochrome P450 46A1-dependent efavirenz effects included changes in the levels of brain sterols, steroid hormones, and such proteins as glial fibrillary acidic protein, Iba1, Munc13-1, post-synaptic density-95, gephyrin, synaptophysin and synapsin-1. Changes in the expression of genes involved in neuroprotection, neurogenesis, synaptic function, inflammation, oxidative stress and apoptosis were also cytochrome P450 46A1-dependent. The total amyloid β load was the same in all groups of animals, except lack of cytochrome P450 46A1 decreased the production of the amyloid β40 species independent of treatment. In contrast, altered transcription of genes from cholinergic, monoaminergic, and peptidergic neurotransmission, steroid sulfation and production as well as vitamin D3 activation was the main CYP46A1-independent efavirenz effect. Collectively, the data obtained reveal that CYP46A1 controls cholesterol availability for the production of steroid hormones in the brain and the levels of biologically active neurosteroids. In addition, cytochrome P450 46A1 activity also seems to affect the levels of post-synaptic density-95, the main postsynaptic density protein, possibly by altering the calcium/calmodulin-dependent protein kinase II inhibitor 1 expression and activity of glycogen synthase kinase 3β. Even at a small dose, efavirenz likely acts as a transcriptional regulator, yet this regulation may not necessarily lead to functional effects. This study further confirmed that cytochrome P450 46A1 is a key enzyme for cholesterol homeostasis in the brain and that the therapeutic efavirenz effects on 5XFAD mice are likely realized via cytochrome P450 46A1 activation.

Human cytochrome P450 enzymes 5-51 as targets of drugs and natural and environmental compounds: mechanisms, induction, and inhibition - toxic effects and benefits

Cytochrome P450 (P450, CYP) enzymes have long been of interest due to their roles in the metabolism of drugs, pesticides, pro-carcinogens, and other xenobiotic chemicals. They have also been of interest due to their very critical roles in the biosynthesis and metabolism of steroids, vitamins, and certain eicosanoids. This review covers the 22 (of the total of 57) human P450s in Families 5-51 and their substrate selectivity. Furthermore, included is information and references regarding inducibility, inhibition, and (in some cases) stimulation by chemicals. We update and discuss important aspects of each of these 22 P450s and questions that remain open.

Retinal Vascular Abnormalities and Microglia Activation in Mice with Deficiency in Cytochrome P450 46A1-Mediated Cholesterol Removal

CYP46A1 is the cytochrome P450 enzyme that converts cholesterol to 24-hydroxycholesterol, a cholesterol elimination product and a potent liver X receptor (LXR) ligand. We conducted retinal characterizations of Cyp46a1^-/- mice that had normal fasting blood glucose levels but up to a 1.8-fold increase in retinal cholesterol. The retina of Cyp46a1^-/- mice exhibited venous beading and tortuosity, microglia/macrophage activation, and increased vascular permeability, features commonly associated with diabetic retinopathy. The expression of Lxrα and Lxrβ was increased in both the whole Cyp46a1^-/- retina and retinal macroglia/macrophages. The LXR-target genes were affected as well, primarily in activated microglial cells and macrophages. In the latter, the LXR-transactivated genes (Abca1, Abcg1, Apod, Apoe, Mylip, and Arg2) were up-regulated; similarly, there was an up-regulation of the LXR-transrepressed genes (Ccl2, Ptgs2, Cxcl1, Il1b, Il6, Nos2, and Tnfa). For comparison, gene expression was investigated in bone marrow-derived macrophages from Cyp46a1^-/- mice as well as retinal and bone marrow-derived macrophages from Cyp27a1^-/- and Cyp27a1^-/-Cyp46a1^-/- mice. CYP46A1 expression was detected in retinal endothelial cells, and this expression was increased in the proinflammatory environment. Retinal Cyp46a1^-/- phosphoproteome revealed altered phosphorylation of 30 different proteins, including tight junction protein zonula occludens 1 and aquaporin 4. Collectively, the data obtained establish metabolic and regulatory significance of CYP46A1 for the retina and suggest pharmacologic activation of CYP46A1 as a potential therapeutic approach to dyslipidemia-induced retinal damage.

Drugs and Scaffold That Inhibit Cytochrome P450 27A1 In Vitro and In Vivo

Cytochrome P450 27A1 (CYP27A1) is a ubiquitous enzyme that hydroxylates cholesterol and other sterols. Complete CYP27A1 deficiency owing to genetic mutations is detrimental to human health, whereas 50% of activity retention is not and does not affect the whole body cholesterol levels. CYP27A1 is considered a potential therapeutic target in breast cancer and age-related neurodegenerative diseases; however, CYP27A1 inhibition should be ≤50%. Herein, 131 pharmaceuticals were tested for their effect on CYP27A1-mediated cholesterol 27-hydroxylation by in vitro enzyme assay. Of them, 14 drugs inhibited CYP27A1 by ≥75% and were evaluated for in vitro binding to the enzyme active site and for inhibition constants. All drugs except one (dasatinib) elicited a spectral response in CYP27A1 and had K_i values for cholesterol 27-hydroxylation either in the submicromolar (clevidipine, delavirdine, etravirine, felodipine, nicardipine, nilotinib, and sorafenib) or low micromolar range (abiratone, candesartan, celecoxib, dasatinib, nilvadipine, nimodipine, and regorafenib). Clevidipine, felodipine, nicardipine, nilvadipine, and nimodipine have the same 1,4-dihydropyridine scaffold and are indicated for hypertension. We used two of these antihypertensives (felodipine and nilvadipine) for administration to mice at a 1-mg/kg of body weight dose, daily, for 7 days. Mouse 27-hydroxycholesterol levels in the plasma, brain, and liver were reduced, whereas tissue levels of total cholesterol were unchanged. Structure-activity relationships within the 1,4-dihydropyridine scaffold were investigated, and features important for CY27A1 inhibition were identified. We confirmed our previous finding that CYP27A1 is a druggable enzyme and found additional drugs as well as the scaffold with potential for partial CYP27A1 inhibition in humans.

Cytochrome P450 27A1 Deficiency and Regional Differences in Brain Sterol Metabolism Cause Preferential Cholestanol Accumulation in the Cerebellum

Cytochrome P450 27A1 (CYP27A1 or sterol 27-hydroxylase) is a ubiquitous, multifunctional enzyme catalyzing regio- and stereospecific hydroxylation of different sterols. In humans, complete CYP27A1 deficiency leads to cerebrotendinous xanthomatosis or nodule formation in tendons and brain (preferentially in the cerebellum) rich in cholesterol and cholestanol, the 5α-saturated analog of cholesterol. In Cyp27a1^-/- mice, xanthomas are not formed, despite a significant cholestanol increase in the brain and cerebellum. The mechanism behind cholestanol production has been clarified, yet little is known about its metabolism, except that CYP27A1 might metabolize cholestanol. It also is unclear why CYP27A1 deficiency results in preferential cholestanol accumulation in the cerebellum. We hypothesized that cholestanol might be metabolized by CYP46A1, the principal cholesterol 24-hydroxylase in the brain. We quantified sterols along with CYP27A1 and CYP46A1 in mouse models (Cyp27a1^-/-, Cyp46a1^-/-, Cyp27a1^-/-Cyp46a1^-/-, and two wild type strains) and human brain specimens. In vitro experiments with purified P450s were conducted as well. We demonstrate that CYP46A1 is involved in cholestanol removal from the brain and that several factors contribute to the preferential increase in cholestanol in the cerebellum arising from CYP27A1 deficiency. These factors include (i) low cerebellar abundance of CYP46A1 and high cerebellar abundance of CYP27A1, the lack of which probably selectively increases the cerebellar cholestanol production; (ii) spatial separation in the cerebellum of cholesterol/cholestanol-metabolizing P450s from a pool of metabolically available cholestanol; and (iii) weak cerebellar regulation of cholesterol biosynthesis. We identified a new physiological role of CYP46A1, an important brain enzyme and cytochrome P450 that could be activated pharmacologically.

Marketed Drugs Can Inhibit Cytochrome P450 27A1, a Potential New Target for Breast Cancer Adjuvant Therapy

Cytochrome P450 CYP27A1 is the only enzyme in humans converting cholesterol to 27-hydroxycholesterol, an oxysterol of multiple functions, including tissue-specific modulation of estrogen and liver X receptors. Both receptors seem to mediate adverse effects of 27-hydroxycholesterol in breast cancer when the levels of this oxysterol are elevated. The present work assessed druggability of CYP27A1 as a potential antibreast cancer target. We selected 26 anticancer and noncancer medications, most approved by the Food and Drug Administration, and evaluated them first in vitro for inhibition of purified recombinant CYP27A1 and binding to the enzyme active site. Six strong CYP27A1 inhibitors/binders were identified. These were the two antibreast cancer pharmaceuticals anastrozole and fadrozole, antiprostate cancer drug bicalutamide, sedative dexmedetomidine, and two antifungals ravuconazole and posaconazole. Anastrozole was then tested in vivo on mice, which received subcutaneous drug injections for 1 week. Mouse plasma and hepatic 27-hydroxycholesterol levels were decreased 2.6- and 1.6-fold, respectively, whereas plasma and hepatic cholesterol content remained unchanged. Thus, pharmacologic CYP27A1 inhibition is possible in the whole body and individual organs, but does not negatively affect cholesterol elimination. Our results enhance the potential of CYP27A1 as an antibreast cancer target, could be of importance for the interpretation of Femara versus Anastrozole Clinical Evaluation Trial, and bring attention to posaconazole as a potential complementary anti-breast cancer medication. More medications on the US market may have unanticipated off-target inhibition of CYP27A1, and we propose strategies for their identification.

Pathways of cholesterol homeostasis in mouse retina responsive to dietary and pharmacologic treatments

Effects of serum cholesterol on cholesterol content in the retina are currently unknown. It is also unclear how cholesterol levels are controlled in the retina. High-cholesterol diet and oral administrations of simvastatin were used to modulate serum cholesterol in mice. These treatments only modestly affected cholesterol content in the retina and had no significant effect on retinal expression of the major cholesterol- and vision-related genes; the sterol-regulatory element binding protein pathway of transcriptional regulation does not seem to be operative in the retina under the experimental conditions used. Evidence is obtained that posttranslational mechanisms play a role in the control of retinal cholesterol. Retinal genes were only upregulated by oral administrations of TO901317 activating liver X receptors. Three of the upregulated genes could be of particular importance (apoD, Idol, and Rpe65) and have not yet been considered in the context of cholesterol homeostasis in the retina. Collectively, the data obtained identify specific features of retinal cholesterol maintenance and suggest additional therapies for age-related macular degeneration, a blinding disease characterized by cholesterol and lipid accumulations in chorioretinal tissues.

Retinal and nonocular abnormalities in Cyp27a1(-/-)Cyp46a1(-/-) mice with dysfunctional metabolism of cholesterol

Cholesterol elimination from nonhepatic cells involves metabolism to side-chain oxysterols, which serve as transport forms of cholesterol and bioactive molecules modulating a variety of cellular processes. Cholesterol metabolism is tissue specific, and its significance has not yet been established for the retina, where cytochromes P450 (CYP27A1 and CYP46A1) are the major cholesterol-metabolizing enzymes. We generated Cyp27a1(-/-)Cyp46a1(-/-) mice, which were lean and had normal serum cholesterol and glucose levels. These animals, however, had changes in the retinal vasculature, retina, and several nonocular organs (lungs, liver, and spleen). Changes in the retinal vasculature included structural abnormalities (retinal-choroidal anastomoses, arteriovenous shunts, increased permeability, dilation, nonperfusion, and capillary degeneration) and cholesterol deposition and oxidation in the vascular wall, which also exhibited increased adhesion of leukocytes and activation of the complement pathway. Changes in the retina included increased content of cholesterol and its metabolite, cholestanol, which were focally deposited at the apical and basal sides of the retinal pigment epithelium. Retinal macrophages of Cyp27a1(-/-)Cyp46a1(-/-) mice were activated, and oxidative stress was noted in their photoreceptor inner segments. Our findings demonstrate the importance of retinal cholesterol metabolism for maintenance of the normal retina, and suggest new targets for diseases affecting the retinal vasculature.

A comprehensive machine-readable view of the mammalian cholesterol biosynthesis pathway

Cholesterol biosynthesis serves as a central metabolic hub for numerous biological processes in health and disease. A detailed, integrative single-view description of how the cholesterol pathway is structured and how it interacts with other pathway systems is lacking in the existing literature. Here we provide a systematic review of the existing literature and present a detailed pathway diagram that describes the cholesterol biosynthesis pathway (the mevalonate, the Kandutch-Russell and the Bloch pathway) and shunt pathway that leads to 24(S),25-epoxycholesterol synthesis. The diagram has been produced using the Systems Biology Graphical Notation (SBGN) and is available in the SBGN-ML format, a human readable and machine semantically parsable open community file format.

Oxysterol generation and liver X receptor-dependent reverse cholesterol transport: not all roads lead to Rome

Cell cholesterol metabolism is a tightly regulated process, dependent in part on activation of nuclear liver X receptors (LXRs) to increase expression of genes mediating removal of excess cholesterol from cells in the reverse cholesterol transport pathway. LXRs are thought to be activated predominantly by oxysterols generated enzymatically from cholesterol in different cell organelles. Defects resulting in slowed release of cholesterol from late endosomes and lysosomes or reduction in sterol-27-hydroxylase activity lead to specific blocks in oxysterol production and impaired LXR-dependent gene activation. This block does not appear to be compensated by oxysterol production in other cell compartments. The purpose of this review is to summarize current knowledge about oxysterol-dependent activation by LXR of genes involved in reverse cholesterol transport, and what these defects of cell cholesterol homeostasis can teach us about the critical pathways of oxysterol generation for expression of LXR-dependent genes.

Posttranslational modification by an isolevuglandin diminishes activity of the mitochondrial cytochrome P450 27A1

Posttranslational modification by isolevuglandins (isoLGs), arachidonate oxidation products, is an important yet understudied process associated with altered protein properties. This type of modification is detected in cytochrome P450 27A1 (CYP27A1), a multifunction enzyme expressed in almost every cell and involved in the metabolism of cholesterol and other sterols. Previously, the CYP27A1 Lys(358)-isoLG adduct was found in human retina afflicted with age-related macular degeneration. Yet, the effect of Lys(358) modification on enzyme activity was not investigated. Herein, we characterized catalytic properties of Lys(358) as well as Lys(476) CYP27A1 mutants before and after isoLG treatment and quantified the extent of modification by multiple reaction monitoring. The K358R mutant was less susceptible to isoLG-induced loss of catalytic activity than the wild type (WT), whereas the K476R mutant was nearly as vulnerable as the WT. Both mutants showed less isoLG modification than WT. Thus, modification of Lys(358), a residue involved in redox partner interactions, is the major contributor to isoLG-associated loss of CYP27A1 activity. Our data show the specificity of isoLG modification, provide direct evidence that isoLG adduction impairs enzyme activity, and support our hypothesis that isoLG modification in the retina is detrimental to CYP27A1 enzyme activity, potentially disrupting cholesterol homeostasis.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Human cytochromes P450 in health and disease

There are 18 mammalian cytochrome P450 (CYP) families, which encode 57 genes in the human genome. CYP2, CYP3 and CYP4 families contain far more genes than the other 15 families; these three families are also the ones that are dramatically larger in rodent genomes. Most (if not all) genes in the CYP1, CYP2, CYP3 and CYP4 families encode enzymes involved in eicosanoid metabolism and are inducible by various environmental stimuli (i.e. diet, chemical inducers, drugs, pheromones, etc.), whereas the other 14 gene families often have only a single member, and are rarely if ever inducible or redundant. Although the CYP2 and CYP3 families can be regarded as largely redundant and promiscuous, mutations or other defects in one or more genes of the remaining 16 gene families are primarily the ones responsible for P450-specific diseases-confirming these genes are not superfluous or promiscuous but rather are more directly involved in critical life functions. P450-mediated diseases comprise those caused by: aberrant steroidogenesis; defects in fatty acid, cholesterol and bile acid pathways; vitamin D dysregulation and retinoid (as well as putative eicosanoid) dysregulation during fertilization, implantation, embryogenesis, foetogenesis and neonatal development.

Conversion of 7-ketocholesterol to oxysterol metabolites by recombinant CYP27A1 and retinal pigment epithelial cells

Of the different oxygenated cholesterol metabolites, 7-ketocholesterol (7KCh) is considered a noxious oxy-sterol implicated in the development of certain pathologies, including those found in the eye. Here we elucidated whether sterol 27-hydroxylase cytochrome P450 27A1 (CYP27A1) is involved in elimination of 7KCh from the posterior part of the eye: the neural retina and underlying retinal pigment epithelium (RPE). We first established that the affinities of purified recombinant CYP27A1 for 7KCh and its endogenous substrate cholesterol are similar, yet 7KCh is metabolized at a 4-fold higher rate than cholesterol in the reconstituted system in vitro. Lipid extracts from bovine neural retina and RPE were then analyzed by isotope dilution GC-MS for the presence of the 7KCh-derived oxysterols. Two metabolites, 3β,27-dihydroxy-5-cholesten-7-one (7KCh-27OH) and 3β-hydroxy-5-cholesten-7-one-26-oic acid (7KCh-27COOH), were detected in the RPE but not in the neural retina. 7KCh-27OH was also formed when RPE homogenates were supplemented with NADPH and the mitochondrial redox system. Quantifications in human RPE showed that CYP27A1 is indeed expressed in the RPE at 2-4-fold higher levels than in the neural retina. The data obtained represent evidence for the role of CYP27A1 in retinal metabolism of 7KCh and suggest that, in addition to cholesterol removal, the functions of this enzyme could also include elimination of toxic endogenous compounds.

Fifty years of advances in bile acid synthesis and metabolism

There are two major pathways that mammalian cells use to supply themselves with cholesterol, one involving the synthesis of sterols from acetyl-CoA and the other the metabolism of cholesterol-rich lipoprotein particles via receptor-mediated endocytosis. There also are several pathways that mammalian cells use to break down cholesterol, and these disposal pathways are equal in physiological importance to the supply pathways. A major catabolic route involves conversion of cholesterol into conjugated bile salts, a transformation mediated by 16 or more liver enzymes. This review highlights findings in cholesterol catabolism from the last five decades with special emphasis on advances in bile acid synthesis, transport, and regulation.

Metabolism of a novel side chain modified Delta8(14)-15-ketosterol, a potential cholesterol lowering drug: 28-hydroxylation by CYP27A1

The synthetic inhibitors of sterol biosynthesis, 3beta-hydroxy-5alpha-cholest-8(14)-en-15-one and 3beta-hydroxy-24S-methyl-5alpha-cholesta-8(14),22-dien-15-one, are of interest as potential cholesterol lowering drugs. Rapid metabolism of synthetic 15-ketosterols may lead to a decrease, or loss, of their potency to affect lipid metabolism. 3beta-Hydroxy-5alpha-cholest-8(14)-en-15-one is reported to be rapidly side chain oxygenated by rat liver mitochondria. In an attempt to reduce this metabolism, the novel side chain modified 15-ketosterol 3beta-Hydroxy-24S-methyl-5alpha-cholesta-8(14),22-dien-15-one was synthesized. We have examined the metabolism by recombinant human CYP27A1 of this novel side chain modified 3beta-hydroxy-24S-methyl-5alpha-cholesta-8(14),22-dien-15-one and compared the rate of metabolism with that of the previously described 3beta-hydroxy-5alpha-cholest-8(14)-en-15-one. Both sterols were found to be efficiently metabolized by recombinant human CYP27A1. None of the two 15-ketosterols was significantly metabolized by microsomal 7alpha-hydroxylation. Interestingly, CYP27A1-mediated product formation was much lower with the side chain modified 3beta-hydroxy-24S-methyl-5alpha-cholesta-8(14),22-dien-15-one than with the previously described 3beta-hydroxy-5alpha-cholest-8(14)-en-15-one. A surprising finding was that this novel side chain modified sterol was metabolized mainly in the C-28 position by CYP27A1. The data on 28-hydroxylation by human CYP27A1 provide new insights on the catalytic properties and substrate specificity of this enzyme. The finding that 3beta-hydroxy-24S-methyl-5alpha-cholesta-8(14),22-dien-15-one with a modified side chain is metabolized at a dramatically slower rate than the previously described 15-ketosterol with unmodified side chain may be important for future development of synthetic cholesterol lowering sterols.

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.

Distinct binding of cholesterol and 5beta-cholestane-3alpha,7alpha,12alpha-triol to cytochrome P450 27A1: evidence from modeling and site-directed mutagenesis studies

Cytochrome P450 27A1 (P450 27A1 or CYP27A1) is an important enzyme that participates in different pathways of cholesterol degradation as well as in the activation of vitamin D(3). Several approaches were utilized to investigate how two physiological substrates, cholesterol and 5beta-cholestane-3alpha,7alpha,12alpha-triol, interact with CYP27A1. The enzyme active site was first probed spectrally by assessing binding of the two substrates and five substrate analogues followed by computer modeling and site-directed mutagenesis. The computer models suggest that the spatial positions and orientations of cholesterol and 5beta-cholestane-3alpha,7alpha,12alpha-triol are different in the enzyme active site. As a result, some of the active site residues interact with both substrates, although they are situated differently relative to each steroid, and some residues bind only one substrate. Mutation of the overlapping substrate-contact residues (W100, H103, T110, M301C, V367, I481, and V482) affected CYP27A1 binding and enzyme activity in a substrate-dependent manner and allowed identification of several important side chains. T110 is proposed to interact with the 12alpha-hydroxyl of 5beta-cholestane-3alpha,7alpha,12alpha-triol, whereas V367 seems to be crucial for correct positioning of the cholesterol C26 methyl group and for regioselective hydroxylation of this substrate. Distinct binding of the CYP27A1 substrates may provide insight into why phenotypic manifestations of cerebrotendinous xanthomatosis, a disease associated with CYP27A1 deficiency, are so diverse.

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.

CHARACTERIZATION AND PARTIAL PURIFICATION OF THE RAT AND HUMAN ENZYME SYSTEMS ACTIVE IN THE REDUCTION OFN-HYDROXYMELAGATRAN AND BENZAMIDOXIME

The enzymic basis for intracellular reduction of N-hydroxylated amidines to their corresponding amidines, and hydroxylamines to their corresponding amines, is unknown. The hydroxylated amidines can be used as prodrug moieties, and an understanding of the enzyme system active in the reduction can contribute to more efficient drug development. In this study, we examined the properties of this enzyme system using benzamidoxime and N-hydroxymelagatran as substrates. In rats and humans, the hepatic enzyme system was localized in mitochondria as well as in microsomes, using preferably NADH as cofactor. Potassium cyanide, N-methylhydroxylamine, p-hydroxymercuribenzoate, and desferrioxamine were efficient inhibitors, whereas typical cytochrome P450 (P450) inhibitors were ineffective. In rats, the highest specific activity was found in liver, adipose tissue, and kidneys, whereas in humans, the specific activity in the preparations of adipose tissue examined was lower. A sex difference was observed in rat liver, where 4-fold higher activity was seen in microsomes from female rats. No gender differences were present in any other tissue investigated. Partial purification of the hepatic system was achieved using polyethylene glycol fractionation followed by Octyl Sepharose chromatography at low detergent concentrations, whereas the enzyme was denatured after complete solubilization. The unique appearance of the enzyme activity in adipose tissue, together with the cyanide sensitivity and the failure of typical P450 inhibitors to impede the reaction, indicates that the enzyme system active in reduction of benzamidoxime and N-hydroxymelagatran formation is not of cytochrome P450 origin, but likely consists of an NADH-dependent electron transfer chain with a cyanide-sensitive protein as the terminal component.

Phospholipids modify substrate binding and enzyme activity of human cytochrome P450 27A1

Cytochrome P450 27A1 (P450 27A1) is an important metabolic enzyme involved in bile acid biosynthesis and the activation of vitamin D3 in mammals. Recombinant P450 27A1 heterologously expressed in Escherichia coli was found to be copurified with phospholipids (PLs). The PL content varied in different preparations and was dependent on the purification protocol. A link between the increased amounts of PLs and deterioration of the enzyme substrate binding properties was also observed. Tandem negative ionization mass spectrometry identified phosphatidylglycerol (PG) as the major PL copurified with P450 27A1. Subsequent reconstitution of P450 into exogenous PG vesicles assessed the effect of this contamination on substrate binding and enzyme activity. Two other PLs, phosphatidylethanolamine (PE) and phosphatidylserine (PS), were also tested. PG and PE increased the Kd for 5beta-cholestane-3alpha,7alpha,12alpha-triol and cholesterol binding, whereas PS had no effect on either substrate binding. PG and PE did not significantly alter 5beta-cholestane-3alpha,7alpha,12alpha-triol hydroxylase activity and even stimulated cholesterol hydroxylase activity. PS inhibited 5beta-cholestane-3alpha,7alpha,12alpha-triol hydrolyase activity and had no effect on cholesterol hydroxylase activity. Our study shows the potential for PLs to regulate the activity of P450 27A1 in vivo and alter the amount of cholesterol degraded through the "classical" and "alternative" bile acid biosynthetic pathways.

Genetic evidence that the human CYP2R1 enzyme is a key vitamin D 25-hydroxylase

The synthesis of bioactive vitamin D requires hydroxylation at the 1 alpha and 25 positions by cytochrome P450 enzymes in the kidney and liver, respectively. The mitochondrial enzyme CYP27B1 catalyzes 1 alpha-hydroxylation in the kidney but the identity of the hepatic 25-hydroxylase has remained unclear for >30 years. We previously identified the microsomal CYP2R1 protein as a potential candidate for the liver vitamin D 25-hydroxylase based on the enzyme's biochemical properties, conservation, and expression pattern. Here, we report a molecular analysis of a patient with low circulating levels of 25-hydroxyvitamin D and classic symptoms of vitamin D deficiency. This individual was found to be homozygous for a transition mutation in exon 2 of the CYP2R1 gene on chromosome 11p15.2. The inherited mutation caused the substitution of a proline for an evolutionarily conserved leucine at amino acid 99 in the CYP2R1 protein and eliminated vitamin D 25-hydroxylase enzyme activity. These data identify CYP2R1 as a biologically relevant vitamin D 25-hydroxylase and reveal the molecular basis of a human genetic disease, selective 25-hydroxyvitamin D deficiency.

Assay method for mitochondrial sterol 27-hydroxylase with 7alpha-hydroxy-4-cholesten-3-one as a substrate in the rat liver

Mitochondrial sterol 27-hydroxylase (EC 1.14.13.15) is an important enzyme, not only in the formation of bile acids from cholesterol intermediates in the liver but also in the removal of cholesterol by side chain hydroxylation in extrahepatic tissues. The enzyme has been assayed by complicated methods using radiolabeled substrates or deuterium-labeled tracers. These methods may be inaccurate for measuring enzyme activity, because the amount of electron-transferring proteins may be insufficient for maximal velocity. To solve this problem, after solubilization of the enzyme from rat liver mitochondria with n-octyl-beta-d-glucopyranoside (OGP), we measured the enzyme activity by incubating the solubilized enzyme with saturated amounts of electron-transferring proteins. In our assay system, using 7alpha-hydroxy-4-cholesten-3-one (HCO) as a substrate, we could easily measure the product, 7alpha,27-dihydroxy-4-cholesten-3-one, with HPLC monitoring absorbance at 240 nm. The product formation was proportionate to the time up to 5 min and the protein concentration up to 0.5 mg of protein/ml. The maximal velocity of the enzyme was 1.1 nmol/min/mg of protein, which was 4- to 16-fold higher than previously reported values. A simple and accurate assay method for sterol 27-hydroxylase in rat liver mitochondria is herein described.

On the substrate specificity of human CYP27A1: implications for bile acid and cholestanol formation

The mitochondrial sterol 27-hydroxylase (CYP27A1) is required for degradation of the C27-sterol side chain in bile acid biosynthesis. CYP27A1 seems, however, to have roles beyond this, as illustrated by patients with a deficient sterol 27-hydroxylase due to mutations of the CYP27A1 gene [cerebrotendinous xanthomatosis (CTX)]. These subjects have symptoms ranging from accumulation of bile alcohols and cholestanol to accelerated atherosclerosis and progressive neurologic impairment. The present work describes a detailed investigation on the substrate specificity of recombinant human CYP27A1. In accordance with some previous work with rat liver mitochondria, the activity in general increased with the polarity of the substrate. An obvious example was the finding that cholesterol was 27-hydroxylated more efficiently than cholesterol oleate but less efficiently than cholesterol sulfate. The oxysterols 24S-hydroxycholesterol and 25-hydroxycholesterol were 27-hydroxylated less efficiently than cholesterol, possibly due to steric hindrance. Surprisingly, sterols with a 3-oxo-Delta4 structure were found to be hydroxylated at a much higher rate than the corresponding sterols with a 3beta-hydroxy-Delta5 structure. The rates of hydroxylation of the sterols were: 7alpha-hydroxy-4-cholesten-3-one>4-cholesten-3-one>7alpha-hydroxycholesterol>24-hydroxy-4-cholesten-3-one> cholesterol>25-hydroxy-4-cholesten-3-one>24-hydroxycholesterol>or=25-hydroxycholesterol. The possibility is discussed that the findings may have implications for oxysterol-mediated regulation of gene expression. The very high activity of CYP27A1 towards the cholestanol precursor 4-cholesten-3-one may be of importance in connection with the accumulation of cholestanol in patients with CTX.

Metabolism of 25-hydroxyvitamin D3 by microsomal and mitochondrial vitamin D3 25-hydroxylases (CYP2D25 and CYP27A1): a novel reaction by CYP27A1

The metabolism of 25-hydroxyvitamin D(3) was studied with a crude mitochondrial cytochrome P450 extract from pig kidney and with recombinant human CYP27A1 (mitochondrial vitamin D(3) 25-hydroxylase) and porcine CYP2D25 (microsomal vitamin D(3) 25-hydroxylase). The kidney mitochondrial cytochrome P450 catalyzed the formation of 1alpha,25-dihydroxyvitamin D(3), 24,25-dihydroxyvitamin D(3) and 25,27-dihydroxyvitamin D(3). An additional metabolite that was separated from the other hydroxylated products on HPLC was also formed. The formation of this 25-hydroxyvitamin D(3) metabolite was dependent on NADPH and the mitochondrial electron transferring protein components. A monoclonal antibody directed against purified pig liver CYP27A1 immunoprecipitated the 1alpha- and 27-hydroxylase activities towards 25-hydroxyvitamin D(3) as well as the formation of the unknown metabolite. These results together with substrate inhibition experiments indicate that CYP27A1 is responsible for the formation of the unknown 25-hydroxyvitamin D(3) metabolite in kidney. Recombinant human CYP27A1 was found to convert 25-hydroxyvitamin D(3) into 1alpha,25-dihydroxyvitamin D(3), 25,27-dihydroxyvitamin D(3) and a major metabolite with the same retention time on HPLC as that formed by kidney mitochondrial cytochrome P450. Gas chromatography-mass spectrometry (GC-MS) analysis of the unknown enzymatic product revealed it to be a triol different from other known hydroxylated 25-hydroxyvitamin D(3) metabolites such as 1alpha,25-, 23,25-, 24,25-, 25,26- or 25,27-dihydroxyvitamin D(3). The product had the mass spectrometic properties expected for 4beta,25-dihydroxyvitamin D(3). Recombinant porcine CYP2D25 converted 25-hydroxyvitamin D(3) into 1alpha,25-dihydroxyvitamin D(3) and 25,26-dihydroxyvitamin D(3). It can be concluded that both CYP27A1 and CYP2D25 are able to carry out multiple hydroxylations of 25-hydroxyvitamin D(3).

25-Hydroxylation of vitamin D3 in primary cultures of pig hepatocytes: evidence for a role of both CYP2D25 and CYP27A1

There has been some controversy over whether the 25-hydroxylation of vitamin D(3) is carried out by one enzyme or two and whether this cytochrome P450 enzyme is found in the mitochondrial or microsomal fractions of liver. The pig is currently the only species in which both the microsomal 25-hydroxylase (CYP2D25) and the mitochondrial 25-hydroxylase (CYP27A1) have been cloned and characterized. In this paper, the roles of the two enzymes in 25-hydroxylation of vitamin D(3) are examined in primary cultures of hepatocytes. Inhibition experiments indicated that tolterodine and 7 alpha-hydroxy-4-cholesten-3-one were selective inhibitors of the CYP2D25- and CYP27A-mediated 25-hydroxylation of vitamin D(3), respectively. Addition of each inhibitor to primary hepatocytes decreased the total 25-hydroxylation of vitamin D(3) to about the same extent. No inhibition of other hydroxylase activities tested was found. Phorbol 12-myristate 13-acetate down-regulated the expression of both CYP2D25 and CYP27A1 as well as the 25-hydroxylase activity of the hepatocytes. The results implicate that both CYP2D25 and CYP27A1 contribute to the 25-hydroxylation in hepatocytes and are important in the bioactivation of vitamin D(3).

Two novel mutations in the sterol 27-hydroxylase gene causing cerebrotendinous xanthomatosis

Cerebrotendinous xanthomatosis (CTX) is a rare recessive autosomal disease caused by mutations of the sterol 27-hydroxylase gene. Clinically, CTX is characterized by tendon xanthomas, cataracts and progressive neurological deficits. Because of the disruption of the 27-hydroxylase activity, CTX patients have elevated plasma levels of cholestanol, a by-product of abnormal bile acid synthesis. The present authors describe a female patient with CTX. The proband in this study presented with elevated cholestanol levels, markedly reduced mitochondrial 27-hydroxylase activity and altered bile acid composition. The 27-hydroxylase gene was analysed for mutations by polymerase chain reaction amplification of the exons and the splice-junction regions of the gene. The proband was found to be a compound heterozygote for two different mutations which have not been previously described: (1) a G --> A transition at nucleotide 455 that is responsible for converting a glycine to a glutamic acid residue at amino acid position 112 (G112E); and (2) a five-nucleotide deletion in exon 5 (from nucleotide 965 to 969) that is responsible for a shift in the reading frame and the insertion of a premature codon at position 296, and consequently, the synthesis of a truncated protein lacking the heme-binding and andrenodoxin-binding domains. Long-term (18-year) treatment of the proband with chenodeoxycholic acid (750 mg day-1) has been effective in preventing any progression of the disease.

Genetics of vitamin D biosynthesis and its disorders

The principal hormone regulator of bone mineralization is vitamin D, which must be activated by a metabolic pathway consisting of a 25-hydroxylase and a 1alpha-hydroxylase to yield 1,25 (OH)(2)D. The hormonal regulation of vitamin D activation is at the level of the 1alpha-hydroxylase. We review the biology of vitamin D, the biochemistry of its activation and the molecular biology of the vitamin D-metabolizing enzymes. Recent advances have resulted in the cloning of the human vitamin D 1alpha-hydroxylase and the identification of mutations in its gene that cause Vitamin D Dependent Rickets type I.

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.

Vitamin D 1 alpha-hydroxylase

The rate-limiting, hormonally regulated step in the bioactivation of vitamin D is renal 1 alpha-hydroxylation by P450c1 alpha. In late 1997, we reported the cloning of the human cDNA and gene from keratinocytes, and established that P450c1 alpha mutations cause vitamin D-dependent rickets, type I, while three other groups reported the cloning of the rodent enzyme. The genetics of P450c1 alpha are well established, with studies of over 30 patients, but the molecular mechanisms for the hormonal regulation of P450c1 alpha are still under investigation.

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.

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.

Genetic disorders of vitamin D biosynthesis

The principal hormonal regulator of bone mineralization is vitamin D, which must be activated by a metabolic pathway consisting of a 25-hydroxylase and a 1 alpha-hydroxylase to yield 1,25 dihydroxy vitamin D. The hormonal regulation of vitamin D activation is at the level of the 1 alpha-hydroxylase. This article reviews the biology of vitamin D, and describes the biochemistry of its activation and the molecular biology of the vitamin D-metabolizing enzymes. Recent advances, principally in the authors' laboratories, have resulted in the cloning of the human vitamin D 1 alpha-hydroxylase and the identification of mutations in its gene that cause Vitamin D Dependent Rickets type I.

Biochemical characterization of a truncated form of CYP27A purified from rabbit liver mitochondria

During purification of CYP27A from rabbit liver mitochondria, a cytochrome P450 of different molecular size was co-isolated. The latter enzyme has an apparent M(r) 51,000 which is slightly lower than that of CYP27A. The 51,000-M(r) protein was found to be present in mitochondria from liver, small intestine, kidney, and spleen but not in lung, testis, heart, or brain mitochondria. Determination of the N-terminal sequence revealed that the 51,000-M(r) protein is a truncated form of CYP27A lacking the first 12 residues. The truncated enzyme was less efficient than the full-length CYP27A in the 27-hydroxylation of C(27)-sterols and much less efficient in the 25-hydroxylation of 1alpha-hydroxyvitamin D(3). The K(m) values for cholesterol and 5beta-cholestane-3alpha,7alpha,12alpha-triol were about the same with both enzymes whereas the K(m) for 1alpha-hydroxyvitamin D(3) was much higher with the truncated CYP27A. The results strongly indicate that the 51,000-M(r) protein is formed via proteolytic processing of CYP27A by endogenous protease(s) in some of the tissues examined. The truncation at the N terminus markedly impairs the ability of CYP27A to use 1alpha-hydroxyvitamin D(3) as substrate and to catalyze 25-hydroxylation in the bioactivation of vitamin D(3).

26-cholesterol hydroxylase in rat corpora lutea: A negative regulator of progesterone secretion

From a subtracted cDNA library of rat luteal tissue, where cDNA fragments in functional luteal tissue were subtracted from those in regressing luteal tissue, a cDNA clone corresponding to 26-cholesterol hydroxylase (P450(C26)) was obtained. It is known that P450(C26) catalyzes the conversion of cholesterol to 26-hydroxycholesterol, which blocks cholesterol utilization in the cell, and that 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD) catalyzes the conversion of progesterone to an inactive steroid, 20alpha-dihydroprogesterone (20alpha-OHP). Thus, using pseudopregnant rats as a model, physiological cooperation of P450(C26) and 20alpha-HSD in the reduction of progesterone release toward the end of the luteal phase was evaluated. Levels of P450(C26) and 20alpha-HSD mRNA were examined in corpora lutea from pseudopregnant rats by Northern blot or reverse transcription-polymerase chain reaction or both. P450(C26) mRNA was ubiquitously expressed in corpora lutea, and its expression increased toward the end of pseudopregnancy, while 20alpha-HSD was expressed in all corpora lutea on Day 16 (Day 0 = the day of after cervical stimulation) but not detected before Day 10. An inhibitor of 20alpha-HSD, STZ26 (D-homo-16-oxa-4-androstene-3,16alpha-dione), was administered at various doses to rats from Day 12 to 20, effectively suppressing the elevation of 20alpha-OHP in a dose-dependent manner but not the depletion of progesterone completely. The expression of P450(C26) mRNA was increased as STZ26 dose increased, which negatively correlated with the progesterone levels. These results strongly suggest that P450(C26) cooperated with 20alpha-HSD in the reduction of progesterone release from the rat luteal tissue at the end of the functional luteal phase.

Modulation of cholesterol 7alpha-hydroxylase and sterol 27-hydroxylase activities by steroids and physiological conditions in hamster

Our purpose was to examine the in vitro modulation of liver mitochondrial sterol 27-hydroxylase (S27OHase) and microsomal cholesterol 7alpha-hydroxylase (CH7alphaOHase) activities by certain drugs, sterols, oxysterols and bile acids, and to compare the influence of sex, age, diet and cholestyramine on these activities, in the hamster. In vitro, 7beta-hydroxycholesterol and 5alpha-cholestan-3beta-ol (cholestanol) were strong inhibitors (at 2 microM) of both enzyme activities, while 5beta-cholestan-3alpha-ol (epicoprostanol, 2 microM) and cyclosporin A (20 microM) inhibited S27OHase, but not CH7alphaOHase. These data suggest that a hydroxyl group at the 7alpha position is not required to inhibit CH7alphaOHase and that the presence of an aliphatic CH2-CH-(CH3)2 chain appears to be structurally important for S27OHase activity. Both enzyme activities remained unchanged by hyodeoxycholic acid (40 or 80 microM) while epicoprostanol inhibited only S27OHase and chenodeoxycholic acid only CH7alphaOHase. Adult (9-week old) male or female hamsters displayed similar S27OHase activity but the CH7alphaOHase activity was lower in females than in males, suggesting that the neutral bile acid pathway has a less important role in females. In male hamsters, S27OHase activity did not change with age, while CH7alphaOHase activity significantly increased (one-year vs 9-week old). A semi-purified sucrose-rich (lithogenic) diet significantly lowered both enzyme activities compared to the commercial diet. Cholestyramine induced a stimulation of both enzymes, slightly more vigorously however for the key enzyme involved in the neutral pathway. Taken together, these data indicate that the two enzymes are separately regulated and that certain drugs or steroid compounds can be useful for specifically inhibiting or stimulating the neutral or acidic bile acid pathway.

Identification of CYP3A4 as the major enzyme responsible for 25-hydroxylation of 5beta-cholestane-3alpha,7alpha,12alpha-triol in human liver microsomes

Human liver microsomes catalyze an efficient 25-hydroxylation of 5beta-cholestane-3alpha,7alpha,12alpha-triol. The hydroxylation is involved in a minor, alternative pathway for side-chain degradation in the biosynthesis of cholic acid. The enzyme responsible for the microsomal 25-hydroxylation has been unidentified. In the present study, recombinant expressed human P-450 enzymes have been used to screen for 25-hydroxylase activity towards 5beta-cholestane-3alpha, 7alpha,12alpha-triol. High activity was found with CYP3A4, but also with CYP3A5 and to a minor extent with CYP2C19 and CYP2B6. Small amounts of 23- and 24-hydroxylated products were also formed by CYP3A4. The Vmax for 25-hydroxylation by CYP3A4 and CYP3A5 was 16 and 4.5 nmol/(nmolxmin), respectively. The Km was 6 microM for CYP3A4 and 32 microM for CYP3A5. Cytochrome b5 increased the hydroxylase activities. Human liver microsomes from ten different donors, in which different P-450 marker activities had been determined, were incubated with 5beta-cholestane-3alpha,7alpha, 12alpha-triol. A strong correlation was observed between formation of 25-hydroxylated 5beta-cholestane-3alpha,7alpha,12alpha-triol and CYP3A levels (r2=0.96). No correlation was observed with the levels of CYP2C19. Troleandomycin, a specific inhibitor of CYP3A4 and 3A5, inhibited the 25-hydroxylase activity of pooled human liver microsomes by more than 90% at 50 microM. Tranylcypromine, an inhibitor of CYP2C19, had very little effect on the conversion. From these results, it can be concluded that CYP3A4 is the predominant enzyme responsible for 25-hydroxylation of 5beta-cholestane-3alpha, 7alpha,12alpha-triol in human liver microsomes.